CdSe/CdTe type-II superlattices grown on GaSb (001) substrates by molecular beam
epitaxy
Jing-Jing Li, Xinyu Liu, Shi Liu, Shumin Wang, David J. Smith, Ding Ding, Shane R. Johnson, Jacek K. Furdyna,
and Yong-Hang Zhang
Citation: Applied Physics Letters 100, 121908 (2012); doi: 10.1063/1.3697676
View online: http://dx.doi.org/10.1063/1.3697676
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/12?ver=pdfcov
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APPLIED PHYSICS LETTERS 100, 121908 (2012)

CdSe/CdTe type-II superlattices grown on GaSb (001) substrates
by molecular beam epitaxy
Jing-Jing Li,1,2 Xinyu Liu,3 Shi Liu,1,2 Shumin Wang,1,2 David J. Smith,1,4 Ding Ding,1,2
Shane R. Johnson,1,2 Jacek K. Furdyna,3 and Yong-Hang Zhang1,2,a)
1

Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287, USA
School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe,
Arizona 85287, USA
3
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA
4
Department of Physics, Arizona State University, Tempe, Arizona 85287, USA
2

(Received 21 February 2012; accepted 5 March 2012; published online 22 March 2012)
CdSe/CdTe superlattices are grown on GaSb substrates using molecular beam epitaxy. X-ray
diffraction measurements and cross-sectional transmission electron microscopy images indicate
high crystalline quality. Photoluminescence (PL) measurements show the effective bandgap varies
with the superlattice layer thicknesses and confirm the CdSe/CdTe heterostructure has a type-II
band edge alignment. The valence band offset between unstrained CdTe and CdSe is determined as
0.63 6 0.06 eV by fitting the measured PL peak positions using the envelope function
approximation and the Kronig-Penney model. These results suggest that CdSe/CdTe superlattices
are promising candidates for multi-junction solar cells and other optoelectronic devices based on
C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697676]
GaSb substrates. V
Monolithic integration of II-VI (MgZnCd)(SeTe) and
III-V (AlGaIn)(AsSb) semiconductors on commercially
available GaSb or InAs substrates has been proposed as a
platform for many electronic and optoelectronic devices.1–4
These two families of semiconductor materials have identical crystalline structures and very similar thermal expansion
coefficients, and they can be grown lattice-matched on GaSb
or InAs substrates with low defect densities. Furthermore,
these materials are ideal for multi-junction solar cells, as
their direct bandgaps allow the use of thin absorbing layers
that can be current-matched to efficiently capture the entire
solar spectrum. Several multi-junction solar cell structures
have been designed with the potential to reach ultra-high
conversion efficiency.1,4
In the proposed solar cell designs, a zinc blende CdSeTe
alloy lattice matched to the GaSb substrates is used as one of
the subcells. However, there have been only a few successful
attempts reported at growing zinc-blende CdSeTe random
alloys on Si substrates, and it is difficult to achieve large Se
compositions because of the high vapor pressure of group VI
elements and the low sticking probability of Se atoms.5–8 In
the current work, CdSe/CdTe superlattices are proposed as an
alternative to the random CdSeTe alloys, and a series of these
superlattice structures are grown on GaSb (001) substrates
using molecular beam epitaxy (MBE). While the ZnSe/ZnTe
system has been intensively investigated,9,10 little work has so
far been done for the CdSe/CdTe system.11–13 In particular,
there was no experimental papers reporting the properties of
CdSe/CdTe superlattices or the valence band offset between
CdSe and CdTe. In order to address these issues, this work
reports the growth and the structural and optical characterization of a series of CdSe/CdTe superlattices.
The epitaxial growth of the superlattice samples was
carried out using a dual-chamber Riber 32P MBE system
a)

Electronic mail: yhzhang@asu.edu.

