Mobilities of a series of compensated a-Si:H samples, measured earlier by the time-of-flight technique [1], were determined by the technique of photomixing. We have found that both the extended state and the drift mobilities decrease as the compensation increases. Modelling these transport processes in the context of the photomixing technique, it is shown that long-range potential fluctuations can account for the decrease in the extended state mobility in compensated samples.

We report on a comparative study, employing the TOF-technique for the characterization of the properties of a-Si:H/a-SiGe:H-heterojunctions. Both the simulated and experimental electron current transients exhibit a pronounced difference depending on the direction of movement of the excess carriers (from the a-Si:H into the a-SiGe:H or vice versa). For the movement from the low bandgap a-SiGe:H into the a-Si:H we find a char¬acteristic increase in the transient current at room temperature, which, as the simulation can reveal, is attributed to the higher drift mobility in the a-Si:H. A drop in the current is observed when the direction for the transit is from a-Si:H to a-SiGe:H. The post-transit behaviour is dominated by the large amount of trapped carriers which remain on the SiGe side of the sample. The situation for a high barrier between a-Si:H and a-SiC:H and the influence of various parameters on the shape of the current transient are discussed.

We have measured the temperature-dependent electron drift mobility in a series of hydrogenated amorphous silicon-carbon alloys using time-of-flight. The specimens were prepared at Solarex using the gas mixture procedures which have recently yielded improvement in the solar conversion efficiency of wide bandgap solar cells. As the bandgap increased due to carbon alloying the electron drift mobility decreased by as much as a factor 30 at some temperatures. The cells with 1.75 eV, 1.81 eV, and 1.87 eV bandgaps had thermally activated drift mobilities over the temperature range 120 K ‐ 200 K; this is associated with simple multiple-trapping behavior. Specimens with bandgaps near 1.90 eV did not have simply activated drift mobilities; we have not accounted for this behavior, but it suggests that the bandtail broadening description used to account for the effects of germanium alloying on the electron drift mobility may not be simply applicable to carbon alloying.

We have measured the electron drift mobility in a-Si:H at high electric fields (E ≤ 3.6 x 105 V%cm). The a-Si:Hpin structure was prepared at Palaiseau, and incorporated a thickp+ layer to retard high field breakdown. The drift mobility was obtained from transient photocurrent measurements from 1 ns - 1 ms following a laser pulse. Mobility increases as large as a factor of 30 were observed; at 77 K the high field mobility de¬pended exponentially upon field (exp(E/Eu), where E u= 1.1 x 105 V%cm). The same field dependence was observed in the time range 10 ns – 1 μs, indicating that the dispersion parameter change with field was negligible. This latter result appears to exclude hopping in the exponential conduction bandtail as the fundamental transport mechanism in a-Si:H above 77 K; alternate models are briefly discussed.

We have deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD), under particular conditions of high H2 dilution and high power density, undoped and phosphorus doped (µc‐SiC:H films. We have obtained n‐doped films with a high band‐gap (E04 up to 2.1 eV) and a high dark conductivity (σ4 up to 10 S cm‐1) which are promising materials for window layers in solar cells. Thermo Electric Power (TEP) measurements allowed to identify the type of majority carriers. The dark conductivity and the Hall mobility have been obtained as a function of temperature in the range 80‐480 K.

Hall experiments performed on intrinsic, phosphorus and boron doped a-Si:H and a-SiC:H in the temperature regime 200 K ≤ T ≤400 K are introduced and discussed. The data confirm the double sign anomaly meaning the Hall coefficient is p-type on intrinsic and phosphorus doped a-Si:H and a-SiC:H, and n-type on boron doped a-Si:H. The Hall mobilities, μH, are significantly smaller than the drift mobilities, decreasing with increasing doping and/or carbon content. μH of holes is about half that for electron which indicate that μH scales approximately with 1/Eg where Eg is the tail slope. The presented interpretation of the Hall coefficient, that is introduced to be a function of Hall- and drift-mobility, enables the accurate determination of the carrier density in intrinsic and doped amorphous silicon and alloys from Hall data.

