Group-III Nitride Nanowires

An Bao

Department of Materials Science and Metallurgy, University of Cambridge

(Declaration: This review is submitted for the Materials Science and Technology Literature Review Prize 2016. The review is the candidate’s own work and any assistance received has been fully acknowledged.)

Abstract

Group III-nitride nanowires have attracted a lot of research interest in the past decade. They contain both the intrinsic properties of III-nitride materials and some unique properties induced by the nanowire structures. This article reviews the growth methods to obtain III-nitride nanowires, and discusses the pros and cons of both top-down and bottom-up approaches, with detailed discussions on diﬀerent epitaxy methods. The most widely used catalyst-induced epitaxy and extrinsic particle free epitaxy to grow III-nitride nanowires are compared. The properties of those nanowires make them promising candidates for a broad range of applications, including optoelectronic, electronic and electromechanical devices, which are also presented, with a focus on the current challenges and recent progresses.

Keywords: group III-nitride; nanowire; epitaxy; optoelectronics; electronics; electromechanical devices Group III-nitride Nanowires

1 Group III-nitride Nanowires List of ﬁgures

Fig 1. Schematic of a top-down method to obtain nanowires

Fig 2. Schematic of the growth of Si nanowires by VLS

Fig 3. Au-Si phase diagram

Fig 4. The wurtzite crystal structure and the polarity of diﬀerent facets of GaN

Fig 5. The bandgap energies of group III-nitrides and their alloys, with comparisons with the other materials

Fig 6. The schematic of nanowire radial and axial heterostructures

Fig 7. (a) is the schematic of a cavity formed by random spacings between the nanowires. The red arrows illustrate the path of photons. (b) is the schematic of the nanowire heterostructure used for the laser.

Fig 8. The structure of a single nanowire based FET

Fig 9. (a) is a cross-sectional scanning transmission electron microscope image of the radial nanowire heterostructure. (b) shows the HEMT structure to access the 2DEG in the nanowire.

1. Introduction

Semiconductor nanowires have significant potential across a broad range of applications. Among them, III-V nanowires have attracted particular interest for optoelectronic applications in the past decade. Most of these materials have a direct band gap, making them very promising for optoelectronic device uses, such as solar cells [1, 2], light emitting diodes (LEDs) [3, 4], lasers [5, 6] and single photon devices [7, 8]. The nanowires can also be used for electronic applications such as field effect transistors (FETs) [9-11] and single electron transistors [12, 13] etc.

Group-III nitrides are a unique category in the entire III-V family, with direct bandgaps over a broad range of energies, which enables III-nitride-based lighting devices to achieve high eﬃciencies and emission wavelengths that are diﬃcult to achieve using other III-V materials. For example, the light emitting regions of most blue LEDs consist of GaN/ InGaN quantum wells. AlGaN ternary alloys enable ultra violet emission, which can be

2 Group III-nitride Nanowires used for water [14] and air puriﬁcation [15]. Moreover, III-nitride optoelectronic devices can have high performances even with high dislocation densities (1010 cm-2), whereas other III-V materials, such as arsenides and phosphides, require much lower dislocation densities (105 cm-2) to have acceptable eﬃciencies [16, 17].

2. Review of the growth methods

2.1.Top-down methods Growth methods for group III-nitride nanowires can be divided into two categories: top down and bottom up. The top down methods refer to lithography-related synthesis approaches, such as electron beam lithography [18] and optical lithography [19]. In addition, nanowires fabricated by focused ion beam (FIB) micromachining [20], plasma etching [21] and wet etching [22] also belong to the top-down category. The schematic of the concept of top-down is presented in Fig 1. The primary advantages of the top-down methods are position control and uniformity of the as grown nanowires. This is important for device fabrication. It is known that one challenge for nanowire-based applications is to integrate the nanowires into devices. By using the top-down growth methods, the positions of the nanowires can be controlled precisely by patterning, and single nanowire fabrication is also achievable by FIB, which enables easy separation of a single nanowire for device integration and post processing. The top-down methods also provide good uniformity of the nanowires, and this is particularly useful to maintain reasonable reproductivity for nanowire array-based devices. However, the drawbacks of the nanowires fabricated by top-down methods are surface damage and larger sizes than self assembled nanowires. Surface damage of the nanowires is a typical problem for nanowires grown by dry etching methods, such as electron beam lithography, FIB and plasma etching. As the nanowires are achieved by removing redundant materials by particle bombardment, the surfaces of the nanowires may contain amorphous regions and implantation of the source particle is another problem. The sizes of the top-down nanowires are limited by the resolution of the patterning techniques, which is generally larger than 100 nm. However, the quantum conﬁned eﬀects normally emerge only if the

3 Group III-nitride Nanowires nanowires are thin enough, resulting in another impediment for the uses of top-down growth methods.

Fig 1. Schematic of a top-down method to obtain nanowires. The etching here can be achieved by various methods, e.g. inductively coupled plasma (ICP) etch. The remaining mask can be removed via various materials depending on the mask material. The mask is redundant if a directional etching source, e.g. electron beam lithography or FIB, is used.

2.2.Bottom-up methods The bottom up growth methods are referred as self assembly, and the most common method for nanowire growth is epitaxy, which refers to the extended growth of single crystal structures on single crystal substrates, with their lattices aligned. However, some reactions in a furnace can also form group-III nitride nanowires, for example the direct reaction of Ga metal vapour with ammonia [23], carbon nanotube conﬁned reaction of

Ga2O with ammonia [24], and oxide-assisted method by laser ablating a target of GaN mixed with Ga2O3 [25]. This kind of methods has no directional or positional control of the nanowires, although they have the potential for large-scale production with low cost. Epitaxy, by contrast, provides more control on the directions and crystalline structures of the nanowires, which is preferable for device uses.

2.2.1.Epitaxy There are two basic types of epitaxy homoepitaxy and heteroepitaxy. Homoepitaxy means the substrate is the same material as the epitaxial material. By contrast, substrate and the epitaxial material are diﬀerent in the case of heteroepitaxy. Heteroepitaxy is more common for the growth of group III-nitride nanowires, because it is currently very diﬃcult and costly to achieve bulk group III nitride substrates, and the formation of 4 Group III-nitride Nanowires nanowires usually requires the strain induced by the hetero-substrates. The most widely used substrates are silicon carbide (SiC) [26], sapphire (Al2O3) [27, 28], and silicon (Si) [29, 30]. The heteroepitaxy methods can be divided into two categories: catalyst-induced or extrinsic particle free (sometimes called catalyst-free or self-catalytic) methods.

Catalyst-induced epitaxy Vapour-liquid-solid (VLS) growth is the fundamental mechanism for catalyst-induced growth of the nanowires. It was ﬁrst used to grow silicon (Si) nanowires, using gold (Au) as the catalytic particles [31], the schematic of which is presented in Fig 2. At suitable temperatures, the Au particle and supplied Si atoms can form a liquid alloy, as shown in the Au-Si phase diagram (Fig 3). If the Au-Si system is in a hypereutectic condition, namely the ratio of Si is higher than the eutectic ratio, then Si will start to precipitate. In this way, if Si is continuously supplied to the environment, the Au-Si system can stay steadily at the hypereutectic condition, i.e. Si remains supersaturated, while the Si atoms can keep precipitating to form nanowires. The VLS concept has been widely used for the growth of III-V nanowires, although the mechanisms are more complex than the growth of Si nanowires because a ternary rather than a binary system is formed at the growth interface.

Fig 2. Schematic of the growth of Si nanowires by VLS. a is the initial condition and b is that during the growth the metal droplet stays on the tip. Figure from [31] (reuse permitted).

5 Group III-nitride Nanowires

Fig 3. Au-Si phase diagram, re-plotted with data from [32] (reuse permitted).

In the case of group III-nitride nanowire, the catalysts are not limited to be Au [33], but can also be iron (Fe) [34], nickel (Ni) [35], and lanthanum (La) [36]. Unlike the growth of other III-V nanowires, which requires a III-rich condition during the epitaxy process, a N-rich condition is necessary for the growth of group III-nitride nanowires. The requirement of the N-rich condition for the catalyst-induced growth methods can be explained as follows. The growth rate is determined by available group III atoms under N- rich conditions [37], and group III atoms accumulate in the extrinsic metal seeds, so more group III atoms are below the seeds than on the substrate. Therefore, nanowires form below the seeds rather than form ﬁlms on the substrate.

The state of the catalytic particles during the growth process, however, remains controversial. Although the mechanism has been explained as a vapour-liquid-solid (VLS) system catalyst in the liquid form, some studies [38, 39] argued that the catalytic particle can stay in the solid state during the growth and therefore form a vapour-solid-solid (VSS) system. The state of the catalytic particles can be observed by in-situ transmission electron microscopy (TEM). The evidences suggest that the catalytic particles can probably be either solid or liquid depending on the growth conditions, and their catalytic function remains regardless of the state of the particles. This means the VLS concept may not be suﬃcient to explain the growth of catalyst-induced nanowire growth. Some kinetic

6 Group III-nitride Nanowires models illustrate that the particle-nanowire interface is kinetically preferable for the material to precipitate, which can to some extent explain the VSS mechanism [40].

