Low-temperature GaAs-based plasmonic photoconductive terahertz detector with Au nano-islands Download: 662次
1. INTRODUCTION
Terahertz time-domain spectroscopy (THz-TDS) is one of the powerful tools to evaluate a variety of samples in noninvasive or label-free fashion [1
There are several kinds of THz emitters using the physical phenomena based on such as nonlinear frequency conversion of fs-lasers, ultrafast carrier dynamics in semiconductors, and the photo-Dember effect [10
Furthermore, LT-GaAs grown at a temperature lower than 300°C (instead of the usual 600°C) has a high density of As-antisite defects, forming deep donors’ states (mid-gap states) with energies close to mid-gap level (about 0.75 eV below the conduction band) [20]. Therefore, direct photoexcitation using a 1560 nm laser is also possible, but the photoexcitation efficiency by 1560 nm fs-laser excitation is only about 10% compared with that of 800 nm fs-laser excitation [8]. For this reason, the wavelength conversion method using a second-harmonic generation (SHG) in a nonlinear optical crystal is generally utilized as one of the simple methods to improve efficiency. However, for the conversion efficiency in SHG due to second-order nonlinear optical effects, the power density of the laser incident into an SHG crystal must be very high to obtain high conversion efficiency of 40%–50%. For example, in the case of beta-barium borate (BBO) crystals, a power density of is required to achieve a conversion efficiency of 47% [21]. In our laboratory, we also perform wavelength conversion using a BBO crystal, but the power conversion efficiency to 780 nm is only 10% even if a 1560 nm fs-laser with a power of 100 mW is incident into the BBO crystal at a narrowed focus of about 5 μm diameter (corresponding to a power density of ).
Therefore, many efforts are underway to improve the THz detection (and THz radiation) efficiency at 1560 nm excitation. As one of the approaches, the use of InGaAs PCD, which has a small bandgap and can be directly excited at 1560 nm, has been tried. However, there is a problem that its detection sensitivity is not so high due to its low resistance and high dark current [22,23]. Apart from the excitation wavelength, as another approach, we have optimized the antenna geometry of the dipole PCD to improve the terahertz detection sensitivity [24]. In that study, we fabricated PCDs with wide dipole electrodes to suppress the shielding of the electric field by photoexcited electron–hole pairs and to increase the reception efficiency of THz waves. As a result, we have succeeded in increasing the detection sensitivity by about 220% and increasing the dynamic range from 56 to 66 dB by using a PCD with wide dipole electrodes of 500 μm width, without reducing the observable THz bandwidth.
On the other hand, recent advances in nanofabrication technologies have led to the rapid development of plasmonic detectors based on metal nanostructures [25
However, the preparation of these nanostructured plasmonic PCDs requires a high-precision electron beam lithography system with nano-resolution, a focused ion beam (FIB) system, etc., which are often not available in a typical laboratory in university.
Therefore, in this paper we propose a plasmonic PCD that is not so highly sensitive but can be easily fabricated. Specifically, we deposited gold nanoparticles by RF sputtering between the electrodes of LT-GaAs-based PCDs and investigated the plasmonic effect at 800 nm and 1560 nm fs-laser excitations. In addition, the detectable bandwidth is also important for THz applications. Therefore, we have also checked the bandwidth associated with gold nanoparticle deposition. These results are reported here.
2. EXPERIMENTS
2.1 A. Optical System Setup
Figure
Fig. 1. Schematic illustration of THz time-domain spectroscopy (THz-TDS) system to evaluate the fabricated PCDs by using different fs-lasers with wavelengths of 800 nm and 1560 nm.
2.2 B. Preparation of LT-GaAs PCD with Au Nano-Islands
In this study we tried to fabricate a plasmonic antenna by forming Au nano-islands in the dipole gap region between the electrodes of LT-GaAs PCD. Before fabrication, Au ultrathin film deposition had been examined by using Si substrates under different RF sputtering conditions, and its electrical characteristics were evaluated. As a result, it turned out that an RF power condition of 20 W is optimal for Au ultrathin film deposition using our RF sputtering system. Figure
Fig. 2. Representative I -V characteristics of Si wafer deposited with gold ultrathin film. RF sputtering of gold was carried out for 30, 60, and 120 s at an RF power of 20 W.
