Wide Band Gap Semiconductor Nanowires 1 E-Book

Vincent Consonni

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 713

Veröffentlichungsjahr: 2014

Beschreibung

GaN and ZnO nanowires can by grown using a wide variety of methods from physical vapor deposition to wet chemistry for optical devices. This book starts by presenting the similarities and differences between GaN and ZnO materials, as well as the assets and current limitations of nanowires for their use in optical devices, including feasibility and perspectives. It then focuses on the nucleation and growth mechanisms of ZnO and GaN nanowires, grown by various chemical and physical methods. Finally, it describes the formation of nanowire heterostructures applied to optical devices.

Leseprobe

Contents

Preface

PART 1: GaN and ZnO Nanowires: Low-dimensionality Effects

1: Quantum and Optical Confinement

1.1. Introduction

1.2. All-optical integrated circuits with Bose exciton polaritons

1.3. High efficiency single photon sources

1.4. High efficiency solar photovoltaics

1.5. Conclusion

1.6. Bibliography

2: Stress Relaxation in Nanowires with Heterostructures

2.1. Introduction

2.2. Calculation and measurement of elastic strain in nanowires

2.3. Core-shell heterostructures

2.4. Axial heterostructures

2.5. Other possible modes of stress relaxation in nanowires with heterostructures

2.6. Summary and conclusions

2.7. Bibliography

3: Surface-related Optical Properties of GaN-Based Nanowires

3.1. Introduction

3.2. Specific exciton and donor states related to surfaces

3.3. Non-radiative surface recombination

3.4. Influence of surface photochemical activity on nitride nanowire optical properties

3.5. Summary

3.6. Bibliography

4: Surface Related Optical Properties of ZnO Nanowires

4.1. Introduction

4.2. Surface excitons in ZnO nanowires

4.3. Surface-related defect luminescence in ZnO nanowires

4.4. Surface functionalization of ZnO nanowires with colloidal quantum dots

4.5. Other surface-related effects in ZnO nanowires

4.6. Conclusion

4.7. Bibliography

5: Doping and Transport

5.1. Introduction

5.2. Advanced lithography processes for direct wide band gap nanowire and microwire devices

5.3. Electrical transport properties of single wire: ZnO nanowire and GaN microwire

5.4. Local probe and mapping of the electric field: cathodoluminescence

5.5. Conclusion and perspectives

5.6. Bibliography

6: Microstructure of Group III-N Nanowires

6.1. Introduction

6.2. Structural properties

6.3. Polarity

6.4. Extended defects in nanowires

6.5. Interfaces and heterostructures

6.6. Conclusions

6.7. Bibliography

PART 2: Nucleation and Growth Mechanisms of GaN and ZnO Nanowires

7: Ni Collector-Induced Growth of GaN Nanowires on C-Plane Sapphire by Plasma-Assisted Molecular Beam Epitaxy

7.1. Introduction

7.2. Experimental description

7.3. Ni-induced GaN nanowire nucleation

7.4. Ni-induced GaN nanowire growth mechanism

7.5. Ni-induced GaN nanowire structural and optical properties

7.6. Conclusion

7.7. Acknowledgments

7.8. Bibliography

8: Self-Induced Growth of GaN Nanowires by Plasma-assisted Molecular Beam Epitaxy

8.1. Introduction

8.2. General principles

8.3. Nucleation phase

8.4. Growth phase

8.5. Conclusion

8.6. Acknowledgments

8.7. Bibliography

9: Selective Area Growth of GaN Nanowires by Plasma-Assisted Molecular Beam Epitaxy

