No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Stability of fully deuterated amorphous silicon thin-film transistors
Abstract
The threshold voltage stability of fully deuterated (a- Si : D ) and hydrogenated amorphous silicon (a- Si : H ) thin-film transistors (TFTs) is compared. The difference in the kinetic energy of D <sup>+</sup> and H <sup>+</sup> ions upon impact with the growing surface during radio-frequency plasma-enhanced chemical vapor deposition leads to material having different physical properties for the same nominal deposition conditions. However, a- Si : D and a- Si : H grown at the same growth rate by adjusting the gas pressure have almost identical properties. By using the growth rate as a normalizing parameter for comparing a- Si : H and a- Si : D TFTs, it is shown that there is no difference in the stability of a- Si : D compared with a- Si : H TFTs. This study rules out the possibility of a giant isotopic effect in amorphous silicon TFTs, and supports the model for Si dangling bond defect creation in a- Si : H where the breaking of weak Si–Si bonds is the rate-limiting step.
... The threshold voltage will tend to shift towards the stressing voltage after long periods of time. Therefore, the time taken for the threshold voltage to shift by half of this maximum, t 0.5 , can be taken as a very simple means of quantitatively comparing the defect creation rate in different samples tested under the same temperature and voltage stressing conditions [13]. If TFTs fabricated from a-Si:D and a-Si:H grown at the same pressure are compared, then the deuterated material generally appears to have a lower defect creation rate as shown in the left hand side of Figure 2. ...
... It has previously been reported that ν is ~10 13 Hz for defect removal, which is the same order of magnitude as most bond oscillation frequencies. It is also believed that the breaking of an Si-H bond is the rate limiting step to defect removal. ...
It has been widely observed that thin film transistors (TFTs) incorporating an hydrogenated amorphous silicon (a-Si:H) channel exhibit a progressive shift in their threshold voltage with time upon application of a gate bias. This is attributed to the creation of metastable defects in the a-Si:H which can be removed by annealing the device at elevated temperatures with no bias applied to the gate, causing the threshold voltage to return to its original value. In this work, the defect creation and removal process has been investigated using both fully hydrogenated and fully deuterated amorphous silicon (a-Si:D) TFTs. In both cases, material was deposited by rf plasma enhanced chemical vapour deposition over a range of gas pressures to cover the a-g transition. The variation in threshold voltage as a function of gate bias stressing time, and annealing time with no gate bias, was measured. Using the thermalisation energy concept, it has been possible to quantitatively determine the distribution of energies required for defect creation and removal as well as the associated attempt-to-escape frequencies. The defect creation and removal process in a-Si:H is then discussed in the light of these results.
... These metastable defects can be removed by annealing the device at temperatures over $400 K [2]. The nature of the defect creation and removal processes have been the subject of previous studies [3][4][5]. It is generally accepted that metastable degradation of a-Si:H is due to the creation of dangling bond defects and that hydrogen mediates the passivation of these defects during annealing. ...
... In this study, the variation in the threshold voltage with annealing time was measured for fully hydrogenated and fully deuterated bottom gate, inverted staggered structure TFTs described in [3]. Measurements were performed on an enclosed probe station with a heating stage which is resistively heated by a low noise dc power supply (HP6642A). ...
Thin film transistors (TFTs) utilizing an hydrogenated amorphous silicon (a-Si:H) channel layer exhibit a shift in the threshold volt-age with time under the application of a gate bias voltage due to the creation of metastable defects. These defects are removed by anneal-ing the device with zero gate bias applied. The defect removal process can be characterized by a thermalization energy which is, in turn, dependent upon an attempt-to-escape frequency for defect removal. The threshold voltage of both hydrogenated and deuterated amor-phous silicon (a-Si:D) TFTs has been measured as a function of annealing time and temperature. Using a molecular dynamics simulation of hydrogen and deuterium in a silicon network in the H Ã 2 configuration, it is shown that the experimental results are consistent with an attempt-to-escape frequency of (4.4 ± 0.3) · 10 13 Hz and (5.7 ± 0.3) · 10 13 Hz for a-Si:H and a-Si:D respectively which is attributed to the oscillation of the Si–H and Si–D bonds. Using this approach, it becomes possible to describe defect removal in hydrogenated and deuterated material by the thermalization energies of (1.552 ± 0.003) eV and (1.559 ± 0.003) eV respectively. This correlates with the energy per atom of the Si–H and Si–D bonds.
