Effects of Light-Soaking in a-Si:H Studied by Photoconductivity Response Time
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EFFECTS OF LIGHT-SOAKING IN a-Si:H STUDIED BY PHOTOCONDUCTIVITY RESPONSE TIME CHANGHUA QIU, SHENFA QIAN AND DAXING HAN, Institute of Physics, Chinese Academy of Sciences, P.O.Box 603, Beijing, China JACQUES PANKOVE, SERI and Dept. of Elect. & Comput. Engin., Univ. of Colorado, Boulder, CO 80309 ABSTRACT The temperature and light intensity dependences of photoconductivity(PC) and PC response time(tr) were investigated in undoped a-Si:H. The effects of light-soaking at 110K and 300K on PC and tr are compared. The experimental results suggest that two kinds of metastable centers are induced by lightsoaking: one corresponds to increase in the subband-gap absorption, the other corresponds to the degradation of pitproduct.
1. Introduction Light-soaking induces degradation of PC and increases in subband-gap absorption and neutral dangling bonds(DO) density. 1- 3 These changes seem to be closely correlated since DO are important recombination centers. However, 4 Han and Fritzsche showed that the degradation of the UT product and the increase in subband-gap absorption, c(l.OeV), are -ot correlated. This obser5 vation is further supported by recent studies.Kocka et al. found that light-soaking under anapplied stress induced a much larger increase of a and a smaller decrease of PT than by light-soaking without stress. Kazanskii et 6 al. observed that the decrease of VT is faster than the increase of a in the early stage of light-soaking. However, these authors suggested that the small increase of a and large decrease of PT may result from a shift of the Fermi level, Ef, which induces charge redistribution among the differently charged dangling bonds(DB) states. From annealing recovery studies, we observed that the recovery behaviour of PTis different from that of usual metastable DO. 7 The degradation of PT product was attributed to the creation of "T-centers", which are more efficient recombination centers than the usual DO states. In this paper, we present further support for the two kinds of metastable centers by PC and PC response time measurements on undoped a-Si:H. Large degradation of VT product and little increase in a were observed after 110K light-soaking with negligible change of dark conductivity or shift of Fermi level.
2.
Experimental results and discussions
The samples were glow-discharge undoped a-Si:H with coplanar electrodes. State AO was reached by annealing at 480K for two hours. In order to avoid light-soaking effects during measurements, the state AO was first soaked by 1mW/cm light for 1/2 hour to reach state Al. This procedure led to decreases of dark conductivity(od) and PC by factors 3 and 2, respectively, with no observable change in subband-gap absorption nor the activation energy(Ea) of Gd. From state Al, the sample was soaked by AMI light at either 300K or 110K for 4 hours to reach state BI or state B2. For both state Al and state B2, Ea was 0.79eV, ud at 300K was 7.4x0l- 1 ( cm)-. Light soaking at 110K induces little increase in subband-gap absorption. Thy value of a(l.OeV) for states Al, BI
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