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ZnO Schottky ultraviolet photodetectors
Journal of Crystal Growth 225 (2001) 110–113
ZnO Schottky ultraviolet photodetectors S. Lianga, H. Shenga, Y. Liua, Z. Huoa, Y. Lua,*, H. Shenb a
Department of Electrical and Computer Engineering, Rutgers University, 94 Brett Road, Piscataway, NJ 08854-8058, USA b U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USA
Abstract We present the results of Schottky UV photodetectors fabricated on n-type ZnO epitaxial films. The ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The metal–semiconductor–metal (MSM) photodetectors were fabricated by using Ag as Schottky contact metal. For comparison, ZnO photoconductive detectors were also fabricated by using Al as ohmic contact metal. I–V characteristics of these devices were analyzed. At a reverse bias of 1 V, the circular Schottky photodiode exhibits a leakage current approximately 5 orders of magnitude smaller than that of its photoconductive counterpart. The photoresponsivity of the ZnO Schottky type MSM UV detector is 1.5 A/W and the leakage current is about 1 nA at 5 V bias. The detector shows a fast photoresponse component with a rise time of 12 ns and a fall time of 50 ns. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Dz; 81.15.Gh; 78.66.Hf; 85.60.Gz Keywords: A3. Metalorganic chemical vapor deposition; B1. ZnO; B2. Semiconducting II–VI materials; B3. Photodetector
1. Introduction Recently, there has been increasing interest in high quality ZnO films. Optical pumped lasing in the UV range [1], and an optically addressed UV light modulator [2] have been reported. Epitaxial ZnO films are promising for UV photodetector applications due to their wide and direct band gap and large photoresponse. We previously reported the epitaxial ZnO UV photoconductive type detector, which was fabricated using aluminum as the ohmic metal for a metal–semiconductor– metal (MSM) device structure [3]. The device showed fast photoresponse time (1 ms) due to *Corresponding author. E-mail address: [email protected] (Y. Lu).
high material quality of a ZnO epilayer. However, Schottky type photodetectors are more attractive due to their high speed and low noise performance. Till today, there has been no Schottky type of photodetector fabricated on ZnO epitaxial films. Fabricius et al. made Schottky barrier type UV sensitive photodiodes using Au on a thin sputtered layer of polycrystalline ZnO [4]. These photodiodes exhibited slow operating speed (rise time 20 ms and decay time 30 ms) and low quantum efficiency (1%) due to a large amount of recombination centers in the polycrystalline ZnO layers. In this paper, we report on Schottky type photodetectors fabricated on high quality ZnO epilayers, which are grown on the R-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD).
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 3 0 - 2
S. Liang et al. / Journal of Crystal Growth 225 (2001) 110–113
2. Experimental procedure (1 1 2 0) ZnO epitaxial films were grown on R-plane ð0 1 1 2Þ sapphire substrates in a lowpressure MOCVD reactor. Diethyl zinc (DEZn) and O2 were used as the source materials. The reactants were introduced into the reactor by two separate injectors to reduce gas phase reaction. DEZn flow rate was 50–100 sccm, and O2 flow rate was 1000–2000 sccm. Growth temperature range was from 3808C to 4208C. Typical growth rate was 1–2 mm/h. The details of the system and growth conditions were reported in Ref. [5]. Because unintentionally doped ZnO film prepared by MOCVD generally exhibits n-type conduction due to oxygen vacancies or Zn interstitials, NH3 (500 sccm) was used as a nitrogen compensation doping source to reduce the electron concentration. A 500 nm ZnO epilayer was grown with carrier concentration of about 1016 cm3. Structural characterizations of the deposited films were performed by X-ray, SEM and TEM. It was confirmed that ð1 1 2 0Þ oriented ZnO films were epitaxially grown on ð0 1 1 2Þ Al2O3 substrates with an atomically sharp interface [6]. UV photodetectors were designed and fabricated using circular and interdigital (IDT) MSM structures. For the circular structure, the outer diameter is 320 mm, the inner diameter is 150 mm, and the gap is 10 mm. For the IDT structure, the fingers are 10 mm wide and 180 mm long, with a 10 mm gap. Aluminum and silver were used to form ohmic and Schottky contacts, respectively. The Ag–ZnO–Al circular structures were used for Schottky diode studies, while the Ag–ZnO–Ag structures served as Schottky type MSM photodetectors. For comparison, the Al–ZnO–Al structures were fabricated as the MSM photoconductive detectors. E-beam evaporation was utilized for the metal deposition. The thickness of Al and Ag layers was 2000 A˚. For the Schottky contact, a 500 A˚ thick Au layer was finally deposited on the top of the Ag layer, to act as an oxide-resistant layer. It is well known that in fabrication of Schottky contacts, the surface states, contaminants and defects of the surface layer affect the barrier height and leakage current. Oxygen plasma was used to clean the surface
111
before and after the metallization process in order to minimize such effects. Photoresponse measurements were performed using an Oriel optical system. A Xe-arc lamp and monochromator combination provided the light source. The photoresponse speed of the detector was also measured. The optical excitation source was the 337.1 nm line of a N2 pulse laser, with a pulse width of 2 ns at a repetition rate of 40 Hz. Neutral density filters were used to control the optical power on the detector. Optical energy on the detector was about 10 nJ per pulse. The signal from a 50Oload resistor was recorded by a digital scope with a time resolution better than 1 ns. The bias voltage was 9 V.
