This PDF file contains the front matter associated with SPIE Proceedings Volume 10532, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

To fabricate such deep UV devices, a high-quality underlying AlN film is desirable because of its wide bandgap. In this work, we studied the effects of thermally annealing sputtered AlN films grown on sapphire substrates on the crystallinity and surface morphology, where the thermal annealing was performed in nitrogen ambient. The effects of annealing were investigated as a function of the sputtered AlN film thickness and the thermal annealing temperature. The thicknesses of the sputtered AlN films were 170 and 340 nm. Subsequently, the sputtered AlN films were thermally annealed in N2 at 1600 - 1700 oC for 1 h. Both the (0002) and (10-12) XRCs of the AlN films had a single sharp peak after thermal annealing owing to the elimination of the tilt and twist components from the sputtered AlN films by high-temperature thermal annealing. The improved crystallinity is related to the solid-phase reactions that occur at high annealing temperatures. The FWHMs of the (0002) and (10-12) XRCs were markedly reduced from 532 to 49 and 6031 to 287 arcsec, respectively. Cross-sectional STEM images of sputtered AlN on sapphire substrate after annealing were observed. Columnar structures are observed in grown layer before annealing while they disappear after annealing. With annealing, hexagonal shaped voids pointed by white arrows form and the polarity invert from N-polar to Al-polar about 10 nm away from AlN/sapphire interface.

In this study, we developed an annealing furnace for 4 inch wafers and used it to anneal AlN on a 4 inch c-plane sapphire substrate at 1700°C for 1 h in a pure N2 atmosphere by using the annealing furnace (STA1800, Taiyo Nippon Sanso). FWHM of the XRC of the (0002) of 48.6 arcsec, and that of the (10-12) of 278.2 arcsec was obtained.

The main objective of this paper is crystallization of semi-insulating material with resistivity ~109 Ωcm in temperature range between 296 K and 1000 K. No free carriers should be activated at elevated temperature. Source of Mn dopant will be metallic manganese. Hydrochloride flow will be set above the Mn source and as a result of reaction MnCl2 will form. Manganese dichloride will be transported to the growth zone of GaN. The following growth parameters will be established and analyzed: i/ growth temperature, ii/ flows of gas reagents (HCl above gallium, HCl above metallic Mn, ammonia), iii/ carrier gas composition (N2, H2, mixture of N2 + H2, or nonreactive gas), iv/ temperature of metallic Mn source. Determining proper parameters should result in a stable growth of HVPE-GaN:Mn crystals with a desired morphology (hillocks). Distribution of manganese dopant will be uniform in the grown layer. HVPE-GaN:Mn will be thicker than 1 mm. Their diameter will depend on the used seed – up to 2-inch. The layers will be removed from the seeds by slicing procedure and as a result free-standing HVPE-GaN:Mn will be obtained. Structural, optical and electrical properties of this material will be examined and presented.

The main objective of this paper is crystallization of AlGaN by HVPE method. Source of Al will be metallic aluminum. Hydrochloride flow will be set above the Al source at temperature of 500ºC and as a result of reaction AlCl will form. Aluminum monochloride will be transported to the growth zone of AlGaN. The following growth parameters will be established and analyzed: i/ growth temperature, ii/ flows of gas reagents (HCl above gallium, HCl above metallic Al, ammonia), iii/ carrier gas composition (N2 or nonreactive gas). Determining proper parameters should result in a stable growth of HVPE-AlGaN layers with a desired composition of aluminum (Al content from 1 to 25%). Distribution of aluminum will be uniform in the grown layers. HVPE-AlGaN will be thick up to 100 µm. Their diameter will depend on the used seed – up to 2-inch. Structural, optical and electrical properties of HVPE-AlGaN will be examined and presented in this paper.

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.

Defect incorporation in AlGaN is dependent on the defect formation energy and hence on associated chemical potentials and the Fermi level. For example, the formation energy of CN in Al/GaN varies as chemical potential difference (µN- µC) and -EF (Fermi level). Here, we demonstrate a systematic point defect control by employing the defect formation energy as tool by (a) chemical potential control and (b) Fermi level control. Chemical potential control (µN and µC) with a case study of C in MOCVD GaN is reported. We derive a relationship between growth parameters, metal supersaturation (i.e. input and equilibrium partial pressures) and chemical potentials of III/N and impurity atoms demonstrating successful quantitative predictions of C incorporation as a function of growth conditions in GaN. Hence growth environment necessary for minimal C incorporation within any specified constraints may be determined and C is shown to be controlled from >1E19cm-3 to ~1E15 cm-3. Fermi level control based point defect reduction is demonstrated by modifying the Fermi level describing the probability of the defect level being occupied/unoccupied i.e. defect quasi Fermi level (DQFL). The DQFL is modified by introducing excess minority carriers (by above bandgap illumination). A predictable (and significant) reduction in compensating point defects (CN, H, VN) in (Si, Mg) doped AlGaN measured by electrical measurements, photoluminescence and secondary ion mass spectroscopy (SIMS) provides experimental corroboration. Further, experiments with varying steady state minority carrier densities at constant illumination prove the role of minority carriers and DQFL in defect reduction over other influences of illumination that are kept constant.

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. We present results on the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.

Nonpolar a-plane GaN (a-GaN) grown on r-plane sapphire substrate is one of the promising materials for eliminating an internal field in III-nitride devices. Thus, a high performance light-emitting diode can be expected by using a high crystalline quality a-GaN. In our study, we realized a high crystalline quality a-GaN by using both patterned sapphire substrate (PSS) and sputtered AlN buffer layer (sp-AlN). The PSS had conical patterns with a diameter of 900 nm and a height of 600 nm. The patterns placed with triangular arrangement and an interval of 1000 nm. The 30-nm-thick sp-AlN was deposited on the PSS at 300 oC. Approximately 3.5-um-thick a-GaN was grown by using metal-organic vapor phase epitaxy with optimized growth conditions. The crystalline qualities of the a-GaN were evaluated by X-ray rocking curves full width at half maximum (XRC-FWHM) for both on- and off-axis planes. Moreover, the growth behavior of a-GaN on PSS was characterized by in-situ reflectance and scanning electron microscope. For the on-axis GaN (11-20) plane, the XRC-FWHM in the c-axis direction of the a-GaN was 462 arcsec, whereas it was 647 arcsec in the m-axis direction. For the off-axis GaN (10-12) plane, the XRC-FWHM was 990 arcsec. These XRC-FWHMs were significantly decreased compared with that of a-GaN grown on nitridated r-plane flat sapphire. It was suggested the density of defects in a-GaN were decreased by both PSS and sp-AlN. To clarify how to defects in a-GaN decrease by using the PSS and sp-AlN the transmission electron microscope observation was performed.

