This PDF file contains the front matter associated with SPIE Proceedings Volume 10104, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Recent progress in our hydride vapor-phase epitaxy (HVPE) technique is discussed. First, the void-assisted separation (VAS)-method for freestanding GaN fabrication is introduced and its recent progress is described. When conventional HVPE conditions are used in the VAS method, the reduction in threading dislocation density (TDD) stops when growth exceeds a certain critical thickness. This limitation was overcome by controlling the crystal hardness by appropriate HVPE conditions, resulting in GaN bulk crystals with the highest nanoindentation hardness value reported to date (22 GPa). Based on this, the TDD had been markedly reduced to the mid-10⁵/cm² range. With regard to the fabrication of large wafers, a 4" GaN substrate was successfully fabricated by using the VAS method. The hardness control was also found to be beneficial for fabrication of 4" GaN substrates with small off-angle variations. Additionally, we succeeded in fabricating a 7" GaN substrate by using a newly developed tiling method. With respect to the material purity, extremely pure GaN crystals were grown by using a quartz-free HVPE system. All impurity concentrations measured by secondary-ion mass spectrometry, including those of Si, O, and C, were below the detection limit. The high-purity layers displayed an insulating nature in the absence of a dopant; by Si doping, the electron concentration could be controlled over a wide range, down to 1 × 10¹⁵/cm³, with a high mobility of over 1100 cm²/Vs. This indicates that HVPE has potential applications as a tool for the epitaxial growth of power-device structures.

Hydride Vapor Phase Epitaxy (HVPE) is the most popular method for fabrication of high structural quality and high-purity GaN substrates. The technology of obtaining a low level of impurities together with high crystallographic quality of HVPE-GaN crystals enables the next step, namely introducing intentional doping to the growth process and obtaining semi-insulating crystals. This work describes developing a method for incorporation of acceptors (carbon or iron) into HVPE-grown GaN while maintaining high structural quality and low level of other impurities in the material. Ammonothermally grown GaN crystals and substrates will be used as seeds. All growth processes will be carried out in a home-built quartz horizontal HVPE reactor. Methane (CH4) will be used as the precursor of carbon. The FeCl2 precursor will be created inside the reactor chamber by an HCl-stream over elemental iron. HVPE crystallization runs with different flows of acceptor precursors will be performed. HVPE-GaN:C and HVPE-GaN:Fe crystals will be characterized with X-ray diffraction, Raman spectroscopy, low-temperature photoluminescence, optical as well as transmission electron microscopies, Hall measurements, and Secondary Ion Mass Spectrometry. The properties of crystallized HVPE-GaN:C and HVPE-GaN:Fe will be compared in detail. It will be shown that concentrations of impurities (carbon or iron) in the new-grown material is always very uniform across the (0001) surface and along the c-direction. This result together with a high crystalline quality of the crystallized material will allow to obtain the semi-insulating HVPE-GaN crystals with resistivity of the order of 109 Ω.cm at room temperature.

Vertical-cavity surface-emitting lasers (VCSELs) are known to have advantages of lower threshold current operation, circular and low-divergence output beam, and lower temperature sensitivity compared to edge-emitting laser diodes. In conventional VCSELs, the formation of a current aperture plays a vital role in the device characteristics. Low laser thresholds and single-transverse-mode operation would not be possible without a well-defined current aperture to confine carriers to generate photons between the two distributed Bragg reflectors. Since the introduction of the controlled oxidation process for the AlxGa1-xAs material system by Dallesasse and Holonyak in 1989, most VCSELs have employed oxidation for current aperture formation as well as optical confinement and this technique has become one of the most commonly used fabrication techniques for traditional III-V compound semiconductor infrared VCSELs. However, for III-N emitters operating at wavelengths in the ultraviolet to green wavelength range, the formation of Al-based native oxide layers has not proven feasible. As a result, various current-confinement techniques have been studied such as, selective-area growth of buried AlN, oxidizing AlInN, and selective activation of acceptors. In this work, we report an ion-implantation process which is effective for carrier confinement and defines a current aperture for our III-N ultraviolet microcavity light-emitting diodes (MCLEDs). The devices have peak emission wavelength of ~371.4 nm with the spectral linewidth of 5.1 nm at the highest pulsed current injection level of 15 kA/cm2. Further discussion on the material growth, material characterization, implantation parameters, as well as numerical simulation for structural design will be presented in the conference.

Effective, easy-to-use and physically meaningful analytical predictive models are developed for the evaluation the lattice-misfit stresses (LMS) in a semiconductor film grown on a circular substrate (wafer). The two-dimensional (plane-stress) theory-of-elasticity approximation (TEA) is employed. First of all, the interfacial shearing stresses are evaluated. These stresses might lead to the occurrence and growth of dislocations, as well as to possible delaminations (adhesive strength of the assembly) and the elevated stress and strain in the buffering material, if any (cohesive strength of the assembly). Second of all, the normal radial and circumferential (tangential) stresses acting in the film cross-sections are determined. These stresses determine the short- and long-term strength (fracture toughness) of the film material. It is shown that while the normal stresses in the semiconductor film are independent of its thickness, the interfacial shearing stresses increase with an increase in the induced force (not stress!) acting in the film cross-sections, and that this force increases with an increase in the film thickness. This leads, for a thick enough film, to the occurrence, growth and propagation of dislocations. These start at the assembly ends and propagate, when the film thickness increases, inwards the structure. The TEA data are compared with the results obtained using a simplified strength-of-materials approach (SMA). This approach considers, instead of an actual circular assembly, an elongated bi-material rectangular strip of unit width and of finite length equal to the wafer diameter. The analysis, although applicable to any semiconductor crystal growth (SCG) technology is geared in this analysis to the Gallium-Nitride (GaN) technology. The numerical example is carried out for a GaN film grown on a Silicon Carbide (SiC) substrate. It is concluded that the SMA model is acceptable for understanding the physics of the state of stress and for the prediction of the normal stresses acting in the major mid-portion of the assembly. The SMA model underestimates, however, the maximum interfacial shearing stress at the assembly periphery, and, because of the very nature of the SMA, is unable to address the circumferential stress. This stress can be quite high at the circular boundary of the assembly. At the assembly edge the circumferential stress is as high as σθ = (2–ν₁)σ₁, i.e., by the factor of 2–ν₁ higher than the normal stress, σ₁, in the mid-portion of the film. In this formula, ν₁ is Poisson’s ratio of the film material.

Internal quantum efficiency (IQE) of radiative recombination for photo-excited carriers in compound semiconductor materials is usually estimated from temperature dependence of photoluminescence (PL) intensity by assuming that the IQE at cryogenic temperature is unity. III-nitride semiconductors, however, usually have large defect-density, and the assumption is not necessarily valid. In this study, we developed a new method to estimate accurate IQE values by simultaneous PL and photo-acoustic (PA) measurements, and demonstratively evaluated the IQE values for an InGaN quantum-well sample. The results show that the conventional method cannot give accurate IQE values, and that our method is a promising way for accurate estimation of absolute IQE values, which could lead to the accurate estimation of radiative and nonradiative recombination lifetimes in carrier dynamics studies.

