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Role of the wetting layer in the carrier relaxation in quantum dots

July 2002
Applied Physics Letters 81(4):613-615

July 2002
81(4):613-615

DOI:10.1063/1.1495525

Authors:

Stefano Sanguinetti

Università degli Studi di Milano-Bicocca

Moriaki Wakaki

Tokai University

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We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum dot structures—grown by modified droplet epitaxy—where no wetting layer is connecting the dots. We find a fast carrier relaxation time (30 ps) to the dot ground state, which becomes even faster for increasing the photogenerated carrier injection. This shows that the two–dimensional character of the wetting layer is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in literature. We discuss the role of the barrier states as well as the possibility of Auger processes involving the zero-dimensional levels of the quantum dots. © 2002 American Institute of Physics.

͑ a ͒ Four different PL spectra taken at increasing P exc at steps of one

…

͑ a ͒ PL traces of the fundamental QD transition at different P exc :

…

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Role of the wetting layer in the carrier relaxation in quantum dots

S. Sanguinetti,a) K. Watanabe, T. Tateno, M. Wakaki,b) and N. Koguchi

National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan

T. Kuroda and F. Minami

Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan

M. Gurioli

I.N.F.M. - Department of Materials Science; Via Cozzi 53, I-20125 Milan, Italy and L.E.N.S.,

Via Sansone 1, I–50019, Sesto Fiorentino, Italy

共Received 8 March 2002; accepted for publication 29 May 2002兲

We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum

dot structures—grown by modiﬁed droplet epitaxy—where no wetting layer is connecting the dots.

We ﬁnd a fast carrier relaxation time 共30 ps兲to the dot ground state, which becomes even faster for

increasing the photogenerated carrier injection. This shows that the two–dimensional character of

the wetting layer is not relevant in determining the quantum dot capture, in contrast with the

conclusions of several models so far presented in literature. We discuss the role of the barrier states

as well as the possibility of Auger processes involving the zero-dimensional levels of the quantum

The nature of the mechanisms underlying carrier relax-

ation phenomena in quantum dot 共QD兲materials has been

largely debated in the last years.1–5 These phenomena con-

tinue to attract much attention since they involve fundamen-

tal physical aspects of the zero-dimensional semiconductor

systems, and also due to their relevance for device applica-

tions. In QD lasers, the carriers are injected into the barriers

embedding the QDs. After energy dissipation processes, the

carriers are captured by the QDs and then relax to the fun-

damental lasing state. The efﬁciency of the relaxation cas-

cade directly affects the device performances, such as thresh-

old current, temperature stability, and so on. The

achievement of a very fast capture rate is therefore a relevant

aspect of the device optimization study.

The self-aggregated dots, grown by Stranski–Krastanov

epitaxy on a two-dimensional 共2D兲wetting layer 共WL兲, sat-

isfy this requirement. In general, the characteristic carrier

relaxation time from the barrier states to the fundamental QD

level is on the order of a few tens of picoseconds and be-

comes even shorter for large injection of carriers. The nature

of the fast carrier relaxation in QDs is not yet completely

understood. The typical energy separation of the QD elec-

tronic levels, which matches neither the longitudinal acoustic

phonons nor the longitudinal optical phonons, suggested the

presence of the well known phonon bottleneck effect.1Sev-

eral mechanisms have been invoked to explain the lack of

phonon bottleneck. The following picture has recently

emerged. In the low injection regime, the fast relaxation time

共30–70 ps兲has been interpreted as a consequence of the

presence of a continuum tail of WL defect states to which the

carriers easily relax the excess energy.4In addition, resonant

multiphonon processes can mediate the relaxation between

the localized states.6By increasing the carrier injection, the

carrier–carrier interaction speeds up the energy relaxation

rate, via Auger-type processes.7,8 In particular, the Auger

processes involving carriers localized in the 2D-WL states

have been claimed to increase the carrier relaxation rate in-

side the QD.9,10 The WL is therefore assumed to have a key

role in the QD carrier relaxation.8

In this letter, we intend to test this picture by means of

picosecond time-resolved photoluminescence 共PL兲in a

GaAs/AlGaAs QD grown by modiﬁed droplet epitaxy

共MDE兲.11,12 MDE is a nonconventional growth method for

self–assembling semiconductor QDs even in lattice matched

systems, such as GaAs/AlGaAs.12 Among other interesting

features, this alternative method leads to the fabrication of

defect free dots without any WL in the structure.13 Therefore,

we eliminate a priori the role of the WL and investigate the

resulting carrier relaxation processes. We found very similar

results with respect to standard QD with a WL, that is a fast

relaxation time 共30 ps兲which becomes even faster for in-

creasing the photogenerated carrier injection. These results

show that the 2D character of the WL is not relevant in

determining the QD capture. We discuss the role of the bar-

rier states as well as the possibility of Auger processes in-

volving the zero–dimensional levels of the QDs.

