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Zinc nonstoichiometry in ZnO
Solid State Ionics 173 (2004) 29 – 33 www.elsevier.com/locate/ssi
Zinc nonstoichiometry in ZnO K. Lotta,*, S. Shinkarenkoa, T. Kirsanovab, L. Tqrna, E. Gorohovac, A. Grebennikb, A. Vishnjakovb b
a Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Department of Physical Chemistry, D. Mendelejev University of Chemical Technology of Russia, Miusskaya Sq.9, 125047 Moscow, Russia c S.I.Vavilov State Optical Institute, Babushkina Street 36-1, 193171 St. Petersburg, Russia
Received 1 May 2004; received in revised form 16 July 2004; accepted 28 July 2004
Abstract Excess Zn in powder and in ceramic ZnO is investigated by the atomic absorption photometry (AAP) method. All the ZnO samples tested were previously heat treated at temperature 900 8C and at fixed Zn pressures from 0.1 to 0.6 of saturated p Zn at given treatment temperature. The vapour–crystal equilibrium in ZnO:Zn crystal was established in 20–40 min. To determine the excess zinc in ZnO samples, the AAP of zinc vapour was used in the conditions of solid–vapour equilibrium. Optical absorbance of zinc, proportional to the concentration of zinc atoms in the vapour phase, was registered photoelectrically on the Zn resonance line. The analysis of temperature dependence of zinc pressure indicated that the value of zinc excess lies in the concentration interval of 1018–1019 cm3 and depends on the sample biography and the conditions of preliminary heat treatment. Zn adsorption on powder ZnO grain surfaces and the reactions of impurity metals with excess Zn in ZnO ceramics were observed in Zn AAP experiments. D 2004 Elsevier B.V. All rights reserved. PACS: 61.50.Nw; 61.72.-y; 61.72.Bb; 61.72.Ji; 64.70.Hz Keywords: Zinc oxide; Nonstoichiometry; Impurities
1. Introduction The physical properties of ZnO crystals depend strongly on the concentration of native defects caused by the deviation from the stoichiometric composition. Reviews on this topic by using defect chemistry methods are made in Refs. [1–4]. Pure ZnO is an n-type semiconductor due to the incorporation of excess Zn. The excess Zn causes a donor band conduction at low temperatures. Caused by donors, paramagnetic centers were observed by EPR [5,6]. Zn nonstoichiometry may be defined by definitions of two types [2]. The definition of the first type is a truly chemical definition where Zn nonstoichiometry is expressed in terms of the gross concentrations of the lattice constituents Zn and O. Analytic–chemical, electrochemical and X-ray investigations of the stoichiometry deviation (d) in ZnO are * Corresponding author. Tel.: +372 53462242; fax: +3726202020. E-mail address: [email protected] (K. Lott). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.048
reviewed in Ref. [1]. The definition of the second type of Zn nonstoichiometry in ZnO is expressed in terms of the concentrations of lattice imperfections as vacancy of oxygen (VO ) or interstitial zinc (Zni ) [2]. Better results in determination of excess Zn gave electrochemical method and Hall effect measurements [7], although no distinguishing surface and bulk effects of Zn excess were observed. According to Ref. [8], adsorption of oxygen on the surface of polycrystalline ZnO can lower the concentration of chemically determined excess Zn more than 30%. The investigations of atomic absorption photometry (AAP) of Zn nonstoichiometry in polycrystalline ZnS and ZnSe showed that the analytically determined concentration of Zn excess appears to be bigger than the concentration of electrically active defects and that Zn adsorption on crystallite surfaces must be taken into account [9,10]. Zn excess in ZnO is built up automatically at high temperatures and growth of stoichiometric material is promoted by lowtemperature methods [11]. At high Zn pressure, Hagemark
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K. Lott et al. / Solid State Ionics 173 (2004) 29–33
[12] found in ZnO monocrystals and Tomlins et al. [13] found in ZnO ceramics the component vapour pressure, p Zn1/3 relationship or corresponding to a ( p O2)1/6 dependence on concentration of electrons n. On the other hand, in polycrystalline ZnO films [14], the measured oxygen pressure dependence of electrical conductivity (r) leads to the value ( p O2)1/4 in the high-temperature region. In the low-temperature region, the oxygen chemosorption dominates the conductivity behaviour. The measured conductivity of nano-ZnO exhibits ( p O2)1/6 dependence and the absolute values of conductivity of nano-ZnO were similar to those measured in a sintered pellet of ZnO powder [15], although the space-charge conductivity does not necessarily depend exactly on oxygen vapour pressure as ( p O2)1/6 [16]. High zinc diffusivity was revealed and explained by fast-moving zinc interstitials; however, experimental data of the contraversy role of grain boundaries appears in the selfdiffusion of ZnO components [17,18]. The properties of polycrystalline ZnO depend upon the presence of charged native point defects in the space charge region adjacent to grain boundaries. Here, the migration and annihilation of excess Zn in the form Zni (or VO) during the frozen-in and oxydation processes take place. The oxydation of excess Zn in ZnO ceramics is the important technological method in production of ZnO varistors for decreasing the concentration of shallow interstitial Zn donors below the deeper ones [19]. It is a long discussion about the excess Zn caused by native donors in ZnO. Although various totalenergy calculation methods has been applied by many researchers to calculate high temperature defect equilibrium (HTDE) models for ZnO, the role of native defects in the results of calculations are interpreted differently [20–23]. Zn AAP allows to eliminate excess Zn connected energetically differentially in ZnO samples. Here, we describe our Zn AAP experiments in the determination of the excess Zn in ZnO of polycrystalline powder and of ceramic samples in the purpose to distinguish excess Zn in different forms of existence.
