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Towards a deterministic cavity-integrated III-nitride single-photon source with two-photon resonant control

Towards a deterministic cavity-integrated III-nitride single-photon source with two-photon resonant control

Considering that the emission of indistinguishable photons has not yet been observed for any IIInitride system, we will concentrate on the potential for high temperature QKD for the remainder of the section, noting only that the development of IIInitride single–photon platforms will certainly benefit from the progress made in other material systems. The fact that QKD applications are well–within the grasp of state–of–the–art IIInitride QDs has been proven in Ref. [29], where a deconvolved g (2) (0) of 0.02 ± 0.05 is observed under off–resonant excitation at 10 K. Similar low–temperature values have recently been measured for Stranski–Krastanov QDs in the group of N. Grandjean (to be published). Since tunable excitation sources in the near–ultraviolet spectral range are not as readily available as in the near–infrared, most experiments with III–nitrides are still conducted under (far–)above–band–gap excitation. For instance, a common continuous–wave excita- tion source for GaN/AlN QDs is a frequency–doubled argon–ion laser, allowing excitation at 244 nm and 266 nm. These wavelengths, however, also efficiently excite defect bands in the AlN matrix [65], and measurements often suffer from defect–related broadband background luminescence (cf. Table 5 in [66] for a summary), limiting single–photon purity to g (2) (0) ∼ 0.2 − 0.3. On the plus side, these values can persist up to high tempera- tures without degrading [26, 27], given that the phonon–broadened exciton and biexciton emission lines are well–separated, i. e. the biexciton binding energy exceeds the thermal energy. As was shown in Ref. [67], the strong carrier confinement in GaN/AlN QDs leads to almost constant luminescence lifetimes up to room temperature, suggesting possible ∼ GHz repetition rates under pulsed resonant excitation. A primary objective of this thesis is to introduce an alternative resonant excitation scheme, exploiting the carrier separation inherent to polar IIInitride systems (cf. chapter 3).
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Polarization-optimized heterostructures with quaternary AlInGaN layers for novel group III nitride devices

Polarization-optimized heterostructures with quaternary AlInGaN layers for novel group III nitride devices

From an industry point of view, III nitride growth on Si is very attractive ow- ing to several advantages, e.g. low-cost, availability of large-size substrates and all types of conductivities [127]. Further, one could envision an, although challenging, integration of III nitride devices with Si technology. The mainstream approach is to grow GaN on planar Si(111) substrates for the reason of the trigonal plane sym- metry [128]. The large mismatch between the thermal expansion coefficients is a severe obstacle for GaN epitaxy on Si(111) and demands for sophisticated strain- engineering methods [129, 130]. By patterning, the coalesced film deposited by selective area growth can be reduced, which mitigates the necessity for excessive strain-compensation [131–133]. Another approach is to use Si(100) substrates, in which non-planar Si{111} facets can be uncovered by e.g. wet-chemical anisotropic etching in Si. Here, selective area growth is performed on the exposed Si{111} facets. All heterostructures investigated in this work were deposited on two inch c-plane sapphire substrates with an offcut between 0.2 % and 0.3 % towards m-plane. With the above mentioned substrates for GaN epitaxy, this substrate choice is the most common one for research due to several reasons: First, it is reasonably cheap 2 .
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Advanced group III-nitride nanowire heterostructures - self-assembly and position-controlled growth

Advanced group III-nitride nanowire heterostructures - self-assembly and position-controlled growth

Semiconductor NWs were introduced by Wagner and Ellis who fabricated Si NWs using a newly developed vapour-liquid-solid (VLS) process in 1964 [56]. For this growth method, metal droplets are required as nucleation sites liquid at the desired growth temperature. These metal droplets absorb the precursors from the gas phase which leads to the formation of a liquid alloy. Nucleation (growth) of the NWs commences at the liquid-solid interface to the substrate (NW) upon supersaturation of the formed alloy droplet. A VLS process can also be applied for the PAMBE growth of compound semiconductors, exempli gratia, group III-V NWs. Self-catalytic growth using a liquid Ga-droplet is feasible for the realization of GaAs NWs [57, 58] while GaN NWs can be grown from Ni-droplets [59]. In contrast, GaN NWs can also be grown self-assembled on Si(111) by PAMBE, in absence of any additional catalyst material [4, 5, 60]. The self-assembled growth mode diers from the self-catalytic growth mode as no metal droplets form to initiate growth [61]. A direct comparison of self-assembled and catalytically grown GaN NWs revealed that the use of Ni as a catalyst has a detrimental eect on the optical and structural properties of the NWs [62]. The self-assembled growth has, therefore, been established as the standard process for the fabrication of group III-nitride NWs by PAMBE and has also been used for the NWs presented in the current chapter as well as the NWHs studied in chapters 5 and 6.
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III-nitride heterostructures as infrared emitters / submitted by Dipl.-Ing. Dmytro Kysylychyn