... The performances of the ns-Si:H TFTs analyzed using different dielectrics and channel lengths are summarized in Tables 4 and 5 and Figure 10. Overall, we notice a significant progress since the first device reported by LeComber et al [11] and the present values are amongst the highest found in the literature for a-Si:H [73][74][75][76][77][78][79] or nanostructured silicon TFTs [80][81][82]. The improvement observed on the TFTs performance is mainly related to two factors: lower growth rate used and the reduced hydrogen bombardment at the growing surface, which reduce defects. ...
This paper reports the performance of small area solar cells, 128 linear integrated position sensitive detector arrays and thin film transistors based on nanostructured silicon thin films produced by plasma-enhanced chemical vapour deposition technique, close to the onset of dusty plasma conditions, within the transition region from amorphous to microcrystalline. The small area solar cells, produced in a modified single chamber reactor, exhibited very good electrical characteristics with a conversion efficiency exceeding 9%. The 128 integrated position sensitive detector arrays, based on a similar pin structure, allow real-time 3D object imaging with a resolution higher than 901 p/mm. The thin film transistors produced exhibited field effect mobility of 2.47 cm(2)/V/s, threshold voltage of 2V, on/off ratio larger than 10(7) and sub-threshold slopes of 0.32 V/decade, which are amongst the best results reported for this type of device.
... The devices were operated in an accumulation Previous studies have shown that hafnium oxide tends to produce small, positive threshold voltages with zinc oxide [7], whereas silicon oxide tends to produce negative threshold voltages [18]. Device stability was investigated by heating the TFTs up to between 100 and 120 • C (the maximum processing temperature that the samples had been exposed to during fabrication) in air and then applying a high gate bias to accelerate any degradation mechanisms, allowing measurements to be made in a reasonable timescale, in a similar fashion to measurements previously reported for amorphous silicon TFTs [23]. ...
A novel rf sputtering technology in which a high density plasma is created in a remote chamber has been used to reactively deposit zinc oxide (ZnO) and indium zinc oxide (IZO) thin films at room temperature from metallic sputtering targets at deposition rates ~50 nm min−1, which is approximately an order of magnitude greater than that of rf magnetron sputtering. Thin film transistors have been fabricated using IZO with a maximum processing temperature of 120 °C, which is defined by the curing of the photoresist used in patterning. Devices have a saturated field effect mobility of 10 cm2 V−1 s−1 and a switching ratio in excess of 106. Gate bias stress experiments performed at elevated temperatures show a consistent apparent increase in the field effect mobility with time, which is attributed to a charge trapping phenomenon.
The stable hydrogen isotope deuterium (D), which is released during the annealing of deuterated silicon nitride films, diffuses through the crystalline silicon and is captured by a thin, amorphous layer of silicon sputtered on the rear surface. We report on the measurement of the concentration of “penetrated” D by secondary ion mass spectrometry to monitor the flux of D diffusing through single-crystalline silicon wafers. The penetrated D content in the trapping layer increases with the annealing time. However, the flux of D injected into the silicon from the silicon nitride layer decreases as annealing time increases.
