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silver micro thin film deposited on nanoparticles aggregation substrate
Journal Pre-proofs Full Length Article Strong Terahertz Emission from Copper Oxides/Silver Micro Thin Film Deposited on Nanoparticles Aggregation Substrate Xu Lu, Ming Qin, Youqing Wang, Jing Zhou, Qiao Zhu, Ping Peng, Yani Zhang, Hongjing Wu PII: DOI: Reference:
S0169-4332(19)34036-X https://doi.org/10.1016/j.apsusc.2019.145219 APSUSC 145219
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
24 October 2019 3 December 2019 27 December 2019
Please cite this article as: X. Lu, M. Qin, Y. Wang, J. Zhou, Q. Zhu, P. Peng, Y. Zhang, H. Wu, Strong Terahertz Emission from Copper Oxides/Silver Micro Thin Film Deposited on Nanoparticles Aggregation Substrate, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145219
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Strong Terahertz Emission from Copper Oxides/Silver Micro Thin Film Deposited on Nanoparticles Aggregation Substrate Xu Lu Conceptualization Writing - original draftb, Ming Qin Methodologya, Youqing Wang Methodology Funding acquisitionb,*,[email protected], Jing Zhou Visualizationb, Qiao Zhu Investigationb, Ping Peng Investigationb, Yani Zhangb Resources, Hongjing Wu Funding acquisition Supervision Writing - review & editinga,*,[email protected] aSchool
of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710072, China
bDepartment
of Physics, School of Arts and Sciences, Shaanxi University of Science
and Technology, Xi'an Weiyang University Park, Xi'an 710021, China *Corresponding
author. Graphical abstract
Terahertz emission was observed from copper oxides/silver micro film, which was comparable to that from gallium arsenide. Highlights
THz emission from the samples was comparable to that from the traditional emitter; Enhanced THz emission was observed from nanoparticles aggregation substrate;
A potential THz emitter, which is economical and simple processing.
Abstract Terahertz emission properties of oxidized silver/copper micro thin film were investigated. Enhanced terahertz emission was observed from the oxidized micro thin film deposited on nanoparticles aggregation substrate due to the various directions of transient current surge. The amplitude of terahertz emission from the film was comparable to that from traditional non-external gallium arsenide. The sample, firstly, was synthesized by hydrothermal method to fabricate nanoparticles aggregation 1
substrate. Next, the etched sample was treated by physical vapor deposition to grow silver/copper micro thin film and oxidized at 200 ℃ atmosphere environment. The space charge region was the dominant area for terahertz emission, built-in field at depletion layer played a key role in terahertz emission, and the contribution arising from optical rectification and photogalvanic was negligible due to the centrosymmetry structure; high band gap, low carriers concentration and mobility leads to the photon drag effect was not obvious; Because the differences of mobility and mass between hole and electron were not distinct, the Dember effect was also ineffective. The intensity of the terahertz emission was related to the thickness of the silver film because the silver film has a close relationship with the oxidation rate of the copper layer, leading to a dramatic influence on band gap structure, which produced a direct response on optical absorption at red zone. Compared with the planar substrate, the radiation of the nanoparticles substrate is 1.2 times that of the planar substrate, and the enhancement of the terahertz emission was ascribed to the various vibrated directions of photo-generated carriers. Our results could provide a potential portable terahertz emitter, which is economical, simple processing and operatable at room temperature. Keywords: terahertz emission, thin films, oxide materials, interface 1. Introduction Terahertz (THz) is in a range from 0.1 THz to 10 THz in electromagnetic spectrum at far infrared region, which is a transition zone between classical electromagnetics and photonics. Due to its special position, terahertz frequency can be applied in numerous fields in prospect [1–7], but it has not been widely used so far
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because of the lack of efficient, economical and convenient emitters. Some narrow bang gap semiconductors like gallium arsenide (GaAs), Indium phosphide (InP) and crystals with high nonlinear coefficient such as zinc telluride (ZnTe) are commonly used terahertz generators. However, the emission power of ZnTe, toxicity of InP as well as the cost of GaAs hinder the further applications of terahertz technology. In order to improve the performance of terahertz emitters, quantum cascade emitter [8–10], relativistic emitter [11–13], air plasmon [14–18], superconductor emitter [19, 20], metamaterials [21–25] and spin ordering materials [26–28] are proposed and some remarkable achievements have been made. Therefore, the efficient, economical, room temperature operatable, and portable terahertz emitter still attracts considerable research attentions by now. Among the existing methods, terahertz emission arises from the urgent separation of photo-generated carriers at semiconductor space charge region is a promising way to develop such terahertz emitters mentioned above. Copper oxides (cuprous oxide Cu2O and cupric oxide CuO) are conventional p-type semiconductor materials used in modern electric technology with widespread applications including terahertz emitters in potentially, providing a new choice for efficient, safe and economical terahertz sources instead of arsenides. In order to strengthen terahertz emission from copper oxides, a deal of efforts has been devoted to this issue, and the results indicated that cuprous oxide could generate terahertz radiation by intra-excitonic transitions under low temperature environment [29]; impressive terahertz emissions were detected from cuprous oxide/gold interface [30, 31]. These works promote the practicality of copper oxides terahertz emitters, but
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their emitting performances are still obviously weaker than that of traditional arsenides. There are three main reasons for this weakness. One is that the band gap energy is higher than the pump energy; the other two are longer carrier lifetime and low out-coupling efficiency. In order to solve the problems mentioned above for obtaining high-performance terahertz emitters, two methods were commonly proposed: artificial surface micro-structures [32–36] and interface optimization [37, 38]. In this paper, an oxidized silver/copper micro thin film on nanoparticles aggregation substrate was designed to improve the terahertz emission from copper oxides interface depletion layer. Compared with the previous reports, a nanoparticles aggregation substrate was utilized to enhance out-coupling efficiency and employed silver to form space charge region. Strong terahertz radiation was detected from our samples which were comparable to that from GaAs; nearly 1.2 times that from the planar substrate. The 1.2 times enhancement is attributed to the variety of vibrating directions of photo-generated carriers due to the nanoparticles aggregation substrate, which both reduces the energy loss caused by internal reflection and increases the collection range of emitted waves. Simultaneously, in comparison with gold, the position of the Fermi level in silver that strengthens the interface electric field also contributes to the terahertz emission significantly. Compared with the traditional terahertz emitters, our sample is economical and simple processing, which offers a promising choice for terahertz emitter in the future. 2. Experimental method
