Improving the reflectance and color contrasts of phase-change materials by vacancy reduction for optical-storage and display applications

High-reflectivity/color contrast is crucial for phase-change materials (PCMs) in optical-storage and display applications. However, the relationship between phase-change and reflectivity/color contrast and how to improve the contrast has not been well explored. Here we find that phase-change-induced reflectivity/color contrast arise from electron delocalization and subsequent extinction coefficient enhancement, rather than refractive coefficient enhancement which we thought previously. Reducing vacancies can significantly increase the reflectivity contrast and color contrast of PCM films by 83.2% and 38.5%, respectively. These new insights are demonstrated through our experiments, theoretical calculations, spectral fittings, and simulations on ${{\rm Ge}_1}{{\rm Sb}_4}{{\rm Te}_7}$Ge1Sb4Te7, ${{\rm Ge}_2}{{\rm Sb}_2}{{\rm Te}_5}$Ge2Sb2Te5, ${{\rm Ge}_8}{{\rm Sb}_2}{{\rm Te}_{11}}$Ge8Sb2Te11, and GeTe.