OPTOELECTRONIC COMPONENTS

Progress made with gallium nitride (GaN) based light-emitting diodes (LEDs) and lasers was discussed at the Materials Research Society`s first international symposium on these and related materials held as part of its fall meeting in November in Boston, MA.

Jan 1st, 1996

OPTOELECTRONIC COMPONENTS

Gallium nitride LEDs shine over visible spectrum

Rick DeMei

Progress made with gallium nitride (GaN) based light-emitting diodes (LEDs) and lasers was discussed at the Materials Research Society`s first international symposium on these and related materials held as part of its fall meeting in November in Boston, MA.

George Craford of the Hewlett-Packard Optoelectronics Division (San Jose, CA) said that, of the $4.4 billion 1994 compound-semiconductor market, visible-wavelength LEDs accounted for 33% of this business and infrared LEDs for 26%. The possibility of a spectrum of GaN quantum-well LEDs with eventual prospects of high-efficiency (more than 30%) operation and narrow emission peaks will enable use in true-color outdoor displays, traffic signs, and vehicle exterior and dash lighting.

Advantages of long-life LEDs will include power-consumption savings and elimination of replacement lighting-element costs. But even after successful development of these devices, Craford says, widespread use will not be realized until current costs of $2-$5 per LED dro¥closer to that of common low-performance red LEDs, which run $0.05-$0.10 per unit.

Advanced LEDs were reviewed by Shuji Nakamura from Nichia Chemical Industries (Tokushima, Japan) in an update on the company`s research on single-quantum-well LEDs. These devices feature a roughly 30-Å InGaN active layer, grown by metal-organic chemical-vapor deposition on a sapphire substrate, which facilitates high-power output from blue to yellow wavelengths with narrow emission spectra (see photo). Nakamura says the LED lifetimes are about 106 hours.

Blue quantum-well (QW) LEDs have a peak wavelength of 450 nm and a full width at half maximum (FWHM) of 20 nm. This compares favorably to current double-heterostructure InGaN/ AlGaN LEDs having high power (more than 1 mW) that is spread over a much broader 70-nm FWHM. For a forward current of 20 mA, blue QW devices produce 5-mW output power and a luminous intensity of 2 cd at an efficiency of 9.1%. A typical green QW LED peaks at 520 nm with a FWHM of 30 nm. For the same forward current as the blue, the green LED produces 3 mW and 12 cd at an efficiency of 6.3%. The luminous intensity is two orders of magnitude higher than for current yellow-green (555 nm) Ga¥LEDs that produce about 0.1 cd. Performance falls off, however, for the yellow QW LEDs, which peak at 590 nm with 50 nm FWHM and produce 1 mW at 20 mA with an efficiency of only around 1%.

Researchers from Meijo University (Nagoya, Japan) reported on stimulated emission of AlGaN/GaInN double heterostructures by optical pumping at room temperature. A silicon carbide substrate allows a 40-kW/cm2 threshold, which the investigators say is the lowest thus far for nitrides, being two to four times lower than the emission threshold with a sapphire substrate. They also claim to have observed lasing action which, however, currently lacks reproducibility.

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