Phys. Status Solidi B 247, No. 2, 303–307 (2010) / DOI 10.1002/pssb.200945415 p s sb

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Investigation of antimony for arsenicexchange at the GaSb covered GaAs(001) surface

Min Xiong1, Meicheng Li*,1, Yongxin Qiu2, Yu Zhao1, Lu Wang1, and Liancheng Zhao1

1School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China2Suzhou Institute of NanoTech and NanoBionics, Chinese Academy of Sciences, Suzhou 215125, P.R. China

Received 4 September 2009, revised 11 November 2009, accepted 11 December 2009

Published online 11 January 2010

PACS 68.43.Bc, 71.15.Mb, 82.30.Hk

*Corresponding author: e-mail mcli@hit.edu.cn, Phone: þ86 451 86418745, Fax: þ86 451 86418745

GaAs/GaAsSb surperlattices with different Sb soak time have

been grown, which were followed by characterizations of high

resolution X-ray diffraction. We suggest that the Sb incorpo-

ration into the GaAs surface contributes to the GaSb formation

on the surface. The Sb-for-As exchange at the GaSb covered

GaAs (GaSb/GaAs) surface has been investigated using first

principle calculations. The results reveal that the Sb substitution

for subsurface As atoms with weak Ga–As bonding arrange-

ments are energetically favored at the strained GaSb/GaAs

surface. After the Sb-for-As exchange, the formed GaSb layer

can be stabilized against degradation from As-for-Sb exchange

in the growth of GaAs/GaAsSb surperlattices.

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1 Introduction Semiconductor low dimensionalstructures that contain both arsenides and antimonides, suchas InAs/GaSb superlattice (SL), GaAs/GaAsSb quantumwell and GaSb/GaAs quantum dot, have attracted muchattention for type-II band alignments and its applications ininfrared detectors and long-wavelength lasers. For thesemixed anion heterostructures, intermixing of the anionspecies would spoil the interface abruptness and degradedevice performance. While the As-for-Sb and Sb-for-Asanion exchange under control can be utilized to obtainexpected interface structures. Compared to As-for-Sb, theSb-for-As exchange is not favorable energetically due toweaker bond strengths of antimonides than that of arsenides[1]. However, the Sb-for-As exchange reaction indeed takeplace at Sb soaked InAs and GaAs surface [2, 3].

TheGaAs/GaAsSb SL formed by Sb exposure have beengrown to estimate the incorporation of Sb in the Sb-exposedGaAs SL [3]. The resulting X-ray simulations indicate thatthe GaAsSb exchange layer of one SL period is only about 1monolayer (ML). A common explanation to such littleexchange is Sb surface segregation. Actually, the study of Sbsurface segregation in InAs/GaInSb SL suggests that the Sbsegregation originates from the surface reconstructionincluding Sb-bilayer [4]. Besides, the Sb-rich surface

reconstructions have been observed in an Sb-exposedGaAs (001) surface [5, 6]. Therefore, it is necessary to takeaccount of the surface reconstruction in the investigation ofSb-for-As exchange in the GaAs/GaAsSb SL. In this workwe grown a set of GaAs/GaAsSb SL with different Sb soaktime, and the GaSb component in the GaAsSb layer wasobtained by the high resolution X-ray diffraction (HRXRD)with strain analysis. First-principles calculations on Sb/Assubstitutions at GaSb covered GaAs surface were performedto give an atomic-scale description of the Sb-for-Asexchange in the SL growth.

2 Experiments and calculation methods Thesamples used in the present work were grown on semi-insulating GaAs(001) substrates using a V80H molecularbeam epitaxy system. For one period of the SL, the finishedGaAs layer was exposed to Sb4 flux for subsequent GaAsSbgrowth. The GaAsSb was formed under Sb flux of2� 10�6 Torr and substrate temperature of 600 8C. ThreeSL samples A, B, and C of 40-periods were grown with Sbsoak time 10, 30, and 60 s, respectively. After MBE growth,the period thickness and strain conditions of the SL sampleswere characterized by the HRXRD.

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304 M. Xiong et al.: Antimony for arsenic exchange at the GaSb covered GaAs (001) surfacep

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Total energy calculations similar to our previous workswere performed to study the exchange reaction at the surfacereconstruction [7]. A model involving Sb substituionfor subsurface As atoms at the GaSb covered GaAs (GaSb/GaAs) (001) surface was established. The first-principlescalculations are in the framework of the DFT, within thelocal density approximation using the Ceperley–Alderexchange and correlation potential and parameterized byPerdew and Zunger [8]. The electron–ion interaction wastreated using norm-conserving pseudopotentials [9], and theelectronic wave functions were expanded in plane waves,with a kinetic energy cutoff of 18Ry. The resulting bulkequilibrium lattice constant of GaAs and GaSb are 5.55 and6.01 A with corresponding Ga–As and Ga–Sb bulk bondlengths of 2.40 and 2.60 A, respectively. All the first-principles calculations were performed using the Quantum-Espresso package [10].

