Thin Solid Films 520 (2012) 6734–6739

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X-ray photoelectron spectroscopy and conducting atomic force microscopyinvestigations on dual ion beam sputtered MgO ultrathin films

Braj Bhusan Singh a, Vikash Agrawal b, Amish G. Joshi b, Sujeet Chaudhary a,⁎a Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110 016, Indiab CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India

⁎ Corresponding author at: Department of Physics, IndiaHauz Khas, New Delhi-110016, India. Tel.: +91 11 26591

E-mail address: sujeetc@physics.iitd.ac.in (S. Chaudh

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.06.078

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 September 2011Received in revised form 3 June 2012Accepted 25 June 2012Available online 2 July 2012

Keywords:Magnesium oxideThin filmsDual ion beam sputteringX-ray photoelectron spectroscopyConductive atomic force microscopy

Ultrathin films of MgO (~6 nm) were deposited on Si(100) using dual ion beam sputtering in different partialpressures of oxygen. These thin films were characterized by X-ray photoelectron spectroscopy (XPS) forchemical state analysis and conducting atomic force microscopy for topography and local conductivitymap. No trace of metal Mg was evidenced in these MgO films. The XPS analysis clearly brought out the for-mation of oxygen interstitials and Mg(OH)2 primarily due to the presence of residual water vapors in thechamber. An optimum value of oxygen partial pressure of ~4.4×10−2 Pa is identified with regard to homo-geneity of film and stoichiometry across the film thickness (O:Mg::0.93–0.97). The local conductivity map-ping investigations also established the film homogeneity in respect of electrical resistivity. Non-linearlocal current–voltage curves revealed typical tunneling characteristics with barrier width of ~5.6 nm andbarrier height of ~0.92 eV.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

MgO based magnetic tunnel junctions (MTJs) are being extensive-ly investigated due to their potential applications in developingmemory architecture, namely, magnetic random access memoriesread-head sensors for ultra-high density data storage, sensitive mag-netic switches, etc. [1–3]. MgO is awide bandgap ionic insulatorwith anenergy gap of 7.8 eV [4,5]. In the tunneling thickness regime (typically,1–6 nm), MgO grows in crystalline form at room temperature, which isan important requirement for the coherent spin dependent tunneling.This is due to the fact that in suchMTJs, the lateralmomentumof electronacross the interface is totally conserved. In such MgO based MTJs, thetunneling barrier height is ~3.7 eV [1,2,6,7]. On theother hand, themajor-ity of the reports reveal considerably lower barrier height values, whichlie typically in the range of 0.5 to 1.1 eV. This discrepancy is attributedto structural defects (oxygen and magnesium vacancies) implying thepresence of charge trapping states inside the barrier and variation of stoi-chiometry in the MgO thin films [2,7–10]. Nature and density of suchstructural defects in MgO are sensitive to the fabrication methods. SinceMgO is hygroscopic in nature, presence of water vapor in the backgroundvacuum results in formation of several defects/impurities states, whichare localized within the forbidden band of the MgO barrier. These local-ized states lead to inelastic tunneling processes, and often dominateover the elastic spin-dependent tunneling. This eventually suppressesthe tunnel magnetoresistance of the MTJs. It is evident that deposition

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conditions play a critical role during the growth of tunneling barrier.While several physical vapor deposition based techniques havebeen employed to deposit MgO thin films for barrier layers, suchas reactive sputtering of Mg target in the presence of Ar/O2 atmo-sphere, rf-sputtering from MgO target, e-beam evaporation ofMgO or rf-sputtering of MgO in the presence of O2 (molecular ox-ygen), etc. [1,11,12], there exist very few reports on the depositionof the MgO films using dual ion beam sputtering (DIBS) [13]. Theability to independently tune the deposition parameters, viz., O2

ion beam energy, Ar ion beam energy, working pressure, ion den-sity, rf-power to Ar-source and to the oxygen source, etc. makesDIBS an appropriate technique for deposition of ultrathin MgObarrier layers for MTJs. In this paper, we report the deposition ofultrathin MgO films employing dual ion beam sputtering in differentoxygen partial pressure ambients and their investigation by X-ray pho-toelectron spectroscopy (XPS) and conducting atomic force microscopy(CAFM).

