J.A. Schmidt

Density of states in the gap of amorphous semiconductors determined from modulated photocurrent measurements in the recombination regime

An experimental technique to study the energy profile of localized states in the gap of amorphous semiconductors is proposed. The method is based on the relationship between the recombination lifetime and the density of states (DOS) at the quasi-Fermi level for trapped carriers. We use the modulated photocurrent experiment in the recombination-limited regime as a convenient method to measure the recombination lifetime. Measurements performed as a function of temperature allow the DOS above the Fermi energy to be determined. The accuracy and limitations of the method are studied by means of computer simulations. The experimental technique is applied to obtain the density of defect states of a hydrogenated amorphous silicon sample.

Limitations of the constant photocurrent method: A comprehensive experimental and modeling study

We present experimental and modeling results for the subgap absorption coefficient of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) in order to explore the limitations of the constant photocurrent method (CPM). To properly model the subgap absorption coefficient, we have developed a simulation computer program including all the possible optical and thermal transitions between gap and extended states. Tail states are assumed to be either donor- or acceptorlike and midgap states are assumed to be amphoteric. The defect-pool model is also incorporated in our analysis. We have fitted several experimental dc-CPM spectra by using a standard set of parameters for the density of states. Our analysis demonstrates that in undoped samples the true absorption coefficient and the density of midgap states are underestimated by the CPM measurement. This is due to a nonconstant electron lifetime in the energy range between 0.8 and 1.2 eV. On the other hand, we show that in n-doped or p-doped samples, the CP...

Determination of hydrogenated amorphous silicon electronic transport parameters and density of states using several photoconductivity techniques

In this paper, we show that the combination of different characterization techniques based on the photoconductivity of hydrogenated amorphous silicon can be a tool to investigate on the density of states and transport parameters of this material. We insist mainly on two techniques in which one records a photocurrent resulting from the movement of an interference grating onto a sample. We describe the experimental set-ups and provide a theoretical explanation of the observed behaviors of these photocurrents. We demonstrate that a density of state spectroscopy can be done with these techniques. Additionally, comparing this spectroscopy to that performed with modulated photocurrent experiment, we show that it is possible to derive a good order of magnitude estimate of the electron capture coefficient of the conduction band tail states as well as the electronic extended states mobility. The derived parameters are compared with previous results.

Energy-resolved photon flux dependence of the steady state photoconductivity in hydrogenated amorphous silicon: implications for the constant photocurrent method

We have studied the photon flux dependence of the steady state photoconductivity in intrinsic and lightly boron-doped hydrogenated amorphous silicon samples illuminated with monochromatic light of sub-bandgap energy. We also report measurements of the sub-bandgap absorption coefficient carried out by using the dc constant photocurrent method (CPM) under different constant values of the photocurrent. We show that the exponent of the power law relation between the photocurrent and the photon flux (which we name λ) depends on the photon energy. This result contradicts one basic hypothesis of the CPM. As a consequence of this fact, the absorption coefficient measured with the CPM is dependent on the constant photocurrent chosen to perform the measurement. Computer simulations based on a complete model for the absorption process reproduce both of these experimental results. We conclude that this model can be used to handle CPM data in order to obtain the actual sub-gap absorption spectra.

Low frequency modulated photoconductivity in semiconductors having multiple species of traps

Modulated photoconductivity has proved to be an excellent tool to probe the density of states of semiconductors. However, though a great deal of attention has been devoted to the high frequency regime for its simplicity of application, the modulated photoconductivity in the low frequency (LF-MPC) regime has been the object of researches only recently. In the case of a dominant species of states present in the forbidden gap, it was shown that LF-MPC is a good complementary technique to the high frequency method to extract different transport parameters. In this paper, we present a complete theoretical analysis of the LF-MPC experiment for the case when different species of traps are present. We solve the complete system of equations that describe the experiment, and we apply simplifying assumptions to deduce a simple formula relating the photocurrent phase shift to the density of states (DOS) at the majority carriers’ quasi-Fermi level. By means of numerical calculations, we discuss the accuracy of our dev...

