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Schottky contact on hydrothermally grown a-plane ZnO for hydrogen sensing and UV detection
Current Applied Physics 16 (2016) 221e225
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Schottky contact on hydrothermally grown a-plane ZnO for hydrogen sensing and UV detection Jimin Kim a, Kwang Hyeon Baik b, Soohwan Jang a, c, * a
Department of Chemical Engineering, Dankook University, Yongin, 448-701, South Korea Department of Materials Science and Engineering, Hongik University, Jochiwon, 339-701, South Korea c Department of Creative Convergent Manufacturing Engineering, Dankook University, Yongin, 448-701, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2015 Received in revised form 9 November 2015 Accepted 18 November 2015 Available online 2 December 2015
Pt Schottky contact on nonpolar a-plane ZnO film grown by a simple hydrothermal method on a-plane GaN was investigated. The Schottky barrier height was measured to be 0.64 eV at room temperature and atmospheric pressure. The effect of ZnO crystal polarity on Schottky barrier height was studied, and the barrier height of a-plane ZnO was compared to the values of c-plane in the literature. Also, hydrogen sensing and UV detection characteristics of a-plane ZnO Schottky diode were evaluated. After exposure of 4% hydrogen in nitrogen, the diode showed current increase due to Schottky barrier height reduction, and 2006% of maximum sensitivity to hydrogen gas was observed. For repeated UV illumination, the Schottky contact presented stable and recoverable current response. © 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO Nonpolar a-Plane Schottky Sensing
1. Introduction There has been great interest in ZnO as alternative new material for optoelectronics and sensor devices. It has wide bandgap of 3.4 eV, large exciton binding energy of 60 meV over GaN, irradiation hardness, facile etching property with wet chemicals, and biocompatibility [1e5]. The crystal structure of ZnO is the same wurtzite structure as GaN, and ZnO epitaxial film is grown along polar [0001] direction in general. In this conventional c-plane multi-quantum-well (MQW) light emitting diode (LED) structure, there exists strong internal electric field induced by the spontaneous and piezoelectric polarizations which result in spatial separation of electron and hole wave functions [2,6]. This quantum confined stark effect (QCSE) causes the reduction in radiative recombination efficiency in MQWs of LEDs [7e9]. To eliminate or reduce the QCSE in LED, nonpolar a-plane ð1120Þ and m-plane ð1100Þ crystal structures, where Zn cations and O anions are equivalently positioned on the surface and form zero net dipole in the plane, are extensively researched as an alternative approach [2,4,5,8,10,11].
* Corresponding author. Department of Creative Convergent Manufacturing Engineering, Dankook University, Yongin, 448-701, South Korea. E-mail address: [email protected] (S. Jang). http://dx.doi.org/10.1016/j.cap.2015.11.014 1567-1739/© 2015 Elsevier B.V. All rights reserved.
Metal Schottky contact on semiconductor is one of the most important basic components in the electronic device structure. The reported Schottky barrier height on ZnO ranges from 0 to 1.2 eV. Such variable barrier heights depend on metal species, surface treatment, crystal quality, defect density, carrier type, carrier density, and ZnO polarity [1]. ZnO epitaxial layer is deposited by various methods such as metal organic chemical vapor deposition, molecular beam epitaxy, pulsed laser deposition, and hydrothermal synthesis [2,5,12e15]. Among these deposition techniques, hydrothermal growth is one of the most facile and cost-effective scalable methods due to its atmospheric growth pressure and significantly lower process temperature compared to the others [2,4,16]. Previously, we have successfully grown nonpolar a-plane ZnO (a-ZnO) film on a-plane GaN (a-GaN) template by simple hydrothermal method, which acts as a light-emitting active layer of nonpolar heterostructure LED [2]. In addition to optoelectronic applications, a-ZnO can be also applied for gas sensor with proper Schottky contact. In the case of GaN, nonpolar a-GaN and semipolar ð1122Þ GaN based Schottky diode hydrogen sensors showed dramatically improved sensing characteristics compared to conventional polar c-plane GaN due to the stronger affinity of hydrogen to nitrogen on the GaN surface than gallium [6,17]. In this work, nonpolar a-ZnO film was grown by facile, low-cost, seed-layer free and non-toxic hydrothermal method at the
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temperature of 90 C and atmospheric pressure, and Pt Schottky metal based a-ZnO diode was fabricated. The device showed distinct rectifying currentevoltage characteristics and the Schottky barrier height of Pt contact to a-ZnO was 0.64 eV. Also, hydrogen sensing and UV detection of a-ZnO Schottky diode was investigated for its potential applications.
