Materials Research Lecture - Caltech Young Investigator Lecture Series
The spectrum of 2D and layered materials "beyond graphene" has been continually expanding. The realization of wide bandgap (Eg) 2D materials "beyond hexagonal boron nitride (hBN)", however, has been limited. Along similar lines to initial theoretical discovery and subsequent experimental synthesis of "beyond graphene" 2D materials (i.e. silicene and borophene), theoretical studies have suggested that indium nitride (InN), gallium nitride (GaN), and aluminum nitride (AlN) take on a 2D graphitic structure with a thickness tunable energy Eg (~0.7-7.0 eV) due to quantum confinement. Despite the extensive computational discovery of 2D materials, the experimental synthesis of wide Eg 2D nitrides "beyond hBN" on technologically relevant substrates still remains elusive. We have developed a novel growth scheme, known as Migration Enhanced Encapsulated Growth (MEEG)1, which utilizes the mechanism of intercalation via defects in graphene to stabilize wide Eg 2D materials that are not layered in bulk crystals. We demonstrate for the first time that 2D GaN not only can be stabilized, but also exhibits unique structural, optical and electrical properties from that of bulk material.
Here we elucidate the mechanism of 2D nitride formation and discuss the ability of the interface of quasi-free standing epitaxial graphene (QFEG) in providing sufficient thermodynamic stabilization of the (direct Eg ~5 eV) 2D buckled structure of GaN (R3m space group symmetry). In the case of 2D GaN, a layer of gallium intercalates between the hydrogenated QFEG and the SiC substrate. The intercalated bilayer of gallium is converted to a quintuple monolayer of 2D GaN via nitrogen intercalation from decomposed NH3. Our density functional theory (DFT) calculations suggest that the atomic structure in 2D nitrides considerably impacts the stability and bandstructure. We verify the atomic structure by directly resolving the nitrogen and gallium atomic columns in 2D GaN using aberration corrected scanning TEM (STEM) in annular bright field (ABF) mode with supported ABF-STEM simulations. Our DFT calculations predict an energy Eg for 2D GaN in the range of 4.79-4.89 eV which correlates well with experimental results from UV-visible reflectance, absorption coefficient and low loss EELS measurements. Vertical transport measurements suggest 2D GaN acts as a Schottky barrier between graphene and SiC. High resolution x-ray photoelectron spectroscopy demonstrates that 2D GaN is stable in air for at least 24 hours after removal of the graphene cap.
We expand our novel growth scheme to the formation of other 2D nitrides, such as 2D InN, broadening the range of accessible Eg energies of 2D materials well into the deep UV. Recognizing the impact of 2D nitrides, it can be expected that the addition of 2D GaN will enable new avenues for scientific exploration and electronic/optoelectronic device development.
1. Al Balushi, Z.Y. et al. Nature Materials 15, 1166–1171 (2016) doi:10.1038/nmat4742