3 Results and discussion3.1 Analysis of the GaSb component in the

GaAs/GaAsSb superlattice Figure 1 displays (004)diffraction patterns of the SL samples. High order satellitepeaks (denoted by 0, �1, �2, etc.) and its very periodicarrangement indicates periodic crystal structure of the SL.The SL zeroth-order peak lying to the left of the GaAs peakindicate a compressive strain and a greater average latticeconstant than that of GaAs.

The average strain e of the SL can be calculated withrespect to the angle difference Du between the SL zeroth-order peak and GaAs peak:

Figudiffe

� 20

e ¼ �Du cot uB; (1)

re 1 HRXRD curves of GaAs/GaAsSb superlattice grown atrent Sb soak times.

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where uB stands for the Bragg angle of GaAs. Since the SLwith low Sb composition is fully strained to the GaAssubstrate, the average Sb composition x can be estimatedthrough the expression using elastic theory:

TabGaA

sam

ABC

e ¼ 1þ n

1� nxaGaSb � aGaAs

aGaAs; (2)

v represents the Possion ratio of the SL, and approximatelyequal to the value of GaAs bulk. We simply assume that theSb composition distribute uniformly at the SL interfaceforming an equivalent pure GaSb layer with the thicknesstGaSb:

tGaSb ¼ xD; (3)

D is the period of the SL, which can be acquired using theangular spacing between the first-order satellite peaks.Although the actual distribution of Sb element, primarilyattributing to the surface segregation [11], is far morecomplicated than the assumption above, these treatmentscan determine the total amount of GaSb component formedby Sb exposure in the GaAs/GaAsSb SL. The calculatedresults and related growth conditions are reported inTable 1.

Table 1 lists GaSb component thicknesses of threesamples prepared under almost the same growth conditionsexcept for different Sb soak time during GaAsSb growth.The amount of GaSb in the sample C is 2.8 A, which getsclose to the results inRef. [3]. The thickness ofGaSb does notincrease to a significant extent more than soak time of 10 s.These indicate that the stable amount of GaSb component inGaAsSb layer is about 1ML of unstrained bulk GaSb(�3.0 A), and most of the GaSb forms rapidly upon Sbexposure. Sb incorporation and Sb-for-As exchange possiblycontribute to the GaSb formation.

The Sb incorporation and distribution of Sb-exposedGaAs surface have been extensively studied [12–15]. Duringthe initial stage of Sb deposition on the arsenide surface, theSb atoms prefer staying in the missing cation row [12] andbonding with the cations beneath the As dimers [13]. Thefollowing Sb adatoms would bond with the incorporated Sbatoms and accumulate to form the structure of multiple Sblayer due to stronger bonding of Sb–Sb than Ga–Sb [14]. Afull GaSb layer of 1ML can be identified from the structureof multilayer coverage of Sb on top of GaAs [5, 6]. Due toabsence of distinct mass transport, the process of Sbincorporation and accumulation is very rapid [12]. On thebasis of the investigations above, we consider that the GaSb

le 1 The effects of Sb flux on the GaSb component in GaAs/sSb superlattice.

ple soak time (s) period (A) tGaAs (A) tGaSb (A)

10 232 230 2.330 229 226 2.760 236 233 2.8

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formation in theGaAs/GaAsSb SL primarily originates fromthe Sb incorporation into GaAs surface during the Sb soak.

3.2 Sb-for-As exchange at the GaSb coveredGaAs surface We introduced reconstructed GaSb/GaAs(001) surface to investigate the Sb-for-As exchange. Thesurface was modeled with a (2� 3) surface unit, followingthe (2� 3)-dimer row model with Sb–Sb homodimer row inRef. [6]. Taking account of existence of heterodimer on theGaSb(001) [16] and Sb-exposed GaAs (001) surface [5], weproposed another model with Ga–Sb heterodimer on top. Infollowing descriptions, GaSb/GaAs-I denotes the modelwith two Ga–Sb dimers in the outermost layer (ad-dimers)and GaSb/GaAs-II for the one with two Sb–Sb ad-dimers.Besides the ad-dimers, each of the surfaces involves a Sb–Sbdimer located in the second Sb layer (in-dimer). Surfacecalculationswere performed using slabs of nine layers of III–V material, which consists of six layers of GaAs and threelayers of GaSb. The cation dangling bonds at bottom layerwere passivated by pseudohydrogen atoms of 1.25 electroncharge. Slabs were separated by 14 A of vacuum tominimizeinteractions. Both the pseudohydrogen and the bottombilayer of GaAs were kept fixed to simulate the constraintcaused by the underlying bulk material. The Brillouin-zone integration was carried out using a set of four specialk-points.

