In this study, the carrier mobility and density for solid phase crystallized (SPC) silicon films fabricated at 600 °C for 48 hours are analyzed by free carrier optical absorption. The carrier mobility is 40 cm2/Vs for SPC films doped with 6×1019-cm−3-phosphorus atoms. This analysis suggests the SPC films have fine crystalline grains closed to single crystalline silicon. In addition, initial carrier density was 3×1019 cm−3, which increased to 6×1019 cm−3by XeCl excimer laser irradiation of 500mJ/cm2. The inactivated regions in SPC films are reduced by laser irradiation. However, the electrical conductivity after laser irradiation for SPC films doped with 6×1018-cm−3-phosphorus atoms decreased from 3.3 to 0.018 S/cm as laser energy density increased to 500mJ/cm2. On the other hand, the electrical conductivity increased from 14.7 to 31.3 S/cm with similar increase of laser energy density after H2O vapor heat treatment at 260°C for 3 hours with 1.3 MPa. Furthermore, the characteristics of n-channel TFTs fabricated with initial SPC films as well as SPC films which was irradiated by laser at 425mJ/cm2 are also researched. The threshold voltage is decreased from 3.8 to 2.0 V by laser irradiation. Threshold voltages of both cases are decreased from 3.8 to 2.4 V for no-laser irradiation and from 2.0 to 0.8 V for laser irradiation, after H2O vapor heat treatment at 310°C for 1 hour with 9.0MPa. Based on the above trial, the defect reduction method combining laser irradiation and H2O vapor heat treatment has proved to be very effective for SPC films and SPC TFTs.

Light induced changes to 1 sun degraded steady state (DSS) have been investigated on hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells and corresponding films fabricated with and without hydrogen dilution of silane. Striking similarities are found for the degradation kinetics, between the electron mobility lifetime (μτ) products and the corresponding fill factors (FF). These correlations that exist for both intrinsic materials at temperatures between 25 and 100°C, are present for the DSS as well as in the kinetics, which exihibit distinctly different dependence on temperature. No such correlations are present between μτ, FF and densities of D0 defects, measured with subgap absorption α(E) at 1.2eV, and electron spin resonance (ESR). The creation of non-D0 defects is also clearly indicated by the temperature dependence of the kinetics and the changes in the shape of α(E) with the results suggesting the presence of more than one mechanism for the creation of light induced defects associated with the Staebler-Wronski effect (SWE).

Si based nanostructures such as Ge or Si1-xGex dots embedded in Si receive a lot of attention. This interest is driven by the reduction of device sizes as well as by their possible use in opto-electronic applications, as a possible solution to improve the radiative light emission. In this paper we give a detailed overview of the growth kinetics as observed for Ge growth in a standard production oriented chemical vapor deposition system. The island morphology and density are controlled by varying the growth conditions or by applying a thermal anneal after the island growth. Island densities up to 2.3x1010 cm−2 have been obtained for depositions at 650°C. With increasing deposition time, the usual change-over from monomodal to bimodal island distribution is pointed out and this change-over depends on the critical island diameter, which decreases with decreasing growth temperature. Applying a thermal budget after the island growth initiates Ge surface diffusion and Si diffusion from the substrate through the islands. This results in an enhancement of the island diameter and height, and also in a reduction of island density. Furthermore, depending on the island distribution after Ge deposition, a transition from pyramid to dome or visa versa is observed after the in-situ anneal. Optical device structures require a Si cap layer. However, Si capping at 700°C, leads to a nearly total dissolution of small islands and a truncation of bigger dome-shaped islands. This can be prevented by reducing the deposition temperature and by changing the Si gas source. Clear island luminescence, is observed up to 200K and lies in the spectral range of 1.35-1.50μm, which is interesting from the point of view of applications. In particular, this shows the potential of this material system for opto-electronic device applications. In spite of the fact that the observed PL intensity is comparable to the best reported values, we could further enhance it by a thermal treatment in a H2 plasma.

Hydrogen dilution is used to promote the nucleation and growth of microcrystalline Si (μc-Si) by plasma enhanced chemical vapour deposition (PECVD). The free energy of μc-Si and hydrogenated amorphous silicon (a-Si:H) is analysed as a function of Si:H composition in order to derive the effect of hydrogen dilution. It is shown that increasing the hydrogen content of the a-SiHx precursor phase increases the relative stability of μc-Si slightly, but strongly increases the driving force for nucleation. The higher stability of μc-Si is the fundamental origin of the higher etch rates of a-Si:H, while surface mobility models do not account for sub-surface nucleation of μc-Si.

