Optical, electrical and surface properties of bare and Al-doped nanocrystalline ZnO (AZO) thin films grown by pulsed laser deposition (PLD) on commercial interdigital electrodes (IDE)

Physica Scripta PAPER • OPEN ACCESS Optical, electrical and surface properties of bare and Al-doped nanocrystalline ZnO (AZO) thin films grown by pulsed laser deposition (PLD) on commercial interdigital electrodes (IDE) To cite this article: Zdeněk Remeš et al 2025 Phys. Scr. 100 115910 View the article online for updates and enhancements. You may also like Advancing Al-doped ZnO thin films structural, optical and electrical properties of low temperature PET substrates via flash lamp annealing Juwon Lee, Chang-Hyeon Jo, Gwangseop Lee et al. - Ultraviolet and visible photo-response of transparent conductive Al-doped ZnO (AZO)/n-Silicon isotype heterojunction device Betül Ceviz akar, Zeynep Orhan, Fatma Yldrm et al. - Improvement in n-ZnO/p-Si diode properties using ZnO/AZO homogeneous metal contact Jin Un An, Ho Jin Yun, Kwang Seok Jeong et al. - This content was downloaded from IP address 147.32.209.98 on 07/11/2025 at 09:44 Phys. Scr. 100 (2025)115910 https://doi.org/10.1088/1402-4896/ae1897 PAPER Optical, electrical and surface properties of bare and Al-doped nanocrystalline ZnO (AZO) thin films grown by pulsed laser deposition (PLD) on commercial interdigital electrodes (IDE) ZdeněkRemeš 1,∗ aa,ŠtěpánRemeš 1 ,NedaNeykova 2 aa, Naini Jain 2 aa, Rupendra Kumar Sharma 2 aa, JakubHolovský 2 aa,EgorUkraintsev 2 aa,JaroslavKuliček 2 aa, Bohuslav Rezek 2 aa andHuaShuHsu 3 aa 1 Institute of Physics of the Czech Academy of Sciences, Na Slovance 2,182 00, Prague, Czech Republic 2 Faculty for Electrical Engineering, Czech Technical University in Prague, Technická2, 166 27 Prague, Czech Republic 3 National Pingtung University, Department of Applied Physics, No. 4-18, Minsheng Road, Pingtung, Taiwan ∗ Author to whom any correspondence should be addressed. E-mail: [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 (AZO)thin films were deposited by pulsed laser deposition (PLD)on commercial gold-based interdigitated electrodes (IDE)on glass substrates. 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. Longer deposition resulted in the growth of larger crystals and thicker ZnO layers, while Al doping only slightly affected grain size and reduced growth rate. Optical and optoelectronic properties wereinvestigated 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. 1. Introduction Low cost, non-toxicity, wide band gap (~3.37 eV)and high exciton binding energy (~60 meV)make ZnO an attractive material for a broad range of applications includingoptoelectronics, 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, Gd, Cr, In, Sn and others—has been explored to achieve conductive ZnO films [5–8]. 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 [9]. Consequently, Al-doped ZnO (AZO)films have emerged as promising, low-cost alternatives to indium tin oxide in photovoltaic and transparent conductive applications [10,11]. ZnO thin films are also employed in acetone sensors used for environmental monitoring, industrial air quality control, and medicaldiagnostics [12–14]as well as microscale sensing platforms requiring high surface sensitivity and reproducibility [15]. 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 [16]. At low temperatures(~4 K), excitonic and defect-related emission bands were observed at ~365 nm and ~653nm, with a surface-related blue PL band appearing in some samples [17,18]. This blue emission disappeared after aaaaaaaaaaaaa OPEN ACCESS RECEIVED 16 June 2025 REVISED 26 August 2025 ACCEPTED FOR PUBLICATION 28 October 2025 PUBLISHED 6November2025 Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s)and the title of the work, journal citation and DOI. © 2025 The Author(s). Published by IOP Publishing Ltd annealing at 500 °C in air [19]. 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. In this work, we extend our investigation to ZnO films deposited directly on commercial gold-based interdigitated electrodes (IDE). We integrate correlative microscopy (AFM and SEM), photothermaldeflection spectroscopy (PDS), and dual-beam photocurrent spectroscopy (PCS)to correlate surface morphology with optoelectronic performance. This study aims to provide insights into trap-induced transport phenomena, particularly space-charge-limited current and persistent photoconductivityin undoped ZnO and AZO films.The commercially available IDE significantlysimplify sample preparations avoiding cornerstone post-processing (photolithography)by providing high quality samples, both doped as well as undoped thin conductive oxides (TCO)thin films. 