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GaN heterostructures
Sensors and Actuators B 222 (2016) 43–47
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hydrogen sensitive Schottky diode using semipolar (1 1 2¯ 2) AlGaN/GaN heterostructures Soohwan Jang a , Pyunghee Son b , Jimin Kim a , Sung-Nam Lee c , Kwang Hyeon Baik b,∗ a b c
Department of Chemical Engineering, Dankook University, Yongin, 448-701, Republic of Korea School of Materials Science and Engineering, Hongik University, Sejong 339-701, Republic of Korea Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung, Gyeonggi 429-793, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 January 2015 Received in revised form 14 July 2015 Accepted 12 August 2015 Available online 15 August 2015 Keywords: Hydrogen Gas sensor GaN Semipolar crystal
a b s t r a c t 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 30 mA at 1 V upon the introduction of 4% hydrogen in nitrogen gas with an accompanying Schottky barrier reduction of 90 meV 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 H2 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. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, hydrogen has received considerable attention due to its potential for alternative energy sources. This has led to extensive studies on the various aspects of commercially viable hydrogen production, particularly in hydrogen-fueled vehicles, aircraft, and fuel cells [1,2]. Unfortunately, such applications have been severely limited due to hydrogen gas-based explosive hazards in the products being developed [3,4]. Also, since hydrogen gas generated from a wide range of industries can be harmful to the environment, the development of highly-sensitive hydrogen gas sensors able to detect even minute amounts of hydrogen has become critically urgent. Thus, quick responsiveness, reliability, improved sensitivity, selectivity, and reliable reproducibility are essential to hydrogen detection if efficiency, affordability and safety are to be achieved in the industry. Presently, one of the most widely used detection apparatuses is the electrochemical gas sensor which relies on an electrochemical reaction with the electrolyte in the device. Electrochemical gas sensors are advantageous for their reliability and specific gas sensitivity. By contrast, semiconductor-type gas sensors utilize a change in the electrical conductivity of the device when gas is in contact with the semiconductor surface. This, in turn, makes them highly compatible with smart screen and wireless monitoring
∗ Corresponding author. Tel.: +82 1052431539; fax: +82 448668493. E-mail address: [email protected] (K.H. Baik). http://dx.doi.org/10.1016/j.snb.2015.08.056 0925-4005/© 2015 Elsevier B.V. All rights reserved.
technologies. For these reasons, gas sensors with catalytic Pd and Pt electrodes had been widely researched based on Schottky diodes, field effect transistors, and metal-oxide-semiconductor (MOS) structures [5–7]. The gallium nitride (GaN)-based materials system is also suitable for use in semiconductor-type gas sensors involving hydrogen sensing. It enables high temperature operations due to low intrinsic carrier concentration, as well as reliable gas detection because of its mechanical and chemical robustness. A wide variety of hydrogen gas sensors based on GaN Schottky diodes, MOS diodes, and AlGaN/GaN high electron mobility transistors (HEMTs) have been developed [8–15]. Song et al. reported Schottky diodes on AlGaN/GaN heterostructures with Pt catalytic metal capable of operating at high temperatures up to 800 ◦ C [11]. According to a recent report by Kim et al., the sensitivity of AlGaN/GaN diodes could be enhanced using Pt nano-networks with a high surfaceto-volume ratio [16,17]. Hydrogen gas sensors using nonpolar or semipolar GaN crystal planes are also of great interest because each crystal plane has its own surface atomic arrangement, which shows different reactivity to hydrogen [18,19]. Wang et al. reported that the c-plane N-polar 0 0 0 1¯ GaN Schottky diodes exhibited much higher sensitivity for hydrogen detection than conventional Gapolar (0 0 0 1) ones, which is consistent with the previous density functional theory noting a much higher affinity of hydrogen to nitrogen in GaN surfaces [20–22]. In this work, we investigated the hydrogen sensing characteristics of Pt Schottky diodes on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures. We find that Pt Schottky diodes on semipolar AlGaN/GaN structures hold great promise for
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Fig. 1. (a) A schematic cross-sectional view and (b) a top-view image under optical microscope of semipolar (1 1 2¯ 2) AlGaN/GaN HEMT sensor with Pt Schottky contact, respectively.
highly-sensitive hydrogen sensors due to a favorable surface atomic configuration.
Fig. 2. (a) –2 high-resolution XRD scan for a semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure grown on m-plane sapphire substrates; (b) the on-axis rocking curve of the AlGaN layer with the full width at half maximum of 1050 arcsec.
