Evaluation of strain balancing layer thickness for InAs/GaAs quantum dot arrays using high resolution x-ray diffraction and photoluminescence

Abstract

The impact of strain-balancing quantum dot superlattice arrays is critical to device performance. InAs/GaAs/GaP strain-balanced quantum dot arrays embedded in p-i-n diodes were investigated via high resolution x-ray diffraction ͑HRXRD͒ and photoluminescence ͑PL͒ as a function of the GaP thickness. A three-dimensional modification of the continuum elasticity theory was proposed and an optimal thickness was determined to be 3.8 ML. HRXRD-determined in-plane strain in superlattices with this range of GaP thickness gave an empirical value for the GaP thickness to be 4.5 ML. Optical characterization indicated the highest integrated PL intensity for the sample at the optimal strain balanced condition.Epitaxial quantum dots ͑QDs͒ have generated much interest from the III-V semiconductor device field for their potential to enhance performance in particular device characteristics. For example, semiconductor lasers can exhibit improved threshold current densities using QDs; 1,2 QD infrared photodetectors have also shown incident absorption enhancements over quantum well ͑QW͒ detectors 3,4 and a number of approaches either have been proposed or were demonstrated for improvement in photovoltaic conversion efficiency. 5-8 The use of epitaxial QDs have been proposed as the most probable candidate for the realization of the intermediate band solar cell. 5 Recently, Stranski-Krastinow ͑SK͒ grown InAs/GaAs QDs have demonstrated improved single junction short-circuit currents. 6 Due to the straindriven nature of the SK growth mechanism of epitaxial QDs, correct strain-balancing is essential to growing the significant numbers of QD layers necessary for these devices. With greater numbers of layers, designs which neglect strain can lead to dislocations and poor QD uniformity which can ultimately degrade photovoltaic parameters such as open circuit voltage 7,8 and laser operation. 9 Strain-balancing, first shown to increase critical thickness by Katsuyama et al., 10 has since advanced in application and has been shown useful in decreasing misfit and threading defects in many superlattice ͑SL͒ structures, improving various optical, mechanical, and device properties. 7,[11][12][13][14] Alternating layers of compressively strained and lattice matched epitaxial material on a substrate have inherent individual layer thickness limits imposed by the Matthews and Blakeslee formulation. 15 Miller et al. 11 and Ekins-Daukes et al. 12 have suggested means of relieving strain in a QW SL by introducing an alternating tensilely strained material to balance the compressively strained layer. Consideration of these approaches has lead to the present work in which they are applied to a multiple-layer array ͑SL͒ of InAs QDs in a GaAs host with GaP strain-balancing layers.Five test structures with 10ϫ layer stacks of InAs ͑wetting layer + QD͒/GaAs/GaP/GaAs were grown on ͑100͒ GaAs substrates using an organometallic vapor-phase epitaxy reactor using traditional III-V metal-organic and hydride gases. Further details of growth are described in Ref.16. InAs...

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