2. Experimental details

Ultrathin films of MgO (~6 nm) were deposited on (100) orientedsilicon (native oxide was not removed) substrates by ion-assisted ionbeam sputter growth employing a NORDIKO 3450 ion beam sputteringsystem. A turbomolecular pump was employed for obtaining an ulti-mate vacuum of ~9.3×10−5 Pa. The film thickness was accuratelydetermined based on calibration performed on a number of thin films(deposited for different durations) using X-ray reflectivity measure-ments. Here, the oxygen ion beam was extracted from a 10 cm diarf-ion source through two-grid assembly (operating parameters:

6735B.B. Singh et al. / Thin Solid Films 520 (2012) 6734–6739

V+ (positive voltage at the inner grid)=50 V, V− (negative voltageat the outer grid)=−30 V and rf-power in the range of 0–100 W).The oxygen beam was directed toward the substrate at 45° withsimultaneous sputtering of 6 in. dia Mg metal target (target–substratedistance~27 cm) by high-energy Ar ion beam at 45°. The high-energyAr ion beam was extracted from another high energy rf‐ion source(operated at V+=500 V, V−=−225 V, rf-power 125 W, ion cur-rent density 1.33 mA/cm2, Ar gas flow 2.0 sccm). The 6 in. dia Mgtarget was mounted on remote controlled water-cooled cubicalturret.

A preliminary round of sample optimization was initially carried outby growing two series of MTJs—Si/SiO2/Co60Fe40(10 nm)/MgO(6 nm)/Ni78Fe22(10 nm) [Junction area~100×100 μm2 defined through in situshadow masking] by varying rf-power applied to oxygen ion source(viz., 0 W, 50 W, 70 W, 80 W, and 100 W) and O2-ion energy(0 eV, 20 eV, 40 eV, 50 eV, and 70 eV), while keeping other depositionparameters identical. The criteria for the performance of optimumbarrier vis-à-vis the deposition-parameter were the reproducibility oftunneling-like non-linear current–voltage (I–V) behavior and most im-portantly the temperature-dependence of junction resistance (R–T),i.e., observance of non-metallic junction characteristics down to 20 K.It is emphasized here that as a consequence of pinholes, unreacted Mg,etc., the inferior quality barrier often showed different I–V behaviorson repetition and/or typical metal like behavior on cooling below240–260 K. Subsequently, another series of MTJs was prepared inwhich MgO barrier was deposited in three different oxygen partialpressure (PO2

) values, (3.7×10−2 Pa/10 sccm, 4.4×10−2 Pa/15 sccm,and 5.2×10−2 Pa/20 sccm), while choosing the optimum values ofrf-power of 70 W and O2-ion energy of 50 eV as fixed growth parame-ters. These three MTJs exhibited both the optimum I–V behavior andweak-insulator like R–T behavior. Finally, three MgO films (each 6 nmthin) were deposited corresponding to these three different PO2

values(henceforth, referred to as O10, O15, and O20, respectively). Thesethree samples were chosen for identifying the optimal oxygen partialpressure appropriate to uniformly stoichiometric MgO via the detailedXPS and CAFM investigations.

The XPS measurements were carried out using a Perkin Elmer1257 model, operating at a base pressure of ∼3.5×10−6 Pa at 300 Kwith a non-monochromatized Al–Kα line at 1486.6 eV and a hemi-spherical sector analyzer. The overall instrumental resolution was≥300 meV. Pass energies for general survey-scan and core-level spectrawere kept at 143.05 and 71.55 eV with measurement steps of 0.50 and0.05 eV, respectively. Initially, the datawere acquiredwithout sputtering.TheAr-ion sputtering experimentswere carried out at 1.3×10−4 Pawithionization energy and emission current set to 4 keV and 20 mA, respec-tively. The sputtering was done for 5 min. After sputtering, a setof XPS spectra was acquired again at the same location. Subse-quently 2nd and 3rd sputtering were also performed for the sameduration.