2. Experiment 2.4 m-thick semipolar (1 1 2¯ 2) GaN epitaxial films were grown on m-plane sapphire substrates using a high temperature, twostep growth method with an 11 × 2 in. AIX 2400 G3 metal–organic chemical vapor deposition system. Trimethylgallium and ammonia were used as gallium and nitrogen sources, respectively. Prior to growing the seed GaN layer, the sapphire substrate was thermally annealed at 1030 ◦ C in H2 and NH3 ambient to remove surface contamination. Semipolar (1 1 2¯ 2) GaN films were grown at 1030 ◦ C with a V/III ratio of 1165. The HEMT structure consisted of a 2.4 m un-doped GaN layer and a 30-nm-thick Al0.32 Ga0.68 N layer with a doping concentration of 4 × 1018 cm−3 . The film surface orientation and crystalline quality of the semipolar (1 1 2¯ 2) GaN films were characterized using the high resolution X-ray diffraction (XRD) method. The Ohmic metal stack of Ti/Al/Pt/Au was deposited by ebeam evaporation, patterned by lift-off, and annealed at 750 ◦ C for 45 s under a N2 ambient. A 200 nm thick SiNx passivation layer was formed for diode isolation using a plasma enhanced chemical vapor deposition. The windows for the active sensing area opening were achieved through buffered oxide etchant etching. A 10 nm Pt film was evaporated on the diode’s Schottky contact area with the diameter of 100 m by e-beam evaporation, followed by Ti/Au contact pads for probing and wire bonding. Fig. 1(a) and (b) shows a schematic illustration of device cross-sectional view and a top-view image under optical microscope of semipolar AlGaN/GaN HEMT sensor with Pt Schottky contact, respectively. Current–voltage characteristics for the Pt Schottky diode sensors on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures exposed to the flammable limit of hydrogen at 1 atm of 4% hydrogen balanced with
nitrogen were measured at room temperature in a gas test chamber using an Agilent 4155C semiconductor parameter analyzer. 3. Results and discussion Fig. 2(a) shows the –2 high-resolution XRD scan for a semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure grown on m-plane sapphire substrates. The AlGaN thickness was estimated to be 30 nm with an Al content of 0.32 for this particular growth condition. Two sharp diffraction peaks at 2 = 68.05◦ and 69.1◦ corresponded to (1 1 2¯ 2) GaN and AlGaN, respectively. As can be seen in Fig. 1(b), the full width at half maximum of the on-axis rocking curve was measured to be ∼1050 arcsec, indicating a good crystalline and epitaxial quality for the semipolar AlGaN layer formed on the semipolar (1 1 2¯ 2) GaN film. Fig. 3(a) shows current–voltage (I–V) characteristics for the Pt Schottky diode on the semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure before and after exposure to 4% H2 in N2 at 25 ◦ C. The Pt Schottky diode exhibited considerable current changes in the sweeping bias range, reaching a change in forward current of more than 30 mA at 1 V. The Schottky behavior gradually shifted to Ohmic-like property with the introduction of hydrogen. This reaction suggests that the H2 molecules dissociated into atoms on the catalytic Pt film, and formed H-induced dipolar layers, thus leading to a decrease in the effective Schottky barrier height [8–11]. As described in the introduction, it is of special interest here that the nitrogen atoms on the GaN surface along the specific crystal orientations displayed strong affinities for the hydrogen atoms, as reported in the previous literature [18–22]. The authors also reported a large response to hydrogen for the Pt Schottky diode on
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Fig. 4. Hydrogen detection sensitivity of the Pt Schottky diodes as a function of bias voltage.
where IN2 and IH2 are the diode currents measured under nitrogen and 4% hydrogen in nitrogen, respectively. Fig. 4 shows the hydrogen sensitivity of the Pt Schottky diode on the semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure. The maximum sensitivity for the semipolar HEMT diode was 37 at the zero bias voltage. This value slowly decreased, however, with the applied bias voltage. Liu
Fig. 3. (a) I–V characteristics of Pt Schottky diodes on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures before and after exposure to 4% H2 in N2 and (b) recovery of Schottky barrier height in the semipolar (1 1 2¯ 2) diode after exposure to 4% H2 in N2 before switching back to an N2 ambient.