The catalyst-induced growth methods provide good control of the nanowire diameter by tuning the size of the catalytic particles. Methods that can achieve this are, for example, in-situ annealing of a thin metal film, or depositing the metal seeds using well-defined masks. The position of the nanowires can also be controlled by positioning the catalytic particles using various patterning methods. However, the particles remain on the top of nanowires after growth. Also, material from the catalytic seed can be incorporated into the nanowire body leading to nanowire tapering during the growth [41]. As the particles are normally foreign materials compared with the nanowires, they may adversely affect the nanowire properties. In addition, the catalytic particles may induce stacking faults during the growth, due to the strain between the particle and the nanowire growth interface [42]. The stacking faults in nanowires can cause unintentional emission peaks for optoelectronic devices and also broaden the emission linewidths. This drives the investigation of catalyst-free nanowire growth.

Extrinsic particle free epitaxy The extrinsic particle free methods to grow nanowires can avoid the use of foreign catalytic particles. These are sometimes called catalyst-free or self-catalytic methods, although whether the growth process involves a catalysis process or only kinetics is still to some extent controversial. The extrinsic particle free heteroepitaxial method for the growth of group-III nitride nanowires is slightly diﬀerent from the other III-V materials, although there are some similarities between the two groups. As the growth of other III-V nanowires are better understood than the growth of III-nitride nanowires, it is helpful to explain it before moving to the extrinsic particle free methods for III-nitride nanowires. For the other III-V nanowires such as arsenides and phosphides, the extrinsic particle free growth methods involves Ga-assisted growth [43] and selective area epitaxy [44, 45]. The Ga-assisted growth has a similar mechanism as catalyst-induced growth mentioned above, providing good quality GaAs [46-49] nanowires. The process is to form Ga droplets on suitable substrates, such as GaAs (100) [43] and Si (111) [50], prior to the growth of nanowires. If a Ga-rich condition is then maintained at suitable temperatures, 7 Group III-nitride Nanowires the VLS or VSS mechanism will be initialised and nanowires start to grow. Likewise, InAs and InP nanowires can be formed by In-assisted growth methods, using In droplets to initialise the nanowire growth [51]. Although the Ga or In droplets usually remain on the top of the nanowires after growth, they have less eﬀects on the nanowire properties compared with the other foreign metal droplets used in catalyst-induced growth methods.

Selective area epitaxy, by contrast, can entirely eliminate the use of any metal droplets, providing nanowires with clean tips. This technique has actually existed for a long time, while not for the growth of nanowires, but to deposit thin films on selected areas on a substrate [52]. The selected areas are made by patterning openings on a sacrificial layer on the substrate, usually SiO2 on III-V [44, 45, 53] or Si substrates [54]. This method involves some lithography techniques to define the openings, so it is to some extent a combination of top-down and bottom-up. A group III-rich growth condition is again needed, similar to Ga-assisted growth. The formation of nanowires does not involve a VLS mechanism, but is probably dominated by the formation of different facets with different growth rates under certain growth conditions [53]. For instance, the growth along (111)B direction is faster than <110> directions by lowering arsine partial pressure for the growth of GaAs nanowires, leading to the formation of nanowires rather than 3D islands [55].

The extrinsic particle free growth for group III-nitride nanowires involves direct epitaxy [42] or selective area epitaxy [26]. Similar to the catalyst-induced growth for group III- nitride nanowires, a N-rich condition is also required for the extrinsic particle free methods, although the mechanisms are very diﬀerent and remain controversial. Direct epitaxy means no catalyst is involved and the nanowires are obtained only by certain growth conditions with suitable substrates using molecular beam epitaxy (MBE) or metalorganic chemical vapour phase deposition (MOCVD). Selective area epitaxy involves a patterning step prior to the nanowire growth, similar to the methods used to grow other III-V nanowires. However, the Ga-assisted growth method used for other III-V materials is not likely to be able to form group III-nitride nanowires. The reason for this is that if Ga droplets are used, it is diﬃcult to achieve a N-rich condition at the growth interface.

8 Group III-nitride Nanowires

The formation of GaN nanowires occurs probably because of the constrained diffusivity of Ga atoms on the substrate under a N-rich condition [56], preventing the coalescence of the nanowires during the growth process. This is consistent with the fact that under a reduced V/III ratio, the growth tends towards the formation of films with more pronounced coalescence. Another effect relevant to the formation of nanowires is that the growth rate along [0001] axis, which is called axial growth rate for hexagonal nanowires, is higher than the nanowire radial growth, due to a longer diffusion length on the m- planes [57], or a higher sticking coefficient on the c-plane [58], under certain growth conditions. The substrate used also has effects on the formation of nanowires. The lattice mismatch between the substrate and the nanowire materials could possibly initialise a Volmer–Weber mechanism [59] at the beginning of the growth process [57], promoting the formation of nanowires rather than films. The most common substrates for extrinsic particle free GaN and InN nanowires are sapphire (Al2O3)(0001) [60-62] and Si (111) [63-66]. It has been found from literature that if sapphire is used as the substrate, a buffer layer, usually AlN or SiNx, is needed prior to the growth of nanowires, otherwise the nanowires cannot be formed. The buffer layer is supposed to help initialise the nanowire nucleation step at the beginning of the growth process. By contrast, the presence of a buffer layer does not affect the formation of nanowires if Si substrates are used.

Epitaxy techniques The most widely used epitaxy techniques for the growth of nanowires are molecular beam epitaxy (MBE) and metalorganic chemical vapour phase deposition (MOCVD). The ﬁrst two reports about the growth of GaN nanowires by MBE were given by Sánchez-García et al. [67] and Yoshizawa et al. [68] in 1998, and the growth by MOCVD was achieved more recently in 2004 by Kim et al. [69]. Some general features of MBE and MOCVD may aﬀect the structure and properties of the as-grown nanowires, and they both have some advantages and disadvantages, which are discussed as follows.

There are some similarities and diﬀerences between MBE and MOCVD. To be simpliﬁed, both of them achieve epitaxy by controlling the temperature and V/III ratio under high

9 Group III-nitride Nanowires vacuum. Pure elements are used as sources for MBE growth, for example Ga atoms, In atoms, and N plasma for the growth of GaN and InN nanowires. However, metalorganic molecules are sources (called precursors) for group III elements in MOCVD, such as

Ga(CH3)3 (TMGa) for Ga, In(CH3)3 (TMIn) for In, and NH3 is the source of N. Some growers use ethyl rather than methyl precursors. A carrier gas, such as H2, N2 or sometimes their mixture, is required for MOCVD growth to bring the metalorganic molecules to the growth chamber. Consequently, the reactions happening in MOCVD are more complex than those in MBE. In order to break the bonds in the molecules to create reactive elements for epitaxy, a higher growth temperature is generally needed in MOCVD than in MBE. If the growth temperature is not high enough in MOCVD, the number of reactive elements may saturate, posing a limit for the growth rate even with increased gas ﬂows. An advantage of MOCVD over MBE is the much higher growth rate, which is industry-preferred, although a slower growth rate in MBE can provide more control during the growth process. As the reactions in MBE are simpler, it needs an ultra high vacuum (UHV) condition, generally 10-8 Pa, while MOCVD only needs a moderate pressure. Due to this, the impurity level in MOCVD is higher than in MBE. For instance, unintentional doping from oxygen is a common problem for the epitaxial growth of group III-nitride materials by MOCVD [70], leading to unintentionally n-type materials. In addition, the diﬀusion length of adatoms in MBE is generally longer than that in MOCVD due to UHV, making kinetics more considerable when interpreting the growth mechanisms in MBE. It also makes MBE perhaps more preferable for the extrinsic particle free growth of nitride nanowires, as current evidence discussed previously suggests that the growth is more likely to be kinetically driven.

3. Properties of group III-nitride nanowires

The group III-nitride nanowires contain both the intrinsic properties of III-nitride materials and some unique properties induced by the nanowire geometry. This section starts with a brief introduction of the crystal structures and bandgaps of group III-nitride materials, followed by discussions of the properties induced by the nanowire structures.

10 Group III-nitride Nanowires

Those properties can be used in various applications which are discussed in the next section.

The crystal structures of group III-nitrides can be either wurtzite or zincblende. Wurtzite is a hexagonal structure and it is the most stable phase for the group III-nitrides, while zincblende is a cubic structure and is meta-stable. A schematic of the wurtzite crystal structure of GaN is shown in Fig 4. At room temperature, the widely accepted lattice parameters for GaN are a=3.186Å and c=5.185Å [71]. From the ﬁgure, the +c-plane (0001) is Ga-polar and the -c-plane (0001) is N-polar.

Fig 4. (a) is the wurtzite crystal structure of GaN, where the [0001] direction is labeled. (b) shows the polar c-plane, the non polar a- and m-planes and the semi-polar (11-22) plane. Figures from [70] (reuse permitted).

