The variation in surface electronic properties of undoped c-plane InxAl1−xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed

Adikimenakis, A.

Bailey, L. R.

Georgakilas, A.

Iliopoulos, E.

King, Philip David

Lu, H.

McConville, C. F.

Schaff, William Joseph

Veal, T. D.

English

Warwick Research Archives Portal Repository

  University of Warwick institutional repository: http://go.warwick.ac.uk/wrapThis paper is made available online in accordance with publisher policies. Please scroll down to view the document itself. Please refer to the repository record for this item and our policy information available from the repository home page for further information.  To see the final version of this paper please visit the publisher’s website. Access to the published version may require a subscription.  Author(s): P. D. C. King, T. D. Veal, A. Adikimenakis, Hai Lu, L. R. Bailey, E. Iliopoulos, A. Georgakilas, W. J. Schaff, and C. F. McConvilleArticle Title: Surface electronic properties of undoped InAlN alloys Year of publication: 2008 Link to published version:  http://dx.doi.org/10.1063/1.2913765 Publisher statement: None    Surface electronic properties of undoped InAlN alloysP. D. C. King,1 T. D. Veal,1 A. Adikimenakis,2 Hai Lu,3,b L. R. Bailey,1 E. Iliopoulos,2A. Georgakilas,2 W. J. Schaff,3 and C. F. McConville1,a1Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom2Microelectronics Research Group, Department of Physics, University of Crete, P.O. Box 2208,71003 Heraklion-Crete, Greece3Department of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USAReceived 21 February 2008; accepted 4 April 2008; published online 1 May 2008The variation in surface electronic properties of undoped c-plane InxAl1−xN alloys has beeninvestigated across the composition range using a combination of high-resolution x-rayphotoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys,electron accumulation layers, accompanied by a downward band bending, are present at the surface,with a decrease to approximately flatband conditions with increasing Al composition. However, forthe Al-rich alloys, the undoped samples were found to be insulating with approximate midgappinning of the surface Fermi level observed. © 2008 American Institute of Physics.DOI: 10.1063/1.2913765The majority of n-type III-V semiconductors investi-gated to date exhibit a depletion of electrons in the near-surface region due to the presence of occupied acceptor sur-face states causing an upward bending of the conduction andvalence bands at the surface. In contrast, however, electronaccumulation layers occur at InN Refs. 1 and 2 and InAsRefs. 3 and 4 surfaces, whereby the Fermi level at thesurface is pinned above that of the bulk, resulting in an in-crease in the near-surface electron concentration compared tothe bulk value. The overriding mechanism driving this accu-mulation of electrons is the particularly low energetic posi-tion of the conduction band minimum CBM, well belowthe charge neutrality level CNL in both materials, resultingin unoccupied, positively charged donor surface states whichinduce a downward bending of the bands and electron accu-mulation at the surface.4,5 The electron accumulation is stillpresent at the surface of p-type material, causing an inver-sion layer to form where an n-type surface region is sepa-rated from the p-type bulk by a depletion layer.6,7As electron accumulation is uncommon among III-Vsemiconductors, when InN is alloyed with other materials, atransition from electron accumulation inversion to electronhole depletion for n-type p-type material would be ex-pected. This has previously been investigated for n- andp-InGaN via x-ray photoemission spectroscopy XPS,8–10where such a transition was observed. This was explained bythe CNL lying significantly above the CBM for In-rich al-loys, but below the CBM for Ga-rich alloys.8–10 The bandlineup relative to the CNL of the III-N semiconductors isshown in Fig. 1a. The CNL lies close to the middle of thedirect band gap in AlN, similar to the situation in GaN, al-though much further below the CBM. A