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Fringe projection profilometry in combination with other optical measuring technologies has established itself over the last decades as an essential complement to conventional, tactile measuring devices. The non-contact, holistic reconstruction of complex geometries within fractions of a second in conjunction with the lightweight and transportable sensor design open up many fields of application in production metrology. Furthermore, triangulation-based measuring principles feature good scalability, which has led to 3D scanners for various scale ranges. Innovative and modern production processes, such as sheet-bulk metal forming, thus, utilize fringe projection profilometry in many respects to monitor the process, quantify possible wear and improve production technology. Therefore, it is essential to identify the appropriate 3D scanner for each application and to properly evaluate the acquired data. Through precise knowledge of the measurement volume and the relative uncertainty with respect to the specimen and scanner position, adapted measurement strategies and integrated production concepts can be realized. Although there are extensive industrial standards and guidelines for the quantification of sensor performance, evaluation and tolerancing is mainly global and can, therefore, neither provide assistance in the correct, application-specific positioning and alignment of the sensor nor reflect the local characteristics within the measuring volume. Therefore, this article compares fringe projection systems across various scale ranges by positioning and scanning a calibrated sphere in a high resolution grid.

Shorter production times, associated with sheet-bulk metal forming (SBMF), require a demand-adapted monitoring and measurement of the formed geometries. In the present work, a multi-scale and multi-sensor setup was developed to meet these requirements. The difficulty in measuring workpieces formed during SBMF are highly reflective areas with varying demands on the measurement resolution of the functional elements. With the developed measurement setup and the integrated sensors used, the functional elements can be measured with sufficient resolution. The registration of the sensors is done with a specially constructed calibration plate. A parallel kinematic hexapod positioning unit provides six degrees of freedom to precisely position the workpiece in front of the individual measurement sensors. The hexapod is therefore registered in a global coordinate system with the new calibration plate. After the calibration, the measured datasets can be merged together automatically. The rigid connection of the calibration plate to the hexapod enables a more demand-oriented verification of the registration between the individual measurements of the sensors. For overexposure due to directly reflecting areas, it is possible to estimate the quality of each reconstructed data point. A deflectometry system, which is attached to the existing fringe projection sensor, makes it possible to detect strongly directionally reflecting areas and thus provide a holistic reconstruction of the workpiece. The influence of remaining lubricant on the workpiece is simulated by a developed thin film thickness standard. This is to adapt the measuring procedure for reconstructions under harsh conditions in industrial environment. This standard allows the measurement of thin films with the existing optical measurement systems in a range

For providing holistic and complete dataset of sheet bulk metal-formed parts, an optical detection with a fringe projection system is useful. By combining several sensors with varying measuring ranges, the workpiece is captured with adapted spatial resolution depending on the forming zone and geometric requirements. The sensors are registered by a two-dimensional point calibration and registration process. The registration process has already been shown in.1 In this article, the calibration procedure is transferred to the measurement setup with three fringe projection sensors with different measurement resolution. A calibration plate with dot patterns is applied to the positioning unit, which is a high precision hexapod in this set-up. This extended calibration plate allows a continuous control of the registration quality as the sensors are able to capture the calibration plate at any time. For determining the positioning accuracy of the hexapod, a procedure following the DIN EN ISO 10360-3 procedure with three spheres mounted on the hexapod was investigated. Therefore the hexapod was fixed at a coordinate measuring machine Zeiss UPMC 1200 and moved in all 6 degrees of freedom. After each movement, the spheres were measured with a probe, which allows the absolute position precision to be calculated. Comparison of the measurements with the specified translation and rotation movement of the hexapod show a positioning precision of less than 10 µm in the positioning directions x and y.

Systematic deviations due to remaining lubricant on the workpiece have a significant influence on the measurement of sheet-bulk metal formed parts. The expected layer thickness for the workpieces after the forming process is less than 35 µm. For the determination of the refractive index of the lubricant and thus the effects of the lubricant on optical measurement techniques, a lubricant thin-film thickness standard was developed which represents a continuous measuring range from 6 to 100 µm. To determine the refractive index, the thin-film thickness standard was measured with a coaxial interferometric measurement system in various thickness ranges. Due to the knowledge of the optical and the geometrical path length, the refractive index can then be determined approximately. In addition, an XY stage was used to scan the entire thin-film area of the standard. The measurement setup in a temperature box allows for determining the effects of temperature changes on the optical properties of the lubricant.

Microcavities with InGaN quantum wells or GaN-based quantum dots as active medium are building blocks of electrically-driven, low-threshold surface-emitting lasers or single photon emitters in the visible-to-UV spectral range. In this chapter, we highlight essential developments in epitaxial growth techniques of such nitride-based microcavities and their active regions. Modern analytical techniques for structural and optical characterization of these complex heterostructures as presented in this chapter are essential to solve remaining challenges.

