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Optimization of Cuprous Oxides Thin Films to be used as Thermoelectric Touch Detectors Joana Figueira, Joana Loureiro, José Marques, Catarina Bianchi, Paulo Duarte, Mikko Ruoho, Ilkka Juhani Tittonen, and Isabel Ferreira ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12753 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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ACS Applied Materials & Interfaces

Optimization of Cuprous Oxides Thin Films to be used as Thermoelectric Touch Detectors Joana Figueira1, Joana Loureiro1,*, José Marques1, Catarina Bianchi1, Paulo Duarte1, Mikko Ruoho2, Ilkka Tittonen2 and Isabel Ferreira1 1

CENIMAT/I3N and UNINOVA, Departamento de Ciência dos Materiais, Faculdade de Ciências

e Tecnologia, Universidade NOVA de Lisboa, 2829-516, Portugal 2

Department of Micro and Nanosciences, Aalto University, P.O. Box 13500, FI-00076 Aalto,

Finland *

[email protected]

Keywords — Copper Oxide, Seebeck, Thermal Evaporation, Thin Films, Post-deposition Annealing

Abstract The electronic and optical properties of p-type copper oxides (CO) strongly depend on the production technique as it influences the obtained phases: cuprous oxide (Cu2O) or cupric oxide (CuO), the most common ones. Cu films deposited by thermal evaporation have been annealed in air atmosphere, with temperature between 225-375 °C and time between 1 to 4 hours. The resultant CO films have been studied in order to understand the influence of processing parameters in the thermoelectric, electrical, optical, morphological and structural properties. Films with a Cu2O single phase are formed when annealing at 225°C, while CuO single phase films can be obtained

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at 375 °C. In between, both phases are obtained in proportions that depend on the film thickness and annealing time. The positive sign of the Seebeck coefficient (S), measured at room temperature (RT), confirms the p-type behavior of both oxides, showing values up to 1.2 mV.°C-1 and conductivity up to 2.9 (Ω.m)-1. A simple detector using Cu2O have been fabricated and tested with fast finger touch events. Introduction The thermoelectric (TE) properties of metal oxides (MO) have gained relevance due to its abundance, facile synthesis and non-toxicity. N-type MO showing high electrical performances are very common (AZO 1, GZO 2, ITO 3, among others) but p-type MO are limited, mainly because of their low electrical conductivity, being difficult to achieve similar performances to n-type MO. Some of the most commonly studied p-type MO are compounds of copper, tin or nickel 4. Copper oxides (CO) have been extensively studied and applied in a wide range of electronic devices: solar cells, gas sensors, transistors, super-capacitors, biosensors and antimicrobial usage 5–11. However, a limited number of studies reveals thermoelectric properties and, most of them, just determine the Seebeck potential to identify the carriers type. The great interest on these oxides is in fact their p-type conduction 12, with values of resistivity as low as 0.01 .m, carrier concentration between 1014 - 1021 cm-3 and mobility between 1 and 90 cm2.V-1.s-1

13–18

. Different studies have been performed with these oxides and by changing the

thickness, doping or alloying it is possible to optimize their properties 12,19,20. The p-type conduction in Cu based oxides arises from its electronic structure in which the top of the valence band is composed of fully occupied Cu 3d states. When the energy level of the uppermost closed shell of the metallic cations (in this case d10 of the Cu+) is almost equivalent to


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those of the 2p levels of the oxide ions, the resultant hybridization can delocalize any holes of the valence band and a p-type conductive oxide is achieved 8,21 (Figure 1). In general, copper oxide (CO) occurs in two common phases known as cuprite (copper (I) oxide, Cu2O), of cubic structure, and tenorite (copper (II) oxide, CuO), of monoclinic structure. However, copper oxide can appear also as paramelaconite (Cu4O3) or Cu2O3 and Cu3O2 8,12. All these phases are greatly dependent of the production process. CO films can be obtained through several methods, like depositing thin films of metallic Cu by thermal evaporation and performing a postdeposition annealing to promote its oxidation 8,22 or depositing at the outset CuxO films through Atomic Layer Deposition

23

, Reactive Ion Plating

14

, rf-Plasma Enhanced Reactive Thermal

Evaporation 24, RF or DC Sputtering 16,17, which may, or not, require a post-deposition annealing step.

