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Journal of Crystal Growth 276 (2005) 121–127

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Microstructural, optical, and magnetic properties of(Zn1�xMnx)O thin films grown on (0 0 0 1) Al2O3 substrates

S.J. Leea, H.S. Leea, D.Y. Kima,�, T.W. Kimb

aDepartment of Semiconductor Science, Dongguk University, 3-26 Phil-dong, Chung-gu, Seoul 100-715, Republic of KoreabAdvanced Semiconductor Research Center, Division of Electrical and Computer Engineering, Hanyang University, 17 Haengdang-dong,

Seongdong-gu, Seoul 133-791, Republic of Korea

Received 6 September 2004; accepted 1 November 2004

Communicated by M. Schieber

Available online 24 December 2004

Abstract

The microstructural, optical, and magnetic properties of (Zn1�xMnx)O thin films grown on (0 0 0 1) Al2O3 substrates

by using a radio-frequency magnetron sputtering were investigated. X-ray diffraction, atomic force microscopy, and

scanning electron microscopy measurements showed that the microstructural properties of the (Zn1�xMnx)O thin films

were improved by increasing the thickness of the (Zn1�xMnx)O buffer layer, which might originate from the

suppression of the columnar-growth mode at the initial growth stage because the (Zn1�xMnx)O buffer layer grown at a

lower temperature had a relaxed c-axis preference. The photoluminescence peak at 419 nm from the (Zn0.91Mn0.09)O

thin films could be attributed to the activation of Mn2+ ions substituting for Zn2+ ions, indicative of the existence of a

diluted magnetic semiconductors. The magnetization curve as a function of the magnetic field at 15K indicated that

ferromagnetism existed in the (Zn0.91Mn0.09)O thin films, and the magnetization curve as a function of the temperature

showed that the Tc value of the (Zn0.91Mn0.09)O thin films was 110K. These results can help improve understanding of

the effects of the microstructural properties on the magnetic properties of (Zn1�xMnx)O thin films grown on (0 0 0 1)

Al2O3 substrates.

r 2004 Elsevier B.V. All rights reserved.

PACS: 75.50.Pp; 75.70.Ak; 75.60.Ej

Keywords: A1. (Zn1�xMnx)O thin film; A2. Sputtering; B2. Diluted magnetic semiconductor

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.11.300

ng author. Tel.: +822 2260 3802; fax:

.

ss: dykim@dongguk.edu (D.Y. Kim).

1. Introduction

Diluted magnetic semiconductor (DMS) materi-als, which can utilize both the spin and the chargeproperties of carriers, have become particularly

d.

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S.J. Lee et al. / Journal of Crystal Growth 276 (2005) 121–127122

attractive because of their potential applications inspintronic devices [1,2]. Among the various kindsof DMS materials, (Zn1�xMnx)O DMS thin filmsare of great interest because they are theoreticallyexpected to have a value of the ferromagnetictransition temperature (Tc) above room tempera-ture as a consequence of the large energy gap andthe large effective mass of the ZnO host material[3]. Since the ZnO host material in (Zn1�xMnx)Othin films has a large exciton binding energy of60meV [4], (Zn1�xMnx)O thin films hold promisefor promising applications in spin-photonicdevices. Recently, many studies concerning theformation and the characterization of ZnO-basedDMSs have been performed to obtain high Tc

values [5–8]. Even though a few worksconcerning Mn+-implanted ZnO bulks with ahigh Tc of 250 1C have been reported [9], theformation of (Zn1�xMnx)O thin films with a highTc has not succeeded due to delicate problemsduring sample growth [10,11]. Since the micro-structural properties of (Zn1�xMnx)O thin filmssignificantly affect the optical and the magneticproperties of the thin films, systematic correlationsof the microstructural properties with the opticaland the magnetic properties in (Zn1�xMnx)O thinfilms are very important for increasing the Tc

