We report on development of plasma-assisted molecular beam epitaxy (PA MBE) of AlGaN-based quantum well (QW) heterostructures with high Al-content (>30%), exhibiting spontaneous and stimulated emission within mid-ultraviolet (UV) spectral range. The threading dislocations  density in the active regions of the QW structures grown on standard c-Al₂O₃ substrates have been reduced to 10⁸-10⁹ cm^-2 through the optimization of initial growth stage and growth conditions of several-μm-thick AlN buffer layers, incorporation into them ultrathin comressively strained GaN layers, and employing both intentionally and spontaneously formed AlGaN-based superlattices. Special attention is paid to threading dislocation filtering mechanism originated from the latter. Metal-rich PA MBE conditions provided atomically smooth (rms~1 nm) morphology of  Al_yGa_1-yN (y=0.4-1) barier layers, while the Al_xGa_1-xN (y-x=0.1-0.15) QWs were formed by sub-monolayer digital alloying technique. The structures emitted within the spectral range of 230-320 nm under both electron-beam and optical pumping. In addition, it was observed that 2D PA MBE growh led to pseudomorphic comressively-strained AlGaN QW active regions over the relaxed AlN buffer layers, which demonstrated TE-polarized spontaneus and stimulated emissions at minimum wavelengths of 254 and 259 nm,  respectively. The compressive stress value achieved in the QWs was found to exceed the calculated elastic stress necessary for preventing the polarization switching from TE to TM mode in the Al_xGa_1-xN QW structures with x>0.7. The paper also discusses peculiarities of stimulated emission in the mid-UV spectral range from 259 to ~303 nm, namely its peak position relative to spontaneous emission spectra, the polarization type and degree, the threshold power densities ranging from several MW/cm² to few hundreds kW/cm², as well as spectral-angular characteristics.

1. PRESENTATION: Plasma-assisted molecular beam epitaxy of AlGaN-based quantum well structures demonstrating spontaneous and stimulated emission in mid-ultraviolet spectral range, PDF document, version 1.4, 0.2MB

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1. Growth of atomically-smooth droplet-free Al_xGa_1-xN (x>0.5) layers by plasma assisted molecular beam epitaxy under strong metal-rich conditions