650
23 Electricity-to-Light Conversion
(b)
(c)
(d)
(e)
(a)
transparent
metal
mirror
opaque
active
Fig. 23.8 a Standard LED layer sequence with opaque substrate (grey), active layer (checkered) and transparent top,
b thick window design with thick top layer (50–70 µm). c Transparent substrate (by rebonding, see Fig. 23.9, d chip
shaping (cf. also Fig. 23.11). e Thin-film LED with metal mirror (black) and rebonding (cf. also Fig. 23.12)
Fig. 23.9 Comparison of light paths in a GaAsP-based LED with a opaque (GaAs) and b transparent (GaP) substrate
(side facets roughened). Adapted from [1947]
GaAs
GaP
GaAs
GaP window
AlGaInP DH
(a)
substrate
(b)
(c)
AlAs
Fig. 23.10 Scheme of fabrication for red high brightness LED: a AlGaInP double heterostructure (DH) with GaP
window on GaAs substrate (growth with MOCVD). b Lift-off using HF etch of sacrificial AlAs layer. c Wafer bonding
on GaP (transparent for red light)
Transparent Substrate
Reflection of photons is not so detrimental if they are not lost later due to absorption in the substrate.
In Fig. 23.8, the evolution of LED chip design is shown schematically. In Fig. 23.9, the light path is
compared for opaque and transparent substrates. The latter provides higher light extraction efficiency
due to the ‘photon recycling’ effect. Efficiencies of 20–25% are possible. In Fig. 23.10, the technological
steps are shown to fabricate a GaP LED with an AlGaInP active layer. The active layer is initially grown
on GaAs due to lattice-match conditions.
Nonrectangular Chip Geometry
If the chip is made with an inverted structure and mounted on a mirror, a high light extraction efficiency
(> 50%) can be achieved. Typical commercial designs are shown in Fig. 23.11.
The increase in quantum efficiency allows the devices to run on much higher output power. While
initially LEDs delivered power only in the mW regime, now output power in the ∼ 1 W regime is
possible (high brightness LEDs). The higher currents made a redesign of the LED mount towards better
heat sinks necessary (Fig. 23.7b). While the standard case has a thermal resistance of 220 K/W (chip
size (0.25 mm)
2 for 0.05–0.1 W and 0.2–2 lm), the high-power case has 15 K/W (chip size (0.5 mm)
2 for
23 Electricity-to-Light Conversion
(b)
(c)
(d)
(e)
(a)
transparent
metal
mirror
opaque
active
Fig. 23.8 a Standard LED layer sequence with opaque substrate (grey), active layer (checkered) and transparent top,
b thick window design with thick top layer (50–70 µm). c Transparent substrate (by rebonding, see Fig. 23.9, d chip
shaping (cf. also Fig. 23.11). e Thin-film LED with metal mirror (black) and rebonding (cf. also Fig. 23.12)
Fig. 23.9 Comparison of light paths in a GaAsP-based LED with a opaque (GaAs) and b transparent (GaP) substrate
(side facets roughened). Adapted from [1947]
GaAs
GaP
GaAs
GaP window
AlGaInP DH
(a)
substrate
(b)
(c)
AlAs
Fig. 23.10 Scheme of fabrication for red high brightness LED: a AlGaInP double heterostructure (DH) with GaP
window on GaAs substrate (growth with MOCVD). b Lift-off using HF etch of sacrificial AlAs layer. c Wafer bonding
on GaP (transparent for red light)
Transparent Substrate
Reflection of photons is not so detrimental if they are not lost later due to absorption in the substrate.
In Fig. 23.8, the evolution of LED chip design is shown schematically. In Fig. 23.9, the light path is
compared for opaque and transparent substrates. The latter provides higher light extraction efficiency
due to the ‘photon recycling’ effect. Efficiencies of 20–25% are possible. In Fig. 23.10, the technological
steps are shown to fabricate a GaP LED with an AlGaInP active layer. The active layer is initially grown
on GaAs due to lattice-match conditions.
Nonrectangular Chip Geometry
If the chip is made with an inverted structure and mounted on a mirror, a high light extraction efficiency
(> 50%) can be achieved. Typical commercial designs are shown in Fig. 23.11.
The increase in quantum efficiency allows the devices to run on much higher output power. While
initially LEDs delivered power only in the mW regime, now output power in the ∼ 1 W regime is
possible (high brightness LEDs). The higher currents made a redesign of the LED mount towards better
heat sinks necessary (Fig. 23.7b). While the standard case has a thermal resistance of 220 K/W (chip
size (0.25 mm)
2 for 0.05–0.1 W and 0.2–2 lm), the high-power case has 15 K/W (chip size (0.5 mm)
2 for