ForewordSince the invention of the first semiconductor transistor in 1947 by the scientistsof Bell Labs, the semiconductor industry has grown at an incredible pace, fabricat-ing faster, smaller, more powerful devices while manufacturing in larger volume atlower costs. Even though the very first semiconductor transistor was made fromger-manium(Ge), silicon (Si) became the semiconductor of choice as a result of the lowmelting point of Ge that limits high temperature processes and the lack of a naturaloccurring germanium oxide to prevent the surface from electrical leakage. Due tothe maturity of its fabrication technology, silicon continues to dominate the presentcommercial market in discrete devices and integrated circuits for computing, powerswitching, data storage and communication. For high-speed and optoelectronicdevices such as high-speed integrated circuits and laser diodes, gallium arsenide(GaAs) is the material of choice. It exhibits superior electron transport propertiesand special optical properties. GaAs has higher carrier mobility and higher effec-tive carrier velocity than Si, which translate to faster devices. GaAs is a directbandgap semiconductor, whereas Si is indirect, hence making GaAs better suitedfor optoelectronic devices. However, physical properties required for high power,high temperature electronics and UV/blue light emitter applications are beyond thelimits of Si and GaAs. It is essential to investigate alternative materials and theirgrowth and processing techniques in order to achieve these devices. Wide bandgapsemiconductors exhibit inherent properties such as larger bandgap, higher electronmobility and higher breakdown field strength. Therefore, they are suitable for highpower, high temperature electronic devices and short wavelength optoelectronics.Zinc oxide is a direct, wide bandgap semiconductor material with many promis-ing properties for blue/UV optoelectronics, transparent electronics, spintronicdevices and sensor applications. ZnOhas been commonly used in its polycrystallineform for over a hundred years in a wide range of applications: facial powders, oint-ments, sunscreens, catalysts, lubricant additives, paint pigmentation, piezoelectrictransducers, varistors, and as transparent conducting electrodes. Its research inter-est has waxed and waned as new prospective applications revive interest in thematerial, but the applications have been limited by the technology available at thetime. ZnO has numerous attractive characteristics for electronics and optoelectron-ics devices. It has direct bandgap energy of 3.37 eV, which makes it transparentin visible light and operates in the UV to blue wavelengths. The exciton bindingviii Forewordenergy is 60 meV for ZnO, as compared to GaN 25 meV; the higher excitonbinding energy enhances the luminescence efficiency of light emission. The roomtemperature electron Hall mobility in single crystal ZnO is 200 cm2V1, slightlylower than that of GaN, but ZnO has higher saturation velocity. ZnO has exhibitedbetter radiation resistance than GaN for possible devices used in space and nuclearapplications. ZnO can be grown on inexpensive substrate, such as glass, at rela-tively low temperatures. Nanostructures, such as nanowires and nanorods, havebeen demonstrated. These structures are ideal for detection applications due to itslarge surface area to volume ratio. Recent work on ZnO has shown ferromagnetismin ZnO by doping with transition metal, e.g. Mn, with practical Curie temperaturesfor spintronic devices. One main attractive feature of ZnO is the ability to bandgaptuning via divalent substitution on the cation site to form heterostructures. Bandgapenergy of 3.0 eV can be achieved by doping with Cd2+, while Mg2+ increasesthe bandgap energy to 4.0 eV. ZnO has a hexagonal wurtzite crystal structure,with lattice parameters a =3.25 and c =5.12 . The Zn atoms are tetrahedrallycoordinated with four O atoms, where the Zn d-electrons hybridize with the Op-electrons. The bonding between the Zn atoms and O atoms are highly ionic, dueto the large difference in their electronegative values (1.65 for Zn and 3.44 for O).Alternating Zn and O layers form the crystal structure.The first utilization of ZnO for its semiconductor properties was detectorsin build-your-own radio sets in the 1920s. A thin copper wire, known ascats whisker, is placed in contact to sensitive spots on a ZnO crystal. Themetal/semiconductor junction allows current to flow only in one direction, con-verting the incoming radio waves from alternating current to direct current in theradiocircuit. In1957, the NewJerseyZinc Companypublisheda bookentitledZincOxide Rediscovered to promote the materials frontier properties (semiconduc-tor, luminescent, catalytic, ferrite, photoconductive, and photochemical properties)and illustrative applications. Research focused mainly on growth, characterizationand applications that do not require single crystals such as varistors, surface acousticwave devices and transparent conductive films.Recent improvements in the growth of high quality, single crystalline ZnOin bothbulk and epitaxial forms has renewed interest in this material. Originally, researchefforts in ZnO growth were intended for gallium nitride (GaN) epitaxy. GaN isanother wide, direct bandgap semiconductor that has been the focus of intensiveresearch for high power, high frequency electronics that can operate at elevatedtemperatures and UV/blue optoelectronics. The lack of a native substrate has ledto a search for suitable choices of substrate in other materials, including sapphire,silicon carbide (SiC) and ZnO. The work of Look and his colleagues played amajor role in the revival of interest in ZnO research. They also organized the FirstZinc Oxide Workshop in 1999 that brought together researchers from all over theForeword ixworld to disseminate their findings and to exchange ideas. Furthermore, Look et al.were the first to publish convincing results of carefully characterized p-type ZnOhomoepitaxial film grown by molecular beam epitaxy (MBE), a critical step inachieving p-n junctions for light emitting devices. Subsequent ZnO Workshops, in2002 and 2004, continue to encourage research efforts on ZnO.Significant efforts in the last few years have been aimed at controlling conduc-tivity and improving crystal quality. However, in order to fully realize ZnOdevices,additional material and process development issues must be overcome. The pur-pose of this book is to provide an overview of recent progress in ZnO research andidentify future areas that need work.Chennupati JagadishCanberra, ACT, AustraliaStephen PeartonGainesville, FL, USAChapter 1Basic Properties and Applications of ZnOV. A. Coleman and C. JagadishDepartment of Electronic Materials Engineering, Research School of Physical Sciences andEngineering, The Australian National University, Canberra, ACT 0200, Australia1.1 IntroductionRecently, zinc oxide (ZnO) has attracted much attention within the scientific com-munity as a future material. This is however, somewhat of a misnomer, as ZnOhas been widely studied since 1935 [1], with much of our current industry andday-to-day lives critically reliant upon this compound. The renewed interest in thismaterial has arisen out of the development of growth technologies for the fabrica-tion of high quality single crystals and epitaxial layers, allowing for the realizationof ZnO-based electronic and optoelectronic devices.With a wide bandgap of 3.4 eV and a large exciton binding energy of 60 meV atroomtemperature, ZnO, like GaN, will be important for blue and ultra-violet opticaldevices. ZnO has several advantages over GaN in this application range however,the most important being its larger exciton binding energy and the ability to growsingle crystal substrates. Other favorable aspects of ZnO include its broad chem-istry leading to many opportunities for wet chemical etching, low power thresholdfor optical pumping, radiation hardness and biocompatibility. Together, these prop-erties of ZnO make it an ideal candidate for a variety of devices ranging fromsensors through to ultra-violet laser diodes and nanotechnology-based devices suchas displays.As fervent research into ZnO continues, difficulties such as the fabrication ofp-type ZnO that have so far stalled the development of devices are being overcome[2]. We are thus moving ever closer to the future in which ZnO will be a viable andintegral part of many functional and exotic devices.In this chapter, an overview of the basic properties of ZnO, including the crystalstructure, energy band structure and thermal properties is presented, as well as anZinc Oxide Bulk, Thin Films and NanostructuresC. Jagadish and S. Pearton (Editors) 2006 Elsevier Limited. All rights reserved2 V. A. Coleman and C. Jagadishintroduction to the mechanical properties, basic electronic and optical propertiesand potential applications of ZnO.1.2 Crystal structure and lattice parametersAt ambient pressure and temperature, ZnO crystallizes in the wurtzite (B4 type)structure, as shown in figure 1.1. This is a hexagonal lattice, belonging to the spacegroup P63mc, and is characterized by two interconnecting sublattices of Zn2+ andO2, such that each Zn ion is surrounded by a tetrahedra of O ions, and vice-versa.This tetrahedral coordination gives rise to polar symmetry along the hexagonalaxis. This polarity is responsible for a number of the properties of ZnO, includingits piezoelectricity and spontaneous polarization, and is also a key factor in crystalgrowth, etching and defect generation. The four most common face terminationsof wurtzite ZnO are the polar Zn terminated (0001) and O terminated (0001) faces(c-axis oriented), and the non-polar (1120) (a-axis) and (1010) faces which bothcontain an equal number of Zn and O atoms. The polar faces are known to possesdifferent chemical and physical properties, and the O-terminated face possess aFigure 1.1: The hexagonal wurtzite structure of ZnO. O atoms are shown as large whitespheres, Zn atoms as smaller black spheres. One unit cell is outlined for clarity.Basic Properties and Applications of ZnO 3slightly different electronic structure to the other three faces [3]. Additionally, thepolar surfaces and the (1010) surface are found to be stable, however the (1120)face is less stable and generally has a higher level of surface roughness than itscounterparts. The (0001) plane is also basal.Aside from causing the inherent polarity in the ZnO crystal, the tetrahedral coor-dination of this compound is also a common indicator of sp3covalent bonding.However, the ZnO bond also possesses very strong ionic character, and thus ZnOlies on the borderline between being classed as a covalent and ionic compound, withan ionicity of fi=0.616 on the Phillips ionicity scale [4]. The lattice parametersof the hexagonal unit cell are a =3.2495 and c =5.2069 , and the density is5.605 g cm3[5].In an ideal wurtzite crystal, the axial ratio c/a and the u parameter (which is ameasure of the amount bywhicheachatomis displacedwithrespect tothe next alongthe c-axis) are correlated by the relationship uc/a =(3/8)1/2, where c/a =(8/3)1/2and u =3/8 for an ideal crystal. ZnO crystals deviate from this ideal arrangementby changing both of these values. This deviation occurs such that the tetrahedraldistances are kept roughly constant in the lattice. Experimentally, for wurtzite ZnO,the real values of u and c/a were determined in the range u =0.38170.3856 andc/a =1.5931.6035 [68].Additional to the wurtzite phase, ZnO is also known to crystallize in the cubiczincblende and rocksalt (NaCl) structures, which are illustrated in figure 1.2.Figure 1.2: The rock salt (left) and zincblende (right) phases of ZnO. O atoms are shownas white spheres, Zn atoms as black spheres. Only one unit cell is illustrated for clarity.4 V. A. Coleman and C. JagadishFigure 1.3: The LDAband structure of bulk wurtzite ZnOcalculated using dominant atomicself-interaction-corrected pseudopotentials (SIC-PP). This method is much more efficientat treating the d-bands than the standard LDA method. [Reprinted with permission fromD. Vogel, P. Krger and J. Pollmann, Phys. Rev. B 52, R14316 (1995). Copyright 1995 bythe American Physical Society.]Zincblende ZnO is stable only by growth on cubic structures [911], whilst therocksalt structure is a high-pressure metastable phase forming at 10 GPa, andcan not be epitaxially stabilized [12]. Theoretical calculations indicate that a fourthphase, cubic cesium chloride, may be possible at extremely high temperatures,however, this phase has yet to be experimentally observed [13].1.3 Energy band gapThe electronic band structure of ZnO has been calculated by a number of groups[1319]. The results of a band structure calculation using the Local DensityApprox-imation (LDA) and incorporating atomic self-interaction corrected pseudopotentials(SIC-PP) to accurately account for the Zn 3d electrons is shown in figure 1.3 [19].The band structure is shown along high symmetry lines in the hexagonal Brillouinzone. Both the valence band maxima and the lowest conduction band minima occurat the point k =0 indicating that ZnO is a direct band gap semiconductor. Thebottom 10 bands (occurring around 9 eV) correspond to Zn 3d levels. The next6 bands from 5 eV to 0 eV correspond to O 2p bonding states. The first two con-duction band states are strongly Zn localized and correspond to empty Zn 3s levels.The higher conduction bands (not illustrated here) are free-electron-like. The O2s bands (also not illustrated here) associated with core-like energy states, occuraround 20 eV. The band gap as determined from this calculation is 3.77 eV. ThisBasic Properties and Applications of ZnO 5Figure 1.4: Wave-vector-resolved LDOSs on the first three layers of the (0001)-Zn (leftpanel) and (0001)-O (right panel) surfaces. The bulk LDOS is given by the dashed lines andsurface induced positive changes to the LDOS are shown as hatched. The letters A, B, Pand S represent anti-back bonds, back bonds, P resonances and S resonances respectively.[Reprinted with permission from I. Ivanov and J. Pollmann, Phys. Rev. B 24, 7275 (1981).Copyright 1981 by the American Physical Society.]correlates reasonably well with the experimental value of 3.4 eV, and is much closerthan the value obtained from standard LDA calculations, which tend to underes-timate the band gap by 3 eV due to its failure in accurately modeling the Zn 3delectrons.In addition to calculations for the band structure of bulk ZnO, Ivanov andPollmann have also carried out an extensive study on the electronic structureof the surfaces of wurtzite ZnO [18]. Using the empirical tight-binding method(ETBM) to determine a Hamiltonian for the bulk states, the scattering theoreticalmethod was applied to determine the nature of the surface states. The calculateddata was found to be in very good agreement with experimental data obtainedfromelectron-energy-loss spectroscopy(EELS) andultra-violet photoelectronspec-troscopy (UPS). Figure 1.4 shows the wave-vector-resolved local density of states6 V. A. Coleman and C. JagadishFigure 1.5: Schematic diagram representing the crystal-field and spin-orbit splitting of thevalence band of ZnO into 3 subband states A, B and C at 4.2 K.(LDOSs) on the first three layers of the (0001)-Zn (left panel) and (0001)-O (rightpanel) surfaces, for the , M and K points of the surface Brillouin zone. The bulkLDOS (calculated using the ETBM) is given by the dashed lines. Surface inducedpositive changes to the LDOS are shown as hatched. As discussed in section 1.2, theproperties of the two polar faces are expected to be different, and this is reflected inthis data. Whilst it indicates that no surface states are present in the band gap, theZn surface shows an increase in back bonds (denoted by Bin fig. 1.4) and anti-backbonds (denoted by A) surface states, while the O face simply shows an increasein P resonances and states. This result suggests that the Zn face possesses morecovalent character, arising from the Zn 4sO 2p states, whilst the O face is moreionic.Experimentally, the ZnO valence band is split into three band states, A, B andC by spin-orbit and crystal-field splitting. This splitting is schematically illustratedin figure 1.5. The A and C subbands are known to posses 7 symmetry, whilst themiddle band, B, has 9 symmetry [20]. The band gap has a temperature dependenceBasic Properties and Applications of ZnO 7Table 1.1: Key properties of the binary II-VI oxides [35,57].ZnO MgO CdOEg (eV) 3.4 7.8 2.2me (m0) 0.28 0.35 mhh (m0) 0.78 1.60 [001], 2.77 [111] mlh (m0) 0.35 [001], 0.31 [111] a () 3.2 4.2 4.7c () 5.2 Stable crystal structure Wurtzite Rocksalt Rocksaltup to 300 K given by the relationship:Eg(T) = Eg(T = 0)5.05 104T2900 T (1.1)These properties, combined with the lattice dynamics (discussed in section 1.5) ofZnOgive rise to interesting optical properties which will be discussed in section 1.8.1.3.1 Opportunities for band gap engineeringFor a semiconductor to be useful, particularly in reference to optoelectronic devices;band gap engineering is a crucial step in device development. By alloying thestarting semiconductor with another material of different band gap, the band gapof the resultant alloy material can be fine tuned, thus affecting the wavelength ofexciton emissions. In the case of ZnO, alloying with MgO and CdO is an effectivemeans of increasing or decreasing the energy band gap respectively [2123]. Someof the key properties of ZnO, MgO and CdO are shown in table 1.1 whilst availablerelevant parameters for the alloy semiconductors Zn(1x)MgxO and Zn(1y)CdyOare shown in table 1.2.Currently however, due to the relative newness of the field, only limited exper-imental and theoretical work has been done for these materials, and thus theinformation available is both incomplete and not well verified.1.4 Mechanical propertiesTable 1.3 gives a brief overview of the well accepted and experimentally usefulparameters describing the mechanical properties of ZnO. As seen from the table,ZnO is a relatively soft material, with a hardness of 5 GPa at a plastic penetration8 V. A. Coleman and C. JagadishTable 1.2: Important parameters for the alloy semiconductors Zn(1x)MgxO andZn(1y)CdyO.Parameter Zn(1x)MgxO Zn(1y)CdyOTypical energy range (eV) 3.374 2.93.37Maximum percentage alloy 43% [58] 70% [59]incorporationa-axis length expansion () 3.250 +0.036x [60] 3.252 +0.143y 0.147y2[61]c-axis length expansion () 3.34 0.063x [60] 5.204 +0.956y 5.42y2[61]Eg (eV) 3.37 +2.51x [58] 3.29 4.40y +5.93y2[61]Ec/Ev 70/30 [62] Table 1.3: Key mechanical properties of c-axis oriented wurtzite ZnO, as determined byexperiment and theory.Parameter Experimental TheoreticalBulk Youngs modulus, E (GPa) 111.2 4.7aBulk hardness, H (GPa) 5.0 0.1aEpitaxial Youngs modulus, E (GPa) 310 40bEpitaxial hardness, H (GPa) 5.75 0.8bBulk modulus, B (GPa) 142.4c156.8ddB/dP 3.6c3.6de33 (C m2) 0.96e1.19fe31 (C m2) 0.62e0.55fe15 (C m2) 0.37e0.46fSpontaneous polarization (C m2) 0.047gc11 (GPa) 209h246fc33 (GPa) 216h246fc12 (GPa) 120h127fc13 (GPa) 104h105fc44 (GPa) 44h56fBorn effective charge, Z* 2.1gaSpherical indentation on bulk ZnO (ref. [24]).bSpherical indentation on epitaxial ZnO (ref. [26]).cSee ref. [63].dAb initio Hartree Fock calculation (ref. [7]).eSee refs. [30,31].fAb initio Hartree Fock calculation (ref. [29]).gCalculation using LDAand Hartree Fock (ref. [28]).hSee ref. [64].Basic Properties and Applications of ZnO 9Figure 1.6: A bright-field XTEM image of a spherical indent in bulk, c-axis oriented ZnOcreated using a 4.2 m indenter tip. The maximum load is 15 mN. Extensive damage canbe seen in the region directly under the indent, extending well beyond the volume undercontact. Slip bands occurring along the basal planes can also be clearly seen and are indicatedby arrows.depth of 300 nm (for c-axis oriented bulk ZnO) [24]. This needs to be taken intoconsideration when processing and designing ZnO-based devices. Nanoindentationstudies conducted by Bradby et al. [25] of ZnO with a spherical indenter of radius4.2 m have shown that the primary mechanism for deformation in this semicon-ductor is the nucleation of slip on the basal and pyramidal planes. For loads of up to50 mN, there has been no observation of phase transformations or cracking in thismaterial. Nanoindentation is a useful technique for probing the mechanical proper-ties