APPLIED PHYSICS REVIEWS—FOCUSED REVIEW

Defects in ZnOM. D. McCluskey1,2,a� and S. J. Jokela1

1Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164-2814,USA2Institute for Shock Physics, Washington State University, Pullman, Washington 99164-2816, USA

�Received 8 May 2009; accepted 8 June 2009; published online 5 October 2009�

Zinc oxide �ZnO� is a wide band gap semiconductor with potential applications in optoelectronics,transparent electronics, and spintronics. The high efficiency of UV emission in this material couldbe harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptordopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis onthe physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a propertythat was historically ascribed to native defects. However, experiments and theory have shown thatO vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature.Group-III �B, Al, Ga, and In� and H impurities account for most of the n-type conductivity in ZnOsamples. Interstitial H donors have been observed with IR spectroscopy, while substitutional Hdonors have been predicted from first-principles calculations but not observed directly. Despitenumerous reports, reliable p-type conductivity has not been achieved. Ferromagnetism iscomplicated by the presence of secondary phases, grain boundaries, and native defects. The famousgreen luminescence has several possible origins, including Cu impurities and Zn vacancies. Theproperties of group-I �Cu, Li, and Na� and group-V �N, P, As, and Sb� acceptors, and theircomplexes with H, are discussed. In the future, doping of ZnO nanocrystals will rely on anunderstanding of these fundamental properties. © 2009 American Institute of Physics.�doi:10.1063/1.3216464�

TABLE OF CONTENTS

I. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1II. INTRINSIC PROPERTIES. . . . . . . . . . . . . . . . . . . . 2III. NATIVE DEFECTS. . . . . . . . . . . . . . . . . . . . . . . . . 2

A. Zinc interstitials. . . . . . . . . . . . . . . . . . . . . . . . 2B. Oxygen vacancies. . . . . . . . . . . . . . . . . . . . . . . 2C. Zinc vacancies. . . . . . . . . . . . . . . . . . . . . . . . . 3D. Damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

IV. GROUP-III DONORS. . . . . . . . . . . . . . . . . . . . . . . 4V. GROUP-IV IMPURITIES. . . . . . . . . . . . . . . . . . . . . 5VI. GROUP-V ACCEPTORS. . . . . . . . . . . . . . . . . . . . . 5

A. Substitutional nitrogen. . . . . . . . . . . . . . . . . . . 5B. Nitrogen molecules. . . . . . . . . . . . . . . . . . . . . . 5C. Phosphorus, arsenic, and antimony. . . . . . . . . 5D. The p-type controversy. . . . . . . . . . . . . . . . . . 5

VII. GROUP-I ACCEPTORS. . . . . . . . . . . . . . . . . . . . . 6A. Lithium and sodium. . . . . . . . . . . . . . . . . . . . . 6B. Copper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

VIII. HYDROGEN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7A. Isolated hydrogen. . . . . . . . . . . . . . . . . . . . . . . 7

1. Diffusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 72. Hydrogen donors. . . . . . . . . . . . . . . . . . . . . 73. Interstitial hydrogen. . . . . . . . . . . . . . . . . . . 7

4. Hidden hydrogen. . . . . . . . . . . . . . . . . . . . . 85. Substitutional hydrogen. . . . . . . . . . . . . . . . 8

B. Hydrogen complexes. . . . . . . . . . . . . . . . . . . . 81. Passivation of defects. . . . . . . . . . . . . . . . . 82. Zinc vacancy–hydrogen. . . . . . . . . . . . . . . . 83. Nitrogen–hydrogen. . . . . . . . . . . . . . . . . . . 94. Copper–hydrogen. . . . . . . . . . . . . . . . . . . . . 95. Lithium–hydrogen. . . . . . . . . . . . . . . . . . . . 9

IX. TRANSITION METALS. . . . . . . . . . . . . . . . . . . . . 9A. Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . 9B. Ferromagnetism. . . . . . . . . . . . . . . . . . . . . . . . 9

X. EXTENDED DEFECTS. . . . . . . . . . . . . . . . . . . . . . 10A. Surface conduction. . . . . . . . . . . . . . . . . . . . . . 10B. Dislocations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10C. Stacking faults. . . . . . . . . . . . . . . . . . . . . . . . . 10D. Grain boundaries. . . . . . . . . . . . . . . . . . . . . . . . 10

XI. CONCLUSIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

I. INTRODUCTION

The past decade has seen dramatic advances in the de-velopment of wide band gap semiconductors in devices rang-ing from blue lasers to solar cells.1,2 Zinc oxide �ZnO� is an“old” semiconductor that has attracted resurgent interest asan electronic material for numerous applications.3 With a di-rect gap of 3.4 eV at room temperature,4 ZnO is a wide bandgap semiconductor that emits light in the near-UV region ofa�Electronic mail: mattmcc@wsu.edu.

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the spectrum. The high efficiency of the emission5 makesZnO a strong candidate for solid-state white lighting. ZnO iscurrently used as a transparent conductor6 in solar cells.7 It isalso an UV-absorbing material in sunscreens8 and the activematerial in varistors.9 ZnO and related alloys are used fortransparent transistors, devices that could be incorporated inproducts such as liquid-crystal displays.10

In addition to optoelectronic and electronic devices, ZnOmay become a key material for spintronic applications.11–13

In nonvolatile memory systems, ferromagnetism is used tostore data for extended periods of time.14 Semiconductorssuch as ZnO offer the possibility of devices that combineoptical and magnetic effects, such as spin light-emitting di-odes �LEDs�,15 spin-polarized solar cells,16 and magneto-optical switches.17 The development of room-temperatureferromagnetic materials will be essential for the widespreadrealization of these technologies. Theoretical work predictedferromagnetism above room temperature for Mn-doped ZnO,given a sufficiently high hole concentration.18 Spurred bythat prediction, research into ZnO crystals for spintronic ap-plications is an active area of experimental research.19

ZnO has practical benefits that make it an attractive ma-terial from an industrial point of view. In contrast to GaN,large single crystals can be grown relatively easily.20 Theenvironmental impact and toxicity are lower than most othersemiconductors—ZnO is actually used as a dietary supple-ment in animal feed.21 The low cost of Zn as compared to Inmakes ZnO economically competitive for transparent con-ductor applications.