9.1. Introduction

9.2. Mask preparation

9.3. Selectivity, nucleation mechanism and morphology control of the nanocolumns

9.4. Growth of ordered NCs for LEDs applications

9.5. Growth of ordered GaN nanocolumns on non-polar and semi-polar directions

9.6. Summary

9.7. Bibliography

10: Metal-Organic Vapor Phase Epitaxy Growth of GaN Nanorods

10.1. Introduction

10.2. Catalyst-assisted growth

10.3. Catalyst-free and self-organized growth

10.4. Selected-area growth

10.5. Discussion and conclusion

10.6. Bibliography

11: Metal-Organic Chemical Vapor Deposition Growth of ZnO Nanowires

11.1. Introduction

11.2. Thermodynamics

11.3. Growth of ZnO nanowires

11.4. Spontaneous growth of ZnO nanowires: growth condition effects

11.5. Selective area growth of ZnO nanowires

11.6. Catalyst-assisted growth of ZnO NWs

11.7. Acknowledgements

11.8. Bibliography

12: Pulsed-Laser Deposition of ZnO Nanowires

12.1. Introduction

12.2. Principles of high-pressure and hot-walled pulsed-laser deposition

12.3. Tuning the nanowire morphology

12.4. Doped binary nanowires and ternary alloy nanowires

12.5. Fabrication of nanowire heterostructures

12.6. Summary and outlook

12.7. Bibliography

13: Preparation of ZnO Nanorods and Nanowires by Wet Chemistry

13.1. Introduction

13.2. Preparation of ZnO nanorods and nanowires by chemical bath deposition and hydrothermal techniques

13.3. Preparation of ZnO nanorods and nanowires by electrodeposition

13.4. Applications of ZnO nanorods/nanowires prepared by wet chemistry and by electrochemistry

13.5. Conclusions

13.6. Bibliography

List of Authors

First published 2014 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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© ISTE Ltd 2014The rights of Vincent Consonni and Guy Feuillet to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2014941789

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-597-9

Preface

This book is devoted to the specific case of wires obtained from a given kind of semiconductors, namely the semiconducting materials with a direct and wide band gap (WBG). In short, semiconductors are considered as WBG semiconducting materials if their band gap energy is typically above 1.5/1.6 eV. The interest of these materials for optoelectronic devices lies in the fact that they are well-adapted for emission, detection or absorption processes in most of the visible range, and part of the UV range as well. From the more basic point of view, the large refractive index and high exciton binding energy as well as the strong photon/exciton interactions give rise to long sought effects such as polariton lasing at room temperature for instance. The two main materials composing the family of WBG semiconducting materials are GaN and ZnO. They have close band gap energy in the near UV region (i.e., around 3.3/3.4 eV), and have in common that their cationic alloys span the visible as well as the UV range (and also part of the near IR region for In-rich GaInN alloys). More importantly, they both crystallize, in standard conditions, in the strongly anisotropic wurtzite crystalline phase, leading to a large number of similar physical quantities such as lattice parameters and piezoelectric constants and of similar physical processes related for instance to polarity.

GaN and its alloys are now well-mastered and used in a flurry of industrial applications as optoelectronic devices. On the other hand, ZnO is less advanced in terms of industrial applications and its development is mainly hampered by the difficulty for controlling p-type doping. However, ZnO has a stronger exciton binding energy than GaN (60 meV vs. 25 meV) and also a stronger oscillator strength. GaN and related alloys are generally heteroepitaxially grown on foreign substrates since low-cost nitride substrates with large dimensions are still not available. In contrast, ZnO and related alloys can homoepitaxially be grown onto ZnO substrates with excellent structural properties but still with limited availability and sizes. Therefore, epitaxial growth is mostly carried out heteroepitaxially for both kinds of materials, typically yielding epitaxial planar layers with a high density of structural defects. If such WBG semiconducting materials with a rather poor structural quality are actually used for some optoelectronic devices such as commercial LEDs for the moment, the improvement of their overall structure would certainly be beneficial for additional potential optoelectronic devices but also for the understanding of the physical processes at stake in these devices.

The need for WBG semiconducting materials with better structural quality is one of the main reasons that propelled (nano)wires to their present day status in the field of semiconductor research: when grown onto foreign substrates, and as for the case of planar layers, wires can relax the elastic strain energy originating from large lattice mismatch by forming misfit dislocations. But these lie in the basal plane or bend towards the nearby lateral surfaces of wires, thus leaving defect-free materials in their core. This process whereby dislocations can bend towards the lateral growth front had been demonstrated beforehand in epitaxial lateral overgrowth (ELO).

The second reason behind the development of WBG semiconductor wires – considered for a long time as the unwanted result of wrong growth conditions when trying to synthesize 2-dimensional (2D) epitaxial layers– is related to the increasing interest for low-dimensionality objects, typically of sub-micron or nanometer size. The specific structural, optical, and electronic properties of these low-dimensionality objects open new opportunities for nanoscale optoelectronic devices, especially to fully exploit the strong photon/exciton interactions. As an example, wires allow for a full confinement of light in their section with free propagation along their axis. Such physics and the related optoelectronic applications are nonetheless limited by the large developed surfaces of the wires, for which surface passivation is for instance required in order to prevent light diffusion. Because of the presence of surface states, Fermi level pinning also leads to band bending affecting the carrier mobility along the wires and resulting in possible carrier trapping. In return, this specific property makes wires very invaluable objects to investigate surface effects in WBG semiconductors and can also be beneficial in photodetection applications.

