Reflection high-energy electron diffraction during substrate rotation: A new dimensionfor in situ characterizationW. Braun, H. Möller, and Y.-H. Zhang Citation: Journal of Vacuum Science & Technology B 16, 1507 (1998); doi: 10.1116/1.589976 View online: http://dx.doi.org/10.1116/1.589976 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/16/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Reflection high-energy electron diffraction and scanning tunneling microscopy study of InP(001) surfacereconstructions J. Vac. Sci. Technol. A 18, 1492 (2000); 10.1116/1.582373 Reflection high-energy electron diffraction oscillations on rotating substrates J. Vac. Sci. Technol. B 17, 474 (1999); 10.1116/1.590579 New model for reflection high-energy electron diffraction intensity oscillations J. Vac. Sci. Technol. B 16, 2404 (1998); 10.1116/1.590182 Reflection high-energy electron diffraction studies of wurtzite GaN grown by molecular beam epitaxy J. Vac. Sci. Technol. B 16, 2237 (1998); 10.1116/1.590155 Molecular-beam epitaxy of BeTe layers on GaAs substrates studied via reflection high-energy electron diffraction Appl. Phys. Lett. 72, 2859 (1998); 10.1063/1.121482

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Reflection high-energy electron diffraction during substrate rotation: A newdimension for in situ characterization

W. Brauna)

Center for Solid State Electronics Research and Department of Electrical Engineering,Arizona State University, Tempe, Arizona 85287-6206

H. MollerFraunhofer Institut fu¨r Integrierte Schaltungen-A, D-91058 Erlangen, Germany

Y.-H. ZhangCenter for Solid State Electronics Research and Department of Electrical Engineering,Arizona State University, Tempe, Arizona 85287-6206

~Received 5 October 1997; accepted 12 December 1997!

We present two methods to access reciprocal space with reflection high-energy electron diffraction~RHEED! during substrate rotation. The extraction of an arbitrary number of still frames from acontinuously changing RHEED pattern is realized by triggering the substrate rotation and it allowsanalysis of quasistatic RHEED patterns that are updated every revolution. At the same time, theintensity along a line parallel to the shadow edge can be used to reconstruct a planar cut through thereciprocal lattice similar to a low-energy electron diffraction pattern. This RHEED pattern directlyreveals the symmetry of the surface reconstruction and its changes during the deposition process.© 1998 American Vacuum Society.@S0734-211X~98!02503-7#

I. INTRODUCTION

Although almost every molecular beam epitaxy~MBE!machine is equipped with a reflection high-energy electrondiffraction ~RHEED! system, the information contained inthe diffraction pattern is generally not used during samplerotation. This limits the use of RHEED as a truein situtechnique. Most layers for device or scientific applicationshave to be grown with substrate rotation because layer uni-formity is of paramount importance. It is therefore highlydesirable to develop methods that allow access to the recip-rocal lattice of rotating substrates.

Several approaches using RHEED during rotation havefocused on the specular spot shape and intensity during sub-strate rotation.1–4 The results demonstrate that it is possibleto extract the growth rate by extracting the RHEED oscilla-tion signal. These approaches are either based on intensitymeasurements combined with Fourier filtering,1,5 and highfrequency rotation2 or measurements of the specular spotsize combined with Fourier filtering.3 Even if the RHEEDoscillations are not evaluated, the specular spot intensity var-ies depending on the growth conditions used and can be usedto characterize MBE growth.4

RHEED measurements on nonrotating substrates, apartfrom growth rate determination6 and characterization ofinterfaces7 using RHEED oscillations, also use the informa-tion contained in the geometry and shape of the diffractionstreaks to assess the surface structure in MBE.8 In this work,we examine ways of accessing this diffraction geometry in-formation during substrate rotation. We demonstrate twomethods that exploit the rotating substrate to obtain recipro-cal lattice scans that are impossible with a static RHEEDexperiment. These new modes of operation expand the capa-

bilities of RHEED beyond the scope of traditional RHEEDexperiments and allow assessment of almost the entire acces-sible reciprocal space.

