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The great enhancement of light-induced transverse thermoelectric effect in c-axis inclined BiCuSeO thin films has been achieved by Pb doping. Large open-circuit thermoelectric voltage signals are all observed in the inclined Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) films when the film surface is irradiated by a 308nm pulsed laser. As the Pb doping content increases, the magnitude Vp of the induced voltage signal increases greatly while the response time τ decrease obviously. A large figure of merit Vp/τ of about 236.7 mV/ns is obtained in the 8% Pb-doped film sample under the 308 nm pulsed irradiation with energy density of 0.5mJ/mm2, which is about 25 times larger than that in the undoped film. Possible mechanism is proposed to explain the experiment results. This work might pave the way for the practical application of BiCuSeO-based thin films for high sensitive and fast response ultraviolet pulsed photodectors.
© 2016 Optical Society of America
1. Introduction
Transverse thermoelectric (TE) effect, also called the off-diagonal TE effect, is an unconventional TE effect in which heat and electric energy flow through a material perpendicular to each other [1–15]. The origin of this effect is suggested to be related with the off-diagonal element of the Seebeck tensor, which becomes nonzero in anisotropic materials when an inclined structure is constructed. Under this condition, a lateral TE voltage signal is generated along the surface of the inclined sample when applying a temperature difference ∆T across the thickness of the sample, which can be quantitatively described by the equation of
Where ΔS = Sab-Sc is the difference of the Seebeck coefficient in the ab-plane and along the c-axis of the sample, α is the inclination angle of the sample with respect to the surface normal, l and d are the length and thickness of the sample, respectively [4]. Recently, the light-induced transverse TE effect in c-axis inclined thin film samples, in which the temperature difference ∆T is produced by absorption of the incident light irradiation, has received increasing attention due to the potential applications in fast response and high sensitive photodectors [3–16]. Extensive studies has been performed in many materials such as high temperature superconductors (YBa2Cu3O7-δ, Bi2Sr2CaCu2O8 and La2-xSrxCuO4) [4–6], colossal magnetoresistance manganites (La1-xCaxMnO and La1-xPbxMnO) [7–9], oxide TE materials (La1-xSrxCoO3, SrTi1-xNbxO3, CaxCoO2, Bi2Sr2Co2Oy) and etc [10–16].BiCuSeO is a new promising oxide-based TE material and its conventional longitudinal TE effect has been extensively studied over the past five years [17–23]. This material has a layered tetragonal crystal structure with P4/nmm space group, which consists of the insulating (Bi2O2)2+ layers and the conductive (Cu2Se2)2- layers alternatively stacked along the c-axis. This anisotropic structure suggests the possible presence of transverse TE effect in this material if we can construct an inclined structure. Very recently, we have reported the successful growth of high-quality c-axis inclined BiCuSeO thin films by pulsed laser deposition (PLD) and the observation of the light-induced transverse TE effect in the films [24]. However, both the optical detection sensitivity and the response time obtained in the pure BiCuSeO thin films are not yet sufficient for practical use. In this paper, based on our previous work, we report the great enhancement of the transverse TE effect in c-axis inclined BiCuSeO thin films via Pb doping. We found that both the voltage sensitivity and the response time of the light-induced transverse TE voltage signals were optimized significantly when doping Pb into the films. A very large and fast voltage signal with the magnitude of about 23 V and the response time of about 80 ns was achieved in the 8% Pb-doped thin films, suggesting that Pb doping is a very effective way to improve the performance of light-induced transverse TE effect in BiCuSeO thin films.
