Photovoltaic and flexible deep ultraviolet wavelength detector based on novel β-Ga2O3/muscovite heteroepitaxy

Flexible and self-powered deep ultraviolet (UV) photodetectors are pivotal for next-generation electronic skins to enrich human life quality. The fabrication of epitaxial β-Ga2O3 thin films is challenging on flexible substrates due to high-temperature growth requirements. Herein, β-Ga2O3 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\stackrel{-}{2}$$\end{document}2- 0 1) films are hetero-epitaxially grown on ultra-thin and environment-friendly muscovite mica which is the first time β-Ga2O3 epitaxy growth on any flexible substrate. Integration of Gallium oxide with muscovite enables high-temperature processing as well as excellent flexibility compared to polymer substrates. Additionally, the metal–semiconductor-metal (MSM) photodetector on β-Ga2O3 layer shows an ultra-low dark current of 800 fA at zero bias. The photovoltaic peak responsivity of 11.6 µA/W is obtained corresponding to very weak illumination of 75 μW/cm2 of 265 nm wavelength. Thermally stimulated current (TSC) measurements are employed to investigate the optically active trap states. Among these traps, trap with an activation energy of 166 meV dominates the persistence photocurrent in the devices. Finally, photovoltaic detectors have shown excellent photocurrent stability under bending induced stress up to 0.32%. Hence, this novel heteroepitaxy opens the new way for flexible deep UV photodetectors.

www.nature.com/scientificreports/ depletion region due to opposite polarity of semiconductor materials as well as band offsets (for heterojunctions) 20 whereas in Schottky diodes, the built-in electric field exists at the metal-semiconductor junction. Nowadays, most of the flexible UV photodetectors are fabricated on polymer substrates such as PET, PDMS, and PEN [21][22][23] . Generally, devices fabricated on epitaxial materials exhibit better performance than amorphous materials. However, the fabrication of epitaxial thin films on flexible substrates is impossible due to the degradation of these substrates at high-temperatures as required for the epitaxial growth. Muscovite mica is a natural substrate that can resolve both the aforementioned challenges of high-temperature processing and environment friendliness 24 . The muscovite has high-temperature stability up to 650 °C. Ultra-thin mica substrate with few nanometer thickness have been reported 25 . The thickness of flexible substrates must be less than 100 μm to function as an e-skin 26 . Therefore, mica is also a strong contender for e-skins. Moreover, muscovite owes superior Young's modulus, thermal conductivity, and tensile strength than polymer substrates such as PET, PDMS, PEN, etc 24 . Being two-dimensional (2D) and layered substrate, the surface of muscovite is free from the dangling bonds. According to Koma et al., the thin film growth progresses on 2D materials with weak interactions rather than strong chemical bonding between thin film and substrate 27,28 . Thus, the growth method is called as van der Waals epitaxy. Even with large lattice mismatch, van der Waals epitaxy growth mechanism provides the excellent heteroepitaxial growth. Earlier, many high-quality functional oxides such as Fe 2 O 3 , MoO 2 , VO 2 , yttria-stabilized zirconia (YSZ), CoFe 2 O 4 have been epitaxially grown on muscovite mica. These functional oxides had lattice mismatch with muscovite ranging from 8 to 36% but it didn't affect the material quality due to van der Waals epitaxy growth [29][30][31][32][33] .
Nowadays, Ga 2 O 3 and AlGaN are the most widely used wide bandgap semiconductors for deep UV photodetector applications 14,[34][35][36][37][38][39][40][41] . Recently, Ga 2 O 3 has emerged as a better candidate than AlGaN due to its intrinsic deep UV absorption which eliminates the complexity of heavy doping 42 . Only a few research works are performed on the growth of gallium oxide thin films on flexible substrates. Thin films used in these reports were amorphous 23,43 . So far, the epitaxial deposition of β-Ga 2 O 3 thin films on flexible substrates has been a major challenge. To fill this research gap, self-powered and flexible deep UV photodetectors based on epitaxial β-Ga 2 O 3 thin films are reported here. The optically active trap states have also been identified, which contribute towards the persistent photocurrent observed in the devices.

