# Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm

^{1}

^{2}

^{*}

## Abstract

**:**

_{1.9}Sb

_{0.1}Te

_{2}Se antenna was shown to be no worse than those of a semiconductor photoconductive antenna, which is an order of magnitude thicker. The current–voltage characteristics were studied for the photo and dark currents in Bi

_{1.9}Sb

_{0.1}Te

_{2}Se. The possible mechanisms for generating terahertz waves were analyzed by comparing the characteristics of terahertz radiation of an electrically biased and unbiased topological insulator.

## 1. Introduction

_{2}Se

_{3}, which proves optical coupling with topological surface states [10].

_{2}Te

_{3}and Sb

_{2}Te

_{3}was given in [14]. A number of conclusions were drawn regarding the prevailing mechanisms of the generation of THz radiation, due to the excitation of photocurrents in the epitaxial films of topological insulators in the absence of any external electric bias field. In [15], it was shown that the contribution to the photocurrent from the PDE is an order of magnitude greater than the contribution from the diffusion current, and two times higher than the contribution from the drift current.

_{2−x}Sb

_{x}Te

_{3−y}Se

_{y}are actively studied in connection with the possibility of smooth changes in their properties over a wide range. In [16], it was shown that there is a certain optimal curve (the “Ren’s curve”) in the composition-structure diagram y(x), where the properties of electronic surface states are most pronounced. This is associated with a significant suppression of the bulk contribution to the conductivity, as the actions of acceptors and donors cancel each other, which leads to the predominance of surface electron transport. Bi

_{2−x}Sb

_{x}Te

_{3−y}Se

_{y}films with compositions located under the Ren’s curve, as a rule, predominantly have p-type conductivity; while compositions located above this curve have conductivity of the n-type.

_{0.53}Ga

_{0.47}As showed a record peak dynamic range of 105 dB and a bandwidth up to 6.5 THz [21]. A significant improvement in the characteristics of these devices was achieved with the help of such nanotechnological tools as plasmonic contact gratings, arrays of optical nanoantennas, optical nanocavities [22,23], and non-standard orientation of the substrates [24,25].

## 2. Proposed System Design

_{1.9}Sb

_{0.1}Te

_{2}Se (BSTS) was prepared with a composition near the Ren’s curve at the composition-structure diagram. It was grown by metalorganic vapor-phase epitaxy (MOVPE) on a (0001) sapphire substrate with a 10-nm zinc telluride (ZnTe) buffer layer of orientation (111) at the atmospheric pressure of hydrogen in a horizontal quartz reactor. The sources of organometallic compounds of bismuth, antimony, zinc, tellurium, selenium, trimethyl bismuth, trimethyl antimony, diethyl zinc, diethyl tellurium, and diethyl selenium were used. The ZnTe buffer layer was grown in one technological cycle with a BSTS film at a temperature of 463 °C. To determine the elemental composition of the films, we used X-MaxN energy dispersive X-ray spectrometer with an electron microscope. A thin film of BSTS, 40 nm thick and of necessary composition, was selected for THz generation (Figure 1a).

_{2}O

_{3}substrate to collect the generated THz radiation (Figure 1c).

_{0.5}Ga

_{0.5}As/In

_{0.5}Al

_{0.5}As superlattice, was used as a second antenna for reference. The parameters of similar spiral antenna were previously studied and reported in [33]. The multilayer heterostructure InGaAs/InAlAs for semiconductor PCA was grown on an InP (111) substrate (Figure 1b). Such structures are usually prepared by molecular beam epitaxy and exhibit a high conversion efficiency of optical radiation into the broadband THz radiation. The overall thickness of the InGaAs/InAlAs heterostructure was 2000 nm, and the total thickness of all InGaAs layers was 1200 nm. An impedance semiconductor PCA was 12.8 kΩ.

^{−1}for our InGaAs layer lattice and 120,000 cm

^{−1}for the BSTS sample. In both cases, the thickness of the antenna material was less than the pump field penetration depth. Thus, the selected film thicknesses, 40 nm in the case of BSTS and 1200 nm of the total thickness of the InGaAs layers, provided almost uniform pumping illumination throughout the depth of the samples.

^{3+}fiber laser was used as a source for optical pulsed pumping at a wavelength of 1560 nm, pulse repetition rate of 70 MHz, and pulse duration of 100 fs. Eighty percent of the radiation power (approximately 100 mW in the average intensity) was directed onto the antenna under study, and the last 20% of the radiation was used to run a commercial (Menlo Systems) semiconductor heterostructure photoconductive antenna. The latter was used for the detection of the waveforms of emitted THz fields. The pump beam was focused on the TI PCA1 with a short-focus (5 mm) lens. The parabolic mirrors collected the THz radiation and focused it on the input silicon lens of the antenna-detector PCA2.