The direct bandgaps of group III nitrides allow the electrons and holes to recombine without phonons in an ideal lattice. For optoelectronic devices, the emission wavelength is correlated to the bandgap of the materials, which can be tuned by using diﬀerent compositions, according to Vegard’s Law [72]. For group III-nitrides and their alloys, the emission wavelengths can vary from ultraviolet (UV) to infrared. Fig 5 shows a comparison of the ranges of emission wavelength in terms of III-nitrides and some other light emitting materials.

11 Group III-nitride Nanowires

The bowing factor needs to be considered when calculating the bandgaps of group III- nitride ternary alloys. For example, the bandgap of the InGaN ternary alloy can be determined by the following equation:

where b is referred as the bowing parameter. Theoretical calculation suggested that b is approximately 1eV for InxGa1-xN [73]. However, the calculation assumed that the material is unstrained, while InGaN is usually grown as a heteroepitaxial layer, in which case strain normally exists. Some authors have argued that the strain and defects in InGaN aﬀect the value of its bowing parameter [74].

Fig 5. (a) shows the bandgap energies of group III-nitrides and their alloys, with corresponding colours in the rainbow. (b) shows some other light emitting materials (solid circles denote direct- bandgap materials, and open circles denote indirect-bandgap materials; solid lines denote direct- bandgap alloys, and dashed lines denote indirect-bandgap alloys). Figures from [75] (reuse permitted).

The geometry of nanowires adds extra promising properties to the nitride materials. For instance, the nanowires grown by heteroepitaxy could eliminate the strain induced by the lattice mismatched substrate [42]. Currently, a lattice matched substrate to grow GaN is very diﬃcult to achieve, and the free standing bulk GaN substrates with lattice parameters similar to the upper layers and with much lower dislocation densities are very

12 Group III-nitride Nanowires costly. The strain within the epitaxial layers increases the number of dislocations in the epitaxial layer, which can act as non-radiative recombination centres [76] and impede the eﬃciencies of optoelectronic devices. Nevertheless, some studies have illustrated that epitaxial GaN nanowire arrays are almost free of dislocations, which can theoretically reduce the non-radiative recombination rate in nanowire-based optoelectronic devices [69]. However, the large surface-to-volume ratio of nanowires introduce more pronounced surface states for non-radiative recombinations, which is not an issue for planar structures. The surface states can be reduced by various surface passivation processes [77]. Additionally, the presence of stacking faults in group III-nitride nanowires, grown by either catalyst-induced or extrinsic particle free methods, remains a problem [28, 78]. The stacking faults are generally basal plane stacking faults for nanowires with the growth direction along [0001]. They adversely aﬀect the mechanical properties of the nanowires, hindering the use in electromechanical applications [79].

Nowadays, most GaN/InGaN quantum well based devices are grown on c-plane (0001) sapphires. The polarised material interface and piezoelectric field in the strained epitaxial layer raise an electric field within the QWs, which is called quantum confined Stark effect (QCSE) [80]. Due to the electric field, the allowed electron states in conduction band shift to lower energies and the allowed hole states in valence band shift to higher states, resulting in lower energies of the emitted photon. The QCSE also shifts the electrons and holes to opposite directions, which decreases the overlap of the electron and hole wavefunctions. This reduces the carrier recombination rate of the system [81], impeding device performances. QCSE can be eliminated by growing the quantum wells on nonpolar facets, such as m-planes and a-planes (see Fig 4). Although this can be achieved by using non-polar substrates [82], it can also be achieved by radial heterostructures (i.e. core-shell structures), as shown in Fig 6 (a), on GaN nanowires grown on normal c-plane substrates [3]. This provides the opportunity to build non-polar nitride based optoelectronic devices using nanowires.

13 Group III-nitride Nanowires

Fig 6. (a) is the schematic of a nanowire radial heterostructure (core-shell structure), and (b) is the schematic of an axial nanowire heterostructure.

In addition, axially heterostructured nanowires, presented in Fig 6 (b), also provide extra freedom for the use of the materials. For example, the p-n junction based on the axial heterostructure of a single GaN nanowire [83] can be used as the fundamental component in GaN-based electrical devices. The electron mobility in nanowires increases signiﬁcantly due to reduced density of scattering centres compared to bulk materials [84]. This combines the advantages of both the material itself and the small size of nanowires, enabling the use of group III-nitride nanowires in nano electronic devices. In addition, embedded quantum wells along the nanowires are another type of axial heterostructures. Due to the quantum conﬁnement induced by the small diameter of the nanowires, the embedded axial quantum wells could have quantum dot properties [85], providing extra freedom for device applications, which is discussed later.

The nanowire structures of group III-nitride materials can also improve the doping quality, for p-type in particular [86-88], although the reason for this is still unclear. For instance, the measured free hole concentration in Mg-doped AlN nanowires has reached 1016 cm -3 [88], whereas the value is around 1012 cm -3 in AlN epilayers [89]. n-type doping usually uses Si for group III-nitride materials, which is a shallow donor. However, p-type doping of GaN is historically diﬃcult to achieve, because the ionisation energies of acceptor dopants are relatively high in GaN [90, 91]. If the energy level of acceptors is deep within the bandgap, then a relatively high energy, or a high temperature, is necessary to activate the dopants. The most successful p-type dopant for GaN so far is Mg, but the conductivity and carrier density of p-type GaN is still low compared to n-type. This is one of the reasons that lead to the eﬃciency droop problem in group-III nitride

14 Group III-nitride Nanowires based lighting devices, which refers to the phenomenon of the reduction in LED efficiency at high current density injection [92]. The difficulty of p-type doping also increases the device resistance for group III-nitride based electronic applications, such as field effect transistors (FETs). Nevertheless, nanowires could possibly provide a solution for the p- type doping problem.

4. Review of Applications

The applications of group III-nitride nanowires can be divided into three categories: optoelectronic applications [93], electronic applications [9, 94-96] and electromechanical applications [18].

4.1.Optoelectronic applications The optoelectronic properties of group III-nitride materials make them promising for use in lighting devices such as LEDs and lasers. For example, InGaN/GaN quantum well based structures are widely used as the active region in LEDs and lasers. The group III nitride nanowire structures have also been investigated in LED [69, 97, 98] and laser [99-105] devices more recently.

One of the motivations of studying nanowire-based LEDs is to obtain a high light extraction eﬃciency compared to the conventional planar quantum well based lighting devices. Also, the feature of density of states in a 1D nanowire system can provide a better control of the colour purity than a 2D quantum well-based system. However, the large surface-to-volume ratio of the nanowires could exacerbate nonradiative surface recombination [106, 107], because of the Shockley–Read–Hall (SRH) process associated with the presence of surface states. Consequently, the carrier injection eﬃciency is further hindered by the nonradiative surface recombination [108, 109]. Therefore, the light output power of nanowire-based LEDs is generally lower than for planar quantum well based LEDs. Moreover, the nonradiative surface recombination can result in temperature increases, degrading the device performance. Nowadays, the best reported planar quantum well based blue LEDs can already achieve satisfactory internal quantum

15 Group III-nitride Nanowires eﬃciencies (IQE). Those factors mentioned above pose a bottleneck for the nanowire- based LEDs to compete with the planar ones especially for blue emission [110].

However, the nanowire structures of the group III nitride materials have shown some promising properties in terms of ultraviolet and infrared emissions. It is still difficult to obtain high efficiency ultraviolet emission using planar AlGaN/GaN or AlGaN/AlN quantum well structures. One reason for this is the difficulty to achieve good quality p- type doping for group-III nitride materials, as discussed above, leading to poor hole injection and high resistances. The p-type doping of AlN or AlGaN is more difficult than InGaN, due to an even higher activation energy of Mg in those materials [111], resulting in the difficulties to achieve high-performance ultraviolet LEDs. The AlN nanowires, however, have shown an enhanced Mg incorporation as compared with planar AlN based LEDs [89, 112], possibly due to the reduced formation energy for Al-substitutional Mg [87, 88]. As for infrared emission, although with high indium content the emission of InGaN can reach red and infrared wavelengths theoretically, as seen in Fig 5, the material quality is generally poor with high In content. Moreover, electron accumulation at the surface of InGaN layers is more pronounced with high In content, leading to band bending and hindering the device performances [113, 114]. However, the In-rich InGaN nanowire structures have been reported to have enhanced In incorporation with an emission wavelength at 1.46 µm [115].

Intensive studies have been made on group III-nitride based solid-state electrically pumped lasers, since first GaN-based blue laser diode was invented in 1996 [116, 117]. The planar quantum well-based laser diodes show promising performances within the visible spectrum [118-121]. However, it is still challenging to obtain high efficiency ultraviolet lasers, where AlGaN [122] or AlN [123] are normally used. The main difficulties for that are firstly the inefficient p-type doping and secondly the high dislocation densities when using AlGaN or AlN, resulting in significantly increased threshold currents for ultraviolet emitting lasers [93]. Nanowire structures show some advantages in this regard. As discussed previously, the Mg incorporation can be enhanced in nanowire structures, and it is possible to grow nearly defect free nanowires to reduce

16 Group III-nitride Nanowires the required threshold current [88]. For example, AlGaN nanowire-based lasers have shown a much lower threshold in the ultraviolet spectrum [124, 125] compared to the AlGaN quantum well based structures [122]. The laser cavity can be formed by the random spacings between the nanowires, as illustrated in Fig 7.

