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Abstract
An all-optical tunable terahertz modulator based on a BiFeO3/Si heterostructure is proposed. Optical and transmission properties of the BiFeO3/Si sample are characterized by the terahertz time-domain spectrometer. Under an external optical pumping, the modulator demonstrates an optical power-dependent modulation effect. A maximum modulation depth of 91.13% can be acquired when the optical pumping power is 700mW in the observed frequency. Due to the separation and localization of photogenerated carriers caused by the BiFeO3/Si heterostructure, the conductivity of the device can be changed and finally resulting in a modulation of the incident terahertz wave. In addition, the photoconductive property of the BiFeO3 thin film on Si substrate is investigated to further explore and interpret the working mechanism of the proposed modulator.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) waves generally refer to electromagnetic waves with a frequency of 0.1–10 THz in the electromagnetic spectrum [1]. In recent years, terahertz wave has attracted widespread concern and reveals potentials in numerous applications, especially in homeland security and fundamental research [2–5]. THz science and technology have been developed rapidly over the past decade due to its superiority in imaging, spectroscopy, security, and biomedical research [3,6–13]. Progress in THz science and technology requires proper THz components such as sources, detectors and modulators [14–19]. Among them, THz modulators are always a research hotspot because of its urgent demand and playing a key role in THz communication system [20]. In order to promote the development of the THz technology, many new ways to control and manipulate THz waves have been invented and created by researchers, which include modulation based on semiconductor materials, ferroelectric materials, liquid crystal materials, novel 2D materials and metamaterials [21–30]. These THz modulators can be classified by various approaches such as mechanical, thermal, optical, electronic, photoelectric, and magnetic modulation. Exploring new ways or looking for new materials or their combination may be a suitable breakthrough for the THz modulation. In addition, terahertz technology has been widely used for research on the properties of various materials [5,31–34].
Bismuth ferrite (BiFeO3), also commonly referred to as BFO in materials science, is an inorganic chemical compound with perovskite structure and one of the most promising multiferroic materials. The room-temperature phase of BFO is classed as rhombohedral belonging to the space group R3c [35]. It is synthesized in bulk and thin film form and both its antiferromagnetic (G-type ordering) Neel temperature and ferroelectric Curie temperature are well above room temperature (approximately 653 K and 1100 K, respectively) [36,37]. Due to its fascinating intrinsic properties such as the ferroelectric/photovoltaic effect [38–41], and potential applications in functional devices such as solar cells, new sensor, memory and photoelectric devices [38,42–48], BFO has attracted a lot of scientific interest. So far, many interesting phenomena and important properties have been found in BFO-based heterostructures including magnetic, optical, electrical, and valley properties and the interactions among them [49]. Since the discovery of the photovoltaic effect in bulk BFO [38,50], the photovoltaic BFO-based heterostructures have been extensively studied. BFO films with various morphologies have been fabricated to realize various functional devices. However, most studies focus on the photovoltaic effects and ferroelectric switchable diodes of the BFO crystals or BFO thin films in the visible wavelengths [38,41,51–53], some basic properties of BFO have not been fully characterized and systematically studied in the terahertz range. Thus, it is worth and necessary to explore the basic properties of BFO in the THz range, which can be easily studied and characterized by the terahertz time-domain spectrometer.
In this paper, we demonstrate an all-optical tunable terahertz modulator based on BFO/Si heterostructure under the effect of an external 532-nm solid laser pumping. A terahertz time-domain spectrometer is used to investigate the transmission and optical properties of the BFO/Si sample. Furthermore, the photoconductive properties of the BFO thin film on Si substrate are also studied under the external optical pumping at room temperature.
2. Experiment details
2.1 Preparation
The THz modulator based on BFO/Si heterostructure, as shown in Fig. 1(b), is prepared by the following steps:
- 1) The BFO film is deposited on a (100)-oriented 20×20×0.5 mm3 p-type Si single crystal using the radio frequency magnetron sputtering method. The deposition parameters of the BFO film are summarized in Table 1.
- 2) The deposited BFO film is annealed by the traditional thermal annealing at 650℃ for about 30 minutes.
- 3) The thickness of the BFO film measured by the step profiler is about 40 nm.
Fig. 1. The experimental design and illustration of the sample. (a) Principle diagram of the THz-TDS system, (b) Structure of the THz modulator based on BiFeO3 and Si.
Table 1. Sputtering parameters of BFO thin film
2.2 Measurement
Crystal phase analysis is performed on a PANalytical X-ray diffractometer (XRD, PANalytical PW3040-60 MRD). A UV-visible-near-infrared spectrophotometer (SolidSpec-3700, Shimadzu, Japan) is used to measure the optical absorption spectra. In order to study the transmission characteristics and optical properties of the proposed BFO/Si sample, a terahertz time-domain spectrometer (THz-TDS) system produced by Zomega Terahertz Corporation (USA) is used to measure the transmission spectra of the sample under an external optical pumping. An all-solid-state CW laser (center wavelength, 532 nm) is used as the external optical pumping in this experiment. As shown in Fig. 1(a), the incident terahertz beam is used to vertically illuminate on the sample while the pumping light is obliquely incident upon the sample at 30°. The spot size of the incident terahertz beam and the external pumping laser is 4 mm and 8 mm, respectively. The complex transmission of the BFO/Si sample can be calculated from the ratio of the two waveforms via the formula: $\textrm{t}(\mathrm{\omega } )= {E_s}(\omega )/{E_r}(\omega )$, where ${E_s}(\omega )$ is the frequency waveform of the modulator and ${E_r}(\omega )$ is the frequency waveform without the sample.
3. Results and discussion
It is well known that the crystal structure determines the properties of crystal, or lattice structure orientation is vital in determining fundamental properties of materials, such as the mechanical, thermal, optical and electrical properties. This is also true for BFO, which is a well-known room-temperature multiferroic material. Figure 2(a) shows the measured XRD pattern of the Si substrate at room temperature. By comparing the measured XRD pattern with a standard reference of XRD pattern for polymorph BFO thin film, it can be observed that the measured results show a good correspondence with the reference data, which indicates that the deposited BFO thin film is polymorph.
Fig. 2. (a) The XRD pattern of the BFO film on Si substrate at room temperature. (b) The value of the optical band-gap of the BFO thin film calculated by linear extrapolation (Tauc’s plot).
The optical band-gap of the thin film is crucially significant to the optoelectronic properties and its application. It is known that BFO films with different substrates have different energy band gap ranging from 2.2 to 2.8 eV [54]. In this experiment, the optical band-gap of the BFO thin film is characterized by the UV–Visible absorption spectroscopy. The optical band-gap of the BFO thin film is calculated to be 2.44 eV through the linear extrapolation of the Tauc’s plots [55,56] from the UV–Visible absorption spectroscopy as shown in Fig. 2(b). The calculated value is slightly smaller than the reported value for the epitaxial BFO thin film [44,57], which might be because of the grain boundaries in the polymorph thin film.
The terahertz time-domain spectrum of the BFO/Si sample is directly measured by the THz-TDS (see in Fig. 1(a)). Figure 3(a) shows the typical experiment results of the THz time-domain signal waveforms passing through the sample. Considering the influence of environmental factors, such as humidity and temperature, the time-domain spectrum of air (black line in Fig. 3(a)) is measured as a reference. Meanwhile, the time-domain spectrum of Si (red line in Fig. 3(a)) is measured as another reference. Furthermore, the time-domain spectrum of the BFO/Si sample (lines in dotted frame in Fig. 3(a)) are measured under the same conditions as Si. With increasing optical pumping power, there is a significant reduction in the time-domain signal waveforms of the BFO/Si sample. To illustrate this tendency more clearly, Fig. 3(b) plots the peak-to-peak value of the time-domain signal waveforms for the sample as function of optical power. For example, when the optical power is 700 mW, the peak-to-peak value of the time-domain spectrum decreases from 595 to 141 compared to that without the optical pumping. Therefore, it can be found that the BFO/Si sample shows a modulation effect for the incident terahertz wave under the external optical pumping.
Fig. 3. (a) Time-domain signal waveforms of air, Si substrate and the BFO/Si sample at room temperature. (b) Peak-to-peak value of the time-domain signal waveforms for the BFO/Si sample as function of optical power.
