Magnetic field tuning of photoelectric and photoluminescence effects in BiFe_0.9Co_0.1O₃ thin film

Recently, multiferroic materials have drawn much attention due to their abundant physical properties and potential application value in the fields of information storage, ^1–3) photoelectric detection, ^4–8) self-powered sensing, ^9–11) and light energy conversion. ^12–15) Semiconductor photoelectric multifunctional devices based on multiferroic BiFeO₃ have become a research hotspot in the field of magnetoelectrics. ^16–18) BiFeO₃ material presents a bulk photovoltaic effect, in which photovoltage generated by the internal electric domain of the material is not astricted by the band gap width. ¹⁹⁾ It has been experimentally confirmed that higher photovoltage than traditional photovoltaic cells (such as Si, GaAs, CuInGaSe, etc) can be obtained in BiFeO₃. ^12,20–22) However, the large resistance in BiFeO₃ makes the photogenerated current small. Therefore, it is difficult to be directly applied in the field of photocells. Alternative research routes in applying multiferroics on photoelectric fields need to be further explored.

In recent years, the field of photoelectric sensors has become an important research direction with multi-field tuning, low cost, high sensitivity and high stability based on the photoelectric effect and external field controllable physics effects. ^23–25) It has been reported that photoelectric performance in BFO-based heterostructure films can be flexibly and effectively adjusted by modulating its magnetic and electric properties. ^26,27) It is an alternative and effective way to research photoelectric properties by introducing the effect of the magnetic field on ferroelectric photovoltaics, since this has a mild influence on the domain structure of the ferroelectric photovoltaic materials. The study of magnetic field effects provides a new idea for the application of multifunctional materials in the field of optoelectronics. ^17,28–31)

In this letter, the BiFe_0.9Co_0.1O₃ (BFCO) thin film was chosen to be a candidate photoelectric thin film since Co doping can enhance the piezoelectric effect of BiFeO₃ by enhancing the polarization rotation space, which makes it possible to obtain strong magnetic and ferroelectric properties by Co doping BFO. ^32–36) Firstly, BFCO was grown on a conductive transparent substrate glass (SnO₂:F, FTO) by PLD method, and the solar cell type heterojunction was prepared with Ag as the back electrode. The photoelectric effect and photoluminescence processes of the BFCO thin film were studied under the condition of different external magnetic fields, and the mechanism of magnetic field tunning photoelectric properties in the thin film were analysed based on the spintronic theory.

100 nm BFCO thin film was grown on conductive FTO substrate by pulsed laser deposition method. The substrate temperature was 650 °C and the deposition atmosphere was an oxygen environment with the pressure of 10 Pa. After deposition, the BFCO thin film was kept in pure oxygen with 10 Pa at 650 °C for 60 mins, and then slowly cooled to room temperature at 10 °C min⁻¹. A square hole mask was added above the sample and a 50 nm thickness Ag layer was deposited on the top surface of the BFCO film by pulsed laser deposition.

Figure 1 presents the XRD pattern of the BFCO thin film, no other hetero-peaks were indexed except for the diffraction peaks corresponding to the standard BFCO phase and the FTO substrate phase. The inset is a magnification of the vicinity of 31° to 32.5°, visually showing that there are two peaks (012) and (110) in this angle range. The main peaks at 22.42° and 31.76° are obvious, indicating that the BFCO thin film on the FTO substrate is well crystallified and has a rhombohedral perovskite phase structure. ^32–36) The results show that the BFCO film have a pure phase structure without any other heterophase.

