Sintering Behavior of Nano- and Micro-Sized ZnO Powder Targetsfor rf Magnetron Sputtering Applications

Nuno Neves,‡,§,† Raquel Barros,‡,§ Elsa Antunes,‡ Isabel Ferreira,§ Joao Calado,‡

Elvira Fortunato,§ and Rodrigo Martins§

‡INNOVNANO, Materiais Avancados, SA, 7600-095, Aljustrel, Portugal

§CENIMAT-I3N, Departamento de Ciencia dos Materiais and CEMOP/UNINOVA, Faculdade de Ciencias e Tecnologia, FCT,Universidade Nova de Lisboa, 2829-516, Caparica, Portugal, 2829-516, Caparica, Portugal

In this work, the nonisothermal sintering behavior of as-received commercial high purity ZnO micrometric (m_ZnO),

submicrometric (sm_ZnO) and nanometric (n_ZnO) powders

was studied. The sintering behavior for sputtering target pro-duction was evaluated by changing the green density of samples

from 62% of theoretical density (TD) to 35%. We observed

that for n_ZnO powder, the maximum shrinkage rate (MSR)

temperature (TMSR) was not affected by the green density, andthat it was reached at lower temperatures (~710°C) compared

with m_ZnO and sm_ZnO powders. For these powders, the

temperature of MSR increased from 803°C to 934°C and from

719°C to 803°C as TD changed from 62% to 35% TD,respectively. Small grain size (~0.560 lm) and high density tar-

gets were obtained for n_ZnO when sintered at temperatures

below the TMSR. Heating rate from 1°C to 15°C/min led tolower activation energy for n_ZnO (~201 ± 3 kJ/mol) than for

the submicrometric (sm_ZnO) (~332 ± 20 kJ/mol) and micro-

metric (m_ZnO) (~273 ± 9 kJ/mol) powders. Using the model

proposed by Bannister and Woolfrey, an n value of 0.75 wasfound, which was correlated with a combination of viscous flow

and volume diffusion mechanisms that should control the initial

stage of n_ZnO sintering. No significant differences were

observed for n_ZnO powder in terms of density when the sizeof targets (scale-up effect) was increased, while in the case of

m_ZnO and sm_ZnO, a delay in the densification was

observed, which was related to the higher sinterability ofn_ZnO powder. Two inches ZnO ceramic targets with different

particle sizes and final densities were used in an rf magnetron

sputtering system to produce ZnO films under the same deposi-

tion conditions. Films with thickness around 100 nm and gooduniformity were produced using those targets, and no variation

was observed in the optical and morphological properties. How-

ever, low electrical resistivity (1.4 Ω·cm) films were obtained

with n_ZnO targets, which could be explained in terms of anonstoichiometric Zn:O composition of the started powders.

I. Introduction

ZINC oxide (ZnO) is currently a widespread material usedin many applications. In this study, we envisaged the

optoelectronic applications namely in device fabrication suchas, solar cells, gas sensors, varistors, thin film transistors,

transparent electronics and piezoelectric devices.1–8 Most ofthe referred devices could be produced using ZnO thin films(intrinsic or doped) as active layer and/or passive layer. TheZnO thin films can be obtained by several methods. At anindustrial level, the sputtering is one of the most commontechniques,9 where the optical and electrical performances offilms are highly dependent on the deposition conditions used,mainly the oxygen partial pressure and rf power densityused.10,11 By doing so, the electrical characteristics can be con-trolled over more than 14 orders of magnitude, going fromhighly resistive films to conducting films.5,9–11 In this tech-nique, the material of the target is sputtered by an ionized inertgas and deposited into a substrate, forming a thin film. In par-ticular, for the ZnO thin film production, ceramic or metal tar-gets can be used, depending on whether we are using rfmagnetron or dc sputtering, respectively. In this study, thefocus was the influence of micro-, submicro-, and nano-ZnOpowders on the sintering processes of small targets. Over thelast years, many studies have related sintering and graingrowth of microcrystalline ZnO.12–15 High-target densifica-tions (95%–98%) were reported by using high purity (~99.9%)ZnO micro powder and sintering temperatures between 1100°C and 1200°C.13–15 Recently, especially during the last decade,with the strong development of nanotechnology, the interest inthe sintering and grain growth of nano-oxides has beenrenewed. Several studies of nanometric ZnO with an averageparticle size from 5 to 100 nm were reported.16–22 Resultsshowed that samples with 95%–98% of theoretical density at950°C16 and 1050°C17 with final grain sizes respectively of 3and 5–10 lm can be obtained. Hynes et al.18 on the otherhand, obtained high density ZnO samples using pressurelessisothermal sintering at 650°C–700°C, whereas Roy et al.19