the semipolar (1 1 2¯ 2) GaN film, most likely due to its favorable surface atomic configuration [23]. Whereas the surface of conventional c-plane GaN epilayer is usually terminated with only Ga atoms, semipolar (1 1 2¯ 2) GaN surface has a large density of neighboring nitrogen atoms having dangling bonds exposed beneath the Ga atoms on the (1 1 2¯ 2) GaN surface, which are prone to respond strongly to hydrogen and result in a reduction of the Schottky barrier height and large current changes. Based on the forward I–V characteristics over the low bias range, the barrier height for the Pt Schottky diode was extracted using the thermionic emission model. Fig. 3(b) shows the reduction in Schottky barrier height upon 4% hydrogen exposure and its subsequent recovery in pure nitrogen ambient. The initial reduction of the Schottky barrier height was 90 meV, resulting in a forward current increase. The Pt Schottky diode on the semipolar HEMT structure showed a recovery in barrier height within 5 min at 25 ◦ C, as well as a full recovery within 15 min when switched back to pure nitrogen ambient. It is important to note that barrier height recovery usually requires a high temperature desorption process, which induces immediate desorption of hydrogen due to decreased partial pressure. Hydrogen detection sensitivity (S) is usually represented by the relative current change to hydrogen exposure as a percentage value. S is defined as S=
IH2 − IN2 IN2
,
Fig. 5. (a) The on–off electrical responses with time when exposed to different hydrogen flow rate from 0.5 ∼ 4% with a step of 0.5% and (b) a linear relationship between the responses and H2 concentration. Data fitting resulted in the following equation for responses to hydrogen: R = 10.09[C] + 88.79.
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et al. [24] recently examined bias voltage dependency for the sensitivity of Schottky-type gas sensors. The analytical method based on the exponential parameter ˛(V) = [d(ln I)/d(ln V)] from forward I–V characteristics can provide the useful parameters such as barrier height change, maximum sensitivity, and bias voltage. In this case, the barrier height change of 90 meV, which was calculated by the y-intercept of the linear region in the semi-logarithm scale [d(ln S)/dV], was also in good agreement with the experimental value presented in Fig. 3(b). Fig. 5(a) shows the electrical response of the forward current at a fixed bias of 1 V for the Pt Schottky diode on the semipolar HEMT structure as a function of H2 concentration. The forward current increased with a reasonable linearity when increasing the concentration of H2 from 0.5 ∼ 4% with a step of 0.5%. After switching off the hydrogen-containing ambient at 25 ◦ C, the forward current decayed exponentially back to its initial value. Fig. 5(b) shows a linear relationship between the responses and H2 concentration, which was calculated from Fig. 5(a). The hydrogen sensors also showed stable repeatability in current changes and the ability to cycle this current in response to repeated introductions of hydrogen into the ambient. Very recently, Chen et al. reported 9.3% hydrogen response to 1% hydrogen for c-plane polar AlGaN/GaN heterostructure field effect transistor with electroless-plated gate electrode [25]. In the case of semipolar AlGaN/GaN heterostructure diode, aforementioned 3700% of the response to 4% hydrogen was obtained. The hydrogen sensitivity is expected to be significantly increased when the quality of semipolar AlGaN/GaN epitaxial layers would be more improved. Semipolar AlGaN/GaN HEMT sensor can be also integrated in monolithic amplifier circuits of electronic devices such as high power and high speed devices [25,26]. Thus, it is demonstrated herein that a Pt Schottky diode on a semipolar AlGaN/GaN HEMT structure is capable of effectively sensing hydrogen. 4. Conclusion We examined the hydrogen sensing characteristics of Pt Schottky diodes using semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures. The 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 H2 concentrations from 0.5 ∼ 4% with a step of 0.5%. The hydrogen sensitivity of Pt Schottky diodes on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures is the result of highly dense uncovered nitrogen atoms bearing a high affinity for hydrogen neighboring the Ga-terminated AlGaN (1 1 2¯ 2) surface. The hydrogen sensitivity of the Pt Schottky diodes peaked at the zero bias voltage, and slowly decreased with applied bias voltage. These results show that Pt Schottky diodes on semipolar AlGaN/GaN structures have great potential for hydrogen detection. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2014R1A1A4A01008877 and 2012R1A1B4002649). This work was also supported by 2015 Hongik University Research Fund. References [1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [2] P. Agnolucci, Economics and market prospects of portable fuel cells, Int. J. Hydrogen Energy 32 (2007) 4319–4328.