of a material, whilst also providing information on the behavior of a materialunder contact induced damage, such as that experienced during device processing.For ZnO, indentation results in significant quenching of the excitonic luminescence.Additionally, extensive damage is created in the ZnO material, with defects prop-agating far beyond the volume under contact [25]. Figure 1.6 shows a bright fieldcross sectional transmission electron microscopy image of indented ZnO. Fromthis image, the extent to which damage propagates well beyond the volume under10 V. A. Coleman and C. JagadishFigure 1.7: Loadunload curves of 500 nm thick c-axis oriented ZnO grown on sapphire.The indents were made using a 4.2 m spherical indenter. Closed circles indicate a loadof 10 mN whilst open circles are for a 50 mN load. No pop-in events are seen, suggestingthat slip along the basal planes has been suppressed in this system.contact can be clearly seen. Slip bands along the basal planes which give rise to socalled pop-in events during indentation [25] loading can also be clearly seen.There is also some indication that the crystal orientation of ZnO influences themechanical properties due to the orientation of the basal planes [26,27]. A-axisoriented bulk ZnO is significantly softer than c-axis material, with a hardness of2 GPa at a plastic penetration depth of 50 nmbelowcontact. In a-axis material, thebasal planes lie perpendicular to the surface and are thus more susceptible to slip.This is reflected in the occurrence of only one large pop-in event for indentationin this material [26]. As with c-axis ZnO, a-axis material does not show any evi-dence of phase transformation or cracking under indentation, however the excitonicluminescence is quenched and very large propagation of defects is seen [27].In addition to bulk material, epitaxial ZnO will be important for ZnO devices,and thus it is important to understand differences in the mechanical properties of thissystem. Studies showthat epitaxial ZnOgrown on sapphire is slightly harder than itsbulk counterpart, with a hardness of 5.7 GPa for c-axis epitaxial layers [26]. Thisincrease in hardness is due to the presence of the underlying layer which inhibits theslip mechanism along the basal planes. This is evidenced by the absence of pop-inevents during indentation loading in epitaxial material. This is illustrated in fig-ure 1.7 which shows an indentation loadunload curve made in a 500 nm thickc-axis oriented ZnO layer grown on sapphire. As-grown threading disloca-tions in epitaxial material also increase its hardness by inhibiting via straincompensation [26].Basic Properties and Applications of ZnO 11Table 1.4: Experimentally determined principalphonon modes of wurtzite ZnO at 300 K [20].Phonon mode Value (cm1)Elow2 101Ehigh2 437TO (A1) 380LO (A1) 574TO (E1) 591Piezoelectricity is also an important mechanical property. ZnO is believed tohave a piezoelectric tensor equal to or even greater than that of GaN andAlN whichmeans that ZnO is a suitable candidate for device applications requiring a largeelectromechanical coupling [28]. Many studies, both theoretical and experimental,have been made to determine the three piezoelectric stress coefficients, eik forwurtzite ZnO [2833], and a selection of these values are also listed in table 1.3,along with the elastic constants chk.1.5 Lattice dynamicsIn single crystal wurtzite ZnO, there are 4 atoms per unit cell, giving rise to12 phonon modes. These modes are important for understanding the thermal,electrical and optical properties of the crystal, and are as follows: one longitudinal-acoustic (LA), two transverse-acoustic (TA), three longitudinal-optical (LO) andsix transverse-optical (TO) branches. The A1 and E1 branches are Raman and infra-red active, while the two E2 branches (non-polar) are only Raman active. The Elow2mode is associated with the vibrations of the Zn sub-lattice, whilst the Ehigh2 modeis associated with the oxygen atoms only. The B1 branches are always inactive. Thephonon modes of ZnO have been extensively studied and modelled (see ref. [34]and references therein). Table 1.4 gives a list of the experimental values for themost common phonon modes visible at 300 K [20].1.6 Thermal propertiesIn this section, the thermal properties of wurtzite ZnO will be presented, includingthe thermal expansion coefficients, thermal conductivity and specific heat.12 V. A. Coleman and C. JagadishFigure 1.8: Graph of the ZnO thermal expansion coefficient th as a function of tem-perature. [Reprinted with permission from H. Ibach, Phys. Stat. Sol. (b) 33, 257 (1969).Copyright 1969 by WILEY-VCH.]1.6.1 Thermal expansion coefficientsThe thermal expansion coefficient of a material describes lattice deforma-tion as a function of temperature. For ZnO, these coefficients are given asa=4.31 106K1and c=2.49 106K1at 300 K [35]. Additionally, fig-ure 1.8 shows a plot of the ZnO thermal expansion coefficient, th, as a function oftemperature.1.6.2 Thermal conductivityThe thermal conductivity, (Wcm1K1) of a semiconductor is an importantproperty when considering high-power/high temperature devices. It is a kineticproperty influenced by the vibrational, rotational and electronic degrees of freedom,and is predominately limited by phonon-phonon scattering in a pure crystal. ZnO,like most other semiconductors, contains a large number of point defects, and thesehave a significant effect on the thermal conductivity. The highest measured valuesof thermal conductivity come from a study done on vapour-phase grown sampleswhich measured the conductivity on the polar faces of ZnO [36]. This study givesthe values of =1.02 0.07 and 1.16 0.08Wcm1K1fromthe Zn face of twodifferent samples, and =1.10 0.09 and 0.98 0.08Wcm1K1from the OBasic Properties and Applications of ZnO 13Figure 1.9: Specific heat data for pure (bulk) and varistor ZnO measured between 1.7 and25 K. [Reprinted with permission from W. N. Lawless and T. K. Gupta, J. Appl. Phys. 60,607 (1986). Copyright 1986, American Institute of Physics.]face of the same twosamples. These values are considerablyhigher thanother valuesmeasured from ZnO which typically fall in the range =0.61Wcm1K1[34].1.6.3 Specific heatThe specific heat of a material is influenced by the lattice vibrations, free carriers anddefects within the material. In crystals of high quality and purity, the specific heat ismainly influenced by the lattice vibrations. Unfortunately, there is very limited dataavailable in the literature for specific heat measurements on ZnO. The Handbookof Chemistry and Physics ([5]) gives a value of the specific heat capacity of ZnOat constant pressure as Cp=40.3 J mol1K1. Astudy by Lawless and Gupta [37]measures the specific heat for pure and varistor ZnO between 1.7 and 25 K. Thedata obtained in this study is shown in figure 1.9. From this figure, it can be seenthat the specific heat of pure ZnO diverges from that of the varistor below 20 Kas a result of impurities and defects incorporated in the varistor grain boundaries.14 V. A. Coleman and C. JagadishAdditionally, the specific heat data presented here for ZnOis unlikely to be accuratefor modern ZnO crystals, as dramatic improvements in growth technology havetaken place since this study was undertaken, which in turn will influence the specificheat measurements. However, at the time of publication, no further specific heatmeasurements on ZnO single crystals could be found in the literature.1.7 Electrical propertiesThe electrical properties of ZnO are hard to quantify due to large variance of thequality of samples available. The background carrier concentration varies a lotaccording to the quality of the layers but is usually 1016cm3. The largest reportedn-type doping is 1020electrons cm3and largest reported p-type doping is 1019holes cm3, however such high levels of p-conductivity are questionable and havenot been experimentally verified [38]. The exciton binding energy is 60 meV at300 K, and is one of the reasons why ZnO is so attractive for optoelectronic deviceapplications. As mentioned in section 1.3.1, the electron effective mass is 0.24m0,and the hole effective mass is 0.59m0. The corresponding electron Hall mobilityat 300 K for low n-type conductivity is =200 cm2V1s1, and for low p-typeconductivity is 550 cm2V1s1[39].1.8 Optical propertiesAs mentioned in section 1.3, the optical properties of ZnO are heavily influencedby the energy band structure and lattice dynamics. For a comprehensive review ofthe optical properties of excitonic recombinations in bulk, n-type ZnO, please referto the work of B. K. Meyer et al. [20]. This work gives a comprehensive treatmentand analysis of the excitonic spectra obtained from ZnO, and assigns many defectrelated spectral features, as well as donoracceptor pair (DAP) emission. A broaddefect related peak extending from1.9 to 2.8 eVis also a common optical featureof ZnO. Known as the green band, the origin of its luminescence is still not wellunderstood and has in the past been attributed to a variety of different impuritiesand defects. Figure 1.10 shows a typical photoluminescence spectra of n-type ZnOmeasured at 4.2 K. The excitonic, DAP and extended green band emission can allbe clearly seen, as can the phonon replicas produced from the longitudinal opticalphonons (LO). Due to the lack of available data on p-type ZnO, a correspondingspectrum is not shown here.In terms of the more fundamental optical properties of ZnO, there have been anumber of comprehensive studies to determine the refractive index and dielectricBasic Properties and Applications of ZnO 15Figure 1.10: Photoluminescence spectrum of n-type bulk ZnO (HeCd excitation) showingexcitonic, donor acceptor pair and green-band emission. The longitudinal optical phononswith the corresponding phonon replicas are indicated on the figure. [Reprinted with permis-sion from B. K. Meyer, H. Alves, D. M. Hofmann, W. Kriegseis, D. Forster, F. Bertram,J. Christen, A. Hoffmann, M. Straburg, M. Dworzak, U. Haboeck and A. V. Rodina, Phys.Stat. Sol. (b) 241, 231 (2004). Copyright 2004 by WILEY-VCH.]Table 1.5: Static (0) and high frequency dielectric constant () data forZnO [40,41].Film [41] Bulk [41] Bulk [40]

