This PDF file contains the front matter associated with SPIE Proceedings Volume 10918, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

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

Kinetics of solid-phase epitaxy of amorphous alumina deposited by atomic layer deposition will be reported. Also, discrete micro-LED fabrication and mechanical lift-off the micro-LED arrays will be reported in this presentation. [1] D. Moon et al., “An ultra-thin compliant sapphire membrane for the growth of less strained, less defective GaN,” J. Crystal Growth 441, 52-57 (2016). [2] J. Jeong et al., “Solid-phase epitaxy of a cavity-shaped amorphous alumina nanomembrane structure on a sapphire substrate,” J. Crystal Growth, 498, 130-136 (2018).

A review on doping with acceptors of pure and structurally perfect HVPE-GaN single crystals grown on the native Ammono-GaN seeds will be described in this paper. Solid iron (Fe), manganese (Mn), magnesium (Mg) or methane (CH4, precursor of carbon) were used as dopant source to crystallize semi-insulating HVPE-GaN. Carbon-doped GaN was highly resistive at room temperature (exceeding 1×108 Ω.cm at 296 K) and became p-type at high temperature. Activation energy of 1 eV was an experimental confirmation of theoretical calculations for CN (deep acceptor). Doping with manganese also led to very high values of resistivity. In this case the activation energy was close to 1.8 eV. Resistivity of GaN with Mn concentration of 1017 cm-3 exceeded 108 Ω.cm at room temperature. Hall measurements revealed n-type conductivity at high temperature. Co-doping of HVPE-GaN with Mn and Mg led to highly resistive material at room temperature (exceeding 1×108 Ω.cm) and p-type at high temperature. The activation energy was 1.2 eV above the maximum of the valence band. GaN doped with Fe was also highly resistive at room temperature (3×107 Ω.cm with free electron concentration of 5×108 cm-3). It showed n-type properties at high temperature and activation energy of around 0.6 eV below the minimum of the conduction band. Structural, optical, and electrical properties of the resulting semi-insulating HVPE-GaN will be examined, presented, and compared in this paper.

HVPE can be used for growing thin, up to 200 µm, GaN layers of high purity and low free carrier concentration. Deposition of such material on conductive n-type GaN seeds results in a structure which is the basis of some vertically operating electronic devices. It should be stressed that thickness of this GaN with low free carrier concentration influences the breakdown voltage of the devices. Therefore, HVPE becomes the main epitaxial technology for crystallizing such layers. The method allows to crystalize GaN with a relatively high growth rate of about 100 µm/h. It makes this technology crucial for preparing transistor structures with breakdown voltage higher than a few or several kV. The main goal of this paper is to investigate implantation of beryllium (Be) acceptors into thin (10-100 µm) unintentionally doped layers of GaN crystallized by HVPE on native seeds. A nitride structure comprising of an n-type layer of low free carrier concentration with implanted regions with p-type conductivity or semi-insulating and a highly conductive n-type substrate will be obtained. Basic parameters of HVPE-GaN growth processes (reagent flows, growth temperature) as well as parameters of ion implantation will be determined. Post-implantation damage, which occurs in implanted layers, will be removed by high-temperature (1400-1480°C) annealing at high nitrogen pressure (1 GPa). Basic structural, optical, and electrical parameters of implanted and annealed GaN will be investigated. The samples will be characterized prior to and after ion implantation.

In this work, we have proposed two ex-situ treatments to annihilate bulk defects and non-radiative surface states in MBE grown AlGaN nanowires on silicon substrate. Vertically aligned nanowires were grown by molecular beam epitaxy (Veeco Gen II) system, equipped with RF plasma source for incorporating nitrogen, at a substrate temperature of 800⁰C. The nanowires were structurally characterized by SEM images and an areal density of 3.8×10⁹ nanowires-cm^-2 was calculated. The peak emission wavelength was measured to be 343nm at 19K from photoluminescence (PL) measurements. The as-grown nanowires were implanted with H- ions at various energies and fluences. A 2-fold increase in PL intensity without any wavelength shift was found in sample with irradiated energy of 3 MeV and a fluency of 1×10¹² ions-cm^-2. X-Ray diffraction measurements on (002) diffraction peak give an FWHM of 1440 arc-sec for ionimplanted samples as compared to 1872 arc-sec for as-grown nanowires indicating an improvement in crystalline quality. The nanowires were further treated in UV ozone environment for 25 minutes and an increase of 1.6 times in PL intensity was found without any wavelength shift. Both recipes were combined to get an effective increase of 3 times in PL intensity indicating the annihilation of bulk defects and non-radiative surface states. Hence, these treatments can be used to reduce non-radiative states and can be used to increase in radiative efficiency of AlGaN nanowires for applications in UV LEDs.

Growing III-nitride materials on unconventional substrates is attractive since it gives the possibility to fabricate novel devices at a potentially reduced cost. Spontaneously grown III-nitride nanowire structure is capable of achieving this goal, as they provide the capability to epitaxially grow high-quality single crystalline structures without global lattice and thermal matching requirement between the material and the substrate. In this work, we present the growth and characterization of GaN nanowire using plasma-assisted molecular beam epitaxy (PA-MBE) using an indium tin oxide (ITO)-coated silica substrate. The nanowires are shown to grow in the [0001] direction perpendicular to the substrate plane, with n-polar polarity. We found that the lateral size of the nanowires closely follows the grain size of the underlying ITO layer. Temperature-dependent photoluminescence measurement indicates a high-quality GaN material as indicated by the high internal quantum efficiency value. Conductive AFM measurement was performed to evaluate the feasibility of utilizing the GaN nanowire on ITO for device fabrication. By growing GaN nanowires on top of ITO-coated silica substrate, we open up the possibility of fabricating transparent nitride-based devices using transparent scalable substrate.

III-V nitride semiconductors, driven by solid-state lighting applications, are nowadays widely spread in optoelectronics industry. These materials exhibit exceptional figures of merit, which make them well suited for high performance light emitters. For instance, blue light emitting diodes with 80% wall-plug-efficiency have been demonstrated, as well as blue and green laser diodes with impressive performance. This is primarily due to the very large material gain of InGaN/GaN quantum wells (QWs). On the other hand, III-nitride semiconductors are also promising for near-infrared applications thanks to negligible absorption in this spectral range related to low free carrier concentration and reduced two-photon process. Furthermore, those materials can be epitaxially grown on silicon substrates offering thereby a potential platform for integrated photonics. In this presentation, we will focus on GaN photonic crystal cavities, which are fabricated from GaN-on-silicon wafers. We will show that quality factors in excess of 40’000 can be obtained in two-dimensional photonic crystal cavities designed at 1.5 m wavelength. This allowed us achieving efficient second and third harmonic generations. We also fabricated 1D nanobeam photonic structures in which a single InGaN QW is embedded. Thanks to high  emission coupling factor, inherent to such a geometry, and to the large material gain of InGaN/GaN QW, we observed lasing action under continuous wave optical pumping at room-temperature. We will eventually highlight the importance of surface states in GaN photonic structures through a careful study of microdisk optical resonators.

