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Nanoscience II: Semiconductor nanostructures
11.11.2010
Markku SopanenMICRONOVA
Department of Micro- and NanosciencesAalto University School of Science andTechnology
Acknowledgments: Prof. Harri Lipsanen, Dr. Mikael Mulot, Dr. Marco Mattila, Dr. Teppo Hakkarainen
page 2
1 Semiconductor nanostructures2 Quantum dots3 Photonic crystals
Outline
page 3
1 Semiconductor nanostructures
page 4
What is a ”semiconductor nanostructure”?
Classification by properties Electronic tailoring (quantum dots, wires) Optical tailoring (photonic crystals)
Classification by nanostructure dimensionality 1D (quantum wells, superlattices, Bragg mirrors) 2D (quantum wires, nanowaveguides, planar photonic crystal) 3D (quantum dot, nanoparticle, photonic crystal)
[Charge carrier system dimensionality is the opposite way.]
Semiconductors do not usually play a crucial role in metamaterials .
Obviously a structure containing at least one semiconductor material and having at least one dimension in nanometer scale. However, usually one-dimensional structures are not considered as ”nano”.
page 5
Covalent bonds in semiconductorsElectronic structure of Si: 1s22s22p63s23p2
4 valence electrons, 4 electrons missing to fill the outer shell
Some semiconductors have more ionic bonds (II-VI, etc.).Electrons involved in the bonds are trapped in the bonds, and are not available for conduction.
Pure semiconductor is a poor conductor
But free carriers can be easily created by doping.
Ga
Ga
Ga Ga
Ga
Ga
As
As
AsAs
As
As
Electronic structure of Ga: 1s22s22p63s2p63d104s24p1
3 valence electrons, 5 electrons missingElectronic structure of As: 1s22s22p63s2p63d104s24p3
5 valence electrons, 3 electrons missing
page 6
Phosphorus impurity atom (extra valence electron ) in silicon lattice: the extra valence atom is weakly bond: an energy Ec - Ed << Eg is required to create a free electron. This type is called donor defect/impurity => n-type semiconductor
Doping
Ev
Ec
Eg
Filled valence band
Ed
Ev
Ec
Eg
Filled valence band
Ea
Boron impurity atom: acceptor defect/impurity => p-type semiconductor
page 7
Diamond structureDiamond structure = FCC lattice + 2 identical
atoms in the primitive cell: (0,0,0) and (a/4, a/4, a/4)
– Examples: Si, Ge and diamond
Crystal viewer (diamond and Zinc blende structure):http://jas2.eng.buffalo.edu/applets/education/solid/unitCell/home.html
Zinc-blende lattice = FCC lattice + 2 different atoms in the primitive cell
– Examples: GaAs, InP, GaP, GaSb, InSb, ZnS, ZnSe, …
(GaN, SiC and ZnO are difficult to manufacture in zinc-blende structure)
page 8
Semiconductor band structure
Electronic structure of Si: 1s22s22p63s23p2
N Si atoms: 2N electrons in 3s orbital, 2N electrons in 3p orbitals
3s
3p
Empty upper bands
Filled lower bands
Conduction band
• Energy states of Si atoms expand into the energy bands of Si crystal• The lower bands are filled and higher bands are empty• The highest totally filled band is the valence band• The lowest empty band is the conduction band
2N electronsValence band
2N electrons
4N electronsEne
rgy
N Si atoms in crystal formN isolated Si atoms
page 9
GaAs band structure (E-k diagram)
Eg
Valence band
Conduction band
X-valleyL-valley
page 10
Direct and indirect bandgap
Direct band gap: The conduction band is formed only by overlap of s-orbitals
Indirect band gap: The conduction band is a mix of p- and s-orbitals
page 11
Quantum well
yz
xInP
InP
• Quantum well: a thin semiconductor layer (Lz<20nm) embedded between two semiconductors with larger bandgaps.
• Electrons and holes trapped in the well are free to move in the x-yplane, but are strongly confined in the z-direction = 2D electron gas.