Investigation of temperature distribution on the facet of the device, with high spatial and temperature resolution, is crucial to gain insight into thermally activated degradation modes in GaN-based lasers. This work undertakes the problem of temperature distribution measurement on the facet of the nitride lasers. Thermal investigation of the nitride devices is mainly based on thermal imaging. However, this approach is characterized by inherently low spatial resolution, as well as the fact, that the registered image, is averaged over the volume of the device, limiting the ability to observe the enhanced thermal processes occurring at the vicinity of the surface (front facet). Thermoreflectance spectroscopy, provides the possibility of registering of high spatial and temperature resolution images of the surface of the device operating in quasi-CW or pulsed mode. In this work we present development of the unique experimental setup and procedure, devoted to the thermal characterization of the nitride lasers. Thermal characterization of series of devices was performed, providing a mode for comparing different operating conditions, geometries and device designs. Measurement of the temperature profile and high-resolution temperature distribution maps on the front facet of AlGaN/GaN via thermoreflectance spectroscopy will be demonstrated. The results indicate the direction to take in order to improve the laser reliability and performance. Additionally, the degradation mechanisms induced by temperature increase are discussed.

Research on III-nitride intersubband (ISB) transitions in the THz spectral range is motivated by the large LO-phonon energy of GaN, which should permit device operation with limited thermal interference, and at infrared wavelengths inaccessible to other III-V compounds due to Reststrahlen absorption. A main challenge to extend the polar GaN-ISB technology towards the THz region comes from the polarization-induced internal electric field, which imposes an additional confinement that increases the energetic distance between the electronic levels. In order to surmount this constraint, we propose alternative multi-layer quantum well designs that create a pseudo-square potential profile with symmetric wavefunctions [1]. The robustness of these designs and their integration in device architectures requiring tunneling transport will be discussed. An alternative approach to obtain square potential profiles is the use of nonpolar crystallographic orientations. In this contribution, we present an experimental study of THz ISB transitions in m-plane GaN/AlGaN quantum wells grown on free-standing m-GaN [2]. For Al contents below 15%, such structures can be grown without epitaxially-induced extended defects. We demonstrate nonpolar quantum wells which display ISB transitions in the 7-10 THz band, and we will discuss the effect of the doping density in the quantum wells on the transition energy and line width. Finally, we will present a comparative study using silicon and germanium as n-type dopants. [1] M. Beeler, et al., Appl. Phys. Lett. 105, 131106 (2014) [2] C.B. Lim, et al., Nanotechnology 26, 435201 (2015); Nanotechnology 27, 145201 (2016).

There are two physical phenomena governing the light emission in InGaN quantum structures: the internal electric fields and the In composition fluctuations. Both these effects manifest through the blue shift of the wavelength emission with the excitation intensity and both of them have the pronounced influence on the light emitting properties of these structures. In order to discriminate between these two effects, we fabricated two identical structures: one with the quantum barriers doped with silicon (method for internal electric field screening) and the other with an undoped active region. Under the optical excitation the emission peak shifts by almost 35 nm (Si doped) and 50nm (without Si). Additionally, we studied temperature dependence of the emission peak position. In case of low temperatures and at RT and high pumping energy, emission energy position is almost the same for both samples. Our observations lead us to the conclusion that at low temperatures and at high pumping regime the Quantum Confined Stark Effect (QCSE) is totally suppressed. While this is understandable that at high carrier injection QCSE is screened, the origin of the low temperature effect is much less clear. We can speculate that at the lowest temperature the carriers are localized eliminating the spatial separation of holes and electrons wavefunctions. Measured cathodoluminescence (CL) maps show the same level of the indium fluctuations for both samples. At higher excitation the fluctuations starts to be less visible suggesting band filling of states. Finally we compare recombination times by means of time resolved photoluminescence.

InGaN based LEDs are known to be very efficient in the blue range. However, although InGaN can theoretically cover all visible range, quantum efficiency drops when emission wavelength emission is increased due to quantum confined Stark effect. Furthermore, indium incorporation is hindered by compressive strain induced by lattice mismatch between InGaN and GaN. To tackle the lattice mismatch problem, a full InGaN structure on a relaxed InGaN substrate is proposed. The structure consisting of five InxGa1-xN / InyGa1-yN multi quantum wells on top of an InyGa1-yN buffer layer is grown by MOVPE on an InGaNOs substrate from Soitec company. Three InGaNOs substrates of lattice parameters of 3.190, 3.200 and 3.205 Ångströms were co-loaded in order to compare their ability to incorporate indium for the same growth conditions. For reference, a sample grown on GaN template will allow us to compare the wavelength red-shift resulting of the use of InGaNOs template. The samples were characterized by photoluminescence at room temperature using 375 nm and 405 nm laser diodes. It is shown that long wavelengths can be reached thanks to the use of InGaNOs substrates. For same active region growth conditions as reference sample, a red-shift up to 65 nm (from 445 to 510 nm) is observed, demonstrating InGaNOs potential for easier In incorporation. Using different growth conditions, wavelengths up to 600 nm have been reached. First internal quantum efficiency measurements demonstrate a good quality material. InGaNOs seems promising for emission in the “green gap”and beyond.

The effects of the structure design of AlGaN-based quantum wells (QWs) on the optical properties are discussed. We demonstrate that to achieve efficient emission in the germicidal wavelength range (250 – 280 nm), Al_xGa_1−xN QWs in an Al_yGa_1−yN matrix (x < y) is quite effective, compared with those in an AlN matrix: Time-resolved photoluminescence and cathodoluminescence spectroscopies show that the Al_yGa_1−yN matrix can enhance the radiative recombination process and can prevent misfit dislocations, which act as non-radiative recombination centers, from being induced in the QW interface. As a result, the emission intensity at room temperature is about 2.7 times larger for the Al_xGa_1−xN QW in the Al_yGa_1−yN matrix than that in the AlN matrix. We also point out that further reduction of point defects is crucial to achieve an even higher emission efficiency.

Solar blind Al_0.5Ga_0.5N/AlN metal-semiconductor-metal photodetectors (MSM PDs) are characterized by means of photocurrent spectroscopy. In order to enhance the external quantum efficiency (EQE) at low bias voltages several strategies have been adopted including absorber layer thicknesses, electrode layout and metallization scheme. Analysis of experimental EQE-bias characteristics under top and bottom illumination conditions reveals (1) a correlation between EQE and electrode pair density for symmetric electrode designs and (2) a slight asymmetry of the EQE with respect to bias polarity for bottom-illuminated MSM PD consisting of electrode pairs with different electrode widths (asymmetric design) and (3) zero-bias operation for a-MSM PD consisting of electrode pairs with different metallization schemes. In addition, the combination of thin absorber layer and asymmetric electrode design leads to high EQE values under bottom illumination at very low voltages and zero-bias operation is achieved for the a-MSM detector. The zero-bias EQE of the a-MSM is further enhanced by combining the symmetric detector design with a high electrode pair density.

