Charge Transport in the Transition From Hydrogenated Amorphous Silicon to Microcrystalline Silicon
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vapor deposition (HW-CVD) [3-7], the decomposition of silane and hydrogen gas mixture allows one to prepare material with a transition from amorphous to microcrystalline growth by variation of the silane to hydrogen ratio. By varying the grain size, an enhanced absorption of microcrystalline compared to crystalline silicon has been observed [8]. The (HW-CVD) method allows a continuous preparation of material with a smooth transition from amorphous hydrogenated silicon to microcrystalline silicon. The photoconductivity in microcrystalline silicon has been studied as a function of Fermi-level to determine the mobility-lifetime product (4,r) [9] with the observation that (4-r) increased significantly by shifting the Fermi level from the mid-gap towards the conduction or valence band; this was attributed to the increase in the majority carrier lifetime due to a change in the thermal occupation of defect centers by the shift of the Fermi level. It is of interest to separately determine the mobility and lifetime in the transition from the amorphous to the microcrystalline state; dc photoconductivity measurements determine the 4,r product. The independent determination of la and T was accomplished by employing the photoconductive frequency mixing technique [10-15]. By observing the increase in the drift mobility as a function of electric field, the range and the depth of the long-range potential fluctuations [13] as a function of hydrogen content and hence the change in these quantities in the transition from amorphous to microcrystalline states were determined.
543 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
EXPERIMENTAL A series of samples was deposited on 1" x I" Coming 7059 glass at a substrate temperature of about 2400 C using the HWCVD technique. The details of the HWCVD reactor are reported elsewhere [16]. A tungsten filament was used with a diameter of 0.5 mm and 6 inches in length and placed 5 cm below the substrate. The filament was heated by an ac current to about 20000 C. The substrate was heated using a resistive heater and monitored using a thermocouple meter. All the samples were grown at the same pressure of 30mTorr while varying the hydrogen to silane ratio (RH) from I to 20. Table I summarizes the sample codes, film thicknesses, and deposition rates. The thickness of the i-layer was measured with a Tencor Instrument Alfa-step 200 profilmeter. The deposition rate was calculated from the film thickness and deposition time. Table 1: Summary of the sample characterization results Sample ID T516 T517 T518 T519 T528 T529 T530 T531 T532 T534
H/Sill4
XRD Structure
1
a-Si jtc Si itc Si ltc Si Pc Si [Ic Si a-Si+ltc Si a-Si a-Si+pc Si a-Si
5 10 20 5 4 3 2 3 2
(gLc) av. Grain size(nm) 12 13 14 18 12 9 11
Sample Thickness
Hcontent
(A)
(%)
(A Is)
41,400 21,350 14,300 11,200 25,400 24,000 16,400 25,000 17,000 30,000
13.3 5.2 4.7 3.9 3.1 4.2 4.4 8.9 4.0 10.9
17.25 5.93 3.97 2.33 7.05 8.00 7.80 10.41 7.08 12.50
Deposition rate
The charge transport parameters were det
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