In this paper we present new results for very thin <p> μc-Si:H films (< 350 Å) deposited at low temperature (170 C) by the Very High Frequency - Glow Discharge technique (VHF-GD) at 70 MHz. First, the effect of boron doping on the growth and electrical properties of μc-Si:H very thin films is investigated, leading to an optimised value of about 0.6 % (B2H6/SiH4). Structural properties of an optimised thickness series ranging from 100 to 350 Å are studied using TEM, Raman, grazing angle X-ray diffraction/reflection and spectroscopie ellipsometry. Further, a columnar structure growth model for these very thin <p>-type μc-Si:H films will be proposed.

AMorphous silicon carbide films were deposited by the PECVD technique in SiH4+CH4 gas mixtures at various CH4 flow rates with and without H2 dilution of the reactive Mixture. A detailed analysis of defect distribution in the gap has been performed by PDS and the results have been correlated with the structural (IR) and the compositional properties (RBS, ERDA). The effect of hydrogen dilution on the electronic properties of the films was investigated by dark and photo electrical conductivity Measurements.

We present a summary of our recent measurements on hole drift mobilities in hydro-genated amorphous silicon-carbon alloys (a-Si1−xCx:H). Increasing the bandgap has a vastly smaller effect for the hole mobility than for electrons. In conjunction with previous drift Mobility Measurements in a-Si1−xCx:H and a-Si1−xGex:H, these hole measurements complete a simple pattern for the effects of bandgap modification on drift Mobilities: electron mobilities decline as the bandgap is increased beyond 1.72 eV or decreased below 1.72 eV, but hole mobilities are relatively unaffected.

We have deposited films of a-Si,N:H by remote PECVD from N2 and SiH4 for N-concentrations, [N], to about 12 atomic percent (at. %). Bonded-H concentrations were ∼7–10 at. %, Mostly in Si-H groups. The films with [N] = 9–12 at. % have εθ4 bandgaps of ∼2.0 to 2.2 eV, which makes them potentially useful as wide bandgap photo-active materials in tandem PV cells. Several properties are of special interest for PV applications. First, like many other a-Si:H-based alloys, the photoconductivity relative to a-Si:H is degraded by alloying, but less than for a-Si,C:H alloys with the same bandgaps. Second, the ambipolar diffusion lengths (Ld) obtained with the Steady State Photocarrier Grating (SSPG) technique for films with [N] = 10 at. % and εθ4 = 2.1eV, are comparable to those of a-Si:H. For lightly-nitrided films to [N] ∼5 at. %, Ld first decreases with respect to a-Si:H and then increases as [N] increases from ∼7 at.% to 10–12 at. %. These trends follow the dark conductivity activation energy, Ea, which initially drops due to doping, and then increases into an alloy regime for [N] > 5 at. %. Films with [N1=10 at. % have dark conductivities and Ea's comparable to those of undoped a-Si:H. Third the magnitude of the Staebler-Wronski effect, as monitored by the photo- to dark conductivity ratio after a 1000 Minute lightsoak, was about the same as in a-Si:H. Finally, we contrast the properties of these films prepared from N2 with a-Si,N:H alloys with the same [N] and E04, but prepared from an ammonia N-atom source gas and attribute differences in their photoelectronic behavior such as a significantly enhanced Staebler-Wronski effect.to the presence of Si-NH bonding arrangements in the films grown from NH3.

In this work we present Thermal Modulation Electron Spin Resonance Measurements performed on a-SiC:H films prepared by Plasma Enhanced Chemical Vapour Deposition with energy gap in the range 1.8–2.5 eV. The results have been compared with previously obtained photothermal and photoconductive ones and have been interpreted by means of a defect distribution which takes into account both silicon based and carbon based defects.

A novel organometallic germylene precursor has been synthesized and used for the deposition of amorphous germanium by thermal OM CVD at temperatures below 300°C. The films are of high purity with an oxygen content below the present detection limit of about < 1018 cm−3. Preliminary results on the electronic properties are reported.

Extensive studies were made of GeH4+H2 and SiH4+H2 rf discharges used in depositing a-Ge:H and a-Si:H films. Residual Gas Analysis (RGA) Measurements were made of the gaseous species generated by these discharges. Data from the measurements of SiH4 + H4 discharges were entirely consistent with the generally accepted models of silane discharge chemistry and a-Si:H film deposition. Identical experiments for GeH4 + H2 discharges show that the chemistry for these discharges cannot be interpreted using a model similar to that proposed for silane. A review of the literature reveals differences between the stability of GeH2 and SiH2 radicals. These differences, usually overlooked by other workers, can account for the observed differences in the RGA results, as well as offer an explanation as to why it is difficult to deposit structurally homogeneous a-Ge:H.

Other RGA Measurements and electrode potential measurements are compared to the properties of a-Ge:H films that are deposited under different deposition conditions. Overall, the results strongly suggest that the chemistry which is beneficial for homogeneous film growth is promoted by a high electron temperature in the discharge. This conclusion is shown to be consistent with the results summarized above.

