Polar vs. nonpolar InGaN/GaN quantum heterostructures: Opposite quantum confined electroabsorption and carrier dynamics behavior

Abstract

Abstract-We present a study of quantum confined electroabsorption and carrier dynamics in polar c-plane and nonpolar a-plane InGaN/GaN quantum heterostructures. We demonstrate red-shifting absorption edge, due to quantum confined Stark effect, in nonpolar InGaN/GaN quantum structures in response to increased electric field, while we show the opposite effect with blue-shifting absorption spectra in polar quantum structures. Moreover, confirmed by timeresolved photoluminescence measurements, we prove that carrier lifetimes increase with increasing electric field for nonpolar structures, whereas the opposite occurs for polar ones.Since the demonstration of blue light emitting diodes [1] and laser diodes [2], optoelectronic devices based on IIINitride quantum structures have found important applications in consumer electronics and lighting industries through display, data storage and white-LED applications [3]. Moreover, in this semiconductor device platform, there is a great potential for even further expansion in optoelectronic device applications through innovation and development [4][5]. To widen their use, to improve their performances, and to expand their applications, physics of these devices should be understood well and materials quality and device efficiencies should be increased. In our research group, we have been working on the physics and applications of optoelectronic devices based on InGaN/GaN quantum structures. Here in this paper, we review the basic physics of InGaN/GaN quantum structures and present our studies on their external electric field dependent optical absorption and time resolved decay kinetics behavior. Polarization-induced built-in electrostatic fieldsIn III-Nitride quantum heterostructures, two-dimensional built-in charges of alternating signs are induced at the interfaces due to the discontinuities of the polarization fields throughout the structure [6]. These charges cause built-in electrostatic fields, which are perpendicular to the interfaces and have alternating directions in quantum well and barrier layers in a multiple quantum well structure. This effect is maximal when the growth is performed on polar c-plane of III-N wurtzite crystal. As a consequence of this effect, electrons and holes are pulled in the opposite directions inside the well layers. This results with a reduced electronhole overlap and thus a reduced electron-hole recombination and generation rate compared to the no polarization induced electrostatic field case [4].However, these fields and this effect are absent when the structure is grown on its m-or a-plane [6]. These two planes are referred to as nonpolar planes. One of the drawbacks for growing III-Nitrides on nonpolar planes is that, dislocation densities in nonpolar-grown structures are much higher than those on polar structures, mainly due to the absence of a native substrate. And this makes them unfavorable for device applications. In a recent study, metal organic chemical vapor deposition (MOCVD) growth of a-plane GaN on r-plane sapphire substrate was...