One of the samples, i.e., O15 was also investigated by CAFM(Digital Instruments/VeecoMetrology Group, Nanoscope IIIa instrument)in contact mode for local conductivity mapping. For CAFM, the MgO thinfilm (~6 nm)with 11×5 mm2was deposited on a 70 nm thick Fe film asbottom electrode (~100 μm×12 mm) on Si substrate. The CAFM isequipped with a sensor of 1 nA/V current sensitivity and amplifier gainof 109. In this CAFMsystem, a siliconnitride tip (diameter~50 nm) coatedwith ~50 nm thin Co film is employed as a conducting tip. The surface ofthe ultrathin MgO film was scanned by this tip, which is subjected to aconstant atomic force maintained through a feedback mechanism. Thus,a topographical real time image is generated. By applying a volt-age between the substrate surface and the tip, both electrical andtopographical images could be obtained simultaneously. An elec-trical image can provide information of hot spots on the surface(pinholes or low resistive paths). The I–V characteristics wererecorded at several local points on the surface of the insulatinglayer (MgO) in a constant-force mode.

3. Results and discussion

3.1. XPS analysis

Fig. 1(a) shows the XPS spectra corresponding to O‐1s orbital of thethree MgO thin film samples after first sputtering (SI). All the spectrawere corrected with the C-1s (284.6 eV). The main peak wasdeconvoluted into three peaks at 530.3±0.1, 531.8±0.1 and 533.2±0.1 eV using XPS-Peak 4.1 software with Lorentzian–Gaussian peakshape function and by subtracting linear background. While the peakobserved at 530.4 eV corresponds to O2− in the lattice of MgO, thehigher binding energy peak observed at 533.2 eV corresponds to OH−

in Mg(OH)2. In between these two peaks, another peak at 531.9 eVwas identified as the consequence of O− ions occupying interstitialpositions of MgO lattice [14].

Also, Mg-2p spectra (SI) are shown in Fig. 1(b) for these threesamples. The main peak was deconvoluted into two peaks centeredat 50.3 and 51.8 eV binding energy values. While the peak at51.8 eV was identified due to magnesium hydroxide present, themain peak at 50.4 eV corresponded with the Mg forming MgO, asreported in literature [14,15]. We did not observe any indication ofun-oxidized metal Mg in Mg-2p XPS spectra. This ensured completeoxidation of Mg in these ultrathin films. Since the spectra of Fig. 1(b)were recorded after first sputtering of 5 min, there was a clear evidenceof Mg(OH)2 formation during growth as confirmed from theMg-2p andO-1s SI-spectra. It is well known that MgO reacts with the water vaporand forms its hydroxide with spinel structure. In our case, this watervapor may come from the residual background pressure of the deposi-tion chamber since the turbo-molecular pump has relatively lesserpumping speed for water vapors.

After the second sputtering of the MgO thin film for 5 min, theO-1s and Mg-2p core level SII-spectra were recorded as shown inFig. 1(c) and (d). The deconvoluted peaks at 50.4–50.6 eV and52.0 eV in the Mg-2p spectra were identified as the O2− lattice oxygenin MgO and OH− ions in Mg(OH)2, respectively The peaks in O-1sspectra at 530.4–530.8 eV, 531.9–532.3 eV and 533.3–533.8 eV,respectively correspond to lattice oxygen in MgO, interstitial O− ions,and OH− ions in Mg(OH)2 as identified previously in the SI-spectra. Itwas observed that in the SII‐spectra, there was a shift in the peak posi-tion of 0.4–0.5 eV energy toward higher binding energy relative to thefirst sputtering observations for all three samples. It may be stressedhere that the intensity of the peaks corresponding to the Mg-2p andO-1s spectra in O10 was smaller than that in the samples O15 andO20, which have nearly similar intensities. We further discuss thisissue in the following paragraph.