III-nitride heterostructures as infrared emitters / submitted by Dipl.-Ing. Dmytro Kysylychyn

with Mn and Mg have been studied in combination with dedicated photonic heterostruc- tures targeted at enhancing the emission in the perspective of realizing a III-nitride IR laser structure without the need of alloying with In. The growth parameters for the DBR het- erostructures have been designed by reflectivity simulations employing the transfer matrix method and then fabricated by MOVPE monitored via in situ reflectometry measurements. All structures studied in this work are structurally characterized: (i) surface by AFM, (ii) crystal quality and strained state of the films by HRXRD and HRTEM, respectively. Even- tually, the optical studies are performed by employing spectroscopic ellipsometry, Raman and PL spectroscopy. In addition, the thermal conductivity of the DBR structures is de- termined through the differential 3ω-method.
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Polarization-optimized heterostructures with quaternary AlInGaN layers for novel group III nitride devices

Polarization-optimized heterostructures with quaternary AlInGaN layers for novel group III nitride devices

From an industry point of view, III nitride growth on Si is very attractive ow- ing to several advantages, e.g. low-cost, availability of large-size substrates and all types of conductivities [127]. Further, one could envision an, although challenging, integration of III nitride devices with Si technology. The mainstream approach is to grow GaN on planar Si(111) substrates for the reason of the trigonal plane sym- metry [128]. The large mismatch between the thermal expansion coefficients is a severe obstacle for GaN epitaxy on Si(111) and demands for sophisticated strain- engineering methods [129, 130]. By patterning, the coalesced film deposited by selective area growth can be reduced, which mitigates the necessity for excessive strain-compensation [131–133]. Another approach is to use Si(100) substrates, in which non-planar Si{111} facets can be uncovered by e.g. wet-chemical anisotropic etching in Si. Here, selective area growth is performed on the exposed Si{111} facets. All heterostructures investigated in this work were deposited on two inch c-plane sapphire substrates with an offcut between 0.2 % and 0.3 % towards m-plane. With the above mentioned substrates for GaN epitaxy, this substrate choice is the most common one for research due to several reasons: First, it is reasonably cheap 2 . Second, for LED industry, sapphire has been developed for over 20 years and still
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Magneto Transport of III nitride Systems / submitted by Markus Aiglinger, BSc

Magneto Transport of III nitride Systems / submitted by Markus Aiglinger, BSc

The samples studied in this work are heteroepitaxially grown on 2-inch c-plane sapphire (Al 2 O 3 ) substrates by means of metal organic vapour phase epitaxy (MOVPE) in an AIXTRON 200RF horizontal-tube MOVPE reactor. The work- ing principle of III-nitride growth by MOVPE is to introduce the group III-metals and the dopands into the reactor in the form of vapour of metal-organic (MO) molecules like trimethyl-gallium (TMGa) and the nitrogen in form of ammonia NH 3 . As carrier gas for the vapour, high purity N 2 and H 2 are used. Inside the reactor the substrate is placed on a temperature controlled rotating stage, called susceptor. The flow of MO vapour and the NH 3 are directed towards the substrate surface. Molecules adsorb on the substrate and decay into metal ions, nitride ions and residuals. The metal ions and the nitride participate in the nucleation and the growth process, while the residuals have a strong tendency to desorb. The residuals, the carrier gas and the wasted MO molecules and NH 3 are then pumped out of the reactor and sent to a disposal system. The delivery of fresh gases and vapour, and the pumping of the waste gas are continuous processes which ensure a steady flow and a stable reactor pressure.
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Linear optical properties of III-nitride semiconductors between 3 and 30eV

