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%.
Due to their large conduction-band offsets, GaN/AlGaN quantum wells can accommodate intersubband transitions at
record short wavelengths throughout the mid-infrared and into the near-infrared spectral regions. As a result, they are
currently the subject of extensive research efforts aimed at extending the spectral reach and functionality of intersubband
optoelectronic devices. Here we review our recent work in this area, based on GaN/AlGaN quantum-well samples
grown by molecular beam epitaxy on sapphire substrates. In particular, we have investigated the intersubband
absorption properties of a wide range of structures, including isolated and coupled quantum wells. Furthermore, we
have developed a new class of ultrafast all-optical switching devices, based on intersubband cross-absorption saturation
in GaN/AlGaN quantum-well waveguides operating at fiber-optic communication wavelengths. Strong self-phase
modulation of ultrafast optical pulses has also been measured in these waveguides, revealing a large refractive-index
nonlinearity which is related to the same intersubband carrier dynamics. Finally, we have demonstrated optically
pumped intersubband light emission from GaN/AlN quantum wells resonantly excited with a pulsed OPO. The
measured room-temperature output spectra are peaked near 2 μm, which represents a new record for the shortest
intersubband emission wavelength from any quantum-well materials system.
Nitride semiconductor quantum structures feature some unique properties for intersubband device development,
including a record large conduction-band offset that allows extending the operating wavelength to the near-infrared
spectral region, and large optical phonon energies that are advantageous for the development of THz devices. In this
paper we review our recent work aimed at the demonstration of novel intersubband device functionalities using these
materials. In particular, we have developed ultrafast all-optical switching devices operating at fiber-optic
communication wavelengths, based on intersubband cross-absorption saturation in GaN/AlN quantum-well waveguides.
Strong self-phase modulation of ultrafast optical pulses has also been measured in these waveguides, revealing a large
intersubband refractive-index nonlinearity which is also promising for all-optical switching applications. Furthermore,
we have demonstrated optically pumped intersubband light emission from GaN/AlN quantum wells at the record short
wavelength of about 2 μm. Finally, we have used a rigorous Monte Carlo model to show that GaN/AlGaN quantum
wells are promising for the development of THz quantum cascade lasers capable in principle of operation without
cryogenic cooling.
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