The stability of fully deuterated amorphous silicon a-Si: D thin-film transistors is compared with their hydrogenated equivalent a-Si: H in terms of gate bias stress. The amorphous silicon channel and silicon nitride gate insulator layers were deposited by radio-frequency plasma-enhanced chemical-vapor deposition. The use of SiD 4 rather than SiH 4 for the deposition of a-Si: D changes the physical properties of the plasma given the same conditions of rf power, pressure, and gas flow rates. Consequently, a higher gas pressure is required to produce a-Si: D at the same growth rate and with similar bulk properties as a-Si: H. It is shown that a-Si: H and a-Si: D deposited at the same growth rate have very similar structural properties. Therefore transistors deposited at the same growth rate may be more sensibly compared to determine the effect of replacing H with D in amorphous silicon without significantly changing the silicon continuous random network. Using this criterion for comparison, no detectable difference is observed between hydrogenated and deuterated transistors in terms of stability under the application of a gate bias. The experimental results rule out the possibility of a giant isotopic effect in amorphous silicon. Furthermore, this result supports the idea that the rate-limiting step for dangling-bond defect creation in amorphous silicon is the breaking of a weak Si–Si bond, rather than breaking of a Si–H bond. © 2005 American Institute of Physics.
Various types of vegetable oil-based organic solvents (VOS), i.e. vegetable oils (corn, canola, sunflower and soybean oils) with and without extractants (di-2-ethylhexylphosphoric acid (D2EHPA) and tributylphosphate (TBP)), were investigated into their potentiality as greener substitutes for the conventional petroleum-based organic solvents to extract Cu(II) from aqueous solutions. The pH-extraction isotherms of Cu(II) using various vegetable oils loaded with both D2EHPA and TBP were investigated and the percentage extraction (%E) of Cu(II) achieved by different types of VOS was determined. Vegetable oils without extractants and those loaded with TBP alone showed a poor extractability for Cu(II). Vegetable oils loaded with both D2EHPA and TBP were found to be the most effective VOS for Cu(II) extraction and, thus, are potential greener substitutes for the conventional petroleum-based organic solvents.
The overall goal of this research is to improve fundamental understanding of the hydrogen passivation of defects in low-cost silicon and the fabrication of high-efficiency solar cells on thin crystalline silicon through low-cost technology development. A novel method was developed to estimate the flux of hydrogen, released from amorphous silicon nitride film, into the silicon. Rapid-firing-induced higher flux of hydrogen was found to be important for higher defect passivation. This was followed by the fabrication of solar cell efficiencies of ~ 17% on low-cost, planar cast multicrystalline silicon. Solar cell efficiencies and lifetime enhancement in the top, middle, and bottom regions of cast multicrystalline silicon ingots were explained on the basis of impurities and defects generally found in those regions. In an attempt to further reduce the cost, high-efficiency solar cells were fabricated on thin crystalline silicon wafers with full area aluminum-back surface field. In spite of loss in efficiency, wafer thinning reduced the module cost. Device modeling was performed to establish a roadmap towards high-efficiency thin cells and back surface recombination velocity and back surface reflectance were identified as critical parameters for high-efficiency thin cells. Screen-printed solar cells on float zone material, with efficiencies > 19% on 300 μm and > 18% on 140 μm were fabricated using a novel low-cost fabrication sequence that involved dielectric rear passivation along with local contacts and back surface field.
We present a new microscopic model for metastable Si dangling bond defect creation in hydrogenated amorphous silicon. Carrier localisation on short, weak Si–Si bonds causes the bond to break. One hydrogen atom from a nearby doubly hydrogenated Si–Si bond (SiHHSi) moves to the Td site of the broken Si–Si bond, leaving the remaining H atom also in a Td site. The net reaction produces two SiHD defects (intimate Si dangling bond and Si–H bond), where both the H atoms are in the Td site. The Td site is energetically favoured over the BC site, for short, weak bonds. The distance between the dangling bonds and the H atoms, in the Td sites, is in the range 4–5 Å. The vast majority of silicon dangling bonds, both metastable and stable, are SiHD, with the H atom in the Td site.
The first hydrogen dilution study of ammonia/silane plasmas tuned to an aminosilane plasma regime is reported. The use of formal statistical experimental design techniques have enabled us to determine the effects that hydrogen dilution level, rf power density and substrate growth temperature have on various film properties. Hydrogen dilution of the ammonia/silane plasma reduced the amino (NH2) content of the deposited nitride films while also driving the intrinsic film stress to compressive values. Increasing the substrate temperature also reduced the amino concentration, but drove the film stress tensile. It was therefore possible to control the film stress and produce nitride films that contained only NH and SiN bonds (with no detectable NH2 or SiH bonds) by choosing a suitable combination of H2 dilution level and growth temperature. Exodiffusion experiments on such ‘bond optimised’ films revealed only one hydrogen evolution peak at temperatures in excess of 900°C, with no ammonia exodiffusion detected.
