Electrochemical and Solid-State Letters, 10 共12兲 J158-J160 共2007兲
J158
1099-0062/2007/10共12兲/J158/3/$20.00 © The Electrochemical Society
Improved Hydrogen-Sensing Properties of a Pt/SiO2 /GaN Schottky Diode Tsung-Han Tsai,a Jun-Rui Huang,a Kun-Wei Lin,b Chin-Wen Hung,a Wei-Chou Hsu,a,z Huey-Ing Chen,c and Wen-Chau Liua a Institute of Microelectronics, Department of Electrical Engineering, and cDepartment of Chemical Engineering, National Cheng-Kung University, Tainan, Taiwan 70101 b Department of Computer Science and Information Engineering, Chaoyang University of Technology, Taichung County, Taiwan
Pt/SiO2 /GaN 共MIS兲 and Pt/GaN 共MS兲 Schottky diodes exhibit hydrogen-sensing characteristics, including forward sensitivity ratios 共SF兲 of 14685 and 603.9, reverse sensitivity ratios 共SR兲 of 44636 and 5626 共in 9970 ppm H2 /air兲, and Schottky barrier height variations 共⌬b兲 of 231.6 and 195.9 meV 共in 9970 ppm H2 /air兲 at 300 K, respectively. As compared with the MS device, the studied MIS device manifests a larger hydrogen-detection capability. This result implies that the SiO2 insulator surface plays a strikingly important role in producing the excellent hydrogen-sensing response. Furthermore, reproducible responses at various temperatures are also realized. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2787873兴 All rights reserved. Manuscript submitted July 23, 2007; revised manuscript received August 10, 2007. Available electronically October 8, 2007.
Recently, hydrogen gas has been considered as a clean energy source instead of petroleum. However, hydrogen is dangerous for its character of autoignition and explosion in air. The determination of hazardous hydrogen concentration in air 共4.65–93.9 vol %兲 is an important issue in many areas of human activity.1 Due to the progressive and severe requirements in industrial safety and environmental protection, the use of hydrogen sensors has attracted considerable attention in gas detection.2,3 Furthermore, there is a strong interest in developing wide bandgap semiconductor 关e.g., GaN 共3.4 eV兲兴-based hydrogen gas sensors which retain their semiconducting properties and can be operated at high temperature ranges.4-8 Based on characteristics of high electron saturation velocity, high breakdown electric field, superior thermal and chemical stability, and weak pinning of Fermi levels,9 GaN-based hydrogen sensors with an intermediate insulator layer have become a promising candidate to improve hydrogen-sensing performance. The sensing properties, such as sensitivity, response time, and thermal stability, of the catalytic metal/insulator/semiconductor 共MIS兲 hydrogen sensors are significantly better than those of the catalytic metal/semiconductor 共MS兲 hydrogen sensors.5,6,10-12 Therefore, the development of a high-quality thin insulator for the catalytic metal/insulator/GaN Schottky diodes becomes a key issue. In this work, a Pt/insulator/GaN 共MIS兲 Schottky diode hydrogen sensor with a thin SiO2 insulator layer is developed. Improved hydrogen-sensing performance is obtained as compared with the Pt/GaN Schottky diode. The relevant hydrogen sensing and response characteristics at various temperatures are studied. Experimental The GaN samples were grown by a metallorganic chemical vapor deposition 共MOCVD兲 on a 2 in. c-plane sapphire substrate. The epitaxial structure consisted of a 2 m thick undoped GaN buffer layer and a 0.5 m Si-doped 共n-type兲 GaN active layer with a carrier concentration of 1 ⫻ 1018 cm−3. After epitaxial growth, the devices were etched using an inductively-coupled-plasma reactive ion etching 共ICP-RIE兲 system for mesa isolation. The native oxide layer on the wafer was removed by a solution of HCl:H2O = 1:1. Ohmic contacts were made by evaporating 100 nm thick Ti/Al metals and subsequently annealing them by a rapid thermal treatment at 900°C for 90 s in a N2 atmosphere. Then, the insulator layer was deposited by plasma-enhanced chemical vapor deposition 共PECVD兲 at 573 K to form a thin 7.5 nm thick SiO2 layer. Finally, the Schottky contacts were produced by evaporating a 7.5 nm thick catalytic Pt metal. The available Schottky contact area was 2.05 ⫻ 10−3 cm2.
