Ultrafast photoelectric effects and high-sensitive photovoltages in perovskite oxides and heterojunctions

Abstract

Perovskite oxides and heterojunctions have attracted much attention due to their multifunctional properties of electricity and optics and magnetic as well as the very good chemical and thermal stability. In this brief review, we describe the novel progress of researches in the optical characteristic, including ultrafast photoelectric effects of picosecond order in perovskite oxide single crystals, thin-films and heterojunctions, high-sensitive photovoltages, the enhanced transient lateral photovoltages in perovskite oxide thin-films and heterojunctions, and the high-sensitive ultraviolet (UV) photodetectors based on perovskite oxides. The recent advances present in this paper not only could stimulate theoretical studies on the mechanism but also would open up the possibilities in the developments of optoelectronic devices based on perovskite oxides and heterojunctions.

Graphical abstract

Keywords

perovskite oxides / photoelectric effect / ultraviolet (UV) photodetector

1 Introduction

Perovskite oxides, as one of the leading research topics, have been intensively studied due to their intrinsic pro-perties such as insulating [1], ferroelectric [2, 3], superconducting [4, 5], colossal magnetoresistance (MR) [6, 7], optical properties [8–10] and so on. The fabrication of artificial crystalline materials through layer-by-layer epitaxial growth with full control over the composition and structure at the atomic-scale level has become one of the most exciting areas of research in condensed-matter physics and materials science [11–14]. Heterojunctions and multilayer structures based on the perovskite oxides have emerged as a leading research topic because many novel phenomena and unusual properties have been observed, for instance, unusual positive MR in the heterojunctions of a non-magnetic SrNb_0.01Ti_0.99O₃ (SNTO1) and a negative MR La_0.9Sr_0.1MnO₃ (LSMO1) [15], electrical modulation of MR in multi-p–n heterostructures of SrTiO_3-δ/LSMO1/SrTiO_3-δ/LSMO1/Si [16], enhancement of second-harmonic generation in BaTiO₃/ SrTiO₃ superlattice [17], polarization enhancement in BaTiO₃/SrTiO₃/CaTiO₃ superlattice [14], and a high-mobility electron gas or even superconductivity in LaAlO₃/SrTiO₃ heterointerfaces [18, 19]. Optical characteristic, as one of the most important properties in perovskite oxides, has been intensively investigated and made a great progress in recent years. The ultrafast photoelectric effects in perovskite oxide single crystals [20–23], thin films [24] and heterojunctions [25–29], the high-sensitive photovoltaic effects in the heterojunctions [30–33], the transient lateral photovoltage (LPV) induced by Dember effect in p–n heterojunctions [34], and ultraviolet (UV) high-sensitive photodetectors [22, 23, 35, 36] have been reported. In this paper, we describe the recent advances and the future prospects of ultrafast photoelectric effects and high-sensitive photovoltages in the perovskite oxides and heterojunctions.

2 Novel progress

2.1 Ultrafast photoelectric effects

Figure 1 shows a typical photovoltage pulse when a tilted 15° SrTiO₃ (STO) single crystal is irradiated by a 355-nm pulsed laser with 25-ps duration at ambient temperature [20]. The inset of Fig. 1 displays the schematic circuit of the measurement. In order to avoid the influences of external fields and obtain the intrinsic property, we chose the tilted STO single crystal without any applied bias and connected a 0.5-Ω resistance in parallel with STO to measure the open-circuit photovoltages. The photoelectric signals are recorded by a 2.5-GHz bandwidth sampling oscilloscope. The rise time (10%–90%) is about 130 ps, and the full width at half maximum (FWHM) is about 230 ps. The signal polarity of photovoltage pulse is reversed when we illuminate the STO wafer through the reverse face, and no photovoltage signal was observed in untilted STO or when the photon energy of incident light is smaller than the bandgap of STO. The mechanism of the photovoltage in tilted STO single crystals can be understood as following. The STO single crystal absorbs the incident photons and generates the electron–hole pairs as well as a temperature gradient in the irradiation orientation. According to the thermoelectric theory [37], the lateral thermoelectric field can be given by E(\alpha )=(S_{ab}-S_c)\sin 2\alpha (\mathrm{d}T/\mathrm{dz})/2, where S_{ab} and S_c are the Seebeck coefficients along the ab plane and c axis, α is the vicinal cut angle, and dT/dz denotes the temperature gradient in the direction of irradiation. The thermoelectric field leads to the separation of photogenerate carriers in the lateral direction and results the photovoltage we observed.

Figure 2 shows the peak photovoltage V_{\mathrm{L}}^{\mathrm{P}} and peak photocurrent magnitude I_{\mathrm{L}}^{\mathrm{P}} dependence of the STO tilting angle α [20]. The experimental procedures are the same as shown in Fig. 1. The signal amplitude increases with α until {20}^{\circ } and then decreases. At \alpha ={45}^{\circ }, the detected signal, similar to that at \alpha =0^{\circ }, is almost 0.

Similar to the STO single crystal, we also observed that the response time is 120 ps for LiNbO₃ (LNO) single crystal [21], 143 ps for LiTaO₃ (LTO) single crystal [22], and 13 ns for LaAlO₃ (LAO) single crystal [23].

Figure 3 shows a typical open-circuit photovoltage pulse when a La_0.67Ca_0.33MnO₃ (LCMO) film grown on tilted STO substrate is irradiated by a laser pulse of 25-ps duration and 1.064-μm wavelength at ambient temperature [24]. The rise time and the FWHM are as short as 300 and 700 ps, respectively. The photovoltage sensitivity is as large as 0.45 V/mJ, and the photocurrent sensitivity is calculated to 0.25 A/mJ. Similar to the STO single crystal, the mechanism is proposed as the combination of a photoelectron and a Seebeck processes, the Seebeck coefficient we obtained, \sim 4.04 μV/K, is one order larger than the previous reports.

We epitaxial growth LaAlO_3-δ, SrTiO_3-δ and La_0.7Sr_0.3MnO₃(LSMO3) thin films on Si substrates by a computer-controlled laser molecular-beam epitaxy (laser-MBE) system and observed the picosecond and nanosecond photoelectric characteristic in the heterojunctions of oxide and Si [26–28]. Figure 4 shows a typical open-circuit photovoltaic pulse of LaAlO_3-δ/Si heterostructure when the LaAlO_3-δ thin film is irradiated by a 355-nm pulsed laser with 25-ps duration [26]. The inset shows the schematic circuit of the measurement. The waveform was recorded by an oscilloscope with 20 GHz bandwidth. In order to reduce the influences of the measuring system and the capacitance of the p–n heterojunction, a 0.1-Ω resistance is connected in parallel with the LaAlO_3-δ/Si heterojunction. The 10%–90% rise time is 86 ps, and the FWHM is 178 ps. The mechanism of the photovoltage in LaAlO_3-δ/Si heterojunction can be understood as follows. An n-type LaAlO_3-δ thin film grown onto p-type Si substrate could form a p–n heterojunction. The electrons with higher density in n-type LaAlO_3-δ film than those in Si should diffuse into Si, and the holes with higher density in Si than those in LaAlO_3-δ should diffuse into LaAlO_3-δ. The diffusion causes a built-in electric field in the space charge region around the interface. With the illumination of light, photogenerated carriers are separated by the built-in field at the interface and caused the photovoltage we measured.

Similar to LaAlO_3-δ/Si heterostructure, we observed that the response time is 210 ps for LSMO3/Si heterostructure [27], 9 ns for SrTiO_3-δ/Si heterostructure [28], and 23 ns for LSMO1/SNTO1 heterostructure [29]. It is noteworthy that all of our experimental results prove that similar to that in the traditional Si semiconductor p–n junction, the photoelectrical process is in a picosecond or nanosecond order in the p–n heterostructures consisting of complex oxides and Si or all oxides.

As mentioned above, the observation of ultrafast photoelectric effects in perovskite oxide single crystals, thin films, and heterojunctions not only proves that the ultrafast photoelectric effect is an intrinsic property of perovskite oxides and heterojunctions but also opens up the application in the development of multifunctional devices.

2.2 High-sensitive photovoltaic effects in the heterojunctions

We observed not only ultrafast photoelectric effects but also high-sensitive photovoltaic effects in the heterojunctions of perovskite oxides. The open-circuit photovoltaic sensitivities are 435 mV/mJ for LSMO3/Si p–n junction [27], 126 mV/mJ for SrTiO_3-δ/Si p–n junction [28], and 85.6 V/W for LaAlO_3-δ/Si p–n junction [26]. The open-circuit photovoltaic sensitivity can enhance 40 times when a He–Ne laser illuminated the LSMO1/Si p–n junction from the cross-section of the junction than that of from the surface of LSMO1 thin film [31].

