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The main results of a complete study by Transmission and Scanning-Transmission Electron Microscopies ((S)TEM) are described for (i) InGaN/Si (111) heterostructures in the whole compositional range of the alloys, and (ii) InN quantum dots (InN QDs) directly grown on Si wafers or on relatively rough InGaN/Si (111) templates. The combination of many (S)TEM-based techniques allowed to evaluate different characteristics of the systems under study: (InN QD/) InGaN/Si and InN QD/Si interfaces and crystal qualities, structural and chemical imperfections and other important features. InxGa1-xN thin films are often identified as single-crystalline, very homogeneous in composition, and mostly wurtzite-type, remarkably at any value of x. Also, (S)TEM techniques revealed that the InN nanostructures were hexagonal single crystals, mostly epitaxial to the supporting lattice. The InN crystals also exhibited partially cubic arrangements when allocated onto In-rich InxGa1-xN (i.e. x > 0.7).

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO

InN quantum dots (QDs) are considered to be promising nanostructures for different device applications. For any hexagonal AB-stacking semiconductor system, polarity is an important feature which affects the electronic properties. Therefore, the determination of this characteristic on any wurtzite (semi)polar III-N compound or alloy is essential for defining its applicability. In this paper, the polarity of InN QDs grown on silicon by indium droplet epitaxy plus nitridation and annealing was determined by a modified approach combining exit wave reconstruction with negative-spherical-aberration high-resolution lattice imaging using TEM. Comparing the micrographs of two QDs from the same TEM specimen with the simulated images of InN slab structures generated under the same conditions as of the experiments, it was confirmed that the QDs of the present study are N polar. Given that the settlement of material's polarity has always been a tedious, indirect and controversial issue, the major value of our proposal is to provide a straightforward procedure to determine the polar direction from atomic-resolution focal series images.

This work presents an experimental and theoretical insight into formation mechanisms of single crystalline wurtzite InN quantum dots (QDs) fabricated via metal droplet epitaxy (DE) by employing plasma assisted molecular beam epitaxy. The applied procedure consists of two fabrication stages. During the first stage, the cold substrate (T ≈ 15 °C) is exposed to an impinging In flux, resulting in formation of metallic In droplets on the substrate surface, and then to an impinging active nitrogen flux, resulting in In conversion into polycrystalline InN islands. During the second stage, the substrate, which is still kept exposed to active nitrogen, is heated up to T ≈ 300 °C, to allow for the reorganization of extended polycrystalline InN islands into groups of independent single-crystalline wurtzite InN QDs. This work provides a detailed experimental insight into both fabrication stages and their qualitative explanations within the scopes of adatom surface kinetics (stage I) and total energy per unit crystal volume minimization (stage II). Finally, the formation mechanisms of InN QDs on the three different substrates (Si(1 1 1), Si(0 0 1) and In0.3Ga0.7N/Si(1 1 1)) are compared, and also linked to the formation mechanisms of other more studied nanostructures, such as self-assembled GaN/AlN QDs and self-assembled and selective-area-grown GaN nanowires.

The three-dimensional structure of GaN/(In,Ga)N core-shell nanowires with multi-faceted pencil-shaped apex is analyzed by electron tomography using high-angle annular dark-field mode in a scanning transmission electron microscope. Selective area growth on GaN-on-sapphire templates using a patterned mask is performed by molecular beam epitaxy to obtain ordered arrays of uniform nanowires. Our results of the tomographic reconstruction allow the detailed determination of the complex morphology of the inner (In,Ga)N multi-faceted shell structure and its deviation from the perfect hexagonal symmetry. The tomogram unambiguously identifies a dot-in-a-wire configuration at the nanowire apex including the exact shape and size, as well as the spatial distribution of its chemical composition. Electronic supplementary material The online version of this article (10.1186/s11671-019-3072-1) contains supplementary material, which is available to authorized users.

