Exciton Diamagnetic Shifts and Magnetic Field Dependent Linewidths in Ordered and Disordered InGaP Alloys
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Exciton Diamagnetic Shifts and Magnetic Field Dependent Linewidths in Ordered and Disordered InGaP Alloys
E. D. Jones1, K. K. Bajaj2, G. Coli2, S. A. Crooker3, Yong Zhang4, A. Mascarenhas4, and J. M. Olsen4 1Sandia
National Laboratories, Albuquerque, New Mexico 87185 2 Physics Department, Emory University, Atlanta, Georgia 30322 3 National High Magnetic Field Laboratory - Los Alamos National Laboratory Los Alamos, New Mexico 87545 4 National Renewable Energy Laboratory, Golden, Colorado 80401 ABSTRACT We have measured the diamagnetic shifts and photoluminescence linewidths of excitonic transitions in ordered and disordered In0.48Ga0.52P alloys, lattice matched to GaAs, in pulsed magnetic fields at 4 and 76K. The pulsed magnetic field ranged between 0 and 50T. The variations diamagnetic shifts with magnetic field in disordered and weakly ordered samples are considerably smaller than those calculated using a free exciton model. For a given magnetic field, the value of the diamagnetic shifts are found to increase with increasing order parameter. Furthermore, for all samples, the diamagnetic shifts at 76K are larger than at 4K suggesting that the excitons are strongly localized. INTRODUCTION
An understanding and appreciation of the material parameters for the InGaP/GaAs system are important for laser and solar cell applications. In particular, this material system’s bandgap energy can be tuned by the phenomenon known as spontaneous ordering, where, depending on the growth conditions, the InGaP crystal structure can be a randomly disordered alloy with the zincblende structure or can form a long-ranged ordered monolayer (AC)1/(BC)1 superlattice in the (111) orientation, known as the CuPt2-like crystal structure. The degree of ordering has been cast in terms of an order parameter h. For h = 0, i.e., disordered zincblende structure, the 4-K bandgap energy is nearly 2 eV while for h ª 0.5 (CuPt2-like) the 4-K bandgap energy is smaller, ~1.9 eV. An excellent review describing this phenomena can be found in the article by Zunger and Mahajan [1]. Low-temperature photoluminescence (PL) is routinely used to asses the quality of semiconductor alloys. At liquid helium temperatures, the PL full-width-at-half-maximum (FWHM) linewidth of an excitonic transition is generally larger than those observed in their binary components. This broadening is proportional to the amount of compositional disorder [2-9] which H6.37.1
is inevitably present in these systems. Because alloy fluctuations are reduced in ordered structures, the PL FWHM linewidth decreases with increasing order parameter h [10]. In this paper we present PL data at 4 and 76K for the diamagnetic shifts of the excitonic transition and FWHM as a function of magnetic field in two InGaP samples, a weakly ordered and disordered In0.48Ga0.52P alloy. For both the weakly ordered and disordered samples, we find that the observed variations of the FWHM with magnetic field are smaller than those calculated by Lee and Bajaj[7] using a free exciton model. The value of the exciton diam
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