A fraction of a SiNx mono-layer is formed on a GaN layer by exposing the surface to a Si flux. When the sample is heated under vacuum at high temperature (900°C), we observe the sublimation of GaN in the regions uncovered by the thermally resistant SiNx mask. This selective area sublimation (SAS) process can be used for the formation of nanopyramids and nanowires with a diameter down to 4 nm. Also, if InGaN quantum wells are included in the structures before sublimation, InGaN quantum disks with quasi identical sizes in the 3 dimensions of space can be formed using SAS.

Research studies on the growth and properties of hexagonal boron nitride hBN have recently attracted a lot of attention. The lattice parameter of h-BN is very close to that of the recently discovered graphene. The surface of hBN is atomically flat and will provide an ideal chemically inert dielectric substrate for 2D-structures. Secondly, the band gap of h-BN is about 6 eV and that has fuelled the interest in hBN as a wide gap material for deep-ultraviolet device (DUV) applications. Currently there are world-wide attempts to develop a reproducible technology for the growth of large area h-BN layers by chemical vapour deposition (CVD), metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE). We have recently demonstrated growth of hBN layers using MBE at extremely high growth temperatures from 1390o to 1690oC [1]. Atomic force microscopy (AFM) shows mono- and few-layer hBN island growth, while conducting atomic force microscopy demonstrates that the MBE grown hBN has electrical properties comparable to exfoliated hBN. The high optical quality of hBN grown at high temperatures on highly oriented pyrolytic graphite (HOPG) has been confirmed by both spectroscopic ellipsometry and photoluminescence spectroscopy [2]. In this work we present our recent results on the high-temperature MBE of hBN monolayers with atomically controlled thicknesses for 2D applications and on the growth of significantly thicker hBN layers for potential DUV applications. [1] Y.J. Cho, et al. Scientific Reports 6 (2016) 34474. [2] T. Q.P. Vuong, et al. 2D Materials 4 (2017) 021023.

Ga₂O₃ is gaining attention for high breakdown electronics. The β-polymorph is air-stable, has a wide bandgap (~4.6 eV) and is available in both bulk and epitaxial form. Different types of power diodes and transistors fabricated on Ga₂O₃ have shown impressive performance. Etching processes for Ga₂O₃ are needed for patterning for mesa isolation, threshold adjustment in transistors, thinning of nano-belts and selective area contact formation. Electrical damage in the near-surface region was found through barrier height changes of Schottky diodes on the etched surface. The damage is created by energetic ion bombardment, but may also consist of changes to near-surface stoichiometry through loss of lattice elements or deposition of etch residues. Annealing at 450°C removes this damage. We also discuss recent results on damage introduction by proton and electron irradiation. In this case, the carrier removal rates are found to be similar to those reported for GaN under similar conditions of dose and energy of the radiation.

In this paper the properties of excitons and phonons in doped GaN is reviewed. We demonstrate that in heavy Ge doped GaN new quasi particle can be stabilized. Furthermore, we discuss and use the observation of local phonon modes to clarify the incorporation of germanium, silicon, carbon, and transition metal ions on different lattice places in the nitride material.

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.

The linewidth of the luminescence should be the reflection of the system disorder, and the natural way of thinking is to associate this disorder with local variation of In composition and/or quantum wells thickness. Problem of the emission linewidth of InGaN quantum structures has been discussed over last two decades. In the early work of O’Donnel et al.1, the remarkable, universal character of the emission broadening in InGaN quantum wells and epilayers was reported. Authors argued that since the line broadening is apparently independent of the growth method, it must reflect an inherent feature of these structures. Indeed, up to our knowledge there is presently no data showing a direct relation between the emission linewidth and microscopic landscape of InGaN structures. To look more closely on the origin of emission line broadening we performed cathodoluminescence (CL) mapping, using Hitachi SU-70 scanning electron microscope equipped with Horiba Jobin Yvon optical detection system. We analyzed InGaN/GaN quantum wells in wide range of In content. Our measuments show that over large span of image diameter, the cathodoluminescence linewidth remains almost constant, meaning that what we perceive as the visible In fluctuation landscape is practically irrelevant. Most likely the observed source of broadening is related to submicron size effects related to strong carriers localization. Reference: 1. K. P. O’Donnell, T. Breitkopf, H. Kalt, W. Van der Stricht, I. Moerman, P. Demeester, and P. G. Middleton, “Optical linewidths of InGaN light emitting diodes and epilayers”, Appl. Phys. Lett. 70, 1843 (1997)

Band potential fluctuations in InGaN/GaN quantum wells (QWs) induce carrier localization that affects emission linewidth and carrier recombination rate. Alloy composition and well width variations are considered as main sources of the potential fluctuations and are often treated indiscriminately. However, their impact on the emission linewidth and the carrier lifetimes may be different. Besides, the impact of the QW width fluctuations on the linewidth could possibly be reduced via optimization of growth, while random alloy composition fluctuations can hardly be avoided. In this work, we have studied these effects in green-emitting semipolar (20-21) plane InGaN/GaN single QW structures of different well widths (2, 4 and 6 nm) and in structures with different number of QWs (1, 5 and 10). Experiments have been performed by scanning near-field photoluminescence (PL) spectroscopy. It has been found that the well width fluctuations, compared to the InGaN alloy composition variations, play a negligible role in defining the PL linewidth. In multiple QW structures, the alloy composition fluctuations are spatially uncorrelated between the wells. Despite that the 10 QW structure exceeds the critical thickness, no PL linewidth changes related to a structural relaxation have been detected. On the other hand, the well width fluctuations have a large impact on the recombination times. In-plane electric fields, caused by the nonplanarity of QW interfaces, separate electrons and holes into different potential minima increasing the lifetimes in wide QWs.

(Al,In)GaN laser diodes are used as the blue and green light source in RGB laser projection, e.g. head-up displays, laser (cinema) projection, and augmented virtual reality. For these applications, a modulation frequency bandwidth in the 100 MHz to 1 GHz range is necessary. We investigate the spectral and temporal dynamics of state-of-the-art 450 nm and 530 nm (Al,In)GaN laser diodes upon modulation with single pulses and pulse patterns, with and without pre-bias, and relate the observed dynamic characteristics to the optical gain spectra. Firstly, in the few MHz range, longitudinal mode competition causes a modulation of the modes of the laser diode’s longitudinal mode frequency comb. Mode competition is caused by variation in the gain smaller than 1/cm. Direct electrical modulation of the laser diode driving current in the MHz frequency range therefore interferes with mode competition. While this is not observable in spectrally integrated time traces of the optical intensity, it does result in a MHz intensity modulation in laser projection if wavelength sensitive elements are in the light path. Secondly, turn-on delay and relaxation oscillations affect the intensity trace on the high frequency side of the modulation frequency spectrum and are caused by large > 10/cm gain variations. Pre-bias and multiple pulses with pulse spacing in the 10 ns range affect intensity of first and consecutive pulses. Furthermore, the spectro-temporal behavior shows longitudinal mode bunching, caused by irregular modulation of the wavelength dependent gain spectrum.