While ZnO has many inherent advantages, the lack ofcontrol over dopants and defects presents an obstacle to therealization of practical devices. This focused review presentsa summary of defects in ZnO, with an emphasis on thephysical properties of point defects in bulk crystals. The lit-erature in this area is vast, and only a representative sampleis presented here. Interested readers are directed to compre-hensive review articles on ZnO materials and devices,22

processing,23 and ceramics.24

II. INTRINSIC PROPERTIES

ZnO has the wurtzite crystal structure, with lattice pa-rameters a=3.24 Å and c=5.21 Å.25 Its bulk modulus is142 GPa.26 Bulk ZnO undergoes a transition to the rocksaltphase at a pressure of �9 GPa.27 Room-temperature Ramanscattering spectra showed transverse optical phonons at378 cm−1 �A1� and 410 cm−1 �E1�, longitudinal optical �LO�phonons at 576 cm−1 �A1� and 588 cm−1 �E1�, and E2

phonons at 98 cm−1 �“low”� and 438 cm−1 �“high”�.28 Dueto spin-orbit and crystal-field splitting, the valence bandsplits into three levels at the center of the Brillouin zone,resulting in A, B, and C excitons. The A free exciton is theone with the lowest energy. Photoluminescence �PL� mea-surements showed that the A free exciton has a binding en-ergy of 60 meV and that the band gap at 10 K is 3.437 eV.29

III. NATIVE DEFECTS

Defects such as O vacancies have low formation enthal-pies in p-type ZnO and are therefore predicted to form

readily.30 While they may play a significant role as compen-sating centers, O vacancies and other native point defects areunlikely to be the predominant cause of unintentional n-typeconductivity in ZnO.31 As discussed in Secs. III A and III B,the O vacancy is a deep donor, while Zn interstitials areunstable at room temperature. Zn vacancies are present ascompensating acceptors in n-type ZnO. Other native defects,such as Zn antisites, O antisites, and O interstitials, have highformation energies. Under normal conditions, they are prob-ably not present in large concentrations, although O intersti-tials may be important in O self-diffusion.32

Native defects affect the performance of Ohmic andSchottky contacts. Depth-resolved cathodoluminescence�CL� revealed the formation of point defects at metal-ZnOinterfaces, leading to changes in the properties of theSchottky barriers.33 The deposition of metals caused the for-mation of point defects with CL energies near 2.1, 2.5, and 3eV. Processing conditions are important; for example, theformation of a AuZn eutectic at 625 °C may cause Zn defi-ciency in the adjacent ZnO material.34

A. Zinc interstitials

Vlasenko and Watkins35 produced defects in ZnO byelectron irradiation at liquid-helium temperatures. Opticallydetected magnetic resonance �ODMR� was used to identifyO vacancies, Zn interstitials, and Zn vacancy–Zn interstitialpairs. The Zn interstitials are shallow donors, with a charac-teristic “effective mass” �EM� g-factor of g�1.96.36 Theyanneal out at a temperature of 170 K. First-principles calcu-lations also show that Zn interstitials are shallow donors.They have high formation energies in n-type ZnO and arefast diffusers with migration barriers as low as 0.57 eV.32

Therefore, Zn interstitials are not stable at roomtemperature,37 in agreement with the experimental work ofVlasenko and Watkins.35

The possibility that Zn interstitials cause n-type conduc-tivity was further challenged by positron annihilation mea-surements, which showed that annealing in Zn vapor did notresult in a decrease in Zn vacancies.38 If Zn interstitials haddiffused into the crystal, then one would have expected theopen-volume defects to have been filled. Instead, the authorsposited that the Zn vapor caused an increase in O vacanciesin the bulk by growing new ZnO on the surface. While thereis consensus that Zn interstitials are not stable, it has beenproposed that complexes involving N impurities and Zn in-terstitials could be stable, shallow donors.39

B. Oxygen vacancies

First-principles calculations consistently show that the Ovacancy is a deep, negative-U donor, where the 1+ chargestate is thermodynamically unstable. When the Fermi energyis above the “0 /2+ level,” the defect is in the neutral chargestate. When the Fermi energy is below the 0 /2+ level, thedefect has a charge of +2e. The 0 /2+ level of the O vacancywas calculated to be 0.5–0.8 eV above the valence-bandmaximum.40–42 Calculations that attempt to correct for theband gap error of the local density approximation �LDA�estimated the 0 /2+ level to be 1–2 eV below the conduction-

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band minimum.43–46 The calculated migration barrier for theneutral O vacancy is �2 eV.47,32 Theorists disagree onwhether O vacancies should be present in n-type ZnO. Forexample, Lany and Zunger44 predicted concentrations of�1017 cm−3 or higher, whereas Janotti and Van de Walle32

claimed that the formation energy is much too high for sig-nificant concentrations to exist at equilibrium.

Electron paramagnetic resonance �EPR� measurementson electron-irradiated ZnO show that neutral O vacancies areconverted to the 1+ charge state by illumination at low tem-perature �Fig. 1�. This results in a g-factor of g�1.995 andspin S= 1

2 �Table I�.35 The O vacancies are stable up to400 °C.48 From the optical threshold for producing the 1+charge state, Evans et al.49 estimated that the neutral O va-cancy ground state lies �2.1 eV below the conduction-bandminimum. An electronic transition results in a broad absorp-tion band near 3.0 eV �409 nm�, giving crystals a yellowishor reddish color.50,51

The famous green luminescence band centered around2.4 eV �510 nm� has been attributed to O vacancies,52 due to

an excited-to-ground state transition.53 Annealing in O2 orZn vapor produces PL lines at 2.35 and 2.53 eV, respectively,and annealing in ZnO powder removes both lines. Theseobservations suggested that the 2.35 eV emission is due toZn vacancies and the 2.53 eV emission is due to Ovacancies.54 It should be noted, however, that numerous de-fects have been blamed for green luminescence.55 Electronirradiation, which produces O vacancies and other defects,leads to a reduction in green luminescence and an increase inPL bands near 600 and 700 nm.48,51 Also, the configuration-coordinate diagram calculated for the O vacancy does notshow any transitions consistent with green emission.56 Forthese reasons, one should never assume that green PL is dueto O vacancies.

