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In this study, electrical properties of four metals (W, Mo, Au, Ni) as Schottky contacts on n-type (100)-oriented β-Ga2O3 substrates grown by the Czochralski method are reported. The Schottky barrier heights for each metal contact were calculated from I-V and/or C-V measurements. Two methods were used to cross check the Schottky barrier heights (B) and ideality factors (n) calculated from I-V measurements. The Schottky barrier height values calculated from C-V and I-V measurements showed excellent agreement with each other and increased with an increase in the metal work functions. Some anomalous behavior of Au contacts, which is similar to behavior reported on (010)-oriented β-Ga2O3, is also described.

The adhesion force between metal-based antibacterial coatings and polymer substrate decides the effectiveness and durability of antibacterial performance. In this study, the silicone rubber substrate was modified by molecular bridge, i.e., polyvinylpyrrolidone, chitosan and (3-mercaptopropyl) trimethoxysilane, to provide “anchors” which captured Ag+ during electroless plating, and Ag coatings were resultantly produced. Different molecular bridge offered different adhesion. The spread plate counting, fluorescent staining assay and bacterial growth kinetics test showed that the antibacterial and anti-biofilm performance against Escherichia coli and Staphylococcus aureus was directly related to coating–substrate adhesion strength. A relative weaker adhesion resulted in rapid bactericidal and bacteria growth inhibition effect, and vice versa. The inductively coupled plasma atomic emission spectrometry indicated that relative weak adhesion provided fast Ag+ release, and strong adhesion gave lower Ag+ cumulative concentration, which may be attributed to different bonding strength. The coatings displayed sustained-release kinetics with the maximum cumulative Ag+ concentration of only 0.45 mg/L after 45 days of immersion in PBS solution, which was far below the human cell toxicity concentration. Nevertheless, all tested Ag coatings exhibited effective and long-lasting antibacterial properties. The tailored adhesion and consequent different antibacterial effect provided a choice for clinical applications.

The Schottky barriers of Ti, Mo, Co, Ni, Pd, and Au on (100) β-Ga2O3 substrates were analyzed using a combination of current-voltage (J-V), capacitance-voltage (C-V), and current-voltage-temperature (J-V-T) measurements. Near-ideal, average ideality factors for Ti, Mo, Co, and Ni were 1.05–1.15, whereas higher ideality factors (∼1.3) were observed for Pd and Au contacts. Barrier heights ranging from 0.60 to 1.20 eV were calculated from J-V measurements for the metals with low ideality factors. C-V measurements of all Schottky metals were conducted and yielded average barrier heights ranging from 0.78 to 1.98 eV. J-V-T measurements of Ti and Co diodes yielded barrier heights of 0.81 and 1.35 eV, respectively. The results reveal a strong positive correlation between the calculated Schottky barrier heights and the metal work functions: the index of interface behavior, S = 0.70, 0.97, and 0.81 for J-V, C-V, and J-V-T data, respectively.

Due to its large bandgap (~4.6-4.8 eV), wide range of n-type doping, and single-crystal wafer availability, β-Ga2O3 holds promise for high-efficiency power devices. R&D of β-Ga2O3 has been conducted in Japan for more than a decade; however, funded research programs on this semiconductor material began in the U.S. only five years ago, highlighting its early stage of development. Device development of a semiconductor technology requires the ability to produce suitable ohmic and rectifying (Schottky) contacts. Whereas ohmic and Schottky contacts have been demonstrated on n-type β-Ga2O3, certain characteristics of contacts to β-Ga2O3 differ from contacts to other wide bandgap semiconductors. For example, we found that morphology was problem with many ohmic contact metals, and it was concluded that metal work function is not a dominant predictive factor for forming an ohmic contact to β-Ga2O3 [1]. Furthermore, thermal instability of the standard ohmic contact metallization (Ti/Au) is a concern for operation of devices at elevated temperatures. Schottky contacts to β-Ga2O3 have shown dependence on the orientation, growth method, and choice of metal. For example, on (-201) β-Ga2O3 Schottky barrier heights calculated from I-V measurements were typically between 0.9 and 1.3 eV and displayed little dependence on the metal work function [2]. Our preliminary measurements of selected metals on (100) β-Ga2O3 substrates yielded lower ideality factors and a correlation between barrier height and metal workfunction. The results for Schottky contacts on the (100) surface show some similarities to those previously reported on the (010) surface. In this presentation specific examples of ohmic and Schottky contact metallizations to β-Ga2O3 will provide a platform for more detailed discussion. [1] Y. Yao, R.F. Davis, and L.M. Porter, J. Electron. Mater. 46(4), 2053-2060 (2016). [2] Y. Yao, R. Gangireddy, J. Kim, K. Das, R.F. Davis, and L.M. Porter, J. Vac. Sci. Technol. B 35(3), 03D113.1-7 (2017).