0003-6951/2012/100(12)/121908/3/$30.00

consisting of III-V and II-VI chambers connected by an ultra
high-vacuum transfer module. The structures were grown on
undoped epi-ready GaSb (001) substrates. First, the substrate
oxide was thermally removed by heating up to 510  C under
an antimony flux in the III-V chamber, while the process was
monitored using reflection high-energy electron diffraction
(RHEED). Next, a 100 nm GaSb buffer layer was grown at
470  C. The substrate was then transferred to the II-VI chamber, where a ZnTe buffer layer was grown at 320  C under a
Zn flux initiated prior to the growth.2 The CdSe/CdTe superlattice was then grown at 320  C using the modulation of Te
and Se shutters while keeping the Cd shutter open. During
the growth, the beam equivalent pressure (BEP) ratio of Te/
Cd and Se/Cd were kept at 2:1 and 4:1, respectively.
Three CdSe/CdTe superlattice structures (labeled A, B,
and C) were grown with the various layer thicknesses shown
in Table I and are expected to have different effective
bandgap energies. During the growth of sample B, the Cd
shutter remained open for 5 s while all the other shutters
were closed after the growth of the CdSe layer, in order to
reduce the intermixing of Se and Te at the CdSe/CdTe interface. The RHEED patterns of samples A and C changed
from streaky to spotty during the growth, suggesting a 3D
growth mode at the end of the growth. In contrast, the
RHEED pattern of sample B remained streaky through to the
end, which suggests that this sample has better crystalline
quality and layer uniformity than the other two samples.
Note that all the CdSe/CdTe superlattices have the same zinc
blende structure as the substrate, as shown by the RHEED
patterns and confirmed by the x-ray diffraction (XRD) and
transmission electron microscopy (TEM) observations.
High-resolution XRD measurements of the samples
were performed for the symmetric (004) and asymmetric
(113) reflections using the Cu Ka1 radiation on a PANalytical X’Pert PRO Materials Research Diffractometer. Figures
1(a)–1(c) show the (004) x-2h diffraction patterns of

100, 121908-1

C 2012 American Institute of Physics
V

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Li et al.

Appl. Phys. Lett. 100, 121908 (2012)

TABLE I. Structures and optical transition energies of the studied samples.
Thickness (nm)
Sample

a// (Å)

a\ (Å)

CdSe

CdTe

PL peak position at 10 K
(eV)

Calculated Eg at 0 K
(eV)

A
B
C

6.103
6.097
6.096

6.101
6.120
6.096

10.52
5.47
5.99

1.51
0.88
0.75

1.04
1.18
1.30

1.05
1.22
1.26

FIG. 1. X-ray (004) x/2h diffraction patterns of samples A (a), B (b), and
C (c).

samples A, B, and C, which respectively consists of 40, 40,
and 50 periods of CdTe and CdSe layers. The lattice constants of GaSb and ZnTe are 6.096 and 6.105 Å, respectively,
and the lattice constants of CdSe and CdTe are 6.052 and
6.482 Å, respectively. Therefore, the CdTe layer is compres-

FIG. 2. Structural characterization of sample B using TEM. (a) Crosssectional image showing regular superlattice periodicity and the absence of
major structural defects, as confirmed by the selected area electron diffraction pattern shown in the inset. (b) High-resolution lattice image showing
excellent crystallinity.

sively strained to the GaSb substrate or the ZnTe buffer
layer, while the CdSe layer is tensilely strained. The high
structural qualities of the samples are evidenced by the sharp
satellite peaks and the absence of appreciable peak broadening. The lateral and average vertical lattice constants a// and
a\ of the superlattices shown in Table I are determined from
the zeroth superlattice peaks in the (004) and (113) XRD patterns, and the layer thicknesses are determined from the separation of the first-order superlattice peaks in the (004) XRD
patterns. Sample A has a ZnTe buffer layer of approximately
200 nm, and the superlattice is strained as shown by its lateral and vertical lattice constants. The ZnTe buffer layers of
samples B and C are about 10 nm thick, and the superlattice
of sample B is strained while that of sample C is almost perfectly lattice matched to the substrate. It can also be seen in
Fig. 1 that the superlattice peaks of sample B have the narrowest FWHM, indicating that this sample has minimal
interface roughness as well as the fewest defects.
Specimens of sample B suitable for cross-sectional
TEM observation were prepared by standard mechanical polishing, dimpling, and a final argon-ion-milling at reduced
energy (2-2.5 keV), with the sample being held at liquidnitrogen temperature to minimize artifacts due to thermal or
ion-beam damage during milling. The low magnification
image shown in Fig. 2(a) demonstrates the overall high
structural-quality and regularity of the CdSe/CdTe superlattice. In addition, the satellite diffraction spot adjacent to the
major diffraction spot visible in selected area electron diffraction pattern, as shown in the inset of Fig. 2(a), confirms
the uniformity of the layer thicknesses. The high-resolution
lattice image of this same specimen shown in Fig. 2(b)

FIG. 3. Photoluminescence spectra of samples A, B, and C measured at
10 K. The difference in the PL peak positions is a result of the different layer
thicknesses in each superlattice.