The samples were grown by MDE12 with the following

procedure.14,15 After the growth of 300 nm GaAs buffer layer

and 500 nm Al0.3Ga0.7As barrier layer at 580 °C, the sub-

strate temperature was lowered to 180 °C, the As valve was

closed and the As pressure in the growth chamber was de-

pleted. 1.75 monolayers of Al0.3Ga0.7 alloy was supplied in

order to form a group-III stabilized surface on the c4⫻4

reconstruction. The subsequent deposition of 3 monolayers

of Ga at this temperature gives rise to the formation of tiny

Ga droplets on the substrate. Following the deposition of the

droplets, an As4molecular beam was irradiated on the sur-

a兲Permanent address: Department of Materials Science, Via Cozzi 53,

I-20125 Milan, Italy; electronic mail: stefano.sanguinetti@mater.unimib.it

b兲Also at: Department of Electro–Photo Optics, Tokai University, Hiratsuka,

Kanagawa 259-1292, Japan.

APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 4 22 JULY 2002

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face. After the complete change of reﬂection high-energy

electron diffraction pattern from halo to spots, an

Al0.3Ga0.7As barrier layer of 10 nm was grown by migration

enhanced epitaxy at the same temperature of 180 °C. This

temperature was chosen in order to prevent 2D regrowth of

the naked GaAs microcrystals.12 Then, the growth tempera-

ture was raised back to 580 °C and 90 nm of Al0.3Ga0.7As

and 10 nm of GaAs as cap layer were grown by ordinary

molecular-beam epitaxy. The sample was reloaded in the

growth chamber and annealed in 1.5⫻10⫺5Torr As4atmo-

sphere at 640 °C. This annealing procedure was shown to

improve the sample quality healing the defects formed in the

low-temperature growth of the Al0.3Ga0.7As barrier layer.13,16

Before the deposition of the capping layer, surface and cross

section high-resolution scanning electron microscope images

of the complete annealed structure demonstrate the formation

of pyramidal shape nanocrystals with typical base sizes of

16 nm⫻20 nm and the absence of the WL.15

Continuous wave 共cw兲PL spectra were measured at 17

K and excited with an Ar⫹laser in multiline mode. PL spec-

tra were measured by a grating monochromator operating

with a GaAs photomultiplier. Time-resolved PL measure-

ments, have been performed by using the second-harmonic

(␭exc⫽400 nm) of a fs mode-locked Ti-Sapphire laser with

76 MHz repetition rate. The PL signal was dispersed by a

polychromator, and time resolved by a streak camera with a

resolution of 1 meV and 9 ps, respectively. All time-resolved

measurements were performed at 5 K.

Figure 1 共a兲shows the cw PL spectra for different exci-

tation power densities. At low-power density, the PL spec-

trum consists in a broad QD band 共160 meV兲, reﬂecting the

large inhomogeneous distribution of the QD size, and in a

narrower band at ⬇2 eV associated with recombination in

the Al0.3Ga0.7As barriers. This latter consists in a doublet

with intrinsic exciton recombination at 1.97 eV and extrinsic

recombination at 1.94 eV. For Pexc larger that 60 W/cm2, the

QD band shows in the high-energy tail 共at ⬇1.82 eV兲the

appearance of a broad emission associated with the carrier

population of the QD excited levels.

The analysis of the time evolution of the different spec-

tral components in the PL spectra 关Fig. 1 共b兲兴 shows an al-

most constant decay 共400 ps兲and rise time 共35 ps兲of the QD

of different size within the inhomogeneously broadened PL

band. On the wing of the high-energy tail of the QD-PL band

(⬎1.84 eV), the decay time becomes shorter 共200 ps兲and it

is associated with the recombination from the QD excited

states. The comparison between the time evolution of the QD

ground state (Eem⫽1.65 eV) and the time evolution of the

excited-state emission (Eem⫽1.85 eV) is reported in Fig.

