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Time-resolved photoluminescence measurements have been performed on Mg-doped GaN for Mg concentrations in the low- to mid-1019 cm−3. As-grown and annealed (600–675 °C) samples were studied. In the as-grown samples, the nonradiative carrier lifetime was found to be about 200 ps and nearly independent of the Mg concentration. Upon annealing, the carrier lifetimes shorten to ∼150 ps but, again, show little dependence on the annealing temperature. The analysis of possible Shockley–Read–Hall recombination centers and their behavior during doping and annealing suggests that the main nonradiative recombination center is the Mg–nitrogen vacancy complex. The weak dependence of the PL decay times on temperature indicates that carrier capture into this center has a very low potential barrier, and the nonradiative recombination dominates even at low temperatures. [ABSTRACT FROM AUTHOR]

The efficiency of high-power operation of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on the realization of uniform hole distribution between the QWs. In long wavelength InGaN/GaN QW LEDs, the thermionic interwell hole transport is hindered by high GaN barriers. However, in polar LED structures, these barriers may be circumvented by the lateral hole injection via semipolar 10 1 ¯ 1 QWs that form on the facets of V-defects. The efficiency of such carrier transfer depends on the transport time since transport in the semipolar QWs is competed by recombination. In this work, we study the carrier transfer from the semipolar to polar QWs by time-resolved photoluminescence in long wavelength (green to red) LEDs. We find that the carrier transfer through the semipolar QWs is fast, a few tens of picoseconds with the estimated room temperature ambipolar diffusion coefficient of ∼5.5 cm2/s. With diffusion much faster than recombination, the hole transport from the p-side of the structure to the polar QWs should proceed without a substantial loss, contributing to the high efficiency of long wavelength GaN LEDs. [ABSTRACT FROM AUTHOR]

We report on the measurement of the lateral distribution of the junction current of an electrical biased p-n GaN diode by electron emission microscopy using a low-energy electron microscope. The vacuum level at the surface of the diode was lowered by deposition of cesium to achieve negative electron affinity, allowing overflow electrons at the surface of the biased diodes to be emitted and their spatial distribution imaged. The results were compared to the literature, and a good match with analytical solutions by Joyce and Wemple [J. Appl. Phys. 41, 3818 (1970)] was obtained. [ABSTRACT FROM AUTHOR]

We report long-cavity (60.5 λ) GaN-based vertical-cavity surface-emitting lasers with a topside monolithic GaN concave mirror, a buried tunnel junction current aperture, and a bottomside nanoporous GaN distributed Bragg reflector. Under pulsed operation, a VCSEL with a 9 µm aperture had a threshold current density of 6.6 kA/cm2, a differential efficiency of 0.7%, and a maximum output power of 290 µW for a lasing mode at 411 nm and a divergence angle of 8.4°. Under CW operation, the threshold current density increased to 7.3 kA/cm2, the differential efficiency decreased to 0.4%, and a peak output power of 130 µW was reached at a current density of 23 kA/cm2. [ABSTRACT FROM AUTHOR]

The heterogeneous integration of III-nitride materials with other semiconductor systems for electronic devices is attractive because it combines the excellent electrical properties of the III-nitrides with other device platforms. Pursuing integration through metalorganic chemical vapor deposition (MOCVD) is desirable because of the scalability of the technique, but the high temperatures required for the MOCVD growth of III-nitrides (>1000 °C) are incompatible with direct heteroepitaxy on some semiconductor systems and fabricated wafers. Thus, the MOCVD growth temperature of III-nitride films must be lowered to combine them with other systems. In this work, 16 nm-thick Si:GaN films were grown by MOCVD at 550 °C using a flow modulation epitaxy scheme. By optimizing the disilane flow conditions, electron concentrations up to 5.9 × 1019 cm−3 were achieved, resulting in sheet resistances as low as 1070 Ω/□. Film mobilities ranged from 34 to 119 cm2 V−1 s−1. These results are promising for III-nitride integration and expand device design and process options for III-nitride-based electronic devices. [ABSTRACT FROM AUTHOR]

The electrical performances of III-nitride blue micro-light-emitting diodes (µLEDs) with different tunnel junction (TJ) epitaxial architectures grown by metalorganic chemical vapor deposition are investigated. A new TJ structure that employs AlGaN is introduced. The current density–voltage characteristic is improved by incorporating AlGaN layer above the n-side of the TJ layer, and the effects of the AlGaN/GaN superlattices is examined. Based upon the data from band diagram simulation, net positive polarization charge is formed at the AlGaN/GaN interface, which results in a reduction in tunneling distance and increase in tunneling probability. Moreover, similar electrical improvement is observed in various device dimensions and is independent of operating current density, suggesting that AlGaN/GaN biaxial tensile strain or current spreading is not the main contribution for the improvement. Finally, the effects on the efficiency performances are determined. While the maximum external quantum efficiency of the TJ devices remains identical, the wall-plug efficiency of µLEDs is enhanced significantly by the proposed AlGaN-enhanced TJ design. This work reveals the possibility of energy efficient TJ contact with high transparency in the visible wavelength range. [ABSTRACT FROM AUTHOR]