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2.1 preparation of the nanoparticles aggregation substrate Nanoparticles aggregation substrates were prepared by hydrothermal method. A heavy boron-doped <111>-oriented single polished Si wafer with a doping concentration of approximately 1 × 1019 cm3 was washed with ethanol and acetone by ultrasonic oscillation for 5 min, and then it was rinsed with purified water (electrical resistivity > 18 MΩ∙cm). The hydrothermal etchant was composed of hydrofluoric acid (HF, 40%), ferric nitrate (Fe(NO3)3, 0.04 M), silver nitrate (AgNO3, 0.0001 M) and purified water. The volume ratio of HF and purified water was 7:5 and the corrosion etchant was placed at drying oven at 110 ℃ for 15 min. 2.2 preparation of the oxidized silver/copper micro thin film Silver thin film was firstly deposited on the surface of as-prepared nanoparticles aggregation substrates by using a magnetron sputtering method. The pressure of the sputtering vacuum chamber was less than 3.0 × 10−3 Pa and then Ar gas was purged into the chamber at 1.0 Pa. The distance between the sample and the sputtering target was maintained at 125 mm. The sputtering power and durations were set to 19.44 W and 30 s, 60 s and 120 s, respectively. The thickness of the deposited silver layer was estimated at ~15 nm per second. Then, the copper thin film was deposited on the silver surface under the same sputtering conditions except that the sputtering power was at the value of 20 W and sputtering period was set to 300 s. The deposition rate of copper was estimated as same as that of silver. Next, placed the obtained Cu/Ag thin film in a drying oven at 200 ℃ for 2.0 h and annealed it gradually at ambient environment in the oven, kept it in air for three weeks.
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2.3 Characterizations The details of surface structures and element mapping of the as-prepared nanoparticles
substrates
were
surveyed
by
a
TESCAN
Corporation
VEGA-3-SBH/Octane Prime type scanning electron microscope (SEM). The composition of the deposited silver and the oxidized copper layers were investigated using a RIGAKU D/max 2200pc type X-ray diffractometer (XRD). Optical absorption properties were analyzed with an Agilent Cary-5000 type UN-VIS-NIR spectrophotometer. X-ray photoelectron spectroscopy (XPS) was carried out using a Shimadzu Kratos Axis Supra type device equipped with a monochromatic Al Kα source operating at 150 W under ultrahigh vacuum (~1.0 × 10−7 Pa) condition; the obtained spectra were calibrated by the C 1s peak. Terahertz emission properties were obtained by THz time-domain system shown in Fig. 1, femtosecond (fs) pulse with 800 nm central wavelength was emitted by a mode-locked Ti: sapphire laser (Spectra-Physics, Spitfire), and the duration time, power, repetition rate were set to 35 fs, 20 mW, 1000 Hz, respectively. The femtosecond-pulse was split into two parts: probe pulse and pump pulse. The pump one illuminated the sample surface with a tilted angle of 45 ° to produce terahertz radiations. The generated terahertz emissions were collected by parabolic mirrors and converged with the probe light. These two beams passed through an electro-optical sampling crystal (ZnTe), and the amplitudes of terahertz signal data were stored by a computer. Here, use the spectra of traditional non-external terahertz emitter GaAs as the reference.
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3. Results and discussion Surface structures and morphology of the as-prepared nanoparticles aggregation substrate and the heated silver/copper micro thin film are shown in Fig. 2. Giant discrete micro-island is observed with a diameter of several tens micrometers with a uniform distribution. The magnified image of the micro-island indicated that the micro-island consisted of a large number of nanoparticles with diameters in a rage of hundreds of nanometers. According to the SEM mapping result (see Fig. S1), the major composition of the nanoparticles was silver, which contained a very small amount of iron, while the content of silicon and oxygen were negligible. The formation mechanism of the nanoparticles aggregation substrate is related to the surface cracks, Ag+, Fe3+ ions were easier to be reduced to silver and iron in sharp point areas, finally, a nanoparticles aggregation structure was synthesized. The subsequent physical deposition layers of silver and copper were characterized by SEM lateral observation (see Fig. S2). The height of the deposited metal layers was several micro meters. Fig. 2c–e show the surface structures of the oxidized silver/copper thin film on nanoparticles substrate, and the protrusions correspond to the nanoparticle regions in Fig. 2b. Element mapping in Fig. 2f–h demonstrate that copper oxides dominate the surface and a small amount of silver was also detected. The source of the silver is due to the cracks of the film covering the nanoparticles during the process of heating, which makes some silver particles expose to air directly after heating.
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The compositions of the heated metal micro films were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) as shown in Fig. 3. In comparison with standard diffraction files, the two remarkable peaks are attributed to the diffraction peaks of silver (111) and silver (200), and the two shoulders on the left of silver (111) from near to far are assigned to cuprous oxide (111), and cupric oxide (002), respectively. Simultaneously, cupric oxide (111) is located on the right of silver (111) peak, which partially overlapped with silver (111) peak. In addition, although the silver film was the bottom layer, the crystallinity of silver is better than that of the copper oxides, so the peak intensity of silver is stronger and due to the experiment conditions. X-ray cannot reach the silicon area effectively because of the micro metal layer obstructions, so there was also no diffraction peak of silicon in the pattern. From the XRD result, the deposited copper/silver layers were eventually oxidized to copper oxides (cuprous oxide and cupric oxide, semiconductor)/silver (metal), which can also be confirmed by XPS spectra in Fig. 3b–c. It is notable that a suitable composition ratio of cuprous oxide and cupric oxide on the surface plays a non-negligible role in optical absorption. Cuprous oxide and cupric oxide obtained by heating copper are natural p-type semiconductors with band gaps of 2.1 (direct type) and 1.2 eV (indirect type), respectively, which are higher and lower than the energy of the central wavelength of the femtosecond laser (1.55 eV). Therefore, a suitable composition ratio of the copper oxides that matches the energy of the incident light is essential for light absorption and consequent lifetime of photon-generated carriers.
