The greater value of refractive index for high-index layers in thin-film interference filters operating in the infrared has an incomparable advantage. Lead telluride (PbTe), which is much superior to other infrared high-index coating materials due to its high index and advantage of fundamental absorption edges, has played an important role in filters employed in the infrared radiometer and other instruments launched in space atmosphere sounding research projects. In this chapter, we summarized some recent achievements in the investigations into another infrared high-index coating material—lead germanium telluride (Pb1−xGexTe), a pseudo-binary alloy of PbTe and GeTe. It can be revealed that the layers of Pb1−xGexTe exhibit the tunable optical properties, such as temperature coefficient of refractive index and fundamental absorption edge, as well as mechanical properties, such as the hardness and Young’s modulus, corresponding to its intrinsic ferroelectric phase transition. Some important applications in thin-film interference filters were also demonstrated for its tremendous potential, such as a stable narrow bandpass interference filter without temperature-induced wavelength shift and a tunable infrared short wavelength cutoff filter. Furthermore, it is also revealed that electron beam evaporation is a more effective congruent-transfer technique to deposit the layers of Pb1−xGexTe.
- thin-film interference filter
The rapid detection of the dynamites or explosives is of increasing importance in the anti-terror campaign. A technology to detect the trace dynamites can be developed using the electromagnetic radiation in the mid-wavelength infrared region, because many strong fundamental absorption bands with regard to “fingerprint” are included in this range . Although, in recent years, a significant progress has been achieved toward the miniaturization of infrared spectrometers, which can play an important role in the homeland security and law enforcement, accompanied with the development of thermoelectrically cooled detectors and quantum cascade lasers, a thin-film narrow bandpass interference filter, which has a passband coinciding with and as narrow as the typical absorption line of “fingerprint,” is urgently needed to increase sensitivities of the spectrometers. An order of magnitude larger angular dispersion than gratings can be carried out due to the characteristic lines assigned to the trace dynamites being distinguished using a thin-film bandpass filter. Therefore, the spectral fluctuation will be reduced considerably and a greater level of miniaturization can be completely obtained without the clumsy gratings . Furthermore, more profits can also be gained out of thin-film filters, because of their cost-efficient mass-production compared with the gratings.
According to Macleod , the simplest type one-cavity all-dielectric Fabry-Perot filter has the form of [
and Eq. (2) for low-refractive-index spacer,
Although, conveniently, germanium (Ge) is a preferred choice of coating material for the high-index layers in the spectral region of mid-wavelength infrared due to its higher value (round 4.0) of refractive index, it is still expected that another material with an even higher index is available to substitute for Ge in order to further reduce the halfwidth of a filter.
2. Properties and applications of PbTe
Lead telluride (PbTe) is one of lead chalcogenides, which has been widely investigated as a conventional semiconducting material for many decades. The mineralogical name of PbTe is Altaite, a yellowish white mineral with an isometric crystal structure, which was discovered in 1845 in the Altai Mountains .
PbTe is a promising material candidate for mid-wave infrared detection because of their superior chemical and mechanical stability over HgCdTe alloys [5, 6, 7]. In addition, as a simple
However, PbTe is also one of high-index infrared coating materials. Currently, it dominates the material selection for the design of infrared thin-film interference filters operating in the long wavelength infrared both at room and reduced temperature. The combination of its high index (above 5.5 in the spectral range of long wavelength infrared at room temperature) and its advantage of a negative temperature coefficient of refractive index (−2.0 × 10−3 K−1) make it much superior to other infrared coating materials [11, 12, 13, 14, 15, 16, 17].
In Figure 1(a), it can be illustrated that a layer of PbTe has a very high value of refractive index , although it is lower than that of bulk single crystal of PbTe . In Figure 1(b), it can be also revealed that the foundational absorption edge of the layer of PbTe will shift toward the longer wavelength with the decreasing ambient temperature. Therefore, a single layer of PbTe can be regarded as a natural selective absorption longwave-pass cutoff filter to omit the auxiliary filters which are necessary to block the unfavorable Planck emission from hot resources.
