Mechanistic studies of oxidation and iodization of PbSe thin film sensitization for mid-infrared detection

The primary fabrication methods for forming thin films of mid-wave infrared (MWIR) sensitive PbSe are chemical bath deposition (CBD) and chemical vapor deposition (CVD). Both these methods have been used for 70+ years to form polycrystalline PbSe and other lead chalcogenide thin films.

PbSe thin films require additional thermal processing (sensitization) of oxidation followed by iodization to enable an MWIR photoresponse by incorporating and activating iodine in the polycrystalline PbSe film within and in-between the individual PbSe grain.¹ The oxidation followed by iodization significantly improves the PbSe thin film photoresponse but the mechanism is not well understood. The combination of oxygen and iodine incorporation in PbSe thin films is considered important.^2,3 The controlled formation of lead selenite (PbSeO₃) is also believed to enable the photoresponse while iodine is understood to impact the recrystallization of PbSe grains, reduce activation energy in forming PbSeO₃, and may also contribute to the photoresponse.^4,5 Golubchenko et al. reported that the presence of iodine in PbSe reduces the formation temperature of oxyselenites by 100 °C and assists in the recrystallization of underlying PbSe grains.^2,3 Some groups have demonstrated reversing the order in a hybrid sensitization process where iodization is performed before oxidation, although oxygen is used as a carrier gas for the iodine.^5–8 The high MWIR photoresponse achieved by many researchers using vastly different methods supports the claim that (1) oxygen and iodine play a very interactive symbiotic role and (2) the as-deposited PbSe film morphology and microstructure determine the sensitization conditions necessary for maximizing the MWIR photoresponse.

The generic sensitization process for PbSe is broken down into two thermal process steps. Most often, the first step is thermal oxidation or annealing in air at a temperature between 280 and 480 °C followed by a second thermal step of iodization performed at a lower temperature of 285–325 °C. During this step, iodine interacts with the previously formed oxidized PbSe (lead selenite), and a surface layer of PbI₂ is formed. The PbI₂ layer that forms on the surface during iodization necessitates this temperature to remain below the melting point for PbI₂ of 410 °C. The oxidation and iodization process is shown in Fig. 1.

The current understanding is that the fully sensitized PbSe films (500–1500 nm thick) are most often comprised of polycrystalline equiaxial grains (200–500 nm). The film is composed of a top layer of PbI₂, a transition layer of unknown composition with little understanding of its influence on the MWIR mechanism, and a polycrystalline PbSe base-layer. The base-layer grains are suspected of being coated with lead selenite which enhances the photoconductivity by producing electron traps.^2,3

Little is understood about the transition layer between the PbI₂ and the PbSe base-layer; and however, some groups have reported Pb:Se:I-based or Pb:Se:O:I-based compound in this transition layer that may impact the photoconduction of PbSe thin films.^2,9 These trinary and quaternary phases are reported in the cited literature as being crystalline in the structure with molar fractions varying greatly from the surface of the film down to the beginning of the PbSe grains. Jang et al. did not report oxygen in his transition layer but describe how the trinary phase influences photoconduction by diffusing electrons down into the PbSe to suppress holes under dark conditions, leading to high dark resistance.² Upon IR illumination, the electrons from this ternary phase are trapped at grain boundary trap sites along with photogenerated electrons. Kumar et al. reported significant high-resolution transmission electron microscopy (HRTEM) analysis with electron diffraction data.⁹ Kumar reported the crystalline properties and the molar fraction through the quaternary phases layer revealing Pb:Se:O:I diffraction patterns and lattice spacing.

Previous works for the sensitization of PbSe-based photodetectors report varying conditions for oxidation and iodization and report the conditions that maximize the photoresponse for their particular film. The goal of this work is to provide a detailed fundamental understanding of the impact of oxidation on iodization and provide data on changes in morphology, chemical composition, and structure of the film under different sensitization conditions. Furthermore, this work aims to provide a general guideline to optimize the IR photoresponse as well as conditions that are detrimental to IR photoresponse. The IR photoresponse mechanism is complex, and the literature shows various proposed mechanisms. Our paper aims to show the optimum material configuration for high photoresponse and provides some insight. Our results show that the oxidation and sensitization process provide higher photoresponse due to (a) the formation of PbI₂ surface passivation layer, (b) infrared absorption by the PbSe thin film, (c) the formation of PbSeO₃ and Pb₃Se₂O₆I₂ controls the necessary PbSe film thickness for efficient infrared light absorption and controls the iodine concentration in the PbSe, and (d) low levels of iodine may be present in the PbSe thin films which may induce electron traps.

