The large infrared lateral photovoltaic effect (LPE) based on semiconductor structures has been a challenge for a long time because the light in this region is hard to be absorbed. In this study, we report an unusual infrared laser induced LPE observed in sputtered Cu2O thin films. The maximum open-circuit lateral photovoltage can reach up to a remarkable value of 30.6mV under irradiation of Ti: Sapphire laser emitting 100 fs pulses at 2000 nm with pulse energy of 50 μJ. Temperature gradient induced by infrared laser is introduced to interpret this infrared induced LPV effect. The high position sensitivity reaching 15.3mV/mm and easier fabrication techniques suggests this oxidized film a potential candidate for the novel infrared photodetectors.
©2010 Optical Society of America
The lateral photovoltaic effect (LPE) was found 70 years ago and has been boosted in many different systems , such as Ti/Si amorphous superlattices , hydrogenated amorphous silicon Schottky barrier structures , perovskite materials , semiconducting polymer system , and metal-semiconductor structures , modulation doped AlGaAs/GaAs heterostructures . Due to its output voltage, lateral photovoltage (LPV), changes with laser position linearly, the LPE has been used in a variety of optical transducers and sensors, such as position sensitive detectors [8,9]. In preliminary studies, however, the attention was paid largely on LPE induced by laser in the ultraviolet or visible range. Practicable LPE with good linearity induced by laser in infrared (IR) range has rarely been reported [5,8,10–17]. Moreover, several classical MS or MOS structures like Co/SiO2/Si, Ti/Si, Co/Si, Cu/Si, which show remarkable the visible light induced LPE [18–21], do not have response in infrared range.
In this study, we report a large infrared laser induced LPE in Cu2O thin film and the output values of lateral photovoltage (LPV) vary very linearly with the position of the infrared laser spot. In fact, Cu2O is a prototypical p-type conducting oxide with a bandgap of 2.0 eV, series of exciton level in the range of neV, and some trap levels at 0.45 or 0.25 eV from the valance band edge [22–26]. Due to many merits like nontoxic nature, abundant availability and low-cost production, Cu2O thin films had been widely studied in many applications, such as heterojunction solar cells, electrochromic devices, and oxygen and humidity sensors. However, using Cu2O as infrared photodetector is new. We present the relation between the LPV output and IR laser position and discuss the possible mechanism of LPE in terms of temperature gradient caused by thermal effect of IR laser.
Easier fabrication techniques and the large LPE induced by infrared illumination are expected to make the Cu2O a candidate for new type of position-sensitive photodetectors (PSD), as current commercial PSD devices are based on small area diffused junction crystalline silicon devices in which the fabrication techniques require high temperature processing stages. The system produced in this work are based upon sputtered thin film which require no high temperature diffusion stages and are readily reproducible and can be adapted for large area devices.
2. Experimental results
We deposited several Cu2O films with various thicknesses ranging from 10nm to 100nm, and 67 nm Cu2O film shows the largest LPV value. And if the film is too thick, there is no LPV. The 67nm Cu2O thin films were deposited on SiO2 layer which naturally grows on the surface of Si substrates (0.3mm), at room temperature by DC magnetron reactive sputtering, because reactive sputtering has advantage over conventional (r.f. or d.c.) sputtering from oxide targets, due to the fact that the plasma density would be better in the former case due to the high conductivity of elemental targets or metal powders, leading to the better uniformity of the films.
The base pressure of the vacuum system prior to deposition was 2.3 × 10−4 Pa. High pure Cu target (60 mm diameter) was used. An argon gas pressure of 2.3Pa an O2 gas pressure of 0.7 Pa were maintained during deposition. The deposition rate, determined by stylus profile meter on thick calibration samples, was 3.7 Å/s. The nominal compositions of Cu2O samples were confirmed by further X-ray diffractometer (XRD).