Since the middle of the twentieth century, using PbTe as the infrared high-index coating materials, Infrared Multilayer Laboratory at the University of Reading, Reading, United Kingdom, have completed spectral design and manufacture of high-quality infrared thin-film interference filters for complex infrared radiometer and ground-based astronomical instruments in over 30 major UK and international space and astronomical research projects . In Figure 2, the space and astronomical research projects being launched in the recent 5 years, in which infrared thin-film interference filters were manufactured in Infrared Multilayer Laboratory using PbTe as the infrared high-index coating materials, were listed.
However, if the conventional PbTe materials are used as the evaporants, which are prepared from the stoichiometric proportions of pure constituents, a strong
Since the partial pressures of Pb and Te2 strongly depend on the properties of the evaporants, it is possible to shift the characteristics of the deposited layers by using a PbTe material with a Te dopant. Therefore, a kind of evaporable PbTe material with “mild” characteristics has been developed in Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, China [21, 22]. By “mild,” we mean that, in a rather broad region of substrate-temperature, the concentration of free carriers in the layers from such a material can be 25–40 times lower than in normal PbTe materials. In Figure 3, some products of “mild” PbTe evaporation materials were exhibited.
3. Properties and applications of Pb1
Lead germanium telluride (Pb1
Like some IV–VI compound semiconductors, for example, the tellurides of Sn and Ge and their alloys, Pb1
In this chapter, some investigations into the optical and mechanical properties of the layers of Pb1
3.1. Low-temperature dependence of mid-infrared optical constants of layers of Pb1
Although many investigations, both theoretical and experimental, have been carried out on the mechanism of ferroelectric phase transition of Pb1−xGexTe, the investigation into the optical constants (refractive index
In our investigation, a layer of Pb1−xGexTe was deposited on a silicon wafer using molybdenum boat heating the ingot of Pb1−xGexTe (
As a consequence, the temperature dependence of optical constants can be obtained at low-temperature in the spectral range of 2.5–8.5 μm. It can be found that the layer of Pb1−xGexTe has a refractive index with a value of 5.350–6.000 corresponding to different measured temperatures in the spectral range of 4.0–8.5 μm, in which dispersion originated from the Reststrahlen and the absorption edge can be negligible. At room temperature, the layers of Pb1−xGexTe have a value of refractive index approaching to that of layers of PbTe. A conclusion can be drawn that Pb1−xGexTe is also an infrared high-index coating material.
In Figure 4(a), the change of refractive index of the layers of Pb0.94Ge0.06Te as a function of both wavelength and temperature was shown. It can be seen that the maximum value of refractive index occurs at 150 K, which can be regarded as the results of increased lattice polarizability that is an indication of the ferroelectric nature of the phase transition. Therefore, a conclusion can be drawn that anomalies in the refractive index, similar to those in the electrical resistivity and specific heat, emerge at the Curie temperature
An empirical formula for the temperature coefficient of refractive index in the spectral range of 4.0–8.5 μm can be expressed as Eq. (3):
; ; .
3.2. The stable narrow bandpass interference filters without temperature induced wavelength shift
When the ambient temperature varies, the performance of an optical thin-film interference filter will be also changed, such as the shift of center wavelength and the deterioration of peak transmission [34, 35, 36]. In particular, when a long wavelength infrared narrow bandpass filter is used in the spaceborne remote sensing instruments, the change of its spectral characteristics, which will lead to the difficulty to sustain the precision radiometric measurements from space, is not acceptable . Addition of an auxiliary temperature control to the filters is not practical in order to maintain its stable optical performance in spaceborne remote sensing systems.
There are two factors that cause the shift of wavelength accompanied by the change of ambient temperature. One is the temperature-induced variations in the indices of refraction of the layers, and another is the variations in the physical thicknesses of the layers. Since the bulk temperature coefficients of linear expansion are an order of magnitude smaller than the temperature coefficients of the indices of refraction for substances similar to those usually employed for thin-film interference filters, it may be speculated that the shift of wavelength should be ascribed to the variations of temperature coefficients of the indices of refraction of the layers .
