Variable temperature electron beam induced current technique was employed for the profiling of ZnO p-n homojunctions and the extraction of minority electron diffusion length values in the Sb-doped p-type ZnO region. A thermally induced increase for diffusion length of minority electrons was determined to have an activation energy of similar to 145 meV. The latter parameter likely represents carrier delocalization energy and determines the increase of the diffusion length due to the reduction in recombination efficiency. (c) 2008 American Institute of Physics

Chernyak, L.

Chu, S.

Flitsiyan, E. S.

Gartsman, K.

Liu, J. L.

Schwarz, C.

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

University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2008 Electron beam induced current profiling of ZnO p-n homojunctions L. Chernyak University of Central Florida C. Schwarz University of Central Florida E. S. Flitsiyan University of Central Florida S. Chu University of Central Florida J. L. Liu See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib2000 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 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Chernyak, L.; Schwarz, C.; Flitsiyan, E. S.; Chu, S.; Liu, J. L.; and Gartsman, K., "Electron beam induced current profiling of ZnO p-n homojunctions" (2008). Faculty Bibliography 2000s. 217. https://stars.library.ucf.edu/facultybib2000/217 Authors L. Chernyak, C. Schwarz, E. S. Flitsiyan, S. Chu, J. L. Liu, and K. Gartsman This article is available at STARS: https://stars.library.ucf.edu/facultybib2000/217 Appl. Phys. Lett. 92, 102106 (2008); https://doi.org/10.1063/1.2896613 92, 102106© 2008 American Institute of Physics.Electron beam induced current profiling ofZnO  homojunctionsCite as: Appl. Phys. Lett. 92, 102106 (2008); https://doi.org/10.1063/1.2896613Submitted: 14 January 2008 . Accepted: 22 February 2008 . Published Online: 13 March 2008L. Chernyak, C. Schwarz, E. S. Flitsiyan, S. Chu, J. L. Liu, and K. GartsmanARTICLES YOU MAY BE INTERESTED INElectron beam induced current measurements of minority carrier diffusion length in galliumnitrideApplied Physics Letters 69, 2531 (1996); https://doi.org/10.1063/1.117729Minority carrier transport in p-ZnO nanowiresJournal of Applied Physics 109, 016107 (2011); https://doi.org/10.1063/1.3530732Electron-beam-induced current and cathodoluminescence studies of thermally activatedincrease for carrier diffusion length and lifetime in -type ZnOApplied Physics Letters 87, 162103 (2005); https://doi.org/10.1063/1.2106001Electron beam induced current profiling of ZnO p-n homojunctionsL. Chernyak,1,a C. Schwarz,1 E. S. Flitsiyan,1 S. Chu,2 J. L. Liu,2 and K. Gartsman31Department of Physics, University of Central Florida, Orlando, Florida 32816-2385, USA2Department of Electrical Engineering, University of California, Riverside, California 92521, USA3Weizmann Institute of Science, Rehovot 76100, IsraelReceived 14 January 2008; accepted 22 February 2008; published online 13 March 2008Variable temperature electron beam induced current technique was employed for the profilingof ZnO p-n homojunctions and the extraction of minority electron diffusion length valuesin the Sb-doped p-type ZnO region. A thermally induced increase for diffusion length of minorityelectrons was determined to have an activation energy of 145 meV. The latter parameterlikely represents carrier delocalization energy and determines the increase of the diffusionlength due to the reduction in recombination efficiency. © 2008 American Institute of Physics.DOI: 10.1063/1.2896613Until recently, p-type doping of ZnO has been a majorchallenge for the implementation of bipolar devices.Light emitting diodes LEDs with ZnO active region havebeen successfully demonstrated, however, utilizingp-AlGaN,1 p-SiC,2 p-diamond,3 as well as p-AlGaN / p-GaNheterostructure4,5 for hole emitters. Due to chemical andcrystallographic differences between ZnO and AlGaN SiCor diamond, the formation of defects at the heterointerfacecan negatively impact optical and electrical properties.Therefore, devices formed from the heterostructures are ex-pected to be inferior to all ZnO-based devices.With p-type doping of ZnO becoming robust, it is verylikely that minority carrier bipolar devices such as LEDs,laser diodes, and transparent p-n junctions can be achievedin the near future.6–8 In such devices, a diffusion length ofminority carriers is one of the critical parameters definingtheir performance as it is closely related to their carrier life-time hence, radiative recombination intensity—LED’s fig-ure of merit and charge collection efficiency crucial for p-njunction photovoltaic detectors.9 Since, due to its wide bandgap 3.3 eV at room temperature, ZnO-based devices showpromise for applications at high temperatures, the impact oflatter parameter on minority carrier transport is of primaryimportance. In this paper, we present results