Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emitting p-n junction was subsequently formed by postimplantation annealing in air. Deep level transient spectroscopy was used to investigate deep level defects induced by N+ implantation and the effect of air annealing. The N+ implantation enhanced the electron trap at E C -(0.31±0.01) eV (E3) and introduced another one at E C -(0.95±0.02) eV (D1), which were removed after annealing at 900 and 750 °C, respectively. Another trap D2 (Ea =0.17±0.01 eV) was formed after the 750 °C annealing and persisted at 1200 °C. © 2008 American Institute of Physics.published_or_final_versio

Anwand, W

Brauer, G

Djurišić, AB

Fung, S

Gu, QL

Hsu, YF

Ling, CC

Lu, LW

Skorupa, W

Zhu, CY

Applied Physics Letters

HKU Scholars Hub

Title Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junctionAuthor(s) Gu, QL; Ling, CC; Brauer, G; Anwand, W; Skorupa, W; Hsu, YF;Djuriši, AB; Zhu, CY; Fung, S; Lu, LWCitation Applied Physics Letters, 2008, v. 92 n. 22Issued Date 2008URL http://hdl.handle.net/10722/80769Rights Applied Physics Letters. Copyright © American Institute ofPhysics.Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junctionQ. L. Gu,1 C. C. Ling,1,a G. Brauer,2 W. Anwand,2 W. Skorupa,2 Y. F. Hsu,1 A. B. Djurišić,1C. Y. Zhu,1 S. Fung,1 and L. W. Lu11Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong,People’s Republic of China2Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Dresden-Rossendorf,Postfach 510119, D-01314 Dresden, GermanyReceived 15 January 2008; accepted 13 May 2008; published online 5 June 2008Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emittingp-n junction was subsequently formed by postimplantation annealing in air. Deep level transientspectroscopy was used to investigate deep level defects induced by N+ implantation andthe effect of air annealing. The N+ implantation enhanced the electron trap at EC− 0.310.01 eV E3 and introduced another one at EC− 0.950.02 eV D1, which wereremoved after annealing at 900 and 750 °C, respectively. Another trap D2 Ea=0.170.01 eV wasformed after the 750 °C annealing and persisted at 1200 °C. © 2008 American Institute of Physics.DOI: 10.1063/1.2940204ZnO has recently received attention because of its appli-cations in optoelectronics and spintronics.1,2 Undoped ZnO isof n type. Asymmetry difficulty in p-type doping of ZnO is amajor obstacle for device applications, although progress hasrecently been made with N, As, P doping, and codoping withgroup III and V elements.2 Ion implantation is useful for aselective region doping, but it inevitably introduces undesir-able defect. This is particularly important for ZnO as thecompensating intrinsic donors have low formation energyand are energetically stable.3,4 There have been only a fewreports of p-ZnO induced by ion implantation N+ Refs.5–8 and As+ implantations Ref. 9 and the ion-implantation induced defects were poorly understood. Be-cause of difficulties in making a junction, deep level tran-sient spectroscopy DLTS studies have also been veryfew.8,10–16In this study, N+ ions were implanted into the n-typemelt grown MG ZnO single crystal and no rectifying prop-erty was observed in the as-implanted sample. A homoge-neous p-n junction which emitted light at room temperatureRT was formed after the postimplantation annealing. Deeplevel defects in the structure and also their thermal evolutionwere studied by DLTS.The starting material was one-side polished undopedn-type MG ZnO 0001 substrate n=51016 cm−3 and=203 cm2 V−1 s−1 from Cermet Inc. N+ ions were im-planted into the polished front side F-side at 300 °C withenergy of 150 keV fluence of 1014 cm−2. This correspondsto a N-implantation profile peaking at 270 nm calculatedby the program TRIM Ref. 17 and measured by secondary-ion-mass