Semiconductor nanostructures for optoelectronic applications artech

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Semiconductor Nanostructures for
Optoelectronic Applications

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For a listing of recent titles in the Artech House
Semiconductor Materials and Devices Library, turn to the back of this book.

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Semiconductor Nanostructures for
Optoelectronic Applications
Todd Steiner
Editor

Artech House, Inc.
Boston • London
www.artechhouse.com

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Library of Congress Cataloging-in-Publication Data
A catalog record of this book is available from the U.S. Library of Congress.

British Library Cataloguing in Publication Data
Semiconductor nanostructures for optoelectronic applications
—(Artech House semiconductor materials and devices library)
1. Semiconductors 2. Nanostructured materials 3. Optoelectronic devices
I. Steiner, Todd
621.3’8152
ISBN

1-58053-751-0

Cover design by Gary Ragaglia
© 2004 ARTECH HOUSE, INC.
685 Canton Street
Norwood, MA 02062
All rights reserved. Printed and bound in the United States of America. No part of this book
may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without
permission in writing from the publisher.
All terms mentioned in this book that are known to be trademarks or service marks have
been appropriately capitalized. Artech House cannot attest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of any trademark or service mark.
International Standard Book Number: 1-58053-751-0

10 9 8 7 6 5 4 3 2 1

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Contents
CHAPTER 1
Introduction

1

1.1
1.2
1.3
1.4
1.5

1
1
2
2
3

Synopsis
Growth
Optoelectronic Devices Based on Semiconductor Nanostructures
Materials for Semiconductor Nanostructures
Summary

CHAPTER 2
Review of Crystal, Thin-Film, and Nanostructure Growth Technologies
2.1
2.2

2.3

2.4

2.5

2.6

Introduction
Review of Thermodynamics
2.2.1 Chemical Reactions
2.2.2 Phase Diagrams
Bulk Crystal Growth Techniques
2.3.1 Czochralski Method
2.3.2 Bridgman Method
2.3.3 Float-Zone Method
2.3.4 Lely Growth Methods
Epitaxial Growth Techniques
2.4.1 Liquid Phase Epitaxy
2.4.2 Vapor Phase Epitaxy
2.4.3 Molecular Beam Epitaxy
2.4.4 Metalorganic Chemical Vapor Deposition
2.4.5 Atomic Layer Epitaxy
Thin-Film Deposition Techniques
2.5.1 Plasma-Enhanced Chemical Vapor Deposition
2.5.2 Vacuum Evaporation
2.5.3 Sputtering
Growth of Nanostructures
2.6.1 Properties and Requirements of Quantum Dot Devices
2.6.2 Growth Techniques
References

5
5
6

7
7
8
8
11
13
14
16
16
17
20
24
29
29
29
31
33
34
35
36
41

v

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vi

Contents

CHAPTER 3
Quantum Dot Infrared Photodetectors

45

3.1
3.2

45
49
50
57

Introduction
QD and QDIP Structure Growth and Characterization
3.2.1 GaAs Capped Large and Small InAs QDs
3.2.2 AlGaAs Capped Large InAs MQD QDIP Structures
3.2.3 InxGa1-xAs Capped Small and Large InAs MQD-Based QDIP
Structures
3.3 QDIP Device Characteristics
3.3.1 Device Structures
3.3.2 Unintentionally Doped Large (PIG) InAs/GaAs MQD-Based
Detectors
3.3.3 QDIPs with AlGaAs Blocking Layers
3.3.4 InAs/InGaAs/GaAs QDIPs
3.3.5 Dual-Color QDIPs
3.4 Prognosis
Acknowledgments
References

64
76
76
77
87
92
102
107
109
109

CHAPTER 4
Quantum Dot Lasers: Theoretical Overview

113

4.1
4.2
4.3
4.4

113
115
115
116
117
126
129
131

132
134
139
142
143

Introduction: Dimensionality and Laser Performance
Advantages of an Idealized QD Laser
Progress in Fabricating QD Lasers
State-of-the-Art Complications
4.4.1 Nonuniformity of QDs
4.4.2 Parasitic Recombination Outside QDs
4.4.3 Violation of Local Neutrality in QDs
4.4.4 Excited States
4.4.5 Spatial Discreteness of Active Elements: Hole Burning
4.4.6 Intrinsic Nonlinearity of the Light-Current Characteristic
4.4.7 Critical Sensitivity to Structure Parameters
4.4.8 Dependence of the Maximum Gain on the QD Shape
4.4.9 Internal Optical Loss
4.5 Novel Designs of QD Lasers with Improved Threshold and Power
Characteristics
4.5.1 Temperature-Insensitive Threshold
4.5.2 Enhanced Power Performance
4.6 Other Perspectives
References