0 Ec 7.46 7.77E c 8.59 8.91

Ec 3.7 3.6 3.68E c 3.78 3.66 3.72constants of this material [4042]. The measurements were all carried out usingspectroscopic ellipsometry. The values determined for the dielectric constants ofZnO are shown in table 1.5 whilst the refractive index dispersion for both Ec andE c as measured and calculated by Yoshikawa and Adachi [40] is shown in figure1.11. The refractive index of wurtzite ZnO is commonly given as n=2.008 andne=2.029 [39].16 V. A. Coleman and C. JagadishFigure 1.11: Refractive index dispersion of ZnO for Ec and E c below the fundamentalabsorption edge. The solid circles represent the spectroscopic ellipsometry data whilst thesolid line is calculated data. [Reprinted with permission from H. Yoshikawa and S. Adachi,Jpn. J. Appl. Phys. 36, 6237 (1997). Copyright 2004 by Jpn. J. Appl. Phys.]1.9 IssuesThe main issue currently limiting the production of ZnO-based devices is that ofthe achievement of p-type ZnO. This has been the case for most wide-band gapsemiconductors, including GaN. ZnO is predicted to be an intrinsic semiconductor,however it almost exclusively occurs naturally as n-type. The cause of this inherentdoping, combined with the difficulty in achieving p-type material is still not directlyunderstood. Native defects such as Zn interstitials and O vacancies [43] are thoughtto compensate the donors and give rise to the native conductivity. Backgroundimpurities such as H introduced during growth could also play a role, as couldother deep impurity levels [44]. Finally, it has been suggested that low solubility ofdopants within this material could also be responsible [45]. Recent improvementsin the as-grown quality of ZnO material as well as successes with dopant atomsindicate that p-type doping of ZnO is an achievable goal.Another issue important for the realization of ZnO devices is that of contactsto ZnO. While Ohmic contacts to n-type ZnO are relatively easily obtained, theproduction of good reliable Schottky diodes still remains an issue [39]. A recentreport by Grossner et al. [46] showed that Pd contacts on clean ZnO surfacesproduced Schottky behavior with a barrier height of 0.83 eV, which is in goodagreement with the value for the barrier height of Pd on ZnO predicted by theory.Basic Properties and Applications of ZnO 17Whilst other issues within ZnO exist, these two are by far the most importantand currently play a limiting role in the realization of ZnO devices.1.10 ApplicationsIn chapters 1016, some of the principal applications for ZnO will be discussed indetail, however a brief overview of these applications is provided here. As men-tioned previously, ZnO is already widely used in our society, and indeed it is a keyelement in many industrial manufacturing processes including paints, cosmetics,pharmaceuticals, plastics, batteries, electrical equipment, rubber, soap, textiles,floor coverings to name just a few. With improvements in growth technology ofZnO nanostructures, epitaxial layers, single crystals and nanoparticles, we are nowmoving into an era where ZnO devices will become increasingly functional andexotic.ZnO-based nanostructures including nanowire arrays hold a host of opportunitiesfor flat screen displays, field emission sources, gas, chemical [47] and biologicalsensors, and as UV light emitters and switches [4750].Epitaxial layers and single crystals will be important for the development ofoptoelectronic (blue and ultraviolet light emitters and detectors) [51], piezoelectric[52] and spintronic [53] devices, and together with GaNmay formthe light source ofthe 21st century [54]. Epitaxial ZnO also holds much promise as a semi-conductingtransparent thin film[55], which again will be important for solar cells, gas sensors,displays and wavelength selective applications.Existing technologies are also being revolutionized with ZnO nanoparticles,which have led to the development of improved sunscreens, paints and coatingsto name just a few.Additionally, the radiation hardness of ZnO to MeV proton irradiation makes itan ideal candidate for space applications [56].Thus ZnO whilst already possessing a wide application base, has enormousopportunities for society and industry alike due to its unique properties which arenow being explored and applied. The future in which ZnO devices become part ofour everyday lives is already approaching reality.AcknowledgmentsWe would like to thank Dr. Jodie Bradby for helpful discussions, Mr. Aert vande Hulsbeek for his assistance with the figures presented in this chapter, and theAustralian Research Council for financial support.18 V. 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Phys. 83 (1998) 7844.[53] D. P. Norton, S. J. Pearton, A. F. Hebard, N. Theodoropoulou, L. A. Boatner,R. G. Wilson, Appl. Phys. Lett. 82 (2003) 239.[54] J. Nause, Comp. Semicond. 11 (2005) 29.[55] H. Hartnagel, A. L. Dawar, A. K. Jain, C. Jagadish, Semiconducting transparent thinfilms, Institute of Physics Publishing, Bristol and Philadelphia, 1995.[56] D. C. Look, D. C. Reynolds, J. W. Hemsky, R. L. Jones, J. R. Sizelove, Appl. Phys.Lett. 75 (1999) 811.20 V. A. Coleman and C. Jagadish[57] T. Makino, Y. Segawa, M. Kawasaki, H. Koinuma, Semicond. Sci. Technol. 20(2005) S78.[58] K. Koike, K. Hama, I. Nakashima, G. Takada, K. Ogata, S. Sasa, M. Inoue, M. Yano,J. Cryst. Growth 278 (2005) 288.[59] S. Shigemori, A. Nakamura, T. Aoki, J. Temmyo, Jpn. J. Appl. Phys. 43 (2004) L1088.[60] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai,Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466.[61] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda,H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237.[62] G. Coli, K. K. Bajaj, Appl. Phys. Lett. 78 (2001) 2861.[63] S. Desgreniers, Phys. Rev. B 58 (1996) 11425.[64] Landolt-Brnstein, O. Madelung (Eds.), New Series, Group III: Solid State Physics,LowFrequency Properties of Dielectric Crystals: Elastic Constants, vol. 29a, Springer,Berlin, 1993.Chapter 2Doping and Defects in ZnODavid C. LookSemiconductor Research Center, Wright State University, Dayton, OH 45435, USA;Materials and Manufacturing Directorate, Air Force Research Laboratory,Wright-Patterson Air Force Base, OH 45433, USAAbstract: ZnOis a wide bandgap semiconductor material with numerous present applica-tions, such as varistors and surface acoustic wave devices, and future applications, includ-ingUVlight-emittingdiodes andtransparent field-effect transistors. However, all of theseapplications are either dependent upon, or are affected by, impurities and defects. We willconsider donor-type impurities H, Al, Ga, and In; and acceptor-type impurities N, P, As,and Sb. Among defects, we will concentrate on Zn interstitials, Zn vacancies, O vacan-cies, and complexes of each. The main experimental techniques discussed here includetemperature-dependent Hall-effect and low-temperature photoluminescence measure-ments, because they alone can provide donor and acceptor concentrations, and donorenergies. Surface conduction is important in the Hall-effect analysis of annealed samples,and is included by means of a two-layer analysis. The important topic of p-type ZnO isconsidered in some detail, especially in connection with the most useful acceptor dopants.2.1 IntroductionThe last decade has witnessed an enormous growth of ZnOrelated research, largelybecause of the possibilities of new or improved types of electronic and photonicdevices [13]. One device that has great commercial potential is a UVlight-emittingdiode (LED), which could be combined with phosphors to produce solid-state whitelighting [46]. Another is a transparent field-effect transistor, which could serve asan active element in large-area displays [79]. In each of these cases, ZnOpossessesfundamental properties that give it advantages over competitive materials, such asGaN. For example, the free exciton is ZnO has a binding energy of about 60 meV,whereas that of GaN is about 24 meV. Thus, efficient excitonic emission processescan persist in ZnO at room temperature and higher, and in fact, prototype ZnO-based LEDs have been shown to operate at nearly 400C. ZnO also has many otheradvantages, including the availability of large-area native substrates, amenabilityZinc Oxide Bulk, Thin Films and NanostructuresC. Jagadish and S. Pearton (Editors) 2006 Elsevier Limited. All rights reserved22 D. C. Lookto low-temperature growth and wet chemical etching, and high radiation resistance.But in regard to LED development, ZnO also has one major disadvantage, namely,the lack of a reliable technology for producing p-type material. Fortunately, in recentyears, this situation has improved, and now several groups have reported p-typeZnO[6,1043], and a fewhave even produced light-emitting p-n junctions [46,44].