Diffusion processes of hydrogen and lithium in amorphous silicon are compared. While similarities in the diffusion energies and in a doping dependence exist, the charge state of the diffusing particles and the type of bonding of diffusing particles in traps differ. In case of Li diffusion, oxygen contamination plays an important role.

We report the first measurements of hydrogen diffusion kinetics in a-Si in the regime of low H concentration (<2×l019 cm−3). The results differ substantially from the diffusion behavior typically observed in hydrogenated a-Si:H at H concentrations >1020 cm−3. The activation energy and pre-exponential factor for low-concentration H diffusion are found to be 2.70 ± 0.02 eV and 2.2 × 104 cm2s−1, respectively, and are shown to be independent of both annealing time and H concentration. It is difficult to reconcile the combination of high activation energy and large pre-exponential factor with a simple deep-trap-limited diffusion model. Consequently, an alternative mechanism for H diffusion involving the migration of dangling bonds coupled with a H bond-switching step is proposed.

1H NMR dipolar echo measurements have been performed on a series of samples of phosphorus- and boron-doped a-Si:H. The dipolar echo sequence consists of three rf pulses followed by an echo in the form: (π/2)x - τ1 - (π /4)y - π2- (π /4)y - echo. The echo height is plotted against π2 and the slope yields the dipolar spin-lattice relaxation time (T1D). T1D is a measure of fluctuations in the local dipolar field surrounding each hydrogen atom in a-Si:H, and measurement of this quantity can be employed as a probe of hydrogen motion on a microscopic scale. The T1D measurements of 10-5 B-doped, 10-3 P-doped and undoped a-Si:H are compared to the previously measured T1D of 10-4 B-doped a-Si:H. The T1D values for 10-4B-doped, 10-4 B-doped and undoped a-Si:H are, respectively, 1.7 ms, 11 ms and 22 ms at 300 K. The T1D for 10-3 P-doped is found to be the same as for 10-5 B-doped within experimental error. These trends are similar to the variation of the macroscopic diffusion of hydrogen with respect to various doping levels, but the details of the local motion are very different from those of the macroscopic diffusion.

Hydrogenated amorphous silicon (a-Si:H) films were irradiated with 1-MeV Si+ ions. The accumulation and annealing of ion damage was investigated by Raman scattering, optical reflection and transmission, and conductivity measurements. For damage levels up to 0.003 displacements per atom, electrical defects are created with no measurable effect on the structural properties. These defects can be completely annealed out at 180°C. Further irradiation results in an increase in the average bond-angle variation in the films. This structural disorder causes a decrease of the optical band gap with 0.46 eV. The structural changes caused by high-dose implantation can not be reversed by annealing at 180° C, which results in the formation of anneal-stable electrical defects.

The coplanar current in n-lype doped hydrogenated amorphous silicon (a-Si:H) displays random telegraph switching noise, indicating the presence of inhomogeneous current filaments whose conductance varies with time. There are strong correlations of the 1/f noise power spectra across differing frequency octaves which are much larger than expected if the magnitudes of the fluctuators are varied in parallel. The scale invariant second spectra and the temperature dependence of the spectral slope indicate that hydrogen motion is involved in the cooperative dynamics between noise sources. A model is described wherein the properties of the current filaments are modulated by hydrogen-hydrogen interactions which are mediated by the Si strain fields.

A straightforward analytical expression of the density-of-states (DOS) of a-Si:H valid in non-equilibrium steady state situation has been obtained. The model is based on a statistical-mechanical treatment of the hydrogen occupation for different defect sites. The broadening of available defect energy levels (defect pool) and the possibility of charged defects are taken into account. This leads to a new explanation of the Staebler-Wronski effect, based on the "conversion" of shallow charge centers to neutrals near the middle of the gap as a consequence of hydrogen redistribution induced by electron-hole recombination.

In this paper we show that the degree of order of the Si network in a-Si:H is increasing with two length scales from the surface into the bulk. The major manifestation of the disorder is the variation in the Si-Si bond-stretching rather than the variation in the width of the dihedral angle distribution. The results are interpreted in terms of the decrease of the hydrogen concentration from the free surface into the bulk.

The experimental results about defect relaxation in a-Si:H have been reported recently on several transient measurements, such as forward-bias-transient-current,1 optical bias effect on electron drift,2 and photo-capacitance transient.3 In this paper we report the steady state results of the bias effect on sub-band-gap absorption and photon-emission spectra measurements in undoped samples.

