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Band alignment of Si/SiO 2 , SiC/SiO 2 , and GaN/SiO 2 interfaces. The dashed line at the top of the figure corresponds to the electron affinity of the SiO 2 surface, which is common in all three interfaces. The deviation from the electron affinity model is shown as , and the charge neutrality level is indicated as a dashed line within the band gap. The VBO is determined from the measurements.
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The band alignment at the SiO2-GaN interface is important for passivation of high voltage devices and for gate insulator applications. X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy have been used to observe the interface electronic states as SiO2 was deposited on clean GaN(0001) surfaces. The substrates, grown by metal...
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Citations
... One major problem when comparing DFT results to experiment is the uncertainty in the energy offsets. Literature values of the VBM band offset of SiO 2 and Si from spectroscopy experiments are in the range of 4.3 to 4.7 eV [56][57][58], depending on the measurement techniques used and the sample growth conditions. In the following, we set the Si/SiO 2 VBM band offset to 4.5 eV for all calculations involving the electronic structure. ...
Experiments as well as theoretical calculations indicate that point defects in the amorphous SiO2 layer of electronic devices as well as in optical fibers are responsible for numerous stability and reliability phenomena, including noise, hysteresis, bias temperature instabilities and decreasing transmission efficiency. In addition to the well-known oxygen vacancy, hydrogen related defects such as the hydrogen bridge and the hydroxyl-E′ center have gained a considerable amount of attention in the recent past, as they are suspected to negatively influence the device performance by capturing charge carriers from e.g. both Si and SiC substrates in field effect transistors. Here, we present a thorough ab initio study of these oxide defects and develop a unified description of electron and hole capture processes in a single multi-state model, supported by a comprehensive analysis of various defect parameters like relaxation energies, charge transition levels, (meta-)stability and transition barriers. We show that a single oxide defect can have two different trap levels for both electron and hole capturing processes, which might be the cause of anomalous device degradation phenomena. Based on its low formation energy compared to other defect types, we find that the hydroxyl-E′ center is the most promising defect candidate to explain charge capture processes in Si/SiO2 systems. Furthermore, we argue that the reduced effect of positive bias temperature instability (PBTI) observed in MOS devices compared to its negative counterpart (NBTI) can be explained by the locations of the hydroxyl-E′ centers charge transition levels.
... VBO and CBO values obtained from XPS measurements for the NiO/Si heterojunction are considerably different from those estimated from EAM, specially the VBO value. These differences could provide from the fact that the EAM considers only ideal interfaces, where chemical interactions and inter diffusions of the atoms/ions on both sides of the interface are totally neglected [50]. However, in reality, interfaces should undergo chemical reactions and reconstruction, and might be strained, thus, an heterojunction is far to be an ideal system [50]. ...
... These differences could provide from the fact that the EAM considers only ideal interfaces, where chemical interactions and inter diffusions of the atoms/ions on both sides of the interface are totally neglected [50]. However, in reality, interfaces should undergo chemical reactions and reconstruction, and might be strained, thus, an heterojunction is far to be an ideal system [50]. After EAM, other models have been developed to estimate the band offset at the heterojunctions [51], suggesting that the band alignment is controlled by the charge transfer across the interface and the resulting dipoles. ...
... After EAM, other models have been developed to estimate the band offset at the heterojunctions [51], suggesting that the band alignment is controlled by the charge transfer across the interface and the resulting dipoles. Thus, it is understandable that our experimentally measured band offset values deviate from the EAM estimated values due to the effect of induced dipole formation at the interface, upon charge transfer [50]. ...
