1 Introduction

In recent years, the fields like missile tracking, ozone layer monitoring, flame detection, UV astronomy, optoelectronics, etc., require UV radiation measurement which is attracting the research community [1]. Conventional silicon-based photodetectors are commonly available in the market but require an additional power source to function, often a battery. This external power supply is not suitable for next-generation smart sensor systems, because (i) the battery components are hazardous to human health and are not environmental friendly; (ii) the network contains many UV sensors and replacing the individual batteries in each circuit would be a time-consuming task. As a result, there is a demand for self-sustaining, maintenance-free, low-cost UV sensors in a variety of applications, such as forest fire prevention and monitoring oil leaking in submarines [2]. The development of a built-in electric field is essential for designing a sensor that can function in a self-sustaining mode. This may be accomplished by making lattice-matched p–n junctions or a metal–semiconductor Schottky junctions. The Schottky junctions have recently acquired popularity due to their rapid switching speed operations and less manufacturing complexity. The choice of metal contacts is crucial in producing a high-quality Schottky barrier at the metal–semiconductor (MS) interface. The metals Au, Pd, Ni, Ir, Pt, and Cu have work functions (> 4.7 eV) that are thought to be suitable Schottky contacts for wide-band-gap n-type semiconductor metal oxides [3, 4]. The metal oxides, such as ZnO, SnO2, TiO2, etc., are ideally suited for exploring UV-A radiation in the electromagnetic spectrum due to their large band gaps. Out of these metal oxides, ZnO is renowned for having low-temperature growth, being inexpensive, being non-toxic, exhibiting steady n-type behavior, and having a high exciton binding energy about 60 meV. The pure ZnO offers high resistivity, and it can be reduced by doping with higher valence elements, such as Ga3+ [5], Al3+ [6], Gd2+ [7], Y3+ [8], Sn4+ [9], etc. Out of all these dopants, Sn4+ is reported to have an equivalent ionic radius to Zn2+ (i.e., Zn2+  = 74 pm and Sn4+  = 69 pm) [10], and Sn atom can be easily substituted in the Zn sites without significantly distorting the crystal structure and can provide two more electrons for electrical conduction. Dhananjay et al. [11] worked on Au/ZnO Schottky diodes and obtained 3 orders of rectification, and barrier height of 1.02 eV. Similarly Rajan et al. [12] worked on Au/ZnO Schottky diodes and observed ideality factor of 6.28 with the barrier height of 0.804 eV. Mei Shen et al. [13] worked on Cu/ZnO Schottky diodes, and noticed an ideality factor of 2.7 with barrier height of 0.55 eV. Although many articles related to Au/ZnO-based Schottky diodes are explored, \(\mathrm{Cu}/{\mathrm{Zn}}_{\left(1-\mathrm{x}\right)}{\mathrm{Sn}}_{\left(\mathrm{x}\right)}\mathrm{O }\left(\mathrm{TZO}\right)\)-based Schottky diodes for UV sensing application are yet to be explored. In this work, we report fabrication and characterization of Cu/ZnO and Cu/TZO Schottky diodes for UV sensing applications, achieved using direct current (DC) magnetron reactive sputtering technique.

2 Experimental details

The Cu/ZnO/ITO/Glass and Cu/TZO/ITO/Glass Schottky-based UV-Photodetectors were fabricated via dc magnetron reactive sputtering technique. ITO-coated glasses served as a back contact for the devices. The cleaning of ITO substrates was done using acetone, isopropyl alcohol solvents ultrasonicated for 10 min, and were dried with nitrogen gas. The dried substrates were subjected to UV-Ozone cleaning to remove organic contaminants. High pure (99.99%) Zn and Sn metal targets having the diameter of 2″ were used for coating. The ultimate base pressure of 9.60 × 10–6 mbar was achieved with the support of a diffusion pump and backed by a rotary pump. The sputtering power about 35W for Zn target was maintained for ZnO coating and similarly for preparing TZO thin films 35W power for Zn target and 5W sputtering power for Sn target was maintained for in co-sputtering mode. The oxygen-to-argon flow ratio \(\left( {{\raise0.7ex\hbox{${{\text{O}}_{2} }$} \!\mathord{\left/ {\vphantom {{{\text{O}}_{2} } {{\text{O}}_{2} + Ar}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${{\text{O}}_{2} + Ar}$}} \times 100} \right)\) was fixed about 20% (i.e., Ar = 5.6 SCCM and O2 = 1.4 SCCM) with a help of mass flow controller (ALICAT-Scientific) for ZnO and TZO coating. All the samples were maintained at constant thickness of 250 ± 05 nm. Finally, deposited samples (pristine and doped) were ex situ air annealed at 100 °C, 150 °C, and 200 °C on a hot plate (IKA-C-MAG HS-7) for 20 min and the samples denoted as TZO-100, TZO-150, and TZO-200, respectively.

