Current Noise Measurements of Surface Defect States in Amorphous Silicon
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of 100 Watts. Ion milling of the surface was performed in a T chnics Ion Mill using Argon and an acceleration voltage of 100 Volts, giving an etch rate of -13 Angstroms/min. Typical etch times for the ion mill were greater than 30 min., since it was necessary to remove the 500 Angstroms of n+, or alternately to insure that all effects of the reactive ion etch were removed from the surface. Wet chemical etching with a mixture of 98% HNO 3 and 2% HF near room temperature for 3-25 seconds was also used. This was a very fast uncontrolled etch and was not used extensively but was employed as an alternative etching method as described below. After removal of the n+ layer, the samples were patterned into a Van der Pauw four-probe structure using standard photolithographic techniques, giving an effective sample volume of - 10-6 cm 3 . For coplanar conductance fluctuation measurements, the samples are placed inside an integrated circuit package, and gold thread is wire bonded from the gold contact pad to the IC package pins. The IC package resides under vacuum in thermal contact with a temperature controlled copper block in the measurement chamber. The sample is annealed at 430 K for one hour to remove any surface adsorbates which might influence the conduction [3] and to remove any effects of prior light exposure [4]. By using the diagonal contacts of the four-probe structure,
measurements are performed in a two-probe configuration commonly used for low level noise studies of high impedance films [5,6]. An Ithaco 564 low-noise current preamplifier is used to amplify the current fluctuations, while a HP 3561 spectrum analyzer digitizes and calculates the power spectral density from the amplified current fluctuations. The third and fourth contacts of the four-probe structure are utilized to insure that noise from the metal contact/a-Si:H region is not present. By contact detection methods outlined elsewhere [7], all noise results reported were verified to be free of contact noise. RESULTS The current noise from these a-Si:H films typically shows a power spectral density which varies as f'Y for frequency f, with y - 0.9-1.2. However, when the top surface of the a-Si:H is reactive ion etched, the noise power spectrum changes dramatically, as illustrated in Figure 1, developing a sharp "Lorentzian" feature (S(f)-4,T/(l+(27rft) 2 )) in addition to the usual 1/f noise component. Shown in Fig. 1 for comparison is a power law 1/f as well as a Lorentzian frequency dependence. The frequency dependence of the spectral density clearly deviates from a pure 1/f power law, but is not described by a pure Lorentzian spectrum either. It is difficult to determine from Figure 1 whether the spectral feature near 100 Hz is broadened, or whether a 1/f component in addition to a pure Lorentzian accounts for the broadened appearance. It is this feature in the noise spectral density that is the focus of this paper, which appears only after a RIE treatment of the thin film surface. This feature is identified as arising from the near surface region,
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