Nitride based light emitting diodes (LEDs) have demonstrated very high efficiency in the blue and green wavelength ranges. For full color LED based displays, efficient and small sized devices emitting in the three primary colors - red, green and blue are required. While phosphide based red LEDs are highly efficient, high surface recombination velocities limit their scaling to small dimensions. Nitride emitters can be scaled to smaller sizes, however for longer emission wavelength such as yellow, orange, and red, the indium content in the active region of the devices needs to be >30%. Due to the large lattice mismatch between GaN and InN, the needed high indium content introduces a large strain in the structure, often resulting in poor material quality. Recently, a novel technique to mitigate this lattice mismatch induced strain was demonstrated with the use of compliant porous GaN underlayer based substrates. In our previous reports, we have discussed the surface morphology, strain state evolution and device demonstrations using these substrates, however a detailed discussion regarding the pore cross-sections and dislocation propagation for each generation of these porous GaN substrates was yet to be reported. In this work, we will discuss the cross-sectional transmission electron microscopy (TEM) results of these substrates across multiple stages of the substrate technology development. [ABSTRACT FROM AUTHOR]

The optical and electrical characteristics of InGaN blue and green micro-light-emitting diodes (μLEDs) with GaN tunnel junction (TJ) contacts grown by metalorganic chemical vapor deposition (MOCVD) were compared at different activation temperatures among three activation methods from the literature, namely, sidewall activation, selective area growth (SAG), and chemical treatment before sidewall activation. The devices with chemical treatment before activation resulted in uniform electroluminescence and higher light output power, compared to the devices with sidewall activation and SAG. Moreover, the green μLEDs showed greater optical degradation at elevated activation temperatures, whereas the blue μLEDs yielded trivial difference with activation temperatures from 670 to 790 °C. The 5 × 5 μm2 devices with chemical treatment before activation and SAG yielded almost identical voltage at 20 A/cm2, and the voltage penalty significantly decreased with activation temperature in the case of devices with sidewall activation. The devices with chemical treatment before activation resulted in higher external quantum efficiency (EQE) and wall-plug efficiency (WPE) in low current density range compared to the devices with SAG. The enhancements in EQE and WPE were observed in different μLED sizes, suggesting that chemical treatment before sidewall activation enables the use of TJ contacts grown by MOCVD and is advantageous for applications that require high brightness and efficiency. [ABSTRACT FROM AUTHOR]

Investigations of the use of patterned porous GaN underlayers to achieve elastic relaxation of lattice‐mismatched top layers such as InGaN and AlGaN are reviewed. Thereby, the degree of relaxation of the top layers increases with the porosity and associated decrease in mechanical hardness of the porous GaN underlayers as well as with decreasing pattern size and increase in thickness of the lattice‐mismatched top layers, following the trends observed for nanostructures. Micrometer‐sized, high‐fill‐factor GaN‐on‐porous‐GaN tile arrays are used as universal substrate for the deposition of InGaN and AlGaN. Due to the elastic nature of the relaxation process, the threading dislocation density in the epitaxial material grown on top of the GaN‐on‐porous‐GaN tiles is equivalent to that in the GaN base material. InGaN and AlGaN layers grown on top of relaxed or partially relaxed InGaN or AlGaN underlayers by metal–organic chemical vapor deposition display a higher indium or aluminum content compared to their counterparts deposited on coloaded planar GaN reference wafers due to the decreased lattice mismatch. First applications of the patterned porous GaN‐based technology for optoelectronic and electronic devices are presented and serve as a pathway toward future implementations. [ABSTRACT FROM AUTHOR]

A highly relaxed InGaN buffer layer was demonstrated over a full two-inch c-plane sapphire substrate by metalorganic chemical vapor deposition. The InGaN buffer layer was grown on a 100 nm GaN decomposition stop layer with a 3 nm thick high indium composition InGaN underlayer. After thermal decomposition of the underlayer at 1000 °C, a 200 nm thick In0.04Ga0.96N buffer showed 85% biaxial relaxation measured by a high resolution x-ray diffraction reciprocal space map. When used as a pseudo-substrate for the regrowth of InGaN/InGaN multi-quantum wells, the sample showed a 75 nm red-shift in room temperature photoluminescence when compared to a co-loaded GaN template reference. The longer emission wavelength is associated with higher indium incorporation in the InGaN layers from the lessening of the compositional pulling effect caused by compressive strain. Using this technique, a simple red light emitting diode was demonstrated with an active layer growth temperature of 825 °C and a peak wavelength of 622 nm at a current density of 20 A cm−2. This work represents a unique method to relax a III-nitride based layer over a full substrate. [ABSTRACT FROM AUTHOR]

