Room Temperature Ferromagnetism in Ga1−xHoxN (x=0.0 and 0.05) Diluted Magnetic

Semiconductor Thin Films

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CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 25, NUMBER 3 JUNE 27, 2012

ARTICLE

Room Temperature Ferromagnetism in Ga1−xHoxN (x=0.0 and 0.05)Diluted Magnetic Semiconductor Thin Films

Ghulam Murtaza Raia∗, Muhammad Azhar Iqbala, Yong-bing Xub, Iain Gordon Willb,Qasim Mahmooda

a. Department of Physics, University of the Punjab, Lahore 54590, Pakistanb. Spintronics Laboratory, Department of Electronics, The University of York, York YO10 5DD, UK

(Dated: Received on November 24, 2011; Accepted on May 4, 2012)

Holmium doped GaN diluted magnetic semiconductor thin films have been prepared bythermal evaporation technique and subsequent ammonia annealing. X-ray diffraction mea-surements reveal all peaks belong to the purely hexagonal wurtzite structure. Surface mor-phology and composition analysis were carried out by scanning electron microscopy andenergy dispersive spectroscopy respectively. The room temperature ferromagnetic proper-ties of Ga1−xHoxN (x=0.0, 0.05) films were analyzed using vibrating sample magnetometerat room temperature. Magnetic measurements showed that the undoped films (i.e. GaN)exhibited diamagnetic behavior, while the Ho-doped (Ga0.95Ho0.05N) film exhibited a ferro-magnetic behavior.

Key words: Diluted magnetic semiconductor, Holmium doping, X-ray diffraction, Scan-ning electron microscopy, Room temperature ferromagnetism

I. INTRODUCTION

Spintronics is a novel multidisciplinary field that haschanged the scenario of semiconductor physics whichwas based on charge properties of electrons and holesin devices like transistors, microchips, photodetectorsetc., The tale of these applications of semiconductors,spintronics requires techniques for the injection, detec-tion, manipulation, transport and storage of spins. Un-der the shadow of spintronics study, the dilute mag-netic semiconductor (DMS) materials are of intense in-terest because of the prospect of their application inthe emerging field of spin-dependent photonic and elec-tronic devices [1, 2].

The study of DMS is based on the replacement ofrare earth or transition metals ions with a considerablenumber of the native ions in the semiconductors lattice[3, 4]. A theoretical study by the first principles calcula-tion predicted the possibility of ferromagnetic DMSs atroom temperature [5]. The theoretical results showedthat transition metals V, Mn, and Cr doped GaN arepromising candidates for room temperature ferromag-netic DMSs. Another theoretical prediction based on amean field model was carried out by Dietl et al., andexhibited a Curie temperature exceeding room temper-ature [6]. Recently, the growth and characterization ofIII-V nitride-based and II-VI DMSs have been reportedby several groups [7−14]. Sonoda et al. reported a

∗Author to whom correspondence should be addressed. E-mail:gm rai786@yahoo.com, Tel.: +92-42-99239238, FAX: +92-42-35856892

surprisingly high Curie temperature (TC=940 K) forMn-doped GaN [7, 8]. Rare earth (RE) doped GaNfilms exhibiting room-temperature ferromagnetism hasattracted great attention due to their potential appli-cations in spintronic devices [15−19]. GaN (wide bandgap semiconductor) is one candidate in which RE ele-ment can be doped to study/explore ferromagnetic be-haviour. In addition to fulfilling the criteria for REhosts, GaN has also been shown to be producing out-standing “intrinsic” light emitting diodes and lasers[20]. Furthermore, the wide band gap and the abil-ity to tailor the band-gap through III-N alloying enablethis materials system matching to RE3+.

In this work we studied the effect of small amountof Ho doping on the structure and magnetic behaviourof Ga1−xHoxN (x=0.0, and 0.05) films, as the samplewith high rare earth concentration shows clear signa-tures of phase segregation [21, 22]. The structural, mor-phological and magnetic characteristics of the thin filmswere studied using X-ray diffractometer (XRD), scan-ning electron microscopy (SEM), and vibrating samplemagnetometer (VSM) respectively.

II. EXPERIMENTS

The targets of Ga1−xHoxN (x=0.0 and 0.05) wereprepared by standard solid state reaction technique.Stoichiometric amount by weight percent of precursorsGa2O3 and Ho2O3 powders (99.99%, Sigma Aldrich)were calculated according to the following chemical re-action:

(1−x)Ga2 O3+ x(Ho2O3)→(Ga1−xHox)2O3

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314 Chin. J. Chem. Phys., Vol. 25, No. 3 Ghulam Murtaza Rai et al.

