Au-modified ZnO thin films for higher-performance ultraviolet photodetectors

Abstract

ZnO-based ultraviolet (UV) photodetectors are widely studied due to their wide bandgap and higher photosensitivity. However, the ZnO thin-film UV photodetectors face higher dark current and limited gain despite their wide bandgap advantage. Here, Au overlayers (5–40 nm) were deposited on optimized 221 nm ZnO (prepared via sol–gel method) films using cost-effective spin-coating (ZnO film) and RF sputtering (Au overlayers). Au induces Schottky barriers (ϕ_Au = 5.47 eV > ϕ_ZnO = 4.45 eV) that suppress dark current down to 7.29 nA. Meanwhile, plasmonic Au nanoparticles generate localized surface plasmon resonance (LSPR) hot electrons that enhance UV absorption and photocurrent. The device with a 10 nm Au overlayer achieves an excellent photoresponse of 7.79 × 10³ and a photocurrent of 50 µA under ultra-low 24 µW cm⁻² UV illumination (365 nm). This performance surpasses literature values at lower UV intensity, also among the highest reported for ZnO detectors prepared using the sol–gel method. The optimized Au–ZnO devices also exhibit fast response (T_r = 15 s) and scalable fabrication routes. These results reveal the promise of Au/ZnO hybrid thin films for sensitive, lower-intensity UV detection with applications in flame sensing, wearable optoelectronics, and environmental monitoring.

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1. Introduction

Ultraviolet (UV) photodetection is vital for environmental monitoring, flame sensing, wearable electronics, and security applications due to the distinct signatures of UV radiation.^1–3 Among various semiconductor materials, ZnO has garnered significant interest as a UV photodetector candidate for its wide direct bandgap (∼3.37 eV), higher exciton binding energy, and chemical stability.⁴ However, ZnO-based devices still struggle with higher dark current and relatively lower photocurrent-to-dark current (I_on/I_off) ratios, limiting their practical sensitivity and energy efficiency.^5,6 Recent research on ZnO-based UV photodetectors has focused on attaining high I_on/I_off ratios and shorter rise and fall times (T_r and T_f).^7,8 These performance metrics can be substantially improved through the incorporation of metals either on the surface or within the bulk of the ZnO detection layer.^9–12 Metals can be deposited as nanostructured overlayers of controlled thickness and morphology or introduced into the ZnO lattice as dopants to tailor the defect profile. Incorporation of metals such as Mg,¹³ Al,¹⁴ Cu, Li,¹⁵ Ga,¹⁶ Sn, Cr, Te,¹⁷ Au,¹⁸ Pt,¹⁹ and Ti²⁰ in forms including dopants, nanoparticles, and overlayers has been reported to alter the structural, optical, and electrical properties of ZnO, thereby enhancing photodetector performance. For example, Ga doping has been employed to generate localized defect states at lattice discontinuities, improving response speed,²¹ while Cu and Li doping has been shown to introduce acceptor states via Cu⁺ ions and Li_i or Li_Zn–Li_i complexes, increasing film resistivity and reducing dark current.¹⁵

Noble metals such as Ag, Au, and Pt possess unique structural, optical, and electronic properties that make them attractive modifiers for ZnO.²² Their ionic radii Ag²⁺ (1.2 Å), Au²⁺ (0.9 Å), and Pt²⁺ (0.9 Å) are comparable to that of Zn²⁺ (0.74 Å),^21,26 enabling potential lattice incorporation. Their work functions ϕ_Au = 5.47 eV, ϕ_Ag = 4.74 eV, and ϕ_Pt = 5.93 eV are higher than that of ZnO (ϕ_ZnO = 4.45 eV),^27,28 facilitating Schottky barrier formation at the metal–ZnO interface, which is known to suppress dark current. Furthermore, in nanoparticulate form, these metals (Ag, Au, and Pt) exhibit localized surface plasmon resonance (LSPR), enabling strong light–matter interactions when their size is smaller than the incident wavelength.^29,30 Although such plasmonic properties have been extensively exploited in photovoltaic and photonic devices, their application to enhancing UV photon absorption in ZnO photodetectors remains limited.²³ Oxygen affinity further influences device performance; Pt is overpriced and Ag exhibits higher oxygen affinity, readily forming silver oxides that can increase dark current.³¹ Consequently, Au is identified as optimal choices due to their stability, lower oxygen reactivity, and ability to suppress dark current while enhancing photocurrent.²⁴ Thus, the integration of Au nanoparticles into the ZnO thin films improves performance by enhancing the photoinduced charge separation and reducing recombination losses, thereby boosting the photocurrent.^32,33 This modification leverages the favorable electron transfer dynamics between Au and ZnO and the plasmonic enhancement effects, which collectively allow for higher responsivity and detection ability in UV photodetectors.²⁵ Additionally, Au-modified ZnO devices show promise for flexible, wearable, and highly sensitive optoelectronic applications due to their improved stability and tunable spectral response.^33,34