2. Experiments
Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) thin films with thickness of about 150 nm were grown on 10° c-axis miscut LaAlO3 single crystal substrates by PLD method, and the detailed film fabrication process can be found in Ref. 23-24. Here, the room temperature thermal conductivity of the LaAlO3 single crystals is about 13.6 W/mK [25], and the thickness of the LaAlO3 single crystal substrates used in this work is about 0.5mm. As the room temperature thermal conductivity of the present BCSO-based compound is very small (< 1W/mK) [20], the LaAlO3 substrate can be regarded as a perfect heat sink [5]. The crystal structure and microstructure of the inclined films were analyzed by x-ray diffraction (XRD, Bruker AXS D8 advanced) and transmission electron microscopy (TEM, Tecnai G2 F20), respectively. Figure 1(a) is the sketch map for measuring the light-induced transverse TE effect in Bi1-xPbxCuSeO thin films. Two indium electrodes separated about 8 mm were deposited on the film surface along the x-axis direction. Good ohmic contact between the films and the electrodes is confirmed by the linear current-voltage (I-V) curves shown in Fig. 1(b). A 308 nm pulsed laser was used as light source to generate the temperature difference ∆T between the film surface and film bottom. The irradiation spot on the film, with the area of 6mm × 5mm, was located on the center position between two electrodes. The induced TE voltage signals were recorded by a digital oscilloscope terminated into 1 MΩ (Agilent DSO9254A).
Fig. 1 (a) Schematic illustration of the transverse TE effect measurements, (b) I-V curve of the two In electrodes on the Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) thin films.
3. Results and discussion
Figure 2(a) shows XRD θ-2θ scans of these Bi1−xPbxCuSeO thin films grown on 10° c-axis miscut LaAlO3 (001) substrates, and the inset is a sketch map of the XRD θ-2θ measurement for films with inclined crystal orientation in an “Offset-Coupled” mode. Here, the offset angle ω is set as the inclined angle α of LaAlO3 substrate, that is 10°, to satisfy the Bragg diffraction condition. Apart from the substrate peaks, all peaks in the figure can be indexed to the (00l) diffractions of BiCuSeO, indicating that phase-pure and c-axis oriented BiCuSeO-based films are obtained and the c axis of the films is tilted about 10° away from its surface normal. Figure 2(b) displays the magnified (003) diffraction peak of these films. With the increase of Pb doping content x, the (003) diffraction peak systematically shifts toward a small angle, suggesting a lattice expansion in the film due to the substitution of Pb2+ (1.29Ǻ) for Bi3+(1.17Ǻ) [19]. Meanwhile, the intensity of the diffraction peak becomes weak with Pb doping, implying a deterioration in the crystalline quality of the doped films. Figure 3 presents the high-resolution TEM (HRTEM) image of the c-axis inclined Bi0.96Pb0.04CuSeO film. Well-ordered layered structure stacked along the c axis can be clearly observed in the image. Moreover, the ab-plane of the film is inclined about 10° away from the substrate surface, which agrees well with the XRD result.
Fig. 2 (a) XRD θ-2θ scan of the Pb-doped BiCuSeO films grown on 10° miscut LaAlO3 (001) single crystal substrates. The inset is a sketch map for the XRD measurement for samples with inclined crystal orientation in a “Offset Coupled” mode. Here, the offset angle ω is set as 10°; (b) demonstrates the magnified curves of the Bi1−xPbxCuSeO (003) diffraction peak.
Fig. 3 HRTEM of the Bi0.96Pb0.04CuSeO thin film grown on 10° miscut LaAlO3 (001) single crystal substrate.
Figure 4 shows the room temperature ab-plane Seebeck coefficient Sab and resistivity ρab of the c-axis oriented Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) films without tilted geometry, which were grown on the LaAlO3 (001) substrates under the same deposition conditions as the tilted film samples. As the Pb doping content increases, both the Seebeck coefficient and the resistivity decrease, which is mainly due to the increase of hole carriers concentration induced by the substitution of Pb2+ to Bi3+. Moreover,the resistivity of all films is smaller than that of corresponding polycrystalline bulks, especially for the undoped sample [19–21]. Two factors are suggested to be responsible for the reduction in resistivity of the films. One is that the carrier concentration of the films is larger than that of the corresponding bulks which is induced by the different growth process between the films and bulks, and another is that the films are highly c-axis orientated. It has been known that the electrical transport in the BiCuSeO system is anisotropic and the resistivity in the ab-plane is lower than that along c-axis direction [22]. The Seebeck coefficient of the films is smaller than those of bulks, which is mainly caused by the reduced carrier concentration of the films.
Fig. 4 The room temperature ab-plane Seebeck coefficient Sab and resistivity ρab of the c-axis oriented Bi1-xPbxCuSeO films without tilted geometry.