Higher indium incorporation in self-organized triangular nanoprisms at the edges of InGaN/GaN core-shell nanorods is directly evidenced by spectral cathodoluminescence microscopy in a scanning transmission electron microscope. The nanoprisms are terminated by three 46 nm wide a-plane nanofacets with sharp interfaces forming a well-defined equilateral triangular base in the basal plane. Redshifted InGaN luminescence and brighter Z-contrast are resolved for these structures compared to the InGaN layers on the nanorod sidewalls, which is attributed to at least 4 % higher indium content. Detailed analysis of the inner optical and structural properties reveals luminescence contributions from 417 nm up to 500 nm peak wavelength proving the increasing indium concentration inside the nanoprism towards the nanorod surface.

Zusammenfassung Im Zuge der Erforschung und Weiterentwicklung des Fertigungsverfahrens Blechmassivumformung wird ein geeignetes Messverfahren zur Qualitätsbeurteilung benötigt. Dieses soll aufgrund der Prozessanforderungen eine ganzheitliche fertigungsnahe Prüfung der Bauteile ermöglichen. Dementsprechend ist ein Messsystem erforderlich, das auch in unmittelbarer Prozessnähe zuverlässig Ergebnisse von allen relevanten Bauteilmerkmalen liefert, welche sich signifikant in ihrer Größe und Form unterscheiden. Durch den mehrskaligen Multisensor-Ansatz, der die Kombination mehrerer Sensoren unterschiedlicher Messverfahren, -auflösungen und -bereichen erlaubt, konnte ein Messsystem für die Anforderungen der neuen Fertigungstechnologie entwickelt werden. Ein eigens angepasstes methodisches Vorgehen erlaubte eine strukturierte Messsystementwicklung mit verschiedenen Entwicklungszielen als Zwischenergebnis. Das resultierende prototypische mehrskalige Multisensor-Messsystem, basierend auf drei verschiedenen Streifenprojektionssensoren erlaubt dimensionelle Messungen von Merkmalen, deren Größe den Messbereich eines Sensors überschreitet. Hierdurch werden ganzheitliche Messungen von Bauteilen der neuen Fertigungstechnologie ermöglicht.

For the requirements of newly developed production processes like sheet bulk metal forming a previously developed measuring method exists for dimensional measurement of these manufactured parts. The presented multi-sensor approach allows the combination of several fringe projection sensors with different measurement resolutions and ranges to meet the requirements of the new production technology. The measurement setup includes a high precision hexapod to position the investigated workpiece inside the measuring volume. The measurement devices utilized are high precision fringe projection systems with 17 µm lateral and 1 µm vertical resolution. An additional overview sensor captures the whole measurement range of the hexapod. This paper presents two approaches that can be used for camera calibration and sensor registration of the fringe projection systems in a global coordinate system. A speckle pattern and a random dot pattern are placed on the hexapod to calculate the intrinsic parameters of the sensor camera and extrinsic parameters of the fringe sensor in one single step. To obtain metric lengths with the calibration process, a defined scale is included additionally on the pattern. The calibration process is executed by an automatic and random movement of the hexapod in the recording area of the sensors. Measurements show a direct transferability from the camera to the sensor coordinate system. This allows the measurement datasets to be directly merged together without the use of a separate registration routine.

In this work we show successful metalorganic vapor phase epitaxy (MOVPE) of an AlN/AlGaN distributed Bragg reflector (DBR) that is wavelength matched to GaN quantum dots (QDs) in an AlGaN lambda cavity on top. Full insight into the growth of these structures enables the epitaxy of resonant cavity deep UV single photon emitters. The DBR was grown on an AlN/sapphire template. In order to obtain a high reflectivity as well as a sufficiently large stopband width, the refractive index contrast needs to be maximized. Additionally, the absorption of QD emission in the high gallium containing layer needs to be minimized. A compromise was found for nominal Al-concentration of 70 % in the AlGaN layers. The resulting DBR splits up into self-organized AlN/Al(X)Ga(1-X)N/Al(Y)Ga(1-Y)N trilayers, which add up to desired lambda/2-periods. Therefore, the stopband at 272 nm with a width of 6 nm shows a maximum reflectivity of 99.7 %. GaN QDs were obtained by growth of GaN on AlGaN for 10 s with a V/III-ratio of 30 followed by a growth interruption of 30 s. The QDs exhibit sharp emission lines with a FWHM down to 1 meV in µ-PL measurements. The main intensity of the QD ensemble emission is in the range of 250 nm to 275 nm. Finally, spatially resolved low temperature CL measurements show resonant DBR-enhanced GaN QD emission at 271 nm showing successful wavelength match between a AlN/AlGaN deep UV DBR and GaN QDs in an AlGaN lambda-cavity on top.

