ZnO p-n homojunctions based on Sb-doped p-type nanowire array and n-type film were grown by combining chemical vapor deposition (for nanowires) with molecular-beam epitaxy (for film). Indium tin oxide and Ti/Au were used as contacts to the ZnO nanowires and film, respectively. Characteristics of field-effect transistors using ZnO nanowires as channels indicate p-type conductivity of the nanowires. Electron beam induced current profiling confirmed the existence of ZnO p-n homojunction. Rectifying I-V characteristic showed a turn-on voltage of around 3 V. Very good response to ultraviolet light illumination was observed from photocurrent measurements

Chernyak, Leonid

Chu, Sheng

Lin, Yuqing

Liu, Jianlin

Wang, Guoping

Zhan, Ning

English

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2011 ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection Guoping Wang Sheng Chu Ning Zhan Yuqing Lin University of Central Florida Leonid Chernyak University of Central Florida See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Wang, Guoping; Chu, Sheng; Zhan, Ning; Lin, Yuqing; Chernyak, Leonid; and Liu, Jianlin, "ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection" (2011). Faculty Bibliography 2010s. 2069. https://stars.library.ucf.edu/facultybib2010/2069 Authors Guoping Wang, Sheng Chu, Ning Zhan, Yuqing Lin, Leonid Chernyak, and Jianlin Liu This article is available at STARS: https://stars.library.ucf.edu/facultybib2010/2069 Appl. Phys. Lett. 98, 041107 (2011); https://doi.org/10.1063/1.3551628 98, 041107© 2011 American Institute of Physics.ZnO homojunction photodiodes based onSb-doped p-type nanowire array and n-typefilm for ultraviolet detectionCite as: Appl. Phys. Lett. 98, 041107 (2011); https://doi.org/10.1063/1.3551628Submitted: 23 November 2010 . Accepted: 13 January 2011 . Published Online: 28 January 2011Guoping Wang, Sheng Chu, Ning Zhan, Yuqing Lin, Leonid Chernyak, and Jianlin LiuARTICLES YOU MAY BE INTERESTED INHomojunction photodiodes based on  -type  for ultraviolet detectionApplied Physics Letters 88, 092103 (2006); https://doi.org/10.1063/1.2178470Opto-electrical properties of Sb-doped p-type ZnO nanowiresApplied Physics Letters 104, 111909 (2014); https://doi.org/10.1063/1.4869355ZnO nanowires array  homojunction and its application as a visible-blind ultravioletphotodetectorApplied Physics Letters 96, 053102 (2010); https://doi.org/10.1063/1.3299269ZnO homojunction photodiodes based on Sb-doped p-type nanowire arrayand n-type film for ultraviolet detectionGuoping Wang,1 Sheng Chu,1 Ning Zhan,1 Yuqing Lin,2 Leonid Chernyak,2 andJianlin Liu1,a1Department of Electrical Engineering, Quantum Structures Laboratory, University of Californiaat Riverside, Riverside, California 92521, USA2Department of Physics, University of Central Florida, Orlando, Florida 32816-2385, USAReceived 23 November 2010; accepted 13 January 2011; published online 28 January 2011ZnO p-n homojunctions based on Sb-doped p-type nanowire array and n-type film were grown bycombining chemical vapor deposition for nanowires with molecular-beam epitaxy for film.Indium tin oxide and Ti/Au were used as contacts to the ZnO nanowires and film, respectively.Characteristics of field-effect transistors using ZnO nanowires as channels indicate p-typeconductivity of the nanowires. Electron beam induced current profiling confirmed the existence ofZnO p-n homojunction. Rectifying I-V characteristic showed a turn-on voltage of around 3 V. Verygood response to ultraviolet light illumination was observed from photocurrent measurements.