The group III nitride nanowires were also investigated for single photon sources [126-128], which are a fundamental requirement for quantum cryptography and can also be used in quantum computing. With the availability of high-speed blue single photon detectors and less attenuation in this spectral region for free space transmission, the III- nitrides have potential advantages over group III-arsenides and -phosphides. As the single photons are emitted by quantum dot structures, a dot or disk is often embedded in a single nanowire, forming axial heterostructures as in Fig 6(b). Although the quantum dots can also be grown within 2D layers, on the top of islands or mesas etc., the nanowire structure could provide an easy separation of the dots, providing access to a single dot or disk, which is preferable for device applications.

Apart from light sources, the group III nitride nanowires can also be used in solar cells [129-132] and photodetectors [133-139]. The nanowire structure can introduce light

17 Group III-nitride Nanowires trapping eﬀects, enhancing the light absorption [93]. Additionally, electron-hole pair generation and charge carrier separation can also be enhanced by the nanowire structure due to the large surface-to-volume ratio [131, 140]. Their wide bandgaps make it possible to achieve photovoltaics in the ultraviolet wavelength range with much less energy loss compared with other materials used in similar spectral range [141, 142]. However, it is still challenging to make high-eﬃciency red or near-infrared photovoltaics using InGaN with high In content [143-145]. This is probably because of the In segregation problem and poor material quality, which remains to be one of the main issues of InGaN materials and devices [146].

4.2.Electronic applications Group III nitride materials are also attractive candidates for high power and high temperature electronic applications, due to their wide bandgaps, high break-down fields and good transport properties [95]. Nanowire structures are promising for assembling small-size electronic components for nano-devices, such as single nanowire-based p-n junctions [27], Schottky diodes [147] and field effect transistors (FETs) (see Fig 8) [9]. The electron mobility could be further enhanced by the 1D confinement effects. It has been demonstrated that the electron mobilities of GaN nanowire-based FETs are comparable to or larger than thin film materials with similar carrier concentration [9].

Fig 8. The structure of a single nanowire based FET. Figure from [9] (reuse permitted).

However, good quality doping is required for those electronic components, which remains a challenge at nanoscale, for p-type doping in particular [148, 149]. Although nanowires can to some extent improve the quality of p-type doping [86-88], an alternative solution to by-pass the doping problem is to use undoped heterostructures to form 2D electron gas by band engineering, which can be applied in high electron mobility transistors

18 Group III-nitride Nanowires

(HEMTs). For example, Li et al. reported the presence of conﬁned electron gas with a high electron mobility in a GaN/AlN/AlGaN radial nanowire heterostructure [10], the structure of which is presented in Fig 9.

Fig 9. (a) is a cross-sectional scanning transmission electron microscope image of the radial nanowire heterostructure. (b) shows the HEMT structure to access the 2DEG in the nanowire. Figures from [10] (reuse permitted).

4.3.Electromechanical applications Group III-nitride nanowires have attracted increasing interest in the area of optoelectronics and electronics, as discussed above. However, their mechanical properties have not been explored in detail, whereas the rigidity, thermal stability, thermal conductivity and piezoelectric properties of the materials make them interesting candidates for nanoscale electromechanical systems, such as resonators, oscillators, strain sensors and energy harvesters [18, 79, 150-152].

The challenge for using group III-nitride nanowires in the electromechanical applications is the size eﬀect on the value of Young’s modulus. The dependence of Young’s modulus on the size of the nanowires remains controversial. Theoretically Young’s modulus is expected to increase with reduced diameters of the nanowires, which applies to ZnO nanowires [153]. For GaN nanowires, by contrast, a signiﬁcantly lower Young’s modulus was found for thinner nanowires [154], which means the diameter of the nanowires have

19 Group III-nitride Nanowires to exceed a certain value to have comparable Young’s modulus as bulk GaN [155], posing extra requirements on the growth and limiting the use of ultra thin nanowires.

Another challenge is to maintain the high mechanical quality factors (Q) at the nanoscale. The Q factor is deﬁned as the ratio of the resonance frequency to the resonance full width at half maximum power. A high Q indicates small energy loss for resonators, oscillators and piezoelectric energy harvesters. However, the Q factors decrease with decreasing nanowire diameters [156, 157]. GaN can provide a material solution for this, as GaN nanowires have demonstrated a much higher Q factor than the widely used Si nanowires at similar sizes [79, 154], although improvement is still required for real device uses.

5. Conclusions

Group III nitride nanowires have been extensively studied in optoelectronic, electronic and electromechanical systems. The advantages of their nanowire structures include both the intrinsic material properties, and extra unique properties induced by the nanowire geometry, making them promising candidates for the broad applications mentioned above. Nowadays, MBE and MOCVD are the main growth techniques to obtain nanowires, and extrinsic particle free epitaxy is usually preferable to catalyst-induced epitaxy, as it can avoid the adverse effects from the foreign catalytic metal particles. However, some top-down growth methods also exist, with their own advantages for specific device uses. In general, group III-nitride nanowires remain a relatively new field as compared with other thoroughly studied nanowires such as III-arsenides, ZnO and Si. Therefore, more effort is still needed to expand the fundamental knowledge of the materials as well as to explore their potential applications. For example, solving the problem induced by nanowire surface states, further improvement of p-type doping in III- nitride nanowires, and obtaining high quality nanowires grown by extrinsic particle free approaches, are the next research focus.

20 Group III-nitride Nanowires Acknowledgement

The author would like to thank RA Oliver in Department of Materials Science and Metallurgy, University of Cambridge, for useful discussions.

References

[1] M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, "Nanowire dye- sensitized solar cells," Nature Materials, vol. 4, pp. 455-459, 2005.

[2] R. LaPierre, "Numerical model of current-voltage characteristics and eﬃciency of GaAs nanowire solar cells," Journal of Applied Physics, vol. 109, p. 034311, 2011.

[3] F. Qian, S. Gradecak, Y. Li, C.-Y. Wen, and C. M. Lieber, "Core/multishell nanowire heterostructures as multicolor, high-eﬃciency light-emitting diodes," Nano Letters, vol. 5, pp. 2287-2291, 2005.

[4] E. D. Minot, F. Kelkensberg, M. Van Kouwen, J. A. Van Dam, L. P. Kouwenhoven, V. Zwiller, et al., "Single quantum dot nanowire LEDs," Nano Letters, vol. 7, pp. 367-371, 2007.

[5] A. Chin, S. Vaddiraju, A. Maslov, C. Ning, M. Sunkara, and M. Meyyappan, "Near- infrared semiconductor subwavelength-wire lasers," Applied Physics Letters, vol. 88, p. 163115, 2006.

[6] B. Hua, J. Motohisa, Y. Kobayashi, S. Hara, and T. Fukui, "Single GaAs/GaAsP coaxial core−shell nanowire lasers," Nano Letters, vol. 9, pp. 112-116, 2008.

[7] M. T. Borgström, V. Zwiller, E. Müller, and A. Imamoglu, "Optically bright quantum dots in single nanowires," Nano Letters, vol. 5, pp. 1439-1443, 2005.

21 Group III-nitride Nanowires

[8] I. Friedler, C. Sauvan, J.-P. Hugonin, P. Lalanne, J. Claudon, and J.-M. Gérard, "Solid-state single photon sources: the nanowire antenna," Optics Express, vol. 17, pp. 2095-2110, 2009.

[9] Y. Huang, X. Duan, Y. Cui, and C. M. Lieber, "Gallium nitride nanowire nanodevices," Nano Letters, vol. 2, pp. 101-104, 2002.

[10] Y. Li, J. Xiang, F. Qian, S. Gradecak, Y. Wu, H. Yan, et al., "Dopant-free GaN/AlN/ AlGaN radial nanowire heterostructures as high electron mobility transistors," Nano Letters, vol. 6, pp. 1468-1473, 2006.

[11] A. Motayed, M. Vaudin, A. V. Davydov, J. Melngailis, M. He, and S. Mohammad, "Diameter dependent transport properties of gallium nitride nanowire ﬁeld eﬀect transistors," Applied Physics Letters, vol. 90, p. 043104, 2007.

[12] X. Duan, Y. Huang, Y. Cui, J. Wang, and C. M. Lieber, "Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices," Nature, vol. 409, pp. 66-69, 2001.

[13] C. Thelander, T. Mårtensson, M. Björk, B. Ohlsson, M. Larsson, L. Wallenberg, et al., "Single-electron transistors in heterostructure nanowires," Applied Physics Letters, vol. 83, pp. 2052-2054, 2003.

[14] M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis, B. J. Marinas, and A. M. Mayes, "Science and technology for water puriﬁcation in the coming decades," Nature, vol. 452, pp. 301-310, 2008.

