Investigation of temperature-dependent photoluminescence in multi-quantum wells

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Abstract Photoluminescence (PL) is a nondestructive and powerful method to investigate carrier recombination and transport characteristics in semiconductor materials. In this study, the temperature dependences of photoluminescence of GaAs-Al x Ga 1-x As multi-quantum wells samples with and without p-n junction were measured under both resonant and non-resonant excitation modes. An obvious increase of photoluminescence(PL) intensity as the rising of temperature in low temperature range (T &lt; 50 K), is observed only for GaAs-Al x Ga 1-x As quantum wells sample with p-n junction under non-resonant excitation. The origin of the anomalous increase of integrated PL intensity proved to be associated with the enhancement of carrier drifting because of the increase of carrier mobility in the temperature range from 15 K to 100 K. For non-resonant excitation, carriers supplied from the barriers will influence the temperature dependence of integrated PL intensity of quantum wells, which makes the traditional methods to acquire photoluminescence characters from the temperature dependence of integrated PL intensity unavailable. For resonant excitation, carriers are generated only in the wells and the temperature dependence of integrated PL intensity is very suitable to analysis the photoluminescence characters of quantum wells.

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received: 30 March 2015

accepted: 03 July 2015

Published: 31 July 2015

# Investigation of temperature- dependent photoluminescence in multi-quantum wells

Yutao Fang, Lu Wang, Qingling Sun, Taiping Lu, Zhen Deng, Ziguang Ma, Yang Jiang,
Haiqiang Jia, Wenxin Wang, Junming Zhou & Hong Chen

Photoluminescence (PL) is a nondestructive and powerful method to investigate carrier
recombination and transport characteristics in semiconductor materials. In this study, the
temperature dependences of photoluminescence of GaAs-AlxGa1-xAs multi-quantum wells samples
with and without p-n junction were measured under both resonant and non-resonant excitation
modes. An obvious increase of photoluminescence(PL) intensity as the rising of temperature in
low temperature range (T < 50 K), is observed only for GaAs-AlxGa1-xAs quantum wells sample with
p-n junction under non-resonant excitation. The origin of the anomalous increase of integrated PL
intensity proved to be associated with the enhancement of carrier drifting because of the increase
of carrier mobility in the temperature range from 15 K to 100 K. For non-resonant excitation, carriers
supplied from the barriers will influence the temperature dependence of integrated PL intensity of
quantum wells, which makes the traditional methods to acquire photoluminescence characters from
the temperature dependence of integrated PL intensity unavailable. For resonant excitation, carriers
are generated only in the wells and the temperature dependence of integrated PL intensity is very
suitable to analysis the photoluminescence characters of quantum wells.

Photoluminescence (PL) technique, a nondestructive and powerful method, has been widely used for
the investigation of optical and electrical characters of semiconductors, such as carriers lifetime 1,2, luminescence of impurities 3,4 and band structures 5,6,7 . Moreover, the PL technique is also an effective method
for investigating the novel characters of low-dimensional structure, such as quantum wells 8,9,10, quantum
wires 11,12,13 and quantum dots 14,15 and an useful tool for exploring new materials for optical and electrical
devices 16,17 . Because of the importance of the PL technique, it is crucial to get a deeper understanding
of the PL process, especially the transport of photogenerated carriers in the PL process. Resonant and
non-resonant excitation modes are two widely used PL methods, and different carrier transport processes
may occur under above excitation conditions 18,19 . Therefore, PL measurements under different excitation
modes may reflect different aspects of the studied materials 20, and the application ranges of the different
excitation modes are needed to be further clarified.

In our previous paper 20, we have shown that resonant excitation PL is suitable for investing the intrinsic PL characters of the InGaN quantum wells, while non-resonant excitation PL provides certain advantages for investigating the carrier transport dynamics and evaluating the internal quantum efficiency.
However, because of the intrinsic large polarization field existing in the InGaN/GaN quantum wells, this
structure is not an ideal structure for distinguishing the different effects of carrier drifting and carrier diffusion on the PL characters under non-resonant excitation mode. In order to further illustrate the carrier
transport mechanism in the PL process under non-resonant excitation, GaAs/AlGaAs multiple quantum
well structure is introduced. One advantage of GaAs/AlGaAs structure is that high-quality AlGaAs/GaAs

Key Laboratory for Renewable Energy, Chinese Academy of Sciences; Beijing Key Laboratory for New Energy
Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese
Academy of Sciences, Beijing 100190, China. Correspondence and requests for materials should be addressed to
[H.C. (email: hchen@iphy.ac.cn)](mailto:hchen@iphy.ac.cn)

quantum wells can be grown either by molecular-beam epitaxy (MBE ) or by metal-organic chemical
vapour deposition 21,22, because of the low lattice mismatch between AlAs and GaAs. The other more
important advantage is that there is no polarization field and inhomogeneous structures in GaAs quantum wells grown on GaAs (001) substrates 23 . Due to the above advantages of GaAs/AlGaAs quantum
wells, the specific roles of drifting and diffusion played in the PL process under non-resonant excitation
can be distinguished in this structure.