The energy difference between the lowest conduction band valleys is a fundamental semiconductor parameter affecting performance of electronic devices via intervalley electron scattering. Surprisingly, the intervalley energy (IVE) value in GaN is still disputed. Recent photoemission experiments showed that IVE is 0.90 – 0.95 eV, which is considerably smaller than the >2 eV values obtained by ab initio calculations. One of the suitable techniques to measure IVE is time-resolved spectroscopy. Excitation wavelength dependent photoluminescence and pump-probe transients allow pinpointing the onset of the intervalley scattering by increase of the electron relaxation time towards the bottom of the conduction band. In this work, we apply this approach by performing differential transmission (DT) and reflection (DR) measurements on n-GaN crystal. In DR, ultraviolet (UV) pump creates electrons in the Γ valley at energies around the scattering threshold, and the onset energy is determined by the change of the electron relaxation time towards the bottom of the conduction band. However, IVE evaluated using this technique is affected by the poor knowledge of the valence band dispersion at large k values. This problem is circumvented in the DT measurements, in which only conduction band states are involved. The DT decay time spectrum provided the IVE value of 0.97 ± 0.02 eV, close to the photoemission data. Comparison of DT and DR intervalley scattering onsets allowed estimating the hole mass as 1.4m0. Modelling of the DR transients with rate equations produced intra-and intervalley electron - LO phonon scattering times of 30 and 15 fs, respectively.

Deep space exploration, use of sensors and devices at harsh climate conditions or in remote, hardly accessible areas, requires solutions, which will be modern and reliable on one hand and have a long lifetime on the other hand. One of the ideas, which meet these requirements is betavoltaic battery. This concept is known since the 50s of the twentieth century and is based on the conversion of energy from radioactive elements using semiconductor p-n junctions. However, the rapid development of semiconductors of wide energy gap (mainly GaN), opened new possibilities of more efficient energy conversion. The idea of the betavoltaic battery is based on fabricating the semiconductor p-n junction, in a similar but not identical way to standard photovoltaic cells. The design of the structure has to be combined with the choice of the most appropriate source of beta particles. Within the present work we demonstrate the modelling, design and the first tests of betavoltaic structure based on GaN p-n junction. The betavoltaic structures were fabricated by MOVPE on sapphire or gallium nitride substrate. The structures were tested by irradiating with electrons inside SEM. The signal detection was performed using EBIC (electron beam induced current) system. The use of electron beam in SEM, made possible to tune the acceleration voltage and the kinetic energy of electrons, allowing the simulation of irradiation using various radioactive sources. We also test various types of top metallization schemes to optimize the overall design of the device. Acknowledgment: The research was supported by the National Science Center, Poland, through grant no. 2014/15/D/ST7/05288.

It emerges that LEDs properties are strongly impacted by intrinsic disorder induced by random In compositional fluctuations. They obviously impact the light emission spectrum and carrier mobilities. The quantitative evaluation of their impact in full heterostructures is made difficult by the extreme demand on computing resources when calculating solutions of the Schrödinger equation for a disordered 3D potential map. Calculations are then limited to small volumes and to the first few quantum states, not allowing for simulations of transport properties in full devices. It was recently shown in a simplified model that disorder can account for a turn-on voltage of LEDs smaller by 1V compared to standard simulations. We will present novel theoretical and modeling tools of disorder, namely the Filoche-Mayboroda 3D localization landscape theory, which from the original disordered energy map provides an effective potential which allows to use standard transport equations while accounting for microscopic disorder. We thus gain a deep understanding of various effects of disorder in nitride heterostructures on their electrical and optoelectronic properties. As a first application of the new tool we model our detailed measurements of the absorption edge of InGaN/GaN quantum wells with varying In composition. The tool is then applied to carrier transport in full LED structures. The effective potential increases current at a given bias voltage by accounting for two quantum effects of disorder, tunneling and confinement, which together smooth out potential discontinuities in the heterostructure. Quantum efficiency, Auger and leakage droop, ideality factor of the LED will be discussed.

III-Nitride-based nanowire LEDs have shown high internal quantum efficiency and stable light emission over a wide current range to enable white phosphor-free white-light emission. The structures provide a unique advantage for flexible electronics where the out-of-plane three-dimensional nanowires are invariant to applied bending. Testing this concept, InGaN dot-in-wire light-emitting diodes on sapphire substrates were transferred onto flexible polyethylene terephthalate (PET) substrates using a bonding and laser-liftoff process. In0.15Ga0.85N nanowire blue LEDs were grown on sapphire substrates having Ni/Au contacts applied to the top p-doped region and a lateral insulating polyimide layer applied between the nanowires. The nanowire structures were then bonded onto a transfer wafer and separated from its sapphire growth substrate by laser-liftoff (LLO) using a 266 nm KrF laser. A double-transfer technique, where the inverted LED structures were then transferred and bonded onto the PET with a silver-based adhesive that formed the final bottom contact. The LEDs before and after the transfer process did not show measurable degradation in the I-V and optical characteristics. The 425 nm luminescence peak was found to remain constant during applied mechanical strain on the flexible substrate suggesting the nanowire LEDs did not experience any plane-strain during bending. A constant 2.5 V turn-on voltage, and a forward current of 0.4 mA at 4 V was measured under concave and convex bending. Atomic force and scanning electron microscopy characterization will also be shown of the nanowire device before and after double transfer as well as numerical simulation of the mechanical motion of the nanowire structures during bending.

Efficient radiative recombination is one of the key properties enabling high performance light emitting devices. We have performed an in-depth experimental analysis of radiative recombination in polar, nonpolar, and semipolar III-nitride quantum wells (QWs), which allows us to elucidate and quantify its mechanisms. We are able to distinguish between localized and free exciton recombination, we clearly see the effect of polarization fields via the quantum-confined Stark effect, and we observe the effect of the valence band structure associated with crystal orientation and strain.

Despite an already existing abundant literature dedicated to report of lot of experimental investigations towards the understanting of the mechanisms that rule the limitation of intense light emission in nitride-based heterostructures, there are still some issues that are not fully elucidated. This is probably related to the lack of investigations away from the blue and aquamarine light regions. In this communication we cover the 480 nm to 620 nm range by using a series of samples with different designs: single and multiple GaInN-GaN quantum wells. This paper is limited to heterostructures grown along the polar orientation. By changing the well width, and the indium content we could tune in the one hand the Quantum Confined Stark Effect that is to say the intrinsic radiative recombination rate while we could tune the crystalline quality by using strain-compensated GaInN-GaN-AlGaN designs. Finally, by changing our optical pump density we modified the intrinsic non radiative Auger. Given an emission wavelength, we find that the photoexcitation density P for the onset of substantial Auger effect to increase with the number of wells. Using an ABC-type modelling we find a clear 3/2 power law correlation between parameters B and C. This behavior is discussed in terms of electron-hole coulomb interaction and electron-electron repulsion in photo-excited samples. In condition of efficient Auger recombination, the variation of the internal quantum efficiency with photoexcitation density is ruled by a universal power law independent of the design: IQE = IQE0 – a log10 P with a =16 %cm2/Watt.

GaN/InGaN multiple quantum wells (MQW) and GaN nanorods have been widely studied as a candidate material for high-performance light emitting diodes. In this study, GaN/InGaN MQW on top of GaN nanorods are characterized in nanoscale using confocal microscopy associated with photoluminescence spectroscopy, including steady-state PL, timeresolved PL and fluorescence lifetime imaging (FLIM). Nanorods are fabricated by etching planar GaN/InGaN MQWs on top of a GaN layer on a c-plane sapphire substrate. Photoluminescence efficiency from the GaN/InGaN nanorods is evidently higher than that of the planar structure, indicating the emission improvement. Time-resolved photoluminescence (TRPL) prove that surface defects on GaN nanorod sidewalls have a strong influence on the luminescence property of the GaN/InGaN MWQs. Such surface defects can be eliminated by proper surface passivation. Moreover, densely packed nanorod array and sparsely standing nanorods have been studied for better understanding the individual property and collective effects from adjacent nanorods. The combination of the optical characterization techniques guides optoelectronic materials and device fabrication.

White light based on blue laser – YAG: Ce³⁺ phosphor has the advantage of implementing solid-state lighting and optical wireless communications combined-functionalities in a single lamp. However, the blue light was found to disrupt melatonin production, and therefore the human circadian rhythm in general; while the yellow phosphor is susceptible to degradation by laser irradiation and also lack tunability in color rendering index (CRI). In this investigation, by using a violet laser, which has 50% less impact on circadian response, as compared to blue light, and an InGaN-quantum-disks nanowires-based light-emitting diode (NWs-LED), we address both issues simultaneously. The white light is therefore generated using violet-green-red lasers, in conjunction with a yellow NWs-LED realized using molecular beam epitaxy technique, on titanium-coated silicon substrates. Unlike the conventional quantum-well-based LED, the NWs-LED showed efficiency-droop free behavior up to 9.8 A/cm² with peak output power of 400 μW. A low turn-on voltage of ~2.1 V was attributed to the formation of conducting titanium nitride layer at NWs nucleation site and improved fabrication process in the presence of relatively uniform height distribution. The 3D quantum confinement and the reduced band bending improve carriers-wavefunctions overlap, resulting in an IQE of ~39 %. By changing the relative intensities of the individual color components, CRI of >85 was achieved with tunable correlated color temperature (CCT), thus covering the desired room lighting conditions. Our architecture provides important considerations in designing smart solid-state lighting while addressing the harmful effect of blue light.

AlxGa1-xN nanowires (NWs) are considered to be a promising solution for solid-state ultraviolet (UV) emission. The basic physics of ternary alloys in NWs, and in particular of AlGaN NWs is however still to be explored. Here we report on the study of the structural and optical properties of AlGaN sections grown on top of GaN NWs on Si (111) substrates by plasma-assisted molecular beam epitaxy, in particular as a function of AlGaN growth temperature and ternary alloy composition Several series of samples with AlxGa1-xN sections on top of GaN NWs were grown in N-rich conditions, at different average AlN molar fractions (x in the 0.3-0.6 range) and various growth temperatures. Microphotoluminescence of single nanowires reveals a broad spectrum made of sharp lines with linewidths in the 1-5 meV range. This is similar to what one obtains when probing in luminescence an ensemble of quantum dots. We thereafter performed photon correlation measurements in a Hanbury-Brown and Twiss set-up, which showed that indeed, antibunching is observed when probing a single line. Such a single AlGaN nanowire thus behaves as a collection of quantum dots, which we attribute to localization centers due to Ga-richer regions in the AlGaN matrix. The mere counting of the number of sharp lines observed thus allows to determine the spatial density of such localized emission centers. We will also discuss time-resolved photoluminescence on such sharp emission lines, which allows to directly probe the size homogeneity of these localization centers in the AlGaN alloy.

Point-like defects in wide-bandgap materials are at the heart of a broad range of emerging applications including quantum information processing and metrology [1]. A well-known example is the nitrogen-vacancy (NV) defect in diamond, which can be used as a solid-state qubit to perform elaborate quantum information protocols [2] and highly sensitive magnetic field sensing [3]. These results motivate the search of new defects in other wide-bandgap materials, which would offer an expanded range of functionalities compared to NV defects in diamond. In that context, hexagonal boron nitride (hBN) appears as an appealing material. First, it has a 6-eV bandgap, which is ideally suited to host optically active defects with energy levels deeply buried between the valence band and the conduction band. Second, hBN is an electrical insulator with a two-dimensional (2D) honeycomb structure, which is a key element of Van der Waals heterostructures. Such “artificial” materials are currently attracting a great interest owing to their unique mechanical, electrical and optical properties [4]. Combining these properties with individual quantum systems would likely open new perspectives in quantum technologies. In this talk, I will report on the optical detection of individual defects hosted in a high-purity hBN crystal. Stable single photon emission is demonstrated under ambient conditions by means of photon correlation measurements [5]. A detailed analysis of the photophysical properties of the defect reveals a highly efficient radiative transition, leading to one of the brightest single photon source reported to date from a bulk, unpatterned, material. These results make a bridge between the physics of 2D materials and quantum technologies, and pave the way towards applications of van der Waals heterostructures in photonic-based quantum information science, metrology and optoelectronics. References [1] D. D. Awschalom, L. C. Bassett, A. S. Dzurak, E. L. Hu, and J. R. Petta, Science 339, 1174 (2013). [2] B. Hensen et al., Nature 526, 682-686 (2015). [3] J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, and V. Jacques, Rep. Prog. Phys. 77, 056503 (2014). [4] A. K. Geim and I. V. Grigorieva, Nature 499, 419-425 (2013). [5] L. J. Martinez, T. Pelini, V. Waselowski, J. R. Maze, B. Gil, G. Cassabois, and V. Jacques, preprint arXiv:1606.04124.

In a core-shell quantum-well (QW) nanorod (NR) structure, because of the non-uniform constituent atom supply for QW growth at different heights on a sidewall, the QW thickness and indium content vary with NR height. Multi-section NRs can be grown by controlling the supply duration of Ga source for decreasing the size of catalytic Ga droplet and hence tapering the NR cross-sectional size. The sidewall QWs of such a multi-section NR can emit light of a broad spectrum due to the larger variation ranges of QW thickness and indium content between sections of different cross-sectional sizes. In this paper, besides the growth processes of the aforementioned NR structures are reported, two models are built for simulating the sidewall QW growth and the tapering process of NR. Based on one of the models, the theories show the consistent results of increasing QW thickness and indium content in increasing NR height with the experimental observations. Based on another model, Ga adatoms diffuse on the slant facets from the Ga droplet on the NR top to deposit GaN on the slant facets for forming a gradient layer. In this situation, the angle of the slant facet increases from ~43 to ~62 degrees during the tapering process. The results are consistent with what observed in experiment. In this paper, besides NRs grown from patterned circular holes, the growth results of NRs from patterned elliptical holes are illustrated.

