SrTiO3–Nb heterojunction under ultraviolet light

Solid State Communications 187 (2014) 10–12

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Time response of photovoltage in La0.9Li0.1MnO3/SrTiO3–Nb heterojunction under ultraviolet light J.Y. Wang n, W. Zhai, B.C. Luo, K.X. Jin, C.L. Chen Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, Northwestern Polytechnical University, Xi’an 710072, China

ar t ic l e i nf o

a b s t r a c t

Article history: Received 13 November 2013 Received in revised form 29 January 2014 Accepted 10 February 2014 by Xianhui Chen Available online 15 February 2014

We focus on the time response of photovoltage property in heterojunction composed of univalent doped manganite La0.9Li0.1MnO3(LIMO) and SrTiO3–Nb(STON). Under the irradiation of 248 nm laser pulse, the photovoltage shows a second order exponential rising process with time constants τ1( ¼27.7–93.0 μs) and τ2 (o 9.84 μs). After the light source is shut down, the photovoltage exhibits a first order exponential decreasing stage. The carrier lifetime τ at the order of millisecond in this p–n junction is much longer than that in similar p–n junctions based on manganites. The maximum values of both τ1 and τ appear at around 250 K, which coincides with the metallic-insulator transition temperature for LIMO. These imply that the time response property of heterojunction is partly controlled by the inherent transportation mechanisms in the LIMO layer. & 2014 Elsevier Ltd. All rights reserved.

Keywords: A. Manganite C. Heterojunction D. Photovoltage

1. Introduction The photoelectric properties of p-manganite/n-titanate junction such as La0.8Sr0.2MnO3/Nb–SrTiO3, La0.67Ca0.33MnO3/Nb– SrTiO3, La0.7Ce0.3MnO3  δ/Nb–SrTiO3 [1–4] attracted much scientific interest in recent years. For these materials, the photovoltage effect versus temperature, light wavelength, light power has been studied [4–6]. The published literatures mainly reported on the photoelectric conversion efficiency whereas the photoelectric response speed [7,8] has always been neglected. In fact, time response of the photovoltage is an important demonstration of the generating, transportation and annihilating processes for the nonequilibrium carriers. From this point of view, the properties on time response of photovoltage in p-manganite/n-titanate junction need to be systematically studied. On the other hand, most investigators have chosen the bivalent and tetravalent ions (Ca2 þ , Ba2 þ ,Sr2 þ and Ce4 þ ) doped manganite to form the heterojunction with titanate, but the photoelectric properties of the p–n junction based on univalent ion (Li þ , Na þ and K þ ) doped-manganite have not be known. In fact, the simple layer film of the univalent ion doping manganate exhibits some unique properties relative to other valence state, such as temperature dependent electroresistance effect and magnetic inhomogeneity [9,10]. Therefore, the photovoltage response behaviors in a wide temperature range in the univalent doped p-manganite/n-titanate are worth studying. In present work, we fabricate the univalent Li-doped LaMnO3/ n

Corresponding author. Tel.: þ 86 2988431670. E-mail address: [email protected] (J.Y. Wang).

http://dx.doi.org/10.1016/j.ssc.2014.02.008 0038-1098 & 2014 Elsevier Ltd. All rights reserved.

SrTiO3–0.5%Nb heterojunction and focus on the time response of the photovoltage under pulse laser. The temperature related mechanisms are discussed in detail.

2. Experimental The La0.9Li0.1MnO3 powder is synthesized from analytically pure LiNO3, Mn(NO3)2 La2O3 using a sol–gel method according to the molecular formula. The additives are citric acid and polyethylene glycol. The powder is sintered at 1200 1C for 24 h in air to create a bulk target. The LIMO film was deposited on single crystal Nb-0.5 wt%-doped SrTiO3 (100) substrate by the pulse laser deposition (PLD) method. The wavelength of laser is 248 nm and the pulse energy is 120 mJ. A substrate temperature of 1023 K and an oxygen pressure of 2 Pa were maintained throughout the deposition. The as-deposited LIMO/STON heterostructure was annealed at 1023 K in an oxygen atmosphere for 30 min. The thickness of the film was evaluated to be 110 nm by a SpecEI2000-VIS ellipsometer. The indium (In) electrodes were placed on the surface of LIMO and STON layers. The in-plane resistance of LIMO and the voltage current characteristic of the p–n junction were measured in Janis CCS-300 closed-cycle refrigerator cryostat system (temperature range from 20 to 300 K) by a KEITHLEY electrical measurement system. The time response of the ultraviolet photovoltage was measured in Janis VPF-475 liquid nitrogen thermostat by Tektronix 500 MHz oscilloscope. The light resource was pulse laser with a wavelength of 248 nm and a pulse width for 20 ns.

