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Thin Solid Films 515 (2

A comparison between ZnO films doped with 3d and 4f magnetic ions

Mariana Ungureanu a,⁎, Heidemarie Schmidt a, Holger von Wenckstern a, Holger Hochmuth a,Michael Lorenz a, Marius Grundmann a, Marian Fecioru-Morariu b, Gernot Güntherodt b

a Universität Leipzig, Institut für Experimentelle Physik II, Abteilung Halbleiterphysik, Linnestrasse 5, 04103 Leipzig, Germanyb II. Physikalisches Institut, RWTH Aachen, Physikzentrum Melaten, Huyskensweg Turm 28B, 52074 Aachen, Germany

Available online 6 April 2007

Abstract

We present electrical and magnetic properties of ZnO films doped with 3d (Mn) and 4f (Gd or Nd) magnetic ions grown on a-plane Al2O3

substrates. Both for films doped with 3d magnetic ions and for films doped with 4f magnetic ions, Hall investigations revealed that the carrierconcentration decreases and the resistivity increases with increasing the oxygen partial pressure during the pulsed laser deposition growth,probably because the formation of oxygen vacancies is hindered. Measurements of magnetic properties revealed ferromagnetism above roomtemperature with magnetic moments up to 0.2 μB/Mn ion in insulating ZnO:Mn films co-doped with 0.1% P and up to 0.3 μB/Gd ion in n-conducting ZnO:Gd films co-doped with 0.2% Al.© 2007 Elsevier B.V. All rights reserved.

Keywords: Diluted magnetic semiconductors; ZnO

1. Introduction

The field of spintronics is based on the idea to use theelectron spin rather than, or in addition to charge for newelectronic devices [1]. Diluted magnetic semiconductors(DMS), systems formed by substituting a fraction of a hostsemiconductor cations with magnetic ions, are believed to besuited materials for spintronics applications. We chose ZnO assemiconductor host because is widely used in electronicdevices, for example in gas sensors, transparent conductinglayers and green or white light-emitting devices, due to its wideband gap of 3.3 eV. A DMS that could find practicalapplications should be ferromagnetic above room temperature.Theoretical computations predicted that transition metal (TM)ions substituted in ZnO could produce ferromagnetism above300 K [2–4]. Experimentally, ferromagnetism was obtainedabove 300 K with small magnetic moments for 3d-TM in ZnO[5,6]. An approach to use rare earth (RE) elements as dopingmagnetic ions in the GaN host, produced large magneticmoments and ferromagnetism above room temperature [7].

In this paper we present structural, electrical and magneticproperties of ZnO films doped with 3d-TM (Mn) or 4f-RE (Gd,

⁎ Corresponding author. Tel.: +49 341 97 32666; fax: +49 341 9732668.E-mail address: ungureanu_mary@yahoo.de (M. Ungureanu).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.04.010

Nd) magnetic ions grown by pulsed laser deposition (PLD) ona-plane Al2O3 substrates. Our work indicated alike behaviors inZnO:Gd films and ZnO:Nd films, for this reason in theremainder of this paper we will make no distinction betweenthese materials.

Hall investigations revealed that the carrier concentrationdecreases and the resistivity increases with increasing theoxygen partial pressure during the PLD growth, probablybecause the formation of oxygen vacancies is hindered [8].Magnetic behavior of films was studied in a superconductingquantum interference device (SQUID) magnetometer andrevealed ferromagnetic behavior above 300 K in insulatingZnO:Mn and ZnO:(Mn, P) films with around 5% Mn and in n-conducting ZnO:(Gd, Al) films with around 1% Gd.