In this study, the GaAs/AlGaAs multi-quantum well structure is used to clarify the specific origin
of the abnormal increase of PL intensity and the application range of different excitation modes. Two
samples of GaAs/AlGaAs multi-quantum wells(MQWs) with and without the p-n junction, grown on
(001) GaAs substrates by MBE are used as the particular samples for the investigation of carrier transport mechanism in the PL process. An increase of integrated PL intensity as the rising of temperature
is observed only for the sample with the p-n junction under non-resonant excitation mode, while for
the sample without p-n junction or under resonant excitation mode, such an anomalous PL character
is not observed. Because of the temperature-dependent carrier supply from the barriers to wells under
resonant excitation, the temperature-dependent integrated PL intensity is not suitable to acquire the
characters of nonradiative characters of GaAs wells. For PL measured under resonant excitation, carriers
are generated only in the wells and the temperature-dependent integrated PL intensity can reflect the
characters of nonradiative channel in the GaAs wells. A modified Arrhenius formula taking into account
the temperature-dependent of radiative lifetime is proposed to fit the integrated PL intensities in this
study. The specific origin of the anomalous temperature dependence of photoluminescence is clarified
and nonradiative characters of GaAs quantum wells are investigated using a modified Arrhenius equation
under resonant excitation.

Results
Photoluminescence mechanism under resonant and non-resonant excitation. The schematic
diagram of sample structures is shown in Fig. 1(a) for sample A and sample B. The only difference
between the two samples is that sample A has a n-doped AlGaAs layer below the quantum wells and
a p-doped AlGaAs layer above the quantum wells, while sample B has two undoped AlGaAs layers.
Therefore, sample A is the sample with p-n junction and sample B is the one without p-n junction. The
energy band structures of sample A and sample B are shown schematically in Fig. 1(b), along the growth
direction. As shown in Fig. 1(b), carrier transport from barriers to wells occurs when non-resonant
excitation is used. Diffusion and drifting are two main mechanisms, which may account for the transport
of carriers in PL process. The carrier concentration gradient is the driving force for diffusion and the
electric field is the driving force for drifting. For sample A, because of the existing of built-in electric
field, carriers transport from barriers to wells can be both diffusion and drifting, while for sample B,
carriers generated in barriers can transport to wells only through diffusion under non-resonant excitation. Under resonant excitation, carriers are only generated in the wells and the barriers are not optically
excited because the energy of excitation photon is lower than the energy of barrier band gap. Hence, no
carriers transport from barriers to wells will happen under the resonant excitation mode. Consequently,
the temperature dependence of integrated PL intensity measured under resonant excitation mode can
reflect the thermal activation characters of non-radiative combination channels in the quantum wells.
However, under the non-resonant excitation, temperature dependent supply of carriers from barriers
through diffusion and drift will influence the PL intensity of quantum wells. As a result, the temperature
dependence of integrated PL intensity measured under non-resonant excitation reflects the combination
effects of carrier transport from barriers to wells and carriers recombination in the wells, which can not
be used to analysis the non-radiative mechanism in the quantum wells by the traditional Arrhenius plot.