III-nitride-based vertical-cavity surface-emitting lasers have so far used intracavity contacting schemes since electrically conductive distributed Bragg reflectors (DBRs) have been difficult to achieve. A promising material combination for conductive DBRs is ZnO/GaN due to the small conduction band offset and ease of n-type doping. In addition, this combination offers a small lattice mismatch and high refractive index contrast, which could yield a mirror with a broad stopband and a high peak reflectivity using less than 20 DBR-pairs. A crack-free ZnO/GaN DBR was grown by hybrid plasma-assisted molecular beam epitaxy. The ZnO layers were approximately 20 nm thick and had an electron concentration of 1×10¹⁹ cm^-3, while the GaN layers were 80-110 nm thick with an electron concentration of 1.8×10¹⁸ cm^-3. In order to measure the resistance, mesa structures were formed by dry etching through the top 3 DBR-pairs and depositing non-annealed Al contacts on the GaN-layers at the top and next to the mesas. The measured specific series resistance was dominated by the lateral and contact contributions and gave an upper limit of ~10^-3Ωcm² for the vertical resistance. Simulations show that the ZnO electron concentration and the cancellation of piezoelectric and spontaneous polarization in strained ZnO have a large impact on the vertical resistance and that it could be orders of magnitudes lower than what was measured. This is the first report on electrically conductive ZnO/GaN DBRs and the upper limit of the resistance reported here is close to the lowest values reported for III-nitride-based DBRs.

Over the past decade, the performance of GaN power devices has rapidly improved. There are two types of devices currently being developed, with either a lateral or a vertical structure. Though mainstream GaN power devices have still a lateral structure, vertical structure devices that is on GaN power devices have recently attracted additional research attention. The vertical structure has the advantages of a small chip size, easy wiring, a high breakdown voltage, and current-collapse-free operation. These characteristics are highly suited for high-power applications, for example, to control high-power motors used in electric automobiles. Though recent reports showed high performance of the vertical structure, the development issues of the fabrication process still remain. The main issue is the quality of GaN substrates. Recent substrates have sufficient quality for high-voltage experiments like fabricating high-voltage devices, which make possible to obtain high-voltage devices over 1kV. However, the entire GaN substrate area does not yet have a uniform quality. It is the large issue. Other important issue is a normally-off gate structure and the gate insulator. Gate structure of the threshold voltage of > 3V like inverted type gate must be developed. The gate insulator which has low interface state density and high reliability is also required. Progress of the gate insulator is very rapid and low interface state density was established using SiO2 and resulted in the inverted MOSFET operation. The recent progress of the on GaN power devices will inspire the researches more.

In this paper, we develop a physically motivated compact model of the charge-voltage (Q-V) characteristics in various III-nitride high-electron mobility transistors (HEMTs) operating under highly non-equilibrium transport conditions, i.e. high drain-source current. By solving the coupled Schrödinger-Poisson equation and incorporating the two-dimensional electrostatics in the channel, we obtain the charge at the top-of-the-barrier for various applied terminal voltages. The Q-V model accounts for cutting off of the negative momenta states from the drain terminal under high drain-source bias and when the transmission in the channel is quasi-ballistic. We specifically focus on AlGaN and AlInN as barrier materials and InGaN and GaN as the channel material in the heterostructure. The Q-V model is verified and calibrated against numerical results using the commercial TCAD simulator Sentaurus from Synopsys for a 20-nm channel length III-nitride HEMT. With 10 fitting parameters, most of which have a physical origin and can easily be obtained from numerical or experimental calibration, the compact Q-V model allows us to study the limits and opportunities of III-nitride technology. We also identify optimal material and geometrical parameters of the device that maximize the carrier concentration in the HEMT channel in order to achieve superior RF performance. Additionally, the compact charge model can be easily integrated in a hierarchical circuit simulator, such as Keysight ADS and CADENCE, to facilitate circuit design and optimization of various technology parameters.

GaN-HEMTs with p-GaN gate have recently demonstrated to be excellent normally-off devices for application in power conversion systems, thanks to the high and robust threshold voltage (V_TH>1 V), the high breakdown voltage, and the low dynamic Ron increase. For this reason, studying the stability and reliability of these devices under high stress conditions is of high importance. This paper reports on our most recent results on the field- and time-dependent degradation of GaN-HEMTs with p-GaN gate submitted to stress with positive gate bias. Based on combined step-stress experiments, constant voltage stress and electroluminescence testing we demonstrated that: (i) when submitted to high/positive gate stress, the transistors may show a negative threshold voltage shift, that is ascribed to the injection of holes from the gate metal towards the p-GaN/AlGaN interface; (ii) in a step-stress experiment, the analyzed commercial devices fail at gate voltages higher than 9-10 V, due to the extremely high electric field over the p-GaN/AlGaN stack; (iii) constant voltage stress tests indicate that the failure is also time-dependent and Weibull distributed. The several processes that can explain the time-dependent failure are discussed in the following.

AlGaN/GaN high electron mobility transistors (HEMTs) showed its promising performance in high power and high frequency, which can be used for applications such as satellite-based communication networks, inverter units in hybrid electric vehicles and advanced radar systems. However, intrinsic defects and defects generated during the device fabrication degraded HEMT performance, such as drain current collapse, high gate leakage, and lower rf power density and power add efficiency. Furthermore, subsequent electrical stressing of the HEMTs during operation leads to creation of more traps and further device degradation through various mechanisms, including gate contact sinking, shallow trap formations, and the inverse piezoelectric effect. It is highly desirable to have non-destructive methods available to identify the activation energies of the defects and spatial location of trap states in HEMT. A sub-bandgap optical pumping technique was developed to identify trap locations in AlGaN/GaN HEMTs. By varying photon fluxes, the traps with different activation energies appeared at different photon flux levels. This implies that the defects originate at different physical locations in the HEMT. The locations of the traps identified with the sub-bandgap optical pumping methods confirmed by gate pulse measurements under optical pumping.

High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are finding an increasing number of commercial and military applications that require high voltage, high power, and high efficiency operation. In recent years, leading GaN HEMT manufacturers have reported excellent RF power characteristics and encouraging reliability, but long-term reliability in the space environment still remains a major concern due to a large number of defects and traps present both in the bulk as well as at the surface, leading to undesirable characteristics including current collapse. Furthermore, degradation mechanisms in GaN HEMTs are still not well understood. Thus, reliability and radiation effects of GaN HEMTs should be studied before solid state power amplifiers (SSPAs) based on GaN HEMT technology are successfully deployed in space satellite systems. For the present study, we investigated electrical characteristics of high-power GaN HEMTs irradiated with protons and heavy ions under various irradiation and biasing conditions.

