Han Daxing

Study of trapped non-equilibrium carriers in a-Si:H by a two-beam experiment

A new method of measuring the density of states in amorphous semiconductors by applying a two-beam photoconductivity experiment is reported. In the twobeam experiment, an intensive bandgap pump light is used to create non-equilibrium carriers and most of these carriers are trapped in the gap states. Then at a delay time t d after turning off the pump light, these trapped carriers are re-excited by an infrared probe light. A photoconductivity overshoot, which depends on the delay time and temperature, is observed in the onset process of IR excited photoconductivity. The results are discussed using the multiple trapping theory. The distribution of the density of gap states is deduced from the experiment.

Theory of Steady State Photoconductivity in Amorphous Semiconductors—Sensitization Due to the Presence of Different Types Recombination Centres

The present work attempts to connect Simmons and Taylors theory of steady state photoconductivity with Rose model of sensitization. A generalized formula of photoconductivity for a system with several recombination centres is given. An iteration procedure is suggested to solve the related rate equations. Using this formula and combining the particular energy band model of a-Si:H, the following result is obtained: when two types of recombination centres in the material exist and the electron capture coefficient for a centre of the second type is much smaller than that of the first type, and when the state density of the former is greater than that of the latter, the free electron lifetime will be greatly increased, therefore, the material will be sensitized. This theoretical result is consistent with that of the Rose model .The dependence of the photocurrent on temperature and incident photon flux are also given. Comparisons with the experimental results on hydrogenated amorphous silicon are made.

Theoretical analysis of infrared stimulated current in a-Si:H

Abstract The infrared stimulated current (IRSC) in amorphous semiconductors is interpreted in terms of a model of two-step excited processes. The photoconductivity behavior with the excitation of two successive light beams, a visible light beam at an early time and an i.r. beam at a later time, is also analyzed based on the same model.

Photoinduced defect optical absorption in a-Si:H

The photoinduced defect absorption in a-Si:H excited by bandgap light was studied. Energy position of light-induced metastable defects is different from that of native dangling bonds.

The Effect of Light Soaking on the Photoconductivity Response in A-SI:H

The temperature dependences of response time tr and steady state photoconductivity (PC) were used to deduce the DOS at energies above the dark Fermi level. The t r and PC of annealled state A and light soaked state B were measured from 115K to 300K. Light soaking causes degradation of PC. Compared with state A, the PC response of state B is faster at low temperatures, but is slower at high temperatures. The difference between state A and state B was interpreted by a decrease of shallow traps and an increase of deep traps.

Generation rate of the light-induced defects and the temperature dependence of photoconductivity in a-Si:H

The time dependence of the creation process of the light-induced defects is observed in a-Si:H. It is found that the density of defects increases with time of light exposure in two steps: at first ΔNs ∞ t0.7~1.0 and then ΔNs ∞ t1/3. Photoconductivity excited by photons of 1.0eV between 130K and 200K shows an activation energy which decreases with prolonged light exposure.

Photo-induced reversible change of Urbach edge in hydrogenated amorphous silicon

Photoconductivity spectra temperature dependence combining with transmission measurements have been formed on hydrogenated amorphous silicon. After exposure by AM1 light a reversible increase of 6% of the Urbach parameter E0(T,X) has been observed. This is a direct evidence for the fact that photo-induced changes in a-Si:H create not only metastable defects but also affect the topological disorder.

KINETIC BEHAVIORS OF THE TIME-DEPENDENCE OF PHOTOCONDUCTIVITY IN A-SI-H

Abstract The behavior of the time dependence of photoconductivity have been calculated on the basis of a model for the kinetics of the defect creation process under the illumination in a-Si:H. Under the adiabatic approximation, the theory of the steady state photoconductivity together with spin generation rate equation is used to derive the time dependence of photoconductivity. The results are in agreement with experimental data.

The exponential absorption edge and the low absorption range shoulder of a-Si:H

We have used optical and photoconductivity measurements on a-Si:H to acquire information about the band tail and gap states. The experimental results show that annealing and exposure do not change the slope of the absorption edge at certain temperatures but change the sub-gap excitation.

Thermal quenching, infrared quenching and supralinearity in a-Si:H

We have tested the temperature and incident flux dependence of photoconductivity and dual beam infrared photoconductivity spectra of a-Si:H samples in annealed states A* and A, and the light soaked states B1 and B2; and observed that they were different for the 4 different states. We believe that light exposures create two kinds of centers, which we name α and τ. A model is suggested to explain the experimental results.