The effect of substrate temperature and oxygen partial pressure on the properties of nanocrystalline copper oxide thin films grown by pulsed laser deposition

The data presented in this paper are related to the research article entitled “Pulsed laser deposition of single phase n- and p-type Cu2O thin films with low resistivity” (S.F.U. Farhad et al., 2020) [1]. The detailed processing conditions of copper oxide thin films and a variety of characterization techniques used are described in the same ref. [1]https://doi.org/10.1016/j.matdes.2020.108848. Thin films need to grow on different substrates to elucidate various properties of the individual layer for attaining optimum processing conditions required for devising efficient optoelectronic junctions as well as thin film stacks for different sensing applications. This article describes the effect of substrate temperature and oxygen partial pressure on the structural, morphological, optical, and electrical properties of pulsed laser deposited (PLD) nanocrystalline copper oxide thin films on quartz glass, ITO, NaCl(100), Si(100), ZnO coated FTO substrates. The low temperature grown copper oxide and zinc oxide thin films by PLD were used for devising solid n-ZnO/p-Cu2O junction and investigated their photovoltaic and interface properties using dynamic photo-transient current measurement at zero bias voltage and TEM/EDX respectively. These datasets are made publicly available for enabling extended analyses and as a guide for further research.


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The data presented in this paper are related to the research article entitled "Pulsed laser deposition of single phase nand p-type Cu 2 O thin films with low resistivity" (S.F.U. Farhad et al., 2020) [1] . The detailed processing conditions of copper oxide thin films and a variety of characterization techniques used are described in the same ref. [1] https://doi. org/10.1016/j.matdes.2020.108848 . Thin films need to grow on different substrates to elucidate various properties of the individual layer for attaining optimum processing conditions required for devising efficient optoelectronic junctions as well as thin film stacks for different sensing applications. This article describes the effect of substrate temperature and oxygen partial pressure on the structural, morphological, optical, and electrical properties of pulsed laser deposited (PLD) nanocrystalline copper oxide thin films on quartz glass, ITO, NaCl(100), Si(100), ZnO coated FTO substrates. The low temperature grown copper oxide and zinc oxide thin films by PLD were used for devising solid n-ZnO/p-Cu 2 O junction and investigated their photovoltaic and interface properties using dynamic photo-transient current measurement at zero bias voltage and TEM/EDX respectively. These datasets are made publicly available for enabling extended analyses and as a guide for further research.
© 2020 The Author(s

Value of the Data
• Nanocrystalline copper oxide thin films were grown by PLD on amorphous, polycrystalline, and crystalline substrates at relatively low temperatures ( ≤ 300 °C) and wide range of oxygen partial pressures to attain thin films with tuneable structural, optical, and electrical properties. • Good quality copper oxide thin films attainable at low processing temperatures are desirable for realizing optoelectronic devices requiring low thermal budget. Researchers who are interested in Cu 2 O based thin film solar cell, p-channel thin film transistor (TFT), and different sensing devices can also be benefited from these data. In literature, most of Cu 2 O thin films grown by physical vapor deposition with processing temperature 500 °C and above to attain desired structural, optical and electrical properties. Such high processing temperature may not be suitable for practical applications and would be difficult to integrate the film stacks on conventional soda lime glass (SLG) and other technologically important substrates. Therefore, the methodology used to deposit and characterize the pristine and processed samples as well as the base values of physical properties attained through the optimization of wide parameter space can be used for comparison to data reported by others.
• The room-temperature grown Cu 2 O thin films showed rectification and a photovoltaic (PV) response while making a junction with ZnO previously grown by PLD at 300 °C. • The cross-sectional thin foils of the ZnO/Cu 2 O interface was made by a focused ion beam (FIB) assisted FE-SEM using the in situ liftout technique and investigated by TEM and EDX. Table 1 summarizes the important PLD processing conditions (substrate temperature (T sub ), oxygen partial pressure (O 2pp ), and Laser energy per pulse (LP)) for the growth of single phase copper (I) oxide (Cu 2 O) thin film with controlling exposed crystal surfaces as well as type and level of conductivity. Fig. 1 shows the PLD setup with the arrangement of substrate holder to deposit copper oxide (and also zinc oxide) thin films simultaneously on two different substrates   . The estimated phase fraction [3] of copper oxide films based on XRD data are summarized in Table 2 [2] . No standard samples are used for quantification, rather the relative amounts of phase fraction were estimated using 'Inorganic Crystal Structures Database (ICSD)' patterns as the basis for phase identification. The variation of average crystallite domain size and lattice constant of nanocrystalline copper oxides thin films as a function of substrate temperature are shown in Fig. 2 a and 2 b. For Cu x O y phase, a defect structure of Cu 2 O [2] , lattice parameters calculated using both (111) (denoted by ) and (200) (denoted by ) orientation found to be higher than the bulk (see Fig. 2 b) and the estimated various strains were found to be ∼1% or more for films deposited at 100 °C ≤ T sub ≤ 300 °C. The Bi-axial strain-stress of as-grown PLD films on quartz substrate were calculated from ex-situ XRD data analyses using the following relations [3][4][5] : Elongation strain ( ε ⊥ ) perpendicular to the plane of thin film,

