Optical Contrast Of Optical-storage And Display Coatings Based On Phase Change Materials

Chalcogenide phase-change materials?PCMs?can be rapidly and cyclically converted between the amorphous and the crystalline state,accompanied by a significant change in optical properties.The unique optical contrast enables PCMs as logical switching for optical-storage and dynamic color switching for nano-pixel displays.However,it's still exists a lot of deficiencies:?1?For optical-storage coatings,although the high optical contrast and thermal stability of PCMs play key roles in achieving non-volatile optical devices with a high signal-to-noise ratio and high stability,recent researches mainly focused on the simulation and manufacture of optical devices based on PCMs as well as the development of novel functionalities.The microscopic origins of these two optical properties have not been explored well.Moreover,there has not yet been a comparative study on the optical properties of GeTe,Ge₂Sb₂Te₅ and Sb₂Te₃,the three most typical PCMs in?Ge–Te?_x?Sb₂Te₃?_1-x pseudo-binary compounds.?2?For display coatings,researchers mainly focused on the color-change performance improvement by optimizing the thickness of dielectric and PCMs layer.However,since it is difficult to understand the relationship among the phase change of PCMs-optical constants?n,k?change-color change,designing appropriate PCMs to further improve the color-change performance is still challenging.To solve these problems,we prepared optical-storage and display coatings with different vacancy concentrations by magnetron sputtering.The following two aspects are studied via a series of simulations,experiments,theoretical calculations and spectral fittings:?1?The optical contrast,thermal stability of GeTe,Ge₂Sb₂Te₅,Sb₂Te₃ and GeTe₄films and physical mechanism.This paper shows that structural disorder and structural stiffness play key roles in improving the optical contrast and thermal stability of PCMs.We obtain high optical contrast and thermal stability in one PCM by analyzing the differences in optical properties and electronic structures among the three most typical PCMs?Ge-Te,Sb-Te and Ge-Sb-Te?.Therefore,these results elucidate the dominant factors and physical mechanisms influencing the optical contrast and thermal stability of PCMs,which sheds new light on designing high-performance non-volatile optics based on PCMs.?2?The optical contrast of ITO/PCMs/ITO/Ag stacks and physical mechanism.Here,the five typical PCMs(GeTe,Ge₈Sb₂Te₁₁,Ge₃Sb₂Te₆,Ge₂Sb₂Te₅ and Ge₁Sb₂Te₄)are investigated for their vacancy concentrations,electronic structures,optical constants,and their effects on the display coatings.We have revealed the color-change mechanism and proposed the new design principles for PCMs.We have found that the color change caused by the phase transitions is derived from the structural order.The structural order causes the electron delocalization,which leads to the evolution of the extinction coefficient and reflectivity.We believe that designing PCMs with less vacancy concentration or no vacancies is an effective means to obtain high color-change display coatings.Suitable PCMs including GeTe,Ge₄Sb₁Te₅,Sb and Ge₄Bi_0.5Sb_0.5Te₅ deserve further research to validate and optimize.PCMs with less or no vacancies,like GeTe,are ideal choice for optical storages with high signal-to-noise ratio and nano-pixel displays with good color-change ability.This new design principle of PCMs with high optical contrast provides a direction for material selection for optical storage and display coatings.