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148
148

150
151
153


Contents

vii

CHAPTER 5
High-Speed Quantum Dot Lasers

159

5.1 Introduction
5.2 MBE Growth of Self-Organized QDs and Their Electronic
Properties
5.2.1 Self-Organized Growth of In(Ga)As QDs
5.2.2 Electronic Spectra of In(Ga)As/GaAs QDs
5.3 Separate Confinement Heterostructure QD Lasers and Their
Limitations
5.3.1 Carrier Relaxation and Phonon Bottleneck in
Self-Organized QDs
5.3.2 Hot Carrier Effects in SCH QD Lasers
5.4 Tunnel Injection of Carriers in QDs
5.4.1 Tunneling-Injection Laser Heterostructure Design and
MBE Growth
5.4.2 Measurement of Phonon-Assisted Tunneling Times
5.5 Characteristics of High-Speed Tunneling-Injection QD Lasers
5.5.1 Room Temperature DC Characteristics

5.5.2 Temperature-Dependent DC Characteristics
5.5.3 High-Speed Modulation Characteristics
5.6 Conclusion
Acknowledgments
References

159
160
160
161
163
164
167
168
169
170
172
172
172
174
183
183
183

CHAPTER 6
Zinc Oxide-Based Nanostructures

187

6.1

187
187
189
191
191
194
210
211
211
215
219
219
221
224

Introduction
6.1.1 General Properties of ZnO
6.1.2 ZnO One-Dimensional Nanostructures
6.2 Growth Techniques
6.2.1 Growth Mechanisms
6.2.2 Growth Techniques
6.2.3 Summary
6.3 Characterizations
6.3.1 Structural Characterizations
6.3.2 Optical Characterizations
6.4 Device Applications
6.4.1 Optical Devices
6.4.2 Electronic Devices
References

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viii

Contents

CHAPTER 7
Antimony-Based Materials for Electro-Optics

229

7.1

229
229
230
232
232
235
235
239
242
250
250
253
253
256
259

259
262
262
262
265
266
267
268
269
271
273
273
275
284
285

7.2

7.3

7.4

7.5

7.6
7.7

7.8

Introduction

7.1.1 Antimony
7.1.2 Sb-Based III-V Semiconductor Alloys
7.1.3 Bulk Single-Crystal Growth
7.1.4 Applications
III-Sb Binary Compounds: GaSb, AlSb, and InSb
7.2.1 GaSb
7.2.2 AlSb
7.2.3 InSb
InAsSb
7.3.1 Physical Properties
7.3.2 Growth of InAsSb
7.3.3 Characterizations
7.3.4 Device Measurement
InTlSb
7.4.1 MOCVD Growth of InTlSb
7.4.2 InTlSb Photodetectors
InBiSb
7.5.1 MOCVD Growth of InSbBi
7.5.2 InSbBi Photodetectors
InTlAsSb
InAsSb/InAsSbP for IR Lasers
7.7.1 Growth and Characterization of InAsSb and InAsSbP
7.7.2 Strained-Layer Superlattices
7.7.3 Device Results
GaSb/InAs Type II Superlattice for IR Photodetectors
7.8.1 Introduction
7.8.2 Experimental Results for Type II Photodetectors
Acknowledgments
References

CHAPTER 8
Growth, Structures, and Optical Properties of III-Nitride Quantum Dots

289

8.1
8.2

289
291
292
314
317
318

Introduction
Growth of III-Nitride QDs
8.2.1 MBE Growth of III-Nitride QDs
8.2.2 Other Techniques
8.3 Optical Properties of III-Nitride QDs
8.3.1 Effects of Quantum Confinement, Strain, and Polarization