3 Characterization methods

3.1 Material characterizations

The crystalline nature of ZnO and TZO thin films deposited on ITO-coated glasses was evaluated using GXRD Rigaku smart lab \(\left( {{\text{Cu}}\;K\alpha , \lambda = 1.54 \mathop {\text{A}}\limits^{ \circ } } \right)\) with a glancing angle of 0.5°. SHIMADZU UV-1800 double beam spectrophotometer was used to determine the optical transmittance and absorbance of the films. Dektak XT stylus profiler was used to measure the thickness of grown thin films. For determining the cross-sectional image and elemental composition in the TZO sample, the Field-Emission Scanning Electron Microscopy (FESEM) with energy-dispersive spectroscopy (EDS) was utilized [EVO MA 18 with oxford EDS (X-act)]. The surface morphology of sputtered thin films examined by atomic force microscope (AFM-Innova SPM). X-Ray Photoelectron Spectroscopy (XPS) was used to determine the chemical composition of the samples (AXIS 165 ULTRA DLD-Kartos, Analytical Limited Instrument). The instrument is outfitted with a monochromatic (Al\({K}_{\alpha }=1486.69\mathrm{ eV}\)) X-ray source with a 15-micron small spot capability.

3.2 Device characterizations

To measure the electrical properties of the device, thermally evaporated copper (Cu) contacts (top electrode) with a thickness of 120 nm were deposited using a shadow mask having a diameter of 500 µm. Chemical etching (HCl) was used to pattern the bottom ITO electrode. The electrodes with a strip of width ~ 3 mm and thickness ~ 400 nm were used. The Keithley 2636-B Source meter was used to measure the current versus voltage. A 60 µW/cm2 calibrated source was used to provide monochromatic 365 nm UV light. UV light was chopped by an optical chopper and the photocurrent signals were recorded by Keithley 2636-B source meter. Figure 1 shows the schematic representation of ultraviolet sensors and fabricated Schottky diode; likewise, Fig. 2 displays the band diagram of constructed Schottky diodes.

Fig. 1
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Ultraviolet sensor: a schematic image of Schottky-based UV sensor and b fabricated Schottky diode

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Fig. 2
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Band diagram of metal–semiconductor: a band diagram of Cu and ZnO before contact, b band diagram of Cu and ZnO after contact, c band diagram of Cu and TZO before contact, and d band diagram of Cu and ZnO after contact

