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Abstract
We demonstrate a novel technique to achieve a highly efficient terahertz (THz) modulation based on hybrid structures of organic layers (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) fabricated on both sides of a silicon (Si) substrate. The organic layer generating an optically induced electron (or hole) transfer is deposited on the back (or front) side of the Si substrate. The spatial charge separation improved owing to the transferred photo-excited electrons or holes at both interfaces of PCBM/Si and TIPS-pentacene/Si, enables a highly efficient THz wave modulation. The photoexcitation on the hole-transfer organic layer (TIPS-pentacene/Si) further improves the modulation efficiency, as the diffusion of electrons through the Si substrate is faster than that of photo-excited holes.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A Terahertz (THz) technologies attract a significant attention owing to their applications in various fields including spectroscopy, imaging, and wireless communication [1,2]. The demand for active THz devices such as modulators, switches, transistors, beam splitters, waveguides, and filters has been constantly growing owing to the development of integrated systems of wireless communication and multi-functional THz spectroscopy/imaging systems [3,4]. Over the past few years, various technologies have been proposed to realize efficient active THz modulators, using various types of structures and materials including plasmonic structures, vanadium dioxide (VO2), integrated graphene layers, two-dimensional semiconductor structures, metamaterials, and their composite structures [5–15].
The latest and highly efficient technology for active THz modulation is based on organic-based hybrid structures. Since Yoo et al. reported active THz modulators using CuPc/Si hybrid structures in 2011, several research groups have concentrated on the improvement of the properties of modulation efficiency, broadband modulation, structural simplicity for a simple fabrication, functionality, and compatibility with existing silicon-based technologies [16–19]. These efforts have led to the fabrication of active THz modulators with an almost perfect modulation efficiency using various organic molecules, perovskites, or polymers such as polyvinyl alcohol [20–25]. In order to reduce the operating power for an active control of the THz modulators, novel methods to achieve a very high modulation efficiency at a lower excitation power are required.
In this study, we demonstrate a novel method for a significant improvement of the modulation efficiency of active THz modulators. Trilayer hybrid structures consisting of organic layers (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) thin films) deposited on both surfaces of a silicon (Si) substrate were fabricated to improve the spatial charge separation caused by the transfer of both photo-excited electrons and holes into the two organic layers. Considering that the diffusion of photo-excited electrons through the Si substrate is faster than that of the photo-excited holes, the photoexcitation at the interface between Si and TIPS–pentacene, acting as a hole-transfer organic layer, enables a very high modulation efficiency of the THz wave transmission.
2. Experimental details
Figure 1 shows the highest-occupied-molecular-orbital–lowest-unoccupied-molecular-orbital (HOMO–LUMO) energy level diagram of the organic-based hybrid structures and their molecular structures. The organics used in the structures are TIPS-pentacene (>99%, HPLC) and PCBM (sublimed, 99.9%) purchased from Sigma-Aldrich (product numbers 716006 and 572500). A 300-μm-thick high-resistivity (larger than 1.0 × 105 Ω·cm) Si was used as a base substrate in order to generate photo-excited carriers. TIPS-pentacene and PCBM thin films were deposited by spin-coating on the front and back sides of the Si wafer, respectively. The TIPS-pentacene thin layer with a thickness of approximately 100 nm was fabricated under the conditions of 2 wt% solution in chlorobenzene, spin speed of 1,000 rpm, and annealing temperature of 180 °C. The PCBM thin layer with a thickness of approximately 100 nm was deposited on the other side of the Si wafer under the conditions of 2 wt% solution in chloroform, spin speed of 1,000 rpm, and annealing temperature of 150 °C. The thicknesses of the organic layers were measured using a depth profiler (Alpha-Step 200). Four types of samples were prepared: 1) bare Si substrate; 2) two-layer structure of TIPS-pentacene and Si; 3) two-layer structure of PCBM and Si, and 4) three-layer structure of TIPS-pentacene, Si, and PCBM.
Fig. 1 (a) HOMO–LUMO energy level diagram of the TIPS-pentacene/Si/PCBM hybrid structure. The insets show the molecular structures of a TIPS-pentacene and PCBM molecules.
We measured time-domain traces of THz waves transmitted through the samples, using a THz time-domain spectroscopy system. Coherent THz waves are generated using a (100) p-type InAs wafer under illumination by femtosecond optical pulses, and detected using a photoconductive antenna [26]. A continuous-wave (CW) laser diode with a center wavelength of 785 nm was employed as an optical source for photoexcitation. The wavelength of 785 nm was chosen as it provides a strong excitation of the photo-carriers in the Si substrate, while the absorption owing to the organic thin layers is negligible. The optical beams were irradiated at an incident angle of 90° on a pinhole with a diameter of 3 mm placed at the center of the sample. The time-domain traces were measured with and without optical excitation, and the corresponding spectral amplitudes were obtained using the fast Fourier-transform method.