The laser sintering method, as described in the previous work, was used to fabricate the PbSe thin films.¹⁰ Prior to laser sintering, the film is a loosely packed layer of ball-milled PbSe crystallites with a huge surface-to-volume ratio. A small dose of KI is introduced by drip-casting KI dissolved in methanol (0.18 μM). All samples in this work were deposited by drip-casting KI prior to laser sintering as described in the previous work;¹⁰ and however, KI incorporation was omitted from experiments when appropriate, such as for control samples (no iodine) when exploring the impact iodization has on grain growth and morphology.

The thermal sensitization processing was performed using a customized INSTEC HCS602SG-PM (INSTEC Corp.) temperature-controlled probe system and chamber, capable of both cooling to −190 °C and heating to 600 °C (Figs. S3(a)–S3(c) in the supplementary material). The oxidation step was performed in the INSTEC chamber at 460 °C for varying durations with ambient air as an oxygen source. The iodization process was performed by sublimating solid iodine pellets heated to 105 °C. Argon gas was used to transport the iodine vapor through a heated glass tube to a custom chamber insert milled from a solid silver block fabricated by INSTEC. The PbSe sample was heated to 325 °C inside the custom iodization chamber for varying durations [Figs. S3(d) and S3(e) in the supplementary material].

The deposited thin film characteristics were analyzed at each stage of the process through extensive top-down and cross-sectional scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), x-ray diffraction (XRD), and Raman spectroscopy. The transformation from as-deposited (laser sintered), oxidized, and iodized was compared under varying conditions and analyzed to better understand the interactions between these process steps and their impact in producing highly MWIR-sensitive thin films. The following equipment was used for this characterization provided by the University of Virginia Nanomaterials Characterization Facility (NMCF): Helios Dual-Beam FIB/SEM/EDS/STEM, Rigaku SmartLab X-ray Diffractometer (XRD), and Renishaw INVIA Confocal Raman. The Helios SEM was used for high-resolution inspection of the films (top-down and cross-section) as well as energy-dispersive spectroscopy (EDS) for film composition. The SEM was operated at 8 KV and 1.9 nA. The XRD system is equipped with a Cu anode (1.54 Å wavelength) and operates at 40 kV and 44 mA. Both 514 nm and 785 nm lasers were evaluated in the Raman spectroscopy analysis; and however, the 514 nm was found to provide ideal penetration depth and better sensitivity to the surface of the film.

Once the PbSe film is fully sensitized (oxidized and iodized), the MWIR photoresponse was measured. The figure of merit for measuring the photoresponse was obtained using an I–V sweep test often called Delta-R or DelR (ΔR). DelR is the percent difference in film resistance between dark (R_D) and illuminated (R_L) conditions $((RD−RL)RD)$⁠. For this test, an 18 mW/cm² MWIR light source illuminated the sample through a 3–5 μm optical filter. The data were collected using Keithley 2400 source meter.¹⁰ A diagram of the circuit setup is shown in Fig. S2 in the supplementary material.

Thin films of polycrystalline PbSe (1.2–1.4 μm) were prepared on oxidized silicon substrates. We then carried out a series of experiments with different oxidation times followed by extensive materials characterization to understand the effects on morphology, composition, and structure. We then carried out a series of additional time studies with both a fixed iodization time (4 min) as well as varying iodization times. Extensive materials characterization was performed to identify the different compositions and crystal structures to understand the interactions between oxidation and iodization. The results of the optimal conditions for maximizing the MWIR photoresponse are reported. Finally, we composed a general model describing the interaction between oxidation and iodization, including regimes to maximize photoresponse as well as several regimes that are detrimental to achieving an optimal photoresponse from the PbSe film.

