We proposed recently a new RF-MBE method called droplet elimination by radical-beam irradiation (DERI) for growing high-quality InN-based III-nitride films. DERI consists of two growth processes: a metal-rich growth process (MRGP) and a droplet elimination process (DEP). In InGaN growth, Ga was preferentially and selectively captured from the Ga/In wetting layer and droplets during MRGP. Then, excess In was swept out from the growing InGaN surface. The swept In was transformed to InN, epitaxially grown on top of InGaN during DEP using nitrogen radical beam irradiation. By repeating this process, an InN/InGaN MQW structure was successfully fabricated. Thick and uniform InGaN films were also successfully obtained by additionally irradiating the same Ga beam flux as that in MRGP even during DEP. When the irradiated Ga/N* beam flux ratio in DEP was changed from that in MRGP, an In_xGa_1-xN/In_yGa_1-yN MQW structure was successfully fabricated.

This paper presents the growth of thick semipolar {10-11}, {11-22}, and {20-21} GaN layers on n, r, and {22-43} patterned sapphire substrates (PSSs), respectively, by hydride vapor phase epitaxy. The reduction rate of the dislocation density varied with growth planes. For {10-11} GaN layers, the dislocation density drastically decreased at over 100 μm, which was as fast the reduction rate as in the case of the c-plane. It was revealed that the reduction rate of the dislocation density could be controlled by the proper selection of the growth plane. We obtained a freestanding GaN of 2 inch diameter. Thick GaN growth led to the self-separation of the GaN layer from the PSS during cooling process. The separation plane formed at the interface between GaN and PSS, which is different from the case of a conventional c-plane GaN/sapphire. The separationability of the GaN layer from the PSS depended on the selective growth area of the sapphire sidewall.

We review a new approach to the bowing management of sapphire substrates for III-nitride epitaxy based on the internally focused laser processing. The laser process modifies the phase of the sapphire, inducing a volume expansion effect that enables the bow to be managed. Bowing control is required in two main areas: 1) control of the initial bow of the sapphire substrate, and 2) reduction in the bow after the epitaxy. The initial bow control was demonstrated for ~250 μm pre-bowed convex and concave sapphire substrates. The effect of the pre-bowed substrate on III-nitride epitaxy was also experimentally verified with an in situ curvature monitoring system during the III-nitride epitaxy; the possibility to accommodate substrate bowing to any target values by applying the initial offset to the substrate was also confirmed. When applied to the substrate after the epitaxy, the same technique was successful in flattening of the substrate for a subsequent chip-fabrication process. This new approach provides wide flexibility in the design engineering of epitaxial and device fabrication processes. Thus, it accelerates the realization of larger diameter device processes with III-nitride/sapphire.

Role and influence of impurities like: oxygen, indium and magnesium, on GaN crystals grown from liquid solution under high nitrogen pressure in multi-feed-seed configuration is shown. The properties of differently doped GaN crystals are presented. The crystallization method and the technology based on it (for obtaining high quality GaN substrates) are described in details. Some electronic and optoelectronic devices built on those GaN substrates are demonstrated.

HVPE crystallization on ammonothermaly grown GaN crystals (A-GaN) is described. Preparation of the (0001) surface of the A-GaN crystals to the epi-ready state is presented. The HVPE initial growth conditions are determined and demonstrated. An influence of a thickness and a free carrier concentration in the initial substrate on quality and mode of growth by the HVPE is examined. Smooth GaN layers of excellent crystalline quality, without cracks, and with low dislocation density are obtained.

Mg-doped In_xGa_1-xN films are investigated using electron paramagnetic resonance (EPR) spectroscopy. Surprisingly, the number of EPR-detected Mg-related acceptors decreases as x increases from 0.021 to 0.112, but the hole density increases as expected. The observation is attributed to the loss of a magnetic resonance signal from Mg impurities in the vicinity of an In-induced perturbing field. Analysis shows that the number of Mg affected is greater than the amount predicted by considering only next nearest neighbors, but does not extend to all the Mg atoms. The results support the model in which In-induced potential fluctuations perturb the Mg dopant.

In-grown group III (cation) vacancies (V_Ga, V_Al, V_In) in GaN, AlN and InN tend to be complexed with donor-type defects These donor defects may in principle be residual impurities such as O or H, n-type dopants such as Si, or intrinsic defects such as the N vacancy (V_N). The cation vacancies and their complexes are generally deep acceptors, and hence they compensate for the n-type conductivity and add to the scattering centers limiting the carrier mobility in these materials. Mg doping reduces the group III vacancy concentrations, but other kinds of vacancy defects emerge. This work presents results obtained with positron annihilation spectroscopy in GaN, AlN, and InN. The vacancy-donor complexes are different in these three materials, and their importance in determining the opto-electronic properties of the material varies as well. The formation of these defects is discussed in the light of the differences in the growth methods.

Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer, and with different numbers of GaN monolayers (n from 1 to 40). The emission energies, E_PL, measured at ambient pressure, are close to the value of the band gap, E_g, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dE_PL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both E_PL and dE_PL/dp are significantly higher than those obtained from abinitio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested...Detailed analysis of the electronic band structure of 1InN/5GaN superlattice is performed showing that the built-in electric field plays an important role in the mInN/nGaN structures. It strongly influences the valence- and conduction-band profiles and thus determines the effective band gap.

The behavior of threading dislocations and stacking faults were investigated in a (1-101) GaN grown on a patterned (001)Si substrate by selective metal-organic-vapor-phase-epitaxy with an AlN buffer layer. Cross sectional transmission electron microscopy images showed that threading dislocations are generated at the hetero-interface of GaN/AlN/Si induced by misfit dislocations, while stacking faults are generated when two crystals with different crystal axes coalesce. We found some of them are annihilated making a loop, where two stacking faults have been generated at a short distance. This suggests a rout to decrease the density of stacking faults in III nitrides.

