New Optical Transition at Thermal Donors in Silicon
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NEW OPTICAL TRANSITION ATTHERMAL DONORS IN SILICON J.WEBER ANDH.J. QWEISSER Max-Planck-Institut fUr FestkUrperforschung, Heisenbergstr. 1, D-7000 Stuttgart 1, Federal Republic of Germany ABSTRACT We detect new luminescence lines in Czochralski-grown silicon after prolonged heat treatments at 4500C. The properties of the new optical transition are well explained by a thermal donor-free hole recombination. From the IR-absorption spectra measured on the same samples, we have evidence for an inhomogeneous distribution of the thermal donors. The spatial fluctuations of the thermal donors appear to be necessary for the new luminescence spectrum to emerge. INTRODUCT ION
Heat treatment around 450 0 C, produces in oxygen-rich Czochralski (CZ) silicon, the so called "thermal" donor (TD) states. [I] Although observed first by resistivity changes in the heat-treated samples, a variety of additional experimental techniques gave further insight in the properties of these thermal defects [2]: During heat treatment at 450 0 C, interstitial oxygen forms several successively shallower double donors with decreasing binding energies. [3] Uniaxial stress measurements on the corresponding IR absorption lines [4], as well as on the associated NL8-EPR spectrum [5], established the C2 v-symmetry of the defects. The low symmetry and the Effective-Mass like behavior of the donors led to a model of a core consisting of an electron wavefunction derived only from one pair of conduction band valleys.
It is still unclear how several different donors with very
similar properties are formed during the heat treatment.
Photoluminescence (PL) measurements were performed on heat-treated samples. [6] Several new luminescence lines were discovered, but there was no apparent connection with recombination at the TDs. In the present work we compare PL and IR-absorption measurements for the same samples but no relationship between the TD absorption spectrum and known luminescence li7es (e.g. P-line at 0.767eV, H-line at 0.926eV) is found. However, after long heat treatments, up to 100hours (h), carbon-lean samples exhibit a new luminescence spectrum. The nature of the lines and the correlation with the TD IR-absorption spectra is presented in this paper. EXPERIMENTAL In this study we use three different starting materials: 3 (a)Si(Cz) 25acm p-type, Oi1-0 18 Ccm, C
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