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Fig. 4 provides the ultraviolet-visible-near infrared (UV-VIS-NIR) reflectance spectra of the samples prepared under different deposition conditions. From the spectra, it is very obvious that the thickness of the silver film can influence the optical absorption to some extent and the signal to noise ratio is quite low in long wavelength areas, which arises from the switch of light source around 800 nm. In details, for the thicker silver deposited sample (red line), there are four valleys in the spectra at around 655 nm, 487 nm, 364 nm, 254 nm, respectively, while the thinner silver deposited sample (black line) showed four valleys at 583 nm, 466 nm, 359 nm and 256 nm. By comparison, the positions of valleys in blue and ultraviolet regions are very close, but those in yellow and red zones display an obvious deviation. The reason for the deviation is that the composition ratio of copper oxides is different, which is closely related to the thickness of silver film. Cuprous oxide has a direct band gap at the value of 2.1 eV (590 nm), so the valley at 583 nm corresponds to the optical absorption of cuprous oxide. Meanwhile, one factor that should be noted that copper has an excellent diffusivity, which means considerable copper atoms diffused into silver layer during the heating process. The thicker the silver film is, the more copper atoms diffuse into the silver film, that is, the less copper atoms are left. Therefore, under same heating conditions, oxidation rate of the copper layer in the sample with thicker silver layer was higher than that of the thinner ones, resulting in the deviation in the spectra. As the samples are solid and about 500 μm in thickness (corroded silicon substrate plus silver and copper oxides films), the reflectance valleys imply
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absorption peaks. The positions of the absorption peaks have a significant impact on terahertz emission abilities, which will be discussed below. Terahertz emission spectra of the various prepared samples are shown in Fig. 5. The comparison of emission capacities of traditional GaAs and the prepared sample are displayed in Fig. 5a (frequency domain spectra of GaAs and sample S3 are shown in Fig. S3), the silver, copper deposition durations of the sample were 120 s and 300 s, respectively. Terahertz emission amplitude from the prepared sample is comparable to that from GaAs, and the detected amplitude is 1.35 times that from GaAs, and the central frequency is 0.69 THz from the frequency spectrum of sample S3 shown in Fig. S3. Simultaneously, a new frequency peak centered at 1.2 THz appears. The main reason for the strong terahertz emission is the transient current surge induced by photon-generated carriers driven by the built-in field in the space charge region at the interface of the silver and copper oxides, especially in the copper oxides side (schematically represented in Fig. 6) and terahertz emission from the nanoparticles aggregation substrate can be ignored (see Fig. S4). In this case, the contact of the silver and copper oxides is determined by their Fermi levels. Copper oxides are p-type semiconductors, the work function of silver (4.26 eV) is lower than that of copper oxides (4.6 eV, 5.1 eV for cuprous oxide, cupric oxide), and electrons will flow into the copper oxides from the silver owing to the electric potential gradient. Finally, the depletion layer is formed at the interface copper oxides side where the density of carriers is much lower than in the bulk. Under equilibrium, the induced built-in electric field overcomes the influence of the electron flow and no special optical
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phenomenon occurs. However, when the incident light illuminates the sample, photon-generated carriers can break the balance resulting in a transient current that radiates electromagnetic waves. And the phase opposition of terahertz waveforms of GaAs and S3 is attributed to the opposite direction of built-in electric field at depletion layer. Opposite direction of built-in electric field in GaAs and S3 leads to the different directions of transient current, resulting in the phase opposition. Terahertz emission properties of various samples with different deposition conditions were investigated and shown in Fig. 5b. The intensities of the terahertz radiations are proportional to the thickness of silver film. As mentioned above, the diffusivity of copper has a close relationship with the proportional phenomenon. A thicker silver film resulted in an efficient oxidation rate. In other words, more cupric oxide presented in the space charge region. The presence of cuprous oxide and cupric oxide formed CuxO (1
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film sample shows stronger terahertz emission. On the other hand, the composition of the oxidized copper film also affects the thickness of the depletion layer. The thickness of semiconductor depletion layer can be expressed as below [39, 40]:
Ld 2U qN a (1) Where ε is the permittivity of the semiconductor, U is the built-in field potential, q is the elementary charge, Na is the acceptor concentration. The potential of the built-in electric field is related to the Fermi level, i.e., it is positively related to Fermi level (work function) difference at the interface. As mentioned above, the work function of silver, cuprous oxide, cupric oxide is 4.26 eV, 4.6 eV, 5.1 eV [41, 42], respectively; the acceptor concentrations of heated cuprous and cupric are almost the same; the permittivity of CuO is also stronger than that of Cu2O, so that an effective oxidation of copper film can increase the width of depletion layer, enlarging terahertz radiation range. Note that cupric oxide is an indirect semiconductor, and excessive cupric oxide in the space charge region will increase the relaxation time, causing a negative impact on terahertz efficiency. Based on the above results and analysis, a suitable ratio of cuprous oxide and cupric oxide is the crucial factor for terahertz emission, and in this study, heated at 200 ℃ for 2.0 h, 120 s silver deposited (the thickness was estimated to 1.8 μm) sample shows the best performance. In addition, the copper diffused into silver can also be oxidized, forming space charge region in the silver layer, contributing to a part of terahertz radiation, which is also proportional to the thickness of silver film. The effect of the substrate structures on terahertz radiation characteristics is 12
shown in Fig. 5c, terahertz emission from nanoparticles aggregation substrate is 1.2 times that from the planar one, which originates from the various separating directions of photon-generated carriers. In most case, when the incident light illuminates the sample, photon-generated carriers will be driven by the built-in field, forming transient current for terahertz emission and the separation of carriers is parallel to the normal direction. As a result, the radiated electromagnetic wave is parallel to the surface, that is, considerable terahertz radiation was absorbed in the copper oxides due to the total reflection. Whereas, if the surface has special texture that processed by artificial control, the direction of transient current will become more diverse, not just parallel to the normal, reducing the energy loss caused by total reflection to enhance the generation. In addition to interface transient current surge, optical rectification is another efficient way for terahertz generation, and terahertz emission from some traditional terahertz materials like GaAs should be attributed to both the surface filed (including Dember effect) and optical rectification. However, in this study, terahertz emission arose from optical rectification can be ignored due to the result shown in Fig. 7. The reason for the phase shifts of terahertz waveforms in Fig. 7b is thickness difference, as the silicon substrate was treated by hydrothermal method before metal film deposition. Terahertz emission from optical rectification displays a strong dependence on azimuthal angle, while the emission test of our samples gives a non-fluctuating response with the azimuthal angle in Fig. 7a–b, which confirms that the transient 13
current surge is the primary reason for terahertz radiations. In addition, photogalvanic effect, photon drag effect as well as photo Dember effect can also generate considerable terahertz radiation in some cases, but in our samples, terahertz emission from other second order nonlinear phenomenon like photogalvanic effect can also be excluded due to the centrosymmetry structure in copper oxides [35, 43]. Although built-in field presented at interface, the optical rectification was still very weak (implies that second-order nonlinear susceptibility is nearly zero), which has been confirmed by figure 7, indicated that second order nonlinear phenomena were negligible. As for the photon drag effect, it requires the tested material has high carrier concentration and mobility, usually it is the primary reason for terahertz emission in some gapless materials. However, for copper oxides, the band gap is more than 1.0 eV and the carrier concentration and mobility are about 1014–1015 /cm3+ [44, 45] and 0.1 – 10 cm2/v/s [44, 46], which are too low to acquire obvious photon drag effect. The contribution of photo-Dember effect to terahertz radiation is also negligible. Effective photo-Dember drift current usually in a narrow band gap semiconductor like InAs with huge differences in mobility and mass between hole and electron. The band gap of InAs is 0.36 eV and the mobility and mass of hole and electron in InAs are 240 cm2/v/s, 30000 cm2/v/s, 0.33 me, 0.027 me [25], respectively. However, for copper oxides, the band gap is more than 1.0 eV, which has an adverse effect on the gradient of carrier concentration along normal direction and the mobility and mass of hole and electron in copper oxides are on the same order of magnitude [44,47], resulting in a weak drift current. Furthermore, the alignment error caused by
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the nanoparticle aggregation surface can also be evaluated by the azimuth measurement. Non-planar surface will result in errors in the amplitude measurement, i.e., if the samples are rotated, the detected intensity will change synchronously. From the results shown in Fig. 7c, the terahertz radiation intensity fluctuates slightly for different angles, but the slightly difference is within acceptable range. That is, in this experiment, the influence of surface roughness on terahertz measurement can be ignored. 4. Conclusion In this study, terahertz emission properties from the oxidized silver/copper micro thin film deposited on nanoparticles aggregation substrate were investigated. The nanoparticles consisted of silver and few of iron and terahertz emission from the nanoparticles aggregation substrate can be ignored. The formation mechanism of the substrate depended on the cracks on the surface. Terahertz emission properties of the prepared samples showed strong radiation and the best performance was comparable to that of GaAs. Thickness of silver layer as well as surface microstructure of the substrate can influence the radiation amplitude significantly. Transient current surge in space charge region driven by built-in field was responsible for the terahertz emission. The results in this study may provide a potential terahertz emitter, which is economical, simple processing and room-temperature operatable. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 21806129, 51872238 and 61804092); the Fundamental Research Funds for the
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Fig. 1 Schematic diagram of terahertz detection system. Fig. 2 Surface morphology of the prepared nanoparticles aggregation substrate, (a) scanning electron microscope (SEM) image of the prepared nanoparticles aggregation substrate; (b) detailed information of the nanoparticles aggregation shown in (a); (c) SEM image of the oxidized silver/copper thin film on nanoparticles aggregation substrate and the thickness of the silver and copper films were 1.8 μm and 4.5 μm, respectively; (d) magnified image of the mountain structure shown in (c); (e) the details of the mountain structure shown in (c); (f)-(h) the mapping result of the oxidized surface.
Fig. 3 (a) X-ray diffraction (XRD) pattern of the oxidized metal micro layers, heated in ambient air at 200 ℃ for 2.0 h. The deposition duration of silver and copper were 30 s and 5 min and the thickness of the silver and copper films were 1.8 μm and 4.5 μm, respectively; (b) XPS wide scan of the oxidized metal micro layers shown in (a); (c) XPS narrow scan of Cu 2p of the oxidized metal micro layers shown in (a).
Fig. 4 Ultraviolet-visible-near infrared (UV-VIS-NIR) reflectance spectra of the prepared samples with differently deposited conditions, black line: the deposition duration of silver and copper were 30 s and 300 s; red line: the deposition duration of silver and copper were 120 s and 300 s, respectively.
Fig. 5 Terahertz emission properties of various prepared samples, (a) the comparison of GaAs and the prepared sample whose deposition conditions of silver and copper were 120 s, 300 s (Sample S3); (b) the comparison of the prepared samples under 23
different silver deposition conditions, the deposition durations of silver were set to 30 s, 60 s and 120 s (Sample S1, Sample S2, Sample S3, respectively and the deposition durations of copper are same); (c) the comparison of the samples deposited on different substrates (the deposition conditions of Sample S3 and Sample S4 are identical but S4 was on a planar substrate). Fig. 6 Schematic diagrams of THz emission caused by transient current surge driven by built-in field at space charge region, (a) before equilibrium and (b) equilibrium.
Fig. 7 The correspondence between terahertz amplitude and azimuth angle, (a) terahertz emission spectra were obtained by rotating the samples S1, S2, S3 90 degrees; (b) terahertz emission spectra from the sample with various azimuth angles (the thicknesses of the silver and copper layers of the tested sample were estimated to be 150 nm and 4.5 μm respectively); (c) correspondence of terahertz amplitude value versus azimuth angle in Fig. 7b (the amplitude value 1.0 corresponds to the terahertz emission from GaAs).
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S0169-4332(19)34036-X https://doi.org/10.1016/j.apsusc.2019.145219 APSUSC 145219
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
24 October 2019 3 December 2019 27 December 2019
Please cite this article as: X. Lu, M. Qin, Y. Wang, J. Zhou, Q. Zhu, P. Peng, Y. Zhang, H. Wu, Strong Terahertz Emission from Copper Oxides/Silver Micro Thin Film Deposited on Nanoparticles Aggregation Substrate, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145219
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Strong Terahertz Emission from Copper Oxides/Silver Micro Thin Film Deposited on Nanoparticles Aggregation Substrate Xu Lu Conceptualization Writing - original draftb, Ming Qin Methodologya, Youqing Wang Methodology Funding acquisitionb,*,[email protected], Jing Zhou Visualizationb, Qiao Zhu Investigationb, Ping Peng Investigationb, Yani Zhangb Resources, Hongjing Wu Funding acquisition Supervision Writing - review & editinga,*,[email protected] aSchool
of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710072, China
bDepartment
of Physics, School of Arts and Sciences, Shaanxi University of Science
and Technology, Xi'an Weiyang University Park, Xi'an 710021, China *Corresponding
author. Graphical abstract
Terahertz emission was observed from copper oxides/silver micro film, which was comparable to that from gallium arsenide. Highlights
THz emission from the samples was comparable to that from the traditional emitter; Enhanced THz emission was observed from nanoparticles aggregation substrate;
A potential THz emitter, which is economical and simple processing.
Abstract Terahertz emission properties of oxidized silver/copper micro thin film were investigated. Enhanced terahertz emission was observed from the oxidized micro thin film deposited on nanoparticles aggregation substrate due to the various directions of transient current surge. The amplitude of terahertz emission from the film was comparable to that from traditional non-external gallium arsenide. The sample, firstly, was synthesized by hydrothermal method to fabricate nanoparticles aggregation 1
substrate. Next, the etched sample was treated by physical vapor deposition to grow silver/copper micro thin film and oxidized at 200 ℃ atmosphere environment. The space charge region was the dominant area for terahertz emission, built-in field at depletion layer played a key role in terahertz emission, and the contribution arising from optical rectification and photogalvanic was negligible due to the centrosymmetry structure; high band gap, low carriers concentration and mobility leads to the photon drag effect was not obvious; Because the differences of mobility and mass between hole and electron were not distinct, the Dember effect was also ineffective. The intensity of the terahertz emission was related to the thickness of the silver film because the silver film has a close relationship with the oxidation rate of the copper layer, leading to a dramatic influence on band gap structure, which produced a direct response on optical absorption at red zone. Compared with the planar substrate, the radiation of the nanoparticles substrate is 1.2 times that of the planar substrate, and the enhancement of the terahertz emission was ascribed to the various vibrated directions of photo-generated carriers. Our results could provide a potential portable terahertz emitter, which is economical, simple processing and operatable at room temperature. Keywords: terahertz emission, thin films, oxide materials, interface 1. Introduction Terahertz (THz) is in a range from 0.1 THz to 10 THz in electromagnetic spectrum at far infrared region, which is a transition zone between classical electromagnetics and photonics. Due to its special position, terahertz frequency can be applied in numerous fields in prospect [1–7], but it has not been widely used so far
2
because of the lack of efficient, economical and convenient emitters. Some narrow bang gap semiconductors like gallium arsenide (GaAs), Indium phosphide (InP) and crystals with high nonlinear coefficient such as zinc telluride (ZnTe) are commonly used terahertz generators. However, the emission power of ZnTe, toxicity of InP as well as the cost of GaAs hinder the further applications of terahertz technology. In order to improve the performance of terahertz emitters, quantum cascade emitter [8–10], relativistic emitter [11–13], air plasmon [14–18], superconductor emitter [19, 20], metamaterials [21–25] and spin ordering materials [26–28] are proposed and some remarkable achievements have been made. Therefore, the efficient, economical, room temperature operatable, and portable terahertz emitter still attracts considerable research attentions by now. Among the existing methods, terahertz emission arises from the urgent separation of photo-generated carriers at semiconductor space charge region is a promising way to develop such terahertz emitters mentioned above. Copper oxides (cuprous oxide Cu2O and cupric oxide CuO) are conventional p-type semiconductor materials used in modern electric technology with widespread applications including terahertz emitters in potentially, providing a new choice for efficient, safe and economical terahertz sources instead of arsenides. In order to strengthen terahertz emission from copper oxides, a deal of efforts has been devoted to this issue, and the results indicated that cuprous oxide could generate terahertz radiation by intra-excitonic transitions under low temperature environment [29]; impressive terahertz emissions were detected from cuprous oxide/gold interface [30, 31]. These works promote the practicality of copper oxides terahertz emitters, but
3
their emitting performances are still obviously weaker than that of traditional arsenides. There are three main reasons for this weakness. One is that the band gap energy is higher than the pump energy; the other two are longer carrier lifetime and low out-coupling efficiency. In order to solve the problems mentioned above for obtaining high-performance terahertz emitters, two methods were commonly proposed: artificial surface micro-structures [32–36] and interface optimization [37, 38]. In this paper, an oxidized silver/copper micro thin film on nanoparticles aggregation substrate was designed to improve the terahertz emission from copper oxides interface depletion layer. Compared with the previous reports, a nanoparticles aggregation substrate was utilized to enhance out-coupling efficiency and employed silver to form space charge region. Strong terahertz radiation was detected from our samples which were comparable to that from GaAs; nearly 1.2 times that from the planar substrate. The 1.2 times enhancement is attributed to the variety of vibrating directions of photo-generated carriers due to the nanoparticles aggregation substrate, which both reduces the energy loss caused by internal reflection and increases the collection range of emitted waves. Simultaneously, in comparison with gold, the position of the Fermi level in silver that strengthens the interface electric field also contributes to the terahertz emission significantly. Compared with the traditional terahertz emitters, our sample is economical and simple processing, which offers a promising choice for terahertz emitter in the future. 2. Experimental method
4
2.1 preparation of the nanoparticles aggregation substrate Nanoparticles aggregation substrates were prepared by hydrothermal method. A heavy boron-doped <111>-oriented single polished Si wafer with a doping concentration of approximately 1 × 1019 cm3 was washed with ethanol and acetone by ultrasonic oscillation for 5 min, and then it was rinsed with purified water (electrical resistivity > 18 MΩ∙cm). The hydrothermal etchant was composed of hydrofluoric acid (HF, 40%), ferric nitrate (Fe(NO3)3, 0.04 M), silver nitrate (AgNO3, 0.0001 M) and purified water. The volume ratio of HF and purified water was 7:5 and the corrosion etchant was placed at drying oven at 110 ℃ for 15 min. 2.2 preparation of the oxidized silver/copper micro thin film Silver thin film was firstly deposited on the surface of as-prepared nanoparticles aggregation substrates by using a magnetron sputtering method. The pressure of the sputtering vacuum chamber was less than 3.0 × 10−3 Pa and then Ar gas was purged into the chamber at 1.0 Pa. The distance between the sample and the sputtering target was maintained at 125 mm. The sputtering power and durations were set to 19.44 W and 30 s, 60 s and 120 s, respectively. The thickness of the deposited silver layer was estimated at ~15 nm per second. Then, the copper thin film was deposited on the silver surface under the same sputtering conditions except that the sputtering power was at the value of 20 W and sputtering period was set to 300 s. The deposition rate of copper was estimated as same as that of silver. Next, placed the obtained Cu/Ag thin film in a drying oven at 200 ℃ for 2.0 h and annealed it gradually at ambient environment in the oven, kept it in air for three weeks.
5
2.3 Characterizations The details of surface structures and element mapping of the as-prepared nanoparticles
substrates
were
surveyed
by
a
TESCAN
Corporation
VEGA-3-SBH/Octane Prime type scanning electron microscope (SEM). The composition of the deposited silver and the oxidized copper layers were investigated using a RIGAKU D/max 2200pc type X-ray diffractometer (XRD). Optical absorption properties were analyzed with an Agilent Cary-5000 type UN-VIS-NIR spectrophotometer. X-ray photoelectron spectroscopy (XPS) was carried out using a Shimadzu Kratos Axis Supra type device equipped with a monochromatic Al Kα source operating at 150 W under ultrahigh vacuum (~1.0 × 10−7 Pa) condition; the obtained spectra were calibrated by the C 1s peak. Terahertz emission properties were obtained by THz time-domain system shown in Fig. 1, femtosecond (fs) pulse with 800 nm central wavelength was emitted by a mode-locked Ti: sapphire laser (Spectra-Physics, Spitfire), and the duration time, power, repetition rate were set to 35 fs, 20 mW, 1000 Hz, respectively. The femtosecond-pulse was split into two parts: probe pulse and pump pulse. The pump one illuminated the sample surface with a tilted angle of 45 ° to produce terahertz radiations. The generated terahertz emissions were collected by parabolic mirrors and converged with the probe light. These two beams passed through an electro-optical sampling crystal (ZnTe), and the amplitudes of terahertz signal data were stored by a computer. Here, use the spectra of traditional non-external terahertz emitter GaAs as the reference.
6
3. Results and discussion Surface structures and morphology of the as-prepared nanoparticles aggregation substrate and the heated silver/copper micro thin film are shown in Fig. 2. Giant discrete micro-island is observed with a diameter of several tens micrometers with a uniform distribution. The magnified image of the micro-island indicated that the micro-island consisted of a large number of nanoparticles with diameters in a rage of hundreds of nanometers. According to the SEM mapping result (see Fig. S1), the major composition of the nanoparticles was silver, which contained a very small amount of iron, while the content of silicon and oxygen were negligible. The formation mechanism of the nanoparticles aggregation substrate is related to the surface cracks, Ag+, Fe3+ ions were easier to be reduced to silver and iron in sharp point areas, finally, a nanoparticles aggregation structure was synthesized. The subsequent physical deposition layers of silver and copper were characterized by SEM lateral observation (see Fig. S2). The height of the deposited metal layers was several micro meters. Fig. 2c–e show the surface structures of the oxidized silver/copper thin film on nanoparticles substrate, and the protrusions correspond to the nanoparticle regions in Fig. 2b. Element mapping in Fig. 2f–h demonstrate that copper oxides dominate the surface and a small amount of silver was also detected. The source of the silver is due to the cracks of the film covering the nanoparticles during the process of heating, which makes some silver particles expose to air directly after heating.
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The compositions of the heated metal micro films were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) as shown in Fig. 3. In comparison with standard diffraction files, the two remarkable peaks are attributed to the diffraction peaks of silver (111) and silver (200), and the two shoulders on the left of silver (111) from near to far are assigned to cuprous oxide (111), and cupric oxide (002), respectively. Simultaneously, cupric oxide (111) is located on the right of silver (111) peak, which partially overlapped with silver (111) peak. In addition, although the silver film was the bottom layer, the crystallinity of silver is better than that of the copper oxides, so the peak intensity of silver is stronger and due to the experiment conditions. X-ray cannot reach the silicon area effectively because of the micro metal layer obstructions, so there was also no diffraction peak of silicon in the pattern. From the XRD result, the deposited copper/silver layers were eventually oxidized to copper oxides (cuprous oxide and cupric oxide, semiconductor)/silver (metal), which can also be confirmed by XPS spectra in Fig. 3b–c. It is notable that a suitable composition ratio of cuprous oxide and cupric oxide on the surface plays a non-negligible role in optical absorption. Cuprous oxide and cupric oxide obtained by heating copper are natural p-type semiconductors with band gaps of 2.1 (direct type) and 1.2 eV (indirect type), respectively, which are higher and lower than the energy of the central wavelength of the femtosecond laser (1.55 eV). Therefore, a suitable composition ratio of the copper oxides that matches the energy of the incident light is essential for light absorption and consequent lifetime of photon-generated carriers.
8
Fig. 4 provides the ultraviolet-visible-near infrared (UV-VIS-NIR) reflectance spectra of the samples prepared under different deposition conditions. From the spectra, it is very obvious that the thickness of the silver film can influence the optical absorption to some extent and the signal to noise ratio is quite low in long wavelength areas, which arises from the switch of light source around 800 nm. In details, for the thicker silver deposited sample (red line), there are four valleys in the spectra at around 655 nm, 487 nm, 364 nm, 254 nm, respectively, while the thinner silver deposited sample (black line) showed four valleys at 583 nm, 466 nm, 359 nm and 256 nm. By comparison, the positions of valleys in blue and ultraviolet regions are very close, but those in yellow and red zones display an obvious deviation. The reason for the deviation is that the composition ratio of copper oxides is different, which is closely related to the thickness of silver film. Cuprous oxide has a direct band gap at the value of 2.1 eV (590 nm), so the valley at 583 nm corresponds to the optical absorption of cuprous oxide. Meanwhile, one factor that should be noted that copper has an excellent diffusivity, which means considerable copper atoms diffused into silver layer during the heating process. The thicker the silver film is, the more copper atoms diffuse into the silver film, that is, the less copper atoms are left. Therefore, under same heating conditions, oxidation rate of the copper layer in the sample with thicker silver layer was higher than that of the thinner ones, resulting in the deviation in the spectra. As the samples are solid and about 500 μm in thickness (corroded silicon substrate plus silver and copper oxides films), the reflectance valleys imply
9
absorption peaks. The positions of the absorption peaks have a significant impact on terahertz emission abilities, which will be discussed below. Terahertz emission spectra of the various prepared samples are shown in Fig. 5. The comparison of emission capacities of traditional GaAs and the prepared sample are displayed in Fig. 5a (frequency domain spectra of GaAs and sample S3 are shown in Fig. S3), the silver, copper deposition durations of the sample were 120 s and 300 s, respectively. Terahertz emission amplitude from the prepared sample is comparable to that from GaAs, and the detected amplitude is 1.35 times that from GaAs, and the central frequency is 0.69 THz from the frequency spectrum of sample S3 shown in Fig. S3. Simultaneously, a new frequency peak centered at 1.2 THz appears. The main reason for the strong terahertz emission is the transient current surge induced by photon-generated carriers driven by the built-in field in the space charge region at the interface of the silver and copper oxides, especially in the copper oxides side (schematically represented in Fig. 6) and terahertz emission from the nanoparticles aggregation substrate can be ignored (see Fig. S4). In this case, the contact of the silver and copper oxides is determined by their Fermi levels. Copper oxides are p-type semiconductors, the work function of silver (4.26 eV) is lower than that of copper oxides (4.6 eV, 5.1 eV for cuprous oxide, cupric oxide), and electrons will flow into the copper oxides from the silver owing to the electric potential gradient. Finally, the depletion layer is formed at the interface copper oxides side where the density of carriers is much lower than in the bulk. Under equilibrium, the induced built-in electric field overcomes the influence of the electron flow and no special optical
10
phenomenon occurs. However, when the incident light illuminates the sample, photon-generated carriers can break the balance resulting in a transient current that radiates electromagnetic waves. And the phase opposition of terahertz waveforms of GaAs and S3 is attributed to the opposite direction of built-in electric field at depletion layer. Opposite direction of built-in electric field in GaAs and S3 leads to the different directions of transient current, resulting in the phase opposition. Terahertz emission properties of various samples with different deposition conditions were investigated and shown in Fig. 5b. The intensities of the terahertz radiations are proportional to the thickness of silver film. As mentioned above, the diffusivity of copper has a close relationship with the proportional phenomenon. A thicker silver film resulted in an efficient oxidation rate. In other words, more cupric oxide presented in the space charge region. The presence of cuprous oxide and cupric oxide formed CuxO (1
11
film sample shows stronger terahertz emission. On the other hand, the composition of the oxidized copper film also affects the thickness of the depletion layer. The thickness of semiconductor depletion layer can be expressed as below [39, 40]:
Ld 2U qN a (1) Where ε is the permittivity of the semiconductor, U is the built-in field potential, q is the elementary charge, Na is the acceptor concentration. The potential of the built-in electric field is related to the Fermi level, i.e., it is positively related to Fermi level (work function) difference at the interface. As mentioned above, the work function of silver, cuprous oxide, cupric oxide is 4.26 eV, 4.6 eV, 5.1 eV [41, 42], respectively; the acceptor concentrations of heated cuprous and cupric are almost the same; the permittivity of CuO is also stronger than that of Cu2O, so that an effective oxidation of copper film can increase the width of depletion layer, enlarging terahertz radiation range. Note that cupric oxide is an indirect semiconductor, and excessive cupric oxide in the space charge region will increase the relaxation time, causing a negative impact on terahertz efficiency. Based on the above results and analysis, a suitable ratio of cuprous oxide and cupric oxide is the crucial factor for terahertz emission, and in this study, heated at 200 ℃ for 2.0 h, 120 s silver deposited (the thickness was estimated to 1.8 μm) sample shows the best performance. In addition, the copper diffused into silver can also be oxidized, forming space charge region in the silver layer, contributing to a part of terahertz radiation, which is also proportional to the thickness of silver film. The effect of the substrate structures on terahertz radiation characteristics is 12
shown in Fig. 5c, terahertz emission from nanoparticles aggregation substrate is 1.2 times that from the planar one, which originates from the various separating directions of photon-generated carriers. In most case, when the incident light illuminates the sample, photon-generated carriers will be driven by the built-in field, forming transient current for terahertz emission and the separation of carriers is parallel to the normal direction. As a result, the radiated electromagnetic wave is parallel to the surface, that is, considerable terahertz radiation was absorbed in the copper oxides due to the total reflection. Whereas, if the surface has special texture that processed by artificial control, the direction of transient current will become more diverse, not just parallel to the normal, reducing the energy loss caused by total reflection to enhance the generation. In addition to interface transient current surge, optical rectification is another efficient way for terahertz generation, and terahertz emission from some traditional terahertz materials like GaAs should be attributed to both the surface filed (including Dember effect) and optical rectification. However, in this study, terahertz emission arose from optical rectification can be ignored due to the result shown in Fig. 7. The reason for the phase shifts of terahertz waveforms in Fig. 7b is thickness difference, as the silicon substrate was treated by hydrothermal method before metal film deposition. Terahertz emission from optical rectification displays a strong dependence on azimuthal angle, while the emission test of our samples gives a non-fluctuating response with the azimuthal angle in Fig. 7a–b, which confirms that the transient 13
current surge is the primary reason for terahertz radiations. In addition, photogalvanic effect, photon drag effect as well as photo Dember effect can also generate considerable terahertz radiation in some cases, but in our samples, terahertz emission from other second order nonlinear phenomenon like photogalvanic effect can also be excluded due to the centrosymmetry structure in copper oxides [35, 43]. Although built-in field presented at interface, the optical rectification was still very weak (implies that second-order nonlinear susceptibility is nearly zero), which has been confirmed by figure 7, indicated that second order nonlinear phenomena were negligible. As for the photon drag effect, it requires the tested material has high carrier concentration and mobility, usually it is the primary reason for terahertz emission in some gapless materials. However, for copper oxides, the band gap is more than 1.0 eV and the carrier concentration and mobility are about 1014–1015 /cm3+ [44, 45] and 0.1 – 10 cm2/v/s [44, 46], which are too low to acquire obvious photon drag effect. The contribution of photo-Dember effect to terahertz radiation is also negligible. Effective photo-Dember drift current usually in a narrow band gap semiconductor like InAs with huge differences in mobility and mass between hole and electron. The band gap of InAs is 0.36 eV and the mobility and mass of hole and electron in InAs are 240 cm2/v/s, 30000 cm2/v/s, 0.33 me, 0.027 me [25], respectively. However, for copper oxides, the band gap is more than 1.0 eV, which has an adverse effect on the gradient of carrier concentration along normal direction and the mobility and mass of hole and electron in copper oxides are on the same order of magnitude [44,47], resulting in a weak drift current. Furthermore, the alignment error caused by
14
the nanoparticle aggregation surface can also be evaluated by the azimuth measurement. Non-planar surface will result in errors in the amplitude measurement, i.e., if the samples are rotated, the detected intensity will change synchronously. From the results shown in Fig. 7c, the terahertz radiation intensity fluctuates slightly for different angles, but the slightly difference is within acceptable range. That is, in this experiment, the influence of surface roughness on terahertz measurement can be ignored. 4. Conclusion In this study, terahertz emission properties from the oxidized silver/copper micro thin film deposited on nanoparticles aggregation substrate were investigated. The nanoparticles consisted of silver and few of iron and terahertz emission from the nanoparticles aggregation substrate can be ignored. The formation mechanism of the substrate depended on the cracks on the surface. Terahertz emission properties of the prepared samples showed strong radiation and the best performance was comparable to that of GaAs. Thickness of silver layer as well as surface microstructure of the substrate can influence the radiation amplitude significantly. Transient current surge in space charge region driven by built-in field was responsible for the terahertz emission. The results in this study may provide a potential terahertz emitter, which is economical, simple processing and room-temperature operatable. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 21806129, 51872238 and 61804092); the Fundamental Research Funds for the
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Central Universities (Nos. 3102018zy045 and 3102019AX11); and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2017JQ5116). References [1] M.C. Beard, G.M. Turner, C.A. Schmuttenmaer, Transient photoconductivity in GaAs as measured by time-resolved terahertz spectroscopy, Physical Review B, 62 (2000) 15764–15777. [2] B.B. Hu, M.C. Nuss, Imaging with terahertz waves, Optics Letters, 20 (1995) 1716. [3] L. Ning, J.L. Shen, J.H. Sun, L.S. Liang, X.Y. Xu, M.H. Lu, J. Yan, Study on the THz spectrum of methamphetamine, Optics Express, 13 (2005) 6750–6755. [4] Y. Huang, Z. Yao, F. Hu, C. Liu, L. Yu, Y. Jin, X. Xu, Tunable circular polarization
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Fig. 1 Schematic diagram of terahertz detection system. Fig. 2 Surface morphology of the prepared nanoparticles aggregation substrate, (a) scanning electron microscope (SEM) image of the prepared nanoparticles aggregation substrate; (b) detailed information of the nanoparticles aggregation shown in (a); (c) SEM image of the oxidized silver/copper thin film on nanoparticles aggregation substrate and the thickness of the silver and copper films were 1.8 μm and 4.5 μm, respectively; (d) magnified image of the mountain structure shown in (c); (e) the details of the mountain structure shown in (c); (f)-(h) the mapping result of the oxidized surface.
Fig. 3 (a) X-ray diffraction (XRD) pattern of the oxidized metal micro layers, heated in ambient air at 200 ℃ for 2.0 h. The deposition duration of silver and copper were 30 s and 5 min and the thickness of the silver and copper films were 1.8 μm and 4.5 μm, respectively; (b) XPS wide scan of the oxidized metal micro layers shown in (a); (c) XPS narrow scan of Cu 2p of the oxidized metal micro layers shown in (a).
Fig. 4 Ultraviolet-visible-near infrared (UV-VIS-NIR) reflectance spectra of the prepared samples with differently deposited conditions, black line: the deposition duration of silver and copper were 30 s and 300 s; red line: the deposition duration of silver and copper were 120 s and 300 s, respectively.
Fig. 5 Terahertz emission properties of various prepared samples, (a) the comparison of GaAs and the prepared sample whose deposition conditions of silver and copper were 120 s, 300 s (Sample S3); (b) the comparison of the prepared samples under 23
different silver deposition conditions, the deposition durations of silver were set to 30 s, 60 s and 120 s (Sample S1, Sample S2, Sample S3, respectively and the deposition durations of copper are same); (c) the comparison of the samples deposited on different substrates (the deposition conditions of Sample S3 and Sample S4 are identical but S4 was on a planar substrate). Fig. 6 Schematic diagrams of THz emission caused by transient current surge driven by built-in field at space charge region, (a) before equilibrium and (b) equilibrium.
Fig. 7 The correspondence between terahertz amplitude and azimuth angle, (a) terahertz emission spectra were obtained by rotating the samples S1, S2, S3 90 degrees; (b) terahertz emission spectra from the sample with various azimuth angles (the thicknesses of the silver and copper layers of the tested sample were estimated to be 150 nm and 4.5 μm respectively); (c) correspondence of terahertz amplitude value versus azimuth angle in Fig. 7b (the amplitude value 1.0 corresponds to the terahertz emission from GaAs).
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