We report the wavelength-dependent nonlinear absorption (NLA) of InN film grown on anr-plane sapphire by molecular beam epitaxy technique. In order to understand the nonlinear optical properties of InN, the Z-scan measurement was performed at two different wavelengths, the photon energies of which are near (resonant excitation) and much higher (non-resonant excitation) than the bandgap of InN, respectively. Under non-resonant excitation, band-filling effect leads the dominant saturable absorption, while under resonant excitation, reverse saturable absorption dominates the nonlinear absorption. From the open-aperture Z-scan measurement under resonant excitation, we found that InN exhibits more than one nonlinear absorption process simultaneously. Particularly, under relatively weak resonant excitation, the transformation from saturable absorption to 2PA through the intermediate excitation state absorption was observed as the sample approaches the beam focus. The close-aperture Z-scan signals of InN show valley-peak response, implying that the nonlinear refraction in InN is caused by the self-focusing of the Gaussian laser beam. Using the Z-scan theory, the corresponding nonlinear parameters, such as saturation intensity, 2PA coefficient, and nonlinear refractive index, of InN were estimated.

Self-assembled GaN nanowires (NWs) currently are a subject of sustained interest in the scientific community motivated by both their potential applications for new LEDs, which should take benefit of the improved crystalline quality of those nano-objects, due to a strongly reduced defects density. In addition, interest of the scientific community for these 1D nano-systems is also related to the new fundamental questions opened by their strongly anisotropic geometry, and to their potential as possible building blocks for future nano-electronic devices. In this context, Raman spectroscopy has been increasingly used to study nitride NWs and several new phenomena have been reported to date with respect to these one-dimensional structures. In this work, both GaN and AlGaN nanowires grown by plasma-assisted Molecular Beam Epitaxy (MBE) have been experimentally investigated by scanning electron microscopy, atomic force microscopy and micro-Raman spectroscopy. Experimental results are analyzed and compared to theoretical ones obtained by dielectric models and Discrete Dipole Approximation (DDA) method. Evidence is given for original surface effects in the optical phonon physics related to both structural anisotropy of the material and 1D geometry of the GaN NWs. By using UV resonant excitation for AlGaN NWs in the whole range of composition, we demonstrate the selective excitation of AlGaN with the Al composition matching the energy of the exciting photons. Finally, we analyzed Raman data from single GaN NW after deposition on a flat substrate and we discuss the nature of strongly polarized A₁(TO) phonon as a function of the NWs aspect ratio.

We review the recent developments to enhance the external quantum efficiency (EQE) in GaN based vertical light-emitting diodes (V-LEDs). The controlling of side-wall angle by SiO2 nanosphere lithography significantly improved the light extraction efficiency (LEE) of V-LEDs; this result is 6% higher than the photo chemical etching (PCE) method, which is known to have the highest light extraction, and 300% higher than flat surface V-LEDs. Nanostructured V-LEDs with a very dense forest of vertically aligned ZnO nanowires on the surface of N-face n-GaN induce the dramtic improvement in LEE. The structural transformation at the nanolevel by the UV radiation and Ozone (UV-O) treatment contributes the high density of Zn seed on GaN, and then this approach shows an extreme enhancement in LEE (>2.8x) compared to flat V-LEDs. The enhanced LEE was also demonstrated by depositing a spontaneously formed MgO nano-pyramids and ZnO refractive-index modulation layer on the surface of V-LEDs, resulting in the increase of output power by 49 %, comparing with the V-LEDs with a flat n-GaN surface. The enhancement of light output power by the nanotexturing of n-face n-GaN was remarkably influenced by 3-dimentional nanostructures.

We report a highly efficient GaN-based blue light-emitting diodes (LEDs) structure with an emitting wavelength of 450nm on flat sapphire substrate by utilizing a nano-porous (NP) GaN insertion layer. Unlike the LED on patterned sapphire substrates (PSS), the presented substrate has a new morphology which not only can generate an embedded nano-dimensional void structure as a mirror layer to reflect the light from active layers for enhancing the light extraction, but can also easily enlarge the wafer size to a large scale, such as wafer diameter larger than 6 inches. With a chip size of 45 mil × 45 mil under a driving current of 350 mA, the light output powers of the NP GaN LEDs without and with encapsulation are 455 and 554 mW respectively. The light output power is improved about 2 -fold comparing to the LED on a flat sapphire substrate, and even comparable to the LED on PSS which all of them have a flat p-type GaN surface. The characterization and performance of this newly NP LED structure will be discussed in detail.

Porous GaN crystals have been grown on Pt- and Au- coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH3 gas in a CVD system, intermetallic metal-Ga alloys formed at the interface allow the seeding and growth of porous GaN by vapor-solid-solid processes. Current-voltage and capacitance-voltage measurements confirm that the intermetallic seed layers result in near-ohmic contacts to porous n-GaN with low contact resistivities.

GaN is an attractive material for high performance power devices. Vertical GaN power devices are suitable for high current operation, on the other hand, lateral GaN power devices, namely GaN lateral HEMTs have both low on-resistance and low parasitic capacitance. In addition, the GaN lateral HEMTs can be fabricated on Si substrate. We can get low conduction loss and low switching loss devices with low cost. So the GaN lateral HEMTs are suitable for subsystems like an air conditioner and an electric power steering. Serious technical issues about GaN power devices are a normally-off operation, a current collapse, and a high quality gate insulator. Several normally-off operation techniques have been proposed but there is no decisive method. An NH₃ surface treatment and a SiO₂ passivation are useful to suppress the current collapse. An Al₂O₃ deposited by ALD is excellent for gate insulator in breakdown and it has enough TDDB reliability under room temperature and 150°C.

