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Study of interfaces chemistry in type-II GaSb/InAs superlattice structures

E. Papis-Polakowska a,⁎, J. Kaniewski a, J. Szade b, W. Rzodkiewicz a, A. Jasik a, K. Reginski a, A. Wawro c

a Institute of Electron Technology, Al. Lotnikow 32/46, 02‐668 Warsaw, Polandb A. Chełkowski Institute of Physics, University of Silesia ul. Uniwersytecka 4, 40‐007 Katowice, Polandc Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02‐668 Warsaw, Poland

a b s t r a c ta r t i c l e i n f o

Article history:Received 19 August 2011Received in revised form 30 August 2012Accepted 4 September 2012Available online 12 September 2012

Keywords:SuperlatticeGaSbSurfaceInterfaceX-ray photoelectron spectroscopy

There is a considerable interest in type-II GaSb/InAs superlattice system due to several modern applicationsincluding infrared detectors. In these studies X-ray photoelectron spectroscopy (XPS) and spectroscopicellipsometry (SE) have been used for extensive characterization of the surface and interface of GaSb/InAssuperlattice. Application of XPS and SE techniques provides precise information from topmost layers of struc-ture and allows excluding presence of GaAs-type interfaces in GaSb/InAs superlattices. It means thatSb-for-As anion exchange does not exist during the molecular beam epitaxial growth of superlattice struc-tures. Simultaneously, these results indicate that InSb-type or GaInSb-type interfaces have been detected inthe structures studied.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

GaSb-based superlattices (SLs) are well recognized for obtaininghigh-performance modern micro- and optoelectronic devices includ-ing infrared detectors, long-wavelength lasers and high-frequency,low-voltage field effect transistors. In particular, the GaSb/InAs sys-tem is promising for the next generation mid-infrared photodetectorfocal plane arrays as an alternative material to usually used HgCdTealloy for better uniformity, reduced tunneling currents and suppressedAuger recombination [1–3]. For many of type-II GaSb/InAs superlatticeapplications the band alignment plays a critical role in the device perfor-mance, however, a determination of band alignments in mixed anion/cation systems is complicated by the fact that two different types of inter-faces can be formed. In the case of an InAs/GaSb heterojunction, bothInSb-like and GaAs-like interfaces can be formed [4] and it is very impor-tant to determine what effect changing the interface composition onband alignments in the GaSb/InAs. Because the InSb-like interface intro-duces compressive strain, but GaAs-like interface initiates tensile strain,the strain of a multilayered complex superlattice structure can be modi-fied and compensated by using proper sequence of interfaces in devices.Several studies have investigated interfacial control in mixed anionheterostructures, and have indicated that the exchange reaction is diffi-cult to suppress [5,6]. In particular, it has been found that structurescontaining only In–Sb interface bonds have increased average lattice con-stants and reduced band gaps compared to structures containing onlyGa–As interface bands. While a number of studies have been performedin order to present the competitive incorporation behavior of arsenic

and antimony during the molecular beam epitaxial (MBE) growth ofGaSb-based superlattices [7,8], little is knownon the chemistry andkinet-ics of the As-for-Sb and Sb-for-As anion exchange reactions, when GaAssurface is exposed to Sb flux and when GaSb surface is exposed to anAs flux respectively. Specific complex multilayered construction of InAs/GaSb SLs requires high quality of surface and interface with good mor-phology and controlled composition. On the other hand, because of thecritical dependence of the electronic and optical properties of thesuperlattice on the thickness and composition of individual layers,as well as of interfaces, non-destructive and atomic-level resolutiontechniques are required for characterization of surface/interface in SLsstructures. Generally, transmission electron microscopy (TEM) [9,10],high-resolution X-ray diffraction (HR XRD) [11], cross-sectional scanningtunneling microscopy (STM) [12,13] and X-ray photoelectron spectros-copy (XPS) [14], have been reported in literature as prominentmethodsused to study thickness and composition of superlattice surface andinterfaces.