On the other hand, we fabricated PCDs of dipole antenna structure on LT-GaAs, which is commercially supplied by BATOP GmbH, as follows. First, AuSn (Au: 90%, Sn: 10%) was deposited on the LT-GaAs at a thickness of 100 nm by thermal evaporation using a joule heated tungsten boat. After the deposition, the LT-GaAs/AuSn sample was in situ heated by the radiation from a joule heated tungsten boat for 60 s in a vacuum chamber, because the existence of the resistance due to the Schottky barrier at the interface of LT-GaAs/AuSn inhibits the photocurrent from flowing into the electrode. Therefore, it is not preferable for observation of THz time-domain signals and plasmonic enhancement effects [5]. Finally, it was patterned into a dipole antenna structure with a gap distance of 6 μm using electron beam lithography. Figure
Fig. 3. Optical microscope image of basic LT-GaAs PCD utilized in the present study. It has the structural parameters of antenna length , width of the transmission line , and width of dipole electrode .
3. EXPERIMENTAL RESULTS AND DISCUSSION
Figure
Fig. 4. AFM images of the dipole gap region of LT-GaAs PCDs. Here (a) PCD-N, (b) PCD-A, and (c) PCD-B correspond to the PCD with RF sputtering time of gold for 0, 30, and 60 s, respectively.
First, the resistance between the electrodes in the dipole gap region was measured under a bias voltage of 1 V. As a result, it was found that the resistances of PCD-N, PCD-A, and PCD-B were 82.4, 84.5, and 77.8 MΩ, respectively, and it was confirmed that the Au particles did not cause short circuit between the electrodes. On the other hand, the resistance of a PCD after RF sputtering of Au for 90 s was also measured, but it was useless with a resistance of less than 1 MΩ.
Using the PCD-N, PCD-A, and PCD-B, the photo-induced increasing current caused by fs-laser irradiation onto the dipole gap region was observed in the I-V measurement of each PCD. Here, for the sake of simplicity, the increasing currents by fs-laser irradiation in the I-V measurements of PCD-A, PCD-B, and PCD-N are defined as , , and , respectively. Figure
Fig. 5. Ratios of increasing current and observed in the I -V characteristics at 800 nm fs-laser irradiation onto the dipole gap region of PCD at a power of 10 mW.
Fig. 6. THz time-domain waveforms detected by (a) PCD-A and (b) PCD-B compared with that of PCD-N at 800 nm fs-laser excitation. Here THz pulse was emitted from the p-InAs excited by 800 nm fs-laser.
Based on these results, we compared the ratio and the THz detection sensitivity of PCD-B at 1560 nm fs-laser excitation with those at 800 nm fs-laser excitation. Figure
Fig. 7. Ratios of increasing current in the I -V characteristics under 800 nm and 1560 nm fs-laser excitations. In the measurements fs-lasers were illuminated onto the dipole gap regions of PCD-B at a power of 10 mW for 800 nm fs-laser and 30 mW for 1560 nm fs-laser.
As shown above, it is noteworthy that the Au nano-islands thus formed in the dipole gap region of PCD should be useful to increase photoexcitation carriers by LSPR and shows a possibility to improve the THz wave detection efficiency at both 800 nm and 1560 nm fs-laser excitations. In fact, Fig.
Fig. 8. THz time-domain waveforms detected by PCD-B and PCD-N at 1560 nm fs-laser excitation. Here THz pulse was emitted from the p-InAs excited by a 1560 nm fs-laser.
On the other hand, Fig.
Fig. 9. FFT spectra of THz time-domain waveforms detected by PCD-B at (a) 800 nm and (b) 1560 nm fs-laser excitation corresponding to the waveforms in Fig. 6(b) and Fig. 8 , respectively. The inserted spectra in the bottom figures show those observed by PCD-N.
In order to discuss the effect of LSPR in more detail, Fig.
The reason for the lack of two-photon absorption is that the spot size of the irradiating laser is about 12–13 μm in the present experiment. Therefore, the power density is quite low. Furthermore, although the pulse width of the 1560 nm fs-laser is originally 110 fs, a cube type beam splitter was used to operate the 800 nm and 1560 nm fs-lasers in the present optical system. Therefore, there is a large possibility that the pulse width is much widened. It can be also seen that the terahertz bandwidth of the FFT spectrum of 1560 nm fs-laser excitation is about 2.5 THz, which is considerably narrow compared with about 4 THz for 800 nm fs-laser excitation in Fig.
Here we discuss the possibility of some other incremental factors for PCD-B, because the difference between the two -factors indicates that there is some other effects by LSPR for PCD-B. Considering the enhancement of the electric field between the Au nano-islands by LSPR, there is a possibility that the electrons near the Fermi level of Au nano-islands can be directly excited into the conduction band in GaAs by tunneling process. To be more specific, the strongly enhanced electric field between the nano-islands reduces the substantial thickness of the tunneling barrier that must be passed through and also allows the electron tunneling.
