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S lf A bl d Q RiSelf Assembled Quantum Rings

110

MBE LAB, Instituto de Microelectrónica de Madrid ,CNM (CSIC), C/Isaac Newton 8 PTM 28760 Tres Cantos (Madrid) Spain

Dr. Jorge M. García

Newton 8, PTM, 28760 Tres Cantos (Madrid),Spain

Present address: Bell Labs, Mountain Avenue, 600-700, Murray Hill, New Jersey, USA

1889 1900 1921 1939 19641889 1900 1921

19691995

2006

60 years of basic research,6 nobel prizes

“Bell Labs bottoms out” (Institute pulls plug on basic research )20 August 2008 | Nature 454, 927 (2008)

p

Bell Labs nobel pri es1937, Clinton Davisson The wave nature of matter

Bell Labs nobel prizes

1956, Shockley, Bardeen and Brattain Transistor

1977, Philip Anderson Electronic structure of glass and magnetic mat.

, y,

, p g g

1978, Penzias and Wilson Cosmic microwave background radiationg

1997, Chu Cool and trap atoms with laser light

1998, Stormer, Laughlin, and Tsui Fractional quantum Hall

Other inventions: MBE, Unix, C++, etc…

S lf A bl d Q t RiSelf Assembled Quantum RingsOutline

• Intro...G h f S lf bl d Q Ri• Growth of Self assembled Quantum Ring

• Structural Properties: XTEM & XSTM• Optical properties: PL• Magnetic Fields

– Magneto-torsion– PL

• Conclusions

History of self assembled RingsI A /G A• InAs/GaAs– QR at UCSB (study of QD in RTD)

• J M Garcia et al APL 71 (1997) 2014• J.M. Garcia et al. APL, 71 (1997) 2014– QR at IMM (fundamental growth studies)

• D.Granados et al APL, 82 (2003) 2401• InAs/InP

• T. Raz et al. APL 82 (2003) 1706, …• SiGe• SiGe

– J.Cui, APL, 83 (2003) 2907, …• Droplet epitaxy growth• Droplet epitaxy growth

– S. HUANG et al, Solid state data. B, vol. 121-23 (2007), 541, ...

MotivationQ t i f ti• Quantum information manipulation

A Quantum Dot Single PhotonA Quantum Dot Single-Photon Turnstile Device,

P. Michler, A. Kiraz, C. Becher, W. V. Schoenfeld, P. M. Petroff, Lidong Zhang,E. Hu, A. Imamogùlu,

SCIENCE 290, p 2282 (2000)

Fl ibili f d i i d f d i i h• Flexibility of design required for devices with nanostructures in the active region.

Lasers solid state near RT Q bits operation etc– Lasers, solid state near RT Q-bits operation, etc…

• Non trivial quantum geometry. Rings under Magnetic Field





• Conclusions

GROWTH - From Dots to Rings: In(Ga)As/GaAs(001)g ( ) ( )

Quantum DotsTQD=540ºC,

QR In(Ga)/AsGaAs (001)

ρ≈ 5x109 cm-2

QD

2 ML InAs

Partial cap

2 nm GaAs

+

anneal

110

AFM 1x1 micron2

110

10

12

110

AFM 1x1 micron2SIZE of capped QR2.0

4

6

8

10

Z (n

m) SIZE of capped QD

From X-TEM and X-STM

H ~6nm

D ~30nm

X-TEM and X-STM

H ~3.5 nm

D ~20nm along [1-10], 0.4

0.8

1.2

1.6

Z (n

m)

-60-45-30-15 0 15 30 45 600

2

X (nm)

D ~30nm

20% GaAs~25nm along [110]

-60 -45 -30 -15 0 15 30 45 60

0.0

X (nm)

Large Ga-In mixing Vertical size reduction

Requierements for Ring formationq g• QD

– Very uniform size distribution– Large QD. Height > 10 nmg Q g– (3.5x10-6 mbar As2, Ts=540ºC)

• Ring formation• Ring formation– GaAs Capping with 20% height QD– Lower T (500ºC) and As2

in situ, reak time CharacterizationMorphology and relaxation processes during MBE epitaxial growth

SPECIAL TECHNICS @ IMMSPECIAL TECHNICS @ IMMa b SPECIAL TECHNICS @ IMM@

•ACCUMULATED STRESS•Reflectance Difference (RD)

hMs

a b

(monitoring of sample curvature measuring the deflection of two laser beam)

hsMs

Deposition of epitaxial stressed layer or surface stress

ED

y

M

RHEE

Rh

R

Δ (1/R) ∝ Δ (surface tension)Δ (1/R) ∝ Δ (surface tension)[ Δ (1/R) = 6 Δ (σ h) / (Ms hs

2) ]

1ML InAs/GaAs(001)1ML InAs/GaAs(001)Observed features 1ML InAs/GaAs(001) + GaAs CAP

2BEP(As )=2.2x10-6

mbarTs=414ºC(N/m

)

4

•1ML In, As4 continously suppliedIsotropic effect, [110] and [110]

Linear incorporation rate<σcoh

-( )

