Geometrical Characteristics and Surface Polarity of Inclined Crystallographic Planes of the Wurtzite and Zincblende Structures

HISASHI MASUI masui@engineering 0 1 SAMANTHA C. CRUZ 0 1 SHUJI NAKAMURA 0 1 STEVEN P. DENBAARS 0 1 0 1.Solid State Lighting and Energy Center, Materials Department, College of Engineering, University of California , Santa Barbara, CA 93106-5055, USA. 2. 1 Masui, Cruz, Nakamura, and DenBaars Inclined crystallographic planes of the wurtzite structure were investigated in comparison with the zincblende structure in terms of surface geometry characteristics. The ball-stick model indicates that the semipolar 1101 surface possesses a surface polarity resembling the anion polarity, which agrees with the common experimental observations of epitaxial growth preference for the cation-polarity 1101 surface over the 1101 surface. The wurtzite 1122 surface was found to share geometrical similarities with the zincblende {100} surface uniquely among the possible semipolar planes. This finding encourages epitaxial growth on the 1122 plane of wurtzite semiconductors, e.g., GaN, with the potential of avoiding atomic step formations typically associated with off-axis crystallographic planes. - Since the successful growth of high-quality GaN crystals and the emergence of GaN-based semiconductor devices, the wurtzite structure (P63mc) has become very familiar to semiconductor scientists. In addition to the traditional c-plane growth, nonpolar and semipolar planes have been proposed to be growth planes for optoelectronic use.1,2 There are many possible choices of semipolar planes: Recent research has been successful in growing GaN-based crystals on 1011 ; 1013 ; and 1122 planes.39 Especially, 1122 has attracted vast attention due to its strong ability to incorporate In, aiming for green-light emitters.6,7 Ueda et al. (the group led by Kawakami at Kyoto University) have been growing exclusively on the 1122 GaN substrates.10 Stimulated emission in the green spectral region was recently achieved.11 Light emission from stable semipolar facets grown on c-plane substrates have also been studied.1215 Here an interesting fact is that metalorganic chemical vapor deposition (Received September 22, 2008; accepted March 23, 2009; published online April 10, 2009) (MOCVD) is more successful on 1011 and 1013 than on 1011 and 1013 ;16 while (0001) and 1122 are preferred to 0001 and 1122 ; respectively. What is believed regarding conventional c-plane GaN growth via MOCVD is that the (0001) plane is preferable partially because of the alkyl Ga and gaseous N precursors. A simple adatom model illustrates that a monomethyl Ga species is adsorbed on the (0001) surface, while the 0001 surface adsorbs trimethyl Ga species and can be crowded with massive CH3 alkyls. Although zincblende F43m GaN has not been used in optoelectronic devices, GaAs- and InP-based IIIV semiconductors form zincblende structures and the (100) plane is the favored orientation for epitaxial growth. The question then arises of whether the (100) plane has a counterpart in hexagonal close packing (hcp). In the close-packed ionic structures, the crystallographic planes perpendicular to the stacking axis, {111} in zincblende and {0001} in wurtzite, are electrically polar planes. It is also widely known that the zincblende {110} and wurtzite 1010 and 1120 planes are electrically nonpolar planes due to the fact that the anion and cation appear on the surface in pairs. In the present report, the zincblende and wurtzite structures are compared to investigate these important crystallographic planes by using the ball stick model. The surface polarity of the semipolar planes is discussed. It is shown that the 1122 plane is unique among the wurtzite semipolar planes for the reason that the 1122 surface shares several geometrical similarities with the (100) surface. It is the close-packed stacking sequence that differentiates the face-centered cubic (fcc) (the ABC sequence) from the hcp (the AB sequence) structure. Figure 1 shows the ballstick model that compares zincblende (fcc) and wurtzite (hcp) structures with the polar axes drawn parallel to each other. The horizontal planes are the polar planes: {111} in zincblende and {0001} in wurtzite. There are two types of popular nonpolar planes in each structure: {110} and {112} in zincblende and the a-plane 1120 and m-plane 1010 in wurtzite. The two nonpolar planes are orthogonal to each other. The first type (called type I hereafter) contains two bonds of the four tetrahedral bonds (drawn in blue in Fig. 1), that is, f110g and 1120 . This vertical plane occurs every 60 deg around the polar axis (sixfold symmetry). The second type (type II) contains parallel bonds (drawn in red in Fig. 1), that is, {112} and 1010 : This plane occurs also every 60 deg around the polar axis. Any other vertical planes (type III) are high-index planes. The atomic arrangements of the four nonpolarplane projections are sketched in Fig. 2. Broken Fig. 1. Perspective view of (a) zincblende and (b) wurtzite structures drawn with the polar axis parallel to each other. The planes drawn in blue are type I nonpolar and those in red are type II nonpolar. lines indicate major inclined planes perpendicular to the projection. In Fig. 2a, in addition to commonly noticed planes, attention needs to be paid to their surface polarity. Two important facts need to be mentioned. First, when the (111) plane is rotated counterclockwise by 70 deg around the zone axis (normal to the page), it becomes 111 ; which is an anion plane, commonly labeled {111}B. It is important to note that this polarity flip occurs before the (111) plane reaches the nonpolar 112 plane by a 90 deg rotation. The mechanism is discussed in the section Surface Polarity below. Second, the (001) plane is an inclined plane at 35.26 deg; it can be considered as a semipolar plane. The piezoelectric polarization appears to be zero as a consequence of the symmetry17; yet {100} does not belong to any of the nonpolar plane types. In Fig. 2b, it can be noticed that {112} is relatively highly symmetrical. In Fig. 2c, these semipolar planes are inclined m-planes. An important fact here is that none of the low-index inclined planes intersects atoms except the corner atoms. In Fig. 2d, it is recognized that 1122 is a nice semipolar plane in the sense that the plane intersects a noncorner atom: a finding that runs counter to previous speculations regarding the atomic step formation upon epitaxial growth on semipolar planes.8 This is a consequence of adjacent atoms of the cation sublattice residing at (0, 0, 0, 0) and 13 ; 0; 13 ; 12 atomic positions. (The space coordinate notation here is consistent with the Miller-Bravais indices and convention, e.g., see Refs. 18 and 19.) Wurtzite planes, which are similar to {100} zincblende planes in including noncorner atoms, are discussed further in the section Surface Geometry below. It is conventional to label the wurtzite GaN (0001) plane the Ga-terminated face20 (or simply Ga face) or Ga polarity. The 0001 plane is then characterized by the (...truncated)