Modulated photocurrent (MPC) was employed to study metastable deep defect creation and annealing in low Ge fraction a-Si,Ge:H alloys (Ge fractions below 10at.%). These MPC spectra reveal two distinct bands of deep defects identified as neutral Si and neutral Ge dangling bonds. Upon heating in the dark from a strongly light degraded state, these are reduced in direct competition to each other, therefore implying a global reconfiguration mechanism. We have extended our studies to compare purely thermal annealing with light-induced annealing in these alloys. We have found that the relative anneal rate for the two types of defects differs from the case of purely thermal annealing. This therefore tends to rule out models in which the rate limiting step during annealing comes from the release of the mediating entity from a remote site.

It is usually assumed that optical and electrical properties of amorphous semiconductors are determined by the short-range-order (SRO) parameters of atomic structure. The SRO parameters behavior is considered here as governed by morphology geometrical parameters (MGP) variations. Generalized Skettrup model is developed for a quantitative description of the MGP influence on the density of electron states distribution N(E). Common N(E) relation for the power, exponential and the ‘defect’ regions of the dependence is derived both for adiabatic (optical) and non-adiabatic (thermal) electron excitation processes. The simulated (by the N(E) convolution) spectral dependence of the opti-cal absorption coefficient contains the Tauc, Urbach and the ‘defect’ parts. The optical gap and the Urbach tail slope energies are typical for the ‘device quality’ a-Si:H films at the average morphology geometrical parameters values of order of 1 μm. Nearly linear the optical gap versus the Urbach tail slope energies dependence is obtained at the mor-phology geometrical parameters and temperature changes. Good agreement of experi-mental and the simulation results is achieved for a-Si:H films, prepared both by the RF sputtering and by the silane decomposition.

A systematic study has been made of the annealing kinetics of amorphous silicon (a-Si) alloy solar cells. The cells were deposited at various rates using H2 dilution with radio frequency (RF) and modified very high frequency (MVHF) glow discharge. In order to minimize the effect of annealing during light soaking, the solar cells were degraded under 30 suns at room temperature to quickly reach their saturated states. The samples were then annealed at an elevated temperature. The J-V characteristics were recorded as a function of annealing time. The correlation of solar cell performance and defect density in the intrinsic layer was obtained by computer simulation. Finally, the annealing activation energy distribution (Ea) was deduced by fitting the experimental data to a theoretical model. The results show that the RF low rate solar cell with high H2 dilution has the lowest Ea and the narrowest distribution, while the RF cell with no H2 dilution has the highest Ea and the broadest distribution. The MVHF cell made at 8Å/s withhigh H2 dilution shows a lower Ea and a narrower distribution than the RF cell made at 3 Å/s, despite the higher rate. We conclude that different annealing kinetics plays an important role in determining the stabilized performance of a-Si alloy solar cells.

The production of amorphous and microcrystalline silicon, e.g. for solar cells, requires large area, high-deposition rate plasma reactors. Increasing the frequency from the conventional 13.56MHz up to VHF has demonstrated higher deposition and etch rates and lower particle generation, a reduced ion bombardement and lower breakdown, process and bias voltages. But the use of VHF for large area systems leads to some problems. The non-uniformity of deposition rate increases due to the generation of standing waves and evanescent waveguide modes at the electrode surface. One possibility to process large area substrates is the use of a one-dimensional extended, homogeneous plasma source in combination with a moving substrate. The requirements, which result from the deposition process and from the RF-engineering, corresponds with the developed plasma source, using deposition frequencies in the VHF-range (50-100 MHz), almost perfectly.

Using a source of 550mm length experiments were done with 81.36MHz at RF power densities of 70-180mW/cm2, silane/ hydrogen pressures of 5-30Pa and flow rates of 10-300sccm. The measured potential distribution error was ±2%. Optical emission spectroscopy delivered discharge intensity errors of ±3-10%. Deposition rates up to 20µm/h for amorphous silicon (60Å/s) and film thickness inhomogenities less than ±5% were achieved (with an area of the moved substrate of 30cm–30cm). Experimental results of the film properties will be discussed in relation to the deposition parameters and compared with complementary experiments, carried out on a small scale equipment with excitation frequencies up to 165 MHz.

For many years it has been assumed widely that hydrogen is involved in some way in the formation of light induced defects. However recently some doubt has been cast on this because of experimental evidence that there is little H near light induced dangling bonds. In this paper we present a number of model calculations comparing ESR spectra of dangling bonds with and without correlations with H positions. The above models include different distributions of H and correlations or anti correlations of nearby H. In all cases the spectra are compared to those with no correlations or clustering. Our results coupled with published experimental data strongly suggest that dangling bonds are not correlated positively with the presence of nearby H and, in fact, it appears that light induced dangling bonds are negatively correlated with nearby H.

