Polarity Control of GaN Films Grown by Metal Organic Chemical Vapor Deposition on (0001) Sapphire Substrates
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E3.20.1
Polarity Control of GaN Films Grown by Metal Organic Chemical Vapor Deposition on (0001) Sapphire Substrates Seiji Mita1, Ramon Collazo1, Raoul Schlesser1, and Zlatko Sitar1 1 Dept. of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, U.S.A. ABSTRACT The polarity control of GaN films grown on c-plane sapphire substrates by low pressure metal organic chemical vapor deposition (MOCVD) was achieved by using N2 as a diluent and transport gas. The type of polarity was governed by the substrate treatment prior to the GaN growth. N-face (-c) GaN films were only obtained by pre-nitridation of the sapphire substrate after a H2 anneal, while Ga-face (+c) GaN films were grown directly on the substrates or on properly annealed AlN buffer layers. In addition, GaN films on improperly annealed AlN buffer layers, that is, under- or over-annealed buffer layers, yielded films with mixed polarity. Smooth N-face GaN films with 2.5 nm RMS roughness, as determined by atomic force microscopy (AFM), were obtained with shorter nitridation times (less than 2 min). Wet chemical etching in an aqueous solution of potassium hydroxide (KOH) was used to determine the polarity type. INTRODUCTION III-nitride semiconductors crystallizing in a wurtzite crystal structure possess a polar axis oriented along the direction. This polarity is a consequence of the non-centrosymmetric crystal structure of the III-nitrides. The most common growth direction of the III-nitrides is normal to the {0001} basal plane, where the atoms are arranged in bilayers consisting of two closely spaced hexagonal layers, one containing the cations and the other anions, so that the bilayers have polar faces, as shown in Figure 1.
Figure 1. Crystal structure and partial unit cell of a wurtzite III-nitride semiconductor.
E3.20.2
The polar faces in GaN are commonly referred to as the Ga- (+c orientation) or N-face (-c orientation). Ga-face indicates the polar structure with three bonds of the nitrogen that point up, while N-face indicates single bond that points upward. Polar orientation should not be confused with surface termination, as each orientation may be terminated with either one of the species. In short, the two polarities are related by an inversion operation, which is a true symmetry operation. The surface atomic configuration and neighboring atom arrangement are different between the Ga-face and N-face. This difference in surface structure has an influence on the growth mode of wurtzite GaN, as well as on the development of potential GaN materials for device applications. Therefore, it is very important to be able to determine and control the polarity of GaN films. Polarity control of MOCVD GaN films can be managed by systematic substrate treatments consisting of nitridation, buffer layer deposition, and buffer layer anneal time [1]. Each step has an influence on the subsequent process. N-face films are obtained when GaN films are grown on nitrided substrates, even after nitridation lasting only a few seconds. As fo
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