Applying PDS in the energy range from 3000 to 9000 cm-1 considerably improves the sensitivity of vibrational spectroscopy of thin films. In a-SiC:H we have found a large number of absorption features not seen before, which give new insight in structural and compositional properties of these alloys.

In a-SiC:H, in addition to the fundamental and the first and second overtone of C-H stretching modes, O-H vibrations occur at 3500 cm-1 and allow a very sensitive detection of chemical degradation of films due to in-diffusion of water. Combined excitation of C-H stretching and C-H wagging or bending modes give rise to a modified interpretation of the origin of the C-H related absorption features observed in methane based a-SiC:H. The induced absorption of molecular hydrogen together with a band at 7000 cm-1, which results from a combined excitation of C-H bonds and H2 molecules colliding with the C-H, indicates that the amount of H2 increases with C alloying. The C-H + H2 combination mode is very strong compared to the corresponding Si-H + H2 peak observed in pure a-Si:H and signalizes mat a considerable amount of the molecular hydrogen enclosed in voids, containing CH3 or CH2 groups.

We report the deposition parameters for optimized a-Si,Ge:H,F alloys in the range of optical (Taue) gap of 1.22eV to 1.65eV. These deposition parameters were optimized using the photosensitivity and initial defect density as figures of merit. We observe two distinct regimes of film growth rate, dependent on the choice of source gases. Growth from fluoride source gases results in a growth rate of less than 0.6 Ås--1. Growth from a mixture of fluorides and silane gives a range of growth rates from 2 Ås-l to 5.5Ås1. Alloys in both regimes display the low defect densities and the high photosensitivities required for devices.

We report TOF measurements on high quality intrinsic a-Ge:H and a-SiGe:H films of E04=1–4.eV in temperature ranges of 200 to 280 and 230 to 300K, respectively. Complete charge collection is achieved in all measurements. For a-Ge:H films, the (μτ)e product obtained from the Hecht plot is (5±3)×10-8 cm2/V above 240K and decreases at lower temperatures. The electron transit signal is dispersive at all temperatures. The a obtained from ttV-1/αis 0.23 at 200K and approaches 1.0 at 260K. The electron drift mobility μd shows activated behavior, with an energy of 0.37±0.05eV, and has an extrapolated room temperature value of 0.03 cm2/Vs. Compared to a-Ge:H, μd of a-SiGe:H alloy samples is lower by one order of magnitude but has a similar activation energy. These results are consistent with a band tail hopping transport model.

Prototype memory cells of a proposed three-bit memory device, whose optical read-out is based on the ellipsometric principle, nas been fabricated by deposition of 5 nm layers of amorphous silicon and amorphous germanium on crystalline silicon substrates. The differences in the ellipsometric parameters of memory cells corresponding to different logical states were much larger than the instrumental resolution. The possibility to optimize with respect to photon energy and angle of incidence is demonstrated.

Urbach tails and midgap absorption, as determined by PDS, and temperature dependent coplanar and sandwich conductivity were systematically investigated on compositional multilayers of a-Si:H and its alloys with Ge and C, under the main aspect of a critical evaluation with respect to a possible existence of quantum size effects. Both electronic well and barrier widths were varied, with and without keeping the mean composition of the multilayers constant. Neither the overall dependences of the Urbach slopes, nor of the conductivity activation energies and room temperature αdark values are consistent with a quantization picture. Other effects, as imposing on each other the own sublayer's degree of structural disorder, and adjustment of the Fermi level by charge transfer, play a dominant role and may feign quantum size effects, if in a multilayer series only variation of the well widths is considered.

We compare films of a-Ge:H deposited by plasma enhanced chemical vapor deposition (PECVD) using GeFLi diluted with H2, He and Ar. The nominal deposition conditions used were those which have produced high quality a-Ge:H with H2 dilution. Using H2 and He as diluents, three studies were done in which the parameters of substrate temperature, diluent gas flow and electrode gap spacing were independently varied. The photoelectronic properties of the films made with He dilution were quite good, although never better than those made with H2 dilution. The overall results indicated that for our nominal conditions hydrogen plays an important and crucial role in producing high quality films, and we believe that this role is mainly to reduce the probability of germane radical incorporation. In contrast to He and H2 dilution, dilution with Ar produced films with much poorer microstructure and a photoresponse two orders of magnitude lower. We attribute this to polymerization of the germane radicals which led to observed dust particles and a reduction in ion bombardment of the growing film.

