Communication—Annealing Strategies for Spray Deposited Precursor Films of p-Type CuCr1-xMgxO2

Annealing of sprayed pure and Mg doped CuCrO2 thin films by high intensity, short time light irradiation leads to a single delafossite phase at comparatively low temperatures compared with traditional furnace annealing. P-type crystalline undoped and Mg-doped CuCrO2 films were obtained within few minutes by annealing with halogen lamp between 550 °C and 650 °C in Ar atmosphere. Transport properties of Mg-doped thin films were comparable to furnace annealed samples despite much shorter annealing time. The results demonstrate that post-annealing of chemically deposited samples using light irradiation is an effective and fast method for obtaining transparent conducting delafossite thin films.

Transparent conductive oxides (TCOs) are materials having good electrical conductivity while retaining high transparency in the visible region.][3][4][5][6][7][8] Except for photocatalysis, 9 in this type of application, semiconductors are heavily doped (degenerate).On the other hand, there is a great quest for transparent electronics which requires fabrication of transparent p-n junctions.All TCOs used nowadays in industrial applications are n-type.The p-type TCOs reported to date have conductivities at least two orders of magnitude lower than their ntype counterparts and suffer from non-optimal transparency.If ptype oxides with high conductivities and controlled transparencies could be manufactured industrially, a variety of new applications would open up.Hence, at present, considerable efforts are made for the search of p-type TCOs and improvement of their conductivity and optical transparency. 10ne of the most promising and widely studied p-type TCO is Mg-doped CuCrO 2 with delafossite crystal structure. 11By substituting trivalent Cr ions by divalent Mg ions p-type conductivity is induced while maintaining optical transparency.3][14] Spray pyrolysis deposition of CuCr 1-x Mg x O 2 precursor films at 400 °C leads to an amorphous mixture of the constituting oxides, requiring high temperature annealing (above 800 °C if air is the ambient) to form the delafossite phase. 12,14The high temperatures and long annealing times are major drawbacks for wide application of CuCr 1-x Mg x O 2 thin films in practical devices.Therefore, it is desirable to develop an alternative annealing process to ensure fast synthesis of CuCr 1-x Mg x O 2 thin films at low temperatures (around 600 °C), which also opens the possibility of using cheap substrates with low softening point (e.g.glass) or polymers.An example is given in Ref. 15.
Solid state reaction in oxide materials was demonstrated and can be significantly accelerated by light irradiation. 16,17The essence of this method which was called "Photostimulated Solid State Reaction" (PSSR) is the irradiation of the mixture of starting oxides by light in a broad spectral range from infrared to ultraviolet with sufficient intensity for starting the solid state reaction between the reagents.The rate of the resulting reaction exceeds the conventional thermal solid state reaction rate in a furnace by about two-three orders of magnitude.This allows dramatic increase of the reaction speed and significantly reduces the synthesis temperature of advanced oxide materials.
A fast process to prepare transparent conductive Mg-doped CuCrO 2 thin films was reported. 18This process involves annealing of samples with atmospheric pressure plasma and the synthesis was performed within 30 min, which is indeed much faster than conventional annealing.Our presented method of annealing by light irradiation shows even faster as it takes only 3-5 min.It should be pointed out that the annealing of samples in a furnace under same temperatures requires at least an order of magnitude more time compared to light annealing.This indicates that some non-thermal processes accelerate solid state synthesis of delafossite thin films as a result of light irradiation similar to other reported oxide compounds. 13,14While the mechanism of the PSSR is still under discussion, it was shown previously that the higher energy photons accelerate the PSSR process. 16,17his paper reports the preparation of spray pyrolysis (SP) deposited CuCr 1-x Mg x O 2 thin films by the PSSR process and characterization of their structural and electrical properties.Emphasis was given to low temperature and fast synthesis of these p-type TCO thin films which is important for their practical application.

Experimental
In the chemical spray pyrolysis technique, a fine spray of a precursor solution is formed in a pneumatic nozzle driven by compressed gas and directed towards a heated substrate, where upon thermal decomposition thin films are formed.For deposition of CuCrO 2 films, an aqueous solution of the precursors copper (II) chloride dihydrate and chromium (III) chloride hexahydrate with the atomic ratio [Cu]/[Cr] = 1 was used.Doping of the films was achieved by adding the corresponding quantity of magnesium chloride hexahydrate to the starting solution with [Mg/(Mg+Cr)] ratios of up to 2 at%.The solution was sprayed onto Corning glass and fused silica substrates held at 400 °C.The distance between nozzle and substrate, the pressure of the carrier gas, and the spray rate were optimized for obtaining good quality thin films.Before deposition, the substrates were ultrasonically cleaned in acetone, methanol and distilled water.
z E-mail: boshta2000@yahoo.com a Present address: Department of Inorganic Technology, University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic.
The light irradiation of as deposited thin films was performed under argon gas flow in a MILA-5000-P-N rapid thermal processing setup from ULVAC Inc. (Japan) which contains 4 halogen lamps, each with 1 kW power.
The crystallographic structure of the films was analyzed by X-ray diffraction (XRD) with a Siemens D-5000 diffractometer using Cu-K α radiation (λ = 1.54 Å).
Four electrical contacts were attached to the thin film samples using conductive silver paint.Contact ohmicity was systematically verified by recording I-V characteristics.DC-resistivity was measured in the Van-der-Pauw configuration at room temperature with a home-made high impedance set-up.The carrier type was determined using a home-made Seebeck effect measurement setup.

Results and Discussion
Films up to a thickness of 1400 nm were deposited under optimized spray conditions and a substrate temperature of 400 °C, using fused silica and glass substrates.

