Da escala micro para a escala nano

As técnicas de crescimento epitaxial permitiram a miniaturização

Como são produzidos os semicondutores ?

MBE – Molecular Beam Epitaxy

CBE – Chemical Beam Epitaxy

MOVPE – Metalorganic Vapor Phase Epitaxy

MBEAlto vácuo

Pressão 10-10 Torr

MOVPE

• MOCVD - Metalorganic Chemical Vapor Deposition

• OMCVD - Organometallic

Chemical Vapor Deposition

• OMVPE - Organometallic Vapor Phase Epitaxy

Princípio de deposição

• (CH3)3Ga + AsH3 → GaAs + 3 CH4

• (1-x) (CH3)3Ga + x(CH3)3Al + AsH3 → AlxGa1-xAs + 3 CH4

TMGa

AsH3

Epitaxial Growth

GaAs Substrate

GaAs

AlAsInP InAs

InxGa1-xAsGaxAl1-xAs

GaP

InxGa1-xP InxAl1-xAs

a a

Lattice matched Strained layers

GaAs

AlGaAs

GaAs

a’ > a InAs

Strained InAs

3D a 0D

De 3D a 0D

• 3DE = Eg + h2k2/8pm

Density of states r(E) = 21/28mc

3/2 (E-Eg)1/2/h3

• 2DE = Eg + Eqz + h2k//

2/8p2mEqz= qz

2h2/8md2

Density of states r(E) = 4pm/h2

• 1DE = Eg+Eqz+Eqy+h2kx

2/8p2mEqz,y= qz,y

2h2/8md2

Density of states r(E) = 8Lm1/2/h2 ½(E-Eq)1/2

• 0DE = Eg+Eqz+Eqy+Eqx

Eq(z,y.x)= qz,y,x2h2/8md2

Density of states r(E) = # of dots g /Vol

Pontos quânticos

• Estruturas com confinamento 3D numa escala menor que o raio de Bohr levando a uma quantização 3D.

• Comportamento atômico.• 1980 foram fabricados os

primeiros pontos quânticos de ZnS em vidro.

• Existem várias maneiras de produzí-los.

O que são estas estruturas 0D?

Estrutura de banda

Sintonia de estruturas de PQs

Fafard 2003

Top-down vs bottom-up

Top-down: PhotolithographyElectron beam lithographyX-raysExtreme ultraviolet lightScanning probe methods

Bottom-up:Self-assembled quantum dotsScanning probe methods

Comparando os métodos

•LithographyAdvantage: The electronics industry is already familiar with this technology.Disadvantage: The necessary modifications will be expensive. UV-light and x-rays can damage the equipment.

•Scanning ProbeAdvantage: STM and AFM are very versatile, they can move particles in a patterned fashion.Disadvantage: Too slow for mass production.

•Bottom-up MethodsAdvantage: Controlled chemical reactions can cheaply and “easily” produce nanostructures.Disadvantage: Cannot produce designed, interconnected patterns.

Pontos quânticos auto-organizados

Métodos diferentes de crescimento

Stranski-Krastanow

Princípio de formação de pontos quânticos por MOVPE

• Uma diferença importante no parâmetro de rede numa heteroestrutura, leva a um aumento na energia elástica que será aliviada com a formação de ilhas de dimensões que podem ser inferiores ao raio de Bohr.

• Para materiais descasados um aumento na tensão elástica com o aumento na espessura torna a superfície rugosa. O crescimento 2D camada a camada é interrompido e num segundo passo, a nucleação 3D se inicia. Numa terceira etapa as ilhas 3D se desenvolvem em tamanho consumindo o material que está móvel na superfície.

Seifert 2000

Espessura da wetting layer

Dots’ parameters

• Dot density108 to 1011 cm2

• Dot size4 – 20 nm height, 20 – 50 nm base width

• Dot shapePyramidal, truncated pyramidal, lens- and

cone-shaped

How to determine these parameters?

Scanning Tunneling Microscopy

(Nobel Prize to Rohrer and Binnig in 1986)

Atomic Force Microscopy

Determination of size distribution and density of quantum dots

4 5 6 7 8 9 10 11 12 13 140

50

100

150

(8.2 ± 1.5) nmnormal curve

(8.2 ± 1.5) nm

830

density = 1.48 1010

QD/cm2

400nm

InAs / InGaAs

520oC5.5 s66 sccm

830

Example of AFM Results

Transmission Electron Microscopy

Two geometries: Plain view Cross section

Cross section gives information about shape, size and composition.

Samples are thinned down to a thickness of the order of 1mm.

