Evidence for Different Kinds of Dangling Bond Defects in a-Si:H
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EVIDENCE FOR DIFFERENT KINDS OF DANGLING BOND DEFECTS IN a-Si:H
MINH TRAN, H. FRITZSCHE AND P. STRADINS University of Chicago, The James Franck Institute, Chicago, IL, U.S.A.
ABSTRACT Comparing the photoconductivity ap of undoped samples with different native defect concentrations we find that light-induced metastable defects decrease the electron lifetime more strongly than the native defects. We discuss the differences between native and metastable dangling bond defects. We find that the constant photocurrent method underestimates the defect concentration in undoped a-Si:H.
INTRODUCTION The principal defect in a-Si:H appears to be a three-fold coordinated Si atom, the Si dangling bond.
The first evidence for defects with different gap energies came from determinations of the densities of states in differently doped a-Si:H by means of the constant photocurrent method (CPM)' and total yield electron emission spectroscopy. 2 In n-type material the defect energies were found at lower and in p-type samples at higher gap energies relative to undoped samples. These differences in energy were attributed to pairing of defects with phosphorus or boron dopant atoms. Alternatively, the defect pool model 3 explains the different defect energies thermodynamically in terms of the Fermi energy at the equilibration temperature. Chen et al. 4 have studied also metastable defects produced by light or by quenching. They found evidence that the gap energies of these metastable defects depend on doping. In this paper we report that even in undoped a-Si:H one notices strong differences between stable and light-induced metastable defects in their effect on the photoconductivity and its temperature dependence. The experimental details are described in another paper at this conference. 5
Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
196
DETERMINATION OF DEFECT CONCENTRATION We will show that light-induced defects have a more profound effect in reducing the electron lifetime and hence the photoconductivity e1p near 300K than the same concentration of native dangling bond defects. In order to do that we have to compare the defect concentration ND of different undoped samples in their annealed state (A) and after light-soaking (B). Among different methods of determiningND the most commonly used one is CPM, because it has been shown to be largely independent of surface states.6 One 7 obtains ND from an integral over the infrared absorption air(hv) below hv < 1.4 eV. One determines air(hv) from the ratio of the light flux F 2 say at hv = 2 eV and the light flux Fir for hv < 1.4 eVwhich produce the same photocurrent:
O~rFir dd2[1=
exp(-a 2d)]
(1)
The absorption coefficient a 2 at hv = 2 eV and the thickness d are obtained independently. Eq. (1) neglects the photon-energy dependence of the reflectance as well as thin film interference terms which are not important for the present discussion. In Fig. I we show our measurements of F2 / Fir and also of a'p of sample I as a function of T after annealing (state A)
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