## 3. Results

_{b}= 20 V. Figure 5b demonstrates the dependence of the average photocurrent on the bias voltage. It was also well described by a linear law over the entire range of U

_{b}. For comparison, a similar dependence was shown for the semiconductor antenna, which lies well below the curve for the TI PCA. Figure 5c shows the obtained dependence of the TI PCA photocurrent on the pump power. It is linear; this is a typical situation for photoconductive antennas operating far from breakdown voltages and carrier screening effects.

_{b}=15 V was reversed. If we summarize the two signals obtained at inverse voltages, we realise practically the same waveform as in the absence of an external voltage (Figure 6b). Similar results were observed at a higher voltage of 20 V.

## 4. Discussion

_{0}. Time t

_{1}was of the order of a few picoseconds and, within this interval, a relaxation of bulk electrons took place due to the phonon emission [37]. The second type of relaxation process was an interband transition from the bulk conduction band to the surface states at the Dirac cone. As the Fermi level E

_{f}lies in the band of surface states (SSs) in a BSTS with a composition near the Ren’s curve, SSs above E

_{f}were primarily unoccupied. The characteristic time t

_{2}of this process is rather long, approximately 5–10 ps [37,38]. Finally, the relaxation of SSs was the third relaxation mechanism, with time t

_{3}as hundreds of femtoseconds or several picoseconds [9,39]. The direct interband transitions from the conduction band to the valence band are rather long and usually last from hundreds to thousands of picoseconds. These slow processes do not significantly contribute to generation at THz frequencies. Our estimate of the relaxation time corresponds to t

_{2}. Thus, we concluded that the process of population of the edge states by photo-induced bulk electrons made up the main contribution to the formation of the THz generation spectrum of the BSTS TI PCA.

## 5. Conclusions

_{2−x}Sb

_{x}Te

_{3−y}Se

_{y}. In addition to the known nonlinear effects, the photoconductive mechanism of THz generation was realized in TIs because of the relaxation of bulk electrons through the surface states. A comparison of photoconductive antennas based on TI and semiconductor heterostructure showed that, despite the great difference in the thicknesses by more than one order, the THz fields of both PCAs were comparable in amplitude. The time-averaged photocurrent and dark currents arising in the TI PCA were measured and their linear dependences on the bias voltage were demonstrated.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**The (

**a**) atomic force microscopy image of BSTS sample surface; (

**b**) sketch of the InGaAs/InAlAs heterostructure; and (

**c**) sketch of the antenna emitter.

**Figure 2.**A (

**a**) photo of the circuit board assembly; (

**b**) sketch of the electrodes, with the gray circle representing the projection of the silicon lens onto the plane of the electrodes; and (

**c**) image of the inter-electrode gap under a microscope. The gap width is 20 μm.

**Figure 4.**The (

**a**) time-domain waveforms of dipole antennas based on BSTS (red) and InGaAs/InAlAs (blue); (

**b**) fast Fourier transform spectra of BSTS (red) and InGaAs/InAlAs (blue) antennas; and (

**c**) THz electric field amplitude versus bias voltage for BSTS antenna.

**Figure 5.**The (

**a**) I-V characteristic of TI PCA measured without pump radiation; (

**b**) filled circles: voltage dependence of the time-averaged photo-excited current of TI PCA; open circles: the same dependence for InGaAs/InAlAs PCA; and (

**c**) dependence of the time-averaged photo-excited current on the pump power for TI PCA.

**Figure 6.**The (

**a**) THz pulses at opposite bias voltages; and (

**b**) comparison of the unbiased THz pulse (blue) with the result of averaging of two waveforms from oppositely biased antennas (magenta).

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**MDPI and ACS Style**

Kuznetsov, K.A.; Safronenkov, D.A.; Kuznetsov, P.I.; Kitaeva, G.K. Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm. *Appl. Sci.* **2021**, *11*, 5580.
https://doi.org/10.3390/app11125580

**AMA Style**

Kuznetsov KA, Safronenkov DA, Kuznetsov PI, Kitaeva GK. Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm. *Applied Sciences*. 2021; 11(12):5580.
https://doi.org/10.3390/app11125580

**Chicago/Turabian Style**

Kuznetsov, Kirill A., Daniil A. Safronenkov, Petr I. Kuznetsov, and Galiya Kh. Kitaeva. 2021. "Terahertz Photoconductive Antenna Based on a Topological Insulator Nanofilm" *Applied Sciences* 11, no. 12: 5580.
https://doi.org/10.3390/app11125580