Temperature-dependent integrated photoluminescence intensities measured under
non-resonant excitation modes. Figure 2 shows the temperature dependences of integrated PL
intensity obtained by non-resonant excitation under varied powers for sample A and sample B. The
energy of excitation photon is 2.33 eV, above the band gap of Al0.28Ga0.72As barrier, so the barriers are
optically excited. As a result, there is a temperature dependent carrier supply from barriers to wells under
non-resonant excitation mode. It is clearly revealed in Fig. 2 that the integrated PL intensities show a
gradually increase for the sample A as the temperature increased from 10 K to about 50 K, while for
the sample B such an increase is not observed. This anomalous phenomenon can be interpreted as the
increase of carriers captured into the wells from the barriers as rising of temperature in low temperature
range. Drifting and diffusion are the two possible ways of carrier transport from the barriers into wells.
Diffusion exists in both samples under the driving force of carrier concentration gradient and drifting
occurs only in sample A under the driving force of built-in field. From the comparison of sample A and
B, we can conclude that the enhancement of carrier drifting from barriers is the major contribution to
the exceptional increase of PL intensities. Because of the low activation energies of the Si and Be dopants
in AlGaAs layer 24, the temperature dependence of built-in electric field can be neglected in the temperature range of measurement. The low-temperature carrier mobility is governed by ionized-impurity
scattering and space-charge scattering and shows a monotonous increase in the low temperature range
(~15 K-100 K) 25,26 . This increasing of carrier mobility will enhance the carrier drifting under the built-in
filed and the amount of carriers captured by the GaAs wells is increase as rising of temperature in the low

Figure 1. Multi-quantum well structures with and without p-n junction. ( a ) The structure diagram of
sample A with p-n junction and sample B without p-n junction. ( b )Schematic of carrier transport in the PL
process for sample A and sample B taking into account the non-resonant excitation(ħν - EQB) and resonant
excitation(EQW < ħν < EQB). Carrier transport from barriers to wells occurs only under non-resonant
excitation mode and both carrier drifting and diffusion occur in sample A while only diffusion occurs in
sample B. EQB is the energy gap of barriers, EQW is the energy gap of wells and ħν is the energy of excitation
photon.

temperature range. As a consequence, the increasing of PL intensity is observed in the low temperature
range for the sample A, while there is no observable abnormal increase of PL intensity for the sample
B. There is no built-in electronic field in sample B, so the carrier transport from the barriers to wells is
diffusion. Therefore, the mechanism responsible for the anomalous increase of PL intensity is the carrier
drifting, not the carrier diffusion in our experiments.

For sample A, the enhancement of carriers captured by the wells and decreasing of internal quantum
efficiency as the rising of temperature result in a peak of PL intensity at the temperature around 50 K.
Moreover, as the increase of excitation power, the temperature for the onset of PL intensity decrease
moves to higher temperature, shown in Fig. 2 (a). The shift of the peak temperature of integrated PL
intensity may be attribute to the increase of the carrier lifetime in the barriers when increasing the
excitation power, because of the saturation of recombination centers 27 . When the lifetime of carriers in
the barriers increase, more carriers will be captured into the wells, which will compensate the decreasing
of radiative recombination efficiency at higher temperature. As a result, the inflection temperature of PL
intensity moves to higher temperature as the increasing of excitation power, shown in Fig. 2(a). From the
non-resonant excitation measurement results, we can conclude that the enhancement of carrier drifting
as the rising of temperature is caused by the increasing of carrier mobility and is responsible for the
abnormal increase of PL intensity for sample A.

Temperature-dependent integrated PL intensities measured under resonant excitation
modes. In order to further illustrate the cause of the anomalous increase of PL intensities, we measured the temperature-dependent photoluminescence of sample A and sample B under resonant excitation with the excited energy of 1.71 eV below the band gap of Al0.28Ga0.72As barrier. Under resonant
excitation, carriers are generated only in the wells and the effects of carrier supply from the barriers can
be eliminated. The temperature-dependent integrated PL intensities of sample A and sample B are shown

Figure 2. Temperature-dependent integrated PL intensities of GaAs quantum wells obtained by nonresonant excitation under various powers. The dashed lines represent the intensities equal to the intensities
at 10 K for each excitation powers. ( a ) The temperature-dependent integrated PL intensities for Sample A
with p-n junction under non-resonant excitation. The anomalous increase of integrated PL intensity are
observed in the low temperature range (below 50 K) under different excitation powers. ( b ) The temperaturedependent integrated PL intensities for Sample B without p-n junction under non-resonant excitation. The
monotonous decrease of the integrated PL intensities are observed in whole measured temperature range
under different excitation powers.

in Fig. 3, obtained by resonant excitation with varied excitation powers. The exceptional increase of PL
intensities is not observed in Fig. 3 for both of sample A and sample B, which further proves that the
supply of photogenerated carriers from barriers to GaAs wells under non-resonant excitation is the cause
of the anomalous increase of integrated PL intensity.

To clarify the mechanism of PL quenching as the rising of temperature, the temperature-dependent
integrated PL intensities measured under resonant excitation are fitted using a modified Arrhenius formula. Typically, for one nonradiative recombination channel, the Arrhenius formula is given by 28 .