This work presents a comparative study, based mainly on X-ray diffraction analysis, of compact (~100 nm thick) and uniform single crystal InGaN layers (In content

We study the impact of increased active nitrogen flux (N-flux) on the indium content and structural properties of InAlN layers grown by plasma-assisted molecular beam epitaxy. It is shown that high N-flux can stabilize In-N bonds, so that In0.18Al0.82N is grown at 605 °C, which is the highest reported temperature so far for the composition lattice-matched (LM) to GaN. A diagram of InAlN indium content is shown as a function of growth temperature and N-flux. The InAlN layers grown using low and high N-flux had grainy surface morphology typical for N-rich conditions. Inhomogeneity in indium distribution on nanometer scale, i.e. typical honeycomb microstructure, is found for InAlN layers grown using both: low and high N-flux. An increase of average cell size is observed for LM-InAlN when the N-flux and growth temperature are increased.

Although it is well known that structural disorder has a strong impact on a resonant optical device's performance, the exact relationship still needs clarification. This article presents the effective refractive-index approximation, an elegant method for simple, quantitative, and comprehensive insight into the link between a resonator's structural disorder and the consequent deterioration of its optical performance. The proposed method is validated both theoretically, against transfer-matrix simulations, and experimentally, by comparison to measured properties of highly disordered Bragg reflectors.

The impact of using thin GaAs(Sb)(N) capping layers (CLs) on InAs/GaAs quantum dots (QDs) is investigated for their application in solar cell devices. We demonstrate the ability to combine strain-balancing techniques with band engineering approaches through the application of such CLs. Extended photoresponse is attainable by means of an independent tunability of the electron and hole confinements in the QD. Moreover, the CL acts itself as a quantum well (QW), providing an additional photoresponse, so that the devices work as hybrid QD–QW solar cells. The use of a GaAsSb CL is particularly beneficial, providing devices with efficiencies under AM1.5 conditions 20% higher than standard GaAs-capped QDs. This is mainly due to a significant increase in photocurrent beyond the GaAs bandgap, leading to an enhanced short-circuit current density ( J sc ). The addition of N to the CLs, however, produces a strong reduction in J sc . This is found to be related to carrier collection problems, namely, hindered electron extraction and retrapping in the CLs. Nevertheless, the application of reverse biases induces a release of the trapped carriers assisted by a sequential tunneling mechanism. In the case of GaAsN CLs, this leads to a complete carrier collection and reveals an even higher QD–QW-related photocurrent than when using a GaAsSb CL. The hindered carrier collection is stronger in the case of the quaternary CLs, likely due to the faster recombination rates in the type-I GaAsSbN/GaAs QW structure as compared to the type-II ternary counterparts. Nevertheless, alternative approaches, such as the use of a thinner CL or a short-period superlattice CL, lead to significant improvements, demonstrating a great potential for the quaternary CLs under a proper device design.

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.

The composition, strain and surface morphology of (0001)InGaN layers are investigated as a function of growth temperature (460–645 °C) and impinging In flux. Three different growth regimes: nitrogen-rich, metal-rich and intermediate metal-rich, are clearly identified and found to be in correlation with surface morphology and strain relaxation. Best epilayers’ quality is obtained when growing under intermediate metal-rich conditions, with 1–2 monolayers thick In ad-coverage. For a given In flux, the In incorporation decreases with increasing growth temperature due to InN thermal decomposition that follows an Arrhenius behavior with 1.84±0.12 eV activation energy.

High-resolution monochromated electron energy loss spectroscopy (EELS) at subnanometric spatial resolution and

This work reports on the benefits from using thin GaAsSb capping layers (CLs) on InAs/GaAs quantum dot (QD) solar cells. The application of such CLs allows the tunability of the QD ground state, switching the QD-CL band alignment from type I to type II for high Sb contents and extending the photoresponse beyond 1.5 µm. Two different structures with ~10% and ~20% Sb contents in the CL (type-I and type-II band alignments, respectively) are explored, leading to efficiency improvements over a reference InAs/GaAs QD solar cell of 20% and 10%, respectively. In general, a significant increase in short-circuit current density (Jsc) is observed, partially due to the extended photocurrent spectrum and the additional contribution of the CL itself. Particularly, for a moderate Sb content, an improved carrier collection efficiency is also found to be a main reason for the Jsc increase. Calculations from an 8×8 k·p method suggest the attribution of such an improvement to longer carrier lifetimes in the wetting layer-CL structure due to the transition to a type-II band alignment. Open-circuit voltages (Voc) exceeding that of a reference QD solar cell are demonstrated under light concentration using GaAsSb CLs, which proves that the Voc is not limited by the low bandgap CLs. Moreover, the highest value is obtained for the high Sb content type-II structure, despite the higher accumulation of strain and the lower effective bandgap. Indeed, the faster Voc increase with light power found in the latter case leads to an Voc even larger than the effective bandgap., We acknowledge the Spanish MICINN-MINECO for funding through project MAT2013-47102-C2-2-R. JMLL and IA acknowledge the financial support of the Comunidad Autónoma de Madrid (Grant S2013/MAE-2780), TEC2015-64189-C3-2-R (MINECO/FEDER) and MINECO (Grant AIC-B_2011-0806).