The variation in ratios of the integrated-area of peaks due tointerstitial-oxygen to that due to lattice oxygen in MgO, [i.e., O(I)/O(MgO)], oxygen in the Mg(OH)2 to that due to lattice oxygen [i.e.,O(Mg(OH)2)/O(MgO)] and the stoichiometry of the film (i.e., O:Mg),with the change in partial pressure of oxygen for both the spectra (SIand SII) for 3 samples are presented in Table 1. The stoichiometry ofMgO was determined from the ratio of integrated-area of peaks appe-aring at 530.4–530.8 eV in the O-1s spectra to that appearing at 50.3–50.8 eV in the Mg-2p spectra by considering atomic sensitivity factors1 for O-1s and 0.13 for Mg-2p electrons [16]. It is clear that as partialpressure of oxygen is increased, the concentration of oxygen interstitialsrelative to lattice oxygen increased (irrespective of SI or SII). A similartrend for hydroxide formation was also observed with increase in oxy-gen partial pressure. Compared to these two ratios, the change infilm-stoichiometry with oxygen partial pressure was found to be rela-tively quite small, and was close to the ideal value of 1:1 in O15 sample.The increased formation of oxygen interstitials at higher oxygen partialpressures can be understood by considering the fact that, as the beamdensity of oxygen ions is increased, re-sputtering of the oxygen in thegrowing MgO film also increases. This will release the bonded oxygenions in MgO lattice to the nearby interstitial sites. It is envisaged that

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56 54 52 50 48 46

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OH-Lattice O

Expt. Fitted curve

Background Lattice O

OH-

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O15

Inte

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/s)

O20

B.E. (eV)

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4000

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Background Lattice O

O-

OH-

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O15

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(co

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/s)

O20

B.E. (eV)

Lattice O

OH-O-

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538 534 534 532 530 528 526 524

538 534 534 532 530 528 526 524

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O-(a)

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B.E. (eV)

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Expt. Fitted curve

Background

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O10

(d)In

tens

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coun

ts/s

)

O15

B.E. (eV)

O20

OH-Lattice O

Fig. 1. Deconvoluted core-level XPS spectra corresponding to O-1s (a, c) and Mg-2p (b, d) recorded on the MgO thin films deposited in the presence of different oxygen partialpressures (a, b: first sputtering SI-spectra, and c, d: second sputtering SII-spectra).

6736 B.B. Singh et al. / Thin Solid Films 520 (2012) 6734–6739

the apparently excess quantity of oxygen over and above that requiredfor 1:1 stoichiometry could also possibly originate as a result of dissoci-ation of the residual OH− ions by the plasma near the growing film sur-face (2OH−→2O−+H2). These oxygen-interstitials in these ultrathinMgO films would eventually create additional defect states whichwould be localized below the conduction band of MgO [10]. These

Table 1Dependence on oxygen partial pressure of the different ratios of integrated areas of the XPS

Ratio of integratedareas of the peaks

Observation(SI or SII)

O10(3.7×10

O(Interstitial)/O(MgO) SI 0.45SII 0.40

O(Mg(OH)2)/O(MgO) SI 0.30SII 0.16

O(MgO)/Mg(MgO)(Stoichiometry)

SI 0.83SII 1.01

localized states can cause inelastic tunneling of electrons through MgObarrier enhancing the conductance [17] and thus reducing the averagebarrier height.

Now,we turn to the formation ofMg(OH)2 in our process. The higheroxygen-ions density (as in the case of O20 film) could lead to more dis-sociation of background water vapor present in the chamber, resulting

peak, due to oxygen interstitials, lattice oxygen, oxygen in Mg(OH)2, and Mg in MgO.

−2 Pa)O15(4.4×10−2 Pa)

O20(5.2×10−2 Pa)

0.50 0.670.56 0.710.33 0.480.34 0.340.97 0.810.93 0.90

6737B.B. Singh et al. / Thin Solid Films 520 (2012) 6734–6739

in relatively more formation of Mg(OH)2. In order to quantify the watervapor content in the chamber, the partial pressures of background gaseswere estimated by RGA spectra recorded using quadrupole mass spec-trometer (QMS) just before the deposition of the MgO thin films.These spectra (Fig. 2) revealed a partial pressure of water vapor of~54% of the total background pressure. As can be seen from Fig. 2, thevolume fraction of water vapors, compared to any other residual gases,was found to be highest in the chamber. This, therefore, accounts forthe readily formation of Mg(OH)2 in our deposition process. Also, it isnoted here that a significant difference in the ratio of O(Mg(OH)2)/O(MgO) was observed in the two spectra (SI and SII) of all the threesamples. This difference can be attributed to the non-uniform distribu-tion of hydroxide content in the MgO matrix along the thickness,which can lead to some density variation along the film-thickness.