In order to better clarify the modulation effect of the BFO/Si sample, the transmission characteristics of the sample under different optical pumping powers are investigated. Figure 4(a) shows the transmission spectra of the BFO/Si sample as pumping power changing from 0mW to 700mW. The transmission of the sample obviously decreases with increasing the pumping power. Figure 4(b) shows the transmission variation of the BFO/Si sample at the frequency of 0.2 THz, 0.4 THz, 0.6 THz, 0.8 THz and 0.95 THz with different optical powers, respectively. Without illuminating the transmission of the sample at 0.95 THz reaches 0.55. However, this value remarkably decreases to 0.05 when the pumping power reaches 700mW. Meanwhile, the transmission of the sample at 0.2 THz, 0.4 THz, 0.6 THz and 0.8 THz all significantly decrease with increasing the pumping power. It is apparent that the optical pumping effects cause a significant reduction in the transmission of the BFO/Si sample. The modulation depth of the THz modulator is obtained from $|{\Delta T/T} |= |{({T_P} - {T_0})/{T_0}} |$, where ${T_0}$ is the transmission without the pumping power and ${T_P}$ is the transmission with the pumping power P. As shown in Fig. 4(c), with the pumping power increasing to 700 mW, the modulation depth of the modulator reaches to 80.42%, 80.76%, 82.71%, 87.08% and 91.13% at 0.2 THz, 0.4 THz, 0.6 THz, 0.8 THz, and 0.95 THz, respectively. As a result, with changing the optical pumping power, the BFO/Si sample has a significant modulation effect for the incident terahertz wave.
Fig. 4. (a) The transmission spectra of the BFO/Si sample under the effect of the external optical pumping. (b) Transmission variation and (c) modulation depth of the BFO/Si sample under different optical powers at 0.2 THz, 0.4 THz, 0.6 THz, 0.8 THz and 0.95 THz, respectively.
As we know, the transmission of the terahertz wave in materials is influenced by the conductivity of materials. The conductivity of the THz modulator based on BFO/Si heterostructure, is affected by the behavior of carriers, which determines the modulation effect of the device. Therefore, the critical point of understanding the physical mechanism of the device is exploring and understanding the behavior of carriers. In this experiment, the transmission of the BFO/Si sample decreases with increasing the optical pumping power, which indicates the conductivity of the modulator changes due to the photogenerated carriers excited by the optical pumping. Since the optical band-gap of Si and BFO thin films are 1.12 eV and 2.44 eV respectively, the pumping light (532 nm, 2.33 eV) passes through the BFO thin film and is mainly absorbed by the Si substrate. Thus, the photogenerated carriers are primarily generated in Si. Without the BFO thin film, the photogenerated carriers will quickly recombine due to the shorter carrier lifetime compared with the BFO/Si sample, which leads to a decrease in the number of the photogenerated carriers, thus restricting the variation of the conductivity. In order to break this limitation, an efficient way is to separate photogenerated electrons and holes into different regions to inhibit the electron–hole recombination. In the BFO/Si heterojunction the built-in electric field just acts this role, which could facilitate the separation of electrons and holes. It is the key point in the BFO/Si-based THz modulator. To better understand this process, Fig. 5 demonstrates the energy band structure of the BFO/Si system [58]. Before contact, the silicon and the BFO have their own band structures as shown in Fig. 5(a), where the optical band-gap of Si and BFO are 1.12 eV and 2.44 eV and the electron affinities are 4.01 eV [59] and 3.30 eV [54,60], respectively. Here we suppose that the Fermi energy level of BFO thin film is close to the middle of the energy gap. After contact as shown in Fig. 5(b), due to the diffusion and drift of carriers the BFO/Si heterojunction shows a band bending behavior, and a space charge region (gray areas in Fig. 5(b)) at the interface of Si and BFO is formed where producing a built-in electric field. It is just because of the existence of the built-in electric field that the photo-excited carriers can be separated. This is the key point of the working mechanism for the device. In Fig. 5(c), when pumping light is irradiated on the sample, the photogenerated carriers are separated under the force of the built-in electric field where the photogenerated electrons are transferred from Si to BFO and restricted in BFO thin film. Meanwhile, the photogenerated holes move in the opposite direction and are restricted in Si. Therefore, due to the electrons and holes being separated and restricted in different regions of the device, the low collision probability between electrons and holes leads to a long-life time of the free carriers. As increasing the pumping power, more electrons are generated and limited in BFO thin film, the conductivity of the BFO thin film increases. When the terahertz wave passes through the sample, most THz wave is absorbed and reflected by the sample, thus the transmission decreases.
Fig. 5. The energy band structure of BFO/Si system. Band structure of the p-type silicon and BFO (a) before contact and (b) after contact. (c) Band structure of the BFO/Si sample under the optical pumping power.
