History of AlGaInP visible-spectrum LEDs
The AlGaInP material system is suited for high-brightness emission in the red (625 nm), orange (610 nm) and yellow (590 nm) spectral range and today is the dominant material system for high-brightness emitters in that wavelength range. Figure 1.13 shows some of the common signage applications of red and yellow AlGaInP LEDs.
The AlGaInP material system was first developed in Japan for visible-spectrum lasers (Kobayashi et al., 1985; Ohba et al., 1986; Ikeda et al., 1986; Itaya et al., 1990). Efforts started with AlGaInP/GaInP double-heterostructure lasers using Ga0.5In0.5P as the active material, which is lattice matched to GaAs substrates. The bandgap energy of lattice-matched GaInP is approximately 1.9 eV (650 nm), making the material suitable for visible lasers emitting in the red. These lasers are used, for example, in laser pointers and in digital video disc (DVD) players.
The addition of Al to the GaInP active region allows one to attain shorter emission wavelengths including the orange and yellow spectral region. However, (AlxGa1-x)05In05P becomes an indirect semiconductor at Al compositions of x « 0.53, so that the radiative efficiency strongly decreases at wavelengths near and, in particular, below 600 nm. Consequently, AlGaInP is not suited for high-efficiency emission at wavelengths below 570 nm.
Subsequent to the AlGaInP laser development that occurred in the early 1980s, AlGaInP LED development started at the end of the 1980s (Kuo et al., 1990; Fletcher et al., 1991; Sugawara et al., 1991). In contrast to the AlGaInP laser structures, the LED structures typically employ current-spreading layers so that the entire p-n junction plane of the LED chip lights up and not just the region below the top ohmic contact. Further improvements were attained by using multiple quantum well (MQW) active regions (Huang and Chen, 1997), coherently strained MQW active regions (Chang and Chang, 1998a, 1998b), distributed Bragg reflectors (Huang and Chen, 1997; Chang et al., 1997), transparent GaP substrate technology (Kish and Fletcher, 1997), and chip-shaping (Krames et al., 1999). Comprehensive reviews of the AlGaInP material system and AlGaInP LEDs were published by Stringfellow and Craford (1997), Mueller (2000), and Krames et al., 2002).