Gallium adlayer adsorption and desorption studies with real-time analysis by spectroscopic ellipsometry and RHEED on A-, M-, and C-plane GaN grown by PAMBE

The absence of spontaneous and piezoelectric polarization in nonpolar-oriented III nitrides can improve emission efficiency in light-emitting devices due to the absence of electrostatic fields. It is therefore of interest to study the growth processes necessary to achieve smooth morphology for nonpolar-oriented films. In this paper, we compare and contrast gallium adsorption and desorption on C-, M-, and A-plane GaN epilayers by plasma-assisted molecular beam epitaxy (PAMBE) coupled with in-situ spectroscopic ellipsometry (SE). Adsorption/desorption of Ga under vacuum and under active nitrogen flux were investigated. Previous studies on C- and M-plane materials have suggested that optimum growth conditions with respect to surface morphology are those giving rise to a Ga bilayer and trilayer, respectively. Under vacuum, the stable Ga coverage was 0.48–0.50 nm for the C-, M-, and A-planes. While under N plasma, C-plane GaN maintained similar Ga coverage, while a thicker Ga-stable coverage of 0.7 nm was determined for M-plane. This is in contrast to A-plane GaN, which did not have a consistent Ga adlayer under plasma. We observed that Ga-rich growth of A-plane GaN leads to rough surface morphologies and N-rich conditions are necessary for smooth two-dimensional growth.