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Contents

ix

8.3.2 GaN QDs

8.3.3 InGaN QDs
8.4 Summary
References

323
337
343
344

CHAPTER 9
Self-Assembled Germanium Nano-Islands on Silicon and
Potential Applications
9.1
9.2
9.3
9.4
9.5

Introduction
Heteroepitaxy Mechanisms
Uniform Ge Islands
Registration and Regimentation of Ge Islands
Novel Device Applications
9.5.1 Optoelectronics
9.5.2 Thermoelectricity
9.5.3 Electronics Applications
9.5.4 Quantum Information Applications
9.6 Conclusion
References

349
349
349
350
355
362
362
365
366
366
367
367

CHAPTER 10
Carbon Nanotube Engineering and Physics

371

10.1 Introduction
10.2 Controlled Fabrication of Uniform Nanotubes in a Highly
Ordered Array
10.3 Interfacing with Biomolecules and Cells
10.4 Intrinsic Quantum Electromechanical Couplings
10.5 Extrinsic Coupling to Radiation Fields
10.6 Heterojunction Nanotubes
10.7 Prospects for Future Advances
Acknowledgments
References

371

373
379
382
391
392
396
398
398

Acronyms

403

About the Editor

407

Index

409

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CHAPTER 1

Introduction
Todd Steiner, Air Force Office of Scientific Research

1.1

Synopsis
As we begin the twenty-first century, nanoscience and technology are advancing at a
rapid pace and making revolutionary contributions in many fields including electronics, materials science, chemistry, biology, structures and mechanics, and optoelectronics. Although nanoscience and technology are progressing along many
fronts, the most impressive progress has been made in the area of semiconductor
technology. This book reviews recent progress in semiconductor nanostructure
growth and materials development and also reviews progress in semiconductor
devices using nanostructures, with a particular emphasis on 3D nanostructures that
have emerged during the last 10 years.

1.2

Growth
Semiconductor nanostructures have been enabled by the advancements in epitaxial
growth techniques, which are now capable of growing epilayers as thin as one
atomic layer and with interface roughnesses that are a mere fraction of a monolayer.
The development of advanced crystal and thin-film growth technologies capable of
realizing high crystalline quality and purity of materials is an enabling step in bringing semiconductor devices to reality. These growth techniques are reviewed in
Chapter 2. Chapter 2 starts with an overview of the bulk crystal growth techniques
that are required for obtaining high-quality substrates, then looks at the primary
means for producing high-quality epilayers, including liquid phase epitaxy, vapor
phase epitaxy, molecular beam epitaxy, metalorganic chemical vapor deposition
(MOCVD), and atomic layer epitaxy (ALE), as well as techniques for thin-film
deposition including plasma-enhanced chemical vapor deposition, electron cyclotron resonance, vacuum evaporation, and sputtering. Chapter 2 then discusses the
different growth modes of low-dimensional structures such as quantum wires and

quantum dots.

1

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2

1.3

Introduction

Optoelectronic Devices Based on Semiconductor Nanostructures
Since the successful development of quantum well lasers in the 1970s, one of the
richest areas of application of semiconductor nanostructures has been in the area of
optoelectronic devices, with the two most important areas being semiconductor
lasers and detectors. Early efforts focused on band-to-band transitions and have
progressed more recently to intersubband devices. In addition, the early devices utilized 2D nanostructures, either superlattices or quantum wells. In recent years, the
growth of quantum dots and their integration into working devices has revolutionized semiconductor devices. This book highlights results in semiconductor devices
based on quantum dots (QDs).
In Chapter 3, we review progress on quantum dot infrared detectors (QDIPs) by
providing a comprehensive discussion of the growth, structural and optical characterization, and device figures of merit. We discuss the QD and the QDIP structure
growth, QD size distribution, and the tailoring of the QD electronic energy levels
and wave functions via manipulation of the QD confinement potential. We also
show how to take advantage of stress manipulation to realize multiple-color QDIPs.
One section focuses on the QDIP device characteristics (dark current, responsivity,
noise, photoconductive gain, detectivity) for each of three classes of QDIPs discussed: InAs/GaAs/AlGaAs, InAs/InGaAs/GaAs, and dual-color InAs/InGaAs/GaAs
QDIPs.
In Chapter 4, we provide a theoretical overview of QD lasers, including the