We have investigated the effect of proton irradiation on reliability of InAlN/GaN high electron mobility transistors (HEMTs). Devices were subjected to 5-15 MeV proton irradiations with a fixed dose of 5 × 10¹⁵ cm^-2, or to a different doses of 2 × 10¹¹, 5 × 10¹³ or 2 × 10¹⁵ cm^-2 of protons at a fixed energy of 5 MeV. During off-state electrical stressing, the typical critical voltage for un-irradiated devices was 45 to 55 V. By sharp contrast, no critical voltage was detected for proton irradiated HEMTs up to 100 V, which was instrument-limited. After electrical stressing, no degradation was observed for the drain or gate current-voltage characteristics of the proton-irradiated HEMTs. However, the drain current decreased ~12%, and the reverse bias gate leakage current increased more than two orders of magnitude for un-irradiated HEMTs as a result of electrical stressing.

AlGaN/GaN field effect transistors (FETs) have shown tremendous advances in performance and reliability over recent years. They are unique in that they operate under the presence of a high density of defects, imperfect surfaces and interfaces. We review key challenges related to defects in these transistors, and recent novel characterization techniques and approaches to study the impact of these imperfections on the device thermal characteristics and reliability, as basis for developing devices with an increased safe operating area (SOA). This includes the development of a nanometer resolution junction temperature analysis using SiC solid immersion lenses, results on hot electron effects and on the role of dislocations and point defects for device reliability. In addition techniques such as dynamic transconductance to access traps near the channel are presented. The approaches shown take advantage of the complementary nature of electrical, optical and microstructural device analysis, combined with thermal and electrical device simulations.

High electron mobility transistors (HEMTs) based on AlGaN-GaN hetero-structures are promising for both commercial and military applications that require high voltage, high power, and high efficiency operation. Study of reliability and radiation effects of AlGaN-GaN HEMTs is necessary before solid state power amplifiers based on GaN HEMT technology are successfully deployed in satellite communication systems. Several AlGaN HEMT manufacturers have recently reported encouraging reliability data, but long-term reliability of these devices in the space environment still remains a major concern because a large number of traps and defects are present both in the bulk as well as at the surface leading to undesirable characteristics. This study is to investigate the effects of the AlGaN-GaN HEMTs and AlGaN Schottky diodes irradiated with protons.

Crystal orientation effects on electronic and optical properties of wurtzite (WZ) InGaN/GaN quantum wells (QWs) with piezoelectric (PZ) and spontaneous (SP) polarizations are investigated using the multiband effective-mass theory and non-Markovian optical model. Also, the electron overflow in non-polar InGaN/GaN QW structures with a superlattice (SL)-like electron injector (EI) layer is investigated using a simple model. The effective mass along k'_y of the topmost valence band greatly decreases with increasing crystal angle while the y'-polarized optical matrix element significantly increases with increasing crystal angle. In particular, matrix elements of non-polar (1120)-oriented a-plane QW structure with a relatively higher In composition of 0.4 are about three and half times bigger than those of the (0001)-oriented c-plane QW structure. On the other hand, in the case of the QW structure with a relatively smaller In composition, the difference of matrix elements between the (0001)- and (1120)-oriented QW structures is smaller than that of the QW structure with a relatively higher In composition. With increasing crystal angle, the optical gain peak for the x'-polarization gradually decreases while that for the y'-polarization significantly increases. As a result, the in-plane optical anisotropy increases with increasing crystal angle. The in-plan optical anisotropy of non-polar a-plane QW structure gradually increases with increasing transition wavelength or In composition. The optical anisotropy is ranging from 0.50 at 400nm to 0.80 at 530 nm for the QW structure with L_w = 30 Å. It is found that the electron overflow is found to be greatly reduced by using the SL-like EI laye and rapidly decreases with increasing the number of EI layer. Hence, we expect that the droop phenomenon can be reduced by using the EI layers.

We present recent progress in the growth of nitride based laser diodes (LDs) made by Plasma Assisted Molecular Beam Epitaxy (PAMBE). In this work we demonstrate LDs grown by PAMBE operating in the range 450 – 460 nm. The LDs were grown on c-plane bulk GaN substrates with threading dislocation density (TDD) ranging from 10³ cm^-2 to 10⁴ cm^-2. The low TDD allowed us to fabricate cw LDs with the lifetime exceeding 2000 h at 10 mW of optical output power. The maximum output power for 3 LDs array in cw mode was 280 mW and 1W in pulse mode. The low temperature growth mode in PAMBE allow for growth of AlGaN-free LDs with high In content InGaN waveguides. The key element to achieve lasing wavelengths above 450 nm was the substantial increase of the nitrogen flux available during the growth by PAMBE.

We demonstrate the possibility of fabrication of InGaN laser diode with an extremely thin lower AlGaN cladding (200 nm) by using high electron concentration, plasmonic GaN substrate. The plasmonic substrates were fabricated by one of high-pressure methods – ammonothermal method or multi-feed-seed growth method and have an electron concentration from 5x10¹⁹ cm^-3 up to 1x10²⁰ cm^-3. New plasmonic substrate devices, in spite of their extremely thin lower AlGaN cladding, showed identical properties to these manufactured with traditional, thick lower cladding design. They were characterized by identical threshold current density, slope efficiency and differential gain. Thin AlGaN devices are additionally characterized by low wafer bow and very low density of dislocations (<10⁴ cm^-2).