[15] J. Zhao and X. Yang, "Photocatalytic oxidation for indoor air puriﬁcation: a literature review," Building and Environment, vol. 38, pp. 645-654, 2003.

[16] J. Elsner, R. Jones, P. Sitch, V. Porezag, M. Elstner, T. Frauenheim, et al., "Theory of threading edge and screw dislocations in GaN," Physical review letters, vol. 79, p. 3672, 1997.

22 Group III-nitride Nanowires

[17] F. Ponce and D. Bour, "Nitride-based semiconductors for blue and green light- emitting devices," Nature, 1997.

[18] C. B. Maliakkal, J. P. Mathew, N. Hatui, A. A. Rahman, M. M. Deshmukh, and A. Bhattacharya, "Fabrication and characterization of GaN nanowire doubly clamped resonators," Journal of Applied Physics, vol. 118, p. 114301, 2015.

[19] G. Liu, B. Wen, T. Xie, A. Castillo, J.-Y. Ha, N. Sullivan, et al., "Top–down fabrication of horizontally-aligned gallium nitride nanowire arrays for sensor development," Microelectronic Engineering, vol. 142, pp. 58-63, 2015.

[20] S. Dhara, C. Lu, C. Wu, C. Hsu, W. Tu, K. Chen, et al., "Focused ion beam induced nanojunction and defect doping as a building block for nanoscale electronics in gan nanowires," The Journal of Physical Chemistry C, vol. 114, pp. 15260-15265, 2010.

[21] R. Debnath, J.-Y. Ha, B. Wen, D. Paramanik, A. Motayed, M. R. King, et al., "Top- down fabrication of large-area GaN micro-and nanopillars," Journal of Vacuum Science & Technology B, vol. 32, p. 021204, 2014.

[22] G. T. Wang and Q. Li, "Method of fabricating vertically aligned group III-V nanowires," ed: Google Patents, 2014.

[23] M. He, I. Minus, P. Zhou, S. N. Mohammed, J. B. Halpern, R. Jacobs, et al., "Growth of large-scale GaN nanowires and tubes by direct reaction of Ga with NH3," Applied Physics Letters, vol. 77, pp. 3731-3733, 2000.

[24] W. Han, S. Fan, Q. Li, and Y. Hu, "Synthesis of gallium nitride nanorods through a carbon nanotube-conﬁned reaction," Science, vol. 277, pp. 1287-1289, 1997.

[25] W. Shi, Y. Zheng, N. Wang, C. Lee, and S. Lee, "Microstructures of gallium nitride nanowires synthesized by oxide-assisted method," Chemical Physics Letters, vol. 345, pp. 377-380, 2001.

[26] S. D. Hersee, X. Sun, and X. Wang, "The controlled growth of GaN nanowires," Nano letters, vol. 6, pp. 1808-1811, 2006.

23 Group III-nitride Nanowires

[27] Z. Zhong, F. Qian, D. Wang, and C. M. Lieber, "Synthesis of p-type gallium nitride nanowires for electronic and photonic nanodevices," Nano Letters, vol. 3, pp. 343-346, 2003.

[28] G. T. Wang, A. A. Talin, D. J. Werder, J. R. Creighton, E. Lai, R. J. Anderson, et al., "Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal–organic chemical vapour deposition," Nanotechnology, vol. 17, p. 5773, 2006.

[29] C.-C. Chen, C.-C. Yeh, C.-H. Chen, M.-Y. Yu, H.-L. Liu, J.-J. Wu, et al., "Catalytic growth and characterization of gallium nitride nanowires," Journal of the American Chemical Society, vol. 123, pp. 2791-2798, 2001.

[30] J. Liu, X.-M. Meng, Y. Jiang, C.-S. Lee, I. Bello, and S.-T. Lee, "Gallium nitride nanowires doped with silicon," Applied Physics Letters, vol. 83, pp. 4241-4243, 2003.

[31] R. Wagner and W. Ellis, "Vapor-liquid-solid mechanism of single crystal growth," Applied Physics Letters, pp. 89-90, 1964.

[32] H. Okamoto and T. Massalski, "The Au− Si (Gold-Silicon) system," Bulletin of Alloy Phase Diagrams, vol. 4, pp. 190-198, 1983.

[33] W.-C. Hou, L.-Y. Chen, W.-C. Tang, and F. C. Hong, "Control of seed detachment in Au-assisted GaN nanowire growths," Crystal Growth & Design, vol. 11, pp. 990-994, 2011.

[34] X. Duan and C. M. Lieber, "Laser-assisted catalytic growth of single crystal GaN nanowires," Journal of the American Chemical Society, vol. 122, pp. 188-189, 2000.

[35] C. Cheze, L. Geelhaar, A. Trampert, O. Brandt, and H. Riechert, "Collector phase transitions during vapor− solid− solid nucleation of GaN nanowires," Nano letters, vol. 10, pp. 3426-3431, 2010.

[36] J. Chen and C. Xue, "Catalytic growth of large-scale GaN nanowires," Journal of materials engineering and performance, vol. 19, pp. 1054-1057, 2010.

24 Group III-nitride Nanowires

[37] L. Geelhaar, C. Cheze, W. Weber, R. Averbeck, H. Riechert, T. Kehagias, et al., "Axial and radial growth of Ni-induced GaN nanowires," Applied Physics Letters, vol. 91, p. 093113, 2007.

[38] A. I. Persson, M. W. Larsson, S. Stenström, B. J. Ohlsson, L. Samuelson, and L. R. Wallenberg, "Solid-phase diﬀusion mechanism for GaAs nanowire growth," Nature materials, vol. 3, pp. 677-681, 2004.

[39] S. Kodambaka, J. Tersoﬀ, M. Reuter, and F. Ross, "Germanium nanowire growth below the eutectic temperature," Science, vol. 316, pp. 729-732, 2007.

[40] K. W. Kolasinski, "Catalytic growth of nanowires: vapor–liquid–solid, vapor–solid– solid, solution–liquid–solid and solid–liquid–solid growth," Current Opinion in Solid State and Materials Science, vol. 10, pp. 182-191, 2006.

[41] L. Lari, T. Walther, M. Gass, L. Geelhaar, C. Chèze, H. Riechert, et al., "Direct observation by transmission electron microscopy of the inﬂuence of Ni catalyst-seeds on the growth of GaN–AlGaN axial heterostructure nanowires," Journal of Crystal Growth, vol. 327, pp. 27-34, 2011.

[42] C. Chèze, L. Geelhaar, O. Brandt, W. M. Weber, H. Riechert, S. Münch, et al., "Direct comparison of catalyst-free and catalyst-induced GaN nanowires," Nano Research, vol. 3, pp. 528-536, 2010.

[43] S. Ambrosini, M. Fanetti, V. Grillo, A. Franciosi, and S. Rubini, "Self-catalyzed GaAs nanowire growth on Si-treated GaAs (100) substrates," Journal of Applied Physics, vol. 109, p. 094306, 2011.

[44] P. Mohan, J. Motohisa, and T. Fukui, "Controlled growth of highly uniform, axial/ radial direction-deﬁned, individually addressable InP nanowire arrays," Nanotechnology, vol. 16, p. 2903, 2005.

[45] B. Mandl, J. Stangl, T. Mårtensson, A. Mikkelsen, J. Eriksson, L. S. Karlsson, et al., "Au-free epitaxial growth of InAs nanowires," Nano letters, vol. 6, pp. 1817-1821, 2006.

25 Group III-nitride Nanowires

[46] C. Colombo, D. Spirkoska, M. Frimmer, G. Abstreiter, and A. F. i Morral, "Ga- assisted catalyst-free growth mechanism of GaAs nanowires by molecular beam epitaxy," Physical Review B, vol. 77, p. 155326, 2008.

[47] A. Fontcuberta i Morral, C. Colombo, G. Abstreiter, J. Arbiol, and J. Morante, "Nucleation mechanism of gallium-assisted molecular beam epitaxy growth of gallium arsenide nanowires," Applied Physics Letters, vol. 92, p. 063112, 2008.

[48] B. Bauer, A. Rudolph, M. Soda, A. F. i Morral, J. Zweck, D. Schuh, et al., "Position controlled self-catalyzed growth of GaAs nanowires by molecular beam epitaxy," Nanotechnology, vol. 21, p. 435601, 2010.

[49] T. Rieger, S. Heiderich, S. Lenk, M. I. Lepsa, and D. Grützmacher, "Ga-assisted MBE growth of GaAs nanowires using thin HSQ layer," Journal of Crystal Growth, vol. 353, pp. 39-46, 2012.

[50] G. Cirlin, V. Dubrovskii, Y. B. Samsonenko, A. Bouravleuv, K. Durose, Y. Y. Proskuryakov, et al., "Self-catalyzed, pure zincblende GaAs nanowires grown on Si (111) by molecular beam epitaxy," Physical Review B, vol. 82, p. 035302, 2010.

[51] M. Mattila, T. Hakkarainen, H. Lipsanen, H. Jiang, and E. Kauppinen, "Catalyst-free growth of In (As) P nanowires on silicon," Applied physics letters, vol. 89, p. 063119, 2006.