Zoom In Zoom Out Reset image size

A multiferroic characteristic of the BFCO thin film was detected. Figure 2(a) shows the magnetic hysteresis loop of the BFCO film at 300 K, where the direction of the external magnetic field is along the in-plane of the film. From the hysteresis loop, the remanence of the sample is 52 emu cm⁻³, and the coercivity is 250 Oe. The magnetic moments of two adjacent iron atoms in the BFCO cell are rotated at a certain angle relative to the axis [111], resulting in a net magnetic moment in plane (111). Coupled with the lattice distortion caused by the substitution of Fe position by doped Co in the BFCO film, the ambient temperature is lower than the anti-ferromagnetic Néel temperature, BFCO becomes ferromagnetic materials. ³⁷⁾

Zoom In Zoom Out Reset image size

The electric hysteresis loops of the BFCO film at 300 K are shown in Fig. 2(b). As the electric field strength between the electrodes of the film increases from 50 kV cm⁻¹ to 300 kV cm⁻¹, the polarization strength of the sample gradually increases from 1.61 μC cm⁻² to 11.15 μC cm⁻². The residual polarization intensity increased from 0.12 μC cm⁻² to 4.56 μC cm⁻², and the coercive field increased from 3.25 kV cm⁻¹ to 119.41 kV cm⁻¹, indicating an obvious ferroelectric property of the thin film and the result is consistent with the conclusion of other researchers that BFCO films with a rhombohedral perovskite structure have ferroelectric properties. ^33–36) The above results show that the BFCO film presents multiferroic properties, which is conducive to the study of the ferroelectric photovoltaic effect of the BFCO film and its magnetic field modulation.

The BFCO thin film is placed in the sample platform under the solar simulator (XES-40S1) light source, so that the light can reach the BFCO layer through the FTO layer. The two copper wires were connected to the semiconductor characterization system (Keithley 4200-SCS) in the FTO layer and the Ag layer. The Gaussimeter magnetic sensing probe was fixed near the sample to measure the strength of the magnetic field. The J-V test module of the semiconductor characterization system was used, and the response curves of the J-V characteristics of the BFCO thin film under different magnetic fields were tested. The size of the magnetic field at the sample can be adjusted by changing the position of the NbFeB permanent magnet placed at the bottom of the test platform.

Figure 3 shows the J-V curves of the BFCO film under dark and light conditions. The upper left inset shows the sample structure and the schematic diagram of the measurement experiment. It can be observed that the J-V curves present a typical diode characteristic. This is due to the fact that the electric domains in the ferroelectrically polarized BFCO film are connected in series, forming a directional internal electric field, resulting in a bulk effect. The difference of the J-V curves under dark and light conditions indicate that the BFCO film has an obvious response to the light field. The lower right inset presents the curves enlarged near the origin. The short-circuit current density reaches 3.85 μA cm⁻². The photoconductivity of the sample is greatly enhanced under light.

Zoom In Zoom Out Reset image size

The sample photoconductance can be expressed as

$$\begin{eqnarray}&&\sigma =e{\rm{\cdot }}n{\rm{\cdot }}{\mu }_{{\rm{e}}+{\rm{h}}}\end{eqnarray} \tag{ 1 }$$

Where n and μ_e+h are the carrier concentration and the carrier mobility of the sample respectively. From Eq. (1), it is obvious that carrier concentration n is one of the determining factors of the photoconductivity of the sample. Under light condition, the photons whose energy is greater than the band gap of BFCO can generate non-equilibrium carriers (photogenerated carriers) to achieve light injection into the BFCO film. The conductivity of the BFCO film obviously increases with the increase of carrier concentration, indicating a clear photoconductivity effect in the BFCO film.

Figure 4 presents the J-V curves under different external magnetic fields. The inset is a local enlarged figure near 5 V. It can be seen that the photocurrent increases with the external magnetic field increasing. Under light condition, the photocurrent with the 5 V bias voltage gradually increases from 445 mA cm⁻² to 995 mA cm⁻² when the external magnetic field increases from zero to 400 Oe, presenting an interesting magnetic field enhancing photocurrent effect. This important result portends an effective way to obtain enhanced photoelectric conversion efficiency and provides an alternative candidate material for photoelectric detecting and photoelectric conversing fields.

