The authors report inductively coupled plasma (ICP) dry etching of nanoscale trenches with feature sizes of approximately 140 nm wide by 20 μm long by 3 μm deep in InP with and without quantum wells, based on HBr chemistry. Both focused ion beam and electron beam lithography nanopatterned features are presented with high aspect ratios in excess of 30:1. Importantly, with HBr, hybrid GaInAsP/InP heterostructures can be anisotropically and vertically etched through with reasonable smoothness at 165 °C, which is comparatively lower than what chlorine-based chemistry requires and is therefore beneficial for achieving reliable InP devices. The phenomenon of aspect ratio dependent etching, or reactive ion etch lag, begins to manifest itself when the etch aspect ratio of InP approaches 30:1 using this ICP technique. The application of interest is a novel coupler for integrated photonics.