ZnTe/GaSb distributed Bragg reflectors grown on GaSb for mid-wave infrared
optoelectronic applications
J. Fan, X. Liu, J. K. Furdyna, and Y.-H. Zhang
Citation: Applied Physics Letters 101, 121909 (2012); doi: 10.1063/1.4753819
View online: http://dx.doi.org/10.1063/1.4753819
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APPLIED PHYSICS LETTERS 101, 121909 (2012)

ZnTe/GaSb distributed Bragg reflectors grown on GaSb for mid-wave
infrared optoelectronic applications
J. Fan,1,2 X. Liu,3 J. K. Furdyna,3 and Y.-H. Zhang1,4,a)
1

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

(Received 23 August 2012; accepted 4 September 2012; published online 19 September 2012)
ZnTe/GaSb distributed Bragg reflectors (DBRs) are proposed and demonstrated for mid-wave
infrared (2–5 lm) optoelectronic applications. The reflectance spectra of ZnTe/GaSb DBRs are
simulated using the transmission matrix method, indicating a peak reflectance higher than 99.9%
for a DBR of 10 quarter-wavelength (k/4) pairs. A series of ZnTe/GaSb DBR structures have been
successfully grown on GaSb (001) substrates using molecular beam epitaxy. X-ray diffraction
results reveal smooth interfaces, uniform thicknesses, and low defect density. The DBR sample of
seven k/4 pairs has a peak reflectance as high as 99.0% centered at 2.5 lm with a 480-nm wide
C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4753819]
stopband. V

The mid-wave infrared (MWIR) wavelength range
between 2 and 5 lm contains absorption lines of several
atmospheric pollutants, such as CO, CH4, NO2, NH3, and
HF. Therefore, semiconductor lasers in this spectral range
are highly attractive light sources for gas detection and spectroscopy applications. Since vertical-cavity surface-emitting
lasers (VCSELs) offer low power consumption and ease of
monolithic integration with microelectromechanical system
(MEMS) structure for wavelength tuning,1–4 much research
effort has been devoted to InP-based and GaSb-based nearinfrared (NIR) and MWIR VCSELs.5–8 While InP-based
VCSELs have reached emission wavelengths up to 2.3 lm,9
GaSb-based VCSELs have covered a spectral range beyond
2.6 lm.10 As is well known, the optical cavity in a VCSEL
needs the use of high reflectivity mirrors, usually in the form
of distributed Bragg reflectors (DBRs). Unfortunately,
almost all the III-V semiconductors lattice-matched to InP
and GaSb offer very small refractive index differences,
which are crucial for the realization of thin DBRs with high
reflectivity to reduce the threshold current density. It is therefore highly desirable to develop DBR structures that are both
lattice-matched to GaSb and have a high refractive index
contrast along with broad wavelength tunability for monolithically integrated tunable VCSELs technology.
Recently, a new materials platform consisting of 6.1-Å
semiconductors grown on GaSb and InAs substrates was proposed for optoelectronic devices.11,12 This materials platform
consists of both II–VI (MgZnCdHg)(SeTe) and III–V
(InGaAl)(AsSb) compound semiconductors, which have
direct bandgaps spanning the entire energy spectrum from
far-infrared (0 eV) up to ultraviolet (3.4 eV). The broad
range of bandgaps and material properties make it very
attractive for a wide range of applications in optoelectronics,
such as photodetectors, solar cells, laser diodes, and light
emitting diodes. Among materials in this platform, ZnTe and
a)

Author to whom correspondence should be addressed. Electronic mail:
yhzhang@asu.edu.

