High external quantum efficiency (6.5%) InGaN V-defect LEDs at 600†nm on patterned sapphire substrates.

Highly efficient long-wavelength InGaN LEDs have been a research focus in nitride LEDs for their potential applications in displays and solid-state lighting. A key breakthrough has been the use of laterally injected quantum wells via naturally occurring V-defects which promote hole injection through semipolar sidewalls and help to overcome the barriers to carrier injection that plague long wavelength nitride LEDs. In this article, we study V-defect engineered LEDs on (0001) patterned sapphire substrates (PSS) and GaN on (111) Si. V-defects were formed using a 40-period InGaN/GaN superlattice and we report a packaged external quantum efficiency (EQE) of 6.5% for standard 0.1 mm2. LEDs on PSS at 600 nm. We attribute the high EQE in these LEDs to lateral injection via V-defects.

Metasurface Light-Emitting Diodes with Directional and Focused Emission.

Phased-array metasurfaces enable the imprinting of complex beam structures onto coherent incident light. Recent demonstrations of photoluminescent phased-array metasurfaces highlight possibilities for achieving similar control in electroluminescent light-emitting diodes (LEDs). However, phased-array metasurface LEDs have not yet been demonstrated owing to the complexities of integrating device stacks and electrodes within nanopatterned metasurfaces. Here, we demonstrate metasurface LEDs that emit directional or focused light. We first design nanoribbon elements that achieve the requisite phase control within typical LED device constraints. Subsequently, we demonstrate unidirectional emission that can be engineered at will via phased-array concepts. This control is further exhibited in metasurface LEDs that directly emit focused beams. Finally, we show that these metasurface LEDs exhibit external quantum efficiencies (EQEs) superior to those of unpatterned LEDs. These results demonstrate metasurface designs that are compatible with high-EQE metal-free LED devices and portend opportunities for new classes of metasurface LEDs that directly produce complex beam structures.

High external quantum efficiency (6.8%) UV-A LEDs on AlN templates with quantum barrier optimization.

AlGaN-based UV-A LEDs have wide applications in medical treatment and chemical sensing; however, their efficiencies are still far behind visible LEDs or even shorter wavelengths UV-C counterparts because of the large lattice mismatch between the low-Al-content active region and the AlN substrate. In this report, we investigated the composition and thickness of the quantum barrier in the active region in terms of LED performance. Due to the improved strain management and better carrier confinement, efficient UV-A LEDs (320 nm - 330 nm) with EQEs up to 6.8% were demonstrated, among the highest efficiencies at this wavelength range.

Hybrid tunnel junction enabled independent junction control of cascaded InGaN blue/green micro-light-emitting diodes.

We demonstrate vertical integration of nitride-based blue/green micro-light-emitting diodes (µLEDs) stacks with independent junctions control using hybrid tunnel junction (TJ). The hybrid TJ was gown by metal organic chemical vapor deposition (p + GaN) and molecular-beam epitaxy (n + GaN). Uniform blue, green and blue/green emission can be generated from different junction diodes. The peak external quantum efficiency (EQE) of the TJ blue µLEDs and green µLEDs with indium tin oxide contact is 30% and 12%, respectively. The carrier transportation between different junction diodes was discussed. This work suggests a promising approach for vertical µLEDs integration to enhance the output power of single LEDs chip and monolithic µLEDs with different emission colors with independent junction control.

Designing Highly Directional Luminescent Phased-Array Metasurfaces with Reciprocity-Based Simulations.

Phased-array metasurfaces grant the ability to arbitrarily shape the wavefront of light. As such, they have been used as various optical elements including waveplates, lenses, and beam deflectors. Luminescent metasurfaces, on the other hand, have largely comprised uniform arrays and are therefore unable to provide the same control over the wavefront of emitted light. Recently, phased-array control of the wavefront of spontaneous emission has been experimentally demonstrated in luminescent phased-array metalenses and beam deflectors. However, current luminescent metasurface beam deflectors exhibit unidirectional emission for only p-polarized light. In this paper, we use a reciprocal simulation strategy to explain the polarization disparity and improve the directionality of incoherent emission from current quantum-well emitting phased-array metasurfaces. We also design complementary metasurfaces to direct emission from systems where emission originates from alternate quantum mechanical processes.

Computational design and optimization of nanostructured AlN deep-UV grating reflectors.

