Nanobumps in In x Al 1-x N/AlN/Sapphire System: A New Kind of Quantum Dots?
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Nanobumps in InxAl1-xN/AlN/Sapphire System:A New Kind of Quantum Dots? Yuri Danylyuk, Dmitri Romanov, and Gregory Auner Department of Electrical and Computer Engineering,Wayne State University, Detroit, MI 48202, U.S.A. ABSTRACT We have investigated InxAl1-xN layers grown by plasma source molecular beam epitaxy (PSMBE) on c-plane sapphire (0001) substrates with thin (about 100 Å) buffer layers of AlN. The value of x varied from 0 to 1. For all these layers, high resolution X-ray diffraction scans (XRD) show no indication of face segregation, while the atomic force microscope (AFM) images of the structures demonstrate a large number of nanobumps. Our spectroscopic measurements (UV/VIS optical absorption and reflection spectroscopy) of these bumpy structures indicate additional peaks that can be only associated with additional energy levels. We ascribe these levels to electrons, which are confined and quantized near the nanobump tips by strong piezoelectric field caused mainly by biaxial strain of the AlN layer. The calculated energies of these quantized states are in a good agreement with the spectroscopic data. INTRODUCTION The ternary alloys of nitride semiconductors have been extensively studied and found useful in laser applications [1,2]. In particular, InxAl1-xN alloys whose energy band gap can be tuned from 1.9 eV (InN) to 6.2 eV (AlN) have many potential applications in light-emitting diodes and lasers based on quantum dots and operating in the ultraviolet to visible wavelength region. However, to date there have been only few studies [3-6] of InxAl1-xN films, mainly because of the difficulties associated with the film growth (the thermal instability due to spinodal phase separation leads to a difficulty in solubility between AlN and InN [7]). In the present work, we have studied a series of InxAl1-xN alloy films with thickness ~ 150 nm and In concentration ranging from 0 to 1. The films exhibit wurtzite crystal structure with no alloy segregation. We have investigated the morphology and the optical properties of the films at room temperatures. EXPERIMENT Al1-xInxN samples have been deposited via plasma source molecular beam epitaxy. The PSMBE system operates at base pressures in the range 10-9 – 10-10 torr and dynamic pressure ~ 3-4 mtorr. It utilizes a unique hollow cathode plasma sources lined with MBE grade Al and In. Low temperature plasma is created inside the hollow cathode, utilizing r.f. power sources and purified Ar and N2 . The Ar and N2 flow rates for all depositions were 40 and 10 sccm, respectively. The substrates were kept at negative bias of 10 V, to accelerate the species emerging from the hollow cathode. The In concentrations of the films were determined by applying Vegard’s law to the measured lattice constants using XRD. Al1-xInxN films with x = 0, 0.12, 0.28, 0.34, 0.55, and 0.67 were used in this study. During the growth, the films were monitored using an in-situ reflection high-energy electron diffraction (RHEED) system, and θ-2θ X-ray diffraction (XRD) was used for ex-situ I6.29.1
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