Characterisation of p-type ZnS:Cu transparent conducting films fabricated by high-temperature pulsed laser deposition
Katherine S. Duncan, Joseph D. Taylor, Martin Jonak, Kayleigh O. E. Derricutt, Alexander G. J. Tallon, Christopher E. Wilshaw, James A. Smith, Neil A. Fox
CCharacterisation of p-type ZnS:Cu transparent conducting films fabricated byhigh-temperature pulsed laser deposition
K. S. Duncan, J. D. Taylor,
1, 2
M. Jonak, K. O. E. Derricutt, A. G. J. Tallon, C. E. Wilshaw, J. A. Smith, and N. A. Fox
1, 3, ∗ School of Physics, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL United Kingdom Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
Copper-doped zinc sulphide (ZnS:Cu) thin films were synthesized through pulsed laser ablation in an in-ert background gas on stationary and rotating substrates, and a comprehensive opto-electrical characterisationis presented. The Cu x Zn − x S films demonstrated comparable conductivity and transparency to other leadingp-type transparent conducting materials, with a peak conductivity of 49.0 Scm − and a hole mobility of 1.22cm V − s − for films alloyed with an x = 0.33 copper content. The most conducting films displayed a trans-parency of 71.8 % over the visible range at a thickness of 100 nm, and band gaps were found in the range3.22-3.52 eV, which showed a strong negative correlation with copper content. The effects of sulphur-richrapid thermal annealing on the synthesized compound are reported, with films reliably displaying an increasein conductivity and carrier mobility. Films grown using a stationary substrate possessed large spatial thicknessdistributions and displayed sub-band gap absorption, which is discussed with respect to inhomogeneous coppersubstitution. Films deposited at 450 ○ C were found to be in the zincblende phase before and after annealing,with no occurrence of a phase change to wurtzite structure.
I. INTRODUCTION
Transparent conducting materials (TCMs) are wide bandgap semiconducting materials with high optical transparen-cies and low resistivities. TCMs play a central role withinopto-electronics, and have passive uses as front contacts inLCD displays, as components in LEDs, and as window layersof solar panels [1, 2].The most prevalent TCM, indium tin oxide (ITO), has92.1 % optical transparency, and average conductivities of1400 Scm − , but is increasingly expensive to fabricate dueto the scarcity of indium [3]. Various sustainable alterna-tives to ITO have been developed [4, 5]. The develop-ment of efficient TCMs is also being driven by the needto harness renewable energy sources effectively. Structuresthat could utilise TCMs, such as heterojunctions, metal-insulator-semiconductor (MIS), or semiconductor-insulator-semiconductor (SIS) structures, would enable the fabricationof low-cost, efficient, and transparent photovoltaic devices.Most TCMs, such as ITO, are n-type, thus comparativelyless literature exists on p-type TCMs. The first reported in-stance of a p-type conductivity in a TCM was in delafossiteCuAlO which displayed hole conductivities of up to 1 Scm − and an optical transmission of 70 % [6]. More recent workhas found that among the best performing p-type materials areCuAlS with a transmittance of 80 % in the visible range andconductivity of 63.5 Scm − [7], and BaCu S with a trans-parency of up to 90 % and a conductivity of 17 Scm − [8].Many n-type TCMs significantly outperform existing p-typeTCMs, and so fewer p-type TCMs have been developed forcommercial use. One reason for the higher performance ofn-type TCMs is the higher mobility of electrons compared tothat of holes. The hole effective mass of ZnS, however, is ∗ [email protected] , a value which is relatively low compared to that ofmost metal oxides [9, 10], and as such should not constitutean obstacle to achieving high mobilities.ZnS is a weakly n-type wide band gap (3.6-3.9 eV) II-VI semiconductor, and exists in either zincblende or wurtzitephase. Both phases are of interest due to their UV range bandgaps, ideal for transparent opto-electronic applications. Dop-ing