An AlGaN/GaN HEMT on Si has received significant attention due to the availability of large sized Si substrate at low cost. The limiting factors for high quality GaN/Si are large lattice and thermal expansion-coefficient mismatches between GaN and Si, which lead to high dislocation densities, wafer bowing and crack formation. Therefore, it is imperative to grow high quality GaN/Si with minimum wafer bowing and without crack in order to improve the device performances. The AlGaN/GaN HEMTs were grown on 8-inch Si substrates using MOCVD technique. The HEMT structure consisted of the high-temperature-grown AlN nucleation layer (HT-AlN NL), the HT-Al0.3Ga0.7N intermediate layer (HT-AlGaN IL), the AlGaN/AlN strained layer superlattice (SLS), the GaN layer and the Al0.26Ga0.74N barrier layer. The HT-AlN NL was effective in avoiding the reaction between Ga and Si, which resulted in the specular surface morphology. The characteristic of the HT-AlN NL affected the vertical breakdown characteristics. The wafer b owing can be minimized by use of SLS and GaN because of counter-balance of thermal and lattice mismatches between SLS and GaN. The AlGaN/GaN HEMT exhibited a Hall mobility of 1730 cm2/vs, a sheet carrier density of 7.4x1012 cm-2 and the wafer bowing value of 42 um. The vertical voltage at 1 uA/mm was between 950 V and 1000 V across the wafer. The normally-off devices were fabricated by using gate-recess and MOS technology. The devices exhibited good dc characteristics with drain current maximum of 300 mA/mm, threshold voltage of +2.4 V and 3-terminal off-breakdown voltage of 1650 V.

Surface voltage response to pulses of piezoelectric polarization is measured with a Kelvin-probe providing a unique means for investigation of the dynamics of polarization induced sheet charge and 2DEG. Combined with biasing of the surface with a corona-deposited charge from accumulation to deep depletion and corresponding non-contact C-V type characterization, the technique identifies surface band bending and interface traps as key factors that affect the magnitude and time decay of piezoelectric polarization. For 2DEG structures, surface potential pinning is observed when the 2DEG is fully populated. Pinning is released by negative corona charging to fully deplete the 2DEG. These results are consistent with the role of surface states. Presently demonstrated polarization modulation and wafer scale measurements shall impact the in-depth characterization and fundamental understanding of AlGaN/GaN 2DEG structures.

In this paper, we utilize the fin-shaped channel to form the AlGaN/GaN HEMTs which can be considered as “Fin- HEMTs” to adjust the threshold voltage (V_TH) toward positive value. The gate metal here is deposited directly on the AlGaN/GaN semiconductor to form the Schottky contact. Although the fin-widths are in the level of micron-scale, the shifts of VTH are still observable and the V_TH becomes more positive with the smaller fin width. This is attributed to the assistant of the side-gate control which can be regarded as the depletion layer formed by Schottky contact at side-gate will deplete the 2DEG in the channel. Therefore, with the smaller fin width, the channel can be pinched off faster which is similar to the double gate MESFETs. The V_TH of planar device is shifted from -3.81 V to -3.37 V with 2-μm-fin-width. On the other hand, unlike to carrier transportation in the conventional FinFET with nano-scale fin width which is dominated by the surface scattering, our Fin-HEMTs with micron-scale fin width exhibit higher drain current than planar device and this is because of the smaller thermal resistance (R_TH) for the fin-HEMT. We extract the R_TH by varying the measured temperature and the R_TH of the device with 4-μm-fin-width and planar device is 58.5 K/W and 249.5 K/W, respectively.

We report our recent improvement of watt class blue and green GaN based LDs. These LDs were grown on c-face GaN substrates by metal organic chemical deposition. The laser chip was mounted on the heat sink by the junction down method in TO-ø9 mm package for the suppression of the thermal resistance. The optical output power of 455nm blue LDs was obtained above 4.7 W at injection current of 3A. The average lifetime was estimated to be over 30,000 hours at case temperature of 65 degree C under 3A. In green LDs, 1 watt class 532 nm green LDs as same wavelength as second harmonic generation (SHG) green laser was developed and the wall plug efficiency was 12.1 %. And the longer lasing wavelength was achieved to 537 nm.

The UV semiconductor-based laser sources are important for a variety of fields, including medical, mechanical processing, chemical processing, biology, and photonics. However, the development of UV-B and UV-C laser diodes is strongly hampered because of the difficulties with current injection technology such as the realization of both a high hole concentration and low resistivity p-type AlGaN with a high AlN molar fraction. Because laser oscillation from AlGaN, with a high AlN molar fraction, can be obtained under optical pumping, UV lasers with controllable wavelengths should be realized if this problem can be solved. One promising technique for avoiding these problems is the use of electron beam excitation. Till date, nitride semiconductor-based lasers have been designed to achieve population inversion of the carrier and to oscillate due to current injection. However, as previously discussed, it is difficult to achieve wavelengths in UV-B and UV-C using this method. The conductivity control of nitride semiconductors is unnecessary using electron beam excitation. Therefore, it would be possible to expand the wavelength region for the laser action of nitride semiconductor-based lasers from deep UV to infrared if a nitride semiconductor-based laser could be oscillated via electron beam excitation. In this study, nitride-based lasers excited by electron beam were investigated, and laser emission was observed for the first time from a GaInN/GaN and GaN/AlGaN-based MQWs excited by an electron beam.

In this paper, we present numerical simulations of different types of nitride VCSELs. We analyzed structures with different DBR mirrors and electrical confinements. We compare threshold parameters, including threshold current, threshold temperature and optical field distribution for structures with an ITO contact and structures with tunnel junctions. Lasers emitting blue/violet and green radiation are analyzed from the point of view of their thermal properties.

Optical clocks have demonstrated an improvement in temporal accuracy of several orders of magnitude over existing time standards based on caesium. Such systems hold great promise in many industrial sectors including financial time stamping, GPS-free navigation and network synchronisation. Atom interferometry has proven to be a reliable method of precision gravity sensing and finds application in geological studies, including earthquake warning systems and oil exploration. Such systems require a number of sophisticated lasers in a compact and reliable format for use outside of a laboratory environment, suitable for commercialisation and user transportation. Of particular interest, is emerging AlGaInN laser diode technology that has the potential to provide practical solutions for next generation optical clock technology.