2. Experiments 4.3 mm thick a-GaN epitaxial layer was grown on r-plane ð1102Þ sapphire substrates utilizing a two-step growth method by MOCVD system [2,5]. High-temperature buffer layer was deposited at 1050 C with high V/III ratio to promote island-like growth. Subsequent second buffer layer was grown with higher lateral growth rate to improve the crystal quality and surface flatness. Afterwards, 4.15 mm-thick a-GaN epilayer was successively grown to promote a lateral growth mode with a V/III ratio of 1000 at 1100 C. The full width at half maximum values of x-ray rocking curve for the a-GaN were in the range of ~450 and ~620 arcsec for the c-axis and the maxis, respectively. Nonpolar a-ZnO film was hydrothermally grown on a-GaN template by a simple aqueous solution method. 25 mM of zinc nitrate hexahydrate was dissolved in deionized water, and equimolar methenamine solution was mixed vigorously together in Teflonlined autoclave. The a-GaN substrate was floated upside down on the growth solution at 90 C and atmospheric pressure for 90 min. This growth cycle was repeated to produce 1.2 mm thick a-ZnO film on a-GaN [2]. Before metallization, the sample was ultrasonically cleaned with acetone, isopropyl alcohol, and deionized water repeatedly, and then blow-dried with nitrogen. Ti/Au (20/100 nm) was deposited on the a-ZnO by electron beam evaporation followed by rapid thermal annealing at 600 C for 1 min under oxygen ambient for ohmic contact. Contact patterns were formed by convention
photolithography and lift-off techniques. Finally, 10 nm Pt was evaporated on the diode Schottky contact area. The surface morphology of the a-ZnO film was examined by field emission scanning electron microscope (SEM) and atomic force spectroscopy (AFM). The crystallographic and optical characterizations were performed by high resolution X-ray diffraction (XRD) system with a Cu Ka1 X-ray target source and photoluminescence (PL) with 325 nm light source. Currentevoltage characteristics of the a-ZnO diode sensor exposed to 4% hydrogen balanced with nitrogen which is the lowest flammable limit of hydrogen at 1 atm was measured at room temperature in a gas test chamber using an Agilent 4155C semiconductor parameter analyzer. For photoresponse measurement of the diode, a 120 W mercury lamp was used as an illumination source.
3. Results and discussion Fig. 1 (a) shows the SEM image of hydrothermally grown a-ZnO. The tapered rough surface exposing m-plane facets of triangular prisms with a-plane base is observed, while hexagonal nanorods with 6 m-plane facets grows along c-axis for hydrothermal growth of ZnO on c-plane GaN [16,18]. The growth rate of ZnO in the c-axis direction is much faster than the a-axis and m-axis directions due to the higher surface energy of c-plane [19]. At the initial stage of aZnO growth on a-GaN, triangular prismatic ZnO islands having aplane basal plane and m-plane inclined facets grows along the fastest c-axis direction. As the growth process continues, individual ZnO prisms coalesce together with adjacent prisms following VolmereWeber growth model, and then nonpolar a-ZnO film with mplane facets having 120 angle is formed eventually [2,4]. The striated surface morphology consisting of ridges and valleys parallel to c-axis is clearly observed as shown in Fig. 1 (b) of AFM image, which is due to the anisotropic fast growth rate along the caxis crystallographic direction. Surface root mean square roughness
Fig. 1. (a) SEM image and (b) AFM image of a-plane ZnO grown by hydrothermal method. (c) XRD q-2q scan of a-plane ZnO grown on a-plane GaN substrate. (d) PL spectrum of hydrothermally grown a-plane ZnO film.