reached full density at 800°C and a final grain size of ~1.5 lm.In this work, the nonisothermal sintering behavior of three

different types of as-received commercial ZnO powders,(micrometric [m_ZnO], submicrometric [sm_ZnO] and nano-metric powder [n_ZnO]), were studied by dilatometry tech-nique at constant heating rates (1°C, 5°C, and 15°C/min).The study of sintering kinetics for the early stages can be con-ducted using various kinetic models of sintering.17,23–25 Tostudy the activation energy for densification during the earlystage of sintering, the classical model developed by Johnson23

for constant heating rate (CHR) sintering was chosen. Severalmodels were developed to understand the sintering behaviorduring the initial stage. The general equation for shrinkagerate (dY/dt) in this stage can be written as in Eq. (1)23:

dY

dt¼ A0

1

GmYn

expð� QRTÞ

T(1)

where Y = Dl/l0 is the linear shrinkage, A0 is a constantdepending only on the material and on sintering mechanism,

W. Mullins—contributing editor

Manuscript No. 29656. Received April 27, 2011; approved August 31, 2011.This work was supported by the Portuguese Agency of Innovation (Adi) under pro-

ject QREN/3435-Nanoxides, and partially supported by the European Commissionunder project INVISIBLE (Advanced Grant from ERC no. 228144).

†Author to whom correspondence should be addressed. e-mail: nuno.neves@fct.unl.pt

204

J. Am. Ceram. Soc., 95 [1] 204–210 (2012)

DOI: 10.1111/j.1551-2916.2011.04874.x

© 2011 The American Ceramic Society

Journal

G is the particle size, Q is the activation energy for sintering,R is the gas constant, T is the absolute temperature, t is thetime and the exponents m and n can take values of m = 1and n = 0 for viscous flow, m = 3 and n = 1 for volume dif-fusion and m = 4 and n = 2 for grain boundary diffusionmechanism. Applying logarithms in both terms, we candetermine the activation energy value in the initial stage, ofan Arrhenius plot of ln (T·dY/dt) vs 1/T. As grain growthdoes not occur in the initial stage of sintering, the generalequation for sintering proposed by Woolfrey and Bannister24

stands as in Eq. (2):

T2 dY

dt¼ aQ

ðnþ 1ÞR� �

� ðYÞ (2)

where T, Y, Q, R, and n have the same meaning as beforeand a is the heating rate. Based on this equation, under con-ditions of CHR, a plot of T2dY/dt versus Y provides astraight line with a slope of a·Q/(n + 1)R, where Q and n canbe determined from Q/(n + 1). Also, the influence of greendensity under nonisothermal conditions was considered forthe three powders.

To verify the influence of particle size, sintering tempera-ture and holding time on densification and grain growth, thesamples were submitted to a CHR of 5°C/min at tempera-tures between 400°C and 1200°C and holding times between1 and 8 h. The scale-up effect on densification was also eval-uated, as well as the influence of the started powder and finalrelative density of several 2 in. ZnO ceramic targets (includ-ing a commercial one) prepared and used in the productionof ZnO thin films by rf magnetron sputtering.

II. Experimental Procedure

Commercial Aldrich ZnO powders (purity >99.9%) withthree different particle sizes, were used in this study. Particlesize distributions of m_ZnO (Ref. 96479) and sm_ZnO (Ref.205532) were calculated by light scattering using Malvernzeta seizer equipment (Malvern Instruments Ltd., Malvern,U.K.). Each powder (0.1 g) was dispersed in distilled water(100 mL) with dispersant (Dolapix PC67; Zschimmer &Schwarz GmbH & Co KG Chemische Fabriken, Lahnstein,Germany) and followed by ultrasonic bath for 5 min. Theaverage particle size of m_ZnO and sm_ZnO powders wascalculated using the results obtained on three different sus-pensions of each powder. For n_ZnO (Ref. 544906), theaverage particle size (D) was calculated based on the Eq. (3),assuming that the particles have a spherical shape:

D ¼ 6

Sq(3)

where S is the specific surface area and ρ is the theoreticaldensity (TD) of the powder (5.61 g/cm3).