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4 Results and discussion

4.1 Structural properties

Figure 3 shows schematic representation of ultraviolet photo sensor and fabricated Schottky device. Figure 4 indicates the X-ray diffraction (XRD) pattern of ZnO and TZO samples synthesized on ITO-coated glass substrate. The diffraction pattern reveals a polycrystalline nature of the samples with typical hexagonal wurtzite structure of ZnO. The strong c-axis orientation along (002) plane noticed in the pristine ZnO, and other diffraction planes like (100), (101), and (110) also can be seen in the Sn doped ZnO (TZO) annealed samples. All the diffraction peaks are in accordance with JCPDS data (card no. 36-1451) [14]. In XRD pattern, there are no diffraction peaks related to element Sn were observed, which confirms that proper substitution of Sn into ZnO crystal matrix. Moreover, it is observed the marginal shift in the (002) plane towards higher angle after Sn doping, which is believed due to difference in ionic radii (i.e., Zn2+ = 74 pm, Sn4+ = 69 pm). Also, which confirms the substitution of Sn4+ ion rather than Sn2+ ion (Sn2+ = 93 pm) in the ZnO crystal matrix [15]. The development of broad-diffraction peak along (110) at 150 °C and 200 °C annealed samples may be due to grain coalescence at higher thermal energy [16]. To find the crystallite sizes, the Scherer’s formula was used [17], and the calculated structural parameters are mentioned in Table 1. A slight decrement in the crystallite sizes after Sn doping compared to pristine ZnO was observed that may be due to development of strain in the crystal matrix because of difference in the ionic radii. Likewise, the dislocation density (\(\delta\)) and micro-strain (\(\varepsilon\)) were determined using the relations as mentioned below [17]

$$\varepsilon = \frac{\lambda }{D\sin \theta } - \frac{\beta }{\tan \theta }$$
(1)
$$\delta =\frac{1}{{D}^{2}}.$$
(2)

where \(\lambda\) is the X-ray wavelength (Cu Kα = 1.5406 Å), \(\beta\) is Full-Width Half Maximum (FWHM), θ is the Bragg angle, and D is crystallite sizes obtained from Scherrer’s formula. Initially, the increment in the micro-strain and dislocation densities were observed in TZO-RT sample; later, after post-annealing treatment, these quantities were relaxed as expected.

Fig. 3
figure 3

X-ray diffraction pattern of ZnO and TZO thin films

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Fig. 4
figure 4

FESEM images of ZnO and TZO thin films, a ZnO, b TZO-RT, c TZO-100, d TZO-200, and e EDS spectrum of TZO-100

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Table 1 Structural parameters of ZnO and TZO thin films
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4.2 Morphological study

Figure 4a–d depicts the FESEM image of DC sputtered ZnO and TZO samples. It is noticed that samples are uniform, homogeneous free of pinholes and cracks, and which validates the good adhesion nature of the film with the substrate. The TZO samples exhibit a slight change in the surface structure which depend on post-annealing temperature. Figure 5a, b shows cross-sectional images of Cu/TZO/ITO/Glass device. From these images, dotted contact (Cu) and TZO layers are clearly visible with well-aligned lateral homogeneity. Figure 6a–e depicts AFM topological 3D images of ZnO and TZO samples, which shows uniform grain distribution. Initially, the slight increment of the root-mean-square roughness (RMS) value was observed after Sn doping, which may be because of development of lattice strain in the crystal matrix after Sn doping [18]. Likewise, the samples TZO-150 (i.e., post-annealed at 150 °C) and TZO-200 (i.e., post-annealed at 200 °C) have exhibited higher in the RMS values, which is due to recrystallisation after 100 °C as seen in the structural analysis. It is known that surface roughness influences the propagation loss, and hence, the films annealed at higher temperatures (> 150 ˚C) are not much suitable for device fabrication. To find the elemental composition and cationic ratio (i.e., \({\raise0.7ex\hbox{${{\text{Sn}}}$} \!\mathord{\left/ {\vphantom {{{\text{Sn}}} {{\text{Sn}} + {\text{Zn}}}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${{\text{Sn}} + {\text{Zn}}}$}}\)) in TZO sample, the EDS was performed for TZO-RT sample (Fig. 4e). The observed cationic ratio was 0.141 in the sample.