Looking back in time, the first demonstration of semiconductor wire growth was achieved by the pioneering work of Wagner and Ellis in 1964 according to the vapor-liquid-solid (VLS) mechanism [WAG 64]. In the field of WBG semiconducting materials for optoelectronic devices, which are the materials that we are interested in in this book, one of the first “nano”objects that were looked into were dots, named quantum dots when the typical dimensions are smaller than the De Broglie’s wavelength, inserted as they were in 2D epitaxial layers. For instance, the dots can be grown according to the so-called Stransky-Krastanov mode owing to the elastic stress relaxation processes at play in lattice mismatched heteroepitaxial systems. This is nevertheless limited somehow to heteroepitaxial layers in a state of compressive strain, and of medium lattice mismatch range (typically a few percent). For one heteroepitaxial system, such dots have once and for all a fixed size given by the nature of the involved materials. Thus, one had to think of other possibilities for making sub-micron or nano objects with an easier control over their sizes and shapes. Instead of playing for instance with strain to form dots, the easier way to grow low-dimensionality structures is to try and depart from the 2D growth conditions, thereby changing the atomic diffusion and incorporation processes, hence using growth modes different from the usual 2D mode. This time, this leads to the controlled formation of 1D objects, now referred to as nanowires, microwires or more generally wires, depending on their lateral dimensions, or also as nanocolumns, nanorods or microrods.

Interestingly, in terms of growth conditions, while most of the semiconductor (i.e., Si, Ge, arsenides, phosphides, …) wires can exclusively be grown by VLS or vapor-solid-solid mechanisms in the bottom-up approach, one of the most amazing properties of GaN and ZnO is their ability to grow in the form of wires following catalyst-free approaches (i.e., self-induced growth, spontaneous growth, …). These catalyst-free approaches are expected to reduce potential contamination into the wires and, more importantly, offer new valuable growth modes with great potentiality for optoelectronic devices. The first demonstrations of GaN and ZnO wire growth were shown in 1998 by molecular beam epitaxy [YOS 97, SAN 98] and in 2001 by vapor phase transport [HUA 01, PAN 01] and in solution [VAY 01], respectively. Basically, GaN wires can mainly be grown by molecular beam epitaxy and metal-organic chemical vapor deposition. In contrast, ZnO wires can additionally be deposited by vapor phase transport, pulsed-laser deposition or more specifically in solution via the low-cost and low-temperature chemical bath deposition technique for instance.

As discussed above, growing wires with dedicated properties in a reproducible way requires a good control of the growth conditions. When it comes to radial as well as axial heterostructures grown around or on top of the wires, things are somehow more complicated, since growth conditions very often have to be moved from the initial 1D case in order to stack the layers on top of each other. As in the case of any kind of heterostructures, managing the lattice mismatch issue may also be essential. This does depend upon the sizes involved and may potentially lead to the generation of misfit dislocations at the interfaces between the constituting layers. Moreover, owing to the specific geometry of the wires, other types of defects may also be introduced, such as stacking faults or inversion domain boundaries for instance, the origin of which has to be identified in order to better limit their occurrence. In return, identifying the right conditions for growing heterostructures with a good structural quality opens up a flurry of applications in the field of optoelectronics. These will benefit not only from the wave guiding properties of the wires (i.e., specific optical modes) but also from the control over the density of defects into the wires, leading to a decrease in the number of non-radiative recombination centers. These applications also take advantage of the larger surface to volume ratio at low-scale dimensions, leading for instance to much larger emitting or absorbing surfaces than in 2D layers or to efficient photodetectors.

The book has been organized along the lines of these introductory remarks.

Accordingly, it is the aim of the first part of volume 1 to focus on the specific properties of WBG semiconductor wires, in order to point out what differentiates these objects from their 2D counterparts. This appears as a necessary step in order to point out what these specificities could bring for the physics and applications of WBG semiconductors in the field of optoelectronics. It is nonetheless also the aim of this first part to try and pin-point the present day limitations associated with the use of WBG semiconductor wires, in order to draw possible solutions for a thorough use of these 1D objects. As for the second part of volume 1, it is dedicated to the different growth methods for the deposition of GaN and ZnO wires, stressing the mechanisms at play for the nucleation and growth of these 1D objects. The most interesting growth methods are discussed in detail with a special emphasis on the necessary ingredients to spontaneously grow GaN and ZnO wires. In volume 2, the first part aims at reviewing the different axial or radial heterostructures that can be integrated into GaN and ZnO wires. This is done to address relevant potential optoelectronic applications including LEDs, lasers, UV photodetectors and solar cells, which are presented and discussed in the second part of volume 2.