2. Film growth and structural properties

We prepared and analyzed ZnO films doped with 3% and 5%Mn or with 0.1% and 1% Gd or Nd. We used the PLD techniqueto grow films with thickness from 20 nm to 1 μm on a-planeAl2O3 substrates. The PLD growth temperature was variedbetween 450 and 850 °C. The films were grown in O2 or N2

atmosphere at pressures between 2E-5 and 0.3 mbar. A part ofthe films containing Mn were co-doped with 0.1% and 0.5% Por 0.05% Sn in order to investigate if their magnetic properties

Fig. 1. XRD spectra for ZnO films doped with 3d or 4f magnetic ions and P or Alas co-dopants. The XRD intensity is shown on a logarithmic scale. No secondaryphases were detected.

Fig. 3. Temperature dependence of mobility (empty symbols) and carrierconcentration (full symbols) for films with different thicknesses (100 and 500 nm).

8762 M. Ungureanu et al. / Thin Solid Films 515 (2007) 8761–8763

are improved as compared to the weak magnetic momentsobtained in ZnO:Mn films [6]. Also, Al was used as co-dopantfor some films in order to increase their electrical conductivity.

X-ray diffraction (XRD) indicated the absence of detectablesecondary phases (example for two films in Fig. 1) and largeinternal tensions, shifting the ZnO peaks, for films containing1% RE elements (inset Fig. 1).

3. Electrical and magnetic properties

The electrical properties of films with different thicknesseswere investigated by Hall effect measurements using the van derPauw geometry. In Fig. 2 we present the carrier concentration,measured at room temperature, for ZnO:Mn and ZnO:Gd filmsgrown at 600 °C and 700 °C in O2 atmosphere. With increasingthe O2 partial pressure in the PLD growth chamber, the carrierconcentration is decreasing (Fig. 2) and the film resistivity isincreasing. This may be explained that by increasing the PLDoxygen partial pressure, the formation of oxygen vacancies is

Fig. 2. Carrier concentration measured at room temperature for ZnO:Mn andZnO:Gd films grown at 600 °C and 700 °C in O2 atmosphere. The inset presentsthe carrier concentration for films grown in O2 and in N2 atmosphere.

hindered [8,9]. The inset of Fig. 2 presents a comparisonbetween Hall data for films grown in O2 and in N2 atmosphere.For films grown in N2, the film carrier concentration andresistivity does not change significantly when changing the N2

partial pressure, which can be explained by the formation ofoxygen vacancies at any N2 pressure.

By Hall measurements on films with thicknesses between10 nm and 1 μmwe obtained increase of resistivity and decreaseof carrier concentration with increasing films thickness,contrary to our expectations. To clarify this effect we performedmore investigations, we measured Hall at different temperaturesand obtained, for example, that the carrier concentration isalmost independent of temperature. Fig. 3 shows, as example,the dependence on temperature of mobility and carrierconcentration for two films with different thicknesses. Wecould model the temperature dependence of mobility byassuming the presence of a thin, degenerate, highly conducting,interface layer between substrate and film and considering thefilm being formed by two sublayers with different mobilitiesand carrier concentrations [10]. This highly conductinginterface layer may be due to Al diffusion from substrate.

Fig. 4. Field dependent magnetization curves at 5 K and 300 K. The insetcontains a zoom-out of the low fields region, showing the coercive fields of90 Oe at 5 K and 71 Oe at 300 K for the ZnO:(5% Mn, 0.1% P) film and of117 Oe at 5 K and 88 Oe at 300 K for the ZnO:(1% Gd, 0.2% Al) film.

8763M. Ungureanu et al. / Thin Solid Films 515 (2007) 8761–8763

Magnetic properties were investigated at different tempera-tures using a SQUID magnetometer. The substrates weremeasured separately after etching off the films by HCl dip andthe substrate contribution was subtracted from the magnetiza-tion curves.

