Chih-Chien Pan

High-Power, Low-Efficiency-Droop Semipolar (20\bar{2}\bar{1}) Single-Quantum-Well Blue Light-Emitting Diodes

We demonstrate a small-area (0.1 mm2) semipolar (2021) blue (447 nm) light-emitting diode (LED) with high light output power (LOP) and external quantum efficiency (EQE) by utilizing a single 12-nm-thick InGaN quantum well. The LED had pulsed LOPs of 140, 253, 361, and 460 mW, and EQEs of 50.1, 45.3, 43.0, and 41.2%, at current densities of 100, 200, 300, and 400 A/cm2, respectively. The device showed little blue shift and had a narrow full width at half maximum (FWHM). Micro-electroluminescence (µ-EL) and scanning transmission electron microscope (STEM) images indicate a high-quality InGaN quantum well (QW) layer.

High efficiency white LEDs with single-crystal ZnO current spreading layers deposited by aqueous solution epitaxy

Heteroepitaxial ZnO transparent current spreading layers with low sheet resistances were deposited on GaN-based light emitting diodes using aqueous solution phase epitaxy at temperatures below 90°C. The performance of the LEDs was analyzed and compared to identical devices using electron-beam evaporated indium tin oxide transparent current spreading layers. White LEDs with ZnO layers provided high luminous efficacy–157 lm/W at 0.5A/cm2, and 84.8 lm/W at 35A/cm2, 24% and 50% higher, respectively, than devices with ITO layers. The improvement appears to be due to the enhanced current spreading and low optical absorption provided by the ZnO.

The reduction of efficiency droop by Al0.82In0.18N/GaN superlattice electron blocking layer in (0001) oriented GaN-based light emitting diodes

To investigate the effect of Al0.82In0.18N electron blocking layer (EBL) on the efficiency droop, (0001) oriented InGaN light emitting diodes (LEDs) were grown with two different types of EBLs—single Al0.82In0.18N:Mg layer and Al0.82In0.18N:Mg (2 nm)/GaN:Mg (2 nm) superlattice (SL) structure with 7 periods. It was found that the output power and operating voltage of single Al0.82In0.18N EBL LED were sensitive to EBL thickness due to the difficulty in growing high quality Mg doped Al0.82In0.18N. On the other hand, LED with SL EBL showed no deterioration of optical power and operating voltage while its efficiency droop (17% at 300 A/cm2) reduced by more than a half compared to a conventional Al0.2Ga0.8N (20 nm) EBL LED (36% at 300 A/cm2).

Vertical Stand Transparent Light-Emitting Diode Architecture for High-Efficiency and High-Power Light-Emitting Diodes

Using a transparent ZnO vertical stand as a submount, a novel Light-emitting diode architecture, which is similar to conventional lighting bulbs, was proposed. The emission power of a blue LED based on c-plane (0001) bulk GaN was increased by 14.2 and 5.1% compared with those of conventional and suspended die packages, respectively. The output power and external quantum efficiency of LEDs respectively reached 31.7 mW and 57.1% at a forward current of 20 mA under direct current conditions. The high thermal conductivity and refractive index of the transparent submount simultaneously resulted in high current operation and high external efficiency.

Semipolar Single-Crystal ZnO Films Deposited by Low-Temperature Aqueous Solution Phase Epitaxy on GaN Light-Emitting Diodes

Low-temperature aqueous solution deposition has been used for the first time to produce epitaxial ZnO layers on the semipolar (1011) surface of bulk GaN substrates and LEDs. Although the ZnO films have single in-plane and out-of-plane orientations, which are nominally the same as those of the (1011) GaN substrate, the ZnO lattice is observed to be slightly tilted with respect to that of the substrate. A (1011) light-emitting diode using an epitaxial ZnO film as a transparent current-spreading layer achieved a high external quantum efficiency of 48%.

High Power and High Efficiency Blue InGaN Light Emitting Diodes on Free-Standing Semipolar (3031) Bulk GaN Substrate

High power and high efficiency semipolar (3031) nitride light emitting diodes (LEDs), fabricated on low extended defect bulk GaN substrates, are reported for the first time. The LEDs were grown by metal organic chemical vapor deposition (MOCVD) at atmospheric pressure. The peak wavelength was 452 nm, and a minimal redshift of <1 nm was observed between 5–100 mA, in comparison to large blueshifts in c-plane LEDs. The output power and external quantum efficiency (EQE) of the packaged 200 ×500 µm2 was 14.48 mW and 26.5%, respectively, at 20 mA.

The polarization field dependence of Ti/Al based Ohmic contacts on N-type semipolar GaN

We report on the polarization field dependence of the Ti/Al based n-type contacts on various polar and semipolar GaN planes. The contact resistance of the metal contacts on N-face GaN could be lowered by photoelectrochemical (PEC) etching, which formed a pyramidal feature on the surface with {101¯1¯} facets. The contact resistance on the semipolar (101¯1¯) surface was 6.3 × 10−5 Ω cm2, showing even lower contact resistance than the PEC etched N-face when annealed at 450 °C. Among few semipolar planes, (101¯1¯), (112¯2¯), and (112¯2), the contact resistance was the lowest with (112¯2¯) plane, which is the plane with the stronger polarization field than (101¯1¯) and more Ga rich surface than (112¯2) plane.

High light extraction efficiency light-emitting diodes grown on bulk GaN and sapphire substrates using vertical transparent package

High light extraction efficiency (LEE) of 75.86% and 77% for light-emitting diodes (LEDs) grown on free-standing GaN and patterned sapphire substrates, respectively, are demonstrated using novel vertical transparent LED (VT-LED) package. Compared to conventional silver header package with consistent external quantum efficiency (EQE) and LEE, enhancement of ~14% and ~12% for GaN and sapphire chips, respectively, are also demonstrated up to 100 A/cm2 under DC operations without any current crowding issues. Also, using high refractive index materials (n~2.1) between LED and ZnO stand, LEE of 81% and 82% for blue and green GaN chips can be achieved.