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Chemical Bonding and Fermi Level Pinning at Metal-Semiconductor Interfaces
Abstract
Since the time of Bardeen, Fermi level pinning at metal-semiconductor interfaces has traditionally been attributed to interface gap states. The present work shows that polarized chemical bonds at metal-semiconductor interfaces can lead to the apparent Fermi level pinning effect. Good agreement with various systematics of polycrystalline Schottky barrier height experiments has been found. These findings suggest that chemical bonding is a primary mechanism of the Schottky barrier height.
... Clearly, in all heterostructures, the band structures of MoSi 2 N 4 and MXenes are nearly intact upon the formation of contacts, and no mid-gap states can be seen, indicating typical characters of junctions. 41 According to the formulas 42,43 where U e V and U h V are the SBH for electron and hole, respectively, E F is the Fermi level, and the E CBM and E VBM are the CBM and VBM, respectively, the SBHs are obtained and the corresponding results are listed in Table S1. It turns out that the MXene/MoSi 2 N 4 vertical contacts can be divided into three groups according to the feature of the Schottky barriers, i.e., the n-type Ohmic contact, p-type Ohmic contact, and Schottky contact. ...
... The pinning factor S is an important parameter that is widely used to evaluate the FLP strength for the M/S contacts. 43,61 Figure 5 shows SBH as a function of the work function, from which the pinning factor is calculated as the slope S e/h ¼ jdU e/h /dW M j with S e and S h being the electron-type and hole-type slope, respectively. 42,43,61 Accordingly, the S e and S h in the vertical direction are determined to be 0.53 and 0.55, respectively, which is consistent with previous results without introducing any correction factors in SBH. ...
... 43,61 Figure 5 shows SBH as a function of the work function, from which the pinning factor is calculated as the slope S e/h ¼ jdU e/h /dW M j with S e and S h being the electron-type and hole-type slope, respectively. 42,43,61 Accordingly, the S e and S h in the vertical direction are determined to be 0.53 and 0.55, respectively, which is consistent with previous results without introducing any correction factors in SBH. 48 As for the lateral direction, S e and S h are 0.49 and 0.48, respectively, which are slightly smaller than the corresponding values in the vertical direction, indicating a stronger FLP. ...
Efficient Ohmic contacts are highly preferred in metal/semiconductor (M/S) junctions to achieve the exceptional intrinsic characteristics of the two-dimensional (2D) semiconductor channel. However, due to the strong Fermi level pinning effect, it is difficult to predict the Schottky barrier heights of heterojunctions, especially those between the M/S electrode and the channel region (i.e., the lateral Schottky barrier heights), which severely hampers the rational design of Ohmic contact. Herein, by using first-principles quantum transport simulations, it is found that the difference between the intrinsic band edges of pristine MoSi2N4 and the work function of pristine MXenes plays a major role in the Schottky barrier heights of vertical contacts. Furthermore, phase diagrams of Schottky barrier heights dependent on metal work function are established, which can facilitate the screening of Ohmic contacts. By selecting nine MXene/MoSi2N4 heterojunctions as demos, it is found that W3N2(OH)2 and V2C(OH)2 form n-type Ohmic contacts to MoSi2N4 in both vertical and lateral directions with 100% tunneling probabilities due to their ultralow work functions, while p-type Ohmic contacts are formed between MoSi2N4 and V3C2O2, V4C3O2, or Ti4N3O2 due to their relatively large work functions. Our findings not only demonstrate great potential of fabricating coherent dopant-free all-2D devices based on MXene/MoSi2N4 contacts but also more importantly deliver a general strategy for fast screening efficient Ohmic contacts.
... On the other hand, theoretical approaches-especially parameter-free AIMD and first-principles methods -have also been applied to investigate the interfaces/heterojunctions between crystalline silicon and metals [7,[14][15][16][17][30][31][32]. For example, in the 1970s, Louie et al investigated the interface between crystalline silicon and Al using density-functional theory (DFT) within the local density approximation (LDA) and observed interfacial states in the Si gap [31,32]. ...
... The Schottky-Mott model has been widely used to study the Schottky barrier height (SBH) of metalsemiconductor junctions based on the work function of the metal (f m ) and electron affinity of the semiconductor (χ Si ) [5,7,30,37]. It is defined as follows: ...
... p-Si. With the calculated a-In work function and the experimental Si electron affinity and band gap [5,7,30,37,55,57] we can apply the Schottky-Mott model. According to figures 9(b) and (a) and the Schottky-Mott model [1], crystalline p-Si/a-In heterojunction is intrinsically a Schottky diode with a Schottky barrier height of Φ B,p = ΔE g − (Φ M − χ Si ) = 1.12 − (3.82 − 4.05) = 1.35 eV for a very lightly doped p-Si and 1.25 when we assume Fermi level of Si is pinned slightly higher (0.1 eV) for a moderately doped Si. ...
Metal-Semiconductor (M/S) heterojunctions, better known as Schottky junctions play a crucial role in modern electronics. At present, the mechanisms behind the M/S junctions are still a subject of discussion. In this work, we investigate the interfaces between semiconducting crystalline Si and amorphous metallic indium, Si{0 0 1}/a-In and Si{1 1 1}/a-In using both ab initio molecular dynamics simulations and a Schottky-Mott approach. The simulations reveal the formation of a distinct border between the Si substrates and amorphous In at the interfaces. The In atoms adjacent to the interfaces exhibit atomic ordering. Charge transfer occurs from In to Si, forming c-Si-q/a-In+q charge barriers at the interfaces. This indicates that a crystalline p-Si/a-In heterojunction will have rectifying properties, which agrees with an analysis using the Schottky-Mott model which predicts a Schottky barrier height of 1.3 eV for crystalline p-Si/a-In using the calculated work function for a-In (3.82 eV). We further discuss the interfacial charge transfer, related hole-depletion regions in Si adjacent to the interfaces and the Schottky-Mott approximations.
... 53 Ga 0 . 47 As) exhibits much higher electron mobility than Ge. However, there are other challenges such as small capacitance of inversion channel due to lower DOS, process compatibility with Si, control of stoichiometry, etc. ...
... Here, "intrinsic FLP" means that the FLP is caused by an intrinsic charge transfer due to the metal/semiconductor interface formation itself, while "extrinsic FLP" means that the FLP is caused by a charge transfer through the actual interface states, such as defects introduced by the interface formation process. Metal-induced gap states (MIGS) [42][43][44][45][46] and chemical bond models [47,48] are typical intrinsic FLP mechanisms. In the MIGS model, a charge transfer is caused by a wave function tailing from the metal into the finite semiconductor band gap. ...
Germanium (Ge) is a promising semiconductor as an alternative channel material to enhance performance in scaled silicon (Si) field-effect transistor (FET) devices. The gate stack of Ge FETs has been much improved based on extensive research thus far, demonstrating that the performance of Ge FETs is much superior to that of Si FETs in terms of the on-state current. However, to suppress the performance degradation due to parasitic contact resistance at the metal/Ge interface in advanced nodes, the reduction of the Schottky barrier height (SBH) at the metal/Ge interface is indispensable, yet the SBH at the common metal/Ge interface is difficult to control by the work function of metal due to strong Fermi level pinning (FLP) close to the valence band edge of Ge. However, the strong FLP could be alleviated by an ultrathin interface layer or a low free-electron-density metal, which makes it possible to lower the SBH for the conduction band edge of Ge to less than 0.3 eV. The FLP alleviation is reasonably understandable by weakening the intrinsic metal-induced gap states at the metal/Ge interface and might be a key solution for designing scaled Ge n-FETs.
... It is worth noting that, because of the complex interfacial behavior, the actual Schottky barrier height in the experiment is different from that in equation (1) [49,50]. Therefore, the Schottky barrier height obtained from equation (1) is called the ideal Schottky barrier height. ...
... Tung [49] Interfacial chemical bonds lead to Fermi-level pinning ...