J. Kim et al. / Current Applied Physics 16 (2016) 221e225
was 71.52 nm over 5 5 mm2 scanning range. This tapered rough surface morphology can enhance the sensitivity of a-ZnO based hydrogen Schottky diode sensor by increasing active surface area for hydrogen adsorption. As shown in Fig. 1 (c), XRD analysis clearly shows distinct a-plane crystallinity of ZnO grown by hydrothermal method at the low temperature of 90 C on a-GaN. The symmetric a-plane ð1120Þ reflection peaks of ZnO and GaN are 56.8 and 57.9 respectively in q-2q scan. Fig. 1 (d) presents PL spectrum of a-ZnO at room temperature. The maximum peak is centered at 376.7 nm, and the full width at half-maximum is 17.9 nm. It is notable that any significant peaks in green and yellow regions from deep level defects which are typically found in hydrothermally grown c-plane ZnO were not observed at the room temperature PL [20e22]. This result demonstrates high quality of low temperature
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hydrothermally grown a-plane ZnO film with very low interface states on a-GaN [23]. Schematic diagram of a-ZnO based Pt metal Schottky diode is shown in Fig. 2 (a). The diameter of Schottky contact is 100 mm, and the gap between Schottky and ohmic contacts is 50 mm. The carrier concentration of a-ZnO film was measured to be 9.4 1018 cm3 with the mobility of 87.3 cm2/V$s by the Hall measurements using van der Pauw method. The specific contact resistance of ohmic contact was determined to be 1.06 105 U cm2 by transmission line measurement. Currentevoltage characteristics of a-ZnO diode were evaluated at room temperature as shown in Fig. 2 (b). Considering high carrier concentration, the diode with Pt Schottky contact shows apparent rectifying behavior with an on/off ratio of 31.2 at ±5 V bias. The Schottky barrier height of Pt to a-ZnO was obtained by thermionic emission model in the forward bias range of 0.1e0.5 V [24].
eØ eV J ¼ A*T2 exp b exp nkT kT
(1)
where J is the current density, A* the Richardson's constant for ZnO, T the absolute temperature, e the electronic charge, Øb the barrier height, k the Boltzmann's constant, n the ideality factor, and V the applied voltage. The barrier height was 0.64 eV with the ideality factor of 1.33. The larger ideality factor than unity may be ascribed
Fig. 2. (a) Schematic diagram of a-plane ZnO Schottky diode. (b) IeV characteristics of a-plane ZnO Schottky diode. Inset is IeV in linear scale. (c) Richardson plot of a-plane ZnO Schottky diode measured at V ¼ 0.2 V.
Fig. 3. (a) IeV characteristics before and after exposure to 4% H2 and (b) time dependence of current changes for cyclic 4% H2 exposures in a-plane ZnO Schottky diode.
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to the tunneling effect from high carrier concentration of hydrothermally grown a-ZnO film [25]. The Schottky barrier height was also calculated from the Richardson plot measured at 0.2 V in temperature range from 25 to 125 C as shown in Fig. 2 (c). 0.63 eV of barrier height which was derived by the slop of ½dðln ð I = T2 Þ = d ð1 = TÞ also confirms the value obtained by Equation (1). Polarity of ZnO film plays an important role in determining Schottky barrier height. Allen et al. reported barrier height for Pt metal on Zn-polar (0001) ZnO is 0.55 eV while 0.68 eV for Pt on O-polar ð0001Þ ZnO [26]. Kim et al. also found more increased barrier height of O-polar ZnO compared to Zn-polar for Ag Schottky contact, which is attributed to insulating partial oxidation between Schottky metal and O-polar ZnO [27,28]. For nonpolar ZnO including a-plane and m-plane, Zn cations and O anions exist in balanced stoichiometric ratio on the surface. Therefore, it is reasonable that Schottky barrier height of hydrothermally grown a-ZnO is higher than Zn-polar c-plane ZnO and lower than O-polar ZnO. Hydrogen sensing and UV detection characteristic of hydrothermally grown nonpolar a-ZnO Schottky diode were investigated. Fig. 3 (a) shows currentevoltage plots of the diode before and after exposure to 4% hydrogen in nitrogen which is the lowest flammable limit at 1 atm. Under the hydrogen ambient, the current increased with the Schottky barrier reduction of 56 meV at room temperaIH IN ture, and the maximum sensitivity defined as 2IN 2 100% was 2 2006%. The device shows good repeatability in its changes of current and the ability to cycle these currents in response to repeated introductions of 4% hydrogen into the ambient at a fixed bias of 1 V as shown in Fig. 3(b). The hydrogen gas molecules are