In particular, Losurdo et al. presented the extensive results of thesuperlattice surface and interface study carried out by XPS, XRD, TEMcombined with spectroscopic ellipsometry (SE) and atomic force mi-croscopy (AFM) complementary techniques [15].

The fractional derivative spectrum (FDS) model in connection withthe SE technique has been used for optical analysis of the superlatticelayers as a helpful technique to extract basic information on relevantphysical quantities from the εE2(E) spectra [16,17]. In the present study,30-period GaSb/InAs superlattice structureswere grown. The interfaceswere formed by exposure of the GaSb and InAs layer to As2 and Sb2 fluxrespectively. The XPS, SE and AFM complementary methods were usedfor extensive characterization of the surface and interface in type-IIGaSb/InAs SLs.

Thin Solid Films 522 (2012) 223–227

⁎ Corresponding author. Tel.: +48 22 5487873; fax: +48 22 5487925.E-mail address: papis@ite.waw.pl (E. Papis-Polakowska).

0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.tsf.2012.09.014

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2. Experimental details

The surface study was performed on a superlattice structureconsisting of 30 period 10 monolayers (MLs) InAs/9 ML GaSb. The SLstructures were grown on undoped (100) GaSb epi-ready substrates ina Riber 32PMBE system equippedwith group V solid sources specificallycracked As and Sb to provide dimmer As2 and Sb2 species. The thicknessof individual InAs and GaSb layers was dInAs=3.02 Å and dGaSb=3.05 Årespectively. One pair of InAs/GaSb system together with the interfacescreated during epitaxial growth characterized by 6.3 nm thickness andthe overall thickness of InAs/GaSb SLs structure was about 180 nm. TheGaAs- and InSb-like interfaces in superlattices were formed by the timecontrolled As2 exposure of GaSb before InAs layer growth and Sb2 ex-posure of InAs before GaSb layer formation, respectively. The HRXRD PANalytical X'Pert PRO Diffractometer and high-resolution (HR)TEM JEOL JEM-2100 systemwere used to provide preliminary informa-tion on the quality and latticemismatch in GaSb/InAs periodical layeredstructures [18]. XRD measurements were carried out by using Cu Kα1

radiation (λ≈1.5406 Å). The LaB6 electron source and operating volt-age 200 kV allowed one to obtain high resolution TEM images. Inparticular, HR XRD system was equipped with X-ray Cu tube, fourbounce, hybrid Ge (400) monochromator, four-circle cradle for sam-ple, three bounce Ge (220) analyzer and a high-accuracy incrementalencoder for accurate angle reading of theω position. The chemistry ofsurface and interfaces in InAs/GaSb SLs was analyzed by XPS using aPhysical Electronics PHI 5700 spectrometer with monochromatized AlKα radiation system. The energy resolution of the spectra presentedwithin this work can be estimated as 0.35 eV. The detection limit formost elements is about 0.1 at.%. The XPS database from PhysicalElectronics Handbook of Photoelectron Spectroscopy [19] and Na-tional Institute of Standards and Technology database were applied

as the preliminary references for identifying the chemical states of ele-ments. Additionally, the reference compounds GaAs, GaSb and InAshave been studied at the same parameters of the XPS spectrometer. TheMULTIPAK software from Physical Electronics was used to deconvolutethe XPS spectra and calculate atomic concentrations. The XPS depth pro-file was obtained during the sputter etching sequences of GaSb/InAsstructures using Ar+ ion beam energy of 1 keV. The SL pseudodielectricfunction (ε)=(ε1)+i(ε2)wasmeasured in the 1.1–5.5 eVphoton energyrange with the energy resolution of 0.01 eV using J. A Woollam VariableAngle Spectroscopic Ellipsometer (VASE) at multiple angles of incidencein the range of 55–75°. The surface topography of the grownmultilayerswas investigated by AFM (Nanoscope IIIa) operating in tapping mode inambient conditions and at room temperature. The radius of the tip probe(OTESPA) was nominally below 10 nm. Nominal spring constant of thecantilever was about 40 N/m. The scan size of representative images ofthe surfacewas equal to 3×3 μm. The rootmean square (rms)was deter-mined from the recorded topography images by a standard procedure.