2. Equipment and theoretical ZnO powder samples were industrial-grade (xl) purity and also ZnO ceramic samples were processed by hightemperature compression of industry-grade purity (Xl) ZnO powder. The equipment for analytical determination of excess Zn by AAP is similar to the equipment described in Ref. [24]. Concentration of excess Zn was measured directly in ZnO powder and in ZnO polycrystalline ceramics after thermal treatment at 900 8C in equilibrium with Zn vapour partial pressure, p Zn (from 0.2 to 0.6 of saturated p Zn at a given treatment temperature; see Fig. 1). The equilibrium p Zn is reached more faster by using powder ZnO objects than ceramic ZnO objects because of the difference in surface area of crystallites. The starting material (ZnO saturated with Zn) was placed into a
Fig. 1. The equipment for the measurement of partial pressure by the method of statical photometry: 1—source of light; 2—condenser; 3— electric stove; 4—optical part of the cuvette; 5—the side part of the cuvette; 6—diaphragm for comparison; 7—photomultiplier; 8—slots; 9—shutter; 10,11—mirrors; 12—diffraction grating.
T-shaped quartz vessel with quartz windows. The volume of the quartz vessel was sealed into high vacuum. A lamp with the atomic spectrum (also containing the resonance line for zinc) was used as a light source. The optical absorption proportional to the concentration of zinc atoms in the vapour phase was registered photoelectrically on Zn line 307.6 nm. This AAP equipment requires an optimum p Zn from 0.5 to 300 Pa. The amount of Zn (as well p Zn), extracted into the vapour phase from ZnO samples, represents the sum of Zn, situated both on surface and in the volume of the crystal. p Zn values above the ZnO objects were determined in the temperature range from 550 to 850 8C. The characteristic feature in experimental investigation of Zn nonstoichiometry is the determination of experimental conditions when the removal of excess Zn from the crystal is almost complete (N99%). These conditions depend on the temperature T, on zinc vapour pressure p Zn, on the ratio of the volume of the crystal and the vessel, on the distribution coefficient of Zn between the vapour phase and the crystal and on the HTDE isotherms. To create donor defects in ZnO samples, we propose the following mechanism Znð g Þ ¼ Znxi
ð1Þ
Znxi ¼ ZnSi þ e= SS ZnSi ¼ Zn i þ e=
ð2Þ ð3Þ
or the sum of these expressions:
SS Znð g Þ ¼ Zn i þ 2e=
ð4Þ
with the equilibrium constant K¼
SS ½Zn i ½eV2 pZn
ð5Þ
K. Lott et al. / Solid State Ionics 173 (2004) 29–33
31
If more constituents occur in the Zn material balance: [Zni]TOTAL=[Znxi ]+[ZnSi] + [ZnSSi ], and the concentrations are close to each other as presented in Ref. [24], then the complicated character of p Zn curve analysis must be realized [25]. Experimentally, it was also found that at constant temperature, log n~1/3 log p Zn, indicating that a doubly ionized donor is involved [7]. In the conditions of additional zinc vapour pressure, the high temperature electroneutrality condition of dominating defects may be presumably approximated here, as in many II–VI crystals: h i SS e= ¼ 2 Zn i ð6Þ Under these conditions, after combination of Eq. (6) to Eq. (5), we obtain SS 4½Zn i 3 ð7Þ K¼ pZn The deviation from stoichiometry can be approximated as dc[ZniSS] and rffiffiffiffiffiffiffiffiffiffiffiffi 3 KpZn d¼ 4
ð8Þ
In our experiment, the relation for calculation of the degree of Zn removed from nonstoichiometric ZnO is: N vapour Nvapour ¼ Ntotal Ncrystal þ Nvapour pZn ðVvessel Vcrystal Þ RT ¼ qffiffiffiffiffiffiffiffi pZn ðVvessel Vcrystal Þ m ZnO 3 KpZn þ 4 RT MZnO
ð9Þ
where N vapour, N crystal—the amount of Zn in vapour and in the crystal, V vessel, V crystal—volumes of vessel and the sample, p Zn—experimentally determined zinc vapour pressure, m ZnO—the mass of the crystal, M ZnO—molar mass of ZnO, K—the equilibrium constant of the reaction of incorporation of Zn into ZnO. The criterion of the complete removal of Zn from ZnO sample at given conditions is the constancy of light absorbance signal with rising of temperature.
3. Results of Zn AAP measurements in ZnO Fig. 2 shows the results of experiments. Data of curves 3 and 4 represent Zn vapour pressure above powder ZnO samples of 49.4 and 50.0 mg in the optical vessel with volume 20 cm3. Data of curve 5 represent Zn vapour pressure above the ZnO ceramic sample of 550.5 mg in the optical vessel with volume 20 cm3. Line 1 represents the temperature dependence of saturated Zn vapour pressure of pure Zn. Line 2 describes stoichiometric composition of vapour under condition p Zn=2p O2.
