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THz pulse detection by a multi-layered GeTe/SbTe

Kotaro Makino, Shota Kuromiya, Keisuke Takano, Kosaku Kato, Makoto Nakajima, Yuta Saito, Junji Tominaga, Hitoshi Iida, Moto Kinoshita, and Takashi Nakano ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11418 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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ACS Applied Materials & Interfaces

THz pulse detection by a multi-layered GeTe/Sb2Te3 Kotaro Makino,∗,† Shota Kuromiya,‡ Keisuke Takano,‡ Kosaku Kato,‡ Makoto Nakajima,‡ Yuta Saito,† Junji Tominaga,† Hitoshi Iida,¶ Moto Kinoshita,¶ and Takashi Nakano† Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan, Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan, and National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8563, Japan E-mail: [email protected]

KEYWORDS: THz detention, THz spectroscopy, topological insulator, phase change material, chalcogenide superlattice, multilayer system Abstract We proposed and demonstrated terahertz (THz) pulse detection by means of multi-layered GeTe/Sb2 Te3 phase change memory materials which are also known as multilayer topological insulator / normal insulator (MTN) system. THz time-domain spectroscopy measurement was performed for MTN films with different multi-layer repetitions as well as a conventional asgrown Ge-Te-Sb alloy film. It was found that MTN absorb THz wave and the absorption ∗

To whom correspondence should be addressed Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ‡ Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan ¶ National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8563, Japan † Nanoelectronics

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coefficient depends on the number of layers while the as-grown GST alloy film was almost transparent for THz wave. Simple MTN-based THz detection devices were fabricated and the THz-induced change in the current signal was measured when a DC bias voltage was applied between the electrodes. We confirmed that irradiation of THz pulse causes a decrease in the resistance of the MTNs. This result indicates that our devices are capable of THz detection.

INTRODUCTION Development of terahertz (THz) wave detection techniques is crucial for further advancement in THz applications. 1 THz imaging, 2–4 for example, requires a high-sensitivity two dimensional (2D) array imaging sensor that can operate at room temperature. Conventional semiconductor-based visible light detectors are not suitable for THz detection since the photon energy of THz wave is basically too faint to overcome band-gap energy of semiconductors. So far, considerable efforts have been devoted to the development of high sensitivity THz detection techniques and some are now available. Coherent optical detection techniques such as photoconductive antennas and electrooptical sampling are most commonly used in imaging and sensor applications yet the terahertz waves are required to be coherent and synchronized with a probe femtosecond laser pulse. On the one hand, a 2D-arrayed bolometer cameras have already been commercialized, but the sensitivity is relatively low. Recent noticeable developments of semiconductor and superconductor quantum wells and quantum dots 5–7 as well as nano carbon-based detectors 8–10 have realized very high sensitivity. However, fabrication of such detector devices is problematic from the industrial point of view, and some of them require low temperatures and/or high magnetic fields. Alternatively, another promising detection technique utilizing topological insulator (TI) was proposed. 11,12 TI is a new class of materials, in which the bulk is insulating but the surfaces are conductive and spin-helical Dirac cones are formed on the surfaces. 13,14 Due to these features, TIs have been anticipated as alternative materials for infrared and THz wave detectors similar to graphene, since Dirac cones can absorb low energy radiation 11,12,15 and induce electric conduction by the photoexcitation of carriers as well as lattice heating via carrier-phonon interaction. Although the use of 2 ACS Paragon Plus Environment

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surface conductive bands is highly attractive, single surface (for graphene) or two surfaces (3DTIs) can only be used for optical detection. To increase the optical absorption, utilization of a multi-layer system 16,17 or a Dirac semimetal is essential. 18 In this study, we propose and demonstrate THz pulse detection using a multilayer chalcogenide consisting of TI and normal insulator (NI). We used Sb2 Te3 as the TI and GeTe as the NI, respectively. The GeTe/Sb2 Te3 multilayer system, known as interfacial phase change memory, has drawn wide attentions not only as a non-volatile phase change memory, 19 but also as the platform to study multilayer TI/NI (MTN) system. 20 Although GeTe/Sb2 Te3 was originally designed and applied to reduce the phase switching energy between low resistance SET and high resistance RESET state in phase change memories, soon later unusual magnetoresistance 21 and anomalous magneto-optical Kerr-rotation 22 were discovered at room temperature, despite the fact that interfacial phase change memory material consists only of non-magnetic elements. Although the fact that MTNs do not always have topological states because of interactions between layers which can break topological property, it was reported that the RESET state has Dirac cones on both surfaces and at TI/NI interfaces based upon ab-initio computer simulations. 23 In addition, injecting a current pulse and controlling the power and duration are expected to result in the phase transition from RESET into the SET state reversibly, in which the topological state is lost, resulting in an open band gap. Therefore, this system is promising for the reversible control of the topological phase transition which leads to the expansion of THz applications such as frequency-selective detection and modulation as well as electrical switching. Furthermore, the matured production technique and thickness control of the interfacial phase change memory material as the memory devices are beneficial for practical THz detector production. Two types of ideas for TI-based photoconductor-type THz detector have been theoretically predicted thus far. One is the simple photoconductor-type 11 which is the same as commonly used visible and infrared light sensors, and the other is based on the TI-ferromagnet heterostructures in which modification of electronic structure is induced to generate photocurrent by application of magnetic field. 12 It is worth mentioning that a different approach for THz detection device that



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is the elaborate TI field effect transistor (TI-FET) utilizing plasma-wave excitation have been reported. 24 Compared with the TI-FET, our device is easy-to-fabricate and suitable for evaluating the intrinsic properties of MTN materials. In order to evaluate the potential of this material, we employed the simple photo-conductive type device structure without any field-enhancement structures such as antennas, cavities, or metamaterial as a first step of the experimental realization of the THz detection. The THz-induced change in electrical resistance was investigated by measurement of current change when a static bias voltage was applied to the devices. In the original idea of THz detection by Zhang et al., 11 interband optical transition in an intrinsic TI is assumed although they discussed the further possibility of resonant detection with trivial TI. In reality, however, density of states (DOS) around the Dirac point is tiny and not suitable for efficient detection. In addition, it is not always true that the energy level of Dirac point corresponds to Fermi energy (EF ) in many TIs. Since both Sb2 Te3 and GeTe are p-type semiconductors, multilayered system of these two materials should also be p-type in which the Dirac points locate above the EF . In this situation, irradiation of THz wave gives rise to intraband transition and heat Dirac electrons by taking advantage of the linear band dispersion in the surface and interface states.

EXPERIMENT The as-deposited (RESET state) MTN used were thin films of [(GeTe)2 (Sb2 Te3 )1 ]3 (3-MTN) and [(GeTe)2 (Sb2 Te3 )1 ]20 (20-MTN) composed of 0.85 nm GeTe sub-layers and 1.0 nm Sb2 Te3 sublayers with the different repetition number of the multi-layer. These films were grown using an RF magnetron sputtering system 25 on glass substrates covered with a 3 nm-thick Sb2 Te3 seed layer and a 5 nm-thick Si initial layer that ensured fabrication of a highly ordered superlattices. 26,27 The substrate temperature was set to 230 ◦ C during the growth. A conventional RESET state (asdeposited) Ge2 Sb2 Te5 (GST) alloy sample with the same thickness of the 20-MTN, and a glass substrate were also used for comparison. 20-nm-thick ZnS-SiO2 oxidation protection layers were deposited on all samples. For the THz detector devices, tungsten electrodes were deposited. Stencil


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masks were used for device fabrication. Before the main THz-detection measurement, we carried out THz time-domain spectroscopy (THz-TDS) for the two MIT films, the GST alloy film, and the glass substrate to investigate the response of films to THz wave (see the experimental details in the Supporting Information). We obtained time-domains signals and their Fourier transformed (FT) spectra. Based on the FT spectra, we calculated the absorption coefficients of the samples in the THz frequency range used in the THz detection experiment. Then, a THz-detection experiment was performed. Figure 1a depicts the schematic illustration of our THz detection measurement setup. Tungsten electrodes were fabricated on the MTN films and the size of the effective active area was 1 × 2 mm as shown in Figure 1b. Ohmic I-V characteristics were confirmed as shown in Figure S1 (Supporting Information). THz pulses were focused onto the center of the active area at normal incidence. The typical beam profile on the focal plane measured by commercially available THz imager, the time-domain electric field shape obtained by electro-optic (EO) sampling measurement, and FT spectra are shown in Figure S2 (Supporting Information). A DC bias voltage (VB ) was applied between the electrodes. We varied the THz incident power (PTHz ), and the THz-induced change in the current (ITHz ) was recorded after subtraction of the dark current (ID ) (see the experimental details in the Supporting Information).

RESULTS AND DISCUSSION THz time domain spectroscopy Figure 2a shows the observed time domain electrical field shape of THz pulses transmitted through the samples. It was found that the MTN samples reduce the field intensity without changing the shape of the THz pulse. Consistent with the previous report, 28 on the other hand, the GST alloy sample did not affect the THz field and the temporal shape of the transmitted THz wave for the alloy sample was basically same as that for the substrate. It should be noted that the substrates are almost transparent and the shape of the transmitted THz pulse was confirmed to remain intact. 5 ACS Paragon Plus Environment


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MTN, and the 20-MTN were 0 %, 5.6 %, and 15.7 %, respectively. This result strongly suggests that GeTe/Sb2 Te3 multilayer systems realize THz absorption due to the topological surface and interface states and the absorption coefficient depends on the number of layers as described above. Note that in-plane rotation of sample with respect to the polarization of THz pulse does not affect on the THz response under the normal incidence condition. This result is consistent with the fact that our samples are noncrystalline where the a-b planes of each grain randomly oriented in the THz spot. 25,29 Also note that thermally excited carriers must absorb THz wave at room temperature. Although it is difficult to distinguish between topological and thermal carrier origin, both of them are expected to contribute the THz-induced electrical signal.

THz detection Figure 3a shows the time-resolved ITHz response from the 20-MTN detector observed on a millisecond time scale. Here, VB and the PTHz were 8.0 V and 820 µ W, respectively. The time delay zero indicates the arrival of the THz pulse and the next pulse arrives at 2 ms because the repetition rate of THz pulse is 500 Hz. Just after the irradiation of THz pulses, ITHz sharply increased and subsequently decayed within the time interval between THz pulses. The THz-induced change in the current response ITHz was ≃ 62 nA which corresponds to a ≃ 0.01 % change in ID . As discussed later, the effective ratio between ITHz and ID in the THz-irradiated area is estimated to be 0.03 %. Note that we have confirmed the ITHz signal intensity reached a maximum when the THz pulses were irradiated at the center of the active area and hence the contribution of thermoelectric effect can be negligible. 30 Figure 3b shows the microsecond-scale response of the ITHz . The rise time of the observed signal was ≃ 0.6 µ s which is faster than the nominal time resolution of the amplifiers (≃ 0.8 µ s). Therefore, the leading edge of the ITHz is thought to be suppressed and the measured intensity and rise time were certainly affected. On the other hand, the decay process of ITHz is relatively slow compared with the leading edge and the trace can be fitted with linear combination of exponential


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functions, ITHz = A1 exp(−1/τ1 (t − t0 )) + A2 exp(−1/τ2 (t − t0 ))

(1)

where A, and τ are the amplitude and the relaxation time of the signals, respectively. The subscripts 1 and 2 indicate the faster and slower components of the signals. By the fitting, we obtained τ1 = 0.73 µ s and τ2 = 27.8 µ s when VB = 8 V and PTHz = 820 µ W. Here, τ1 is also shorter than the time resolution of the system and therefore the actual relaxation time should be faster than the experimentally obtained τ1 value. The faster response with the relaxation time of τ1 is likely to consist of both non-thermal (photoconductive) and thermal (bolometric) components. On the other hand, the slower component is likely to correspond to the thermally-relaxation process because the time scale of the dynamics of photo-excited electrons is mush faster than τ2 . Both A1 and A2 linearly increase with increasing VB and PTHz (Figure S3 in Supporting Information). It should be mentioned about another slow response, which persists for 1 ms (Figure 3a). This component can be attributed to the increase in sample temperature leading to an additional slow decay channel. The area of the ellipsoidally focused THz pulse on the sample surface is 0.68 mm2 that is 34 % of the active area of our device of 2 mm2 (Figure 2b and Figure S2). Approximately a three times larger sensitivity can be potentially expected if the shape of the active area matches the THzirradiated area. For the future, improvements of device structure such as fabrication of antenna, cavity, and metamaterial structures are promising for high performance THz detectors. 10,31,32 Indeed, more than three orders of magnitude sensitivity improvements have been achieved with an antenna and silicon lense in graphene-based THz detector. 32 To obtain deeper insight into the THz-induced electrical response, the dependences of PTHz and VB on ITHz were evaluated. We found that ITHz linearly increases with increasing PTHz ( Figure 4a). The energy poured into the electron system by THz excitation Eel is described as 33

Eel ∝ σ PTHz

(2)

where σ is the conductivity. Eq 2 implies that the the Dirac electron with high mobility is favorable



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Figure 3: (a) Time-resolved THz-induced current signal for the 20-MTN device in millisecond time scale. (b) Magnified view of (a). Red solid curve is the experimental signal and Black dotted curve is the double-exponential function fitting curve. for THz wave absorption. After electronic excitation, the energy transfers from electron system to lattice system. Since the width of THz pulse is constant, the increase in the lattice temperature would also be proportional to PTHz , consistent with the linear function fit (Figure 4a). The energy transferred from THz pulse is expected to initially increase the lattice temperature at the topological surfaces and interfaces. Then the whole MTN film can be heated efficiently owing to its multilayered structure. Similar to the PTHz dependence, ITHz was confirmed to enhance in a linear manner with increases in VB (Figure 4b). The linearity over high ITHz regime is suitable for intense sources. The linear VB dependence on ITHz is a typical characteristic of a photoconductive photodetector. This result also suggests that the application of VB does not significantly affect on the position of EF and the conductivity of the MTN film. Both of these good linearities are favorable characteristics as a detector. Finally, we carried out the same measurement in 3-MTN detector to prove the effectiveness of MTN. In Figure 4c, ITHz /Id for 3- and 20-MTN are compared. A ≃ 9.2-fold enhancement of the sensitivity as well as a significant improvement in the signal-to-noise ratio (SNR) were realized 10 ACS Paragon Plus Environment

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Figure 4: (a) PTHz dependence on ITHz for VB = 5.0 V. (b) VB dependence on ITHz for PTHz = 820 µ W. The black solid lines in (a) and (b) are linear function fits. (c) Comparison of ITHz /ID for 20-MTN and 3-MTN. by increasing the number of multi-layer repetitions. The resistance of 3-MTN was ≃ 1.64 times higher than that of 20-MTN because of the difference in layer thickness. On the other hand, the THz absorption for the 20-MTN film was 2.8 times larger than that of the 3-MTN film as discussed previously. Therefore, the expected factor of improvement in the ITHz /ID for the 20-MTN film compared with the 3-MTN film is 4.6 which is smaller than the experimentally obtained value of ≃ 9.2. The rest of the contributions can be partly attributed to the multiple reflection between the MTN materials and the substrates which become prominent with the increase in sample thickness. By stacking TI materials without breaking the topological property, the optical absorption coefficient in MTN simply is likely to be enhanced proportional to the number of TI layers. For instance, inserting a NI with a sufficient layer thickness between two TIs results in four independent TI states. An ideal single intrinsic TI with top and bottom metallic surface states, in which interband transition can be induced even by THz wave, is theoretically predicted to absorb 1.15 % of incident THz wave when bulk band gap is larger than energy of THz wave. 11,15 Although



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the MTN samples are p-type, the absorption coefficients obtained by THz-TDS (Figure 2c) are roughly comparable to this theoretical value of intrinsic MTN. According to eq S2 (Supporting Information), the photo absorption coefficient of the sample must be lower than that of the film itself due to the existence of the substrate with a refractive index greater than 1. For further improvement in the sensitivity of the detector device, the combination of the MTN and substrate with low refractive index should be considered. The 2.8-fold increase in absorption coefficient between the two MTNs is smaller than the difference of number of layers. This can be understood by the fact that our MTN materials are not single crystal and the structures are not perfect. This is supported by the recent high-resolution transmission electron microscopy and extended x-ray absorption fine structure spectroscopy measurements in the epitaxial GeTe/Sb2 Te3 multilayer films. 34,35

CONCLUSION In conclusion, we have demonstrated THz pulse detection with p-type MTN-based photodetectors. THz-TDS measurement revealed that increase in the number of layers of MTN enhances the absorption of THz waves whose photon energy is smaller than the bulk band gap, while the asgrown GST alloy was almost transparent. Simple THz detection devices were fabricated and the THz-induced change in the current was measured when a DC bias voltage was applied between the electrodes. We confirmed that irradiation of THz pulse causes a decrease in the resistance of the MTN. This result indicates that our devices are capable of THz detection. The sensitivity as well as the SNR of the detectors can be improved by the alternative stacking of TI and NI blocks.

Acknowledgement This work was supported in part by KAKENHI-26790063, 16H03886, 25286063 from JSPS, Japan and CREST JST, Japan. We acknowledge R. Kondou for sample preparation.


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Supporting Information Available Experimental details; I-V characteristic of the 20-MTN device; special profile, electrical field shape, and spectra of THz pulse; Variation of the amplitude of output signal This material is available free of charge via the Internet at http://pubs.acs.org/.

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