Strain-Related Excitonic In-Plane Optical Anisotropy in (100) InGaAs/InAlAs/InP MQW
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ABSTRACT Strained (x=0.48) and lattice-matched (x=0.53) InxGal-xAs/InAlAs/InP (100) MQWs have been investigated by photoreflectance. In the strained sample the relative intensities of the light-hole and heavy-hole excitonic transitions is different for the two different polarizations of the incident light parallel and perpendicular to the [0-1-1 ] direction. This polarization anisotropy is explained in terms of the spontaneous formation of "quantum wires" and the presence of anisotropic strain due to spinodal-like phase decomposition of the InGaAs alloy in In-rich and Ga-rich regions. INTRODUCTION Multiple quantum wells (MQW) grown on (100) substrates possess tetragonal symmetry and are expected to exhibit isotropic optical properties with respect to the in-plane polarization of light. However, in the presence of ordering [1] or under the influence of an externally applied inplane uniaxial strain [2] the symmetry is lowered and in such cases polarization anisotropy has been observed [1,2]. An understanding of the optical anisotropy may help to control the polarization instabilities [3] commonly observed in Vertical Cavity Surface Emitting Lasers (VCSELs) and may lead to new applications based on polarization-sensitive, very high contrast [2], very high speed spatial light modulators. In most of the cases [1] evidence of optical anisotropy has been obtained by using luminescence at low temperatures where only the lowest energy lying transition (either heavy-hole (hh) or light-hole (li)) was observed. In the present study, by using photoreflectance, an absorption-based technique, we report on the optical anisotropy of both the hh and lh transitions as well as the higher excited transitions in strained QWs. This may help to better understand the reason that causes the anisotropy and may lead to useful applications. MQW STRUCTURE AND EXPERIMENTAL SET-UP The samples were grown by MBE at Tg = 540 °C and had the following structure : 10 nm In 0.53 Ga 0.47As cap layer/1 pim MQW multilayer structure/100 nm In0.52A10.4sAs buffer layer/(100) S.I. InP substrate. The MQW region for the lattice-matched (x=053) sample HDL597 consisted of 33 periods of 15 nm InxGalixAs wells followed by 15 nm InAlAs barriers. In this sample the wells were tensile strained while the barriers were under compression with a barrier composition such that the net strain was zero. Sample HDL377 (x=0.48), had 50 periods of 10 nrm wells followed by 10 nm barriers. All layers were undoped, while the substrates were of exact (100) orientation. Modulation Spectroscopy (photoreflectance) was used to probe the polarization anisotropy. This technique utilizes a monochromatic beam to probe small reflectivity changes AR/R caused by a chopped He-Ne laser light [4]. These changes occur only at the vicinity of interband optical 463
Mat. Res. Soc. Symp. Proc. Vol. 448 01997 Materials Research Society
transitions so that the method provides a means for a room temperature, high spectral resolution characterization of ground and excited states in QWs. A Glan Thomson p
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