The changes in the crystal structure as well as the electrical transport properties of BiCuSeO thin films after doping Pb element should have a significant influence on its light-induced transverse TE effect. Figure 5(a) presents typical transverse TE voltage signals of the c-axis inclined Bi1-xPbxCuSeO films in response to the irradiation of a 308 nm pulsed laser with energy density Ed of 0.5mJ/mm2. Large open-circuit voltage signals can be clearly observed in all these films under the laser irradiation. With the increase of the Pb doping content, the magnitude of the voltage signal Vp increases greatly while the response time τ (the full-width at half maximum of the voltage signal) decreases obviously, which can be seen in Table 1. It is worth mentioning that although the Seebeck anisotropy (ΔS) in BiCuSeO is not obvious [22], large voltage signals still can be obtained because the value of (l/2d)ΔT in equ.1 could be very large when considering l is in mm, d is in nm and ∆T can reach several hundreds Kelvin upon the illumination of an UV pulsed laser [16, 26].
Fig. 5 (a) The typical light-induced transverse TE voltage signals of 10° c-axis inclined Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) thin films under the 308 nm pulsed irradiation; (b)Variation of the rise time τr and the decay time τd of these voltage singles with Pb doping content; (c) Dependence of the magnitude of the voltage signal, Vp, on the incident laser energy density for each film; (d) The voltage response of a commercial light detector to the 308 nm pulsed irradiation with the same irradiation condition as that of Fig. 5(a).
Table 1. Comparison of the measured voltage peak, time response τ, and the calculated figure of merit of Fm for the 10° inclined BiCuSeO thin films upon the UV pulsed irradiation
To evaluate the performance of transverse TE effect of different Pb-doped films, we calculate the figure of merit Fm of these films, here Fm is defined as Fm = Vp/τ [8,9]. A large Fm of 236.7 mV/ns is achieved in the 8% Pb-doped film sample Bi0.92Pb0.08CuSeO, which is about 25 times larger than that obtained in the undoped sample. Moreover, other critical parameters, including voltage sensitivity R (R = Vp/E, here E is the laser energy on the film surface), that can evaluate the performance of transverse TE effect are also listed in Table 2 and compared with the results reported in literatures. It can be clearly seen that the values of R and Fm obtained in the Bi0.92Pb0.08CuSeO film are all larger than those in manganese oxides or SrTiO3 films [8,11,12,15,27–29]. It needs to be mentioned here that when we further increase Pb doping content to 12%, the c-axis texture degree of the film decreases and some impurities appear, which leads to a deterioration in the performance of transverse TE voltage of the film. For example, the values of Vp and τ observed in the Bi0.88Pb0.12CuSeO film sample are about 8.78 V and 117 ns respectively, resulting in a Fm of about 75.0 mV/ns. This result is very similar to that of the conventional TE effect, in which the best TE performance has been obtained in the 6-8% Pb doped samples [19–21].
