AlN is a promising substrate materials for AlGaN-based ultraviolet light emitting diodes, because of its high thermal conductivity, high ultraviolet transmittance and small lattice mismatch with AlGaN layers. Heteroepitaxial growth techniques of AlN layer on sapphire substrates should be improved to satisfy the increasing demand for large-size sapphire substrates. Recently, we have developed a new liquid phase epitaxial growth technique to grow AlN films on the nitrided sapphire using Ga-Al fluxes. Influential factors such as flux composition, growth temperature and crystal orientation of sapphire were studied. Details of the experimental results of the Ga-Al flux technique and its thermodynamic principle are introduced in this article.
Homo-epitaxial growth of thick AlN layers by hydride vapor phase epitaxy (HVPE) was investigated on low dislocation density (< 10^3 cm^<-2>) AlN wafers prepared by physical vapor transport (PVT). AlN wafers prepared from HVPE layers had high structural qualityidentical to that of the PVT-AlN wafers and deep-UV transparency with an optical cutoff at 206.5 nm and below. The development of deep-UV transparency was found to be related to lower concentration of carbon impurity in the HVPE-AlN wafers. Strong electroluminescence (EL) peaking at 268 nm from deep-UV LEDs fabricated by metalorganic chemical vapor deposition (MOCVD) on the HVPE-AlN wafers could be extracted through the HVPE-AlN wafers. The continuous wave output power reached 28 mW at an injection current of 250 mA, where external quantum efficiency of 2.4% was obtained.
Material parameters in nitride semiconductors are still controversial. Previously reported values are highly scattered and several parameters have yet to be experimentally deduced. Furthermore, the quasicubic approximation has been conventionally used although its quantitative validity has not been supported. Herein, we performed reflectance spectroscopy under uniaxial stress for nonpolar and semipolar GaN and AlN bulk substrates. All the excitonic deformation potentials were experimentally determined for the first time, and we found that the quasicubic approximation breaks in GaN and AlN.
Influences and control techniques of polarization charges generated in AlGaN heterostructure are described. Firstly the polarization charge density and its influences to carrier density and potential profiles were theoretically investigated. The density, 1×10^<13>cm^<-2>, typically generated in an Al_<0.2>Ga_<0.8>N/GaN interface resulted in a few 100 meV potential barrier. Secondly as an example showing adverse effects of the large potential barrier on device performances, a p-AlGaN electron blocking layer in a LED was described. A technique to suppress the adverse effects, diluting the polarization charges with graded layer, was then mentioned. Finally a new layer structure of a deep UV-LED accelerating the use of the polarization charges even in the case of the Al-face epitaxial growth was proposed. Such large polarization charge densities should be widely recognized, and the polarization charge engineering must be established along with the bandgap engineering.
Aluminum nitride (AlN) is a key material for deep-ultraviolet (deep-UV) light-emitting diodes (LEDs). In AlN-based materials, the light emission intensity varies largely depending on the crystal plane because of its strong light polarization property as well as the quantum-confined Stark effect (QCSE). In this article, we introduce nonpolar plane AlN-based heterostructures working toward high-efficiency deep-UV LEDs.
Plasmonics is very useful to increase the light emission efficiencies of InGaN/GaN-based quantum wells (QWs) and the other various materials. By using this technique, highefficiency and high-speed light emission is predicted for optically as well as electrically pumped light-emitting devices. Flexible tuning of the plasmonic resonance within the visible wavelength range was achieved by controlling the metallic nano-structures. To extend the tuning range into UV regions, we fabricated the nanograin structures of Al and succeeded in remarkable enhancements of UV light emissions from AlGaN/AlN-based QWs. We found that Al is very useful also for wider tuning of the plasmonic resonance at the visible wavelength region. The plasmonic tuning at the IR regions were also discussed by using Ta nanoparticle. These tunable plasmonics over the UV-IR range should bring new possibilities of applications to plasmonics and lead to new class of several photonic and electronic technologies.