1

Common Lattice Matched Systems

GaAs AlAs

InP InGaAs

GaSb InAs

GaN AlN (wurzite)

MBE Growth of III-V Semiconductors

2 x 4 (As rich)

4 x 2 (Ga-rich)

0,0 1,0 0,10,0

[110] [110]

2

Latticed Matched MQWs

quantum cascade lasers

Ishida et al, JJAP 2005

Band Offset and Schottky Barrier Height

Semiconductor Heterojunction Schottky Barrier

3

Heterojunction Band Offset

ISRABBA

VBO e∆+−=Φ − µµ

bulk terms interface term

Heterojunction Band Offset Theories

Anderson: Electron-affinity ruleFensley & Kroemer: Interst. Poten. Harrison: Atomic orbitalVan de Walle & Martin: Model solidBaldereschi: Wigner-Seitz cell

Tersoff: Branch PointCardona & Christensen:

Dielectric Midgag EnergyHarrison & Tersoff: Hybrid SP3

“charge neutrality level”

4

Band Offset Theories

Yu, McCaldin, McGill, Solid State Physics 1992

CNL - based theory Charge-density based theory

“There is no single theory that yields band offset values that are substantially more accurate than those predicted by any other theory.”

Multiple Quantum Wells

Faist et al, PRL 1996

Gmachl et al, APL 2000

5

Interface Sharpness

1. Shape (abruptness) of band offset

2. Interface width

3. Layer thickness and uniformity

4. Electronic structure, optical properties,

5. ...

Interface sharpness directly impacts:

Considering that: in straight MBE growth at constant rates, interface sharpness is best at a intermediate temperature to minimize interdiffusion and surface roughness....

How do we achieve high interface sharpness?

Atomic Layer Epitaxy (ALE) and Migration Enhanced Epitaxy (MEE)

Atomic layer epitaxy (ALE) is based on the alternate saturation adsorption of constituent atoms which guarantees complete 1 monolayer (ML) coverage. ALE requires that both constituent atoms display self-limited adsorption, e.g. II-VI semiconductors.

Migration enhanced epitaxy (MEE) supply the constituent atoms alternately, but does not require saturation adsorption from both constituent species (saturation adsorption of one of the speciesis enough for MEE).

Advantage of ALE and MEE: interface sharpness, low growth temperature, heterovalent epitaxy, selective deposition.

6

Atomic Layer Epitaxy

Zhang et al, APL 2001 Volkman et al, PRB 2004AlInGaN

* ALE is popular for II-VI &oxides

* Electrochemical ALE

Kim et al, APL 2003

Migration Enhanced Epitaxy

Horikoshi, JCG 1999Yamaguchi et al, JJAP 1989

7

Growth Interruption

Horikoshi, JCG 1999

Growth Interruption

Yamakawa et al, APL 2004

∆: roughness amplitude

MOVPE: TMIn, TMGa, TBP, TBA, H2.

8

Growth Interruption & Interface Roughness

AFM images of etch-exposed Al.3Ga.7As-GaAs interface

with growth interruptions

w/o 2nd growth interruption

Gottwald et al, JAP 2003

Tilted Superlattice (TSL)

Miller et al. PRL 1992 Serpentine Superlattice

Tsuchiya et al. PRL 1989

Gaines et al. JVST 1988

9

Quantum Confinement

Particle In A Box

Infinite Potential Well

ground state energy increases by h2/8mL2

Effective Mass Approximation

++= 2

33222

211

2

min111

8 LmLmLmhEhttp://www.falstad.

com/qm2dbox/

free electrons

Quantum Confinement

http://www.mtmi.vu.lt/pfk/funkc_dariniai/nanostructures/quant_structures.htm

dkLdN

=πdkkLdN π

π2

2 2

2

=dkkLdN 2

3

3

44

ππ

=

h

)(2 0EEmk

−=

)(2 0EEmdEdk−

=h

10

Strained Structure & Strain Relief

misfit dislocations

Heteroepitaxial Growth Morphology

Frank-van der MerweVolmer-Weber Stranski-Krastanow

11

Critical Thickness: Continuum Theories

Van Der Merwe proposed that the critical thickness is when the strain energy equals the interface energy.

Mathews and Blakeslee proposed that the critical thickness is when the misfit stress on an existing threading dislocation equals the line tension of the dislocation, or equivalently, when a dislocation half-loop is stable against the misfit stress. (Any higher misfit stress will start to move the dislocations).

