Thermal defect generation processes are investigated in heat‐treated (250 to 600 °C, 1 to 500 h) silicon crystals by a low‐temperature photoluminescence method. In the spectral range from 0.75 to 1.20 eV, a number of emission bands are found. Some of these bands have been already observed in the luminescence spectra of irradiated silicon. The bands are found to cause the thermallyinduced defects, incorporating oxygen and carbon impurity atoms. Experimental data on the uniaxial stress splitting of the 0.7667 and 0.9259 eV zero‐phonon luminescence lines associated with the oxygen thermal donors are presented.

The behaviour of noble gas atoms implanted in silicon is studied by the luminescence method. The energy position of Mossbauer-type luminescence bands with zero-phonon lines 1.0148, 1.0120, 1.0097, 1.0048 eV and others connected with implanted atoms of neon, helium, argon, krypton, respectively, indicates the formation of deep energy levels in the forbidden gap of silicon. Implantation of the noble gas isotopes confirms their participation in formation processes of the luminescence centers in silicon. The temperature range of existence and the symmetry of defects incorporating the noble gas atoms are found. It is noted that noble gas atoms form impurity complexes with deep energy levels and their behaviour in crystals does not differ from that of main doped or residual technological impurity atoms.-~ 1) Leninskii prospekt 65, Minsk 220027, USSR.

Irradiation of silicon crystals by high-energy electrons, f -rays, neutrons, o r ions is known to lead t o the generation of radiation defects which cause the appearance of a number of electron-vibrational bands in luminescence spectra /1 to 4/. The presence of narrow zero-phonon emission lines in these spectra permits an effective use of uniaxial s t r e s s effects for determining the symmet r y of the corresponding luminescence centres /5/. Knowledge of this important microscopic characteristic is, in most cases, a decisive factor for the choice of geometrical and physical models and ultimately for the establishment of the irradiation-induced defect nature.The luminescence band with the zero-phonon line at 1.0186 eV has been observed in pulled and float-zone silicon crystals after irradiation by neutrons /3/ and various ions /2, 4/ and annealing in the temperature range 50 to 350 OC. Previously /2/, the centres responsible for this band, considering the apparent impurity independence, annealing properties, and comparison with the E P R data, have been tentatively identified with the five vacancy cluster defects (Si-P1 centre),which have Clh(= Cs) symmetry /6/.In this note we report the results of uniaxial stress studies on the 1.0186 eV emission line performed with the purpose to determine the symmetry of the corresponding centres. F r o m the analysis of uniaxial s t r e s s splitting of the line and the relative intensity of the stress-split components under a polarized light, it is found that the centres have a trigonal symmetry with a circular magnetic dipole oscillator in the plane normal to the (111) centre axis. The preliminary data of these investigations have been reported in /?/.

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