Efficient room-temperature terahertz detection via bolometric and photothermoelectric effects in EuBiTe<sub>3</sub> crystal

Yingying Niu; Yingying Niu; Yingxin Wang; Yingxin Wang; Weidong Wu; Jianguo Wen; Yayun Cheng; Meng Chen; Shoulu Jiang; Dong Wu; Dong Wu; Ziran Zhao; Ziran Zhao

doi:10.1364/OME.387795

1. Introduction

Comparing with other bands in the electromagnetic wave spectrum, terahertz (THz) waves have some distinctive properties and thus has attracted much attention in recent years. THz technologies have broad application prospects in various fields such as security imaging, astronomy, biomedicine and communication [1–5]. However, high-performance THz source and room temperature (RT) THz detector are the major bottlenecks for the further development of THz technology. Hence, fast and reliable detection of THz radiation has become an important issue to develop the THz technology. To improve THz detection technologies, on one hand, researchers focus on improving the performance indicators of traditional thermal detector, such as thermal radiometer, and pyroelectric THz detection technology [6,7]. However, the performance of thermal radiation detectors usually suffer a lot from the low detection sensitivity or need to work with expensive cryogenic systems.[8] On the other hand, many researchers are constantly developing detectors based on new materials and new mechanisms, such as graphene-based THz photodetectors and THz field-effect transistor detectors [9–11]. Benefiting from the gapless nature, graphene can serve as a good candidate for THz detection, but monolayer grapheme-based photodetectors are plagued seriously with low absorption, which limits the photoresponsivity to just several mA/W [12,13]. Although considerable efforts have been made to improve the responsivities of graphene-based photodetectors, such as forming heterostructure with silicon nanowire array, making quantum dot arrays and so on, they all suffer from the complex device fabrication processes [11,14]. Besides, THz field-effect transistors based on Si, topological insulators, black phosphorus (BP), carbon nanotubes (CNTs) and other new materials, [10,15–20] usually facing the problems of low operating wavelength (below 1 THz) and low responsivity, not appropriate to practical applications. Thus, exploring new materials which are favorable for both THz spectrum and high performance photodetection is becoming increasingly eminent. Considering about the detection mechanisms, conventional photovoltaic and photoconductive effects are not suitable for THz detection for the low photon energy of THz radiation cannot create carriers. Alternatively, band gap-independent effects, including bolometric and photothermoelectric (PTE) effects show great potential for THz detection, and the devices based on these effects have the advantages of simple device geometry, broadband operation, low cost and good stability. Recently, alloy compound of EuBiTe₃ has been discovered with high absorptivity in THz range at room-temperature, [21] which holds potential for THz detection. Owing to the excellent physical properties, EuBiTe₃ crystal can achieve sensitive THz detection through bolometric and PTE effects. Here we demonstrate high performance EuBiTe₃-based THz detectors at room temperature under both the bolometric and PTE effects. The bolometric and PTE response of EuBiTe₃-based THz detectors are characterized in this paper, respectively. The contributions of the absorption, thermal resistance and temperature coefficient of resistance (TCR) to bolometric responses are investigated. Our results, theoretically and experimentally, prove that EuBiTe₃-based THz detectors are efficient choice for room temperature THz photodetection.