II. EXPERIMENT

The experiments were performed using a DCA MBE sys-tem equipped with a low wobble substrate manipulator, astandard RHEED e-gun operated at 20 kV and a P20 phos-phorus screen. Intensities were recorded at a rate of 50 Hzusing a charge coupled device~CCD! camera and PC-basedanalysis system.9 A straightforward way to obtain staticRHEED images from a rotating substrate is a triggered col-lection of images along high symmetry azimuths during ro-tation. The CCD camera taking images of the RHEED screenand the rotation motor are therefore interlocked via triggersignals. Images can then be extracted at arbitrary azimuthsduring the continuous rotation. The windows displaying theresults for the different azimuths are updated once per revo-lution, allowing a quasisimultaneous display of the differentquasistatic displays. An example of the implementation inFig. 1 shows the capture of the four in-plane^110& azimuthsof a ~001! GaAs b(234) reconstructed surface at 580 °C.The source area on the live image is marked by a brightframe on the continuous video image in the background.While the two @110# azimuths exhibiting the 23 pattern~left! are almost identical, the position of the specular spot inthe two patterns along the@110# azimuth~43, right! is dif-ferent, indicating substrate wobble with a misorientation to-wards@110#. For the 20 keV electrons used in this measure-ment the distance between the~00! and ~01! streakscorresponds to an angle of 1.23°. This yields a value of 0.5°for the substrate wobble. The cyclic recording of imagesalong several azimuths corresponds to the situation of sev-eral RHEED guns installed at the corresponding angles. Ita!Electronic mail: braun@enpop2.eas.asu.edu

1507 1507J. Vac. Sci. Technol. B 16 „3…, May/Jun 1998 0734-211X/98/16 „3…/1507/4/$15.00 ©1998 American Vacuum Society

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therefore allows a very detailed assessment of the reciprocallattice. All the standard measurements like evolution of in-tensities, peak shapes, etc. can be performed with a maxi-mum rate corresponding to the revolution frequency by sub-sequent analysis extracting data from the individual displays.

The triggered recording of images during rotation allowsaccess to several discrete cuts of the reciprocal lattice. Acomplete scan of the accessible reciprocal space can beachieved by recording continuously during sample rotation.The basic diffraction geometry of RHEED is shown in Fig.2~a!. For any incidence angleu, the Ewald sphere construc-tion requires bothk0 andk8 to end on a reciprocal lattice rod

for diffraction to occur. During sample rotation, the recipro-cal lattice revolves around the~00! rod, while the Ewaldsphere stays fixed in space if we neglect wobble. The Ewaldsphere therefore scans almost the entire upper half of recip-rocal space as indicated by the shaded area in Fig. 2~b!. Thescanned volume is largest for the smallest incidence anglesu, where the wave vector component perpendicular to thesurface is smallest, ensuring high surface sensitivity. Thisallows us to reconstruct practically the entire upper half ofreciprocal space during one sample revolution.

In a practical situation, this is not possible in real time dueto the lack of capability of current computers. We thereforepresently restrict ourselves to a plane parallel to the surfaceand choose the plane containing the specular spot, since it isthe only plane that is continuous across the~00! rod. Wedenote this plane as the specular plane~SP!. Its position isindicated in Fig. 2~b! by a dashed line. The measurementtakes place by recording the intensities along a line parallelto the shadow edge through the specular spot as shown inFig. 3~a!. The specular plane is then constructed by plottingthe lines bent with radiusr and the specular spot centered on~00! according to their azimuthal angle. This is schematicallyplotted in Fig. 3~b!. The radiusr is given by r 5k0 sinugiven by the geometry in Fig. 2~a!. As the rotation is com-pleted, the lines add up to a complete cut through the recip-rocal lattice at a distance 2k0 cosu from the origin of thereciprocal lattice. Such a cut is similar to a low-energy elec-tron diffraction ~LEED! pattern and represents the two-dimensional~2D! symmetry of the surface reconstruction ina top view. By varying the incidence angle, a wide range of

FIG. 1. Snapshot of RHEED patterns taken along the four^110& azimuths during continuous rotation of the substrate. The source area of the data is indicatedby a bright frame in the background image. The measurement was made at 580 °C on a GaAs~001! surface reconstructed in theb(234) structure using 20keV electrons. Left:@110# and @110# azimuths, right:@110# and @110# azimuths.

FIG. 2. Reciprocal space geometry for~a! static RHEED and~b! duringrotation. The shaded area in~b!, rotated around~00!, represents the volumeof reciprocal space accessible during a single sample rotation. The onlysurface parallel plane without a void in the center, the specular plane, inter-sects the specular spot and is indicated by a dashed line.

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wave vectors perpendicular to the surface can be measured,similar to aI –V measurement in LEED.

III. RESULTS

To demonstrate the method, we have investigated theb(234) andc(434) surface reconstructions of GaAs~001!.The accuracy in both cases was impeded by charging effectsclose to the RHEED screen leading to a distorted RHEEDpattern and unsteady rotation that resulted from the gears notbeing driven precisely by a magnetically coupledfeedthrough. Both factors contributed to a slight distortion ofthe processed scan. The results are shown in Fig. 4 for theb(234) and in Fig. 5 for thec(434) surface reconstruc-tions, respectively. The rotation speed was approximately 3rpm. The intensity profiles were recorded at a rate of 50 Hz,resulting in an angular resolution of about 0.4°. The inci-

dence angleu was 1.5° using 20 keV electrons. In both im-ages, the underlying 131 mesh of the surface reciprocallattice is indicated by a white square. The scan spans morethan six unit cells in each direction. This size is only limitedby the electron energy~higher electron energyE reduces thedistance between the reciprocal lattice rods;E21/2! and thesize of the RHEED screen. In our setup, this limit was ap-proximately 16 unit cells for GaAs. The eight in-plane^001&and ^110& azimuths can easily be identified by the adjacenthigh intensity radial streaks. These correspond to surfaceresonance conditions10–12 where the Laue circle tangentiallytouches reciprocal lattice rods, leading to increased reflectiv-ity.