Deep-ultraviolet (DUV) optoelectronics require innovative light collimation and extraction schemes for wall-plug efficiency improvements. In this work, we computationally survey material limitations and opportunities for intense, wavelength-tunable DUV reflection using AlN-based periodic hole and pillar arrays. Refractive-index limitations for underlayer materials supporting reflection were identified, and MgF2 was chosen as a suitable low-index underlayer for further study. Optical resonances giving rise to intense reflection were then analyzed in AlN/MgF2 nanostructures by varying film thickness, duty cycle, and illumination incidence angle, and were categorized by the emergence of Fano modes sustained by guided mode resonances (holes) or Mie-like dipole resonances (pillars). The phase-offset conditions between complementary modes that sustain high reflectance (%R) were related to a thickness-to-pitch ratio (TPR) parameter, which depended on the geometry-specific resonant mechanism involved (e.g., guided mode vs. Mie dipole resonances) and yielded nearly wavelength-invariant behavior. A rational design space was constructed by pointwise TPR optimization for the entire DUV range (200-320 nm). As a proof of concept, this optimized phase space was used to design reflectors for key DUV wavelengths and achieved corresponding maximum %R of 85% at λ = 211 nm to >97% at λ = 320 nm.

Fully transparent metal organic chemical vapor deposition-grown cascaded InGaN micro-light-emitting diodes with independent junction control.

In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown InGaN cascaded micro-light-emitting diodes (µLEDs) with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a tunnel junction (TJ), a green emitting diode, and a TJ, without using any conductive oxide layer. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently, which show high optical and electrical performance. The forward voltage (Vf) at 20 A/cm2 for the TJ blue µLEDs and TJ green µLEDs is 4.06 and 3.13 V, respectively. These results demonstrate the efficient TJs and fully activated p-type GaN in the cascaded µLEDs. Such demonstration shows the important application of TJs for the integration of µLEDs with multiple color emissions.

Metalorganic chemical vapor deposition of InN quantum dots and nanostructures.

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This III-N material system, famous for enabling blue and white solid-state lighting, has been pushing towards longer wavelengths in more recent years. With a bandgap of about 0.7 eV, InN can emit light in the near infrared, potentially overlapping with the part of the electromagnetic spectrum currently dominated by III-As and III-P technology. As has been the case in these other III-V material systems, nanostructures such as quantum dots and quantum dashes provide additional benefits towards optoelectronic devices. In the case of InN, these nanostructures have been in the development stage for some time, with more recent developments allowing for InN quantum dots and dashes to be incorporated into larger device structures. This review will detail the current state of metalorganic chemical vapor deposition of InN nanostructures, focusing on how precursor choices, crystallographic orientation, and other growth parameters affect the deposition. The optical properties of InN nanostructures will also be assessed, with an eye towards the fabrication of optoelectronic devices such as light-emitting diodes, laser diodes, and photodetectors.

Light-emitting metalenses and meta-axicons for focusing and beaming of spontaneous emission.

Phased-array metasurfaces have been extensively used for wavefront shaping of coherent incident light. Due to the incoherent nature of spontaneous emission, the ability to similarly tailor photoluminescence remains largely unexplored. Recently, unidirectional photoluminescence from InGaN/GaN quantum-well metasurfaces incorporating one-dimensional phase profiles has been shown. However, the possibility of generating arbitrary two-dimensional waveforms-such as focused beams-is not yet realized. Here, we demonstrate two-dimensional metasurface axicons and lenses that emit collimated and focused beams, respectively. First, we develop off-axis meta-axicon/metalens equations designed to redirect surface-guided waves that dominate the natural emission pattern of quantum wells. Next, we show that photoluminescence properties are well predicted by passive transmission results using suitably engineered incident light sources. Finally, we compare collimating and focusing performances across a variety of different light-emitting metasurface axicons and lenses. These generated two-dimensional phased-array photoluminescence waveforms facilitate future development of light sources with arbitrary functionalities.

Fabrication and chemical lift-off of sub-micron scale III-nitride LED structures.