The effect of microstructure in undoped a-Si:H films on carrier transport, recombination, densities of midgap states and solar cell characteristics has been investigated. Extended state mobilities of electrons were obtained from photo and dark conductivity measurements between 40° C and 190° C and the gap states characterized using Dual Beam Photoconductivity. In these films the estimated room temperature electron mobilities increase from about 1 to 30 cm2/V sec as the dihydride concentrations and void volume fractions decrease. It is found that the carrier mobility-lifetime products are not solely determined by the dangling bond states. The effects of changes in the mobilities and midgap states on p-i-n homojunction solar cell characteristics are presented and discussed.

Intrinsic deep defect-related recombination process has been studied in a series of undoped hydrogenated amorphous silicon(a-Si:H) films grown under different deposition conditions. Steady-state photoconductivity (σph) was measured as a function of deep defect density Nd, Urbach energy Eu, and dark Fermi energy Ef. It was found that σph strongly depends on these parameters while Ef- stays at the energy levels lower than 0.82 eV below Ec, but it is nearly independent of those while Ef stays at above 0.82 eV. These behaviors were found to be independent of the sample deposition conditions. These results indicates that subgap defect states enclosed by E=0.82 eV and Ef are the dominant recombination centers.

Excess electron-hole recombination at high excitation densities in a-Si:H is investigated by comparison of the transient photoconductivity during and just after excitation in the nanosecond time range to numerical models. A simple one parameter recombination model fits the experimental data satisfactorily. An extension of the model where deep trapping is taken into account explains also the transient photoconductivity in Boron doped a-Si:H.

A new Monte-Carlo simulation algorithm has been applied to verify the concept of the effective temperature recently suggested as a description of energy relaxation of carriers in band tails of amorphous semiconductors under the presence of an applied electric field. The algorithm allows the simulation with arbitrary applied field and finite temperature. The results of the simulation agree favourably with the theoretical prediction.

It is shown that the anomalous band in dual-beam CPM (DBCPM) spectra results from a combination of two processes: photocurrent enhancement due to excitations by the probe light from the valence baud to theD+ defect states, and photocurrent reduction due to excitations by the probe light from the D° defect states to the conduction band. From the DBCPM spectra, the electron correlation energy for the (filled)D - defects is determined to be 0.25 eV.

Comparative µτ-product measurements performed on a series of slightly doped (micro- doped) a-Si:H films by steady-state photocarrier grating (SSPG), steady-state photoconductivity (SSPC) and time of flight (TOF) are presented. The observed discrepancy between transient and steady-state majority carriers µτ-products is discussed within the framework of a dangling bond recombination model. Furthermore, results on the variation of µτ with doping and light-soaking are reported and ratio of capture cross-sections of charged to neutral defects are deduced.

Aim of this work is to investigate the opto-electronic properties of amorphous hydrogenated silicon (a-Si:H). The deposition temperature nas been used as a driving force to modify the morphology and bonded hydrogen distribution. The influence of the hydrogen microstructure on the carriers μτ product has been examined. The majority and minority carrier μτ have been evaluated from the diffusion length measurement, by using the Steady State Photocarrier Grating (SSPG) technique, and from the photoconductivity in the steady state condition (SSPC). The μτ values have been correlated with the defect density and the Fermi level position. Some considerations are proposed to explain the carrier transport in terms of the compositional inhomogeneities in Si:H alloys due to the morphological variations.

The transport properties of a-Si:H based thin film transistors have been characterized by the ambipolar diffusion length (Lamb) which was measured by the steady-state photocarrier grating technique (SSPG). The results show that with increasing positive gate voltage (Vg) the photoconductivity increases, but Lamb decreases. The above results can be consistently explained in terms of the shift of the Fermi level towards the conduction band edge with increasing V g. On the other hand, from the analysis of the SSPG results it is found that the factor γS, which is related to the interface recombination velocity, increases with V g.This indicates that when narrowing the conductive channel the recombination of the carriers at the interface increases.