Purpose: This study aims to focus on roughening N-face (backside) GaN substrate prior to GaN-on-GaN light-emitting diode (LED) growth as an attempt to improve the LED performance. Design/methodology/approach: The N-face of GaN substrate was roughened by three different etchants; ammonium hydroxide (NH4OH), a mixture of NH4OH and H2O2 (NH4OH: H2O2) and potassium hydroxide (KOH). Hexagonal pyramids were successfully formed on the surface when the substrate was subjected to the etching in all cases. Findings: Under 30 min of etching, the highest density of pyramids was obtained by NH4OH: H2O2 etching, which was 5 × 109 cm–2. The density by KOH and NH4OH etchings was 3.6 × 109 and 5 × 108 cm–2, respectively. At standard operation of current density at 20 A/cm2, the optical power and external quantum efficiency of the LED on the roughened GaN substrate by NH4OH: H2O2 were 12.3 mW and 22%, respectively, which are higher than its counterparts. Originality/value: This study demonstrated NH4OH: H2O2 is a new etchant for roughening the N-face GaN substrate. The results showed that such etchant increased the density of the pyramids on the N-face GaN substrate, which subsequently resulted in higher optical power and external quantum efficiency to the LED as compared to KOH and NH4OH. [ABSTRACT FROM AUTHOR]

Integration of nitrides with other material systems has recently become of interest due to the high performance of GaN-based high-electron mobility transistors. However, the elevated growth temperatures often used to grow high quality AlN pose challenges toward metalorganic chemical vapor deposition (MOCVD) on temperature sensitive substrates such as processed wafers. In this work, the growth of AlN was conducted at temperatures below 550 °C via MOCVD using a flow-modulated epitaxy scheme, and their morphological, compositional, and electronic properties of these films were investigated. Sheet charges up to 2.1 × 1013 cm−2 and mobilities on the order of 400 cm2/V s were measured for two dimensional electron gases, which formed at the interface between the low temperature grown AlN layers and the semi-insulating GaN base layers deposited at high temperatures. Despite their low growth temperatures, nominally pure AlN barrier layers exhibited measurable unintentional gallium incorporation adjacent to the GaN interface. The result sets the stage for the integration of nitride-based electronics via epitaxy-based schemes on temperature sensitive substrates. [ABSTRACT FROM AUTHOR]

Crack-free 1.3 μm thick elastically relaxed AlxGa1−xN layers were demonstrated on compliant high fill-factor 10 × 10 μm2 tile patterned GaN-on-porous-GaN pseudo-substrates (PSs). Porous GaN was utilized as a semi-flexible underlayer. The AlxGa1−xN layer was grown in steps of 200, 300, 400, and 400 nm. While the AlxGa1−xN layer regrown on a co-loaded GaN-on-sapphire template cracked after deposition of only 200 nm AlGaN, on the GaN-on-porous GaN patterned substrates, no cracks were observed, and smooth films were achieved. In addition, an enhanced aluminum uptake was observed as the AlGaN growth progressed on the GaN-on-porous-GaN PS, owing to the composition pulling effect, until the point when the tiles started to coalesce. Upon further regrowth on these coalesced tiles, the aluminum uptake saturated, while the surface remained crack-free and smooth. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of III-nitride materials are of interest due to their potential use for high temperature power generation applications and the increasing commercial importance of the material system; however, the very large parameter space of different alloy compositions, carrier densities, and range of operating temperatures makes a complete experimental exploration of this material system difficult. In order to predict thermoelectric performances and identify the most promising compositions and carrier densities, the thermoelectric properties of InxGa1-xN, InxAl1-xN, and AlxGa1-xN are modeled. The Boltzmann transport equation is used to calculate the Seebeck coefficient, electrical conductivity, and the electron component of thermal conductivity. Scattering mechanisms considered for electronic properties include ionized impurity, alloy potential, polar optical phonon, deformation potential, piezoelectric, and charged dislocation scattering. The Callaway model is used to calculate the phonon component of thermal conductivity with Normal, Umklapp, mass defect, and dislocation scattering mechanisms included. Thermal and electrical results are combined to calculate ZT values. InxGa1-xN is identified as the most promising of the three ternary alloys investigated, with a calculated ZT of 0.85 at 1200 K for In0.1Ga0.9N at an optimized carrier density. AlxGa1-xN is predicted to have a ZT of 0.57 at 1200 K under optimized composition and carrier density. InxAl1-xN is predicted to have a ZT of 0.33 at 1200 K at optimized composition and carrier density. Calculated Seebeck coefficients, electrical conductivities, thermal conductivities, and ZTs are compared with experimental data where such data are available. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) has drawn significant attention in recent years. Though high-speed visible-light sources have seen significant advances, commercially available photodetectors have low wavelength selectivity and modulation bandwidth in the near-violet-blue wavelengths, making them a bottleneck in VLC links. Here we show a record 7.4-Gbit/s visible-light communication link using a wavelength-selective, ($\mathbf {20}\overline {21}$)-oriented, semipolar InGaN/GaN multiple-quantum-well micro-photodetector ($\mu $ PD) on GaN substrate. This is achieved by leveraging on the unique photodetection properties of semipolar $\mu $ PDs, combined with an optimized communication system utilizing bit- and power-loading schemes in orthogonal frequency-division multiplexing (OFDM) modulation over a 2-GHz bandwidth. We used a 405-nm violet laser diode transmitter as the responsivity of the $\mu $ PD was highest within the responsivity range of 360 - 420 nm. The investigation fully demonstrated the feasibility and favorable choice of semipolar InGaN/GaN $\mu $ PDs for multi-Gbit/s optical wireless communication. [ABSTRACT FROM AUTHOR]