similar transitionfrom electron accumulation to depletion may therefore beexpected on moving from In-rich to Al-rich n-InAlN alloys,motivating the current study. In this letter, high-resolutionXPS is utilized to determine the variation of the surfaceFermi level pinning position for undoped InAlN alloysacross the composition range, and the surface electronicproperties of these alloys are discussed.Undoped InAlN alloys were grown by plasma-assistedmolecular beam epitaxy on c-plane sapphire substrates incor-porating AlN buffer layers. The InAlN layer thicknessesranged from 190 to 4600 nm. The AlN sample was grown toa thickness of 1000 nm on a c-plane sapphire substrate bymetal-organic chemical-vapor deposition. Details of thegrowth methodologies are reported elsewhere.15–17 The alloycompositions were determined by x-ray diffraction XRDusing the AlN 0002 peak as a reference peak. Additionally,Rutherford backscattering RBS measurements were per-formed on the thinnest sample 190 nm. The composition ofthis sample was determined to be In0.60Al0.40N from RBSmeasurements, in very good agreement with the compositionof In0.59Al0.41N determined from XRD measurements. High-resolution XPS measurements were performed using a Sci-enta ESCA300 spectrometer at the National Centre forElectron Spectroscopy and Surface Analysis, DaresburyLaboratory, UK. Details of the spectrometer and its arrange-ment are reported elsewhere.8 The binding energy scale isaElectronic mail: c.f.mcconville@warwick.ac.uk.bPresent address: Department of Physics, Nanjing University, Nanjing,210093, China.FIG. 1. Color online a Band lineup relative to the CNL of the III-Nsemiconductors, determined from measured valence band offsets Refs. 11and 12 band gaps Refs. 13 and 14 and the measured VBM to CNLseparation for InN Ref. 5. b Valence band photoemission spectra withrespect to the Fermi level EF of undoped InxAl1−xN alloys across the com-position range.APPLIED PHYSICS LETTERS 92, 172105 20080003-6951/2008/9217/172105/3/$23.00 © 2008 American Institute of Physics92, 172105-1Downloaded 30 Jun 2009 to 137.205.202.8. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspgiven with respect to the Fermi level. For the In-richsamples, this was calibrated using the Fermi edge of an ion-bombarded silver reference sample. However, the Al-richsamples were insulating and so required charge compensa-tion via a low-energy electron flood gun. For these alloys, thecalibration of the Fermi level was achieved via a referencingof the binding energy of the C 1s core-level peak fromphysisorbed carbon to the average C 1s binding energy forthe In-rich samples, where no flood gun had been used. Theposition of the valence band maximum VBM at the surfaceis determined by extrapolating a linear fit of the leading edgeof the valence band photoemission to the background level toaccount for the broadening of the photoemission spectra.18The valence band photoemission is shown in Fig. 1b.The leading edge of this valence band photoemission shiftsto higher binding energies with increasing Al content, as waspreviously observed for increasing Ga content in n-InGaNalloys,8,10 indicating an increase in the VBM to surfaceFermi level separation with increasing Al content. The sur-face Fermi level positions, determined from the XPS mea-surements, relative to the semiconductor band edges and theCNL are shown in Fig. 2. Additionally, the carrier densitywas measured by the single-field Hall effect, and these val-ues were used to calculate via carrier statistics calculationsincorporating a Kane19 nonparabolic conduction band anupper limit of the bulk Fermi level. This is shown in Fig. 2for the In-rich samples and is an upper limit of the bulkFermi level as the electron accumulation present for In-richsamples leads to a slight overestimation of the bulk carrierdensity by single-field Hall effect measurements. The Al-richsamples were insulating by single-field Hall effect measure-ments, and so, although a precise value of the bulk Fermilevel could not be determined, it is expected to lie signifi-cantly below the CBM, approaching