The electron energy band diagram at the (100)Si/MgO interface is characterized using internal photoemission of electrons and holes from Si into the oxide. For the as-deposited amorphous MgO the interface barriers correspond to a band gap width of 6.1 eV, i.e., much lower than the conventionally assumed bulk crystal value (7.83 eV). The annealing-induced crystallization of MgO mostly affects the energy of the valence band while the conduction band bottom retains its energy position at 3.37 +/- 0.05 eV above the top of the silicon valence band.(C) 2010 American Institute of Physics. (doi:10.1063/1.3294328

Afanas'ev, V. V.

Cherkaoui, Karim

Hurley, Paul K.

Stesmans, A.

Applied Physics Letters

Afanas\u27ev, V. V.

Irish Universities

Electron energy band alignment at the (100)Si/MgO interface

Cork Open Research Archive

Title Electron energy band alignment at the (100)Si/MgO interfaceAuthor(s) Afanas'ev, V. V.; Stesmans, A.; Cherkaoui, Karim; Hurley, Paul K.Publication date 2010Original citation Afanas’ev, V. V., Stesmans, A., Cherkaoui, K. and Hurley, P. K. (2010)'Electron energy band alignment at the (100)Si/MgO interface', AppliedPhysics Letters, 96(5), pp. 052103. doi: 10.1063/1.3294328Type of publication Article (peer-reviewed)Link to publisher'sversionhttp://aip.scitation.org/doi/abs/10.1063/1.3294328http://dx.doi.org/10.1063/1.3294328Access to the full text of the published version may require asubscription.Rights © 2010 American Institute of Physics.This article may bedownloaded for personal use only. Any other use requires priorpermission of the author and AIP Publishing. The following articleappeared in Afanas’ev, V. V., Stesmans, A., Cherkaoui, K. andHurley, P. K. (2010) 'Electron energy band alignment at the(100)Si/MgO interface', Applied Physics Letters, 96(5), pp. 052103and may be found athttp://aip.scitation.org/doi/abs/10.1063/1.3294328Item downloadedfromhttp://hdl.handle.net/10468/4347Downloaded on 2018-08-23T18:50:02ZElectron energy band alignment at the (100)Si/MgO interfaceV. V. Afanas’ev, , A. Stesmans, K. Cherkaoui, and P. K. HurleyCitation: Appl. Phys. Lett. 96, 052103 (2010); doi: 10.1063/1.3294328View online: http://dx.doi.org/10.1063/1.3294328View Table of Contents: http://aip.scitation.org/toc/apl/96/5Published by the American Institute of PhysicsElectron energy band alignment at the „100…Si/MgO interfaceV. V. Afanas’ev,1,a A. Stesmans,1 K. Cherkaoui,2 and P. K. Hurley21Department of Physics, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium2Tyndall National Institute, University College Cork, Lee Maltings, Prospect Row, Cork, IrelandReceived 5 November 2009; accepted 27 December 2009; published online 1 February 2010The electron energy band diagram at the 100Si/MgO interface is characterized using internalphotoemission of electrons and holes from Si into the oxide. For the as-deposited amorphous MgOthe interface barriers correspond to a band gap width of 6.1 eV, i.e., much lower than theconventionally assumed bulk crystal value 7.83 eV. The annealing-induced crystallization of MgOmostly affects the energy of the valence band while the conduction band bottom retains its energyposition at 3.370.05 eV above the top of the silicon valence band. © 2010 American Institute ofPhysics. doi:10.1063/1.3294328Thanks to the ease of epitaxial growth on various crystalsubstrates1 and the wide bulk crystal band gap 7.83 eV2magnesium oxide, MgO, has attracted considerable interestas a material for a broad spectrum of electronic applicationsranging from gate insulators in metal-oxide-semiconductorMOS transistors3–5 to a buffer layer enabling integration ofother functional oxides.6,7 In particular, MgO is widely usedas a barrier in magnetic tunnel junctions MTJs8–10 with therecently demonstrated possibility of spin injection into thesilicon crystal.11,12 In all these applications, MgO is supposedto serve as an insulator providing high energy barriers forelectrons and holes. Though the knowledge of the barriers isof particular importance for MTJs as they determine the typeof charge carriers that provide the dominant contribution tothe tunneling current,11 the band alignment between MgOand silicon, the most relevant for device fabrication, has notbeen addressed yet experimentally. One of the reasons forthis lack of information concerns the strong impact of thex-ray induced insulator charging13 which in the case of MgOhas