Lead Selenide Polycrystalline Coatings Sensitized Using Diffusion and Ion Beam Methods for Uncooled Mid-Infrared Photodetection

Polycrystalline lead selenide material that is processed after a sensitization technology offers the additional physical effects of carrier recombination suppression and carrier transport manipulation, making it sufficiently sensitive to mid-infrared radiation at room temperature. Low-cost and large-scale integration with existing electronic platforms such as complementary metal–oxide–semiconductor (CMOS) technology and multi-pixel readout electronics enable a photodetector based on polycrystalline lead selenide coating to work in high-speed, low-cost, and low-power consumption applications. It also shows huge potential to compound with other materials or structures, such as the metasurface for novel optoelectronic devices and more marvelous properties. Here, we provide an overview and evaluation of the preparations, physical effects, properties, and potential applications, as well as the optoelectronic enhancement mechanism, of lead selenide polycrystalline coatings.


Introduction
The photoelectric conversion is one of the important interaction effects between semiconductors and light, and has promoted the development of society and affected our daily life for decades.Numerous applications such as the optical communication, photography, biomedical imaging, security/defense, industry monitoring, gas sensing, and solar cells have been explored due to the developments of photosensitive semiconductor materials and relevant device integration technologies such as the complementary metal-oxide-semiconductor (CMOS).However, higher requirements on the optical and optoelectronic sensitivity, response speed, portability, sensitive wavelength range, as well as the ability to integrate with CMOS for the photoelectric devices, have been put forward due to the developments of industry and our society.
The PbSe nanocrystalline and polycrystalline coatings are becoming more and more attractive in many potential applicable fields such as infrared optoelectronics [29,30], optical nonlinearity [15,16], mode-locked lasers [17], thermoelectric material [21][22][23], solar cells [18][19][20], and biomedical labeling [31,32].Various fabrication methods, including chemical bath deposition (CBD) [33], electrochemical deposition [34,35], thermal evaporation [13,36], electron beam evaporation (EBE) [37], molecular beam epitaxy (MBE) [38,39] and RF magnetron sputtering [40,41], as well as other preparation methods [42,43], have been adopted to prepare PbSe nanocrystalline and polycrystalline coatings.Likewise, great efforts have been devoted to the structural, compositional, optical, electrical, and optoelectronic properties of this material, as well as the relevant physical mechanisms such as crystal growth and carrier transport [44][45][46][47].Among these, high performance infrared-sensitive photodetectors based on the quantum dot [29,30], polycrystalline [48,49], and composite material [50], as well as the relevant heterojunctions [51], have attracted wider attention.However, absorbing and detecting mid-infrared photons should usually performed at low temperatures because of the thermally excited carriers and density fluctuations of carriers due to the narrow bandgap of the material.Chalcogens and halogens were discovered to be able to reduce the noise signal and enhance the photosensitivity of PbSe coatings by doping and incorporating them into a PbSe crystal lattice [52].This technology, which is known as sensitization, consequently leads to the material being structurally polycrystalline and completely uncooled for mid-infrared photodetection.Compared with the thermal detector, the uncooled photodetector with sensitized PbSe polycrystalline coatings exhibits a variety of advantages, such as a high speed response, large dynamic range, the ability of miniaturization, and high spatial resolution, and can be a promising candidate in numerous midinfrared applicable fields, as shown in Figure 1.In this review, we present various material fabrication technologies using diffusion and ion beam techniques toward photosensitive PbSe polycrystalline coatings, and the resulting uncooled photodetectors in the mid-infrared range (1.5-5.5 µm).First, we introduce the initial motivation of Coatings 2018, 8, 444 3 of 21 optoelectronic sensitization and the relationship between the photosensitivity enhancement and it in detail.Then, the recent progress on the optical, electrical, and optoelectronic properties, as well as the structural and compositional dependences of PbSe polycrystalline coatings fabricated by currently different diffusion and ion beam technologies, is presented and discussed.Finally, the future development direction, as well as the possible interesting areas, challenges, and perspectives, are also discussed.
How Can the Optoelectronic Sensitization Enable Photosensitivity Enhancement Without Cooling?
Generally, the physical properties-including optical, electrical, and optoelectronic-of a photodetector are highly dominated by the structural and compositional properties of the material, which are determined by the preparation conditions and parameters.The optoelectronic performance of a photodetector is usually defined using the responsivity and the specific detectivity.The responsivity of a photoconductor can be expressed as where η is the internal quantum efficiency (IQE), λ is the wavelength, τ is the minority carrier lifetime, V b is the bias voltage, A is the detector area, d is the thickness of the material, h is the Planck constant, c is light speed in vacuum, and n 0 is the free carrier concentration [53].Thus, the specific detectivity can be calculated by where ∆f is the frequency bandwidth, and V n is the rms noise voltage [51].To improve the optoelectronic performance, the signal-to-noise ratio should be improved, but is hard to achieve at room temperature, since thermally excited carrier concentration and the carrier fluctuations are significant and comparable with the photogenerated carrier concentration at room temperature due to the narrow bandgap.An easy approach is cooling the photodetector platform to reduce the thermally induced noise signal and free carrier concentration, which are usually positively correlated.This cooling method uses thermoelectric cooler (TEC) or another cooling platform, which requires ultra-power, and makes the detector non-portable and complicated.Another method is to improve the minority carrier lifetime that can be determined by another parameter: the photoconductive gain factor where τ n and τ p are the lifetime of the electrons and holes, respecitvely; µ n and µ p are the mobility of electrons and holes, respectively; and l denotes the electrode distance.It is seen that the G factor is affected by the carrier transport properties and the design of detector electrodes.An easy approach to improve the carrier lifetime is doping the near deep energy-level elements into the material to construct the minority carrier traps, which capture minority carriers and then release them.Oxygen has been found to be the proper one, and it can be diffused into the PbSe crystal lattice and activated by annealing at about 400 • C. Another effect of annealing in oxygen atmosphere is the material passivation by which the structural defects are passivated, and the free carrier concentration (n 0 ) and the dark current are reduced.Therefore, the responsivity is highly enhanced to enable the photodetector to be much more sensitive to the infrared radiation without cooling.This is the initial motivation of the optoelectronic sensitization of the lead chalcogenides.The PbSe material after sensitization exhibits a polycrystalline structure with an average crystallite size of hundreds of nanometers.Another sensitization technology is halogen treatment, which can further passivate the material to reduce the dark current, and is proven to be prone to highly improve the potential barrier height between crystallites, as well as be superior to oxygen [54].A high potential barrier height between crystallites can reduce the dark current and the noise signal to further improve the specific detectivity of the photodetector, since the thermally excited carriers don't have enough energy to move across the crystallite boundary barrier.These effects make the PbSe polycrystalline coatings able to work at room temperature (300 K) without cooling.

