Modifying the Barrier Height of β-Ga2O3 Schottky Contacts Using Covalently Grafted Organic Layers

Beta gallium oxide (beta-Ga2O3) is a transparent semiconductor that is attracting significant interest as researchers attempt to harness its ultrawide bandgap (similar to 4.8 eV at 300 K) and high breakdown strength (similar to 8 MV/cm) for applications in high-efficiency power electronics, deep ultraviolet sensing, and transparent electronics. We have recently shown (Carroll et al., ACS Appl. Electron. Mater. 2021, 3, 5608-5620) that the covalent modification of beta-Ga2O3 surfaces with organic layers, in particular, aryl and phosphonic acid molecules, can produce significant changes in near-surface band bending and electron density. However, it is not known whether these changes will persist during electronic device fabrication and influence the performance of the resulting devices. In this work, we compare the barrier heights of Pd Schottky contacts (SCs) fabricated on the (001), (010), and ((2) over bar 01) surfaces of beta-Ga2O3 following the electrografting of 4-nitrophenyl (NP) layers and the spontaneous grafting of octadecylphosphonic acid (ODPA) molecules. We show that NP modification consistently produces significant increases in the barrier height of Pd:beta-Ga2O3 SCs of between 0.3 and 0.8 eV on each of the commonly used crystallographic surfaces of beta-Ga2O3. In contrast, only a small decrease in barrier height of similar to 0.1 eV on the (001) beta-Ga2O3 surface was observed following ODPA modification, which may be due to the much thinner nature of the ODPA layers compared to those produced by NP modification. However, we have conclusively shown that the surface modification of beta-Ga2O3 with electrografted NP layers is a viable strategy for significantly increasing the barrier height of beta-Ga2O3 SCs, a finding that may have significant consequences for the fabrication of high-performance beta-Ga2O3 electronic devices.