Hydrogen sensors based on AlGaN/AlN/GaN HEMT
ARTICLE IN PRESS
Microelectronics Journal 39 (2008) 20–23 www.elsevier.com/locate/mejo
Hydrogen sensors based on AlGaN/AlN/GaN HEMT X.H. Wang, X.L. Wang, C. Feng, C.B. Yang, B.Z. Wang, J.X. Ran, H.L. Xiao, C.M. Wang, J.X. Wang Novel Semiconductor Material Laboratory, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China Received 4 September 2007; accepted 27 October 2007
Abstract Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) were fabricated and characterized for hydrogen sensing. Pt and Ti/Al/Ni/Au metals were evaporated to form the Schottky contact and the ohmic contact, respectively. The sensors can be operated in either the field effect transistor (FET) mode or the Schottky diode mode. Current changes and time dependence of the sensors under the FET and diode modes were compared. When the sensor was operated in the FET mode, the sensor can have larger current change of 8 mA, but its sensitivity is only about 0.2. In the diode mode, the current change was very small under the reverse bias but it increased greatly and gradually saturated at 0.8 mA under the forward bias. The sensor had much higher sensitivity when operated in the diode mode than in the FET mode. The oxygen in the air could accelerate the desorption of the hydrogen and the recovery of the sensor. r 2007 Elsevier Ltd. All rights reserved. Keywords: AlGaN/AlN/GaN HEMT; Hydrogen sensor
1. Introduction Gallium nitride-based materials are very promising candidates for high power, high temperature, and high frequency applications [1–3]. In the meantime, detection of hydrogen leaks at room temperature is necessary for applications in hydrogen-fueled automobiles, protonexchange membrane, solid oxide fuel cells for space craft, and other long-term sensing applications [4]. Gallium nitridebased materials are started to be used as hydrogen sensors in recent years and a lot of progress has been made in this area [5–12]. AlGaN/GaN heterostructures have some special advantages in the application of gas sensing. A high electron sheet carrier concentration can be easily obtained without intentional doping. The two-dimensional electron gas (2DEG) is near the surface and very sensitive to the change of the atmosphere. The sensors based on the heterostructure can provide higher sensitivity than the devices fabricated on GaN layers. By employing AlN interlayer, AlGaN/AlN/ GaN HEMT structures with higher mobility and electron concentration of the 2DEG can be obtained compared with Corresponding author. Tel.: +86 1082304132; fax: +86 10 82304232.
E-mail address: [email protected] (X.H. Wang). 0026-2692/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2007.10.022
conventional AlGaN/GaN structure. This kind of structures has not been used in the application of gas sensing. In this letter, Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) have been fabricated and measured for H2 sensing. The current–voltage (I–V) characteristics of the HEMT under two different modes (FET and Schottky diode) in N2 and 10% H2 in N2 ambient are given. The time response of the sensor in the diode mode at 300 K is also measured. The relationship between the sensitivity and the voltage applied is finally discussed. 2. Device structure and fabricating process Figs. 1(a) and (b) illustrate the schematic cross sections and microscope picture of the Pt/AlGaN/AlN/GaN HEMT studied. The sample was grown by metal-organic chemical-vapor deposition (MOCVD) on 2 in c-plane sapphire substrate. The epitaxial structure consisted of a 3.6 mm thick undoped GaN buffer layer, 100 nm GaN layer of high mobility, 1 nm AlN layer and 16.8 nm AlGaN layer without intentional doping, as shown in Fig. 1(a). More details could be obtained in [13–15]. After epitaxial growth, the devices were processed by B ion implantation for mesa isolation. A four-layer Ti/Al/Ni/Au for ohmic contacts was
ARTICLE IN PRESS X.H. Wang et al. / Microelectronics Journal 39 (2008) 20–23
evaporated on the wafer, followed by annealing at 860 1C for 30 s in N2 atmosphere. The Schottky contact was produced by evaporating 20 nm thick catalytic Pt metal with the help of the Si3N4 medium. The gate dimension of the device is 5 400 mm2. Finally, Ti/Au was evaporated as interconnection contacts.
21
The gas sensing experiments were performed in a stainless tube that contained electrical feed-throughs connecting to Keithley 2602 source meter to perform I–V measurement. Most of the experiments were performed at 300 K, with flowing gas of N2, H2 diluted in N2 and air.
