We used Fourier Transform Infra-Red (FTIR) analysis of bi-layers of plasma-grown hy-drogenated amorphous silicon-carbide films to investigate the role of the material structure in the hydrogen diffusion process. In the bi-layers one layer was deposited using CH4/SiH4 and in the other layer CD4/SiD4 was applied. The carbon concentration was 20 at.%. In previous work we showed, using Elastic Recoil Detection (ERD) and Thermal Desorption Spectrometry (TDS), that the hydrogen moves molecular through these films in the temperature range 325 < T < 450 °C [1]. Using FTIR we obtained information about the number of Si-H and Si-D bonds and their change upon annealing. The FTIR data indicate a structural change during annealing. A comparison with the TDS spectra led us to the conclusion that at higher temperatures the out-diffusion of hydrogen stops because of the hindrance of the molecular transport.

Hydrogen plays an important role in the electronic behavior, structure and stability of amorphous silicon films. Therefore, determination of the hydrogen density of states (DOS) and correlation of the hydrogen DOS with the electronic film properties are important research goals. We have developed a novel method for determination of hydrogen DOS in silicon films, based on fractional evolution experiments. Fractional evolution experiments are performed by subjecting a silicon film to a series of linear, alternating heating and cooling ramps, while monitoring the hydrogen evolution rate. The fractional evolution data can be analyzed using two complementary memods, the fixed frequency factor approach and Arrhenius analysis. Using a rigorous, mean-field evolution model, we demonstrate the applicability of the two approaches to obtaining the hydrogen DOS in silicon films. We further validate both methods by analyzing experimental fractional evolution data foran amorphous silicon carbide film. Both types of analysis yield a similar double peaked density of states for the a-Si:C:H:D film.

It is demonstrated that the exposure of polycrystalline silicon (poly-Si) to monatomic hydrogen results in the formation of H clusters. These H stabilized platelets appear in the near-surface region ( 100 nm) and are predominantly oriented along {111} crystallographic planes. Platelet concentrations of ≈5×1015, 1.5×1016-cm−3, and 2.4×1017 cm−3 were observed in nominally undoped poly-Si, phosphorous doped poly-Si (P=1017 cm−3), and phosphorous doped single crystal silicon (P>3×1018 cm−3), respectively. Results obtained on doped c-Si demonstrate that platelet generation occurs only at Fermi-level positions of Ec -EF < 0.4 eV.

In this work the characteristics of hydrogen clusters in hot filament assisted CVD and conventional glow discharge a-Si:H films are discussed. Computer simulations of the observed free-induction decays of the 1H NMR signals indicate that the distribution of the nearest-neighbor distances between H atoms in the H clusters is quite narrow in hot filament assisted CVD a-Si:H whereas the distribution is larger in glow discharge a-Si:H. This is clear evidence of improved structural order in hot filament assisted CVD a-Si:H. The relaxed hydrogenated divacancy and multi-vacancy models reproduce the main features of the observed free-induction decay in hotwire a-Si:H very well. Computer simulations of the multiple-quantum NMR spectra indicate that a relaxed hydrogenated divacancy configuration leads to good agreement with experimental observations in device quality glow discharge a-Si:H. Results of simulations based on other H cluster configurations are also discussed. These results provide restrictions on the possible models for H clusters in a-Si:H.

This paper discusses the techniques we have developed to analyse the photo-response of amorphous semiconductors to transient optical excitation to determine the energy distribution of gap states (DOS). We highlight the difficulties arising from a direct ‘single point’ approach using the instantaneous transient photocurrent i(t) when there is significant structure in the DOS or in the presence of recombination. Frequency domain methods which involve measurement of the amplitude I(ω) and phase φ(ω)of the steady ac photocurrent in response to ac excitation are shown to overcome these problems, but include other, experimental shortcomings. The paper describes a solution which combines the best features of time and frequency domain methods. The method involves numerical Fourier transformation of the time sampled impulse response i(tk) into a complex ac response (ω n), followed by analysis for gap-state distribution. Essentially, the method succeeds since it folds in information from the whole of the measured response for each energy considered. Other advantageous features include applicability to semiconductors exhibiting dispersive or non-dispersive transport, and to pre- and post- recombination regimes of the transient photocurrent, without modification. The features of this analytical method are demonstrated with computer simulated and experimental transient photocurrent data for a-Si:H.

