Hall-Effect Studies on Microcrystalline Silicon with Different Structural Composition and Doping
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7 6XC> 90%, 8=293AS
5
C
-
=103%
SIMS
0n
In
2
1HALL
Xc < 20%, 5=50A S= 6.1% 0 .1 . . . . . . . . . . . 25 5 10 15 20 3
3
10 /T [10 /K] Fig. 1 Conductivity and carrier density for two samples with high and low Xc and grain size 5.
01
. . .
1
3
. . . . . 5
. 7
S [%] Fig. 2 Carrier density and phosphorous concentration as a function of the silane concentration in the gas phase.
concentration in the gas phase was 2% PH3 in SiH 4. The Sil 4 concentration S (S=SiH4S(SiH 4 +H 2)) is one of the crucial parameters for the structural composition of pc-Si:H. With increasing S the grain size decreases (here: S=2%: 8=350,k->S=6.1%: 5=50A) and the structure changes from crystalline to amorphous (here: S=2%: Xc>90%--S=6.1%: Xc90%) and various phosphorous doping levels between 2% and 20 ppm. For the structural characterization we performed Raman scattering (excitation wavelength: 488 nm of an Ar-Laser) and X-ray diffraction (grazing incidence). For the determination of the phosphorous concentration we carried out secondary ion mass spectroscopy (SIMS) using nearnormal oxygen bombardment at an energy of 6kV and detecting positive secondary ions. The phosphorous concentrations were calibrated by comparing the integrated SIMS ion signals of a phosphorous implanted sample with the implanted ion dose. RESULTS In Fig. 1 the conductivity and the carrier density for two highly doped samples (series I) with different crystalline volume fractions Xc and grain sizes 5 are plotted as a function of temperature. The conductivity of the sample with a high crystalline volume fraction above 90% (see also Ref. [3], sample A) exhibits the non-singly activated behaviour which is typical for highly doped pcSi:H. Decreasing the crystalline volume fraction and the grain size (here: Xc 5%, X, < 50%) differences between the SIMS data and the carrier density appear. The saturation of the phosphorous concentration for S>4% means that the build-in coefficient of the doping atoms does not change very much. Instead the carrier density changes strongly for S>5%, i.e. in the range where the strongest changes of X, can be observed. We recall that the conductivity of the sample with the smallest crystalline volume fraction (Fig. 1, Xc < 20%) is at least two orders of magnitude higher as for amorphous material with the same doping level. Therefore it is proposed that the conductivity is determined by the crystalline phase. This means further that the data obtained from the Hall-effect measurements have to be corrected with the crystalline volume fraction (see arrow in Fig. 2). In this picture the entire crystalline phase contributes to transport, dead ends and not connected grains are neglected. Despite the uncertainty of the exact crystalline volume fraction (depending on the method 100 1021 Xc differs by a factor of three for samples with low X, [6]) a correction of the data seems not appropriate. But we can conclude 00 ppm from the results of the SIMS and the Halleffect investigation that the doping efficiency in our highly doped pc-Si:H films
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