Cryst. Res. Technol. 50, No. 3, 268–275 (2015) / DOI 10.1002/crat.201500005

Orig

inal

Pape

r

Epitaxial growth of magnetic semiconductor EuO on silicon bymolecular beam epitaxyDmitry V. Averyanov, Peter E. Teterin, Yuri G. Sadofyev∗, Igor A. Likhachev, Alexey E. Primenko,Andrey M. Tokmachev, and Vyacheslav G. Storchak∗

Received 6 January 2015, accepted 7 January 2015Published online 6 February 2015

Functional oxides demonstrate a wide range of magnetic,optical and transport properties. Their integration with sil-icon promises significant advances in electronics. An im-portant key in enabling brand-new oxide technologies isthe utilization of silicon/oxide epitaxy, thus making qual-ity of the interface a critical issue. The progress dependson our ability to avoid formation of impurity phases at theinterface and to tackle structural mismatch of the oxideand Si. We design a novel chemical protection of Si (001)surface on the submonolayer scale based on the surfacemetal silicide with the (1×5) reconstruction. This new tech-nique is applied to the long-standing problem of integra-tion of a ferromagnetic semiconductor with Si. Direct epi-taxial growth of EuO on Si without any buffer layer, so far in-accessible, is achieved by molecular beam epitaxy. The nu-cleation step, comprising first 10 monolayers of EuO, is fol-lowed by a distillation-controlled growth. An alternative tostandard capping procedures for EuO, based on controlledformation of an amorphous Eu2O3 layer, is devised. Crystalperfection of the films is established ex situ by x-ray diffrac-tion and Rutherford backscattering. Magnetic properties ofthe EuO films match those of the bulk.

1 Introduction

Potential for functional thin-film oxide devices utilizingferroelectricity, superconductivity and/or magnetism isenormous. A wealth of physical properties of functionaloxides comes from their diversity and ensures their lead-ing role in the research focused on emergent interfacialphenomena in heterojunctions. This is a relatively newarea with both exciting possibilities and difficult chal-lenges. Industrial applications strongly depend on thequality of the interface between structurally dissimilar

substrates, thus placing a barrier between our expecta-tions and the reality. Silicon is a technological platformof the modern electronics. Therefore, integration of func-tional oxides with silicon is of the utmost importance.This is usually a formidable task: joining covalent andionic systems with a large thermal and lattice mismatchis never easy and straightforward. Another challenge isthe formation of impurity phases at the interface: siliconsurface is not inert; it reacts with oxygen forming oxidesand with metals forming bulk silicides. These phases aredetrimental to most applications, and special measuresare required to avoid their formation.

A major demand for integration of functional oxideswith Si comes from semiconductor spintronics – anemerging alternative to traditional electronics. Employ-ment of electron spin in addition to its charge mayenormously increase functionality of semiconductordevices. Optical creation of spin-polarized carriers insilicon is rather ineffective, and their electrical injectionfrom ferromagnetic contacts is probably the only viableroute [1]. Attempts to inject spin-polarized electronsfrom ferromagnetic metals directly into Si invariablyfail. In theory, it is due to a very small ratio of electricconductivities of Si and the ferromagnetic metal [2],although chemical processes at the interface can be alsoof importance. This problem can be avoided by injec-tion of spin-polarized electrons from a ferromagneticsemiconductor – conductivity mismatch is effectivelyeliminated by the doping.

Europium (II) oxide is widely considered to be themost promising ferromagnetic semiconductor for siliconspintronics. First, it is thermodynamically stable in di-rect contact with silicon [3]. Stoichiometric EuO has the

∗ Corresponding author: e-mail: yugsad49@gmail.com;mussr@triumf.caNational Research Center “Kurchatov Institute”, Kurchatov Square 1,Moscow, 123182, Russia

268 C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cryst. Res. Technol. 50, No. 3, 268–275 (2015)

Orig

inal

Pape

r

room-temperature band gap 1.12 eV [4], perfectly match-ing that of Si. Fm3m (rock-salt) crystal structure of EuO iscompatible with the cubic diamond structure of silicon,although mismatch of lattice parameters of EuO (5.144 A)and Si (5.431 A) is rather large (+5.6%). The spin splittingof the conduction band of EuO below its Curie temper-ature Tc = 69 K is enormous (0.6 eV), leading to almost100% spin polarization of the carriers [5]. Magnetizationof EuO is 7 μB per Eu2+ ion [4] which corresponds to half-filled f-bands. When EuO is doped by other rare-earthelements (La, Lu, or Gd) or departs from the stoichiom-etry due to oxygen vacancies (EuO1-x), its conductivityand Curie temperature increase dramatically [6–10]. Thiscombination of properties makes EuO a unique materialfor design of semiconductor contacts – spin injectors.