3. Results and discussion Shown in Fig. 1 are the I–V characteristics of ZnO MSM circular devices, including both Schottky and ohmic types. The linear I–V relation from the Al–ZnO–Al structure clearly indicates the ohmic behavior of the Al on n-type ZnO contact. On the other hand, the rectified I–V relation from the Ag–ZnO–Al confirms Schottky junction formation between Ag and n-type ZnO. In a Schottky diode, the general I–V characteristics are
Fig. 1. I–V characteristics of ZnO diodes with the circular pattern. The electrode gap is 10 mm.
112
represented by [7] qV J ¼ Js exp 1 ; nkT
S. Liang et al. / Journal of Crystal Growth 225 (2001) 110–113
ð1Þ
where Js ¼ A * T 2 expðFB =kT Þ is the saturation current density based on thermionic emission theory, n the ideality factor, k the Boltzmann’s constant, T the absolute temperature, A * the effective Richardson coefficient, and FB the barrier height. The Schottky contact area is 1.77 104 cm2. The values of n ¼ 1:5 and Js ¼ 2:4 108 A/cm2 were obtained from curve fitting. The barrier height FB is estimated to be about 0.84 eV by using A*32 A/cm2 K2 (A * ¼ 4pqm * k2 =h3 , where m * 0:27m0 [8]). At a reverse bias of 1 V, the leakage current (0.1 nA) of the Ag–ZnO–Al device is about 5 orders of magnitude smaller than that of the Al–ZnO–Al device (10 mA). The Ag–ZnO–Ag MSM structure with IDT configuration was used to evaluate the UV detector performance. Shown in Fig. 2 are the dark and photo-illuminated I–V characteristics of a Schottky detector. The wavelength and power of the illuminated light are 368 nm and 0.1 mW, respectively. The leakage current of the photodetector is about 1 nA at a bias of 5 V. The breakdown starts at about 8 V. The early and soft breakdown is mainly due to the non-uniform field distribution, especially at the corners of the finger
Fig. 2. I–V characteristics of a ZnO Schottky photodetector with an IDT Structure. The inset shows a SEM picture of the top view of the device.
electrodes in the IDT configuration. The lowfrequency photoresponsivity is about 1.5 A/W, which corresponds to a gain of 2.5. We believe that the photoconductive effect which occurs at the high field contributes to the large gain (>1). The spectra-photoresponse of the IDT Schottky device is shown in Fig. 3. The responsivity drops more than 3 orders of magnitude from 370 to 390 nm. The sharp cut-off at wavelength of 370 nm agrees with the ZnO energy band gap of 3.35 eV. The responsivity decreases at the shorter wavelength range due to decrease of the penetrating depth of the light, resulting in an increase of the surface recombination. Shown in Fig. 4 is the photocurrent as a function of time from an IDT ZnO Schottky detector. The photoresponse has a fast component, which rises within 12 ns and falls to 66% of its peak value within 50 ns as shown in the inset of Fig. 4. After that a slow process follows lasting about 5 ms (not shown in the figure). The fast response in a MSM detector is usually related to the transit time of the photo-generated carriers, while the slow response in a ZnO MSM detector is usually attributed to the oxygen adsorption at the surface and grain boundaries [9,10]. We used the high quality epitaxial ZnO for both Schottky and photoconductive detectors. As reported in Ref. [3], we did not observe such a slow process in the photoconductive devices with the same material quality. Therefore, we attribute this slow process
Fig. 3. Spectral-photoresponse of a ZnO Schottky photodetector with an IDT structure.