(For a review of ZnO-based LEDs, 20012003, see Ref. 45.) However, the opticaland electrical properties of ZnOare still not well understood, and this means that theroles of various impurities and defects in ZnOare not well understood, because as iswell known, impurities and defects generally control such properties. Thus, in thischapter, we will attempt to summarize our present knowledge, both experimentaland theoretical, of the roles of defects and impurities in controlling the optical andelectrical properties of ZnO. Indeed, our understanding of some of these mattershas undergone major changes in the last few years.It is useful to summarize some of these changes. As far as we know, as-grownZnO has always been found to be n-type, and temperature-dependent Hall-effect(T-Hall) measurements have usually yielded donor energies of 30 to 70 meV [46].Since most samples are grown under Zn-rich conditions, it was always natural inthe past to assume that the dominant donor was either the O vacancy VO, or the Zninterstitial ZnI. In the year 2000, this conclusion was strongly challenged by Kohanet al. [47] who showed theoretically that both VO and ZnI have high formationenergies in n-type ZnO, and that furthermore, they are deep, not shallow, donors.Thus, it was concluded that neither VO nor ZnI would exist in measurable quantities,and that even if one or the other were present, its ionization energy would be toohigh to produce free electrons. Other theoretical analyses have concluded that ZnIis actually a shallow donor, rather than deep [48,49], as has also been shown byelectron-irradiation experiments [50]; however, the high formation energy of ZnImentioned earlier would still limit its ability to influence the conductivity of n-typematerial. Also in the year 2000, the assignments of VO and ZnI as the dominantdonors in as-grown ZnOwas further challenged by Van de Walles theoretical resultthat H is always a donor in ZnO, that it is easily ionized, and that it has a lowenough formation energy to be abundant; thus, Van de Walle suggested that it waslikely to be a dominant background donor in ZnO materials that were exposed toH during growth [51]. This proposal has been subjected to testing, because H-containing, high-quality, bulk ZnO, grown by a seeded chemical vapor transport(SCVT) technique, has been commercially available for the last few years [46]. Ingeneral, these tests have confirmed that a shallow donor due to H exists in SCVTZnO [5257] and can contribute significantly to its conductivity. This fact, coupledwith the theoretical evidence of high formation energies for the native donors hasled to a prevailing opinion that native donors do not play a significant role inthe conductivity of as-grown ZnO. However, we will challenge this conclusionDoping and Defects in ZnO 2305103.32 3.33 3.34 3.35 3.36 3.37 3.38(1) As-grown(2) AnnealedHTESAl,GaTESHD0XDefectD0XInD0XAl, GaD0XXA1010E (eV)PL intensity (arb. units) Figure 2.1: Photoluminscence at 4 K for a SCVT-grown ZnO sample, #1: (1) as-grown;and (2) annealed at 715C for 12 hr in flowing N2 gas.and show that native donors, especially ZnI, do indeed contribute significantly toconduction in ZnO, but as complexes, rather than isolated elements. It should bepointed out that there is really no conflict with the theoretical results of Kohan et al.and Van de Walle, because those theories deal with isolated defects, not complexes.Besides H, several other donor impurities are known to be important in ZnO.In particular, the Group III elements Al, Ga, and In, substitute easily for Zn, andeach can be incorporated to very high concentrations, >1020cm3[58,59]. In fact,photoluminescence (PL) spectral features assigned to Al, Ga, or In, are seen inalmost every ZnO sample (cf. Fig. 2.1), although of course, PL is not quantitative.The Group VII elements, in particular F and Cl, also are known to have donoractivity in ZnO [60]. However, there is not much electrical or optical evidencethat they are significant background dopants, so we will not discuss them in detailhere. It is also possible that the Group IV elements, such as C, Si, and Ge couldbe electrically active as either donors or acceptors. However, there is no strongevidence that any of these elements forms a shallow donor or acceptor [61], so wewill also not consider them here.Acceptor dopants are of great interest in ZnO, because the realization of high-conductivity p-type ZnO could lead to a viable p-n-junction UV LED, which couldhave an enormous impact on the solid-state white-lighting industry. In this regard,the Group I elements, substituting for Zn, and Group V elements, substituting forO, all must be considered. Interestingly, theory predicts shallow acceptor levelsfor both LiZn and NaZn [62,63], but neither dopant produces high-conductivity,p-type material [64]. The reason, also suggested by theory, is that Li and Na can beincorporated either as donors or as acceptors. In contrast, theory predicts relatively24 D. C. Lookdeep levels for PO, AsO, and SbO [62], but each of these dopants has been used toproduce good p-type ZnO [12,24,32,65]. Again, theory offers an explanation [66],which we will discuss. Among defect-type acceptors, the Zn vacancy is definitelypresent in much of the ZnO being grown today, and in fact is the dominant acceptorin some cases [67].Up to now, all of the defects mentioned, such as VZn, VO, or ZnI-X, are quitesmall and qualify as point defects. However, there are also much larger defects thatare present and important. For example, many ZnO samples contain line defects,such as threading dislocations; and surface defects, such as stacking faults. Wewill not consider such extended defects here, because their optical and electricalproperties are not well known as yet. Much more information on dislocations andstacking faults is available in the GaN literature, and it is likely that their respectivebehaviors in ZnO will be similar.The main experimental techniques discussed here will be T-Hall and low-temperature PL measurements. The reasons are: (1) these are the main charac-terization methods that have been applied to ZnO in the authors laboratory; and(2) they are the only common techniques that can deliver quantitative informationabout donor and acceptor concentrations and energies. Many of the impurities anddefects mentioned above have had fingerprints established, or at least suggested,by T-Hall and PL measurements. Other characterization techniques, such as trans-mission electron microscopy (TEM), x-ray diffraction (XPD), secondary-ion massspectroscopy (SIMS), and optical absorption spectroscopy (OAS) have also con-tributed much to our understanding of ZnO, and will be mentioned here as needed.However, detailed discussions of these and other techniques will have to be soughtin other chapters in this volume or in the literature.The establishment of T-Hall and PLfingerprints of impurity donors and acceptorshas benefited from doping experiments, and we will refer to some of those reportedin the literature. However, concentrations of impurities can often be changed byhigh-temperature annealing, and we will employ that technique here. For defect-related donors and acceptors, high-energy electron irradiation is by far the best wayto introduce such species, and we also make extensive use of this technique. Thus,our experimental program here involves irradiation and annealing steps, and T-Halland PL measurements after each step.2.2 Samples and apparatusThe samples discussed here are 5 5 0.5-mm squares cut from larger wafers thathad been sliced from (0001) boules grown by the seeded chemical vapor transport(SCVT) method at ZN Technology [68]. The SCVT technique produces very highDoping and Defects in ZnO 25quality material. Ohmic In dots were soldered on the corners of the samples, and vander Pauw Hall-effect measurements were performed with a LakeShore Model 7507apparatus, including a closed-cycle He cooling system operating from 15 to 320 K.From measurements of Hall coefficient R and conductivity , the Hall mobilityH=R and the Hall concentration nH=1/eR could be calculated at each tem-perature. The true carrier concentration n is related to nH by n =rnH, where r isthe so-called Hall r-factor [46,69]. The r-factor was calculated at each temperature,but typically was so close to unity that it was ignored in the plots. Thus, we presentonly nH and H data in the plots.Photoluminescence measurements were performed at 4.2 K. Excitation, disper-sion, and detection were accomplished, respectively, with a 45-mW HeCd laser,a 1.25-m spectrometer, and a photomultiplier detector. Resolution was better than0.01 meV in the spectral range important for this study.Electron irradiations were carried out at room temperature with a 2-MeV vande Graaff accelerator. A 1.0-MeV beam, of current about 2 A/cm2, was directedonto the (0001) Zn-face. Calculations show that a 1-MeV electron beam in the[0001] direction should produce O and Zn displacements at rates of about 0.2and 0.3 cm1, respectively [70]. Thus, e.g., a fluence of 3 1017cm2would beexpected to produce about 6 1016cm3O displacements, and 9 1016cm3Zndisplacements.2.3 Hall-effect theoryThe basic equations for Hall-effect analysis, allowing for the energy dependence ofthe electrons, are as follows [69]:jx = ne2m Ex necEx (2.1)RH = EyjxB = 1ne2

