OPTOELECTRONIC COMPONENTS: Gallium nitride LEDs shine over visible spectrum

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 drop 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) GaN 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.