physics and control of valence states in zno by codoping method

Physica B 302–303 (2001) 155–162

Physics and control of valence states in ZnOby codoping method

Tetsuya Yamamotoa,*, Hiroshi Katayama-Yoshidab

aDepartment of Electronic and Photonic Systems Engineering, Kochi University of Technology, 185 Miyanokuchi,

Tosayamada-cho, Kamigun, Kochi 782-8502, JapanbDepartment of Condensed Matter Physics, The Institute of Scientific and Industrial Research, Osaka University,

8-1 Mihogaoka, Osaka 567-0047, Japan

Abstract

We investigate unipolarity in ZnO based on ab initio electronic band structure calculations. We find that p-typedoping using Li, N or As species causes an increase in Madelung energy, n-type doping using B, Al, Ga, In or F speciesgives rise to a decrease in Madelung energy. In order to solve the unipolarity, to fabricate p-type ZnO, we propose a

codoping method using acceptors and reactive donors simultaneously. A codopant pair including Ga-reactive donorsand N-acceptors is eminently suitable for use in the codoping. For Li-acceptors, F species is a good candidate forreactive donors. For As-acceptors, Ga species is a preferable codopant.# 2001 Elsevier Science B.V. All rights reserved.

PACS: 68.44.L

Keywords: ZnO; Codoping; Ab initio electronic band structure calculations

1. Introduction

Wurtzitic zinc oxide (ZnO) is a wide-band gap(3.437 eV at 2K) semiconductor which has manyapplications, such as piezoelectric transducers,varistors, highly optically transparent conductinglayers in place of expensive Sn-doped In2O3 [1] indisplays and photovoltaic devices, especially forthe transparent n-type window layer of CuInSe2(CuInS2)-based-thin-film solar cells [2]. Most ofthese applications require only polycrystalline.

Very recently, ZnO which has been producedin single crystals has also attracted attentionbecause of its possible application in short-wavelength light-emitting devices [3,4]. The mainadvantage of ZnO as the light emitter are its largeexciton binding energy (60meV) even at roomtemperature (25meV). In order to develop suchoptoelectronic devices such as efficient electrically-pumped lazing, one important issue that should beresolved is the fabrication of low-resistivity p-typeZnO.ZnO exhibits unipolarity, or asymmetry in its

ability to be doped n-type or p-type. ZnO isnaturally an n-type semiconductor because of adeviation from stoichiometry due to the presenceof intrinsic defects such as Zn interstitials (Zni)

*Corresponding author. Tel.: +81-887-57-2112; fax: +81-

887-57-2120.

E-mail address: [email protected]

(T. Yamamoto).

0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 2 1 - 5

and O vacancies (VO). In general, the case of metalexcess due to interstitial cations is more commonthan that of metal excess due to anion vacancies.We have found that while the Zni causes anincrease in the lattice energy, Madelung energy,the VO gives rise to a substantial decrease from abinitio electronic band structure calculations: Thepresence of Zni results in the instabilization of theionic charge distribution in ZnO. Considering thatthe wurtzite structure tends to occur in semicon-ductors of great polarity, Frenkel defects, espe-cially nearest neighbor Frenkel pairs such Znvacancy}Zni, are not stable. At this moment, wehave little information of the dominant nativedonor. Under the assumption that the dominantnative donor is the Zni or Zni-related defect, theabove findings and relatively high ionizationenergy of Zn with a completed ns2 subshell suggestthat n-type doping using extrinsic donors is moreeffective in realizing stable n-type ZnO with highelectron carrier densities. In fact, several authorsreported the fabrication of low-resistivity n-typeZnO using group III elements [5–10], Si, a Zn-substituting species [11], or a group VII element,F [12].ZnO has proven to be difficult to dope as p-type:

ZnO has unipolarity. The problem of p-typedoping in ZnO can arise for several reasons suchas high activation energy of acceptors, lowsolubility of acceptor dopants and inducing self-compensating process on doping as well as theother wide-band gap semiconductors. For p-typedoping, few studies have been reported. Recently,we have been proposing materials design of thecodoping method for the realization of low-resistivity p-type wide-band gap semiconductorssuch as GaN [13–15], ZnSe [16,17] and AlN [18].For p-type ZnO, we reported that three pairsincluding (N and Ga), (N and Al) [19] and (Li andF) [20,21] are suitable candidates for use in thecodoping. Based on the analysis on a change in thelattice energy and electronic structures by thecodoping, we find that two pairs, (N and Ga) and(N, Al) are more effective.Very recently, Osaka’s group has succeeded in

fabricating low-resistivity p-type ZnO by thecodoping method using N as acceptors and Gaas reactive codopants for the first time in the world