The changes in morphology as a result of the oxidation time of as-deposited PbSe thin films (Fig. 2) were investigated. The oxidation process induces both chemical and morphological changes; and however, the primary morphological changes are a result of the thermal anneal as opposed to the result of oxidation. As-deposited PbSe thin film samples, without any KI content (no potassium iodide), were annealed at 460 °C for 0, 6, and 12 min, and the top-down and cross-sectional morphology and grain size was measured. The as-deposited PbSe thin film morphology shows round/spherical grains in a porous and columnar form. The average grain size is 42 nm. Following 6 and 12 min of oxidation, the morphology shows round/spherical grains but densified from porous and columnar to equiaxial merged grains. The average grain size increased to 242 nm. Cross-sectional SEM shows that regardless of oxygen incorporation, which primarily occurred in the top half of the film, the grain size, and morphology changed consistently throughout the film. This suggests the dominant transformation in morphology is due to thermalization rather than oxidation.

A composition study was performed on oxidized samples followed by detection of the elemental distribution throughout the film thickness using cross-sectional EDS line-scans and complemented with Raman spectroscopy. The relative elemental distribution data are correlated with Raman spectroscopy measurements to better understand the material composition distribution.

The EDS line-scans (Fig. 3) show a nominal amount of oxygen near the surface in the as-deposited PbSe film prior to any thermal oxidation (0 min). Although the laser sinter deposition (LSD) is performed in an inert gas-filled (argon) chamber, some level of surface oxidation occurs during laser sintering. Oxidation for up to 6 min at 460 °C forms a uniform PbSeO₃ layer (400–500 nm) and preserves the Pb:Se stoichiometric ratio throughout the entire thin film. The primary reaction is shown in Eq. (1). Oxidation durations beyond 6 min begin to volatilize free selenium from the surface of the film to produce PbO_x, and the Pb:Se ratio begins to increase as selenium escapes the film as a gas. These reactions are shown in Eqs. (2)–(4). Notice the greater selenium loss as a result of extended oxidation duration (yellow arrows) in Fig. 3.

Raman spectroscopy of the same samples was then performed with a 514 nm laser source to determine the bond type and compositions of the PbSe thin film following the various oxidation times. The 514 nm laser was specifically chosen due to the shallow penetration depth in PbSe, as observed by the lack of a silicon substrate peak (520 cm⁻¹) and the presence of a silicon peak when illuminated with a 785 nm laser. The Raman study (Fig. 4) confirms the EDS data showing that initially, PbSeO₃ forms and oxidation durations longer than 6 min begin to lose Se and form Se-poor lead selenite and PbO_x. The as-deposited film initially exhibits characteristic Raman peaks for PbSe at 136, 274, and 793 cm⁻¹.^11,12 The weak characteristic PbSeO₃ peak at 94 cm⁻¹, for the as-deposited sample, is likely the nominal surface oxidation from sample preparation. The signal below 100 cm⁻¹ is obscured by the grating lower limit. Following progressively longer oxidation durations, peaks at 118, 361, and 790 cm⁻¹ confirm the presence of PbSeO₃.¹³ The resolution of the Raman system is approximately 4 cm⁻¹; and however, deconvolution was used to delineate between the PbSe peak at 793 cm⁻¹ and the PbSeO₃ peak at 790 cm⁻¹. Finally, as excessively long oxidization duration occurs, the presence of PbO peaks appear at 145 and 807 cm⁻¹.¹⁴ The 807 cm⁻¹ peak is subtle and was only seen as a shoulder for the sample oxidized for 30 min. X-ray photoelectron spectroscopy (XPS) confirmed the presence of PbO on the surface of oxidized PbSe.¹⁵ 

X-ray diffraction (XRD) spectroscopy of as-deposited PbSe shows a stoichiometrically balanced (Pb:Se = 1:1) forms of lead selenite PbSeO₃ following oxidization for 6 min at 460 °C (Fig. 5). This is consistent with the EDS line-scan results from the previous section. As oxidation durations extend beyond 14 min, selenium-poor forms of lead selenite such as Pb₃O₂(SeO₃) begin to dominate, as reported previously by Jang et al., and a transition to a polycrystalline form of PbO_x can occur with excessively long oxidation times.¹¹ 

