Role of Metallic Adlayer in Limiting Ge Incorporation into GaN

Abstract

Atomically thin metal adlayers are used as surfactants in semiconductor crystal growth. The role of the adlayer in the incorporation of dopants in GaN is completely unexplored, probably because n-type doping of GaN with Si is relatively straightforward and can be scaled up with available Si atomic flux in a wide range of dopant concentrations. However, a surprisingly different behavior of the Ge dopant is observed, and the presence of atomically thin gallium or an indium layer dramatically affects Ge incorporation, hindering the fabrication of GaN:Ge structures with abrupt doping profiles. Here, we show an experimental study presenting a striking improvement in sharpness of the Ge doping profile obtained for indium as compared to the gallium surfactant layer during GaN-plasma-assisted molecular beam epitaxy. We show that the atomically thin indium surfactant layer promotes the incorporation of Ge in contrast to the gallium surfactant layer, which promotes segregation of Ge to the surface and Ge crystallite formation. Understanding the role of the surfactant is essential to control GaN doping and to obtain extremely high n-type doped III-nitride layers using Ge, because doping levels >1020 cm−3 are not easily available with Si.

Nanostars in Highly Si-Doped GaN

“…Ge doping is advantageous in this regard as it allows to reach much higher concentrations than with Si, but it is not easy to avoid macro step formation of thick MOVPE grown GaN:Ge layers . In the case of MBE, smooth GaN:Ge layers are reported even for a doping level as high as 5 × 10 20 cm –3 . The concentration of Ge also scales linearly with the supplied atomic flux up to the point of surface degradation, and formation of Ge x N y precipitates occurs .…”

“…When GaN:Ge is grown with gallium as a surfactant, there is a tendency for Ge to stay in the gallium surface adlayer. Then, a memory effect for Ge doping is observed in the GaN layer grown on top even though the Ge flux is no longer supplied from the effusion cell . Abrupt doping profiles are achieved only for InGaN:Ge.…”

“…The concentration of Ge also scales linearly with the supplied atomic flux up to the point of surface degradation, and formation of Ge x N y precipitates occurs . However, for MBE-grown Ge-doped layers that are grown under metal-rich conditions, it is important to note that the surfactant type, whether it is gallium or indium, dramatically impacts the doping interface abruptness . When GaN:Ge is grown with gallium as a surfactant, there is a tendency for Ge to stay in the gallium surface adlayer.…”

Harnessing III-Nitride Built-In Field in Multi-Quantum Well LEDs

“…Each TJ consists of a 15 nm In 0.17 Ga 0.83 N layer with Ge-and Mgdoped halves. Doping levels are 2 × 10 20 cm −3 for Ge and 1 × 10 20 cm −3 for Mg. 26 It was estimated based on separate calibration samples investigated by secondary ion mass spectrometry. The TJ allows termination of all the studied LED structures with the same n-type layer that provides low resistivity and ensures uniform current spreading.…”

“…Each TJ consists of a 15 nm In 0.17 Ga 0.83 N layer with Ge- and Mg-doped halves. Doping levels are 2 × 10 20 cm –3 for Ge and 1 × 10 20 cm –3 for Mg . It was estimated based on separate calibration samples investigated by secondary ion mass spectrometry.…”