[22]. They observed a room temperature resistivityof 2O cm and a hole concentration of 4�1019 cm�3. Their method using N2O gas throughan ECR plasma source for N doping and Ga2O3for Ga codoping are found to be very good inreproducibility [23]. On the other hand, Minegishiet al. succeeded in realizing p-type ZnO bychemical vapor deposition, by using NH3 andexcess Zn, where they found the resistivity of100O cm and lower carrier concentration(� 1� 1016 cm�3) [24]. In addition, p-type ZnOincorporated with As and Ga, which was depos-ited on (0 0 1)GaAs substrate, was reported [25].In this paper, first, we discuss the unipolarity of

ZnO and the codoping method in more detail.Then we study the effects of the codoping usingcodopant pairs except for the pair, N and Ga.

2. Methodology

The results of our band structure calculationsfor ZnO crystals were based on the local-densityapproximation (LDA) treatment of electronicexchange and correlation [26–28] and on theaugmented spherical wave (ASW) formalism forthe solution of effective single-particle equations[29]. For the calculations, the atomic sphereapproximation (ASA) with a correction term wasadopted. For undoped ZnO crystals, Brillouinzone integration was carried out for 84-k points inan irreducible wedge and for 24-k points for dopedand codoped ZnO crystals. For valence electrons,we employed outermost s, p and d orbitals for Znand As atoms and s and p orbitals for the otheratoms. The Madelung energy, which reflects long-range electrostatic interaction in the system, wasassumed to be restricted to a sum over themonopoles.We studied the crystal structures of doped and

codoped ZnO with periodic boundary conditionsby generating supercells that contain the object ofinterest. (1) For n-type ZnO doped with group IIIelements (III=Al, Ga or In), a Zn-substitutingspecies, we replace one of the 16 sites of Zn atomsby a donor site in model supercells, as shownFig. 1. (2) For n-type ZnO doped with a group VIIelement, F species, an O-substituting one, we

T. Yamamoto, H. Katayama-Yoshida / Physica B 302–303 (2001) 155–162156

replace one of the 16 sites of O atoms by a donorsite in model supercells. (3) For p-type ZnO dopedwith N or As alone, we replace one of the 16 sitesof O atoms by an N- or As-acceptor site. (4) For p-type ZnO doped with Li, we replace one of the 16sites of Zn atoms by a Li-acceptor site. (5) For p-type ZnO codoped with the reactive donors usingthe group III elements and 2N, (ZnO:(III, 2N)), wereplace two of the 16 sites of the O atoms by the Natom sites and one of the 16 sites of the Zn atomsby the donor site. (6) For p-type ZnO codopedwith the reactive donors using the F species and2Li, (ZnO:(F, 2Li)), we replace two of the 16 sitesof the Zn atoms by the Li atom sites and one of the16 sites of the O atoms by the donor site. (7) ForZn-rich ZnO, we determine the crystal structure ofZnO doped with Zn interstitials by minimizing thetotal energy.In this work, for codoped ZnO crystals, we

decided the crystal structures under the conditionthat the total energy is minimized from all atomicconfigurations. We neglect the effects of relaxationdue to dopants whose covalent radii are similar tothose of substituted atoms on the lattice constantsand displacement of atoms in the vicinity of thedopants because the magnitude of change inthe total energy due to the relaxation, almost onthe order of 102meV, is smaller than those of theMadelung energy calculated below.