We report III-N surface-emitting resonant-cavity light-emitting diodes (RCLEDs) at λ = 375 nm using a novel hybridmirror approach. The hybrid mirrors consist of 5 pairs of air-gap/AlGaN distributed Bragg reflector (DBR) at the bottom side of the vertical cavity and HfO₂/SiO₂ dielectric DBR (DDBR) on the top to facilitate the formation of a resonant cavity for nitride-based surface light emitting diodes. The air-gap/AlGaN DBR replaces the conventional thick stack of semiconductor DBR to achieve high reflectivity. Hybrid-mirror III-N RCLEDs with airgap/AlGaN DBR mirror were fabricated and the results showed that the III-N RCLEDs achieved high current density operation up to 40 kA/cm² with a peak emission wavelength atλ = 375 nm and a full-width-half-maximum (FWHM) of 9.3 nm at room temperature.

Nitride semiconductors with large optical nonlinearity and high optical damage tolerance have potential for high-efficiency wavelength conversion. However, it is difficult to form the quasi-phase matched (QPM) periodic polarity-inverted structure, and a long interaction length of the order of cm is usually required for achieving high SHG efficiency in QPM devices. Then we propose a GaN monolithic microcavity SHG device with high-reflectivity Bragg reflectors located on both sides of the GaN resonator to enhance the excitation light intensity within the resonator significantly. By appropriate phase control at reflection, phase mismatch can be compensated even in the absence of the polarity-inverted structure. Due to a very short cavity length of ~ 10 μm, it is necessary to secure a laser beam path by etching the substrate outside of the cavity. In this work, the monolithic microcavity on the Si trapezoidal structure was fabricated using a thick GaN film on a Si substrate. The Cr/Ni masks with microcavity pattern were formed by EB drawing, EB evaporation and lift-off process. After the dry etching by ICP-RIE using Cl2 gas, KOH wet etching was performed in order to improve the sidewall verticality and flatness. The structure was covered by a Ni mask again. After removing the remaining GaN film and the underlying buffer layer, the Si substrate outside of the microcavity was deeply etched by ICP-RIE using SF6 gas. Finally, the Ni mask was removed by wet etching. We will report on the optical characteristics of the fabricated device in the presentation.

We report here an optically pumped deep UV edge emitting laser with AlGaN MQWs active region grown on AlN substrate by low pressure organometallic vapor phase epitaxy (LP-OMVPE) in a high-temperature reactor. The 21- period Al_0.53Ga_0.47N/Al_0.7Ga_0.3N MQWs laser structure was optically pumped using 193 nm deep UV Excimer laser source. A laser peak was achieved from the cleaved facets at 280.3 nm with linewidth of 0.08 nm at room temperature with threshold power density of 320 kW/cm². The emission is predominantly TE polarized with a polarization suppression ratio of 24 dB. The side mode suppression ration (SMSR) is measured to be around 14 dB at 465 kW/cm².

Waveguide Mach-Zehnder interferometers (MZIs) composed of two directional couplers (DCs) are widely used as basic building blocks in the optical communications systems and photonic quantum circuits. Since nitride semiconductors have a strong electro-optic effect, they are suitable for implementation of the fast electric-field driven phase shifters in MZIs. In addition, based on its strong optical nonlinearity, the nitride semiconductors can be applied to the quantum light source of 800 nm band via an optical parametric down conversion process. By integration of the nitride-based waveguide MZI devices and the InGaN laser diodes pumped high-efficiency quantum light sources, novel quantum information processing systems made of single material can be expected. In this work, we report on the design, fabrication and evaluation of two types of waveguide DCs: rib waveguide and strip waveguide. As for the design the DC which splits an incident wave at a wavelength of 810 nm into two waves with equal power, 3D beam propagation method simulations were performed. The DCs with different coupled waveguide lengths were fabricated by RIE using Ni hard masks. After a SiO2 cladding layer deposition, the devices were diced and end facets were polished. The splitting characteristics of the rib waveguide DCs were evaluated using a DFB laser, and almost 1:1 splitting ratio was successfully obtained. Measured data shows that we can control the splitting ratio by changing coupled waveguide length. In addition to the above results on rib waveguide DC, we will report on the fabrication and evaluation of the strip waveguide DCs.

InGaN alloys have gained considerable interest over the past due to their tunable band gap extending the operation wavelengths of optoelectronic devices to green–red and IR regions. However, the realization of high In-content InGaN materials is still limited by their material properties. Despite encouraging achievements in InGaN based devices, it is difficult to achieve high quality InGaN with high indium composition. Up to now, there are only few reports about high indium content InGaN films, in particular with indium content > 50%. Successfully grown In-rich InGaN layers with 300 nm thickness and nominally [In] = 70% deposited on GaN template by MBE were comprehensively investigated by highly spatially-resolved cathodoluminescence. The surface morphology has been investigated by atomic force microscope (AFM) and scanning electron microscopy (SEM) and shows grain-like features. The lateral as well as the vertical luminescence distribution yields a detailed insight in the [In] homogeneity. The thick InGaN films, free of droplets, have a quite homogenous emission at 1.035 eV (~1200 nm) laterally with full-width at half maximum of only 68 meV. Determined from the emission peak, the indium composition is about 75%, which is slightly higher than the nominally intended indium composition. The evolution in growth direction will be presented.

We report in this paper electro-optical results on InGaN/GaN based green micro light-emitting diodes (μLEDs). Currentlight- voltage measurements reveal that the external quantum efficiency (EQE) behavior versus charge injection does not follow the ABC model prediction. Light-emission homogeneity investigation, carried out by photoluminescence mapping, shows that the Quantum Confinement Starck Effect (QCSE) is less significant at the edges of μLEDs. Electroluminescence shows a subsequent color green-to-blue deviation at high carrier injection levels. The extracted spectra at different current injection levels tend to show the appearance of discrete wavelength emissions. These observations may enhance the hypothesis that higher-energy excited-levels in InGaN quantum wells may also contribute to the blue shift, solely attributed to QCSE lessening under intense electric field magnitudes. We hereby present first results dealing with green μLEDs electro-optical performances with regards to their size.

Commercial LEDs with different colors (ex. InGaN and AlInGaP) were studied using spatially and time resolved Electroluminescence (EL) and Photoluminescence (PL) measurements. A novel experimental setup was used enabling performance of all these techniques at the same point with high temporal resolution, speed and accuracy. LEDs under the study demonstrated inhomogeneous broadening of the luminescence peak. PL and EL spectra observed for these LEDs were different as a result of different radiative transitions attributed to each phenomenon. Time-resolved data for fixed wavelengths demonstrated multiple time spectra at different photon emission energies due to inhomogeneous distribution of Gallium and Indium for InGaN or Gallium and aluminum for AlGaInN.

