Atomic Layer Deposition of Transparent and Conducting p-Type Cu

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C: Physical Processes in Nanomaterials and Nanostructures

Atomic Layer Deposition of Transparent and Conducting pType Cu Incorporated ZnS Thin Films: Unravelling the Role of Compositional Heterogeneity on Optical and Carrier Transport Properties Neha Mahuli, Debabrata Saha, Sandeep Kumar Maurya, Soumyadeep Sinha, Nirmalendu Patra, Balasubramaniam Kavaipatti, and Shaibal K. Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03027 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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The Journal of Physical Chemistry

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 Neha Mahuli1, Debabrata Saha2, Sandep Kumar Maurya2, Soumyadeep Sinha2, Nirmalendu Patra3, Balasubramaniam Kavaipatti2 and Shaibal K Sarkar*,2 1

Center for Research in Nanotechnology and Sciences, Indian Institute of Technology Bombay, Powai 400076 Mumbai India

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Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai 400076 Mumbai India 3

Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India

ACS Paragon Plus Environment

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ABSTRACT

Optically transparent and highly conducting p-type Cu(I) incorporated ZnS (Cu:ZnS) films are deposited by stacking individual layers of CuS and ZnS using atomic layer deposition (ALD). The deposition chemistry and growth mechanism are studied by in-situ quartz crystal microbalance (QCM). Compositional disorder in atomic scale is observed with increasing Cu incorporation in the films that results systematic decrease in the optical transmittance in the visible spectrum. Again the conductivity also emphatically depends on the volume fraction of phase segregated conducting covellite phase. An illustrious correlation prevailing the interplay between the optical transparency and the charge transport mechanism is established. The hole transport mechanism that indicates insulator to metal transition (IMT) with increasing Cu incorporation in the composite is explained in terms of inhomogeneously disordered system. Under optimized condition the material having moderately high optical transmission with degenerate carrier concentration lies exactly at the confluence between the metallic and insulating regime. The lowest resistivity that is obtained here (1.3x10-3 Ω-cm) with >90% (after reflection correction) transmission is highly comparable to the best ones that are reported in the field and probably analogous to the commercially available n-type transparent conductors.

INTRODUCTION Transparent conductors (TCs) have wide range applicability in major field of optoelectronics including photovoltaic devices,1-3 LEDs,1,2 organic electronics1 etc. The p-type TCs which have been experimentally realized so far, exhibit much lower electrical conductivity as compared to that of extensively studied and commercially available n-type TCs. Strategically, developing ptype transparent conducting materials (predominantly, oxides) are generally found challenging as


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often they are limited either by the doping rule4 or charge compensation effects5,6 or by the hole mobility due to localized valance band7 structure. Incorporating metal cations like Cu2+ Ag2+

8,13,14

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in the AIIBIIIO2 structure or partial replacement of oxygen by chalcogenides15-17 have

substantial effect in the delocalization of the upper valance band in delafossites leading to relatively higher hole mobility however often limited by the free carrier concentration.11 Recently non-stoichiometric Cu incorporated ZnS (Cu:ZnS), often referred as doped18,19 or alloyed20,21,24, is explored as promising p-type transparent conductor;18-24 though the material is otherwise known for a while.25-28 The wide bandgap of the host ZnS is blended proportionately with the p-type CuS, resulting in a good interplay between transparency and electrical conductivity. Majority of the current available literature is apparently limited exploring deposition techniques; be it vacuum or popular solution phase synthesis. The optoelectronic properties of this material are found varied and vastly dependent on deposition methods20-23 and sometimes on post deposition treatments.18 However, it is always scientifically motivating to find a distinct correlation between the optical properties, charge transport mechanism and chemical nature of the constituent elements. Solution route deposition, being considered as facile and relatively cost effective technique with a provision for large scale deposition, is well explored to deposit thin films of Cu:ZnS.29,30,26 Co-precipitation or chemical bath deposition (CBD) is a commonly preferred process for incorporation of Cu concentration in ZnS host. Due to the limited solubility of CuS in ZnS, 23,31,6 homogeneously doped Cu:ZnS system is apparently unrealistic and most often results in phase segregated materials. However the relative stoichiometry can be further improved, though slightly, with the co-dopants like halogens.31,25 In recent times, Ortiz-Ramos et al.19 reported Cu incorporated ZnS by CBD resulting in ca. 70% transmittivity but high enough (ca.


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105 ohm-cm) resistivity that eradicates its applicability as transparent conductor. Following the similar method with modified solution composition, later, Xu et al.22 demonstrated near degenerate carrier concentration without compromising much in the optical properties. Here carrier concentration or conductivity is proportionate to the volume fraction of phase segregated covellite CuS. In comparison to electroless deposition, electrodeposited Cu:ZnS exhibits moderate (ca. 70%) transmission properties but at a cost of high resistance.29 Among many vacuum based techniques pulsed laser deposition (PLD) shows promising results in Cu incorporated ZnS thin films with reasonable tradeoff between conductivity and transmittivity that found dependent on the process parameters.20,21 Woods-Robinson et al.21 recently demonstrated Cu-alloyed ZnS thin films deposited at room temperature by PLD with ca. 50% transmission in visible region with conductivity of ca. 42 S.cm-1 as optimum TC performance. Among the other PVD techniques, radio-frequency sputtering is also explored to deposit the same material at 200oC, however resulting a relatively lower transmittivity due to high Cu concentration.23 As a general remark, the thermal budget associated with physical vapor deposition (PVD) processes, in comparison to solution processed ones, restrains its applicability where organic substrates or active materials are involved. In recent years, Atomic Layer deposition (ALD) has emerged to be one of the versatile most deposition techniques for thin film growth. The surface limited deposition process in ALD ensures conformality and pinhole free film formation, virtually on any substrate including high aspect ratio structures.32,33 A notable advantage of ALD is the formation of thin films with very low surface roughness which is frequently the criteria for many organic electronic devices. While the development of binary materials using ALD is largely explored to its potential,33 there are only a very few reports so far that demonstrates the development of doped materials.34-38 Here, a


