Influences of post-annealing conditions on the formation of delafossite-CuCrO2 films

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P76 ECS Journal of Solid State Science and Technology, 2 (3) P76-P80 (2013) 2162-8769/2013/2(3)/P76/5/$28.00©The Electrochemical Society

Influence of Post-Annealing Conditions on the Formation

of Delafossite-CuCrO

2

Films

Hong-Ying Chenz_{and Kuei-Ping Chang}

Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan

The influence of post-annealing conditions, the partial oxygen pressures (pO2) and temperatures, on the formation of CuCrO2films is examined in this study. The sol-gel derived films were annealed at 500◦C in air and post-annealed at 600◦C to 850◦C in pO2 = 10−3_{atm to pO2} _{= 0.21 atm. The CuO and CuCr}_2O4_{phases appeared above pO2}_{= 10}−2_{atm and 600}◦_{C of post-annealing,} whereas a pure delafossite-CuCrO2phase was detected when the films were post-annealed in pO2= 10−3atm above 600◦C. The binding energies of Cu-2p3/2at 932.2 and 934.3 eV, and satellites were observed because the post-annealed films had CuO and CuCr2O4phases. The binding energy of Cu-2p3/2was 932.2 eV because the post-annealed films had a delafossite-CuCrO2phase. A binding energy of Cr-2p3/2at 576.2 eV was observed when the post-annealed films had CuCr2O4or delafossite-CuCrO2phases. The formation of a delafossite-CuCrO2 phase in the post-annealing processing is in good agreement with thermodynamics. The transmittance of delafossite-CuCrO2films was 60−75% in the visible region and the direct optical bandgap of delafossite-CuCrO2 films was 3.0–3.05 eV. The positive Seebeck coefficients revealed the p-type characteristics in the delafossite-CuCrO2films. The electrical conductivity of the delafossite-CuCrO2films was (2.0−3.8) × 10−2Scm−1with the carrier concentrations of (1.3−1.9) × 1017_cm−3_.

Manuscript submitted November 5, 2012; revised manuscript received December 13, 2012. Published January 3, 2013.

Wide-bandgap oxide semiconductors have become desirable re-cent years because of the coexistence of electrical conductivity and optical transparency in the visible region of a single material. Transpar-ent conducting oxides (TCOs) are wide-bandgap oxide semiconduc-tors that combine electrical conductivity with optical transparency.1 This combination has numerous optoelectronic applications, such as in solar cells, flat-panel displays, electromagnetic shielding devices, light-emitting diodes, and transparent heat sources.1,2_{Common TCOs,} such as ZnO, In2O3, and SnO2, mostly exhibit n-type characteristics.

However, p-type TCOs are not extensively examined.1,2_Delafossites are p-type TCOs with a layered oxide crystal structure and have the basic formula AI_MIII_O

2, where A are monovalent cations such as Cu

or Ag, and M are trivalent metals ranging from Al to La.1_CuCrO

2has

a relative high electrical conductivity compared to the other Cu-based delafossites.1

Various film deposition techniques have been employed for the deposition of delafossite compounds2 _{because of success in the} preparation of p-type CuAlO2 films.3 CuCrO2 films can be

de-posited using pulsed laser deposition,4–8_sputtering,9,10_{chemical vapor} deposition,11–13_{and chemical solution methods.}14–19 _Vacuum-based processes are complex and time-consuming. Conversely, the chemi-cal solution-based methods have become an effective alternative tech-nique for depositing delafossite films. They are more economical, easily set-up, have a large area of coating, and can be mass pro-duced. Previous studies showed that the sol-gel method is a powerful technique for preparing delafossite films.14–19_{A two-step annealing} processing is frequently used in this method. The first annealing step is applied to oxidize the films to form precursor phases and the second annealing step (post-annealing step) is employed to form a desir-able delafossite phase and improve the crystallinity of the resulting films.