with Cu is one method of fabricating a ZnS-based p-typealloy, whilst preserving optical transparency. Valency con-siderations of Cu + and Zn + suggest that where dopant Cureplaces Zn on lattice sites, it should act as an acceptor. Thealloy is expected to remain n-type for low Cu contents, and tobecome p-type at a critical percentage of 0.5-1.0 % [11], wherethere is enough Cu present to modify the entire band structure.Extensive doping is limited by the low solubility of Cu in ZnSdue to dissimilarity in the crystal structures of ZnS and Cu S.Consequently, Cu x S phases are commonly observed as pre-cipitates on the surface of films [11]. A number of methodshave recently been employed to synthesise ZnS:Cu, includingsputtering [12] and solution-based approaches [13, 14].Diamond et al. recently synthesised ZnS:Cu thin films us-ing pulsed laser deposition (PLD), and reported conductivi-ties of 54.4 Scm − and a transmission of 65 % for films ofapproximately 100 nm thickness [15]. Hole mobility and car-rier density for high-temperature PLD-fabricated ZnS:Cu thinfilms remain unreported. More recently, Woods-Robinson etal. synthesised ZnS:Cu at low temperature (T < ○ C) alsousing PLD, achieving a maximum conductivity of 42 Scm − ,and hole concentrations of 1-2 × cm − in films with a Cucontent of 30 % [16]. Noting the relatively high dopant con-centration, the small free carrier concentrations suggest partialimpurity-related compensation. Our preliminary work foundthat post-deposition annealing in vacuum can render sulphidethin films S-deficient, with increased S vacancies resulting inhigh levels of compensation. Despite the noted problems, itis evident that ZnS:Cu has the potential to be a versatile anduseful p-type TCM for use in opto-electronics. a r X i v : . [ c ond - m a t . m t r l - s c i ] N ov This paper is organised as follows. In Sec. II, the experi-mental details of the fabrication and characterisation are pro-vided. Sec. III presents an optical and morphological charac-terisation of zincblende pulsed-laser-deposited ZnS:Cu beforeexploring the opto-electrical properties of the films and the ef-fects of doping on hole mobility and carrier density. The paperconcludes with a summary in Sec. IV.
II. EXPERIMENTAL DETAILS
ZnS:Cu thin films were grown on fused quartz substrates(10 x 10 mm ) by PLD, using an ArF UV excimer laser(coherent, model COMPexPRO 102 F-Version), with wave-length λ = 193 nm. In-house ceramic targets comprised of99.9 % purity ZnS:Cu S were used, and a deposition chambertemperature of 450 ○ C was maintained throughout the deposi-tions. Prior to film fabrication, the deposition chamber wasevacuated to a pressure below 10 − Torr by a rotary vacuumpump, after which argon gas was introduced at a pressure of30 mTorr. The substrate-target distance was 45 mm. Full de-position parameters are given in Table I. Post-deposition, thesamples underwent rapid thermal annealing (RTA) inside thevacuum chamber. The annealing process lasted 15 minutes,during which the films were raised to a temperature of 550 ○ Cin a background gas of H S, before being rapidly broughtdown to room temperature.The morphology of the samples was investigated by atomicforce microscopy (AFM) and scanning electron microscopy(SEM, HITACHI S2300), and the microstructure was de-termined from X-ray diffraction analysis (XRD, PANalyti-cal X’Pert PRO) using a Cu K α source. The film composi-tion and thicknesses were found with energy-dispersive X-rayanalysis and focused ion beam spectroscopy (EDX and FIB,FEI Helios NanoLab 600 DualBeam). Hall-effect measure-ments were carried out to determine the conductivity, carriermobility, and carrier concentration, using the van der Pauwmethod. The optical properties of the films were obtainedwith ultraviolet-visible spectroscopy (UV-Vis, Shimadzu UV-2600). Parameter ValueArgon pressure/mTorr 30Laser pulse frequency/Hz 10Laser fluence/Jcm − ○ C 450Substrate-target distance/mm 45TABLE I. Experimental deposition parameters for the pulsed-laserdeposition of the Cu-doped ZnS films.
