Journal of Science: Advanced Materials and Devices (2016) 290e294 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article New insights into CoFe/n-Si interfacial structure as probed by X-ray photoelectron spectroscopy Arvind Kumar a, b, *, T Shripathi c, P.C Srivastava a a Department of Physics, Banaras Hindu University, Varanasi 221005, UP, India Department of Physics, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi 110021, India c UGC-DAE Consortium for Scientific Research, University Campus, Indore 452017, MP, India b a r t i c l e i n f o a b s t r a c t Article history: Received 11 June 2016 Accepted 24 July 2016 Available online 28 July 2016 X-ray photoelectron spectroscopy (XPS) is a well known tool in studying the physical and chemical properties of surface/interfaces which provides the element specific, non-destructive and quantitative information In the present study, information about the surface chemical states of interfacial structure of CoFe thin films on n-Si substrates has been studied from XPS technique The surface of the samples has also been cleaned from ion beam etching for 30 with Arỵ ions to record the XPS spectra The observation shows that the Si atoms are present within the probed surface layer due to interfacial intermixing across the interface which is due to strong chemical reactivity of n-Si substrate A shift in the binding energy peaks of Fe2p and Co2p has also been observed which could be due to the formation of silicide phases as a result of interfacial intermixing XPS results have indicated the formation of silicide phases across the interfaces which poses interfacial antiferromagnetic coupling across CoFe/n-Si interface to affect the magnetic behaviour It has been found that the present XPS results are in well support with our earlier study © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: XPS Interfacial intermixing Surface states Spintronics Silicides Introduction Recently, a new field of electronics, spintronics has attracted much attention which is based on the fact that electrons have spins as well as charge [1] At present, two different approaches have been reported to realize the spintronic based devices, i.e., the use of (i) a magnetic semiconductor [2e7], and (ii) a hybrid structure of ferromagnetic (FM) metal and semiconductor (SC) [8e12] A basic obstacle of spintronic devices for its application at room temperature (RT), is the relatively low Curie temperature of existing ferromagnetic semiconductor (~100 K) [13] On the other hand, ferromagnetic metals such as Fe, Co, Ni and their alloys have the Curie temperature high enough for operation at RT, as required for application The combination of FM with semiconducting materials is promising from both perspectives, i.e., with respect to application and basic research as it merges the physical properties of two technologically important material classes FM/SC interfaces can also be used as a spin injectors and spin analyzers for polarised * Corresponding author Department of Physics, Atma Ram Sanatan Dharma College, University of Delhi, New Delhi 110021, India E-mail address: bhuarvind2512@gmail.com (A Kumar) Peer review under responsibility of Vietnam National University, Hanoi currents from ferromagnetic metal into semiconductors Except the transition metals such as Fe, Co and Ni etc., magnetic alloy of CoFe have also attracted much attention towards spintronic devices because of its various important properties such as soft magnetic properties, high Curie temperature (~1500 K), low magnetic anisotropy, high saturation magnetization (~15% greater than Fe), low coercivity and high permeability [14e16] The electronic and magnetic properties of such heterostructures depend on the nature of the metal/semiconductor interface which in turn, also affects the magneto-transport Due to interfacial chemistry such as interfacial intermixing across the interfaces at RT, there is a change in the chemical states of the constituent elements X-ray photoelectron spectroscopy (XPS) is a versatile surface sensitive technique and is widely being used to characterize the surface chemical compositions and surface electronic states of such structures [17] XPS is a very useful technique to probe the interface of FM/SC structure to study the interfacial reaction occurred In our earlier study [18], we reported the magnetic, morphological and structural investigations of