The optical bias effects the sub-band absorption. In additional to an enhancement of the sub-band gap absorption, the absorption threshold moved from 0.8 eV to 0.6 eV for undoped samples under optical bias at room temperature. The effects are larger in the light-soaked-state than in the annealed-state.

We also report on the generation rate dependence on photoluminescence (PL) spectra line shape. We found that the energy position of the defect PL band shifted from 1.1 eV to 0.9 eV when the excitation level increased 4,000 times.

While motion of the quasi-Fermi level as a explanation cannot be eliminated, defect relaxation seems more plausible.

We report studies of the spin-dependent recombination processes in device-quality a-Si:H by optically detected ESR (ODESR) with the excitation energy (Ex) varied from above-gap to sub-gap. The photoluminescence (PL) transients induced by the chopping of the microwaves are recorded as functions of the magnetic field to yield the lineshapes. This scheme, to some extent, circumvents the problem of interferences between "enhancing" and "quenching" signals encountered in conventional phase-sensitive detection. The PL below 1.1 eV is monitored. For Ex above 1.5 eV, we find all the lineshapes are similar: a broad, slightly asymmetric enhancing line peaking at g = 2.008 and an asymmetric quenching line peaking at g = 2.005. Lineshape changes are noticeable for Ex below 1.5 eV. The relative signal intensity AI/I changes dramatically with varying excitation energy. Light-soaking of the sample induces an additional enhancing signal which peaks at g = 2.006.

The effect of incorporation of SiH2 on a-Si:H matrix on the optical properties has been investigated on some details. It is shown that SiH2 incorporation is responsible for a further increase of the optical gap, and also for an increase in disorder. For concentrations larger than 15% approximately, microvoids are probably created and SiH2, which is covering their inner surfaces, seems not to affect the optical properties further on. The results indicate also clearly that the effect of autoannealing of the film during its growth cannot be neglected and that a new parameter should be retained, i.e. the total deposition time.

The distribution of occupied states in a-Si:H has been inferred by applying a new self- consisting iterative method to the absorption coefficient spectra. This procedure does not require any assumption about the localized states below the Fermi level, and provides a more accurate insight with respect to the simple derivative method. Numerical simulations have been made in order to probe the reliability of our method. The optical spectra have been obtained by means of Photothermal Deflection Spectroscopy (PDS) and Constant Photocurrent Method (CPM); the comparison between the results as obtained by the two techniques suggests that different sensitivities to electronic transitions are involved; this can be used to infer information about the unoccupied defects.

Pulsed measurements on the solid-electrolyte system, which proved very useful in the study of crystalline semiconductors, have been found to be equally effective when applied to hydrogenated amorphous Si films. Here, as well, the aSi:H/electrolyte interface is essentially blocking to current flow and, as a result, surface space-charge layers, ranging from large depletion to very strong accumulation conditions, can be induced and studied. In particular, valuable information can be gained on the density of the localized bulk states. Measurements in the depletion range under illumination yield directly the total density of occupied states in the entire energy gap. This is very useful in obtaining a quick and reliable assessment of the quality of the amorphous films. In high-grade films we find that the total density of occupied states is around 1018 cm−3. The data in the accumulation range, on the other hand, provide useful information on unoccupied states near the conduction band edge. The blocking nature of the amorphous Si/electrolyte interface is utilized also to apply a sweep-out technique for an accurate determination of μτ the product of the electron mobility and lifetime, even when this value is very low. In a rather poor-quality film, for example, we find μτ to be 5 × 10−8 cm2/V.

The transport and recombination of injected electrons and holes in a-Si:H p-i-n-solar cells is studied by simultaneous detection of electroluminescence and forward current, both steady state and time resolved. We have calculated the spatial distribution of the radiative recombination in the layer using an optical model that takes into account all layers of the solar cell as well as the substrate and the back contact. The validity of the results has successfully been tested by photoluminescence measurements using different excitation energies (penetration depths). By comparison between measured EL spectra and calculated spectra it is found that in a 0.6μm thick cell at room temperature the radiative recombination occurs in a narrow region (90 nm) in the vicinity of the p-layer. Time resolved electroluminescence and forward current measurements show that at room temperature the forward current is first determined by the space charge limited current of the electrons subsequently followed by the much larger recombination current. The recombination current is determined by the injection of holes from the p-contact in agreement with results from device modelling. Radiative and non-radiative lifetimes of the injected carriers are determined.