Figure 1. Schematic illustration of the chemical bond between an oxide ion and a cation that has a closed shell electronic configuration (like Cu+).



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Most of the TE applications require materials with high Seebeck coefficient (S) and high electrical conductivity (), maximizing the Power Factor, S2, while keeping the thermal conductivity low. Since the electrical conductivity of CO is expected to be low, even if high S values are achieved, the resultant PF value will be insufficient to make these materials suitable for TEG devices, when compared to other oxides. Nonetheless, there are other type of thermoelectric applications, where the PF is not so important, provided that the Seebeck coefficient is high, for example temperature sensors25. Since CO have been extensively applied in a wide range of electronic devices, in this work, CO thin films have been studied in order to evaluate their thermoelectric potential and possible application as touch detector. For that purpose, the influence of the fabrication conditions (annealing time and temperature and films thickness) on the CO films phase and their structural, morphologic, electro-optical and thermoelectric properties (both Seebeck coefficient and Power Factor) has been studied and the results are here presented.

Experimental Section Copper thin films with thicknesses between 30 and 160 nm have been deposited by thermal evaporation, on Corning glass substrates. The source material (Cu pellets from CERAC, 99.999% pure) was evaporated from a tungsten boat, without substrate heating, under pressure of 1x10-5 mbar. The post-annealing process was done in a furnace (Nabertherm L 2/11/B180), in air atmosphere, with a ramp of 5 °C.min-1 until it reaches the annealing target temperature, varying between 225 °C and 375 °C, and a holding time between 1 and 4 hours. The thicknesses of the films, as-deposited and after annealing, were measured in a KLA TENCOR D-600 Profilometer.


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The structural properties of the deposited films have been ascertained by X-ray diffraction (XRD), using a PANalytical X’Pert PRO with CuKα radiation at 45 kV and 40 mA, equipped with an X’Celerator. XRD patterns were collected with a scanning step of 0.03 over the angular range 2060, and total acquisition time of 10 min. The in-situ XRD annealing was performed under a heating rate of 5 °C.min-1, a plateau of 15 min and a pattern acquisition during the last 5 min, at each selected temperature (from room temperature to 325ºC). The Raman analysis was performed in a Labram 300 Jobin Yvon spectrometer, equipped with a solid state laser operating at 532 nm. The laser beam was focused with a 50× Olympus objective lens. The surface morphology was analysed using scanning electron microscopy (Zeiss Supra 40). The transmittance measurements were performed in a JASCO V-770 spectrophotometer, for wavelengths in the range of 190-2500 nm, with a step of 0.5 nm. The measurements of Seebeck coefficient and Power Factor (PF) were determined as described in previous studies1. The carrier Hall mobility, electrical conductivity and carrier concentration were measured with a Ecopia HMS-5300 measurement system, using van der Pauw contact geometry. The samples for Hall measurement were covered with Al contacts at the corners and the size of the rectangular samples was about 10×10 mm2. The electrical conductivity (calculated with the films thickness measured after annealing treatment) versus temperature was measured in samples with in-plane Al contacts (2mm apart), placed on a Peltier module to induce a temperature variation on the samples, reaching temperatures between 20-35 °C, with a step of 2.5 °C, using a custom-made software and a KEITHLEY 6487 Picoammeter/Voltage Source. To test the material as a thermal detector, the thermovoltage when a finger touches one of the metallic contacts was continuously monitored by a nano-voltmeter Agilent 34420A connected to a computer and a custom-made software.



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Results and Discussion In order to preview how the annealing temperature influences the CO phases formation, starting from metallic copper thin films, an in situ XRD annealing analysis was performed. This preliminary study provides a prediction of the phase formation relation with temperature, although the annealing kinetics was different from the one used in the subsequent study (where each sample undergoes an annealing step in an oven, at a single temperature). The resultant XRD patterns are shown in Figure 2 and the typical peaks of Cu, Cu2O and CuO phases are clearly visible. Since the literature reports a correlation between the films thickness and its oxidation kinetics, this analysis was done for a thinner sample, ~50nm, Figure 2-a), and a thicker sample, ~110nm, Figure 2-b).