3. Unipolarity and codoping method

3.1. Unipolarity

Wide-band-gap semiconductors except for dia-mond have large polarity. In general, theredecreases cohesive energy per bond, Ecoh, with anincrease in the ionicity in chemical bonds incompound semiconductors:Experimental data ofthe Ecoh of AlN, GaN, and ZnO are 2.88, 2.24 and1.89 eV, respectively. We focus on the latticeenergy, Madelung energy, as an important para-meter on the stabilization of the ionic chargedistributions in n- or p-type doped semiconductorsin order to investigate unipolarity of wide-band-gap materials. In previous work, we established itfor GaN [13–15] and ZnSe [16,17].We summarize the calculated differences in the

Madelung energy between undoped and p-typeusing group V elements (=V and As) or a group Ielement, Li species, n-type using group IIIelements (=B, Al Ga and In) or a group VIIelement, F species, doped ZnO in Table 1.Table 1 shows that n-type doping causes a

decrease in the Madelung energy while p-typedoping gives rise to an increase in the Madelungenergy. From the analysis of electronic structure ofn-type doped ZnO, we find that a weight of pstates at O sites in the vicinity of donor sites shiftstowards lower energy regions. The same shift isfound also for n-type GaN doped with Si or O[13–15]. It should be noted that n-type doping inZnO causes the stabilization of the ionic chargedistribution in the material, especially, at O sites.For the group III elements (where a p electron

is being removed: the electron configuration is

Fig. 1. Crystal structure of a supercell for ZnO doped with

group III elements (III=B, Al, Ga and In).

Table 1

Calculated differences in Madelung energy between undoped

and n- or p-type doped ZnOa

n-type doping p-type doping

A1 �6.44 N +0.79

Ga �13.72 Li +13.56

In �9.73 As +12.61

F �1.86

aUnits: eV.

T. Yamamoto, H. Katayama-Yoshida / Physica B 302–303 (2001) 155–162 157

ns2np), the main advantage is lower ionizationenergy than that for the adjacent group II element,Zn with a completed ns2 subshell. From the site-decomposed density of states for ZnO :Al, we findsubstantial delocalized s states near the bottom ofthe conduction band. In such a case, we find veryweak repulsive interactions between the Al donorsin the supercells. It results in the stabilization ofthe Al donors in n-type ZnO crystal. Then the Alspecies is eminently suitable for a practical use inn-type doping. On the other hand, for the Gadonors, we find slight localization of its impuritystates compared with that for Al-doped ZnO. This,however, probably gives little effects on thesolubility. Considering a remarkable decrease inthe Madelung energy, the Ga species is a prefer-able element for donors in order to fabricate stablen-type ZnO.For p-type doping, we find an increase in the

Madelung energy for ZnO doped with N (ZnO :N)and a substantial increase in the Madelung energyfor both ZnO :Li and ZnO :As. For Li doping,high solubility is reported by Onodera et al. [30]We show the total and site-decomposed density ofstates (DOS) for undoped as a standard reference,ZnO :N, ZnO :Li and ZnO :As in Fig. 2. The O 2 sstates are included in the calculation as valencestates, but those located around –18 eV areomitted in Fig. 2 because such electrons form anarrow band and have little interaction with otherstates. Energy is measured relative to the Fermilevel (EF).In Fig. 2(a), we observe two groups in

the valence band. (1) From �6.5 eV to �4 eV,there are bands with a strong d characteroriginating mostly from d states at Zn sites.(2) The upper valence band located aboveapproximately �4.0 eV originates mainly fromthe O 2p states. The lowest conduction bandshave a strong Zn 4 s contribution; there arecharge transfers from Zn 4 s to O 2p due to themixing between the s and p states at O sites, andthe s and p states of the surrounding Zn shifts thecenter of gravity of the local DOS at the O sitestowards lower-energy regions. This means thatthe chemical bonds in ZnO with a wurtzitestructure have an ionic rather than a covalentcharacter.

Fig. 2. (a) Total density of states (DOS) for undoped ZnO,

(b) total DOS and (c) Li-site-decomposed DOS for ZnO : Li,

(d) total DOS and (e) N-site-decomposed DOS for ZnO :N and

(f) total DOS and (g) As-site-decomposed DOS for ZnO :As.


