Determination of CdTe bulk carrier lifetime and interface recombination velocity of
CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy
Xin-Hao Zhao, Michael J. DiNezza, Shi Liu, Calli M. Campbell, Yuan Zhao, and Yong-Hang Zhang
Citation: Applied Physics Letters 105, 252101 (2014); doi: 10.1063/1.4904993
View online: http://dx.doi.org/10.1063/1.4904993
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APPLIED PHYSICS LETTERS 105, 252101 (2014)

Determination of CdTe bulk carrier lifetime and interface recombination
velocity of CdTe/MgCdTe double heterostructures grown by molecular
beam epitaxy
Xin-Hao Zhao,1,2 Michael J. DiNezza,1,3 Shi Liu,1,3 Calli M. Campbell,1,2 Yuan Zhao,1,3
and Yong-Hang Zhang1,3,a)
1

Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287, USA
School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe,
Arizona 85287, USA
3
School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe,
Arizona 85287, USA
2

(Received 17 October 2014; accepted 12 December 2014; published online 22 December 2014)
The bulk Shockley-Read-Hall carrier lifetime of CdTe and interface recombination velocity at the
CdTe/Mg0.24Cd0.76Te heterointerface are estimated to be around 0.5 ls and (4.7 6 0.4)  102 cm/s,
respectively, using time-resolved photoluminescence (PL) measurements. Four CdTe/MgCdTe
double heterostructures (DHs) with varying CdTe layer thicknesses were grown on nearly
lattice-matched InSb (001) substrates using molecular beam epitaxy. The longest lifetime of 179 ns
is observed in the DH with a 2 lm thick CdTe layer. It is also shown that the photon recycling
effect has a strong influence on the bulk radiative lifetime, and the reabsorption process affects the
C 2014 AIP Publishing LLC.
measured PL spectrum shape and intensity. V
[http://dx.doi.org/10.1063/1.4904993]

Reducing surface and interface recombination is important for minority carrier devices such as solar cells and infrared detectors. GaAs and CdTe (Ref. 1) are two popular
materials for high efficiency solar cells. It has been found
that many materials, such as AlGaAs and GaInP,2–6 provide
sufficient carrier confinement to GaAs, which prevents carriers from reaching the top surface of the epilayers and thus
effectively reduces the surface recombination rate by providing a heterojunction interface. The interface recombination
velocity (IRV) of a high quality GaAs/Al0.5Ga0.5As interface
has been demonstrated to be as low as 18 cm/s,3 whereas the
recombination velocity of a GaAs free surface is on the order
of 107 cm/s.7 Similarly, the surface recombination velocity
of CdTe was found to be on the order of 105 cm/s.8 Research
efforts of reducing CdTe surface recombination include
using chemical passivation which reduces the surface recombination velocity down to 200 cm/s (Ref. 9) and using a
CdS/CdTe heterojunction with an interface recombination
velocity in the range of 103 cm/s–106 cm/s.10,11 It has been
reported that MgCdTe and CdTe form a type-I band edge
alignment,12 suggesting that MgCdTe is good for electron
and hole confinement and is expected to reduce the surface
recombination rate of CdTe. Recently, we reported the
growth, structural, and optical properties of CdTe/MgCdTe
double heterostructures (DHs) grown on InSb (001) substrates by Molecular Beam Epitaxy (MBE).13,14 It was found
that CdTe/MgCdTe DH samples show a three order of magnitude improvement in the photoluminescence (PL) intensity
compared to plain CdTe layers grown on InSb, therefore,
indicating qualitatively that the MgCdTe layers effectively
confine carriers and that CdTe/MgCdTe heterointerface has
a lower recombination velocity in comparison to the CdTe
a)