2θ (º)

2θ (º)

Figure 2. X-ray diffraction patterns obtained with in-situ annealing and XRD measurement, from room temperature up to 325 °C, a) for a 50 nm thick Cu metallic film and b) for a 110 nm thick Cu metallic film.


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Through the diffractograms analysis, we quickly realize that the metallic copper phase is first converted to Cu2O, at lower temperatures, and later, with the increase in the annealing temperature, there is a transformation of the Cu2O phase into CuO. This sequence of oxides appearance was the expected one, since we begin with a metallic copper thin film and then, due time and temperature, the oxygen content starts to grow, wherein the last phase appearing, CuO, is the one with more oxygen in its structure. From this first test, it is also perceptible that the films thickness interferes with the oxides formation through annealing. The appearance of the Cu2O phase, as well as the metallic copper total consumption, occurs at lower annealing temperature and time for the thinner sample. Looking to the 175ºC spectrum, in Figure 2-a), there is no longer metallic copper, only Cu2O, whereas, in Figure 2-b), not all the metallic copper was yet transformed into oxide. This is also logic, since it is known that this type of oxidation process starts at the film surface and then the oxygen atoms have to diffuse through the films depth to continue the oxidation, which takes more time and/or more thermal energy for thicker films. Analysing now the 325ºC spectra of Figure 2, we can apply the same reasoning. In the thinner sample, there is only CuO because all the Cu 2O had already time and temperature to convert, whereas in the thicker sample, both phases are present. In conclusion, based on literature and our results, the CO phase formation kinetics depends not only on the annealing technique and temperature, but also on the annealing time and film thickness. In order to evaluate the optical, electrical and thermoelectrical properties of these oxide phases, Cu thin films with different as-deposited thicknesses were annealed in a furnace, under several annealing conditions (as described in the experimental section). Figure 3 shows the XRD measurements of Cu films with as-deposited thicknesses of dCu≈ 50 nm and dCu≈ 110 nm, subjected to different annealing conditions.



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2θ (º)

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2θ (º)

Figure 3. X-ray diffraction patterns of CO thin films: a) dCu~ 50 nm, tann= 1h, varying Tann; b) dCu~ 110 nm, tann= 1h, varying Tann; c) Tann= 225ºC, orange lines dCu ≈ 50 nm and green lines dCu ≈ 110 nm, varying the tann and d) Tann= 275ºC, orange lines dCu ≈ 50 nm and green lines dCu ≈ 110 nm, varying the tann.

Figure 3-a) and b) confirm that the Cu2O transformation to CuO occurs between 225 and 275 °C and is thickness dependent since, the higher the thickness, the higher the temperature and/or time to have the phase transformation. Figure 3-c) shows that despite the time (and for both thicknesses), when annealing at 225 °C a clear diffraction peak at 2 = 36.6° is visible, corresponding to Cu2O with a cubic structure and main difraction planes (111) (ICDD-01-078-


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2076). Films with 110 nm annealead only for 1 h still have Cu metallic present. On the other hand, Figure 3-d) shows that the annealing at 275°C is more thickness and time sensitive, for annealing times above 1 h, the Cu2O peak starts to vanish and two new peaks appear, (11-1) and (111) planes of CuO monoclinic structure, at 2 = 35.6° and 2 = 38.7°, respectively (ICDD-00-048-1548). This transformation is even more evident at 325 °C, however, in order to obtain films with single CuO phase (in the considered thickness range), 1h of annealing at 375 °C is needed (as predicted by the in situ annealing and XRD measurement). These results reflect the oxidation process of films evaporated from pure copper pellets, without substrate heating and with post-deposition annealing steps in atmospheric conditions and uncontrolled levels of oxygen or other gases. Those facts may explain differences between these annealing time/temperature dependence and others reported in literature 15,26. The crystallites sizes were calculated for the main peaks of each phase, by the Debye-Sherrer’s formula (eq. 1), where  is the wavelength of the X-ray,  is the full width at half maximum and  is the diffraction/Bragg’s angle. 𝐷=

0.9𝜆 𝛽𝑐𝑜𝑠𝜃

(1)

The crystallites size of Cu2O is around 10 nm, increasing with the growth of the CuO phase, up to 18.8 nm for single CuO films. The SEM images from Figure 4 reveal CuO samples with smoother surfaces while Cu2O samples show nano features and grains with 50-100 nm. As the crystallite size of samples are below 19 nm those grains/domains must be composed by several crystallites.