We investigated alloy fluctuations at dislocations in III-Nitride alloys (InGaN and AlGaN). We found that in both alloys, atom segregation (In segregation in InGaN and Ga segregation in AlGaN) occurs in the tensile part of dislocations with an edge component. In InGaN, In atom segregation leads to an enhanced formation of In-N chains and atomic condensates which act as carrier localization centers. This feature results in a bright spot at the position of the dislocation in the CL images, suggesting that non-radiative recombination at dislocations is impaired. On the other hand, Ga atom segregation at dislocations in AlGaN does not seem to noticeably affect the intensity recorded by CL at the dislocation. This study sheds light on why InGaN-based devices are more resilient to dislocations than AlGaN-based devices. An interesting approach to hinder non-radiative recombination at dislocations may therefore be to dope AlGaN with In.

We present a model of carrier distribution and transport accounting for quantum localization effects in disordered semiconductor alloys. It is based on a recent mathematical theory of quantum localization which introduces a spatial function called localization landscape for carriers. These landscapes allow us to predict the localization of electron and hole quantum states, their energies, and the local densities of states. The various outputs of these landscapes can be directly implemented into a drift-diffusion model of carrier transport and into the calculation of absorption/emission transitions. This model captures the two major effects of quantum mechanics of disordered systems: the reduction of barrier height (tunneling) and lifting of energy ground states (quantum confinement), without having to solve the Schrödinger equation. Comparison with exact Schrödinger calculations in several one-dimensional structures demonstrates the excellent accuracy of the approximation provided by the landscape theory [1]. This approach is then used to describe the absorption Urbach tail in InGaN alloy quantum wells of solar cells and LEDs. The broadening of the absorption edge for quantum wells emitting from violet to green (indium content ranging from 0% to 28%) corresponds to a typical Urbach energy of 20 meV and is closely reproduced by the 3D sub-bandgap absorption based on the localization landscape theory [2]. This agreement demonstrates the applicability of the localization theory to compositional disorder effects in semiconductors. [1] M. Filoche et al., Phys. Rev. B 95, 144204 (2017) [2] M. Piccardo et al., Phys. Rev. B 95, 144205 (2017)

Due to its large band gap and excellent electrical properties, nitride-based heterostructures are rapidly becoming a material of choice for RF and power switching applications. However, these devices require a carbon or iron doped semi-insulating buffer to deliver high breakdown voltages and suppress off-state leakage currents. We have grown semi-insulating GaN using precursor-based metal-organic chemical vapor phase epitaxy by intentionally introducing carbon and iron impurities with doping concentration ranging from 1x10^17cm-3 to 5x10^18cm-3 to compensate residual donors. Scanning probe microscopy techniques, scanning surface potential microscopy (SSPM) and bias dependent electric force microscopy (EFM) are mainly used to compare contact potential differences and local potential mapping at the vicinity of dislocation regions. For reference n-type GaN layers doped with Si and Ge, and p-type GaN layers doped with Mg are also investigated. Skew and edge type dislocation densities are estimated from tilt and twist x-ray diffraction measurements using omega-scans for the (0002) reflection and grazing incidence in-plane geometry for the (101 ̅0) reflection. The obtained values are in the range of low 108 cm-2 for screw-type and low 109 cm-2 for edge-type dislocations, independent of doping type and concentration. Locally probing dislocations by SSPM reveals a negative charge contrast with respect to the surrounding areas in C-doped samples increasing with doping concentration whereas Fe-doped samples exhibit no contrast. By investigating the contact potential by EFM, the combined effects of Fermi-level position and surface band bending due to surface states are determined. With the references of n-type and p-type GaN samples, the acceptor states introduced by carbon cause Fermi-level pinning below midgap position whereas acceptor-states by Fe impurities have to be energetically above midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6x10^18 cm-3 exhibit an up to 5 orders of magnitude lower dark current at room temperature and significantly higher thermal activations than Fe-doped samples with a comparable dopant concentration. In conclusion C-doped samples show superior properties in comparison to Fe-doped samples.

p-GaN/u-GaN alternating-layer nanostructures are grown with molecular beam epitaxy to show a low p-type resistivity level of 0.04 Ohm-cm. The obtained low resistivity is due to the high hole mobility in the u-GaN layers, which serve as effective transport channels of holes diffused from the neighboring p-GaN layers. A model for estimating the current penetration behavior in a p-GaN/u-GaN alternating-layer structure when its I-V characteristics is to be measured is proposed. In this model, an exponential decay with a characteristic penetration depth is assumed for a layered structure of low effective conductivity. In a high-conductivity structure, this penetration depth is regarded as infinity such that the depth-dependent current distribution is uniform. By growing p-type structures with an upper portion of a layered structure of unknown effective conductivity and a lower portion of high conductivity, which can be a layered or a uniform structure, of different individual thicknesses, we can have a sequence of sheet conductance data for best-fitting with the proposed model to simultaneously obtain the characteristic penetration depth and effective conductivity of the upper layered structure. Simulation studies are performed to provide the results supporting the proposed model. From the simulation results, it is found that the key factor hindering the current penetration is the low conductivity and finite thickness of a sub-layer around the middle of a u-GaN layer, which is not covered by the hole diffusion range from the neighboring p-GaN layers.

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.

III-nitride ultraviolet (UV) light emitting diodes (LEDs) with emission wavelengths in the range of 250-280 nm have attracted considerable interest for applications such as germicidal disinfection and biological detection. However, the widely-used AlGaN quantum well (QW)-based LEDs at such wavelengths suffer from low quantum efficiencies. One main factor that limits the AlGaN QW LED efficiency at ~250-280 nm is the suffering of the severe band mixing effect caused by the valence subbands crossover, as well as the Quantum Confined Stark Effect (QCSE). Therefore, the novel AlGaN-delta-GaN QW design was proposed to address these issues in order to realize high-efficiency deep-UV LEDs.

Here, we proposed a novel Al_0.9Ga_0.1N-delta-GaN QW by inserting an ultra-thin delta-GaN layer into a conventional Al_0.9Ga_0.1N QW active region. The physics from such QW design was investigated by 6-band k·p model and the structure was experimentally demonstrated by Plasma-assisted Molecular Beam Epitaxy (PAMBE). The calculated results show that the insertion of delta-GaN layer could successfully address the band mixing issue and QCSE, leading to a significant improvement in spontaneous emission rate as compared to that of Al_0.55Ga_0.45N QW at 260 nm. The 5-period Al_0.9Ga_0.1N-delta-GaN QW with 3-nm AlN barrier was grown on AlN/sapphire substrate by MBE with ~2-monolayer delta-GaN layer, which was evidenced by the cross-sectional transmission electron microscope. The two-photon photoluminescence spectrum presented a single peak emission centered at 260 nm from the grown Al_0.9Ga_0.1N-deltaGaN QW with a full width at half maximum of 12 nm, which shows that the demonstrated QW would be promising for high-efficiency UV LEDs.