The effects of carrier concentration, composition, and temperature on the thermoelectric properties of high quality n-type InGaN grown by metal organic chemical vapor deposition (MOCVD) were systematically investigated. The Seebeck coefficient was found to decrease and electrical conductivity increase with increasing carrier concentration, while both were found to decrease with increasing indium composition. Additionally, thermal conductivity was found to decrease by over an order of magnitude as indium composition was increased from 0 to 19%. These trends resulted in optimum carrier concentration and indium composition of 1.1×1019 cm-3 and 17%, respectively, with a room temperature ZT of 0.04. Increasing temperature resulted in a rapidly increasing ZT, reaching a maximum value of 0.34 at 875 K. This significantly improved ZT demonstrates the potential of InGaN and other III-Nitride materials for high temperature thermoelectric applications. [ABSTRACT FROM AUTHOR]

This article presents a theoretical analysis of dislocation behavior and stress relaxation in semipolar III-nitride heteroepitaxy, e.g., for AlxGa1-xN and InyGa1-yN layers grown on {hh2hm}- or {h0hm}-type semipolar planes of GaN substrates. We demonstrate that the shear stresses on the unique inclined basal (0001) plane do not vanish for such growth geometries. This leads to the onset of relaxation processes in semipolar III-nitride heterostructures via dislocation glide in the basal slip systems <1120>(0001) and to the formation of misfit dislocations (MDs) with Burgers vectors of (a/3)<1120>-type at the semipolar heterointerface. Next we calculate the Matthews-Blakeslee critical thickness for MD formation in semipolar III-nitride layers together with the MD equilibrium spacings for complete misfit relaxation. The component of the MD Burgers vector normal to the film/substrate interface will cause a crystal lattice tilt in the epilayer with respect to the GaN substrate. The calculated magnitudes of the tilt angles are 0.62° and 0.67° for AlxGa1-xN and InyGa1-yN alloys with compositions of x = 0.20 and y = 0.07, respectively, grown in the (1122) semipolar orientation. The modeling results are discussed in light of recent experimental observations [A. Tyagi et al., Appl Phys. Lett. 95, 251905 (2009); E. Young et al., Appl. Phys. Express 3, 011004 (2010); and F. Wu et al., J. Appl. Phys. 109, 033505 (2011)] of MDs and crystal lattice tilt in semipolar III-nitride heteroepitaxial layers. [ABSTRACT FROM AUTHOR]

The realization of the first high-brightness blue-light-emitting diodes (LEDs) in 1993 sparked a more than twenty-year period of intensive research to improve their efficiency. Solutions to critical challenges related to material quality, light extraction, and internal quantum efficiency have now enabled highly efficient blue LEDs that are used to generate white light in solid-state lighting systems that surpass the efficiency of conventional incandescent lighting by 15–20×. Here we discuss the initial invention of blue LEDs, historical developments that led to their current state-of-the-art performance, and potential future directions for blue LEDs and solid-state lighting. [ABSTRACT FROM AUTHOR]

The finite element method was used to determine the thermal resistance of a flip-chip-bonded GaN-based vertical-cavity surface-emitting laser (VCSEL) with substrate removal and dielectric distributed Bragg reflectors (DBRs). In this work, we investigate the effects of the DBR configuration, mask layer alignment tolerances, aperture diameters, and cladding layer thicknesses on the thermal properties of the flip-chip design. The current flip-chip design suffers from high thermal resistance (∼4600 K W−1). By reducing the mask layer alignment tolerances and increasing the cladding thickness, the thermal resistance values could be made comparable to devices reported in the literature that achieved CW operation at room temperature (∼3000 K W−1). Low thermal resistance designs are critical to achieve CW operation and mitigate the effects of thermal roll-over in flip-chip GaN-based VCSELs. [ABSTRACT FROM AUTHOR]

The superior low-efficiency droop performance of semipolar (20\bar2\bar1) InGaN light-emitting diodes (LEDs) makes it a hot candidate for efficient solid-state lighting and full-color displays. To unveil the mystery of this low droop and high efficiency, the emission dynamics of semipolar (20\bar2\bar1) LEDs is investigated by time-resolved and steady-state photoluminescence (PL) measurements. Much smaller carrier lifetimes (radiative and nonradiative lifetime) were obtained from semipolar (20\bar2\bar1) InGaN QWs compared with those on the c-plane samples, possibly due to the reduced quantum-confined Stark effects and smaller indium fluctuation on semipolar InGaN samples. The experimental findings indicate a much reduced excess carrier density in semipolar (20\bar2\bar1) InGaN LEDs, which will impact the device performance. Based on this, a modified ABC equation with weak phase-space-filling (PSF) effect was used to model the droop characteristics of semipolar (20\bar2\bar1) LEDs. [ABSTRACT FROM PUBLISHER]

