Gallium Nitride Materials and Devices XIX

In this presentation, seeded growths of large diameter GaN crystals using the low-pressure acidic ammonothermal (LPAAT) method operated at around 100 MPa will be demonstrated. Nearly bowing- and mosaic-free GaN crystals exhibiting the full-width at half-maximum values for the 0002 X-ray rocking curves below 20 arcsec were achieved on high lattice coherency c-plane AAT seeds with gross dislocation densities in the order of 104 cm−2. The photoluminescence spectra of the grown crystals exhibited a predominant near-band-edge emission at 295 K, of which intensity was one order of magnitude higher than the characteristic deep-state emission called "yellow luminescence band". A nearly bowing-free large diameter c-plane GaN crystal was eventually obtained.

Gallium nitride (GaN) crystals of the best structural quality are grown by ammonothermal method in a supercritical ammonia solution inside high pressure autoclaves. This lecture will focus on the basic ammonothermal growth. The growth mechanism in different crystallographic directions, growth morphology and structural quality of GaN crystals, will be discussed. Structural properties and shape of the seeds will be shown. The influence of the crystallization run parameters, the internal configuration of the autoclave and the structural quality of the obtained GaN will be discussed. The path from bulk crystal to finished substrate of GaN will be presented. All challenges and difficulties to grown bulk GaN from ammonothermal method will be demonstrated. Scenarios for the further development of bulk GaN crystallization will be presented.

Practical application of devices using GaN substrates has been slow to progress. One of the big reasons is that the price of the GaN substrate is very expensive. To solve this issue, we propose a new GaN substrate slicing method using laser as a method that can contribute much more to the cost reduction of GaN substrate. With this technology, it is possible to reduce kerf loss to almost zero. Under the current slicing conditions, the damage depth after laser slicing is about 14um, and device epilayers could be grown on the sliced substrate by removing this thickness. In addition, this slicing does not require water or coolant and does not contaminate the workpiece. Therefore, laser slicing can also be used to thin the devices. By using laser slicing instead of backgrinding at the end of the device fabrication process, device layers can be thinned while leaving the substrate reusable.

The basic characteristics of group-III nitride films prepared by pulsed sputtering have been investigated. We have found that films prepared by sputtering shows high electron and hole mobilities. We have also found that Heavily donor-doped GaN and AlGaN showed record low resistivity. and were successfully applied to various devices such as the source/drain of AlN/AlGaN HEMTs and tunnel contacts for optical devices.

ScAlMgO4 (0001) is a substrate material suitable for heteroepitaxy of In0.17Ga0.83N because lattice matching can be achieved. Although direct growth of In0.17Ga0.83N layers on ScAlMgO4 (0001) substrates is ideal, direct growth by metalorganic vapor phase epitaxy results in nitrogen polarity. In contrast, In0.17Ga0.83N low temperature buffer layers can flip the polarity. On the group-III-polar In0.17Ga0.83N/ScAlMgO4 templates red-emitting quantum wells (QWs) and LEDs were fabricated. The QWs had much better internal quantum efficiencies than those on conventional GaN/sapphire templates, and electrical drive of LEDs was demonstrated.

High-speed growth (>1um/min) of highly-pure homo-epitaxial GaN layers ([C], [O] < mid-1e14/cm3) suitable for a drift layer of vertical-type power device became possible by using a quartz-free HVPE. This highly pure GaN crystal exhibited a record high room temperature and low temperature electron mobilities of 1480 and 14,300 cm2/Vs, respectively. Difficulty in alloy composition control of AlGaN crystals in HVPE-method due to parasitic vapor phase reaction was suppressed by careful adjustment of the growth conditions. As a result, AlGaN layers with good surface and crystal qualities were successfully prepared within almost the entire Al-fraction range by the HVPE method.

We have demonstrated the operation of UV-B laser diodes at room temperature by current injection using lattice-relaxed high-quality AlGaN fabricated on a sapphire substrate. Meanwhile, there are still many issues to be solved for practical use, especially the high threshold current required for laser oscillation and the low light output obtained. In this presentation, we report the results of our study on the reduction of lateral optical loss by applying a refractive index waveguide structure to reduce the threshold current. Specifically, device design and process technology development were carried out, and as a result, laser oscillation with an oscillation wavelength of 298 nm and a threshold current of 85 mA was obtained under room temperature pulse driving. In addition, optimization of the device layer structure for increasing the output power will be discussed.

The growth of high In content InGaN material is notorious for being challenging because of high mechanical strain and thermodynamic instability of the system. It has been shown that one can improve the growth quality by using variable surface miscut. In this study, we demonstrate the use of micropatterning of bulk GaN substrates in order to improve the quality of high In content layers. During MOVPE growth the quality of the InGaN layers and the In content depends on the local shape of the substrate surface, reaching the highest emission intensity at the top of every pattern. We study patterns with characteristic sizes ranging from 1 to 6 µm created using two methods: binary photolithography combined with a thermal reflow process as well as multilevel photolithography. The latter allows for achieving lower inclination of the sidewall of the pattern which in turn supports a more stable growth process. The properties of the samples are studied using fluorescence microscopy, microphotoluminescence mapping and carrier diffusion measurements.