Previous studies recognized that post-deposition annealing temperature and detection layer thickness critically affect the physical and electrical properties of sol–gel-derived ZnO thin films.³⁵ Optimizing a 221 nm ZnO film annealed at 575 °C tailored its structure for UV sensing. However, higher dark current and limited I_on/I_off ratio restrict practical applications. Incorporating metals, especially Au, has shown promise for suppressing dark current via Schottky barrier formation while enhancing photocurrent through plasmon–exciton interactions.³⁶ Unlike prior works focused on higher-intensity UV (>mW cm⁻²) with incomplete performance analyses, this study systematically evaluates Au–ZnO photodetectors under lower-intensity 24 µW cm⁻² UV.^37,38 The optimized devices determine ultra-lower dark current (∼nA), significantly enhanced photocurrent (mA scale) at 365 nm with 5 V bias, and the highest photoresponse gain (K) reported for sol–gel ZnO systems, outperforming literature benchmarks summarized in Table S1. The cost-effective sol–gel spin-coating combined with Au sputtering provides a scalable platform for higher-performance, lower-intensity UV detection. This work pioneers plasmonic Au overlayer optimization for ZnO UV photodetectors, evaluating physical, optical, electrical, and photoresponse properties systematically for the first time as per our nest knowledge.

2. Experimental details

2.1. Preparation of UV photodetector

ZnO and Au-overlayer/ZnO UV photodetectors were fabricated on Pt interdigital electrodes (IDEs) patterned on corning glass substrates using standard photolithography (Scheme 1). ZnO thin films were prepared through the spin-coating method using a ZnO solution prepared via sol–gel process. A 0.227 M zinc acetate dihydrate solution in methanol, stabilized with monoethanolamine (MEA) in a 1 : 1 molar ratio, was spin coated at 3000 rpm, followed by pyrolysis at 300 °C for 5 min after each coating. The films were subsequently annealed at 575 °C for 1 h to improve crystallinity. Before the spin coating; the corning glass substrates were cleaned by ultrasonic agitation in acetone, and deionized water for 10 minutes each to remove organic and particulate contaminants. This multistep cleaning ensures uniform film adhesion and reproducibility. Au overlayers of varying thicknesses (5–40 nm) were deposited by RF sputtering (sputtering rate ∼0.16 nm s⁻¹, 99.9% pure Au target, 0.0027 kPa Ar pressure, 10 W power, 4 cm target-substrate distance). Post deposition annealing was carried out at 400 °C for 2 h in oxygen. For patterned Au deposition, a mesh with 200 µm pores was used during sputtering. Structural and optical properties of the films were characterized, while electrical and UV photoresponse measurements were performed on devices fabricated on the corning glass substrates. The ZnO thickness was fixed at its optimized value of 221 nm for maximum efficiency.³⁵ The effect of Au overlayer thickness on photoresponse parameters (I_off, I_on, gain, rise time, and fall time) was systematically examined and correlated with the structural and optical results.

Scheme 1 Systematic presentation of process of preparation of UV photodetector.

2.2. Characterization techniques

The ZnO films were characterized for their physical and optical properties using Bruker D8 Discover X-ray diffraction system and Perkin Elmer lambda 35 UV-visible spectrophotometer respectively. Measurements of thickness were taken using Veeco Dektak 150 surface profiler having a vertical resolution of 2 nm. Scanning electron microscopy (SEM) has been done using Quanta 200 FEG SEM system. Steady-state UV photoresponse of the deposited ZnO films of varying thickness was recorded by illuminating the samples with 365 nm radiation (intensity of 24 µW cm⁻²). The intensity of UV radiations was measured using a Newport optical power meter (model no. 1830 C). The semiconductor characterization system Keithley 4200 was used to acquire the photoresponse transients.

