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RESEARCH ARTICLE |  FEBRUARY 07 2024
Epitaxial growth and characterization of multi-layer site-
controlled InGaAs quantum dots based on the buried
stressor method
Imad Limame  ; Ching-Wen Shih  ; Alexej Koltchanov  ; Fabian Heisinger  ; Felix Nippert  ;
Moritz Plattner  ; Johannes Schall  ; Markus R. Wagner  ; Sven Rodt  ; Petr Klenovsky   ;
Stephan Reitzenstein  
Appl. Phys. Lett. 124, 061102 (2024)
https://doi.org/10.1063/5.0187074
 11 March 2024 13:29:00

Epitaxial growth and characterization

of multi-layer site-controlled InGaAs quantum

dots based on the buried stressor method

Cite as: Appl. Phys. Lett. 124, 061102 (2024); doi: 10.1063/5.0187074

Submitted: 21 November 2023 .Accepted: 19 January 2024 .

Published Online: 7 February 2024

Imad Limame,

Ching-Wen Shih,

Alexej Koltchanov,

Fabian Heisinger,

Felix Nippert,

Moritz Plattner,

Johannes Schall,

Markus R. Wagner,

1,2

Sven Rodt,

Petr Klenovsky,

3,4,a)

and Stephan Reitzenstein

1,a)

AFFILIATIONS

Institute for Solid State Physics, Technical University of Berlin, Hardenbergstraße 36, D-10623 Berlin, Germany

Paul-Drude-Institut f€

ur Festk€

orperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., 10117 Berlin, Germany

Department of Condensed Matter Physics, Masaryk University, Kotl

a

rsk

a 267/2, 611 37 Brno, Czech Republic

Czech Metrology Institute, Okru

zní 31, 63800 Brno, Czech Republic

Authors to whom correspondence should be addressed: [email protected]

and stephan.reitzenstein@physik.tu-berlin.de

ABSTRACT

We report on the epitaxial growth, theoretical modeling, and structural as well as optical investigation of multi-layer, site-controlled quantum

dots fabricated using the buried stressor method. This deterministic growth technique utilizes the strain from a partially oxidized AlAs layer

to induce site-selective nucleation of InGaAs quantum dots. By implementing strain-induced spectral nano-engineering, we achieve spectral

control of emission and a local increase in the emitter density. Furthermore, we achieve a threefold increase in the optical intensity and

reduce the inhomogeneous broadening of the ensemble emission by 20% via stacking three layers of site-controlled emitters, which is valuable

for using the SCQDs as a gain medium in microlaser applications. Our optimization of site-controlled growth of quantum dots enables the

development of high-bmicrolasers with increased confinement factor.

C2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://

creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/5.0187074

The scalability of nanophotonic systems is a key to unlocking

their full potential and realizing their widespread applications in vari-

ous fields, such as optoelectronics and emerging photonic quantum

information technologies based on single quantum emitters.

1–3

Furthermore, the emission properties of cavity-enhanced microlasers

are also improved by a controlled integration of quasi-zero-

dimensional gain centers.

In this regard, the deterministic fabrication

of nanophotonic devices with a controlled number and position of

semiconductor quantum dots (QDs) is an important step forward in

achieving that goal. In the last two decades, innovative methods to

control the position of QDs have been developed, including the usage

of etched nanoholes, pre-patterned substrates, and cleaved-edge over-

growth.

5–12

Another attractive growth approach for site-controlled

QDs (SCQDs) is the buried stressor method.

13,14

This deterministic

growth technique leads to high-quality SCQDs with narrow emission

linewidth and high multi-photon suppression as well as pronounced

resonance fluorescence,

In this method the stressor layer below the

growth surface is used to control the nucleation site, and local number

of the QDs.

13,14

This allows for the deterministic and scalable fabrica-

tion of single-photon sources (SPSs)

and high-bmicropillar lasers.

However, despite its proven potential, systematic studies on the

employment of the buried-stressor method for the optimization of

site-controlled QDs (SCQDs) for usage as a gain medium in microlas-

ers are still lacking.

Here, we report on the investigation of the buried stressor growth

method to engineer the QD gain medium for increasing the confine-

ment (C)andb-factors of microcavity lasers, while at the same time

reducing the absorption losses due to non-positioned QDs. This paves

the way to the energy-efficient operation of those devices, which is of

great interest, for instance, in photonic reservoir computing, where

hundreds of microlasers need to be pumped above threshold.