Fig. 1. Schematic diagram of (a) cross-section and (b) microscopy image of the studied Pt/AlGaN/AlN/GaN HEMT.
0.06
Current change (A)
0.05 Current (A)
0.010
N2 10% H2
0.04 0.03 0.02 0.01 0.00
0
6
3
9
0.006 0.004 0.002 0.000
12
experient fit
0.008
2
0
4
Voltage(V)
6
8
10
12
Voltage(V)
Fig. 2. (a) I–V characteristics from the HEMT(FET mode) in N2 or 10% H2 in N2 ambient at 300 K and (b) current change as a function of the applied voltage.
0.0010
Current (A)
0.005
Current change (A)
0.006 N2 10% H2
0.004 0.003 0.002 0.001 0.000 -3
-2
-1 0 1 Voltage (V)
2
3
0.0008 0.0006 0.0004 0.0002 0.0000 -0.0002
-3
-2
-1 0 1 Voltage(V)
2
3
Fig. 3. (a) I–V characteristics from the HEMT(diode mode) in N2 or 10% H2 in N2 ambient at 300 K and (b) current change versus voltage.
ARTICLE IN PRESS X.H. Wang et al. / Microelectronics Journal 39 (2008) 20–23
22
3. Device performances AlGaN/GaN HEMT can have two modes of operation for the detection of hydrogen [16]. In the mode of field effect transistor (FET), the change of the drain–source current at different gate biases is the signal; in the mode of Schottky diode, the change of the gate current at zero–drain–source bias is measured. The I–V characteristics of the HEMT operated in FET mode exposed to pure N2 and 10% H2 in N2 at 300 K are shown in Fig. 2(a). It is obvious that the hydrogen has made the current larger. Fig. 2(b) gives the current change as a function of the applied voltage. As the voltage increases, the current change increases proportionally with the applied voltage. When the voltage is above 6 V, the current change saturates gradually at 0.008 A. Fig. 3(a) gives the I–V characteristics of the HEMT operated in diode mode exposed to pure N2 and 10% H2 in N2 at 300 K. A shift of 0.4 V at 300 K is obtained at a fixed forward current for switching from N2 to 10% H2 in N2. Fig. 3(b) gives the current change as the function of the applied voltage. It can be seen that under the reverse bias, 0.0008 0.0007
10% H2
N2
Current (A)
0.0006
H2 off
0.0005 0.0004
air
H2 on
0.0003
S ¼ ðI H2 I N2 Þ=I N2 ,
N2
0.0002 0.0001
0
the current change is very small; under the forward bias, the current change increase greatly between 0.5 and 1.5 V, and finally saturates at 0.8 mA. Fig. 4 shows the time response of forward current of the sensor (diode mode) biased at 1 V in different atmosphere (N2, 10% H2 in N2 and air) at 300 K. Even at room temperature, the change of the forward current under a fixed bias of 1 V is significant and shows there is sufficient cracking of the H2 for the diode to be a gas sensor. It could be seen that the turn-on response of the sensor is very quick (in tens of seconds); the turn-off response in N2 is rather slow (in several hours), while the turn-off response in synthetic air is relatively quicker. The current after exposed to the air is found to be still high compared to the current level at the initial stage, and this could be attributed to the remaining of the H-induced dipoles in the device. The oxygen in the air can react with the hydrogen atom to produce water and inhibit the hydrogen to adsorb at the metal’s surface, so the number of the hydrogen covered at the interface of the metal and the semiconductor is reduced [12]. So the change of the barrier height and the current is reduced. Hydrogen atoms adsorbed at the interface are polarized and create a dipolar layer [6]. The observed Schottky barrier height lowering can be explained in terms of the appearance of a dipolar layer at the interface between Pt metal and AlGaN surface. The dipolar layer at the interface leads to a shift of electrostatic potential of AlGaN and consequently causes a significant lowering of the Schottky barrier height. The sensitivity of the sensor can be defined by
400
200
800
600
1000
1200
Time (s)
Fig. 4. Time response at 300 K of the forward current of the sensor (diode mode) biased at 1 V.
where I H2 and I N2 are the current levels in H2 containing ambient and in N2 ambient, respectively. From the I–V curves in Figs. 2(a) and 3(a), we can calculate the sensitivity under the two different modes. Figs. 5(a) and (b) show the sensitivity versus the applied voltage in the FET and diode mode, respectively. In the FET mode, the sensitivity is only about 0.2 through the whole voltages. But the sensitivity of the diode mode is much higher and there exists a peak
120 100
0.3
Sensitivity (S)
Sensitivity (S)
0.4
0.2 0.1
80
experient Lorentz fit
60 40 20
0.0
0
2
4
6
8
Voltage (V)
10
12
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Voltage (V)
Fig. 5. Sensitivity as a function of the voltage applied to the sensor (a) FET mode and (b) diode mode.