InAs0.65P0.35 (5nm)
Lx
Lz
Ly
Cross-sectional TEM picture of a GaInNAs QW grown on GaAs.
page 12
2
222
2 zee Lm
E ∗=
π
∗
+++=
e
yxeC m
kkEEE
2)( 222
EC
EV
Eg,1 Eg,2
Lz
z
E
Lz
EC
In the infinite well approximation, the energy levels are given by:
e2 (ℓ=2)e1 (ℓ=1)
e3 (ℓ=3)
e4 (ℓ=4)
Electron energy:
Energy levels for electrons
page 13
Energy levels for holes
Lz
2
222
2 zhhhh Lm
E ∗=
π
EC
EV
Eg,1 Eg,2
EV
Lzz
E
hh2 (ℓ=2)lh2 (ℓ=2)
lh1 (ℓ=1)hh1 (ℓ=1)
In the infinite well approximation, the energy levels are given by:
2
222
2 zlhlh Lm
E ∗=
π
∗
+++=
hh
yxhhV m
kkEEE
2)( 222
Heavy hole energy:
page 14
E
Density of states (electrons): 2D vs. 3D
D(E)
Ee1 Ee2 Ee3EC
3D2D
( )*
2( ) el
l
mD E H E E dEπ
= −∑
( )1, when E E0, when E < E
ll
l
H E E≥
− =
page 15
Superlattices
Intersubband emission
Superlattice consists of two (or more) different materials in alternating layers. The periodicity induces subbands within the conduction band and the valence band.
For electronic effects layer thicknesses are 1-10 nm and for optical effects 10-100 nm.
Superlattice structure
page 16
Microelectronics and -photonics
Microcavity LED
There are already nm-scale layers in present devices.
E.g., the QW’s are 2-3 nm thick in white LEDs.
Transistor + pin-photodiode
Integrated optics
page 17
Quantum wire
yz
x
• Quantum wire: 1D electronic system (confinement in 2D)
• Electrons and holes trapped in the wires are free to move only along the y-direction
InP
InP
InAs0.65P0.35
Ly
LzLx (110) cross-section TEM picture of
stacked InAs QWires in InAlAs matrix lattice matched to InP.
page 18
E
Density of states: 3D, 2D and 1D
D(E)
E1e E2e E3eEC
3D2D1D
Note: At the absorption edge, the density of states is 0 in the bulk (3D) case. However, it is very large in quantum wires (1D).
page 19
Fabrication of quantum wires
Top-down methods: wires, e.g., defined by lithography and consequent etching
Bottom-up methods: wires, e.g., grown by VLS (vapor-liquid-solid) method using metal particles as seeds
page 20
Example: InP nanowires on InP by MOVPE
VLS growth of InP using In droplets
SEM image of InP nanowires on InP
TEM image of InP nanowires: the metal droplet can be seen at the end of the wire
page 21
Applications of quantum wires
- Nanowire transistors, logic elements, electronic waveguides
- Optical waveguides,
optical emitters
- Sensors utilizing functionalized surface
page 22
Density of states in QDsDensity of states in • 3-dimensional (bulk), • 2-dimensional (well), • 1-dimensional (wire) and • 0-dimensional (dot) semiconductors
page 23
2 Quantum dots
page 24
QD classification
Classification by structure
Particles
Composites
Single crystals
Classification by confinement potential Strongly confinedWeakly confined
Classification by fabrication Homogeneous nucleation Heterogeneous nucleation Kinetically confined synthesis Physical techniques (lithography, nanoimprinting, etc.)
Quantum dots (QDs): nanosize structures of crystalline nature, confined in three dimensionsClassification of quantum dots by various criteria:
page 25
QD nanoparticles
QD band gap is effectively shifted in proportion to 1/R2. The size causes different colors in optical absorption and emission.
Fluorescence (emission) of CdTe quantum dots in solution. Color variation is due to diameter from 2 nm (green) to 5 nm (red).
page 26
Core-shell QD- core-shell structure has a core QD surrounded by a thin shell of another material
- surface consists of a large fraction of the atoms in the quantum dot
=> surface structure important factor for the properties, e.g. biotin activated quantum dots (Evident Technologies)
page 27
Examples of the fabrication methods of quantum dots
8 nm
CdSe
Smält kiseldioxid
Mask
AlGaAsGaAs
AlGaAs
-e
EtsningKvantpunkt
GaAs
InAs2 Självorganiserad tillväxt InAs
Homogeneous nucleation:nanoclusters in glass
Physical technique: patterning of heterostructures- e-beam lithography- maskless FIB lithography
Heterogeneous nucleation: self-assembled growth
SiO2 (insulator)=> optical color filters
GaAs QDetching
~20 nm
no artificial patterning!
