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## Creator

[Martin Markwitz](https://orcid.org/0009-0007-6516-3571), [Peter P. Murmu](https://orcid.org/0000-0002-0109-1798), [Takao Mori](https://orcid.org/0000-0003-2682-1846), [John V. Kennedy](https://orcid.org/0000-0002-9126-4997), [Ben J. Ruck](https://orcid.org/0000-0002-3719-7375)

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This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Martin Markwitz, Peter P. Murmu, Takao Mori, John V. Kennedy, Ben J. Ruck; Defect engineering-induced Seebeck coefficient and carrier concentration decoupling in CuI by noble gas ion implantation. Appl. Phys. Lett. 18 November 2024; 125 (21): 213901 and may be found at https://doi.org/10.1063/5.0233754.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Defect engineering-induced Seebeck coefficient and carrier concentration decoupling in CuI by noble gas ion implantation](https://mdr.nims.go.jp/datasets/29535d0d-158d-43c6-a647-59ce95e74b6f)

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Defect engineering-induced Seebeck coefficient and carrier concentration decoupling in                                               CuI by noble gas ion implantationMartin Markwitz,1, 2, 3, a) Peter P. Murmu,2 Takao Mori,4, 5 John V. Kennedy,2, 3 and Ben J. Ruck1, 31)School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140,New Zealand2)National Isotope Centre, GNS Science, PO Box 30368, Lower Hutt 5010, New Zealand3)The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington,PO Box 600, Wellington 6140, New Zealand4)International Center for Materials Nanoarchitectonics (WPI-MANA), NationalInstitute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044,Japan5)Graduate School of Pure and Applied Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305–8671,Japan(Dated: 9 August 2024)Copper(I) iodide, CuI, is the leading p-type non-toxic and earth-abundant semiconducting material for trans-parent electronics and thermoelectric generators. The power factor of CuI thin films was increased from332±32 µWm-1K-2 to 578±58 µWm-1K-2 after implantation with noble gas ions (Ne, Ar, Xe). The increasedpower factor is due to a decoupling of the Seebeck coefficient and carrier concentration identified through achanging scattering mechanism. Ion implantation causes the abundant production of Frenkel pairs, whichwere found to to suppress compensating donors in CuI, studied using density functional theory calculations.The compensating donor suppression led to a significantly improved Hall carrier concentration, increasingfrom 6.5 × 1019 ± 0.1 × 1019 cm-3 to 11.5 × 1019 ± 0.4 × 1019 cm-3. This work provides an important stepforward in the development of CuI as a transparent conducting material for electronics and thermoelectricgenerators by introducing beneficial point defects with ion implantation.Energy harvesting through the thermoelectric effect isa rapidly growing low carbon-emission technology. Ma-terials facilitating such energy conversion are becom-ing increasingly technologically important1. Thermoelec-tric devices convert heat flux into electrical power bythe Seebeck effect at a conversion efficiency which de-pends on the figure of merit ZT = α2σT/κ where α,σ, κ, and T are the Seebeck coefficient, electrical con-ductivity, total thermal conductivity, and absolute tem-perature, respectively2. The highest-performing roomtemperature thermoelectric materials are degeneratelydoped semiconductors, usually with small band gaps,such as the tetradymite alloys based on (Bi,Sb)2(Te,Se)3composition, with up to ZT = 1.5. Unfortunately,these compounds are composed of expensive and toxicprecursors2,3. Heavily doped wide band gap semicon-ductors provide a much wider application scope thantheir non-transparent counterparts4. This advantage isparticularly useful at room temperature, but, before de-vice integration can be considered on grounds of cost-effectiveness, an increase in performance is still required.The state of the art n-type transparent conductorsfor near room-temperature thermoelectric applicationsare In2O3:Sn and ZnO:Al, each with electrical conduc-tivities near σ ∼ 10000 Scm-15, whilst for p-type con-ductors, CuI possesses a moderate conductivity of σ ∼100 Scm-16,7. The other p-type transparent conductorsare doped copper oxides, the best of which is CuAlO2with σ = 0.01 