Continuous-wave blue semipolar(2021) III-nitride laser diodes were fabricated with highly vertical, smooth, and uniform mirror facets produced by chemically assisted ion beam etching. Uniform mirror facets are a requirement for accurate experimental determination of internal laser parameters, including internal loss and injection efficiency, which were determined to be 9 cm-1 and 73%, respectively, using the cavity length dependent method. The cavity length of the uncoated devices was varied from 900 µm to 1800 µm, with threshold current densities ranging from 3 kA/cm² to 9 kA/cm² and threshold voltages ranging from 5.5V to 7 V. The experimentally determined internal loss was found to be in good agreement with a calculated value of 9.5 cm-1 using a 1D mode solver. The loss in each layer was calculated and in light of the analysis several modifications to the laser design are proposed. [ABSTRACT FROM AUTHOR]

We investigate thermoelectric pumping in wide-bandgap GaN based light-emitting diodes (LEDs) to take advantage of high junction temperature rather than avoiding the problem of temperatureinduced efficiency droop through external cooling. We experimentally demonstrate a thermally enhanced 450 nm GaN LED, in which nearly fourfold light output power is achieved at 615K (compared to 295K room temperature operation), with nearly no reduction in the wall-plug efficiency (i.e., electrical-optical energy conversion efficiency) at bias V < hw=q. The LED is shown to work in a mode similar to a thermodynamic heat engine operating with charged carriers pumped into the active region by a combination of electrical work and Peltier heat (phonons) drawn from the lattice. In this optimal operating regime at 615 K, the LED injection current (3.26 A/cm²) is of similar magnitude to the operating point of common high power GaN based LEDs (5-35 A/cm²). This result suggests the possibility of removing bulky heat sinks in current high power LED products thus realizing a significant cost reduction for solid-state lighting. [ABSTRACT FROM AUTHOR]

The author discusses the story behind the invention of blue light-emitting diode .Topics include the progress made in combining white LED to blue LED using yellow phosphor, the material properties of indium gallium nitride as well as metal organic chemical vapor deposition as candidates for the development of blue LED and the future challenges of blue LED in the field of electronic. The author Shuji Nakamura, a Japanese physicist and the inventor of blue LED presents his autobiography.

Shuji Nakamura discovered p-type doping in Gallium Nitride (GaN) and developed blue, green, and white 'InGaN-based' light emitting diodes (LEDs) and blue laser diodes (LDs). His inventions made possible energy efficient, solid-state lighting systems and enabled the next generation of optical storage. In this biography, Shuji Nakamura tells the story of his personal life and his scientific career. [ABSTRACT FROM AUTHOR]

Devices grown on nonpolar and semipolar planes of GaN offer key performance advantages over devices grown on the conventional c-plane, including reduced polarization fields. This allows for a wider design space on semipolar planes for light emitting diodes (LEDs) to address the problem of efficiency droop at high current densities. LED structures with very thick (10-100 nm) InGaN single-quantum-well/double heterostructure active regions were grown using conventional metal organic chemical vapor deposition on semipolar (3031) free-standing GaN substrates and processed and packaged using conventional techniques. Simulated band diagrams showed reduced polarization fields on the (3031) plane. The calculated critical thickness for misfit dislocation formation is higher on the (3031) plane than on other semipolar planes, such as (2021), allowing for thicker active regions than our previous work to further reduce droop. The higher critical thickness was confirmed with defect characterization via cathodoluminescence. A trend is demonstrated in lower efficiency droop for devices with thicker active regions. Thermal droop characteristics of these devices are also presented. These observed results were utilized to demonstrate over 1W of output power at a current density of 1 kA/cm² from a single 0.1mm² LED device. [ABSTRACT FROM AUTHOR]

Dark triangle defects (DTDs) are common nonradiative defects in semipolar (202̅1) oriented green quantum wells (QWs), commonly used in green laser diodes (LDs). We show that DTDs do not appear “as-grown,” and DTD size depends strongly on post-QW-growth annealing time and temperature. Using low temperature p-GaN, we prevent catastrophic QW damage and directly compare LDs with GaN and AlGaN containing barriers. The GaN barrier LD exhibited a lasing wavelength of 511 nm, reduced operating voltage, and the lowest threshold current density, likely due to enhanced optical confinement factor and the elimination of very low growth temperature AlGaN in the active region. [ABSTRACT FROM PUBLISHER]

The history of development for gallium–nitride-based light-emitting diodes (LEDs) is reviewed. We identify two broad developments in GaN-based LED technology: first, the key breakthroughs that enabled the development of GaN-based devices on foreign substrates like sapphire (first-generation LEDs), and, second, a new wave of devices benefiting from developments in GaN substrate manufacturing, which has led to native bulk-GaN-based LEDs with unprecedented performance characteristics that portend a disruptive shift in LED output power density and the corresponding cost of generating light. [ABSTRACT FROM PUBLISHER]

Edge-emitting laser diodes (LDs) were fabricated on a reduced dislocation density epitaxial lateral overgrown (ELO) wing of a semipolar { 20 2 ¯ 1 } GaN substrate, termed an ELO wing LD. Two types of facet feasibility studies were conducted: (1) "handmade" facets, wherein lifted-off ELO wing LDs were cleaved manually, and (2) facets formed on wafers through reactive ion etching (RIE). Pulsed operation electrical and optical measurements confirmed the laser action in the RIE facet LDs with a threshold current of ~19 kAcm−2 and maximum light output power of 20 mW from a single uncoated facet. Handmade facet devices showed spontaneous, LED-like emission, confirming device layers remain intact after mechanical liftoff. [ABSTRACT FROM AUTHOR]

