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Abstract
β-FeSi2 is of interest for Si-based optoelectronic applications in the past decades. We fabricated β-FeSi2 thin films on the SrTiO3 single crystal by KrF-pulsed laser deposition to open a new view of integrating β-FeSi2 with non-silicon functional materials. After investigating the lateral photovoltaic effect of β-FeSi2/SrTiO3 under the illumination of the 808 nm and 1064 nm steady lasers, we found that the position detection sensitivity can reach 2.68 mVmW−1mm−1 and 2.24 mVmW−1mm−1, respectively. The low degree of nonlinearities of position-sensitive and power-sensitive characteristics provide a promising application of SrTiO3-based β-FeSi2 thin films on position-sensitive detection devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the middle of the 1980s, the semiconducting β-FeSi2 initially aroused researchers’ attention for high temperature thermoelectric applications [1]. The multiple physical properties of β-FeSi2 have been of interests since the film fabrication techniques were improved recently. Birkholz et al. used the solid phase epitaxy technology to grow β-FeSi2 films on Si substrate and indicated that the as-grown β-FeSi2 was an n-type semiconductor with the direct-band gap of 0.89 eV [2], which was confirmed theoretically and experimentally by Geserich et al. [3–5]. Izumi and Shaban et al. investigated the near-infrared photoelectric performance of β-FeSi2/Si heterojunction under low-temperature and found its rectification characteristic could be modified by 1.31 μm-infrared light radiation and be improved as the measurement temperature decreased [6]. The rectification characteristic of β-FeSi2/Si heterojunction presented an unsaturated current leakage due to the diffusion of Fe atoms toward the interface of β-FeSi2/Si [7,8]. These studies provide the promising applications of β-FeSi2 films in photonic and microelectronic devices. However, the rapid diffusion of Fe/Si atoms to Si-based substrates during the silicide formation can form deep energy levels (~0.1 eV) above the valence band of β-FeSi2, acting as traps for minority carriers and considerably reducing the carrier lifetime [9]. These Fe/Si atoms-induced defects can result in a high resistivity of β-FeSi2 films and low photoelectric conversion efficiency. The photovoltaic effect of β-FeSi2/Si heterostructures was degraded due to the vague interfaces between β-FeSi2 and Si substrate [10–16], which stimulates the need for non-silicon substrate integrating with β-FeSi2. However, rarely research has been reported on the synthesis and photoelectric characteristics of β-FeSi2 thin films on non-silicon substrates.
The lattice parameters a, b, and c of orthorhombic β-FeSi2 (9.683 Å, 7.884 Å, and 7.791 Å) [17] could be in good match with the doubling c-lattice parameter of cubic SrTiO3 (3.942 Å) [18]. The wide energy bandgap of SrTiO3 (3.75 eV) eliminates its impact on the near-infrared (NIR) optoelectronic response [19]. Hence, we can identify the particular photovoltaic effect of β-FeSi2 thin films based on the SrTiO3 substrate at near-infrared wavelength. More importantly, SrTiO3 substrate, as a typical ABO3 perovskite oxide, has been attractive in integrated microelectronic and microwave devices due to its high dielectric constant (high breakdown strength) and low dielectric loss (low leakage current density) [20].
Most importantly, the current studies on the photoelectric properties of β-FeSi2 films and β-FeSi2/Si heterostructures are mostly focused on their photovoltaic effect for solar cell applications. The area of the lateral photovoltaic effect of β-FeSi2 is empty. The main motivation of this work is to study the lateral photovoltaic effect of β-FeSi2 films on non-silicon substrates to fill this emptiness. It is expected to promote the potentiality of β-FeSi2 for integration with multi-functional materials, and to exploit its promising applications besides solar cell devices. In our study, β-FeSi2 thin films were prepared on the SrTiO3 single crystal by pulsed laser deposition and short-term thermal annealing. The potential application of β-FeSi2 films in position detection was discussed based on its lateral photovoltaic effect.