Electronic mail: yhzhang@asu.edu

0003-6951/2014/105(25)/252101/4/$30.00

surface. In this letter, we quantify the CdTe/MgCdTe interface recombination velocity by measuring the carrier lifetime
of several CdTe/MgCdTe DHs with various CdTe layer
thicknesses using time-resolved photoluminescence (TRPL).
The bulk Shockley–Read–Hall (SRH) lifetime of CdTe is
also extracted.
The CdTe/MgCdTe DHs are grown on closely latticematched 2-in. InSb (001) substrates using a dual-chamber
MBE system. The system consists of a II-VI chamber and a
III-V chamber inter-connected by an ultra-high vacuum
preparation chamber. The InSb substrate is at first thermally
deoxidized in the III-V chamber and a 500 nm InSb buffer
layer is grown. After that the substrate is transferred through
the preparation chamber to the II-VI chamber to grow a
500 nm CdTe buffer layer, followed by the growth of
CdTe/MgCdTe DH with a 10 nm thick CdTe cap layer.
Detailed growth conditions and sample structure were
reported previously.13 To determine the interface recombination velocity, the CdTe middle layers in the DHs are
designed with different thicknesses of 0.3 lm, 0.5 lm, 1 lm,
and 2 lm. The 30 nm thick MgCdTe barrier layers have a
Mg composition of 24%, as determined by high resolution
X-ray diffraction (XRD) measurements. All the epilayers are
undoped and the background doping level is estimated to be
lower than 1015 cm3 in the CdTe middle layer based on
temperature dependent carrier lifetime measurements.
TRPL measurements are carried out using a timecorrelated single photon counting system as reported previously.14 The excitation source is a pulsed Ti:Sapphire laser
operating at 750 nm wavelength with 0.8 MHz repetition
rate. The laser power is 2 mW and the beam radius is about
1 mm. It is estimated that for samples of different thickness
the initial excited carrier density is on the order of
1015 cm3. Steady-state PL spectra are measured using a

105, 252101-1

C 2014 AIP Publishing LLC
V

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Zhao et al.

Appl. Phys. Lett. 105, 252101 (2014)

spectrometer equipped with a photomultiplier tube. A
532 nm diode pumped solid-state laser is used as an excitation source. The laser power is set to 0.92 mW and the beam
radius is 0.54 mm.
PL decay measurements can be used to measure the carrier lifetime of a sample. However, this lifetime can be
affected by carrier diffusion, surface recombination, etc. The
use of DHs simplifies the carrier lifetime model. In our case,
the MgCdTe barrier layers confine the carriers inside the
middle CdTe layer and it is reasonable to assume that the
excess carriers distribute uniformly in CdTe due to the long
diffusion length of minority carriers. The effective carrier
lifetime seff of a CdTe/MgCdTe DH sample can then be
expressed using the following equation:15
1
sef f

¼

1
sbulk

þ

1
sinterf ace

¼

1
sbulk

þ

2S
;
d

(1)

where sbulk is the bulk carrier lifetime, sinterface is the interface recombination lifetime, S is the interface recombination
velocity, and d is the thickness of the sample. The above
equation is valid when S is relatively small15 and the diffusion length of minority carriers is much longer than the middle layer thickness.
Fig. 1 shows the room temperature PL decays of the
CdTe/MgCdTe DHs with different CdTe middle layer thicknesses, where the initial PL intensity has been normalized.
The carrier lifetime is determined by fitting near the tail of
the decay curve. It is found that the carrier lifetimes vary
across the wafer, which is probably due to non-uniformity in
the substrate temperature and beam flux distribution during
MBE growth, and the lifetimes shown here are measured
near the center of the wafer. It is also found that the carrier
lifetimes of CdTe/MgCdTe DHs with a thin 10 nm cap layer
gradually degrade with time, and the lifetime measurements
were carried out within days after the samples were taken
out of the MBE chamber. Fig. 1 shows that the thinner samples have shorter decay times, suggesting a non-zero recombination rate at the CdTe/MgCdTe interface. The longest
lifetime measured at room temperature is 179 ns for the sample with a 2 lm thick middle layer.

FIG. 1. Time-resolved photoluminescence decay of CdTe/MgCdTe double
heterostructures with different CdTe middle layer thicknesses. The thinner
the layer, the shorter the lifetime, indicating that interface recombination
lifetime plays an important role in the effective carrier lifetime.

