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METANANO 2019
IOP Conf. Series: Journal of Physics: Conf. Series 1461 (2020) 012028
IOP Publishing
doi:10.1088/1742-6596/1461/1/012028
1
Quantum light sources based on deterministic microlenses
structures w ith (111) In(Ga)As and AlInAs QDs.

I A Derebezov 1 , V A Hais ler 1 , A V Hais ler 1 , D

V Dmitriev 1 , A I Toropov 1 , S Rodt 2 ,
M von He lversen 2 , C de la Hay e 2 , S Boun ouar 2 , S Reitz enstein 2

1 Rzhanov Institute of Sem iconductor Physics, Sib erian Branch, Russian Academ y of
Sciences, pr. Ac adem ika Lavrent’ eva 13, Nov osibirsk, 630090 Russia
2

Technische Univ ersitaet Berlin, Hardenb ergstras se 36, Berl in, D-10623 Germ any

Abstract

The results of the deve lopment and implementatio n of a single photon source based on a bottom semiconductor
Bragg ref lector, top deterministic Ga As microlens structures and a si ngle (111) In(Ga)As QD are presented. The
structure of the microcavity ensures effective pumping of

a single (111) In(Ga)A s QD and high emission output
efficiency, a clear single – photon emissi on w as detected with a second – orde r c orrelation fu nction at zero delay
g (2) (0) = 0.07. A system of QD’s on the basis of Al X In 1-X As/Al Y Ga 1-Y As solid sol utions has been studied. T he usage
of broadband A l X In 1-X As solid solutions as the basis of quantum dots makes it possible to expand considerably the
spectral emis sion rang e into the sh ort-wave region, in cluding the wa velength reg ion near 770 nm being of interest
for the desig n of aerospace systems of quantum cr y ptography. The optical characteristics of sing le Al X In 1-X As
quantum dots gro wn according to the Stranski

–Krastano v mechanism are studied by the cryogenic
microphotolum inescence m ethod.

1. INTRODUCTION

Non-classic al ligh t source s emitt ing s ingle photons or entang led pho ton pa irs on d emand
constitute k ey building blo cks towards the re alizat ion of adv anced quantum com m unication netwo rks [1 –
5]. I n recent years, s ingle sel f assem bled quantum dots (QDs) integrated into photonic m icrostructure s
have turn ed out to b e very prom ising cand idate s for realiz ing su ch qu antum

- light sourc es [6–10]. S ing le
QDs can emit o f entang led photon pairs in the proces s of ca scade recom bination of biexc itons (XX) an d
excitons ( X) in the case where excit on state s are e nergy degenera ted or their splitt ing ∆E FS does not
exceed the natur al width of the ex citon levels Γ X = ħ/τ X , where τ X is the life time o f an excito n. In this cas e,
the study touches up on the entang led pho ton pair. I n re al QDs
grown on substrates with orie ntation (001),

the splitting of exciton states ∆E FS usually exceed s the natura l width of th e excito n levels Γ X many tim es,
which is du e to devia tion o f the QD fo rm from a perfect on e and th e presence o f a piez opoten tial induced
by em bedded mechani cal stress es. This is the ma in obs tacle in dev eloping QD based em itters o f
entangle d photon p airs. One of the so lutions of th is prob lem is to use QDs grown on substrate s with
orienta tion (111 ). I n this case, we have QDs o f the symm etry C 3V , in which, accordi ng to [11], the
splitting o f exciton states ∆E
FS

can be supp ressed t o ze ro valu es. The sy stem of I nAs/GaAs q uantum dots
with the un ique wide spectral range r eaching ~400 nm including the firs t and se cond te lecomm unicatio n
standards ( ~0.9 and 1.3 μ m , respective ly) is the m ost stud ied to da te. The sin gle -photon em ission and
emission of pa irs o f enta ng led photons a t waveleng ths near the fi rst telecom munica tion standard were

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IOP Conf. Series: Journal of Physics: Conf. Series 1461 (2020) 012028
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demonstrated us ing sing le I nAs QDs [ 12, 13]. The ex pansion of the spec tral ra n ge of quantum dots into
the sho rt-wav elength reg ion i s of interest f or studyin g physics of new low-dim ensional s emiconduc tor
system s and creating emission so urces fo r system s of atmospheric or a erospa ce quantum cryp tography.
The waveleng th reg ion nea r 770 nm [14], where th e s ensitivi ty of s ilicon pho tod etecto rs is m axim al, an d
the absorption o f the a tmospher ic lay er and fluctuation s of the local r efractiv e index are m inim al, which is
necessary for the m aintena nce of th e photon po lariz ation, is opt imal fo r th ese system s.
I n this work, we realiz e a sing le photon sou rce on (111)B GaAs s ubstra te based on bottom DBR, a
determ inistic ally int egrate d sing le-QD and top m icrolens s tructu re. Probing the photon statistic s of th e
emission o f a single QD-state we dem onstrate si ng le-photon em ission of the QD m icrolens with g (2) (0) =
0.07. In additional we con sider m echanism s of the fo rm ation and the op tical ch aracte ristics o f quantum
dots on th e basis of Al