β-Gallium oxide (β-Ga2O3) has been studied extensively in the past decades due to its excellent ability in fabricating variety of devices, such as solar-blind photodetectors and power devices. However, as an important part in device, the related investigation of β-Ga2O3-metal contact, especially for Schottky contact, are rare. In this review, we summarized the recent research progresses on β-Ga2O3-metal contact, including related theories, measurements, fabrication processes, controlment methods, etc. This review will provide insights for both theoretical understanding of the metal/semiconductor interface, and the fabrication process in engineering applications of Ga2O3 based devices.
... The increase in both the low-bias device conductance and the saturation current in Figure 2a suggests that not only does the NW resistance decrease but also the Schottky barriers at the electrical contacts are lowered by strain. The saturation current of a Schottky diode has an exponential dependence on the effective barrier height, 45 ...
... This intrinsic pinning is a natural phenomenon for all semiconductors, not only those of types III-V but also silicon-based semiconductors. [58][59][60][61] Therefore, to suppress sidewall surface recombination, both sidewall treatment and the reduction of the diffusion length of carriers should be pursued. ...
Display technology has developed rapidly in recent years, with III-V system-based micro-light-emitting diodes (uLEDs) attracting attention as a means to overcome the physical limitations of current display systems related to their lifetime, brightness, contrast ratio, response time, and pixel size. However, for uLED displays to be successfully commercialized, their technical shortcomings need to be addressed. This review comprehensively discusses important issues associated with uLEDs, including the use of the ABC model for interpreting their behavior, size-dependent degradation mechanisms, methods for improving their efficiency, novel epitaxial structures, the development of red uLEDs, advanced transfer techniques for production, and the detection and repair of defects. Finally, industrial efforts to commercialize uLED displays are summarized. This review thus provides important insights into the potential realization of next-generation display systems based on uLEDs.
... These transport features indicate the metal-like character of ultrathin PtSe 2 due to defect-induced mid-gap states 24 . This can also explain why photovoltage at the interface is not prominent in the defective sample: the photovoltaic effect is negligible due to a smaller Schottky barrier height as a result of Fermi-level pinning and defect states 25 . ...
Sunlight is widely seen as one of the most abundant forms of renewable energy, with photovoltaic cells based on pn junctions being the most commonly used platform attempting to harness it. Unlike in conventional photovoltaic cells, the bulk photovoltaic effect (BPVE) allows for the generation of photocurrent and photovoltage in a single material without the need to engineer a pn junction and create a built-in electric field, thus offering a solution that can potentially exceed the Shockley–Queisser efficiency limit. However, it requires a material with no inversion symmetry and is therefore absent in centrosymmetric materials. Here, we demonstrate that breaking the inversion symmetry by structural disorder can induce BPVE in ultrathin PtSe 2 , a centrosymmetric semiconducting van der Waals material. Homogenous illumination of defective PtSe 2 by linearly and circularly polarized light results in a photoresponse termed as linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE), which is mostly absent in the pristine crystal. First-principles calculations reveal that LPGE originates from Se vacancies that act as asymmetric scattering centers for the photo-generated electron-hole pairs. Our work emphasizes the importance of defects to induce photovoltaic functionality in centrosymmetric materials and shows how the range of materials suitable for light sensing and energy-harvesting applications can be extended.
... The average surface potential value of the reference was 65 meV, whereas for the buried interface with TPATC, it reached the value of 546 meV. We attribute this significant increase in surface potential to the influence of Fermi Level Pinning (FLP) [77][78][79] This long-debated topic plays a crucial role in aligning energy levels at semiconductor interfaces with metals, insulators, and heterostructures [80,81]. The FLP process can be elucidated by disorder-induced gap states (DIGS) originating from the electronic properties of semiconductors at the contact surface [81]. ...
... Unfortunately, the contact between conventional bulk metal electrodes and a 2D channel material has inherent problems. Usually, a Schottky barrier is formed in their interfacial region due to the Fermi level pinning [19], mainly caused by the generation of metal-induced gap states (MIGS) [20] and the formation of interface dipole. As a result, it gives rise to high contact resistance, which dramatically affects the efficiency of the carrier injection and eventually leads to a degradation of device performance. ...
Monolayer blue phosphorene (BlueP) has attracted much interest as a potential channel material in electronic devices. Searching for suitable two-dimensional (2D) metal materials to use as electrodes is critical to fabricating high-performance nanoscale channel BlueP-based field effect transistors (FETs). In this paper, we adopted first-principles calculations to explore binding energies, phonon calculations and electronic structures of 2D metal-BlueP heterojunctions, including Ti3C2-, NbTe2-, Ga(110)- and NbS2-BlueP, and thermal stability of Ti3C2-BlueP heterojunction at room temperature. We also used density functional theory coupled with the nonequilibrium Green function method to investigate the transport properties of sub-5 nm BlueP-based FETs with Ti3C2-BlueP electrodes. Our calculated results indicate that Ti3C2-BlueP has excellent thermal stability and may be used as a candidate electrode material for BlueP-based FETs. The double-gate can more effectively improve the device performance compared with the single-gate. The estimated source leakage current of sub-5 nm transistors reaches up to 369 µA µm⁻¹, which is expected to meet the requirements of the international technology roadmap for semiconductors for LP (low-power) devices. Our results imply that 2D Ti3C2 may act as an appropriate electrode material for LP BlueP-based FETs, thus providing guidance for the design of future short-gate-length BlueP-based FETs.
... Even though this is a simple method for reducing subthreshold slope in several transistor architectures [21,22], it comes with a serious disadvantage of increasing the leakage current at the metal/semiconductor diodes of the source and drain regions, which is even worse at higher temperatures [23,24]. Additionally, the SBH at a metal/semiconductor interface is not easy to control given the Fermi level pinning effect that occurs when the Fermi level of the metal is fixed somewhere in between the gap of the semiconductor [25] and that is caused by the many interfacial defects present at this interface [26]. Even though there have been immediate solutions to minimize Fermi level pinning at metal/semiconductor interfaces (mainly by the introduction of ultra-thin high-dielectric constant metal oxides [27]), these solutions complicate the transistor processing while increasing its total thermal budget, thus preventing introduction of these devices in advanced BEOL stages of an integrated circuit [27]. ...
By using a simple device architecture along with a simple process design and a low thermal-budget of a maximum of 100°C for passivating metal/semiconductor interfaces, a Schottky barrier MOSFET device with a low subthreshold slope of 70 mV/dec could be developed. This device is enabled after passivation of the metal/silicon interface (found at the source/drain regions) with ultra-thin SiOx films, followed by the e-beam evaporation of high- quality aluminum and by using atomic-layer deposition for HfO 2 as a gate oxide. All of these fabrication steps were designed in a sequential process so that a gate-last recipe could minimize the defect density at the aluminum/silicon and HfO 2 /silicon interfaces, thus preserving the Schottky barrier height and ultimately, the outstanding performance of the transistor. This device is fully integrated into silicon after standard CMOS-compatible processing, so that it could be easily adopted into Front-End-Of-Line or even in Back-End-Of-Line stages of an integrated circuit, where low thermal budget is required and where its functionality could be increased by developing additional and fast logic.
... Consequently, the electron transfer to the carrier is increasingly impeded with growing surface charge until the newly formed H* species tend to become practically electrically ineffective. The Fermi energy can be tied to the electronic level of the surface donor, which is also referred to as Fermi level pinning [52,53]. The observed decrease in the relative sensitivity of the Pt/TiO 2 system with increasing H 2 content is thus a further indication of the postulated mechanism of action of the spillover sensor. ...
Die Feintrocknung spielt eine Schlüsselrolle für den Einsatz und insbesondere die Einspeisung von Wasserstoff in die Erdgasinfrastruktur. Zur thermischen Regenerierung des Trockenmittels können Radiowellen für eine direkte dielektrische Erwärmung des Schüttbettes eingesetzt werden. In Abhängigkeit von den verwendeten Zeolithen, NaX oder NaY, sind dabei sowohl eine homogene Erwärmung als auch eine selektive Erwärmung der wasserbeladenen Trockenmittelschüttungen möglich. Nach entsprechenden Tests steht nunmehr ein einsatzfähiger druckfester Prototyp für die Wasserstofftrocknung zur Verfügung.
