Defect Structure and Network Disorder in Boron Doped Amorphous Silicon
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DEFECT STRUCTURE AND NETWORK DISORDER IN BORON DOPED AMORPHOUS SILICON M.B. Schubert, G. Schumm, E. Lotter, K. Eberhardt, G.H. Bauer Institut fir Physikalische Elektronik, Universitilt Stuttgart Pfaffenwaldring 47, 7000 Stuttgart 80, Germany ABSTRACT A series of boron doped a-Si:H films have been characterized by PDS, FTIR, Raman, and SIMS in order to evaluate the effects of boron incorporation on structural properties and hydrogen bonding. Doping by B2 H6 or B(CH 3)3 does not significantly enhance the overall disorder of the silicon network showing up in the TO-like Raman halfwidth whereas remarkable changes in local, defect related structures are evident from PDS. An analysis of the data suggests two bands of defects in the pseudogap at low boron concentration and only one band for higher concentration. To account for Fermi level positions, shifts of the hole transport path well into the valence band tail upon doping must be invoked. INTRODUCTION Several thermodynamical models have recently been discussed [1][2] which for the first time consistently explain the vast majority of experimental data on defect structure and defect kinetics in a-Si:H. In the framework of these 'defect pool' calculations structural properties of the samples directly affect the defect density and gap state distribution via changes in band tails and/or bond angle fluctuations. A well known problem common to photovoltaic applications of a-Si:H-based semiconductors is, that diborane doping remarkably deteriorates the optical quality of p-doped window layers [3]. We examine structural properties of B2 H6 - and trimethylborane (TMB)- doped a-Si:H in terms of bond angle distribution from Raman backscattering, Urbach energy from Photothermal Deflection Spectroscopy (PDS), and hydrogen content as well as H bonding structure from Raman, Secondary Ion Mass Spectrometry (SIMS), and Fourier Transfrom Infrared Absorption (FTIR). To correlate the overall network structure with distributions and densities of gap states, PDS-, CPM-, and in some cases ESR-measurements have been performed. NETWORK DISORDER Since hydrogen atoms are directly bound to about half of the boron atoms in B2 H6 doped a-Si:H [4] network ordering will be influenced by two competitive effects: * Twofold (BH)-incorporation is expected to relax bond angle distortions in conjunction with an increase in void density; * high B2 H6 collision cross sections at lower energy compared to SiH 4 give rise to changes in plasma decomposition of SiH 4 and in SiHl surface reactions, which presumably deteriorate network structure [3] [5]. Fig. 1 compares different order parameters from Raman experiments like the full width at half maximum (FWHM) of transverse optic (TO) contributions (measured from the high energy side of the TO-peak) [6] [7] or the ratio of integrals taken over the transverse acoustic (TA) and TO-peaks of the first order silicon spectra [8]. The increase in TO-FWHM by no more than 5 cm"1 resulting from diborane doping is not larger than changes in the Raman Mat. Res. Soc. Symp. Pro
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