III. RESULTS AND DISCUSSIONA. Structural and morphological characterisation
The θ -2 θ XRD patterns of the ZnS:Cu thin films wererecorded in the range 2 θ = 25 ○ -45 ○ , and a representativeset of data are presented in Fig. 1. The results indicatethat films deposited at 450 ○ C have the polycrystalline struc-ture, while room-temperature depositions lead to amorphousZnS:Cu. Diffraction peaks appeared at the 2 θ value of 29 ○ ,corresponding to reflections from the the (111) lattice plane ofthe zincblende phase of ZnS. The crystal structure of the filmsis in good agreement with (JCPDS 05-566) zincblende ZnSdata, with no trace of any minority wurtzite phase in eitherthe as-deposited films or the annealed samples. No significantshift for the doped ZnS:Cu film is observed in the (111) peakin comparison to the undoped ZnS film, as can be expectedfrom the similar crystal radii of Cu and Zn. FIG. 1. XRD spectra plotted as a function of Cu K α θ diffrac-tion angle. Diffraction standards of the zincblende crystal phase arerepresented by the vertical lines in the lower subplot. Top: XRDspectrum of a post-annealed Cu . Zn . S film displaying the char-acteristic (111) peak of zincblende ZnS. Middle: XRD spectrumof an as-deposited Cu . Zn . S film. Bottom: XRD pattern of athin film deposited at room temperature. Inset: AFM image of aCu . Zn . S film deposited under the rotation setup.
Using the Scherrer equation, the lower bound on crystallitesize was found to vary in the range 22-72 nm, with the sizeappearing to depend heavily on the deposition conditions tosuch an extent that correlations with both Cu content and post-
FIG. 2. SEM images of film surfaces: (a) 5000x magnification of film surface showing Cu S precipitates; (b) 35,000x magnification offilm surface showing surface contaminants and overall uniformity; (c) Cross-sectional film displaying clear columnar growth; (d) Particulatedeposited on a substrate during a deposition with a low-density target; (e) Image of Cu S nanoflowers on the surface of a film post-deposition;(f) Cu S nanoflowers synthesised by Meng et al. for comparison [17].Sample ID (hkl) FWHM (degrees) D (nm) δ × − (nm − ) (cid:15) × − CuZnS 1 (111) β β β β δ and strain (cid:15) of conducting ZnS:Cu films. deposition annealing were not apparent. In addition to thegrain size, the dislocation density and strain of the films wereobtained using the Scherrer method on the dominant peak inorder to compare the data with literature values. These re-sults are presented in Table II and are found to resemble thosein previous studies [18, 19]. The films were not found to besubject to any considerable strain, a result which is furthercorroborated by a lack of film cracking, as seen in Fig. 2.The surface morphology of the as-grown ZnS:Cu films wascharacterized by SEM and FIB. The films display clear colum-nar growth, with dense structures possessing a high degreeof binding within the columns and at the boundaries betweenthem, and the formation of well-defined grain borders (Fig.2 (c)). Given that the normalised temperature during deposi-tion was approximately 0.5, this structure is consistent withtheory [19, 20]. AFM showed approximate crystallite size inthe range of 10-20 nm (Fig. 1, inset). This is in accordancewith the XRD findings. Surface spots, examples of whichare present in Fig. 2 (a,b) were found by EDX measurementand SEM to be Cu S surface defects. This is expected, asCu is only soluble in ZnS up to a concentration of around400 ppm, and upon reaching this limit, Cu is often found nearthe surface of the ZnS films or in phase-separated Cu x S pre-cipitates [11, 21]. After annealing, these precipitates becamesparser, and it was observed that they displayed the character- istic structure of Cu S nanoflowers (Fig. 2 (e)). Aside fromthese defects, the film surfaces possessed high uniformity andappeared to be continuous, dense, and well-adhered to thesubstrate. FIB measurements determined the thicknesses ofthe films to be 100 ± B. Composition and elemental distribution