value.This article reports data for the microstructural,

optical, and magnetic properties of (Zn1�xMnx)Othin films grown on (0 0 0 1) Al2O3 substrates byusing a radio-frequency (rf) magnetron sputtering.X-ray diffraction (XRD), atomic force microscopy(AFM), and scanning electron microscopy (SEM)measurements were performed to characterize thestructures and the surfaces of the grown(Zn1�xMnx)O buffer and active layers, and en-ergy-dispersive X-ray spectroscopy (EDX) mea-surements were performed to investigate thestoichiometry of the (Zn1�xMnx)O thin films.Hall-effect measurements were performed in orderto determine their carrier types and carrierconcentrations. Photoluminescence (PL) measure-ments were performed to investigate the opticalproperties of the (Zn1�xMnx)O thin films, andsuperconducting quantum interference device(SQUID) measurements were carried out tocharacterize their magnetic properties.

2. Experimental details

The Zn1–xMnxO thin films used in this studywere grown on (0 0 0 1) Al2O3 substrates by usingan rf magnetron sputtering method. The(Zn1�xMnx)O targets were prepared by using thestandard ceramic synthesis technique. The deposi-tion of a (Zn1�xMnx)O active layer onto a 10-, 20-,30-, or 40-nm-thick (Zn1�xMnx)O thin film, whichhad been grown on a (0 0 0 1) Al2O3 substrate at200 1C, was performed at a substrate temperatureof 450 1C in a mixture of Ar and O2 gases(15:15 sccm) at an rf power of 120W. Thisrelatively low growth temperature of the(Zn1�xMnx)O buffer layer was used to improvethe crystallinity of the film, and the growthtemperature of the (Zn1�xMnx)O active layer wasused to increase the solubility of Mn+ ions in theZnO thin films. The O2 overpressure effectivelyreduced the number of oxygen vacancies in the(Zn1�xMnx)O thin films. The carrier type and thecarrier concentration of the (Zn1�xMnx)O thinfilms were controlled by changing the O2 partialpressure [12]. The carrier type of the (Zn1�xMnx)Othin films grown with an O2 overpressure in thisstudy, determined from the Hall effect measure-ments, was p-type, and the carrier concentrationswere about �1015 cm�3, which might be consid-ered to be the limit of the intrinsic levels. As soonas the growth process for the (Zn1�xMnx)O thinfilms had been completed, the samples werepromptly characterized in order to avoid transmu-tation of the physical properties.XRD measurements were performed by using

the CuKa source from Bede D3 system, and AFMmeasurements were carried out by using aDigital Instruments Nanoscope IIIa system. SEMand EDX measurements were performed byusing an FE SEM XL-30 system and an EDAXNEW XL-30 system, respectively. PL measure-ments were performed by using a 75-cm mono-chromator equipped with a GaAs photomultipliertube. The excitation source was the 3250-A line ofa He-Cd laser, and the sample temperature wascontrolled between 10 and 300K by using a Hedisplex system. SQUID measurements were per-formed using a Quantum Design MPMSXLsystem.

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S.J. Lee et al. / Journal of Crystal Growth 276 (2005) 121–127 123

3. Results and discussion

The (Zn1�xMnx)O thin films had mirror-likesurfaces with no indications of pinholes, whichwas confirmed by using Normarski optical micro-scopy measurements. XRD measurements on the(Zn1�xMnx)O thin films grown at several tem-peratures were performed to determine the opti-mum growth condition. Fig. 1 shows the XRDpatterns of (Zn1�xMnx)O thin films grown on(0 0 0 1) Al2O3 substrates at: (a) 200, (b) 300, (c)400, (d) 500, and (e) 600 1C. The (0 0 0 2) and(0 0 0 4) Ka1 diffraction peaks corresponding to the(Zn1�xMnx)O thin film were clearly observed in allof the (Zn1�xMnx)O films. These results indicatethat the (Zn1�xMnx)O films grown on (0 0 0 1)Al2O3 substrates have a strong c-axis preferencenormal to the substrate and that the crystal qualityof the (Zn1�xMnx)O films is improved by increas-ing the substrate temperature. However, aninordinate c-axis preference of the (Zn1�xMnx)O