dissociated on catalytic Pt film, diffuse into ZnO surface, and form dipoles, thus reducing the Schottky barrier height [6,17,29]. Hydrogen atom is more prone to be attached to oxygen than zinc, which results in high responsivity of nonpolar ZnO where oxygen atoms position on the surface with zinc in equivalent ratio at room temperature [27,28,30]. Also, UV response of a-ZnO Schottky diode with same device structure as illustrated in Fig. 2 (a) was evaluated under ambient condition with light source of mercury lamp emitting 365 nm peak in the UV region. Once UV light shined the diode, the current rose in the range of bias sweep due to increase of free carrier density from photon-generated electron and hole pairs in the interface region (Fig. 4 (a), (b)). Also, the photon induced desorption of oxygen atoms on the surface modifies the surface defect density, which lowers the Schottky barrier height with increased free carriers [31]. Response of the device to visible light from microscope was nominal. The a-ZnO Schottky diode shows stable and full recovery for the repeated UV exposure with long recovery time. This may be reduced by surface passivation with UV transparent polymers such as poly(diallyldimethylammonium) and poly(sodium 4-styrenesulfonate) [31]. 4. Conclusion Pt Schottky contact of nonpolar a-ZnO grown by facile, low-cost, seed-layer free, and wafer level scalable hydrothermal method at low temperature of 90 C was studied. The ideality factor was 1.33, and Schottky barrier height was 0.64 eV which is higher than Znpolar c-plane ZnO and lower than O-polar ZnO due to its equivalent atomic configuration in the plane. Also, hydrogen sensing and UV detection characteristic were investigated. The diode showed 2006% of maximum sensitivity to 4% hydrogen in nitrogen, and stable and recoverable response to UV light illumination resulting from barrier height variation. The hydrothermally grown nonpolar a-ZnO appears to be a promising vehicle for sensor and optoelectronic applications. Acknowledgment This research was supported by the Ministry of Trade, Industry and Energy, Korea through the education program for creative and industrial convergence (grant number N0000717). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 4. (a) IeV characteristics before and after exposure to UV light and (b) time dependence of current changes for cyclic UV exposures in a-plane ZnO Schottky diode.
[17]
L.J. Brillson, Y. Lu, J. Appl. Phys. 109 (2011) 121301. K.H. Baik, H. Kim, J. Kim, S. Jung, S. Jang, Appl. Phys. Lett. 103 (2013) 091107. S. Jang, P. Son, J. Kim, S. Lee, K.H. Baik, Opt. Mater. Express 5 (2015) 1621. P. Son, K.H. Baik, S. Jang, ECS Solid State Lett. 4 (2015) Q1. K.H. Baik, H. Kim, S. Jang, Thin Solid Films 569 (2014) 1. K.H. Baik, H. Kim, S. Lee, E. Lim, S.J. Pearton, F. Ren, S. Jang, Appl. Phys. Lett. 104 (2014) 072103. T. Detchprohm, M. Zhu, Y. Li, Y. Xia, C. Wetzel, E.A. Preble, L. Liu, T. Paskova, D. Hanser, Appl. Phys. Lett. 92 (2008) 241109. S. Hwang, Y.G. Seo, K.H. Baik, I. Cho, J.H. Baek, S. Jung, T.G. Kim, M. Cho, Appl. Phys. Lett. 95 (2009) 071101. A. Chakraborty, B.A. Haskell, S. Keller, J.S. Speck, S.P. DenBarrs, S. Nakamura, U.K. Mishra, Jpn. J. Appl. Phys. Part 2 44 (2005) L173. K.H. Baik, Y.G. Seo, J. Kim, S. Hwang, W. Lim, C.Y. Chang, S.J. Pearton, F. Ren, S. Jang, J. Phys. D. Appl. Phys. 43 (2010) 295102. X. Ni, Y. Fu, Y.T. Moon, N. Biyikli, H. Morkoc, J. Cryst. Growth 290 (2006) 166. J.G.E. Gardeniers, Z.M. Rittersma, G.J. Burger, J. Appl. Phys. 83 (1998) 7844. K. Minegishi, Y. Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, A. Shimizu, Jpn. J. Appl. Phys. 36 (1997) L1453. D. Andeen, J.H. Kim, F.F. Lange, G.K.L. Goh, S. Tripathy, Adv. Funct. Mater. 16 (2006) 799. , L. Chow, G. Chai, B. Viana, V.V. Ursaki, E. Monaico, O. Lupan, T. Pauporte I.M. Tiginyanu, Appl. Surf. Sci. 259 (2012) 399. L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. 115 (2003) 3139. Y. Wang, F. Ren, W. Lim, S.J. Pearton, K.H. Baik, S. Hwang, Y.G. Seo, S. Jang, Curr. Appl. Phys. 10 (2010) 1029.
