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The reaction of GaN to ion implantation was studied in thin films grown on the a-plane (non-polar) and on the c-plane (polar). 300 keV argon ions were implanted at room temperature to fluences ranging from 5 × 1012 atoms/cm2 to 8 × 1015 atoms/cm2. The structural analysis was performed using Rutherford Backscattering/Channeling and X-Ray Diffraction. The results allow to reinforce the suggestion that perpendicular strain caused by ion implantation is the driving force behind defect transformation processes inside the lattice. Furthermore, they confirm a lower relative damage level for a-plane GaN implanted with the highest fluence, in comparison with c-plane GaN, as reported previously for low temperature implantation.

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments., publishedVersion

Group III-nitride quantum well (QW) structures have recently been proved as promising (bio) chemical transducers with purely optical read-out [1]. The InGaN QW structures with pure optical read-out overcome the limitations imposed by electrical devices with contacts vulnerable to aggressive chemicals and the high cost and fabrication difficulties of sophisticated lithographic and epitaxial methods [2, 3]. The (bio) chemical sensing mechanism relies on spectral shift of the quantum well photoluminescence that can be observed with varying surface modification. The spectral shift can be attributed to an externally modulated Quantum Confined Stark Effect (QCSE) caused by the adsorbed species deposited on the quantum well structure surface. Optimization of this spectral shift experimentally requires extensive time and cost that can be truly reduced by simulating the structure using Technology Computer Aided Design (TCAD) tools.

In this paper, investigations on gallium indium nitride (GaInN) quantum well structures as optochemical transducers in (bio)chemical sensing are presented. In contrast to the conventional electrical read-out of III-nitride-based sensors, a purely optical photoluminescence read-out is performed. A significant spectral shift of the quantum well photoluminescence is observed with varying surface modification. The spectral photoluminescence shift can be attributed to an externally induced quantum confined Stark effect caused by the adsorbed species deposited on the quantum well surface. In order to improve the sensitivity of the transducer elements, different chemical surface treatments are studied. In particular, optical sensing experiments with reducing and oxidizing gases are performed in order to investigate the quantum well photoluminescence response. Additionally, optical investigations of the iron-storage molecule ferritin with varying iron load are presented. The iron load of this molecule is generally considered as a superior biomarker for severe illnesses, such as Alzheimer's disease. In contrast to conventional fluorescent labels, GaInN quantum wells provide a much more stable luminescence signal, and hence, are promising candidates for next generation bioanalytical sensor structures.

International audience; Epitaxial GaN layers with a-, c- and m-plane surface orientations were implanted with 300 keV Ar-ions at 15 K with fluences ranging from 2 × 1012 to 4 × 1016 at/cm2. Damage build-up proceeds in three steps separated by wide fluence regions where the maximum damage level, measured by in situ Rutherford Backscattering Spectrometry/Channelling, saturates. The three steps occur at similar fluences for the three crystal orientations and similar defect formation rates for the lowest fluences are observed. Surprisingly, the second saturation regime reveals a significantly lower damage level in a-plane layers while m- and c-plane samples suffer more than 4 times higher damage levels. The strong radiation resistance of a-plane GaN was attributed to very efficient dynamic annealing of point defects during the implantation even at 15 K. The migration and aggregation of point defects also lead to distinct defect microstructures as evidenced by transmission electron microscopy. Besides point defects and their clusters the dominant extended defects caused by implantation in c-plane GaN are basal stacking faults while dislocation loops are formed in a-plane material. m-plane GaN presents a mixture of planar defects and dislocation loops after implantation.

In order to clarify the reasons for the fairly poor electroluminescence (EL) performance of semipolar LED structures grown on patterned sapphire wafers, we have analyzed both, pure photoluminescence (PL) test structures without doping only containing 5 GaInN quantum wells and full EL test structures, all emitting at a wavelength of about 510 nm. Evaluating the PL intensity over a wide range of temperatures and excitation powers, we conclude that such quantum wells possess a fairly large internal quantum efficiency of about 20%. However, on EL test structures containing nominally the same quantum wells, we obtained an optical output power of only about 150μW at an applied current of 20 mA. This may be due partly to some thermal destruction of the quantum wells by the overgrowth with p-GaN. Even more important seems to be the not yet finally optimized p-doping of these structures.

