Computer device transport simulation can be a very powerful tool for device physics understanding, design, and optimization. This is especially true when it is done with a PC version that can be easily run on the desktop. However, like any powerful tool, computer simulation must be used with care and appreciation. Using our newest version of our AMPS (Analysis of Microelectronic and Photonic Structures) [1,2] transport physics code, AMPS for Windows 95/NT, we discuss using solar cell simulations for increased understanding, for materials evaluation, and for device design and optimization.

In this work, gap states in doped and undoped a-SiGe:H alloys are examined by numerical simulations of sub-bandgap absorption spectra measured by the constant photocurrent method and photothermal deflection spectroscopy. Deconvolution methods, neglecting the condition of charge neutrality, can be used for a rough estimate of the defect density value but not for ob- taining detailed information on the distribution of gap states in undoped samples. Our numerical analysis uses adapted occupation statistics and takes into account the condition of charge neutrality. Good agreement between measured and simulated PDS and CPM spectra is obtained. For a certain composition, i.e. a certain bandgap, the investigation of doped films yields infor- mation on the density and the position of charged defect states in the bandgap. In addition, the density of neutral defect states can be derived from a comparison of CPM and PDS spectra. The results reveal the coexistence of charged and neutral defects. In doped as well as in undoped films, charged defect states dominate the defect density. In the investigated range of compo- sitions the defect distribution of a-SiGe:H is similar to those found in a-Si:H. The width of the defect distributions does not decrease with decreasing bandgap. No evidence for a different be- havior of Si- and Ge-related defect states can be found in sub-bandgap absorption spectra.

A calibration procedure for determining the model input parameters of standard a-Si:H layers, which comprise a single junction a-Si:H solar cell, is presented. The calibration procedure consists of: i) deposition of the separate layers, ii) measurement of the material properties, iii) fitting the model parameters to match the measured properties, iv) simulation of test devices and comparison with experimental results. The inverse modeling procedure was used to extract values of the most influential model parameters by fitting the simulated material properties to the measured ones. In case of doped layers the extracted values of the characteristic energies of exponentially decaying tail states are much higher than the values reported in literature. Using the extracted values of model parameters a good agreement between the measured and calculated characteristics of a reference solar cell was reached. The presented procedure could not solve directly an important issue concerning a value of the mobility gap in a-Si:H alloys.

We present a SPICE model that takes into account the different mechanisms contributing to leakage current in hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). The main sources of leakage current in these devices have been identified to be the parasitic reverse-biased p-i-n diode at the vicinity of the drain as well as diffusion of phosphorous atoms from micro-crystalline (n+ μc-Si:H) contact layer into the intrinsic a-Si:H region. The latter gives rise to ohmic conduction which dominates at very low drain voltages (< I V) and very low gate voltages (< 5 V). At higher gate voltages (5V ≤ VG ≤ 20 V), the reverse current of the parasitic p-i-n diode can be attributed to thermal generation of electrons from the valence to conduction bands through the mid-gap states in the a-Si:H. At even higher gate voltages (> 20 V), the reverse current is due to trap-assisted tunneling, whereby the electrons tunnel to the conduction band via the mid-gap states. A systematic characterization of TFTs with different a-Si:H layer thicknesses shows that the optimal thickness for low leakage current is around 50 nm. The bias dependent leakage current behavior has been modeled and implemented in SPICE using simple circuit elements based on voltage controlled current sources (VCCS). Simulated and measured reverse leakage current characteristics are in reasonable agreement.

The growth mechanism of Si film at low temperature on Si(100) by photo-CVD was theoretically analyzed by using reaction models both in the gas phase and on the growing surface. We introduced three surface reactions; the growth of Si from SiH3 radicals, the dangling bond termination by atomic hydrogen and the abstraction of bonding hydrogen by SiH3 radicals. We assumed that the film structure is determined by the hydrogen surface coverage ratio “ø” and the parameters of the surface reaction model were determined from the experimental results. The theoretical analysis explained well the experimental data on the growth rate.

