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An advanced approach to control the electro-optical properties of LT-GaAs-based terahertz photoconductive antenna
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An advanced approach to control the electro-optical properties of LT-GaAsbased terahertz photoconductive antenna A.M. Buryakova,*, M.S. Ivanova,b,*, S.A. Nomoevc, D.I. Khusyainova, E.D. Mishinaa, V.A. Khomchenkob, I.S. Vasil’evskiic, A.N. Vinichenkoc, K.I. Kozlovskiic, A.A. Chistyakovc, J.A. Paixãob MIREA - Russian Technological University “RTU MIREA”, 119454 Moscow, Russia CFisUC, Department of Physics, University of Coimbra, 3004-516 Coimbra, Portugal c National Research Nuclear University “MEPhI”, 115409 Moscow, Russia a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Low temperature-grown gallium arsenide (LTGaAs) Photoconductive antenna (PCA) Terahertz (THz) radiation THz-antenna THz-spectroscopy
This work reports on an advanced approach to the design of THz photoconductive. antenna (PCA). The LT-GaAs thin films used for the PCA fabrication were synthesized by MBE method on GaAs (100) substrate by adjusting the As pressure, As/Ga fluxes ratio, growth/annealing temperatures and annealing time. These parameters crucially affect electro-optical properties of the PCA samples as evidenced by the THz radiation power and timedomain spectroscopy measurements. The annealing temperature of 670 °C was found to be optimal for constructing a PCA possessing high amplitude of the THz radiation over the spectral range up to 1 THz at the resonance of 0.1 THz. The comparison of this PCA with the reference ZnTe crystal reveals a 2-fold increase in THz power. Furthermore, this antenna attains a 1.5-, 3-, and 2-fold increase in THz power, photocurrent efficiency, and actuating dc BV, as compared with the commercial ZOMEGA antenna. These results pave the way towards the creation of highly efficient LT-GaAs-based PCAs.
1. Introduction Sources and detectors of terahertz (THz) radiation are the decisive components used in high-speed wireless communication systems, medical and pharmaceutical diagnostics, non-contact/remote material quality control and hazardous objects detection [1–3]. The key element allowing to generate the THz radiation is a photoconductive antenna (PCA). Generally, PCA is an optoelectronic key consisting of a semiconductor covered by the contact electrodes, both significantly impacting on the final characteristics of the fabricated antenna [4–6]. One of the most commonly used semiconductors for the generation of the THz radiation is a low-temperature-grown gallium arsenide (LT-GaAs) demonstrating a short lifetime (1−10 ps) [7] and relatively high mobility (120–150 cm2/(V∙s) ) [7] of photoexcited charge carriers. The main reasons underlying the described properties are associated with the formation of structural defects during the growth and annealing procedures. These defects play the role of deep centers, thus providing an extraordinary short lifetime of charge carriers [8]. Because of that, an optically excited LT-GaAs generates pico- and femtosecond electric pulses with the maximum in THz spectra [9–13]. As compared with
⁎
other widely-used semiconductor materials for PCAs fabrication (for example, semi-insulating gallium arsenide (SI-GaAs) doped by chromium), the LT-GaAs-based antennas possess a tremendously shorter lifetime of charge carriers which contributes to their higher breakdown dc voltage, decreased dark current and the ability of the material to be simultaneously used as both an emitter and a detector [14–16]. Post-growth annealing of LT-GaAs results in an increase in the frequency of the generated THz radiation (up to 3 THz) as compared with non-annealed LT-GaAs [17]. The studies of post-growth annealing of epitaxial LT-GaAs report on the standard temperature of about 600 °C applied for several minutes at the As overpressure of about 0.5% [17,18]. Despite the significant number of experimental investigations devoted to finding the most appropriate parameters for post-growth LTGaAs annealing, they have not been adequately determined yet. The current work continues the study started in Ref [19]. and describes the optimal growth and annealing procedure to design PCAs possessing high output THz radiation power operating at high frequency.
Corresponding authors at: Moscow Technological University "MIREA", 119454 Moscow, Russia. E-mail addresses: [email protected] (A.M. Buryakov), [email protected] (M.S. Ivanov).