The last decade witnessed numerous attemptsto grow EuO on chemically inert substrates such asyttrium aluminate YAlO3 [11], yttria-stabilized cubiczirconia [12], MgO [13], and SrTiO3 [14]. It is firmlyestablished that epitaxial growth of stoichiometric EuOlayers is possible in the so-called distillation growthmode [12]. It requires two conditions. First, the flux of Euatoms coming to the substrate surface exceeds that ofoxygen atoms. Second, the temperature of the substrateis relatively high (at least 400 °C) for the excess of Euatoms which avoid reaction with oxygen to evaporatefrom the substrate.

Silicon is different: its surface is chemically reactiveat these temperatures and even small flux of oxygenleads to formation of a layer of amorphous SiO2. Itsuppresses epitaxial growth and leads to polycrystallineor amorphous EuO layers. Decreased temperature of thegrowth slows down the oxidation of silicon surface butinevitable accumulation of excessive Eu atoms againinduces formation of polycrystalline and then amor-phous layers. The reverse ratio of fluxes with an excessof molecular oxygen affects the chemical composition ofthe film due to emerging phases of non-magnetic stablehigher oxides Eu3O4 and Eu2O3.

The integration of EuO and silicon is possible witha spacer. Growth of EuO on Si with a buffer layer ofSrO is well developed [15, 16]. Strontium does not formhigher oxides and the lattice parameter of SrO (5.159 A)is close to that of EuO. Epitaxial growth of SrO on siliconis possible even at the room temperature making it aleading contender to be a buffer layer in the integrationof EuO with Si. One of the drawbacks is that SrO is notchemically stable and reacts with water vapor in theair producing hydroxide Sr(OH)2. More important isthat SrO is a dielectric material with the band gap �6eV [17], which sets up a potential barrier for injection ofspin-polarized electrons from EuO into Si.

The EuO/Si system is very appealing. It promisesbreakthroughs in spin injection into Si and has a hugepotential for industrial applications. Despite significantefforts worldwide, successful epitaxy of ferromagneticEuO directly on Si has not been demonstrated so far [18,19]. The epitaxial quality of the films and the absenceof impurity phases at the interface are most essentialfor injection of spin-polarized carriers into silicon: thepresence of an intermediate interfacial layer reduces theprobability of spin injection exponentially, while impu-rities prevent spin injection due to spin-flip scattering.An attempt of growth of EuO directly on Si has been re-ported in Ref.7 but subsequent analysis of the films [18]shows that the distance between the Si surface and thelayer of EuO1-x composition exceeds 10 nm making theiruse for spintronics applications problematic. The issuecan be pinpointed to an insufficient chemical control ofthe interface [18, 19]. Here we report a novel approachto the Si surface protection and successful growth ofEuO/Si heterojunctions as witnessed by a combinationof analytic techniques.

2 Results and discussion

MBE growth of EuO directly on Si is a complex, multi-step process. It consists of 5 major technological stages:(i) preparation of Si surface, (ii) chemical protection of Sisurface, (iii) nucleation of EuO films, (iv) regular growthof EuO, (v) chemical protection of EuO films. Below wedescribe these stages in detail. The growth process iscontrolled in situ with RHEED while the overall qual-ity of the films is established with a number of ex situtechniques.

2.1 Preparation of Si surface

Silicon is a workhorse of modern electronics, and a num-ber of Si substrates are available on the market. Our sub-strates are high-ohmic Si (001) wafers with miscut anglesnot exceeding 0.5°. Their surfaces are covered with amor-phous oxide SiO2. This natural layer can be removedchemically or by heating. We employ a mixed technique:the substrate heated in vacuum to 950 °C is exposed toa flux of Eu or Sr atoms. At these conditions, a chemicalreaction takes place:

SiO2 + M → SiO ↑ +MO ↑; (M = Eu, Sr).