Linear optical properties of III-nitride semiconductors between 3 and 30eV

Nevertheless, we should mention again that spectroscopic ellipsometry is based on the re- flection of the probing light at the sample surface. Due to the broken symmetry at the sample boundaries, surface effects have to be taken into account even if the sample would provide a ”perfect” surface. In the strict sense the electronic states and, thus, the optical properties of a bulk semiconductor are only well defined in an infinite crystal. In some additional experi- ments, which are not shown in this work, we could determine the pseudodielectric function measured by ellipsometry on different surface reconstructions and termination, respectively [115, 116]. These differences, on the one hand, can be used for a detailed analysis of the re- spective surface properties e. g. during the growth of III-nitride layers. But on the other hand, if the bulk optical properties are investigated by means of spectroscopic ellipsometry, these effects has to be considered in the evaluation of the measurements. However, the biggest changes in the measured pseudodielectric function are obtained for the very special situation of a very Ga rich surface, where Ga double layer constitutes the special surface properties. But also for N-rich surfaces smaller differences depending on the exact surface stoichiometry (on the specific surface reconstruction) are found. Finally we conclude from these measure- ments that the GaN spectra measured on samples stored in air (e. g. fig. 6.2 ) are very close to the results obtained on very defined N-rich surfaces.
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Gruppe-III-Nitride: Phononen, Plasmonen, Polarität

Gruppe-III-Nitride: Phononen, Plasmonen, Polarität

Gruppe-III-Nitride haben eine erstaunliche und rasante Entwicklung innerhalb der letzten 20 Jahre erlebt, wobei die Erfolge in den ersten Jahren vor allem durch die großen Fortschritte in GaN getrieben wurden. Nachdem in der Mitte des letzten Jahrhunderts bis Mitte der 1980er Jahre hauptsächlich die grundlegenden Eigenschaften von GaN und das Wachstum mittels Metallorganischer-Gasphasenepitaxie (MOCVD) erforscht wurden, erfolgte ein wichtiger Durchbruch beim Wachstum 1986, mit der erstmaligen Verwendung eines AlN buffer-layers zur Defektreduktion. 1-3 Als es nur drei Jahre später durch Amano et al. gelang erfolgreiche p- Dotierung in GaN zu erzielen, führte diese Entdeckung zu einer wahrhaften Explosion von Publikationen im Bereich der Nitride von einigen zehn (1989) auf über 2000 pro Jahr (2010). 4,5 Kurz darauf wurden erste LEDs und Laser basierend auf InGaN hergestellt und kommerzialisiert, wodurch sich auch das wissenschaftliche Interesse an AlN und InN bzw. den ternären Mischkristallen AlGaN und InGaN nochmals verstärkte. 6,7 Nach 2002 wurde der Forschungsdrang nochmalig durch die Entdeckung der geringen Bandlücke von InN angetrieben. 8 Der Vorteil der Gruppe-III-Nitride ist dabei unübersehbar: Während AlN eine sehr große Bandlücke von 6.2 eV hat, besitzt InN mit 0.6 eV eine äußerst geringe Bandlücke im IR- Bereich. 9,10 Das quaternäre System AlGaInN überspannt damit also einen extrem weiten Bogen für optische Bauelemente. Diese Vorteile von AlGaInN drücken sich auch in ihrer zunehmenden kommerziellen Bedeutung aus. Firmen wie OSRAM Opto Semiconductors, Cree oder Philips Lumileds vermelden inzwischen Umsätze im Milliarden-Dollar-Bereich für LEDs und Laserdioden basierend auf Gruppe-III-Nitriden. 11-13
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Analysis of the green gap problem in III-nitride LEDs

Analysis of the green gap problem in III-nitride LEDs

It is not clear whether these piezoelectric fields are desirable. It is agreed that the reduction of wavefunction overlap is deleterious to the IQE. However, without the QCSE, more indium would be needed to reach the same wavelength, and QWs with a higher indium content may be less efficient for reasons other than the piezoelectric fields (see § 2.6). Therefore, there still is the concrete possibility that the simultaneous removal of the piezoelectric fields and increase of the indium content will actually reduce the overall efficiency. The consensus in the scientific community seems to be that the performance of the IIInitride LEDs would improve if the piezoelectric fields were removed [Wal00], and this has driven a lot of research on the growth of GaN in non-polar and semipolar directions [Rom06, Spe09, Mas10, Sch12b].
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Investigation of AlN/GaN superlattices and their application to group III nitride devices

