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Jang, Soohwan,Lee, Sohyun,Baik, Kwang Hyeon Institute of Pure and Applied Physics 2017 Japanese Journal of Applied Physics Vol. No.
<P>We studied the anisotropy of electrical conductivity in surface-roughened semipolar (11 (2) over bar2) GaN (s-GaN) films. Highly crystalline s-GaN films were obtained using asymmetric lateral epitaxy on oxide-patterned m-plane sapphire substrates. The in-plane structural anisotropy of the s-GaN films was confirmed by anisotropic peak broadening in X-ray rocking curves (XRC) with the in-beam orientations. The XRC full-width at half maximum (FWHM) values were measured to be 454 and 615 arcsec along the [(11 (2) over bar3)](GaN) and [1 (1) over bar 00](GaN) directions, respectively. The s-GaN surface was roughened using photo-chemical etching, and the electrical anisotropy was then investigated as a function of azimuth angles with the transmission line method. The Ohmic contact properties on the roughened s-GaN surface did not depend on the azimuth angle or annealing temperatures between 750 and 950 degrees C. The sheet resistances parallel to the [1 (1) over bar 00](GaN) direction on roughened s-GaN were found to be approximately half of the resistance parallel to the [11 (2) over bar3](GaN) direction, showing that anisotropic electrical conductivity is maintained for surface-roughened s-GaN due to charged carrier scattering induced by basal-plane stacking faults. (C) 2017 The Japan Society of Applied Physics</P>
Terahertz Dielectric Response of Nonpolar <i>a</i>-plane GaN Films
Jang, Soohwan,Jung, Sunwoo,Park, Jaehun,Kim, Seungheun,Baik, Kwang Hyeon The Electrochemical Society 2017 ECS transactions Vol.77 No.6
<P>We report on the dielectric properties of non-polar <I>a</I>-plane (11-20) GaN (<I>a</I>-GaN) films in the terahertz (THz) frequency region, using THz time domain spectroscopy. We measured the transmitted THz time-domain waveforms by rotating the phi angle for 6μm-thick <I>a</I>-GaN films, which were lifted off from sapphire substrates. THz transmittance of <I>a</I>-GaN films clearly showed the angular dependence on azimuth angle, possibly due to the birefringence and optical phonon modes in <I>a</I>-GaN crystal. It is believed that the discrepancy in the refractive index results from the difference of the transverse-optical and the longitudinal optical phonon branches.</P>
Dielectric Response of Nonpolar a-plane (1120) ZnO Crystals in the Terahertz Frequency
Jang, Soohwan,Park, Jaehun,Kim, Seongheun,Baik, Kwang Hyeon American Scientific Publishers 2016 Journal of Nanoscience and Nanotechnology Vol.16 No.11
<P>We investigated the dielectric response of nonpolar a-plane (11 (2) over bar0) ZnO (a-ZnO) bulk crystals in the terahertz (THz) frequency using femto-second THz time-domain spectroscopy. The peak amplitude of the transmitted output pulse for nonpolar a-ZnO crystal decreased down to about 25% of c-plane ZnO (c-ZnO), showing strong absorptions in a-ZnO when compared with polar c-ZnO. While the polar c-ZnO did not show any azimuthal angle dependence, nonpolar a-ZnO crystals exhibited significant variation as a function of azimuthal angle, presumably due to the birefringence in a-ZnO crystal. We believe that the discrepancy in the refractive index originates from the difference of the transverse-optical and the longitudinal optical phonon splitting between the optical phonon branches.</P>
Hydrogen sensitive Schottky diode using semipolar ( 1 1 2 ¯ 2 ) AlGaN/GaN heterostructures
Jang, Soohwan,Son, Pyunghee,Kim, Jimin,Lee, Sung-Nam,Baik, Kwang Hyeon Elsevier 2016 Sensors and actuators. B, Chemical Vol.222 No.-
<P><B>Abstract</B></P> <P>In this work, we investigated the hydrogen sensing characteristics of Pt Schottky diodes using semipolar ( 1 1 2 ¯ 2 ) AlGaN/GaN structures. First, these diodes showed a large current change of 30mA at 1V upon the introduction of 4% hydrogen in nitrogen gas with an accompanying Schottky barrier reduction of 90meV at 25°C. Second, their hydrogen detection sensitivity peaked at the zero bias voltage, and slowly decreased with applied bias voltage. Third, they demonstrated stable and reproducible current changes with a reasonable linearity in response to H<SUB>2</SUB> concentrations from 0.5∼4% with a step of 0.5%. As such, Pt Schottky diodes on semipolar AlGaN/GaN structures hold great promise for highly-sensitive hydrogen sensors due to their surface polarity and atomic configuration.</P>
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>
Jang, Soohwan,Kim, Jimin,Baik, Kwang Hyeon The Electrochemical Society 2016 Journal of the Electrochemical Society Vol.163 No.8
<P>We report on the enhanced hydrogen sensing characteristics of surface-etched Pt Schottky diodes fabricated on semipolar (11 (2) over bar2) GaN films using photo-electrochemical wet etching. The surface-etched Pt Schottky diodes showed a rapid sensing response to 4% hydrogen, as well as a full recovery to their initial current level after removing the hydrogen from the ambient. They also demonstrated stable and reproducible current changes with a reasonable linearity in response to H-2 concentrations of 0.5 similar to 4% in increments of 0.5%. The hydrogen sensitivity of Pt Schottky diodes on semipolar (11 (2) over bar2) GaN could therefore be improved by incorporating surface etching on the Schottky contact area using KOH solutions. This rough surface is expected to improve hydrogen detection sensitivity due to the presence of more available adsorption sites, resulting in effective variations of the Schottky barrier height. (C) 2016 The Electrochemical Society. All rights reserved.</P>
Jang, Soohwan,Jung, Sunwoo,Kim, Jihyun,Ren, Fan,Pearton, Stephen J.,Baik, Kwang Hyeon The Electrochemical Society 2018 ECS journal of solid state science and technology Vol.7 No.7
<P>We investigated the hydrogen sensing characteristics of Pt Schottky diodes on ((2) over bar 01) and (010) beta-Ga2O3 bulk crystals. The Pt Schottky diodes on beta-Ga2O3 wafer exhibited the fast, reversible, and cyclic response upon hydrogen exposure. The maximum value of the relative current change of the ((2) over bar 01) Ga2O3 diode sensor was as high as 7.86 x 10(7) (%) at 0.8 V, which is slightly higher than that of the (010) Ga2O3 diode. The hydrogen responses of both beta-Ga2O3 diode sensors are believed to result from oxygen and gallium atomic configurations of Ga2O3 surfaces for hydrogen adsorption. The Pt Schottky diodes of Ga2O3 wafers did not show any clear response to other gases, such as N-2, CO, CO2, O-2, CH4, NO2, and NH3. Our finding suggests that the Pt Schottky diodes on beta-Ga2O3 hold great potential for the applications of hydrogen gas sensors with high sensitivity and selectivity. (C) The Author(s) 2018. Published by ECS.</P>