2. Methods
The SrTiO3 (001) single crystal was used as substrate and a FeSi2 target with the purity of 4N was selected as the source of deposition in this study. Before the β-FeSi2 films deposition, the SrTiO3 substrate was treated by 75% acetone and 98% ethanol, respectively, for five minutes in ultrasonic bath. After cleaning and airing, the substrate was attached to a substrate layer with silver paste, and then mounted in the sample holder of the deposition chamber. Subsequently, the FeSi2 target was loaded into the target chuck, which was adjusted to 55 mm opposite the sample holder. The base pressure of the vacuum system prior to films deposition was better than 4 × 10−6 mbar. The output energy and pulse frequency of the KrF pulsed laser deposition system was kept being 400 mJ and 4 Hz. β-FeSi2 thin films (thickness: 200 nm) were then grown on SrTiO3 at a substrate temperature of 800 °C and successively annealed in situ for 30 minutes. The growth process of β-FeSi2 thin films was monitored by an in situ reflection high-energy electron diffraction (RHEED) system and charge coupled device (CCD) camera.
After that, one prepared β-FeSi2/SrTiO3 (001) sample with 5 × 6 mm2 of size was measured. The crystal orientations of the sample were confirmed by X-ray diffraction (XRD). The electrical properties of the sample were characterized by current-voltage characteristic and sheet resistance-temperature curves, which were measured by the four-dot method in the dark and under illumination of a1064 nm and an 808 nm stable laser. The laser power intensity was kept being 45 mW/mm2.
The near-infrared lateral photovoltaic effect of β-FeSi2 thin films was investigated by laterally scanning the sample with an incident light spot of 0.2 mm diameter. Two silver electrodes were fixed on the surface of β-FeSi2 films with a distance of 4 mm. The middle dot between these two electrodes was chosen as the incident zero point and the sign of the point coordinates corresponded with the polarity of electrodes, which means the position coordinate near the positive electrode should be positive (e.g. the position of “+1.6 mm” is the position close to the positive electrode) and the position coordinates toward the negative electrode should be negative. The measurement interval was 0.2 mm and there were in total 20 measurement points. The schematic of the lateral photovoltaic effect measurement is presented as Fig. 1. The corresponding output of the photovoltaic signal was recorded by the digital oscilloscope (Tektronix DPO 4032).
Fig. 1 Schematic of lateral photovoltaic effect measurement setup. The surface of β-FeSi2 thin films was equally zoning for laser illumination and marked with x-axis position.
3. Results and discussion
The time-resolved RHEED recorded the β-FeSi2 thin films formation process as shown in Figs. 2(a)-2(c). The SrTiO3 substrate displayed the clean single crystal diffraction spots before films deposition (Fig. 2(a)). An evolution of RHEED patterns at the β-FeSi2 films deposition is shown along with increasing pulse numbers (Figs. 2(b) and 2(c)). The typical Debye rings indicate that the deposited β-FeSi2 films were polycrystalline [21].
Fig. 2 RHEED images of films deposited with (a) 0 pulse, (b) 300 pulses and (c) 2000 pulses; XRD patterns of β-FeSi2 films deposited on SrTiO3 (001) substrate (d); SEM images of (e) SrTiO3 substrate and (f, g) β-FeSi2 films on SrTiO3 substrate.
As shown in Fig. 2(d), the XRD patterns of β-FeSi2/SrTiO3 suggested the single β phase of the FeSi2 films. There was a diffraction peak near the 18.4 °, indicating the β-FeSi2 (111) crystal direction. And a diffraction peak near the 46.5 ° implied the (004) and (040)-orientations of β-FeSi2. It was found that off-diagonal thermoelectricity, due to domain-structural anisotropy, can generate the laser-induced photovoltage in La2/3Ca1/3MnO3 films on miscut-SrTiO3 and miscut- LaSrAlO4 (001) substrates [22,23]. The anomalous photovoltaic effect was detected in La0.8Sr0.2MnO3 films grown on untilted SrTiO3 (100) substrate due to the (101)-oriented microdomains of La0.8Sr0.2MnO3 [24]. These facts indicate that a laser-induced photovoltaic signal may be detected in the anisotropic β-FeSi2 films on SrTiO3 (100) substrate due to the Seebeck effect [24–26]. The surface morphology of SrTiO3 substrate and β-FeSi2 films were studied by SEM as shown in Figs. 2(e)-2(g). The β-FeSi2 films were uniformly deposited on SrTiO3 (100) substrate. The average grain size of the β-FeSi2 layer was about 100 nm-diameter.
