The continuum state in photoluminescence of type-II In0.46Al0.54As/Al0.54Ga0.46Asquantum dots

Linlin Su, Baolai Liang, , Ying Wang, Qinglin Guo, Xiaowei Li, Shufang Wang, Guangsheng Fu, Yuriy I. Mazur,Morgan E. Ware, and Gregory J. Salamo

Citation: Appl. Phys. Lett. 109, 183103 (2016); doi: 10.1063/1.4966895View online: http://dx.doi.org/10.1063/1.4966895View Table of Contents: http://aip.scitation.org/toc/apl/109/18Published by the American Institute of Physics

The continuum state in photoluminescence of type-II In0.46Al0.54As/Al0.54Ga0.46As quantum dots

Linlin Su,1 Baolai Liang,1,a) Ying Wang,1 Qinglin Guo,1 Xiaowei Li,1 Shufang Wang,1

Guangsheng Fu,1 Yuriy I. Mazur,2 Morgan E. Ware,3 and Gregory J. Salamo2

1College of Physics Science and Technology, Hebei University, Baoding 071002, People’s Republic of China2Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA3Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA

(Received 21 June 2016; accepted 20 October 2016; published online 31 October 2016)

The continuum state associated with type-II In0.46Al0.54As/Al0.54Ga0.46As quantum dots (QDs) is

investigated. Emission from the continuum states of the QDs is directly observed in photolumines-

cence (PL) spectra. The PL excitation and time-resolved PL spectra reveal an efficient carrier relax-

ation from the AlGaAs barrier into the InAlAs QD ground state via the continuum states. The

temperature dependence of the PL spectra shows a decreasing PL linewidth and a strong redshift of

the peak energy at low temperature, indicating that carriers are redistributed through the continuum

states by thermal activation and lateral transfer. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4966895]

Self-assembled quantum dots (QDs) have been investi-

gated extensively not only for their fundamental physical

properties but also for their possible optoelectronic device

applications, such as high quality lasers and high efficiency

detectors.1–10 These QDs generally form via the Stranski-

Krastanow (S-K) growth mode after a transition from two-

dimensional (2D) to three-dimensional (3D) growth. The

remaining film that does not make the transition into 3D

islands is then considered a 2D wetting layer (WL), on which

the QDs reside.

Theoretically, there should be no electronic states in the

energy range between the confined state of the WL and the

highest confined state of the QDs. It has been shown recently,

however, that there is a significant amount of absorption

observed in such an energy range for self-assembled QDs.11–13

The energy states into which this light absorption occurs have

been named the continuum states. Toda et al.11 first used the

concept of the continuum states to describe the mechanism

behind efficient intra-dot relaxation and the intense photolumi-

nescence (PL) spectra. They observed a 2D-like continuum

state from the PL excitation (PLE) spectra of a single QD

between a zero-absorption region and the 2D WL absorption

edge, revealing that carriers can relax efficiently through the

continuum states and transfer to the excitonic ground state by

the resonant emission of localized phonons. After that, there

have been many attempts to explain the mechanism and ori-

gins of the continuum states in terms of both intrinsic proper-

ties and structural imperfections.14–21

The existing studies of the continuum states generally

focus on In(Ga)As/GaAs QDs with a type-I band alignment.

These have revealed that the existence of the continuum state

greatly modifies the optical performance and the carrier

dynamics in self-assembled QDs. In comparison, type-II

QDs may also be useful for optical memory, photovoltaic,

and laser applications owing to their longer decay times

caused by spatially separated electrons and holes. The carrier

dynamics, particular continuum states related properties,

would be very important for the photonics device applica-

tions for type-II QDs. In this letter, we explore the contin-

uum states of type-II In0.46Al0.54As/Al0.54Ga0.46As QDs. The

PL, PLE, and time-resolved PL (TRPL) spectra have been

measured considering the contribution of the continuum

state. The results support an efficient mechanism of carrier

transition via the continuum state coupling of the bound QD

ground state with the surrounding AlGaAs barrier.

The sample was grown on a semi-insulated GaAs (100)

substrate by solid source molecular beam epitaxy (MBE).22

The sample structure is as follows: first, there is a 500 nm

GaAs buffer layer and a 75 nm Al0.54Ga0.46As layer grown at

620 �C, then a 10 ML In0.46Al0.54As QD layer grown at

540 �C, followed by 16 nm of Al0.54Ga0.46As grown at 540 �Cand 60 nm of Al0.54Ga0.46As grown at 620 �C, and finally a

9 nm GaAs capping layer was grown at 600 �C to protect the

In0.46Al0.54As/Al0.54Ga0.46As QD structure from oxidation.

