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Lateral photovoltaic effect has been studied in p-La0.67Ca0.33MnO3/n-Si heterojunction. Under illumination of continuous 808 nm laser beam on the film surface, a transient photovoltaic overshoot accompanied with the steady signal was observed when the laser turned off and on. The open-circuit photovoltage had a linear dependence on illuminated position, and the sensitivity reached 0.75 mVmW−1mm−1 for steady value and 6.25 mVmW−1mm−1 for the transient peak value. Especially, an enhancement in position detecting sensitivity was observed when the interface of this heterojunction was irradiated, which were 1.25 mVmW−1mm−1 (steady value) and 26.0 mVmW−1mm−1 (peak value). This work demonstrates a novel way to increase sensitivity for manganite-based position sensitive detectors.
©2011 Optical Society of America
1. Introduction
Lateral photovoltaic effect (LPVE) was first discovered by Schottky in 1930 and proposed as position sensitive detectors (PSDs) sixty years ago [1,2]. Since PSDs have a wide range of applications in the field requiring precision measurements, such as robotic vision, remote optical alignment, machine tool alignment and medical instrumentation, etc, many studies have been carried out to improve the sensitivity and linearity of PSDs in various kinds of systems, from conventional p-n junctions to hydrogenated amorphous silicon based structures [3], porous silicon [4], Ti/Si amorphous superlattices [5], semiconducting polymer [6], metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) structures [7–9], modulation-doped AlGaAs/GaAs heterostructure [10], Cu2O nanoscale film [11], etc. However, the fabrication processes for the PSDs mentioned above are complex and high cost.
Recently perovskite-type manganite thin films were discovered to have photoresponse above their forbidden gap. By coating such a film on a chosen substrate to form a heterostructure, one obtains an ultrafast photodetector [12,13]. This kind of material with good chemical stability is insensitive to harsh physical environment such as fluctuations of temperature and pressure, and can meet the needs of oil and gas optical engineering. However, a drawback of this material is its relatively low sensitivity. As a result, researchers tried methods to improve their photo-response sensitivity, such as applying an bias voltage [14], changing oxygen contents in the film and film thickness [15,16], etc.
In the present letter, we reported an enhanced LPVE by irradiating the interface of La0.67Ca0.33MnO3 (LCMO)/Si heterojunction. Under illumination of continuous 808 nm laser beam, a transient photovoltaic overshoot accompanied with the steady signal was observed when the laser turned off and on, and an enhancement in position sensitive detection was observed when the junction interface was irradiated.
2. Experimental details
LCMO thin film with a thickness of about 100 nm was deposited on 0.5-mm-thick n-type Si (001) substrate by facing target sputtering technique from stoichiometry targets [17]. The substrate temperature was kept at 680 °C, and the oxygen partial pressure at 30 mTorr during deposition. Immediately after each deposition, the vacuum chamber was back-filled with 1 atm oxygen gas. The deposited film was then cooled to room temperature with the substrate heater power cut off.
The sample used in our experiment was 4.0 mm × 7.0 mm in area and was carefully cleaned using alcohol and acetone. Two silver electrodes of 4.0 mm × 1.0 mm in area, separated by about 5.0 mm, were fixed on the sample to form ohmic contact with LCMO film or Si substrate. We used two ways of electrodes for photoresponse measurements as shown in Fig. 1 . Mode 1 and mode 2 denote different contact mode (mode 1: both contacts on film surface; mode 2: one contact on film surface with the other on substrate surface).
Continuum solid state laser with 808 nm in wavelength was used to scan the sample in the experiment without any background illumination and a slit was placed to narrow the laser spot width to be 0.2 mm. The effective irradiated area was 4.0 mm × 0.2 mm for the film illumination case and about 0.5 mm × 0.2 mm for the side illumination case. The open-circuit photovoltaic signal was recorded by a sampling oscilloscope terminated into 1 MΩ at room temperature. The laser beam was chopped at 12.5 Hz before it reached the sample and the power density was kept to be 2.7 mW/mm2.
3. Results and discussion
The typical current-voltage (I-V) curve of the LCMO/Si heterojunction was measured by tuning the applied voltage with a pulse-modulated voltage source at room temperature in dark and under the 808 nm laser irradiation (see Fig. 2 ). The forward bias is defined as the current flowing from the LCMO film to the Si substrate. The p-n rectification characteristic was ascribed to the presence of LCMO/Si interfacial potential due to carrier diffusion.
Fig. 2 I-V characteristics of the LCMO/Si heterojunction in dark and under irradiation of the 808 nm laser and inset shows the schematic measurement setup.
Two typical waveforms recorded by oscilloscope were listed in Fig. 3 when 808 nm laser spot irradiated two positions which were the nearest to each electrode in Mode 1 as shown in the insets of Fig. 3. There existed both transient and continuous processes in photovoltaic pulse. After the laser was switched on, the photovoltaic signal increased suddenly to a transient maximum Vp1 followed by a steady value Vs. On turning off the laser, there was an immediate transient signal Vp2 in the opposite direction, which then decayed slowly to zero. These changes occurred quite reproducibly, and no degradation was observed after switching for many times. Otherwise, the Vp1 always has the same sign with Vs.
