Secondary ion mass spectroscopy (SIMS) and capacitance-voltage measurements were combined to thoroughly study Mg doping in GaN layers grown by metal organic vapor phase epitaxy. First we found that the Mg steady-state incorporation regime occurs for a surface coverage of 0.3 monolayer. Additionally SIMS indicates that H incorporates proportionally with Mg until a certain [Mg] where [H] saturates. After thermal activation, [H] while being much lower still scales with [Mg]. These results suggest that H combines with Mg to form two different types of Mg–H complexes: a metastable one leading to the Mg acceptor after annealing, the other one (dominating at high [Mg]) being stable and electrically inactive. The obtained results allowed us optimizing doping conditions for blue laser diodes.

Role of stable and metastable Mg-H complexes in p-type GaN for cw blue laser diodes

A hole injection layer (HIL) is designed in GaN-based light emitting diodes (LEDs) between multiple quantum wells and p-AlGaN electron blocking layer (EBL). Based on numerical simulation by apsys, the band diagram is adjusted by HIL, leading to the improved hole-injection efficiency. The designed HIL is a p-GaN buffer layer grown at low temperature (LT_pGaN) on last quantum barrier before p-AlGaN EBL. The output power of the fabricated GaN-based LED device with LT_pGaN HIL is enhanced by 128% at 100 A/cm2, while the efficiency droop is reduced by 33% compared to the conventional LED.

Enhanced performance of GaN based light-emitting diodes with a low temperature p-GaN hole injection layer

We have studied the properties of Mg-doped GaN epilayers grown by molecular beam epitaxy (MBE) with ammonia as nitrogen source. GaN p-n homojunctions has been developed to determine the optoelectronic characteristics of the junctions as a function of the p-type GaN growth conditions. It is shown that the electrical characteristics strongly depend on the Mg flux and the growth temperature. As a result, the junction characteristics have been drastically improved and state of the art MBE-grown p-type layers have been obtained: the hole concentration, the mobility, and the resistivity are 1×1018cm−3, 9cm2∕Vs, and 0.75Ωcm, respectively. These characteristics lead to an increase of the homojunction light emitting diode (LED) optical output power by two orders of magnitude. To further assess the quality of these MBE-grown p-type layers, we have prepared a hybrid LED which consists of an InGaN∕GaN quantum well active structure grown by metal organic vapor phase epitaxy followed by a p-type region grown by MBE. An...

High doping level in Mg-doped GaN layers grown at low temperature

Two and three-dimensional (2D/3D) hybrid materials have the potential to advance communication and sensing technologies by enabling new or improved device functionality. To date, most 2D/3D hybrid devices utilize mechanical exfoliation or post-synthesis transfer, which can be fundamentally different from directly synthesized layers that are compatible with large scale industrial needs. Therefore, understanding the process/property relationship of synthetic heterostructures is priority for industrially relevant material architectures. Here we demonstrate the scalable synthesis of molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) via metal organic chemical vapor deposition (MOCVD) on gallium nitride (GaN), and elucidate the structure, chemistry, and vertical transport properties of the 2D/3D hybrid. We find that the 2D layer thickness and transition metal dichalcogenide (TMD) choice plays an important role in the transport properties of the hybrid structure, where monolayer TMDs exhibit direct tunneling through the layer, while transport in few layer TMDs on GaN is dominated by p–n diode behavior and varies with the 2D/3D hybrid structure. Kelvin probe force microscopy (KPFM), low energy electron microscopy (LEEM) and X-ray photoelectron spectroscopy (XPS) reveal a strong intrinsic dipole and charge transfer between n-MoS2 and p-GaN, leading to a degraded interface and high p-type leakage current. Finally, we demonstrate integration of heterogeneous 2D layer stacks of MoS2/WSe2 on GaN with atomically sharp interface. Monolayer MoS2/WSe2/n-GaN stacks lead to near Ohmic transport due to the tunneling and non-degenerated doping, while few layer stacking is Schottky barrier dominated.

Large scale 2D/3D hybrids based on gallium nitride and transition metal dichalcogenides

Wide range doping control and defect characterization of GaN layers with various Mg concentrations

We have investigated the thermal activation process of Mg dopant in Mg-doped GaN by secondary ion mass spectrometry (SIMS) and Hall measurement with the van der Pauw configuration. We have found a systematic relationship between the amount of the hydrogen dissociated from the Mg-doped GaN layer by thermal annealing and the electronic conductive properties of the layer, i.e., Hall mobility, Hall carrier density and resistivity of the layer. The hydrogen in the Mg-doped GaN layer has been classified into at least two different modes. The first mode involves relatively larger activation energy of the dissociation process from the Mg-doped GaN layer, and the dissociation energy from the Mg-doped GaN to the atmosphere is 0.8–1.5 eV. The second mode has the dissociation energy of 0.2–0.5 eV. The former mode may be associate with the hydrogen passivation of the Mg dopant in GaN and the latter one may be closely related to the passivation of the electrical compensation mechanism for the positive hole carriers in the Mg-doped GaN.

https://iopscience.iop.org/article/10.1143/JJAP.43.23/pdf

Takae Takehara

Papers

Hydrogen Dissociation from Mg-doped GaN