advantages of QD lasers over quantum well lasers, the recent progress in fabricating
QD lasers, and a theoretical treatment of many issues of practical importance in
developing QD lasers, such as the nonuniformity of QDs, parasitic recombination
outside of QDs, threshold and power characteristics, and nonlinear properties. The
chapter also includes novel designs for QD lasers with improved threshold and
power characteristics.
In Chapter 5, we provide an overview of InGaAs tunnel injection QD lasers,
which have demonstrated the lowest thresholds for QD lasers and the highest modulation bandwidths. This chapter describes the growth of these QD lasers, the unique
carrier dynamics observed in self-organized QDs, their effect on high-frequency performance of QD lasers, and the novel injection technique whereby electrons are
injected into the QD ground state by tunneling. The enhanced performance of these
tunnel injection QD lasers is also described and discussed.

1.4

Materials for Semiconductor Nanostructures
Progress in semiconductor nanostructures is advancing to a wide variety of material
systems. In this book we highlight the progress in five important material systems of
technological importance. Each of these material systems has demonstrated 2D and
3D nanostructures and has had varying degrees of success in the fabrication of
optoelectronic devices.
In Chapter 6 we review progress in zinc oxide-based nanostructures, including
the Zno/ZnMgO system. Zinc oxide is emerging as an important material for

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1.5

Summary

3

ultraviolet and visible optoelectronic applications, due to the ease with which light
emission can be obtained. In Chapter 7 we review progress in antimony-based
nanostructures, including the binary compounds GaSb, InSb, and AlSb; the tertiary
compounds InAsSb, InAsP, InTlSb, and InSbBi; and the quaternary compounds
InTlAsSb and InAsSbP. Devices based on these materials are also discussed. In
Chapter 8 we review recent advances in the growth of III-nitride quantum dots and
their unique properties. The growth techniques and the structural and optical properties associated with quantum confinement, strain, and polarization in GaN and
InxGa1–xN quantum dots are discussed in detail.
In Chapter 9 we review the progress of nanostructures in the silicon/germanium
material system, which has the potential for bringing optoelectronics and photonics
to silicon. Specifically, we review issues of Ge island formation on Si. We show uniform Ge island formation on planar Si and ordered island formation on prepatterned mesa structures. We discuss the effect of growth conditions such as growth
temperature, deposition rate, deposition coverage, and substrate patterning on the
formation of the islands. We discuss the potential applications of Ge islands in the
fields of optoelectronics, thermoelectricity, electronics, and quantum information.
In Chapter 10, we present a review of carbon nanotubes, especially for optoelectronics applications. The field of carbon nanotubes has advanced quickly and
widely on many fronts during the past decade. Controlled fabrication of carbon
nanotubes of uniform diameter, length, and spacing is now feasible. Real and perceived potential applications in electronics, sensing, molecular biology, actuation,
composite material, and energy storage have been demonstrated. We introduce
some of these advances and some of the fundamental properties of the carbon nanotubes, discuss the underlying physics of new effects and phenomena observed or
anticipated, and describe the controllable fabrication processes of new forms of
nanotubes, as well as some interesting and relatively new and unconventional directions of potential applications.

1.5

Summary
As we enter the twenty-first century, semiconductor nanostructures are revolutionizing many areas of electronics, optoelectronics, and photonics. We present in this
volume some of the more interesting results that are leading the revolution in the
area of optoelectronics. It is in this area that the real benefits of 3D structures are

being realized for practical devices. These achievements will serve to enhance the
contributions of semiconductor nanostructures in other areas, helping to maintain
the leading position of semiconductor nanotechnology in the more general world of
nanoscience and technology.

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.