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dCu~ 50nm

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dCu~ 110nm

Cu2O

Cu2O

CuO

CuO

3h, 225ºC

1h, 375ºC

Figure 4. SEM images show the top surface morphology after the annealing step leading to pure Cu2O and CuO phase, for dCu~ 50nm and dCu~ 110nm.

In order to better evaluate the thickness influence on the oxidation process, the same annealing conditions were applied to films with different dCu and the phases were analyzed by XRD. Two annealing conditions were chosen, one with a long annealing time and lower temperature (4h, 225°C, Figure 5-a), where previously only Cu2O has been observed, and a second one with a short annealing time and higher temperature (1h, 325°C, Figure 5-b), which is expected to lead to mixed phase films. From Figure 5-a) and Figure 3-c) patterns, we can conclude that at 225 ºC there is no CuO present, despite the Cu film thickness and annealing time, the only thing that is affected is the speed of the Cu metallic film transformation to Cu2O. At 325ºC, Figure 5-b), depending on the thickness,


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different phases can be obtained: single CuO phase, mixed CuO and Cu2O phase, either with higher Cu2O content or higher CuO. No other CO phase has been observed by XRD.

2θ (º)

Figure 5. XRD patterns of CO films obtained from Cu films with different thicknesses, a) annealed 4 h at 225 °C and b) 1h at 325 °C. All XRD patterns have been normalized by the as-deposited fim thickness.



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The films’ thickness acts like a kinetic barrier to the annealing. Thicker films need higher temperature or time to be fully oxidized and thinner films are easily transformed from Cu to CuO (always passing through the intermediate Cu2O phase). The film thickness influence on the crystallites size has also been evaluated, however no significant changes have been found in samples with the same CO phase (less than 2 nm variation for samples in the 40 nm to 150 nm range). As the annealing step in air atmosphere promotes the oxidation of Cu films, it causes changes in the films structures, from the metallic Cu films (body centred cubic structure) to Cu2O (cubic structure) and CuO (monoclinic structure). For that reason the film thickness is expected to increase due to different unit cell volumes of the formed phases 8. Although this fact has already been mentioned in literature 8, a detailed study on the thickness variation has not been performed. After the different annealing processes, the film thicknesses were re-measured for the CO films and compared with the initial thickness of the Cu films, dCu. Overall, the thickness variation (d/dCu %) was above 100% for all films, being higher for the thinner films and for CuO phase transformation (reaching a maximum of 150% for dCu≈ 50 nm films annealed 1 hour at 325 °C). This behaviour is a consequence of the oxidation process from the surface towards the film depth, so, thinner films oxidize faster and the transformation temperature is lower. On the other hand, as the unit cell of Cu2O is smaller than CuO, the phase transformation to the CuO latter leads to an increase of the cell volume and therefore film thickness. It also important to emphasize that the CO films roughness is below 10 nm for all films, despite their initial dCu and CO phase. The electrical conductivity was, therefore, calculated using the CO films thickness. To further confirm the phases and components, Raman analysis was conducted in films were single Cu2O, single CuO and mixed phases films are expected (Figure 6).


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Figure 6. Raman spectra for CO films, corresponding to samples of dCu~ 50 nm annealed under different conditions.

From theory, a single peak in the Raman spectrum of Cu2O is expected, corresponding to the T2g mode (~505 cm-1). This mode is in fact observed, however, the experimental spectrum of the film annealed 4h at 225 ºC contains several extra peaks, at 107, 147, 216, 413 and 638 cm−1 (Figure 6). This apparent disagreement between theory and Raman experiments is a breakdown of the selection rules and has been ascribed to defects, non-stoichiometry and resonant excitation27. A clear difference between the first spectrum (4h, 225ºC) and the other spectra is visible and is justified by the Cu2O phase transformation to CuO, as expected from the previous analysis. Among the nine optical modes of CuO, three (Ag + 2Bg) are Raman-active and are visible in Figure 6 at 293, 342 and 628 cm-1. In conclusion, the Raman spectra confirm that there are no other CO phases than the ones shown in DRX diffractograms and, even if these two techniques can’t precise the CO films oxygen content



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exactly (stoichiometry), they confirm that we can obtain Cu2O single phase structure, as well as CuO single phase structure.