_I_ ( _T_ ) = _I_ 0
1 + _a_ exp(−E _a_ /k _BT_ ) (1)

where _I_ (T) represents the integrated PL intensity at temperature of T. The parameter _I_ 0 is the integrated
PL intensity when the temperature approaching 0 K. E _a_ is the activation energy of nonradiative channel
and _kB_ is the Boltzmann’s constant. The parameter _a_ is obtained by

_τR_

_R_

_a_ = _R_
_τ_ 0 (2)

where τ _R_ is radiative lifetime of carriers and τ0 is the constant related to nonradiative lifetime of carriers. Traditionally the temperature dependence of τ _R_ is neglected in the Arrhenius formula and the
parameter a is treated as a constant 28,29 . However, for the GaAs quantum well the radiative lifetime of

Figure 3. Temperature-dependent integrated PL intensities of GaAs quantum wells obtained by resonant
excitation under various powers. The solid curves represent the best fits for the PL intensities as a function
of temperature, using the modified Arrhenius formula described in the text. ( a ) The temperature-dependent
integrated PL intensities for Sample A with p-n junction under resonant excitation. ( b ) The temperaturedependent integrated PL intensities for Sample B without p-n junction under non-resonant excitation.

carriers approximately linearly depends on temperature 30, so parameter _a_ can not be treated as constant.
Consequently, the Arrhenius formula can be revised as

_I_ ( _T_ ) = _I_ 0
1 + _a_ 0Texp(−E _a_ /k _BT_ ) (3)

0

with _a_ 0 =

with _a_ 0 = _ττR_ 0 _R_ = _τR_ 0 _T_ . In this paper, we propose that there are two nonradiative recombination

0 and _τ_
channels related to the quenching mechanism of PL intensities for the GaAs quantum wells. Then the
modified Arrhenius formula used to fit our experiment data can be written as

_τ_

_a_ 0 = _ττR_

_τR_ 0

_I_ ( _T_ ) = _I_ 0
1 + _a_ 01Texp(− E _a_ 1 / _kB_ _T_ ) + _a_ 02 _T_ exp(−E _a_ 2 /k _BT_ ) (4)

0

01Texp(− E _a_ 1 / _kB_ _T_ ) + _a_ 02 _T_ exp(−E _a_ 2

where _a_ 01 and E _a_ 1 are related to the nonradiative recombination channel in the GaAs wells and _a_ 02 and
E _a_ 2 are related to the nonradiative recombination channel in AlxGa1-xAs barriers 31 . Unlike the traditional
Arrhenius plot fitting, our modified Arrhenius formula can be used to fit the integrated PL intensities
very well in the whole measured temperature range, as the solid line shown in Fig. 3. All fitting results
for both samples under different powers, are listed in Table 1. As the increase of photogenerated carrier
density, the exciton binding energy decreases because of the enhancement of carrier screening effect.
Therefore, the activation energies for thermal quenching of the exciton luminescence decrease at high
excitation power, which is consistent with the fitting results of activation energies, shown in Table 1.
Moreover the decreases of _a_ 01 and _a_ 02 as the increase of excitation power can be interpreted as the consequence of increase of τ0. Because of the saturation of the nonradiative centers, the constant τ0 increases
as the increasing of excitation power which in turn leads to the decrease of _a_ 01 and _a_ 02. The differences
of the coefficients _a_ 01, _a_ 02 between sample A and sample B, are proposed to be attribute to the increase
of carrier life time under electrical field 32,33 . The increase of radiative lifetime dominates for _a_ 01, leading
to a larger _a_ 01 of sample A, while the increase of non-radiative lifetime dominates for _a_ 02, resulting in a
smaller _a_ 02 of sample A. The fitting activation energies of E _a_ 2 are around 220 m eV, which are comparable

Excitation power

726 nm laser-sample A 726 nm laser-sample B

1.2 mW 6 mW 30 mW 1.2 mW 6 mW 30 mW

_a_ 01 0.1129 0.0428 0.0330 0.0251 0.0179 0.0197

_Ea_ 1 (m eV) 3.66 3.01 0.060 2.55 0.74 0.17

_a_ 02 5277110 324333 19796 30675100 2370330 75011

_Ea_ 2 (m eV) 236.83 227.96 198.18 241.49 230.11 207.54

Table 1. The obtained fitting parameters: activation energies (E _a_ 1 and E _a_ 2) and constants ( _a_ 01 and _a_ 02).

to the barrier height of the exciton in the quantum wells. These fitting results further prove that the photoluminescence quenching at high temperature is caused by the nonradiative recombination of carriers
escaped from the well to barrier, which is consistent with previously reported results 34,35 .