1共b兲. Finally, the barrier PL time evolution 共not shown兲is

characterized by a fast rise time (⬍10 ps) and a relatively

long decay time 共250 ps兲. Therefore, the barrier PL kinetics

is not determined by the QD capture mechanisms. This can

be explained by assuming that the carriers which recombine

in the barrier are localized in a spatial region far from the QD

layers.

Figure 2 共a兲reports the PL time evolution of the funda-

mental optical transition in the QDs for three different exci-

tation power densities. The time evolution of the PL band has

been ﬁtted with a phenomenological model based on the dif-

ference between two exponential decays after a convolution

with the experimental response function. The best ﬁts are

reported in Fig. 2 共a兲as solid lines. The decay time

␶

Dis

almost constant for a large range of Pexc 共nearly three orders

of magnitude兲. This shows that

␶

Dstems from the enhanced

optical matrix element of the ground-state transition in

GaAs/Al0.3Ga0.7As QD and not from nonradiative competi-

tive channels. At Pexc higher than 300 W/cm2, a lengthening

is observed due to the QD ﬁlling. Aclear reduction of the PL

rise time

␶

Ris also observed when increasing the excitation

power, showing that the carrier capture into the QDs strongly

depends of on the carrier injection. Asummary of the PL rise

time is reported in Fig. 2 共b兲. A sharp decrease of

␶

Ris ob-

served for Pexc larger than ⬇10 W/cm2. At the highest ex-

citation power, the rise time of the PL from the low-lying

transition is almost the same of the rise time of the PL from

the excited state. The solid 共dashed兲line in Fig. 2共b兲refers

to a best ﬁt with 1/Pexc (1/Pexc

2) dependence. Despite the

small amount of data, the comparison indicates that the

1/Pexc dependence gives a better agreement with the experi-

FIG. 1. 共a兲Four different PL spectra taken at increasing Pexc at steps of one

optical density in the range 10 W/cm2–10 kW/cm2.共b兲PL traces recorded

at 1.65 eV 共ground-state emission—circles兲and 1.85 eV 共excited-state

emission—squares兲with Pexc⫽1 kW/cm2.

FIG. 2. 共a兲PL traces of the fundamental QD transition at different Pexc :

3300 W/cm2共squares兲, 330 W/cm2共circles兲, and 3 W/cm2共triangles兲. The

ﬁts are shown by continuous lines. 共b兲PL rise time as a function of Pexc

共circles兲. Continuous and dashed lines show the best 共in ps兲1/Pexc and

1/Pexc

2ﬁts, respectively.

614 Appl. Phys. Lett., Vol. 81, No. 4, 22 July 2002 Sanguinetti

et al.

Downloaded 16 Apr 2004 to 144.213.253.14. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

mental data. Finally, we note that in the range of Pexc corre-

sponding to the speeding up of

␶

R共Ref. 17兲, the QD PL

spectrum shows the insurgence of the recombination from

excited levels.

Let us now compare our results with the ﬁndings re-

ported in literature on standard InAs/GaAs QDs which nucle-

ate on a 2D InAs WL. As already discussed, several recent

papers assume that the presence of the 2D layer connecting

the QDs has relevant consequences on the QD carrier relax-

ation. In particular, Morris et al.8reports a PL rise time

␶

⫽35 ps, for Pexc⬍10 W/cm2, and a decrease of

␶

Rtoward a

value of 10 ps when Pexc⬎100 W/cm2共with a 1/Pexc depen-

dence兲. The 1/Pexc rise time behavior has been attributed to

carrier relaxation processes involving two-particle Coulomb

scattering 共pseudo–Auger process兲between carriers in the

QDs and in the WL.9,10,8 The very same results are observed

here in QDs that are not connected by the 2D WL. We,

therefore, conclude that the picture in which the WL states

play a relevant role in speeding up the QD relaxation time is

not completely correct. Our data show that both the fast car-

rier relaxation time, and the dependence of

␶

Ron Pexc , are

general properties of self-aggregated QDs.

