Gallium nitride (GaN) ultraviolet (UV) laser diodes (LDs) show tremendous promise for optical communications, data storage, and medical applications due to their compact size and higher efficiency compared to gas lasers. Typically, GaN UV LDs utilize a symmetric waveguide structure surrounding a multiple quantum well (MQW) active region for optical confinement. By increasing the thickness of these waveguides, device performance can be enhanced by reducing absorption losses. However, thin waveguides offer decreased carrier losses and improved electrical performance. These two competing effects can be balanced through the use of an asymmetric waveguide structure, composed of a thin upper waveguide and thick lower waveguide, in order to minimize both carrier (hole) losses as well as optical losses. Here, we demonstrated an edge-emitting ridge waveguide UV GaN LD emitting at ~392 nm. Mirror facets were fabricated through reactive ion etch and potassium hydroxide wet etch. These LD structures with InGaN/GaN MQWs and AlGaN cladding layers were grown via metalorganic chemical vapor deposition on a patterned sapphire substrate and utilize an asymmetric 100 nm thick upper unintentionally doped GaN (uGaN) waveguide and 500 nm thick lower uGaN waveguide structure. We have successfully demonstrated a LD device with 1000 μm cavity length with a lasing threshold of 2.2 A, and 111.8 mW per facet peak optical output power with a differential efficiency of 3%. This demonstration paves the way for GaN LDs with improved differential efficiency at high current densities through the use of optimized asymmetric waveguide structures.
Surface properties are important for structures such as micropillars and nanowires, which are critical for emerging devices including μLEDs, nano-lasers, and vertical power transistors due to increased surface to volume ratios. Fabrication of III-Nitride micropillars can be realized through a top-down approach, where structures are defined through lithography and reactive ion etching (RIE). While effective at forming these micropillar structures, RIE etching leaves behind roughened, non-vertical sidewalls. This surface damage increases non-radiative recombination, forms current leakage paths, and can severely degrade device performance. However, damage can be removed through a follow-up wet etch in potassium hydroxide (KOH) solution. KOH acts as a crystallographic etchant, preferentially exposing vertical <1-100> m-planes, producing smooth, vertical sidewalls. Here, we investigate KOH wet etch passivation for 2.5 μm diameter top-down fabricated GaN micropillars utilizing different temperatures and solution concentrations, and the effects of a Ni etch mask present during wet etching. We observed an average etch rate of 11.67 nm/min for micropillars etched in 60% AZ400k solution compared to 9.44 nm/min for micropillars etched in 20% AZ400k solution, both at a temperature of 80°C. At a constant 40% AZ400k concentration, an average etch rate of 14.39 nm/min for micropillars etched at 90°C are observed compared to 9.89 nm/min for micropillars etched at 70°C. Micropillars with a Ni etch mask present during KOH etching have an average etch rate of 9.46 nm/min compared to 12.83 nm/min for those without a Ni mask. The effects of KOH etching work to further optimize the performance of GaN-based micropillar and nanowire devices.
Micro-Light-Emitting Diode (μLED) displays have seen increasing interest over the past decade due to their promising advantages over other display technologies, especially in applications requiring extremely high resolutions such as virtual and alternate reality headsets. Most modern full-color μLED displays rely on red, green, and blue (RGB) pixels based on different material systems combined together on a thin-film transistor back panel, which is often costly and has poor yield. An alternative approach is to create a monolithic display in the GaN/InGaN material system, capable of covering the entire visible spectrum through tuning of quantum well (QW) Indium content or phosphor down conversion. However, monolithic GaN displays present the issue of pixel isolation, as the lack of truly insulating undoped GaN (u-GaN) makes it difficult to electrically isolate rows or columns of μLEDs from one another. In this work, we demonstrate a novel solution to this issue which utilizes photoresist to fill deep trench isolation features, enabling interconnection of μLED p-contacts in a single-color passive matrix display. A photoresist layer is used to fill deep trenches isolating columns of μLEDs from one another. This photoresist layer is patterned and baked at 250 °C for 30 minutes, crosslinking it and making it extremely durable. This process allows for formation of p-interconnects using liftoff, and avoids the issues involved in bridging high aspect ratio trenches. This photoresist planarized trench isolation process could contribute to creation of improved monolithic full color μLED displays which require multiple deep isolation features to be bridged by conductive interconnects.