Freshly introduced a-screw dislocations in gallium nitride are an effective source of ultraviolet radiation, characterized by intense emission of narrow luminescence doublet lines in the spectral range of 3.1-3.2 eV. Furthermore, an additional narrow spectral line with an energy of 3.3 eV has been found at the points of intersection of such dislocations, where extended dislocation nodes were formed. In this communication, we report on the spectral properties of the characteristic luminescence of such nodes, which were obtained for the (0001) gallium nitride samples with dislocations introduced by nanoindentation. The spectral position of the dislocation-related luminescence doublet experiences a redshift with increasing distance from the indentation site. It follows the spectral shift of the excitonic near-bandgap emission, associated with stress relaxation. The luminescence of the intersection points exhibits a similar tendency. At certain local positions, its doublet fine structure is observed, which has a spectral linewidth of the order of or even less than that of the exciton. In this case, the spectral splitting between components of the doublet varies irregularly depending on the position of the exciton (i.e., on the mechanical stress). We see a clear indication of quantum dot-like emission. The fine structure of the luminescence of the intersection points can be easily explained by the energy dependence of emission on their size, as well as on their density, in particular, by the formation of paired nodes, which were previously observed in experiments in a transmission electron microscope.

We report the growth of green emitting InGaN quantum wells (QWs) by metal-organic vapor-phase epitaxy (MOVPE) on three-dimensional GaN templates. The {10 (1) over bar1} facets of GaN pyramids, fabricated by selective-area growth (SAG), reduce the influence of the quantum-confined Stark effect (QCSE) on the emission properties of the QW. The luminescence properties of a QW are a good indicator for the crystalline quality of the GaN layer beneath. Especially the presence of voids inside the pyramids, as well as stacking faults (SFs) and threading dislocations (TDs) in the wing region strongly influence the strain situation and the incorporation of In into the overlying InGaN layer. Thus, the crystal quality of the GaN pyramid has a strong influence on the efficiency and the emission properties of the active region. Therefore, a low-temperature nucleation on the GaN buffer in conjunction with a decreasing Ga-flux taking the decreasing c-plane growth surface of the pyramid into account was introduced. The low-temperature photoluminescence (PL) properties of the InGaN QW reveal the differences between the standard formation of the pyramid and this modified growth. Cathodoluminescence (CL) and transmission electron microscopy (TEM) measurements confirm the differences between the two growth modes in the crystal quality of the inner part of the pyramid. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Due to recent developments in sheet-bulk metal forming processes, holistic measuring systems are required which provide reliable results for the measurement of all relevant formed part features. The presented multi scale multisensor approach, which combines several fringe projection sensors of different measuring resolutions and ranges, accomplishes the requirements of the new production technology. Calibration of the single measurement systems to a global coordinate system is realized with a specially developed calibration object. A high-precision hexapod positions the calibration object and the workpiece within the measuring range. The result is a holistic dimensional measurement for characteristics whose size exceeds the measuring range of a single sensor. It is also shown that the distance of the measurement sensor to the suface of the measurement object is relevant for the uncertainty of the obtained point cloud. By positioning the workpiece in the focus of camera of the fringe projection system, the standard deviation of the measured point cloud can be significantly reduced.

We systematically studied the desorption induced GaN/AlN quantum dot formation using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM). The GaN films were grown by metal organic vapor phase epitaxy (MOVPE) on top of an AlN/sapphire-template. After the deposition of a few monolayers GaN at 960°C a growth interruption (GRI) without ammonia supply was applied to allow for quantum dot formation. A sample series with GRI durations from 0 s to 60 s was prepared to analyze the temporal evolution systematically. Each quantum dot (QD) structure was capped with AlN grown at 1195°C. Without GRI the cross-sectional STEM images of the reference sample reveal a continuous GaN layer with additional hexagonally-shaped truncated pyramids of 20 nm height and ~100 nm lateral diameter covering dislocation bundles. Spatially averaged spectra exhibit a broad emission band between 260 nm and 310 nm corresponding to the continuous GaN layer. The truncated pyramids exhibit only drastically reduced CL intensity in panchromatic images. Growth interruption leads to desorption of GaN resulting in smaller islands without definite form located in close vicinity to threading dislocations. Now the emission band of the continuous GaN layer is shifted to shorter wavelengths indicating a reduction of GaN layer thickness. By applying 30 s GRI these islands exhibit quantum dot emission in the spectral range from 220 nm to 310 nm with ultra narrow line widths. For longer growth interruptions the QD ensemble luminescence is shifted to lower wavelengths accompanied by intensity reduction indicating a reduced QD density.

We present low temperature cathodoluminescence (CL) characterization of non-polar GaN epitaxial lateral overgrowth (ELO) structures at various growth stages. The a-plane GaN ELO was grown on a-plane GaN template on r-plane sapphire by metal organic chemical vapor deposition (MOCVD). A 50 nm SiO2 mask with 4 µm mask / 6 µm window regions was used for selective growth aligned along the c-direction. Growth was promoted vertically out of the mask openings with a shift to lateral promoting growth by halving the V/III ratio of precursors. Finally, the structures were capped by an AlGaN layer. The distinctly different growth domains of a-plane ELO GaN on stripe masks oriented along c-direction were directly visualized by highly spatially and spectrally resolved cathodoluminescence microscopy. Distinct microscopic regions dominated by differing individual peak wavelengths originating from either basal plane stacking faults, prismatic stacking faults, impurity related donor-acceptor pair or (D0,X) emission as well as yellow luminescence are explicitly correlated to the different growth domains. A strong increase in luminescence intensity from the ELO wings in comparison to the coherently grown region is observed. A 70 nm AlGaN film of 30% Al-concentration was deposited on a coalesced GaN ELO sample and hydride vapor phase epitaxy (HVPE) grown bulk GaN film by MOCVD. A comparison of the luminescence properties was made to probe the growth quality of the overgrown layer and AlGaN/GaN interface. Acknowledgement: This work was supported by the National Science Foundation under Grant no. DMR-1309535.