We report on the design, demonstration and current status of tunnel-injected ultra-violet light emitting diodes (UV LEDs). III-Nitride ultraviolet light emitting diodes (UV LEDs) are promising in various applications including sterilization, water purification and medical sensing. However, both the light extraction efficiency and electrical efficiency face fundamental challenges for the conventional UV LED structures. This stems from the poor p-type conductivity and high p-type contact resistance. Hole injection using interband tunneling provides an elegant solution to the fundamental issues of UV LEDs, and can resolve both the hole injection and light extraction issues that have been the primary problems for UV LEDs. In this talk, we will discuss in detail the heterostructure design and demonstration through polarization engineering to realize efficient interband tunneling in ultra-wide band gap AlGaN material. We will then outline some of the growth and fabrication challenges, and discuss our approaches to overcome these. Finally, we will present our results on tunnel-injected UV LEDs that have enabled us to achieve efficient UV light emission in the UVA and UVB wavelength ranges with on-wafer efficiencies comparable to state-of-the-art values [1,2,3]. References: 1. Yuewei Zhang, et al. ''Interband tunneling for hole injection in III-nitride ultraviolet emitters", Appl. Phys. Lett. 106, 141103 (2015); 2. Yuewei Zhang, et al. "Design of p-type cladding layers for tunnel-injected UV-A light emitting diodes", Appl. Phys. Lett. 109, 191105 (2016); 3. Yuewei Zhang, et al. “Tunnel-injected sub-260 nm ultraviolet light emitting diodes”, Appl. Phys. Lett. 110, 201102 (2017).

We propose a normally-off vertical GaN-based transistor on a bulk GaN substrate with low specific on-state resistance of 1.0 mΩ·cm² and high off-state breakdown voltage of 1.7 kV. P-GaN/AlGaN/GaN triple layers are epitaxially regrown over V-shaped grooves formed over the drift layer. The channel utilizes so-called semi-polar face with reduced sheet carrier concentration at the AlGaN/GaN interface, which enables high threshold voltage of 2.5 V and stable switching operations. The employed p-type gate does not give any concern of the gate instability. Note that formation of carbon doped insulating GaN layer formed on p-GaN well layer underneath the channel suppresses the punch-through current at off-state between the source and drain, which enables good off-state characteristics. The fabricated high-current vertical transistor achieves successful fast switching at 400V/15A. We also propose a novel vertical GaN-based junction barrier Schottky (JBS) diode with trenched p-GaN region on a bulk GaN substrate. A specific differential on-resistance of the GaN JBS diode is 0.9 mΩ·cm² while keeping high breakdown voltage of 1.6 kV. These results indicate that the demonstrated vertical GaN devices are very promising for future high power switching applications.

We present novel approaches for the development of Ga₂O₃ Schottky barrier and heterojunction diodes. Samples of β- Ga₂O₃ were first annealed in N₂ and O₂ to demonstrate the effect of annealing on the carrier concentration. Cathodoluminescence and electron spin resonance measurements were also performed. Schottky barrier diodes on asgrown and O₂-annealed epitaxial Ga₂O₃ films were fabricated and breakdown voltages were compared. Lower reverse current and a breakdown voltage of about 857 V were measured on the O₂-annealed device. Finally, we report preliminary results from the development of anisotype heterojunctions between n-type Ga₂O₃ with a sputtered NiO layer. Rectifying current-voltage characteristics were obtained when the NiO was deposited both at room temperature and at 450 °C.

AlxInyGa(1-x-y)N/GaN heterostructure is gaining increasing popularity for high power millimetre (mm)-wave high electron mobility transistors (HEMTs) because of its superior electron transport properties. For HEMTs to be able to operate in the mm-wave range, greatly scaled devices with gate length less than 100 nm is necessary. Consequently a sub-10 nm barrier, i.e. a small gate to channel distance, is required to mitigate short channel effects. For this, AlInN/GaN HEMT, which offers high two-dimensional electron gas (2DEG) density, is an alternative to its AlGaN/GaN counterpart. Adding Ga to AlInN might allows the engineering of total polarization, bandgap, and strain of the quaternary alloy independently, providing additional degrees of freedom in device design for performance optimization. Since there have not been many reports on this subject, this study aims at growing high electron mobility, low sheet resistance and high bandgap sub-10 nm AlInGaN/AlN/GaN heterostructures on 150 mm silicon substrates by investigating the role of barrier composition in the transport properties at the heterointerface. Electron mobility of 1,910 cm2/v.s and 2DEG density of 1.33x1013 cm-2, resulting a sheet resistance of 246 ohm/sq. is achieved on an AlInGaN/AlN/GaN HEMT with a 8.7 nm-thick Al0.73In0.08Ga0.19N barrier on a 150 mm Silicon (111) substrate. To our knowledge, this is one of the best electron mobility obtained on AlInGaN HEMTs grown on Si. As indicated by transmission electron microscopy images, it could be attributed to the smooth hetero-interface, which leads to reduced interface roughness scattering.

Nitride based laser diodes utilize as an active medium extremely strained InGaN quantum wells. As the nitride materials are piezoelectric in their nature, this strain is reflected in a strong piezoelectric field. This tilts the energy bands and shifts the emission spectrum position through the Quantum Confined Stark Effect (QCSE). As the laser diode is operated at elevated currents, the built-in electric field is reduced due to the screening by injected carriers. This leads to the increase of emission energy (blue-shift). It has been a subject of many discussions whether the field is preserved at lasing, or is it completely screened. In this work we compare the emission wavelength shift of nitride laser diodes and superluminescent diodes having different QW compositions. The superluminescent diodes allow us to study the emission spectrum at higher carrier densities than for laser diodes. In laser structures, we clearly see the saturation of the blue-shift at threshold current. While, on the other hand, we see the continuous shift of the emission wavelength in case of superluminescent diodes. This suggests, that the piezoelectric fields are not fully screened at threshold current. We also see that for UV laser diodes the emission line shift is much smaller than for blue wavelength devices. This implies practically complete screening of the electric field for UV laser while the lasing of blue laser diodes occurs at high electric field conditions.

We report on the design and study of solid state laser sources for lighting applications. While LEDs are affected by droop, limiting efficacy at higher currents, a possible solution is represented by solid state laser lighting, where a blue (450nm) laser is exciting a luminescent material thus achieving white light. With this work we designed and tested several LARP (Laser-Activated-Remote-Phosphors) test structures, both diffused lighting and focused applications will be discussed. Results indicates that good efficiency are achievable, without any sensible droop also at high injection currents. Phosphors have also been subjected to thermal stability tests up to 550°C.

