Combinatorial Tuning of Structural and Optoelectronic Properties in CuxZn1−xS

Abstract

•CuxZn1−xS structure-property relations are mapped across full cation composition space•A metastable wurtzite CuxZn1−xS alloy forms between two cubic binary endpoints•The wurtzite structure correlates with increased p-type conduction and transparency•A p-type transparent region is observed and contextualized with literature reports Advancing renewable energy technologies requires development of new materials. One missing link in photovoltaic (PV) solar cells are semiconductors that are optically transparent and electrically conductive, a combination of properties that is rare in nature. In particular, so-called “p-type” transparent conductors (TCs) could enable more efficient charge extraction in PV, so extensive research has ensued to find suitable p-type TCs. This work elucidates the evolution of crystal structure and physical properties across a non-oxide p-type TC system formed by alloying transparent ZnS with p-type conductive CuyS. The method of selecting two binary endpoints and tailoring their properties is a promising pathway to discover and optimize new p-type TCs. By coupling combinatorial experiments with theoretical calculations, we demonstrate a high-throughput approach to design properties of PV materials. P-type transparent conductors (TCs) are important enabling materials for optoelectronics and photovoltaics, but their performance still lags behind n-type counterparts. Recently, semiconductor CuxZn1−xS has demonstrated potential as a p-type TC, but it remains unclear how properties vary with composition. Here, we investigate CuxZn1−xS across the entire alloy space (0 ≤ x ≤ 1) using combinatorial sputtering and high-throughput characterization. First, we find a metastable wurtzite alloy at an intermediate composition between cubic endpoint compounds, contrasting with solid solutions or cubic composites (ZnS:CuyS) from the literature. Second, structural transformations correlate with shifts in hole conductivity and absorption; specifically, conductivity increases at the wurtzite phase transformation (x ≈ 0.19). Third, conductivity and optical transparency are optimized within a "TC regime" of 0.10 < x < 0.40. This investigation reaffirms CuxZn1−xS as a promising, tunable, multifunctional semiconductor alloy, provides new insight into composition-dependent evolution of structure and properties, and informs future research into device applications. P-type transparent conductors (TCs) are important enabling materials for optoelectronics and photovoltaics, but their performance still lags behind n-type counterparts. Recently, semiconductor CuxZn1−xS has demonstrated potential as a p-type TC, but it remains unclear how properties vary with composition. Here, we investigate CuxZn1−xS across the entire alloy space (0 ≤ x ≤ 1) using combinatorial sputtering and high-throughput characterization. First, we find a metastable wurtzite alloy at an intermediate composition between cubic endpoint compounds, contrasting with solid solutions or cubic composites (ZnS:CuyS) from the literature. Second, structural transformations correlate with shifts in hole conductivity and absorption; specifically, conductivity increases at the wurtzite phase transformation (x ≈ 0.19). Third, conductivity and optical transparency are optimized within a "TC regime" of 0.10 < x < 0.40. This investigation reaffirms CuxZn1−xS as a promising, tunable, multifunctional semiconductor alloy, provides new insight into composition-dependent evolution of structure and properties, and informs future research into device applications. Achieving a p-type transparent conductor (TC) with properties similar to n-type TCs such as tin-doped