Conversion of the Aun-ZnO contact from Ohmic to rectifying with H2 O2 pretreatment was studied systematically using I-V measurements, x-ray photoemission spectroscopy, positron annihilation spectroscopy, and deep level transient spectroscopy. H2 O2 treatment did not affect the carbon surface contamination or the EC -0.31 eV deep level, but it resulted in a significant decrease of the surface OH contamination and the formation of vacancy-type defects (Zn vacancy or vacancy cluster) close to the surface. The formation of a rectifying contact can be attributed to the reduced conductivity of the surface region due to the removal of OH and the formation of vacancy-type defects. © 2007 American Institute of Physics.published_or_final_versio

Beling, CD

Brauer, G

Chen, XD

Cheng, CK

Djurišić, AB

Fung, S

Gu, QL

Ling, CC

Lu, LW

Ng, AMC

Ong, HC

Applied Physics Letters

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Title Hydrogen peroxide treatment induced rectifying behavior ofAun-ZnO contactAuthor(s) Gu, QL; Ling, CC; Chen, XD; Cheng, CK; Ng, AMC; Beling, CD;Fung, S; Djuriši, AB; Lu, LW; Brauer, G; Ong, HCCitation Applied Physics Letters, 2007, v. 90 n. 12, article no. 122101Issued Date 2007URL http://hdl.handle.net/10722/80600Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Hydrogen peroxide treatment induced rectifying behavior of Au/n-ZnOcontactQ. L. Gu, C. C. Ling,a X. D. Chen, C. K. Cheng, A. M. C. Ng, C. D. Beling, S. Fung,A. B. Djurišić, and L. W. LuDepartment of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic ofChinaG. BrauerInstitut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf, Postfach510119, D-01314 Dresden, GermanyH. C. OngDepartment of Physics, Chinese University of Hong Kong, Shatin, Hong Kong, People’s Republic of ChinaReceived 26 January 2007; accepted 13 February 2007; published online 19 March 2007Conversion of the Au/n-ZnO contact from Ohmic to rectifying with H2O2 pretreatment was studiedsystematically using I-V measurements, x-ray photoemission spectroscopy, positron annihilationspectroscopy, and deep level transient spectroscopy. H2O2 treatment did not affect the carbonsurface contamination or the EC–0.31 eV deep level, but it resulted in a significant decrease of thesurface OH contamination and the formation of vacancy-type defects Zn vacancy or vacancycluster close to the surface. The formation of a rectifying contact can be attributed to the reducedconductivity of the surface region due to the removal of OH and the formation of vacancy-typedefects. © 2007 American Institute of Physics. DOI: 10.1063/1.2715025Zinc oxide is a wide band gap semiconductor which hasrecently attracted extensive attention because of its potentialfor a wide variety of applications, such as optoelectronic,high frequency, gas sensing, piezoelectric and transparent de-vices, etc.1 Fabricating high quality metal contact to ZnO isan important step for successful device fabrication because ametal-semiconductor structure is one of the essential struc-tures for many devices. However, reports of successful rec-tifying metal/n-ZnO contact have been scarce. The metalsused include Au,2–9 Ag,6,10 Pt,11–13 and Pd.2,3,14,15 It has alsobeen shown that surface treatments such as H2O2,12 sulfide,13ozone,11and plasma4,9 can improve the rectifying contactquality increase the barrier height, decrease the reverse biasleakage current, and achieve an ideality factor approachingunity. While several explanations, such as good surfacemorphology, removal of C and OH contamination, and re-duction in surface conductivity, have been proposed, themechanisms behind contact quality improvements are stillcontroversial and not well understood.We have systematically studied the influence of theH2O2 pretreatment duration and temperature on the proper-ties of Au/n-ZnO contacts. As the observed phenomenonmay involve more than one physical process, x-ray photo-emission spectroscopy XPS, scanning electron microscopySEM, positron annihilation spectroscopy PAS, and deeplevel transient spectroscopy DLTS were used to investigatethe influence of contamination, morphology, and defects onthe electrical properties of the fabricated Au/ZnO contacts.The ZnO sample used was melt-grown undoped n-type ZnOobtained from the Cermet Inc., U.S. The carrier concentra-tion of the as-received material was found to be 51016 cm−3. Au contacts having a diameter of 0.5 mm and athickness of 50 nm were fabricated by thermal evaporationunder a vacuum of 10−6 Torr. The time and temperature ofthe H2O2 pretreatment were systematically varied in theranges of 1–30 min and from room temperature to the boil-ing temperature of H2O2, respectively.The I-V data of the Au/n-ZnO contacts without H2O2pretreatment shown in Fig. 1 clearly showed the nonrectify-ing property of the contact. On the other hand, all thesamples with H2O2 pretreatment became rectifying, but theideality factor n, the Schottky barrier height S, and the leak-age current Ileak were dependent on the pretreatment tem-perature and time, having values in the ranges of n=1.15–2.89, S=0.35–0.65 eV, and Ileak=10−9–10−4 A asthe reverse bias VR=−1 V. The best performing diode I-Vcurve shown in Fig. 1 was obtained by preetching thesample in H2O2 at 100 °C for 3 min. Its ideality factor,Schottky barrier height, and leakage current at −1 V wereaAuthor to whom correspondence should be addressed; electronic mail:ccling@hku.hkFIG. 