GaN laser diodes have the potential to be a key enabling technology since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. Novel applications include high power laser bars for optical pumping, to laser sources for quantum technologies based on atom interferometry, such as next generation optical clocks and gravity sensors.

We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and atom interferometry systems. This includes the [5s²S_1/2-5p²P_1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s₂ ¹S₀-5p¹P₁] cooling transition in neutral strontium clocks at 461 nm and the [5s²s_1/2 – 6p²P_3/2] transition in rubidium at 420 nm.

In addition, we report our latest results on tapered high power GaN laser diodes, for i) optical amplifiers, and ii) optimising the optical power of the device by reducing filamentation and hence avoiding catastrophic optical mirror damage (COMD).

We have realised InGaN/GaN distributed feedback laser diodes emitting at a single wavelength in the 42X nm wavelength range. Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow line width blue laser source can be used to target the atomic cooling transition. Previously, GaN DFB lasers have been realised using buried or surface gratings. Buried gratings require complex overgrowth steps which can introduce epi-defects. Surface gratings designs, can compromise the quality of the p-type contact due to dry etch damage and are prone to increased optical losses in the grating regions. In our approach the grating is etched into the sidewall of the ridge. Advantages include a simpler fabrication route and design freedom over the grating coupling strength.Our intended application for these devices is cooling of the Sr+ ion and for this objective the laser characteristics of SMSR, linewidth, and power are critical. We investigate how these characteristics are affected by adjusting laser design parameters such as grating coupling coefficient and cavity length.

In this paper, we present a novel design of a nitride-based VCSEL emitting at 414 nm and perform numerical analysis of optical, electrical and thermal phenomena. The bottom mirror of the laser is a Al(In)N/GaN DBR (Distributed Bragg Reflector), whereas the top mirror is realized as a semiconductor-metal subwavelength-grating, etched in GaN with silver stripes deposited between the stripes of the semiconductor grating. In this monolithic structure simulations show a uniform active-region current density on the level of 5.5 kA/cm² for the apertures as large as 10 μm. In the case of a broader apertures, e.g. 40 μm, we showed that, assuming a homogeneous current injection at the level of 5.5 kA/cm² , the temperature inside the laser should not exceed 360 K, which gives promise to improve thermal management by uniformisation of the current injection.

This is the first demonstration of continuous-wave (CW) operation of nonpolar GaN-based VCSELs. These devices had a dual-dielectric distributed Bragg reflector (DBR) design with ion implanted apertures and III-nitride tunnel junction (TJ) intracavity contacts. Unlike c-plane devices, nonpolar GaN-based VCSELs have anisotropic gain that leads to a 100% polarization ratio and polarization-locked VCSEL arrays. Previous nonpolar devices were unable to lase under CW operation, notably due to the thermally-insulating bottom dielectric DBR. Based on thermal modeling using COMSOL, the main thermal pathway was restricted to a thin p-side metal contact that goes around the bottom DBR to the submount. Heat flow was further impaired as the Au-Au thermocompression flip-chip bond created cracks and voids in the p-side metal. The thermal performance was improved in our latest VCSELs by increasing the cavity length to 23λ and utilizing Au-In solid liquid interdiffusion bonding to create a more robust pathway for heat transport. This led to stable CW VCSEL operation for over 20 minutes. The peak output powers for a 6 μm aperture VCSEL under CW and pulsed operation were 150 μW and 700 μW, respectively. Lasing wavelengths were observed at 406 nm, 412 nm, and 419 nm. The fundamental transverse mode was observed without the presence of filamentary lasing.

Coaxial core-shell nanowire heterostructures can consist of different materials with varying lattice parameters, electronic band energetics, opposite doping types, and also of different crystal structures. Based on this high degree of freedom, core-shell nanowires open up a large variety of new concepts for applications on the nanoscale. Here, we demonstrate the controlled tuning of the GaN band gap within coaxial nanowire heterostructures by up to 240 meV towards higher, as well as towards lower energies. This is realized by the epitaxial overgrowth of GaN nanowires by (Al,Ga)N and (In,Ga)N shells inducing compressive and tensile strain in the GaN cores, respectively. Long-term stability tests are performed on GaN-(Al,Ga)N core-shell nanowires, revealing coherently strained crystals despite high strain fields of up to -3.4%. A reduction of the radiative recombination rate in GaN-(Al,Ga)N core-shell nanowires compared to pure GaN nanowires measured under optical excitation is discussed by means of strain-dependent dipole transition matrix elements. A study on the thermal dissociation of excitonic recombination is performed via temperature-dependent photoluminescence spectroscopy, indicating the passivation of nonradiative surface defect centers in the presence of an (Al,Ga)N shell. For the case of GaN-(In,Ga)N core-shell nanowires, complementary strain fields are measured within the core and the shell, which provides a new concept for band gap tuning in the visible spectral range. In addition, specific shapes of the (In,Ga)N top facets of the core-shell nanowires imply a new and simple method for evaluating the wurtzite crystal polarity of individual GaN nanowires.

We present a nanometer-scale correlation of the structural, optical, and chemical properties of InGaN/GaN core-shell microrods. The core-shell microrods have been fabricated by metal organic vapor phase epitaxy (MOVPE) on c-plane GaN/sapphire templates covered with a SiO2-mask. The MOVPE process results in a homogeneous selective area growth of n-doped GaN microrods out of the mask openings. Surrounding the n-GaN core, a nominal 5 nm thick GaN shell and 30 nm thick InGaN layer were deposited. Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the selective Indium incorporation in the thick InGaN shell and the luminescence properties of the individual layers. Cross-sectional STEM analysis reveal a hexagonal geometry of the GaN-core with m-plane side-walls. Directly at the corners of the hexagon a-plane nano-facets with a length of 45 nm are formed. The overgrowth of the GaN core with InGaN leads to a selective formation of Indium-rich domains with triangular cross-section exactly at these nano-facets as evidenced by Z-contrast imaging. Probing the local luminescence properties, the most intense CL emission appears at the m-plane side-facets with 392 nm peak wavelength. As expected, the Indium-rich triangles emit a red-shifted luminescence around 500 nm.

The monolithic integration of InGaN-based multi-color light-emitting diodes (LEDs) exerts a great impact on the full-color application field. The two dimensional arrangement of three primary colors micro-LED is expected to be used as a semiconductor video panel [1]. The emission color of InGaN/GaN triangular latticed nanocolumn arrays with the same lattice constant (L) is controlled by the nanocolumn diameter (D) [2]. For a blue light emission, the narrow nanocolumns should be utilized, resulting in a low filling factor of nanocolumns. However, the use of high filling factor of nanocolumn system is suitable for a stable device fabrication. In this study, therefore, we fabricated the densely-packed regularly arranged InGaN nanocolumns (D/L > 0.9). For the nanocolumn arrays with high filling factors, it was found that the emission color shifted from blue to red with increasing L from 80 to 350 nm. The emission color change is attributed to the different mechanism from Ref. [2], which is investigated to be clarified. Using the emission color change for the high filling factor nanocolumn system, four-color (red, green, blue, and yellow; RGBY) micro light emitting diodes (LEDs) were integrated in a 20×20 μm2 area (hereinafter called “unit”) and the units were two-dimensionally arrayed in a 16×16 square lattice in 400×400 mm2 area. These nanocolumn micro-LEDs were independently driven using matrix wiring electrodes, exhibiting RGBY light emissions. [1] K. Kishino et al., Appl. Phys. Express 6, 012101 (2013). [2] H. Sekiguchi et al., Appl. Phys. Lett 96, 231104 (2008).

