Visible-spectrum (AlxGa1-x)05In05P LEDs with typical operating wavelengths of 560-660 nm are grown lattice matched on GaAs substrates. Since the energy gap of GaAs is Eg = 1.424 eV
(Xg = 870 nm) at room temperature, GaAs substrates are absorbing at these emission wavelengths. As a result, the light emitted towards the substrate will be absorbed by the thick GaAs substrate. Thus, the extraction efficiency of AlGalnP/GaAs LEDs grown on GaAs substrates is low.

The extraction efficiency of AlGaInP LEDs can be increased substantially by removal of the GaAs substrate and bonding of the epitaxial layer to a GaP substrate (Kish et al., 1994). GaP is an indirect-gap semiconductor with Eg = 2.24 eV (Xg = 553 nm). Thus, GaP does not absorb light with X > 553 nm emitted by the AlGaInP active region.

The fabrication process of the AlGaInP LEDs wafer-bonded to a GaP substrate is illustrated schematically in Fig. 9.12. The AlGaInP double heterostructure (DH) is initially grown by OMVPE on a GaAs substrate. Subsequently, a thick GaP window layer (~ 50 ^m) is grown on the DH by chloride VPE, a growth technique that allows for the low-cost growth of thick epitaxial layers. The GaAs substrate is then removed using a wet chemical selective etching process (Adachi and Oe, 1983; Kish et al., 1994). During the removal of the GaAs substrate, the thick GaP window layer serves as a temporary mechanical support for the DH. The DH with the GaP window is then wafer-bonded to a GaP substrate.

Wafer-bonding processes require a high degree of cleanliness, the lack of any particles between the wafers, and the absence of native surface oxides on both wafers. Frequently, the gap between two wafers is filled with a contact liquid. Rotating the wafers at a high rate spins out the contact liquid. Kish et al. (1995) and Hoefler et al. (1996) reported an AlGaInP-to-GaP wafer-
bonding process, suitable for 50 mm (2 inch) GaP substrates. Uniaxial pressure and elevated temperatures (750-1000 °C) are used in this process (Hoefler et al., 1996). Kish et al. (1995) showed that the achievement of low-resistance ohmic conduction across wafer-bonded interfaces is critically dependent upon the crystallographic alignment of the bonded wafer surfaces, irrespective of the lattice mismatch between the surfaces. Furthermore, Kish et al. (1995) showed that the crystallographic surface orientation of the bonded surfaces must be nominally matched while simultaneously maintaining rotational alignment of the wafers. Low diode forward voltages of 2.2 V for AlGaInP/GaP LEDs are routinely achieved under high-volume manufacturing conditions with the process. The reliability of wafer-bonded LEDs is comparable to monolithic AlGaInP/GaAs LEDs. Usually the technical details of wafer-bonding processes are proprietary and not known to the general public.

doping concentration in the GaP substrate is kept moderately low, to minimize free-carrier absorption.