Fig. 2. The temperature dependence of p Zn: 1—above pure liquid zinc, 2— above stoichiometric ZnO. Experimental p Zn data: 3—above powdered ZnO of mass 49.4 mg, 4—above powdered ZnO of mass 50.0 mg (both sample 4 and sample 5 were in the optical vessel with volume 20 cm3), 5— above ceramic ZnO of mass 550.5 mg and in the optical vessel with volume 20 cm3. Calculated p Zn data: 6—under the thermodynamic conditions of Ref. [2], 7—under the thermodynamic conditions of Ref. [12].
3.1. Zn AAP of powder ZnO Excess Zn in polycrystalline ZnO is concentrated in grains by different forms. Here, the role of processes between the crystal surface and the vapour phase becomes important. The extracted amount of Zn from ZnO powder is the sum of adsorbed Zn on polycrystalline powder surface and incorporated into the ZnO grain volume Zn. The character of temperature dependence of p Zn above powdered ZnO samples indicates in Fig. 2 the dominating role of adsorption in the total concentration of excess zinc. At specific surface values in the order of 1 m2/g, the calculated concentration of adsorbed zinc can exceed 0.04 mol% and depends significantly on temperature. The chemical analysis of samples 3 and 4 by Norman’s method [8] gave Zn concentration values 4.9103 and 6.6103 mol%. 3.2. Zn AAP of ceramic ZnO The behaviour of ceramic samples was different from powder ones. For ceramic ZnO, the absolute amount of extracted Zn into the vapour phase was an order of magnitude lower than for ZnO powder. The specific surface for ceramic ZnO sample is more than two orders of magnitude lower than for ZnO powder sample and we can avoid surface effects in ceramic object at the first stage. Line 5 on Fig. 2 shows the dependence of p Zn in the ceramic sample after heat treatment at p Zn value equal to half of the saturated p Zn at T=1173 K. The results of AAP measurements indicate the complicated mechanism of excess Zn, bounded with ZnO crystals. During the heating up of the ZnO ceramic sample, the precipitated-into-inner-surfaces metallic Zn causes constant p Zn value. In this temperature
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K. Lott et al. / Solid State Ionics 173 (2004) 29–33
region, the amount of Zn removed from the sample remains almost constant. Beginning from 890 K, p Zn values begin to rise with rising of the temperature. At first sight, we can assume the existence of two forms of Zn localization in ceramics: weakly bounded form at low temperatures and tightly bounded form at high temperatures. The values of p Zn in the low-temperature region are very close to predicted from the equilibrium constants of quasi-chemical reactions of Ref. [7] (line 7 in Fig. 2) for creation of donor defects in ZnO. If we assume all excess Zn is extracted from ceramic samples into vapour phase at T=890 K and p Zn=32 Pa, then the calculations give the concentration of donor defects 5.61017 (or 13.3 ppma). At maximum p Zn of the experiment, we will have 0.004 mol% (21018 cm3 or 47.5 ppma). This value is higher than the concentration of ionized defects in the most reviewed HTDE models of Ref. [4] but is significantly lower than the concentrations of defects in Krfger’s [2] model (line 6 in Fig. 2). In the following analysis, we try to prove that in the temperature region from 890 to 1000 K, the removed from ZnO ceramic sample Zn does not originate only from Zn nonstoichiometry but also is the result of the reduction–oxidation reactions of ZnO with contamination of metallic impurities and their oxides which exist in the starting ZnO material. We analyse here three cases in relation with the impurity metals or metal oxides: the starting powder, the high-temperature exposure of p Zn for creation of Zn nonstoichiometry, and the heating up of ZnO with nonstoichiometry of Zn. In the first case, the industrygrade purity (xl) ZnO powder was used for the ceramic sample preparation. It contains the impurity metal oxides (Al2O3, SiO2, FeO, PbO, CdO, CuO, TiO2). The additional contamination occurs during the process of formation of high-temperature ceramics. These impurity oxides exist in ZnO ceramic samples used for experiments. In the second case, during the exposure of ceramic ZnO samples at fixed p Zn (for creation of Zn nonstoichiometry), the impurity metal oxides reduce to metal form.
