A new self-aligned poly-Si TFT has been fabricated by employing a photoresist backside exposure technique. A pre-patterned aluminum (Al) layer on a-Si film not only induces the lateral grain growth (∼1.6 μm) in excimer laser crystallization but also implements the selfalignment of the gate region with the lateral grain region. Photoresist backside exposure through poly-Si film has been successfully performed because crystallized poly-Si has a fairly high UV transmittance. A self-aligned poly-Si TFT with a single grain boundary within the 2 μm channel was successfully fabricated and high on/off current ratio (∼107) was obtained.

X-Ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and spectral ellipsometry have been used to study sub-35 Å low temperature oxynitrides of SiGe. The oxynitridations steps have been performed at 550°C and 650°C, while the oxynitridation feed gases have been preheated to 900°C and 1000°C, respectively, before entering the reaction zone. XPS and SIMS data suggests that NO-assisted oxynitridation incorporates more nitrogen than the N2O-assisted one, while there is minimal Ge segregation towards the dielectric/substrate interface in both oxynitridation processes. SIMS data suggests that the nitrogen is distributed throughout the film in contrast to high temperature Si oxynitridation where nitrogen incorporation takes place near the dielectric/substrate interface. Spectral Ellipsometry has been used to measure the final thickness of the oxynitrides film. These results are discussed in the context of an overall mechanism of the oxynitridation of SiGe.

The microstructure of amorphous alloys of silicon with oxygen, carbon, nitrogen and germanium is studied by effusion measurements of hydrogen and of implanted rare gases helium, neon, argon. The results suggest a general widening of network openings with increasing alloy concentration. Isolated voids disappear for wide bandgap alloys already at low alloy concentration of 5–10 at.% presumably because interconnected voids are formed. At an alloy concentration near 20–30 at.%, Ne and H2 become mobile at T < 500°C and at higher temperature/alloy concentration Ar becomes mobile, too. Void formation is mainly attributed to an increase in hydrogen incorporation.

We investigated influence of the beam irradiation conditions with oblique incidence on crystallization of an Si film by a linearly polarized pulse laser in order to enlarge the periodic width of grain boundary. The irradiation conditions are fluence, pulse number and film thickness. We can obtain the periodic width of about 900 nm by increasing the incident angle to 25°. The experimental results suggest that the pulse number and the film thickness should be controlled properly as well as fluence in order to produce large grain stably for the oblique incidence. The detail of these conditions was discussed.

This paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.

Recent electron spin resonance (ESR) results relating to (1) recombination processes for optically excited electrons and holes in tetrahedrally coordinated amorphous semiconductors and (2) kinetics of metastable defects (dangling bonds associated with the Staebler-Wronski effect) in hydrogenated amorphous silicon (a-Si:H). With regard to recombination processes, ESR measurements have been performed over a wide range of excitation intensities (nW/cm2 to W/cm2) on hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous germanium (a-Ge:H). The kinetics can be studied down to carrier densities as low as 1014 cm-3. The longtime decay curves show that at large carrier separation (1) the random distribution of optically excited electrons and holes is subject to the condition of charge neutrality, and (2) the decays are universal and independent of the densities of localized, band-tail states. With regard to the metastable defects in a-Si:H, the kinetics of the production and thermal annealing of silicon dangling bonds have been measured at temperatures between 25 and 480 K using ESR. Below about 150 K the measurement of the dangling bonds is masked by long-lived, band tail carriers that accumulate with time. The production rate for silicon dangling bonds decreases with decreasing temperature and is nearly temperature independent below approximately 100 K. Defects created by 10 hours of irradiation below 100 K anneal almost completely at 300 K. In a- Ge:H, the first measurements of optically induced, metastable germanium dangling bonds have been made.

The properties of hydrogen in disordered silicon are of great interest due to its beneficial and deleterious aspects. This paper highlights some of the manifold properties of hydrogen in silicon. The influence of dopants on hydrogen diffusion in poly-Si, laser-induced dehydrogenation and crystallization of amorphous silicon, and hydrogen bonding in a-Si:H, poly-Si, and c-Si are reviewed.

A metal-free germanium-induced crystallization technique is reported to realize poly-silicon films on glass at temperatures as low as 480°C. The reduction in crystallization temperature is achieved by using ultra-violet illumination during thermal treatment of the amorphous silicon layer. The annealed samples have been studied using SEM and XRD analyses. Also the electrical conductivity of samples with various annealing conditions has been examined. Conductivity of the samples annealed in the presence of ultra-violet exposure shows more than three orders of magnitude improvement compared to that of amorphous films. Samples treated without UV exposure do not show much enhancement. Size of poly-silicon grains is 0.3μm, as revealed using SEM study.