C. Zinc vacancies

Zn vacancies are double acceptors. First-principles cal-culations find the 0/1– and 1–/2– acceptor levels to be 0.1–0.2 and 0.9–1.2 eV above the valence-band maximum,respectively.31,32,57 Under illumination at low temperatures,Zn vacancies can be transformed to the 1– or neutral chargestate, both of which are EPR active �Table II�. In the 1–charge state, the defect has a spin S= 1

2 and an anisotropicg-tensor.58,35 A trigonal Jahn–Teller distortion causes the holeto be localized on one of the neighboring O sites, with non-axial sites ��110° to the c axis� being energetically preferred�Fig. 1�. In the neutral charge state, two holes are trapped ondifferent nonaxial O sites, resulting in a spin S=1.58,59 Fromthe wavelength of light required to pump Zn vacancies fromthe 1– to the neutral charge state, Evans et al.49 estimatedthat the ground state for the 1– charge state lies 0.9 eV abovethe valence band.

TABLE I. EPR parameters for donors in ZnO, where � and � refer todirections parallel and perpendicular to the c axis, respectively. The electronspins are S= 1

2 .

Donor g� g� Ref.

Shallow �EM� donor 1.9570 1.9551 36Zn interstitial �1+� 1.9605 1.9595 35O vacancy �1+� 1.9946 1.9960 35Pb3+ 2.0133 2.0135 92

FIG. 1. EPR spectra of electron-irradiated ZnO, taken at 30 K and 9.48GHz, with the magnetic field perpendicular to the c axis. �a� In the dark. �b�After illumination with 325 nm light. The Zn vacancies are in the 1– chargestate and the O vacancies are in the 2+ charge state. Reprinted with permis-sion from L. A. Kappers, O. R. Gilliam, S. M. Evans, L. E. Halliburton andN. C. Giles, Nucl. Instrum. Methods Phys. Res. B, 266, 2953 �2008�. Copy-right © 2008, Elsevier.

TABLE II. EPR parameters for acceptors in ZnO. For centers with axial symmetry, � and � refer to directionsparallel and perpendicular to the c axis, respectively. Deep acceptors can have their holes localized on axial ornonaxial O atoms. � is the angle between the c and z axes. For Li and the singly ionized Zn vacancy, z pointsroughly along the Zn–O bond direction. For the neutral Zn vacancy, z lies in the basal plane and points from oneO atom to an adjacent O atom.

Acceptor

Axial Nonaxial�

�deg� Ref.S g� g� gxx gyy gzz

N �neutral� 1/2 1.9953 1.9633 36N2 �1–� 1/2 2.0036 1.9935 98Li �neutral� 1/2 2.0028 2.0253 2.0223 2.0254 2.0040 113 136Zn vacancy �1–� 1/2 2.0024 2.0193 2.0173 2.0183 2.0028 111 58Zn vacancy �neutral� 1 2.0132 2.0187 2.0088 90 58

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Zn vacancies may account for the green luminescence insome ZnO samples. As discussed by Janotti and Van deWalle,32 the transition from a shallow donor level to the1–/2– acceptor level could give rise to PL around 2.5 eV.Also, exposure of ZnO samples to H plasma reduced theintensity of the green luminescence,60 consistent with thepassivation of Zn vacancies. However, as noted in Sec. III B,electron irradiation leads to a decrease in green lumines-cence, even though defects such as Zn vacancies are created.

Positron annihilation studies have yielded useful infor-mation about vacancies. In bulk ZnO, positrons have a life-time of 169 ps at room temperature.61 O vacancies do notresult in a significant increase in open volume, due to relax-ation of the neighboring Zn atoms.62,63 Therefore, while pos-itron annihilation is sensitive to Zn vacancies, it cannot de-tect O vacancies. Tuomisto et al.64 showed that Zn vacanciesare important deep-level acceptors in as-grown and irradiatedZnO, with trapped positrons having a lifetime of �230 ps.Zn vacancies formed by electron irradiation are annealed at200–300 °C.65,66 As grown, hydrothermal ZnO containsopen-volume defects that give rise to a positron lifetime of182 ps.67,68 It has been suggested that these defects areH-decorated Zn vacancies,67 although this assignment is ten-tative. In ZnO ceramics, the concentration of vacancy defectsdecreases steadily with increasing sintering temperature.69

D. Damage

Along with electron irradiation, defects produced by ionimplantation have been investigated extensively.70 For ZnOimplanted with N ions at a fluence of 1014 cm−2, deep-leveltransient spectroscopy showed an increase in electron trapstates at 0.31 and 0.95 eV below the conduction-bandminimum.71 Implantation with 2 MeV O ions produces Znvacancies for an ion fluence of 1015 cm−2. At higher flu-ences, vacancy clusters are created.72 Implantation with 220keV N ions in the 1015 cm−2 fluence range produces vacancyclusters containing two to four Zn vacancies.73 Even largervacancy clusters, containing as many as eight Zn vacancies,were reported to form upon implantation with 1015 cm−2 Alions at energies up to 380 keV. After implantation, annealingfrom 200 to 600 °C causes further agglomeration of vacancyclusters to voids.74 In ZnO implanted with protons, the Znvacancies agglomerated into voids filled with H2.75 At dosesof 4�1016 cm−2, implantation with Au ions does not makeZnO amorphous. However, similar doses of Si ions do causeZnO to become amorphous, presumably due to a chemicaleffect of the Si impurities.76

Irradiation of ZnO with a 193 nm excimer laser causesthe formation of defects in a near-surface region��200 nm�. The irradiation promotes the diffusion of Znfrom the ZnO to the surface, where Zn nanoparticles 10–20nm in diameter are formed.77 Open-volume defects, extend-ing �30 �m into the bulk, can also be created by simplemechanical polishing.38

IV. GROUP-III DONORS

As grown, whether by design or by accident, ZnO isusually n-type. For bulk samples with n�1017 cm−3, the

room-temperature electron mobility is typically ��150 cm2 /V s.22 Electrons from shallow donors, measuredby EPR, have a g-factor of g�1.96.78–80 From a hydrogenic�EM� donor model, the calculated electron binding energy is50 meV.81 Variable-temperature Hall-effect measurementsshow values consistent with that estimate; for example, Looket al.82 measured a 60 meV donor.

