Contact Materials and Joining Technologies for High Temperature Operation of Laser Diodes

After the discovery of laser technology in the 1960s by Theodore H. Maiman at Hughes Research Laboratories [1], high power laser diode systems became popular in material processing, medical applications, and the pumping of solid-state lasers in recent years [2]. High power laser diode systems are typically made of an array of diode laser units a.k.a. laser bars, whose beams are coupled in order to achieve high output power [3]. However, when laser diodes are operated at a higher temperature, the performance and reliability deteriorate. This is due to the strong dependence of the laser properties on the joining technology and contact materials. For instance, GaAs laser bar (6.2*10-6 K-1) with indium solder material on copper (16.8*10-6 K-1) heat sink material faces serious thermo-mechanical stress, at high temperature, due to the thermal expansion coefficient (CTE) mismatch (between GaAs and Copper) during the cooling process. A ductile solder material, like indium, can be chosen to compensate for the stress but indium itself is chemically unstable, undergoes oxidation and the ultimate fate is cracking due to creeping and as a result, short operation lifetime [4] or catastrophic failure [8]. Higher CTE miss-match relates directly to the higher operating temperature of the laser. High operation temperature must be possible for higher output power of the laser and therefore how high the output power of the laser will be determined by the operability of the laser diode at high temperature. This implies that the overall performance of the laser diode at high temperatures is determined by the inherent property of materials and technologies used for joining. Therefore, the core task of this report is to find out a good combination of joining technology and contact materials that offer mechanical stability, high thermal conductivity, and high electrical conductivity to make sure better reliability and performance of laser diodes at high temperatures