CoFe/Si interfaces Structural investigations (from XRD) have shown the formation of more silicide phases for CoFe/n-Si interfacial structure as compared to CoFe/ p-Si sturucture Magnetic property of CoFe/n-Si interfacial structure has shown the presence of antiferromagnetic coupled phase which http://dx.doi.org/10.1016/j.jsamd.2016.07.008 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) A Kumar et al / Journal of Science: Advanced Materials and Devices (2016) 290e294 Fig shows the recorded survey scan spectra of CoFe films over n-Si substrates for as-deposited (i.e., e0) and 30 sputter etched (i.e., e30), respectively It can be seen that the spectra contain only photoemission peaks due to Fe and Co at the binding energy (B.E.) positions of ~711.5 eV and ~794.0 eV, respectively The presence of Si signal has also been detected but it is not very prominent In addition to this, the presence of absorbed carbon (C) and oxygen (O) signals has also been detected on the sample's surface The observed contribution of carbon and oxygen in survey scan could originate due to atmospheric exposure of the samples during transfer to the chamber Moreover, after 30 sputter etching of the surface (with low energy Arỵ ions), the signals due to carbon and oxygen are getting weaker in intensity whereas signals due to Fe, Co and Si are emerging out with a prominent intensity This suggests that the some of the atmospheric impurities (like C and O) which were present at top of the surface are getting removed after sputter etching To gain more insight about the observed elements (Co, Fe and Si) in survey scan spectra, separate detailed scans have been recorded for each element, i e, for Co2p, Fe2p and Si2p The detail scan spectrum for Si has been recorded only after sputter cleaning (e30) because for the asdeposited sample (e0), the signal due to Si was not significant Furthermore, to analyze the variation in content of Co and Fe either in form of silicide or oxide, the detail scan spectra of Co2p, Fe2p and Si2p were further deconvoluted 3.1.1 Detail scan spectra of Fe2p Fig shows the core level XPS spectra of Fe2p peak for asdeposited (e0) and 30 sputter etched sample, recorded in a narrow scan between ~685 eV and ~750 eV The Fe2p spectrum recorded for as-deposited sample (e0) contains photoemission peaks of Fe2p3/2 and Fe2p1/2 at binding energy positions of ~707.4 eV and 720.4 eV, respectively which corresponds to the metallic phase of iron The other observed B.E peaks ~712.7 eV and 723.4 eV seem to correspond to formation of iron oxide phase, i e, Fe2O3 phase Moreover, after 30 sputter etching (e30), the B.E positions are observed at ~708.1 eV (for Fe02p3/2) and 721.4 eV (for Fe02p1/2) along with a satellite peak at ~713.4 eV The B.E peaks at ~708.1 eV and 721.4 eV corresponds to metallic phase of Fe which is observed to be shifted towards higher B.E side as compared to asdeposited sample (e0) It is noteworthy to mention here that after sputter cleaning (e30) the contribution due to oxide phase get reduced and metallic nature has improved 40000 40000 e0 e30 35000 35000 30000 30000 20000 20000 15000 10000 5000 5000 Si2p 10000 C1s 15000 0 1400 Intensity (cps) 25000 25000 O1s n-Si (100) substrates of resistivity 8e10 U-cm and a thickness of ~400 mm have been used for the sample preparation Prior to metallization, Si substrates were ultrasonically cleaned in trichloroethylene (TCE) solution to remove the organic contamination Subsequently, Si substrates were chemically etched in a solution of HF and HNO3 ~ (1:30 ratio) to remove the native oxide layer followed by rinsing in distilled water and then dried in a clean vacuum chamber To realize the CoFe/n-Si interfacial structure, a thin film of CoFe (of thickness ~50 nm) has been deposited on etched and cleaned n-Si substrates by electron beam evaporation technique under the base pressure of ~10À6 torr XPS measurement was performed using VSW-ESCA photoelectron spectrometer at a base pressure of  10À9 torr at room temperature AlKa unmonochromatized X-rays (energy, hn ~1486.6 eV) with the source operated at an emission current 10 mA and an anode voltage of 10 kV was used for analysis Samples were properly grounded to avoid any charging effect Hemispherical energy analyzer was used in the fixed analyzer transmission mode with the pass energy (~40 eV) to give an instrument resolution of ~0.9 eV Graphitic C1s ~284.6 eV has been used