For a series of a-Si:H/a-SiC:H quantum well structures and superlattices the diffusion coefficient for the lateral ambipolar motion of optically excited free carriers was measured using the transient grating technique. A significant dependence of the diffusion coefficient on the well layer thickness was found. With decreasing quantum well thickness the lateral mobility decreases. These observations may be explained assuming that scattering due to interface roughness is the dominant scattering process.

The interdiffusion of carbon in a-Si:H and a-Si1-xCx:H multilayers is measured by analyzing the time evolution of the X-ray diffraction from the superlattices. A time dependent diffusivity D(t)which follows a power law is found. The theoretical analysis shows that such a time dependence, referred to as “dispersive” or “anomalous” diffusion and studied for hydrogen diffusion in the literature, is fundamentally related to the amorphous structure of the material.

The recombination processes in a-Si:H and a-Si:H/a-SiCo.3s:H multilayers at T=10 K are studied by transient photoluminescence on a time scale from 2 ns to 100 ms. Our investigations yield a lifetime distribution with two peaks at ∼1 ms and ∼1 /μS for bulk a-Si:H. We present measurements on samples with varying defect density ND. It is shown that there is no significant shift of the two lifetimes as a function of ND. However, the relative contribution of the μs-peak is enhanced with increasing ND. In a-Si:H/a-SiC:H multilayers with a varying number of interfaces and well layer thicknesses dsi there is also no discernible change of the lifetimes as a function of the overall defect density. An additional peak at ∼20 ns is observed for samples with dsi smaller than ∼30 Å. It may be attributed to recombination in the interface layers.

Amorphous Ge/Si superlattices have been grown by dual-target dc magnetron sputtering at ambient temperature under different negative substrate bias voltages Vs. The films were studied with cross-sectional transmission electron microscopy (XTEM). All films were amorphous, but for Vs ≤ 110 V the superlattices exhibited a columnar structure. Within each column, the compositional modulation was clearly resolved except in the vicinity of the column boundaries. For 110<VS≤250V, dense films with homogeneous and well-defined layers were obtained. High-resolution XTEM studies showed that superlattices with individual layer thicknesses less than 1.5 nm and smooth interfaces could be fabricated. Increasing Vs > 250 V resulted in ion induced intermixing such that at Vs ≥ 450 no compositional modulation was observed. The obtained structural results are discussed in terms of available models for ion-irradiation effects during low-temperature ballistic growth.

We report the deposition parameters for optimized a-Si,Ge:H,F alloys in the range of optical (Taue) gap 1.5 leV to 1.62eV. These deposition parameters were optimized using the saturated defect density as a figure of merit. We report initial defect densities at or below 2.5×1016 cm-3, saturated defect densities below 9×1016 cm-3, photoconductivities (at G = 1021 cm-3s-1) between 8.7×10-6 Scm-1 and 7×10-5 Scm-land photosensitivities between 104 and 105 for alloys in this range of optical gap.

We have investigated the difference in the electronic energies of neutral Si and Ge dangling bond states in undoped a-Si1-xGex alloys as a function of the alloy composition, x, and local bond-angle distortions. The local density of states, LDOS, in a-Si1-xGex alloys has been calculated using nearest-neighbor interactions, and employing the Cluster Bethe Lattice method. We conclude that for ideal, tetrahedrally bonded amorphous semiconductors alloys, the Ge dangling bond energy is lower than that of Si dangling bonds by ∼ 0.13 eV, independent of the specific nearest neighbors to the dangling bond (3 Si-atoms, 2 Si-atoms and 1 Ge-atom, etc.), but that the spread in dangling bond energies associated bond-angle variations of the order of 6–8 degrees can be larger than this energy difference (∼0.3 eV or greater). This means that structural disorder, rather than chemical disorder causes Si and Ge-atom dangling bond states to overlap in their energy distributions.

It is a question as yet unresolved whether the density of light-induced defects in a-Si:H reaches a saturated value that cannot fundamentally be exceeded, or whether the defect density is in all conditions a steady-state value that reflects carrier concentration and temperature. In our experiments on a-Si:H we have observed defect saturation at low temperature and high light intensity; on the other hand, data exhibiting no saturation have also been published. To learn more about this question we have carried out saturation experiments on a-SiGe:H(F) alloys. These alloys have lower defect freeze-in temperatures than a-Si:H and, presumably, lower annealing energies. Therefore, saturation should be more difficult to achieve in the alloys than in a-Si:H.