Structural Characterization
XRD studies were used to determine the crystalline phase of the spray-deposited Mg-doped CuCrO 2 films.Generally, spray deposited films at 400 °C were black and amorphous and probably contained CuO and Cr 2 O 3 , but not CuCrO 2 . 14e phase boundary CuCrO 2 /(CuCr 2 O 4 + CuO) in air (Eq. 1) lies around 800 °C according to the phase diagram: 19 At lower partial oxygen pressures, the phase boundary CuCrO 2 /(CuCr 2 O 4 + CuO) is shifted to lower temperatures, however at the expense of reaction rate.Short annealing of spray deposited precursor layers in N 2 showed only Cr 2 O 3 diffraction peaks.After annealing of CuCrO 2 film deposited on a glass substrate at 600 °C in N 2 ambient for 4 d, the presence of CuCrO 2 became evident as shown in Fig. 1a.Indexing of the diffraction planes based on the 2θ positions was carried out using the JCPSD data reference base. 20The delafossite phase (R3m) showed a good degree of crystallinity and all the diffraction lines ((003), ( 006), ( 101), ( 012), (104), etc.) were discernible.Additional diffraction lines belonging to the constituting oxides, CuO and Cr 2 O 3 , and the spinel (CuCr 2 O 4 ), were absent after annealing.
In the next step, instead of long time furnace annealing, rapid light annealing as described in the experimental section was used to decrease the processing time.Figure 1b shows the XRD pattern of a light annealed CuCrO 2 :2%Mg sample deposited on a fused silica substrate at 650 °C for 5 min.The diffraction peaks are again related to the delafossite phase, JCPDS 39-247.All other possible phases, including the Mg-spinel which appeared at high Mg doping densities, were not present.The time saved with respect to conventional annealing is considerable.We further decreased the light annealing time to 3 min at the same temperature of 650 °C for the CuCrO 2 sample deposited on glass substrate.As can be seen in Fig. 1c, again the delafossite phase was observed with all peaks of the XRD pattern indexed according to JCPDS 39-247.Finally, the temperature was decreased to 550 °C and light annealing was carried out during 10 min for the CuCrO 2 :2%Mg sample deposited on a glass substrate.As shown in Fig. 1d, the delafossite peaks were still discernible, but the XRD peak intensities were low due to incomplete conversion of the amorphous material.Therefore, it can be concluded that 550 °C is the lower limit of temperature to effectively anneal CuCrO 2 :Mg thin films with halogen light irradiation.In all cases, there was no warping of the glass substrates although the temperatures used were in the region where float glass starts to soften.
SEM pictures showed similar patterns for the tube furnace annealed and the PSSR annealed samples, especially as to the 150 to 200 nm long bar-like crystallites (Fig. 2).

Electrical Properties
The positive values of the Seebeck coefficient confirmed the ptype conductivity in Mg-doped CuCrO 2 thin films annealed by light irradiation.Hall effect measurements of a 2% Mg doped thin film grown on a quartz substrate and annealed by light at 650 °C during 5 min yielded the following values of hole carrier density p = 1.5 × 10 18 cm −3 and a mobility μ = 0.14 cm 2 V −1 s −1 at room temperature.These values are close to results reported in a previous publication for samples synthesized by conventional thermal processing. 14In light annealed thin films with 5% Mg doping, the room temperature conductivity measured by the linear four point probe was 1.1 S cm −1 , which compares well with the values reported in the literature for similarly doped thin films. 12,14 for the present example, the savings achieved by using PSSR instead of low temperature annealing were considerable: instead of an energy requirement of 500 W × 4 d (48000 Wh), RTP needed only 4000 W for 3 min (200 Wh), leading to a factor of 240 in energy saving and a factor of 1900 in saving processing time.

Conclusions
Single phase p-type undoped and Mg-doped CuCrO 2 thin films were successfully obtained by spray pyrolysis deposition followed by the PSSR process with halogen lamp irradiation in inert atmosphere.It was shown that the light irradiation enabled the significant acceleration of delafossite crystal structure formation compared to conventional annealing.As a result, pure and Mg-doped CuCrO 2 films formed by solid state reaction between the starting Cu and Cr oxides nucleated on the substrate, upon annealing between 550 °C and 650 °C in Ar atmosphere for as short as 3-5 min.No softening of glass substrates was noticed during this time span and temperature range.
The positive sign of the measured Seebeck coefficient and the positive voltage of the Hall effect confirmed that these films were ptype semiconductors and that holes were the majority carriers.The room temperature conductivity, carrier density and hole mobility of Mg-doped samples were comparable to furnace annealed samples reported in the literature despite much smaller annealing time used in the present work.
Therefore, we expect that the light annealing by using a combination of UV and halogen lamps might further decrease synthesis time and temperature of transparent conducting delafossite thin films.

Figure 1 .
Figure 1.XRD patterns for (a) CuCrO 2 thin film on glass after annealing at 600 °C in N 2 atmosphere for 4 d, (b) CuCrO 2 :2%Mg film on fused silica after light irradiation at 650 °C for 5 min, (c) CuCrO 2 film on glass after light irradiation at 650 °C for 3 min, (d) CuCrO 2 :2%Mg film on glass after light irradiation at 550 °C for 10 min.All light irradiation synthesis (PSSR) was performed in Ar atmosphere.

Figure 2 .
Figure 2. SEM pictures of CuCrO 2 films annealed in inert atmosphere.(a) thin film on glass after annealing at 600 °C in N 2 atmosphere for 4 d, (b) PSSR annealing of sample on fused silica under Ar at 750 °C for 5 min.