In our sample, the continuum of states associated with

the barriers may play the role attributed to the WL in Refs. 4

and 8. That is, the presence of defect states in the

Al0.3Ga0.7As barrier can facilitate the carrier relaxation inside

the QDs.3Similarly, the pseudo–Auger processes could in-

volve the electronic states of the barriers. The most relevant

difference between the barrier and the WL states is, a priori,

the dimensionality. The barrier states have a three–

dimensional character while the electronic states of the WL

are 2D states. Our results show that the 2D character of the

continuum of states over the QD does not play any role in

the carrier capture and relaxation. This can be justiﬁed, in

principle, by considering that the electronic wave functions

associated with the WL states largely penetrate into the bar-

riers thus washing out a predominant 2D character of the WL

states.

On the other hand, the observed behavior may stem from

intrinsic relaxation mechanisms in a QD. Fast relaxation pro-

cesses, promoted by polaronic 共electron–phonon兲, anhar-

monic 共phonon–phonon兲,18 and resonant multiphonon

processes6have been proposed. As far as the dependence of

the relaxation rate on the carrier injection is concerned, we

ﬁnd that the speeding up of the relaxation rate with Pexc

corresponds to a large QD ﬁlling, which leads to PL emission

from the QD excited states. We remark that the same effect

was also present in the previous studies,7,8,19 even if the au-

thors did not stress it. The onset of the Auger effect therefore

occurs when the QD is ﬁlled by a large number of carriers.

This strongly suggests that the nonlinear mechanisms that

are effective in the QD carrier relaxation processes are pro-

moted by the carrier population inside the QD rather than

involving the carrier population in the continuum of states

above the QD. Note also that the 1/Pexc dependence of the

carrier relaxation time is a common feature to any two-

particle scattering mechanisms. Fast intraband relaxation

processes due to two–particle Coulomb scattering 共intraband

Auger兲, active in a QD for dot carrier population larger than

one, have been recently suggested.5,20 We, therefore, believe

that the Auger-type relaxation in QDs has to be associated

with Coulomb scattering processes involving carriers con-

ﬁned in the QD. This is also supported by the recent obser-

vation of fast relaxation processes in isolated CdSe dots.21

In conclusion, we have studied the carrier relaxation dy-

namics in GaAs QDs embedded in three–dimensional

Al0.3Ga0.7As barriers. The removal of the 2D WL does not

modify the carrier relaxation processes. We believe that the

possibility of growing, and therefore investigate, a defect

free QD without a WL gives a further relevant degree of

freedom for obtaining deeper understanding of the carrier

dynamics and optical properties of self–assembled dots.

One of the authors 共S.S.兲acknowledges partial ﬁnancial

support from INFM-LENS.

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共1996兲.

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4114 共1999兲.

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16 S. Sanguinetti, K. Watanabe, T. Kuroda, F. Minami, Y. Gotoh, and N.

Koguchi, J. Cryst. Growth 共to be published兲.

17 The ratio between the carrier injection in cw and time-resolved 共TR兲con-

ditions is given by Ncw /NTR⫽

␯

rep

␶

PLPcw /PTR , where Pcw (PTR)isthe

excitation power density in the cw the 共TR兲condition,

␶

PL is the PL decay

time and

␯

rep is the laser repetition rate.

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et al.

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Chapter

Mar 2024

The review of carrier dynamics in quantum dots (QDs) in this chapter often refers to ensembles of QDs, but we will highlight the properties single quantum dots wherever possible. The discussion starts with the capture of carriers into and their relaxation within the QD potential. We examine the phonon-bottleneck problem and continue with the role of Coulomb scattering. This review of ultra-fast relaxation at low densities closes with pointing out the importance of considering polaron formation. With increasing excitation density charged excitons, multi-excitons and Pauli blocking affect the relaxation dynamics. The treatment of the recombination dynamics of electron-hole pairs has to consider first the mostly unwanted effects of blinking, photodarkening and spectral diffusion. Then the focus is on the interpretation of the decay of the photoluminescence signal. A quantitative discussion of intrinsic parameters like lifetime, oscillator strength and dipole moment of various excitonic species is given. We compare these quantities for the various types of quantum dots. Important for the carrier dynamics in QDs is the role of dark states which will be considered in detail. Finally we will highlight the generation of single photons using semiconductor QDs for possible applications in quantum information. This chapter also includes the description of experimental techniques like charge loading of quantum dots or the Hanbury-Brown and Twiss (HBT) correlation experiment.