... Consequently, the electron transfer to the carrier is increasingly impeded with growing surface charge until the newly formed H* species tend to become practically electrically ineffective. The Fermi energy can be tied to the electronic level of the surface donor, which is also referred to as Fermi level pinning [52,53]. The observed decrease in the relative sensitivity of the Pt/TiO 2 system with increasing H 2 content is thus a further indication of the postulated mechanism of action of the spillover sensor. ...
Selective measurement of hydrogen in the natural gas grid as large transport and storage infrastructure is a crucial requirement for establishing a hydrogen economy. Cost-effective sensors must face mainly two critical conditions: (i) absence of oxygen contents capable for ensuring the sensor function and (ii) presence of a large surplus of another reducing gas, methane. The developed Pt/TiO2 sensor is based on the combination of the catalytic spillover effect and an impedance measurement. It allows hydrogen detection in a wide concentration range and it is specific for hydrogen while other gases such as methane or carbon dioxide behave like inert gases. The sensor curve with decreasing relative sensitivity for higher hydrogen concentrations and the temperature dependence of the sensor effect are in agreement with the spillover-based sensor principle, where hydrogen is activated on Pt and then spills over onto the TiO2 where it acts as electron donor.
... Their decay lengths, for example for the Au/MoS 2 junction, vary from ∼0.55 nm for midgap states to 2 nm for bandedge states [4], which are named metal-induced gap states (MIGSs) [5][6][7][8][9][10]. Even for the high-quality ideal metal/2D semiconductor interface, MIGSs remain for 3D bulk metal contacts, which lead to Fermi-level pinning (FLP) [8,[11][12][13]. The strength of FLP is characterized by the slope parameter, S = |dϕ/dW |, where ϕ is the Schottky barrier height (SBH) in the MSJ and W is the metal work function [14,15]. ...
The van der Waals interface and interfacial reconstruction between two-dimensional (2D) transition-metal dichalcogenides (TMDCs) and metal electrodes significantly influence their device performance. During the growth of MoS2 and WS2 on metals, the participation of sulfur atoms leads to the Au(100) surface forming a [email protected](100)-(22×22)R45∘ reconstructed phase [Luo, Nat. Commun. 11, 1011 (2020)10.1038/s41467-020-14753-8]. To reveal the nature of the reconstructed interface interactions, here we perform a comparative study for Au/TMDCs junctions with and without Au(100) surface reconstruction by density-functional theory calculations. Metal-induced gap states are apparent in the unreconstructed junction, while with reconstruction, significant quasi-bonding-induced gap states (QBIGSs) appear above the valence band maximum of TMDCs, which are antibonding states from the quasi-bonding interaction at the Au4S4/TMDCs interface. The QBIGS favors the formation of p-type contacts and significantly reduce the p-type Schottky barrier height (SBH). It is anticipated that QBIGS commonly exist at the chalcogenide-reconstructed metal/TMDCs junctions. This study opens a different route for p-type SBH reduction in metal/2D TMDCs junctions.
... Now, one may rightly argue that the Schottky-Mott rule does not always tell the full story. This is because the chemical interactions at the metal-semiconductor interface also contribute to the resulting barrier as opposed to the difference between their work functions alone: for example, Fermi level pinning as a consequence of metal-induced gap states [104][105][106][107][108][109]. As a starting point, photoelectron spectroscopy is a good tool to ascertain the Schottky-barrier formation by measuring band bending at the semiconductor interfaces [101,110,111]. ...
Transition metal trichalcogenides (TMTs) are two-dimensional (2D) systems with quasi-one-dimensional (quasi-1D) chains. These 2D materials are less susceptible to undesirable edge defects, which enhances their promise for low-dimensional optical and electronic device applications. However, so far, the performance of 2D devices based on TMTs has been hampered by contact-related issues. Therefore, in this review, a diligent effort has been made to both elucidate and summarize the interfacial interactions between gold and various TMTs, namely, In 4 Se 3 , TiS 3 , ZrS 3 , HfS 3 , and HfSe 3 . X-ray photoemission spectroscopy data, supported by the results of electrical transport measurements, provide insights into the nature of interactions at the Au/In 4 Se 3 , Au/TiS 3 , Au/ZrS 3 , Au/HfS 3 , and Au/HfSe 3 interfaces. This may help identify and pave a path toward resolving the contemporary contact-related problems that have plagued the performance of TMT-based nanodevices.
Graphical abstract
I – V characteristics of (a) TiS3, (b) ZrS3, and (c) HfS3
... The defined channel has a length of 4 µm, and the width of the electrode is 6 µm. Bi 2 Se 3 flakes are obtained by mechanical exfoliation and then transferred onto the Au electrode to form an ideal Schottky contact without the pinning effect [19]. Subsequently, the SnSe 2 flake is transferred onto the Bi 2 Se 3 flake and Au electrode to form the SnSe 2 /Bi 2 Se 3 heterostructure. ...
Using the inherent properties of a heterostructure, ultrafast photodetectors with high sensitivity can be progressively developed that have the potential to carve a niche among the optoelectronic devices. In this Letter, a heterojunction photodetector based on SnSe2/Bi2Se3 is constructed, and a visible–infrared photoresponse with good sensitivity at room temperature is obtained. The SnSe2/Bi2Se3 photodetector demonstrates a high Iph/Id ratio of 1.2 × 10⁴ at 0 V. Moreover, the high responsivity of 2.3 A/W, detectivity of 1.6 × 10¹¹ Jones, and fast response time of 40 µs are simultaneously achieved. The presented results offer an alternative route for ultrafast photodetectors with high sensitivity.
... 1a, b, respectively. Fermilevel pinning effects and Schottky barriers naturally arise when defect-induced gap states occur at the interface 7,8 . These interfacial barriers dramatically suppress the carrier-injection efficiency by capping the available carrier mobility and presenting large contact resistance in TMDSC FETs. ...
Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the low contact resistance (down to 90 Ωµm in MoS 2 , towards the quantum limit), the high field-effect mobility (up to 358,000 cm ² V ⁻¹ s ⁻¹ in WSe 2 ), and the prominent transport characteristics at cryogenic temperatures. This method also offers possibilities of the local manipulation of atomic structures and electronic properties for TMDSC device design.
... Table 2 shows variation of On/Off ratio with light intensity, which confirms increasing the On/Off ratio with light intensity indicating the photodetectors have good linearity characteristics. The value of interface index S, which is given by S = Φ b/Φm [18] (where Φm is metal work function), was 0.18 and 0.14 for Cd/PSi/c-Si and Ni/PSi/ c-Si, respectively, which indicates the large concentration of structural defects and Fermi level pinning effect in Ni/PSi/c-Si, which results in reducing the photocurrent. The spectral responsivity (R λ ) of the photodetector is given by [19]: ...
In this work, nanocrystalline porous silicon (PSi) was prepared by the photo-electrochemical etching (PECE) technique. A comparison study between the optoelectronic properties of double junctions Ni/PSi/c-Si and Cd/PSi/c-Si photodetectors is reported. The Ni and Cd thin films were deposited on the porous silicon layers by the thermal evaporation technique. The structural, electrical, and optoelectronic properties of Cd/PSi/n-Si and Ni/PSi/c-Si devices were examined at room temperature. The XRD analysis confirmed the formation of the nanocrystalline structure of the PSi layer. Scanning electron microscope (SEM) studies reveal the formation of circular pores with an average diameter of 250 nm. The dark and illuminated I-V characteristics of the photodetectors are investigated at room temperature, and the junction characteristics of the Cd/PSi/c-Si junction are better than those of the Ni/PSi/c-Si junction. The maximum responsivity of the Cd/PSi/c-Si photodetector is 1.47AW⁻¹ at 400 nm, while the maximum responsivity of Ni/PSi/c-Si was about 1.45AW⁻¹ at 540 nm. The external quantum efficiency (EQE) of Cd/PSi/c-Si and Ni/PSi/c-Si photodetectors was 450% and 330% at 400 nm, respectively.
