Energy states and carrier transport processes in metamorphic InAsquantum dotsL. Seravalli, G. Trevisi, P. Frigeri, R. J. Royce, and D. J. Mowbray Citation: J. Appl. Phys. 112, 034309 (2012); doi: 10.1063/1.4744981 View online: http://dx.doi.org/10.1063/1.4744981 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i3 Published by the American Institute of Physics. Related ArticlesSpontaneous emission control of single quantum dots by electromechanical tuning of a photonic crystal cavity Appl. Phys. Lett. 101, 091106 (2012) High-frequency gate manipulation of a bilayer graphene quantum dot Appl. Phys. Lett. 101, 043107 (2012) Magnetovoltaic effect in a quantum dot undergoing Jahn-Teller transition Appl. Phys. Lett. 101, 023120 (2012) Fast detection of single-charge tunneling to a graphene quantum dot in a multi-level regime Appl. Phys. Lett. 101, 012104 (2012) Refrigeration effect in a single-level quantum dot with thermal bias Appl. Phys. Lett. 100, 233106 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Energy states and carrier transport processes in metamorphicInAs quantum dots

L. Seravalli,1 G. Trevisi,1 P. Frigeri,1 R. J. Royce,2 and D. J. Mowbray2

1CNR-IMEM Institute, Parco Area delle Scienze, 37/a, 43124 Parma, Italy2Department of Physics and Astronomy, University of Sheffield, Hicks Building, Sheffield S3 7RH,United Kingdom

(Received 1 March 2012; accepted 11 July 2012; published online 6 August 2012)

Photoluminescence excitation spectroscopy is used to probe energy states and carrier transport in

InAs quantum dot structures grown on InGaAs metamorphic layers, designed for room temperature

emission at 1.3, 1.4, or 1.5 lm. The dominant spectral feature is shown to arise from the partially

relaxed InGaAs confining layer. In structures with a low indium composition or thin InGaAs layer,

a clear wetting layer feature is observed which acts as the dominant reservoir for carriers thermally

excited from the quantum dots. Structures with high indium composition and/or thick InGaAs lack

a wetting layer and carriers escape directly to the InGaAs layers. VC 2012 American Institute ofPhysics. [http://dx.doi.org/10.1063/1.4744981]

I. INTRODUCTION

InAs quantum dots (QDs) grown on GaAs substrates

now allow the fabrication of 1.3 lm emitting lasers with sig-

nificantly improved performance in comparison to quantum

well lasers.1,2 A possible approach to extend emission

towards the main telecommunications wavelength of

1.55 lm is QD growth on a metamorphic InGaAs layer.3,4

Here, the relaxation of the InGaAs layer results in a decrease

of the lattice mismatch between the QDs and confining

layers (CLs) and hence of the QD strain (compared to growth

on GaAs), producing a large emission redshift. By varying

the composition and thickness of the metamorphic layer, it is

possible to separately control both the QD emission energy

and confinement potential.5 However, the precise nature of

the energy states related to the wetting layer (WL) and CLs,

and the processes of carrier transport in these complex struc-

tures remain unclear. An understanding of these factors is

vital for the optimisation of these structures. In addition, the

nature of the QD formation as the composition and thickness

of the InGaAs CL increases is unclear. In this paper, low

temperature photoluminescence (PL) and photoluminescence

excitation (PLE) spectroscopy are used to study a series of

InAs/InGaAs metamorphic QD structures. By studying struc-

tures with a systematic variation in both the thickness and

composition of the InGaAs CL, it is possible to obtain a

detailed understanding of the critical energy states, carrier

transport processes and the structural environment of the

QDs.

II. EXPERIMENTAL DETAILS

The samples were grown by molecular beam epitaxy

(MBE) on (100) GaAs substrates. Following a GaAs buffer

layer, an InxGa1� xAs metamorphic lower confining layer

(LCL) of thickness t and In composition x was deposited at

490 �C, followed by a single layer of InAs QDs deposited at

460 �C and capped with a 20 nm-thick InxGa1� xAs cap

layer. The LCL and capping layer have the same In composi-

tion. Sample parameters are given in Table I, with growth

and structural details are given in Refs. 5 and 6. By varying

the composition and thickness of the InGaAs LCL, it is pos-

sible to separately control both the strain state and confining

potential of the QDs. Optical spectroscopy was performed at

6 K in a He flow cryostat with PL excited with a red diode

laser and PLE excited by light from a 150 W tungsten-

halogen bulb dispersed by a 0.22 m monochromator. Detec-

tion was via a 0.75 m spectrometer and nitrogen cooled Ge

detector.