Figure 3(a) shows the current-voltage (I-V) characterization of β-FeSi2 films grown on SrTiO3 (001) substrate at room temperature. The insert graph shows the I-V curves in logarithmic scale. The difference between I-V curves under dark and 808 nm (1064 nm)-infrared light illumination indicated that the conductivity of β-FeSi2 films can be modified by the external radiation. Upon infrared light illumination, electron-hole pairs were generated, separated by applied bias and collected by the electrodes, leading to a decrease of sheet resistance of β-FeSi2 films. However, the photocurrent at 0 V was as small as 50 nA which could be due to the surface defects, trapping mobile carries.
Fig. 3 Current-voltage (a) and resistance-temperature (b, c) characteristics of β-FeSi2 films under dark (black square) and illumination of 1064 nm (blue sphere) and 808 nm (red circle) stable laser.
The temperature-dependent resistance can be used to obtain the defects information in β-FeSi2 films. Since the not-intentionally doped β-FeSi2 exhibits n-type conductivity [27], the Fe vacancies, acting as donors for β-FeSi2 films could be the origin of the defect-level [28]. Figures 3(b) and 3(c) display the sheet resistance-temperature (R-T) characteristics of β-FeSi2 films from 30 K to 300 K. Based on the following relations, we can extract the defect amounts and energy levels in the β-FeSi2 films by Arrhenius’ plot [29]. It is noted that these equations are valid for semiconductors in equilibrium, thus they cannot be used in light-induced non-equilibrium cases.
where ρ is resistivity, σ is conductivity, n is carrier concentration, NV and Nd are the effective density of states in the valence band and defect level, respectively. Ea is the activation energy, μ is the carrier mobility, k is Boltzmann constant, T is temperature, q is unit charge, Rsheet and d are the sheet resistance and thickness of semiconductor films, respectively. The activation energies of the donor (defect) levels were equal to 33 meV, 94 meV and 197 meV, forming the impurity band near the conduction band. These defect levels, resulting from crystalline defects, can be recovered by annealing process [30].The temperature dependence of resistivity in β-FeSi2 films under dark shows the similar regularity with previous report [27]. According to the R-T curves, the resistivity of β-FeSi2 films at room temperature were 0.17 Ω∙cm, 0.07 Ω∙cm, and 0.05 Ω∙cm under dark, 1064 nm and 808 nm-infrared light illumination, respectively. The reduction in resistivity upon infrared light-radiation can be due to the increase of carrier concentration because of photon- and thermally-excitation. This observation stimulated the further studies of photovoltaic effect of β-FeSi2 films.
Figure 4. shows the lateral photovoltaic signals of β-FeSi2 films under the illumination of 1064 nm and 808 nm-infrared light at different positions. The photovoltaic signals represented the obvious symmetry of location and the open circuit voltages reversed through the zero point as shown in Figs. 4(a2) and 4(b2). In order to investigate the capability of β-FeSi2 films to detect weak light signals, its position-sensitive characteristics at different light power intensities are measured. The light was focused at “-0.8 mm”, with different irradiation power intensities from 7.6 mW/mm2 to 131.0 mW/mm2 for 808-nm steady laser and from 7.8 mW/mm2 to 97.9 mW/mm2 for 1064-nm steady laser (Figs. 4(a1) and 4(b1)). A detectable photovoltage can be obtained when the incident infrared light was as weak as ~8 mW/mm2. Considering the small light spot area (0.2 mm-diameter), the β-FeSi2 films-based position-sensitive detectors (PSDs) can be used for weak signal detection down to ~0.25 mW. (Fig. 5(c)),
Fig. 4 Lateral photovoltaic signals of β-FeSi2 films under illumination of 808 nm (a1, a2) and 1064 nm (b1, b2) stable laser. (a1) and (b1) are the power dependence of photovoltage at incident position “-0.8mm”; (a2) and (b2) are the position dependence of photovoltage at light power intensity of 45 mW/mm2.