It may be pointed out that in sample O15, the ratios O(I)/O(MgO)and O(Mg(OH)2)/O(MgO) and the film stoichiometry (O:Mg) changedvery little with respect to SI and SII. These results suggest that opti-mum oxygen partial pressure should be between 3.7×10−2 Paand 4.4×10−2 Pa. The MgO thin film sample O10 deposited at loweroxygen partial pressure (3.7×10−2 Pa) was stoichiometric, i.e., O:Mgfor SII is 1.01. For SI, the O:Mg is quite lower at ~0.83. This variation ofstoichiometry could be due to either the non-uniform distribution of Oand Mg in the MgO thin film or degradation at the top few monolayersinMgO. It is noted that in sample O15 (i.e., grown at oxygen partial pres-sure of ~4.4×10−2 Pa), no appreciable change in stoichiometry wasobserved after sputtering of the MgO thin film.

The valence band spectra of all the three samples for SI and SII areshown in Fig. 3(a) and (b), respectively. The Fermi-level defined at0 eV is shown as dashed line. The weak peaks around the Fermilevel, EF, are estimated to be a kind of non-local states in the bandgap possibly caused by defects. Broad peak at ~6 eV mainly involvesO-2p orbitals. The part of O-2p orbitals may hybridize with Mg-2pand 3s orbitals. The binding energy shift in valence band maximum(VBM) with respect to EF is obtained by linearly extrapolating theleading edge to the base line. The tangent to the fitted curve wasdrawn and its intersection with the base line is defined as VBM.Valence band edge corresponding to the SI spectra is found to occurat 3.8±0.1 eV, 2.8±0.1 eV and 3.4±0.1 eV, respectively and at 3.3±0.1 eV, 3.0±0.1 eV and 3.6±0.1 eV corresponding to the SII spectrafor oxygen partial pressure of 3.7×10−2, 4.4×10−2 and 5.2×10−2 Pa,respectively in the three samples. In both the observations, the smallestvalue of a VBM was observed corresponding to the employed oxygenpartial pressure of 4.4×10−2 Pa during the deposition. Hence, nomonotonic pattern was observed in VBM in the investigatedrange of oxygen partial pressure variation. In our opinion, thiscould be due to the combined effect of different oxygen partialpressures, effect of Ar-sputtering (prior to SI and SII), and the fact that

5.3x10-5

4.0x10-5

2.7x10-5

1.3x10-5

5.9x10-9

H2O C

xH

yN

2/CO O

2HeH

2CO

2

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(Pa)

Residual gases

Ar

Fig. 2. Background RGA spectrum recorded just before deposition of the MgO films.

no correction in these VBM spectra due to underlying substrate wasconsidered.

3.2. CAFM investigations

Considering the nearly same stoichiometry along the film thicknessand moderately lower ratios of O(I)/O(MgO) and O(Mg(OH)2)/O(MgO),we chose sample O15 for the local conductivity mapping and point-contact tunneling investigations. Fig. 4(a) shows the topographicalimage of MgO surface in the scanned area of 5×5 μm2. Therms-roughness was found to be 1.2 nm. The 2D‐electrical imagesof the corresponding area at a bias voltage of 2 and 3 V are shown inFig. 4(b), (c), respectively. It is clear from Fig. 4(b) and (c) that as thebias voltage is increased from 2 to 3 V, the density of hot-spots signifi-cantly increases. In addition, the size of hot-spots also increases. Thehot-spots could be either low resistive networks like pinholes orsome unoxidized Mg, in the barrier. Since we did not observe anyindication of unoxidized Mg in our XPS analysis, these hot-spotscould be the result of the presence of pinholes. A mixed charactergrowth (Stranski–Krastanov type) of MgO could result in the for-mation of such pinholes. While, the background current is of theorder of 200 nA (15 MΩ resistance), the current corresponding tospikes is 400 nA (5 MΩ resistance). This indicates that such re-gions are relatively less resistive. In contrast to this, very fewsuch current spikes (bright spot) are observed in case of 2 V bias.

