This PDF file contains the front matter associated with SPIE Proceedings Volume 11686

Introduction to SPIE Photonics West OPTO conference 11686: Gallium Nitride Materials and Devices XVI.

We demonstrated a room-temperature continuous-wave (CW) operation of the milliwatt-class single-mode blue and green VCSELs with epitaxially grown AlInN/GaN DBRs on c-plane GaN substrates. The emission wavelength and the threshold current of the blue VCSEL were 442.3 nm and 0.40 mA, respectively. The wall plug efficiency of the blue VCSEL was 13.6%, which is the highest value ever reported. Moreover, stable CW operation beyond 1,000 hours has been confirmed under 0.6 mW at 25 °C. Lasing yield in a 2 in. wafer was more than 80 percent at an average threshold current of 1 mA. Furthermore, we also succeeded in providing high performance green VCSELs. The emission wavelength and the threshold current of the green VCSELs were 514.9 nm and 2.8 mA, respectively. The optical output power was over 1.5 mW and the wall plug efficiency achieved 3.7%, which is the best record for GaN-based green VCSELs. These results will enhance the possibility of the practical use of blue and green VCSELs in the near future.

We will give an overview of the progress in ultraviolet-emitting vertical-cavity surface-emitting lasers (VCSELs) and their potential applications in areas such as disinfection and medical therapy. This includes our demonstration of the shortest wavelength VCSEL, emitting at 310 nm under optical pumping, and a detailed analysis of its filamentary lasing characteristics. The UVB-emitting AlGaN-based VCSEL was realized by substrate removal using electrochemical etching, enabling the use of two high-reflectivity dielectric distributed Bragg reflectors. The potential of using this or alternative methods to push the emission to shorter wavelengths will be examined as well as concepts to realize electrically injected devices.

We will show our two recent results related to GaN-based VCSELs with AlInN/GaN DBRs. One is in-situ wafer curvature control of AlInN/GaN DBRs. Curvature data successfully provides us the strain condition in the AlInN layers, which leads to a high reproducibility for fabricating the AlInN/GaN DBRs. The other is laser operation of the VCSELs with large current apertures. While our previous VCSELs with only 8-15 µm apertures showed RT CW operations, our recent VCSELs now show RT CW operations with 5-30 µm apertures. such larger apertures will eventually provide much higher LOPs in the future.

The development of GaN-based vertical-cavity surface-emitting lasers (VCSELs) has been progressing rather fast in the past few years. Owing to the special device structures of GaN-based VCSELs, novel designs have to be incorporated in order to achieve high performance in terms of low threshold, high efficiency, stable mode profile and polarization direction. We demonstrate an easy and cost-effective way to realize index-guiding structure that significantly lowers the threshold and observe the transverse mode spectra of guiding and antiguiding VCSELs. We also realize the GaN-based VCSEL with high-contrast grating (HCG) as the top mirror and achieve polarization control over manipulation of HCG parameters.

GaN-based vertical-cavity surface emitting lasers (VCSELs) have attracted significant attention in recent years for their potential applications in solid-state lighting, optical communications, laser printing, and display technologies. The main challenge in the development and commercialization of GaN-based VCSELs is the absence of electrically conductive epitaxial distributed Bragg reflectors (DBRs) with high crystal quality and near-unity reflection. The GaN VCSELs based on the epitaxial (AlGaN/GaN or AlInN/GaN) DBRs and dielectric DBRs have been actively studied and demonstrated by several research groups. Besides, the blue VCSELs using the lattice-matched and electrically conductive nanoporous (NP) GaN DBR, composed of GaN/NP GaN alternating layers fabricated in a manufacturable way, have been recently demonstrated by our group and University of New Mexico. We will discuss these DBR mirror technologies as well as the nano-engineering based on the electrochemistry to create NP (Al)GaN as a low index medium. The selective nanopore formation/fabrication, mainly depending on the electrical conductivity contrast between the alternating layers, can be carried out on a large scale and barely necessitates complex growth techniques and structural limitations, resulting in manufacturable DBR fabrication for III-nitrides from green to UV.

III-nitride membranes offer novel device designs in photonics, electronics and optomechanics. However, substrate removal often leads to a rough back surface, which degrades device performance. Here, we demonstrate GaN membranes with atomically smooth etched surfaces by electrochemical lift-off, through the implementation of a built-in polarization field in the sacrificial layer. This leads to a faster reduction in the sacrificial layer free carrier density during etching and thus an abrupter etch stop, reducing the root-mean-square roughness down to 0.4 nm over 5×5 µm2. These results open interesting perspectives on high-quality optical cavities and waveguides in the ultraviolet and visible.

Blue InGaN/GaN broad-ridge laser diodes show rich lateral mode dynamics and filamentation effects in addition to the longitudinal mode dynamics that are known from narrow-ridge laser diodes. We find a complex spectral, spatial and temporal behavior caused by the interplay between lateral and longitudinal mode competition mechanisms. In the experiment we measure the spectral-temporal dynamics with a streak camera, and the lateral resolution is achieved by scanning through a magnified near-field image. Additional time-averaged measurements with a high-resolution spectrometer indicate the presence of multiple longitudinal mode combs and the lateral scans show that there are different lateral modes in each mode comb. But also in separate regions of the laser spectrum, lateral modes of different order can be found.

For certain, growing, class of applications it is necessary to have better suiting combination of beam and spectral quality. This specific combination of beam quality and spectrally broad emission can be delivered by superluminescent diodes (SLEDs). Additionally, the expected advent of nitride integrated photonic circuits and visible light communication call for new active optical elements, such as optical semiconductor amplifiers (SOA). Within this work we will present the progress in the development of broad spectral emission (>15 nm and OSA amplifiers which reach the gain of 10-20 dB at violet-blue wavelength range.