[52] J. Hong, S. Wang, T. Sands, J. Washburn, J. Flood, J. Merz, et al., "Selective-area epitaxy of GaAs through silicon dioxide windows by molecular beam epitaxy," Applied physics letters, vol. 48, pp. 142-144, 1986.

[53] K. Ikejiri, J. Noborisaka, S. Hara, J. Motohisa, and T. Fukui, "Mechanism of catalyst- free growth of GaAs nanowires by selective area MOVPE," Journal of Crystal Growth, vol. 298, pp. 616-619, 2007.

[54] K. Tomioka, Y. Kobayashi, J. Motohisa, S. Hara, and T. Fukui, "Selective-area growth of vertically aligned GaAs and GaAs/AlGaAs core–shell nanowires on Si (111) substrate," Nanotechnology, vol. 20, p. 145302, 2009.

26 Group III-nitride Nanowires

[55] A. F. i Morral, "Gold-free GaAs nanowire synthesis and optical properties," IEEE Journal of Selected Topics in Quantum Electronics, vol. 17, p. 819, 2011.

[56] T. Zywietz, J. Neugebauer, and M. Scheﬄer, "Adatom diﬀusion at GaN (0001) and (0001) surfaces," Applied Physics Letters, vol. 73, pp. 487-489, 1998.

[57] J. Ristić, E. Calleja, S. Fernandez-Garrido, L. Cerutti, A. Trampert, U. Jahn, et al., "On the mechanisms of spontaneous growth of III-nitride nanocolumns by plasma- assisted molecular beam epitaxy," Journal of crystal growth, vol. 310, pp. 4035-4045, 2008.

[58] K. Bertness, A. Roshko, L. Mansﬁeld, T. Harvey, and N. Sanford, "Mechanism for spontaneous growth of GaN nanowires with molecular beam epitaxy," Journal of Crystal Growth, vol. 310, pp. 3154-3158, 2008.

[59] M. Volmer and A. Weber, "Novel growth mechanism in heteroepitaxial semiconductor growth," Z. Phys. Chem, vol. 119, p. 277, 1926.

[60] M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, and K. Kishino, "Growth of self- organized GaN nanostructures on Al2O3 (0001) by RF-radical source molecular beam epitaxy," Japanese journal of applied physics, vol. 36, p. L459, 1997.

[61] C. Chao, J.-I. Chyi, C. Hsiao, C. Kei, S. Kuo, H.-S. Chang, et al., "Catalyst-free growth of indium nitride nanorods by chemical-beam epitaxy," Applied physics letters, vol. 88, p. 233111, 2006.

[62] H. Sekiguchi, T. Nakazato, A. Kikuchi, and K. Kishino, "Structural and optical properties of GaN nanocolumns grown on (0001) sapphire substrates by rf-plasma- assisted molecular-beam epitaxy," Journal of crystal growth, vol. 300, pp. 259-262, 2007.

[63] K. Bertness, A. Roshko, L. Mansﬁeld, T. Harvey, and N. Sanford, "Nucleation conditions for catalyst-free GaN nanowires," Journal of crystal growth, vol. 300, pp. 94-99, 2007.

27 Group III-nitride Nanowires

[64] L. Largeau, D. Dheeraj, M. Tchernycheva, G. Cirlin, and J. Harmand, "Facet and in- plane crystallographic orientations of GaN nanowires grown on Si (111)," Nanotechnology, vol. 19, p. 155704, 2008.

[65] Y.-L. Chang, F. Li, A. Fatehi, and Z. Mi, "Molecular beam epitaxial growth and characterization of non-tapered InN nanowires on Si (111)," Nanotechnology, vol. 20, p. 345203, 2009.

[66] K. Cui, S. Fathololoumi, M. G. Kibria, G. A. Botton, and Z. Mi, "Molecular beam epitaxial growth and characterization of catalyst-free InN/InxGa1− xN core/shell nanowire heterostructures on Si (111) substrates," Nanotechnology, vol. 23, p. 085205, 2012.

[67] M. Sanchez-Garcia, E. Calleja, E. Monroy, F. Sanchez, F. Calle, E. Munoz, et al., "The eﬀect of the III/V ratio and substrate temperature on the morphology and properties of GaN-and AlN-layers grown by molecular beam epitaxy on Si (1 1 1)," Journal of crystal growth, vol. 183, pp. 23-30, 1998.

[68] M. Yoshizawa, A. Kikuchi, N. Fujita, K. Kushi, H. Sasamoto, and K. Kishino, "Self- organization of GaN/Al 0.18 Ga 0.82 N multi-layer nano-columns on (0001) Al 2 O 3 by RF molecular beam epitaxy for fabricating GaN quantum disks," Journal of crystal growth, vol. 189, pp. 138-141, 1998.

[69] H.-M. Kim, Y.-H. Cho, H. Lee, S. I. Kim, S. R. Ryu, D. Y. Kim, et al., "High- brightness light emitting diodes using dislocation-free indium gallium nitride/gallium nitride multiquantum-well nanorod arrays," Nano letters, vol. 4, pp. 1059-1062, 2004.

[70] T. Zhu and R. A. Oliver, "Unintentional doping in GaN," Physical Chemistry Chemical Physics, vol. 14, pp. 9558-9573, 2012.

[71] H. P. Maruska and J. Tietjen, "The preparation and properties of Vapor-Deposited single-crystal-line GaN," Applied Physics Letters, vol. 15, pp. 327-329, 1969.

[72] A. R. Denton and N. W. Ashcroft, "Vegard’s law," Physical Review A, vol. 43, p. 3161, 1991.

28 Group III-nitride Nanowires

[73] A. Wright and J. Nelson, "Bowing parameters for zinc-blende Al1− xGaxN and Ga1− xInxN," Applied physics letters, vol. 66, pp. 3051-3053, 1995.

[74] C. G. Van de Walle, M. McCluskey, C. Master, L. Romano, and N. Johnson, "Large and composition-dependent band gap bowing in In x Ga 1− x N alloys," Materials Science and Engineering: B, vol. 59, pp. 274-278, 1999.

[75] C. J. Humphreys, "Solid-State Lighting," MRS Bulletin, vol. 33, pp. 459-470, 2008.

[76] X. Ning, F. Chien, P. Pirouz, J. Yang, and M. A. Khan, "Growth defects in GaN ﬁlms on sapphire: The probable origin of threading dislocations," Journal of Materials Research, vol. 11, pp. 580-592, 1996.

[77] C. Zhao, T. K. Ng, A. Prabaswara, M. Conroy, S. Jahangir, T. Frost, et al., "An enhanced surface passivation eﬀect in InGaN/GaN disk-in-nanowire light emitting diodes for mitigating Shockley–Read–Hall recombination," Nanoscale, vol. 7, pp. 16658-16665, 2015.

[78] K. Bertness, N. Sanford, J. Barker, J. Schlager, A. Roshko, A. Davydov, et al., "Catalyst-free growth of GaN nanowires," Journal of Electronic Materials, vol. 35, pp. 576-580, 2006.

[79] S. M. Tanner, J. Gray, C. Rogers, K. Bertness, and N. Sanford, "High-Q GaN nanowire resonators and oscillators," Applied Physics Letters, vol. 91, p. 203117, 2007.

[80] T. Takeuchi, S. Sota, M. Katsuragawa, M. Komori, H. Takeuchi, H. Amano, et al., "Quantum-confined Stark effect due to piezoelectric fields in GaInN strained quantum wells," Japanese Journal of Applied Physics, vol. 36, p. L382, 1997.

[81] D. Miller, D. Chemla, T. Damen, A. Gossard, W. Wiegmann, T. Wood, et al., "Band- edge electroabsorption in quantum well structures: the quantum-conﬁned Stark eﬀect," Physical Review Letters, vol. 53, p. 2173, 1984.

[82] J. Speck and S. Chichibu, "Nonpolar and semipolar group III nitride-based materials," MRS bulletin, vol. 34, pp. 304-312, 2009.

29 Group III-nitride Nanowires

[83] G. Cheng, A. Kolmakov, Y. Zhang, M. Moskovits, R. Munden, M. A. Reed, et al., "Current rectiﬁcation in a single GaN nanowire with a well-deﬁned p–n junction," Applied Physics Letters, vol. 83, pp. 1578-1580, 2003.

[84] H. Sakaki, "Scattering suppression and high-mobility eﬀect of size-quantized electrons in ultraﬁne semiconductor wire structures," Japanese Journal of Applied Physics, vol. 19, p. L735, 1980.

[85] H. P. T. Nguyen, S. Zhang, K. Cui, X. Han, S. Fathololoumi, M. Couillard, et al., "p- Type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si (111)," Nano letters, vol. 11, pp. 1919-1924, 2011.

[86] S. Zhao, B. Le, D. Liu, X. Liu, M. Kibria, T. Szkopek, et al., "p-Type InN nanowires," Nano letters, vol. 13, pp. 5509-5513, 2013.