Light extraction efficiency (η_extraction) remains as a big challenge for high-efficiency deep-ultraviolet (UV) lightemitting diodes (LEDs) due to the large refractive index contrast at the AlN(sapphire)/air interface. Various surface patterning approaches such as microdome design and patterned sapphire substrates have been proposed to address the low η_extraction issue. Nevertheless, these previously proposed methods all involved additional complicated fabrication steps and the polarization-dependent analysis for these devices has not been investigated experimentally. In this work, we investigate the feasibility of using 700-nm SiO₂ microsphere array on 280 nm flip-chip UV LEDs to improve the η_extraction. Angle- and polarization-dependent electroluminescence measurements have been performed to compare the 280 nm LEDs with and without the SiO₂ microsphere array. The UV LED with microsphere array showed enhancement for transverse-electric (TE)-polarized light intensities at small angles while decreased intensities at large angles with respect to c-axis, as compared to the device without SiO₂ microspheres For instance, up to 7.4% enhancement is observed at θ = 0°. However, for transverse-magnetic (TM)-polarized light, the intensities largely remain the same at small angles while decrease at large angles. Cross-sectional near-field electric field distribution from three-dimensional finite-difference time-domain simulation has confirmed that the use of SiO₂ microspheres array resulted in scattering of photons at the sapphire/SiO₂ microspheres interface, which eventually leads to enhanced TE-photons extraction at small-angles. From simulation, the light radiation patterns from the UV LED with SiO₂ spheres are reshaped to a small-angle-favored pattern without reducing the total output power, showing great consistency with the measurement results.

consumption and financial burden of the multiple light sources required for such systems. The AlGaInN material system allows for single transverse mode laser diodes to be fabricated with optical powers up to 100 mW over a wide range from ~380 nm up to ~530 nm. By tuning the indium content and thickness of the GaInN quantum well, we have developed a range of AlGaInN diode-lasers targeted to meet the wavelength and power requirements suitable for optical clocks and atom interferometry systems. One of the major limiting factors in nitride laser diode development has been the lack of a suitable low defectivity and uniform GaN substrate. Recently, single crystal growth of large area, very low dislocation-density and uniform GaN substrates are grown using a combination of high temperature and high pressure enabling a range of AlGaInN laser technology to be developed. This direct light generation at the required wavelength is crucial to reduce complexity and size of the overall system, and to ensure a high wall-plug efficiency that is critical for space and mobile applications. We will present our development of GaN based, low SWaP, frequency-stabilised external-cavity seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising the optical performance and minimising surface reflectivity to identify suitable working parameters.

Structural investigation of InGaN/GaN heterostructures and quantum wells for long wavelength emission P. Ruterana CIMAP, 6 boulevard du Marechal Juin, Université de Caen, Caen, France InGaN/GaN multiple quantum wells (MQW) heterostructures are at the basis of highly efficient light-emitting diodes (LED) and laser diodes in the near UV to the green range [1]. However, bridging the green gap still remains an important challenge [2]. Different mechanisms have been reported in the literature in order to explain the decrease of emission efficiency when the indium composition is increased in order to reach longer wavelengths. One of them is the quantum confined Stark effect (QCSE) which introduces a large spatial separation between electrons and holes [3]. Others are the Auger effect which impacts the internal quantum efficiency (IQE) mainly under high excitation regime [4], and the non-radiative recombination centers which could be predominant for highest indium contents due to possible generation of high densities of defects [5]. In this alloy, reports have also shown that there could be strain relaxation by generation of defects such as stacking faults, dislocations [6] or trenches which may constitute non-radiative recombination centers [7]. More recently, it was reported that the relaxation of the local strain through I1 basal stacking faults (BSF) was at the origin of a type threading dislocations (TDs) from QW layers to the surface [8]. In this work, we investigate the local structure of indium rich heterostructures produced by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) using transmission electron microscopy (TEM) in order to explain the strain relaxation mechanisms. Our observations point out to important differences between the two growth techniques, surfaces play a critical role in MOVPE whereas dislocation glide is found to be acting in MBE. For the indium rich QWs, we find new hexagonal defects that are at the origin of a type threading dislocations going from the quantum well to the surface. Interestingly, they become predominant when the nominal indium composition is beyond 20%. For these native defects, the measured displacement vector is along <10-10> directions and it is very small. Therefore, they do not correspond to the conventional stacking faults of the wurtzite structure. This work is supported by the Laboratory of excellence GaNex and Région Normandie. [1] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, J. Disp. Technol. 3, 160 (2007). [2] T. Mukai, M. Yamada, and S. Nakamura, Jpn. J. Appl. Phys. 38, 3976 (1999). [3] W. Liu, D. G. Zhao, D. S. Jiang, P. Chen, Z. S. Liu, J. J. Zhu, X. Li, F. Liang, J. P. Liu, S. M. Zhang, H. Yang, Y. T. Zhang, and G. T. Du, Superlattices Microstruct. 88, 50 (2015). [4] C. Weisbuch, M. Piccardo, L. Martinelli, J. Iveland, J. Peretti, and J. S. Speck, Physica Status Solidi (A) 212, 899 (2015). [5] T. Li, A. M. Fischer, Q. Y. Wei, F. A. Ponce, T. Detchprohm, and C. Wetzel, Appl. Phys. Lett. 96, 031906 (2010). [6] M. Zhu, S. You, T. Detchprohm, T. Paskova, E. A. Preble, D. Hanser, and C. Wetzel, Phys. Rev. B 81, (2010). [7] F. Wu, Y.-D. Lin, A. Chakraborty, H. Ohta, S. P. DenBaars, S. Nakamura, and J. S. Speck, Appl. Phys. Lett. 96, 231912 (2010). [8] J. Smalc-Koziorowska, C. Bazioti, M. Albrecht, and G. P. Dimitrakopulos, Appl. Phys. Lett. 108, 051901 (2016).

In-rich InGaN/GaN nanowires (NWs) are key optoelectronic materials, which can close the green gap of the light emitting diodes and can be used in efficient high-bandgap solar cells for integration in tandem devices. Realization of these devices requires as a first step the optimization of the NW structure and their electrical parameters. Electron Beam Induced Current (EBIC) microscopy is well suited to probe nanoscale devices with a high resolution and to extract the material parameters. Here, we analyze the electrical properties of axial GaN and InGaN/GaN n-p and p-n junction NWs using EBIC microscopy. III-N NWs were grown on Si(111) substrates by molecular beam epitaxy using Mg as a p-dopant and Si as an n-dopant. The growth conditions were adjusted to optimize the doping order with an abrupt axial junction without a parasitic radial overgrowth. From the EBIC analysis of the GaN p-n junctions, the doping level and the minorities carrier diffusion lengths were extracted. Next, a p-GaN/i-InGaN/n-GaN junction containing an In-rich InGaN segment [1] was grown yielding a flat and strong EBIC signal in the InGaN NW portion. NW arrays were then contacted and their behavior under visible light was analyzed. [1] Morassi et al., Cryst. Growth Des., 2545, 18 2018