3. Results and discussion

3.1. X-ray diffraction studies

The XRD patterns of pure ZnO and Au-overlayer/ZnO thin films with Au thicknesses varying from 5 to 40 nm are shown in Fig. 1. The patterns show that all deposited thin films exhibit peaks corresponding to the hexagonal wurtzite structure of ZnO, confirming the growth of polycrystalline thin films.³⁹ The (002) diffraction peak is found to be significant, while the (100) and (101) peaks are relatively weak, indicating that the films are preferentially oriented along the (002) plane with the c-axis normal to the substrate. With the deposition of continuous Au overlayers, the diffraction peak corresponding to the (101) plane becomes dominant over the (002) and (100) peaks, as observed from Fig. 1. In addition, an intense peak at 2θ ≈ 38°, corresponding to the Au (111) plane, is observed, confirming the presence of Au on the ZnO thin film surface.²³ The relative intensity of the Au (111) peak (I₁₁₁), calculated using the formula described in 35, increases continuously with Au overlayer thickness. This increase is attributed to the increase in number of Au atoms (act as diffracting centre) present on the ZnO surface with increasing thickness. Furthermore, an additional peak at ∼44.5° is observed in all Au-overlayer/ZnO films. This peak can be assigned to the (440) diffraction plane of Au₂O₃, indicating the formation of gold oxide due to post-annealing of the films in an oxygen atmosphere. These oxygen vacancies produced additional electronic states in the forbidden gap of ZnO lattice and increased with the number of Au atoms. However, this peak may also correspond to the Au (200) reflection, generally located at ∼44.3°, further supporting the presence of Au on the ZnO surface.

Fig. 1 XRD spectra of pure ZnO thin film and Au overlayer/ZnO films with Au thickness varying from 5 nm to 40 nm.

The structural parameters estimated from the XRD patterns are summarized in Table 1. These parameters are calculated on account of two significant peaks w.r.t. (002) & (101) planes using the Debye–Scherrer formula.^40,41 For the pure ZnO thin film (221 nm), the crystallite size was calculated to be 27 nm. With the deposition of Au, the crystallite size initially decreased to 23–24 nm for 5–10 nm Au thickness, indicating slight lattice distortion and strain at the ZnO surface due to Au incorporation. At intermediate thicknesses (20–30 nm), the crystallite size increased to 26–29 nm, suggesting partial grain coalescence and improved crystallinity. However, for the thickest overlayer (40 nm), the size decreased again to 26 nm, possibly due to increased scattering from Au agglomerates and the formation of Au₂O₃. The relative intensity of the Au (111) reflection (I₁₁₁) increased consistently with Au thickness, confirming progressive Au deposition. Also, the (440) reflection associated with Au₂O₃ became more pronounced, consistent with oxide formation during post-annealing in oxygen.

Table 1 Various parameters calculated for pure ZnO (221 nm) and Au overlayer/ZnO thin films of varying Au thickness from 5 nm to 40 nm from XRD analysis

Thickness of Au overlayer (nm)	Relative intensity (I111)	Relative intensity (I440)	Bragg's angle (2θ°)	FWHM	Crystallite size (nm)
0	—	—	34.36	0.309	27
5	0.17	0.03	36.28	0.37	23
10	0.50	0.07	36.30	0.351	24
20	0.51	0.08	36.32	0.319	26
30	0.82	0.11	36.38	0.289	29
40	1.71	0.13	36.46	0.323	26

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The Bragg angle shows a slight shift from 36.28° to 36.46° with increasing Au thickness, indicating minor lattice strain in ZnO. The variation in crystallite size and strain is observed from variation in FWHM values; which signify the combined effects of both these parameters. Thus, the results confirm that Au overlayers affect the structural characteristics of ZnO thin films through the formation of Au₂O₃, along with modest changes in crystallite size and strain.