We report on the optical properties and the electronic structure of the p-NiO/n-Si heterostructure system, established for optoelectronic applications in the UV spectral region. Pulsed laser deposition was used to deposit p-NiO onto n-Si to form the p-NiO/n-Si heterojunction system. The optical constants (n, k) and the morphological properties were obtained by using a spectroscopic ellipsometry performed in the energy range of 1-5 eV, while the band-offset measurements of the p-NiO/n-Si heterostructure were performed using X-ray photoelectron spectroscopy analysis. Our results show valence and conduction band offsets values between NiO and Si of about 0.34 ± 0.2 eV and 1.68 ± 0.2 eV, respectively. Valence and conduction band offset values assume that a type I band alignment is formed at the p-NiO/n-Si interface, with the conduction band offset being higher than valence band offset. Based on these findings, an energy-band diagram of the heterojunction is constructed to predict the charge transport properties of the p-NiO/n-Si system. We infer that photogenerated holes should experience a lower potential barrier for their transfer across the p-NiO/n-Si interface than photoelectrons, which should experience a larger barrier. Our results pave the way towards the development of these p-NiO/n-Si heterojunctions for UV detectors, solar cells, and self-biased device applications.
... [12][13][14][15][16][17] UPS measurements have been often used to investigate the valence band structure, work function, and electron affinity of the sample surface. [18][19][20] Since the UPS measurements under HeI (21.2 eV) and HeII (40.8 eV) are the surface sensitive analysis as long as a few nm from the surface, 21,22) evaluation of the electronic states at the interface is not so easy, and the removal of the surface contaminants is one of the key issues to measure the electronic states precisely. From the theoretical calculations and the experiment results, it has been discussed that the mean free path for photoelectrons is increased by decreasing the incident photon energy in the region below several tens of eV. ...
... The conduction band offset ΔE C is calculated as the difference between electron affinities χ of ZnO and 6H-SiC: ΔE C =χZnO−χ SiC. The reported electron affinity χ values for 6H -SiC varies from 3.3 to 4.2 eV[45][46][47] and for ZnO it is 4.2-4.52 eV[48][49][50]. ...
In this work properties of ZnO:N/n‐SiC heterojunctions (HJs) and light emitting diodes based on them are studied. The HJs were grown by Molecular Beam Epitaxy. Active nitrogen generated by the rf plasma source was used for p‐type doping. The location of the space charge area on the ZnO:N/n‐SiC interface was revealed by Electron Beam Induced Current (E‐BIC) scans. The diffusion lengths of holes and electrons have been calculated. This paper presents characterization of ZnO:N/n‐SiC HJs and the presence of tunneling related current transport in them as well as contribution of exponentially distributed traps in large voltage bias. Electroluminescence (EL) was measured at ambient pressure by a standard EL system and also at 77K in vacuum by a system utilizing E‐BIC in a scanning electron microscope. Analysis of the light output power at higher current level indicates the limited effect of nonradiative defects in this structure. EL results have been compared with cathodoluminescence spectra. Color temperature for HJs, based on the EL spectra have also been calculated.
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... Of analyzed wide band gap oxides, sufficient data exist to conclude that ALD oxides consisting of Al 2 O 3 /HfO 2 grown at low temperature were stable in contact with investigated semiconductor materials (see Refs. [17,23]), which gave a suitable barrier to the flow of electrical charges from the semiconductor to the inside of the device. The Al 2 O 3 /HfO 2 very thin layer should be a good electron blocking and hole transporting layer for the p-GaN/n-ZnO LED because these dielectrics were a combination of high-k nature and the high conduction-band offset for Al 2 O 3 /ZnO interface was about 3 eV [24] and the relatively low valence-band offset for HfO 2 /GaN interface was about 0.3 eV [25]. ...
The aim of this work was an analysis of structural, electrical and optical properties of n-type ZnO/i/p-type (called as n-i-p) GaN heterostructure with interlayer between these semiconductors. Such junctions are promising structures for light-emitting diodes, light detectors in the range of ultraviolet radiation and photovoltaic cells. This work concentrates on the investigation of the influence of oxide interlayers in optoelectronic structures, and optimization of growth parameters (in the atomic layer deposition — ALD process) of all oxide layers. Composition of HfO2, TiO2, ZrO2 and Al2O3 films was also optimized. The layers were used as intrinsic barriers to improve the performance and realize color shift of the n-ZnO/p-GaN heterojunction LEDs. The rectification ratio of n-i-p junctions was Ion/Ioff = 1 ×10²–10³ for the voltage of 20 V. The color emission of these devices was adjusted from violet to white light in a room temperature depending on the growth parameters and composition of the ALD oxide layers.