The integration of different electronic materials systems together has gained increasing interest in recent years, with the III-nitrides being a favorable choice for a variety of electronic applications. To increase flexibility in integration options, growing nitrides material directly on semi-processed wafers would be advantageous, necessitating low temperature (LT) growth schemes. In this work, the growth of AlN and GaN was conducted via metalorganic chemical vapor deposition (MOCVD) using both NH3 and DMHy as N-precursors. The relationships between growth rate versus temperature were determined within the range of 300 to 550 °C. The growth of AlN/GaN heterostructures was also investigated herein, employing flow modulation epitaxy MOCVD at 550 °C. Subsequent samples were studied via atomic force microscopy, X-ray diffraction, TEM, and Hall measurements. Two-dimensional electron gases were found in samples where the LT AlN layer was grown with NH3, with one sample showing high electron mobility and sheet charge of 540 cm2/V∙s and 3.76 × 1013 cm−2, respectively. Inserting a LT GaN layer under the LT AlN layer caused the mobility and charge to marginally decrease while still maintaining sufficiently high values. This sets the groundwork towards use of LT nitrides MOCVD in future electronic devices integrating III-nitrides with other materials. [ABSTRACT FROM AUTHOR]

This work examines the effects of polarization-related electric fields on the energy band diagrams, wavelength shift, wave function overlap, and efficiency droop for InGaN quantum wells on various crystal orientations, including polar (0001) (c-plane), semipolar (\20\bar\2\1), semipolar (\20\bar\2\bar\1), and nonpolar (\10\bar\1\0) (m-plane). Based on simulations, we show that the semipolar (\20\bar\2\bar\1) orientation exhibits excellent potential for the development of high-efficiency, low-droop light-emitting diodes (LEDs). We then present recent advancements in crystal growth, optical performance, and thermal performance of semipolar (\20\bar\2\bar\1) LEDs. Finally, we demonstrate a low-droop, high-efficiency single-quantum-well blue semipolar (\20\bar\2\bar\1) LED with an external quantum efficiency of more than 50% at 100 \A/cm^2. [ABSTRACT FROM PUBLISHER]

Light-emitting diodes (LEDs) were fabricated in gallium nitride with two embedded photonic crystals (PhCs), creating a waveguide with a highly confined, strongly excited, and well-extracted fundamental mode. This structure improves upon previous PhC LED designs by reducing the extraction length of the fundamental mode and establishing a path to designs with very low absorption losses. Optical output was measured with angular-resolved electroluminescence. The extraction length of the fundamental mode was measured to be 21-39 μm along the PhCs' Γ-M directions, which is much shorter than values reported for single-PhC devices. This structure opens the way to more efficient LEDs and lasers. [ABSTRACT FROM AUTHOR]

It has been several years since InGaN/GaN light-emitting diodes (LEDs) on nonpolar and semipolar orientations were first demonstrated. Prominent performance and inherent potential of these crystallographic orientations have been revealed as bulk-GaN substrates of arbitrary orientations became available for epitaxial device growth. At this point in time, intend to survey the progress made to date and prospect the future requirements for further device improvements. The discussion begins with a historical background: how nonpolar/semipolar orientations were introduced to III-nitride LEDs and why they are beneficial. The discussion then provides information on elementary crystallography and piezoelectricity in addition to the electronic band structure of wurtzite crystals. Later in this paper, LED reports are collected to develop comprehensive knowledge of the past research efforts and trends. Nonpolar and semipolar orientations provide not only high LED performances, e.g., optical output power and wavelength ranges, but also unique functions, e.g., polarized light emission, which will explore new fields of applications. [ABSTRACT FROM AUTHOR]

The realization of high-performance optoelectronic devices, based on GaN and other nitride semiconductors, requires the existence of a high-quality substrate. Non-polar or semipolar substrates have recently been proven to provide superior optical devices to those on conventional c-plane substrates. Bulk GaN growth enables GaN substrates sliced along various favourable crystal orientations. Ammonothermal growth is an attractive method for bulk GaN growth owing to its potential to grow GaN ingots at low cost. Here we report on improvement in the structural quality of GaN grown by the ammonothermal method. The threading dislocation densities estimated by plan-view transmission electron microscopy observations were less than 1×106 cm−2 for the Ga face and 1×107 cm−2 for the N face. No dislocation generation at the interface was observed on the Ga face, although a few defects were generated at the interface on the N face. The improvement in the structural quality, together with the previous report on growth rate and scalability, demonstrates the commercial feasibility of the ammonothermal GaN growth. [ABSTRACT FROM AUTHOR]

Microdisk lasers feature low-loss, high-quality whispering gallery modes that offer the potential for ultralow-threshold lasing that is not limited by challenges in mirror fabrication. Here we report fabrication and optical measurements of GaN-based microdisk lasers with a very low threshold of 300 W cm−2—orders of magnitude lower than any previous GaN microdisk laser. This is also the first report of room-temperature, continuous-wave (CW) lasing in a GaN microdisk. [ABSTRACT FROM AUTHOR]

