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Bias induced cutoff redshift of photocurrent in ZnO ultraviolet photodetectors
Applied Surface Science 359 (2015) 432–434
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Bias induced cutoff redshift of photocurrent in ZnO ultraviolet photodetectors Man Zhao a,b , Xin Wang a,∗ , Guang Yang b , Mai-Yu Zhou b , Wen-Jing Liu b , Tian-Wen Luo b , Hai-Feng Tan b , Xiao-Rui Sun b a b
School of Science, Changchun University of Science and Technology, Changchun 130022, P. R. China Aviation & Spaceflight intelligence Department, Aviation University of Air Force, Changchun 130022, P. R. China
a r t i c l e
i n f o
Article history: Received 8 May 2015 Received in revised form 14 October 2015 Accepted 17 October 2015 Available online 19 October 2015 Keywords: ZnO Redshift Photocurrent
a b s t r a c t A ZnO film with a c-axis preferred orientation was prepared on quartz using the radio frequency (RF) magnetron sputtering technique. Then, a metal-semiconductor-metal (MSM)-structured ultraviolet (UV) photodetector was fabricated on the film. It was found that the cutoff wavelength of the photocurrent redshifted from 361 to 379 nm when the bias increased from 5 to 30 V. The origin of the redshift has been interpreted in terms of a qualitative model considering the declining band gap caused by the bias. This method opens up the possibility of tuning the cutoff redshift of ZnO UV photodetectors. © 2015 Elsevier B.V. All rights reserved.
1. Introduction To date, many different applications for UV detection have been described, including missile warning and tracking, engine/flame monitoring, chemical/biological agent detection, and covert spaceto-space communication [1–4]. This growing interest has prompted research into new materials and devices for photo detection in the UV range [5–7]. ZnO is a wide band gap semiconductor which has a large absorption coefficient in the UV spectrum while being transparent to visible light. It appears to be an ideal material for UV detectors as it does not require a heavy and expensive filter to avoid interference from visible light [8–10]. The dependence of the responsivity of UV photodetectors on applied bias has been widely studied [11–14]. However, only a few researchers have pointed out that the responsivity cutoff shifts to longer wavelengths as the bias increases [15]. The Schottky photodiode discussed by Biyikli et al. showed a cutoff wavelength redshift from 266 to 274 nm as the bias increased from 0 to 50 V [15]. This phenomenon was also observed in our MSM-structured photodetectors. By making use of this redshift, the cutoff of the photodetectors may be adjusted by varying the applied bias. Unfortunately, the origin of this interesting phenomenon has not been studied in detail, although it is of great significance in studying the
∗ Corresponding author. Tel.: +86 431 85583407; fax: +86 431 85583015. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apsusc.2015.10.109 0169-4332/© 2015 Elsevier B.V. All rights reserved.
stability of the photodetectors. In this work, we discuss the redshift observed in a ZnO-based photodetector, and attribute it to the applied bias.
2. Experiment The growth of the ZnO film was carried out on a SiO2 substrate using an RF magnetron sputtering system. The background vacuum in the growth chamber was about 5 × 10−4 Pa before the sputtering gas was introduced. The ultrapure (5 N) O2 and Ar gases were introduced into the sputtering chamber through two separate mass flow controllers with flow rates of 15 sccm and 45 sccm (standard cubic centimeter per minute), respectively. The working pressure in the chamber was kept at 3 Pa with an RF power of 120 W. The growth was performed at a substrate temperature of 683 K. The rate of deposition was adjusted to achieve a thickness of nearly 600 nm in a growth time of 120 min. X-ray diffraction (XRD) spectra were collected using a D/max − RA x − ray spectrometer (Rigaku International Corp., Japan) with CuK␣ radiation of 1.543 A˚ to obtain the structural information of the film. A PerkinElmer Lambda 950 UV/VIS spectrometer was used to detect absorption spectra in the wavelength ranging from 350 to 700 nm. The current–voltage (I–V) characteristics of the photodetectors were measured using a semiconductor analyzer (Agilent 16442A Test Fixture). The spectral responsivity was measured using a Zolix DR800-CUST testing system.