S. Liang et al. / Journal of Crystal Growth 225 (2001) 110–113
113
photoresponsivity of 1.5 A/W at a bias of 5 V was obtained. The leakage current of the device at 5 V bias was in the order of 1 nA. The photoresponse of the detector showed a fast component with a rise time of 12 ns, and a fall time of 50 ns.
Acknowledgements The work has been supported by Rutgers University under SROA grant. Fig. 4. Photocurrent as a function of response time of a ZnO Schottky photodetector with an IDT structure.
to the trapping and emission of photo-generated carriers in the ZnO surface of the Schottky detector. It should be noted that oxygen plasma was used to treat the ZnO surface only when making the Schottky detectors. The detailed study will be reported elsewhere.
4. Conclusion We have demonstrated Schottky type UV photodetectors based on ZnO epitaxial films grown on R-plane Al2O3 by MOCVD. Ag was used to form Schottky contact on n-type ZnO with a barrier height about 0.84 eV. A low frequency
References [1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [2] M. Wraback, H. Shen, S. Liang, Y. Lu, Appl. Phys. Lett. 74 (1999) 507. [3] Ying Liu, C.R. Gorla, S. Liang, N. Emanetoglu, Y. Lu, J. Electron. Mater. 29 (2000) 69. [4] H. Fabricius, T. Skettrup, P. Bisgaard, Appl. Opt. 25 (1986) 2764. [5] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, J. Appl. Phys. 85 (1999) 2595. [6] S. Liang, C.R. Gorla, N. Emanetoglu, Y. Liu, W.E. Mayo, Y. Lu, J. Electron. Mater. 27 (1998) L72. [7] S.M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, Berlin, NewYork, 1981, p. 264 (Chapter 5). [8] Rolf E. Hummel, Electronic Properties of Materials, Springer, Berlin, 1992, p. 373. [9] Y. Takahashi, M. Kanamori, A. Kondoh, H. Minoura, Y. Ohya, Jpn. J. Appl. Phys. 33 (1994) 6611. [10] D.H. Zhang, J. Phys. D 28 (1995) 1273.
ZnO Schottky ultraviolet photodetectors S. Lianga, H. Shenga, Y. Liua, Z. Huoa, Y. Lua,*, H. Shenb a
Department of Electrical and Computer Engineering, Rutgers University, 94 Brett Road, Piscataway, NJ 08854-8058, USA b U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USA
Abstract We present the results of Schottky UV photodetectors fabricated on n-type ZnO epitaxial films. The ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The metal–semiconductor–metal (MSM) photodetectors were fabricated by using Ag as Schottky contact metal. For comparison, ZnO photoconductive detectors were also fabricated by using Al as ohmic contact metal. I–V characteristics of these devices were analyzed. At a reverse bias of 1 V, the circular Schottky photodiode exhibits a leakage current approximately 5 orders of magnitude smaller than that of its photoconductive counterpart. The photoresponsivity of the ZnO Schottky type MSM UV detector is 1.5 A/W and the leakage current is about 1 nA at 5 V bias. The detector shows a fast photoresponse component with a rise time of 12 ns and a fall time of 50 ns. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.Dz; 81.15.Gh; 78.66.Hf; 85.60.Gz Keywords: A3. Metalorganic chemical vapor deposition; B1. ZnO; B2. Semiconducting II–VI materials; B3. Photodetector
1. Introduction Recently, there has been increasing interest in high quality ZnO films. Optical pumped lasing in the UV range [1], and an optically addressed UV light modulator [2] have been reported. Epitaxial ZnO films are promising for UV photodetector applications due to their wide and direct band gap and large photoresponse. We previously reported the epitaxial ZnO UV photoconductive type detector, which was fabricated using aluminum as the ohmic metal for a metal–semiconductor– metal (MSM) device structure [3]. The device showed fast photoresponse time (1 ms) due to *Corresponding author. E-mail address: [email protected] (Y. Lu).
high material quality of a ZnO epilayer. However, Schottky type photodetectors are more attractive due to their high speed and low noise performance. Till today, there has been no Schottky type of photodetector fabricated on ZnO epitaxial films. Fabricius et al. made Schottky barrier type UV sensitive photodiodes using Au on a thin sputtered layer of polycrystalline ZnO [4]. These photodiodes exhibited slow operating speed (rise time 20 ms and decay time 30 ms) and low quantum efficiency (1%) due to a large amount of recombination centers in the polycrystalline ZnO layers. In this paper, we report on Schottky type photodetectors fabricated on high quality ZnO epilayers, which are grown on the R-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD).
