A. Lunev

AlGaN Deep-Ultraviolet Light-Emitting Diodes with External Quantum Efficiency above 10%

Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 10.4% at 20 mA CW current with the output power up to 9.3 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates.

Selective area deposited blue GaN–InGaN multiple-quantum well light emitting diodes over silicon substrates

We report on fabrication and characterization of blue GaN–InGaN multi-quantum well (MQW) light-emitting diodes (LEDs) over (111) silicon substrates. Device epilayers were fabricated using unique combination of molecular beam epitaxy and low-pressure metalorganic chemical vapor deposition growth procedure in selective areas defined by openings in a SiO2 mask over the substrates. This selective area deposition procedure in principle can produce multicolor devices using a very simple fabrication procedure. The LEDs had a peak emission wavelength of 465 nm with a full width at half maximum of 40 nm. We also present the spectral emission data with the diodes operating up to 250 °C. The peak emission wavelengths are measured as a function of both dc and pulse bias current and plate temperature to estimate the thermal impedance.

SiO2-passivated lateral-geometry GaN transparent Schottky-barrier detectors

We report on a transparent Schottky-barrier ultraviolet detector on GaN layers over sapphire substrates. Using SiO2 surface passivation, reverse leakage currents were reduced to a value as low as 1 pA at 5 V reverse bias for 200 μm diameter device. The device exhibits a high internal gain, about 50, at low forward biases. The response time (about 15 ns) is RC limited, even in the internal gain regime. A record low level of the noise spectral density, 5×10−23 A2/Hz, was measured at 10 Hz. We attribute this low noise level to the reduced reverse leakage current.

AlGaN-based 280nm light-emitting diodes with continuous-wave power exceeding 1mW at 25mA

Optimization of the migration-enhanced metalorganic chemical vapor deposition and further optimization of the contact and active layer design for 280nm light-emitting diodes resulted in large improvement of cw and pulsed output power and in a superior spectrum purity. The ratio of the main peak to the background luminescence determined by the detection system is higher than 2000:1 at 20mA dc. The on-wafer cw power was measured to be 255μW at 20mA dc. The power popped up exceeding 1mW for a packaged device under 25mA dc and 9mW under pulse 200mA. The maximum wall-plug-efficiency of 0.67% was obtained for the packaged device at 25mA dc.

High power AlGaN ultraviolet light emitters

We present the analysis of the external quantum efficiency in AlGaN deep ultraviolet (DUV) light-emitting diodes (LEDs) on sapphire substrates and discuss factors affecting the output power of DUV LEDs. Performance of the LED is related to optimization of the device structure design and improvements of the epitaxial material quality.

AlGaN Deep-Ultraviolet Light-Emitting Diodes

We report on the development of AlGaN based deep ultraviolet (DUV) light-emitting diodes (LEDs) grown by migration-enhanced metalorganic chemical vapor deposition (MEMOCVD). Improved quality of AlGaN has allowed us to achieve milliwatt-power at wavelengths ranging from 365 to 265 nm. For 295 and 280 nm LEDs, record CW powers with wall-plug-efficiency approaching 1.0% were realized. The CW power reached 1.2 and 1.0 mW at 20 mA for 280 and 295 nm LEDs, respectively. A multiple-chip package (UV lamp) emitted CW power of 11 mW at the wavelength of 280 nm. Under pulse operation, the 280 nm UV lamp produced power as high as 56 mW. The CW power levels at 20 mA were 0.5, 0.25 and 0.15 mW for a single-chip 275, 270 and 265 nm LEDs, respectively. A 265 nm UV lamp exhibited a record high CW power exceeding 1.5 mW. The applications of these DUV LEDs in bio-agents detection have been demonstrated and the preliminary results will be presented.

Efficiency droop in 245-247 nm AlGaN light-emitting diodes with continuous wave 2 mW output power

We report on 245–247 nm AlGaN-based deep ultraviolet (DUV) light-emitting diodes with continuous wave output power up to 2 mW. DUV diodes with peak emission wavelength of 245 and 247 nm exhibit turn-on voltage less than 10 V. At room temperature and cw operation the maximum external quantum efficiency was close to 0.18%, which is the highest value published to date for devices with peak emission wavelength shorter than 250 nm. A large external efficiency droop observed at current densities above 100 A/cm2 is attributed to self-heating, carrier spillover from the QWs into the barrier layers or the p-type cladding layer, and/or Auger recombination. A semiempirical equation was proposed to describe the efficiency droop in DUV diodes at a high current injection.

High-quality p–n junctions with quaternary AlInGaN/InGaN quantum wells

We report on quaternary AlInGaN/InGaN multiple quantum well (MQW) light emitting diode structures grown on sapphire substrates. The structures demonstrate high quality of the p–n junctions with quaternary MQW. At low forward bias (below 2 V), the temperature dependent of current–voltage characteristics are exponential with the ideality factor of 2.28, which is in a good agreement with the model of the injected carrier recombination in the space charge region. This ideality factor value is approximately three times lower than for conventional GaN/InGaN light emitting diodes (LEDs). The obtained data indicate the recombination in p–n junction space charge region to be responsible for a current transport in LED structures with quaternary quantum wells. This is in contrast to InGaN based LEDs, where carrier tunneling dominates either because of high doping of the active layer or due to the high density of localized states.

Migration enhanced lateral epitaxial overgrowth of AlN and AlGaN for high reliability deep ultraviolet light emitting diodes

We report on the growth of low-defect thick films of AlN and AlGaN on trenched AlGaN/sapphire templates using migration enhanced lateral epitaxial overgrowth. Incoherent coalescence-related defects were alleviated by controlling the tilt angle of growth fronts and by allowing Al adatoms sufficient residence time to incorporate at the most energetically favorable lattice sites. Deep ultraviolet light emitting diode structures (310nm) deposited over fully coalesced thick AlN films exhibited cw output power of 1.6mW at 50mA current with extrapolated lifetime in excess of 5000hours. The results demonstrate substantial improvement in the device lifetime, primarily due to the reduced density of growth defects.

10 Milliwatt Pulse Operation of 265 nm AlGaN Light Emitting Diodes

We report on the development of solid-state deep ultraviolet light sources optimized for the germicidal applications. Pulsed power levels in excess of 10 mW were achieved for AlGaN based 265 nm light emitting diodes by improving the material quality using Migration-Enhanced Metal Organic Chemical Vapor Deposition. Packaged devices reached the continuous-wave power of 237 µW at 30 mA and a pulse power exceeding 10 mW for 1.2 A driving current.