M. Zhao et al. / Applied Surface Science 359 (2015) 432–434
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Fig. 1. XRD spectra of the ZnO film prepared on SiO2 .
Fig. 3. Dark current versus applied bias of the ZnO MSM photodetector. The inset shows the interdigitated electrode configuration of our MSM device.
Fig. 2. UV–visible absorption and transmission spectra of the ZnO film.
3. Results and discussion Fig. 1 shows a typical XRD pattern from the ZnO thin film. The peak at 34.73◦ can be indexed to the diffraction from the (0 0 2) facet of the wurtzite ZnO. Notably, a peak at 72.70◦ is also visible, which can be identified as diffraction from the (0 0 4) facet. The XRD data reveal that the ZnO has a hexagonal structure with a c-axis preferred orientation. Fig. 2 shows the absorption and transmission spectra of the ZnO film. The sample has a sharp absorption edge and high transmittance of about 80% in the visible spectrum region. Additionally, absorption is prominent for wavelengths smaller than about 390 nm, indicating the quantity of the photon-generated carriers starts to increase significantly around 390 nm. This increase should be responsible for the photocurrent shifting range that we discuss below. The MSM-structured photodetector was fabricated on ZnO thin film using an interdigitated electrode mask set. A clear-cut MSM structure with an interdigitated configuration was obtained by conventional UV lithography and wet etching. Au was chosen as the contact metal and In was used for the bonding pads. The schematic of the device with interdigitated electrodes is shown in the inset of Fig. 3. The electrode fingers are 5 m wide and 500 m long and the spacing between the fingers is 2 m. The I–V curve of the photodetector was measured in darkness, and a linear curve was obtained, as shown in Fig. 3. For a 5 V bias, the measured average dark current is 21.1 nA, which is close to the calculated dark current based on the resistivity of ZnO. Fig. 4 shows the room temperature spectral photocurrent of the ZnO photodetector at different biases. Note that all the photocurrent curves are normalized for comparison. As shown in Fig. 4, the cutoff of the photocurrent undergoes an obvious shift to longer wavelengths as the bias increases. The inset of Fig. 4 shows a close
Fig. 4. Normalized spectral photocurrent of the ZnO photodetector under applied biases from 5 to 30 V. The inset shows a close up of the photocurrent cutoff.
Fig. 5. The cutoff wavelength of the photocurrent as a function of the bias voltage.
up of the cutoff of photocurrent. To reveal the nature of these interesting phenomena, the cutoff wavelength of the photocurrent versus bias voltage is plotted in Fig. 5. The cutoff of the photocurrent shifts from 361 to 379 nm as the bias voltage increases from 5 to 30 V. It is also noteworthy that the photocurrent shift starts at about 390 nm wavelength rather than 410 nm (the onset of the spectral photocurrent). In order to understand this phenomenon, a qualitative model is proposed, as described below. To investigate the redshift of the photocurrent shutoff in ZnO UV photodetectors, a schematic band diagram for ZnO is shown in Fig. 6. Under UV light illumination, some electrons will be excited to the conduction band from the valence band. Since the
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M. Zhao et al. / Applied Surface Science 359 (2015) 432–434
photodetector at a 5 V bias is plotted in Fig. 7. A peak responsivity of 3.6 × 10−4 A/W is realized with a cutoff of 392 nm. This is in excellent agreement with the absorption edge shown in Fig. 2. Furthermore, the UV-to-visible rejection is more than four orders of magnitude, which indicates that the photodetector has a relatively high signal-to-noise ratio. 4. Conclusion
Fig. 6. Schematic diagram of a qualitative model of the declining band caused by the applied bias. E is energy and k is wave number.