z
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Results and Discussion Figures 1 and 2 show the current-voltage 共I-V兲 characteristics of the studied Pt/GaN 共MS兲 and Pt/SiO2 /GaN 共MIS兲 Schottky diodes measured under atmospheric conditions and when exposed to different-concentration hydrogen gases balanced with air at 300 and 850 K, respectively. At hydrogen concentrations as low as 4.3 ppm H2 /air, the current variation of both studied devices can be detected. As compared with the MS device, the studied MIS device manifests a larger hydrogen-detection capability in either forward or reverse bias at 300 K. When the temperature is elevated up to 850 K, the turn-on voltage and current variations of both studied devices drastically decrease in an air atmosphere, as shown in Fig. 2. Even at 850 K, the reverse-bias current variations of both studied devices for different hydrogen concentrations in an air atmosphere are larger than the forward-bias current variations. Moreover, a higher Schottky barrier height results in a lower current under a reversebias condition than that under a forward bias.13 Hence, the sensing current induced by H2 increases due to the larger Schottky barrier height variation under a reverse-bias condition. The hydrogen sensing mechanism can be briefly described as follows. When hydrogen gases are introduced, some hydrogen molecules are dissociated on the catalytic Pt metal surface and become hydrogen atoms.14 These hydrogen atoms are then adsorbed on the Pt metal surface and subsequently diffuse through the thin Pt metal film until they are adsorbed at the Pt/GaN or Pt/SiO2 interface.15,16 The hydrogen atoms adsorbed at the interface are polarized and form a dipolar layer near the interface. The dipolar layer causes a considerable change of the electrical field across the depletion region near the GaN surface. Subsequently, this leads to a modulation of the effective Schottky barrier height and a change of the forwardand reverse-bias currents.17 Besides, the trapping sites for the atomic hydrogen at the Pt–SiO2 interface are located at the oxygen atoms of the SiO2 surface and not by any catalytic reactions on the metal surface.16 Figure 3 shows the forward sensitivity ratio 共SF兲, reverse sensitivity ratio 共SR兲, and Schottky barrier height variation 共⌬b兲 as a function of hydrogen concentration at 300 and 850 K. The applied voltages of both studied devices are fixed at VF = 0.5 V and VR = −2 V. The forward or reverse sensitivity ratio is defined as7,10-12 SF or SR =
IH2 − Iair Iair
关1兴
where Iair and IH2 are currents measured in air and a hydrogencontaining ambience, respectively. Experimentally, the SF 共SR兲 of the studied MS device is increased from 0.5 共0.95兲 共in 4.3 ppm H2 /air兲 to 603.9 共5626兲 共in 9970 ppm H2 /air兲 at 300 K and from
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Electrochemical and Solid-State Letters, 10 共12兲 J158-J160 共2007兲
J159
Figure 1. I-V characteristics of the studied Pt/GaN 共MS兲 and Pt/SiO2 /GaN 共MIS兲 Schottky diodes measured under atmospheric conditions and when exposed to different-concentration hydrogen gases balanced with air at 300 K.
Figure 2. I-V characteristics of the studied MS and MIS Schottky diodes measured under atmospheric conditions and when exposed to different-concentration hydrogen gases balanced with air at 850 K.