2. Methods 2.1. Pulsed laser deposition (PLD) Polycrystalline ZnO and 1 wt% AZO layers were deposited on commercially available gold-based Interdigitated ElectrodesIDE1-Au—10/10 μm(90 pairs of electrodes width 10 μm, distance 10 μm, Ti/Au layer thicknesses 50 nm and 150 nm)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% Al-doped 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 cm −2 , 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 focused onto a spot area of 2mm 2 and operated at a repetition rate of 20 Hz. The number of pulses varied between 20,000 and100,000. During deposition, oxygen was introduced at a process pressure of 10 Pa and aflow rate of 10 sccm, see table 1. 2.2. Microscopy 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 [20,21], designed for the visualization and analysis of data from scanning probe microscopy techniques [22,23]. The surface properties were also studied by (AFM)using Dimension ICON AFM (BrukerCorporation, 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 ×1μm 2 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 μm 2 spots were studied on each samples too. 2.3. Photothermal deflection (PDS)and photocurrent (PCS)spectroscopy The transmittance, reflectance, absorptance and photocurrent spectra were measured simultaneously in the 230–1400 nm spectral range by photothermal deflection spectroscopy (PDS)setup with 150 W Xe lamp SpectraPro-150 monochromator (150-mm focal length, f/4-aperture), filter wheel and mechanical chopper, Table 1. Summary of nanocrystalline ZnO and AZO 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 (R). Sample Target Pulses d(nm)nα(cm −1 )R(Ω) A ZnO 20 000 85 1.92 <10 2 ×10 6 B ZnO & 1 wt % Al 40 000 77 1.80 730 50 C ZnO 100 000 300 1.92 <3 6 ×10 6 2 Phys. Scr. 100 (2025)115910 Z Remeš et al see figure 1. Monochromator was equipped with two gratings: a UV holographic (1200 /mm)and a ruled (600/mm)blazed at 500 nm. Gratings were changedat 375 nm. The spectral resolution was 5 nm with the UV grating and 10 nm with the ruled grating when using 1 mm wide input and output slits. The filter wheel contained eSource Optics 25250FBB (230 −270 nm), Thorlabs FGUV11-UV(Schott UG 11, 270 −375 nm), Thorlabs FGB37-A (BG40, 375 −580 nm), combined EdmundOptics #66-052 & #19-321(Schott OG560 & KG2, 580 −780 nm)and Schott RG780 (780 −1400nm)filters. The monochromator output slit image 1×4mm 2 was rotated by 90°from vertical to horizontal direction by flat Al mirror attached to 90°off-axis focusing mirror M1 to be parallel with red laser beam (not shown in figure 1). Samples were immersed into liquid(Florinert FC72)and the relative temperature of the illuminated sample was measured via deflection of probe laser beam. The PDS was spectrally calibrated using black carbon sample. Figure 2shows the dual beam photocurrent (PCS)setup using the same monochromator as shown in figure1. UV LED 365 nm (5 mW)operating in a continuous mode was used as a secondary light source. The voltage bias (U)was provided by Keithley 6487 Picoammeter & Voltage Source that was also used to monitor the dc current and to provide ac current amplification for a second lock-in amplifier (L2)via analog voltage output of Keithley 6487 proportional to measured current. 3. Results The choice of laser pulses and their fluence is responsible for optimizing the thickness of deposited layers by PLD. For the present analysis, we wanted to achieve a layerthickness of about 80 −100 nm for both Al-doped and standard ZnO targets. After comprehensive optimization, we achieved 85 nm thick layer for ZnO and 77 nm for Al-doped ZnO targets at 20 000 and 40 000 pulses, respectively. Furthermore, we prepareda 300 nm Figure 1. Photothermal deflection (PDS)setup: light source (LS), monochromator (MCH), filter wheel (FW), mechanical chopper (CH), 90°off-axis focusing mirror (M1), spherical focusing mirror (M2), beamsplitter (BS), combined UV-NIR detectors (D1 −D3), lock-in amplifiers (L1 −L4), probe beam (LASER), position detector (P), cuvette (C), sample (S). Figure 2. Photocurrent (PCS)setup: light source (LS), monochromator (MCH), filter wheel (FW), mechanical chopper (CH),90° off-axis focusing mirrors (M1, M2), beamsplitter (BS), detector (D1), long pass filter (LP), LED, lock-in amplifiers (L1, L2), current amplifier (CA), picoammeter & voltage source (IV). 