[87] A. T. Connie, S. Zhao, S. M. Sadaf, I. Shih, Z. Mi, X. Du, et al., "Optical and electrical properties of Mg-doped AlN nanowires grown by molecular beam epitaxy," Applied Physics Letters, vol. 106, p. 213105, 2015.

[88] S. Zhao, A. Connie, M. Dastjerdi, X. Kong, Q. Wang, M. Djavid, et al., "Aluminum nitride nanowire light emitting diodes: Breaking the fundamental bottleneck of deep ultraviolet light sources," Scientiﬁc reports, vol. 5, 2015.

[89] Y. Taniyasu, M. Kasu, and T. Makimoto, "An aluminium nitride light-emitting diode with a wavelength of 210 nanometres," Nature, vol. 441, pp. 325-328, 2006.

[90] S. Fischer, C. Wetzel, E. Haller, and B. Meyer, "On p-type doping in GaN—acceptor binding energies," Applied physics letters, vol. 67, pp. 1298-1300, 1995.

[91] J.-K. Sheu and G. Chi, "The doping process and dopant characteristics of GaN," Journal of Physics: Condensed Matter, vol. 14, p. R657, 2002.

[92] T. Mukai, M. Yamada, and S. Nakamura, "Characteristics of InGaN-based UV/blue/ green/amber/red light-emitting diodes," Japanese Journal of Applied Physics, vol. 38, p. 3976, 1999.

30 Group III-nitride Nanowires

[93] S. Zhao, H. P. Nguyen, M. G. Kibria, and Z. Mi, "III-Nitride nanowire optoelectronics," Progress in Quantum Electronics, vol. 44, pp. 14-68, 2015.

[94] S. N. Mohammad, A. Salvador, and H. Morkoc, "Emerging gallium nitride based devices," Proceedings of the IEEE, vol. 83, pp. 1306-1355, 1995.

[95] S. J. Pearton and F. Ren, "GaN electronics," Advanced Materials, vol. 12, pp. 1571-1580, 2000.

[96] J. H. Seo, Y. J. Yoon, H. G. Lee, G. M. Yoo, Y.-W. Jo, D.-H. Son, et al., "Design and Analysis of Vertical-Channel Gallium Nitride (GaN) Junctionless Nanowire Transistors (JNT)," Journal of Nanoscience and Nanotechnology, vol. 14, pp. 8130-8135, 2014.

[97] A. Kikuchi, M. Kawai, M. Tada, and K. Kishino, "InGaN/GaN multiple quantum disk nanocolumn light-emitting diodes grown on (111) Si substrate," Japanese Journal of Applied Physics, vol. 43, p. L1524, 2004.

[98] H. Sekiguchi, K. Kato, J. Tanaka, A. Kikuchi, and K. Kishino, "Ultraviolet GaN- based nanocolumn light-emitting diodes grown on n-(111) Si substrates by rf-plasma- assisted molecular beam epitaxy," Physica Status Solidi (a), vol. 205, pp. 1067-1069, 2008.

[99] J. C. Johnson, H.-J. Choi, K. P. Knutsen, R. D. Schaller, P. Yang, and R. J. Saykally, "Single gallium nitride nanowire lasers," Nature Materials, vol. 1, pp. 106-110, 2002.

[100] F. Qian, Y. Li, S. Gradečak, H.-G. Park, Y. Dong, Y. Ding, et al., "Multi-quantum-well nanowire heterostructures for wavelength-controlled lasers," Nature Materials, vol. 7, pp. 701-706, 2008.

[101] R. Chen, H. Sun, T. Wang, K. Hui, and H. Choi, "Optically pumped ultraviolet lasing from nitride nanopillars at room temperature," Applied Physics Letters, vol. 96, 2010.

[102] M. Sakai, Y. Inose, K. Ema, T. Ohtsuki, H. Sekiguchi, A. Kikuchi, et al., "Random laser action in GaN nanocolumns," Applied Physics Letters, vol. 97, p. 151109, 2010.

31 Group III-nitride Nanowires

[103] S. Ishizawa, K. Kishino, R. Araki, A. Kikuchi, and S. Sugimoto, "Optically pumped green (530–560 nm) stimulated emissions from InGaN/GaN multiple-quantum-well triangular-lattice nanocolumn arrays," Applied physics express, vol. 4, p. 055001, 2011.

[104] Q. Li, J. B. Wright, W. W. Chow, T. S. Luk, I. Brener, L. F. Lester, et al., "Single-mode GaN nanowire lasers," Optics express, vol. 20, pp. 17873-17879, 2012.

[105] J. B. Wright, S. Campione, S. Liu, J. A. Martinez, H. Xu, T. S. Luk, et al., "Distributed feedback gallium nitride nanowire lasers," Applied Physics Letters, vol. 104, p. 041107, 2014.

[106] W. Guo, A. Banerjee, P. Bhattacharya, and B. S. Ooi, "InGaN/GaN disk-in-nanowire white light emitting diodes on (001) silicon," Applied Physics Letters, vol. 98, p. 193102, 2011.

[107] H. P. T. Nguyen, K. Cui, S. Zhang, M. Djavid, A. Korinek, G. A. Botton, et al., "Controlling electron overﬂow in phosphor-free InGaN/GaN nanowire white light- emitting diodes," Nano letters, vol. 12, pp. 1317-1323, 2012.

[108] H. P. T. Nguyen, S. Zhang, A. T. Connie, M. G. Kibria, Q. Wang, I. Shih, et al., "Breaking the carrier injection bottleneck of phosphor-free nanowire white light-emitting diodes," Nano letters, vol. 13, pp. 5437-5442, 2013.

[109] S. Zhang, A. T. Connie, D. Laleyan, H. P. T. Nguyen, Q. Wang, J. Song, et al., "On the carrier injection eﬃciency and thermal property of InGaN/GaN axial nanowire light emitting diodes," Quantum Electronics, IEEE Journal of, vol. 50, pp. 483-490, 2014.

[110] Y. J. Hong, C. H. Lee, A. Yoon, M. Kim, H. K. Seong, H. J. Chung, et al., "Visible-

Color-Tunable Light-Emitting Diodes," Advanced Materials, vol. 23, pp. 3284-3288, 2011.

[111] M. Nakarmi, N. Nepal, J. Lin, and H. Jiang, "Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys," Applied Physics Letters, vol. 94, p. 91903, 2009.

32 Group III-nitride Nanowires

[112] Y. Taniyasu and M. Kasu, "Surface 210 nm light emission from an AlN p–n junction light-emitting diode enhanced by A-plane growth orientation," Applied physics letters, vol. 96, p. 221110, 2010.

[113] H. Lu, W. J. Schaﬀ, L. F. Eastman, and C. Stutz, "Surface charge accumulation of InN ﬁlms grown by molecular-beam epitaxy," Applied physics letters, vol. 82, pp. 1736-1738, 2003.

[114] I. Mahboob, T. Veal, C. McConville, H. Lu, and W. Schaﬀ, "Intrinsic electron accumulation at clean InN surfaces," Physical review letters, vol. 92, p. 036804, 2004.

[115] K. Kishino, J. Kamimura, and K. Kamiyama, "Near-Infrared InGaN Nanocolumn Light-Emitting Diodes Operated at 1.46 µm," Applied Physics Express, vol. 5, p. 031001, 2012.

[116] S. Nakamura, M. Senoh, S.-i. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, et al., "InGaN-based multi-quantum-well-structure laser diodes," Japanese Journal of Applied Physics, vol. 35, p. L74, 1996.

[117] S. Nakamura, M. Senoh, S. i. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, et al., "Room-temperature continuous-wave operation of InGaN multi-quantum-well structure laser diodes," Applied Physics Letters, vol. 69, pp. 4056-4058, 1996.

[118] Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi, et al., "531 nm green lasing of InGaN based laser diodes on semi-polar {2021} free-standing GaN substrates," Applied Physics Express, vol. 2, p. 082101, 2009.

[119] M. Adachi, Y. Yoshizumi, Y. Enya, T. Kyono, T. Sumitomo, S. Tokuyama, et al., "Low threshold current density InGaN based 520–530 nm green laser diodes on semi-polar {2021} free-standing GaN substrates," Applied physics express, vol. 3, p. 121001, 2010.

[120] C. Holder, J. S. Speck, S. P. DenBaars, S. Nakamura, and D. Feezell, "Demonstration of nonpolar GaN-based vertical-cavity surface-emitting lasers," Applied Physics Express, vol. 5, p. 092104, 2012.

33 Group III-nitride Nanowires

[121] K. Katayama, N. Saga, M. Ueno, T. Ikegami, and T. Nakamura, "High-Power True Green Laser Diodes on Semipolar {202¯ 1} GaN Substrates," Electronics and Communications in Japan, vol. 98, pp. 9-14, 2015.

[122] H. Yoshida, Y. Yamashita, M. Kuwabara, and H. Kan, "Demonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diode," Applied Physics Letters, vol. 93, p. 241106, 2008.

[123] M. Kneissl, Z. Yang, M. Teepe, C. Knollenberg, O. Schmidt, P. Kiesel, et al., "Ultraviolet semiconductor laser diodes on bulk AlN," Journal of Applied Physics, vol. 101, p. 123103, 2007.

