Polarity control during molecular beam epitaxy growth of Mg-doped GaN

Abstract

Mg doping has been found in some situations to invert growth on Ga-face GaN to N-face. In this study, we clarified the role the Ga wetting layer plays in rf plasma molecular beam epitaxy of GaN when Mg doping, for [Mg] from ∼2×1019 to ∼1×1020 cm−3 corresponding to the useful, accessible range of hole concentrations of p∼1017–1018 cm−3. Structures were grown in the N-rich and Ga-rich growth regime for single Mg doping layers and for multilayer structures with a range of Mg concentrations. Samples were characterized in situ by reflection high-energy electron diffraction and ex situ by atomic force microscopy, transmission electron microscopy, convergent beam electron diffraction, and secondary ion mass spectroscopy. Growth on “dry” surfaces (without a Ga wetting layer) in the N-rich regime completely inverted to N-face upon exposure to Mg. No reinversion to Ga-face was detected for subsequent layers. Additionally, Mg was seen to serve as a surfactant during this N-rich growth, as has been reported by others. Growth initiated in the Ga-rich regime contained inversion domains that nucleated with the initiation of Mg doping. No new inversion domains were found as the Mg concentration was increased through the useful doping levels. Thus the Ga wetting layer was found to inhibit nucleation of N-face GaN, though a complete wetting layer took time to develop. Finally, by establishing a complete Ga wetting layer on the surface prior to growth, we confirmed this finding and demonstrated Mg-doped GaN completely free from inversion domains to a doping level of [Mg]∼2×1020.

Metal modulation epitaxy growth for extremely high hole concentrations above 1019cm−3 in GaN

“…15 Unfortunately, however, the formation energy of nitrogen vacancies is lowest in this condition 17 and as mentioned above, these defects will compensate free holes.…”

“…18 Attempts to solve this problem with higher Mg fluxes cause the local polarity to invert from Ga-polar to N-polar, 3,15 and results in reduced Mg incorporation. Furthermore, the formation energy of nitrogen vacancies decreases under slightly Ga-rich conditions and as a result more nitrogen vacancies are available to compensate the free holes.…”

Dependence of local electronic structure in p-type GaN on crystal polarity and presence of inversion domain boundaries

“…Specifically, the presence of a Ga wetting layer on the surface during growth is found to lead to Ga-face polarity, while the absence of such a wetting layer results in N-face growth. 7 By adjusting the Ga:N flux ratio during MBE growth, the presence and spatial distribution of a Ga wetting layer on the surface can be controlled, and hence fully Ga-face growth, fully N-face growth, or an intermediate situation with regions of each polarity can be achieved. For these studies, three samples were employed, grown with Ga fluxes of 3.3 ϫ 10 −7 , 3.1ϫ 10 −7 , and 2.9ϫ 10 −7 Torr.…”

“…5 Finally, the crystal polarity during growth of GaN by molecular-beam epitaxy ͑MBE͒ has been observed to invert from Ga face to N face upon exposure to Mg; 6 the observed polarity inversion has been found to be dependent on the Ga:N flux ratio, with decreased Ga flux leading to inversion of polarity from Ga face to N face. 7 Given the difficulties associated with efficient acceptor doping of GaN, an understanding of the relationships among active acceptor concentration, the existence and spatial distribution of defects, crystal polarity, and other factors is critical. In this article we describe spatially resolved studies of local electronic structure in p-type GaN as a function of crystal polarity and in the vicinity of inversion domain boundaries.…”

Efficiency optimization of p-type doping in GaN:Mg layers grown by molecular-beam epitaxy

“…A surfactant effect [5] is observed for coverages up to a 1 4 monolayer (ML), while a 1 4 to 1 2 ML coverage yields p-type doping, yet limited [6]. A coverage beyond 1 ML generates structural defects and a very rough growth front [7], that may cause a polarity inversion (Npolarity) [7][8][9][10], leading to a sharp reduction of the Mg incorporation [11].…”