We propose a new Fabry-Perot (FP) GaN laser fabrication method utilizing an epitaxial lateral overgrowth (ELO) technique and m-plane cleavability of the GaN crystal. The removed m-plane InGaN laser having a lasing wavelength 408. 1 nm operated at a low threshold current density as low as 2.15 kA/cm². First, unlike a conventional ELO growth technique, we avoid coalescence between adjacent ELO layers, thus forming island-like ELO layer bars, which were later used as the base for FP GaN laser. Then, a laser device epilayers were epitaxially grown and laser ridge structure was fabricated on each of these non-coalesced island-like ELO base layers. Island-like ELO laser bar formation facilitates an easy removal of the laser bars mechanically using a commercially available adhesive polymer film. Our investigation found that cleavable m-plane of the GaN crystal assists in the liftoff of the fabricated m-plane InGaN FP lasers. We further confirm that the reported fabrication method can be adopted to semi-polar crystal plane orientations of GaN.

Narrow linewidth laser diodes (LDs) emitting in the near-UV (NUV) are gaining attention for applications ranging from spectroscopy to atom cooling and interferometry or other applications requiring high spectral purity. InGaN edge-emitting LDs can exhibit a power of hundreds of mW in an unstable multimode regime detrimental to aforementioned uses. In this paper we report on a compact and robust design based on a low-cost blue LD, a beam shaping optical system and a fiber Bragg grating (FBG) acting as a wavelength selective reflector. One longitudinal mode of the non-antireflection coated laser diode is selected by a close to 30 pm bandwidth FBG allowing a few mW output power around 400 nm and a sidemode- suppression-ratio approaching 50 dB exceeding our last published results. Our previous studies showed that a single-frequency regime with a sub-MHz integrated linewidth and an estimated intrinsic linewidth of 16 kHz was possible by a carefully engineered external cavity. We will study the influence of the cavity length with different fiber types (SM or PM). Assessment will focus on the linewidth and a detailed intensity and frequency noise analysis of the emission. We will also investigate for the first-time the stability of several types of UV-FBG submitted to tens of mW of 400 nm light guided into the fiber core. This work demonstrates state-of-the-art performances by connecting low-cost components and opens the way to the fabrication of highly coherent laser sources that could meet the markets for the NUV applications.

Blue multisection laser diodes are being used before for short pulse applications with high output powers. Hysteresis effects in the current-optical output power (IP) characteristics due to absorption and saturation are known. We additionally observed hysteresis effects in the current-forward voltage (IV) characteristics of these diodes. The voltage increases at the lasing onset current which is different from some observations before. We explain this rise with the shielding of the piezoelectric field, which exists due the stress of the material, due to an increased carrier density below the onset current.

UV-C laser diodes (LDs) have not been realized for many years owing to the problems of crystal quality and p-type conductivity control. In our group, AlGaN-based LD structures with low dislocation density were fabricated using AlN single-crystal substrates, and a p-type cladding layer with sufficient hole concentration was realized without impurities doped by distributed polarization doping (DPD). As a result, we have demonstrated pulsed current injection UV-C LDs at room temperature. We have also developed an on-wafer process technology to solve the problems of LDs manufactured by the conventional cleavage method. The key points of this method are the flatness of mirror facets, their angle to the cavity, and the coating of distributed Bragg reflector (DBR) on the mirror facets formed perpendicular to the wafer. The method is a combination of dry etching and TMAH wet etching to selectively expose the m-plane, and the DBR fabrication by atomic layer deposition (ALD) which provides good coverage. The LDs fabricated by the on-wafer method were observed to lase in the UV-C region when a pulsed current was injected similarly to the LDs fabricated by the cleavage method. These results indicate the potential of not only the realization of UV-C LDs but also for the fabrication of devices using high-Al-composition AlGaN with p-type conductivity and optical integrated circuits.

Ultraviolet (UV) semiconductor lasers are widely used in medical and industrial applications, and their commercialization is strongly desired. Recently, laser oscillation by current injection in the UV-C and UV-B regions has been reported . From now on, it is necessary to demonstrate CW operation, which is indispensable for practical applications. In order to realize this, it is important to understand the internal loss and optical gain of the current devices. In this presentation, we report the details of our UV-B lasers. Specifically, we would like to discuss the results of the variable stripe length analysis of the internal loss of the obtained device and the performance of the UV-B lasers. The results show that the internal losses of the optimized UV-B laser are relatively low and good values are obtained. We also discuss the details of polarization doping, which is very effective in the realization of these UV lasers.

Here, we present the design and fabrication of AlGaN/GaN heterostructures that constitute the active element of electron-beam pumped UV lasers. The design of multi-quantum-well separate confinement heterostructures (SCHs) and graded-index separate confinement heterostructures (GRINSCHs) was adapted based on Monte Carlo simulations of the electron penetration depth. The structures were synthesized by plasma-assisted molecular beam epitaxy on bulk GaN substrates, and validated using transmission electron microscopy and X-ray diffraction. Mirror facets were fabricated by cleaving. Cathodoluminescence studies prove the benefits of the GRINSCH design in terms of carrier collection efficiency. The threshold power density under optical pumping with an Nd-YAG laser (266 nm) was below 200 kW/cm² at room temperature.

Deep ultra-violet (DUV) light emitting diodes (LED) are expected to be the next generation of UV sources, offering significant advantages such as compactness, low consumption and long lifetimes. Yet, improvements of their performances are still required and the potential of Al_yGa_1-yN quantum dots as DUV emitters is investigated in this study. Using a stress induced growth mode transition, quantum dots (QD) are spontaneously formed on Al_0.7Ga_0.3N/AlN heterostructures grown on sapphire substrates by molecular beam epitaxy. By increasing the QD Al composition, a large shift of the QD photoluminescence in the UV range is observed, going from an emission in the near UV for GaN QD down to the UVC region for Al_0.4Ga_0.6N QD. A similar behavior is observed for electroluminescence (EL) measurements performed on LED structures and an emission ranging from the UVA (320-340 nm) down to the UVC (265-280 nm) has been obtained. The main performances of Al_0.7Ga_0.3N based QD LED are presented in terms of electrical and optical characteristics. In particular, the emission dependence on the input current density, including the emitted wavelength, the optical power and the external quantum efficiency are shown and discussed.

