Alp Osman Kodolbaş

Light-induced metastable defects in a-Si:H studied by constant photocurrent method at 120 K

Abstract We have employed the constant photocurrent method at 120 K to study the low temperature creation and annealing kinetics of light induced metastable defects in a good quality undoped a-Si:H. Infrared selfquenching influences the CPM spectra taken at 120 K and therefore optical absorption coefficient αCPM measured at this temperature underestimates the defect concentration. However, relative concentration changes of the metastable defects in various light exposed or annealed states can be deduced from these measurements. We have obtained satisfactory results for the light induced defect creation kinetics at 120 K, and from a systematic annealing experiment we have deduced the activation energy distribution function for the metastable light induced defects created at 120 K. The result is a broad, resembling a two component distribution of defect annealing activation energies. Comparing this distribution with the one obtained by Q. Zhang, H. Takashima, J.H. Zhou, M. Kumeda, T. Shimuzu [Phys. Rev. B 50 (1994) 1551] we have come to the conclusion that defects with low annealing activation energies are firstly created at the earlier stage of the illumination.

Empirical calibration of the optical gap in a-Si1−xCx:H (x<0.20) alloys

The optical gap of tetrahedrally coordinated hydrogenated amorphous semiconductors is defined in various ways in the literature. These differently defined optical gaps are the Tauc gap, the Cody gap, the Sokolov gap, and the two-isoabsorption gaps. In this work, these differently defined optical gaps are determined for a set of good quality a-Si1−xCx:H with x<0.2 by measuring optical absorption coefficients. From these measurements empirical relations were derived between these differently defined optical gaps and carbon content (x) in the alloys. Variation of optical gaps with hydrogen content in the films is also discussed in correlation with structural disorder in the alloys.

Comparison of the methods used to calculate the density of states from measured subbandgap absorption in a-Si:H and related alloys

Different methods used to calculate the density of states (DOS) from optical absorption spectra measured by the constant photocurrent method in a-Si:H and in the related alloys are examined. We have carried out both numerical simulations and used measured spectra to distinguish the differences between the methods. Although it depends on the shape of the measured absorption spectra, the methods using the derivative of the measured subbandgap absorption spectra give most correct DOS.

Light-induced metastable defect formation and annealing kinetics in a-Si:H

We have investigated the kinetics of creation and annealing of light-induced metastable defects and their effect on photocarrier lifetime in two good quality, hydrogenated amorphous silicon samples using the constant photocurrent method (CPM) and steady state photoconductivity measurements at room temperature. Narrow distributions of annealing activation energies that peaked at about 1 eV, account for the annealing of created metastable defects for both samples. A hysteresis-like relation is observed between light-induced defect density and photoconductivity. This relationship is different for the two samples investigated and also depends on light intensity used for degradation. Calculated distributions of annealing activation energies do not provide additional information in the explanation of the observed differences in the hysteresis-like relationship between the samples. Comparing the deduced annealing activation energies distributions with the one obtained by Zhang et al. [Mater. Res. Soc. Symp. Proc. 336 (1994) 269] we have come to the conclusion that, more stable light-induced defects are created at longer illumination times or at higher light intensities, shifting the peak value of distribution of annealing activation energies from about 1 to 1.1 eV.

A comparative study for metastable defect creation and annealing kinetics and their relation to photoconductivity in a-Si1−xCx:H

Abstract We have utilised the constant photocurrent method (CPM) and steady state photoconductivity measurements to investigate the creation and annealing kinetics of light induced metastable defects, and their effect on photoconductivity, in a set of good quality a-Si 1− x C x :H ( x ⩽0.11) at room temperature. The annealing activation energy distribution for the alloys was deduced using the method proposed by Hata and Wagner [J. App. Phys. 72 (1992) 2857]. A narrow Gaussian distribution of annealing activation energies, peaking at about 1 eV, accounts for the observed annealing behaviour for the unalloyed sample. For the alloys, the peak positions of the Gaussian distributions shift to 1.1 eV, with increasing carbon content. The relationship between the inverse mobility–lifetime product, determined from steady state photoconductivity measurements, and the light induced metastable defect density during the creation and annealing cycles was also investigated for these alloys.

Electrical and optical properties of glow-discharge a-SiOx : H (x<2)

Hydrogenated amorphous silicon–oxygen alloy (a-SiOx : H) films were deposited by r.f. glow-discharge decomposition of a SiH4+CO2 gas mixture at a substrate temperature of 300 °C. The optical band gaps of the samples were found to change between 1.65 and 2.73 eV by varying the oxygen content from 4.5 to 64.2 at %. While both the room-temperature photo- and dark-conductivity decrease with oxygen alloying, for oxygen contents below 11.9 at % the measured conductivities are comparable to those of unalloyed a-Si : H. In contrast, the deep-defect density and Urbach parameter continuously increase with oxygen alloying.