Oxidation reactions:

We measured the morphology changes of a PbSe thin film following a standard sensitization process proven to produce a high MWIR response (6 min oxidation at 460 °C + 4 min iodization at 325 °C). In Sec. III A 1, it was shown that oxidation (alone) increases grain size by nearly 6× (42–242 nm) and densifies the entire film stack to reduce porosity. Other researchers have reported that iodine promotes recrystallization of PbSe and reduces the formation temperature of oxyselenite by 100 °C.^4,5

Both top-down and cross-sectional SEM results revealed similar structural changes following the standard sensitization recipe described above. Although the iodine must diffuse top-down in what is now a multi-layered film (PbSeO₃/PbSe) and iodization is performed at 325 °C (135 °C lower than oxidation), the entire film stacks morphology and grain structure are transformed by 4 min of iodization [Fig. 6(a)]. SEM cross sections show a 260% increase (242 → 628 nm) in grain size, but most notable is that significant structural changes occur due to iodization, transitioning from a round/spherical grain shape to a highly planar faceted grain interface. This is directly a result of the iodine and appears to influence the full film stack despite iodization occurring primarily in the upper part of the film. Given the grain boundaries are suspected of playing an important role in improving photoresponse, the planar faceting likely improves the inter-grain coupling and raises the question as to how iodization influences the grain boundaries of the base-layer of PbSe and if iodine is present at these grain boundaries.

Previous studies revealed the high (>30% ΔR) MWIR photoresponse for PbSe thin films occurs when oxidized for 6 min at 460 °C and iodized for 4 min at 325 °C. For this reason, the same samples were taken directly from the oxidation time study (0, 6, 14, and 30 min of oxidation at 460 °C), and all samples were iodized for 4 min at 325 °C to determine the impact of varying oxidization times. Figure 6 reveals the EDS line-scan results for the different oxidation conditions with a fixed iodization duration of 4 min at 325 °C. The sample without any oxidation showed signs of rapid iodization, and iodine diffused through nearly the entire film in just 4 min. The regions in this sample where iodine concentrations were highest showed nearly zero selenium suggesting the reaction in Eqs. (5) and (6) rapidly occurs when pure PbSe is exposed to iodine at 325 °C, and the film is converted to PbI₂.

Iodization under the same conditions through a protective lead selenite (PbSe_(1−x)O_x) layer formed during the prior oxidation process [Figs. 6(b)–6(d)] results in a surface layer of PbI₂ and a gradient of iodine concentration. Recall from the oxidation time study (Fig. 3), extended oxidation duration of greater than 6 min produces a selenium-poor lead selenite layer. The PbI₂ layer becomes proportionately thicker than would occur in a stoichiometric PbSeO₃ layer [Fig. 6(b)] under the same iodization conditions [Figs. 6(c) and 6(d)]. This suggests that selenium-poor lead selenite (PbSe_(1−x)O_x) and lead oxide (PbO_x) will iodize much more rapidly, similar to that of pure PbSe, to produce a thick PbI₂ layer. The thick PbI₂ results in a thin PbSe base-layer and is detrimental to achieving a high MWIR photoresponse. This reaction is represented in Eqs. (7) and (8).

A second-time study similar to that which was done for oxidation was performed for iodization following a 6 min oxidation step at 460 °C to form the previously determined 400–500 nm PbSeO₃ layer necessary to adequately protect the underlying PbSe base-layer. Equation (9) represents the chemical reaction for PbSeO₃ during iodization which is believed to be a slower, more uniform, and thus more controlled reaction. Recall the 6 min oxidation recipe also best preserves the Pb:Se stoichiometric balance throughout the film stack. The oxidized PbSe thin films were iodized at 325 °C for 0, 1, 2, 4, 6, and 8 min. For this study, Raman spectroscopy was used to better understand the composition transition of the film with a fixed oxidation time and varying iodization durations. The results are shown in Fig. 7. Here, we show that initially, the characteristic PbSe peaks at 136 and 793 cm⁻¹ are present; and however, the PbSe and PbSeO₃ peaks reduce in intensity as the PbI₂ layer thickness increases due to the absorption of the 514 nm laser wavelength used for Raman measurements.^11,12 With just a short iodization duration of 1–2 min, the peaks at 790 cm⁻¹ from the PbSeO₃ layer begin to diminish below the background, and characteristic PbI₂ peaks increase at 94, 110, and 214 cm⁻¹.^17,18 The sample processed with 6 min of oxidation and just 1 min iodization showed what appears to be a PbO_x peak at 804 cm⁻¹.¹⁴ This suggests that even 6 min of oxidation can produce some PbO_x on the surface; however, it rapidly disappears with iodization durations longer than 1 min at 325 °C.

