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porous GaN nanonetwork metal-semiconductor Schottky diode for room temperature hydrogen sensor
Sensors and Actuators A 209 (2014) 52–56
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Platinum/porous GaN nanonetwork metal-semiconductor Schottky diode for room temperature hydrogen sensor Aihua Zhong ∗ , Takashi Sasaki, Kazuhiro Hane Department of Nanomechanics, Tohoku University, Sendai 980-8579, Japan
a r t i c l e
i n f o
Article history: Received 7 September 2013 Received in revised form 24 December 2013 Accepted 9 January 2014 Available online 18 January 2014 Keywords: Honeycomb GaN nanonetwork Nano-Schottky diode H2 sensor
a b s t r a c t An in-plane electrically conductive honeycomb GaN nanonetwork grown by molecular beam epitaxy was used to fabricate a platinum (Pt)/porous GaN nanonetwork Schottky diode type hydrogen sensor. The Pt Schottky contact is a nanonetwork with typical width of 40 nm. Both the scanning electron microscopy image and current–voltage curve indicates that the Pt/porous GaN nanonetwork Schottky diode with barrier height of 0.497 eV and ideality factor of 38.5 is comprised of parallel nano-Schotttky diodes. The operating temperature of this Schottky diode hydrogen sensor on the porous GaN nanonetwork is successfully decreased to room temperature and it performs well in detecting hydrogen gas with various concentrations from 320 to 10,000 ppm. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It is of great interest to develop a hydrogen (H2 ) sensor that is capable to operate in harsh environmental conditions such as chemically corrosive ambient. Because of its high resistance to acids and alkalis as well as large band gap (3.4 eV) [1], H2 sensor based on GaN semiconductor could be used for many harsh applications. These include gas sensing operations in a chemical reactor processing, fuel leak detections in space vehicles as well as automobiles, and emissions from industrial process [2,3]. For these sensors, it is important to operate with minimum power consumption near room temperature, especially for a long-term H2 monitor. H2 sensors on the planar GaN film have been investigated. A resistive type H2 sensor on a GaN film has been demonstrated in 2005 [3]. The primary researches are focused on the platinum (Pt) or palladium (Pd)/planar GaN film Schottky diode type H2 sensors, of which the catalytic metal Pt or Pd dissociates the H2 molecules to H atoms [4,5]. Schottky diodes are well formed on the planar GaN film, exhibiting high sensitivity and stability. Additionally, a Pt/SiO2 /GaN (MIS, metal–insulator–semiconductor) Schottky diode type H2 sensor also has been investigated by Tsai and coworkers, which improves the sensitivity, response time, and thermal stability [6,7]. Both the resistive and Schottky type H2 sensors on the GaN film, however, are required to be heated, typically to 200 ◦ C [3,5]. The response time [5] is about 7 min in 980 ppm H2 at 60 ◦ C. On the other hand, nanostructures are promising in improving the performance of H2 sensor due to its nanosize effect and large
∗ Corresponding author. Tel.: +81 22 795 6965. E-mail address: [email protected] (A. Zhong). 0924-4247/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2014.01.014
surface area to volume ratio [8,9]. To date, there are only a few investigations of H2 sensors on GaN nanostructures [2,10] partly due to the difficulty of nano-sensor fabrication. Lim and coworkers [2] have reported a resistive type H2 sensor on the GaN nanowire, which could lower the operating temperature to room temperature. In comparison with a resistive type H2 sensor, a Schottky diode type H2 sensor exhibits higher sensitivity [3,5]. Moreover, when the Schottky diode is downscaled to nanoscale regime, the nanoSchottky diode is different from the common large Schottky diode [11,12] and may exhibit promising performance. To our knowledge, there has been no report on a Schottky diode type H2 sensor made from a GaN nanostructure. In this work, we demonstrate a Schottky diode type H2 sensor on a porous GaN nanonetwork. The Ga-polar porous GaN nanonetwork, which is of high quality like a GaN nanowire [13–15], was epitaxially grown on a (1 1 1) Si substrate. Different from a separated nanowire or nanotube, the porous GaN nanonetwork is continuous for electric current in the lateral direction. Because of its in-plane electrical conductivity, fabrication of an electrical device on the porous GaN nanonetwork is expected to be as easy as that on a planar GaN film. The characteristic of a Schottky diode on the porous GaN nanonetwork and its performance in sensing H2 gas at room temperature was investigated. Using a Si wafer as the substrate, Si-based micromachining as well as integrated circuit (IC) can be applied to fabricate an integrated sensor. 2. Experimental details A slightly Mg doped porous GaN nanonetwork was epitaxially grown on a 3-in. Si (1 1 1) wafer by a molecular beam epitaxy system under a nitrogen-rich condition. The growth process in detail
A. Zhong et al. / Sensors and Actuators A 209 (2014) 52–56
53
Fig. 1. Two-mask fabrication process of the H2 sensor on a porous GaN nanonetwork.
was similar to the previous publication [13]. For Mg doping, an Mg cell was heated with pressure in the order of 10−9 Torr. The porous GaN nanonetwork was 400 nm thick with electron concentration of about 8 × 1016 cm−3 measured by Hall Effect measurement system. The native oxide on the porous GaN nanonetwork was removed by a solution of HCl:H2 O = 1:1. Then a Schottky diode type H2 sensor was fabricated on this porous GaN nanonetwork by a two-mask process, as shown in Fig. 1. Pt Schottky contact with a thickness of 50 nm and an area of 0.16 mm2 was deposited on the porous GaN nanonetwork by a sputtering machine using a patterned Si wafer as a mask. Another electrode Ti (20 nm)/Al (80 nm) was deposited by an e-beam evaporator and was patterned by photolithography and lift off. After fabrication, the Schottky diode was exposed to various
H2 concentrations diluted in dry air in a steel chamber. The flow rate was kept at 100 sccm. 3. Results and discussions 3.1. Morphology and schematic profile Under a nitrogen-rich growth condition, GaN grows in the three-dimensional model to a honeycomb GaN nanowall network, namely porous GaN nanonetwork structure as shown in Fig. 2(a), which was measured by a field-emission scanning electron microscopy (FESEM). The GaN nanowalls overlap and interlace with one another, forming an in-plane electrically conductive porous GaN nanonetwork [14,15]. The typical width of the GaN nanowall
Fig. 2. (a) FESEM image of a porous GaN nanonetwork; (b) optical image of a Pt/porous GaN nanonetwork Schottky diode; (c) FESEM image of the Pt Schottky contact; (d) cross-sectional image of GaN nanonetwork coated with Pt electrode; (e) EDX mapping; and (f) schematic profile of the Schottky diode on the GaN nanowall network.