[3] G.R. Astbury, S.J. Hawksworth, Spontaneous ignition of hydrogen leaks: a review of postulated mechanisms, Int. J. Hydrogen Energy 32 (2007) 2178–2185. [4] K.L. Cashdollar, I.A. Zlochower, G.M. Green, R.A. Thomas, M. Hertzberg, Flammability of methane, propane, and hydrogen gases, J. Loss Prev. Process Ind. 13 (2000) 327–340. [5] M. Eriksson, L.-G. Ekedahl, Hydrogen adsorption states at the Pd-SiO2 interface and simulation of the response of a Pd metal-oxide-semiconductor hydrogen sensor, J. Appl. Phys. 83 (1998) 3947. [6] I. Lundstrom, H. Sundren, F. Winquist, M. Eriksson, C. Krantz-Rulcker, A. Lloyd-Spetz, Twenty-five years of field effect gas sensor research in Linköping, Sens. Actuators B 121 (2007) 247–262. [7] M. Eickhoff, J. Schalwig, G. Steinhoff, O. Weidmann, L. Gorgens, R. Neuberger, M. Hermann, B. Baur, G. Muller, O. Ambacher, M. Stutzmann, Electronics and sensors based on pyroelectric AlGaN/GaN heterostructures, Phys. Status Solidi C 6 (2003) 1908–1918. [8] J. Kim, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, Reversible barrier height changes in hydrogen-sensitive Pd/GaN and Pt/GaN diodes, Appl. Phys. Lett. 82 (2003) 739. [9] B.S. Kang, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, AlGaN/GaN-based metal-oxide-semiconductor diode-based hydrogen gas sensor, Appl. Phys. Lett. 84 (2004) 1123. [10] B.S. Kang, S. Kim, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, Comparison of MOS and Schottky W/Pt-GaN diodes for hydrogen detection, Sens. Actuators B 104 (2005) 232–236. [11] J. Song, W. Lu, J.S. Flynn, G.R. Brandes, Pt-AlGaN/GaN Schottky diodes operated at 800 ◦ C for hydrogen sensing, Appl. Phys. Lett. 87 (2005) 133501. [12] H. Wang, T.J. Anderson, F. Ren, C. Li, Z. Low, J. Lin, B.P. Gila, S.J. Pearton, A. Osinsky, A. Dabiran, Robust detection of hydrogen using differential AlGaN/GaN high electronmobility transistor sensing diodes, Appl. Phys. Lett. 89 (2006) 242111. [13] S.-T. Hung, C.-J. Chang, C.-H. Hsu, B.H. Chu, C.F. Lo, C.-C. Hsu, S.J. Pearton, M.R. Holzworth, P.G. Whiting, N.G. Rudawski, K.S. Jones, A. Dabiran, P. Chow, F. Ren, SnO2 functionalized AlGaN/GaN high electron mobility transistor for hydrogen sensing applications, Int. J. Hydrogen Energy 37 (2012) 13783–13788. [14] C.-S. Hsu, H.-I. Chen, C.-F. Chang, T.-Y. Chen, C.-C. Huang, P.-C. Chou, W.-C. Liu, On the hydrogen sensing characteristics of a Pd/AlGaN/GaN heterostructure field-effect transistor (HFET), Sens. Actuators B 165 (2012) 19–23. [15] T. Anderson, F. Ren, S.J. Pearton, B.S. Kang, H.-T. Wang, C.-Y. Chang, J. Lin, Advances in hydrogen, carbon dioxide, and hydrocarbon gas sensor technology using GaN and ZnO based devices, Sensors 9 (2009) 4669–4694. [16] H. Kim, W. Lim, J.-H. Lee, S.J. Pearton, F. Ren, S. Jang, Highly sensitive AlGaN/GaN diode-based hydrogen sensors using platinum nanonetworks, Sens. Actuators B 164 (2012) 64–68. [17] H. Kim, S. Jang, AlGaN/GaN HEMT based hydrogen sensor with platinum nanonetwork gate electrode, Curr. Appl. Phys. 13 (2013) 1746–1750. [18] J.E. Northrup, J. Neugebauer, Strong affinity of hydrogen for the GaN (000-1) surface: implications for molecular beam epitaxy and metalorganic chemical vapor deposition, Appl. Phys. Lett. 85 (2004) 3429. [19] M. Mayumi, F. Satoh, Y. Kumagai, K. Takemoto, A. Koukitu, Influence of polarity on surface reaction between GaN {0001} and hydrogen, Phys. Status Solidi B 228 (2001) 537–541. [20] Y.-L. Wang, F. Ren, U. Zhang, Q. Sun, C.D. Yerino, T.S. Ko, Y.S. Cho, I.H. Lee, J. Han, S.J. Pearton, Improved hydrogen detection sensitivity in N-polar GaN schottky diodes, Appl. Phys. Lett. 94 (2009) 212108. [21] Y.-L. Wang, F. Ren, W. Lim, S.J. Pearton, K.H. Baik, S.-M. Hwang, Y.G. Seo, S. Jang, Hydrogen sensing characteristics of non-polar a-plane GaN Schottky diodes, Curr. Appl. Phys. 10 (2010) 1029–1032. [22] Y.-L. Wang, B.H. Chu, C.Y. Chang, K.H. Chen, Y. Zhang, Q. Sun, J. Han, S.J. Pearton, F. Ren, Hydrogen sensing of N-polar and Ga-polar GaN Schottky diodes, Sens. Actuators B 142 (2009) 175–178. [23] K.H. Baik, H. Kim, S.-N. Lee, E. Lim, S.J. Pearton, F. Ren, S. Jang, Hydrogen sensing characteristics of semipolar (11-22) GaN Schottky diode, Appl. Phys. Lett. 104 (2014) 072103. [24] Y. Liu, J. Yu, W.M. Tang, P.T. Lai, On the voltage dependence of sensitivity for Schottky-type gas sensor, Appl. Phys. Lett. 105 (2014) 223503. [25] C-C. Chen, H.-I. Chen, I.-P. Liu, P.-C. Chou, J.-K. Liou, C.-C. Huang, W.-C. Liu, Hydrogen sensing characteristics of a Pt/AlGaN/GaN heterostructure field-effect transistor (HFET) prepared by sensitization, activation, and electroless plating (EP) approaches, Sens. Actuators B 212 (2015) 127–136. [26] C.W. Hung, K.W. Lin, R.C. Liu, Y.Y. Tsai, P.H. Lai, S.I. Fu, T.P. Chen, H.I. Chen, W.C. Liu, On the hydrogen sensing properties of a Pd/GaAs transistor-type gas sensor in a nitrogen ambiance, Sens. Actuators B 125 (2007) 22–29.
