Molecular Beam Epitaxy of 2H-TaS$_2$ few-layers on GaN(0001)

TL;DR

This study demonstrates the epitaxial growth of 2H-TaS2 few-layers on GaN(0001) by molecular beam epitaxy and analyzes the structural and electronic interface properties in-situ. RHEED and LEED confirm van der Waals epitaxy with the overlayer unstrained from the first monolayer, while the lattice constant matches bulk TaS2 and domain size increases with growth temperature. XPS reveals electron transfer from n-GaN to TaS2 and a secondary S* component associated with sulfur in distinct coordination, whereas STEM shows GaN surface pits from thermal decomposition but no Ta–Ga intermixing. Low-temperature LEED detects no CDW signature down to 30 K, suggesting charge transfer or other factors suppress the CDW; overall GaN is a promising substrate for wafer-scale 2D/3D integration, with growth optimization under N2 to prevent decomposition and preserve interfacial properties.

Abstract

2H-TaS$_2$ few layers have been grown epitaxially onto GaN(0001). A high substrate growth temperature of 825$^{\circ}$C induces best structural properties of the overlayer, as revealed by in-situ electron diffraction (RHEED and LEED). The 2D-overlayer grows unstrained right after deposition of a monolayer. However, evidence of pits at the interface is provided by scanning transmission electron microscopy, most probably due to GaN thermal decomposition at the high growth temperature. In-situ x-ray photoemission spectroscopy shows core level shifts that are consistently related to electron transfer from the n-GaN(0001) to the 2H-TaS$_2$ epitaxial layer as well as the formation of a high concentration of nitrogen vacancies close to the interface. Further, no chemical reaction at the interface between the substrate and the grown TaS$_2$ overlayer is deduced from XPS, which corroborates the possibility of integration of 2D 2H-TaS$_2$ with an important 3D semiconducting material like GaN.

Figures (9)

• Figure 1: AFM topography measurements (1 µ m $\times$ 1 µ m) of (a) the GaN substrate, (b) after growth of 2H-TaS$_2$ (6 ML) at $T_g = 620$ ℃ and (c) after growth at $T_g=825$ ℃. The scale bar length is 100 nm. Corresponding LEED patterns measured at a beam energy of 100 eV are reported in the insets. The slight rotation of the LEED pattern in (c) is due to a rotation of the sample holder.
• Figure 2: (a) RHEED pattern along GaN [$11\bar{2}0$] at different stages of 2H-TaS$_2$ growth, showing the transition to a streaky pattern of the epitaxial layer. (b) Line profiles along the marked directions in (a) (see text). The intensity profiles have been normalized.
• Figure 3: XPS spectra of the Ta-4f (a) and S-2p (b) core levels for 2 ML and 6 ML 2H-TaS$_2$ grown on GaN (dots). Signal from the substrate is detected. The intensity of the spectra is normalized and shifted for clarity. The fitted components are shaded and the black line is the result of the best fit. The S-2p fit required two doublets (blue, short dash and red, long dash). The results of the fit are summarized in table \ref{['tab:1']}. Refer to the text for more details.
• Figure 4: (a) Cross section ADF-STEM image of an 18 ML thick 2H-TaS$_2$ layer grown on GaN. Below, EELS signals from the Ga L-edge and Ta M-edge after background subtraction are shown. (b) Higher magnification image with an intensity profile along the marked direction on the right.
• Figure 5: Evolution of XPS spectra measured in-situ on the bare GaN substrate and after deposition of 2H-TaS$_2$. (a) Ga-2p$_{3/2}$ and (b) Ga-3d. The fitted parameters are reported in table \ref{['tab:1']}. The background corrected intensity of the spectra is normalized and shifted for clarity.