In this manuscript we carry out a comparative analysis of p-i-n junction solar cells based on 10 stacks of InAs/GaAs quantum dots (QDs) capped with GaAs(Sb)(N) capping layers (CLs). The application of such CLs allows to significantly extend the photoresponse beyond 1.3 μm. Moreover, a strong photocurrent from the CLs is observed so that the devices work as QD-quantum well solar cells. The GaAsSb CL leads to the best results, providing a strong sub-band-gap contribution, which is higher than that in a sample containing standard GaAs-capped QDs, despite giving rise to the highest accumulated strain. The use of a GaAsN CL reduces the photocurrent originating from GaAs, pointing to electron retrapping and hindered extraction and/or the introduction of point defects as possible reasons for this. Nevertheless, the addition of N helps to balance the accumulated strain, necessary to stack a higher number of QD layers. In addition, the possibility to independently tune the hole and electron confinements by the simultaneous presence of Sb and N in the CL is also confirmed for 10 stacked QD layers. This not only allows to further extend the QD ground state and, therefore, the photoresponse, but also offers the possibility to design an optimized structure facilitating carrier extraction from the QDs. Nevertheless, carrier losses seem to be stronger under the simultaneous presence of N and Sb in the CL.

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.

In this work, through a comparative study of self-assembled (SA) and selective area grown (SAG) (In)GaN nanocolumn (NC) ensembles, we first give a detailed insight into improved crystallographic uniformity (homogeneity of crystallographic tilts and twists) of the latter ones. The study, performed making use of: reflective high energy electron diffraction, X-ray diffraction and scanning electron microscopy, reveals that unlike their SA counterparts, the ensembles of SAG NCs show single epitaxial relationship to both sapphire(0001) and Si(111) underlying substrates. In the second part of the article, making use of X-ray diffraction, we directly show that the selective area growth leads to improved compositional uniformity of InGaN NC ensembles. This further leads to improved spectral purity of their luminescence, as confirmed by comparative macro-photoluminescence measurements performed on SA and SAG InGaN NC ensembles. An improved crystallographic uniformity of NC ensembles facilitates their integration into optoelectronic devices, whereas their improved compositional uniformity allows for their employment in single-color optoelectronic applications.

III-V nitride (AlGa)N distributed Bragg reflector devices are characterized by combined high-angle annular dark-field (HAADF) and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Besides the complete structural characterization of the AlN and GaN layers, the formation of AlGaN transient layers is revealed using Vegard law on profiles of the position of the bulk plasmon peak maximum. This result is confirmed by comparison of experimental and simulated HAADF intensities. In addition, we present an advantageous method for the characterization of nano-feature structures using low-loss EELS spectrum image (EEL-SI) analysis. Information from the materials in the sample is extracted from these EEL-SI at high spatial resolution.The log-ratio formula is used to calculate the relative thickness, related to the electron inelastic mean free path. Fitting of the bulk plasmon is performed using a damped plasmon model (DPM) equation. The maximum of this peak is related to the chemical composition variation using the previous Vegard law analysis. In addition, within the context of the DPM, information regarding the structural properties of the material can be obtained from the lifetime of the oscillation. Three anomalous segregation regions are characterized, revealing formation of metallic Al islands.