We have studied photoexcitation dynamics in undoped a-Si:H from 80 K to 300 K by the techniques of photoluminescence (PL), photoinduced absorption (PA) and their respective versions of optically detected magnetic resonance, namely PLDMR and PADMR. Both PL and PA spectra are composed of low and high energy bands, respectively. Using their respective temperature dependences we correlate the low energy PA band (∼ 0.4 eV) to the high energy PL band (∼1.3 eV) and the high energy PA band (∼ 1 eV) to the low energy PL band (∼ 0.8 eV). We also found that the PADMR spectrum is composed of three main contributions: a narrow resonance at g ∼ 2 (FWHM = 15 G), a broad resonance at g ∼ 2 (160 G) and a narrow asymmetric resonance at g ∼ 4 (20 G). By measuring their photon energy spectrum we found that the g ∼ 4 and the broad g ∼ 2 resonances are correlated with each other and therefore are due to trapped electron-hole pairs in the triplet spin configuration.

We have studied the electric field dependence of the electron-hole thermal generation process in hydrogenated amorphous silicon. A model was developed which takes into account the Poole-Frenkel effect and the thermally assisted tunneling. In order to explain the experimental results it was necessary to consider a strong electron-lattice interaction describing the carrier tunneling mechanism. Deep defects relaxation is also discussed.

We have measured the low temperature internal friction (Q−1) of amorphous silicon (a-Si) films. Electron-beam evaporation leads to the well-known temperature-independent plateau common to all amorphous solids. For hydrogenated amorphous silicon (a-Si:H) with about 1 at.

% H produced by hot wire chemical vapor deposition, however, the value of is over two hundred times smaller than for e-beam a-Si. This is the first observation of an amorphous solid without any significant low energy excitations. This finding offers the opportunity to study amorphous solids containing controlled densities of tunneling defects, and thus to explore their nature.

Modulated photocurrent (MPC) measurements in intrinsic a-Si:H reveal a prominent band of electron deep traps with a thermal emission energy near 0.6eV. We have identified this defect band by directly comparing MPC and ESR spectra for both an intrinsic and a lightly n-type doped sample for a various metastable states such that the Fermi level, EF, ranges from less than 0.5eV to more than 0.7eV below Ec. This comparsion unambiguously demonstrates that the MPC band arises from the Do charge state of the defects (specifically, the D−&Do transition). This identification is also confirmed when the quasi-Fermi level is varied by the application of light bias even though the peak emission rate from the MPC defect band is changed by more than a factor of 100. These observations specifically rule out the possibility of large populations of charged defects in intrinsic samples predicted by proponents of the defect pool model. Instead, observed behaviors have a natural explanation in terms of a defect relaxation process.

By comparison of transient photoconductivity (TPC) and transient photoinduced absorption (PA) the influence of the density of states in the bandgap on excess charge carrier kinetics is studied for a-Si:H films deposited at different temperatures and for state of the art a-Si:H films in two different states of light soaking. In both series the rising deep defect density leads to an enhancement of electron trapping rather than recombination via deep defects. The samples deposited at temperatures lower than 250°C additionally show a lower effective electron mobility, i.e. a broader conduction band tail.

We have studied the four phototransport properties as a function of temperature in undoped a-Si:H films deposited with different substrate temperatures in the range 150 – 225°C. The analysis of the results indicates how Ts determines the densities of the various defects. The general trend of decrease of both the density of dangling bonds and the Urbach energy is in agreement with the weak bond breaking model. However, we conclude that a slight modification of this model is required.

The electronic properties of a series of n-type doped hydrogenated amorphous silicon (a-Si:H) films grown with deposition rates ranging from 2 Å/s to 33 Å/s have been studied. Infrared absorption spectroscopy shows an increase in S1-H2 content with deposition rate, concurrent with a decreasing conductivity, increasing thermal equilibration relaxation time, and increasing disorder at the mobility edge as measured by the difference in thermopower and dark conductivity activation energies. The current 1/f noise properties become highly nonstationary, with increased variability and inapplicability of statistical analysis as the deposition rate increases.

Resistance fluctuations have been studied in hydrogenated amorphous silicon in the temperature range between 300 K and 450 K. The primary noise source has a power spectrum of approximately 1/f and is ascribed to hydrogen motion. Hopping of weakly bound hydrogen is thermally activated at such low temperatures with an average activation energy of 0.85 eV. The attempt rate amounts to 7 · 1012 s−1.

The dependence of the transient photoconductivity induced by pulsed excitation (TPC) on the excitation density is discussed with the help of numerical simulations. It is shown that recombination between excess mobile electrons and all excess holes (mainly localized) can explain the excitation density dependence of the TPC amplitude of standard a-Si:H at room temperature using a rate parameter kBB of 10−8cm3/s. This model leads to a decay faster than experimentally observed in the time range from 40ns to 1 μs. A variation of the recombination model is presented that gives a better fit for the longer time range still showing the correct excitation density dependence in the short time range. Moreover comparison of the simulations with experimental data yields limits for the parameters of the conduction band tail. In particular, the time necessary to establish a dynamic equilibrium of excess electrons between delocalized states in the conduction band and localized states in the tail appears to be very informative.

The photoconductivity and subband gap absorption measurements over a wide range of generation rate(G) have been carried out ondiluted and undiluted a-Si:H films. It is found that in these high quality films there are significant differences in the functional dependence of mobility-lifetime(μτ) products on G. In additionto the different values of subgap absorption(a) there are also distinct differences in the dependence of a on photon energy (E) as well as G. It is difficult to selfconsistently analyze the results on theundiluted film with the previously used three gaussian distribution, particularly at high generation rates. Selfconsistent analysis isobtained when the (+/0) transitions of negative charged defects andthe (0/-) transitions of positive charged defects are introducedrespectively closer to the valence and conduction bands. This new gapstate distribution is a better representation for the defectpool model and potential fluctuation model.

It has been reported in previous works that using of RF 55 kHz PECVD method allows to deposit microstructural inhomogeneous a-Si:H films at high deposition rate (10–20Å/c) and with high photoconductivity The structural analysis with using of IR spectroscopy and atomic force microscopy (AFM) performed in this work have shown that these films possess a relatively regular microstructure consisting of “grains” with characteristic size of∼300–500Å. The regular microstructure of investigated films differs them from inhomogeneous a-Si:H with deteriorate electronic properties. At the same time the diffraction analysis didn't reveal the presence of microcrystals. Therefore, we denoted our films as “microstructural” a-Si:H films In this work we performed the modeling of the photoconductivity of “microstructural” a-Si:H films to analyze the density-of-states (DOS) responsible for recombination kinetics For this purpose different approaches to photoconductivity modeling have been used to simulate the experimentally measured temperature dependence of photoconductivity The comparative analysis of results of these simulations and ESR measurements have shown that recombination in high photoconductive ‘microstructural’ films is controlled by deep neutral states

Measurements of the cooling rate of hot carriers in amorphous silicon are made with a two-pump, one-probe technique. The experiment is simulated with a rate-equation model describing the energy transfer between a population of hot carriers and the lattice. An energy transfer rate proportional to the temperature difference is found to be consistent with the experimental data while an energy transfer independent of the temperature difference is not. This contrasts with the situation in crystalline silicon. The measured cooling rates are sufficient to explain the difficulty in observing avalanche effects in amorphous silicon.

Space charges build up near one or both electrodes of a photoconductor unless the thermoionic current balances the photocurrent. Space charges built up also when a voltage U is applied at low T to a semiconductor containing a non-equilibrium distribution of carriers. The presence of a space charge is observed as a relaxation-current transient when the sample is illuminated at zero bias. If one type of carrier is immobile the steady state photocurrent becomes zero.

Recent experimental developments have cast doubt on the validity of the common assumption that the distribution of tail states of hydrogenated amorphous silicon exhibits a single exponential functional form. We employ transient photocurrent decay measurements to determine this distribution of tail states. In our approach, however, we determine the distribution of tail states directly from the experimental data, without assuming, a priori, a specific functional form. It is found that these experimental results are consistent with other more recent experimental determinations of the distribution of tail states, suggesting the possibility of deviations from a single exponential distribution of tail states in hydrogenated amorphous silicon.

A study is presented of a series of high quality PECVD a-Si0.33Ge0.67 films, produced by cathodic deposition, in which small concentrations of PH3, B2H6 or air impurities were added during deposition. The quantum efficiency-mobility-lifetime product (ημτ) increases, and the ambipolar diffusion length (Lamb) decreases monotonically with dopant concentration for both PH3 and B2H6. This result is strong evidence that for these films neither photocarrier is dominant (μτe= μτh) at zero doping. This result is very different from what has been typically observed by other researchers, that the electron is the dominant photocarrier for undoped a-SiGe:H.

Drive level capacitance (DLC) measurements of these alloys show an unusual behavior of being temperature-independent, and the dark conductivity activation energy is maximum for zero doping. It is proposed that all of these unusual properties are due to the unusually low impurity concentration of these films, and that these properties are, in fact, the intrinsic properties for a-SiGe:H alloys. To verify this, films were prepared with a calibrated and controlled air leak introduced during deposition. As the air leak was increased, the film properties changed to typical behavior. Even for air concentrations as low as 2 ppm (gas phase), the transport measurements showed changes consistent with a shift in the Fermi level toward the conduction band.