3.2. SEM analysis

SEM micrographs of pure ZnO and Au overlayer/ZnO thin films with varying Au thickness are presented in Fig. 2. The pure ZnO film (Fig. 2a) exhibits a dense and uniform surface morphology composed of nanosized grains, consistent with the crystallite sizes calculated from XRD. Upon deposition of Au, the surface features change markedly. At 5 nm, small bright granules, attributed to Au nanoparticles, are sparsely distributed across the ZnO surface, indicating isolated nucleation sites. With increasing thickness to 10 nm and 20 nm, the Au granules grow in size and density, beginning to coalesce into larger clusters. At higher Au thicknesses (Fig. 2e and f), the Au nanoparticles agglomerate significantly, forming large, interconnected clusters that progressively cover the underlying ZnO grains. The continuity of the Au overlayer is more pronounced at 40 nm, where most ZnO surface sites appear masked by the metallic layer. This morphological evolution reveals that Au growth progresses from isolated nanoparticles i.e., island-like growth, at low thickness to nearly continuous overlayers at higher thickness, in agreement with the changes in crystallite size and strain observed in XRD.

Fig. 2 Surface microstructure of (a) pure ZnO thin film and (b–f) Au overlayer/ZnO thin films having Au thicknesses: 5 nm, 10 nm, 20 nm, 30 nm and 40 nm, respectively. Scale is 1 µm.

This morphological growth is critical for UV photoresponse. At lower Au thicknesses (5–10 nm), isolated Au nanoparticles act as plasmonic hot spots, enhancing light absorption and exciton plasmon coupling while maintaining sufficient surface exposure of ZnO for oxygen adsorption–desorption. At intermediate thicknesses (20–30 nm), partial coalescence improves carrier transport by forming conductive pathways, while still retaining plasmonic enhancement. However, at higher thickness (40 nm), the nearly continuous Au film suppresses surface activity of ZnO and can increases dark current. Therefore, SEM analysis confirms that Au thickness directly influences the plasmonic enhancement and oxygen-mediated surface interactions, which govern the overall UV photodetector performance.

3.3. UV-visible spectroscopy

The UV-visible transmittance spectra of pure ZnO and Au overlayer/ZnO thin films with Au thickness varying from 5 nm to 40 nm, deposited on Corning glass substrates, are presented in Fig. 3a. Pure ZnO thin film exhibits high optical transparency (∼80%) in the visible region, with a sharp absorption edge near 380 nm.⁴² Deposition of the Au overlayer leads to a systematic reduction in transmittance. As the Au thickness increases from 5 nm to 40 nm, transmittance decreases significantly, from ∼80% to ∼5%.

Fig. 3 (a) Optical transmittance spectra of pure ZnO thin film and Au overlayer/ZnO thin films with varying Au thickness. (b) Tauc plots used to determine the optical band gap of the thin films.

The optical band gap (E_g) of the thin films was determined using Tauc plots,⁴³ obtained by extrapolating the linear portion of (αhν)² versus hν curves (Fig. 3b). The variation in E_g with Au thickness is summarized in Fig. 4. A clear band gap narrowing is observed, from 3.276 eV for pure ZnO to 3.155 eV for the 40 nm Au overlayer/ZnO thin film. The narrowing of the ZnO bandgap upon Au overlayer deposition, primarily attributed to lattice strain and defect states such as oxygen vacancies (due to formation of Au₂O₃), leads to shifts in the band alignment at the Au–ZnO interface.²² This results in a downward shift of both the conduction band minimum and valence band maximum, effectively reducing the Schottky barrier height. Consequently, the modified band alignment facilitates improved charge carrier injection and separation under UV illumination, enhancing photocurrent generation while maintaining low dark current. Thus, bandgap narrowing plays a crucial role in tuning the electronic properties and improving the overall photoresponse of the Au–ZnO photodetectors. The progressive reduction in band gap with increasing Au thickness confirms the strong influence of Au deposition on the optical properties of ZnO thin films.

Fig. 4 Variation in optical band gap of pure ZnO and Au overlayer/ZnO thin films as a function of Au thickness.

The optical findings are consistent with the XRD analysis, where the emergence and strengthening of the Au₂O₃ (440) diffraction peak with increasing Au thickness indicated oxide formation. Thus, both structural and optical analyses corroborate that Au₂O₃ formation, along with increasing Au coverage, governs the observed reduction in transparency and band gap narrowing.

