Smooth etching by Cl-2 reactive ion beam etching is performed on various III/V or II/VI compound semiconductors, such as InP, GaAs, ZnSe, and ZnTe. These can be properly etched only under one set of conditions : high substrate temperature (similar to 200 degrees C), high ion energy (similar to 1 keV), and low gas pressure (similar to 3x10(-5) Torr). The smoothness of a 1 mu m etched surface was estimated by surface tunneling microscopy to be only 1.4 nm, comparable to its smoothness before etching, 0.9 nm. The etched surface was clean and without C or O contamination as estimated by in situ Auger electron spectroscopy. Although etching damage from high ion energy was a concern, maximum damage depth was found to be less than 150 nm, and damage to the etched sidewall was negligible. Laser diodes and photodetectors were fabricated using this etching technique, and these had reliability and other characteristics comparable to conventional devices.