The structural, electrical and optical properties of amorphous Si-Ge alloys prepared either from mixtures of hydrides or of fluorides will be compared. It will be shown that the majority of many properties studied are essentially the same, the major exceptions being the structure of thin films used for TEM investigation, and the photocurrents developed between contacts in a coplanar configuration. Possible explanations for the result of including fluorine in the preparation plasma will be discussed.

Results of a detailed study of photoluminescence (PL) in the a-Si1−xGex:H system are presented. Many samples exhibit a low energy “tail” to the PL efficiency which is of constant magnitude independent of x. There is a departure from this behavior when a low energy PL peak near 0.8–0.9 eV is present. The position of the low energy PL peak is independent of Ge concentration. It has been suggested that this PL transition is from an electron in the conduction band tail into a silicon dangling bond state. As Ge is added to a-Si:H it is the edge of the conduction band which decreases in energy while the valence band remains relatively constant in energy. It is therefore unlikely that the low energy PL is due to a transition from the conduction band into a silicon dangling bond state because the energy of the silicon dangling bond with respect to the valence band is probably essentially independent of Ge content. If the PL which peaks near 0.8 eV results from a transition which involves a silicon dangling bond, then the transition may be from the dangling bond to the valence band.

a-S1−xCx:H and a-Si1− Ge :H films are prepared by magnetron sputtering in an argon-hydrogen-methane/germane atmosphere. The hydrogen content of the carbon alloy increases with x, saturating near 40 at% for the maximum C-content (x = 0.88). From a detailed analysis of the infrared absorption modes we conclude that the formation of (CHm)n C/Si-groups (n = 1,2,3, m = 2,3) is responsible for the growth of hy r gen rich and inhomogeneous films. Structural, electrical and optical properties of the films show characteristic changes near an alloy composition of x = 0.5 which indicate the transition to a structure with threefold coordinated carbon. For the germanium alloy system lower DOS and improved photoelectrical properties have been measured for y ≤ 0.01. Decreasing H-content and SiH2 /GeH2 bonding configurations impair the alloy properties for higher y.

The structure of three series of hydrogenated amorphous Si-based alloys was studied by EXAFS at the Si K-edge. Average bond-lengths and first shell compositions were determined. It is shown that the two atomic species are randomly distributed in a-Si1−xGex:H whereas a chemical ordered composition is favored in a-Si1−xNx:H

In order to improve the conversion efficiency of a-Si solar cells, high-quality a-Si based alloys of both narrow handgap and wide bandgap were studied.

Concerning the narrow bandgap material, we found a particular dependence of film qualities on substrate temperature. In addition, high-quality a-SiGe:H films were obtained by using a super chamber (separated ultra-high vacuum reaction chamber).

As for the high-quality wide bandgap material, a-Si/a-SiC superlattice structure films fabricated by a photo-CVD method were studied for the first time. From the analysis of their properties, we found that the superlattice structure p-layer was an active layer for photovoltaic effect. A conversion efficiency of 11.2% has been obtained for a pin a-Si solar cell whose player was of the superlattice structure.

Plasma-deposited a-Si:C:H and a-Si:N:H alloys both show a rapidly rising hydrogen incorporation with increasing carbon or nitrogen content. For hydrogen concentrations exceeding ∼ 20 at.%, the host material starts to lose its connectiveness leading to the formation of a void structure as evidenced by hydrogen evolution and infrared absorption. Raising the substrate temperature leads to a reduction of the hydrogen concentration, to an increase of the Si-C and Si-N bond concentration and to a more compact naterial. However, for a-Si:C:H films it leads also to graphitic carbon bonds. The widening of the optical bandgap of a-Si:C:H films up to about 50 at.% of carbon is almost entirely due to the increased hydrogen incorporation whereas for a-Si:N:H films both hydrogen and nitrogen incorporation plays a role.