indium oxide (ITO) and aluminum-doped zinc oxide (AZO) remains an outstanding challenge in optoelectronic applications, and in particular for photovoltaics.1Delahoy A.E. Guo S. Handbook of Photovoltaic Science and Engineering.Second Edition. Wiley Online Library, 2011: 716-796Crossref Scopus (53) Google Scholar Although n-type TCs are usually oxides, recently there has been an upsurge of interest in non-oxide chalcogenide materials as potential p-type TCs.2Morales-Masis M. De Wolf S. Woods-Robinson R. Ager J.W. Ballif C. Transparent electrodes for efficient optoelectronics.Adv. Electron. Mater. 2017; 3: 1600529Crossref Scopus (250) Google Scholar Despite their lower gaps and stability challenges, sulfides offer several distinct advantages over oxides that directly address the challenges of achieving p-dopable TCs. First, due to the position of S-3p orbitals, the valence bands of sulfides could have greater hybridization and delocalization than oxides.3Hosono H. Recent progress in transparent oxide semiconductors: materials and device application.Thin Solid Films. 2007; 515: 6000-6014Crossref Scopus (593) Google Scholar This could lead to lower hole effective masses which could correlate to elevated hole mobilities. Second, the higher position of their valence bands suggests higher p-type dopability than oxides.3Hosono H. Recent progress in transparent oxide semiconductors: materials and device application.Thin Solid Films. 2007; 515: 6000-6014Crossref Scopus (593) Google Scholar These advantages have been demonstrated in the development of high-performing TC chalcogenides such as self-doped and Zn-doped CuAlS2 (with one of the highest conductivities of p-type TCs in the literature reported at 250 S cm−1),4Huang F.-Q. Liu M.-L. Yang C. Highly enhanced p-type electrical conduction in wide band gap Cu1+x Al1-xS2 polycrystals.Sol. Energy Mater. Sol. Cells. 2011; 95: 2924-2927Crossref Scopus (28) Google Scholar, 5Liu M.-L. Huang F.-Q. Chen L.-D. Wang Y.-M. Wang Y.-H. Li G.-F. Zhang Q. p-type transparent conductor: Zn-doped CuAlS2.Appl. Phys. Lett. 2007; 90: 072109Crossref Scopus (55) Google Scholar BaCu2S2,6Park S. Keszler D.A. Valencia M.M. Hoffman R.L. Bender J.P. Wager J.F. Transparent p-type conducting BaCu2S2 films.Appl. Phys. Lett. 2002; 80: 4393-4394Crossref Scopus (63) Google Scholar, 7Han Y. Siol S. Zhang Q. Zakutayev A. Optoelectronic properties of strontium and barium copper sulfides prepared by combinatorial sputtering.Chem. Mater. 2017; 29: 8239-8248Crossref Scopus (26) Google Scholar and Cu3TaS4,8Tate J. Newhouse P.F. Kykyneshi R. Hersh P.A. Kinney J. McIntyre D.H. Keszler D.A. Chalcogen-based transparent conductors.Thin Solid Films. 2008; 516: 5795-5799Crossref Scopus (47) Google Scholar and mixed anion chalcogenides such as LaCuO(S,Se)9Hiramatsu H. Ueda K. Ohta H. Hirano M. Kamiya T. Hosono H. Degenerate p-type conductivity in wide-gap LaCuOS1-xSex (x= 0-1) epitaxial films.Appl. Phys. Lett. 2003; 82: 1048-1050Crossref Scopus (160) Google Scholar and BaCu(S,Se,Te)F.10Yanagi H. Tate J. Park S. Park C.-H. Keszler D.A. p-type conductivity in wide-band-gap BaCuQF (Q=S, Se).Appl. Phys. Lett. 2003; 82: 2814-2816Crossref Scopus (73) Google Scholar More recently, several TC chalcogenides have been predicted using high-throughput computational screenings, and remain to be achieved experimentally.11Shi J. Cerqueira T.F. Cui W. Nogueira F. Botti S. Marques M.A. High-throughput search of ternary chalcogenides for p-type transparent electrodes.Sci. Rep. 2017; 7: 43179Crossref PubMed Scopus (41) Google Scholar, 12Raghupathy R.K.M. Kühne T.D. Felser C. Mirhosseini H. Rational design of transparent p-type conducting non-oxide materials from high-throughput calculations.J. Mater. Chem. C. 