Scm-18. The reason for the conductivitya)Electronic mail: m.markwitz@gns.cri.nzdifference is the high hole mobility in CuI, believed topossibly to reach 30 cm2V-1s-1 at carrier concentrationsof 1020 cm-39. Presently, hole mobilities are limited to≈ 10 cm2V-1s-1 at carrier concentrations near 1020 cm-3.The high carrier mobilities in-part originate from theband-degenerated light and heavy hole bands with effec-tive masses 0.3m0 and 2.4m0, where m0 is the free elec-tron mass10. The capability for CuI as a functional ma-terial has already been demonstrated in laboratory scalecomponents such as in thin film transistor11,12, opticalmemory element13,14, and as p-type legs of transparentthermoelectric generator6,15,16 applications, the perfor-mance of which in one way or another relies on a highelectrical conductivity. Further investigation of its prop-erties and improvements in electrical conductivity toward1000 Scm-1 will likely follow with commercial applicationsin commonplace electronic devices. The further advan-tages of CuI over alternative p-type transparent conduc-tors is its non-toxic and earth-abundant constituents inaddition to its facile fabrication procedures6.Ion implantation is an alternative method to annealingby which to modify the electrical and thermal propertiesof a thin film or the surface of a bulk material chemicallyor structurally. Ion implantation has seen recent inter-est for thermoelectric material thin films, especially thecase of ScN17–19 or Bi2Te320,21 for nanostructuring andchemical doping. Ion implantation is an energetic pro-cess which displaces atoms from their lattice sites intointerstitial sites and antisites in a collision cascade, lead-ing to the formation of a large concentration of Frenkelpairs within a small volume. The displacement of atomsfrom their lattice sites usually reduces the phonon ther-mal conductivity as the long-range crystal order is sup-mailto:m.markwitz@gns.cri.nz2TABLE I. Relationship between implantation fluence F , dis-placements per ion D, and (70 nm) depth-averaged DPA.Species Energy [keV] F [ions cm-2] D [disp. ion−1] DPANe 13 0 200 0Ne 13 1.6 × 1015 200 1.3Ne 13 3.2 × 1015 200 2.5Ne 13 9.6 × 1015 200 7.4Ne 13 1.92 × 1016 200 14.1Ar 27 0 442 0Ar 27 7.2 × 1014 442 1.3Ar 27 1.45 × 1015 442 2.5Ar 27 4.34 × 1015 442 7.5Ar 27 8.69 × 1015 442 14.7Xe 70 0 1331 0Xe 70 5 × 1014 1331 2.7Xe 70 1 × 1015 1331 5.3Xe 70 2 × 1015 1331 10.5Xe 70 3 × 1015 1331 15.7pressed, beneficial for thermoelectric materials22. Recentstudies have investigated the effects of ion implantation ofS, Se, and Te as dopants in CuI, but the effect of the im-plantation damage itself remained unclear23–25. Implant-ing noble gas ions into a material is an excellent testingground by which to investigate the properties of intrin-sic point defects therein. Additionally, it is a processwhich can be applied without the need for high tempera-ture processing, which generally causes a loss in electricalconductivity of CuI26–30.Noble gas ions provide the opportunity to modify thestructure of thin films without also performing chemicalmodification, allowing the effects to be studied indepen-dently. In this work we implanted Ne, Ar, or Xe ions tomodify the electronic properties of conducting transpar-ent CuI thin films by introducing point defects. Overall,this process results in an enhanced p-type conductivityin CuI. This was identified to be due to an increasedHall carrier concentration with only a minor reduction inHall carrier mobility. Further investigation on its ther-moelectric properties suggest a variation of the scatter-ing process toward ionized impurity scattering. The fastand ambient temperature implementation of this modifi-cation process makes it a low cost step during the devicefabrication process.A previous publication outlines the 60 − 70 nm CuIfilm deposition by ion beam sputtering process31. TheCuI films were implanted with neon, argon, or xenon, at13 keV, 27 keV, or 70 keV, respectively, to provide equiva-lent projected ranges (26 nm), with the implantation flu-ences scaled to approximately match the displacementsper atom (DPA) calculated with the software StoppingRange of Ions in Matter (SRIM) in the Detailed Cal-culation with full Damage Cascades mode32. Due to thestrong texture of the CuI films, the implantation was con-ducted at 7 ° from the sample normal. The relationshipbetween