This paper reviews semi-polar GaN surfaces, of interest for light emitting devices, from both theoretical and experimental perspectives. Theoretical results on polarization charges at InGaN/GaN heterointerfaces and In incorporation into InGaN films are presented for polar (0001), semi-polar (1122) and nonpolar (1100) surfaces. Specific features of semi-polar InGaN/ GaN structures are emphasized which can be beneficial for improving optical and transport properties of quantum-wellbased light emitting devices. The analysis favours semi-polar surfaces such as the (1122) surface as growth plane for longwavelength light emitters. Therefore, the experimental sections emphasize progress towards long-wavelength LEDs and lasers by growth of InGaN/AlGaN/GaN(1122) heterostructures on large-area GaN(1122)/m-sapphire templates. The current status of such templates as grown by hydride vapour phase epitaxy is presented. The implementation of an epitaxial lateral overgrowth method on such templates to improve device performances is demonstrated.

Specially designed intermediate carrier blocking layers (ICBLs) in multi-active regions of III-nitride LEDs were shown to be effective in controlling the carrier injection distribution across the active regions. In principle, the majority of carriers, both holes and electrons, can be guided into targeted quantum wells and recombine to generate light of specific wavelengths at controlled current-densities. Accordingly we proposed and demonstrated a novel monolithic InGaN-based LED to achieve three primary colors of light from one device at selected current densities. This LED structure, which has three different sets of quantum wells separated with ICBLs for three primary red-green-blue (RGB) colors, was grown by metal-organic chemical vapor deposition (MOCVD). Results show that this LED can emit light ranging from 460 to 650 nm to cover the entire visible spectrum. The emission wavelength starts at 650 nm and then decreases to 460 nm or lower as the injection current increases. In addition to three primary colors, many other colors can be obtained by color mixing techniques. To the best of our knowledge, this is the first demonstration of monolithic full-color LED grown by a simple growth technique without using re-growth process.

QPI is a new generation of spatial light modulator (SLM) that combines light source and light modulation at the pixel level, thus eliminating the waste in most existing SLMs where light is separately generated and modulated. This paper explains the QPI architecture and its 3D application and beyond.

A color-temperature tunable white light-emitting diode (LED) based on a newly developed monolithic color-tunable LED structure was demonstrated. The color-tunable LED structure consists of three different sets of quantum wells separated by intermediate carrier blocking layers that can independently emit visible lights from 460 to 650 nm under different injection currents. To generate white light, the color-tunable LED is operated under pulsed conditions with each pulse consisting of multiple steps of different current amplitudes and widths emitting different colors. The combined spectrum of different colors is aimed to mimic that of the blackbody radiation light source. The pulse rate is designed to be higher than the human eye response rate, so the human eye will not discern the emission of successive colors but a singular emission of white light. Results of a two-step pulse design show this method is able to generate white light from 2700 K – 6500 K. Moreover, their color coordinates fall within the 4-step MacAdam ellipses about the Planckian locus while achieving the Color Rendering Index (CRI) in the 80-90 range. Finally, simulations show improvement of CRI into the 90-100 range is possible with further optimization to the color-tunable LED spectral emission and use of three-step pulses.

Articles you may be interested inDetermination of the composition and thickness of semi-polar and non-polar III-nitride films and quantum wells using X-ray scattering Carrier recombination mechanisms in nitride single quantum well light-emitting diodes revealed by photo-and electroluminescence J. Appl. Phys.Carrier confinement and injection characteristics of polar and nonpolar III-nitride quantum well ͑QW͒ light-emitting diode or laser diode structures are compared. We demonstrate that strongly inhomogeneous QW injection in multiple-QW ͑MQW͒ active region is one of the possible reasons holding back the advance of nonpolar laser structures. In polar structures, strong interface polarization charges induce the nonuniform carrier distribution among the active QWs so that the extreme p-side QW always dominates the optical emission. On the contrary, in nonpolar MQW structures, the inhomogeneity of QW populations is supported mainly by QW residual charges and the prevailing QW is the one closest to the n-side of the diode. For both polar and nonpolar structures, the QW injection inhomogeneity is strongly affected by the QW carrier confinement and becomes more pronounced in longer wavelength emitters with deeper active QWs. We show that in nonpolar structures indium incorporation into optical waveguide layers improves the uniformity of QW injection. On the contrary, QW injection in polar structures remains inhomogeneous even at high-indium waveguide layer compositions. We show, however, that polarization-matched design of the electron-blocking layer can noticeably improve the injection uniformity in polar MQW structure and enhance the structure internal quantum efficiency.