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.

N-type amorphous silicon films were grown using a mixture of silane and tertiarybutylphosphine (TBP-C4H11P) vapor in a plasma enhanced chemical vapor deposition system. The concentration of TBP in silane was varied from 0 to 3% by volume. As expected, at low doping levels, the photoluminescence (PL) intensity associated with both the band-tail recombination (peak at 1.3 eV) and deep-defect recombination (peak at 0.8 eV) decreased as the impurity concentration increased, but for TBP concentrations > 0.1% the PL intensity increased again. For moderate doping levels the activation energy for conductivity leveled off at ∼ 0.2 eV. For concentrations of TBP > 0.1% the activation energy for dark conductivity increased. A shift in the optical gap was observed for the highest impurity concentrations due to the incorporation of carbon from the TBP. These results are interpreted as a pronounced decrease in the doping efficiency for heavily doped films (> 0.1%) perhaps influenced by the increased carbon concentration.

Boron and phosphorus doped high conductivity microcrystalline thin films were deposited in a PECVD reactor. We report conductivities as high as 3 and 41 S/cm for B and P doped materials respectively on films deposited at 210 °C. The conductivity as well as the microcrystalline fraction increase for the n layer with decreasing RF power, while, for the p material, an increase of power is needed to improve the film characteristics. The conductivity prefactor (σo) as well as the conductivity (σ) itself as a function of the activation energy (Ea) show a slope inversion for both n and p materials at an activation energy or about 40 meV and 80 meV respectively. Different possible transport mechanisms are examined in order to explain the experimental data.

We report results on a study on μc-Si:H and (μc-SiC:H films deposited by PECVD. The crystallinity fraction and the crystal sizes have been evaluated by X-ray diffractometry, Raman spectroscopy and Transmission Electron Microscopy(TEM). Infrared vibrational spectra of both μc-Si:H and |μc-SiC:H samples have been studied to obtain information on their structure. A comparison between the structure of the amorphous and diphasic amorphous-microcrystalline samples has been performed. Optical properties were obtained by transmission-reflectance and PDS measurements. Electronic transport mechanisms through the conductivity measurements in a wide range of temperatures (50-500 K) have been defined. Structural and electron density models have been discussed and used to interpret the experimental results.

Reactive magnetron sputtering of a c-Si target in an atmosphere containing argon and a varying partial pressure of hydrogen has been used to prepared a series of samples which span the transition from amorphous to microcrystalline silicon. Samples prepared at a substrate temperature of 200 °C, power of 100 W, and Ar partial pressure of 2.10 mtorr can be divided according to the partial pressure of H2, PH2, used in the deposition. Samples deposited using PH2 ≤ 1.00 mtorr are amorphous, those deposited using P H2≥ 1.25 mtorr are microcrystalline. Raman scattering is used to confirm amorphousity/microcrystallinity. Two types of changes are observed using infrared, IR, spectroscopy. First, the IR absorption spectrum changes at the onset of microcrystallinity. The absorption centered at 630 cm-1 changes shape while the feature at 2000 cm-1 appears to shift upward to 2090 cm-1. Second, microcrystalline samples deposited using PH2 ≥ 1.50 mtorr show significant post-deposition contamination upon atmospheric exposure. This is most clearly noted by the strengthening, over time, of Si02 related absorption features which appear at 1050 and 1150 cm-1, and a weakening of a narrow component of the 630 cm-1 absorption. This narrow component is not observed in amorphous material. The strength of the O-related absorption suggests that the oxide is a bulk property. The porous, low density network which is necessary for this contamination to occur is confirmed using HRTEM.

A thermodynamic calculation of Si-X alloys has been carried out, where X is any element from group III or V. Free-energy diagrams for those systems have been established. A comparison between thermodynamic predictions and experimental results is carried out for the Si-B and the Si-Sb alloys. It is found that the agreement between theory and experimental results for the free- energy diagrams and for the crystallization temperature predictions are good. The model and the different features for the various elements are described in detail.