We describe the optoelectronic characteristics of hydrogenated amorphous silicon germanium carbon (a.Si1-x-yGexCy:H) alloys prepared by plasma deposition from SiH4/GeH4/CH4/H2 gas mixtures. a-Si1-x-yGexCy:H is a homogeneous random alloy having a variable optical gap depending on composition, with properties similar to those of amorphous Si-Ge alloys of the same optical gap but with improved thermal stability. Calculations show that if the ratio of Ge/C atomic fractions is 8.2, the average bond length matches that of unalloyed amorphous a-Si:H with the possibility of reduced defect densities at heterointerfaces. After light-soaking with high intensity white light, a sample having a 1.3 eV optical gap exhibited no Staebler-Wronski change in its properties.

The properties of amorphous silicon nitride films (a-SiNx:H) prepared by PECVD from SiH4-NH3 and SiH4-N2 gas mixtures have been determined by spectroscopie ellipsometry and FTIR spectroscopy as a function of the nitrogen concentration measured by XPS. The films are transparent for silane ratios [SiH4]/([SiH4] + [NH3]) < 20% and [SiH4]/([SiH4] + [N,]) < 1.5%. The refractive index shows a wide range of progressive variation from 3.2 for high silane concentrations to 1.8 for low silane concentrations. The hydrogen content of the low-absorbing films has much lower values for those obtained by SiH4 + N2 plasma than for those obtained by SiH4 + NH3 plasma. The results are discussed in terms of growth models of PECVD a-SiNx:H films from SiH4-NH3 and SiH4-N2 mixtures.

We study the high temperature (≤1300°C) thermal evolution from hydrogenated amorphous silicon nitride (a-SiNx:H) films prepared by plasma enhanced chemical vapor deposition. Two principle peaks are found, one at 525–750°C associated with hydrogen release, and one at ≥950°C from hydrogen and nitrogen release. In nitrogen-rich films (x>4/3), the low temperature (525°C) peak intensity is smaller compared to silicon-rich films (x≤4/3), implying that hydrogen is more thermally stable in N-rich films. Helium dilution during film growth further reduces the low temperature peak intensity, producing the most thermally stable N-rich material, with the onset of hydrogen evolution occurring at ∼600°C. For a nitrogen-rich film, high temperature hydrogen evolution began at ∼900°C and was accompanied by nitrogen evolution starting at ∼950°C. UV-illumination of N-rich samples prior to thermal evolution produced no observable changes in the evolution spectra.

The electro-optical properties of hydrogenated amorphous silicon nitride films (a-SiNx:H) prepared by rf glow discharge of SiH4 and N2 have been determined as a function of the silicon content in the alloy. The stoichiometry and structure of the layers have been studied by ellipsometry, infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Two different electrical behaviours have been found. The samples with x>0.8 show conductivity based on the Frenkel-Poole mechanism, while the samples with x<0.8 show quasi-ohmic conductivity. Both kinds of conduction and the transition between them are analyzed in the framework of the percolation theory. In this context, the correlation between the stoichiometry and structure of the layers with their electrical behaviour indicate that the transition from the Frenkel-Poole to the quasi-ohmic conduction is a consequence of the formation of conducting paths as the percolation threshold of Si-Si bonds is reached.

We report the measurement of photoluminescence (PL) from nitrogen-rich hydrogenated amorphous silicon nitride (a-SiN1.6:H) films. Excitation at 4.8 eV gives a PL spectrum of approximately Gaussian lineshape centered at 2.9 eV with a full width at half maximum of 1.4 eV. The PL intensity at 2.9 eV fatigues under continuous 4.8 eV excitation, but may be restored by thermal annealing or photobleaching with light of energy ≥ 2 eV. For emission at 2.9 eV, the PL excitation spectrum in the range 3.0 to 5.2 eV follows the a-SiN1.6:H film optical absorption edge at low energy and then decreases with a maximum at approximately 5.4 eV. The PL results are discussed within the framework of a negative effective electron-electron correlation energy (negative Ueff) for the dominant defect (silicon atom bonded to three nitrogen atoms).