As far as the infrared filters employed in the spaceborne remote sensing systems are concerned, the convenient materials used for the low-index layers are either ZnS (2.2) or ZnSe (2.3). Moreover, Seeley et al.  modeled the sensitivity of the narrow bandpass filter to the change of temperature, showing that the spacer and the next two adjacent layers are dominant contributors relative to the other layers (and stacks). Therefore, when a negative shift in PbTe resulting from the negative temperature coefficient of refractive index is suitably combined with a positive shift in ZnS (or ZnSe) from its positive coefficient, temperature-invariant compensation becomes possible; namely, to achieve a negligible wavelength shift with temperature.
However, as a matter of fact, the temperature coefficient of refractive index of PbTe cannot exactly compensate for that of ZnS (ZnSe). Therefore, another solution to the problem is to seek a coating material of which the temperature coefficient of refractive index can be tuned.
Since the maximum value of refractive index of layers of Pb1−xGexTe occurs corresponding to the structural phase transition, as a consequence, at the designated low-temperature, the temperature coefficient of refractive index of layers of Pb1−xGexTe can be tuned from negative to positive by varying the Ge composition, that is, the layers of Pb1−xGexTe with the specific composition may be used as the high-index layers in the thin-film interference filters.
Since the component elements in a multicomponent alloy system will evaporate at a different rate, which causes changes in compositions of layers relative to the evaporants, and it is in great necessity to designate directly the compositions in evaporated layers of Pb1−xGexTe. For example, the corresponding stoichiometry of evaporated layers can be expressed by the formula (Pb1−xGex)1-yTey.
In Figure 5, the spectral characteristics of a simple one-cavity Fabry-Perot filter on a Ge substrate was demonstrated in the temperature range of 85–300 K, which was designed with peak wavelength of 11.30 μm and fabricated using ZnSe as the low-index layer. For its high-index layers, the ingots of Pb1−xGexTe (
3.3. Homogeneity of composition in evaporated layers of Pb1−xGexTe
It is commonly believed that the existence of inhomogeneity of composition in layers of Pb1−xGexTe will have a disadvantageous influence on the performance of thin-film interference filters, because the existence of graded Ge concentration profile in Pb1−xGexTe layers will lead into the coexistence of ferroelectric and paraelectric phases at a fixed temperature associated with phase transition as a function of Ge concentration and temperature . Although Partin  observed the Ge concentration profile in the layers of Pb1−xGexTe grown on (100) oriented PbTe single crystal by molecular beam epitaxy from PbTe, GeTe, and Te source ovens, to the author’s best knowledge, it has still not been clarified whether or not a Ge concentration gradient exists in the layers of Pb1−xGexTe evaporated directly from bulk alloys. Therefore, the investigation on compositional depth profile in evaporated layers of Pb1−xGexTe is of a great significance.
In our investigation, the layers were deposited on silicon wafers using molybdenum boat heating the ingots of Pb1−xGexTe, of which compositions were analyzed using energy-dispersive X-ray analysis (EDAX) in a Hitachi S-520 scanning electron microscope. Depth distribution of elements was measured by using a Microlab 301F Scanning Auger Microprobe (SAM) system combined with a discontinuous ion sputtering mode at a base pressure below 8.0 × 10−8 Pa.
The results from our investigation are not in agreement with those reported by Partin. In Figure 6, a representative compositional depth profile was illustrated for an evaporated layer of Pb1−xGexTe, of which the measured elemental concentrations are 42.92 ± 0.55 for Pb, 54.96 ± 0.90 for Te, and 2.12 ± 0.74 for Ge, respectively. It can be observed that the Ge concentration at the surface is lower than that of the stoichiometry, meanwhile, the Pb concentration was distinctly higher. However, the Ge concentration increases from a Ge-deficient state at near surface to the Ge-rich one at near substrate. At same depth, all concentrations of elements remain balanced, that is, the increase of the Ge concentration must be accompanied with a decrease of the Pb concentration. After removal of the upper layers with a thickness of about 300 nm, the layer of Pb1−xGexTe transforms to the Te-deficient characteristic from the Te-rich one. In addition, a stepwise change of elemental concentrations in the depth profile cannot be detected in the layer.