We have studied the temperature and intensity dependence (130K to 300K) of photo- and dark conductivity in a series of low-gap a-Si,Ge:H,F alloys (Eopt=1.25 to 1.33 eV) prepared under different deposition conditions. Electron time of flight experiments were conducted between 300K and 400K. Results reveal an increase in the slope of the exponential conduction band tail to ∼ 50 meV and a peak in electron trapping states at 0.3 to 0.4 eV below the conduction band edge, leading to a transition from extended to hopping conduction by electrons at slightly below room temperature. The alloys have midgap defect densities in the low 1017 cm−3eV−1 range.

We present the results of a carbon-13 nuclear magnetic resonance (NMR) study of well-characterized thin films of amorphous hydrogenated silicon carbide. The NMR data detail the distribution of carbon local bonding configurations in films which have carbon-to-silicon ratios less than one. In particular, we show data which clearly identify and quantify non-hydrogenated sp2, or unsaturated, carbon bonding environments.

It is intended to throw some high lights on the photoconductivity properties of narrow band semiconducting CSn alloys. The feasability, the structure and the usual optical properties of dielectric constants and optical gap have been predicted in a previous paper /1/. Starting with a number of well acknowledged models for semiconductors, properties, the values of the ratio between the light and the dark conductivities of a material having an o9tifal gap of 1.2 eV at 300 °K, under an illumination of 1016 photons cm−2s−1 are found between 1 and 103, but a density of gap states lower than 1018 cm−3 eV−1 is required.

We have studied the photoelectronic properities of a- Si(x),Ge(1−x):H alloy films and have concluded that there are depletion layers at the film surface and film/oxide interface that effect the determination of bulk quantum effieciencymobility- lifetime (nuT) products. From changes in dark conductivity activation energy with film thickness and the nuT product with wavelength of incident light we have estimated defeci sta:e lensitles. Our best x=0.5 film has an nuT value of 9×10−8 cm2 V−1 andaj defect state density near the Fermi level of approximately 5×10 cm−3 eV−1.

Amorphous hydrogenated carbon nitride thin films have been grown by plasma decomposition of a feedstock of CH4 and N2. In the films with higher nitrogen concentration, the infrared absorption spectra are dominated by NH2 modes and give strong evidence of a polymeric structure. The optical absorption and photoluminescence spectra show that nitrogen incorporation decreases the bandgap and increases the structural order of these thin films. The undoped material is an insulator with resistivities up to 1015Q cm, but when doped with iron, it becomes a p-type degenerate semiconductor.

We report the temperature dependence of several optical parameters of thin films of hydrogenated and fluorinated amorphous silicon alloys (a-SiGe:H,F) between 5 and 95 °C. The absorption coefficient near the optical band gap Eg increases with temperature. From this increase we calculate a temperature coefficient for Eg of −4.5×10−4 eV/K, which is essentially independent of band gap The concomittant change of index of refraction n was determined in the near infrared region from the interference pattern in the reflection spectra. The temperature coefficient dn/n/dT is 0.9×10−4 K−1 for un-alloyed a-Si:H,F and increases with increasing Ge atomic fraction. The changes of Eg and n with temperature are consistent with a simple quantummechanical description of the complex dielectric constant. We also report the temperature dependence of the minority carrier diffusion length in a-SiGe:H,F.

Infrared, photoconductivity, and photothermal deflection spectroscopy measurements have been used to probe the relationship between material quality and the amount of microstructure in glow discharge deposited a-SiC:H and a-SiGe:T films. We find that the microstructure is directly responsible for the decrease in photoconductivity observed in both alloys as a function of increased alloy content. The microstructure does this by causing a decrease in steepness of the Urbach tail, thus allowing for an increase in both carrier trapping at the wider band edges and carrier recombination at or near the band tails.

We study the dark conductivity σd, dark conductivity activation energy Ea and photoconductivity σph of a-Si:H,F/a-Si,Ge:H,F superlattices both perpendicular and parallel to the plane of the layers. In parallel transport, both the σph and σd are dominated by the alloy layer characteristics with the superposition of carrier confinement quantum effects. In perpendicular transport, the σd shows an interplay of quantum mechanical tunneling through the barriers and of classical thermal emission over the barrier layer and the σph is controlled by the decreasing absorption by the silicon barrier layer as the optical gap Eopt of the structure decreases.

