Synchrotron X-ray topographic study of dislocations and stacking faults in InAs

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Journal of Crystal Growth 283 (2005) 320–327

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Synchrotron X-ray topographic study of dislocations andstacking faults in InAs

A. Lankinena,�, T. Tuomia, J. Riikonena, L. Knuuttilaa, H. Lipsanena,M. Sopanena, A. Danilewskyb, P.J. McNallyc, L. O’Reillyc, Y. Zhilyaevd,L. Fedorovd, H. Sipilae, S. Vaijarvie, R. Simonf, D. Lumbg, A. Owensg

aOptoelectronics Laboratory, Helsinki University of Technology, P.O. Box 3500, FIN-02015 TKK, FinlandbKristallographisches Institut, Universitat Freiburg, D-79104 Freiburg, Germany

cResearch Institute for Networks and Communications Engineering (RINCE), Dublin City University, Dublin 9, IrelanddA.F. Ioffe Physico-Technical Institute, St Petersburg 194021 Russia

eOxford Instruments Analytical Oy, P.O. Box 85, FI-02631 Espoo, FinlandfANKA, Institute for Synchrotron Radiation, Forschungszentrum Karlsruhe, Germany

gScience Payload and Advanced Concepts Office, ESA/ESTEC, 2200 AG, Noordwijk, The Netherlands

Received 1 October 2004; received in revised form 15 March 2005; accepted 10 June 2005

Available online 20 July 2005

Communicated by M.S. Goorsky

Abstract

X-ray diffraction topographs made with synchrotron radiation of an epitaxial InAs structure show images of

dislocations and stacking faults. Three types of dislocations are identified and their Burgers vectors are determined from

a number of topographs having different diffraction vectors and recorded on the same film at a time. Straight

dislocations are found to be edge dislocations and their Burgers vector is h1 1 0i. Also mixed dislocations are found. The

overall dislocation density is about 2000 cm�2. Large stacking faults are limited by long straight dislocations, the

Burgers vector of which is h1 1 0i. Only a few threading dislocations are observed in the epitaxial layer grown by vapour-

phase epitaxy. Their density is about 500 cm�2. Small circular dots found are interpreted as indium-rich inclusions.

r 2005 Elsevier B.V. All rights reserved.

PACS: 81.05.Ea; 61.72.Ff; 81.10.Bk

Keywords: A1. Crystal defects; A1. X-ray topography; A3. Chloride vapor-phase epitaxy; B2. Semiconducting indium compounds; B3.

X-ray detectors

e front matter r 2005 Elsevier B.V. All rights reserved.

ysgro.2005.06.009

ng author. Tel.: +35894513120; fax: +35894513128.

ss: [email protected] (A. Lankinen).

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1. Introduction

Indium arsenide is a semiconductor having adirect room-temperature bandgap of 0:35 eV andan electron mobility as large as 33 000 cm2 V�1 s�1

[1]. These properties make it an interesting anduseful material for infrared diode lasers andphotodetectors. Because InAs strongly absorbsnot only infrared radiation but also X-rays, it is asemiconductor with potential similar to GaAs forthe development of p–i–n diodes for detectors inthe X-ray spectral range of photon energies.Optimal detector performance should be achievedby fabricating devices on a nearly perfect and pureintrinsic InAs wafer and then by growing thedoped epitaxial layers on the substrate as defectfree as possible. The results of this work will serveas a quality assessment of the InAs wafers after theepitaxial growth process, especially for X-raydetector applications. Particular attention is paidto samples consisting of rather thick layers grownby open tube chloride hydride vapour-phaseepitaxy (HVPE) from indium, arsenic chlorideand solid arsenic using hydrogen as a carrier gason InAs substrates acquired from commercial andother sources.X-ray diffraction topography using synchrotron

radiation is a non-destructive technique ideallysuited for studying extended defects in electronicmaterials by imaging the defects and analysing theimage contrast [2]. Because of the strong X-rayabsorption of III–V semiconductors like InP andGaAs, synchrotron X-ray topography has turnedout to be the only practical means of getting alarge number of transmission topographs inreasonable exposure times on the same high-resolution film [3–7]. This imaging technique alsoallowed the determination of the Burgers vectorand resulted, e.g. in finding straight screw andmixed dislocations in InP [3] and similar straightdislocations in GaAs [5–7] all possessing a Burgersvector along h1 1 0i.InAs has been studied in an earlier work with

