Incorporation of Mg into thick free-standing HVPE GaN
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Incorporation of Mg into thick free-standing HVPE GaN M.E. Zvanut1, J. Dashdorj1, J.A. Freitas Jr.2, E.R. Glaser2, J.H. Leach3, and K. Udwary3 1
Physics Department, University of Alabama at Birmingham, AL 35294, USA Naval Research Laboratory, Washington, DC 20375, USA 3 Kyma Technologies Inc., Raleigh, NC 27617, USA 2
ABSTRACT Mg, the only effective p-type dopant for nitrides, is well-studied in thin films due to the important role the impurity plays in light emitting diodes and high power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here we evaluate the material quality and point defects in GaN grown by hydride vapor phase epitaxy (HVPE) using metallic Mg as the doping source. The crystal quality is typical of commercially grown HVPE substrates, and the photoluminescence measurements reveal distinctively sharp excitonic and shallow-donor shallow-acceptor features. Secondary ion mass spectroscopy indicates total Mg concentrations between 7x1016 and 6x1018 cm-3 in the four separate samples studied but, more significantly, photoluminescence and electron paramagnetic resonance spectroscopy show that the Mg is incorporated as a shallow acceptor. INTRODUCTION GaN and related alloys are presently used to produce the energy efficient white-light LEDs which are increasingly available to consumers. Presently, much effort is being made to expand the nitride market to high power, high frequency electronics because such devices will benefit greatly from the high saturation velocity and breakdown field of nitride semiconductors [1]. While the success of the material is based largely on the availability of n- and p-type films, future development will depend strongly on the ability to minimize non-uniform strain and dislocations caused by the commonly used heteroepitaxial growth methods. Homoepitaxial GaN films have only recently been made feasible with the availability of high quality substrates grown by either hydride vapor phase epitaxy (HVPE) or various solution methods. The new structures show promising results for device applications [2,3]. Indeed, the world record GaN LED and p-n junction devices were obtained on bulk GaN substrates [4,5]. Despite the success of nitride technology, the types of devices produced will always be limited by the range of conductivity achievable in GaN substrates. For example, only lightly ptype bulk GaN is presently available, yet the emergence of a wide conductivity range would open interesting possibilities for alternative complimentary logic. The lightly p-type GaN substrates on the market are grown by a solution method and, although Kruszewski and coworkers report successful device fabrication utilizing these substrates, there is no published assessment of the material [6]. Typically, solution-grown GaN suffers from high concentrations of oxygen and yield n-type substrates [7], so it would be interesting to understand the mechanism for the p-type conductivity. Presently, only n-type and semi-insulating substrates are produced commercially using H
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