III. RESULTS AND DISCUSSION

PL spectra of the samples are shown in Figure 1 for tem-

peratures of 6 and 300 K (RT). Peak emission energies

extracted from the PL measurements at 6 K are given in

Table I. Samples A-D emit around 1.03 eV, with line widths

of �42 meV; samples E and F emit at 0.95 eV, with line

widths �65 meV. Sample G has a much larger line width

and exhibits a multimodal emission with a maximum at

0.91 eV. PL emission from the QDs is observed at RT for

samples A-D around 1.3 lm and at 1.42 lm for sample E, as

shown in Fig. 1. However, for samples F and G, the QD

emission is more strongly quenched and is no longer

observed at RT. This behaviour was previously studied and

interpreted as due to the thermal escape of confined carriers

out of the QDs due to the reduced confinement which occur

as the In composition, x, of the confining layers is

increased.5,6 For samples F and G, QD PL is observed up to

temperatures of 200 and 170 K, respectively, allowing ex-

trapolated RT band gaps of 0.885 eV for sample F and

0.825 eV for sample G to be inferred. These band gaps would

correspond to emission wavelengths of 1.4 lm and 1.5 lm at

RT if thermal carrier escape from the dots was reduced. This

can be achieved, for example, by embedding metamorphic

QDs in InAlAs barriers where we have shown previously

that it is possible to obtain RT emission as long as 1.59 lm.4

Figure 2 shows 6K PLE spectra for detection at the peak

of the QD PL. For comparison, the PLE of a reference

0021-8979/2012/112(3)/034309/5/$30.00 VC 2012 American Institute of Physics112, 034309-1

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sample with a layer of InAs QDs grown within an

In0.15Ga0.85As-GaAs quantum well (RT emission at 1.3 lm)

is included.7 Between 1.05 and 1.25 eV, the PLE of the refer-

ence sample exhibits features that shift rigidly with the

detection energy and are therefore attributed to carrier relax-

ation via the emission of multiple LO phonons, while no fea-

tures directly attributable to absorption into excited QD

states are observed.8 Elastically scattered light and the ab-

sence of a Stokes’ shift prevents the observation of the QD

ground state absorption. At higher energies (�1.35–

1.45 eV), two features are attributed to absorption by the

InGaAs QW and/or InAs WL. These two features are

believed to represent the light and heavy hole (LH and HH)

states which are split by confinement and strain effects. At

�1.5 eV, the step in the PLE represents the onset of strong

absorption by the bulk GaAs. Absorption at and above the

GaAs band gap gives a large positive feature in the PLE for

detection on the QD emission demonstrating, for this refer-

ence sample, the efficient transfer of carriers initially created

in the GaAs to the QDs.

The PLE spectra of the QDs grown on the metamorphic

layers, which are broadly similar to the only other reported

PLE characterization of metamorphic QD structures,9 show

some similarities with that of the reference sample but also

some noticeable differences. In three of the samples designed

to emit at 1.3 lm at RT (samples A, B—not clearly observed

in the spectrum used in Fig. 2—and D) multiple-LO phonon

features are observed. For example, in sample A, these are

observed to occur at fixed energies of 64, 96, and 132 meV

from the detection energy, suggesting 2, 3, and 4 multiples

of a �32 meV phonon energy (elastically scattered light pre-

vents the observation of any feature at the fundamental

energy of 32 meV). A similar fundamental energy is obtained

for samples B and D and also the reference sample. How-

ever, the resolution of the current PLE spectra does not allow

any small shifts in phonon energy between the different

TABLE I. Sample LCL parameters plus calculated values of the residual CL strain, QD-CL lattice mismatch, CL energy, and LH-HH splitting and experimen-

tally determined values of the PL energy and EQDþEact.