ARTICLE IN PRESS X.H. Wang et al. / Microelectronics Journal 39 (2008) 20–23
around 0.4 V. If the sensor is operated at the voltage near the peak, we can get higher sensitivity. It seems that the peak locates in the range of the thermionic emission region and small variation of the barrier height induced by the hydrogen can have significant current change of the device. 4. Conclusions Pt/AlGaN/AlN/GaN HEMT have been fabricated and characterized for H2 sensing. In the FET mode, the current change increases proportionally with the applied voltage. We can get a large current change as high as 8 mA, while the sensitivity of the FET mode is only about 0.2. In the diode mode, the current change is very small under reverse bias, but it increases rapidly between 0.5 and 1.5 V, and finally saturates at 0.8 mA under forward bias. There is a peak of the sensitivity of the sensor in the diode mode as the applied voltage changes and higher sensitivity can be obtained at 0.4 V biasing. Time response of the sensor in the diode mode under a fixed bias of 1 V is also measured and the oxygen in the air can accelerate the recovery of the sensor. Acknowledgments This work was supported by the Knowledge Innovation Project of Chinese Academy of Sciences under Grant no. YYYJ-0701-02, the National Natural Sciences Foundation of China under Grant nos. 60576046 and 60606002, and the National Basic Research Program of China under Grant nos. 2002CB311903, 2006CB604905 and 513270505. The authors acknowledge Prof. Tangsheng Chen and
23
Chunjiang Ren in Nanjing Electronic Devices Institute for the fabrication of the AlGaN/AlN/GaN gas sensors. References [1] M.A. Khan, A. Bhattarai, J.N. Kuznia, et al., Appl. Phys. Lett. 63 (1993) 1214. [2] M. Miyoshi, H. Ishikawa, T. Egawa, et al., Appl. Phys. Lett. 85 (2004) 1710. [3] X.L. Wang, C.M. Wang, G.X. Hu, et al., Phys. Stat. Sol. (c) 3 (2006) 607. [4] S.J. Pearton, B.S. Kang, S. Kim, et al., J. Phys.: Condens. Matter 16 (2004) R961. [5] B.P. Luther, S.D. Wolter, S.E. Mohney, Sensors Actuators B 56 (1999) 164. [6] J. Schalwig, G. Mu¨ller, M. Eickhoff, et al., Mater. Sci. Eng. B 93 (2002) 207. [7] G. Steinhoff, M. Hermann, W.J. Schaff, et al., Appl. Phys. Lett. 83 (2003) 177. [8] M. Eickhoff, J. Schalwig, G. Steinhoff, et al., Phys. Stat. Sol. (c) 6 (2003) 1908. [9] J. Kim, B.P. Gila, C.R. Abernathy, et al., Solid State Electron. 47 (2003) 1487. [10] J. Song, W. Lu, J.S. Flynn, et al., Appl. Phys. Lett. 87 (2005) 133501-1. [11] K. Matsuo, N. Negoro, J. Kotani, et al., Appl. Surf. Sci. 244 (2005) 273. [12] J.R. Huang, W.Ch. Hsu, Y.J. Chen, et al., Sensors Actuators B 117 (2006) 151. [13] C.M. Wang, X.L. Wang, G.X. Hu, et al., J. Cryst. Growth 289 (2006) 415. [14] X.L. Wang, G.X. Hu, Z.Y. Ma, et al., J. Cryst. Growth 298 (2007) 835. [15] Z.Y. MA, X.L. Wang, G.X. Hu, et al., Chinese Phys. Lett. 24 (2007) 1705. [16] H.T. Wang, B.S. Kang, F. Ren, et al., Appl. Phys. Lett. 87 (2005) 172105.