To prepare targets, the appropriate composition ofthe constituents was mixed and grinded properly toachieve better homogeneity. The mixture was then cal-cined at 500 ◦C for 6 h in nitrogen atmosphere. Thecalcined powder thus obtained was again ground for15 min in agate mortar and pestle and then pressed toform dense pellets. The pressed pellets were sintered for12 h at 1000 ◦C in nitrogen atmosphere to discover thefinest conditions for making a mechanically robust tar-get. Films were deposited on Si(100) substrate by ther-mal evaporation technique at 100 ◦C substrate tempera-ture under base pressure of 0.4 mPa. Cleaned substrateswere held at a distance 15 cm above the targets and thethickness of the films were measured using Intellemet-rics IL150 thickness meter in the range of 200 nm. Theprepared films on Si(100) substrate was set in plasmaenhanced chemical vapour deposition (PECVD) ovenand heated under a flow of ammonia (Showadenko Co.,99.999%) at a temperature of 950 ◦C for 3 h. The flowrate of ammonia was 50 mL/min and after heating thefilms were allowed to cool down to room temperature.The yellow GaN and Ga0.95Ho0.05N films were then ob-tained for further characterizations.

The structure analysis of undoped and holmiumdoped GaN films were analyzed by using the D8 DIS-COVER X-ray diffractometer using Cu Kα radiations.The grain size and composition distribution were calcu-lated using high resolution FEIXL30 Sirion SEM withfull EDX chemical microanalysis respectively. Magneticanalyses were carried out using a VSM model 10 mea-suring system under the applied field of 8 kOe at roomtemperature.

III. RESULTS AND DISCUSSION

A. XRD analysis

The XRD patterns of GaN and Ga0.95Ho0.05N films,which are shown in Fig.1, were indexed to a wurtzitestructure. Sharp peaks of relatively high intensity spec-ify well crystallization of the samples. Both the filmswere annealed at 950 ◦C in vacuum chamber underthe continuous flow of NH3. The very high and sharpdiffraction peaks present at 2θ=34.590◦, 36.906◦ and73.039◦ in the XRD pattern of GaN at x=0.0, whichcorresponds to the (002), (101) and (004) plane of GaNthin films besides the peak of Si(100) substrate as shownin Fig.1. These diffraction peaks match to the hexag-onal GaN wurtzite structure by comparing with stan-dard data (JCPDS No.02-1078). Moreover, the latticeparameters a and c have been calculated using the fol-lowing relation [23]:

1d2

=4(h2 + hk + k2)

3a2+

l2

c2(1)

where h, k, and l are the corresponding indices of eachline in the pattern and d is the interplanar distance in

FIG. 1 XRD spectra of undoped GaN and Ho-doped GaNfilm with on Si(100) substrate after exposure to flowing am-monia for 3 h at 950 ◦C.

the hkl planes. The calculated values of lattice parame-ters for GaN with x=0.0 thin film are c=0.5188 nm anda=0.319 nm, while no secondary phases were detectedin the XRD patterns. The lattice parameters werealso confirmed by unit cell software (with uncertainty±2.3 pm). The lattice parameters of Ga0.95Ho0.05N at2θ=32.276◦, 34.488◦, and 36.805◦ were calculated us-ing the program Cell Ver.5.0 (copyright by K. Dwight,l986). The calculated values of lattice constant c and aare 519.1±2.3 pm and 319.6±2.3 pm respectively. XRDanalysis revealed that at x=0.05 a slight shift towardthe lower angle was observed. A slight shift towardlower angle of GaN(002) is the evidence of incorpora-tion of Ho into GaN matrix. The shifting on lower angleof GaN(002) peak might be the difference in the ionicradii of gallium and holmium (Ho has larger ionic radiusthan Ga). This behavior of Ho doped GaN is consistentwith the reported results in Ref.[24].

B. SEM and EDS analysis

Figure 2 shows microstructure of the Ga1−xHoxN(x=0.0, 0.05) thin films sintered at 1000 ◦C in NH3

atmosphere for 3 h. SEM images of the GaN andGa0.95Ho0.05N reveal the well define grains. SEM re-sults exhibit decrease in grain size with the dopingconcentration (x=0.05) of Ho contents. The averagegrain size was measured 10 and 8 nm for Ga1−xHoxN(x=0.0 and 0.05) films. The average grain size calcu-lated from the SEM micrographs, corresponding to Hodoped sample, shows decreasing behavior with the in-crease of Holmium contents, which exhibit that rareearth element Ho3+ precede as grain growth inhibitor.A similar effect was observed in literature [25] and ourprevious work [26, 27].

The EDS method was used to determine the chemicalcomposition of the GaN and Ga0.95Ho0.05N thin films.Figure 3 shows the typical EDS spectra of Ga1−xHoxN(x=0.0, 0.05) thin films sintered at 1000 ◦C in NH3 at-

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Chin. J. Chem. Phys., Vol. 25, No. 3 Ga1−xHoxN Diluted Magnetic Semiconductor Thin Films 315

200 nm (a)

200 nm (b)

FIG. 2 SEM micrographs of Ga1−xHoxN films on Si(100)substrate at 950 ◦C NH3 atmosphere. (a) x=0.0 and (b)x=0.05.

