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Jang, Soohwan,Jung, Sunwoo,Beers, Kimberly,Yang, Jiancheng,Ren, Fan,Kuramata, A.,Pearton, S.J.,Baik, Kwang Hyeon Elsevier 2018 Journal of alloys and compounds Vol.731 No.-
<P><B>Abstract</B></P> <P>We report on the effect of β-Ga<SUB>2</SUB>O<SUB>3</SUB> crystal orientation on wet etching and Ohmic contact formation. The photochemical etching rate in KOH solutions of ( 2 ¯ 01 ) oriented, n-type bulk single crystals grown by the edge-defined film-fed growth method is ∼3–4 times higher than for the (010) planes. The activation energy for etching was 0.498 eV and 0.424 eV for ( 2 ¯ 01 ) and (010) orientations, respectively, suggesting the etching is reaction-limited with the same rate-limiting step. Ti (200 Å)/Au (1500 Å) metallization deposited on the two different orientations and annealed at 450 °C showed Ohmic current-voltage (<I>I-V</I>) behavior for ( 2 ¯ 01 ) but rectifying characteristics for (010). For (010) Ga<SUB>2</SUB>O<SUB>3</SUB>, there exists 2 types of surfaces having Ga and O atomic densities of 0.58 and 0.87 × 10<SUP>15</SUP> cm<SUP>−2</SUP>, respectively. By contrast, for ( 2 ¯ 01 ) Ga<SUB>2</SUB>O<SUB>3</SUB> surfaces, there exist 2 types of surface, with each type terminated with only Ga or O. If the surface is terminated with O, the dangling bond densities of O are 1.78 and 2.68 × 10<SUP>15</SUP> cm<SUP>−2</SUP>, respectively. We found that ( 2 ¯ 01 ) –oriented Ga<SUB>2</SUB>O<SUB>3</SUB> is etched at higher rates and is easier to form Ohmic contacts than (010) Ga<SUB>2</SUB>O<SUB>3</SUB>. The higher density of oxygen dangling bonds on the ( 2 ¯ 01 ) plane correlates with the faster etch rates and pronounced Ohmic behavior from deposited metals.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We report on the effect of β-Ga<SUB>2</SUB>O<SUB>3</SUB> crystal orientation on wet etching and Ohmic contact formation. </LI> <LI> The photochemical etching rate in KOH solutions of ( 2 ¯ 01 ) oriented, n-type bulk single crystals grown by the edge-defined film-fed growth method is ∼3–4 times higher than for the (010) planes. </LI> <LI> Ti (200 Å)/Au (1500 Å) metallization deposited on the two different orientations and annealed at 450 °C showed Ohmic current-voltage (<I>I-V</I>) behavior for ( 2 ¯ 01 ) but rectifying characteristics for (010). </LI> <LI> We found that ( 2 ¯ 01 ) –oriented Ga<SUB>2</SUB>O<SUB>3</SUB> is etched at higher rates and is easier to form Ohmic contacts to than (010) Ga<SUB>2</SUB>O<SUB>3</SUB>. The higher density of oxygen dangling bonds on the ( 2 ¯ 01 ) plane correlates with the faster etch rates and pronounced Ohmic behavior from deposited metals. </LI> </UL> </P>
High Breakdown Voltage (−201) <tex> $\beta $</tex> -Ga2O3 Schottky Rectifiers
Yang, Jiancheng,Ahn, Shihyun,Ren, F.,Pearton, S. J.,Jang, Soohwan,Kuramata, A. IEEE 2017 IEEE electron device letters Vol.38 No.7
<P>beta-Ga2O3 Schottky barrier diodes were fabricated in a vertical geometry structure consisting of Ni/Au rectifying contacts without edge termination on Si-doped epitaxial layers (10 mu m, n similar to 4x10(15) cm(-3)) on Sn-doped bulk Ga2O3 substrates with full-area Ti/Au back Ohmic contacts. The reverse breakdown voltage, V-BR, was a function of rectifying contact area, ranging from 1600 V at 3.1x10(-6) cm(2) (20-mu mdiameter) to similar to 250V at 2.2x10(-3) cm(-2) (0.53-mm diameter). The current density near breakdown was not strongly dependent on contact circumference but did scale with contact area, indicating that the bulk current contributionwas dominant. The lowest ON-state resistance, R-on, was 1.6 m Omega.cm(2) for the largest diode and 25 m Omega.cm(2) for the 1600-V rectifier, leading to a Baliga figure-of-merit (V-BR(2)/R-on) for the latter of approximately 102.4 MW.cm(-2). The ON-OFF ratio was measured at a forward voltage of 1.3 V and ranged from 3x10(7) to 2.5x10(6) for reverse biases from -5 to -40 V and showed only a small dependence on temperature in the range from 25 degrees C to 100 degrees C.</P>
Band offsets in ITO/Ga<sub>2</sub>O<sub>3</sub> heterostructures
Carey IV, Patrick H.,Ren IV, F.,Hays IV, David C.,Gila IV, B.P,Pearton IV, S.J.,Jang IV, Soohwan,Kuramata IV, Akito Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.422 No.-
<P><B>Abstract</B></P> <P>The valence band offsets in rf-sputtered Indium Tin Oxide (ITO)/single crystal β-Ga<SUB>2</SUB>O<SUB>3</SUB> (ITO/Ga<SUB>2</SUB>O<SUB>3</SUB>) heterostructures were measured with X-Ray Photoelectron Spectroscopy using the Kraut method. The bandgaps of the component materials in the heterostructure were determined by Reflection Electron Energy Loss Spectroscopy as 4.6eV for Ga<SUB>2</SUB>O<SUB>3</SUB> and 3.5eV for ITO. The valence band offset was determined to be −0.78±0.30eV, while the conduction band offset was determined to be −0.32±0.13eV. The ITO/Ga<SUB>2</SUB>O<SUB>3</SUB> system has a nested gap (type I) alignment. The use of a thin layer of ITO between a metal and the Ga<SUB>2</SUB>O<SUB>3</SUB> is an attractive approach for reducing contact resistance on Ga<SUB>2</SUB>O<SUB>3</SUB>-based power electronic devices and solar-blind photodetectors.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We measured the band offsets of ITO on Ga<SUB>2</SUB>O<SUB>3</SUB>, a promising wide bandgap semiconductor for solar-blind detectors and power electronics. </LI> <LI> The band alignment is nested (type I), with a valence band offset of −0.78eV </LI> <LI> The results show the use of ITO interlayers will be beneficial to improving Ohmic contacts on Ga<SUB>2</SUB>O<SUB>3</SUB>. </LI> </UL> </P>