Electrical and thermoelectric properties The annealing time and temperature influence in electric and thermoelectrical properties is presented in Figure 7, for the same samples analysed in Figure 3. Figure 7-a), corresponding to the thinner samples, shows a decrease in both electric and thermoelectrical properties for annealing temperatures above 275 ºC, with  falling from 2.9 to 0.03 .m-1. At these high annealing temperatures, the samples become so resistive that their unstable thermoelectric potential do not allow a proper Seebeck coefficient measurement. In Figure 3-a), for 225 ºC and 275 ºC the films main phase is Cu2O, for 325 ºC is CuO and for 375ºC there is a single CuO phase so, it seems that the CuO phase affects negatively the electric properties. In the same way, analysing Figure 7-b) and comparing with Figure 3-b), there is a decrease in  and S values when CuO becomes the main phase (Tann > 325ºC).


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S σ

S, 50nm and 110nm σ, 50nm and 110nm

S, 50nm and 110nm σ, 50nm and 110nm

Annealing Temperature (ºC)

Annealing Time (h)

Figure 7. Electrical conductivity and Seebeck coefficient of CO films: a) dCu~ 50 nm, tann= 1h, varying Tann; b) dCu~ 110 nm, tann= 1h, varying Tann; c) Tann= 225ºC, orange lines dCu ≈ 50 nm and green lines dCu ≈ 110 nm, varying the tann and d) Tann= 275ºC, orange lines dCu ≈ 50 nm and green lines dCu ≈ 110 nm, varying the tann.

Looking at Figure 7-c), we can see that there is no relevant variation neither in electric conductivity nor Seebeck coefficient value with annealing time increment. This is consistent with the DRX analysis of Figure 3-c), which shows that for Tann = 225 ºC, independently of the annealing time or thickness, there is always a single phase of Cu2O. Similar electric and thermoelectrical properties are in agreement with similar diffractograms. Likewise, in Figure 7d) both properties are mainly constant, which may be because a single CuO phase is not obtained for these temperatures.


Electrical Conductivity (Ω.m)-1

S σ

Seebeck Coefficient (μV.ºC-1)

Electrical Conductivity (Ω.m)-1

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Seebeck Coefficient (μV.ºC-1)

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The optimized  value is obtained for dCu 50 nm, annealed 4 h at 225 ºC. As for the S values, it reaches a maximum value for a sample of dCu 110 nm, annealed 1 h at 325 ºC. The positive sign of the Seebeck coefficients confirms the p-type conductivity, both for Cu2O and CuO films, which is also supported by the Hall measurements. As the power factor combines electrical conductivity and Seebeck coefficient, the highest PF films are those annealed 4 hours at 225 °C (dCu 55nm): PF= 2.8 W.m-1.°C-2, and 1 hour at 325 °C (dCu 107 nm): PF= 2.7 W.m-1.°C-2. Figure 8 summarizes the obtained PF and indicates the corresponding CO phase, showing that Cu2O is the main phase of CO samples with optimized PF. These values are above the state of art for Cu2O thin films since the maximum value found in literature, at room temperature, is of the order of 0.4 W.m-1.°C-2

17

. For the two optimized

conditions, the crystallite size is below 12 nm. Most of the room temperature S values found in literature for Cu2O and CuO powders are in the range of 650 V.°C-1 28,12, where some dependence on film thicknesses is also mentioned12. Recently, in a study regarding the influence of the oxygen flux on the CO structure and TE properties of films deposited by magnetron sputtering, high S values, 900 V.°C-1, for Cu2O films with higher thicknesses (between 0.7 -1.3 m) have been reported. In that study, by tuning the oxygen flux and the holes concentration (up to 1021 cm-3) CuO films have reached PF of 2.2 W.m-1.°C-2 17. Although being state of art, the PF values for these CO thin films are still very low and even with small thermal conductivities (as expected for these thin films

29,30

) their figure of merit will be

extremely low, in the order of 10-5, not allowing their usage as energy harvesters. What make them interesting is their high Seebeck values at room temperature.