Vacuum-enhanced nickel-induced crystallization of hydrogenated amorphous silicon

We report the results of enhanced nickel induced crystallization of intrinsic hydrogenated amorphous silicon thin films under vacuum conditions. Crystallization was performed by conventional furnace annealing at both atmospheric pressure and vacuum or low pressure conditions (≈10–6 Torr) for comparison. We have investigated the influence of low pressure during annealing on the resulting polycrystalline films by means of optical microscopy, ultraviolet reflectance, and photoacoustic spectrometry measurements. A faster crystallization and a smaller grain size were observed when the process is carried out under vacuum, with an annealing time reduction of more than 50%. We discuss, from a thermodynamical viewpoint, some possible causes by which vacuum annealing influences incubation and nucleation stages due to the presence of mobile hydrogen atoms inside the amorphous silicon matrix. Large grains with diameters of 30 and 100 μm were obtained at vacuum and atmospheric pressure, respectively.

Analysis of the oscillating photocarrier grating technique

In this paper we present a complete theoretical analysis of the oscillating photocarrier grating (OPG) method, starting from the generalized equations that describe charge transport and recombination under oscillating grating illumination conditions. The solution of these equations allows us to implement a calculation reproducing the experimental OPG curves. We study both experimentally and from our calculations the dependence of the OPG curves on different external parameters, such as the applied electric field, grating period and illumination intensity. We find that the response of the sample is linked to a characteristic time of the material, which could be the dielectric relaxation time or the small signal lifetime depending on the regime at which the experiment is performed. Therefore, the OPG technique provides a simple method to estimate these parameters. In addition, we demonstrate that the small signal lifetime provides information on the density of states of the material.

Nickel-induced crystallization of amorphous silicon

The nickel-induced crystallization of hydrogenated amorphous silicon (a-Si:H) is used to obtain large grained polycrystalline silicon thin films on glass substrates. a-Si:H is deposited by plasma enhanced chemical vapour deposition at 200 °C, preparing intrinsic and slightly p-doped samples. Each sample was divided in several pieces, over which increasing Ni concentrations were sputtered. Two crystallization methods are compared, conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The crystallization was followed by optical microscopy and scanning electron microscopy observations, X-ray diffraction, and reflectance measurements in the UV region. The large grain sizes obtained – larger than 100 μm for the samples crystallized by CFA – are very encouraging for the preparation of low-cost thin film polycrystalline silicon solar cells.

On the validity of defect density determinations by the recombination-regime modulated photoconductivity technique

In this work we study the modulated photocurrent arising from experiments performed in the recombination regime on amorphous and microcrystalline semiconductor samples. The influence of the illumination intensity on the results obtained from the recombination-regime modulated photocurrent (RRMPC) technique is studied both from measurements and computer simulations. A wide range of density of states (DOS) distributions is used to simulate different material qualities. Applying a computer code that takes into account all thermal and optical transitions involving gap states, modulated photoconductivity experimental data are simulated. The DOS is then reconstructed from the simulated data following the RRMPC method. Simulations performed under different light intensities lead to the empiric definition of an indicator parameter, which allows us to evaluate the validity of the hypotheses of the RRMPC method for a particular experiment. Measurements for microcrystalline and amorphous hydrogenated silicon samples are presented as examples.

Further insight on recombination losses in the intrinsic layer of a-Si:H solar cells using computer modeling tools

Recombination losses of a-Si:H based p-i-n solar cells in the annealed state are analyzed with device computer modeling. Under AM1.5 illumination, the recombination rate in the intrinsic layer is shown to be controlled by a combination of losses through defect and tail states. The influence of the defect concentration on the characteristic parameters of a solar cell is analyzed. The impact on the light current-voltage characteristic curve of adopting very low free carrier mobilities and a high density of states at the band edge is explored under red and AM1.5 illumination. The distribution of trapped charge, electric field, and recombination loses inside the intrinsic layer is examined, and their influence on the solar cell performance is discussed. Solar cells with intrinsic layers deposited with and without hydrogen dilution are examined. It is found that the photocurrent at −2 V is not always a good approximation of the saturated reverse-bias photocurrent in a-Si:H p-i-n solar cells at room temperature...