PHYSICAL REVIEW 8 VOLUME 50, NUMBER 24 15 DECEMBER 1994-II Picosecond radiative decay of free and bound excitons and excitonic molecules in Cdse S. S. Prabhu, A. S. Vengurlekar, and S. K. Roy Tata Institute of Fundamental Research, Bombay $00005, India 3. Shah ATST Bell Ia,born, tories, Holmdel, New Jersey 0775'5' (Received 27 July 1994; revised manuscript received 7 September 1994) Band-edge luminescence in photoexcited CdSe is known to show several interesting features due to radiative decay of free and bound excitons and excitonic molecules (biexcitons). In this paper, we report our experiments on the study of the time evolution of some of these processes in CdSe, using up-conversion luminescence spectroscopy with a time resolution of 2.5 ps. We obtain time- dependent luminescence spectra at two e-h pair excitation densities (estimated to be no — — 8 x 10' and 8 x 10 cm ). The role of hot biexciton dynamics in determining the time evolution of luminescence is evident. At low density (nz = 8 x 10 cm ), the luminescence spectrum also reveals the time evolution of the bound exciton emission at long delays () 200 ps). In addition, we obtain time-resolved luminescence of the longitudinal-optical phonon-assisted Stokes sidebands of the free exciton. The exciton and biexciton lifetimes are deduced to be 600 and 10 ps, respectively, on the basis of a theoretical model describing their coupled dynamics. I. INTRODUCTION Radiative recombination of photoexcited electron-hole (e-h) pairs in semiconductors such as CdS has been stud- ied extensively in. the past. Depending upon the excita- tion density, different processes dominate the recombina- tion luminescence of these e-h pairs. At low temperatures and low excitation densities, the edge emission spectrum is dominated by exciton recombination and exciton in- teraction. s with phonons and impurities. At higher ex- citation densities, the mutual interactions of the exci- tons become important. One should then take into account the exciton-exciton collisions, formation of ex- citon rnolecules (biexcitons), and biexciton-biexciton in- teractions. The dynamics of processes such as energy relaxation of hot e-6 pairs towards their respective band edges, formation of excitons and biexcitons &om the ini- tially excited hot &ee carriers, thermalization and cool- ing of exciton- and biexciton-energy distributions and the radiative decay of these excitations has been a topic of much interest. Considerable efforts have been devoted in the case of materials such as CdS and CdSe to identify luminescence due to excitons, exciton-exciton collisions, biexcitons, electron-exciton interactions, etc. , following the transformation of the free e-h pairs into excitons. At still higher densities, exceeding the Mott density, the e- 6 plasma effects dominate, screening out the excitonic states, at least near the surface, where the density due to one-photon generation is the highest. Rapid density reduction due to plasma expansion, recombination, and diffusion can make excitonic states possible once again, within 100 — 200 ps in some cases, as shown by Saito for CdS and Frigo et aL and Fujimoto et al. for CdSe at high densities of 10 to 10 cm . The study of the dynamics of these processes in II-VI semiconductors, es- pecially in GdS, using time-resolved (TR) luminescence measurements has been the subject of intense research in the past . In recent years, resonant optical nonlin- earities in CdSe due to biexcitons and exciton-biexciton tran. sitions have attracted attention. In addition to the processes listed above, it is well known that the exciton dynamics under low-density exci- tations is greatly infIuenced by its capture in an electro- static potential well due to an impurity complex present in the semiconductor. In II-VI semiconductors such as CdS and CdSe, for example, an. exciton bound to a neutral donor (I2) is known to have giant oscillator strength. Optical bistability due to the bound exciton saturation and quantum beats~ due to the split states of the bound exciton have been observed in recent years. Several studies have been performed to determine the ra- diative lifetime of the I2 bound exciton in CdS and CdSe using different techniques. 6' Although the bound ex- citon recombination (I2) usually dominates the lumines- cence in CdS and CdSe at low excitation densities, the time evolution of the bound exciton luminescence with time &om an initial low-density photoinjected e-6 assem- bly has not been clearly demonstrated. The picosecond time evolution of luminescence spec- trum due to radiative annihilation of &ee and bound ex- citons and biexcitons, following ultrafast generation of high-density e-6 pairs has been of great interest for some time. ' The biexciton binding energy in CdSe is about 4 to 5 meV. This may be compared with the value of 8. 5 meV deduced for CdS, 0.1 meV for GaAs and 1. 0 meV for GaAs (Ref. 19) quantum wells. These val- ues make studies of biexcitons possible in CdS and CdSe, but not in bulk GaAs. Such studies are of much relevance now with the renewed interest in the applications of II-VI semiconductors and their heterostructures. 0163-1829/94/50(24)/18098(8)/$06. 00 18 098 1994 The American Physical Society