VPE生长CdS外延膜的光学性质与光学双稳态

OPTICAL PROPERTY AND BISTABILITY OF CdS EPILAYERS GROWN BY VAPOUR PHASE EPITAXY

本文报导了在透明的CaF_2衬底上采用气相外延(VPE)方法生长的高质量CdS薄膜。通过对其发射和吸收光谱以及光双稳特性的研究,表明这种外延膜的性质与体单晶相似。

Optical bistability(OB) in semiconductors is of increasing interest in both basic aod applied lesearch. Much is known about optical nonlinearity and OB, and several kinds of optical bistability have been observed in CdS crystals and films. This paper leports the optical properly and bistability in CdS epilayers grown on CaF2 tiansparent substrates by vapour phase epitaxy(VPE) for getting large area, uniform and high quality samples. The source material v as the high quality CdS single crystal and the superpurity H2 gas yas used as carrier gas. The freshly cleaved CaF2 substrate was put on the deposit range, then raised the lemperature of source material to 700-800℃ and that of substrate to 500-600℃ in H2 flow. The emission and transimission spectra were measured by 44W grating monochro-meter and recorded by cooled pholomultiplier. The 5145A line of Ar+ laser chopped into 6ms light pulses was focused to the radius of about 50um for OB measurement at room temperature.The crystallographic paremeters of CdS and CaF2 sho ed that c-axis of CdS epilayers gro n on (111) surface of CaF2 was parallel to [111] direction of substrates. Because of the great lattic mismatching between CdS and CaF2,high density of mismatching dislocations would be produced in the epilayers when the CdS films were relatively thick. The surface morphology of almost all the films was smooth and featureless with 10-50um hexagonal flat tops. Fig. 1 sho s the emission and transmission spectra of CdS cpilaycr and bulk CdS crystals at 77K. At emission spectra dominant luminescence peaks at 4890A are obscrved in both of epila er and bulk cr stals. This band (labeled I2) is due to an exciton band bound to a neutral donor . Other kinds of emission, such as Ex-nLO and DA pairs are extremely v eak v hich could only be recognized at the locer energy side of I2. No deep level luminescence has been measured from the, epila er. A steep absorption edge of CdS epila er is found at 4870A at 77K and its blue-shift is a little bit less than that of bulk CdS crystals oaing to the influence of thickness of samples. These results are much beltei than Humenberger's . At room temperature, the green emission band at 5190A,which orginaled from free exciton recombination following scattering from free elections in the conducting band, has been observed and no deep center emission appeals. Like the transmission spectra at 77K, the absorption edge at about 5100 A is also steep at room temperature. The optical bistability of 8μm CdS epilayer is measured by 5145A line of Ar+ laser modulated into 6ms squaie light pulses at room temperature. Fig.3 shows the t pical time traces of incident (a) and tiansmitted (b) light at 20mW powet level. T o spikes appear at the front and back edges of transmitted light pulses. The s ilching time is about 200us, which is smaller than that in CdS bulk crystals at the same condition. The mechanism of the OB is due to the increasing absorption and thermal red shift of the absorption edge.