2 = rne (2.2)where electric E and magnetic B fields are imposed in the x and z directions,respectively, andn(E) =

0 n(E)E3/2 f0E dE

0 E3/2 f0E dE

0 n(E)E3/2eE/kTdE

0 E3/2eE/kTdE (2.3)26 D. C. Lookwhere the integration is over energy E. (Although these equations are written forelectron current, they hold equally well for hole current with the simple trans-formation n p and e e.) This formulation is called the relaxation-timeapproximation (RTA) to the Boltzmann Transport Equation (BTE). Here f0 is theFermi-Dirac distribution function and the second equality in Eqn. 2.3 holds for non-degenerate electrons, i.e., those describable by Boltzmann statistics. The relaxationtime, (E), depends upon how the electrons interact with the lattice vibrations aswell as with extrinsic elements, such as charged impurities and defects. For exam-ple, acoustical-mode lattice vibrations scatter electrons through the deformationpotential (ac) and piezoelectric potential (pe); optical-mode vibrations through thepolar potential (po); ionized impurities and defects through the screened coulombpotential (ii); and charged dislocations, also through the coulomb potential (dis).The strengths of these various scattering mechanisms depend upon certain latticeparameters, such as dielectric constants and deformation potentials, and extrinsicfactors, such as donor, acceptor, and dislocation concentrations, ND, NA, and Ndis,respectively. The total momentum scattering rate, or inverse relaxation time, is1(E) = 1ac (E) +1pe (E) +1po (E) +1ii (E) +1dis(E) (2.4)and this expression is then used to determine n(E) via Eqn. 2.3, and thence,H=e2/m*. Formulas for ac, pe, po, ii, and dis, can be found in theliterature [69,71]. Note that H, the Hall mobility, is related to c, the conductivitymobility (Eqn. 2.1), by H=rc.The fittingof H vs. T data, describedabove, should be carriedout in conjunctionwith the fitting of n vs. T, which is derived fromthe charge-balance equation (CBE):n +NA =

iNDi1 +n/Di(2.5)where the subscript i denotes a particular donor and whereDi = g0ig1ieDi/kN

CT3/2eED0i/kT(2.6)For each donor, g0/g1 is a degeneracy factor, N

C=2(2mn * k)3/2/h3is the effectiveconduction-band density of states at 1 K, h is Plancks constant, ED is the donorenergy, and ED0 and D are defined by ED=ED0DT. Eqn. 2.5 describes thesimplest type of charge balance, in which each of the one or more donors has onlyone charge-state transition within a fewkT of the Fermi energy. An example of suchDoping and Defects in ZnO 27a donor is Ga on a Zn site in ZnO. If there are double or triple donors, or more thanone acceptor, proper variations of Eqn. 2.5 can be found in the literature [69].For a p-type sample, the simple CBE becomesp +ND =

iNAi1 +n/Ai(2.7)where N

V =2(2mp * k)3/2/h3, A=(g1/g0)N

V exp(A/k)T3/2exp(EA0/kT), andEA=EA0AT, for each acceptor. Note that A has the term g1/g0, whichis inverted from the first term in Eqn. 2.6 because the degeneracies still refer toelectrons, not holes. From Eqn. 2.7, it can be shown that the hole concentration ina nondegenerate, single-donor/single-acceptor model, will be given by [69]p = 12 (A+ND)