Growth of nonpolar and semi-polar GaN and GaN-based structures offers the opportunity to reduce quantum confined Stark effect and possibly increase indium incorporation, as compared to polar structures, for enhanced performance in green and longer wavelength light emitters. However, the development of the nonpolar and semi-polar GaN growth is hampered by the lack of suitable substrates. Silicon, despite its large thermal-expansion and lattice mismatch with GaN, provides the advantages of the availability of large-size wafers with high crystalline quality at low cost, good electrical conductivity, and feasibility of its removal through chemical etching for better light extraction and heat transfer. In this article, we overview the recent progress in epitaxial growth of nonpolar and semi-polar GaN-based structures on patterned Si substrates. Also discussed are structural and optical properties of the resulting material.

Polarization-field reduction in c-plane InGaN multi-quantum well (MQW) structures is achieved by pulsed-flow growth of quaternary AlInGaN barriers using metalorganic vapor phase epitaxy (MOVPE). The pulsed-flow growth allows for precise control of the quaternary composition at very low growth rate. In photoluminescence (PL) experiments a blue-shift of the MQW emission wavelength is observed by successive adjustment of the AlInGaN barriers toward reduced polarization mismatch to the InGaN quantum wells. Accordingly, we find a decreasing radiative lifetime by time resolved cathodoluminescence (TRCL). The application of AlInGaN as barrier material instead of conventional GaN is limited by plastic relaxation. While for InGaN-QWs emitting in the violet-blue spectral region, fully polarization-matched AlInGaN barriers can be realized, for long-wavelength (green spectral region) severe lattice relaxation is observed leading to inferior optical properties as compared to binary GaN barriers.

Spatially resolved cathodoluminescence (CL) microscopy is used to study semipolar gallium nitride (GaN) grown out of trenches with c-plane-like sidewalls etched into the r-plane sapphire substrate. The beneficial effect of growing locally c-GaN on c-sapphire and subsequent switching to the semipolar growth mode manifests in bending of threading dislocations (TDs) and a +c-wing of excellent material properties represented by pure donor-bound-exciton emission. The − c-wing, on the other hand, is strongly affected by a huge number of stacking faults and other structural defects. An optimized growth process leads to a delayed coalescence of the single GaN stripes which allows to block the complete − c-wing including all the defects by a void, and coalescence occurs only between the +c-wings consisting of GaN with excellent material quality. The final semipolar GaN surface shows exclusively CL of the donor-bound exciton (D0, X) recombination and no other spectral contribution of structural defects and a reduced TD density. After coalescence and further growth a significant increase of the homogeneity of the CL intensity and emission wavelength is observed for thick samples. CL of an InGaN/GaN active region deposited on top reveals homogeneous luminescence properties over the coalesced region showing no modulation related to the pre-structured growth mode as the defect containing growth domains are effectively blocked.

The optical and crystalline properties of a c-plane GaN-based LED structure with embedded semipolar InGaN quantum wells (QW) were investigated using highly spatially resolved cathodoluminescence spectroscopy (CL) directly performed in a scanning transmission electron microscope (STEM). Direct correlation of the cross-sectional STEM image with the simultaneously recorded spatially resolved CL mapping at room-temperature reveals the most intense emission coming from the semipolar InGaN QWs. We observe an inhomogeneous wavelength distribution due to local indium fluctuations and varying QW thickness. In contrast, the donor-acceptor pair recombination (DAP) becomes the dominating luminescence process at 16 K resulting in a superposition of the DAP luminescence and the InGaN QW emission.

GaN based laser diodes with semipolar quantum wells are typically grown on free-standing pseudo-substrates of small size. We present an approach to create a distributed-feedback (DFB) laser with semipolar quantum wells (QWs) on c-oriented templates. The templates are based on 2-inch sapphire wafers, the method could easily be adapted to larger diameters which are available commercially. GaN nanostripes with triangular cross-section are grown by selective area epitaxy (SAE) and QWs are grown on their semipolar side facets. The nanostripes are completely embedded and can be sandwiched inside a waveguide. For optical pumping, open waveguide structures with only a bottom cladding are used. Using nanoimprint lithography, stripe masks with 250 nm periodicity were fabricated over the whole wafer area. The periodicity corresponds to a 3rd order DFB structure for a laser emitting in the blue wavelength regime. These samples were analyzed structurally by high-resolution transmission electron microscopy (HRTEM), and spatio-spectrally by cathodoluminescence (CL) inside a scanning transmission electron microscope (STEM). Samples with an undoped cap are pumped optically for stimulated emission. To prove the feasibility of realizing a 2nd order DFB structure with this approach, stripes with a 170 nm periodicity are fabricated by electron beam lithography and SAE.

