Full text of DOI:10.1007/BF02665873

The Effect of High Energy Electrons During the Growth
Epitaxy
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Special Issue P
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The Effect of High Energy Electrons During the Growth of
ZnSe and ZnMgSe by Molecular Beam Epitaxy
B.L. VANMIL, A.J. PTAK, N.C. GILES, T.H. MYERS, P.J. TREADO,2
1
1
1
1,3
2
2
2
M.P. NELSON, J.M. RIBAR, and R.D. SMITH
1
2
.—West Virginia University, Department of Physics, Morgantown, WV 26506.
.—ChemIcon, Inc. Pittsburgh, PA 15208. 3.—e-mail: tmyers@wvu.edu
Electron irradiation during reflection high-energy electron diffraction is shown
to affect the growth of ZnSe and ZnMgSe by molecular beam epitaxy. The high-
energyelectronsproduceanelectronstimulateddesorptioneffectduringgrowth
of ZnSe which primarily affects adsorbed Se. Se desorption rates under electron
irradiation are shown to be significantly larger than thermal desorption rates.
Electron irradiation also decreases ZnSe growth rates under Zn-rich conditions.
The decrease in growth rate can be suppressed by either growth under Se-rich
conditions or by using high-index substrate orientations, in this case (211)B.
High-energy electron irradiation does not alter composition during the growth
of ZnMgSe.
Key words: ZnSe, ZnMgSe, reflection high energy electron diffraction,
electron stimulated desorption
by careful analysis using techniques such as spatial/
spectral reflectance imaging. This special issue paper
details the preliminary results of our investigation of
howhighenergyelectronirradiationaffectsthegrowth
of ZnSe and ZnMgSe.
INTRODUCTION
Reflectionhigh-energyelectrondiffraction(RHEED)
is one of the most useful and undoubtedly the most
widespread of the various techniques used for in-situ
monitoring during molecular beam epitaxy (MBE)
EXPERIMENTAL
1
–4
growth. While it was recognized early that RHEED
could affect growing material, it has proven to
be fairly benign for growth of GaAs and related
III-Vmaterials.Therehavebeenfewinvestigationsof
how RHEED actually influences growth kinetics,
impurity incorporation, or point defect formation.
ZnSe and ZnMgSe were grown by MBE at West
Virginia University. Details of the growth have been
7
described elsewhere. High-energy electron diffrac-
tion and irradiation studies were performed using a
VE-026 electron gun from Vieetech Japan Co. Ltd.
operating between 10 and 13 keV, with an electron
current of ~25 mA. Studies were done in a typical
RHEED geometry. RHEED images were captured
and analyzed using a Si charge-coupled device (CCD)
camera under the control of software from k-Space
Associates, Inc. (Ann Arbor, MI). Film growth rates
weremonitoredin-situusingtheinterferencepattern
observed in reflected light from a 680-nm diode laser.
A Janis Super-Varitemp Dewar was used to obtain
photoluminescencedatausing325nmexcitationfrom
a helium-cadmium laser with emitted light collected
in a near-backscattering geometry using a 0.64 m
grating spectrometer and a GaAs photomultiplier.
5
6
Farrell et al. and Wu et al. have reported that the
high-energy electron irradiation occurring during
RHEED affects the stoichiometry of static ZnSe and
CdTe surfaces, respectively. Electron irradiation was
alsoshowntoinfluencesurfacereconstructionduring
the growth of CdTe.6
We have recently observed that RHEED can have a
dramatic effect on the growth kinetics in ZnSe. A
stripe” associated with the position of the RHEED
beam can be seen on many samples, while the effect of
electronirradiationcanbeseenonothersamplesonly
“
(
Received December 5, 2000; accepted March 1, 2001)
785

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VanMil, Ptak, Giles, Myers, Treado, Nelson, Ribar, and Smith
Imaging reflectance experiments were performed at
ChemIcon, Inc. (Pittsburgh, PA).
the form of a stripe visible to the eye. Such a stripe is
readily apparent for the sample shown in Fig. 1a,
which is a reflectance micrograph taken at a wave-
length of 900 nm. Figure 1b shows the spectral reflec-
tance taken both inside and outside the RHEED
stripe. Analysis of the interference pattern indicates
a slight decrease in growth rate with electron irradia-
tion. This leads to a phase difference in the interfer-
ence of the reflected light in the two regions, and
provides the contrast making the stripe apparent.