Microelectronics Journal 39 (2008) 20–23 www.elsevier.com/locate/mejo
Hydrogen sensors based on AlGaN/AlN/GaN HEMT X.H. Wang, X.L. Wang, C. Feng, C.B. Yang, B.Z. Wang, J.X. Ran, H.L. Xiao, C.M. Wang, J.X. Wang Novel Semiconductor Material Laboratory, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People’s Republic of China Received 4 September 2007; accepted 27 October 2007
Abstract Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) were fabricated and characterized for hydrogen sensing. Pt and Ti/Al/Ni/Au metals were evaporated to form the Schottky contact and the ohmic contact, respectively. The sensors can be operated in either the field effect transistor (FET) mode or the Schottky diode mode. Current changes and time dependence of the sensors under the FET and diode modes were compared. When the sensor was operated in the FET mode, the sensor can have larger current change of 8 mA, but its sensitivity is only about 0.2. In the diode mode, the current change was very small under the reverse bias but it increased greatly and gradually saturated at 0.8 mA under the forward bias. The sensor had much higher sensitivity when operated in the diode mode than in the FET mode. The oxygen in the air could accelerate the desorption of the hydrogen and the recovery of the sensor. r 2007 Elsevier Ltd. All rights reserved. Keywords: AlGaN/AlN/GaN HEMT; Hydrogen sensor
1. Introduction Gallium nitride-based materials are very promising candidates for high power, high temperature, and high frequency applications [1–3]. In the meantime, detection of hydrogen leaks at room temperature is necessary for applications in hydrogen-fueled automobiles, protonexchange membrane, solid oxide fuel cells for space craft, and other long-term sensing applications [4]. Gallium nitridebased materials are started to be used as hydrogen sensors in recent years and a lot of progress has been made in this area [5–12]. AlGaN/GaN heterostructures have some special advantages in the application of gas sensing. A high electron sheet carrier concentration can be easily obtained without intentional doping. The two-dimensional electron gas (2DEG) is near the surface and very sensitive to the change of the atmosphere. The sensors based on the heterostructure can provide higher sensitivity than the devices fabricated on GaN layers. By employing AlN interlayer, AlGaN/AlN/ GaN HEMT structures with higher mobility and electron concentration of the 2DEG can be obtained compared with Corresponding author. Tel.: +86 1082304132; fax: +86 10 82304232.
E-mail address: [email protected] (X.H. Wang). 0026-2692/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2007.10.022
conventional AlGaN/GaN structure. This kind of structures has not been used in the application of gas sensing. In this letter, Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) have been fabricated and measured for H2 sensing. The current–voltage (I–V) characteristics of the HEMT under two different modes (FET and Schottky diode) in N2 and 10% H2 in N2 ambient are given. The time response of the sensor in the diode mode at 300 K is also measured. The relationship between the sensitivity and the voltage applied is finally discussed. 2. Device structure and fabricating process Figs. 1(a) and (b) illustrate the schematic cross sections and microscope picture of the Pt/AlGaN/AlN/GaN HEMT studied. The sample was grown by metal-organic chemical-vapor deposition (MOCVD) on 2 in c-plane sapphire substrate. The epitaxial structure consisted of a 3.6 mm thick undoped GaN buffer layer, 100 nm GaN layer of high mobility, 1 nm AlN layer and 16.8 nm AlGaN layer without intentional doping, as shown in Fig. 1(a). More details could be obtained in [13–15]. After epitaxial growth, the devices were processed by B ion implantation for mesa isolation. A four-layer Ti/Al/Ni/Au for ohmic contacts was
ARTICLE IN PRESS X.H. Wang et al. / Microelectronics Journal 39 (2008) 20–23
evaporated on the wafer, followed by annealing at 860 1C for 30 s in N2 atmosphere. The Schottky contact was produced by evaporating 20 nm thick catalytic Pt metal with the help of the Si3N4 medium. The gate dimension of the device is 5 400 mm2. Finally, Ti/Au was evaporated as interconnection contacts.
21
The gas sensing experiments were performed in a stainless tube that contained electrical feed-throughs connecting to Keithley 2602 source meter to perform I–V measurement. Most of the experiments were performed at 300 K, with flowing gas of N2, H2 diluted in N2 and air.