The monoxides are volatile at high temperatures and thesurface reaction is accompanied by sublimation of theproducts.

www.crt-journal.org 269C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

D. V. Averyanov et al.: Epitaxial growth of magnetic semiconductor EuO on silicon by molecular beam epitaxy

Orig

inal

Pape

r

A fingerprint of a clean Si (001) surface is a surfaceatomic reconstruction characterized by the (2×1) struc-ture. It comes from dimerization of neighboring Si atoms(coupling of dangling bonds) along a [110] azimuth withformation of surface atomic chains. If the miscut angleof Si (001) substrate is small (like in our experiments), itis manifested in the single-atom steps on the surface. Di-rections of Si dimers are orthogonal on the neighboringterraces. When the primary RHEED beam crosses a largenumber of terraces, not only principal but also fractionalreflexes of the order 1/2 are observed on the RHEED pic-tures along all [110] azimuths. In contrast, fractional re-flexes are absent for RHEED measurements along [100]azimuths. This is a mark of the two-domain reconstruc-tion (2×1) + (1×2) Si, which is observed in all our experi-ments after removal of natural oxides from the Si surface.This surface is a template for the growth of epitaxial EuOlayers.

2.2 Chemical Protection of Si Surface

Dimerization of surface Si atoms only partially saturatestheir chemical valences. There is still a dangling bondper surface Si atom, inviting surface chemical reactions.Thus, chemical protection of the Si surface is an essen-tial part of the recipes for epitaxial growth of functionaloxides on Si. In particular, epitaxial growth of SrO is pre-ceded by exposure of the clean Si surface to a flux of Sratoms till a stable (1 × 2) reconstruction is reached [15,16]. According to the phase diagram of Sr on Si (001)[20], the reconstructions depend on the Sr coverage andchange in the following sequence: (2 × 1) Si → (2×3) Sr→ (1×2) Sr → (1×5) Sr → (1×3) Sr. The first of Sr recon-structions is observed at �1/4 monolayers (ML) cover-age, while the next reconstruction (1×2) Sr correspondsto 1/2 ML coverage. The latter is associated with the sur-face silicide SrSi2 [15, 16]. (It should be not confused withthe bulk phase of the same stoichiometry.) The other re-constructions are observed for a larger Sr coverage. Thesurface phases of Eu on Si are not sufficiently studied butan educated guess is that the phase diagrams for Eu andSr are very similar – almost identical radii of Eu2+ andSr2+ lead to structural isomorphism of their compounds.

The silicon surface covered with the MSi2 silicideis believed to be ready for the growth of oxides: all thedangling bonds are saturated and the surface reactivityis drastically reduced. This protection is, indeed, suffi-cient for the growth of some functional oxides, like SrOwhich grows at the room temperature. EuO is a notableexception because it requires higher temperatures forevaporation of excess Eu atoms [18]. One can expect that

surface structures with a higher coverage of the metalwould provide a better protection: not only the danglingbonds would be saturated through formation of thesurface silicide, but also some Si-Si dimers susceptibleto oxidation would be replaced by Si-M-Si structures,providing additional protection from formation of Si-Obonds at the beginning of the growth.

Our search for surface phases with a higher cov-erage of Eu has led to observation of a stable (1×5)Eu reconstruction, which can be obtained by expos-ing the Si substrate to a flux of Eu atoms. The opti-mal temperature of the substrate is determined to be660 °C. A sequence of surface transformations is de-tected with RHEED. First, the initial (2×1) + (1×2) Si im-age transforms into the (2×3) + (3×2) Eu reconstruction(figure 1a): a superposition of reflexes of orders 1/2 and1/3 is observed for all [110] azimuths. Next, the (1×2) +(2×1) Eu surface phase is formed (figure 1b). Our sub-strates have only single-atom steps between neighboringterraces, and any changes of directions of dimer chainsare not detected. In contrast, experiments for Sr adsorp-tion on Si (001) surface with miscut angle 4° and, as aconsequence, diatomic steps clearly demonstrate thatthe (2×1) Si reconstruction is transformed into (1×2)Sr [21]. It is likely that the same is true for the Eu sur-face silicides. Further exposure of the surface to a flux ofEu atoms produces the (1×5) + (5×1) Eu surface phase(figure 1c). No further transformations are observed, atleast at the given above substrate temperature. The re-construction remains stable when the flux of Eu atoms isterminated and the substrate temperature is decreasedto 360±10 °C – the initial temperature of the EuO growth.