of variable tem-perature electron beam induced current EBIC measure-ments of minority electron diffusion length L in the p-typeregion of ZnO p-n homojunction and propose a possiblemechanism for L increase with temperature.ZnO p-n junctions were grown by molecular-beam epi-taxy, as described in Ref. 10. The homojunction was formedby growing a 425 nm n-type ZnO:Ga layer on highly resis-tive 200  cm p-Si substrate, followed by a 425 nm p-typeZnO:Sb layer. Device structures were processed using stan-dard photolithography techniques,8 and its device schematicis shown in Fig. 1a. Au /NiO and Au /Ti Ohmic contactswere formed on p-ZnO and n-ZnO, respectively. Hall effectmeasurements showed that Ga-doped ZnO layer had an elec-tron concentration, mobility, and resistivity of 2.81019 cm−3, 8.7 cm2 V−1 s−1, 0.02  cm, respectively. TheHall effect measurements were carried out on a n-type layerafter the top p layer see Fig. 1a was etched off. Therefore,surface roughness due to etching and Si-substrate effect mayimpact the obtained values of carrier concentration and car-rier mobility in n-ZnO. In addition, very large density ofionized donors in n-ZnO may cause stronger impurity scat-tering in the n layer and, as a result, lower carrier mobility.Electrical properties of p-ZnO cannot be reliably obtained byHall effect measurements due to the impact from n-ZnOlayer underneath. Nevertheless, the p-type behavior of Sb-doped ZnO was confirmed from diode characteristics. Figure1b shows a typical current-voltage I-V curve of ZnO p-njunction diode in semilogarithmic scale. From Fig. 1b, theturn-on voltage has been determined to be more than 3.4 V.It should be noted that although n+-n− junctions may alsoexhibit rectifying characteristics, their turn-on voltage wouldbe normally less than 1.7 V assuming good Ohmic contacts.Thus, it is confirmed that the diode under test is a p-n ratherthan a n+-n− junction.Minority carrier diffusion length measurements wereconducted on the structures which were cleaved perpendicu-lar to the growth plane cf. Figs. 1 and 2 in situ in PhilipsXL30 scanning electron microscope under a 30 kV electronbeam accelerating voltage using the EBIC method. EBICsignal line scans were recorded using homemade software,Stanford Research System low noise current amplifier and aKeithley 2000 digital multimeter, employed as digitizer. De-tailed description of the EBIC technique can be foundelsewhere11 and references therein. Note that due to an “in-aElectronic mail: chernyak@physics.ucf.edu.FIG. 1. a Architecture of ZnO p-n homojunction. Metal thickness forcontact layers are as follows: Au /NiO, 300 /30 nm; Au /Ti, 150 /30 nm.ZnO p- and n-epitaxial layers are 425 nm thick each. Si substrate shownnot to scale is p type with resistivity of 200  cm. b I-V curve for ZnOp-n junction at room temperature.APPLIED PHYSICS LETTERS 92, 102106 20080003-6951/2008/9210/102106/3/$23.00 © 2008 American Institute of Physics92, 102106-1finite” sample depth in the direction perpendicular to theplain of view cf. Figs. 1 and 2, a relatively large1.5 m electron range penetration depth in ZnO underthe above-referenced accelerating voltage does not affect theaccuracy of the measurements. For temperature-dependentEBIC measurements, the sample temperature was variedin situ from 25 to 125 °C using a specially designed hotstage and an external temperature controller Gatan. At eachtemperature, the EBIC measurements were conducted atseven to ten different locations and the mean and standarddeviation values for minority carrier diffusion length werecalculated from the EBIC line scan see Fig. 2. While theEBIC signal in p-ZnO layer exhibits an exponential decay tothe left from the p-n junction interface, the electron beaminduced current in the n-ZnO layer increases with scanningelectron microscopy electron beam moving to the right of thep-n junction toward Si substrate ideally the EBIC signalshould decay to zero at n-ZnO /Si interface. The latter in-crease is related to the additional nonequilibrium electroncollection from the substrate. Although the Ohmic contactsto p- and n-ZnO layers are used for EBIC measurements, theelectrons from Si substrate, due to their very long diffusionlength 100 m, are swept by the built-in field of thespace charge region at n-ZnO /Si interface this region ispredominantly located in the highly resistive Si substrateand are collected at the contact to n-ZnO layer, thus, contrib-uting to the EBIC signal and making determination of minor-ity carrier diffusion length in n-ZnO impossible under thepresent configuration. On the contrary, a pronounced expo-nential decay of the EBIC signal in p-ZnO layer allows ex-traction of minority carrier diffusion length from the EBICline scan.11Figure 3 shows the dependence of mean value for elec-tron diffusion length in p-ZnO layer on temperature. As itcan be seen, the value of L increases exponentially as thetemperature is raised. This