We investigate the emission from confined excitons in the structure of a single-monolayer-thick quasi-two-dimensional (quasi-2D) InxGa1−xN layer inserted in GaN matrix. This quasi-2D InGaN layer was successfully achieved by molecular beam epitaxy (MBE), and an excellent in-plane uniformity in this layer was confirmed by cathodoluminescence mapping study. The carrier dynamics have also been investigated by time-resolved and excitation-power-dependent photoluminescence, proving that the recombination occurs via confined excitons within the ultrathin quasi-2D InGaN layer even at high temperature up to ~220 K due to the enhanced exciton binding energy. This work indicates that such structure affords an interesting opportunity for developing high-performance photonic devices.

This work reports on the growth and characterization of InGaN/GaN core–shell structures by plasma-assisted molecular beam epitaxy on an ordered array of top-down patterned GaN microrods fabricated on a GaN/sapphire substrate. Growth of InGaN/GaN is conformal, with axial and radial growth components leading to core–shell structures with clear hexagonal facets. The radial InGaN growth (shell) was confirmed by spatially resolved cathodoluminescence performed using scanning electron microscopy as well as scanning transmission electron microscopy. The In content of the InGaN shell is controlled by means of the growth temperature and the III/V ratio.

Ordered arrays of very high quality, defect-free GaN nanocolumns were achieved by selective area growth following a two step process involving nanopillar dry etching (top down) and overgrowth by Molecular Beam Epitaxy (bottom up). A study by transmission electron microscopy, over more than 50 individual nanocolumns, confirmed the absence of extended defects, such as dislocations, polarity inversion domain boundaries and stacking faults. Low temperature (10 K) photoluminescence spectrum is dominated by a donor-bound exciton emission line at 3.472 eV with a line width of 0.5 meV. In addition, a distinct emission line from the free-exciton A is observed at 3.479 eV. No traces of emission lines, either at 2.3 eV (Yellow Band); 3.45 eV (also labeled as UX line and recently linked to polarity inversion domain boundaries); or 3.42 eV (stacking faults) were observed.

We present semi-polar GaN light-emitting diode (LED) structures grown on non-patterned Si(112) and Si(113) substrates by metal organic vapor phase epitaxy. Cathodoluminescence and field emission scanning electron microscopy are used for sample characterization. A correlation between the structural and optical properties of the semi-polar GaN LED structures is observed. In samples, which were simultaneously grown on Si(112) and Si(113) under growth conditions optimized for Si(112), we observed that structures on Si(112) consist of a relatively smooth surface but those on Si(113) have a large number of surface pits with a three dimensional growth mode of the GaN layers resulting in a rough GaN surface. The growth conditions were further optimized to obtain smooth GaN layers on Si(113) and compared to the sample on Si(112).

Nitride-based three-dimensional core-shell nanorods (NRs) are promising candidates for the achievement of highly efficient optoelectronic devices. For a detailed understanding of the complex core-shell layer structure of InGaN/GaN NRs, a systematic determination and correlation of the structural, compositional, and optical properties on a nanometer-scale is essential. In particular, the combination of low-temperature cathodoluminescence (CL) spectroscopy directly performed in a scanning transmission electron microscope (STEM), and quantitative high-angle annular dark field imaging enables a comprehensive study of the nanoscopic attributes of the individual shell layers. The investigated InGaN/GaN core-shell NRs, which were grown by metal-organic vapor-phase epitaxy using selective-area growth exhibit an exceptionally low density of extended defects. Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core-shell layers. Most interesting, we observe a red shift of the InGaN single quantum well from 410 to 471 nm along the nonpolar side wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the single quantum well (SQW) from 6 to 13 nm, accompanied by a slight increase of the indium concentration along the nonpolar side wall from 11% to 13%. Both effects, the increased quantum-well thickness and the higher indium incorporation, are responsible for the observed energetic shift of the InGaN SQW luminescence. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich regions with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings that show strong localization effects of the excitonic SQW emission.

Can, Nuri (Balikesir Author), Among the major obstacles for development of non-polar and semipolar GaN structures on foreign substrates are stacking faults which deteriorate the structural and optical quality of the material. In this work, an in-situ SiNx nano-network has been employed to achieve high quality heteroepitaxial semipolar (11 (2) over bar2) GaN on m-plane sapphire with reduced stacking fault density. This approach involves in-situ deposition of a porous SiNx interlayer on GaN that serves as a nano-mask for the subsequent growth, which starts in the nanometer-sized pores (window regions) and then progresses laterally as well, as in the case of conventional epitaxial lateral overgrowth (ELO). The inserted SiNx nano-mask effectively prevents the propagation of defects, such as dislocations and stacking faults, in the growth direction and thus reduces their density in the overgrown layers. The resulting semipolar (11 (2) over bar2) GaN layers exhibit relatively smooth surface morphology and improved optical properties (PL intensity enhanced by a factor of 5 and carrier lifetimes by 35% to 85% compared to the reference semipolar (11 (2) over bar2) GaN layer) which approach to those of the c-plane in-situ nano-ELO GaN reference and, therefore, holds promise for light emitting and detecting devices., Materials World Network grant from the National Science Foundation - DMR-1210282 German Research Foundation, DFG -FOR 957 Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK)