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A. Zhong et al. / Sensors and Actuators A 209 (2014) 52–56
and size of pores are 30 and 50 nm, respectively. In the gas sensing process, the diffusivity of gases into the sensor and their reactivity with sensitive material together determine the performance of a gas sensor [16,17]. Because of its porous structure, the porous GaN nanonetwork is promising in terms of diffusivity of gas. Fig. 2(b) is an optical image of a device on a GaN nanonetwork. After deposition of a 50 nm thick Pt electrode, the surface still keeps the nanonetwork morphology as shown in Fig. 2(c). This means the Schottky contact Pt electrode is also a nanonetwork consisted of Pt stripes with large surface area to volume area. It is reported that both the response and recover times of the H2 sensor are linearly correlated with the surface area/volume ratio [18,19]. From the cross-sectional image in Fig. 2(d), we could observe that the Pt stripe with typical width of 40 nm was deposited on the top of the GaN nanowall. Fig. 2(e) shows the distribution of Pt, Ga, and Si elements measured by an energy dispersive X-ray spectroscopy (EDX) equipped in the FESEM. The bottom is the Si substrate and the middle part is GaN. For Pt, it locates on the top of the GaN nanowall. Based on the images above, we made a schematic profile of the Schottky diode on the GaN nanowall network as shown in Fig. 2(f). The contacts between Pt and GaN nanowalls are stripes with typical width of 40 nm. These stripes connect together, comprising the Pt electrode. When the device is forward biased, the current flows through these Pt/GaN nanowall stripes and then arrives the other electrode Ti/Al by means of the continuous GaN nanowalls between the two electrodes.
3.2. Current–voltage curve measurement We measured the current–voltage (I–V) curves of the Pt/porous GaN nanonetwork Schottky diode at room temperature and show them in Fig. 3(a) and (b). For a Schottky diode the I–V curve is described by the thermionic emission theory [20] expressed as follows:
I = I0 exp
qV nkT
qϕ B
− 1 = AA∗ T 2 exp −
kT
exp
qV nkT
−1
where I0 is the saturation current, A is the area of Pt electrode, A* is the effective Richardson constant (24 A cm−2 K−2 for GaN), q is the electron charge, V is the applied voltage, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, and ϕB is the zero-biased Schottky barrier height. Through fitting the measured I–V curves to the thermionic emission theory as shown in Fig. 3(a), the Schottky barrier height ϕB in air and in 1% H2 are determined to be 0.497 and 0.454 eV, respectively. The ideality factor n are also determined, 38.5 and 136.8, respectively. The fitting voltage range is from 0.25 to 0.85 V. Similar results [21] were reported for Au/individual GaN nanowire nano-Schottky diode with the Schottky barrier height of 0.48 eV and ideality factor of 12. The large ideality factor of the nano-Schottky diode is mainly attributed to the large tunneling current, which is resulted from the thin depletion region of a small Schottky diode [22,23]. Different from a large Schottky diode on a GaN film, the reverse current of the Pt/porous GaN nanonetwork Schottky diode is large as shown in Fig. 3(b). Smit and coworkers [11,12] suggested that in case of a small nano-Schottky diode where the Schottky contact was smaller than the characteristic length lc , the reverse current would be profoundly affected by the size and shape of the Schottky contact. For a very small Schottky contact, the reverse current is comparable with the forward current. The characteristic length lc is defined as follows [12,20]:
lc =
2εs Vs qNd
(2)
Fig. 3. (a) Fitting the I–V curves of the Pt/porous GaN nanonetwork Schottky diode with the thermionic emission theory and (b) I–V curves measured in air and in 1% H2 gas at room temperature.
where the total potential drop over the space charge region Vs is expressed by Vs =
ϕB − kT ln (NC /Nd ) −V q
(3)
Here εs is the dielectric constant, q is the electron charge, Nd is the carrier concentration, ϕB is the Schottky barrier, V is the applied voltage, and NC is the effective density of states in the conduction band. For GaN, the characteristic length lc is calculated to be 77 nm when parameters εs = 10.4 [24], Nd = 8 × 1016 cm−3 , ϕB = 0.497 eV, and NC = 2.3 × 1018 cm−3 [25] are used. From the SEM results above, the typical width of the Pt stripe is about 40 nm, smaller than the characteristic length lc of 77 nm. Hence the 40 nm Pt Schottky contacts could be considered as small nano-Schottky contacts and the large reverse current in Fig. 3(b) is a characteristic of these small Pt nano-Schottky contacts. Therefore, the Pt/porous GaN nanonetwork Schottky diode could be regarded as one Schottky diode type sensor element comprised of multiple nano-Schottky diodes in parallel. By utilizing the multiple parallel nano-Schottky diodes as a sensor element, the uniformity and reliability of the sensor would be improved [26]. For an undoped porous GaN nanonetwork with electron concentration of 2 × 1017 cm−3 , we did not observe a rectified I–V curve (not shown here). Thus, we grew a slightly Mg doped porous GaN nanonetwork with lower electron concentration of 8 × 1016 cm−3 .