Hot-Wire deposited amorphous silicon is an excellent material for the incorporation as the absorbing layer in n-i-p solar cells. We decreased the deposition temperature from 430 °C to 250 °C, keeping device quality (opto-)electrical properties of the a-Si:H layers. This enables application of Hot-Wire deposited a-Si:H in p-i-n structures and tandem solar cells. Layers deposited at 250 °C have been applied in efficient n-i-p and n-i-p/n-i-p solar cells. The deposition rate of the intrinsic layer was about 10 Å/s. No optical enhancements, like texturing or back reflectors, were used. Single-junction cells with a fill factor of 0.72 and an open-circuit voltage of 0.89 V have been produced. On a flexible stainless steel substrate, an initial efficiency of 7.2 % was recorded. Tandem cells also show a high fill factor (0.71) and open-circuit voltage (1.70 V), resulting in an initial efficiency of 8.5 %.

New features of the ASA software package are described that allow the analyses of the performance of hydrogenated amorphous silicon (a-Si:H) based solar cells with rough interfaces and buffer and graded layers. Using the updated version of ASA improved understanding of i) the effect of the front and back contact roughness on the external quantum efficiency and the absorption of light in the individual layers of a-Si:H solar cells and ii) the role of band gap grading of the intrinsic a-SiGe:H layer in a solar cell is obtained. The simulated results using the ASA package agree well with measured data on solar cells and are used to give insight into the behavior of the internal parameters of the solar cells.

A phenomenological model for the light-induced metastability of a-Si:H is proposed in which a two-domain model of the amorphous network plays a central role. Boundaries between high and low density domains are associated with a significant fraction of the clustered Si-H in aSi:H. Weakly bonded hydrogen at these boundaries catalyzes metastable local configuration changes in the Si network due to non-radiative carrier recombination, leading qualitatively to both the gross structural changes and the increase in electronic defect density that are observed experimentally.

We have developed a plasma enhanced chemical vapor deposition (PECVD) technique for high-rate growth of µc-Si:H at low temperatures using hydrogen diluted monosilane source gas under high-pressure depletion conditions. It was found that material qualities deteriorate, e.g. crystallinity decreases and defect density increases with increasing growth rate mainly due to ion damage from the plasma. We have found that deuterium dilution improves not only the crystallinity but also defect density as compared to hydrogen dilution and that deuterium to hydrogen ratio incorporated in the film has a good correlation with crystallinity. The advantages of the deuterium dilution are ascribed to lower ion bombardment due to slower ambipolar diffusion of deuterium ion from the plasma. Further improvement of material quality has been achieved using a triode technique where a mesh electrode inserted between cathode and anode electrodes prevents from ion bombardment. In combination with a shower head cathode, the triode technique remarkably improves the crystallinity as well as defect density at a high growth rate. As a consequence, we have succeeded to obtain much better crystallinity and uniformity at 5.8 nm/s with a defect density of 2.6×1016cm−3. We also discuss the limiting factors of growth rate and material quality for µc-Si solar cells.

We have studied photoluminescence (PL) of surface oxidized nanocrystalline silicon quantum dots (QDs) for various oxidation periods and temperatures. With increasing oxidation period, the surface oxide grows and the Si QD core shrinks initially, then retardation of the oxidation process occurs which is ascribed to compressive stress at the interface between Si QD core and oxide. Upon oxidation, the PL spectrum peak shifts toward the shorter wavelength side followed by retardation of the blueshift or even manifestation of the redshift. The origin of PL is due to the localized excitons at the interface between Si QD core and oxide or amorphous SiOx (a-SiOx) formed at the interface. The blueshift is associated with the increased quantum con.nement or increased bandgap of a-SiOx. The redshift is due to the stress e.ect of the bandgap of Si QD core or a-SiOx. We have successfully confirmed the effect of compressive stress associated with the self-limiting oxidation by PL measurement.