implantation fluence (F ), the displacements perion (D), the atomic density (ρn = 3.385 × 1022 at cm-3),FIG. 1. (a) Angle-symmetric X-ray diffractograms, verticallyoffset for visual clarity. (b) Out-of-plane lattice constantsderived from the XRD measurement, the dashed line is usedas a guide.and the resultant DPA (averaged over the 70 nm initialfilm thickness) are written as DPA = FD/ρn, and sum-marized in Table I. The samples were not visibly affectedby implantation. Throughout this work the samples arelabelled by the implantation species with the associatedDPA (see Table S1 for a summary of sample details). Theimplantation and DPA depth profiles are included as Fig-ures S1(a-f). More details pertaining to the implantationmethod are covered in previous publications24,25.The films’ composition and thicknesses were inves-tigated with Rutherford backscattering spectrometry(RBS). The RBS measurements were conducted with a2.0 MeV 4He+ beam with a 165 ° backscattering angle,a current density of 10 nA and an integrated charge of20 µC with a surface barrier detector33. The backscat-tering spectra are shown in Figures S2(a-c), which sug-gest that the as-deposited films possessed stoichiomet-ric [Cu]/[I] ratios of 1.02 ± 0.02. The implanted sam-ples exhibited increasing [Cu]/[I] ratios to an average of1.16±0.07 when implanted to an average DPA of 15, theratios of which are included in Figure S3. Excess copperin CuI is commonly observed for thin films29,30,34,35. Theloss of iodine, and excess of copper is a result of prefer-ential halide sputtering, subsequently oxidising the filmsurface upon exposure to air36–38.The films’ structural properties were studied with X-ray diffraction using a Rigaku SmartLabs diffractometer,employing a Cu X-ray source. The angle-symmetric mea-surements are shown for the argon implanted samples inFigure 1a and Figures S4(a-b) for the others, resulting3FIG. 2. (a) Electrical conductivity, (b) Seebeck coefficient, (c) Hall coefficient, (d) power factor, (e) Hall carrier concentration,and (f) Hall carrier mobility of CuI films implanted with noble gas ions. The dashed lines are used as a guide.in the identification of the zincblende CuI, strongly tex-tured along the 〈111〉 direction10. The lattice constantsof the as-deposited films are 6.070 ± 0.002 Å, settling to6.048±0.004 Å when implanted to an average DPA of 15,calculated using the goniometer error function30. Theseare shown in Figure 1b, and the fits themselves depictedin Figures S5(a-c). The out-of-plane lattice constant ofCuI thin films are well known to be greater than the bulkvalue of 6.054 Å, but is known to not be strongly de-pendant on the [Cu]/[I] ratio10,30. CuI thin films grownalong the (111) plane is known to be trigonally distorted,associated with a contraction (an expansion) of the out-of-plane (in-plane) lattice constants39.To evaluate the thermoelectric and carrier propertiesof the CuI thin films, the Seebeck coefficient α, electri-cal conductivity σ, and Hall coefficient RH were mea-sured. Room temperature Seebeck coefficient measure-ments were conducted with an ADVANCE RIKO ULVACZEM-3 with pressed nickel contacts. Hall effect measure-ments were conducted with an HMS-3000 after sputter-ing gold contacts on the corners of the samples. The sys-tematic measurement errors for the Seebeck effect mea-surements are 6 % due to systematic uncertainties41,42,and 5 % for and Hall effect measurements. The resultsof those measurements are summarized in Figure 2(a-f). The Seebeck and Hall coefficients were positive forall samples. The electrical conductivity increased from76 ± 2 Scm-1 to 116 ± 3 Scm-1 for the unimplanted, andsamples with highest DPA, respectively, an increase of53 ± 6 %. The Seebeck coefficient slightly increased be-tween the unimplanted samples (206± 7 µVK-1) and thesamples with highest DPA (223±8 µVK-1). The increas-ing electrical conductivity results in the improvement ofthe power factor, calculated from α2σ, the values rangingfrom 322 ± 32 µWm-1K-2 to 578 ± 58 µWm-1K-2. Over-all, the highest power factor was observed for the Xe-10.5 sample with a power factor of 743± 128 µWm-1K-2.This result exceeds the highest CuI thin film powerfactors of Coroa et al.15 (467 µWm-1K-2), Bae et al.35(681 µWm-1K-2), and Mirza et al.40 (481 µWm-1K-2), butlower than those of Almasoudi et al.28 (1632 µWm-1K-2).Concomitant with