Quantitative and semi-quantitative EDX analysis was per-formed on several films in order to determine stoichiometryand the distribution of the constituent elements. The first set offabricated films were deposited on stationary substrates. Thegeometry of the setup and the plume dynamics resulted in thesubstrates being subject to differential stoichiometry acrosstheir areas, and the consequent growth was found to be highlysensitive to initial conditions (Fig. 3). The atomic percentagesof the more volatile element, S, were found to be relatively sta-ble across the films, however the films often possessed manyatomic-percent of non-stoichiometry with respect to the heav-ier, more directional, Cu and Zn cations. In contrast, filmsdeposited under the rotating setup were found to have highuniformity of elemental distribution, even when stoichiome-try with respect to the target material was low (see Fig. 3(c-e)). Post-deposition annealing in a S-rich background gas
FIG. 3. Elemental mapping showing the distribution of depositedelements in representative conducting ZnS:Cu films. Mapping wasperformed on a central area of the films. Top: elemental maps of astationary-substrate-grown film. Note the non-uniform distributionfrom left to right in each elemental map. Bottom: elemental mapsfor a film grown under the rotation setup, showing high levels ofuniformity across the film. prevented sulphur vacancy (V S ) formation during the heatingprocess and otherwise kept the stoichiometry stable. C. Electrical characterisation
P-type behaviour of the ZnS:Cu films was confirmed byHall measurement from which sheet resistance, hole concen-tration, and carrier mobility in the annealed samples werecalculated. The highest recorded mobility of 49 Scm − wasfound at a Cu concentration of x = 0.33 (Fig. 4). Consistentwith previous studies, it was found that an increase in con-ductivity was correlated with an increasing Cu concentration.For values of x = 0.05 and below, the films were found to benon-conducting. A significant barrier to high conductivity isthe compensatory action of defect formation. As a result ofcompensating V S native defects, ZnS cannot be naturally p-type, and therefore extrinsic Cu-acceptors are the main sourceof free carriers [10]. The total hole concentration in ZnS:Cutherefore depends both on the extent of defect formation andon the number of Cu ions that successfully substitute for Znions in the ZnS lattice. At copper concentrations of x = 0.10and above, the films experience sufficient cation substitutionto display p-type conducting behaviour. Conductivity, holemobility, and carrier density are plotted as a function of cop-per concentration in Fig. 4, and a full electrical characterisa-tion of the films is presented in Table III. The hole mobility inthe peak conductivity films was found to be 1.22 cm V − s − with a corresponding free carrier density of 2.20 × cm − .These data are consistent with the XRD findings, whichshow evidence of some secondary phase Cu S. Given theseeming surface localisation and relative sparsity of the Cu S(see Fig. 2 (a,b)), however, we instead believe that ineffectivedoping of Cu within the film is the origin of these defect states. The lack of sub-band gap absorption in the rotating-substratefilms gives further weight to this hypothesis.Post-deposition, the films underwent RTA. If the annealingprocess leads to S-poor conditions, spontaneous formation ofzinc interstitials (Zn i ) will compensate free holes, making p-type doping unobtainable. Under S-rich conditions, the for-mation energy of this defect increases significantly, and thuscopper doping should lead to p-type behaviour. The RTAwas therefore performed in the presence of H S to reduce S-outgassing. The conductivity of the films was found to haveincreased post-deposition annealing by a factor of 1.88 ± D. Optical characterisation