Semipolar InGaN/GaN single quantum wells (SQWs) grown on {1122} planes of an inverted pyramid surface and {1011} facets of single self assembled pyramids have been studied by spatially, spectrally, and time-resolved cathodoluminescence (CL) microscopy. Mappings of local spectra and local transients provide the distribution of spectral and time-resolved lurninescence properties by peak wavelength images, time delayed CL images (TDCLIs), and initial lifetime maps. The SQW on inverse pyramids exhibit strong local differences in recombination kinetics ― two orders of magnitude change in initial lifetime ― correlated with a giant shift in emission energy of ∼1 eV along afacet. For single pyramids a migration process of indium adatoms from the upper facet to the edges leads to an emission of longer wavelengths at the edges and shorter wavelengths at the upper facet with respect to the base.

m -Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m -plane GaN films. The epitaxial relationship was found to be GaN(1 1 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1 1 2 0] || Si[1 1 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.

Pyramidal GaN structures were deposited by selective metal-organic vapor phase epitaxy (MOVPE) to create semipolar facets. On top of these pyramids InGaN was deposited forming nanostructures of different dimensionality according to the varying strain regions along the pyramid. We have studied the emission properties of pyramid ensembles using low-temperature time-resolved photoluminescence (PL) spectroscopy. The observed PL spectra in conjunction with the revealed decay times indicate already the expectations of regions with different confinement on the pyramids. Changing the size of the pyramids and the indium supply lead to variations in the emission wavelength of the ensemble. Spatially resolved cathodoluminescence (CL) experiments were performed to analyze locally the emission properties all over the pyramidal structures. We have found that indeed the indium accumulates at the edges and the apex of the pyramids. Furthermore the emission of the quantum well (QW) sidewalls exhibit also a shift in emission energy which can be attributed again to a change of the In-content or a change of the QW thickness.

Selective-area epitaxy is used to form three-dimensional (3D) GaN structures providing semipolar crystal facets. On full 2-in. sapphire wafers we demonstrate the realization of excellent semipolar material quality by introducing inverse GaN pyramids. When depositing InGaN quantum wells on such a surface, the specific geometry influences thickness and composition of the films and can be nicely modeled by gas phase diffusion processes. Various investigation methods are used to confirm the drastically reduced piezoelectric polarization on the semipolar planes. Complete electrically driven light-emitting diode test structures emitting in the blue and blue/ green spectral regions show reasonable output powers in the milliwatt regime. Finally, first results of the integration of the 3D structures into a conventional laser design are presented.

GaInN/GaN multiple quantum well structures grown on polar and nonpolar surfaces have been compared with respect to the indium incorporation efficiency in the quantum wells (QWs). Under the same growth conditions X-ray diffraction measurements reveal similar growth rates and In concentrations for c-plane, a-plane, and m-plane with In contents up to 40%. These results are in good agreement with optical experiments, in particular for homoepitaxial growth. However, there is strong evidence that the optical properties of the nonpolar heteroepitaxial GaInN QWs are dominated by the high density of stacking faults in those samples.

Semipolar (1122) oriented GaN has been grown on a prestructured r-plane sapphire substrate. By using silicon doped marker layers (MLs) we have been able to monitor the growth evolution of the stripes until coalescence. With that technique we correlated the growth type (direction) with the results of cathodoluminescence (CL) and transmission electron microscopy. Both characterization methods show only a few defects for the major part of the structure and a relatively high defect density for material grown in a-direction at one side of the stripes. It is shown that during coalescence these defects are mainly terminated resulting in a flat, planar (1122) GaN layer with strongly reduced defect density. Additionally, X-ray diffraction (XRD) measurements show the high quality of these layers.

Excellent semipolar GaN material quality can be obtained by growing inverse GaN pyramids on full 2 inch c-plane sapphire when combining the advantages of defect reduction via FACELO and selective area growth for faceted surfaces. The nearly defect free material is obtained by structuring the mask into a honeycomb pattern. When realizing full GaInN/GaN LED structures on those templates a relatively broad emission is observed during electroluminescence measurements. Furthermore, the dominant wavelength is found to be dependent on the applied current density. The pronounced shift is in good agreement with the wavelength shift along one single facet. Gas phase diffusion during the active area growth is believed to be the main reason for the varying indium content. The homogeneity could be influenced by changing the reactor pressure during the active area growth (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InxGa1� xN/GaN quantum well (QW) structures grown on c-plane and m-plane surfaces have been investigated intended for long wavelength light emitters. On c-plane InxGa1� xN QWs reached indium concentrations of xInX35% with good optical and structural quality. For QW thicknesses dQWp 2n m a fully strained layer structure is observed and the indium concentration is quite homogenous. Under the same growth conditions of the QW region we find similar or even slightly larger indium concentrations on m-plane surfaces. QWs of such high indium concentrations, however, are very sensitive to the growth conditions of the subsequent layers and we observe degradation such as indium outdiffusion or partial relaxation for high growth temperatures.