Nitride LEDs can emit over a wide spectral range with particularly high efficiency in the blue. The active regions of these devices are InGaN/GaN quantum wells (QWs) which exhibit emission spectra that are much broader than expected. This broadening has been widely debated in the literature and is often attributed to spatial fluctuations in the emission energy due either to the intrinsic compositional disorder of the ternary alloy or to extrinsic growth inhomogeneities and structural defects. These different causes of disorder occur at different scales, ranging from a few nm to several hundred nm. To study the effects of disorder on the electroluminescence processes at the relevant scales, we have developed a novel approach based on Scanning Tunneling (Electro-)Luminescence Microscopy. We have applied this technique for the simultaneous mapping of the surface topography and the electroluminescence of an operational InGaN/GaN LED. Significant changes in the local electroluminescence spectrum are observed at the scale of alloy disorder and spectacular effects on the emission energy and intensity are evidenced in the vicinity of V-pits that result from emerging dislocations.

Isotopic purification allows to improve some intrinsic properties of hexagonal boron nitride (hBN), such as the phonon-polariton lifetime [1] and the thermal conductivity [2], however the impact of isotopic purification on the defects properties has been scarcely studied. Here, we extend the class of isotopically-purified hBN crystals to 15N and report experimental results obtained by UV-photoluminescence spectroscopy on the intrinsic band-edge and the 4.1 eV emissions in isotopically purified hBN crystals. The isotopic bandgap shift has been estimated to -4.4 meV for 10B14N and +3.9 meV for 11B15N [3], whereas, we evaluate the ZPL shift to -1.8 meV for 10B14N and +1.1 meV for 11B15N. Moreover, the LO3(Γ) phonon mode is shifted by +4.5 meV for 10B14N and -3.9 meV for 11B15N compared to NatBN, while we estimate the splitting between the ZPL and the 1st phonon replica is shifted to +0.9 meV for 10B14N and -1.3 meV for 11B15N. [1] Giles et al, Nature Mat. 17, 134. (2018). [2] Cai et al, Phys. Rev. Lett. 125, 085902 (2020) [3] Janzen et al, Advanced Materials, submitted (June 2023) arXiv:2306.13358

Proton irradiation-induced point defects acting as Shockley-Read-Hall (SRH) recombination centers in homoepitaxial GaN p-n junctions were characterized based on analyses of recombination current. Positron annihilation spectroscopy (PAS) data indicated that the vacancies in the GaN specimens comprised Ga vacancies and divacancies. The SRH lifetimes were decreased with the increase of the 4.2 MeV proton dose. For the same dose, the carrier concentrations and the SRH lifetimes for p-/n+ junctions were significantly reduced compared with those for p+/n- junction. The results suggest the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes. The authors thank Mr. Takahide Yagi and Mr. Joji Ito of SHIATEX Co., Ltd., for performing proton irradiation and irradiation simulations. The authors thank Dr. Akira Uedono of Tsukuba Materials Research Co., Ltd., who is also a professor at the University of Tsukuba, for assisting in the assessment of vacancy types using PAS.The authors thank C-TEFs at Nagoya University for fabricating the devices used in this work.

We have conducted simultaneous photoacoustic (PA) and photoluminescence (PL) measurements to accurately estimate the internal quantum efficiency (IQE). The method detects light from radiative recombination through PL measurement and heat from non-radiative recombination through PA measurement. In this study, we have applied the method to an InGaN-QW sample on a “stripe-core” GaN substrate in which the dislocation density periodically changes. Considering that photo-excited carriers recombine either radiatively or non-radiatively, the heat generation will increase in the defective region where emission efficiency is weak. In the line-scan measurement, the position-dependent complementary relationship between the PA and PL intensity is clearly observed.

Recent carrier diffusion length measurements in InGaN quantum wells (QWs) revealed the potential for carriers to travel tens of micrometers before recombination. These observations are consistent with the efficiency loss in InGaN micro-Light-Emitting-Diodes (µLEDs) with size reduction down to a few microns. From micro-photoluminescence and cathodoluminescence measurements, a QW-width-dependent study on InGaN QWs grown on various substrates show a diffusion length reduction with QW thickness. This is consistent with the fact that carrier lifetime decreases with QW-width in c-plane InGaN QWs, due to a Quantum-Confined-Stark-Effect (QCSE) reduction. Additionally, a study on the effects of carrier density and substrate-type will be presented.

Gallium nitride (GaN) micropillar structures show tremendous promise for light emitting diodes (LEDs) and power electronics due to their small size and high performance. However, reactive ion etching (RIE) used for top-down fabrication of these structures results in surface damage in the form of roughened, non-vertical sidewalls, diminishing performance. In this study, we demonstrate the effectiveness of wet etching in KOH solution at removing this damage and passivating the microstructure surface. We have found that increasing the etch solution temperature and concentration can lead to a 20% and 45% increase in etch rate respectively and effectively passivate the structure surface.

In this work, strain relaxation in green-red emitting InGaN/GaN quantum well (QW) structures are investigated by transmission electron microscopy. In these structures, high indium content QW strain relaxation takes place through hexagonal domains formation inside GaN barrier just on QW top. With In increases, domains become limited by I1 stacking faults (SF) which can terminate by Frank partial dislocations at the junction when folding to prismatic planes. Closed domain defects two configurations are observed with rotated or parallel I1 basal SF. Two possible origins of a-type threading dislocations emanating from such defects and propagating to free surface, are then identified.

