Adam Moldawer

Development of AlGaN-based graded-index-separate-confinement-heterostructure deep UV emitters by molecular beam epitaxy

The authors report on the growth, structure, and emission properties of AlGaN double heterostructures having a graded-index-separate-confinement-heterostructure design. These devices were grown on the Si-face of 6H-SiC substrates by plasma-assisted molecular-beam epitaxy. The active region of the device consists of 75-nm thick Al0.72Ga0.28N film, confined by two 50-nm thick compositionally graded AlxGa1−xN films (x = 1–0.8 and x = 0.8–1) and two AlN cladding layers. X-ray diffraction and transmission electron microscopy provide evidence that the compositionally graded AlGaN layer may also be serving as a strain transition buffer, by blocking threading defects in the vicinity of the AlN/AlGaN heterointerface. Polarization dependent photoluminescence studies indicate that the emission from these structures at 257 nm is transverse magnetic polarized. Simulation studies indicate that the vertical confinement of the optical mode in these structures is 32.5% and simulations of the band structure indicate the formation of a p-n junction resulting from polarization-induced doping. Electron-beam pumping of these structures provides evidence of the onset of stimulated emission at room temperature.

Molecular beam epitaxy growth of AlGaN quantum wells on 6H-SiC substrates with high internal quantum efficiency

The authors report the development of high internal quantum efficiency AlN/AlGaN/AlN double heterostructures and AlGaN/AlN multiple quantum wells (MQWs) grown on 6H-SiC and 4H-SiC substrates of various miscuts by plasma-assisted molecular-beam epitaxy. The authors find that the luminescence spectra for identical MQWs show a single peak across the gap, with a wavelength that is redshifted by ∼20 nm as the excess Ga during growth of the wells increases. The internal quantum efficiency of the double heterostructures emitting at 250 nm is found to be 43%, and that of the multiple quantum wells emitting at 245 nm is 68%. These results suggest that AlGaN alloys on SiC substrates are capable of producing deep-ultraviolet emitters with high efficiency. The authors propose that these results can be accounted for by the introduction of lateral band structure potential fluctuations due to the changing of the growth mode from physical vapor phase epitaxy to liquid phase epitaxy (LPE) as the excess gallium increases. ...

Deep UV-LEDs with high IQE based on AlGaN alloys with strong band structure potential fluctuations

In this paper we review our progress in developing AlGaN-based deep UV LEDs with internal quantum efficiency (IQE) in excess of 50%. This is accomplished by growing the active region of the LEDs by plasma-assisted MBE under a growth mode which promotes the introduction of deep band structure potential fluctuations in the wells beyond the statistical ones due alloy disorder. AlGaN-based deep UV-LEDs emitting in the wavelength range from 320 nm to 265 were grown by this method and fabricated into devices. By combining high IQE AlGaN QWs in the active region with polarization field enhanced carrier injection layers, unpackaged deep UV-LEDs emitting at 295 nm and 273 nm were obtained with optical output power of 0.35 mW and 1.8 mW at 20 mA continuous wave and 100 mA pulsed drive current, respectively. The maximum external quantum efficiency of these devices was calculated to be 0.4%, a result consistent with the low extraction efficiency of only 1-2%.

Development of Milliwatt Power AlGaN-based Deep UV-LEDs by Plasma-assisted Molecular Beam Epitaxy

In this paper, we report the development of AlGaN-based deep ultraviolet LEDs by rf plasma-assisted molecular beam epitaxy (MBE) emitting between 277 and 300 nm. Some of these devices were evaluated after fabrication at bare-die and some at wafer-level configurations. Devices with total optical output of 1.3 mW at injection current of 200 mA were produced, with maximum external quantum efficiency (EQE) of 0.16%. These performance values are equivalent to those reported for deep UV-LEDs grown by the Metalorganic chemical vapor deposition (MOCVD) method and measured at bare-die configuration. In parallel, we have evaluated the internal quantum efficiency (IQE) of AlGaN quantum wells, and found that such wells emitting at 250 nm have an IQE of 50%. From the analysis of these data, we concluded that the efficiency of deep UV LEDs is not limited by the IQE but by the light extraction efficiency, injection efficiency or a combination of both.