We develop a monolithic picosecond laser pulse generator, based on the classical design of a group-III-nitride Fabry-Pérot laser diode with electrically separated ridge sections. We use two different multi-section design variants, with the absorber section placed either in the center or at the end of the ridge. Profiting from the very low lateral conductivity in the p-type GaN top contact layer, we implement the multi-section concept just by etching off small sections of the top metalization on the ridge. The physical mechanism underlying short pulse generation within such system, operating in the 400 - 435nm wavelength range, strongly depends both on the reverse bias applied to the absorber and the forward current in the gain section. Varying the applied reverse bias affects both the absorption and the carrier lifetime in the absorber section through changes in the QW internal field. In consequence we can distinguish between different modes of operation. For moderately long carrier lifetimes the absorber stabilizes relaxation oscillations in the GHz frequency range and self-pulsation occurs, of relatively long duration. With increasing reverse bias, and thus decreasing carrier lifetime, we observe a transition to self-Q-switching. Finally, at large enough negative bias, the carrier life time in the absorber is so short that the laser diode operates in a passive self-mode-locking regime with a repetition rate of 87 GHz and pulse duration of 2 ps for a cavity length of 540 μm.

We studied on terahertz-quantum cascade lasers (THz-QCLs) using III-Nitride semiconductors, which are promising materials for the realization of the unexplored frequency range from 5 to 12 THz and the higher temperature operation on THz-QCLs, because these compounds have much larger longitudinal optical phonon energies (> 18 THz) than those of conventional GaAs-based materials (~ 9 THz). Firstly, we showed clearly that it is possible to design a GaN-based quantum cascade (QC) structure which operates in the THz range in which population inversion can be obtained, by performing numerical calculations based on a self-consistent rate equation model. Secondly, we succeeded in the stack of QC structure with a large number of periods and the drastic improvement of structural properties of QC structure, by introducing a new growth technique named "a droplet elimination by thermal annealing (DETA)" in which utilized the differences of the properties between metals (Al, Ga) and Nitrides (AlN, GaN) into molecular beam epitaxy. Finally, we for the first time successfully observed spontaneous electroluminescence due to intersubband transitions with peaks at frequencies from 1.4 to 2.8 THz from GaN/AlGaN QCL devices fabricated with using the DETA technique grown on a GaN substrate and a metal organic chemical vapor deposition (MOCVD)-AlN template on a sapphire substrate. In this paper, we demonstrate recent achievements on the quantum design, fabrication technique, and electroluminescence properties of GaN-based QCL structures.

The latest developments in AlGaInN laser diode technology are reviewed. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v. to the visible, i.e., 380-530nm, by tuning the indium content of the laser GaInN quantum well. Ridge waveguide laser diode structures are fabricated to achieve single mode operation with optical powers of >100mW in the 400-420nm wavelength range with high reliability. Low defectivity and highly uniform TopGaN and Ammono GaN-substrates allow arrays and bars of nitride lasers to be fabricated. In addition, high power operation of AlGaInN laser diodes is demonstrated with the operation of a single chip, ‘mini-array’ consisting of a 3 stripe common p-contact at powers up to 2.5W cw in the 408-412 nm wavelength range and a 16 stripe common p-contact laser array at powers over 4W cw.

Data are presented for AlGaN-AlN multiple-quantum-well optically pumped lasers operating at 300K. The structures were grown by MOCVD on bulk AlN substrates and were fabricated into cleaved bars with a cavity length ~1mm. The epitaxial structures consist of ten 3 nm AlGaN quantum wells with 5 nm AlGaN barriers and an AlN buffer layer deposited on a (0001) AlN substrate at a growth temperature 1155 ºC. The bars were photopumped under pulsed conditions at 300K with a 193nm excimer laser. The threshold optical pump power is 455 kW/cm² and laser emission is observed at 247 nm.

GaN based violet Laser Diode has been applying for the industrial market with unique high potential characters. It has possibility Replacing Gas lasers, Dye Lasers, SHG lasers and Solid-state Lasers and more. Diode based laser extreme small and low costs at the high volume range. In addition GaN Laser has high quality with long lifetime and has possibility to cover the wide wavelength range as between 375 to 520nm. However, in general, diode based laser could only lase with Longitudinal Multi Mode. Therefore applicable application field should be limited and it was difficult to apply for the analysis. Recently, Single Longitudinal Mode laser with GaN diode has also be accomplished with external cavity by Nichia Corporation. External cavity laser achieved at least much higher than 20dB SMSR. The feature of installing laser is that Laser on the front facet with AR coating to avoid chip mode lasing. In general, external cavity laser has been required precision of mechanical assembly and Retention Capability. Nichia has gotten rid of the issue with Intelligence Cavity and YAG Laser welding assembly technique. This laser has also been installed unique feature that the longitudinal mode could be maintained to Single Mode lasing with installing internal functional sensors in the tunable laser.^*1,*2 This tunable laser source could lock a particular wavelength optionally between 390 to 465nm wavelength range. As the results, researcher will have benefit own study and it will be generated new market with the laser in the near future.

We report the enhanced electroluminescence (EL) of GaN light-emitting diodes (LEDs) on glass substrates. We found that GaN morphology affected the EL and achieved enhanced EL of GaN-LEDs on glass by identifying the optimal GaN morphology having both high crystallinity and compatibility for device fabrication. At proper growth temperature, GaN crystallinity was improved with increasing GaN crystal size irrespective of the GaN crystallographic orientation, as determined by spatially resolved cathodoluminescent spectroscopy. The optimized GaN LEDs on glass composed of the nearly single-crystalline GaN pyramid arrays exhibited excellent microscopic EL uniformity and luminance values of ~ 9100 cd/m² at the peak wavelength of 495 nm. The EL color could be adjusted mainly by varying the quantum well temperature. In addition, new growth methods for achieving high GaN crystallinity at a low growth temperature (e.g. ~700°C) were briefly reviewed and attempted by adopting selective heating. We expect that performance of the GaN LEDs on glass can be much enhanced by enhancing GaN crystallinity and p-GaN coating, and evolvement of low-temperature growth of high-quality GaN might even customize ordinary glass as a substrate, which enables high-performance, low-cost lighting or display.