A. Zhong et al. / Sensors and Actuators A 209 (2014) 52–56
Fig. 4. Changes of voltage (reverse current fixed at 22 A) as a function of time from the Pt/porous GaN nanonetwork Schottky diode exposed to 10,000 ppm H2 gas diluted in air at room temperature. The measurement was repeated for 4 cycles.
Mg was used as a compensated dopant to decrease the electron concentration in the porous GaN nanonetwork in order to increase the depletion region width. According to the Hall effect measurement, the conduction type is not changed (n type) since the porous GaN nanonetwork is slightly doped. By Mg compensatory doping, we fabricated a Schottky diode with a rectified I–V curve. Compared with a Pt/planar GaN film Schottky diode [27], the Schottky barrier in our work is about 0.2 eV smaller. The reduced Schottky barriers are often discovered in nano-Schottky diodes [21]. Because of the in-plane electrical conduction of the porous GaN nanonetwork, device fabrication is as easy as that on a film. No nanofabrication equipment is needed in the fabrication of the parallel nano-Schottky diodes. As shown in Fig. 3(b), the reversed current differs a lot from that in air after exposing to 1% H2 gas and the forward current changes a little. 3.3. Sensing measurement Fig. 4 shows the changes of voltage (left Y-axis) of the Pt/porous GaN nanonetwork Schottky diode at a fixed reverse current of 22 A as a function of time in 10,000 ppm H2 gas diluted in air at room temperature. To describe the hydrogen detection, the relative response of both V [28,29] and I/I [4,6] are used. In Fig. 4, the right Y-axis indicates the relative response V/V = (Vair − VH2 )/Vair , where V stands for the voltage in air (Vair , the steady value before exposing to H2 ) and V is defined as Vair − VH2 . The voltage V keeps in constant of 1.92 V in air when the current is fixed at 22 A. Once the Schottky diode was exposed to 10,000 ppm H2 , the measured voltage decreased to 1.52 V. It is well known that the catalytic metal Pt would dissociate the H2 molecules to H atoms. Some of these H atoms diffuse and adsorb in the metal–semiconductor interface, forming a dipole layer and hence leads to a decrease of Schottky barrier [4,30]. As discussed in Fig. 3(a), we observed a reduction of Schottky barrier from 0.497 to 0.454 eV. That is the reason why the voltage decreases after exposing to H2. The response time (defined as the time duration over which 90% of the voltage change occurred) is 1 min. If the Schottky diode was measured in H2 gas in N2 , the response time would be shorter due to the remove of the affection of oxygen in the air [5]. The measured voltage recovers to 97% of the original voltage when the Schottky diode is exposed to the air for 10 min. If the diode is heated to 100 ◦ C, it could fully recover to the original voltage within several minutes (not shown here). The recovery is due to the consumption of the dissociated H atoms by O2 in the air through
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Fig. 5. Changes of voltage as a function of time in various H2 concentrations ranging from 320 to 10,000 ppm diluted in air at room temperature. The inset is the V versus H2 concentrations.
chemical reaction [31]. The measurement was repeated for 4 cycles with almost same performance, indicating good repeatability and reversibility. Since the Schottky diode H2 sensor operates at room temperature, the power consumption is only caused by measurement with value of about 0.045 mW. This is promising in energy saving, especially for a long-term H2 monitor. The performance of the Schottky diode in various H2 concentrations ranging from 320 to 10,000 ppm was also carried out and shown in Fig. 5. In all H2 concentrations, after exposing to the H2 gas the voltage decreases and then keeps at a constant value (steady state) within 3 min. The relative response V/V is about 23% in 10,000 ppm H2 . The changed values of the voltage V, however, depend on the H2 concentrations exposed to, since the V is proportional to the amount of H atoms ni (per unit area) adsorbed in the metal–semiconductor interface, which is determined by the hydrogen concentrations in the ambient [32]. It has been an important topic but difficult problem to detect low H2 concentration, such as <500 ppm. The limit of H2 detection (LODH2 ) is 320 ppm for the porous GaN nanonetwork reported here. Minimizing the leakage current or the noise (such as caused by temperature fluctuation) may benefit the improvement of LODH2 . To make it clear, we extracted and plotted the voltage change V versus H2 concentrations in the inset. Although the V increases with the increase of H2 concentration, the graph in the inset could be divided into two regimes around 1000 ppm H2 according to the changed rate of V. In low H2 concentration regime, the V is low but changes fast while in high H2 concentration regime it is large but changes slowly. 4. Conclusions In conclusion, we have fabricated a nano-Schottky diode type H2 sensor on a porous GaN nanonetwork. Based on the SEM images and I–V measurements, the Pt/porous GaN nanonetwork Schottky diode could be regarded as one H2 sensor element comprised of nanoSchottky diodes in parallel. This Schottky diode on porous GaN nanonetwork is able to perform well at room temperature in detecting hydrogen gas with concentrations from 320 to 10,000 ppm. Because of its in-plane electrical conductivity, a two-mask process without any nanofabrication equipment is enough for the fabrication of H2 nano-sensors on the porous GaN nanonetwork, which is as easy as that on a GaN film. The in-plane electrical conductive GaN nanonetwork may provide a novel material and technology for the fabrication of nano-sensors.