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 3 0 - 2
S. Liang et al. / Journal of Crystal Growth 225 (2001) 110–113
2. Experimental procedure (1 1 2 0) ZnO epitaxial films were grown on R-plane ð0 1 1 2Þ sapphire substrates in a lowpressure MOCVD reactor. Diethyl zinc (DEZn) and O2 were used as the source materials. The reactants were introduced into the reactor by two separate injectors to reduce gas phase reaction. DEZn flow rate was 50–100 sccm, and O2 flow rate was 1000–2000 sccm. Growth temperature range was from 3808C to 4208C. Typical growth rate was 1–2 mm/h. The details of the system and growth conditions were reported in Ref. [5]. Because unintentionally doped ZnO film prepared by MOCVD generally exhibits n-type conduction due to oxygen vacancies or Zn interstitials, NH3 (500 sccm) was used as a nitrogen compensation doping source to reduce the electron concentration. A 500 nm ZnO epilayer was grown with carrier concentration of about 1016 cm3. Structural characterizations of the deposited films were performed by X-ray, SEM and TEM. It was confirmed that ð1 1 2 0Þ oriented ZnO films were epitaxially grown on ð0 1 1 2Þ Al2O3 substrates with an atomically sharp interface [6]. UV photodetectors were designed and fabricated using circular and interdigital (IDT) MSM structures. For the circular structure, the outer diameter is 320 mm, the inner diameter is 150 mm, and the gap is 10 mm. For the IDT structure, the fingers are 10 mm wide and 180 mm long, with a 10 mm gap. Aluminum and silver were used to form ohmic and Schottky contacts, respectively. The Ag–ZnO–Al circular structures were used for Schottky diode studies, while the Ag–ZnO–Ag structures served as Schottky type MSM photodetectors. For comparison, the Al–ZnO–Al structures were fabricated as the MSM photoconductive detectors. E-beam evaporation was utilized for the metal deposition. The thickness of Al and Ag layers was 2000 A˚. For the Schottky contact, a 500 A˚ thick Au layer was finally deposited on the top of the Ag layer, to act as an oxide-resistant layer. It is well known that in fabrication of Schottky contacts, the surface states, contaminants and defects of the surface layer affect the barrier height and leakage current. Oxygen plasma was used to clean the surface
111
before and after the metallization process in order to minimize such effects. Photoresponse measurements were performed using an Oriel optical system. A Xe-arc lamp and monochromator combination provided the light source. The photoresponse speed of the detector was also measured. The optical excitation source was the 337.1 nm line of a N2 pulse laser, with a pulse width of 2 ns at a repetition rate of 40 Hz. Neutral density filters were used to control the optical power on the detector. Optical energy on the detector was about 10 nJ per pulse. The signal from a 50Oload resistor was recorded by a digital scope with a time resolution better than 1 ns. The bias voltage was 9 V.
3. Results and discussion Shown in Fig. 1 are the I–V characteristics of ZnO MSM circular devices, including both Schottky and ohmic types. The linear I–V relation from the Al–ZnO–Al structure clearly indicates the ohmic behavior of the Al on n-type ZnO contact. On the other hand, the rectified I–V relation from the Ag–ZnO–Al confirms Schottky junction formation between Ag and n-type ZnO. In a Schottky diode, the general I–V characteristics are
Fig. 1. I–V characteristics of ZnO diodes with the circular pattern. The electrode gap is 10 mm.