AlN is a UWBG material (Eg=6.1 eV) for the realization of UV optoelectronics and high-power, high-frequency electronics. Although most challenges in crystal growth and epitaxy of AlN have been overcome, achieving controlled electrical conductivity of technological interest has proven to be challenging. While controlling the chemical and electrochemical potentials during doping has been crucial for achieving doping and compensation control, the obtained conductivity was still modest. The presumed DX formation has been considered an insurmountable killer defect as it pins the Fermi level and imposes a low limit on the achievable free carrier concentration. We have developed several novel equilibrium and non-equilibrium approaches for doping and point defect management in AlN and shown that donors in AlN do not undergo a DX transition but rather are distributed between a shallow and deep state. We have shown that the shallow state can be kinetically stabilized to obtain highly-conducting n-type AlN with mobilities approaching 400 cm2/Vs. These results have enabled the first demonstration of AlN Schottky diodes capable of >3 kA/cm2 with a critical breakdown field exceeding 10 MV/cm.

Ion implantation (I/I) is a key technology for preparing semiconductor devices. In the case of GaN, I/I is still under development. The formation of n-type and p-type regions remains a major challenge. In this paper, we will focus on analyzing the effect of structural quality, represented by the threading dislocation density (TDD), on the diffusion of implanted silicon (Si; donor) and magnesium (Mg; acceptor) in GaN. Four (0001) GaN substrates with different TDD, varied from 103 cm-2 to 1010 cm-2 will be used. Substrates with different TDD will be implanted with Si and Mg. The samples will then be annealed at a few temperatures at the same high nitrogen pressure and time. Analysis of the diffusion profiles of the implanted dopants will allow, using the finite element analysis (FEA), to determine D and the activation energies for GaN as a function of TDD.

Positron annihilation spectroscopy is a powerful set of methods for the detection, identification and quantification of vacancy-type defects in semiconductors. In the past decades, it has been used to reveal the relationship between (opto-)electronic properties and specific defects in a wide variety of elemental and compound semiconductors. In typical binary compound semiconductors, the selective sensitivity of the technique is rather strongly limited to cation vacancies that possess significant open volume and suitable charge: negative of neutral. I will present recent advances in combining state-of-the-art positron annihilation experiments and ab initio computational approaches. The latter can be used to model both the positron lifetime and the electron-positron momentum distribution based on electronic structure calculations. These quantities can be directly compared with experimental results. These methods have been applied to study a wide variety of nitride semiconductors. The focus will be on recent advances in identifying an quantifying defects in AlGaN alloys and GaN/AlGaN device structures.

To fabricate deep-ultraviolet(DUV)-LEDs with high efficiency, the crystallinity of AlGaN must be improved, and is significantly influenced by that of the underlying AlN template. The face-to-face annealed sputter-deposited AlN templates (FFA Sp-AlN) on sapphire have achieved screw- and edge-dislocation densities (TDDs) of 10^4 cm^-2 and 10^7cm^-2. Reduction of TDDs in FFA Sp-AlN and surface flattening of AlxGa1−xN grown on the FFA Sp-AlN play important roles to achieve a high external quantum efficiency (EQE). After the MOVPE homoepitaxial growth of AlN, the FFA Sp-AlN exhibits ideally smooth surface morphology. The EQE of the UV-C LED fabricated on FFA Sp-AlN increased with the TDD reduction of the FFA Sp-AlN. Maximum EQE of 8.0% and output power of 6.6 mW at a 20-mA input were achieved with the peak emission wavelength of 263 nm. We also fabricated 230 nm and 236 nm LEDs on the FFA Sp-AlN templates.

We have obtained longitudinal optical-like phonon resonant mid-infrared emission (LORE) and absorption peaks using micro-surface line-and-space structures of metallic plates on III-nitrides with stronger interaction of longitudinal optical phonon and radiation than conventional III-V compounds. We show the mechanism dominating the emission peak energy in the range of 670 – 900 /cm from line and space structure of metallic plates on AlGaN films in a temperature range of 450 – 630 K. Particularly, this LORE shows stronger thermal intensity than black-body-like emission and independent of the emission direction, which is quite different from the property of widely studied surface phonon polariton. This mechanism shows the feasibility for high-efficiency mid-infrared emission devices.

This research delves into the promising field of Visible Light Communication (VLC), focusing on InGaN-based micro-LEDs' potential for high-speed applications. It examines yellow-green micro-LEDs with nanoporous distributed Bragg reflector (NP-DBR) and red InGaN micro-LEDs, which achieved significant quantum efficiency, bandwidth, and data rates. It also spotlights the superior performance of red micro-LEDs with single quantum well (SQW) structures over double quantum wells (DQWs), noting higher efficiency, bandwidth, and faster transmission rates. These findings suggest a significant potential for full-color micro-display and high-speed VLC applications, promising a breakthrough in next-generation communication technology.