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Acknowledgements The authors are grateful to thank Y. Kanamori, T. Wu, and B. Thubthimthong for scientific discussion. This work was supported by a research project, Grant-in-Aid for Scientific Research (A 24246019). One of the authors (A.Z.) appreciates the China Scholarship Council (CSC) for financial support. References [1] J.L. Rouviere, J.L. Weyher, M. Seelmann-Eggebert, S. Porowski, Polarity determination for GaN films grown on (0 0 0 1) sapphire and high-pressure-grown GaN single crystals, Appl. Phys. Lett. 73 (1998) 668–670. [2] W. Lim, J.S. Wright, B.P. Gila, J.L. Johnson, A. Ural, T. Anderson, F. Ren, S.J. Pearton, Room temperature hydrogen detection using Pd-coated GaN nanowires, Appl. Phys. Lett. 93 (2008) 072109-1–072109-3. [3] F. Yun, S. Chevtchenko, Y.-T. Moon, H. Morkoc, T.J. Fawcett, J.T. Wolan, GaN resistive hydrogen gas sensors, Appl. Phys. Lett. 87 (2005) 073507-1–073507-3. [4] J.-R. Huang, W.-C. Hsu, Y.-J. Chen, T.-B. Wang, K.-W. Lin, H.-I. Chen, W.C. Liu, Comparison of hydrogen sensing characteristics for Pd/GaN and Pd/Al0.3Ga0.7As Schottky diodes, Sens. Actuators B: Chem. 117 (2006) 151–158. [5] J.-R. Huang, W.-C. Hsu, H.-I. Chen, W.-C. Liu, Comparative study of hydrogen sensing characteristics of a Pd/GaN Schottky diode in air and N2 atmospheres, Sens. Actuators B: Chem. 123 (2007) 1040–1048. [6] T.-H. Tsai, J.-R. Huang, K.-W. Lin, W.-C. Hsu, H.-I. Chen, W.-C. Liu, Improved hydrogen sensing characteristics of a Pt/SiO2 /GaN Schottky diode, Sens. Actuators B: Chem. 129 (2008) 292–302. [7] T.-H. Tsai, J.-R. Huang, K.-W. Lin, C.-W. Hung, W.-C. Hsu, H.-I. Chen, W.-C. Liu, Improved hydrogen-sensing properties of a Pt/SiO2 /GaN schottky diode, Electrochem. Solid-State Lett. 10 (2007) J158–J160. [8] M. Zhao, J.X. Huang, C.W. Ong, Room-temperature resistive H2 sensing response of Pd/WO3 nanocluster-based highly porous film, Nanotechnology 23 (2012) 315503. [9] H. Liu, D. Ding, C. Ning, Z. Li, Wide-range hydrogen sensing with Nb-doped TiO2 nanotubes, Nanotechnology 23 (2012) 015502. [10] J.S. Wright, W. Lim, D.P. Norton, S.J. Pearton, F. Ren, J.L. Johnson, A. Ural, Nitride and oxide semiconductor nanostructured hydrogen gas sensors, Semicond. Sci. Technol. 25 (2010) 024002. [11] G.D.J. Smit, S. Rogge, T.M. Klapwijk, Enhanced tunneling across nanometerscale metal–semiconductor interfaces, Appl. Phys. Lett. 80 (2002) 2568–2570. [12] G.D.J. Smit, S. Rogge, T.M. Klapwijk, Scaling of nano-Schottky-diodes, Appl. Phys. Lett. 81 (2002) 3852–3854. [13] A. Zhong, K. Hane, Characterization of GaN nanowall network and optical property of InGaN/GaN quantum wells by molecular beam epitaxy, Jpn. J. Appl. Phys. 52 (2013) 08JE13. [14] A. Zhong, K. Hane, Growth of GaN nanowall network on Si (1 1 1) substrate by molecular beam epitaxy, Nanoscale Res. Lett. 7 (2012) 686. [15] M. Kesaria, S. Shetty, S.M. Shivaprasad, Evidence for dislocation induced spontaneous formation of GaN nanowalls and nanocolumns on bare C-plane sapphire, Cryst. Growth Des. 11 (2011) 4900–4903. [16] Y. Shimizu, Y. Nakamura, M. Egashira, Effects of diffusivity of hydrogen and oxygen through pores of thick film SnO2 -based sensors on their sensing properties, Sens. Actuators B: Chem. 13 (1993) 128–131. [17] Y. Shimizu, T. Maekawa, Y. Nakamura, M. Egashira, Effects of gas diffusivity and reactivity on sensing properties of thick film SnO2 -based sensors, Sens. Actuators B: Chem. 46 (1998) 163–168. [18] F. Yang, S.-C. Kung, M. Cheng, J.C. Hemminger, R.M. Penner, Smaller is faster and more sensitive: the effect of wire size on the detection of hydrogen by single palladium nanowires, ACS Nano 4 (2010) 5233–5244. [19] B.K. Duan, P.W. Bohn, Response of nanostructured Pt/GaN Schottky barriers to carbon monoxide, Sens. Actuators Phys. 194 (2013) 220–227. [20] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley-Interscience, Hoboken, NJ, 2006. [21] S.-Y. Lee, C.-O. Jang, J.-H. Hyung, T.-H. Kim, S.-K. Lee, High-temperature characteristics of GaN nano-Schottky diodes, Physica E: Low-Dimens. Syst. Nanostruct. 40 (2008) 3092–3096. [22] R.T. Tung, Electron transport at metal–semiconductor interfaces: general theory, Phys. Rev. B 45 (1992) 13509–13523.
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Biographies
Aihua Zhong was born in Fujian, China, 1985. He received his B.S. and M.S. degrees in Material Science and Engineering, China University of Geosciences, Hubei, China, in 2008 and 2011, respectively. He has been a Ph.D. student in Nanomechanics, Graduate School of Mechanical Engineering, Tohoku University, Japan, since 2011. His current research interests include GaN epitaxial growth, semiconductor device, and Microelectromechanical systems (MEMS).
Takashi Sasaki received the Dr. E. degree in mechanical engineering from Tohoku University, Sendai, Japan, in 2012. Since 2012, he has been an Assistant Professor with the Graduate School of Mechanical Engineering, Tohoku University, Sendai, Japan, where he is currently engaged in research and development of microelectromechanical systems for sensor applications.
