High-quality focused-ion-beam-made mirrors for InGaP/InGaAlP visible-laser diodes

Abstract

We employed a focused ion beam (FIB) to sputter the end mirrors on InGaP/InGaAlP laser diodes. This particular diode operates at a wavelength of about 670 nm. For this wavelength the quality of the sputtered mirrors is far more critical than for longer wavelength devices. The present lasers do, in fact, show a relatively large increase in their threshold current after FIB micromachining. In this article, we investigate the origins of this increase and how to prevent it. After optimization, the lasers with a FIB-made end mirror have a threshold current comparable to that of cleaved-facet devices. We have seen that the polycrystalline structure of the device metallization on top of the laser diode causes roughening of the mirror during sputtering, which results in severe scatter losses of the laser light. A method to decrease the surface roughness is given. A further reason for the increase in threshold current is optical absorption at the mirror, presumably by a Ga-rich layer. This can be circumvented by chemical etching after FIB treatment to remove the absorbing layer. Finally, to demonstrate the quality and the applicability of the FIB-made mirrors, the influence of the tilt angle of the end mirror on the threshold current of the laser has been investigated. This dependency can be excellently described by theory.