2. Experimental section

2.1. Preparation and characterization of EuBiTe₃ crystals

The EuBiTe₃ crystals were crystallized by the flux BiTey (y ≈ 12). The Eu (99.95%), Bi(Sb) (99.999%), and Te (99.999%) pure elements were mixed in a 1:2.5:13 molar ratio and loaded into the fused-silica ampoule, which were performed in a glovebox under an argon environment. The ampoule was sealed under vacuum (<10⁻³ Pa), and the EuBiTe₃ crystals were synthesized following a temperature controlled process that was described in detail elsewhere [21]. After characterization, the desired EuBiTe₃ single crystal were picked out for device fabrication (see the inset in Fig. 1(a)). In order to evaluate the quality of the synthesized crystals, the EuBiTe₃ flakes with smooth and shining surfaces were picked up to perform the X-ray diffraction (XRD) measurement. Figure 1(a) shows the XRD pattern of the EuBiTe₃ crystal flake. The sharp diffraction patterns of (00k) series indicates that the EuBiTe₃ crystal samples were in high crystal quality. The results of XRD were highly consistent with the measurement in [22]. Figure 1(b) shows the optical absorption spectrum of EuBiTe₃ crystal in THz range at room temperature and the inset is the crystal structure of EuBiTe₃. The thickness of the sample for the absorption spectrum measurement was about 100 µm and the probe light incidented along the c axis and measured the results of the ab plane of EuBiTe₃. The optical absorption of EuBiTe₃ crystal increases as the frequency increasing from 1.6 THz to 3.2 THz. Since the bandgap of the EuBiTe₃ is 0.3 eV, [21] the absorption mechanism of the EuBiTe₃ in the terahertz band should be free carrier absorption. In addition, obvious absorption peaks can be seen from Fig. 2(b), indicating that EuBiTe₃ has strong phonon absorption in the THz range. The high absorptivity in THz range at room temperature will hold potential for THz detection

Fig. 1. (a) XRD pattern of the EuBiTe₃ crystal sample. The inset is the optical image of the as-grown EuBiTe₃ single crystal. (b) The intrinsic THz absorbance spectrum of the EuBiTe₃ crystal. The inset shows the crystal structure of EuBiTe₃.

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Fig. 2. (a) Schematic of the bolometric device. (b) Current−voltage characteristics of our device in air. The insets in Fig. 1. (b) shows the photocurrent in air. (c) The photoresponse curves of bolometric device under illumination at different THz frequencies. The bias voltage is 0.4 V. The THz power levels are presented in the legends. The inset is a single on-off cycle of EuBiTe₃ detector under 1.84 THz laser illumination at a bias voltage of 0.4 V in air. The response time of our detectors at 1.84 THz (d), 2.52 THz (e), and 3.11 THz (f).

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2.2. Device fabrication

First, the photodetectors were fabricated using the about 40 to 50µm thick EuBiTe₃ crystal flakes that exfoliated from the bulk crystals by means of scotch tapes method and the flakes were subsequently transferred onto a piece of sapphire. We selected sapphire as the substrate, which is semitransparent to THz waves, to avoid the thermal effect of terahertz wave absorbed by the substrate. Sapphire plays a supporting role to facilitate later device fabrication. Next, two gold electrodes were deposited on the EuBiTe₃ crystal flake by thermal evaporation to enable a good ohmic contact and then the EuBiTe₃ flake was processed into strip with the photosensitive area of l × w × t = 300 × 100 × 70 µm for bolometric measurements and l × w × t = 900 × 110 × 50 µm for PTE measurements, where l is the distance between the electrodes, w is the width of sample and t is the thickness, respectively. Finally, the silver paste was used to provide conductive contacts between the gold electrodes and the conducting wires. When the conducting wires were extracted from the devices, due to the stresses of the wires, a small air gap was formed between the sample flake and sapphire. Therefore, the photoelectric characteristics of the EuBiTe₃ we measured were the intrinsic properties and not influenced by transfer methods or sapphire substrate. The schematic of the device structure is shown in Fig. 2(a).

2.3. Photoresponse measurements

A Keithley 2602B SourceMeter unit was used to measure the current−voltage (I−V) characteristics and the photoresponse of EuBiTe₃ crystal flake at room-temperature. The experiments in vacuum were conducted at a pressure below 10⁻³ Torr. A far-infrared gas laser generating THz waves (1.84, 2.52, 3.11 THz, Edinburgh Instruments Ltd.) served as the excitation light sources. The light beam was focused onto the device with the beam spot size being evaluated by the scanning knife-edge method [23]. All of the light spots of the 1.84, 2.5 and 3.11 THz are with diameters of ∼2 mm.

3. Results and discussion

3.1. THz bolometric response

The bolometric effect derives from the resistivity change of the active material, which is induced by the heating effect of incident radiation. For bolometric device, the spot should illuminate on the middle of the channel material as shown in the Fig. 2(a). Figure 2(b) plots the current−voltage (I−V) characteristic curves of bolometric device under dark condition and steady-state 2.52 THz light illumination at RT. The high conductivity of the EuBiTe₃ detector can be seen from the Fig. 2(b), which will directly reflect the charge transport and the collection of charges inside the detector as well as the detector dark current (${I_{\textrm{dark}}}$). Obviously, the device displays linear characteristics within a bias voltage range of ± 0.5 V. Linear behavior of the I-V relationship indicates that the channel material has an ohmic contact with the electrodes. The photocurrent (${I_{\textrm{ph}}}$) in air is depicted in the inset of Fig. 2(b). And the photocurrent ${I_{\textrm{ph}}}$ also displays a linear relationship with the bias voltage range of ± 0.5 V. Figure 2(c) shows the photoresponse curves of bolometric device under illumination of THz waves with different frequencies (1.84, 2.52 and 3.11 THz) at bias 0.4 V in air. The inset shows the single on-off cycle photoresponse curve under illumination of 1.84 THz. We can see that the photocurrents at different frequencies all rise to a high level under the illumination and decay after removing the illumination, suggesting a good switching behavior of our device under the periodic THz illumination (1.84, 2.52, 3.11 THz). Figure 2(d)-f show the single on-off cycle to evaluate the response time of our detectors at 1.84 THz, 2.52 THz and 3.11 THz, respectively. The time interval between 10% and 90% of the ${I_{\textrm{ph}}}$ peak value at the rise edge refers to the rise time. The rise time of our device are around 100ms at all three frequencies mentioned above. The current responsivity (${R_\textrm{I}}$) is one of the key figures of merit to evaluate the sensitivity of THz bolometric detectors, which is given as the ${I_{\textrm{ph}}}$ generated by unit power of the incident light illuminate on the effective area of the detector, defined by

(1)$${R_\textrm{I}} = \frac{{{I_{\textrm{ph}}}}}{{{P_{{\mathop{\rm in}\nolimits} }}}}$$

where ${I_{\textrm{ph}}}$ is the photoinduced current and ${P_{\textrm{in}}}$ is the incident power (the practical received portion of power over the effective area normalized to the light beam spot size). Figure 3(a) shows the ${R_\textrm{I}}$ at different frequencies. The ${R_\textrm{I}}$ at 1.84, 2.52, 3.11 THz is 0.35, 0.88, 1.32 A/W, respectively. The ${R_\textrm{I}}$ of our device in THz range is much larger than those reported devices without optimization based on other materials, such as 8nA/W for graphene, [12] 11.7 mA/W for carbon nanotube films, [18] 28 mA/W for RGO [24]. As shown in Fig. 1(b), from the 1.5 to 3.2 THz range, the absorbance monotonically increases with the increase of frequency. It is shown that the ${R_\textrm{I}}$ of our device at different frequencies are well consistent with the intrinsic absorption of the EuBiTe₃. The sensitivity of the EuBiTe₃-based THz detector is closely related to the conductivity and intrinsic THz absorbance of the EuBiTe₃ flakes, because the former affects the transport and the collection of charges inside the detector and the latter determines the ability of the detector to absorb THz waves. Figure 3(b) shows that the photocurrent response of our device which is linearly dependent on the power at 2.52 THz, indicating our device can effectively distinguish different incident light intensities and has good tunability in many applications. Bolometric effect originates from the changes in conductance of the channel material, which induced by heating associated with incident photons. The temperature coefficient of resistance (TCR) is known as a key figure of merit to evaluate the bolometric property, defined by

Fig. 3. (a) Photocurrent responsivities at three examined frequencies measured in air. (b) Power dependent photocurrent under 2.52 THz light illumination. (c) Temperature dependence of the resistance of a EuBiTe₃ crystal. ${R_0}$ is the resistance at room temperature. (d) The noise current as a function of frequency for EuBiTe₃ photodetectors at bias 10 mV.

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(2)$$\alpha = \frac{{\textrm{d}R}}{{R\textrm{d}T}}$$

Where $\alpha$ is the TCR, R is the resistance of sample in vacuum, and T is the absolute temperature, respectively. We measured the resistance dependence of the temperature of a EuBiTe₃ flake from 200K to 340K (see Fig. 3(c)). In the region from 300 K to 340 K, the resistance is nearly linear to temperature (see the inset in Fig. 3(c)). Hence, we can get a TCR equal to −0.309%/K. Surprisingly, the value of TCR is not as large as expected because this level is lower than other materials with good bolometric performance such as graphene, [11] CNTs [25] and vanadium oxide thin film [26]. For an ideal bolometer, the responsivity is expressed as [26]:

(3)$${R_\textrm{I}} = \frac{{\alpha \eta {U_{\textrm{DC}}}{R_{\textrm{th}}}}}{{{R_{\textrm{channel}}}}}$$

Where $\eta$ and ${R_{\textrm{th}}}$ are the absorption efficiency and thermal resistance, respectively. We can see that the highly sensitive bolometer is related not only to a lager TCR, but also to a large thermal resistance and high absorption efficiency. The absorptivity of EuBiTe₃ crystal at 3.11 THz is 24.63% as shown in Fig. 1(b), ${R_{\textrm{channel}}}$ is about 10Ω and the value of TCR is −3.09×10³ K⁻¹ as mentioned above. In air, the bias applied to our device is 0.4 V, and the ${R_\textrm{I}}$ is 2.33 A/W at 3.11 THz. Combining formula (3), we can figure out that the thermal resistance is ∼4×10⁴ K/W. Although the high thermal resistance contributes to the high ${R_\textrm{I}}$ of our devices, it leads to also a relatively long response time (about 100 ms in air). Therefore, future improvements should be to optimize the device structure (design antenna to enhance absorption) to ensure high ${R_\textrm{I}}$, and fabricate nanoscale sheets of EuBiTe₃ (thinner samples, micro-nano devices) to reduce thermal resistance aiming for shorter response time. Noise-equivalent power (NEP) is another important parameter for a photodetector. The noise current spectral density of our photodetector was measured by a network signal analyzer (SR780, Stanford Research Systems) and is shown in Fig. 3(d). The noise current as a function of frequency for EuBiTe₃ photodetectors at bias 10 mV. As shown, the major contribution to the noise figure was given by the 1/f flicker noise [27,28]. To estimate NEP values, noise current ${i_\textrm{n}}$ was extracted from the noise power spectra at frequency of 10 Hz. The NEP can be defined as [29]:

(4)$$\textrm{NEP = }\frac{{{i_\textrm{n}}}}{{{R_\textrm{I}}}}$$

According to the inset in Fig. 2(b), under 10 mV bias condition, the values of current responsivity (${R_\textrm{I}}$) at 1.84, 2.52, 3.11 THz are 0.009, 0.022, 0.034 A/W, respectively. As can be seen from Fig. 3(d), the value of ${i_\textrm{n}}$ at 10 mV is 3.6×10⁻¹⁰ A/Hz^1/2. Combining the formula (4), we can calculate the values of NEP at 1.84, 2.52, 3.11 THz are 1.1, 0.41, 0.27 nW/Hz^1/2 at 10 mV bias, respectively. Because the major contribution to the noise figure was given by the 1/f flicker noise, the NEP can be defined as $\textrm{NEP} = k{R_{\textrm{channel}}}{U_{\textrm{DC}}}/{f^{\frac{1}{2}}}{R_\textrm{I}}$, where k, and f are dimensionless constant and frequency. We can see that NEP is proportional to the bias voltage. Therefore, the values of NEP at 1.84, 2.52, 3.11 THz are 43.6, 16.4, 10.9 nW/Hz^1/2 at 0.4 V bias, respectively. In case of biased detectors operated in steady state, the major contribution to the noise figure is given by the flicker noise, which can be two orders of magnitude larger than Johnson noise. The bias can probably cause a slight self-heating of the device via the joule effect and this effect is discussed in several scientific papers about bolometers. [27,30] Therefore, there is a strong effort towards zero-bias (self-powered) devices. PTE-based detectors can serve as good candidates for the requirement of THz response with low NEP (the noise is limited by the Johnson noise) [31] because of their advantages of zero-bias operation. PTE detection based on EuBiTe₃ crystal will be reported in details below.

3.2. THz PTE response

The PTE effect arises when the laser causes local heating and a temperature difference between the two electrodes, leading to a photovoltage. The photovoltage generated in the device is given as [32]:

(5)$${V_{\textrm{ph}}} = \int_{{T_0}}^T {({S_{\textrm{EBT}}}} - {S_{\textrm{Au}}})\textrm{d}T \approx {S_{\textrm{EBT}}}\Delta T$$

Where ${V_{\textrm{ph}}}$ is the photovoltage, ${S_{\textrm{EBT}}}$ and ${S_{\textrm{Au}}}$ are the Seebeck coefficients of the channel material EuBiTe₃ crystal and Au electrodes, ${S_{Au}} \ll {S_{EBT}}$. $\Delta T$ is the temperature difference between the two electrodes. In order to measure the ${S_{\textrm{EBT}}}$, we use silver paint to anchor the two pairs of T-type thermocouples at the ends of the EuBiTe₃ crystals. In addition to recording the temperature of the two end of the EuBiTe₃ crystals, the Cu leads of the thermocouples are used to measure the voltage. Therefore, the temperature between the two ends of the sample and the voltage can be measured simultaneously. Figure 4(a) shows the temperature difference dependence of the voltage of a EuBiTe₃ crystal at room temperature. The red line is the linear fit to the experimental data and the value of the ${S_{\textrm{EBT}}}$ is −104 µV/ K. Generally, the sign of S is determined by the potential of the cold end relative to the hot end. It should be noted that carriers always diffuse from the hot end to the cold end, which means the majority carrier is supposed to accumulate in the cold end. The negative ${S_{\textrm{EBT}}}$ indicates that the potential of cold end is lower than that of hot end, and electrons accumulation in the cold end. The sign of ${S_{\textrm{EBT}}}$ is negative and electron diffusion plays a dominant role in the PTE process of the sample, which means that crystal EuBiTe₃ is a kind of n-type material. We chose the device with longer channel material (110 × 900 × 50 µm) for PTE measurements, in order to avoid the influence of radiation spot size on PTE performance as much as possible. Figure 4(b) shows the I-V characteristic curves under dark condition (dark line) and steady-state 2.52 THz light illumination at the positive (blue line) and negative (red line) ends of the PTE device in air. We define the end of device that connected to the positive channel of the sourcemeter as the positive end. Obviously, the red and blue lines exhibit obvious upward and downward shift with respect to the dark I–V curve when the negative and the positive ends were exposed to the 2.52 THz beam with a power of about 33 mW, which are typical PTE signatures. In order to further study the PTE characteristics of the EuBiTe₃ crystals, as a typical example, Fig. 5(a) and 5(d) show the photoresponse curves under illumination of a 2.52 THz laser in air and vacuum, respectively. Without special instructions, the THz beam spots are localized at the positive end of the PTE device. Our device shows a good switching behavior under the periodic 2.52 THz illumination both in air and vacuum. The responsivity ${R_\textrm{V}}$ is one of the key figures of merit needed to be evaluated for PTE photodetectors and is defined as:

Fig. 4. (a) Voltage across the EuBiTe₃ versus the corresponding temperature difference to determine the room-temperature Seebeck coefficient ($S$) of EuBiTe₃. The red line is the linear fit to the experimental data. (b) I-V characteristics of PTE device in air.

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(6)$${R_\textrm{V}} = \frac{{{V_{\textrm{ph}}}}}{{{P_{\textrm{in}}}}}$$

${R_\textrm{V}}$ at 2.52 THz are 0.3 V/W in air and 4.2 V/W in vacuum at the same excitation conditions (2.52 THz laser, constant incident power). As we all know, NEP is the main performance index of THz detector. For PTE THz photodetector, the dark noise is limited by the Johnson noise because our device generates ${V_{\textrm{ph}}}$ at zero bias [31]. Thus, the NEP can be calculated as:

(7)$$\textrm{NEP = }\frac{{\sqrt {4{k_\textrm{B}}T{R_{\textrm{channel}}}} }}{{{R_\textrm{V}}}}$$

Fig. 5. (a) The photoresponse curves of PTE device under illumination at 2.52 THz in air. (b) The response time of our detector in air. (c) Power dependent photocurrent under 2.52 THz light illumination in air. (d) The photoresponse curves of PTE device under illumination at 2.52 THz in vacuum. (e) The response time of our detector in vacuum. (f) Power dependent photocurrent under 2.52 THz light illumination in vacuum.

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The values of NEP at 2.52 THz are 4.3 nW/Hz^1/2 in air and 300 pW/Hz^1/2 in vacuum, respectively. Such low NEP is 1-2 orders of magnitude lower than previously reported PTE detectors based on millimetric-scale graphene and CNT films without optimization [33,34]. In other words, our device can efficiently convert THz light energy into thermal energy, which can be measured by an electrical signal. The response time in air and vacuum shown in Fig. 5(b) and 5(e). We observed the same experimental phenomenon, that is, the response time in vacuum (∼276 ms) is slower than that in air (∼42 ms). Compared with the above bolometric device, the response time of PTE device is faster, because PTE device is thinner. This means that we can get faster response time by making thinner devices to optimize detection performance. Figure 5(c) and 5f show that the photovoltage response of PTE device which are linearly dependent on the power at 2.52THz both in air and vacuum.

4. Conclusion

The EuBiTe₃ crystals were prepared for serving as the sensing element of the THz photodetector. Devices consisting of EuBiTe₃-metal contacts were fabricated and the bolometric and PTE responses of EuBiTe₃ devices were systematically investigated at THz frequencies. We demonstrate the bolometric response device with stable photoresponse from 1.84 THz to 3.11THz with good reproducibility at room temperature. For bolometric device, the high responsivity up to 1.33 A/W with a NEP of 10 nW/Hz^1/2 at 3.11 THz had been observed in air, even without coupling antenna and optimization of the thermal environment. The high bolometric responsivity originates from the large thermal resistance and high THz absorbance of EuBiTe₃ crystal. The high self-powered responsivities up to around 0.3 V/W in air (4.2 V/W in vacuum) with fast response time around 42 ms in air (270 ms in vacuum) has been observed based on PTE device. Since the zero-bias operation, the PTE device has low NEP of 4.3 nW/Hz^1/2 in air and 300 pW/Hz^1/2 in vacuum. In addition, our devices can effectively distinguish different light intensities under both mechanisms of bolometric and PTE effects because of the linear dependence on the incident power. Moreover, on account of the layered structure of the EuBiTe₃ crystal, we could manufacture two-dimensional structural devices with coupling antennas for optimizing performance. EuBiTe₃ photodetectors provide a promising way to develop efficient room temperature THz photodetectors.

Funding

National Natural Science Foundation of China (61901242, U1633202); Ministry of Science and Technology of the People's Republic of China (2017YFC0803601).

Disclosures

The authors declare no conflicts of interest.

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Efficient room-temperature terahertz detection via bolometric and photothermoelectric effects in EuBiTe₃ crystal

Abstract

1. Introduction