Sample

LCL In

composition (x)

LCL thickness

(nm)

Residual CL

strain (%)

QD-CL lattice

mismatch (%)

6 K PL

energy (eV)

LH-HH splitting

(eV)

CL energy

(eV)

EQDþEact

(eV)

A 0.09 1000 �0.232 6.74 1.049 0.015 1.389 1.319 6 0.014

B 0.12 165 �0.547 6.84 1.021 0.034 1.347 1.296 6 0.014

C 0.15 60 �0.876 6.96 1.028 0.053 1.332 1.285 6 0.018

D 0.18 31 �1.185 7.08 1.023 0.070 1.300 1.191 6 0.010

E 0.24 145 �0.517 5.92 0.943 0.033 1.183 1.175 6 0.028

F 0.28 37 �0.998 6.14 0.954 0.061 1.168 1.110 6 0.010

G 0.31 1000 �0.188 5.04 0.911 0.012 1.089 1.083 6 0.031

FIG. 1. PL spectra at 6 K (samples A-G) and RT (samples A-D) showing the

QD emission.

FIG. 2. PLE spectra recorded for a sample temperature at 6 K and detection

at the peak of the QD emission. A spectrum of a reference InAs-InGaAs dot-

in-a-well sample is included for comparison. Solid boxes indicate the calcu-

lated CL heavy and light hole energies. Solid horizontal lines indicate the

values of EQDþEact with associated errors. Vertical arrows indicate features

attributed to WL absorption. P indicates multiple phonon features.

034309-2 Seravalli et al. J. Appl. Phys. 112, 034309 (2012)

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samples to be determined. Phonon energies between 28 and

34 meV have been reported previously in PLE studies of

InAs QDs grown within a GaAs matrix.10 The majority of

the samples also show a broad WL related feature which will

be discussed in detail below.

One significant difference between the reference sample

and the metamorphic structures is the absence of a GaAs-

related feature in any of the PLE spectra. All samples contain

a 100 nm GaAs buffer layer and the thickness of the metamor-

phic InGaAs layers in all but samples A and G should be suffi-

ciently thin to permit optical excitation of the GaAs buffer;

under high power laser excitation at 6 K GaAs emission is

observed for samples B-F. The absence of a GaAs signal in

the PLE indicates that carriers excited in the GaAs buffer are

unable to migrate efficiently to the QDs, suggesting that they

are either trapped in the GaAs or recombine non-radiatively at

misfit dislocations within the InGaAs layer; these are known

to exist at high concentrations at the InGaAs-GaAs interface.

The strain relaxation of the InGaAs capping layer in the

present samples has been analyzed in detail by x-ray diffrac-

tion (XRD).11 It is found that the capping layer is pseudo-

morphic to the InGaAs LCL, hence in terms of energy levels

the upper CL and LCL can be effectively considered as iden-

tical in the following discussions.

The left hand edge of the solid boxes in Fig. 2 is the cal-

culated CL band gaps as given by the model of Ref. 5, based

on the approach of strain relaxation developed by Maree

et al.12 These results have been confirmed by x-ray diffrac-

tion, photo-reflectance (PR), and Raman characterization of

the current samples.11 The calculated CL band gap energies

agree very well with the onset or initial maximum of the

strongest features in the PLE spectra, allowing these to be

attributed to absorption by the uniformly strained InGaAs

layer. The calculated and experimentally determined ener-

gies are plotted as a function of In composition in Fig. 3. The

residual strain in the CL will split the LH and HH states.

