Molecular beam epitaxial growth of high-reflectivity and broad-bandwidth ZnTe/GaSb
distributed Bragg reflectors
Jin Fan, Xinyu Liu, Lu Ouyang, Richard E. Pimpinella, Margaret Dobrowolska, Jacek K. Furdyna, David J. Smith,
and Yong-Hang Zhang
Citation: Journal of Vacuum Science & Technology B 31, 03C109 (2013); doi: 10.1116/1.4793475
View online: http://dx.doi.org/10.1116/1.4793475
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Molecular beam epitaxial growth of high-reflectivity and broad-bandwidth
ZnTe/GaSb distributed Bragg reflectors
Jin Fan
Center for Photonics Innovation, Arizona State University,Tempe, Arizona 85287 and School of Electrical,
Computer, and Energy Engineering, Arizona State University, Tempe, Arizona 85287

Xinyu Liu
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556

Lu Ouyang
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and Department of Physics,
Arizona State University, Tempe, Arizona 85287

Richard E. Pimpinella, Margaret Dobrowolska, and Jacek K. Furdyna
Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556

David J. Smith
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and Department of Physics,
Arizona State University, Tempe, Arizona 85287

Yong-Hang Zhanga)
Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287 and School of Electrical,
Computer, and Energy Engineering, Arizona State University, Tempe, Arizona 85287

(Received 15 December 2012; accepted 12 February 2013; published 25 February 2013)
This paper reports the molecular beam epitaxial growth and characterization of high-reflectivity
and broad-bandwidth distributed Bragg reflectors (DBRs) made of ZnTe/GaSb quarter-wavelength
(k/4) layers for optoelectronic applications in the midwave infrared spectral range (2–5 lm). A
series of ZnTe/GaSb DBRs has been successfully grown on GaSb (001) substrates using molecular
beam epitaxy (MBE). During the MBE growth, a temperature ramp was applied to the initial
growth of GaSb layers on ZnTe to protect the ZnTe underneath from damage due to thermal
evaporation. Post-growth characterization using high-resolution x-ray diffraction, atomic force
microscopy, and transmission electron microscopy reveals smooth surface morphology, low defect
density, and coherent interfaces. Reflectance spectroscopy results show that a DBR sample of
seven k/4 pairs has a peak reflectance as high as 99.0% centered at 2.56 lm with a bandwidth of
C 2013 American Vacuum Society. [http://dx.doi.org/10.1116/1.4793475]
517 nm. V
I. INTRODUCTION
Midwave infrared (MWIR) laser diodes are of much interest for gas sensing and spectroscopic applications due to
the strong absorption lines of various gases in the MWIR
spectral range from 2 to 5 lm. It has been found that
vertical-cavity surface-emitting lasers (VCSELs) are desirable laser sources offering many advantages, such as small
beam divergence, single-mode operation, low threshold, and
ease of monolithic integration with microelectromechanical
system structure for wide wavelength tunability.1–4 Much
research has been done on InP- and GaSb-based VCSELs
emitting in the near-infrared (NIR) and MWIR range.5–8
While InP-based heterostructures have shown an emission
wavelength limit up to 2.3 lm,9 GaSb-based heterostructures
allow coverage of a major part of the 2–4 lm range. Typical
VCSELs use high-reflectivity mirrors, usually in the form of
distributed Bragg reflectors (DBRs). However, it has been
found that almost all III-V semiconductors lattice-matched
to InP or GaSb have very small refractive index contrast,
which makes it challenging to achieve high reflectivity using
thin DBRs to reduce the VCSEL threshold current density. It
a)

Electronic mail: yhzhang@asu.edu

03C109-1 J. Vac. Sci. Technol. B 31(3), May/Jun 2013

is, therefore, highly desirable to develop a DBR structure
lattice-matched to GaSb, which has high material quality,
large refractive index contrast, and broad wavelength tunability, for monolithically integrated tunable VCSELs.
A DBR structure consisting of ZnTe and GaSb k/4 layers
has recently been proposed for the use in MWIR VCSELs
(Ref. 10) and other optoelectronic applications. The ZnTe
and GaSb semiconductors have lattice constants very close
to 6.1 Å and a lattice mismatch of only 0.13%, which potentially offers unlimited degrees of freedom for monolithically
integrating the ZnTe/GaSb DBR structure with other GaSbbased materials without generating large amounts of misfit
dislocations, thus ensuring the best possible materials quality. More importantly, GaSb and ZnTe have a large refractive index contrast in the MWIR range [for example,
Dn ¼ 1.18 at 0.6 eV (Ref. 11)], which is significantly higher
than that of other widely used DBRs in the MWIR range, as
shown in Table I.9,10,12–14 It was previously demonstrated
that the ZnTe/GaSb DBRs could achieve a peak reflectivity
of 99.0% centered at 2.5 lm with 480-nm-wide stopband.10
In this paper, the growth of ZnTe/GaSb DBRs using
molecular beam epitaxy (MBE) is described. Post-growth
characterization using x-ray diffraction (XRD), atomic force
microscopy (AFM), transmission electron microscopy