Traditionally the bulk carrier lifetime is treated as thickness independent at low injection levels. However, the bulk
carrier lifetime can vary with thickness of the sample, as it
consists of both SRH and radiative lifetime and the latter is
related to photon recycling factor c as shown below2
1
sbulk

¼

1
sSRH

þ

1
srad

¼

1
sSRH

þ ð1  cÞBNdoping ;

(2)

where the photon recycling factor c is defined as the percentage of photons created by radiative recombination that are
reabsorbed within the sample.16 For CdTe, the absorption
coefficient near the band edge is on the order of 104 cm1
resulting in a short absorption length of the photons. The value
of c for CdTe middle layer is calculated using the ray-tracing
method17 as shown in Fig. 2 and it increases as a function
CdTe layer thickness. Thus, the radiative lifetime increases
with increasing CdTe layer thickness. The material radiative
recombination coefficient B was determined previously from
excitation-dependent PL measurements.14 It is calculated
from Eq. (2) that the radiative lifetimes for the 0.3 lm,
0.5 lm, 1 lm, and 2 lm thick DH samples are 0.7 ls, 0.9 ls,
1.6 ls, and 3.1 ls, respectively, by assuming a doping concentration of 1015 cm3. These values are much longer than the
measured effective carrier lifetime for each sample. Hence, it
is reasonable to assume that radiative lifetime does not affect
the effective carrier lifetime at room temperature. This
assumption is further supported by temperature-dependent
and excitation-dependent PL measurement, which show that
non-radiative recombination dominates at room temperature
and under low injection levels.14 Therefore, the measured lifetime is only related to the SRH bulk carrier lifetime and the
interface recombination lifetime as shown below
1
1
1
1
2S

þ
¼
þ :
sef f sSRH sinterf ace sSRH
d

(3)

High-resolution XRD measurements show that all the
CdTe layers in the studied samples are coherently strained
even when the thickness reaches 2 lm. Thus, we can assume
that the bulk SRH carrier lifetime is the same for CdTe DHs
with different CdTe layer thicknesses. By linearly fitting
1/seff versus 2/d, both the bulk SRH lifetime and the interface

FIG. 2. Calculated photon recycling factor for the CdTe middle layer of the
double heterostructure using ray-tracing method.

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Zhao et al.

recombination velocity can be extracted. Figure 3 shows the
carrier lifetime as a function of CdTe layer thickness and the
fitted curve of Eq. (3). Based on this fitting, the interface
recombination velocity at the CdTe/MgCdTe interfaces and
the bulk SRH lifetime of CdTe are extracted to be
(4.7 6 0.4)  102 cm/s and 0.5 ls, respectively. This interface
recombination velocity is much smaller than that of a free
CdTe surface and comparable to that of a typical GaAs/
AlGaAs interface, suggesting that MgCdTe is an excellent
barrier layer for CdTe based solar cells. The long bulk SRH
carrier lifetime indicates that the CdTe epilayer grown on
InSb substrates is of high quality, which is in agreement with
the low defect densities of 104 cm2 measured using confocal PL mapping.18 If it is assumed that the effective carrier
lifetime of the 0.3 lm sample is limited only by interface
recombination, an upper limit of the interface recombination
velocity can be obtained by using seff  sinterface ¼ d/2 S.
Using d ¼ 0.3 lm and seff ¼ 31 ns, the upper limit of S is
determined to be 484 cm/s, which is close to the value
obtained by the linear fitting. Therefore, the recombination
process in the sample with 0.3 lm thickness is dominated by
interface recombination, and the fitting provides a relatively
accurate measurement of S. However, it should be noted that
the extracted bulk SRH lifetime is very sensitive to this fitting method and 0.5 ls is only a rough estimation.
PL spectra of the DH samples with different thicknesses
are measured under the same conditions. As shown in Fig. 4,
the PL peak shifts to longer wavelengths when the CdTe
layer is thicker, which is an indication of photon reabsorption. The photons generated deep inside the CdTe layer can
be reabsorbed before escaping the front surface of the CdTe
layer. As the absorption coefficient of longer wavelength
photons is smaller than that of shorter wavelength photons,
the probability for longer wavelength photons to be reabsorbed is lower and thus the measured PL spectra shape
changes and the PL peak shifts to a longer wavelength.
Kuciauskas et al. reported a similar effect on single crystalline CdTe using subbandgap two-photon excitation PL measurements. The measured PL peak moves significantly to
longer wavelengths when the excitation region is a few mm
below the surface of the sample.8

FIG. 3. Effective carrier lifetime seff as a function of sample thickness d.
The IRV and the bulk Shockley–Read–Hall carrier lifetime are extracted to
be (4.7 6 0.4)  102 cm/s and 0.5 ls, respectively.

