Dissociation Kinetics of Shallow-Acceptor-Hydrogen Pairs in Silicon
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DISSOCIATION KINETICS OF SHALLOW-ACCEPTOR-HYDROGEN PAIRS IN SILICON
ZUNDEL T. and WEBER J. Max-Planck-Institut fuir Festk5rperforschung, Heisenbergstr.1, D-7000 Stuttgart 80, Federal Republic of Germany
ABSTRACT Annealing of hydrogenated p-type silicon with a reverse bias applied to a Schottky diode allows us to precisely determine the dissociation frequency VA of shallow acceptorhydrogen pairs (AH with A = B, Al, Ga, and In). The temperature dependent values of vA satisfy the relation vA = VoA exp (-EA/kT), with VoB = 2.8.1014 s-1, VoAl = 3.1 .1013 s- 1 , voG 0 = 6.9 . 1013 s-1, and VoJ = 8.4 •1013 s-1. The dissociation energies EA depend only weakly on the acceptors: EB - (1.28+0.03)eV, EAl = (1.44+0.02) eV, EG0 = (1.401+0.03) eV, and E1 n = (1.42+0.05) eV. The dissociation frequency of BH pairs shifts to a lower value when H is replaced by the deuterium isotope.
INTRODUCTION Atomic hydrogen forms electrically neutral complexes with group-III shallow acceptors (B, Al, Ga, In, and TI) in silicon. [1,2] Despite many studies on the thermal stability of the acceptor neutralization [1,3,4,5,6], there are no reliable data on the dissociation energies EA of these pairs. Capizzi et al. [6] tried to fit deuterium diffusion profiles in boron-doped Si by taking into account the D-diffusion and the formation and dissociation of BD pairs. The fit requires many unknown parameters and is poor in the near surface region. In the isochronal annealing experiments of Ref. [1] and [3], a first-order kinetic is implicitly assumed to derive EA from a plot of the AH complex concentration versus annealing temperature. However, isothermal anneals [4,5,7] give evidence of much more complex reactivation kinetics of the acceptor, which makes the determination of EA inaccurate [4,5] or even impossible [7]. The reversibility of the reaction AH +-p A + H is the main reason for the departure from first-order kinetics. In our experiment, the drift property of positively charged hydrogen under an electric field [8] is used to remove the atomic hydrogen released by the dissociation of the AH complexes. We anneal hydrogenated samples with a reverse bias applied to a Schottky diode, and observe a reactivation of the acceptor which rigorously follows first-order kinetics in the high-field space-charge region.
EXPERIMENTAL DETAILS AND RESULTS We use B-doped (100) float-zone silicon, Al-, or Ga-doped (111) Czochralski silicon, and In-doped (111) float zone silicon of 10 Qlcm resistivity. The samples are exposed to a radio-frequency (13 MHz) hydrogen plasma for 2h at temperatures in the range of 120°C 250°C . A layer of thickness d (d = 2-4 pm) below the irradiated surface is then chemically removed, and titanium Schottky contacts (1 mm diameter) are evaporated. The ohmic
Mat. Res. Soc. Symp. Proc. Vol. 163. 1990 Materials Research Society
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contact consists of an indium-gallium alloy scratched onto the back surface. Anneals at a temperature T, in the range 40°C to 2200C with a reverse-bias VR applied to the Schottky diode are performed in a cr
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