In summary, we have described an MSM-structured UV photodetector based on a ZnO thin film. A bias-induced redshift of the photocurrent was observed in the photodetector. The cutoff wavelength of the photocurrent could be tuned from 361 to 379 nm by increasing the applied bias from 5 to 35 V. The declining band gap was proposed as the dominant cause of the redshift phenomenon. A schematic model of the declining band resulting from the bias could be employed to understand the redshift. Acknowledgements This work is supported by the Scientific and Technological Development Project of Jilin Province, China (Grant Nos. 201201121, 20120435, 20130204033GX). References
Fig. 7. Responsivity as a function of wavelength for the ZnO photodetector under a 5 V bias.
electrode spacing is 2 m, the strength of the electric field is about 106 V/m. Therefore, we believe that the redshift outlined above can be understood using a model where the band gap bends due to the high intensity of the electric field, as shown in the Fig. 6. Subsequently, under illumination, many electrons are excited to the conduction band along the vertical orientation, which is immediately evident as the principle transition path. This results in the excitation wavelength becoming longer for larger electric fields. The redshift phenomenon is therefore satisfactorily explained by this model. As for the fact that the photocurrent shift starts around 390 nm rather than at the onset of the spectral photocurrent, since the origin for the shift is still not very clear, we speculate that it might be related to absorption performance. Firstly, the absorption of the ZnO film is prominent for wavelengths less than about 390 nm, indicating that the quantity of photon-generated carrier starts to increase significantly at about 390 nm. Therefore, according to the proposed model, the more remarkable redshift should be at wavelengths less than 390 nm, as shown in the Fig. 6. Secondly, a low absorption shoulder (390–410 nm) is observed in Fig. 2, which is strong evidence of the existence of defects. Therefore, the photocurrent range 390–410 nm is irrelevant to the proposed model, which could be responsible for the redshift not being observed in this range. It also confirms that the proposed model is correct. The intensity of the xenon lamp is measured using a calibrated Si UV photodetector so as to estimate the responsivity of the ZnO photodetector precisely. The spectral responsivity of the ZnO
[1] M. Razeghi, A. Rogalski, Semiconductor ultraviolet detectors, J. Appl. Phys. 79 (1996) 7433. [2] M.P. Ulmer, M. Razeghi, E. Bigan, Ultraviolet detectors for astrophysics: present and future, SPIE 2397 (1995) 210. [3] A. Rogalski, M. Razeghi, Semiconductor ultraviolet photodetectors, Opto-electr. Rev. 4 (1996) 13. [4] A.M. Streltsov, K.D. Moll, A.L. Gaeta, P. Kung, M.D. Walker, M. Razeghi, Pulse autocorrelation measurements based on two-and three-photon conductivity in a GaN photodiode, Appl. Phys. Lett. 75 (1999) 3378. [5] W. Yang, S.S. Hullavarad, B. Nagaraj, I. Takeuchi, R.P. Sharma, T. Venkatesan, R.D. Vispute, H. Shen, Compositionally-tuned epitaxial cubic Mgx Zn1-x O on Si (100) for deep ultraviolet photodetectors, Appl. Phys. Lett. 82 (2003) 3424. [6] A. Soltani, H.A. Barkad, M. Mattalah, B. Benbakhti, J.C. De