Fig. 1. X-ray diffraction patterns of (Zn1�xMnx)O thin films

grown on (0 0 0 1) Al2O3 substrates at (a) 200, (b) 300, (c) 400,

(d) 500 and (e) 600 1C. The inset indicates the integrated (0 0 0 2)

intensity determined from X-ray diffraction patterns as a

function of the reciprocal growth temperature.

film causes the film to form into a columnarstructure, which deteriorates the efficiency offabricated devices such as lasers, light emittingdiodes, photodetectors, and microelectronic tran-sistors. The kinetics for the growth of(Zn1�xMnx)O thin films with high crystallinitywere elaborately investigated to remove theinherent problems of the columnar structure. Theintegrated (0 0 0 2) peak intensity, indicative of thedegree of c-axis preference, as a function of thereciprocal growth temperature is shown in theinset of Fig. 1. Two modes with different slopesintegrated are observed in the inset of Fig. 1, andthese growth modes indicate that the activation ortransformation of the solid state occurs at a criticaltemperature. The slope of the growth mode in thegrowth temperature range between 200 and 350 1Cindicates a relatively inactive aspect. This resultsuggests that the growth of the (Zn1�xMnx)O thinfilms is related to a relaxation mode of the c-axispreferential growth. However, the steep slope inthe growth temperature range between 400 and600 1C can be attributed to an enhancement modeof the c-axis preferential growth.Based on the XRD results, we introduced a two-

step growth of a buffer layer at a lower tempera-ture and an active layer at a higher temperature, toobtain high-quality (Zn1�xMnx)O thin films withgood crystallinities. The (Zn1�xMnx)O bufferlayers with the relaxed c-axis preference grownon (0 0 0 1) Al2O3 substrates at 200 1C wereemployed in order to eliminate the formation ofcolumnar structures during the initial growth. The(Zn1�xMnx)O active layers were grown on(Zn1�xMnx)O buffer layers at 450 1C because wehad previously found that the (Zn1�xMnx)O thinfilms grown on (0 0 0 1) Al2O3 substrates at atemperature of 400 1C in a mixture of Ar, O2, andN2 gases (5:5:10) at an rf power of 80W had thebest surface morphology [13]. In this study, the(Zn1�xMnx)O thin films, which were grown underdifferent growth conditions, also had the bestsurface morphologies.Fig. 2 shows AFM images of: (a) 10-, (b) 20-, (c)

30-, and (d) 40-nm (Zn1�xMnx)O buffer layersgrown on (0 0 0 1) Al2O3 substrates at 200 1C. Theimages in Fig. 2 show that the grain size of the(Zn1�xMnx)O buffer layer is increased with

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Fig. 2. Atomic force microscopy images of: (a) 10-, (b) 20-, (c) 30- and (d) 40-nm (Zn1�xMnx)O buffer layers grown on (0 0 0 1) Al2O3

substrates at 200 1C.

S.J. Lee et al. / Journal of Crystal Growth 276 (2005) 121–127124

increasing thickness of the (Zn1�xMnx)O bufferlayer. When the thickness of the (Zn1�xMnx)Obuffer layer is above 40 nm, the buffer layer hasalmost the same high-quality surface morphologyas that of the film with a thickness of 40 nm. Theincrease in the lateral grain size of the(Zn1�xMnx)O buffer layer with increasing thick-ness of the buffer layer is attributed to the relaxedc-axis preference.