We report on 3D micro-LEDs consisting of a nitride-based core and a p-doped PEDOT polymer shell. The doped polymer shell is deposited from the gas phase using oxidative chemical vapor deposition (oCVD). oCVD can be directly integrated with state-of-the-art semiconductor processing technologies and allows for a gentle deposition from the gas phase at moderate vacuum conditions and temperatures below 150°C. We characterize the electrical properties of the hybrid organic/inorganic interface with temperature-dependent current-voltage measurements. Applying a Schottky model, we identify thermionic emission of electrons as the dominant conduction mechanism across the heterojunction and extract a mean barrier height of 1.4 eV with an ideality factors as low as 2.0 at room temperature indicating a low defect density at the interface. The results give a conclusive picture of the chances of oCVD-PEDOT/nitride heterojunctions for the use in 3D micro-LEDs.

We discuss the device performances of triboelectric pressure sensors (TEPSs) using GaN nanowires (NWs) as a response medium. TEPSs were fabricated by stacking polydimethylsiloxane directly on GaN NWs formed on Si(111) and forming an electrode underneath the substrate. When hammering the TEPS with a finger, the output voltage was measured to be 14.7 V, which is 2.5 times higher than that (5.9 V) of the TEPS without GaN NWs. The output voltage of TEPSs rarely changed even after the 100-cyclic measurements. These results will be discussed using an analytical model with the spatial electrostatic induction of GaN NWs.

Laser power beaming technology has been expected as a promising power supply system due to its advantages of small size, long range. However, it is necessary to solve the safety problem for the human eye. In this study, we focused on InGaN photovoltaic cells absorbing light below 400 nm, with little light absorption in the retina. We investigated the conversion efficiency under 394 nm laser light using InGaN cells with different grid electrode spacing on a p-GaN layer. As a result, InGaN photovoltaic cell with 70 μm grid electrode spacing showed conversion efficiencies of 21.8% and 16.5% at 80 mW/cm2 and 800 mW/cm2 irradiation, respectively. However, InGaN cell with 95 μm grid spacing showed efficiencies of 20.4% and 14.3% under 90 mW/cm2 and 816 mW/cm2 irradiation, respectively. These results suggested that reducing lateral resistive loss of a p-GaN layer was essential to achieving high efficiency under high-power irradiation.

Nonlinear optical microcavity structures within GaN meta-surfaces monolithically integrated with ZnO nanoantennae form an ideal platform for realizing tunable nanophotonic sources and directional emitters. Highly efficient second harmonic generation (SHG) is demonstrated from GaN micro-pyramids selectively grown on a wafer-scale substrate to form a meta-surface with ZnO nanoantennae at the tip of each pyramid. The nonlinear light generation from this cavity-based antenna depends on the relative competition of the second and third-order nonlinear process. The selective excitation of a single nanoantenna within the meta-structure can control the nonlinear light generation process. The energy density and frequency of the excitation source compared to the semiconductor's bandgap influences the coherent scattering and directionality of the emission process from the excitation of the meta-structure. The efficiency and directionality of the SHG signal depend on the localized spatial excitation of the ZnO nanorod and GaN micro-pyramid.

Gallium nitride (GaN) laser diodes (LDs) show great promise for industrial and medical applications. In this work, we demonstrate an edge-emitting GaN LD with etched mirror facets emitting in the UV-A range utilizing an asymmetric waveguide structure. The use of a thick (500 nm) lower waveguide offers reduced optical absorption losses while a thin (100 nm) upper waveguide minimizes carrier (hole) losses. LDs fabricated from this epitaxial structure with a 1000 µm long cavity show a lasing threshold of 2.2 A, and 111.8 mW per facet peak output with a differential efficiency of 3%.

Wide bandgap semiconductor Gallium nitride (GaN) has a great potential for various applications in optoelectronics, UV and visible lasers.In particular, the formation of exciton-polariton, which is one of the candidates for the next optoelectronics, is possible at room temperature conditions owing to the large exciton binding energy with strong oscillator strength in this semiconductor. Exciton polariton can be applied to various applications such as energy efficient polariton laser[ref] with ultra-low threshold density and all optical logic gate. However, to enter this intriguing quantum electrodynamics, robust exciton with high oscillator strength and cavity photon with high Q factor is required. Here, we have created GaN trianular cavity structure with loss controllable post by metal-organic chemical vapor deposition. We measure power dependent angle-resolved PL and confirm room temperature polariton lasing in our sample by intensity is increased super linearly and there are linewidth broadening and blue shift that evidence of polariton lasing.

Aluminum nitride (AlN) exhibits large breakdown electric fields and high thermal conductivity which allows for excellent miniaturization and power density in high-power electronics. In this talk, we investigate normally-off vertical n-channel trench MISFETs, with the channel consisting of nominally undoped graded AlGaN. The graded AlGaN layer creates immobile volume charges, and the lack of impurities reduces impurity scattering. The n-doped drift layer is composed of AlN for optimum electric field management. Contacts are placed on AlGaN for low ohmic resistance. Using TCAD simulations, the physics of device operation is studied. For comparison, a conventional impurity-doped FET without PID is taken as reference device. The simulations encompass calculation of local strain, solution of the Poisson-equation and electron and hole continuity equation on a 2-dimensional cross-section of the device. Transfer characteristics, threshold voltage, on-resistance and electric field are discussed. Surface states and interface charges at the nonpolar trench sidewall are included in the study. Finally, technological implementation and experimental results are discussed.