The specific surface area was determined by the Brunauer–Emmett–Teller (BET) method using nitrogen adsorption witha Quantachrome Nova 1000E equipment (QuantachromeInstruments, Boynton Beach, FL) series that uses He as car-rier gas. Crystallite size of the powders/films and structuralcharacterization were carried out using a Bruker D8 AdvanceX-ray diffractometer (Bruker AXS GmbH, Karlsruhe,Germany) with CuKa radiation (k = 1.5406 A).

Cylindrical powder compacts of ~6.4 mm diameter and~6 mm height were uniaxially cold pressed in a hydraulicpress without binders or additives. The obtained green bodieshad a relative density of 62% ± 1% (geometric density) ofTD (5.61 g/cm3). The study of sintering behavior was madein a vertical dilatometer Linseis L-75 Platinum Series (LinseisMessgerate GmbH, Selb, Germany) in air. For each type ofpowder, we used different heating rates, sintering tempera-tures, and holding times at maximum temperature. The cool-

ing rate was constant at 10°C/min. The shrinkage of analumina sample was also measured under identical conditionsto correct the differences of shrinkage between the dilatome-ter rod and the sample holder. Densities of the sintered sam-ples were measured by the Archimedes liquid immersiontechnique with distilled water. The final densities wereobtained by the average of three samples sintered under thesame conditions.

Morphological and microstructural characterizations werecarried out by scanning electron microscopy (SEM) using aHitachi SU 70 microscope (Hitachi High-TechnologiesCanada, Inc., Toronto, Canada), which runs at 30 kV. Imageanalyzer program was used to calculate the mean grain size ofthe samples. Over 300 grains were measured for each sample.

The scale-up effect on densification was also studied usingdifferent diameter dies (1.27, 2.5, and 6 cm) to prepare thepressed samples. The samples used in this study had a greendensity of 52% ± 1% and were sintered in a muffle labora-tory furnace at different temperatures with 2 h of holdingtime and CHR of 5°C/min.

Several ~5 cm (2 in.) ZnO ceramic targets were sinteredwith n_ZnO, sm_ZnO, and m_ZnO powders. Apart from ahigh-density n_ZnO target (n_ZnO (HD)), which was submit-ted to cold isostatic pressing (CIP) at 300 MPa to enhancegreen density before sintering, all the targets were uniaxiallycold pressed and sintered at different temperatures for 2h ofholding time and CHR of 5°C/min. A commercial targetwith a density of ~80% TD was also used. The targets wereplaced in an AJA, Model ATC ORION8 RF magnetronsputtering system (AJA International, Inc., North Scituate,MA), at 15 cm from the substrate (soda-lime glass) and thebase pressure of the system was 3.4 9 10�4 Pa. The samedeposition conditions were used to all the ZnO films grownwith the different targets. These conditions were chosenaccording to our previous works concerning the role of depo-sition parameters on the films’ performances10,11: argon pres-sure (PAr) of 0.6Pa; RF power of 100 W, without addition ofoxygen gas and unheated substrate, to avoid extra processparameters influence on the films process.

The optical transmission of the films deposited on glasssubstrates was measured in a double beam spectrophotome-ter (Shimadzu UV-VIS-NIR 3100 PC; Shimadzu EuropaGmbH, Duisburg, Germany), the thickness was determinedin a profilometer (Ambios XP-Plus 200 Stylus; Ambios Tech-nology, Inc., Santa Cruz, CA) and the morphology wasobserved with an atomic force microscope (AFM, AsylumMFP 3D; Asylum Research, Santa Barbara, CA) operated inac mode. Electrical transport properties were examined byHall Effect (Biorad HL 5500; Biorad, Hercules, CA) usingsamples with the van der Pauw configuration. Analyses ofthe O content of the powders were made by X-ray photoelec-tron spectroscopy (XPS).