In fact, the mechanism of evaporating from an alloy is much more complicated than that from a single metal because of the different vapor pressures of their individual components. It is much more possible that Ge partial pressure will change with the increasing of deposition time because Ge is more volatile than other components, Pb and Te, in the Pb1−xGexTe alloy. Thereby, Ge concentration in the evaporants will be gradually depleted and the deposition process will result in a compositional gradient in the layers. As a consequence, the results obtained in the compositional depth profiles in evaporated layers of Pb1−xGexTe can be reasonably explained.
Because it is often problematic to maintain a desired alloy in layers and over larger substrate surfaces, therefore, an empirical procedure may be used to determine how to control the composition of the layers by adjusting the composition of the bulk alloy.
3.4. Compositional dependence of absorption edges in evaporated layers of Pb1−xGexTe and tunable infrared short wavelength cutoff filters
An ideal cutoff filter should have small losses in the transmission region and high attenuation or reflectance in the rejection region over an extended spectral range, which can be carried out depending on interference or absorption . Therefore, a cutoff filter may take a number of different forms, such as interference cutoff filters and thin-film absorption filters.
A thin-film absorption filter usually has very high rejection in the stop region and consists of a layer of material which has an absorption edge at the required wavelength. It is usually short wavelength cutoff in character. A layer of semiconductor that exhibits a very rapid transition from opacity to transparency at the intrinsic absorption edge is a good example to make an excellent thin-film short wavelength cutoff absorption filter. Nevertheless, as far as a layer of a certain semiconductor, such as Ge or PbTe, is concerned, the absorption filter is inflexible in character and the cutoff wavelength cannot be tuned, because of their fundamental optical properties. In order to take advantage of the characteristic of deep rejection of thin-film absorption filters, it will be of great significance to find out a semiconductor material, of which the absorption edge position can be tuned by means of controlling the composition of its layers, to fabricate the absorption filter.
In our investigation, the layers were deposited on silicon wafers using molybdenum boat heating the ingots of Pb1−xGexTe, of which compositions were analyzed using energy-dispersive X-ray analysis (EDAX) in a Hitachi S-520 scanning electron microscope. The optical transmission spectra of the layers were measured in the spectral range of 2.5–25 μm using a Perkin Elmer Spectrum GX Fourier-Transform Infrared Spectrometer with a resolution of 8 cm−1 at normal incidence. The crystallographic structures of the layers were investigated by X-ray-diffraction (XRD) using Cu Kα radiation on a D/max 2550 V diffractometer with an accuracy of 0.02°. The single-phase nature and polycrystalline of the layers were revealed. The composition dependence of positions of the fundamental absorption edges in the evaporated layers of Pb1−xGexTe was explored. The aim is to elucidate that the tunability of the cutoff wavelength in thin-film absorption filters can be reached if the controllability of the composition of constituent layers can be carried out.
It is revealed that for the evaporated layers of Pb1−xGexTe with an identical Te concentration, the absorption edges will shift toward short wavelength with the increase of Ge concentration
Our investigation indicates that if the controllability of the composition of evaporated layers of Pb1−xGexTe can be carried out, it will be possible to fabricate an infrared single-layer thin-film absorption filter with the short wavelength cutoff at the required wavelength.
3.5. Compositional congruency, correlation and high-pressure polymorphism in electron-beam evaporated layers of Pb1−xGexTe
Currently, evaporation, as a physical vapor deposition process, is still principally used in optical coating industry, because of its simplicity, flexibility and relatively low cost; moreover, the enormous number of existing deposition systems . However, it may be undesirable from a practical viewpoint to evaporate compound semiconductors from a single source because the vapor compositions of compound semiconductors are usually different from their nominal compositions. As a consequence, the stoichiometry of the layers will differ generally from the evaporants .