In this article, the influence of the trench period and depth of pre-structured r-plane sapphire substrates on the metalorganic vapor phase epitaxy (MOVPE) growth of ( 11 2 ¯ 2 ) GaN is investigated. We found that a larger trench period is beneficial for a small basal plane stacking fault (BSF) and threading dislocation (TD) density on the wafer surface, because it facilitates a better formation of a coalescence gap, which effectively blocks defects from penetrating to the surface. Further, the amount of BSFs emerging from the −c-wings of the uncoalesced GaN stripes is directly related to the trench period. With the help of in situ deposited marker layers we studied the development of the individual GaN stripes and observed that the trench depth and hence the ratio of the sapphire c-plane area relative to the total surface area heavily influences the coalescence process and defect development. Moreover, it is observed that the parasitic donor concentration increases for samples with smoother wafer surface.

A stack of five Al(Ga)N-based quantum wells is investigated by combined laterally and depth resolved cathodoluminescence (CL) spectroscopy in order to distinguish lateral and vertical inhomogeneities of these wells. Transmission electron microscopy (TEM) micrographs provide data for the real sample structure, which enters into the Monte-Carlo simulation of the depth-resolved CL measurements to refine the depth resolution. The comparison of these CL measurements to the results of electron energy loss spectra (EELS) allows to identify local thickness variations of the lower three quantum wells to be the origin of two different luminescence contributions to the overall spectrum. The differentiation of the two groups of quantum wells by depth-resolved CL is demonstrated.

AlGaN-based LEDs for UV-C-light emission still suffer from relatively poor efficiency. Besides problems with carrier injection and light extraction, strong piezoelectric fields in the optically active region originating from lattice mismatch between quantum wells and barrier material are a major issue. Mixing only few percent of boron into the AlGaN active region may be sufficient to achieve lattice matched conditions, thus decreasing the influence of the quantum-confined Stark effect on the radiative recombination efficiency. However, the epitaxial growth of AlBGaN layers with sufficient crystalline quality is still a challenge, particularly due to the low solubility of boron in AlGaN and the low mobility of boron ad-atoms on the surface. Consequently, only extremely weak luminescence has been reported on layers containing few percents of boron. By thoroughly optimizing the metalorganic vapor phase epitaxial growth of AlBGaN layers with a boron content of some percent, we could achieve similar luminescence intensities as for reference AlGaN layers along with smooth hetero-interfaces and low surface roughness as measured by TEM and AFM. Besides studying the influence of basic growth parameters like temperature, V-III ratio etc., we investigate possible improvements by an optimized pulsed precursor supply sequence. To reduce the unintentional doping with impurities like oxygen or carbon, typically attributed to the standard boron precursor tri-ethyl boron (TEB), we investigate the novel metalorganic precursor tri-isopropyl-boron (TiPB). Its lower vapor pressure as compared to TEB facilitates a controlled incorporation of small B amounts. First PL spectra of AlBGaN layers grown with TiPB show promising data.

By utilizing a novel three-layer resist process, InAIN/AIN/GaN T - and T -gate high electron mobility transistors with 0.1 μm gate lengths and below have been demonstrated. This process is based on direct electron-beam lithography with a single exposure step. Furthermore, the effect of different T -gate shapes on RF power performance is reported. A T -gate shift to the source side of the Ohmic contact, results in lower gate-to-drain capacitance and in higher transistor RF transducer gain G t .

The electroluminescence (EL) output power of c-plane InGaN/GaN-based light-emitting diodes (LEDs) is much higher than that of semipolar and LEDs at the same operation current. In order to elucidate the reasons for this behavior, we have fitted the pulsed EL data by the well-known ABC model to extract the internal quantum efficiency (IQE) and the carrier injection efficiency (CIE) to clarify which parameter weighs more for the poor EL output power of the semipolar LEDs. The CIE shows large differences, 78%, 4%, and 4% for the c-plane, and LEDs, respectively, whereas the IQE values are fairly the same for all three structures. The fit of resonant photoluminescence (PL) data at room temperature confirms the similar IQE values for all three structures. The CIE was increased from 4% to 10% for the planar LED with better electrical conductivity of the p-GaN layer.