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.

GaN-based multi-quantum well devices are promising candidates as photodetectors in the UV to visible spectral range. Their complex structure and the extreme input optical power density still poses problems of reliability. In the devices under test, degradation takes place when the optical power density reaches values higher than 44 W/cm² , and consists in a reduction in the efficiency of the device and in its output current. This degradation process is not sudden and is caused by a gradual increase in the defect concentration, detected by means of photocurrent spectroscopy experiments, that suggest the role of gallium vacancies and/or their complexes as the physical origin. A secondary effect is the reduction in open circuit voltage, likely originating from an improvement in dopant and/or contact quality.

Group-III-nitride nanophotonics on silicon is a booming field, from the near-IR to the UV spectral range. The main interest of III-nitride nanophotonic circuits is the integration of active structures and laser sources. Laser sources with a small footprint can be obtained with microresonators formed by photonic crystals or microdisks, exhibiting quality factors up to a few thousand down to the UV-C. So far, single microdisk laser devices have been demonstrated, mostly under optical pumping. Combining microdisk lasers under electrical injection with passive devices represents a major challenge in realizing a viable III-nitride nanophotonic platform on silicon. As a first step to realize this goal, we have separately demonstrated electroluminescence from microdisks and side-coupling of microdisks to bus waveguides with outcoupling gratings in the blue spectral range. We have developed the fabrication of electrically injected microdisks with a circular p-contact on top of the disk that is connected to a larger pad via a mechanically stable metal microbridge. Blue electroluminescence is observed under current injection at room temperature. We also demonstrated high Q factor (Q > 2000) WGMs in the blue spectral range from microdisks side-coupled to bus waveguides, as measured from the luminescence of embedded InGaN quantum wells. The WGM resonances are clearly observed through outcoupling gratings following propagation in partially etched waveguides to remove quantum well absorption. Small gaps between microdisks and bus waveguides of around 100 nm are necessary for efficient coupling in the blue spectral range, which represents a major fabrication challenge. We will discuss the progress brought by these building blocks to generate future III-nitride photonic circuits.

We report the high-speed performance of semipolar GaN ridge laser diodes at 410 nm and the dynamic characteristics including differential gain, damping, and the intrinsic maximum bandwidth. To the best of our knowledge, the achieved modulation bandwidth of 6.8 GHz is the highest reported value in the blue-violet spectrum. The calculated differential gain of ~3 x 10^-16 cm², which is a critical factor in high-speed modulation, proved theoretical predictions of higher gain in semipolar GaN laser diodes than the conventional c-plane counterparts. In addition, we demonstrate the first novel white lighting communication system by using our near-ultraviolet (NUV) LDs and pumping red-, green-, and blueemitting phosphors. This system satisfies both purposes of high-speed communication and high-quality white light illumination. A high data rate of 1.5 Gbit/s using on-off keying (OOK) modulation together with a high color rendering index (CRI) of 80 has been measured.

The internal quantum efficiency (IQE) is a key property of light-emitting semiconductor structures. We critically review the most popular methods for determining the IQE. In particular, we discuss the impact of low- temperature non-radiative recombination on temperature-dependent CW photoluminescence measurements. Using temperature-dependent time-resolved photoluminescence we establish a method to verify 100 % IQE at low temperature and thus to obtain absolute internal quantum efficiencies at all temperatures.

Measurement of carrier lifetime is very important to understand the physics in light-emitting diodes (LEDs), as it builds a link between carrier concentration and excitation power or current density. In this paper, we present our study on optical and electrical characterizations on carrier lifetimes in polar InGaN-based LEDs. First, a carrier rate equation model is proposed to explain the non-exponential nature of time-resolved photoluminescence (TRPL) decay curves, wherein exciton recombination is replaced by bimolecular recombination, considering the influence of polarization field on electron-hole pairs. Then, nonradiative recombination and radiative recombination coefficients can be deduced from fitting and used to calculate the radiative recombination efficiency. By comparing with the temperature-dependent photoluminescence (TDPL) and power-dependent photoluminescence (PDPL), it is found these three methods provide the consistent results. Second, differential carrier lifetimes depending on injection current are measured in commercial near-ultraviolet (NUV), blue and green LEDs. It is found that carrier lifetime is longer in green one and shorter in NUV one, which is attributed to the influence of polarization-induced quantum confined Stark effect (QCSE). This result implies the carrier density is higher in green LED while lower NUV LED, even the injection current is the same. By ignoring Auger recombination and fitting the efficiency–current and carrier lifetime–current curves simultaneously, the dependence of injection efficiency on carrier concentration in different LED samples are plotted. The NUV LED, which has the shallowest InGaN quantum well, actually exhibits the most serious efficiency droop versus carrier concentration. Then, the approaches to overcome the efficiency droop are discussed.

Recent progress of growth techniques of bulk GaN crystals is remarkable for realizing GaN-based power switching devices with a high breakdown voltage. Point defects play important role to characterize the high quality GaN, because structural defects like threading dislocations (TDs) and stacking faults are nearly disappeared. We have been investigating the relation between the near-band-edge (NBE) photoluminescence (PL) lifetime observed at room temperature and the concentration of intrinsic nonradiative recombination centers (NRCs) in n-GaN, of which origins are point defect complexes containing Ga vacancy, by combining positron annihilation spectroscopy (PAS) and time-resolved PL methods [S. F. Chichibu, et al., APL86, 021914 (2005)]. However, PAS becomes less sensitive below the concentration of Ga vacancy ([VGa]) of 10^16 cm^-3. Thus, the development of an alternative way to detect such dilute point defects (<10^16 cm^-3) in high quality GaN crystals is essential. In this presentation, we will show the quantification of absolute quantum efficiency of radiation (AQE) by employing the omnidirectional PL (ODPL) technique to determine internal quantum efficiency (IQE) of the emission in GaN crystals with different excitation conditions. A high AQE of 8.22% corresponding to IQE of 70.9% was measured at room temperature for the NBE emission of a freestanding-GaN crystal grown by hydride vapor phase epitaxy on a GaN seed crystal manufactured with the acidic ammonothermal method, when cw photo pumping density was 66 W/cm^2.

