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We have investigated on the effect of different substrate surfaces in changing the microstructure of μc-SixGe1-x:H films prepared by reactive magnetron sputtering. Films were deposited on hydrogen terminated Si(111), Si(100) surfaces, and surfaces chemical and plasma oxides. The thin film microstructure was characterized by Fourier transform infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman scattering.

Amorphous and microcrystalline silicon-germanium alloys, a-SixGel-x:H and μc-SixGel-x:H, respectively, have been prepared by reactive magnetron sputtering (RMS) from pure crystalline Si and Ge targets in a hydrogen ambient using argon as the sputtering gas. We have investigated the structural, optical, and electronic properties of the as-deposited films. The optical and electrical properties, e.g., the ambipolar diffusion length, photoconductivity, and photosensitivity, were found to be comparable to those of device-grade a-SixGe1-x:H alloys, e.g., films with x ∼ 0.5, and band-gaps ∼ 1.3–1.4 eV. In contrast to the behavior of the a-SixGel-x:H alloys, the μc-SixGe1-x:H alloys do not display a Staebler-Wronski effect, as manifested by a decay of the photoconductivity under intense illumination.

Many photoelectronic properties of a-Si,N:H alloys prepared by remote PECVD (RPECVD) from two N-atom source gases - N2 and NH3 - are the same; however, the photo-induced changes in the electrical properties in alloys with -2.1 eV bandgaps are ∼3 to 5 times greater in alloys deposited from NH3, which display Si-NH, as well as SiH bonding. Based on this result, we show that bonding groups important in the Staebler-Wronski effect include (i) ≡SiH, and nearest-neighbor (ii) ≡Si-NH-Sis and/or ≡Si-O-Sis in which the respective N and O-atoms make H-bonds with the sSiH group. The model, based on ab-initio calculations, includes a H-exchange reaction in which trapping of photo-generated holes promotes a transfer of the H-atom from the ^SiH group to a nearest-neighbor ≡Si-NH-Si≡ creating (i) a Si-dangling bond (Si*) and (ii) a Metastable (≡Si-NH2-Si≡) + group. Calculations indicate that neutral (≡Si-NH2≡Sis) ° is unstable, so that relaxation of (≡Si-NH2-Si≡) + groups can occur by trapping of a thermally-released (trapped) electron during a post-light-soaking thermal-anneal. The same type of model is developed for hole/electron trapping-induced H-atom transfer between ≡SiH and ≡Si-0-Si≡ groups in other a-Si:H Materials.

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 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.

SixGei1−x:H alloys which span the transition from amorphous to microcrystalline structures have been prepared by reactive magnetron sputtering (RMS) from pure crystalline Si and Ge targets in different partial pressures of hydrogen, using argon as the sputtering gas. Film properties were studied as a function of H2 flow and partial pressure. X-ray diffraction (XRD), Raman scattering, Fourier transform infrared spectroscopy (FTIR), reflection high-energy electron diffraction (RHEED), and high resolution transmission electron microscopy (HRTEM) have been used for microstructural characterization. Films prepared by RMS at a partial pressure of hydrogen (PH2) < ∼ 4 mTorr were amorphous, while those prepared with PH2 > ∼ 6 mTorr were microcrystalline.

We have incorporated N-atoms into hydrogenated amorphous silicon in the Si-rich alloy regime to ∼12 at.% N, and have observed a transition from n-type doping to alloying as the concentration of N-atoms is increased above about 5 at.%. By analogy with the local bonding arrangements of P-donors in n-doped a-Si:H, we attribute the doping to four-fold coordinated N-atoms with second neighbor H-atoms as in N+-Si-H linkages. The occurrence of these arrangements is supported by (i) IR studies which indicate a non-statistical association of N and H-atoms bonded to the same Si-atom, and (ii) a chemical bonding model in which the large effective electronegativies of four-fold coordinated N+ atoms and neutral O-atoms promote similar bonding properties with respect to their nearest-neighbor arrangements with Si and H atoms such as N+ (O) -Si-H linkages

We have investigated a-Si,N:H alloys as an alternative wide band-gap, photo-active material. The entire alloy range between a-Si:H and a-Si3N4:H can be formed by a remote plasma-enhanced chemical-vapor deposition (PECVD) process. Other studies have demonstrated that a-Si,N:H alloys could be doped to form window materials for p-i-n devices. This paper focuses on alloy materials with E04 bandgaps to about 2.2 eV. We have prepared these a-Si,N:H alloys, characterized their microstructure, and studied their photoconductivity, sensitivity to light-soaking and transport properties. For example, with increased alloying we show that i) the white-light photoconductivity and ii) the kinetics and magnitude of the decay of photoconducitivity under intense illumination (the Staebler-Wronski effect), are about the same as for PV-grade a-Si:H.

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.