the midgap position.The insulating nature of the Al-rich samples was confirmedby the necessity of using a low-energy electron flood gun toachieve charge compensation in the XPS measurements.For InN, the surface Fermi level lies significantly abovethe bulk Fermi level, indicating that electron accumulation ispresent at the surface. This can be understood within theconcept of virtual gap states ViGS:22 As the bulk Fermilevel lies below the CNL, the surface Fermi level also pinsslightly below the CNL, causing a number of donor ViGS tobe unoccupied, donating their electrons into theaccumulation layer and causing a large downward bandbending, as has previously been discussed in detail for InNaccumulation layers.2,5 The band bending and carrier concen-tration profiles in the near-surface region can be calculatedfrom the surface and bulk Fermi levels via a solution ofPoisson’s equation within a modified Thomas–Fermiapproximation23 MTFA as described elsewhere.24 The re-sults of such a calculation are shown in Figs. 3a and 3bfor InN revealing the large downward band bending and ex-treme electron accumulation present for this sample.Upon moving to slightly more Al-rich compositions, thesurface Fermi level still remains pinned above the bulkFermi level, indicating electron accumulation is still presentat these surfaces. This is consistent with the surface Fermilevel lying below the CNL and hence causing some donorViGS to be unoccupied as for InN. However, the surface andbulk Fermi levels move closer together with increasing Alcontent, indicating a reduction in the amount of band bend-ing and hence, a reduction in the degree of electron accumu-lation at the surface. This is clearly evident from the Poisson-MTFA calculations for the In0.83Al0.17N sample, as shown inFigs. 3c and 3d. On further increasing the Al content toFIG. 2. Color online Surface filled circles and bulk open squares Fermilevel EF for undoped InxAl1−xN alloys relative to the CNL ECNL, andInAlN band edges EC ,EV and midgap position Emid. The CNL is located1.83 eV above the VBM in InN from Ref. 5. The variation in valence bandedge is assumed to be linear after Mönch Ref. 20, with the measuredvalence band offset of 1.52 eV Ref. 12 and a band gap bowing parameterof 4 eV Ref. 21, entirely in the conduction band due to the linear variationof the valence band edge, used to define the band edges relative to the CNL.The equivalent plot for InxGa1−xN alloys, adapted from Ref. 10, is showninset, indicating the general trend in surface Fermi level pinning position forn-type dot-dash line and p-type dot-dot-dash line alloys.FIG. 3. Color online Band bending and carrier concentration profiles as afunction of depth from Poisson-MTFA calculations for a and b InN, cand d In0.83Al0.17N, and e and f In0.59Al0.41N.172105-2 King et al. Appl. Phys. Lett. 92, 172105 2008Downloaded 30 Jun 2009 to 137.205.202.8. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp41% In0.59Al0.41N, the bulk Fermi level and surface Fermilevel are virtually coincident in energy, leading to almost noelectron accumulation, as shown in Figs. 3e and 3f. Areduction of the amount of electron accumulation upon in-creasing the Al content in In-rich n-InAlN alloys has there-fore been observed, as was previously observed upon in-creasing the Ga content in In-rich n-InGaN alloys.8,10With further increase in Al content, however, thesamples become insulating, as revealed by single-field Halleffect measurements. This can be largely understood withinthe amphoteric defect model,25 whereby the favorable chargestate of native defects is determined by the position of theFermi level relative to the CNL, so that native defects act todrive the Fermi level toward the CNL. As the Al compositionincreases, the CNL moves closer toward the middle of thedirect band gap, causing the Fermi level to become lower inthe band gap, leading to insulating samples. The surfaceFermi levels also tend toward the mid gap energy, suggestingapproximately flatband behavior, for the Al-rich samples.Small amounts of band bending may be present at these sur-faces