x In 1−x As/Al y Ga 1−y As ternary solid solutions. Th e optical c haracte ristics of sing le
Al x In 1−x As/ Al y Ga 1−y As QD’s grown acco rding to the Stran ski–Krastanov m echanism were stu died f or th e
first tim e by the c ryog enic m icr ophotoluminescence m ethod. Hanb ury Brown and Twis s experim ent has
been ca rried out to m easure th e photon sta tistics. Phot on correlation function dem onstrates a cl ear pho ton
antibunching effect, what i s a d irect eviden ce of a singl e photon emission by Al

x In 1−x As a single q uantum
dot.

2. SAMPLE P REPARATION

Al x In 1−x As/ Al y Ga 1−y As structures were grown on GaAs(001) substr ates by m olecular-beam
epitaxy (MBE) system Riber – C21
. The

full structure containes two 40- nm thick Al 0. 7 Ga 0.3 As la yers
inhibiting the di ffusion of photoe xcited cha rge carri ers and a 200- nm-thick Al y Ga 1–y As laye r sa nd wiche d
between th e abovem entioned two layers. I n the middle of t he 200-nm
thick Al

y Ga 1–y As layer absorb ing
the ma jor part o f the lase r excita tion powe r, there was a l ayer of Al x In 1–x As QDs. We studie d Al x In 1– x As
QDs whose com position param eter was in the range of x= 0–0.3 with a step o f 0.05. I n this c ase, the
composition of the Al

y Ga 1–y As lay ers was specified to prov ide the re latio n y≈1.7 x. The lay er of Al x In 1–
x As QDs was g rown by the Stransk i–Krastanov m ec hanism at the tem perature T= 505°C. On the Al y Ga 1–
y As surface, an Al x In 1–x As layer was grown to the
critical t hicknes s (~2 single la y ers (SL )). A s the crit ical

thickness was a ttain ed, an array of selfass emb led Al x In 1–x As QDs beg an to emerg e. The growth rate of the
Al x In 1–x As layer was 0.045 SL s –1 . The trans ition fro m the two- dimensi onal m echanism of growth to the
three- dim ensional m echanism was m onitored by th e high-energy electron- diffract ion m ethod. As th e
critica l thickness wa s attai ned, the pro cess of g rowth of Al x In 1–x As stopped, and in th e time τ GI , an array
of Al x In 1–x As QDs was form ed by the Ostwald m e chanism [15-17], after which the Al x In 1–x As layer was
overg rown by Al y Ga 1 –y As. T he t ime of g rowth inte rrup tion w as τ GI = 10 s.
The stru ctu re conta ining (111 )-oriented I n (Ga)As QDs was synthe sized by MBE on 2°-
misorien ted n+ GaAs(111) B substrate. The ini tial stru ct ure of the micro cavity consis ts of a sem iconductor
distributed Br agg reflector (DBR) and a GaAs la yer contain ing an (111) I nGaAs QD lay er.. The DBR
contains 23 perio ds of alter nating quarter- wave
layers of GaAs and Al

0.9 Ga 0. 1 As, which ensures the high
reflection coe fficient (R ≥ 0.99) at the ope rating wa veleng ths near 930 nm . The GaAs layer grown on the
DBR surface has thickness 2λ. There is a QD layer locat ed ins ide th is layer a t dist ance λ/2 from the DBR.
There is also the 10-nm thick Al 0.4 Ga 0.6 As layer locate d inside the GaAs l ayer at distance λ f rom the DBR
and preventing the diffusion of photo excit ed carriers to the s tructural surfa ce.

The GaAs microlen s
structu res were form ed by a device c reated on an el ectron m icro scope and c om bin ing the capab ilit ies of
me asuring the cry ogenic ca thode-lum i nescence (CL) spect ra with high spat ial res olution and car rying out
electron l ithogra phy. The firs t step comes down to de te rm ining the lateral coord inate s of suitable sing le
QDs, which are of interest in dev elop ing non-classical light sou rces. For this pur pose, the elec tron beam
is used to sca n the s truct ural reg ion and m easure the c ryog enic CL spectra. The nex t ste p is thr ee -
dim ensional e lectron li thog raphy carr ied out in ord er to fo rm a lens
-

shaped res istiv e mask placed
according to certain coordinates of the single QD. To perfo rm thi s proc edure, the m icroscope electron
beam with a ca reful ly controlled power c ircumscrib es th e conce ntric circle s whose c enter is d eterm ined
by the single QD co ordin ates. The r emo val of the unexposed re sist is foll owed by pl asma-chemical
etching of the structure. As the e tching rate s of the len s - shaped resistiv e m ask and GaAs are
approxim ately equal, the e tchi ng causes the shape of the semiconduct or m aterial to becom e i dentical to
that of the m ask. As a resul t, there is a GaAs m icrol ens of a controlled siz e that is formed on the stru cture
surface and locate d over t he chosen s ingle QD. The l ens di ameter D an d heig ht H could v ary with in
certain lim its, with ty pical values being ~2.5 and ~0.5 μm, respect ively . In a case is no GaAs m icrolen s

