Effect of Contacts on Capacitance Transient Measurements in N-Type Hydrogenated Amorphous Silicon
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times: as the trap occupation time increases, the defect energy becomes deeper. The emission times therefore increase as the occupation time of the trap increases. Because of the long time scale (1-100 msec) for the observed effect, the relaxation is too slow for the usual lattice relaxation which occurs on orders of magnitude shorter time scales. The long time scale of the relaxation phenomena was attributed to special properties of the amorphous network.5 Because these previous capacitance studies supporting the existence of bulk defect relaxation were performed on devices with non-Ohmic, non-standard electrical contacts in a short filling pulse time regime, it is important to examine the effect of contacts on the capacitance transient experiments. The work reported here extends the previous transient capacitance filling pulse measurements to samples possessing standard, Ohmic contacts. The underlying approach taken in this investigation was to fabricate a set of devices with identically prepared bulk material but different types of contacts. If defect relaxation is truly a property of bulk n-type a-Si:H, the anomalous filling pulse behavior should be observable in diodes with alternative contacts. Since the anomalous capacitance transients were observed only in devices with non-Ohmic contacts, the anomalous effects are most likely associated with the non-Ohmic contacts rather than a property of bulk n-type a-Si:H.
233 Mat. Res. Soc. Symp. Proc. Vol. 377 01995 Materials Research Society
EXPERIMENT Two sets of devices were fabricated with identical bulk material but different contacts. The bulk material in both sets of devices consisted of a 2.3 pm layer of conventional glow discharge deposited amorphous silicon from silane mixed with 20 ppm PH 3. One set of devices was fabricated with standard Ohmic contacts produced by depositing 100nm of Cr onto a polished, degenerately B doped, c-Si wafer with an Al film deposited on the back surface. A heavily P-doped (1% PH 3 in SiH 4) layer 100 nm thick was deposited onto the Cr layer forming the Ohmic n+ a-Si:H/Cr contact. This standard Ohmic contact will be referred to as an Cr/n+ contact. The lightly doped bulk layer of amorphous silicon was deposited onto this contact. In the second type of device, the lightly P doped bulk a-Si:H layer was deposited directly onto a polished, degenerately doped c-Si wafer which had been chemically 220 stripped of a native oxide. An Al film was deposited on the back surface of the 200 (c) --- ' wafer as well. These types of contacts n+/Cr illuminated 180 will be referred to as p+ c-Si contacts. 330K The P concentration in the bulk near the 10 kHz 160 contacts in both types of samples was found to be identical from secondary ion 130 (b) mass spectrometry. Portions of both of 120 these wafers were exposed to IR filtered n+/Cr white light with an intensity of 500 U- 110 ,* 'unilluminated mW/cm2 for three days. The sample 330K, 10 kHz 100 temperature was held near room temperature by flowing air over the (a) 130 sample during the illuminat
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