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multilayer stack of individual materials with varied thicknesses are deposited that allow interdiffusion of constituents under favorable conditions.34,37,38 Thus considering the advantages like uniformity, control over composition, thickness and scalability we choose to explore ALD for development of Cu(I) incorporated ZnS as transparent conductor. In this report, we aim to achieve high performance p-type TCs through its rational designing, based on the fundamental understanding on the compositional dependent evolution of the underlying charge transport mechanisms and optical properties. Here we demonstrate degenerate p-type conductivity in an optically transparent Cu:ZnS thin films grown by ALD. A nanolaminate structure comprising of individual layers of constituent metal sulfides is deposited. Inter diffusion of Cu+ ions in II-VI materials enables uniform elemental profile without any postdeposition treatment. Under optimized condition, films with thickness of ca. 40 nm exhibits >90% transmission throughout the visible range with free hole carrier concentration of ca. 6.5x1021 cm-3 as estimated from the Hall effect measurements. We investigate the extent of ptype conductivity in these materials where a composition dependent insulator to metal transition (IMT) is achieved. A correlation between the local chemical nature, as determined by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) measurements, of the constituent elements and the charge transport mechanism is established that leads to an understanding behind the role of the Cu+/0 in this composite material. EXPERIMENTAL SECITON A custom-built hot wall viscous flow ALD reactor is used to deposit Cu:ZnS thin films at 150°C. The details of our customized ALD deposition set-up along with in-situ quartz crystal microbalance (QCM) assembly have been described elsewhere.39,40 Considering the Cu diffusion in most metal chalcogenides, the nano-laminate type growth of alternate ZnS and CuS layers is


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practiced to achieve efficient uniform incorporation in ZnS. Diethylzinc (DEZ, Sigma Aldrich Inc.) and Copper(II) hexafluoroacetylacetonate (Cu(hfac)2, Gelest Inc.) serve as the metal precursors for ZnS and CuS layers respectively, while hydrogen sulfide (H2S gas, 99.99% purity, Asia Advanced Gas, Hong Kong) is used as the chalcogen source. DEZ and H2S are stored at room temperature and dosed directly to the deposition chamber. To enable sufficient vapor pressure, Cu(hfac)2 is heated to ca. 95°C inside a vacuum sealed stainless steel container while an overhead N2 is used as the carrier gas. Overall pulsing sequence for both the ZnS and CuS ALD reactions is denoted as (n x t1 - t2 - m x t3 - t4), where ‘n’ and ‘m’ represent the number of pulses of precursors in first and second half cycles respectively. Times ‘t1’ and ‘t3’ are the dosing times of precursors while ‘t2’ and ‘t4’ are the N2 purge times in the consecutive first and second half cycles. The growth rates and the film thicknesses are monitored by in-situ QCM (Inficon) measurements. It provides information about the changes in the frequency upon mass deposit on the Au coated AT cut quartz crystal having resonance frequency of 6 MHz. The change in the frequency is converted to the mass deposited using the Saurbrey equation. X-Ray diffraction either with grazing incidence (GIXRD) or under Bragg’s Brentano configuration are recorded with Rigaku Smartlab X-ray Diffractometer, equipped with Cu-Kɑ source with a wavelength of 1.5418 Å. High resolution transmission electron microscopy (HRTEM) imaging and selected area electron diffraction (SAED) patterns are acquired using Tecnai G2, F30 model from FEI using 300 kV beam energy. X-ray photoelectron spectroscopy (XPS) is used to understand the chemical nature of the constituent elements that are performed with AXIS Supra by Kratos Analytical, UK (SHIMADZU group). The samples are analysed using Al 600 W (λ = 1486.6 eV) X-ray source.


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XPS survey and high resolution scans are acquired for both the as-deposited sample and after 30 sec of 2 keV Ar+ sputtering to analyse surface contamination if any. Ultraviolet photoelectron spectroscopy (UPS) measurements are also executed with the same instrument using He-I (λ = 21.2 eV) as source to study the valence band onset (VBS spectra) of these films. Electronic structure of the material, valence state of the ions and the local structural environment is studied using synchrotron based X-ray absorption spectroscopy (XAS). X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements are carried out at Zn and Cu K-edges at the Energy Scanning EXAFS beamline (BL-9) at the Indus-2 Synchrotron source (2.5 GeV, 250 mA) at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.41,42 The EXAFS measurement at the Zn K-edge (9659 eV) and Cu K-edge (8979 eV) are performed in the fluorescence mode by placing the sample at an angle 45o to the direction of the incident beam. An ionization chamber filled with predetermined gas mixture is used to measure the incident flux (I0) while a Si drift detector placed at 90° to the incident beam has been employed to measure the fluorescence signal (If) from the sample. The X-ray absorption co-efficient of the sample is determined by µ = [If/I0] that is obtained as a function of energy by scanning the monochromator over a specific energy range. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements are obtained using PHI TRIFT V nanoTOFTM (ΦULVAC-Physical Electronics, MN, USA). Cu:ZnS films obtained on glass substrate are used to study dopant and contamination profile throughout the depth of the sample. Positive mode of SIMS is used to maintain high sensitivity factors for all the elements including Cu, Zn, S, O, C and Si. The depth profile analysis is performed using 3 keV Cs+ (600 µm x 600 µm raster size) and 30 kV Ga+ (40 µm x 40 µm raster size) as the sputtering


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and analysis gun respectively. Due to the comparatively low thickness of the films, a low sputtering speed of 2 sec with 6 frames per cycle are used for data acquisition and analysis. Total transmission (% T) and total reflection (% R) for all the Cu:ZnS films obtained on quartz substrates, are acquired using a Lambda 950 UV-NIR visible spectrometer (Perkin-Elmer) equipped with an integrating sphere assembly. The reflection-corrected transmission data are used to compare the optical transparency of different samples. Electrical measurements are performed in four-point probe van der Pauw geometry using Lakeshore 8404 AC/DC Hall Measurement System equipped with a helium atmosphere-based closed cycle refrigerator. AC Hall measurement technique is used for reliable extraction of the low Hall voltage signal. An AC magnetic field of frequency 100 mHz with amplitude of 1.21 T is used and the desired AC Hall voltage is measured using a lock-in amplifier.

RESULTS AND DISCUSSION Copper incorporated ZnS (Cu:ZnS) films are deposited by stacking individual ALD grown layers of ZnS and CuS without any post-deposition treatment. One super-cycle consists of n numbers of ZnS layers and one CuS layer. The concentration of Cu in the ZnS host is varied by changing the number of ZnS layers (i.e. “n”) in the constituent super-cycle while keeping the single CuS layer fixed. Henceforth, “n:1” (where n=100, 90, 80, 70, 60, 50) is the nomenclature used to signify the varying composition of the material. Super-cycles are then repeated to obtain desired film thickness. To retain consistency, we choose to maintain the thickness in the range of 40-45 nm for all the physical property measurements. Despite such multilayer deposition scheme, uniform distribution of the constituent elements is achieved due to the inter diffusion of


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Cu+ in ZnS as determined by the secondary ion mass spectrometry (ToF-SIMS) as described in the supplementary information (see Figure S1). ALD grown ZnS and CuS layers are deposited using diethylzinc (DEZ) and copper (II) hexafluroacetyleacetonate (Cu(hfac)2) as metal sources and hydrogen sulfide (H2S) gas as mentioned in the experimental section. The ALD of ZnS (at 150oC), involving the overall chemical reaction (C2H5)2Zn + H2S ZnS + 2 C2H6 ↑ is described in the supplementary information (Figure S2). Based on the already reported (Metal)(hfac)x chemistries,43 the binary reaction for CuS ALD can be proposed as follows: Cu(hfac)2 + H2S CuS + 2 H(hfac) ↑ The above reaction can be splitted into two half-cycle reactions as follows: (A) Cu-SH* + Cu(hfac)2 Cu-S-Cu-(hfac)* + H-(hfac) ↑ (B) Cu-(hfac)* + H2S Cu-SH* + H-(hfac) ↑ The ‘*’ asterisks represents the respective surface species. The repetition of ABAB… sequence leads to growth of CuS.


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Figure 1. (a) Nucleation and linear growth characteristics of CuS ALD on ZnS surface at 150°C alongwith the mass changes corresponding to first CuS monolayer in the inset and (b) arbitrarily chosen 2 cycles from linear regime of CuS with corresponding reactor pressures. Figure 1(a) illustrates mass gain versus deposition time during the first 75 cycles of CuS ALD at 150°C on ZnS surface with a pulsing sequence of (1x1-15-1x1-15). A slender non-linearity during the initial deposition cycles is observed which can be attributed to the changes in the surface properties, both chemical and physical. Beyond this regime which often extends to first 20-30 ALD cycles, a linear growth characteristic can be observed. Arbitrarily chosen two representative cycles from linear growth regime are presented in Figure 1(b). A positive mass gain in the first half cycle followed by a mass loss in the H2S half cycle is measured during a single CuS ALD cycle. The positive mass gain of ∆m1 = 62-63 ng/cm2 during the first half cycle can be attributed to surface adsorption of Cu(hfac)* species, as anticipated from equation (A). Upon H2S exposure, a distinct mass loss of ∆m2 = 30-31 ng/cm2 is observed. The resultant mass loss during the second half cycle can be a contributing and competitive effect of the addition of SH ligands and removal of H-(hfac) as expected from equation (B). A growth rate of ca. 0.4


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Å/cycle is hence estimated from the net mass change (∆m = ∆m1-∆m2 = 30-31 ng/cm2) considering the density of the material as 4.76 gm/cm3.

Figure 2. Self-limiting growth chracteristics for (a) Cu(hfac)2 and (b) H2S in the respective half cycles during the CuS ALD at 150°C alongwith the representative mass changes of saturated ALD cycle meaured in-situ by QCM in the inset. Here, ‘n’ is the number of variable precursor exposures. Self terminated growth mechanism, a characteristic feature of ALD is then studied further with in-situ QCM as shown in Figure 2(a) and 2(b) with varying precursor exposures. The surface saturated growth is indicative that every constituent half cycle results in a complete surface coverage, however limited by steric hindrance. It is evident that a pulsing sequence of (2x1-151x1-15) is sufficient to form a complete monolayer of CuS with the saturated mass gain of 36-37 ng/cm2 corresponding to the growth rate of ca. 0.5 Å/cycle at 150°C.


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Figure 3. Mass changes per cycle measured in-situ by QCM versus deposition temperature for CuS ALD. The formation of a self-assembled monolayer in each half cycle follows the Langmuir isotherm. Thus keeping the reactant partial pressures during the growth as constant (2x1-15-1x115), we extend the study to understand the temperature dependency. Figure 3 depicts mass changes per cycle versus deposition temperatures for CuS ALD within the temperature range of 130-200°C. The lower temperature is chosen as 130oC due to two limiting factors; (a) Cu(hfac)2 is a solid source that is heated at 95oC to obtain sufficient vapor pressure and (b) the vapor is dosed to the reactor with differentially heated flow path to avoid condensation. The mass change per cycle of ca. 36 ng/cm2 is found constant over the temperature range of 130-150°C. Beyond this temperature, we see a substantial drop in the growth rate reaching a value of ca. 20 ng/cm2 at ca. 180oC. Thus 130-150°C can be termed as the ALD temperature window while the decrease in growth rate beyond 150oC is attributed to the increased rate of desorption. However interestingly, the films deposited beyond 250oC do not satisfy the characteristic selflimiting behavior. At this temperature, the growth rate per cycle is found increasing with


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precursor dosages, as shown in the Figure S3(a) in supplementary information. Such feature signifies CVD type growth mechanism aided by the precursor decomposition. An indicative two cycles with a pulsing sequence of (3x1-15-1x1-15) is shown in Figure S3(b) depicting almost equivalent mass changes with consecutive Cu(hfac)2 exposures. It is important to mention here that the material thus formed at 250°C is found Cu2S and not CuS as determined by the XRD measurements (Figure S4).