2018; 6: 541-549Crossref Google Scholar One particularly promising p-type chalcogenide is the ternary compound CuxZn1−xS (also notated Cu-Zn-S, CuZnS, ZnS:CuyS, and CuxZn1−xS1−δ). CuxZn1−xS combines the properties of a wide-gap insulator, ZnS, with an absorbing p-type semiconductor, CuyS. Zinc sulfide typically crystallizes in a low-temperature zinc blende (ZB) ZnS phase (F4¯3m space group, band gap Eg ≈ 3.7 eV) or a high-temperature wurtzite (WZ) ZnS phase in a variety of polytype stackings (P63mc, Eg ≈ 3.9 eV).13Madelung O. Semiconductors: Data Handbook. Springer Science & Business Media, 2012Google Scholar Copper sulfide crystallizes in a variety of stoichiometries and nominal oxidation states, most commonly covellite CuS (space group Cmcm, Eg ≈ 0.6–1 eV) or chalcocite Cu2S (space group P21/c, Eg ≈ 1.2–2 eV), although compounds CuyS (y = 1.6, 1.8, etc.) with mixed nominal Cu oxidation states are also common, so we will use the notation CuyS herein. At dilute dopings (x < 0.005), Cu-doped ZnS nanoparticles and films have been studied as electrochromic materials and photocatalysts,14Corrado C. Cooper J.K. Hawker M. Hensel J. Livingston G. Gul S. Vollbrecht B. Bridges F. Zhang J.Z. Synthesis and characterization of organically soluble Cu-doped ZnS nanocrystals with Br co-activator.J. Phys. Chem. C. 2011; 115: 14559-14570Crossref Scopus (26) Google Scholar, 15Ummartyotin S. Bunnak N. Juntaro J. Sain M. Manuspiya H. Synthesis and luminescence properties of ZnS and metal (Mn, Cu)-doped-ZnS ceramic powder.Solid State Sci. 2012; 14: 299-304Crossref Scopus (220) Google Scholar, 16Ummartyotin S. Infahsaeng Y. A comprehensive review on ZnS: From synthesis to an approach on solar cell.Renew. Sustain. Energy Rev. 2016; 55: 17-24Crossref Scopus (100) Google Scholar and at higher concentrations alloys have been explored. CuxZn1−xS was first demonstrated as a p-type TC in 2011 using electrochemical deposition, although previous studies of doping ZnS with Cu had alluded to its application.17Yang K. Ichimura M. Fabrication of transparent p-Type CuxZnyS thin films by the electrochemical deposition method.Jpn. J. Appl. Phys. 2011; 50: 040202Crossref Google Scholar, 18Fang X. Zhai T. Gautam U.K. Li L. Wu L. Bando Y. Golberg D. ZnS nanostructures: from synthesis to applications.Prog. Mater. Sci. 2011; 56: 175-287Crossref Scopus (805) Google Scholar The CuxZn1−xS p-type TC alloy gained prominence in 2012, when pulsed laser deposition (PLD) yielded thin films with simultaneous hole conductivities up to 56 S cm−1 and elevated optical transparencies.19Diamond A.M. Corbellini L. Balasubramaniam K. Chen S. Wang S. Matthews T.S. Wang L.-W. Ramesh R. Ager J.W. Copper-alloyed ZnS as ap-type transparent conducting material.Phys. Status Solidi A. 2012; 209: 2101-2107Crossref Scopus (74) Google Scholar Additional studies demonstrated synthesis at ambient temperature using PLD can retain crystallinity, high transparencies, and similar conductivities,20Woods-Robinson R. Cooper J.K. Xu X. Schelhas L.T. Pool V.L. Faghaninia A. Lo C.S. Toney M.F. Sharp I.D. Ager J.W. P-Type transparent Cu-alloyed ZnS deposited at room temperature.Adv. Electron. Mater. 2016; 2: 1500396Crossref Scopus (35) Google Scholar and that annealing can further increase conductivity.21Feng M. Zhou H. Guo W. Zhang D. Ye L. Li W. Ma J. Wang G. Chen S. Fabrication of P-type transparent conducting CuxZn1-xS films on glass substrates with high conductivity and optical transparency.J. Alloys Compd. 