Compound semiconductor alloys have limited miscibility, which can lead to composition segregation during growth. This effect produces periodic lateral composition modulation in epitaxial InAs-AlAs [1] and other alloys [2]. Intense modulation is found in vertical (InAs) n /(AlAs) m superlattices (n ≈ m ≈ 2) grown by molecular beam epitaxy [3]. Growth on (001) InP substrates leads to two modulation directions with only short InAs-enriched regions (~0.1 µm) [4]. Superlattices with global-average compositions slightly rich in InAs (n > m) have modulations ~8° from the in-plane <100> directions. Growth on substrates miscut 2° toward (100) selects the modulation near [100], resulting in vertical sheets enriched in InAs and extending nearly parallel to [010] with lengths approaching ~1 µm.