Photoinduced Changes in the Charge States of Native Donors and Acceptors in ZnGeP 2
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EXPERIMENT The ZnGeP2 crystals were grown by the horizontal gradient-freeze technique at Sanders and were nominally undoped. Sample dimensions were approximately 3 x 3 x 3 mm3 along the [1001, [010], and [001 ] dimensions, respectively. The [001 ] dimension is also referred to as the c axis. EPR data were taken using a Bruker ESP 300 spectrometer operating at 9.45 GHz. An Oxford Instruments ESR-900 helium gas flow system maintained the sample at cryogenic temperatures. EPR spectra were taken at different temperatures for the three different native defects being studied. The microwave cavity allows optical access to the sample so that in-situ illumination is possible. The output from a HeNe laser (633 nm) or a cw Nd:YAG laser (1064 nm) was used in the photo-EPR experiments. The time decay of photoinduced changes in the EPR signal intensity were also monitored. The EPR intensities were quantified in order to obtain defect concentrations. A spectrometer detection limit of 5 x 10° spins (for a 1 gauss linewidth and a temperature of 8 K) was used. The uncertainty in the spectrometer sensitivity is no greater than a factor of 2 (this would only change absolute concentration values and would not affect determination of relative concentrations between samples. A set of opposing [100] faces were optically polished for each of the samples. Optical absorption data were taken at room temperature using a Perkin Elmer Lambda 9 spectrophotometer [7]. The spectral dependence of a photo-induced absorption was measured using a Cary spectrophotometer that has been upgraded and computerized. The ZGP sample was kept at 5 K for the photoinduced absorption study. RESULTS AND DISCUSSION Singly Ionized Zinc Vacancy Acceptor All of our bulk ZGP crystals contain large (>10IS cm"3) concentrations of singly ionized zinc vacancies (Vs,). With H IIc, the EPR spectrum reveals a triplet of lines (1:2:1 intensity ratio) as a result of hyperfine interactions with two equivalent phosphorus nuclei [1-3]. Because of long spin-lattice relaxation at the lowest temperatures, this center is best observed between 20 and 30 K. In a recent study of 23 bulk ZGP samples, the absorption coefficient for o-polarized light at I jitm was found to correlate with the Vz, EPR signal [7]. An interesting effect was observed when the ZGP samples were illuminated with either 633-nm light or 1064-nm light while being held in the microwave cavity. The intensity of the V• EPR spectrum increased with the shorter wavelength. The photon energy for 633-nm light is slightly less than the band gap energy and electronic transitions from the valence band to donor states or electronic transitions from acceptors to the conduction band can take place. Care was taken to maintain the appropriate sample temperature so that heating effects could not be responsible for the increase in the observed signal. A temperature of 20 K was chosen so that any sample heating would actually decrease the EPR signal, not produce an increase. The increase in intensity was usually a factor of 2, but some sam
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