Investigation of AlN/GaN superlattices and their application to group III nitride devices

Despite their great success, group-III nitride heterostructures suffer from a major drawback which is the occurrence of large stress leading to straining and relaxation of the material. This is unwanted because defects are formed which deteriorate the crystal quality. Also the material surface can roughen or the material can crack. The large amount of stress in nitride epitaxy originates from large differences in lattice parameters between the binary compounds GaN, AlN and InN [7]. The lattice mismatch between GaN and AlN is 2.4 % [7], which is very large compared to GaAs and AlAs (-0.14 % [8]). This causes relaxation processes which affect material properties like the band gap energy [9]. Especially ternary alloys as AlGaN, InGaN and AlInN pose a challenge to process control because stress, strain and process parameter fluctuations can cause the composition to vary. If relaxation starts in ternary alloys and the lattice parameter is changed gradually, the energy barrier for the incorporation of certain atomic species into the solid phase can be lowered or increased. This causes the composition of ternary alloys to change during the relaxation process and is termed as compositional pulling [10], [11]. Binary materials as GaN or AlN are much more stable and robust regarding their epitaxial processes. Compositional pulling cannot occur for these materials. To combine the robustness
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Group III-Nitride Nanowires as Multifunctional Optical Biosensors

Group III-Nitride Nanowires as Multifunctional Optical Biosensors

Group-III nitride materials exhibit a high electrochemical stability [13–15]. In addition, AlGaN surfaces reveal non-toxic properties in contact with living cells [15]. This qual- ifies the electrodes for biological applications and in vivo studies. Other nanomaterials that are commonly used for optical biomolecule detection are nanoparticles (NPs), such as gold NPs or surface modified NPs like surface modified quantum dots (QDs). The detection mechanism here is based on the quenching of the nanoparticles´ PL intensity due to hole transfer from the QDs to the molecules [89]. In contrast to the InGaN/GaN NWH electrodes, the working point of such NPs cannot be defined. Thus, the applicabil- ity for the detection of one individual biomolecule is limited. Besides, quantum dots have been reported to possess electrochemical stability only in a small pH range in the neutral range of 6 < pH < 9 [90] or 6 < pH < 8 [91]. For application in different environments, an irreversible decrease of the photoluminescence intensity was observed.
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Group III-nitride based UVC LEDs and lasers with transparent AlGaN:Mg layers and tunnel junctions grown by MOVPE

Group III-nitride based UVC LEDs and lasers with transparent AlGaN:Mg layers and tunnel junctions grown by MOVPE

temperature of 960 ℃ and should not contribute to re-passivation. In the case of the presented AlGaN-based TJ-LEDs, the discussed SIMS data have been obtained from the full heterostructure (i. e. after AlGaN:Si overgrowth) and no detrimental re-passivation within these structures has been observed. For MOVPE-grown GaN-based TJ-LEDs presented in literature, however, severe Mg acceptor passivation has been reported as well as strategies for post-growth activation [164]: For instance, a post-growth thermal annealing step of the wafers, e. g., within a rapid thermal annealing (RTA) furnace is carried out after mesa definition, i. e. when the p-layer can be laterally accessed. It was theoretically described and experimentally verified that hydrogen diffusion is strongly suppressed within n-type III-nitride layers compared to p-type layers [192– 194]. Therefore, hydrogen in-diffusion from the gas phase into AlGaN:Mg layers during AlGaN:Si overgrowth is reduced, but ex-situ hydrogen removal through the surface of TJ-LEDs featuring an n-type top current spreading layer is prevented. Lateral activation within an RTA system after mesa etching was demonstrated as a successful activation concept. Here, the higher mobility of hydrogen atom diffusion within p-type layers is exploited to remove hydrogen through the open side walls of the etched mesa [164]. At a fixed annealing temperature, a square-root dependency of the hydrogen diffusion length on the annealing time has to be considered when designing an appropriate lateral mesa size in order to achieve very low hydrogen concentrations close to complete activation [164].
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Investigation of AlN/GaN superlattices and their application to group III nitride devices

