When temperature varies, the performance of an optical thin-film interference filter changes, such as the shift of center wavelength and the deterioration of peak transmission. In particular, when the filter is used in light-wave communication systems or spaceborne remote sensing instruments, the performance variations are not acceptable. It becomes difficult to sustain the functional performance of non-dispersive narrow-bandpass interference filters for precision spectroradiometric measurements from space [1].

In the last decade, some new evaporation techniques based on the ion-assisted deposition (IAD) or the plasma-IAD have been developed, which make it possible to grow dense films of near-unity packing density, in order to avoid the moisture-induced wavelength shift. These techniques to date are applied to fiber-optics communication systems in UV and visible region at an elevated temperature [2–4]. However, as far as infrared thin-film interference filters employed at low-temperature are concerned, it is difficult to eliminate completely the shift of center wavelength of a narrow-bandpass filter induced by temperature. It is not practical to add an auxiliary temperature control to a narrow-bandpass filter 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 temperature. One is the temperature-induced variation in the refractive index of the film, and another is the variation in the physical thickness of the film. Since the bulk temperature coefficients of linear expansion are an order of magnitude smaller than the temperature coefficients of refractive index for substances similar to those usually employed for interference filters, it may be speculated that the shift of wavelength should be ascribed to the variations of temperature coefficient of refractive index of the film [5].

By classical design, thin-film interference narrow-bandpass filters usually consist of two materials that possess high refractive index and low refractive index, respectively. As far as the infrared filters employed in the spaceborne remote sensing systems are concerned, a convenient material used for high-index layers is PbTe (5.5), and for low-index layers the materials are either ZnS (2.2) or ZnSe (2.3). All materials are fully transparent over the infrared. Seeley et al. [6] 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). The temperature shift of the peak wavelength and the loss of transmittance could then be predicted. 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 does not exactly compensate for that of ZnS (ZnSe). Therefore, another solution to the problem is to seek a material where temperature coefficient of refractive index can be tuned.

Among the IV-VI narrow-gap semiconductors, the tellurides of Sn and Ge, also their alloys show a ferroelectric phase transition from a high-temperature rocksalt (O_h) structure above a critical temperature T_c to a low-temperature rhombohedral, arsenic-like (C_3v) phase. The rhombohedral structure originates from a displacement of two sublattices along a <111> direction that becomes the c-axis. In particular, for the pseudobinary alloy lead germanium telluride (Pb_1-xGe_xTe), the transition temperature T_c increases steeply with the increasing Ge concentration. The phase transition is driven by off-center site occupation of Pb ion sites by Ge ions [7]. Anomalies occur in the electrical resistivity and specific heat of Pb_1-xGe_xTe alloys corresponding to the ferroelectric phase transition. Our previous investigations [8,9] revealed that a maximum refractive index of Pb_1-xGe_xTe films also occurs corresponding to the structural phase transition, which reflects an increase of lattice polarizability. With the consequence that at the designated low-temperature, the temperature coefficient of refractive index of Pb_1-xGe_xTe thin films can be tunable from negative to positive by varying the Ge composition, the specific composition may be use as the high-index layers in the thin-film interference filters,.

Bulk Pb_1-xGe_xTe was grown in our crystal-growth laboratory using a modified Bridgman method from Pb, Ge and Te of purity 99.99999% in an excess of Te (<1 mol.%) proportion sealed in evacuated (~10^-6 mbar), carbon-coated quartz ampoules with a tip at the bottom. Then the ingots were crushed into small pieces and were used as the evaporable starting materials. The films were prepared by conventional thermal evaporation in a homemade system with a background vacuum 2.0×10^-5 torr. The Φ10×0.8mm silicon and germanium wafers polished on both sides were used as substrates. Each batch the films were simultaneously deposited on Si substrates for the composition analyses and on Ge substrates for transmission measurement. The substrate temperatures for all batches were kept at 130±2.0°C. The thicknesses of the films were monitored using an optical system during deposition, and determined by fitting the measured transmission spectra. The thicknesses for all samples were 2.10±0.3µm. The composition for all films was investigated by using an energy-dispersive analysis by X-ray (EDAX) with a sensitivity limit for element detection 0.1 wt.% equipped in Hitachi S-520 scanning electron microscope. The optical transmission spectra were measured in the spectral range of 2.5–25µm using a Perkin Elmer Spectrum GX Fourier-transform infrared spectrometer (FTIR) with a resolution of 8cm-1 at normal incidence.

Subsequently, Pb_1-xGe_xTe evaporable materials with appropriate composition were selected to fabricate the thin-film interference narrow-bandpass filters. In order to make a comparison, equivalent filter was also fabricated using PbTe material under completely identical manufacturing conditions. The design of thin-film interference narrow-bandpass filters is a simple 8-layer Fabry-Perot type, that is, one half-wave film immersed in quarter wavers is deposited on a germanium substrate of index 4.0 without rear surface antireflection, as follows:

substrate|LHLHLLHLH|air

where L and H represent, respectively, quarter-wave layers of ZnSe and Pb_1-xGe_xTe (or PbTe), and the underlining signifies a spacer.

The spectral characteristics of fabricated narrow-bandpass filters were measured in the temperature range of 80–300K using a bath cryostat (Oxford, DN1704). The measurements were reproducible, and no indication of hysteresis effect was observed. The characteristics for filters fabricated with Pb_0.94Ge_0.06Te and with PbTe are shown in the Figs. 1(a) and (b), respectively. A comparison was made for the values of center wavelength and peak transmittance versus temperature is given in the Fig. 2.

It can be inferred that, if further improvements on the accurate control of the Ge composition in the Pb_1-xGe_xTe consistent layers can be carried out, it was possible to ultimately realize the temperature-invariant characteristics for infrared thin-film interference filters. Usually the component elements in the multicomponent alloy will evaporate at a different rate causing a change in thin-film composition relative to the initial bulk alloy, therefore, an empirical procedures may be used to determine how to control the composition of the film by adjusting the composition of the bulk alloy, or by using nonequilibrium evaporation conditions. If these procedures are unsuitable, alternative deposition processes, such as sputtering, may be used.

 

Fig. 1. (a) Low-temperature spectral characteristics of the filter fabricated with Pb_0.94Ge_0.06Te; (b) Low-temperature spectral characteristics of the filter fabricated with PbTe.

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Fig. 2. Comparison of center wavelength and peak transmittance versus temperature for two filters (square and up-triangle are for the filter fabricated with Pb_0.94Ge_0.06Te; circle and down-triangle are for that fabricated with PbTe)

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