To further explore the carriers transport properties in the BFO/Si sample, it is necessary to investigate the conductivity of the BFO thin film on silicon, especially under the effect of external optical pumping. The conductivity of the BFO thin film can be calculated from $\textrm{t}(\omega )= ({1 + {n_{sub}}} )/({1 + {n_{sub}} + {Z_0}\sigma (\omega ){t_{film}}} )$ [26], where $\textrm{t}(\omega )$ is the transmission of the BFO thin film, ${n_{sub}}$ is the complex refractive index of substrate Si, ${Z_0} = 377\mathrm{\Omega }$ is the impedance of free space, and ${t_{film}}$ is the thickness of the BFO thin film. Figure 6(a) shows the conductivity of BFO thin film in the terahertz band under different optical powers. In the absence of external optical pumping, the conductivity changes little because large band-gap inhibits the generation of the photogenerated carriers (or electron-hole pairs). When optical pumping is applied, a significant increment in the conductivity of the BFO thin film originates from a dramatic increase of the photogenerated carrier concentration. With increasing the optical power, more photo-excited electrons are generated and transferred from Si to BFO and finally limited in BFO thin film, which leads to the increments of the BFO conductivity. The Drude model is employed to fit the experimental data to understand the mechanism underlying this case. The real part of the Drude conductivity can be expressed as $\textrm{Re}({\mathrm{\sigma }(\omega )} )= ({\gamma \omega_p^2} )/({\pi ({{\omega^2} + {\gamma^2}} )} )$, where $\mathrm{\gamma }$ is the damping coefficient and ${\omega _p} = \sqrt {N{e^2}/({{\varepsilon_0}m} )} $ is the plasma frequency. In the expression of the plasma frequency, N is the number density of carrier, e is the electronic charge, ${\varepsilon _0}$ is the free-space permittivity, and m is the effective carrier mass. As shown in Fig. 6(a), the fitting results indicate that the carrier density increases from $1.56 \times {10^{17}}c{m^{ - 3}}$ to $1.21 \times {10^{22}}c{m^{ - 3}}$ with the optical pumping power increasing from 0 mW to 700 mW in the BFO. This means that the photogenerated carriers obviously increase with the excitation of the external optical pumping, which has a significant contribution to the BFO conductivity as shown in Fig. 6(b). However, for the BFO/SiO2/Si sample, the carrier density increases from $1.95 \times {10^{17}}c{m^{ - 3}}$ to $3.56 \times {10^{18}}c{m^{ - 3}}$ when the optical pumping power is increased from 0 to 700 mW. Due to the SiO2 intermediate layer between Si and BFO, the photogenerated electron-hole pairs can’t be effectively separated and restricted in different regions, thus the change of the conductivity in the BFO thin film is much smaller. Through comparative analysis, it can be concluded that a large number of photogenerated carriers are excited by the optical pumping in BFO/Si sample, which leads to a large increase in the BFO thin film’s conductivity. As a result, the transmission of THz wave decreases with enhancing the pumping power when the THz signal passes through the BFO/Si sample.
Fig. 6. (a) Experimental and fitting results of the real part of conductivity for the BFO thin film on the Si substrate with different optical power at room temperature. (b) Experimental results of the real part of conductivity for the BFO thin film at 0.6 THz in different samples. the black spheres represent the Si substrate and the red spheres represent the SiO2/Si substrate.
4. Conclusion
In summary, a terahertz modulator based on BFO/Si heterostructure is proposed in this work. Under the effect of an external optical pumping, the transmission and optical properties of the BFO/Si sample are characterized and investigated by a THz-TDS system. The sample exhibits an optical power-dependent modulation effect for the incident terahertz wave with a maximum modulation depth of 91.13%. The energy band theory of semiconductor is used to account for the work mechanism of the THz modulator based on the BFO/Si heterostructure. The key point is that the built-in electric field in the BFO/Si heterostructure promotes the separation of the photogenerated carriers, which leads to change the conductivity of the device, thus realizing a modulation of the incident THz waves. In addition, the photoconductive characteristic of the BFO thin film on Si substrate is studied to further understand the carrier transport property in the BFO/Si sample. This work provides a new path for studying the multiferroic material BFO and extends the practical application of BFO thin films in the terahertz region.
Funding
National Safety Academic Fund (U1930117).
Acknowledgments
We thank Guang Huang, engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO, for support in preparation of the samples, and thank the Analytical and Testing Center (ATC) of WNLO for support in testing of the samples.
Disclosures
The authors declare no conflicts of interest.
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