Nanowire array LEDs rely on an interlayer spacer dielectric to enable connection of many nanowires in parallel. Conventional solutions use spin-coatable materials such as polydimethylsiloxane (PDMS) or spin-on-glass (SOG), which are thermally and mechanically unstable. Alternatively, more stable dielectric materials such as SiO2 or Si3N4 can be used, however these materials deposit conformally, leading to significant surface topology above the nanowires prior to the etch back step. In this work, we present a method for removing this surface topology by utilizing a self-planarizing photoresist layer and a plasma etch which removes photoresist and SiO2 at the same rate. By performing this planarization process several times, surface features of height h > 1 μm can be reduced to less than 50 nm, allowing further etching of the SiO2 to expose the tips the nanowires and allow for reliable p-contact formation. Unlike CMP, this process only involves dielectric deposition and dry etching, and places no limitations on sample size and shape, making it ideal for research settings.
AlGaN light emitting diodes (LEDs) emitting in the deep ultraviolet (DUV) range typically suffer from poor light extraction efficiency (LEE). In this study, we determine the effects of nanostructure height, diameter, and emission wavelength on LEE. Changes to device morphology influencing surface to volume ratio (SVR) are studied in order to optimize device dimensions to maximize LEE. Simulations show improvements in LEE of up to 300% and 60% for structures with increased height and decreased diameter respectively, which is predicted for higher SVR structures. These results shows that engineering of nanostructure SVR could be used to improve DUV LED efficiency.
Multi-color red, green, and blue (RGB) micro-light emitting diode (LED) displays often require the use different material systems to obtain high-efficiency RGB pixels. While blue and green micro-LEDs can be fabricated with sufficiently high external quantum efficiencies (ηEQE) using the InGaN materials system, red InGaN emitters currently exhibit much lower ηEQE. The reduction of InGaN LED efficiency at longer wavelengths can be attributed to the Quantum Confined Stark Effect (QCSE), which reduces the electron-hole wavefunction overlap (Γe_hh) and radiative recombination rate (𝑅𝑠𝑝), and worsens at longer wavelengths with increasing QW In-content. Consequently, higher efficiency AlInGaP LEDs are usually used for the red pixels in micro-LEDs, complicating the fabrication process. In this work, InGaN-delta-InN quantum well (QW) LEDs with InGaN quantum barriers on InGaN substrates are shown to produce significant enhancement in electronhole wavefunction overlap and 𝑅𝑠𝑝 over the entire red emission regime and into the near-infrared. Analysis and comparison of various InGaN-delta-InN QWs with InGaN barriers and InGaN/InGaN QWs emitting at 630 nm was performed using self-consistent six-band 𝑘 ∙ 𝑝 formalism. Wavefunction overlap from InGaN-delta-InN QWs was shown to increase by 3.5x when compared to an InGaN/InGaN QW at 630 nm. These improvements in wavefunction overlap were shown to lead to ~5 - 7x and ~3 - 10x enhancements in 𝑅𝑠𝑝 and IQE, respectively. With growth of InN monolayers on InGaN now readily achievable, this novel delta-InN active region on InGaN substrate design could pave the way for high efficiency, native red-emitting InGaN LEDs and allow for monolithic fabrication of InGaN micro-LED displays.
