AlGaN-based Light-emitting Diodes Research Articles

Characteristics of a Non-Phosphor White LED Grown by Using Mixed-Source HVPE

The growth of an AlGaN-based non-phosphor white light emitting diode (LED) on a templated n-GaN (0001) sapphire substrate is performed by using mixed-source hydride vapor phase epitaxy (HVPE) with a multi-sliding boat. The DH (doublehetero) structure is grown by using a selective area growth (SAG) method. The non-phosphor white LED structure consists of a Te-doped AlGaN cladding layer, an AlGaN active layer, a Mg-doped AlGaN cladding layer and a Mg-doped GaN capping layer. The gas ow rates of HCl and NH3 are maintained at 20 sccm and 800 sccm, respectively. The growth temperatures of the source zone and the growth zone are 900 C and 1090 C, respectively. The electroluminescence (EL) main peak of the AlGaN-based white LED is observed at 400 nm and minor peaks are emitted in a broad spectrum from 450 nm to 600 nm. A combination of blue and broadband yellow peaks creates a white light. The non-phosphor white LED has a measured color rendering index (CRI) from 76 to 87. These results show mixed-source HVPE with a multi-sliding boat can be used to fabricate a non-phosphor white LED. Additionally, optical module measurements and X-ray di raction (XRD) veri ed the optical and the crystal quality of the white LED.

230-270nm深紫外AlGaN系LEDの進展

We demonstrated AlGaN multi-quantum well (MQW) deep-ultraviolet (UV) light-emitting diodes (LEDs) with wavelengths in the range of 227.5 to 273 nm fabricated on high-quality AlN buffers on sapphire substrates grown by metal-organic chemical vapor deposition (MOCVD). We realized crack-free, thick AlN buffers on sapphire with a low threading dislocation density (TDD) and an atomically flat surface by using the ammonia (NH3) pulse-flow multilayer (ML) growth technique. We obtained single-peaked operation of an AlGaN-MQW LED with a wavelength of 227.5 nm, which is the shortest wavelength of AlGaN-based LED on sapphire. The maximum output power and the external quantum efficiency (EQE) of the 261- and 227.5-nm LEDs were 1.65 mW and 0.23% in room-temperature (RT) continuous-wave (CW) operation, and 0.15 mW and 0.2% in RT pulsed operation, respectively. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(3): 24–33, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10197

High-efficiency AlGaN-based UV light-emitting diode on laterally overgrown AlN

Ultraviolet light-emitting diodes (UV-LEDs) with a peak wavelength of 335 nm were fabricated on AlN/sapphire templates. As templates for the fabrication of UV-LEDs, planar AlN and epitaxial laterally overgrown (ELO) AlN on sapphire (0 0 0 1) substrates were compared. The output power of UV-LEDs grown on ELO-AlN was 27 times higher than that of UV-LEDs on the planar AlN template.

231–261nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphire

The authors demonstrated AlGaN multiquantum-well (MQW) deep-ultraviolet light-emitting diodes (LEDs) with wavelengths in the range of 231–261nm, fabricated on low threading dislocation density AlN buffers formed through an ammonia (NH3) pulse-flow multilayer growth technique. The authors obtained a single-peaked operation of the AlGaN-MQW LED with a wavelength of 231nm, which is the shortest wavelength of AlGaN-based LED on sapphire. The maximum output power and external quantum efficiency of the 261 and 231nm LEDs were 1.65mW and 0.23% under room-temperature (RT) continuous-wave (cw) operation, and approximately 5μW and 0.001% under RT pulsed operation, respectively.