=> degradation of optical properties due to processing steps
=> defect-free
structure
large surface/volume ratio
page 28
Colloidal growth (kinetically controlled synthesis)
- monodisperse nanocrystals (diameter variation <5%) needed
- chemical synthesis (fig.): reagents are rapidly injected into hot solvent, colloids are formed in the supersaturated solution
page 29
Group II-VI semiconductor nanocrystals
- group II-VI semiconductors ME, where M = Zn, Cd, Hg and E = S, Se, Te are the most common nanocrystals due to their ease of chemical synthesis (CdSe, ZnS...)
- more complex coated nanocrystals, such as CdSe/ZnS core-shell structure important (Evident Technologies)
page 30
Group III-V semiconductors
- group III-V semiconductor nanocrystals such as InP and InAs can be produced similarly as the II-VI structures
- not very useful in applications
Epitaxial growth:
Fabrication of nanocrystals on surface by epitaxy (layer growth)
- growth from vapor phase (CVD), molecular beam epitaxy (MBE), laser ablation etc.
- good control of growth conditions required (amount of material, choice of materials, temperature)
- typically mismatch of lattice constants between deposited thin layer and substrate causes nucleation into nanoscale islands (quantum dots)
page 31
Modern epitaxial techniques
- good control of layer thickness d(∆d < 1Å) and composition needed
MBE (molecular beam epitaxy)- ultra-high vacuum- like vacuum evaporation- often solid sources- several systems in Tampere, one in Micronova
MOVPE or MOCVD (metalorganic vapor phase epitaxy)- sources: vapors or gases- two systems in
Optoelectronics Lab., Micronova
P
As Al InGa
In
As
page 32
Growth modes in epitaxy
Frank-van der Merwe (2-d) Volmer-Weber (3-d)
Stranski-Krastanow (2-d + 3-d)
Transition to 3-d growth after ultrathin strained wetting layer
page 33
Coherent Stranski-Krastanow growth mode
Ge islands on Si not dislocatedEaglesham, Cerullo, Phys. Rev. Lett. 64, 1943–1946 (1990)
Stress is not released by dislocation formation. Strain energy is accumulated both in the island and in the substrate.
TEM image of Ge island on Si
page 34
Self-assembled growth of III-V QDs
- InAs has 8% larger lattice constant than GaAs- after deposition of >1.7 monolayers of InAs,
small islands (~10 nm wide) are formed (energetically favorable) on a very thin 2D layer (wetting layer)
- islands are defect-free and act as quantum dots with a high density (~1010 cm-2)
E.g., InAs island formation on GaAs surface
Stranski-Krastanow growth mode AFM
page 35
Example: self-assembled InP islands on GaAs
- from vapor phase or molecular beam at 500 - 650°C
AFM images of InP nanocrystals on GaAs surface. InP layer thickness is 3 monolayers (~0.9 nm). Density of 20 nm high nanocrystals is about 109 cm-2.
GaAs
InP
InP
ultrathin strained layer,~3 ML InP on GaAs
Tg=635°C
page 36
Shape engineering of quantum dots
TEM cross section of InAs nanocrystal on GaAs surface.
AFM image of InAs(P) quantum rings fabricated at our laboratory.
Annealing of InAs dots in P atmoshere results in shape change.
- nanocrystals can be capped (e.g. with GaAs) to form buried quantum dots
- the shape can be altered either by the capping process or by annealing
page 37
Stacked quantum dots
TEM cross section of stacked InAs quantum dots.
Multilayer stacks of quantum dots can also be grown- the quantum dots have laterally statistical distribution in position- vertical coupling due to strain fields causes vertical ordering- size and shape of dots can be tuned by GaAs
barrier layer thickness
page 38
Pyramidal InAs QDs
40x40 nm2 cross-section STM current image of cleaved InAs quantum dot and the wetting layer.
