Study on thickness-dependence characteristics of bismuth ferrite (BFO) for ultraviolet (UV) photodetector application

The present research work reports on the fabrication of ultraviolet (UV) photodetectors using bismuth ferrite (BiFeO3, BFO) thin films with varying thickness. Using the spray pyrolysis technique, BFO thin films were deposited on the glass substrate at 673 K. The deposited BFO thin films were characterized by Raman and FTIR spectroscopic analysis. The morphological analysis reveals uniform grain distribution for the prepared BFO samples. The optical analysis reveals that transmittance value decreases upon an increase in the thickness of BFO thin films and the calculated optical band gap value lies between 2.0 to 2.3 eV. The varying thickness of the BFO active layer was stacked between ITO and Al electrodes and the current–voltage (I–V) characteristics of the fabricated ITO/BFO/Al devices were studied under dark and UV illumination (λ = 365 nm). It was observed that BFO with an optimum thickness (365 nm) exhibits higher photoresponsivity of 110 mA/W with an external quantum efficiency (EQE) of 37.30%. The impact of different thickness of the BFO active layer, the role of adsorption and desorption of oxygen (O2) molecules upon the surface of BFO layers towards UV photoresponse characteristics were investigated.


Introduction
A photodetector is an optoelectronic device that converts the light energy into electrical energy and has potential applications in optical communication [1], spectroscopic instruments [2], defense operations [3], detection of moving objects [4], and also in the biological field [5]. An efficient photodetector can be classified in terms of higher photoresponsivity, large ON/OFF ratio, and low operating voltage [6]. Commercially available silicon (Si) based photodiodes have the potential in the detection of visible light. However, such Si-based photodiodes have demerits such as low sensitivity towards ultra-violet (UV) light and high production costs involved in the fabrication process [7,8]. Wide bandgap semiconductors such as ZnO [9], NiO [10], and TiO 2 [11] based photodetectors were extensively studied towards UV light detection applications. Recent research includes study on ferroelectric materials such as BaTiO 3 [12], BiFeO 3 [13], Bi 0.8 Pr 0.2 FeO 3 [14] and Bi(Fe,Mn)O 3 [15] towards fabrication of UV photodetectors [16]. In such ferroelectric materials, the presence of an internal electric field prevents the recombination process between electron-hole (e-h) pairs and facilitates the separation of charge carriers [17]. Among various ferroelectric materials, bismuth ferrite (BiFeO 3 , BFO) has attracted intensive interest in the fabrication of photonic devices [18]. BFO has a perovskite rhombohedral structure, energy bandgap (E g ) of 2.1 to 2.8 eV, a large absorption coefficient and higher remanent polarization [18]. The realization of high-quality BFO thin films towards device applications has been successfully reported by various physical methods such as r.f. sputtering [19], pulsed vapour deposition (PLD) [20], molecular vapour epitaxy (MBE) [21]. However, the deposition of BFO thin films using non-vacuum chemical route is of particular interest due to its unique advantages such as low-cost and large-area device processing [22].
In the present research work, the BFO thin films with varying thickness were deposited using spray pyrolysis technique on a pre-cleaned ITO coated glass substrate. The effect of BFO thin film thickness on structural, morphological and optical properties was analyzed. The current-voltage (I-V) characteristics under dark and UV illumination were investigated. The role of adsorption and desorption of oxygen molecules towards the photoresponse switching behavior of the fabricated BFObased UV photodetector were investigated.