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.

It is demonstrated that the threading dislocation density in GaN is considerably reduced by Facet Assisted Epitaxial Lateral Overgrowth (FACELO) on a hexagonal honeycomb grid structure. We observed a more isotropic strain and curvature development in the GaN layer by such a mask geometry. Here, we describe how to achieve a nearly dislocation-free surface by a fairly complex variation of the epitaxial growth parameters. Eventually, dislocation analysis of epitaxially grown GaN using an HCl vapor phase etching process resulted in dislocation densities below 10 6 cm − 2 .

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.

This paper reviews our recent investigations about semipolar GaN-based optoelectronic heterostructures grown on foreign substrates. Two basically different approaches are discussed, both making use of epitaxial growth in the polar c-direction to minimize any crystalline defects. By selective area growth, stripes with triangular cross-section have been formed with semipolar side-facets, on which quantum well and electroluminescence test structures have been deposited. By careful optimisation of many growth parameters, we could drastically increase the growth temperature of GaInN quantum wells emitting beyond 500 nm. In the second approach, the GaN growth starts on inclined sapphire c-planes, which form the side facets of trenches etched into the substrates. After coalescence, planar semipolar GaN layers can be achieved. We investigated various sapphire wafer orientations leading to {112‾2}, {101‾1}, and {202‾1} layers. After careful optimisation with a major focus on the decrease of the stacking fault density, we have also investigated the doping behaviour of such semipolar structures. Eventually, full electroluminescence test structures could be grown.

In this contribution, we demonstrate that for non-and semipolar AlInN one-dimensionally lattice-matched to GaN, controlled relaxation in different in-plane directions of the growth surface can be induced. All of our samples were grown on (11 (2) over bar2) GaN templates on patterned r-plane sapphire as well as on moriented 6H-SiC substrates and free-standing pseudo-bulk GaN substrates. The InN mole fraction of the Al1-xInxN is varied from 18% up to 28% corresponding to one-dimensional lattice matching in various in-plane directions of the growth surface. By controlling the composition as well as the thickness, we realized one-dimensional relaxation of the AlInN layers along the nonlattice-matched direction. This opens new perspectives for strain and polarization engineering in optically active layers grown subsequently and gives new possibilities to design high indium-containing GaInN quantum well structures for efficient long wavelength light emitters. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Group III nitrides are promising materials for light emitting diodes (LEDs). The occurrence of structural defects strongly affects the efficiency of these LEDs. We investigate the optical properties of basal plane stacking faults (BFSs), and the assignment of specific spectral features to distinct defect types by direct correlation of localized emission bands measured by cathodoluminescence in a scanning electron microscope with defects found in high resolution (scanning) transmission electron microscopy and electron beam induced current at identical sample spots. Thus, we are able to model the electronic structure of BSFs addressing I1, I2, and E type BSFs in GaN and AlGaN with low Al content. We find hints that BSFs in semipolar AlGaN layers cause local changes of the Al content, which strongly affects the usability of AlGaN as an electron blocking layer in nitride based LEDs.

Polar and semipolar LED structures with InGaN quantum wells (QWs) were investigated in a scanning electron microscope (SEM) using simultaneously electron beam induced current (EBIC) and cathodoluminescence (CL). EBIC yields important information about depletion region, leakages, and the overall functionality of the pn-junction and allows to determine the diffusion length of the generated minority carriers on both sides of the pn-junction. Spectrally and spatially resolved CL measurements yield both, information about the QW quality and about centers of non-radiative recombination. EBIC measurements were carried out in top-view and in cross-section between and room temperature. The diffusion lengths measured for polar and semipolar structures prove the better crystal quality of the polar structures in terms of the higher carrier diffusion length and its increase for lower temperatures, as expected. On the semipolar structures, top-view EBIC measurements detect specific areas, where the pn-junction seems to be disturbed. The comparison of EBIC and CL measurements shows interesting correlations of the EBIC signal and the emission wavelengths of the quantum wells.