Series of GaN nanowires (NW) with controlled diameters (160–500 nm) and heights (420–1100 nm) were homoepitaxially grown on three different templates: GaN/Si(111), GaN/AlN/Si(111), and GaN/sapphire(0001). Transmission electron microscopy reveals a strong influence of the NW diameter on dislocation filtering effect, whereas photoluminescence measurements further relate this effect to the GaN NWs near-bandgap emission efficiency. Although the templates' quality has some effects on the GaN NWs optical and structural properties, the NW diameter reduction drives the dislocation filtering effect to the point where a poor GaN template quality becomes negligible. Thus, by a proper optimization of the homoepitaxial GaN NWs growth, the propagation of dislocations into the NWs can be greatly prevented, leading to an exceptional crystal quality and a total dominance of the near-bandgap emission over sub-bandgap, defect-related lines, such as basal stacking faults and so called unknown exciton (UX) emission. In additi...

We report on properties of high quality ~60 nm thick InAlN layers nearly in-plane lattice-matched to GaN, grown on c-plane GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy. Excellent crystalline quality and low surface roughness are confirmed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. High annular dark field observations reveal a periodic in-plane indium content variation (8 nm period), whereas optical measurements evidence certain residual absorption below the band-gap. The indium fluctuation is estimated to be +/- 1.2% around the nominal 17% indium content via plasmon energy oscillations assessed by electron energy loss spectroscopy with sub-nanometric spatial resolution.

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 (similar to 10(10) cm(-3)). 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.

We demonstrate six to ten period lattice-matched In0.18Al0.82N/GaN distributed Bragg reflectors with peak reflectivity centred around 400 nm, grown by molecular beam epitaxy. Thanks to the well-tuned ternary alloy composition crack-free layers have been obtained as confirmed by both optical and scanning electron microscopy. In addition, cross-sectional analysis by high resolution transmission electron microscopy reveals highly periodic structure with abrupt interfaces. When the number of DBRs periods increased from six to ten, peak reflectivity increased from 45% to 60%. This increase was found to be in reasonable agreement with theoretical simulations. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Indium incorporation and surface morphology of InAlN layers grown on (0001) 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.

A uniformly n-type doped GaN:Si nanowire (NW), with a diameter of d = 90 nm and a length of 1.2 μm, is processed into a metal-semiconductor field effect transistor (MESFET) with a semi-cylindrical top Ti/Au Schottky gate. The FET is in a normally-ON mode, with the threshold at −0.7 V and transconductance of gm ∼ 2 μS (the transconductance normalized with NW diameter gm/d > 22 mS/mm). It enters the saturation mode at VDS ∼ 4.5 V, with the maximum measured drain current IDS = 5.0 μA and the current density exceeding JDS > 78 kA/cm2.

This work gives a detailed insight into how the formation of (Al,Ga)N transient layers (TLs) at the interfaces of AlN/GaN Bragg reflectors modifies their structural and optical properties. While abrupt AlN/GaN interfaces are typically characterized with a network of microcracks, those with TLs are characterized with a network of nanocracks. Transmission electron microscopy reveals a strong correlation between strain and the TLs thickness, identifying thus the strain as the driving force for TLs formation. The AlN/GaN intermixing preserves the targeted stopband position (∼410 nm), whereas the peak reflectivity and the stopband width are both reduced, but still significantly high: >90% and >30 nm, respectively. To model their optical properties, a reduced refractive index contrast approximation is used, a novel method which yields an excellent agreement with the experiment.

Developer-based wet chemical etch of nearly lattice-matched InAlGaN/GaN heterostructures (HEMT-like) has been studied in detail by means of Rutherford backscattering spectroscopy, x-ray diffraction, atomic force microscopy and reciprocal space mapping (RSM). Etch isotropy depended on the rms surface roughness of the as-grown material. The profiles of etched samples varied in crack density, giving rise to island-like structures. We found that a possible reason for the preferential etching can be ascribed to the dislocations present in the quaternary layers originating in the underlying GaN. Moreover, the etched material suffers crystal relaxation as confirmed by RSM.

