Orientation disparity in GaN/graphene/$m$-sapphire: control-based re-examination of thru-hole epitaxy

TL;DR

This study challenges the idea that crystallographic orientation alone can distinguish remote epitaxy from thru-hole epitaxy in GaN on $m$-plane sapphire. By comparing four templates under identical GaN growth within a single process window, including bare, graphene-grown, anneal-only, and GO-reduced surfaces, the authors demonstrate that nuclei form on exposed sapphire in all cases, and that the resulting GaN orientation is governed primarily by substrate pre-treatment rather than mask continuity. The results show orientation can vary within the thru-hole regime, providing a concrete counter-example to attributing orientation disparities solely to RE. The work emphasizes the need to explicitly decouple mask presence from substrate surface conditioning when using orientation as evidence for growth mechanisms, with implications for interpreting 2D-masked epitaxy data.

Abstract

The crystallographic orientation of films grown on 2D-masked substrates is often used to infer the pathway among remote, van der Waals, and thru-hole (pinhole-seeded) epitaxy. However, attribution of a specific growth mechanism based on orientation can be ambiguous unless mask continuity and substrate pre-treatment are evaluated within a single process window. We compare GaN grown under identical conditions on four m-plane sapphire templates: (i) bare, (ii) "graphene-grown" (high-temperature Ar/H2 with CH4 on), (iii) "anneal-only" (high-temperature Ar/H2 with CH4 off), and (iv) graphene oxide spin-coated and reduced on pristine sapphire. GaN selects (103) on graphene-grown and anneal-only m-plane sapphire, selects (100) on bare m-plane sapphire, and is predominantly (100) with a minority (103) on graphene oxide spin-coated and reduced/pristine m-plane sapphire. High-resolution TEM shows that, on partly graphene-covered samples, nucleation occurs on exposed sapphire (thru-hole), not on graphene, providing mechanism evidence independent of orientation. Within this window, the substrate surface state set by high-temperature Ar/H2 pre-treatment (rather than mask continuity) primarily governs orientation, while open-area effects can play a secondary role. Thus, preferred orientation alone may not determine the growth mechanism; mask continuity and substrate pre-treatment must be explicitly controlled when using orientation as evidence for mechanism assignment.

Figures (7)

• Figure 1: (a) AFM topography of graphene directly grown on an $m$-plane sapphire substrate. Shown below is a height profile of graphene/$m$-plane sapphire along the blue line. (b) Raman spectroscopy of graphene grown on $m$-plane sapphire substrate S2.
• Figure 2: SEM images of GaN domains grown on (a) a bare $m$-plane sapphire or (b) a partly graphene-covered $m$-plane sapphire substrate. The characteristic shapes of GaN domains on substrate S1 and S2 are associated with (100)- and (103)-oriented domains, respectively. Schematic diagrams of (c) (103)-oriented and (d) (100)-oriented GaN domains. (e) Raman spectroscopy of graphene after GaN was grown on a partly graphene-covered $m$-plane sapphire substrate (S2).
• Figure 3: (a) The reciprocal space map of X-ray diffraction of GaN grown on a bare $m$-plane sapphire substrate, S1. (b)$\sim$(c) Magnified reciprocal space maps clearly show that (100) Bragg peak at $q_z=2.270$ Å$^{-1}$ and (200) Bragg peak at $q_z=4.547$ Å$^{-1}$ are the only Bragg peaks of GaN observed when grown on substrate S1. Of course, sapphire (300) Bragg peak at $q_z=4.570$ Å$^{-1}$ is also seen.
• Figure 4: The reciprocal space map of X-ray diffraction of GaN grown on substrate S2. (b)$\sim$(c) Magnified reciprocal space maps clearly show that (103) Bragg peak at $q_z=4.270$ Å$^{-1}$ is the only Bragg peak of GaN observed when grown on substrate S2 while sapphire (300) Bragg peak at $q_z=4.570$ Å$^{-1}$ is of course observed. GaN (103) Bragg peak is split because of the slight tilt of twins in opposite directions.Jue-APL-104-092110 Note that neither (100) nor (200) GaN Bragg peak is observed when grown on substrate S2.
• Figure 5: High-resolution cross-sectional TEM image of the interfacial region between GaN and $m$-plane sapphire for the case when grown on (a) substrate S1 and (d) substrate S2, respectively. FFT of the GaN and sapphire regions marked by blue and red squares are shown in (b) and (c) for case I and (e) and (f) for case II, respectively.