The lateral lattice parameter of the surface region of Sisubstrate and later the epitaxial layer of EuO is controlledwith a standard procedure based on measurements ofthe distances between the intensity maxima of the re-ciprocal lattice streaks along the [110] azimuth in theRHEED pictures. The data are calibrated according tothe RHEED images for clean Si surface with well-knownstructure. Figure 2 shows dynamics of lattice parameterchanges. Left part describes a sequence of Eu on Si re-constructions starting with Si as a reference point whilethe right part describes formation of EuO layers with thevertical line in the middle corresponding to the start ofEuO growth. We observe a reproducible small decreaseof the lateral lattice parameter for all Eu on Si reconstruc-tions (left part of figure 2). It points at some decreaseof distances between atomic chains of surface europiumsilicides with respect to silicon. It should be noticed thatthe lateral lattice parameters for the surface phases (1×2)Eu and (1×5) Eu are very close. Both are smaller than thelattice parameter of Si (5.43 A) by about 2%.

270 www.crt-journal.orgC© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cryst. Res. Technol. 50, No. 3, 268–275 (2015)

Orig

inal

Pape

r

Fig. 1 RHEED images along the [110] azimuth of reconstructed sur-face phases of Eu-Si: (a) (2×3) + (3×2); (b) (1×2) + (2×1); (c) (1×5)+ (5×1)

2.3 Nucleation of EuO films

Despite a better protection of the Si surface, thedistillation-controlled epitaxial growth of EuO directlyon Si is still not possible: the temperature is too high and

Fig. 2 Changes of the lattice parameter of surface reconstructionphases (blue) and during the low-temperature stage of EuO growth(red)

surface reactions are unavoidable. A low-temperaturenucleation step with a strict control of EuO stoichiom-etry should be introduced. MBE growth of binarysemiconductors with deviations from the stoichiometryis often characterized by the equivalent beam pressure(EBP) ratio. In our case, it can be used for estimates ofEu and O2 pressures required for the growth: EBP = 1.0corresponds to the stoichiometric supply of reactants;distillation regime requires EBP > 1.0.

The initial stage of the growth is implemented at EBPonly slightly larger than 1.0. First, it suppresses formationof higher oxides. Second, it does not lead to significantamounts of physically adsorbed excess Eu atoms on thesurface. The oxygen pressure at the substrate (measuredby ionization manometer) is kept constant at 5 ×·10−9

Torr. In order to keep EBP close to 1.0 the pressure of Eu(which still evaporates from the surface, though slowly,even at this low temperature) beam is kept at 5 ×·10−8

Torr, which is achieved at the temperature of the Eu effu-sion source 475 °C. The growth rate at these pressures isdetermined to be �2 ML per minute.

Supply of Eu and O2 to the substrate surface be-gins simultaneously. RHEED images mirror the surfacechanges: the fractional reflexes of the order 1/5 aredimming, while the elongation of streaks for the (1×1)lattice points at smoothing of the surface microrelief.Additional reflexes may indicate an accumulation of Euatoms on the surface. Any attempt to increase EBP leadsto RHEED images with a superposition of streaks fromthe epitaxial EuO film and a system of semi-rings frompolycrystalline Eu on the surface. This is a precursor forthe failed epitaxial growth.

The RHEED spectra do not change their form till theend of the nucleation, i.e. �10 ML of EuO. We established

www.crt-journal.org 271C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

D. V. Averyanov et al.: Epitaxial growth of magnetic semiconductor EuO on silicon by molecular beam epitaxy

Orig

inal

Pape

r

empirically that this thickness of the film provides a re-liable protection of the substrate from the oxidation atlater stages of the process. The relaxation of EuO filmsis controlled with RHEED (right part of figure 2). Theinitial moment of EuO nucleation brings the laterallattice parameter back to the Si value. It happens atless than 1 ML EuO and can be explained by formationof a wetting (pseudomorphic) layer, according to theStranski-Krastanov model. Then, plastic relaxation ofheteroepitaxial strains leads to a smooth decrease ofthe lateral lattice parameter to the bulk EuO value. Thelargest part of the relaxation process takes place at thegrowth of first 4 ML EuO. It is remarkable that aftercompletion of the plastic relaxation phase there is notransformation of RHEED images to those typical for 3Dgrowth mechanisms. The only effect is a weak intensitymodulation along the streaks.

These results can be compared with those for the epi-taxial growth of SrO on silicon [22]. It turns out that theplastic relaxation of SrO begins at the 1 ML thickness ofthe wetting layer. The layer of SrO becomes almost fullyrelaxed after reaching the 2.5 ML thickness. The condi-tions for the SrO epitaxy [22] (growth rate �0.6 ML/min,temperature �50 °C) are different from those in our ex-periments. In combination with different elastic moduliof EuO and SrO, it can be a reason for the observed dif-ferences in the plastic relaxation characteristics.

