Optically Stimulated Luminescence from Ag-doped Lithium Tetraborate (Li2B4O7)

Silver-doped lithium tetraborate (Li2B4O7) crystals emit optically stimulated luminescence (OSL) in response to stimulating light around 400 nm. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) were used to identify the defects in the crystal that cause this OSL. Lithium tetraborate crystals have Ag+ ions at Li+ sites and at interstitial sites. Upon ionization at room temperature via x rays, electron-hole pairs are generated. The electrons are trapped at Ag+ occupying interstitial sites, while the holes are trapped at Ag+ at lithium sites. The trapped electron centers become Ag0 (4d105s1) and the trapped hole centers, or recombination centers, become Ag2+ (4d9). Evidence for these centers is seen in EPR at room temperature. Optical absorption of the irradiated crystal showed a broad peak near 370 nm. Bleaching with 400 nm light decreased the EPR signals of the Ag0 and Ag2+ centers. When the crystal was stimulated with 400 nm light, OSL was produced with 270 nm emission. The effects of the stimulating light’s flux on OSL were observed by using xenon lamp and diode laser sources. OSL, with the laser at 405 nm, decayed faster, and provided insight on the existence of a competing electron trap, oxygen vacancies

Identification of Native Defects (Vacancies and Antisites) in CdSiP2 Crystals

Electron paramagnetic resonance (EPR) is used to identify four native defects in single crystals of CdSiP2. This nonlinear optical material is used in optical parametric oscillators to generate tunable output in the mid-infrared. The performance of these frequency-conversion devices is limited when infrared absorption bands associated with native defects overlap a pump wavelength. Cadmium, silicon, and phosphorus vacancies and also silicon-on-cadmium antisites are present in the as-grown undoped CdSiP2 crystals. Using near-band-edge 632.8 nm light from a He-Ne laser, a paramagnetic charge state, and thus an EPR spectrum, is formed at liquid-helium temperatures for three of the four defects. The EPR spectrum from the singly ionized silicon vacancy (V-Si) is present without light and has five hyperfine lines due to equal interactions with the four neighboring 31P nuclei. In contrast, the photoinduced EPR spectrum from the singly ionized cadmium vacancy (V-Cd) has a three-line hyperfine pattern due to equal interactions with only two of its four neighboring 31P nuclei. The light-induced spectrum from the singly ionized silicon-on-cadmium antisite (Si+Cd) also has a three-line hyperfine pattern, thus indicating that the unpaired spin interacts primarily with only two 31P neighbors. For the neutral phosphorus vacancy (V0P), the unpaired spin is primarily localized on the nearest-neighbor silicon ions and the photoinduced EPR spectrum has no resolved 31P hyperfine interactions. The silicon and cadmium vacancies are acceptors, and the silicon-on-cadmium antisite and the phosphorus vacancy are donors

Optically Stimulated Luminescence (OSL) from Ag-doped Li\u3csub\u3e2\u3c/sub\u3eB\u3csub\u3e4\u3c/sub\u3eO\u3csub\u3e7\u3c/sub\u3e Crystals

Optically stimulated luminescence (CW-OSL) is observed from Ag-doped lithium tetraborate (Li2B4O7) crystals. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) are used to identify the defects participating in the OSL process. As-grown crystals have Ag+ ions substituting for Li++ ions occupying interstitial sites. During a room-temperature exposure to ionizing radiation, holes are trapped at the Ag+ ions that replace Li++ ions, i.e., the radiation forms Ag2+ (4d9 ) ions and Ag0 (4d105s1 ) atoms. These Ag2+ and Ag0 centers have characteristic EPR spectra. The Ag0 centers also have a broad optical absorption band peaking near 370 nm. An OSL response is observed when the stimulation wavelength overlaps this absorption band. Specifically, stimulation with 400 nm light produces an intense OSL response when emission is monitored near 270 nm. Electrons optically released from the Ag0 centers recombine with holes trapped at Ag2+ ions to produce the ultraviolet emission. The OSL response is progressively smaller as the stimulation light is moved to longer wavelengths (i.e., away from the 370 nm peak of the absorption band of the Ag0 electron traps). Oxygen vacancies are also present in the Ag-doped Li2B4O7 crystals, and their role in the OSL process as a secondary relatively short-lived electron trap is described