1. Color online I-V data of the Au/n-ZnO samples with differentH2O2 pretreatments.APPLIED PHYSICS LETTERS 90, 122101 20070003-6951/2007/9012/122101/3/$23.00 © 2007 American Institute of Physics90, 122101-1Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsfound to be 1.15, 0.63 eV and 4.210−9 A, respectively.The SEM images not included showed that the as-grownZnO sample had a smooth surface, and the 3 min 100 °CH2O2 treatment increased the surface roughness and inducedappearance of submicron grains on the surface. The worstperforming diode was the one pretreated with the longesttime period i.e., 30 min and the highest temperature i.e.,boiling H2O2, which had n=2.89, S=0.53 eV, and leakagecurrent equal to 7.810−8 A at −1 V. The SEM imageshowed that the surface was damaged after the 30 min boil-ing H2O2 treatment.XPS data revealed that all the samples contained carbonimpurities. The C 1s 285.36 keV signal, which disappearedafter Ar sputtering, corresponded to a thickness of about50 Å implying that the detected C impurity resided at thesample surface. As shown in Table I, the carbon concentra-tion could not be removed with the H2O2 treatments. Thesurface carbon contamination therefore does not seem tohave any significant adverse influence on the formation ofAu Schottky contact. The O 1s signal was well fitted by atwo-peak fit having binding energies of 531.25 and532.76 eV, which corresponded to oxygen bound to the lat-tice and oxygen in the OH impurity.4 3 min H2O2 treatmentresults in a significant reduction of the OH intensity from26.1% to 1.3%, as well as the loss of Zn and the reduction ofthe Zn:O ratio from 1.85 to 0.40. OH leads to the formationof a high conductivity accumulation layer on both the polarfaces of ZnO.4 The correlation between the OH removal andthe improvement of the metal-ZnO Schottky contacts hasalso been observed by Coppa et al.4 and Mosbackeret al.9PAS was employed to study the vacancy-type defectsclose to the surface.16,17 Monoenergetic positrons were im-planted into the sample with variable positron energy up to30 keV, hence varying the positron implantation depth.The implanted positrons will then be thermalized, undergodiffusion, possibly be trapped by vacancy-type defects, andfinally annihilate with an electron emitting two gamma pho-tons with energies of 511 keV, providing informationabout the electronic environment at which the positron anni-hilates. The Doppler broadening of the annihilation radiationwas parametrized by the S parameter, defined as the ratio ofthe central region count i.e., AC in Fig. 2 to the total countand is thus associated with the fraction of positrons annihi-lating with the valance electrons. The W parameter wingparameter is defined as the ratio of a fixed pair of highenergy windows i.e., AW1 and AW2 in Fig. 2 to the totalcounts, and is thus related to fraction annihilating core elec-trons. As a positron trapped in the vacancy state has lessprobability to annihilate with the core electrons havinghigher momentum relative to the valance electrons, annihila-tion events originated from the vacancy state would be lessDoppler broadened and would thus contribute a larger S pa-rameter smaller W parameter as compared to the bulk state.The S parameter as a function of the positron implantingenergy of three ZnO is shown in Fig. 2. The SE data werefitted by the source code VEPFIT,18 where the dynamics of theimplanted positrons were described by the diffusion-trapping-annihilation equation considering positron implan-tation, positron diffusion, positron trapping into vacancy, andsubsequent annihilation at different states. The resultant Sparameter measured at different implanting energy is givenby SE=f iSi, where f i and Si are the fraction and the char-acteristic S parameter of positron annihilating at site i. Forthe untreated sample, a single layer model was found to givea good fitting to the data. The positron diffusion length Leffwas found to be 72±2 nm. The SE data of the 3 min100 °C and the 30 min boiling treated samples in Fig. 2clearly show shoulders implying the existence of at least onedefective layer corresponding to the high value of the S pa-rameter. Finally, the SE curves of these two samples werewell fitted by the three layer model with the results shown inTable