Furthermore, these structural and optical modifications are expected to strongly influence the electrical transport and UV photoresponse behavior of the Au/ZnO photodetectors, which are analysed in the following section.

3.4. UV sensing response

The UV photoresponse characteristics of pure ZnO and Au overlayer/ZnO thin film-based photodetectors were investigated at a bias voltage of 5 V towards UV radiation of λ = 365 nm with an intensity of 24 µW cm⁻². Fig. 5 shows the time-dependent on/off photoconduction measurements of Au overlayer/ZnO thin film-based detectors.

Fig. 5 Time-dependent on/off photoconduction measurements of pure ZnO and Au overlayer/ZnO thin film UV photodetector.

For the 221 nm pure ZnO film, the dark current (I_off) is 0.73 µA, and the photocurrent (I_on) is 0.111 mA, nearly three orders of magnitude higher than the dark current. This UV response has already been reported³⁵ as the maximum among ZnO thin films of different thicknesses. Fig. 6a and b show the variation of I_off and I_on, respectively, with increasing Au overlayer thickness on the ZnO surface. Initially, I_off decreases drastically from 0.73 µA to 1.29 nA with the deposition of a 5 nm Au layer. However, with further increase in Au thickness (5–40 nm), I_off increases significantly, eventually reaching the µA range (Fig. 6a).

Fig. 6 (a) The variation in dark current (I_off) (b) and photocurrent (I_on) of the prepared UV photodetectors.

From Fig. 6a and Table 2, it is evident that dark current decreases sharply when a thin 5 nm Au layer is deposited, but increases progressively with further increase in Au thickness. The variation in dark current can be explained by three main factors: firstly, formation of Au–ZnO Schottky barrier at the interface; secondly, formation of Au₂O₃ phase, which increases oxygen vacancies and enhances I_off; and third is the availability of free ZnO sites, which adsorb oxygen and reduce I_off. The maximum suppression of dark current (1.29 × 10⁻⁹ A) occurs at 5 nm Au thickness. This reduction is attributed to the Schottky barrier formed at the Au–ZnO interface due to the higher work function of Au (ϕ_Au ≈ 5.47 eV) compared to ZnO (ϕ_ZnO ≈ 4.45 eV).^44,45 Scheme 2 shows the band alignment of isolated n-type ZnO and Au, Schottky barrier formation at the interface.

Table 2 UV photoresponse characteristics (dark current (I_off), photocurrent (I_on) and photoresponse (K), rise time (T_r) and fall time (T_f)) of pure ZnO thin film and Au overlayer/ZnO based UV photodetectors having different thickness of Au overlayer on ZnO surface

Thickness of Au	Dark current (Ioff) (A)	Photocurrent (Ion) (A)	Photoresponse (K)	Rise time (Tr) (s)	Fall time (Tf) (s)
0	0.729 × 10^−6	0.11 × 10^−3	1.52 × 10^2	11.95	3.62
5	1.29 × 10^−9	0.01 × 10^−3	5.74 × 10^3	31.23	11.74
10	7.29 × 10^−9	0.05 × 10^−3	7.79 × 10^3	15.22	11.77
20	19.8 × 10^−9	0.06 × 10^−3	3.03 × 10^3	20.54	11.63
30	106 × 10^−9	0.15 × 10^−3	1.45 × 10^3	22.52	21.57
40	1.5 × 10^−6	0.80 × 10^−3	5.33 × 10^2	22.32	24.02

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Scheme 2 Schematics energy band diagram of Au/ZnO and visualisation of photoresponse mechanism.

Under thermal equilibrium, electrons from the conduction band of ZnO transfer to Au, leading to downward band bending and the formation of a barrier potential (ϕ_B). This reduces dark current since electron flow from Au atomic layer to ZnO conduction band is suppressed by the barrier qϕ_B. Moreover, SEM analysis confirms that the 5 nm Au film is not continuous, leaving ZnO surface sites available for oxygen adsorption, further lowering I_off. XRD results also support this, as the Au₂O₃ (440) peak shows the lowest intensity (I₄₄₀ = 0.03) for 5 nm Au, indicating that factors (1) and (3) dominate over factor (2) as mentioned above. At higher Au thicknesses (10–40 nm), I_off increases significantly (up to 1.5 µA at 40 nm). This trend can be due to; enhanced formation of Au₂O₃, confirmed by the growing intensity of the (440) peak in XRD, leading to higher oxygen vacancy concentrations; in addition, improved continuity and coverage of the Au film, as seen in SEM, which reduces the number of ZnO sites available for oxygen adsorption. Thus, while the Schottky barrier should reduce I_off, oxygen vacancy formation and loss of adsorption sites dominate at higher thicknesses, leading to enhanced dark current.