We have performed photoluminescence measurements (PL) for green-yellow InGaN/GaN multiple quantum wells (MQWs) in which InGaN layer was embedded in the barrier as a strain-controlling interlayer to verify the effect on the crystal quality and the internal electric field (F0). From the analysis of both the time-resolved PL and the excitation-power dependence of PL, it has been revealed that the PL intensities and the decay time were enhanced for the MQWs with the InGaN interlayer although the wavelength dependence of the F0 scarcely changes. This indicates that the InGaN interlayer suppresses the degradation of InGaN quantum well rather than the quantum- confinement Stark effect, suggesting that optical properties can be improved by improving the crystal quality through optimizing the local structure as well as the growth conditions. From the light-emitting diodes using the interlayer, we obtained an output power and external quantum efficiency of 10.2 mW and 23.5% at 20 mA with the wavelength of 552 nm.

For high efficiency at high current injection InGaN light emitting diodes (LEDs) necessitate active regions that can mitigate the aggravating electron overflow. Multi double-heterostructures (DHs), 3D active regions separated by low energy barriers, were investigated as optimum solutions for high efficiency as they can accommodate a larger number of states compared to multiple quantum wells (MQWs). However, the number of DH active regions is limited as the material degrades with increasing thickness; therefore, carrier cooling should be partially achieved before the active region using staircase electron injector (SEI) layers. Using electroluminescence (EL) efficiency measurements supported by simulations, active regions and electron injectors were optimized to minimize the electron overflow and the associated efficiency drop at high injection. For a single 3 nm DH LED, the electron overflow was nearly eliminated by increasing the two-step staircase electron injector layer thickness from 4+4 nm to 20+20 nm, whereas the change in SEI thickness had nearly no effect for the DH LEDs with thicker active region. Temperature and excitation density dependent photoluminescence (PL) spectroscopy allowed determination of the material quality and the internal quantum efficiency of device structures with varying active region and SEI thickness.

Fully microscopic many-body models are used to calculate the radiative losses in GaN-based light emitting devices. It is shown how simpler models under-estimate these losses significantly. Using the high accuracy of the models allows to eliminate the corresponding loss parameter (B) and its density- and temperature dependence from the space of parameters that are used to fit efficiency data. This allows to study the dependencies of the remaining processes with high accuracy. Using this model, it is show that many processes that have been proposed as causes for the efficiency droop either have wrong dependencies, magnitudes or require unreasonable assumptions to explain the phenomena in general. The most plausible droop model appears to be a combination of carrier delocalization at very low temperatures and pump powers, density- activated defect-recombination at low to medium pumping and injection/escape losses at strong pumping.

The efficiency of the injection and recombination processes in InGaN/GaN LEDs is governed by the properties of the active region of the devices, which strongly depend on the conditions used for the growth of the epitaxial material. To improve device quality, it is very important to understand how the high temperatures used during the growth process can modify the quality of the epitaxial material. With this paper we present a study of the modifications in the properties of InGaN/GaN LED structures induced by high temperature annealing: thermal stress tests were carried out at 900 °C, in nitrogen atmosphere, on selected samples. The efficiency and the recombination dynamics were evaluated by photoluminescence measurements (both integrated and time-resolved), while the properties of the epitaxial material were studied by Secondary Ion Mass Spectroscopy (SIMS) and Rutherford Backscattering (RBS) channeling measurements. Results indicate that exposure to high temperatures may lead to: (i) a significant increase in the photoluminescence efficiency of the devices; (ii) a decrease in the parasitic emission bands located between 380 nm and 400 nm; (iii) an increase in carrier lifetime, as detected by time-resolved photoluminescence measurements. The increase in device efficiency is tentatively ascribed to an improvement in the crystallographic quality of the samples.

Deep-ultraviolet (DUV) light-emitting diodes (LEDs) have a wide range of potential applications, such as sterilization, water purification, and medicine. In recent years, the external quantum efficiency (EQE) and the performance of AlGaNbased DUV LEDs on sapphire substrates have increased markedly due to improvements in the crystalline-quality of high Al-content AlGaN layers, and the optimization of LED structures. On the other hand, DUV LEDs fabricated on Si substrates are very promising as a low-cost DUV light-source in the near future. However, AlN layers on Si have suffered from cracking induced by the large mismatch in lattice constants and thermal expansion coefficients between AlN and Si. In this paper, DUV LEDs on Si were realized by a combination of a reduction in the number of cracks and of the threading dislocation density (TDD) of AlN templates by using the epitaxial lateral overgrowth (ELO) method. The ELO-AlN templates were successfully coalesced on trench-patterned substrates, with the stripes running along the <1-100> direction of AlN. The density of cracks was greatly reduced in 4- μm-thick ELO-AlN templates, because voids formed by the ELO process relaxed the tensile stress in the AlN layer. Furthermore, the AlN templates showed low-TDD. The full-width-at-half-maximum values of the (0002) and (10-12) X-ray rocking curves were 780 and 980 arcsec, respectively. DUV LEDs fabricated on these high-quality ELO-AlN/Si substrates showed single peak emission at 256- 278 nm in electroluminescence measurements. It is expected that we will be able to realize low-cost DUV LEDs on Si substrates by using ELO-AlN templates.

