Interface Properties of MoS2 van der Waals Heterojunctions with GaN

Abstract

The combination of the unique physical properties of molybdenum disulfide (MoS2) with those of gallium nitride (GaN) and related group-III nitride semiconductors have recently attracted increasing scientific interest for the realization of innovative electronic and optoelectronic devices. A deep understanding of MoS2/GaN interface properties represents the key to properly tailor the electronic and optical behavior of devices based on this heterostructure. In this study, monolayer (1L) MoS2 was grown on GaN-on-sapphire substrates by chemical vapor deposition (CVD) at 700 °C. The structural, chemical, vibrational, and light emission properties of the MoS2/GaN heterostructure were investigated in detail by the combination of microscopic/spectroscopic techniques and ab initio calculations. XPS analyses on as-grown samples showed the formation of stoichiometric MoS2. According to micro-Raman spectroscopy, monolayer MoS2 domains on GaN exhibit an average n-type doping of (0.11 ± 0.12) × 1013 cm−2 and a small tensile strain (ε ≈ 0.25%), whereas an intense light emission at 1.87 eV was revealed by PL analyses. Furthermore, a gap at the interface was shown by cross-sectional TEM analysis, confirming the van der Waals (vdW) bond between MoS2 and GaN. Finally, density functional theory (DFT) calculations of the heterostructure were carried out, considering three different configurations of the interface, i.e., (i) an ideal Ga-terminated GaN surface, (ii) the passivation of Ga surface by a monolayer of oxygen (O), and (iii) the presence of an ultrathin Ga2O3 layer. This latter model predicts the formation of a vdW interface and a strong n-type doping of MoS2, in closer agreement with the experimental observations.

Tailoring MoS2 domains size, doping, and light emission by the sulfurization temperature of ultra-thin MoOx films on sapphire

“…17 This method proved to be effective to obtain 1L MoS 2 flakes, typically with triangular shape and variable size (from $1 to $100 lm), on different substrates, including SiO 2 , 17 sapphire, 18 SiC, 19,20 and GaN. 21,22 However, the difficulty to simultaneously control S and MoO 3 vapor fluxes on the target substrate 23,24 makes it challenging to achieve uniform MoS 2 coverage on wafer scale, whereas variations in the size, shape, and thickness of the flakes are typically observed in different wafer regions. 17,25 In this context, metal organic chemical vapor deposition (MOCVD) emerged in the last few years as the most advanced approach for wafer scale growth of MoS 2 (and other TMDs), thanks to the excellent control of the process parameters, such as the gas precursors fluxes within the reaction chamber.…”