Reduced electric field in semipolar (1122) GaN/InGaN heterostructures makes this orientation attractive for high efficiency light emitting diodes. In this work, we investigated indium incorporation in semipolar (1122) GaN grown by metal-organic chemical vapor deposition on planar m-plane sapphire substrates. Indium content in the semipolar material was compared with that in polar c-plane samples grown under the same conditions simultaneously side by side on the same holder. The investigated samples incorporated dual GaN/InGaN/GaN double heterostructures with 3nm wide wells. In order to improve optical quality, both polar and semipolar templates were grown using an in-situ epitaxial lateral overgrowth (ELO) technique. Indium incorporation efficiency was derived from the comparison of PL spectra measured on the semipolar and polar structures at the highest excitation density, which allowed us to minimize the effect of quantum confined Stark effect on the emission wavelength. Our data suggests increased indium content in the semipolar material by up to 3.0%, from 15% In in c- GaN to 18% In in (1122) GaN.

Enhancement of optical and structural quality of semipolar (11‾22) GaN grown by metal-organic chemical vapor deposition on planar m-sapphire substrates was achieved by using an in-situ epitaxial lateral overgrowth (ELO) technique with nanoporous SiNx layers employed as masks. In order to optimize the procedure, the effect of SiNx deposition time was studied by steady-state photoluminescence (PL), and X-ray diffraction. The intensity of room temperature PL for the (11‾22) GaN layers grown under optimized conditions was about three times higher compared to those for the reference samples having the same thickness but no SiNx interlayers. This finding is attributed to the blockage of extended defect propagation toward the surface by the SiNx interlayers as evidenced from the suppression of emissions associated with basal-plane and prismatic stacking faults with regard to the intensity of donor bound excitons (D 0 X) in lowtemperature PL spectra. In agreement with the optical data, full width at half maximum values of (11‾22) X-ray rocking curves measured for two different in-plane rotational orientations of [1‾100] and [11‾23] reduced from 0.33o and 0.26o for the reference samples to 0.2o and 0.16o for the nano-ELO structures grown under optimized conditions, respectively.

Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) and cross-sectional cathodoluminescence (CL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p-GaN or 1500 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photo-generation near the surface region by above bandgap excitation. Taking into consideration the absorption in the top GaN layer as well as active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be 93 ± 7 nm and 70 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively, at photogenerated carrier densities of 4.2 × 1018 cm−3 using PL spectroscopy. CL measurem...

Pyramidal GaN structures were deposited by selective metal-organic vapor phase epitaxy (MOVPE) to create semipolar facets. On top of these pyramids InGaN was deposited as active region. We have studied the emission properties of these pyramids using low-temperature time-resolved photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. The observed PL spectra revealed that changing the size of the pyramids and the Indium supply leads to variations in the emission wavelength of the ensemble. Spatially resolved cathodoluminescence (CL) experiments were performed to analyze locally the emission properties all over the pyramidal structures. We have found that the Indium accumulates at the edges and the apex of the pyramids for small pyramids and high Indium flows. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The optical quality of semipolar (1 101)GaN layers was explored by time- and polarization-resolved photoluminescence spectroscopy. High intensity bandedge emission was observed in +c-wing regions of the stripes as a result of better structural quality, while -c-wing regions were found to be of poorer optical quality due to basal plane and prismatic stacking faults (BSFs and PSFs) in addition to a high density of TDs. The high optical quality region formed on the +cwings was evidenced also from the much slower biexponential PL decays (0.22 ns and 1.70 ns) and an order of magnitude smaller amplitude ratio of the fast decay (nonradiative origin) to the slow decay component (radiative origin) compared to the -c-wing regions. In regard to defect-related emission, decay times for the BSF and PSF emission lines at 25 K (~ 0.80 ns and ~ 3.5 ns, respectively) were independent of the excitation density within the range employed (5 – 420 W/cm2), and much longer than that for the donor bound excitons (0.13 ns at 5 W/cm2 and 0.22 ns at 420 W/cm2). It was also found that the emission from BSFs had lower polarization degree (0.22) than that from donor bound excitons (0.35). The diminution of the polarization degree when photogenerated carriers recombine within the BSFs is another indication of the negative effects of stacking faults on the optical quality of the semipolar (1101)GaN. In addition, spatial distribution of defects in semipolar (1101)-oriented InGaN active region layers grown on stripe patterned Si substrates was investigated using near-field scanning optical microscopy. The optical quality of -c- wing regions was found to be worse compared to +c-wing regions due to the presence of higher density of stacking faults and threading dislocations. The emission from the +c-wings was very bright and relatively uniform across the sample, which is indicative of a homogeneous In distribution.

