Albert Davydov Metallurgy D

Outline: NW research in our Division Nano-LEDs case study: 

 Design: Core-shell vs. Axial Heterostruct Growth: VLS vs. VS (nucleation, phase dia

 Quantum Discs: InGaN phase separation, 

 Summary

Semiconductor Nanowires: Opportun

Semiconductor Nanowires: Opportun

Semiconductor Nanowires: Opportun
Denis Tsvetkov, Sergiy Krylyuk – GaN and Si CVD growth

Kris Bertness (Boulder) – GaN MBE growth

Igor Levin, Vladimir Oleshko – TEM

Abhishek Motayed – nanodevicesJohn Schlager, Norman Sanford (Boulder) – opt. spectros

Acknowledgeme

Acknowledgem

Acknowledgem
Key Issues: 

• uncontrolled NW dimensions, orientation an

• lack of metrology: new measurements & mo

AlN by HVPE (D. Tsvetkov)

1 μm 2 μm

ZnO by CVD (B. Nikoobakht)

1 μm 

Semiconductor NW Growth at NIST

Si by CVD (S. Krylyuk)

GaN by MBE (K. Bertness)

1 μm
HVPE Systems for GaN and Si NW Growth

System Features: Variable pressure (3 mTorr

– 760 Torr)

 Fast growth interruption Growth of alloys (AlGaN, InGaN) and heterostructure

etc.)

 In situ

n‐ and

p‐ doping

T

g=700

°

C ‐ 1050

°C; P=450 torr;

t

= 20s – 9

0 min;

V

/III

= 20‐30 (VS)

& 0

Als + H

Clv 

→ AlCl

x

v

NH

3

v 

Halide

Vapor

Phase

Epitaxy:

I II III IV

Sample tray

Inlet
Motivation: NWs for Optoelectronics, PV, batterie

Advantages of NW arrays over thin‐film devices:

• high efficiency (no defects, high surface/volume ratio)

• high selectivity (multiple materials and/or devices on

a sing

• new types of devices (quantization effects)

Bio LEDs, lasers, detectors ‐/chemi‐ sensors

Li‐ion batteries

F
GaN NW Gas Sensors (Chemiresistors)

GaN

TiO2

Abhishe
Si NW Photovoltaics

n-GaN cored~120 nm

(111)Si n++

n-Si

p-Si

Photovoltaics

[09Fan]

IMS Pro
AlN and Si NW Field-Emitters

Field‐emitters
n/p Core-Shell GaN NWs for LEDs

5 μm

5 μm

MBE n-GaN NWs on Si HVPE p-shells over n-cores

5 μm

p-shelln-core

LEDs, lasers, detectors

IMS program w/Boul
GaN NW LEDs

p‐GaN

GaN NW LED

APL (2007) 90, 183120 

PL from InGaN/GaN NWs [10Kishino]

Schematic NW array LED [10Kim]
GaN NW LEDs: case study

• Design: Core

‐shell vs. Axial Heterostructures

• Growth: VLS vs. VS

• phase diagrams; nucleation, defect format

• Quantum Disks:

• strain, InGaN phase separation, defects

LEDs, lasers, detectors

Sophia Univ.
VS Growth Status: well understood?

• Spontaneous formation of wetting layer: not understood yet (no 

Court

Our work
[Lieber]

Au Mol. % GaAs

(Au, Ga, As)

vapor

Au

GaAs<Ga>, <As>

I II

vapor

III IV

Does VLS path lie in the Au‐GaAs binary?

• Growth path can not be traced on the Au‐GaAs section!

Not possible to:

• define growth temperature or trace compositions

• choose metal for catalyst Need te

?

VLS status: misuse of phase diagrams for compo

• Thermodynamic description of VLS growth for compounds is neede
Effect of VLS and VS Growth on Defects

VS growth (MBE) NIST

• Higher density of extended defects in VLS NWs: growth instabilities at 

• Contamination of NWs with catalyst (indirect evidence, PL): inside ma

VLS Si

NWs

SFs
Competition of VS vs. VLS Growth

Complications:

• Heterostructure integrity

• Doping inhomogeneity• Unified model to explain morphology?

VLS growthVS growth

10 nm

GaN

AlGaN

GaN

GaN

GaN

GaN

AlGaN/GaN QDs (Courtesy 

GaN

GaN

J. Ristic et al., Phys. Rev. B68, 1
Quantum Discs (QDs): InGaN phase separation, strain, 

• QD inhomogeneity 

• Charge localization 

n‐G

a

N

p‐G

a

N

InGaN QDs

[10Kishino]

In incorporation – functio

n of NW diameter!

Effect of biaxial strain during layer growth
Summary

Modeling opportunities:• VS growth:

• understanding wetting layer formation (Strans

• defect formation

• VLS growth:

• thermodynamic description of ternary system

• combine with VS model

• explain NW morphologies and defects

• Core‐shell and axial heterostructures:

• strain and its relaxation – effect on morpholog

• doping issues

• Surface states and their effect on opto/electronic p