The micro-morphologies of PbSe films fabricated from CBD, traditional VPD and modified VPD technologies were shown in Fig. 3. The side-view of CBD-PbSe films in Fig. 3(a) displayed a rough and compact layer constituted by the oriented pyramidal-shape micro-polycrystalline, which was confirmed by the top-view FESEM image in Fig. 1(c). Based on the appearance of a strong peak at 29.14° in its XRD pattern showing in Fig. 4, corresponding to PbSe (200) facet, it sees that the pyramidal-shape micro-polycrystalline were mostly oriented along [111] direction. Higher resolution side-view FESEM in Fig. 3(b) revealed that every micron-size pyramidal crystallite was self-assembly of very small nanocrystals. The size of nanocrystals is of 8–12 nm. This structural feature was attributed to the synergistic effect of initial ion-by-ion (IBI) growth model and later Oriented-Attachment (OA) growth model, considering lower surface energy of (100) facets.³⁹ Through the contrast various microstructures obtained from diverse methods, we figured that the self-assembly structure occurring in as-grown films was a most important factor for PbSe IR-response. No any infrared response was observed from the PbSe films composed of micro-size pyramidal crystal without self-assembly nanoparticles by using hydrothermal method at 180 °C (Fig. 5(b)) or the films packed by disordered nanoparticles synthesized at 3 °C CBD growth temperature (Fig. 5(c)), regardless of the sensitized conditions.

The recent studies^{9,18,33–37,41} have shown that iodine is key element to trigger the IR-response of PbSe by forming PbI₂, PbSeI_x or Pb–Se–O–I phase at the surface and interfaces between PbSe grains. Due to hard diffusion of iodine in the large PbSe single-crystal grains, it is difficult to form a continuous PbI₂, PbSeI_x or Pb–Se–O–I phases transporting channels for photo-generated electrons in the whole PbSe film during sensation process. At the same time, large single-crystal grain was disadvantageous for oxygen passivation to eliminate the defects in the PbSe crystalline. There were lots of defects in an amorphous nanoparticle PbSe film synthesized at 3 °C.³⁸ These defects can't be completely eliminated by the sensitization process. The surviving defects become the recombination centres for photo-generated electrons and holes, then decreased the carrier lifetime, resulting in unsensitive for IR radiation.

No infrared responsivity was detected in sensitized traditional-VPD method, confirming that the key role of self-assembly microstructure of CBD as-grown PbSe films on the infrared response. A typical compact columnar structure of traditional VPD-PbSe films was shown in Fig. 3(c), and one sharp and narrow XRD peak corresponding to PbSe (200) facet in Fig. 4 indicates that columnar crystals with 1 μm size grow preferentially along [200] orientation. By introducing O₂ into the VPD deposition chamber and depositing at 25 °C substrate temperature, a microstructure similar with CBD-PbSe film was achieved, as shown in Fig. 3(e) and (f). It was a smooth and compact film constituted by (200) oriented PbSe nanorod-like clusters with a uniform distribution of 120 nm. High-resolution side-view FESEM in Fig. 3(f) obviously displayed that the nanorods were built by self-assembly of nanoparticles, which is originated from OA growth mechanism.

The influence of vacuum degree and substrate temperature on the microstructures of VPD-PbSe films were studied, shown in Fig. 5(d) and (e) show the typical microstructural features of VPD-PbSe films by various O₂ pressure at 130 °C growth temperature. With the increase of the O₂ pressure from 10⁻⁴ to 5 × 10⁻³ Pa, the size of columnar crystals decreased from 1 μm to 50 nm, presenting a single crystal rode-like crystals as shown in Fig. 5(d). With further increasing the O₂ pressure to 4 × 10⁻² Pa, one-dimensionally ordered self-assembly of nanocrystals could be obviously observed from Fig. 5(e) and (f). Therefore, there seemed to be no doubt that high O₂ pressure and low growth temperature contribute to the formation of microstructure of nanorods composed of self-assembled nanocrystals. It is well known that low growth temperature inhibits the spontaneous growth of columnar PbSe grain originated from dynamic driving. At the same time, the absorbed oxygen on the surfaces of PbSe nanoparticles lead to a transformation of growth mechanism from IBI to OA model.⁴⁰ Subsequent, detectivity texting results proved that the optimized substrate temperature is at the range of 25–130 °C under O₂ pressure of 10⁻² Pa. More typical morphologies of VPD-PbSe as-grown films could be obtained from ESI.†

Beside of microstructure, the modified VPD technology also duplicates the phase composition of PbSe film fabricated from CBD technology, identified by XRD patterns showing in Fig. 4. Although a slight difference in intensities, the all PbSe facets in CBD-film XRD pattern appeared in the pattern of modified VPD film.

It is well known that once deposited, lead-salt film is alive to IR radiation after suffering thermal-sensitization process in O₂ and/or I₂ atmosphere.^18,32 By now, the sensitization processes vary with the individual group. In our case, an optimized two-step sensitization process was employed, including oxidization at 420 °C for 1 h, followed by iodization at 380 °C for 10 min.

The responsivity R and specific detectivity D* were obtained as described in our previous work by using the definition as shown below:where I_s and I_n are the measured detector output signal and noise currents, A is the device detection area, Δf is the noise bandwidth, and P_i is the incident radiant power. Peak responsivity R_peak and peak detectivity D* are then calculated through a conversion factor defined in ref. 17.

Finally, a room-temperature peak detectivity D* of 1.6 × 10¹⁰ cm Hz^1/2 W⁻¹ was achieved at blackbody 500 K under 750 Hz chopping frequency modulation, which outperforms the VPD-PbSe detectors reported by NIT, and approaches the D* of 2.8 × 10¹⁰ cm Hz^1/2 W⁻¹ of the CBD-PbSe detector.¹⁷

Fig. 6 presented the microstructures of CBD-and VPD-PbSe detectors. The side- and top-view of FESEM image of CBD detector showing in Fig. 6(a) and (b) demonstrated the sensitized CBD-PbSe grains was 0.5–1 μm, and have blunted edges and a surface roughness of 0.2–0.3 μm. We deem that the morphological evolution before and after sensitization was attributed to in sequence (1) the oxygen passivation of intrinsic and defects within PbSe nano-crystals; (2) the recrystallization of PbSe nano-crystals and (3) the formation of PbSeO₃ or Pb–Se–O–I phase around the PbSe crystals.^9,18 The smaller the nanocrystal is, more easily the oxygen diffuses and more complete the passivation, the better the nanocrystal quality. Subsequently, the nanocrystals with high crystal quality recrystallize to form large microcrystals powered by the thermodynamic driving force (420 °C). The two above-mentioned reasons could be vitrified by the enhanced peak intensities and the narrowed full width at half maximum (FWHM) related to PbSe facets in the XRD patterns, showing in Fig. 7.

An exactly the same evolvements in morphology and phase composition were observed in the sensation process of VPD-PbSe film, as shown in Fig. 6(c)–(e) and 7. The morphology of sensitized VPD-PbSe showed a small PbSe grains of 150–200 nm and a smooth surface roughness of less than 100 nm. The substantial improvement in the distribution of PbSe microcrystal size and the surface roughness can help to improve the performance uniformity and yield of lead-salt IR-sensitive films, which is key for FPA imaging system. In comparation with compact tissue of CBD-PbSe sensitized film, there were many voids in the sensitized VPD-PbSe films, which was considered as the reason for the D* of VPD-PbSe detector lower than that of CBD-PbSe detector in our case. Therefore, detectivity D* of VPD-PbSe detectors has great developing opportunity by further optimizing the sensitization process to eliminate the voids in VPD-PbSe detectors.

Note, there were only (200) and (400) peaks surviving in the sensitized VPD-PbSe XRD pattern in Fig. 7. This is caused by a mixed Oriented Attachment and Ostwald Ripening (OR) growth pathway during the recrystallization process.³⁸ First, adjacent small PbSe particles with common crystallographic orientations combine to form large one by OA mechanism in initial annealing stage. Then larger PbSe grains with high orientation are formed by an interface-reaction-limited mass transfer from smaller particles. The PbSe (100) facet is significantly lower in energy than the (111) facet, PbSe shows a tendency to form clusters with a rectangular symmetry with (100) orientation on the glass at low temperature, or rods with a cubic symmetry at high temperature, as shown in Fig. 3(e) and (d), respectively. After high-temperature recrystallization, the PbSe particles with (100) orientation merges into big grains by OA growth mechanism, companying with significant increase of (100) intensity contributed from recrystallization and oxygen passivation. At the same time, other PbSe small particles with (220), (311) orientation slowly disappear for the interface-reaction-limited mass transfer, and merge into large PbSe grains with (100) orientation. Finally, there are only (200) and (400) facets with strong intensities observed in XRD patterns.

The influence of resistance on the detectivity was analysed, and was summarized in Table 1. Based on the p–n charge separation model for the enhanced photoconduction detection mechanism,³³ the resistance of PbSe detector was regulated by the thickness of PbSeO₃,¹⁶ PbSeI_x (ref. 35) or PbI₂ (ref. 34–37) wrapping in PbSe microcrystals, which was dominated by the iodine-sensitization time. With increasing the iodine-sensitization time to 30 min, the resistance of detector significantly increases by almost four orders of magnitude up to 8 MΩ. The best detectivity D* was achieved at resistance of 0.2 MΩ with a 0.2 μV noise and a 9150 signal/noise (S/N). It is found that the signal and noise reach the maximum as the resistance kept on increasing to 0.4 MΩ. However, the increased multiple in noise (3) was more than the multiple in signal (1.4), resulting in an order of magnitude reduction in the detectivity D* (2.0 × 10⁹ cm Hz^1/2 W⁻¹). Subsequently, the signal and noise drop synchronously with further increasing resistance to 8 MΩ. Based on the morphological evolution, there was a reasonable prospect that the size and number of voids mount with increase of the resistance (iodine-sensitization time), resulting in increasing of the noise. The maximum signal of 2490 μV at 0.4 MΩ for VPD-PbSe detector almost approached to the signal of best CBD-PbSe detector obtained at 4 MΩ, indicating that the future research of VPD-PbSe technology is focus on the noise reduction while maintaining the maximum signal.

The typical frequency response of the optimized VPD-PbSe detector was shown in Fig. 8. It's always been known that the lead-salt detector has a 1/f noise characteristic, so the lower frequencies have higher noise and consequently a lower D* value. The detectivity D* relatively constant after the chopping frequency was increased to 950 Hz. For PbSe detectors, 1 kHz is chosen as a standard chopping frequency because it is away from the 1/f noise but lower than the time constant response limit.

Base on the dependency of the detectivities on the microstructures of PbSe films obtained from various fabrication technologies, a qualitative explanation for the optimum microstructure is given. It is obvious that the performance of PbSe photoconductors is dominated by the crystal quality, orientation and size of PbSe particles, which depends on the fabrication technology and deposition temperature.^44,45 The crystal quality determinates the life time of the photogenerated carriers. Higher crystal quality is, fewer defects inside crystal, lower number of recombination centres of photogenerated electrons and holes, longer the lifetime of photogenerated carriers, resulting in an enhanced detectivity. The ordered crystal orientation of PbSe particles contributes to the glide of the defects or dislocations inside crystals⁴⁶ during the oxidization step, resulting in an improved carrier lifetime. Based on the p–n charge separation model, the particle size is critical to the formation of p–n heterojunction interface. Larger particle is, denser the structure, harder the iodine diffuses, fewer the p–n heterojunction interface, the more difficult for the photogenerated carriers, resulting in a lower infrared responsivity. Suffering from the process modifying, the microstructure of VPT PbSe film changes from typical microrods (Fig. 3(d)) into nanorods comprised of self-assembled nanoparticles (Fig. 3(f)), which helps to eliminate the defects and dislocation during the first-step sensitization, resulting in a longer carrier lifetime. At the same time, small size and loose structure make the iodine diffusion easier to form larger number of p–n heterojunction interface during the second-step sensitization, speeding the separation of photogenerated carriers.