We present ultraviolet InGaN superluminescent diodes fabricated in a “j-shape” waveguide geometry. Under CW operation at room temperatures, devices emit optical power up to 80 mW at 395 nm with no tendency for lasing. The chip length was 1.5 mm. Emitted optical power was very sensitive to the device temperature. This effect limited the maximum optical power obtained in CW operation. With better packaging scheme better performance in CW regime should be achieved.

The potential of GaN for X-ray detection in the range from 5 to 40 keV has been assessed. The absorption coefficient has been measured as a fonction of photon energy. Various detectors have been fabricated including MSM and Schottky diodes. They were tested under polychromatic X-ray illumination and under monochromatic irradiation from 6 to 22 keV in the Soleil synchrotron facility. The vertical Schottky diodes perform better as their geometry is better suited to the thick layers required by the low absorption coefficient. The operation mode is discussed in terms of photoconductive and photovoltaic behaviors. Some parasitic effects related to the electrical activation of defects by high energy photons and to the tunnel effect in lightly doped Schottky diodes have been evidenced. These effects disappear in diodes where the doping profile has been optimized. The spectral response is found to be very consistent with the spectral absorption coefficient. The sensitivity of GaN Schottky diodes is evaluated and found to be on the order of 40 photons per second. The response is fast nd linear.

We present the determination of the index variation in the GaN/AlN heterostructures related to the population/depletion of the quantum wells fundamental state leading to the intersubband (ISB) absorption variation in the spectral domain around 1.5μm. The experiments were performed using wide-strip waveguide structure. It is shown that the determination of the refraction index in a wide-strip structure is possible when the waveguide is multimode in the vertical direction with a small number of higher order modes. The variation of the refractive index is then deduced from the shift of the position of the beating interference maxima of different order modes. The obtained index variation with bias from complete depletion to full population of the quantum wells is around -5×10^-3. This value is similar to the typical index variation achieved in InP and is an order of magnitude higher than the index variation obtained in silicon. The remarkable feature is that maximum index variation is obtained at the wings of the ISB transition line where absorption is reduced with respect to the peak value. This index variation mechanism opens prospects for the realization of ISB phase modulators by inserting the active region in a Mach-Zehnder interferometer.

Switches are at the heart of all pulsed power and directed energy systems, which find utility in a number of applications. At present, those applications requiring the highest power levels tend to employ spark-gap switches, but these suffer from relatively high delay-times (~10^-8 sec), significant jitter (variation in delay time), and large size. That said, optically-triggered GaN-based photoconductive semiconductor switches (PCSS) offer a suitably small form factor and are a cost-effective, versatile solution in which delay times and jitter can be extremely short. Furthermore, the optical control of the switch means that they are electrically isolated from the environment and from any other system circuitry, making them immune from electrical noise, eliminating the potential for inadvertent switch triggering. Our recent work shows great promise to extend high-voltage GaN-based extrinsic PCSS state-of-the-art performance in terms of subnanosecond response times, low on-resistance, high current carrying capacity and high blocking voltages. We discuss our recent results in this work.

Junction temperature of a laser diode (LD) determines the value of threshold current, maximum achievable power and device lifetime. In this work we studied this parameter by a method of comparing current-voltage characteristics measured under pulse bias (at various temperatures) with DC characteristic obtained at room temperature. As exemplary devices we chose various laser diode arrays and single emitter laser with different substrate thickness. The results show, that the primary factor determining thermal resistance of the device is the chip’s surface, which means, that a dominating mechanism is related with a heat transfer between the chip and the heat sink.

In this contribution, we quantitatively investigate nonradiative recombination due to argon implantation induced point defects in GaInN/GaN quantum wells via time-resolved photoluminescence spectroscopy. A significant reduction of carrier lifetimes in the QW is observed already for implantation doses of 1 × 10¹¹ cm^-2 and higher due to nonradiative recombination at implantation defects. These new nonradiative processes exhibit thermal activation energies below 40 meV, therefore being a dominant loss mechanism at room temperature. The thermal stability of the defects has been analyzed using rapid thermal annealing (RTA) at 800°C and 850°C. We find a partial recovery of the nonradiative lifetimes after RTA indicating an elimination of some defects.

We demonstrate light color conversion in patterned InGaN light-emitting diodes (LEDs), which is enhanced via non-radiative exciton resonant energy transfer (RET) from the electrically driven diode to colloidal semiconductor nanocrystals (NCs). Patterning of the diode is essential for the coupling between a quantum well (QW) and NCs, because the distance between the QW and NCs is a main and very critical factor of RET. Moreover, a proper design of the pattern can enhance light extraction.

We reported the influence of free-standing (FS) GaN substrate on ultraviolet light-emitting-diodes (UV LEDs) by atmospheric-pressure metal-organic chemical vapor deposition (APMOCVD). The Raman spectrum shows the in-plane compressive stress of the GaN epitaxial structures grown on FS GaN substrate. Besides, the Raman spectrum reveals the relation between the crystal quality and the carrier localization degree in multi-quantum wells (MQWs). High resolution X-ray diffraction (HRXRD) analysis results show that the In_0.025Ga_0.975N/Al_0.08Ga_0.92N MQWs grown on FS GaN substrate has higher indium mole fraction than sapphire at the same growth conditions. The higher indium incorporation is corresponding with the red-shift 6 nm (387 nm) of the room temperature photoluminescence (PL) peak. The full widths at half maximum (FWHM) of omega-scan rocking curve in (002) and (102) reflectance on FS GaN substrate (83 arcsec and 77 arcsec) are narrower than UV LEDs grown on sapphire (288 arcsec and 446 arcsec). This superior quality may attribute to homoepitaxial growth structure and better strain relaxation in the FS GaN substrate. An anomalous temperature behavior of PL in UV LEDs designated as an S-shaped peak position dependence and W-shaped linewidth dependence indicate that exciton/carrier motion occurs via photon-assisted tunneling through localized states, what results in incomplete thermalization of localized excitons at low temperature. The Gaussian broadening parameters of carrier localization is about 16.98 meV from the temperature dependent photoluminescence (TDPL) measurement. The saturation temperature from the TDPL linewidth of UV LEDs on FS GaN substrate at about 175 K represents a crossover from a nonthermalized to thermalized energy distribution of excitons.