Ferromagnetism above room temperature was obtained ininsulating ZnO:Mn films with around 5% Mn, grown at largeO2 pressures, with a maximum magnetic moment of 0.06 μB/Mn ion at 5 K. Al addition to ZnO:Mn films leaded to decreaseof ferromagnetism, while by growing the films in N2 themagnetic moment could be increased up to 0.09 μB/Mn ion at5 K. An improvement in the ferromagnetic behavior wasobtained by adding P to the ZnO:Mn films, increasing themagnetic moment up to 0.2 μB/Mn ion for 0.1% P incorpora-tion, ferromagnetism still being present above 300 K. Byincreasing the P content from 0.1 to 0.5%, the ferromagnetismdecreases [11], an indication that it does not originate in smallMnP clusters [12], possibly to be present and not detected inXRD. The presence of P may provide holes that mediateexchange in ZnO:Mn films, as predicted by Dietl et al. [2], andas experimentally observed in the case of As incorporation inZnO:Mn films [13]. For ZnO:Mn films co-doped with 0.05%Sn, no ferromagnetism could be obtained above 250 K. ForZnO:Gd or ZnO:Nd films with around 0.1% RE content onlyparamagnetic behavior was obtained at all temperatures,indicating that the magnetic ions are too far apart for exchangeinteractions. For ZnO:Gd or ZnO:Nd films with around 1% REcontent ferromagnetism could be observed below 150 K. Byadding 0.2% Al to ZnO:Gd films with around 1% Gd,ferromagnetism above room temperature was obtained, with amaximum magnetic moment around 0.3 μB/Gd ion at 5 K. Thismight indicate that exchange in ZnO:Gd films is mediated byelectrons.

Temperature dependent magnetization curves were mea-sured as field cooled (FC) and zero field cooled (ZFC) curves.For all the investigated films, the superparamagnetic and spin-glass behaviors could be excluded from the shape of thetemperature dependent magnetization curves, no peak beingpresent on the ZFC curve.

In Fig. 4 we show, as example, the field dependent mag-netization curves at 5 K and at 300 K for two films grown ataround 640 °C, an electrically insulating ZnO:(5% Mn, 0.1% P)film grown at 0.3 mbar O2 and an n-conducting ZnO:(1% Gd,0.2% Al) film grown at 1E-3 mbar O2.

4. Conclusions

We analyzed ZnO films doped with Mn, Gd or Nd grown bypulsed laser deposition. Some films were co-doped with Al toincrease their electrical conductivity, while a part of ZnO:Mnfilms were co-doped with P or Sn to bias the magneticinteractions. Without Al doping, films grown in O2 at pressuresbelow 0.05 mbar were n-conducting and films grown at higher

pressures were insulating, while all films grown in N2 at-mosphere, even at 0.3 mbar, were n-conducting. This can beexplained by assuming that with increasing the PLD O2 pressurethe formation of oxygen vacancies is hindered [8,9], while thePLDN2 atmosphere has no visible influence on oxygen vacanciesformation.

Magnetic measurements revealed ferromagnetism above300 K in insulating ZnO:Mn films, with increased magneticmoments when co-doping with 0.1% P, and in n-conductingZnO:Gd films co-doped with 0.2% Al.

The presence of P may provide holes that mediate exchangein ZnO:Mn films, as predicted by Dietl et al. [2], andexperimentally determined in the case of As co-doped ZnO:Mn films by Lim et al. [13]. The presence of ferromagnetism at300 K in ZnO:Gd films with around 1%Gd co-doped with 0.2%Al might indicate electron mediated exchange in ZnO:REsystems.

Acknowledgments

This work was supported (M. U., H. S., and H. H.) by theBundesministerium für Bildung und Forschung in the frame-work of the Young Scientist's group “Nano-Spinelectronics”(Grant No. FKZ 03N8708). We are grateful to Gabriele Rammfor preparing the PLD targets. We thank Daniel Spemann forfilm composition measurements by RSB/PIXE and MatthiasBrandt for contributions to Hall investigations. We thank ChrisSturm and Rüdiger Schmidt-Grund for film thickness determi-nation by spectroscopic ellipsometry.

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