The QD formation was confirmed during the growth by reflec-

tion high-energy election diffraction (RHEED), which showed

a transition from a streaky pattern to a spotty one. An

uncapped QD sample was also prepared for the morphology

study, on which atomic force microscope (AFM) measure-

ments were performed immediately after removal from the

MBE growth chamber.

Figure 1(a) displays an AFM image of the uncapped

In0.46Al0.54As/Al0.54Ga0.46As QDs with the height distribu-

tion shown in Fig. 1(b). The statistical results reveal that the

uncapped In0.46Al0.54As/Al0.54Ga0.46As QDs have an average

diameter, height, and density of �18.5 nm, �1.6 nm, and

�2.1� 1011 cm�2, respectively. We did not find any large

incoherent islands on the surface of the sample, indicating

the good quality of the QDs. The high surface density and

small dimensions of the QDs are likely due to the short sur-

face diffusion length of the Al atom because of its high

bonding energy with As.

a)Author to whom correspondence should be addressed. Electronic mail:

liangbaolai@gmail.com

0003-6951/2016/109(18)/183103/5/$30.00 Published by AIP Publishing.109, 183103-1

APPLIED PHYSICS LETTERS 109, 183103 (2016)

For PL and TRPL measurements, the capped QD sample

was mounted on a closed-cycle cryostat with temperature

variable from 8 K to 300 K, and a 470 nm pulsed laser (pulse

FWHM¼ 87 ps, frequency¼ 20 MHz) was focused on the

sample surface by a 50� long working distance objective

lens to excite the QDs. The emission signal was first dis-

persed by a 50 cm Acton spectrometer, then detected either

by a liquid nitrogen cooled CCD detector array to obtain the

PL spectrum or by a PicoHarp 300 time-correlated-single-

photon-counting (TCSPC) system with a silicon SPAD

detector to get the TRPL spectra. For PLE measurements, a

super-continuum pulsed laser (pulse lengthffi 60 ps) with an

output wavelength tunable from 465 nm � 680 nm was used

to excite the sample.