Nanoscale light emitting diodes (nanoLEDs, diameter < 1 µm), with active and sacrificial multi-quantum well (MQW) layers epitaxially grown via metal organic chemical vapor deposition, were fabricated and released into solution using a combination of colloidal lithography and photoelectrochemical (PEC) etching of the sacrificial MQW layer. PEC etch conditions were optimized to minimize undercut roughness, and thus limit damage to the active MQW layer. NanoLED emission was blue-shifted ∼10 nm from as-grown (unpatterned) LED material, hinting at strain relaxation in the active InGaN MQW layer. X-ray diffraction also suggests that strain relaxation occurs upon nanopatterning, which likely results in less quantum confined Stark effect. Internal quantum efficiency of the lifted nanoLEDs was estimated at 29% by comparing photoluminescence at 292K and 14K. This work suggests that colloidal lithography, combined with chemical release, could be a viable route to produce solution-processable, high efficiency nanoscale light emitters.

Violet semipolar (20-2-1) InGaN microcavity light-emitting diode with a 200†nm ultra-short cavity length.

Violet semipolar (20-2-1) InGaN microcavity light-emitting diodes (MC-LED) with a 200 nm ultra-short cavity length were demonstrated. The emission wavelength was 419 nm with a spectrum width of 20 nm. The external quantum efficiency (EQE) of MC-LED was constant at 0.8% for a forward current from 0.5 to 2 mA with the emitting area of 30×30 µm2. With increasing forward current, the peak wavelength and spectrum width of the emission showed almost no changes. For epitaxial growth, metal-organic chemical vapor deposition (MOCVD) was used. Substrate removal and tunnel-junction with an Ag-based electrode made possible the fabrication of the ultra-short 200 nm thick cavity MC-LED. This is more than a factor of 2 improvement compared to previous MC-LEDs of 450 nm cavity thickness sustaining 5 modes.

High performance of a semipolar InGaN laser with a phase-shifted embedded hydrogen silsesquioxane (HSQ) grating.

Single-frequency blue laser sources are of interest for an increasing number of emerging applications but are still difficult to implement and expensive to fabricate and suffer from poor robustness. Here a novel and universal grating design to realize distributed optical feedback in visible semiconductor laser diodes (LDs) was demonstrated on a semipolar InGaN LD, and its unique effect on the laser performance was investigated. For the first time, to the best of our knowledge, a low threshold voltage, record-high power output, and ultra-narrow single-mode lasing were simultaneously obtained on the new laser structure with a thinner p-GaN layer and a third-order phase-shifted embedded dielectric grating. Under continuous-wave operation, such 450 nm lasers achieved 35 dB side-mode suppression ratio, less than 2 pm FWHM, and near 400 mW total output power at room temperature.

Color-changing refractive index sensor based on Fano-resonant filtering of optical modes in a porous dielectric Fabry-Pérot microcavity.

Refractometry is a ubiquitous technique for process control and substance identification in the chemical and biomedical fields. Herein, we present an all-dielectric, wafer-scalable, and compact Fabry-Pérot microcavity (FPMC) device for refractive index (RI) sensing. The FPMC consists of a highly porous SiO2 microcavity capped with a thin, quasi-periodically patterned TiO2 hole array partial reflector that enables rapid, nanoliter-scale analyte transport to and from the sensor. Liquid (alcohols) or condensed-vapor (water from human breath) infiltration resulted in spectral redshifts up to 100 nm, highly apparent visible color change, rapid recovery (< 20 s), and RI sensitivity of up to 680 nm/RIU. The sensor can also be used in spectral or single-wavelength detection modes. Effective-medium and finite-difference time-domain optical simulations identified that Fano-resonant scattering modes induced by the quasi-periodic TiO2 outcoupling layer effectively filter higher-order Fabry-Pérot cavity modes and thereby confer an easily identifiable red-to-green color transition during analyte infiltration.

Dependence of carrier escape lifetimes on quantum barrier thickness in InGaN/GaN multiple quantum well photodetectors.

We reported significant improvements in device speed by reducing the quantum barrier (QB) thicknesses in the InGaN/GaN multiple quantum well (MQW) photodetectors (PDs). A 3-dB bandwidth of 700 MHz was achieved with a reverse bias of -6 V. Carrier escape lifetimes due to carrier trapping in the quantum wells (QWs) were obtained from both simulation and experimental fitting, identifying carrier trapping as the major speed limiting factor in the InGaN/GaN MQW PDs.

Size-independent low voltage of InGaN micro-light-emitting diodes with epitaxial tunnel junctions using selective area growth by metalorganic chemical vapor deposition.