Recently, an unusual and rather large LPV has been observed in LSMO1/SNTO1 and LSMO3/Si heterojunctions under the nonuniform irradiation of a pulsed laser [34]. According to the well-established LPV theory of conventional semiconductors, the general understanding of the LPV for semiconductor p–n junctions is that the photogenerated electron–hole pairs balance out a portion of the barrier space charge resulting in a lateral electric field which induces the lateral flow of majority carriers [38]. The LPV effect measured between two random positions on the p side and n side should be reversible because the majority carriers at the p side are holes and those at the n side are electrons. Figure 5(a) and (b) shows the maximum LPV values measured on both sides of the p–n junctions as a function of the laser spot position (x) on the LSMO1 and SNTO1 surfaces in LSMO1/SNTO1 junction, and LSMO3 and Si surfaces in LSMO3/Si junction, respectively [39]. The maximum LPV values were measured on both sides of the p–n junctions; V_{\mathrm{BA}}^{\mathrm{m}} and V_{\mathrm{ED}}^{\mathrm{m}} of the schematic setup for LPV measurements are shown in the insets of Fig. 5(a) and (b). A small area of 0.5 mm in diameter on the p-LSMO1 or LSMO3 surface was irradiated by a 308-nm XeCl excimer laser pulse with a pulse width of 20 ns and energy of 0.15 mJ. However, the LPVs are irreversible for both sides of the LSMO1/ SNTO1 p–n junction or LSMO3/Si p–n junction in Fig. 5(a) and (b). This phenomenon challenges the well-established theory of LPV mentioned above for conventional semiconductors. A mechanism based on the difference between the mobility of electrons and holes, that is, the Dember effect [40], was introduced to explain this interesting phenomenon. It should be noticed that the above model of the laser-induced majority carriers dominating LPV effect is only correct when the amount of laser induced carriers is small in the theory of conventional semiconductors. In our experiment, the pulsed laser used in our experiment is a 308-nm XeCl excimer laser beam with an energy density of 0.76 mJ/mm² and duration of 20 ns. In this case, the power density of the laser pulse is very high, and the amount of photogenerated carriers should be comparable with or even much larger than that of the majority carriers in the oxide semiconductor, so that both electrons and holes generated by phonons should play an important role in LPV. In other words, only strong light can cause the Dember-effect-induced LPV in p–n junctions to be observed. As a comparison, Fig. 5(c) shows the Dember-effect-induced LPV in both substrates of SNTO1 and Si measured under the same conditions in Fig. 5(a) and (b). From Fig. 5 (a), (b) and (c), we can see that the enhanced LPV in p–n junctions is 10 times larger than that of LPV in the bulk materials. Actually, the Dember effect has come to a greater prominence recently as a mechanism for generating THz radiation [41].

Figure 6 displays the transient LPV to the laser pulse that depends on the position of the spot in the x axis and undergoes a sign reversal as the laser spot travels from one electrode to the other for the LSMO3/Si junction. The changeover in sign occurs at halfway between the two electrodes (with x=0). The maximum LPV values, V_{\mathrm{BA}}^{\mathrm{m}}, are plotted in Fig. 7 as a function of the laser spot position (x,y) on the LSMO3 surface. In the region between the contacts, V_{\mathrm{BA}}^{\mathrm{m}} varies quite linearly with x for different y values. The open-circuit position sensitivity, which means the variation of the output photovoltage in V/mJ for a 1 mm displacement of the spot, is about 0.59 (0.39) V/(mJ·mm) for y=0(\pm 1.5) mm, respectively. The response to the irradiation position in the y direction is shown in the inset of Fig. 7. Figure 8 summarizes the spatial distribution of the peak values of LPV in the plane of the junction. The voltage sign reversal is obtained if the spot is moved across the center between the two contacts. The signal is symmetric on the reflection in a plane normal to the y axis at y=0. It is obvious that the large LPV in the p–n junctions is expected to make the candidate for position-sensitive photodetectors.

2.3 High-sensitive UV photodetectors

UV radiation detectors have drawn a great deal of interest in recent years due to many civil and military requirements. UV est in recent years due to many civil and military requirements. UV detection with high discrimination against visible and infrared light is ideal for detection in the background of infrared and visible radiation. In particular, solar-blind deep-UV detectors can work in a harsh environment of sunlight radiation. Based on the ultrafast photoelectric effects of perovskite oxides, we have successfully fabricated the high-sensitive UV photodetectors using STO [35], LAO [36], LNO [22], and LTO [23] single crystals with interdigitated electrodes.