As revealed by the very numerous publications, the subject is far from being closed and new results emerge at a quick pace. With this in mind, this book is intended to give the reader a detailed overview of the current status of research in the field of WBG semiconductor wires for optoelectronic devices. As announced in the very title of this book, the choice was deliberately made to intermix chapters devoted to GaN and ZnO wires: the two materials have a lot in common, and the two communities will gain from mutual exchanges.

We hope that the reviews presented here by pioneering and world-leading scientists in the field, the discussion on the chemistry, physics, and applications of WBG semiconductor wires, together with the comparison between the two kinds of materials and between the different growth methods will be a useful source of information not only for the new comers in the field, but also for the already involved engineers and scientists who seek a detailed overview of the subject to give their work a new impulse.

Finally, we would like to warmly thank all our friends and colleagues who took part in this book project to create a lively, fruitful and high level place on the hot topic of WBG semiconductor wires.

Vincent CONSONNIGuy FEUILLETJune 2014

Bibliography

[WAG 64] WAGNER R.S., ELLIS W.C., Appl. Phys. Lett., 4, 89 (1964).

[YOS 97] YOSHIZAWA M., KIKUCHI A., MORI M., et al., Japanese J. Appl. Phys., 36, L459 (1997).

[SAN 98] SANCHEZ-GARCIA M.A., CALLEJA E., MONROY E., et al., J. Cryst. Growth, 183, 23 (1998).

[HUA 01] HUANG M.H., MAO S., FEICK H., et al., Science, 292, 1897 (2001).

[PAN 01] PAN Z.W., DAI Z.R., WANG Z.L., Science, 291, 1947 (2001).

[VAY 01] VAYSSIERES L., KEIS K., LINDQUIST S.E., et al., J. Phys. Chem., B 105, 3350 (2001).

PART 1

GaN and ZnO Nanowires: Low-dimensionality Effects

1

Quantum and Optical Confinement

1.1. Introduction

GaN and ZnO nanowires (NWs) are a fascinating photonics platform with the combination of one-dimensional (1D) structural geometry and the remarkable electronic and optical properties of wide band gap semiconductors [TAK 07, MOR 08]. GaN and ZnO have a direct gap ~3.4 eV at room temperature and an exciton oscillator strength, roughly two orders of magnitude larger than in GaAs [GIL 97, KLI 10], one of the most popular semiconductors for optoelectronics. Their surface recombination velocity is comparable to most of the other semiconductors, ~ 104–105 cm/s [ZHA 08, LIN 06, ALE 03], therefore, some care should be taken to manage unavoidable surface effects in NW structures. For further discussion on this topic, please refer to Chapter 3 of this book for GaN NWs and to Chapter 4 for ZnO NWs.

GaN and ZnO NWs are usually grown along the c-axis of the wurtzite crystal structure, with six hexagonal facets, tens to hundreds of nm in diameter and around 1 μm in length. These dimensions are orders of magnitude larger than the exciton Bohr radius ~2.8 nm in GaN and ~1.8 nm in ZnO; thus, electronic quantum confinement can only be obtained through material modulation, either along the growth axis with axial heterostructures or along the radial direction with core-shell heterostructures. This will be the subject of the different chapters in Volume 2 [CON 14]. However, NW diameters are of the same order or smaller than optical wavelengths in the near band edge region (~150 nm, assuming 2.5 refractive index); as a result, light is naturally confined in the NW cross-section plane and freely propagative along the NW length. This 1D wave guiding effect turns out to be a highly efficient way to extract or absorb light in a medium of high refractive index such as GaN and ZnO. In this chapter, quantum and optical confinement in NWs will be examined with three photonics topics of great promise: (1) all-optical integrated circuits with Bose exciton polaritons, involving 1D photon modes interacting with bulk excitons in the so-called strong coupling regime of the light-matter interaction; (2) high efficiency single photon sources (SPSs) for quantum information processing, based on single quantum dots (QDs) axially embedded in 1D photonic wires; (3) high efficiency photovoltaics with core–shell NW arrays.

1.2. All-optical integrated circuits with Bose exciton polaritons

All-optical networks were developed more than 20 years ago to overcome the electronic bottleneck of a few gigabit/s, with extra benefits of lower energy consumption and lower loss. For higher speeds, up to terabit/s, the signal must remain photonic all along its path using optical switching and routing. Recently, various all-optical devices based on exciton polaritons have been demonstrated at low temperature ~10 K, with very promising performance in terms of speed and control power [CER 13, BAL 13, NGU 13, STU 14].

