Hae in Yang1,Danial Coyle1,Michelle Wurch1,Prachi Yadav1,Michael Valentin1,Mahesh Neupane1,Kortney Almeida1,Ludwig Bartels1
University of California, Riverside1
Hae in Yang1,Danial Coyle1,Michelle Wurch1,Prachi Yadav1,Michael Valentin1,Mahesh Neupane1,Kortney Almeida1,Ludwig Bartels1
University of California, Riverside1
The existence of bandgaps in transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS<sub>2</sub>) offers an attractive possibility of using these two-dimensional layered materials in various device applications. Gallium nitride (GaN), initially introduced into semiconductor technology as a wide-band-gap material able to provide blue light-emitting diodes, has found a number of additional applications for power conversion. MoS<sub>2</sub> and GaN are almost lattice matched at a mismatch of only ~0.8%, leading to an interest in the epitaxial relationship. Integration of MoS<sub>2</sub> with wide band gap GaN could provide an avenue toward high-performance devices. The interface for MoS2 on GaN plays an important role in determining the properties of heterojunctions.<br/><br/>Here, we demonstrate the optimization of the MoS<sub>2</sub>/GaN interface and MoS<sub>2</sub> growth epitaxially on GaN. We use a high vacuum CVD growth technique. First, we clean the GaN substrate with a broad range of sputter fluence and anneal it with ammonia gas to generate well-defined surface termination. Next, we grow a controlled layer number of MoS<sub>2</sub> by exposing hot metallic molybdenum to H<sub>2</sub>S gas at 650 °C. All process was performed without breaking the vacuum. We grow single-layer as well as 4-layer, which are monitored by colorimetric measurements of the reflection of the filaments from the sample, allowing us to end the deposition process at the desired number of integer layers.<br/><br/>We use low-energy electron diffraction (LEED) and DFT simulation to demonstrate epitaxial MoS<sub>2</sub> grown on GaN and interface quality. We observed 22 patterns resulting from a doubling of the periodicity, which attests to a clean and well-ordered termination of the GaN substrate. X-ray photoelectron spectroscopy (XPS) is used to evaluate the cleanliness of GaN and interpret transport measurements. Atomic force microscopy (AFM) was analyzed to characterize surfaces after MoS<sub>2</sub> growth. We fabricate MoS<sub>2</sub>-GaN diode to test the impact of different interface preparation on electrical properties. We find a high-performance behavior achieved by optimizing in situ interface preparation: low turn-on voltage (~0.3 V) Schottky-diode behavior, on/off current ratio of ~10<sup>5</sup>. This highlights the technological potential of devices and gives us a guideline for interface preparation for further research.