Reflection high-energy electron diffraction during substrate rotation: A new dimension
for in situ characterization
W. 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) surface
reconstructions
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

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 209.147.144.20 On: Fri, 13 Feb 2015 01:30:40

Reflection high-energy electron diffraction during substrate rotation: A new
dimension 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. Möller
Fraunhofer Institut für Integrierte Schaltungen-A, D-91058 Erlangen, Germany

Y.-H. Zhang
Center 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 a
continuously changing RHEED pattern is realized by triggering the substrate rotation and it allows
analysis of quasistatic RHEED patterns that are updated every revolution. At the same time, the
intensity along a line parallel to the shadow edge can be used to reconstruct a planar cut through the
reciprocal lattice similar to a low-energy electron diffraction pattern. This RHEED pattern directly
reveals 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 electron
diffraction ~RHEED! system, the information contained in
the diffraction pattern is generally not used during sample
rotation. This limits the use of RHEED as a true in situ
technique. Most layers for device or scientific applications
have to be grown with substrate rotation because layer uniformity is of paramount importance. It is therefore highly
desirable to develop methods that allow access to the reciprocal lattice of rotating substrates.
Several approaches using RHEED during rotation have
focused on the specular spot shape and intensity during substrate rotation.1–4 The results demonstrate that it is possible
to extract the growth rate by extracting the RHEED oscillation signal. These approaches are either based on intensity
measurements combined with Fourier filtering,1,5 and high
frequency rotation2 or measurements of the specular spot
size combined with Fourier filtering.3 Even if the RHEED
oscillations are not evaluated, the specular spot intensity varies depending on the growth conditions used and can be used
to characterize MBE growth.4
RHEED measurements on nonrotating substrates, apart
from growth rate determination6 and characterization of
interfaces7 using RHEED oscillations, also use the information contained in the geometry and shape of the diffraction
streaks to assess the surface structure in MBE.8 In this work,
we examine ways of accessing this diffraction geometry information during substrate rotation. We demonstrate two
methods that exploit the rotating substrate to obtain reciprocal lattice scans that are impossible with a static RHEED
experiment. These new modes of operation expand the capaa!

Electronic mail: braun@enpop2.eas.asu.edu

1507

J. Vac. Sci. Technol. B 16„3…, May/Jun 1998

bilities of RHEED beyond the scope of traditional RHEED
experiments and allow assessment of almost the entire accessible reciprocal space.
II. EXPERIMENT
The experiments were performed using a DCA MBE system equipped with a low wobble substrate manipulator, a
standard RHEED e-gun operated at 20 kV and a P20 phosphorus screen. Intensities were recorded at a rate of 50 Hz
using a charge coupled device ~CCD! camera and PC-based
analysis system.9 A straightforward way to obtain static
RHEED images from a rotating substrate is a triggered collection of images along high symmetry azimuths during rotation. The CCD camera taking images of the RHEED screen
and the rotation motor are therefore interlocked via trigger
signals. Images can then be extracted at arbitrary azimuths
during the continuous rotation. The windows displaying the
results for the different azimuths are updated once per revolution, allowing a quasisimultaneous display of the different
quasistatic displays. An example of the implementation in
Fig. 1 shows the capture of the four in-plane ^110& azimuths
of a ~001! GaAs b (234) reconstructed surface at 580 °C.
The source area on the live image is marked by a bright
frame 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 in
the two patterns along the @11̄0# azimuth ~43, right! is different, indicating substrate wobble with a misorientation towards @11̄0#. For the 20 keV electrons used in this measurement the distance between the ~00! and ~01! streaks
corresponds to an angle of 1.23°. This yields a value of 0.5°
for the substrate wobble. The cyclic recording of images
along several azimuths corresponds to the situation of several RHEED guns installed at the corresponding angles. It

0734-211X/98/16„3…/1507/4/$15.00

©1998 American Vacuum Society

1507

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 209.147.144.20 On: Fri, 13 Feb 2015 01:30:40

1508

Braun, Möller, and Zhang: RHEED during substrate rotation

1508

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 indicated
by a bright frame in the background image. The measurement was made at 580 °C on a GaAs~001! surface reconstructed in the b (234) structure using 20
keV electrons. Left: @110# and @1̄1̄0# azimuths, right: @1̄10# and @11̄0# azimuths.

therefore allows a very detailed assessment of the reciprocal
lattice. All the standard measurements like evolution of intensities, peak shapes, etc. can be performed with a maximum rate corresponding to the revolution frequency by subsequent analysis extracting data from the individual displays.
The triggered recording of images during rotation allows
access to several discrete cuts of the reciprocal lattice. A
complete scan of the accessible reciprocal space can be
achieved by recording continuously during sample rotation.
The basic diffraction geometry of RHEED is shown in Fig.
2~a!. For any incidence angle u, the Ewald sphere construction requires both k 0 and k 8 to end on a reciprocal lattice rod

FIG. 2. Reciprocal space geometry for ~a! static RHEED and ~b! during
rotation. The shaded area in ~b!, rotated around ~00!, represents the volume
of reciprocal space accessible during a single sample rotation. The only
surface parallel plane without a void in the center, the specular plane, intersects the specular spot and is indicated by a dashed line.

