Gallium Selenide: A Novel Optoelectronic 2D Material

N. Briggs, J. Robinson
Pennsylvania State University,
United States

Keywords: 2D materials, optoelectronics, synthesis

Summary:

Since its industrial emergence in the 1960s, silicon has dominated the electronic market and enabled the creation of revolutionary computing technology. Now, nearly 60 years later, we must turn to new materials and engineering to expand upon silicon technologies. One promising class of materials is that of the group III chalcogenides (GIIICs) – a group of naturally layered materials that exhibit high electron mobility, wide band gaps, and promise in photodetectors and memory technologies[1], [2]. Gallium selenide (GaSe) – one member of the GIIIC family – exhibits a bulk band gap of 2eV[3] which increases to roughly 3eV[2] for monolayers of GaSe. This wide band gap, coupled with inherent photoconducting properties makes GaSe an interesting candidate for optoelectronic and silicon technologies. Here we show that mono and few-layer GaSe can be synthesized on SiO2/Si and epitaxial graphene substrates through a powder vaporization method. Current work aims to investigate the relationship between substrate choice and GaSe nucleation, morphology, and properties. To synthesize mono and few-layer GaSe, substrates are placed face-down over a GaSe powder on a quartz boat. This setup is then heated to 700°C in an argon atmosphere, resulting in vaporization of the GaSe powder and nucleation of GaSe on the overlying substrate. Atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the resulting layers are monolayers of stoichiometric GaSe. However, the morphology and deposition density of the GaSe layers has been found to vary largely with substrate. When grown on SiO2/Si, monolayers of GaSe exhibit a more rotund, triangular shape compared with bi and trilayers that have nucleated on preexisting monolayers (Figure 1). This indicates an interaction between the oxide layer and GaSe monolayer which could effect the crystallinity of the grown monolayers. When grown on epitaxial graphene, GaSe domains with sharper edges are observed, however, nucleation is often limited to scratches or defects in the graphene. To further investigate the effect of defects on GaSe nucleation, epitaxial graphene substrates have been etched with oxygen plasma for 15s – 60s. GaSe growth on these substrates has shown that increased etch times (and as a result, increasingly defective graphene) leads to a drastic increase in GaSe nucleation, which occurs primarily at and along graphene step edges (Figure 2). Current studies are being carried out to investigate the electronic properties of GaSe a function of layer thickness and graphene quality via Kelvin probe force microscopy. Future work will investigate the synthesis of GaSe via metal organic chemical vapor deposition (MOCVD) through the 2D Crystal Consortium at Penn State. These studies will utilize in-situ mass spectrometry and spectroscopic ellipsometry to monitor GaSe properties in real time, throughout the growth process. Additionally, indium selenide, another GIIIC, will be synthesized and investigated through the described powder vaporization process, as well as through MOCVD with the Penn State 2D Crystal Consortium.