Cross-sectional scanning tunneling microscopy (STM) of cleaved InAs quantum dots shows structural and compositional information with atomic resolution (fig.)
- the typical structure for capped dots is a truncated pyramide (below)
5 nm high and 15 nm wide InAs quantum dot
page 39
Optical properties of self-assembled quantum dots
Modeling of the self-assembled quantum dot potential using a hemispherical cap of InAs on top of an InAs wetting layer embedded in a GaAs substrate and cap layer.
- density of state of quantum dots resemble that of atoms: sharp energy levels
- modeling of the quantum dot can be approximately done by using a simple structure (fig.)
Schematic of the energy levels in an InAs/GaAs self-assembled quantum dot having 5 electron and hole shells (s, p, d, f, g) with a degeneracy (2,4,6,8,10 particles / energy). The shells here are partially filled (state-filling process).
page 40
PL spectra- ideal quantum dot system would
give narrow lines in optical spectra
- in real systems the size and shape fluctuation of the quantum dots broadens the spectra (fig. below)
- typical photoluminescence (PL) spectra of >>103 dots consists of Gaussian peaks (note state-filling)
State-filling of the quantum dot shells with increasing excitation intensity in low temperature photoluminescence (PL) spectroscopy. The inset shows a Gaussian fit used to deconvolute the contributions from the various shells.
PLpumpλ
ground state
excited states
page 41
Stressor quantum dot structure- strain field of a self-assembled island causes local decrease of bandgap of a quantum well just below the island. The quantum dot has nearly parabolic potential for electrons and holes.
- almost perfect crystal structure
=> narrow intense PL peaks
1.20 1.25 1.30 1.35
Energy (eV)
QW
QD
CB
VB
BAND DIAGRAM
PL SPECTRUM
self-assembled island
quantum well
high excitation
low excitation( )substratea a>
AFM (1 x 1 µm)
page 42
QD applications
Semiconductor quantum structures are already commonly used in optoelectronic applications such as telecom lasers, CD & DVD read-write heads, light emitting diodes (LEDs) etc.
QD structures are expected to improve performance, e.g, in near-infrared QD lasers (900 - 1300 nm), QD vertical cavity surface-emitting lasers (VCSEL), QD photodetectors. They might also enable new devices in, e.g., quantum computing.
QD VCSEL
page 43
Photonic crystals
page 44Sea mouse
Natural photonic crystals
a = 510nm
a < 100nm
page 45
Natural opals
2 μm
page 46
Photonic crystal classification
Photonic Crystals (PhCs)
1D PhCsBragg, 1887
2D PhCs 3D PhCsYablonovitch et al., 1991
PhC fibersRussel et al., 1995
Planar PhCsKrauss et al., 1996
page 47
• Studied by Lord Rayleigh in 1887
• Quarter wave layers: ,
Bragg grating mirror (1D PhC)
LL n
d4
0λ=
nLnH
dL dH
λ0
HH n
d4
0λ=
page 48
Bragg grating mirror example: SiN/SiO2 mirror
When the incidence angle decreases, the reflection band becomes narrower and eventually vanishes
∆λ
λ0
Wavelength (nm)
Rel
fect
ivity
« stop band »
page 49
From Bragg mirrors to photonic crystals
• Photonic crystal: generalization of the Bragg mirror concept to 2D and 3D periodic structures
• A 3D photonic crystal can have a full bandgap: it then reflects light for any incident angle.
• Full bandgap requires a large refractive index contrast in the structure.
Joannopoulos et al., MIT
page 50
120°
120°
35°35°35°
120°
The ”Yablonovite”
• Manufactured by the Yablonovitch group at MIT in 1991• First 3D PhC with a full photonic bandgap in microwave range• Consists of a periodic pattern of holes drilled into plexiglas. Each
hole is drilled three times in three different directions• The obtained 3D pattern reproduces the diamond structure
page 51
Artificial opals
Vos et al. Nature 430 654
• Opal can be manufactured by sedimentation of SiO2 spheres of controlled size (Left picture).
• Only inverted opals with refractive index above 2.2 exhibit a full photonic bandgap (Right picture).