J.Y. Wang et al. / Solid State Communications 187 (2014) 10–12

3. Results and discussions Fig. 1(a) illustrates the current–voltage (I–U) characteristics of the LIMO/STON heterojunction in the temperature range from 40 to 300 K. The positive bias voltage is defined by the current flowing from LIMO to STON. It is clear that positive bias voltage exhibits a tendency to saturation whereas the backward voltage results in low leakage current. This p–n structure shows a good rectifying behavior. The variation of Ud versus temperature is displayed as the inset in Fig. 1(a). It is apparent that Ud–T curve becomes steep at first as the temperature rises and then exhibits a flat tendency around 300 K. The inflection point in the Ud–T curve (maximum|dUd/dT|) is around 250 K. These results demonstrate that the transport property of this junction is significantly affected by temperature. Fig. 1(b) presents the resistance variation of the upper layer versus temperature, which shows that the metallic-insulator transition (MIT) of the LIMO film occurs at TP  250 K. This coincides with the inflection point of Ud–T curve, indicating that the transport property of this junction should be mainly related to the changes of electrical conducting mechanism and magnetism of LIMO layer. In the metallic temperature region lower than 250 K, double-exchange effect describes the electrical conducting mechanism. In the higher temperature region, the mechanism should be small-polaritons transportation. For the doped manganite LIMO, the spin-up and spin-down eg bands act as the valence and conduction bands. The rise in temperature leads to the decrease of ferromagnetic order of the manganites. This increases the possibility of valence electronics reaching to the conduction bands [7]. As a result, the diffusion voltage decreases with the rise of temperature. Around Tp, there exists the transition from ferromagnetic state to paramagnetic state, which leads to the great change of Ud. Fig. 2 shows the induced photovoltaic pulse as a function of time at different temperatures. The light source is a pulse ultraviolet laser with an energy density of 0.1 mJ/mm2, and the resistance of external circuit RE is fixed at 1 MΩ. As seen in Fig. 2 (a), the maximum photovoltage UP decreases with the rise of temperature. This is because that a larger voltage is required to counterbalance the photocurrent for the reduction of the concentration of thermal charge carriers at low temperature. In addition, UP exhibits same changing trend versus time at various temperatures. It has a sharp rise at the very beginning, and then gradually decreases to zero after several milliseconds. Mathematical analysis suggests that the falling edge is fitted well by a first order exponential function U p ¼ Aexpð  t=τÞ

ð1Þ

Here, A is a constant, t is time, and τ is time constant. In general, two mechanisms are responsible for the decay of Up. On one hand,

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once the light source is shut down, new holes and electrons cannot be produced, and the annihilating between holes and electrons inside the p–n junction leads to the sharp decrease of Up. On the other hand, the collective carriers at the two sides of the junction may neutralize through the external circuit. Obviously, for the two mechanisms, the former one is worth concerning. However, a large enough resistance of external circuit should be determined so that the external discharge can be ignored. The inset of Fig. 2(b) shows the values of τ during the decaying processes at different external impedances RE. As RE increases, τ rises swiftly and then becomes stable gradually when RE 4 200 kΩ. Therefore, RE ¼1 MΩ is suitable to make the p–n junction in an approximate open-circuit state. Thus, we claim that the falling edge in Fig. 2(a) mainly contains the information on the annihilation process of the nonequilibrium carriers, and τ corresponds to the lifetime of nonequilibrium carriers. Quantity statistics of τ at different temperatures is displayed in Fig. 2(b). The lifetime τ extends from 1.80 ms to the peak value of 7.10 ms as temperature rises from 80 to 250 K, and then decreases monotonically to 4.57 ms at 300 K. Compared with the p–n junctions based on other doped manganites, the lifetime τ at the order of millisecond in this p–n junction is much longer [11–13]. The rising stage of Up is shown in Fig. 3(a). Unlike the falling edge, the relationship between Up and t can no longer be described by a first order but a second order exponential function U p ¼ U 1 ½1 expð  t=τ1 Þ þ U 2 ½1  expð t=τ2 Þ;