0003-6951/2012/101(12)/121909/3/$30.00

GaSb are known to be closely lattice-matched with a lattice
mismatch of only 0.13%. High quality ZnTe/GaSb and
GaSb/ZnTe heterostructures have already been successfully
demonstrated with very low density of misfit dislocations.13,14 Furthermore, GaSb and ZnTe have a large refractive index contrast in the MWIR range (for example,
Dn ¼ 1.18 at 0.6 eV15). This refractive index contrast is significantly higher than those of InGaAs/InAlAs (Dn ¼ 0.27)
and AlAsSb/GaSb (Dn ¼ 0.6), which have been widely used
for DBRs in VCSELs emitting in the MWIR spectral
range.9,16 As a result, DBR structures consisting of ZnTe and
GaSb can provide very high reflectivity with significantly
fewer pairs of quarter-wavelength (k/4) layers. Consequently, the overall thicknesses of DBR structures can be
greatly reduced. In this work, we propose and demonstrate
the ZnTe/GaSb DBRs that can be potentially used in
VCSELs for MWIR optoelectronic applications.
To properly design the DBR structures, a quantitative
study of the reflectivity of the proposed ZnTe/GaSb k/4
structures was carried out using the transmission matrix
method.17 The transmission matrix formulation is expressed
as follows:
 
B
¼
C

2N 
Y
q

cosDq
i  sinðDq Þ  nq

i  sinðDq Þ=nq
cosDq

! 

1

;
nsub
(1)

Dq ¼ 2pnq  dq =k;
dq ¼

kpeak
;
4  nq

(2)
(3)

where N is the number of k/4 pairs, nq and nsub are the refractive indices of the qth layer and the substrate, respectively, dq
is the thickness of the corresponding k/4 layer, and kpeak is
the peak wavelength of the high reflectance band, which is
set as 2.3 lm for the simulation. The reflectance is given by

101, 121909-1

C 2012 American Institute of Physics
V

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121909-2

Fan et al.

Appl. Phys. Lett. 101, 121909 (2012)

FIG. 1. Simulated reflectance spectra for ZnTe/GaSb DBRs with different
numbers of k/4 pairs.


R¼

 

1  C=B
1  C=B

;
1 þ C=B
1 þ C=B

FIG. 2. Peak reflectance versus number of k/4 pairs.

tionally doped GaSb buffer layer. During the growth, ZnTe
epilayers were deposited at 320  C in the II–VI chamber,
while GaSb epilayers were grown in the III–V chamber
using a temperature ramp method to protect the ZnTe layer
surfaces and to achieve high material quality.14 The wafer
was transferred between II–VI and III–V chamber repeatedly
to complete the whole DBR structure. The detailed growth
conditions, such as growth temperatures, temperature ramping rate, BEP ratios, and growth rates were reported
previously.14
After completion of the growth, x-ray diffraction (XRD)
patterns of the samples were measured in the vicinity of the
(004) diffraction peak of the GaSb substrate using a PANalytical X’Pert PRO MRD x-ray diffractometer with multicrystal monochromator. The copper Ka1 line (1.54 Å) was
used as the incident beam. The (004) x-2h curve for the
DBR sample of four k/4 pairs is shown in Figure 3. Pendell€osung fringes are clearly observed, indicating high interface smoothness, as well as excellent composition and
thickness uniformities of all the layers. The (004) XRD pattern is also simulated using X’Pert Epitaxy software. The
ZnTe and GaSb layer thicknesses are set equal to 190 and
135 nm, respectively, as estimated from the growth rates.
The simulated results show excellent agreement with the experimental curve.
The reflectance measurements were carried out at normal
incidence using a Globar as the light source. The incident light
passed through an optical microscope and was focused on the
samples surface. The reflectivity spectra were measured by a
Fourier transform infrared (FTIR) spectrometer equipped with
a CaF2 beam-splitter and a liquid-nitrogen-cooled HgCdTe
detector. As seen in Figure 4, the measurement result for the
DBR sample of seven k/4 pairs shows a peak reflectance of
99.0% with a wide stopband of 480 nm centered at 2.5 lm.