Material Institute of Madrid
page 52
Band diagram
Wavevector k
Nor
mal
ized
freq
uenc
y a/λ
Wav
elen
gth λ
(μm
)
full bandgap, transmission forbidden in all directions
no transmission in Γ−L (111) direction → reflectance maximum
page 53
Self-assembled opals• Made by self-assembly of SiO2, PMMA or polystyrene nanospheres.
• Structure must be inverted with Si to obtain a complete bandgap
• Typical sphere size for bandgap around 1.5µm: 900nm
• Possibily to sediment nanospheres onto Si patterned substrates.
Material Institute of Madrid
3D Photonic crystals
Difficult to insert defects in the lattice… 2 µm
VTT+Tyndall (Cork)
page 54
Lithography defined structuresTime consuming and complex…
Sandia Nat. Lab
3D Photonic crystals
D. N. Sharp et al., Opt. Quant. Elec. 34, 3 (2002)
10µmM. Qi, H. Smith, MIT
…or difficult to add defects
page 55
Planar 2D PhCs
2D array of holesVertical structureConfines light in the
vertical directionControls light propagation
in the plane
2D PhCs• Relatively simple structure • Have most of the properties of 3D PhCs• Existing technologies can be directly applied or developed further• Compatible with planar optoelectronics
n1
n1
n2 > n1
page 56
The InP/GaInAsP/InP system
xy
z
GaInAsPInP
InP substrate
Provides light confinement in the vertical direction
Hx
Hy
EzTM
2 polarizations: Transverse Magnetic like (TM)
Hz ~ 0
TE
Ex
HzEy
Tranverse Electric like (TE)Ez ~ 0
GaInAsP (n=3.35)
-2
-1
0AirInP (n=3.17)
InP (n=3.17)z
(µm
)
Low index contrast system (∆n = 0.18)
Weak confinement in the core
Field profile
Active system
page 57
The Silicon-on-Insulator (SOI) system
xy
z
SiO2
Si
Si substrate
Provides light confinement in the vertical direction
Hx
Hy
EzTM
2 polarizations: Transverse Magnetic like (TM)
Hz ~ 0
TE
Ex
HzEy
Tranverse Electric like (TE)Ez ~ 0
High index contrast system (∆n = 1.95)
Strong confinement in the core
Si (n=3.4)
Air
SiO2 (n=1.45)
1
0
Field profile
Passive system
page 58
W1 waveguide
2D PhCs etched in InP membranes
600 nm InGaAs
300 nm InP
Facet view
Top view
InP membrane = high index contrast system (∆n = 2.17)improved light confinement compared to InP/GaInAsP/InP
Sample facet
M. Mulot, M. Swillo, M. Qiu, M. Strassner, M. Hede, S. Anand,J. Appl. Phys. 95, p.5928, 2004
page 59
1 µm
PhC waveguides
W1 waveguide
W1
wav
egu
ide
• Line defects in PhCs can be used to guide light
• 1 line defect = W1 waveguide, 3-line defect = W3 waveguide
• PhC waveguides are essential building blocks of a PhC integrated circuit
page 60
Single defect resonant
wavelength: λi
λ1, λ2, ...,λi
λi
λ1, λ2, ...,λi-1
Filter combining cavity and waveguide
GaInAsP membraneNoda et al., Nature 2000
page 61
Point-defect cavity
• One hole removed = defect in the PhC lattice• Simulation by 2D Finite Difference Time Domain method
detector
Normalized frequency (a/λ)
Det
ecto
r sig
nal (
a.u.
)bandgap
page 62
Point-defect cavity
At the resonance wavelength, light is trapped in the defect
The point-defect defect acts as a trap for photons. Lightcannot escape the structure due to the surrounding bandgap.
Normalized frequency (a/λ)
Det
ecto
r sig
nal (
a.u.
)
page 63
Single-cell photonic crystal laser
Q = 2500 (measured)
Hong-Gyu Park et al., Science 305, p. 1444 (2004)
Ith = 260 μAMax power: a few nW
page 64
Fabrication – the stacking method
© Crystal Fibre A/S
Photonic crystal fibers
page 65
• Large mode area fibers
• Nonlinear fibers
• Polarization maintaining fibers
• High numerical aperture fibers
• Double cladding active fibers
• Air-guiding fibers
Photonic crystal fibers: Applications