Figure 4. (a) Mass changes recorded in-situ by QCM during the successive ZnS, CuS and ZnS ALD growth from a Cu:ZnS super-cycle at 150oC with (inset) a zoom-in section depicting the transition between ZnS last cycle to solitary CuS layer to first ZnS cycle and (b) ZnS mass changes per cycle with respect to ALD cycles during the ZnS/CuS/ZnS transition in a supercycle as indicated in (a). The Cu:ZnS nanolaminate deposition is hence achieved at 150oC using the saturating dosages for both the constituent metal sulfides. Figure 4(a) shows the QCM study of an arbitrarily chosen section of a super-cycle depicting the mass gain during Cu:ZnS growth with single pulses of the respective precursors. Here we would like to drag the attention to couple of noticeable observations, which are as follows,


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(a) A slender non-linearity in the mass changes per cycle during the ZnS growth after the solitary cycle of CuS is observed as shown in the Figure 4(b). Beyond the first 10-15 cycles where the growth rate is lower than the steady-state growth rate, it eventually converges to 67-70 ng/cm2, similar to the steady state mass gain in the linear growth regime. (b) The mass gain in the single CuS cycle (inset of Figure 4(a)) is also found substantially higher (ca. 115 ng/cm2) in comparison to the saturated mass gain (36-37 ng/cm2, see Figure 2). However, this is similar to the one that is observed during the very first cycle of the CuS growth on ZnS terminated surface as shown in the inset of the Figure 1(a). This is indeed embracing the fact that the ALD growth rate is inherently sensitive to the substrate’s physiochemical properties. For all the physical characterizations, Cu:ZnS films are grown with saturating pulsing sequence of (1x1-15-3x1-15) and (1x1-15-1x1-15) for ZnS and CuS respectively at 150oC. To be noted here that a single pulse of Cu(hfac)2 is used instead of two dosages that are required for saturation (see Figure 2(a)). However, the vapor pressure of Cu(hfac)2 that is accumulated during the two consecutive super-cycles is found sufficient enough to obtain self-saturation.


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Figure 5. Normalized XRD patterns of as-deposited Cu:ZnS thin films (n:1, n=100, 90, 80, 70, 60, 50) along with ALD grown pristine ZnS for reference, deposited at 150oC on glass substrates. Here, peaks indicated with ‘o’ denotes ZnS: JCPDS file no: 00-036-1450, ‘*’ denotes cubic metallic Cu: JCPDS file no: 00-004-0836 and ‘#’ denotes CuS: JCPDS file no: 01-078-2122. Normalized grazing angle X-ray diffraction (GIXRD) patterns of as-deposited Cu:ZnS thin films with varied Cu incorporation are shown in Figure 5 (non-normalized scans are shown in Figure S5). Pristine ZnS films are wurtzite (JCPDS file no. 00-036-1450) in structure with preferred orientation along the [002] direction. However apart from the (002) peak at 28.57°, three other peaks are also observed at ca. 47.6°, ca. 51.8°, and ca. 56.3° corresponding to (120), (013) and (111) planes of wurtzite ZnS. At low (100:1 and 90:1) Cu incorporation in ZnS, no significant structural change is observed. Here the preferred [002] orientation of the pristine ZnS is retained without any notable shift or any consistent change in FWHM (full width at half maxima), indicating that the coherence length also remains unaffected. It is rational to believe that if Cu1+ is substituted at the Zn2+ site, any structural disorder may not be distinctly observable as the ionic radii for Cu1+ and that of Zn2+ with IV co-ordination are both ca. 0.74Å.23,44


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However with further increase in CuS incorporation (from 80:1 onwards) a small shoulder peak alongside the (120) peak of host ZnS starts emerging. This peak prominently increases with Cu incorporation (70:1, 60:1 and 50:1) indicating the coexistence of orthorhombic CuS (JCPDS file number: 01-078-2122) and host ZnS. The appearance of CuS peaks can possibly be attributed to the phase segregation at the grain boundaries due to the limited solubility of CuS in ZnS.6,23,31 However, with utter surprise, a clear signature of metallic Cu (cubic, JCPDS number: 00-0040836) is also detected when the Cu incorporation in ZnS exceeds a certain limit (60:1 onwards). The formation of the metallic Cu is probably similar to the one as reported earlier45.

Figure 6. Reflection corrected total optical transmittance spectra of as-deposited Cu:ZnS thin films (n:1, n=100, 90, 80, 70, 60, 50) along with ALD grown pristine ZnS. All films are deposited at 150oC on quartz substrates. Figure 6 shows the reflection-corrected transmittance spectra of Cu:ZnS films deposited on quartz substrates. The reflection correction applied here [T=Tmeasured/(1-Rmeasured)] is mainly considering the losses arising from the front surface, which is true only for ultra thin films.46 The raw transmission and reflection spectra are also shown in Figure S6(a) and S6(b) respectively. Pristine ZnS, having band gap of ca. 3.62 eV, shows an average optical transparency >90%


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throughout the visible range of the spectra. Low incorporation of CuS in ZnS host (100:1, 90:1 and 80:1) does not significantly change the transmission property and the band gap of the material. However, certainly a non-negligible monotonous shift at the onset of the fundamental absorption edge can be clearly noticed. In this set of samples, the calculated Urbach energy tail also get extended from ca. 200 meV (for ZnS) to ca. 600 meV for Cu:ZnS (80:1). Subsequently, from the ultraviolet photoelectron spectroscopy (UPS) measurements a substantial change in the valance band spectra (see Figure S7) is also evident. Thus we may conclude that the changes at the onset of the fundamental absorption edge are essentially due to the reformation of the valance band probably due to the structural or point defects in the material aided by the minor incorporation of CuS in ZnS host. However, further increase in the CuS incorporation results significant reduction in the optical transmission properties in the visible spectral range. Such changes in the optical properties can be well attributed to the relative increase in the volume fraction of CuS in the composite and scattering from the metallic Cu. These observations are well correlated with the XRD analysis. However, in the interest of current study, aiming for transparent conductor, we preferentially restrict Cu incorporation till 80:1 that satisfy maximum Cu incorporation without much compromise in the optical transparency.


