Infrared Absorption Studies of Thermal Donors in Silicon
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INFRARED ABSORPTION STUDIES OF THERMAL DONORS IN SILICON PETER WAGNER Heliotronic GmbH D-8263 Burghausen, Federal Republic of Germany
ABSTRACT Infrared absorption investigations of oxygen containing silicon reveal that the thermal donors generated by annealing around 450 °C consist of a series of shallow double donors with slightly differing ground state energies. The kinetics of formation and decay of these donors in samples with varying oxygen content annealed at temperatures between 460 and 600 0C support the relation of the thermal donors to aggregates of oxygen atoms. A preferential orientation of thermal donors induced by uniaxial stress during their growth demonstrates their low symmetry and extended defect-like nature. Optical cross sections evaluated by comparing IR-absorption spectra with resistivity measurements allow to estimate the concentrations of the different donors.
By applying the knowledge about shallow donors to the problems of thermal donors (TDs) /I/ generated in CZ - Si by heat treatments around 4600 C it was demonstrated that the TDs are a series double donors with slightly different binding energies /2, been identified so far /4,
3/. Up to nine different TDs have
5, 6/. The ground-states of the neutral TDs are
shifted due to central cell corrections, as compared to Effective Mass Theory (E4r) /7/, but the ground states of the singly ionized TDs behave almost exactly as
EMT-donors /8/.
The binding energies evaluated by
optical excitation spectroscopy agree very well with values determined by capacitance transient spectroscopy /9,
10, 11/ and Hall effect /12,
13/.
The relation of TDs with paramagnetic centers, especially NL 8, was supposed /14/
and recently corroborated /15,
symmetry group of the TDs being C2v /14,
16/ as well as the point
17/. The TD with the largest
binding energy is labelled as TDI, the other ones following in the order of the decreasing binding energies. The optical transitions are labelled according to their respective final states, e.g. 2p ,
2
p+ etc. Tables ± containing the wavelengths of the various absorption lines can be found in 0
/6, 15/. The splitting of the p+-1ines of the singly ionized TDs is thought to be due to an anisotropic potential /6/. The shallow ground states of the same TDs might arise from a repulsive contribution to the Mat. Res. Soc. Symp. Proc. Vol. 59. 1 1986 Materials Research Society
126
potential or a screening effect of the highly electronegative agglomerated oxygen atoms /18/.
MODELS
In the course of the long history of the TDs in silicon since 1954 /I/ various models were suggested in order to explain their formation kinetics and electrical activity after the correlation of TDs with oxygen was established by /19/. More or less complete accounts of models for TDs and criteria for them can be found in /8, 20/. A common feature of these models is the aggregation of oxygen atoms, the electrically active part being related either to oxygen /10, sic defect /15,
20,
24, 25,
21,
22, 23/ or an incorporated intrin-
26/.
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