synchrotron X-ray topography using the transmis-sion geometry [8]. In Ref. [8] the main interest wasin the evaluation of the heterostructure consistingof an In0:97Ga0:03As layer grown by liquid-phaseepitaxy on an InAs substrate rather than in the

characterisation of the defects in the bulk. TheInGaAs/InAs structure was intended for use in aninfrared detector working in the wavelength rangefrom 1:8 to 5mm. Two mutually perpendicular setsof misfit dislocations along h1 1 0i were observed.The n-type InAs substrate of the InGaAs/InAsheterostructure had a rather uniform and densedislocation network close to the upper detectionlimit of about 104 dislocations/cm2 at whichindividual dislocation images can be discerned inthe X-ray diffraction topographs.

2. Experimental procedures

The X-ray diffraction topographs were made atthe Topo Beamline at the ANKA (Angstroem-quelle Karlsruhe) synchrotron radiation facility inKarlsruhe and at HASYLAB-DESY (HamburgerSynchrotronstrahlungslabor am Deutschen Elek-tronen-Synchrotron) in Hamburg. Topographswere made on 100� 100mm Slavich VPR-Mhigh-resolution films both in the transmissionand in the back-reflection geometry. At ANKAthe film-to-sample distance was as long as70–85mm whereas at HASYLAB-DESY it was30–70mm. At the ANKA topography station thesource of radiation is a bending magnet. AtHASYLAB-DESY most of the topographs weremade at the topography F1 beam line using theradiation of the DORIS bending magnet sourcehaving a continuous spectrum of wavelengths. Theother source at HASYLAB-DESY was an X-rayundulator of the BW1 beamline. The undulatorwas tuned to emit a rather narrow spectrum whosemaximum occurs at a photon energy of about10 keV. This was achieved by adjusting theundulator magnet gap and by using two grazingincidence gold-coated silicon mirrors downstreamof the beam.The storage ring of the ANKA facility had an

electron momentum of 2:5GeV=c and a typicalbeam current of 100–160mA. The positron ring atDORIS III had a particle momentum of4:445GeV=c and a beam current of 80–130mA.Due to the large particle momentum of theDORIS positron ring compared to the ANKAelectron ring the spectrum of radiation extended to

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A. Lankinen et al. / Journal of Crystal Growth 283 (2005) 320–327322

larger photon energies at HASYLAB F1 than atANKA.The InAs samples investigated in this work were

grown at A. F. Ioffe Physico-Technical Instituteusing HVPE [9]. The substrate was a (0 0 1)oriented Czochralski-grown n+ -type wafer hav-ing an electron concentration of 1017 cm�3. Thesubstrate temperature in epitaxial growth wasabout 750 �C, and the HVPE reactor high-tem-perature zone was at 800 �C. The epitaxial layerhad a thickness of 40 mm and an electronconcentration of ð229Þ � 1016 cm�3 at room tem-perature. The sample thickness, including theepilayer, was 360 mm. Another sample was grownon a 4� toward h1 1 1i misoriented (0 0 1) substratehaving a rather large electron concentration of2� 1018 cm�3. The epitaxial layer on this samplehad a thickness of 10 mm and an electronconcentration of 1017 cm�3.The (0 0 1) surfaces of the sample wafers were set

perpendicular to the incident beam or in certaincases at a tilt angle of 14�218� measured from thevertical ½1 1 0 direction.

3. Burgers vector analysis

An analysis of the Burgers vectors of disloca-tions is possible with the aid of the topograhicimages of the dislocations. The type of thedislocation can be resolved if both the Burgersvector ~b of the dislocation and the direction vector~l of the dislocation line are known [10]. If ~b and ~lare parallel, the dislocation is a pure screwdislocation. If ~b and ~l are perpendicular to eachother, the dislocation is an edge dislocation. Quitefrequently ~b and~l are neither parallel nor mutuallyperpendicular and the dislocation is called a mixeddislocation.The image of a dislocation disappears in an X-

ray topograph, if

~g ~b ¼ 0, (1)

where ~g is the diffraction vector ½h k l. Upon theidentification of at least two topographs, in whichthe dislocation is not visible, ~b can be solved byusing Eq. (1). Consequently, a mixed dislocationnever completely disappears in any topograph.

It is often difficult to find the direction ~l of thedislocation. One could assume that the dislocationgoes through the sample from the front to the backsurface. By knowing the thickness of the sampleand the length of the image in a given topographthe direction ~l can be calculated. Another way tofind ~l is to determine it from stereo pairtopographs. Any two topographs recorded onthe same film form such a pair. Sometimes, as inthe case of an edge dislocation, it is not necessaryto know the exact direction of the dislocation. It issufficient to determine whether ~b is perpendicularto ~l.

4. Results and discussion

4.1. Dislocations

Fig. 1 shows four transmission topographs ofthe InAs sample, enlarged from the same film. Theproduct of the linear absorption coefficient m andthe sample thickness t is mt � 6:6, 6:6, 5:6 and 8:4for the reflections 331, 331, 602, and 331,respectively. Because all these values of mt aresignificantly larger than unity, anomalous trans-mission occurs and dynamical images of disloca-tions are seen. They appear as white lines againstthe perfect crystal background. The white disloca-tion lines that are marked with dashed ovals arevisible in the 331 and 602 topographs anddisappear in the 331 and 331 topographs. Apply-ing Eq. (1) a straightforward calculation gives theresult that the Burgers vector ~b of these disloca-tions is parallel to ½1 1 0.The sample was oriented so that ½1 1 0 points to

the right in Fig. 1. Therefore, the white markeddislocation lines are perpendicular to ½1 1 0 andconsequently they are edge dislocations. It is worthnoting that similar edge dislocations whose Bur-gers vectors were aligned to h1 1 0i were observedin undoped GaAs grown by the vapour pressurecontrolled Czochralski (VCz) method [7], suggest-ing that these two III–V materials have commonproperties in their dislocation structures. However,the relationship between the different growthtechniques and the effect of substrate doping needfurther research efforts.

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Fig. 1. (a) 331, (b) 331, (c) 602, and (d) 331 synchrotron transmission topographs of InAs. Image width is 2mm. Dislocation images

are marked with arrows and dashed ovals. S0 marks a stacking fault. Diffraction vector projections are marked with ~g.


Fig. 2 shows a stereo pair of 151 and 151transmission topographs. These two topographswere enlarged from the same film as thosepresented in Fig. 1. In Fig. 2 151 and 151reflections had mt � 2:8, which implies that bothkinematical and dynamical images can co-exist.The dislocation images of Fig. 2 consist predomi-nantly of black lines indicating a kinematicalimage contrast, but the dynamical images can alsobe seen, albeit very faintly. The white images ofFig. 1 are dynamical images of the same disloca-tions. In addition to the linear edge dislocations inboth Figs. 1 and 2 there are circular arc disloca-tions. Analogous to similar dislocations in GaAs[6,7] they are identified as mixed dislocations, theBurgers vectors of which are along h1 0 1i. Theproportion of the circular arc dislocations is larger

in semi-insulating GaAs grown by VGF than inslightly doped InAs studied in this work. Theoverall dislocation density of the sample is about2000 cm�2, which is slightly more than the densityof 1500 cm�2 found in VGF and VCz GaAs [6,7].About half of the dislocations are circular arcdislocations. The exact number of circular arcdislocations is not clear, because some of the lineardislocation images might actually be arc disloca-tions seen from the plane they lie in. Due to thedifferent growth techniques commonly used infabricating substrates of these III–V materials, it isnot clear whether the differences in the dislocationstructures and densities are material based orgrowth related.Fig. 3 shows the 117 back-reflection topograph

of the epitaxial-layer side of the same region of the

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Fig. 2. Stereo pair of (a) 151 and (b) 151 transmission topographs of InAs. Image width is 2mm. Stacking faults S0 are marked with

arrows and diffraction vector projections with ~g.

Fig. 3. 117 back reflection topograph from epitaxial layer side

of InAs. T marks a threading dislocation, C marks a circular

arc dislocation and ~g the projection of the diffraction vector.

Image width is 2mm.


sample as shown in the transmission topographs ofFigs. 1 and 2. The X-ray attenuation length is 9; 79and 58mm for the contributing reflections 117,3321 and 4428, respectively, which implies that theimage is mainly from the epitaxial layer of thesample. The short, nearly horizontal, parallel andpredominantly white lines are interpreted as theends of threading dislocations, one of which is

marked with T. The dislocation density of theepilayer is about 500 cm�2, which is clearly lessthan the overall dislocation density of the wholesample. This shows that most of the dislocationspresent in the substrate do not penetrate into theepitaxial layer. Because Fig. 3 shows only oneclearly visible circular arc dislocation image,marked with C, most of the circular arc disloca-tions are confined into the substrate. Thus, theepilayer threading dislocations originate likelyfrom the linear substrate dislocations. The opentube HVPE grown epitaxial InAs layer had fewerdislocations than the InAs substrate it was grownon, indicating that the HVPE growth method canbe used to grow thick epitaxial layers with lowdislocation density on InAs, which is of impor-tance for the fabrication of the p–i–n X-raydetector structures.The topographs of Figs. 1–3 are from the