Jaeger, Y.M. Chong, Y.S. Zou, W.J. Zhang, S.T. Lee, A. BenMoussa, B. Giordanengo, J.F. Hochedez, 193nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors, Appl. Phys. Lett. 92 (2008) 053501. [7] C.J. Collins, U. Chowdhury, M.M. Wong, B. Yang, A.L. Beck, R.D. Dupuis, J.C. Campbell, Improved solar-blind detectivity using an Alx Ga1-x N heterojunction pin photodiode, Appl. Phys. Lett. 80 (2002) 3754. [8] C.H. Park, I.S. Jeong, J.H. Kim, S. Im, Spectral responsivity and quantum efficiency of n-ZnO/p-Si photodiode fully isolated by ion-beam treatment, Appl. Phys. Lett. 82 (2003) 3973. [9] D. Basak, G. Amin, B. Mallik, G.K. Paul, S.K. Sen, Photoconductive UV detectors on sol-gel-synthesized ZnO films, J. Cryst. Growth 256 (2003) 73. [10] Z.G. Ju, C.X. Shan, D.Y. Jiang, J.Y. Zhang, B. Yao, D.X. Zhao, D.Z. Shen, X.W. Fan, Mgx Zn1-x O-based photodetectors covering the whole solar-blind spectrum range, Appl. Phys. Lett. 93 (2008) 173505. [11] S.K. Zhang, W.B. Wang, I. Shtau, F. Yun, L. He, H. Morkoc, X. Zhou, M. Tamargo, R.R. Alfano, Backilluminated GaN/AlGaN heterojunction ultraviolet photodetector with high internal gain, Appl. Phys. Lett. 81 (2002) 4862. [12] W. Yang, R.D. Vispute, S. Choopun, R.P. Sharma, T. Venkatesan, H. Shen, Ultraviolet photoconductive detector based on epitaxial Mg0.34 Zn0.66 O thin films, Appl. Phys. Lett. 78 (2001) 2787. [13] S.W. Seo, K.K. Lee, S. Sangbeom Kang, Huang, A. William, N.M. Doolittle, A.S. Jokerst, Brown, GaN metal-semiconductor-metal photodetectors grown on lithium gallate substrates by molecular-beam epitaxy, Appl. Phys. Lett. 79 (2001) 1372. [14] J. Li, Z.Y. Fan, R. Dahal, M. Nakarmi, J.Y. Lin, H.X. Jiang, 200nm deep ultraviolet photodetectors based on AlN, Appl. Phys. Lett. 89 (2006) 213510. [15] E. Ozbay, N. Biyikli, I. Kimukin, T. Kartaloglu, T. Tut, O. Aytur, High-performance solar-blind photodetectors based on Alx Ga1-x N heterostructures, IEEE J. Sel. Top. Quant. 10 (2004) 742.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Bias induced cutoff redshift of photocurrent in ZnO ultraviolet photodetectors Man Zhao a,b , Xin Wang a,∗ , Guang Yang b , Mai-Yu Zhou b , Wen-Jing Liu b , Tian-Wen Luo b , Hai-Feng Tan b , Xiao-Rui Sun b a b
School of Science, Changchun University of Science and Technology, Changchun 130022, P. R. China Aviation & Spaceflight intelligence Department, Aviation University of Air Force, Changchun 130022, P. R. China
a r t i c l e
i n f o
Article history: Received 8 May 2015 Received in revised form 14 October 2015 Accepted 17 October 2015 Available online 19 October 2015 Keywords: ZnO Redshift Photocurrent
a b s t r a c t A ZnO film with a c-axis preferred orientation was prepared on quartz using the radio frequency (RF) magnetron sputtering technique. Then, a metal-semiconductor-metal (MSM)-structured ultraviolet (UV) photodetector was fabricated on the film. It was found that the cutoff wavelength of the photocurrent redshifted from 361 to 379 nm when the bias increased from 5 to 30 V. The origin of the redshift has been interpreted in terms of a qualitative model considering the declining band gap caused by the bias. This method opens up the possibility of tuning the cutoff redshift of ZnO UV photodetectors. © 2015 Elsevier B.V. All rights reserved.