Biographies Soohwan Jang received the B.S. degree in Department of Chemical Engineering from Seoul National University, Seoul, Korea in 2003. He also received the Ph.D. degree in Department of Chemical Engineering from University of Florida, Gainesville, Florida in 2007. From 2007 to 2009, he worked in Samsung Electronics. He has been a professor at the Department of Chemical Engineering at Dankook University since 2009. His research interests include nitride and oxide based sensors, light emitting diodes, and integrated electronic devices.
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Pyunghee Son received the B.S. degree in School of Materials Science and Engineering at Hongik University, Sejong, Korea in 2013. He is currently pursuing Master’s Degree School of Materials Science and Engineering at Hongik University. His research has focused on the growth and the characterization of nonpolar and semipolar ZnO crystals.
he worked at Samsung Electro-Mechanics Co., where he developed high power GaN-based light-emitting diodes. Since 2009, he has been with Korea Polytechnic University as an associate professor. His current research interests include the growth and characterization of GaN and ZnO-based light-emitting diodes (LEDs) and hydrogen gas sensors, and III–V semiconductor-based electronic devices.
Jimin Kim received the B.S. degree in Department of Chemical Engineering at Dankook University in 2014. She is currently pursuing Master’s Degree in Chemical Engineering at Dankook University. Her research has focused on ZnO hydrothermal growth and GaN-based semiconductor gas sensors.
Kwang Hyeon Baik received the B.S. degree at Yonsei University, Seoul, Korea in 1998, and Ph.D. degree in Materials Science and Engineering from University of Florida, Gainesville, Florida in 2004. He worked at the Photonics Lab at Samsung Advanced Institute of Technology from 2005 to 2008, and also worked at the Korea Electronics Technology Institute from 2009 to 2011. Since 2012, he has been with Hongik University, Sejong, Korea as an assistant professor. His current research interests include the growth and characterization of GaN and ZnO-based light-emitting diodes and hydrogen gas sensors, and III–V semiconductor-based electronic devices.
Sung-Nam Lee received the B.S. in Metallurgical Engineering from SungKyunKwan University, M.S. and Ph.D. degrees in Material Science Engineering from Gwangju Institute of Science and Seoul National University, respectively. In 2000, he joined Samsung Advanced Institute of Technology, where he worked on GaN-based ultraviolet, blue and green laser diodes for blue-ray disc and laser display systems. And,
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hydrogen sensitive Schottky diode using semipolar (1 1 2¯ 2) AlGaN/GaN heterostructures Soohwan Jang a , Pyunghee Son b , Jimin Kim a , Sung-Nam Lee c , Kwang Hyeon Baik b,∗ a b c
Department of Chemical Engineering, Dankook University, Yongin, 448-701, Republic of Korea School of Materials Science and Engineering, Hongik University, Sejong 339-701, Republic of Korea Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung, Gyeonggi 429-793, Republic of Korea
a r t i c l e
i n f o
Article history: Received 25 January 2015 Received in revised form 14 July 2015 Accepted 12 August 2015 Available online 15 August 2015 Keywords: Hydrogen Gas sensor GaN Semipolar crystal
a b s t r a c t 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 30 mA at 1 V upon the introduction of 4% hydrogen in nitrogen gas with an accompanying Schottky barrier reduction of 90 meV 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 H2 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. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, hydrogen has received considerable attention due to its potential for alternative energy sources. This has led to extensive studies on the various aspects of commercially viable hydrogen production, particularly in hydrogen-fueled vehicles, aircraft, and fuel cells [1,2]. Unfortunately, such applications have been severely limited due to hydrogen gas-based explosive hazards in the products being developed [3,4]. Also, since hydrogen gas generated from a wide range of industries can be harmful to the environment, the development of highly-sensitive hydrogen gas sensors able to detect even minute amounts of hydrogen has become critically urgent. Thus, quick responsiveness, reliability, improved sensitivity, selectivity, and reliable reproducibility are essential to hydrogen detection if efficiency, affordability and safety are to be achieved in the industry. Presently, one of the most widely used detection apparatuses is the electrochemical gas sensor which relies on an electrochemical reaction with the electrolyte in the device. Electrochemical gas sensors are advantageous for their reliability and specific gas sensitivity. By contrast, semiconductor-type gas sensors utilize a change in the electrical conductivity of the device when gas is in contact with the semiconductor surface. This, in turn, makes them highly compatible with smart screen and wireless monitoring
∗ Corresponding author. Tel.: +82 1052431539; fax: +82 448668493. E-mail address: [email protected] (K.H. Baik). http://dx.doi.org/10.1016/j.snb.2015.08.056 0925-4005/© 2015 Elsevier B.V. All rights reserved.
