Activating photosensitivity in polycrystalline PbSe via in-situ oxygen ion beam induction
Introduction
From infrared imaging to biomedicine and other fields, the great potential of application has constantly aroused people's enthusiasm for mid-infrared radiation research [1,2]. Unfortunately, photodetectors working in the mid-infrared region, manufactured with narrow band gap semiconductors, always suffer from the quantum efficiency degradation and the large thermal noise due to the thermally activated carriers. Although narrow band gap semiconductors such as HgCdTe and InSb, as well as quantum superlattices [3], have been proved to be suitable for the application in mid-infrared optoelectronics. However, in order to realize the effective detection of mid infrared photons at room temperature (300 K), the narrow band gap and high operating temperature are still extremely demanding requirements. In the application of mid-infrared optoelectronics, lead salt materials show unique advantages such as high dielectric constant, large exciton Bohr radius, high optical nonlinear effect, good chemical stability and easy processing [4,5]. Polycrystalline lead selenide (PbSe) provides strong mid-infrared photosensitivity enhancement via noise suppression and carrier transport manipulation by incorporation of oxygen/halogens, making it an attractive candidate in uncooled mid-infrared sensing and optoelectronics [[6], [7], [8], [9], [10], [11], [12]]. Oxygen can affect the optical properties of specific materials and enhance the photosensitivity of the materials, which has been gradually demonstrated in the lead chalcogenides and metal oxides [[13], [14], [15]]. This material can be prepared by many fabrication methods, among which the physical vapor phase deposition (VPD) provides extra merits such as large area homogeneity and CMOS compatibility. For decades, the thermal diffusion method based on VPD has been the main method to prepare infrared sensitive lead chalcogenides, which indeed provides an easy-processing method. However, that method has its limitations, such as the inability to adjust the depth distribution of dopants and control the doping dose precisely. The large optical band gap of thermally diffused polycrystalline PbSe films (about 1.6 eV or higher) leads to a weak absorption in mid-infrared region and therefore a low internal quantum efficiency [[16], [17], [18]]. We reported an ion beam implantation based preparation method to improve optical and electrical properties [19,20]. However, that method also has drawbacks such as high cost and high induced free carrier concentration of target material [21].
The atmosphere diffusion method widely used by researchers diffuses oxygen/iodine into the surface of PbSe at high temperature. Unlike atmosphere thermal diffusion, the in situ ion beam method makes the low energy ion beam grow together with the material in the deposition process, and then through annealing to activate the incorporated oxygen ions into the PbSe lattice. The interaction between the low energy ion beam and the material can adjust the surface and bulk properties of the material, exhibiting unique electrical, optical and photoelectric properties [22,23]. Additionally, the ion beam doping can adjust the dose precisely, change the spatial distribution in the depth direction, separate the doping from the polycrystallization process, and increase the extra freedom of the preparation process optimization. Furthermore, low energy ion beam will not damage the crystal structure and is fully compatible with CMOS technology.
In this letter, a low-cost in situ O+ beam preparation method for mid-infrared sensitive polycrystalline PbSe films was demonstrated. A relatively effective mid-infrared absorption and low structural disorder were both produced via low energy O+ beam based modulations and modifications. The morphological, structural, optical and optoelectronic properties were characterized. The dependences of responsivity on structural and optical properties were discussed. The mechanism on how the photosensitivity was activated through O+ beam was also discussed based on the induced hierarchical structure and the effect of oxygen on PbSe.
Section snippets
Experimental
The sample was deposited on an ITO glass substrate by the vacuum thermal evaporation with the substrate temperature of 393 K and the vacuum pressure of 4.5 × 10−3 Pa. The high pure (99.99%) PbSe powder was used for preparation. The thickness of the epitaxial layer and the deposition rates were set to 1.0 μm and 2.0 Å/s, respectively. Since the absorption and photoelectric effect mainly occur around film surface, and the filament of ion source is quickly oxidized using oxygen, a step modulation
Results and discussion
Unlike the rock-like morphology of thermally diffused PbSe films [9], a layered strip-like microstructure of this material was observed, as shown in Fig. 1(b). The strip-like crystallites are dominated by the predominant growth along PbSe(200) determined by the preparation conditions such as substrate temperature [24]. The formation of layered microstructure can be attributed to the minimization of surface and strain energies which distributions are affected by film thickness and temperature [24
Conclusion
In conclusion, an in situ O+ beam preparation method was presented to fabricate mid-infrared sensitive polycrystalline PbSe thin film. The XRD pattern and the low Urbach energy suggested a high crystallinity of this material. XPS transitions confirmed the incorporation of O+ into PbSe lattice. A suitable optical band gap of 0.79 eV was obtained for mid-infrared optoelectronics. Preliminary optoelectronic characterizations confirmed the mid-infrared photosensitivity of this material. A
Funding
This work is supported by the National Natural Science Foundation of China [grant No. 61505166]; Natural Science Basic Research Plan in Shaanxi Province of China [program No. 2019JM-246]; and the Fundamental Research Funds for the Central Universities [grant No. 3102015ZY098].
CRediT authorship contribution statement
Hao Yang: Conceptualization, Formal analysis, Writing - original draft. Guodong Wang: Investigation, Methodology. Xiaojiang Li: Investigation. Jianbang Zheng: Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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