G¨otzend¨orfer and his coworkers14 _{fabricated CuCrO}

2 thin films

using sol-gel processing. Their sol-gel derived films were annealed in air and post-annealed at 700◦C in an Ar atmosphere. Bywalez et al.16 fabricated CuCrO2 thin films using sol-gel processing. The sol-gel

derived films were annealed at 400–600◦C in air and post-annealed in 400–700◦C in an Ar atmosphere. Pure CuCrO2phase was formed

as the films post-annealed at 700◦C in Ar. Wang et al.19 _showed CuCrO2 thin films that were prepared onto a sapphire substrate.

The spin-coated films were annealed at 400◦C in air and CuCrO2

films were obtained as the films post-annealed under a pressure of 0.13 Pa at 750◦C for 1 h. Wang et al.18_{synthesized CuCrO}

2thin films

z_E-mail:_{[email protected]}

using sol-gel process and two-step annealing. The sol-gel derived films were annealed at 300–700◦C in air and the CuCrO2 phase was

formed when the films were post-annealed at 900◦C in flowing N2.

Our previous studies20,21_{indicated that delafossite-CuAlO}

2thin films

can be obtained through the annealing of sputtered amorphous Cu-Al-O thin films in pO2∼ 5 × 10−5atm above 800◦C, but the CuO and

CuAl2O4phase were archived at 700◦C. Furthermore, the

thermody-namics can explain the formation of the delafossite-CuAlO2phase.20

Sol-gel derived Cu-Fe-O films post-annealed in pO2∼ 5 × 10−5atm

above 700◦C leads to the formation of a pure delafossite-CuFeO2

phase during sol-gel processing. Moreover, this phase formation can be successfully explained using thermodynamics.21_Therefore, appro-priate pO2levels and temperatures during the post-annealing

process-ing are critical factors for the formation of a desirable delafossite phase.

Studies regarding post-annealing conditions on the formation of delafossite-CuCrO2 films have not been performed systematically.

Therefore, this study examines the effect of various partial oxygen pressure (pO2) and temperatures during post-annealing processing on

the formation of delafossite-CuCrO2films prepared using sol-gel

pro-cessing. The microstructures of the post-annealed films are identified using the grazing incident X-ray diffraction (GIXRD), X-ray photo-electron spectroscopy (XPS), and field-emission scanning photo-electron mi-croscope (FE-SEM). The optical properties of the post-annealed films are measured using an ultraviolet-visible (UV-VIS) spectrometer, the electrical properties are examined using the Hall-effect measurement, and the carrier type of conduction is examined using the Seebeck coefficient measurement. Moreover, the formation of the delafossite-CuCrO2phase during post-annealing is also discussed with regards to

a thermodynamic analysis.

Experimental

The Cu-Cr-O films on 1 mm-thick quartz substrates (Tochance Technology CO., LTD, Taiwan) were prepared using spin-coating followed by two-step annealing. The Cu(CH3COO)2· H2O (0.02 mol,

purity 98%, Showa) and Cr(NO3)3· 9H2O (0.02 mol, purity 99%,

ACROS) were initially dissolved in 80 mL ethanol, and 0.04 mol triethanolamine (purity 95%+, Tedia, USA) was added to the solution. Thereafter, this precursor was spin-coated onto quartz substrates at 1500 rpm for 15 s. The specimens were annealed at 500◦C in air for 1 h at a ramp rate of 5◦C/min before the next cycle. Two cycles were performed in this study. The specimens were post-annealed between 600◦C and 850◦C in flowing gas under various pO2levels that ranged

from 10−3 atm to 0.21 atm to examine the influences of pO2 and

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ECS Journal of Solid State Science and Technology, 2 (3) P76-P80 (2013) P77

temperatures on the formation of the delafossite-CuCrO2phase. The

pO2 in the flowing gas was achieved by mixing mixed O2 (purity

99.7%) into N2(purity 99.9%, pO2∼ 10−3atm). The annealing was

maintained for 2 h at a ramp rate of 5◦C/min.