Fig. 5
figure 5

FESEM cross-section image of the device a at 20 µm and b at 200 nm

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Fig. 6
figure 6

AFM images of ZnO and TZO thin films: a ZnO, b TZO-RT, c TZO-100, d TZO-150, and e TZO-200

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4.3 Optical properties

Figure 7a indicates the transmission spectra of ZnO and TZO thin films on ITO-coated glass substrate. The observed band edge in the range of 350–375 nm for all the samples. The significant changes in the transmittance after doping and annealing were observed, which is due to phonon scattering with free carriers, since the post-annealing treatment results increment in the carrier concentration as mentioned in our previous study [10]. The interference pattern in the transmittance graph, which suggests that the films are smooth and good interface with ITO substrate. Figure 7b shows the absorption spectra, and which confirms the maximum absorbance in UV-A wavelength region. Furthermore, to find the optical energy band gap, the Tauc’s plot was adopted by considering direct allowed transition [19]. The ZnO sample possesses band gap of 3.22 eV and TZO-RT sample possesses band gap of 3.29 eV, and this increment in the optical band gap after Sn doping due to Burstein–Moss effect [20]. Likewise, a slight decrement in optical band gap in TZO samples after annealing was noticed because of change in the Urbach energy as mentioned in Table 2. Urbach Energy \(({E}_{u})\) talks about irregularities caused by defects and is given by

Fig. 7
figure 7

Optical parameters by UV spectroscopy: a absorbance nature of TZO thin films (inset: absorbance nature of ZnO), b transmittance nature of TZO thin films (inset: transmittance of ZnO), and c optical band gap of TZO thin films (inset: band gap of ZnO)

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Table 2 Optical parameters of ZnO and TZO thin films
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$$ln\alpha =ln{\alpha }_{o}+\left(\frac{h\nu -{E}_{0}}{{E}_{U}}\right).$$
(3)

By drawing lnα vs. hν & from the inverse slope of the linear part of this curve, the \({E}_{U}\) can be estimated. The tabulated values are mentioned in Table 2.

The static refractive index of the film corresponds to direct bandgap can be found using Moss relation [21]

$${n}^{4}{E}_{g}=95 eV,$$
(4)

where n and \({E}_{g}\) are refractive index and energy band gap. The slight variations in the RI values were noticed after doping and annealing, which is due to scattering of incident photons because of changes in the surface roughness after doping and annealing.

4.4 Chemical analysis

The X-ray photoelectron spectroscopy (XPS) were performed to check the chemical composition of the sample. To analyse the raw spectra, Casa XPS software was adopted. The Shirley-type background correction was applied to all the spectra, and binding energy peaks were deconvoluted by utilizing mixed Gaussian–Lorentzian function. Before analysis of actual data, the standard C1s peak at 284.80 eV was used for the charge correction.

Figure 8a shows the Zn 2p core spectra of ZnO, TZO-RT, and TZO-200 samples. The samples are represented peak doublets known to be Zn 2p3/2 and Zn 2p1/2 due to spin–orbit coupling. The binding energy peaks and their percentage contents are mentioned in Table 3. Figure represents the deconvoluted Sn 3d spectrum, which are related to Sn 3d3/2. The minor peak situated at 494.45 eV for TZO-RT and 494.31 eV for TZO-200 sample can be designated to Sn2+ ions. Likewise, major peak located at 497.77 eV for TZO-RT and 497.65 eV for TZO-200 sample can be assigned to Sn4+ ions, in the crystalline matrix. From the table, we can clearly see that increment in the percentage composition of Sn4+ ions compared to Sn2+ ions, after annealing, and which signifies the increment in the donor concentrations in the sample [22]. Figure 8c shows the core spectra of O1s for ZnO, TZO-RT, and TZO-200 samples, and the first peak (OI) in all the samples represents the lattice oxygen related to Zn or Sn atoms [23], and likewise, the second peak (OII) indicates the oxygen vacancy; similarly, the last peak (OIII) corresponds to adsorbed water or moisture content in the sample [24]. It is notice that after Sn doping and post-annealing, a slight elevation in the second peak of TZO-RT and TZO-200 samples compared to pristine ZnO, and which indicates that increment oxygen vacancies. Subsequently, this donor defect enhances the conductivity of the samples.