A combination of the sputtering deposition and high-temperature annealing is a promising technique for preparing low-dislocation-density AlN templates. In this talk, MOVPE growth behavior of AlGaN films grown on the annealed AlN templates and on conventional MOVPE-grown AlN templates was comprehensively discussed. The low density of screw- and mixed-type dislocations of the annealed AlN templates invoked the formation of hillock structures. By adjusting the MOVPE growth conditions and utilizing sapphire substrates with appropriate surface off-cut, dislocation-induced hillock structures were suppressed. Improved surface flatness resulted in higher EQE and better wavelength uniformity of the DUV-LED fabricated on the annealed AlN templates.

Using Si for n-type doping and Mg/In co-doping for p-type, AlN nanowire p-n junctions were demonstrated [1]. Optimal electrical activation of acceptor impurities was achieved by electron irradiation. Current voltage characteristics in forward bias conditions have established that the current was varying as Vn, (with n larger than 6) before activation while a space charge limited current regime was observed after activation. The formation of AlN NW p-n junction was assessed by electron beam induced current (EBIC) experiments, putting in evidence the electrical field associated with the junction. [1] A. M. Siladie et al, Nano Lett. 2019, 19, 8357−8364

GaN/AlN ultrathin quantum wells (QWs) were grown by metalorganic vapor phase epitaxy using a self-limiting process of GaN thickness to the monolayer (ML) level. 1 ML GaN/AlN QWs emit at 225 nm. The photoluminescence (PL) intensity ratio between low and room temperatures is improved from 0.1% for a conventional AlGaN-based QW emitting at 235 nm to 5% the 1 ML GaN QW. Further improvement to 50% was achieved by an ultrathin GaN QW on r-plane, showing promise of ultrathin GaN QWs as efficient UV emitters.

We demonstrated an enhancement of light-extraction efficiency (LEE) in an AlGaN UVC light-emitting diode (LED) using photonic-crystal (PhC)-reflector fabricated on a p-GaN contact layer, which was introduced to realize both of low resistivity and high-reflectivity in p-contact layer. We fabricated an AlGaN UVC-LED with PhC-reflector on p-GaN contact layer, and confirmed that the external quantum efficiency (EQE) was increased by 1.7 times by introducing PhC-reflector. We also fabricated flip-chip UVC LED with PhC-reflector on p-contact layer and obtained more than 50 mW output power.

Driven by applications like monitoring of combustion engines, toxic gases, nitrates in water, as well as the inactivation of multi-drug-resistant germs, the development of AlGaN-based light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) has markedly intensified. This paper will provide a review of recent advances in development of DUV-LEDs, including the realization of low defect density AlGaN heterostructures on sapphire substrates. The performance characteristics of DUV LEDs emitting in the wavelength range between 260 nm and 217 nm will be discussed and milli-Watt power LEDs near 233 nm will be demonstrated.

For the realization of highly efficient deep-ultraviolet light-emitting diodes (DUV-LEDs) based on III-nitride semiconductors, it is essential to improve the crystalline quality of the AlN templates for crystal growth of AlGaN. Our group has suggested sputtering deposition and post-deposition high-temperature face-to-face annealing (FFA) as a fabrication method of AlN films with low threading dislocation density (TDD) on sapphire substrates. Although the FFA enables reduction of TDDs, it possibly causes a cracking for AlN films due to a large thermal expansion coefficient mismatch between AlN and sapphire. In this work, we controlled the residual stress in AlN films by modifying the sputtering conditions. Consequently, we achieved crack-free AlN films with low TDDs.

Heavy reliance on extensively studied AlGaN based light emitting diodes (LEDs) to replace environmentally hazardous mercury based ultraviolet (UV) lamps is inevitable. However, external quantum efficiency (EQE) for AlGaN based deep UV emitters remains poor. Dislocation induced nonradiative recombination centers and poor electron-hole wavefunction overlap due to the large polarization field induced quantum confined stark effect (QCSE) in “Al” rich AlGaN are some of the key factors responsible for poor EQE. In addition, the transverse electric polarized light is extremely suppressed in “Al”-rich AlGaN quantum wells (QWs) because of the undesired crossing over among the light hole (LH), heavy hole (HH) and crystal-field split-off (SH) states. Here, optical and structural integrities of dislocation-free ultrathin GaN quantum disk (QDisk) (~ 1.2 nm) embedded in AlN barrier (~ 3 nm) grown employing plasma-assisted molecular beam epitaxy (PAMBE) are investigated considering it as a novel nanostructure to realize highly efficient TE polarized deep UV emitters. The structural and chemical integrities of thus grown QDisks are investigated by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We, particularly, emphasize the polarization dependent photoluminescence (PL) study of the GaN Disks to accomplish almost purely TE polarized UV (~ 260 nm) light. In addition, we observed significantly high internal quantum efficiency (IQE) of ~ 80 %, which is attributed to the enhanced overlap of the electron-hole wavefunction in extremely quantum confined ultrathin GaN QDisks, thereby presenting GaN QDisks embedded in AlN nanowires as a practical pathway towards the efficient deep UV emitters.

The aim of this paper is the definition of a model for the degradation of an AlGaN-based ultraviolet (UV) Light Emitting Diode (LED) with a nominal wavelength of 285 nm (UV-C). These devices are widely used in disinfection, sterilization, water purification, medical devices, in plant lighting and as insect traps; moreover, UV antiviral treatments are being developed recently, under the push of the current COVID-19 emergency. We analyzed the behavior of the devices during a constant current stress at the current of 250 mA, through electrical (I-V), optical (L-I) and spectral (PSD) measurements and steady state photocapacitance (SSPC) analysis. By investigating the optical measurements, we found out the presence of two different degradation mechanisms, one before 1000 min of stress and one after 1000 min of stress. We ascribed the first one to a decrease in the injection efficiency and we modeled it with a system of three differential rate equations to describe the dynamics of the de-hydrogenation of gallium vacancies, that lead to a defects generation. On the other hand, the second degradation mechanism is well correlated to the generation of midgap defects (Ec-2.15 eV), as detected from the SSPC analysis, that indicates the generation of non-radiative centers induced by the stress.