Ion implantation (I/I) and annealing techniques to enable high carrier doping into selected regions are one of important research fields for realization of GaN power devices. However, particularly difficult research challenges are present in Mg-I/I into GaN as p-type doping technique. First problem is related to nitrogen vacancies (VN), crystal defects introduced by Mg-I/I . [1] Second issue is connected to degeneration of GaN surface by pyrolysis reaction during high-temperature annealing process. We examined Mg/N co-implantation into GaN as p-doping in order to compensate of VN defects. Research on ultra-high pressure thermal activation process to maintain equilibrium conditions at high temperature was conducted to avoid degradation of GaN surface. We prepared Mg/N co-implanted GaN-on-GaN samples with 300-nm-deep Mg-box-profile of 1E19 cm-3 and with N-box-profile of various concentrations in the range 0 ~ 1E20 cm-3. The samples were annealed without a protection cap layer at temperature between 1300 °C and 1480 °C under 1 GPa in N2 atmosphere. Samples capped with sputter-AlN were treated by conventional lamp annealing at 1300 °C for a comparison purpose. We obtained the dominant Green Luminescence (GL) emission at 2.3 eV and the recessive Donor Acceptor Pair (DAP) emission at 3.2 eV from a low-temperature cathodoluminescence (LT-CL) spectra in Mg-I/I samples without N co-implantation. This is in good agreement with results published before.[1][2] In addition to this, we found that the Mg/N co-implantation, for optimized N-ion dose, suppresses the GL intensity while maintaining the DAP intensity. Furthermore, in the ultra-high pressure thermal activation process, the DAP intensity markedly increases with anneal temperature and GL intensity suppression is also visible. These results strongly suggest that the origin of GL is related to VN, and the VN defect compensation occurs by N co-implantation. We also compare the AFM images of GaN surface roughness. The ultra-high pressure annealed surfaces were as smooth (RMS = 0.2~0.3 nm) as the as-implanted samples (0.3 nm), whereas surface treated with conventional lamp was rough (1.9 nm). We can, therefore, conclude that Mg/N co-implantation and ultra-high pressure thermal activation process allows activation of the Mg acceptor and recovery of p-GaN crystal quality. Such treatment results in VN defect compensation and a smooth surface of the annealed sample. This work was supported by MEXT “Program for research and development of next-generation semiconductor to realize energy-saving society.” [1] A. Uedono et al., Phys. Status Solidi B 252, No. 12 (2015) [2] K. Kojima et al., Appl. Phys. Express 10, 061002 (2017).

The advancement in potential selective placement of p-GaN regions places the gallium nitride (GaN) material system on the forefront of next generation power semiconductor devices. The ion implantation technique is commonly used in fabrication of semiconductor devices to achieve high conductivity regions with successful demonstration for n-GaN. This technique has shown little success for post-Mg-implanted activation anneal to achieve p-GaN [1], largely due to the formation of substantial point defects as a result of the implant process. To activate Mg and repair these defects, high annealing temperatures of > 900 °C are required. Considering that GaN dissociates at 840 °C at atmospheric pressure, higher temperature annealing should be performed under a high overpressure of nitrogen and in combination with a protective cap layer. We report on the results of using a novel Gyrotron annealing technique for Mg implant-activation. Mg implanted GaN layers have been annealed at temperatures and pressures as high as 1300 °C and 40 bar respectively with and without a protective cap. It is observed that Gyrotron annealing at 1100 ˚C for 30 seconds eliminates secondary GaN (0004) Bragg peaks, due to stress relaxation. In addition, skew-symmetric x-ray rocking curves show no stress induced by annealing in the GaN (10-12) peak with FWHM of 59’’, although the GaN (30-32) peak is observed to broaden. We will present an extensive array of characterizations detailing results from Gyrotron annealing, conventional RTA, and high temperature (1300 °C) RTA on similar Mg-implanted, homoepitaxially grown GaN samples. [1] F. J. Kub et al. Electron. Lett., 50, 3 (2014): 197–198

Polarization engineering of III-Nitride materials in the nitrogen polarity has allowed for improved 2DEG confinement in GaN/AlGaN HEMTs and reduction in efficiency droop in MQW LEDs. Achieving p-type material through Mg doping continues to be a challenge in the nitrogen polarity compared to the gallium polarity with lower stability for substitutional replacement of Mg for Ga under nitrogen-rich conditions as calculated by density functional theory [1] and shown experimentally [2]. High conductivity p-type material in the nitrogen polar orientation is needed for improved efficiency of a variety of III-nitride emitters, detectors and high-hole-mobility transistors (HHMTs). In this work, GaN:Mg was overgrown on N-polar GaN templates on on-axis sapphire substrates by MOCVD. The underlying GaN template growths were optimized to achieve control over hillock density. Low and high hillock density templates were then used for overgrowth of GaN:Mg to study dependency of Mg incorporation efficiency on hillock density. Similar N-polar uGaN/GaN:Mg were epitaxially grown on high hillock density and low hillock density N-polar GaN/sapphire to form Cs-free photocathode detector devices. Photocathodes with high hillock density produced a quantum efficiency of 24.4% whereas low hillock density device produced an efficiency of only 0.8%, larger than 30X difference. To understand the origin of this difference, material quality and composition of these devices were studied. Secondary ion mass spectroscopy (SIMS) showed Mg concentration of 4x10^22 atoms/cm3 and 5x10^19 atoms/cm3 for high and low hillock density devices respectively. Atom probe tomography was performed to study incorporation and distribution of Mg within and at sidewalls of hillock structures. [1] Q. Sun et al., Phys. Rev. B 73, 155337 (2006). [2] J. Marini et al., J. Elec. Mat. 26, 5820 (2017).

The outstanding material properties of III-Nitride semiconductors, has prompted intense research efforts in order to engineer resonant tunneling transport within this revolutionary family of wide-bandgap semiconductors. From resonant tunneling diode (RTD) oscillators to quantum cascade lasers (QCLs), III-Nitride heterostructures hold the promise for the realization of high-power ultra-fast sources of terahertz (THz) radiation. However, despite the considerable efforts over last two decades, it is only during the last three years that room temperature resonant tunneling transport has been demonstrated within the III-Nitride family of semiconductors. In this paper we present an overview of our current understanding of resonant tunneling transport in polar heterostructures. In particular we focus on double-barrier III-Nitride RTDs which represents the simplest device in which the dramatic effects of the internal polarization fields can be studied. Tunneling transport within III-heterostructures is strongly influenced by the presence of the intense spontaneous and piezoelectric polarization fields which result from the non-centrosymmetric crystal structure of III-Nitride semiconductors. Advances in heterostructure design, epitaxial growth and device fabrication have led to the first unequivocal demonstration of robust and reliable negative differential conductance. which has been employed for the generation of microwave power from III-Nitride RTD oscillator. These significant advances allowed us to shed light into the physics of resonant tunneling transport in polar semiconductors which had remained hidden until now.

