Amorphous Silicon-based Alloys Research Articles

High quality amorphous silicon materials and cells grown with hydrogen dilution

Hydrogen dilution of the active gas during deposition has been found to be a very effective way to improve the quality of amorphous silicon-based materials and solar cells. With increasing hydrogen dilution, the material is characterized by an improved order, and at a certain threshold dilution, the amorphous to microcrystalline transition takes place. The best material is grown just below the threshold and is heterogeneous consisting of tiny crystallites embedded in an amorphous matrix of improved order. In this paper, we discuss the effects of hydrogen dilution on the material and cell properties of amorphous silicon-based alloys and provide an explanation for their improved stability against light-induced degradation. We also discuss some special properties of the on-the-edge materials that are not seen in the conventional amorphous or microcrystalline alloys.

Wide band gap amorphous silicon-based alloys

Wide band gap a-SiC:H and a-SiN:H films have been deposited by ultra high vacuum plasma enhanced chemical vapor deposition. For a-SiC:H films SiH 4 + CH 4, SiH 4 + CH 4 + H 2 and SiH 4 + C 2H 2 gas mixtures have been used, whereas for a-SiN:H films SiH 4 + NH 3 gas mixtures have been employed. The deposition conditions have been chosen for all the feedstocks to allow the growth of high electronic quality materials, in order to reach a complete understanding of analogies and differences in technological and physical characteristics of the two alloys, which is necessary for a more efficient use in electronic devices. Compositional, optical, structural, defective and electrical properties have been investigated for both a-SiC:H films, having optical gap in the range 1.9–3.25 eV, and a-SiN:H films, having optical gap in the range 1.9–5.2 eV. By Raman spectroscopy a lower structural disorder has been evidenced in a-SiC:H films compared to a-SiN:H ones and by electron spin resonance a lower spin density have been found in a-SiN:H films. The a-SiC:H and a-SiN:H films have similar electrical properties at the same band gap.

Preparation and properties of anodized amorphous silicon

Boron-doped amorphous silicon layers deposited by glow-discharge on conductive substrates have been electrochemically etched in the same hydrofluoric acid solutions that yield intense visible light-emitting porous layers from crystalline silicon. We show that both as-etched and electro-oxidized a-Si:H films are luminescent at room temperature in the visible and near infrared and we report electroluminescence spectra recorded during the anodic oxidation. The room-temperature photoluminescence decay times of a few tens of μs are longer at larger emission wavelengths and comparable to those of porous wafers obtained in the same way. In both cases, the decay pattern is shown to be qualitatively distinct from that observed in amorphous silicon-based alloys.

Ambipolar Diffusion Coefficients in a-SiC:H Alloys in Steady- State and Transient Grating Measurements

Recently we have determined surprisingly large values of the ambipolar diffusion coefficient D of 3 − 9 cm2/s in amorphous silicon-based alloys and a-Si:H/a-SiC:H multilayer structures from transient grating decays in the psec time domain. The steady-state photocarrier grating method, however, resulted in much lower D values (∼10−4 cm2/s) in the same samples. Since high carrier densities of typically 1019 cm−3 are reached in the psec domain, the Einstein relation may no longer be valid. The large diffusivity of non-equilibrium carriers decreases, however, rapidly in time due to energy relaxation and carrier recombination until a stable trap occupation under steady-state condition is reached.