InAs/GaAs

104 – 106 atoms per dot

TEM image of an InAs/InGaAs/InP dot

Landi et al 2005

HREMimages

Photoluminescence• The laser beam usually probes an ensemble

of quantum dots. The FWHM gives information on the uniformity of the dot size distribution.

• For a density of 1010 cm-2, one probes about 106 dots for a 100 mm laser spot.

• Single dot spectroscopy requires low dot density and processing to isolate one dot.

spd

f

Luminescence of an ensemble of dotswith resolved excited states.Linewidths of the order of 20-30 meV.

Fafard et al 2000

Single dot spectroscopy.Linewidths of the order of meV. Signal is time averaged.

Examples of Photoluminescence of Dots

Finley et al 2001

Electroluminescence for two injection levels reveals the Pauli principle.

Photocurrent measurements show absorption to the ground state (s) and to three excited states (p, d, f).

Mowbray et al 2005

Growth parameters• TemperatureHigher temperature, lower density, larger size.

• Deposition timeLonger times, more material, larger dots.

• Fluxes of gases/ Growth rateHigher growth rates, smaller dots, higher density.

• Annealing time

For the same amount of material the dot density and the dot size show inverse behavior

3 6 9 120

20

40

60

80

cou

nt

QD height

(7.8 ± 1.8) nm

density = 8.0 109 QD/cm2

Tgrowth = 500°C

3 6 9 120

40

80

120

QD height

cou

nt

(9.0 ± 1.4) nm

density = 9.05 109 QD/cm2

Tgrowth = 520°C

• Height increases

• FWHM decreases

Effect of temperature on InAs/InGaAs/InP

0.60 0.63 0.66 0.69 0.72 0.75 0.78 0.81

0.0

0.2

0.4

0.6

0.8

1.0

1.277 K60 mW

Tgrowth

= 500 °C, FWHM =106meV T

growth = 510 °C, FWHM =70meV

Tgrowth

= 520 °C, FWHM =47meV

no

rma

lize

d P

L (

arb

. u

nits)

energy (eV)

Reduction of the PL FWHM in agreement with AFM results

PL intensity for higher energies decreases → larger dots

Effect of temperature on InAs/InGaAs/InP

1.0µm 1.0µm1.0µm

Deposition time increases → Dot density increases

InAs/InGaAs/InP

22 23 24 25 26 27 280

2

4

6

8

(25 ± 1) nm

<density> = 4.8 107 QD/cm

2744

1.0µm 1.0µm 1.0µm 1.0µm

5 10 15 20 25 30 350

20

40

60

80

100

(35.6 ± 3.5) nm

normal curve(18.5 ± 3.4) nm

(18.7 ± 2.8) nm

738density = 7.8 10

9 QD/cm

2

10 20 30 400

50

100

normal curve(16.9 ± 4.3) nm

(34.2 ± 3.9) nm

(16.5 ± 2.4) nm

density = 8.05 109 QD/cm

2

743 AFM image A

5 10 15 20 25 30 350

20

40

60

normal curve(14.5 ± 4.7) nm

753

(30.8 ± 3.3) nm

(13.8 ± 2.8) nm

density = 5.2 109 QD/cm

2

In flux: 30 sccm 60 sccm 66 sccm 76 sccm

Tgrowth: 520 oC tgrowth: 4.2 sInAs / InP

In flux / growth rate dependence

1.0µm

1.0µm

InAs / InGaAs / InP

400nm

400nm

Attempting to reach higher densities

InAs /InP

200nm

Same scale: from 2.0 108 to 2.0 1010 dots cm-2

InAs/InP Tg = 490oCH 12 nm

InAs/InGaAs Tg =490oCH 9 nm

Stacks of quantum dots

• For device applications it is important to have several layers of dots.

• Nature has helped. In general dots spontaneously grow on top of each other.

200 nm

Surface QDs

200nm

Multi-layers of quantum dots

20 nm

AFM image

TEM Images of Stacked Quantum Dots

Landi et al 2005

Red-shift with increasing number of stacks

0 2 4 6 8 10 12

16

32

48

64

Q D

den

sity

( c

m -

2 )

number of stacks

0 2 4 6 8 10 124

8

12

16

Q D

heig

ht (n

m)

number of stacks

0,54 0,57 0,60 0,63 0,66 0,69 0,72

0,0

0,2

0,4

0,6

0,8

1,0

1,2

PL 12.5 K

1 QD layer 10 QD layers

norm

aliz

ed P

L (a

rb. u

nits

)

energy (eV)

Vertical coupling increases the average dot height

Effect of number of stacks on dots’ properties

Landi et al 2004

14µm400nm1.5µm

Controlled site deposition of quantum dots on a patterned surface

Patterned substrateusing AFM

Dots’ formation on designated sites

Dots grown away from the patterned region

Fonseca Filho et al 2005