III. Results and Discussion

The mean particle size obtained for the powders as well asits specific surface area and crystallite size are presented inTable I.

The data show specific surface area for n_ZnO superior tothe one obtained for sm_ZnO and n_ZnO. The crystallitesize obtained by DRX for n_ZnO is similar to the particlesize for the same powder, which allows us to conclude that

Table I. Physical Characteristics of ZnO Powders

Powder

Average particle

size (nm)

Specific surface

area (m2/g)

Crystallite size

(DRX) (nm)

m_ZnO 1129 6.869 198sm_ZnO 283 7.354 137n_ZnO 81 (BET) 13.280 80

January 2012 Sintering of Nano- and Micro-Sized ZnO Powder Targets 205

the n_ZnO is composed of single crystals. X-ray diffractionmeasurements show Zincite (ZnO) as the single phase presentin all three powders.

To verify the influence of green density on linear shrinkageand shrinkage rate of the ZnO nanopowder (n_ZnO), sam-ples prepared with different green densities (35%, 44%, and62% of TD) were sintered at a CHR of 5°C/min. Asexpected, the linear shrinkage decreases for samples preparedwith higher green densities [Fig. 1(A)].

As shown in Fig. 1(B), no significant change was observedfor the maximum value of shrinkage rate of n_ZnO powder,which is approximately 710°C for all samples. These resultswere not observed for m_ZnO and sm_ZnO powders(Table II) where the increase in green density substantiallydecreases the temperature of MSR (TMSR), especially for thepowder with larger particle size (m_ZnO). The MSR valuestend to decrease for higher green densities and increase forsmall particle size powders. Several factors can contribute tothese results. Firstly n_ZnO presents higher surface energyrelated to smaller particle size and higher specific surface

area than the ones in the microsize range.21,26 Furthermorethere are an increased fraction of mass transport by grain-boundary diffusion, as a consequence of the large boundaryarea associated to nanoparticles.18,20

The effect of the heating rate on the nonisothermal sinter-ing behavior of n_ZnO is shown in Fig. 2. Linear shrinkagecurves obtained shows a delay in the onset temperature forthe densification [Fig. 2(A)]. The shrinkage rate dY/dt islower for slower heating rates, and the higher values ofshrinkage rate are close to 700°C for all heating rates[Fig. 2(B)]. This is of high importance to the study of theearly sintering stages. As suggested by Lange,27 for tempera-tures below the one of the greater linear shrinkage rate, thesintering kinetic is dominated by densification mechanisms,and above this temperature, nondensification mechanismssuch as grain growth will prevail.

Particle size measurements from SEM micrographs showthat they do not increase significantly for linear shrinkage upto 3%. Therefore, the activation energies for the initial stageof sintering were calculated considering the slopes in Fig. 3,according to Eq. (1). For n_ZnO powder, the activationenergy for densification calculated was ~201 ± 3 kJ/mol, ingood agreement with the theoretical values of pure ZnO (200–300 kJ/mol) obtained by other authors,13,14,18,20 and the val-ues obtained in this work for sm_ZnO (~332 ± 20 kJ/mol)and m_ZnO (~273 ± 9 kJ/mol).

On the basis of model proposed by Woolfrey and Bannis-ter,24 we can determine the sintering mechanism for the ini-tial sintering stage with a plot of ln (Y) as a function of ln(a) (Fig. 4) at a constant temperature and considering theslope equal to �1/(n + 1). The temperature range used in thecalculation of the slope for n_ZnO powder (500°C–550°C) isthe one which ensures no significant grain growth during the

(A)

(B)

Fig. 1. Linear Shrinkage (A) and shrinkage rate (B) withtemperature of n_ZnO powder as a function of green density for aconstant heating rate of 5°C/min.

Table II. Influence of Green Density on MaximumShrinkage Rate and Corresponding Temperature for Each

Studied Powder

Samples

(%)

m_ZnO sm_ZnO n_ZnO

MSR

(%·min�1)

TMSR

(°C)MSR

(%·min�1)

TMSR

(°C)MSR

(%·min�1)

TMSR

(°C)

35 �0.441 934 �0.505 803 �0.537 71244 �0.394 885 �0.443 770 �0.465 70962 �0.322 803 �0.294 719 �0.363 711

(A)

(B)

Fig. 2. Linear shrinkage (A) and shrinkage rate (B) of n_ZnOpowder (~62% TD) for heating rates of 1°C, 5°C, and 15°C/min.