Nonpolar m -plane GaN layers were grown on patterned Si (112) substrates by metal-organic chemical vapor deposition (MOCVD). A two-stage growth procedure involving a low-pressure (30 Torr) first stage to ensure formation of the m -plane facets and a high-pressure stage (200 Torr) for improvement of optical quality was employed. The layers grown in two steps show improvement of the optical quality: the near-bandedge photoluminescence (PL) intensity is about 3 times higher than that for the layers grown at low pressure, and deep emission is considerably weaker. However, emission intensity from m- GaN is still lower than that of polar and semipolar ( 1100 ) reference samples grown under the same conditions. To shed light on this problem, spatial distribution of optical emi ssion over the c+ and ci wings of the nonpolar GaN/Si was studied by spatially resolved cathodoluminescence and near-field scanning optical microscopy. Keywords: GaN, nonpolar, m-plane, Si substrate, MOCVD, time-resolved photoluminescence, near-field scanning optical microscopy

Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In 0.15 Ga 0.85 N active layer capped with either 500 nm p-GaN or 1300 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a f unction of the top GaN thickness upon pho togeneration near the surface region by above bandgap excitation. Taking into consideration the absorption in the active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be about 92 7r nm and 68 7r nm for Mg-doped p-type GaN and 432 30r nm and 316 30r nm for unintentionally doped n-type GaN, respectively. Cross-sectional cathodoluminescence line-scan measurement was performed on a separate sample and the diffusion length in n-type GaN was measured to be 280 nm. Keywords: Diffusion length, GaN, Photoluminescence, Cathodoluminescence.

Semipolar (11macron01) GaN layers and GaN/InGaN LED structures were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. Optical properties of the semipolar samples were studied by steady-state and time-resolved photoluminescence (PL). Photon energies and intensities of emission lines from steady-state PL as well as carrier decay times from time-resolved PL were correlated with the distributions of extended defects studied by spatially resolved cathodoluminescence and nearfield scanning optical microscopy. Intensity of donor-bound exciton (DX) emission from both coalesced and non-coalesced semipolar layers is comparable to that of state-of-art c-plane GaN template. To gain insight into the contribution from near surface region and deeper portion of the layers to carrier dynamics in polar c-plane and semipolar (11macron01) GaN, time-resolved PL was measured with two different excitation wavelengths of 267 and 353 nm, which provide different excitation depths of about 50 nm and 100 nm, respectively. Time-resolve PL data indicate that the near-surface layer is relatively free from nonradiative centers (point and/or extended defects), while deeper region of the semipolar film (beyond of ~100 nm in depth) is more defective, giving rise to shorter decay times.

Large arrays of GaN core–shell microrods were fabricated on Si(111) substrates applying a combined bottom-up and top-down approach which includes inductively coupled plasma (ICP) etching of patterned GaN films grown by metal–organic vapor phase epitaxy (MOVPE) and selective overgrowth of obtained GaN/Si pillars using hydride vapor phase epitaxy (HVPE). The structural and optical properties of individual core–shell microrods have been studied with a nanometer scale spatial resolution using low-temperature cathodoluminescence spectroscopy (CL) directly performed in a scanning electron microscope (SEM) and in a scanning transmission electron microscope (STEM). SEM, TEM, and CL measurements reveal the formation of distinct growth domains during the HVPE overgrowth. A high free-carrier concentration observed in the non-polar HVPE shells is assigned to in-diffusion of silicon atoms from the substrate. In contrast, the HVPE shells directly grown on top of the c-plane of the GaN pillars reveal a lower free-carrier concentration.

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Semipolar (1-101) GaN layers were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. The effects of reactor pressure and substrate temperature on optical properties of (1-101) GaN were studied by steadystate and time-resolved photoluminescence. The optical measurements revealed that the optical quality of (1-101)- oriented GaN is comparable to that of c-plane GaN film grown on sapphire. Slow decay time constants, representative of the radiative recombination, for semipolar (1-101)GaN grown at 200 Torr are found to be very long (~1.8 ns), comparable to those for the state-of-art c-plane GaN templates grown using in situ epitaxial lateral overgrowth through silicon nitride nano-network. Defect distribution in the GaN stripes was studied by spatially resolved cathodeluminescence measurements. The c+-wing regions of the GaN stripes were found to be dominated by a (D0,X) emission. Only a thin slice of emission around 3.42 eV related to basal stacking faults was revealed in c--wing regions.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Non-polar (1-100 ) and semipolar (1-101)GaN layers were grown on (112) and (001) Si substrates, respectively, by metalorganic chemical vapor deposition. In both cases, grooves aligned parallel to the Si direction were formed by anisotropic wet etching to expose vertical {111}Si facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied. It was found that low pressure and low ammonia flow rate are beneficial for m-facet formation, while high ammonia flow rate promotes formation of (1-101) facets. Steady-state and time-resolved photoluminescence measurements revealed that the optical quality of (1-101) oriented GaN is comparable to that of c-plane GaN film grown on sapphire. The nonpolar (1-100 ) GaN shows only weak emission and fast non-radiative recombination rate. The poor optical quality of the mplane GaN can be explained by carbon incorporation during the growth under low pressure. Although further optimization of the growth conditions for better optical quality is required, preliminary results obtained for semipolar (1-101) -oriented GaN are encouraging.