Wet Etching of Bulk AlN Crystals
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Wet Etching of Bulk AlN Crystals Dejin Zhuang, Ziad G. Herro, Xianglin Li, Raoul Schlesser, and Zlatko Sitar Materials Science and Engineering, North Carolina State University, Campus Box 7919, Raleigh, NC, 27695-7919 Abstract AlN bulk crystals of different crystallographic orientations were etched in aqueous KOH solution (general etchant) and molten KOH/NaOH eutectic alloy (defect selective etchant). In the general etchant, the aluminum polar surface (0001) was found to be stable while the nitrogen −
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polar ( 0001 ) surface and non-polar m-plane ( 101 0 ) surfaces showed significant etching. Etch patterns on nitrogen polar surfaces consisted of complete coverage of hexagonal pyramids while the m-plane surfaces were etched in a layer-by-layer mode. TEM studies revealed that the observed etch hillocks on nitrogen polar surfaces were not associated with extended structural defects. When etched in a defect-selective etchant, the aluminum polar surface showed welldefined hexagonal etch pits, with an etch pit density (EPD) of 2×104 cm-2. The nitrogen polar surfaces and m-plane surfaces showed etch features similar to the one obtained during general etching, however, the etch feature density was significantly lower. The etch mechanisms, etch kinetics, as well as the correlation between the observed etch features and structural defects in bulk crystals will be discussed. Introduction Single crystalline aluminum nitride (AlN) is a promising semiconductor material for advanced optoelectronic and microelectronic device fabrication [1] due to its unique properties [2], such as direct wide bandgap (6.1 eV), high thermal conductivity, and high electrical resistivity. AlN is also an ideal substrate material for AlGaN-based device fabrication as it shares the same crystal structure and close lattice and thermal match to these device layers [3]. Driven by exciting progress in AlN sublimation growth [4, 5], further investigation of AlN etch behavior is required to develop routes for device processing. Although high etch rates have been achieved by dry etching [6], wet chemical etching of AlN offers the advantages of high selectivity, low surface damage and low capital investment. In addition, defect-selective wet etching is widely employed to characterize the density and distribution of defects in wide bandgap semiconductors [7]. As a non-centrosymmetric crystal, wurtzite AlN possesses different properties on the two polar surfaces, as well as on the non-polar surfaces. In order to capture these differences, two different etchants were used in the present studies: (1) aqueous potassium hydroxide (KOH) solution as a general etchant and (2) potassium hydroxide/sodium hydroxide (KOH/NaOH) eutectic alloy as a defect-selective etchant. Schowalter et al. found that the aluminum polar surface remains intact after etching in both phosphoric/sulphuric acid mixture and KOH aqueous solution, while the nitrogen polar surface develops pyramidal structures after etching [8]. Zhuang et al. [9] further confirmed polarity-selective e
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