Preprint of "Optical, electrical and surface properties of bare and doped nanocrystalline ZnO thin films on interdigital electrodes "

Journal of Materials Science: Materials in Electronics Optical, electrical and surface properties of bare and Al-doped nanocrystalline ZnO thin films prepared by pulsed laser deposition on interdigital electrodes --Manuscript Draft-- Manuscript Number: Full Title: Optical, electrical and surface properties of bare and Al-doped nanocrystalline ZnO thin films prepared by pulsed laser deposition on interdigital electrodes Article Type: Original Research Keywords: ZnO thin films; photocurrent; pulsed laser deposition; interdigitated electrodes Corresponding Author: Neda Neykova Czech technical University in Prague: Ceske Vysoke Uceni Technicke v Praze Prague, CZECHIA Corresponding Author Secondary Information: Corresponding Author's Institution: Czech technical University in Prague: Ceske Vysoke Uceni Technicke v Praze Corresponding Author's Secondary Institution: First Author: Zdeněk Remeš First Author Secondary Information: Order of Authors: Zdeněk Remeš Neda Neykova Naini Jain Rupendra Kumar Sharma Jakub Holovský Egor Ukraintsev Jaroslav Kuliček Bohuslav Rezek Hua Shu Hsu Order of Authors Secondary Information: Funding Information: Grantová Agentura České Republiky (24-10607J) Dr. Zdeněk Remeš Ministerstvo Školství, Mládeže a Tělovýchovy (CZ.02.01.01/00/22_008/0004617) Dr. Jakub Holovský National Science and Technology Council (NSTC 113-2923-M-153 -001 -MY3) Prof. Hua Shu Hsu Abstract: Zinc oxide (ZnO) is a low-cost, environmentally friendly material with unique optical properties and a wide variety of nanoand microstructures, making it attractive for applications in energy conversion, scintillators, photocatalytic wastewater treatment, electrochemical energy storage, and sensing. In this work, nominally undoped and Aldoped nanocrystalline ZnO thin films were deposited by pulsed laser deposition (PLD) on commercial gold-based interdigitated electrodes (IDEs). Surface morphology and structure were characterized using Correlative Probe and Electron Microscopy (CPEM), combining Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to perform spatially correlated measurements. PLD resulted in the formation of larger crystals in thicker ZnO layers, while Al doping only slightly affected grain size. Optical and optoelectronic properties were investigated by photothermal deflection (PDS) and photocurrent spectroscopy (PCS). The mobility edge was found to lie approximately 0.3 eV below the band gap. Space-charge-limited currents were Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation observed at low voltages. Persistent photoconductivity indicates potential issues with charge transport. Additional Information: Question Response Journal of Materials Science: Materials in Electronics considers only outstanding papers that make a distinct contribution to the field of experimental electronic materials. This includes optoelectronic and photonic materials as well. Please explain in point form why your work is outstanding and what distinguishes this work from past work. What are the outstanding and exceptional contributions of this paper? How does it contribute to the state-of-the art? This study integrates pulsed laser deposition-grown bare and Al-doped ZnO films with commercial IDEs for low-cost device prototyping. Using correlative AFM-in-SEM (CPEM), it links morphology with electronic properties. A mobility edge ~0.3eV below the band gap is identified, revealing trap-related transport. Space-charge-limited conduction and persistent photoconductivity are observed. The work offers a comprehensive structure-property analysis, advancing ZnO-based optoelectronic applications. Is this manuscript part of an ongoing Special Issue (SI), and were you asked to submit to this Special Issue? No Were you asked by one of our editors to resubmit your manuscript because of textual overlap or not complying with scientific guidelines? No Does your manuscript comply with the journal's standards and instructions to authors on notation, significant figures, experimental errors and the use of the Scherrer equation? Instructions to authors can be accessed at “https://www.springer.com/journal/10854/s ubmissionguidelines#linksAndDownloads”. Manuscripts that do not comply with instructions to authors will be rejected. Yes How many of your own papers have you self-cited in this manuscript? (Your paper may be rejected if the Editor feels self-citations are excessive or not related to the work in the paper) 3 Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Optical, electrical and surface properties of bare and Al-doped nanocrystalline ZnO thin films