Investigation of AlN/GaN superlattices and their application to group III nitride devices

16 nm is caused by the low refractive index contrast in group III nitrides. Experimental results from several groups are summarized in the paper from Carlin et al. [111]. Compared to these data, a maximum reflectivity around 75 % with 10 pairs is ranked acceptable for nitride-based DBR. The reflectivity data from our sample are even slightly higher than the given data of AlGaN/GaN-DBR but this can be attributed to the very small stopband wavelength close to the absorption edge of GaN for the SL-DBR sample. This increases the refractive index contrast. The graph in red (in fig. 4.22) shows the reflectivity spectrum of a buffer structure (sapphire substrate, AlN, GaN) comparable to the one in the SL-DBR sample. The reflectivity of the buffer structure shows no stopband peak but also periodic oscillations between 400 and 800 nm. By comparison of the two graphs, it becomes clear that the modulation observed for the SL-DBR on the long-wavelength side of the stopband peak originates from the buffer structure. Between 550 and 800 nm, the reflectivity graphs of SL- DBR (black) and buffer structure (red) look quite identical and only the oscillation frequency of the buffer structure is higher. This could be related to differences in layer thicknesses. Between 400 and 550 nm, the spectra for SL-DBR and buffer structure look more different. This can be attributed to a convolution of DBR reflectivity ripples from the adjacent stopband with the thickness-related oscillation from the buffer for the SL-DBR structure. The node at approximately 530 nm is also related to the layer thicknesses in the buffer stack. A simulational analysis of the reflectivity will be given at the end of this chapter.
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Chip designs for high efficiency III-nitride based ultraviolet light emitting diodes with enhanced light extraction

Chip designs for high efficiency III-nitride based ultraviolet light emitting diodes with enhanced light extraction

Kneissl, “Enhancement of light extraction in ultraviolet light-emitting diodes using nanopixel contact design with Al reflector,” Applied Physics Letters, vol.. Hartmann, “PROXECCO-Proxim[r]

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Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

For PL experiments, the samples were mounted in a cold-finger cryostat with the temperature controlled from T ¼ 7 K to RT. PL was excited with a frequency-doubled argon laser (k ¼ 244 nm) and collected into a Jobin- Yvon HR460 monochromator equipped with an ultraviolet- enhanced charge-coupled device (CCD) camera. The diame- ter of the excitation spot on the sample was about 100 lm. The excitation power was kept around 100 lW, low enough to avoid screening of the internal electric field. The low- temperature (T ¼ 7 K) PL spectra of the samples under study are presented in Fig. 5 . In the case of the GaN/AlN QWs, the spectral structure of the emission is due to monolayer thick- ness fluctuations in the QWs, as described elsewhere. 33 In the rest of the structures (GaN/AlN QDs and InGaN/GaN QDs and QWs), the broader linewidth makes it possible to observe the superimposition of a Fabry-Perot interference pattern related to the total nitride thickness.
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III-nitride-based optochemical transducers for gas detection

III-nitride-based optochemical transducers for gas detection

The first promising research successes date back to some 50 years ago where first works on GaN based transistors were published [13]. As GaN materials have to be produced via epitaxial growth processes, appropriate substrates are mandatory. As bulk GaN crystals were not available then, the progress of research on GaN was very slow in the beginning. It was only in the 1980s that the nitride material science rapidly progressed as these materials could then be grown on sapphire substrates using GaN and AlN as nucleation layers [14,15]. In the 1990-ties p-type GaN was discovered us- ing magnesium as a dopant [16]. Some years later, after high quality GaN films could be manufactured, the successful commercialization of GaN devices set in. GaN based light emitting (LED) and laser diodes found their way into consumer products such as Blue Ray discs and lighting bulbs. The inventors of high efficient GaN based LEDs were rewarded with the Nobel prize in 2014 [17]. Other successful products were high- electron-mobility transistors (HEMT) for high frequency applications [18].
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Characterization of group III–nitride nanowires for bio–electrochemical sensors

