This work provides experimental evidence and theoretical explanations regarding the formation mechanisms of GaN nanowires grown by selective area growth on GaN-on-sapphire templates. The first growth stage, driven by selective area growth kinetics, consists of initial nucleation (along the nanohole inner periphery), coalescence onset and full coalescence, producing a single nanocrystal within each nanohole. In the second growth stage, driven by free-surface-energy minimization, the formed nanocrystal undergoes morphological evolution, exhibiting initial cylindrical-like shape, intermediate dodecagonal shape and a final, thermodynamically stable hexagonal shape. From this point on, the nanowire vertical growth proceeds while keeping the stable hexagonal form.

Indium incorporation and surface morphology of InAlN layers grown on ͑0001͒ GaN by plasma-assisted molecular beam epitaxy were investigated as a function of the impinging In flux and the substrate temperature in the 450-610°C range. In incorporation was found to decrease with substrate temperature due to thermal decomposition of the growing layer, while for a given temperature it increased with the impinging In flux until stoichiometry was reached at the growth front. The InN losses during growth followed an Arrhenius behavior characterized by an activation energy of 2.0 eV. A growth diagram highly instrumental to identify optimum growth conditions was established. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3026541͔ InAlN alloys have a direct band gap tunable from 0.7 to 6.2 eV, and for a 17% In there is lattice match ͑in-plane͒ to GaN. Heterostructures and devices including InAlN layers, such as resonant cavities, multiquantum wells for high-speed intersubband devices, or high electron mobility transistors, have been recently reported. [1][2][3][4] Growth diagrams, useful to identify optimum conditions, have been established for binaries ͑AlN, GaN, InN͒, grown by plasma-assisted molecular beam epitaxy ͑PA-MBE͒, from the surface morphology dependence on growth temperature and impinging fluxes. [5][6][7][8] However, the growth of InAl͑Ga͒N alloys poses much more difficulties due to InN thermal decomposition 9-12 and strong differences between InN and AlN.This work reports on the growth and characterization of metal-face InAlN layers on GaN templates. Indium incorporation and surface morphology are analyzed as a function of growth temperature and metal fluxes to build up a growth diagram.InAlN thin layers ͑ϳ80 nm thick͒ were grown by PA-MBE on ͑0001͒ GaN templates ͑ϳ3.6 m thick͒ grown by metal-organic vapor phase epitaxy on sapphire ͑Lumilog͒. Growth temperature was measured with an Ircon Modline3 optical pyrometer. Metal fluxes ͑⌽ Ga , ⌽ Al , ⌽ In ͒, measured as beam equivalent pressure ͑Bayard Alpert͒, were calibrated in atoms/ s cm 2 using cross-sectional scanning electron microscopy ͑SEM͒ data from N-rich GaN, AlN, and InN thick layers grown at temperatures where thermal decomposition and adatom desorption are negligible. 5 Similarly, the active N flux ⌽ N was calibrated in atoms/ s cm 2 using cross-sectional SEM data from Ga-rich GaN thick layers grown at low temperatures ͑680°C͒. Prior to the InAlN growth, a 100 nm thick GaN buffer layer was grown at 700°C under intermediate Ga-rich conditions 5 to obtain a smooth and flat surface. Alloy compositions were assessed by high resolution x-ray diffraction ͑HR-XRD͒ and surface morphologies were characterized by SEM ͑JEOL JSM-5800͒ and by atomic force microscopy ͑AFM͒ ͑Digital Instruments MMAFM-2͒.To analyze separately the effects of growth temperature and impinging In flux on In incorporation, two sets of samples were grown. In a first set ͑series A͒ all impinging fluxes were kept constant and the growth temperature varied between 450 and 610°C, a range...