The development of group-III nitride materials has started a new era of GaN-based high-power devices, which have achieved a remarkable progress since then. However, the current large gap between theoretical performance predictions based on material properties and device physics on one side and practically achievable device figures of merit on the other requires a deeper understanding of the complex heterostructures, their inherent electrical fields, doping properties, interface quality and crystal defects. In this study, we will present the nano-scale correlation of structural, electronic and optical properties of a GaN-based lateral p-n+ superjunction and the two-dimensional electron gas (2DEG) of a lateral AlGaN/GaN field-effect transistor by cathodoluminescence directly performed in a scanning transmission electron microscope.

The sublimation of GaN is a powerful alternative etching technique to avoid the electrical traps usually induced by dry etching. It is selective towards Al containing alloys such as AlGaN and towards dielectric materials like silicon oxide or silicon nitride so that patterns can be defined to fabricate devices based on GaN/AlGaN heterostructures. In the present work, we report on the fabrication of enhancement mode p-GaN/Al(Ga)N/GaN high electron mobility transistors (HEMTs) with selective area sublimation under vacuum of the p-GaN cap layer used to define the gate. Furthermore, we show that sublimation can be combined with the regrowth of AlGaN, which is a key to increase the maximum drain current in the transistors and enables the co-integration of enhancement mode devices with depletion mode ones.

Gallium Nitride (GaN) transistors are quickly revolutionizing RF amplifiers and power converters thanks to its large critical electric field and electron mobility. Until now, all these transistors have been based on electron conduction (i.e. n-type transistors). Although this is common in field effect transistors based on compound semiconductors, the use of n-type-only devices severely limits the design options available to circuit designers. This paper will discuss several new GaN-based transistor structures that enable p-type current conduction with a performance similar to what it is found on n-type GaN transistors. In addition, the careful design of a p-GaN cap layer on top of the AlGaN barrier allows for the fabrication of both enhancement-mode (E-mode) n-channel and p-channel transistors next to each other. We will show how this new structure is compatible with 8” Si substrates and, in combination with the appropriate device fabrication technology, can significantly improve the linearity of RF amplifiers and increase the efficiency of power converters.

AlGaN-based UV-C laser diodes (LDs) are expected to be applied to various applications as a low-cost, environmentally friendly, and highly efficient laser light source. Although it has been difficult to realize DUV LDs due to problems with AlGaN crystals, we have achieved pulsed lasing at room temperature (R.T.) by improving crystal quality and demonstrating hole injection technology based on polarized doping technology. Furthermore, by suppressing process-induced dislocations and improving optical confinement, the threshold gain and drive voltage were improved, and continuous-wave lasing of a UV-C laser at R.T. with a threshold current density of 4.2 kA/cm2 and a threshold voltage of 8.7 V was successfully achieved.

Novel wavelength conversion devices with a transverse quasi-phase matching principle, which can be operated even in the absence of birefringence and ferroelectricity, is proposed. The devices include polarity-inverted stacked multilayer of AlN waveguide fabricated using epitaxial polarity inversion techniques. The report on the successful demonstration of far-UV 229-nm-wavelength second harmonic generation as well as a variation of similar structures will be given in the presentation.

Ultraviolet-C vertical-cavity surface-emitting lasers (UVC VCSELs) are promising candidates for applications in disinfection, UV curing, sensing, and optical communication. Here we present optically pumped 10λ cavity UVC VCSELs with SiO2/HfO2 DBRs, enabled by substrate removal through electrochemical etching. This resulted in lasers with an accurate cavity length control where the main lasing wavelength varied ± 1.2 nm between different VCSELs across a 2 mm x 3 mm area. Post-growth cavity detuning by a HfO2 spacer layer before the DBRs decreased the threshold pump power density from ~ 10 MW/cm2 to ~ 5 MW/cm2.

As underlayer substrates for laser diodes (LDs), two types of GaN with different carbon concentration grown by the epitaxial lateral overgrowth (ELO) method on Si substrates were investigated. In the sample with lower carbon concentration, many line defects were observed in the wing region, whereas in the sample with higher carbon concentration, such defects were not observed. Analysis by Raman spectroscopy and XRD revealed that the tensile stress applied to GaN decreased in the sample with higher carbon concentration, which is thought to be the reason for suppression of defects. The threshold current density was significantly improved operation by using the stress-relaxed template with no defects, and CW lasing was successfully achieved with a threshold current density of 5.6 kA/cm2. Both in terms of performance and cost, we believe that this short cavity LDs created from Si substrate is suitable for mobile applications such as augmented-reality glasses.

GaN-based VCSELs have been developed towards light sources in retinal scanning displays and so on. One of the biggest issues in VCSEL fabrication is to align three wavelengths, (1) a DBR center wavelength, (2) a cavity resonance wavelength, and (3) a gain peak wavelength. Typically, all of them are determined during the epitaxial growth. Therefore, in-situ measurements during the growth could be a key to solve the issue. In order to fabricate AlInN/GaN DBRs as designed, an InN mole fraction in AlInN must be controlled. We have used an in-situ wafer curvature measurement to monitor the InN mole fraction during the DBR growth, leading to a precise determination of the InN mole faction value within ±0.1% range. Next, we developed a control of the cavity resonance wavelength (cavity length). We used an in-situ reflectivity spectra measurement. As a result, a difference between a measured resonance wavelength and the designed wavelength was only 2 nm, corresponding to a 0.5% error. The above two in-situ measurements are powerful tools for in-situ controls of alloy compositions and thicknesses, drastically improving the reproducibility of the VCSEL performances.