Interdiffusion At a-Ge:H/Al and a-Si:H/Al Interfaces

The degree of interdiffusion at the amorphous semiconductor/bulk Al interface was studied using Auger electron spectroscopy analysis. 300-500 Å thick a-Si:H and a-Ge:H films were deposited onto high-purity Al and 5052 Al alloy substrates and subsequently annealed to various temperatures up to 500 °C for 6 hrs. The high-purity Al is used as a substrate for our small-angle x-ray scattering studies of amorphous silicon-based alloys. For all the films deposited on the pure Al, little or no interdiffusion was noted at or below anneal temperatures of 400°C. This result is contrary to those commonly found for samples produced by evaporating Al onto the previously deposited amorphous semiconductor without breaking the vacuum where interdiffusion has been noted at temperatures at or below 200°C. We suggest interdiffusion in the amorphous semiconductor/bulk Al samples is hindered by the presence of a 150-300 Å Al oxide on the Al. A large amount of interdiffusion and partial crystallization is noted in the a-Ge:H sample after an anneal of 450°C while a temperature between 450 and 500°C is required for interdiffusion and crystallization to occur in the a-Si:H sample. In the case of the a-Ge:H films deposited on the 5052 Al alloy, interdiffusion occurs after 300°C anneals due possibly to the migration of Mg and other components of the alloy into the amorphous semiconductors or structural defects in the alloy which enhance interdiffusion.

Small-Angle X-Ray Scattering from a-Si:H and a-SiC:H Alloys Prepared by Reactive DC Magnetron Sputtering

ABSTRACTThe microstructure of amorphous silicon-based alloys prepared by reactive magnetron sputter (RMS) deposition has been examined on a scale from 1 to 25 nm using small-angle x-ray scattering (SAXS) and compared to that from films prepared by standard glow-discharge (GD) technology. Device-quality RMS a-Si:H material is found to have a larger microvoid fraction than device-quality GD a-Si:H and to have a significantly different size distribution. Addition of by the RMS technique using CH produces enhanced SAXS similar to the introduction of C via CH4 in GD material. A significant difference in the SAXS from RMS a-Si:H and a-SiC:H films compared to GD films is the observation of some oriented microstructure, most likely columnar in nature. Flotation density measurements of the same films examined by SAXS support the assumption that the SAXS originates primarily from microvoids.

Hydrogen Stability in Hydrogenated Amorphous Silicon-Based Alloys

ABSTRACTThe thermal stability of hydrogen in amorphous silicon-based alloy films was studied by deuterium/hydrogen interdiffusion and hydrogen effusion experiments. Depending on the film structure, hydrogen stability is limited by hydrogen surface desorption or hydrogen diffusion. The hydrogen surface desorption energy is found to decrease with rising germanium content and to increase with rising nitrogen and carbon content. At T = 400°C, hydrogen diffusion is found to proceed in the germanium subnetwork for a-SiGe alloys and in the silicon subnetwork for a-SiN and a-SiC alloys.

Relation between thermal equilibrium temperature and bandgap in undoped hydrogenated amorphous silicon-based alloys

Abstract This Letter demonstrates that there is a good correlation between the thermal equilibrium temperature T E and the optical gap E opt for undoped hydrogenated amorphous silicon (a-Si: H) and a-Si-based alloys: T E, is proportional to E opt for both wide-bandgap a-Si-based alloys, such as a-Si1–x Cx: H and a-Si1–x Nx: H, and narrow-bandgap a-Si1–xCx: H alloys. It is also found that the relaxation process of all thermally-induced metastable changes in these alloys follows a stretched exponential form.

Optical absorption and defects in amorphous SiNx and SiOx

Abstract Photothermal deflection spectroscopy has been used to measure optical absorption for two series of amorphous silicon-based alloys prepared by the decomposition of silane/ammonia or silane/nitrous oxide mixtures to produce a-SiNx or a-SiOx respectively. Measurements have been made in the energy range 1 to 5 e V. Each sample shows a well-defined exponential edge and a low-absorption region. The inverse slope of the exponential edge increases monotonically with nitrogen or oxygen content. The low-absorption region is analysed and discussed in terms of the defect-state density. For the silicon nitride samples the results can be interpreted in terms of silicon dangling-bond defect states. On the incorporation of some nitrogen the density of defect states increases, but the defect remains at a certain energy above the valence band edge. As stoichiometry is approached the number of such states decreases. For the silicon oxide samples, oxygen incorporation results in a decrease in the density of silicon ...