Two-dimensional transition metal chalcogenides (2D-TMDs) have attracted much attention because of their unique layered structure and physical properties for transistor applications. Mechanically transferred metal contacts on these low-dimensional materials or their homogeneous and heterogeneous multilayers have generated huge interest to avoid deposition damages. In this paper, we show that there are large physical gaps at both the edge contact and surface contact between the transferred electrodes and the 2D materials. A method called laser shock induced superplastic deformation (LSISD) is proposed to tackle this issue and enhance the performance of the transistors. The enhancement mechanism was investigated by molecular dynamics (MD) simulations of the nanoforming process, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) characterizations of the interfaces, and density functional theory (DFT) modeling. The force effect of laser shock can reduce the contact gap between metals and semiconductors. The electrical performances of the transistors before and after LSISD, along with MD simulations, are used to find the optimal process parameters. In addition, this paper applies the LSISD method to the short-channel MoS2/graphene vertical transistors to show potential improvement in interface contact and electrical properties. This paper demonstrates the first report on using mechanical force induced by laser shock to enhance metal–semiconductor interfaces and transistor performances.
Two-dimensional (2D) tungsten selenide (WSe2) is promising candidate material for future electronic applications, owing to its potential for ultimate device scaling. For improving the electronic performance of WSe2-based field-effect transistors (FETs), the modification of surface properties is essential. In this study, the seamless structural phase transition in WSe2 lattice is achieved by soft oxygen plasma, regulating the electrical conductance of WSe2-based FETs. We found that during the soft oxygen plasma treatment with optimal processing time, the generated oxygen ions can substitute some selenium atoms and thus locally modify the bond length, inducing 2H → 1T phase transition in WSe2 with seamless interfaces. The mosaic structures have been proven to tailor the electronic structure and increase the hole carrier concentration inside WSe2, significantly increasing the channel conductance of WSe2 FETs. With the further increase of the oxygen plasma treatment time, the creation of more selenium vacancy defects leads to the electronic doping, resulting in the reduction of conductance. Benefiting from the hexagonal boron nitride (h-BN) encapsulation to interrupt the partial structural relaxation from 1T to 2H phase, our WSe2 FET exhibits high electronic stability with conductance of 6.8 × 10−4 S, which is about four orders of magnitude higher than 2H WSe2 (5.8 × 10−8 S). This study could further broaden the WSe2 FETs in applications for functionalization and integration in electronics.
The main focus of this framework is the preparation of CdTe nanocrystalline thin films (similar to 120 nm) on single crystal p-Si wafers (270 mu m) with Miller index (100) using thermal evaporation. Then, the In/n-CdTe/p-Si/Al solar cell was successfully fabricated. The dark I-V characteristics for the fabricated solar cell have been determined in range of 300-375 K and an applied voltage range of - 2 to 2 V. The fabricated solar cell's behavior was thoroughly explained. As a result, the important parameters for the fabricated solar cell such as the rectification ratio RR, the junction resistance R-J, ideality factor of solar cell n, the shunt resistance R-sh, the series resistance R-s, the barrier height created at the interface between the CdTe thin film and the p-Si wafer phi(b), the energy of trap level E-t and the activation energy of carriers's recombination in the depletion region Delta E were determined. Finally, the Poole-Frenkel beta(PF) and Schottky beta(S) parameters were computed.
2D materials have exceptional physical and chemical characteristics, which makes them attractive for wearable technology. These characteristics include high carrier mobility, outstanding mechanical performance, abundant chemistry, and excellent electrostatic tunability. However, due to the high electron doping effect of interfacial charge impurities and intrinsic defects, most reported 2D materials are n‐type. Complementary electronic devices and high‐performance wearable sensors necessitate the development of p‐type 2D semiconductors, which have superior electrocatalytic performance in oxidative processes compared to their n‐type counterparts. This review paper thoroughly accounts for recent advancements in 2D p‐type semiconductor‐based wearable sensors, covering basic understandings, synthesis and fabrication, functional devices, and sensor performance insights. Finally, challenges and future opportunities for 2D p‐type semiconductor‐based wearable sensors are discussed.
Abstract Semiconducting molybdenum ditelluride (MoTe2) is widely reported owing to its favorable electronic and optoelectronic properties. The effective modulation of its electrical characteristics has garnered growing attention in regard to building high‐performance MoTe2‐based complementary devices. However, the inherent Schottky barrier (SB) in MoTe2‐based devices severely inhibits the charge‐carrier injection efficiency, leading to a high contact resistance between MoTe2 and contact metals. Here, an efficient method is presented for reducing the SB height of field‐effect transistors (FETs) based on MoTe2 by in situ potassium modification. Interestingly, the electrons transported from K continuously change the electrical characteristics of MoTe2 FET from ambipolar to n‐type with an improvement of electron mobility of over one order of magnitude. Meanwhile, the contact resistance of MoTe2 FET is significantly decreased from 11.5 to 0.4 kΩ µm. By regulating the modification region spatially, it is possible to create a complementary inverter with a high gain of ≈32 at VDD = 3 V. This research demonstrates a relatively simple method for optimizing the contact for MoTe2‐based devices and tuning the electrical properties of MoTe2 for future high‐performance complementary electronics.
van der Waals heterostructures (vdWHs) open the possibility of creating novel semiconductor materials at the atomic scale that demonstrate totally new physics and enable unique functionalities, and have therefore attracted great interest in the fields of advanced electronic and optoelectronic devices. However, the interactions between metals and vdWHs semiconductors require further investigation as they directly affect or limit the advancement of high-performance electronic devices. Here we study the contact behavior of MoS2/WSe2 vdWHs in contact with a series of bulk metals using ab initio electronic structure calculations and quantum transport simulations. Our study shows that dual transmission paths for electrons and holes exist at the metal-MoS2/WSe2 hetero-bilayer interfaces. In addition, the metal-induced bandgap state (MIGS) of the original monolayer disappears due to the creation of the heterolayer, which weakens the Fermi level pinning (FLP) effect. We also find that the creation of the heterolayer causes a change in the Schottky barrier height (SBH) of the non-ohmic contact systems, whilst this does not occur so easily in the ohmic contact systems. In addition, our results indicate that when Al, Ag and Au are in contact with a MoS2/WSe2 hetero-bilayer semiconductor, a low contact barrier exists throughout the whole transmission process causing the charge to tunnel to the MoS2 layer, irrespective of whether the MoS2 is in contact with the metals as the nearest layer or as the next-nearest layer. Our work not only offers new insights into electrical contact issues between metals and hetero-bilayer semiconductors, but also provides guidance for the design of high-performance vdWHs semiconductor devices.
Glass/Mo/CIGS was produced by the physical vapor deposition method and its morphological and structural characterizations were performed. The forbidden band gap and electron affinity values of the CIGS material were calculated depending on the element ratio of the EDS result obtained for CIGS thin films. The current–voltage (I–V) measurements of Cu/CIGS/Mo, Al/CIGS/Mo, and Zr/CIGS/Mo structures were taken at room temperature. While the Cu/CIGS structure exhibited ohmic behavior, rectifying behavior was observed for Al and Zr contacts. The zero-bias barrier height values for Al/p-CIGS and Zr/p-CIGS devices were calculated as 0.78 and 0.72 eV, respectively. The barrier heights determined from the experimental I−V measurements were lower than the barrier heights calculated using the Schottky–Mott model. When the contact characteristics of Al/CIGS and Zr/CIGS Schottky diodes were compared, Al performed better than Zr.