From Figs. 2(a) and (b), we find that forZnO :Li, we find almost same shape of the totaldensity of states in the valence band as that forundoped ZnO. It means that the impurity statesintroduced by Li doping as p-type doping are verydelocalized, suggesting that the Li-acceptor levelsin the band gap are very shallow. On the otherhand, as shown in Figs. 2(a), (d) and (f), we findlocalized impurity states near the top of valenceband for both ZnO :N and ZnO :As. From theanalysis of Figs. 2(e) and (g), the main contribu-tions to the impurity states are p states at the sitesof the acceptors, N and As. For ZnO :As,especially, we find substantial localized impuritystates near the top of the valence band caused bythe strong repulsive potential. From the abovefindings concerning the electronic structures forZnO :N and ZnO :As, we predict that theiracceptor levels in the band gap are deeper thanthat for ZnO :Li.For Li doping as p-type, we have a crucial

problem to be solved from Table 1, as pointed outin our previous work [20,21]: An increase in theMadelung energy caused the formation of Ovacancy, which exhibit donors, near the Li sites,as shown in Fig. 3 [20]. It suggests a drastic changein the lattice constants without the conduction-type conversion from n- to p-type, for ZnO heavilydoped with Li alone.

For p-type doping using N or As species, wecontrol the valence states in order to realize notonly the delocalization of their impurity states, orshallow acceptor level, but also a decrease in theMadelung energy.

3.2. Codoping method

We summarize the significant physics of ourcodoping method using acceptors and donors asreactive dopants simultaneously for the fabrica-tion of low-resistivity of p-type wide-band-gapsemiconductors. We find that the codoping meth-od causes the formation of the complexes includ-ing acceptors and donors in the crystals, andcontributes (i) to reduce the Madelung energiesand enhance the incorporation of acceptorsbecause the strong acceptor–donor attractiveinteraction overcomes the repulsive interactionsbetween the acceptors, and (ii) to lower the energylevels of the acceptors and raise them of the donorsin the band gap due to the strong interactionbetween the acceptors and donors with forming anacceptor–donor–acceptor complex in Fig. 4, and(iii) to increase the carrier mobility due to theshort-range dipole-like scattering mechanism(long-range Coulomb scattering one is dominatedin the case of doping of acceptors alone). Thus, thep-type codoped semiconductors exhibit low-resis-tivity with high carrier density and high mobility.Very recently, a successful experiment on the

fabrication of low-resistivity p-type ZnO using the

Fig. 3. Crystal structure of ZnO with Li and O-vacancy (VO).

Fig. 4. A change in acceptor and donor levels in the band gap

due to the strong interactions between the acceptors and

reactive donors.


codoping method has been reported, conducted byOsaka group [22]. They used N as acceptors andGa as reactive donors. They observed the resistiv-ity of 2O cm and a hole concentration of 4�1019 cm�3 at a room temperature. In the next sec-tion, we study two cases by codoping the otherpairs.

4. Application of codoping

4.1. Case I: Codoping of Zn interstials and N

Let us consider the effects of Zni on theincorporation of N and a change in the Madelungenergy in ZnO. In this work, we assume that theexcess Zn incorporated would form interstitial Zn(Zni) in ZnO crystals. The reduction of the metalions, Zn, leaves extra electrons in ZnO. In otherwords, the Zn locates as lower-valent ions ininterstitial positions: ZnO :Zni behaves n-type.From this viewpoint, the doping method used inthe report by Minegishi et al. [24] corresponds tothe codoping method using acceptor and donorsimultaneously.We show the crystal structure of ZnO :

Zni, which was determined using ab initio electro-nic band structure calculations under the

condition of the minimization of the total energyin Fig. 5.From the calculations, we find that the inter-

stitial Zn is surrounded by three oxygen atoms asthe first neighbors with the distance of 2.005 (Abetween the Zni and the O sites, being almost samevalue as that between Zn and O in the hostmaterial, ZnO. It should be noted that the Zni bydoping of excess Zn causes a remarkable increasein the Madelung energy of 11.38 eV, which is anopposite change in the Madelung energy due to theother n-type doping. It suggests that for n-typeZnO, the dominant native donor is not Zni but aZni-related defect. We study the effects of only Znion the incorporation of N acceptors into ZnO forthe simplicity below.Then, we show the crystal structure of ZnO