Nonpolar (1100) m-plane gallium nitride has been grown heteroepitaxially on (100) γ-LiAlO2 by several groups. Previous attempts to grow m-plane GaN by hydride vapor phase epitaxy (HVPE) yielded films unsuitable for subsequent device regrowth because of the high densities of faceted voids intersecting the films' free surfaces. We report here on the growth of planar m-plane GaN films on (100) γ-LiAlO2 and elimination of bulk and surface defects. The morphology achieved is smooth enough to allow for fabrication of m-plane GaN templates and free-standing substrates for nonpolar device regrowth. The GaN films were grown in a horizontal HVPE reactor at 860-890°C. Growth rates ranged from 30 µm/h to 240 µm/h, yielding free-standing films up to 250-µm thickness. The m-plane GaN films were optically specular and mirror-like, with undulations having 50-200-nm peak-to-valley heights over millimeter length scales. Atomic force microscopy revealed a striated surface morphology, similar to that observed in m-plane GaN films grown by molecular beam epitaxy (MBE). Root-mean-square (RMS) roughness was 0.636 nm over 25-µm² areas. Transmission electron microscopy (TEM) was performed on the m-plane GaN films to quantify microstructural defect densities. Basal-plane stacking faults of 1 x 105 cm-1 were observed, while 4 x 109 cm-2 threading dislocations were observed in the g = 0002 diffraction condition. [ABSTRACT FROM AUTHOR]

By means of time-resolved emission spectroscopy, the emission dynamics of InGaN quantum well light-emitting devices is evaluated. The localization, radiation, and nonradiating recombination processes of the exciton are evaluated. It is found that there exist random potential fluctuations in the quantum well in samples with less than about 10% In in the InGaN activation layer. By means of these fluctuations, localization of the excitons can be attained. It is also found that the degree of localization increases as the In composition is increased, and that quantum dot-like regions are naturally formed in samples with an In content of more than about 20%. It is found that the capture of excitons by nonradiating recombination centers is suppressed and that a high-emission quantum efficiency is realized by such a pronounced localization. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(7): 45–56, 1998 [ABSTRACT FROM AUTHOR]

We compare facet morphology, device characteristics, and far field patterns (FFPs) for semipolar (11\bar22) laser diodes fabricated with mechanically polished and dry etched mirror facets. Facets formed by Cl2-based dry etching produced inclined and heavily striated facets. Mechanically polished facets, in contrast, provided vertical and smooth facets (rms~roughness=5.2~nm). The threshold currents of polished facet devices were on average \sim100 and \sim200~mA lower than etched facet devices (2\,\times\,1200 \mum^2 and 4\,\times\,1200 \mum^2 dimension devices, respectively). FFPs from etched facets were also shown to be obscured due to substrate reflections. [ABSTRACT FROM PUBLISHER]

We demonstrate GaN-based semipolar ([formula]) laser diodes (LD) in the green region of the spectrum. 505 nm lasing was observed under pulsed operation, with a threshold current density (Jth) of 27.5 kA/cm2 and a threshold voltage (Vth) of 15.5 V. The blueshift of spontaneous emission was less than 5 nm when varying the injection current level from 125 A/cm2 to threshold. Simulations show that the reduced blueshift can be attributed to the minimal dependence of the energy potential profile in the active region on the applied bias and current injection level. [ABSTRACT FROM AUTHOR]

The polarization-preserving property of photonic crystals (PhCs) is combined to the superior extraction efficiency of embedded PhCs to enhance polarized light emission in m-plane GaN light-emitting diodes. As guided modes in m-plane GaN are mostly polarized along the a-direction (Evector ∥avector ), their efficient extraction is achieved by one-dimensional embedded PhCs also aligned to the a-direction. A better diffraction efficiency is obtained by air-gap PhCs which required the growth of high quality, defect-free, m-plane GaN coalesced over the embedded air-grooves. [ABSTRACT FROM AUTHOR]

High internal and external quantum efficiency GaN/InGaN solar cells are demonstrated. The internal quantum efficiency was assessed through the combination of absorption and external quantum efficiency measurements. The measured internal quantum efficiency, as high as 97%, revealed an efficient conversion of absorbed photons into electrons and holes and an efficient transport of these carriers outside the device. Improved light incoupling into the solar cells was achieved by texturing the surface. A peak external quantum efficiency of 72%, a fill factor of 79%, a short-circuit current density of 1.06 mA/cm2, and an open circuit voltage of 1.89 V were achieved under 1 sun air-mass 1.5 global spectrum illumination conditions. [ABSTRACT FROM AUTHOR]

The polarization of spontaneous emission was investigated for various indium compositions and quantum wells on m-plane oriented gallium nitride (GaN) light emitting diodes (LEDs) grown on bulk-GaN substrates. Internal light scattering and depolarization was mitigated with application of absorber materials to the LED die. The polarization ratio (ρ) was measured under electrical injection for devices with InGaN active regions emitting up to 520 nm and observed as high as 96%. Values of ρ were independent of drive current. The valence band energy separation (ΔE) was characterized using spectral measurement and temperature dependent optical analysis of valence band hole distributions. [ABSTRACT FROM AUTHOR]