Fig. 1. Schematic diagram of (a) cross-section and (b) microscopy image of the studied Pt/AlGaN/AlN/GaN HEMT.
0.06
Current change (A)
0.05 Current (A)
0.010
N2 10% H2
0.04 0.03 0.02 0.01 0.00
0
6
3
9
0.006 0.004 0.002 0.000
12
experient fit
0.008
2
0
4
Voltage(V)
6
8
10
12
Voltage(V)
Fig. 2. (a) I–V characteristics from the HEMT(FET mode) in N2 or 10% H2 in N2 ambient at 300 K and (b) current change as a function of the applied voltage.
0.0010
Current (A)
0.005
Current change (A)
0.006 N2 10% H2
0.004 0.003 0.002 0.001 0.000 -3
-2
-1 0 1 Voltage (V)
2
3
0.0008 0.0006 0.0004 0.0002 0.0000 -0.0002
-3
-2
-1 0 1 Voltage(V)
2
3
Fig. 3. (a) I–V characteristics from the HEMT(diode mode) in N2 or 10% H2 in N2 ambient at 300 K and (b) current change versus voltage.
ARTICLE IN PRESS X.H. Wang et al. / Microelectronics Journal 39 (2008) 20–23
22
3. Device performances AlGaN/GaN HEMT can have two modes of operation for the detection of hydrogen [16]. In the mode of field effect transistor (FET), the change of the drain–source current at different gate biases is the signal; in the mode of Schottky diode, the change of the gate current at zero–drain–source bias is measured. The I–V characteristics of the HEMT operated in FET mode exposed to pure N2 and 10% H2 in N2 at 300 K are shown in Fig. 2(a). It is obvious that the hydrogen has made the current larger. Fig. 2(b) gives the current change as a function of the applied voltage. As the voltage increases, the current change increases proportionally with the applied voltage. When the voltage is above 6 V, the current change saturates gradually at 0.008 A. Fig. 3(a) gives the I–V characteristics of the HEMT operated in diode mode exposed to pure N2 and 10% H2 in N2 at 300 K. A shift of 0.4 V at 300 K is obtained at a fixed forward current for switching from N2 to 10% H2 in N2. Fig. 3(b) gives the current change as the function of the applied voltage. It can be seen that under the reverse bias, 0.0008 0.0007
10% H2
N2
Current (A)
0.0006
H2 off
0.0005 0.0004
air
H2 on
0.0003
S ¼ ðI H2 I N2 Þ=I N2 ,
N2
0.0002 0.0001
0
the current change is very small; under the forward bias, the current change increase greatly between 0.5 and 1.5 V, and finally saturates at 0.8 mA. Fig. 4 shows the time response of forward current of the sensor (diode mode) biased at 1 V in different atmosphere (N2, 10% H2 in N2 and air) at 300 K. Even at room temperature, the change of the forward current under a fixed bias of 1 V is significant and shows there is sufficient cracking of the H2 for the diode to be a gas sensor. It could be seen that the turn-on response of the sensor is very quick (in tens of seconds); the turn-off response in N2 is rather slow (in several hours), while the turn-off response in synthetic air is relatively quicker. The current after exposed to the air is found to be still high compared to the current level at the initial stage, and this could be attributed to the remaining of the H-induced dipoles in the device. The oxygen in the air can react with the hydrogen atom to produce water and inhibit the hydrogen to adsorb at the metal’s surface, so the number of the hydrogen covered at the interface of the metal and the semiconductor is reduced [12]. So the change of the barrier height and the current is reduced. Hydrogen atoms adsorbed at the interface are polarized and create a dipolar layer [6]. The observed Schottky barrier height lowering can be explained in terms of the appearance of a dipolar layer at the interface between Pt metal and AlGaN surface. The dipolar layer at the interface leads to a shift of electrostatic potential of AlGaN and consequently causes a significant lowering of the Schottky barrier height. The sensitivity of the sensor can be defined by
400
200
800
600
1000
1200
Time (s)
Fig. 4. Time response at 300 K of the forward current of the sensor (diode mode) biased at 1 V.
where I H2 and I N2 are the current levels in H2 containing ambient and in N2 ambient, respectively. From the I–V curves in Figs. 2(a) and 3(a), we can calculate the sensitivity under the two different modes. Figs. 5(a) and (b) show the sensitivity versus the applied voltage in the FET and diode mode, respectively. In the FET mode, the sensitivity is only about 0.2 through the whole voltages. But the sensitivity of the diode mode is much higher and there exists a peak
120 100
0.3
Sensitivity (S)
Sensitivity (S)
0.4
0.2 0.1
80
experient Lorentz fit
60 40 20
0.0
0
2
4
6
8
Voltage (V)
10
12
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Voltage (V)
Fig. 5. Sensitivity as a function of the voltage applied to the sensor (a) FET mode and (b) diode mode.