J. Kim et al. / Current Applied Physics 16 (2016) 221e225 [18] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nat. Mater. 4 (2005) 455. [19] H.Q. Lea, S.J. Chua, Y.W. Koh, K.P. Loh, E.A. Fitzgerald, J. Cryst. Growth 293 (2006) 36. [20] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. 42 (2003) 3031. [21] Y. Sun, N.G. Ndifor-Angwafor, D.J. Riley, M.N.R. Ashfold, Chem. Phys. Lett. 431 (2006) 352. [22] C.B. Tay, S.J. Chua, K. P.Loh, J. Cryst. Growth 311 (2009) 1278. [23] O. Lupan, T. Pauporte, B. Viana, Adv. Mater. 22 (2010) 3298. [24] H. Kim, W. Lim, J.-H. Lee, S.J. Pearton, F. Ren, S. Jang, Sens. Actuators B 164 (2012) 64.
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[25] K. Ip, G.T. Thaler, H. Yang, S.Y. Han, Y. Li, D.P. Norton, S.J. Peartona, S. Jang, F. Ren, J. Cryst. Growth 287 (2006) 149. [26] M.W. Allen, M.M. Alkaisi, S.M. Durbin, Appl. Phys. Lett. 89 (2006) 103520. [27] H. Kim, H. Kim, D. Kim, J. Appl. Phys. 108 (2010) 074514. [28] H. Kim, H. Kim, D. Kim, J. Korean Phys. Soc. 61 (2012) 1314. [29] H. Kim, W. Lim, J.-H. Lee, S.J. Pearton, F. Ren, S. Jang, Sens. Actuators B 164 (2012) 64. [30] M. Allen, D. Zemlyanov, G. Waterhouse, J. Metson, T. Veal, C. McConville, S. Durbin, Appl. Phys. Lett. 98 (2011) 101906. [31] J. Zhou, Y. Gu, Y. Hu, W. Mai, P. Yeh, G. Bao, A.K. Sood, D.L. Polla, Z.L. Wang, Appl. Phys. Lett. 94 (2009) 191103.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Schottky contact on hydrothermally grown a-plane ZnO for hydrogen sensing and UV detection Jimin Kim a, Kwang Hyeon Baik b, Soohwan Jang a, c, * a
Department of Chemical Engineering, Dankook University, Yongin, 448-701, South Korea Department of Materials Science and Engineering, Hongik University, Jochiwon, 339-701, South Korea c Department of Creative Convergent Manufacturing Engineering, Dankook University, Yongin, 448-701, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 September 2015 Received in revised form 9 November 2015 Accepted 18 November 2015 Available online 2 December 2015
Pt Schottky contact on nonpolar a-plane ZnO film grown by a simple hydrothermal method on a-plane GaN was investigated. The Schottky barrier height was measured to be 0.64 eV at room temperature and atmospheric pressure. The effect of ZnO crystal polarity on Schottky barrier height was studied, and the barrier height of a-plane ZnO was compared to the values of c-plane in the literature. Also, hydrogen sensing and UV detection characteristics of a-plane ZnO Schottky diode were evaluated. After exposure of 4% hydrogen in nitrogen, the diode showed current increase due to Schottky barrier height reduction, and 2006% of maximum sensitivity to hydrogen gas was observed. For repeated UV illumination, the Schottky contact presented stable and recoverable current response. © 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO Nonpolar a-Plane Schottky Sensing
1. Introduction There has been great interest in ZnO as alternative new material for optoelectronics and sensor devices. It has wide bandgap of 3.4 eV, large exciton binding energy of 60 meV over GaN, irradiation hardness, facile etching property with wet chemicals, and biocompatibility [1e5]. The crystal structure of ZnO is the same wurtzite structure as GaN, and ZnO epitaxial film is grown along polar [0001] direction in general. In this conventional c-plane multi-quantum-well (MQW) light emitting diode (LED) structure, there exists strong internal electric field induced by the spontaneous and piezoelectric polarizations which result in spatial separation of electron and hole wave functions [2,6]. This quantum confined stark effect (QCSE) causes the reduction in radiative recombination efficiency in MQWs of LEDs [7e9]. To eliminate or reduce the QCSE in LED, nonpolar a-plane ð1120Þ and m-plane ð1100Þ crystal structures, where Zn cations and O anions are equivalently positioned on the surface and form zero net dipole in the plane, are extensively researched as an alternative approach [2,4,5,8,10,11].