Planar deep-ultraviolet (DUV) light emitting diodes (LEDs) suffer from extremely low external quantum efficiencies (EQEs) due to poor light extraction efficiencies (LEE) which are often less than 1%, hindering their widespread use. In AlGaN DUV LEDs with high Al-content, the positioning of the valence subbands leads to dominant transverse magnetic (TM)-polarized emission which is difficult to extract from planar devices. To improve the LEE of DUV LEDs, techniques such as surface roughening and nanowire formation have been used. Nanowires are especially promising for DUV LEDs because they allow for very efficient extraction of TM-polarized light through their sidewalls. In this work, we demonstrate a novel “inverse taper” profile in AlGaN nanowires, in which the base of the nanowire can be narrowed to have a smaller diameter than the top through a KOH-based wet etch process. Hydroxyl-based chemistries are known to have a lower etch rate against the c-plane of wurtzite AlGaN alloys. Here, we report on observations of 0.8% KOH at 80℃ exhibiting a unique selectivity to a different wurtzite crystal plane, believed to be the (202̅ 1) plane, allowing for formation of an inverse taper structure. Finite difference time domain (FDTD) simulations at 280 nm reveal that AlGaN nanowire LEDs with high sidewall inverse taper angles can have greater than 75% and 90% LEE for TE and TM-polarized light respectively, ~2.5x higher than the LEE of vertical sidewall nanowires. This novel phenomenon may allow for significant improvements in the LEE of DUV nanowire LEDs.
Ultraviolet (UV) light-emitting diodes (LEDs) are useful in applications such as water/air purification, sterilization, and biosensing. However, due to the low external quantum efficiencies (ηEQE) of III-Nitride semiconductor UV LEDs, the technology has struggled to achieve penetration into many of these potential applications. While the active regions of UV LEDs have been well optimized, allowing for internal quantum efficiencies of greater than 60%, light extraction efficiency (ηEXT) remains a significant obstacle, and is limited to less than 10% in conventional UV LEDs, limiting their ηEQEs to around 1% for wavelengths below 300 nm. Surface texturing of the p-GaN or p-AlGaN layer in top-emitting UV LEDs has allowed for improvements in ηEQE at the expense of hole injection efficiency. Etching of the sapphire or AlN substrates to form lenses avoids this tradeoff in bottom-emitting LEDs, but is exceptionally time and resource intensive. Here, we investigated a novel method of enhancing ηEXT of AlGaN multiple quantum well UV LEDs at 280 nm using self-aligned monolayers of SiO2 microspheres and microlenses. Finite-difference time-domain simulations were utilized to investigate the effects of these nanostructure monolayers on the ηEXT of DUV LEDs emitting at 280 nm, and predicted up to 2.31x times enhancement of ηEXT. Electroluminescence (EL) measurements were performed in tandem with our simulations of UV LEDs. At normal incidence, up to 6.1% and 12.7% EL intensity enhancements were observed using 700 nm SiO2 microspheres and microlenses, respectively. These promising enhancements in output power may allow for high ηEQE in UV LEDs.
Light extraction efficiency (ηextraction) remains as a big challenge for high-efficiency deep-ultraviolet (UV) lightemitting diodes (LEDs) due to the large refractive index contrast at the AlN(sapphire)/air interface. Various surface patterning approaches such as microdome design and patterned sapphire substrates have been proposed to address the low ηextraction issue. Nevertheless, these previously proposed methods all involved additional complicated fabrication steps and the polarization-dependent analysis for these devices has not been investigated experimentally. In this work, we investigate the feasibility of using 700-nm SiO2 microsphere array on 280 nm flip-chip UV LEDs to improve the ηextraction. Angle- and polarization-dependent electroluminescence measurements have been performed to compare the 280 nm LEDs with and without the SiO2 microsphere array. The UV LED with microsphere array showed enhancement for transverse-electric (TE)-polarized light intensities at small angles while decreased intensities at large angles with respect to c-axis, as compared to the device without SiO2 microspheres For instance, up to 7.4% enhancement is observed at θ = 0°. However, for transverse-magnetic (TM)-polarized light, the intensities largely remain the same at small angles while decrease at large angles. Cross-sectional near-field electric field distribution from three-dimensional finite-difference time-domain simulation has confirmed that the use of SiO2 microspheres array resulted in scattering of photons at the sapphire/SiO2 microspheres interface, which eventually leads to enhanced TE-photons extraction at small-angles. From simulation, the light radiation patterns from the UV LED with SiO2 spheres are reshaped to a small-angle-favored pattern without reducing the total output power, showing great consistency with the measurement results.
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