18,19

Furthermore, we show that the buried stressor technique could serve

Appl. Phys. Lett. 124, 061102 (2024); doi: 10.1063/5.0187074 124, 061102-1

CAuthor(s) 2024

Applied Physics Letters ARTICLE pubs.aip.org/aip/apl

11 March 2024 13:29:00

as an alternative approach for growing high-quality QDs emitting pro-

spectively in the telecom O-band, to complement existing non-

positioning methods, such as strain-reducing layers (SRLs) and meta-

morphic buffer layers (MB).

20,21

The buried stressor method takes advantage of the strain induced

by a partially oxidized AlAs layer, stemming from a lattice constant

difference between AlAs (5:660 ˚

A) and Al

(4:785 ˚

A). The resulting

tensile strain at the center of the etched mesa protrudes to the surface,

where it leads to the accumulation of indium atoms at the most tensile

strained positions (largest lattice constant) above the aperture, which

results in the site-controlled nucleation of QDs.

In addition, control

over the emission wavelength is achieved by optimizing the growth

interruption time. To further increase the optical gain for laser applica-

tions, we stack multiple layers of SCQD (ML-SCQDs). Compared to

conventional SK QDs, the stacking of SCQDs results in higher optical

gain and better spectral homogeneity, as well as positional control of

QD formation.

Our samples were fabricated through a two-step growth process

using metal-organic chemical vapor deposition (MOCVD). After the

first growth step, UV-lithography was employed to pattern an array of

square mesas with approximately 20 lm side length. These mesas were

subsequently dry-etched to expose the AlAs layer, which was then

selectively oxidized to create the AlAs aperture at the mesa’scenter,

before performing the second growth step of the site-controlled QDs.

The layer design and process steps are displayed in the supplementary

material Fig. S1. Atomic force microscopy (AFM) was employed to

investigate the strain profile at the QD growth surface and to measure

the QD density. The microscopic analysis is complemented by Raman

spectroscopy above the aperture, which is used to monitor the strain

profile by measuring the strain-induced spectral shift of the longitudi-

nal optical phonon (LO) line of GaAs. Moreover, we applied the finite-

difference method to simulate the strength and shape of induced stress

and its effect on the SCQDs using continuum elasticity theory.

Finally, high-resolution cathodoluminescence (CL) was utilized to

characterize the emission properties of the emitters. Our approach

allows us to shift the energy of the site-controlled emitters compared to

the non-positioned QDs, enabling, thus, the growth of SCQD with high

density without compromising the site-controlled nature of the buried

stressor technique. Moreover, compared to non-positioned QDs, we

enhance the optical gain threefold while reducing the inhomogeneous

broadening of the ensemble by stacking three layers of SCQDs.

We first discuss the growth and emission properties of a single

layer of SCQDs. Figure 1(a) shows a CL intensity color map of a single

layer of SCQDs with a growth interruption time of 40 s in the wave-

length range between 1000 and 1150 nm overlapped with an SEM

image of the mesa, demonstrating high position accuracy (below

100nm) of the buried stressor method. Noteworthy, while SCQDs

based on nanohole arrays and inverted pyramids feature an alignment

accuracy in the tens of nm and are suitable for integration into pho-

tonic crystal cavities,

24,25

buried stressor SCQDs, with usually better

optical properties, are more suitable for integration in high Q-factor

cavities with relaxed mode-matching requirements on the order of a

few 100 nm. CL emission of on- and off-aperture positions from the

mesa shown in Fig. 1(a) is displayed in Fig. 1(b).TheSCQDshavea

maximum emission wavelength of 1070 nm [red curve in Fig. 1(b)]

and exhibit a strongly redshifted emission in comparison with the

non-positioned QDs of 950 nm [blue curve in Fig. 1(b)]. For

comparison, the wetting layer emission occurs at 920 nm (see the sup-

plementary material, Fig. S2). The shift of the emission wavelength is

achieved by higher localized tensile surface strain above the aperture

combined with a longer growth interruption (of 40 s as discussed

below) time. The observed magnitude of the redshift is compatible

with an increased indium content in the SCQDs as well as with larger

QDs. We interpret the observed behavior by buried stressor-induced

migration of the indium atoms to the most tensile strained position

directly above the aperture, resulting in a higher indium concentration

in the center of the mesa. This together with a higher InGaAs growth

rate, hence, leads to an increase in the density, size, and indium con-

tent of SCQDs compared to non-positioned QDs, thus contributing to

the observed redshift of SCQD emission. Intriguing, apart from the

achieved spectral redshift, the peak intensity of the SCQDs is main-

tained compared to the non-positioned QDs. We attribute that to the

high quality of the growth surface and low defect density compared to

other methods for redshifting QD emission using a SRL or a MB.