Fig. 3 shows SEM images of the (Zn1�xMnx)Oactive layers grown on (a) 10-, (b) 20-, (c) 30-, and(d) 40-nm-thick buffer layers at 450 1C. Themagnitudes of the pinholes and the columnsexisting in the (Zn1�xMnx)O films are significantlydecreased with increasing thickness of the bufferlayer. The (Zn1�xMnx)O active layer grown on a40-nm-thick buffer layer has a mirror-like surfacewithout any indications of pinholes and columns,

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Fig. 3. Scanning electron microscopy images of (Zn1�xMnx)O

active layer films grown on: (a) 10-, (b) 20-, (c) 30- and (d) 40-

nm-thick buffer layers at 450 1C.

Fig. 4. Microstructural parameters of the (Zn0.91Mn0.09)O

active and buffer layers as functions of the buffer layer

thickness. The lateral grain size of the (Zn0.91Mn0.09)O buffer

layer and the sprouted column density of the (Zn0.91Mn0.09)O

active layer were determined from microstructural analyses of

atomic force microscopy and scanning electron microscopy

images, respectively.

S.J. Lee et al. / Journal of Crystal Growth 276 (2005) 121–127 125

as shown in Fig. 3(d), and the improvement in thesurface topology for the (Zn1�xMnx)O active layergrown on the 40-nm-thick buffer layer might beattributed to the high-quality surface morphologyof the (Zn1�xMnx)O buffer layer. The Mn contentof the grown (Zn1�xMnx)O thin film, determinedfrom the in situ EDX measurements, was approxi-mately 9%.

The microstructural parameters of the(Zn0.91Mn0.09)O active and buffer layers as func-tions of the buffer layer thickness were investi-gated in order to clarify the effect of the bufferlayers on the structural properties of(Zn0.91Mn0.09)O thin films. The lateral grain sizesof the (Zn0.91Mn0.09)O buffer layers, determinedfrom the AFM images shown in Fig. 2, and thesprouted column densities of the active layers,determined from the SEM images shown in Fig. 3,are plotted in Fig. 4. The density of sproutedcolumns for the (Zn0.91Mn0.09)O active layer isdramatically decreased with increasing buffer layerthickness, and the lateral grain size at the top edgeof the (Zn0.91Mn0.09)O buffer layer is graduallyincreased. This result indicates that the increase in

the lateral size at the top edge of the buffer grainsmay lead to a decrease in the density of nucleationsites for the growth of columns because theprecursors for columnar structures during theinitial growth occur at the grain boundaries.The optical properties of (Zn0.91Mn0.09)O films

with 40-nm-thick buffer layers grown on (0 0 0 1)Al2O3 substrates were investigated by usingtemperature-dependent PL measurements. Fig. 5shows PL spectra at various temperatures for the(Zn0.91Mn0.09)O thin films grown on (0 0 0 1) Al2O3

substrates. Two well-resolved PL peaks, one at 388and the other at 411 nm, together with a broadhump at 517 nm, were observed for the(Zn0.91Mn0.09)O thin film. The PL peak at388 nm is considered to be from near-band-edgeemission in the ZnO host material [14], and thepeak at 411 nm is attributed to Zn vacancies [14]

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Fig. 5. Photoluminescence spectra at various temperatures for

the (Zn0.91Mn0.09)O thin films grown on (0 0 0 1) Al2O3

substrates.

Fig. 6. Magnetization curve as a function of the magnetic field

at 15K for (Zn0.91Mn0.09)O thin films grown on (0 0 0 1) Al2O3

substrates.

S.J. Lee et al. / Journal of Crystal Growth 276 (2005) 121–127126

due to the substitution of Mn2+ ions for the Zn2+

ions resulting from the Mn incorporation. PLspectra at low temperatures depict that a shoulderat 419 nm related to the peak at 411 nm appears, asshown in Fig. 5. The shoulder 419 nm is attributedto atomic level transitions of the Mn2+ ions[15,16], and the appearance of the peak due tothe Mn2+ atomic transition clarifies the effectiveincorporation of the Mn2+ with ZnO hostmaterials [16]. The broad hump at 517 nm isrelated to an emission originating from oxygenvacancies [14].