Discussion
The PL characters of GaAs-AlGaAs MQWs were measured for the samples with and without p-n junction under both non-resonant and resonant excitation. The exceptional increase of integrate PL intensities as rising of temperature, is found to be mainly caused by the drifting of photogenerated carriers from
barriers to wells under non-resonant excited. The increasing of carrier mobility in the low temperature
(~15 K-100 K) is believed to be the specific cause of the anomalous temperature dependence PL intensity. The temperature-dependent integrated PL intensities under resonant excitation can be well fitted by
the modified Arrhenius formula with two nonradiative recombination channels in the whole measured
temperature. The activation energies of PL thermal quenching at high temperature fitted by the modified
Arrhenius formula are compare to the barrier height for the exciton escaping out of well, which confirms
that the non-radiative channel at high temperature is related to the non-radiative centers in barriers.

Because of the superiority of GaAs/AlGaAs structure, we are able to distinguish the roles of carrier
diffusion and drifting played in PL process of quantum wells measured under non-resonant excitation.
The origin of the increase of PL intensity as the rising of temperature is elucidated and the application
range of PL measured under different excitation modes are confirmed. The resonant excitation mode is
suitable to investigate the luminescence characters of quantum wells, while the non-resonant excitation
mode reflects a combining effect of carrier transport and carrier recombination and is not a proper way
to investigate the optical characters of quantum wells directly. The modified Arrhenius formula taking
into account the temperature dependence of radiative lifetime of quantum well has been used to fit the
temperature dependent integrate PL intensities under the resonant excitation. The results acquired in
this work will help to get a comprehensive picture of PL process and serve as a guiding role for the PL
measurement of low dimensional semiconductor structures.

Methods
Sample growth. The samples studied in this paper are grown on the semi-insulating (001) GaAs
substrate at 580 ° C by VG-80H solid-source MBE system in which solid elements (Ga, Al, As, Si and
Be) are used as sources. The epitaxial films were grown in a As2-rich condition with a valved arsenic
cracker cell at temperature of 850 ° C. For the sample A, firstly, a 400 nm GaAs layer was grown on GaAs
(001) surface followed by a 300 nm Si-doped AlGaAs layer and a 130 nm undoped AlGaAs layer . Then
3 periods GaAs/AlGaAs MQWs with 5 nmGaAs Wells separated by 30 nm AlGaAs barriers were grown.
Finally, a 100 nm AlGaAs layer and a 150 nm Be-doped AlGaAs layer were grown on the MQW with a
8 nm GaAs capping layer on the top. Sample B has the same grown structure as sample A except that
there is no dopant used during the growth of AlGaAs layers for sample B.

Photoluminescence measurements. Temperature-dependent PL measurements are performed
with the sample mounted in a closed-loop He gas cryostat, providing the measurement temperature
range from 10 K to 300 K. A 532 nm semiconductor laser diode is used as the non-resonant excitation
and a 726 nm semiconductor laser diode is used as resonant excitation. The size of the light spot focused
on the samples was approximately 0.1 mm. The excitation power using in this paper are adjusted by a
neutral optical attenuator. The PL signal is analysis with a triple-grating 50-cm monochromator and
detector by Si photomultiplier tube using the conventional lock-in technique.

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Acknowledgments
This work was supported by the National Nature Science Foundation (Grant No. 11374340), the National
Hi-tech Research, Development Program of China (Grant No. 2013AA031903) and the Science &
Technology Innovation Platform of Industry-Academia-Research Union of Guangdong ProvinceMinistry Cooperation Special Fund of China (Grant No. 2012B090600038).

Author Contributions
Y.F., H.C. and L.W. conceived the study. Y.F. and Q.S. conducted the sample-growth experiments. Y.F.
and T.Lu. performed the variable-temperature photoluminescence measurements. H.C. supervised the
entire research project. Y.F., H.C., Z.D., L.W., Z.M., Y.J., J.Z., H.J. and W.W. analysed the data. All authors
discussed the results and wrote the manuscript.

Additional Information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article : Fang, Y. _et al._ Investigation of temperature-dependent photoluminescence in
multi-quantum wells. _Sci. Rep._ 5, 12718; doi: 10.1038/srep12718 (2015).

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