The optimized atomic structures of GaSb/GaAs-I and IIstructures are shown in Fig. 2 with some bond length andbonds annotated. In the GaSb/GaAs-I structure, the topmostlayer features twoGa–Sb heteroad-dimers which correspondto 1/3MLof Sb and 1/3MLofGa on top of the second full Sblayer. All of the Ga–Sb ad-dimers show buckling, and the Gaatoms of the Ga–Sb ad-dimers are displaced downward by aconsiderable amount of �0.74 A to form a planar sp2-likearrangement with their Sb nearest neighbors. The bucklingvalue is consistent with experimental measurements of 0.6–

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0.8 A, which is distance between first and second layer of Sb-exposed GaAs (001) surface determined by in situ X-raycrystal truncation rod (CTR) scattering [15]. However, thesecond layer in our model is Ga layer of 1/3ML coveragerather than 2/3MLof Sb layer deduced from theCTR results.This discrepancy deserves further investigations.

The bond length of Ga–Sb ad-dimer is 2.59 A, which isnearly identical to the equilibrium bulk Ga–Sb bond length.While the Ga–Sb inward bond with the second Sb layer is2.52 A, which is in compression because the atom positionsare constrained by the subsurface GaAs layers. The bondlength is shorter than the Ga–Sb bulk bond by 3.1%. Thethreefold Sb atom in the Sb–Sb in-dimer is in a p3-typebonding configuration with an average bond angle of 84.68,which is much smaller than the ideal angle of 908 [17]. Thisdeviation primarily arises from that the Sb atoms tend tomove upward to relax more compression strain between theGaSb layer and GaAs sublayers. With the Ga–Sb hetero-dimers replaced by two Sb–Sb homoad-dimers, Ga atom isabsent at the topmost layer in the GaSb/GaAs-II structure.The Sb–Sb ad-dimers are parallel to the surface with bondlength 2.87 A,which are nearly identical to the bond length inrhombohedral Sb. The Sb–Sb inward bond with the secondSb layer is in compression and the length is 2.81 A.

To study the Sb-for-As exchange at the reconstructedGaSb/GaAs (001) surfaces, we simulated the exchange bysubstituting single and two As atoms belonging to thesubsurface GaAs with surface Sb atoms of same number. Asshown in the Fig. 2, the surface Sb atoms indicated by (blue)numbers and the subsurface As atoms by (black) letters.Since the Sb in-dimer breaks the mirror symmetry slightlywith respect to the (110) layer passing through the ad-dimers,we consider a (1� 3)-like unit cell to reduce the number ofpossible exchanging configurations. The areas for calcu-lation are shaded in the Fig. 2. In the following, notations like‘‘1B’’ are employed to denote the permutation of Sb atoms 1

Figure 2 (online color at: www.pss-b.com)Representation of the optimized geometriesof the GaSb/GaAs-I (left) and GaSb/GaAs-II(right). Dark balls correspond to Ga atoms andlight large (small) balls to Sb (As) atoms. Thesurface antimony atoms indicated by numbersand the subsurface arsenic atoms by letters.

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Table 2 The calculated energy differences of different substi-tution configurations from GaSb/GaAs-I and GaSb/GaAs-II struc-tures.

GaSb/GaAs-I GaSb/GaAs-II

configuration energy (eV) configuration energy (eV)

10A0 �0.02 1A 0.2810B0 �0.23 1B �0.0110C0 �0.02 1C 0.2820B0 0.08 2B 0.0330B0 0.19 3B 0.2140B0 0.33 4B 0.46– – 5B 0.701020A0C0 0.10 12AC 0.401020A0B0 0.12 12AB 0.401020 B0C0 �0.09 12BC 0.131030B0C0 0.08 13BC 0.31

Figure 3 (online color at: www.pss-b.com) Cross-sectional viewof charge density through the dimer atoms of (a) 10B0 and (b) 1Bconfigurations. Isocontour step is 0.001 atomic units (au).

and As atom B in each GaSb/GaAs structure investigated.Each of the test configurationswere allowed to relax after Sb/As substitutions. The energy difference DE was calculatedby comparing the total energyEex of the surfaces after the Sb/As substitutions with that of the reference surfaces, E0. Thetest exchange configurations and corresponding energydifference values DE¼Eex�E0 are given in Table 2.