2.4 Regular growth of EuO

The nucleation step is followed by the growth of the mainbody of the EuO film. We studied two possible routes.The first one is based on a simultaneous gradual increaseof substrate and Eu source temperatures without any in-terruption of the growth. After 5 minutes, the substratetemperature is stabilized at 470 °C, and the Eu effusionsource temperature – at 500 °C. The Eu beam pressure in-creases to �1.5 ×·10−7 Torr, in accordance with the tem-perature dependence of the equilibrium vapor pressure(an order of magnitude increase for each 65 °C). Largerinflux of Eu atoms to the substrate surface is necessarydue to (i) increased re-evaporation of Eu atoms at highertemperatures, (ii) distillation regime of the growth withEBP > 1.0. This set of conditions is maintained for about60 min leading to 50 nm EuO films. This step is charac-terized by disappearance of RHEED reflexes originatingfrom the excess of Eu atoms on the surface. At the end ofthe growth, RHEED images exhibit sets of bright streakswithout any collateral reflexes of surface phases. The ob-served Kikuchi lines are fingerprints of perfect crystals(figure 3). Short annealing of the samples at 530 °C in a

Fig. 3 Typical RHEED image along the [110] azimuth of 70 nm thickEuO film grown on Si

vacuum chamber enhances the brightness of RHEED re-flexes.

An alternative scenario includes termination of thegrowth and annealing of the nucleation layer of EuOwithout any influx of the reactants. At temperaturesless than 510 °C RHEED reflexes from the excess of Eugradually disappear. Annealing at higher temperaturesleads to rugged surfaces due to interdiffusion of EuO andSi at the interface and formation of 3D islands on thesurface. The following EuO growth is unchanged withthe substrate and Eu effusion source temperatures keptat 470 °C and 500 °C, respectively. Again, the resultingRHEED image contains bright streaks, corresponding tothe surface cell (1×1), accompanied by a set of Kikuchilines (figure 3). Although EuO is known for its thermody-namic stability in contact with silicon [3], the very mildannealing temperature is an additional factor preventingchemical interactions between EuO and substrate.

2.5 Chemical protection of EuO films

The surface of EuO film is highly reactive. In contactwith the air, it forms higher oxides Eu2O3 and Eu3O4. Eu-ropium hydroxide is a byproduct due to interaction withthe water vapor. Degradation of the film is fast, com-plete and irreversible. The standard way to avoid theseunwanted reactions is a protective capping of the film.A wide range of materials can be used for this purpose.Our experiments show that commonly used capping lay-ers formed by 5 nm of Al or SiOx provide sufficient pro-tection of EuO films. An alternative idea stems from thewell-known protection of metal surfaces by natural ox-ides (for example, Al2O3 on Al): chemically stable Eu2O3

can be used for protection of EuO. As stated above, expo-sure of EuO to the air oxygen does not lead to formation

272 www.crt-journal.orgC© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cryst. Res. Technol. 50, No. 3, 268–275 (2015)

Orig

inal

Pape

r

of a protective layer, but oxidation in a controlled fash-ion can do the trick. Previous attempts to protect EuOby growing a surface layer of Eu2O3 are inconclusive: insome cases the film degrades over a period of severaldays [23], in another case the durability experiments arelimited to very small (20 hours) periods of time [24]. Anissue is to make the protective layer of Eu2O3 ultrathin:it has been recently reported that in the 60 nm EuO film45 nm are oxidized at the capping Eu2O3 formation [25].In our experiments, postgrowth exposure of EuO film to asmall oxygen flux in the MBE machine leads to formationof Eu2O3 protective layer with the thickness 2–3 nm: theRHEED reflexes from EuO are still seen after controlledoxidation of the topmost layer of EuO. Magnetic mea-surements of thin EuO films set the upper limit on thewidth of the Eu2O3 layer to about 2 nm. Our ex situ ex-periments show that the quality of thus protected EuOfilms does not deteriorate after months-long exposure tothe atmosphere. It should be noted that the results of ourex situ experiments do not depend on the type of cappinglayer used (Al, SiOx or Eu2O3).

