Lead sulfide (PbS) is an important semiconductor material in group Ⅳ?Ⅵ semiconductors with a narrow band gap (0.41 eV) and a large exciton Bohr radius (18 nm) at room temperature. These characteristics make PbS highly sensitive to infrared (IR) radiation, which has led to the widespread use of PbS films in IR detectors, solar cells, gas sensors, and other fields. The as-grown PbS thin films respond poorly to IR and can only be used as highly sensitive IR detectors after sensitization. Annealing in oxygen is an effective method to enhance the photosensitivity of PbS detectors, and oxygen has been proven to be the best sensitizer. Although numerous studies have been carried out on one-step annealing in an oxygen-contained atmosphere, the development of new methods to further improve thin films' responsiveness to IR continues to plague academia and industry. In this work, a new two-step sensitization treatment process for as-grown PbS thin films by the chemical bath deposition (CBD) method is investigated, in which the films are first annealed in oxygen at a low temperature and then re-annealed in nitrogen at a higher temperature. It is shown that this process can effectively improve the optoelectronic properties of PbS thin films compared with pure oxygen annealing.

PbS thin films are fabricated on glass substrates by the CBD method. The reaction solution is composed of lead acetate [Pb(CH₃COO)₂·3H₂O], trisodium citrate (C₆H₅Na₃O₇), potassium hydroxide (KOH), and thiourea (CH₄N₂S). The cleaned glass substrates are immersed in the deposition bath. After deposition, the samples are rinsed with deionized water and dried. The as-grown PbS films are uniform, mirror-reflective films, and subsequently, they are annealed in oxygen for 1 h at 550 ℃. Lastly, the oxygen-annealed thin films are re-annealed in nitrogen at 600 ℃ for 10 min, 50 min, 80 min, and 110 min. The Cr/Au electrodes are realized by the magnetron sputter, and the phase and crystal structure of these samples are characterized by the X-ray diffractometer. The surface morphology of the samples is observed by optical microscope and scanning electron microscope (SEM), and the thicknesses of the films are measured by the step profiler. The photoelectric properties are evaluated by a photoelectric test system with a source meter connected to a probe station at room temperature. Lasers are used as excitation sources, and a waveform generator is employed to control the switching of the lasers.

The as-grown PbS thin film shows a dense, compact surface morphology. After oxygen annealing, the new oxidation phase is formed (Fig. 1). Some holes are observed on the surface of the annealed thin films as the re-annealing continues (Fig. 2). The grains of re-annealed thin films recrystallize to form larger nanocrystals powered by the thermodynamic driving force at high temperature (Fig. 3). The study of photoelectric properties shows that the photocurrent of samples increases before it declines with the re-annealing time at different power densities [Fig. 4 (b)]. The responsivity and specific detectivity of thin films drop as the light power density increases [Fig. 4(c) and Fig. 4(d)]. Although the number of photogenerated carriers increases with the rise in light power density, the conversion efficiency of photons into photocurrent is reduced. As the re-annealing time goes by, the values of specific detectivity and responsivity both increase first and then decrease. For a short period of re-annealing, the grains recrystallize to form microcrystals with high crystal quality, which allows the specific detectivity and responsivity values to rise. As the re-annealing time further increases, over re-annealing not only reduces the thicknesses of the films but also creates holes, which leads to lower specific detectivity and responsivity. The optimum value of specific detectivity is obtained at a re-annealing time of 80 min when the specific detectivity and responsivity values are 236% (5×10⁹ cm·Hz^1/2·W^-1 to 1.18×10¹⁰ cm·Hz^1/2·W^-1) and 259% (0.90 A·W^-1 to 2.33 A·W^-1) higher than those of thin films only annealed in oxygen, respectively[Fig. 5 (c) and Fig. 5(d)]. In addition, the sample re-annealed for 80 min exhibits high-frequency switching behavior and excellent stability at 4 kHz (Fig. 6).

In this study, polycrystalline PbS thin films are prepared by the CBD method. The thin films are sensitized with the two-step annealing method, in which the thin films are first annealed in oxygen at a low temperature and then re-annealed in nitrogen at a higher temperature. Appropriate nitrogen re-annealing time improves photoelectric properties by improving the crystal quality of thin films. The results show that the photoelectric properties of the PbS thin films sensitized with the two-step annealing method are significantly enhanced. Compared with the case of one-step annealing, the responsivity of thin films annealed in two steps with a re-annealing time of 80 min can be increased to 2.33 A·W^-1, an increase of about 259%,and the specific detectivity is raised to 1.18×10¹⁰ cm·Hz^1/2·W^-1, a growth rate of about 236%,under the light power density of 0.2 mW·mm^-2 and incident wavelength of 1550 nm. Additionally, the sample re-annealed for 80 min shows high-frequency switching behavior and excellent stability at 4 kHz. More importantly, two-step annealing can improve the photoelectric properties of the photodetectors on the basis of the traditional oxygen-sensitized thin films.