prepared by pulsed laser deposition on interdigital electrodes Zdeněk Remeš1*, Neda Neykova1,2*, Naini Jain2, Rupendra Kumar Sharma2, Jakub Holovský1,2, Egor Ukraintsev2, Jaroslav Kuliček2, Bohuslav Rezek2, and Hua Shu Hsu3 1Institute of Physics of the Czech Academy of Sciences, Cukrovarnická 10, 162 00, Prague, Czech Republic 2Faculty for Electrical Engineering, Czech Technical University in Prague, Technická 2, 166 27 Prague, Czech Republic 3National Pingtung University, Department of Applied Physics, Taiwan *Corresponding authors: Z. Remeš, N. Neykova (e-mail: [email protected], [email protected] ) Keywords: ZnO thin films, photocurrent, pulsed laser deposition, interdigitated electrodes Abstract Zinc oxide (ZnO) is a low-cost, environmentally friendly material with unique optical properties and a wide variety of nanoand microstructures, making it attractive for applications in energy conversion, scintillators, photocatalytic wastewater treatment, electrochemical energy storage, and sensing. In this work, nominally undoped and Al-doped nanocrystalline ZnO thin films were deposited by pulsed laser deposition (PLD) on commercial gold-based interdigitated electrodes (IDEs). Surface morphology and structure were characterized using Correlative Probe and Electron Microscopy (CPEM), combining Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to perform spatially correlated measurements. PLD resulted in the formation of larger crystals in thicker ZnO layers, while Al doping only slightly affected grain size. Optical and optoelectronic properties were investigated by photothermal deflection (PDS) and photocurrent spectroscopy (PCS). The mobility edge was found to lie approximately 0.3 eV below the band gap. Space-charge-limited currents were observed at low voltages. Persistent photoconductivity indicates potential issues with charge transport. Introduction Zinc oxide (ZnO) is a well-established semiconductor material recognized for its low cost, nontoxicity, wide band gap (~3.37 eV) and high exciton binding energy (~60 meV). These properties make ZnO an attractive material for a broad range of applications including optoelectronics, sensors, transparent conductors, and photocatalysis [1–3]. The material's versatility is further enhanced by its tunability through doping and nano structuring, which allow for controlled manipulation of electrical conductivity and optical absorption [4]. A broad range of dopants—such as F, B, Al, Ga, In, and Sn—has been explored to achieve conductive ZnO films [5]. Among these, Al stands out as a cost-effective, abundant, and non-toxic option. Its small ionic radius enables effective substitution without significantly disrupting the ZnO lattice structure, thereby enhancing carrier concentration [6]. Consequently, Al-doped ZnO films have emerged as promising, low-cost alternatives to indium tin oxide in photovoltaic and transparent conductive applications [7,8]. ZnO thin films are also employed in acetone sensors used for environmental monitoring, industrial air quality control, and medical diagnostics [9– 11]. Click here to access/download;Manuscript;manuscript_ Remes_Neykova.pdf 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Pulsed laser deposition (PLD) has gained recognition in recent years as a precise method for fabricating nanocrystalline ZnO thin films. The technique allows fine control over film thickness, composition, and stoichiometry—parameters critical for incorporating ZnO into microelectronic devices. When ZnO is deposited on patterned structures such as interdigital electrodes (IDEs), it shows potential for microscale sensing platforms requiring high surface sensitivity and reproducibility[12]. Our earlier work focused on nanocrystalline ZnO films (<100 nm thick) deposited on fused silica by PLD, examining how Al doping influences optical absorbance and photoluminescence (PL) properties [13]. At low temperatures (~4 K), excitonic and defect-related emission bands were observed at ~365 nm and ~650 nm, with a surface-related blue PL band appearing in some samples [14,15]. This blue emission disappeared after annealing at 500 °C in air [16]. Hall effect measurements confirmed n-type conductivity, albeit with relatively low mobility, indicative of defect-related scattering. Additionally, persistent photoconductivity under UV illumination pointed to deep trap states influencing carrier transport—highlighting potential applications in UV detectors and transparent conductive oxides. Simultaneously, advancements in microscopy have enabled more comprehensive analysis of nanostructured materials. Correlative Probe and Electron Microscopy (CPEM)—which integrates AFM and SEM—permits concurrent topographic and electronic