Crystal phase engineering is an exciting pathway to enhance the properties of conventional semiconductors. Metastable SiGe presents a direct band gap well suited for optical devices whereas wurtzite (WZ) phosphide alloys enable efficient light emission in the green spectral range. Availability of these materials could profoundly impact electronics as well as solid-state lighting industries. However, their synthesis in high quality and beyond nanowire size constraints remained elusive, hampering both scientific and commercial exploitation. In this thesis, we develop new approaches for metastable film growth to enable a platform for robust material characterization and device fabrication. Metal-organic vapor phase epitaxy is used to grow pure WZ nanowires and fins. To obtain planar layers, we explore two extensions of this well-known technique. The first one is based on conformal epitaxy and enables the use of standard (001)-oriented substrates. In this way, we achieve the controlled concurrent epitaxy of stable and metastable InP. We further propose a variation of this approach by using a nanowire as seed. This establishes a clear route for the synthesis of a large range of materials as is exemplarily demonstrated for WZ GaAs films. For the second technique we explore a method to enforce epitaxial lateral overgrowth on (111)A-oriented substrates. This allows to deposit pure WZ InP layers on insulator exceeding areas of 100 µm2, the largest ever demonstrated. We conceive a nucleation-based model and argue on a fundamental size limitation of metastable film epitaxy. Further advancement of this process allows to directly grow metastable micro- and nanostructures, which are optically isolated from the substrate. The high quality and shape-control of these crystals enables room-temperature lasing in WZ InP microdisks. This thesis demonstrates the successful development of new concepts to synthesize metastable semiconductors with pure crystalline phase and dimensions tailored towards planar device fabrication. We expect our findings to significantly contribute to the realization of crystal phase engineered electronic and optoelectronic devices.