We here demonstrate thin-film flip-chip (TFFC) ultraviolet-B light-emitting diodes (LEDs) fabricated by a standard LED process and followed by a substrate removal based on selective electrochemical etching of an n-doped multilayered Al0.11Ga0.89N/Al0.37Ga0.63N sacrificial layer. The integration of the LEDs to a Si carrier using thermocompression bonding allowed roughening of the N-polar AlGaN side of the TFFC LEDs using TMAH-etching, which increased the light extraction efficiency by approximately 45% without negatively affecting the I-V-characteristics. This resulted in an optical output power of 0.47 mW at 10 mA for an LED with a p-contact area of 0.03 mm2.

Front-illuminated p-i-n GaN-based ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition (MOCVD) on 25 mm dia. bulk Ammono® n-GaN substrate having a low etch pit density (EPD) less than 5 × 10⁴ [cm^-2] and processed into 6×6 APD arrays. The devices employed N-ion implantation to achieve sidewall passivation. Evaluation of these 6×6 arrays will help to confirm the uniformity of the epitaxial materials and device processing. The maximum avalanche gain reached ~ 3×10⁵ at the breakdown (current limited). The dark current density was 10^-9 A/cm² at reverse bias up to -20 V and the APDs exhibited a reverse breakdown voltage of 81 ± 1 V for all 36 devices without any leaky devices, confirming a high uniformity of the growth and fabrication processes.

Development of an efficient red LED based on GaN is pivotal to ultra-small-size, full-color, and high-resolution micro-LED displays. In a red LED using Eu-doped GaN (GaN:Eu), the peak position of the emission is extremely stable against ambient temperature and injected current. Photoluminescence quantum efficiency of the Eu emission was investigated as a function of chip size of square structures. Even for sizes smaller than 24 µm, an influence of sidewall-related non-radiative recombination of carriers on the quantum efficiency was only minor as a result of limited carrier diffusion lengths in GaN:Eu. We also demonstrated monolithic vertically stacked full-color LEDs consisting of GaN:Eu and InGaN quantum wells. These results indicate a high potential of the GaN:Eu LED for the micro-LED applications.

III-Nitride based light-emitting diodes based on InGaN active layers formed on GaN, are fine for the blue and green emitting LEDs. However, the large lattice mis-match between red-emitting active layers and the GaN substrate still limits the efficiencies to very low values, typically <5%. We propose to use seeding techniques originally developed for nanowire growth, to seed the formation of ternary InGaN pyramids which later are converted to thin c-facet platelets of InGaN. I will in this presentation show that such relaxed, and dislocation-free, InGaN platelets with In-composition about 20%, have the potential as ideal templates for red-emitting microLEDs.

Two types of novel wavelength conversion device structure are proposed: a transverse quasi-phase matched waveguide and a monolithic microcavity. The former includes polarity-inverted stacked multilayer of AlN or GaN artificially fabricated by wafer bonding or epitaxial polarity inversion techniques. And the latter is a microcavity composed of a GaN domain with one coherence length sandwiched with two DBRs. The report on the successful demonstration of second harmonic generation and the discussion on the exploration for the novel nonlinear optical materials, even in the absence of birefringence and ferroelectricity, will be given in the presentation.

We obtained the EL intensity enhancement by a factor of 1.3 with increasing of n-GaN thickness from 2 to 8 µm. We achieved a light output, forward voltage, FWHM and external quantum efficiency of 0.64 mW, 3.3 V, 59 nm, and 1.6% at 20 mA, respectively. Particularly, the wall plug efficiency was 1.0%, which is comparable with the state-of-the-art InGaN-based red LEDs. The reduction of the in-plane compressive stress by the GaN underlying layers appears to be crucial for enhancing the light output of InGaN-based red LEDs on conventional sapphire substrates.

We present a critical study of LEDs on h-BN compared to the conventional LEDs on sapphire from materials characterizations, device fabrication to the device performances measurements performed before and after liftoff and transfer with and without intermediary adhesion layer to arbitrary substrates

GaInN/GaN multiple quantum shells (MQS) nanowires and p-GaN shells were embedded with n-GaN layers through tunnel junction (TJ) shells using metalorganic chemical vapor deposition (MOCVD) method. The MQS nanowires were selectively grown on n-GaN/sapphire or GaN substrates. The fabrication process of laser structures with different resonators of 600500, 750, 1000 μm, and cavity widths of 7, 12, and 17 μm were investigated with insulating layer on the sidewalls of the ridge. The structures of the fabricated devices were characterized by scanning electron microscope (SEM) and current-voltage-light output characteristics were evaluated. Two different methods for mirror formation, etching and cleavage, were developed for the laser devices. During the investigation, a superior mirror formation suffered from the difference in etching rate between GaInN and GaN, generating concaves in the MQS region. Bluegreen light emission was observed from the entire ridge surface of the MQS index-guided laser structures. A maximum current density of emission at 17.9 kA/cm² has been confirmed in the devices. The electroluminescence and cathodoluminescence measurements demonstrated that the r-plane and c-plane at the top of the MQS are dominant at low current densities, and the m-plane emission becomes stronger as the current density increases.

We report here our latest results of achieving high quantum efficiency (QE) N-polar GaN photocathodes through engineering of bulk and interfacial impurity concentrations. The photocathode structures studied consist of an N-polar p-GaN absorbing layer with 10 nm u-GaN cap layer. An increase in QE from 0.27% to 10.1% is observed for photocathodes grown without a regrowth interface between p-GaN and u-GaN cap layers compared to an identical structure with a regrown interface. SIMS is used to identify impurities at the regrowth interface, which cause lower QE due to scattering and modification of the electric field. The QE is further increased to 26.6% through a 2x increase in hole concentration when the structure is grown on a high hillock density template. This work demonstrates a combined approach that has the promise of advancing toward and surpassing performance of current state-of-the-art photocathodes in the UV range.

Solid-state, vacuum nanoelectronic devices have the potential to combine the advantages of vacuum electron devices, such as robustness in harsh environments and high frequency operation, and solid-state devices, such as size, integrability, and low-power operation. In this work, we demonstrate novel GaN nanogap field emission diodes that operate in air and exhibit ultra-low turn-on voltage, high field emission current, and excellent on-off ratio. We present experimental and modeling results on the field emission characteristics of these devices at various nanogap sizes and operating pressures. These results provide critical new insights into the behavior of this new class of devices and point to future challenges and opportunities. Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

III-Nitride semiconductors are attractive materials for optoelectronic devices including LEDs and LDs. To enhance the device performances, it is important to prevent impurity incorporation during thin film growth. The knowledge of atomistic-scale phenomena is indispensable to prevent contamination of thin films. In the present work, we model elementary growth processes in GaN MOVPE such as (1) vapor phase reaction, (2) surface reaction and (3) solid-phase diffusion during epitaxy. In this presentation, we discuss incorporation mechanisms of carbon in GaN(0001) and (000-1) MOVPE, and of oxygen in vicinal GaN(10-10) MOVPE.