dependent on the exact relative position of the bulk andsurface Fermi levels, but this is not expected to be a largeeffect. This can be understood by analogy with the behaviorobserved at n- and p-type InGaN surfaces,8,10 shown in theinset of Fig. 2. For n-type InGaN alloys, an increase in sur-face Fermi level to above the CNL although below the CBMfor the most Ga-rich samples with increasing Ga contentwas observed, leading to electron depletion at Ga-rich sur-faces. In contrast, for p-type InGaN alloys, an increase in Gacontent was accompanied by a lowering of the surface Fermilevel position further below the CNL, toward the VBM, lead-ing to hole depletion layers. For the insulating Al-rich InAlNsamples, the bulk Fermi levels will lie between that of the n-and p-type cases, leading to approximately midgap pinningof the surface Fermi level, somewhat between the two ex-tremes observed for Ga-rich InGaN alloys, and approxi-mately flatband conditions. However, if the Al-rich sampleswere doped to become n-type p - type, electron holedepletion layers would still be expected to occur at the sur-face, by analogy with InGaN.In conclusion, the surface electronic properties of un-doped InxAl1−xN alloys have been investigated using high-resolution x-ray photoemission spectroscopy and single-fieldHall effect measurements. For InN, an extreme electron ac-cumulation was observed, consistent with previous observa-tions. This was reduced with increasing Al content for theIn-rich alloys, with approximately flatbands observed forx=0.59. Upon further increase in Al content, the samplesbecame insulating, resulting in the surface Fermi level pin-ning close to the midgap energy with little surface space-charge characteristics. The charge neutrality level was iden-tified as important for determining both the bulk and surfaceelectronic properties of the InAlN alloys across the compo-sition range.We are grateful to D. Law and G. Beamson of NCESSfor their assistance with XPS measurements and to T. Wangand P. Parbrook of the University of Sheffield, UK, for pro-viding the AlN sample. Also, we acknowledge the Engineer-ing and Physical Sciences Research Council, UK, for finan-cial support under Grant No. EP/C535553/1 and for access tothe NCESS facility under Grant No. EP/E025722/1.1H. Lu, W. J. Schaff, L. F. Eastman, and C. E. Stutz, Appl. Phys. Lett. 82,1736 2003.2I. Mahboob, T. D. Veal, C. F. McConville, H. Lu, and W. J. Schaff, Phys.Rev. Lett. 92, 036804 2004.3D. C. Tsui, Phys. Rev. B 4, 4438 1971.4M. Noguchi, K. Hirakawa, and T. Ikoma, Phys. Rev. Lett. 66, 22431991.5P. D. C. King, T. D. Veal, P. H. Jefferson, S. A. Hatfield, L. F. J. Piper, C.F. McConville, F. Fuchs, J. Furthmüller, F. Bechstedt, H. Lu, and W. J.Schaff, Phys. Rev. B 77, 045316 2008.6C. A. Mead and W. G. Spitzer, Phys. Rev. Lett. 10, 471 1963.7R. E. Jones, K. M. Yu, S. X. Li, W. Walukiewicz, J. W. Ager III, E. E.Haller, H. Lu, and W. J. Schaff, Phys. Rev. Lett. 96, 125505 2006.8T. D. Veal, P. H. Jefferson, L. F. J. Piper, C. F. McConville, T. B. Joyce, P.R. Chalker, L. Considine, H. Lu, and W. J. Schaff, Appl. Phys. Lett. 89,202110 2006.9P. D. C. King, T. D. Veal, P. H. Jefferson, C. F. McConville, H. Lu, and W.J. Schaff, Phys. Rev. B 75, 115312 2007.10P. D. C. King, T. D. Veal, H. Lu, P. H. Jefferson, S. A. Hatfield, W. J.Schaff, and C. F. McConville, Phys. Status Solidi B 245, 881 2008.11C. F. Shih, N. C. Chen, P. H. 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Surface electronic properties of undoped InAlN alloys

King, PDC

Veal, TD

Adikimenakis, A

Lu, H

Bailey, LR

Iliopoulos, E

Georgakilas, A

Schaff, WJ

McConville, Christopher

Deakin Research Online

P. D. C. King

T. D. Veal

A. Adikimenakis

Hai Lu

L. R. Bailey

E. Iliopoulos

A. Georgakilas

W. J. Schaff

C. F. McConville

Crossref

The variation in surface electronic properties of undoped c-plane InxAl1-xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed. (C) 2008 American Institute of Physics.</p

Surface electronic properties of undoped InAlN alloys

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