The current voltage characteristics were recorded at some localpoints by positioning the tip at these points, and varying the biasfrom negative to positive. We have selected three regions, i.e., darkregion (background), intermediate region (dull bright portion) andbright region (brightest portion) for performing the I–V measure-ments. The I–V curves corresponding to these regions are shown inFig. 4(d), (e) and (f), respectively. The observed asymmetry in theI–V curves could be due to different work-functions of the Co(5.0 eV) coated tip and that of the bottom Fe electrode (4.5 eV)[18]. But, in the case of I–V curve recorded on the brightest region(Fig. 4(f)), current increases linearly with voltage, and becomes ex-tremely large (so much so that it goes out of range of the system'ssensitivity), which is a clear indication of junction breakdown atthat point. By fitting the I–V curves using Simmon's model [19], wehave estimated the values of average barrier height (q) and thickness(s) for both intermediate and dark regions, as shown in Table 2. Thedata within ±1.5 V was used for fitting using Simmon's equation. Itmay be pointed out that the I–V data could not be fitted for the brightregion due to the breakdown of the junction. The thickness andbarrier height exhibited similar values for q and s in the other two re-gions implying better film homogeneity. The barrier height q was de-termined to be 0.92 eV. Although, this is small compared to the idealvalue of barrier height of 3.7 eV for MgO, but it is relatively higherthan that reported for MgO grown by other techniques [1,2]. Thelower barrier height in the present case could be due in part to thepresence of interstitial ionic defects (as conclude by XPS analysis) aswell as to the surface states due to the absence of top ferromagneticelectrode. The ionic defects create localized defect states within the for-bidden region at ~1.2–1.4 eV below the bottom of the conduction bandof MgO [10]. Together with the XPS investigations (i.e., on sample O15),the studies thus suggest that the optimum value of oxygen partial pres-sure is ≤4.4×10−2 Pa for uniform and homogeneous MgO ultrathinfilms by dual ion beam deposition.

4. Conclusion

Ultrathin films of ion-assisted ion-beam sputteredMgO deposited indifferent oxygen partial pressure environments have been investigatedusing X-ray photoelectron spectroscopy and conducting atomic forcemicroscopy. The analyses of the Mg-2p and O-1s spectra establishedthe absence of metallic Mg in these ultra-thin films. In addition to the

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Fig. 3. Valence band maximum (VBM) spectra—SI (a) and SII (b)—recorded on the MgO thin films sputtered at different oxygen partial pressures. The VBM was evaluated byextrapolation of linear fitting (refer to the text for details).

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Fig. 4. (a) Topographic image (V=0), (b) and (c) current images recorded at biases of 2 V and 3 V of the O15 film sample. The (d), (e) and (f) are the I–V characteristics of this MgOfilm recorded at dark, intermediate and bright regions, respectively.

6738 B.B. Singh et al. / Thin Solid Films 520 (2012) 6734–6739

Table 2Barrier height (q) and barrier width (s) values obtained for the MgO film (O15) byfitting its I–V data in the range of 0–1.5 V using Simmon's model [19].

Region Barrier heightq (eV)

Barrier widths (nm)

Dark 0.92 5.6Intermediate 0.91 5.5

6739B.B. Singh et al. / Thin Solid Films 520 (2012) 6734–6739

main MgO phase, formation of oxygen interstitials and Mg(OH)2 is alsoobserved. The latter is ascribed due to higher residual partial pressure ofwater vapors in the vacuum chamber, as confirmed from the RGA mea-surements. As concentration of oxygen interstitials increased with oxy-gen partial pressure, the oxygen partial pressure of ≤4.4×10−2 Pa isfound to be conducive for optimum quality of MgO possessing ho-mogeneous stoichiometry across the film thickness. The XPS find-ings corroborated satisfactorily with the conducting AFM mappinginvestigations and local I–V tunneling like characteristics.

Acknowledgments

Theworkwas fundedby theMinistry of Communication and Informa-tion Technology (MCIT), Govt. of India (Grant No. 1(14)/2007—M & C).BBS acknowledges the Council of Scientific and Industrial Research(CSIR) India for a research fellowship grant. We also acknowledge

Dr. Deepak Varandani for conducting AFM measurements and Prof. D.K.Pandya for useful discussion.

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