Effective densities of states in conduction and valence band, Nc and Nv, are usually set to a fixed value of 1019 to 1020 cm−3 in all computer simulations of a-Si structures. In this contribution the densities Nc and Nv are analytically expressed for different selected extended states distributions. The derivatives d(lnNc)/dx and d(lnNv)/dx, representing specific terms in transport equations, are expressed as function of position dependent band structure. The effect of an increased disorder in heterojunction is simulated by Gaussian distribution of states in a linearly graded heterojunction region.

In an earlier study of 37 a-Si:H films we identified positive correlations between the saturated light-induced defect density Nsat, the hydrogen content CH measured by IR absorption, and the optical gap. Impurities also have been implicated in the production of metastable defects. Yet no conclusive evidence has been produced that non-dopant impurities cause defects. This situation led us to analyze our samples for impurity content. Our present 42 samples were made in six laboratories by ten combinations of deposition technique. The concentrations of H, B, C, N, O, F, Na, Cr, and Ge were determined by secondary ion mass spectroscopy. Na, Cr, or Ge were not detected. The content of C, N and O varies by a factor ∼1,000 and depends on deposition system and source gas. We report a comprehensive evaluation of our data with emphasis on their correlation with optoelectronic properties. We find the annealed state defect density Nann, and the saturated light-induced defect density Nsat surprisingly independent of impurity content.

The effect of C impurities in a-Si:H in levels of 0.4 to 2.6 at. % were studied over a wide range of metastable defect densities. Three complimentary experimental techniques [electron spin resonance (ESR), drive-level capacitance (DLC) and photo-current] were employed to track the material's defect density with light soaking and annealing, as well as Urbach energies, midgap absorption and mobility gaps energies as a function of the C content. Our results show C impurities have a definite effect on the initial and saturated defect densities, as well as the midgap absorption and Urbach energies at levels 1 at. % and above. The results indicate that C acts mainly as a center for increased disorder in the material which results in an increase in the bandtail widths, and consequently an increase in intrinsic defects. Comparison to the ESR and drive-level data show an excellent agreement between these two techniques in determining the bulk defect densities in a-Si:H.

Nanocrystalline silicon (nc-Si:H) films with an average grain size ranging between 2 and 10 nm are prepared in a modified plasma chemical vapor deposition system. X-ray diffraction, transmission electron diffraction, and high resolution electron microscope are used to elucidate the structural nature in the deposited nc-Si:H films. The lattice network of the deposited silicon films changes from disordered structure to ordered structure with the increasing of plasma energy. The results also show that the crystal lattice of the nc-Si:H film is distorted from those of microcrystalline silicon and crystalline silicon. In the observed x-ray diffraction, there are two anomalous peaks at 2θ = 29.5° and 32.5° of Si besides the normal peaks at 2θ = 28.5° of Si(111) and 2θ = 47.3°of Si(220). By the high resolution electron microscopy study, a new crystallography structure with distorted Si(111) crystallites in nc- Si:H films is found. Based on our results, a structure model of nc-Si:H films is proposed.

The effect of the non-uniform defect profile in a-Si:H films on the subgap photothermal spectra is investigated. A comprehensive theoretical model is suggested. By means of this model it is possible to extract quantitative information about the defects inhomogeneity in the film.

Exact formulas describing reflectance and transmittance R and T of the system absorbing film%substrate%back ambient are examined. Basing on the consideration that, unlike R and T, the quantity T%(l-R) does not oscillate with wavelength, the entity of terms entering the full equation are evaluated, and some approximations are made. As a result, a non-oscillating analytical equation is obtained, that can be inverted and therefore utilized to compute the film absorption coefficient with an accuracy that is better than 3 %for α>103cm-1. The effect of further approximations, like fixing the refractive index of the film and the substrate, is also evaluated. A comparison with a normally used, simplified formula, as well as an example of application, are also reported.

Irregularities in the current-voltage-characteristics of a-Si:H based potential barriers have been investigated experimentally, and are discussed theoretically with respect to different transport mechanisms. The investigated samples were different series of double and single barrier a-Si:H - a-Si1-xCx:H - heterostructures, as well as homogeneous samples without heterostructure barrier. Current-voltage-(I-V)-characteristics showing a wide variety of features, from complete smoothness of the curves, to bumps and even accidental step-like switching behaviour, as well as different forms of noise, were recorded at different temperatures. Resonant tunnelling as an explaining transport mechanism for the anomalies was excluded because of inconsistency between experiment and calculations partially including special amorphous features. Instead it is argued that all observed irregularities, i. e. bumps in I-V-curves, switching-like behaviour, and appearance of noise, are related to current transport via trap-assisted tunnelling through locally strongly confined transport paths, leading to the meta-stable formation, change and break-down of conductory filaments.