“Photonics Multiannual Strategic Roadmap 2014-2020” mentions flexible electronics, light sources, displays, sensors and solar cells as key emerging technologies with a high expected growth of the market share. Technologies based on organic semiconductors still suffer from a short lifetime and low efficacy as compared to their inorganic counterparts. To make a flexible device from inorganic semiconductors one should shrink the size of the active elements and to integrate them on mechanically-flexible substrates. This can be achieved using control-by-design nanowires. In this work, we address the growth of nitride nanowires on novel substrates and the fabrication and characterization of flexible devices based on nitride nanowires. First, we will discuss the epitaxy of GaN nanowires on graphene-on-SiO2 substrates. We show that without any catalyst or intermediate layer, the nanowires grow on graphene with an excellent selectivity compared to the uncovered SiO2 surface. Taking advantage of this selectivity, we demonstrate that organized arrays of nanowires can be synthesized by structuring the graphene layer. Next, we will discuss the approach for nanowire lift-off, transfer into polymer-embedded membranes and flexible contacting. The realization and characterization of flexible light sources, photodetectors and piezogenerators will be presented.

We investigate the formation of GaN nanowires in plasma-assisted molecular beam epitaxy on epitaxial graphene prepared on SiC(0001) using the surface graphitization method in an inductively heated furnace. The pristine graphene layer structure is characterized by the presence of atomically flat terraces and steps which are covered by single-layer and bi-layer graphene, respectively. The formation of GaN is investigated under N-rich growth conditions for substrate temperatures between 725 and 800°C. Regardless of the substrate temperature, graphene is found to degrade during GaN growth due to its exposure to the N plasma. The morphology of the samples varies significantly between the regions originally covered with single-layer and bi-layer graphene. Specifically, on the terraces GaN grows as a Ga-polar layer, while along the step edges it forms meandering rows of N-polar nanowires. The formation of N-polar GaN nanowires on the cation-polar SiC substrate is explained in terms of a C-induced polarity inversion. Due to the superior thermal stability of N-polar material, it is possible to exclusively form nanowires along the step edges when using elevated substrate temperatures. Therefore, the investigated graphene layer structure enables the self-assembled formation of well-separated rows of GaN nanowires.

Here we propose that hybrid heterostructures, composed of inorganic nanostructures grown directly on 2-dimentional layered materials such as graphene, are the most promising material system for flexible device applications. In particular, the hybrid heterostructures composed of high-quality GaN thin films or nanostructures grown directly on graphene offer a novel material system for transferable and/or flexible optoelectronics. The inorganic nanostructures in the hybrid nanomaterials exhibit excellent electrical and optical characteristics, including high carrier mobility, radiative recombination rate, and long-term stability. Meanwhile, for the flexible devices based on the hybrid structures, the graphene layers, which have excellent electrical and thermal conductivity, high mechanical strength and elasticity, and/or optical transparency, act as a novel substrate offering new functionalities such as transferability or flexibility. Here I will present on position- and morphology-controlled growths of ZnO nanostructures using catalyst-free metal-organic vapor phase epitaxy and describe the methods to fabricate flexible LEDs based on nitride coated ZnO nanostructures grown on graphene, which exhibit strong light emission after the transfer onto foreign substrates, such as metal, glass, and plastic. We believe that our unique technology to make hybrid nanomaterials will make paradigm shift from rigid to flexible and planar to three-dimensional inorganic semiconductor structures and devices.

Superluminescent light emitting diodes (SLEDs) have beam-like optical output similar to laser diodes (LDs) while offering a broader emission wavelength spectrum. They represent, therefore, an interesting alternative to conventional LDs for applications where a short coherence length or low speckle noise are required. Visible SLEDs emitting in the red, blue, and green are ideal candidates for the manufacturing of speckle-free light sources in portable or wearable compact projection systems. In this paper, we review the current status of EXALOS’ GaN-based SLED technology in the violet-blue spectral range and report on our recent progress in terms of performance for devices with 440-460 nm emission. Furthermore, we discuss the challenges in achieving light output at even longer wavelengths. As a matter of fact, lower refractive index contrast between the waveguiding and cladding layers, decreased p-type doping efficiency when growing at low temperatures, low crystal quality and thermal stability of the active region have to be addressed and solved in order to achieve green emission. The epitaxial structures were grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane freestanding GaN substrates. Growth was followed by standard fabrication of SLEDs with a ridge waveguide design. A record CW output power of 150 mW (at an operating current of 330 mA) and a wall-plug efficiency (WPE) of 8% have been obtained at an emission wavelength >440 nm.

Patterned sapphire substrate (PSS) technique has been widely used to improve the performance of the GaN-based LEDs, because it is helpful for both internal quantum efficiency and light extraction efficiency. In this study, patterned sapphire substrates with different pattern morphology are fabricated by wet etching. The light angular distribution of LED can be adjusted as the pattern morphology on sapphire surface of LED is changed. We perform the white LEDs packaging with the chessboard phosphor structures. The blue light emitted from GaN die chip can be converted into white light more efficiently, due to the chessboard phosphor structures. We can select specific light angular distribution of LED fitting the chessboard phosphor structure to obtain the optimal white light packaging efficiency. The highest packaging efficiency of the chessboard packaging is 70.87 % at color temperature of daylight 6500 K +/- 500 K. The increasing packaging efficiency is contributed by well geometric matching between the light angular distribution of LED and the the chessboard phosphor structure. Thus, our experiment almost achieves the limitation of the chessboard packaging efficiency in simulation: 74.30 %.

The optical performance of red-light emitters grown along polar orientation InGaN/(In)GaN multiple quantum well (MQW) with semi-polar structure are examined and compared. Given a colour of the emitted light, time-resolved photoluminescence (TRPL) measurements show a large difference of decay times between polar and semi-polar structures, when temperature varies in 8 K to 300 K range. The TRPL results evidence a weak internal electric field for the semi-polar structure as the decay time in this structure is slightly wavelength-dependent and is, at a given wavelength, two orders of magnitude smaller than for the polar sample. The Auger non-radiative recombination is probed by the evolution of the PL intensity with changing photo-excitation power density. In the semi-polar structure, the Auger non-radiative recombination is observed at a threshold PT of photo-excitation density 200 times smaller than in the polar oriented sample. This observation is linked to the difference in efficiency of the localization effect ( different indium compositions) and impact of the quantum confined Stark effect (QCSE) for polar and semi-polar samples. Both localization effect and the QCSE facilitate the establishment of carrier-carrier repulsions before the radiative recombination of electrons and holes occurs. This favor Auger non-radiative recombination process and hence leads to the decrease of the IQE. The smaller threshold PT of the semi-polar oriented structure indicates that the QCSE dominates the reduction of the IQE at high injection level rather than the localization. The semi-polar oriented structure is one promising structure for growth red-light emitters with strong luminescence.