ARTICLE IN PRESS X.H. Wang et al. / Microelectronics Journal 39 (2008) 20–23
around 0.4 V. If the sensor is operated at the voltage near the peak, we can get higher sensitivity. It seems that the peak locates in the range of the thermionic emission region and small variation of the barrier height induced by the hydrogen can have significant current change of the device. 4. Conclusions Pt/AlGaN/AlN/GaN HEMT have been fabricated and characterized for H2 sensing. In the FET mode, the current change increases proportionally with the applied voltage. We can get a large current change as high as 8 mA, while the sensitivity of the FET mode is only about 0.2. In the diode mode, the current change is very small under reverse bias, but it increases rapidly between 0.5 and 1.5 V, and finally saturates at 0.8 mA under forward bias. There is a peak of the sensitivity of the sensor in the diode mode as the applied voltage changes and higher sensitivity can be obtained at 0.4 V biasing. Time response of the sensor in the diode mode under a fixed bias of 1 V is also measured and the oxygen in the air can accelerate the recovery of the sensor. Acknowledgments This work was supported by the Knowledge Innovation Project of Chinese Academy of Sciences under Grant no. YYYJ-0701-02, the National Natural Sciences Foundation of China under Grant nos. 60576046 and 60606002, and the National Basic Research Program of China under Grant nos. 2002CB311903, 2006CB604905 and 513270505. The authors acknowledge Prof. Tangsheng Chen and
23
Chunjiang Ren in Nanjing Electronic Devices Institute for the fabrication of the AlGaN/AlN/GaN gas sensors. References [1] M.A. Khan, A. Bhattarai, J.N. Kuznia, et al., Appl. Phys. Lett. 63 (1993) 1214. [2] M. Miyoshi, H. Ishikawa, T. Egawa, et al., Appl. Phys. Lett. 85 (2004) 1710. [3] X.L. Wang, C.M. Wang, G.X. Hu, et al., Phys. Stat. Sol. (c) 3 (2006) 607. [4] S.J. Pearton, B.S. Kang, S. Kim, et al., J. Phys.: Condens. Matter 16 (2004) R961. [5] B.P. Luther, S.D. Wolter, S.E. Mohney, Sensors Actuators B 56 (1999) 164. [6] J. Schalwig, G. Mu¨ller, M. Eickhoff, et al., Mater. Sci. Eng. B 93 (2002) 207. [7] G. Steinhoff, M. Hermann, W.J. Schaff, et al., Appl. Phys. Lett. 83 (2003) 177. [8] M. Eickhoff, J. Schalwig, G. Steinhoff, et al., Phys. Stat. Sol. (c) 6 (2003) 1908. [9] J. Kim, B.P. Gila, C.R. Abernathy, et al., Solid State Electron. 47 (2003) 1487. [10] J. Song, W. Lu, J.S. Flynn, et al., Appl. Phys. Lett. 87 (2005) 133501-1. [11] K. Matsuo, N. Negoro, J. Kotani, et al., Appl. Surf. Sci. 244 (2005) 273. [12] J.R. Huang, W.Ch. Hsu, Y.J. Chen, et al., Sensors Actuators B 117 (2006) 151. [13] C.M. Wang, X.L. Wang, G.X. Hu, et al., J. Cryst. Growth 289 (2006) 415. [14] X.L. Wang, G.X. Hu, Z.Y. Ma, et al., J. Cryst. Growth 298 (2007) 835. [15] Z.Y. MA, X.L. Wang, G.X. Hu, et al., Chinese Phys. Lett. 24 (2007) 1705. [16] H.T. Wang, B.S. Kang, F. Ren, et al., Appl. Phys. Lett. 87 (2005) 172105.