This splitting has been calculated using the equation given in

Ref. 13 and material parameters from Ref. 14, with relevant

values given in Table I. The LH energies are indicated by

the right hand edge of the solid boxes in Fig. 2 so that the

width of these boxes indicates the calculated LH-HH split-

ting. None of the PLE spectra exhibit a feature that correlates

with the expected position of the LH state (assuming the

onset represents the HH state) and in general the width of the

main PLE feature is significantly broader than the calculated

LH-HH splitting. As PLE represents not only the sample

absorption but also the relaxation efficiency for carriers from

their initial energy to the detection energy, the decrease of

the PLE strength at high energies indicates that carriers cre-

ated with energies high in the CL relax inefficiently into the

QDs; instead they are preferentially lost to non-radiative

centres. Evidence for such centres in QD structures grown

on metamorphic InGaAs layers has been reported in Refs. 15

and 16. It is noticeable that in the majority of structures the

CL PLE signal falls to zero above �1.48 eV, indicating effi-

cient carrier transport to non-radiative centres above this

energy.

Previous studies of the temperature dependence of the

PL intensity have allowed the activation energy, Eact, to be

extracted for the PL quenching processes.5,6 This energy rep-

resents the potential barrier for carrier escape from the QDs,

thus the sum of this energy and the QD emission energy,

EQD, gives the energy of the state to which carriers first

escape. EQDþEact values are given in the final column of

Table I and are indicated in Fig. 2 by the horizontal lines, the

length of which indicates the experimental errors in the

determined values.

For samples A-D and F, the values of EQDþEact coin-

cide fairly well with an obvious feature in the PLE (indicated

by the vertical arrows in Fig. 2). This feature is spectrally

distinct from the CL feature and is hence attributed to the

WL which has previously been identified as the critical state

controlling carrier escape from the QDs. The EQDþEact

values and energies of features observed in the PLE spectra

are plotted in Fig. 3. Also, included in Fig. 3 is PR data for

one sample (data of Ref. 6) which has been used to deter-

mine the WL energy of the x¼ 0.15 sample C: this agrees

well with the feature observed in the PLE spectrum of this

sample. Hence, it appears that for samples with a low In

composition or small thickness, the PLE exhibits a feature

that can be attributed to the WL and that it is this state that

acts as the reservoir to which carriers are thermally excited

from the QDs.

The exceptions to the observation of a probable WL

PLE feature corresponding to the EQDþEact energy are the

two samples with thick and high In composition LCLs; sam-

ples E and G. In both samples, the calculated EQDþEact val-

ues are close to the onset of the feature attributed to the CL

absorption. This agrees with previous preliminary studies6

which suggested that WL states may not be effective for

structures with high In composition CLs. It is found that in

structures with x� 0.28 and a LCL thickness exceeding

100 nm (consistent with the parameters for samples E and

G), the value of EQDþEact is significantly larger than for

FIG. 3. Experimentally determined CL energies (open squares �) and WL

energies (open upward triangles D) by PLE, and EQDþEact (open circles O)

by PL, with calculated CL energies (open downward triangles r) as a func-

tion of the In composition x of the InGaAs CLs. The solid diamond indicates

PR data from Ref. 6.

034309-3 Seravalli et al. J. Appl. Phys. 112, 034309 (2012)

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structures with LCLs of the same composition but smaller

thicknesses. This suggests a different state acts as the reser-

voir for carrier escape in these structures. However, in the

previous PR spectroscopic study of high energy states, spec-

tra were not reported for samples with x> 0.15,6 leaving

open, the question of what happens to the WL in metamor-

phic samples with large x.