Sidewall lateral epitaxial overgrowth (SLEO) is demonstrated by metal organic chemical vapor deposition for nonpolar [formula] m-plane GaN films. m-plane GaN films were grown by metal organic chemical vapor deposition on m-plane 6H SiC substrates with an AlN initiation layer. Subsequently, an SiO2 stripe dielectric pattern was formed with 2 μm window openings parallel to the [1120] a-direction and an 8 μm mask and then the m-plane GaN was etched through the window openings to reveal Ga-face (0001) and N-face (0001) sidewalls. The SLEO growth was achieved in two growth steps—lateral growth from the sidewalls and subsequent growth through and then over the dielectric mask openings. In comparison to planar m-plane GaN films grown on 6H SiC, the threading dislocation density was reduced from low 1010 to 3×108 cm-2 and the stacking fault density was reduced by one order of magnitude. [ABSTRACT FROM AUTHOR]

We introduce GaN/InGaN light emitting diodes with a dielectric photonic crystal embedded in the epitaxial layer by lateral epitaxial overgrowth on a patterned GaN template. Overgrowth, coalescence, and epitaxial growth of the pn junction within a thickness of 500 nm is obtained using metal-organic chemical vapor deposition. This design strongly modifies the distribution of guided modes, as confirmed by angle-resolved measurements. The regime of operation and potential efficiency of such structures are discussed. [ABSTRACT FROM AUTHOR]

We report on the realization of highly efficient InGaN microcavity light-emitting diodes incorporating a high index contrast air-gap distributed Bragg reflector (DBR). Detailed analysis deduces an effective cavity length of ∼500 nm and cavity mode orders of 5 and 6 for measured Fabry-Pérot fringes. A value reflectivity of ∼70% was determined for the 4.5 period air/Al0.08Ga0.92N DBR through the analysis of cavity finesse based on the angle-resolved photoluminescence (PL) data. A fivefold improvement in light extraction efficiency was verified by electrical probing as well as angle-resolved PL measurements. [ABSTRACT FROM AUTHOR]

Direct photoelectrolysis of water was observed using a structured n-GaN photoanode, aqueous NaOH solution, and platinum cathode. The structured n-GaN was formed by selective area regrowth using metal stripes defined by photolithography. Gas evolution from both the n-GaN anode and the platinum cathode was observed under UV irradiation without external bias. The metal stripes eliminated current crowding and the n-GaN regrowth increased specific surface area. As a result, distinct improvements in photocurrent were achieved. The origin of the photocurrent and the conversion efficiency are discussed based on water-splitting reactions and etching reactions. [ABSTRACT FROM AUTHOR]

Gallium nitride based microcavity light emitting diodes less than 400 nm thick emitting at a peak wavelength of 455 nm have been fabricated. The epitaxial structure was grown by metal organic chemical vapor deposition, and the device was fabricated using a laser lift-off process. Cavity thinning was carried out using inductively coupled plasma etching until a cavity length of roughly 2λ (∼375 nm, corresponding to a cavity order of 4 for λ=455 nm in GaN) was achieved. Devices are presented that show perfectly detuned angular emission and perfectly resonant emission between the cavity length and emission wavelength. [ABSTRACT FROM AUTHOR]

The compliant behavior of densely packed 10 × 10 µm2 square patterned InGaN layers on top of porous GaN is demonstrated. The elastic relaxation of the InGaN layers is enabled by the low stiffness of the porous GaN under layer. High resolution X-ray diffraction measurements show that upon InGaN re-growths on these InGaN-on-porous GaN pseudo-substrates, not only was the regrown layer partially relaxed, but the degree of relaxation of the InGaN pseudo-substrate layer on top of the porous GaN also showed an increase in the a-lattice constant. Furthermore, methods to improve the surface morphology of the InGaN layers grown by metal-organic chemical vapor deposition (MOCVD) were explored in order to fabricate InGaN pseudo-substrates for future optoelectronic and electronic devices. The largest a-lattice constant demonstrated in this study using this improved method was 3.209 Å, corresponding to a fully relaxed InGaN film with an indium composition of 0.056. [ABSTRACT FROM AUTHOR]

We use photoreflectance (PR) spectroscopy to study the electronic band structure modification of GaN films grown along different nonpolar orientations due to biaxial, anisotropic in-plane strain. The exciton transition energies of an unstrained, high-quality C-plane GaN film are used to accurately determine the crystal-field and spin-orbit splitting energies. For films with a nonpolar orientation, the resonant features observed in the PR spectra exhibit a strong in-plane polarization anisotropy and different transition energies from the ones measured in the C-plane GaN film. The deformation potential D5 is accurately determined from four GaN films with a nonpolar orientation using the measured energies together with the polarization properties and out-of-plane strain. [ABSTRACT FROM AUTHOR]

We report on the fabrication and study of laser lift-off GaN-based light-emitting diodes, thinned down to the microcavity regime, incorporating two-dimensional photonic crystal diffraction gratings. Angle-resolved measurements reveal the photonic behavior of the devices, which strongly depends on the GaN thickness. Data point out the detrimental role of metal absorption. We explore theoretically the possibility to limit this loss channel. [ABSTRACT FROM AUTHOR]