as an internal reference to correct the shifts in binding energy of core levels due to charging effect The depth profiling of the samples was done by ion beam (attached to the spectrometer at oblique incidence) etching of the surface using low energy (~3.5 keV) Arỵ ion gun Quantitative analysis of the composition of the films have been performed by collecting the integrated intensities of C1s, O1s, Fe2p, Co2p and Si2p signals present in XPS using Wagner's sensitivity factors The relative concentration of Co and Fe has been calculated and found to be of 60% and 40%, respectively XPSPEAK4.1 software [22] has been used for deconvolution (i.e., curve fitting) of the XPS spectra Prior to curve fitting, Shirley background was subtracted and then peaks were deconvoluted 3.1 Core level (XPS) study of CoFe/n-Si interfacial structure Co2p Fe2p Experimental details Results and discussion Intensity (cps) was understood due to various magnetic silicide phases of Fe3Si, FeSi2 and ε-FeSi to result in observed exchange bias for n-Si structure, large magnetoresistance and distinct behaviour as compared to CoFe/p-Si structures The results of structural, morphological and transport studies across the interfaces were interpreted in the realm of strong chemical reactivity of n-type silicon wafer to result in more silicide phases Our earlier study [18] has shown that the interfacial chemistry across CoFe/n-Si interface plays a significant role in determining the structural, magnetic and transport properties Exchange bias is an interfacial phenomenon Fan, Y et al [19] studied the Fe/MgO system and found the significant shift in the magnetization loop which is a classic signature of exchange bias In our present study, we have also found such effect of exchange bias which is an interfacial effect due to formation of silicide phases at the interface Other studies [20,21], made on permalloyeCoO and composites systems also found significant exchange bias So, the motivation behind the present study is to investigate the presence of surface chemical states/chemical phases across CoFe/nSi interfacial structure using core level X-ray photoelectron spectroscopy (XPS) technique In this report, XPS analysis of the structure has been detailed XPS analysis has shown the formation of various phases of silicides of Fe Moreover, the XPS spectra have also shown the presence of metallic phases of Fe and Co The chemical interaction and surface chemical states of the CoFe/n-Si interface has been described in detail and confirmed the presence of metallic and silicide phases (of CoFe, Fe3Si, FeSi) which was proposed in our earlier study [18] and found to affect the magnetic and transport behaviour of such interfaces 291 -5000 1200 1000 800 600 400 200 Binding Energy (eV) Fig Survey scan XPS spectra of CoFe/n-Si interfacial structure for as prepared (e0) and after 30 sputter etching (e30) of the surface 292 A Kumar et al / Journal of Science: Advanced Materials and Devices (2016) 290e294 9000 721.4 8000 713.4 708.1 e30 Intensity (cps) 7000 6000 699.5 5000 723.4 720.4 e0 4000 712.7 707.4 3000 699.7 2000 750 740 730 720 710 700 690 680 Binding Energy (eV) Fig Detail scan XPS spectra of Fe2p of CoFe/n-Si interfacial structure for as prepared (e0) and after 30 sputter etching (e30) of the surface Deconvoluted spectra of Fe2p Fig (a) and (b) shows the deconvoluted spectra of Fe2p peak for as-deposited (e0) and sputter etched (e30) sample, respectively For the as-deposited sample, the spectrum has been deconvoluted into five peaks at B.E positions of ~707.4 eV, 710.8 eV, 715.2 eV, 720.4 eV and 723.4 eV The peak positions at ~707.4 eV (Fe2p3/2) and 720.4 eV (Fe2p1/2) correspond to the metallic phase of iron The observed B.E difference between Fe2p doublet spectra, i.e., between 2p3/2 and 2p1/2 is 13.0 eV which is close to the reported value [23] The other observed B.E peak positions at ~710.8 eV (Fe2p3/2) and 723.4 eV (Fe2p1/2) seems to 4500 (a) 723.4 720.4 715.2 e0 710.8 707.4 Intensity (cps) 4000 3500 3000 correspond to oxide phase of iron Such observation of iron oxide phase has earlier been reported [24e26] A satellite peak at ~715.2 eV (~7.8 eV higher than the metallic phase of Fe ~707.4 eV) is likely due to oxide phase of iron The observation of such oxide and metallic phases of Fe2p doublet at the above mentioned binding energies along with the satellite peaks has also been observed by other groups [25e28] They observed Fe2p3/2 peak at binding energy position of 706.73 eV, 707.33 eV, and 710.26 eV Peak at binding energy position of 706.73 eV was explained due to metallic nature of Fe whereas peak at binding energy position of 710.26 eV was due to the formation of iron oxide [29] Moreover, for sputter etched (e30) sample (Fig 3b), the spectrum has been deconvoluted into three peaks at the B.E positions of ~708.1 eV, 713.4 eV and 721.4 eV The observed B.E peak shifting of Fe2p doublet spectra due to Fe2p3/2 and Fe2p1/2 signals towards higher binding energies of ~708.1 eV, and ~721.4 eV (for e30) as compared to B.E positions at ~707.4 eV and 720.4 eV (for e0) seems to be due to formation of iron silicide (Fe3Si/FeSi/FeSi2) phase [30] Such shifting of binding energy position of Fe 2p peaks towards higher energy which seems due to silicide formation were also reported by other researchers [31e34] Similar phases of silcides were also observed in our XRD data [18] which is now confirmed from present XPS study Such observed silicide phases are responsible for the observed distinct behaviour of magnetic, structural, morphological and transport properties for CoFe/n-Si interfacial structre as compared to CoFe/pSi interfacial structure As, it is also clear from the survey scan spectra (Fig 1) that we have also observed Si2p signal so the observed silicide phase could be likely due to the interfacial intermixing between CoFe alloy and Si substrate The formation of such iron silicide phases has been observed in our earlier study [18] which is now confirmed from our XPS data The formation of such silicide phases could also result due to the strong chemical reactivity of n-type Si substrate [35] The other observed peak at binding energy ~713.4 eV is due to Co auger signal bind with Fe atoms and thus confirms the formation of CoFe alloy phase [36] as also observed in our XRD result Thus, the confirmation of metallic phase of CoFe and iron silicide phases (of Fe3Si, FeSi) is in support to get an exchange bias due to interfacial antiferromagnetic coupled grains of FeeSi compounds or spacer layers [37] 699.7 3.1.2 Detail scan spectra of Co2p Fig shows the core level XPS spectra of Co2p peak for asdeposited (e0) and after 30 sputter etched (e30) sample, recorded in a narrow scan between 820 eV and 760 eV The recorded Co2p spectrum for as-deposited (e0) sample contains 2500 2000 730 720 710 700 690 Binding Energy (eV) 6000 (b) 708.1 713.4 721.4 779.5 11000 e30 5500 10000 794.8 5000 4500 Intensity (cps) Intensity (cps) 9000 4000 3500 699.5 3000 e30 784.5 8000 7000 772.6 6000 779 e0 793.8 5000 2500 4000 773.4 2000 3000 1500 730 720 710 700 690 Binding Energy (eV) Fig Deconvoluted XPS spectra of Fe2p for (a) as prepared (e0) and (b) after 30 sputter etching (e30) of the surface 830 820 810 800 790 780 770 760 750 Binding Energy (eV) Fig Detail scan XPS spectra of Co2p of CoFe/n-Si interfacial structure for as prepared (e0) and after 30 sputter etching (e30) of the surface A Kumar et al / Journal of Science: Advanced Materials and Devices (2016) 290e294 photoemission peaks due to Co2p3/2 and Co2p1/2 at binding energy positions of ~779.0 eV and 793.8 eV, respectively which corresponds to metallic phase of Co [38] Whereas, after 30 sputter etching (e30) the B.E peak positions are observed to be shifted slightly towards higher B.E side of ~779.5 eV and 794.8 eV as compared to prior to sputter etching (e0) The difference between the observed doublet spectra of Co2p is found to be of ~14.8 eV (for e0) and ~15.3 eV (for e30) which is very close to the reported value ~15 eV for metallic phase of Co [32] The other observed peak at B.E position of ~784.5 eV seems to correspond Fe auger peak or may be due to the satellite peak of Co2p3/2 Deconvoluted spectra of Co2p Fig (a) and (b) shows the deconvoluted spectra of Co2p for as-deposited and 30 sputter etched sample, respectively The spectrum of as-deposited sample has been deconvoluted into six peaks at B.E positions of ~773.4, 779.0, 782.3, 787.5, 794.0 and 797.3 eV Peaks at binding energy positions of ~779.0 eV (for Co2p3/2) and 794.0 eV (for Co2p1/2) are due to metallic phase of Co2p Binding energy difference between Co2p doublet spectra i.e., Co2p3/2 and 2p1/2 is ~15.0 eV which is close to the standard separation between the doublet spectra [38] The other observed peaks at binding energy positions of ~782.3 eV and 797.3 eV