In metamorphic structures, it is possible to study the

separate effects of QD-CL mismatch (proportional to the QD

strain) and the confinement potential for QD carriers on the

relevant sample parameters. In Refs. 5 and 6, we have dis-

cussed in detail how the change in the QD-CL mismatch

affects the activation energies when x is kept constant. In the

current work, we focus on the effect of varying the confine-

ment. QD confinement potentials were calculated as the sum

of the electron and heavy hole band discontinuities between

the InAs QDs and InGaAs CLs. In Fig. 4, the experimentally

determined PLE values of the CL and WL are plotted against

these calculated confinement potentials, in addition, to the

QD peak energy emission at 6 K derived from the PL spectra

of Fig. 1. As the confinement is reduced, due to the decrease

of the InGaAs band gap (as experimentally verified by the

CL PLE), the levels to which carriers initially in the QDs are

thermally excited to are lowered, for both escape to WL

states (samples A–D and F) and escape to CL states (samples

E and G). Fig. 4 shows that the QD emission in samples A-D

(all engineered for RT emission at 1.3 lm) is not affected by

a reduction in confinement as x is increased, due to the con-

comitant increase of the QD mismatch, obtained by using

thinner LCLs (see Table I). In contrast, it is noticeable that

the WL energy in samples A-D is much more sensitive to the

QD-CL mismatch than is the QD emission. A similar behav-

ior is observed for samples E and F (engineered for RT emis-

sion at 1.4 lm). This pair of samples demonstrates an

additional interesting behavior in that for sample E (smaller

x, thicker LCL) no WL feature is observed, resulting in ther-

mally activated carriers escaping directly to the CL, as veri-

fied by the coincidence of EQDþEact with the energy of the

CL (see Figure 3). This results in a larger thermal activation

energy for the QD PL in this sample, allowing the observa-

tion of emission up to RT (Figure 1). In contrast in sample F

carrier’s escape to the WL which results in a smaller activa-

tion energy and the complete quenching of the PL at 200 K.

Metamorphic samples, hence, allow QD structures emitting

at the same wavelength but with different WL and CL

energies.

Using the present data, obtained from PLE spectra, it is

possible to conclude more directly that WL states in samples

with high-x and thick LCLs appear to be absent. This behav-

ior cannot be due to the band discontinuities becoming too

small so as not to confine the WL states as in Fig. 4 it is evi-

dent that the WL is observed for sample F despite having a

lower confinement potential than sample E where the WL is

absent. This possible absence of WL states is tentatively

attributed to an increase of surface roughness with increasing

metamorphic layer thickness. This would affect the WL

properties and possibly inhibit the formation of a two-

dimensional system. It should be noted that QDs are still

well formed in structures with high In composition, as they

have been experimentally observed by AFM and other tech-

niques, as widely discussed in Refs. 5 and 17. It should also

be noted that this conclusion of an absence of WL states act-

ing as an escape channel for QD confined carriers is based

not only on the absence of a feature in the PLE spectra but

also, more importantly, on the concomitant agreement of

EQDþEact with the CL energy states.

The WL is expected to affect not only the thermal

escape of carriers but also confined carrier properties, for

example, oscillator strengths, inter-dot coupling, and dipole

polarization via Coulomb interactions.18 A modified or

absent WL could have important consequences in metamor-

phic InGaAs/InAs structures with a low density of QDs, as it

has been recently shown that such structures with high In

composition can act as single photon sources with emission

at long wavelengths.19

IV. CONCLUSIONS

In conclusion, PLE has been used to identify the main

energy levels in InAs QDs grown on metamorphic InGaAs

layers. The dominant feature is attributed to the partially

relaxed InGaAs CL. Carriers generated with high energy in

the CLs do not relax into the QDs but are preferentially lost

to non-radiative centres, most probably related to disloca-

tions in the metamorphic layer. By comparing PLE spectra

with data from temperature-dependent PL it is found that in

samples with thin or low-In composition InGaAs layers, WL

states are the main channel for thermal escape of confined

carriers. In contrast, in samples with thick and high In com-

position InGaAs layers there are no WL states acting as

escape channels for confined carriers or relaxation paths for

excited carries. By analyzing data as a function of the QD

FIG. 4. Experimentally determined 6 K CL energies (open squares �) and

WL energies (open upward triangles D) by PLE, and QD emission (open

circles O) by PL as a function of the calculated confinement potentials (the

sum of the QD-CL band discontinuities for electrons and heavy holes).

034309-4 Seravalli et al. J. Appl. Phys. 112, 034309 (2012)

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confinement energy, it has been shown how in metamorphic

QD structures it is possible to control the WL and CL levels

whilst keeping the QD emission constant, with important

consequences in terms of thermally activated carrier trans-

port processes.

ACKNOWLEDGMENTS

The work has been supported by the Italian Ministry of

University and Research through FIRB Contract No.

RBAP06L4S5, by the UE through the “SANDiE” Network

of Excellence, Contract No. NMP4-CT-2004-500101 and by

the U.K. Engineering and Physical Sciences Research Coun-

cil (EPSRC), Grant No. GR/S49308/01.

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