2.6 Ex Situ characterization of EuO films

X-ray diffraction of the samples demonstrates that EuOfilms are single crystal with the (001) orientation, thesame as Si substrate (figure 4). EuO reflexes witness thatits structure is Fm3m with the lattice parameter a =5.138 A, similar to Ref.16. No signs of higher Eu oxides orother unwanted phases are present in the XRD spectra.XRD measurements for the (200) reflex show a series ofstrong oscillations at both sides of the EuO signal. They

Fig. 4 X-ray θ -2θ diffraction spectrum of SiOx/EuO/Si structure.Thickness of EuO layer is 40 nm

Fig. 5 Simulated RBS and channeling spectra of SiOx/EuO/Si het-erostructure. Thickness of EuO and SiOx layers are 40 nm and 4 nm,respectively

point at atomically sharp interfaces EuO/Si and cappinglayer/EuO.

Rutherford backscattering (RBS) spectra (figure 5)were recorded in two regimes: random (angle of severaldegrees between the incoming beam and the normal)and channeling (the incoming beam is aligned with amajor crystallographic direction of the sample). A sig-nificant decrease of the backscattered yield due to ionbeam channeling (the minimum channeling yield ascompared to the random spectrum, determined at the Eusignal is 23%) is a mark of the crystal structure of a highquality. The analysis of the RBS spectrum presented infigure 5 produces the surface concentration of Eu atoms1.2 ×·1017 atoms·cm−2, which corresponds to the thick-ness of the film �41 nm. The RBS measurement fitsdemonstrate that the films are very close to stoichiom-etry: the difference between the numbers of Eu and Oatoms in our thicker films (> 60 nm) does not exceed 1–2%, i.e. within the experimental error of the technique.

Magnetic properties of thick EuO films were deter-mined with the SQUID magnetometer. Temperature de-pendence of the DC magnetization (1 Oe) of EuO layer(field-cool mode) is shown in figure 6. The saturationmagnetization 6.9±0.2 μB per Eu atom at 2K comes fromthe half-field f-shells of Eu2+ ions, in full accordance withthe bulk EuO value. The accuracy of the estimate is gov-erned by the accuracy of the number of Eu ions in thefilm which is determined from RBS data. The correctvalue of the saturated magnetic moment is so difficult toachieve for EuO films grown on Si (the values reportedso far are 4.3 μB per Eu atom [26], 5 μB per Eu atom[19], 6 μB per Eu atom (with a buffer layer) [18]) that itbecomes a fingerprint of the highest quality of the film.The Curie temperature 68±1 K is also in good agreement

www.crt-journal.org 273C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

D. V. Averyanov et al.: Epitaxial growth of magnetic semiconductor EuO on silicon by molecular beam epitaxy

Orig

inal

Pape

r

Fig. 6 Temperature dependence of DC magnetization of 31 nmthick EuO film in FC mode and magnetic field 1 Oe applied parallelto the surface

with the bulk value. Magnetization measurements of ul-trathin films can provide information on the presence ofinterfacial impurities. Our films as thin as 2.5 nm showno reduction of the magnetic moment per Eu atom withrespect to the bulk EuO value. Precision of better than10% in our measurements of the magnetic moment perEu atom leaves no room to formation of unwanted Euphases at the interface thicker than 0.25 nm.

3 Conclusion

Integration of a ferromagnetic semiconductor with sili-con opens new avenues for developments in electronics.The intrinsic difficulties come from competing processesat the interface: the range of conditions for a successfulgrowth is very narrow and demands to the equipmentare high, especially with respect to stability of the growthconditions. In the present paper, we have demonstratedthe possibility of a growth of thin EuO films directly on Si.The thickness of the films is up to 80 nm, and their prop-erties, including magnetism, match those of the bulk. Amost helpful feature of the growth procedure is forma-tion of a protective layer of Eu on Si, corresponding to the(1×5) reconstruction in the RHEED spectra. It ensuresan epitaxial growth at elevated temperatures without for-mation of unwanted phases. It should be stressed thatprotection layers based on Si-M or Si-H surface bondingare thermodynamically not stable in contact with oxy-gen. The protection is likely to be determined by kinetics.Therefore, any additional protection of the surface cansignificantly shift a fine balance of chemical and physi-cal processes at the interface. We foresee a large impactof such Si surface protection on the direct integration

of functional oxides with Si, especially nonisostructuralheteroepitaxy.

