Short-period AlGaN based superlattices for deep UV light emitting diodes grown by gas source molecular beam epitaxy
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Short-period AlGaN based superlattices for deep UV light emitting diodes grown by gas source molecular beam epitaxy S. A. Nikishin, B. A. Borisov, V. V. Kuryatkov, M. Holtz, and H. Temkin Nano Tech Center, Texas Tech University, Lubbock, Texas 79409
ABSTRACT We report the results of two studies of the growth and physical properties of AlGaNbased short-period superlattices (SPSLs), each aimed at improving light emission. In the first experiment, we grow structures on bulk AlN substrates. We observe ~ 3 times higher luminescence efficiency than identically grown structures on sapphire. In the second experiment, we grow structures on sapphire while controlling the growth mode. We observe a significant improvement in the room temperature cathodoluminescence efficiency (at least by factor of 10) of AlGaN quantum wells when the 3D growth mode is induced by reduced flux of ammonia over identically prepared structures grown in the 2D mode.
INTRODUCTION There has been significant recent progress towards sub-300 nm light emitting diodes (LEDs) based on AlxGa1-xN alloys and various approaches to active LED structures have been reported [1-9]. These designs typically use a transparent sapphire substrate overgrown by a complicated AlxGa1-xN buffer layer to reduce the defect and dislocation densities in device structure grown on this buffer. The typical device structure consists single quantum well or multiple quantum well active regions sandwiched between wide bandgap n- and p-type cladding layers selected to be transparent to the desired emission. Heavily doped p-type contact layers of GaN or AlxGa1-xN with small x are grown last. Device preparation is limited by the difficulty of preparing p-type layers of large bandgap AlGaN. We have shown that p-type limitation can be overcome by the use of short period superlattices (SPSLs) consisting of AlN barriers (3-5 monolayers thick) and AlxGa1-xN wells (2-3 monolayers thick) [1, 4, 6, 9]. The external quantum efficiency is also poor due to two major causes. The first is a high dislocation in the LED active region originating from the substrate-structure interface. Many approaches have been used to reduce the defect and dislocation densities. One obvious choice is to grow on bulk AlN substrates. Close lattice match between bulk AlN and SPSL layers makes AlN substrates very attractive for use in devices. We describe here recent results concerning the use of such substrates to grow SPSLs with high average AlN content. A second important cause of low efficiency is a non-optimal design of interior LED device structure. In particular, the active region must be designed in order to achieve high internal quantum efficiency. This can be achieved using cladding layers for carrier confinement and by the growth of quantum dots in the active region to enhance
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electron-hole localization and radiative recombination efficiency. It is well known that the internal efficiency of quantum wells of InGaN is enhanced by formation of InN-rich quantum dots (QDs) [10]. QD f
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