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Figure 7. (a) XPS survey scans for the optimized 80:1 Cu:ZnS thin film acquired as-deposited as well as after Ar+ sputtering and the de-convoluted high resolution spectra for (b) Zn 2p, (c) S 2p and (d) Cu 2p core levels. Representative survey scans for Cu:ZnS samples (80:1) before and after Ar+ sputtering are shown in Figure 7(a). The traces of elemental C are found below the detection limit of the system after sputtering indicating the absence of any unreacted organometallic species in the sample. We refrained calculating the elemental composition from the XPS even after sputtering to avoid any misleading conclusions due to preferential etching rates. The Zn 2p spectrum is shown in Figure


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7(b). The two distinct peaks at ca. 1021.7 eV and ca. 1044.7 eV are observed that correspond to Zn 2p3/2 and Zn 2p1/2 peaks respectively.47 Approximately 23 eV splitting between the two peaks due to the spin-orbit coupling is consistent with the literature Zn 2p spectra. Analogous to XRD, Zn 2p XPS also supports that incorporation of Cu within ZnS matrix does not alter the bonding environment adversely (see Figure S8(a) for ZnS and 80:1 comparison). The S 2p spectrum of the same sample is shown in Figure 7(c). The de-convolution of spectra reveals contribution from two distinct peaks located at binding energies ca. 161.05 eV and ca. 162.2 eV. The peak position of S 2p depends on the degree of ionicity of the Mn+-S2- and generally falls within the binding energy range of 160-163 eV with higher to lower degree of ionicities48,49. In Cu:ZnS, the electronegativity difference between Zn2+ and S2- is greater than that between Cu+ and S2-, implying higher ionicity of the Zn2+-S2- bond. Hence, the S 2p peak at binding energy ca. 161.1 eV can be assigned to that contributed from the ZnS. To be noted here the fact that the S 2p peak from the ALD grown pristine ZnS is broad in nature that can be de-convoluted to two peaks at ca. 161.02 eV and ca. 161.9 eV corresponding to S 2p3/2 and 2p1/2 respectively (see Figure S8(b)). While for pure ALD grown CuS, the peak positions corresponding to the S 2p3/2 and 2p1/2 peaks are found at ca. 161.7 eV and ca. 162.9 eV respectively [see Figure S8(c)]. A noticeable difference in the S 2p peaks in these pristine samples is highly distinguished splitting (ca. 1.2 eV) between the 2p3/2 and 2p1/2 peaks in CuS that is missing in ZnS, independent of the scan speed and the excitation beam energy. A Cu 2p spectrum with CuS like splitting (ca. 1.2 eV) in the 2p3/2 and 2p1/2 peaks is also seen in the Cu:ZnS samples as shown in Figure 7(c). We believe that the S 2p1/2 peak at binding energy ca. 162.2 eV (with a shift of 0.3 eV to the higher energy with respect to S 2p1/2 peak in ZnS) is predominantly contributed by the sulfur from Cu-S and the ZnS bonds. The binding energies for Cu 2p XPS spectrum at ca. 932.15 eV and ca. 952.05 eV


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correspond to the Cu 2p3/2 and Cu 2p1/2 peaks respectively as shown in Figure 7(d) (see Figure S8(d) for CuS and 80:1 comparison). This is indicative of the monovalent state of copper (Cu+) in CuS configuration48-53. Even though, Cu2S chalcocite also exhibits monovalent state of copper, the Cu 2p3/2 peak in chalcocite Cu2S is ca. 0.6 eV higher than that observed in covellite CuS, as the Cu-S bond length in chalcocite is larger than that found in covellite.48,49 The spinorbit coupling of ca. 19.9 eV and the absence of the “shake-up” peaks again confirms the presence of monovalent Cu+ in CuS structure in these samples. For reference, the evolution of the chemical state of Cu 2p3/2 in the Cu:ZnS with varied composition (n:1, n=100, 90, 80, 70, 60, 50) along with the pristine CuS is indicated and discussed in supplementary information (see Figure S9).

Figure 8. Normalized (a) Zn K-edge and (b) Cu K-edge XANES spectra collected from ALD grown pristine metal sulfides (ZnS and CuS resspectively) along with as-deposited Cu:ZnS thin films (n:1, n=100, 90, 80, 70, 60, 50). The reference metal spectra Zn0 and Cu0 are also recorded. All spectra are normalized by subtracting the pre-edge and post-edge regions for getting the edge jump to unity.


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Since the X-ray absorption features near the K-edge of the respective material is sensitive to the ionization state, chemical environment, local and structural disorders, we analyze the individual XANES spectra for Zn and Cu with varied composition (n:1, n=100, 90, 80, 70, 60, 50) as shown in the Figure 8(a) and 8(b) respectively. Spectra obtained from the individual metals (Zn0 and Cu0) and metal sulfides are also shown for reference and energy calibration. The Zn K-edge XANES spectra for lower Cu incorporated (100:1, 90:1, 80:1 and 70:1) ZnS films are found similar to that of the pristine ALD grown ZnS thin film. The absorption onset is studied precisely with 1st order derivative of normalized XANES spectra as shown in Figure S10(a) where metallic Zn K-edge is observed at ca. 9658 eV. The distinctive shift in the absorption edge in these materials to higher energy than that for Zn0 is indicative that the oxidized state of all the samples corresponds to standard Zn2+. However, the fundamental absorption edge is slightly shifted further and the features just after the edge also get blurred upon higher (60:1, 50:1) Cu incorporation. From the crystallographic and the optical spectroscopic data, we intend to infer that such changes are probably due to the structural distortion and local disorder in ZnS lattice aided by the increased Cu incorporation. The Cu K-edge XANES spectra also show some significant changes in the near edge absorption features as shown in Figure 8(b). Since we do not see any narrow peak at or near the edge, we unambiguously infer that the transition is from 1s to the continuum beyond the 4p states for all the Cu:ZnS samples. The 1st order derivative of normalized XANES spectra at Cu K-edge as shown in Figure S10(b) indicates the right shift in the absorption edge for all the samples in general as compared to the absorption edge of Cu0 (ca. 8979 eV) indicating possible Cu1+ charge state in all the samples. It has been observed from the above Figure that for the samples with relatively lower Cu concentration (80:1, 70:1 and 60:1), the XANES features