Ten-period InAlN/GaN distributed Bragg reflectors are examined by aberration corrected scanning transmission electron microscopy and by valence electron energy-loss spectroscopy (VEELS) with sub-nanometric spatial resolution and sub-eV energy dispersion. Deconvolution and peak subtraction methods, implemented in Matlab routines, are applied to the low loss region of the obtained VEEL spectra to retrieve information about the band gap energy and chemical composition, whereas a Kramers-Kronig transformation is used to retrieve the complex dielectric function of the examined material. The VEEL measurements reveal significant compositional variations in InAlN layers and show a ~2nm thick InAlN layer with high indium content at each GaN/InAlN interface.

We report on molecular beam epitaxy growth and characterization of ten-period lattice-matched InAlN/GaN distributed Bragg reflectors (DBRs), with peak reflectivity centered around 400 nm. Thanks to the well tuned ternary alloy composition, crack-free surfaces have been obtained, as confirmed by both optical and transmission electron microscopy (TEM). Their good periodicity and well-defined interfaces have been confirmed by both x-ray diffraction and TEM measurements. Peak reflectivity values as high as 60% with stop bands of 30 nm have been demonstrated. Optical measurements revealed that discrepancy between the obtained (60%) and the theoretically expected (∼75%) reflectivity is a consequence of significant residual absorption (∼35%). TEM measurements revealed the coexistence of zinc-blende and wurtzite phases, as well as planar defects, mainly in GaN. These defects are suggested as the potential source of the undesired absorption and/or scattering effects that lowered the DBRs’ peak reflectivity.

We present a detailed examination of a multiple InxGa1−xN quantum well (QW) structure for optoelectronic applications. The characterization is carried out using scanning transmission electron microscopy (STEM), combining high-angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS). Fluctuations in the QW thickness and composition are observed in atomic resolution images. The impact of these small changes on the electronic properties of the semiconductor material is measured through spatially localized low-loss EELS, obtaining band gap and plasmon energy values. Because of the small size of the InGaN QW layers additional effects hinder the analysis. Hence, additional parameters were explored, which can be assessed using the same EELS data and give further information. For instance, plasmon width was studied using a model-based fit approach to the plasmon peak; observing a broadening of this peak can be related to the chemical and structural inhomogeneity in the InGaN QW layers. Additionally, Kramers-Kronig analysis (KKA) was used to calculate the complex dielectric function (CDF) from the EELS spectrum images (SIs). After this analysis, the electron effective mass and the sample absolute thickness were obtained, and an alternative method for the assessment of plasmon energy was demonstrated. Also after KKA, the normalization of the energy-loss spectrum allows us to analyze the Ga 3d transition, which provides additional chemical information at great spatial resolution. Each one of these methods is presented in this work together with a critical discussion of their advantages and drawbacks.

InxGa1−xN is a promising material for flexible and efficient water-splitting photoelectrodes since the bandgap is tunable by modifying the indium content. We investigate the potential of an InxGa1−xN/Si tandem used as a water-splitting photoelectrode. We predict a maximum theoretical photogeneration efficiency of 27% for InxGa1−xN/Si tandem photoelectrodes by computing electromagnetic wave propagation and absorption. This maximum is obtained for an indium content between 50% and 60% (i.e., a bandgap between 1.4 eV and 1.2 eV, respectively) and a film thickness between 280 nm and 560 nm. We then experimentally assess InxGa1−xN photoanodes with the indium content varying between 9.5% and 41.4%. A Mott–Schottky analysis indicates doping concentrations (which effectively represent defect density, given there was no intentional doping) above 8.1 × 1020 cm−3 (with a maximum doping concentration of 1.9 × 1022 cm−3 for an indium content of 9.5%) and flatband potentials between −0.33 VRHE for x = 9.5% and −0.06 VRHE for x = 33.3%. Photocurrent–voltage curves of InxGa1−xN photoanodes are measured in 1M H2SO4 and 1M Na2SO4, and the incident photon-to-current efficiency spectra in 1M Na2SO4. The incident photon-to-current efficiency spectra are used to computationally determine the diffusion length, the diffusion optical number, as well as surface recombination and transfer currents. A maximum diffusion length of 262 nm is obtained for an indium content of 23.5%, in part resulting from the relatively low doping concentration (9.8 × 1020 cm−3 at x = 23.5%). Nevertheless, the relatively high surface roughness (rms of 7.2 nm) and low flatband potential (−0.1 VRHE) at x = 23.5% cause high surface recombination and affect negatively the overall photoelectrode performance. Thus, the performance of InxGa1−xN photoelectrodes appears to be a tradeoff between surface recombination (affected by surface roughness and flatband potential) and diffusion length (affected by doping concentration/defect density). The performance improvements of the InxGa1−xN photoanodes are most likely achieved through modification of the doping concentration (defect density) and reduction of the surface recombination (e.g., by the deposition of a passivation layer and co-catalysts). The investigations of the ability to reach high performance by nanostructuring indicate that reasonable improvements through nanostructuring might be very challenging.