Existing growth mechanisms of hydrogenated amorphous silicon (a-Si:H) and micro-crystalline silicon assume that the growth species SiH3 can diffuse over the hydrogen-saturated Si surface. However, recent calculations suggest that this could not happen. Local density formalism pseudopotential calculations have been carried out of binding of SiH3 to hydrogen terminated (111)Si surfaces. The bound site is not the three-centre Si-H-Si bridging site previously assumed. It has a direct Si-Si bond between the SiH3 and the surface Si, and the surface hydrogen is displaced to a bond centre of a surface Si-Si bond. A bound site confirms conventional models of growth of a-Si:H and microcrystalline Si, in which a mobile growth species creates smooth surfaces.

Hydrogenated microcrystalline silicon (μc-Si:H) solar cells are made using modified veryhigh-frequency (MVHF) glow discharge at deposition rates ∼3-5 Å/s. We find that the solar cells made under certain conditions show degradation in air without intentional light soaking. The short-circuit current drops significantly within a few days after deposition, and then stabilizes. We believe that post-deposition oxygen diffusion along the grain boundaries or cracks is the origin of the ambient degradation. By optimizing the deposition conditions, we have found a plasma regime in which the μc-Si:H solar cells do not show such ambient degradation. The best a-Si:H/μc-Si:H double-junction solar cell has an initial active-area efficiency of 10.9% and is stable against the ambient degradation. The stability data of the solar cells after light soaking are also presented.

In-situ real-time spectroscopic ellipsometry (RTSE) provides detailed information on the evolution of the structural and optical properties of Si:H films during film growth. We have used in-situ RTSE to characterize the film morphology and crystallinity of hot-wire CVD (HWCVD) Si:H films as a function of hydrogen dilution R=[H]/[H+SiH4], substrate temperature Ts, and film thickness db. Transitions from one mode of film growth to another are indicated by abrupt changes in the magnitude of the surface roughness during film growth. The degree of crystallinity of the film can be determined from the bulk dielectric function. We have studied the growth parameter space consisting of R from 0 to 12, Ts from 150°C to 550°C, and db from 0 to 1 um. For each set of R and Ts values, the structural evolution of the film can be characterized by the shape of the surface roughness thickness ds versus bulk thickness db curve. In contrast to studies done by Collins et al on PECVD growth of Si:H films, our studies of HWCVD growth find no conditions where ds remains constant after coalescence of the initial nucleation centers. Most of the films grown within the range of parameters studied exhibit a secondary nucleation and coalescence signature. The transition between a-Si:H and uc-Si:H growth is near the R=3 to R=4 dividing line. Initial coalescence of purely uc-Si:H material doesn't occur until R>8. We have verified the RTsE crystallinity classification using ex-situ Raman scattering.

Thin film transistors(TFT) in microcrystalline and amorphous Si require high quality gate insulators that can be grown at low temperatures. In this paper, we show that one can oxidize Si wafers to produce high quality fluorinated silicon dioxide gate insulator using a low pressure remote plasma. The insulating film was grown on c-Si substrates using a low pressure ECR oxygen plasma, with small quantities of fluorine added to the mixture. Helium was used as the carrier gas for both oxygen and fluorine. The growth temperatures were in the range of 400–450 C. MOS type capacitors were made to judge the quality of the oxide/semiconductor interface, and interface defect densities were measured using capacitance-voltage techniques. It was found that when no fluorine was present in the oxide, the interface defect density was ∼1-2 x 1011/cm2 eV. The addition of F2 to oxygen immediately reduced the defect density by an order of magnitude, to ∼1.5 x 1010/cm2eV. The addition of more F2 slowly increased the defect density. Thermal cycling measurements showed that the semiconductor/oxide interface is very stable under cycling.

This paper presents a fabrication process for vertical thin film transistors (VTFT) based on hydrogenated amorphous silicon (a-Si:H) technology. This process yields VTFTs with an ON/OFF ratio of 105 and a leakage current of the order of 10-13A for a channel length of 1 μm. The device structure, because of significant undercutting formed after dry-etch process (reactive ion etching or RIE), has a channel profile that is skewed as opposed to vertical. This serves to compromise the structural integrity and the electrical performance of the device. Therefore, an anisotropic dry-etch process for this channel profile is being developed. It is found that a CF4/20%H2 gas mixture yields a sharp vertical sidewall profile. In addition, a modified masking process has been developed to produce a rectangular photoresist profile so as to achieve an anisotropic etch profile for the channel region. The effects of the photoresist geometry on the anisotropic dry-etch process will be discussed.

a-Si:H intrinsic films and their solar cells were prepared by DC-PECVD with growth rates of 1, 3, and 10 Å/s. Raman, IR, and PL spectroscopies were used to study the i-layer properties in relation to the solar cell performance. Raman shows an identical TO mode for all the samples, which indicates the local silicon-bonding configuration does not change with the growth rate from 1 to 10 Å/s. IR results show that the hydrogen bonding configuration is monohydride (Si-H) dominated, and the hydrogen content obtained from the 630 cm-1 wagging mode is 12-14 at.%. Surprisingly, a very small absorption strength for the stretching 2000-2100 cm-1 mode was found for some samples deposited at the higher growth rates (3, 10 Å/s). For these same samples, the PL spectra exhibit a red shift. Both the IR and PL results might be related to the same microstructures formed at high growth rates. We found that although the properties of the i-layer varied as the growth rate increased from 1 to 10 Å/s, the performances of the cells were comparable (within about 4%).