Data Description
where, a and a 0 ( ∼0.427 nm [3] ) are the lattice constants for thin film and bulk crystals respectively, C ij are the elastic stiffness of the material of interest, for Cu 2 O: C 11 ∼ 116.5 GPa and C 12 ∼105.3 GPa; the corresponding stress, σ , is related by Hooke's law [5] : ≈ 31.5 GPa. The in-plane strain (as well as stress) induced in the PLD grown copper oxide films at various growth temperatures were calculated using equations (1) -(4) and summarized in Table 3 . It is to be noted that XRD data were recorded at room temperature using Cu K α ( λ ≈ 1.5406 Ǻ) radiation. A step size of ∼0.025 0 with 18 s per step was used and during scanning, samples were rotated to homogenize the measurements [1] .
The electrical and optical properties of the deposited thin films on quartz substrate can be found in ref. [1] . The room temperature Photoluminescence and Raman spectra of copper oxide thin films grown at T sub ≈ 25 °C -300 °C onto quartz substrate with a constant laser pulse energy (LP ≈ 25 ±4 mJ) and O 2pp ≈ 3 mTorr are shown in Fig. 4 a and 4 b respectively. Table 3 Bi-axial strain-stress related calculations for cubic crystals of (111) and (200)     The XRD patterns of room temperature grown Cu 2 O films on amorphous, polycrystalline and crystalline substrates can be found in the supplementary material of ref. [1] . The FE-SEM investigated surface morphologies of Copper-Oxide and Zinc Oxide thin films grown on different substrates at various deposition and processing conditions are presented respectively in Fig. 5 and Fig. 6 below.

Low temperature PLD grown ZnO/Cu 2 O based solar cell
A set of four thin film solid heterojunctions were fabricated on commercially available FTO coated glass substrates by successive deposition of ZnO layer (T sub ≈ 300 °C) followed by a Cu 2 O thin layer (T sub ≈ 25 °C). Prior to the deposition of Cu 2 O, PLD grown ZnO layer was subject to anneal at 366 °C inside the PLD chamber (Vacuum < 10 −6 mBar) for 40 min. (The vacuum of PLD chamber was interrupted in order to change ablation target). Six circular gold (Au) pads ( ∼100 nm thick and 2 mm dia, 2 mm distance apart, see the picture in the Fig. 7 a) were de-posited by a thermal evaporator under high vacuum ( < 10 −6 mTorr) through a patterned shadow mask to make good ohmic contacts. Therefore, the final device has an architecture that comprised the Au/FTO/ZnO/Cu 2 O/Au thin film stacks. The electrical connection of the p-and n-type metal oxide sides were made to the source meter (e.g., Potentiostat) probes via Au coated springloaded pins (tip dia ∼1 mm) to avoid scratching film surface and/or short circuiting between the metal oxide layers. A typical device structure based on Cu 2 O/ZnO heterojunction and its electrical contact with the potentiostat is shown in Fig. 7 b. Current-Voltage and Transient current measurements were performed in the Dark, and under 528 nm LED illumination trough the FTO side (see Fig. 7 b and 7 c). An IVIUM-CompactStat potentiostat was used as a source measure unit (SMU). The working electrode (WE) and short-circuited counter electrode/reference electrode (CE/RE) of the potentiostat were treated as positive and negative end of a voltage source meter respectively [3] . Fig. 7 a shows a photograph of a typical PLD grown solar cell with Au-FTO/ZnO/Cu 2 O/Au thin film stacks. The Au pads on FTO and Cu 2 O layers are numbered to assist the reader. Two adjucent gold (Au ∼2 mm dia, 100 nm thick) pads in the same layer were used for measuring contact behaviour of FTO (1 and 2 on white part in 7a) and p-Cu 2 O (yellow part in 7a) layer with Au (see supplimentary materials of ref. [1] for details).
A schematic of the J-V curve measurement setup is shown in Fig. 7 b. In Fig. 7  2 eV (564 nm)) layer) by a pulse width of 20 s exhibited very low and noisy photocurrent yet distinguishable from the dark current (see Fig. 7 d). The photo responses of the cell were seen to degrade over time (see scan 2, scan 3, and scan 4 in Fig. 7 d).

