Defect Distributions in Doped and Undoped A-SiGe:H Alloys
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Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
germane (GeH 4) were diluted in hydrogen ([H2]/[SiH 4 + GeH 4] > 10). Phosphine and Diborane were used for doping. The thickness of the samples was between 0.7 urm and 1.3 ýtm. Compositional information was obtained from Rutherford backscattering. The optical bandgap EG = E03 .5 was deduced from transmission and reflection measurements. Further information on the absorption coefficient a were obtained from CPM and PDS spectra which were taken by conventional set-ups at room temperature [3,4]. The Fermi level EF was derived from the room temperature dark conductivity ad = 150 (K2cm)-1 exp((Ec - EF)/kT). MODEL Our simulation of CPM and PDS spectra is based on a density of localized states consisting of exponentially increasing band tails and a broad defect distribution. A correlation energy U = + 0.2 eV is taken into account. A spatially homogeneous defect density without interface states is assumed. Our model includes the full set of optical transitions between localized and extended states, capture and emission processes of carriers into and out of localized states as well as the position of the Fermi level. A single set of parameters was used for successful modeling of CPM and PDS spectra and for calculating the temperature dependent photo conductivity of n-type a-Si:H as well as device characteristics [5]. For capture cross sections for electrons and holes into charged and neutral defect states a ratio of 16 was chosen. For a-SiGe:H alloys the model assumes capture cross sections and carrier mobilities to be independent of the composition. The consistent description of simulations of sub-bandgap absorption spectra as well as modeling of solar cell characteristics allows a variation of the capture cross sections only within a factor of 2. Detailed information on the occupation statistics is given elsewhere [6]. To simulate CPM spectra, we calculate the photon flux ýCPM, required to keep the photocurrent constant, and obtain the absorption coefficient (xcpM - 1/OCPM. Based on the defect distributions derived from CPM simulations, PDS spectra are calculated by summing up all optical transitions. RESULTS Figure 1 shows CPM spectra of undoped a-SiI1 xGex:H for compositions x between 0.11 and 0.51. With increasing Ge content the bandgap decreases and the Urbach edge shifts towards lower energies. With decreasing bandgap the onset of the defect absorption shifts to lower energies. However, the inverse logarithmic slope of the Urbach tail E 0 shows no significant dependence on the composition. Only for x = 0.51 a slightly enhanced E 0 is found. In contrast to E0 , the defect density rises from 3.1016 cm-3 for x = 0.11 to 8.1017 cm-3 for x = 0.51. In Fig. 2 the Fermi level, derived from dark conductivity measurements, is plotted versus the bandgap for a wide range of undoped samples, i.e. a-SiGe:H and a-Si:H. In addition, data taken from literature are shown for a-Ge:H, a-SiGe:H and a-SiC:H [7,8], respectively. It should be noted that regardless of the
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