This paper reports the influence of carbon impurities on electrical properties of AlGaN: Mg layer which is used in InGaN-based blue/green laser diodes (LDs) as the cladding layer. AlGaN layer grown by MOCVD usually contains more carbon impurity than GaN, especially when AlGaN is grown at a low temperature to avoid the degradation of InGaN/GaN quantum wells in green LD with high indium content. However, no experimental study on the effect of carbon impurity on the electrical properties of the AlGaN:Mg layer has been reported. All AlGaN:Mg samples were grown on c-plane GaN/sapphire template at various growth pressure, growth rate and V/III ratio to suppress the carbon impurity concentration. These sample were then activated at 950℃ for 90s in nitrogen ambient. The hole concentration and resistivity of Al0.07Ga0.93N:Mg samples depend on carbon concentration. By reducing carbon concentration from 2×1018 to 5×1016 cm-3, the hole concentration increase from 7.5×1016 to 3.5×1017 cm-3, and thus the resistivity of p-Al0.07Ga0.93N decreases from 7.4 to 2.2 Ω·cm . Based on the analysis of the charge neutrality equation, we believe that the compensation effect of CGa(Al) as a shallow donor in AlGaN:Mg explains the dependence of the hole concentration and the resisitivity on the carbon concentration in our samples, which will be discussed in detail in this report. By applying the optimized AlGaN:Mg grown at low temperature as the cladding layer, we have obtained green LD structures without thermal degradation in the InGaN active region. The differential resistance is 2.4 Ω leading to V= 4.9 V at 4 kA/cm2. It lases at 508nm with Jth= 1.8 kA/cm2, and has a output power of 58 mW at a current density of 6 kA/cm2.

Deep ultraviolet light emitting diodes (UV-C LEDs) are taking significant interest in varying applications such as disinfection and purification for air, water, and surface. Therefore, many reports have been published regarding the development and applications of UV-C LEDs, which have been adopted a home appliances recently. However, UV-C LEDs still have a number of challenges such as output power, cost, reliability, and manufacturability. In this talk, recent advances in epitaxial quality, device design, and reliability for high current driven UV-C LEDs will be presented.

AlGaN deep ultraviolet light-emitting diodes (DUV-LEDs) are attracting much attention for a wide variety of applications, however, the efficiency of DUV-LED is still low suppressed by low light-extraction efficiency (LEE). Transparent contact layer is considered to be necessary in order to obtain high LEE in AlGaN DUV LEDs. In this work, we demonstrate over 10% external quantum efficiency (EQE) in an AlGaN DUV-LED by using transparent p-AlGaN contact layer and highly reflective p-type electrode. We fabricated AlGaN quantum well (QW) DUV LEDs with transparent p-AlGaN contact layers on AlN/sapphire templates. EQEs were compared between LEDs with Ni/Al highly reflective electrode and with conventional Ni/Au electrode. The transparency of the p-AlGaN contact layer was confirmed to be more than 97 %. The maximum EQE for 261 nm LEDs with Ni/Al and Ni/Au electrodes were approximately 2 and 3.3%, respectively. We confirmed that the LEE was increased by about 1.7 times. We also fabricated flip-chip (FC) UVC LED module with transparent p-AlGaN contact layer and reflective electrode. The FC LED module was encapsulated to increase LEE. The emission wavelengths were 276 nm. The EQE value under the forward current of 120 mA was increased from 2.7 to 8.6% by increasing an LEE. The output power of approximately 60 mW was obtained under the forward current of 150 mA. The EQE value was maximally increased up to 10.8%. LEE was estimated to be increased from 8.6 % to 25.5 % by introducing LEE enhancement structure.

III-nitride based ultraviolet (UV) light emitting diodes (LEDs) are of considerable interest in replacing gas lasers and mercury lamps for numerous applications. Specifically, AlGaN quantum well (QW) based LEDs have been developed extensively but the external quantum efficiencies of which remain less than 10% for wavelengths <300 nm due to high dislocation density, difficult p-type doping and most importantly, the physics and band structure from the three degeneration valence subbands. One solution to address this issue at deep UV wavelengths is by the use of the AlGaN-delta-GaN QW where the insertion of the delta-GaN layer can ensure the dominant conduction band (C) - heavyhole (HH) transition, leading to large transverse-electric (TE) optical output. Here, we proposed and investigated the physics and polarization-dependent optical characterizations of AlN-delta- GaN QW UV LED at ~300 nm. The LED structure is grown by Molecular Beam Epitaxy (MBE) where the delta-GaN layer is ~3-4 monolayer (QW-like) sandwiched by 2.5-nm AlN sub-QW layers. The physics analysis shows that the use of AlN-delta-GaN QW ensures a larger separation between the top HH subband and lower-energy bands, and strongly localizes the electron and HH wave functions toward the QW center and hence resulting in ~30-time enhancement in TEpolarized spontaneous emission rate, compared to that of a conventional Al_0.35Ga_0.65N QW. The polarization-dependent electroluminescence measurements confirm our theoretical analysis; a dominant TE-polarized emission was obtained at 298 nm with a minimum transverse-magnetic (TM) polarized emission, indicating the feasibility of high-efficiency TEpolarized UV emitters based on our proposed QW structure.

Light emitting diodes (LEDs) in the UVB (280 nm – 315 nm) spectral range are of particular interest for applications such as plant growth lighting or phototherapy. In fact, LEDs offer numerous advantages compared to conventional ultraviolet light sources such as a tunable emission wavelength, a small form factor, and a minimal environmental impact. State-of-the-art devices utilize p-GaN and low aluminum mole fraction p-AlGaN layers to enable good ohmic contacts and low series resistances. However, these layers are also not transparent to UVB light thus limiting the light extraction efficiency (LEE). The exploitation of UV-transparent p-AlGaN layers together with high reflective metal contacts may significantly increase the LEE. In this paper, the output power of LEDs emitting at 310 nm with a UV-transparent and absorbing Mg-doped AlGaN superlattice is compared. A three-fold increase of the output power was observed for LEDs with UV-transparent p-AlGaN layers. To investigate these findings, LEDs with low reflective Ni/Au and high reflective Al contacts are fabricated and characterized. Together with ray tracing simulations and detailed measurements of the metal reflectivities, we were able to determine the LEE and the internal quantum efficiency (IQE). According to on-wafer measurements, the external quantum efficiency (EQE) increases from 0.3% for an absorbing p-Al0.2Ga0.8N/Al0.4Ga0.6N-superlattice with Ni/Au contacts to 0.9% for a UV-transparent p-Al0.4Ga0.6N/Al0.6Ga0.4N-superlattice with Al contacts. This 3× enhancement of the EQE can be partially ascribed to an improved LEE (from 4.5% to 7.5%) in combination with a 1.8× increase of the IQE when using a p-Al0.4Ga0.6N/Al0.6Ga0.4N-superlattice instead of a p-Al0.2Ga0.8N/Al0.4Ga0.6N-superlattice.

III-nitride light emitters, such as light-emitting diodes (LEDs) and laser diodes (LDs), have been demonstrated and studied for solid-state lighting (SSL) and visible-light communication (VLC) applications. However, for III-nitride LEDbased SSL-VLC system, its efficiency is limited by the “efficiency droop” effect and the high-speed performance is limited by a relatively small -3 dB modulation bandwidth (<100 MHz). InGaN-based LDs were recently studied as a droop-free, high-speed emitter; yet it is associated with speckle-noise and safety concerns. In this paper, we presented the semipolar InGaN-based violet-blue emitting superluminescent diodes (SLDs) as a high-brightness and high-speed light source, combining the advantages of LEDs and LDs. Utilizing the integrated passive absorber configuration, an InGaN/GaN quantum well (QW) based SLD was fabricated on semipolar GaN substrate. Using SLD to excite a YAG:Ce phosphor, white light can be generated, exhibiting a color rendering index of 68.9 and a color temperature of 4340 K. Besides, the opto-electrical properties of the SLD, the emission pattern of the phosphor-converted white light, and the high-speed (Gb/s) visible light communication link using SLD as the transmitter have been presented and discussed in this paper.