2018; 750: 750-756Crossref Scopus (9) Google Scholar A significant step was development of a facile, low-temperature chemical bath deposition (CBD) approach, yielding ZnS:CuyS composites with conductivities on the order of 103 S cm−1, albeit with drops in transparency.22Xu X. Bullock J. Schelhas L.T. Stutz E.Z. Fonseca J.J. Hettick M. Pool V.L. Tai K.F. Toney M.F. Fang X. Ager J. Chemical bath deposition of p-type transparent, highly conducting (CuS)x:(ZnS)1-x nanocomposite thin films and fabrication of Si heterojunction solar cells.Nano Lett. 2016; 16: 1925-1932Crossref PubMed Scopus (87) Google Scholar Subsequently, this compound has been synthesized via various chemical and physical methods, e.g., sol gel,23Goktas A. Aslan F. Tumbul A. Nanostructured Cu-doped {ZnS} polycrystalline thin films produced by a wet chemical route: the influences of Cu doping and film thickness on the structural, optical and electrical properties.J. Solgel Sci. Technol. 2015; 75: 45-53Crossref Scopus (55) Google Scholar spray pyrolysis,24Sreejith M. Deepu D. Kartha C.S. Vijayakumar K. Tuning of properties of sprayed CuZnS films.AIP Conf. Proc. 2014; 1591: 1741-1743Crossref Scopus (6) Google Scholar, 25Jubimol J. Sreejith M. Kartha C.S. Vijayakumar K. Louis G. Analysis of spray pyrolysed copper zinc sulfide thin films using photoluminescence.J. Lumin. 2018; 203: 436-440Crossref Scopus (8) Google Scholar atomic layer deposition (ALD),26Di Benedetto F. Cinotti S. D’Acapito F. Vizza F. Foresti M.L. Guerri A. Lavacchi A. Montegrossi G. Romanelli M. Cioffi N. Innocenti M. Electrodeposited semiconductors at room temperature: an X-ray absorption spectroscopy study of Cu-, Zn-, S-bearing thin films.Electrochim. Acta. 2015; 179: 495-503Crossref Scopus (11) Google Scholar, 27Mahuli N. Saha D. Maurya S.K. Sinha S. Patra N. Kavaipatti B. Sarkar S.K. Atomic layer deposition of transparent and conducting p-Type Cu(I) incorporated {ZnS} thin films: unravelling the role of compositional heterogeneity on optical and carrier transport properties.J. Phys. Chem. C. 2018; 122: 16356-16367Crossref Scopus (12) Google Scholar and sputtering,28Chamorro W. Shyju T. Boulet P. Migot S. Ghanbaja J. Miska P. Kuppusami P. Pierson J. Role of Cu+ on ZnS: Cu p-type semiconductor films grown by sputtering: influence of substitutional Cu in the structural, optical and electronic properties.RSC Adv. 2016; 6: 43480-43488Crossref Google Scholar, 29Maurya S.K. Liu Y. Xu X. Woods-Robinson R. Das C. Ager J.W. Balasubramaniam K.R. High figure-of-merit p-type transparent conductor, Cu alloyed {ZnS} via radio frequency magnetron sputtering.J. Phys. D Appl. Phys. 2017; 50: 505107Crossref Scopus (17) Google Scholar among others (see Table S1). We emphasize that remarkable properties and crystalline films have been achieved even at low deposition temperatures; this is particularly advantageous to applications in optoelectronic devices with low thermal budgets, such as CdTe and perovskite photovoltaics (PV).30Ellmer K. Past achievements and future challenges in the development of optically transparent electrodes.Nat. Photonics. 2012; 6: 809Crossref Scopus (1476) Google Scholar, 31Krebs F.C. Jørgensen M. Polymer and organic solar cells viewed as thin film technologies: what it will take for them to become a success outside academia.Sol. Energy Mater. Sol. Cells. 2013; 119: 73-76Crossref Scopus (29) Google Scholar Accordingly, CuxZn1−xS films of various crystallinity and microstructure have been recently demonstrated as, e.g., a transparent electrode in np+:Si PV devices with a demonstrated maximum open-circuit voltage of 535 mV,22Xu X. Bullock J. Schelhas L.T. Stutz E.Z. Fonseca J.J. Hettick M. Pool V.L. Tai K.F. Toney M.F. Fang X. Ager J. Chemical bath deposition of p-type transparent, highly conducting (CuS)x:(ZnS)1-x nanocomposite thin films and fabrication of Si heterojunction solar cells.Nano Lett. 2016; 16: 1925-1932Crossref PubMed Scopus (87) Google Scholar a back contact on CdTe solar cells to enable bifacial PV,32Subedi K.K. Bastola E. Subedi I. Song Z. Bhandari K.P. Phillips A.B. Podraza N.J. Heben M.J. Ellingson R.J. Nanocomposite (CuS)x(ZnS)1-x thin film back contact for CdTe solar cells: Toward a bifacial device.Sol. Energy Mater. Sol. Cells. 