Optoelectronic Sensitization Models toward Physical Mechanisms
To exploit the physical mechanism of optoelectronic sensitization, many models have been presented.These models can be generally divided into four classes: the minority carrier trap model, the barrier model, the generalized model, and charge separation junction models.Each model describes a potential mechanism of carrier transport, although there may be multiple mechanisms in the PbSe polycrystalline coatings.Furthermore, the model referred to physical issues that are hard to be verified by experimental observations or theoretical calculations.Nevertheless, the specific parameters of the physical properties calculated by the models can be almost consistent with the experimental characterizations.Therefore, more experimental observations and verifications on a micro level such as structural defects, carrier transport, and crystal growth, and theoretical calculations on band structure using density functional theory (DFT) and the Monte Carlo method, should be performed to exploit or verify the physical mechanisms in the PbSe polycrystalline coatings.

The Minority Carrier Trap Model
By doping near deep-level oxygen atoms into the crystal lattice of PbSe, a minority carrier trap level could be introduced into the band gap.Humphrey and Petritz suggested that a (PbO) + generated by a (PbO) ++ acceptor level after sensitization with oxygen may serve as an electron trap [55].When the photons are absorbed, the electron-hole pairs are excited.Some electron-hole pairs recombine immediately, while some other electrons are trapped by the trap level, as shown in Figure 2a.The untrapped electrons will recombine with holes at the recombination center in a very short time of about tens of nanoseconds (process I), which is known as Shockley-Read-Hall (SRH) recombination.The trapped electrons should be released to the conduction band before recombination (process II).However, the release process is much slower than recombination within about serval microseconds.So, the minority carrier (electron) lifetime will be improved and enhance the responsivity of the photodetector, since the surface layer of the PbSe polycrystalline coating will be converted from n-type to p-type after sensitization with oxygen.This model can coherently explain the carrier lifetime improvements, but the minority carrier trap energy level and the composition have not been verified by experimental observations.Recently, a preliminary theoretical band diagram calculation by DFT was performed for the oxygen/iodine-doped crystal cell, showing that an acceptor level exists and may act as a hole trap by accepting an electron [54].However, this calculation result exhibits a difference in carrier trap type.Any refined theoretical or experimental attempts should be made to verify or refute the carrier trapping mechanism.of the photodetector, since the thermally excited carriers don't have enough energy to move across the crystallite boundary barrier.These effects make the PbSe polycrystalline coatings able to work at room temperature (300 K) without cooling.

Optoelectronic Sensitization Models Toward Physical Mechanisms
To exploit the physical mechanism of optoelectronic sensitization, many models have been presented.These models can be generally divided into four classes: the minority carrier trap model, the barrier model, the generalized model, and charge separation junction models.Each model describes a potential mechanism of carrier transport, although there may be multiple mechanisms in the PbSe polycrystalline coatings.Furthermore, the model referred to physical issues that are hard to be verified by experimental observations or theoretical calculations.Nevertheless, the specific parameters of the physical properties calculated by the models can be almost consistent with the experimental characterizations.Therefore, more experimental observations and verifications on a micro level such as structural defects, carrier transport, and crystal growth, and theoretical calculations on band structure using density functional theory (DFT) and the Monte Carlo method, should be performed to exploit or verify the physical mechanisms in the PbSe polycrystalline coatings.

The Minority Carrier Trap Model
By doping near deep-level oxygen atoms into the crystal lattice of PbSe, a minority carrier trap level could be introduced into the band gap.Humphrey and Petritz suggested that a (PbO) + generated by a (PbO) ++ acceptor level after sensitization with oxygen may serve as an electron trap [55].When the photons are absorbed, the electron-hole pairs are excited.Some electron-hole pairs recombine immediately, while some other electrons are trapped by the trap level, as shown in Figure 2a.The untrapped electrons will recombine with holes at the recombination center in a very short time of about tens of nanoseconds (process I), which is known as Shockley-Read-Hall (SRH) recombination.The trapped electrons should be released to the conduction band before recombination (process II).However, the release process is much slower than recombination within about serval microseconds.So, the minority carrier (electron) lifetime will be improved and enhance the responsivity of the photodetector, since the surface layer of the PbSe polycrystalline coating will be converted from ntype to p-type after sensitization with oxygen.This model can coherently explain the carrier lifetime improvements, but the minority carrier trap energy level and the composition have not been verified by experimental observations.Recently, a preliminary theoretical band diagram calculation by DFT was performed for the oxygen/iodine-doped crystal cell, showing that an acceptor level exists and may act as a hole trap by accepting an electron [54].However, this calculation result exhibits a difference in carrier trap type.Any refined theoretical or experimental attempts should be made to verify or refute the carrier trapping mechanism.

The Barrier Model
Slater found that the temperature dependence of the electrical conductivity follows a linear relationship between lnσ and 1/T (Arrhenius behavior) [56].Hence, he claimed that the grain boundary barrier exists between neighboring grains, and is caused by an n-p-n type region around the grain boundary, as shown in Figure 2b.Horn also pointed out that the grain boundary is surrounded by PbO to form a p-n heterojunction between grains in the PbSe polycrystalline coatings after sensitization [57].This model suggests that the high resistance after sensitization is attributed to the grain boundary barrier, since free electrons are hard to move from a grain to another under a bias voltage, as shown in Figure 2b.However, the increase in photoconductivity is attributed to the decline of the barrier height due to the photogenerated carriers.The study on carrier transport from the recent literature found that the carrier transport indeed obeys the Arrhenius thermally-driven behavior at high temperature of >210 K, while an obvious deviation from the Arrhenius behavior occurs at low temperature of <210 K [54].Therefore, there should be another transport mechanism that dominates the carrier transport and electrical conductivity at low temperatures.

The Generalized Model
Considering the density of photogenerated carriers and the grain boundary barrier, Petrtiz presented a model that is similar to that of bulk material [58].It suggests the electrical conductivity of the PbSe polycrystalline coating is the sum of that of every crystallite, and the photoconductivity enhancement is attributed to the variation of carrier mobility that depends on the grain boundary barrier.Likewise, the noise and sensitivity limit were also deduced.This model is mathematically coherent, and similar with that for polycrystalline silicon [59][60][61].Unfortunately, no one has claimed that the experimental data is always in good agreement with this model until now.Some oriented experiments including measuring the minority carrier concentration and lifetime, quantum efficiency, majority carrier concentration, and mobility by the Van der Pauw method, responsivity, and noise voltage can perhaps be carried out to verify this model.

The Charge Separation Junction Model
This model was presented by Zhao et al. and has been accepted by many researchers, including us [62].This model also supports the p-n junction region around the surface of the crystallites, which contains the oxides and iodides of PbSe, exhibiting an n-type conductive nature.On the contrary, it claims that the photogenerated carriers are just separated by the p-n junction into the corresponding channels (n-channel for electrons, p-channel for holes), and few carriers choose to move across the grain boundary barrier when biased [63], which is different from the previous p-n junction and barrier models [56,57].Electrons move along the surface of crystallites, and holes move along the core of crystallites, since the neighboring crystallites will fuse and connect together, as shown in Figure 3.The separation of the photogenerated carrier leads to an improvement of the minority carrier lifetime.According to this model, the carrier concentration distribution in the crystallites, minority carrier lifetime, and resistance after sensitization were calculated.The shell microlayer of the crystallites was designated to be n-type, because iodine was assumed to be an n-type dopant for PbSe, which is completely different from the oxygen/halogen-induced p-type conductivity reported in the literature [52].Furthermore, the previous works on the conductive types of PbSe polycrystalline coatings also revealed that only the as-grown PbSe thin film in n-type can trigger optoelectronic enhancement after sensitization, which implicitly supports the existence of the charge separation p-n junction, combining with the p-type shell of crystallites [64].Unfortunately, the core/shell p-n junction structure and the carrier transport channels seem to be hard to verify experimentally.Perhaps the high-resolution scanning photocurrent imaging may be promising to observe the carrier transport channels [65,66].

