Influence of Boron-Doping on Hydrogen Diffusion and Effusion in a-Si:H and a-Si-Alloys
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INFLUENCE OF BORON-DOPING ON HYDROGEN DIFFUSION AND EFFUSION IN a-Si:H AND a-Si-ALLOYS W, BEYER*, J. HERION*, H. MELL** AND H. WAGNER* Institut fUr Grenzfldchenforschung und Vakuumphysik, Kernforschungsanlage JUlich GmbH, D-5170 JUlich, Federal Republic 2 Germany Fachbereich Physik, Universitat Marburg, Renthof 5, D-3550 Marburg, Federal Republic of Germany ABSTRACT The change of the hydrogen effusion spectra and of deuterium in-diffusion profiles in a-Si:H and a-Si alloys by boron doping is attributed to a Fermi level dependent change of the Si-H bond rupture energy. For low and high substrate temperatures additional effects occur due to structural heterogeneity. INTRODUCTION The thermal stability of hydrogenated amorphous silicon (a-Si:H) and a-Si:H alloy films is known to depend largely on the diffusion and effusion of hydrogen. Since boron-doping has been found to change the hydrogen diffusion and effusion in a-Si:H considerably [1,2], it is the aim of this article to study the boron doping effect in greater detail, using compensated films, a-Si:H alloys and films of different substrate temperatures. EXPERIMENTAL The films were prepared in capacitively coupled glow-discharge systems from pure SiH4 , SiD4 and CH 4 gases. Typical deposition conditions involved a vacuum pressure of 0.2-0.4 mbar, a gas flow of 2-10 sccm and an rf power of 5-25 W at 13.5 MHz resulting in deposition rates of 1-5 A/s. The films of 0.2-2 pm thickness were deposited on sapphire or crystalline silicon substrates. For hydrogen effusion, the samples were inserted into a quartz tube evacuated by a turbomolecular pump and they were heated to about 1100 0 C at a heating rate of 20 K/min. The partial pressure of hydrogen monitored by a quadrupole mass analyzer was used as a measure of the hydrogen effusion rate. For studying the hydrogen diffusion, thin layers of deuterated amorphous silicon (a-Si:D) were deposited on top of the hydrogenated layers and the penetration of deuterium into the hydrogenated layer after one hour annealing steps at various temperatures was monitored by secondary ion mass spectrometry (SIMS) profiling [3] employing an 9 keV oxygen sputtering beam. Hydrogen and deuterium diffusion in silicon are expected to differ only slightly [3]. RESULTS AND DISCUSSION Typical effusion results for both hydrogenated and deuterated amorphous silicon films are shown in Figs. la and b for the deposition temperatures T. = 40 and 300 0 C, respectively. Plotted is the hydrogen (and deuterium) effusion rate dN/dt versus temperature T. While little difference between hydrogenated and deuterated films is observed as expected [3], boron doping is found to Mat. Res. Soc. Symp. Proc. Vol. 118.
1988 Materials Research Society
292
TS=40C 10 1%B 2 H6D
Sa-Si:H
a-Si:H zr a-Si:D
10Wo;z. a-Si:D d-0.7Tim
W
TSi300°C
1% B2H6oea=.e•r MBA.
617
z 1001 Ib
107 200
400
600
800
10 17
200
400
T(°C) Fig.
600 T(0C)
800
1. Effusion spectra of boron-doped (1% B H ) and undoped (UD) 0 a-Si:H and a-Si:D films (a) for T $ = 46-8, (b) for Ts = 300
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