technologies. For these reasons, gas sensors with catalytic Pd and Pt electrodes had been widely researched based on Schottky diodes, field effect transistors, and metal-oxide-semiconductor (MOS) structures [5–7]. The gallium nitride (GaN)-based materials system is also suitable for use in semiconductor-type gas sensors involving hydrogen sensing. It enables high temperature operations due to low intrinsic carrier concentration, as well as reliable gas detection because of its mechanical and chemical robustness. A wide variety of hydrogen gas sensors based on GaN Schottky diodes, MOS diodes, and AlGaN/GaN high electron mobility transistors (HEMTs) have been developed [8–15]. Song et al. reported Schottky diodes on AlGaN/GaN heterostructures with Pt catalytic metal capable of operating at high temperatures up to 800 ◦ C [11]. According to a recent report by Kim et al., the sensitivity of AlGaN/GaN diodes could be enhanced using Pt nano-networks with a high surfaceto-volume ratio [16,17]. Hydrogen gas sensors using nonpolar or semipolar GaN crystal planes are also of great interest because each crystal plane has its own surface atomic arrangement, which shows different reactivity to hydrogen [18,19]. Wang et al. reported that the c-plane N-polar 0 0 0 1¯ GaN Schottky diodes exhibited much higher sensitivity for hydrogen detection than conventional Gapolar (0 0 0 1) ones, which is consistent with the previous density functional theory noting a much higher affinity of hydrogen to nitrogen in GaN surfaces [20–22]. In this work, we investigated the hydrogen sensing characteristics of Pt Schottky diodes on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures. We find that Pt Schottky diodes on semipolar AlGaN/GaN structures hold great promise for
44
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Fig. 1. (a) A schematic cross-sectional view and (b) a top-view image under optical microscope of semipolar (1 1 2¯ 2) AlGaN/GaN HEMT sensor with Pt Schottky contact, respectively.
highly-sensitive hydrogen sensors due to a favorable surface atomic configuration.
Fig. 2. (a) –2 high-resolution XRD scan for a semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure grown on m-plane sapphire substrates; (b) the on-axis rocking curve of the AlGaN layer with the full width at half maximum of 1050 arcsec.
2. Experiment 2.4 m-thick semipolar (1 1 2¯ 2) GaN epitaxial films were grown on m-plane sapphire substrates using a high temperature, twostep growth method with an 11 × 2 in. AIX 2400 G3 metal–organic chemical vapor deposition system. Trimethylgallium and ammonia were used as gallium and nitrogen sources, respectively. Prior to growing the seed GaN layer, the sapphire substrate was thermally annealed at 1030 ◦ C in H2 and NH3 ambient to remove surface contamination. Semipolar (1 1 2¯ 2) GaN films were grown at 1030 ◦ C with a V/III ratio of 1165. The HEMT structure consisted of a 2.4 m un-doped GaN layer and a 30-nm-thick Al0.32 Ga0.68 N layer with a doping concentration of 4 × 1018 cm−3 . The film surface orientation and crystalline quality of the semipolar (1 1 2¯ 2) GaN films were characterized using the high resolution X-ray diffraction (XRD) method. The Ohmic metal stack of Ti/Al/Pt/Au was deposited by ebeam evaporation, patterned by lift-off, and annealed at 750 ◦ C for 45 s under a N2 ambient. A 200 nm thick SiNx passivation layer was formed for diode isolation using a plasma enhanced chemical vapor deposition. The windows for the active sensing area opening were achieved through buffered oxide etchant etching. A 10 nm Pt film was evaporated on the diode’s Schottky contact area with the diameter of 100 m by e-beam evaporation, followed by Ti/Au contact pads for probing and wire bonding. Fig. 1(a) and (b) shows a schematic illustration of device cross-sectional view and a top-view image under optical microscope of semipolar AlGaN/GaN HEMT sensor with Pt Schottky contact, respectively. Current–voltage characteristics for the Pt Schottky diode sensors on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures exposed to the flammable limit of hydrogen at 1 atm of 4% hydrogen balanced with