characterization of identical regions, making it invaluable for studies of heterogeneous thin films where surface features significantly impact functionality. In this study, we investigate nominally undoped and Al-doped ZnO thin films deposited via PLD on commercial gold-based IDEs. We employ PDS and PCS for optical and photoelectrical characterization and use AFM and CPEM to correlate morphology with electrical behavior. This integrated approach provides new insights into the interplay between structure and performance in doped ZnO films, supporting their development for next generation sensing and electronic devices. Table 1 Summary of nanocrystalline ZnO films deposited by PLD on commercial IDEs (10 µm electrode spacing), detailing target composition, number of laser pulses, film thickness (d), refractive index (n), absorption coefficient at 1 eV (α), and electrical resistivity (ρ). Sample target pulses d (nm) n  () A ZnO 20 000 85 1.92 2×106 B ZnO & 1 wt % Al 40 000 77 1.80 50 C ZnO 100 000 300 1.92 6×106 Experiment Polycrystalline ZnO and 1 wt.% Al-doped ZnO layers were deposited on commercially available gold-based Interdigitated Electrodes IDE1-Au – 10/10 µm (90 pairs of electrodes, electrode width 10 µm, electrode distance 10 µm) on a glass substrate (MicruX Fluidic, S.L., Gijón, Asturias) by pulsed laser deposition (PLD) setup from TSST B.V. equipped with KrF (λ = 248 nm) excimer laser COMPex 50. For the deposition, stoichiometric ZnO and 1 wt.% Aldoped ZnO targets were ablated using a laser pulse energy of approximately 24 mJ (measured inside the chamber), corresponding to a fluence of 1.2 J/cm2, respectively. The substrates were mounted on a rotating holder (5 rpm) and maintained at room temperature (25 °C) during deposition. The base pressure before the deposition was 10–4 Pa. The excimer laser beam was 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 focused onto a spot area of 2 mm2 and operated at a repetition rate of 20 Hz. The number of pulses varied between 20,000 and 100,000 to achieve different film thicknesses. During deposition, oxygen was introduced at a process pressure of 10 Pa and a flow rate of 10 cm3/min. Three polycrystalline ZnO layers were deposited, and their thickness, optical, and electrical properties are summarized in Table 1. The samples were studied by LiteScope AFM (NenoVision Ltd., Brno, Czechia) placed inside EVO 10 SEM (Carl Zeiss AG, Oberkochen, Germany) using nose type self-sensing Akiyama cantilever (NenoVision) to perform Correlative Probe and Electron Microscopy (CPEM), that integrates Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) to perform simultaneous correlated measurements of the same sample region for 3D visualization. SEM images were obtained with 10 kV voltage, 10 pA current and 9.6 mm working distance. CPEM image was created using MATLAB script for aligning and cropping and Gwyddion software[17,18], designed for the visualization and analysis of data from scanning probe microscopy techniques [19,20]. The surface properties were also studied by (AFM) using Dimension ICON AFM (Bruker Corporation, Billerica, MA, USA) with untreated Budget Sensors Multi75Al cantilever (Inovative Solutions Bulgaria Ltd., Sofia, Bulgaria) in Peak Force Quantitative Nanomechanical Mapping (PFQNM) mode. In PFQNM mode, the AFM probe taps the surface at a fixed frequency (much lower than conventional tapping mode) and measures the force– distance curve at every pixel. Unlike tapping mode, where resonance dynamics dominate, PFQNM directly controls the peak force applied during each tap. Two 1 × 1 µm2 spots were studied on each sample, one spot on the gold part and one spot on the glass part of the MicruX substrate. Two 50 × 50 µm2 spots were studied on each samples too. The transmittance, reflectance and absorptance spectra were measured simultaneously in the 300–1400 nm spectral range by photothermal deflection spectroscopy (PDS) setup with 150 W Xe lamp, SpectraPro-150 monochromator (150-mm focal length, f/4-aperture, slits 1/1mm) equipped with two gratings: a UV holographic (1200 /mm) and a ruled (600 /mm) blazed at 500 nm, see Figure 1. The spectral resolution was 5 nm with the UV grating and 10 nm with the ruled grating. Samples were immersed during PDS into liquid (Florinert FC72) to measure the relative temperature of the illuminated sample independently for selected photon energies using deflection of probe laser beam. The spectra were spectrally calibrated by measuring PDS of a black carbon sample. Figure 2 shows the dual beam photocurrent (DBP) setup. The monochromatic light was provided by Xe lamps and Acton Research SpectraPro-275 monochromator equipped with three gratings: 1 (600 gr/mm, blazed at 300 