Kazuhiro Hane received the M.S. and Dr. Eng. degrees from Nagoya University, Nagoya, Japan, in 1980 and 1983, respectively. From 1983 to 1994, he was in the Department of Electrical Engineering, Nagoya University. From 1985 to 1986, he was a Visiting Researcher with the National Research Council of Canada. Since 1994, he has been a Professor in the Gradual School of Mechanical Engineering, Tohoku University, Sendai, Japan, where he is currently engaged in research and development of optical microsensors and optical microelectromechanical systems.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Platinum/porous GaN nanonetwork metal-semiconductor Schottky diode for room temperature hydrogen sensor Aihua Zhong ∗ , Takashi Sasaki, Kazuhiro Hane Department of Nanomechanics, Tohoku University, Sendai 980-8579, Japan
a r t i c l e
i n f o
Article history: Received 7 September 2013 Received in revised form 24 December 2013 Accepted 9 January 2014 Available online 18 January 2014 Keywords: Honeycomb GaN nanonetwork Nano-Schottky diode H2 sensor
a b s t r a c t An in-plane electrically conductive honeycomb GaN nanonetwork grown by molecular beam epitaxy was used to fabricate a platinum (Pt)/porous GaN nanonetwork Schottky diode type hydrogen sensor. The Pt Schottky contact is a nanonetwork with typical width of 40 nm. Both the scanning electron microscopy image and current–voltage curve indicates that the Pt/porous GaN nanonetwork Schottky diode with barrier height of 0.497 eV and ideality factor of 38.5 is comprised of parallel nano-Schotttky diodes. The operating temperature of this Schottky diode hydrogen sensor on the porous GaN nanonetwork is successfully decreased to room temperature and it performs well in detecting hydrogen gas with various concentrations from 320 to 10,000 ppm. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It is of great interest to develop a hydrogen (H2 ) sensor that is capable to operate in harsh environmental conditions such as chemically corrosive ambient. Because of its high resistance to acids and alkalis as well as large band gap (3.4 eV) [1], H2 sensor based on GaN semiconductor could be used for many harsh applications. These include gas sensing operations in a chemical reactor processing, fuel leak detections in space vehicles as well as automobiles, and emissions from industrial process [2,3]. For these sensors, it is important to operate with minimum power consumption near room temperature, especially for a long-term H2 monitor. H2 sensors on the planar GaN film have been investigated. A resistive type H2 sensor on a GaN film has been demonstrated in 2005 [3]. The primary researches are focused on the platinum (Pt) or palladium (Pd)/planar GaN film Schottky diode type H2 sensors, of which the catalytic metal Pt or Pd dissociates the H2 molecules to H atoms [4,5]. Schottky diodes are well formed on the planar GaN film, exhibiting high sensitivity and stability. Additionally, a Pt/SiO2 /GaN (MIS, metal–insulator–semiconductor) Schottky diode type H2 sensor also has been investigated by Tsai and coworkers, which improves the sensitivity, response time, and thermal stability [6,7]. Both the resistive and Schottky type H2 sensors on the GaN film, however, are required to be heated, typically to 200 ◦ C [3,5]. The response time [5] is about 7 min in 980 ppm H2 at 60 ◦ C. On the other hand, nanostructures are promising in improving the performance of H2 sensor due to its nanosize effect and large
∗ Corresponding author. Tel.: +81 22 795 6965. E-mail address: [email protected] (A. Zhong). 0924-4247/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2014.01.014
surface area to volume ratio [8,9]. To date, there are only a few investigations of H2 sensors on GaN nanostructures [2,10] partly due to the difficulty of nano-sensor fabrication. Lim and coworkers [2] have reported a resistive type H2 sensor on the GaN nanowire, which could lower the operating temperature to room temperature. In comparison with a resistive type H2 sensor, a Schottky diode type H2 sensor exhibits higher sensitivity [3,5]. Moreover, when the Schottky diode is downscaled to nanoscale regime, the nanoSchottky diode is different from the common large Schottky diode [11,12] and may exhibit promising performance. To our knowledge, there has been no report on a Schottky diode type H2 sensor made from a GaN nanostructure. In this work, we demonstrate a Schottky diode type H2 sensor on a porous GaN nanonetwork. The Ga-polar porous GaN nanonetwork, which is of high quality like a GaN nanowire [13–15], was epitaxially grown on a (1 1 1) Si substrate. Different from a separated nanowire or nanotube, the porous GaN nanonetwork is continuous for electric current in the lateral direction. Because of its in-plane electrical conductivity, fabrication of an electrical device on the porous GaN nanonetwork is expected to be as easy as that on a planar GaN film. The characteristic of a Schottky diode on the porous GaN nanonetwork and its performance in sensing H2 gas at room temperature was investigated. Using a Si wafer as the substrate, Si-based micromachining as well as integrated circuit (IC) can be applied to fabricate an integrated sensor. 2. Experimental details A slightly Mg doped porous GaN nanonetwork was epitaxially grown on a 3-in. Si (1 1 1) wafer by a molecular beam epitaxy system under a nitrogen-rich condition. The growth process in detail
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Fig. 1. Two-mask fabrication process of the H2 sensor on a porous GaN nanonetwork.
was similar to the previous publication [13]. For Mg doping, an Mg cell was heated with pressure in the order of 10−9 Torr. The porous GaN nanonetwork was 400 nm thick with electron concentration of about 8 × 1016 cm−3 measured by Hall Effect measurement system. The native oxide on the porous GaN nanonetwork was removed by a solution of HCl:H2 O = 1:1. Then a Schottky diode type H2 sensor was fabricated on this porous GaN nanonetwork by a two-mask process, as shown in Fig. 1. Pt Schottky contact with a thickness of 50 nm and an area of 0.16 mm2 was deposited on the porous GaN nanonetwork by a sputtering machine using a patterned Si wafer as a mask. Another electrode Ti (20 nm)/Al (80 nm) was deposited by an e-beam evaporator and was patterned by photolithography and lift off. After fabrication, the Schottky diode was exposed to various
H2 concentrations diluted in dry air in a steel chamber. The flow rate was kept at 100 sccm. 3. Results and discussions 3.1. Morphology and schematic profile Under a nitrogen-rich growth condition, GaN grows in the three-dimensional model to a honeycomb GaN nanowall network, namely porous GaN nanonetwork structure as shown in Fig. 2(a), which was measured by a field-emission scanning electron microscopy (FESEM). The GaN nanowalls overlap and interlace with one another, forming an in-plane electrically conductive porous GaN nanonetwork [14,15]. The typical width of the GaN nanowall
Fig. 2. (a) FESEM image of a porous GaN nanonetwork; (b) optical image of a Pt/porous GaN nanonetwork Schottky diode; (c) FESEM image of the Pt Schottky contact; (d) cross-sectional image of GaN nanonetwork coated with Pt electrode; (e) EDX mapping; and (f) schematic profile of the Schottky diode on the GaN nanowall network.