Doping levels of n�1020 cm−3 are used for transparentconductor applications. Sputtered, polycrystalline thin filmshave mobilities as high as 60 cm2 /V s and resistivities aslow as 2�10−4 � cm.83 The group-III dopants Al and Gaare preferred donors for transparent ZnO conductors.6 Ex-amples of intentional doping include ZnO:Al grown by pho-toassisted metalorganic chemical vapor deposition�MOCVD�,84 ZnO:Ga grown by chemical vapor deposition�CVD�,85 and ZnO:Ga grown on GaN substrates by plasma-assisted molecular beam epitaxy �MBE�.86 Heavily n-typeZnO:Al shows an increase in the band gap due to the fillingof the conduction band.87

As reviewed by Meyer et al.,81 PL spectra provide infor-mation about donors in ZnO �Fig. 2�. Excitons bound todonors have PL energies slightly below those of free exci-tons. At liquid-helium temperatures, PL peaks labeled I4

�3.3628 eV�, I6 �3.3608 eV�, I8 �3.3598 eV�, and I9 �3.3567eV� arise from excitons bound to H, Al, Ga, and In, respec-tively �Fig. 3�. A donor-acceptor pair transition was observedat 3.217 eV, with phonon replicas at 3.145, 3.073, and 3.001eV.29 The spacing of the replicas is 72 meV �580 cm−1�,which corresponds to the LO phonon frequency.28

As discussed in Sec. III, native defects such as O vacan-cies and Zn interstitials are unlikely to account for n-typeconductivity in ZnO. Instead, impurities play the largest role.Secondary ion mass spectrometry �SIMS� measurements re-vealed significant concentrations of Al in samples of bulksingle-crystal ZnO obtained from Cermet Inc., Ga and B insamples from Eagle-Picher, and Si in both.88 H and thegroup-III elements account for essentially all of the free car-riers in bulk, n-type ZnO grown by Eagle-Picher andCermet.89,88

FIG. 2. PL spectrum of ZnO, showing exciton lines �3.3–3.4 eV�, donor-acceptor pair transitions �DAxP�, and their phonon replicas �ELO and 2�ELO�. The broad “green luminescence” is centered at �2.45 eV. Re-printed with permission from B. K. Meyer, H. Alves, D. M. Hoffman, W.Kriegseis, D. Forster, F. Bertram, J. Christen, A. Hoffman, M. Strassburg,M. Dworzak, U. Haboeck, and A. V. Rodina, Phys. Status Solidi B, 241, 231�2009�. Copyright © 2009, Wiley-VCH Verlag GmbH & Co. KgaA.

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V. GROUP-IV IMPURITIES

Very little is known about group-IV impurities in ZnO.Although C might be expected to be an impurity in ZnOgrown by MOCVD or chemical vapor transport �CVT� withgraphite, there is not much evidence that it incorporates inthe crystal. Nickel and Fleischer90 did observe a series ofRaman lines in the C–H bond-stretching region. Si impuritieswere observed in bulk ZnO from SIMS, but their chemicalform is unknown.88 Pb donors were observed using ODMR,with Pb3+ ions having a g-factor of g�2.013.91,92

VI. GROUP-V ACCEPTORS

A. Substitutional nitrogen

According to first-principles calculations, N substitutesfor an O and is an acceptor with a hole binding energy of 400meV.93 PL experiments on n-type ZnO:N showed a morepromising hole binding energy of �200 meV,81,94,95 a valuethat is comparable to that of Mg acceptors in GaN.96 Theorysuggests that the binding energy might be lowered by codop-ing with isovalent Mg or Be.97 Magnetic resonance studies ofZnO identified N acceptors,36,98 and x-ray absorption spec-troscopy verified that N occupies the O substitutional site.99

In the absence of laser excitation, the acceptors are nega-tively ionized due to compensating donors such as group-IIIimpurities.98 N acceptors may also be compensated via theformation of defects such as O vacancies, complexes with Zninterstitials, or N2 molecules.93,100,101 Theory102 andexperiment103 have investigated the possibility of using NOor NO2 gas to introduce sufficient concentrations of N accep-tors to obtain p-type conductivity. The key challenge is tointroduce acceptors without being overwhelmed by compen-sating donors.

Reports of p-type ZnO using NH3 in CVD �Ref. 104�and an atomic N source in MBE �Ref. 105� were followed bya jökulhlaup of papers that described various means toachieve p-type conductivity. Doping methods includedMOCVD with NO,106 N2O,107 diallylamine,108 and NH3

�Ref. 109� as N doping sources; metalorganic MBE;110 andplasma-assisted MBE using a NO source111 or a mixture of

N2 and O2.112 The oxidation of sputtered Zn3N2 thin filmswas reported to yield p-type ZnO,113 as was N implantationof sputtered ZnO thin films.114 Other studies, however, re-ported high N concentrations �1019 cm−3� but n-typeconductivity.115 As discussed in Sec. VI D, the evidence forp-type conductivity in ZnO is not conclusive.

B. Nitrogen molecules

X-ray photoelectron spectroscopy on sputtered ZnO:Nfilms showed that N exists in two distinct chemical environ-ments, attributed to N acceptors and N2 molecules on the Osite.116,117 The experimental observations are in agreementwith first-principles calculations that predict N2 molecules inO vacancies to be compensating double donors.100,101 It wasspeculated that such N2 molecules may be removed fromZnO by thermal annealing.118 EPR experiments show evi-dence for a N2

− acceptor.98 N2 impurities in ZnO nanowiresgive rise to a Raman line at 2306 cm−1 at roomtemperature.119 In the future, Raman spectroscopy performedon bulk ZnO may provide needed microscopic informationabout N2 defects.

C. Phosphorus, arsenic, and antimony

First-principles calculations predict that P, As, and Sb aredeep acceptors, owing to their large ionic radii as comparedto O.93,120 It is therefore surprising that some researchershave reported p-type conductivity using these dopants.ZnO:P films grown by sputtering121 and MBE �Ref. 122�were reported to be p-type. Other groups found that ZnO:Pfilms grown by pulsed lased deposition �PLD� were semi-insulating, consistent with P acting as a deep acceptor.123

Device structures using ZnO:P yielded indeterminate Hallvoltages.124 Experiments that measured the radioactive decayof implanted arsenic-73 determined that the majority of Asatoms reside on the Zn site,125 making it ineffective as anacceptor. Nonetheless, there are reports of p-type ZnO:As�Refs. 126 and 127� as well as ZnO:Sb.128

D. The p-type controversy

Reports of p-type ZnO are controversial. Hall-effectmeasurements on inhomogeneous samples can yield thewrong carrier type.129 As pointed out by Barnes et al.,130

many studies that claimed to have measured hole conductionwere based on a small number of samples, and the resultswere not subsequently reproduced by other groups. Interfaceand near-surface states131 may be responsible for the appar-ent p-type conductivity measured by some experiments.Many of the allegedly p-type thin films were grown on mis-matched substrates such as sapphire, silicon, or glass,132

which undoubtedly led to high interface defect densities.The properties of these acceptor-doped samples are often

unstable. ZnO:N films that exhibited p-type conductivitywent n-type after a few days, accompanied by a relaxation ofthe lattice constant to its undoped value.130 ZnO:N sputteredon glass shows p-type conductivity that reverts to n-typeafter repeated measurements in the dark, only to becomep-type again after exposure to sunlight.118

FIG. 3. PL spectrum of donor-bound excitons in ZnO at 4.2 K. Reprintedwith permission from B. K. Meyer, H. Alves, D. M. Hoffman, W. Kriegseis,D. Forster, F. Bertram, J. Christen, A. Hoffman, M. Strassburg, M. Dworzak,U. Haboeck, and A. V. Rodina, Phys. Status Solidi B, 241, 231 �2009�.Copyright © 2009, Wiley-VCH Verlag GmbH & Co. KgaA.

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Optical emission in ZnO LEDs �Ref. 133� may indicatesuccessful p-type doping. However, p-n junctions are notrequired for a light-emitting device—it is well known thatSchottky diodes can emit light and rectify current.134 A PLband at 3.31 eV was originally attributed a transition fromfree electrons to group-V acceptors with a hole binding en-ergy of 130 meV.105 Subsequent spatially resolved lumines-cence experiments showed that the emission is localized atstacking faults, not the bulk of the ZnO.135 Given the doubtsraised so far, reports of p-type ZnO should generally beviewed with skepticism.

VII. GROUP-I ACCEPTORS

A. Lithium and sodium

EPR measurements showed that ZnO:Li contains Li ac-ceptors with a hole binding energy of 0.8 eV.136 The Li atomsreside on the substitutional Zn site and cause significant re-laxation of the neighboring O atoms. At low temperatures,exposure to light excites a hole, which becomes trapped onone of the O atoms.137 The hole prefers to sit on the O atomalong the c axis �axial site�, where it has an energy of 15meV lower than the nonaxial sites. Li �Ref. 138� and Na�Ref. 139� give rise to PL emission near 570 nm, attributed tothe transition from a shallow donor to the deep acceptorstate.

Recent PL experiments by Meyer et al.140 suggest thatNa or Li diffused into bulk ZnO introduce acceptors with a

hole binding energy near 0.3 eV, in qualitative agreementwith first-principles calculations that show Li and Na shouldbe shallow acceptors.93 However, the acceptors LiZn andNaZn are compensated by the interstitial donors Lii and Nai,respectively.141–143 Such compensating defects would need tobe eliminated in order to achieve p-type conductivity.

B. Copper

Cu substitutes for Zn and introduces electronic levelsinto the band gap.144 The Cu2+ ion is in the 3d9 state. Sincethere is one unfilled d orbital, the hole can undergo intra-dtransitions. In a cubic crystal, the lowest-energy transition isfrom the T2 to the E state. Since ZnO is wurtzite, however,the defect has C3v symmetry and the states split into severalcomponents.145 This splitting gives rise to sharp IR absorp-tion peaks at 5782 and 5820 cm−1 �Fig. 4�.

Cu is a common trace impurity in bulk ZnO crystals andshould be a candidate for green luminescence in almost anyZnO sample.146 Green emission due to Cu2+ impurities wasfirst observed by Dingle.147 Under illumination, an electronfrom a neighboring O atom can be transferred to Cu2+, re-sulting in Cu+ plus a hole. This state, denoted �Cu+,h�, is atransient shallow acceptor.148 The green emission was attrib-uted to the transition from �Cu+,h� to the Cu2+ T2 state,149

where the hole is “recaptured” by the Cu ion core, leading tothe emission of a photon. The zero-phonon line for this tran-sition is 2.86 eV �434 nm� at low temperatures �Fig. 4�.Phonon replicas occur at lower energies, resulting in a struc-

Conduction band

Valence band

T2

ECu2+

(Cu+,h)

5740 5760 5780 5800 5820 5840 5860

Absorption(arb.units)

Wave numbers (cm-1)

FIG. 4. �Color online� Schematic of Cu energy levels in ZnO �see Ref. 148�. Intra-d transitions of Cu2+ result in IR absorption peaks at 10 K, shown in thetop spectrum. The transition of a hole from the �Cu+,h� state to the T2 state produces a no-phonon emission line at 2.86 eV, shown in the expanded spectrum�shifted to the left�. Phonon replicas result in the structured green luminescence band centered around 510 nm. Reprinted with permission from R. Dingle,Phys. Rev. Lett. 23, 579 �1969�. Copyright © 1969, American Physical Society.

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tured band centered at 2.4 eV �510 nm�. While green lumi-nescence is enhanced by Cu2+,146,150 the band-edge PL inheavily doped samples is suppressed.151

In semiconductor terminology, the Cu acceptor level isdenoted �0/–�. When the Fermi level is above the �0/–� level,the equilibrium charge state is Cu1+, i.e., a negativelycharged acceptor. Electrical measurements on ZnO:Cu indi-cate that the �0/–� level lies 0.2 eV below the conduction-band minimum.144,152 The �0/–� level is the one labeled T2 inFig. 4. Garces et al.146 reported that annealing ZnO:Cu in airtransformed Cu1+ into Cu2+. The authors proposed that an-nealing lowered the Fermi level, pushing it below the �0/–�level.

Yan et al.153 calculated the �0/–� levels for Cu, Ag, andAu at 0.7, 0.4, and 0.5 eV above the valence-band maximum,respectively. However, one must take into account the factthat LDA calculations underestimate the band gap. Wardle etal.57 calculated the Cu �0/–� level at 1.0 eV above thevalence-band maximum, which was only �0.3 eV belowthe calculated conduction-band minimum.

VIII. HYDROGEN

A. Isolated hydrogen

1. Diffusion

Studies performed in the 1950s by Mollwo154 and Tho-mas and Lander155 concerned H diffusion into ZnO crystals.Both studies found that annealing ZnO crystals in a H2 am-bient increased their electrical conductivity. However, theirconclusions were somewhat different from each other.Mollwo154 considered that the reduction in surface ZnO by Hleft elemental Zn to diffuse into the crystal and act as inter-stitial donors. Thomas and Lander155 concluded that H dif-fuses into the ZnO crystal lattice and is directly responsiblefor producing donors.

From the electrical conductivity measurements, both re-search groups determined the diffusion rate of H into ZnOcrystals at various temperatures. Mollwo154 obtained a diffu-sion activation energy of 1.12 eV while Thomas andLander155 determined the value to be 0.91 eV. However, re-cent experiments that exposed ZnO to H plasmas yielded anactivation energy for diffusion of only 0.17 eV.156 This lowactivation energy seemed to support calculations that ob-tained a low migration barrier �0.4–0.5 eV� for interstitialH.157,158

The discrepancy between the experimental activation en-ergies was resolved by Nickel,159 who measured H and Ddepth profiles as a function of passivation temperature forpolycrystalline and single-crystal ZnO. The low activationenergies measured in H plasma experiments were ascribed tolarge effects from the H chemical potential. The migrationbarrier was found to be �1.0 eV, in agreement with theearlier work. It was proposed that the migration was limitedby H traps. Experiments on D-implanted ZnO confirmed thatdiffusion is described by a trap-limited model, with an acti-vation energy of 0.85�0.19 eV.160

2. Hydrogen donors

Theoretical work by Van de Walle161 showed that H is ashallow donor in ZnO, stimulating new research into theelectrical and structural properties of H donors. The low en-ergy of the conduction-band minimum relative to the vacuumresults in the H�+ /−� level being in the band.162,163 Experi-mental results on muonium164,165 implanted into ZnO andelectron-nuclear resonance measurements89 on lightly doped,n-type ZnO provided evidence that, as predicted, H is a shal-low donor. In order to determine the microscopic structure ofH donors, IR spectroscopy was used to measure O–H vibra-tional modes arising from these complexes.166–168

Thin films grown by MOCVD experience a factor-of-3increase in free-electron concentration after exposure to a Hplasma.169 ZnO ceramics implanted with H also show anincrease in conductivity.170 Films grown by PLD in a H2 orD2 ambient have carrier concentrations three orders of mag-nitude greater than films grown in an O2 ambient.171 Grow-ing in H2 or D2 appeared to form a shallow donor �a fewmeV� that could not be produced via postgrowth annealing inH2 gas.172

3. Interstitial hydrogen

In Fig. 5, two proposed models161 for interstitial H do-nors are shown. In one model, H is in the antibonding �AB��orientation, attached to a host O atom and pointing awayfrom the Zn–O bond. Another model is the bond-centered�BC�� configuration, where H sits between the hosts Zn andO. By measuring the pressure and polarization dependenceof the O–H vibrational mode, and comparing it to the pre-dictions of first-principles calculations,173,174 Jokela andMcCluskey175 attributed an IR peak at 3326 cm−1 �10 K� toH donors in the AB� configuration �Table III�.

There were problems with this assignment, however.Muon-implantation experiments176 and ab initiocalculations168 indicated that H donors should reside in theBC� configuration, not the AB� configuration. Experimentsby Lavrov et al.168 on CVT-grown ZnO samples revealed apeak at 3611 cm−1 �10 K�, attributed to H in the BC� con-figuration. Seager and Myers177 observed a similar peak, al-though shifted and broadened, at room temperatures. Thework by Shi et al.178 showed that samples annealed in Hgenerally show the 3611 and 3326 cm−1 peaks, but in differ-ent ratios depending on the type of sample. For example,Eagle-Picher samples show a strong 3611 cm−1 peakwhereas Cermet samples show a strong 3326 cm−1 peak�Fig. 6�. This sample-dependent variation suggests that one

Zn

O

H

c

FIG. 5. �Color online� Models for interstitial H donors in ZnO. Left: anti-bonding �AB�� configuration. Right: bond-centered �BC�� configuration.The dashed lines indicate broken Zn–O bonds. See Ref. 161.

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of the peaks actually arises from H paired with anotherimpurity.158

Subsequent SIMS analysis showed that Cermet samplescontained Ca impurities at concentrations of �4�1016 cm−3, whereas Eagle-Picher samples contained�1015 cm−3.88 This difference led us to propose that the3326 cm−1 line is due to Ca–H complexes. First-principlescalculations support this idea.179 They show that the elec-tropositive Ca atom donates extra electric charge to theneighboring O atoms, one of which traps a H atom. The Hatom resides in an AB� configuration and acts as a shallowdonor, in agreement with experiment. As of this writing,however, there is no direct confirmation of the Ca–H model.

4. Hidden hydrogen

The IR peaks at 3326 and 3611 cm−1 are unstable, de-caying at room temperature with a time constant of several

weeks.175 They form a species of “hidden hydrogen” thatremains in the crystal. Shi et al.180 proposed that the H do-nors combine to form H2 molecules, which are electricallyneutral and nearly invisible to IR. When the crystal is re-heated, the H2 molecules dissociate and form H donorsagain. Experimental verification for H2 molecules in ZnOwas achieved using low-temperature Raman spectroscopy.181

Theoretical work by Karazhanov and Ulyashin182 sug-gests that two interstitial H atoms can form a H2

� defect,where one H attaches to an O while the other attaches to aZn. Unlike H2 molecules, however, the H2

� defect shouldhave clear IR signatures. To our knowledge, H2

� has not beenobserved experimentally in ZnO.

5. Substitutional hydrogen

First-principles calculations by Janotti and Van deWalle45 showed that H can occupy an O substitutional siteand act as a shallow donor. One can think of “substitutionalH” as an O vacancy with a single H atom near the center.The predicted vibrational frequency is �800 cm−1. Themode has not been seen, perhaps because free-carrier andtwo-photon absorptions are too strong in that region of theIR spectrum. Unlike interstitial H, substitutional H is stable,with a calculated migration energy of 1.7 eV.157 This makesit a strong candidate for H-related donors in as-grown ZnO.89

The H-related I4 �3.363 eV� line183,184 may be due to substi-tutional H. The calculated electron density suggests that Hbonds equally with its four Zn neighbors.45 However, Tak-enaka and Singh62 claimed that substitutional H is ionic �H−�and does not form Zn–H bonds.

