Basics
Crystal Structures
Junctions
Finite Barrier
Radiative Recombination
Density of States
Lasers
Photonic Crystals
 Singe Crystal Growth
Contacts

Figure 1. Liquid Encapsulated Czochrazski crystal pulling chamber

The first step in making any device is to start with a wafer of semiconductor material. These wafers are sliced from a large single crystal known as a boule . Figure 1 shows the apparatus used to grow the boule. A small seed crystal of the material to be grown is lowered to the surface of the melt and then drawn upwards, slowly. As the seed crystal is pulled from the melt, it draws with it a layer of molten material. This material cools gradually, taking on the same crystalline structure as the seed crystal. Pellets of dopant material are added to the melt if extrinsic semiconductor material is required. Making compound semiconductor crystals is somewhat more difficult because the vapour pressures of the constituent materials are different. At the temperature required to melt the higher temperature material, the lower melting-point material has evaporated. Evaporation can prevented by the use of a liquid lid or encapsulate. The encapsulate must of course be made of a material that is less dense than the material in the crucible and must not be absorbed in the melt. For making Indium Phosphide crystals, a Boric-Oxide (B 2 O 3 ) encapsulate, coupled with a high pressure of inert gas in the chamber, prevents the volatile phosphorous from vaporising from the melt.

Wafer Bonding

Figure 6. A schematic diagram of a wafer bonding vessel.

Wafer bonding is a method of fusing two semiconductor materials at the atomic level. This method is especially useful in bonding materials such as GaAs and InP which have different lattice constants. The technique has been used successful in creating long wavelength VCSELs with InGaAsP active regions and InP cladding layers bonded to GaAs/AlAs DBRs. ^,, . The difference in refractive index of InP and GaInAsP, used is in epitaxial DBR mirrors is very small. At least 30-periods are required to achieve the desired reflectivity. GaAs/AlAs has a greater contrast in refractive index. Therefore fewer periods are required to achieve the same reflectivity but have slightly different lattice constants to InP. (Approx. 5.8Å for InP compared to approx. 5.6Å for GaAs/AlAs.) The wafers to be fused are placed within the quartz reactor. When heated the difference in the thermal expansion of the graphite and the quartz reactor cause the wafers to be compressed together. For successful fusion to occur, the wafers must be optically flat and free of contaminants. VCSELs created with the method showed normal device characteristics indicating the integrity of the fusion boundary. However, the effects of strain between the two kind of semiconductor at the boundary make the long-term reliability uncertain.