0.38 共0.74兲 共in 4.3 ppm H2 /air兲 to 8.4 共244兲 共in 9970 ppm H2 /air兲 at 850 K, respectively. The SF 共SR兲 of the studied MIS device is increased from 1.24 共1.71兲 共in 4.3 ppm H2 /air兲 to 14685 共44636兲 共in 9970 ppm H2 /air兲 at 300 K and from 0.46 共0.83兲 共in 4.3 ppm H2 /air兲 to 60.84 共906兲 共in 9970 ppm H2 /air兲 at 850 K, respectively. Based on the thermionic-emission transport mechanism, when the applied forward voltage V is larger than 3kT/q, the Schottky barrier height can be calculated as18
Figure 3. Forward sensitivity ratio 共SF兲, reverse sensitivity ratio 共SR兲, and Schottky barrier height variation 共⌬b兲 as a function of hydrogen concentration at 300 and 850 K. The applied voltages of both studied devices are fixed at VF = 0.5 V and VR = −2 V.
b =
冉 冊冉
kT T2 ln AA** q I0
冊
关2兴
where k is the Boltzmann constant, T the absolute temperature, A the Schottky contact area, I0 the saturation current, and A** the effective Richardson constant 共24 A cm−2 K−2 for n-GaN兲19. Experimentally, the ⌬b of the studied MS 共MIS兲 device is increased from 7.2 共20.1兲 共in 4.3 ppm H2 /air兲 to 195.9 共231.6兲 meV 共in 9970 ppm H2 /air兲 at 300 K and from 6.8 共7.4兲 共in 4.3 ppm H2 /air兲 to 53.9 共76.6兲 meV 共in 9970 ppm H2 /air兲 at 850 K, respectively. Accordingly, the large modulation in the Schottky barrier heights and current variations are observed for the studied MIS device Figure 4 shows the transient response curves of the studied 共a兲 MIS and 共b兲 MS devices upon introduction and removal of a 9970 ppm H2 /air gas measured at various temperatures under the forward bias of 1 and 0.35 V, respectively. The studied MIS device demonstrates excellent reproducibility, rapid responses, and large magnitudes of the current changes, as shown in Fig. 4a. Yet, the studied MS device reveals a long recovery time from the final steady-state current back to its initial value, particularly at lower temperatures, as shown in Fig. 4b. The hydrogen-detection adsorption time constant 共a兲 and hydrogen-detection desorption time constant 共b兲 are defined as the time to reach the inverse exponential 共e−1兲 value of the final steady-state current as shown in the inset of Fig. 4b. As compared with the MS device, the studied MIS device exhibits rapid response and recovery time.
Conclusion The hydrogen-sensing and response characteristics of the Pt/SiO2 /GaN and Pt/GaN Schottky diodes under different concen-
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J160
Electrochemical and Solid-State Letters, 10 共12兲 J158-J160 共2007兲 of 44636 共in 9970 ppm H2 /air兲, ⌬b of 231.6 meV 共in 9970 ppm H2 /air兲, and temperature 共300–850 K兲 and voltage-operating range under forward- and reverse-bias conditions. This result implies that the SiO2 insulator surface provides the higher activity of hydrogenadsorption reaction which can improve hydrogen-detection capability. Moreover, the studied MIS device exhibits reproducible responses at various temperatures. Therefore, the studied MIS device exhibits a promise for high-performance hydrogen-sensor applications. Acknowledgments This work was supported by the National Science Council of the Republic of China under contract no. NSC 95-2221-E-006-352MY3. National Cheng-Kung University assisted in meeting the publication costs of this article.
References
Figure 4. Transient response curves of the studied 共a兲 Pt/SiO2 /GaN 共MIS兲 and 共b兲 Pt/GaN 共MS兲 Schottky diodes upon introduction and removal of a 9970 ppm H2 /air gas measured at various temperatures. The applied forward bias of the studied MIS and MS devices are kept at 1 and 0.35 V, respectively. The inset shows the hydrogen detection adsorption time constant 共a兲 and hydrogen detection desorption time constant 共b兲 upon the switching actions as a function of temperature.
trations of hydrogen gases are studied. The studied MIS device exhibits hydrogen-sensing performance, including SF of 14685 and SR
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