Hydrogen Passivation and Reactivation of Bistable Thermal Donors in Silicon
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HYDROGEN PASSIVATION AND REACTIVATION OF BISTABLE THERMAL DONORS IN SILICON D.I. BOHNE, P. DEAK* AND J. WEBER, Max-Planck-Institut fiir Festkorperforschung, Heisenbergstr. 1, D-7000 Stuttgart, Federal Republic of Germany Abstract The dissociation of Thermal Donor-hydrogen complexes is studied by infra-red absorption, CV and DLTS profiling. Silicon samples are prepared to contain only TD1 and TD2 and in particular only TD1 is exhibiting bistable properties. After hydrogen passivation, annealing in the dark under reverse bias leads to a first order reactivation of TD1 and TD2. A marked difference in the dissociation energies is found (ED(TD1 - H) = 1.67eV, ED(TD2 - H) • 1.9eV). Our results suggest that TD1 and TD2 have a different microscopic structure. Introduction
The Thermal Donors (TD) have been under investigation for nearly 40 years [1].
Although different models for TD were develioped, the microscopic structure and the
origin of the electrical activity are still unknown (for a review see Ref. [2]). Thermal donors are generated in oxygen-rich silicon by thermal annealing of the samples at temperatures from 300 - 5000C. These defects are double donors, with binding energies close to the conduction band edge. Eleven different species have been reported (TD1,...,TDl1) [3], successively appearing during the heat treatment with decreasing binding energies. The accepted model for TD is the oxygen chain model, which assumes the existence of one common electrically active core for all TD and the slight differences in binding energies arise from successive incorporation of additional oxygen atoms (or self-interstials) in chains along a < 110 >-direction of the crystal [2]. Thermal Donors 1 and 2 are negative U like defects and exhibit bistable properties. In Fig.la, we show a schematic configuration coordinate diagramm for TD1/2 as suggested by Latushko et al. [4]. There are two different configurations of TD1/2, the stable configuration S which is electrically inactive whereas the metastable configuration M reveals the two electronic levels of the double donors. The configurational state is controlled by the charge state of TD(M)* * 2e
_____________E
eD(S)
0
"_j 0
o 0 .07eV + 0.e+eV
TD(M) +e0 - D(M)
TD(s)o M
S++
0.22eV
0,+
0.32eV
S
CONFIGURATION COORDINATE Figure la Schematic configuration coordinate diagram for TD1/2
Figure lb Electronic levels for TD1/2 and occupancy levels for TD1 at Ec - 0.32eV and TD2 at EC - 0.22eV
Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
396
the defects. Cooling the sample from room temperature in the dark (IR) or without applied reverse bias (DLTS) freezes TD1/2 in the stable configuration (S) due to the lower energy for the neutral charge state, and no signal can be detected. The metastable configuration (M) is accessible only from the ionized charge state. Freezing-in of the metastable configuration (M) is achieved by cooling the sample from room temperature under illumination (IR) or by applying a reverse bias to the Schottky diode (DLTS). No
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