Morphology, polarity, and lateral molecular beam epitaxy growth of GaN on sapphire

Abstract

Gallium nitride was grown on sapphire ͑0001͒ substrates by radio frequency plasma assisted molecular beam epitaxy. The surface morphology was characterized during growth by reflection high energy electron diffraction, and ex situ by scanning electron microscopy ͑SEM͒, atomic force microscopy ͑AFM͒ and x-ray diffraction. It is found that surface morphological features are linked to domains of specific wurtzite crystal polarity, ͑0001͒Ga face or (0001 )N face, for Ga-rich growth. For growth on AlN buffer layers, we commonly observe films which consist of largely ͑0001͒Ga polarity material, as confirmed by selective etch tests, with a varying coverage of (0001 )N-face inversion domains threading along the growth direction. For growth near stoichiometric conditions, the growth rate of the N-face domains is slightly lower than that for the Ga-face matrix, which results in the formation of pits with inversion domains at their centers. For samples grown by first depositing GaN under N-rich conditions, followed by growth under Ga-rich conditions, a different morphology is obtained, exhibiting large hexagonal flat terraces observable by SEM and AFM. The apparent grain size of these films is increased substantially over films grown using a single step approach. The cross sectional SEM images of the two-step films show a network of voids and columns at the interface between the N-rich and the Ga-rich layers, above which micron-scale islands form and coalesce via lateral growth. Lateral growth may result in reduced defect density and improved crystal quality. The asymmetric x-ray peak (112 4) width is reduced to approximately 280 arcsec in the two-stage GaN films.

Evaluation of GaN and In0.2Ga0.8N Semiconductors as Potentiometric Anion Selective Electrodes

“…Furthermore, similar surface features were found by Piquette [23] in SEM images of mixed polarity GaN surfaces. Mixed polarity surfaces can be formed due to the difference in growth rates between the Ga-faces and N-faces in the GaN crystal [23].…”

Inversion domains in AlN grown on (0001) sapphire

“…"House" shaped domains have been reported for GaN on sapphire films grown by MOCVD [6,8]; they have also been reported for some films grown by hydride vapour phase epitaxy (HVPE) [8] and have been shown to have Npolarity [13]. Because GaN having N-polarity grows slower than GaN having Ga-polarity [14,15], "house" shaped domains are overgrown by the matrix material and are localized near the GaN/sapphire interface.…”

Optical study of gallium and nitrogen polarity layers of GaN grown on sapphire

“…Different types of defects have been reported, which may introduce electronic traps or be optically and electronically inert [2]. Among the posible defects, inversion domain boundaries (IDB's) are particularly interesting [3], since they facilitate the coexistence of crystalline regions of inverse polarity [4][5][6] (Ga-polarity when grown along the (0001) direction or N-polarity for the (000 1 1) direction) [7]. The origin of the inversion domains stems from the different stable ordered atomic arrangements, [5] or configurations, [8] of Ga and N atoms, which depend upon the substrate, buffer layer and growth conditions.…”