* Corresponding author. Department of Creative Convergent Manufacturing Engineering, Dankook University, Yongin, 448-701, South Korea. E-mail address: [email protected] (S. Jang). http://dx.doi.org/10.1016/j.cap.2015.11.014 1567-1739/© 2015 Elsevier B.V. All rights reserved.
Metal Schottky contact on semiconductor is one of the most important basic components in the electronic device structure. The reported Schottky barrier height on ZnO ranges from 0 to 1.2 eV. Such variable barrier heights depend on metal species, surface treatment, crystal quality, defect density, carrier type, carrier density, and ZnO polarity [1]. ZnO epitaxial layer is deposited by various methods such as metal organic chemical vapor deposition, molecular beam epitaxy, pulsed laser deposition, and hydrothermal synthesis [2,5,12e15]. Among these deposition techniques, hydrothermal growth is one of the most facile and cost-effective scalable methods due to its atmospheric growth pressure and significantly lower process temperature compared to the others [2,4,16]. Previously, we have successfully grown nonpolar a-plane ZnO (a-ZnO) film on a-plane GaN (a-GaN) template by simple hydrothermal method, which acts as a light-emitting active layer of nonpolar heterostructure LED [2]. In addition to optoelectronic applications, a-ZnO can be also applied for gas sensor with proper Schottky contact. In the case of GaN, nonpolar a-GaN and semipolar ð1122Þ GaN based Schottky diode hydrogen sensors showed dramatically improved sensing characteristics compared to conventional polar c-plane GaN due to the stronger affinity of hydrogen to nitrogen on the GaN surface than gallium [6,17]. In this work, nonpolar a-ZnO film was grown by facile, low-cost, seed-layer free and non-toxic hydrothermal method at the
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temperature of 90 C and atmospheric pressure, and Pt Schottky metal based a-ZnO diode was fabricated. The device showed distinct rectifying currentevoltage characteristics and the Schottky barrier height of Pt contact to a-ZnO was 0.64 eV. Also, hydrogen sensing and UV detection of a-ZnO Schottky diode was investigated for its potential applications.
2. Experiments 4.3 mm thick a-GaN epitaxial layer was grown on r-plane ð1102Þ sapphire substrates utilizing a two-step growth method by MOCVD system [2,5]. High-temperature buffer layer was deposited at 1050 C with high V/III ratio to promote island-like growth. Subsequent second buffer layer was grown with higher lateral growth rate to improve the crystal quality and surface flatness. Afterwards, 4.15 mm-thick a-GaN epilayer was successively grown to promote a lateral growth mode with a V/III ratio of 1000 at 1100 C. The full width at half maximum values of x-ray rocking curve for the a-GaN were in the range of ~450 and ~620 arcsec for the c-axis and the maxis, respectively. Nonpolar a-ZnO film was hydrothermally grown on a-GaN template by a simple aqueous solution method. 25 mM of zinc nitrate hexahydrate was dissolved in deionized water, and equimolar methenamine solution was mixed vigorously together in Teflonlined autoclave. The a-GaN substrate was floated upside down on the growth solution at 90 C and atmospheric pressure for 90 min. This growth cycle was repeated to produce 1.2 mm thick a-ZnO film on a-GaN [2]. Before metallization, the sample was ultrasonically cleaned with acetone, isopropyl alcohol, and deionized water repeatedly, and then blow-dried with nitrogen. Ti/Au (20/100 nm) was deposited on the a-ZnO by electron beam evaporation followed by rapid thermal annealing at 600 C for 1 min under oxygen ambient for ohmic contact. Contact patterns were formed by convention
photolithography and lift-off techniques. Finally, 10 nm Pt was evaporated on the diode Schottky contact area. The surface morphology of the a-ZnO film was examined by field emission scanning electron microscope (SEM) and atomic force spectroscopy (AFM). The crystallographic and optical characterizations were performed by high resolution X-ray diffraction (XRD) system with a Cu Ka1 X-ray target source and photoluminescence (PL) with 325 nm light source. Currentevoltage characteristics of the a-ZnO diode sensor exposed to 4% hydrogen balanced with nitrogen which is the lowest flammable limit of hydrogen at 1 atm was measured at room temperature in a gas test chamber using an Agilent 4155C semiconductor parameter analyzer. For photoresponse measurement of the diode, a 120 W mercury lamp was used as an illumination source.