1. Introduction To date, many different applications for UV detection have been described, including missile warning and tracking, engine/flame monitoring, chemical/biological agent detection, and covert spaceto-space communication [1–4]. This growing interest has prompted research into new materials and devices for photo detection in the UV range [5–7]. ZnO is a wide band gap semiconductor which has a large absorption coefficient in the UV spectrum while being transparent to visible light. It appears to be an ideal material for UV detectors as it does not require a heavy and expensive filter to avoid interference from visible light [8–10]. The dependence of the responsivity of UV photodetectors on applied bias has been widely studied [11–14]. However, only a few researchers have pointed out that the responsivity cutoff shifts to longer wavelengths as the bias increases [15]. The Schottky photodiode discussed by Biyikli et al. showed a cutoff wavelength redshift from 266 to 274 nm as the bias increased from 0 to 50 V [15]. This phenomenon was also observed in our MSM-structured photodetectors. By making use of this redshift, the cutoff of the photodetectors may be adjusted by varying the applied bias. Unfortunately, the origin of this interesting phenomenon has not been studied in detail, although it is of great significance in studying the
∗ Corresponding author. Tel.: +86 431 85583407; fax: +86 431 85583015. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apsusc.2015.10.109 0169-4332/© 2015 Elsevier B.V. All rights reserved.
stability of the photodetectors. In this work, we discuss the redshift observed in a ZnO-based photodetector, and attribute it to the applied bias.
2. Experiment The growth of the ZnO film was carried out on a SiO2 substrate using an RF magnetron sputtering system. The background vacuum in the growth chamber was about 5 × 10−4 Pa before the sputtering gas was introduced. The ultrapure (5 N) O2 and Ar gases were introduced into the sputtering chamber through two separate mass flow controllers with flow rates of 15 sccm and 45 sccm (standard cubic centimeter per minute), respectively. The working pressure in the chamber was kept at 3 Pa with an RF power of 120 W. The growth was performed at a substrate temperature of 683 K. The rate of deposition was adjusted to achieve a thickness of nearly 600 nm in a growth time of 120 min. X-ray diffraction (XRD) spectra were collected using a D/max − RA x − ray spectrometer (Rigaku International Corp., Japan) with CuK␣ radiation of 1.543 A˚ to obtain the structural information of the film. A PerkinElmer Lambda 950 UV/VIS spectrometer was used to detect absorption spectra in the wavelength ranging from 350 to 700 nm. The current–voltage (I–V) characteristics of the photodetectors were measured using a semiconductor analyzer (Agilent 16442A Test Fixture). The spectral responsivity was measured using a Zolix DR800-CUST testing system.
M. Zhao et al. / Applied Surface Science 359 (2015) 432–434
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Fig. 1. XRD spectra of the ZnO film prepared on SiO2 .
Fig. 3. Dark current versus applied bias of the ZnO MSM photodetector. The inset shows the interdigitated electrode configuration of our MSM device.
Fig. 2. UV–visible absorption and transmission spectra of the ZnO film.
3. Results and discussion Fig. 1 shows a typical XRD pattern from the ZnO thin film. The peak at 34.73◦ can be indexed to the diffraction from the (0 0 2) facet of the wurtzite ZnO. Notably, a peak at 72.70◦ is also visible, which can be identified as diffraction from the (0 0 4) facet. The XRD data reveal that the ZnO has a hexagonal structure with a c-axis preferred orientation. Fig. 2 shows the absorption and transmission spectra of the ZnO film. The sample has a sharp absorption edge and high transmittance of about 80% in the visible spectrum region. Additionally, absorption is prominent for wavelengths smaller than about 390 nm, indicating the quantity of the photon-generated carriers starts to increase significantly around 390 nm. This increase should be responsible for the photocurrent shifting range that we discuss below. The MSM-structured photodetector was fabricated on ZnO thin film using an interdigitated electrode mask set. A clear-cut MSM structure with an interdigitated configuration was obtained by conventional UV lithography and wet etching. Au was chosen as the contact metal and In was used for the bonding pads. The schematic of the device with interdigitated electrodes is shown in the inset of Fig. 3. The electrode fingers are 5 m wide and 500 m long and the spacing between the fingers is 2 m. The I–V curve of the photodetector was measured in darkness, and a linear curve was obtained, as shown in Fig. 3. For a 5 V bias, the measured average dark current is 21.1 nA, which is close to the calculated dark current based on the resistivity of ZnO. Fig. 4 shows the room temperature spectral photocurrent of the ZnO photodetector at different biases. Note that all the photocurrent curves are normalized for comparison. As shown in Fig. 4, the cutoff of the photocurrent undergoes an obvious shift to longer wavelengths as the bias increases. The inset of Fig. 4 shows a close
Fig. 4. Normalized spectral photocurrent of the ZnO photodetector under applied biases from 5 to 30 V. The inset shows a close up of the photocurrent cutoff.