All these samples were scanned spatially with a mode-locked Ti: Sapphire laser emitting 100 fs pulses at 2000 nm with a pulse energy of 50 μJ focused on a roughly 50 μm diameter spot at the surface and without any spurious illumination (e.g., background light, etc.) reaching the samples, and all the contacts (less than 1 mm in diameter) to the films were formed by alloying indium and showed no measurable rectifying behavior (very perfect ohmic contact). The schematic picture of the experimental set-up for the LPV measurement is shown in the inset of Fig. 1 . The lateral photovoltage between the indium electrodes was measured. The electrodes were always kept in the dark to prevent the generation of any electrical contact photovoltage.
The appearance of Cu2O films was yellowish in our experiment. The topography of the samples was measured using atomic force microscopy (AFM) in tapping mode, as shown in Fig. 1, from which we can see that the films are continuous and approximately uniform. This is important because the degree of uniformity is a key factor for high performance of LPV . As the LPV measurement shown in the bottom inset of Fig. 1, the LPV was observed on the Cu2O surface between two indium electrodes. We find that the LPV is the largest when the incident radiation spot is closest to the measurement electrodes and shows a monotonic linear change as the spot is scanned away from the contacts, becoming null at the midpoint of these two contacts. Due to the scattering of the substrate, the LPV with the laser illumination on Cu2O was bigger than that on the substrate at the same position. The position sensitivities, which means the variation of LPV for a 1mm displacement of the spot, were about 15.3mV/mm and11.2mV/mm for the laser irradiating on the Cu2O surface and the substrate, respectively. The correlation coefficient [6,27], which measures the linearity of the device output, is close to 1.000 in the linear region. This indicates a perfect linearity for our sample. It is clear that the LPV depends on the position of the spot on the x-axis. When the light spot is at the center between two electrodes, the LPV values are zero which can attribute to the diffusion symmetry of the carriers. If the light position is positive, the LPV is positive and vice versa. Nevertheless, when the spot reaches past the contacts, the LPV decays rapidly to zero. The nonlinearity occurring close to the contacts is consistent with previous report, we ascribed it to the contact effects [6,27–29].
Response to the IR laser position in the y direction is recorded, and the measurement schematic is shown in the inset of Fig. 2 (a) .The LPV values, plotted as a function of the laser spot position x with different y on the Cu2O surface, are shown in Fig. 2 (a). It is clear that the LPV still varies linearly with the distance between the electrodes. The position sensitivity was different for each scan. The highest sensitivity can be get at y = 0. Figure 2 (b) summarizes the spatial distribution of the LPV in the plane of the Cu2O surface. The voltage sign reversal is obtained when the spot moves across the centre between the two contacts A and B. The signal is roughly plane symmetric on a plane normal to the y axis at y = 0.
We also find IR induced LPE strongly depends on the distance of two contacts placing in the Cu2O film. LPVs recorded with different distance of AB are plotted in the Fig. 3 . Clearly, the smaller distance of two electrodes, the larger change ratio of LPV is obtained, which means the short distance of AB can lead to the high sensitivity of laser position. When the distance of AB ( = 2L) is too large, L = 5.5 (mm), the lateral photovoltage is no longer linear with the light spot position. That is to say, linearity of LPV versus laser position can be only obtained in a relatively small area (less than 5.5mm) on the surface of the Cu2O film.
To understand the mechanism causing the infrared laser induced LPE in the Cu2O film, the electrodes were also placed in the middle of another two opposite sides of Cu2O surface, and a similar spatial distribution map was obtained, which indicates that the system is isotropic. The voltage output when two electrodes put on substrate without Cu2O film was also measured, and there is no LPE observed in the SiO2/Si substrate under the illumination of infrared laser. Furthermore, we used infrared laser with different wavelength and observed the LPV on the Cu2O surface. The output of lateral photovoltage ΠLPV) (∯Π〉〉 changes with laser position linearly. The sensitivities of laser spot position are 17.2mV/mm and 16.8mV/mm for 1600 nm, 1300nm IR laser, respectively. The value of LPV didn’t show significant dependence on incident IR photon energy, but we find when the sample is heated, the maximum value of LPV increases. This increase may be explained by hopping transport of carriers, which is common in disorder semiconductors like amorphous Si and organic semiconductors . The LPE dependence on wavelength and temperature needs further investigation. Clearly, the photon energies of the laser pulse with the wavelength of 1300nm, 1600nm, 2000 nm are far below the band gap of Cu2O (about 2.0eV) or the substrate Si (1.1eV). Besides, copper oxide has series of exciton level with resonance widths in the range of neV , which suggests Cu2O film has potential application in terahertz.
