RADIATIVE RECOMBINATION OF LOCALIZED EXCITONS IN AMORPHOUS AND CRYSTALLINE As2S3

Two photo-luminescence bands characterized by their excitation in c-As2S3 shows a surprising similarity to those in a-AsZS3. In addition, a band in c-As2S3 can be interpreted as due to the recombination of localized excitons as well as a luminescence band in a-As2S3. The luminescence bands in a-As2S3 are discussed on the basis of the luminescence processes in c-As S 2 3' In a-As S it is suggested from the time resolved and the polari2 3' zation memory of lumine~cence~)~) that two kinds of luminescence processes exist. One process produces a broad luminescence AI5) with the peak energy of about 1.15eV. The other process produces another broad luminescence A, with the peak energy, 1.4l.?cV, depending on the excitation energy 1)3)6). Some isc cuss ions about these luminescence processes are made on the basis of the model specific to amorphous materials but it is also important to compare these luminescence phenomena with the luminescence in the corresponding crystalline materials, c-As2S3. In c-As2S3, Kolmiets et a15) and Mollot et a17) observe a spectrum of luminescence (C,) with the peak energy of about 1.2eV while Street et a18) and Bosch et a19) obseLe another spectrum of luminescence (C ) with the peak energy of about 2 1.6eV. The lumirlescence CL is compared with luminescence A1 by Kolomiets et a1 5) and they suggest that the luminescence A1 is responsible for a deep center similar to the center of C1. The authors find in detailed measurements that the luminescence, C and C2, in 1 c-As2S3 are due to the different recombination processes depending on the excitation energylO). In this paper, we will report the two luminescence processes in c-As2S3 and compare these processes with those in a-As S 2 3High quality natural crystals of orpiment were cleaved into sheets prior to measurements of the luminescence and optical absorption. An Ar ion laser provided the excitation light for the luminescence, which was analyzed by a monochrometer and detected with a photo-multiplier or a Ge photo diode. The absorption spectrum in c-As2S3 observed at 2 K is shown in Fig. l(a). We observed a shoulder at 2.5eV superimposed on the steep rise of the absorption at 11) higher energies which is due to the band to band transitions. Zakis and Fritzsche also report a large shoulder at the same position. They propose that the process is due to the high energy transition between a band and a shallow level produced by an impurity. The luminescence spectra obtained from the same sample with two different