Figure 2(a) presents the PL spectra of the In0.46Al0.54As/

Al0.54Ga0.46As QDs measured at T¼ 8 K with an average

laser excitation intensity varying between 0.075 W/cm2 and

75 W/cm2. The peaks at �820 nm and �840 nm are identified

as the emission from bulk GaAs. The PL peak at �715 nm is

attributed to the emission from the In0.46Al0.54As QDs.22–25

There is a blue-shift of this emission from 1.709 eV to 1.732 eV

as the laser excitation intensity increases from 0.075 W/cm2 to

75 W/cm2, i.e., a blue-shift of �23 meV is observed as the exci-

tation intensity increases the three orders of magnitude. Such a

blue-shift is regarded as a signature of the In0.46Al0.54As QDs

with a type-II band alignment.22 Specifically, a type-II band

alignment of QDs is known to result in a cube root power law

blue-shift with the excitation laser intensity. This is shown in

Fig. 2(b) for our In0.46Al0.54As/Al0.54Ga0.46As QDs. These

observations indicate that our In0.46Al0.54As/Al0.54Ga0.46As QDs

have a type-II band alignment as illustrated by the band diagram

in Fig. 2(c). The quantum-confinement lifts the ground state of

the electron in the In0.46Al0.54As QDs above the X-valley of the

Al0.54Ga0.46As barrier. This gives rise to a type-II band structure

and a spatial separation of the electrons in the Al0.54Ga0.46As

barrier from the holes in the In0.46Al0.54As QDs.22

Figure 3 shows the PL and PLE spectra from the

In0.46Al0.54As/Al0.54Ga0.46As QDs measured at T¼ 77 K and

excited with the super-continuum laser power maintained at

0.1 mW. In the PL spectrum, other than the In0.46Al0.54As QD

peak at �710 nm, we observe a broad PL band between

560 nm and 650 nm, which we suggest is from the continuum

states associated with the In0.46Al0.54As QD. The PLE spec-

trum is detected at the position of the QD PL maximum and

distinctly displays heavy hole and light hole absorptions from

the Al0.54Ga0.46As barrier at �490 nm and �540 nm, respec-

tively. A continuum-like absorption band is seen to continu-

ously expand from �560 nm to the emission energy of the

QD ground state. We did not find a sharp WL signal in either

the PL spectrum or the PLE spectrum. We suggest that the

WL of the In0.46Al0.54As QDs is sufficiently thin that the reso-

nant signal in the PLE spectrum due to the WL is very close

to the Al0.54Ga0.46As absorption band edge. Another possibil-

ity is that intermixing at the In0.46Al0.54As/Al0.54Ga0.46As

interface creates a very rough WL region and consequently a

very broad WL band. In both cases, the WL resonant signal in

the PLE spectrum would be so close to the Al0.54Ga0.46As

absorption band edge that we cannot identify it due to the

�10 nm linewidth of our excitation laser. Therefore, we assign

the broad band signal centered at �600 nm in both the PL and

the PLE spectrum to be the “continuum state,” which corre-

sponds to the energy states between the Al0.54Ga0.46As band

FIG. 1. (a) An AFM image (1 lm � 1 lm) of the In0.46Al0.54As/

Ga0.46Al0.54As QDs, the inset shows a zoomed three-dimensional image of

the QDs; (b) a histogram of the QD heights along with a Gaussian fit to the

height distribution.

FIG. 2. (a) The PL spectra at T¼ 8 K measured as a function of the average

laser excitation intensity using a 470 nm pulsed laser; (b) the PL peak energy

with respect to the cube root of the laser excitation intensity; (c) the diagram

of the type-II band alignment of the QDs.

FIG. 3. The PL spectrum and the PLE spectrum detected at the position of

the QD PL maximum to show the QD emission and the continuum state.

183103-2 Su et al. Appl. Phys. Lett. 109, 183103 (2016)

edge (or WL) and the confined state of the QDs. Some other

mechanisms, such as submonolayer growth of QDs,26,27 may

possibly lead to a broad band signal similar to the “continuum

state.” In particular, the phase separation in AlGaAs epi-

layers, giving a local enhancement of the Al concentration

areas with other areas where Ga concentration is enhanced,

provides another possibility that the Ga concentration

enhanced areas contribute to the broad band of 560–650 nm in

PL and PLE. In addition, the 560 nm PLE band edge fits very

well with the X-valley of AlAs. However, the indirect transi-

tion nature of the X-valley of AlAs and the very small effec-

tive spatial volume of the Al concentration enhanced areas

generated through phase separation in the AlGaAs, make it

unlikely to observe a strong PLE absorption edge from the

AlAs material. Therefore, it is reasonable to attribute the

560 nm edge observed in the PLE spectrum to the direct tran-

sition absorption from the C-valley of the Al0.54Ga0.46As layer

at low temperature.

It is very interesting that in Fig. 3, we can directly

observe the emission signal from the continuum state associ-

ated with the QDs. This could be a result of the high peak

power of the pulsed laser excitation. However, it is most

likely that the photo-generated electrons, which are forced to

remain in the AlGaAs barrier due to the type-II band align-

ment, live long enough to recombine with a subsequent hole,

which has not yet been captured by a QD. In other words,

carrier recombination in the continuum states can compete

with that in the QDs due to the long recombination lifetime

in the type-II In0.46Al0.54As QDs. Clearly, this feature of

continuum state emission cannot be observed for normal

InGaAs/GaAs QDs with type I band structures.

In order to further study the continuum states, we have

measured the PL lifetimes as functions of the detection

wavelength. Fig. 4(a) shows the PL decay profile detected at

several wavelengths from the GaAs, through the QDs and

the continuum states. For comparison, the pulse shape of the

excitation laser is shown as well with a FWHM of �58 ps

and a decay time constant of �35 ps. This demonstrates the

overall system resolution. The GaAs shows a mono-

exponential decay behavior with a decay constant of �11 ns,

which is common for the intrinsic bulk GaAs material. For

the QD emission, it is clearly observed that the PL decay

curves contain both slow and fast decay components. The

slow component is attributed to the type-II nature of the

recombination found in our InAlAs/AlGaAs QDs, on

account of the small wave function overlap between the elec-

trons in the AlGaAs matrix and the holes in InAlAs QDs.

This slow time component is shown in Fig. 4(b) as the red

circles along with the PL spectrum. Here, we see that the life-

time of the QDs decreases with the detection wavelength from

the QD peak center at 720 nm, which is consistent with previous

reports.28–30 This can be understood by considering the distribu-

tion of the QD sizes and energies. For the smaller higher energy

QDs, the confined carriers (holes) will have a more significant

wave-function overlap with the electrons in the barrier due to a

reduced effective confinement. Therefore, the higher energy

(shorter wavelength) QDs should have reduced lifetimes.

