ZnO tetrapod p - n junction diodesMarcus C. Newton and Rais Shaikhaidarov Citation: Applied Physics Letters 94, 153112 (2009); doi: 10.1063/1.3119630 View online: http://dx.doi.org/10.1063/1.3119630 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Analog and bipolar resistive switching in pn junction of n-type ZnO nanowires on p-type Si substrate J. Appl. Phys. 114, 064502 (2013); 10.1063/1.4817838 ZnO based organic-inorganic hybrid p-n junction diode AIP Conf. Proc. 1512, 468 (2013); 10.1063/1.4791114 Hybrid p - n junction light-emitting diodes based on sputtered ZnO and organic semiconductors Appl. Phys. Lett. 95, 253303 (2009); 10.1063/1.3275802 Dominant ultraviolet light emissions in packed ZnO columnar homojunction diodes Appl. Phys. Lett. 93, 132113 (2008); 10.1063/1.2992629 Experimental analysis and theoretical model for anomalously high ideality factors in ZnO/diamond p-njunction diode Appl. Phys. Lett. 84, 2427 (2004); 10.1063/1.1689397

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

202.28.191.34 On: Sat, 20 Dec 2014 04:58:45

ZnO tetrapod p-n junction diodesMarcus C. Newton1,a� and Rais Shaikhaidarov2

1London Centre for Nanotechnology, University College London, London, EnglandWC1H 0AH, United Kingdom2Department of Physics, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom

�Received 1 March 2009; accepted 23 March 2009; published online 16 April 2009�

ZnO nanocrystals hold the potential for use in a wide range of applications particularly inoptoelectronics. We report on the fabrication of a highly sensitive p-n junction diode structure basedon a single ZnO tetrapod shaped nanocrystal. This device shows a noted response to ultraviolet lightwith high internal gain. The high reponsivities we have observed exceed 104 A /W and are likelydue to impact-ionization effects at the p-n junction interface. © 2009 American Institute of Physics.�DOI: 10.1063/1.3119630�

Semiconducting nanoscale crystalline structures such asnanowires and nanotubes are extensively studied as theirunique optical and electronic properties have potential use invarious device applications. The increasing need to makesmaller device structures with enhanced functionality has ledthe move away from conventional top-down lithographic ap-proaches toward the utilization of self assembled nanoscalestructures. The unique morphologies exhibited by manynanocrystals make them ideal for various applications in-cluding gas sensors,1 photodetectors,2–5 light emitters,6–8

transistors,9,10 high frequency oscillators,11 interconnects,7

and waveguides.12 In particular, nanocrystal optics and pho-tonics is an interesting field with the promise of creatingphotonic devices for generation and detection at the nano-scale. Thus far, light emitting diodes, electrically drivennanowire lasers, waveguides, and photodetectors made fromsingle nanostructures have been demonstrated. The ability tocoassemble various device structures such as detectors andemitters widens the possibility for integration with siliconbased electronics.

Zinc oxide �ZnO� is a polar group II-VI semiconductormaterial with a direct band gap of 3.37 eV. ZnO also hasseveral advantages over other wide band gap semiconductorsin that it is biocompatible, amenable to wet chemical etching,and highly resistive to high-energy radiation. Over the pastdecade, progress in developing single crystal bulk ZnO andrecent results in growing p-type material have brought ZnO’spromise as a wide band gap semiconductor to the forefront.There is currently a vibrant research effort into ZnO with thehope of creating new kinds of devices such a spin polarizedlight emitting diodes and carrier mediated ferromagnetismfor magnetic storage. Özgür et al.13 have compiled a reviewof current trends in ZnO research. ZnO films show n-typeconductivity often attributed to the presence of H ions andZn interstitials. The exciton binding energy of 60 meV ismore than twice that of GaN. This being larger than thethermal energy, allows ZnO to provide stable band edge ul-traviolet �UV� emission at room temperature via the signifi-cantly more efficient exciton recombination process as op-posed to the electron-hole plasma process employed incurrent GaN based devices. ZnO has high chemical resis-tance and thermal stability up to 1200 °C. Applications