Exciton polaritons (or polaritons) in semiconductors are bosonic quasi-particles resulting from the strong interaction between photon and exciton modes [HOP 65, KAV 03]. Due to their composite half-light half-matter nature, polaritons possess a unique combination of physical properties. From the half-light part, they can be easily manipulated by standard optical spectroscopy techniques. They can travel at high speed in semiconductors (~1% speed of light, [FRE 00]), and feature an ultralight mass (~10-4–10-5 the electron mass), favoring transition to quantum-condensed phases at elevated temperatures, e.g. Bose–Einstein condensation [KAS 06]. On the other hand, the half-exciton part brings in Coulombic interaction, spin polarization, and optical nonlinearities that could initiate many physical situations of interest for photonics, e.g. parametric scattering, quantum correlation and entanglement [POR 10].

For practical control and manipulation, polaritons are preferably generated in heterostructures combining both photonic and electronic confinements, the most popular one being a (two-dimensional) 2D photonic microcavity embedding a few 2D quantum wells [KAV 03, WEI 92]. Such a device has been used to generate the first low-dimensional polaritons in 1992 [WEI 92] and to achieve their Bose–Einstein condensation in 2006 [POR 10]. Shortly after, a new “polaritronics” field emerged prospecting for innovative photonic applications based on exciton polaritons [DEV 08, ESP 13]. Indeed, polariton potentialities for all-optical operations have been convincingly demonstrated for ultrafast (on the picosecond time scale) and low power (on the fJ energy scale) spin switch [CER 13], transistor [BAL 13], resonant tunneling diode [NGU 13] and giant phase shift in Mach–Zehnder interferometer [STU 14]. The proof of principle of the polariton transistor by Ballarini et al. is depicted in Figure 1.1 [BAL 13]. The sample used is a GaAs-based planar microcavity embedding three InGaN quantum wells. A monomode laser is tuned to inject (at low level) two types of polaritons in the microcavity: control polaritons with wave vector KC by in-resonance pumping and address polaritons with wave vector KA by slightly off-resonance pumping, as shown in Figure 1.1(b). The low population of address polaritons corresponds to the OFF state of the transistor. By increasing the laser power, one can induce a global polariton blue shift through the polariton-polariton interaction [KAV 03], and bring address polaritons into resonance with the laser. This triggers a sharp increase in their population (Figure 1.1(c)), switching the transistor to the ON state. In this demonstration device, the gain IA/IC, defined as the population ratio of address and control polaritons, is around 15. Furthermore, Ballarini et al. show that their polariton all-optical transistor is fully cascadable, and can be used as a building block for AND/OR logic gates [BAL 13]. With a fast switching time ~10 ps (polariton lifetime) and a control energy as low as ~1 fJ, this work is a very important step toward the development of all-optical networks based on exciton polaritons.

Figure 1.1.Demonstration of all-optical polariton transistor at 10 K. a) Experimental configuration: polaritons are optically injected into the planar Fabry–Perot microcavity using a cw-laser. During their lifetime ~10 ps, they propagate in the microcavity plane over 50–100 μm. The (relative) polariton population can be extracted by measuring its photoluminescence intensity. b) Bottom panel: polariton dispersion (E, Kx) visualized by photoluminescence imaging at low pumping laser power. The horizontal dotted line indicates the pumping laser energy used in this experiment, and the two vertical solid lines the wave vectors KC and KA of control and address polaritons injected into the microcavity, respectively. The laser is in resonance with control polaritons and slightly detuned with address polaritons. Top panel: same data as in bottom panel, but displayed in wave vector space. c) Photoluminescence intensity of address polaritons as a function of the pumping laser power, featuring the ON and OFF states of the polariton transistor. (Reprinted with permission from Ballarini et al. [BAL 13]. Copyright 2013 Nature Publishing Group). For a color version of this figure, see www.iste.co.uk/consonni/nanowires1.zip

Interests of wide band gap semiconductors

All the above “polaritronics” works have been carried out at low temperatures (~10 K), using GaAs-based 2D microcavities and 1D wave guiding structures etched out of planar microcavities. Extension to higher temperatures calls for similar structures made of materials with larger exciton binding energies, such as wide band gap semiconductors. In fact, GaN- and ZnO-based 2D microcavities have been successfully used to realize polariton “lasing” at room temperature [CHR 08, LU 12, LI 13]. However, their spatial homogeneities are still too limited for polariton propagation, a desirable feature for practical applications. In this context, readily available photonic alternatives for all-optical polariton networks are as grown NWs of wide band gap semiconductors.

NWs with wavelength size diameters can sustain ID optical modes, with full confinement of light in the NW cross-section plane and free propagation along the NW length axis. Owing to the large refractive index ~2.5 in GaN and ZnO, standing optical waves called whispering gallery modes (WGMs) can be formed by total internal reflection at the semiconductor–air interface, as sketched in Figure 1.2.