Fig. 3 Two typical waveforms recorded by oscilloscope with 1 MΩ input impedance without any bias for Mode 1a when laser spot fixed at (a) x = 2.4 mm and (b) x = −2.4 mm, respectively.
Figure 4 shows selected waveforms for each electrode and illumination mode when the laser beam scanned the sample in x axis direction. The 10%-90% rise time of transient overshoot in each case to be around 2 ms. Both the transient overshoot and steady photovoltaic signal change with the laser spot position. The signals reach a minimum when the laser spot is fixed at x = 0 for Mode 1a and 1b, while the photovoltages rise up in an opposite direction when the laser spot travels through the zero point (Fig. 4(a) and (b)). For Mode 2a and 2b the photovoltaic responses Rp1 monotonically decreased when laser spot moved from the anode on LCMO surface to cathode on Si substrate (Fig. 4(c) and (d)).
Fig. 4 Waveform and detailed profile of a PSD for Mode 1 and Mode 2. (a) Mode 1a, (b) Mode 1b, (c) Mode 2a, (d) Mode 2b. The vertical arrow x leads to the laser spot moving direction.
The steady responsivity Rs and peak responsivity Rp1 and Rp2 as functions of laser spot position are presented in Fig. 5 . In mode 1, Rs was amplified by five times for the convenience of observation, and we can see that Rp1, Rs and Rp2 all vary quite linearly with x. Rp1 is much larger than Rs in each illuminated position and Rp2 is approximately as large as Rp1 with an opposite sign. According to the slope of each line we can calculate the position detecting sensitivities to be 0.75 and 1.25 mVmW−1mm−1 for Vs and 6.25 and 26.0 mVmW−1mm−1 for Vp1. Thus, there is nearly 1.7 times in Vs and 4.2 times in Vp1 higher sensitivities for side illumination.
As the photon energy for 808 nm is above the band gap of LCMO (~1eV) and Si (~1.1eV), when the illumination occurred on the sample, electrons in the illuminated region absorbed photon energy and were excited from valance band to conduction band, becoming non-equilibrium carriers. We propose the reason for higher lateral photovoltage in side illumination case to be more efficient charge-separating mechanism by built-in electric field in the interface. After non-equilibrium hole-electron pairs are generated, electrons near the space charge region in LCMO film are swept to Si substrate and holes in Si are forced into LCMO film. When there are more holes in LCMO and more electrons in Si under side illumination, carriers reaching each electrode were of higher quantity. According to the carrier concentration distribution for nonuniform illumination in semiconductor-based heterojunctions, the potential difference between two lateral electrodes can be presented as [18,19]
Here L and –L are the positions of the two electrodes, Kf is proportional coefficient, λf is the holes’ diffusion length as the carriers in p-type LCMO film are holes, N 0 is the transition electrons from LCMO to Si substrate at position x. Ideally, Eq. (1) can be simplified aswhere, p is laser power, τ is the life time of diffusion carriers, and n 0 is the density of light-excited carrier. P represents the possibility for electrons to recombine with holes in the film, so excited electrons have a possibility of 1-P to transit from LCMO film to Si substrate. Understandably, when P is smaller, more electrons go through the interface to Si and in the meantime more holes are left in the film. When illumination occurs in the interface, photoexciting process occurs in the region of both LCMO thin film and the substrate. Different from film surface illumination, in the case of side illumination, photoexcited hole-electron pairs disperse in the region of both thin film and substrate, and this increases the possibility for holes and electrons to be separated by electric field in the space charge region before recombination happens. A larger quantity of electron-hole pairs are separated, so the final N 0 is larger. From Eq. (2), we know that Vs is in proportion to N 0, so the photovoltaic difference between two electrodes is larger for side illumination.For Mode 2a and 2b, in which charges selected by electrodes are relative to both lateral and transverse direction, the responsivity has non-linear dependence on spot position. We can see from the Fig. 5 that the photovoltage signal does not change signs when laser spot travels from one electrode to the other. From the photoelectric process in the sample demonstrated above, we know that non-equilibrium charges in LCMO are holes and in Si are electrons, so it is natural that carriers near the electrode on the film surface are holes and ones near the Si electrode are electrons and thus potential of the electrode on the film plane is always higher than the one of the other electrode.
4. Conclusions
In conclusion, enhanced lateral photovoltaic effect has been observed in La0.67Ca0.33MnO3/Si p-n heterojunction by side illumination. The transient and steady open-circuit photovoltages were observed by chopping mechanically 808 nm continuous laser beam. Both of them showed a linear relationship with irradiated position, and the position detecting sensitivities reached 1.25 and 26.0 mVmW−1mm−1 for steady and transient signals, respectively. This method for sensitivity enhancement exhibits the potential application for this material as high sensitive PSDs.
Acknowledgments
This work has been supported by NCET, RFDP, NSFC, Direct Grant from the Research Grants Council of the Hong Kong Special Administrative Region (Grant No. C001-2060295), and Foresight Fund Program from China University of Petroleum.
References and links
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