nm), #2 (300 gr/mm, blazed at 750 nm) and #3 (150 gr/mm, blazed at 1200 nm). The spectral resolution was 5 nm (10 nm, 20 nm) with the grating #1 (#2, #3). The monochromator covered the spectral range 260 – 1500 nm with the Xe lamp and 500 – 2400 nm with the halogen lamp used as a light source. The intensity of light was modulated by a mechanical chopper (CH) placed just behind the filter wheel (FW). FW contains the following band pass and long pass filters: eSource Optics 25270FBB (260−280 nm), Thorlabs FGUV11-UV (280−380 nm), Thorlabs FGB39 (380-560 nm), Thorlabs FGL550S & FGS550 (560−750 nm), FGL715 (750−1300 nm) and 3 mm thick Si wafer (1300−2400 nm). The light was collimated and focused by 90-degrees off-axis mirrors (focal length 150 mm). The light intensity is monitored via beamsplitter (BS) by Si photodiode used as an auxiliary detector (D) connected to the lock-in-amplifier (L1) referenced to the chopper frequency. UV 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 LED 365 nm (5mW) operating in a continuous mode was used as a secondary light source. The constant voltage bias (U) was provided by Keithley 6487 Picoammeter & Voltage Source that was used to monitor the dc current and to provide ac current amplification for a second lock-in amplifier (L2) via analog voltage output proportional to measured current. Results and discussion Two 1 × 1 µm2 AFM spots were studied on each sample, one spot on the gold part and one spot on the glass part of the MicruX substrate. AFM topography images were similar on gold and on glass part. Two 50 × 50 µm2 spots were studied on each samples too. The morphologies of three ZnO films are presented in Figure 3 a-c. Growth of a thicker ZnO film (sample C) led to formation of larger ZnO crystals. In these experiments Al doping of ZnO film affect the grain size only slightly. Comparison between ZnO films grown on glass and on gold parts of MicruX substrates and analysis of 50 × 50 µm2 images did not reveal any changes neither in morphology nor in thickness of ZnO films. Figure 3d shows the homogeneous coverage of MicruX substrate with ZnO film (sample C). Figure 3e shows the CPEM (AFM-in-SEM) image of the gold-glass border of 10 µm MicruX substrate, covered with the undoped ZnO film (sample A). 3D morphology corresponds to AFM morphology, while the colors correspond to SE intensity. Brighter colors above gold compared to darker colors above glass are caused by better conductivity of gold compared to (nonconductive) glass. As expected, the resistivity of Al-doped layer (sample B) is much lower than the resistivity of undoped layers (samples A and C), see Table 1. However, the dark volt-ampere characteristic of undoped ZnO is not ohmic at low voltages, but follows the Mott-Gurney law [21], see Figure 4. In the low-voltage regime, the injected carriers control space charge and the electric-field profile is dominated by the drift component of the injected carriers. This results in feedback where the electric field drives the current, which in turn sets up the field. This regime is called the space-charge-limited current [22]. The space-charge effect is common in nominally undoped or lightly doped semiconductors, and it can occur outside the depletion region. Under UV illumination, the undoped ZnO was about three orders more conductive. Thus, undoped ZnO was photosensitive, but the photo-response was slow due to a high concentration of electron traps, see Figure 5. In materials exhibiting persistent conductivity, charge carriers are thermally released from traps long after the light source is removed [16,23]. The persistence of photocurrent in ZnO is related to oxygen vacancies [24]. According to the theory, photoexcitation forms a doubly ionized oxygen vacancy from the neutral oxygen vacancy. The deionization of this vacancy involves a large lattice relaxation which accounts for the photocurrent. It should be noted that the photocurrent decay to original (dark) current value took significantly longer (several hours) then the saturation of the photocurrent that occurred within 1 hour. Since the high dark current prevents photoconductivity measurements, only the photocurrent spectra of the undoped samples were measurable. Figure 6 shows the transmittance (T), reflectance (R) and absorptance (A) spectra of the undoped ZnO layer (sample C) as well as its photocurrent spectra. Absorptance spectrum was measured using PDS and put into the absolute scale using 1-R-T spectrum. PC spectra were normalized on A spectra at 3.1 eV. Reflectance interferometry was used to evaluate thin film thickness from interferences in reflectance spectra [13,25,26], see Table 1. The PC spectrum decreases above the optical absorption edge at about 