In our investigation, the ingots with different Ge concentrations were chosen and evaporated using the molybdenum boat heating; meanwhile, the ingots with five Ge concentrations,
In Figure 8(a), for the layers of Pb1−xGexTe evaporated using electron beam heating, the change of Ge and Te concentration, that is,
It can be observed that Ge concentration
Furthermore, with an increasing of Ge concentration, it can be obviously observed that concentration of tellurium
Perhaps, the congruency in the process of evaporation of Pb1−xGexTe using electron beam heating of ingots can be attributed to the “ablation” characteristics of electron beam evaporation and lower thermal conductivity of the PbTe-based alloy. A significant feature of an electron beam evaporator is its ability to concentrate a large amount of power onto a small area of the surface of evaporants, independent of materials being heated. Usually when the evaporants are heated, water cooling is supplied to the crucible to remove the heat which escapes by conduction through the evaporants and liner. In addition, it has been well accepted that PbTe-based alloys constitute a category of materials with excellent thermoelectric figure of merit,
Furthermore, in our investigation, an assumption can also be proposed to explain the compositional correlation observed in the layers of Pb1−xGexTe. It has been well known that the ionic radii of Ge and Pb are 0.73 and 1.2 Å, respectively; therefore, Pb1−xGexTe belongs to a class of alloys in which a substitutional atom has a size significantly smaller than that of the host atom it replaces. In such a “diluted” alloy, the addition of even a very small number of substitutional atoms will lead to a substantial change in their physical properties. For example, although PbTe itself is not ferroelectric, the addition of even 0.05% Ge to PbTe will induce a structural transition . It is obvious that in such a ternary alloy, due to the substitution of Ge ions for Pb ions, two types of nearest-neighbor bonds, Pb—Te and Ge—Te, must be concerned. Thus, if only the strength of a Ge—Te bond is weaker than that of Pb—Te bond, the amount of Te ions which are incorporated into the system of Pb1−xGexTe will decrease with the increasing of Ge concentration in the layers.
Our assumption can be supported from the experimental data of bond energies reported by Rao and Mohan . It is pointed out that Ge—Te has a bond energy of 1.87 eV, whereas Pb—Te has a bond energy of 1.90 eV; therefore, Ge ions cannot “hold” Te ions as tight as Pb ions do. When the more Ge ions are placed on the Pb site, the more Te ions will “escape.” Therefore, the gradual decreasing of Te concentration will make a Te-rich characteristic in layers shift into a Te-deficient one.
In Figure 9, the patterns of XRD analysis were demonstrated for the layers evaporated using electron beam heating the ingots with Ge concentration
3.6. The mechanical properties of evaporated layers of Pb1−xGexTe
As far as the single-crystal of PbTe is concerned, the microhardness is relatively a constant of ∼30 HV for the various carrier concentrations [46, 47]. To the author’s best knowledge, no data is reported on hardness of layer of PbTe. However, a layer of PbTe is so soft that it can be scratched easily. As a consequence, an infrared thin-film interference filters consisting of the layers of PbTe is not robust enough to withstand the damage originated from standard wafer dicing processing, such as “from wafer to chips,” even if more robust low-index materials, like ZnSe or ZnS, are chosen as an outermost layer.
Therefore, as a solution to the problem in integrating of the standard semiconductor process into the mass-production of infrared thin-film interference filters, a new infrared high-index coating material is needed to deposit the more robust high-index layers to withstand the damages in wafer dicing processing. It is necessary to investigate the mechanical properties of evaporated layers of Pb1−xGexTe.
In our investigation, the layers of Pb1−xGexTe were deposited on silicon wafers using electron beam evaporation, of which compositions were analyzed using energy-dispersive X-ray analysis (EDAX) in a Horiba EX-220 energy-dispersive X-ray microanalyzer (model 6853-H) attached to the FE-SEM without coating the surfaces of the layers. Nanoindentation measurements were performed using a Nano Indenter G200 with a three-side pyramidal Berkovich diamond indenter of 50 nm radius under the continuous stiffness measurement (CSM) option. At least 10 indents were performed on each layer with a maximum load of 13 mN, accompanied with a corresponding indentation depth no more than 500 nm. Following the analytic method proposed by Oliver and Pharr , the average values and standard deviations of the hardness and Young’s modulus of thin films were extracted from the load–displacement results.