Fig. 8
figure 8

XPS spectra of ZnO and TZO samples: a Zn 2p spectra, b Sn 3d spectra, and c O1s spectra

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Table 3 The XPS spectral peaks of sputtered ZnO and TZO thin-film samples
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4.5 Electrical studies

Figure 9a shows current–voltage (I–V) behavior of Cu/ZnO/ITO/Glass and Cu/TZO/ITO/Glass devices in dark condition, and it demonstrates the rectification behavior due to Schottky characteristics of Cu/TZO junction. Initially, we noticed that near ohmic junction formation in Cu/ZnO-based device (fig. S1 ESI), and after Sn doping the good rectification behavior was seen in the Cu/TZO-based device (Fig. 9c–f). The shift in the near ohmic nature to Schottky nature after Sn doping might be due to increment in the carrier concentration after Sn doping, which was reflected as increment in the oxygen vacancies as seen in the XPS analysis. Since the work function of Cu differs from that of ZnO and TZO, their energy band can bend at the interface due to electron migration from semiconductor (TZO) to metal (Cu) and develops built-in electric field as shown in Fig. 2c–d. This built-in electric field supports the separation and transportation of photogenerated carriers.

Fig. 9
figure 9

Current–voltage nature of Schottky diodes at dark and under UV illumination: a current–voltage nature of Cu/ZnO and Cu/TZO at dark condition, b Cu/ZnO under UV illumination, c Cu/TZO-RT, d Cu/TZO-100, e Cu/TZO-150, and f Cu/TZO-200

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To find the junction parameters like ideality factor and barrier height, the thermionic emission model was used by assuming series resistance as zero (\(\mathrm{i}.\mathrm{e}., {R}_{s}=0)\). The current (I) through the diode is given by

$$I = I_{O} \left( {e^{{{\raise0.7ex\hbox{${qV}$} \!\mathord{\left/ {\vphantom {{qV} {nkT}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${nkT}$}}}} - 1} \right),$$
(5)

where \({I}_{O}\) is the saturation current, n is the ideality factor, T is temperature in Kelvin, k is the Boltzmann’s constant, and q is the elementary charge. The ideality factor can be determined from the slope of linear portion of forward bias in I–V graph. Similarly, the barrier height \((\Phi_{b} )\) at metal–semiconductor junction can be calculated as follows [25]:

$$q\Phi_{b} = kT\ln \left( {{\raise0.7ex\hbox{${AA^{*} T^{2} }$} \!\mathord{\left/ {\vphantom {{AA^{*} T^{2} } { I_{O} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${ I_{O} }$}}} \right),$$
(6)

where A is the area of the Cu electrode (\(1.96\times {10}^{-03}{\mathrm{cm}}^{2}\)) and \({A}^{*}\) (\(32 A {\mathrm{cm}}^{-2}{\mathrm{K}}^{-2})\) is known as Richardson’s constant for ZnO. The ideality factor of Cu/ZnO and Cu/TZO was determined from the I–V characteristics measured under dark condition. It was observed that, after Sn doping, the drastic decrement in n value was noted as mentioned in Table 4. The obtained values of ideality factor (n) for all the devices are significantly above the unity (n = 1 for ideal diode) which could be attributed to voltage drop across metal/semiconductor interface and also the possible series resistance of the active layer [26]. We have observed a decrease in the ideality factor (n) for the Cu/TZO junction, which could be attributed to improved carrier concertation following Sn doping. Furthermore, the increase in donor concentration might tune the work function of TZO films, and eventually, the migration of more electrons from semiconductor to metal may result in band bending. Subsequently there is a formation of a Schottky barrier at the interface. We have observed a minimum ideality factor in the Cu/TZO-100 device (Table 4), and later, slight increases in the ideality factor values were noticed at higher annealing temperatures (i.e., at 150 °C and 200 °C). It could be due to an increase in the carrier concentration of TZO films at these temperatures, which could lead to additional electron migration from semiconductors to metal. It eventually decreases the thickness of the Schottky barrier due to sharp band bending at the metal semiconductor interface, resulting in the possibility of tunnelling current.