for diffraction to occur. During sample rotation, the reciprocal lattice revolves around the ~00! rod, while the Ewald
sphere stays fixed in space if we neglect wobble. The Ewald
sphere therefore scans almost the entire upper half of reciprocal space as indicated by the shaded area in Fig. 2~b!. The
scanned volume is largest for the smallest incidence angles
u, where the wave vector component perpendicular to the
surface is smallest, ensuring high surface sensitivity. This
allows us to reconstruct practically the entire upper half of
reciprocal space during one sample revolution.
In a practical situation, this is not possible in real time due
to the lack of capability of current computers. We therefore
presently restrict ourselves to a plane parallel to the surface
and choose the plane containing the specular spot, since it is
the only plane that is continuous across the ~00! rod. We
denote this plane as the specular plane ~SP!. Its position is
indicated in Fig. 2~b! by a dashed line. The measurement
takes place by recording the intensities along a line parallel
to the shadow edge through the specular spot as shown in
Fig. 3~a!. The specular plane is then constructed by plotting
the lines bent with radius r and the specular spot centered on
~00! according to their azimuthal angle. This is schematically
plotted in Fig. 3~b!. The radius r is given by r5k 0 sin u
given by the geometry in Fig. 2~a!. As the rotation is completed, the lines add up to a complete cut through the reciprocal lattice at a distance 2 k 0 cos u from the origin of the
reciprocal lattice. Such a cut is similar to a low-energy electron diffraction ~LEED! pattern and represents the twodimensional ~2D! symmetry of the surface reconstruction in
a top view. By varying the incidence angle, a wide range of

J. Vac. Sci. Technol. B, Vol. 16, No. 3, May/Jun 1998

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 209.147.144.20 On: Fri, 13 Feb 2015 01:30:40

1509

Braun, Möller, and Zhang: RHEED during substrate rotation

1509

FIG. 3. Measurement geometry and top view of the azimuthal scan. Experimental conditions in ~a! are similar to those in Fig. 1, close to the @11̄0#
azimuth. The intensity distribution in the specular plane is obtained by plotting the lines according to their azimuthal angle with the specular spot
centered at ~00! and radius r ~see Fig. 2!.

wave vectors perpendicular to the surface can be measured,
similar to a I – V measurement in LEED.
III. RESULTS
To demonstrate the method, we have investigated the
b (234) and c(434) surface reconstructions of GaAs~001!.
The accuracy in both cases was impeded by charging effects
close to the RHEED screen leading to a distorted RHEED
pattern and unsteady rotation that resulted from the gears not
being driven precisely by a magnetically coupled
feedthrough. Both factors contributed to a slight distortion of
the processed scan. The results are shown in Fig. 4 for the
b (234) and in Fig. 5 for the c(434) surface reconstructions, respectively. The rotation speed was approximately 3
rpm. The intensity profiles were recorded at a rate of 50 Hz,
resulting in an angular resolution of about 0.4°. The inci-

FIG. 5. Azimuthal scan of a GaAs c(434)-reconstructed surface. The main
azimuths and the surface unit cell are indicated.

dence angle u was 1.5° using 20 keV electrons. In both images, the underlying 131 mesh of the surface reciprocal
lattice is indicated by a white square. The scan spans more
than six unit cells in each direction. This size is only limited
by the electron energy ~higher electron energy E reduces the
distance between the reciprocal lattice rods ;E 21/2! and the
size of the RHEED screen. In our setup, this limit was approximately 16 unit cells for GaAs. The eight in-plane ^001&
and ^110& azimuths can easily be identified by the adjacent
high intensity radial streaks. These correspond to surface
resonance conditions10–12 where the Laue circle tangentially
touches reciprocal lattice rods, leading to increased reflectivity.
Spots in the azimuthal scan correspond to intersections of
reciprocal lattice rods with the specular plane. The spots are
of elliptic shape with the longer axis along the radial direction. The anisotropy is a direct measure of the different transfer widths t ~Ref. 13! of RHEED parallel and perpendicular
to the beam that are not proportional to the normalized full
width at half maximum ~FWHM! of the spot in the respective direction:
t5

FIG. 4. Azimuthal scan of a GaAs b (234)-reconstructed surface. The main
azimuths and the surface unit cell are indicated.

a i jgi j
,
h

where h denotes the FWHM of the spot and a i j and g i j are
the real and reciprocal lattice constants in the direction the
FWHM is measured. In Fig. 4, the b (234) structure is
clearly identified by the rows of closely spaced spots directed
along @110#. This distinguishes it from the c(434) structure
in Fig. 5 that exhibits a centered mesh. A display of the full
2D symmetry of the reciprocal lattice allows an immediate
distinction, e.g., between c(434) and (232) structures to

JVST B - Microelectronics and Nanometer Structures

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 209.147.144.20 On: Fri, 13 Feb 2015 01:30:40