3. Results and discussion Fig. 1 (a) shows the SEM image of hydrothermally grown a-ZnO. The tapered rough surface exposing m-plane facets of triangular prisms with a-plane base is observed, while hexagonal nanorods with 6 m-plane facets grows along c-axis for hydrothermal growth of ZnO on c-plane GaN [16,18]. The growth rate of ZnO in the c-axis direction is much faster than the a-axis and m-axis directions due to the higher surface energy of c-plane [19]. At the initial stage of aZnO growth on a-GaN, triangular prismatic ZnO islands having aplane basal plane and m-plane inclined facets grows along the fastest c-axis direction. As the growth process continues, individual ZnO prisms coalesce together with adjacent prisms following VolmereWeber growth model, and then nonpolar a-ZnO film with mplane facets having 120 angle is formed eventually [2,4]. The striated surface morphology consisting of ridges and valleys parallel to c-axis is clearly observed as shown in Fig. 1 (b) of AFM image, which is due to the anisotropic fast growth rate along the caxis crystallographic direction. Surface root mean square roughness
Fig. 1. (a) SEM image and (b) AFM image of a-plane ZnO grown by hydrothermal method. (c) XRD q-2q scan of a-plane ZnO grown on a-plane GaN substrate. (d) PL spectrum of hydrothermally grown a-plane ZnO film.
J. Kim et al. / Current Applied Physics 16 (2016) 221e225
was 71.52 nm over 5 5 mm2 scanning range. This tapered rough surface morphology can enhance the sensitivity of a-ZnO based hydrogen Schottky diode sensor by increasing active surface area for hydrogen adsorption. As shown in Fig. 1 (c), XRD analysis clearly shows distinct a-plane crystallinity of ZnO grown by hydrothermal method at the low temperature of 90 C on a-GaN. The symmetric a-plane ð1120Þ reflection peaks of ZnO and GaN are 56.8 and 57.9 respectively in q-2q scan. Fig. 1 (d) presents PL spectrum of a-ZnO at room temperature. The maximum peak is centered at 376.7 nm, and the full width at half-maximum is 17.9 nm. It is notable that any significant peaks in green and yellow regions from deep level defects which are typically found in hydrothermally grown c-plane ZnO were not observed at the room temperature PL [20e22]. This result demonstrates high quality of low temperature
223
hydrothermally grown a-plane ZnO film with very low interface states on a-GaN [23]. Schematic diagram of a-ZnO based Pt metal Schottky diode is shown in Fig. 2 (a). The diameter of Schottky contact is 100 mm, and the gap between Schottky and ohmic contacts is 50 mm. The carrier concentration of a-ZnO film was measured to be 9.4 1018 cm3 with the mobility of 87.3 cm2/V$s by the Hall measurements using van der Pauw method. The specific contact resistance of ohmic contact was determined to be 1.06 105 U cm2 by transmission line measurement. Currentevoltage characteristics of a-ZnO diode were evaluated at room temperature as shown in Fig. 2 (b). Considering high carrier concentration, the diode with Pt Schottky contact shows apparent rectifying behavior with an on/off ratio of 31.2 at ±5 V bias. The Schottky barrier height of Pt to a-ZnO was obtained by thermionic emission model in the forward bias range of 0.1e0.5 V [24].
eØ eV J ¼ A*T2 exp b exp nkT kT
(1)
where J is the current density, A* the Richardson's constant for ZnO, T the absolute temperature, e the electronic charge, Øb the barrier height, k the Boltzmann's constant, n the ideality factor, and V the applied voltage. The barrier height was 0.64 eV with the ideality factor of 1.33. The larger ideality factor than unity may be ascribed
Fig. 2. (a) Schematic diagram of a-plane ZnO Schottky diode. (b) IeV characteristics of a-plane ZnO Schottky diode. Inset is IeV in linear scale. (c) Richardson plot of a-plane ZnO Schottky diode measured at V ¼ 0.2 V.