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.

In this article, the growth and coalescence of semi-polar oriented GaN layers, deposited on pre-structured r-plane sapphire substrates, is studied with the help of Si-doped marker layers. It has been found to be very important to adjust the shape of the initial GaN stripes by varying the growth temperature to obtain not only a smooth surface, but also a small density of basal plane stacking faults (BSFs) and threading dislocations (TDs) on the wafer surface. With the help of transmission electron microscopy (TEM) and cathodoluminescence measurements (CL), we can conclude that during growth, we need to achieve a compromise between small BSF density, small TD density, and perfect coalescence with smooth surface, free of fissures, and other growth artifacts. Also the formation of arrow-head-shaped surface artifacts called “chevrons” [1, 2] can be understood to be caused by imperfect coalescence. We observe with the help of the marker layers that the growth rate fluctuates between neighboring stripes. This effect strongly increases for higher growth temperature.

Via temperature-dependent time-resolved photoluminescence spectroscopy, we investigate the radiative and nonradiative recombination processes in thin (quantum well widths of about 1.5 nm) a-plane, m-plane, () and () GaInN/GaN fivefold quantum well structures of varying indium content grown on low defect density GaN substrates and GaN templates. At room temperature, we observe surprisingly short radiative lifetimes in the range from 100 ps to 1 ns for these structures, being about one to two orders of magnitude shorter than for similar c-plane quantum wells. This large difference cannot solely be explained by the larger overlap matrix element in non- and semipolar wells. Higher exciton binding energies and lower effective density of states masses may contribute to an enhanced radiative probability. The nonradiative recombination exhibits a thermally activated behavior with activation energies of about 10 meV for () and around 25 meV for nonpolar quantum wells. These values are lower than the quantum well barrier height and the exciton binding energy, but in a similar range as the localization energies estimated from the radiative recombination.

authoren We present results of the investigation on the doping behavior of planar semipolar -oriented GaN grown on patterned sapphire substrates mainly focusing on the magnesium incorporation. We observed that Mg is incorporated with much lower efficiency into the plane as compared to polar c-plane GaN. This problem could be decreased by varying the growth temperature. We found higher Mg concentrations with constant Mg flow for reduced growth temperature. Simultaneously, the parasitic background carrier concentration was reduced. Using this optimization, a planar semipolar InGaN/GaN-LED on -oriented GaN was grown. Electroluminescence measurements show reasonable electrical and optical performance.

authoren InGaN multiple quantum well light-emitting diodes (LEDs) were grown on chemically–mechanically polished (112―2) GaN templates (up to 100 mm diameter wafers) by metalorganic vapour phase epitaxy. Initial GaN overgrowth on the polished templates in nitrogen ambient maintained the polished surface. The peak emission wavelength of the LEDs varied from 445 to 550 nm. In contrast to the simultaneously grown LEDs on as-grown templates, the LEDs on polished templates have very smooth surface morphology, uniform luminescence, and higher output power. Fluorescence images of 445 nm LEDs grown on (a) as-grown and (b) polished (112―2) GaN templates.

