Terahertz emission enhancing indium tin oxide film and preparation method thereof

Abstract

The invention provides an indium tin oxide thin film for enhancing terahertz emission and a preparation method of the indium tin oxide thin film, and belongs to the technical field of nonlinear optics and terahertz. The method comprises the following steps: depositing an indium tin oxide (ITO) film on a substrate by adopting a magnetron sputtering process; and the thin film is placed in a vacuum environment to be subjected to rapid thermal annealing treatment. The core of the method is that the annealing time and the annealing temperature are accurately controlled, and the thin film is regulated and controlled to be in a middle semitransparent state which is converted from opaque to full-transparent. Experiments show that the ITO thin film in the semitransparent state maintains high nonlinear absorptivity in a near-infrared pumping wave band while ensuring certain crystallization quality, so that the emission intensity of terahertz generated by the ITO thin film is obviously higher than that of a full-transparent thin film which is annealed for a long time. According to the terahertz emission source, the problem that the nonlinear optical conversion efficiency is reduced due to excessive pursuit of film transparency in the prior art is solved, and a terahertz emission source preparation scheme which is simple in process, compatible with a CMOS (complementary metal oxide semiconductor) process and efficient is provided.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic materials and nonlinear optics technology, specifically relating to a method for preparing an indium tin oxide thin film to enhance terahertz emission, the resulting thin film, and its application in a terahertz light source. Background Technology

[0002] Indium tin oxide (ITO) is a typical heavily doped transparent conductive oxide (TCO). Due to its high visible light transmittance, excellent electrical conductivity, and good compatibility with complementary metal-oxide-semiconductor (CMOS) processes, it is widely used in optoelectronic devices such as flat panel displays, photovoltaic cells, light-emitting diodes, and photodetectors.

[0003] In recent years, with the development of nanophotonics, the application potential of ITO thin films in nonlinear optics has attracted great attention. Studies have shown that ITO thin films exhibit near-zero dielectric constant (ENZ) in the near-infrared spectral region, which endows them with extremely strong optical field localization capabilities and significantly enhanced nonlinear optical responses. Based on second- or third-order nonlinear optical processes, ITO thin films can be excited by femtosecond lasers to generate broadband terahertz (THz) emission. Given the enormous application value of terahertz waves in nondestructive testing, broadband communication, and biomedical imaging, the development of efficient, integrated terahertz emission sources based on ITO has become a current research hotspot.

[0004] In terms of fabrication processes, magnetron sputtering is widely used due to its good controllability and large film area. Typically, freshly deposited ITO films have poor crystallinity and contain a large amount of In₄Sn₃O₂. 12 Intermediate phases with defects often appear dark or opaque and have poor conductivity. Therefore, the standard practice in the prior art is to perform long-term high-temperature annealing on the deposited ITO film to promote grain growth, eliminate defects, and completely decompose the intermediate phase, thereby obtaining a high-quality film that is fully transparent and highly conductive.

[0005] However, existing ITO thin film preparation and post-processing techniques primarily focus on improving the film's linear visible light transmittance and electrical conductivity. While thorough long-term annealing significantly enhances film transparency, it is not optimal for terahertz emission applications based on nonlinear optics principles. The physical mechanism indicates that the terahertz emission efficiency of ITO thin films is closely related to their nonlinear absorption capability for near-infrared pump light. "Fully transparent" ITO thin films that have undergone long-term, thorough annealing exhibit a significantly reduced absorption coefficient in the near-infrared band (especially the commonly used pump band of 1200nm-1600nm), leading to a weakened interaction between the pump light and the material, thus greatly limiting the excitation efficiency of terahertz waves.

[0006] In summary, the current research strategy of pursuing complete transparency in fabrication methods comes at the cost of sacrificing nonlinear terahertz emission performance. Currently, the industry has not yet established a fabrication strategy specifically optimized for nonlinear terahertz emission performance. Therefore, there is an urgent need to develop a novel fabrication method that breaks away from the traditional mindset that "complete transparency is optimal," and precisely controls the light absorption characteristics of the thin film to significantly enhance terahertz emission while ensuring a certain level of crystallinity. Summary of the Invention

[0007] Purpose of the invention: The purpose of this invention is to provide an indium tin oxide (ITO) thin film that enhances terahertz emission and its preparation method. By precisely controlling the rapid thermal annealing process, while ensuring a certain crystal quality of the film, its specific semi-transparent state is maintained, thereby significantly improving the nonlinear terahertz emission intensity of the ITO thin film.

[0008] Technical solution: A method for preparing an indium tin oxide thin film to enhance terahertz emission, characterized by comprising the following steps: Step S1: Deposit an initial state indium tin oxide (ITO) thin film on the substrate using a magnetron sputtering process; Step S2: Place the deposited film in a vacuum environment for rapid thermal annealing; In step S2, by controlling the annealing process parameters, the resulting indium tin oxide film is placed in a crystalline transition state, and its microstructure includes partially decomposed In4Sn3O. 12 The film contains both the In2O3 crystalline phase and the In2O3 crystalline phase, and the light absorption rate of the film in the near-infrared pump band (1200nm-1600nm) is between 15% and 30%, so as to improve its nonlinear absorption and terahertz emission intensity.

[0009] Further, in step S1, the specific parameters of the magnetron sputtering process include: using an ITO target with an In2O3 to SnO2 mass ratio of 90:10 (wt%); sputtering power of 40W-60W, preferably 50W; argon flow rate of 20sccm-40sccm, preferably 30sccm; substrate temperature maintained at 280°C-320°C, preferably 300°C; and deposited film thickness controlled at 100nm-150nm, preferably 120nm.

[0010] Furthermore, in step S2, the annealing time of the rapid thermal annealing process is controlled between 1 minute and 30 minutes; preferably, in order to obtain the best terahertz emission efficiency, the annealing time is controlled between 2.5 minutes and 10 minutes.

[0011] Further, in step S2, the specific process parameters of the rapid thermal annealing treatment include: the annealing temperature is controlled between 280°C and 320°C, preferably 300°C; the heating rate is controlled between 200°C / min and 400°C / min, preferably 300°C / min; and the gas pressure in the vacuum chamber is maintained between 1.0×10⁻¹ Pa and 2.0×10⁻¹ Pa, preferably 1.3×10⁻¹ Pa.

[0012] Furthermore, the present invention provides an indium tin oxide thin film with high nonlinear terahertz emission efficiency prepared by the above method. The film is macroscopically semi-transparent, and microscopically contains deposited In4Sn3O. 12 The phase partially decomposes and dissolves in the In2O3 lattice, forming a mixed phase structure between the sedimentary state and the highly crystalline state.

[0013] Furthermore, when the indium tin oxide film is excited by a femtosecond laser with a center wavelength of 1200nm-1600nm, it generates broadband terahertz emission, and the peak-to-peak intensity of its terahertz emission signal is at least 1.5 times that of a fully annealed transparent indium tin oxide film of the same thickness.

[0014] The beneficial effects and mechanisms of this invention are as follows:

[0015] 1. Precise Balance of Photoelectric Properties: Existing technologies have shown that terahertz emission efficiency depends on the local field enhancement effect. This invention controls the annealing process to a specific intermediate state, maintaining the light absorption rate of the thin film in the near-infrared band (1200-1600 nm) within a preferred range of 15%-30%. This avoids the low nonlinear polarization caused by poor crystallinity in unannealed samples, and also avoids the absorption loss caused by excessive carrier activation in fully transparent samples, thus maximizing the third-order nonlinear effect.

[0016] 2. Significant nonlinear enhancement: Experimental data show that the terahertz emission intensity of the thin film prepared by this invention under 1550nm excitation can reach more than twice that of the fully transparent standard sample, and the preferred time window (2.5-10 minutes) exhibits a stable high emission plateau region, with good process robustness. Attached Figure Description

[0017] Figure 1 These are photographs and visible light transmittance spectra of ITO films after different annealing times according to the present invention; wherein (a) is a comparison of the appearance of the actual products, showing the change in transparency of the film as the annealing time increases; and (b) is the transmittance spectrum in the 300-1000nm wavelength band.

[0018] Figure 2 These are the optical and electrical property curves of ITO thin films after different annealing times according to the present invention; wherein: (a), (b), and (c) are the transmittance, reflectance, and absorptivity spectra in the near-infrared band (1200-2400nm), respectively; (d) is the AC impedance spectrum under different annealing times, reflecting the change in the conductivity of the thin film; (e) and (f) are the curves showing the change of the real and imaginary parts of the dielectric constant of the thin film with wavelength, respectively, demonstrating the modulating effect of annealing on the dielectric properties of the thin film.

[0019] Figure 3 This is a schematic diagram of the experimental setup used to test the nonlinear terahertz emission performance of the ITO thin film of this invention.

[0020] Figure 4 The results of the terahertz emission test of the ITO thin film of the present invention under different pump wavelengths and the correlation analysis diagram are shown. Among them, (a), (c) and (e) are the terahertz time-domain waveforms under excitation wavelengths of 1300nm, 1400nm and 1550nm, respectively; (b), (d) and (f) are the comparison diagrams of the changes in near-infrared absorptivity and terahertz signal peak value with annealing time at the corresponding wavelengths, which intuitively show the positive correlation between terahertz emission intensity and linear absorptivity.

[0021] Figure 5 These are X-ray diffraction (XRD) patterns of ITO thin films after different annealing times according to the present invention. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0023] I. Sample Preparation and Experimental Conditions

[0024] Example 1: Preparation of translucent ITO film by short-time rapid thermal annealing (2.5 minutes)

[0025] 1. Thin Film Deposition: A 1mm thick flat quartz glass substrate was selected. A DC magnetron sputtering system was used with a 99.99% pure ITO ceramic target (In₂O₃:SnO₂ = 90:10 by mass ratio) as the target material. The sputtering power was 50W, the argon flow rate was 30sccm, the substrate temperature was 300°C, and the vacuum chamber background pressure was better than 6.6×10⁻⁻⁻⁻⁶. 4 Under Pa conditions, a dark brown, opaque deposited indium tin oxide (ITO) film with a thickness of approximately 120 nm was prepared on a substrate by controlling the deposition time.

[0026] 2. Rapid thermal annealing: The deposited film is placed in a vacuum rapid thermal treatment furnace, the vacuum is evacuated to 1.3×10⁻¹ Pa, and the temperature is rapidly increased to 300°C at a heating rate of 300°C / min. The film is held at this temperature for 2.5 minutes and then allowed to cool naturally.

[0027] 3. Film morphology: The obtained ITO film is semi-transparent.

[0028] Example 2: Preparation of translucent ITO film by short-time rapid thermal annealing (5 minutes)

[0029] Except for adjusting the annealing holding time to 5 minutes, the other steps and parameters are exactly the same as in Example 1. The transmittance of the obtained film is slightly increased compared with Example 1, but it is still in the semi-transparent range.

[0030] Comparative Example 1: Unannealed sample (As-deposited)

[0031] That is, the original ITO film after step 1 deposition without any subsequent annealing treatment.

[0032] Comparative Example 2: Preparation of fully transparent ITO films by conventional long-time annealing (1 hour)

[0033] Except for extending the annealing holding time to 1 hour, the other steps and parameters are the same as in Example 1. This is a typical process in the prior art to obtain a high-transmittance TCO film, and the resulting film has a clear and transparent appearance.

[0034] Comparative Example 3: Preparation of fully transparent ITO films by long-term annealing (2 hours)

[0035] Except for extending the annealing holding time to 2 hours, the other steps and parameters are the same as in Example 1.

[0036] II. Performance Testing and Result Analysis

[0037] To verify the effectiveness of the technical solution of the present invention, the samples prepared in the above embodiments and comparative examples were subjected to microstructure characterization, linear optical testing and nonlinear terahertz emission testing.

[0038] 1. Microstructure evolution (XRD analysis)

[0039] like Figure 5 As shown, the XRD pattern of Comparative Example 1 (unannealed) contains In4Sn3O, which causes the film to appear dark and opaque. 12 Phase diffraction peaks; and after the rapid thermal annealing treatment described in this invention (Examples 1 and 2), In4Sn3O 12 The phase decomposes and Sn atoms gradually dissolve into the In2O3 lattice, which increases the intensity of the (222) crystal plane diffraction peak and narrows the half-peak width. This indicates that short-time annealing effectively improves the crystal quality and puts the film in a transitional stage from a defect-rich phase to a highly crystalline In2O3 phase.

[0040] 2. Linear Optical Properties Analysis

[0041] Figure 1 (a) Visually demonstrates the appearance evolution from the opaque state of Comparative Example 1, through the semi-transparent states of Examples 1 and 2 (2.5 min, 5 min), to the fully transparent states of Comparative Examples 2 and 3 (1 h, 2 h). Figure 2 This further reveals the synergistic regulatory effect of linear optical and electrical properties in this process.

[0042] Specifically, such as Figure 2 As shown in (a)-(c), both Examples 1 and 2 exhibit unique optical properties within a broad spectral range of 1200 nm–2400 nm. Particularly in the terahertz pump band (1200 nm–1600 nm) of interest in this invention, the absorbance of Examples 1 (2.5 min) and 2 (5 min) remains at a high level. Figure 2 c) The absorption rate did not drop significantly below 20% due to over-annealing as in Comparative Examples 2 and 3 (1h and 2h); Meanwhile, Figure 2 The AC impedance spectra and dielectric constant evolution curves from (d) to (f) show that the thin film impedance value (Z) decreases with increasing annealing time, reflecting that the carrier concentration and mobility increase with the improvement of crystallinity. This change in dielectric environment directly reflects the change in the density of free electron gas inside the thin film.

[0043] Based on the combined optical and electrical data, Examples 1 and 2 are at a critical balance point, namely, the impedance was reduced and In4Sn3O was eliminated through appropriate annealing. 12 This avoids the adverse effects of the phase while preventing the blue shift of the plasma edge and near-infrared absorption loss caused by excessive activation of charge carriers in long-term annealed samples.

[0044] 3. Nonlinear Terahertz Emission Performance

[0045] use Figure 3The transmission terahertz time-domain spectroscopy system shown was tested, and the results are as follows: Figure 4 As shown. Observation Figure 4 The time-domain waveforms (a), (c), and (e) show that the terahertz signal amplitudes of Example 1 (2.5 min) and Example 2 (5 min) are significantly higher than those of Comparative Examples 2 and 3 at wavelengths of 1300 nm, 1400 nm, and 1550 nm; combined with Figure 4 The correlation analysis of (b), (d), and (f) shows that as the annealing time increases from 2.5 min to 2 h, the near-infrared absorptivity and terahertz emission intensity both show a highly consistent trend of "maintaining a high value first and then decreasing significantly". In particular, at 1550 nm, the terahertz emission intensity of Example 1 reaches its peak, which is significantly higher than that of the fully transparent comparative example 3.

[0046] III. Conclusion

[0047] The above experimental results confirm that by controlling the rapid thermal annealing time (2.5-10 minutes) to bring the ITO film to a specific semi-transparent state, the present invention can maximize the retention of the film's nonlinear absorption of near-infrared pump light while ensuring a certain level of crystal quality and conductivity (confirmed by XRD and impedance spectroscopy). This optimized near-infrared absorption has been proven to be the decisive factor in achieving high-efficiency nonlinear terahertz emission.

Claims

1. A method of preparing an indium tin oxide film for enhancing terahertz emission, characterized by, The method comprises the following steps: Step S1: depositing an initial-state indium tin oxide (ITO) thin film on a substrate by a magnetron sputtering process; Step S2: performing rapid thermal annealing on the deposited thin film in a vacuum environment; In step S2, the obtained indium tin oxide thin film is in a crystalline transition state by controlling annealing process parameters, and the microstructure thereof contains partially decomposed In4Sn3O 12 phases and In2O3 crystalline phase, and the light absorption rate of the thin film in the near-infrared pump waveband (1200 nm-1600 nm) is between 15% and 30%, so as to improve the nonlinear absorption and terahertz emission intensity thereof.

2. The production method according to claim 1, characterized by, In step S1, the specific parameters of the magnetron sputtering process include: using an ITO target material with a mass ratio of In2O3 and SnO2 of 90:10 (wt%); a sputtering power of 40W-60W; an argon flow rate of 20sccm-40sccm; a substrate temperature maintained at 280°C-320°C; and a deposited thin film thickness controlled at 100nm-150nm.

3. The preparation method according to claim 1, characterized in that, In step S2, the annealing time of the rapid thermal annealing process is controlled between 1 minute and 30 minutes; preferably, in order to obtain the best terahertz emission efficiency, the annealing time is controlled between 2.5 minutes and 10 minutes.

4. The method of claim 1, wherein, In step S2, the specific process parameters of the rapid thermal annealing process include: an annealing temperature of 280°C to 320°C; a temperature rising rate of 200°C / min to 400°C / min; and a vacuum chamber pressure maintained between 1.0×10⁻¹ Pa and 2.0×10⁻¹ Pa.

5. The preparation method according to claim 1, characterized in that, The ITO thin film produces broadband terahertz emission when excited by a femtosecond laser with a center wavelength of 1200nm-1600nm, and the peak-to-peak intensity of its terahertz emission signal is at least 1.5 times that of a fully annealed fully transparent ITO thin film of the same thickness.

6. An indium tin oxide film having high nonlinear terahertz emission efficiency, prepared by the method according to any one of claims 1 to 4, characterized in that, The thin film has a mixed-phase structure between the deposition state and the high-crystalline state, and exhibits a synergistic enhancement effect of near-zero dielectric constant (ENZ) characteristics and high nonlinear absorption in the near-infrared waveband.

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