To obtain better insight into the buried stressor growth of

SCQDs, the surface strain distribution at the growth surface was mod-

eled using the continuum elasticity theory. Simulations of the strain

profile at the GaAs surface vs the position on the mesa for different

aperture sizes are depicted in Fig. 1(c). The difference in lattice con-

stants between AlAs and Al

results in lateral strain, which propa-

gates in the growth direction to the surface [see the supplementary

material, Figs. S3(a) and S3(b)], where the InGaAs wetting layer is

deposited. The buried stressor creates tensile and compressive stress

with surface strain extrema of 0.4% and 0.12%, respectively. The

strain profile depends in this configuration solely on the size of the

aperture as seen in Fig. 1(c) (see the supplementary material, Fig. S4),

whereas the strain magnitude depends on the size of the aperture as

can be observed in Fig. 1(c),thestressorlayerthickness,andtheaper-

ture to surface distance [see the supplementary material, Fig. S6(a)].

Corresponding AFM measurements in Fig. 1(d) show a smooth height

profile after the growth of a single uncapped QD layer for different

aperture sizes. Here, the migration of indium atoms from the compres-

sive to the tensile position leads to a change in the growth rate directly

above and surrounding the aperture. As shown in Fig. 1(d),thisresults

in a thickness difference in the growth direction, whereas

the unstrained GaAs on the mesa remains unchanged. We denote

the height difference between most tensile positions located above the

aperture and the GaAs surface as Dt, a parameter that characterizes the

magnitude of the strain-induced growth rate change above the stressor.

As predicted by simulations, the smallest aperture exhibits the highest

Dtdue to an increase in the surface strain with a smaller aperture size.

As the compressively strained position is depleted of indium, no QD

growth is observed in related areas in the AFM images (see the supple-

mentary material, Fig. S4). Indium atoms that migrate to the nearby

tensile strained positions above the aperture result in SCQDs with

higher indium content, which explains the redshift of the SCQD emis-

sion in Fig. 1(b). The associated lack of QD growth around the aper-

ture can also be observed in the CL maps [see white ring surrounding

the aperture in Fig. 1(a) right], as the luminescence from the surround-

ing area of the aperture is weak to non-existent.

The strain distribution is monitored by measuring the Raman

emission in the surrounding GaAs above the unoxidized AlAs layer.

For this purpose, we performed systematic Raman spectroscopy mea-

surements. A corresponding color map of the GaAs LO-phonon

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Raman band recomputed to strain for a mesa with an aperture size of

approximately 1000 nm is shown in Fig. 1(e). The stressor-induced

surface strain change is associated with a change in the bond

lengths, resulting in a shift of the GaAs LO-phonon band, in this case

resulting in a Raman shift of approximately 1.7 cm

1

, corresponding

to a strain magnitude of ð0:350 60:001Þ%. The induced strain is evi-

dent in the line scans displayed in Fig. 1(f) for three different-sized

apertures. The noteworthy observation in this context is the

excellent quantitative agreement between the computed strain

(approximately 0.3%) and the measured strain [ð0:350 60:001Þ%], as

evident in Figs. 1(c) and 1(f). This substantiates the predictive nature

of the simulation.

Next, we study the stacking multiple QD layers using the buried

stressor method. In Fig. 2(a),wepresenttheAFMheightprofilesatthe

center of the mesa for one, two, and three layers of SCQD structures

with similar aperture sizes (approximately 2 lm). We observe that

stacking of multiple QD layers does not alter the overall shape of the

surface strain profile and that QDs in the upper layers are aligned by

the strain field of buried stressor as well (see, e.g., Fig. S5). However,

the height difference Dtbetween the most tensile and compressive

positions increases systematically with the number of deposited QD

layers, as indicated by the red data points in Fig. 2(b). The gradual

increase in Dtwith the number of QD layers is attributed to the larger

tensile strain due to the increased distance between the aperture and

FIG. 1. Optical and strain analysis of SCQDs. (a) Low temperature (20 K) CL intensity map of SCQD emission ranging from 1000 to 1150 nm is overlaid with an SEM image of

the quadratic mesa, including the crystallographic orientation of the mesa edges. The high position accuracy of the growth technique is visually demonstrated by the presence

of two yellow dotted lines. Inset: Zoom-in CL image taken at the center of the mesa. (b) The CL spectrum is shown in red and blue for the center and off-center regions of the

mesa at 20 K, respectively. (c) Calculated surface strain profile exx þeyy along the cross section of the mesa, as a function of the aperture size. exx þeyy ¼0 corresponds to

unstrained GaAs. (d) AFM height profile of the aperture for various aperture sizes. (e) Strain taken at the center of the mesa, obtained by Raman measurements. (f) A line cut

to the center of the mesa of the Raman map in (e) for three different aperture sizes (800, 1100, and 1900 nm).