High performance InGaN micro-size light-emitting diodes (µLEDs) with epitaxial tunnel junctions (TJs) were successfully demonstrated using selective area growth (SAG) by metalorganic chemical vapor deposition (MOCVD). Patterned n + GaN/n-GaN layers with small holes were grown on top of standard InGaN blue LEDs to form TJs using SAG. TJ µLEDs with squared mesa ranging from 10×10 to 100×100 µm2 were fabricated. The forward voltage (Vf) in the reference TJ µLEDs without SAG is very high and decreases linearly from 4.6 to 3.7 V at 20 A/cm2 with reduction in area from 10000 to 100 µm2, which is caused by the lateral out diffusion of hydrogen through sidewall. By contrast, the Vf at 20 A/cm2 in the TJ µLEDs utilizing SAG is significantly reduced to be 3.24 to 3.31 V. Moreover, the Vf in the SAG TJ µLEDs is independent on sizes, suggesting that the hydrogen is effectively removed through the holes on top of the p-GaN surface by SAG. The output power of SAG TJ µLEDs is ∼10% higher than the common µLEDs with indium tin oxide (ITO) contact.

560†nm InGaN micro-LEDs on low-defect-density and scalable (20-21) semipolar GaN on patterned sapphire substrates.

We demonstrate InGaN-based semipolar 560 nm micro-light-emitting diodes with 2.5% EQE on high-quality and low-defect-density (20-21) GaN templates grown on scalable and low-cost sapphire substrates. Through transmission electron microscopy observations, we discuss how the management of misfit dislocations and their confinement in areas away from the active light-emitting region is necessary for improving device performance. We also discuss how the patterning of semipolar GaN on sapphire influences material properties in terms of surface roughness and undesired faceting in addition to indium segregation at the proximity of defected areas.

Electrically driven, polarized, phosphor-free white semipolar (20-21) InGaN light-emitting diodes grown on semipolar bulk GaN substrate.

We demonstrate a simple method to fabricate efficient, electrically driven, polarized, and phosphor-free white semipolar (20-21) InGaN light-emitting diodes (LEDs) by adopting a top blue quantum well (QW) and a bottom yellow QW directly grown on (20-21) semipolar bulk GaN substrate. At an injection current of 20 mA, the fabricated 0.1 mm2 size regular LEDs show an output power of 0.9 mW tested on wafer without any backside roughing, a forward voltage of 3.1 V and two emission peaks located at 427 and 560 nm. A high polarization ratio of 0.40 was measured in the semipolar monolithic white LEDs, making them promising candidates for backlighting sources in liquid crystal displays (LCDs). Furthermore, a 3dB modulation bandwidth of 410 MHz in visible light communication (VLC) was obtained in the micro-size LEDs (µLEDs) with a size of 20×20 µm2 and 40×40 µm2, which could overcome the limitation of slow frequency response of yellow phosphor in commercial white LEDs combing blue LEDs and yellow phosphor.

Improved performance of AlGaInP red micro-light-emitting diodes with sidewall treatments.

The electrical and optical improvements of AlGaInP micro-light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) sidewall passivation were demonstrated. Due to the high surface recombination velocity and minority carrier diffusion length of the AlGaInP material system, devices without sidewall passivation suffered from high leakage and severe drop in external quantum efficiency (EQE). By employing ALD sidewall treatments, the 20×20 µm2 µLEDs resulted in greater light output power, size-independent leakage current density, and lower ideality factor. The forward current-voltage characteristic was enhanced by using surface pretreatment. Furthermore, ALD sidewall treatments recovered the EQE of the 20×20 µm2 devices more than 150%. This indicated that AlGaInP µLEDs with ALD sidewall treatments can be used as the red emitter for full-color µLED display applications.

Compliant Micron-Sized Patterned InGaN Pseudo-Substrates Utilizing Porous GaN.

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.

Demonstration of GaN-based vertical-cavity surface-emitting lasers with buried tunnel junction contacts.

We report III-nitride vertical-cavity surface-emitting lasers (VCSELs) with buried tunnel junction (BTJ) contacts. To form the BTJs, GaN TJ contacts were etched away outside the aperture followed by n-GaN regrowth for current spreading. Under pulsed operation, a BTJ VCSEL with a 14 µm diameter aperture showed a lasing wavelength of 430 nm, a threshold current of ∼20 mA (12 kA/cm2), and a maximum output power of 2.8 mW. Under CW operation, an 8 µm aperture VCSEL showed a differential efficiency of 11% and a peak output power of ∼0.72 mW.