Fabrications and Properties of the PbSe Polycrystalline Coatings
We now discuss the physical properties of PbSe polycrystalline coatings prepared by different fabrication technologies, and their possible connections with the physical mechanisms of structural evolution, carrier transport, optical absorbance, and optoelectronic enhancement.

The Functional Effects Induced by Oxygen Treatment
The oxygen treatments on the monocrystalline PbSe coating fabricated by molecular beam epitaxy (MBE) at different temperatures were performed by Zhao et al. [67].They discovered that the oxygen treatment has a passivation effect on the carrier recombination in the PbSe polycrystalline coating, revealed by using the photoluminescence (PL) spectra operated at room temperature and 77 K, and the X-ray photoelectron spectroscopy (XPS) spectra, as shown in Figure 4.A high PL intensity revealed that the surface recombination and non-radiative recombination mechanisms, including Auger and SRH, were significantly inhibited after oxygen treatment, indicating that the surface states and the deep energy level recombination centers were passivated to improve the minority carrier lifetime and optoelectronic performance eventually.These results may be very useful for understanding the structural modification of the surface functional layer of the polycrystalline lead chalcogenides, and manufacturing ultrasensitive mid-infrared uncooled photodetectors [68,69].Likewise, another study on the effect of oxygen treatment on the evaporated n-type PbSe coatings was also carried out, and focused on the impact of electrical conductive type on the structural, compositional, electrical, and optoelectronic properties [64].Another effect of electrical conductivetype dependence of optoelectronic performance was discovered, and could be a direct experimental support for the charge separation model [61], as shown in Figure 5.

Fabrications and Properties of the PbSe Polycrystalline Coatings
We now discuss the physical properties of PbSe polycrystalline coatings prepared by different fabrication technologies, and their possible connections with the physical mechanisms of structural evolution, carrier transport, optical absorbance, and optoelectronic enhancement.

The Functional Effects Induced by Oxygen Treatment
The oxygen treatments on the monocrystalline PbSe coating fabricated by molecular beam epitaxy (MBE) at different temperatures were performed by Zhao et al. [67].They discovered that the oxygen treatment has a passivation effect on the carrier recombination in the PbSe polycrystalline coating, revealed by using the photoluminescence (PL) spectra operated at room temperature and 77 K, and the X-ray photoelectron spectroscopy (XPS) spectra, as shown in Figure 4.A high PL intensity revealed that the surface recombination and non-radiative recombination mechanisms, including Auger and SRH, were significantly inhibited after oxygen treatment, indicating that the surface states and the deep energy level recombination centers were passivated to improve the minority carrier lifetime and optoelectronic performance eventually.These results may be very useful for understanding the structural modification of the surface functional layer of the polycrystalline lead chalcogenides, and manufacturing ultrasensitive mid-infrared uncooled photodetectors [68,69].Likewise, another study on the effect of oxygen treatment on the evaporated n-type PbSe coatings was also carried out, and focused on the impact of electrical conductive type on the structural, compositional, electrical, and optoelectronic properties [64].Another effect of electrical conductive-type dependence of optoelectronic performance was discovered, and could be a direct experimental support for the charge separation model [62], as shown in Figure 5.

The Iodination Impact on the Photosensitivity
Iodine was verified to be another sensitizer for lead chalcogenides.Torquemada et al. reported that halogen only serves as a transport improver during the PbSe recrystallization in their work of thermally deposited PbSe treated with iodine years ago, which promotes the in situ incorporation of oxygen into the PbSe crystal lattice in an electrically active position [70][71][72][73][74].However, Qiu et al. pointed out that it is iodine that triggers the photosensitivity of PbSe [75], compared with oxygen, by performing a variety of annealing experiments using nitrogen/oxygen/iodine single or hybrid atmosphere, as shown in Figure 6.A more blunted and coalescent boundary structure was observed after the iodination process, as shown in Figure 6a.The presence of PbSeO3 (301), as a result of the oxygen passivation, was revealed to be responsible for the optoelectronic enhancement of PbSe polycrystalline coatings, as shown in Figure 6b.It was stated that iodine is prone to decrease the PL intensity and increase the resistance of the p-type PbSe polycrystalline coatings, indicating that iodine will incorporate into the PbSe lattice, but the incorporated concentration could be very low [75].Reprinted with permission from [64].Copyright Springer 2016.

The Iodination Impact on the Photosensitivity
Iodine was verified to be another sensitizer for lead chalcogenides.Torquemada et al. reported that halogen only serves as a transport improver during the PbSe recrystallization in their work of thermally deposited PbSe treated with iodine years ago, which promotes the in situ incorporation of oxygen into the PbSe crystal lattice in an electrically active position [70][71][72][73][74].However, Qiu et al. pointed out that it is iodine that triggers the photosensitivity of PbSe [75], compared with oxygen, by performing a variety of annealing experiments using nitrogen/oxygen/iodine single or hybrid atmosphere, as shown in Figure 6.A more blunted and coalescent boundary structure was observed after the iodination process, as shown in Figure 6a.The presence of PbSeO3 (301), as a result of the oxygen passivation, was revealed to be responsible for the optoelectronic enhancement of PbSe polycrystalline coatings, as shown in Figure 6b.It was stated that iodine is prone to decrease the PL intensity and increase the resistance of the p-type PbSe polycrystalline coatings, indicating that iodine will incorporate into the PbSe lattice, but the incorporated concentration could be very low [75].