Morphology, structure, and chemical composition of the ZnO/ Cu 2 O interface
The interface quality between ZnO and Cu 2 O layer of the PLD grown n-ZnO/p-Cu 2 O stack was also investigated by SEM and TEM. The SEM micrograph of the FIB assisted cross-sectional Cu 2 O/ZnO specimen (see Fig. 8 a) revealed a very thin but continuous Cu 2 O ( ∼53 nm) and ZnO ( ∼114 nm) layer across the specimen (see Fig. 8

Experimental Design, Materials and Methods
All experimental design, materials, and methods were based on reported paper [1] .

Target materials
The target material in PLD was commercially available sintered ceramic Cu 2 O (purity ∼99.95%). Although, single crystals are more preferable in the case of choosing target material, however, for many materials, they are difficult to obtain. The majority of the reported literature on copper oxide thin film deposition used CuO ceramic target, although Cu 2 O as a target material also reported by few researchers. We prefered Cu 2 O over CuO as a target material for the following reasons: Cu 2 O has higher absorption coefficient, lower thermal expansion coefficient, lower boiling point, and lower melting point than that of CuO (see Table 4 below). Target-material having high thermal expansion coefficient and high melting point has been reported to have high probability of 'exfoliation' during laser ablation due to the fact that "the thermal oscillations induced by repeated laser excitation do not exceed melting point" of the target-material ( [3] and refs. therein). For ZnO and Al-doped ZnO (AZO) thin films, ceramic tragets with purity ∼99.999% and ∼99.999% (composed of 99 wt% ZnO and 1 wt% Al 2 O 3 ) were used respectively.

Laser wavelength and processing parameters of the PLD setup
Bulk of the reported literature on copper oxide thin film deposition is focused on the use of KrF: λ = 248 nm excimer laser whereas use of the visible lasers (e.g., Nd: YAG laser operated at λ = 532 nm) is really scarce ( [3] and refs. therein). This is due to the fact that thin film deposition using a longer wavelength ( λ) laser is found to generate more droplets than a shorter wavelength laser. Furthermore, droplet density on the surface of the deposited films has been reported to be reduced as the optical absorption coefficient ( α) increases. And as α increases with Table 4 Crystallographic and physical properties of Cu 2 O and CuO ([ 3 ,[6][7][8][9][10][11][12] [11] decreasing λ, therefore use of UV lasers (e.g., ArF: λ = 193 nm) generally encourages smoother film morphologies. Therefore, following laser and processing conditions are chosen for depositing copper oxide thin films: UV-ArF Excimer Laser ( λ: 193 nm), repetition rate: 10 Hz, pulsed width: 20 ns; typical pulse energy: ∼25-30 mJ which, when focused onto the target, produced a laser fluence (LF) of ∼1.5 J/cm 2 -2.0 J/cm 2 . The oxygen partial pressure (O 2pp) and substrate temperature (T sub ) were varied to allow film growth in a stable regime, where no decomposition of the films is observed. The same PLD setup and operation conditions but with a fixed O 2pp = 10 mTorr; LF ∼2.0 J/cm 2 were used for depositing both ZnO and AZO thin films on various substrates.

Credit Author Statement
Syed Farid Uddin Farhad : Conceptualization, Investigation, Data curation, Writing, editing and reviewing the manuscript.

Declaration of Competing Interest
The author(s) declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.
Bristol, UK. The author also acknowledges the experimental support of the Energy Conversion and Storage Research (ECSR) section, Industrial Physics Division (IPD), BCSIR Labs, Dhaka 1205, Bangladesh Council of Scientific and Industrial Research (BCSIR), Bangladesh.