High-temperature AlN homoepitaxial growth up to 1600℃ by HVPE was investigated. High-purity AlN with excellent crystallinity was reported where HVPE-AlN was homoepitaxially grown on the PVT-AlN substrate. However, quartz-derived impurities incorporation was slightly problematic in the HVPE-AlN. In this study, a new reactor was introduced that is a quartz glass reactor including a high-temperature growth zone constructed with heat-resistant materials. O and Si impurity incorporation was reduced with high growth rates of around 150 μm/h by the newly introduced high-temperature growth system. This work was partially supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan.

Combined photonic and electronic systems require diverse devices to be co-integrated on a common platform. This heterogeneous integration is made possible through several separation and transfer methods where the functioning epilayers are essentially released from their growth substrate. The use of 2D layered h-BN as a mechanical release layer has been demonstrated to be a promising technique for the hybrid integration of III-nitride devices. In this talk we will give an overview of our results on wafer-scale van der Waals epitaxy by MOVPE of different III-N heterostructure devices such as LEDs, HEMTs, solar cells, sensors and photodetectors. Furthermore, mechanical release and transfer techniques of crack-free III-N devices on foreign substrates will be presented along with a comparison between the device performances before and after transfer.

In this paper we would like to report the recent progress in basic ammonothermal GaN crystal growth. Growth on ammonothermal GaN seeds is presented. Structural properties of the seeds are shown and discussed. A new shape of seed crystals is proposed. It allows to eliminate the appearance of cracks in the growing GaN. The unwanted lateral growth is restricted and controlled. A higher growth rate, both in lateral and vertical directions, is obtained. Generally, the high structural quality of native seeds is maintained. In case of crystallization in lateral directions the structural quality of the seeds can even be improved. The influence of the crystallization run parameters, growth direction and configuration on the structural quality of the obtained GaN will be discussed.

The selective-monolithic growth of coaxial GaInN/GaN NWs was investigated by changing the TEG flow rate, barrier and well growth temperature during MQS growth. In incorporation increased with a higher TEG flow rate. However, In-rich flakes were formed the NWs resulting in the deterioration of crystal quality. Using a higher growth temperature of quantum barriers, abnormal growth at the top of NWs was eliminated. As a result, the CL emission intensity was enhanced. Furthermore, the occurrence of In desorption was suppressed by decreasing the growth temperature of quantum wells. Therefore, these results are promising for NW-based white LEDs.

GaN nanowire (NW) arrays are investigated with the main focus on their uniformity, which is a key requirement for the performance of photonic crystals. The NWs are fabricated by two established top-down approaches which are directly compared and critically discussed in terms of their individual merits and drawbacks. A statistical analysis of NW arrays with a nominal NW spacing of 200 nm and NW diameters of 30–70 nm yields distributions of the spacing (diameter) peaking around their nominal values within standard deviations of 2–6 (3–4) nm, a circularity of 0.88–0.89 and a yield of 99.9%.

We examine the MOCVD growth conditions on c-plane semi-relaxed InGaN substrates necessary for morphological improvement, defect reduction, and elimination of V-pits during epitaxy prior to the active region growth. V-pit defects can propagate through the crystal as epitaxy continues, causing serious morphological degradation. These defects may also be a source of leakage current if they form a low-resistance path through p-n junction. By employing an InGaN/GaN periodic structure, thick base layers can be grown with the morphology improving as the epitaxy proceeds, allowing for high quality layers to be achieved. High temperature (HT) GaN interlayers in the InGaN/GaN base layer structure then continue to reduce defects significantly, notably eliminating the V-pit type defect, and significantly improving growth morphology. Resulting microLEDs on these improved base layers exhibit a nearly three order of magnitude reduction in leakage current density at 1V, far below the μLED turn-on threshold, and significantly lower dynamic resistance. This result indicates the reduction in base layer defects and layer morphology improvement results in significant improvement in electrical performance and enables production of viable LEDs.

Efficient p-type doping of III-nitride materials is notoriously difficult due to their large band gaps, intrinsic n-type doping, and the large ionization energy of acceptors. Dilute-anion III-nitride materials are a promising solution for addressing this issue and increasing the activation efficiency of p-type dopants. Upward movement of the valence bands reduces the ionization energy of the dopants, allowing for enhanced p-type conductivity in comparison to the conventional nitrides. Incorporation of a dilute-arsenic impurity into AlN significantly reduces the ionization energy of Mg-acceptors from 500 meV to 286 meV, allowing for a two-order magnitude increase in activation efficiency in 6.25%-As AlNAs.

Vacancies in Mg-implanted GaN were probed using positron annihilation technique. Mg was implanted into GaN with [Mg] = 1E19 /cm3. For an as-implanted sample, the major defect species was identified as Ga-vacancy related defects. The sample was annealed under a nitrogen pressure of 1 GPa in a temperature range of 1000–1480C without a protective capping layer. Comparing with the sample annealed with the capping layer, although no large difference in the defect spices was observed, their concentration was decreased by the cap-less annealing. The diffusion of Mg during annealing was influenced by the presence of residual vacancies. H was unintentionally incorporated into the sample during annealing, and its diffusion property were also affected by vacancies and Mg. A part of this work was supported by MEXT “Research and development of next-generation semiconductor to realize energy-saving society (JPJ005357)” and the Polish National Science Centre through project No 2018/29/B/ST5/00338.