2018; 186: 227-235Crossref Scopus (25) Google Scholar a top contact on perovskite PV,33Li J. Kuang C. Zhao M. Zhao C. Liu L. Lu F. Wang N. Huang C. Duan C. Jian H. et al.Ternary CuZnS nanocrystals: synthesis, characterization, and interfacial application in perovskite solar cells.Inorg. Chem. 2018; 57: 8375-8381Crossref PubMed Scopus (12) Google Scholar and a junction partner in self-powered UV photodetectors,34Xu X. Chen J. Cai S. Long Z. Zhang Y. Su L. He S. Tang C. Liu P. Peng H. et al.A real-time wearable UV-radiation monitor based on a high-performance p-CuZnS/n-TiO2 photodetector.Adv. Mater. 2018; 30: 1803165Crossref PubMed Scopus (277) Google Scholar, 35Cai J. Xu X. Su L. Yang W. Chen H. Zhang Y. Fang X. Self-powered n-SnO2/p-CuZnS core-shell microwire UV photodetector with optimized performance.Adv. Opt. Mater. 2018; 6: 1800213Crossref Scopus (69) Google Scholar among other applications. As this material space gains prominence in the photovoltaic community and beyond, it is important to understand the structural driving forces of high conductivity and stability. Among the previously mentioned studies, this compound tends to either (1) phase segregate into a nanocomposite material (ZnS and CuyS), usually with chemical synthesis methods, (2) form a heterostructural, heterovalent alloy to varying degrees, usually with physical deposition methods, or (3) some combination of the first two (see Perspective). However, it is still not fully understood what exactly is responsible for elevated conductivity in this material: a conducting network of semimetallic CuyS or metallic Cu phases, doping of Cu+1 cations into ZnS, realization of a unique heterovalent ternary alloy, or some combination of these effects?36Xu X. Li S. Chen J. Cai S. Long Z. Fang X. Design principles and material engineering of ZnS for optoelectronic devices and catalysis.Adv. Funct. Mater. 2018; 28: 1802029Crossref Scopus (69) Google Scholar In particular, why can high performance be achieved at such low synthesis temperatures? Additionally, to our knowledge there has been no exploration yet of structure-property relations across full cation composition space, which could provide insight into the compound's formation conditions and thermodynamic stability. In this study, we investigate the CuxZn1−xS1−δ phase space using combinatorial sputtering at low deposition temperature, as depicted in Figure 1A. We focus on the role of Cu concentration x across its entire composition space (0 ≤ x ≤ 1), since the tuning of cations is primarily responsible for the shift between insulating, semiconducting, and metallic properties. Thus, we refer to this ternary space simply as “CuxZn1−xS” herein, as is common in the literature, yet we acknowledge sulfur concentration 1 − δ is also relevant. Using customized high-throughput combinatorial characterization tools, e.g., synchrotron diffraction, UV-visible near-infrared (UV-vis-NIR) spectrophotometry, and four-point probe resistivity measurements, as depicted in Figure 1B, we demonstrate that heterovalent, heterostructural alloying occurs within this compound, and observe the stabilization of the metastable WZ ZnS phase in the approximate region 0.19 < x < 0.50. The transition from the ZB to WZ phase around x = 0.19 is found to correlate with increased conductivity and a decrease in absorption coefficient, and understanding these structure-property relations provides guidelines on how to improve transparency and conductivity in future work. We