Physics and Material Science of Semiconductor Nanostructures
Transcript of Physics and Material Science of Semiconductor Nanostructures
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Physics and Material Science of Semiconductor Nanostructures
PHYS 570P
Prof. Oana MalisEmail: [email protected]
Course website:http://www.physics.purdue.edu/academic_programs/courses/phys570P/
Introduction to semiconductor nanostructures
• Review of semiconductors• Classification of semiconductors • Low-dimensional semiconductors: from 3D to 2D, 1D and 0D• Applications of semiconductor nanostructures
Ref. Ihn Chapter 1
insSCmetal
Conductor(Cu, Ag..)
Semiconductor(Si, GaAs..)
Insulator(SiO2,..)
Resistivity(Ohm.cm)
26 10~10 92 10~10 10 2210 ~ 10
Metal, Insulator, and Semiconductor
Phenomenology
Semiconductors Conductivity/Resistivity Definition
Semimetals
Metals
Phenomenology – cont.
Temperature dependence of resistivity and absorption
Metal Metal
Semiconductor Semiconductor
Band Diagram of Solids
1s
2s
2p
3s
2N
2N
6N
N
Single atom Solid
Valence band
conduction band
Energy
position
Metal, Insulator, and Semiconductor
Valence Band (VB)
Conduction Band (CB)
metal insulator semiconductor
T>0 doping
+ + + + + +
Energy gap (Eg)
insSCmetal RRR
Semiconductor ~ A small bandgap insulatorStrictly speaking, it must also be capable of being doped.
Typical BandgapsSemiconductors: 0 ~ ≤ Eg ≤ ~ 3 eV
Metals & Semimetals: Eg = 0 eVInsulators: Eg ≥ 3 eV
Exceptions AlN, with Eg = ~ 6 eV, is usually an insulator, but it can be doped & used as a semiconductor!
Also, sometimes there is confusing terminology likeGaAs: Eg = 1.5 eV is sometimes called semi-insulating!
Semiconductors: Bandgap Definition
Classification of semiconductors
Jan 2006
On semiconductor technology, the concept of randomly mixing two or moresemiconductors has two main objectives:
Altering the gap energy to a previously determined value (e.g.laser/detectors)
ex. HgCdTe – IR detectors; ex. InGaAsP – lasers ex. AlGaAs – laser layer confinement; ex. InGaN; AlGaN
Creating a material with an adequate lattice constant that matches theavailable substrates
e.g. In0.53Ga0.47As – matches InP
Ternary and quaternary semiconductors
Ternary and quaternary semiconductors are alloys
InSb
E.g. Solid Solution of type AxB1-xC x atom elements A and (1-x) atoms of element B,randomly distributed over one of the sublattices(e.g. In the one of group III); Element C occupies the other sub lattice (e.g. GroupV); x varies between 0 and 1 E.g.. AlxGa1-xAs, GaAs1-xPx, InxGa1-xN, AlxGa1-xN
Jan 2006
Alloys
When two semiconductors A and B are mixed using a propergrowth technique, the following alloy information should beobtained:
The lattice crystalline structure: on most semiconductorsthe two (or more) alloy components have the same crystallinestructure in a way that the final alloy has the same structure.For materials having the same structure, the lattice constantobeys the:
Vergard law
Jan 2006
In the case of direct gapsemiconductors, the gap energies arealso linearly weighted in accordanceto:
BgAgligag ExxEE ,,, )1(
Energy gap of Ga1-xInxAs
Eg(Ga1-xInx As)For x=0.48, Eg=0.8 eV (1.5 m) lasers.: excellent for optical fiber communications
Bowing (C) pictures the deviation from the truly random behavior
Eg (InAs)=0.4 eV
Eg (GaAs)=1.4 eV
)(8.1239)(
nmehceVEg
Intrinsic Semiconductor
Extrinsic Semiconductor
Donor impurities – provide extra electrons to conduction(type n)
Acceptor impurities – provide excess holes to conduction(type p)
inpn
e -
+
B
e+
B
e+
Si:As
Si:B
Doping
16 Jan 2006
At 0 K, the energy level is filled. Littlethermal energy is needed in order toexcite these electrons up to the CB. So,above 50-100K, electrons are virtually“donated” to the CB.
Likewise, acceptor levels can bethermally occupied with VB electrons,therefore generating holes.
Donors and acceptors
Other materials that are semiconductors
Many interesting semiconductor materials:Have crystal lattice structures Diamond or Zincblende
• In these structures, each atom is tetrahedrally coordinated with four (4) nearest-neighbors.
• The bonding between neighbors is (mostly) sp3 hybrid bonding(strongly covalent).