https://doi.org/10.1016/j.materresbull.2019.110688 Received 22 May 2019; Received in revised form 18 September 2019; Accepted 9 November 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: A.M. Buryakov, et al., Materials Research Bulletin, https://doi.org/10.1016/j.materresbull.2019.110688
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2.2. Characterization methods
Table 1 Growth and annealing conditions for LT-GaAs samples, output THz radiation power at 160 V and resistivity at room temperature. Sample
Tg (°C)
Tann (°C)
tann (min)
P, μW
ρ, Ω⋅cm
A1 A2 A3 A4 B1 B2
230 230 230 230 260 260
670 700 720 600 670 600
6 6 6 15 6 15
5 1.4 4.2 0.65 0.05 0.65
4*106 7*107 3*108 4*105 2*106 1*105
The integral spectra collected over the range of 0.1–3.5 THz were measured using the optical setup based on a femtosecond titaniumsapphire laser (Lighthouse Photonics) with λ=800 nm, an average laser power of ∼ 2 W, a pulse duration of 100 fs, and a pulse repetition frequency of 80 MHz. This setup allows the simultaneous generation and detection of the THz radiation (the absolute value of the generated power was measured by a pyroelectric detector). The results obtained were compared with the parameters of a manufactured THz antenna (ZOMEGA 16-PCA-GAAS-006, maximum power of 3.25 μW at working dc voltage of 90 V) [28]. The terahertz time-domain spectroscopy technique [26,29,30], was employed for the spectral characteristics of output THz radiation of PCAs. Briefly, the source laser beam (a femtosecond Ti:sapphire laser (MaiTai, Spectra Physics) with λ=800 nm, an average laser power of ∼ 2.7 W, a pulse duration of 100 fs, a pulse repetition frequency of 80 MHz) was separated by a beam splitter into two components: a 300 mW (0.5 mJ/cm2) pump beam was focused on the sample surface in a 30 μm-diameter spot, and a 150 mW probe beam was focused on the detector surface (the plasmon-enhanced logarithmic spiral antenna) in a 8 μm-diameter spot. The pump beam power of 300 mW was experimentally found to ensure the optimal conditions for the maximum generation of THz radiation and prevent the damage of the functional layer of PCAs. The output THz radiation was probed by a femtosecond laser pulse with a temporal resolution tuned by a delay line setup (an optical paths difference between the pump-probe beams). The purchased commercial antenna (Zomega) was measured using the same experimental setup along with the fabricated PCA samples. All the spectra obtained were compared with the reference ZnTe nonlinear single crystal (thickness of about 1 mm) in balanced detection mode [31]. Sample morphology was studied with a NTEGRA Prima (NT-MDT Spectrum Instruments) scanning probe microscope operating in semicontact atomic force microscopy (AFM) mode (the NSG01 probes with a force constant ∼ 5.1 N/m and a resonant frequency∼ 150 kHz were used). The results for ZOMEGA PCA, ZnTe and fabricated samples were obtained at equal experimental conditions.
2. Experimental 2.1. Synthesis of the samples Thin films of LT-GaAs with a thickness of 1.2 μm were grown on GaAs (100) substrates by the molecular beam epitaxy (MBE) method using As4 flux. The growth temperature (Tg) varied between 230 °C (marked as A series samples) and 260 °C (marked as B series samples) in order to form a high density of point defects (over-stoichiometric metallic As embedded into the GaAs lattice) [20–25]. The arsenic/gallium flux ratio during the growth (γ = PAs PGa ), annealing procedure (performed under As-pressure in a high vacuum of 1.3 × 10−5 Pa), annealing temperature (Tann) and annealing time (tann) were adjusted for the controlled fabrication of the defective (possessing deep centers, i.e. nonequilibrium states) GaAs, thus ensuring the control of the charge carriers lifetime (Table 1). Taking into account the previous investigations describing the role of flux ratio in the formation of lattice defects during the GaAs film growth [20,25,26], the γ ∼ 19 (2) was chosen to create a high concentration of As defects. For the B1 and B2 samples, the growth temperature was increased in order to follow the effect of the excessive As in GaAs lattice (the conditions result in a reduction of non-stoichiometric defects in LT-GaAs). During the annealing procedure, partial evaporation of As from the surface of the sample was prevented by GaAs wafer canopy. The absence of the As voids in fabricated LT-GaAs samples was confirmed via EDX analysis. The resistivity measured as a function of the annealing temperature (600 °C–720 °C) was found to be in the range of 1*105(Ω⋅cm) 3*108(Ω⋅cm) (Table 1), which is consistent with the semiquantitative model based on Ostwald ripening and aimed at explaining the observed trends in both the carrier trapping lifetime and bulk resistivity, when low-temperature-grown gallium arsenide is partially annealed [27]. PCA samples were fabricated by deposition of the Ni/AuGe/Ni/Au strip lines electrodes (with a gap width of 200 μm) on top of LT-GaAs films by using contact photolithography and physical vapor deposition methods. Additionally, an anti-reflective coating (100 nm of singlelayer Si3N4 ) was applied to the A1 sample by using plasma-enhanced chemical vapor deposition method in order to decrease the laser beam reflection (∼ 1%) in nonlinear optical experiments.