II. The characteristic bulk S parameters and the effec-tive positron diffusion length of the three samples werefound to be Sb=0.5096 nm and LB=72 nm. The first and thesecond layers of the two H2O2 treated samples had values ofS parameters higher than the bulk and L+ smaller than thebulk indicative of the creation of vacancy-type defects inthese two layers by the H2O2 treatment. Moreover, the factthat S1S2 and L1L2 implied that the first layer was moredefective than the second layer. The defective region ex-tended to widths of 393 and 422 nm for the 3 min 100 °Ctreated and the 30 min boiling treated samples, respectively.The SW plot of the data accumulated from the 3 min 100 °C,30 min 100 °C, and 30 min boiling H2O2 treated samples isshown in Fig. 3, which clearly shows a straight line. ThisTABLE I. Intensities of XPS signals in atomic ratio found in the nontreated and the 3 min 100 °C H2O2treated n-type ZnO samples.C%O%OH%Zn%Zn:O ratioon surfaceNontreated 14.2 20.9 26.1 38.8 1.86100 °C, 3 min 14.2 55.2 1.3 22 0.40FIG. 2. Color online SE data of the untreated circle, the 3 min 100 °CH2O2 treated triangle, and the 30 min boiling H2O2 treated n-ZnOsamples. The inset illustrates the definition of the S parameter and the Wparameter. The solid lines are the fitted curves.122101-2 Gu et al. Appl. Phys. Lett. 90, 122101 2007Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsimplies that the vacancies created in the H2O2 treatment ir-respective of time and temperature were of the identicaltype.16 Since O vacancy has low binding energy and thusdoes not trap positrons at room temperature,19–23 the vacancydefects created by H2O2 treatment are Zn-vacancy-relateddefects and vacancy clusters, although the Doppler techniqueused in the present study cannot offer an unambiguous dis-tinction between the two possibilities. XPS data in Table Ishowed that after the H2O2 treatment, the Zn:O ratio signifi-cantly reduced to a Zn insufficient value of 0.40. It is thusspeculated that the vacancy-type defects formed are Zn va-cancy or Zn-vacancy cluster. As Zn vacancy is known to bean acceptor, the H2O2 etching would thus possibly have theeffect of forming a defective layer having a relatively lowconductivity.In the DLTS measurements, a deep level having the ac-tivation energy of Ea=0.31 eV was identified in all of thediodes tested. This defect was also found in some othern-type ZnO structure not pretreated by H2O2.24,25 The deeplevel concentrations found in the 3 min 100 °C, the 30 min100 °C, the 3 min 185 °C, and the 30 min 185 °C H2O2pretreated samples were observed to be independent of thereverse bias applied during the DLTS measurement, the treat-ment temperature, or the treatment duration with values of8.81014, 1.01015, 1.21015, and 8.61014 cm−3, re-spectively. This implies that the deep level is 1 a uni-formly distributed bulk defect, 2 not associated with thevacancy-type defect as detected by PAS, and 3 thus notrelated to the H2O2 treatment.In conclusion, Ohmic to rectifying conversion was ob-served on the Au/n-ZnO contact with the pretreatment ofH2O2. A good quality Schottky contact was obtained withn=1.15, S=0.63 eV, and IleakVR=−1 V=4.210−9 A.The good performance of a Au Schottky contact was associ-ated with the removal of OH surface contamination and cre-ation of a defective surface region, containing zinc vacancyor vacancy cluster. Both of these effects result in the overallreduction of the conductivity of the surface layer. A deeplevel with EA=0.31 eV was observed in all the samples butwas suggested to be unrelated to the H2O2 treatment.This project was supported by the CERG, RGC, HKSARNos 7032/04 and 7036/07, and the University DevelopmentFund Grant, The University of Hong Kong.1Ü. Özgür, Yu. I. Alivov, C. Liu, A. Teke, M. A. 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The indices S, 1,2, and B in the S parameter and the diffusion length L denote the surface, thefirst layer, the second layer and the bulk, respectively, while X1 and X2 arethe boundary positions of the first and the second layers, respectively.Surface First layer Second layer BulkUntreated SS=0.5373 NA NA SB=0.5109LB=72 nm3 min SS=0.5453 S1=0.5224 S2=0.5212 SB=0.5092100 °C L1=35 nm L2=48 nm LB=70 nmX1=56 nm X2=393 nm30 min SS=0.5465 S1=0.5401 S2=0.5202 SB=0.5086185 °C L1=29 nm L2=37 nm LB=72 nmX1=223 nm X2=422 nmFIG. 3. Color online SE plot of the data obtained from the 3 min100 °C, the 30 min 100 °C, and the 30 min boiling H2O2 treated n-ZnOsamples.122101-3 Gu et al. Appl. Phys. Lett. 90, 122101 2007Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Hydrogen peroxide treatment induced rectifying behavior of Aun-ZnO contact

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Hydrogen peroxide treatment induced rectifying behavior of Aun-ZnO contact

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