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CHAPTER 2

Review of Crystal, Thin-Film, and
Nanostructure Growth Technologies
Alireza Yasan and Manijeh Razeghi, Northwestern University

2.1

Introduction
During the latter half of the twentieth century, in an effort to increase integration,
enhance functionality, and reduce energy consumption, the major focus of the
development of semiconductor devices was on miniaturization. As a result, semiconductor devices have evolved from millimeter-sized devices capable of manipulating electricity (e.g., transistors) into micrometer-sized devices that can handle both
electricity and light (e.g., light-emitting diodes). As we enter the twenty-first century, we envision nanometer-sized semiconductor devices that can directly interact
with individual atoms and molecules at the nanometer level (e.g., quantum sensors).
In this regard, the development of advanced crystal and thin-film synthesis technologies capable of realizing high crystalline quality and purity of materials is an
enabling step toward making such semiconductor devices a reality.
We begin this chapter by giving an overview of thermodynamics. Chemical

reactions and phase diagrams are the subject of this first section, after which we
move on to a discussion of crystal growth techniques.
The earliest crystal growth techniques consisted of growing semiconductor
crystals in bulk form using one of the bulk crystal growth techniques: Czochralski,
Bridgman, or float zone. These methods are appropriate for the synthesis of largevolume semiconductor crystals under thermodynamic equilibrium conditions, but
offer nearly no flexibility in terms of alloy composition or the heterostructures
needed for advanced semiconductor devices. Nevertheless, these are excellent techniques for manufacturing high-purity, near perfect, single-crystal wafers to be used
as substrates for epitaxial growth.
Epitaxial growth techniques have been specifically developed to enable the
growth of high-quality semiconductor alloys under controlled conditions. Using
these techniques, single-crystal semiconductor thin films are synthesized on a substrate. As the need for even more complex semiconductor devices increased, several
techniques have been successively developed and refined to satisfy these everevolving needs. Liquid phase epitaxy is the oldest epitaxial growth technique.
Although still used in some instances, this technique is losing momentum because of
5

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6

Review of Crystal, Thin-Film, and Nanostructure Growth Technologies

its poor thickness uniformity and control and poor interface. The second technique, vapor phase epitaxy, has enjoyed broader success, but the material generally suffers from surface defects. It is nevertheless gaining interest in the case of
GaN-based semiconductors. Two other techniques, molecular beam epitaxy and
metalorganic chemical vapor deposition, are the most widely used techniques and
have demonstrated unsurpassed capabilities in the epitaxial growth of numerous
semiconductor structures, in terms of material quality, process control, and
reliability.
Other thin-film deposition techniques exist that are primarily used for the deposition of dielectric films, but can also be used for the deposition of semiconductors in
a polycrystalline form. These techniques include plasma-enhanced chemical vapor

deposition, electron cyclotron resonance, vacuum evaporation, and sputtering.
They are much simpler and cheaper than the epitaxial growth techniques, but are
not as flexible and do not yield material that is as high in quality. Nevertheless, they
are well suited for the deposition of the dielectric films commonly employed in the
manufacturing process used for semiconductor devices.
Finally, we conclude by discussing low-dimensional structures such as quantum
wires and quantum dots. Different growth modes and possible growth techniques
are presented. The requirements for room-temperature operation of devices based
on nanostructures are briefly discussed.

2.2

Review of Thermodynamics
In this section we briefly review the thermodynamics of materials. Thermodynamics
tells us whether or not a reaction is possible. It can also determine, to some extent,
the feasibility of a chemical reaction. To get such information the free-energy function, G, is often used:
G=H−TS

(2.1)

where H is the enthalpy, S is the entropy, and T is the absolute temperature. Let’s
assume that the initial state of the system (i) changes to a final state (f) due to a
chemical reaction while the temperature is kept constant. The free-energy change
can be written as
∆G = Gf − Gi = ∆H − T∆S

(2.2)

The second law of thermodynamics states that “in all energy exchanges, if no
energy enters or leaves the system, the potential energy of the state will always be less

than that of the initial state (∆G < 0).” This implies that systems tend to minimize
the free energy to a lower value than the initial value. After the system has achieved
the equilibrium, ∆G = 0. For a process that cannot occur, ∆G > 0. Therefore, the
possibility of occurrence of a particular reaction can be determined through the sign
of ∆G.