Front-illuminated GaN p-i-p-i-n separate-absorption and multiplication avalanche photodiode (SAM-APD) epitaxial structures were grown by metalorganic chemical vapor deposition (MOCVD) on n-type bulk GaN substrates and fabricated into 4×4 arrays with a large detection area of 100×100 μm². The SAM-APD array showed a uniform distribution of dark current density of J_Dark<(5.1±0.8)×10^-8 A/cm² at reverse bias (V_R) of 44 V except for two of them. In addition, the average onset points of breakdown voltages (V_BR) of the SAM-APD array was 73.1±0.21 V, and no microplasmas were visually observed after multiple times I-V scans.

Vertical GaN power devices have become recognized as a strong candidate of high power devices because several reports having over 1kV breakdown voltage and low on-resistance have been published. However, these data still include some issues to be solved for practical applications. A merit of GaN is potential of high channel mobility which results in low on-resistance compared with SiC. Therefore, channel structure having high channel mobility is essential for vertical GaN devices. Most useful property of GaN for high channel mobility is that AlGaN/GaN heterostructure can be used. Panasonic Group developed high performance vertical GaN device with 1.7kV withstand voltage and 1mΩcm2 on resistance in 2016. The device had an AlGaN / GaN channel with a p-GaN gate of which channel mobility was 500-1000cm2/Vs. This performance is beyond the performance of SiC-MOSFET for the first time. Though AlGaN/GaN channel is ideal as high mobility channel, it, however, is difficult to fabricate the normally-off channel with high threshold voltage. If conventional MOS channel is possible, simple structure normally-off with high threshold voltage will be possible. In recent years, a high channel mobility exceeding 100 cm2/Vs of MOS channel has been reported by the two groups. Fuji Electric Group showed high mobility of 120 cm2/Vs in the inverted channel of the MOSFET in 2017. And UC Davis group reported 185 cm2/Vs with a GaN channel regrown on the trench sidewall in the trench gate MOSFET in 2017. These data make the expectation of the possibility of higher channel mobility of MOS structure by improving the interface state.

The realization of next-generation vertical GaN devices relies heavily on advances in both epitaxial growth of GaN drift layers on commercially available GaN substrates and selective area n-type and p-type doping in a planar process. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates span defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. . In many other semiconductor materials such as Si and SiC, ion implantation is a routine step in most processing sequences for selective area doping and greatly facilitates manufacturing by avoiding the complicated etch/regrowth process. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge though. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been shown to activate up to ~10% of the implanted Mg dopant atoms using a combination of a temporary thermally stable capping layer, annealing in a nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. This paper will present an assessment of substrate-dependent effects on the quality of homoepitaxial GaN films, evaluate ion implantation processing for selective area doping, address basic vertical devices to identify process module development toward practical MOSFET devices.

This paper reviews the most relevant mechanisms responsible for the degradation of GaN-based lateral and vertical electron devices. These components are almost ideal for application in power electronics, but the presence of semiconductor defects and the existence of degradation processes may limit their stability and lifetime. In this paper we focus on the following aspects: (i) the degradation processes induced by off-state conditions and leading to a time-dependent and/or catastrophic breakdown of the devices; (ii) the stability of the gate stack; (iii) the degradation of the electrical performance of vertical GaN transistors and diodes. To discuss these topics, we refer to case studies carried out in our laboratories.

AlGaN/GaN High Electron Mobility Transistors (HEMTs) are capable of achieving high breakdown voltage, low operating resistance and large switching speed due to the excellent performance shown by III-N structures. The paper presents selected details of technological experimental work on high voltage (HV) AlGaN/GaN-on-Si HEMTs fabricated with multifinger structures and gate widths of up to 40×1 mm. The electrical isolation of individual devices was elaborated using Al+ implantation. The ions were implanted up to a depth of 200 nm in order to produce an effective damage and isolation up to the non-conducting AlGaN buffer layer. The influence of the ion energy (in the range 208-385 kV) and the ion dose (in the range 8.5x1012-1.4x1013cm-2) on the effectiveness of the fabricated isolation was found. The properties of the fabricated ohmic contacts (using Ti/Al/Mo/Au and Ti/Al/TiN/Cu metallization schemes) with emphasis put on the technology of recess etching were studied. The impact of various pretreatment, applied before deposition of the gate metallization, on electrical parameters of multifinger devices was analysed. The tested pretreatment methods included oxide removal in HCl-based solution, and O2 or BCl3 plasma treatment, with the lowest gate leakage current obtained for the latter. The results of fabrication of the HV HEMTs with single field-plate structures with various dielectrics (Si3N4 or Al2O3) are discussed. The characterization results within the paper cover electrical (I-V characteristics), structural (TEM, XRD), topographical (AFM) and elemental (EDS mapping) analyses. This work was supported by The National Centre for Research and Development under Agreement nr TECHMATSTRATEG1/346922/4/NCBR/2017 for project "Technologies of semiconductor materials for high power and high frequency electronics"

Low-resistance ohmic contacts on AlGaN/GaN HEMT devices presently require annealing at temperatures up to 850°C, which can adversely affect material properties. Here we investigate contacts with the metal directly contacting the 2DEG in the GaN, about 20 nm below the top surface. For convenience, we employed a 10-mm × 10-mm sample composed of 3-nm-GaN/16-nm-Al_0.27Ga_0.73N/1-nm-AlN/1.8-μm-GaN (Fe-doped). Four, 2-mm-long, 2-μm deep lines were scribed near the corners of the sample and filled with indium metal from a soldering iron. Hall measurements were then performed from 10 to 320 K at a current of 1 mA and with a magnetic field strength of 10 kG. At 10K (300K) the mobility was 1.96 × 10⁴ (1.88 × 10³) cm²·V^-1·s^-1; the sheet concentration, 9.39 × 10¹² (9.35 × 10¹²) cm^-2, and the sheet resistance, 33.9 (353) Ω/sq = r_s. The contact resistance R_c was calculated from the average total resistance R_tot across each pair of contacts: R_tot = 2R_c + r_s. At 10 K (300 K), R_c ≈ 1 (2) kΩ. Also, R_c has a much smaller temperature dependence than r_s, implying tunneling, rather than thermionic current. From a Schrödinger-Poisson calculation, the peak volume carrier concentration in the 2DEG is n ≈ 3.7 × 10¹⁹ cm^-3. The tunneling probability is P = exp[e(V – V_bi)/ε₀₀] and for ε = 9.9ε_vac and m* = 0.22m₀, ε₀₀ = 0.077 eV = 894 K; thus, ε₀₀ < kT, further suggesting the dominance of tunneling current. This technique is immediately applicable to any HEMT-type structure, including AlScN/GaN.