crystallites (dislocations) or form complexes with native donors. In this manner, the concentration of native donors (as well as Zn nonstoichiometry) depends on the contamination impurities of ZnO sample. The concentration of nonstoichiometric Zn in crystals decreases with lowering of temperature of the sample. It is caused by the creation of supersaturated states and as result the segregation of excess Zn on dislocations occurs. During the heating up of ZnO ceramic sample in the optical vessel in the course of the Zn AAP measurement process, the precipitated-into-the-innersurfaces nonstoichiometric Zn is removed into vapour phase and causes constant p Zn value during the temperature rising from 840 to 890 K. At the temperatures TN890 K, the p Zn value in the vessel is many times less than p Zn value above the liquid zinc and the impurity metals can react with ZnO by the endothermic process, reversible to the reaction (10):
MeO þ Znð1Þ ¼ Me þ ZnO
Me þ ZnO ¼ MeO þ ZnðgÞ
ð10Þ
The conclusion about the reduction of the oxides with zinc is based on the comparison of equilibrium constants of the presented in Fig. 3 exothermic reactions with the equilibrium constant at the Zn boundary of the nonstoichiometric phase: K¼
aMe aZnO aMe ¼ aMeO aZn aMeO
ð11Þ
It characterizes the relation between the activity of the reduced and the oxidized forms of impurity metals. The temperature dependence of the constants K in Fig. 3 are calculated by the program HSC [26]. The K value increases with lowering of T. As a result, the cooling of the crystal after high-temperature exposure of p Zn accompanies the rising of the concentration of impurities in metallic form. These impurity metals segregate on the inner surfaces of the
Fig. 3. The equilibrium constants K for reactions: (1) FeO+Zn(l)=Fe+ZnO; (2) PbO+Zn(l)=Pb+ZnO; (3) CdO+Zn(l)=Cd+ZnO; (4) NiO+Zn(l)= Ni+ZnO; (5) CuO+Zn(l)=Cu+ZnO and K rev for reactions (6) Fe+ZnO= FeO+Zn(g); (7) Pb+ZnO=PbO+Zn(g); (8) Cd+ZnO=CdO+Zn(g); (9) Ni+ZnO=NiO+Zn(g); (10) Cu+ZnO=CuO+Zn(g) are calculated by the program HSC [26].
ð12Þ
with Krev c
cMeO ½MeO cMe ½Me
ð13Þ
where c MeO and c Me are the activity coefficients of impurity metal oxides and of the impurity metal, [MeO] and [Me] are the concentrations of impurity metal oxides and impurity metals. As a result, we observe the increasing of p Zn during the rise of temperature from 890 to 1000 K. The values of equilibrium constant K rev are shown in Fig. 3. From our analysis, it can be concluded that at the p Zn range investigated ( p Zn=32–150 Pa), only Fe can reduce Zn from ZnO because the activities of impurity metals change as Fe NNCdcNiNPb NN Cu. The slope of the high temperature (890–1000 K) branch characterizes the sum of three enthalpies: enthalpy DH 1 of reaction (12) of reducing of ZnO at the presence of impurity metals, plus enthalpy DH 2
K. Lott et al. / Solid State Ionics 173 (2004) 29–33
of solution of oxidized form, minus enthalpy of solution of the reduced form DH 3. The determined summary enthalpy, DH=159 kJ/mol can be compared with the enthalpy of reduction of ZnO by Fe with DH=202 kJ/mol at T=1073 K [25]. To know the values of DH 2 and DH 3, one must know the ratio a MeO/a Me values through the investigation of solubilities of the oxidized and the reduced forms of impurity metals in ZnO. As we lack this information, we can estimate the relation a MeO/a Me values through the ratio of equilibrium constants to the determined p Znvalue, i.e., K/ p Zn in the relation K ¼ pZn ðaMeO =aMe Þ
ð14Þ
This relation is in favour to oxide formation only in the case of Fe. For other impurity metals (Ni, Cu, Cd, Pb), the dominating form of existence in this temperature region will be atomic.
4. Conclusions Zn AAP experiments in the determination of the excess Zn in ZnO of polycrystalline powder and ceramic samples distinguished the nonstoichiometric Zn in different forms of existence. The character of temperature dependence of p Zn above powdered ZnO samples indicates the dominating role of Zn adsorption in the total concentration of excess zinc. Impurity metals take active role in the oxydation–reduction reactions during cooling down and heating up of ZnO ceramic samples and influence on the determination of nonstoichiometric zinc.
Acknowledgements The authors would like to thank the Estonian Science Foundation for financial support (Grant 5156).
33
References [1] G. Neumann, The quantitative determination of excess zinc, Chap. 3.3, in: E. Kaldis (Ed.), Current Topics in Materials Science, vol. 7, North-Holland, Amsterdam, 1981. [2] F.A. Krfger, The Chemistry of Imperfect Crystals, North-Holland, Amsterdam, 1974, p. 743. [3] M.H. Sukkar, H.L. Tuller, Defect equilibria in ZnO varistor materials, in: M.F. Yan, A.H. Heuer (Eds.), Advances in Ceramics, vol. 7, The American Ceramic Society, 1983, p. 71. [4] J. Han, P. Mantas, A.M.R. Senos, J. Eur. Ceram. Soc. 22 (2002) 49. [5] K.A. Mqller, J. Schneider, Phys. Lett. 4 (1963) 288. [6] J.M. Smith, W.E. Vehse, Phys. Lett. 31A (1970) 147. [7] K.I. Hagemark, P.E. Toren, J. Electrochem. Soc. 122 (1975) 992. [8] V.J. Norman, Austr. J. Chem. 20 (1967) 85. [9] A.V. Grebennik, A.V. Vishnjakov, O.M. Mezhujev, I.A. Mironov, G.T. Petrovskii, J. Opt. Technol. 