It can be revealed that the layers of Pb1−xGexTe have greater values of hardness and Young’s modulus compared with those of PbTe. For example, from Figure 10(a), it can be found that the hardness of the layer of Pb0.83Ge0.17Te is three times as great as that of PbTe; meanwhile, Young’s modulus is twice greater than that of PbTe, as seen in Figure 10(b). Therefore, a conclusion can be drawn that a mechanically robust infrared high-index layer can be obtained using Pb1−xGexTe as evaporation materials.
These mechanical behaviors of layers of Pb1−xGexTe can be linked to the ferroelectric phase transition. Moreover, the strength loss in the layers can be also explained in light of strong localized elastic-strain fields in concentrated solid solutions.
Since Seeley et al. begun the employment of PbTe into the design and manufacture of infrared thin-film interference filters in Infrared Multilayer Laboratory at the University of Reading in 1960s, half of a century has passed. Nowadays, PbTe is still the first choice for the design of infrared thin-film interference filters operating in the long wavelength infrared both at room and cryogenic temperature. In the beginning of this century, aiming at the further improvement of the performance of PbTe, the investigations into Pb1−xGexTe were started up in Shanghai Institute of Technical Physics, Chinese Academy of Sciences. Nowadays, many fruits have been harvested after more than a decade passed.
First of all, it can be concluded that the electron beam evaporation can prove itself a promising powerful tool to make sure the congruent-deposition of the layers of Pb1−xGexTe directly from original Pb1−xGexTe evaporants. Therefore, because the controllability of the composition of evaporated layers of Pb1−xGexTe can be carried out, Pb1−xGexTe will be a prospective infrared high-index material in thin-film interference filters, due to its tunable optical properties corresponding to its intrinsic ferroelectric phase transition, such as temperature coefficient of refractive index and fundamental absorption edge. Furthermore, because the layers of Pb1−xGexTe have superior mechanical properties, such as the hardness and Young’s modulus, to those of PbTe, an infrared thin-film interference filters consisting of them will be robust enough to withstand the damage originated from standard wafer dicing processing. As a consequence, the integration of the standard semiconductor process into the mass-production of infrared thin-film interference filters can be also realized.
In addition, one main challenge that needs to be addressed is the toxicity of lead and tellurium. In particular, some issues are concerned regarding the massive use of them in technology due to the toxicity, high costs, and scarcity. In fact, as far as an infrared thin-film interference filter is concerned, the low-index materials, like ZnSe or ZnS, are chosen as an outermost layer so that the layers of PbTe or Pb1−xGexTe are extremely well encapsulated between two adjacent layers of ZnSe or ZnS, followed by careful edge sealing, in order to reduce the hazards of Pb and Te exposure. Furthermore, at the end of the module lifetime, it is important to ensure that all materials be recycled, as already happens for all of the products of infrared thin-film interference filters. However, it would be desirable to find alternatives which retain the unique optical properties of PbTe and Pb1−xGexTe. Currently, an investigation is in progress in Shanghai Institute of Technical Physics, Chinese Academy of Sciences to seek an environmentally-friendly, cost-efficient alternative to PbTe-based infrared high-index coating materials.
All authors are sincerely grateful to contributions from Prof. Fengshan Zhang and Mr. Ling Zhang (Shanghai Institute of Technical Physics, Chinese Academy of Sciences), Dr. Bin Fan (Optorun Co., Ltd.) and Prof. Jinchun Jiang (East China Normal University). Many thanks to Mrs. Ling Yu (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the AES analyzes; to Mr. Yuehong Hong and Dr. Guofeng Cheng (Shanghai Institute of Ceramics, Chinese Academy of Sciences) for the XRD analyzes; to Prof. Xianghua Nan (Shanghai Jiao Tong University) and Prof. Xiangming Meng (Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) for the EDAX analyzes; to Dr. Jinlong Li (Ningbo Institute of Industrial Technology, Chinese Academy of Sciences) for his support in the nanoindentation measurements.
Conflict of interest
We declare that we have no conflict of interest.