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2.2

Review of Thermodynamics

2.2.1

7

Chemical Reactions

For a typical chemical reaction involving materials X, Y, and Z in equilibrium with
x, y, and z as the stoichiometric coefficients,
xX + yY → zZ

(2.3)

The free-energy change of the reaction is given by
∆G = zGZ − xGX − yGY

(2.4)

Free energy of individual reactants is often written as
Gi = Gi0 + RT ln a i

(2.5)

0

where Gi is the free energy of the species in its standard state and ai is a term called
activity that reflects the change in free energy when the material is not in its standard
state. The standard state is typically 1 atmosphere partial pressure for a gas at 25°C.
A pure liquid or solid is the standard state of the relevant substance. Table 2.1 lists
the standard values of enthalpy and entropy for various substances [1]. Substitution
of (2.5) into (2.4) and letting ∆G = 0 yields
−∆G 0 = RT ln K

(2.6)

where
K=

2.2.2

a Zz ( eq )
y

a Xx ( eq ) aY ( eq )

(2.7)

Phase Diagrams

Phase diagrams are the primary visualization tools in materials science because they
allow one to predict and interpret changes in the composition of a material from

Table 2.1 Standard Values of Enthalpy and Entropy for Various Species
Species

State

∆Hf (kJ/mol)

CO2

Gas

–393.51 ± 0.13

213.785 ± 0.010

Cl2

Gas

0

223.081 ± 0.010

S (J/mol⋅K)

H

Gas

217.998 ± 0.006

114.717 ± 0.002

H+

Aqueous

0

0

H2O

Liquid

–285.830 ± 0.040

69.95 ± 0.03

H2O

Gas

–241.826 ± 0.040

188.835 ± 0.010

N

Gas

472.68 ± 0.40

153.301 ± 0.003

NH3

Gas

–45.94 ± 0.35

192.77 ± 0.05

O2

Gas

0

205.152 ± 0.005

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8

Review of Crystal, Thin-Film, and Nanostructure Growth Technologies

phase to phase. As a result, phase diagrams have been proven to provide an immense
understanding of how a material forms microstructures within itself, leading to an
understanding of its chemical and physical properties. However, in some instances
materials have failed to perform to their proposed potential. One can deduce, by
referring to a material’s phase diagram, what may have happened to the material
when it was made to cause failure. In these instances, one can use thermodynamic
relations to go into the phase diagrams and extrapolate the data.
A few simple rules are associated with phase diagrams with the most important
of them being the Gibb’s phase rule. The phase rule describes the possible number of
degrees of freedom in a (closed) system at equilibrium, in terms of the number of
separate phases and the number of chemical constituents in the system. It can be simply written as follows:
f = C − P +2

(2.8)

where C is the number of components, P is the number of phases, and f is the number
of degrees of freedom in the system. The number of degrees of freedom (f) is the
number of independent intensive variables (i.e., those that are independent of the
quantity of material present) that need to be specified in value to fully determine the
state of the system. Typical such variables might be temperature, pressure, or concentration. This rule states that for a two-component, one-phase system, there are
two degrees of freedom. For example, on a P-T diagram, pressure and temperature
can be chosen independently. On the other hand, for a two-phase system, there is
only one degree of freedom and there is only one pressure possible for each temperature. Finally, for a three-phase system, there exists only one point with fixed pressure
and temperature (Figure 2.1). As a real-world example, the P-T-x phase diagram of
the Ga-N system at a fixed pressure of 1 atm is shown in Figure 2.2 [2].

2.3

Bulk Crystal Growth Techniques
The historical starting point for virtually all semiconductor devices has been in the
synthesis of single crystals. Today, three major methods have been developed to
realize large-volume semiconductor crystals under thermodynamic equilibrium conditions: the Czochralski, Bridgman, and float-zone methods, which are discussed in
the following subsections.
2.3.1

Czochralski Method

The Czochralski (CZ) crystal growth method was developed in 1916 by accident.
Jan Czochralski, an engineer at the AEG Company in Berlin at that time, accidentally dipped his pen into a crucible containing molten tin and withdrew it quickly.
He observed a thin wire of solidified metal hanging at the tip. This small observation
later led to development of the Czochralski method for obtaining single crystals [3].
The Czochralski method is by far the most popular crystal growth method,

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Bulk Crystal Growth Techniques

9

Liquid
(P = 1, f = 2)
Solid
(P = 1, f = 2)

Liquid + solid
(P = 2, f = 1)

Pressure

2.3

Gas + liquid
(P = 2, f = 1)

Solid + gas
(P = 2, f = 1)

Solid + gas + liquid
(P = 3, f = 0)
Gas
(P = 1, f = 2)
Temperature

Figure 2.1 P-T diagram of a one-component system showing degrees of freedom for a different
number of phases.