A normally-off InAlN/GaN high electron mobility transistor (HEMT) on Si substrate with a p-GaN gate is reported. Devices are fabricated on two different epitaxial structures, one containing a high resistive GaN buffer layer and one containing an AlGaN back-barrier, and the threshold voltage, drain current density, and buffer leakage current are compared. With the epitaxial structure containing a high resistive GaN layer, normally-off operation with a threshold voltage of +0.5 V is achieved. The threshold voltage is further increased to +2 V with the AlGaN back-barrier, and the buffer leakage current was improved by over an order of magnitude.

Leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability. However, long-term reliability in the space environment still remains a major concern due to a number of defects and traps as well as unknown degradation mechanisms. Thus, careful study of reliability and radiation effects of GaN HEMTs should be performed before GaN HEMT technology based solid state power amplifiers (SSPAs) are successfully deployed in space satellite systems. We studied both RF GaN HEMTs fabricated per our design and commercial high-power GaN HEMTs, both grown on SiC substrates. Our RF devices had a nominal Ni-Au Schottky gate length of 0.25 μm, a total gate width of 6 × 150 μm periphery, and a field plate, while high-power devices had a Ni-Au Schottky gate length of 0.4 μm, a total gate width of 10 × 350 μm periphery, and a field plate. First, DC and RF characteristics of RF devices were compared before and after they were aged under different conditions (DC and temperature). Focused-ion-beam was employed to prepare TEM cross sections from degraded devices for defect analysis using a high-resolution TEM. Also, we performed strain analysis on pristine and degraded devices using TEM-based techniques. Second, DC characteristics of high-power devices were compared before and after they were irradiated with protons and heavy ions. Some of devices were exposed while they were unbiased, DC biased, and DC and RF biased.

We present latest development results of GaN based high power blue and green Laser Diodes (LDs). The epitaxial structures of LDs including n-type, active and p-type layers were grown by metal organic chemical vapor deposition (MOCVD) on C-plane free-standing GaN substrates. And a ridge type structure and Electrodes of the n-type and p-type were formed. Front and rear mirror facets were obtained by cleavage at the m-plane surface. We optimized the epitaxial and the device structures for high efficiency, high optical output power and reliability. Every LD chip was mounted on a heat sink using a junction down method in a TO-Φ9 mm package for suppressing thermal resistance. A New developed blue LD showed the optical output power and the voltage of 5.25 W and 4.03 V at the forward current of 3 A under Continuous Wave (CW) operation. The wall plug efficiency of the blue LD was 43.4% at 3A. And pure green LDs at 532 nm showed the optical output power of 1.19 W and the wall plug efficiency of 17.1 % at the forward current of 1.6A. Furthermore, 543 nm green LDs were fabricated on C-plane GaN substrates.

GaN-based high-power laser diodes (LDs) have attracted tremendous interests in next-generation lighting applications, such as laser display, car laser light. However, high injection current usually brings inevitable drawbacks, including the well-known efficiency droop, Auger recombination and self-heating which obstruct further improvements of GaN-based optoelectrical devices. In this paper, influence of hole overflow at high injection current in an asymmetric GaN-based highpower blue LD has been comprehensively investigated and successfully suppressed by employing a new sandwiched GaN/AlGaN/GaN lower quantum barrier (GAG-LQB). Systematical simulations and measurements of structural and optical properties are carried out. As a result, the V-shaped defects induced by thick n-InGaN waveguide layer are apparently eliminated, which provides a more growth-friendly platform for deposition of the rest epitaxial layers and thus a better crystalline quality is obtained. On the other hand, the modified LD exhibits better photo-electrical properties with slope efficiency (SE) increasing from 0.98 to 1.24 and wall-plug efficiency (WPE) increasing from 18.7% to 20.5% at a high current of 1.5 A and no obvious efficiency droop is observed at a current as high as 2 A compared with the conventional one, because the middle-inserted AlGaN layer could form an extra barrier on the valence band to weaken the hole overflow and enhance the radiative recombination. Furthermore, the in-plane compressive strain induced by InGaN quantum wells (QWs) is also partially compensated by the tensile strain induced by the AlGaN layer. Therefore, the piezoelectric fieldinduced polarization is effectively alleviated and the wavelength blueshift is reduced from 7 nm to 1.6 nm.

Green vertical-cavity surface-emitting lasers (VCSELs) were fabricated with two different kinds of gain medium, a green-emitting InGaN quantum dot (QD) active region and a normally blue-emitting quantum well (QW) active region. The VCSELs have dual dielectric DBRs and room temperature (RT) continuous wave (CW) lasing was observed in both type VCSELs. For the QD VCSELs, lasing at different wavelengths from 491.8 to 565.7 nm was obtained, covering most of the “green gap”. The lasing wavelength could be controlled by adjusting the cavity length, and the devices were featured with low threshold current of less than 1 kA/cm². For the QW VCSELs, the emission peak of active layer is around 445nm, dominantly in the blue. However, lasing was observed at around 493 nm, locating at the emission edge and approaching to the green region. The green emission comes from the fluctuation-induced localization centers. And the cavity-enhanced recombination played an important role in realization of lasing action. These results open up opportunities to design and fabricate semiconductor green VCSELs that are useful for wide-gamut, low consumption power and compact displays and projectors.

The recent progress of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with incorporated curved mirrors at one end of their cavities is reviewed. GaN-VCSELs consisting of 3 InGaN/GaN quantum wells, current apertures formed by boron ion implantation, and curved mirrors at one end of the cavities were fabricated. The near-field and far-field patterns exhibited Gaussian-like profiles, and their divergences agreed well with the theoretical values calculated from the radius of curvature of the curved mirror and the cavity length. The near-field beam waist for a GaNVCSEL with a 6-μm current aperture was as small as 1.4 μm (half width at 1/e²), which indicated that the light was laterally confined by the incorporated curved mirror and that the current aperture could be made smaller than 6 μm. For a GaN-VCSEL with a current aperture of 4μm, a threshold current of as low as 0.56 mA (J_th = 4.5 kA/cm²) was obtained at room temperature under CW operation at a wavelength of 451.8 nm. To the best of our knowledge, this is the lowest threshold current reported among all-dielectric DBR type GaN-VCSELs and is comparable with the lowest value reported for GaN-VCSELs that have a two-dimensional quantum well structure. The authors believe this result to be a milestone for the realization of GaN-VCSELs with extremely low power consumption.

In this paper, we present numerical simulations of nitride tunnel junction VCSELs (Vertical-Cavity Surface-Emitting Laser). This analysis concerned lasers emitting 405 nm wavelength. The simulated VCSEL is similar to the structure fabricated at University of California, Santa Barbara (UCSB). This structure has an Al-ion implantation applied for outer regions of the cavity. The results show how threshold parameters (threshold temperature and threshold current) and emitted power depend on the contrast of the refractive index in this implantation. We also analyze the influence of the implantation thickness and dimensions of the electrical aperture of the laser on capacitance phenomena occurring in the laser.