The Iodination Impact on the Photosensitivity
Iodine was verified to be another sensitizer for lead chalcogenides.Torquemada et al. reported that halogen only serves as a transport improver during the PbSe recrystallization in their work of thermally deposited PbSe treated with iodine years ago, which promotes the in situ incorporation of oxygen into the PbSe crystal lattice in an electrically active position [70][71][72][73][74].However, Qiu et al. pointed out that it is iodine that triggers the photosensitivity of PbSe [75], compared with oxygen, by performing a variety of annealing experiments using nitrogen/oxygen/iodine single or hybrid atmosphere, as shown in Figure 6.A more blunted and coalescent boundary structure was observed after the iodination process, as shown in Figure 6a.The presence of PbSeO 3 (301), as a result of the oxygen passivation, was revealed to be responsible for the optoelectronic enhancement of PbSe polycrystalline coatings, as shown in Figure 6b.It was stated that iodine is prone to decrease the PL intensity and increase the resistance of the p-type PbSe polycrystalline coatings, indicating that iodine will incorporate into the PbSe lattice, but the incorporated concentration could be very low [75].The optoelectronic measurements indicated that the highest specific detectivity of 2.8 × 10 10 Jones was obtained from their sample treated by 25 min of oxygen followed by five minutes of iodine, and was attributed to the combination of oxygen passivation and iodine incorporation [76].Oxygen otherwise was revealed to serve as an improver to aid iodine incorporating into the PbSe lattice and a passivation agent to passivate structural defects such as Pb vacancies, as shown in Figure 6b,c.Low PL intensities in the iodized PbSe polycrystalline coatings may be attributed to the high surface recombination rate of minority carriers; this may be caused by the coalescent boundary in morphology, which introduces a lot of surface states again after iodination.These results promote a new understanding of the relationship between oxidation and iodination in the PbSe polycrystalline coatings.

Stoichiometric Optimization via Iodine Concentration Regulation
Another work on iodination effect was carried out to fabricate the material by changing the iodine concentration, as shown in Figure 7, indicating that it is possible to manipulate the inherent stoichiometry of PbSe polycrystalline coatings via controlling the iodine concentration during the sensitization process [77].It was revealed that the Pb/Se atomic ratio can be modulated by the I/O atomic ratio to change the stoichiometry of the material, as shown in Figure 7e.As is seen, the high responsivity of 3.97 A/W was achieved at the iodine concentration of 9.1 × 10 11 mm −3 , and a low iodine concentration of <1.5 × 10 12 mm −3 was necessary to obtain the high photoelectric performance, as shown in Figure 7d.It was indicated that the significant deviation of the Pb/Se atomic ratio from 1:1 due to over-iodination was responsible for the significant decline of responsivity [73,74].These results present a new role for halogens in the sensitization mechanism, and provide a direct approach to optimize the photosensitivity of the PbSe polycrystalline coatings at room temperature.The optoelectronic measurements indicated that the highest specific detectivity of 2.8 × 10 10 Jones was obtained from their sample treated by 25 min of oxygen followed by five minutes of iodine, and was attributed to the combination of oxygen passivation and iodine incorporation [76].Oxygen otherwise was revealed to serve as an improver to aid iodine incorporating into the PbSe lattice and a passivation agent to passivate structural defects such as Pb vacancies, as shown in Figure 6b,c.Low PL intensities in the iodized PbSe polycrystalline coatings may be attributed to the high surface recombination rate of minority carriers; this may be caused by the coalescent boundary in morphology, which introduces a lot of surface states again after iodination.These results promote a new understanding of the relationship between oxidation and iodination in the PbSe polycrystalline coatings.

Stoichiometric Optimization via Iodine Concentration Regulation
Another work on iodination effect was carried out to fabricate the material by changing the iodine concentration, as shown in Figure 7, indicating that it is possible to manipulate the inherent stoichiometry of PbSe polycrystalline coatings via controlling the iodine concentration during the sensitization process [77].It was revealed that the Pb/Se atomic ratio can be modulated by the I/O atomic ratio to change the stoichiometry of the material, as shown in Figure 7e.As is seen, the high responsivity of 3.97 A/W was achieved at the iodine concentration of 9.1 × 10 11 mm −3 , and a low iodine concentration of <1.5 × 10 12 mm −3 was necessary to obtain the high photoelectric performance, as shown in Figure 7d.It was indicated that the significant deviation of the Pb/Se atomic ratio from 1:1 due to over-iodination was responsible for the significant decline of responsivity [73,74].These results present a new role for halogens in the sensitization mechanism, and provide a direct approach to optimize the photosensitivity of the PbSe polycrystalline coatings at room temperature.

The Properties of Surface Functional Layer
The surface structure of PbSe polycrystalline coatings prepared by oxygen/iodine diffusion is an inexplicable issue due to the complexity of phases and crystalline structures.In the following sections, we introduce the characterization results and discoveries from the work reported by Kumar et al., since they made a comprehensive study on the phase and structural properties.

Phase Analysis
A further investigation on the surface phase analysis of PbSe polycrystalline coatings was made using selected area electron diffraction patterns (SAED), electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDX) by Kumar et al. [78].They showed in detail that the PbSe polycrystalline coating after oxygen/iodine sensitization should consist of a bottom layer of PbSe and a top layer of Pb-Se-O-I, which can be divided into a polycrystalline and a nanocrystalline phase, as shown in Figure 8.The high iodine mole fraction of ~20 at % in the polycrystalline phase and that of <10 at % in the nanocrystalline phase were revealed by EDX characterizations via the Cliff-Lorimer method [79,80].The nanocrystalline and polycrystalline phases in the top layer were confirmed by SAED, and exhibited large unit cells and d spacings of 0.892 and 1.069 nm, respectively, as shown in Figure 9.By EELS spectroscopes, the nanocrystalline Pb-Se-O-I phase with a broad plasmon peak centered at about 23.2 eV and the polycrystalline phase with two sharp plasmon peaks at 17.2 and 23.8 eV were distinguished from PbSe with plasmon energy of 15 eV, as shown in Figure 9.They suggested that the Pb-Se-O-I phase is responsible for the high photosensitivity of iodized PbSe polycrystalline coatings.

The Properties of Surface Functional Layer
The surface structure of PbSe polycrystalline coatings prepared by oxygen/iodine diffusion is an inexplicable issue due to the complexity of phases and crystalline structures.In the following sections, we introduce the characterization results and discoveries from the work reported by Kumar et al., since they made a comprehensive study on the phase and structural properties.

Phase Analysis
A further investigation on the surface phase analysis of PbSe polycrystalline coatings was made using selected area electron diffraction patterns (SAED), electron energy-loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDX) by Kumar et al. [78].They showed in detail that the PbSe polycrystalline coating after oxygen/iodine sensitization should consist of a bottom layer of PbSe and a top layer of Pb-Se-O-I, which can be divided into a polycrystalline and a nanocrystalline phase, as shown in Figure 8.The high iodine mole fraction of ~20 at % in the polycrystalline phase and that of <10 at % in the nanocrystalline phase were revealed by EDX characterizations via the Cliff-Lorimer method [79,80].The nanocrystalline and polycrystalline phases in the top layer were confirmed by SAED, and exhibited large unit cells and d spacings of 0.892 and 1.069 nm, respectively, as shown in Figure 9.By EELS spectroscopes, the nanocrystalline Pb-Se-O-I phase with a broad plasmon peak centered at about 23.2 eV and the polycrystalline phase with two sharp plasmon peaks at 17.2 and 23.8 eV were distinguished from PbSe with plasmon energy of 15 eV, as shown in Figure 9.They suggested that the Pb-Se-O-I phase is responsible for the high photosensitivity of iodized PbSe polycrystalline coatings.

