Study of Trapping and Recombination in a-Si:H by Means of Infrared Enhancement Spectra of Photoconductivity
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STUDY OF TRAPPING AND RECOMBINATION IN a-Si:H BY MEANS OF INFRARED ENHANCEMENT SPECTRA OF PHOTOCONDUCTIVITY
WU WENHAO, QIU CHANGHUA, ZHAO SHIFU AND HAN DAXING Institute of Physics, Academia Sinica, P.O. Box 603,
Beijing, China
Abstract IR re-excitation of non-equilibrium carriers in undoped a-Si:H has been used to probe the profile of the distribution of deep traps. In a dualbeam experiment, after turning off a pump light, the trapped carriers are re-excited by an IR probe light which causes the photoconductivity(PC) to pass through a maximum, Umax, before settling down towards its steady state value, as. Omax depends on the time interval td between turning off the pump light and turning on the probe light in a manner Aa=Omax-Osrtd-a(T) Based on the multiple trapping (MT) model, the distribution of deep traps has been deduced from the temperature dependences of a(T).
Introduction To make further understanding of the transport properties of a-Si:H, it is necessary to make out the detailed information about the distribution of the density of gap states (DOS) g(E). A variety of techniques have been used to measure the DOS such as the transport methods including photoconductivity (PC) (1), space charge limited currents (SCLC) (2), thermostimulated currents (TSC) (3), the field effects (FE) (4), and the junction capacitance methods including deep level transient spectroscopy (DLTS) (5) and thermally stimulated capacitance (6). However, these methods have given very different DOS curves (7). In this paper, we present the measurement of DOS by a time resolved dual-beam photoconductivity experiment. It is widely accepted in a-Si:H that photoexcited carriers are trapped in band tail states and deep trap states. As Fig.1 shows, at first the sample is illuminated by an intense pump light (with photon energy of 2.0eV 16 2 and flux of 10 photons/cm .s) to fill the traps with electrons. Then at a time delay td after turning off the pump light, an IR probe light (with 4 photon energy of 1.0eV and flux of 101 photons/cm2.s) is turned on and reexcites these trapped carriers to the conduction band. These re-excited carriers will contribute to PC, resulting in an overshoot in PC onset.
FLUX 2
.OeV
on off
Gmax
ats
on
off
on
TIME
/tqr 0
td
TIME
Mat. Res. SoC. Symp. Proc. Vol. 70. ý 1986 Materials Research Society
Fig.1 The sample is illuminated by a pump light at first, then the subsequent probe light excites a PC overshoot before getting to steady state PC. Turn on the probe light secondary, no PC overshoot occurs. (a) Time dependent excitations (b) Time dependent PC
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To discuss the decay of trapped carriers after turning off the pump light, we employ the multiple trapping (MT) model proposed by TROK (8,9) Set the electron mobility and follow the idea of Monroe and Kastner (10). edge at Ec=O and assume the constant trapping coefficient for all states. The condiThe thermal release rate for traps are given by v=Voexp(E/kT). tion vt=l defines a demarcation energy Ed(t)=-kTln(vot),
(1) 2
1
where vo is the attemp
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