Electron-stimulated desorption (ESD) effects have
THE EFFECT OF ELECTRON IRRADIATION
ON STATIC ZnSe SURFACES
During our studies of the growth of ZnSe, we often
observe features on the grown layers that correlate
with the location of the electron beam during RHEED
measurements. We typically grow on small samples,
1
.5 cm ¥ 1.5 cm, and have sacrificed sample rotation
for better knowledge and control of the sample tem-
perature during growth. If RHEED is performed
during growth, the samples contain a discoloration in
5
been previously reported by Farrell et al. for ZnSe,
6
and for CdTe by Wu et al. Electron irradiation was
shown to increase Se-desorption from ZnSe, and
Te-desorption from CdTe static surfaces. In particu-
lar, the Se-stabilized (100) ZnSe surface exhibits
either a (2¥1) or a (1¥1) reconstruction with an addi-
tional two-fold reconstruction along the [011] direc-
tion. The Zn-stabilized surface always exhibits a
8
c(2¥2) reconstruction. Thus, either the disappear-
ance of the reconstruction along [011] or the emer-
gence of the two-fold reconstruction diffraction along
the [010] direction can be used to monitor Se desorp-
tion. We chose to monitor the emergence of the [010]
reconstruction to indicate the attainment of a
Zn-stable surface. Approximately 0.3 mm of ZnSe was
grown under conditions known to produce high qual-
ity material. The growth was interrupted and the
static surface exposed to a Se-flux for about 5 sec. The
Se shutter was closed and the RHEED pattern was
monitored using a CCD camera. Figure 2 indicates
the times required after the Se shutter was closed for
the emergence of the [010] reconstruction pattern for
several substrate temperatures for surfaces under
electron irradiation. Each point represents the aver-
age of five measurements. The temperature-depen-
Fig. 1. (a) Reflectance image of a ZnSe layer at a wavelength of
Fig. 2. TimeintervalsrequiredforaSe-stablesurfacetoevolvetoaZn-
stable surface at various temperatures, with and without electron
irradiation.
900 nm, (b) spectral reflectance from the locations indicated by the
rectangles.

The Effect of High Energy Electrons During the Growth
of ZnSe and ZnMgSe by Molecular Beam Epitaxy
787
dence is indicative of a thermally-activated process,
with an activation energy between 0.8 and 1.0 eV,
possibly depending on electron energy. The desorp-
tion times for a strictly thermal process, one without
electron irradiation, are also shown. For the latter
case, the ZnSe surface was exposed to excess Se as
before at the various substrate temperatures, but
without the presence of electron irradiation. After a
time, the sample was probed with the electron beam
to see if a Zn-stable surface had emerged. The surface
was then exposed to Se once again to produce a
Se-stable surface. The process was repeated for vari-
ous waiting times prior to electron beam exposure to
bracket the Se thermal desorption time. The signifi-
cant difference in activation energy between the two
cases, 2.1 eV vs. ~1.0 eV indicates a large electron
stimulated desorption effect is present.
THE EFFECT OF ELECTRON IRRADIATION
DURING GROWTH OF ZnSe
The electron-stimulated desorption (ESD) of Se
could lead to a decrease in growth rate. To investigate
this, the RHEED beam was defocused to give a stripe
approximately 3 mm wide. The laser used for growth
rate measurements was focused to coincide with this
stripe. By turning the electron beam on and off while
monitoring the film thickness evolution, the effect on
growth rate could be determined at various substrate
temperatures as indicated in Fig. 3a for growth on a
(100) surface. While increasing the substrate tem-
perature lowers the growth rate for both Zn- and Se-
Fig. 3. ZnSe growth rates with and without electron irradiation for (a)
Zn-stable (100) growth, (b) Se-stable (100) growth and Zn-stable
Fig. 4. (a) Reflectance image of a ZnMgSe layer at a wavelength of
900 nm, (b) spectral reflectance from the locations indicated by the
rectangles.