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and size of pores are 30 and 50 nm, respectively. In the gas sensing process, the diffusivity of gases into the sensor and their reactivity with sensitive material together determine the performance of a gas sensor [16,17]. Because of its porous structure, the porous GaN nanonetwork is promising in terms of diffusivity of gas. Fig. 2(b) is an optical image of a device on a GaN nanonetwork. After deposition of a 50 nm thick Pt electrode, the surface still keeps the nanonetwork morphology as shown in Fig. 2(c). This means the Schottky contact Pt electrode is also a nanonetwork consisted of Pt stripes with large surface area to volume area. It is reported that both the response and recover times of the H2 sensor are linearly correlated with the surface area/volume ratio [18,19]. From the cross-sectional image in Fig. 2(d), we could observe that the Pt stripe with typical width of 40 nm was deposited on the top of the GaN nanowall. Fig. 2(e) shows the distribution of Pt, Ga, and Si elements measured by an energy dispersive X-ray spectroscopy (EDX) equipped in the FESEM. The bottom is the Si substrate and the middle part is GaN. For Pt, it locates on the top of the GaN nanowall. Based on the images above, we made a schematic profile of the Schottky diode on the GaN nanowall network as shown in Fig. 2(f). The contacts between Pt and GaN nanowalls are stripes with typical width of 40 nm. These stripes connect together, comprising the Pt electrode. When the device is forward biased, the current flows through these Pt/GaN nanowall stripes and then arrives the other electrode Ti/Al by means of the continuous GaN nanowalls between the two electrodes.
3.2. Current–voltage curve measurement We measured the current–voltage (I–V) curves of the Pt/porous GaN nanonetwork Schottky diode at room temperature and show them in Fig. 3(a) and (b). For a Schottky diode the I–V curve is described by the thermionic emission theory [20] expressed as follows:
I = I0 exp
qV nkT
qϕ B
− 1 = AA∗ T 2 exp −
kT
exp
qV nkT
−1
where I0 is the saturation current, A is the area of Pt electrode, A* is the effective Richardson constant (24 A cm−2 K−2 for GaN), q is the electron charge, V is the applied voltage, n is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, and ϕB is the zero-biased Schottky barrier height. Through fitting the measured I–V curves to the thermionic emission theory as shown in Fig. 3(a), the Schottky barrier height ϕB in air and in 1% H2 are determined to be 0.497 and 0.454 eV, respectively. The ideality factor n are also determined, 38.5 and 136.8, respectively. The fitting voltage range is from 0.25 to 0.85 V. Similar results [21] were reported for Au/individual GaN nanowire nano-Schottky diode with the Schottky barrier height of 0.48 eV and ideality factor of 12. The large ideality factor of the nano-Schottky diode is mainly attributed to the large tunneling current, which is resulted from the thin depletion region of a small Schottky diode [22,23]. Different from a large Schottky diode on a GaN film, the reverse current of the Pt/porous GaN nanonetwork Schottky diode is large as shown in Fig. 3(b). Smit and coworkers [11,12] suggested that in case of a small nano-Schottky diode where the Schottky contact was smaller than the characteristic length lc , the reverse current would be profoundly affected by the size and shape of the Schottky contact. For a very small Schottky contact, the reverse current is comparable with the forward current. The characteristic length lc is defined as follows [12,20]:
lc =
2εs Vs qNd
(2)
Fig. 3. (a) Fitting the I–V curves of the Pt/porous GaN nanonetwork Schottky diode with the thermionic emission theory and (b) I–V curves measured in air and in 1% H2 gas at room temperature.
where the total potential drop over the space charge region Vs is expressed by Vs =
ϕB − kT ln (NC /Nd ) −V q
(3)
Here εs is the dielectric constant, q is the electron charge, Nd is the carrier concentration, ϕB is the Schottky barrier, V is the applied voltage, and NC is the effective density of states in the conduction band. For GaN, the characteristic length lc is calculated to be 77 nm when parameters εs = 10.4 [24], Nd = 8 × 1016 cm−3 , ϕB = 0.497 eV, and NC = 2.3 × 1018 cm−3 [25] are used. From the SEM results above, the typical width of the Pt stripe is about 40 nm, smaller than the characteristic length lc of 77 nm. Hence the 40 nm Pt Schottky contacts could be considered as small nano-Schottky contacts and the large reverse current in Fig. 3(b) is a characteristic of these small Pt nano-Schottky contacts. Therefore, the Pt/porous GaN nanonetwork Schottky diode could be regarded as one Schottky diode type sensor element comprised of multiple nano-Schottky diodes in parallel. By utilizing the multiple parallel nano-Schottky diodes as a sensor element, the uniformity and reliability of the sensor would be improved [26]. For an undoped porous GaN nanonetwork with electron concentration of 2 × 1017 cm−3 , we did not observe a rectified I–V curve (not shown here). Thus, we grew a slightly Mg doped porous GaN nanonetwork with lower electron concentration of 8 × 1016 cm−3 .
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Fig. 4. Changes of voltage (reverse current fixed at 22 A) as a function of time from the Pt/porous GaN nanonetwork Schottky diode exposed to 10,000 ppm H2 gas diluted in air at room temperature. The measurement was repeated for 4 cycles.
