Benine, E.

Berenice Anina Dedavid

Cusatis, C.

Eleani Maria da Costa

Huang, X. R.

Hönnicke, M. G.

Manica, J.

Mazzaro, I.

Journal of Electronic Materials

Repositório Institucional da PUCRS

Structural Characterization of Doped GaSb Single Crystalsby X-ray TopographyM.G. HÖNNICKE,1,5 I. MAZZARO,2 J. MANICA,3 E. BENINE,2E.M. DA COSTA,4 B.A. DEDAVID,4 C. CUSATIS,2 and X.R. HUANG11.—NSLS II, Brookhaven National Laboratory, Upton, NY, USA. 2.—LORXI, Departamento deFisica, Universidade Federal do Parana, Curitiba, PR, Brazil. 3.—Departamento de EngenhariaMecanica, Universidade Federal do Parana, Curitiba, PR, Brazil. 4.—Departamento deEngenharia Mecanica e Mecatronica, PUC-RS, Porto Alegre, RS, Brazil. 5.—e-mail: mhonnicke@bnl.govWe characterized GaSb single crystals containing different dopants (Al, Cd,and Te), grown by the Czochralski method, using x-ray topography and high-angular-resolution x-ray diffraction. Lang topography revealed dislocationsparallel and perpendicular to the crystal surface. Double-crystal GaSb 333x-ray topography showed dislocations and vertical stripes that could beassociated with circular growth bands. We compared our high-angular-resolution x-ray diffraction measurements (rocking curves) with findingspredicted by the dynamical theory of x-ray diffraction. These measurementsshow that our GaSb single crystals have a relative variation in the latticeparameter (Dd/d) on the order of 105. This means that they can be used aselectronic devices (e.g., detectors) and as x-ray monochromators.Key words: X-ray imaging, x-ray topography, Lang topography,double-crystal topographyINTRODUCTIONGaSb single crystals are group III–V semicon-ductors. They possess suitable properties for elec-tronic devices (viz. transistors, detectors, infraredsources, and high-efficiency thermo-photovoltaiccells) especially due to their high electron mobilityand small bandgap (0.82 eV).1,2 Furthermore, thecombination of band offsets and lattice matchingwith other semiconductor materials makes GaSbvery valuable as a substrate for different electronicdevices that are based on epitaxial thin films.However, defects that can be present in its crystal-line structure reduce electron mobility and canintroduce stress/strain, and even defects, in theepitaxial film. The Czochralski technique is usuallyemployed to grow GaSb crystals.1,3–7 Imperfectionsare expected (e.g., swirls, inclusions, and disloca-tions) during growth and will affect the crystalstructure and its electronic properties. IntrinsicGaSb single crystals are always p-type. Dopantssuch as Al and Cd retain its p-type conductivity,while Te at concentrations over 1017 atoms/cm3changes its conductivity to n-type. The advantageof Al and Cd dopants is that they reduce antisitedefects.8X-ray topography9 is a technique well suited tostudy structural defects in single crystals; it issensitive to variations in the lattice parameter(Dd/d) because it is based on x-ray diffractionimaging of a single crystal. Variant techniques arereported in the literature: Berg-Barrett, Lang, anddouble-crystal topography (or plane-wave topogra-phy). More recently, synchrotron-based x-raytopography techniques (white-beam topographyThis manuscript has been authored by employees of BrookhavenScience Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisherby accepting the manuscript for publication acknowledges thatthe United States Government retains a non-exclusive, paid-up,irrevocable, world-wide license to publish or reproduce the pub-lished form of this manuscript, or allow others to do so, for UnitedStates Government purposes. This preprint is intended for pub-lication in a journal or proceedings. Since changes may be madebefore publication, it may not be cited or reproduced without theauthor’s permission.