Light-Induced Change of Si-H Bond Absorption in Hydrogenated Amorphous Silicon
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___o)
where A- cn.o( _ -, N Ah =2 2e*2'
(1)
where a(0o) is the absorption coefficient, (o is the oscillator frequency, NH is the number of the Si-H bonds, c is the speed of light, n is the refractive index, m is the reduced mass of the SiH species, and e* is the effective charge for the oscillator in the a-Si matrix [7]. The sensitivity of the commercial IR spectrometer, including Fourier transform IR, is too low to detect any hydrogen-bonding concentration change less than 1021/cm 3 in a few gm thick a-Si:H film. Light-induced changes in IR spectra were first reported by Carlson et. al. using multiple passages of light through a-Si:H films deposited on prisms [8]. The detected change in IR absorption near 2000 cm- 1 was about 1-2%. However, the films employed in their experiment were contaminated by oxygen. More recently, R. Dawich et al. [9] reported evidence for participation of atomic hydrogen in the light-induced metastability by FTIR measurement at 6 K on poor-quality a-Si:H. They observed an evolution (first increased and then recovered) of the bending band at - 870 cm- 1 as well as that of the band at - 1720 cm- 1 , which they attributed 685 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
to the formation of three center Si-H-Si bonds under illumination. Until very recently, there was no reliable technique which has high enough sensitivity to detect the metastable changes of hydrogen configurations in device-quality a-Si:H. The invention of the differential infrared absorption (DIR) technique [10,11] has changed this situation. Using DIR, Kong et al. found [10,11 ] that the IR absorption around the wave number 2000 cm-1 in a device quality GD a-Si:H increases with light-soaking time; this change is reversible upon annealing. It is our goal in this work to compare subtle features of H environments between the a-Si:H samples with varied photoinduced metastability. SAMPLE AND EXPERIMENT a-Si:H films were deposited on intrinsic crystalline Si wafer (p>100 Q2-cm) by both glow discharge and hot-wire CVD at Solarex and the National Renewable Energy Lab (NREL), respectively. The defect density is < 1016 cm- 3 . In order to eliminate interference effects, the silicon wafer was single side polished and the a-Si:H film was deposited on the rough side. The sample preparation conditions, thickness, and hydrogen content are listed in Table I. Both infrared (IR) and differential infrared (DIR) absorption spectra were measured by a home-made spectrometer. The measurements were focused on the wave number range from 1800 cm- 1 to 2300 cm- 1 where the characteristic peak of the Si-H stretching vibration mode is located. The initial state of the sample was reached by 160'C annealing in vacuum to remove surface adsorption and metastable defects in the a-Si:H film. Then the measurements were carried out after step-by-step light-soaking. Finally, the sample was annealed at 200 oC in vacuum for one hour to remove the photo-induced defects in the a-Si:H film. Table I Sample preparation conditions and pa
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