We also found that the multilayer structure allows to grow lower gap a-Si,Ge:H,F alloys than achievable under the same deposition conditions for bulk materials. This stabilizing effect allowed us to study low-gap superlattice structures and extract information about these very low gap (<1.2 eV) a- Si,Ge:H,F alloys.

We report measurements of Schottky barrier heights and minority carrier mobilitylifetime products of multilayer structures composed of a-Si:H,F and a-Si,Ge:H,F. These layers are grown by r.f. glow discharge decompostion of SiF4, GeF4, and H2 in the a-Si,Ge:H,F (well) layer and of SiF4 and H2 in the a-Si:H,F (barrier) layer.

Schottky barrier height ΦB of Pt is measured using internal photoemission measurements. The minority carrier mobility-lifetime product (μτ)p is extracted from a fit of the voltage dependence of internal quantum efficiency to the Hecht expression. Both ΦB and (μτ)p are measured as a function of barrier and well thicknesses.

Results are presented on the electron transport, parallel and perpendicular to the layers, in a Ge:H/a-Si:H superlattices. The dependence of this carrier transport on the optical gap shifts, the heterojunction band alignment, as well as the bulk properties and thickness of the Ge,Si layers is established. No significant change in the transport mechanisms at the band edge or Ge with dr was detected.

Amorphous super lattices composed of a-Si:H and a-SiC:H:F layers were continuously prepared from a gas miture of CF4 and either SiH4 or Si2 H6 by the pulsed plasma and photo (PP&P) CVD method. The method utilizes the difference in photochemical reactivity of the source gases for the formation of layered structure. The layered films prepared by using Si2H6 as silicon source under the condition that they had optical gaps up to 2.0 eV were found to show photoconductivities higher not only than those of a-SiC:H:F films prepared from the same gas mixture by the continuous plasma and photo hybrid CVD but also than those of a-Si:H films by the photo CVD. From the comparison of optical and electrical properties of super lattices prepared, the photoconductivity of the films is suggested to be primarily dependent on the quality of barrier layer rather than on the quality of well layer.

A method for determining the density of states [N(E)] in the upper half of the energy gap in hydrogenated amorphous silicon (a-Si:H) is proposed and experimental results are simulated. The method involves the growth and measurement of the planar conductivity in multilayer films in which each layer is separated by a thermally grown oxide. Band-bending occurs at each interface throughout the film thickness. The conductivity parallel to the layers in the films is a function of the band-bending, which in turn depends on N(E), in the energy range through which the Fermi level is shifted. Computer simulations of the oxide-induced band-bending have been used to generate curves of the conductivity as a function of layer-thickness for various N(E). By matching experimental results with the simulation curves, the N(E) may be deduced. The simulations have also been used to show the difference between the bulk conductivity activation energy and effective activation energy which is measured in films influenced by a surface oxide.

The contact properties between different metals and hydrogenated amorphous silicon, prepared by various deposition techniques in different laboratories, are reviewed. From these studies the appropriate metallizations have been established for the achievement of Schottky diode, quasi-ohmic or ohmic contact to undoped and doped films. The various characteristic parameters describing Schottky barrier interfaces such as ideality factor, current saturation, contact resistance and barrier height are discussed. The dependence of Schottky barrier height upon the metal work function, measuring and annealing temperature, and optical band-gap are also reported. The minority-carrier injection and series resistance effects on the contact properties of a-Si:H diodes are described. All the results are interpreted in terms of a self-consistent model that exhibits an electrode-limited to bulk-limited transition.

Ellipsometry measurements of hydrogenated amorphous silicon (a-Si:H)-based heterostructures have been used to characterize surfaces and single interfaces. From spectroscopic studies before and after plasma oxidation of a-Si:H, surface roughness on the atomic scal; (in the plane of the surface) can be distinguished from larger scale (> 100 Å) modulation. We also show how in situ studies of heterostructure preparation can be used to set deposition conditions to minimize near-interface layers owing to thickness dependent film properties. When these are eliminated, the discrepancies in the data from planar layer-bylayer growth models can be attributed to the larger scale surface and interface modulation. Finally, extensive deviations from planar growth for a-Si:H/μc-Si:H have been characterized in terms of specific heterogeneities.