We investigated the optical and electronic properties of amorphous silicon carbide (a-Si1−xCx:H) films produced by plasma enhanced chemical vapor deposition from admixtures of silane and ditertiarybutylsilane [SiH2 (C4H9) 2 or DTBS] using photothermal deflection spectroscopy, electrical conductivity and its temperature dependence as well as photoconductivity. These a-Si1−xCx:H films exhibit low Urbach energies and high photoconductivities similar to films produced with other carbon feedstock sources. We also present our results for hydrogen diluted a-Si1−xCx:H films using DTBS as the carbon feedstock source.

Alloying a-Si:H with small amounts of sulfur (≤ 4 × 1020 cm−3) results in a significant enhancement in the photoconductivity and a suppression of the optically-induced degradation of the photoconductivity (Staebler-Wronski effect). The Magnitudes of the activation energies for conductivity imply that these films remain essentially intrinsic in nature. When the sulfur concentration is increased to about 2 to 3 at. %, the Fermi level rises by about 0.25 eV, a fact that suggests that sulfur may act as a very inefficient dopant in a-Si:H.

a-Si:H p-layers doped by trimethylboron (TMB) were obtained by PECVD in a monochamber reactor with a rotating substrate holder. The influence of the substrate temperature (Ts) on the film properties was systematically studied for two different doping gas concentrations. The incorporation of boron, hydrogen and carbon was studied by Secondary Ion Mass Spectrometry (SIMS). Optical properties were determined by means of Photothermal Deplection Spectroscopy (PDS) and optical transmission. Dark conductivity (aj and activation energy (Eact) were measured electrically. Our results show that Σd has a marked dependence on substrate temperature, although boron atom concentration depends only slightly on Ts. The optical gap for samples obtained at the higher concentration also depends on Ts and its dependence is related to the hydrogen content, as boron content does not change. P-i-n diodes were obtained with the p-layer deposited from TMB.

In this paper we report results on the optoelectronic and structural properties of device-quality a-SixC1−x:H intrinsic films with energy gap of 1.94 eV and Urbach energy of 70 MeV, grown by PECVD of SiH4+CH4 Mixture, which have been doped by means of boron or phosphorus ion implantation. Doping levels varied in the range 1018 to 5×1020 atoms/cm3. The behaviour of electrical characteristics, as well as energy gap and defect density, Measured on samples annealed in the range 150–400°C, showed that the optimum annealing temperature for the recovery of radiation damage is in the range 250–270°C independent of the implanted dose. Our results also show that after ion implantation and annealing, an increase of the SiH bonds concentration is detected, which is associated to a decrease of the contribution of SiH2, SiCH3, and SiCH vibrations in IR spectra.

In order to study the dependence of the atomic fine structure and optical band gap of the amorphous alloy on concentration and annealing temperature, thin Si1−XBX alloy films were grown and then annealed at temperatures from 400 to 1050 °C. The films were characterized by Extended Energy Loss Fine Structure spectroscopy (EXELFS), High Resolution transmission Electron Microscopy (HREM), Auger Electron Spectroscopy (AES), and light absorption spectro-photometry. It is shown that all the amorphous Si1−XBX alloys are thermally stable (e.g., >1050 °C for x=0.6) as compared to a-Si, and that the optical band gap of the alloys increases gradually with annealing temperatures up to 700 – 900 °C. When annealed at higher temperatures the band gap increased rapidly, corresponding to a phase transformation between two amorphous phases.

We have compared the photo-transport properties of thin films of B-compensated Μc-Si and device-grade a-Si:H prepared by remote plasma-enhanced chemical-vapor deposition (PECVD). The steady state photocarrier grating technique was used to determine the ambipolar diffusion lengths, Lamb. The conductivity of the μc-Si:B thin films changed from n-type to p-type as the B2H6 fraction in the B2H6/SiH4 source gas mixture was increased. The steady state photoconductivity, σph, decreased as B2H6 was initially introduced and the material changed from n-type to intrinsic; σph then increased as the material converted to p-type. Lamb displayed a complementary behavior with a maximum value at the approximate compensation point between the n-type and p-type conductivity regions. The steady-state photoconductivity and Lamb did not show any significant photo-induced degradations under intense illumination. Finally, the photoconductivity and Lamb displayed a strong dependence on sample thickness which is associated with Fermi level pinning at the surface.

We have deposited p+ a-Si,C:H films by reactive magnetron sputtering from boron doped target with 1 at.% B. We have investigated the influence of H2 pressure on the electrical and microstructural properties of the doped a-Si,C:H films. The boron concentration in the film is ∼2–4×1020cm−3. The incorporation of B atoms decreases by a factor of 2 at the highest H2 pressure. We have obtained films of Taue bandgap ∼1.8–1.84eV with dark conductivity 2–8×10− 6Ω−1cm−1, thermal activation energy ∼0.28–0.33eV and νμτe ∼1–3×10−8cm2/V. This result is comparable with glow discharge B doped a-Si,C:H film. We discuss the film Microstructure, as reflected in infra-red & thermal H evolution spectra, vs. the absolute composition of sputtered boron doped a-Si,C:H films.