The development of efficient (In)AlGaN light emitting diodes (LEDs) in the ultraviolet B (UVB) spectral region (280nm-320nm) is essential due to their vast commercial potential. UVB LEDs are expected to not only replace traditional mercury lamps in applications such as curing of materials and phototherapy but also to establish new applications in the fields of plant growth and sensing. Although a lot of progress has been made on the performance of the UVB LEDs, the efficiency of the devices as well as the lifetime still needs to be improved. In this study the influence of the heterostructure design and package on the efficiency of UVB LEDs, grown by metalorganic vapor phase epitaxy on c-plane sapphire substrates, will be presented. Firstly, the performance of UVB AlGaN and InAlGaN multiple quantum well LEDs were studied and the influence of the material composition on the emission characteristics was analyzed. Secondly, the performance of LEDs with different electron blocking layer (EBL) designs and doping concentrations was compared. The highest internal quantum efficiency and emission power were obtained for LEDs with a gradient-like EBL, with decreasing aluminum content, because of the improved carrier injection. Additionally, the output power of the LEDs was found to increase with the p-doping level in the EBL. Finally, investigations on the influence of the metal contacts and insulator as well as the device packaging on the performance of UVB LEDs will be presented. Based on these optimizations, 315nm LEDs with output powers up to 10mW at 100mA were realized

In this study, a highly conductive and transparent AlN–based glass electrode, fabricated by either DC or AC-pulse-based electrical breakdown processes, is introduced, and applied to AlGaN–based UV-A and UV-C light-emitting diodes with p-AlxGa1-xN contact layers (x = 0.05, 0.1, 0.4). This AlN–based glass electrode with a conducting filament exhibited high transmittance in the deep-UV region (over 95.6 % at 280 nm) and low contact resistance with a p-Al0.4Ga0.6N layer (ρc = 3.2 × 10-2 Ω·cm2). The ohmic conduction mechanism at the interface between the AlN film and p-Al0.4Ga0.6N layers was then examined using various analytical tools. One of the 280-nm top-emitting LEDs with the AlN-based glass electrodes operated stably with a forward voltage of 7.7 V at 20 mA and a light-output power of 7.49 mW at 100 mA after packaging. The external quantum efficiency was measured to be a record-high 2.8. This report is the first demonstration of top emission from DUV LEDs, and the proposed method may be used extensively in various areas of optoelectronic devices and sensors.

In AlGaN, the dominating emission polarization depends on the Al content. Generally speaking, a higher Al content leads to a stronger TM-polarized emission. Normally, the dominating emission polarization of an AlGaN layer changes from the TE polarization into the TM polarization when the emission wavelength is shorter than 300 nm. Because a TM-polarized photon propagate along the lateral dimension of a c-axis grown LED sample, its light extraction efficiency is lower, when compared with a TE-polarized photon. In this study, the material characterization techniques of transmission electron microscopy observation, reciprocal space mapping and omega-2theta scan in X-ray diffraction measurement, and geometric phase analysis are used for first identifying the existence of the high-Al layers (HALs) on both sides of a quantum well (QW) in three 3-period AlGaN QW structures of different deep-UV emission wavelengths. Then, optical analyses, including transmission and photoluminescence (PL) measurements, particularly the PL measurements under an applied stress along the sample c-axis, are undertaken for understanding the effects of such HALs on the band structures and hence the polarized emission behaviors of the samples. Simulation studies are also performed for providing the favorable comparisons with the experimental data. Basically, the HALs produce an extra compressive strain in the c-plane for lowering the heavy-hole (HH) band edge (lower than the edge of the split-off band) such that the TE-polarized emission through the electron transition between the conduction and HH band becomes dominating. In this situation, the light extraction efficiency of such a deep-UV light-emitting diode can be enhanced.

GaN material holds an advantageous position in the fabrication of power devices. This advantage is manifested by the possibility to perform GaN based devices working in high voltage, high current, high frequency and high temperature conditions. However, despite these theoretical forecasts, trapping mechanisms limit the performances of the GaN based devices revealed by the so-called “drain current collapse”. Our study is based on a methodology to understand trapping mechanisms in GaN metal-insulator-semiconductor high-electron mobility transistors. This work was achieved by means of electrical and optical characterization techniques such as Fourier transform deep level transient spectroscopy and photoluminescence. The activation energy and the apparent capture cross section of eight traps were extracted in normally-off (Ids=0A when Vgs=0V) AlGaN/GaN heterostructure technology used for power conversion. Six of these traps, E1=0.16eV, E2=0.31eV, E3=0.46eV, E4=0.5eV, E5=0.64eV and E6=0.79eV are electron traps located in the channel. An identification has been proposed for each trap. Two hole-like traps, H1=0.17eV and H2=0.74eV were assigned to the Mg and C doping of the GaN buffer layers, respectively. These traps might play a role in the current collapse which appears after the application of a large reverse voltage on the gate of the device. Furthermore, the results obtained using electrical and optical techniques allowed concluding that oxygen atoms and dislocations are incorporated in GaN layers during the growth.

GaN and related III-nitrides find great application in optoelectronic devices like light-emitting diodes (LEDs) and hence there is an increase in their demand. Although sapphire is still considered to be the most apte substrate for the growth of GaN based LEDs, but it is certainly not the most ideal one due to its low thermal conductivity and a large lattice mismatch with GaN. Hence, for the production of high performance and highpower LEDs, sapphire is being substituted by unconventional substrates having low lattice mismatch with GaN and a relatively larger thermal conductivity. The work focuses on the performance of such GaN-based LED materials and devices on unconventional substrates. It involves the detailing of the reasons responsible for the difficulty in developing GaN-based LEDs on the unconventional substrates. The solutions to outdo these difficulties and enhance the III-nitride growth are also elaborated along with the defect control, and chip processing for each type of unconventional substrate. The techniques developed to achieve such highperformance GaN LEDs are also discussed. With this it becomes quite easy to study the progress made in this particular field of physics. We strongly believe that with constant efforts in this field the quality of GaN based LEDs on unconventional substrates can improve progressively. And in the near future, with the usage of such unconventional substrates, we can produce LEDs commercially with a varied set of applications.

III-N-based edge-emitting lasers suffer from low refractive index contrast between GaN, AlGaN and InGaN layers, conventionally used in their epitaxial structures. This issue becomes more severe with an increase in wavelength at which those devices operate when tuning from blue-violet to real blue and green light. To overcome this issue and to increase the refractive index contrast other materials must be employed within the epitaxial structures replacing the standard nitride layers with materials with lower refractive index. We demonstrate results of effective-index numerical calculations performed for the state-of-the-art semipolar real blue (471 nm) and green (518 nm) edge-emitting lasers with structural modifications that include ITO, AlInN, plasmonic GaN:Ge and nanoporous GaN layers. Such solutions are extensively investigated for III-N-based EELs operating in blue-violet region but only separately. Using combination of these solutions we managed to increase optical confinement factor over twice in blue- and over 3.5-times in green-EELs.