Spots in the azimuthal scan correspond to intersections ofreciprocal lattice rods with the specular plane. The spots areof elliptic shape with the longer axis along the radial direc-tion. The anisotropy is a direct measure of the different trans-fer widths t ~Ref. 13! of RHEED parallel and perpendicularto the beam that are not proportional to the normalized fullwidth at half maximum~FWHM! of the spot in the respec-tive direction:

t5ai j gi j

h,

whereh denotes the FWHM of the spot andai j andgi j arethe real and reciprocal lattice constants in the direction theFWHM is measured. In Fig. 4, theb(234) structure isclearly identified by the rows of closely spaced spots directedalong@110#. This distinguishes it from thec(434) structurein Fig. 5 that exhibits a centered mesh. A display of the full2D symmetry of the reciprocal lattice allows an immediatedistinction, e.g., betweenc(434) and (232) structures to

FIG. 3. Measurement geometry and top view of the azimuthal scan. Experi-mental conditions in~a! are similar to those in Fig. 1, close to the@110#azimuth. The intensity distribution in the specular plane is obtained by plot-ting the lines according to their azimuthal angle with the specular spotcentered at~00! and radiusr ~see Fig. 2!.

FIG. 4. Azimuthal scan of a GaAsb(234)-reconstructed surface. The mainazimuths and the surface unit cell are indicated.

FIG. 5. Azimuthal scan of a GaAsc(434)-reconstructed surface. The mainazimuths and the surface unit cell are indicated.

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be made that is not evident from observation along the^110&azimuths, where both would exhibit the same symmetry.

The c(434) pattern in Fig. 5 does not have a fourfoldrotational symmetry. If we look, for example, at the spots inthe row directly adjacent to the@110# direction, we observehigh intensity on the second, third, and fifth spots from~00!.In the corresponding row-along@110#, he third spot is veryweak. This deviation from the fourfold, symmetry is presenteven on (131) reconstructed surfaces and allows identifica-tion of the two different azimuths even if the static RHEEDpatterns along both azimuths differ only very little. It is dueto asymmetries in the structure like the orientation of dimers.Therefore, azimuthal scans are promising candidates to com-pare against theoretical models. The ease of acquisition to-gether with the simple geometry allow the acquisition ofhighly accurate data. At the same time, the access to a largeregion of reciprocal space spanning many unit cells makesthe comparison with models easier since many points can befitted simultaneously. Since in the kinematical approximationthe structure of reciprocal space far from the~00! rod con-tains the most detailed information about the atomicpositions,14 even structure optimization seems feasible andcan be expected to be more reliable than previous optimiza-tions using data sets obtained from conventional RHEEDscans.15–18

The comparison of Figs. 4 and 5 suggests a reliablemethod for in situ control of surface reconstruction. If thegrowth process requires the surface to maintain a certain re-construction, the respective spots in the azimuthal scan canbe monitored during rotation. The two-dimensional nature ofthe measurement permits the simultaneous monitoring ofmany spots, resulting in increased accuracy and allowingconsistency checks. In this way, a control algorithm could,for example, distinguish whether the desired surface recon-struction changes to the adjacent low- or high-temperatureconfiguration if it controlled the sample temperature andchange it accordingly.

IV. SUMMARY

In this work, we presented new ways to access reciprocalspace during substrate rotation. Both gated detection and azi-

muthal scans provide direct access to the surface reconstruc-tion in real time during growth, allowingin situ control ofthe growth front. Neither approach requires modifications tothe MBE machine except for the generation of a trigger sig-nal by the rotation motor and either is reliable enough towork on standard MBE machines. In addition to conven-tional recording techniques, azimuthal scans provide full 2Dsymmetry of the reciprocal lattice, a direct ratio of the trans-fer width of the instrument both parallel and perpendicular tothe beam, and access to large portions of reciprocal spacewith a single scan. These properties make azimuthal scansideal candidates for comparison with theoretical models ofRHEED.

ACKNOWLEDGMENTS

The authors would like to thank C.-H. Kuo for the MBEgrowth. This work was supported by DARPA under ContractNo. MDA972-95-1-0016.

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