shows Co2p3/2 and 2p1/2 spineorbit doublet due to formation of Co-oxide phases (of CoO or Co3O4) Similar observation has also been reported by Tan et al [39] where they found the separation between spineorbit doublet due to oxide phases of Co2p is ~15.0 eV The presence of shake up satellite peak at B.E ~787.5 eV (~5.2 eV higher than the Co2ỵ2p3/2e782.3 eV) could also be due to the oxide phase of cobalt (CoO/Co3O4) The observation of such shake up satellite peak ~6.0 eV higher than Co2p3/2 peak has 293 also been reported by other groups [40] So, it looks that some trace amount of oxide phases are present along with the metallic phase of Co peak The observed peaks of oxide phases can originate due to atmospheric exposure of the sample Moreover, the deconvoluted spectrum of 30 sputter etched sample (Fig 5b) shows five distinct peaks at B.E positions of ~772.6, 779.5, 783.0, 794.5 and 796.5 eV The doublet spectra of Co2p is observed to be at B.E positions of ~779.5 eV (for Co02p3/2) and ~794.5 eV (for Co02p1/2) which is 0.5 eV shifted to higher B.E side as compared to prior to sputter etching (e0) The observed shifting of B.E positions is likely due to the formation of oxide/silicide phases across the interface The B.E position at ~783.2 eV seems to correspond to Fe auger peak bind with Co atoms which again confirms the formation of the CoFe alloy phase [31,36] as also earlier observed by us in XRD data [18] Peak at binding energy position of ~796.5 eV (Co2p1/2) could be due to the presence of trace amount of CoO/Co3O4 formed during the atmospheric exposure It is also interesting to observe that after 30 sputter cleaning (e30) the signal due to oxide phase get suppressed and metallic nature of Co is prominent 3.1.3 Detail scan spectra of Si2p As discussed earlier, the Si signal was not significant for asdeposited (e0) sample so we have only recorded the detail scan spectra after sputter etching Fig shows the core level XPS spectra of Si2p peak recorded only after 30 sputter etched (e30) 740 100.5 720 5500 779.0 (a) 794.0 e0 782.3 797.3 787.5 Intensity (cps) 5000 Intensity (cps) 700 680 660 640 620 600 4500 580 773.4 4000 560 540 110 3500 105 100 95 90 Binding Energy (eV) 3000 800 790 780 770 Fig Detail Scan XPS spectra of Si2p of CoFe/n-Si interfacial structure after 30 sputter etching (e30) of the surface 760 Binding Energy (eV) 12000 779.5 (b) 740 e30 11000 700 Intensity (cps) 783.0 794.5 796.5 10000 Intensity (cps) e30 101.0 720 9000 8000 772.6 7000 680 660 640 104.1 620 600 6000 580 5000 560 540 4000 800 790 780 770 760 Binding Energy (eV) Fig Deconvoluted XPS spectra of Co2p for (a) as prepared (e0) and (b) after 30 sputter etching (e30) of the surface 110 108 106 104 102 100 98 96 94 Binding Energy (eV) Fig Deconvoluted XPS spectra of Si2p after 30 sputter etching (e30) of the surface 294 A Kumar et al / Journal of Science: Advanced Materials and Devices (2016) 290e294 sample in a narrow scan between 110 eV and 90 eV The recorded detail scan spectra of Si2p shows the B.E peak position at ~101.0 eV which corresponds to the presence of Si Almand (~101.8 eV) or oxide phase [25] Deconvoluted spectra of Si2p Fig shows the deconvoluted spectra of Si2p signal The spectrum has been deconvoluted into two peaks at B.E positions of ~101.0 eV and 104.1 eV The peak at binding energy ~101.0 eV corresponds to Si Almand (~101.8 eV)/and or Co3s whereas the other peak at ~104.1 eV corresponds to oxide phase of Si [25] Conclusions In conclusion, we investigated about the chemical interactions and surface chemical states of CoFe/n-Si interfacial structures which are responsible for affecting the electronic and magnetic behaviour of layered structures It has been observed that after sputter etching (or cleaning) there is a shifting in the B.E 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XPS spectra of Co2p of CoFe/n-Si interfacial structure for as prepared (e0) and after 30 sputter etching (e30) of the surface A Kumar et al / Journal of Science: Advanced Materials and Devices... chemical interaction and surface chemical states of the CoFe/n-Si interface has been described in detail and confirmed the presence of metallic and silicide phases (of CoFe, Fe3Si, FeSi) which was... formation of CoFe alloy phase [36] as also observed in our XRD result Thus, the confirmation of metallic phase of CoFe and iron silicide phases (of Fe3Si, FeSi) is in support to get an exchange