spectroscopy. Ohmic contacts were fabricated byevaporating 50 nm Al film onto the samples. The postim-plantation annealing was performed in air for 30 min. C-V,I-V, and DLTS measurements were made using a HP4155Asemiconductor parameter analyzer and a Sula TechnologiesDLTS system, respectively. The DLTS measurements em-ployed a reverse bias and a filling pulse voltage of VR=−2 V and VP=0 V. The activation energy Ea, capture crosssection , and concentration NT of the traps were calculatedby the Arrhenius plot.18 To fabricate a light-emitting diodeLED for electroluminescence EL measurement, four-folded N+ implantations with energies of 80, 180, 310, and500 keV with fluences of 1014 cm−2 were carried out onthe same raw ZnO sample. This resulted in a 1 m deepbox-shaped region with N concentration 61018 cm−3.Then a 750 °C air annealing was performed. A 50 nmthick ITO film was evaporated on the implanted side usingan electron beam evaporator while keeping the sample at250 °C.19 The EL spectra were collected using a monochro-mator Acton SpectraPro 500i and a Peltier-cooled photo-multiplier detector Hamamatsu R636-10.I-V data in Fig. 1 showed that the as-implanted samplehas no rectifying property. However, rectifying behaviorwas observed for samples annealed at 650, 750, 900, and1200 °C in air, which indicated the formation of p layer inthe N+-implanting region. Their leakage currents at −1.5 Vwere 1270, 43, 12, and 140 nA, respectively. Referring to theC-V measurement scheme in Ref. 20 the hole concentrationsof the p-type layers were estimated to be 1017 cm−3. Thethermoelectric probe measurements performed on the an-nealed N+-implanted samples also confirmed the p-layerformation.aAuthor to whom correspondence should be addressed. Electronic mail:ccling@hku.hk.FIG. 1. Color online I-V data of the N+-implanted undoped MG ZnOsamples with different postimplantation annealing temperatures in air. TheI-V measurements were performed across the AlF-AlB connection.APPLIED PHYSICS LETTERS 92, 222109 20080003-6951/2008/9222/222109/3/$23.00 © 2008 American Institute of Physics92, 222109-1Downloaded 08 Sep 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsThe RT EL spectra of the forward biased LED Fig. 2shows two broad peaks at 530 nm 2.34 eV and740 nm 1.68 eV. Their peak intensities increase withforward bias voltage. The observed light emission clearlydemonstrated the formation of a p-n junction in the annealedN-implanted ZnO samples.To carry out DLTS study on the nonrectifying as-implanted and nonimplanted samples, Au rectifying contactswere fabricated on the F-side of the samples pretreated withH2O2.21 For the as-grown sample DLTS spectrum, a majoritypeak i.e., e− trap with Ea=0.31 eV, n10−16 cm−2, andNT1015 cm−3 usually referred to E3 Refs. 8 and 10–16and another one Ea=0.10 eV, n10−17 cm−2, and NT1013 cm−3 with a much weaker intensity less than 20times of E3 were identified. The E3 intensity dropped withannealing temperature, but persisted at 1200 °C annealing.The Ea=0.10 eV level anneals at 900 °C. The DLTS spec-trum of the as-N+-implanted sample is shown in Fig. 3 theas-grown sample spectrum also included for reference,which shows that N implantation enhances the E3 intensityand introduces another e− trap at 360 K D1 having Ea=0.94 eV and n10−14 cm−2.Figure 3 shows the DLTS spectra of the annealedN+-implanted p-n junctions. Biasing at VR=−2 V during theDLTS measurements corresponded to a depletion of 30 and100 nm extending to the p and n sides, respectively. It is thusdifficult to determine the nature i.e., e− trap or h+ trap of thesignals identified in these spectra. However, the two majoritypeaks at 160 and 360 K in these annealed sample spectrahave Ea and n as shown in Table I agreeing well to the E3and D1 already identified in the as-grown