The Influence of Defects and Piezoelectric Fields on the Luminescence from InGaN/GaN Single Quantum Wells
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THE INFLUENCE OF DEFECTS AND PIEZOELECTRIC FIELDS ON THE LUMINESCENCE FROM InGaN/GaN SINGLE QUANTUM WELLS S. J. Henley and D. Cherns H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, U.K. ABSTRACT High spatial resolution cathodoluminescence (CL) studies have been carried out on GaN/InGaN/(0001)GaN single quantum well (SQW) structures in a field emission scanning electron microscope at 5kV and temperatures down to 8K. Direct comparison of QW CL maps with transmission electron microscope studies of plan-view samples showed that edge type threading dislocations act as non-radiative recombination centers. Spectra taken from extended areas showed a progressive blue shift in the QW emission from around 460nm at low beam intensities to about 445nm as the beam intensity was increased. This effect which correlated with a decrease in the spatial resolution is interpreted as due to an increase in the diffusion length of carriers in the SQW due to a combination of screening of the piezoelectric field and band filling effects.
INTRODUCTION Optoelectronic devices such as light emitting diodes and lasers based on InGaN/GaN quantum well(QW) structures are now a commercial reality. Despite the rapid technological advances, the reasons why these devices operate as well as they do is still unclear given the presence of a high density of threading dislocations (as high as 1010cm-2 in some cases[1]). Cathodoluminescence (CL) studies have shown recently that dislocations act as non-radiative recombination centers[2-4]. From these studies the minority carrier diffusion length (MCDL) in GaN has been estimated. The results indicate that this diffusion lengths is are in the range 50250nm depending on doping.[3-4] In InGaN layers values for the MCDL of ~50-60nm have been estimated where quantum dots are present[5]. The role of different types of threading dislocation is still unclear. Three types of dislocation are common in GaN, those with Burgers vector b = a (edge type), those with b = c (screw type) and those with b = c + a (mixed type.) Some theoretical studies have suggested that edge type dislocations will be electrically inactive and that the screw type will be electrically active[6]. There is some indirect experimental evidence to support the theoretical calculations[7]. In this report we present high spatial resolution CL studies of an InGaN/GaN single quantum well (SQW) sample carried out in a scanning electron microscope (SEM). Comparison between the CL studies and transmission electron microscopy (TEM) studies on plan-view and cross-sectional samples shows a direct correlation between dark-spots in the QW luminescence and the location of edge type dislocations. We identify other sorts of defects present in this sample and estimate the effect that these defects as well as the edge dislocations have on the luminescence efficiency of the sample. We also present results that show evidence for a change in the MCDL after high dosage SEM electron beam irradiation of the sample. This change is acco
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