First observation is reported of the deposition rate dependence of Raman-detected structural disorder in a-Si:H/a-SiNx semiconductor superlattices as well as a-Si/a-SiNx ones. FWHM of TO-like Raman peak of a-Si:H in the multilayer structure rapidly decreases as deposition rate decreases, while that of a- Si decreases more slowly. The results demonstrate that the structural disorder of a-Si:H/a-SiNx decreases as time to grow a monolayer (TGM) increases in a time range of a second, and also suggest that hydrogen covering the growing surface enhances the structural-relaxation velocity of disordered amorphous network.

Random Telegraphic Noise (RTN) with an amplitude of about 1% has been observed in the current through a-Si:H/a-Si1−xNx.:H double barrier structures. The area of the devices, 0.25 mm2, is five orders of magnitude larger than the area of the devices where RTN has previously been observed. The power spectra of the noise can be fitted by a superposition of Lorentzians from which average switching times are deduced. The switching times are thermally activated with activation energies between 0.2 and 0.6 eV and depend on bias voltage. Assuming that the current through these structures is confined to microchannels of cross section area < lμm2 the RTN and its characteristics are explained by the random charging and discharging of individual traps placed in the vicinity of the current channel.

We present an in-situ Kelvin probe study of the effects of hydrogen and argon plasma treatments on fluorine doped tin-oxide (SnO2) substrates, as well as the consequences of these treatments on the subsequent growth of a-Si:H. We found that the effect of both hydrogen and argon plasmas is to enhance the reactivity of SnO2 substrates in a silane plasma. Moreover, the, doping level (Fermi level) of boron doped a-Si:H layers deposited on SnO2 is reached more quickly when the substrate has been treated previously with an argon plasma. As a consequence, we expect a higher open circuit voltage (Voc) for SnO2/p-i-n solar cells in which the substrate has been treated by an argon plasma. The Kelvin probe results have been confirmed by Voc measurements on solar cells deposited on treated and untreated substrates.

a-Si:H/a-Si1−x Cx:H double-barrier structure imbedded in the i layer were fabricated by glow discharge. By using the built-in field in the i layer and the external applied bias, the effects of photo-excited carriers driven by electric field in the i layer tunneling through the double-barrier quantum well were examined. Distinct current bumps are distinctly observed when the quantum well structure is imbedded in the middle section of the i layer. A simple theoretical analysis was developed to study the quantum transport behavior.

The optical properties of random multilayer films are presented. Very small disorder in the well layer thickness (3 A standard deviation in 20 A) change the optical properties drastically. These optical spectrum are found to be useful to detect the disorder in the multilayers and also are very suitable to investigate the properties of randomness.

A novel a-Si:H/c-Si Schottky diode heterostructure device premits the study of the capacitive response to relatively low temperatures (25 K) and also allows fast pulse filling capture measurements of electrons from the c-Si substrate into a-Si:H defect states. These latter measurements, as well as capacitance vs. temperature measurements on these diodes, indicate a nearly zero conduction band offset (50±50 meV). We also have observed trapping of holes at the a-Si:H/c-Si valence band discontinuity ΔEv. A clear threshold for the subsequent optical release of these trapped holes by sub-bandgap light yields a value of ΔEv = 0.58±0.02 eV. Finally, photocapacitance spectra along with thermally stimulated capacitance (TSCAP) measurements indicate an anomalously large (l×1018/cm3) Gaussian-shaped defect band located a Ec - 0.88 eV with a FWHM of 0.46 eV. Model calculations of the high temperature capacitance-voltage dependence indicate these defects lie predominately within 350 Å of the a-Si:H/c-Si interface.

We describe a detailed study of surface states on a-Si:H, F films deposited by DC-plasma from SiF4 and H2. In the a-Si:H, F growth from these source gases, the density of surface states (Nss) and the Urbach Energy (Eu) show a reciprocal relation, meaning that the film grown with less bond distortion is accompanied by a higher density of surface states. These surface states can be reduced by keeping the films at growth temperature after termination of growth; immediate cooling produces a high density. This suggests that bonds can rearrange on or near the film surface at growth temperature, after growth termination. Nss can be made as high as l×1014 cm−2. The surface states can be measured by optical subgap absorption, but do not give an ESR signal. A high density of these surface states lowers the Pd-Schottky barrier height. Time-of flight electron collection data show that the surface states act as deep electron traps, located at about 0.4eV below the conduction band edge. We present and discuss other experimental results to help understand these nature of the surface state on a-Si:H, F.

We discuss high-resolution x-ray diffraction measurements on a-Si:H/a-Ge:H periodic amorphous multilayers. Analysis of the data using the dynamical theory yields information on layer thicknesses and densities, interface and surface roughness, and structural defects such as layer thickness fluctuations.

Comparative studies of low-temperature (T ∼ 30 K) optically induced ESR using above gap red (633 nm) and below gap i.r. (1060 nm) light have been performed on several samples of a-Si:H. Experiments were performed on both films on quartz substrates and on powdered samples which were removed from the substrates. Excitation with red light always yields the well-known light induced ESR (LESR) spectrum which is ascribed to a superposition of two resonances: electrons trapped in conduction band-tail states and holes trapped in valence band tail states in a 1:1 ratio. On the other hand, excitation with i.r. light results in a different LESR spectrum with an enhanced feature overlapping with the signal attributed to trapped band-tail electrons. Although observed on all films studied on quartz substrates, this asymmetry is not observed in a powdered sample where only the band tail electron and hole resonances are observed in the ratio 1:1. This result suggests that interface states are responsible for the excess LESR spin density in the films on quartz substrates. Experiments performed as a function of film thickness confirm this interpretation.

The electron and hole transport perpendicular to the plane of the layers in a-Si:H, F/a-Si, Ge:H, F multilayers is analyzed. We measure the electron dark conductivity σd and its activation energy Ea, d, the photo conductivity σph and its exponent γ, the electron and hole mobility-deep trapping lifetime (μτd)e, h and the hole mobility-recombination lifetime (μτr)h. We identify three regions of barrier thickness ds with very different transport properties: (a) ds≲50Å, dominated by tunnelling between quantum confined states in the bottom of the wells; (b) 50Å ≲ds≲200Å, in which the well acts as the provider of an extra, controllable tail state density; and (c) ds≳200Å, where the individual layers are essentially decoupled.

We report on our investigation of the silicon nitride on hydrogenated amorphous silicon interface by high frequency capacitance-voltage measurements. We show evidence for the existence of a large number of interface states at energies less than or equal to 0.36 eV below the hydrogenated amorphous silicon conduction band. In addition, we demonstrate that the states responsible for the hysteresis in capacitance-voltage characteristics are located in the silicon nitride layer. Finally, we argue that the ability of hydrogenated amorphous silicon to act as hole diffusion barrier is responsible for the observed flat-band voltage shift.