We present a nanometer-scale correlation of the structural, optical, and electronic properties of InGaN/GaN core-shell microrod LEDs: The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subsequently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. The Si-doped GaN core was finally encased by an InGaN single quantum well (SQW) followed by an intrinsic GaN layer and a thick Mg-doped p-GaN shell. Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the free-carrier concentration within the Si-doped GaN core and the luminescence properties of the individual functional layers. The CL was supported by Raman spectroscopy directly carried out at the same microrod on the thin TEM-lamella. The cross-sectional CL of a single microrod resolves the emission of the single layers. CL and Raman measurements reveal a high free-carrier concentration of 7x1019 cm 3 in the bottom part and a decreasing doping level towards the tip of the microrod. Moreover, structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense emission coming from the InGaN QW on the non-polar side walls, which shows a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium concentration right at the intersection of generated defects and InGaN QW, a red shifted emission appears, which indicates indium clustering.

Group III-nitride (Al-, In-, Ga-, N) material system has been well studied and widely applied in optoelectronics such as light emitting diodes (LEDs) for solid state lighting. In contrast, the group II-IV-nitride is rarely studied, yet it can expand the material properties provided by III-nitrides. For example, ZnGeN2 has a similar bandgap and lattice constant as those of GaN. Recently, theoretical studies based on first principle calculation indicate a large band offset between GaN and ZnGeN2 (Delta_Ec=1.4 eV; Delta_Ev=1.5 eV). Utilizing the novel heterostructures of GaN (InGaN)/ZnGeN2, we studied the following two types of device structures: 1) Type-II InGaN-ZnGeN2 quantum wells (QWs) for high efficiency blue and green LEDs; 2) Lattice-matched GaN-ZnGeN2 coupled QWs for near-IR intersubband transitions. The design of type-II InGaN-ZnGeN2 QWs leads to a significant enhancement of the electron-hole wavefunction overlap due to the strong confinement of the holes in the ZnGeN2 layer as well as the engineered band bending. Simulation studies based on a self-consistent 6-band k∙p method indicate an enhancement of 5-7 times of spontaneous emission rate for an appropriately designed type-II InGaN-ZnGeN2 QWs for LED applications. For the coupled QW structure, it is comprised of two GaN QWs separated by a thin ZnGeN2 barrier layer, with thick ZnGeN2 layers as outer barriers surrounding the QWs. Our studies indicate that with optimized ZnGeN2 barrier thickness, the energy separation between E1 and E2 can be tuned to 92 meV for the resonance of the electron and LO-phonon scattering.

The advent of optical coherence tomography (OCT) has permitted high-resolution, non-invasive, in vivo imaging of the eye, skin and other biological tissue. The axial resolution is limited by source bandwidth and central wavelength. With the growing demand for short wavelength imaging, super-continuum sources and non-linear fibre-based light sources have been demonstrated in tissue imaging applications exploiting the near-UV and visible spectrum. Whilst the potential has been identified of using gallium nitride devices due to relative maturity of laser technology, there have been limited reports on using such low cost, robust devices in imaging systems.

A GaN super-luminescent light emitting diode (SLED) was first reported in 2009, using tilted facets to suppress lasing, with the focus since on high power, low speckle and relatively low bandwidth applications. In this paper we discuss a method of producing a GaN based broadband source, including a passive absorber to suppress lasing. The merits of this passive absorber are then discussed with regards to broad-bandwidth applications, rather than power applications. For the first time in GaN devices, the performance of the light sources developed are assessed though the point spread function (PSF) (which describes an imaging systems response to a point source), calculated from the emission spectra. We show a sub-7μm resolution is possible without the use of special epitaxial techniques, ultimately outlining the suitability of these short wavelength, broadband, GaN devices for use in OCT applications.

Novel layer release and transfer technology of single-crystalline GaN semiconductors is attractive for enabling many novel applications including flexible photonics and hybrid device integration. To date, light emitting diode (LED) research has been primarily focused on rigid devices due to the thick growth substrate. This prevented fundamental research in flexible inorganic LEDs, and limited the applications of LEDs in the solid state lighting (due to the substrate cost) and in biophotonics (i.e. optogenetics) (due to LED rigidness). In the literature, a number of methods to achieve layer transfer have been reported including the laser lift-off, chemical lift-off, and Smartcut. However, the release of films of LED layers (i.e. GaN semiconductors) has been challenging since their elastic moduli and chemical resistivity are much higher than most conventional semiconductors. In this talk, we are going to review the existing technologies and new mechanical release techniques (i.e. spalling) to overcome these problems.

To reduce the $/lm for GaN-based LEDs, most LED makers are adopting flip-chip based Chip Scale Packaging (CSP) technology. However, it is difficult to realize true wafer-level (WL) CSP technology with conventional sapphire substrates caused by “blue light leak” issue. On the other hand, thin-film flip-chip technology practically eliminates blue light leak and allows a simpler process of phosphor coating and dicing. To maximize the advantages of WL-CSP, large diameter process is necessary for it to be cost effective. In this sense, 8 inch Si wafer is the best candidate. GaN-on-Si based LEDs, however, have seen little growth in the lighting market due to several issues that hamper the efficiency and reliability related to the quality of GaN films grown on Si. However, we have overcome all drawbacks including yield, device reliability and LM-80 by proprietary stress-managed buffer and optimized LED epitaxial structure. In addition to the comparable efficiency and reliability performance, we discovered several other advantages of using 8 inch Si substrates such as the wavelength uniformity, low thermal droop and low compressive strain of MQW. Wafer-level chip scale package (WL-CSP) on silicon substrates has its own set of advantages such as relatively easier to texture the GaN or phosphor surface at the wafer level to maximize photon extraction efficiency and multi-layer phosphor coating to further push the efficiency upwards.

We report the fabrication process and characterization of high resolution 873 x 500 pixels emissive arrays based on blue or green GaN/InGaN light emitting diodes (LEDs) at a reduced pixel pitch of 10 μm. A self-aligned process along with a combination of damascene metallization steps is presented as the key to create a common cathode which is expected to provide good thermal dissipation and prevent voltage drops between center and side of the micro LED matrix. We will discuss the challenges of a self-aligned technology related to the choice of a good P contact metal and will present our solutions for the realization of the metallic interconnections between the GaN contacts and the higher levels of metallization at such a small pixel pitch. Enhanced control of each technological step allows scalability of the process up to 4 inch LED wafers and production of high quality LED arrays. The very high brightness (up to 10⁷ cd.m^-2) and good external quantum efficiency (EQE) of the resulting device make these kind of micro displays suitable for augmented reality or head up display applications.