ð2Þ

where U1 and U2 are constants, τ1 and τ2 are time constant. This second order exponential function reveals two different mechanisms during the rising stage of Up. In order to make a distinction between the two different mechanisms at all temperatures, the bigger weight coefficient is called U1, and the smaller one is called U2. The variations of τ1 and τ2 with the changes of temperatures are shown in Fig. 3(b). τ1 locates in a wide region from 27.7 to 93.0 μs. By contrast, τ2 is relatively stable and ranges between 4.08 and 9.84 μs. Apparently, τ1 is about one order of magnitude higher than τ2. Hence, it can be deduced that the τ2 item corresponds to a rapid growth process, while the τ1 item correlates to a process of relatively slow saturation. Considering that the speed of production of photo-induced carriers is very fast and the carriers are generated simultaneously when the light is irradiated on the sample; τ1 and τ2 relate to the migration towards the junction and being swept to the two sides of the p–n junction. Since U1 is larger than U2, the rising speed of the photovoltage reveals mainly on the time constant τ1 of the slower process. As the temperature rises, τ1 first increases and then decreases, whose maximum value occurs at 250 K, which is the metallic-insulator transition temperature of LIMO. The charge migration process in LIMO is influenced by the inherent transportation

Fig. 1. (Color online) (a) Current–voltage characteristics at different temperatures. The inset is the diffusion voltage versus temperature; (b) In-plane resistance of the LIMO layer.

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J.Y. Wang et al. / Solid State Communications 187 (2014) 10–12

Fig. 2. (Color online) (a) Time response of the photovoltage in LIMO/STON heterojunction under 248 nm pulse ultraviolet laser (RE ¼ 1 MΩ). The inset is the Lifetime τ in different RE at 300 K; (b) Lifetime τ at different temperatures.

Fig. 3. (Color online) (a) The rising processes of Up. (RE ¼1 MΩ) at different temperatures; (b) Time constants τ1 and τ2 as functions of temperature.

mechanisms induced by temperature variation. At metallic region (ToTp), the migration of carriers is controlled by the Mn3 þ –O2 – Mn4 þ double exchange which is spin correlated. The rising of the temperature weakens the ferromagnetism and aggravates the lack of parallelism of the local spin. Thus, the carrier mobility needs much time. Beyond Tp, small-polaritons transportation is the dominant mechanism. The rising of the temperature promotes the delocalization of the carriers and increases the transition probability. Consequently, τ1 decreases. 4. Conclusion In summary, we fabricate the univalent Li-doped LaMnO3/ SrTiO3–0.5%Nb heterojunction and report on its time response of photovoltage property. Under the irradiation of 248 nm laser pulse, the approximate open-circuit photovoltage shows a second order exponential rising and a first order exponential decreasing process. Compared with similar p–n junctions based on manganites, the lifetime τ at the order of millisecond in this p–n junction is much longer. The second order exponential type rising edge with time constants τ1(27.7–93.0 μs) and τ2 (o9.84 μs) indicates that there exists two different processes during the charge production and transportation. τ and τ1 are remarkably affected by temperature and their maximum values appear at around 250 K, which is the temperature of metallic-insulator transition in LIMO. These indicate that the time response property of

heterojunction is partly controlled by the inherent transportation mechanisms in the LIMO layer.

Acknowledgments We acknowledge the support by the National Natural Science Foundation of China (Nos. 51201136 and 51172183), the National Natural Science Foundation of Shaanxi Province in China (No. 2012JQ8013), Aviation Foundation of China (No. 2011ZF53065), and the NPU Foundation for Fundamental Research (No. JC201155). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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