A perfect invisible lens (within the Geometrical Optics approximation) made of isotropic spherical-graded-index material is introduced. This unique lens is compared with other known invisible lenses (Pendry-Schurig-Smith’s and Leonhart’s).

A 3 lambda/2 (In,Ga)N/GaN resonant cavity, designed for similar to 415nm operation, is grown by molecular beam epitaxy and is sandwiched between a 39.5-period (In,Al)N/GaN distributed Bragg reflector (DBR), grown on c-plane GaN-on-sapphire pseudo-substrate by metal-organic vapor phase epitaxy and an 8-period SiO2/ZrO2 DBR, deposited by electron beam evaporation. Optical characterization reveals an improvement in the cavity emission spectral purity of approximately one order of magnitude due to resonance effects. The combination of spectrophotometric and micro-reflectivity measurements confirms the strong quality (Q)-factor dependence on the excitation spot size. We derive simple analytical formulas to estimate leak and residual absorption losses and propose a simple approach to model the Q-factor and to give a quantitative estimation of the weight of cavity disorder. The model is in good agreement with both transfer-matrix simulation and the experimental findings. We point out that the realization of high Q-factor (In,Ga)N containing microcavities on GaN pseudo-substrates is likely to be limited by the cavity disorder. (C) 2013 AIP Publishing LLC.

This work reports an experimental and theoretical insight into phenomena of two-color emission and different electron-hole recombination dynamics in InGaN nanodisks, incorporated into pencil-like GaN nanowires. The studied nanodisks consist of one polar (on $c$ facet) and six (nominally) identical semipolar (on $r$ facets) sections, as confirmed by transmission electron microscopy. The combination of cathodoluminescence with scanning electron microscopy spatially resolves the nanodisk two-color emission, the low-energy emission (\ensuremath{\sim}500 nm) originating from the polar section, and the high-energy emission (\ensuremath{\sim}400 nm) originating from the semipolar section. This result has been directly linked to a ``facet-dependent'' nanodisk composition, the In content being significantly higher in the polar ($\ensuremath{\sim}20%$) vs semipolar ($\ensuremath{\sim}10%$) section (as quantified by energy dispersive x-ray spectroscopy), further leading to a strong facet-dependent strain anisotropy. Time-resolved cathodoluminescence reveals significantly different electron-hole recombination times in the two sections, moderately fast (\ensuremath{\sim}1.3 ns) vs fast (\ensuremath{\sim}0.5 ns) in polar/semipolar sections, respectively, the difference being linked to a strong anisotropy in the nanodisk internal electric fields. To determine the influence of each of the three contributing ``facet-related'' anisotropies (composition, strain, and electric field) on the two-color emission, a proper simulation [relying on virtual crystal approximation and involving three-dimensional (3D) continuum mechanical modeling, a 3D Poisson equation, and a one-dimensional Schr\"odinger equation] has been performed. The theoretical simulations allow the three effects to be quantitatively disentangled, revealing a clear hierarchy among their contributing weights, the facet-dependent composition inhomogeneity being identified as the dominant one (and the strain inhomogeneity being identified as the least significant one). As for different recombination times, while it is mainly linked to the internal electric field anisotropy, we also suggest that it is, very likely, influenced by gradually increasing In content along the nanodisk growth direction (lattice-pulling effect); the latter mechanism keeps electrons and holes in (relative) proximity within the polar section, enabling their relatively fast and efficient radiative recombination.