Positive and Negative Photoconductivity in Lead Telluride Doped With Gallium Epitaxial Films
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accompanied by formation of impurity states with negative correlation energy U [8]. The problem of mixed valence impurity reveals some specific features for lead telluride and related materials. First, the present state of theory gives no possibility to predict correctly the mixed valence behavior for an impurity in lead telluride or its solid solutions. Second, in contrast to classic DX-centers, the FL in doped PbTe may be pinned in allowed bands as well as within the gap. Third, DX-center formation in llI-V results in appearance of a deep (ground) and a split-off (metastable) level. Study of photoconductivity transient in lead telluride doped with Ga proved that the impurity levels associated both with the ground and the metastable states of impurity centers are separated from the extended ones with a barrier [9], so both levels are deep. EXPERIMENTAL RESULTS Typical temperature dependence of film resistivity p in darkness (Pfk) and under continuous illumination by the thermal source is shown in Fig. 1. pda(T) curves are characterized by rather slight growth of resistivity at cooling from the room temperature down to T - 50 K. The following cooling results in appearance of a saturation region [curves 1 and 2] for the samples with slightly higher resistivity values. For the mostly low resistivity sample [curve 3] pdak even decreases with temperature reduction. The observed behavior of pdak(T) curves differs significantly from that of semi-insulating films [9]. 327 Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
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0.02
0
0.2
,01
0.2
0.01
4.2K
-0.4-
0.0050
1
2
3
4
K526
Time t (s)
Reverse temperature 100/T (K-)
FIG. 2. Photoconductivity transient measured under illumination by a thermal source in nPbTe(Ga) film at 77K (curve 1) and 4.2K (curve 2). Arrows mark the moments of illumination switching off.
FIG. 1.Temperature dependence of resistivity p for n-PbTe(Ga) films with slightly different conductivity values. Curves 1-3 were measured in the darkness, 1'-3' - under illumination by a thermal source.
Photoresponse of low resistivity n-PbTe(Ga) films is composed of negative and positive parts which contribution to the PC process depends on temperature and resistivity of a film in darkness. The films are sensitive to IR illumination at temperatures Tc < 100 + 110 K. The coexistence of NPC and PPC processes does not allow us to determine the amplitudes for each of the effects but it is possible to carry out qualitative estimations. It should be mentioned that the temperature Tc corresponding to appearance of the PPC signal in the investigated low resistivity films coincides with the value obtained for semi-insulating n-PbTe(Ga) samples where the amplitude of the PPC response increases at cooling and saturates at temperatures lower 50 K. In the case of low resistivity films, one more characteristic temperature TN corresponding to the beginning of positive photoresponse reduction appears. At T=TN the contributions of the NPC and the PPC to the photoresponse value become comparabl
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