206 Journal of the American Ceramic Society—Neves et al. Vol. 95, No. 1

initial stage of sintering. The obtained n value was 0.75,which lies between the theoretical values for the viscous flowand volume diffusion. These values are consistent with thoseobtained by Han et al.28 (0.6 for pure ZnO with a meanparticle size of 0.260 lm similar to sm_ZnO), who found twodifferent regions for ZnO and Mn-doped ZnO: in the firstone a viscous flow mechanism predominates, whereas in thesecond one the volume diffusion predominates.

The effect of sintering temperature on the densification ofthe samples prepared with different commercial ZnO powderswas studied in compacts with the same green density (~62%

TD) sintered in a dilatometer at different temperatures andconstant holding time of 2 h. As shown in Fig. 5, consider-able higher densities at lower temperatures can be achievedin the case of n_ZnO powder. For a sintering temperature of700°C, the relative density of samples prepared with n_ZnOpowder is ~97%, which is higher than that obtained for thesm_ZnO powder (~88% TD). On the other hand, for them_ZnO powder sintered at this temperature, the increase indensity is quite low (~68% TD) when compared with the oneobtained by uniaxial cold pressing, 62% TD. Above 800°C,the targets made with n_ZnO and sm_ZnO powders havereached final densities near TD, while for the ones made withm_ZnO, it happens only at 1200°C. These results show thatsintering becomes more favorable as particle size of the pow-ders reduces, which is related to the increase in the specificsurface area and the surface energy of the smaller particles.

The influence of holding time in densification and graingrowth during sintering of n_ZnO was studied using severaldifferent holding times at constant temperature, which waschosen according to the MSR temperature (TMSR) observedfor n_ZnO (~710°C). The selected temperature is below theTMSR to avoid a grain overgrowth. The samples with a greendensity of ~62% TD were sintered at a CHR of 5°C/minwith different holding times at 680°C. Results for the relativedensity and grain size are presented in Table III.

The results shown in Table III reveal a moderate increasein grain size as the holding time for a temperature below TMSR

raises. Samples with densities higher than 93% were obtained

Fig. 4. ln (Y) vs ln (a) plot for a temperature range between 500°Cand 550°C.

Fig. 5. Influence of sintering temperature on relative density ofn_ZnO, sm_ZnO and m_ZnO with constant heating rate of 5°C/min.

Fig. 3. Arrhenius plot ln(T·dY/dt) vs 1/T for 1%, 2%, and 3%constant linear shrinkage values for n_ZnO sintered at differentheating rates (1°C, 5°C, and 15°C/min).

Table III. Influence of Holding Time on Sintering and

Relative Density of n_ZnO Powder

Powder Sintering cycle

Relative

density (%)

Grain size

(lm)

n_ZnO 680°C 1 h 93.19 0.412 h 94.92 0.564 h 97.86 0.678 h 98.47 0.75

(A)

(B)

Fig. 6. SEM micrographs of n_ZnO samples sintered at 680°C: (A)1 h and (B) 8 h.

January 2012 Sintering of Nano- and Micro-Sized ZnO Powder Targets 207

with grain size between 410 and 750 nm [Figs. 6(A) and (B)].At this temperature, high densities without grain overgrowthwere achieved. A bimodal microstructure with a wide disper-sion of grain sizes is observed in SEM micrographs. Hyneset al.18 also identified two families of grains during sinteringof nanocrystalline ZnO. A decrease in porosity with theincrease in density from 93.19% to 98.47% TD was alsoobserved.

Figure 7 shows the influence of temperature on the relativedensity and grain growth for n_ZnO sintered at differenttemperatures.