For the two-layer structures of PCBM/Si and TIPS-pentacene/Si, the THz waves and optical beam are incident on the surface of the organic thin layers. The understanding of the HOMO–LUMO energy levels of the materials is essential, as it determines the injection of photo-carriers across the organic/Si interfaces. As shown in Fig. 1, at the interface of the TIPS/Si hybrid structure, the photo-excited holes excited in the Si substrate are injected into the TIPS-pentacene thin layer, whereas at the interface of PCBM/Si, the PCBM thin film acts as an electron acceptor layer. For the three-layer structure of TIPS-pentacene/Si/PCBM, the injection of photo-excited electrons and holes occurs simultaneously on both surfaces of the Si substrate. When an optical beam is incident on the TIPS-pentacene layer, the photo-excited holes are immediately injected into the layer and slightly later the electrons are also injected into the PCBM layer after moving through the Si substrate. On the other hand, when the optical beam is incident on the PCBM layer, the photo-excited holes may pass through the Si substrate.
3. Results and discussion
Figure 2(b) shows the Fourier-transformed spectral amplitudes measured with and without photoexcitation for the different types of samples and geometries described in Fig. 2(a). As shown in the inset of Fig. 2(b), the samples do not have considerably different spectral amplitudes in the case without photoexcitation. However, upon the irradiation with the optical beam for photoexcitation, the THz transmissions significantly decrease in all of the samples. The degree of transmission decrease depends on the organics and geometry. The PCBM/Si hybrid structure has a larger transmission decrease than the TIPS-pentacene/Si structure, which could be attributed to the PCBM thin film, which, acting as an electron acceptor layer, improves the charge transfer at the interface owing to its higher electron mobility. The increase of photoexcited carrier density due to the improved charge transfer process may deduce carriers assembling at the interface of silicon and organics, decreasing the transmission of THz waves at a broad THz frequency range [16,17,25,27,28]. Especially, the photo-excited carriers accumulated in the organic thin layers becomes dominant because of their higher carrier concentration and the effect of transferring both photo-excited electrons and holes at both sides of Si
Fig. 2 (a) Structures of the samples of bare Si, PCBM/Si, TIPS-pentacene/Si, and TIPS-pentacene/Si/PCBM, denoted as A, B, C, D, and E, respectively. The optical beams for photoexcitation are incident in the forward (D) and backward (E) directions of the three-layer structure of TIPS-pentacene/Si/PCBM. The arrows represent the direction of the optical beams. (b) Fourier-transformed spectral amplitudes for the samples measured under photoexcitation. The inset shows the spectral amplitudes measured without photoexcitation.
The trilayer hybrid structure of PCBM/Si/TIPS-pentacene exhibits a larger decrease compared with those of the bilayer hybrid structures. In addition, the transmission decrease depends on the direction of the incident irradiation of the optical beam, represented with the signals D and E in Fig. 2(b). Figures 3(a) and 3(b) show the measured transmission spectra as a function of the laser power for photoexcitation for the bilayer (TIPS-pentacene/Si) and trilayer (TIPS-pentacene/Si/PCBM) structures, respectively. When the laser beam for photoexcitation is incident on the side of the TIPS-pentacene thin film, the trilayer structure exhibits a higher modulation efficiency, compared with that of the bilayer structure. The experimental results reveal that the modulation efficiency depends on both structures of the organic layers and incidence direction of the optical beam.
Fig. 3 Contour plots of the spectral amplitudes transmitted through the (a) bilayer (TIPS-pentacene/Si) and (b) trilayer (TIPS-pentacene/Si/PCBM) structures, as a function of the THz frequency and excitation laser power. (c) Modulation efficiencies, as a function of the laser intensity, for the samples of bare Si (black squares), bilayer structures of TIPS-pentacene/Si (blue circles) and PCBM/Si (red triangles), and trilayer structures of PCBM/Si/TIPS-pentacene (green reverse triangles) and TIPS-pentacene/Si/PCBM (magenta diamonds).