We have studied saturation for a-SiGe:H(F) samples to temperatures above the onset of thermal annealing and have observed that its behavior is similar to that seen in a-Si:H.

The optical properties of solar cell grade hydrogenated amorphous silicon (a-Si:H), silicon germanium (a-SiGe:H) and silicon carbon (a-SiC:H) alloy thin films have been investigated over a wide photon energy range (0.8–4.8 eV) using a combination of subgap photoconductivity, reflection and transmission, and spectroscopie ellipsometry techniques in order to obtain accurate optical functions for solar cell modelling. Studies on films with thicknesses ranging from a few hundred Å to a few microns show that the optical spectra obtained by the different techniques agree closely over the energy ranges of overlap and display no thickness dependence from the Urbach tail energies and above. Thus, the results appear to be free of measurement and sample related artifacts. Three different methods provide a common value for the optical gap within ±0.02 eV and the result for a-Si:H is ∼0.1 eV below the mobility gap.

We have prepared silicon carbon alloys with Tauc's gap of 2.1 eV, low defect density (≃ 3–1015cm-3) and large photoconductivity (αPhoto/αdark=105 in AM 1.5 illumination). On these samples light soaking induces a large number of metastable gap defects which are annealed out at 250 °C. We have studied the kinetics of defect formation varying the duration of light exposure and the light intensity. The experimental data are consistent with a bond breaking model (conversion of tail weak bonds into dangling bonds), provided that the actual occupation of tail states is taken into account.

Microcrystalline films of SiC.H have been deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) of SiH4+CH4+H2 mixtures with high power density and high H2 dilution, at variable CH4/SiH4 ratios. Their elemental composition, structural, optical and electrical properties have been investigated in order to evidence the influence of hydrogen and carbon on microcrystallinity.

In this paper we report on the properties of a-SiC:H alloys with reduced density of CH3 radicals obtained by hydrogen dilution of gases. A reduced density of voids, hydrogen content and density of carbon bonded hydrogen atoms, were obtained; while carbon content and density of Si-H bonds are not affected much. A reduction of the optical gap and an increase of the refractive index were observed and related to the reduced densities of CH3 groups and voids. Dark- and photo- conductivity measurements showed that a performance comparable to the best undiluted films may be easily achieved, associated with a shift of the dark Fermi level towards the conduction band.

We report a new method of depositing amorphous hydrogenated silicon carbide thin films with low IR-detected microstructure in a plasma-enhanced chemical vapor deposition reactor. Films prepared at various conditions are studied with Fourier-transform infrared absorption. Their optical band gaps and photoconductivities are also measured. The amount of microstructure can be controlled by adjusting the powered-electrode potential during deposition, and the microstructural changes are reflected in the film properties. By applying an external dc voltage to the rf-excited powered electrode, we can shift the optimal deposition temperature from 250 °C to as low as 100 °C. We find that films deposited at positive powered-electrode potential and low substrate temperature exhibit less microstructure, wider optical band gaps, and faster deposition rates than films deposited at conventional conditions.

The optoelectronic properties of a-SiC:H alloys with different carbon content are investigated with time resolved photoconductivity and photoinduced absorption measurements. It is shown that the electron drift mobility decreases with increasing carbon content. Also a weak increase of deep electron trapping with increasing carbon content is suggested by the experimental data. The recombination does not seem to be strongly changed by the presence of carbon at least at low carbon content.

Using standard rf glow discharge deposition, we compare the properties of a-Si1-xCx:H films prepared with different carbon sources, namely methane and di-, tri- and tetrasilylme-thane (DSM, TSM and TetraSM). The optical bandgap EG was varied between 1.8 eV and 2.4 eV. All oua-Si1-xCx:H films exhibit low Urbach energies (EQ < 60 meV for EG< 2.0 eV) and low defect absorption, both determined by PDS. Using the silylmetha-nes as feedstocks, we do not find significant differences relative to our methane-based material. Independently of the carbon source material, the energy gap dependences of the film properties (photoconductivity, activation energy of the dark conductivity, Urbach energy and subbandgap defect absorption) show a distinct change around EG = 2.1 eV, which is interpreted as a consequence of structural changes with increasing carbon content.