(Received September 15, 2009; accepted February 23, 2010;published online March 23, 2010)Journal of ELECTRONIC MATERIALS, Vol. 39, No. 6, 2010DOI: 10.1007/s11664-010-1154-z 2010 TMS727and monochromatic-beam topography) have beenwidely used.10–14 The differences between theseapproaches are their sensitivity to Dd/d and thetype of defect detected.High-angular-resolution x-ray diffraction is acomplementary measurement method that canhelp to determine crystal quality. Such measure-ments are taken by fine angular rotation of thecrystal around the diffraction condition, so-calledrocking-curve measurement. These assessmentscan be made using conventional x-ray sources orsynchrotron sources. Different setups are employedfor rocking-curve measurements:15 (a) a dispersivesetup using a four-crystal monochromator (conven-tional sources)16 or a monochromatic beam of asynchrotron source; and (b) a highly sensitivenondispersive setup based on high diffraction order.In the present work, we used Lang topography,x-ray double-crystal topography, and rocking-curvemeasurements to characterize GaSb single crys-tals with different dopants [Al (p-type), Cd and Te(n-type)], grown by liquid-encapsulated Czochral-ski (LEC) in the [111] direction. We evaluatedthe quality of those crystals in relation to theirpractical application as electronic devices andx-ray monochromators, and discuss our findingsherein.EXPERIMENTAL PROCEDURESThin (300 lm) and thick (2 mm) crystals fromGaSb ingots were oriented, cut, polished, and etchedfor analysis by Lang topography, double-crystaltopography, and rocking-curve measurements. Foursamples were used: (a) two (samples 1 and 2) ori-ented and cut in the [111] direction (300 lm thick,both Al doped at 1017 atoms/cm3), and (b) two(samples 3 and 4) oriented and cut in the [110]direction (2 mm thick, one Cd doped at 1017 atoms/cm3 and the other Te doped at 1018 atoms/cm3).All were etched (HF:HNO3:CH3COOH, 1:20:1) for1 min three times, to remove the damaged layer dueto cutting and polishing.Lang topography, in the horizontal scatteringplane, was carried out using a conventional x-raysource (long fine focus Mo target) (Fig. 1a). Thex-ray tube was set in the line focus geometry, so thatthe source’s horizontal width was 40 lm (for atakeoff angle of 6) and vertically it was 12 mmlong. A 1.1-m-long collimator with a 150-lm slitallowed us to select, by crystal diffraction, theenergy of Mo Ka1 (17.48 keV). The crystal wasmounted on a Lang camera based on elastic trans-lation.17,18 The images were acquired with high-spatial-resolution Kodak Industrex M5 film.Fig. 1. Experimental setups for structural characterization of GaSb single crystals: (a) Lang topography (transmission geometry—Laue case);(b) double-crystal topography; (c) high-resolution x-ray diffraction based on four-crystal Ge 220 monochromator; and (d) ultrahigh-resolutionx-ray diffraction for high-order reflections.Hönnicke, Mazzaro, Manica, Benine, da Costa, Dedavid, Cusatis, and Huang728We also carried out high-resolution double-crystalx-ray topography (Fig. 1b) to better characterizethese crystals. The x-ray tube (long fine focus) alsowas set in the line focus geometry. We employed thesame 1.1-m-long collimator, but with a slit of0.5 mm. As the first crystal, we employed anasymmetrically cut Si 333 monochromator (floatingzone, b = 0.04; magnification m = 25) at 8.05 keV(Cu Ka1). The second crystal was the GaSb singlecrystal in different orientations of [111] and [110].The images again were acquired with high-spatial-resolution Kodak Industrex M5 film.Rocking-curve measurements were taken withtwo different setups. For low-order reflections (111and 220), a Ge 220 four-crystal monochromator wasemployed (Fig. 1c) at 8.05 keV. For high-orderreflections (333 and 440) we chose an ultrahigh-resolution nondispersive setup, more sensitive toDd/d (Fig. 1d), also at 8.05 keV.X-RAY TOPOGRAPHY ANDROCKING-CURVE MEASUREMENTSThe Lang topography (Fig. 2a) was obtained fromthe thin GaSb 111 single crystals (samples 1 and 2,initially a single piece); however, for the 220 diffrac-tion on transmission geometry (Laue case, Fig. 1a) at17.48 keV. The resulting image shows several linesand small spots that, respectively, could reflect dislo-cations parallel or almost parallel to the surface, andnormal or almost normal to the surface. The featuresresembling ‘‘holes’’ in the image might be dislocationsor even other defects such as precipitates. However,most of the spots and stripes are diffuse, an appear-ance that could be caused by insufficient etching dur-ing removal of the disturbed layer, or even the verticaldivergence of the current setup that might blur theimage. More comprehensive studies are needed toclarify the origins of this effect.To better characterize the sample, we undertookdouble-crystal x-ray topography (Fig. 1b) on theselected dashed area of Fig. 2a, at 8.05 keV for theGaSb 333 diffraction in reflection geometry (Braggcase). Such topography (Fig. 2b) does not showimages of dislocations or any other