Time-resolved photoluminescence and light-induced transient grating techniques were applied for the comparative investigation of the evolution of the internal quantum efficiency and of the carrier diffusion in AlxGa1-xN (x > 0.6) heterostructures in 80 – 550 K temperature range and for excitations from 1 µJcm-2 to 1 mJcm-2. The decrease of the photoluminescence efficiency measured at high excitations and temperatures is quantitatively correlated to the increase of the diffusion coefficients of carriers and to the increase of their non-radiative recombination rate on vacancies. Complex numerical modelling provides a full set of carrier and exciton recombination parameters.

The current progress of the omnidirectional photoluminescence (ODPL) spectroscopy for quantifying the internal quantum efficiency (IQE) of a near-band-edge emission in GaN will be reviewed. IQE reflects the concentration of nonradiative recombination centers and deep impurities such as carbon. A prototype of the ODPL spectroscopy was proposed with “2pi-configuration”. However, the 2pi-configuration has a limitation on the sample size due to the integrating sphere used. Therefore, "phi-configuration" was introduced, where the crystal was placed outside the sphere. This allows to cool the sample down to 12 K in vacuum, and to perform a mapping experiment of IQE across the GaN wafers.

Photoluminescence spectroscopy using a scanning near-field optical microscope (SNOM) is a powerful technique to study luminescence properties of III-Nitride semiconductors. To date, we have developed a SNOM with an excitation and a detection wavelength of 210 nm and >220 nm, respectively.[1] The deep-ultraviolet (DUV) SNOM has the shortest operation wavelength ever reported and visualizes the localized emission nature of Al-rich AlGaN quantum wells with a spatial resolution exceeding 150 nm. In the presentation, recent progresses of our study using the DUV-SNOM are given. [1] Ishii et al., APL Photonics 4, 070801 (2019).

InGaN/GaN core-shell wire-based LEDs are a promising alternative to the c-plane two-dimensional LEDs for µLED display technologies. Developing correlative doping characterization techniques adapted to wires is essential to optimize the epitaxial structure. The impact of different LED growth conditions was investigated through current-voltage (I-V) and capacitance–voltage (C–V) characteristics on both single wire and assembly of wires. A statistical C-V study was carried out on ~70 single wires to evaluate the agreement in depletion width and effective doping level Neff values between single and assembly of connected wires. To corroborate the results, EBIC measurement were also performed to determine the position of the p-n junction and depletion region width.

Diffusion of Be was investigated for the main crystallographic directions in HVPE-GaN: c [0001], m [10-10], and a [11-20]. Be was implanted into the samples at room temperature with a dose of 2.9e15 cm-2 with energy of 200 keV. Ultra-high pressure annealing (UHPA) was performed to repair the post-implantation damage and activate the dopant. The annealing was performed at different time (15 and 30 minutes) and temperature (1200 – 1400°C). Depth profiles of Be were measured by secondary ion mass spectrometry (SIMS). Diffusion coefficients were calculated using the complementary error function (erfc) or Boltzmann-Matano analysis. The determined diffusion coefficients were summarized in an Arrhenius plot: D(1/T). From this relation the pre-exponential factor D0 and the activation energy were calculated. In addition, it was possible to calculate the formation energy for interstitial Be as well as gallium vacancy defects.

A novel force sensor capable of determining both the direction and magnitude of the force is proposed and demonstrated. The sensor structure is based on two orthogonally oriented arrays of GaN (Gallium Nitride) elliptical nanopillars and a digital CMOS (Complementary Metal Oxide Semiconductor) image sensor. The pixelated array also enables twodimensional force mapping. The directional sensitivity originates from the breaking of the nanopillar symmetry and the orthogonal orientation of the arrays. Each array is more sensitive to stress along the long axis of the nanopillars as opposed to the short axis of the nanopillars. Shear stress on the nanopillars causes a decrease in the overlap of the electron and hole wavefunctions which is proportional to the intensity of the light emitted from the nanopillars. Only a common bias, either optical or electrical, is needed across the entire device, making two-dimensional mapping simple and easily completed using a common digital CMOS camera. No stringent uniformity requirement across the sensor array is necessary. The data from an unstressed array is compared to the data from the same array under stress so that the relative change of emission intensity can be determined. With a combination two arrays, we showed that the direction and magnitude of the force can be determined by comparing the emission intensity change between two orthogonal arrays.

We study the impact of non-radiative defects on Auger recombination in c-plane InGaN/GaN single quantum wells (SQWs) in the efficiency droop regime using high injection time-resolved photoluminescence. The defect density in the active layer is tuned by varying the thickness of an InAlN underlayer. When the defect density is increased, apart from Shockley–Read–Hall (SRH) and standard Auger recombination, introducing an extra defect-assisted Auger process is required to reconcile the discrepancy observed between the usual ABC model and experimental data. We derive a linear dependence between the SRH coefficient and the bimolecular defect-assisted Auger coefficient, which suggests that the generated defects can act as scattering centers responsible for indirect Auger processes. Our results further suggest that the defect-assisted Auger recombination is expected to be all the more critical in green to red III-nitride light-emitting diodes due to their reduced radiative rate.

Recent reports indicated that the use of an InAlN underlayer (UL) can significantly improve the efficiency of InGaN/GaN quantum well (QW) LEDs. Currently, this result is explained by considering that the UL reduces the density of nonradiative recombination centers in the QWs. However, an experimental proof of the reduction of defects in the QWs is not straightforward. In this paper, we use combined electrical (I-V), optical (L-I), capacitance (C-V), steady-state photocapacitance (SSPC) and light-assisted capacitance-voltage (LCV) measurements to explain why devices with UL have a much higher efficiency than identical LEDs without UL. Specifically, we demonstrated an improvement in both electrical and optical characteristic for the sample with the UL; SSPC measurements revealed a higher concentration of defects in the active region for the sample without the UL (9.2x10¹⁵ cm^‑3), compared to the sample with UL (0.8x10¹⁵ cm^-3). In addition, we demonstrated that dominant defects are located near the midgap (EC ≈ 1.8 eV), thus acting as non-radiative recombination centers. Finally, we proposed a model to find the traps distribution in the active region. By comparing model and experimental results, we demonstrate the effectiveness of the UL in blocking the growth of defects in the bulk of the device, preventing their propagation towards the QW. The results presented in this paper give a proof of the effectiveness of the UL in limiting the propagation of defects towards the QWs and an experimental characterization of the related traps.