3. Results and discussion Fig. 1 shows the AFM topography of the LT-GaAs A1 sample measured before and after annealing. The root means square (RMS) surface roughness and the average grain size were found to diminish after annealing procedure. In particular, the annealing at Tann=670 °C results in the decrease of these parameters from 60 nm to 1.2 nm and from 400 nm to 100 nm, respectively. This trend is similar for all LT-GaAs samples studied in the present work. The dependences of the output THz radiation power spectra, P, on applied dc BV, V, measured for the PCA samples are presented in Fig. 2 (a) to be compared with a commercially manufactured THz antenna
Fig. 1. The AFM images of the A1 sample morphology captured before (a) and after (b) annealing procedure. 2
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Fig. 2. The voltage (a) and photocurrent (b) dependences of the output THz radiation power spectra for PCA samples and ZOMEGA antenna.
growth of the maximum amplitude of the THz radiation power with increasing applied dc BV and/or decreasing squared length of the contact gap. Both parameters (V and d) can be used simultaneously or independently to control and tune the output amplitude of the THz radiation power of a PCA. The charge carrier mobility (μ) parameter can be derived from the Boltzmann approach for the time relaxation process, thus linking it with the charge carrier relaxation time (τ is the momentum relaxation time of electrons):
(ZOMEGA). The A1 and A3 PCAs demonstrate a similar nonlinear increase in the THz radiation power as a function of applied dc BV. This result is in good agreement with the theoretical prediction [32] suggesting that
P (V ) ∼ A⋅V 2
(1)
where A is determined by material parameters (THz resonance frequency, carrier mobility, carrier lifetime, resistance, capacitance). The effect of annealing temperature Tann on the output THz radiation power is summarized in Table 1. One can see that the maximum power is attained for the A1 sample annealed at 670 °C. The THz radiation power of the PCA A1 sample was found to be higher than that specific to the commercial THz antenna (5.0 μW vs. 3.25 μW), when operating at the higher actuating dc BV (160 V vs. 90 V, the comparison was carried out at the maximum working voltage for the A1 sample and ZOMEGA antenna). The obtained results are consistent with the scenario of formation of metallic As clusters (with the typical size of 3–10 nm) in the GaAs lattice [33]. Indeed, it has been previously reported [15] that the increase in the effective generation of THz radiation is mainly attributed to the clear-cut crystal boundaries (between metallic As clusters and the GaAs lattice) possessing high resistivity (108 Ohm·cm) and extremely small charge carrier lifetime (< 1 ps). The decrease in the output THz radiation power observed for the B1 and B2 PCA samples (Table 1) can be associated with the significant reduction of the charge carrier’s mobility caused by an excessively high concentration of As defects formed during the sample preparation. Fig. 2 (b) compares the dependences of the output THz radiation power spectra on photocurrent for the PCA samples and ZOMEGA antenna. One can see that the maximum current efficiency is attained for the A1 PCA sample (5 μW at 14 mA) having the current-to-power conversion factor (0.36 mW/A) three times exceeding the value specific to a commercial THz antenna (0.13 mW/A) (Fig. 2 (b)) (the comparison was made at the maximum current efficiency values for the A1 sample and ZOMEGA antenna). It is worth noting that the comparison of the current characteristic of the fabricated LT-GaAs (14 mA at 160 V) with that of the commercial antenna Zomega (24 mA at 90 V) yields the difference of 10 mA, thus indicating that the samples under study should be more stable. In order to evaluate the maximum amplitude of the THz radiation power for the fabricated PCAs, the ratio between applied dc BV (V) and squared length of the contact gap (d2) was used [34]:
ETHz ( max ) ∼ eμtint
(1 − R) pV hvd 2
μ=
e ⋅τ m*
(3)
Taking into account this equation, one can see that the decrease in the μ or τ leads to a decrease in the output THz radiation power. This fact establishes the limitations for the PCAs to operate in the high-frequency THz region since the balance between high mobility and short lifetime of photo-excited charge carriers should be provided [27]. Our experimental data indicate that the power of the generated THz radiation grows for the annealing temperatures up to ∼ 670 °C. However, temperatures exceeding 700 °C do not affect significantly the PCA output THz radiation power. In accordance with [13], the charge carries lifetime parameter is strongly dependent on annealing temperature (rising with its increase). A similar effect was found in LT-GaAs samples grown at 250 °C and 310 °C as measured by double optical pump terahertz time-domain emission spectroscopy [35]. Moreover, there is no final conclusion concerning the “negative” reflection coefficient that appears in structures fabricated at the annealing temperature less than 600 °C [36–39]. Accordingly, we believe that the annealing temperature of 670 °C corresponds to the optimal conditions for the generation of the THz radiation possessing the maximum output power and operating at the higher THz frequency as compared with the other reference samples [23,26], and manufactured THz antenna (ZOMEGA). The difference observed between the output THz radiation power of the PCA