Excitonic Diamagnetic Shifts and Magnetic Field Dependent Linewidths in Al x Ga 1-x As Alloys
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Excitonic Diamagnetic Shifts and Magnetic Field Dependent Linewidths in AlxGa1-xAs Alloys
G. Coli1, K. K. Bajaj1, J. L. Reno2, and E. D. Jones2 1Physics Department, Emory University, Atlanta, Georgia 30322 2Sandia National Laboratories, Albuquerque, New Mexico 87185 ABSTRACT We report measurements of both the diamagnetic shifts and the linewidths of excitonic transitions in AlxGa1-xAs alloys as a function of Al concentration and magnetic field at 1.4 K using photoluminescence spectroscopy. The magnetic field was varied from 0 to 13 tesla and Al composition in our samples ranged from 0 to 30%. The samples were grown on GaAs substrates oriented along [001] direction using molecular beam epitaxy at 590ºC. We find that for a given value of alloy composition, both the diamagnetic shift and excitonic linewidth increase as a function of magnetic field. To explain our experimental data we propose that the excitons are localized in a very specific manner. To simulate this localization, we assume that the exciton reduced mass is effectively increased and is obtained by using the alloy dependent heavy-hole mass along (001) direction treated isotropically. The calculated values of the variations of the diamagnetic shift and excitonic linewidth as a function of magnetic field obtained using this model agree very well with those reported here. INTRODUCTION One of the most commonly used optical characterization techniques to assess the quality of a semiconductor alloy is the low-temperature photoluminescence (PL) spectroscopy. At liquid helium temperatures, the linewidth of an excitonic transition defined as the full-width-at-halfmaximum (s) in semiconductor alloys as determined by PL spectroscopy is considerably larger than those observed in their components. This broadening has been attributed to the compositional disorder [1-8] which is inevitably present in these systems. In addition, the value of s can be controlled by the application of an external magnetic field as was first pointed out independently by Raikh and Éfros [8], and Singh and Bajaj [2]. This is due to the fact that the application of a magnetic field shrinks the excitonic wave function and thus enhances the value of s. This effect was first observed by Jones et al [9] in InGaP lattice matched to GaAs. During the past twenty years several groups[10] have studied the variation of s as a function of alloy composition in AlGaAs. In this paper we present an observation of the variations of the diamagnetic shift of the excitonic transition (d) and s as a function of magnetic field in AlGaAs at 1.4 K using PL spectroscopy. The Al composition x ranged from 0 to 0.30 and the magnetic field was varied from 0 to 13 tesla. We find that for a given value of alloy composition, both d and s increase as a function of magnetic field. The observed variations of d and s with magnetic field H7.4.1
are considerably smaller than those calculated by Lee and Bajaj[6] using a free exciton model. To explain our experimental data we propose that the excitons are localized in a very spe
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