Compared with conventional edge-emitting laser diodes, vertical-cavity surface-emitting lasers (VCSELs) have numerous advantages such as the compatibility with mainstream semiconductor microfabrication, the possibility of on-wafer testing, a better beam profile, the feasibility of arrayed and coherent operation and the possibility of low-power and single-mode operation. While, in the short wavelength, from green and blue to near ultraviolet (530 to 360 nm), only edge-emitting laser diodes are commercially available, the commercialization of III-nitride VCSELs are expected to enable unique applications in mobile display and projection, virtual, augmented, and/or mixed reality, adaptive beam steering, and high-resolution laser printing. The main challenge in the technology of III-nitride VCSELs, however, is the fabrication of planar distributed Bragg reflectors (DBRs) that provide near-unity reflection and thus support the formation of high-quality vertical cavity. While, over the past decade, numerous groups reported VCSEL operations (only in visible spectrum) using epitaxial DBRs (AlGaN/GaN, AlN/GaN, AlInN/GaN) and dielectric DBRs (involving lift-offs or epitaxial lateral overgrowth), the devices commercialization is still hampered by challenging epitaxial growth, complicated fabrication or low thermal management. We report in this paper the use of a nanoporous (Al)GaN medium as an alternative method in the formation of III-nitride VCSELs. Specifically, we will focus on the development of VCSELs with an emission wavelength at 369 nm which is crucial for the realization of next-generation compact atomic clocks. We note that VCSEL operation below 400 nm has never been demonstrated, and we will discuss factors of importance toward this realization, including the formation of near-unity DBR mirrors using nanoporous AlGaN/AlGaN structures, the investigation and control of cavity loss including absorption and scattering, and the formation of intra-cavity current injection, optically-transparent pathways. So far clear single-mode spontaneous emission at 369 nm (linewidth < 2 nm) is observed. We will report the latest result including pulsed characterizations under high level injection.

Gallium nitride based micro-displays comprise LED epi-structures pixelated on a micro-scale (pixels of a few microns to a few 10's of microns in dimension on a similar pitch) such that the pixels can be electronically addressed either individually or in matrix formats. Such displays can be implemented across the full wavelength range covered by gallium nitride alloys, from the deep ultraviolet to the amber/red. Although typically operated on their growth substrates (usually c-plane sapphire) with flip-chip bonding to an electronic backplane, it is possible to transfer the devices to other substrates such as glass via pick-and-place techniques including micro-transfer printing, to facilitate novel formats, array scaling and multi-colour operation. These emissive devices are robust and operate at brightness levels that can exceed other formats of micro-display. In addition to direct view and projection display applications, the technology also enables a host of other applications. These include optrodes for optogenetics and neuroscience; mask-free photolithography and direct writing; visible light communications; and structured light imaging. For optical communications, the benefits include not only the ability to modulate the individual pixels at data rates that can exceed 1Gb/s (enabled by the high current densities of above 1kA per square cm that the pixels can sustain) but also the capability to implement novel forms of spatial modulation and spatial multiplexing, such as space-shift keying and MIMO [1]. In structured lighting, very high frame rate (kHz to potentially MHz) projection of binary mask pattern sequences is possible, facilitating novel forms of optical navigation, tracking and imaging functions [2]. [1] S. Rajbhandari et al., "A review of gallium nitride LEDS for multi-Gb/s visible light data communications", Semicond. Sci. Technol, , 32, 023001 (2017). [2] J. Herrnsdorf et al., "Positioning and space-division multiple access enabled by structured illumination with light-emitting diodes", J. Lightwave Technol., 35, 2339 (2017).

Light-emitting diodes based on InGaN/GaN quantum wells are widely used for lighting and display applications. However, such LEDs are monochromatic with relatively narrow spectral line-widths and without color-tuning capabilities. In order to use them for lighting, additional color-conversion material, typically phosphor, is needed. For display applications, multiple chips are required for color-tuning. The highly-strained nature of the InGaN/GaN wells due to lattice and thermal mismatch, especially for those of longer emission wavelengths, offer a viable way of wavelength tuning. Through dimensional downsizing of the emitters, the emission wavelengths can be blue-shifted via the strain-relaxation effect. Such wavelength tuning techniques can be exploited for the development of monolithic broadband LEDs for lighting, as well as RGB pixelated arrays for display applications, potentially offering improvements to performances, as well as manufacturing costs and yields.

We review the main challenges for fabricating high performance microLED displays for direct view applications from watch to large area TV. The main one is the transfer process of the microLEDs on the large area TFT backplane. We show how the microtube technology developed at LETI can ensure in one step both the mechanical and electrical connection between the two parts. Driving electronics is also a strong challenge. We also report a new approach providing full CMOS driving of the MicroLED display and a more efficient transfer solution. This new approach is paving the way for high-performance MicroLED displays, and in a much wider range of applications.

In this study, we demonstrated a monolithic InGaN-based LED to achieve three primary colors of light. Three primary red-green-blue (RGB) colors can be independently obtained from this device at different current densities. This LED structure, which contains three different sets of quantum wells separated with intermediate carrier blocking layers (ICBLs), was grown by metal-organic chemical vapor deposition (MOCVD). Results show that this LED can emit light ranging from 460 to 650 nm to cover the entire visible spectrum. In addition to three primary colors, many other colors can be obtained by color mixing techniques. Based on this novel structure, we also achieved, for the first time, a monolithic white-light LED with tunable color-temperatures from 2700 to 6500 Kelvin (K). To generate white light, a full-color LED (or tricolor LED) is operated under pulsed conditions with each pulse consisting of multiple steps of different current amplitudes and widths emitting different colors. The combined spectrum of different colors is aimed to mimic that of the blackbody radiation light source. The pulse rate is designed to be higher than the human eye response rate, so the human eye will not discern the emission of successive colors but a singular emission of white light. Results of a two-step pulse design show this method is able to generate white light from warm color temperature (2700 K) to cold color temperature (6500 K).

We demonstrate the fabrication of 100 x 100µm2 nitride-based tricolor micro-LEDs. While the optical properties of the devices are very promising, the electrical characterizations show large penalty voltage due to the introduction of compensating defects in the p-GaN layers upon plasma etching. Therefore, we consider replacing the p-GaN layers by p+/n+ tunnel junctions. We analyzed the surface morphology and the Mg, Si, H and C concentration of different p/n junctions grown at different temperature. SIMS results show the C concentration drastically increases from 10^16/cm3 at 1100°C to about 10^17/cm3 at 975°C without significantly affecting the electrical properties of both p- and n-type layers. However, the decomposition of tetramethylsilane (TMSi) strongly depends on the growth temperature, and thus the Si/Ga ratio has to be carefully controlled with temperature. The Mg shows a strong memory effect in the n-side since its concentration is slowly decaying of only one decade after 50nm from the p/n interface. The pining of the Fermi level, close to the conduction band in the n-GaN, destabilizes the Mg-H complex and allows the Mg to be activated (Mg/H<<1). In addition, at the high Mg/Ga and Si/Ga ratio typically required in p+/n+ tunnel junction, we observed a change of the GaN surface morphology with the promotion of inversion domains and 3D growth respectively, whatever the growth temperature. Based on these data, we show that a p+/n+ tunnel junction is a trade off between high structural and electrical quality.