3 Phys. Scr. 100 (2025)115910 Z Remeš et al thick undoped layer to analyze the effect of their grain size, which was achieved at 100 000 pulses while keeping the same fluence. Two 1 ×1μm 2 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 both the gold and glass parts. Two 50 ×50 μm 2 spots were also studiedon each sample. The morphologies ofthe three ZnO films arepresented in figures 3(a)–(c). Surface RMS roughness values are 4.8 nm, 16.6 nm and 6.5 nm respectively. Growth of a thicker ZnO film (sample C)led to the formation of larger ZnO crystals. Al doping of ZnO film affected the grain size only slightly. Comparison between ZnO films grown on glass and gold parts of MicruX substrates and analysis of 50 ×50 μm 2 images did not reveal any changes in morphology or film thickness. Figure 3(d)shows homogeneous coverage of the MicruX substrate with ZnO film (sample C).Figure 3(e)shows the CPEM (AFMin-SEM)image of the gold-glass border of the 10 μm MicruX substrate, covered with undoped ZnO film (sampleA). 3D morphology corresponds to AFM morphology, while the colors correspond to SE intensity— brighter over gold due to better conductivity compared to the darker glass. As expected, the electric resistivity of the Al-doped layer (sampleB)was much lower than that of undoped layers (samples A and C), see table 1. Moreover, the dark volt-ampere characteristic of undoped ZnO is nonohmic at low voltages and follows the Mott-Gurney law, see figure 4. The law describes space-charge-limited current(SCLC)in insulators or semiconductors when carrier injection from electrodes dominates [24]. Under UV illumination, the undoped ZnO became about three orders of magnitude more conductive, but the photoresponse was very slow, see figure 5. Since high dark current in AZO films prevents photoconductivity measurements, only the photocurrent spectra of undoped samples were measured. Figure 6shows the transmittance (T), reflectance (R), absorptance (A), and photocurrent (PC)spectra of the undoped ZnO layer (sample C). Absorptance was measured using PDS and scaled with 1−R−T. Reflectance interferometry was used to 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 topographymap of the AZO film(sample B); Zscale =50 nm; (d)AFM topography map of a 300 nm thick undoped ZnO film, height difference between ZnO film on gold and ZnO film on glass is 245 nm, Zscale =400 nm; (e)CPEM image of a 100 nm thick undoped ZnO film, height difference between ZnO film on gold and ZnO film on glass is 220 nm. 4 Phys. Scr. 100 (2025)115910 Z Remeš et al determine the index of refraction and film thickness [16,25–27]. Once the index of refraction and the film thickness were known, the optical absorption coefficient αof thin films was evaluated from PDS spectra independently, see table. 4. Discussions AZO is a degenerate TCO with electrical resitivity (ρ)about 10 −4 Ω·cm corresponding to sheet resitance ρ/d about 10 Ωsq −1 for thin film thicness d=100 nm [28]In our AZO film grown at room temperature in oxygen from a 1 wt% Al target on IDE substrate exhibits electrical resistance R≈50 Ωand near infrared free carrrier absorption coefficient at 1 eV about 730 cm −1 (table 1). By contrast, the resistivity of undoped ZnO films is 5–6 orders of magnitude higher showing no measurable free carrier optical absorption in near infrared spectra region, which is consistent with (i)room-temperature growth, (ii)low residual Al content in undoped samples (excluding possible contamination of the PLD chamber),(iii)high oxygen partial pressure (which drastically increasesρand suppresses electrically active Al), These process-condition trends are well documented for PLDAZO. 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 a feedback where the electric field drives the current, which in turn sets up the field. This regime is called the space-charge-limited current[29]. The spacecharge effect is common in nominally undoped or lightly doped semiconductors, and it can occur outside the Figure 4. Volt-ampere characteristics of the undoped ZnO film (sample A)measured in dc mode in dark and under UV illumination (LED364, 5 mW). Figure 5. Time decay of photocurrent (sample A)measured in dc mode at voltage 1 V after UV light was switched on/off at 20 s. 5 Phys. Scr. 100 (2025)115910 Z Remeš et al 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, see figure 5. In materials exhibiting persistent conductivity, charge carriersare thermally released from trapslong after the light sourceis removed [19,30]. The persistence of photocurrent in ZnO is related to oxygen vacancies [31]. 