Growth of Nonpolar GaN(1100) Films and Heterostructures by Plasma-Assisted Molecular Beam Epitaxy
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Growth of Nonpolar GaN(1100) Films and Heterostructures by Plasma-Assisted Molecular Beam Epitaxy Oliver Brandt, Yue Jun Sun, and Klaus H. Ploog Paul-Drude-Institut f¨ur Festk¨orperelektronik, Hausvogteiplatz 5–7, D-10117 Berlin, Germany ABSTRACT We discuss the growth of M-plane GaN films and (In,Ga)N/GaN multiple quantum well (MQW) structures on LiAlO2 (100) substrates by plasma-assisted molecular beam epitaxy. The adsorption and desorption kinetics of Ga on M-plane GaN is studied by reflection high-energy electron diffraction, allowing us to identify the optimum growth conditions with regard to surface morphology. Furthermore, we investigate the compositional profile of M-plane (In,Ga)N/GaN MQWs grown under conditions resulting in comparatively abrupt interfaces. The results demonstrate that significant In surface segregation occurs for the case of M-plane (In,Ga)N. The dependence of the transition energies of the M-plane MQWs on the actual well thickness reveals, however, that the structures are indeed free of electrostatic fields along the growth direction. INTRODUCTION Group-III nitrides are inherently piezo- and pyroelectric materials due to their wurtzite structure. Since the singular polar axis in the wurtzite lattice is the [0001] direction (the c-axis), large electrostatic fields may arise in heterostructures grown along this orientation. However, structures grown along any direction perpendicular to [0001], such as [1100] (M-plane) and [1120] (A-plane), are free of both piezo- and pyroelectric polarization [1]. The resulting absence of electrostatic fields along these directions constitutes a distinct advantage for fabricating highefficiency GaN-based LEDs over corresponding [0001]-oriented structures on conventional substrates such as Al2 O3 (0001) and SiC(0001). Additionally, since the c-axis of GaN lies in the growth plane for the nonpolar directions, the optical properties of these structures are inherently strongly anisotropic [2], making them promising candidates for polarization-sensitive photodetectors [3]. Consequently, the growth of group III-nitrides along nonpolar orientations, such as [1100] [4–7] and [1120] [8–10] has recently attracted considerable attention. However, the optimum growth conditions for any of these nonpolar orientations have yet to be explored. This fact is in contrast to the case of GaN(0001), for which several studies established these conditions and related them to the existence of a Ga bilayer on the growth front [11–15]. Empirically, we have found that Ga-rich conditions [16] results in GaN(1100) layers with comparatively smooth surface morphology, whereas N-rich conditions lead to a roughening of the growth front. Recently, Lee et al. [17] observed a reconstruction of approximate symmetry ”(4×5)” on Ga-rich grown GaN(1100) on ZnO by scanning tunneling microscopy. The surface was found to be metallic in nature, and the authors proposed that it consists of ≥2 monolayer (ML) of Ga based on the fact that Ga-adlayer structures with coverages between 2 and 3 ML on Ga
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