Christopher D. Pynn

High-power low-droop violet semipolar (303¯1¯) InGaN/GaN light-emitting diodes with thick active layer design

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 (303¯1¯) free-standing GaN substrates and processed and packaged using conventional techniques. Simulated band diagrams showed reduced polarization fields on the (303¯1¯) plane. The calculated critical thickness for misfit dislocation formation is higher on the (303¯1¯) plane than on other semipolar planes, such as (202¯1¯), allowing for thicker active regions than our previous work to further reduce droop. The higher critical thickness was confirmed with defect characterization via cathodol...

Sustained high external quantum efficiency in ultrasmall blue III–nitride micro-LEDs

Ultrasmall blue InGaN micro-light-emitting diodes (µLEDs) with areas from 10−4 to 0.01 mm2 were fabricated to study their optical and electrical properties. The peak external quantum efficiencies (EQEs) of the smallest and largest µLEDs were 40.2 and 48.6%, respectively. The difference in EQE was from nonradiative recombination originating from etching damage. This decrease is less severe than that in red AlInGaP LEDs. The efficiency droop at 900 A/cm2 of the smallest µLED was 45.7%, compared with 56.0% for the largest, and was lower because of improved current spreading. These results show that ultrasmall µLEDs may be fabricated without a significant loss in optical or electrical performance.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl2/N2-based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

High wall-plug efficiency blue III-nitride LEDs designed for low current density operation

Commercial LEDs for solid-state lighting are often designed for operation at current densities in the droop regime (~35 A/cm2) to minimize costly chip area; however, many benefits can be realized by operating at low current density (J ≈1 - 5 A/cm2). Along with mitigation of droop losses and reduction of the operating voltage, low J operation of LEDs opens the design space for high light extraction efficiency (LEE). This work presents detailed ray tracing simulations of an LED design for low J operation with LEE ≈94%. The design is realized experimentally resulting in a peak wall-plug efficiency of 78.1% occurring at 3.45 A/cm2 and producing an output power of 7.2 mW for a 0.1 mm2 emitting area. At this operation point, the photon voltage Vp=hνq exceeds the forward voltage (V), corresponding to a Vp/V = 103%.

Using band engineering to tailor the emission spectra of trichromatic semipolar InGaN light-emitting diodes for phosphor-free polarized white light emission

We report a polarized white light-emitting device that monolithically integrates an electrically injected blue light-emitting diode grown on the (202¯1¯) face of a bulk GaN substrate and optically pumped InGaN quantum wells (QWs) with green and red light emission grown on the (202¯1) face. To overcome the challenges associated with growing high indium content InGaN QWs for long wavelength emission, a p-i-n doping profile was used to red-shift the emission wavelength of one of the optically pumped QWs by creating a built-in electric field in the same direction as the polarization-induced electric field. Emission peaks were observed at 450 nm from the electrically injected QW and at 520 nm and 590 nm from the optically pumped QWs, which were situated in n-i-n and p-i-n structures, respectively. The optically pumped QW in the p-i-n structure was grown at a growth temperature that was 10 °C colder compared to the QW in the n-i-n structure, so the emission from the QW in the p-i-n structure was red-shifted due...

On the optical polarization properties of semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells

In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in ( 20 2 ¯ 1 ), by the larger sensitivity of holes to this upper interface due to the polarization field in ( 20 2 ¯ 1 ), and/or by the different degrees of localization of holes.In the framework of k · p-theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with...

Using tunnel junctions to grow monolithically integrated optically pumped semipolar III-nitride yellow quantum wells on top of electrically injected blue quantum wells

We report a device that monolithically integrates optically pumped (20-21) III-nitride quantum wells (QWs) with 560 nm emission on top of electrically injected QWs with 450 nm emission. The higher temperature growth of the blue light-emitting diode (LED) was performed first, which prevented thermal damage to the higher indium content InGaN of the optically pumped QWs. A tunnel junction (TJ) was incorporated between the optically pumped and electrically injected QWs; this TJ enabled current spreading in the buried LED. Metalorganic chemical vapor deposition enabled the growth of InGaN QWs with high radiative efficiency, while molecular beam epitaxy was leveraged to achieve activated buried p-type GaN and the TJ. This initial device exhibited dichromatic optically polarized emission with a polarization ratio of 0.28. Future improvements in spectral distribution should enable phosphor-free polarized white light emission.

Enhancing light extraction from III-nitride devices using moth-eye nanostructures formed by colloidal lithography

Summary form only given. We report a novel, reliable, and scalable technique for the surface nanostructuring of GaN by colloidal lithography. Moth-eye protuberances of varying dimensions have been formed on bulk GaN substrates and on samples containing an InGaN multiple-quantum-well active layer, grown by metal organic chemical vapor deposition. Angle-resolved optical characterization and photoluminescence (PL) indicates enhanced transmission through these samples compared to flat, unstructured surfaces. We analyze the potential mechanisms causing this enhancement, such as recycling of PL pump light, improved first-pass extraction, and improved diffuse scattering. The impact of these effects is evaluated using finite-difference time-domain and effective medium theory simulations. Light extraction with moth-eye nanostructures is compared to state-of-the-art methods for c-plane and semipolar GaN surface roughening.

Tunnel junction devices with monolithic optically pumped and electrically injected InGaN quantum wells for polarized white light emission

Summary form only given. We demonstrate polarized white light emission from a semipolar device in which a tunnel junction is used to monolithically integrate a blue electrically injected light-emitting diode (LED) and yellow-emitting InGaN quantum wells (QWs) that are optically pumped by the LED emission. Polarized white light has important applications, for example, in backlighting liquid crystal displays. The use of a tunnel junction allows the optically pumped QWs for long wavelength emission to be grown after the blue LED, which protects the high indium content InGaN layers from high temperature growth steps and thermal damage. The tunnel junction allows n-GaN above the tunnel junction to be used for current spreading across the LED area, and our molecular beam epitaxy (MBE) tunnel junction growth technique enables buried p-GaN. Optically pumping-rather than electrically injecting-offers several advantages for realizing long wavelength emission from InGaN QWs, and growth on (2021) enables optically polarized emission.

Designing optically pumped InGaN quantum wells with long wavelength emission for a phosphor-free device with polarized white-light emission

We report a semipolar III-nitride device in which an electrically injected blue light emitting diode optically pumps monolithic long wavelength emitting quantum wells (QWs) to create polarized white light. We have demonstrated an initial device with emission peaks at 440 nm and 560 nm from the electrically injected and optically pumped QWs, respectively. By tuning the ratio of blue to yellow, white light was measured with a polarization ratio of 0.40. High indium content InGaN is required for long wavelength emission but is difficult to achieve because it requires low growth temperatures and has a large lattice mismatch with GaN. This device design incorporates optically pumped QWs for long wavelength emission because they offer advantages over using electrically injected QWs. Optically pumped QWs do not have to be confined within a p-n junction, and carrier transport is not a concern. Thus, thick GaN barriers can be incorporated between multiple InGaN QWs to manage stress. Optically pumping long wavelength emitting QWs also eliminates high temperature steps that degrade high indium content InGaN but are required when growing p-GaN for an LED structure. Additionally, by eliminating electrical injection, the doping profile can instead be engineered to affect the emission wavelength. We discuss ongoing work focused on improving polarized white light emission by optimizing the optically pumped QWs. We consider the effects of growth conditions, including: trimethylindium (TMI) flow rate, InGaN growth rate, and growth temperature. We also examine the effects of epitaxial design, including: QW width, number of QWs, and doping.