The growth of EuO is implemented as a two-step pro-cedure, which is common for heteroepitaxy of materialswith a significant lattice mismatch. In our particular caseof EuO/Si interface the purpose of the low-temperaturenucleation step is two-fold: 1) formation of a continuouscoverage with a low roughness of the microrelief and adecreased density of dislocations; 2) an additional pro-tection of the substrate at the next, high-temperature,distillation-controlled stage of the growth. Annealing atmoderate temperatures improves the epitaxial quality ofEuO film as a whole. The interplay between the quality ofthe EuO bulk and the EuO/Si interface places restrictionson the annealing procedures, depending on further ap-plications of the films. The proposed protection of EuOfilms by oxidation of their top layer is a clean, easy to im-plement, and free of any additional chemicals procedure.

4 Experimental section

Thin films of EuO are grown in Riber Compact 12 sys-tem for molecular beam epitaxy (MBE). Ultra-high vac-uum system is based on 200 l s−1 Gamma Vacuum TitanIon Pump, cryopump Cryo-Torr 8 (Brooks CTI Cryogen-ics), titanium sublimation pump and cryopanels cooleddown by liquid nitrogen. The capacity of the systemis sufficient for maintaining stable ratios of Eu and O2

fluxes without any accumulation of oxygen in the vac-uum chamber. The limit for the pressure of residual gasesis less than 10−10 Torr. The system is equipped with 5Knudsen cell effusion sources and one gas injector foran oxygen beam with stable and uniform intensity. Ma-terials for the effusion sources are 4N Eu, Sr, Gd, Al, andSiO. The last two chemicals are routinely used for thecapping layer formation on the EuO surface. The inten-sity of the molecular oxygen (6N) flux is tuned with thegas flow system based on the mass flow controller andBaratron manometer. It provides stable oxygen pressureat the level �5 ×·10−10 Torr.

The growth is controlled in situ with reflection high-energy electron diffractometer (RHEED) fitted with kSA400 Analytical RHEED System (k-Space Associates, Inc.)and residual gas analyzer RGA 200 (Stanford ResearchSystems). Thermocouples are used for cell temperaturemeasurements, and as sensors in the circuits control-ling the substrate temperature. The absolute substratetemperature is determined with PhotriX ML-AAPX/090infra-red pyrometer (LumaSense Technologies) operat-ing at the 0.9 μm wavelength and registration tempera-tures higher than 270 °C.

274 www.crt-journal.orgC© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cryst. Res. Technol. 50, No. 3, 268–275 (2015)

Orig

inal

Pape

r

Intensity of molecular beams is measured withBayard-Alpert ionization gauge fitted at the substratesite. It provides reliable information on the oxygenflux intensity but lacks selectivity. Gases extracted fromheated rare-earth metals are counted as a part of the re-actant flux, potentially leading to noticeable errors in theestimates. Therefore, calibration of Eu flux is performedonly after an extended gas removal from the loaded Eusample with hydrogen being the major impurity. Besides,the differences in the first ionization potentials for thematerials (2.35, 2.54 and 12.2 eV for Sr, Eu and O2, re-spectively) lead to uncertainties in the estimated ratiosof molecular beams. Quartz-crystal monitor would be abetter instrument for rare earths but it is not suitable formeasurements of oxygen fluxes.

Ex situ control of the quality of the grown films isperformed by a combination of physical methods. Spec-trometers Bruker D8 Advance and Rigaku SmartLab 9kWare used for x-ray measurements. Both spectrometersare equipped with CuKα x-ray sources with the wave-length λ = 1.5418 A. Rutherford backscattering spectraare recorded for He ions with the energy 1.7 MeV. De-pendence of magnetic properties on the temperatureand magnetic field is studied with SQUID magnetome-ter Quantum Design MPMS XL-7.

Acknowledgements. The work is financially supported by NRC“Kurchatov Institute”, Russian Foundation for Basic Researchthrough grant 13-07-00095, and Russian Science Foundationthrough grant 14-19-00062.

Key words. magnetic semiconductors, europium monoxide, sili-con, heteroepitaxy, spintronics.

References

[1] R. Jansen, Nature Mater. 11, 400 (2012).[2] G. Schmidt, D. Ferrand, L. W. Molenkamp, A. T. Filip,

and B. J. vanWees, Phys. Rev. B 62, R4790 (2000).[3] K. J. Hubbard and D. G. Schlom, J. Mater. Res. 11, 2757

(1996).[4] A. Mauger and C. Godart, Phys. Rep. 141, 51 (1986).[5] P. G. Steeneken, L. H. Tjeng, I. Elfimov, G. A. Sawatzky,

G. Ghiringhelli, N. B. Brookes, and D.-J. Huang, Phys.Rev. Lett. 88, 047201 (2002).

[6] A. Melville, T. Mairoser, A. Schmehl, T. Birol, T. Heeg, B.Hollander, J. Schubert, C. J. Fennie, and D. G. Schlom,Appl. Phys. Lett. 102, 062404 (2013).