X-ray diffraction (XRD) was performed for thin films prepared under two different iodization conditions. First, the iodization of an as-deposited thin film was carried out to confirm the rapid transformation of pure PbSe to PbI₂. XRD confirms the film has very little crystallinity, and only PbI₂ peaks were observed.

The second condition of iodization for XRD is the standard sensitization recipe (described above) and reveals the intensity of the PbSe and PbSeO₃ peaks diminishes as the PbI₂ peaks increase. This is expected; and however, one new finding is the identification of characteristic XRD peaks for Pb₃Se₂O₆I₂ which has not been reported in the scientific literature for IR-sensitized PbSe-based detectors. Several researchers have reported a Pb:Se:I-based or Pb:Se:O:I-based compound in this transition layer that may impact the photoconduction of PbSe thin films.^2,9 This compound is only present in PbSe thin films showing a highly MWIR photoresponse. This compound is within the transition layer material between the top PbI₂ and PbSe base-layer. Jang et al. report that this iodine-incorporated PbSe transition layer on top of the PbSe base-layer provides diffused electrons that cancel out hole concentrations under dark conditions due to electron–hole recombination.² The Pb₃Se₂O₆I₂ is shown in Fig. 5 in the XRD spectrum labeled Iodized. Equation (10) represents the reaction necessary to produce this compound.

Based on the above analysis, the overall sensitization mechanism that occurs in thin films of PbSe to maximize MWIR photoresponse can be described as follows:

• A 1.2 μm thin film of polycrystalline PbSe requires 6 min of oxidation (in the air) at 460 °C to produce a 400–500 nm layer of PbSeO₃ and similar thickness of PbSe.

• A 400–500 nm layer of formed PbSeO₃ serves multiple purposes by (1) protecting the underlying PbSe base-layer from rapid iodization and (2) allowing trace levels of oxygen and iodine to diffuse along grain boundaries to recrystallize the PbSe base-layer into a highly ordered and planar faceted grain structure. This incorporates both oxygen and iodine at the grain barriers and increases inter-grain coupling during photoconduction.¹⁵ 

• Shorter oxidation durations result in an insufficient PbSeO₃ layer thickness to protect the PbSe base-layer from the subsequent iodization.

• Longer oxidation durations result in an excessively thick lead selenite layer which is selenium-poor (PbSe_(1−x)O_x) and a PbO_x surface layer. Both these conditions negatively impact the MWIR photoresponse following iodization by (1) producing conditions for a thick and uncontrollable PbI₂ layer and (2) reducing the PbSe base-layer thickness, causing exceptionally high dark resistance (R_D) and lower infrared light absorption.

• Optimum iodization of 4 min at 325 °C was determined for its ability to simultaneously perform several roles. Iodization needs to sufficiently diffuse iodine along grain boundaries to recrystallize the PbSe base-layer while simultaneously forming a 100–200 nm surface layer of PbI₂ and a 300 nm transition layer (containing Pb₃Se₂O₆I₂) on top of a 400–500 nm base-layer of PbSe. The above configuration produces a film that is passivated by PbI₂ and is composed of a highly conductive stoichiometrically balanced polycrystalline PbSe base-layer. This further supports the claim that iodine directly influences the photoconductive effect for PbSe.

• The KI likely contributes to the recrystallization but the thermal iodization process is very important.