Table 2. Comparison of the transverse TE properties in some typical films
The enhanced magnitude of the transverse TE voltage signals observed in Pb-doped BiCuSeO thin film are very likely due to its low resistivity. It is known that the optical absorption coefficient γ of a semiconductor increases with the reduction of resistivity ρ [11]. For a film with fixed thickness d, a larger optical absorption coefficient γ will lead to a larger ΔT according to ΔT∝(1-e-γd) [15]. With the increase of the Pb doping content, the resistivity of Bi1-xPbxCuSeO films decreases, and the absorption coefficient of the films to the incident 308nm irradiation becomes large, resulting in an increase in ΔT. Therefore, the magnitude of the TE voltage signals is enhanced in the Pb-doped thin films. The quantitative discussion on the large voltage values of BiCuSeO-based films is very difficult at present since many data of this new TE material is still not available in literatures (for example, the data of ∆S, the thermal conductivity of the films κ, the volumetric specific heat of the films Cp and the thermal boundary resistance of the film-substrate boundary Rbd all are unavailable, the last three of which determine the value of ∆T [5,14]). More clear explanation is definitely needed and will be explored in our coming work. In addition to the larger magnitude, the transverse TE voltage signals obtained in the Pb-doped film samples exhibits faster response time, i.e. faster rise and decay time than that of the undoped sample, as shown in Fig. 5(b). In the light-induced transverse TE effect, the rise time τr of the voltage signals represents how fast the TE field establish and is related to the resistivity of the films [10]. Low resistivity indicates that it is easy to establish the TE field through the movement of carriers in the films. So the voltage signals of the Pb-doped thin films exhibit a faster rise time. As for the decay time of a light-induced transverse TE signal, it is governed by the heat diffusion within the film when the film thickness is larger than the penetration depth of the incident light in the film, and is inverse proportional to the thermal conductivity κ of the film, i.e. τ∝1/κ [30]. Generally, the total thermal conductivity κ can be expressed as a sum of lattice and electronic terms, i.e., κ = κl+κe. We calculate the electronic term κe of different Bi1-xPbxCuSeO films by using the Wiedemann-Franz law κe = LT/ρ, where L = 2.44 × 10 8 W Ω/K2 is the Lorenz number, ρ is the electrical resistivity, and T is the absolute temperature [31]. The κe is calculated to be about 0.06, 0.25 and 0.52 W m/K for the 0, 4% and 8% Pb-doped BiCuSeO films, respectively. Although Pb-doping can suppress the lattice thermal conductivity κl of the BiCuSeO thin films due to the enhancement in the phonons scatterings induced by the lattice point defects, the significant increase in the electronic thermal conductivity κe might lead to an increase in the total thermal conductivity κ when considering the very low intrinsic thermal conductivity of this material [17,18]. Therefore, the fast decay time observed in the Pb-doped BiCuSeO thin films could be attributed to the increased thermal conductivity κ of the film induced by the small resistivity. Detailed experimental measurements on κ are needed in our coming work to confirm the above explanation.
Figure 5(c) displays the variation of Vp with incident laser energy density Ed on the Bi1-xPbxCuSeO films. For each sample, Vp increases almost linearly with Ed, which is beneficial for the practical usage of light detection. For comparison, we also provide the voltage response of a commercial laser detector (IP-550, Physcience Opto-electronics, Beijing) to the 308nm pulsed irradiation under the same irradiation condition as Fig. 5(a). (spot area: 6mm × 5mm, laser energy density:0.5mJ/mm2). A open-circuit voltage signal with the Vp of 0.037 V and τ of 8.8 ms is obtained by using the commercial laser detector. Both the voltage sensitivity and the response of this commercial laser detector are much inferior to that of the present Bi1-xPbxCuSeO detectors based on the transverse TE effect. This result demonstrates the great potential applications of Bi1-xPbxCuSeO in the high sensitive and fast response UV pulsed photodectors.
4. Conclusion
In summary, we investigated the effect of Pb-doping on the light-induced transverse TE effect in c-axis inclined BiCuSeO thin films. Large transverse TE voltage signals can be all clearly detected in these Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) films when the films surface was irradiated by the 308 nm pulsed laser irradiation and the voltage magnitude of these signals all increases linearly with the laser energy on the film surface. It is found that doping Pb can greatly improve the voltage sensitivity of the BiCuSeO films to the 308 nm pulsed irradiation while obviously decrease the response time. A very large figure of merit Fm of 236.7 mV/ns is achieved in the Bi0.92Pb0.08CuSeO film, which is about 25 times larger than that reported in the undoped film and also much larger than that reported for other materials based on the same transverse TE effect. The high sensitivity and fast response transverse TE voltage signals observed in the Pb-doped BiCuSeO thin film are suggest to be mainly related with its low resistivity due to the increased carrier concentration induced by the substitution of Pb2+ to Bi3+. This work demonstrates that Pb-doped BiCuSeO film has great potential applications in high sensitive and fast response UV pulsed photodectors.
Acknowledgments
This project was supported by the National Natural Science Foundation of China (No. 51372064), the Nature Science Foundation for Distinguished Young Scholars of Hebei Province, China (No. 2013201249), the Science and Technology Research Projects of Colleges and Universities in Hebei Province (No. ZD2016036) and the Nature Science Foundation for Distinguished Young Scholars of Hebei University (No. 2015JQ03).
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