Photonic crystal surface-emitting lasers (PCSELs) offer high-power single-mode emission with a low-divergent beam shape and extending such lasers’ emission wavelengths into the ultraviolet (UV) is therefore of interest. Here we demonstrate optically pumped PCSELs in the UVB (280-320 nm) and UVC (<280 nm). The photonic crystal is dry etched into the top AlN cladding layer and different photonic crystal lattice symmetries and structural dimensions are investigated. Unwanted emission bands in the far-field can be diminished by proper choice of parameters and this results in a narrow-linewidth emission with a divergence angle below 1°, when pumped above threshold (>7 MW/cm2).

InGaN quantum well (QW) based vertical-cavity surface-emitting lasers, which are usually fabricated on the c-plane GaN substrates, have a problem of unstable polarization since the c-plane has high symmetry. A candidate for solving this problem is to introduce optical anisotropy by breaking the six-fold symmetry of c-plane InGaN-QWs. In this study, we proposed that the anisotropic strain can be introduced by bending InGaN-QWs so that lasing polarization can be stabilized, and we have performed demonstrative optical measurements of such polarization control in InGaN-QWs. From the results of the experiment, we have determined the deformation potentials of InGaN alloy materials.

The goal of this work lies in expanding the integrated circuit technology to short wavelengths with the use of nitride emitters. We propose an approach that allows monolithic fabrication of lasers and waveguides using the same epitaxial structure. This is achieved by increasing the misorientation of the substrate locally, prior to the epitaxy, which allows local modification of the indium incorporation into the InGaN layers. Such areas are then used for etching down waveguides with low absorption. Within this work, we develop our technology for the fabrication of waveguide combiners, which involves creating waveguides with bends that bring two or more optical modes into close proximity. We compare systems consisting of 1 mm long laser diodes coupled to 1 mm bent waveguides with bend angles from 2.5° to 45° and different bend radiuses. We estimate the losses based on the optoelectrical parameters of the working system, treating it as a laser diode with a passive region introducing optical losses.

We will show the latest development results of GaN-based blue (455nm) and green (525nm) edge-emitting laser diodes (EELs). The epitaxial layers were grown on C-plane free-standing GaN substrates. With current injection under continuous wave (CW) operation, the wall-plug efficiencies (WPEs) of blue and green lasers have reached to over 52% and 24%, respectively. These are the highest values of blue and green EELs with CW operation. We also confirmed that optical output powers of blue and green LDs were more than 5.9 W and 1.9 W, respectively.

The potential of InAlN for edge-emitting AlInGaN devices is remarkable. For a 18% indium composition InAlN is lattice-matched to GaN and offers a 4-times larger refractive index contrast to waveguiding layers if compared to Al0.06Ga0.94N. These properties make InAlN an ideal cladding candidate. However, while being proposed already 20 years ago, multiple crystal growth challenges have hampered its implementation in commercial devices, above all its rapid morphological degradation. By implementing a n-type cladding design based on multiple GaN/InAlN pairs to overcome the morphological degradation, LDs and SLEDs with increased optical confinement factors are demonstrated. This results in devices with improved power efficiency. For example, LDs with threshold currents as low as 3 mA in the blue and 12 mA in the green spectral range are obtained. On the other hand, an operating current decrease of 85 mA is reported for state-of-the-art green SLEDs.

We present a method of in-plane modification of the refractive index using ion implantation and electrochemical etching of GaN layers. Proposed method allows for the fabrication of embedded air-GaN channels that can be periodically arranged inside III-nitride heterostructures. Importantly, a flat top surface is preserved for further regrowth. High refractive index contrast between air and GaN makes the proposed technology attractive for the fabrication of embedded photonic structures such as diffraction gratings for distributed feedback laser diodes (DFB LDs). We discuss the impact of the different design of air-GaN channels on the properties of DFB LDs.

Nitride semiconductor-based light emitters (LEDs and laser diodes) are influenced by magnesium (Mg) acceptors, limiting conductivity and operational temperature due to high ionization energy. Mg also causes strong optical absorption, reducing laser diode efficiency. Dielectric polarization engineering using wurtzite nitride lattice symmetry (polarization doping) has been proposed to manipulate electrical properties. Our study demonstrates low threshold current density (2.5 kA/cm2), low internal losses (around 5 cm-1), and good thermal stability in fabricated laser diodes, enabling operation at cryogenic temperatures. Notably, polarization-doped p-layers yield lower voltage than Mg-doped ones. Understanding hole injection from polarization-doped layers remains a challenge.