3. Results and discussion

3.1. X-ray photoelectron spectroscopy results

Preliminary characterization of 30 period 10 GaSb MLs/9 InAs MLssuperlattices carried out by HR XRD and HR TEM techniques indicatedwell-defined high quality multilayered SL structures with a smoothand sharp interfaces between InAs and GaSb layers. The results ofthese studies will be presented in a separate paper. The XPS analysiswas focused on the spectra of Sb 4d, As 3d, Ga 3d levels with similarbinding energy (BE) in order to assure similar surface sensitivity causedby the effect of energy dependent inelastic mean free path (IMFP) ofphotoelectrons. The value of IMFP for such electrons can be estimated

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to be about 30 Ǻ [20]. Only In 3d line is characterized by higher BE(above 400 eV) and slightly higher surface sensitivity. The results ofXPS measurements of GaSb/InAs superlattice structures (multilayer)compared to reference compounds GaSb, GaAs and InAs are shown inFig. 1 where the spectra of the most prominent Sb, As, Ga and In linesand valence band are displayed. Analysis of Sb 4d and As 3d XPS spectra(Fig. 1a, b) indicates to the presence of GaSb and InAs, as expected forthe main components of the multilayer although the In lines show dif-ferent features as discussed later in the text.

The broad line with maximum at the energy of 48 eV correspondsto a satellite of the Sb 4d photoemission line. Similar feature is seen inthe spectrum obtained for pure GaSb (Fig. 1a). Therefore, this struc-ture cannot rather be ascribed to formation of AsSby compounds assuggested by Losurdo et al. [15]. Moreover, satellites separated byabout 16 eV observed in pure GaSb are accompanied by other Sblines. The presence and shape of satellites depend on the Sb chemicalstate, since we did not find them in strongly oxidized GaSb surface.The spectral line at about 44–46 eV is related to arsenic oxides. Analysisof As 3d and Ga 3d spectra (Fig. 1b, c) proves that GaAs does not exist inthe SL structure. Themore detailed analysis of the Ga 3d line is presentedby Papis et al. [21]. One has to remember that Ga lines have strong con-tribution from the surface oxides. Interpretation of the In 3d spectrum is

more complex. The shift of the In 3d line towards lower binding energywith respect to pure InAs (Fig. 1d) suggests a modification of the chem-ical bonding of indium in the superlattice. One of the reasons can be re-lated to the presence of InSb or GaInSb in the superlattice structures.Formation of such compounds was reported previously. Simultaneouslythe As 3d linefits well to the ones related to InAs—see Fig. 1a, b. It may berelated to the structure of the last interface where Sb2 exposure of InAsbefore GaSb layer formation was used during superlattice growth. Inorder to clarify this problem we performed the depth profile for thesuperlattice of 30 periods 10 GaSb MLs/9 InAs MLs. The low energy andlow dose of Ar+ ions and short sputtering cycle enabled detailed analysisof the topmost layers including the oxidized surface. The sputtering ratewas estimated to be about 2–4 Ǻ/min. The composition of the main ele-ments as a function of sputtering time is presented in Fig. 2. One has toremember that the value of IMFP causes that information at everypoint comes from several atomic layers and sputtering may cause someinterlayer mixing. Information is thus partly integrated for the depth ofthe order of 3–4 nm. The evolution of the particular XPS spectra isshown in Figs. 3–5. The topmost layers are influenced by oxidation. Anexact XPS characterization of GaSb surface has been previously reportedby Papis-Polakowska [22].

The Ga oxides and Sb oxides are well visible in the front spectra. Aslight shift of the Sb lines was observed during sputtering which proba-bly can be attributed to some mixing of Sb and In in the region of inter-face. Also the composition changes in the region 10–25 min of sputteringcan be assigned to the first interface (Fig. 3). The excess of In and Sbwithrespect to their standard neighbors from the main layers may indicateformation of InSb or GaInSb at the interface. The In 3d line (Fig. 5)shows a slight shift of the peak maximum position with sputteringtime, that can be related to the reactions at the interface. Generally,after 45 min of sputtering the profile shows a relatively uniform compo-sition with two main components GaSb and InAs, which may be causedby partial mixing of layers due to sputtering.