Structural Analysis
Another investigation of the surface structural properties was carried out by SAED, high-resolution transmission electron microscopy (HRTEM), EDX, and low loss EELS [81].A pseudo-orthorhombic Bravais lattice of the PbSe x I y O z phase of the top layer with lattice parameters of a = 0.833 nm, b = 0.662 nm, and c = 1.8 nm was eventually identified via energy-filtered SAED patterns in the [100], [010], and [111] poles, as shown in Figure 10a.The lattice parameter of c = 1.8 nm was confirmed by the high-resolution transmission electron microscopy (HRTEM) images, as shown in Figure 10b.The chemical composition of the quaternary phase PbSe x I y O z was determined by EDX spectroscopy as 0.17 ≤ x ≤ 0.5 and 0.2 ≤ y ≤ 0.5 in atomic ratio.EELS spectra also proved the presence of the new phase with a double plasmon peak at 15.7-16.3eV (peak 1) and 23.1-23.2eV (peak 2), respectively [82], as shown in Figure 10c.These results promote the understanding of surface structure and stoichiometry.However, the role of this new phase in the photosensitivity enhancement needs further verifications experimentally.

Structural Analysis
Another investigation of the surface structural properties was carried out by SAED, highresolution transmission electron microscopy (HRTEM), EDX, and low loss EELS [81].A pseudoorthorhombic Bravais lattice of the PbSexIyOz phase of the top layer with lattice parameters of a = 0.833 nm, b = 0.662 nm, and c = 1.8 nm was eventually identified via energy-filtered SAED patterns in the [100], [010], and [111] poles, as shown in Figure 10a.The lattice parameter of c = 1.8 nm was confirmed by the high-resolution transmission electron microscopy (HRTEM) images, as shown in Figure 10b.The chemical composition of the quaternary phase PbSexIyOz was determined by EDX spectroscopy as 0.17 ≤ x ≤ 0.5 and 0.2 ≤ y ≤ 0.5 in atomic ratio.EELS spectra also proved the presence of the new phase with a double plasmon peak at 15.7-16.3eV (peak 1) and 23.1-23.2eV (peak 2), respectively [82], as shown in Figure 10c.These results promote the understanding of surface structure and stoichiometry.However, the role of this new phase in the photosensitivity enhancement needs further verifications experimentally.

Growth with the Nanostructured Antireflective Coatings
Another approach to further enhance the photoresponsivity of PbSe polycrystalline coatings, as presented by Weng et al., is to use an alkaline earth metal fluoride coating such as CaF2 and BaF2 as an antireflective surface layer [83].One reason to use it is the epitaxial compatibility with PbSe due to the similar crystal structure and lattice parameters, which can reduce the structural defects caused by lattice mismatch.Another reason is alkaline earth metal fluorides are transparent (~95% transmittance) in a wide range of 0.2-12 μm [84].Figure 11a-c shows the PbSe polycrystalline film coated by a 200 nm thick CaF2 flower-like nanostructured coating with a tapered shape, which may be attributed to the predominant growth at the (111) plane according to the lowest surface energy mechanism [45,46].It was revealed that light propagation will be bent by the sub-wavelength

Growth with the Nanostructured Antireflective Coatings
Another approach to further enhance the photoresponsivity of PbSe polycrystalline coatings, as presented by Weng et al., is to use an alkaline earth metal fluoride coating such as CaF 2 and BaF 2 as an antireflective surface layer [83].One reason to use it is the epitaxial compatibility with PbSe due to the similar crystal structure and lattice parameters, which can reduce the structural defects caused by lattice mismatch.Another reason is alkaline earth metal fluorides are transparent (~95% transmittance) in a wide range of 0.2-12 µm [84].Figure 11a-c shows the PbSe polycrystalline film coated by a 200 nm thick CaF 2 flower-like nanostructured coating with a tapered shape, which may be attributed to the predominant growth at the (111) plane according to the lowest surface energy mechanism [45,46].It was revealed that light propagation will be bent by the sub-wavelength dimensional tapered structure to improve the light collection efficiency due to the induced gradient refractive index profile and a convergence effect as convex optical lens [85][86][87][88][89], as shown in Figure 11d.
The optical and the optoelectronic characterizations, including PL, optical reflectance, and responsivity spectra, were carried out by the Fourier transform infrared spectroscopy (FTIR), demonstrating that the photoresponsivity was enhanced by the CaF 2 antireflective nanostructured coating to about 200% due to the broadband antireflective effect caused by light bending, as shown in Figure 12a,b.Another effect induced by the CaF 2 nanostructured coating is surface passivation, which could decrease the surface recombination of minority carriers [90,91], as indicated by the PL spectra shown in Figure 12c.These effects and mechanisms were predicted to be responsible for the optoelectronic enhancement of this composite structure.These results show a good structural coupling between polycrystalline PbSe and alkaline earth metal fluorides, and provide an effective approach to further improve the optical and optoelectronic performances of photodetectors and light-emitting devices (LEDs) [92,93].
Coatings 2018, 8, x FOR PEER REVIEW 12 of 21 dimensional tapered structure to improve the light collection efficiency due to the induced gradient refractive index profile and a convergence effect as convex optical lens [85][86][87][88][89], as shown in Figure 11d.
The optical and the optoelectronic characterizations, including PL, optical reflectance, and responsivity spectra, were carried out by the Fourier transform infrared spectroscopy (FTIR), demonstrating that the photoresponsivity was enhanced by the CaF2 antireflective nanostructured coating to about 200% due to the broadband antireflective effect caused by light bending, as shown in Figure 12a,b.Another effect induced by the CaF2 nanostructured coating is surface passivation, which could decrease the surface recombination of minority carriers [90,91], as indicated by the PL spectra shown in Figure 12c.These effects and mechanisms were predicted to be responsible for the optoelectronic enhancement of this composite structure.These results show a good structural coupling between polycrystalline PbSe and alkaline earth metal fluorides, and provide an effective approach to further improve the optical and optoelectronic performances of photodetectors and lightemitting devices (LEDs) [92,93].dimensional tapered structure to improve the light collection efficiency due to the induced gradient refractive index profile and a convergence effect as convex optical lens [85][86][87][88][89], as shown in Figure 11d.
The optical and the optoelectronic characterizations, including PL, optical reflectance, and responsivity spectra, were carried out by the Fourier transform infrared spectroscopy (FTIR), demonstrating that the photoresponsivity was enhanced by the CaF2 antireflective nanostructured coating to about 200% due to the broadband antireflective effect caused by light bending, as shown in Figure 12a,b.Another effect induced by the CaF2 nanostructured coating is surface passivation, which could decrease the surface recombination of minority carriers [90,91], as indicated by the PL spectra shown in Figure 12c.These effects and mechanisms were predicted to be responsible for the optoelectronic enhancement of this composite structure.These results show a good structural coupling between polycrystalline PbSe and alkaline earth metal fluorides, and provide an effective approach to further improve the optical and optoelectronic performances of photodetectors and lightemitting devices (LEDs) [92,93].

