Telecom-wavelength-transparent Ge-Sb-Se-Te Nonvolatile Optical Phase Change Materials And Devices

Phase change materials（PCMs）could achieve ultra-fast reversible phase change and exhibit huge electrical/optical contrast via the change of atomic arrangement between different phases.Numerous studies have been done on properties,structures and applications of phase change materials in academia and industry worldwide.Chalcogenide compounds are commonly used nonvolatile phase change materials with the self-hold nonvolatile ordered/unordered atomic arrangement.They are of great importancea due to their unique role in the information storage including optical disks and phase change memories.The development of the photonic integration technology gave birth to the concepts of photonic computer and optical logic.As the fundamental components,the integrated optical switches/memories gradually began to be studied.However,when applying the PCMs into integrated optical devices,researchers found that all of them were quite lossy at the most important 1550 nm telecom wavelength.The optical absorption induced by PCMs is the major obstacle when leveraging the optical contrast between different phases of PCMs in integrated optical devices,thus impairs the performance of the optical chips.The purpose of this dissertation is to put forward an optical phase change material（O-PCM）which is transparent at 1550 nm telecom wavelength and meanwhile owns large optical contrast between phases.Ge₂Sb₂Te₅ is the most widely used and studied nonvolatile PCM which had been commercialized already.The ultrafast phase transition,large optical contrast and good stability of Ge₂Sb₂Te₅ are satisfying.Therefore,to inherit these good properties,we started from Ge₂Sb₂Te₅ and stepped into Ge-Sb-Se-Te material system.The optical properties and applications of Ge-Sb-Se-Te were studied.The main research contents are as follows:1)The Ge₂Sb₂Se_xTe_（5-x）（x=0,1,2,3,4,5）thin films were deposited,and their crystallization behavior and optical properties were studied.The phase evolution of Ge₂Sb₂Se_xTe_（5-x）with the temperature were characterized with X-ray diffraction,and the influence of Se-Te substitution on the crystallization temperature and thermal stability were investigated.The optical constants（refractive index and extinction coefficient）of amorphous/crystalline Ge₂Sb₂Se_xTe_（5-x）thin films were measured,modeled and fitted using ellipsometry,and the role Se played in the optical constants and optical contrast was analyzed.2)Making a balance among different characteristics of Ge₂Sb₂Se_xTe_（5-x）materials,Ge₂Sb₂Se₄Te₁ became a candidate of the new O-PCM we were looking for.The selected area electron diffraction result suggests that Ge₂Sb₂Se₄Te₁ exhibits a hexagonal lattice structure,and is believed to be related to its special phase change properties.3)Using cut-back method in integrated waveguides,the upper limit of the material loss of Ge₂Sb₂Se₄Te₁ was further determined to be 705 dB/cm（extinction coefficient<0.002）at 1550 nm wavelength.Ge₂Sb₂Se₄Te₁ was put forward as the first 1550 nm wavelength-transparent O-PCM with a figure-of-merit more than 22 times higher than Ge₂Sb₂Te₅.4)In order to demonstrate the applications of Ge₂Sb₂Se₄Te₁ in integrated optical devices,the Ge₂Sb₂Se₄Te₁ based SiN micro-ring resonator optical switch was fabricated,achieving an on/off contrast of 41 dB and an insertion loss of 0.15 dB.Both of the performances represent significant improvements compared to state-of-the-art Ge₂Sb₂Te₅-based devices.The credit was given to the combination of ultra-low loss and large-enough optical contrast of Ge₂Sb₂Se₄Te₁.5)The potential unconventional application of O-PCM was explored.To make advantage of the mixing state between the fully amorphous state and the crystalline one,and to leverage the broadband operation of chalcogenide-compound O-PCMs,phase change tunable filters for wavelength multiplexing multispectral infrared imaging were designed.The principle of the high-throughput and non-scanning infrared imaging system was introduced.Ge₂Sb₂Se₄Te₁ was employed in the design and simulation of the tunable broadband filter in the near-infrared wavelength.The filter transmittance spectra were calculated by transfer-matrix-method and optimized using singular value decomposition.The signal reconstruction of the system accompanied by the random detector noise or the state noise was analyzed.