© 2011 American Institute of Physics. doi:10.1063/1.3551628ZnO has a wide band gap of 3.37 eV and a large excitonbinding energy of 60 meV at room temperature,1,2 whichmake it a promising candidate for optoelectronic devicessuch as blue-light emitting diodes,3,4 ultraviolet UV laserdiodes,5–7 and photodiodes.8–10 Recently, there have been tre-mendous interests in ZnO nanowire arrays. Although hetero-junction optoelectronic devices11–13 have been fabricatedbased on vertically aligned ZnO nanowire arrays, there stilllacks the homojunction type devices based on p-type nano-wire array/n-type ZnO film. It is well known that one of thebiggest challenges toward good ZnO-based optoelectronicdevices is the difficulty of reliably fabricating p-type ZnOdue to the self-compensating effect from native defects forexample, oxygen vacancy Vo and zinc interstitial Zni and/orH incorporation.14 Nevertheless, there has already been agreat deal of efforts on the fabrication of p-type ZnO bydoping group V elements, such as N, P, and As.15–17 Previ-ously, our group showed that Sb, another group V element, isan effective dopant for reproducible p-type ZnO thin films18on Si and sapphire substrates. Heterojunction8 andhomojunction9 light emitting diodes as well as photodiodesbased on these films were achieved. In this study, we reportZnO p-n homojunctions based on Sb-doped p-type ZnOnanowire arrays grown by chemical vapor deposition CVDon n-type ZnO films on sapphire substrates grown bymolecular-beam epitaxy MBE. The homojunction diode ex-hibits a clear rectification characteristic as well as very goodultraviolet light absorption characteristics.The device fabrication process of the p-type ZnOnanowire/n-type ZnO film structure is described as follows.A highly oriented vertical ZnO nanowire array was synthe-sized via seed growth method.19 First, n-type ZnO seed film,which also acted as n-type component of the junction, wasgrown on a 2 in. c-plane sapphire substrate using plasma-assisted MBE. The growth process began with a thin MgO/ZnO buffer layer of less than 10 nm at a growth temperatureof 550 °C for improving the subsequent ZnO film quality.Then the seed ZnO film was grown at 700 °C for 5 h, yield-ing a total thickness of around 1050 nm. Elemental Zn wasevaporated at an effusion cell temperature of 360 °C. Theoxygen plasma was generated using a radio-frequencyplasma source. The oxygen flow rate was kept at 5 sccm.Hall effect measurements were carried out on the MBE-grown ZnO thin film. The film was found to have n-typeconductivity with an electron concentration of 3.741017 cm−3, a mobility of 23.04 cm2 V−1 s−1, and a resis-tivity of 0.73  cm, respectively. The ZnO seed film waspartially covered for exposing the film for later n-type con-tact deposition and then subsequently transferred into aquartz tube furnace system. The growth of nanowire arraywas carried out at 650 °C for 15 min with a gas flow of 1000sccm nitrogen and 200 sccm mixture gas of argon/oxygen99.5:0.5. A Zn powder source 99.999% in a glass bottlewas placed in the center of the quartz tube. At the place of5 cm upstream to the Zn powder source, Sb powder99.99% was placed in an open glass boat. The ZnO seedfilm was kept around 10 cm away from the Zn source on thedownstream side. After a vertical nanowire array wasachieved in CVD, polymethyl-methacrylate PMMA wasspun on the sample to separate the bottom ZnO film and thesubsequent top indium tin oxide ITO contact layer. Thespin speed was 2000 rpm for 30 s, and this process wasrepeated for five times. After the sample was dried, a thinlayer of ITO was sputtered by a sputter evaporator on the topof ZnO nanowire array. The sputtering process was per-formed at room temperature and the pressure was maintainedat 10−2 Torr. The sputtering power and time were 180 W and10 min, respectively. After the growth of ITO, acetone wasused to remove the PMMA layer. The device was finished bydepositing Ti/Au 20 nm/100 nm on the n-type ZnO filmusing an electron-beam e-beam evaporator and rapidly an-nealed under 600 °C for 1 min to improve conductivity. An-other low-resistant ITO glass slide was placed on the top ofthe ZnO nanowire array when I-V and photoresponse char-acteristics were measured. Figure 1 shows a schematic dia-gram of the device.Figures 2a and 2b show scanning electron micros-copy SEM images of the nanowire array from the top andaElectronic mail: jianlin@ee.ucr.edu.APPLIED PHYSICS LETTERS 98, 041107 20110003-6951/2011/984/041107/3/$30.00 © 2011 American Institute of Physics98, 041107-1the side, respectively. Typical