4 (1994) 111. [10] K. Lott, A. Grebennik, A. Dontsova, M. Raukas, A. Vishnjakov, Proc. Est. Acad. Sci., Chem. 44 (1995) 114. [11] H. Hartmann, R. Mach, B. Selle, Wide gap II–VI compounds as electronic materials, in: E. Kaldis (Ed.), Current Topics in Materials Science, vol. 7, North-Holland, Amsterdam, 1982, p. 335. [12] K.I. Hagemark, J. Solid State Chem. 16 (1976) 293. [13] G. Tomlins, J. Routbort, T. Mason, J. Appl. Phys. 87 (2000) 117. [14] P. Bonasewicz, W. Hirschwald, G. Neumann, J. Electrochem. Soc. 133 (1986) 2270. [15] S. Kim, J. Maier, Electrochem. Solid-State Lett. 6 (2003) J7. [16] S. Kim, J. Maier, J. Electrochem. Soc. 149 (2002) J73. [17] M. Nogueira, W. Ferraz, A. Saboni, Mater. Res. 6 (2003) 167. [18] H. Haneda, I. Sakaguchi, A. Watanabe, J. Tanaka, Defect Diffus. Forum 143–147 (1997) 1219. [19] J. Han, A.M.R. Senos, P.Q. Mantas, J. Eur. Ceram. Soc. 22 (2002) 1653. [20] A.F. Kohan, G. Ceder, D. Morgan, C.G. Van de Walle, Phys. Rev., B 61 (2000) 15019. [21] S.B. Zhang, S.-H. Wei, A. Zunger, Phys. Rev., B 63 (2001) 075205. [22] S. Yuming, X. Pengshou, S. Chaoshu, X. Faqiang, P. Haibin, L. Erdong, J. Electron Spectrosc. Relat. Phenom. 114–116 (2001) 1123. [23] F. Oba, S.R. Nishitani, S. Isotani, H. Adachi, J. Appl. Phys. 90 (2001) 824. [24] K. Lott, O. Volobujeva, M. Raukas, L. Tqrn, A. Grebennik, A. Vishnjakov, Phys. status solidi (c) 0 (2003) 622. [25] K. Lott, L. Tqrn, J. Cryst. Growth 197 (1999) 493. [26] HSC Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database by Antti Soine, Outokumpu, Pori, Finland.
Zinc nonstoichiometry in ZnO K. Lotta,*, S. Shinkarenkoa, T. Kirsanovab, L. Tqrna, E. Gorohovac, A. Grebennikb, A. Vishnjakovb b
a Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia Department of Physical Chemistry, D. Mendelejev University of Chemical Technology of Russia, Miusskaya Sq.9, 125047 Moscow, Russia c S.I.Vavilov State Optical Institute, Babushkina Street 36-1, 193171 St. Petersburg, Russia
Received 1 May 2004; received in revised form 16 July 2004; accepted 28 July 2004
Abstract Excess Zn in powder and in ceramic ZnO is investigated by the atomic absorption photometry (AAP) method. All the ZnO samples tested were previously heat treated at temperature 900 8C and at fixed Zn pressures from 0.1 to 0.6 of saturated p Zn at given treatment temperature. The vapour–crystal equilibrium in ZnO:Zn crystal was established in 20–40 min. To determine the excess zinc in ZnO samples, the AAP of zinc vapour was used in the conditions of solid–vapour equilibrium. Optical absorbance of zinc, proportional to the concentration of zinc atoms in the vapour phase, was registered photoelectrically on the Zn resonance line. The analysis of temperature dependence of zinc pressure indicated that the value of zinc excess lies in the concentration interval of 1018–1019 cm3 and depends on the sample biography and the conditions of preliminary heat treatment. Zn adsorption on powder ZnO grain surfaces and the reactions of impurity metals with excess Zn in ZnO ceramics were observed in Zn AAP experiments. D 2004 Elsevier B.V. All rights reserved. PACS: 61.50.Nw; 61.72.-y; 61.72.Bb; 61.72.Ji; 64.70.Hz Keywords: Zinc oxide; Nonstoichiometry; Impurities
1. Introduction The physical properties of ZnO crystals depend strongly on the concentration of native defects caused by the deviation from the stoichiometric composition. Reviews on this topic by using defect chemistry methods are made in Refs. [1–4]. Pure ZnO is an n-type semiconductor due to the incorporation of excess Zn. The excess Zn causes a donor band conduction at low temperatures. Caused by donors, paramagnetic centers were observed by EPR [5,6]. Zn nonstoichiometry may be defined by definitions of two types [2]. The definition of the first type is a truly chemical definition where Zn nonstoichiometry is expressed in terms of the gross concentrations of the lattice constituents Zn and O. Analytic–chemical, electrochemical and X-ray investigations of the stoichiometry deviation (d) in ZnO are * Corresponding author. Tel.: +372 53462242; fax: +3726202020. E-mail address: [email protected] (K. Lott). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.048
reviewed in Ref. [1]. The definition of the second type of Zn nonstoichiometry in ZnO is expressed in terms of the concentrations of lattice imperfections as vacancy of oxygen (VO ) or interstitial zinc (Zni ) [2]. Better results in determination of excess Zn gave electrochemical method and Hall effect measurements [7], although no distinguishing surface and bulk effects of Zn excess were observed. According to Ref. [8], adsorption of oxygen on the surface of polycrystalline ZnO can lower the concentration of chemically determined excess Zn more than 30%. The investigations of atomic absorption photometry (AAP) of Zn nonstoichiometry in polycrystalline ZnS and ZnSe showed that the analytically determined concentration of Zn excess appears to be bigger than the concentration of electrically active defects and that Zn adsorption on crystallite surfaces must be taken into account [9,10]. Zn excess in ZnO is built up automatically at high temperatures and growth of stoichiometric material is promoted by lowtemperature methods [11]. At high Zn pressure, Hagemark
30
K. Lott et al. / Solid State Ionics 173 (2004) 29–33