samples annealed at 700 °C and 720 °C is due to positive/negative balance transients associated with the conductive band electron population decrease/increase (the conditions are determined by recombination of holes with electrons occupying the deep-donor states) [39]. The other processes, such as a recombination of carriers on As precipitates, a cluster volume fraction excess of As, a variation of the deep-donor states due to a change in the defect environment, can also play a significant role [39], [40]. The spectral characteristics of the output THz radiation from the PCA samples were determined by using the THz time-domain spectroscopy described in details in Ref. [26]. Fig. 3 (b) compares the frequency dependences of the amplitude of the output THz radiation for the A-series samples (possessing the maximum THz power) with that characteristic of the reference ZnTe nonlinear crystal. For all PCA samples, a spectral maximum at 0–1.5 THz and two resonant peaks at ∼ 0.1 THz and ∼ 0.5 THz were observed (Fig. 3(b)). The obtained resonance frequencies obey the law [41]:
(2)
where R is the reflection coefficient, hν is the laser pulse energy, p is the average laser radiation power, and the tint is the time interval between neighboring laser pump pulses. While the values of R, hν, and p are constants, the parameter (V/d2) can be taken as a variable. Fig. 3 (a) shows the dependencies of ETHz(max) on V/d2 for the A1, A2 and A3 samples (as for the most powerful PCAs) demonstrating a linear-like 3
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Fig. 3. (a) Maximum amplitude of the THz radiation power (ETHz(max)) as a function of V/ d2 parameter for the A1, A2, and A3 samples; (b) the spectral amplitude of output THz radiation of the fabricated PCAs, commercial antenna Zomega, and reference nonlinear ZnTe crystal measured in the frequency range from 0 to 4 THz at the dc BV of 100 V and pump/ probe laser power of 300 mW and 150 mW, respectively.
νres =
c 2⋅Le ⋅n
Russian Foundation for Basic Research (project No. 16-29-14029 ofi_m). M. S. I. is grateful to the Fundaçao para a Ciencia e Tecnologia (FCT) for financial support through the project MATIS - Materiais e Tecnologias Industriais Sustentáveis (CENTRO-01-0145-FEDER000014). V. A. K. is grateful to FCT for financial support through the FCT Investigator Programme (project IF/00819/2014/CP1223/ CT0011). The Coimbra Physics Centre is supported by FCT through the project UID/FIS/04564/2016, also co-funded by COMPETE- UE.
(4)
where c is the velocity of light, n is the semiconductor refractive index at the generated wavelength (n∼ 13 for the GaAs), and Le is the effective length of antenna: Le = L + 2⋅D (where L (∼200 μm) is the distance between the anode and cathode, D (∼100 μm) is the width of the electrode). It worth noting that the design of the electrode structure undoubtedly influences the THz spectra being the crucial task of vast studies [42,43]. Apart from these investigations, the main idea of our work is to explore the influence of material`s modification on the functional properties of PCAs. In accordance with Ref. [43], the only variable parameter defining the electrode structure (the effective length of the antenna) was chosen to verify the efficiency of the approaches proposed. The resonance frequency calculated using Eq. 4 is equal to 0.104 THz, which is consistent with the experimental data (Fig. 3 (b)). The positions of the peaks of resonance frequency are very similar for all PCA samples since the effective length of antennas was the same (and quite long). This technical aspect makes impossible to distinguish the difference in charge carrier relaxation time for the PCA samples in high-frequency THz region. Potentially, the Le decrease and simultaneous charge carrier mobility increase should create the conditions for the ballistic mechanism of the current flow allowing the effective generation of high-frequency THz radiation to be attained [44]. The peak at 0.5-0.7 THz corresponds to the inherent LT-GaAs contribution to the generated THz wave amplitude [45]. It is important to note that the only difference in the design of the samples under study and the commercial antenna (Zomega) is the gap between the anode and cathode (200 μm vs. 100 μm). This explains the shift of the bandwidth of THz spectra to low-frequency range, as was estimated theoretically and measured experimentally (Fig. 3 (b)). To sum up, we have described a modified approach to the design of PCAs based on LT-GaAs films. The THz radiation power detection and THz time-domain spectroscopy measurements reveal that electro-optical properties of the PCAs are dramatically dependent on fabrication parameters such as growing and annealing temperatures, arsenic/gallium flux ratio and annealing time. The annealing of LT-GaAs at 670 °C–700 °C was found to ensure the optimal conditions for obtaining a powerful generation of high-frequency THz radiation. The fabricated PCA samples demonstrate the generation of the THz radiation in the spectral range up to 1 THz with a resonant maximum at 0.1 THz. The comparison of the spectral amplitude of output THz radiation specific to the PCAs with that characteristic of the reference ZnTe nonlinear crystal reveals a 2-fold increase in the THz power. Moreover, the PCA samples demonstrate a 1.5-fold increase in output THz radiation power, a 3-fold increase in photocurrent efficiency, and a 2-fold increase in actuating dc BV as compared with a commercial THz antenna ZOMEGA.