Results and discussion
To begin with, structural properties of the Ga 2 O 3 thin film are investigated by high-resolution X-ray diffraction (HRXRD). Out of plane X-ray 2θ scan of Ga 2 O 3 /muscovite heterostructure is shown in Fig. 1(a). The red color plot depicts the diffraction pattern of muscovite whereas the green color plot represents the diffraction pattern of Ga 2 O 3 /muscovite heterostructure. It shows that ( − 2 0 1) orientation (JCPDS No.-431012) of monoclinic β-Ga 2 O 3 was grown on muscovite. Moreover, phi (ϕ)-scan of β-Ga 2 O 3 ( − 4 0 1) diffraction is recorded for in-plane structural correlation which is shown in Figure S1. Six peaks are observed in the ϕ-scan of ( − 4 0 1) plane at the interval of 60°. The ( − 4 0 1) reflection plane of β-Ga 2 O 3 possesses two-fold symmetry. Therefore, observation of six peaks in the ϕ-scan represents the three-fold domains in the β-Ga 2 O 3 epitaxy. Furthermore, the crystalline quality of the thin film is analyzed using the rocking curve measurement of ( − 2 0 1) plane giving full width at half maximum (FWHM) of 1.3° as depicted in Fig. 1(b). This FWHM of β-Ga 2 O 3 epitaxy is much lower than the reported MBE grown thin films (1.38-1.99°) 44,45 . HRXRD reveals the epitaxial relationship between β-Ga 2 O 3 and mica as (  Figure 1(c) shows the surface topography of thin film recorded by atomic force microscopy in 2 × 2 µm 2 scan area. The root mean square roughness of 0.5 nm is obtained which revealed that the thin film is quite smooth and can be used for device fabrications.
To further confirm the epitaxial relationship and interfacial characterization, a cross-sectional imaging of the β-Ga 2 O 3 /mica interface is done using a high-resolution transmission microscope (HRTEM) and is shown in Fig. 2(a). The sharp interface between Ga 2 O 3 thin film and mica is also indicated in Fig. 2(a). The fast Fourier transforms (FFT) of the Ga 2 O 3 thin film and mica in the reciprocal space are indicated in Fig. 2(b,c) simultaneously. The FFT images also confirm the epitaxial relationship as β-Ga 2 O 3 (  www.nature.com/scientificreports/ 265 nm illumination, as shown in Fig. 3(a). The photocurrent is 45 times larger than the dark current. Here photovoltaic response i.e. zero bias photoresponse is also noticed under illumination, which is known as a self-powered behaviour in photodetectors. This self-powered behaviour is advantageous for the sensors in terms of power consumption which is the need of the current technology. The responsivity of a photodetector is defined as 14,46 where I d and I p denote dark and photocurrent respectively. P λ represents the power density at wavelength λ and A is the effective area of the device under illumination. The voltage-dependent peak responsivity under 265 nm illumination is shown in Fig. 3(b). The zero-bias peak responsivity of 11.6 µA/W was obtained at a weak power density of 75 μW/cm 2 . For a better performance evaluation of the current work, photovoltaic UV-C detectors are compared in Table 1 with different device architectures. The epitaxial growth of β-Ga 2 O 3 is also compared due to importance of channel layer in the photodetector performance. Thus, low temperature grown β-Ga 2 O 3 /muscovite heterostructures provided UV-C photovoltaic detector with simple MSM architecture and ultra-low dark current. Flexibility of Ga 2 O 3 /muscovite heterostructure is another important merit over reported photovoltaic UV-C wavelength detectors.
Further, the photovoltaic current obtained under the illumination of 265 nm wavelength is displayed in Fig. 3(c). The photocurrent increases/decreases exponentially with time during switching on/off the illumination. It is observed that photocurrent persisted for several seconds after switching off the illumination. The photocurrent rise and decay curves can be fitted with biexponential Eq. 2 and 3, respectively: 14,23,40 where I s represents the steady-state current and A and B are the fitting constants. τ r1 and τ r2 are fast and slow components of rise times whereas τ d1 and τ d2 denote the fast and slow components of decay times. Two rise/decay times are obtained by fitting of time response curve using Eqs. (2) and (3). The rise times τ r1 and τ r2 of 1.78 s and 14.80 s are obtained whereas the decay times of 2.09 s and 33.06 s are acquired. The slow component of rise/ decay time is attributed to charge carrier trapping/de-trapping owing to the presence of bandgap states. Larger values of slow components represent the persistent photocurrent in the detector. In this case, high persistent photocurrent is obtained in the time response curve due to the existence of bandgap states in the β-Ga 2 O 3 films. These trap levels may get charged due to trapping of carriers generated by light.