A comprehensive summary and model of the various oxidation and iodization conditions studied in this work as well as the IR photoresponse is shown in Fig. 8. This model was constructed by correlating the elemental distribution analysis (spatial dependent) with the compositional and crystallographic analysis techniques. A comparison between the process conditions and the MWIR photoresponse reveals the parameters necessary to maximize the MWIR photoresponse of PbSe thin films. The conditions for the highest photoresponse were found to be ΔR = 29%, corresponding to 6 min oxidation + 4 min iodization (2nd column of Fig. 8). This study also shows that there is a narrow process window for oxidation and iodization that would lead to high photoresponse. This study also provides a scientific explanation as to why IR photoresponse decreases outside the optimum process condition.

Iodization reactions:

The goal of this study is to provide a fundamental understanding of oxidation and iodization processes used for PbSe thin film sensitization for mid-infrared detection. This study shows that there is a narrow process window for oxidation that leads to a high IR photoresponse. This study also provides a scientific explanation of why IR photoresponse decreases outside the optimum process conditions. The optimum sensitization process that led to the highest IR photoresponse converted the original PbSe thin film of 1.2 μm thickness into a multi-layered film with a 200–300 nm thick top layer of PbI₂ nm, a transition layer partially containing Pb₃Se₂O₆I₂, and a bottom layer of PbSe of thickness 400–500 nm. Both oxygen and iodine were observed between PbSe grains with TEM in previous works.¹⁵ 

The oxidation of PbSe thin films at 460 °C produces a surface layer of lead selenite (PbSeO₃) with a base-layer of polycrystalline PbSe. This study provides evidence that a targeted PbSeO₃ layer thickness of 400 nm is needed to (1) restrict the PbI₂ surface layer thickness to less than 200 nm, (2) regulate the iodization process, allowing only trace amounts of iodine to diffuse along grain boundaries and recrystallize the PbSe base-layer, and (3) preserve the stoichiometric balance of the underlying PbSe layer. The 1.2 μm PbSe thin films used in this study required 6 min of oxidation at 460 °C to produce a 400–500 nm layer of PbSeO₃. An optimum process window was observed for the oxidation step whereby initially, 400 nm of PbSeO₃ forms with minimal selenium loss in the first six minutes at 460 °C; however, significant selenium loss begins to occur with longer oxidation times and PbO_x forms on the surface. The subsequent iodization process is found to be highly dependent on the thickness and type of oxides formed during oxidation. This work shows that PbSeO₃ iodizes to form PbI₂ at a rate of ∼100 nm/min. Both pure PbSe and selenium-poor lead selenite rapidly transform into PbI₂ at nearly 5× the rate of PbSeO₃. Consequently, PbSeO₃ regulates the diffusion of iodine into the film, allowing the iodine to impact the morphology of the entire film and prevent the rapid conversion of PbSe into PbI₂. As such, a narrow window exists in the oxidation process whereby too much, as well as too little oxidation, both result in a thick PbI₂ layer and a thin PbSe base-layer. Both conditions result in an excessively high dark resistance (R_D > 500 MΩ) and a reduced MWIR photoresponse due to lower infrared light absorption by thinner PbSe.

This work shows a diffused gradient of iodine reaching the PbSe base-layer to recrystallize it and increases inter-grain coupling by transforming the PbSe structure to a highly planar faceted morphology throughout the entire film stack. While iodine concentrations vary with depth, the iodization process uniformly transforms morphology and increases grain size by 260% (242 → 628 nm) for the entire film. This work also identifies a new compound (Pb₃Se₂O₆I₂) that is formed between the PbI₂ surface and the PbSe base-layer, which is only present in samples with a high MWIR photoresponse. The presence of this new compound (Pb₃Se₂O₆I₂) is observed in highly photoresponsive PbSe thin films and supports the notion that iodine does indeed play a direct role in the photoconduction mechanism.

It is concluded that to maximize the MWIR photoresponse, the overall sensitization processing (oxidation and iodization) must restrict PbI₂ to the surface yet allow oxygen and iodine to diffuse to PbSe base-layer grain boundaries while maintaining the stoichiometric balance of the underlying PbSe layer.

Supplementary material figures are made available in a separate document at the link provided.

Joel T. Harrison would like to thank the Naval Surface Warfare Center, Dahlgren Division (NSWCDD) for the support of his graduate research under the Office of Naval Research Grant No. N00014-21-WX-0-0334.

There are no conflicts of interests.

Data supporting the findings of this study are available from the corresponding author on request.