The Influence of a-Si:H/Substrate Interface on Electronic Properties Of Me-a-Si:H - Substrate Structures
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substrates (glass, oxynitride SiON, polyimide). The substrates prior to deposition were cleaned using a standard ammonia-hydrogen peroxide solution (NH 4 OH:H 20 2 :H20=l:l:5) and then rinsed in deionized water. After the deposition the films were annealed at 240 0C during 0. 5 hour in the vacuum of 10-' Torr to eliminate the residual impurity contamination. The presence of residual impurities (O,N, and C) in the films was controlled by depth profile Auger analysis. The impurities were found to be concentrated in the surface layer less than 30 A thick. This layer was then removed by dilute HF(I-IF:H 20=1:50) for 20 sec before formation of aluminium contacts for conductivity measurements. The aluminum coplanar electrodes separated by 0.6 mm were deposited on the top of aSi:H films and annealed at 2000 C during 0.5 hour. The measurements of temperature dependence of dark conductivity were performed in the temperature range of 25-2500 C. For this purpose the current, I, was measured as a function of temperature, T, at fixed applied electric field during heating with the heating rate of 5 'C/min and cooling with the cooling rate of 10 'C/min. The value of applied electric field was varied from 10 V/cm to 2.102 V/cm. RESULTS AND DISCUSSION Figure 1 shows the results of measurement of dark conductivity Gd temperature dependencies for a-Si:H films deposited on glass substrate. As one can see, strong "kink" on lgad(1/T) at the first heating was observed while there was no "kink" during cooling and at the second heating. Measurements of ad(T) after prolonged storing show that "kink" is reproduced in the same temperature range. If the first heating was performed without applied electric field the conductivity measurements at a second heating again revealed the presence of "kink". These results suggest a crucial role of electric field in this phenomenon. As it is seen from Figl. there is a threshold value of electric field, F_ , below which "kink" has never been observed. Iga -3
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Fig.2. The temperature dependencies of dark conductivity for a-Si:H deposited on glass substrate (#), oxynitride SiON (2) and polyimide film (6)
Fig. 1. The temperature dependencies of dark conductivity for a-Si:H deposited on glass substrate at 2102.V/cm for: the first heating (0), cooling (A), a second heating (0), after prolonged storing (#); and at 10 V/cm for the first heating (0)
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Results of lgad(1/T) measurements for a-Si:H films deposited on different substrates (glass, SiON, polyimide film) performed at electric field of 2 102 V/cm are presented in Fig.2. One can see that "kink" appears for the samples of a-Si:H both on glass and on SiON substrates, and does not appear in the case of a-Si:H on polyimide film. Moreover, "kink" on lgad(1/T) for a-Si:J- deposited on SiON substrate shifted to lower temperatures in comparison with a-Si:J1 on glass. Thus, "kink" appeared in the case of a-Si:H film deposited o
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