[12] found in ZnO monocrystals and Tomlins et al. [13] found in ZnO ceramics the component vapour pressure, p Zn1/3 relationship or corresponding to a ( p O2)1/6 dependence on concentration of electrons n. On the other hand, in polycrystalline ZnO films [14], the measured oxygen pressure dependence of electrical conductivity (r) leads to the value ( p O2)1/4 in the high-temperature region. In the low-temperature region, the oxygen chemosorption dominates the conductivity behaviour. The measured conductivity of nano-ZnO exhibits ( p O2)1/6 dependence and the absolute values of conductivity of nano-ZnO were similar to those measured in a sintered pellet of ZnO powder [15], although the space-charge conductivity does not necessarily depend exactly on oxygen vapour pressure as ( p O2)1/6 [16]. High zinc diffusivity was revealed and explained by fast-moving zinc interstitials; however, experimental data of the contraversy role of grain boundaries appears in the selfdiffusion of ZnO components [17,18]. The properties of polycrystalline ZnO depend upon the presence of charged native point defects in the space charge region adjacent to grain boundaries. Here, the migration and annihilation of excess Zn in the form Zni (or VO) during the frozen-in and oxydation processes take place. The oxydation of excess Zn in ZnO ceramics is the important technological method in production of ZnO varistors for decreasing the concentration of shallow interstitial Zn donors below the deeper ones [19]. It is a long discussion about the excess Zn caused by native donors in ZnO. Although various totalenergy calculation methods has been applied by many researchers to calculate high temperature defect equilibrium (HTDE) models for ZnO, the role of native defects in the results of calculations are interpreted differently [20–23]. Zn AAP allows to eliminate excess Zn connected energetically differentially in ZnO samples. Here, we describe our Zn AAP experiments in the determination of the excess Zn in ZnO of polycrystalline powder and of ceramic samples in the purpose to distinguish excess Zn in different forms of existence.
2. Equipment and theoretical ZnO powder samples were industrial-grade (xl) purity and also ZnO ceramic samples were processed by hightemperature compression of industry-grade purity (Xl) ZnO powder. The equipment for analytical determination of excess Zn by AAP is similar to the equipment described in Ref. [24]. Concentration of excess Zn was measured directly in ZnO powder and in ZnO polycrystalline ceramics after thermal treatment at 900 8C in equilibrium with Zn vapour partial pressure, p Zn (from 0.2 to 0.6 of saturated p Zn at a given treatment temperature; see Fig. 1). The equilibrium p Zn is reached more faster by using powder ZnO objects than ceramic ZnO objects because of the difference in surface area of crystallites. The starting material (ZnO saturated with Zn) was placed into a
Fig. 1. The equipment for the measurement of partial pressure by the method of statical photometry: 1—source of light; 2—condenser; 3— electric stove; 4—optical part of the cuvette; 5—the side part of the cuvette; 6—diaphragm for comparison; 7—photomultiplier; 8—slots; 9—shutter; 10,11—mirrors; 12—diffraction grating.
T-shaped quartz vessel with quartz windows. The volume of the quartz vessel was sealed into high vacuum. A lamp with the atomic spectrum (also containing the resonance line for zinc) was used as a light source. The optical absorption proportional to the concentration of zinc atoms in the vapour phase was registered photoelectrically on Zn line 307.6 nm. This AAP equipment requires an optimum p Zn from 0.5 to 300 Pa. The amount of Zn (as well p Zn), extracted into the vapour phase from ZnO samples, represents the sum of Zn, situated both on surface and in the volume of the crystal. p Zn values above the ZnO objects were determined in the temperature range from 550 to 850 8C. The characteristic feature in experimental investigation of Zn nonstoichiometry is the determination of experimental conditions when the removal of excess Zn from the crystal is almost complete (N99%). These conditions depend on the temperature T, on zinc vapour pressure p Zn, on the ratio of the volume of the crystal and the vessel, on the distribution coefficient of Zn between the vapour phase and the crystal and on the HTDE isotherms. To create donor defects in ZnO samples, we propose the following mechanism Znð g Þ ¼ Znxi
ð1Þ
Znxi ¼ ZnSi þ e= SS ZnSi ¼ Zn i þ e=
ð2Þ ð3Þ
or the sum of these expressions:
SS Znð g Þ ¼ Zn i þ 2e=
ð4Þ
with the equilibrium constant K¼
SS ½Zn i ½eV2 pZn
ð5Þ
K. Lott et al. / Solid State Ionics 173 (2004) 29–33
31
If more constituents occur in the Zn material balance: [Zni]TOTAL=[Znxi ]+[ZnSi] + [ZnSSi ], and the concentrations are close to each other as presented in Ref. [24], then the complicated character of p Zn curve analysis must be realized [25]. Experimentally, it was also found that at constant temperature, log n~1/3 log p Zn, indicating that a doubly ionized donor is involved [7]. In the conditions of additional zinc vapour pressure, the high temperature electroneutrality condition of dominating defects may be presumably approximated here, as in many II–VI crystals: h i SS e= ¼ 2 Zn i ð6Þ Under these conditions, after combination of Eq. (6) to Eq. (5), we obtain SS 4½Zn i 3 ð7Þ K¼ pZn The deviation from stoichiometry can be approximated as dc[ZniSS] and rffiffiffiffiffiffiffiffiffiffiffiffi 3 KpZn d¼ 4
ð8Þ
In our experiment, the relation for calculation of the degree of Zn removed from nonstoichiometric ZnO is: N vapour Nvapour ¼ Ntotal Ncrystal þ Nvapour pZn ðVvessel Vcrystal Þ RT ¼ qffiffiffiffiffiffiffiffi pZn ðVvessel Vcrystal Þ m ZnO 3 KpZn þ 4 RT MZnO
ð9Þ
where N vapour, N crystal—the amount of Zn in vapour and in the crystal, V vessel, V crystal—volumes of vessel and the sample, p Zn—experimentally determined zinc vapour pressure, m ZnO—the mass of the crystal, M ZnO—molar mass of ZnO, K—the equilibrium constant of the reaction of incorporation of Zn into ZnO. The criterion of the complete removal of Zn from ZnO sample at given conditions is the constancy of light absorbance signal with rising of temperature.