... 8) Also, the depth profile of the defect states in dielectric layer on semiconductor can also be evaluated from the change in PYS intensity with dielectric thinning. [9][10][11] Taking into account the reported energy level of valence band tops and the conduction band bottoms from the vacuum level for GaN and related materials, [12][13][14][15] we previously improved a PYS system with a wide measurement energy range from 3.0 to 10.0 eV using two light sources, namely, highly brilliant Xe-arc and D 2 lamps. 16) In this work, the PYS technique was used to evaluate the energy distribution of electronic state density at the wetcleaned epitaxial GaN surface and SiO 2 =GaN interface. ...
... Because the energy level of the valence band top from the vacuum level can be determined from the differences in threshold energies between the highest and lowest kinetic energies of the measured signals with the X-ray excitation energy taken into account, the charge-up effect of the sample surface during the measurement is smaller than that in PYS analysis. 14,15,23) The measured spectra of secondary photoelectrons and valence band signals of the wet-cleaned epitaxial GaN surface with a carrier concentration of ∼2.0 × 10 16 cm −3 are shown in Fig. 4. The result of cleaned Ag(111) is also shown as a reference. During the measurements of secondary photoelectrons, a negative bias of −25.0 V was applied to the sample in order to detect such a low kinetic energy photoelectron with high sensitivity. ...
The energy distribution of the electronic state density of wet-cleaned epitaxial GaN surfaces and SiO2/GaN structures has been studied by total photoelectron yield spectroscopy (PYS). By X-ray photoelectron spectroscopy (XPS) analysis, the energy band diagram for a wet-cleaned epitaxial GaN surface such as the energy level of the valence band top and electron affinity has been determined to obtain a better understanding of the measured PYS signals. The electronic state density of GaN surface with different carrier concentrations in the energy region corresponding to the GaN bandgap has been evaluated. Also, the interface defect state density of SiO2/GaN structures was also estimated by not only PYS analysis but also capacitance–voltage (C–V) characteristics. We have demonstrated that PYS analysis enables the evaluation of defect state density filled with electrons at the SiO2/GaN interface in the energy region corresponding to the GaN midgap, which is difficult to estimate by C–V measurement of MOS capacitors.
... However, this is difficult to assess because there is a significant scatter in experimental IP values. For example, typical photoemission measurements of IP or electron affinity yield the values of IP for GaN between 6.2 and 7.2 eV [39][40][41][42][43][44][45][46], evidently due to varying surface conditions. Therefore, we also calculated the bulk IP of GaN for a range of HSE parameters, following the approach presented by Hinuma et al. [47], using 36-atom GaN slabs and 20-Å layers of vacuum, with surfaces along the nonpolar [1120] direction. ...
The problem of magnesium acceptor in gallium nitride is that experimental photoluminescence measurements clearly reveal a shallow defect state, while most theoretical predictions favor a localized polaronic defect state. To resolve this contradiction, we calculate properties of magnesium acceptor using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, tuned to fulfill the generalized Koopmans condition. We test Koopmans tuning of HSE for defect calculations in GaN using two contrasting test cases: a deep state of gallium vacancy and a shallow state of magnesium acceptor. The obtained parametrization of HSE allows calculations of optical properties of acceptors using neutral defect-state eigenvalues, without relying on corrections due to charged defects in periodic supercells. Optical transitions and vibrational properties of MgGa defect are analyzed to bring the dual (shallow and deep) nature of this defect into accord with experimental photoluminescence measurements of the ultraviolet band in Mg-doped GaN samples.