3.3 eV because UV light is absorbed at the surface and therefore does not penetrate to IDE. Thus, the photoexcited carriers at the sample surface cannot contribute to the photocurrent. The photocurrent is also lower than the optical absorption below 3 eV. This indicates that the states 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 below the optical absorption edge are defect-related localized states that do not contribute to the electrical conductivity. The mobility edge in semiconductors is defined as the energy level that separates localized states from extended (or delocalized) states in a disordered system [27]. Electrons are trapped in localized states—they cannot move freely through the material. These are typically found in the band tails caused by disorder and defects. We can conclude that the mobility edge is about 0.3 eV below the band gap of polycrystalline ZnO prepared by PLD. Conclusion Commercial metal-based IDE are two individually addressable strips of microelectrode arrays on a glass substrate designed for impedance, capacitance, and conductivity measurements. In this paper IDE substrates were used for photoconductivity testing of nanocrystalline ZnO layers prepared by PLD. The advantages of using commercial IDE include easy-to-use, time and cost effectiveness. Moreover, since the IDE can be easily covered by PLD, the electrical properties of the layer deposited on IDE can be measured outside clean rooms. In this paper we have shown that growth of a thicker ZnO film led to formation of larger ZnO crystals whereas Al doping of ZnO film affected the grain size only slightly. IDE substrates were used for photoconductivity testing of polycrystalline ZnO layers deposited on IDE by PLD. We have found that the mobility edge was about 0.3 eV below the band gap of polycrystalline ZnO prepared by PLD. Space-charge-limited currents were observed at low voltages. Persistent photoconductivity indicates potential issues with charge transport due to a high concentration of electron traps below the mobility edge. Author contributions ZR: Resources, Writing—original draft, Writing— review & editing. NN: Writing—review & editing. NJ: Writing—review & editing. RKS: Writing—review & editing. JH: Writing—review & editing. EU: Writing—review & editing. JK: Writing—review & editing. BR: Writing—review & editing. HSH: Writing—review & editing Funding The work was supported by the Czech Science Foundation project 24-10607J and by Czech Ministry of Education, Youth and Sports grant no. CZ.02.01.01/00/22_008/0004617 - “Energy conversion and storage”. This work was supported by the National Science and Technology Council, Taiwan, for financially supporting this research under Contract No. ：NSTC 113-2923-M-153 -001 -MY3. Data availability The data are available in a public ASEP repository of the Czech Academy of Sciences (https://doi.org/...) accessed on 2025. Conflicts of Interest The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. References 1. C. F. Klingshirn, editor , Zinc Oxide: From Fundamental Properties towards Novel Applications (Springer, 2010). 2. C. 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C: Solid State Phys. 20, 3075 (1987). 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Figures Figure 1 Photothermal deflection (PDS) setup: Xe lamp (Xe), monochromator (MCH), filter wheel (FW), mechanical chopper (CH), 90 off-axis focusing mirror (M1), spherical focusing mirror (M2), beamsplitter (BS), detectors (D1−D3), lock-in amplifiers (L1− L4), HeNe laser (LASER), position detector (P), sample (S) Figure 2 Dual beam photocurrent (DBP) setup: light source (LS), monochromator (MCH), filter wheel (FW), 90 off-axis focusing mirrors (M1, M2), mechanical chopper (CH), beamsplitter (BS), long pass filter (LP), LED, lock-in amplifiers (L1, L2), current amplifier (CA), picoammeter & voltage source (K6487). Figure 3. a) AFM topography map of the thinner undoped ZnO film (sample A); Zscale=50 nm; b) AFM topography map of the thicker undoped ZnO film (sample C); Zscale=100 nm; c) AFM topography map of the Al-doped ZnO film (sample B); Zscale=50 nm; d) AFM topography map of a 500 nm thick undoped ZnO film. Zscale=400 nm; e) CPEM image of a 100 nm thick undoped ZnO film. Figure 4 Volt-ampere characteristics of the undoped ZnO film (sample A) measured in dark and under UV illumination (LED364, 5mW). Figure 5 Time decay of photocurrent (sample A) measured at voltage 1 V after UV light was switched on/off at 20 s. Figure 6 The transmittance (T), reflectance (R), absorptance (A) and photocurrent (PC) spectra of the undoped ZnO layer (sample C). PC spectra were normalized on A spectra at 3.1 eV. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65