A Bruker D8 Discover SSS X-ray diffractometer operating with Cu-Kα radiation (λ = 0.154 nm) at 40 kV and 40 mA was used to determine the changes in crystal structure. The grazing incidence op-erating mode was used with an incidence angle of 1◦and the sampled step size was 0.01◦within 2θ = 10◦−70◦. XPS was performed using an ULVAC PHI-5000 spectrometer with the Al K_α(hν = 1486.6 eV) exciting X-ray source. The surface was sputtered cleaning prior to measurement using an Ar ion gun operated at 2 keV for 2 min. High-resolution spectra of Cu-2p, Cr-2p and O-1s were obtained at an energy interval of 0.2 eV/step. All spectra were calibrated according to the C-1s peak at 284.6 eV. The XPS spectra were fitted using a nonlinear least squares fit with a Gaussian/Lorentzian peak shape (G/L mixing ratio= 0.3) and the background was subtracted prior to each fitting routine. The surface morphology of the films was analyzed using the FE-SEM (JEOL JSM-6700F). Optical properties were measured using a Perkin Elmer Lambda 35 ultraviolet-visible (UV-VIS) spectrome-ter. The electrical resistivity, carrier type, and carrier concentration of the post-annealed films were measured using the standard Hall-effect measurement.

Results and Discussion

GIXRD analysis.— Figure1shows the GIXRD pattern of the sol-gel derived films annealed at 500◦C in air and post-annealed un-der various pO2levels at 600–850◦C. The CuO (JCPDS #89-5899)

and CuCr2O4 (JCPDS #85-2313) are the predominant phases in the

films annealed in air at 500◦C for 1 h, which is shown in the bottom curve of Fig.1d. Moreover, the intense CuO (JCPDS #89-5899) and CuCr2O4(JCPDS #85-2313) phase appeared in the pattern as the films

were post-annealed in pO2 = 0.21 atm at 600–850◦C (Fig.1a), pO2

= 10−1_{atm at 600–800}◦_{C (Fig.}_1b_{), and pO}

2= 10−2atm at 600–750◦C

(Fig.1c) for 2 h. Moreover, post-annealed films exhibited small crys-tallinity when obtained from the large full width at half maximum (FWHM) values.

Conversely, intense delafossite-CuCrO2(R3m, JCPDS #89-6744)

diffraction peaks were well-defined in the films post-annealed in pO2

= 10−3_{atm at 600–700}◦_{C (Fig.}_1d_{). Strong (006), (012), and (110)}

diffraction peaks are observed in the figure, where the relative inten-sities of the diffraction peaks remained unchanged in the films post-annealed in pO2= 10−3atm. The narrow FWHM of the

delafossite-CuCrO2films’ peaks represented a good crystallinity.

The post-annealing conditions and corresponding phases ob-served from GIXRD patterns are summarized in Table I. The

Figure 1. X-ray diffraction pattern of sol-gel derived films annealed in air at 500◦C and post-annealed under various pO2levels: (a) pO2= 0.21 atm, (b) pO2 = 10−1_{atm, (c) pO2}_{= 10}−2_{atm, and (d) pO2}_{= 10}−3_{atm (}_{:CuO, ◦:CuCr}_2O4,_●:CuCrO2_).

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P80 ECS Journal of Solid State Science and Technology, 2 (3) P76-P80 (2013)

Figure 6. Transmittance spectrum of delafossite-CuCrO2films obtained by sol-gel derived films post-annealed under pO2= 10−3atm at 600–700◦C (inset is optical bandgaps).

during the post-annealing process. The equation indicates that the film post-annealed under pO2= 10−3 atm at 700◦C has aG value

of−29.5 kJ/mol, whereas the G value is −10.8 kJ/mol for the film post-annealed under pO2= 10−2atm at 700◦C.

