High-Performance All-Optical Terahertz Modulator Based on Graphene/TiO2/Si Trilayer Heterojunctions
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In this paper, we demonstrate a trilayer hybrid terahertz (THz) modulator made by combining a p-type silicon (p-Si) substrate, TiO2 interlayer, and single-layer graphene. The interface between Si and TiO2 introduced a built-in electric field, which drove the photoelectrons from Si to TiO2, and then the electrons injected into the graphene layer, causing the Fermi level of graphene to shift into a higher conduction band. The conductivity of graphene would increase, resulting in the decrease of transmitted terahertz wave. And the terahertz transmission modulation was realized. We observed a broadband modulation of the terahertz transmission in the frequency range from 0.3 to 1.7 THz and a large modulation depth of 88% with proper optical excitation. The results show that the graphene/TiO2/p-Si hybrid nanostructures exhibit great potential for terahertz broadband applications, such as terahertz imaging and communication.
KeywordsTHz modulator TiO2 interlayer Single-layer graphene Built-in electric field Broadband modulation
Ultraviolet photoemission spectroscopy
Terahertz (THz) imaging technology  and terahertz communication technology [2, 3] are two major research directions in the field of THz. And the THz modulators are the basic components of the technologies, which can modulate the transmission and reflectivity of THz waves by modulating signals (light, electricity, heat, etc.) . Much research has been done on THz modulators [5, 6], mainly focusing on materials. Semiconductor materials, such as Si and Ge, have been used for THz modulators. But the modulation performance is not ideal, and the modulation depth is not high, so many new materials have been proposed [7, 8, 9]. A representative new material is metamaterial. High-speed THz modulators can be realized by combining metamaterial with semiconductors. However, the bandwidth of the modulators based upon metamaterial is still very narrow due to the fixed structure and the fabrication process is complicated [10, 11]. Another typical material is a phase change material, such as VO2. At a certain temperature or voltage, the VO2 can undergo a reversible phase change between the insulating and metal states, and the electromagnetic properties change accordingly. The metallic state can cause an attenuation of the THz wave. But the THz wave can easily penetrate the insulating state of VO2. Therefore, the THz transmission can be modulated by applying external excitation to make the phase change of VO2. But such modulators [12, 13, 14, 15] are based on the change of temperature, and have a slower temperature drop, so the modulation speed is slow.
In recent years, graphene has been gradually applied to THz technology due to its excellent electronic, optical, and mechanical properties [16, 17, 18, 19]. Lee et al. fabricated an electrically controlled THz modulator by integrating graphene with metamaterials . When electrical and optical properties of graphene were enhanced by the strong resonance of metal atoms, the light-matter interaction is enhanced, realizing the amplitude modulation of transmission terahertz wave by 47% and phase modulation by 32.2%. In 2012, Sensale et al. prepared a graphene-based field effect transistor (GFET) THz wave modulator, while the gate voltage tuned the carrier concentration in graphene . However, the modulation depth of this kind of modulator [22, 23, 24] was shallow because of the limited carrier injection. The graphene/n-Si THz modulator prepared by Weis et al. has a modulation depth of up to 99% under the excitation of 808 nm femtosecond pulse laser . Later, the graphene/n-Si THz modulator made by Li et al. achieved a modulation depth of 83% with simultaneous electrical and optical excitations. However, when no electric field was applied, only the light was added, and the modulation effect was not very well . As a low-cost, non-toxic, and chemically stable semiconductor material, titanium dioxide (TiO2) has attracted great attention in the field of energy and environment. It is not only used for photocatalytic degradation of environmental pollutants, but also widely used in solar cells. Recently, Tao et al. prepared MoS2 film on TiO2 surface . The interface introduced a strong built-in electric field, which enhanced the separation of electron-hole pairs, leading to the improvement of its photocatalytic properties. In 2017, Cao et al. made a high-performance perovskite/TiO2/Si photodetectors . They attributed the improvement in performance to increased separation and reduced recombination of photoexcited carriers at the interface between Si and perovskite by the insertion of TiO2 film. Here, a graphene/TiO2/p-Si nanostructured all-optical THz modulator was fabricated. The device we designed has a large modulation depth of maximum 88% in the frequency range from 0.3 to 1.7 THz.
The 500-μm-thick Si (p-type, resistivity ρ ~ 1–10 Ω cm) substrates were sequentially washed with acetone, ethanol, and deionized water for 20 min in an ultrasonic bath, and then immersed into 4.6 M HF solution for 10 min to remove the native oxide layer on the surface. Next, the cleaned Si was immersed into 0.1 M TiCl4 aqueous solution at 343 K for 1 h to obtain 10-nm-thick TiO2 film. Monolayer graphene was grown on copper by chemical vapor deposition . And then, the graphene was transferred onto TiO2 film by using a wet etching method  to form graphene/TiO2/p-Si heterostructure. The entire sample area is 1 cm2. The quality of graphene was characterized by Raman spectroscopy. The absorption spectra were measured by a UV-visible spectrophotometer (Shimadzu, UV-3600). The ultraviolet photoemission spectroscopy (UPS) (Thermo Scientific, Escalab 250Xi) measurements were performed to get the energy band structure. The static modulation was evaluated by Fico THz time-domain system (Zomega Terahertz Corporation).
Results and Discussion
In summary, we have successfully fabricated a high-performance all-optical graphene/TiO2/p-Si terahertz modulator. The modulator exhibits broadband ranging from 0.3 to 1.7 THz, with 88% modulation depth. The inserting of TiO2 film introduced a PN junction with p-Si, and the built-in electric field enhanced the separation of photoexcited carriers in Si. The photoelectrons migrated from Si to TiO2, and then injected into the graphene layer, causing the Fermi level of graphene to shift into a higher conduction band. Therefore, the THz transmission modulation could be realized because of the increase of conductivity in graphene. The device is also very easy to make and low-cost. There is no need to deposit electrodes, and the TiO2 film can be prepared by a chemical solution method. What is more, the laser we used is a semiconductor laser, not necessarily the expensive femtosecond pulse laser as a modulation signal.
This work was supported by the National Key Research and Development Plan (No. 2016YFA0300801), the Sichuan Science and Technology Support Project (No. 2016GZ0250), and the International Cooperation Project 2013HH0003 and 111 Project No. B13042.
Availability of Data and Materials
All data supporting the conclusions of this article are included within the article.
MQW conceived the idea, fabricated the devices, and wrote the paper. DNZ participated in the TiO2 film fabrication. YPL and TLW helped with the THz characterization. LZ contributed to the sample fabrication. LCJ and FMB analyzed the data. HWZ supervised the paper. All authors read and approved the manuscript.
The authors declare that they have no competing interests.
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