B. Hydrogen complexes

1. Passivation of defects

The efficiency of band-edge emission was found to in-crease after exposure to a H plasma,185 presumably due topassivation of deep-level states.60,186 Implantation of H,however, causes damage that reduces the emissionintensity.187 Raman spectra also show changes after hydroge-nation. A Raman line at 582 cm−1, which is correlated withdefects in ZnO powders and thin films,188 diminishes afterexposure to H2 at 400 °C.189

2. Zinc vacancy–hydrogen

As noted in Sec. III C, the green luminescence intensityin ZnO decreases after exposure to a H plasma. This obser-vation suggests that �1� Zn vacancies are responsible forgreen luminescence and �2� H passivates Zn vacancies.60

Evidence for passivation was provided by Lavrov et al.,168

who observed O–H modes at 3312 and 3350 cm−1 �10 K�after exposure of CVT-grown ZnO to H plasma. The fre-quencies of these modes agreed with first-principles calcula-tions for a Zn vacancy with two H atoms. This fully passi-vated vacancy has no electronic levels in the gap. EPRexperiments on electron-irradiated samples provided evi-dence for a Zn vacancy with a single H atom.49 It has beensuggested that the 3326 cm−1 line �Sec. VIII A 3� might ac-tually be due to such a complex.190

TABLE III. Local vibrational modes of H complexes in ZnO, determined byIR spectroscopy at liquid-helium temperatures. Frequencies correspond toO–H or N–H bond-stretching vibrations.

Frequency�cm−1� Assignment Ref.

2783 Ni–H 2083151 N–H 1913192 Cu–H 1963312 Zn vacancy–2H 16833493326 H donor �AB�� 166, 175, and 178

Ca–H donor? 88 and 1793347 Cu–2H 19933743577 Li–H 200 and 2023611 H donor �BC�� 168 and 178

FIG. 6. IR absorption spectra �4.2 K� of O–H complexes in three differentZnO samples. The Eagle-Picher and Cermet samples were c-cut crystalstilted at 45° to the IR beam �inset�. The Erlangen sample was a-cut and wasperpendicular to the IR beam. All samples contained the 3326 and3611 cm−1 peaks, but the intensities of the peaks varied from sample tosample. Reprinted with permission from G. Alvin Shi, M. Stavola, S. J.Pearton, M. Thieme, E. V. Lavrov, and J. Weber, Phys. Rev. B, 72, 195211�2008�. Copyright © 2005, American Physical Society.

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3. Nitrogen–hydrogen

A possible route toward p-type conductivity involves theincorporation of neutral N–H complexes, followed by an-nealing to drive out the H atoms. Jokela and McCluskey191

reported N–H complexes in ZnO grown by CVT in a NH3

ambient. The N–H bond-stretching mode gives rise to an IRabsorption peak at 3151 cm−1 �Fig. 7�. Isotopic substitutionsof D for H and 15N for 14N resulted in the appropriate fre-quency shifts. The N–H complexes were stable up to�700 °C. The N–H mode was also observed by Ramanspectroscopy in ZnO:N films grown by MOCVD.192 First-principles calculations predicted that H attaches to a N atom,nearly perpendicular to the c axis.193 The vibrational modespredicted by the calculations were in good agreement withthe experimental observations. Calculations show that theN–H bond is more stable than the O–H bond for interstitialH, also in agreement with experiment.194 Other researchersreported that N–H modes resulted in vibrations at 2400,195

3000,193 and 3100 cm−1.90 Since none of those studies usedisotopic substitution to verify the assignments, we believethat the reported modes came from C–H or O–H bonds. It ispossible, however, that there may be more than one possiblecomplex involving N and H.

4. Copper–hydrogen

The Cu–H complex gives rise to an O–H bond-stretchingmode at 3192 cm−1 �Ref. 196� and is stable up to 650 °C.168

First-principles calculations show that H attaches to an Oatom and resides in a bond-centered location, nearly perpen-dicular to the c axis.57 Stress-induced dichroism experimentsperformed at various temperatures determined a reorientationactivation barrier for H of 0.5 eV,197,190 in reasonable agree-ment with theory.198 The Cu–H complex can bind an addi-tional H atom, resulting in Cu–2H. This complex consists ofa substitutional Cu atom and two adjacent bond-centered Hatoms, giving rise to O–H modes at 3347 and 3374 cm−1.199

5. Lithium–hydrogen

H-related defects in hydrothermal ZnO exhibit an O–Hbond-stretching vibration at 3577 cm−1 �12 K� due to a Li–Hcomplex.200 The complex is highly stable,143 annealing out

only at temperatures above 1100 °C.201 The O–D line at2645 cm−1 showed isotopic splitting due to 6Li and 7Li.That result, combined with first-principles calculations, pro-vided evidence that H is in a bond-centered location, be-tween Li and O, parallel to the c axis.202 The temperaturedependence of the O–H and O–D modes was explained bycoupling to host phonons.203

IX. TRANSITION METALS

A. Spectroscopy

Transition-metal dopants occupy the substitutional Znsite and are normally in the 2+ or 3+ oxidation state.57,204

ZnO:Co exhibits low-temperature absorption205 andluminescence206 peaks at 3611 and 3616 cm−1, which wereattributed to Co2+ �3d7� transitions between the A2 and T2

levels. ZnO:Ni showed similar peaks at 6091 and6096 cm−1, which Schulz and Thiede206 attributed to thesame transitions in Ni3+ �3d7�. The Ni2+ �3d8� oxidation stategives rise to intra-d absorption peaks near 4200 cm−1.207,208

Luminescence features near 6880 cm−1 were attributed toT2−A2 transitions of V3+ �3d2�.209

Transition metals also affect the visible luminescenceproperties. In ZnO:Ni, Shulz and Thiede206 claimed thatintra-d transitions caused the red PL emission near 661 nm.Electron irradiation transformed Fe impurities from the 2+state to the 3+ state,49 resulting in an EPR-active Fe3+ centerwith spin S=5 /2.210 The structured luminescence around 690nm was attributed to the T1−A1 transitions of Fe3+ �3d5�.211

Mn2+ dopants suppress green luminescence212 and electricalconductivity213 in ZnO, perhaps because they fill Zn vacan-cies or other defects.

