STM CHARACTERIZATION OF INP GRATINGS FOR DFB LASER FABRICATION

Diffraction gratings for distributed feedback (DFB) laser devices were characterized by means of a scanning tunnelling microscope (STM) oper ating in air. The gratings, made of n-doped InP semiconductor, were pr epared using holographic and electron beam lithography (EBL) exposures followed by reactive ion etching (RIE) to transfer the photoresist pa ttern into the wafer. The grating surfaces were partly shadow-coated w ith a thin gold layer, and the line patterns were STM-imaged on both t he coated and non-coated InP surfaces. Pitches of first- and second-or der gratings, including lambda/4 phase shift lines and the depth of th e holographic second-order grating, were measured by means of a precis ely calibrated piezo-capacitive STM head, a UV diffractometer and a sc anning electron microscope (SEM). A precise STM characterization of th e coated line patterns was achieved. As regards the non-coated surface s, reliable STM measurements of InP grating pitches were performed wit hout any sample preparation and surface contamination.