We analyze the formation kinetics and microstructure of hydrogenated vs. deuterated microcrystalline (μc-Si:H or D) thin films using real-time spectroscopic ellipsometry, post- deposition thermal hydrogen evolution, and TEM. The films are deposited by reactive magnetron sputtering of a silicon target in Ar (1.65 mT) with added partial pressures of H2or D2(0-5.5mT) on Coming 7059 glass substrates at 230°C. Amorphous films are deposited when PH2=0. When hydrogen is added to the chamber, the reactive magnetron sputtering process generates a flux of fast neutral H which promotes stc-Si growth. The substitution of D for H varies the kinetics of hydrogen reflection from the target and implantation into the growing film. We analyze the amorphous to microcrystalline transition as a function of the isotope (H2or D2) and pressure used in the deposition process. We find that the films enter the microcrystalline regime at lower D2pressures than H2pressures. Furthermore, the <ε2> data determined by ellipsometry have a different shape for deuterated films, compared to hydrogenated films at similar growth pressures. This indicates changes in band structure which we interpret as evidence for enhanced crystallinity.

Polycrystalline silicon (poly-Si) films are obtained at temperatures lower than 400°C by catalytic chemical vapor deposition (Catalytic CVD = Cat-CVD) method, often called hot-wire CVD method. Structural properties of the Cat-CVD poly-Si films, deposited with various gas pressures, are studied by Raman scattering spectroscopy and X-ray diffraction technique. It is found that there are two recipes for obtaining device quality poly-Si films, that is, such poly-Si films are obtained at low gas pressure around 1 mTorr or less as already reported, and also at high gas pressure around 0.1 to 1 Torr. It is also found that, in addition to catalyzer temperature, the gas pressure is a key factors to obtain device quality poly-Si films at high deposition rates.

We report on the growth and properties of a-(Si,Ge):H films and p-i-n solar cell devices prepared using a remote, low pressure ECR plasma deposition technique. The films and devices were prepared using either He or H2 as the diluent gas. The plasma conditions were controlled so as to induce significant ion bombardment during growth. We find that there is a dramatic influence of plasma chemistry on the growth and properties of a-(Si,Ge):H films and devices. In particular, with hydrogen as the diluent gas, changing the pressure in the reactor dramatically changes both the Germanium incorporation in the film, and the electronic properties. Lower pressures lead to less Ge being incorporated, and higher mobility-lifetime product for holes for a given Tauc gap, as well as better p-i-n devices. In contrast, changing the pressure when He is the diluent gas does not produce such large changes. We speculate that the changes in device and film properties are due to the influence of ion bombardment on growth chemistry, and that both efficient energy and momentum transfer to the growing surface are necessary to achieve the best devices. The differences between He and hydrogen may simply be due to the fact that He plasma is much more energetic than a comparable hydrogen plasma, and there is more efficient momentum transfer when He is used as compared to when hydrogen is used. We have also produced very good single junction a-(Si,Ge) devices using the ECR technique.

We successfully grow high-quality hydrogenated amorphous-silicon-germanium alloys (a-SiGe:H) by the hot-wire chemical-vapor deposition (HWCVD) technique using silane and germane gas mixtures. These alloys display electronic properties as good as those grown by the plasma-enhanced chemical-vapor deposition (PECVD) technique, when comparing materials with the same optical bandgaps. However, we grow materials with good electrical properties at high deposition rates—up to 40 Å/s, compared to 1–4 Å/s for PECVD materials. Our alloys exhibit similar trends with increasing Ge content to alloys grown by PECVD. The defect density, the dark conductivity, and the degree of nanostructural heterogeneity (as measured by small-angle X-ray scattering) all increase with increasing germanium content in the alloy. The nanostructural heterogeneity displays a sharp transition between 9 at.% and 14 at.% germanium. PECVD- grown a-SiGe:H alloys exhibit a similar transition at 20 at.% Ge. The photoconductivity and the ambipolar diffusion length of the alloys decrease with increasing germanium content. For a fixed silane-to-germane gas ratio, all material properties improve substantially when increasing substrate temperature (Tsub) from 220°C to 375°C. Increasing Tsub also narrows the optical bandgap and lowers the hydrogen content in the alloys for the same germane-to-silane gas ratio.