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resemble that of ZnS manifesting efficient Cu substitution in ZnS lattice. However, at higher (50:1) Cu concentration, the spectral features are similar to that of the mixture of metallic Cu0 and Cu1+ from CuS phase. This observation is similar to that of the ones that are already concluded from the XRD studies. On the other extreme for lower Cu incorporated ZnS (100:1) material though, the fundamental absorption edge is found broader and a conclusive argument is rather difficult to put forward.

Figure 9. Fourier transformed experimental EXAFS data (FT-EXAFS) (χ(R) vs R) fitted with the theoretically generated plot in R space at (a) Zn K-edge, (b) Cu K-edge (Considering Cu at Zn site in ZnS and Cu clustering) and (c) Cu K-edge (Considering only CuS). The solid line represents the theoretically fitted data while the scattered plot denotes the experimentally obtained data. The χ(R) vs R plots at the Zn and Cu K-edge for all the ALD grown films are shown in Figure 9. The experimentally recorded normalized EXAFS spectra (µ(E) versus E) and the details of the various fitting parameters at Zn K-edge and Cu K-edge for all the samples are discussed in


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supplementary information (S11). The Fourier transformed EXAFS spectra or χ(R) vs R plot alongwith the theoretically generated fitting at the Zn K-edge (Figure 9(a)) shows an intense peak appearing within the R range 1.4-2.4 Å which can be mainly attributed to the scattering by S atoms in the 1st co-ordination shell where the Zn-S bond length is found to be smaller than the theoretical bond length of ca. 2.35 Å for bulk ZnS.54 However, absence of any significant feature above 2.5 Å in the χ(R) vs R plots indicates substantial structural disorder in as-grown samples. It should be noted here that the best quality fitting for the Cu:ZnS samples with varied Cu incorporation is achieved by considering an extra scattering path of Zn-Cu (CN=6) in-between 1st and 2nd shell in addition to Zn-S and Zn-Zn paths in the model. The details of various parameters from the fitting are tabulated in Table S1 in supplementary information. The increase in the co-ordination number (CN) of this path is observed with the Cu incorporation which suggests the formation of metallic Cu interstitials within the lattice at higher doping concentrations. In addition to CN, increase in Debye-Waller factor σd2 of Zn-Zn and Zn-Cu paths manifests that though the1st co-ordination shell remains almost unaffected, the metallic paths are affected in Cu incorporated samples due to the larger lattice strains or possible redistribution of the Zn atoms. This primarily suggests presence of interstitial Cu along with Cu clustering with the increase in Cu incorporation in Cu:ZnS samples. Figure 9(b) and 9(c) represent the χ(R) vs R plots at Cu K edge for all the Cu:ZnS samples with ALD grown pristine CuS reference sample. Similar to Zn K-edge, a model assuming Cu at Zn site along with Cu clustering (f.c.c. phase of Cu) in wurtzite ZnS is considered for fitting of EXAFS oscillations at Cu K-edge (Figure 9(b)). The clear distinction within all the samples is seen where EXAFS oscillations for a set of samples (100:1, 90:1, 80:1, 70:1 and 60:1) represents a close resemblance with their respective Zn K-edge spectra implying efficient Cu doping at Zn


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sites within these samples as is estimated earlier from Zn K-edge analysis. The CN for Cu-Cu and Cu-Zn paths of the second shell are found to be smaller indicating the enhanced structural disorder due to the Cu doping in all the Cu:ZnS samples. However, for the Cu-rich samples 50:1 the χ(R) vs R plot at Cu K-edge are significantly different from that of their respective χ(R) vs R plots at Zn K-edge. For fitting of FT-EXAFS spectrum of this sample two phases have been considered viz., Cu at Zn site in ZnS matrix as described above and f.c.c Cu phase to take care of Cu clustering in these samples. The fittings have been carried out taking Cu-S and Cu-Cu/Zn paths from the 1st phase and Cu-Cu path from the 2nd phase. A phase fraction has also been considered in the fitting algorithm, whose value has also been obtained from the fit and best fit results have been shown in Table S2 in supplementary information. However, higher Rfactor values of this model indicate a possibility of additional scattering path at the Cu K-edge. Hence a new model considering covellite CuS structure for fitting these EXAFS oscillations is tried based on the earlier work by Car et al.55 alongwith metallic Cu0 path as is observed from XRD and Zn K-edge analysis. The χ(R) vs R plot fittings using this model are represented in Figure 9(c) and the parameters are tabulated in Table S3 in supplementary information. An intense peak within the range of 1.5-2.3 Ǻ is a convolution of the scattering waves from the nearby 3S atoms of covellite CuS and 6Cu atoms from the f.c.c. Cu0. Additionally, the above analysis also suggests that S vacancies are present at one of the neighboring Cu sites as is also pointed out by Car et al.55 earlier in case of Cu doped ZnS nanocrystals synthesized by an aqueous precipitation. Hence, as a combination of both these models at Cu K-edge and Zn K-edge and XANES analysis indicate the co-existence of Cu substitutional doping in ZnS lattice along with CuS and Cu clustering in this composite material as is also observed from XRD measurements. The results of the EXAFS study primarily suggests that


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unlike homogeneously doped system in which dopant atoms introduce disorder in atomic scale due to their statistical distribution in the host, Cu:ZnS samples can be considered as compositionally inhomogeneous disordered system.