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SCQD layer in addition to the accumulation of the indium, which per-

sists across the multiple layers. Therefore, the growth of multiple layers

and the stress induced by the QDs do not seem to modify the surface

strain profile significantly, except of an approximately 0.7% change in

the absolute surface strain between the first and the third layer due to

the larger distance from the aperture. However, we do not expect any

significant effects on the formation of SCQDs, which is confirmed by

the constant QD density distribution observed across the layers [see

the supplementary material, Figs. S5 and S6(b)].

To evaluate the optical properties of the QDs, we fit the spectra

obtained from CL measurements, as shown in Fig. 1(b), by a Gaussian

function, across multiple mesas and variations in the number of layers.

In Fig. 2(b), we present the corresponding emission wavelength of the

SCQDs. Notably, we observed a distinct blue shift in the emission

when two layers were stacked, reducing the wavelength from

(1059 610) nm for a single layer to (1027 622) nm for the two-layer

structure. This shift can be attributed to a decrease in the average

height of the QDs in the second layer analogous to standard SK

QDs.

Furthermore, Fig. 2(c) demonstrates a similar trend regarding

the inhomogeneous broadening of the non-positioned and SCQDs,

which decreases with the number of layers from (80 613) nm and

4366

ðÞ

nm in the first layer to 6769

ðÞ

nm and 2863

ðÞ

nm for

positioned and non-positioned QDs, respectively, indicating improved

uniformity of QD sizes because of strain-coupling.

The stacking of SCQDs also results in a nearly linear increase in

integrated CL intensity, as shown in Fig. 2(d). The intensity in

Fig. 2(d) does not show saturation, which shows that the high optical

quality of the emitters is preserved during the stacking.

Interestingly, the buried stressor method also provides an attractive

opportunity to redshift the emission of SCQD. For example, consider

the strain magnitude or, more precisely, the shift in the GaAs LO band

associated with an aperture size of 1000 nm. The corresponding Raman

frequency shift is approximately 1.65 cm

1

, a value comparable to that

of an In

0.15

0.85

As MB layer with a thickness ranging from approxi-

mately 40–50 nm.

To demonstrate the strain-engineered redshifting of

SCQD emission, we utilize the buried stressor-induced strain profile

above the aperture. Here, the additional tensile strain in the center of the

mesa leads to energetically lower QD equilibrium compared to the

unstrained surface. Second, by increasing the growth interruption time,

we allow for the SCQDs to accumulate more indium atoms, resulting in

bigger dots with increased indium concentration and a corresponding

70nmredshiftoftheemissionasseeninFig. 3(a). The blue (red) spec-

trum represents a sample prepared with a growth interruption time after

the deposition of InGaAs of 20 (40) s. Additionally, our numerical

FIG. 2. Properties of multi-layer SCQDs. (a) Line scan of AFM measurements, presenting the height profile for a varying number of SCQD layers at the center of the mesa with

similar aperture sizes (approximately 2 lm), relative to the GaAs surface. (b) Emission wavelength of the SCQDs (black data points) relative to the number of deposited emitter

layers and height difference (red data points) between the most tensile and compressive positions in the center of the mesa. (c) The CL emission spectra exhibit inhomoge-

neous broadening for different numbers of stacked layers, including the wetting layer (red), non-positioned QDs (blue), and SCQDs (black). (d) The integrated CL intensity is

plotted against the number of grown layers, revealing enhanced intensity with increased layer stacking.

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studies predict that the tensile strain induced by the aperture will further

redshift the emission [see the supplementary material, Fig. S7(a)]. Note

that in the calculations, the surface strain obtained from the continuum

elasticity theory is added to the as-grown QD strain during the k.p com-

putation.