An attempt is made to interpret low-frequency noise measured in a-Si:H n-i-n devices based on the assumption that the noise is due to generation-recombination (g-r) processes. The interpretation is based on a spatial distribution of recombination lifetimes of electrons obtained with the help from the AMPS-1D device simulation program. These lifetimes have the same order of magnitude as that of the noise observed. Further, a distribution of lifetimes naturally results from band bending. Simulations of noise spectra are obtained by multiplication of a phenomenological weight factor to the Lorentzian spectral contribution corresponding to the local electron lifetime. There are, however, two problems with this interpretation: (i) we were not able to come up with a reasonable noise description, where the relatively long electron lifetimes were not overshadowed by much shorter times (ii) the current description, where variance is included in a weight factor, is still incomplete, and therefore we can not explain the shape of the weight factor yet.

We have fabricated high-mobility TFTs with large lateral grains and investigated the variation of drain current with increasing temperature. The activation energy of drain current in large grain TFTs is found to be higher under the off-state and lower under the on-state than that in small grain TFTs. The field-effect mobility during the on-state is reduced with increasing temperature due to a lattice scattering inside grain. Because the proposed device has large lateral grain in the channel region, the lattice scattering inside the grain would be dominant, which is similar to single crystal Si MOSFETs.

We have achieved the best open-circuit voltage (Voc = 0.94 V) to-date in hydrogenated amorphous silicon (a-Si:H) photovoltaic cells deposited entirely by hot-wire chemical vapor deposition. The fill factor (FF = 0.74) remained high and a current density of 8-9 mA/cm2 with about 1800 Å i-layer was obtained in our n-i-p cells on untextured stainless-steel substrates. The Voc improvement of about 60 mV in compared to our previous best Voc was obtained by incorporating materials grown with H-dilution close to the phase transition from amorphous to microcrystalline silicon in the i-layer and at the i-p interface. A low substrate temperature of 150°C for the i-layer was also essential, most likely to widen the bandgap of the i-layer. A brief atomic H-treatment after grown the i-layer increases the Voc further by improving the p-i interface. The last 60 Å of the i-layer before p-layer is extremely close to the transition to microcrystallinity, though it remains mainly amorphous. Our p-layers are also close to the phase transition.

We have revisited the issue of p-layer optimization for amorphous silicon (a-Si:H) solar cells, correlating spectroscopic ellipsometry (SE) measurements of the p-layer in the device configuration with light current-voltage (J-V) characteristics of the completed solar cell. Working with p-layer gas mixtures of H2/SiH4/BF3 in rf plasma-enhanced chemical vapor deposition (PECVD), we have found that the maximum open circuit voltage (Voc) for n-i-p solar cells is obtained using p-layers prepared with the maximum possible hydrogen-dilution gas-flow ratio R=[H2]/[SiH4], but without crossing the thickness-dependent transition from the a-Si:H growth regime into the mixed-phase amorphous + microcrystalline [(a+μc)-Si:H] regime for the ∼200 Å p-layers. As a result, optimum single-step p-layers are obtained under conditions similar to those applied for optimum i-layers, i.e., by operating in the so-called “protocrystalline” Si:H film growth regime. The remarkable dependence of the p-layer phase (amorphous vs. microcrystalline) and n-i-p solar cell Voc on the nature of the underlying i-layer surface also supports this conclusion.

Structural models of amorphous silcon nitride, a-Si3N4. consisting of 112-448 atoms were studied using density functional methods. We used continuous random alterating networks with well-defined topology for the respersentation of chemical order in the material as theoretical precursors. The models were optimized within the DFT framework and compared them to one “ab inito derived” model obtained from quenching a hypothetical melt. The strong chemical order is maintained in the network models even after Car-Parrinello molecular dynamic (CPMD) simulations at elevated temperatures for several pico-seconds, In contrast, the “ab initio derived” model exhibits n-N bonds.

The optimized strutures of Si3N4 have between 2.6 and 3.2 g/vm3 and comprise few topological defects only. The dominant defects are ever over-coordinated Si and N atoms and the 2-connected is averaged over a dozen modles is, averaged over a dozen models, about 1%. Some models are even free of three-connected Si. The calculated bulk moduli decrease with decreasing density of the a-Si3N4 model. We furthermore investigated the properties of the material ater alloying elements such as H and O, espically their capacity to reduces interal strain.

A short channel polycrystalline silicon thin film transistor (poly-Si TFT), which has single grain boundary in the center of channel, is reported. The reported poly-Si TFT employs lateral grain growth method through aluminum patterns, which acts as a selective beam mask and a lateral heat sink during the laser irradiation, on an amorphous silicon layer. The electrical characteristics of the proposed poly-Si TFT have been considerably improved due to grain boundary density lowered. The reported short channel poly-Si TFT with single grain boundary exhibits high mobility as 222 cm2/Vsec and large on/off current ratio exceeding 1 × 108.