Figure 1.2.Cross-section of a hexagonal nanowire of radius R. The gray line represents the path of a whispering gallery mode confined by total internal reflection at the semiconductor-air interface. For a color version of this figure, see www.iste.co.uk/consonni/nanowires1.zip

The gray line represents the path of such WGM. In a simple plane wave model, energies E of WGMs in a hexagonal cavity of radius R ( optical wavelength) are given by [NOB 04]:

The combination of large exciton oscillator strength and strong confinement of WGMs contributes to Rabi splittings up to 115 meV in GaN [TRI 12], and 200 meV in ZnO NWs [TRI 11], generating a kind of potential trap in k-space, as shown in Figure 1.3(b).

Then, in addition to the quenching of the acoustic phonon interaction mentioned above [SAV 97, BOR 00], longitudinal optical (LO) phonon interaction could also be inhibited if the trap is deep enough. Specifically, when the energy separation between the trap bottom and the exciton states at large kz is larger than the LO phonon energy, scatterings of kz ~ 0 polaritons by LO phonon would only involve final polariton states in the trap. The strength of such scattering events is several orders of magnitude weaker than the usual exciton–LO phonon scatterings, because of the very low polariton density of states (~ 10-4 exciton density of states). Quenching of LO phonon scatterings is clearly evidenced in ZnO NWs, in which the trap depth is ~ ΩRabi/2 ~ 100 meV > LO phonon energy ~ 72 meV [TRI 11].

In Figures 1.3(b) and (c), the polariton full-width-at-half maximum is ~ 4 meV at room temperature, as compared to ~ 40 meV broadening of exciton resonance in bulk ZnO [KLI 07]. This complete phonon quenching is shown to persist up to 550 K [ZHA 12]. For GaN NWs, the situation is less favorable with ΩRabl/2 ~ 57 meV < LO phonon energy ~ 92 meV. Nevertheless, WGMs with negative detunings ~ –20 meV with respect to exciton resonances can yield polariton traps that are deep enough to satisfy the LO phonon quenching condition. The full-width-at-half-maximum of these polaritons slightly increases from 6.5meV at 5 K to 7.5 meV at room temperature, attesting the severe reduction of phonon scatterings [TRI 12]. With GaN and ZnO WGM polaritons, we have a unique situation in the solid state of complete decoupling from the surrounding lattice vibrations as a result of ultrastrong coupling to photons.

In summary, the absence of phonon broadening in GaN and ZnO NW polaritons is an invaluable benefit for the development of all-optical integrated circuits and spintronics operating at an elevated temperature [CER 13, BAL 13, NGU 13, STU 14, DEV 08, ESP 13]. It should also allow studies of the very rich 1D Bose physics [CAZ 11] over an exceptionally wide range of temperatures 4-500 K.

1.3. High efficiency single photon sources

Consider a QD inserted along the z-axis of a cylindrical wire of diameter d (see Figure 1.4).

Radiative recombination of electrons and holes confined in the QD can be modeled by a dipole moment parallel to the QD base plane (or (x,y) plane), as reported in GaN- [BAR 08, AML 12] and GaAs-based QDs [SIL 03]. This dipole can couple to the surrounding electromagnetic field formed by either guided modes in the wire or radiation modes in the free space. Usually, a guided mode is selected to collect the QD spontaneous emission because it offers a better control of out-coupling into free space. Then Γ and γ defined above are the QD recombination rates into this selected guided mode and into all other modes, respectively. For sufficiently small diameters, i.e. d/λ 1 (λ being the mode wavelength in vacuum), photonic wires support a single guided mode, the fundamental hybrid HE11 mode. This mode features strong electric field distribution in the transverse (x, y) plane, fully compatible for coupling to the transverse QD dipole moment. On the other hand, dielectric screening effect in thin wires comes into play, inhibiting coupling of transverse dipole to free space radiation modes [BLE 11, CLA 13], leading to γ/Γ 1, and consequently β ~ 1. According to modeling of GaAs-based photonic wire displayed in Figure 1.4, we would get β > 0.90 over a wide range of wavelengths λd/λ ~ 0.25 ± 0.03. This broad coupling bandwidth of the photonic wire concept is a crucial advantage when considering the inherent dispersion of QD emission wavelength. In the alternative approach based on the Purcell effect in a resonant cavity [MOR 01, HEI 10, GIE 13, GAZ 13], the coupling bandwidth ~ 1/Q < 10-3 is too narrow for practical implementation.