include UV diodes, chemical sensors, and magnetic storage.ZnO therefore has many advantages over the industry-standard GaN for UV and blue optoelectronic devices. Inaddition, ZnO nanoparticles have been widely used in paints,rubber processing, and sunscreen lotions, while their poly-crystalline forms have been used for more high-tech usessuch as phosphors, piezoelectric transducers, varistors, andtransparent conducting films.

ZnO also forms a large family of self assembled nano-scale structures.14–17 These nanostructures are interesting asthey often allow us to observe and study physical propertiesthat differ widely from the bulk material. These includequasi-one-dimensional charge transport and photon confine-ment. ZnO nanostructures may be synthesized using a num-ber of techniques, the most common of which involve achemical vapor transport process. Our work has mainly fo-cused on the study of ZnO tetrapod �ZnO-T� nanocrystalsand their devices.18 In this study we report on the fabricationand device performance characteristics of p-n junction diodeformed from a single ZnO-T nanocrystal.

The synthesis of ZnO-T nanocrystals is carried out in ahorizontal tube furnace as described in detail elsewhere.14

Briefly, synthesis is carried out in a tube furnace and pro-ceeds via a carbothermal reduction process which releasessupersaturated Zn vapor into an oxygen containing carriergas causing ZnO nanocrystals to condense downstream.The source material consists of zinc carbonate�ZnCO3·2Zn�OH�2 ·H2O� and graphite powder. The furnace

a�Electronic mail: marcus.newton@ucl.ac.uk.

FIG. 1. �Color online� SEM image of ZnO-T p-n junction diode. The tetra-pod nanocrystal is circled in red. 2 �m scale bar shown. The inset shows anSEM image of the ZnO-T nanocrystal before lithographic processing. 500nm scale bar shown.

APPLIED PHYSICS LETTERS 94, 153112 �2009�

0003-6951/2009/94�15�/153112/3/$25.00 © 2009 American Institute of Physics94, 153112-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

202.28.191.34 On: Sat, 20 Dec 2014 04:58:45

is heated to 900 °C in a flow of argon and oxygen. Thereaction is maintained for 30 min before the system is al-lowed to cool naturally to room temperature �20 °C�. Aftersynthesis, a fluffy white layer is visible on the surface of thesubstrate. The substrate is then submerged and sonicated in aclean beaker of methyl alcohol �CH3OH� to form a nano-crystal suspension.

The inset of Fig. 1 shows a scanning electron micro-graph �SEM� image of a ZnO-T nanocrystal synthesized us-ing this technique. From the image it can be seen that eacharm is well faceted and is uniform in length and diameter.X-ray diffraction analysis confirms that these nanocrystalscondense into the wurtzite crystal structure.14

A number of transparent p-type semiconductor materialsare ideal for deposition onto ZnO with low interfacial strain.Suitable materials include p-type GaN, SrCu2O2, andCuAlO2. These materials however require deposition tech-niques, such as molecular beam epitaxy, chemical vapordeposition, pulsed laser deposition, or sputter deposition,which can be difficult to integrate into the lithographic pro-cess. In the following, we show how NiO can be used as awide band gap transparent p-type semiconducting materialthat can be deposited by thermal evaporation.

The fabrication of tetrapod p-n diodes is carried out asfollows. SiO2 /Si substrates with an oxide thickness of 200nm and patterned with lithographic alignment markers andcontact leads were first cleaned with isopropyl alcohol.ZnO-T nanocrystals were then spin coated from methanolsuspension to form a sparse monodispersion on the surface.E-beam lithography with high precision layer alignment,thermal evaporation, and lift off technique were used to

deposit 300 nm thick NiO and Al contacts. Metallic �Al�leads formed Ohmic contacts, while p-type semiconductor�NiO� leads formed p-n junction with ZnO-T. High purity Nimetal was evaporated in an oxygen atmosphere at a partialpressure of 8�10−5 mbar to form the NiO film. Figure 1shows an SEM image of a ZnO-T p-n diode contacted withboth NiO and Al contacts. In this case the lithography pro-cess is repeated twice for each of the two materials.