1510

Braun, Möller, and Zhang: RHEED during substrate rotation

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 fourfold
rotational symmetry. If we look, for example, at the spots in
the row directly adjacent to the @110# direction, we observe
high intensity on the second, third, and fifth spots from ~00!.
In the corresponding row-along @11̄0#, he third spot is very
weak. This deviation from the fourfold, symmetry is present
even on (131) reconstructed surfaces and allows identification of the two different azimuths even if the static RHEED
patterns along both azimuths differ only very little. It is due
to asymmetries in the structure like the orientation of dimers.
Therefore, azimuthal scans are promising candidates to compare against theoretical models. The ease of acquisition together with the simple geometry allow the acquisition of
highly accurate data. At the same time, the access to a large
region of reciprocal space spanning many unit cells makes
the comparison with models easier since many points can be
fitted simultaneously. Since in the kinematical approximation
the structure of reciprocal space far from the ~00! rod contains the most detailed information about the atomic
positions,14 even structure optimization seems feasible and
can be expected to be more reliable than previous optimizations using data sets obtained from conventional RHEED
scans.15–18
The comparison of Figs. 4 and 5 suggests a reliable
method for in situ control of surface reconstruction. If the
growth process requires the surface to maintain a certain reconstruction, the respective spots in the azimuthal scan can
be monitored during rotation. The two-dimensional nature of
the measurement permits the simultaneous monitoring of
many spots, resulting in increased accuracy and allowing
consistency checks. In this way, a control algorithm could,
for example, distinguish whether the desired surface reconstruction changes to the adjacent low- or high-temperature
configuration if it controlled the sample temperature and
change it accordingly.
IV. SUMMARY
In this work, we presented new ways to access reciprocal
space during substrate rotation. Both gated detection and azi-

1510

muthal scans provide direct access to the surface reconstruction in real time during growth, allowing in situ control of
the growth front. Neither approach requires modifications to
the MBE machine except for the generation of a trigger signal by the rotation motor and either is reliable enough to
work on standard MBE machines. In addition to conventional recording techniques, azimuthal scans provide full 2D
symmetry of the reciprocal lattice, a direct ratio of the transfer width of the instrument both parallel and perpendicular to
the beam, and access to large portions of reciprocal space
with a single scan. These properties make azimuthal scans
ideal candidates for comparison with theoretical models of
RHEED.
ACKNOWLEDGMENTS
The authors would like to thank C.-H. Kuo for the MBE
growth. This work was supported by DARPA under Contract
No. MDA972-95-1-0016.
G. W. Turner and A. J. Isles, J. Vac. Sci. Technol. B 10, 1784 ~1992!.
J. P. A. van der Wagt and J. S. Harris, Jr., J. Vac. Sci. Technol. B 12,
1236 ~1994!.
3
D. A. Collins, G. O. Papa, and T. C. McGill, J. Vac. Sci. Technol. B 13,
1953 ~1995!.
4
R. F. Kromann, R. N. Bicknell-Tassius, A. S. Brown, J. F. Dorsey, K.
Lee, and G. May, J. Cryst. Growth 175/176, 334 ~1997!.
5
G. W. Turner, B. A. Nechay, and S. J. Eglash, J. Vac. Sci. Technol. B 8,
283 ~1990!.
6
J. J. Harris, B. A. Joyce, and P. J. Dobson, Surf. Sci. 103, L90 ~1981!.
7
W. Braun, A. Trampert, L. Däweritz, and K. H. Ploog, Phys. Rev. B 55,
1689 ~1997!.
8
S. Ino, Reflection High-Energy Electron Diffraction and Reflection Electron Imaging of Surfaces, edited by P. K. Larsen and P. J. Dobson ~Plenum, New York, 1988!, p. 3.
9
‘‘Safire’’ by VTS Schwarz/FhG Erlangen.
10
A. Ichimiya, K. Kambe, and G. Lehmpfuhl, J. Phys. Soc. Jpn. 49, 684
~1980!.
11
G. Meyer-Ehmsen, in Ref. 8, p. 99.
12
L.-M. Peng, J. Cowley, and N. Yao, Ultramicroscopy 26, 189 ~1988!.
13
M. Henzler, Appl. Surf. Sci. 11/12, 450 ~1982!.
14
W. Braun, O. Brandt, M. Wassermeier, L. Däweritz, and K. H. Ploog,
Appl. Surf. Sci. 104/105, 35 ~1996!.
15
Y. Horio and A. Ichimiya, Surf. Sci. 219, 128 ~1993!.
16
Y. Ma, S. Lordi, P. K. Larsen, and J. A. Eades, Surf. Sci. 289, 47 ~1993!.
17
J. M. McCoy, U. Korte, P. A. Maksym, and G. Meyer-Ehmsen, Surf.
Sci. 306, 247 ~1994!.
18
Y. Ma, S. Lordi, P. K. Larsen, and J. A. Eades, Surf. Sci. 306, 252 ~1994!.
1
2

J. Vac. Sci. Technol. B, Vol. 16, No. 3, May/Jun 1998

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 209.147.144.20 On: Fri, 13 Feb 2015 01:30:40