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Physics and Material Science of Semiconductor Nanostructures

PHYS 570P

Prof. Oana MalisEmail: omalis@purdue.edu

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