Microscopic Origin of the Effective Spin-Spin Interaction in a Semiconductor Quantum Dot Ensemble

Article

Oct 2022

We present a microscopic model for a singly charged quantum dot (QD) ensemble to reveal the origin of the long-range effective interaction between the electron spins in the QDs. Wilson’s numerical renormalization group (NRG) is used to calculate the magnitude and the spatial dependency of the effective spin-spin interaction mediated by the growth-induced wetting layer. Surprisingly, we found an antiferromagnetic Heisenberg coupling for very short inter-QD distances that is caused by the significant particle-hole asymmetry of the wetting layer band at very low filling. Using the NRG results obtained from realistic parameters as input for a semiclassical simulation for a large QD ensemble, we demonstrate that the experimentally reported phase shifts in the coherent spin dynamics between single- and two-color laser pumping can be reproduced by our model, solving a long-standing open problem of the microscopic origin of the inter-QD electron spin-spin interaction.

Polaron lifetime and energy relaxation in semiconductor quantum dots

Article

Full-text available

Aug 2000

We show that the anharmonicity driven instability of optical phonons leads in semiconductor quantum dots to a decay of polaron states which otherwise would be everlasting. By such a mechanism a single electron in an excited dot state can relax down to the ground state even if the electron energy difference markedly differs from the optical phonon energy.

Photoluminescence study of {GaAs} quantum dots grown by droplet epitaxy

Article

Jul 2001
J CRYST GROWTH

We have investigated post-annealing effects of photoluminescence (PL) properties in GaAs/AlGaAs QDs fabricated by modified droplet epitaxy. The annealing temperatures were changed between 5208C and 7608C. The PL intensity of QDs increased drastically with the increase of annealing temperature. The PL intensity of QDs after the annealing at 7608C was enhanced by two orders of magnitude as compared to that of before post-annealing. This sample showed a distinct PL peak even at the room temperature. With the increase of annealing temperatures, the peak energy shifted from 1.646 to 1.749 eV, continuously. These effects may be caused by improving the crystallinity of QDs systems and the size reduction and/or changing the composition of QDs by the post-annealing. # 2001 Elsevier Science B.V. All rights reserved.

Electron and Hole Relaxation Pathways in Semiconductor Quantum Dots

Article

Nov 1999

Femtosecond (fs) broad-band transient absorption (TA) is used to study the intraband relaxation and depopulation dynamics of electron and hole quantized states in CdSe nanocrystals (NC’s) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a “phonon bottleneck,” we observe a fast subpicosecond 1P-to-1S electron relaxation, with the rate exceeding that due to phonon emission in bulk semiconductors. The energy relaxation is enhanced with reducing the NC’s radius, and does not show any dependence on the NC surface properties (quality of the surface passivation). These data indicate that electron energy relaxation occurs by neither multiphonon emission nor by coupling to surface defects, but is likely meditated by Auger-type electron-hole energy transfer. We use fs infrared TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole relaxation pathways leading to the depopulation of NC quantized states. In contrast to the electron relaxation, which is controlled by NC surface passivation, the depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types of samples, independent of NC surface treatment (including NC’s overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

New MBE growth method for {InSb} quantum boxes

Article

May 1991
J CRYST GROWTH

We propose a new MBE growth method for InSb microcrystals on CdTe which has a nearly equal lattice constant to InSb. The average size of the InSb microcrystals was about 150 nm × 200 nm × 70 nm. This method is based on the Sb incorporation into In droplets and thought to be useful for fabricating quantum well boxes.

Fabrication of GaAs Quantum Dots by Modified Droplet Epitaxy

Article

Feb 2000

We propose a modified droplet epitaxy method for fabricating self-organized GaAs/AlGaAs quantum dots (QDs) with a high As flux irradiation and a low substrate temperature. By our novel method, GaAs QDs were successfully formed, retaining their pyramidal shape, original base size and density of droplets, and preventing layer-by-layer growth. Quantum size effects of the QDs were distinctly observed by photoluminescence measurements. It was confirmed that this new modified droplet epitaxy method is promising for fabricating a high-quality GaAs/AlGaAs QD system.

Efficient Carrier Relaxation Mechanism in InGaAs/GaAs Self-Assembled Quantum Dots Based on the Existence of Continuum States

Article

May 1999

Comparison of near-field and far-field photoluminescence excitation (PLE) spectra gives new insight into the carrier relaxation process in InGaAs/GaAs self-assembled quantum dots. The near-field PLE spectra of single quantum dots clearly show 2D-like continuum states and a number of sharp lines, between a large zero-absorption region due to the quasi-0D density of states and the 2D wetting layer absorption edge. The results reveal an efficient intradot relaxation mechanism, proceeding as follows: The carriers can relax easily within continuum states, and make transitions to the excitonic ground state by resonant emission of localized phonons.

