Zn1−xMnxO (0 ≤ x ≤ 0.1) thin films were grown on sapphire substrates by pulsed laser
deposition and rf magnetron sputtering methods. As a target source ceramic pellets prepared by
mixing ZnO and Mn3O4 powders were used. The shift of the absorption edge due to increase the
energy band gap with increasing Mn content was observed in optical absorption spectra of the
Zn1−xMnxO thin films. Magnetic susceptibility and magneto-optical Faraday rotation experiments
show paramagnetic behavior and absence of ferromagnetic ordering in the studied films.This work has been supported in part by grant No. M/128-2004 from the Ministry of
Education and Science of the Ukraine

Fediv, V.I.

Perrone, A.

Savchuk, A.I.

Savchuk, S.A.

Цифровий Репозиторій - Інтелектуальні Фонди Буковинського державного медичного університету

Superlattices and Microstructures 38 (2005) 421–427www.elsevier.com/locate/superlatticesGrowth and characterization of ZnMnO thin filmsA.I. Savchuka,∗, V.I. Fediva, S.A. Savchuka, A. PerronebaDepartment of Physics of Semiconductors and Nanostructures, Chernivtsi National University, 2 KotsyubynskyStreet, 58012 Chernivtsi, UkrainebUniversity of Lecce, Physics Department and INFN, Via per Arnesano, 73100 Lecce, ItalyAvailable online 16 September 2005AbstractZn1−xMnxO (0 ≤ x ≤ 0.1) thin films were grown on sapphire substrates by pulsed laserdeposition and rf magnetron sputtering methods. As a target source ceramic pellets prepared bymixing ZnO and Mn3O4 powders were used. The shift of the absorption edge due to increase theenergy band gap with increasing Mn content was observed in optical absorption spectra of theZn1−xMnxO thin films. Magnetic susceptibility and magneto-optical Faraday rotation experimentsshow paramagnetic behavior and absence of ferromagnetic ordering in the studied films.© 2005 Elsevier Ltd. All rights reserved.Keywords: Semimagnetic semiconductor; Zn1−xMnxO; Thin film; Pulsed laser deposition; Ferromagnetism;Optical absorption; Faraday rotation1. IntroductionSemimagnetic semiconductors or diluted magnetic semiconductors are attractivematerials because of exhibition of the sp–d exchange interaction between the localizedmagnetic moments and the spins of the band carriers. The unique spin-dependentphenomena in these semiconductors can introduce new functionality to spintronicdevices. Recently the family of semimagnetic semiconductors was increased by additionof the semiconducting oxides [1,2]. Initially interest in oxide-based semimagnetic∗ Corresponding author. Tel.: +380 372 584755; fax: +380 372 515360.E-mail address: savchuk@chnu.cv.ua (A.I. Savchuk).0749-6036/$ - see front matter © 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2005.08.015422 A.I. Savchuk et al. / Superlattices and Microstructures 38 (2005) 421–427semiconductors was stimulated by the theoretical prediction of Dietl et al. [3] thatZnO would exhibit ferromagnetism at room temperature after doping with 5% Mn ions.Ferromagnetism in ZnO doped with magnetic ions was theoretically studied by ab initiocalculations based on the local density approximation [4]. Wang et al. [5] by using firstprinciple calculations based on the density functional theory and generalized gradientapproximation show that the ground state of Mn doped ZnO thin film changes fromantiferromagnetic to ferromagnetic when codoped with N.Until now several research groups attempted to obtain experimental evidence for suchferromagnetism in powder, bulk and thin film forms of ZnO doped with transition metals.So far studied thin films of ZnO-based semimagnetic semiconductors were grownby the spin coating method [6], sol–gel method [7,8], metallorganic chemical vapourdeposition [9], radio frequency magnetron sputtering [10] and pulsed laser depositiontechnique [11–16]. It should be noted that in most of the studies the obtained materialwas the n-type. Fukumora et al. [11] have shown that the Mn content in Zn1−xMnxO canreach x = 0.35 whereas the equilibrium solubility limit is only x = 0.13. For these filmslarge magnetoresistance has been revealed but no evidence for ferromagnetism. On theother