On the other hand, the I_on also exhibits a thickness-dependent behavior as depicted in Fig. 6b. After deposition of 5 nm Au, I_on decreases drastically from 0.111 mA (ZnO) to 0.01 mA, likely due to a reduced number of oxygen vacancies. Although free ZnO sites are available for oxygen adsorption, the simultaneous reduction in vacancy-related defects lowers the net photocurrent. As Au thickness increases from 10 to 40 nm, I_on rises significantly, reaching 0.8 mA. This enhancement is attributed to: firstly, increased oxygen vacancies introduced by Au₂O₃ formation, which act as electron trap centers. Moreover, improved defect profile of the Au/ZnO system, facilitating higher carrier generation under UV illumination.

The photocurrent mechanism can be summarized as follows: oxygen vacancies act as trap centers for electrons, while adsorbed oxygen molecules capture conduction electrons, forming O₂⁻ species. Upon UV illumination, photogenerated holes neutralize trapped O₂⁻ ions, releasing electrons back to the conduction band, thereby increase in photocurrent.

The influence of Au overlayers on the photoresponse characteristics of ZnO-based UV photodetectors can be correlated with Schottky barrier modulation, oxygen vacancy concentration, and plasmonic effects. As shown in Fig. 7, the photoresponse (K) of pure ZnO (1.52 × 10²) increases drastically to 5.74 × 10³ upon deposition of a 5 nm Au layer, owing to the formation of a Schottky barrier at the Au/ZnO interface which suppresses the dark current and enhances the I_on/I_off ratio. A further increase to 10 nm thickness yields the maximum K value (7.79 × 10³), attributed to the optimum balance between Schottky barrier height, limited Au₂O₃ formation, and plasmonic coupling effects that promote efficient carrier generation. At higher Au thicknesses (>10 nm), K decreases due to improved surface coverage by Au grains, which reduces the number of ZnO adsorption sites and increases oxygen vacancy defects through Au₂O₃ formation, thereby enhancing dark current. However, K remains significantly higher than that of pure ZnO.

Fig. 7 Variation in photoresponse (K) of the prepared UV photodetectors.

The dynamic response parameters (Fig. 8) further highlight the role of Au overlayers in modulating charge transport and trapping. The rise time (T_r) increases to 31.23 s for 5 nm Au due to reduced oxygen vacancy concentration that slows down carrier release, but decreases to 15.22 s at 10 nm thickness as optimized defect density and plasmonic enhancement accelerate charge transport. For thicker overlayers, T_r again increases due to excessive oxygen vacancies acting as trapping centers. In contrast, the fall time (T_f) shows non-systematic variation with Au thickness, reflecting the complex interplay of recombination dynamics at defect states and the Au/ZnO interface. Thus, the integration of Au overlayers significantly enhances the photoresponse while offering tunable response times, with 10 nm Au providing the optimum balance of performance parameters.

Fig. 8 Variation in (a) rise time (T_r) and (b) fall time (T_f) of the prepared UV photodetectors.

From the figure it is observed that the T_r exceeds T_f at initial thickness, due to the oxygen-mediated photoconductivity mechanism dominant in ZnO: during UV “on”, photogenerated holes must desorb surface O₂⁻ species (formed by pre-adsorbed O₂ capturing electrons), a process slowed by limited hole diffusion and Schottky barrier trapping at Au–ZnO interfaces; conversely, “off” recovery is faster as residual carriers recombine rapidly via defect states and plasmon-enhanced pathways. This T_r > T_f trend is common in surface-controlled ZnO UV photodetectors, contrasting bulk-dominated devices, and Au modification balances it by optimizing defect density excess Au₂O₃ vacancies at >20 nm prolong both via trapping. Despite longer T_r, the devices achieve superior I_on/I_off (>7 × 10³) at lower 24 µW cm⁻².