been quoted as “commonly catastrophic.”14To avoid the charging-related errors, here we have ad-dressed the problem using the internal photoemission IPEspectroscopy.15–17 In IPE, the interface barrier height is ob-tained as the spectral threshold of photocurrent generated ina MOS capacitor by charge carriers photoexcited to energiesabove the barrier and subsequently injected into the insulat-ing layer.15 Depending on the orientation of electric field atthe interface, one may observe IPE of electrons or holes anddetermine the corresponding barrier heights.16,17 By combin-ing the two barrier values, we have found that amorphousa- MgO on 100Si has a much reduced band gap, Eg=6.1 eV, compared to the above mentioned bulk crystalvalue. Comparison between the as-deposited a-MgO layerand that crystallized by additional postdeposition annealPDA indicates that the conduction band CB offset re-mains considerably lower 2.25 eV than the valence bandVB offset 2.7 eV in a-MgO and higher in the crystallizedlayer.Studied samples were prepared on the H-terminated100 surface of low-doped n- or p-type Si nd, na1015 cm−3 by e-beam deposition of MgO from 99.99%purity pellets with the Si temperature kept low 150 °C toobtain amorphous MgO layers.18 Some samples were subse-quently crystallized by 30 min PDA in N2 at 500 °C result-ing in a columnar granular film of rocksalt structure.5,18 Forthe IPE experiments we used relatively thick 39–40 nmMgO layers to prevent the Si/MgO interface fromhydroxylation16 during ex situ metal evaporation. MOS ca-pacitors were fabricated by thermoresistive evaporation froma W boat of semitransparent 15 nm thick Au or Al elec-trodes of 0.5 mm2 area. These were used to measure IPEspectra at room temperature in the photon energy range h=2–6.8 eV with a constant spectral resolution of 2 nm. Thequantum yield Y is defined as the photocurrent normalizedto the incident photon flux and analyzed as a function of hand the strength of the electric field F in the MgO.Logarithmic plots of the yield are shown in Fig. 1 forMOS capacitors with as-deposited amorphous a and crys-tallized b MgO insulators, as measured under the indicatedaElectronic mail: valeri.afanasiev@fys.kuleuven.be.QUANTUMYIELD(RELATIVEUNITS)10-1410-1310-1210-1110-1010-910-810-710-6 +4PHOTON ENERGY (eV)2 3 4 5 610-1410-1310-1210-1110-1010-910-810-710-6+5+8+10-3-4-5-8+2+4+6E1E2(a)(b)-2-4-6-7V (Volt)V (Volt)V (Volt)V (Volt)Si MgO Si MgOEVECECEVFIG. 1. Logarithmic plot of the IPE quantum yield as a function of photonenergy measured in as-deposited a 100Si/MgO/Au samples and b aftercrystallization of the oxide insulators by PDA at 500 °C as measured underthe indicated positive open symbols and negative filled symbols biasapplied to the top Au electrode. Vertical lines mark the onsets of E1 and E2direct optical transitions 3-1 and 2-3, respectively, in the Si substratecrystal. Insets in panel a show schematics of the electron transitions.APPLIED PHYSICS LETTERS 96, 052103 20100003-6951/2010/965/052103/3/$30.00 © 2010 American Institute of Physics96, 052103-1positive open symbols or negative filled symbols bias ap-plied to the top Au electrode. In the photon energy rangeh4.5 eV the curves measured under opposite orientationof electric field diverge suggesting that the photocurrent isdominated by IPE from the electrodes of the MOS capacitor.As replacement of the top electrode material from Au to Alhas no effect on the curves measured under positive metalbias not shown, electron states of the metal are unlikely tocontribute to the IPE current thus indicating the electron IPEat the Si/MgO interface to be the major signal source. Thisconclusion is consistent with the observed modulation of thespectra at photon energies of E1=3.4 eV and E2=4.3 eVcf. vertical lines in Fig. 1 due to the excitation of the directoptical transitions 3-1 and 2-3, respectively, in the Sicrystal.16,17 These transitions cause a significant increase inreflectivity and optical absorption of Si but do not provideelectrons of energy sufficient to overcome the interface bar-rier and, therefore, result in a decrease in the yield with in-creasing photon energy. The spectral threshold of electronIPE corresponds to the energy barrier between the top of theSi VB and the bottom of the MgO CB cf. inset in the upperleft corner in Fig. 1a and can be found from the Y1/3-hplots15,19 shown by the open symbols in Fig. 2 for the