Preparation of Optoelectronic Devices Based on AlN/AlGaN Superlattices
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Preparation of Optoelectronic Devices Based on AlN/AlGaN Superlattices M. Holtz,a,b G. Kipshidze,c A. Chandolu,c J. Yun,c B. Borisov,c V. Kuryatkov,c K. Zhu,c S. N. G. Chu,d S. A. Nikishin,b,c and H. Temkinb,c a
Department of Physics, b Nano Tech Center, c and Department of Electrical Engineering Texas Tech University, Lubbock, Texas 79409 d Agere Systems, Murray Hill, NJ 07974
ABSTRACT We present results on growth and fabrication experiments of AlN/AlGaN superlattices for ultraviolet (UV) optoelectronic devices. Superlattices with extremely short periods have been studied. The AlN “barrier” layers are 0.5 nm thick, and the AlxGa1-xN “wells” are 1.25 nm thick, with x ~ 0.08. This combination gives an average AlN mole fraction of 0.63 across one full period. The superlattice periods, AlN mole fractions, and energy gaps are determined using TEM, X-ray diffraction, and optical reflectance. They are all consistent with each other. For device fabrication, p-i-n structures are grown doped with Si (n-type) and Mg (p-type). The acceptor activation energy of ~ 0.2 eV is found. Mesa structures are plasma etched using chlorine chemistry. Etch rates of AlN are ~ 1/3 those of GaN under identical circumstances. Etch rates of 250 nm/min are used for the device structures. A light emitting diode, with primary emission at 280 nm is reported, and a detector with sensitivity edge at 260 nm are reported. INTRODUCTION There is currently considerable interest in making optoelectronic devices operating in the ultraviolet (UV). Light emitting diodes (LEDs) with emission wavelengths between 340 nm and 280 nm are potentially useful for a number of important applications, from fluorescence excitation to data storage. High performance UV photodetectors having responsivity below ~ 280 nm would be highly useful because they would not respond to the visible solar spectrum, i.e., they would be solar-blind. The most promising material for achieving high performance at short wavelength is the AlxGa1-xN alloy system [1-5], which can be grown epitaxially in the wurtzite structure across the full composition range [6]. There are important problems associated with growing AlxGa1-xN. First, substrates used for the epitaxial growth (sapphire, silicon, and silicon carbide) each induces stress in the layers due to a combination of lattice mismatch and thermal stress [7,8]. Second, to achieve a band gap above 4.4 eV (below 280 nm), a composition of x > 0.5 is needed. While it is possible to dope these alloys n-type (Si), producing p-type (Mg) material with high hole concentrations is inherently difficult because the acceptor level grows deeper with higher Al content. Our recent work has shown that diodes based on superlattices (SLs) of AlN/AlGaInN can be used to produce both LEDs with light emission down to 280 nm [5] and photodetectors with sensitivity edge near 260 nm [9]. This is accomplished using SLs of AlN and AlGaInN with extremely small well and barrier thicknesses. The average AlN mole fraction is very high in these structures, over 0.6,
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