GaAs-based photonic wires have been fabricated using either a top-down [CLA 10, MUN 13] or a bottom-up approach [REI 12]. They include a bottom mirror to redirect backward emitted photons in the forward direction as well as a designed top for out coupling into free space with minimum reflection loss and maximum emission directivity. Out coupling of the guided mode is initially realized with a cone-like top to smoothly increase the mode spatial expansion (“adiabatic” out coupling), which has permitted a record efficiency of 0.72 of single photons collected by standard optics with N.A. ~ 0.75 [CLA 10]. However it was recently shown that similar efficiency can be obtained with a trumpet-like top (Figure 1.5, [MUN 13]), which features additional benefits of Gaussian far-field distribution, of major interest for further manipulation with optical fibers, and weaker sensitivity to geometrical imperfections.

Last but not the least, the purity of the single photon emission is particularly high with photonic wires as assessed by their intensity autocorrelation function g2 (0) < 0.008 [CLA 10] and < 0.02 [MUN 13].

Interests of wide band gap semiconductors

Nitrides are more attractive than oxides because they can emit over a wider range of emission wavelengths, from deep (ultraviolet) UV wavelengths for solar blind transmission in free space to infrared wavelengths for optical fiber transmission. Furthermore, growth and processing technologies of nitrides are far more advanced. In fact, basic building blocks for nitride SPSs are already available, such as QDs embedded in NWs [KIM 13, REN 08, REN 09, SON 11, DES 13, KAK 06, CHO 13a, CHO 13b]. Figures 1.6(a) and (b) show scanning and transmission electron microscopy images of GaN photonic wires fabricated by top-down [KIM 13] and bottom-up techniques [REN 09], respectively. The top-down approach consists of a two-step process: first, formation of nano-obelisks by chemical etching of a thick GaN layer in vapor phase HCl at high temperature; next, MOCVD growth of InGaN/GaN QDs on top of nano-obelisks [KIM 13]. In the bottom-up approach, nitride QDs are directly inserted in NWs using (molecular beam epitaxy) MBE [REN 08, REN 09, SON 11, DES 13] or MOCVD [KAK 06, CHO 13a, CHO 13b].

Optical properties of nitride QDs in NWs are very promising. Figure 1.7 displays photoluminescence results obtained for an ensemble of 1 nm thick GaN/AlN QDs similar to that shown in Figure 1.6(b) [REN 09].

The QD photoluminescence intensity decreases by half between low temperature and room temperature, suggesting an internal quantum efficiency ~ 50% if the radiative recombination process was dominant at low temperature. However, the photoluminescence decay time of these QDs remains nearly constant ~ 300 ps, independent of the temperature. These optical data point to the existence of a thermally activated non-radiative process, which becomes comparable to the radiative process at room temperature. This loss channel could originate from the thin barrier separating the QD from the NW surface (Figure 1.6(b)). Figure 1.8 presents another type of QDs in NWs grown by site-controlled MOCVD [CHO 13b]. These small GaN/AlGaN QDs (height ~ 0.5-1 nm, diameter ~ 10 nm) are inserted in the cone-shaped tops of NWs, surrounded by a thick lateral barrier. They emit at ~4.4 eV, with typical exciton decay time ~ 300 ps at 4 K. Most interestingly, a record binding energy ~ 52 meV is found for the biexciton (Figure 1.8(c)). This is larger than the exciton broadening at room temperature (~ 45 meV), which could ensure reasonably pure single photons with exciton emission at room temperature. Finally, it should be noted that an electrically driven SPS has been demonstrated up to 150 K using InGaN QDs inserted in GaNp-n junction NWs grown by MBE on (111) Si [DES 13]. All these know-hows form a solid foundation for the development of bright SPSs based on nitride materials.

Figure 1.7.GaN/AlN quantum dots in NWs grown by MBE. a) Temperature dependence of photoluminescence spectra of ensemble of quantum dots similar to the one shown in Figure 1.6(b). Inset displays their integrated intensity (circles) and decay time (black squares) as a function of the temperature. Reprinted with permission from [REN 09]. Copyright 2008 American Chemical Society. For a color version of this figure, see www.iste.co.uk/consonni/nanowires1.zip

Figure 1.8.GaN/AlGaN quantum dots in NWs grown by MOCVD. a) Sketch of the quantum dot in nanowire structure. b) High-resolution TEM image of the GaN/AlGaN quantum dot on top of the GaN NW. c) Microphotoluminescence spectra from a single GaN/AlGaN quantum dot as a function of the excitation power measured at ~ 4 K. X and XX denotes radiative recombination lines of exciton and biexciton, respectively. (Reprinted with permission from [LEI 13]. Copyright 2013 American Physical Society). For a color version of this figure, see www.iste.co.uk/consonni/nanowires1.zip

