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Deep-ultraviolet emitting structures based on Al0.65Ga0.35N/Al0.8Ga0.2 N multiple quantum wells (MQWs), embedded in compositionally graded AlxGa1-xN films in the form of graded-index-separate-confinement heterostructure, were grown by molecular beam epitaxy. The graded AlGaN layer blocked threading defects (TDs) in the vicinity of the AlN/AlGaN heterointerface, resulting in a reduction of the TDs in the active layer. The intensity of the transverse-magnetic polarized amplified spontaneous emission spectra with peak emission at 270 nm was found to increase linearly with the number of QWs. Furthermore, while the peak intensity for devices with a single QW varies linearly with the pump fluence, it varies superlinearly for larger number of QWs, suggesting light amplification by stimulated emission. These results are consistent with numerical simulations, which indicate that the confinement of the optical mode in this device structure increases with the number of QWs and simultaneously the threshold under optical pumping decreases with increasing of the number of QWs. Finally, the band structure simulations indicate that the device is in the form of a p-n junction due to polarization-induced p- and n-doping in the compositionally graded AlxGa1-xN films on either side of the MQWs, and, thus, could be used for the development of an electrically pumped deep-ultraviolet laser by further optimizing the structures.

The effects of 2-D hexagonal-lattice TiO2 photonic crystal layers on the directional light extraction efficiency of low-scattering (Y1-xCex)3Al5O12 (YAG:Ce 3+) ceramic converters have been investigated. The hexagonal-lattice TiO2 patterns were fabricated by an electron beam lithography process. The optimized yellow directional light extraction enhancement by a factor of 4.4 is achieved with a 2-D photonic crystal layer having a diameter of 430 nm, a lattice constant of 590 nm, and a height of 350 nm. A 3-D finite-difference time-domain model has also been developed to study the geometric parameters of the photonic crystal layer, yielding results that are consistent with our measured data.

This paper reviews the device physics and technology of optoelectronic devices based on semiconductors of the GaN family, operating in the spectral regions from deep UV to Terahertz. Such devices include LEDs, lasers, detectors, electroabsorption modulators and devices based on intersubband transitions in AlGaN quantum wells (QWs). After a brief history of the development of the field, we describe how the unique crystal structure, chemical bonding, and resulting spontaneous and piezoelectric polarizations in heterostructures affect the design, fabrication and performance of devices based on these materials. The heteroepitaxial growth and the formation and role of extended defects are addressed. The role of the chemical bonding in the formation of metallic contacts to this class of materials is also addressed. A detailed discussion is then presented on potential origins of the high performance of blue LEDs and poorer performance of green LEDs (green gap), as well as of the efficiency reduction of both blue and green LEDs at high injection current (efficiency droop). The relatively poor performance of deep-UV LEDs based on AlGaN alloys and methods to address the materials issues responsible are similarly addressed. Other devices whose state-of-the-art performance and materials-related issues are reviewed include violet-blue lasers, ‘visible blind’ and ‘solar blind’ detectors based on photoconductive and photovoltaic designs, and electroabsorption modulators based on bulk GaN or GaN/AlGaN QWs. Finally, we describe the basic physics of intersubband transitions in AlGaN QWs, and their applications to near-infrared and terahertz devices. [ABSTRACT FROM AUTHOR]