Intensity dependence of the minority-carrier diffusion length in amorphous silicon based alloys

In this paper we present results showing the intensity dependence of the minority-carrier diffusion length in an amorphous silicon-based alloy. A theoretical analysis shows that this dependence can be related to the energy dependence of the density of states near the valence band, which assumed to be exponential, has a characteristic energy of approximately 240 mV around 1.2 eV below the conduction band edge.

Effective correlation energy of the dangling bond in amorphous silicon

The current theoretical and experimental situation with regard to the sign of the effective correlation energy of the dangling-bond defect in amorphous silicon-based alloys is reviewed. It is concluded that the defect could well have a negative effective energy if complete atomic relaxations occur.

Normal structural bonding and defects in covalent amorphous solids

Although the 8- N rule of covalent bonding is generally obeyed in amorphous semiconductors, well-defined defect centers exist and these control the electronic properties of the solids. The defects have two distinct reasons for their presence—they can arise from either strains upon material preparation or thermodynamic considerations. The strain-related defects characterize those amorphous solids in which the average coordination number is larger than approximately 2.4; their concentration is ordinarily very sensitive to the preparation techniques. In contrast, thermodynamically induced defects arise because of their low creation energy, and a minimum concentration characterizes any given material. These ideas have led to a resolution of several major puzzles with regard to the electronic properties of the two major classes of amorphous semiconductors—chalcogenide glasses and amorphous silicon-based alloys. Pure amorphous silicon is overconstrained and has large defect densities, but these can be reduced by many orders of magnitude if the material is alloyed with monovalent atoms such as hydrogen or fluorine. On the other hand, amorphous As 2Se 3 always contains a high defect density, for thermodynamic reasons. In addition to the concentration of defects present in a given material, its electronic properties depend critically also on the nature of these defects. In particular, the sign of the effective correlation energy of the defect with the lowest creation energy is of the utmost importance.

Minority-carrier diffusion lengths in amorphous silicon-based alloys

In this paper, we present results of collection efficiency versus wavelength measurements on thick (⩾1 μm) amorphous silicon Schottky barrier photovoltaic devices illuminated through both the ohmic and Schottky contacts. These results enable us to infer the zero-field minority carrier diffusion length in amorphous silicon-based alloys by using an appropriate theoretical model. For intrinsic a-Si:H (hydrogen) films produced by the glow discharge of silane this was found to be ∼2,100 Å. Our novel approach of illuminating the devices through the ohmic contact ensures that the collection efficiency results are extremely sensitive to the minority carrier diffusion length, and we also compare this approach to surface photovoltage type experiments which we show can lead to overestimation of this quantity.

Recombination mechanisms in amorphous silicon-based alloys

Photoconductivity has been measured as a function of temperature, photon energy, and electric field strength for amorphous Si:F:H alloys. The results clearly show that geminate recombination is not important in the visible region of the spectrum in these materials. The significance of these results for efficient solar photovoltaic energy conversion is discussed.

Electronic structure of amorphous semiconductors

The current theory of the electronic structure of amorphous semiconductors is reviewed. It is now clear that the properties of covalent amorphous solids simply represent the predominant mode of chemical bonding in the material. As in crystalline semiconductors, the electrical transport properties are controlled by the nature of the defects present. Two major classes of amorphous semiconductors have been studied intensively: amorphous silicon-based alloys and chalcogenide glasses. The properties of these materials is contrasted and the origin of the striking differences between these groups is analyzed.

Properties of amorphous Si:F:H alloys

Amorphous silicon-based alloys containing hydrogen and fluorine prepared from SiF 4 and H 2 using r.f. glow discharge technique are shown to possess low density of states and high photoconductivity. Because of the inherent low density of states, the material can be doped n type very effectively. Because of the lower state density, the Schottky barrier devices in comparison with a-Si:H alloy exhibit a wide depletion region. The mechanisms of the device performance is also put forward.