codoped with Zni and 2N (ZnO:(2N, Zni) inFig. 6(a), which is determined using ab initioelectronic band structure calculations by minimiz-ing the total energy. For a standard reference, weappend the crystal structure of ZnO : 2N inFig. 6(b), which is determined by minimizing thetotal energy. For Zni-free ZnO in Fig. 6(b), we findstrong repulsive interactions between the two Nacceptors. As a result, we find a most distantdistance between them in the supercell modelstructure. On the other hand, Fig. 6(a) forZnO:(2N, Zni) shows that the formation of thecomplex including N–Zni–N, which occupy near-est-neighbor sites, is energetically favorable. Itindicates that the growth technique using excessZn enhances the incorporation of N speciesinto ZnO crystals. Considering that the abovecomplex behaves not acceptor but neutral, wemust control a partial pressure of N gas in such away that the complex including N–Zn–N–N– or a‘‘chain-like’’ complex, such as N–Zn–N. . .N,which occupy more distant sites from the rest ofthe complex will be formed. In such a case,codoping using N and excess Zn as reactive donorcodopant may cause a problem of reproducibility.The reason is that the calculations show a 13.33 eVincrease in the Madelung energy for intrinsicZnO with the complex, N–Zni–N, compared withthat for undoped ZnO with stoichiometry, incontrast with p-type ZnO codoped with 2N andGa [19].

Fig. 5. Crystal structure of ZnO doped with Zn interstitials

(Zni).


4.2. Case II: codoping of Ga and As

Very recently, Ryu et al., reported synthesis ofp-type ZnO doped with As species [25]. Theydeposited ZnO films on (0 0 1)-GaAs substrated bypulsed laser ablation. According to secondary ionmass spectroscopy (SIMS), the Ga and As atomsdiffuse from the substrate to the ZnO films duringthe post anneal: The As-atom concentration is inthe upper 1017 to upper 1021 atoms/cm3 while theconcentration of the Ga is in the range 1016–1021 atoms/cm3. It should be noted that the SIMSmeasurement shows the concentration of the Gaon the order of 1018–1020 atoms/cm3 is slightlybelow that of As near the surface. Then we studywhat roles the Ga species play on ZnO :As below.Table 1 shows that As-doping as p-type doping

gives rise to a drastic increase in the Madelungenergy. In this case, Ga species, which donateseasily electrons and causes substantial decrease inthe Madelung energy, is eminently suitable for usein codoping. Considering the distance of Ga to Asbonds in GaAs with zinc-blende structure is almost2.52 (A, which is larger than almost 2.0 (A of Zn toO bonds in ZnO, it is easily predicted that theformation of Ga to As bonds in ZnO is notenergetically favorable. We verified that the Gaincorporated with As locates at the third neighbor-cation sites from the As at O sites, as shown inFig. 7. In other words, we predict a weak correla-

tion between the As and Ga species in ZnO. Inaddition, we find a very weak repulsive interactionbetween the As acceptors in ZnO from thecalculations for ZnO : 2As, in contrast with thatfor ZnO : 2N. In such a case, the codoping of Gacauses little effect on the solubility of the Asacceptors and their impurity level in the band gap.Considering the localized impurity states intro-duced by As-doping, it would be suggested that itis very dufficult to realize low-resistivity p-typeZnO by doping of As alone. A role to be noted thecodopant Ga species play on ZnO :As is todecrease the Madelung energy. This will prevent

Fig. 6. Crystal structure of ZnO; (a) codoped with 2N and Zn interstitials (Zni), (b) doped with 2N.

Fig. 7. Crystal structure of ZnO codoped with As and Ga. The

distance between As and Ga site is 3.80 (A.


vacancies of O in the vicinity of the As at O sitesfrom being formed by As-doping.

5. Conclusions

The following conclusions were derived from theresults and discussion. (1) While n-type dopingusing group III elements (=B, Al, Ga and In) anda group VII element, F species, decreases theMadelung energy, p-type doping using a group Ielement, Li species, and group III elements (=Nand As) increases the Madelung energy. (2) Li-doping introduces shallow levels in the band gap,whereas N and As are deeper acceptor. (3)Codoping of Ga and N is most effective in orderto fabricate low-resistivity p-type ZnO. (4) For Lidoping as acceptors, F species is a good candidateas the reactive donors. (5) For As acceptors, Gaspecies is a preferable element for the codopants.

Acknowledgements

One of the authors, T. Yamamoto, thanksDr. J .urgen Stichit for his technical support. Inthis work, we used ESOCS code of MolecularSimulations Inc. This work was partially sup-ported by Regional Consortium Project on Devel-opment of ZnO-based thin-film devices of NewEnergy and Industrial Technology DevelopmentOrganization (NEDO).

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physics and control of valence states in zno by codoping method

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