... It should be noticed that the formation of an interface dipole is usually observed in a GaN-based Schottky barrier. 13,14) In particular, when ITO is used as an electrode, some researchers have also found that the interface dipole could be effectively modulated by performing interface treatment. 15) Finally, in order to make a comparison with previous research from other groups, 7,8,[16][17][18][19][20][21][22][23] the performance of UV LEDs fabricated with MOCVD-ITO TCEs is displayed in Fig. 5. ...
Ultraviolet (UV)-transparent indium tin oxide (ITO) grown by metal–organic chemical vapor deposition (MOCVD) is used as the current-spreading layer for 368 nm AlGaN-based light-emitting diodes (LEDs). By performing in situ contact treatment on the LED/ITO interface, the morphology, resistivity, and contact resistance of electrodes become controllable. Resistivity of 2.64 ' 10%⁴ Ω cm and transmittance at 368 nm of 95.9% are realized for an ITO thin film grown with Sn-purge in situ treatment. Therefore, the high-power operating voltage decreases from 3.94 V (without treatment) to 3.83 V (with treatment). The improved performance is attributed to the lowering of the tunneling barrier at the LED/ITO interface.
... Photoemission measurement techniques, such as XPS and ultraviolet photoelectron spectroscopy (UPS), are powerful for evaluating the energy band alignment results of a gate stack structure such as the valence band (VB) offset at the hetero-interface, the bandgap energy of dielectric thin films, the metal work function, and the electrical dipole moment at the interface. [12][13][14][15][16][17][18] In our previous work, the energy level difference between the vacuum level (VL) and the VB top for Si-based materials [H-terminated Si(100), wet-cleaned SiC(0001) Si-face, and thermally-grown SiO 2 ] has been determined from the dataset of the onset of VB signals and the cut-off energy of secondary photoelectrons in consideration of excitation energy from photoemission. 19) Moreover, electrical dipole moments at SiO 2 =Si, HfO 2 =SiO 2 , Y 2 O 3 =SiO 2 , and Al 2 O 3 =SiO 2 interfaces have been thus far estimated by using this measurement technique, 20,21) and it has also been confirmed that the measured electrical dipole moments of SiO 2 =Si and HfO 2 =SiO 2 interfaces are in good agreement with the electrical properties of C-V characteristics taken for Al-gate MOS capacitors with HfO 2 =SiO 2 stacks. ...
The electrical dipole moment at an ultrathin high-k (HfO2, Al2O3, TiO2, Y2O3, and SrO)/SiO2 interface and its correlation with the oxygen density ratio at the interface have been directly evaluated by X-ray photoelectron spectroscopy (XPS) under monochromatized Al Kα radiation. The electrical dipole moment at the high-k/SiO2 interface has been measured from the change in the cut-off energy of secondary photoelectrons. Moreover, the oxygen density ratio at the interface between high-k and SiO2 has been estimated from cation core-line signals, such as Hf 4f, Al 2p, Y 3d, Ti 2p, Sr 3d, and Si 2p. We have experimentally clarified the relationship between the measured electrical dipole moment and the oxygen density ratio at the high-k/SiO2 interface.
... Cook et al. reported that an n-GaN=SiO 2 interface shows a larger interface dipole than a p-GaN=SiO 2 interface, which is consistent with our theoretical prediction. 38) The other system we deal with is the GaN=Al 2 O 3 system. The formation energy for V O has been reported to be about 8.0 eV, 39) and V O structures that enable electrons to transfer from GaN to Al 2 O 3 have not been reported. ...
We examine the energy band diagram at the n-type GaN (n-GaN)/SiO2 interface and show that electron transfer from n-GaN to SiO2 leads to the formation of negatively charged oxygen vacancies in the SiO2, resulting in the self-formation of an n-GaN/Ga2O3/SiO2 structure. On the other hand, it is difficult to automatically form Ga2O3 at a p-type GaN (p-GaN)/SiO2 interface. This electron-transfer-induced self-formation of Ga2O3 causes an interface dipole, which leads to band bending, resulting in an increase in the conduction band offset between GaN and SiO2. Accordingly, by using this self-forming phenomenon, GaN MOSFETs with lower leakage current can be realized.