Figure 10. (a) Room temperature Hall measurements of as-deposited Cu:ZnS samples (n:1, n=100, 90, 80, 70, 60, 50) revealing the dependence of free carrier density, electrical resistivity and Hall mobility on Cu incorporation in ZnS matrix and (b) Temperature dependent electrical resistivity of as-deposited Cu:ZnS samples (n:1, n=100, 90, 80, 70, 60, 50). Different regimes of carrier transport are also marked as insulating (I), critical (II) and metallic (III). Electrical properties of as-deposited Cu:ZnS films are studied with a commercially available Hall effect measurement system under standard van-der Pauw configuration at room temperature. Figure 10(a) shows the change in resistivity, free carrier density and Hall mobility of Cu:ZnS films (ca. thickness 40-45 nm) with varying Cu concentration. All the conducting films exhibit reproducible p-type nature as confirmed by Hall and Seebeck effect measurements (see Figure S12). It is important to clarify here that due to the instrumental limitation, electrical properties of (apparently, highly resistive) pristine ZnS cannot be measured. Nevertheless, a significant decrease in the electrical resistivity is found with minor CuS incorporation (100:1) in the ZnS host. The measured resistivity is found to be ca. 5 Ω-cm as shown in the Figure 10(a)


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alongside the corresponding carrier concentration (5x1018 cm-3) and Hall mobility (0.25 cm2V-1s1

). Further incorporation of CuS results decrease in the electrical resistivity with the minimum

resistivity reaching a value of ca. 3.7x10-4 Ω-cm for the sample 60:1. Such reduction in electrical resistivity can be attributed to the increase in free carrier (hole) concentration from ca. 5x1018 cm-3 to ca. 1022 cm-3 from sample 100:1 to 60:1. This clearly justifies that incorporation of CuS aids as hole-donor in the insulating ZnS host.22 The in-depth understanding of the local chemical environment of the constituents from EXAFS and XANES measurements clearly indicates that CuS incorporation in ZnS host results in a combination of (a) substitutional doping of Cu+ at the Zn2+ sites of the wurtzite ZnS lattice, (b) phase segregated covellite CuS and (c) formation of metallic Cu clusters. Such observations unambiguously attribute to the fact that enhanced carrier concentration is a resultant property of the composite material. In the context of transparent conductor application, optimized CuS incorporation in ZnS is found to be for Cu:ZnS sample of 80:1 ratio. The electrical resistivity, Hall mobility and carrier concentration in this optimized material are ca. 1.3x10-3 Ω-cm, 0.7 cm2V-1s-1 and 6.5x1021 cm-3 respectively, which is approximately in the degenerate limit. However, it is rather interesting to find that further increase in the CuS incorporation (70:1, 60:1, 50:1) results in slender but nonnegligible escalation of the Hall mobility beyond the degenerate limit. This is indeed in sharp contrast to the other homogeneously doped polycrystalline semiconductor thin films56,57 where the mobility usually increases up to the degenerate level and then levels off, satisfying a rather well known equation µ-1 = µ-1LO + µ-1dis + µ-1imp,


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where µ is the resultant mobility, µLO is the mobility governed by the longitudinal phonon scattering, µdis is the dislocation scattering in a degenerate semiconductor and µimp is the mobility term governed by the impurity scattering.57 We may like to infer the increase in the mobility beyond degenerate limit observed here is due to the reduced grain boundary scattering that often plays a deterministic role in the overall transport particularly in disordered polycrystalline materials. Enhanced inhomogeneous disorder with increased Cu incorporation probably improves the charge percolation that renders the Hall mobility beyond the degenerate limit. Evolution of the composition dependent charge transport properties are further studied with temperature dependent resistivity measurements. Figure 10(b) shows temperature dependent resistivity ρ(T) curves for all the Cu:ZnS films within the temperature range of 10-300 K. There is a characteristic difference observed between the samples with low (100:1, 90:1, 80:1) and high (70:1, 60:1, 50:1) CuS incorporation in ZnS. Materials with relatively lower Cu concentration show ‘semiconductor-like’ resistivity behavior i.e., negative temperature coefficient of resistivity (dρ/dT< 0) in the entire range of the measurement temperature, while metallic resistivity behavior (dρ/dT >0) is observed for higher CuS incorporated materials. The electrical properties of these composite films are fundamentally determined by the variation of the relative volume fraction of the conducting (Cu clusters, covellite CuS and Cu substituted ZnS) and insulating ZnS phases. Thus a compositionally varied insulator to metal transition (IMT) is clearly observed with increase in CuS incorporation. The Cu:ZnS samples are divided in three broad regions namely insulating, critical and metallic regime of insulator to metal transition (IMT) based on their transport mechanisms as shown in Figure 10(b). For low CuS incorporated materials (100:1 and 90:1), the relative resistivity ratio


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[ρr = ρ (10 K)/ρ (300 K)] is found to be in the order of 106 to 105 (respectively). Hence the diverging nature of ρ(T) at T0 and high ρr values justify that these materials are in the insulating regime of the IMT.58,59 The variable range hopping transport equation is used to further investigate the transport mechanism in these samples and can be expressed as ρ(T) = ρ0 exp(T0/T)p

(1)

where the power p is the deterministic factor of the activation process. The resistivity curves for these materials (100:1 and 90:1) are fitted (see Figure S13(a)) well throughout the temperature range for p = ½ indicating it to be a Efros and Shklovskii type variable range hopping (ES-type VRH) transport mechanism observed in polycrystalline films.60-64. In this regime charge transport is mostly governed by the intra-grain conductance with negligible contribution from the phase-segregated CuS. This is well supported by the UV-VIS measurements where little change in the transmission property is noticed and no detectable (XRD analysis, Figure 5) phase segregated CuS is found. Again the contribution from the grainboundary tunneling cannot be neglected that contributes significantly in the temperature dependent resistivity. It is important to note that the resistivity curves for these samples are well explained using equation 1 without any crossover to Arrhenius type activated behavior throughout the temperature range of 10-300 K. This is in contrast to our recently reported p-type Bi2S3 films in which we have clearly observed a crossover from Mott variable range hopping to band-like transport with increasing temperature.39 In contrast to the above, resistivity curves for the samples with higher Cu concentration (70:1, 60:1, 50:1) exhibit typical metallic characteristic (i.e., dρ/dT >0) that broadly implies that samples are well above the critical percolation threshold or are deep inside the metallic regime of the IMT. The resistivity curves for these samples are found to be linear with temperature (T) (see


