Semiconductor lasers have been used in many fields due to their compactness and high efficiency. In this paper, we report on recently realized UV-B AlGaN-based laser diodes, focusing on the perspective of crystal growth techniques. We also discuss the effect of dislocations in UV AlGaN-based lasers by comparing the threshold power densities of optically pumped lasers and the internal losses measured by using the optically pumped variable stripe length method. As a result, it was found that dislocations in AlGaN lead to non-radiative recombination and increased internal losses, and the realization of high quality AlGaN crystals is very important for the realization of high-performance UV lasers. After describing the high current density operation using polarization doping, the characteristics of laser diodes based on these techniques are introduced.
Combination of sputter deposition and high-temperature annealing is a promising technique for preparing AlN templates with a low threading dislocation density (TDD) at a lower film thickness compared to those prepared by the conventional metalorganic vapor phase epitaxy. We controlled the residual stress of the sputter-deposited AlN films by modifying the sputtering conditions. By optimizing the sputtering and annealing conditions, TDDs of 2 × 108 cm−2 and 5 × 107 cm−2 were achieved for the AlN templates with thicknesses of 480 nm and 1200 nm, respectively. By utilizing the substrates with appropriate surface off-cut, dislocation-induced hillock structures on the AlxGa1-xN surfaces grown on the AlN templates were suppressed. The advantages of low-dislocation densities of the AlN templates for achieving high external quantum efficiencies were demonstrated.
UV−C laser diodes (LDs) have potential applications in the fields of healthcare, sensing and processing lasers. Our group has demonstrated UV−C LDs with an AlGaN−based heterostructure by pulsed current injection at room temperature. This is the first report of current−driven LDs in the UV−C wavelength range. The key technologies are the highly conductive p-clad layer using polarization doping and the low defect AlGaN heterostructures using single crystal AlN substrate. The 2-inch based on-wafer LD formation technique was also investigated, such as mirror formation by etching and high-reflection coatings, which are the basis for large-scale integration.
The microscopic structural and optical characteristics of AlGaN light-emitting diodes (LEDs) fabricated on the AlN templates with dense macrosteps are shown to clarify the origin of their high internal quantum efficiency of radiation (IQE). The cross-sectional transmission electron microscopy observations under the high-angle annular dark field scanning mode and microscopic energy dispersive X-ray spectroscopy revealed that the AlGaN cladding layer under the AlGaN quantum well (QW) layer had a microscopic compositional modulation, which originates from the macrosteps at the AlN template surface. Moreover, Ga-rich portions in the cladding layer behaved as current micropaths, and the micropaths are connected with the carrier localization structure formed in QWs. The in-plane cathodoluminescence (CL) spectroscopy showed a significant inhomogeneity of the CL characteristics. The gentle slopes at the sample surface showed brighter emissions with a lower peak photon energy, confirming the carrier localization. This carrier localization structure in the QWs combined with the current micropaths in the cladding layer can increase the IQE as well as external quantum efficiency of the AlGaN LEDs.
Simulation of AlGaN-based UV and deep-UV light-emitting diodes with one-dimensional drift-diffusion method is discussed, with model accounting for polarization charge and ABC recombination. It is shown that the results of simulations with this model agree with experimental results for this class of devices, provided that the parameters of the model are properly matched to the underlying material. In particular, IV characteristics and LI characteristics are taken into account. ABC recombination constants are calculated based on comparison with experimental results. The simulations of the deep-UV LED efficiency in case of significant electron escape are then presented. The impact of EBL length and barrier between quantum wells and the EBL is discussed. Moreover, it is shown that improving p-type conductivity by introducing polarization-doped p-type region leads to significant improvement of the internal quantum efficiency.
III-nitride semiconductors are promising nonlinear optical crystals due to a wide bandgap energy, a high nonlinear optical constant, enabling broadband operation of nonlinear optical effects such as wavelength conversion and optical modulation in the deep ultraviolet (DUV) wavelength. Integration of nitride-based wavelength conversion with blue laser diodes enables a compact and highly-efficient coherent source in the DUV wavelength. This paper describes the principle of wavelength conversion and nonlinear optical crystals used in the ultraviolet region. Subsequently, focusing on nitride semiconductors as nonlinear optical crystals, polarity inversion methods, evaluation methods, and device applications are described.