the increase in electrical conductivity,there was a reduction in the average Hall coefficient from0.096 ± 0.010 cm3C-1 to 0.054 ± 0.005 cm3C-1. The Hallcoefficient is related to the carrier concentration in thesingle parabolic band model by p = (qRH)−1, wherein qis the elementary charge. The derived Hall carrier con-centrations increase from an average of 6.5×1019±0.1×1019 cm-3 to an average of 11.5×1019±0.4×1019 cm-3, anincrease of 77 ± 4 %. Also, the Hall carrier mobility canbe derived from µH = RHσ, reducing slightly from aninitial value of 7.3±0.5 cm2 V-1s-1 to 6.3±0.4 cm2 V-1s-1.Such an effect, where the electrical conductivity increasesby ion irradiation with a −16 ± 14 % reduction in Hallmobility has before been noted in Bi2Te3 which resultedin an improved thermoelectric power20. It is possiblethat the point neutral and ionized disorder introducedby implantation is the cause for the reduction in carriermobility. Willis et al.9 summarize the state-of-the-art re-sults to which the results of this work can be directlycompared.The combined variation in Hall carrier concentration,Seebeck coefficient, and Hall carrier suggest that thereis a change in the carrier scattering process. To furtherinvestigate this, the semiclassical Boltzmann transportmodel in the relaxation time approximation is applied31.To investigate variations in scattering mechanism theSeebeck coefficient is plotted against the Hall carrier con-4FIG. 3. (a) Seebeck coefficient and Hall carrier concentration plot with the corresponding Boltzmann transport equationsolutions by using various scattering mechanisms (r). A survey of original literature data15,28,35,40 (colored circles) comparedto previous23–25,27 is compared to the experimental findings of neon (red, squares), argon (green, diamonds), and xenon (blue,triangles). Inset graph presents a magnified view of the observed Hall carrier concentration and Seebeck coefficients. (b) Powerfactor plotted against Hall carrier concentration for the same data from literature. Arrows are used to imply the progressionof the thermoelectric material properties with increasing DPA.centration in Figure 3a, in addition to the correspondingtheoretical curves. The results are also compared to re-sults from CuI thin films in literature15,23–25,27,28,35,40.The inset Figure 3a presents a magnified view of the re-sults, which suggest a deviation from the conventional re-lationship of decreasing α with increasing p. This can beattributed to a change from deformation potential scat-tering toward ionized impurity scattering, as expecteddue to the increased Hall carrier concentration. Thepower factor is compared to the Hall carrier concentra-tion and the corresponding Boltzmann transport equa-tions in Figure 3b (by using τ0 = 6.5 fs). The scatteringprocess is implied to change as the data does not trackalong any particular r curve. The highest-performingsamples across literature possess Hall carrier concentra-tions of the order of 1020 − 1021 cm-3.The cause for the increased carrier concentration ispresumably due to the increased concentration of cop-per vacancies, or alternatively, an increased concentra-tion of interstitial iodine, although those possess a greaterformation energy43. Darnige et al.29 hypothesized thatduring thin film growth, a large concentration of Frenkelpairs (complexes of vacancies and interstitials of the sametype) are included which can be annealed out, resulting ina reduced carrier concentration after annealing26–28. Inaddition to those produced during growth, Frenkel pairsare produced in abundance through ion implantation, es-pecially in ionic crystals, which could be the reason forthe increased carrier concentration36. A connection be-tween Frenkel pairs and the macroscropic electrical prop-erties can be made by use of density functional theory(DFT) calculations, such as was done for Cd1-xZnxTe44,CeO245, ThO246, and CsPb(I1-xBrx)347.We conduct density functional theory calcula-tions for intrinsic defects in CuI, such as vacan-cies (V Cu/V I), antisites (CuI/ICu), and interstitials(Cui, tet-I/Ii, tet-I/Cui, tet-Cu/Ii, tet-Cu), using the samecomputational setup using the PBE exchange-correlationfunctional as discussed in our previous work24,25. The in-terstitial sites are coordinated tetragonally either by Cuions, or I ions48. The calculated formation and thermo-dynamic transition