Preparation via Ion Beam Implantation
The diffusion technologies mentioned above actually provide an easy approach to dope and activate the oxygen/iodine atoms into lead chalcogenides.However, the profile of oxygen/iodine is exponentially decayed along the depth direction, which is constant and mainly prone to modify the surface properties [94,95].Turning to modify the stoichiometric depth profile of the inner layer is a promising method to modulate/optimize the optical, electrical, and optoelectronic performances of materials [96][97][98][99].
A sensitization technology via O + beam implantation at 50 keV followed by an annealing process in Ar atmosphere at 600 • C was demonstrated by us recently [100], as shown in Figure 13.Compared with the conventional oxygen diffused counterpart, the preliminary characterizations revealed that the ion beam-prepared PbSe material exhibited a polycrystalline structure with crystallite sizes of 300-700 nm, as shown in Figure 13a.The optical bandgap of 0.22 eV is more suitable for detecting mid-infrared photons, and the low Urbach energy of 34.6 meV indicates a well-ordered crystalline structure with low defect density, leading to higher optical absorption in the mid-infrared range and carrier mobility, which were responsible for the high responsivity of 1.23 A/W at 4 µm, as shown in Figure 13b-d.
Other ion beam preparations were performed with a higher following annealing temperature of 990 • C to further repair the damaged crystal structure [101], as shown in Figure 14a,b.It was revealed that the resulting material is nanocrystalline with high mid-infrared absorption and ultrahigh carrier mobility up to the magnitude of 10 3 cm 2 V −1 s −1 due to the low defect density represented from the low Urbach energy of 29.5 meV [102,103], as shown in Figure 14c-f.It was indicated that the PbSe polycrystalline coatings prepared by ion beam implantation exhibited a high optoelectronic performance that can be modulated by the implantation dose.However, a high dark current was also observed, which limits the improvement of the specific detectivity, as shown in Figure 14g-i.The low size-dependent crystallite boundary barrier height may be responsible for the high dark current, since the free carrier concentration is relatively low due to the deep energy level doping of oxygen [104,105], as shown in Figure 14f.These results show a new approach to synthesize and sensitize PbSe polycrystalline coatings for high optical, electrical, and optoelectronic performances.More fundamental researches on how the dark current could be affected by carrier transport and how the electrical performance can be optimized by tuning the ion beam parameters, e.g., ion energy, should be considered to further explore the potential of these ion beam-prepared PbSe polycrystalline coatings.

Preparation via Ion Beam Implantation
The diffusion technologies mentioned above actually provide an easy approach to dope and activate the oxygen/iodine atoms into lead chalcogenides.However, the profile of oxygen/iodine is exponentially decayed along the depth direction, which is constant and mainly prone to modify the surface properties [94,95].Turning to modify the stoichiometric depth profile of the inner layer is a promising method to modulate/optimize the optical, electrical, and optoelectronic performances of materials [96][97][98][99].
A sensitization technology via O + beam implantation at 50 keV followed by an annealing process in Ar atmosphere at 600 °C was demonstrated by us recently [100], as shown in Figure 13.Compared with the conventional oxygen diffused counterpart, the preliminary characterizations revealed that the ion beam-prepared PbSe material exhibited a polycrystalline structure with crystallite sizes of 300-700 nm, as shown in Figure 13a.The optical bandgap of 0.22 eV is more suitable for detecting mid-infrared photons, and the low Urbach energy of 34.6 meV indicates a well-ordered crystalline structure with low defect density, leading to higher optical absorption in the mid-infrared range and carrier mobility, which were responsible for the high responsivity of 1.23 A/W at 4 μm, as shown in Figure 13b-d.
Other ion beam preparations were performed with a higher following annealing temperature of 990 °C to further repair the damaged crystal structure [101], as shown in Figure 14a,b.It was revealed that the resulting material is nanocrystalline with high mid-infrared absorption and ultrahigh carrier mobility up to the magnitude of 10 3 cm 2 V −1 s −1 due to the low defect density represented from the low Urbach energy of 29.5 meV [102,103], as shown in Figure 14c-f.It was indicated that the PbSe polycrystalline coatings prepared by ion beam implantation exhibited a high optoelectronic performance that can be modulated by the implantation dose.However, a high dark current was also observed, which limits the improvement of the specific detectivity, as shown in Figure 14g-i.The low size-dependent crystallite boundary barrier height may be responsible for the high dark current, since the free carrier concentration is relatively low due to the deep energy level doping of oxygen [104,105], as shown in Figure 14f.These results show a new approach to synthesize and sensitize PbSe polycrystalline coatings for high optical, electrical, and optoelectronic performances.More fundamental researches on how the dark current could be affected by carrier transport and how the electrical performance can be optimized by tuning the ion beam parameters, e.g., ion energy, should be considered to further explore the potential of these ion beam-prepared PbSe polycrystalline coatings.

Electrical Properties
The electrical properties have a close relation with the optical and optoelectronic properties.Although there are many comprehensive studies on PbSe polycrystalline coatings, a deep focus on the electrical properties such as the carrier transport mechanism, impurities, defects, and their impact on the carrier transport, and the fine electronic structure has rarely been reported in recent years.Especially, the carrier transport mechanism at low temperature is unclear [54].Furthermore, deeplevel impurities and defects (including vacancies, dislocations, stacking faults, etc.) can form nonequilibrium carrier traps or recombination centers.The characterizations on the concentration distribution, compositional identification, and the capture cross-section of the trap centers are

Electrical Properties
The electrical properties have a close relation with the optical and optoelectronic properties.Although there are many comprehensive studies on PbSe polycrystalline coatings, a deep focus on the electrical properties such as the carrier transport mechanism, impurities, defects, and their impact on the carrier transport, and the fine electronic structure has rarely been reported in recent years.Especially, the carrier transport mechanism at low temperature is unclear [54].Furthermore, deeplevel impurities and defects (including vacancies, dislocations, stacking faults, etc.) can form nonequilibrium carrier traps or recombination centers.The characterizations on the concentration distribution, compositional identification, and the capture cross-section of the trap centers are

Electrical Properties
The electrical properties have a close relation with the optical and optoelectronic properties.Although there are many comprehensive studies on PbSe polycrystalline coatings, a deep focus on the electrical properties such as the carrier transport mechanism, impurities, defects, and their impact on the carrier transport, and the fine electronic structure has rarely been reported in recent years.Especially, the carrier transport mechanism at low temperature is unclear [54].Furthermore, deep-level impurities and defects (including vacancies, dislocations, stacking faults, etc.) can form non-equilibrium carrier traps or recombination centers.The characterizations on the concentration distribution, compositional identification, and the capture cross-section of the trap centers are important to understand the carrier transport and recombination mechanisms [47,106,107].However, these characterizations seem to be difficult to perform, because they need more complex analysis to extract the physical mechanisms from limited experimental characterizations.The thermally stimulated current (TSC) spectroscopy and low temperature Hall effect measurement may be promising to exploit the mysterious electrical properties of PbSe polycrystalline coatings.