Real time spectroellipsometry (RTSE) has been applied to study the growth of a-Si1-xCx:H alloys (x∼0.1; Eg=1.90–2.00 eV) for applications as i- and p-type layers in wide band gap solar cells. Two important material parameters, the optical gap and the relative bond-packing density (or void volume fraction), can be estimated from RTSE data collected during the growth of a sequence of layers onto the same substrate using different plasma-enhanced CVD conditions. In this way, large regions of parameter space have been scanned expeditiously, and an improved understanding of the effects of H2-dilution, substrate temperature (Ts), plasma power, gas pressure, and gas flow on the film properties has been obtained.

A possible alternative to a-SiGe for use in multijunction solar cells is a-GeC alloy. Only preliminary work has been done so far on this alloy, Mostly for use as an IR anti-reflection coating, with little study done on its optoelectronic properties. GeC films were grown by the PECVD Method from a novel gas mixture using GeF4 as the germanium source gas. Methane was used as the carbon source, with H2 being added for additional hydrogenation of the film. It was found that adding silane to the mixture would increase the growth rate from less than 1 Å/s up to 30 Å/s. The films contained no detectable silicon, even at mixture ratios of five SiH4 to one GeF4. Varying the amount of methane in the gas mixture allowed the film optical bandgap to be adjusted from 1.1 eV to 1.8 eV. The effect of other parameters such as RF power level and H2 flow rate on bandgap and growth rate were also explored. Optoelectronic properties in the 1.2 to 1.4 eV bandgap range will also be reported.

We have carried out a comprehensive Raman scattering study of a-S1-xCx:H alloys using three laser excitations: 458, 514, and 581 nm. The alloys studied were prepared with and without H2 dilution, had carbon concentrations, x, between 7 and 20 at %, and Taue optical gaps in the range of 1.8 ≤ Eg ≤ 2.2 eV. The results obtained explain the previous conflicting reports in the literature regarding the Si network disorder and indicate that in the Si-rich compositions, chemical clustering takes place with increasing carbon content.

We report a photoluminescence study on amorphous hydrogenated silicon carbon (a-Si1-xCx:H) alloys with carbon concentration in the range O < x < 0.5, prepared by PECVD in the “low-power” regime, that preserves the tetrahedral coordination of the carbon atoms. These samples have optical gaps higher than conventional “high power” alloys with the same carbon content. For carbon concentrations below x = 0.2 the photoluminescence behaves essentially as in pure a-Si:H with increased gap, Urbach energy and DOS. For higher carbon concentrations there is a change in the recombination process, that we attribute to a change in the dominating diffusion process of the photogenerated carriers. The integrated photoluminescence intensity for carbon-rich samples is very weakly dependent on the temperature, and at room temperature it approaches that of pure a-Si:H at 77K. For all samples, the photoluminescence bandwidth can be well described by a zero-phonon model.

We report measurements in nitrogen-rich a-SiNx:H (x > 1.3) of visible light photobleaching of photoluminescence (PL) that has been fatigued with UV light. We also review the temperature dependence of electron spin resonance (ESR) from a-SiN1.6:H films. The samples were prepared by plasma-enhanced chemical vapor deposition (PECVD) at 400 and 250° C. The optical gap of a-SiN1.6:H is about 5 eV. In PL experiments, the PL intensity fatigues under UV excitation, finally reaching a saturated value. By illuminating with visible light, the UV effect is partially bleached, that is, the PL intensity is partially restored. The bleaching light ranges in energy from 2.0 to 4.8 eV. With increasing bleaching-light energy, the reinduced PL intensity increases, reaches a maximum at a bleaching energy of 3.8 eV, and then quickly decreases. In ESR experiments, there exists a small ESR signal in as-deposited films before UV excitation and the variation of the ESR signal with temperature is similar to that of a-As.

The technique of the steady-state photocarrier grating (SSPG) has been applied to different series of a-Si:H/a-Si1-xCx:H and a-Si:H/a-Si1-xGex:H Multilayers. In dependence on the electronic well and barrier widths of the Multilayers, characteristic changes of the measured ambipolar diffusion lengths Lambi have been found. The general trend of Lambi can be understood in terms of interface recombination and scattering, when the mean composition of the multilayers is kept constant. In some cases, values of Lambi of multilayers were higher than the corresponding value of a homogeneous a-Si:H reference layer, a fact which might be explained by a dimensional effect. The effect in a-Si:H/a-Si1-xCx:H Multilayers was reduced with green HeNe-laser illumination with photon energies above the Taue gap of the barrier Material, instead of a red illumination generating photocarriers only in the potential wells.