The hydrogen collision model of light-induced metastability in hydrogenated amorphous silicon is described in detail. Recombination of photogenerated carriers excites mobile H from Si-H bonds, leaving threefold-coordinated Si dangling-bond defects. When two mobile H atoms collide and associate in a metastable two-H complex, the two dangling bonds from which H was emitted also become metastable. The proposed microscopic mechanism is consistent with electron-spin-resonance experiments. Comprehensive rate equations for the dangling-bond and mobile-H densities are presented; these equations include light-induced creation and annealing. Important regimes are solved analytically and numerically. The model provides explanations for both the t1/3 time dependence of the rise of defect density during continuous illumination and the t1/2 time-dependence during intense laser-pulse illumination. Other consequences and predictions of the H collision model are described.
We report experimental results that replacing hydrogen with deuterium during the final wafer sintering process greatly reduces hot electron degradation effects in metal oxide semiconductor transistors due to a new giant isotope effect. Transistor lifetime improvements by factors of 10–50 are observed. A plausible physical theory suggests that the benefits of deuterium use may be general and also applicable to other areas of semiconductor device processing and fabrication. © 1996 American Institute of Physics.
We have studied light-induced degradation in hydrogenated and deuterated amorphous silicon alloy solar cells. Replacing hydrogen with deuterium in the intrinsic layer of the cell improves stability against light exposure. Possible explanations for the improved stability are discussed. © 1997 American Institute of Physics.
The frequency response and quantum efficiency (QE) of silicon‐on‐insulator (SOI) metal‐semiconductor‐metal photodetectors in the near‐infrared (∼800 nm) are greatly enhanced with a simple reactive ion etching to form electrodes inside the interdigitated trenches. Detectors with 1.25 μm trench spacing were fabricated on a SOI substrate with a 6‐μm‐thick silicon top layer. The unique device structure isolates carriers generated deep inside the semiconductor substrate and at the same time provides a highly uniform electric field throughout the active region of the detector, resulting in an instrumentation limited response time of 23 ps at 5 V bias and a −3 dB bandwidth of 2.3 GHz as measured at 790 nm. The dc responsivity is 0.12 A/W, corresponding to an external QE of 18.7% and an internal QE of 88.5%. The large bandwidth and good responsivity at the wavelength of interest, combined with their low operating voltages, make these detectors attractive for use in short‐distance optical communication systems. © 1996 American Institute of Physics.
To improve the bias-induced degradation in hydrogenated amorphous silicon thin film transistor, the hydrogen in the amorphous silicon film should be replaced by deuterium. The stability of deuterated amorphous silicon thin film transistors, i.e., the shifts of threshold voltage and subthreshold swing, is indeed improved compared to that of the hydrogenated ones. This result is consistent with the improvement of the light-induced degradation in deuterated amorphous silicon films and this improvement can be explained by the efficient coupling between the Si–D wagging mode and the amorphous silicon phonon mode. © 1999 American Institute of Physics.
We have studied light-induced photoconductivity degradation in an intrinsic hydrogenated and deuterated amorphous-silicon (a-Si) alloy. Deuterated a-Si turns out to be more stable under light exposure. A possible mechanism is proposed to explain this phenomenon. It is attributed to the highly efficient coupling between the localized Si–D wagging modes ( ∼ 510 cm−1) and the extended Si–Si lattice vibration modes ( ∼ 495 cm−1). The energy released from electron or hole capture at silicon dangling bonds causes localized vibrations of nearby Si–D bonds. The energy dissipates quickly to the background lattice and a higher recombination rate at local sites is needed in deuterated a-Si than in hydrogenated amorphous silicon to accumulate enough energy to break the nearby weak bonds. © 1997 American Institute of Physics.