Mg was used as a compensated dopant to decrease the electron concentration in the porous GaN nanonetwork in order to increase the depletion region width. According to the Hall effect measurement, the conduction type is not changed (n type) since the porous GaN nanonetwork is slightly doped. By Mg compensatory doping, we fabricated a Schottky diode with a rectified I–V curve. Compared with a Pt/planar GaN film Schottky diode [27], the Schottky barrier in our work is about 0.2 eV smaller. The reduced Schottky barriers are often discovered in nano-Schottky diodes [21]. Because of the in-plane electrical conduction of the porous GaN nanonetwork, device fabrication is as easy as that on a film. No nanofabrication equipment is needed in the fabrication of the parallel nano-Schottky diodes. As shown in Fig. 3(b), the reversed current differs a lot from that in air after exposing to 1% H2 gas and the forward current changes a little. 3.3. Sensing measurement Fig. 4 shows the changes of voltage (left Y-axis) of the Pt/porous GaN nanonetwork Schottky diode at a fixed reverse current of 22 A as a function of time in 10,000 ppm H2 gas diluted in air at room temperature. To describe the hydrogen detection, the relative response of both V [28,29] and I/I [4,6] are used. In Fig. 4, the right Y-axis indicates the relative response V/V = (Vair − VH2 )/Vair , where V stands for the voltage in air (Vair , the steady value before exposing to H2 ) and V is defined as Vair − VH2 . The voltage V keeps in constant of 1.92 V in air when the current is fixed at 22 A. Once the Schottky diode was exposed to 10,000 ppm H2 , the measured voltage decreased to 1.52 V. It is well known that the catalytic metal Pt would dissociate the H2 molecules to H atoms. Some of these H atoms diffuse and adsorb in the metal–semiconductor interface, forming a dipole layer and hence leads to a decrease of Schottky barrier [4,30]. As discussed in Fig. 3(a), we observed a reduction of Schottky barrier from 0.497 to 0.454 eV. That is the reason why the voltage decreases after exposing to H2. The response time (defined as the time duration over which 90% of the voltage change occurred) is 1 min. If the Schottky diode was measured in H2 gas in N2 , the response time would be shorter due to the remove of the affection of oxygen in the air [5]. The measured voltage recovers to 97% of the original voltage when the Schottky diode is exposed to the air for 10 min. If the diode is heated to 100 ◦ C, it could fully recover to the original voltage within several minutes (not shown here). The recovery is due to the consumption of the dissociated H atoms by O2 in the air through
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Fig. 5. Changes of voltage as a function of time in various H2 concentrations ranging from 320 to 10,000 ppm diluted in air at room temperature. The inset is the V versus H2 concentrations.
chemical reaction [31]. The measurement was repeated for 4 cycles with almost same performance, indicating good repeatability and reversibility. Since the Schottky diode H2 sensor operates at room temperature, the power consumption is only caused by measurement with value of about 0.045 mW. This is promising in energy saving, especially for a long-term H2 monitor. The performance of the Schottky diode in various H2 concentrations ranging from 320 to 10,000 ppm was also carried out and shown in Fig. 5. In all H2 concentrations, after exposing to the H2 gas the voltage decreases and then keeps at a constant value (steady state) within 3 min. The relative response V/V is about 23% in 10,000 ppm H2 . The changed values of the voltage V, however, depend on the H2 concentrations exposed to, since the V is proportional to the amount of H atoms ni (per unit area) adsorbed in the metal–semiconductor interface, which is determined by the hydrogen concentrations in the ambient [32]. It has been an important topic but difficult problem to detect low H2 concentration, such as <500 ppm. The limit of H2 detection (LODH2 ) is 320 ppm for the porous GaN nanonetwork reported here. Minimizing the leakage current or the noise (such as caused by temperature fluctuation) may benefit the improvement of LODH2 . To make it clear, we extracted and plotted the voltage change V versus H2 concentrations in the inset. Although the V increases with the increase of H2 concentration, the graph in the inset could be divided into two regimes around 1000 ppm H2 according to the changed rate of V. In low H2 concentration regime, the V is low but changes fast while in high H2 concentration regime it is large but changes slowly. 4. Conclusions In conclusion, we have fabricated a nano-Schottky diode type H2 sensor on a porous GaN nanonetwork. Based on the SEM images and I–V measurements, the Pt/porous GaN nanonetwork Schottky diode could be regarded as one H2 sensor element comprised of nanoSchottky diodes in parallel. This Schottky diode on porous GaN nanonetwork is able to perform well at room temperature in detecting hydrogen gas with concentrations from 320 to 10,000 ppm. Because of its in-plane electrical conductivity, a two-mask process without any nanofabrication equipment is enough for the fabrication of H2 nano-sensors on the porous GaN nanonetwork, which is as easy as that on a GaN film. The in-plane electrical conductive GaN nanonetwork may provide a novel material and technology for the fabrication of nano-sensors.