Transmittance measurements were carried out by UV-Visspectroscopy at room temperature. Transparency of ZnS:Cufilms of 100 nm thickness was found to reach a maximum of82.6 % in an x = 0.05 sample, with this value falling to 71.8 % in the most conducting x = 0.33 films. (Fig. 5). The en-ergy band gaps of Cu x Zn − x S films were calculated by usingoptical transmittance spectra. To determine the energy bandgap values, Tauc plots were produced, from which band gapenergies could be determined by the extrapolation of the lin-ear regions to the energy axis. Fig. 6 (a) shows the shift inabsorption onset with increasing Cu percentage to lower ener-gies, which is indicative of the states introduced at the valenceband maximum through Cu incorporation. For good opticaltransparency, ZnS:Cu must have a band gap of greater than 3eV, so that only light of wavelengths shorter than the visible( <
400 nm) can be absorbed by the material. In the ZnS:Cufilms grown on stationary substrates, significant sub-band gapabsorption was observed, as evidenced in Fig. 6 (b). Thiscould be indicative of valence band tailing or of the presenceof a secondary phase, and occurs to a greater extent in Cu-rich films. This is also consistent with previous work display-ing the reduction in doping efficiency with increasing dopantconcentration [22]. Fig 6 (c) displays the reduction in thissub-band gap absorption behaviour with lowered Cu-content.Fig. 6 (d) shows the smallest observed band gap for vari-ous Cu contents. Band gaps ranged from 3.20-3.53 eV, witha reduction from the zincblende ZnS value of 3.54 eV withincreasing copper doping [23]. Fig. 6 (e) shows typical trans-mittance spectra for three rotated ZnS:Cu films. The inter-
FIG. 4. Electrical properties at room temperature plotted as a function of x. Relation between copper content and conductivity (a), holemobility (b), and free carrier concentration (c). A maximum conductivity of 49.0 Scm − was found at x = 0.33 which corresponds to a holemobility of 1.22 cm V − s − .Cu x Zn − x S R S (k Ω / (cid:50) ) σ (Scm − ) µ (cm V − s − ) p S ( × cm − )x = 0 - - - -x = 0.05 - - - -x = 0.14 30.3 0.21 0.04 0.39x = 0.15 19.0 0.65 0.05 0.42x = 0.22 9.70 9.54 0.25 0.74x = 0.27 6.97 13.3 0.65 1.7x = 0.33 7.80 49.0 1.22 2.2TABLE III. Summary of the electrical properties of a selection of ZnS:Cu films grown at various Cu concentrations with an average thicknessof 105 nm.FIG. 5. Photograph demonstrating the high transparency of a typicalfilm of 100 nm thickness. ference fringes in the spectra indicate that the films preparedunder the rotating conditions display high uniformity.Zn i , V S , and Cu(I) formation are theorised to create deepenergy levels with electron excitations that correspond to visi- ble wavelengths, reducing transparency [24]. It was observedthat annealing did not affect transmission values within thevisible and ultraviolet range, and had no significant effecton the band gap of the films, which suggests that sulphuroutgassing and consequent vacancies were prevented duringH S-rich annealing.
IV. SUMMARY
Optical and electrical properties of PLD-grown transpar-ent p-type Cu-doped ZnS films were investigated and a com-prehensive opto-electrical characterisation of the material ispresented. Hall measurements have shown that ZnS:Cu thinfilms display electrical properties among the best reported fora p-type TCM, and UV-Vis spectroscopy has determined ahigh optical transmittance, with a band gap dependent on Cu-content. The most conducting Cu x Zn − x S films possessed aconductivity of 49 Scm − and a transmittance of 71.8 % inthe visible at x = 0.33. A comparison of films grown on sta-tionary and rotating substrates demonstrated the sensitivity ofCu-doping on the plume geometry, with the stationary filmsdemonstrating differential conductivity across the films andsub-band gap absorption. We conclude that, for compositionsof x = 0.10 and above, copper doping onto Zn sites is sufficientto overcome native compensation, and that Cu undergoes par-tial separation into Cu S precipitates. Application of the de-
