Direct observation of frozen gallium gas on wurtzite gallium nitride (000\underline {1}) using low-temperature scanning tunneling microscopy

Gallium nitride layers are ordinarily grown under gallium-rich growth conditions by molecular beam epitaxy (MBE) to obtain the highest material quality. In 1997, Smith \textit{et al.} reported the family of reconstructions existing on the growth surface at room temperature, the highest-order being the c(6x12).[1] Additional gallium deposition does not lead to new reconstructions. Instead, excess gallium atoms are presumed to exist in a 2-dimensional gas state. Using a custom-built MBE/low-temperature (4.2 K) STM system, we have imaged this gallium gas for the first time by freezing out the motion. The frozen-out gallium atoms are visualized as asymmetric `L-shaped' features, with left-handed and right-handed L's scattered randomly across the surface. Interestingly, on any given atomic terrace we observe a 4x greater probability of left-handed versus right-handed L's (or vice versa), which inverts across bilayer-height steps. The cause of this asymmetry is explored by zooming in with atomic resolution, revealing two inequivalent adsorption sites. [1] A. R. Smith \textit{et al.} Phys. Rev. Lett., \textbf{79}, 3934 (1997).