MXenes emerging as an amazing class of 2D layered materials, have drawn great attention in the past decade. Recent progress suggest that MXene‐based materials have been widely explored as conductive electrodes for printed electronics, including electronic and optoelectronic devices, sensors, and energy storage systems. Here, the critical factors impacting device performance are comprehensively interpreted from the viewpoint of contact engineering, thereby giving a deep understanding of surface microstructures, contact defects, and energy level matching as well as their interaction principles. This review also summarizes the existing challenges of MXene inks and the related printing techniques, aiming at inspiring researchers to develop novel large‐area and high‐resolution printing integration methods. Moreover, to effectually tune the states of contact interface and meet the urgent demands of printed electronics, the significance of MXene contact engineering in reducing defects, matching energy levels, and regulating performance is highlighted. Finally, the printed electronics constructed by the collaborative combination of the printing process and contact engineering are discussed.
Two‐dimensional (2D) monolayer transition metal dichalcogenides (TMDCs) show great promise for the development of snext‐generation light‐emitting devices (LEDs) owning to their unique electronic and optoelectronic properties. The dangling bond‐free surface and direct bandgap structure of monolayer TMDCs allow for near‐unity photoluminescence quantum efficiencies. The excellent mechanical and optical characteristics of 2D TMDCs offer great potentials to fabricate TMDC‐based LEDs featuring good flexibility and transparency. Great progress has been made in the fabrication of bright and efficient LEDs with varying device structures. In this review article, we aim to provide a comprehensive summary on the state‐of‐the‐art progress made in the construction of bright and efficient LEDs based on 2D TMDCs. After a brief introduction on the research background, the preparation of 2D TMDCs used for LEDs is briefly discussed. The requirements and the corresponding challenges to achieve bright and efficient LEDs based on 2D TMDCs are introduced. Thereafter, various strategies to enhance the brightness of monolayer 2D TMDCs are described. Following that, the carrier injection schemes enabling bright and efficient TMDC‐based LEDs along with the device performance are summarized. Finally, the challenges and future prospects regarding the accomplishment of TMDC‐LEDs with ultimate brightness and efficiency are discussed. This article is protected by copyright. All rights reserved
Organic–inorganic or inorganic metal halide materials have emerged as a promising candidate for a resistive switching material owing to their ability to achieve low operating voltage, high on–off ratio, and multi‐level switching. However, the high switching variation, limited endurance, and poor reproducibility of the device hinder practical use of the memristors. In this study, a universal approach to address the issues using a van der Waals metal contact (vdWC) is reported. By transferring the pre‐deposited metal contact onto the active layers, an intact junction between the metal halide and contact layer is formed without unintended damage to the active layer caused by a conventional physical deposition process of the metal contacts. Compared with the thermally evaporated metal contact (EVC), the vdWC does not degrade the optoelectronic quality of the underlying layer to enable memristors with reduced switching variation, significantly enhanced endurance, and reproducibility relative to those based on the EVC. By adopting various metal halide active layers, versatile utility of the vdWC is demonstrated. Thus, this vdWC approach can be a useful platform technology for the development of high‐performance and reliable memristors for future computing.
Developing Ohmic contact systems or achieving low contact resistance is significant for high‐performance semiconductor devices. This work comprehensively investigates the interfacial properties of CrX2N4 (X = C, Si) based field‐effect transistors (FETs) with different metal (Ag, Au, Cu, Ni, Pd, Pt, Ti, and graphene) electrodes by using electronic structure calculations and quantum transport simulations. It is highlighted that the stronger interlayer coupling allows CrC2N4 to form an n‐type Ohmic contact with Ti electrode in the vertical direction. Furthermore, the absence of tunneling barrier at the CrC2N4–Ti interface greatly improves the electron injection efficiency. On the other hand, the studied metals form Schottky contact with CrC2N4 at the lateral interface due to Fermi level pinning (FLP) effects. Surprisingly, the strong FLP effects restrict the Schottky barrier heights of CrSi2N4‐metal contacts to a narrow range. Where Ag, Au, Ni, Pd, Pt, Ti electrodes and Ag, Ti electrodes form ideal ohmic contact with CrSi2N4 in the vertical and lateral directions, respectively, while the other metals form quasi‐ohmic contact. Ti exhibits the highest contact performance as the electrode in both CrC2N4 and CrSi2N4 based FETs. The findings may provide fundamental understanding for designing high‐performance and energy‐efficient FETs based on CrX2N4. Based on first‐principles calculations and nonequilibrium Green's function simulations of CrX2N4 based field‐effect transistors (FETs) with various electrodes, it is found that CrC2N4 forms an Ohmic contact with Ti electrode in the vertical direction. Furthermore, the strong Fermi level pinning effects restrict the Schottky barrier hights of CrSi2N4 based FETs to a narrow range, therefore, quasi‐Ohmic contact or Ohmic contacts are formed.
For emerging wearable chip-based electronics, power loss is a critical concern for micro-nano electronic circuits due to high subthreshold swing (SS) value of 60 mV dec⁻¹ for the conventional transistors. In this review article, a variety of steep slope transistor (SST) architectures based on 2-dimensional (2-D) materials, transition material oxides (TMO) and organic materials for ultra-low SS value and low power consumption are elaborated. Firstly, we have reviewed 2-D materials and their heterostructures for SST applications. The minimum SS value extrapolated from the 2-D materials was 0.25 mV/dec based on avalanche breakdown using InSe/BP heterostructures. Various TMO materials are also explored to optimize the SS value and a minimum SS value of 0.74 mV/dec was recorded for threshold switching devices based on NbO2. Based on organic materials, flexible cellulose memristors reports record low SS value of < 0.24 mV/dec with record low turn-ON voltage thus setting the evolutionary standard for future wearable electronics. Moreover, an overview of memtransistors based on 2-D materials and TMO materials is presented to explore the prominence of neuromorphic devices. The spike-driven switching characteristics, short-term to long-term evolution of the resistance state mimics the efficient learning process in biological synapses. Finally, we have explored the difficulties encountered during the designing of different SST architectures for its industrial applications and future technologies. These explored ideas offer new approaches for developing improved wearable devices with effective carrier manipulation for applications in micro and nanoelectronics.
Yttrium silicide formation and its contact properties on Si(100) have been studied in this paper. By evaporating a yttrium metal layer onto Si(100) wafer in conventional vacuum condition and rapid thermal annealing, we found that YSi2–x begins to form at 350°C, and is stable to 950°C. Atomic force microscopy characterization shows the pinholes formation in the formed YSi2–x film. By current–voltage measurement, the Schottky barrier height (SBH) of YSi2–x diode on p-type Si(100) was shown to be between 0.63 and 0.69eV for annealing temperature from 500 to 900°C. By low temperature current–voltage measurement, the SBH of YSi2–x diode on n-type Si(100) was directly measured and shown to be 0.46, 0.37, 0.32eV for annealing temperature of 500, 600, and 900°C, respectively, and possibly even lower for annealing at 700 or 800°C.
Silicon carbide (SiC) is a promising candidate for applications in high temperature, high voltage, high power, and low-power dissipation devices due to its unique properties like wide band gap, high critical electric field, and high thermal conductivity. However, one of the main bottlenecks hindering the SiC power devices from developing and being put into practical application is the fabrication of good metal/SiC contact. In this review, the research status of Ohmic contact and Schottky contact of SiC device are compared and analyzed. The complicated interface properties and uncontrollable barrier height at metal/SiC interface are revealed. In addition, the research status of metal/SiC contact barrier and interface state properties are analyzed, and the important significance of effective control of interface barrier is highlighted. Furthermore, the research progress of metal/SiC contact interface regulation technology is specially analyzed. The future development directions in the nature of metal/SiC interface states and interface control technology are finally prospected.
Germanium sulfide (GeS) is a layered monochalcogenide semiconductor with a band gap of about 1.6 eV. To verify the suitability of GeS for field-effect-based device applications, a detailed understanding of the electronic transport mechanisms of GeS-metal junctions is required. In this work, we have used conductive atomic force microscopy (c-AFM) to study charge carrier injection in metal-GeS nanocontacts. Using contact current-voltage spectroscopy, we identified three dominant charge carrier injection mechanisms: thermionic emission, direct tunneling, and Fowler-Nordheim tunneling. In the forward-bias regime, thermionic emission is the dominating current injection mechanism, whereas in the reverse-bias regime, the current injection mechanism is quantum mechanical tunneling. Using tips of different materials (platinum, n-type-doped silicon, and highly doped p-type diamond), we found that the Schottky barrier is almost independent of the work function of the metallic tip, which is indicative of a strong Fermi-level pinning. This strong Fermi-level pinning is caused by charged defects and impurities.
