Photoluminescence and EPR of Phosphorus Vacancies in ZnGep 2
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of nine bulk ZnGeP 2 crystals. Two PL bands are resolved in these samples peaking near 1.6 eV and 1.4 eV. The relative intensities of these two bands differ from sample-to-sample, and correlate with the relative concentrations of P and Zn vacancies, as measured by EPR. EXPERIMENT The nine ZnGeP 2 crystals were grown by the horizontal gradient-freeze technique and were nominally undoped. The PL data were obtained from liquid helium (-4.8 K) to near room temperature using a grating spectrometer and either a PMT (GaAs, or S-I response), or a 77-K cooled Ge detector. The linear polarization of the PL was analyzed using a broad-band linear polarizer placed in front of the entrance slit to the spectrometer. Samples were rotated so that data was collected with the electric field vector either parallel or perpendicular to the crystal's c axis (i.e., the polarizer was kept fixed). Spectra have been corrected for the detection-system response using a calibrated source. Photoluminescence excitation (PLE) spectra were recorded at 4.8 K using a 150-Watt short-arc Xenon lamp coupled into a 0.22-m monochromator. The power on the sample surface was about 16 ptW. The PLE spectra are corrected for the variation in output intensity as a function of wavelength from the lamp/monochromator combination. The energy resolution of the PLE data was limited by the slit settings monochromator, and was measured to be about 4 meV. EPR data were taken with a Bruker ESP 300 spectrometer operating at 9.45 GHz. An Oxford Instruments ESR-900 helium gas flow system maintained the sample temperature at 8 K for the P vacancy signal, and 25 K for the Zn vacancy signal. RESULTS AND DISCUSSION The bulk ZnGeP 2 single-crystal samples were first studied using 515-nm argon-ion laser excitation. Representative data are shown in Fig. 1. Emission to 1400 nm was checked using the cooled Ge detector, however, no appreciable signal beyond 1100 nm was observed from any of the bulk crystals included in our study. In Fig. 1(a), we show the PL spectra taken at 4.8 K without a polarizer from two representative samples. Sample A has the emission peak occurring near 800 nm, while Sample B has an emission peak closer to 900 nm. Linearly polarized PL data obtained for Samples A and B are shown in Figs. 1(b) and 1(c), respectively. From the polarization dependence, the presence of two distinct broad emission bands at 1.58 eV (780 nm) and 1.36 eV (910 nm) are detected. These two bands are superimposed in the unpolarized spectra. The two bands have opposite polarization, and their relative intensities differed from sample to sample. Taking the two band positions and allowing the relative intensities and widths to vary for a "best-fit" analysis, the unpolarized PL spectra were reproducedfor each of the nine samples. This verified that the same two bands are present in all samples. (In Fig. 1(b), the emission with E c peaks near 850 nm because there is still a contribution from the lower energy band.) From the fitting, the average FWHM is 0.25 eV for the 1.58-eV band and 0.26 eV
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