According to the Table 2, there is only one stableconfiguration 1BofGaSb/GaAs-II structure,which is similarto the most stable configuration 10B0 of GaSb/GaAs-Istructure. The charge density through the dimer atoms,partially changed bond length and bond angles are illustratedin Fig. 3. The results suggest that the As atom B (B0) is thepossible one to be substituted. There are two reasonscontributing to this favorable substitution. For one thing,the Ga atoms bonding with Sb atom 3 are laterally movedtowards the Sb–Sb in-dimer due to P3 arrangement, leadingto the stretch of Ga–As bond with As atom B. The length ofthe stretched Ga–As bond is 2.44 A (Fig. 2), larger than theGaAs bulk by 2%, indicating a weaker Ga–As bondformation. After substitution, the weak Ga–As bond isreplaced by a shrunkGa–Sb bondwith length 2.57 A (Fig. 3).For another, when As atom B is replaced by Sb atom, thecircled Ga atom is fourfold-coordinated containing threeGa–Sb bonds. The angle of the Sb in in-dimers bonding withthe third Ga layer increases to 87.78 (Fig. 3b), which indicatemore relaxation of strain in the surface GaSb layer due to the‘‘softer’’ Ga–Sb bonds than Ga–As [18].

The substitutions of Sb atom 10 with As atomwould leadto formation of much stronger Ga–As bonds than originalGa–Sb ones. Furthermore, replacing Sb with the smaller Asreduces the bond compression, allowing partial relaxation ofthe strain in the GaSb layer. After substitution, the newlyformed Ga–As inward bond with the As atoms at secondlayer is 2.37 A (Fig. 3a), which shrinks slightly (�1%) whencompared with the Ga–As bulk bond length. Under similarexchange configurations, there are more energeticallyfavorable configurations, including one favorable two Asatoms substitution 1020 B0C0, on GaSb/GaAs-I structure than

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GaSb/GaAs-II surface. These results suggest that the Sb-for-As exchange is more energetically favored when thereconstructed surface is covered with Ga atoms.

As-for-Sb exchange in the growth of GaAs/GaAsSb SLis not negligible due to stronger bond strengths of Ga–Asthan that of Ga–Sb. The formed GaSb on the GaAs layersubjects to degradation from As-for-Sb exchange in the nextGaAs growth. In GaSb (4� 3) surface reconstruction, thephase a with more Ga atoms exhibit higher reactivity of As-for-Sb exchange than phase b [19]. This reference listsvarious favored As-for-Sb exchange configurations justifiedby the DFT-based total energy calculations. We notice thatthe bonding structures of substituted Sb atoms in the mostfavored configuration g nearly the same as the Sb atom 10 inour works. In other words, this type of Sb atoms tend toreplace the subsurface As atoms and can be replaced by theadsorbed As atoms as well. We deduce that, after Sb-for-Asexchange, theAs-for-Sb exchange at theGaSb/GaAs surfacewould be depressed to some extent. While full Sb-for-Asexchange requires relative long time with respect to the Sbincorporation, since the surface Sb substitutions for thesubsurface As atoms require atoms migration and surfacerearrangement. This can be utilized to explain the nearly

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stable amount of the GaSb component about 1ML in the SLfor long Sb soak time in sample B and C.

4 Conclusions In conclusion, the Sb-for-As exchangeon GaSb/GaAs (001)-(2� 3) surface has been investigatedemployingDFT calculations. The calculations reveal that theSb atoms on the strained GaSb covering layer can migrateinto the subsurface GaAs layer by substituting of the Asatoms with weak Ga–As bond. On surface with Ga–Sbheterodimers, there are more energetically favored substi-tution configurations than that with Sb–Sb homodimers.Besides, after full Sb-for-As exchange, the As-for-Sb on theGaSb/GaAs surface would be depressed. These mechanismscan be used to explain the variation of the amount of GaSbcomponent in GaAs/GaAsSb SL with increased Sb soaktime, and provide an possible approach to control the Sb/Asexchange reactions as well.

Acknowledgements Weare grateful toGuojunLiu,Mei Li,YongWang, andZhanguoLi for their experimental assistances. Thestudy was financially supported in part by the NSFC (Under GrantNumbers: 50502014and50972032), 863project (2009AA03Z407),and the program for New Century Excellent Talents in University(NCET-06-0337).

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