112
represented by [7] qV J ¼ Js exp 1 ; nkT
S. Liang et al. / Journal of Crystal Growth 225 (2001) 110–113
ð1Þ
where Js ¼ A * T 2 expðFB =kT Þ is the saturation current density based on thermionic emission theory, n the ideality factor, k the Boltzmann’s constant, T the absolute temperature, A * the effective Richardson coefficient, and FB the barrier height. The Schottky contact area is 1.77 104 cm2. The values of n ¼ 1:5 and Js ¼ 2:4 108 A/cm2 were obtained from curve fitting. The barrier height FB is estimated to be about 0.84 eV by using A*32 A/cm2 K2 (A * ¼ 4pqm * k2 =h3 , where m * 0:27m0 [8]). At a reverse bias of 1 V, the leakage current (0.1 nA) of the Ag–ZnO–Al device is about 5 orders of magnitude smaller than that of the Al–ZnO–Al device (10 mA). The Ag–ZnO–Ag MSM structure with IDT configuration was used to evaluate the UV detector performance. Shown in Fig. 2 are the dark and photo-illuminated I–V characteristics of a Schottky detector. The wavelength and power of the illuminated light are 368 nm and 0.1 mW, respectively. The leakage current of the photodetector is about 1 nA at a bias of 5 V. The breakdown starts at about 8 V. The early and soft breakdown is mainly due to the non-uniform field distribution, especially at the corners of the finger
Fig. 2. I–V characteristics of a ZnO Schottky photodetector with an IDT Structure. The inset shows a SEM picture of the top view of the device.
electrodes in the IDT configuration. The lowfrequency photoresponsivity is about 1.5 A/W, which corresponds to a gain of 2.5. We believe that the photoconductive effect which occurs at the high field contributes to the large gain (>1). The spectra-photoresponse of the IDT Schottky device is shown in Fig. 3. The responsivity drops more than 3 orders of magnitude from 370 to 390 nm. The sharp cut-off at wavelength of 370 nm agrees with the ZnO energy band gap of 3.35 eV. The responsivity decreases at the shorter wavelength range due to decrease of the penetrating depth of the light, resulting in an increase of the surface recombination. Shown in Fig. 4 is the photocurrent as a function of time from an IDT ZnO Schottky detector. The photoresponse has a fast component, which rises within 12 ns and falls to 66% of its peak value within 50 ns as shown in the inset of Fig. 4. After that a slow process follows lasting about 5 ms (not shown in the figure). The fast response in a MSM detector is usually related to the transit time of the photo-generated carriers, while the slow response in a ZnO MSM detector is usually attributed to the oxygen adsorption at the surface and grain boundaries [9,10]. We used the high quality epitaxial ZnO for both Schottky and photoconductive detectors. As reported in Ref. [3], we did not observe such a slow process in the photoconductive devices with the same material quality. Therefore, we attribute this slow process
Fig. 3. Spectral-photoresponse of a ZnO Schottky photodetector with an IDT structure.
S. Liang et al. / Journal of Crystal Growth 225 (2001) 110–113
113
photoresponsivity of 1.5 A/W at a bias of 5 V was obtained. The leakage current of the device at 5 V bias was in the order of 1 nA. The photoresponse of the detector showed a fast component with a rise time of 12 ns, and a fall time of 50 ns.
Acknowledgements The work has been supported by Rutgers University under SROA grant. Fig. 4. Photocurrent as a function of response time of a ZnO Schottky photodetector with an IDT structure.
to the trapping and emission of photo-generated carriers in the ZnO surface of the Schottky detector. It should be noted that oxygen plasma was used to treat the ZnO surface only when making the Schottky detectors. The detailed study will be reported elsewhere.
4. Conclusion We have demonstrated Schottky type UV photodetectors based on ZnO epitaxial films grown on R-plane Al2O3 by MOCVD. Ag was used to form Schottky contact on n-type ZnO with a barrier height about 0.84 eV. A low frequency
References [1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [2] M. Wraback, H. Shen, S. Liang, Y. Lu, Appl. Phys. Lett. 74 (1999) 507. [3] Ying Liu, C.R. Gorla, S. Liang, N. Emanetoglu, Y. Lu, J. Electron. Mater. 29 (2000) 69. [4] H. Fabricius, T. Skettrup, P. Bisgaard, Appl. Opt. 25 (1986) 2764. [5] C.R. Gorla, N.W. Emanetoglu, S. Liang, W.E. Mayo, Y. Lu, J. Appl. Phys. 85 (1999) 2595. [6] S. Liang, C.R. Gorla, N. Emanetoglu, Y. Liu, W.E. Mayo, Y. Lu, J. Electron. Mater. 27 (1998) L72. [7] S.M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, Berlin, NewYork, 1981, p. 264 (Chapter 5). [8] Rolf E. Hummel, Electronic Properties of Materials, Springer, Berlin, 1992, p. 373. [9] Y. Takahashi, M. Kanamori, A. Kondoh, H. Minoura, Y. Ohya, Jpn. J. Appl. Phys. 33 (1994) 6611. [10] D.H. Zhang, J. Phys. D 28 (1995) 1273.