Here we consider the following two simple cases. System.Xml.XmlElementSystem.Xml.XmlElement
Fig. 11. Schematic illustration of excitations of electrons in Au nano-islands to the GaAs conduction band by induced electric field modulation between Au nano-islands by LSPR and change in Fermi level with electron transfer at DC bias .
We should consider the latter case for the present study because of randomly deposited Au nano-islands by RF sputtering, and it may give the results obtained in Fig.
Here we discuss the wavelength dependence obtained in the present study. Since the resonance wavelength is generally limited to a visible light range from 500 to 600 nm wavelength in the LSPR studies using spherical metal nanoparticles [40–
If the length of the major axis of the prolate spheroid is and the length of the minor axis is (), the depolarization factors are defined as where
Here the ratio is the aspect ratio .
We can simulate the absorption spectra for the nanoparticles with different aspect ratios by using Eqs. (
Since the dielectric function of Au is given by using the Drude model as follows: the real part of the dielectric function is given by where is the electron relaxation rate, is the plasma frequency, is the plasma wavelength, and is the speed of light.
The relationship between the longitudinal resonance wavelength and the aspect ratio can be obtained by substituting Eqs. (
Fig. 12. Relationship between resonance wavelength and aspect ratio obtained by the calculation using Gan’s model [44,45].
Here let us consider the experimental results based on the calculation results. In the present study, the deposited Au nano-islands are of various sizes with a width of roughly 100 nm and a length of roughly 100–500 nm. To roughly obtain the aspect ratios of Au nanoparticles, we described the rectangles almost corresponding to the sizes of nano-islands in Fig.
On the other hand, the aspect ratio of the egg-shaped nanoparticles in the PCD-A shown in Fig.
In the present study, it was found that the plasmonic PCD using rod-shaped Au nanoparticles can detect THz waves with high sensitivity by fs-laser with a wavelength in the visible to infrared region. It is also confirmed that the plasmonic effect is strongly dependent on the aspect ratio of the nanorods. Therefore, if we want to further improve the detection sensitivity with an excited fs-laser with a specific wavelength in the infrared wavelength range, we can control the aspect ratio and use Au nanorods with almost the same aspect ratio to significantly improve the detection sensitivity. For example, Fig.
On the other hand, we have already optimized the dipole antenna structure of PCD to improve the terahertz detection sensitivity and succeeded in increasing the detection sensitivity by about 220% and increasing the dynamic range from 56 to 66 dB by using a PCD with wide dipole electrodes of 500 μm width, without reducing the observable THz bandwidth [24]. In the near future, by using the PCD with wide dipole electrodes and Au nanorods, we can expect a significant improvement in THz detection sensitivity of about 300% at 1560 nm fs-laser excitation.
4. CONCLUSIONS
A simple plasmonic PCD for high-sensitivity THz wave detection was proposed. This method is composed of a very simple step of depositing Au nano-islands on the dipole gap region of PCD by RF sputtering for about 1 min at an RF power of 20 W. As a result, 29% and 40% improvements in the THz detection sensitivity were observed at 800 nm and 1560 nm fs-laser excitations, respectively. Furthermore, the Fourier spectra of the observed THz time-domain waveforms also show that the detectable THz bandwidth does not change, but the amplitude of FFT spectrum increases by about 33% and 42% at 800 nm and 1560 nm fs-laser excitation, respectively.
[1] M. Tonouchi. Cutting-edge terahertz technology. Nat. Photonics, 2007, 1: 97-105.
[3] J. B. Baxter, G. W. Guglietta. Terahertz spectroscopy. Anal. Chem., 2004, 83: 4342-4368.
[13] M. Tonouchi. Simplified formulas for the generation of terahertz waves from semiconductor surfaces excited with a femtosecond laser. J. Appl. Phys., 2020, 127.
[14] H. Dember. Über eine photoelektronische Kraft in Kupferoxydul-Kristallen. Z. Phys., 1931, 32: 554-556.
[21] R. S. Adhav, S. R. Adhav, J. M. Pelaprat. BBO’s nonlinear optical phase-matching properties. Laser Focus, 1987, 23: 88-100.
[44] R. Gans. Über die form ultramikroskopischer goldteilchen. Ann. Phys., 1912, 342: 881-900.
[45] R. Gans. Über die Form ultramikroskopischer Silberteilchen. Ann. Phys., 1915, 352: 270-284.
Article Outline
Hironaru Murakami, Tomoya Takarada, Masayoshi Tonouchi. Low-temperature GaAs-based plasmonic photoconductive terahertz detector with Au nano-islands[J]. Photonics Research, 2020, 8(9): 09001448.