da escala micro para a escala nano as técnicas de crescimento epitaxial permitiram a...
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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
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