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121908-3

Li et al.

Appl. Phys. Lett. 100, 121908 (2012)

TABLE II. Material parameters of CdSe and CdTe used in the Kronig-Penney model.
a (Å)

Eg (eV)

me

mzhh

mzlh

CdSe
CdTe

6.052 (Ref. 12)
6.482 (Refs. 12 and 24)

1.76 (Refs. 12 and 20)
1.61 (Refs. 12, 16, and 24)

0.12 (Ref. 15)
0.088 (Ref. 17)

0.83 (Ref. 19)
0.53 (Refs. 17 and 18)

0.13 (Ref. 19)
0.11 (Refs. 17 and 18)

CdSe
CdTe

av (eV)25
1.81
2.14

b (eV)23
0.8
1

demonstrates very-high crystalline-quality at the atomic
length scale.
It has been reported that the unstrained CdTe and CdSe
forms a type-II band edge alignment11,12 with the respective
bandgap energies 1.76 eV (Refs. 12 and 20) and 1.61 eV
(Refs. 12, 16, and 24) at 10 K. In the superlattice structures
studied here, the band edges are modified by a combination
of the hydrostatic and shear strains. The hydrostatic strain
shifts the conduction band and valence band edges, while the
shear strain splits the heavy hole and light hole bands. The
superlattice minibands are formed as the hybridization of the
bound states in the quantum wells coupled through the
barriers.
To study the impact of band alignment on the proposed
superlattice materials, photoluminescence (PL) measurements
of samples A, B, and C were performed using a 660-nm diode
laser with an excitation power density of 180 W/cm2. Photoluminescence is observed at both room temperature and low
temperature. The 10 K measurement results plotted in Fig. 3
show that the PL spectra of the three samples peak at different
wavelengths due to the different superlattice period of each
sample. The ground state transition energy of sample B shows
the strongest PL intensity and the narrowest FWHM. Furthermore, due to the type-II band edge alignment between CdTe
and CdSe (see Fig. 3 inset), the PL peak energies are considerably lower than the bandgap energy of either CdTe or CdSe.
The conduction and valence band ground state energy
levels of the three samples were calculated using the KronigPenney model,14 with the layer thicknesses given in Table I,
the material parameters shown in Table II, and the valence
band offset DEV as a fitting parameter. The valence band offset DEV determined by fitting the calculated ground state
transition energies to the measured PL peak positions is
0.63 6 0.06 eV, which agrees with the theoretical prediction
of 0.57 eV (Ref. 12) within the experimental error. The calculated ground state transition energies of the samples are in
reasonable agreement with the measured PL peak positions
as shown in Table I.
In conclusion, a series of CdSe/CdTe superlattices with
different layer thicknesses are grown on GaSb substrates by
MBE. The superlattices exhibit high structural quality as
shown by the sharp satellite peaks in high-resolution XRD
patterns, smooth interfaces in high-resolution TEM micrographs, and clear electron diffraction patterns. PL measurements show that the ground state transition energies of the
superlattices are smaller than the bandgap of either constituent material and hence establish the existence of a strong

C11 (1011 dyne/cm2)
8.8 (Ref. 22)
5.35 (Refs. 21 and 24)

ac (eV)25
3.77
5.09

C12 (1011 dyne/cm2)
5.3 (Ref. 22)
3.68 (Refs. 21 and 24)

type-II band alignment. The type-II valence band offset
between unstrained CdSe and CdTe is determined as
0.63 6 0.06 eV by fitting the PL peak positions of superlattices with different layer thicknesses to the ground state transition energies calculated using the Kronig-Penney model.
This work is partially supported by the Science Foundation Arizona, Contract No. SRG 0339-08, the Air Force
Research Laboratory/Space Vehicles Directorate, Contract
No. FA9453-08-2-0228, and by NSF grant ECCS-1002072.
Jing-Jing Li would like to thank Jin Fan for discussion of
XRD data analysis.
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