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) operating in air. The gratings, made of n‐doped InP semiconductor, were prepared using holographic and electron beam lithography (EBL) exposures followed by reactive ion etching (RIE) to transfer the photoresist pattern into the wafer. The grating surfaces were partly shadow‐coated with a thin gold layer, and the line patterns were STM‐imaged on both the coated and non‐coated InP surfaces. Pitches of first‐ and second‐order gratings, including λ/4 phase shift lines and the depth of the holographic second‐order grating, were measured by means of a precisely calibrated piezo‐capacitive STM head, a UV diffractometer and a scanning electron microscope (SEM). A precise STM characterization of the coated line patterns was achieved. As regards the non‐coated surfaces, reliable STM measurements of InP grating pitches were performed without any sample preparation and surface contamination.