We report on recent advances in InGaN laser diodes(LDs) for emerging applications in the visible spectrum. KSLD has matured semipolar/non-polar epitaxial growth, epitaxial layer transfer, and wafer-level facet formation technologies enabling a new class of high-performance LDs. We will present the performance of high efficiency, single mode LDs spanning violet to green wavelengths. In addition, we will present progress on developing manufacturable distributed feedback LDs. These new devices can enable advances in applications including: AR/VR, quantum sensing, quantum computing, directed energy, 3D printing, optical wireless communication, biomedical, and life science.

Laser-based visible laser communication (VLC) is an emerging optical wireless communication technology for future 6G wireless network. In this work, we present the design, fabrication, and characterization of high-speed InGaN/GaN multiple quantum-well (MQW) laser diodes on c-plane GaN substrate. The peak emission wavelength is ~ 451 nm. A modulation bandwidth of 5 GHz has been achieved. Using the laser diode as transmitter, a data transmission rate beyond 15 Gbps has been measured in the VLC link using discrete multitone (DMT) modulation.

We report the growth of highly crystalline InN nanowires (NWs) on Si(111) and their application to self-powered and high-sensitive flexible piezoelectric motion sensors (PMSs). Under the strain of 0.8%, the output voltage increased from 0.7 to 3.0 V with increasing the length of NWs from 399 to 1194 nm. With increasing the strain from 0.8 to 2.9%, the output voltage of the PMS fabricated with 1194-nm long InN NWs increased from 3.0 to 7.1 V. These results are attributed to the variation in piezoelectric field according to elastic deformation of NWs depending on the length and mechanical stress. In addition, the device performances hardly changed even after the 1500-cylic bending test.

In this study, ArF laser annealing of an Mg-doped GaN four probe small mesa device is investigated. The dependence of the resistivity to the laser energy density and irradiation time were investigated. Results reveal that laser annealing at 530mJ/cm2 for 25mins (at 150Hz repetition rate) decreases the resistivity from 11.5 (before irradiation) to 5.6 Ω·cm (after irradiation). This value is comparable to resistivity achieved by RTA at 800°C for 2mins, suggesting successful activation.

To fabricate native and monolithic full color micro-displays with a pixel pitch below 10 µm, the three primary colors should be achieved with the InGaN alloy. The prerequisite is to get an efficient red emission with thin InxGa1-xN quantum well (QW) width and an In content of 35%. However, the In content is limited to 25% when grown on GaN. A full InGaN structure combined with different types of relaxed InGaN pseudo-substrates are used to reduce the strain in the active zone. Red electroluminescence was obtained until 650 nm. Homogeneous red emitting InGaN based QWs were also demonstrated.

Porotech's unique gallium nitride (GaN) material engineering and production technique enables the world's first commercial native red LED epiwafer for micro-LED applications. Micro-LED technology offers significant brightness, efficiency, and image definition improvements, crucial for AR and head-mounted displays. Traditional red LEDs faced efficiency challenges due to small device size. Porotech's unique porous GaN semiconductor material extends InGaN LED emission range, meeting red display needs, and scaling for micro-LED technology. This breakthrough enables highly efficient full-color micro-LED displays for large area displays, mobile phones, smartwatches, and more. Positioned for success, Porotech drives micro-LED technology and redefines display possibilities.

The idea of using relaxed InGaN as templates for the growth of InGaN-based devices has emerged in recent years. We investigate the process of plastic strain relaxation to exploit it for the preparation of good quality InGaN/GaN templates. The (0001)-oriented InGaN relaxes by the introduction of (a+c) misfit dislocations. Several peculiar phenomena of this mechanism are observed: unusual dislocation core dissociation, introduction of crystallographic tilt, anisotropic relaxation and formation of the point defect agglomerations as the traces of gliding dislocations. All of the mentioned aspects have to be addressed in order to prepare uniform highly relaxed InGaN/GaN templates.

For the development of efficient red LEDs with high-In-content InGaN quantum wells (QWs), we have developed the micro-flow-channel MOVPE method. This MOVPE can grow high-In-content InGaN at higher growth temperatures, resulting in higher quality. Also, we have introduced the strain compensation method at the QW region. Barrier layers consisting of Al(Ga)N could compensate for a compressive strain induced by InGaN. The strain compensation method has improved LED efficiency and elongated peak EL electroluminescence.

Electrical and optical excitation of the active region of a UVB LED chip was combined while imaging its in-plane lateral light emission by a UV camera. This made it possible to distinguish between spatial distribution in current density and in efficiency of radiative recombination of charge carriers. It is demonstrated that the degradation of the active region is more prominent in the areas where the local current density increased throughout the long-term operation. It will be shown how the spatial intensity distributions in UVB and UVC LEDs are affected by the operation current, chip design, and mesa edges.

We report successful growth of crack-free UVC LEDs on 6-inch sapphire substrates using high-temperature annealed AlN by 6-inch × 7-wafers MOCVD (Taiyo Nippon Sanso UR25K). The UVC LEDs grown on high-temperature annealed AlN templates with thickness of 450 nm were crack-free. Cracks of UVC LEDs were prevented using high-temperature annealed AlN templates with small curvature due to thin AlN film. The in-plane EL wavelength of UVC LEDs was uniform ((Max-Min)/Average<2%). In conclusion, crack-free UVC LEDs epitaxial growth on 6-inch sapphire substrates was demonstrated using high-temperature annealed AlN by production scale MOCVD.