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Acknowledgments The work was supported by the Ministry of Education and Science of the Russian Federation (State Task No. 3.7331.2017/9.10) and the 4
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An advanced approach to control the electro-optical properties of LT-GaAsbased terahertz photoconductive antenna A.M. Buryakova,*, M.S. Ivanova,b,*, S.A. Nomoevc, D.I. Khusyainova, E.D. Mishinaa, V.A. Khomchenkob, I.S. Vasil’evskiic, A.N. Vinichenkoc, K.I. Kozlovskiic, A.A. Chistyakovc, J.A. Paixãob MIREA - Russian Technological University “RTU MIREA”, 119454 Moscow, Russia CFisUC, Department of Physics, University of Coimbra, 3004-516 Coimbra, Portugal c National Research Nuclear University “MEPhI”, 115409 Moscow, Russia a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Low temperature-grown gallium arsenide (LTGaAs) Photoconductive antenna (PCA) Terahertz (THz) radiation THz-antenna THz-spectroscopy
This work reports on an advanced approach to the design of THz photoconductive. antenna (PCA). The LT-GaAs thin films used for the PCA fabrication were synthesized by MBE method on GaAs (100) substrate by adjusting the As pressure, As/Ga fluxes ratio, growth/annealing temperatures and annealing time. These parameters crucially affect electro-optical properties of the PCA samples as evidenced by the THz radiation power and timedomain spectroscopy measurements. The annealing temperature of 670 °C was found to be optimal for constructing a PCA possessing high amplitude of the THz radiation over the spectral range up to 1 THz at the resonance of 0.1 THz. The comparison of this PCA with the reference ZnTe crystal reveals a 2-fold increase in THz power. Furthermore, this antenna attains a 1.5-, 3-, and 2-fold increase in THz power, photocurrent efficiency, and actuating dc BV, as compared with the commercial ZOMEGA antenna. These results pave the way towards the creation of highly efficient LT-GaAs-based PCAs.
1. Introduction Sources and detectors of terahertz (THz) radiation are the decisive components used in high-speed wireless communication systems, medical and pharmaceutical diagnostics, non-contact/remote material quality control and hazardous objects detection [1–3]. The key element allowing to generate the THz radiation is a photoconductive antenna (PCA). Generally, PCA is an optoelectronic key consisting of a semiconductor covered by the contact electrodes, both significantly impacting on the final characteristics of the fabricated antenna [4–6]. One of the most commonly used semiconductors for the generation of the THz radiation is a low-temperature-grown gallium arsenide (LT-GaAs) demonstrating a short lifetime (1−10 ps) [7] and relatively high mobility (120–150 cm2/(V∙s) ) [7] of photoexcited charge carriers. The main reasons underlying the described properties are associated with the formation of structural defects during the growth and annealing procedures. These defects play the role of deep centers, thus providing an extraordinary short lifetime of charge carriers [8]. Because of that, an optically excited LT-GaAs generates pico- and femtosecond electric pulses with the maximum in THz spectra [9–13]. As compared with
⁎
other widely-used semiconductor materials for PCAs fabrication (for example, semi-insulating gallium arsenide (SI-GaAs) doped by chromium), the LT-GaAs-based antennas possess a tremendously shorter lifetime of charge carriers which contributes to their higher breakdown dc voltage, decreased dark current and the ability of the material to be simultaneously used as both an emitter and a detector [14–16]. Post-growth annealing of LT-GaAs results in an increase in the frequency of the generated THz radiation (up to 3 THz) as compared with non-annealed LT-GaAs [17]. The studies of post-growth annealing of epitaxial LT-GaAs report on the standard temperature of about 600 °C applied for several minutes at the As overpressure of about 0.5% [17,18]. Despite the significant number of experimental investigations devoted to finding the most appropriate parameters for post-growth LTGaAs annealing, they have not been adequately determined yet. The current work continues the study started in Ref [19]. and describes the optimal growth and annealing procedure to design PCAs possessing high output THz radiation power operating at high frequency.
Corresponding authors at: Moscow Technological University "MIREA", 119454 Moscow, Russia. E-mail addresses: [email protected] (A.M. Buryakov), [email protected] (M.S. Ivanov).