and the as-implanted samples, and are thus attributed to E3 and D1. TheE3 concentration decreases with annealing temperature andreduces by a factor of 100 after the 900 °C annealing.D1 annealed out at 750 °C. Referring to Fig. 3, another ma-jority peak D2 at 120 K with Ea=0.17 eV and n10−16 cm−2 was introduced after 750 °C annealing. How-ever, it is not certain as to whether this is an e− or a h+ trap.Its concentration drops with annealing temperature, but per-sists at 1200 °C.Although DLTS itself cannot offer definitive informationabout the defect microstructure, it would still be useful toexplore the possible origins of the identified defects E3, D1,and D2. E3 is commonly observed in as-grown ZnO singlecrystals.10 However, its exact microstructure is still contro-versial and has been attributed to VO and VZn-relateddefects.10–12,16 In the N+-implanted sample Fig. 3, E3 be-came undetectable upon 1200 °C annealing. However, inthe nonimplanted sample, E3 persisted concentration1015 cm−3 upon 1200 °C annealing. Thus, the mechanismof the E3 annealing process in the N+-implanted sampleprobably involves another defect created by the N+ implan-tation.DLTS measurements were performed also at the AuSchottky contacts fabricated on the as-C+-implanted and theas-As+-implanted samples fluence=1014 cm−2 each usingFIG. 3. Color online DLTS spectra of the N+-implanted undoped MG ZnOsamples annealed at different temperatures. The DLTS spectra of the as-grown ZnO and the as-C+-implanted ZnO samples are included for refer-ence. The measurements were taken with the rate window, the reverse biasvoltage, the filling pulse width, and the filling pulse voltage equal to t=21.5 ms, VR=−2 V, tP=1 ms, VP=0 V.FIG. 2. Color online EL spectra of the N-implanted ZnO homogeneousp-n junction taken at RT. The IV character of the p-n junction is shown inthe inset at the top right corner. The device schematic diagram and the LEDphoto are shown at the top left corner.TABLE I. Activation energies Ea, trap concentrations NT, and capture cross sections T of the deep level defectsintroduced by the N implantation.Unimplanted As implanted 650 °C 750 °C 900 °C 1200 °CE3 Ea=0.31 eV,NT1015 cm−3,10−16 cm2Ea=0.31 eV,NT1016 cm−3,10−15 cm2Ea=0.32 eV,NT1016 cm−3,10−15 cm2Ea=0.31 eV,NT1015 cm−3,1015 cm2Ea=0.31 eV,NT1014 cm−3,1015 cm2Annealed outD1 ¯ Ea=0.94 eV,NT1016 cm−3,10−14 cm2Ea=0.98 eV,NT1016 cm−3,10−14 cm2Annealed out ¯ ¯D2 ¯ ¯ Ea=0.18 eV,NT1015 cm−3,10−16 cm2Ea=0.17 eV,NT1016 cm−3,10−16 cm2Ea=0.17 eV,NT1016 cm−3,10−16 cm2Ea=0.17 eV,NT1015 cm−3,1016 cm2222109-2 Gu et al. Appl. Phys. Lett. 92, 222109 2008Downloaded 08 Sep 2011 to 147.8.21.150. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsthe same raw materials. D1 is also observed in theas-C+-implanted and the as-As+-implanted samples theformer shown in Fig. 3, hence indicative of being an intrin-sic defect. D1’s intensity and peak position dependences ontp 10−6–0.1 s were also studied while fixing t=43 ms.The D1 intensity increases with tp and saturates at tp=0.01 s. Its peak position is independent of tp, which impliesD1 is a point defect. D1 is close to the calculated energystate 2+ /0 of VO at EC–1.0 eV, which possesses negativeU behavior.22 This theoretical result also allowed a detailedand consistent interpretation of the optically detected elec-tron paramagnetic resonance data.23 We thus tentatively at-tribute D1 to the 2+ /0 state of VO.D2 was only formed in the N+-implanted samples after750 °C annealing but not in the nonimplanted sample. 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Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction

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Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction

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