Zinc sulfide chemical vapor deposition optical ceramic analyzed by XPS

Zinc sulfide (ZnS) ceramics transmit in the infrared wavelength range between 1 and 12 μm (Ref. 1). This material is also semiconducting with a bandgap of 3.67 eV and exhibits unique optical and electrical properties, making it a popular choice for various applications, such as thermal imaging cameras, night vision devices, and lasers (Refs. 2–4).

Chemical vapor deposition (CVD) allows for the deposition of bulk ZnS ceramics with precise control over thickness, uniformity, and crystalline structure. This makes it possible to produce high-quality ZnS with low defects and excellent optical properties, including high optical transparency, low absorption, and high refractive index (2.35 ± 0.01) (Ref. 5). The resulting ZnS are also durable and resistant to environmental degradation. While the ceramics made by CVD are of high quality, the visual clarity of the material is not clear. Upon hot isostatic pressing, the ceramics transform into optically transparent substrates.

There are little to no reports of the x-ray photoelectron spectroscopy (XPS) of ZnS transparent ceramics. The reports available are either incomplete or the available XPS spectral characterization is for ZnS with other form factors such as thin films, powders, and nanoparticles (Refs. 2 and 6–8).

In this study, we analyze ZnS CVD transparent ceramic by XPS to study and quantify the bonding states of observable electron shells for Zn and S, along with minor species C and O. The use of XPS also provides quantification of atomic percentages (i.e., stoichiometry) along with the bonding states of trace level impurities, if present. From the oxygen (O 1s) fine spectra, we see that the material has surface level oxidation. The sulfur fine spectrum (S 2p) does not show evidence of oxidized sulfur, indicating that O is associated primarily with zinc.

Host Material: Zinc Sulfide Bulk Ceramic

CAS Registry #: 1314-98-3

Host Material Characteristics: Homogeneous; solid; polycrystalline; semiconductor; inorganic compound; Ceramic

Chemical Name: Zinc Sulfide

Source: APC American Photonics, 6621 19^th Street East Sarasota, FL 34243, USA

Host Composition: ZnS

Form: CVD Zinc Sulfide Ceramic

Structure: Microcrystalline cubic zinc blend

History & Significance: The specimen was received from APC American Photonics in an optics case with lens tissue and lining and stored in normal atmosphere at room temperature. ZnS is commonly used in infrared windows, domes, and optical elements. While the ZnS nanocrystalline CVD thin film has been cataloged in the past, this work is done on a commercially available ZnS CVD ceramic sample that is optically transparent. The samples were used as received.

As Received Condition: Optically clear ceramic window

Analyzed Region: Same as the host material

Ex Situ Preparation/Mounting: Ceramic was sonicated for 5 min in hexane, then transferred to isopropanol and sonicated for 5 min, and finally transferred to de-ionized water for a final 5 min of sonication. The sample was then dried by compressed air. The sample was mounted on the XPS stage using double-sided carbon tape.

In Situ Preparation: 600 s argon ion sputtering at 500 eV was used to clean the surface before analysis.

Charge Control: Charge compensation is delivered by both an in-lens electrostatic electron flood source (1 eV, 100 μA) and a dual-beam low-energy electron and ion coaxial flood source (2 eV, 100 μA).

Temp. During Analysis: 300 K

Pressure During Analysis: 5 × 10⁻⁸ Pa

Pre-analysis Beam Exposure: 0 s

Manufacturer and Model: Thermo Fisher Scientific ESCALAB 250Xi

Analyzer Type: Spherical sector

Detector: Channeltron

Number of Detector Elements: 6

Analyzer Mode: Constant pass energy

Throughput (T = E^N): Calculated from a polynomial fit to a plot of log [peak area/(PE × XSF)] versus log[KE/PE], where PE is the pass energy, KE is the kinetic energy, and XSF is the relative sensitivity factor.

Excitation Source Window: None

Excitation Source: Al K_α monochromatic

Source Energy: 1486.6 eV

Source Strength: 200 W

Source Beam Size: 650 × 650 μm²

Signal Mode: Single channel direct

Incident Angle: 58°

Source-to-Analyzer Angle: 58°

Emission Angle: 0°

Specimen Azimuthal Angle: 90°

Acceptance Angle from Analyzer Axis: 45°

Analyzer Angular Acceptance Width: 22.5° × 22.5°

Manufacturer and Model: Thermo Fisher Scientific EX03 Ion Gun System

Energy: 500 eV

Current: 0.02 mA

Current Measurement Method: biased stage

Sputtering Species: Ar⁺

Spot Size (unrastered): 500 μm

Raster Size: 4500 × 4500 μm²

Incident Angle: 40°

Polar Angle: 40°

Azimuthal Angle: 270°

Comment: These parameters correspond to ion-cleaning methods used in optical materials requiring surface cleaning.

Energy Scale Correction: Binding energy scale was referenced to C 1s = 284.8 eV fine spectra pre and post sputter

Recommended Energy Scale Shift: Shift +0.15 eV

Peak Shape and Background Method: thermo scientific avantage software version 5.9902 was used for peak shape and background subtraction. The smart (Shirley function) was used to subtract the background for Zn 2p, S 2p, O 1s, and C 1s peaks. Using the smart feature utilizes revised constraints that limit the background from having greater intensity than data from points in the collection region.

Quantitation Method: Atomic percentages were calculated using the thermo scientific avantage software version 5.9902. Sensitivity factors were obtained from the thermo scientific avantage software database and were used in the calculation of elemental atomic percentages. The peak library is ALWAG (Ref. 9).

The authors thank the Lockheed Martin University Engagement and Applied Research organizations, the US Naval Surface Warfare Center, the US Army Research Laboratory, and the US Air Force Research Laboratory for their integral and collaborative support of this research. The authors acknowledge the NSF MRI: ECCS: 1726636 and the MCF-AMPAC facility, MSE and CECS along with Kirk Scammon MCF Research Engineer. This research was supported, in part, by the Florida High Tech Corridor’s Matching Grant Research Program at the University of Central Florida.

The authors have no conflicts to disclose.

Brian Butkus: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (lead); Methodology (lead); Software (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (equal). Alexandros Kostogiannes: Investigation (supporting); Validation (supporting); Writing – review & editing (supporting). Andrew Howe: Investigation (supporting); Visualization (supporting); Writing – review & editing (supporting). Myungkoo Kang: Validation (supporting); Visualization (supporting); Writing – review & editing (supporting). Romain Gaume: Conceptualization (supporting); Formal analysis (supporting); Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (supporting); Validation (supporting); Writing – review & editing (supporting). Kathleen A. Richardson: Conceptualization (supporting); Formal analysis (supporting); Funding acquisition (lead); Project administration (equal); Resources (equal); Supervision (supporting); Validation (supporting); Visualization (supporting); Writing – review & editing (equal). Parag Banerjee: Conceptualization (equal); Formal analysis (supporting); Funding acquisition (equal); Investigation (supporting); Project administration (equal); Resources (equal); Supervision (lead); Validation (equal); Visualization (supporting); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.