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Cu 2 O

Cu 2O+CuO

Cu 2O+CuO Cu 2 O Cu 2 O

Cu 2O

Cu 2 O+CuO

Cu 2 O+CuO CuO

1h 3h 4h 1h 3h 4h 1h 1h 225ºC 225ºC 225ºC 275ºC 275ºC 275ºC 325ºC 375ºC

Figure 8. Power Factor and correspondent oxide phases of CO films, for different annealing conditions, orange symbols corresponding to dCu ≈ 50 nm and green symbols corresponding to dCu ≈ 110 nm.

Hall effect measurements were carried out, although reliable Hall effect measurements are challenging for low mobility and highly resistive samples. This is a problem faced by many researchers in the characterization of p-type semiconductor oxides 8. It was only possible to obtain reliable results with the samples with highest electrical conductivities which have shown p-type carrier concentration of 4x1016 cm-3 and mobility up to 4.1 cm2.V-1.s-1. These values averaged 1020 acceptable measurements and agree with most of the values found in literature where 1016 cm3

is usually the carrier concentration range, while mobilities have been reported up to 90 cm2.V-

1 -1 18

.s

. The smaller mobilities in these films are probably due to a different structure, grains size

and defects. The activation energy (Ea), which corresponds to the acceptor levels, for both Cu2O and CuO samples, are between 0.24-0.38 eV, which also agrees with the literature values for these nonstoichiometric semiconductors in this low range of holes concentration 16,31.



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Optical properties The oxidation of Cu films leads to the formation of copper oxide films with different oxygen content, structure and phases 8,22,32 and consequently the optical properties such as transmittance, reflectance, refraction index, absorption coefficient and optical band gap (Eg) show a large range of values. For instance, optical gap from 1.9 eV to 3.0 eV for Cu2O and from 1.2 eV to 2.1 eV for CuO can be found in literature 8,12,13,15. The variety of methods/processes used to obtain CO films are in the origin of this large variation. Figure 9 shows the transmittance of the CO films referred in the XRD data of Figure 3-a) and b), zoomed between 200 and 1500 nm for a better comprehension of the spectra differences. We can observe the influence of annealing conditions in the transmittance of CO films with d Cu 50 nm and dCu 110 nm. In the infrared region (up to 2500 nm), the transmittance is significantly high (above 60%) independently of the annealing temperature and dCu. In the visible range, the CO samples with single Cu2O phase show much higher average transmittance than CuO, as expected from literature 12. The visible transmittance percentage (VT %) was calculated according to the JIS R3106 standard (eq. 2) 33, and is shown in the inset of Figure 9. 𝜏∨ = 𝛾

∑𝜆 𝐷𝜆 .𝑉𝜆 .𝜏(𝜆) ∑𝜆 𝐷𝜆 .𝑉𝜆

(2)

Dλ is the spectral distribution at CIE15-2004 standard illuminant D65 34 and Vλ is the CIE15:2004 standard photopic luminous efficiency function, which describes the average spectral sensitivity of human visual perception of brightness (standard data have been used from 400 to 700 nm, with a step of 5 nm). The calculated values of VT for the thinner Cu2O films (obtained after annealing 1 h, 225 ºC) reach 52% (dCu 50 nm) and 38% for thicker ones (dCu 110 nm), while CuO films (obtained after


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annealing 1 h, 375 ºC) have values below 27% and 4%, respectively. The decrease in the optical gap of the material, as the CuO films have a lower optical gap, contributes to this VT% difference.

Cu2O

CuO

Cu2O

CuO

Figure 9. Influence of the annealing temperature (during 1 h) in the transmittance of the CO films obtained from: a) Cu film thickness of 50 nm and b) Cu film thickness of 110 nm. The insets show the VT% values and photographs of the samples.

To estimate the optical band gap energy, Eg, a plot of (h)2 versus h was performed, assuming direct allowed transition (based on literature

8,15,23

). Other allowed transitions (direct forbidden,

indirect allowed and indirect forbidden) were also tested but the direct allowed (h)2 was the one giving the best correlation factor. Indeed, Cu2O films have the highest Eg values, 2.5 eV but, with



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the increase of the CuO phase, Eg values decrease to 2.2 eV. These are within the range of values found in literature