The magnetization curve as a function of themagnetic field for the (Zn0.91Mn0.09)O thin filmswith 40-nm buffer layers grown on (0 0 0 1) Al2O3

substrates is shown in Fig. 6. When an externalmagnetic field is applied parallel to the filmsurface, a sharp hysteresis loop, indicative offerromagnetism, clearly appears in the magnetiza-

tion curve, as shown in Fig. 6. This result suggeststhat the easy axis of the magnetization for the(Zn0.91Mn0.09)O thin film is in-plane and that thegrowth process of the (Zn0.91Mn0.09)O thin filmson (0 0 0 1) Al2O3 substrates with 40-nm bufferlayers is well-merged into a lateral growth modebecause the wurtzite lattice structure with a c-axisorientation receives a compressive strain andbecause the easy axis of the in-plane direction forthe (Zn0.91Mn0.09)O thin films originates from acompressive-strain-dependent anisotropy. The re-manent magnetization and the coercive field of the(Zn0.93Mn0.07)O thin films, determined from Fig.6, were 11.2 emu/cm3 and 263Oe, respectively.The magnetization curve as a function of the

temperature for the (Zn0.91Mn0.09)O thin filmsgrown on (0 0 0 1) Al2O3 substrates is shown inFig. 7. Fig. 7 shows that the ferromagneticproperties of the (Zn0.91Mn0.09)O thin films aremaintained until 110K. This Tc value of 110K forthe (Zn0.91Mn0.09)O thin film is the highest valueobserved until now. Although some possibilitiesexist for the presence of secondary phases, such asMnO, MnO2, Mn3O4, and (Mn, Zn)Mn2O4, in the(Zn0.91Mn0.09)O thin film, since MnO and MnO2

films are antiferromagnetic and since the Tc valuesof Mn3O4 and (Mn, Zn)Mn2O4 films are below43K [12,17-18], the ferromagnetic behavior of the(Zn0.91Mn0.09)O thin film grown on a (0 0 0 1)Al2O3 substrate should not be greatly affected by

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Fig. 7. Magnetization curve as a function of the measurement

temperature for (Zn0.91Mn0.09)O thin films grown on (0 0 0 1)

Al2O3 substrates.

S.J. Lee et al. / Journal of Crystal Growth 276 (2005) 121–127 127

the existence of these secondary phases. Further-more, diffraction peaks corresponding to MnO,MnO2, Mn3O4, and (Mn, Zn)Mn2O4, were notobserved in the XRD patterns, as shown in Fig. 1.

4. Summary and conclusions

The microstructural, optical, and magneticproperties of (Zn1�xMnx)O thin films grown on(0 0 0 1) Al2O3 substrates by using the rf magne-tron sputtering system were investigated. XRD,PL, and Hall effect results showed that the(Zn1�xMnx)O thin films were p-type crystallinesemiconductors with single phases, and AFM andSEM images showed that the microstructuralproperties of the (Zn1�xMnx)O buffer and activelayers were improved by the increasing thicknessof the (Zn1�xMnx)O buffer layer. The magnetiza-tion curve as a function of the magnetic field at15K indicated that ferromagnetism existed in the(Zn0.91Mn0.09)O thin film, and the magnetizationcurve as a function of the temperature showed thatthe Tc value of the (Zn0.91Mn0.09)O thin film was110K. The enhancement of the Tc value for the(Zn0.91Mn0.09)O thin film originated from animprovement in the crystallinity of the filmresulting from the suppression of the columnar-growth mode during initial growth. These resultscan help improve understanding the effects of the

microstructural properties on the magnetic proper-ties of (Zn1�xMnx)O thin films grown on (0 0 0 1)Al2O3 substrates.

Acknowledgements

This work was supported by the Korean Scienceand Engineering Foundation through the Quan-tum-functional Semiconductor Research Center atDongguk University.

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