No abstract

Quantum wells (QW) of 111-V semiconductors have been studied extensively not only for fundamental physics but also for infrared QW lasers and other photonic devices. In recent years, studies on QW of wide and direct band gap 11-VI semiconductor materials have been attractive subjects because they have been applied for developing green and blue laser diodes [l to 41. Among them, the ZnSe/ZnSeTe strained-layer QW system, in which Te acts both as an isoelectronic trap and as a constituent of the alloy, has attracted considerable interest for its special optical properties. There are a green and a blue strong exciton emission band which relate to different Te concentrations and are interpreted as the recombination of self-trapped excitons around the sites of Te cluster and single Te, respectively. Its temperature and concentration dependent characteristics have been investigated in detail [5 to 81. In this note, time-resolved and excitation density dependent photoluminescence of ZnSe/ZnSeTe QW is studied. The excitation energy transfer process and the exciton fluorescence kinetics are described. In addition, it has been found that the emission of self-trapped excitons around the sites of Te cluster, single Te, and free exciton in the ZnSe barrier are dominant one after another with increasing pump density. This reveals the relationship between the emission wavelength and the saturable properties of the self-trapped exciton states around Te centers.Our time-resolved photoluminescence experiments have been performed with a frequency tripled single mode-locked Nd : YAG laser as a pump source and a single sweep streak camera (Hamamatsa Model C1587, 2 ps time resolution) connected to a polychromator as a recorder. Thus, the spectral and temporal characteristics can be recorded simultaneously. The pulse duration of the single pulse mode-locked Nd : YAG laser is 30 ps. The QW was grown by the atomic layer epitaxy method. The photoluminescent experiments have been carried out at the sample temperature of 77 K. Fig. 1 shows the pump density dependent photoluminescence spectra of ZnSe/ZnSeTe. When the excitation source is the 365 nm line of a cw Hg lamp, only the green broad band is detected, as shown in Fig. la. When the pump source is a very weak frequency tripled mode-locked Nd : YAG laser, the photoluminescence spectrum is similar to that shown in Fig. la. As the pump density increases, the emission spectrum is blue shifted. When the pump density is 0.4 mJ/cm2, the photoluminescence is dominated by a blue broad band, and a sharp line at 448 nm, which is the emission of a free exciton in ZnSe, appears, as I ) Guangzhou 510275, People's Republic of China.