1 + 4A(NAND)(A+ND)2

1/21

(2.8)where the various symbols were defined earlier.2.4 Multi-layer analysisThere is extensive recent evidence that the ZnO near-surface region can be highlyconductive, possibly due to H donors in this region [7274]. As we will showlater, the samples discussed in this work can be reasonably modeled as having twolayers: (1) a bulk layer, comprising most of the sample; and (2) a surface layer,much thinner than the bulk layer, but containing a high sheet concentration ofelectrons, high enough that they are degenerate (temperature-independent). Underthese conditions, the total mobility and carrier concentration can be written as [74]: = nb2b+ns2snbb+nss(2.9)n = (nbb+nss)2nb2b+ns2s(2.10)Here b denotes a bulk quantity, and s, a surface quantity. However, it must be notedthat we have written ns as a volume concentration, when in reality we can measureonly the sheet concentration ns. The true volume concentration of the near-surfaceelectrons would be ns/ds, but we dont know ds either, in general. Thus, to simplify28 D. C. Lookmatters, we normalize ns to the bulk thickness db; i.e., ns=ns/db. In this way, wecan plot nb and ns on the same axis; however, it must be remembered that thetrue volume concentration of the near-surface electrons is nsdb/ds, a much higherquantity than the plotted ns values presented later.2.5 Hydrogen and Group I impuritiesIt has long been known that H incorporates as a donor in ZnO, even in n-typeZnO [75,76]. This fact is somewhat surprising, because H is amphoteric in mostother semiconductor materials and thus should incorporate mainly as an acceptor inn-type ZnO. Van de Walle explained this paradox by showing theoretically that Hnever has a lower formation energy than H0or H+, so that the donor state is alwaysthe preferred state in thermodynamic equilibrium [51]. Subsequent experimentshave confirmed this assertion [5257], and in some cases H is even the dominantdonor. For example, in Fig. 2.1, the strongest photoluminescence (PL) line is one at3.36270 eV, arising froman exciton bound to interstitial H0[77]. (We will designatesuch a complex particle as a neutral donor-bound exciton, D0X.) Note also inFig. 2.1, and even Fig. 2.2, this line virtually disappears during a 12-hour annealat 715C in N2 gas, because a large share of the H is known to leave the sampleat this temperature [55,78]. We obtain further information about the H donor fromits two-electron satellite (TES) line at 3.32961 eV. A TES line occurs when thedonor is left in an excited (n =2) state upon the collapse of the exciton. From ahydrogenic model, the donor ground state (n =1) energy can then be obtained fromED=4/3(ED0XETES), giving 44 meV for the H donor energy.05103.32 3.33 3.34 3.35 3.36 3.37 3.38(2) Annealed(3) Irradiated1010E (eV)PL intensity (arb. units) IrradD0XIrradTESFigure 2.2: Photoluminscence at 4 K for sample #1: (2) annealed at 715C for 12 hr inflowing N2 gas; and (3) irradiated with 1-MeV electrons to a fluence of 3 1017cm2.Doping and Defects in ZnO 29To determine the concentration of the H donor, we turn to T-Hall measurements,presented in Figs 2.32.6. The solid lines in these figures are theoretical fits usinga three-donor model (cf. Eqn. 2.5, above), with the energies of the three donorschosen as 30, 44, and 75 meV. Although we cannot claim complete uniqueness forthis choice of energies, they are justified fromthe following considerations: (1) theyproduce excellent fits of n vs. T, in most cases; and (2) the 30 and 44 meV valuesare computed from PL TES lines. The 75 meV choice does not have an obvious0500 1000 1500 2000 0 100 200 300As-grownAs-grown: bulk onlyAnnealed 800CAnnealed 800C: bulk only T (K) (cm2/V-s)Figure 2.3: Mobility vs. temperature for a SCVT-grown ZnO sample, #2, as-grown, andannealed at 800C for 12 hr in flowing N2 gas. The solid lines are theoretical fits to the data,and the dashed lines are the predicted mobilities in the absence of surface conduction.1012 10131014 1015 10160 20 40As-grownAs-grown: bulk onlyAnnealed 800C Annealed 800C: bulk only 103/T (K1)n (cm3)Figure 2.4: Carrier concentration vs. inverse temperature for #2, as-grown, and annealed at800C. The solid lines are theoretical fits to the data, and the dashed lines are the predictedcarrier concentrations in the absence of surface conduction.30 D. C. Look05001000150020000 100 200 300(1) As-grown(2) Annealed 800C(3) Irradiated; (4) Annealed 450CT (K) (cm2/V-s)Figure 2.5: Mobility vs. temperature for sample #2: (1) as-grown; (2) annealed at 800C;(3) irradiated with 1 MeVelectrons; and (4) annealed at 450C. The solid lines are theoreticalfits to the data.1014101510160 20 40(1) As-grown(2) Annealed 800C(3) Irradiated; (4) Annealed 450C n (cm3)103/T (K1)Figure 2.6: Carrier concentration vs. inverse temperature for sample #2: (1) as-grown;(2) annealed at 800C; (3) irradiated with 1 MeV electrons; and (4) annealed at 450C. Thesolid lines are theoretical fits to the data.TES origin; instead, there are lines near 3.32 eV, which have been assigned to theTES lines of Al and Ga [77], and which would then give about 55 meV for ED,Aland ED,Ga. Unfortunately, the fits seem to be better with 75 than 55 meV, so thereis some uncertainty connected with the origin of the 75 meVdonor. However, mostof our evidence suggests that it is indeed associated with Al and/or Ga.Before going on, we note that the solid lines in Figs 2.3 and 2.4 are the fitsto the actual experimental data, including the contributions of surface conduction,represented in Eqns 2.9 and 2.10 by parameters s and ns. On the other hand, theDoping and Defects in ZnO 31Table 2.1: Donors and acceptor concentrations in sample #1 after various treatments. Allanneal times were 12 hr. The 400Canneal was preceeded by anneals at 250, 300, and 350C.The 1-MeV-irradiation fluence was 3 1017cm2. The concentration unit is 1016cm3.Treatment ND1 (30 meV) ND2 (44 meV) ND3 (75 meV) NAAs-grown 0.45 3.0 2.0 0.13715C anneal 0.74 0.1 3.2 0.71-MeV irrad. 20 0.1 0.13 20400C anneal 1.35 0.1 0.4 1.2Table 2.2: Donors and acceptor concentrations in sample #2 after various treatments.The 800C anneal was preceeded by anneals at 500, 600, and 700C. The 450C annealwas preceeded by anneals at 300, 350, and 400C. The 1-MeV-irradiation fluence was3 1017cm2. The concentration unit is 1016cm3.Treatment ND1 (30 meV) ND2 (44 meV) ND3 (75 meV) NAAs-grown 0.35 1.9 3.1 0.13800C anneal 0.16 0.1 2.7 0.191-MeV irrad. 30 0.1 30450C anneal 0.26 0.1 0.27 0.27500C anneal 0.22 0.1 0.70 0.23dashed lines are the predictions of what the solid curves would look like in theabsence of surface conduction, i.e., with ns set to 0. It is clear that the surfaceconduction produces large changes in the mobility and carrier concentration curvesat lowtemperatures. This phenomenon has been investigated in detail only recently,and probably arises from H adsorbing on the surface from the ambient, or from Hdiffusing to the surface from the interior of the sample [74]. In any case, it is easyto include the effects of surface conduction in the analysis (Eqns 2.9 and 2.10),because the values of s and ns come directly from the low-temperature and ndata, respectively.Returning to the 44-meV donor concentration, the T-Hall fitting parameters aresummarized in Tables 2.1 and 2.2. Here it is seen that this concentration dropsprecipitously as a result of 800 or 715C anneals, and the same happens to the3.36270-eVD0Xand 3.32961-eVTESlines. Coupled with the studies of Hdiffusionand effusion as a function of temperature, it is entirely reasonable to assign theseT-Hall and PL fingerprints to H.32 D. C. LookAnother valence-1 element of high importance is Li. Although theory predictsthat Li on the Ga site LiGa should be a relatively shallow acceptor [62,63], itturns out that doping with Li almost always produces semi-insulating (SI) material[17,64]. The reason evidently involves the fact that the Li interstitial LiI has a lowerformation energy than LiZn in p-type ZnO, and LiI is a donor. Thus, a sample withhigh concentrations of Li would attain a balance between LiGa and LiI and theFermi level would end up somewhere close to midgap, producing SI material. Itshould be noted that Li also produces a deep state in the gap, as determined by EPRanalysis [79], but this state may arise froma complex [63]. The same considerationsevidently hold for Na, also, although not nearly as much experimental work hasbeen carried out for Na.2.6 Group II elementsThe group II elements Mg and Cd form solid solutions with Zn, and give a poten-tial range of bandgaps from about 2 eV to 8 eV. Heterostructures of ZnO withMgZnO and CdZnO have already been demonstrated, and will undoubtedly beimportant in future optical and electronic devices based on ZnO. In fact, this subjectis emphasized in Chapter 3, and so will not be discussed further here.2.7 Group III elementsThe Group III elements Al, Ga, and In, are well-known donor dopants in ZnO, andeach can produce carrier concentrations in the >1020cm3range [58,59]. In fact,these types of ZnO produce some of the highest-conductivity transparent materialavailable today. However, as background impurities, their roles are somewhat ofa mystery. From doping experiments, the D0X lines at 3.36042, 3.35964, and3.35664 have been assigned to GaZn, AlZn, and InZn, respectively, as seen in Fig.2.1 [77]. Indeed, as noted in Tables 2.1 and 2.2, they are the dominant D0X linesin sample #1, and have intensities second only to that of H in the unannealedversion of sample #2. However, analytical measurements, such as secondary-ionmass spectroscopy (SIMS), and glow-discharge mass spectroscopy (GDMS), oftengive much lower concentrations of these elements than that of the 75-meV donorfound fromT-Hall measurements [80]. In fact, after annealing, the 75-meVdonor isalways dominant, so either the 75-meV donor is not related to Group-III elements,or the analytical measurements are inaccurate. This latter solution is most likely,because concentrations in the low-1016-cm3range are not easy to measure byDoping and Defects in ZnO 33either SIMS or GDMS. More studies on this matter will have to be carried out, butour belief is that the 75 meV donor is indeed associated with Group-III impurities.2.8 Group IV elementsVery little is known of the electrical and optical activity of the Group-IV elements,such as C and Si. It is difficult to get accurate concentrations of C from massspectroscopic techniques because of CO in the spectrometer background and alsobecause of surface contamination from polishing and etching processes. Accordingto density functional theory (DFT), the split-interstitial complex (CO)O, should be adonor [81]; however, its energy is questionable because of the crude DFT treatmentof electron correlation, known as the local density approximation (LDA). If Nis alsopresent, then the complex (NC)O is expected to be a shallower donor. With respectto Si, this element is often present at the 1 1016-cm3level in SCVT-grown ZnO,according to GDMS analysis [80]; however, there is no indication that Si producesshallow levels in the bandgap.2.9 Group V elementsThis group, comprising N, P, As, and Sb, are extremely important for the realizationof p-type ZnO. A glance at the periodic table would suggest that N would make agood acceptor dopant, because it has an electronic core structure and ionic radiussimilar to those of O and so