epitaxial structure that had the smallest densityof defects. For comparison, Fig. 4 shows a 228back-reflection topograph of the other epitaxialsample grown on a substrate having a largeelectron concentration of about 2� 1018 cm�3.This topograph displays a clear misfit dislocationnetwork at the interface between the intrinsic InAsepitaxial layer and the heavily doped substrate.Topographs of an intrinsic epitaxial layer on the

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Fig. 4. 228 back reflection topograph of an InAs epitaxial layer

grown on heavily doped InAs substrate showing misfit

dislocations in the interface. The broad white horizontal line

is an image of a scratch on the sample surface. Diffraction

vector projection is marked with ~g. Image width is 2mm.


less doped (about 1017 cm�3) substrate in Fig. 3 didnot show any signs of misfit dislocations. Inaddition, rather a large number of small (10mmin diameter) white dots are clearly visible in Fig. 4.These dots are believed to be ends of threadingdislocations having density of about 25 000 cm�2,which is approximately 50 times as large as that ofthe best sample.Fig. 5 shows four transmission topographs of

the same sample as the topographs of Figs. 1 and2, but taken at a different location on the wafer.Straight and circular arc dislocations similar tothose in Figs. 1 and 2 are also seen in Fig. 5. In themiddle of the topographs of Fig. 5 there are imagesof small dislocation loops marked with L. They arelocated on the (0 0 1) plane, which is parallel to thesample surface.

4.2. Stacking faults

In addition to the dislocations discussed inSection 4.1, Fig. 5 shows rather large grey areasmarked with S and S0. These areas are identified asimages of stacking faults. The images of the

stacking faults are darker (thus having largerdiffracted intensity) than the background,because a new wavefield generated by the crossingfield at the fault boundary adds intensity to thediffracted beam [11]. The image contrast of thestacking faults of type S0 and S is especially strongin the 511 topograph of Fig. 5(a) and 151topograph of Fig. 5(d), respectively. Two stackingfaults on the right-hand side of the topographsmarked with S are limited by long straight parallellines. These four lines are interpreted as partialdislocations and the enclosed grey areas asstacking faults. The partial dislocations on theleft-hand sides of the stacking faults disappear inthe 151 topograph. Their Burgers vector is mostlikely h1 0 1i.The round stacking faults S0 have a more

complicated structure. These kind of stackingfaults are seen in all topographs of this work.For example, in Fig. 1 they are very common andin some areas they line up in a row as observed inthe 331 topograph of Fig. 1. Other good examplesof stacking faults like S0 may be observed in Fig. 2.The peripheries of the S0 stacking fault imagesseem to be formed by straight dislocations, theBurgers vector of which are along h1 1 0i.

4.3. Precipitates

Fig. 6 shows a 004 transmission topograph ofthe same sample as in Figs. 1–3 and 5, but imagingyet another area of the sample. Apart from thedislocation lines the most striking features in thistopograph are the small circular dots clearly visiblein the two enlargements. The black and whitecontrast of such dots, one of which is marked withP in the left enlargement suggests the presence of aprecipitate or a void. Because the contrast of thedots is black on the positive side of the ~g vector,the black–white dots originate from precipitatesthat compress the surrounding lattice [12,13].Precipitates in bulk III–V semiconductors arecommonly predicted to consist of the group Velements, i.e. in this case of arsenic, similar to thewell-studied GaAs precipitation [14,15]. Accordingto the dynamical theory of X-ray diffraction theprecipitates seen in the topographs are close to thesurface [12,13].

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Fig. 5. 511, 511, 151 and 151 transmission topographs of InAs. Image width is 1.5mm. Stacking faults S and S0, dislocation loops L

and diffraction vector projections ~g are marked with arrows.