We have investigated the effect of depositing amorphous silicon (a-Si:H) under α and γ (or dusty) plasma conditions on hydrogen bonding and TFT performance. By infrared measurements three deposition regimes could be identified due to their distinctive absorption curves in the bending mode range of 800 to 900 cm'1. These corresponded to a films deposited below 300°C, γ films deposited below 300°C, and films deposited in either plasma condition at temperatures of 300°C and above. There was a correlation between the a-Si:H material regimes and TFT performance. When the deposition temperature was below 300°C the a-TFTs had a higher field effect mobility than γ-TFTs, but lower stability. For deposition temperatures of 300°C and above the films had more similar properties regardless of whether they were deposited under a or γ conditions. TFT mobilities were the same, but TFTs containing a-Si:H deposited under γ conditions were still more stable. These results show that the mobility and stability of TFTs are optimised for different growth conditions, and that the overall best conditions for TFT manufacture depends upon the specific application.
We investigate the mechanism for Si dangling bond defect creation in amorphous silicon thin film transistors as a result of bias stress. We show that the rate of defect creation does not depend on the total hydrogen content or the type of hydrogen bonding in the amorphous silicon. However, the rate of defect creation does show a clear correlation with the Urbach energy and the intrinsic stress in the film. These important results support a localized model for defect creation, i.e., where a Si–Si bond breaks and a nearby H atom switches to stabilize the broken bond, as opposed to models involving the long-range diffusion of hydrogen. Our experimental results demonstrate the importance of optimizing the intrinsic stress in the films to obtain maximum stability and mobility. An important implication is that a deposition process where intrinsic stress can be independently controlled, such as an ion-energy controlled deposition should be beneficial, particularly for deposition temperatures below 300 °C. © 2000 American Institute of Physics. S0021-89790010001-5
We compared threshold voltage shifts in amorphous Si, microcrystalline Si and polycrystalline Si thin-film transistors TFTs in terms of a recently developed thermalization energy concept for a dangling-bond defect state creation in amorphous Si TFTs. The rate of the threshold voltage shift in microcrystalline Si TFTs was much lower than in amorphous Si TFTs, but the characteristic energy for the process, which we identified as the mean energy to break a SiSi bond, was virtually the same. This suggests that the same basic SiSi bond breaking process was responsible for the threshold voltage shift in both cases. The lower magnitude in microcrystalline Si TFTs was due to a much lower attempt frequency for the process. We interpreted the attempt frequency in amorphous and microcrystalline silicon in terms of the localization length of the electron wavefunction and the effect of stabilizing H atoms being located only at grain boundaries. 2001 Elsevier Science B.V. All rights reserved.
We present a microscopic model for metastable Si dangling-bond defect creation in hydrogenated amor-phous silicon, which is applicable to both light-induced defect creation in solar cells Staebler-Wronski effect and bias-stress-induced defect creation in thin-film transistors. Light or gate bias causes electron-hole pairs or electrons, respectively, to be localized on short, weak Si-Si bonds, which then break. A hydrogen atom, from a neighboring, doubly hydrogenated weak Si-Si bond SiHHSi moves to the T d site of the broken Si-Si bond. The other H atom from the SiHHSi is also located in the energetically favorable T d site. Overall, the reaction produces two SiHD defects. Each SiHD defect is an intimate Si dangling bond and Si-H bond, where the H atom is in the T d site, not the BC site. The distance between the dangling bond and the H atom in the T d site is in the range 4 –5 Å, in agreement with ESR data. The majority of silicon dangling bonds, both metastable and stable, exist as SiHD, with the H atom in the T d site. The microscopic process for defect creation is fairly well localized, requiring only short-range H motion, which proceeds via bond switching between neighboring T d sites. In contrast, the microscopic process for defect removal during thermal annealing involves reequili-bration of H in the a-Si:H network and is a global process involving a large fraction of H atoms. The rate-limiting step for this process is Si-H bond breaking from SiHHSi sites, which accounts for the maximum activation energy of 1.5 eV. We present a revised hydrogen density of states diagram, in line with this process.
