We report the internal structures and emission properties of GaN/AlN single- and multiple-quantum-well (QW) heterostructures with well widths of dw = 1–4 monolayers (MLs), grown by plasma-assisted molecular-beam epitaxy on c-sapphire at metal-rich conditions and low temperatures (∼700 °C). The formation of plane QWs with abrupt symmetrical interfaces is confirmed by both scanning transmission electron microscopy and X-ray diffraction analysis. Pulse-scanning and continuous-wave output powers of 150 and 28 mW, respectively, at a peak emission wavelength of 235 nm were achieved at 300 K in an electron-beam-pumped deep-ultraviolet (1.5 ML-GaN/5.5 nm-AlN)360 multiple-QW emitter with a maximum efficiency of 0.75%.

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Monolayer (ML)-scale GaN/AlN multiple quantum well (MQW) structures for electron-beam-pumped ultraviolet (UV) emitters are grown on c-sapphire substrates by using plasma-assisted molecular beam epitaxy under controllable metal-rich conditions, which provides the spiral growth of densely packed atomically smooth hillocks without metal droplets. These structures have ML-stepped terrace-like surface topology in the entire QW thickness range from 0.75–7 ML and absence of stress at the well thickness below 2 ML. Satisfactory quantum confinement and mitigating the quantum-confined Stark effect in the stress-free MQW structures enable one to achieve the relatively bright UV cathodoluminescence with a narrow-line (~15 nm) in the sub-250-nm spectral range. The structures with many QWs (up to 400) exhibit the output optical power of ~1 W at 240 nm, when pumped by a standard thermionic-cathode (LaB6) electron gun at an electron energy of 20 keV and a current of 65 mA. This power is increased up to 11.8 W at an average excitation energy of 5 µJ per pulse, generated by the electron gun with a ferroelectric plasma cathode at an electron-beam energy of 12.5 keV and a current of 450 mA.

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.

Phone: þ7 812 292 7124, Fax: þ7 812 297 1017 A 60 mW output power has been achieved in mid-UV (l ¼ 270 nm) spontaneous sources with electron-beam pulsescanning pumping, fabricated from AlGaN MQW heterostructures grown by PA MBE on c-Al 2 O 3 substrates. Under the CW pumping at much lower excitation power density the mid-UV sources demonstrate a 4.7 mW output power. In that regime the power efficiency of the structures is about 0.24%, while their internal quantum efficiency is estimated to be as high as 50%.