Polarity of GaN
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APA Optics, Inc., 2950 N.E. 84th Lane, Blaine, MN 55449, USA Nichia Chemical Industries Ltd, 491 Oka, Kaminaka, Anan, Tokushima 774, Japan
ABSTRACT Convergent beam electron diffraction (CBED) was applied to study polarity of GaN heterolayers grown by MOCVD on sapphire and SiC substrates and also mechano-chemically polished bulk GaN platelet crystals grown from Ga melt under hydrostatic pressure of N. Heterolayers of GaN grown on both types of substrates showed (0001) Ga polarity, e.g. Ga to N along c-axis. Mechano-chemical etching of bulk GaN platelet crystals leaves one surface smooth and the opposite surface rough with visible damage caused by mechanical polishing. CBED studies showed that the smooth surface has (000 T) N polarity, confirming our earlier studies. INTRODUCTION Earlier studies showed that growth rate and structural perfection of homoepitaxial GaN layers and also heteroepitaxial layers are influenced by polarity of the growth direction. However, polarity assignment has been associated with a lot of controversy [1-6]. Convergent beam electron diffraction (CBED) can be used to determine crystal polarity, since for noncentrosymmetric crystals intensity distributions within +g and -g diffraction discs for polar directions are different. This information can be used for determination of atom distribution (stacking sequence) within the unit cell. Rotation angle and the 1800 ambiguity between the image and the diffraction pattern needs to be taken into account. The experimental information need to be verified by comparison with a standard sample where crystal polarity has been previously determined. Information also can be obtained from computer simulations where several parameters including sample thickness, DebyeWaller factor and absorption coefficient contribute to the intensity distribution within the CBED discs. The central disc carries the information about the sample thickness. A good matching between the experimental and simulated pattern is required for at least two or three patterns with different sample thicknesses for accurate interpretation and assignment of polarity, since the pattern changes with crystal thickness. In order to determine crystal polarity for noncentrosymmetric crystals coordinates of A and B atoms need to be determined for a specific crystallographic direction and followed in the diffraction pattern to determine "+" and "-" direction. For the case of GaN usually the [1 TOO] zone axis is chosen, since (0002) and (000 2) diffraction discs carry the necessary asymmetry information. If one assigns Ga to the origin with N connected by a long bond along +c axis, this assignment will be called (0001) Ga polarity. An opposite assignment will give (OOOT) N polarity. The CBED method cannot give information about surface termination, only about stacking sequence, eg. Ga to N along +c direction or the opposite. 363
Mat. Res. Soc. Symp. Proc. Vol. 512 © 1998 Materials Research Society
EXPERIMENTAL AND DISCUSSION In order to avoid the 1800 ambiguity in the microscope GaAs samples g
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