Ordered heterojunction nanocap arrays composed of the bilayer film Ag/ZnS were prepared onto ordered two-dimensional polystyrene bead arrays by magnetron control sputtering, and the surface morphologies were tuned by changing the ZnS thickness. When the ZnS thickness varied from 10 to 30 nm with a Ag thickness of 5 nm, the roughness of the bilayer film Ag/ZnS increased obviously. The UV–VIS spectra showed the shifted LSPR peaks with ZnS thickness, which was attributed to the changes of the electron density as confirmed by Hall effect analysis. SERS observations confirmed the charge transfer process for the varied electromagnetic couplings when the ZnS thickness changed.
We study the effects of interface atomic rearrangement on the band alignments of Ge/a-Al2O3/Au structures as a prototype for metal-amorphous oxide-semiconductor systems with a nanometer-thick oxide from first principles. The significant atomic rearrangement at Ge/a-Al2O3 and a-Al2O3/Au interface regions results in a slanted band alignment in the oxide region, which cannot be described by the numerical Schottky–Mott model. The band alignment is explained by considering the interface dipoles due to the interface-work-function change and the interface effective charge density, which were previously dismissed as unimportant. On the other hand, the effect of the interface dipole on the band alignment originating from the interface-charge-density difference is found to be negligible.
High performance edge states-based quantum piezotronic tunneling transistor with MoS2 nanoribbon device architecture at room temperature is demonstrated. The edge states are identified by the tight-binding band calculations. The Fermi energy position related to carrier concentration and tunneling probability are investigated based on quantum mechanics theory. It is found that the tunneling current can be exponentially controlled by piezotronic effect, and the Schottky barrier height can also be modified. The edge states transport behavior is further elucidated by conductance and electronic density distribution with applied strains. The strain sensitivity of the quantum piezotronic transistor can reach over 10³. This study is capable of advancing the design of new generation of transistor devices based on edge states, and providing prospects of realizing high performance room temperature quantum piezotronic devices.
Low-energy electron-loss spectra reveal that a majority of common metals react with CdS, CdSe, and most other covalent semiconductors. Furthermore, barrier heights of metals on individual compound semiconductors exhibit a sharp transition as a function of heat of reaction, increasing dramatically above an experimentally determined critical heat of reaction.
Most measurement techniques of the Schottky barrier height (SBH) contain an inherent assumption of homogeneity. The vast majority of electrical characteristics experimentally obtained from SB junctions actually display clear evidence for inhomogeneities, contradicting the concept of a unique Fermi‐level position which is an essential part of Fermi‐level pinning models. It is also shown that many other basic assumptions of the pinning models are not born out by recent theoretical calculations. A large body of experimental data from well‐characterized epitaxial metal–semiconductor interfaces has firmly established the critical dependence of the SBH on local structure. Thus, understanding the formation of interface structure is likely a prerequisite of any predictive SB theory.
The unified defect model has been successful in explaining a wide variety of phenomena as oxygen or a metal is added to the III–V surface. These phenomena cover a range from a small fraction of a monolayer of adatoms to practical III–V structures with very thick overlayers. The tenets of the unified defect model are outlined, and the experimental results leading to its formulation are briefly reviewed. InP levels 0.4 and 0.1 eV and GaAs levels 0.7 and 0.9 eV below the conduction‐band minimum (CBM) are associated with either missing column III or V elements. In InP, it has been found possible by a number of workers to ’’switch’’ between the two defect levels by variations in surface processing, temperature, and/or selection of the deposited atom. The need to apply the proper concepts for surface and interface chemistry and metallurgy is recognized, and the danger of using solely bulk concepts is emphasized. The reason for this is examined for certain cases on an atomic level. The need for new fundamental attacks on interface interaction is shown. The importance of semiconductor–oxide chemical stability is also recognized and, drawing on a large body of work from several laboratories, it is suggested that there will be more difficulties with ’’native’’ oxides on GaAs than on InP. It is concluded that ’’scientific engineering’’ of interfaces to give optimum performance should be a goal and test of the fundamental work described here. Specific possibilities are discussed for Schottky barriers on III–V’s.
Chemical interaction and Schottky-barrier formation at the metal-MoS2 interface were studied by evaporating metals (Ag, Al, Au, Co, Fe, In, Mn, Pd, Rh, Ti, and V) onto the (0001) basal-plane surface of cleaved molybdenite, and then analyzing the interface with x-ray photoelectron spectroscopy (XPS). Except for Mn, negligible changes were revealed in the Mo (3d5/2) and S (2p3/2) peak shapes, or widths, after deposition. The shifts in the binding energies did not correlate with the electron configuration of the metal but rather with the metal electronegativity, and are interpreted in terms of band bending at the metal-semiconductor interface, rather than chemical reaction. Plots of both Mo and S binding energies versus metal electronegativity yield approximately linear curves with nonzero (positive) slopes, which provide an average ``index of interface behavior'' of S'=1.28+/-0.22. This value is considerably higher than for other covalent semiconductors, which exhibit S'
A dipole-layer approach is presented, which leads to analytic solutions to the potential and the electronic transport at metal-semiconductor interfaces with arbitrary Schottky-barrier-height profiles. The presence of inhomogeneities in the Schottky-barrier height is shown to lead to a coherent explanation of many anomalies in the experimental results. These results suggest that the formation mechanism of the Schottky barrier is locally nonuniform at common, polycrystalline, metal-semiconductor interfaces.
The electronic structure of seven ideal metal/GaAs interfaces is calculated self-consistently, within the local-density approximation. Calculated pinning positions ranged from 0.3 to 1 eV above the valence-band maximum. The metal d electorns are found to play a significant role in the electronic structure of the ideal interface, and in determining the Schottky-barrier height. These calculations contradict models that invoke intrinsic interface states to explain the experimentally observed Fermi-level pinning.
We have found that the electrical properties of carriers across the metal-semiconductor interface for alloyed Zn based metallizations to n- and p-InP are dominated by nanosized non-barrier inhomogeneities. The effective area covered by the nanosized regions is a small fraction of the contact area resulting in high values of the specific contact resistance to p-InP. For n(-)-InP, thermionic emission across nanosized inhomogeneities dominates the carrier flow when T-ann > 440 degrees C. (C) 1998 Elsevier Science B.V.
Most metal–semiconductor contacts are rectifying. For moderately doped semiconductors, the current transport across such Schottky contacts occurs by thermionic emission over the Schottky barrier. The current–voltage characteristics of real Schottky contacts are described by two fitting parameters that are the effective barrier heights ΦBeff and the ideality factors n. Due to lateral inhomogeneities of the barrier height, both parameters differ from one diode to another. However, their variations are correlated in that ΦBeff becomes smaller with increasing n. Extrapolations of such ΦBeff-versus-n plots to the corresponding image-force-controlled ideality factors nif give the barrier heights of laterally homogeneous contacts. They are then compared with the theoretical predictions for ideal Schottky contacts. Data of Si, GaN, GaAs, and CdTe Schottky contacts reveal that the continuum of metal-induced gap states is the fundamental mechanism that determines the barrier heights. However, there are additional but then secondary mechanisms. As an example, contacts with (7×7)i-reconstructed interfaces have smaller barrier heights than diodes with (1×1)i-unreconstructed interfaces. This lowering of the Schottky barrier is caused by the electric dipole associated with the stacking fault in one of the triangular halves of the (7×7) unit mesh. © 1999 American Vacuum Society.