https://doi.org/10.1016/j.materresbull.2019.110688 Received 22 May 2019; Received in revised form 18 September 2019; Accepted 9 November 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: A.M. Buryakov, et al., Materials Research Bulletin, https://doi.org/10.1016/j.materresbull.2019.110688
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2.2. Characterization methods
Table 1 Growth and annealing conditions for LT-GaAs samples, output THz radiation power at 160 V and resistivity at room temperature. Sample
Tg (°C)
Tann (°C)
tann (min)
P, μW
ρ, Ω⋅cm
A1 A2 A3 A4 B1 B2
230 230 230 230 260 260
670 700 720 600 670 600
6 6 6 15 6 15
5 1.4 4.2 0.65 0.05 0.65
4*106 7*107 3*108 4*105 2*106 1*105
The integral spectra collected over the range of 0.1–3.5 THz were measured using the optical setup based on a femtosecond titaniumsapphire laser (Lighthouse Photonics) with λ=800 nm, an average laser power of ∼ 2 W, a pulse duration of 100 fs, and a pulse repetition frequency of 80 MHz. This setup allows the simultaneous generation and detection of the THz radiation (the absolute value of the generated power was measured by a pyroelectric detector). The results obtained were compared with the parameters of a manufactured THz antenna (ZOMEGA 16-PCA-GAAS-006, maximum power of 3.25 μW at working dc voltage of 90 V) [28]. The terahertz time-domain spectroscopy technique [26,29,30], was employed for the spectral characteristics of output THz radiation of PCAs. Briefly, the source laser beam (a femtosecond Ti:sapphire laser (MaiTai, Spectra Physics) with λ=800 nm, an average laser power of ∼ 2.7 W, a pulse duration of 100 fs, a pulse repetition frequency of 80 MHz) was separated by a beam splitter into two components: a 300 mW (0.5 mJ/cm2) pump beam was focused on the sample surface in a 30 μm-diameter spot, and a 150 mW probe beam was focused on the detector surface (the plasmon-enhanced logarithmic spiral antenna) in a 8 μm-diameter spot. The pump beam power of 300 mW was experimentally found to ensure the optimal conditions for the maximum generation of THz radiation and prevent the damage of the functional layer of PCAs. The output THz radiation was probed by a femtosecond laser pulse with a temporal resolution tuned by a delay line setup (an optical paths difference between the pump-probe beams). The purchased commercial antenna (Zomega) was measured using the same experimental setup along with the fabricated PCA samples. All the spectra obtained were compared with the reference ZnTe nonlinear single crystal (thickness of about 1 mm) in balanced detection mode [31]. Sample morphology was studied with a NTEGRA Prima (NT-MDT Spectrum Instruments) scanning probe microscope operating in semicontact atomic force microscopy (AFM) mode (the NSG01 probes with a force constant ∼ 5.1 N/m and a resonant frequency∼ 150 kHz were used). The results for ZOMEGA PCA, ZnTe and fabricated samples were obtained at equal experimental conditions.
2. Experimental 2.1. Synthesis of the samples Thin films of LT-GaAs with a thickness of 1.2 μm were grown on GaAs (100) substrates by the molecular beam epitaxy (MBE) method using As4 flux. The growth temperature (Tg) varied between 230 °C (marked as A series samples) and 260 °C (marked as B series samples) in order to form a high density of point defects (over-stoichiometric metallic As embedded into the GaAs lattice) [20–25]. The arsenic/gallium flux ratio during the growth (γ = PAs PGa ), annealing procedure (performed under As-pressure in a high vacuum of 1.3 × 10−5 Pa), annealing temperature (Tann) and annealing time (tann) were adjusted for the controlled fabrication of the defective (possessing deep centers, i.e. nonequilibrium states) GaAs, thus ensuring the control of the charge carriers lifetime (Table 1). Taking into account the previous investigations describing the role of flux ratio in the formation of lattice defects during the GaAs film growth [20,25,26], the γ ∼ 19 (2) was chosen to create a high concentration of As defects. For the B1 and B2 samples, the growth temperature was increased in order to follow the effect of the excessive As in GaAs lattice (the conditions result in a reduction of non-stoichiometric defects in LT-GaAs). During the annealing procedure, partial evaporation of As from the surface of the sample was prevented by GaAs wafer canopy. The absence of the As voids in fabricated LT-GaAs samples was confirmed via EDX analysis. The resistivity measured as a function of the annealing temperature (600 °C–720 °C) was found to be in the range of 1*105(Ω⋅cm) 3*108(Ω⋅cm) (Table 1), which is consistent with the semiquantitative model based on Ostwald ripening and aimed at explaining the observed trends in both the carrier trapping lifetime and bulk resistivity, when low-temperature-grown gallium arsenide is partially annealed [27]. PCA samples were fabricated by deposition of the Ni/AuGe/Ni/Au strip lines electrodes (with a gap width of 200 μm) on top of LT-GaAs films by using contact photolithography and physical vapor deposition methods. Additionally, an anti-reflective coating (100 nm of singlelayer Si3N4 ) was applied to the A1 sample by using plasma-enhanced chemical vapor deposition method in order to decrease the laser beam reflection (∼ 1%) in nonlinear optical experiments.
3. Results and discussion Fig. 1 shows the AFM topography of the LT-GaAs A1 sample measured before and after annealing. The root means square (RMS) surface roughness and the average grain size were found to diminish after annealing procedure. In particular, the annealing at Tann=670 °C results in the decrease of these parameters from 60 nm to 1.2 nm and from 400 nm to 100 nm, respectively. This trend is similar for all LT-GaAs samples studied in the present work. The dependences of the output THz radiation power spectra, P, on applied dc BV, V, measured for the PCA samples are presented in Fig. 2 (a) to be compared with a commercially manufactured THz antenna
Fig. 1. The AFM images of the A1 sample morphology captured before (a) and after (b) annealing procedure. 2
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Fig. 2. The voltage (a) and photocurrent (b) dependences of the output THz radiation power spectra for PCA samples and ZOMEGA antenna.
growth of the maximum amplitude of the THz radiation power with increasing applied dc BV and/or decreasing squared length of the contact gap. Both parameters (V and d) can be used simultaneously or independently to control and tune the output amplitude of the THz radiation power of a PCA. The charge carrier mobility (μ) parameter can be derived from the Boltzmann approach for the time relaxation process, thus linking it with the charge carrier relaxation time (τ is the momentum relaxation time of electrons):
(ZOMEGA). The A1 and A3 PCAs demonstrate a similar nonlinear increase in the THz radiation power as a function of applied dc BV. This result is in good agreement with the theoretical prediction [32] suggesting that
P (V ) ∼ A⋅V 2
(1)
where A is determined by material parameters (THz resonance frequency, carrier mobility, carrier lifetime, resistance, capacitance). The effect of annealing temperature Tann on the output THz radiation power is summarized in Table 1. One can see that the maximum power is attained for the A1 sample annealed at 670 °C. The THz radiation power of the PCA A1 sample was found to be higher than that specific to the commercial THz antenna (5.0 μW vs. 3.25 μW), when operating at the higher actuating dc BV (160 V vs. 90 V, the comparison was carried out at the maximum working voltage for the A1 sample and ZOMEGA antenna). The obtained results are consistent with the scenario of formation of metallic As clusters (with the typical size of 3–10 nm) in the GaAs lattice [33]. Indeed, it has been previously reported [15] that the increase in the effective generation of THz radiation is mainly attributed to the clear-cut crystal boundaries (between metallic As clusters and the GaAs lattice) possessing high resistivity (108 Ohm·cm) and extremely small charge carrier lifetime (< 1 ps). The decrease in the output THz radiation power observed for the B1 and B2 PCA samples (Table 1) can be associated with the significant reduction of the charge carrier’s mobility caused by an excessively high concentration of As defects formed during the sample preparation. Fig. 2 (b) compares the dependences of the output THz radiation power spectra on photocurrent for the PCA samples and ZOMEGA antenna. One can see that the maximum current efficiency is attained for the A1 PCA sample (5 μW at 14 mA) having the current-to-power conversion factor (0.36 mW/A) three times exceeding the value specific to a commercial THz antenna (0.13 mW/A) (Fig. 2 (b)) (the comparison was made at the maximum current efficiency values for the A1 sample and ZOMEGA antenna). It is worth noting that the comparison of the current characteristic of the fabricated LT-GaAs (14 mA at 160 V) with that of the commercial antenna Zomega (24 mA at 90 V) yields the difference of 10 mA, thus indicating that the samples under study should be more stable. In order to evaluate the maximum amplitude of the THz radiation power for the fabricated PCAs, the ratio between applied dc BV (V) and squared length of the contact gap (d2) was used [34]:
ETHz ( max ) ∼ eμtint
(1 − R) pV hvd 2
μ=
e ⋅τ m*
(3)
Taking into account this equation, one can see that the decrease in the μ or τ leads to a decrease in the output THz radiation power. This fact establishes the limitations for the PCAs to operate in the high-frequency THz region since the balance between high mobility and short lifetime of photo-excited charge carriers should be provided [27]. Our experimental data indicate that the power of the generated THz radiation grows for the annealing temperatures up to ∼ 670 °C. However, temperatures exceeding 700 °C do not affect significantly the PCA output THz radiation power. In accordance with [13], the charge carries lifetime parameter is strongly dependent on annealing temperature (rising with its increase). A similar effect was found in LT-GaAs samples grown at 250 °C and 310 °C as measured by double optical pump terahertz time-domain emission spectroscopy [35]. Moreover, there is no final conclusion concerning the “negative” reflection coefficient that appears in structures fabricated at the annealing temperature less than 600 °C [36–39]. Accordingly, we believe that the annealing temperature of 670 °C corresponds to the optimal conditions for the generation of the THz radiation possessing the maximum output power and operating at the higher THz frequency as compared with the other reference samples [23,26], and manufactured THz antenna (ZOMEGA). The difference observed between the output THz radiation power of the PCA samples annealed at 700 °C and 720 °C is due to positive/negative balance transients associated with the conductive band electron population decrease/increase (the conditions are determined by recombination of holes with electrons occupying the deep-donor states) [39]. The other processes, such as a recombination of carriers on As precipitates, a cluster volume fraction excess of As, a variation of the deep-donor states due to a change in the defect environment, can also play a significant role [39], [40]. The spectral characteristics of the output THz radiation from the PCA samples were determined by using the THz time-domain spectroscopy described in details in Ref. [26]. Fig. 3 (b) compares the frequency dependences of the amplitude of the output THz radiation for the A-series samples (possessing the maximum THz power) with that characteristic of the reference ZnTe nonlinear crystal. For all PCA samples, a spectral maximum at 0–1.5 THz and two resonant peaks at ∼ 0.1 THz and ∼ 0.5 THz were observed (Fig. 3(b)). The obtained resonance frequencies obey the law [41]:
(2)
where R is the reflection coefficient, hν is the laser pulse energy, p is the average laser radiation power, and the tint is the time interval between neighboring laser pump pulses. While the values of R, hν, and p are constants, the parameter (V/d2) can be taken as a variable. Fig. 3 (a) shows the dependencies of ETHz(max) on V/d2 for the A1, A2 and A3 samples (as for the most powerful PCAs) demonstrating a linear-like 3
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Fig. 3. (a) Maximum amplitude of the THz radiation power (ETHz(max)) as a function of V/ d2 parameter for the A1, A2, and A3 samples; (b) the spectral amplitude of output THz radiation of the fabricated PCAs, commercial antenna Zomega, and reference nonlinear ZnTe crystal measured in the frequency range from 0 to 4 THz at the dc BV of 100 V and pump/ probe laser power of 300 mW and 150 mW, respectively.
νres =
c 2⋅Le ⋅n
Russian Foundation for Basic Research (project No. 16-29-14029 ofi_m). M. S. I. is grateful to the Fundaçao para a Ciencia e Tecnologia (FCT) for financial support through the project MATIS - Materiais e Tecnologias Industriais Sustentáveis (CENTRO-01-0145-FEDER000014). V. A. K. is grateful to FCT for financial support through the FCT Investigator Programme (project IF/00819/2014/CP1223/ CT0011). The Coimbra Physics Centre is supported by FCT through the project UID/FIS/04564/2016, also co-funded by COMPETE- UE.
(4)
where c is the velocity of light, n is the semiconductor refractive index at the generated wavelength (n∼ 13 for the GaAs), and Le is the effective length of antenna: Le = L + 2⋅D (where L (∼200 μm) is the distance between the anode and cathode, D (∼100 μm) is the width of the electrode). It worth noting that the design of the electrode structure undoubtedly influences the THz spectra being the crucial task of vast studies [42,43]. Apart from these investigations, the main idea of our work is to explore the influence of material`s modification on the functional properties of PCAs. In accordance with Ref. [43], the only variable parameter defining the electrode structure (the effective length of the antenna) was chosen to verify the efficiency of the approaches proposed. The resonance frequency calculated using Eq. 4 is equal to 0.104 THz, which is consistent with the experimental data (Fig. 3 (b)). The positions of the peaks of resonance frequency are very similar for all PCA samples since the effective length of antennas was the same (and quite long). This technical aspect makes impossible to distinguish the difference in charge carrier relaxation time for the PCA samples in high-frequency THz region. Potentially, the Le decrease and simultaneous charge carrier mobility increase should create the conditions for the ballistic mechanism of the current flow allowing the effective generation of high-frequency THz radiation to be attained [44]. The peak at 0.5-0.7 THz corresponds to the inherent LT-GaAs contribution to the generated THz wave amplitude [45]. It is important to note that the only difference in the design of the samples under study and the commercial antenna (Zomega) is the gap between the anode and cathode (200 μm vs. 100 μm). This explains the shift of the bandwidth of THz spectra to low-frequency range, as was estimated theoretically and measured experimentally (Fig. 3 (b)). To sum up, we have described a modified approach to the design of PCAs based on LT-GaAs films. The THz radiation power detection and THz time-domain spectroscopy measurements reveal that electro-optical properties of the PCAs are dramatically dependent on fabrication parameters such as growing and annealing temperatures, arsenic/gallium flux ratio and annealing time. The annealing of LT-GaAs at 670 °C–700 °C was found to ensure the optimal conditions for obtaining a powerful generation of high-frequency THz radiation. The fabricated PCA samples demonstrate the generation of the THz radiation in the spectral range up to 1 THz with a resonant maximum at 0.1 THz. The comparison of the spectral amplitude of output THz radiation specific to the PCAs with that characteristic of the reference ZnTe nonlinear crystal reveals a 2-fold increase in the THz power. Moreover, the PCA samples demonstrate a 1.5-fold increase in output THz radiation power, a 3-fold increase in photocurrent efficiency, and a 2-fold increase in actuating dc BV as compared with a commercial THz antenna ZOMEGA.
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Acknowledgments The work was supported by the Ministry of Education and Science of the Russian Federation (State Task No. 3.7331.2017/9.10) and the 4
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