Optoelectronic properties analysis.— The optoelectronic proper-ties of the delafossite-CuCrO2films obtained from the post-annealing

processes are analyzed. Figure6shows the optical properties, trans-mittance spectrum and direct optical bandgap of the delafossite-CuCrO2films that were post-annealed under pO2= 10−3atm at 600◦C

to 700◦C. The absorption edge near 400 nm is shown in this figure. The transmittance of the films in the visible region is approximately 60%−75%, which slightly increases as temperature increases. This increase may be associated with the good crystallinity of the films at high temperatures (cf. Figs.3cand3d). The direct optical bandgap of the delafossite-CuCrO2films deduced from the absorption coefficient

and incident photo energy are shown in the inset of the figure. The direct optical bandgap of the delafossite-CuCrO2films were

approx-imately 3.0 eV (600 and 650◦) and approximately 3.05 eV (700◦C). The values obtained in the direct optical bandgap for the delafossite-CuCrO2films are consistent with those of other studies.18,19

Moreover, the positive Seebeck coefficients confirm the p-type characteristics of the delafossite-CuCrO2 films. The electrical

con-ductivity, carrier concentration, and mobility measurements of the delafossite-CuCrO2 films prepared by post-annealing above 600◦C

under pO2 = 10−3 atm were examined using a standard Hall-effect

analyzer. The electrical conductivities of the delafossite-CuCrO2films

were 3.8× 10−2Scm−1(600◦C), 2.0× 10−2Scm−1(650◦C), and 2.3 × 10−2 _Scm−1 ₍₇₀₀◦_{C). The corresponding carrier concentrations}

were 1.7× 1017_cm−3 ₍₆₀₀◦_{C), 1.3}_{× 10}17 _cm−3 ₍₆₅₀◦_{C), and 1.9}

× 1017 _cm−3 ₍₇₀₀◦_{C), respectively. The electrical conductivities}

of the sol-gel-derived delafossite-CuCrO2 thin films were (3.4−5)

× 10−3 _S/cm18 _{and 1.8}_{× 10}−2 _S/cm.19 _{Moreover, the carrier} con-centration of the sol-gel-derived delafossite-CuCrO2 thin films was

3.14× 1015_cm−3_.19_{Therefore, the electrical conductivities and} car-rier concentrations of the delafossite-CuCrO2films in this study are

in accordance other studies.

Conclusions

The influence of partial oxygen pressure and temperatures during the post-annealing processing on the formation of delafossite-CuCrO2

films were examined. CuO and CuCr2O4phases were observed above

pO2= 10−2atm during post-annealing processing. Pure

delafossite-CuCrO2films were obtained when the films were post-annealed above

600◦C in pO2 = 10−3 atm. Binding energies of Cu-2p3/2 at 932.2

and 934.3 eV, and satellites were observed when the post-annealed films had CuO and CuCr2O4phases. Conversely, the binding energy

of Cu-2p3/2was 932.2 eV as the delafossite-CuCrO2 phase formed.

The binding energy of 576.2 eV for Cr-2p3/2was observed when the

post-annealed films had a CuCr2O4or delafossite-CuCrO2phase. The

compact surface with an average particle size of 100 nm in conjunction with large particles was apparent when the films were post-annealed at 700◦C under pO2= 10−2atm, which contributed to CuO and CuCr2O4

phases. The surface exhibited granular features that resulted from the formation of the delafossite-CuCrO2 phase, which was observed in

films post-annealed at 700◦C in pO2 = 10−3 atm.

Thermodynam-ics can explain the formation of the delafossite-CuCrO2 phase in

the films post-annealed in pO2= 10−3 atm above 600◦C. However,

thermodynamics cannot explain the experimental results near the cal-culated line, which may be caused by the small driving force for the phase transformation. The transmittance of the delafossite-CuCrO2

films was 60%−75% and the estimated direct optical bandgap of the delafossite-CuCrO2 films was 3.0−3.05 eV. The electrical

conduc-tivity of the delafossite-CuCrO2films was (2.0−3.8) × 10−2 Scm−1

with carrier concentrations of (1.3−1.9) × 1017_cm−3_{. Hence}

appro-priate pO2and temperature are major factors in the formation of the

delafossite-CuCrO2 phase during the post-annealing step of sol-gel

processing.

Acknowledgments

We thank the National Science Council of the R.O.C. for financial assistance under grant numbers NSC 100-2221-E-151-027 and NSC 101-2221-E-151-029.

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