Fig. 5 Position-sensitive characteristics of β-FeSi2 films under illumination of 808 nm and 1064 nm-stable laser (a). The photovoltage as a function of incident light power at the position of 0.8 mm away from the electrode (b). The infrared-photovoltaic response time of β-FeSi2 films (c).
In this work, the used figure of merit for β-FeSi2 films-based PSDs are:
- (i) Nonlinearity (δ) is to characterizes the position-detection error and usually expressed as [31]:
- where is the measured position, is the linear-fit position, N is the number of measured position, and L is the distance between the two electrodes. The acceptable PSDs have nonlinearities of less than 15% [32]. As shown in the position-sensitive characteristics (Fig. 5(a)), the low nonlinearities of 1.59% and 0.821% were obtained under the 808 nm and 1064 nm-laser illumination, respectively. In addition, the photovoltage had a good linear relationship with incident light power intensity (Fig. 5(b)), implied by the nonlinearities of 4.30% and 8.57%. These studies support the promising applications of β-FeSi2 films on position-sensitive devices.
- (ii) Correlation coefficient (r) is another parameter to characterize the device linearity, which measures the change in distance (x) corresponding to a unit change in voltage (y) [33]. For an ideal position-sensitive device, its linear fit of position-voltage characteristics can keep the value of r as 1. As shown in Figs. 5(a) and 5(b), both the position-sensitive and power-sensitive characteristics of the β-FeSi2 films had a correlation coefficient approximate to 1.
- (iii) The responsivity (Rs) illustrates the sensitivity of PSDs, which relates the produced photovoltage (photovurrent) to the input power of light [34]. The Rs of β-FeSi2 films is more than 2 mVmW−1mm−1 under the near-infrared light (808 nm and 1064 nm) illumination as shown in Fig. 5(a). This value is acceptable for PSDs application considering the real input power is as small as ~1.4 mW (power intensity: 45mW/mm2, light spot size: 0.2 mm-diameter).
- (iv) The 10-90% rise time (τrise) and 10-90% fall time (τfall) indicate the response speed of output signal to input stimuli. The τrise and τfall are ~800 ms and ~830 ms, respectively, for both 808 nm and 1064 nm-steady laser illumination where the photovoltaic signal curves are fitted using a single exponential function. The relatively large response time may be due to the poor crystalline quality of β-FeSi2 films with extensive defects as trapping centers, which can be improved by selection of well-matched substrates and annealing process.
4. Conclusion
The overall results indicate that we can obtain the single phase β-FeSi2 polycrystalline films on the SrTiO3 (001) single crystal substrate and the position detection sensitivity of our sample can achieve 2.68 mVmW−1mm−1 and 2.24 mVmW−1mm−1 under illumination of 808 nm and 1064 nm-steady leaser, respectively. The low degree of nonlinearities of position-sensitive characteristics can be less than 1%. In addition, our study shows clearly that the open-circuit photovoltaic signals have a good linear relationship with the external laser power. These facts imply that the β-FeSi2 films, grown on SrTiO3 (001) single crystal substrate, have a potential application in the near-infrared position detection device. The further investigation can be proposed to study β-FeSi2-based heterostructures grown on non-silicon substrates. The response spectrum, accurate sensitivity and reliability of β-FeSi2-based position detector can be measured to optimize the device performances [35,36].
Funding
National Natural Science Foundation of China (11504432, 61890964, 61805285); Fundamental Research Funds for the Central Universities (18CX02046A, 19CX05003A-10).
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