Plasma-assisted molecular beam epitaxy of Al(Ga)N layers and quantum well structures for optically pumped mid-UV lasers on c-Al₂O₃

Abstract

This paper reports on novel approaches developed for plasma-assisted molecular beam epitaxy of Al-rich AlGaN epilayers and quantum well heterostructures on c-sapphire, which allowed us to fabricate low-threshold optically-pumped separate confinement heterostructure lasers emitting in the mid-UV spectral range (258-290 nm) with the threshold power density below 600 kW cm −2 . The optimum buffer structure has been developed which provides lowering the near-surface threading dislocation density down to 1.5 × 10 8 and 3 × 10 9 cm −2 for screw and edge types, respectively, and improving the surface morphology (rms < 0.7 nm at the area of 3 × 3 μm −2 ). It comprises the high-temperature (780 °C) migration enhanced epitaxy growth of a (30-70) nm thick AlN nucleation layer on c-Al 2 O 3 , followed by a 2 μm thick AlN buffer grown under the metal-rich conditions in the Al-flux modulation mode and containing several (up to 6) ultra-thin (∼3 nm) GaN interlayers grown at N-rich conditions. Proper strain engineering in AlGaN single quantum well heterostructure grown atop of the AlN buffer layer enables one to preserve dominant TE polarization of both spontaneous and stimulated emission even at shortest obtained wavelength (258 nm). The threshold power density of stimulated emission as low as 150 kW cm −2 at 289 nm for a single quantum well laser structure has been demonstrated.

Demonstration of transverse-magnetic deep-ultraviolet stimulated emission from AlGaN multiple-quantum-well lasers grown on a sapphire substrate

“…8 The stimulated emission peak at 239 nm is the shortest RT lasing wavelength yet reported for AlGaN DUV lasers grown on foreign substrates including sapphire and SiC. 8,11,12 In addition, we have not observed a clear trend, where the thresholds are strongly correlated to the cavity length. The small variation of thresholds can be affected by the on-wafer variations of dislocation density, thickness, composition, and strain as well as facet condition.…”

“…249 nm. 8,11,12 Hence, there has been no observation of dominant TMpolarized stimulated emission for AlGaN DUV lasers grown on AlN or sapphire substrates to date. There was one report for the optical polarization of stimulated emission from a photo-pumped AlGaN DUV laser grown on a c-plane SiC substrate, showing TM-dominance at 240.8 nm.…”

“…In other words, the HH band and CH band crossed over for the laser structure employed in this work. Figure 5 summarizes the reported above-threshold polarization degree, q, of DUV lasers grown on AlN, 1-6 sapphire, 8,11,12 and SiC 10 substrates. The polarization of laser grown on the SiC substrate was TM-dominant at 240.8 nm.…”

Pulsed growth techniques in plasma-assisted molecular beam epitaxy of Al Ga1−N layers with medium Al content (x=0.4–0.6)

“…All AlGaN films were grown by using PA MBE setup Riber21T on c-Al 2 O 3 substrates on top of a 500-nm-thick AlN/GaN buffer structures designed and grown as described earlier [14]. AlGaN layers were grown by both standard and pulsed techniques including MME and DETA ones.…”

AlGaN Nanostructures with Extremely High Room-Temperature Internal Quantum Efficiency of Emission Below 300 nm

“…The typical density of threading dislocations in thus-grown structures was previously estimated as $10 9 cm À2 . [22][23][24] The active region of the structure was grown pseudomorphically under metal-rich conditions. It consisted of a 32-nm-thick Al 0.7 Ga 0.3 N barrier, a 1.5-nm-thick Al 0.4 Ga 0.7 N QW, a 65-nm-thick Al 0.7 fi 1 Ga 0.3 fi 0 N graded barrier layer, and 10-nm-thick AlN capping.…”