Wet etching studies of aluminum nitride bulk crystals and their sublimation growth by microwaves

The research described in this dissertation was motivated by the need of bulk AlN single crystals to improve the quality of group III nitride based devices. In this dissertation, first the evolution of semiconductors is reviewed. Second, historical reviews and recent advances of AlN crystal growth are presented. Third, the experimental setup and characterization methods are described. Finally, four papers regarding wet etching and sublimation growth of AlN are attached: (1) AlN bulk crystal growth using microwaves as heat source; (2) a review of wet etching of GaN and AlN; (3) anisotropic etching technique for identifying AlN crystal polarities; and (4) defect-selective etching to reveal dislocations in Al-polar crystals.; Single crystalline AIN platelets up to 2 x 3 mm2 and needles 3 mm long were successfully grown by directly heating the source materials with microwaves. The grown crystals were characterized by optical microscopy, photoluminescence (PL), Raman spectroscopy, synchrotron white beam X-ray topography (SWBXT), and defect-selective etching. The grown crystals have good structural quality, with etch pit density as low as 103 cm -2. A peak positioned at 5.5 eV in PL spectra was attributed to magnesium impurities, presumably originating from the source materials.; The wet etchings of GaN and AlN by electrochemical etching and defect-selective etching are reviewed. The mechanism of each etching process and etching conditions resulting in highly anisotropic, dopant-type/bandgap selective, defect-selective, and smooth surfaces are discussed. The applications of wet etching techniques in device fabrication and crystal characterization are also reviewed.; The anisotropic etching technique for AlN crystals was successfully developed. Aqueous KOH solution did not attack Al-polar surfaces, but produced hexagonal hillocks on N-polar surfaces. The etching results suggested that freely nucleated AlN crystals predominately have the Al polarity facing the source materials. The hillocks on N-polar after etching were bonded by {lcub}11¯01{rcub} planes.; Defect-selective etching in eutectic alloys resulted in hexagonal etch pits and hillocks on Al-polar and N-polar surfaces, respectively. The reliability of estimating dislocation density in Al-polar crystals by defect-selective chemical etching was confirmed by SWBXT. From the defect density point of view, self-seeded AlN crystals have a better quality than those grown on Si-face 6H-SiC (0001) substrates.