Figure 2 shows the resulting band structure of the de-vice. By considering the band alignments it can be seen thata p-n junction resides at the p-type NiO /n-type ZnO inter-face. The Al forms an Ohmic contact. As the band gap of theNiO is 4.0 eV which is greater than that of ZnO, the p-njunction interface is transparent to visible light with energyup to 3.37 eV or 380 nm �the fundamental energy band gapof ZnO�.

Figure 3 shows the photoresponse of the device whenexcited by an unfocused continuous-wave He–Cd 325 nmlaser beam. In the absence of illumination a reverse biascurrent less than 50 pA is observed. Upon illumination acorresponding reverse bias photocurrent is observed. Withincreasing intensity a proportional increase in the reversebias photocurrent is observed. This current however beginsto saturate suggesting that the photocurrent observed is dueto the finite number of excitons created optically within theZnO material which are separated by the applied electricfield. By considering the dimensions of the diode structureand the incident light from the He–Cd 325 nm laser, weobtain a responsivity of 1.7�104 A /W.

Similar carrier multiplication effects have recently beenobserved in a series of semiconductor nanocrystals using op-tical techniques such as four-wave mixing. The route bywhich this occurs, however, is not fully understood and is atopic of our ongoing research. Carrier multiplication ishowever readily achieved through impact-ionization via mul-tiple exciton-exciton and exciton-electron scatteringprocesses.19–21 This process is responsible for the gain effectsexploited in the avalanche diode structure and are most likelyto be the major contributing factor here.

Figure 4 shows the spectral response in the range of200–500 nm obtained using a xenon lamp and monochro-mator. A peak in response is observed at 393 nm which canbe attributed to band edge absorption in ZnO. An additionalless intense peak is also observed at 240 nm which is mostlikely due to 3D-shell excited state transitions.22,23

FIG. 2. �Color online� Heterostructure showing interface between n-typeZnO and p-type NiO semiconductor materials under reverse bias and UVillumination. Conduction and valence bands only are shown for clarity. Ad-ditional carriers are created at the interface due to impact-ionization of car-riers accelerated by the potential at the interface. The energy gap, electronaffinity, and work function for ZnO and NiO are as follows: Eg�ZnO�=3.37 eV, ��ZnO�=4.35 eV, WF�ZnO�=5.2 eV, Eg�NiO�=4.0 eV,��NiO�=3.7 eV, WF�NiO�=7.5 eV.

FIG. 3. �Color online� Photoresponse of ZnO–NiO p-ndiode to continuous-wave UV excitation at 325 nm withincreasing intensity up to 7�10−4 W /cm2.

153112-2 M. C. Newton and R. Shaikhaidarov Appl. Phys. Lett. 94, 153112 �2009�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

202.28.191.34 On: Sat, 20 Dec 2014 04:58:45

Using a 325 nm laser source focused down to 1 �mpoint, the spatial resolution of the photoresponse is studied.This is acquired by laterally displacing the tetrapod diode intwo normal directions from the microscope center of imag-ing while measuring the photogenerated current. A clear peakwas observed in both directions near to the origin of thenanocrystal. This confirms that the observed photocurrentsare generated locally at the p-n interfacial region. The insetof Fig. 4 shows the displacement in a single direction.

In summary, we have fabricated a highly sensitive p-njunction diode structure that employs a single ZnO-T nano-crystal. The device structure is highly responsive to ultra-violet irradiation and shows good device characteristics. Thiswork marks a significant advancement toward the integrationand utilization of nanoscale structures in device type con-figurations where their unique properties can be exploited.