contextualize our findings across the literature space of CuxZn1−xS, comparing the reported phases, synthesis methods, and achieved properties. Sample stoichiometry of >350 data points in eight combinatorial libraries spans almost all of the cation composition space of CuxZn1−xS, with 0 ≤ x ≤ 1 where x = Cu/(Cu + Zn). Sulfur concentration (1 − δ in CuxZn1−xS1−δ) versus copper concentration x is plotted in Figure 2A (see Supplemental Information for calibration). Some regions have overlapping libraries and others have no data. Endpoint films with x = 0 are S-poor with respect to ZnS, with 1 − δ = 0.92 ± 0.04 rather than 1 − δ = 1. Endpoint x = 1 aligns with a stoichiometry somewhere between Cu1.2S and Cu1.4S, which is S-rich with respect to the nominal Cu2S of the target. In all alloy films with x > 0 and x < 1, 1 − δ appears to align approximately with that expected from an alloy of ZnS:Cu1.6S, within measurement uncertainty. These findings corroborate literature findings of S-poor ZnS37Barman B. Bangera K.V. Shivakumar G. Preparation of thermally deposited Cux(ZnS)1-x thin films for opto-electronic devices.J. Alloys Compd. 2019; 772: 532-536Crossref Scopus (11) Google Scholar and S-rich Cu2S,38Welch A.W. Zawadzki P.P. Lany S. Wolden C.A. Zakutayev A. Self-regulated growth and tunable properties of CuSbS2 solar absorbers.Sol. Energy Mater. Sol. Cells. 2015; 132: 499-506Crossref Scopus (107) Google Scholar, 39Baranowski L.L. Zawadzki P. Christensen S. Nordlund D. Lany S. Tamboli A.C. Gedvilas L. Ginley D.S. Tumas W. Toberer E.S. et al.Control of doping in Cu2SnS3 through defects and alloying.Chem. Mater. 2014; 26: 4951-4959Crossref Scopus (122) Google Scholar although S content is rarely reported. We observe a minor library-dependent effect such that data points from different sample libraries with the same x have slightly offset 1 − δ. These off-stoichiometries may stem from unintentional substrate heating from the plasma, from other non-equilibrium effects, or from a mixed oxidation state of Cu. Although energy-dispersive X-ray spectroscopy (EDS) measurements suggest oxygen contamination of less than 2%, this is presumed to only reside on the surface as a thin oxidized layer, which is expected because of air exposure of films. Bulk oxygen is undetectable via Rutherford backscattering spectrometry (RBS). In Figure 2B, we plot the synchrotron X-ray diffraction intensities (color scale) of the ambient-temperature-synthesized films as a function of cation composition x, with both Q and 2θ on the y axis (calibrated to Cu K-α emission wavelength) for reference. The amorphous halo around Q = 2 Å−1 is due to the glass substrate. Broad peaks indicate a nanocrystalline film. Within the phase space of CuxZn1−xS we observe four distinct binary crystal structures: cubic zinc blende ZnS-type (space group F4¯3m, denoted “ZB”), hexagonal wurtzite ZnS-type (space group P63mc, denoted “WZ”), CuyS-type (1.6 ≤ y ≤ 2; space group F4¯3m), and CuyS-type (1.2 ≤ y ≤ 1.4; space group Fm3¯m). These crystal structures, the Cu concentrations in which they are present, local coordination environments, theoretical GGA band structures (from the Materials Project database),40Jain A. Ong S.P. Hautier G. Chen W. Richards W.D. Dacek S. Cholia S. Gunter D. Skinner D. Ceder G. Persson K.A. The materials project: A materials genome approach to accelerating materials innovation.APL Mater. 2013; 1: 011002Crossref Scopus (3900) Google Scholar and BoltzTraP effective masses from a computational database41Madsen G.K. Singh D.J. BoltzTraP. A code for calculating band-structure dependent quantities.Comput. Phys. Commun. 2006; 175: 67-71Crossref Scopus (3422) Google Scholar, 42Ricci F. Chen W. Aydemir U. Snyder G.J. Rignanese G.