1.4. High efficiency solar photovoltaics

Solar photovoltaics refers to the conversion of sun radiation into electrical power. The potential capacity of this renewable source of energy would largely exceed the global demand. The first generation of solar cells is based on single p-n junctions of bulk Si materials, with an average power conversion efficiency ~ 15%–20%. This is far from the theoretical Shockley–Queisser limit of 34%, mostly due to reflection loss at the front air–Si interface and non-optimum Si band gap. The second generation of solar cells aims at reducing fabrication and materials costs by using thin films of stronger absorbent materials, e.g. CdTe, copper indium gallium selenide (CIGS) or amorphous Si. Nowadays, a third generation of solar cells is emerging with new concepts encompassing nanotechnologies, novel materials (including QDs, NWs and organic materials), and novel concepts (multi-junction (MJ) cells and hot-carrier cells), that could boost efficiency beyond the Shockley–Queisser limit.

Efficiency in single p-n junction solar cells is mostly limited by the non-absorption of photons of energy below the band gap and thermal relaxation of hot carriers generated by the absorption of photons of energy above the band gap. These losses could be minimized in an MJ solar cell, each junction tuned to some specific spectral band of the solar spectrum. In theory, an infinite-junction cell would achieve an efficiency ~87%. The most common approach to the MJ cell structure is to stack subcells by order of decreasing band gaps, with the highest band gap on the top front surface, and connect them together in series by low resistance tunnel junctions. Modeling shows that efficiencies greater than 51% could be achieved with a three-junction solar cell built around InGaAs (0.94 eV), InGaAsP (1.39 eV) and InAlAs (1.93eV) [LEI 13].

Another concept for high efficiency solar cells is to replace conventional thin film devices by NW arrays [KAY 05, LAP 11]. Recent reports on NW-based solar cells strongly support their superior PV performance [KRO 13, WAL 13]. In the following, we examine the various arguments that could justify interest in developing NW solar cells made of wide band gap semiconductors. The reader may also refer to Chapter 10 in Volume 2 for further details on the subject.

1.4.1. Potential photovoltaic benefits of the nanowire geometry

Another benefit of NWs is the possibility of decoupling light absorption from carrier collection. In a planar solar cell structure, light absorption and carrier transport take place along the same material path. For most semiconductor PV materials, the light absorption length (~ 1/α > 1 μm, α being the material absorption coefficient) is usually longer than the minority carrier diffusion length, and a compromise has to be found for the material thickness. However, these two essential PV functionalities can be decoupled in NWs by growing a radial p-n junction: light would be absorbed along the NW length and photogenerated carriers rapidly separated and collected along the radial direction [KAY 05, LAP 11]. Fundamental processes in NW photovoltaics, e.g. carrier collection or surface passivation, can be investigated with single NW devices without being interfered by ensemble averaging or electrical shorting [KRO 13, DON 09, TIA 09, HOL 13], while only NW arrays can provide insights into light scattering and absorption in practical devices [WAL 13]. For a recent review of nanowire photovoltaics, see [LAP 13, GAR 11].

1.4.2. Interests of wide band gap semiconductor photovoltaics

Up until now, the role of ZnO NWs in photovoltaics has been rather limited, usually serving as an anode in dye-sensitized solar cells [LAW 05, PEN 11] (see Chapter 10 by J. Baxter in Volume 2). However, InGaN alloys were developed in the early 2000s as solar cell absorbers. These materials are well suited for MJ cells with a band gap tunability between 0.7 and 3.4 eV, n and p dopability, and large absorption coefficients 105 cm−1. This would ensure complete light absorption in most of the solar radiation spectrum with active thickness ~ 0.5 μm, which is interesting in terms of material costs and ecological footprint.

1.5. Conclusion

GaN and ZnO NWs feature remarkable optical properties deriving from the strong exciton binding energy and large oscillator strength of wide band gap semiconductors and the natural wave guiding effect of the NW geometry, largely exploited in optoelectronics devices (such as lasers, light-emitting diodes (LEDs) and photodetectors as discussed in the related chapters in Vol. 2 of this book). In this chapter, we have discussed three nanophotonics topics, which are less popular, but nevertheless very promising in terms of breaking new ground: (1) all-optical integrated circuits with Bose exciton polaritons, involving 1D photon modes interacting with bulk excitons in the so-called strong coupling regime of the light-matter interaction; (2) high-efficiency single photon sources (SPSs) for quantum information processing, based on single QDs axially embedded in photonic wires; (3) high-efficiency photovoltaics with core-shell InGaN NW arrays. As always, the most challenging task is in the control of NW growth, e.g. growth of photonic wire with trumpet-like top, or NW array with uniform size and radial doping profile.

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