Localized states (Tamm levels), having energies distributed in the "forbidden" range between the filled band and the conduction band, may exist at the surface of a semi-conductor. A condition of no net charge on the surface atoms may correspond to a partial filling of these states. If the density of surface levels is sufficiently high, there will be an appreciable double layer at the free surface of a semi-conductor formed from a net charge from electrons in surface states and a space charge of opposite sign, similar to that at a rectifying junction, extending into the semi-conductor. This double layer tends to make the work function independent of the height of the Fermi level in the interior (which in turn depends on impurity content). If contact is made with a metal, the difference in work function between metal and semi-conductor is compensated by surface states charge, rather than by a space charge as is ordinarily assumed, so that the space charge layer is independent of the metal. Rectification characteristics are then independent of the metal. These ideas are used to explain results of Meyerhof and others on the relation between contact potential differences and rectification.
New results and new microscopic models are presented to describe chemical trends in barrier heights at transition-metal-silicide-silicon interfaces and at nontransition-metal-silicon interfaces, where in the latter case a very thin oxide is present.
The β-SiC/Al interface has been studied using the ab initio pseudopotential method, the conjugate-gradient technique proposed by Bylander, Kleinman, and Lee [Phys. Rev. B 42, 1394 (1990)], and Troullier-Martins soft pseudopotentials [Phys. Rev. B 43, 8861 (1991)]. Ionic and electronic structures at the interface, local density of states, Schottky-barrier heights, and bond adhesion between the two materials were determined for both the silicon-terminated and carbon-terminated interfaces. Results show a distinct difference between the Al-Si and the Al-C interactions effecting all aspects of the chemical bond, as well as bond adhesion. However, bond adhesion for both the Si-terminated and C-terminated interfaces is substantially greater than for nonreactive interfaces such as MgO/Al.
The electronic structures of the two types of NiSi2/Si(111) interface were studied within the local-density approximation using the linear muffin-tin orbitals method in the atomic-sphere approximation. Calculations were done for four supercell sizes. The largest supercell contained 12 Si2 layers and 11 NiSi2 layers. With each large supercell, the difference between the Schottky-barrier heights (SBH’s) of the two types of interface was consistent with experimental values. However, SBH’s depend on the supercell size, although larger supercells have enough layers to screen the interface disturbance. Why SBH’s depend on the cell size is investigated.
We have investigated the role of ionicity in metal-semiconductor Schottky barriers by examining interfaces of increasing semiconductor ionicity. The electronic structure of four separate interfaces consisting of jellium (of A1 density) in contact with the (111) surface of Si and the (110) surfaces of GaAs, ZnSe, and ZnS is investigated through the use of a self-consistent pseudopotential method. The barrier height and the surface density of states in the semiconductor band gap are determined. The phenomenological index of interface behavior S (studied by Kurtin, McGill, and Mead for semiconductors of different ionicity) is discussed in terms of a simple model involving metal-induced states in the semiconductor gap.
Experimental data on metal-semiconductor interfaces are reexamined. It is found that the previously reported abrupt transition between covalent and ionic semiconductors is not that clearly defined and the outcome is diffused by data scattering. Furthermore, the data indicate no saturation of the interface parameter S for S=1. Considering the definition of S, it follows that the true Schottky limit should occur for some number S≈2.0-3.0 rather than for exactly S=1 as previously claimed.
Heteronuclear molecules have electric dipole moments because of electronic charge transfer among the constituent atoms. Quantum mechanical calculations reproduce the values of observed dipoles quite well, but easy-to-use model theories have so far failed to produce dipole moments in agreement with experiment. By combining density-functional theory and classical concepts, we obtain a simple predictive model for charge transfer which overcomes the shortcomings of earlier models based on the concept of electronegativity equalization. It yields dipole moments for many diatomic molecules and for the water molecule that are in satisfactory agreement with experiment. The model has promise as a supplement of classical molecular dynamics simulations for multicomponent polyatomic systems. © 1997 American Institute of Physics.
In this work we investigated the relationship between the integral Schottky barrier height (SBH) obtained from conventional current–voltage (I–V) measurement and the distribution of the local SBH measured by ballistic electron emission microscopy (BEEM) on a nanometer scale length. For this purpose, we investigated inhomogeneous Au/Co/GaAs67P33-Schottky contacts. The samples were prepared by the deposition of a discontinuous Co film on the semiconductor followed by the deposition of a continuous Au film. This provided regions with local presence of one or the other metal (Au or Co) at the metal-semiconductor interface, resulting in mesoscopically extended SBH inhomogeneities. The local SBH distribution as well as the integral SBH depended on the preparation parameter of the Co layer, i.e., on the combination of the substrate temperature (300 or 500 K) and the nominal Co thickness (0, 0.25, 0.5, 0.8, 1.0 nm). For the different preparation parameters, statistical distributions of the local SBH were measured by BEEM. Treating these SBH distributions in terms of a parallel conduction model for the electron transport across the MS interface, we calculated for each preparation parameter an integral SBH and compared it with the measured integral SBH obtained from conventional I–V measurement. The calculated and measured integral SBH’s were in very good agreement, demonstrating clearly the strong influence of the low SBH regions on the electron transport across the interface and therefore on the integral SBH. The SBH values for homogeneous Au/GaAs67P33- and Co/GaAs67P33-Schottky contacts, i.e., with only one sort of metal at the interface, were determined to be ΦSBAu=1180±10 meV and ΦSBCo=1030±10 meV. As with regard to the inhomogeneous Schottky contacts the fraction of area of the MS interface covered by Co increased, the local SBH distributions as well as the integral SBH’s decreased gradually from the value of ΦSBAu to ΦSBCo. © 1998 American Institute of Physics.
The energies of positive and negative ions relative to the neutral atoms are conveniently and accurately expressed for a given atom by a power series in N = n - Z, where n = the number of electrons around the nucleus in a given ionization state and Z = the atomic number of the nucleus. For a neutral atom the electronegativity is defined as χ = (- dE/dN)N = 0 where dE is the energy change which accompanies the change in charge dN and should be expressed in the units energy per electron. Similarly the value of (- dE/dN)N = -1 represents the electronegativity of the singly charged positive ion. The E(N) curve exhibits a discontinuity in slope at N values where there is a transition from one type of atomic orbital to another. If only the first ionization potential and the first electron affinity are known for a given species, χ = (- dE/ dN)N = 0 equivalent to the well-known Mulliken relationship that electronegativity is equal to the average of the ionization potential and the electron affinity.
We report here an approach for predicting charge distributions in molecules for use in molecular dynamics simulations. The input data are experimental atomic ionization potentials, electron affinities, and atomic radii. An atomic chemical potential is constructed by using these quantities plus shielded electrostatic interactions between all charges. Requiring equal chemical potentials leads to equilibrium charges that depend upon geometry. This charge equilibrium (QEq) approach leads to charges in excellent agreement with experimental dipole moments and with the atomic charges obtained from the electrostatic potentials of accurate ab initio calculations. QEq can be used to predict charges for any polymer, ceramic, semiconductor, or biological system, allowing extension of molecular dynamics studies to broad classes of new systems. The charges depend upon environment and change during molecular dynamics calculations. We indicate how this approach can also be used to predict infrared intensities, dielectric constants, and other charge-related properties.
Electrical behaviors at two single-crystal metal-semiconductor interfaces are studied. Schottky-barrier heights of NiSi/sub 2/ layers grown under ultrahigh-vacuum conditions on n-type Si(111) are found to be 0.65 and 0.79 eV for type-A and type-B epitaxial systems, respectively. These results are compared with the proposed theoretical models of Schottky barriers.
The dependence of the barrier height of metal-semiconductor systems upon the metal work function is derived based on the following assumptions: (1) the contact between the metal and the semiconductor has an interfacial layer of the order of atomic dimensions; it is further assumed that this layer is transparent to electrons with energy greater than the potential barrier but can withstand potential across it. (2) The surface state density (per unit area per electron volt) at the interface is a property only of the semiconductor surface and is independent of the metal. The barrier height φBn is defined here as the energy needed by an electron at the Fermi level in the metal to enter the conduction band of the semiconductor.