-M. Jain A. Hautier G. An ab initio electronic transport database for inorganic materials.Sci. Data. 2017; 4: 170085Crossref PubMed Scopus (96) Google Scholar are illustrated in Figure 3. The dominating diffraction peaks of these phases are marked in Figure 2B using colored triangles. Interestingly, the ratio of binary constituents does not change monotonically across x, in contrast to other low-temperature syntheses of CuxZn1−xS, e.g., CBD.22Xu X. Bullock J. Schelhas L.T. Stutz E.Z. Fonseca J.J. Hettick M. Pool V.L. Tai K.F. Toney M.F. Fang X. Ager J. Chemical bath deposition of p-type transparent, highly conducting (CuS)x:(ZnS)1-x nanocomposite thin films and fabrication of Si heterojunction solar cells.Nano Lett. 2016; 16: 1925-1932Crossref PubMed Scopus (87) Google Scholar There is also no evidence of a unique ordered ternary phase of all three elements, corroborated by the lack of such a thermodynamically stable phase in the Inorganic Crystal Structure Database or Materials Project database (although an ordered F4¯3m alloy may be possible around x = 0.6; cf. Supplemental Information for computational structure prediction).40Jain A. Ong S.P. Hautier G. Chen W. Richards W.D. Dacek S. Cholia S. Gunter D. Skinner D. Ceder G. Persson K.A. The materials project: A materials genome approach to accelerating materials innovation.APL Mater. 2013; 1: 011002Crossref Scopus (3900) Google Scholar Rather, this system appears to be a heterovalent, heterostructural alloy, with distinct regions in composition space where the structure transitions either sharply or gradually to another phase. One particular difficulty in identifying these phases arises from the similar symmetry, coordination, atomic radius, and unit cell size of ZB ZnS and cubic CuyS. Coupled with broad peaks, this results in nearly identical X-ray reflection locations, albeit slightly distinct relative intensities. For example, ZnS is less atomically dense in the (111) plane in comparison, so the peak at Q = 2.0 Å−1 is fainter; we will be focusing on this peak subsequently. At x = 0 (far left of Figure 2B) and from 0 ≤ x ≤ 0.18, diffraction peaks correspond to ZB ZnS, with a faint shoulder peak at approximately 2θ = 27° suggestive of WZ ZnS (100). Films are nanocrystalline, yet somewhat oriented in the (111) direction. Between 0.10 < x < 0.19, the (111) peak shifts slightly to higher 2θ by approximately 0.2°, as observed in Figure 2C. This correlates to the contraction of lattice parameter plotted in Figure 2D, which is expected upon Cu incorporation into ZnS, although the peak shift is lower in magnitude than the expected theoretical contraction of complete substitution of CuZn antisites.20Woods-Robinson R. Cooper J.K. Xu X. Schelhas L.T. Pool V.L. Faghaninia A. Lo C.S. Toney M.F. Sharp I.D. Ager J.W. P-Type transparent Cu-alloyed ZnS deposited at room temperature.Adv. Electron. Mater. 2016; 2: 1500396Crossref Scopus (35) Google Scholar However, at approximately x = 0.19, the cubic peaks and the shoulder disappear and sharply transition to peaks indicative of (002) oriented WZ ZnS, as shown most discernibly by the appearance of the (103) WZ peak at approximately Q = 3.5 Å−1 (approximately 2θ = 51°). The large peak around Q = 2.0 Å−1 (2θ = 28.5°) remains dominant, although it shifts noticeably to slightly lower values (see Figure 2C). It aligns with both WZ (002) and ZB (111), but is too broad to resolve. The intensity of the peak suggests (002) orientation, although could also discern some ZB still in the material. The WZ phase is present until x = 0.50, although a cubic F4¯3m phase appears briefly in the middle of this regime from approximately 0.30 < x < 0.40, and could be from ZB ZnS or some ordered ternary F4¯3m structure, e.g., F4¯3m