Investigation On The Growth, Properties And Applications Of Pb_1-xGe_xTe, An Infrared High-index Coating Materials

An atmospheric sounding interferometer designed to measure radiant and reflected energy from areas sampled on the earth using infrared narrow bandpass interference filters to isolate and discriminate accurately between spectral bands to quantify aerosol–cloud–radiation interactions for numerical climate and weather forecast. The design of these filters is established on the basis of appropriate choice of refractive index, layer thickness, and the number of layers. Because the greater the ratio of the high to the low indices of the layers fewer layers is needed, using infrared high-index coating materials can reduce the number of layers and the difficulty in product design and manufacturing, increase the durability to withstand the rigors of use in spaceflight. Therefore, it is of vital significance to investigate infrared high-index coating materials.Currently, lead telluride（Pb Te） is used practically as infrared coating materials with the highest refractive index. Although, for many years, a “mild” Pb Te coating material developed in Shanghai Institute of Technical Physics, CAS, has been used in the manufacturing of infrared narrow bandpass interference filters and played an important role in the spaceflight remote-sensing, the filters consisting of layers of Pb Te is not robust enough to withstand the damage from standard fabrication processing, such as “from wafer to chips”.One of the aims in the investigation is to explore the mechanical properties of thin films of Pb_1-xGe_xTe, which is the pseudo-binary alloy of Pb Te and Ge Te, another IV-VI narrow gap semiconductor compound, in order to improve the robustness of the high-index layers in the filters through solid solution strengthening.It is established on the basic of bulk growth of Pb_1-xGe_xTe for it to be used in the manufacturing of infrared narrow bandpass interference filters as a high-index coating material. Due to a ferroelectric phase transition from a rocksalt paraelectric phase to a rhombohedral ferroelectric phase exists in Pb_1-xGe_xTe, many properties are linked with the Ge concentration x. Therefore, the investigation into the inhomogeneity of Ge Te distribution originated from solute partition and segregation in Bridgman crystal-growth is one of the aims.Nowadays, the evaporation is still a principal physical vapor deposition process. To disclose the difference of the stoichiometry between thin film and the evaporants before Pb_1-xGe_xTe can be used in optical coating industry is also one of the aims. Moreover, the compositional correlation and crystallographic structure in evaporated Pb_1-xGe_xTe thin films are also to be under investigation.The absorption edges of thin films of Pb_1-xGe_xTe can be tuned toward short wavelengths with the increase of the Ge concentration in films. The combination of this tunability and its high index, Pb_1-xGe_xTe can be a potential candidate used in the mid-wavelength infrared narrow bandpass filters, in substituting for the convenient high-index material Ge. One of the aims is to investigate the feasibility of using Pb_1-xGe_xTe as an infrared high-index coating material in the mid-wavelength infrared narrow bandpass filters.Some fruits have been harvested:1.After an analysis of the solute partition and segregation formations in Bridgman growth of multi-component crystals, especially for solid solution type of crystals, a simulation model was proposed for the crystal-growth of Pb_1-xGe_xTe, on which the processes of crystal-growth of Pb_1-xGe_xTe were optimized to obtain the homogeneous distribution of Ge Te along the solidification direction. Two kinds of Pb_1-xGe_xTe were developed: the longwave infrared Pb_1-xGe_xTe, which taking advantage of its mechanical properties superior to Pb Te, provides a solution to the susceptibility of the filters to the damage from standard fabrication processing; and mid-wavelength infrared Pb_1-xGe_xTe, which takes advantage of its combination of this tunability and its high index to substitute for the convenient high-index material Ge.2. It was found in the investigations that the hardness of thin films of Pb1-x Gex Te is five time higher than that of Pb Te, Young’s module is two time higher, and the adhesive strength to Ge substrates is 3 time higher. Moreover, it was confirmed that Hall-Perch equation can be established to describe the relationship between the hardness and the grain size in Pb1-x Gex Te thin films. Furthermore, the mechanical properties of Pb1-x Gex Te thin films were explained in light of the principle of solid solution strengthening, ferroelectric phase transition from a rocksalt paraelectric phase to a rhombohedral ferroelectric phase, and the strong localized elastic-strain fields in concentrated solid solutions.3. The difference of the vapor pressure between Pb Te and Ge Te results into the stoichiometry of the evaporated Pb1-x Gex Te film will differ generally from the evaporants. It was found in the investigations that the incongruency is dependent with substrate temperature. Under an optimized substrate temperature, a compositional congruency exists between the evaporants and thin films of Pb1-x Gex Te evaporated using electron beam heating, which can be attributed to the “ablation” characteristics of e-beam evaporation and lower thermal conductivity of the Pb Te based alloy. It was also revealed that with an increasing of Ge concentration, Te concentration in films decreases gradually. As a consequence, the Te-rich characteristic will shift into the Te-deficient one due to the increasing of Ge concentration. The compositional interdependence can be attributed to the facts that Ge-Te bond energy is smaller than that of Pb-Te when Ge ions substitute for Pb ions; as well as that the distance between a Ge ion and its adjacent Te ions is different due to the displacement of Ge ions from the cell center. Furthermore, the high-pressure phases of Ge Te and Te are presented in Pb1-x Gex Te thin films which have the rhombohedral ferroelectric phase structure at room temperature. It was inferred that the transition from the disorder to the order in the system induces high pressure polymorphism in evaporated thin films.4. It was demonstrated in the investigation that the optimized optical properties in the mid-wavelength window can be obtained for thin films of Pb1-x Gex Te by evaporating from the starting materials with a Ge concentration x of 0.18 under a substrate temperature of 150°C. In order to demonstrate an improvement to the bandwidth of filters using Pb1-x Gex Te as a high-index material to substitute for Ge, a simple Fabry-Perot filter with a central wavelength of 4 μm was designed and manufactured. Compared to those using Ge, the filters using Pb1-x Gex Te have a narrower bandwidth and a superior rejection ratio.