(211)B growth.

7
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VanMil, Ptak, Giles, Myers, Treado, Nelson, Ribar, and Smith
stable growth, the presence of high energy electrons
leads to a much more pronounced decrease in growth
rate at higher temperatures for Zn-stable growth. A
similar effect was not observed for Se-stable growth,
as shown in Fig. 3b, consistent with the belief that the
ESD process is primarily affecting Se adatoms.
Farrell et al. determined a thermal activation en-
ergy of ~0.6 eV for the ESD process from 10 keV
electrons in ZnSe, considerably lower than our mea-
sured value of ~1.0 eV at the same energy. This
difference in activation energy may be related to the
fact that we used (100) substrates off-cut 2∞ towards
minimize the effects of electron irradiation for ZnSe.
RHEED stripes were observed on each sample.
Spatial/spectral reflectance imaging allowed mea-
surement of the thickness both inside and outside the
RHEED stripe on each sample. Typically thickness
changeswerelessthan2%,withthelargestmeasured
change around 8%. The magnitude of the thickness
changes ranged around 30 nm to 150 nm for ~2 mm
thick epilayers. Since this is an optical measurement,
the apparent thickness change could also be due to a
change in the index of refraction and thus composi-
tion. To check this latter point, measurements were
made using a surface profilometer that indicated
thickness variations of this order, but instrumental
resolutionlimitedquantitativecomparison.Low-tem-
perature photoluminescence measurements were
made on the RHEED stripes and surrounding area.
The resulting spectra were indistinguishable for each
sample, presenting strong evidence that no change in
composition occurred from electron irradiation. The
photoluminescenceresultisconsistentwithadsorbed
Se being the primary surface specie affected by high
energy electron irradiation.
[
011]withahigherstepdensitythatmayhavecaused
the Se to be more tightly bound to the surface. To
further test this idea, we investigated growth of ZnSe
on (211)B-oriented GaAs substrates. This orientation
does not exhibit differing reconstruction depending
on Zn or Se-stable conditions, precluding RHEED
desorption studies. However, as illustrated by the
data in Fig. 3b, there was not a measurable difference
in growth rate with and without electron irradiation
onthe(211)Borientationupto375∞CunderZn-stable
conditions, indicating the effects of ESD were much
less pronounced for this highly-vicinal orientation.
SUMMARY
5
Farrell et al. attribute the ESD process to the
generation of holes in the ZnSe, which, according to
The results reported here clearly show that high-
energy electron irradiation can affect the growth of
compound semiconductors. A significant electron
stimulated desorption effect is observed for Se during
the growth of ZnSe. While the effects on growth rate
can be minimized either by growing under Se-stable
conditions or on high index orientations, this may not
give the best conditions for all types of structures. In
addition, it is not yet clear how electron irradiation
affectspointdefectformation.Sincehigh-energyelec-
trons can distinctly alter surface reconstruction and
surface chemistry, RHEED measurements must be
carefully considered on a case-by-case basis in order
to not misinterpret the observations.
9
Marfaing, will affect the more electronegative sur-
10
face specie, in this case Se. Simpson et al. argue that
this same process is the underlying origin of photon-
stimulated effects also observed for above bandgap
light illumination during growth. The observations
reported here do not contradict this proposed mecha-
nism, but also do not unambiguously resolve the
underlying processes. In addition, there is little dis-
cussion in the literature on how the ESD process
affects point defect formation, which is important for
doping of ZnSe. We are extending the studies of the
work on ZnSe reported in this special issue paper to
address these two issues, with particular emphasis to
see if RHEED affects the generation of point defects
in II-VI semiconductors.
ACKNOWLEDGEMENTS
This work was supported by the National Science
Foundation (Grant No. DMR-9806299).
THE EFFECT OF ELECTRON IRRADIATION
DURING GROWTH OF ZnMgSe
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not be the same magnitude for both Zn and Mg. Thus,
a compositional change with high-energy electron
irradiationwouldindicateanESDeffectforthegroup
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