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Figure S13(b)) that could be well attributed to the phonon scattering, mostly observed in metallic samples.65 In the context of the transparent conducting electrode, it is clearly evident that the films with higher Cu concentration do not satisfy due to the optical transmission constraint though having low sheet resistance. On the other hand, eventhough the optical properties satisfy the criteria for TCs with lower Cu incorporated materials, high enough resistivity restricts their application as transparent conducting electrodes. We find that Cu:ZnS films with 80:1 ratio show optimal transmittance (> 90%, after reflection correction) with a sheet resistance as low as ca. 330 Ω/□ and it is in the critical regime differentiating the metal and the insulating side of the IMT. This specific combination reveals highly contrasting though coexisting metallic and insulating electrical transport properties that justifies why it is in the critical regime, as discussed below. a. Metallic behavior: Primarily it shows a considerably low value of ρr (ca. 1.2-1.3) with finite resistivity while extrapolating at T0. Furthermore the reduced activation energy [ w = −d (ln ρ ) / d (ln T )] plot against the temperature (see Figure S13(c)) also shows a positive

slope. These properties unambiguously justify that the material is in the metallic regime of the IMT.58,66 b. Semiconducting nature: In the range of the measurement temperature this material exhibits semiconductor-like behavior with dρ/dT< 0. Such characteristic semiconductor-like behavior in a material with degenerate carrier concentration can be attributed to the Coulombic interaction between the carriers.67,68 The electrical conductivity equation with the correction factors for classical Drude conductivity rising from the Coulombic interaction between the carriers can be expressed as,


29


1/2   k BT  1 α  k BT   ln  σ (T ) = σ 0 1 +    + 2  2π gT d  gT Ec  12π gT  gT δ  

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(2)

The second term in the equation (2) primarily denotes the correction for Coulombic interactions in inhomogeneously disordered system64 while the third term is the Alshuler-Aronov correction from homogeneously disordered materials.64,69 Our experimentally obtained resistivity data perfectly resembles the nature described by the above equation as can be seen in the Figure S13(d). This further justifies the fact that the charge transport characteristics of the optimized Cu:ZnS (80:1, i.e., under optimum transparency and conductivity) can be described as a combination of homogeneously and inhomogeneously disordered systems. CONCLUSION In conclusion, we demonstrate controlled Cu incorporation in ZnS host by laminar growth of CuS and ZnS layers, deposited by ALD. The individual film growth, studied by in-situ QCM, validates distinctive self-saturating nature justifying the ALD mechanism. Inter-diffusion of CuS and ZnS allows uniform compositional distribution throughout the film thickness as determined by SIMS without any post deposition treatment. Our investigations suggest that beyond a certain Cu incorporation in the film, phase segregated CuS get formed along with Cu0 and substituted Cu+1 as detected by X-ray spectroscopy techniques (XRD, XANES and EXAFS). The increased volume fraction of these phase segregated species results decrease in the optical transparency in the visible range due to the absorption in CuS and scattering from metallic Cu clusters. In contrast to the optical transmittance, the conductivity increases with increasing Cu concentration in the materials. A compositionally varied insulator to metal transition (IMT) is observed where the low Cu incorporated materials behave like insulator/semiconductor (dρ/dT < 0) while beyond a particular composition the films behave like metals (dρ/dT < 0). The optimized composition for


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TCs is found at the point of transition between metal and insulating phase. Here the charge transport in this particular composition follows non-Arrhenius type semiconducting behavior but lies on the metallic side of the IMT transition. We believe that the charge transport in these materials is likely to follow inhomogeneously disordered granular system. Under optimized condition, Cu:ZnS shows moderately high transparency (>90%, after reflection correction) and low resistivity value of ca. 1.32x10-3 Ω-cm (carrier concentration as ca. 6.49x1021 cm-3, Hall mobility as ca. 0.72 cm2V-1s-1) that justifies its application as transparent conductor.

ASSOCIATED CONTENT Supporting Information. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) to study the elemental profile across the thickness of Cu:ZnS film, In-situ Quartz crystal microbalance (QCM) studies of ZnS ALD at 150oC, Structural properties of CuxS thin films deposited at 150oC and 250oC as determined by X-ray diffraction (XRD), Non-normalized XRD data of Cu:ZnS samples alongwith pristine ZnS, Optical transmission and reflectance of Cu:ZnS thin films along with pristine ZnS sample, Ultraviolet photoelectron spectroscopy (UPS) analysis, Comparative XPS HR-scans of Zn 2p,Cu 2p and S 2p, First derivative analysis of XANES spectra at Zn and Cu K-edges for all the Cu incorporated samples along with their respective metals and metal sulfides, EXAFS (µ(E) versus E) analysis of Cu:ZnS samples with reference metals and metal sulfides at Zn and Cu K-edges, Confirmation of reproducible p-type conductivity by Seebeck and Hall measurement and Low temperature resistivity analysis to study transport mechanism in Cu:ZnS films.


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AUTHOR INFORMATION Corresponding Author * Prof. Shaibal K. Sarkar, Email: [email protected] ACKNOWLEDGMENT This article is based upon work supported under the US-India Partnership to Advance Clean Energy-Research (PACE-R) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC36-08GO28308 to the National Renewable Energy Laboratory, Golden, Colorado) and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd November 2012. This work is also partially funded by Ministry of New and Renewable Energy, Government of India. Authors thank Bharti Patro for the TEM measurement along with the Sophisticated Analytical Instrument Facility (SAIF) and Central Facility at IIT Bombay for material characterizations. We thank D. Bhattacharyya, S. N. Jha, R. Shah and Raja Ramanna Center for Advance Technology (RRCAT), Indore for their support in synchrotron based measurements.

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Atomic Layer Deposition of Transparent and Conducting p-Type Cu

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