Deposition of bismuth ferrite (BFO) thin films
For the preparation of bismuth ferrite (BFO) thin films, 0.33 M equimolar concentration of bismuth nitrate pentahydrate (Bi(NO 3 ) 3 . 5H 2 O) and ferric nitrate nonahydrate (Fe(NO 3 ) 3 . 9H 2 O) was dissolved in 30 ml deionized water independently and allowed to continuously stir at room temperature for 1 h. Secondly, 10 ml of dilute nitric acid (HNO 3 ) was added dropwise to the prepared (Bi (NO 3 ) 3 . 5H 2 O) base solution and stirred for 15 min. Finally, 0.33 M of citric acid as a chelating agent was added to the solution and stirred for 30 min. The prepared homogeneous solutions were used to deposit BFO thin films by spray pyrolysis method. The glass substrates were cleaned using detergent and deionized water. Further, ultrasonic cleaning was carried out for 30 min using a mixture of isopropyl alcohol (IPA) and HNO 3 solution. The cleaned glass substrate was preheated at 423 K before the deposition of BFO thin film. The substrate temperature was maintained at 673 K with the fixed air pressure of 2 mbar. The solution flow rate was 5 ml min −1 and the nozzle-substrate distance was fixed at 15 cm. The prepared BFO thin films were post-annealed at 623 K for 1 h under ambient atmosphere. To investigate the impact of the different thickness of BFO thin films in the device performance, the volume of the precursor solution while spraying was varied as 10, 15, and 20 ml and the samples were coded as BFO(10), BFO (15) and BFO (20) respectively. The thickness of the deposited BFO thin films was measured using the Stylus profilometer (DEKTAK XT Stylus Profiler-Bruker, USA) and the value was measured as 243 ± 7, 365 ± 15 and 660 ± 12 nm for BFO(10), BFO (15) and BFO (20) respectively. The structural study of BFO thin film samples was investigated by Raman spectra using BRUKER RFS 27: Standalone FT-Raman Spectrometer. The surface morphology BFO thin film samples were performed by High-Resolution Field Emission Electron Microscope system (FEI Quanta FEG 200-High). The Fourier transform infrared spectra (FTIR) spectra of the thin film samples were performed using IR Tracer-100-Shimadzu and optical properties were analyzed using UV Visible Spectrophotometer (Thermo scientific Evolution 201).

Device fabrication
For the fabrication of BFO-based UV photodetector devices, the following steps were processed: indium tin oxide (ITO) was deposited on the ultrasonically cleaned glass substrate (2.5 cm × 2.5 cm) using the r.f magnetron sputtering method. Commercially purchased ITO target (99.9% purity, 2 inch, Able targets, China) was used during the sputtering technique. Highly transparent and electrical conducting ITO thin films (90 Ώ/cm) were used as a back electrode for the proposed device. BFO with different thickness (243, 365 and 660) were deposited upon the ITO layer using the spray pyrolysis method at 673 K. Aluminum (Al) was deposited as a top electrode using an electron beam evaporation (EBE) approach. Figure 1 shows the schematic diagram of the fabricated device structure (ITO/BFO/Al). The current-voltage (I-V) characteristics of the proposed ITO/BFO/Al device under dark and UV illumination were measured using the Agilent B2901A source measuring unit (SMU) using a scan rate of 0.06 V/ms. The source of UV light is the commercially purchased monochromatic UV lamp (Wavelength ƛ = 365 nm and power = 4 mW/cm 2 ). The photograph of the fabricated ITO/BFO/Al photodetector and experimental set up used to study the photosensing characteristics is included as supplementary information (see Additional file).