We have developed an alloy of silicon rich amorphous silicon nitride as a linkable interlevel metal dielectric. This material is thermally stable through a heat treatment of 425°C for 30 minutes. The hydrogen content remains at approximately 21 at.%, even after this heat treatment. The breakdown potential is 1.9 × 106 V/cm at which point a metallic filament link forms between the metal contacts. The index of refraction is 2.43 and the HF permittivity is 9 Co- One important property is its ability to store polarized charge like an electret. This feature could allow it to be used in memory applications like MNOS. The alloy is also photoconductive with a response time of less than 5 μs, allowing it to be used as part of an optically sensitive circuit element.

Infrared (ir) spectroscopy is used to investigate the structural properties of a-SiC:H in a wide compositional range and as a function of film thickness. Hydrogen content NH increases considerably with increasing carbon fraction. For low carbon alloys this is mainly due to an increase of hydrogen bonded to silicon, incorporated in a mono- or dihydride form. Above Eg=2.3eV the proportion of hydrogen incorporated in C-H bonds increases considerably. Oxidation of high C alloys is observed. Converting experimental transmission exactly into absorption data yields thickness independent NH values. It is shown that the previously reported discrepancy between the hydrogen content calculated from ir and nuclear reaction techniques is an artifact of the ir analysis.

We discuss electrostatic random potentials in doped and in compensated amorphous semiconductors. These potentials are caused by the residual inhomogeneity of a random distribution of charged dopants and their compensating charges. Random potentials are also present in undoped material with negative-U defects. A high density of positive-U defects can also give rise to a random potential in undoped material.

We demonstrate with the help of detailed model calculations the effect of such random electrostatic potentials on the transport properties. For transport in extended states the random potential does not give rise to a mere shift of the mobility edge. Instead several new features are observed: the activation energy of the resulting Ohmie dc conductivity is virtually unaffected by the random potential in contrast to the activation energy of the thermoelectric power and that of the Hall effect, respectively. The Ohmie dc current changes at high fields into a superlinear current. The random potential contributes to the dispersion of the transients in time-of-flight experiments but leaves the field dependence of the TOF mobility unaltered. Comparing our results with experimental data we discuss under which circumstances the effect of random potentials can be identified.

To date the photocarrier grating (PCG) technique is widespread among a- Si:H researchers. This technique is presently the only available technique for the determination of the minority carrier diffusion length in materials where this length is in the submicron range. The basic idea or the PCG technique will be presented, and results which follow its fruitful use in the study of a-Si:H materials and devices will be reviewed. Finally some future studies of these materials by the PCG method will be suggested.

General features of the steady-state photocarrier grating technique applied to amorphous semiconductors are investigated by complete numerical simulation. The results are interpreted with an analytical model which delivers a closed-form expression for β(A,E) assuming dominance of one carrier type. The variation of the electric field E instead of the grating period A is suggested as an easier and more accurate tool for the experimental technique.

The total scattering time of free carriers injected optically in the extended states of amorphous semiconductors has been measured using the techniques of femtosecond time-resolved spectroscopy. We find that this scattering time is less than 1 femtosecond, independent of alloy composition (a-Si:H, a-Si,Ge:H, a-Si,C:H) and temperature (from 77 K to 400 K). The extended state mobility deduced from the measurement of the scattering time and of the effective mass is approximately 6 cm2/Vs. A model is proposed to explain these results

The transient characteristics of amorphous silicon hydride (a-Si:H) devices are of great importance as a means of studying the fundamental electrical and photo-electrical processes in the material. To assist in the interpretation of these experiments, several groups have performed simulations to understand better the meaning of transient waveforms. Simulations begin with hypotheses of how the processes of interest affect each other and affect the measured quantity, and then calculate an expected experimental result. Transient experiments on a-Si:H have been simulated by a variety of techniques, including the Monte Carlo, Laplace transform, and time integration methods. Of these methods, time integration is the most powerful but also the most demanding on computer capabilities. Fortunately, recent advances in computer technology have brought the simulation of transient experiments on a-Si:H by time integration within the capabilities of advanced personal computers.

In this paper we report on a range of simulation studies. The transient photocur-rent decay after switching off of steady illumination is investigated and we show that the protracted decay in undegraded pin-solar cells is a hole current. In degraded pin samples a change of sign in the photocurrent during the decay is observed by simulation (as is experimentally) which is attributed to an electron diffusion current in reverse direction. We study the distribution of defects according to the defect pool model in a pin structure. The varying dangling bond density with position of the Fermi level leads to large inhomogeneities in defect density across the i-layer in thermal equilibrium leading to increased band bending and giving increased recombination close to the interfaces under illumination. As a demonstration of another application of the simulation method we show how the quasi-steady state is established in the steady state photocarrier grating experiment in lock-in technique.