Table 4 Electrical parameter of Cu/ZnO and Cu/TZO device under dark and UV illumination
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The decrement in the rectification ratio (103–101) at ± 2 V was observed after post-annealing above 100 °C, which may be due to increase in the tunnelling current because of increased RMS roughness of the films [27]. Likewise, decrease in the barrier height above 100 °C post-annealing was noted, which may be due to reduction in the interfacial defect densities at higher annealing temperature and the modification of defect densities after heat treatment could alter the pinning of Fermi levels [28].

4.6 Photoresponse analysis

Photoresponse study of all the devices was examined by irradiating 365 nm UV light with an optimized power of 60 µW/cm2. The current–voltage relations are shown in Fig. 8b–f. Since the photocurrent contribution can be measured through reverse bias condition, we have plotted the current–voltage characteristics in reverse bias condition, as shown in Fig. 10. The development of photocurrent is clearly visible after illumination and maximum \({\raise0.7ex\hbox{${I_{{{\text{Photo}}}} }$} \!\mathord{\left/ {\vphantom {{I_{{{\text{Photo}}}} } {I_{{{\text{dark}}}} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${I_{{{\text{dark}}}} }$}}\) ratio about 4.33 × 102 was observed at reverse bias of 2 V in TZO-100 sample.

Fig. 10
figure 10

Current–voltage relation of Cu/TZO Schottky diodes at reverse bias condition (inset: current–voltage relation of Cu/ZnO)

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It is widely known that oxygen is an important type of surface states in metal oxides. These oxygen molecules adsorb the electrons near the surface of the semiconductors and forms a depletion layer

$${O}_{2}\left(g\right)+{e}^{-}\to {O}_{2}^{-}\left(ad\right).$$
(7)

Then, the Schottky barrier height \((\Phi_{b} )\) can be expressed as

$$\Phi_{b} = V_{bi} + \varepsilon = \frac{{Q_{s}^{2} }}{{2qN_{sc} \in_{s} A^{2} }} + \varepsilon ,$$
(8)

where \({V}_{bi}\) is the built-in potential, \(\varepsilon\) is the potential difference between minimum of conduction band and fermi level, \({Q}_{s}\) is surface charge, \(\in_{s}\) is dielectric constant of semiconductor material, and A is the contact area (\(1.96\times {10}^{-03}{\mathrm{cm}}^{2}\)). When the device was illuminated with ultraviolet light, the electron–hole pairs are created and separated by the built-in electric field at the Schottky barriers. Some of the created holes captured by the \({O}_{2}^{-}\) at the interface and make it neutral as per the below given relation

$${O}_{2}^{-}\left(ad\right)+{h}^{+}\to {O}_{2}\left(g\right).$$
(9)

Therefore, the desorption of the oxygen leads to decrease in the surface states (\({Q}_{s})\), and which leads to decrease in the barrier height. The relation between barrier height and surface states is given in Eq. (8). The similar observations were derived by other research groups [29,30,31].

Similarly, the figure of merits of UV photosensors like photoresponsivity (R), detectivity (D), linear dynamic range (LDR), and transient photoresponse were evaluated, and the values are mentioned in Table 5.

Table 5 Sensor parameters of Cu/ZnO and Cu/TZO devices
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The response of the sensors towards UV illumination could be determined by calculating the parameter known as responsivity R (A W.−1) and is given by [32]

$$R=\frac{{I}_{\mathrm{illuminated}}-{I}_{\mathrm{dark}}}{{P}_{\mathrm{illuminated}}},$$
(10)

where \({I}_{\mathrm{illuminated}}\), \({I}_{\mathrm{dark}}\), and \({P}_{\mathrm{illuminated}}\) are known as current after illumination, dark current, and power of UV light falling on active area (1 mm2) of the device, respectively. The maximum responsivity of 7.77 mA/W was observed in 100 °C post-annealed sample (i.e., TZO-100). The higher responsivity in this sample compared to TZO-150 and TZO-200 samples could be attributed to a decrease in the dark current at 100 ˚C post-annealing treatment due to the smoother surface of the film and minimum Urbach energy as seen in morphological and optical studies, respectively. Likewise, to check the ability of the detector towards faintest optical input signal detection, the parameter detectivity (D) (in Jones) was calculated as follows:

$$D= \frac{R {A}^{1/2}}{\sqrt{2e{I}_{d}}},$$
(11)

where R is the responsivity, \({I}_{d}\) is the dark current, A is active area where light absorption takes place, and e is the electronic charge. Conferring to above relation, a low background dark current results in greater detectivity. In our case, the minimum dark current of 7.65 × 10–06 A was seen in the sample TZO-100 and the observed detectivity was around 1.39 × 1011 Jones. The parameter linear dynamic range (LDR, typically denoted in dB) for the sensor was also calculated using relation [33]

$$\mathrm{LDR}=20\mathrm{log} \frac{{I}_{\mathrm{illuminated}}}{{I}_{\mathrm{dark}}}.$$
(12)

The maximum LDR ~ 34 dB was noted in TZO-100 sample. Furthermore, transient photocurrent or speed of response was measured to check the switching action of the device, by chopping the optical signal. From I–V graphs (Fig. 8b–f), we can clearly notice that up-shift of current at zero bias after UV illumination, which ensures that the fabricated device can work under zero bias condition, nothing but in self-power condition. Based on this, while measuring the photocurrent versus optical power and photocurrent versus time, the device was not subjected to any external bias. Figure 11 represents the photocurrent and optical power relation for TZO-100 sample; here, the increase in the photocurrent with increment in the optical power of UV-365 nm source was noticed. The maximum photocurrent was observed at optical signal of 60 µW/cm2. Therefore, by fixing this power (60 µW/cm2), transient response (speed of response) measurement was performed.

Fig. 11
figure 11

Optical power and photocurrent relation of Cu/TZO-100/ITO device

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The rise time of 783 ms (i.e., photocurrent varies from 10 to 90% of its maximum value) and the fall time of 876 ms (i.e., photocurrent varies from 90 to 10% of its maximum value) were observed as shown in Fig. 12b.

Fig. 12
figure 12

Transient photocurrent response: a current–time relation of Cu/TZO-100 sample and b current–time relation (single cycle) Cu/TZO-100 sample

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The switching action of the sensor can be explained as follows: under UV illumination, the photogenerated carriers are separated by built-in electric field in space-charge region. The holes migrate towards the surface of the semiconductor and neutralise the oxygen ions as per Eq. (9), which leaves some unpaired electrons on the surface. These unpaired electrons contribute the generation of photocurrent. Similarly, the recovery process can be defined as the adsorption of O2− ions on the surface once the UV signal cuts off, which leads to relaxation of holes remain at the surface and reduces the photocurrent [30]. In Table 6, we compared the performance of our device with reported Schottky-based UV sensors.

Table 6 Performance comparison of Schottky-based UV sensors
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5 Conclusion

In conclusion, Cu/ZnO near ohmic and Cu/TZO-based Schottky diodes were fabricated by dc magnetron sputtering. The structural, morphological, optical, and electrical studies were thoroughly investigated. The XRD analysis revealed that the samples were polycrystalline, with no evidence of secondary phases related to Sn. The morphological study supports an increase in RMS roughness after post-annealing at temperatures above 100 °C. UV–visible spectroscopy confirms that there are no significant changes in the optical band gap after doping and annealing. The increase in oxygen vacancies after Sn doping and post-annealing was confirmed by XPS analysis. The I–V characteristics show a near ohmic junction in Cu/ZnO samples and a Schottky junction in Cu/TZO samples. The three orders of rectification were observed in TZO samples annealed at 100 ˚C. The decrease in the rectification ratio (103–101) at ± 2 V was observed after post-annealing above 100 °C, due to an increase in the tunnelling current caused by an increase in the RMS roughness of the films. All the Schottky diodes responded to a 365 nm UV light of power 60 µW/cm2 even at zero external voltage supply. As a result, sensor parameters, such as responsivity, detectivity, linear dynamic range, and response time, were thoroughly evaluated and analysed. These Schottky-based UV sensors may be useful in large-scale UV-A photosensor applications due to their advantages of being low-temperature processed, self-powered, and visible-light transparent.