Zoom In Zoom Out Reset image size

Here, we attempt to explain this magnetic field modulation effect on the photocurrent. As shown in Fig. 5(a), electrons in the Fe/Co position ions of the BFCO film are split in spin exchange, the energy of the state density of the spin-up (σ = ↑) band is generally lower than that of the spin-down (σ = ↓) band. So the electrons with σ = ↑ become the majority-spinning-electrons, which means that the number of spin-up electrons is higher than that of the spin-down electrons (minority-spinning-electrons). This results in a spontaneous magnetization of the electrons. ^38–40) In the BFCO film, Fe and Co are the activation centres of the photogenerated carriers, and the light absorption makes the spin-up traveling electrons in the Ev band transfer to the Ec band and become majority spin photoelectrons (e_maj). At the same time, the spin-down traveling electrons become minority spin photoelectrons (e_min). In the process of the ferroelectric photovoltaic effect, charges are transferred towards other adjacent ions under the action of the depolarization field or external voltage to form a photogenerated current. The energy dispersion relationship of most of the spin photoelectrons (e_maj, σ = ↑) and a few of the spin photoelectrons (e_min, σ = ↓) is ${E}_{{\rm{maj}}}({\boldsymbol{k}})$ and ${E}_{\min }\left({\boldsymbol{k}}\right),$ then

$$\begin{eqnarray}&&{E}_{{\rm{maj}}}({\boldsymbol{k}})=\frac{{k}^{2}}{2{m}^{* }}-h\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{E}_{\min }\left({\boldsymbol{k}}\right)=\frac{{k}^{2}}{2{m}^{* }}+h\end{eqnarray} \tag{ 3 }$$

Where k is the wave vector of the electrons (the Planck constant has been taken to be 1), m^* is the effective mass of the electrons, and 2 h is the exchange energy. Thus, the energy state density N(E_F) of the electrons at the Fermi level E_F depends on the spin cases:

$$\begin{eqnarray}&&{N}_{{\rm{maj}}}\left({E}_{{\rm{F}}}\right)=\frac{{m}^{* }{k}_{{\rm{F}}}^{{\rm{maj}}}}{{\pi }^{2}}=\frac{{m}^{* }{\left[2{m}^{* }\left({E}_{{\rm{F}}}+h\right)\right]}^{\frac{1}{2}}}{{\pi }^{2}}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{N}_{\min }({E}_{{\rm{F}}})=\frac{{m}^{* }{k}_{{\rm{F}}}^{\min }}{{\pi }^{2}}=\frac{{m}^{* }{\left[2{m}^{* }\left({E}_{{\rm{F}}}-h\right)\right]}^{\frac{1}{2}}}{{\pi }^{2}}\end{eqnarray} \tag{ 5 }$$

Where ${k}_{{\rm{F}}}^{{\rm{maj}}}$ and ${k}_{{\rm{F}}}^{\min }$ are the Fermi wave vectors for the majority and minority spinning-electrons, respectively. Apparently, ${N}_{{\rm{maj}}}\left({E}_{{\rm{F}}}\right)\gt {N}_{\min }({E}_{{\rm{F}}}).$ Therefore, the extent of the scattering (generally coming from grain boundaries and domain walls in the thin film) for the majority-spinning-electrons (excited by light) depends on the electron spin orientation. Then, the effective mass ${m}_{\sigma }^{* }$ of the electron related to the spin orientation. So, the different spin channel resistance and total resistance in the film can be expressed as,

$$\begin{eqnarray}&&{\rho }_{\sigma }=\frac{{m}_{\sigma }^{* }}{{n}_{\sigma }{e}^{2}{\tau }_{\sigma }}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{\rho }_{{\rm{T}}}=\frac{{\rho }_{\uparrow }{\rho }_{\downarrow }}{{\rho }_{\uparrow }+{\rho }_{\downarrow }}\end{eqnarray} \tag{ 7 }$$

Where ${n}_{\sigma }$ is the number of spin-polarized electrons, e is the amount of electron charge, ${\tau }_{\sigma }$ is the relaxation time of the spinning-electron scattering, ρ↑ and ρ↓ are the conductive channel resistances coming from the spin-up and spin-down electrons scattering, respectively. Since most of the spin photoelectrons produced by photoexcitation are spin-up carriers, the total resistance of the BFCO film is positively correlated with the density of the spin-up electron states.