With the above assumptions, the barrier height for n-type semiconductor-metal contacts is found to be a linear combination of the metal work function φm and a quantity φ0 which is defined as the energy below which the surface states must be filled for charge neutrality at the semiconductor surface. The energy φ0 is measured from the edge of the valence band. For constant surface state density the theoretical expression obtained is φBn=γ(φm−χ)+(1−γ)(Eg−φ0)−Δφn,where χ and Eg are electron affinity and the band gap of the semiconductor, respectively, Δφn is the image force barrier lowering, and γ is a weighting factor which depends mainly on the surface state density and the thickness of the interfacial layer.
The theoretical expression is compared to the presently available φBn VS φm data for Si, GaP, GaAs, and CdS, by fitting the data to straight lines using the method of least squares. The best straight-line fit was obtained for the GaP data, with probable error limits on the slope and intercept of ±0.03 and ±0.13 eV, respectively.
The parameter γ in the theoretical expression is found to range from 0.07 for GaAs to almost unity for the CdS data reported by Goodman indicating weak and strong dependence of the surface barrier height on the metal work function, respectively.
The value of φ0 is roughly a third of the respective band gap energies for Si, GaP, and GaAs, and the surface state density for these semiconductors is found to be in the range 1013−1014 states/cm2/eV, for the experiments cited.
Excessive scatter in the data points for the CdS data of Mead and Spitzer casts doubt on the significance of a straight-line fit for this case. The data of Goodman for CdS obey the Schottky theory for a metal-semiconductor barrier, but this agreement requires a value of the electron affinity χ which is different from the vacuum-photothreshold value measured by other authors.
A detailed study of current transport at the Schottky-type n-InP | poly(pyrrole) interface is presented. At room temperature, this interface exhibits an average quality factor of n=1.02±0.02, a C–V barrier height of qΦ<sub>b</sub><sup>CV</sup>=0.78±0.01 eV , and a surface recombination velocity over two orders-of-magnitude slower than at ideal n-InP metal interfaces. These latter two parameters imply an effective barrier height of 0.9 eV, which is among the highest values ever reported for an n-InP Schottky-type diode. The quality factor increases monotonically with decreasing temperature reaching a value of 1.23 at 98 K. Substantial curvature is also observed in a Richardson plot at reduced temperature. These temperature dependencies can be quantitatively modeled using thermionic emission theory in the presence of barrier inhomogeneities. Standard models, including thermionic emission with image force effects, interfacial layer models with and without surface states, and tunneling, do not adequately explain the temperature dependence of the quality factor and the curvature in the Richardson plot. © 1999 American Institute of Physics.
The energy location for the interface state density N ss minimum of the insulator–semiconductor (I–S) interface and the Fermi‐level pinning position at the metal–semiconductor (M–S) interface are shown to coincide and to lie at the same position of 5.0 eV from the vacuum level for major tetrahedral semiconductors. Neither the unified defect model nor the metal induced gap state model can explain the novel striking correlation between the I–S and M–S interfaces. The correlation as well as the observed peculiar photoionization behavior of the I–S interface are explained by the novel unified disorder induced gap state (DIGS) model where DIGS pin or restrict the movement of the surface Fermi level. The above characteristic energy, E HO , is shown to be the Fermi energy of the DIGS spectrum which is given by the hybrid orbital energy of the sp<sup>3</sup> bond of the host. The DIGS model explains remarkably well the behavior of the M–S interface formed on the bare or oxide covered semiconductor surface as well as the various features of N ss distribution of the I–S interface. The correlation between the DIGS‐free heterojunction (S–S) interface and M–S/I–S interface is explained by the fact that E HO is a universal reference energy level of the host which is invariant under any off‐diagonal interactions, as is evidenced by the alignment of transition metal deep levels, DX centers and EL2 with respect to E HO . Band offset at the S–S interface is proposed to be determined by the alignment of E HO which inevitably involves formation of interface dipole when two E HO levels lie at different positions from the vacuum level.
The influence of different interfacial chemical compositions on the Schottky barrier heights across the Al/GaAs(001) interface is studied using first‐principles local density functional calculations. The barrier heights are calculated for seven different interfacial chemical compositions, including the chemically abrupt As‐terminated and Ga‐terminated interfaces, and also several other interfaces related by Al↔Ga place exchange or containing As antisites. We find p‐type barrier heights ϕ p that vary by 0.4 eV, demonstrating a significant influence of the interface composition on the resulting barrier heights. The barrier height variation is explained by the different chemical bonding at the interface in the various cases. The metal induced gap states (MIGS) of two structures with different barrier heights are compared in order to demonstrate why such states do not result in barrier heights independent of interfacial chemical composition. It is thus suggested that the reason an experimental value of ϕ p =0.65 eV is generally found for Al/GaAs is not due to several possible interfacial structures all having their Fermi level pinned by MIGS at the same value, but rather due to the thermodynamic preference for certain structures over others with significantly different barriers. This proposal offers a potential explanation for recent photoemission experiments that find the Fermi level position in the gap varying by several tenths of a volt as a function of the initial surface structure of the GaAs substrate: some of the samples are likely to have interfacial compositions which are metastable with respect to structures that give the standard barrier heights.
Many Schottky barrier heights have less dependence upon metal work function than the simple work function matching model of this interface proposed by Schottky 40 years ago. The conventional explanation for this result involves the assumption that interface states of high density change their occupancy in response to work function differences and thereby reduce the variation of Schottky barrier heights. Such states could also change their occupancy under bias variations, and transport measurements have been interpreted in this fashion. We show that these latter assumptions also imply that the Schottky barrier height should itself be strongly altered by bias—a conclusion inconsistent with the results of capacitance and current transport experiments.
A semi-empirical model of alloy cohesion involving two material constants for each element is introduced by means of the physical ideas underlying the scheme. The resulting expressions for the heat of formation of binary alloys are presented and their applicability in various extreme situations is discussed. The model is shown to reproduce a vast amount of experimental information on the sign of heats of formation. Detailed comparison with experiment for particular classes of alloys will be presented in the sequels to this paper.
We present theoretical evidence that Schottky-barrier heights may, for certain types of interfaces, be far more sensitive to interface geometry than previously suspected. We have identified two classes of diamond/metal interface orientations, one of which leads to Schottky-barrier heights of order 1 eV, the other to barrier heights that are very small or zero. This striking difference can be traced to details of the gap-state band structure that arise from different bonding mechanisms, for which the precise atomic geometry plays a crucial role.
Various models of Schottky-barrier formation suggest Fermi-level pinning in midgap. Elemen- tary band-structure considerations indicate that, for diamond-structure semiconductors, the physically relevant gap is the indirect gap, corrected for spin-orbit splitting. Schottky-barrier heights for elemental and III-V compound semiconductors can be predicted to 0.1 eV from measured indirect gaps and splittings. The dimensionless pinning strength S¯ is given by the optical dielectric constant. Chemical trends are thus simply explained.
Changes in both the heat of formation of transition-metal silicides and the core-level shifts of various transition metals with silicidation are interpreted in terms of the chemical trend of silicide electronic structure. It is shown that the heat of formation and the core-level shifts are dominated by transition metal d-band occupancy which well characterizes silicide electronic structure. Further, it is also found that the d-band occupancy correlates well with Schottky-barrier heights of silicide-silicon interfaces. This suggests that silicide-Si Schottky-barrier heights are influenced by not only interface properties but also bulk silicide electronic properties.
We have performed an ab initio study of the surface energy and the work function for six close-packed surfaces of 40 elemental metals by means of a Green's-function technique, based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. The results are in excellent agreement with a recent full-potential, all-electron, slab-supercell calculation of surface energies and work functions for the 4d metals. The present calculations explain the trend exhibited by the surface energies of the alkali, alkaline earth, divalent rare-earth, 3d, 4d, and 5d transition and noble metals, as derived from the surface tension of liquid metals. In addition, they give work functions which agree with the limited experimental data obtained from single crystals to within 15%, and explain the smooth behavior of the experimental work functions of polycrystalline samples as a function of atomic number. It is argued that the surface energies and work functions calculated by present day ab initio methods are at least as accurate as the experimental values.