Fig. 3. (a) IeV characteristics before and after exposure to 4% H2 and (b) time dependence of current changes for cyclic 4% H2 exposures in a-plane ZnO Schottky diode.
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to the tunneling effect from high carrier concentration of hydrothermally grown a-ZnO film [25]. The Schottky barrier height was also calculated from the Richardson plot measured at 0.2 V in temperature range from 25 to 125 C as shown in Fig. 2 (c). 0.63 eV of barrier height which was derived by the slop of ½dðln ð I = T2 Þ = d ð1 = TÞ also confirms the value obtained by Equation (1). Polarity of ZnO film plays an important role in determining Schottky barrier height. Allen et al. reported barrier height for Pt metal on Zn-polar (0001) ZnO is 0.55 eV while 0.68 eV for Pt on O-polar ð0001Þ ZnO [26]. Kim et al. also found more increased barrier height of O-polar ZnO compared to Zn-polar for Ag Schottky contact, which is attributed to insulating partial oxidation between Schottky metal and O-polar ZnO [27,28]. For nonpolar ZnO including a-plane and m-plane, Zn cations and O anions exist in balanced stoichiometric ratio on the surface. Therefore, it is reasonable that Schottky barrier height of hydrothermally grown a-ZnO is higher than Zn-polar c-plane ZnO and lower than O-polar ZnO. Hydrogen sensing and UV detection characteristic of hydrothermally grown nonpolar a-ZnO Schottky diode were investigated. Fig. 3 (a) shows currentevoltage plots of the diode before and after exposure to 4% hydrogen in nitrogen which is the lowest flammable limit at 1 atm. Under the hydrogen ambient, the current increased with the Schottky barrier reduction of 56 meV at room temperaIH IN ture, and the maximum sensitivity defined as 2IN 2 100% was 2 2006%. The device shows good repeatability in its changes of current and the ability to cycle these currents in response to repeated introductions of 4% hydrogen into the ambient at a fixed bias of 1 V as shown in Fig. 3(b). The hydrogen gas molecules are
dissociated on catalytic Pt film, diffuse into ZnO surface, and form dipoles, thus reducing the Schottky barrier height [6,17,29]. Hydrogen atom is more prone to be attached to oxygen than zinc, which results in high responsivity of nonpolar ZnO where oxygen atoms position on the surface with zinc in equivalent ratio at room temperature [27,28,30]. Also, UV response of a-ZnO Schottky diode with same device structure as illustrated in Fig. 2 (a) was evaluated under ambient condition with light source of mercury lamp emitting 365 nm peak in the UV region. Once UV light shined the diode, the current rose in the range of bias sweep due to increase of free carrier density from photon-generated electron and hole pairs in the interface region (Fig. 4 (a), (b)). Also, the photon induced desorption of oxygen atoms on the surface modifies the surface defect density, which lowers the Schottky barrier height with increased free carriers [31]. Response of the device to visible light from microscope was nominal. The a-ZnO Schottky diode shows stable and full recovery for the repeated UV exposure with long recovery time. This may be reduced by surface passivation with UV transparent polymers such as poly(diallyldimethylammonium) and poly(sodium 4-styrenesulfonate) [31]. 4. Conclusion Pt Schottky contact of nonpolar a-ZnO grown by facile, low-cost, seed-layer free, and wafer level scalable hydrothermal method at low temperature of 90 C was studied. The ideality factor was 1.33, and Schottky barrier height was 0.64 eV which is higher than Znpolar c-plane ZnO and lower than O-polar ZnO due to its equivalent atomic configuration in the plane. Also, hydrogen sensing and UV detection characteristic were investigated. The diode showed 2006% of maximum sensitivity to 4% hydrogen in nitrogen, and stable and recoverable response to UV light illumination resulting from barrier height variation. The hydrothermally grown nonpolar a-ZnO appears to be a promising vehicle for sensor and optoelectronic applications. Acknowledgment This research was supported by the Ministry of Trade, Industry and Energy, Korea through the education program for creative and industrial convergence (grant number N0000717). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Fig. 4. (a) IeV characteristics before and after exposure to UV light and (b) time dependence of current changes for cyclic UV exposures in a-plane ZnO Schottky diode.
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