Si

GaN

Si

GaHo

N0 2 4 6 8 10

0 2 4 6 8 10E / keV

E / keV

Ga2000

4000

6000

8000

2000

4000

6000

8000(a)

(b)

Coun

t Co

unt

FIG. 3 EDS analyses of (a) the GaN film and (b) theGa0.95Ho0.05N film on Si(100) substrate after exposure toflowing ammonia for 3 h at 950 ◦C.

mosphere for 3 h, representing the existence of Ga, Ho,and N in the films. Moreover, the highest peak belongsto Si, which originates from the substrate. EDS resultsreveal that no other impurity was present in undopedGaN and Ho doped GaN thin films, which is consistentwith the XRD results. Quantitative measurements fromEDS given in Table I confirm the required stoichiometryof GaN and the dopant Ho. In addition, EDS resultstaken from different locations of the films confirm thedistribution of holmium uniformly in GaN, which agreewith the reported data [14].

TABLE I Elemental analysis of the Ga1−xHoxN (x=0.0 and0.05) films by EDS.

Composition Element Weight/% Atom/%

x=0.0 N 8.53 17.54

Si 66.12 63.09

Ga 25.35 19.37

x=0.05 N 9.87 20.30

Si 60.13 59.53

Ga 19.88 16.83

Ho 10.12 3.34

FIG. 4 M -H curve of (a) GaN film and (b) Ga0.95Ho0.05Nfilm at 300 K by VSM, after the subtraction of diamagneticcontribution of Si(100) substrate.

C. Magnetic measurements

Magnetic parameters of undoped and Ho-doped GaNfilms are obtained at room temperature, subsequentsintering of the films in NH3 atmosphere. Figure4 exhibits the magnetization versus field behavior at300 K, after subtracting the diamagnetic backgroundof the Si substrate. From the M -H curves, it is pre-dicted that the GaN sample shows a diamagnetic be-havior while the Ho-doped sample, Ga0.95Ho0.05N, ex-hibit clear room-temperature ferromagnetism. The co-ercive field Hc≈94 Oe and saturation magnetizationMs≈32.52 emu/cm3 were calculated from Fig.4(b). Thediamagnetic behavior of GaN is consequences of thepaired electrons of its p orbital, which is liable for thenonexistence of a permanent net magnetic moment per

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316 Chin. J. Chem. Phys., Vol. 25, No. 3 Ghulam Murtaza Rai et al.

atom. Therefore, once electrons are paired together,their opposite spins originate the magnetic fields to can-cel with each other. Consequently, once an applied mag-netic field is performing on this atom, it slightly unbal-ances their orbiting electron and generates small mag-netic dipoles inside the atoms which resist the appliedfield. This accomplishment creates a negative magneticeffect. While at x=0.05, the ferromagnetism behaviormight be due to magnetic coupling between doped Hoions and GaN phases. It is believed that the majority ofrare earth ions substitute Ga upon doping into GaN lat-tice [28, 29]. Lo et al., have already reported the Ho sub-stitution on Ga sites in GaN [30]. Thus, it is assumedthat the majority of doped Ho ions have substituted theGa sites to produce a diluted magnetic semiconductor.The observed ferromagnetic behavior is not due to anysecondary phases of Ho with host atoms Ga and N, sinceHoN exhibits ferromagnetic ordering at low tempera-ture up to 13−18 K [31, 32] and HoGa2 does not showferromagnetic ordering at room temperature [33−35].Absence of any new peak in XRD spectra also gives aclue that Ho doped samples (x=0.05) do not containsecondary phases. These findings indicate that roomtemperature ferromagnetic property in the Ho dopedGaN film is only due to DMS alloy Ga0.95Ho0.05N. Theroom temperature ferromagnetic ordering is in goodagreement with the reported data [30, 36, 37]. Thus theferromagnetic ordering in Ga1−xHoxN (with x=0.05)film is due to weak Ruderman-Kittel-Kasuya-Yoshida(RKKY) interaction between Ho ions content and car-rier electron [38].

IV. CONCLUSION

We have successfully prepared Ga0.95Ho0.05N andGaN films on Si(100) from the ceramic targets of Ga2O3

and (Ga, Ho)2O3 with subsequent annealing of NH3.Structural analysis reveals that no indication of a sec-ondary phase was found in both the samples. The roomtemperature ferromagnetism has been observed in Hodoped GaN (Ga0.95Ho0.05N) sample, showing a well de-fined hysteresis loop indicating ferromagnetic behavior(Hc≈94 Oe). This work provides support to fabricateholmium doped GaN using two routes and observationof ferromagnetic behavior in this film. Further study,for example, using X-ray photoelectron spectroscopy(XPS) and magnetic behavior at different temperaturecould be helpful to develop more understanding aboutthe origin of ferromagnetism in these films.

V. ACKNOWLEDGMENTS

This work was done in the frame of Indigenous Schol-arship scheme project. This work was supported bythe Higher Education Commission Government of Pak-istan. We are equally highly thankful to Mr. Ian Wright

for the use of instruments (SEM and EDX) and staff as-sistance in York JEOL Nanocentre, UK.

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