In order to confirm the existence of defects in the bandgap as well as to identify the peak responsivity, we performed spectral response measurements. The normalized spectral responsivity of the fabricated photodetector www.nature.com/scientificreports/ is shown in Fig. 3(d). Normalization is carried out by dividing maximum responsivity value to whole spectral responses. The spectral photovoltaic response measurements are performed in the wavelength range of 205 to 800 nm at zero bias. The detector shows the peak photoresponse at 265 nm (4.68 eV) which corresponds to  www.nature.com/scientificreports/ the bandgap of β-Ga 2 O 3 . Thus, the peak photoresponse is observed to lie in the solar-blind region (< 280 nm). Further, the photoresponse at lower wavelengths from 265 to 240 nm remains almost constant and then tends to decrease till 205 nm. In the spectral response curve, various defect bands from 320 to 800 nm are observed. The observation of sub-bandgap absorption is due to optically active recombination centers in β-Ga 2 O 3 which tend to generate persistent photocurrent. Thermally stimulated current (TSC) spectroscopy is employed to explore the role of sub-bandgap states in photocurrent transport. In TSC measurements, all the trap levels are charged optically at a relatively lower temperature to mitigate the thermal emission rate of the carriers from the traps. Thereafter, charge carriers are de-trapped by gradually increasing the temperature at a finite rate under dark conditions and simultaneously recording the stimulated current. Figure 4(a) depicts the thermally stimulated current while heating and cooling at 5 K/min respectively after the charging of trap levels at 85 K. The large difference in the heating and cooling curves is indicative of the presence of traps in the material. The net TSC, which is defined as the total stimulated current minus dark current is shown in Fig. 4(b). In the case of thermally activated electron/hole emission, a particular filled trap starts to emit the charge carriers at a characteristic temperature and later on the emission rate continuously decreases with removal of electron/hole from the trap, generating in this way a current peak. In our particular case, two broad current peaks near about 95 and 400 K have appeared in the net TSC spectra. Both the peaks are fitted with Gaussian to deconvolute the trap states as shown in Fig. 4(c,d) corresponding to peak at 95 and 400 K simultaneously. The first peak is deconvoluted in two peaks centered at 93.3 and 125 K whereas the second peak is deconvoluted into three peaks situated at 351.3, 399.7 and 424.7 K. In the variable heating method of TSC, the activation energy (E T ) of a trap state at a heating rate of γ can be defined as 47 www.nature.com/scientificreports/ where T m is the characteristic temperature of TSC peak, v th is the thermal velocity, N c is the density of states in the conduction band, σ n is the capture cross-section of the trap and k is the Boltzmann constant. The second term in Eq. (4) is very small relative to the first term and hence can be neglected 47 . Therefore, Eq. (4) can be approximated for the activation energy of trap state as 47 The activation energies of all the traps corresponding to the aforementioned characteristic temperatures are calculated using Eq. (5). The trap states denoted as E 1 , E 2 , E 3 , E 4 and E 5 correspond to the characteristic temperatures of 93.3, 125, 351.3, 399.7 and 424.7 K respectively. The activation energies of all these traps are tabulated in Table 2 51 . Therefore, the E 2 trap of 234 meV may be originated from unintentional nitrogen doping. The E 3 trap having activation energy 785 meV is the most common trap center in Ga 2 O 3 . It is reported that Fe doping is responsible for this trap 52 . Remaining E 4 and E 5 traps are also common to Ga 2 O 3 . Irmscher et al. reported that Co impurity may be the origin of these deep traps 48 . On the basis of TSC results, the persistence photocurrent of Ga 2 O 3 selfpowered photodetectors is due to five trap levels with activation energies ranging from 0.16 to 0.97 eV. Among them, the E 1 trap with activation energy of 166 meV had the dominant contribution in persistence photocurrent.
