Optical Properties of CdZnS-ZnS Strained-Layer Superiattices
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OPTICAL PROPERTIES OF CdZnS-ZnS STRAINED-LAYER SUPERIATrICES Yasuyuki Endoh and Tsunemasa Taguchi of Electrical Engineering, Department University, Suita, Osaka 565, JAPAN.
Faculty
of
Engineering,
Osaka
ABSTRACT The Cd0 3 Zn0 .7S-ZnS strained-layer superlattices has for the first time been fabricated on (100)GaAs substrates by a low-pressure MOCVD method. Exciton luminescence properties were investigated by the photoluminescence spectroscopies which show that the quantum confinement effect of excitons occurs in the CdZnS alloy layer. Temperature dependence of the exciton properties reveals a dominant scattering process which originates from exciton Nevertheless, the large exciton-phonon interaction(F1 0 =68meV). binding energy of about 166meV makes it possible to produce the exciton peak at room temperature. The effect of external electric field on the exciton intensity and its peak position is found and is tentatively interpreted in terms of a quantum confined Stark effect. 1.INTRODUCTION Of available wide bandgap 1I-VI compounds, it has recently been revealed that quantum well and superlattice structures enhance the exciton effects [1,2] as well as in III-V materials. Among II-VI materials, ZnS and CdS have large exciton binding energies which have been estimated to be 40meV and 28meV [3], respectively. If this alloy, CdxZni-xS is used to form a superlattice with ZnS, then it acts as a well for electrons and holes, and as a result Its electric structure is modified by quantum confinement. However, It Is expected that the CdZnS well Is considerably under compressive strain, while the ZnS barrier receives tensile strain due to the lattice mismatch between them. Namely this structure exhibits a strained-layer superlattice (SLS) in nature within a certain critical thickness [4.51. Both quantum confinement effect and biaxial compression will increase the effective energy bandgap of the CdZnS well, so exciton properties are much affected by the band structure changes which remove the degeneracy of the valence band. Furthermore, cubic symmetry of the alloy material will be lost, which makes it possible to create the anistropy in the susceptibilities. So, the large nonlinear effect of the excitons in the present SLS will appear. Our objective for investigating exciton properties of the CdZnS-ZnS SLS Is as follows: (i) observation of the RT exciton emission, (ii) scattering mechanism and localization of excitons in the alloyed CdZnS well and (iii) quantum confined Stark effect. We are concerned in this paper with the characteristics of excitons in Cd0 3 Zn0 7S-ZnS SLSs grown by low-pressure metalorganlc chemical-vapourdeposition (MOCVD). Our discussion will concentrate on the temperature and external electric-field dependence of photoluminescence properties which will be originated from quasi two-dimensional excitons. 2.FABRICATION The [4,5].
OF
STRAINED-LAYER
SUPERLATTICE
AND
STRUCTURAL
PROPERTIES
CdZnS-ZnS SLSs were grown by all gaseous low-pressure MOCVD Each layer was alternatively deposited on a Cr-doped
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