[124] K. Li, X. Liu, Q. Wang, S. Zhao, and Z. Mi, "Ultralow-threshold electrically injected AlGaN nanowire ultraviolet lasers on Si operating at low temperature," Nature Nanotechnology, vol. 10, pp. 140-144, 2015.

[125] S. Zhao, X. Liu, S. Woo, J. Kang, G. Botton, and Z. Mi, "An electrically injected AlGaN nanowire laser operating in the ultraviolet-C band," Applied Physics Letters, vol. 107, p. 043101, 2015.

[126] S. Deshpande, J. Heo, A. Das, and P. Bhattacharya, "Electrically driven polarized single-photon emission from an InGaN quantum dot in a GaN nanowire," Nature Communications, vol. 4, p. 1675, 2013.

[127] M. J. Holmes, K. Choi, S. Kako, M. Arita, and Y. Arakawa, "Room-temperature triggered single photon emission from a III-nitride site-controlled nanowire quantum dot," Nano Letters, vol. 14, pp. 982-986, 2014.

[128] S. Lazić, E. Chernysheva, Ž. Gačević, N. García-Lepetit, H. P. van der Meulen, M. Müller, et al., "Ordered arrays of InGaN/GaN dot-in-a-wire nanostructures as single- photon emitters," in SPIE OPTO, 2015, pp. 93630U-93630U-8.

[129] B. M. Kayes, H. A. Atwater, and N. S. Lewis, "Comparison of the device physics principles of planar and radial pn junction nanorod solar cells," Journal of Applied Physics, vol. 97, p. 114302, 2005.

34 Group III-nitride Nanowires

[130] Y. Dong, B. Tian, T. J. Kempa, and C. M. Lieber, "Coaxial group III− nitride nanowire photovoltaics," Nano Letters, vol. 9, pp. 2183-2187, 2009.

[131] E. C. Garnett, M. L. Brongersma, Y. Cui, and M. D. McGehee, "Nanowire solar cells," Annual Review of Materials Research, vol. 41, pp. 269-295, 2011.

[132] J. J. Wierer Jr, Q. Li, D. D. Koleske, S. R. Lee, and G. T. Wang, "III-nitride core–shell nanowire arrayed solar cells," Nanotechnology, vol. 23, p. 194007, 2012.

[133] L. Rigutti, M. Tchernycheva, A. De Luna Bugallo, G. Jacopin, F. Julien, L. F. Zagonel, et al., "Ultraviolet photodetector based on GaN/AlN quantum disks in a single nanowire," Nano Letters, vol. 10, pp. 2939-2943, 2010.

[134] A. de Luna Bugallo, M. Tchernycheva, G. Jacopin, L. Rigutti, F. H. Julien, S.-T. Chou, et al., "Visible-blind photodetector based on p–i–n junction GaN nanowire ensembles," Nanotechnology, vol. 21, p. 315201, 2010.

[135] A. D. L. Bugallo, L. Rigutti, G. Jacopin, F. Julien, C. Durand, X. Chen, et al., "Single- wire photodetectors based on InGaN/GaN radial quantum wells in GaN wires grown by catalyst-free metal-organic vapor phase epitaxy," Applied Physics Letters, vol. 98, p. 233107, 2011.

[136] F. González-Posada, R. Songmuang, M. Den Hertog, and E. Monroy, "Room- temperature photodetection dynamics of single GaN nanowires," Nano Letters, vol. 12, pp. 172-176, 2011.

[137] W.-Y. Weng, T.-J. Hsueh, S.-J. Chang, S.-B. Wang, H.-T. Hsueh, and G.-J. Huang, "A high-responsivity GaN nanowire UV photodetector," IEEE Journal of Selected Topics in Quantum Electronics, vol. 17, pp. 996-1001, 2011.

[138] A. Babichev, H. Zhang, P. Lavenus, F. Julien, A. Y. Egorov, Y. Lin, et al., "GaN nanowire ultraviolet photodetector with a graphene transparent contact," Applied Physics Letters, vol. 103, p. 201103, 2013.

35 Group III-nitride Nanowires

[139] M. Tchernycheva, A. Messanvi, A. de Luna Bugallo, G. Jacopin, P. Lavenus, L. Rigutti, et al., "Integrated Photonic Platform Based on InGaN/GaN Nanowire Emitters and Detectors," Nano Letters, vol. 14, pp. 3515-3520, 2014.

[140] K. X. Wang, Z. Yu, V. Liu, Y. Cui, and S. Fan, "Absorption enhancement in ultrathin crystalline silicon solar cells with antireﬂection and light-trapping nanocone gratings," Nano Letters, vol. 12, pp. 1616-1619, 2012.

[141] M. D. Brubaker, P. T. Blanchard, J. B. Schlager, A. W. Sanders, A. Roshko, S. M. Duﬀ, et al., "On-chip optical interconnects made with gallium nitride nanowires," Nano Letters, vol. 13, pp. 374-377, 2013.

[142] X. Wang, Y. Zhang, X. Chen, M. He, C. Liu, Y. Yin, et al., "Ultrafast, superhigh gain visible-blind UV detector and optical logic gates based on nonpolar a-axial GaN nanowire," Nanoscale, vol. 6, pp. 12009-12017, 2014.

[143] R. Dahal, B. Pantha, J. Li, J. Lin, and H. Jiang, "InGaN/GaN multiple quantum well solar cells with long operating wavelengths," Applied Physics Letters, vol. 94, p. 063505, 2009.

[144] R. Dahal, J. Li, K. Aryal, J. Lin, and H. Jiang, "InGaN/GaN multiple quantum well concentrator solar cells," Appl. Phys. Lett, vol. 97, p. 073115, 2010.

[145] J.-H. Song, J.-H. Oh, J.-P. Shim, J.-H. Min, D.-S. Lee, and T.-Y. Seong, "Improved eﬃciency of InGaN/GaN-based multiple quantum well solar cells by reducing contact resistance," Superlattices and Microstructures, vol. 52, pp. 299-305, 2012.

[146] D. Cherns, R. Webster, S. Novikov, C. Foxon, A. Fischer, F. A. Ponce, et al., "Compositional variations in In0. 5Ga0. 5N nanorods grown by molecular beam epitaxy," Nanotechnology, vol. 25, p. 215705, 2014.

[147] J.-R. Kim, H. Oh, H. M. So, J.-J. Kim, J. Kim, C. J. Lee, et al., "Schottky diodes based on a single GaN nanowire," Nanotechnology, vol. 13, p. 701, 2002.

36 Group III-nitride Nanowires

[148] M. Shim and P. Guyot-Sionnest, "N-type colloidal semiconductor nanocrystals," Nature, vol. 407, pp. 981-983, 2000.

[149] S. C. Erwin, L. Zu, M. I. Haftel, A. L. Efros, T. A. Kennedy, and D. J. Norris, "Doping semiconductor nanocrystals," Nature, vol. 436, pp. 91-94, 2005.

[150] T. Henry, K. Kim, Z. Ren, C. Yerino, J. Han, and H. X. Tang, "Directed growth of horizontally aligned gallium nitride nanowires for nanoelectromechanical resonator arrays," Nano Letters, vol. 7, pp. 3315-3319, 2007.

[151] C.-T. Huang, J. Song, W.-F. Lee, Y. Ding, Z. Gao, Y. Hao, et al., "GaN nanowire arrays for high-output nanogenerators," Journal of the American Chemical Society, vol. 132, pp. 4766-4771, 2010.

[152] Z. L. Wang, "Self-powered nanosystem: From nanogenerators to piezotronics," in Solid-State and Integrated Circuit Technology (ICSICT), 2010 10th IEEE International Conference on, 2010, pp. 4-4.

[153] C. Chen, Y. Shi, Y. Zhang, J. Zhu, and Y. Yan, "Size dependence of Young’s modulus in ZnO nanowires," Physical Review Letters, vol. 96, p. 075505, 2006.

[154] C.-Y. Nam, P. Jaroenapibal, D. Tham, D. E. Luzzi, S. Evoy, and J. E. Fischer, "Diameter-dependent electromechanical properties of GaN nanowires," Nano Letters, vol. 6, pp. 153-158, 2006.

[155] R. A. Bernal, R. Agrawal, B. Peng, K. A. Bertness, N. A. Sanford, A. V. Davydov, et al., "Eﬀect of growth orientation and diameter on the elasticity of GaN nanowires. A combined in situ TEM and atomistic modeling investigation," Nano Letters, vol. 11, pp. 548-555, 2010.

[156] D. W. Carr, S. Evoy, L. Sekaric, H. Craighead, and J. Parpia, "Measurement of mechanical resonance and losses in nanometer scale silicon wires," Applied Physics Letters, vol. 75, pp. 920-922, 1999.

37 Group III-nitride Nanowires

[157] K. Ekinci and M. Roukes, "Nanoelectromechanical systems," Review of Scientiﬁc Instruments, vol. 76, p. 061101, 2005.