LASER PROCESSING OF III-V SEMICONDUCTORS

Abstract

Laser-controlled etching of GaAs, Al3Ga7As, Al4Ga6As and Al5Ga5As was investigated in detail for varying laser parameters such as laser power, beam scanning speed, beam duty cycle and number of scans. This was done for materials of different substrate doping types and doping concentrations. An argon ion laser emitting at 514.5 nm was used for this purpose, and the etchant used was aqueous KOH (0.25 M). Etch rates of n-type materials were found to be significantly higher than those for p-type materials. Generally, higher etch rates were also recorded for materials of higher aluminium mole fraction. Disparities in etch rates for the different conditions were accounted for by the differences in the built-in electric field in the etchant/substrate interface as well as differences in the laser-induced temperature rise of the substrates. It was also found from SEM micrographs that etched lines were as smooth as the unetched areas when the spot radius was about 7 µm. However, as the beam is focused more tightly, ripples attributable to the discrete movement of the XY-stage can be seen along the trench. Further, it was found that using a chopped laser beam to scan a line multiple times resulted in the best control of the etch depth for AlGaAs/GaAs structures. This is due to the self-limiting nature of the etch rate as the top AlGaAs layer is thinned down close to the AlGaAs/GaAs interface,