3.2. VASE analysis

The real (ε1) and imaginary (ε2) components of the effective com-plex dielectric function versus photon energy E, and the second energyderivative d2ε2/dE2 of 30 period 10 GaSb MLs/9 InAs MLs superlatticestructures are shown in Fig. 6a, b. Critical Points (CP) are resolved asmin-ima in d2ε2/dE2 at energies of 1.81 eV, 2.08 eV, 2.54 eV and 4.07 eV.These critical points have been assigned to the E1 interband transitionin InSb, to the E1 transition in GaSb, to a superposition of the GaSbE1+Δ1, to E2 transition in GaSb and E1 transition in InAs, respectively.

Fig. 2. The depth profile of GaSb/InAs SLs obtained using Ar+ ion beam etching.

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The critical point at energy of 2.21 eV corresponds to a superposition ofthe GaInSb E1+Δ1 [23]. One can notice that the VASE results do not in-dicate critical point corresponding to GaAs. The detailed description ofoptical analyses of GaAs substrate by EDS model has been reported byKulik et al. [24].

The values of CP in the second energy derivative of GaSb/InAssuperlattices structures were extracted by using FDS and VASE tech-niques for GaSb, InAs and InSb standard samples. The ε(E) spectra ofGaSb and InAs are presented in Fig. 6c, d. The effective pseudodielectricfunction of GaSb/InAs SL was calculated in a multilayer model, describ-ing the dielectric functions of the individual layers by ensemble of threeGaussian oscillators to model to critical point structures.

The VASE measurements of GaSb/InAs superlattice structure arevery sensitive to their surface and interface morphology. The AFMmeasurements show that superlattice structures have high quality,smooth surface with rms=0.2 nm.

In summary, one can conclude, that XPS and SE are suitablemethodsfor superlattice characterization. Using thesemethods, the complemen-tary studies of the GaSb/InAs SLs surface/interface chemistry wereperformed. Anion exchange reaction as a result of exposure of GaSbunder As2 and InAs under Sb2 fluxes, respectively has been investigated.X-ray photoelectron spectroscopy was used to gain knowledge aboutthe electronic structure and chemical composition of the topmost layersfrom the superlattice. The depth profile XPS analysis performed withlow energy Ar+ ions beam allows one to get insight to the interface re-gion. Despite the main components, GaSb and InAs, Ga and Sb oxideswere detected on the surface. Analysis of the As and In lines indicatesthe formation of InSb-type or GaInSb-type compounds at the layerinterface.

The presence of GaAs-type interface has been excluded in the struc-tures studied. It means that Sb-for-As anion exchange does not exist.Moreover, formation of AsSby compounds, suggested in the literature,was not detected in our samples and we believe that the interpretationof the XPS peak at about 48 eV should be revised. The results of SE ac-companied with FDS procedure confirmed these conclusions. Addition-ally, it is shown that using GaSb, GaAs and InAs reference compounds isa goodmethod to study interface chemistry from both XPS and SEmea-surements. This approach to type-II GaSb/InAs superlattice structures isinnovative and has not been applied so far.

Acknowledgments

The authors would like to thank Dr I. Sankowska and Dr J.Kubacka-Traczyk for HR XRD measurements, and Dr. A. Laszcz andDr. M. Wzorek for HR TEM measurements.

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References

[1] N. Gautam, M. Naydenkov, S. Myers, A.V. Barve, E. Plis, T. Rotter, L.R. Dawson, S.Krishna, Appl. Phys. Lett. 98 (2011) 121106.

[2] M. Razeghi, S. Abdollahi Pour, E.K. Huang, G. Chen, A. Haddadi, B.M. Nguyen,Opto-Electron. Rev. 19 (2011) 261.