3.5. Photoresponse mechanism

The superior UV photoresponse of Au-overlayer ZnO photodetectors arises from synergistic oxygen-mediated photoconductivity, Schottky barrier effects, defect engineering, and plasmonic enhancement. The photoresponse mechanism can be summarized as follows: oxygen vacancies (Vo) act as trap centers for electrons, while adsorbed oxygen molecules capture conduction electrons, forming O₂⁻ species. Upon UV illumination, photogenerated holes neutralize trapped O₂⁻ ions, releasing electrons back to the conduction band, thereby increasing photocurrent. The whole mechanism is visualised in Scheme 2.

3.5.1. Dark current suppression (I_off mechanism). In ambient conditions, O₂ molecules adsorb on ZnO surface sites, capturing conduction band electrons to form O₂⁻ species and creating a low-conductivity electron-depleted layer. For pure ZnO, residual free carriers yield I_off = 0.73 µA. Au deposition (work function of Au = 5.47 eV > ϕ_ZnO = 4.45 eV) forms a Schottky barrier at the Au–ZnO interface, causing band bending and depleting electrons from ZnO near the interface (barrier height qϕ_B ≈ 0.5–1.0 eV). At 5–10 nm Au (island morphology), exposed ZnO maintains O₂ adsorption while Schottky blocks carrier injection, yielding I_off = 1.29–7.29 nA. Thicker Au (>20 nm) forms continuous coverage more Au₂O₃, increasing oxygen vacancies (Vo²⁺) that donate electrons, dominating to raise I_off to µA range.^44,45

3.5.2. Photocurrent generation (I_on mechanism). Under 365 nm UV (hν > E_g = 3.27 eV), electron–hole pairs generate across ZnO. Holes drift to surface, neutralizing O₂⁻ → O₂(g) + e⁻(CB), collapsing depletion layer and releasing trapped Vo²⁺ electrons, yielding I_on surge (pure ZnO: 0.111 mA). Au overlayers modulate this: (1) plasmonic enhancement – Au nanoparticles (<20 nm) excite localized surface plasmon resonance (LSPR, Mie theory³⁶), coupling UV photons to hot electrons that inject into ZnO CB via Schottky, boosting exciton generation ∼2–5 times;^29,30 (2) Vo²⁺ traps – from Au₂O₃ amplify gain (K = I_on/I_off = 7.79 × 10³ max); (3) thicker Au increases I_on to 0.8 mA via vacancy proliferation despite coverage losses.

3.5.3. Dynamic response (T_r/T_f). T_r > T_f reflects surface-dominated kinetics. UV-on requires sequential O₂⁻ neutralization and Vo²⁺ detrapping (slowed at 5 nm by lower Vo density); UV-off involves rapid e–h recombination via mid-gap states. Optimal 10 nm Au balances these (T_r = 15.22 s, T_f = 11.77 s).

This multi-mechanism synergy enables the Au/ZnO photodetector's ultra-sensitive detection at 24 µW cm⁻², showing its advantageous over the literature comprised in Table S1.

4. Conclusion

The integration of a gold thin film on ZnO surfaces significantly enhances the photoresponse characteristics of 221 nm ZnO thin film-based UV photodetectors. For the thickness of 10 nm Au overlayer results in substantial improvements in key parameters, including reduced I_off, increased I_on, and enhancement in the photoconductive gain (7.79 × 10³) compared to the pure ZnO device (1.52 × 10²). This improvement is attributed to the simultaneous suppression of dark current and enhancement of photocurrent, leading to superior photoresponse in the Au/ZnO hybrid system. However, increasing the Au overlayer thickness beyond 10 nm leads to performance degradation due to elevated dark current. Interestingly, while the Au thin film improves photoresponse, it also induces slower response and recovery times in the devices. These findings highlight that the higher work function of Au effectively reduces dark current, while the plasmonic effects of Au nanoparticles enhance optical absorption, collectively boosting the performance of ZnO-based UV photodetectors. Therefore, this study reveals the critical role of Au film thickness in tailoring the balance between dark current suppression and photocurrent enhancement, establishing Au-integrated ZnO thin films as promising candidates for high-performance UV photodetection.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data that support the findings of this study are available from the corresponding author up on reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5ra08186f.

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