A modified Schottky-contact gated-four-probe structure was applied to study the stability of the hydrogenated and deuterated amorphous silicon (a-Si:D) thin-film transistors under various bias conditions. It was found that after 10 V bias stress, the density of gap states generated in both the upper and lower part of the mobility gap of deuterated amorphous silicon is two to twenty times less than those of hydrogenated silicon. Besides, less density of states at the lower part of mobility gap of a-Si:D is generated after 20, 10, and 20 V bias stress. © 2003 American Vacuum Society.
The Raman spectra of hydrogenated and deuterated amorphous silicon films (a- Si:H , a- Si:D ) have been investigated. It is suggested that the asymmetrical broadening of the transverse-optical (TO) Raman peak of a- Si:D compared to the TO Raman peak of a- Si:H results from the coupling between the Si–D wagging mode and the Si–Si TO phonon mode rather than the structural difference. © 1999 American Institute of Physics.
We investigate the scanning tunnelling microscopy-induced H and D atom desorption from Si(100)-(2 × 1):H(D). The desorption of both atoms shows the same energy threshold that corresponds well with the computed σ → σ∗ excitation energy of the SiH group. The H desorption yield, however, is much higher than the D yield. We ascribe this to the greater influence of quenching processes on the excited state of the SiD species. We use wavepacket dynamics to follow the motion of H and D atoms, and conclude that desorption occurs, for the most part, from the ‘hot’ ground state populated by the quenching process. Site-selective excitation-induced chemistry is found in the desorption of H from Si(100)-(3 × 1):H.
We present experimental and theoretical results on the STM-induced SiH bond-breaking on the Si(100)-(2 × 1):H surface. First, we examine the character of the STM-induced excitations. Using density functional theory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that SiH bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the σ→σ∗ transition of SiH. The desorption yield is ∼2.4 × 10−6 H-atoms/electron and is independent of the current. We also find that D-atom desorption is much less efficient than H-atom desorption. Using the isotope effect and wavepacket dynamics simulations we deduce that a very fast quenching process, ∼1015 s−1, competes with desorption. Most of the desorbing atoms originate from the “hot” ground state produced by the quenching process. Most interestingly, excitation at energies below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy range is a strong function of the tunneling current. We propose that desorption is now the result of the multiple-vibration excitation of the SiH bond. Such excitation becomes possible because of the very high current densities in the STM, and the long SiH stretch vibrational lifetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process — the ultimate lithographic resolution.
063513 ͑2005͒ This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at
- J Robertson
- Appl
- Lin
Robertson, J. Appl. Phys. 87, 144 ͑2000͒. 063513-3 Lin et al. Appl. Phys. Lett. 86, 063513 ͑2005͒ This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 131.111.164.128 On: Mon, 22 Dec 2014 04:45:22
- R E Avouris
- A R Walkup
- T C Rossi
- G C Shen
- J R Abeln
- J W Tucker
- Lyding
Avouris, R. E. Walkup, A. R. Rossi, T. C. Shen, G. C. Abeln, J. R.
Tucker, and J. W. Lyding, Chem. Phys. Lett. 257, 148 1996.
- J H Wei
- S C Lee
J. H. Wei and S. C. Lee, J. Appl. Phys. 85, 543 1999.
- D Liu
- A Shih
- S D Chen
- S C Lee
D. Liu, A. Shih, S. D. Chen, and S. C. Lee, J. Vac. Sci. Technol. B 21,
677 2003.
- G C Shen
- J W Abeln
- Lyding
Shen, G. C. Abeln, and J. W. Lyding, Surf. Sci. 363, 368 1996.
- I French
- S Deane
- D Murley
- J Hewett
- I Gale
- M Powell
I. French, S. Deane, D. Murley, J. Hewett, I. Gale, and M. Powell, Mater.
Res. Soc. Symp. Proc. 467, 875 1997.