The Study of Oxygen Thermal Donors in Silicon by Photothermal Ionisation Spectroscopy
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THE STUDY OF OXYGEN THERMAL DONORS IN SILICON BY PHOTOTHERMAL IONISATION SPECTROSCOPY JENNIFER A. GRIFFIN , H NAVARRO**, L GENZEL Max Planck Institut fUr Festkdrperforschung, Heisenbergstrasse 1, 7000 Stuttgart 80, Federal Republic of Germany. "*jointly at University of Bath, Bath, Avon, Great Britain. "**Alexander Von Humboldt Stipendiat, on leave from Universidad Autonoma de Puebla, Puebla, Mexico.
ABSTRACT It is well known that Czochralski-grown silicon contains up to 2x10 1 8 cm- 3 oxygen atoms. When this silicon is annealed at 450'C a series of "thermal" donors are formed. In this work we present the results of a study of annealed Czcochralski-silicon samples by Photothermal Ionisation Spectroscopy and IR transmission measurements. All previously 1reported thermal donors, which occur in the spectral region above 330cm- , and hove ionisation energies 53-69.3 meV, are observed by us. Also, our experimental results show the presence of at least three previously unreported thermally formed donor transition series below 330cm-1. Our results indicate that the ionisation energies of these new shallow thermal donors would be 36.3, 37.0 and 37.4 meV. Assignments are made by comparison of the energy differences between excited states to those predicted by the Effective Mass Theory. INTRODUCTION This paper reports on the observation of three new shallow donor centers in annealed Czochralski (Cz) silicon. These donor centers are observed by Photothermal Ionisation Spectroscopy (PTIS) [I1 which has a very high sensitivity to low concentrations of impurity centers in semiconductors. PTIS involves the two stage ionisation of an impurity. Firstly, a photon is absorbed exciting the bound carrier into an excited state. Secondly, a phonon is absorbed thus ionising the impurity. The change in photocurrent due to the ionisation of the impurities is monitored. Impurity concentrations of the order of 107 atoms/cm3 have been observed and identified in ultra-pure Ge [2,3] by this method. PTIS has previously been applied to the study of some elemental impurities in silicon such as phosphorus [4], indium [5], selenium and sulphur [6] but this is the first study of oxygen-rich silicon by this technique.
EXPERIMENTAL PROCEDURE High-ohmic n-type Cz-silicon samples were studied. The free carrier concentration was known to be INd-N-I - 4x10 1 2 c- 3 before heat treatment. Two samples, 1 and 2, were first sugjected to 10 minutes of annealing at
Mat. Res. Soc. Symp. Proc. Vol. 59. c1986 Materials Research Society
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770 0 C. Secondly, sample 2 was annealed for 15 minutes and sample 1 for 4.5 hours at 450'C. This resulted in a donor concentration of -5x10 1 3 cm- 3 for 1 3 sample 2 and -8x1O 4cm- for sample 1 as determined from resistivity measurements. Each sample, a disc 12mm in diameter and 2mm thick, was prepared for contacting by chemically polishing the circular faces. Then two strips of gold-antimony were evaporated onto the surface, 6mm apart. For PTIS it is important that ohmic contacts are achieved. It was found that laser anne
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