In this study, a specific design on the electron blocking layer (EBL) by band engineering is investigated numerically with an aim to improve the output performance and to reduce the efficiency droop in green LEDs. Systematic analyses including the energy band diagrams, carrier distributions in the active region, and electron leakage current are given and the simulation results show that the proposed lattice-compensated superlattice-AlGaN/InGaN EBL can provide better optical and electrical output performances when compared to the conventional rectangular AlGaN EBL. The output power of the green LED can be enhanced by a factor of 52% and the applied voltage can be reduced from 5.08 V to 4.53 V at an injection current of 1500 mA. The internal quantum efficiency is improved and the percentage of the efficiency droop can also be reduced from 58% to 37%, which is mainly attributed to the successful suppression of electron leakage current and improvement in hole injection efficiency.

In this work we study the growth mechanisms of InGaN in plasma-assisted molecular beam epitaxy (PAMBE). We investigate growth of InGaN layers on vicinal GaN (0001) substrates. Indium incorporation as a function of gallium and nitrogen fluxes was examined. We propose microscopic model of InGaN growth by PAMBE postulating different indium adatom incorporation mechanisms on two nonequivalent atomic step edges of wurtzite crystal. The different roles of gallium and nitrogen fluxes during the growth of InGaN layers is discussed.

LED structures based on nanowires (NWR) have recently received much attention as a potential way to increase the output power and efficiency of GaN LEDs. We introduce a diffusion-assisted carrier injection scheme for III-Nitride optoelectronic devices, which may open up new current injection methods e.g. for free-standing nanowire emitters (FSNWR) and other structures where the active region is located outside the pn junction and the conventional current path. We simulate the charge transport numerically in selected InGaN/GaN nanowire structures as well as present a simplified analytical model for the current transport. We also discuss the basic characteristics of the bipolar diffusion injection scheme and the factors that make it more sensitive to the dimensions and materials of the current-spreading layers than the conventional LED injection scheme. Our results show that bipolar diffusion enables high efficiency current injection to free-standing nanowires with no top contacts and may also be beneficial to more conventional quantum well structures.

The present study investigated the structural and optical characterizations of the growth of GaN-based green lightemitting diodes using a TiN buffer layer. The purpose of growing GaN-based green LEDs on the TiN interlayer was to produce the naturally occurring hexagonal pattern structure on the surface of undoped-GaN. Then dislocations of the grown InGaN/GaN MQWs green LEDs structure on the uGaN template with the TiN interlayer produced base plane staking faults through epitaxial lateral overgrowth. Cross-section transmission electron microscope images showed that the dislocation density of green LEDs was decreased from 5 × 10⁸ cm^-2 to 7 × 10⁷ cm^-2, and that the dislocations in the green LEDs structure were reproduced. The full widths at half maximum of the omega-scan rocking curves in (002) and (102) reflectance on the GaN-based green LEDs were 334 and 488 arcsec, respectively. As the injection current was increased from 5 mA to 40 mA, the electroluminescence peak wavelength of the GaN-based green LEDs was shifted from 508 nm to 481 nm, a blue-shift of 27 nm. The forward voltage measured at an injection current of 20 mA was 4.9 V for the GaN-based green LEDs according to the current-voltage characteristics. Due to an increase in the In mole fraction of the GaN-based green LEDs on the uGaN template with the TiN interlayer, the strain and phase separation were increased, and the multiple quantum wells structural quality and device performances of the GaN-based green LEDs were decayed. A yellow band with a wavelength of 551nm was thereby produced according to room temperature photoluminescence measurement. Meanwhile, cross-section transmission electron microscope images indicated V-defects in multiple quantum wells structures of the green LEDs.

In this work we compare electronic transport performance in HFETs based on single channel (SC) GaN/Al_0.30GaN/AlN/GaN (2nm/20nm/1nm/3.5μm) and coupled channel (CC) GaN/Al_0.285GaN/AlN/GaN/AlN/GaN (2nm/20nm/1nm/4nm/1nm/3.5μm) structures. The two structures have similar current gain cut-off frequencies (11.6 GHz for SC and 14 GHz for CC for ~ 1μm gate length) however, the maximum drain current, I_Dmax, is nearly doubled in the CC HFET (0.64 A/mm compared to 0.36 A/mm in SC). HFETs exhibit maximum transconductance (Gm_max) at a bias point close to where maximum f _T occurs: V_GS =-2.25 V and V_DS =12 V and V_GS = -2 V and VDS= 15 V for SC and CC HFETs, respectively. Since threshold voltage (V_th) is ~ -3.75 V for both SC and CC structures, devices are able to work at high frequencies with a high g_m delivering higher ID. This is in contrast with device performance reported by others where f _T is attained at V_GS closer to V_th and therefore with lower ID/I_Dmax ratios and low Gm. Results are consistent in that CC HFET delivers higher I_Dmax because of the higher electron mobility (μ) and higher carrier density (n) in the channel. As the saturation drain current, I_Dsat, is attained at electric fields (~40KV/cm) lower than the critical electric field, E_cr , (~ 150KV/cm for GaN ) the higher f _T in CC HFETs can be attributed, mainly, to a higher μ, which is in agreement with the Hall measurements. A higher μ in CC HFET is attributed to a shorter hot phonon lifetime.