2

stre

ss

σcoh

σcoh=Mf·(af-as)/af ≈2 N/(m·ML)

No sharp interface, In segregationlengh W(T)

1 wum

late

dg ( )

Unexpected increase of StressThickness while capping “pseudomorphic” InAs

0 20 40 60 800

As GaAs capIn1MLA

ccu

0.5ML/s0.053ML/s

pseudomorphic InAs

InAs 0 20 40 60 80Time (sec)GaAs

J.M. García et al.APL 77, 409 (2000)J.P. Silveira et al, J. Cryst. Growth, 227-228, 995, (2001)

W tti L E b ddi LWetting Layer vs. Embedding Layer

Liquid “Floating” IndiumcappingIn deposition capping

dire

ctio

n

GaAs

In deposition

dire

ctio

n

grow

th dW

InAs Fully relaxed

2D compressed

3D compressed

GaAs

grow

th

InAs(solid) + In (liquid) + InAs(liquid)

Dinamical equilibrium in which the stressDinamical equilibrium in which the stress elastic energy plays the main role

Accumulated Stress (AS) during

BIG QUANTUM DOTS

Dots formation

2.0

2.5 T=540 ºCP As2=1.2 x 10 -6torr.

3D-RHEED

ss (N

/m)

BIG QUANTUM DOTSPhase equilibrium

1.0

1.5

ulat

ed S

tres

0.0

0.5 2 ML InAs

Accu

mu

0 10 20 30 40 50 60 70 80Time (s)

•Relaxation of incoming material on top of QD

TRANSFORMATION SCHEMETRANSFORMATION SCHEMESOLID HIGH QD

G ATHIN GaAs CAP

LIQUID InAs

HIGLY STRESSED InAs

GaAs

TS = 500 ºC A2

Dewetting along 1 1 0 AND 1 1 0

= 1.1x10-6 mbar

SOLID HIGH QD

GaAs

WEAK InGaAs Alloying

QR

GaAs

Accumulated stressAccumulated stress Partial sumaryy

• Large fraction of In is in a liquid-like, stress-free state, which “floats” and progressively incorporates into the capping p g y p pp glayers.

• Stress induced melting of QD tip during• Stress induced melting of QD tip during capping Enhancement of mobility nanoeruption





• Conclusions

Ring Shape is preserved after cappingg p p pp g

AFM profileρ≈ 5x109 cm-2

110

AFM 1x1 micron2

08

1.2

1.6

2.0

(nm

)

-60 -45 -30 -15 0 15 30 45 60

0.0

0.4

0.8

X (nm)

Z (

Daniel Granados et al.APL. 82, 2401 (2003), APL 86, 071918 (2005)

X-STM of capped Rings

AFM profile

Dr. Paul Koenraad, Univ. EindhovenP. Offermans et al. Appl. Phys. Lett. 87, 1 (2005)

Parametrization of potential from X STM datafrom X-STM data

Structural CharacterizationStructural Characterization Partial sumaryy

• Ring shape is preserved after capping• Drastic reduction in vertical size• Knowledge of realistic distribution of• Knowledge of realistic distribution of

Indium





• Conclusions

Tuning of emissiong

1 01.1

1400 1300 1200 1100 1000 900 800

Q ant mDots

Wavelength (nm)

15K

0 70.80.91.0 QuantumDots

Quantum Ringsna

l (a.u

.) 15K

0 40.50.60.7

1s t Excited StateGaAs

ed P

L sig

n

0. 20.30.4

WLNorm

alize

0.9 1.0 1.1 1.2 1.3 1.4 1.50.00.1N

Energy (eV)Energy (eV)•Large change in Optical properties due to vertical confinement.

•Tunability of the nanostructures properties…Perfect for device applications!!





• Conclusions

QUANTUM…OK but Rings or small Dots?but… Rings or small Dots?Aharonov – Bohm Effect

The persistent current I(Φ) of a ring as function of magnetic flux Φ. The Aharonov-Bohm effect implies that allphysical properties of a 1D quantum ring of radius R in a magnetic field B are periodic functions of the enclosed magnetic fluxwith period Φ0=h/e (the flux quantum)

At B =0 there is no persistent current in the ring At small B the ground state has zero orbital momentum (ℓ = 0)At B =0 there is no persistent current in the ring. At small B the ground state has zero orbital momentum (ℓ = 0),and to compensate for the magnetic flux that penetrates the ring, a (diamagnetic) persistent current appears in the ring (yellowarrow). At a magnetic field for which Φ = ½Φ0 the orbital momentum of the ground state ℓ changes from 0 to 1, together with anabrupt change in direction of the persistent current. For a ring with radius 11.5 nm the value of ½Φ0 corresponds to a magneticfield of B=5 T. With further increasing B the persistent current decreases linearly and is zero again at Φ = Φ0, when the ringencloses precisely one flux quantum (green arrow). This behaviour repeats itself periodically and each time the flux equals(ℓ+½)Φ0 , ℓ is increased by 1, a jump in the magnetic moment occurs with magnitude µB/m*.