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 h. PCS was normalized in figure 6on PDS at 3.1 eV. The PCS decreases above the optical absorption edge at about 3.3 eV because UV light was absorbed at the surface and therefore did not penetrate to IDE. Thus, the photoexcited carriers at the sample surface could not contribute to the photocurrent. The photocurrent was also lower than the optical absorption below 3 eV. This indicates that the states 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 [32]. 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. 5. Conclusions This work demonstrates the effectiveness of commercial IDE substrates for electrical and optoelectronic testing of nanocrystalline ZnO andAZO layers fabricated by PLD. Themethod allows for cost-effective and straightforward integration of test structures outside of cleanroom environments. PLD allows fine control over film thickness, composition, and stoichiometry—parameters critical for incorporating ZnO into microelectronic devices. Simultaneously, advancements in 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. Morphological analysis confirms that increasing film thickness leads to larger ZnO grains, while Al doping has only a subtle effect on grain size. Electrical measurements of undoped ZnO reveal space-charge-limited conduction and significant persistent photoconductivity, which we attribute to oxygen vacancy-related traps. The photocurrent response further suggests that the mobility edge in polycrystalline ZnO lies approximately 0.3 eV below the optical band gap. These insights into defect-driven charge transport mechanisms provide a foundation for tailoring ZnO-based thin films for sensor and optoelectronic applications. Figure 6. The transmittance (T), reflectance (R), absorptance (A)and ac photocurrent (PC)spectra of the undoped ZnO layer (sample C). PC spectra were normalized on Aspectra at 3.1 eV. 6 Phys. Scr. 100 (2025)115910 Z Remeš et al Acknowledgments The work was supported by the Czech Science Foundation project 24-10607J and by the National Science and Technology Council, Taiwan under Contract No. NSTC 113-2923-M-153 -001 -MY3. The infrastructure was supported by Operational Programme Johannes Amos Comenius financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (grants ECO&Stor—CZ.02.01.01/ 00/22_008/0004617 and LASCIMAT—CZ.02.01.01/00/23_020/0008525). Conflict of interests The authors declare no conflicts of interest. The funders had no role in the designof the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. Data availability statement The data that support the findings of this study are openly available at the following URL/DOI: https://doi. org/10.57680/asep.0636315 [33]. Data access The data are available in a public ASEP repository of the Czech Academy of Sciences accessed on June 13, 2025 (https://doi.org/10.57680/asep.0636315)and in repository Zenodo, accessed on May 13, 2025 (https://doi. org/10.5281/zenodo.15434235) Ethics statement Our research does not include studies on humansubjects, human data or tissue, or animals. Author contributions Zdeněk Remešaa 0000-0002-3512-9256 Conceptualization (lead), Data curation (equal), Formal analysis (equal), Funding acquisition (lead), Investigation (equal), Methodology (equal), Supervision (lead), Writing – original draft (lead) ŠtěpánRemeš Software (equal) NedaNeykovaaa 0000-0002-9992-4988 Conceptualization (equal), Methodology (equal), Writing – original draft (equal) NainiJainaa 0009-0008-0040-8950 Visualization (equal) Rupendra Kumar Sharmaaa 0000-0002-1058-7592 Data curation (equal) Jakub Holovskýaa 0000-0002-4222-6070 Conceptualization (equal) Egor Ukraintsevaa 0000-0002-9768-5119 Conceptualization (equal), Data curation (equal), Methodology (equal), Resources (equal), Visualization (equal) JaroslavKuličekaa 0000-0002-8600-7822 Data curation (equal), Investigation (equal) 7 Phys. Scr. 100 (2025)115910 Z Remeš et al Bohuslav Rezekaa 0000-0002-0378-4598 Conceptualization (equal), Supervision (lead) HuaShu Hsuaa 0000-0002-5773-2451 Conceptualization (equal), Supervision (equal) References [1]Klingshirn C F 2010 Zinc Oxide: From Fundamental Properties Towards Novel Applications (Springer) [2]Klingshirn C 2007 ZnO: material, physics and applications ChemPhysChem 8782–803 [3]Klingshirn C, Fallert J, Zhou H, Sartor J, Thiele C, Maier-Flaig F, Schneider D and Kalt H 2010 65 years of ZnO research - old and very recent results Phys. Stat. Sol. (b 247 1424–47 [4]Janotti A and Van de Walle C G 2009 Fundamentals ofzinc oxide as a semiconductor Rep. Prog. Phys. 72 126501 [5]Özgür Ü, Alivov Y I, Liu C, Teke A, Reshchikov M A, Doğan S, Avrutin V, Cho S-J and Morkoç H 2005 A comprehensive review of ZnO materials and devices J. Appl. 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