as-deposited a and crystallized b MgO. Though the electronIPE threshold is better exposed in the spectra of the crystal-lized layer cf. Fig. 2b than in those of a-MgO panel a,the values obtained by linear extrapolation to the subthresh-old level appear to be the same within experimental accu-racy.The IPE spectra measured under negative metal biasfilled symbols in Figs. 1 and 2 indicate cf. panels a thatin the as-deposited sample the yield exhibits an enhancementat E24.3 eV suggestive of hole IPE from Si.17 However,upon crystallization this feature disappears although a some-what reduced photocurrent remains measurable in the range3.5h4.5 eV. This behavior can be explained by photo-current arising in the same spectral range not only from holeIPE from the CB of Si into the MgO VB cf. insert in thebottom right corner in Fig. 1a but, also from electron IPEfrom Au to the MgO CB since the Au/MgO interface is char-acterized by a barrier height of 4.0 eV.20 To separate electronand hole IPE signals it appears instructive to compare theyield spectra measured under the same bias of 7 V insamples with Au and Al electrodes prior and after MgO crys-tallization. The logarithmic plots of these spectra, shown inFig. 3, reveal that crystallization of the MgO layer has nonoticeable effect on the barrier height between the Fermilevel of Al and the bottom of the MgO CB; the barrier isfound to be 2.70.1 eV the Fowler plots are not shown ingood agreement with the literature.20 By contrast, in the caseof the Au electrode one may notice the disappearance uponannealing of the portion of current characterized by the wellexpressed E2 singularity related feature. Therefore, we con-clude that the crystallization of MgO suppresses the hole IPEfrom Si due to a shift of the MgO VB top. Since the hole IPEin the as-deposited samples provides the dominant photocur-rent in the spectral range 4h5 eV, for this case onestill can evaluate the IPE spectral threshold from the Y1/2-hplots19 as shown by the filled symbols in Fig. 2a.The field dependence of the electron IPE thresholds isshown in Fig. 4 in the form of Schottky plots for the amor-phous  and crystallized  MgO. The measured electronIPE thresholds are seen to obey the classical image-forcebarrier model,15,21 yielding a zero-field barrier height of e=3.370.05 eV and the high-frequency dielectric constantMgO=4.00.2. As one expects in2,15 the latter is in fairagreement with the measured refractive index nMgO=1.8 ath=5.0 eV. From the inferred energy barrier between the SiVB top and the MgO CB bottom the CB offset at the100Si/MgO interface can be calculated as EC=e-EgSi=2.250.05 eV.The top of the image force barrier is located at a distanceof xm= q /16	0MgOF1/2 from the surface of emitter, whereq is the elemental charge and 0 is the vacuumpermittivity.15,17,21 As a result, the barrier height measured byIPE corresponds to the energy position of the MgO CB at thedistance xm from the silicon surface. In our experiments thestrength of the electric field F in MgO increases from 0.1 to2.5 MV/cm, which corresponds to a decrease in xm from 3 to(QUANTUMYIELD)1/3(RELATIVEUNITS)0.00000.00100.0020 +2PHOTON ENERGY (eV)2.0 2.5 3.0 3.5 4.0 4.50.00000.00100.0020+3+4+5+8(QUANTUMYIELD)1/2( RELATIVEUNITS)0.000000.000050.00010-3-4-5-8+2+3+4+5+60.000000.000050.00010(a)(b)-3-4-5-7ΦeΦhΦeV (Volt)V (Volt)FIG. 2. Cube left scale and square root right scale plots of the IPEquantum yield as a function of photon energy measured in 100Si/MgO/Ausamples in the as-deposited state a and after MgO crystallization by PDAat 500 °C b under the indicated voltages applied to the Au electrode.Vertical lines mark the onsets of E1 and E2 direct optical transitions withinthe Si crystal and the spectral thresholds of electron e and hole h IPE.PHOTON ENERGY (eV)2 3 4 5 6QUANTUMYIELD(RELATIVEUNITS)10-1310-1210-1110-1010-910-810-7Au AlPDA 500 °CE2Au AlAs-depositedV= -7 VoltFIG. 