Rapid carrier relaxation in self-assembled In_ {x} Ga_ {1-x} As/GaAs quantum dots

Article

Oct 1996
Phys Rev B

Carrier capture and relaxation processes in self-assembled 15-nm In0.5Ga0.5As/GaAs quantum dots are investigated by means of time-resolved photoluminescence spectroscopy. In a systematic study of photoluminescence rise times and barrier decay times (variation of temperature, excitation energy, and excitation density) we aim to identify the physical mechanisms responsible for fast carrier capture and relaxation in quantum dots. Both processes are separated by using appropriate excitation energies. Carrier capture and relaxation are shown to proceed with rates as high as ∼2×1010 s-1 at low temperature even if less than one electron-hole pair per dot and excitation pulse is created. We interpret our results in terms of multiphonon processes at low excitation densities and in terms of Auger processes at high excitation densities. © 1996 The American Physical Society.

Energy relaxation by multiphonon processes in InAs/GaAs quantum dots

Article

Oct 1997

Carrier relaxation and recombination in self-organized InAs/GaAs quantum dots (QD’s) is investigated by photoluminescence (PL), PL excitation (PLE), and time-resolved PL spectroscopy. We demonstrate inelastic phonon scattering to be the dominant intradot carrier-relaxation mechanism. Multiphonon processes involving up to four LO phonons from either the InAs QD’s, the InAs wetting layer, or the GaAs barrier are resolved. The observation of multiphonon resonances in the PLE spectra of the QD’s is discussed in analogy to hot exciton relaxation in higher-dimensional semiconductor systems and proposed to be intricately bound to the inhomogeneity of the QD ensemble in conjunction with a competing nonradiative recombination channel observed for the excited hole states. Carrier capture is found to be a cascade process with the initial capture into excited states taking less than a few picoseconds and the multiphonon (involving three LO phonons) relaxation time of the first excited hole state being 40 ps. The |001〉 hole state presents a relaxation bottleneck that determines the ground-state population time after nonresonant excitation. For the small self-organized InAs/GaAs QD’s the intradot carrier relaxation is shown to be faster than radiative (>1 ns) and nonradiative (≈100 ps) recombination explaining the absence of a “phonon bottleneck” effect in the PL spectra.

Auger carrier relaxation in self-assembled quantum dots by collisions with two-dimensional carriers

Article

Jun 1997

Carrier relaxation in self-assembled quantum dots due to Coulomb interaction with two dimensional (2D) carriers is studied theoretically. Auger coefficients for carrier relaxation rates are calculated in the dipole approximation for Coulomb interaction. The dipole approximation allows one to derive selection rules for Auger relaxation in a cylindrical quantum dot, and to describe a general picture of Auger relaxation via energy levels in self-assembled quantum dots. A numerical example for InAs/GaAs self-assembled quantum dots demonstrates that the Auger effect may lead to relaxation times in the order of 1–10 ps at 2D carrier densities of 1011–1012 cm−2. This result demonstrates the possibility of fast carrier relaxation in quantum dots if the carrier density in the surrounding barrier is sufficiently high. Analytical formulas for Auger coefficients are derived for moderate temperatures of the 2D carriers. © 1997 American Institute of Physics.

Carrier energy relaxation by means of Auger processes in InAs/GaAs self-assembled quantum dots

Article

Jan 2000
APPL PHYS LETT

Carrier relaxation processes are investigated in self-assembled InAs/GaAs quantum dots using time-resolved photoluminescence spectroscopy. The quantum-dot photoluminescence rise time has been measured as functions of carrier excitation density and excitation wavelengths. The measured relaxation time is about 32 ps at low excitation density and decreases by 1 over the excitation density from about 3 W/cm <sup> 2 </sup>, under nonresonant laser excitation. The threshold of this density-dependent regime occurs at a slightly higher density as the excitation wavelength increases and it disappears when the photon pumping energy is below the wetting layer barrier energy. These results clearly establish the regime where Auger processes become the dominant carrier relaxation mechanism in these self-assembled quantum dots. © 1999 American Institute of Physics.

Role of the wetting layer in the carrier relaxation in quantum dots

Abstract and Figures

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