defects in thecrystal bulk. However, it is strongly sensitive toweak deformations near the crystal surface. Dislo-cations, precipitates, or even structures caused byinsufficient etching when removing the disturbedlayer are apparent in the image as large dark areas.Also, the image displays narrow vertical stripesthat can be associated with circular growth bands.Double-crystal topography of samples 3 and 4 (Cddoped and Te doped, respectively) taken with 440diffraction (not shown here) did not show suchstriations. Their absence might be due to the dif-ferent orientation of the crystal, since all weregrown under the same conditions (pulling velocity5.2 mm/h, seed rotation 5 rpm). The striationsalmost always appear in GaSb crystals grown inthe [111] direction via the Czochralski technique.This occurs because the method encompasses asubstantial nonstationary growth process due tointense convective flows, misalignment of the axis ofcrystal rotation with respect to the axis of thethermal symmetry, concentration overcooling, andother issues.7 Together, these factors cause fluctu-ations in growth rate that are responsible for vari-ations in the crystal composition, entailingformation of growth striations.7 A solution for this isto try to regrow the crystal by the vertical Bridgmanmethod.7We can consider the topography shown in Fig. 2bas plane-wave x-ray topography due to the experi-mental setup described in the ‘‘ExperimentalFig. 2. (a) Lang topography (transmission geometry) of a 300-lm-thick GaSb 220 single crystal (Al doped); long fine focus Mo Ka1(40 kV 9 20 mA), exposure time 30 h. (b) Double-crystal topogra-phy of the dashed area of the GaSb 333 single crystal shown in (a);long fine focus Cu Ka1 (40 kV 9 20 mA), exposure time 6 h.Structural Characterization of Doped GaSb Single Crystals by X-ray Topography 729Procedure’’ section. Accordingly, we can extract aDd/d map from the image. To obtain this map, weacquired the topography on the maximum slope atthe high-angle side of the rocking curve. By con-sidering the rocking curve of the GaSb 333 singlecrystal as a Gaussian profile with the same full-width of half-maximum (FWHM) as the diffractioncurve predicted by the dynamical theory of x-raydiffraction (FWHM = 19.7 lrad), we can employ thefollowing expression to determine the angulardeviation (Dh) of the different points (a pixel-by-pixel approach) on the image:Dh ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiln Ið Þ þ lnðImaxÞpþ x; (1)where r is the width of the Gaussian profile, I isthe intensity at the different points in the image(pixels), Imax is the maximum intensity of the image,and x is the angular position on the rocking curvewhere the image was acquired. From the differentvalues for Dh we can determine the relative varia-tions in the lattice parameter (Dd/d):Ddd¼  cot hð ÞDh; (2)where h is the diffraction angle.The two-dimensional (2D) and a three-dimen-sional (3D) map of Dd/d is shown in Fig. 3a and b.Variations within Dd/d  105 are very apparent.The extinction depth (that is, the depth to whichx-rays penetrate during diffraction)19–21 has valuesvarying from 1 lm to 50 lm. So, the detected Dd/d,caused by dislocations, precipitates, or even struc-tures due to insufficient etching, lies very close tothe surface. Such variations were not detected byrocking-curve measurements; we did not identifyany differences when comparing the experimentaland the theoretical rocking curves [predicted by thedynamical theory of x-ray diffraction (Fig. 4)]. Also,Fig. 3. (a) Two-dimensional map of the relative variations in thelattice parameter (Dd/d) based on the topography shown in Fig. 2b.(b) Three-dimensional map of the relative variations in the latticeparameter (Dd/d), also from the topography in Fig. 2b.Fig. 4. (a) High-resolution x-ray diffraction measurement (rockingcurve) on the GaSb 111 single crystal using the four-crystal Ge 220monochromator (Fig. 1c). Solid line Theoretical rocking curve basedon the dynamical theory of x-ray diffraction; open circles representthe experimental data. (b) Rocking-curve measurement on the GaSb220 single crystal using the four-crystal Ge 220 monochromator(Fig. 1c). Solid line Theoretical rocking curve based on the dynami-cal theory of x-ray diffraction; open circles represent experimentaldata.Hönnicke, Mazzaro, Manica, Benine, da Costa, Dedavid, Cusatis, and Huang730high-order diffraction rocking-curve