Luminescence energy and carrier lifetime as a function of applied biaxial strain in InGaN/GaN quantum-well structures
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Luminescence energy and carrier lifetime as a function of applied biaxial strain in InGaN/GaN quantum-well structures Noad A. Shapiro1, Henning Feick1∗, William Hong1, Nathan F. Gardner2, Werner K. Goetz2, J. W. Yang3, and Eicke R. Weber1 1
Materials Science Division, Lawrence Berkeley National Laboratory and University of California, Berkeley, 1 Cyclotron Road, Berkeley, California 94720 2 LumiLeds Lighting, San Jose, California 95131 3 APA Optics, Blaine, MN 55449 ABSTRACT Photoluminescence (PL) and Time-resolved PL (TR-PL) are used to measure the luminescence energy and carrier lifetime of InGaN/GaN quantum well (QW) structures as a function of biaxial strain and excitation density. A blueshift of the transition energy and a decrease in the carrier lifetime reveal a field-dependent spatial electron-hole (e-h) wavefunction separation. This behavior is observed both under the application of tensile, biaxial strain, which directly affects the piezo-related field, and under increased excitation density, which effectively screens the electric field. Our results show an increased carrier separation with increasing QW thickness.
INTRODUCTION InGaN has emerged in the last years as the most important material for short-wavelength optoelectronics. Devices based on this material are already commercially available, yet the nature of the radiative transitions that occur in these devices is still under debate. Two distinct transition types that have been proposed are the recombination of carriers localized in indiumrich nano-clusters [1] and the recombination of carriers separated due to strong built-in electric fields [2]. Recently, we have developed a method to directly study the effects of the electric field on the radiative transition through the application of biaxial strain [3]. We found that, in structures where the carriers were separated, the strain-induced field-reduction acted to increase the transition energy. Here we show that, in addition to an increase in the transition energy, there is a decrease in the carrier lifetime. This reduction is due to the reduced carrier separation that results from the reduction in the electric field. In addition, we show that we can generate a similar coupling between the transition energy and carrier lifetime by carrier screening resulting from increased excitation density. EXPERIMENTAL DETAILS For the application of biaxial strain, the sample is used as a window of a pressure cell. The biaxial strain varies linearly with pressure (p) and with the square of the ratio of window diameter to sample thickness. Details of the experimental procedure are outlined in ref. [3]. The thickness of the samples (substrate + epilayer) was about 400 µm, and the window radius was ∗
on leave from center of advanced european studies and research (caesar), Bonn, Germany
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