Study of Thin Films Polarity of Group III Nitrides
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K. Dovidenko,* S. Oktyabrsky,** J. Narayan,* M. Razeghi*** *Center for Advanced Materials and Smart Structures, Dept. of Mat. Sci. & Eng., North Carolina State University, Raleigh, NC 27695, [email protected] **NYS Center for Advanced Thin Film Technology, State University of New York, Albany, NY 12203 ***Center for Quantum Devices, Northwestern University, Evanston, Illinois 60208. Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G6.46 (1999) Abstract Thin films of GaN grown by MOCVD on (0001) sapphire were studied by transmission electron microscopy in order to correlate the observed extended defects with crystal polarity of the films. We propose relatively simple and unambiguous method of polarity determination for wurtzite group III nitrides based on the dependence of the intensity of diffracted beams upon thickness of the specimen. Due to the dynamic scattering by polar structure, the convergent beam electron diffraction patterns lose inversion symmetry and become in fact fingerprints of the structure carrying information about crystal polarity. In this study, we have used the thinnest regions of the specimens (< 15 nm) and multiple diffraction spots in high-symmetry orientation for polarity determination. The films were found to have Ga-polar surfaces, either being unipolar, or containing thin (10-30 nm in diameter) columnar inversion domains (IDs) of Npolarity. The occurrence of IDs was correlated with specific types of dislocation distribution in the films. Introduction In recent years the Group III nitride materials have generated tremendous interest due to their potential applications in high-efficiency LEDs (light-emitting diodes) and blue lasers. For device structures, high quality GaN and relative ternary alloy films and heterostructures are needed. The group III nitrides are materials with non-centrosymmetrical crystal structure, therefore, Ga- (Al-) polar or N-polar layers can be grown. Polarity of Ill-V nitride thin films is known to be an important factor in determining the surface roughness and properties of the asgrown material. Results of efforts of many groups on the polarity study are well reviewed by Hellman [1]. The primary focus of our investigations was the absolute polarity determination and correlations between the polarity and extended defects present in the GaN films grown on sapphire. The polarity of the layers was traditionally determined by convergent beam electron diffraction (CBED) [2-4] and multiple dark field transmission electron microscopy (TEM) techniques [5-6]. However, CBED method can only be applied for perfect crystals of considerable volume, while the GaN films often contain thin (of the order of 10 nm in diameter) pipe-like domains. The multiple dark field TEM imaging [5] allows to determine relative, not absolute polarity of such domains. Here we used a new method of polarity determination based on the different dependency of intensity vs. thickness for the specific diffracted beams revealing the non-centrosymmetric nature of the crystal. This allo
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