measurements(based on the nondispersive setup, more sensitive toDd/d) showed no significant differences (Table I) forthe FWHMs of the measured and theoreticallypredicted rocking curves. We anticipated bothresults. For the diffraction planes and the x-rayenergy employed, the rocking-curve measurementsare not sensitive to Dd/d smaller than 105, whiletopography is sensitive. This difference was alreadyproven by several authors by strain mappingand Dd/d mapping acquired with a rocking-curveimaging technique.13,14CONCLUSIONSWe characterized GaSb single crystals with dif-ferent dopants grown by the liquid-encapsulatedCzochralski technique via x-ray topography andhigh-resolution rocking curves.Dislocations, precipitates, and growth bands wereobserved by different topographies. However, rela-tive variations in the lattice parameter of aboutDd/d  105 and the match between the measuredand theoretically predicted rocking curves providegood reasons to try to use those crystals for elec-tronic devices (detectors, for example, Te doped),and for x-ray monochromators (Al and Cd doped) inexperiments that do not require high energy reso-lution. The crystal quality was found to be compat-ible with that of other GaSb single crystals reportedin the literature. However, we were not able todetect that different dopants could help improve thecrystal quality, as already described in the litera-ture.1 The detected growth striations can affect theelectronic properties because, in these cases, thedislocation density can reach values of 102/cm2. Fornon-high energy resolution monochromator appli-cations this is not a serious problem. Finally, therewere no significant variations between the mea-sured and theoretically predicted rocking curves.ACKNOWLEDGEMENTSPart of this work was supported by the U.S.Department of Energy, Office of Science, Office ofBasic Energy Sciences, under Contract No. DE-AC-02-98CD10886. The authors grateful acknowledgeAvril Woodhead (BNL) for manuscript editing.REFERENCES1. E.M. Costa, B.A. Dedavid, and A. Muller, Mater. Sci. Eng.B44, 208 (1997).2. A. Peles, A. Janotti, and C.G. Van de Walle, Phys. Rev. B 78,035204 (2008).3. J. Setak, V. Sestakova, and B. Stepanek, J. Therm. Anal.Calorim. 56, 749 (1999).4. J. Sestakova, B. Stepanek, and J. Sestak, Cryst. Res. Tech-nol. 31, 929 (1996).5. A.E. Voloshin, T. Nishinaga, P. Ge, and C. Huo, J. Cryst.Growth 234, 12 (2002).6. A.E. Voloshin, T. Nishinaga, and P. Ge, Crystallogr. Rep. 47,S136 (2002).7. I.A. Prokhorov, Yu.A. Serebryakov, B.G. Zakharov, I.Zh.Bezbakh, V.V. Ratnikov, and I.L. Shulpina, J. Cryst. Growth310, 5477 (2008).8. P.S. Dutta, H.L. Bhat, and V. Kumar, Appl. Phys. Lett. 81,5821 (1997).9. B.K. Tanner, X-Ray Diffraction Topography (Oxford:Pergamon, 1976).10. A.N. Danilewsky, R. Simon, A. Fauler, M. Fiederle, andK.W. Benz, Nucl. Instrum. Methods B 199, 71 (2003).11. M. Dudley, S.P. Wang, W. Huang, C.H. Carter, V.F.Tsvetkov, and C. Fazi, J. Phys. D: Appl. Phys. 28, A63(1995).12. R. Barrett, J. Baruchel, J. Hartwig, and F. Zontone, J. Phys.D: Appl. Phys. 28, A250 (1995).13. D. Lubbert, C. Ferrari, P. Mikulik, P. Pernot, L. Helfen,N. Verdi, D. Korytar, and T. Baumbach, J. Appl. Cryst. 38,91 (2005).14. D. Lubbert, T. Baumbach, J. Hartwig, E. Boller, andE. Pernot, Nucl. Instrum. Methods B 160, 521 (2000).15. J.W.M. DuMond, Phys. Rev. 52, 872 (1937).16. S.E.G. Slusky and A.T. Macrander, J. Appl. Cryst. 20, 522(1987).17. J.E.A. Miltat (D. Phil. Thesis, University of Oxford, 1971).18. M.G. Honnicke, I. Mazzaro, C. Cusatis, and V.H. Etgens,Jpn. J. Appl. Phys. 43, 5614 (2004).19. A. Authier, Dynamical Theory of X-ray Diffraction (Oxford:Oxford University Press, 2001).20. M.G. Honnicke and C. Cusatis, J. Phys. D: Appl. Phys. 38,A73 (2005).21. M.G. Honnicke, C. Cusatis, and P.C. de Camargo, J. Phys.D: Appl. Phys. 41, 065401 (2008).Table I. Full-width at half-maximum (FWHM) ofrocking curves measured with the nondispersivesetup (Fig. 1d)SampleDiffractionPlaneExperimentalversus TheoreticalFWHM (lrad)1 9 2 (GaSb 111) 333 34 versus 303 9 4 (GaSb 110) 440 37 versus 37Structural Characterization of Doped GaSb Single Crystals by X-ray Topography 731

Structural Characterization of Doped GaSb Single Crystals by X-ray Topography

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Structural Characterization of Doped GaSb Single Crystals by X-ray Topography

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