A rapid nondestructive defect assessment and quantification method based on X-ray diffraction and three-dimensional reciprocal-space mapping has been established. A fast read-out two-dimensional detector with a high dynamic range of 20 bits, in combination with a powerful data analysis software package, is set up to provide fast feedback to crystal growers with the goal of supporting the development of reduced defect density GaN growth techniques. This would contribute strongly to the improvement of the crystal quality of epitaxial structures and therefore of optoelectronic properties. The method of normalized three-dimensional reciprocal-space mapping is found to be a reliable tool which shows clearly the influence of the parameters of the metal–organic vapour phase epitaxial and hydride vapour phase epitaxial (HVPE) growth methods on the extent of the diffuse scattering streak. This method enables determination of the basal stacking faults and an exploration of the presence of other types of defect such as partial dislocations and prismatic stacking faults. Three-dimensional reciprocal-space mapping is specifically used in the manuscript to determine basal stacking faults quantitatively and to discuss the presence of partial dislocations. This newly developed method has been applied to semipolar GaN structures grown on patterned sapphire substrates (PSSs). The fitting of the diffuse scattering intensity profiles along the stacking fault streaks with simulations based on a Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. Three-dimensional reciprocal-space mapping is shown to be a method sensitive to the influence of crystallographic surface orientation on basal stacking fault densities during investigation of semipolar (11{\overline 2}2) GaN grown on an r-plane (1{\overline 1}02) PSS and semipolar (10{\overline 1}1) GaN grown on an n-plane (11{\overline 2}3) PSS. Moreover, the influence of HVPE overgrowth at reduced temperature on the quality of semipolar (11{\overline 2}2) GaN has been studied.

Recently, GaInN has found increasing interest in chemical sensors and biosensors. Such sensors are typically based on changes in the near-surface band bending caused by different adsorbates on the surface. Besides electrical structures, optochemical transducers have been demonstrated, where the sensor response is read out remotely by analyzing the photoluminescence of the sensor structures. Hence, no chemically vulnerable electric contacts are required. In biosensing using optical technologies, the attachment of fluorescent labels to biomolecules is a frequently applied method. Often these fluorophors suffer from photobleaching which limits applications. In our current studies, polar GaInN quantum wells (QWs) are applied for sensing different molecules adsorbed on the transducer surface. Instead of fluoro¬phors, adsorbate-caused changes of the GaInN quantum well photo¬lumi¬nescence (PL) are taken as chemical sensing signal. In particular, the band-bending influences the electric field in a near-surface quantum well and hence changes the emission wavelength owing to the quantum confined Stark effect (QCSE). The sensitivity depends on the design of the hetero structures like QW and cap layer thickness, as evaluated by band structure simulations. Besides gases such as oxygen and hydrogen, also biomolecules can be adsorbed on the semiconductor surface and studied by PL. As an example, we have studied the iron-storage protein ferritin. Ferritins with and without iron-load (the latter corresponds to apoferritin) are immobilized on hydroxylated polar GaInN quantum well surfaces. A spectral shift of the quantum well PL is found depending on the iron-load of the molecules which might enable sensing of ferritin-bound iron.

Recent advances of the performance of GaN based devices with semipolar quantum wells have been realized homoepitaxially on pseudo bulk substrates which are typically small in size and high in cost. These limitations fuel the search for cheap and large area alternatives. Heteroepitaxial growth on sapphire substrates is well established with excellent results for polar GaN structures - the growth of semipolar gallium nitride on sapphire, however, presents unique challenges. In order to profit from our expertise in c-plane samples, our semipolar gallium nitride growth experiments are based on growth in c-direction. Using selective area epitaxy (SAE) on c-oriented templates, we can grow 3D structures with semipolar side facets. These structures are typically several μm in size which constitutes further challenges for device processing. Reducing the size of the structures to a sub-μm scale, we are able to bury our semipolar QWs within planar layers resulting in flat samples with c-plane surfaces. In this contribution, we present our results concerning the structural quality and spectral properties of quantum wells emitting in the blue and green spectral range as well as light emitting diodes. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, the influence of sapphire mis-orientation on the quality of coalesced ( 11 2 ¯ 2 ) GaN layers grown on r -plane prestructured sapphire substrates ( r -PSS) is investigated. It was found that the angle of the GaN ( 11 2 ¯ 2 ) plane towards the surface plane of the sapphire wafer can be adjusted by the mis-orientation of the substrate. Furthermore, we discovered that the c -direction of GaN is tilted by more than 1° towards the c -direction of the sapphire wafer. In a second experiment, the influence of the MOVPE growth temperature, V/III ratio and reactor pressure on the coalesced layer has been studied. While a high temperature and small V/III ratio are beneficial, the reactor pressure did not show any significant impact on the crystal quality and surface roughness.