3. Logarithmic plot of the IPE quantum yield as a function of photonenergy measured in 100Si/MgO/Au samples with as-deposited open sym-bols and crystallized filled symbols MgO under 7 V bias applied to theAu circles or Al squares electrode. The vertical line marks the onset ofthe E2 direct optical transition in the Si substrate crystal.052103-2 Afanas’ev et al. Appl. Phys. Lett. 96, 052103 20100.6 nm. Following the argumentation of DiStefano,21 as nomeasurable deviation from the ideal image-force behavior isobserved, one may conclude that the MgO CB edge remainsat the same energy at distances larger than 0.6 nm from theSi/MgO interface plane. The depth of the thus probed MgOlayer from 0.6 to 3 nm covers the range of the of the tunneloxide thickness typically used in MTJs9–11 making the IPEanalysis relevant not only to the gate insulator stacks but,also, to the thin oxide structures.The inferred spectral threshold of the hole IPE exhibits aweaker field dependence compared to the electron IPE casecf.  in Fig. 4. This effect is consistent with the expectedslow motion of a hole in the oxide VB, possibly as a polaron,thus allowing polarization of the silicon.17 The barrier be-tween the Si CB bottom and the MgO VB top obtained byextrapolation to zero field is equal to h=3.850.10 eV.Combined with the inferred electron IPE barrier the lattervalue allows us to obtain the band gap of a-MgO asEgMgO=e+h−EgSi=6.10.1 eV, i.e., significantlysmaller than the commonly quoted crystal gap of 7.83 eV.2For h5 eV, the photocurrents under positive andnegative metal bias become close. They appear to exhibit thesame spectral distribution cf. Fig. 1, suggesting that thesignal arises from the generation of mobile charge carriers inthe MgO layer. From the Y1/2-h plots not shown the spec-tral thresholds of these currents are found to be about 4.5 and5.3 eV for the as-deposited and the crystallized samples, re-spectively. Yet, in the absence of additional structural orchemical information we refrain from speculations regardingthe atomic origin of electron states responsible for this MgOphotoconductivity.The band gap narrowing in the as-deposited a-MgOcompared to the bulk crystal represents the most intriguingresult of this work. We have noted earlier that the band gapin amorphous light-metal oxides is significantly lower thanin the bulk crystal. In Al2O3 the gap decreases from 8.8 eVin sapphire to 6.0–6.2 eV in deposited amorphous alumina22values as low as 5.6 eV can be found in the literature23. Ina-Sc2O3 on Si the gap is 5.6–5.7 eV, observed to widen to6.0–6.1 eV in the film crystallized by supplemental PDA.Most of the gap variation is associated, again, with a shift ofthe VB top.24 The MgO layers are seen to follow the sametrend: Upon crystallization the gap increases from 6.1 eV toa value likely close to the one of the bulk crystal 7.83 eVwith the result that hole IPE becomes undetectable.The inferred hole IPE barrier h at the 100Si/a-MgOinterface allows us to calculate the VB offset as 2.7 eV,which is likely to increase upon crystallization. With a CBoffset of 2.2 eV the interface band diagram emerges as asym-metric, with a lower barrier for electrons than for holes.However, if a metal with large work function is used asemitter in the MTJ structure, the band gap narrowing in non-crystallized MgO may result in a lower barrier for holes thanfor electrons. For instance, the inferred value EgMgO=6.1 eV would make the latter case relevant to the Au/MgOand Fe/MgO interfaces characterized by electron barriers ex-ceeding EgMgO /2.20The work at KU Leuven was supported by the “FondsWetenschappelijk Onderzoek FWO-Vlaanderen” throughGrant No. 1.5.057.07. The authors from the Tyndall NationalInstitute acknowledge the support of Science Foundation Ire-land SFI under the FORME Strategic Research ClusterAward No. 07/SRC/I1172.1See, e. g., D. P. Norton, Mater. Sci. Eng. R. 43, 139 2004, and referencestherein.2R. C. Whited, C. J. Flaten, and W. C. Walket, Solid State Commun. 13,1903 1973.3L. Yan, C. M. Lopez, R. P. Shrestha, E. A. Irene, A. A. Suvorova, and M.Saunders, Appl. Phys. Lett. 88, 142901 2006.4A. Posadas, F. J. Walker, C. H. Ahn, T. L. Goodrich, Z. Cai, and K. S.Ziemer, Appl. Phys. Lett. 92, 233511 2008.5E. Miranda, E. O’Connor, K. Cherkaoui, S. Monaghan, R. Long, D.O’Connell, P. K. Hurley, G. Hughes, and P. Casey, Appl. Phys. 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Schottky plot of the electron , and hole  IPE thresholdsmeasured in 100Si/MgO/Au samples with as-deposited , and crystal-lized  MgO layers. The insert shows a schematic of the Si/MgO interfaceband diagram with the electron e and hole h IPE barriers indicated byarrows.052103-3 Afanas’ev et al. Appl. Phys. Lett. 96, 052103 2010

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Electron energy band alignment at the (100)Si/MgO interface

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