Indium gallium nitride (InGaN) semiconductor quantum dots are an attractive candidate for scalable room temperature quantum photonics applications owing to their large exciton binding energy and large oscillation strength. Previously, we reported single photon emission from site-controlled InGaN quantum dot structures. However, large homogeneous linewidth and significant non-radiative recombination were thought to be linked to the nearby surface charge centers. These charge centers can lead to spectral diffusion and excessive non-radiative recombinations at high temperature. In this work, ammonium sulfide passivation was investigated. Nitrogen vacancies were successfully passivated by ammonium sulfide ((NH₄)₂S_x) treatment, and the emission linewidth of a single quantum dot was reduced by 5 meV. Furthermore, the linewidth broadening with an increasing temperature was suppressed in the temperature range from 9 K to 95 K in this study. Satellite emission peak believed to be associated with the nitrogen vacancy was observed for un-passivated quantum dots. The satellite peak was 55 ~ 80 meV away from the main InGaN emission peak and was eliminated after sulfide passivation.

The carrier recombination dynamics in bulk m-plane GaN were investigated by excitation and temperature dependent time-resolved photoluminescence (TRPL) spectroscopy. Polarization-resolved measurements of photoluminescence (PRPL) spectra were performed to evaluate the individual contributions of excitons and free carriers to the radiative recombination. The polarization degree and PL lifetime were strongly correlated and dependent on the populations of free carriers and excitons at different excitation density and temperature levels. The free carrier concentration was found to increase due to the dissociation of excitons at high excitation density and temperatures. The excitonic PL life time was found to be ~ 0.7 ns at 10 K at the lowest excitation density used 0.04 μJ/cm², where no exciton screening was present as confirmed by the 100% polarization degree. The polarization degree obtained at different excitation levels and temperatures by comparing the PL decay times to the excitonic PL lifetime correlated very well with the polarization degree obtained from the excitation dependent PRPL measurements. Finally, it was shown that TRPL and PRPL can be used to separate the excitonic and free carrier contributions to the recombination dynamics in m-plane GaN at any temperature and excitation density.

GaN-based vertical cavity structures containing bottom AlN/GaN DBRs with top dielectric DBRs on freestanding c-GaN and all dielectric DBRs on GaN on c-sapphire were investigated. Epitaxial lateral overgrowth (ELO) technique allowed the use of both top and bottom all dielectric reflector stacks without substrate removal and the fabrication of the active region containing InGaN multiple quantum wells entirely on the nearly defect-free laterally grown wing regions to avoid nonradiative centers caused by extended and point defects. Compared with the cavity containing hybrid-DBRs on freestanding GaN, the cavity with all dielectric DBRs exhibited quality factors up to 1200 at high optical excitation and an order of magnitude lower stimulated emission threshold density (nearly 5 μJ/cm²). Vertical to lateral growth ratio for ELO could be enhanced up to 5 by increasing the V/III ratio and employment of NH3 modulation, which minimizes the use of dry etching to reduce the cavity thickness and therefore is promising for high quality vertical cavities with all dielectric DBRs.

Semipolar (11macron01) GaN layers and GaN/InGaN LED structures were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. Optical properties of the semipolar samples were studied by steady-state and time-resolved photoluminescence (PL). Photon energies and intensities of emission lines from steady-state PL as well as carrier decay times from time-resolved PL were correlated with the distributions of extended defects studied by spatially resolved cathodoluminescence and nearfield scanning optical microscopy. Intensity of donor-bound exciton (DX) emission from both coalesced and non-coalesced semipolar layers is comparable to that of state-of-art c-plane GaN template. To gain insight into the contribution from near surface region and deeper portion of the layers to carrier dynamics in polar c-plane and semipolar (11macron01) GaN, time-resolved PL was measured with two different excitation wavelengths of 267 and 353 nm, which provide different excitation depths of about 50 nm and 100 nm, respectively. Time-resolve PL data indicate that the near-surface layer is relatively free from nonradiative centers (point and/or extended defects), while deeper region of the semipolar film (beyond of ~100 nm in depth) is more defective, giving rise to shorter decay times.

In an effort to investigate the particulars of their stability, In_18.5%Al_81.5%N/GaN HFETs were subjected to on-state electrical stress for intervals totaling up to 20 hours. The current gain cutoff frequency f_T showed a constant increase after each incremental stress, which was consistent with the decreased gate lag and the decreased phase noise. Extraction of small-signal circuit parameters demonstrated that the increase of f_T is due to a decrease in the gate-source capacitance (C_gs) and gate-drain capacitance (C_gd) as well as the increased microwave transconductance (g_m). All these behaviors are consistent with the diminishing of the gate extension (“virtual gate”) around the gate area.

Surface structure of the free-standing GaN substrates with polar (000-1), non-polar (1-100), (11-20), and semipolar (20- 21) surface plane were investigated. Clean polar and non-polar GaN surfaces were prepared by annealing under NH₃ atmosphere. (1x1) diffraction patterns were observed by low-energy electron diffraction (LEED) for both polar and non-polar GaN surfaces. The polar GaN surface was found well-ordered, while the non-polar GaN surfaces were found less ordered with atomic steps on the surface. Polar angle dependences of the photoelecton diffraction (PED) intensities exited by MgKα radiation from N 1s level were analyzed for all the GaN surfaces, aiming to determine the polarities of the GaN surfaces with polar and semipolar crystal orientations.