5. Conclusion

Edge dislocations having a Burgers vector ~bparallel to h1 1 0i are found in InAs epitaxialsamples grown by vapour-phase epitaxy. Thedislocations lie in the 1 1 0 plane perpendicular to~b. In addition to these straight edge dislocations,circular arc dislocations are observed in synchro-tron transmission X-ray topographs. The circulararc dislocations are similar to those found

frequently in vertical gradient freeze grown andvapour pressure Czochralski grown GaAs crystals.They are identified as mixed dislocations, whichhave the same Burgers vector ~b parallel to h1 1 0i asthe straight ones.Stacking faults as well as precipitates are also

common defects in InAs. The rather thickundoped epitaxial layer grown on the n-typesubstrate having an electron concentration of1017 cm�3 is well lattice matched. The back-

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Fig. 6. 004 transmission topograph of InAs. Image width is

2mm. P indicates a precipitate. Diffraction vector projection is

marked with ~g.


reflection topographs show only a few images ofthreading dislocations on the (0 0 1) sample sur-face.The crystalline quality of the InAs i–n structure

grown by vapour-phase epitaxy in this work isalmost as good as the similar GaAs i–n structuregrown by the same technique [6]. Although thestacking faults and precipitates may cause pro-blems it is believed that InAs p–i–n diodes arepotentially useful device structures. The diodes areviable candidates for development as X-raydetectors and further investigations are requiredto determine whether their performance is betterthan that exhibited by similar GaAs devices [16].

Acknowledgements

This work was supported by the EuropeanSpace Agency Contract 17356/03/NL/CP and bythe IHP-Contract HPRI-CT-2001-00140 of theEuropean Commission.

References

[1] O. Madelung (Ed.), Data in Science and Technology,

Semiconductors Group IV Elements and III–V Com-

pounds, Springer, Berlin, 1991, pp. 133–141.

[2] T. Tuomi, J. Synchrotron Radiat. 9 (2002) 174.

[3] K. Naukkarinen, T. Tuomi, V.-M. Airaksinen, K.-M.

Laakso, J.A. Lahtinen, J. Crystal Growth 64 (1983) 485.

[4] T. Tuomi, H. Lipsanen, T. Ranta-aho, J. Partanen, J.A.

Lahtinen, E. Monberg, R.A. Logan, J. Crystal Growth 96

(1989) 881.

[5] E. Prieur, T. Tuomi, J. Partanen, E. Yli-Juuti, M. Tilli,

J. Crystal Growth 132 (1993) 599.

[6] T. Tuomi, M. Juvonen, R. Rantamaki, K. Hjelt, M.

Bavdaz, S. Nenonen, M.-A. Gagliardi, P.J. McNally, A.N.

Danilewsky, E. Prieur, M. Taskinen, M. Tuominen, Mat.

Res. Soc. Symp. Proc. 487 (1998) 459.

[7] T. Tuomi, L. Knuuttila, J. Riikonen, P.J. McNally, W.-M.

Chen, J. Kanatharana, M. Neubert, P. Rudolph, J. Crystal

Growth 237–239 (2002) 350.

[8] P.J. McNally, J. Curley, A. Krier, Y. Mao, J. Richardson,

T. Tuomi, M. Taskinen, R. Rantamaki, E. Prieur, A.

Danilewsky, Semicond. Sci. Technol. 13 (1998) 345.

[9] M.J. Cardwell, J. Crystal Growth 70 (1984) 97.

[10] D.K. Bowen, Exercises in diffraction contrast, in: B.K.

Tanner, D.K. Bowen (Eds.), Characterization of Crystal

Growth Defects by X-Ray Methods, Plenum Press, New

York, 1980, Ch. A3, pp. 528–534.

[11] J. Chikawa, Laboratory techniques for transmission X-ray

topography, in: B.K. Tanner, D.K. Bowen (Eds.), Char-

acterization of Crystal Growth Defects by X-Ray Meth-

ods, Plenum Press, New York, 1980, Ch. 15, pp. 368–400.

[12] T. Tuomi, R. Rantamaki, P.J. McNally, D. Lowney, A.N.

Danilewsky, P. Becker, J. Phys. D 34 (2001) A133–A135.

[13] B.K. Tanner, X-ray Diffraction Topography, Pergamon,

Oxford, 1976, pp. 71–77.

[14] A.G. Cullis, P.D. Augustus, D.J. Stirland, J. Appl. Phys.

51 (1980) 2556.

[15] P. Schlossmacher, K. Urban, H. Rufer, J. Appl. Phys. 71

(1992) 620.

[16] A. Owens, M. Bavdaz, A. Peacock, A. Poelaert, H.

Andersson, S. Nenonen, H. Sipila, L. Troger, G. Bertucci,

J. Appl. Phys. 90 (10) (2001) 5376.

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Synchrotron X-ray topographic study of dislocations and stacking faults in InAs