Effect of gamma-irradiation on optical and mechanical properties of Al2O3, SiO2 with deposited films

Solar cells and optical devices based on satellites are exposed to a long term ionizing radiation from the Sun at various temperatures and micro-meteorite impact, therefore they should be made of special radiation stable and mechanically hard materials. Stability of crystal structure to damaging radiation is provided by the balance between rates of defect production and recombination. In the case of ionizing radiation, when point defects are produced mainly by non-irradiative decay of electronic excitations, intensive and fast luminescence (or short afterglow) is believed to ensure radiation stability of structure. One should also expect noticeable change of mechanical properties under combine influence of radiation and micro-meteorite blows. Here we present the results of study of optical absorption, gamma luminescence (GL), photoluminescence (PL) spectra, kinetics of afterglow (AG), and also microhardness of widely used Al2O3 crystals (pure and with Cr impurity) and fused quartz (SiO2) substrate with several oxide films deposited on it. To simulate a long term exposition to ionizing radiation causing structure damages, the samples were irradiated by 60Co gamma-rays (∼1.25 MeV, 1100 R/s) at 80 and 320 K to doses 106-109 R (equivalent to 20-years-orbiting). The gamma-irradiation was expected to charge the existing volume structure defects and to generate additional surface defects at the substrate-film interface. Diamond pyramidal indentor was used both for simulating mechanical damage from micro-meteorites and measuring the microhardness prior and after gamma-irradiation. The element composition of the films and substrates was examined with instrumental activation and X-ray fluorescent analyses. The substrates made of fused quartz were found to be pure at the impurity level of 10-5 %. Two different multilayer oxide films had the following composition: 1) Ba-30%, As-1.4%, Sr-0.35%, Zr-0.9%, Zn-0.5%; 2) Ba-30%, J-10 %, Ti-2 %, Zr-1.15 %, As-1.27 %, Sr-0.36 %. Microhardness Hμ measured with Wicker's technique at different loads on the non-irradiated samples Al2O3 and SiO2 was 12740 and 5360 kg/mm2. Film 1 had 1570, and 2 - 1960 kg/mm2. After gamma-irradiation the values of Hμ decreased by 13% for Al2O3 and 6-25% for SiO2 depending on strains. While the Hμ for film 1 increased by 197-170 % with strain's growth. Thus, the film structure has become harder after the irradiation. Optical absorption spectra were studied to determine the photoexcitation bands and self-absorption of photoluminescence if any. The SiO2 and Al2O3 substrates had the UV absorption edge at 200 nm and did not show any noticeable absorption bands till 900 nm. After gamma-irradiation to 106 R no additional bands were found in SiO2, however Cr impurity in Al2O3 caused absorption at 200-500 nm. The films were characterized by the UV edge at 260 nm and two bands at 290 and 360 nm (the first was weaker). After gamma-irradiation the band at 290 nm increased, and that at 360 nm decreased, and also the weak band at 500-600 nm appeared. UV-pulse laser was used for studying PL spectra and AG kinetics. No PL was excited in the pure substrates either before or after the irradiation to 106 R. The AG time was within 40 ns. At presence of trace Cr3+ in AI2O3 weak PL at 400-500 nm and a strong band at 650-670 nm were found after 106 R. The AG kinetics had two slower components 10 and 40 μs related to Cr centers. The films have intensive PL at 420 nm and weak bands at 510-540 and 650-700 nm. The irradiation to 106 R was found to weaken the PL bands and shorten the AG. A long irradiation of pure Al2O3 109 caused PL at 460 nm with AG within 5 μ. The observed absorption, PL spectra and AG kinetics are indicative of gamma-induced oxygen defect pairs (vacancies trapping electrons and oxygen ions with gangling bonds) responsible both for additional absorption and short re-combinative luminescence