People & Bean, APL (1985), energy balance

Dislocation Free SK Islands

Eaglesham, et al PRL 1990

12

Wetting Layer: Ge on Si(001)

Layer-by-layer growth of the wetting layer. The distance between the trenches decreases with increasing coverage.

Voigtlaender, Forschungzentrum Juelich

300oC

Asaro Tiller Grinfeld (ATG) instability (small lattice mismatch)

Huts

Mo, et al PRL (1990)

{105} facets

13

Huts of Ge on Si(100), 575K

PRL 82 (1999) 2745

Dome - Huts

Pure Ge, Ross et al. PRL (1998)SiGe alloy, Volpi et al. TSF (2000)

14

Domes

Tersoff et al. PRL 2002InAs islands on GaAs

Ge Wetting Layer on Si(111), 500oC

Initial layer-growth of the Stranski-Krastanow wetting layer is observed. Details can be found in: Review of Scientific Instruments 67 (1996) 2568.

Property of Voigtlaender, Forschungzentrum Juelich

15

Ge Islands On Si(111), 350oC

Subsequent island-growth of Stranski-Krastanov islands. The atomic distances in a Ge crystal are larger than in Silicon. The resulting mechanical stress leads to the formation of three dimensional Geislands. The "growth movie" shows the evolution of the three dimensional islands at the same location as function of coverage. The form of the Ge islands is a flat toped tetrahedron. At low coverage the size-fluctuations of the islands are quite large, whereas at higher coverage the size of the islands becomes quite uniform. Typical dimensions of the islands are 700Å base length and 80 Å height. Further analysis shows that an anomaly in the aspect ratio of the islands (height divided by base length) as function of coverage indicates a transition from strained coherent islands (high aspect ratio) to relaxed islands with dislocations (lower aspect ratio) at higher coverage. Details can be found in: Applied Physics Letters 63 (1993) 3055.

Property of Voigtlaender, Forschungzentrum Juelich

Self-Assembled Quantum Dots

Floro, et al, APL (1998)

16

Sign Of Strain - Surface Roughness

Y. H. Xie et al. PRL (1994).

Strain-InducedRoughness & Segregation

Cullis, et al. JVSTA (1994)

17

Self-Organized Growth: InAs on GaAs(100)

Xie et al. PRL (1995).

Stacked Islands

18

Stacked Quantum Dots

Thanh et al. JCG (2000).

Oblique Island Correlation

19

Propagation of Strain

Schmidt et al, PRB 2000

2 ML Ge

Elastic Anisotropy

Heidemeyer et al, PRL 2003

20

Impurity-stabilized Surface Structure

As-terminated Si(111) 1x1

H-terminated Si(111) 1x1 B-stabilized Si(111)oR3033 −×

H and As stay on top, B goes under the surface.

Surfactant Growth: Ge on As-Si(100)

8 ML Ge on clean Si(100)

15 ML Ge on As-Si(100)

Copel et al. PRL (1989).

21

Surfactant Growth

(a) STM image (perspective view) of the Stranski-Krastanow growth of Ge on Si(111) without and (b) with surfactant. (area: 3µmx3µm, coverage 30ML, T=450oC). Forschungzentrum Juelich

Surfactant Growth - Surface Energy Anisotropy

Eaglesham et al. PRL (1993).

UHV anneal H anneal

Sb- anneal

In anneal

22

Positioning Of Quantum Dots

Xie, et al. APL (1997).

Positioning Of Quantum Dots

Kamins & Williams APL (1997).Kamins et al, APL (1999).

Nano-imprinting and dry etching

23

Self-Assembled Nanostructures on Patterned Si

Jin et al, APL (2000).

PL of Ge Nanodots

Denker et al, APL 2003

Schmidt et al, APL 2000

huts

domes

24

Effect Of Strain On SiGe Band Offset

SiGe on Si substrate Si on relaxed SiGe

Effect of Strain On Si/Ge Band Offset

25

Formation of Porous Si

electrochemical process

Abramof et al, JNCS 2004

Dian et al, ASS 2004

Photoluminescence From Porous Si

Dimov et al, JAP 2005 Zhao et al, PBCM 2005Typically, PS contains crystalline skeleton and amorphous Si in pores.

26

Semiconductor Bands

Sze, Phys. of Semicond. Dev.

indirect bandgap

Band Gap of Si Nanostructures

Delley et al, APL 1995

Si CBM

Si VBM