Physical properties of nitride semiconductors are compared with other typical semiconductors. Developments of blue-green LEDs and blue-violet LDS are reviewed. Latest efforts to develop shorter wavelength LEDs and LDS with both shorter and longer wavelength are introduced. Low temperature growth and electrical and optical characterization of high quality InN films on sapphire by RF-MBE aredemonstrated. Using this high quality InN films, it is found by photoluminescence and optical absorption measurements that band-gap of InN is around 0.8 eV, which is much narrower than the reported value of 1.9 eV. Latest developments of high power and high frequency AIGaN/ GaN HFETs are also reported in this paper, which confirms high potential of nitride semiconductors in the application to high power and high frequency electronics fields. Finally, it is emphasized that studies on crystal growth technology and characterization are very important in developing wide variety of new generation electronic and optoelectronic devices using nitride semiconductors.
AlGaN-based UV light emitting diode is fabricated and factors which limits performance of the UV LED is discussed. Microscopic cathodoluminescence suggests that dislocations act as the non-radiative recombination center. For the high-efficiency emission, it is necessary to reduce density of dislocations less than 2×10^7 cm^<-2>.
We report on the growth and optical properties of quaternary InAlGaN for the application of 250-350-nm-band bright ultraviolet (UV) light-emitting diodes (LEDs) or laser diodes (LDs). We demonstrated that the 300-nm-band UV emission is considerably enhanced by the In-segregation effect upon the introduction of 1-5% of In into AlGaN. We fabricated In_<x1>AlylGa_<1-x1-y1>N/In_<x2>Al_<y2> Ga_<1-x2-y2>N multi-quantum wells (MQWs) with various compositions, and obtained intense room-temperature emission in the wavelength range of 290-390 nm. The emission from the InAlGaN-based MQWs was as intense as the blue emission from InGaN-based QWs. We also fabricated UV-LEDs using a quaternary InAlGaN active region and achieved high-intensity 330 nm emission under RT CW operation. The UV intensity from quaternary InAlGaN-LED was more than one order of magnitude larger than that of AlGaN or GaN based LEDs.
III-V nitride semiconductors are useful for LED with colors covering ultraviolet, blue through green. Luminescence of these LEDs has high luminosity and high purity of color, and, therefore, many applications such as full colordisplay and backlight of LCD module were realized by us ing these LEDs. Short wavelength LED is used for an exciter of phosphors that can generate any color including white. The air purifier is also developed by combining short wavelength LED and the photo-catalyst such as TiOx.
The fabrication and evaluation of AlGaN-based ultraviolet light emitting diodes (UV-LEDs) are described. First, the formation of an atomically flat GaN surface and the control of GaN/AlGaN heterointerfaces is confirmed. From the photoluminescence measurement of GaN/AlGaN quantum well structures achieved by this heterointerface control, we found a clear quantum confinement effect and extremely large internal polarization field of nitride heterostructures. In consideration of the importance of this internal polarization field, the superiority of the active layer consisting of AlGaN quantum wells, and of the cladding layers consisting of short-period alloy super lattice (SPASL) are discussed. Finally, a highly efficient UVLED grown on a high quality bulk GaN substrate and its application are demonstrated.
In recent years, nitride materials which can emit red to ultraviolet light have attracted much attention from the viewpoint of wide-gap semiconductors. In this review article, we briefly present (a) low-dislocation density substrate, (b) hi-power 400 nm InGaN-LDs, (c) blue InGaN-LDs, (d) near-ultraviolet LDs, and then near-ultraviolet InGaN-LEDs.
We present the design and fabrication of highly reflective and low loss multilayer dielectric mirrors (SiO_2 /ZrO_2) for GaN based vertical cavity surface emitting lasers (VCSELs). We consider two types of VCSEL structures; one consists of AlN/GaN DBR and SiO_2/ZrO_2 DBR on a sapphire substrate. A resonant emission from a photo-pumped GalnN/GaN vertical cavity structure with a cavity length of 1.9pm has been demonstrated. The other consists of two dielectric mirrors (SiO_2/ZrO_2) with a polished thin sapphire substrate. A resonant emission from a photo-pumped GalnN/GalnN vertical cavity with a spectral line width of 3.8 nm has been demonstrated. Also, we propose a new structure using lateral growth on dielectric mirrors as a bottom reflector. We have demonstrated and characterized the thin GalnN/GaN 10 MQWs fabricated by removing a sapphire substrate with UV light irradiation for making a micro-cavity structure. The PL properties of standing-alone QWs show no noticeable degradation of QWs after the removing process. We pointed out the importance of the flatness of removed interfaces.
A new method has been developed to grow high-quality and large protein crystals. By a slow cooling method, tetragonal lysozyme crystals were grown from spontaneous nucleation at the interface of two liquids. The crystals had no contact with a growth vessel, which made it possible to remove crystals easily without causing any mechanical damage. The long and narrow shape of the crystals indicated that they had grown at a lower supersaturation, leading to high crystallinity. A large lysozyme crystal 3.0 mm in length was obtained in 20 days from a seed crystal kept at the interface. The stir with protein solution accelerated the growth of a protein crystal and prevented additional nucleation due to uniform concentration of the solution. The combination of growth at the interface between two liquids and controlled slow cooling is effective for the growth of high-quality and large protein crystals.