Metastability of Phosphorus- or Boron Doped a-Si:H Films
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light-induced changes in the dark conductivity and in the defect density of thick phosphorus- and boron-doped films as functions of the doping level and the temperature during light-soaking. Sample Dopant Thickness Deposition Optical gap (sccm) (1m) rate (A/s) (eV) Trimethyl-boron doped a-Si:H samples
Sigma dark (S/cm)
Ea (eV)
TE (0C)
1.83
5.10- 8
0.63
110
511102 2 0.53 0.74 511135 8 0.61 0.85 Trimethyl-boron doped a-SiC:H samples 511141 0.5 0.68 0.63 511142 2 0.54 0.75 511155 8 0.68 0.94 Phosphine-doped a-Si:H samples 710081 0.2 0.45 0.45 708082 0.5 0.69 0.64
1.84 1.84
5.10-7 3.10-6
0.39 0.5
67 72
1.94 1.92 1.88
5.10-8 1.10-6 5.10-6
0.55 0.48 0.43
119 104 97
1.78 1.77
2.10-3 5.10-3
0.28 0.17
127 120
708111
1
0.52
---
1.74
9.10-3
0.18
120
708112 708113
2 4
0.61 0.64
0.56 0.59
1.74 1.70
8.10-3 1.10-2
0.21 0.19
120 120
511101
0.5
0.67
0.64
Table L Effects of the dopant gasflow rate on the deposition rate and on the optical and electricalpropertiesof the films used in this study. 649 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
EXPERIMENT Thick (0.5 pm) phosphorus- or boron-doped a-Si:H films were prepared at 150 °C by RF glow discharge decomposition of silane (10 sccm) mixed with either phosphine or thrimethylboron diluted at 2% in hydrogen [11]. Boron-doped silicon-carbon alloys (a-SiC:H) usually used as window layers in solar cells were produced by the decomposition of a mixture of 50% silane and methane. The structure of the films was characterized by complementary techniques: UV-visible ellipsometry, infra-red absorption, and hydrogen evolution. The electrical properties and defect density were deduced from photothermal deflection spectroscopy (PDS), dark conductivity, photoconductivity, and two-point constant photocurrent method (CPM) measurements [12]. Accelerated light-soaking was performed under vacuum by exposing the samples kept at a fixed temperature (0 °C, 40 °C or 80 °C) to the light of a 1 kW arc Xe lamp, which was filtered by a 150 nm thick a-Si:H film to ensure a uniform creation of defects as a function of the film depth. The intensity of the filtered light on the samples was - 350 mW/cm 2 . RESULTS Dark conductivity Figure 1 shows the temperature dependence of the dark conductivity for boron- and phosphorus-doped a-Si:H samples. For each as-deposited sample, the conductivity was recorded during the increase of temperature from ambient up to 170 'C and during the cooling-down. The dark conductivity and activation energy values reported in table I were determined from the cooling-down curve, below the equilibrium temperature TE defined as the temperature above which both curves overlap. The increase of the flow rate of the doping gas above I sccm results in small changes of the conductivity (activation energy) for the p-type films but has practically no effect for the n-type samples. As a matter of fact, the increase of the doping flow rate above 2 sccm induces structural changes of the silicon network and a deterioration of the prope
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