Introduction GaAs and structures made of it are widely used at present. However, in spite of considerable effort made to understand the nature, stability, and energy spectrum of local centres introduced into them by high energy particles, there still remains a field of accumulation of significant facts and of much discussion [l, 21. It is well known at present that the complicated processes due to the interaction of point defects, newly introduced by particle irradiation can result in various changes of the energy spectrum which are observed experimentally. a-particle irradiation constitutes a simple method of defect introduction, permitting to estimate both the fluence and the range within which the primary defects are generated. Experimental techniques Three GaAs specimens No. 1, No. 2, and No. 3 of concentration n = (2.8 to 2.4) x 10L6/cm3an d 0.5 mm thickness were used. Two semicircular gold contacts at a distance of 100 pm were formed on the illuminated surface. The other experimental techniques were the same as in [3]. The total dose of a-particles from a 0.6 mCi source was 4.75 x and 8.11 x 1OI2 particles/cm2 for specimens No. 1, No. 2, and No. 3, respectively. The range of the 2.84 MeV a-particles in the direction vertical to the surface was found equal to 6.4 pm. Specimen No. 1 was irradiated at 300 K, while the other two were irradiated at 200 K. 7.28 x Results and discussion In Fig. 1 we plotted the photocurrent spectra for one ofthe specimens (No. 1) before irradiation at various temperatures. They show a main peak at a wavelength corresponding to the energy gap and a very small, not well resolved, peak at photon energies smaller than E,. This peak appears very clearly in Fig. 2c. As the temperature decreases, the photocurrent I,, at this peak also decreases. The dark current I,, negligible at temperatures T < 300 K, increases rapidly with T reaching the value I = 320 nA at T = 350 K. The same spectra are observed for the other specimens. In Fig. 2 the photocurrent spectra at two temperatures (300 and 200 K) before (I,) and after (I,) irradiation for specimens No. 1, No. 2, and No. 3, respectively, are plotted. It is clear that after irradiation the shape of the I,, curve remains the same but I,, increases in contrast to the case of AlGaAs/GaAs irradiated with electrons [3].