FIG. 6. Tauc plots derived from transmittance spectra. (a) Tauc plot of four Cu x Zn − x S films showing the reduction in absorption onsetwith an increase in copper concentration. (b) Tauc plot of a high Cu-content film grown on a stationary substrate. (c) Tauc plot of a low Cu-content film grown on a stationary substrate. (d) Optical band gap plotted as a function of copper concentration. The band gaps are found tovary between 3.2-3.53 eV. (e) Transmittance spectra for Cu . Zn . S films of approximately 100 nm thickness grown on rotated substrates.Optical transmission at 550 nm displays an appreciable decrease at 140 nm thickness from 71.8 % to 64.0 % . position method in the formation of SIS and MIS heterostruc-tures constitutes a promising approach for the development ofearth-abundant TCM-based opto-electronic devices. ACKNOWLEDGMENTS
We thank our colleagues Jonathan A. Jones and Sean A.Davis who helped in the collection of EDX data. Generousassistance was also provided by Dr. Peter Heard, Dr. RossSpringell, Dr. Robert Harniman and Dr. Benjamin Mills withthe DualBeam, XRD, AFM, and UV-Vis equipment, respec-tively, throughout the course of the research. [1] Y. Wang, X. Zhang, L. Bai, Q. Huang, C. Wei, and Y. Zhao,Applied Physics Letters , 263508 (2012).[2] M. Berginski, J. H¨upkes, M. Schulte, G. Sch¨ope, H. Stiebig,B. Rech, and M. Wuttig, Journal of applied physics , 074903(2007).[3] Z. Chen, W. Li, R. Li, Y. Zhang, G. Xu, and H. Cheng, Lang-muir , 13836 (2013).[4] T. Minami, Thin Solid Films , 5822 (2008).[5] D. S. Hecht, L. Hu, and G. Irvin, Advanced materials , 1482(2011).[6] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi,and H. Hosono, Nature , 939 (1997).[7] M.-L. Liu, F.-Q. Huang, L.-D. Chen, Y.-M. Wang, Y.-H. Wang,G.-F. Li, and Q. Zhang, Applied physics letters , 072109(2007). [8] Y. Wang, M. Liu, F. Huang, L. Chen, H. Li, X. Lin, W. Wang,and Y. Xia, Chemistry of materials , 3102 (2007).[9] N. ¨Uzar and M. C¸ . Arikan, Bulletin of Materials Science ,287 (2011).[10] P. Li, S. Deng, L. Zhang, G. Liu, and J. Yu, Chemical PhysicsLetters , 75 (2012).[11] M. Ichimura and Y. Maeda, Thin Solid Films , 277 (2015).[12] W. Chamorro, T. Shyju, P. Boulet, S. Migot, J. Ghanbaja,P. Miska, P. Kuppusami, and J. Pierson, RSC Advances ,43480 (2016).[13] D. E. Ort´ız-Ramos, L. A. Gonz´alez, and R. Ramirez-Bon, Ma-terials Letters , 267 (2014).[14] M. Dula, K. Yang, and M. Ichimura, Semiconductor Scienceand Technology , 125007 (2012). [15] A. M. Diamond, L. Corbellini, K. Balasubramaniam, S. Chen,S. Wang, T. S. Matthews, L.-W. Wang, R. Ramesh, and J. W.Ager, physica status solidi (a) , 2101 (2012).[16] R. Woods-Robinson, J. K. Cooper, X. Xu, L. T. Schelhas, V. L.Pool, A. Faghaninia, C. S. Lo, M. F. Toney, I. D. Sharp, andJ. W. Ager, Advanced Electronic Materials (2016).[17] K. Meng, P. K. Surolia, O. Byrne, and K. R. Thampi, Journal ofPower Sources , 218 (2014).[18] M. A. Yıldırım, Optics Communications , 1215 (2012).[19] M. A. Yildirim, A. Ates¸, and A. Astam, Physica E: Low-dimensional Systems and Nanostructures , 1365 (2009). [20] I. Petrov, P. Barna, L. Hultman, and J. Greene, Journal of Vac-uum Science & Technology A: Vacuum, Surfaces, and Films , S117 (2003).[21] H. H. Pham, G. T. Barkema, and L.-W. Wang, Physical Chem-istry Chemical Physics , 26270 (2015).[22] K. Byrappa and T. Ohachi, Crystal growth technology (Elsevier,2003).[23] V. Kumar, S. K. Sharma, T. Sharma, and V. Singh, Optical ma-terials , 115 (1999).[24] C. G. Granqvist and A. Hult˚aker, Thin solid films411