We also examine the decay behavior of the continuum

state emission between 560 nm and 650 nm. As an example,

the early decay period of the decay curve at 620 nm is shown

in Fig. 4(c). It also exhibits a double-exponential decay

behavior with a fast lifetime component of 45 ps and a slow

lifetime component of 1.8 ns. The fast component appears to

be limited by the laser pulse, whereas we find that the slow

component of the continuum state emission, while much lon-

ger is still many times shorter than most of the QD emission.

The role of the continuum state during carrier relaxation17,21

can account for this result. For electron-hole pairs generated

inside the barriers, they can relax into either the GaAs bulk

or the QDs through the continuum state.31–33 These phonon

mediated relaxations account for the fast decay and the lon-

ger rise times seen in both the QDs and the GaAs in Fig.

4(c). Subsequently, carriers in the QDs can be either ther-

mally excited back into the continuum state or finally radia-

tively recombine in the QDs.34–37 We believe that this

secondary process of thermal excitation at the relatively ele-

vated temperature of 77 K results in the much longer decay

time for the continuum state seen in Fig. 4(c), following the

initial transient of the laser pulse.

The temperature dependence of the QD emission is

shown in Fig. 5 using an excitation intensity of 75 W/cm2 at

470 nm. Most of the observed behavior is consistent with the

redistribution of thermalized carriers and lateral coupling

through the continuum state. From the normalized PL spectra

plotted in Fig. 5(a), we extract the QD PL peak energy as a

function of the temperature in Fig. 5(b). Both Figs. 5(a) and

5(b) present a redshift for the QD PL peak energy. This red-

shift is partially associated with the decrease in bandgap, and

partially contributed from thermally excited carrier transfer

from small dots to large dots due to the lateral coupling

between the high density InAlAs QDs. The thermally excited

carrier transfer leads to more carriers getting excited out of

small QDs and getting recaptured by large QDs. It gives a

fast redshift of the QD PL peak energy as the temperature

FIG. 4. (a) TRPL decay curves at dif-

ferent detection wavelengths with the

laser pulse as a reference; (b) PL spec-

trum and lifetime measured at different

wavelengths; (c) TRPL early time

decay curves show the transient rise of

emission from the GaAs, the QDs, and

the continuum states.

183103-3 Su et al. Appl. Phys. Lett. 109, 183103 (2016)

increases above 80 K, and also a distinct PL profile transform

from a anisotropic spectrum to a symmetric shape in Fig.

5(a) as the temperature increases. Meanwhile, the observed

slight increase of PL integrated intensity seen in Fig. 5(c)

indicates the absence of nonradiative recombination. Above

70 K, the PL peak is more sensitive to change in temperature

as a result of the carrier redistribution to lower energy QDs.

This leads to increased recombination at nonradiative centers

or escape leading to the reduction of integrated intensity.

This same redistribution explains the decreasing FWHM in

Fig. 5(d) as the smaller dots become depopulated. However,

normal S-K QDs maintain their full linewidth initially at low

temperatures before decreasing from redistribution. This

indicates a lower barrier here for redistribution. With further

increasing temperature, the electron-phonon scattering

becomes a dominant factor, and the FWHM begins to

increase.24 In addition, the lifetime increase seen in Fig. 5(e)

at low temperatures is observed due to the high density of

QDs and is interpreted again as carrier redistribution, where

the shortest lifetime and shallowest and highest energy QDs

are thermally depopulated resulting in an observable increase

in the ensemble lifetime.23,38,39

In conclusion, we have studied the optical properties of

In0.46Al0.54As/Al0.54Ga0.46As QDs grown on a GaAs sub-

strate. From the laser excitation intensity dependence of the

PL spectra, the peak energy follows a cube root power rela-

tionship, which indicates a type-II band alignment between

the QDs and their barrier. The signals from the continuum

state are directly observed in PL, PLE, and TRPL spectra.

From the temperature dependence of the PL spectra, the line-

width decreases at very low temperatures. At the same time,

the peak is more sensitive to temperature (above 70 K) com-

pared with other S-K QDs such as with the obvious redshift

of the PL energy. These phenomena prove that carrier relax-

ation and redistribution among QDs occur through the con-

tinuum state associated with the QDs with type-II band

alignment.

The authors acknowledge the financial support by the

“Hebei Province 100-Talents Program” (Grant No.

E2013100013) and Natural Science Foundation of Hebei

Province (Grant No. A2012201013) of China. This research

is also supported by the National Science Foundation of the

U.S. (Grant No. DMR-1309989 and EPSCoR Grant No.

OIA-1457888).

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