hand, recently Jung et al. [17] found ferromagnetism in Zn1−xMnxO epitaxial filmswith a Curie temperature of 45 K. Sharma et al. [16] reported ferromagnetism at roomtemperature for Zn1−xMnxO films with small content of Mn x = 0.02. Thus, as one cansee, the results have been rather controversial. It is obvious that the magnetic properties foroxide films are extremely sensitive to the sample preparation conditions.In our previous work different methods for growth of the single crystals and Zn1−xMnxOthin films were presented [18]. In the present work, we focus on the preparation andcharacterization of Zn1−xMnxO thin films.2. ExperimentalThe depositions of the Zn1−xMnxO (0 ≤ x ≤ 0.1) thin films were performed in astainless-steel chamber under vacuum of 10−5 Pa. A XeCl pulsed excimer laser (λ =308 nm, τ = 30 ns) with a fluence of (7–10) J/cm2 has been applied. The number ofpulses was set at 30 000 for each deposition. The ablated material was collected on asubstrate located in a plane parallel to the target surface and 30 mm apart from it. Thefilms with a thickness of several hundred nanometers were deposited on sapphire(0001)substrates at 360 ◦C. The Zn1−xMnxO targets were fabricated by mixing the appropriateamount of ZnO and Mn3O4 powders and preparing of ceramic pellets. For this purpose themixture of ZnO and Mn3O4 powders was first calcined at 450 ◦C for 7 h, then the pressedpellet was sintered at 920 ◦C for 10 h in flowing oxygen. Similar targets were appliedalso in the technique of radio frequency (rf) magnetron sputtering. The sputtering wasperformed under a mixture of Ar and O2 gases in order to compensate oxygen vacanciesin the obtained films.The crystal structure of the deposited films was studied by X-ray diffraction (XRD)measurements using a Cu-Kα radiation. The magnetic susceptibility was measured by theFaraday method using an electronic balance at temperatures of 80–300 K and magneticfields up to 1.0 T. Optical absorption and Faraday rotation spectra were measured on aA.I. Savchuk et al. / Superlattices and Microstructures 38 (2005) 421–427 423Fig. 1. XRD patterns of c-oriented Zn1−xMnxO films.set-up with a grating monochromator for wavelengths (200–2200) nm and a water-cooledelectromagnet with maximal magnetic field of 5 T, and temperatures of 5–300 K.3. Results and discussionAs shown in Fig. 1, the grown films showed (002) and (004) peaks of wurtzite structurein X-ray diffraction spectra, indicating that the films are c-axis oriented.Fig. 2 shows the absorption spectrum which was extracted from the transmission spectrafor Zn0.9Mn0.1O crystalline samples of different thickness. The observed absorption bandat 2.8 eV is due to the presence of Mn ions because it is absent for undoped ZnO.This structure is interpreted by d–d crystal-field transitions between multiplets of 3d5configuration of the Mn2+ ions. According to the calculations using the cluster model [21]the ground state of the Mn2+ ions is 6A1. The d–d transitions from 6A1 to the lowest excitedterms 4T1, 4T2, 4E, and 4A1 give energies 2.55, 2.85, 2.97, and 2.99 eV, respectively. Sincethe intensity of 4T1 is weak compared to those of 4T2, 4E, and 4A1 we can assign the broadabsorption band around 2.8 eV to the transitions from 6A1 to 4T2, 4E, and 4A1.Investigation of the Zn1−xMnxO thin films allows us to study absorption spectra in theregion of higher absorption coefficients and at shorter wavelengths. The observed shift ofthe absorption edge to the high photon energy side is due to the increase of the energyband gap Eg with x . In order to determine the energy band gap Eg the data for theZn1−xMnxO thin films with different values of x are plotted in dependence on the square ofthe absorption coefficient α2 versus photon energy, as shown in Fig. 3. In this case the valueof Eg can be estimated from the intersection of the extrapolated lines with the horizontalaxis. The estimated values for the Zn1−xMnxO Eg are 3.26, 3.35, and 3.40 eV for x = 0.0,x = 0.06, and x = 0.1, respectively. The obtained variation with Mn content is in goodagreement with the expression Eg = 3.273+ 1.32x (eV) from [11].424 A.I. Savchuk et al. / Superlattices and Microstructures 38 (2005) 421–427Fig. 2. Absorption spectrum of Zn0.9Mn0.1O extracted from transmission data of single crystal samples withdifferent thickness.Fig. 3. Absorption spectra of Zn1−xMnxO films with different Mn content (the curves 1, 2 and 3 correspond tox = 0.0, x = 0.06, x = 0.1, respectively).The temperature dependence of magnetic susceptibility χ shown in Fig. 4 clearlydemonstrates paramagnetism of the studied material. The diamagnetic contribution of pureZnO is equal to χd = −0.38 × 10−6 emu/g. Magnetic susceptibility data fits well toan expression of χ = CM (T − θp)−1, where CM is the molar Curie constant and θpis the Curie–Weiss temperature. The negative sign of θp indicates on antiferromagneticinteraction between Mn2+ ions in the Zn1−xMnxO.The study of the Faraday effect in semimagnetic semiconductors is useful from thepoint of view of a probe into the microscopic magnetic properties of these materials andA.I. Savchuk et al. / Superlattices and Microstructures 38 (2005) 421–427 425Fig. 4. Temperature dependence of the magnetic susceptibility for Zn1−xMnxO (the curves 1 and 2 correspondto x = 0.06 and x = 0.1, respectively).can thus give additional data to improve their understanding [19,20]. For instance, by usingthe Faraday rotation one can estimate exchange interaction between the Mn2+ ions and theband electrons. In addition, information about type of magnetism can be extracted from themagnetic field dependence of the Faraday rotation. In Fig. 5 the spectral dependence of theVerdet constant is shown for a Zn0.9Mn0.1O single crystal sample. The main feature is thechange in the sign of the rotation from positive to negative when we go from pure ZnO tosemimagnetic oxide. This is a typical situation for semimagnetic semiconductors which isattributed to the domination of the sp–d spin exchange contribution to the Faraday effect.An advantage of using thin films to investigate the Faraday rotation lies in the possibilityto measure the oscillatory behaviour shown in Fig. 6. The oscillatory behaviour can bedescribed by a simple dielectric model which takes into account the fact that light circularlypolarized as σ+ and σ− propagates through a material with velocities correspondingto refractive indices n+ and n−, respectively. For semimagnetic semiconductors thedifference in refractive indices (n− − n+) which gives its main contribution to the Faradayrotation angle is much larger than in nonmagnetic materials. The corresponding largedifference between the refractive indices n+ and n− is due to large excitonic spin splitting(giant Zeeman effect) which is enhanced in semimagnetic semiconductors by the sp–dspin-exchange interaction. According to the recent calculations [21] the magnitude of thep–d exchange constant in Zn1−xMnxO is much larger than that in other II–VI- and III–V-based semimagnetic semiconductors.4. ConclusionIn conclusion, thin films of oxide semimagnetic semiconductor Zn1−xMnxO (x ≤ 0.1)were fabricated by pulsed laser deposition and rf magnetron sputtering. From opticalabsorption spectra the energy band gap was found to increase with Mn content and426 A.I. Savchuk et al. / Superlattices and Microstructures 38 (2005) 421–427Fig. 5. Spectral dependence of the Verdet constant for Zn0.9Mn0.1O single crystals.Fig. 6. Spectral dependence of the Verdet constant for Zn0.9Mn0.1O films.the absorption band is interpreted in the framework of d–d transitions. The temperaturedependence of the magnetic susceptibility suggests paramagnetic behaviour for the studiedmaterial. The magneto-optical Faraday effect of Zn1−xMnxO thin films demonstratesfeatures which are typical for II–VI-based semimagnetic semiconductors.AcknowledgmentsThis work has been supported in part by grant No. M/128-2004 from the Ministry ofEducation and Science of the Ukraine.A.I. 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Growth and characterization of ZnMnO thin films

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Цифровий Репозиторій - Інтелектуальні Фонди Буковинського державного медичного університету