N. N. Ormont

Anomalous relaxation of light-induced states of a-Si:H

Abstract For a large variety of B, P doped and undoped a-Si:H films, we observed the non-monotonic kinetics of structural changes at elevated temperatures (above 100°C), indicating the coexistence of two processes described by stretched exponentials. While the faster process has characteristics corresponding to the conventional metastable dangling bond creation, the characteristics of the slow process are shown to be anomalous (the activation energy for defect creation larger than that for the normal process and the parameter, β, decreasing with temperature). To describe the anomalous process, we propose a phenomenological model based on a three-level configuration–coordinate diagram with correlated activation energies for the light-induced metastable state creation and annealing. The model accounts for the observed features of the anomalous process. A possible underlying microscopic mechanism of structural changes is discussed.

On the photoinduced effect in undoped a-Si:H films

The temperature dependences of the dark conductivity and photoconductivity of annealed and preliminarily illuminated undoped a-Si:H films are studied in different modes of temperature variation. Also, the variation kinetics of the photoconductivity and dark conductivity of the films during and after their exposure to light at different temperatures are analyzed. It is shown that the anomalous nature of the dependences obtained may be due to the formation of two kinds of photoinduced defects that have different energies of formation and thermal annealing and energy levels situated in different parts of the band gap of the films under study.

Specific features of photoelectric properties of layered films of amorphous hydrogenated silicon

Temperature dependences of photoconductivity of layered and conventional undoped films of amorphous hydrogenated silicon have been studied within a wide range of temperatures (130–420 K) and illumination intensities (0.1–60 mW cm−2). It is established that a higher photosensitivity of layered films compared with conventional films is governed by a low dark conductivity of layered films as a consequence of a deeper position of the equilibrium Fermi level in the band gap and the absence of temperature quenching of photoconductivity in these films. It is shown that these specific features of electrical and photoelectric properties of layered films can be attributed to a low concentration of silicon dangling bonds in comparison with the concentration of oxygen-related acceptor centers, which feature a larger capture coefficient for holes.

Light-induced relaxation of the metastable conductivity of undoped a-Si:H films illuminated at elevated temperatures

The kinetics of relaxation of the light-induced (at a temperature above 140°C) dark conductivity of undoped a-Si:H films is studied. The calculated time dependences of the relaxation rate of the dark conductivity are analyzed under the assumption that the thermal rates of the generation and relaxation of metastable defects formed by preliminary illumination are independent of illumination. It is shown that the features of the kinetics of the relaxation rates of dark conductivity under illumination are determined by the presence of light-induced processes of the relaxation and generation of slowly relaxing metastable defects whose energy levels are located in the upper half of the band gap.

On the nature of photo-induced defects and recombination mechanisms in light-soaked a-Si:H films

Abstract Results of measurements of the temperature dependence of the photoconductivity and intensity-current characteristics of undoped a-Si:H films light-soaked at various temperatures are presented. The observations support the assumption of the introduction of τ-centers in addition of the conventional dangling bonds playing an important role in the recombination. The electronic energy spectrum and recombination properties of intimate dangling-bond pairs are discussed and it is argued that such complexes may be the τ-centers introduced at light soaking.

Modification of the thermal relaxation kinetics of the photoinduced (at T = 425 K) metastable dark conductivity of a-Si:H films by weak illumination during the initial stage of relaxation

The effect of weak illumination during the initial stage of relaxation of the dark metastable conductivity of an undoped a-Si:H film, photoinduced at T = 425 K, on the rate of its subsequent thermal relaxation is studied. It is found that the kinetics of relaxation upon illumination or in the absence of illumination is described by stretched exponents with the parameters τ0 and β, which are smaller in the case of illumination. It is shown that a decrease in these parameters increases the rate of thermal relaxation of the dark conductivity of the film. Because the temperature and the illumination intensities at which the study is carried out are low, the changes in the relaxation rate of the metastable conductivity are unlikely associated with significant restructuring of the amorphous network. This may be due to changes in the system of hydrogen bonds, which can result, in particular, from the generation and relaxation of slow photoinduced defects under the influence of illumination.

Specific features of recombination in layered a-Si:H films

The absorbed light spectral composition determines the type of carrier generation: interband generation or mixed generation that also includes the generation of electrons from levels of the valence-band tail. The generation type affects the value and temperature dependence of the electron recombination rate in a-Si:H layered films. This effect is caused by a variation in the occupation of the levels of silicon dangling bonds and the valence band tail with electrons upon a change in the carrier generation type. As a result, in the case of mixed carrier generation in the investigated films with a low concentration of native dangling bonds, electron recombination in the films is slow and recombination at the levels of the valence-band tail can prevail up to room temperature.

Characteristics of recombination processes in doped hydrated amorphous silicon films

The results of measurements of the temperature dependences of the dark conductivity and photoconductivity ofa-Si:H films, doped from the gas phase or by implantation of phosphorus or boron ions, as well as the effect of preillumination with white light with different duration on the photoconductivity are presented. A model is proposed for carrier recombination in doped films, taking into account the broadening of the levels of dangling bonds and the difference in the coefficients trapping of electrons and holes on neutral and charged dangling bonds. The dependence of the stationary interband photoconductivity on the equilibrium Fermi level and the appearance of the temperature-induced quenching of the photoconductivity in doped films after preillumination are studied on the basis of this model.