3. Results of Zn AAP measurements in ZnO Fig. 2 shows the results of experiments. Data of curves 3 and 4 represent Zn vapour pressure above powder ZnO samples of 49.4 and 50.0 mg in the optical vessel with volume 20 cm3. Data of curve 5 represent Zn vapour pressure above the ZnO ceramic sample of 550.5 mg in the optical vessel with volume 20 cm3. Line 1 represents the temperature dependence of saturated Zn vapour pressure of pure Zn. Line 2 describes stoichiometric composition of vapour under condition p Zn=2p O2.
Fig. 2. The temperature dependence of p Zn: 1—above pure liquid zinc, 2— above stoichiometric ZnO. Experimental p Zn data: 3—above powdered ZnO of mass 49.4 mg, 4—above powdered ZnO of mass 50.0 mg (both sample 4 and sample 5 were in the optical vessel with volume 20 cm3), 5— above ceramic ZnO of mass 550.5 mg and in the optical vessel with volume 20 cm3. Calculated p Zn data: 6—under the thermodynamic conditions of Ref. [2], 7—under the thermodynamic conditions of Ref. [12].
3.1. Zn AAP of powder ZnO Excess Zn in polycrystalline ZnO is concentrated in grains by different forms. Here, the role of processes between the crystal surface and the vapour phase becomes important. The extracted amount of Zn from ZnO powder is the sum of adsorbed Zn on polycrystalline powder surface and incorporated into the ZnO grain volume Zn. The character of temperature dependence of p Zn above powdered ZnO samples indicates in Fig. 2 the dominating role of adsorption in the total concentration of excess zinc. At specific surface values in the order of 1 m2/g, the calculated concentration of adsorbed zinc can exceed 0.04 mol% and depends significantly on temperature. The chemical analysis of samples 3 and 4 by Norman’s method [8] gave Zn concentration values 4.9103 and 6.6103 mol%. 3.2. Zn AAP of ceramic ZnO The behaviour of ceramic samples was different from powder ones. For ceramic ZnO, the absolute amount of extracted Zn into the vapour phase was an order of magnitude lower than for ZnO powder. The specific surface for ceramic ZnO sample is more than two orders of magnitude lower than for ZnO powder sample and we can avoid surface effects in ceramic object at the first stage. Line 5 on Fig. 2 shows the dependence of p Zn in the ceramic sample after heat treatment at p Zn value equal to half of the saturated p Zn at T=1173 K. The results of AAP measurements indicate the complicated mechanism of excess Zn, bounded with ZnO crystals. During the heating up of the ZnO ceramic sample, the precipitated-into-inner-surfaces metallic Zn causes constant p Zn value. In this temperature
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K. Lott et al. / Solid State Ionics 173 (2004) 29–33
region, the amount of Zn removed from the sample remains almost constant. Beginning from 890 K, p Zn values begin to rise with rising of the temperature. At first sight, we can assume the existence of two forms of Zn localization in ceramics: weakly bounded form at low temperatures and tightly bounded form at high temperatures. The values of p Zn in the low-temperature region are very close to predicted from the equilibrium constants of quasi-chemical reactions of Ref. [7] (line 7 in Fig. 2) for creation of donor defects in ZnO. If we assume all excess Zn is extracted from ceramic samples into vapour phase at T=890 K and p Zn=32 Pa, then the calculations give the concentration of donor defects 5.61017 (or 13.3 ppma). At maximum p Zn of the experiment, we will have 0.004 mol% (21018 cm3 or 47.5 ppma). This value is higher than the concentration of ionized defects in the most reviewed HTDE models of Ref. [4] but is significantly lower than the concentrations of defects in Krfger’s [2] model (line 6 in Fig. 2). In the following analysis, we try to prove that in the temperature region from 890 to 1000 K, the removed from ZnO ceramic sample Zn does not originate only from Zn nonstoichiometry but also is the result of the reduction–oxidation reactions of ZnO with contamination of metallic impurities and their oxides which exist in the starting ZnO material. We analyse here three cases in relation with the impurity metals or metal oxides: the starting powder, the high-temperature exposure of p Zn for creation of Zn nonstoichiometry, and the heating up of ZnO with nonstoichiometry of Zn. In the first case, the industrygrade purity (xl) ZnO powder was used for the ceramic sample preparation. It contains the impurity metal oxides (Al2O3, SiO2, FeO, PbO, CdO, CuO, TiO2). The additional contamination occurs during the process of formation of high-temperature ceramics. These impurity oxides exist in ZnO ceramic samples used for experiments. In the second case, during the exposure of ceramic ZnO samples at fixed p Zn (for creation of Zn nonstoichiometry), the impurity metal oxides reduce to metal form.