AlGaN UV-C light-emitting diodes (LEDs) are attracting a great deal of attentions for applications of sterilization, water purification, in the medical fields, and so on. The increase in wall-plug efficiency (WPE) is a recent main subject for an AlGaN UVC-LED. The main cause for reducing WPE is a significant reduction in light-extraction efficiency (LEE) owing to a heavy light absorption by p-GaN contact layer. If we introduce transparent p-AlGaN contact layer for increasing LEE, the contact resistance is increased, resulting in the significant increase of operating voltage. In order to achieve both of low contact resistance and high-reflectivity in a p-contact layer, we introduced a photonic crystal (PhC) reflector on a p-contact layer. We predicted by a simulation analysis that an LEE of UVC-LED can be increased by 2.8 times by introducing a PhC reflector on p-GaN contact layer. Previously, we fabricated a 283nm AlGaN DUV-LED with PhC reflector on a p-AlGaN transparent contact layer, and obtained an increase of external quantum efficiency (EQE) from 8 to 10 % by introducing PhC reflector. However, the contact resistance was increased. In this work, we fabricated a 273nm AlGaN UVC-LED with PhC reflector on p-GaN contact layer. The EQE was increased by about 1.7 times by introducing PhC reflector. The operating voltage was not changed even when introducing PhC and remained low value. We confirmed that it is possible to realize high WPE by introducing PhC reflector on the p-GaN contact layer of UVC-LED.

Unprecedented high-temperature operational stability of interfacial silicide-free ultraviolet-A multiple-quantum- disk AlGaN nanowire-based light-emitting diodes on metal is achieved and investigated. Reasonable variations in device operational parameters across a wide range of temperatures demonstrate the high quality of the layer interfaces and efficient carrier injection. We previously presented ultraviolet-A quantum-confined Al_xGa_1-xN/Al_yGa_1-yN nanowire-based light-emitting diodes and studied their steady-state electro- and photo- luminescent characteristics at room temperature. Herein, we significantly expand the scope of our previous work by investigating the operational stability of the device across a wide range of temperatures (-50-100°C) with conformal parylene-C deposition, forming a nanowire forest as a polymer/nanowire three-dimensional composite material. This work constitutes part of a larger study into the operational stability of ultraviolet light-emitting diode chemical sensors at a wide range of temperatures for operation in harsh environments such as in downhole oil exploration.

One of the main reasons that the emission efficiency of GaN-based light emitting diodes (LEDs) decreases significantly as the emission wavelength shorter than 300 nm is the low light-extraction efficiency (LEE). Especially in deep ultra-violet (DUV) LEDs, light propagating outside the escape cone and being reflected back to the semiconductor or substrate layer is absorbed not only by active layers but also by p-type layers with narrower bandgaps and electrodes that is neither transparent nor reflective for the DUV wavelength. In this report, we propose a DUV LED structure with thin indium-tin-oxide (ITO) layer as the p-ohmic contact and DBR as the reflectivity mirror. The DUV LED with the ITO/DBR p-ohmic-contact layer showed the output power of 30% higher than that from the DUV LED with Ni/Au p-ohmic-contact layer.

Donor-acceptor-codoped SiC was recognized as a candidate of blue LED material in 1970s-1980s. However, very few works on optical device applications of SiC are recently conducting, because many scientists and researchers believe that the indirect bandgap material including SiC is not suitable for optical use. In 2006, a concept of fluorescent SiC (f-SiC), which is a thick 6H-SiC epilayer and contains donor and acceptor impurities, was proposed. The f-SiC works as a phosphor and it is not an electrically excited material like LEDs. And a monolithic white LED, composed of the f-SiC substrate and over-grown nitride-based near UV LED stack was demonstrated. A combination of the f-SiC and porous SiC, which is made from the f-SiC by anodic oxidation was reported to show pure white light emission with a color rendering index of 81. Advantages of f-SiC based phosphor might be its chemical stability and the superior temperature characteristics, compared with current powder phosphors used in white LEDs. Thus, in this talk, optical properties of f-SiC and porous f-SiC, and performance of the white LED in combination with the f-SiC substrate and a nitride-based near UV LED are given.

To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (BLD) directly modulated by preleveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity beyond 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bitrate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m. Light scattering induced by impurities in seawater attenuates the blue laser power to degrade the transmission such that the BLD based UWOC enables a 16-QAM OFDM bitrate of up to 7.2 Gbps in seawater more than 6.8 m. To optimize the QAM-OFDM transmission, the sampling rate of the encoded data is compromised to avoid the aliasing and oversampling effects during waveform extraction procedure. The sampling rate is optimized to 3~5 times of the encoded data bandwidth for suppressing peak-to-average power ratio (PAPR). This improves the BLD based UWOC with 16-QAM OFDM data transmission at 14.8 Gbps over 1.7 m. Lengthening the seawater distance to 10.2 m only decreases the transmission data rate by 4 Gbps, Oversampling not only filters out background noise but also attenuates data amplitude to degrade transmission performance. With spectrally filtering out the sidelobes of each OFDM subcarrier, the allowable modulation bandwidth is greatly improved from 1.9 to 2.7 GHz, as the inter-carrier interference induced crosstalk between subbands is relieved to improve the SNR of the carried data with a raw data rate of up to 10.8 Gbps over 10.2 m in seawater channel.

Optical properties of InGaN/GaN multi-quantum-well (MQWs) grown on sapphire and on Si(111) are reported. The tensile strain in the MQW on Si is shown to be beneficial for indium incorporation and Quantum-confined Stark Effect reduction in the multi-quantum wells. Raman spectroscopy reveals compressive strains of -0.107% in MQW on sapphire and tensile strain of +0.088% in MQW on Si. Temperature-dependent photoluminescence shows in MQW on sapphire a strong (30 meV peak-to-peak) S-shaped wavelength shift with decreasing temperature (6 K to 300K), whereas MQW on Si luminescence wavelength is stable and red-shifts monotonically. Micro-photoluminescence mapping over 200 by 200 μm² shows the emission wavelength spatial uniformity of MQW on Si is 2.6 times higher than MQW on sapphire, possibly due to a more uniform indium incorporation in the multi-quantum-wells as a result of the tensile strain in MQW on Si. A positive correlation between emission energy and intensity is observed in MQW on sapphire but not in those on Si. Despite the lower crystal quality of MQW on Si revealed by atomic force microscopy, it exhibits a higher internal quantum efficiency (IQE) than MQW on sapphire from 6 K to 250 K, and equalizes at 300 K. Overall, MQW on Si exhibits a high IQE, higher wavelength spatial uniformity and temperature stability, while providing a much more scalable platform than MQW on sapphire for next generation integrated photonics.