2166-2746/2013/31(3)/03C109/4/$30.00

C 2013 American Vacuum Society
V

03C109-1

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03C109-2 Fan et al.: MBE growth of high-reflectivity and broad-bandwidth ZnTe/GaSb distributed Bragg reflectors
TABLE I. Comparison between commonly used DBRs in NIR/MWIR
VCSELs and ZnTe/GaSb DBRs.9,10,12–14

DBR materials
InGaAsP/InP (Ref. 13)
AlGaInAs/InP (Ref. 13)
AlGaAsSb/AlAsSb (Ref. 13)
AlAs/GaAs (Ref. 13)
AlGaAsSb/AlAsSb (Ref. 12)
AlGaAsSb/InP (Ref. 12)
AlGaInAs/InP (Ref. 12)
AlGaInAs/AlInAs (Ref. 12)
GaInAsP/InP (Ref. 12)
InGaAs/InAlAs (Ref. 9)
AlAsSb/GaSb (Ref. 14)
ZnTe/GaSb (Ref. 10)

TABLE II. List of the ZnTe/GaSb DBR samples with MBE run number and
the numbers of k/4 pairs.
Sample number

Refractive
index
contrast
(Dn)

Number of
pairs needed
for R ¼ 99.9%
(N)

Lasing
wavelength
(lm)

0.18
0.19
0.49
0.50
0.44
0.43
0.34
0.30
0.27
0.27
0.60
1.18

65
63
27
23
31
33
38
43
51
30
22
10

1.3
1.3
1.3
1.3
1.55
1.55
1.55
1.55
1.55
2.3
2.5
2.3–2.5

(TEM), and reflectance spectroscopy are carried out to comprehensively study the material properties of the ZnTe/GaSb
DBRs.
II. EXPERIMENTAL DETAILS
A series of ZnTe/GaSb DBR samples with different numbers of k/4 pairs was designed and grown using an MBE system consisting of two separate II-VI and III-V growth
chambers connected by an ultrahigh-vacuum transfer module. The vacuum of the transfer chamber was kept below
5  109 Torr, preventing any cross contamination during
sample transfer. The DBR structures were grown on a GaSb
(001) substrate with a buffer. The substrate temperatures
were measured using a thermocouple on the back of the substrate holder. During growth, the ZnTe epilayers were deposited at 320  C in the II-VI chamber while the GaSb epilayers
were grown in the III-V chamber.
The normal growth temperature for GaSb (470  C) is
much higher than that for ZnTe (320  C). Therefore, the
interface between ZnTe and GaSb could be severely damaged if the initial growth of GaSb on ZnTe is carried out at
such a high temperature. To address this problem, a temperature ramp was introduced while GaSb was being initially deposited on ZnTe.15 The growth temperature was gradually
ramped up from 360 to 470  C at a rate of 33  C/min to protect the ZnTe layer surfaces and to achieve high material
quality.15 The wafers were transferred repeatedly between
the II-VI and III-V chambers to complete the entire DBR
structure. The growth parameters for the samples described
in this paper, including growth temperatures, temperature
ramping rate, beam equivalent pressure ratios, and growth
rates, were reported previously.15 The number of k/4 pairs
for each grown sample is summarized in Table II.
High-resolution XRD x-2h scans were carried out on a
PANalytical X’Pert PRO MRD x-ray diffractometer with
multicrystal monochromator. Surface morphology was studied using AFM. Cross-sectional TEM studies were carried out
using a JEM-4000EX high-resolution electron microscope

03C109-2

1
2
3
4
5
6

MBE run number

Number of k/4 pairs (N)

120412B
111122C
120412A
120313A
120413A
120314A

1
2
4
7
7
7

operated at 400 keV with a structural resolution of 1.7 Å.
Reflectance spectra were measured using a Fourier transform
infrared spectrometer equipped with a CaF2 beam-splitter and
a liquid-nitrogen-cooled HgCdTe detector.
III. RESULTS AND DISCUSSION
After completion of growth, high-resolution XRD measurements were performed on the grown samples in the vicinity of the (004) diffraction peak of GaSb. The Cu Ka1 line
was used as the incident beam. The (004) x-2h curves for
the DBR samples with different numbers of k/4 pairs (N ¼ 1,
2, 4) are shown in Fig. 1. The diffraction peaks of ZnTe layer
and GaSb substrate are clearly observed. Their peak separations (Dh) for all samples are listed in Table II. It is found
that the Dh value is slightly increased by 15 arc sec as the
number of k/4 pairs increases from N ¼ 1 to N ¼ 4.
Correspondingly, the relative change in the lattice constant
of ZnTe layers between the DBR samples of one and four
k/4 pairs is only about 0.0098%, indicating that there is only
a small effect on lattice parameters and residual strain introduced by growing different numbers of k/4 pairs.
Furthermore, Pendell€osung fringes are observed from all
samples, indicating good interface smoothness, excellent
composition, and thickness uniformities of all layers.
The ZnTe and GaSb layer thicknesses can be estimated
using the separation of the Pendell€osung fringes, as
described by the following equation:

FIG. 1. (Color online) Experimental and simulated x-2h XRD patterns for
the ZnTe/GaSb DBR samples with different number of k/4 pairs (N): (a)
N ¼ 1; (b) N ¼ 2; and (c) N ¼ 4.

J. Vac. Sci. Technol. B, Vol. 31, No. 3, May/Jun 2013

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03C109-3 Fan et al.: MBE growth of high-reflectivity and broad-bandwidth ZnTe/GaSb distributed Bragg reflectors

03C109-3

TABLE III. Summary of XRD measurement results of the ZnTe/GaSb DBR
samples.

Sample number
(k/4 pairs)

Peak separation
between ZnTe
layer and GaSb
substrate (arc sec)

ZnTe layer
thickness
(nm)

GaSb layer
thickness
(nm)

274
270
259

255
265
270

145
140
145

1 (N ¼ 1)
2 (N ¼ 2)
3 (N ¼ 4)

d¼

k
;
2  DhP  cos hB

where d is the layer thickness, k is the incident x-ray wavelength, DhP is peak separation between Pendell€osung fringes,
and hB is the Bragg angle. The thicknesses of ZnTe and
GaSb layers are then calculated for each sample as summarized in Table III. For comparison, the (004) XRD patterns
for each sample are also simulated using X’PERT EPITAXY software and show close agreement with the experimental data.
AFM measurements were carried out to study the surface
morphology of the grown samples. The AFM images for
three samples of different k/4 pairs (N ¼ 1, 4, 7) were
recorded over 3  3 lm2 areas, as shown in Figs. 2(a)–2(c).
A well-defined step-flow growth mode is observed for all
samples. The root-mean-square (RMS) surface roughness is
found to be 0.193 nm (N ¼ 1), 0.221 nm (N ¼ 4), and
0.691 nm (N ¼ 7), indicating smooth surface morphology.
The RMS values show that the surface roughness increases
slightly as the number of k/4 pairs increases. The results
show that the observed surface and interface roughnesses of
these samples have limited impact on the peak reflectivity
because the wavelength of the light being reflected is much
longer (2.5 lm) than the dimensions of the roughnesses.
Cross-sectional specimens were prepared for TEM observation using mechanical polishing and dimpling followed by
Argon ion-milling. As illustrated by Fig. 3, low-magnification
TEM images for the ZnTe/GaSb DBR samples revealed high
quality ZnTe and GaSb epilayers with a low density of interfacial defects. Smooth morphology for all of the ZnTe/GaSb
and GaSb/ZnTe interfaces is clearly observed. Very few misfit dislocations and no stacking faults are visible at the interfaces, indicating highly coherent interfacial configuration and
overall high crystallinity. In addition, a few dark spots are
visible in the region of the GaSb buffer layer and GaSb substrate: these are caused by milling damage during sample
preparation.
Reflectance measurements were performed at normal incidence using a Globar as the light source. The reflectance spectra were measured in the spectral range from NIR (1.7 lm)
to MWIR (6.2 lm), as shown in Fig. 4. The reflectance
spectra show that the peak reflectance increases quickly from
53% to almost unity as the number of k/4 pairs increases from
one to seven. The sample with seven k/4 pairs shows best
results with a peak reflectance of 99.0% at 2.56 lm and a
stopband of 517-nm width. To investigate the reproducibility

FIG. 2. (Color online) AFM images of three ZnTe/GaSb DBR samples with
different number of k/4 pairs (N): (a) N ¼ 1; (b) N ¼ 4; and (c) N ¼ 7.

of the MBE growth of the samples, three different DBR samples with seven k/4 pairs were grown and their reflectance
spectra were measured and compared. The peak reflectance,
stopband center wavelengths, and bandwidths of these samples are summarized in Table IV. The peak reflectance has a
variation of 2.2%, the center wavelengths of stopbands are
within 70 nm, and bandwidths of stopband are within 30 nm,
indicating a good reproducibility.