energies are in good agreement withthe work of Huang et al.43, for both the Cu-rich and Cu-poor chemical potential limits. Taking a step further, theFrenkel pair binding energy is calculated withEqb,D1:D2= ∆HqD1+ ∆HqD2−∆HqD1:D2(1)which relates the formation energy of the isolated defects(∆HqD1and ∆HqD2) with the formation energy of the de-fect complex (∆HqD1:D2)44,49. A positive binding energy(EqD1:D2) indicates that the defects are stable complexesand will remain in proximity of one another, but, it doesnot indicate the likelihood of the formation thereof, which5FIG. 4. (a) Formation energy of isolated and Frenkel defects. (b) Summed formation energies of the relevant isolated defectscompared to their defect complexes. (c) Frenkel pair binding energy diagram. The defects involving the interstitial atomscoordinated by iodine (copper) atoms are drawn as dashed (filled) lines.relies on a low formation energy of the defect complex(∆HqD1:D2). Figure 4a shows the formation energy of theisolated defects and and defect complexes in CuI, Figure4b shows the summed isolated defect formation energies,while Figure 4c shows the Frenkel pair defect bindingenergies calculated using Eq. 1.Due to the low formation energy of V Cu the self-consistently calculated Fermi energy is always near thevalence band edge48. When the Fermi energy is nearthe valence band edge, the pair of isolated defects (V Cuand Cui, tet-I/Cui, tet-Cu) overall exhibit donor status.On the other hand, the charge of the Frenkel pairs re-mains neutral, which acts to passivate the compensat-ing donors. Such a phenomenon has been previouslyused to describe the relation to macroscopic conductivity-switching effect based on different charge states of theisolated defects and their Frenkel pair binding energiesin Cd1-xZnxTe44. Additionally, the effect of hydrogenpassivation of cation vacancies in CuMO2 (M=Al, Ga,In) was similarly investigated49. This donor passivationeffect is exacerbated by ion implantation, where Frenkelpairs are produced in abundance within the CuI matrix,leading to an overall p-type doping effect.In summary, noble gas ion implantation with Ne, Ar,or Xe is a post-deposition technique to modify the trans-parent conducting and thermoelectric properties of CuIthin films by introducing intrinsic point defects. The out-of-plane lattice constant reduced from the as-depositedvalues of 6.070±0.002 Å to 6.048±0.004 Å for the highest-implanted films. Simultaneously, there was a remarkableimprovement in electrical conductivity from an average of76±2 Scm-1 to 116±3 Scm-1 by implantation to an aver-age DPA of 15, regardless of the noble gas ion. This wasmainly attributed to the increase in Hall carrier concen-tration, which increased from 6.5×1019±0.1×1019 cm-3to 11.5× 1019± 0.4× 1019 cm-3. The increasing Hall car-rier concentration could be due to (1) the damage pro-file caused by the implantation resulting in the forma-tion of Frenkel pairs which suppress the concentration ofcompensating donors, or (2) the replenishment of O intoV I near the film surface. It should be noted that O isknown to not be a very shallow acceptor within CuI, andis not expected to drive the carrier concentrations above1020 cm-3.The power factor of CuI thin films increased from anaverage of 322± 32 µWm-1K-2 to 578± 58 µWm-1K-2 byion implantation with noble gas ions, comparable to thestate-of-the-art power factors reported in literature. Thisimprovement in the thermoelectric properties is likelydue to a change in scattering mechanism from phononto ionized impurity scattering, achieved by ambient tem-perature defect engineering with ion implantation.CREDIT AUTHORSHIP CONTRIBUTION STATEMENTMM: conceptualization, data curation, formal analy-sis, investigation, methodology, software, visualization,writing – original draft and editing. PPM: funding ac-quisition, supervision, validation, writing – review. TM:funding acquisition, writing – review. JVK: fundingacquisition, supervision, validation, writing – review.BJR: funding acquisition, supervision, validation, writ-ing – review.ACKNOWLEDGMENTSThis research is funded the Royal Society of NewZealand through Marsden Fund (grant number MFP-GNS2301). 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Beneficial point defects for an enhanced power factor in copper(I) iodide Abstract CRediT authorship contribution statement Acknowledgments