Longitudinal optical (LO) phonon has strong electric interaction with particles and fields. Particularly, the interaction in III-nitrides is more significant than that in conventional III-V materials. We show phonon-exciton interaction properties in experimental PL spectrum analysis and theoretical calculation of population transfers of excitonic levels. Thermally nonequilibrium occupations of LO phonons and other modes generated by the LO-phonon decomposition are thought to shift the population distribution in principal quantum number states and kinetic energy to the higher energy side. The radiative exciton recombination lifetime is determined by the population distribution in the excitonic states, which is determined by the balance of the electronic and phononic elementary processes. The interaction of excitons and phonons releases the excess energy to the thermal bath of the lattice system, which sometimes yields negligible lattice temperature increase in the excited region or the nonequilibrium state between electron and phonon systems. A Raman scattering imaging measure is introduced to exhibit spatial transport of phonons generated by the energy relaxation and nonradiative recombination of the excited electrons and holes, where pump-probe measurements are enabled by the simultaneous irradiation of two laser beams. It is found that the phonon transport is blocked by the misfit dislocations located on a Ga_0.84In_0.16N/GaN heterointerface.

We present principle of defect characterization of GaN using multiphoton-excitation photoluminescence and several experimental results. Threading dislocations and basal plane dislocations in n-type GaN are observed from near-band-edge emission imaging. The diameters of dark spots and dark lines are determined by the minority carrier diffusion length. Three-dimensional feature of dark lines is different by the Burgers vectors of dislocations. Based on these properties, nondestructive classification of dislocations is possible. Simultaneous detection of near-band-edge emission and yellow luminescence can be utilized to visualize three-dimensional growth processes such as facet-initiated epitaxial lateral overgrowth, which is used to obtain GaN substrates with low threading dislocation densities.

The growth of high-quality crystals of hexagonal boron nitride (hBN) in 2004 has revealed that hBN is a promising material for light-emitting devices in the deep ultraviolet domain, as illustrated by the demonstration of lasing at 215 nm by accelerated electron excitation, and also the operation of field emitter display-type devices in the deep ultraviolet. Although it was recently understood that hBN is an indirect bandgap semiconductor, the understanding of the optical response close to the bandgap is still an open debate. In hBN, the direct and indirect excitons are close in energy and the dielectric permittivity around 200 nm is expected to be governed by the interplay between direct and indirect transitions. Here I will present our recent reflectivity experiments in high-quality hBN crystals from single monolayer to bulk hBN. I will explain the originality of the dielectric permittivity close to the bandgap of hBN in comparison with other nitride semiconductors.

We study shallow and deep levels in carbon-doped hexagonal boron nitride crystals precipitated from a molten metal solution in a high-temperature furnace. Reflectance and photoluminescence under deep ultraviolet excitation are complemented by spatially resolved experiments by means of a scanning confocal micro-photoluminescence setup operating in the ultraviolet. Isotopically controlled carbon doping does not induce any energy shift of the well-known deep-level emission at 4.1 eV. Our detailed characterization in a series of carbon-doped crystals reveals that the incorporation of carbon during the growth process results in a distinct class of shallow and deep levels in hexagonal boron nitride, calling into question the exact role of carbon in the growth of hexagonal boron nitride and its direct or indirect influence on the formation of the crystal defects.

We determine the band alignment of ScxAl1-xN/GaN heterojunctions at ScN alloy fractions ranging from x = 0.04 to 0.20 using x-ray photoemission spectroscopy, and determine the band gap from spectroscopic ellipsometry. We find a transition from straddling to staggered gap as ScN alloy fraction increases, and show that the experimental results are consistent with first-principles calculations. This crossover from type-I to type-II band alignment shows a degree of freedom for engineering improved heterostructures in ScAlN/GaN-based electronic and optoelectronic devices.

The band structure and band alignment for ScxAl1-xN /AlN heterojunctions, with Sc-content x up to 25%, are investigated via first principle density functional theory (DFT) calculations. The supercell approach is utilized to build the crystal structure of ScAlN and AlN alloys and we investigate the natural band offsets of ScxAl1-xN relative to AlN through the potential line-up method. Our calculations suggest a type-I ScAlN / GaN heterostructure with large band offsets. The implementation of ScAlN will have impacts for deep ultraviolet, power electronics, and laser devices.

AlGaN is a leading candidate for current and future ultra-wide bandgap electronic and optoelectronic applications. However, 3D etch technologies for AlGaInN remain immature compared to silicon, limiting its full potential for novel devices. Here, we build from the foundation of anisotropic KOH-based wet etchants used to fabricate GaN structures and explore AlGaN alloys etched in acids and bases. We investigate the etch reactivity of AlGaN alloys as a function of Al content in various etchants. We then explore the etch evolution of novel nanostructures observed and discuss possible mechanistic explanations. Lastly, we look at field emission properties of AlGaN alloys.

We have investigated AlGaN/GaN high-electron mobility transistors (HEMTs) with a high κ gate dielectric using hafnium silicate (HfSiO_x). The (HfO₂)/(SiO₂) laminate structure was deposited on the AlGaN surface by a plasma enhanced atomic layer deposition, followed by a post-deposition annealing at 800 °C. The HfSiO_x-gate HEMT showed good transfer characteristics with a high transconductance expected from its κ value and a subthreshold swing of 71 mV/ decade. In addition, we observed excellent capacitance-voltage (C-V) characteristics with negligible frequency dispersion in the metal-oxide-semiconductor (MOS) HEMT diode. The detailed C-V analysis showed low state densities in the order of 10¹¹ cm^-2 eV^-1 at the HfSiO_x/AlGaN interface.

We have performed the operando analysis using synchrotron radiation nanobeam X-ray diffraction (nanoXRD) techniques combined with the pump-probe method to quantitatively analyze strain induced by the inverse piezoelectric effect (IPE) in AlGaN/GaN HEMTs in operation. The c-plane lattice spacing change of the AlGaN barrier depending on applied gate voltage to the device and its transient states were successfully detected with a nanosecond resolution. Since our nanoXRD analysis directly characterizes IPE-induced crystalline lattice structures, it paves the way for novel approaches to elucidate defect formation mechanisms in nitride semiconductor based HEMT devices.