A photomixing technique was employed to determine the lifetime and the mobility in intrinsic a-Si:H, by measuring the power of the beat frequency in conjunction with the dc. photocurrent. Both the mobility and lifetime were found to decrease continuously following the light-soaking process, which indicates the existence of long-range potential fluctuations and a change in the recombination processes due to light-soaking. For the first time, evidence is provided that degradation is accompanied by loss of mobility, a fact that is neglected in all prevailing models for the Staebler-Wronski effect. Intrinsic a-Si:H samples produced by glow-discharge deposition and the hot wire technique were employed in this work.

Statistical analysis of the 1/f noise power spectrum of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) find that the noise arises from a small number of correlated fluctuators. The noise power displays a complicated time dependence with the noise power changing in both magnitude and as a function of frequency. Spectral analysis of these noise power fluctuations display an approximate 1/f frequency dependence. These results are surprising given the effective volume (∼10-7 cm3) of the sample, and indicate that cooperative dynamics govern conductance fluctuations in a-Si:H.

The position of the dark fermi level in hydrogenated amorphous silicon (a-Si:H) is important in determining its electrical properties and is a key parameter in the detailed modelling of materials and devices. The activation energies of conductivities have been investigated on intrinsic a-Si:H films from various laboratories where the slopes of the Arrhenius plots have ranged from 0.27 eV to 0.87 eV. In many cases, marked differences are found between the results obtained from two and four probe measurements, highlighting the importance of the four probe configuration. Results are presented which help to explain the scatter in the measured activation energies between intrinsic films of a-Si:H. Differences in the activation energies are discussed in terms of current limiting processes which act in series with the material bulk resistance. It will be shown that the conductivity of the film and the contact are key factors in assessing whether the position of the fermi level can be accurately determined from a two contact measurement.

Transient and steady-state measurements of the ambipolar diffusion length (L*) in undoped a-Si:H films have been carried out through Flying Spot Technique (FST) and Spectral Photovoltage Technique (SPT) using either Schottky or pin structures. The FST is based on the photovoltage response of a monochromatic light that moves with a constant velocity in the interface direction, while in SPT the optical excitation is achieved by changing the absorption coefficient of the incident light and so, the light depth penetration. In FST the additional photo-effect due to the light spot movement allows to infer separately L* and the effective lifetime, (τ*) while in SPT the curve shape of the spectral response allows to estimate the interface behavior and L*. The obtained results reveal that the transient measurement is useful for determining the transport properties of the bulk while the steady technique seems to be more useful to infer the role of the interface on device performances. Limitations of the above techniques will also be reported.

A specially designed photoconductivity experiment directly shows that, above room temperature, the steady-state recombination lifetime of the free electrons in undoped amorphous silicon can be described by the free electron quasi-Fermi energy alone. The apparent discrepancy on recombination demarcation levels between the Rose model and the Simmons-Taylor model is resolved by including the effect of the occupation function. The significance of the free electron quasi-Fermi level is discussed.

Fermi level and light intensity dependences of electron and hole lifetimes have been calculated using a recombination model which considers positively correlated dangling bonds as the only localized states in the gap. The model equations have been solved numerically taking into account the non-equilibrium statistics of correlated electrons and the Fermi level dependence of the defect density. The results are in agreement with the anticorrelated behavior of the majority' and minority carrier μτ products observed in a-Si:H. The majority carrier lifetime is found to be more sensitive to the photogeneration rate than the minority carrier lifetime. The position of the Fermi level with respect to the energies of the D° and D- centers in the gap is a determinant factor of the (°τ)e/(μτ)h ratio.

Photoluminescence (PL) in a-Si:H has been studied as a function of excitation energy from above the gap down to well below the gap. Although the width of the PL peak shows no strong dependence on the excitation energy at low temperatures, the low energy tail of bandtail PL exhibits an approximately exponential drop with decreasing energy, I ≈ exp(hν/δE), where δE increases with decreasing excitation energy for excitation energy below about 1.65 eV. This behavior is attributed to the slow downward hopping of holes in the valence band tail that are generated below a thermalization threshold at low temperatures.