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Acknowledgements The authors are grateful to thank Y. Kanamori, T. Wu, and B. Thubthimthong for scientific discussion. This work was supported by a research project, Grant-in-Aid for Scientific Research (A 24246019). One of the authors (A.Z.) appreciates the China Scholarship Council (CSC) for financial support. References [1] J.L. Rouviere, J.L. Weyher, M. Seelmann-Eggebert, S. Porowski, Polarity determination for GaN films grown on (0 0 0 1) sapphire and high-pressure-grown GaN single crystals, Appl. Phys. Lett. 73 (1998) 668–670. [2] W. Lim, J.S. Wright, B.P. Gila, J.L. Johnson, A. Ural, T. Anderson, F. Ren, S.J. Pearton, Room temperature hydrogen detection using Pd-coated GaN nanowires, Appl. Phys. Lett. 93 (2008) 072109-1–072109-3. [3] F. Yun, S. Chevtchenko, Y.-T. Moon, H. Morkoc, T.J. Fawcett, J.T. Wolan, GaN resistive hydrogen gas sensors, Appl. Phys. Lett. 87 (2005) 073507-1–073507-3. [4] J.-R. Huang, W.-C. Hsu, Y.-J. Chen, T.-B. Wang, K.-W. Lin, H.-I. Chen, W.C. Liu, Comparison of hydrogen sensing characteristics for Pd/GaN and Pd/Al0.3Ga0.7As Schottky diodes, Sens. Actuators B: Chem. 117 (2006) 151–158. [5] J.-R. Huang, W.-C. Hsu, H.-I. Chen, W.-C. Liu, Comparative study of hydrogen sensing characteristics of a Pd/GaN Schottky diode in air and N2 atmospheres, Sens. Actuators B: Chem. 123 (2007) 1040–1048. [6] T.-H. Tsai, J.-R. Huang, K.-W. Lin, W.-C. Hsu, H.-I. Chen, W.-C. Liu, Improved hydrogen sensing characteristics of a Pt/SiO2 /GaN Schottky diode, Sens. Actuators B: Chem. 129 (2008) 292–302. [7] T.-H. Tsai, J.-R. Huang, K.-W. Lin, C.-W. Hung, W.-C. Hsu, H.-I. Chen, W.-C. Liu, Improved hydrogen-sensing properties of a Pt/SiO2 /GaN schottky diode, Electrochem. Solid-State Lett. 10 (2007) J158–J160. [8] M. Zhao, J.X. Huang, C.W. Ong, Room-temperature resistive H2 sensing response of Pd/WO3 nanocluster-based highly porous film, Nanotechnology 23 (2012) 315503. [9] H. Liu, D. Ding, C. Ning, Z. Li, Wide-range hydrogen sensing with Nb-doped TiO2 nanotubes, Nanotechnology 23 (2012) 015502. [10] J.S. Wright, W. Lim, D.P. Norton, S.J. Pearton, F. Ren, J.L. Johnson, A. Ural, Nitride and oxide semiconductor nanostructured hydrogen gas sensors, Semicond. Sci. Technol. 25 (2010) 024002. [11] G.D.J. Smit, S. Rogge, T.M. Klapwijk, Enhanced tunneling across nanometerscale metal–semiconductor interfaces, Appl. Phys. Lett. 80 (2002) 2568–2570. [12] G.D.J. Smit, S. Rogge, T.M. Klapwijk, Scaling of nano-Schottky-diodes, Appl. Phys. Lett. 81 (2002) 3852–3854. [13] A. Zhong, K. Hane, Characterization of GaN nanowall network and optical property of InGaN/GaN quantum wells by molecular beam epitaxy, Jpn. J. Appl. Phys. 52 (2013) 08JE13. [14] A. Zhong, K. Hane, Growth of GaN nanowall network on Si (1 1 1) substrate by molecular beam epitaxy, Nanoscale Res. Lett. 7 (2012) 686. [15] M. Kesaria, S. Shetty, S.M. Shivaprasad, Evidence for dislocation induced spontaneous formation of GaN nanowalls and nanocolumns on bare C-plane sapphire, Cryst. Growth Des. 11 (2011) 4900–4903. [16] Y. Shimizu, Y. Nakamura, M. Egashira, Effects of diffusivity of hydrogen and oxygen through pores of thick film SnO2 -based sensors on their sensing properties, Sens. Actuators B: Chem. 13 (1993) 128–131. [17] Y. Shimizu, T. Maekawa, Y. Nakamura, M. Egashira, Effects of gas diffusivity and reactivity on sensing properties of thick film SnO2 -based sensors, Sens. Actuators B: Chem. 46 (1998) 163–168. [18] F. Yang, S.-C. Kung, M. Cheng, J.C. Hemminger, R.M. Penner, Smaller is faster and more sensitive: the effect of wire size on the detection of hydrogen by single palladium nanowires, ACS Nano 4 (2010) 5233–5244. [19] B.K. Duan, P.W. Bohn, Response of nanostructured Pt/GaN Schottky barriers to carbon monoxide, Sens. Actuators Phys. 194 (2013) 220–227. [20] S.M. Sze, K.K. Ng, Physics of Semiconductor Devices, 3rd ed., Wiley-Interscience, Hoboken, NJ, 2006. [21] S.-Y. Lee, C.-O. Jang, J.-H. Hyung, T.-H. Kim, S.-K. Lee, High-temperature characteristics of GaN nano-Schottky diodes, Physica E: Low-Dimens. Syst. Nanostruct. 40 (2008) 3092–3096. [22] R.T. Tung, Electron transport at metal–semiconductor interfaces: general theory, Phys. Rev. B 45 (1992) 13509–13523.
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Biographies
Aihua Zhong was born in Fujian, China, 1985. He received his B.S. and M.S. degrees in Material Science and Engineering, China University of Geosciences, Hubei, China, in 2008 and 2011, respectively. He has been a Ph.D. student in Nanomechanics, Graduate School of Mechanical Engineering, Tohoku University, Japan, since 2011. His current research interests include GaN epitaxial growth, semiconductor device, and Microelectromechanical systems (MEMS).
Takashi Sasaki received the Dr. E. degree in mechanical engineering from Tohoku University, Sendai, Japan, in 2012. Since 2012, he has been an Assistant Professor with the Graduate School of Mechanical Engineering, Tohoku University, Sendai, Japan, where he is currently engaged in research and development of microelectromechanical systems for sensor applications.
Kazuhiro Hane received the M.S. and Dr. Eng. degrees from Nagoya University, Nagoya, Japan, in 1980 and 1983, respectively. From 1983 to 1994, he was in the Department of Electrical Engineering, Nagoya University. From 1985 to 1986, he was a Visiting Researcher with the National Research Council of Canada. Since 1994, he has been a Professor in the Gradual School of Mechanical Engineering, Tohoku University, Sendai, Japan, where he is currently engaged in research and development of optical microsensors and optical microelectromechanical systems.