The authors would like to thank all those who have con-tributed positively to this work. Special thanks is given toProfessor Christopher Baker, Professor Arokia Nathan,

and Professor Ian Robinson for their continued support andencouragement.

1C. Xiangfeng, J. Dongli, A. B. Djurisic, and Y. H. Leung, Chem. Phys.Lett. 401, 426 �2005�.

2Z. Fan, P. C. Chang, J. G. Lu, E. C. Walter, R. M. Penner, C. H. Lin, andH. P. Lee, Appl. Phys. Lett. 85, 6128 �2004�.

3Y. Heo, L. Tien, D. Norton, S. Pearton, B. Kang, F. Ren, and J. LaRoche,Appl. Phys. Lett. 85, 3107 �2004�.

4M. C. Newton, S. Firth, and P. A. Warburton, Appl. Phys. Lett. 89,072104 �2006�.

5J. Wang, M. S. Gudiksen, X. Duan, Y. Cui, and C. M. Lieber, Science293, 1455 �2001�.

6X. Duan, Y. Huang, R. Agarwal, and C. Lieber, Nature �London� 421, 241�2003�.

7X. Duan, Y. Huang, Y. Cui, J. Wang, and C. M. Lieber, Nature �London�409, 66 �2001�.

8R. Konenkamp, R. C. Word, and C. Schlegel, Appl. Phys. Lett. 85, 6004�2004�.

9Y. Cui, Z. Zhong, D. Wang, W. Wang, and C. Lieber, Nano Lett. 3, 149�2003�.

10M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo,and P. Yang, Science 292, 1897 �2001�.

11R. S. Friedman, M. C. McAlpine, D. S. Ricketts, D. Ham, and C. M.Lieber, Nature �London� 434, 1085 �2005�.

12M. Law, D. J. Sirbuly, J. C. Johnson, J. Goldberger, R. J. Saykally, and P.Yang, Science 305, 1269 �2004�.

13Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V.Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 �2005�.

14M. Newton, S. Firth, T. Matsuura, and P. Warburton, J. Phys. Conf. Ser.26, 251 �2006�.

15Z. L. Wang, Mater. Today 7, 26 �2004�.16Z. L. Wang, J. Phys.: Condens. Matter 16, R829 �2004�.17Z. L. Wang and J. Song, Science 312, 242 �2006�.18M. C. Newton and P. A. Warburton, Mater. Today 10, 50 �2007�.19J. Luther, M. Beard, Q. Song, M. Law, R. Ellingson, and A. Nozik, Nano

Lett. 7, 1779 �2007�.20R. D. Schaller, V. M. Agranovich, and V. I. Klimov, Nat. Phys. 1, 189

�2005�.21A. Abiyasa, S. Yu, and W. Fan, IEEE J. Sel. Top. Quantum Electron. 42,

455 �2006�.22D. Fritsch, R. Schmidt-Grund, H. Schmidt, C. Herzinger, and M.

Grundmann, Proceedings of the Fifth International Conference onNumerical Simulation of Optoelectronic Devices, 2005 �unpublished�, pp.69–70.

23M. Yin, Y. Gu, I. L. Kuskovsky, T. Andelman, Y. Zhu, G. F. Neumark, andS. O’Brien, J. Am. Chem. Soc. 126, 6206 �2004�.

FIG. 4. Spectral response of ZnO–NiO p-n diode obtained using xenonlamp and monochromator configuration. The inset shows the position de-pendent photoresponse of a ZnO-T p-n diode. The photocurrent is obtainedas a 1 �m 325 nm beam and is linearly scanned across the diode structure.

153112-3 M. C. Newton and R. Shaikhaidarov Appl. Phys. Lett. 94, 153112 �2009�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

202.28.191.34 On: Sat, 20 Dec 2014 04:58:45