Further, flexibility tests of photodetectors are performed using the measurement setup shown in Fig. 5(a). The variation of bending radius of the muscovite substrate having 21 mm length was executed from flat condition to 16 mm radius. Figure 5(b) represents the tensile strain in the film with respect to the bending radius. The strain ɛ induced in the film consisting of thickness t f can be determined as 53 where t s is the thickness of the substrate, η is defined as t f /t s , R c is the radius of the curvature due to bending and χ is denoted as Y f /Y s , Y f and Y s are Young's modulus of thin film and substrate respectively. The value of Young's modulus of Ga 2 O 3 and muscovite are 261 and 190 GPa, respectively 24,54 . The maximum strain of 0.32% is induced in the film for photoelectrical measurements. Both photocurrent and dark current are recorded with varying bending radii which are shown in Fig. 5(e). The CCD camera images of the mica sheet is also recorded corresponding to flat and bending conditions which are shown in the inset of Fig. 5(c,d). The radius of curvature was 4.99 mm analogous to a 16 mm bending radius. The photocurrent and dark current showed no obvious change and remained almost constant under bending tests which is desirable feature for a flexible photodetector. The excellent performance under bending may be attributed to good structural stability towards mechanical strain of thin film. Other than the structural stability, durability of extremely good metal-semiconductor contacts with mechanical strain resulted in stable dark and photocurrent. In the earlier article, amorphous gallium oxide/muscovite-based photodetectors are reported to possess about one order enhancement in dark as well as photocurrent 7 . Table 3 shows the comparisons of flexible UV-C photodetectors based on gallium oxide thin films. Hence, epitaxial Ga 2 O 3 /muscovite photodetectors demonstrate much better performance stability towards mechanical flexibility and also has zero power consumption which are very useful properties for e-skins.

Conclusions
Heteroepitaxy growth of β-Ga 2 O 3 ( − 2 0 1) on layered muscovite mica substrate has been carried out using pulsed laser deposition. The epitaxial relationship between β-Ga 2 O 3 and muscovite is also established using HRXRD and HRTEM. The MSM photodetector made on Ga 2 O 3 thin film shows a photovoltaic response corresponding to 265 nm illumination. It is noticed that the photoresponse is influenced by persistence photocurrent. Thermally

Experimental section
Sample fabrication. The heteroepitaxy of β-Ga 2 O 3 on cleaved muscovite (001) is grown using a pulsed laser deposition system (Nobert's electron beam and laser deposition equipment). A KrF excimer laser having 248 nm wavelength is utilized to ablate the tin-doped (1 wt%) gallium oxide target. The laser fluence is kept at 1.5 J/cm 2 with a repetition rate of 5 Hz. The muscovite substrate is placed at a 50 mm distance from the gallium oxide target. The temperature of 550 °C is maintained during the deposition at an oxygen pressure of 5 × 10 -3 mbar.
Material characterizations. The X-ray diffraction is acquired with Cu K α (λ = 1.54 Å) source using a Panalytical Xpert Pro system. The topography of the thin film is mapped using XE-100 Park AFM system. Nanostructural characterization is performed using field emission transmission electron microscope (FE-TEM) system (Jeol, JEM-F200) with a point resolution of about 0.23 nm.
Device fabrication and characterization. The 600 µm long interdigitated electrodes (IDE) at 200 µm spacing are patterned using a metal mask. E-beam evaporation system (Scientific Vacuum System Ltd.) is used to fabricate Pt (20 nm)/Au (80 nm) metal contacts for IDE. Photoelectrical measurements are carried out in an optical cryostat (Janis VPF-700). For Spectral photoresponse distribution, the wavelength of incident light is tuned using a monochromator and Xenon lamp. At the same time, the output current is measured using an electrometer (Keithley, 6517). Further, the photocurrent spectrum is calibrated by power spectrum of the Xenon lamp for photoresponse measurement. The photoresponse distribution is normalized with respect to the maximum response value.
Thermally stimulated current measurements. For the TSC experiment, the device is cooled down from room temperature to 85 K under the dark conditions in a cryostat. The trap filling in Ga 2 O 3 is performed under 265 nm wavelength illumination at 85 K. To ensure the better trap filling, the device is illuminated at zero bias for 20 min with 20 μW/cm 2 power density of 265 nm light. Further, illumination is turned off and charge carriers were de-trapped by heating the device up to 450 K with a constant heating rate of 5 K/min. Simultaneously, the thermally stimulated current is recorded with increasing temperature. The dark current measurements are also conducted with temperature to ensure the net TSC in the device.

Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. In future, it will be provided on request by corresponding author.