Optical and Microstructural Properties of N- and Ga-Polarity GaN
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Optical and Microstructural Properties of N- and Ga-Polarity GaN A. Bell1, J. L. Smit1, R. Liu1, J. Mei1, F. A. Ponce1, H. M. Ng2, A. Chowdhury2, and N. G. Weimann2. 1 Dept. of Physics and Astronomy, Arizona State University, Tempe, AZ, 85287-1504. 2 Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ. ABSTRACT The effect of polarity on the optical and microstructural properties of GaN is presented. A sample with adjacent domains of Ga- and N-polarity material was grown by varying the nucleation layer. This allows a unique opportunity to study the two different polarities under controlled conditions. We found that the N-polarity material has a much lower dislocation density than the Ga-polarity material. The N-polarity material contains voids that are not present in the Ga-polarity region. The surface roughness of the N-polarity material appears to be caused by Ga-polarity inversion domains which lead the growth. A cathodoluminescence study showed that the N-polarity material is much brighter than the Ga-polarity material, suggesting a higher donor concentration, probably due to increased impurity incorporation in the [000 1 ] growth direction or possibly due to an increase in intrinsic point defects. There is also evidence that the N-polarity region contains two types of material, one is flat and can be etched, the other has an inclined facet and does not etch. INTRODUCTION GaN can grow in either the [0001] or [000 1 ] directions. In the case of GaN grown in the [0001] direction, the Ga atoms at the growth surface will be bonded to three nitrogen atoms. This material is often called Ga-polarity material. GaN grown in the [000 1 ] direction (known as N-polarity material) will have Ga atoms at the growth surface attached to one nitrogen atom. These differences in the Ga bonding at the growth surface lead to differences in impurity incorporation, and therefore the material properties of the two polarities can be very different. N-polarity material tends to incorporate more oxygen [1-4], carbon [1-2] and aluminum [2]. Si incorporation was not found to be effected by polarity [3]. In Mg-doped GaN, it was found that no extra impurities are incorporated in the N-polarity material, but it was also found that p-type doping could not be achieved in N-polarity material [5]. By varying the nucleation conditions during growth, the polarity of GaN can be selected [3-7]. In this paper we report on the optical and structural properties N- and Ga-polarity GaN grown simultaneously using different nucleation layers. EXPERIMENTAL DETAILS A GaN sample with adjacent Ga- (0001) and N-polarity (000 1 ) regions was studied by transmission electron microscopy (TEM) and cathodoluminescence (CL). The sample was grown by plasma-assisted molecular beam epitaxy on a sapphire substrate, with a 20 nm to 30 nm thick AlN buffer layer. A thin layer of GaN (~100 nm) was deposited on the buffer layer to prevent oxidation of the AlN upon removal from the growth chamber. The sample was
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