Excitonic Properties of ZnSe-ZnS Strained-Layer Superlattices and A Fibonacci Sequence
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EXCITONIC PROPERTIES OF ZnSe-ZnS STRAINED-LAYER SUPERLATTICES AND A FIBONACCI SEQUENCE Tsunemasa TAGUCHI and Yoichi YAMADA Department of Electrical Engineering, University, Suita, Osaka 565, Japan
Faculty of Engineering.
Osaka
ABSTRACT Excitonic properties of ZnSe-ZnS strained-layer quantum wells (SLQ~s) with type I band lineups are reviewed on the basis of our recent results of temperature- and strain-dependent photoluminescence and absorption spectra. In order to estimate the conduction and valence band offsets as a function of ZnSe well thickness, we have modified the "model-solid" theory in which the valence bands (heavy-hole band in ZnSe and light-hole band in ZnS) are relatively moved with strains. Temperature and high excitation dependent studies of the n=1 heavy-hole excitons suggest a localization of excitons and reveals the important evidence on scatterings of excitons with acoustic and optical phonons. The thermal quenching of the exciton emission is caused by thermal dissociation of quasi-two-dimensional excitons through electrons and holes, from which the activation energy for this dissociation is 4 times larger than Ea. 3D (a binding energy of bulk exciton) of ZnSe. A new superlattice structure with a quasiperiodic crystal which is derived from a finite Fibonacci sequence, has been fabricated by a low-pressure MOCVD method and its photoluminescence properties are for the first time introduced. 1. INTRODUCTION The most exciting optoelectronics applications for wide bandgap I1-VI materials are semiconductor injection emitters and lasers at wavelengths throughout the visible spectrum [1]. These possibilities are now amplified by the rapid progress in the growth of 1I-VI single and multiple-quantum well (MOW)structures, which can confine electrons and holes in a two-dimensional well,
fabricated by MBE
[21 and MOCVD
[3].
Despite the small amount of
effort devoted to these structures in comparison with their equivalents in Il- V materials, it has been very recently reported the possibilities of optically-pumped blue laser [4], quantum confined Stark effect [31, waveguide [5] and second-harmonic generator (SHG) [6] from ZnSe-ZnS(Se) and CdZnS-ZnS [7] strained-layer superlattices (SLSs). In semiconductor strained-layer superlattices, however, the lattice mismatch can be entirely accommodated by elastic layer strain rather than misfit dislocations. The flexibility in the choice of widegap II-VI SLS materials from green (CdS) to violet (ZnS) in the spectral regions, and the interesting effects of nonlinear properties allow SLS materials to exhibit a wide range of photonics devices which are advantageous for SHG and third-harmonic generator (THG) devices [8). ZnSe-ZnS SLS, exhibiting a type I heterojunction structure, has been most extensively studied using low-temperature excitonic luminescence and absorption measurements to observe the valence band feature of the ZnSe well [3,9]. The lattice mismatch between cubic ZnSe and ZnS is about 4.5 % at RT, so that a critical thickness for each layer along grow
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