In order to quantitatively analyze this phenomenon and its mechanism, the spectral intensity modulation efficiency Meff is defined as the following formula:
where Eun and Eex are the electric field amplitudes measured under the condition with and without photo-excitation, respectively [20]. Figure 3(c) shows the values of the THz modulation efficiency for the samples with different organics and geometries, as a function of the excitation laser power, including the results shown in Figs. 3(a) and 3(b). For all of the samples, the THz modulation efficiency increases with the laser power, as the photo-excited carrier density, increased by the higher power of the optical beam, effectively obstructs the transmission of incident THz waves. The TIPS-pentacene/Si/PCBM structure exhibits a superior modulation efficiency with respect to those of the other structures or geometry, for all excitation laser powers. In particular, the modulation efficiency significantly depends on the incident direction of the optical beam.In order to overcome this issue, we defined an efficiency increment as: ΔMeff = Meff,tri – Meff,bi, where Meff,tri and Meff,bi are the modulation efficiencies for the trilayer and bilayer structures, respectively, as shown in Fig. 4. The efficiency increment at the geometry where the optical beam for photoexcitation is incident on the TIPS-pentacene thin film, ΔMTIPS, is higher than that for incidence on the PCBM thin film, ΔMPCBM, for all laser intensities.
Fig. 4 Efficiency increment owing to the introduction of the organic layer, TIPS-pentacene (blue bars) or PCBM (red bars), on the back side of the Si substrate. The upper figure shows the increment rate, R, between the incidences on the TIPS-pentacene and PCBM layers.
Therefore, the increment rate between the incidences on the TIPS-pentacene and PCBM films, defined as R = ΔMTIPS / ΔMPCBM, is larger than two for all of the cases in spite of the decrease of efficiency increment with increasing the excitation laser power caused by the effect of saturation of modulation efficiency, as shown in the upper panel of Fig. 4.
From a microscopic point of view, the only difference between the two systems is the type of carriers (photo-excited electrons and holes) passing through the structure, from the front side to the back side of the Si substrate. The photo-excited electrons diffusing through the Si substrate, while the photo-excited holes are injected into the TIPS-pentacene layer, are expected to contribute to the higher increase of the modulation efficiency, as the diffusion coefficient of electrons (≤36 cm2/s) in a bare Si wafer is approximately three times larger than that of holes (≤12 cm2/s). The increase in total amount of electrons injected into the PCBM layer successively after passing through the Si substrate leads to the enhancement of the modulation efficiency shown in Fig. 4.
For this interpretation, we provide supporting as follows. In a single crystalline Si wafer used for solar cells, the carrier diffusion length is 200~300 μm [29,30]. In addition, the carrier diffusivity (or diffusion coefficient) of a Si wafer increases with increasing the value of resistivity: 1.011 cm2/s at 0.0001 Ω·cm; 33.275 cm2/s at 1 Ω·cm; 40.430 cm2/s at 10000 Ω·cm [31]. We have used a Si wafer with much higher resistivity (larger than 1.0 × 105 Ω·cm) than that of a Si wafer used for solar cells (0.05~2 Ω·cm) and with intrinsic carrier concentration less than 1.0 × 1012 (cm−3). The diffusion length of the high-resistivity Si wafer used in our experiments may be at least several times higher than that of a typical Si wafer used for solar cells thus. Therefore, the photo-induced electrons may effectively pass through the 300-μm-thick Si wafer. Typically, the mobility and diffusion length of PCBM and TIPS pentacene thin films lie near the values of 10−2 cm2/V•s and 100 nm, respectively [32–34]. Even though the diffusion length and mobility of PCBM and TIPS pentacene thin films are smaller than those of a bare Si wafer, the photo-excited carriers transferred into the organic thin layer from the Si wafer mainly contribute to the modulation of THz waves due to their high carrier concentration formed in the thin organic layers and their long survival time caused by electron-hole separation in the trilayer structure [35,36].
4. Conclusion
In conclusion, we demonstrated a novel method to achieve a higher modulation efficiency of THz wave transmission, using the hybrid structures of organic layers on both sides of a Si substrate. The increase of the spatial charge separation owing to the deposition of organic layers on both sides of the Si substrate enabled a highly efficient THz wave modulation. Compared even to the case of incidence of the optical beam for photoexcitation on a hole acceptor layer (TIPS-pentacene thin film), the trilayer structure generating the photo-excited electrons transferring through the Si substrate offered a better modulation efficiency owing to the higher diffusion coefficient of electrons than that of holes. Owing to the excellent performance and functionality, the presented concept and structural design could be useful in the development of active modulators for THz spectroscopy, imaging, and communications, requiring optimal operating conditions.
Funding
Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (NRF-2016R1D1A1B03935241; NRF-2017R1D1A1B03030669); Chonnam National University (2018); “Research on Advanced Optical Science and Technology” grant; GIST Research Institute (GRI) grant funded by GIST (2018).
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