behavior is common to manysemiconductors11,12 and is modeled by the following rela-tionship:L = L0 exp− EA,T2kT  , 1where L0 is a scaling factor, k is Boltzmann constant, andEA,T is the thermal activation energy. The latter parameterlikely represents carrier delocalization energy and determinesthe increase of the diffusion length due to the reduction inrecombination efficiency.13 Earlier cathodoluminescencestudies on ZnO:Sb epitaxial layers had shown that the re-combination route of nonequilibrium carriers involves tran-sitions to a deep, neutral acceptor level e ,A0.13 An increasein the temperature of the sample leads to a higher ionizationfraction of the acceptors, reducing the concentration of A0and thus inhibiting the recombination rate. Lower recombi-nation rate translates directly to greater lifetime of minorityelectrons in the conduction band cf. Ref. 13 and, conse-quently, to greater diffusion length, since L= D1/2, with Dbeing the diffusivity of the carriers. From the Arrhenius plotshown in the inset of Fig. 3, the value of EA,T was deter-mined to be 14510 meV and is in reasonable agreementwith the activation energy of temperature-induced lifetimeincrease obtained by us in Ref. 13 from the temperature-dependent cathodoluminescence measurements. The latterincrease was attributed to a SbZn–2VZn complex, theoreti-cally predicted by Limpijumnong et al.14 to have the activa-tion energy of about 160 meV. It should be also noted thatthe involvement of other Sb-related defects is highly un-likely. The substitutional defect SbO as well as the single-vacancy complex SbZn–VZn are predicted to have ioniza-tion energies about an order of magnitude greater than thoseobtained experimentally, while other defects can also beruled out based on their electrical behavior and/or high for-mation energies.14In summary, temperature-dependent EBIC measure-ments allowed us to estimate the activation energy for thethermally induced increase of the minority carrier diffusionlength in p-type, Sb-doped ZnO epitaxial layer within a ZnOp-n homojunction. Based on the value of the activation en-ergy, the phenomenon is suggested to occur due to thermalionization of Sb-related acceptor level, which is likely aSbZn–2VZn complex.The work at the University of Central Florida was sup-ported in part by the National Science Foundation ECS0422604 and NATO SfP 981939. The work at UC River-side was supported by DMEA through the center for Nano-Science and Innovation for Defense H94003-07-2-0703.1Ya. I. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev,A. K. Omaev, M. V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83,4719 2003.FIG. 2. Secondary electron image of device structure cleaved perpendicularto the growth plane and the superimposed EBIC line scan. The image in thisfigure is rotated 90° counterclockwise relative to the Fig. 1a.FIG. 3. Diffusion length mean values of minority electrons as a functionof temperature open circles and the fit with Eq. 1 solid line. Standarddeviations of mean L values range from 4 to 8 nm for the measurementtemperatures. Inset: Arrhenius plot of the same data yielding activation en-ergy of 14510 meV.102106-2 Chernyak et al. Appl. Phys. Lett. 92, 102106 20082Ya. I. Alivov, U. Ozgur, S. Dogan, D. Johnstone, V. Avrutin, N. Onojima,C. Liu, J. Xie, Q. Fan, and H. Morkoç, Appl. Phys. Lett. 86, 2411082005.3C. X. Wang, G. W. Yang, C. X. Gao, H. W. Liu, Y. H. Han, J. F. Luo, andG. T. Zou, Carbon 42, 317 2004.4A. Osinsky, J. W. Dong, M. Z. Kauser, B. Hertog, A. M. Dabiran, P. P.Chow, S. J. Pearton, O. Lopatiuk, and L. Chernyak, Appl. Phys. Lett. 85,4272 2004.5J. W. Dong, A. Osinsky, B. Hertog, A. M. Dabiran, P. P. Chow, Y. W. Heo,D. P. Norton, and S. J. Pearton, J. Electron. Mater. 34, 416 2005.6D. C. Look, B. Claflin, Ya. I. Alivov, and S. J. Park, Phys. Status Solidi A201, 2203 2004.7Z. P. Wei, Y. M. Lu, D. Z. Shen, Z. Z. Zhang, B. Yao, B. H. Li, J. Y.Zhang, D. X. Zhao, X. W. Fan, and Z. K. Tang, Appl. Phys. Lett. 90,042113 2007.8L. J. Mandalapu, Z. Yang, F. X. Xiu, D. T. Zhao, and J. L. Liu, Appl. Phys.Lett. 88, 092103 2006.9S. M. Sze, Semiconductor Devices Physics and Technology Wiley, NewYork, 1985.10F. X. Xiu, Z. Yang, L. J. Mandalapu, D. T. Zhao, J. L. Liu, and W. P.Beyermann, Appl. Phys. Lett. 87, 152101 2005.11L. Chernyak, A. Osinsky, H. Temkin, J. W. Yang, Q. Chen, and M. A.Khan, Appl. Phys. Lett. 69, 2531 1996.12M. Eckstein and H. U. Habermeier, J. Phys. IV 1, 23 1991.13O. Lopatiuk, L. Chernyak, A. Osinsky, J. Q. Xie, and P. P. Chow, Appl.Phys. Lett. 87, 162103 2005.14S. Limpijumnong, S. B. Zhang, S. H. Wei, and C. H. Park, Phys. Rev. Lett.92, 155504 2004.102106-3 Chernyak et al. Appl. Phys. Lett. 92, 102106 2008

Electron beam induced current profiling of ZnO p-n homojunctions

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Electron beam induced current profiling of ZnO p-n homojunctions

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