The samples sintered with the n_ZnO powder acquired ahigh density (>97%) at a relatively low temperature of 680°C.The enhancement in density is substantial, especially between600°C and 700°C, with an increase from 79.29% to 97.30%,respectively. This behavior also occurs in terms of grain size,which increases from 0.240 to 0.775 lm. The same trend isobserved as the temperature increases further, going from~1 lm at 750°C to ~5 lm at 1100°C. The densification andgrain growth are both driven by diffusive mechanisms thatare influenced by several parameters like pressure, particle

size, or temperature, resulting in the simultaneous activationof densification and grain growth, particularly in the finalsintering stage.29 Above 90% TD (final sintering stage),grains can grow when the continuous network of poresbreaks down on grain boundaries and grains tend to growrapidly,30 invalidating the advantages of using nano-sizedparticles [Figs. 8(A) and (B)]. It is also remarkable that thedispersion on grains size tends to disappear as the tempera-ture increases, producing compact samples with mean grainsize above 5 lm at 1100°C.

The scale-up effect was studied using several dies with dif-ferent diameters keeping constant the ratio between thicknessand target diameter, to check the influence of sample sizes onthe final relative density. Figure 9 shows the density variationwith temperature for samples with ~52% TD sintered at dif-ferent temperatures with 2 h of holding time and CHR of 5°C/min. As observed, there are no significant variations in thesintering behavior for samples of n_ZnO [Fig. 9(C)] with

Fig. 7. Relative density and grain size of nanocrystalline ZnOcompacts sintered at different temperatures and a constant holdingtime of 2 h.

(A)

(B)

Fig. 8. SEM micrographs of n_ZnO samples sintered at: (A) 900°C/2 h and (B) 1100°C/2 h.

(A)

(B)

(C)

Fig. 9. Relative density versus temperature for samples with agreen density of ~52%TD and different diameters—(A) m_ZnO, (B)sm_ZnO, (C) n_ZnO.

208 Journal of the American Ceramic Society—Neves et al. Vol. 95, No. 1

higher dimensions. However, in sm_ZnO [Fig. 9(B)] andmainly in m_ZnO [Fig. 9(A)], there is a tendency for a delayin densification especially in the samples prepared with the6 cm die. Thus, we can conclude that targets densification isimproved by the use of nanoparticles. This behavior proveswhat was discussed before about the high sinterability ofn_ZnO powder. The lower sintering temperature of nanocrys-talline powders is related to the existence of a large fractionof atoms at grain boundaries, which causes an increase in thediffusivity between particles compared with polycrystallinematerials with larger particle sizes (micrometric powders).Moreover, the higher reactivity associated with the higherspecific surface area of n_ZnO powder acts as a driving forcefor sintering, as the surface diffusion between the particlesincreases, favoring the sintering mechanisms at low tempera-tures.31,32 The nondependence of target size on densificationfor n_ZnO is an advantage compared with sm_ZnO andm_ZnO powders for the production of ZnO sputtering tar-gets as it is possible to achieve good target properties repro-ducibility at substantially much lower temperatures, avoidingsome problems associated with scale-up effect.

Several targets produced with ~5 cm (2 in.) size were usedin a sputtering system for thin films production. The sinter-ing conditions of targets and the properties of the filmsobtained are shown in Table IV.

The targets were optimized to study the role of particlesize and final density on the morphological and electro-opti-cal properties of the sputtered films. Each target was pre-pared to obtain approximately the same relative density ofthe commercial ZnO sputtering target (~80% TD). With80% of relative density, we are at an early stage of densifica-tion (porous structures without significant change in the ini-tial particle size [Fig. 10]). A high density target sinteringn_ZnO at higher temperatures was also prepared after CIP[Fig. 8(B)].

No significant differences were attained in terms of mor-phology and structure, for all the deposited films. The sput-tered ZnO films were polycrystalline, preferentially orientedalong the c axis (002) (Fig. 11), typical of a hexagonal struc-ture, which can be explained in terms of surface energy mini-mization.33 According to the XRD spectra peak intensityshown in Fig. 11, the crystallite sizes of the deposited filmsare 60.3, 45.7 and 42.3 nm, using m_ZnO, sm_ZnO andn_ZnO (LD) targets, respectively. These results show that thehigh crystallite size is obtained for films produced usingm_ZnO targets. Nonetheless, the SEM top view images showa very smooth surface with comparable morphologies (seeSEM images in Fig. 11), which is in good agreement withthe roughness values obtained by AFM (Table IV). The opti-cal properties of the films are independent of the target(approximately 80% in the visible range for all films). HallEffect measurements show that sputtered films obtained fromn_ZnO targets present the lowest resistivity. Minami et al.34