27,28

The increase in the aperture size results in a weaker lateral

strain between AlAs and Al

, which in turn decreases the stress prop-

agation to the surface. The increased magnitude of the tensile strain at

the center of the mesa due to the reduction of the aperture size results in

larger QDs with increased indium content, leading to a redshift of

70 nm in the emission of the SCQDs, as depicted in Fig. 3(b).In

Fig. 3(c), the experimental and theoretical emission wavelengths are

plotted against the aperture size. In both graphs, the emission wave-

length is highest for the smallest aperture size and then drops rapidly

with increasing aperture size. That is due to the reduction in surface

strain with a bigger unoxidized AlAs region. Consequently, the migra-

tion of indium atoms and an increase in growth rate above the aperture

occur. The discussed dependencies are also reflected in the CL line scans

of apertures with varying sizes, as presented in Fig. 3(d). Apertures of

930, 1700, and 5300 nm in width result in peak emission wavelengths of

1070, 1010, and 985 nm, respectively. In order to shift the emission

toward the telecom O-band, we suggest that a thicker AlAs stressor layer

and a top GaAs spacer, leading to an increased tensile surface strain, and

associated increase in Indium content in QDs should be used.

In summary, we introduced an innovative approach based on the

buried stressor technique, to achieve a redshift of SCQDs relative to

non-positioned ones. This approach enables the deposition of high-

density SCQDs within a singular layer, preserving excellent positional

control. This is achieved while concurrently monitoring and simulat-

ing strain distribution to gain deeper insights into the underlying

mechanisms. Furthermore, we demonstrate the stacking of three

SCQD layers, resulting in a threefold increase in optical intensity and a

reduction in ensemble inhomogeneous broadening. Subsequently, we

investigate the strain-induced redshift of the SCQD ensemble, granting

us direct control over emission properties. Combining the buried

stressor technique with strain nano-engineering of emission provides

control over the site, number/density, and optical properties of both

ensembles and single QDs. This achievement can potentially pave the

way not only for high-bfactor and low-threshold microlasers, but also

for single-photon sources operating in the O-band without the need of

a conventional strain reducing layer.

FIG. 3. Strain engineering the emission wavelength of SCQDs. (a) CL spectra of two similar structures containing SCQDs are shown. The blue (red) spectrum corresponds to

QDs obtained with a growth interruption time of 20 (40) s. (b) The CL emission from a structure with 40 s of growth interruption time for two aperture sizes. Increasing the size

of the aperture reduces the tensile surface strain, leading to a blue shift in the emission. (c) The experimental peak position of the emission for the SCQDs (black data points)

and the numerical results. The theory results are provided for the surface strain at the center of mesa (green squares) and for maximum surface strain on the aperture (khaki

squares). (d) Examples of CL line scans displaying the emission properties above the aperture for various aperture sizes are shown, with the red line serving as a reference to

facilitate the observation of the blue shift resulting from the increased aperture size.

Applied Physics Letters ARTICLE pubs.aip.org/aip/apl

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See the supplementary material for details regarding fabrication,

characterization, and simulation methods, as well as additional optical,

structural, and simulation results.

The authors gratefully acknowledge the financial support from

the Volkswagen Foundation via the project NeuroQNet2, the

German Research Foundation via INST 131/795-1 320 FUGG, the

German Federal Ministry of Education and Research (BMBF) via the

project MultiCoreSPS (Grant No. 16KIS1819K), and the SEQUME

projects (20FUN05) and QADeT (20IND05) from the EMPIR

program cofinanced by the Participating States and from the

European Union’s Horizon 2020 research and innovation program.

P.K was partly funded by the Institutional Subsidy for the Long-Term

Conceptual Development of a Research Organization granted to the

Czech Metrology Institute by the Ministry of Industry and Trade of

the Czech Republic and by the project Quantum Materials for

applications in sustainable technologies, CZ.02.01.01/00/22_008/

0004572. The authors further acknowledge Kathrin Schatke, Praphat

Sonka, Lucas Rickert, Aris Koulas-Simos, and Maximilian Ries for

their invaluable technical support and engaging scientific discussions,

which greatly contributed to this research.

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Imad Limame: Conceptualization (equal). Petr Klenovsky: Funding

acquisition (supporting). Stephan Reitzenstein: Supervision (lead).

Ching-Wen Shih: Investigation (equal). Alexej Koltchanov: Data curation

(equal). Fabian Heisinger: Data curation (supporting). Felix Nippert:

Formal analysis (supporting). Moritz Plattner: Data curation (supporting).

Johannes Schall: Methodology (supporting). Markus R. Wagner:

Supervision (supporting). Sven Rodt: Methodology (supporting).

DATA AVAILABILITY

The data that support the findings of this study are available from

the corresponding author upon reasonable request.

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