The creation and annealing kinetics of light-induced defects in a-SiSx:H are studied by ESR and LESR measurements. The dispersion of defect creation after prolonged illumination with white light is greater for a-SiSx:H than that for undoped a-Si:H. In addition, the saturated value of the dark spin density is slightly lower for a-SiSx:H than that for a-Si:H. The annealing behavior can be fitted with a Gaussian distribution of annealing activation energies as is the case for undoped a-Si:H. The incorporation of sulfur decreases the peak energy and increases the width of the distribution of activation energies. Light-soaking does not change the low temperature LESR spectrum and LESR spin density.

Hydrogenated amorphous silicon alloyed with selenium has been made by plasma enhanced chemical vapor deposition (PECVD). The activation energy for electrical conduction is essentially unchanged for selenium concentrations < 1 at.%. The photo conductivity changes for selenium concentrations > 0.5 at. %. Photothermal deflection spectroscopy (PDS) and electron spin resonance (ESR), respectively, show that the width of the valence band tail states and the density of neutral silicon dangling bonds also change for selenium concentrations > 0.5 at. %.

The shape of several waveguide end of samples for photoluminescence absorption spectroscopy (PLAS) was studied by atomic force microscope (AFM), because there was an experimental problem where some samples for PLAS did not work. Using the result of AFM, the waveguide end was reshaped by plasma dry etching. The shortening of the etching time was an effective method to improve the structure of the waveguide end. Secondly, the PLAS method was extended to the other materials from a-Si:H. The PLAS signal of amorphous carbon nitride a-CNx was detected for the first time. Amorphous carbon nitride a-CNx film itself and the interface between a-CNx and a-Si02 are found as good as a-Si:H and the interface between a-Si:H and a-Si 3N4+x:H, respectively.

The effect of SiF4 addition on the deposition characteristics and film property has been studied for the improvement of long term stability of the SiNx:F film in TFT applications. During the deposition with SiF4 addition, atomic hydrogen species in the gas phase significantly decreased. A detailed bonding configuration of fluorine and hydrogen was investigated by XPS and FT-IR. Optical band gap increases with the increment of fluorine concentration in the film. High compressible stress turned to low tensile stress with increasing SiF4 flow rate. The breakdown strength and barrier height for the conduction of trapped electron were improved by SiF4 addition even in Si-rich SiNx:H films due to the stable Si-F bond formation.

Real time spectroscopic ellipsometry (RTSE) has been applied to identify optimal conditions for the nucleation and growth of 120 Å microcrystalline silicon (μc-Si:H) p-layers by rf plasma-enhanced chemical vapor deposition (PECVD) at 200°C on amorphous silicon (a-Si:H) i-layers in the n-i-p solar cell configuration. Analysis of the RTSE data provide the bulk p-layer dielectric function (2.5-4.3 eV), whose amplitude and shape yield insights into the structural quality and crystallinity of the p-layer. Among the deposition parameters varied include the underlying i-layer surface treatment, the p-layer plasma power flux, and the p-layer dopant source gas and flow ratio. Here we focus on the differences between p-layer deposition using trimethyl boron, B(CH3)3, and boron trifluoride, BF3, source gases. We find significant differences attributed to the differing effects of F and CH3radicals in the plasma on silicon crystallite growth.

The role of substrate ion bombardment on the structural and H bonding properties of hydrogenated amorphous germanium (a-Ge:H) films was studied by infrared (ir) spectroscopy. A Kaufman type ion source was used to produce an Ar1 beam directed towards a Ge target for a- Ge:H ion beam sputtering deposition in a H2-containing vacuum chamber. A low energy (100 eV) H2++Ar+ beam obtained from an additional ion source was allowed to impinge directly on the substrate during film growth at various beam currents.

It was found that substrate bombardment by 100 eV ions favors the formation of voids, as deduced from the increasing contribution of the surface-like Ge-H stretching mode to the ir spectrum with increasing ion current. The void density was reduced below the ir detection limit by totally removing the ion beam on the substrate while keeping all other parameters fixed. For this condition, we observe no or very small surface-like contributions to the ir spectra, irrespective of substrate temperature (25-260°C) or growth rate used. A narrowing of the infrared Ge-H stretching mode peak is observed with increasing deposition temperature, indicating a concomitant tendency towards a more ordered structure.