crystallites (dislocations) or form complexes with native donors. In this manner, the concentration of native donors (as well as Zn nonstoichiometry) depends on the contamination impurities of ZnO sample. The concentration of nonstoichiometric Zn in crystals decreases with lowering of temperature of the sample. It is caused by the creation of supersaturated states and as result the segregation of excess Zn on dislocations occurs. During the heating up of ZnO ceramic sample in the optical vessel in the course of the Zn AAP measurement process, the precipitated-into-the-innersurfaces nonstoichiometric Zn is removed into vapour phase and causes constant p Zn value during the temperature rising from 840 to 890 K. At the temperatures TN890 K, the p Zn value in the vessel is many times less than p Zn value above the liquid zinc and the impurity metals can react with ZnO by the endothermic process, reversible to the reaction (10):
MeO þ Znð1Þ ¼ Me þ ZnO
Me þ ZnO ¼ MeO þ ZnðgÞ
ð10Þ
The conclusion about the reduction of the oxides with zinc is based on the comparison of equilibrium constants of the presented in Fig. 3 exothermic reactions with the equilibrium constant at the Zn boundary of the nonstoichiometric phase: K¼
aMe aZnO aMe ¼ aMeO aZn aMeO
ð11Þ
It characterizes the relation between the activity of the reduced and the oxidized forms of impurity metals. The temperature dependence of the constants K in Fig. 3 are calculated by the program HSC [26]. The K value increases with lowering of T. As a result, the cooling of the crystal after high-temperature exposure of p Zn accompanies the rising of the concentration of impurities in metallic form. These impurity metals segregate on the inner surfaces of the
Fig. 3. The equilibrium constants K for reactions: (1) FeO+Zn(l)=Fe+ZnO; (2) PbO+Zn(l)=Pb+ZnO; (3) CdO+Zn(l)=Cd+ZnO; (4) NiO+Zn(l)= Ni+ZnO; (5) CuO+Zn(l)=Cu+ZnO and K rev for reactions (6) Fe+ZnO= FeO+Zn(g); (7) Pb+ZnO=PbO+Zn(g); (8) Cd+ZnO=CdO+Zn(g); (9) Ni+ZnO=NiO+Zn(g); (10) Cu+ZnO=CuO+Zn(g) are calculated by the program HSC [26].
ð12Þ
with Krev c
cMeO ½MeO cMe ½Me
ð13Þ
where c MeO and c Me are the activity coefficients of impurity metal oxides and of the impurity metal, [MeO] and [Me] are the concentrations of impurity metal oxides and impurity metals. As a result, we observe the increasing of p Zn during the rise of temperature from 890 to 1000 K. The values of equilibrium constant K rev are shown in Fig. 3. From our analysis, it can be concluded that at the p Zn range investigated ( p Zn=32–150 Pa), only Fe can reduce Zn from ZnO because the activities of impurity metals change as Fe NNCdcNiNPb NN Cu. The slope of the high temperature (890–1000 K) branch characterizes the sum of three enthalpies: enthalpy DH 1 of reaction (12) of reducing of ZnO at the presence of impurity metals, plus enthalpy DH 2
K. Lott et al. / Solid State Ionics 173 (2004) 29–33
of solution of oxidized form, minus enthalpy of solution of the reduced form DH 3. The determined summary enthalpy, DH=159 kJ/mol can be compared with the enthalpy of reduction of ZnO by Fe with DH=202 kJ/mol at T=1073 K [25]. To know the values of DH 2 and DH 3, one must know the ratio a MeO/a Me values through the investigation of solubilities of the oxidized and the reduced forms of impurity metals in ZnO. As we lack this information, we can estimate the relation a MeO/a Me values through the ratio of equilibrium constants to the determined p Znvalue, i.e., K/ p Zn in the relation K ¼ pZn ðaMeO =aMe Þ
ð14Þ
This relation is in favour to oxide formation only in the case of Fe. For other impurity metals (Ni, Cu, Cd, Pb), the dominating form of existence in this temperature region will be atomic.
4. Conclusions Zn AAP experiments in the determination of the excess Zn in ZnO of polycrystalline powder and ceramic samples distinguished the nonstoichiometric Zn in different forms of existence. The character of temperature dependence of p Zn above powdered ZnO samples indicates the dominating role of Zn adsorption in the total concentration of excess zinc. Impurity metals take active role in the oxydation–reduction reactions during cooling down and heating up of ZnO ceramic samples and influence on the determination of nonstoichiometric zinc.
Acknowledgements The authors would like to thank the Estonian Science Foundation for financial support (Grant 5156).
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