12,15,22

, however they depend on the different models used, e.g., α, α2, (αE)2,

(αE)2/3, and (αE)1/2 as well as the interpretation of the nature of the gap (i.e. direct or indirect) 35. The grain sizes and grain boundaries may cause light dispersion giving rise to small variation in the gap which explain the differences between results found in literature 36. Table 1 compares the results of our Cu2O and CuO thin films with the results found in literature, also for thin films (dCu< 500 nm), produced by different deposition techniques and annealing procedures. Not only the processes influence these results (as it influences the stoichiometry) but, as seen in this work, the as-deposited thickness of Cu is also very important, as it acts like a kinetic barrier during the annealing, and in Table 1 it varies between 50 nm and 1.3 m, therefore a direct comparison is limited. The electrical conductivity, carriers’ concentration and mobility of the films developed in this work are similar to most of the values reported in literature. In this work, the Cu2O conductivity results are slightly higher, most probably due to a higher nonstoichiometry of the CO films. Although CuO films (>10x thicker) with higher PF values (due to higher conductivity) have been reported, the S values are lower than the ones reported in this work.


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Table 1. Comparison of the Cu2O and CuO thin films properties (measured at room temperature) and deposition/annealing methods published in literature with the ones developed in this work

Thermal detector The developed Cu2O thin films exhibit high Seebeck coefficient, above 1mV.°C-1, therefore its high sensitivity is suitable for thermal detector applications. A touch thermal detector has been tested as a proof of concept. The electric response of Cu2O thin film to finger touch was tested using a sample with dCu≈ 60 nm, annealed 4 h at 225 °C (because it’s the less resistive film with the highest S), and two Al contacts in co-planar configuration separated 5 mm apart.



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The real-time voltage signal was measured using a home-made software and a computer connected to a voltmeter. Figure 10 shows the continuous response of the detector and the peaks correspond to the values obtained when a finger covered by a glove is touching instantly the region of the contacts. The fast fingertip touch events (t< 1s, repeated five times) show a rise time below 1s and reach peak values above 1 mV, corresponding to a thermal gradient of about 1 °C, and also a fast fall time (t< 1s). This means that these sensors can be used for fast/real-time human touch electrical trigger/detector using standard electronics (like Arduino) to read the output voltages 37.

0

1 1

0

Figure 10. Continuous voltage response of Cu2O thin film to human touch. Fast rise (0 to 1) and fall (1 to 0) times are observed with output voltages above 1 mV.

Conclusion CO thin films have been obtained through a simple fabrication method (thermal oxidation of Cu metallic films deposited by thermal evaporation) and depending on the as-deposited thickness and annealing conditions (time and temperature), single phases of Cu2O or CuO were obtained. The Cu films annealed at 225 °C show a single Cu2O phase, despite the annealing time, having a cubic


Page 23 of 28

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structure. Further increase of temperature to 375 °C led to Cu2O transformation into CuO monoclinic phase, even for the 110nm thick films. These transformations have been well documented by XRD patterns and Raman spectra. High Seebeck coefficient values (up to 1 mV.°C1

) have been obtained in the Cu2O films reaching a maximum PF of 2.8 W.m-1.°C-2.

All samples, despite having a CuO or Cu2O phase, have major p-type electrical conductivity, with band gap energies between 2.22 eV (for CuO) and 2.53 eV (for Cu2O) and acceptors levels that are about 0.3 eV above the valence band edge with EgCu2O > EgCuO in all samples. The obtained Cu2O films have transmittance in the visible wavelength range higher than CuO, reaching VT% above 50% for thinner Cu2O films while VT% of CuO is as low as 4% for the thicker film, making the Cu2O films suitable for applications where translucency is desired. The developed films do not have competitive PF values for energy harvesting applications at room temperature, although they are, to the best of our knowledge, state of art values for Cu2O thin films. Nevertheless, their high S values make them suitable for temperature detectors and to prove this concept, a simple detector using Cu2O have been fabricated and tested with fast touch events showing rise and fall times below 1 s with signal amplitude above 1 mV.

Acknowledgments This work is funded by H2020-ICT-2014-1, RIA, TransFlexTeg-645241 and by ERC-CoG-2014, ChapTherPV, 647596, and partially funded by FEDER funds through the COMPETE 2020 Program and National Funds through FCT - Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2013. This research was partly performed at the Micronova Nanofabrication Centre, supported by Aalto University. Paulo Duarte thanks FCT for his PhD grant SFRH/BD/113263/2015.



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