wurtzite hexagonal structureZnO nanowires are evident. The length and diameter of theZnO nanowires are on average 3 m and 150 nm, respec-tively. The single-crystalline nature of the ZnO nanowires isconfirmed by high-resolution transmission electron micros-copy TEM analysis, as shown in Fig. 2c and by the se-lected area electron diffraction SAED pattern Fig. 2d.Figure 3a shows spectrum of the electron beam in-duced current EBIC profiling of the homojunction betweenthe ZnO nanowires and ZnO film superimposed on the side-view SEM image of the device. As a versatile tool, EBICwas widely used to identify buried junctions in semiconduc-tors, including characterization of ZnO thin film p-njunctions20 and silicon nanowire p-n junctions.21 In the ex-periment, electron-hole pairs are generated by electron beamirradiation. Then they are separated by built-in electric fieldin the p-n junction depletion region and collected by an ex-ternal amplifier. Here, an accelerating voltage of 30 kV wasapplied, corresponding to the electron penetration depth of1.5 m. Silver paste instead of ITO glass was used as con-tact to the p-type nanowires for facilitating the EBIC experi-ment. As seen from Fig. 3a, an EBIC signal forms a peakon the thin film/nanowire junction, indicating the formationof p-n junction. The second peak on the right is due to theformation of the nanowire/ITO/silver paste semiconductor-metal junction.The p-type conductivity of the Sb-doped ZnO nanowirescan also be proved by field-effect measurement. ZnO nano-wire field-effect transistors NWFETs were fabricated bystandard photolithography. Sb-doped ZnO nanowires weretransferred to a p+-silicon wafer with a 300 nm thick siliconoxide on the surface. Microcontact windows were defined onthe ends of the nanowires, then Ni/Au 20 nm/100 nm elec-trodes were formed by e-beam deposition and subsequentlift-off. The p+-silicon substrate served as the backgate elec-trode of the transistor. The inset in Fig. 3b shows the SEMimage of the NWFET. The drain current Id versus gatevoltage Vg curve under a drain voltage Vd of 10 V and Idversus Vd curves under different gate voltages are shown inFigs. 3b and 3c, respectively. The results show that theSb-doped ZnO nanowires exhibit p-type behavior under gatevoltage ranging from 10 to 30 V. The origination of thep-type conductivity of the Sb-doped ZnO nanowires is fromthe shallow acceptors formed by substitutional SbZn simulta-neously connecting two Zn vacancies.22 The hole concentra-tion P in nanowires can be estimated by using the follow-ing equation:23P = Vthq 	 2roln4h/d 1d2/4 , 1where r, o, h, d, Vth, and q are the effective dielectricconstant 3.9 for SiO2, the dielectric constant in vacuum, thethickness of dielectric layer 300 nm, the nanowire diameterFIG. 1. Color online Schematic view of the photodetector device, whichconsists of ZnO thin film on c-sapphire substrate, vertically aligned ZnOnanowires, ITO contacts, and Ti/Au contacts.FIG. 2. a Top-view and b side-view SEM images of as-grown p-typeZnO nanowires. c High-resolution TEM image of a single nanowire. Thelattice spacing between two atomic layers is measured to be 0.52 nm. dSAED pattern, indicating the single-crystalline characteristic of thenanowire.FIG. 3. Color online a EBIC profile of the device. A clear peak can beseen in the area between the ZnO nanowires and ZnO film, representing theformation of junction. b Id-Vg curve of the ZnO nanowire FET under Vd=10 V. Inset shows the SEM image of the NWFET. c Id-Vd curves of theZnO nanowire FET recorded at different gate voltages.041107-2 Wang et al. Appl. Phys. Lett. 98, 041107 2011200 nm, the threshold voltage of the ZnO NWFET29 V, and the elementary charge constant, respectively.From Eq. 1, the hole concentration is calculated to be 6.91017 cm−3. The mobility can be calculated by using theequation23 =  dIdVg ln4hd 2ro LVd , 2where , L, and Vd are the mobility, the channel length1.8 m, and the drain-source voltage 10 V, respec-tively. dI /dVg=2.55e−009 A /V can be extrapolated fromthe linear region of the Id-Vg curve. From