[3] P. Christol, C. Cervera, J.B. Rodriguez, K. Jaworowicz, I. Ribet-Mohamed, in: SanFrancisco, California, USA, Quantum Sensing and Nanophotonic Devices VIII, 23January 2011, Proceedings of SPIE 7945, 2011, 79451H.

[4] G.J. Sullivan, A. Ikhlassi, J. Bergman, R.E. DeWames, J.R. Waldrop, C. Grein, M.Flatté, K. Mahalingam, H. Yang, M. Zhong, M. Weimer, J. Vac. Sci. Technol., B 23(2005) 1144.

[5] F.S. Juang, Y.K. Su, H.H. Yu, K.J. Liu, Mater. Chem. Phys. 78 (2003) 620.[6] J.T. Zborowski, A. Vigliante, S.C. Moss, T.D. Golding, J. Appl. Phys. 79 (1996) 8379.[7] J. Steinshnider, M. Weimer, R. Kaspi, G.W. Turner, Phys. Rev. Lett. 85 (2000) 2953.[8] M. Pristovsek, M. Zorn, U. Zeimer, M. Weyers, J. Cryst. Growth 276 (2005) 347.[9] F. Szmulowicz, H.J. Haugan, G.J. Brown, K. Mahalingam, B. Ullrich, S.R. Munshi, L.

Grazulis, Opto-Electron. Rev. 14 (2006) 71.[10] B. Satpati, J.B. Rodriguez, A. Trampert, E. Tournié, A. Joullié, P. Christol, J. Cryst.

Growth 301–302 (2007) 889.[11] J. Kubacka-Traczyk, I. Sankowska, J. Kaniewski, Opt. Appl. 39 (2009) 875.[12] S.G. Kim, S.C. Erwin, B.Z. Nosho, L.J. Whitman, Phys. Rev. B 67 (2003) 121306(R).

[13] K. Suzuki, K. Kanisawa, S. Perraud, M. Ueki, K. Takashina, Y. Hirayama, Phys. StatusSolidi C 3 (2006) 643.

[14] M.W. Wang, D.A. Collins, T.C. McGill, R.W. Grant, R.M. Feenstra, Appl. Phys. Lett.66 (1995) 2981.

[15] M. Losurdo, P. Capezzuto, G. Bruno, A.S. Brown, T. Brown, G. May, J. Appl. Phys.100 (2006) 013531.

[16] J. Wagner, J. Schmitz, N. Herres, G. Tränkle, P. Koidl, Appl. Phys. Lett. 70 (1997)1456.

[17] W. Rzodkiewicz, M. Kulik, E. Papis, A. Szerling, Acta Phys. Pol. A S95 (2009) 116.[18] A. Jasik, I. Sankowska, K. Regiński, E. Machowska-Podsiadło, A. Wawro, M.

Wzorek, R. Kruszka, R. Jakieła, J. Kubacka-Traczyk, M. Motyka, J. Kaniewski,in: Nicole A. Mancuso, James P. Isaac (Eds.), Crystal Growth: Theory, Mecha-nisms and Morphology, Nova Science Publishers Inc., New York, 2011, p. 293.

[19] J.F. Mouldr, W.F. Stickle, P.E. Sobol, K.D. Bomben, Phys. Electron. (1995).[20] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal. 20 (1993) 77.[21] E. Papis, A. Baranska, P. Karbownik, A. Szerling, A. Wojcik-Jedlinska, M. Bugajski,

W. Rzodkiewicz, J. Szade, A. Wawro, Opt. Appl. 399 (2009) 787.[22] E. Papis-Polakowska, Electron. Technol. Internet J. 37/38 (4) (2005/2006) 1

http://www.ite.waw.pl/etij/pdf/37_38-04a.[23] P.P. Paskov, J. Appl. Phys. 81 (1997) 1890.[24] M. Kulik, W. Rzodkiewicz, A. Drozdziel, K. Pyszniak, Phys. Status Solidi C 8 (2011)

1315.

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