The application of mismatched heteroepitaxial semiconductors has been quite limited due to the presence of high threading dislocation densities. The conventional approaches to avoiding this problem require pseudomorphic or lattice-matched growth, thus severely limit the choice of material combinations. We have proposed a new approach, the patterned heteroepitaxial processing (PHP), for removal of threading dislocations from mismatched heteroepitaxial layers. ^ This study involves the growth and characterization of ZnSe-based II-VI layers for blue-green lasers, developing new high-resolution x-ray diffraction (HRXRD) methods for the characterization of epitaxial layers, and evaluating the PHP approach in the ZnSe-based II-VI/GaAs material systems. ^ We studied the photoassisted metalorganic vapor phase epitaxy (MOVPE) of ZnSe and ZnSySe1−y/ZnSe on GaAs (001). We have determined the critical layer thickness of ZnSe, achieved the compositional control of ZnSySe1−y, and investigated the relationship between the composition and the structural and optical properties of ZnS ySe1−y. ^ We have developed new HRXRD methods for the determination of the critical layer thickness and for the analysis of tetragonally distorted ternary layers. We have shown theoretically and experimentally that our methods provide higher accuracy than the traditional HRXRD methods. ^ We applied the PHP technique to the ZnSe/GaAs (001) and ZnSySe 1−y/ZnSe/GaAs (001) material systems. For 2000–6000 Å thick ZnSe on GaAs (001), the as-grown layers contained more than 10 7 cm−2 threading dislocations. We have achieved complete removal of threading dislocations from 70 μm by 70 μm square mesas that were thicker than 3000 Å and annealed at 500°C or above. Neither postgrowth annealing alone nor post-growth patterning alone had a significant effect on dislocation reduction. By studying the annealing temperature dependence, we have determined that the dislocation removal by PHP is thermally activated. For ZnS0.02Se0.98 on GaAs (001), we have succeeded in reducing the threading dislocation densities from 1.5 × 107 cm−2 in as-grown layers to less than 104 cm−2 in mesas (thicker than 6000 Å and annealed at 600°C). This corresponds to a 1400-fold reduction. These results show that PHP can be used to completely remove threading dislocations from lattice-relaxed heteroepitaxial layers. In principle this approach should be generally applicable to mismatched heteroepitaxial materials.