Two types of silicon clathrates, NaxSi136 (0≤ × ≤ 24) and Na8Si46 and a mixed clathrate (Na,Ba)Si46 were synthesized and characterized by X-ray diffraction, 23Na NMR and high resolution TEM. Systematic changes in X-ray diffraction intensities enabled the sodium content and site occupancy in the NaxSi136 series to be followed and then refined by Rietveld profile analysis. 23Na NMR spectra of Na8Si46 and NaxSi136 samples reveals two peaks with large paramagnetic shifts (1600-2000 ppm) for each phase, associated with the presence of Na atoms in two different environments. High resolution electron microscopy reveals the clathrate structures of these samples, and allows classes of defects to be characterized.

Internal stress in a-Si:H films prepared by plasma chemical vapor deposition was studied in connection with optical degradation or Staebler-Wronski effect (SWE). It was found that the stress increased with increasing discharge power density (0.03 ∼ 1.5 W/cm2) of silane, accompanied by increase in the introduction rate of SWE. These results imply that highly decomposed radical species due to high discharge power density dominate the surface reaction. Consequently, larger stress and larger introduction rate of SWE are brought about in the films. The stress also slightly increased at lower power density (< 0.03 W/cm2), suggesting that neutral silane molecules take part in the reaction at these power density.

In this work we have used square-wave-modulated (SQWM) low frequency (55kHz) glow discharge plasma for deposition of a-Si:H films and observed a significant stability improvement. To investigate the origin of stability improvement the relation between growth mechanism, film microstructure and metastability was analyzed. It was shown that the use of SQWM-plasma decreases the concentration and average size of inhomogeneities calculated from AFM data while photoconductivity under He-Ne laser illumination (Staebler-Wronski effects) didn't degrade during standard time of experiments. Differential scanning calorimetry (DSC) measurements were carried out and it was found that in SQWM samples hydrogen effusion occurs at higher temperatures than in cw a-Si:H which is connected with increased structural stability of films deposited in SQWM-plasma. Taking into account the high photoconductivity and low defect density of SQWM samples it is concluded that modulating of LF plasma is a perspective method for deposition of stable a-Si:H films and its alloys.

PECVD processes are the most common way of growing amorphous and microcrystalline silicon thin films. These low pressure plasmas are known to generate large amounts of particles in the gas phase which may be a source of contamination in the production of silicon based devices. In this report, however, we show that these particles can be incorporated profitably into a growing film, provided that their size is controlled. The Modulation of the RF power is a direct method of controlling particle size. Plasma characteristics determine the dynamics of particle formation, whereas the duration of the plasma-on period controls the particle's size and atomic structure. The size and atomic structure of particles from two different periods of the same discharge were investigated. Their main characteristics, monodisperse size distribution and ordered atomic structure, are discussed in the context of the latest findings on particle nucleation and coagulation.

A semi-phenomenological model was used to show that, during the first milliseconds of the discharge, particles of 1–2 nm that are electrically neutral may leave the plasma and contribute to the growth of the film. Although these particles could not be observed on the TEM grid, their contribution to film growth might be relevant.

Microcrystalline silicon films were deposited on corning glass substrates both by the standard hydrogen dilution and the layer-by-layer (LBL) technique. In-situ UV-visible spectroscopic ellipsometry measurements were performed to analyze the evolution of the composition of the films.The change of the hydrogen plasma conditions by increasing the pressure in the LBL process leads to a faster kinetic of crystallization and to an increase of the deposition rate by a factor of two. The increase of the pressure and the decrease of the inter-electrode distance allowed to increase the deposition rate from 0.26 to 3 Å/s in the hydrogen dilution technique. Interestingly enough, the crystalline fraction of the films remains higher than 50%. However, as the deposition rate increases the growth process results in a slower kinetic of crystallization with a long range evolution of the film composition (up to 0.5 νm).