should readily substitute for O. Indeed, this is the case,with concentrations of 1020cm3having been measured in many N-doped ZnOsamples. In our SCVT samples, SIMS measurements show an N concentration ofabout 1017cm3[17], and EPR measurements find that it it primarily goes ontothe O site, as an acceptor [82,83]. Interestingly, however, the total, active acceptorconcentration in as-grown SCVT samples is only in the low 1015-cm3range [46],leading to a possible discrepancy. Evidently, either (1) the SIMS measurements arewrong, or (2) most of the N is passivated. Indeed, there is direct evidence that atleast some of the N in SCVT ZnO is passivated with H, because H-N vibrationalmodes have been seen in the IR absorption spectra; furthermore, other evidencesuggests that most of the H in hydrothermally-grown ZnO is associated with thepassivation of acceptors (such as NO), rather than the creation of shallow donors[55,84,85].Many different growth techniques have been used to produce N-dopedZnO, including chemical vapour deposition (CVD), pulsed laser deposition(PLD), molecular-beam epitaxy (MBE), metal-organic chemical vapour deposition34 D. C. Look(MOCVD), and rf or dc sputtering [86]. Recently, a MBE scheme using temper-ature (T) modulation has been employed to produce good p-type material, whichhas been used to fabricate homoepitaxial light emitting diodes (LEDs) [6]. In thisT-modulation method, a thin layer of N-rich ZnO is grown at low temperature, sothat the N will incorporate more readily, and then a thin undoped layer is grownat higher temperatures, so that the crystal quality will be better. This process isrepeated until the full layer is grown. There are reports that this growth method isbeing commercialised in Japan, and only time will tell if it is the preferred way toproduce UV LEDs.Several other attempts to dope ZnO with N have also produced p-type material,but in some cases the p-type nature was weak, and seemed to be unstable andsusceptible to type-change under light. In this connection, great care has to beexercised when measuring the present generation of p-type ZnO samples [86]. Thehole mobilities are small, so that the Hall voltages tend to be small; also, the contactsin p-type ZnO can be noisy, sometimes leading to n-type spikes, as the contactconfigurations are switched during the van der Pauw analysis. Also, even roomlight can sometimes produce persistent electron-dominated photoconductivity, andrender the sample n-type, because the electron mobility is so much larger thanthe hole mobility. These effects are a nuisance, but they are not the same as an actuallattice instability, in which the sample permanently goes from p-type to n-type intime. This latter phenomenon is very rare, in our experience, although it may occurin some cases [42].Conventional wisdom suggests that if it is difficult to make p-type ZnO by Ndoping, then it should be nearly impossible to do so by P, As, or Sb doping. However,the exact opposite is true; that is, good p-type material has been made using allthree dopants [12,24,32,65]. The conventional wisdom is based on acceptor energyand solubility, which are functions of ionic size. Of the Group V elements, NOshould have the smallest acceptor transition energy and highest solubility, and PO,AsO, and SbO, should be deeper and more difficult to incorporate. To explain theexperimental disagreement with this scenario, Limpijumnong et al. examined whatwould happen if As actually went on the Zn site, where it would fit better, rather thanthe O site. Using DFT, they indeed found that a complex, AsZn-2VZn, has a ratherlow formation energy (high solubility), and a relatively shallow acceptor transitionenergy [66]. Two recent studies are consistent with this finding: (1) implanted Astends mainlytogoonthe Znsite [87]; and(2)As-dopedZnOcontains highquantitiesof Zn vacancies [88]. These results dont prove the AsZn-2VZn conjecture, but dolend credence to it.Photoluminescence and T-Hall results on p-type ZnOlayers are presented in Figs2.7 and 2.8, respectively. In Fig. 2.7, a N-doped layer grown by MBE, and P- andAs-doped layers grown by rf sputtering, are compared with a bulk, n-type SCVTDoping and Defects in ZnO 350500 1000 1500 3.30 3.32 3.34 3.36 3.38 Pl Intensity (arb. units)E (eV)3.311 eV3.367 eVZnO:NZnO:As (5)ZnO:P (10)Undoped3.357 eVFigure 2.7: Acomparison of photoluminscence at 4 K for a SCVT-grown, undoped, n-typeZnO sample; a MBE-grown, N-doped, p-type ZnO layer; a rf-sputtered, P-doped, p-typeZnO layer; and a rf-sputtered, As-doped, p-type ZnO layer.12510 20 500 100 200 300ppp (cm2/V-s), p (1017 cm3)T (K)TheoryFigure 2.8: Mobility and carrier concentration vs. temperature for a rf-sputtered, As-doped,p-type ZnO layer.sample. It is seen that PL lines at 3.31, 3.357, and 3.367 eV are relatively strongin all of the p-type materials. The 3.31-eV line has been variously conjecturedto arise from acceptor-bound excitons, donor-acceptor pairs, or isolated acceptors(e+A0A), but at this point, none of these models is certain. The 3.357-eVline is often attributed to In, but another, broader line often appears in the samevicinity after 800C annealing [89]. The 3.367-eV line has been attributed to anionized-donor-bound exciton [77], which indeed might be reasonable in p-typematerial. But at this stage, none of these lines has been unambiguously identified.The T-Hall results in Fig. 2.8 involve theAs-doped sample shown in Fig. 2.7 [32].The 300 K mobility is about 5 cm2/V-s, a good value for present-day p-type ZnO,36 D. C. Lookand close to what is also usually found in p-type GaN. Asimple, one-band fit to thehole-mobility data gives donor and acceptor concentrations in the high 1019cm3range, close to the As concentration found by SIMS. These results are consistentwith the p-type nature being due to As acceptors, whether AsO, or AsZn-2VZn. Ofcourse, some possible secondary phases, such as As metal or Zn3As2, can also bep-type, and must be considered in the analysis of a sample such as this one. NeitherAs metal nor Zn3As2 can explain the data of Fig. 2.8, but potential secondary phasesmust always be in mind when doping with P, As, or Sb.2.10 Group VI elementsAlthough some of the Zn-VI ternary compounds, such as ZnSxSe1x, have beenstudied in great detail, only a fewreports exist on compounds of the type ZnVIxO1x[90]. Thus, we will not review mixtures of ZnO with Group VI elements.2.11 Group VII elementsThe elements F, Cl, Br, andI shouldsubstitute for Oandact as donors inZnO. Indeed,studies have found that Fdoping greatly decreases the resistivity in ZnOfilms grownby chemical spray techniques [60,91]. However, the Group III elements are pre-ferred as donor dopants, and they also seem to be more prevalent as backgroundimpurities than the Group VI elements. In any case, there is little recent literatureon Group-VII doping, and we will not consider this class of elements further.2.12 Donor-type point defectsAs mentioned earlier, the prevalent n-type nature of ZnO has usually, in the past,been attributed to native defect donors, either the Zn interstitial ZnI or the OvacancyVO. Nowadays it is known that H, along with the Group III elements Al and Ga, arealso background donors that can contribute significantly to the conductivity of as-grown ZnO, and we have discussed their optical and electrical fingerprints above.To study the point-defect-related donors, the best way is to create them by high-energy electron irradiation [50,92], and in Fig. 2.2 we present PL data on sample#1, grown by the SCVT method and irradiated with 1 MeV electrons to a fluenceof 3 1017cm2. A new, sharp D0X line pops up at 3.3607 eV, and it also hasTES lines in the region of 3.338 eV. Thus, the defect donor energy is approximatelyED=4/3(ED0XETES) =30 meV. In a separate work, we argue that these lines areDoping and Defects in ZnO 37associated with a ZnI complex, probably ZnI-NO [93]. The changes in mobility andcarrier concentration due to irradiation are shown in Figs 2.5 and 2.6, respectively,and the fits to these curves give the donor and acceptor concentrations shown inTables 2.1 and 2.2. The main results of irradiation are an increase in the 30-meVdonor, and decrease in the 75-meV donor. We believe that the increase in the 30-meVdonor is due to the creation of Zn interstitials, and the decrease of the 75-meVdonor may be due to the complexing of O interstitials (acceptor-like defects) withGroup-III donors. However, this model will have to be tested further.The O vacancy produces a deep donor state, according to theory [4749]. Inagreement, EPR experiments have seen deep states associated with VO [94]. Onthe other hand, positron annihilation spectroscopy (PAS) suggests that a shallow(100 meV) VO-related state may also exist [95]. Thus, more research on thismatter is needed.Besides ZnI and VO, the antisite ZnO should also be a donor, although there issome disagreement among theoreticians on whether it is shallow or deep [4749].In any case, right now there is little strong evidence for any optical or electricalactivity caused by ZnO.2.13 Acceptor-type point defectsAccording to theory, the O interstitial OI and Zn vacancy VZn should behave asacceptors [4749], and VZn, at least, should be prevalent in n-type ZnO, especiallyin material created under O-rich conditions. Experimentally, little is known aboutOI, which can exist in both tetrahedral and octahedral positions. However, muchis known about VZn, because as a negatively charged vacancy, it can easily trappositrons. Here again, electron irradiation has been used to create a large con-centration of VZn and thereby facilitate the establishment of its PAS fingerprints.Then, by comparing the PAS signals with acceptor concentrations, as determinedby T-Hall measurements, it can be shown that the small acceptor concentration of12 1015cm3in as-grown SCVT ZnO (cf. Tables 2.1 and 2.2) can be entirelyexplained by VZn-related defects [67]. As mentioned before, there are also NO-related acceptors in SCVT ZnO, but they are likely neutralized by H unless thesample has been subjected to annealing or irradiation [55,84,85].2.14 SummaryTemperature-dependent Hall-effect (T-Hall) and low-temperature photolumines-cence (PL) measurements have been carried out on as-grown, annealed, and38 D. C. Lookelectron-irradiated ZnO samples, and have yielded the energies and concentrationsof several donors and acceptors. Donors which have been identified by T-Hall, PL,andother techniques, include interstitial H, substitutional Al andGa, a Zn-interstitialcomplex, and the O vacancy. For the acceptors, substitutional N, P, As, and Sb, andthe Zn vacancy seem to be the most important species. Although ZnO research andtechnology has expanded greatly in the last few years, a major problem is still thedevelopment of a reliable means of producing high-conductivity p-type material.A critical aspect of this quest is to understand how P, As, and Sb form acceptorsin ZnO, and, in particular, which lattice site each occupies. Future research willundoubtedly address these and other issues.AcknowledgmentsWe wish to thank T.A. Cooper and W. Rice for technical assistance, and B.B.Claflin for helpful discussions. Support was provided by US Air Force ContractF33615-00-C-5402, and subcontracts with Structured Materials Industries, Inc., andSVTA, Inc. 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