found that the oxygen content into the thin films decreasesas the oxygen content of the target used in the depositiondecreases. As the sintering conditions of the powders anddeposition conditions were similar for all prepared targets,

the lower resistivity obtained with n_ZnO targets (n_ZnO(LD) and n_ZnO (HD)) could be related to the startingpowder properties (namely the depletion of oxygen). Accord-ing to other authors,9,10,35–37 the resistivity of ZnO isstrongly influenced by the quantity of oxygen incorporatedbeing the highest resistivity value (above 109 Ω·cm) obtainedin quasi stoichiometry films. XPS analysis performed on thestarting powders shows that n_ZnO powder presents a non-stoichiometric Zn:O composition (O/Zn = 0.85 atomic ratio),while both sm_ZnO and m_ZnO powders present a stoichi-ometric Zn:O composition (O/Zn = 1.02 and O/Zn = 1.04atomic ratio, respectively). Therefore, the difference in resis-tivity obtained in the films produced with n-ZnO targetscould be related to a nonstoichiometric Zn:O composition ofthe started n_ZnO powder, and consequently influences thedeposition of the thin films. As the ZnO films were processedusing the same base pressure and controlled argon pressure(process gas), it means that the films produced are mainly

Table IV. Sintering Conditions of ZnO Ceramic Targets and Thin Films Properties Obtained by RFMagnetron Sputtering Deposition (RF power = 100 W; Par = 0.6 Pa)

Powder

ZnO targets ZnO sputtering thin films on glass

Green

density (%)

Sintering

temperature (°C)Relative

density (%)

Thickness

(nm)

Roughness

(nm)

Transmittance

(400–800 nm; in %)

Resistivity

(Ω·cm)

m_ZnO 51.6 795 80.6 114 0.42 80 3.7 9 107

sm_ZnO 51.9 705 81.1 90 0.60 77 8.1 9 104

n_ZnO (LD) 51.3 610 80.3 103 0.38 79 1.4 9 100

n_ZnO (HD) 62.1 1100 99.1 98 0.48 83 6.6 9 102

Commercial target n.a. n.a. 79.4 95 0.52 80 1.7 9 106

n.a., not available.

Fig. 10. SEM micrographs of n_ZnO ceramic target sintered at610°C for 2 h.

Fig. 11. XRD pattern of: n_ZnO (LD), sm_ZnO, and (LD) andm_ZnO (LD); Top view SEM images of films produced with RFpower of 100 W and deposition pressure of 0.6 Pa.

January 2012 Sintering of Nano- and Micro-Sized ZnO Powder Targets 209

dependent on the started target characteristics. For each tar-get, the tuning of the electrical properties films could beachieved by introducing oxygen gas during the deposition,with controlled partial pressure, being the subject of furtherstudies. The films obtained show electrical and optical perfor-mances suitable for optical and electronic devices application,such as thin film transistors5,11 or gas sensors.10

IV. Conclusions

Initial stage sintering of three as-received ZnO powders wasstudied under nonisothermal conditions. The temperature ofMSR is independent of the initial density (green density) andheating rate in the case of n_ZnO, which is not verified form_ZnO and sm_ZnO in which a decrease in green densitycauses an increase in MSR temperature. An activation energyof 202 kJ/mol was obtained for n_ZnO densification signifi-cantly below the values obtained for other studied powdersm_ZnO and sm_ZnO (270–320 kJ/mol). A combination ofviscous flow and volume diffusion mechanism was found tobe probably controlling the initial stage of sintering forn_ZnO. It was observed that targets prepared with n_ZnOreach higher densities at significantly lower temperatures(~680°C) compared with sm_ZnO (~800°C) and m_ZnO(>900°C). When sintered at temperatures below TMSR for dif-ferent holding times, submicrometric grains were obtainedwith controlled grain growth. Due to the high reactivity andsinterability of n_ZnO, similar densities were obtained for alldiameters used.

It was shown that the electrical properties of films depos-ited by rf magnetron sputtering under the same conditions,are strongly influenced by the properties of the started pow-der, and consequently the target properties. The n_ZnOpowder was the one leading to the production of nonstoi-chiometric films corresponding to an increase in the electricalconductivity.

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