M t T i i tMagneto-Torsion experimentsThe sample (7x8 mm2) is mountedonto a thin torsion wire (yellow).In a magnetic field B (Bluearrow) the inducedarrow), the inducedmagnetization of the sample M(green arrow) results in a torquewhich tends to rotate the sample,trying to align M and B. Thisoptically detected rotation iscounteracted by sending acurrent through a feedback coilcurrent through a feedback coilof known area, locatedunderneath the sample. Thecurrent through the feedback coilis a quantitative measure of M

•Up to 15 Teslas

•T=1.2 K

Magneto-TorsionDesign and growth of sample

g

MBE Growth at IMM•High uniformity <10% (Critical!)

•High density 29 layers stacked QR g y y Q

•1-2e per QR Modulation doping

Theoretische Fysicavan de Vaste Stoffen

COBRA Inter-University Research Institute

Theoretical approach

Prof. Devreese, V. Fomin ([email protected]) et al, Uni. AnwerpenBased on the structural information from X-STMmeasurements on buried self-assembled InGaAs/GaAs

i l l h l dquantum rings, we calculate the electron energy spectra andthe magnetization as a function of the applied magnetic field.

Since the lateral size of quantum rings substantially exceedstheir height, we consider the lateral motion of electrons to begoverned by the adiabatic potential related to the fast electrongoverned by the adiabatic potential related to the fast electronmotion along the growth axis.

Th l t t t l l t d b di li i thThe electron states are calculated by diagonalizing theadiabatic Hamiltonian in the basis of in-plane wave functions.

Calculations on Energy levelsCalculations on Energy levels

Theory: Magnetization

RESULTS: QR under Magnetic FieldMagnetic moment of electrons in QR.

•Oscillation at 14 T

g

•Theory from realistic experimentallymeasured X- STM data

•Excellent agreement demonstrate

Aharonov-Bohm oscillation 14T

•Excellent agreement demonstratepersistent currents associated withRING geometry

Magnetization

first derivative first derivativeEXPERIMENT THEORY

Magnetic moment per electron, (μ=M/N)

Micro PL set up,A.G.Taboada @ Eindhoven, NL

15 micro eV resolution with a triple monocromator triple

T=4.2, Bmax=11T

Micro PL from one single Quantum Ring

Additional splitting at ~4.4T

“Normal” Zeeman splitting

Power excitation=8μW

Micro PL from one single Quantum Ring: Fitting

12970

1.2972

)

12966

1.2968

1.2970600 Data: A1B024_C

Model: Lorentz Chi 2/DoF = 1124.76666R^2 = 0.98352 y0 -71.76043 ±34.95393xc 957.01704 ±0.00095un

ts (a

.u.)

B=0.2T

12962

1.2964

1.2966

y (e

V)

300

w 0.05877 ±0.00566A 69.23435 ±8.43353

ensi

ty c

ou

12958

1.2960

1.2962

Ene

rgy

0

PL

Inte

12954

1.2956

1.2958E 954 956 958 960

Wavelength (nm)1 pixel=15μeV, FWHM=4-5 pixels

0 2 4 6 8 10

1.2954

B(T)B(T)

Possible explicationsPossible explications

-Aharonov-Bohm effect: Capture of one quanta flux by the exciton due to the different radial sizes of the electron and the hole due to the ring geometry. g g yTransition phole-to-Selectron A.O. Govorov et al, PRB 66 (2002) 081309

M ti fi ld b k th i i t t d t-Magnetic field breaks the ring into two dots.

-Mixture of heavy-hole/ light hole due to breaks of symmetry rules by B. (Bayer et al ) But intensity distribution is very differentet al…) But intensity distribution is very different.





• Conclusions

•Quantum rings preserve shape after capping

•Key role of stress-free-state of In on growth ofKey role of stress free state of In on growth of self assembled nanostructures

•Tunning of properties Reduction in vertical•Tunning of properties. Reduction in vertical size achieved in a controlled way

•Demonstrate the AB effect•Demonstrate the AB effect…

Quantum Rings

• Conclusions

AcknowlegmentsPeople

IMM-CNM-CSIC: A.G. Taboada (Chon), D. Granados, J.M. Ripalda

B Alen F Briones J P Silveira

People

B. Alen, F. Briones, J.P. Silveira,

Uni. Cadiz: S. Molina, T. Ben, N.

Tech. U. Eindhoven (TUE) A. J. M. Kleemans, P. Offermans, J. H. Wolter, ( ) , , ,

P. M. Koenraad,

U. Nijmegen: I. M. A. Bominaar-Silkens, U. Zeitler, P. C. M. Christianen, J. C MC. Maan,

U. Anwerpen: V. M. Fomin, V. N. Gladilin, J. T. Devreese, M.Ediger

• EU– SANDIE, NANOMAT

Funding agencies

– EUROMAGNET• Spain

– NANOSELF II, NANIC, NANOCOMIC,CONSOLIDER 2010 QOIT• Belgium T GOA BOF UA 2000, IUAP, FWO-V projects G.0274.01N, G.0435.03, the g p j

WOG WO.035.04N• Netherlands Cobra Thank you for your attention

slfa bldq riself assembled quantum rings · slfa bldq riself assembled quantum rings 110 mbe lab,...

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