Characterization of group III–nitride nanowires for bio–electrochemical sensors

For a realistic description of the situation it has to be taken into account that due to diffusion from the Si substrate the Si–concentration in the bottom part of the NW is significantly higher than in the rest of the NW [14]. Furthermore, the high Si content related to the high T Si during growth, the NW diameter increases along the c–axis [14]. Both effects have not been taken into consideration in earlier reports [30]. The resulting evolution of the conduction band profile along a highly Si–doped NW is schematically shown in Figure 4.6c. A small depletion width and the presence of car- riers are found in the bottom part of the NW (i). If a homogeneous Si–concentration due to doping is assumed in the middle and upper part, the SBB increases along the growth direction, favoring the presence of carriers in the top part of the NW with a larger diameter (iii) while the middle part (ii) remains resistive. The corresponding effective barrier V fb ∗ for electrons can be calculated according to Eq. 2.3.
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Growth and characterisation of group-III nitride-based nanowires for devices

Growth and characterisation of group-III nitride-based nanowires for devices

Calleja, ‘Morphology and optical properties of InN layers grown by molecular beam epitaxy on silicon substrates’, Phys.. Status Solidi C, vol..[r]

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Germanium doping of aluminum-containing cubic group III-nitride heterostructures / von Michael Deppe

Germanium doping of aluminum-containing cubic group III-nitride heterostructures / von Michael Deppe

Group III-nitrides are nowadays used in many different areas due to their unique properties. By utilizing ternary compounds of AlN, GaN, and InN, the band gap can be varied over a large range, enabling to fabricate optoelectronic devices operating over the entire visible spectrum, as well as in the infrared and ultraviolet spectral range. Especially in the blue and ultraviolet spectral range, nitrides are without any alternative for fabricating devices like LEDs or lasers operating with high efficiency. Only progress in the development of group III-nitrides made the production of white LEDs possible, which are used in almost all lighting applications today. Although the band gap of ternaries containing In can be small enough to cover the infrared spectral range utilizing interband transitions, the efficiency of these structures would be poor due to several difficulties associated with the incorporation of In [ 1 – 3 ]. However, intersubband transitions within the conduction band of GaN-based quantum wells feature energies suited for infrared applications. Conceivable devices are for example quantum cascade lasers operating at high powers or at high frequencies [ 4 , 5 ], or fast infrared detectors [ 6 ].
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Indium gallium nitride nanostructures for optoelectronic applications

Indium gallium nitride nanostructures for optoelectronic applications

time for InN NWs. The crystal structure of the V-shaped NWs was investigated in detail by TEM studies. They were found to crystallise in wurtzite crystal structure. A report on V-shaped NWs for indium arsenide (InAs) material was published by Conesa-Boj et al. [139]. They found a pyramidal-shaped nucleus at the NW bottom enabling the V-shape of their NWs. Here, no such nucleus at the bottom of the InN NWs could be found. In this work, the V-shaped profile of the NW top is caused by a twin-plane reaching through the entire vertical NW axis. This defect separates two opposite crystal planes. A similar V-shaped profile was reported by Gamalski et al. [135] where a twin-plane re-entrant mechanism was found for Ge NWs. The twin- plane is suggested to be a preferential nucleation site, promoting the growth along the [¯110¯1] direction, due to a reduced nucleation barrier at the twin-plane re-entrant groove by the presence of a line energy. Originally this mechanism was proposed by Wagner, Hamilton and Seidensticker [140, 141] based on preferential nucleation at the re-entrant-groove of the surfaces at the twin boundary bounded by a low energy habit plane and the self-perpetuating two-dimensional growth of the nuclei. So far, this model has been applied to the anisotropic growth of various materials with repeated twinned structures, e.g. semiconductor dendrites, [140–142] or face centred cubic met- als [143, 144]. Furthermore, Gamalski et al. [135] noted that the triple phase line (point where vapour, solid and liquid phase are in equilibrium), which is known to be the preferential nucleation site in standard VLS growth [77], is not the preferred nucleation site in the twin-plan re-entrant growth mode. This growth mode was not specifically reported for III-nitride NWs. Nevertheless, their main statements were found to be consistent with the observations in this work presented the first time for InN NWs.
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