We present a study of the electrical and optical characteristics of 280 nm emission deep ultraviolet light emitting diodes (LED) at room and cryogenic temperatures. At low bias the defect assisted carrier tunneling primarily determines the current conduction. The room-temperature spectral performance and optical power are limited mostly by pronounced deep level defect assisted radiative and non-radiative recombination as well as poor electron confinement in the active region. At temperatures below 100 K the electroluminescence peak intensity increases by more than one order of magnitude due to suppression of non-radiative recombination channels indicating that with a proper device design and improved material quality, milliwatt power 280 nm LED are viable.

We report blue-purple pn-junction light-emitting diodes (LEDs) with a-plane GaN–InGaN multiple quantum well active region. The LEDs were grown over r-plane sapphire substrates. Our study has shown the low pump intensity photoluminencence and electroluminescence to be dominated by emission from the band-tail states which then saturates rapidly giving rise to band-edge emission.

We report on AlGaN-based light-emitting diodes over sapphire with peak emission at 280nm. A modified active layer structure consisting of four multiple quantum wells, addition of an electron blocking magnesium doped p-AlGaN layer, improved contacts along with flip-chip packaging resulted in a cw power of 0.7mW at 230mA for a single 200μm×200μm device. Flip-chipping four 100μm×100μm devices in a parallel configuration improved the dc saturation current and enabled us to obtain a cw power of 1.53mW (at 450mA) and a pulse power as high as 24mW (at 1.5A). These powers translate to values of 0.36% and 0.12% for the external quantum efficiency and the wall plug efficiency.

We report on 269 nm emission deep ultraviolet light-emitting diodes (LEDs) over sapphire. The material quality, device design, and contact processing sequence yielded devices with external quantum efficiencies as high as 0.4% for a pumped pulse current of 200 mA and 0.32% for a dc pump current of 10 mA. For a module of two LEDs connected in series, a record continuous-wave power of 0.85 mW (at 40 mA) and a wall plug efficiency of 0.16% (at 10 mA dc) were measured.

Using a micro-pixel design, we report the demonstration of high-power deep UV AlGaN-based light-emitting diodes (LEDs) with peak emission wavelength at 280nm. The design comes in response to lateral current crowding problems, which severely limit the maximum possible active area and the overall performance of ordinary square geometry III-nitride LEDs fabricated on insulating substrates. It is shown that the interconnected micro-pixel geometry significantly reduces both the device series resistance and the thermal impedance, thereby improving heat dissipation and increasing the maximum optical power. The design imparts ever-increasing advantages as the operating wavelength decreases (and the aluminum content increases). The optical power of the 10×10pixel array with an effective area of 222×222μm2 only saturates at dc currents higher than 200mA, which is nearly 50% greater than found for a square geometry LED with identical junction area, fabricated from the same wafer. These 280nm LEDs demonstrated a high on-wafer cw power of 145μW with 200mA of pumping current.

We report on AlGaN multiple-quantum-well light-emitting diodes over sapphire with peak emission at 325 nm. A pulsed-atomic-layer-epitaxy growth process was used to improve the material quality of the AlN buffer and the AlN/AlGaN strain-relief layers for reducing the nonradiative recombination. In addition, a modified device epilayer structure was used to improve the carrier confinement and the hole injection. A 40% improvement of external quantum efficiency is obtained, resulting in record high optical powers of 10.2 mW at a pulsed pump current of 1 A. (C) 2003 American Institute of Physics.

We report on a deep UV light-emitting diode over sapphire substrate with AlGaN multiple-quantum-well active region. Pulsed atomic-layer epitaxy deposited low-defect AlN/AlGaN buffers and an optimized active layer design yielded a sharp quantum-well emission peak at 287 nm and very little long-wave secondary emission. For a 100 μm×100 μm unpackaged device, a power of 27 μW at 20 mA dc and a peak external quantum efficiency of 0.1% at 100 mA pulse pumping were measured. Flip-chip packaging should increase these numbers nearly by a factor of 3.

We present a study on the time evolution of the electroluminescence (EL) spectra of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) under pulsed current pumping. The EL spectra peaks at 285 nm and 330 nm are found to result from recombination involving band-to-band and free carriers to deep acceptor level transitions. The 330 nm long-wavelength transitions to deep acceptor levels in the p-AlGaN layer as well as the nonradiative processes significantly influence the LED internal quantum efficiency.

Flip-chip 325 nm emission LEDs over sapphire with powers of 0.84 and 6.68 mW at 180 mA DC and 1 A pulsed pump currents are reported. A thermal management study shows the DC output power to be limited by the package heat dissipation.

We report on a study of current crowding in AlInGaN multiple quantum well based deep ultra-violet light emitting diodes. For lateral geometry devices on sapphire substrates, our study concludes that the thickness and doping level of the high Al-content buffer and the cladding n-AlGaN layers is a key contributor to the lateral resistance and hence current crowding at the mesa edges. The inhomogeneous pumping of large area devices results in increased differential resistance causing a pronounced localized overheating. This degrades the device performance under high dc current operation. We also show stripe geometry to be a better choice for high power deep ultra-violet light emitting diodes on sapphire.

We report on AlGaN single-quantum-well light-emitting diodes (LEDs) on sapphire with peak emission at 285 nm. A study is presented to identify the key material parameters controlling the device quantum efficiency. At room temperature, for a 200 μm×200 μm square geometry mesa type device, we obtain a power as high as 0.25 mW for 650 mA pulsed pumping. The LEDs show significantly higher output powers at temperatures below 100 K.

We present a study of the electrical and optical characteristics of 285 nm emission deep ultraviolet light-emitting diodes (LED) at temperatures from 10 to 300 K. At low bias, our data show the tunneling carrier transport to be the dominant conduction mechanism. The room-temperature performance is shown to be limited mostly by poor electron confinement in the active region and a pronounced deep level assisted recombination but not by the hole injection into the active region. At temperatures below 100 K, the electroluminescence peak intensity increases by more than one order of magnitude indicating that with a proper device design and improved material quality, milliwatt power 285 nm LED are viable.

We report on AlGaN multiple-quantum-well (MQW)-based deep ultraviolet light-emitting diodes over sapphire with peak emission at 278 nm. A new buffer layer growth process was used to reduce the number of defects and hence the nonradiative recombination. The improved material quality and carrier confinement resulted in pulsed powers as high as 3 mW at 278 nm and a significantly reduced deep-level-assisted long-wavelength emission.

We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement.