Growth and Characterization of AlGaN/GaN Heterostructures With multiple Quantum Wells by PAMBE
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INTRODUCTION The advances in the material growth and processing of III-V nitrides have led to the demonstration of a current injection laser diode (LD) with InGaN-based multiple quantum wells (MQWs) 1' 2. In the attempt to realize AIGaN/GaN QW LDs, there have been a few reports on the growth and characterization of the AlGaN/GaN single quantum well structures3 '4 . Krishnankutty et al. 3 reported the strain-induced energy gap shift, which was deduced from the position of bound excitons in an AIGaN/GaN strained-layer quantum well grown by metalorganic chemical vapor deposition (MOCVD). Recently, Salvador et al. 4 fabricated a Si-doped AIGaN/GaN 60 A thick single quantum well using gas source MBE with ammonia gas as a source of active nitrogen. From the observed bound exciton transitions, they estimated heavy hole and electron effective masses as 0.3 mo and 0.19 ino, respectively. In this paper, we present and analyze charcteristics of AIGaN/GaN MQWs grown by plasma-assisted molecular beam epitaxy (PAMBE), which employs inductively coupled nitrogen plasma as a source of active nitrogen. The optical quality of the present quantum wells is shown to be comparable to that for similar heterostructures fabricated with other growth techniques.
EXPERIMIENTAL A detailed description of the PAMBE system developed at the University of Illnois can be found in our earlier publication 5 and in the companion paper 6 . Here we only emphasize the specific features of the system or process relevant to the growth of quantum wells. Figure 1 shows a schematic of the MQW structures studied in the present work. First, a 30 nm AIN 347
Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
buffer layer was grown on a sapphire substrate. Then, a 0.5 pm thick GaN epilayer was deposited at a substrate temperature of 750 'C. The purpose of this layer was to accommodate stress caused by the sapphire/GaN lattice mismatch. Multiple layers of AlGaN/GaN were grown on top of the GaN layer. In a separate set of experiments, the growth rate and Al mole fraction were determined with sufficient accuracy to precisely control Al concentration and to grow nanolayers of AlGaN and GaN with predetermined thicknesses.
AIN buffer AlGaN barriers
AEc
Sapphire
GaN
'*\ GaN QWs
Figure 1. Diagram of conduction energy band of GaN/AIGaN MQW structures. To avoid formation of dislocations and cracks at the AIGaN - GaN interfaces, the thicknesses of the GaN quantum wells were kept within the pseudomorphic limit 7. The thicknesses of the GaN QW layers were less than the critical thicknesses (486, 208, and 125 A for GaN QWs with AIGaN barriers, XAj = 0.1, 0.2 and 0.3, respectively). The critical thicknesses were calculated using the Matthew and Blankeslee force balance model8 . To study radiative transitions in unstrained GaN MQWs, we prepared the samples with different well thicknesses (25 A, 50 A, 100 A) and the same Al mole fraction (0.2) in the barriers. To prevent substantial coupling between the wells, the barrier thicknesses were maintained at 50
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