Light-induced degradation of photoconductive properties is investigated as a function of lightinduced Si dangling bond defect concentration and subgap absorption. This relation is studied for a variety of sample preparation techniques including a-Si:D, wide range of temperatures during light exposure and annealing, and sample degradation pre-history. Under a given exposure condition, light-induced defects have a fixed effective electron capture coefficient, while its value varies greatly with exposure conditions (temperature during exposure or stepwise anneal) and with sample microstructure. This explains commonly observed non-linear dependences and hysteresis between defect concentration and mobility-lifetime product. Creation efficiency of less-stable defects with large capture coefficients is not influenced by the presence of a large number of stable defects. Structural change accompanying the defect creation, if any, is strongly related to the defect creation and is likely to occur locally around the defects, modifying their capture coefficients. Capture can vary both because of the changes in intrinsic capture probabilities and due to changes in the net defect charge balance. The former may be increased by the presence of H or photocarrier trap (e.g. weak bond tail state) in a close vicinity of Si dangling bond. The latter may be caused by a wide distribution of defect states in the gap or by simultaneous creation of negatively charged floating bond states near VB edge.

We present thin-film transistors with both amorphous silicon and silicon nitride deposited by hot-wire chemical vapor deposition. Hot-wire amorphous silicon with good electrical properties was deposited from the decomposition of silane at a substrate temperature of 250°C. For Hot-wire silicon nitride we used silane and ammonia at a substrate temperature of 340°C. In this paper we address structural and electrical properties of this material. A high ammonia flow results in porous films that exhibit post-deposition oxidation. By limiting the ammonia/silane ratio to 30, compact layers with a hydrogen content of only 10 at.% and a refractive index of 1.95 are obtained. Using this layer as gate dielectric results in thin-film transistors with good switching behavior and a field-effect mobility of 0.3 cm2/Vs.

Plasma deposition using silane and germane is extensively used for depositing amorphous (Si,Ge) films and devices, and has the potential of also being used for making crystalline films. In this paper, we report results on the electrical and optical characterization of germane plasmas using a Langmuir probe set-up and optical emission spectroscopy. The plasma studied was an electron-cyclotron-resonance plasma. For the first time ever, we have detected the characteristic 246 nm GeH emission peak in the plasma. The GeH emission peak is suppressed significantly when small quantities of hydrogen are added as a diluent gas. We have also studied the effect of adding a voltage bias to the substrate on the plasma properties. We find that adding voltage bias does not simply change the effective ion energy impinging on the substrate, as is usually assumed, but also changes all the plasma properties, including the density of neutral, excited H radicals arriving at the growing surface.

Experimental measurements of the Q-function (a comparison of the conductivity and thermopower activation energies) and the non-Gaussian statistical character of the 1/f noise in n-type doped a-Si:H as a function of light soaking (the Staebler-Wronski effect) are reported. There is a significant decrease in the non-Gaussian statistical character of the 1/f noise following light soaking of a device quality film, consistent with a slight increase in the long-range disorder. However, there is no change in the Q-function following light exposure, indicating that there is no significant increase in the long-range disorder.

For application to active matrix liquid crystal displays (AMLCDs), a low temperature (< 600 °C) process for the production of polycrystalline silicon is required to permit the use of inexpensive glass substrates. This would allow the integration of drive electronics onto the display panel. Current low temperature processes include excimer laser annealing, which requires expensive equipment, and solid phase crystallization, which requires high temperatures. It is known that by adding small amounts of metals such as nickel to the amorphous silicon the solid phase crystallization temperature can be significantly reduced. The rate of this solid phase metal induced crystallization is increased in the presence of an electric field. Previous work on field aided crystallization has reported crystal growth that either proceeds towards the positive terminal or is independent of the direction of the electric field. In this work, extensive investigation has consistently revealed directional crystallization, from the positive to the negative terminal, of amorphous silicon thin films during heat treatment in the presence of an electric field. This is the first time that this phenomenon has been reported. Models have been proposed for metal induced crystallization with and without an applied electric field in which a reaction between Ni and Si to produce NiSi is the rate-limiting step. The crystallization rate is increased in the presence of an electric field through the drift of positive Ni ions.

Previously a through the substrate ellipsomtery technique was used to study the high temperature dynamics of light induced reversible changes in amorphous silicon thin films [1]. Since this technique was based on above gap optical changes it is sensitive to the structural aspects of the light induced effects, differently from the below-gap absorption techniques which detect dangling-bond defect states [2-3]. It was found that high intensity light soaking at an elevated temperature causes surprising large, reversible, changes [1]. By comparing these optical changes with the changes in dangling bond concentrations probed by electronic and below gap methods, a fuller picture of temperature dependent light-induced defect creation and annealing dynamics emerges. A high temperature high intensity light soaking method is developed which reduces saturation times, decreases the saturated dangling bond density, as well as decreases the annealing activation energy. These results are discussed in terms of the coupling between network disorder and its relaxation with respect to defect concentrations at high temperature.