Figure 9 shows the photocurrent variation with the power density of an STO photodetector with the interdigitated electrodes of 10 μm finger width and 10 μm interspacing at 10 V bias [35]. The inset is the spectral response of the STO photodetector. The cutoff wavelength at 390 nm is very sharp, which corresponds to photon energy of 3.2 eV, agreeing well with the STO bandgap and demonstrating a bandgap excitation process. The UV/visible contrast ratio reaches more than four orders of magnitude. The peak photocurrent responsivity is 213 mA/W located at 330 nm corresponding to the quantum efficiency of 80.2%. The dark current was 50 pA at 10 V bias. The experimental results demonstrate that STO single crystal has potential and wide application in visible-blind UV detection.

Figure 10 shows the spectral response of the LAO photodetector with a finger width of 5 μm and 5-μm interspacing and an effective detection area of 2\times 2 mm at 10-V bias [36]. The inset is the photocurrent variation with the incident light intensity of a D₂ lamp. The sharp cutoff wavelength of the spectrum is located at 220 nm, which corresponds to photon energy of 5.6 eV, agreeing well with the LAO bandgap and demonstrating a bandgap excitation process. The detectors show a high sensitivity to the wavelength less than 210 nm, and the contrast ratio of 200 versus 290 nm is more than two orders of magnitude, indicating that the LAO detector has an intrinsic solar blindness. At a 200-nm wavelength, its photocurrent response is 71.8 mA/W, while its quantum efficiency reaches 44.6%. As shown in Fig. 11 [36], the noise current of LAO detector is only 77 pA under sunlight at midday outdoors, suggesting that the LAO detector can directly detect deep-UV without any filters to block the sunlight.

For LNO photodetector with an interdigitated electrode of 10 μm finger width and 10-μm interspacing, the absorption peak and the absorption edge appear at about 315 and 330 nm, respectively, corresponding to an optical bandgap of \sim3.8 eV. The UV/visible (300 versus 390 nm) contrast ratio is more than two orders of magnitude. The photocurrent response is 17.1 mA/W under the irradiation of 300 nm wavelength UV light at 10-V bias [21].

For an LTO photodetector with interdigitated electrodes of 10-μm finger width and 10-μm interspacing, the response peak appears at about 235 nm and a sharp cutoff at about 270 nm. The noise current is only 61 pA at 20-V bias under the illumination of sunlight at midday outdoors. Similar to the LAO detector, the LTO detector is a solar-blind UV photodetector [22].

The photodetectors mentioned above are based on commercial STO, LAO, LNO, and LTO single crystals and do not need a complicated fabrication technique or process. Their intrinsic characteristics of visible blind and solar blind as well as the excellent high sensitivity demonstrate that the photodetectors based on perovskite oxides with wide bandgap have potential and attractive application in UV detections.

3 Conclusions

In this paper, we described the novel progress in ultrafast photoelectric effects and high-sensitivity photovoltages in perovskite oxides and heterojunctions, such as ultrafast photoelectric effects in perovskite oxide single crystals, thin films, and heterojunctions, high-sensitive photovoltaic effects in the heterojunctions, Dember-effect-induced LPV in perovskite p–n junctions, as well as UV high-sensitive photodetectors. The ultrafast photoelectric effects prove that similar to that in the traditional semiconductor, the photoelectric process is in picosecond order in complex perovskite oxides and heterojunctions, enhancing the confidence to develop the new devices with the perovskite oxide materials. The open-circuit high-sensitive photovoltages as well as Dember-effect-induced LPV have wide applications, such as passive detections, solar cells, and photoelectric control. On the other hand, the high-sensitive photodetectors based on perovskite oxide single crystals with wide bandgap have the potential and attractive applications in UV detection. It is noteworthy that the p–n junctions of oxides and Si combine the multifunctional properties in oxide materials, such as electricity, optics, and magnetism, with Si electronics; such a new structure related to spin ordering, charge carrying, and electron–photon interaction in the system is stimulating a wide study. We believe that the research results present in this paper not only could stimulate theoretical study on the mechanisms but also would open up possibilities in the development of multifunctional devices.

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