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IOP Conf. Series: Journal of Physics: Conf. Series 1461 (2020) 012028
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structu re on the stru cture s urface, t he exte rnal qu antum efficiency of the emitt er η ext is very low. Due to
the tota l inte rnal re flection (TI R) at the GaAs — air interface, and the critic al ang le of TIR for a g iven
spectral ra nge is ~16˚. The eff ect of TIR is sign if icantly redu ced with the use of GaAs microl ens
structu re. The calculation of the η ex t depending on t he GaAs microle ns structure ch aracte ristic s was
perform ed by num erica l solut ion of the Maxwe ll 3D equatio ns. The calculati on of the
η

ext (H ) for a
microlens with D= 2.4 μm shows that, as the l ens curv ature in creases, the η ext rises up f rom v ery low
values an d reaches the level of 20–25% for H ≥ 0. 5 μm.

3. EXPERIM ENTAL RESULTS

The structure co ntaining Al x In 1− x As QD’s and (111)-
oriented I n (Ga)As QD’s

were studied b y
ma crophotolum inescence a nd microphotol um inescence m ethods. The a rea of the excita tion la ser rad iation
spot at the struc ture sur face was 3000 and 3 μm 2 , respe ctively . The lum inescence signal was exc ited with
Nd:YAG laser radiation at the w avelength 532 nm in the con tinuous -w ave (cw) m ode of operation in a
case of Al x In 1−x As QD’s structures and a tunable Ti:Al 2 O 3 laser operating in the cont inuous reg ime as
well as in the m ode synchroniz ati on regim e with a pulse dura tion of 3 ps in a case of (111)

-or i e nt e d
In(Ga)As QD’s struc tures. The luminescenc e signal was dete cted with a s ingle-g rating m onochromator
equipped with a cooled Ge p–i–n photodiod e (macro lum inescence) o r with a TriVista - 555 trip le-g rating
monochrom ator wit h a cool ed CCD m atri x of Si p hotodetecto rs (m icro luminesce nce). The lum inescence
peaks of single QDs were i nterpreted by apply ing t he dependence s of the inten sities o f the peak s on the
excitatio n laser r adiation power. At the lowest excita tion powers, the exciton pe aks X

become ev ident in
the luminesc ence spectra f irst, and the dependenc e of the inte nsities of the s e peaks on t he excita tion
power is linear. At hig her powers, biexciton peak s XX appear, whose intens ity increas es in accordance
with a square law, as th e lase r power density is increa sed [ 18, 19]. T he statistic al properti es of the
emission o f single QDs w ere studied with the us e of a Hanbury Brown–T w iss (HBT) in terferom eter
assemb led in accordance w ith the classica l schem e [18, 19
]. Em ission from the Q D, when passed th roug h

the first TriVista- 555 m onochromator, was d ire cte d to a 50 /50 CCM 1 -BS014 beam- splitting
nonpolarizing prism that div ided the inc ident lig ht flux into strictly equal pa rts. Em ission in eac h of th e
channels was recorded b y photon counters b ased on PerkinElm er SPCM -AQRH-15 Si avalanche
photodiod es; the tim e charac teristics of em issi on were analyzed with a PicoHarp 300 coincidenc e
counter.
The m acrophotolum inesce nce spec tra of struc tures c onta ining Al x In
1−x

As /Al y Ga 1– y As QD’s with
different com positions are shown in figure 1. The spectra we re recorded a t T= 295 K. The data
demonstra te an extension of the spectr al region of th e emission of QD’s to the short-wavelength reg ion
(to 200 nm),
including the region clo se to 700 nm which is of intere st for the eng ineering of atm ospheri c

system s of quantum cry ptog raphy. The d ata sh own in figure 1 we re obta ined fo r struc ture reg ions
contain ing high density array s of Al x In 1− x As /Al y Ga 1–y As QD’s (with the den sity d ≥10 10 cm –2
). To study

the charac teristics of s ingle QD’s, we need low- density regions with d ≤10 8 cm –2 . To solve thi s problem ,
we use an app roach, in which the Al x In 1− x As layer was grown withou t rotatio n of th e substrate. This
specified t he gradient o f the layer thick ness; as a re sul t, the structure contained QD regions with different
densities in the range from z ero