Recent progress on the development of AlGaN based UVC detectors is discussed. This includes growth, fabrication, and characterization of AlGaN based devices grown on AlN substrates. Special focus is put on the impact of the growth condition and impurity concentration of the epitaxial layers on the dark current. Overall, it is shown AlGaN based detectors have a sensitivity in the range of 130–270 nm while rejecting solar emission. If operated as an Avalanche photodiode (APD), these detectors have an exceptional high linear gain of 300,000 and quantum efficiency <70%. Finally, the potential for 1D and 2D arrays is discussed.

A micro-photoluminescence setup is used to investigate the ambipolar diffusion of charge carriers in InGaN quantum wells (QW) grown by molecular beam epitaxy. The thickness of the active region varies between 2.6 and 25 nm. Our results show for all samples diffusion in the range of a few μm. Additionally, a larger QW thickness is accompanied by a smaller luminescence spot radius in our experiment. However, a larger dark carrier diffusion with increasing QW thickness cannot be excluded due to an increasing carrier lifetime. Moving away from the excitation center leads to a stronger tilt of the QW potential due to lower carrier density, consequently suppressing radiative recombination.

In recent years, point defects (PDs) have been unveiled as critical nonradiative recombination centres in InGaN/GaN quantum wells (QWs). When left unchecked, these nonradiative PDs can lead to at least an order-of-magnitude reduction in the internal quantum efficiency of blue light-emitting diodes. While macroscale studies have provided some information on such critical PDs, much deeper insight could be obtained by directly accessing the nanoscale impact of PDs on QW optical properties. Here, we present a detailed investigation of nonradiative PDs in a series of single InGaN/GaN QWs. Applying time-resolved cathodoluminescence (TRCL), we map the evolution of QW CL intensity spatially and temporally with nanometre and sub-nanosecond resolution, pinpointing individual PD locations. We fit the CL decays around single PDs with a carrier diffusion-recombination model to fully quantify their intrinsic properties, including novel phonon-limited relaxation times.

We investigated the influence of V-pits on the turn-on voltage of GaN-based high periodicity quantum wells device with different p-GaN thickness. By combining electrical analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and 2D simulations we show that sample with the thinnest p-GaN layer present an early turn-on voltage due to insufficient V-pit planarization, which penetrate the junctions, and locally put the quantum well region in closer connection with the p-side contact. Results provide insight on the role of V-pits on the electrical performance of future GaN-based devices with high-periodicity MQWs, helping to ensure a desired turn-on voltage design.

In this paper we will study the influence of InGaN underlayer on efficiency of InGaN-based LEDs grown by plasma-assisted molecular beam epitaxy (PAMBE). We observed that LEDs with the thinnest underlayer have the highest efficiency. This finding agrees with the theory that the defects, which are buried in standard LEDs are in fact generated during the growth of GaN in MOVPE at high temperature. In case of PAMBE, the growth temperature of GaN is 300°C lower, and these defects are not generated in the first place and there is no need for an InGaN underlayer.

Due the reduced dimensions of micro-light-emitting diodes (μLEDs), non-radiative sidewall recombinations are becoming increasingly important and therefore it is necessary to quantify the impact of surface recombinations (SR). Here, we probe the influence of SR on the optical properties of InGaN/GaN μLEDs with a spatial resolution of 100 nm and a time resolution of 50 ps using spatially-resolved time-correlated cathodoluminescence spectroscopy. Variations in carrier lifetime near sidewalls enabled us to quantify the surface recombination velocity. By coupling this technique with a diffusion model, we demonstrate that the combination of KOH treatment and Al2O3 passivation drastically improves the efficiency of μLEDs. We then show that this technique is not limited to InGaN µLED and we will provide examples in other area of applications.

There is a need in commercial imaging technologies for an accurate, solar blind, in-situ beam locator to ensure laser alignment during operation. In this work, fully transparent gallium nitride LED is fabricated and characterized as a two-dimensional position sensitive detector (PSD). Fabricated devices are shown to successfully and repeatably locate 405 nm laser light in two dimensions in the PSD area. These devices then can provide real time spatial light information in advanced imaging technologies for ensuring accurate laser alignment without affecting the system’s operation.

We investigated the carrier localization effect in μLEDs with low (blue) and high (green) indium composition quantum wells through the intensive analysis of the size-dependent forward bias current and electroluminescence mapping. We have observed a negative surface current and high electroluminescence intensity in random spots in green μLEDs, indicating the existence of a converging current to the deep well. On the other hand, in blue μLEDs, we observed a diverging current towards the mesa sidewall. We experimentally confirmed through EQE and Jmax EQE that the converging current caused by carrier localization in green μLEDs is responsible for the size-immunity properties in green μLEDs.

Defects in the active region of light-emitting diodes (LEDs) have a strong correlation with low-frequency electrical and, consequently, optical fluctuations of the light source. We investigate these two types of noise in red, blue, and green commercially available LEDs using a superposition model of stationary (thermal and shot noise) and excess noise (flicker and generation-recombination noise). The dependence of the degree of the low-frequency component on the supply current and the LED material was obtained. The occurrence of Lorentzinana-type spectrum at high currents for blue and green LEDs is also observed.