Fig. 5. The cutoff wavelength of the photocurrent as a function of the bias voltage.
up of the cutoff of photocurrent. To reveal the nature of these interesting phenomena, the cutoff wavelength of the photocurrent versus bias voltage is plotted in Fig. 5. The cutoff of the photocurrent shifts from 361 to 379 nm as the bias voltage increases from 5 to 30 V. It is also noteworthy that the photocurrent shift starts at about 390 nm wavelength rather than 410 nm (the onset of the spectral photocurrent). In order to understand this phenomenon, a qualitative model is proposed, as described below. To investigate the redshift of the photocurrent shutoff in ZnO UV photodetectors, a schematic band diagram for ZnO is shown in Fig. 6. Under UV light illumination, some electrons will be excited to the conduction band from the valence band. Since the
434
M. Zhao et al. / Applied Surface Science 359 (2015) 432–434
photodetector at a 5 V bias is plotted in Fig. 7. A peak responsivity of 3.6 × 10−4 A/W is realized with a cutoff of 392 nm. This is in excellent agreement with the absorption edge shown in Fig. 2. Furthermore, the UV-to-visible rejection is more than four orders of magnitude, which indicates that the photodetector has a relatively high signal-to-noise ratio. 4. Conclusion
Fig. 6. Schematic diagram of a qualitative model of the declining band caused by the applied bias. E is energy and k is wave number.
In summary, we have described an MSM-structured UV photodetector based on a ZnO thin film. A bias-induced redshift of the photocurrent was observed in the photodetector. The cutoff wavelength of the photocurrent could be tuned from 361 to 379 nm by increasing the applied bias from 5 to 35 V. The declining band gap was proposed as the dominant cause of the redshift phenomenon. A schematic model of the declining band resulting from the bias could be employed to understand the redshift. Acknowledgements This work is supported by the Scientific and Technological Development Project of Jilin Province, China (Grant Nos. 201201121, 20120435, 20130204033GX). References
Fig. 7. Responsivity as a function of wavelength for the ZnO photodetector under a 5 V bias.
electrode spacing is 2 m, the strength of the electric field is about 106 V/m. Therefore, we believe that the redshift outlined above can be understood using a model where the band gap bends due to the high intensity of the electric field, as shown in the Fig. 6. Subsequently, under illumination, many electrons are excited to the conduction band along the vertical orientation, which is immediately evident as the principle transition path. This results in the excitation wavelength becoming longer for larger electric fields. The redshift phenomenon is therefore satisfactorily explained by this model. As for the fact that the photocurrent shift starts around 390 nm rather than at the onset of the spectral photocurrent, since the origin for the shift is still not very clear, we speculate that it might be related to absorption performance. Firstly, the absorption of the ZnO film is prominent for wavelengths less than about 390 nm, indicating that the quantity of photon-generated carrier starts to increase significantly at about 390 nm. Therefore, according to the proposed model, the more remarkable redshift should be at wavelengths less than 390 nm, as