Quaternary InAlGaN-based high-efficiency ultraviolet light-emitting diodes

In order to realize 250–350-nm-band high-efficiency deep ultraviolet (UV) emitting devices using group-III-nitride materials, it is necessary to obtain high-efficiency UV emission from wide-band-gap (In)AlGaN. The use of the In-segregation effect, which has already been used for InGaN blue emitting devices, is quite effective for achieving high-efficiency deep UV emission. We have demonstrated high-efficiency UV emission from quaternary InAlGaN-based quantum wells in the wavelength range between 290 and 375 nm at room temperature (RT) using the In-segregation effect. Emission fluctuations in the submicron region due to In segregation were clearly observed for quaternary InAlGaN epitaxial layers. An internal quantum efficiency as high as 15% was estimated for a quaternary InAlGaN-based single quantum well at RT. Such high-efficiency UV emission can even be obtained on high threading-dislocation density buffer layers. A comparison of electroluminescence is made between light-emitting diodes (LEDs) with InAlGaN, AlGaN, and GaN active regions fabricated on SiC substrates with emission wavelengths between 340 and 360 nm. The emission intensity from the quaternary InAlGaN UV-LED was more than one order of magnitude higher than that from the AlGaN or GaN UV-LEDs under RT cw operation. We therefore fabricated 310–350-nm-band deep UV-LEDs with quaternary InAlGaN active regions. We achieved submilliwatt output power under RT pulsed operation for 308–314-nm LEDs. We also demonstrated a high output power of 7.4 mW from a 352-nm quaternary InAlGaN-based LED fabricated on a GaN substrate under RT cw operation. The maximum external quantum efficiency (EQE) of the 352-nm InAlGaN-based LED was higher than that obtained for an AlGaN-based LED with the same geometry. From these results, the advantages of the use of quaternary InAlGaN in 350-nm-band UV emitters were revealed.

High‐efficiency 350 nm‐band quaternary InAlGaN‐based UV‐LED on GaN/sapphire template

We have demonstrated high-efficiency ultraviolet (UV) light-emitting diode (LED) with emission wavelength at 349 nm using quaternary InAlGaN multiple-quantum-well (MQW) fabricated on a GaN/sapphire template. The threading dislocation density of the GaN/sapphire template was approximately 1 × 109 cm–2. The maximum UV output power obtained was as high as 4.1 mW with an injection current of 160 mA under room temperature (RT) CW operation. The maximum external quantum efficiency (EQE) was 1.02% with an injection current of 60 mA, which is higher than the EQE obtained for a 350 nm-band AlGaN-based QW LED fabricated on GaN substrate. From these result, we confirmed the advantage of the use of quaternary InAlGaN for 350 nm-band UV emitters in comparison with the use of AlGaN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 deep ultraviolet light-emitting diodes basedon a micro-pixel design

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.

AlGaN -based 280nm light-emitting diodes with continuous wave powers in excess of 1.5mW

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.

Better Writing and Reading

APPLIED OPTICS Reducing the operating wavelength of laser diodes is desirable for a number of applications. Shorter wavelength enables the writing and reading of optical information at higher storage densities, as seen in the development of CD and DVD players over the past decade, in which the wavelength has decreased from infrared to blue. In addition, ultraviolet (UV) radiation at wavelengths below 390 nm can be used in fluorescence-based chemical and biological detection schemes. The development of suitable materials that can be electrically driven and operated at ambient temperatures has, however, presented a significant challenge. Providing an addition to the small selection of wide-bandgap materials, such as diamond, that emit in the UV, and one that the wafer growers may find more attractive to work with, Fischer et al. present results on UV-emitting AlGaN-based light-emitting diodes. Their devices can operate at room temperature and provide around a milliwatt of 290-nm UV light when biased at around 10 V and with an injection current of 300 mA. — ISO Appl. Phys. Lett. 84 , 3394 (2004).

Indium‐free violet LEDs grown by HVPE

We report on first demonstration of violet light emitting diodes (LED) based on AlGaN/GaN/AlGaN heterostructures grown by hydride vapor phase epitaxy (HVPE). The unique aspects of this technological approach are (i) growth of Al-containing epitaxial material by HVPE and (ii) use of HVPE to fabricate submicron multi-layer epitaxial structures. The LEDs provide light emission at the wavelength of 415–420 nm that did not shift with forward current. External efficiency up to 2.5% is reached at the current of 20 mA. The brightness of LED lamp is as high as 400–500 mcd. This suggests HVPE as an alternative technique for growing AlGaN-based LED structures. Results of the LED modeling and characterization are discussed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-resolved electroluminescence of AlGaN-based light-emitting diodes with emission at 285 nm

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.

Submilliwatt operation of AlGaN-based ultraviolet light-emitting diode using short-period alloy superlattice

Over 0.1 mW ultraviolet output was achieved by an AlGaN-based light-emitting diode. To realize a highly conductive and ultraviolet-transparent layer, a short-period alloy superlattice was introduced. The device was fabricated on SiC substrate. Low electric resistivity due to the short-period alloy superlattice and the high thermal conductivity of the SiC substrate enabled large current injection of up to 1.7 kA/cm2. The emission was monochromatic band-edge emission about 350 nm in wavelength without significant D–A and/or deep emissions.