Growth Mechanism-Related Calculations
Growth mechanism on the morphological evolution, preferred growth orientation, the surface energy minimization, and defect formation, as well as the relation with the influencing factors such as preparation conditions, is another interesting area, which can be important to understand the crystal growth mechanism and effectively control the repeatability and reliability of the preparation technology.Recently, the growth mechanism of evaporated PbSe based on the Gibbs free energy variation minimization was calculated via DFT by Sun et al., which explained the preferred growth orientation of (200) for evaporated PbSe film on the Si substrate [45,46].This is a good beginning for understanding the growth mechanism of PbSe polycrystalline material.

New Preparation Technologies
Another interesting area is developing new preparation methods for exploring new physical properties and mechanisms.As mentioned above, ion beam technology exhibits a variety of advantages, but also has drawbacks such as a high dark current and low performance/price ratio.A novel ion beam-assisted preparation method may be promising.Unlike the ion implantation, ion beam assistance doesn't damage the crystal lattice.Therefore, a high temperature (>600 • C) annealing is unnecessary.The preliminary characterizations demonstrate that this technology exhibits the advantages that ion beam implantation has, as well as a high performance/price ratio.Figure 15 has summarized the properties of this technology with comparisons with currently developed technologies.As is seen, this ion beam-assisted method almost combines the advantages of ion beam implantation and diffusion technologies.Moreover, the improvements of other preparation methods such as CBD and RF magnetron sputtering are also attractive for the preparation of PbSe polycrystalline coatings for high optical, electrical, and optoelectronic performances.
important to understand the carrier transport and recombination mechanisms [47,106,107].However, these characterizations seem to be difficult to perform, because they need more complex analysis to extract the physical mechanisms from limited experimental characterizations.The thermally stimulated current (TSC) spectroscopy and low temperature Hall effect measurement may be promising to exploit the mysterious electrical properties of PbSe polycrystalline coatings.

Growth Mechanism-Related Calculations
Growth mechanism on the morphological evolution, preferred growth orientation, the surface energy minimization, and defect formation, as well as the relation with the influencing factors such as preparation conditions, is another interesting area, which can be important to understand the crystal growth mechanism and effectively control the repeatability and reliability of the preparation technology.Recently, the growth mechanism of evaporated PbSe based on the Gibbs free energy variation minimization was calculated via DFT by Sun et al., which explained the preferred growth orientation of (200) for evaporated PbSe film on the Si substrate [45,46].This is a good beginning for understanding the growth mechanism of PbSe polycrystalline material.

New Preparation Technologies
Another interesting area is developing new preparation methods for exploring new physical properties and mechanisms.As mentioned above, ion beam technology exhibits a variety of advantages, but also has drawbacks such as a high dark current and low performance/price ratio.A novel ion beam-assisted preparation method may be promising.Unlike the ion implantation, ion beam assistance doesn't damage the crystal lattice.Therefore, a high temperature (>600 °C) annealing is unnecessary.The preliminary characterizations demonstrate that this technology exhibits the advantages that ion beam implantation has, as well as a high performance/price ratio.Figure 15 has summarized the properties of this technology with comparisons with currently developed technologies.As is seen, this ion beam-assisted method almost combines the advantages of ion beam implantation and diffusion technologies.Moreover, the improvements of other preparation methods such as CBD and RF magnetron sputtering are also attractive for the preparation of PbSe polycrystalline coatings for high optical, electrical, and optoelectronic performances.

Compounding with Other Materials and Structures
Constructing composite materials and nanostructures with metasurfaces, two-dimensional (2D) materials, and carbon nanotubes is another predictable development area, which enables taking advantage of the specific properties of these materials and structures.Coating with a metasurface can improve the optical absorbance intensively at a specified wavelength according to the induced SPR

Compounding with Other Materials and Structures
Constructing composite materials and nanostructures with metasurfaces, two-dimensional (2D) materials, and carbon nanotubes is another predictable development area, which enables taking advantage of the specific properties of these materials and structures.Coating with a metasurface can improve the optical absorbance intensively at a specified wavelength according to the induced SPR effect, which is very useful to enhance the internal quantum efficiency and the optoelectronic performance.Compounding with 2D materials such as graphene is another attractive technology for high-speed applications due to the high carrier mobility in graphene.These composite materials can highly expand the applications of PbSe polycrystalline coatings for novel properties and devices in optoelectronics.

Summary
In summary, we have discussed the initial motivation to make PbSe bulk material sensitive to mid-infrared radiation without cooling.Then, several physical models are introduced to explain and understand why the optoelectronic enhancement is possible.The structural, compositional, optical, electrical, and optoelectronic properties, as well as the corresponding physical effects of PbSe polycrystalline coating prepared by different diffusion and ion beam technologies, are discussed in detail, which show huge optical and optoelectronic potentials in this material.Finally, the interesting areas, challenges, and perspectives for novel hybrid optoelectronic devices, physical effects, and applications are also discussed.These devices and properties make PbSe polycrystalline material more attractive in the applications of high-speed, low-cost, and high-performance optical and optoelectronic devices.

Figure 2 .
Figure 2. The schematics of (a) the minority carrier trap and (b) the grain boundary barrier models.Figure 2. The schematics of (a) the minority carrier trap and (b) the grain boundary barrier models.

Figure 2 .
Figure 2. The schematics of (a) the minority carrier trap and (b) the grain boundary barrier models.Figure 2. The schematics of (a) the minority carrier trap and (b) the grain boundary barrier models.

Figure 3 .
Figure 3.The schematic of the charge separation model, showing (a) the conversion of crystallites' surfaces from p-to n-type, (b) the charge carrier transport, and (c) the crystallite structure.Reprinted with permission from [61].Copyright AIP 2014.

Figure 3 .
Figure 3.The schematic of the charge separation model, showing (a) the conversion of crystallites' surfaces from p-to n-type, (b) the charge carrier transport, and (c) the crystallite structure.Reprinted with permission from [62].Copyright AIP 2014.

Figure 4 .
Figure 4.The defect passivation effect on the PbSe thin films fabricated by molecular beam epitaxy (MBE), (a) photoluminescence (PL) intensities versus annealing temperature at room temperature and 77 K, (b) a comparison of PL intensities of PbSe thin films treated at 350 °C and an as-grown one at room temperature.Reprinted with permission from [67].Copyright AIP 2008.

Figure 5 .
Figure 5. (a) A schematic of charge separation under mid-infrared (IR) illumination and (b) the corresponding energy diagram in the sensitized PbSe polycrystalline coatings based on electrical conductive type effect, (c) the X-ray diffraction (XRD) patterns of n-type and p-type PbSe thin fims.Reprinted with permission from [64].Copyright Springer 2016.

Figure 4 .
Figure 4.The defect passivation effect on the PbSe thin films fabricated by molecular beam epitaxy (MBE), (a) photoluminescence (PL) intensities versus annealing temperature at room temperature and 77 K, (b) a comparison of PL intensities of PbSe thin films treated at 350 • C and an as-grown one at room temperature.Reprinted with permission from [67].Copyright AIP 2008.

