Photoluminescence and Esr Studies of Localized States in Amorphous Phosphorus

Photoluminescence and electron spin resonance measurements have been performed in bulk a-red P. We observe a photoluminescence band at 1.40 eV which exhibits a sensitivity to the excitation energy employed. Specifically, the peak of this band progresses to lower energies for higher energy excitation. Electron spin resonance measurements indicate -lo1? spins/cm3 for "cold dark" and optically induced (63282) conditions. In contrast to similar measurements in bulk a-As, thermally generated paramagnetism is not apparent in bulk a-red P up to 300K. These results are compared with various defect models proposed for the pnictides. Introduction.Photoluminescence (PL) and electron spin resonance (ESR) measurements in amorphous arsenic (a-As) have been interpreted in terms of localized electronic defect states such as twoor four-fold coordinated pnictide atoms (1,2,3) or "vacancies" (4,5) in the normally three-fold coordinated pnictide network. Semiconducting (E ~2.1 eV (6)) bulk amorphous (a-) red P exhibits an average bond angle of 102O, whicfi is significantly larger than the 98O bond angle characteristic of a-As (1). This difference suggests that bulk a-red P may provide a useful additional test of the various defect models proposed for a-As. We have performed photoluminescence (PL), PL excitation and ESR measurements on bulk a-red P, to further elucidate the nature of defects in the pnictides. Experimental.PL and PL excitation measurements were performed at 4.2K with a SPEX 1701 monochromator. An appropriate choice of filters prevented scattered excitation light from being detected by either the S-1 photomultiplier or PbS detector. A krypton laser and tungsten lamp were employed as excitation sources. The ESR spectra were obtained with a standard X-band (9 GHz) spectrometer with variable temperature capability and an optical access cavity. Samples of bulk a-red P were obtained from Mining and Chemical Products. The preparation procedure has been described elsewhere (7). Results.Shown in Fig. 1 are photoluminescence spectra of bulk a-red P obtained with the excitation energies indicated. Luminescence energies above 1.24 eV were detected with an S-1 photomultiplier, while lower energies were observed with a cooled PbS detector. Excitation with 2.60 eV and 2.38 eV radiation resulted in very weak luminescence signals which could not be observed with the PbS detector. In contrast to that which has previously been observed in the chalcogenide glasses (8), both the position and width of the luminescence band in bulk a-red P exhibit subtle changes with excitation energy. These changes are further documented in Table I where both the peak position and full width at half maximum are shown for various excitation energies. The peak of the luminescence band progresses, although not monotonically, to higher energies with lower energy excitation. The position of the luminescence band in bulk a-As also exhibits a sensitivity to excitation energy (9). Previous luminescence measurements (10) on bulk a-red P are in substantial agreement with those presented here. The previous measurements, which were not corrected for the throughput of the spectrometer, found a narrower half width for the luminescence band of 0.36 eV. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814189