shown in the Fig. 6. Secondly, a low absorption shoulder (390–410 nm) is observed in Fig. 2, which is strong evidence of the existence of defects. Therefore, the photocurrent range 390–410 nm is irrelevant to the proposed model, which could be responsible for the redshift not being observed in this range. It also confirms that the proposed model is correct. The intensity of the xenon lamp is measured using a calibrated Si UV photodetector so as to estimate the responsivity of the ZnO photodetector precisely. The spectral responsivity of the ZnO
[1] M. Razeghi, A. Rogalski, Semiconductor ultraviolet detectors, J. Appl. Phys. 79 (1996) 7433. [2] M.P. Ulmer, M. Razeghi, E. Bigan, Ultraviolet detectors for astrophysics: present and future, SPIE 2397 (1995) 210. [3] A. Rogalski, M. Razeghi, Semiconductor ultraviolet photodetectors, Opto-electr. Rev. 4 (1996) 13. [4] A.M. Streltsov, K.D. Moll, A.L. Gaeta, P. Kung, M.D. Walker, M. Razeghi, Pulse autocorrelation measurements based on two-and three-photon conductivity in a GaN photodiode, Appl. Phys. Lett. 75 (1999) 3378. [5] W. Yang, S.S. Hullavarad, B. Nagaraj, I. Takeuchi, R.P. Sharma, T. Venkatesan, R.D. Vispute, H. Shen, Compositionally-tuned epitaxial cubic Mgx Zn1-x O on Si (100) for deep ultraviolet photodetectors, Appl. Phys. Lett. 82 (2003) 3424. [6] A. Soltani, H.A. Barkad, M. Mattalah, B. Benbakhti, J.C. De Jaeger, Y.M. Chong, Y.S. Zou, W.J. Zhang, S.T. Lee, A. BenMoussa, B. Giordanengo, J.F. Hochedez, 193nm deep-ultraviolet solar-blind cubic boron nitride based photodetectors, Appl. Phys. Lett. 92 (2008) 053501. [7] C.J. Collins, U. Chowdhury, M.M. Wong, B. Yang, A.L. Beck, R.D. Dupuis, J.C. Campbell, Improved solar-blind detectivity using an Alx Ga1-x N heterojunction pin photodiode, Appl. Phys. Lett. 80 (2002) 3754. [8] C.H. Park, I.S. Jeong, J.H. Kim, S. Im, Spectral responsivity and quantum efficiency of n-ZnO/p-Si photodiode fully isolated by ion-beam treatment, Appl. Phys. Lett. 82 (2003) 3973. [9] D. Basak, G. Amin, B. Mallik, G.K. Paul, S.K. Sen, Photoconductive UV detectors on sol-gel-synthesized ZnO films, J. Cryst. Growth 256 (2003) 73. [10] Z.G. Ju, C.X. Shan, D.Y. Jiang, J.Y. Zhang, B. Yao, D.X. Zhao, D.Z. Shen, X.W. Fan, Mgx Zn1-x O-based photodetectors covering the whole solar-blind spectrum range, Appl. Phys. Lett. 93 (2008) 173505. [11] S.K. Zhang, W.B. Wang, I. Shtau, F. Yun, L. He, H. Morkoc, X. Zhou, M. Tamargo, R.R. Alfano, Backilluminated GaN/AlGaN heterojunction ultraviolet photodetector with high internal gain, Appl. Phys. Lett. 81 (2002) 4862. [12] W. Yang, R.D. Vispute, S. Choopun, R.P. Sharma, T. Venkatesan, H. Shen, Ultraviolet photoconductive detector based on epitaxial Mg0.34 Zn0.66 O thin films, Appl. Phys. Lett. 78 (2001) 2787. [13] S.W. Seo, K.K. Lee, S. Sangbeom Kang, Huang, A. William, N.M. Doolittle, A.S. Jokerst, Brown, GaN metal-semiconductor-metal photodetectors grown on lithium gallate substrates by molecular-beam epitaxy, Appl. Phys. Lett. 79 (2001) 1372. [14] J. Li, Z.Y. Fan, R. Dahal, M. Nakarmi, J.Y. Lin, H.X. Jiang, 200nm deep ultraviolet photodetectors based on AlN, Appl. Phys. Lett. 89 (2006) 213510. [15] E. Ozbay, N. Biyikli, I. Kimukin, T. Kartaloglu, T. Tut, O. Aytur, High-performance solar-blind photodetectors based on Alx Ga1-x N heterostructures, IEEE J. Sel. Top. Quant. 10 (2004) 742.