Zoom In Zoom Out Reset image size

As we know, the spin polarization in the BFCO film without the magnetic field is relatively weak and the spin-dependent scattering is small. Therefore, it can be understood from Eq. (7) that the contribution of the two channels (corresponding to the spin-up and spin-down electrons) to the total resistivity is similar. As shown in Fig. 5(b), when the external magnetic field is applied, the spin polarization degree of photoelectrons in the BFCO layer is enhanced, i.e., the difference in the number of spin-up and spin-down electrons is increased, which enhances the degree of spin-dependent scattering in the thin film. Therefore, according to Eq. (6), the resistivity of either channel (spin-up or spin-down) decreases, resulting in a reduction of the total resistance ρ_T expressed by Eq. (7). At the same time, the applied magnetic field makes the spin polarization direction of the photoelectrons consistent, which reduces the grain boundary scattering and domain wall scattering in the BFCO layer, and therefore significantly increases the photocurrent density.

The BFCO thin film was placed on the sample frame of the fluorescence spectrophotometer (FL3-22). The response curves of the PLE spectra and the PL spectra of the BFCO thin film under the magnetic field was tested by inserting or removing NbFeB permanent magnets at a fixed position near the film.

Figure 6(a) shows the excitation spectrum (PLE) of the BFCO film with and without applying a magnetic field. The wavelength (λ) of emitting light is 530 nm. It can be seen that the wavelength of excited light corresponding to the strongest peak is 333 nm. By using 333 nm excited light, the emission spectra (PL) of the BFCO film were tested with and without applying a magnetic field. The results were presented in Fig. 6(b). It can be seen that the emission spectrum mainly consists of a short wavelength (414 nm) peak and a long wavelength (540 nm) peak. When applying a magnetic field to the thin film, the position of the two wavelength peaks in the emission spectrum is almost the same as that without a magnetic field. Although, the intensity of the excitation spectrum and emission spectrum obviously decreases when applying a magnetic field to the thin film, indicating that a magnetic field can inhibit the radiative recombination of photoexcited electrons and valence band holes. ⁴¹⁾ The magnetic field reduces the scattering of spin-photoelectrons during their migration process and increases the lifetime of photogenerated carriers. More carriers participate in the migration process under the magnetic polarization field, which in turn means less carriers are involved in the radiative transition and therefore decreases the recombination rate. This process further verifies the experimental results regarding the enhancement of the photocurrent by a magnetic field presented in Fig. 4.

Zoom In Zoom Out Reset image size

In summary, the BiFe_0.9Co_0.1O₃ thin film was deposited on the FTO substrate by PLD method. The BFCO thin film presents multiferroic properties and exhibits obvious photoelectric response properties. Importantly, magnetic field modulating photocurrent effects and photoluminescence effects are detected in the thin film. When the magnetic field increases from zero to 400 Oe, the photocurrent gradually increases from 445 mA cm⁻² to 995 mA cm⁻². The magnetic field significantly weakens the photoluminescence intensity of the film. The magnetic field tuning of the photocurrent in the BFCO film is affected by the spin-dependent scattering and lattice scattering in the process of conduction band motion due to the excitation of light. The enhancement of spin-dependent scattering caused by the magnetic field inhibits the radiative transition of the photoelectrons, thereby reducing the photoluminescence intensity, and decreasing the recombination rate of the photogenerated carriers, and therefore enhances the photocurrent. Our study provides an important reference for enhancing photoelectric characteristics in multiferroic thin films.

Acknowledgments