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.

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 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 1 W of output power at a current density of 1 kA/cm2 from a single 0.1 mm2 LED device.

The effect of employing an AlGaN cap layer in the active region of green c-plane light-emitting diodes (LEDs) was studied. Each quantum well (QW) and barrier in the active region consisted of an InGaN QW and a thin AlGaN cap layer grown at a relatively low temperature and a GaN barrier grown at a higher temperature. A series of experiments and simulations were carried out to explore the effects of varying the AlGaN cap layer thickness and GaN barrier growth temperature on LED efficiency and electrical performance. We determined that the AlGaN cap layer should be around 2 nm and the growth temperature of the GaN barrier should be approximately 75° C higher than the growth temperature of the InGaN QW to maximize the LED efficiency, minimize the forward voltage, and maintain good morphology. Optimized AlGaN cap growth conditions within the active region resulted in high efficiency green LEDs with a peak external quantum efficiency (EQE) of 40.7% at 3 A/cm. At a normal operating condition of 20 A/cm, output power, EQE, forward voltage, and emission wavelength were 13.8 mW, 29.5%, 3.5 V, and 529.3 nm, respectively.

Commercial LEDs for solid-state lighting are often designed for operation at current densities in the droop regime (~35 A/cm) to minimize costly chip area; however, many benefits can be realized by operating at low current density (J ≈1 - 5 A/cm). 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/cm and producing an output power of 7.2 mW for a 0.1 mm emitting area. At this operation point, the photon voltage V=hνq exceeds the forward voltage (V), corresponding to a Vp/V = 103%.

Stable ferroelectric (FE) phases in nanometer-thick films would enable ultra-high density and fast FE field effect transistors (FeFETs), and the stability of ferroelectricity in ultrathin films has been under intense theoretical and experimental investigation. Here we predict, using density functional theory calculations, that the low-energy epitaxial PbTiO3 (001)/Pt interface strengthens the electrode-oxide bonds by breaking in-plane symmetry and stabilizes a ground state with enhanced polarization in sub-nanometer oxide films, with no critical-size limit. Additionally, we show that such enhancement is related to large work function differences between the P − and P + PbTiO3 surfaces, which gives rise to a net polarizing field in the oxide.