nitrogen were measured at room temperature in a gas test chamber using an Agilent 4155C semiconductor parameter analyzer. 3. Results and discussion Fig. 2(a) shows the –2 high-resolution XRD scan for a semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure grown on m-plane sapphire substrates. The AlGaN thickness was estimated to be 30 nm with an Al content of 0.32 for this particular growth condition. Two sharp diffraction peaks at 2 = 68.05◦ and 69.1◦ corresponded to (1 1 2¯ 2) GaN and AlGaN, respectively. As can be seen in Fig. 1(b), the full width at half maximum of the on-axis rocking curve was measured to be ∼1050 arcsec, indicating a good crystalline and epitaxial quality for the semipolar AlGaN layer formed on the semipolar (1 1 2¯ 2) GaN film. Fig. 3(a) shows current–voltage (I–V) characteristics for the Pt Schottky diode on the semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure before and after exposure to 4% H2 in N2 at 25 ◦ C. The Pt Schottky diode exhibited considerable current changes in the sweeping bias range, reaching a change in forward current of more than 30 mA at 1 V. The Schottky behavior gradually shifted to Ohmic-like property with the introduction of hydrogen. This reaction suggests that the H2 molecules dissociated into atoms on the catalytic Pt film, and formed H-induced dipolar layers, thus leading to a decrease in the effective Schottky barrier height [8–11]. As described in the introduction, it is of special interest here that the nitrogen atoms on the GaN surface along the specific crystal orientations displayed strong affinities for the hydrogen atoms, as reported in the previous literature [18–22]. The authors also reported a large response to hydrogen for the Pt Schottky diode on
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Fig. 4. Hydrogen detection sensitivity of the Pt Schottky diodes as a function of bias voltage.
where IN2 and IH2 are the diode currents measured under nitrogen and 4% hydrogen in nitrogen, respectively. Fig. 4 shows the hydrogen sensitivity of the Pt Schottky diode on the semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structure. The maximum sensitivity for the semipolar HEMT diode was 37 at the zero bias voltage. This value slowly decreased, however, with the applied bias voltage. Liu
Fig. 3. (a) I–V characteristics of Pt Schottky diodes on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures before and after exposure to 4% H2 in N2 and (b) recovery of Schottky barrier height in the semipolar (1 1 2¯ 2) diode after exposure to 4% H2 in N2 before switching back to an N2 ambient.
the semipolar (1 1 2¯ 2) GaN film, most likely due to its favorable surface atomic configuration [23]. Whereas the surface of conventional c-plane GaN epilayer is usually terminated with only Ga atoms, semipolar (1 1 2¯ 2) GaN surface has a large density of neighboring nitrogen atoms having dangling bonds exposed beneath the Ga atoms on the (1 1 2¯ 2) GaN surface, which are prone to respond strongly to hydrogen and result in a reduction of the Schottky barrier height and large current changes. Based on the forward I–V characteristics over the low bias range, the barrier height for the Pt Schottky diode was extracted using the thermionic emission model. Fig. 3(b) shows the reduction in Schottky barrier height upon 4% hydrogen exposure and its subsequent recovery in pure nitrogen ambient. The initial reduction of the Schottky barrier height was 90 meV, resulting in a forward current increase. The Pt Schottky diode on the semipolar HEMT structure showed a recovery in barrier height within 5 min at 25 ◦ C, as well as a full recovery within 15 min when switched back to pure nitrogen ambient. It is important to note that barrier height recovery usually requires a high temperature desorption process, which induces immediate desorption of hydrogen due to decreased partial pressure. Hydrogen detection sensitivity (S) is usually represented by the relative current change to hydrogen exposure as a percentage value. S is defined as S=
IH2 − IN2 IN2
,
Fig. 5. (a) The on–off electrical responses with time when exposed to different hydrogen flow rate from 0.5 ∼ 4% with a step of 0.5% and (b) a linear relationship between the responses and H2 concentration. Data fitting resulted in the following equation for responses to hydrogen: R = 10.09[C] + 88.79.
