Figures

Figure 1

5 µm x 5 µm AFM image of the surface morphology of a (0001) oriented GaN film (Ga-face). The MBE film, grown at 750°C, is about 0.7 µm thick, and was grown on top of a 1 µm thick MOCVD-grown GaN film on sapphire. The grey-scale of the image ranges from 0 (black) to 24 nm (white).

Figure 2

Cross-sectional TEM image of a GaN film, consisting of 160 nm of MBE-grown material (as estimated from stylus profilometry measurements) on a 1 µm thick MOCVD-grown layer. An interface between the MBE and MOCVD layers can be very faintly seen, located 190 nm below the surface; generally this interface between the layers appears to be epitaxial and continuous. The apparent large surface pit seen on the left hand side of the image is due to the specimen thinning used in TEM sample preparation.

Figure 3

RHEED patterns of GaN(0001) surface with electron beam along the (110) direction. (a) Ga flux of 7.5x1014 cm-2s-1 (effusion cell temperature of 1095°C), (b) initial pattern with Ga flux of 6.3x1014 cm-2s-1 (effusion cell temperature of 1085°C), (c) same flux as (b) but after waiting several minutes, and (d) same surface as (a) but after reducing Ga flux to zero.

Figure 4

Critical gallium fluxes corresponding to the transition between rough and smooth surface morphology as seen by RHEED, as a function of sample temperature. The line is drawn as a guide to the eye.

Figure 5

STM image of surface nitrided at 600°C, showing small ordered areas of 2x2 reconstruction. Sample bias = -2.0 V; tunnel current = 0.075 nA; gray scale range = 0.3 nm.

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