JVST B - Microelectronics and Nanometer Structures

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03C109-4 Fan et al.: MBE growth of high-reflectivity and broad-bandwidth ZnTe/GaSb distributed Bragg reflectors

03C109-4

IV. SUMMARY
In summary, high-reflectivity and broad-bandwidth DBRs
based on ZnTe/GaSb quarter-wavelength (k/4) layers have
been successfully grown on GaSb (001) substrates using
MBE. During the MBE growth, GaSb layers were deposited
under a temperature ramp to protect the GaSb/ZnTe interfaces and to achieve good material quality. Post-growth characterization using XRD, AFM, and TEM indicate overall
excellent crystallinity, surface morphology, and highly
coherent interfaces. Reflectance measurements show that a
peak reflectance of 99.0% with a wide stopband of 517 nm
centered at 2.56 lm is experimentally achieved from a sample with seven k/4 pairs.
ACKNOWLEDGMENTS

FIG. 3. Low-magnification TEM image of a ZnTe/GaSb DBR sample with
seven k/4 pairs.

The work at ASU was partially supported by a Science
Foundation Arizona grant (SRG 0339-08) and an AFOSR
(FA9550-10-1-0129). ASU and Notre Dame were also
jointly supported by an NSF grant (ECCS-1002072). The
authors gratefully acknowledge the use of facilities in the
John M. Cowley Center for High Resolution Electron
Microscopy, the Center for Solid State Electronics Research,
and the LeRoyEyring Center for Solid State Science at
Arizona State University.
1

C. J. Chang-Hasnain, IEEE J. Sel. Top. Quantum Electron. 6, 978 (2000).
J. S. Harris, IEEE J. Sel. Top. Quantum Electron. 6, 1145 (2000).
F. Riemenschneider, M. Maute, H. Halbritter, G. Boehm, M.-C. Amann,
and P. Meissner, IEEE Photon. Technol. Lett. 16, 2212 (2004).
4
M. Maute, B. K€
ogel, G. B€
ohm, P. Meissner, and M.-C. Amann, IEEE
Photon. Technol. Lett. 18, 688 (2006).
5
J. J. Dudley, D. I. Babic, R. P. Marin, L. Yang, B. I. Millers, R. J. Ram,
T. E. Reynolds, E. L. Hu, and J. E. Bowers, Appl. Phys. Lett. 64, 1463
(1994).
6
V. Jayaraman, T. J. Goodnough, T. L. Beam, F. M. Ahedo, and R. A.
Maurice, IEEE Photon. Technol. Lett. 12, 1595 (2000).
7
L. Cerutti, A. Garnache, A. Ouvrard, M. Garcia, E. Cerda, and F. Genty,
Electron. Lett. 40, 869 (2004).
8
A. Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer, and M.-C.
Amann, Electron. Lett. 44, 202 (2008).
9
M. Ortsiefer, G. B€
ohm, M. Grau, K. Windhorn, E. R€
onnerberg,
J. Rosskopf, R. Shau, O. Dier, and M.-C. Amann, Electron. Lett. 42, 640
(2006).
10
J. Fan, X. Liu, J. K. Furdyna, and Y.-H. Zhang, Appl. Phys. Lett. 101,
121909 (2012).
11
S. Adachi, Handbook on Physical Properties of Semiconductors Volume
2: III-V Compound Semiconductors (Kluwer, Boston, 2004).
12
M. H. M. Reddy, T. Asano, R. Koda, D. A. Buell, and L. A. Coldren,
Electron. Lett. 38, 1181 (2002).
13
C.-K. Lin, D. P. Bour, J. Zhu, W. H. Perez, M. H. Leary, A. Tandon, S. W.
Corzine, and M. R. T. Tan, IEEE J. Sel. Top. Quantum Electron. 9, 1415
(2003).
14
A. Ducanchez, L. Cerutti, P. Grech, F. Genty, and E. Tournie, Electron.
Lett. 45, 265 (2009).
15
J. Fan, L. Ouyang, X. Liu, D. Ding, J. K. Furdyna, D. J. Smith, and Y.-H.
Zhang, J. Vac. Sci. Technol. B 30, 02B122 (2012).
2
3

FIG. 4. (Color online) Experimental reflectance spectra for the ZnTe/GaSb
DBR samples with different number of k/4 pairs (N ¼ 1, 2, 4, 7).

TABLE IV. Summary of reflectance results of DBR samples with seven k/4
pairs.

Sample number
(k/4 pairs)
4 (N ¼ 7)
5 (N ¼ 7)
6 (N ¼ 7)

Peak
reflectance
(%)

Stopband
center wavelength
(lm)

Stopband
bandwidth
(nm)

98.8
99.0
96.8

2.51
2.56
2.58

495
517
525

J. Vac. Sci. Technol. B, Vol. 31, No. 3, May/Jun 2013

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