Appl. Phys. Lett. 105, 252101 (2014)

FIG. 4. Photoluminescence spectra of CdTe/MgCdTe double heterostructures with different CdTe layer thicknesses. The inset figure shows that the
photoluminescence peak position changes with the CdTe layer thickness,
indicating photon reabsorption effect.

Fig. 4 also shows that PL intensity is a function of CdTe
layer thickness. The PL intensity increases with thickness up
to 1 lm, then decreases. This finding can be explained as follows. In steady state PL measurements, the generation rate G
is equal to the recombination rate R inside the CdTe layer. G
is related to the laser power density P, the thickness of the
sample d, and the photon energy Ephoton as shown in Eq. (4).
The recombination rate R is related to the excess carrier densities Dn and the effective carrier lifetime seff as shown in
Eq. (5). Using Eqs. (4) and (5), the excess carrier densities
Dn can be obtained. PL intensity under low excitation is proportional to the net radiative recombination rate, which takes
into account the photon recycling factor, radiative recombination coefficient, background doping Ndoping, excess carrier
density, and the thickness of the sample, as shown in Eq. (6)
G¼

P
dEphoton

R¼

Dn
;
sef f

;

(4)
(5)

PL / ð1  cÞBNdoping Dnd ¼ ð1  cÞsef f BNdoping P=Ephoton :
(6)
Therefore, under low excitation, PL intensity is simply
proportional to (1  c)seff. The term (1  c) is defined as the
photon extraction factor, which is the percentage of radiatively generated photons that emit into the free space.16 The
effective carrier lifetime can be calculated using the above
thickness dependent carrier lifetime fitting results. Fig. 5
shows the measured PL intensity plotted together with the
calculated (1  c)seff curve. It is observed that the measured
PL intensity varies with the curve as predicted by the theory.
On one hand, with a thicker layer, the effective carrier lifetime increases as a result of smaller interface recombination
rate and thus more excess carriers are generated during
steady state PL measurements. On the other hand, the photon
reabsorption is enhanced in thicker layer samples and those
photons generated by radiative recombination are more
likely to be reabsorbed before escaping the CdTe layer.
Therefore, the PL intensity is observed to follow the trend of

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Appl. Phys. Lett. 105, 252101 (2014)

The authors would like to thank Su Lin at ASU for
assistance with TRPL measurements. This work was
partially supported by AFOSR (Grant No. FA9550-12-10444). This material was also based upon work supported by
the National Science Foundation Graduate Research
Fellowship (Grant No. DGE-0802261).
1

FIG. 5. Comparison between measured photoluminescence intensity and (1
 c)seff as a function of sample thickness.

(1  c)seff and appears to exhibit a peak around a thickness
of 1 lm for the CdTe layer.
In summary, long bulk SRH lifetime and low interface
recombination velocity have been demonstrated in the CdTe/
MgCdTe DHs grown by MBE. The bulk SRH carrier lifetime is approximately 0.5 ls, which shows the high quality
of the epitaxial CdTe layer on InSb. The longest lifetime
observed is 179 ns for a DH sample with a 2 lm thick CdTe
layer. The interface recombination velocity is estimated to
be (4.7 6 0.4)  102 cm/s from the effective carrier lifetimes
of the samples with different CdTe middle layer thicknesses.
It indicates that MgCdTe is a promising barrier layer material for solar cell applications. The photon recycling effect is
discussed and it has a strong influence on the radiative lifetime; however, the radiative lifetime does not play a significant role in these samples since the lifetime is dominated by
interface and bulk SRH recombination. The PL spectra of
different samples show that the peak shifts due to the photon
reabsorption and the PL intensity of the samples varies as a
function of (1  c)seff.

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