Özcan Öktü

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.

Electron drift mobility in hydrogenated amorphous Si1−xC x with a low carbon content

Abstract The temperature dependences of the electron drift mobility μd in hydrogenated amorphous silicon (a-Si:H) and in a set of four hydrogenated amorphous Si1−xC x (a-Si1−xC x :H) alloys with x ≤ 0.1 has been determined from steady-state photoconductivity and response time measurements. The temperature dependence of the μd curve of a-Si: H shows a region with a single activation energy of 0.13 eV between 150 and 300 K, and an electron drift mobility of about 1 cm2 V−1 s−1 at 300 K. For the alloys, the single-activation-energy region is steeper and moves to higher temperatures. The results confirm that the conduction-band tail widens rapidly with increasing x in a-Si1−xC x :H.

The evaluation of Mott parameters by using the field dependent conductivity in amorphous germanium

Abstract The dc conductivity of amorphous germanium films, prepared by thermal evaporation in high vacuum, was investigated between 77 and 300 K as a function of applied electric field. The dc conductivity of these films obeys Motts T − 1 4 variable range hopping conductivity law below 200 K for low ( F 3 V/cm) applied electric fields. The electric field dependent variable range hopping conductivity data was analyzed using the models developed by Apsley and Hughes (1975) and by Pollak and Riess (1976). The two important parameters of hopping conduction, the localized wave function exponent α −1 and the density of localized states at the Fermi level N ( E F ), were calculated by using these models.

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.

An efficient and low-cost computer-controlled programmable duty cycle light-emitting diode pulse generator

The decay of photoconductivity from the steady state is widely utilized to characterize semiconductor materials such as hydrogenated amorphous silicon and its related alloys. We propose an efficient and low-cost light-emitting diode (LED) pulse generator with variable duty cycle and intensity to be used in measurements of photoconductivity response time. It is crucial to switch on/off the light swiftly. The circuit developed allows measurements of decay times ranging down to a few hundred nanoseconds. The LED pulse generator was successfully tested in measurements of the temperature dependence of the response time for an a-Si:H film and an a-Si1−xCx:H film.

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.

Drift mobility measurements in a-Si1-xCx:H

Abstract Steady-state photoconductivity and response-time measurements have been used to determine the influence of carbon alloying on the carrier drift mobility of hydrogenated amorphous silicon samples. While the expected decrease of the mobility with increasing carbon content is generally seen, the opposite sometimes occurs in regions where thermal quenching of the photoconductivity is observed. An explanation can be formulated in terms of recent proposals that a photocarrier sign reversal take place in the quenching region. The implication of this model for the standard interpretations of drift-mobility data sets is pointed out.

Annealing behavior of light-induced metastable defects in a-Si1−xCx : H

We have used the room-temperature constant-photocurrent method and dark-conductivity measurements to study the annealing kinetics of light-induced metastable defects in a set of a-Si1−xCx : H (x≤0.11) films. Light-induced metastable defects created at room temperature started annealing at higher temperatures for alloys with high carbon contents. The annealing activation-energy distribution function was calculated to be a narrow Gaussian peaked at about 1 eV for the unalloyed sample. For the alloys, the peak position shifts to higher energies with increasing carbon content. The variation of the dark conductivity of the samples was measured as a function of annealing time and annealing temperature. A similarity between the observed increase in the dark conductivity and the annealing rate of light-induced defects was identified.