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-Single photon emitters (SPEs) are at the basis of many applications for quantum information management. Semiconductor-based SPEs are best suited for practical implementations because of high design flexibility, scalability and integration potential in practical devices. Single photon emission from ordered arrays of InGaN nano-disks embedded in GaN nanowires is reported. Intense and narrow optical emission lines from quantum dot-like recombination centers are observed in the blue-green spectral range. Characterization by electron microscopy, cathodoluminescence and micro-photoluminescence indicate that single photons are emitted from regions of high In concentration in the nano-disks due to alloy composition fluctuations. Single photon emission is determined by photon correlation measurements showing deep antibunching minima in the second order correlation function. The present results are a promising step towards the realization of on-site/on-demand single photon sources in the blue-green spectral range operating in the GHz frequency range at high temperatures.Introduction. -Single photons are ideal "flying" qubits to convey quantum information between distant nodes of a quantum network. Reliable and controlled generation of single photons is therefore a crucial step to develop applications for quantum communication, quantum information processing and quantum metrology [1,2]. Single photons can be emitted in principle by material entities possessing discrete energy levels, as they need a finite time to "recharge" after emission of one photon. The standard method to assess single photon emission is to measure the second order photon correlation function by Hanbury-Brown and Twiss (HBT) interferometry. As shown in Fig. 1, single photons are either reflected or transmitted by a beam splitter, so that the probability of simultaneous detection in the two detectors of the interferometer is zero. The detection events are stored in a Time-Correlated Single Photon Counter (TCSPC), and the resulting correlation function g 2 (τ) shows an

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency of the QD three-dimensional confinement in circumventing the potential fluctuations related to structural defects or alloy inhomogeneity. PL excitation measurements demonstrate efficient carrier transfer from the wetting layer to the QDs in the GaN/AlN system, even for low QD densities ($10 10 cm À3). In the case of InGaN/GaN QDs, transport losses in the GaN barriers cannot be discarded, but an upper limit to these losses of 15% is deduced from PL measurements as a function of the excitation wavelength.

The optical emission of InGaN quantum dots embedded in GaN nanowires is dynamically controlled by a surface acoustic wave (SAW). The emission energy of both the exciton and biexciton lines is modulated over a 1.5 meV range at ~330 MHz. A small but systematic difference in the exciton and biexciton spectral modulation reveals a linear change of the biexciton binding energy with the SAW amplitude. The present results are relevant for the dynamic control of individual single photon emitters based on nitride semiconductors.Surface acoustic waves induce periodic strain and piezoelectric fields near a semiconductor surface which can dynamically modify their basic properties. The use of SAWs is an expanding research field, which has been widely applied to semiconductor quantum wells (QWs) 1-3 , wires 4-6 and dots (QDs) [7][8][9][10][11] .By controlling the excitonic emission in III-V semiconductor QDs by SAWs, high repetition rate single photon sources (SPSs) 7-9 and periodic laser mode feeding 12 have been reported. Also, dynamic control of individual QDs 11 and on-demand single-electron transfer between distant quantum dots 13 have been demonstrated. In a more general context, a proposal for onchip quantum transducers based on SAWs enabling long-range coupling of many qubits has been recently put forward 14 . In addition to the band-edge modulation, which determines the QD emission wavelength, the tunable strain field of the SAW can be used to modify other properties related to the band structure, as the exciton binding energy, in a similar way as static strain field 15 . While most of the SAW-related work reported in semiconductor structures refers to III-V systems, studies on group III-Nitrides are scarce. Extension of these techniques to group III-Nitride systems is important, as their large gap enables highpower/high-temperature applications and high repetition rate SPSs covering a broad spectral range. Also, the high sound velocities and the stronger electromechanical coupling coefficients of nitrides, as compared to (Al,Ga

High-resolution monochromated electron energy loss spectroscopy~EELS! at subnanometric spatial resolution and ,200 meV energy resolution has been used to assess the valence band properties of a distributed Bragg reflector multilayer heterostructure composed of InAlN lattice matched to GaN. This work thoroughly presents the collection of methods and computational tools put together for this task. Among these are zero-loss-peak subtraction and nonlinear fitting tools, and theoretical modeling of the electron scattering distribution. EELS analysis allows retrieval of a great amount of information: indium concentration in the InAlN layers is monitored through the local plasmon energy position and calculated using a bowing parameter version of Vegard Law. Also a dielectric characterization of the InAlN and GaN layers has been performed through Kramers-Kronig analysis of the Valence-EELS data, allowing band gap energy to be measured and an insight on the polytypism of the GaN layers.

Theoretical modelling of the intraband absorption spectrum in InAs/GaAs quantum dot infrared photodetectors is performed for several typical structures reported in the literature. The calculations are performed within the framework of the two methods: a simple and so far widely used effective mass method with the values of conduction band offset and the effective mass modified to take account of the effects of strain and band mixing on average and the more realistic eight-band k × p method with the strain distribution taken into account via the continuum mechanical model. Both methods give qualitatively the same results; however, the peak positions obtained within the effective mass approach are blue shifted and the absorption cross sections are overestimated, compared to the more accurate k × p approach.