GaN high electron mobility transistors (HEMTs) on SiC substrates are produced for both commercial and defense applications that require high voltage, high power, and high efficiency operation. Although leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability, long-term reliability in the space environment remains a major concern due to unknown degradation mechanisms. For the present study, we continued to investigate degraded RF GaN HEMTs using micro-analytical techniques. Our RF AlGaN-GaN devices grown on SiC substrate had a Ni-Pd-Au Schottky gate length of 0.25 μm, a total gate width of 6 × 150 μm periphery, and a field plate. First, we performed DC bias-temperature stress tests on GaN HEMTs and some GaN HEMTs were thermally stressed as control samples. Second, we employed focused ion beam (FIB) to prepare TEM cross sections from degraded and control devices for defect analysis using a high resolution TEM. Pits, cracks, and pockets of palladium were found at the edge of the drain side of the gate. We report our detailed analysis results including our understanding on the out-diffusion of Pd as a potential degradation mechanism in our RF GaN HEMTs.

The band structures of wurtzite and zincblende III-nitrides are aligned by the electron affinities and the band gaps calculated using a unified hybrid density-functional theory. Based on the Anderson’s electron-affinity rule, the conduction (and valence) band offsets of 1.60 (1.15), 2.47 (0.30), and 4.07 (1.45) eV have been extracted for wurtzite GaN/InN, AlN/GaN, and AlN/InN interfaces, where the conduction (and valence) band offsets of 1.85 (0.89), 1.32 (0.43), and 3.17 (1.32) eV have been procured for zincblende GaN/InN, AlN/GaN, and AlN/InN interfaces, respectively. The valence band edges of both wurtzite and zincblende ternary III-nitrides could be linearly interpolated because the dominant anion compositions at the valence band maximum have a weak dependence on the cation mole fractions. Contrarily, the large bowings on the conduction band edges are attributed to the cation-like nature. Both wurtzite and zincblende AlGaN behaves differently from InGaN and AlInN because (1) the conduction band edges at Γ-valley are composed of anion orbitals, which account for the linear relationship between the conduction band edges and the cation mole fractions, and (2) the conduction band edges of zincblende Al_xGa_1-xN shift from Γ- to X-valley when x > 0.65, which results in an anion-to- cation transition and leads to a large conduction-band-edge bowing.

There is a soaring demand for UV lamps emitting at 220-270 nm for applications in disinfection. These needs are currently met by mercury lamps, hazardous for heath and the environment. Despite intense studies on UV LEDs, their efficiency remains limited by problems related to electrical injection. Here, we propose electron pumped UV lamps as an alternative to LEDs in this spectral range. For this purpose, superlattices of close-packed self-assembled AlGaN quantum dots are particularly promising, due to their high internal quantum efficiency (around 50%) and promising external quantum efficiency (4% in as-grown material, increasing to 7% by dice polishing).

Implantation of Ga ions into ammonothermal GaN crystals is proposed as a method of controlling the concentration of gallium vacancies. Ultra-high pressure annealing (UHPA) is expected to facilitate the diffusion of the implanted Ga ions into the entire volume of the crystal. Gallium vacancies are expected to be replaced by the Ga ions. Since Ga vacancies act as acceptors in GaN, reducing their content will result in a higher free electron concentration in the crystal. Gallium ion implantation and UHPA processes will be presented in detail. Values of the main parameters of UHPA allowing for the complete replacement of the Ga vacancies with Ga ions in GaN crystals will be determined. The morphology as well as structural, optical, and electrical properties will be compared for samples without any treatment and those implanted and annealed. The ultimate goal is to obtain highly conductive GaN crystals with the lowest possible Ga vacancy concentration.

The LEDs were obtained that the peak emission wavelength and FWHM were 665 nm and 67 nm at 20 mA, respectively. It exhibited a large blueshift of the EL peak wavelength from 691 nm at 5 mA to 631 nm at 100 mA. In this range, the blue-shifted value was 60 nm. Besides, we realized the single peak emission LEDs without an additional emission. We obtained a light output, forward voltage, and EQE of 0.07 mW, 2.45 V, and 0.19% at 20 mA, respectively. The LEDs exhibited the temperature stability of EL intensity and peak wavelength.

Fabrication of indium tin oxide (ITO) was optimized for InGaN-based amber/red light-emitting diodes (LEDs). A radiofrequency sputtering reduced the sheet resistivity of ITO at low pressures, and a subsequent two-step annealing resulted in a low sheet resistivity (below 2×10-4 Ωcm) and high transmittance (over 98%) in the amber and red regions between 590 nm to 780 nm. Double ITO layers by sputtering could form an excellent ohmic contact with p-GaN. Application of the double ITO layers on amber and red LEDs enhanced light output power by 15.6% and 13.0%, respectively, compared to those using ITO by e-beam evaporation.

This work found larger red chips exhibited lower forward voltages due to their lower series resistance originated from the device area. Moreover, a larger chip resulted in a longer emission wavelength, narrower full-width at half maximum (FWHM), and higher EQE at high currents. These characteristics are beneficial for InGaN-based red LEDs. On the other hand, smaller chips had merits of a high characteristic temperature of 399K. This characteristic temperature is almost similar level with the world record in AlGaInP-based red LEDs, suggesting that InGaN-based red LEDs with small chip sizes are good candidates for temperature tolerant lighting applications.

A tunnel junction and a n-GaN cap layer grown on the multi-quantum shells (MQS) /nanowires are introduced to decrease the resistivity and optical loss. The selective-area growth of the MQS/nanowire core-shell structures on the template was performed by metalorganic vapour phase epitaxy (MOVPE). Further, the MQS structure was covered with the tunnel junction and the n-GaN cap layer. Here, the growth conditions of the n-GaN cap layer were systemically investigated. The effect of p-GaN shape on the morphology of grown n-GaN cap layer was also assessed.

An optics-free spectrometer is demonstrated based on InGaN semiconductor photodiodes. The design is insensitive to the incident angle of light and delivers decent light absorption across the visible spectrum. In combination with compressive sensing algorithms, a high-performance spectrometer suitable for portable applications as well as sensing in harsh environments is shown.