B. Ferromagnetism

The possibility of room-temperature dilute magneticsemiconductors stimulated a great deal of research into fer-romagnetic doping of ZnO. Reviews of dilute magneticsemiconductor oxides include Refs. 214–219. There are nu-merous reports of ferromagnetism, cited in these reviews.For example, ZnO:Co films were reported to exhibit ferro-magnetism with a Curie temperature higher than 300 K.220

After the initial reports, subsequent studies cast doubt onthe idea that room-temperature ferromagnetism arises fromthe bulk of the ZnO material. Polycrystalline ZnO dopedwith Co and Mn showed no ferromagnetism.221–223 High-quality thin films of n-type ZnO:Co deposited by PLD werealso not ferromagnetic.224 ZnO:Co thin films only showedroom-temperature ferromagnetism for high Co doping levels,and those samples had Co clusters.225 Correct identificationof phases in the material is essential to determine whether theferromagnetism actually comes from the ZnO.226 In ZnO:Cothin films, x-ray photoelectron spectroscopy clearly showedthat CoZn secondary phases were responsible for the ferro-magnetism of the sample.227 Grain boundaries and intrinsicdefects such as oxygen vacancies may also enhanceferromagnetism.228

3130 3140 3150 3160 3170 3180

14N-H

15N-HAbsorbance

Wave numbers (cm-1)

FIG. 7. �Color online� IR spectrum �10 K� of ZnO grown in a mixture of14NH3 �90%� and 15NH3 �10%�; see Ref. 191. Absorption peaks from N–Hbond-stretching vibrational modes are indicated by the arrows. The ball-and-stick model is from first-principles calculations �Ref. 193�.

071101-9 M. D. McCluskey and S. J. Jokela J. Appl. Phys. 106, 071101 �2009�

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X. EXTENDED DEFECTS

A. Surface conduction

Bulk ZnO samples often contain a surface conductionlayer, with a free-electron concentration of �1–3��1012 cm−2.229,230 Since the conduction band of ZnO has alow energy relative to vacuum,162 the Fermi level at the sur-face is probably pinned above the conduction-band mini-mum. This leads to a surface accumulation of electrons,similar to the situation in InN.231,232 Exposure to air229 or anO2 plasma230 reduces the conductivity of the layer, presum-ably due to adsorption of O2 and/or changes in the surfacereconstruction. The surface conduction electrons do notfreeze out and they contribute to the overall conductivity ofan n-type sample. Hall-effect measurements can be inter-preted with a two-layer model, where one layer is the surfaceconducting channel and the second layer is the underlyingbulk.233

B. Dislocations

Heteroepitaxial layers of ZnO typically contain thread-ing dislocation densities of 109 cm−2. Threading dislocationsgenerally run along the c axis, although ZnO grown underZn-rich conditions showed some dislocations 20°–30° fromthe c axis.234 Transmission electron holography indicates thatthe dislocations are negatively charged,235 perhaps due to theaccumulation of Zn vacancies near the dislocation core. Theactivation energy for dislocation motion was estimated to be0.7–1.2 eV in the range 650–850 °C.236 Dislocations createdby plastic deformation actually seem to increase the exci-tonic emission intensity.237 ZnO grown on r-cut sapphire issingle crystal, but the interface contains a high density ofmisfit dislocations.238

C. Stacking faults

Samples grown by MBE on r-cut sapphire showed abasal-plane stacking fault density of 105 cm−1 and partialdislocation density in the mid-1010 cm−2 range.239 Calcula-tions showed that basal-plane stacking faults have low for-mation energies but do not introduce electronic states intothe gap.240 However, spatially resolved CL measurementsshowed that basal-plane stacking faults contain localized ac-ceptors with a hole binding energy of 130 meV.135 The re-combination of free electrons with acceptor-bound holesleads to an emission at 3.31 eV. An emission at 3.333 eV wasattributed to excitons bound to structural defects.81

D. Grain boundaries

Thin films sputtered on c-cut sapphire show columnargrowth, not unlike GaN, with low-energy grainboundaries.241 At the grain boundaries, some atoms havethreefold or fivefold coordination.242 In ZnO grown on silica,several types of �0001�-tilt grain boundaries wereobserved.243

Defects at grain boundaries play an important role inZnO varistors,244 and ceramic semiconductor devices withhighly nonlinear current-voltage characteristics.245 Varistorsare insulating up to a well-defined breakdown field. They are

comprised of conductive grains surrounded by additive cat-ions �e.g., Bi or Pr� that segregate to the grain boundary.Grain sizes are typically around 10 �m and the typical grainresistivity is �1 � cm. A depletion region is believed toform at each grain boundary, where the depletion is whollywithin the ZnO grain,246 resulting in a breakdown voltage of�3 V/grain. First-principles calculations showed that Bimay substitute for Zn atoms at the grain boundary, resultingin a localized acceptor state.247 Such a state could create ap-n junction between the grain boundary and the n-type ZnOgrain.

XI. CONCLUSIONS

While the intrinsic properties of ZnO provide significantadvantages for device applications, the overarching goal ofreliable p-type conductivity has not been met. The evidencesuggests that ferromagnetic ZnO, which probably requireshigh hole concentrations, has not been achieved either. De-spite these roadblocks, researchers have made great strides inunderstanding the fundamental properties of defects and dop-ants in this material. N-type conductivity, historically blamedon native defects, is likely due to impurities such as group-IIIelements and H. Experimental and theoretical investigationshave provided insight into the microscopic structure of Li,Cu, and N acceptors, and their complexes with H. We hope,of course, that these fundamental insights will result in trans-formative technological breakthroughs.

While this review focused on bulk ZnO, the topic ofdefects in ZnO nanocrystals is an emerging field that willimpact many areas of science and technology. The relativeease and reproducibility of nanoparticle synthesis makesZnO a natural system for such investigations. Applicationsinvolving nanoscale ZnO will require a fundamental under-standing of defect phenomena across different length scales.

ACKNOWLEDGMENTS

We gratefully acknowledge the authors who gave per-mission to reprint their figures. We benefited from discus-sions with L. E. Halliburton, E. E. Haller, A. Janotti, S.Limpijumnong, N. H. Nickel, W. M. Hlaing Oo, M. Stavola,M. C. Tarun, S. Teklemichael, C. G. Van de Walle, S. B.Zhang, and many others. The research at WSU was sup-ported primarily by the National Science Foundation �GrantNo. DMR-0704163, LaVerne Hess and Charles Ying� and theDepartment of Energy �Grant No. DE-FG02-07ER46386,Refik Kortan�.

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