IMPROVED GAAS BONDING PROCESS FOR QUASI-PHASE-MATCHED 2ND-HARMONIC GENERATION

A multilayer stack of bonded GaAs wafers, each layer rotated 180 degre es from the adjacent one, has been proposed for quasi-phase-matched se cond harmonic generation. Current bonding technology, however, often l eads to unacceptable optical losses and, therefore, poor device perfor mance. In this study, three sources of optical losses were investigate d: (i) interfacial defects between the wafers, (ii) bulk defects withi n the wafers, and (iii) decomposition at the exposed outer surfaces. S urface losses due to incongruent evaporation were easily eliminated by repolishing the outer surfaces. However, to minimize the losses from interfacial and bulk defects, it was necessary to investigate the rela tionship between these defects and the processing parameters. It was f ound that an increase in temperature and/or time led to a decrease in interfacial defects, but an increase in bulk and surface defects. Opti mized processing conditions were developed which permit the preparatio n of stacks containing over 50 layers of (100) GaAs wafers, and about 40 layers of (110) GaAs wafers. Optical losses as low as 0.1 to 0.3 % per interface (at 5.3 and 10.6 mu m) were observed for the (110) orien ted multilayer structures.