to the m aximum density d≈ 10 11 cm –2 . The presence of are as with a low
density of QD’ s on the sy nthesized structures a llowed us to refe r to s ingle QD’s a nd to stu dy the ir opt ical
characteris tics by the m icrophoto lum inescence te ch niq ue. Figure 2 contains tw o m icrophoto lum inescence
spectra

of t he A l 0. 1 In 0. 9 As/Al 0.24 Ga 0.76 As QD’s structure, spect ra were obtain ed for areas with different
density of QD’s. The botto m spectrum exhi bits a na rrow peak correspond ing t o
recom binatio n

of the
exciton X in a single Al 0.1 In 0. 9 As/Al 0.24 Ga 0.76 As QD. The statistics of em ission of s ingle QDs was
analyz ed by m easuring pa ir photon correla tions ob ta ined with a HBT in terferom eter. Figure 3 shows the
correlation function g 2 (τ) determ ined for the e xciton peak of a sing le Al 0.1 In
0.9

As/Al 0. 24 Ga 0.76 As QD
(Figure 2, bottom spectrum). At τ= 0, the depende nce g 2 (τ) has a well- pronounce d minimum , g 2 (τ) ≈ 0.46,
which is ind icativ e of the sub-Poisson ty pe of em issio n statistic s. This is d irect v erification of th e
possibility of produc ing sin gle-photon em itters on the basis of A l 0.1 In 0.9 As QDs.

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Figure

1. Photo luminesc ence spectra of three structu res containing Al X In 1-X As/Al Y Ga 1-Y
As QDs
at T=295 K.

Figure 2. Microlum inesce nce spectra for tw o
regions o f the Al 0.1 In 0.9 As/Al 0.24 Ga 0.76 As
structure with the QD de nsity d≈10 11 cm -2
(top) an d d=10 8 cm -2 (bottom ) at T=10 K.

Fi gure 3. The second- order photon
correlation function g 2 (τ) for emiss ion of a
single A l 0.1 In 0.9 As/Al 0.24 Ga 0. 76 As QD, 770 nm
spectra l range , T=10 K.

The m icrophoto luminesce nce sp ectra of m icro lens

es struc tures wit h single ( 111) I n(Ga)As QD’s
are presented at F ig ure 4. Em ission peaks of th e only QD clea rly m anifest th em selves in th e
microlum inesc ence spectr a of the c o m pleted micro ca vity structure with the microlens hav ing a diameter
of D= 2.4 μm and heig ht of H=0.4μm . QD luminescence pea k s were inte rpreted using the d ependence s of
the peak int ensities on t he excit ation-laser em issi on power. The peak of excito n X first and for emost
m anifests
themselv es at the lowes t excita tion po wers in lum inescenc e spec tra, an d power dependen ce o f

the intensity is linear. The peak of biexciton XX appe ar in the spectra at larger powers, the intensity of
which inc reases acco rding to quadra tic law dep endi n g on the laser power d ensi ty [ 13, 18]. I n quantum
optical m easurem ents the second-order autoco rrela tion clearly reveals the single pho ton em ission w ith
g (2) (0) = 0.07, (F igure 5).

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Figure 4. Microlum inescen ce spectra of t he structure based on de termini stic micro lens and
(111)
I n(Ga)As QD’s at diffe rent excita tion powe rs at T=10 K.

Figure 5. The second- order
photon correlation fun ction
g 2 (τ) for the s tructu re based
on determinis tic m icrolens
and sing le (111) I n(Ga)A s
QD, measure d at T= 10 K .

4.
CONCLUSIONS

Thus, in this study the system s of AlI nAs- and (111) I n(Ga)As-based QD’s have been ana lyzed. The
use of wide-g ap Al x In 1– x As alloys as a bas is of QD ’s mak e s possible a substantial extension of t he
spectral region of em ission to th e short-
waveleng th reg ion, includ ing the region close t o 770 nm which is

of interes t for the engin eering of aerospace sy st em s of quantum cry ptog raphy. T he structure s based on a
bottom sem iconductor Br agg reflec tor, top dete rm inisti c GaAs m icrolens structures and a sing le (111 )

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IOP Conf. Series: Journal of Physics: Conf. Series 1461 (2020) 012028
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In(Ga)As QD shows a cle ar sing le – photon emission with a second – orde r corr elatio n function at z ero
delay g (2) (0) = 0.07. Both types o f emitt ers are p rom isin g build ing block towards the realiz ation of

advanced qu antum comm unication ne tworks based on a single sem icondu ctor QD.
This wo rk has been pa rtia lly support ed by RFB R, Projects No. 16- 52-12023.

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