Figure 4 .
Figure 4.The defect passivation effect on the PbSe thin films fabricated by molecular beam epitaxy (MBE), (a) photoluminescence (PL) intensities versus annealing temperature at room temperature and 77 K, (b) a comparison of PL intensities of PbSe thin films treated at 350 °C and an as-grown one at room temperature.Reprinted with permission from [67].Copyright AIP 2008.

Figure 5 .
Figure 5. (a) A schematic of charge separation under mid-infrared (IR) illumination and (b) the corresponding energy diagram in the sensitized PbSe polycrystalline coatings based on electrical conductive type effect, (c) the X-ray diffraction (XRD) patterns of n-type and p-type PbSe thin fims.Reprinted with permission from [64].Copyright Springer 2016.

Figure 5 .
Figure 5. (a) A schematic of charge separation under mid-infrared (IR) illumination and (b) the corresponding energy diagram in the sensitized PbSe polycrystalline coatings based on electrical conductive type effect, (c) the X-ray diffraction (XRD) patterns of n-type and p-type PbSe thin fims.Reprinted with permission from [64].Copyright Springer 2016.

Figure 6 .
Figure 6.The (a) morphological, (b) structural, and (c) photoluminescence (PL) optical properties of PbSe polycrystalline coatings fabricated by annealing in single or hybrid atmosphere, and (d) the PL intensity as a function of crystallite size.Reprinted with permission from [75].Copyright AIP 2013.

Figure 6 .
Figure 6.The (a) morphological, (b) structural, and (c) photoluminescence (PL) optical properties of PbSe polycrystalline coatings fabricated by annealing in single or hybrid atmosphere, and (d) the PL intensity as a function of crystallite size.Reprinted with permission from [75].Copyright AIP 2013.

Figure 7 .
Figure 7. (a) Schematic of I-V tests under mid-IR illumination, (b) I-V test results for iodine concentration of 9.1 × 10 11 mm −3 , (c) scanning electron microscopy (SEM) morphologies for as-grown and iodized samples using different iodine concentrations, (d) the responsivity, (e) Pb/Se and I/O atomic ratios, and (f) the atomic proportions versus iodine concentration.Reprinted with permission from [77].Copyright Elsevier 2016.

Figure 7 .
Figure 7. (a) Schematic of I-V tests under mid-IR illumination, (b) I-V test results for iodine concentration of 9.1 × 10 11 mm −3 , (c) scanning electron microscopy (SEM) morphologies for as-grown and iodized samples using different iodine concentrations, (d) the responsivity, (e) Pb/Se and I/O atomic ratios, and (f) the atomic proportions versus iodine concentration.Reprinted with permission from [77].Copyright Elsevier 2016.

Figure 9 .
Figure 9. Selected area electron diffraction patterns (SAED) patterns, energy-dispersive X-ray spectroscopy (EDX) spectra, and electron energy-loss spectroscopy (EELS) spectra of (a) PbSe bottom layer, (b) a nanocrystalline phase of the top layer with the largest d spacing of 0.892 nm, and (c) a polycrystalline phase of the top layer with the largest d spacing of 1.069 nm.Reprinted with permission from [78].Copyright Elsevier 2017.

Figure 9 .
Figure 9. Selected area electron diffraction patterns (SAED) patterns, energy-dispersive X-ray spectroscopy (EDX) spectra, and electron energy-loss spectroscopy (EELS) spectra of (a) PbSe bottom layer, (b) a nanocrystalline phase of the top layer with the largest d spacing of 0.892 nm, and (c) a polycrystalline phase of the top layer with the largest d spacing of 1.069 nm.Reprinted with permission from [78].Copyright Elsevier 2017.

Figure 9 .
Figure 9. Selected area electron diffraction patterns (SAED) patterns, energy-dispersive X-ray spectroscopy (EDX) spectra, and electron energy-loss spectroscopy (EELS) spectra of (a) PbSe bottom layer, (b) a nanocrystalline phase of the top layer with the largest d spacing of 0.892 nm, and (c) a polycrystalline phase of the top layer with the largest d spacing of 1.069 nm.Reprinted with permission from [78].Copyright Elsevier 2017.

Figure 10 .
Figure 10.(a) Energy-filtered selected area diffraction patterns in the [100], [010], and [111] poles, and the corresponding schematics of the pseudo-orthorhombic crystal structure of the iodized PbSe polycrystalline coatings; (b) the high-resolution transmission electron microscopy (HRTEM) images along the c-axis; (c) low-loss electron energy-loss spectroscopy (EELS) spectra for distinguishing the PbSexIyOz phase from PbSe.Reprinted with permission from [81].Copyright Elsevier 2017.

Figure 10 .
Figure 10.(a) Energy-filtered selected area diffraction patterns in the [100], [010], and [111] poles, and the corresponding schematics of the pseudo-orthorhombic crystal structure of the iodized PbSe polycrystalline coatings; (b) the high-resolution transmission electron microscopy (HRTEM) images along the c-axis; (c) low-loss electron energy-loss spectroscopy (EELS) spectra for distinguishing the PbSe x I y O z phase from PbSe.Reprinted with permission from [81].Copyright Elsevier 2017.

Figure 11 .
Figure 11.The morphological and optical properties of CaF2 nanostructure-coated PbSe polycrystalline film; (a) high-resolution field emission scanning electron microcopy (FESEM) image of CaF2 nanostructure arrays; (b) cross-sectional FESEM image; and (c) magnified view of PbSe polycrystalline film with CaF2 nanostructured coating; (d) schematics of light bending effect and gradient refractive index profile of CaF2 nanostructured coating.Reprinted with permission from [83].Copyright AIP 2014.

Figure 11 .
Figure 11.The morphological and optical properties of CaF 2 nanostructure-coated PbSe polycrystalline film; (a) high-resolution field emission scanning electron microcopy (FESEM) image of CaF 2 nanostructure arrays; (b) cross-sectional FESEM image; and (c) magnified view of PbSe polycrystalline film with CaF 2 nanostructured coating; (d) schematics of light bending effect and gradient refractive index profile of CaF 2 nanostructured coating.Reprinted with permission from [83].Copyright AIP 2014.

Figure 11 .
Figure 11.The morphological and optical properties of CaF2 nanostructure-coated PbSe polycrystalline film; (a) high-resolution field emission scanning electron microcopy (FESEM) image of CaF2 nanostructure arrays; (b) cross-sectional FESEM image; and (c) magnified view of PbSe polycrystalline film with CaF2 nanostructured coating; (d) schematics of light bending effect and gradient refractive index profile of CaF2 nanostructured coating.Reprinted with permission from [83].Copyright AIP 2014.

Figure 13 .
Figure 13.Preparations and characterizations of PbSe polycrystalline coatings prepared by oxygen ion beam implantation: (a) morphological and structural; (b) Tauc's plot; (c) Urbach tail; and (d) responsivity spectra with a comparison with the diffused one.Reprinted with permission from [100].Copyright Elsevier 2017.

Figure 15 .
Figure 15.The summaries and comparisons of different sensitization methods.

Figure 15 .
Figure 15.The summaries and comparisons of different sensitization methods.