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et al. [24] recently examined bias voltage dependency for the sensitivity of Schottky-type gas sensors. The analytical method based on the exponential parameter ˛(V) = [d(ln I)/d(ln V)] from forward I–V characteristics can provide the useful parameters such as barrier height change, maximum sensitivity, and bias voltage. In this case, the barrier height change of 90 meV, which was calculated by the y-intercept of the linear region in the semi-logarithm scale [d(ln S)/dV], was also in good agreement with the experimental value presented in Fig. 3(b). Fig. 5(a) shows the electrical response of the forward current at a fixed bias of 1 V for the Pt Schottky diode on the semipolar HEMT structure as a function of H2 concentration. The forward current increased with a reasonable linearity when increasing the concentration of H2 from 0.5 ∼ 4% with a step of 0.5%. After switching off the hydrogen-containing ambient at 25 ◦ C, the forward current decayed exponentially back to its initial value. Fig. 5(b) shows a linear relationship between the responses and H2 concentration, which was calculated from Fig. 5(a). The hydrogen sensors also showed stable repeatability in current changes and the ability to cycle this current in response to repeated introductions of hydrogen into the ambient. Very recently, Chen et al. reported 9.3% hydrogen response to 1% hydrogen for c-plane polar AlGaN/GaN heterostructure field effect transistor with electroless-plated gate electrode [25]. In the case of semipolar AlGaN/GaN heterostructure diode, aforementioned 3700% of the response to 4% hydrogen was obtained. The hydrogen sensitivity is expected to be significantly increased when the quality of semipolar AlGaN/GaN epitaxial layers would be more improved. Semipolar AlGaN/GaN HEMT sensor can be also integrated in monolithic amplifier circuits of electronic devices such as high power and high speed devices [25,26]. Thus, it is demonstrated herein that a Pt Schottky diode on a semipolar AlGaN/GaN HEMT structure is capable of effectively sensing hydrogen. 4. Conclusion We examined the hydrogen sensing characteristics of Pt Schottky diodes using semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures. The 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 H2 concentrations from 0.5 ∼ 4% with a step of 0.5%. The hydrogen sensitivity of Pt Schottky diodes on semipolar (1 1 2¯ 2) AlGaN/GaN HEMT structures is the result of highly dense uncovered nitrogen atoms bearing a high affinity for hydrogen neighboring the Ga-terminated AlGaN (1 1 2¯ 2) surface. The hydrogen sensitivity of the Pt Schottky diodes peaked at the zero bias voltage, and slowly decreased with applied bias voltage. These results show that Pt Schottky diodes on semipolar AlGaN/GaN structures have great potential for hydrogen detection. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2014R1A1A4A01008877 and 2012R1A1B4002649). This work was also supported by 2015 Hongik University Research Fund. References [1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [2] P. Agnolucci, Economics and market prospects of portable fuel cells, Int. J. Hydrogen Energy 32 (2007) 4319–4328.
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Biographies Soohwan Jang received the B.S. degree in Department of Chemical Engineering from Seoul National University, Seoul, Korea in 2003. He also received the Ph.D. degree in Department of Chemical Engineering from University of Florida, Gainesville, Florida in 2007. From 2007 to 2009, he worked in Samsung Electronics. He has been a professor at the Department of Chemical Engineering at Dankook University since 2009. His research interests include nitride and oxide based sensors, light emitting diodes, and integrated electronic devices.
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Pyunghee Son received the B.S. degree in School of Materials Science and Engineering at Hongik University, Sejong, Korea in 2013. He is currently pursuing Master’s Degree School of Materials Science and Engineering at Hongik University. His research has focused on the growth and the characterization of nonpolar and semipolar ZnO crystals.
he worked at Samsung Electro-Mechanics Co., where he developed high power GaN-based light-emitting diodes. Since 2009, he has been with Korea Polytechnic University as an associate professor. His current research interests include the growth and characterization of GaN and ZnO-based light-emitting diodes (LEDs) and hydrogen gas sensors, and III–V semiconductor-based electronic devices.
Jimin Kim received the B.S. degree in Department of Chemical Engineering at Dankook University in 2014. She is currently pursuing Master’s Degree in Chemical Engineering at Dankook University. Her research has focused on ZnO hydrothermal growth and GaN-based semiconductor gas sensors.
Kwang Hyeon Baik received the B.S. degree at Yonsei University, Seoul, Korea in 1998, and Ph.D. degree in Materials Science and Engineering from University of Florida, Gainesville, Florida in 2004. He worked at the Photonics Lab at Samsung Advanced Institute of Technology from 2005 to 2008, and also worked at the Korea Electronics Technology Institute from 2009 to 2011. Since 2012, he has been with Hongik University, Sejong, Korea as an assistant professor. His current research interests include the growth and characterization of GaN and ZnO-based light-emitting diodes and hydrogen gas sensors, and III–V semiconductor-based electronic devices.
Sung-Nam Lee received the B.S. in Metallurgical Engineering from SungKyunKwan University, M.S. and Ph.D. degrees in Material Science Engineering from Gwangju Institute of Science and Seoul National University, respectively. In 2000, he joined Samsung Advanced Institute of Technology, where he worked on GaN-based ultraviolet, blue and green laser diodes for blue-ray disc and laser display systems. And,