[7] A. Schmehl, V. Vaithyanathan, A. Herrnberger, S. Thiel,C. Richter, M. Liberati, T. Heeg, M. Rockerath, L. F.Kourkoutis, S. Muhlbauer, P. Boni, D. A. Muller, Y.Barash, J. Schubert, Y. Idzerda, J. Mannhart, and D. G.Schlom, Nature Mater. 6, 882 (2007).

[8] A. Melville, T. Mairoser, A. Schmehl, D. E. Shai, E. J.Monkman, J. W. Harter, T. Heeg, J. Hollander, J. Schu-bert, K. M. Shen, J. Mannhart, and D. G. Schlom, Appl.Phys. Lett. 100, 222101 (2012).

[9] T. Mairoser, F. Loder, A. Melville, D. G. Schlom, and A.Schmehl, Phys. Rev. B 87, 014416 (2013).

[10] T. Mairoser, A. Schmehl, A. Melville, T. Heeg, L.Canella, P. Boni, W. Zander, J. Schubert, D. E. Shai, E. J.Monkman, K. M. Shen, D. G. Schlom, and J. Mannhart,Phys. Rev. Lett. 105, 257206 (2010).

[11] J. N. Beukers, J. E. Kleibeuker, G. Koster, D. H. A. Blank,G. Rijnders, H. Hilgenkamp, and A. Brinkman, ThinSolid Films 518, 5173 (2010).

[12] R. Sutarto, S. G. Altendorf, B. Coloru, M. MorettiSala, T. Haupricht, C. F. Chang, Z. Hu, C. Schußler-Langeheine, N. Hollmann, H. Kierspel, H.H. Hsieh,H.-J. Lin, C. T. Chen, and L. H. Tjeng, Phys. Rev. B 79,205318 (2009).

[13] A. G. Swartz, J. J. I. Wong, I. V. Pinchuk, and R. K.Kawakami, J. Appl. Phys. 111, 083912 (2012).

[14] N. Iwata, G. Pindoria, T. Morishita, and K. Kohn, J.Phys. Soc. Jpn. 69, 230 (2000).

[15] J. Lettieri, J. H. Haeni, and D. G. Schlom, J. Vac. Sci.Technol. A 20, 1332 (2002).

[16] J. Lettieri, V. Vaithyanathan, S. K. Eah, J. Stephens, V.Sih, D. D. Awschalom, J. Levy, and D. G. Schlom, Appl.Phys. Lett. 83, 975 (2003).

[17] M. Labidi, S. Labidi, S. Ghemid, H. Meradji, and F. ElHaj Hassan, Phys. Scr. 82, 045605 (2010).

[18] J. A. Mundy, D. Hodash, A. Melville, R. Held, T.Mairoser, D. A. Muller, L. F. Kourkoutis, A. Schmehl,and D. G. Schlom, Appl. Phys. Lett. 104, 091601(2014).

[19] C. Caspers, S. Flade, M. Gorgoi, A. Gloskovskii, W.Drube, C. M. Schneider, and M. Muller, J. Appl. Phys.113, 17C505 (2013).

[20] W. C. Fan, N. J. Wu, and A. Ignatiev, Phys. Rev. B 42,1254 (1990).

[21] J. W. Reiner, K. F. Garrity, F. J. Walker, S. Ismail-Beigi,and C. H. Ahn, Phys. Rev. Lett. 101, 105503 (2008).

[22] M. El Kazzi, G. Delhaye, C. Merckling, E. Bergignat, Y.Robach, G. Grenet, and G. Hollinger, J. Vac. Sci. Tech-nol. A 25, 1505 (2007).

[23] K. Y. Ahn, J. Appl. Phys. 40, 3193 (1969).[24] G. Street, IEEE Trans. Magn. 8, 45 (1972).[25] A. J. Melville, Ph.D. Thesis, Cornell University, (2014).[26] R. P. Panguluri, T. S. Santos, E. Negusse, J. Dvorak, Y.

Idzerda, J. S. Moodera, and B. Nadgorny, Phys. Rev. B78, 125307 (2008).

www.crt-journal.org 275C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim