Investigation of optical pump on dielectric tunability in PZT/PT thin film by THz spectroscopy

Jie Ji; Chunya Luo; Yunkun Rao; Furi Ling; Jianquan Yao

doi:10.1364/OE.24.015212

1. Introduction

Optical or electrical control of the propagation of terahertz (THz) radiation has great importance in the current technology, and functional devices in the THz range have recently received considerable attention [1–4]. Ferroelectric materials play an important role in the research of functional devices operating in the THz range because of their response time, dielectric loss and tenability [5]. There is an obvious size effect of ferroelectric materials, and the thickness of ferroelectrics has great effect upon their properties. The electric property of ferroelectric films is intimately associated with the crystal structure and microstructures [6]. In past decades, great effort has been made to improve the behavior of ferroelectric thin films, such as using different substrates, periodicity and compositional gradation [7–9]. The structure of ferroelectric superlattice displays its high efficiency when tuning the electrical properties among these approaches [10]. Generally, the enhancement in the dielectric property of ferroelectric superlattice mainly resulted from the strain effects [11–13].

PbTiO₃ (PT) is a displacive ferroelectric with the ABO₃ perovskite-type structure. The photon structure of PT has been extensively studied by different technologies, including neutron scattering, Raman scattering, and far-infrared spectroscopy. Previous results have indicated that PT is a textbook example of a material that undergoes “soft-mode” displacive ferroelectric phase transition [14–17]. The far-infrared dielectric responses of PT and PbZr_xTi_1-xO₃ (PZT) thin films have been investigated by I. Fedorov and J.Petzel, who found that the soft mode cannot account for the total low-frequency permittivity of the materials because of the occurrence of additional relaxation in the several cm⁻¹ range at room temperature [15]. The soft mode and the central mode also exited in the superlattice owing to the particular compositions [8, 18].

Most studies on the dielectric properties of the soft mode behavior of ferroelectric thin films have been conducted with external electrical and temperature fields [5, 19–21]. In this letter, we investigate the dielectric spectra and properties of single-layer PT, single-layer PZT, and multilayer PZT/PT thin films with different external optical field powers at room temperature (approximately 291 K) in the THz range.

2. Experimental details

An radio frequency magnetron sputtering apparatus was employed to prepare PT and PZT thin ﬁlms. The targets were single-crystal PT and PZT, both of which were (100)-oriented and purchased from Beijing Goodwill Metal Technology Co., Ltd. The sputtering gas was a mixture of Ar and O₂ (Ar:O₂ = 9:1 in pressure), and the RF power was 100 mW. (100)-oriented 20 × 20 × 0.5 mm³ Si crystal with a 20 nm thickness of SiO₂ thin film was used as a substrate, and SiO₂ layer was used as a blocking layer to inhibit interactions between Si and the ferroelectric films. The deposition rate of the films was 50–60 nm/h, and the deposited films were annealed by conventional thermal annealing at 650 °C. Three samples were prepared as follows:

(a) A single bare 100 nm-thick PT layer on the SiO₂/Si substrate.
(b) A single bare 100 nm-thick PZT layer on the SiO2/Si substrate.
(c) A multilayer structure with two PZT/PT bilayers on the SiO₂/Si substrate. The thickness of each layer was 25 nm, and the total thickness of the film was 100 nm.

A THz time-domain spectrometer (THz-TDS) system produced by Zomega Terahertz Corporation (USA) was used to measure the transmittance spectra of the films, as shown in Fig. 1. In the THz-TDS system, M1-M12 was reflective mirror and the delay line was composed of two reflective mirror. L1-L4 were polyethylene lenses. A fiber femtosecond laser beam was divided into two beams by a polarized beam splitter PBS. The two beams were named the pump beam and the probe beam, respectively. A HWP was placed to change the ratio between the pump beam and the probe beam. After passing through the attenuator and reflected by mirrors, the pump beam was focused on a GaAs photoconductive antenna for the generation of THz waves by lens L1. Meanwhile, the probe beam was focused into a GaAs crystal for the detection of the THz wave by lens L2. Confocal polyethylene lenses L3 and L4 were used to collimate and focus the emitted THz radiation onto the sample which was placed at the focus of L3 and L4. The transmitted THz wave was collected and focused on the GaAs crystal with two polyethylene lenses. The detectable frequency range was from 0.1 THz to 3 THz (3.3 cm⁻¹ to 100 cm⁻¹). The frequency resolution was 4.5 GHz. In addition, the diameter of the spot size for probe and pump are 8 mm and the spot radius on the samples was 4 mm. An all-solid-state green CW laser (center wavelength, 532 nm) was employed for external optical pumping in this experiment [22]. The green light was obliquely incident upon the film at 45°, and the temperature of the system was maintained at 18 °C (291 K) during measurement. Before each measurement of increasing the optical power, the sample was exposure to an uniform illumination of 450 nm wavelength for 5 minutes so the samples would return to the initial dynamic state. Redistributions of the electrons would form an internal space charge field but it could shield the external electric field. So optical erasure was taken to remove the built-in electric field before each external optical power was increased and the samples would return to the initial dynamic state. Therefore, the shielding has little effect on this study of dielectric property of the ferroelectric films with the increasing external optical power.

Fig. 1 Schematic diagram of the THz-TDS system. A green laser is obliquely incident upon the surface of the film at 45° with regard to the polar axis.

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3. Experimental results and discussion

Figure 2 shows the time-domain signal waveforms of the three samples obtained with different external optical pumping powers at room temperature. The time lag of the time-domain signal waveforms between the air and the substrate is 4.14 ps, as shown in Fig. 2(a). The signal waveforms transmitted through sample (a) shifted by only 0.12 ps compared with those of the bare substrate. In addition, compared with that without light excitation, the shift time of the signal waveforms of each sample demonstrated minimal variations under increasing optical power; this phenomenon could also be observed from Fig. 2. Furthermore, the signal waveforms of all samples had the largest shift time, when the optical power reached to 450 mW.

Fig. 2 (a) Time-domain signal waveforms of the substrate and air at room temperature. (b), (c), (d) are the time-domain signal waveforms of the single-layer PT, single-layer PZT, and multilayer PZT/PT thin films, respectively. The black lines represent the data of each sample without optical pumping, the red lines represent the data at P = 150 mW, the blue lines represent the data at P = 300 mW, and the magenta lines represent the data at P = 450 mW.

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The complex transmission of the film could be obtained from the ratio of the two waveforms via the formula: $t (ω) = E s (ω, P) / E r (ω, P)$ , where the $E s (ω, P)$ is the frequency waveform of the bulk sample and $E r (ω, P)$ is the frequency waveform of the underlying substrate at a certain external optical field power. According to the Fresnel format, the complex transmittance function can be expressed as [19]:

t (ω) = \frac{2 N_{f} (N_{s} + 1) e x p (i ω (N_{f} - 1) d_{f} / c)}{(1 + N_{f}) (N_{f} + N_{s}) - (1 - N_{f}) (N_{f} - N_{s}) e x p (2 i ω N_{f} d_{f} / c)},

where and

N_{f}

,

N_{s}

are the complex refractive indices of the film and the underlying substrate, respectively.

d_{f}

is the thickness of the ferroelectric film, and c is the speed of light. Hence, the complex refractive index and complex permittivity

ε = ε^{'} + i ε^{″} = N_{f}^{2}

could be retrieved from the THz transmittance spectra.

Figure 3 shows the complex permittivity of the three samples at frequencies ranging from 0.2 THz to 1 THz (6.6 cm⁻¹ to 33 cm⁻¹) with different external optical field powers at room temperature. As shown in Fig. 2, the real part of the dielectric constant $ε^{″} (ω)$ of the films increased with increasing optical power from 0 mW to 450 mW. The imaginary part of the dielectric constant $ε^{″} (ω)$ of the PT thin film sample showed a slight change with increasing optical power. Moreover, the $ε^{″} (ω)$ of the PZT/PT multilayer thin film demonstrated a turning point at around 0.48 THz, increasing before the point with increasing optical power and then decreasing after this point. In the Fig. 3, relaxation (the central mode) was clearly observed in the lower frequency domain; this phenomenon may be caused by the domain wall and polar clusters in the strain-induced ferroelectric phase [4, 23, 24]. Thus, a more general formula was applied to analyze the THz dielectric spectra. This formula describes the coexistence of Debye relaxation (the central mode) and the Lorentz mode (the soft mode) [25].

ε (ω) = \frac{f (1 - i ω / γ) + g (ω_{0}^{2} - ω^{2} - i ω Γ) + 2 δ \sqrt{f g}}{(ω_{0}^{2} - ω^{2} - i ω Γ) (1 - i ω / γ) - δ^{2}},

where ε_∞ is the high-frequency permittivity; this parameter is the cumulative result of different types of excitation, such as that of electrons, and its value was fixed to 6.37 during the fittings [15]. Γ, f, and w₀ are the damping coefficient, oscillator strength, and eigenfrequency of the soft mode, respectively. The central model is modeled by Debye relaxation with frequency γ and strength g; δ is a coupling constant. To determine which factor contribute to the alteration of the dielectric spectrum of each sample, we extracted the parameters of the model described by Eq. (2), as shown in Table 1 and Table 2. From the results it could be found that for the central mode, the parameters (g, γ, δ) of each sample were observed to change minimally with different optical powers, because relaxation response was weak in THz spectroscopy. However for the soft mode, damping coefficient, and oscillator strength of the soft mode show more noticeable increases with increasing external optical power than those of the central mode.

Fig. 3 Measured dielectric spectra of thin films with different optical powers at room temperature. Symbols are experimental data and lines indicate fitting with Eq. (2). Here, Re(ε) is the real part of permittivity and Im(ε) is the imaginary part of permittivity. (a) Single-layer PT thin film; (b) single-layer PZT thin film; (c) multilayer PZT/PT thin film.

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Table 1. Estimated characteristics of the central mode under different optical pumping power

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Table 2. Estimated characteristics of the soft model under different optical pumping power

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In order to illustrate the micro-mechanisms behind the observed results shown in Table 2, it is worthwhile to study the variation of characteristics of the soft model under the appearance of external optical fields. Figure 4 displayed the variation of refractive of the three thin films with external optical pump. $Δ n$ is the variation of refractive, E is the electric field intensity and N is electron concentration forming the electric field. It could be observed the alteration of refractive is proportional to the optical power, which is in line with the relation Eq. (3) described [26].

Δ n \propto E \propto N \propto P

Thus it can be deduced the built-in electric field has been increased with the external optical power which hardened the soft mode [25]. In addition, the multilayer PZT/PT thin film has the largest variation which, indicating that it is more sensitive to the external optical field. The photon energy of the 532 nm CW laser for optical pumping is 2.3eV, and the band gaps of PT and PZT is 3.4 eV and 3.9 eV [27, 28], which are both located above the pump photon energy [29,30]. However, both PT and PZT thin film interfaces are expected to respond to sub-bandgap (1.9eV) light illumination and inject carriers into the thin film. Therefore, there were excited free carriers originated from the hop over the sub-band in the films when with external 532 nm optical pump.

Fig. 4 variation of refractive index dependent on the optical power at 0.4 THz for the three thin films samples. Symbols represent measured data, solid lines are the fits by using Eq. (3).

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To clarify the contribution of excited free carriers from the hop over the sub-band, the dielectric spectra of the multilayer PZT/PT thin film with external 1064 nm optical pump (1.06eV) were measured, as Fig. 5 showed. It could be found that the real part and imaginary part of the dielectric constants were increased with the optical pump power. Figure 6 compared the variation of refractive for the multilayer PZT/PT thin film with external 1064nm and 532 nm optical pump. The variation of the refractive of PZT/PT multilayer thin film with 532 nm optical pump is much larger than that with 1064 nm optical pump, which indicated that more electrons were involved in the process of the diffusion and migration and most were the excited free carriers. Except this, the external optical field drives the electrons in Ti ³⁺ ions to hop to the conduction band, migrate toward a preferred direction in dark areas, and become captured by the trap level of Ti⁴⁺ ions. Such electron migration existed both in the ferroelectric films with external 1064nm and 532 nm optical pump.

Fig. 5 the dielectric spectra of the PZT/PT multilayer thin film with external 1064nm optical power. (a) is the real part of permittivity; (b) is the imaginary part of permittivity.

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Fig. 6 Variation of refractive for the PZT/PT with external 532 nm and 1064 nm optical pump at 0.4THz. The red line represents variation of refractive with 1064 nm optical pump and the black line represents with external 532 nm optical pump. Symbols are experimental data and the lines are fitted with Eq. (3).

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In the Eq. (1), f represents the oscillator strength of the soft mode. it also can be described: $f = \frac{N q^{2}}{μ ε_{0}}$ , where N is the number of oscillators per unit volume and q is the effective charge. $μ$ is reduced mass and $ε_{0}$ is the vacuum permittivity. To make crystal lattice oscillatory quanta, the ideal of phonon was introduced. Phonon is a kind of elementary excitation, excited energy unit of lattice wave formed by collective motion of lattice vibration. The interaction between photons, electrons and vibration lattice could be considered as with phonons in the lattice. Phonon is When lattice is in thermal equilibrium and the temperature is T, the average number of phonons ( $E_{0} = ℏ ω_{q = 0}$ ) in the lattice is:

\bar{n} (q = 0) = \frac{1}{e^{ℏ ω_{q = 0} / k T} - 1} .

From Eq. (4), it can be estimated that with the increasing of the temperature, the number of the phonons would be added. So the oscillator of the soft mode was strengthened. In addition,

Γ \propto \sqrt{T}

, more phonons are excited and they increase the probability of interactions between the electrons and the excited phonons, which could increase the damping coefficient.

Figure 7 displays the tunability of the three thin film samples at different optical powers according to the Eq. (5) below:

t u n a b i l i t y = Δ ε^{'} / ε^{'} = \frac{ε^{'} (P) - ε^{'} (0)}{ε^{'} (0)},

where ε’(0) is the real part of permittivity without an optical field and ε’(P) is the real part of permittivity with an optical field. The tunability of the three thin film samples increased with increasing optical power. The highest tunability (maximum, 170%) was measured in the multilayer PZT/PT thin film, which was highly sensitive to the optical field and reacted swiftly to the change in optical field. These characteristics are probably due to the mismatch of lattices and strains between the two dissimilar layers in the sample.

Fig. 7 Tunability of the ferroelectric thin films under different optical field powers at (a) 0.4 THz, (b) 0.5THz, (c) 0.6THz and (d) 0.8THz. The black, blue, and red symbols represent the experimental data of the single-layer PT, single-layer PZT, and multilayer PZT/PT thin films, respectively.

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4. Conclusion

We characterized the dielectric properties of single-layer PT, single-layer PZT, and multilayer PZT/PT thin films grown on a SiO₂/Si substrate under an external optical field. As light was transmitted through the thin film, some of the external energy was transferred as thermal energy of the lattice, thus contributing to increases in the damping coefficient and oscillator strength of the soft mode. Electron migration caused by the external optical field also formed a built-in electric field, resulting in hardening of the soft mode. However, compared with the soft mode, the external optical field exerts negligible effects on the central mode at room temperature. Owing to its enhancement of dielectric tunability compared to the single layer thin film, the multilayer PZT/PT film was found to be a potential candidate for applications in the THz range, such as THz modulators. The results of the present experiment provide a reference for further development of tunable structures controlled by an external optical field.

Acknowledgments

This work was supported by the National Basic Research Program (973 Program) (Grant No.2015CB755403), the CAEP THz Science and Technology Foundation (No. AEPTHZ201402) and the CAEP THz Science and Technology Foundation (No. AEPTHZ201407).

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	g	γ (cm⁻¹)	δ (cm⁻¹)
PT	4800	3.47~3.98	68
PZT	5000	2.29~2.68	70
PZT/PT	4950	1.85~2.52	65

sample	PT			PZT			PZT/PT
parameters	f (10⁶ cm⁻²)	W₀ (cm⁻¹)	Γ (cm⁻¹)	f (10⁶ cm⁻²)	W₀ (cm⁻¹)	Γ (cm⁻¹)	f (10⁶ cm⁻²)	W₀ (cm⁻¹)	Γ (cm⁻¹)
0 mW	5.60	77.31	24.36	1.02	65.41	7.99	5.3	74.41	29.54
150 mW	5.82	85.41	30.53	1.12	73.48	20.98	11.9	86.47	67.61
200 mW	7.74	85.76	47.33	2.08	74.68	56.51	13.6	92.93	92.17
300 mW	8.17	87.69	70.48	2.38	76.81	127.03	15.6	95.83	95.15
400 mW	9.1	88.02	74.81	2.4	78.81	133.81	16.5	96.75	99.26
450 mW	9.6	88.24	75.25	3.69	81.49	134.72	17	97.22	102.61

	g	γ (cm⁻¹)	δ (cm⁻¹)
PT	4800	3.47~3.98	68
PZT	5000	2.29~2.68	70
PZT/PT	4950	1.85~2.52	65

sample	PT			PZT			PZT/PT
parameters	f (10⁶ cm⁻²)	W₀ (cm⁻¹)	Γ (cm⁻¹)	f (10⁶ cm⁻²)	W₀ (cm⁻¹)	Γ (cm⁻¹)	f (10⁶ cm⁻²)	W₀ (cm⁻¹)	Γ (cm⁻¹)
0 mW	5.60	77.31	24.36	1.02	65.41	7.99	5.3	74.41	29.54
150 mW	5.82	85.41	30.53	1.12	73.48	20.98	11.9	86.47	67.61
200 mW	7.74	85.76	47.33	2.08	74.68	56.51	13.6	92.93	92.17
300 mW	8.17	87.69	70.48	2.38	76.81	127.03	15.6	95.83	95.15
400 mW	9.1	88.02	74.81	2.4	78.81	133.81	16.5	96.75	99.26
450 mW	9.6	88.24	75.25	3.69	81.49	134.72	17	97.22	102.61

Investigation of optical pump on dielectric tunability in PZT/PT thin film by THz spectroscopy

Abstract

1. Introduction

2. Experimental details

3. Experimental results and discussion

4. Conclusion

Acknowledgments

References

Cited By

Figures (7)

Tables (2)

Equations (5)

Optics Express