Eq. 2, the mo-bility is calculated as approximately 0.037 cm2 V−1 s−1based on the experimental data.Figure 4a shows current-voltage characteristics of theZnO p-n homojunction. Clear rectifying behavior can be ob-served with and without UV illumination. The turn-on volt-age is at 3 V. The possible formation of the graded p-njunction due to the impurity diffusion during nanowiregrowth and the formation of the metal/semiconductor nano-wire contact are the reasons for the small rectification ratio ofthe p-n homojunction. Photocurrent PC measurements werecarried out using a homebuilt system. The PC system con-sists of an Oriel Xe arc lamp as the UV source. The lightfrom the lamp passes through an Oriel 0.25 m monochro-mator, which produces a specific wavelength light at its out-put port. After chopping, the light is then cast on the device.The generated PC signal is fed to a lock-in amplifier fromwhere the data are collected. Figure 4b shows PC spectra ofthe homojunction device operated in the photovoltaic modeand also under different reverse biases. The device showsresponse as a result of the photocarriers generated by theabsorption of light in the space-charge region. The responseextends from 280 nm and steadily increases up to 380 nm3.26 eV, which corresponds to the effective band gap ofZnO. The magnitude of photocurrent increases with the in-crease of applied reverse bias 2, 4, and 6 V due toenhanced carrier collection. There is a small peak at 570 nm2.17 eV in the spectra due to the presence of deep levels inZnO.In summary, ZnO p-n homojunctions based on Sb-dopedp-type nanowire arrays and n-type thin films were grown bythe combination of CVD and MBE. The ZnO thin film/nanowire p-n junction was proved by EBIC profiling, I-Vmeasurement, and field-effect measurement. Evident photo-response was observed in the UV region of the PC spectra.This study not only proved that Sb doping can lead to p-typeZnO nanowires, but also demonstrated an ultraviolet homo-junction photodetector device based on p-type nanowire ar-ray and n-type thin film.Guoping Wang and Sheng Chu contributed equally tothis work. This research was partly supported by NationalScience Foundation Grant No. ECCS-0900978.1E. O. Kane, Phys. Rev. B 18, 6849 1978.2C. Klingshirn, Phys. Status Solidi B 244, 3027 2007.3D. C. Look, Mater. Sci. Eng., B 80, 383 2001.4S. Chu, J. H. Lim, L. J. Mandalapu, Z. Yang, L. Li, and J. L. Liu, Appl.Phys. Lett. 92, 152103 2008.5H. Cao, Y. G. Zhao, H. C. Ong, S. T. Ho, J. Y. Dai, J. Y. Wu, and R. P.Chang, Appl. Phys. Lett. 73, 3656 1998.6S. F. Yu, C. Yuan, S. P. Lau, W. I. Park, and G. Yi, Appl. Phys. Lett. 84,3241 2004.7S. Chu, M. Olmedo, Z. Yang, J. Y. Kong, and J. L. Liu, Appl. Phys. Lett.93, 181106 2008.8L. J. Mandalapu, F. X. Xiu, Z. Yang, D. T. 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Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, G. Somor-jai, and P. D. Yang, Nano Lett. 5, 1231 2005.20L. Chernyak, C. Schwarz, E. S. Flitsiyan, S. Chu, J. L. Liu, and K. Garts-man, Appl. Phys. Lett. 92, 102106 2008.21S. Hoffmann, J. Bauer, C. Ronning, Th. Stelzner, J. Michler, C. Ballif, V.Sivakov, and S. H. Christiansen, Nano Lett. 9, 1341 2009.22S. Limpijumnong, S. B. Zhang, S. H. Wei, and C. H. Park, Phys. Rev. Lett.92, 155504 2004.23R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and Ph. Avouris, Appl. Phys.Lett. 73, 2447 1998.FIG. 4. Color online a I-V characteristics of the ZnO nanowire/ZnO filmdevice with and without UV illumination. b PC spectra under differentreverse biases. Good responses in the UV region are evident.041107-3 Wang et al. Appl. Phys. Lett. 98, 041107 2011

ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection

Guoping Wang

Sheng Chu

Ning Zhan

Yuqing Lin

Leonid Chernyak

Jianlin Liu

Crossref

Applied Physics Letters

https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=3068&amp;context=facultybib2010

ZnO homojunction photodiodes based on Sb-doped p-type nanowire array and n-type film for ultraviolet detection

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