(4)

where C=B is also referred to as optical admittance.
In the simulation, measured refractive indices of bulk
ZnTe and GaSb are used.18 Dispersion curves of ZnTe and
GaSb refractive indices in the spectral range from 0.2 to
0.7 eV are parameterized and extracted using the Sellmeier
equation.19 With refractive indices, layer thicknesses, and
the numbers of k/4 pairs as input parameters, reflectance
spectra of the ZnTe/GaSb DBR structures are simulated
using Eq. (4), as shown in Figure 1. The relationship between
the peak reflectance and the number of k/4 pairs is also plotted in Figure 2. It is clearly seen that the reflectance higher
than 99% can be expected from the ZnTe/GaSb DBR consisting of only seven k/4 pairs. For the ZnTe/GaSb DBR of
10 pairs, a reflectance as high as 99.9% can be achieved. It is
worth noting that the reflection spetra show a very wide and
flat stopband, as expected due to the large refractive index
contrast between ZnTe and GaSb. By using ten k/4 pairs, the
bandwidth of primary reflectance band above 99% is
428 nm. Comparisons of main DBR design parameters and
peak reflectance are made among ZnTe/GaSb, AlAsSb/
GaSb, and InGaAs/InAlAs DBRs. As shown in Table I, only
ten k/4 pairs are needed to reach 99.9% reflectance for ZnTe/
GaSb DBRs, while the other two DBRs require as many as
20 and 30 pairs, respectively. As a result, the thickness of
overall ZnTe/GaSb DBR structure can be reduced to 4 lm or
less, vs. 7 to 11 lm for the other two structures.
A set of ZnTe/GaSb DBR samples were grown using a
molecular beam epitaxy (MBE) system consisting of two
separate II–VI and III–V growth chambers connected by an
ultrahigh-vacuum (UHV) transfer module. The DBR structures were grown on a GaSb (001) substrate with an unintenTABLE I. Comparison of different DBRs used for VCSELs emitting at 2.3 lm.

[DBR materials]/substrate
[ZnTe/GaSb]/GaSb
[AlAsSb/GaSb]/GaSb
[InGaAs/InAlAs]/InP

Refractive index
contrast (Dn)

Number of k/4
pairs (N)

Calculated peak
reflectivity (%)

Total thickness
(lm)

1.18
0.6
0.27

10
20
30

99.9
99.7
99.4

3.6
6.6
10.5

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121909-3

Fan et al.

Appl. Phys. Lett. 101, 121909 (2012)

using MBE. High-resolution XRD results show narrow line
widths and distinctive Pendell€osung fringes from ZnTe and
GaSb epilayers, indicating smooth morphology, uniform
thickness, and low defect density. A peak reflectance of
99.0% with a wide stopband of 480 nm centered at 2.5 lm
was experimentally demonstrated for a ZnTe/GaSb DBR
with only seven k/4 pairs.

FIG. 3. Measured and simulated XRD patterns for a ZnTe/GaSb DBR sample of four k/4 pairs.

The work at ASU was partially supported by Science
Foundation Arizona (Contract No. SRG 0339-08) and
AFOSR Grant (FA9550-10-1-0129). ASU and Notre Dame
were also jointly supported by an NSF Grant (ECCS1002072). The authors gratefully acknowledge the use of
facilities in the LeRoyEyring Center for Solid State Science
at Arizona State University.
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FIG. 4. Measured and simulated reflectance spectra for a ZnTe/GaSb DBR
sample of seven k/4 pairs.

Simulation of reflectance spetra was also performed. By comparing the simulation results with the experimental data,
excellent agreement is obtained in terms of peak reflectance,
bandwidth of photonic stopband, and sidelobe positions.
In summary, ZnTe/GaSb DBR structure is proposed for
applications in mid-wave infrared VCSELs and other optoelectronic devices. Numerical simulation using the transmission matrix method shows that a peak reflectance as high as
99.9% can be expected from ZnTe/GaSb DBRs with only 10
pairs k/4 layers. Successful growth of high quality ZnTe/
GaSb DBRs has been demonstrated on GaSb (001) substrates

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