Advanced Nitride Light Emitters

 Dear Colleagues,

1. Advances in UV LEDs and LDs

Despite continuous efforts to develop an AlGaN deep UV LED (emission below 300 nm), their wall-plug efficiency is still as low as 3%, which is 20 times lower than for their blue and green counterparts based on InGaN. The problems to overcome include (i) low internal quantum efficiency of AlGaN and AlGaInN emitters due to high defect density and internal electric fields; (ii) low hole concentration of p-AlGaN, resulting in low carrier injection efficiency and high operating voltage; and (iii) low light extraction efficiency due to light absorption of the p-GaN contact layer.

The first laser diode operating in UVC range (below 280 nm) recently appeared. There are also several demonstrations of optically pumped laser diodes with emission wavelengths down to 237 nm.

2. Nitride-VCSELs

We have recently witnessed rapid progress in the development of nitride-based vertical-cavity surface-emitting lasers (VCSELs). However, it is not yet clear which approach will be the most successful: double dielectric Bragg mirrors, AlInN/GaN monolithic Bragg mirrors, or hybrid solutions.

3. Advances in long-wavelength emitters

Quantum-dot vertical-cavity surface-emitting lasers have reached yellow-green wavelengths (560–565 nm). Both VCSELs and edge-emitting green lasers can be grown on polar and nonpolar substrates, leading to improved parameters. The problem of growing InGaN-based red quantum-dot/quantum-well lasers and efficient red LEDs remains one of the biggest challenges for the near future.

4. Quantum-cascade infrared emitters

Quantum-cascade emission in the near- and mid-infrared range has been achieved in GaN/AlGaN superlattices due to large conduction band offsets (>1 eV) and high longitudinal optical phonon energy (up to 99 meV).

5. Progress in RGB displays and micro and nano LEDs

The logical direction in the development of full-color display is to transit from existing LCD or OLED solutions to those based on inorganic mLEDs. This is because only inorganic (III–V semiconductor) LEDs may show wall-plug efficiency above 80%, outperforming both LCD and OLED. This is critical for daylight visible mobile devices such as smartphones and smartwatches. In order to achieve full-color display, we have to monolithically integrate red, green, and blue emitters on one wafer. The current approaches to accomplish this range from vertically stacking quantum wells to using various combinations of nanocolumnar structures.

6. Physics of QW and QD structures

Interesting physical effects occur in nitride LEDs due to extremely high built-in electric fields. Recombination mechanisms and the origin of droop are debated, and single-photon generation from InGaN quantum dots has been demonstrated. Very wide (15–25 nm) InGaN quantum wells and structures with tunnel junctions show interesting properties.

Prof. Dr. Witold A. Trzeciakowski
Prof. Dr. Perlin Piotr
Guest Editors

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• UV LEDs and LDs
• nitride VCSELs
• quantum-cascade nitride emitters
• RGB displays
• micro LEDs
• physics of QW and QD LEDs