• There are 2 atoms/unit cell(repeated to form an infinite solid).
Zincblende (ZnS) Lattice
Zincblende LatticeThe Cubic Unit Cell.
The Zincblende (ZnS) Lattice
Zincblende Lattice:A Tetrahedral
Bonding Configuration
Zincblende Lattice:The Cubic Unit Cell.
If all atoms are the same,it becomes the
Diamond Lattice!
Zincblende & Diamond Lattices
Diamond LatticeThe Cubic Unit Cell
Zincblende LatticeThe Cubic Unit Cell
Semiconductor Physicists & Engineersneed to know these structures!
Other semiconductor materials of interest:have crystal lattice structures Wurtzite Structure
• This is similar to the Zincblende structure, but it has hexagonal symmetry instead of cubic.
• In these structures, each atom is tetrahedrally coordinated with four (4) nearest-neighbors.
• The bonding between neighbors is (mostly) sp3 hybrid bonding(strongly covalent).
• There are 2 atoms/unit cell(repeated to form an infinite solid).
Wurtzite Lattice
Semiconductor Physicists & Engineersneed to know these structures!
History of semiconductor technology
L. L. Sohn, Nature 394(1998)131
Ge transistor LSI
Quantum corral
Carbon nanotube
Point contact
1950 1970 1980 2000
Low-Dimensional Systems
Quantum Well (quasi-2D)
Quantum Wire (quasi-1D)
Quantum Dot (quasi-0D)
<<100nm, in usual.
Formation of nanostructures
- - - - -- - - - -
-+
etching
~10nm
1m~100nm
Self-assembled dots
Gate-defined dot Pillar dot
1m~100nm
Semiconductor Heterostructures*
AB
Confinementpotential
* 2000 Nobel prize in physics
Quantum Structures & Density of States
Bulk (3D)
Quantum well (2D)
Quantum wire (1D)
Quantum dot (0D)
Energy
DO
S
Energy
DO
S
Energy
DO
SEnergy
DO
S
Quantum Phenomena and Quantum Devices with Semiconductor Nanostructures
ENERGY Quantization
FLUX Quantization
CHARGE Quantization
• Low-Dimen. Elect.• Band Modulation• Resonant Tunneling• Quantum Hall Effect• Ballistic Resistance• Optical Bistability
- HEMT / MODFET- QWIP- Quantum Hall Effect- QWL/QWR/QD Laser
• Elect. Interference-Aharanov-Bohm
effect-universal conduct.
fluctuation• Ballistic Transport
- Quantum Interfer-ence Dev.
- Elect. Wave Device- Ballistic Device
• Single Elect. Effect-electron charging-electron tunneling• Current Standard• Capacitance
Standard
- SET Transistor- Single Electron
Devices
Photoluminescence (PL) from Quantum Wells
Photoluminescence (PL) from (parabolic) Quantum Well
R.C. Miller, et al. Phys. Rev. B 29, 3740 (’84)Also see sec. 4.3 in Davies textbook
40meV
PL from Ensemble of Quantum Dots
Sylvain Raymond and cowokers, NRC, Canada
~20nm
Artificial atoms!!!
PL from Single Quantum Dot
Robin Williams and cowokers, at NRC, Canada
20meV
~20nm
I
V
I
V+ _
w
Current transport through a classical resistance
Conductance (G)
WL
WG
GVI
law sOhm'
Quantum Point Contact
(see also J.H. Davies Fig.5.22/p186)
B.J. van Wees, PRL 60, 848(1988).
Quantum Point Contact
Vg
1
2
3
45
)/2( 2 heG_
: metal (gate): two-dimensional electron gas
h: Planck’s constantI
VgVg~250nm
+V
W
807.25812
resistance sKlitzing' von
2ehRK
*see also quantum Hall effect (Nobel prizes in ’85,’98)
Quantum Point Contact (metal)
Quantized conductance through individual rows of suspended gold atoms H. OHNISHI, et al., Nature 395, p780 (‘98)
F of metal: nm10 10~10
~0.9nm
)( ,, SCFMF
Coulomb Blockade in Quantum Dot (Q.D.)
J. Weis, et al. Phys. Rev. Lett. 71, 4019-4022 (1993)
IG
Vg
Vg Vg
Quantum dot
“single” electron transistor (SET)
G
S D G
S D
(a review article about Q.D.: S.M. Reimann and M. Manninen, Review of Modern Physics, 74,1283 (2000))
U. Banin, Y. Cao,D. Katz, and O. Millo, Nature vol.400, 542 (1999)
InAs NC
Coulomb Blockade spectrum of a Single Nanocrystal