T, K

Ga-N diagram at 1 atm of Nitrogen

2,500

Gas

2,000
Liquid + gas
1,500

1,000
Liquid + GaN

GaN + Gas

500
Ga + GaN
Ga

Figure 2.2

20

40
60
Atomic % Nitrogen

80

Calculated P-T-x phase diagram for Ga-N at atmospheric pressure.

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N


10

Review of Crystal, Thin-Film, and Nanostructure Growth Technologies

accounting for between 80% and 90% of all silicon crystals grown for the semiconductor industry.
The Czochralski method uses a high-purity quartz (SiO2) crucible filled with
pieces of polycrystalline material, called charge, which are heated above their melting point (e.g., 1,415°C for silicon). The crucible, shown in Figure 2.3, is heated
either by induction using radio-frequency (RF) energy or by thermal resistance
methods. A “seed” crystal, about 0.5 cm in diameter and 10 cm long, with the
desired orientation is lowered into molten crystal, termed melt, and then drawn up
at a carefully controlled rate. When the procedure is properly done, the material in
the melt will make a transition into a solid phase crystal at the solid/liquid interface,
so the newly created material accurately replicates the crystal structure of the seed
crystal. The resulting single crystal is called the boule. Modern boules of silicon can
reach diameters of more 300 mm and be up to 2m long.
During the entire growth period, the crucible rotates in one direction at 12 to 14
rotations per minute (rpm), while the seed holder rotates in the opposite direction at
6 to 8 rpm while being pulled up slowly. This constant stirring prevents the formation of local hot or cold regions. The crystal diameter is monitored by an optical
pyrometer that is focused at the interface between the edge of the crystal and the
melt. An automatic diameter control system maintains the correct crystal diameter
through a feedback loop control. Argon is often used as the ambient gas during this

Quartz tube

Pulling rod
Seed crystal
Encapsulant

Heater coil

Melt

Crucible

Crucible holder

Figure 2.3 Cross section of a furnace used for the growth of single-crystal semiconductor boules
by the Czochralski process.

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2.3

Bulk Crystal Growth Techniques

11

crystal-pulling process. By carefully controlling the pull rate, the temperature of the
crucible, and the rotation speed of both the crucible and the rod holding the seed,
precise control over the diameter of the crystal is obtained.
During the Czochralski growth process, several impurities are incorporated into
the crystal. Because the crucibles are made from fused silica (SiO2) and the growth
process takes place at temperatures around 1,500°C, small amounts of oxygen will
be incorporated into the boule. For extremely low concentrations of oxygen impurities, the boule can be grown under magnetic confinement. In this situation, a large
magnetic field is directed perpendicularly to the pull direction and used to create a
Lorentz force. This force will change the motion of the ionized impurities in the melt
in such a manner as to keep them away from the solid/liquid interface and therefore
decrease the impurity concentration. Using this arrangement, the oxygen impurity
concentration can be reduced from about 20 parts per million (ppm) to as low
as 2 ppm.
It is also common to introduce dopant atoms into the melt in order to tailor the
electrical properties of the final crystal: the carrier type and concentration. Simply
weighing the melt and introducing a proportional amount of impurity atoms is all

that is theoretically required to control the carrier concentration. However, impurities tend to segregate at the solid/liquid interface, rather than be uniformly distributed inside the melt. This will in turn affect the amount of dopant incorporated into
the growing solid.
The growth of GaAs with the Czochralski method is far more difficult than for
silicon because of the vast differences in vapor pressure of the constituents at the
growth temperature of ~1,250°C: 0.0001 atm for gallium and 10,000 atm for arsenic. The liquid encapsulated Czochralski (LEC) method utilizes a tightly fitting disk
and sealant around the melt chamber (see the encapsulant in Figure 2.3) to prevent
the out-diffusion of arsenic from the melt. The most commonly used sealant is boric
oxide (B2O3). Additionally, pyrolytic boron nitride (pBN) crucibles are used instead
of quartz (silicon oxide) in order to avoid silicon doping of the GaAs boule. Once
the charge is molten, the seed crystal can be lowered through the boric oxide until it
contacts the charge, at which point it may be pulled.
Because the thermal conductivity of GaAs is about one-third that of silicon, the
GaAs boule is not able to dissipate the latent heat of fusion as readily as silicon. Furthermore, the shear stress required to generate a dislocation in GaAs at the melting
point is about one-fourth that in silicon. Consequently, the poorer thermal and
mechanical properties allow GaAs boules to be only about 8 inches in diameter [4]
and they contain many orders of magnitude larger defect densities than are realized
in silicon.
2.3.2