Results and discussion
Raman spectroscopic analysis Figure 2 shows the Raman spectroscopic analysis for (a) BFO (10) and (b) BFO (15). The space group of the BFO material is R3c and group theory predicts that BFO should have 13 Raman-active phonon modes summarized as Ѓ = 4A1 + 9E; where A 1 and E represent the symmetry in rhombohedral distorted structure [23]. In the present study, four optical transverse A 1 symmetry [A1(TO)] and six optical transverse E-symmetry [E(TO)] phonon modes were recorded. The Raman modes positioned at 122, 171, 221 and 432 cm −1 can be assigned as A 1 -1, A 1 -2, A 1 -3 and A 1 -4 modes respectively. The other six modes positioned at 77.6, 113, 278, 348, 485 and 560 cm −1 can be assigned as E-1, E-2, E-4, E-5, E-7 and E-8 respectively. The low-frequency modes correspond to bismuth (Bi) and oxygen (O 2 ) vibrations and the higher frequency modes correspond to Fe-O vibrations [24]. The Raman active modes observed in the 160 to 365 cm −1 range were related to the atomic motion of Fe and O 2 molecules in the FeO 6 octahedron of BFO [25]. Raman active modes that are identified above 500 cm −1 are ascribed to the stretching vibrations of oxygen atoms [26][27][28].     (20) thin films increases [32]. As a result, the grain size of BFO (20) gets reduces significantly as observed in Fig. 4c. Figure 5a exhibits the optical transmittance spectra of BFO (10), BFO (15), and BFO (20) thin films in the wavelength spectrum of 350-1800 nm. An average transmittance between 25 and 60% in the visible region was observed for all the prepared BFO thin films. The bandgap energy of BFO thin films was determined by extrapolating the linear part of the tauc's graph as shown in Fig. 5b. The calculated optical band gap energy values were found to be 2.3 eV for BFO (10), 2.2 eV for BFO (15), and 2.1 eV for BFO (20). The observed decrease in the value of the bandgap upon an increase in the thickness of the BFO thin films may be due to the increase in grain size of the samples and also due to an increase in the crystallinity of the films [33,34].  [18]. Notable important parameters to determine the performance of UV photodetector includes photoresponsivity (R) and external quantum efficiency (EQE) and were calculated using the following Eqs. (1) and (2) respectively [35] where I UV and I dark signify the value of current measured under UV exposure and dark conditions and P inc represents the power of the incident UV light source (4 mW/cm 2 ). Table 1 reveals the UV photoresponse properties of ITO/BFO(10)/Al, ITO/BFO(15)/Al and ITO/BFO(20)/Al based photodetectors. Variation in the obtained photoresponse properties among the fabricated devices shows that the thickness of the active layer plays one of the vital parameters towards the realization of an efficient photonic device [36]. Among the fabricated devices, ITO/ BFO(15)/Al photodetector with an optimum thickness of BFO (365 nm) showed higher photoresponsivity (R) of 110 mA/W with an external quantum efficiency (EQE) of 37.30%. The reason for the obtained low photoresponse characteristics at higher BFO thickness (~ 20 ml) of the active layer is due to the limitation in the penetration depth of incident UV photons [37]. It is reported that UV photons interact predominantly with the surface of the active layers for an optimal penetration depth (~ 300 nm) and such a thin surface of the active layers contributes towards photoconductivity mechanism [38]. In addition, the photodetector with a thick active layer results in the formation of a small depletion region which tends to capture fewer incident photons. Thus, ITO/BFO(20)/Al photodetector with thick active layer shows low photoresponse characteristics. On the other hand, the obtained less photoresponse property for ITO/BFO(10)/Al photodetector may be attributed due to the formation of a low internal electric field in the BFO(10) active layer [39]. . When the BFO-based photodetector was exposed to UV light, the photocurrent rises dramatically and the photocurrent steadily decreases to its original level when the light is turned off. When the light is irradiated on the BFO based photodetector, electronhole (e-h) pairs are generated and these photogenerated charge carriers move towards the opposite electrode to contribute to the external photocurrent. The rise time and recovery time for ITO/BFO(15)/Al photodetector were calculated as 6 and 17 s respectively. Figure 8 shows the working mechanism of ITO/BFO/Al photodetector based on oxygen adsorption and desorption process under dark and UV illumination. During the dark condition, an oxygen molecule is adsorbed by capturing the free electron from the BFO thin film surface and forms a depletion layer near to the surface of BFO. The formation of the depletion region significantly reduces the electrical conductivity of BFO layers. The electron-hole pairs (e-h) are produced when the ITO/BFO/Al photodetector is illuminated with UV light with the condition E hv > E BFO . Such photogenerated charging carriers are driven into the field path by the external bias and neutralize the adsorbed oxygen. The width of the depletion layer will now decrease and the electrical conductivity of BFO thin surface increases under the UV illumination condition [23,29,[40][41][42]. However, it can be observed that the obtained photoresponse characteristics of ITO/BFO/Al photodetector do not show a steady-state condition. Such an exponential increase/decrease of photocurrent value with an unsaturated response is termed as persistent photoconductivity (PPC) effect [43]. Such effect occurs in unsaturated photoresponse characteristics [46,47]. Similarly, during a dark condition, the slower re-adsorption rate results in prolonged unsaturated recovery time [43,44]. The present analysis correlates the role of oxygen adsorption and desorption process under UV light and

Conclusion
The analysis correlates the role of the thickness of the BFO active layer towards the realization of an efficient UV photodetector. The various thickness of BFO thin films was deposited using spray pyrolysis technique and characterized by Raman spectroscopy, FTIR analysis. The morphological and optical analyses were investigated. The BFO active layer was stacked between Al and ITO electrodes and the current-voltage (I-V) characteristics of the fabricated ITO/BFO/Al were analyzed under dark and UV illumination (λ = 365 nm). The I-V analysis showed that
Additional file 1. The photograph of the fabricated ITO/BFO/Al photodetector and experimental set up used to study the photosensing characteristics is included as supplementary information.