Cu2ZnS2 (see Supplemental Information). Between x = 0.50 and x = 0.60, a gradual shift from WZ to cubic F4¯3m occurs in what appears to be a phase-separated composite of WZ ZnS and cubic CuyS. The peak around Q = 2.0 Å−1 shifts to higher Q. We note the similarity with the ZB ZnS phase, and it is plausible there is mixed ZB ZnS within this regime. The diffraction patterns remain relatively unchanged from 0.60 < x < 0.90, although the cubic (111) peak becomes fainter while the cubic (220) peak gets stronger with x. This could correspond to a shift in orientation. We deduce a phase change from F4¯3m CuyS to what is likely an Fm3¯m structure somewhere between x = 0.90 and x = 1, but the onset is unclear. This phase is highly oriented in the (220) direction and is measured with RBS as CuyS with 1.2 ≤ y ≤ 1.4. This is more S-rich than the Cu2S target. We see no evidence of covellite or chalcocite, the thermodynamic ground-state CuyS structures, nor any unique ternary phase across the entire phase space. In Supplemental Information we show patterns from higher-temperature synthesis conditions. In contrast to ambient-temperature-synthesized films, sputtered films at temperatures 180°C and greater clearly phase segregate into ZB ZnS and cubic Fm3¯m CuyS with larger grain sizes. Coherence length of sample grains is estimated from the wide-angle X-ray scattering (WAXS) patterns using Scherrer analysis in Figure 2E.43Smilgies D.-M. Scherrer grain-size analysis adapted to grazing-incidence scattering with area detectors.J. of Appl. Crystallogr. 2009; 42: 1030-1034Crossref PubMed Scopus (517) Google Scholar Across the full phase space, coherence length ranges from approximately 10 nm to 50 nm, with a slight decrease as x increases. Error bars stem from uncertainty in peak fit. It is highest within the WZ dominated films, suggesting larger grains. Due to peak broadening, Scherrer analysis is useful to estimate a lower limit to grain size. Transmission electron microscopy (TEM) was performed for samples with approximately x = 0.18 and x = 0.21, selected as representative compositions that lie before and after the ZB-to-WZ phase transition, respectively, and within the TC regime. Figures 4A and 4B show TEM bright-field images, Figures 4C and 4D selected area electron diffraction (SAED) patterns, and Figure 4E integrated SAED patterns compared with the WAXS diffraction patterns. We find SAED patterns to corroborate our WAXS measurements, confirming the dominance of the ZB structure at x = 0.18 and WZ structure at x = 0.21. Furthermore, a Rietveld refinement analysis suggests that both samples contain approximately 5%–10% of secondary phases of WZ and ZB, respectively. This could explain the shoulder peak in ZB films. Additionally, both films appear to be polycrystalline. Comparison of Figures 4C and 4D illustrates that the ZB phase contains relatively randomly oriented grains, as indicated by its diffraction rings, but in contrast the oriented diffraction spots in the WZ phase indicate high texturing in the (002) direction, corroborating the diffraction patterns. This is not unusual for WZ thin films in general, but somewhat surprising for thin films synthesized at ambient temperature, especially on glass substrates. TEM micrographs show grain sizes on the order of 20 nm in width for the x = 0.18 and x = 0.21 samples, comparable with WAXS Scherrer analysis. TEM also illustrates the samples' growth in columnar grains (see Supplemental Information), which is typical for sputtered films.44Thornton J.A. High rate thick film growth.Annu. Rev. Mater. Sci. 1977; 7: 239-260Crossref Scopus (1926) Google Scholar We also note that EDS line scans and mapping do not detect any evidence of a phase-segregated CuyS at either