Bridgman Method

The Bridgman crystal growth method is similar to the Czochralski method except
that all of the semiconductor material (melt, seed, crystal) is kept completely
inside the crucible during the entire heating and cooling processes, as shown in
Figure 2.4.

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12

Review of Crystal, Thin-Film, and Nanostructure Growth Technologies
Furnace tube

Crystal

Molten semiconductor

Crystal

Molten area

Seed

Polycrystal

Pull

Pull

Pull
Crucible
(a)

Heater coils
(b)

Figure 2.4 The Bridgman growth method in a crucible: (a) solidification from one end of the
melt and (b) melting and solidification in a moving heated zone.

A quartz crucible filled with polycrystalline material is pulled horizontally
through a furnace tube. As the crucible is drawn slowly from the heated region into a
colder region, the seed crystal induces single-crystal growth. The shape of the resulting crystal is determined by the shape of the crucible. As a variation to this procedure, the heater may move instead of the crucible. As an alternative, the heater may
move instead of the crucible.
A couple of disadvantages are associated with the Bridgman growth method.
They result from the fact that the material is constantly in contact with the crucible.
First, the crucible wall introduces stresses in the solidifying semiconductor. These
stresses will result in deviations from the perfect crystal structure. Also, at the high
temperatures required for bulk crystal growth, silicon tends to adhere to the
crucible.
In the case of compound semiconductors, the process is slightly different from
that for silicon. The solid gallium and arsenic components are loaded onto a fused
silica ampule, which is then sealed. The arsenic in the chamber provides the overpressure necessary to maintain stoichiometry. A tube furnace is then slowly pulled
past the charge. The temperature of the furnace is set to melt the charge when it is
completely inside. As the furnace is pulled past the ampule, the molten GaAs charger
in the bottom of the ampule recrystallizes. A seed crystal may be mounted so as to
contact the melt.
Typical compound semiconductor boules grown by the Bridgman method have
diameters of 2 inches. The growth of larger crystals requires very accurate control of
the stoichiometry and the radial and axial temperature gradients. Dislocation densities of lower than 103 cm–2, compared to 104 cm–2 for boules grown by the CZ
method, are routinely achieved with the Bridgman method. This method produces
the best results for compound semiconductor growth such as GaAs, and approximately 75% of the compound semiconductor boules are grown by the Bridgman
growth method.

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2.3

Bulk Crystal Growth Techniques

2.3.3

13

Float-Zone Method

Unlike the previous two methods, the float-zone (FZ) technique proceeds directly
from a rod of polycrystalline material obtained from the purification process as
shown in Figure 2.5. Moreover, this method does not make use of a crucible. For
this reason, extremely high-purity silicon boules, with carrier concentrations lower
than 1011 cm–3, have been grown by the FZ method. But in general, this method is
not used for compound semiconductor growth.
The principle of the FZ method is as follows. A rod of an appropriate diameter
is held at the top of the growth furnace and placed in the crystal-growing chamber.
A single-crystal seed is clamped in contact at the other end of the rod. The rod and
the seed are enclosed in a vacuum chamber or inert atmosphere, and an inductiveheating coil is placed around the rod outside the chamber. The coil melts a small
length of the rod, starting with part of the single seed crystal. A “float zone” of melt
is formed between the seed crystal and the polysilicon rod. The molten zone is
slowly moved up along the length of the rotating rod by moving the coil upward.
High-purity crystals can be obtained with FZ method.
The molten region that solidifies first remains in contact with the seed crystal
and assumes the same crystal structure as the seed. As the molten region is moved
along the length of the rod, the polycrystalline rod melts and then solidifies along its
Inert gas

Quartz tube

Polycrystalline
rod

Upward
moving
heater coil

Molten zone

Single crystal

Seed crystal

Figure 2.5

Cross section of the FZ crystal growth furnace.

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