Therefore, conventional LPV theory based on the photogenerated carrier effect is inapplicable for our sample. Though Kui-Juan Jin, et al. reported an unusual LPE in La0.9Sr0.1MnO3 /Si oxide epitaxial film , they attribute this phenomenon to Dember effect which is only existent in the condition of induced light with high energy density and short wavelength. Therefore, the Dember effect should be ruled out in the measured voltage of our sample.
3. Mechanism of infrared induced LPV
We explain this unusual infrared LPE based on the thermal effect. When the Cu2O film is illuminated by the laser beam, a temperature gradient, as shown in the Fig. 4 , will form between the illuminated and the nonilluminated areas due to the thermal effect caused by the IR laser. As carrier mobility is much higher in the illuminated zone, these major carriers have tendency to flow from the illuminated zone to the nonilluminated region, which results in the formation of a transient electric field. This should be the intrinsic reason for formation of lateral photovoltage in Cu2O film. If the distances from the centre of the carrier packet to each electrode are different, then the quantity of the collected carriers on the two probe electrodes is different. Therefore, the temperature gradient induced by IR laser in the direction of carrier transfer (along the film surface) causes the LPV in this system. When the light spot is at the centre between electrodes on the film, the photovoltage is zero due to the diffusion symmetry. If the light spot is close to one electrode, the electric potential is higher than the other electrode, for more hot carriers are collected by this electrode. The LPV will form between these two electrodes, as the bottom inset of Fig. 4 shows the schematic of electric field distribution along two electrodes.
Based on this diffusion mechanism we could understand the IR induced LPE phenomenon. Because our Cu2O film is an isotropic system, the temperature gradient along the film surface caused by laser irradiation on the substrate side is the same as that by laser irradiation on the Cu2O side. The shortened distance of electrode AB will let more hot carriers collected and lead to the LPV which is more sensitive to laser position. The results we got when electrodes AB are distant also reflect some complex IR induced LPV characteristics of oxidized metal. Further theoretical and experimental investigations on the relationship between LPV and temperature gradient are urgently needed.
In summary, the Cu2O film has been found to exhibit IR laser-induced photovoltaic effect which is ascribed to the temperature gradient and difference of carrier mobility between the illuminated and the unilluminated regions. When the laser spot scans along the line between the two electrodes, we can observe LPV changes linearly with a position sensitivity of 15.3mV/mm. This large infrared LPE induced by IR laser suggests the potential of Cu2O film to be applied to a wide variety of applications for infrared detection.
We acknowledge the financial support of the National Nature Science Foundation of China (NNSFC) (grants 60776035 and 10974135) and the support of the National Minister of Education Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).
References and links
1. J. T. Wallmark, “A new semiconductor photocell using lateral photoeffect,” Proceedings of the Institute of Radio Engineers (IRE) 45(4), 474–483 (1957) (IRE).
2. B. F. Levine, R. H. Willens, C. G. Bethea, and D. Brasen, “Lateral photoeffect in thin amorphous superlattice films of Si and Ti grown on a Si substrate,” Appl. Phys. Lett. 49(22), 1537–1539 (1986). [CrossRef]
3. J. Henry and J. Livingstone, “Thin-film amorphous silicon position-sensitive detectors,” Adv. Mater. 13(12–13), 1022–1026 (2001). [CrossRef]
4. K. Zhao, K. Jin, H. Lu, Y. Huang, Q. Zhou, M. He, Z. Chen, Y. Zhou, and G. Yang, “Transient lateral photovoltaic effect in p-n heterojunctions of La0.7Sr0.3MnO3 and Si,” Appl. Phys. Lett. 88(14), 141914 (2006). [CrossRef]
5. D. Kabra, T. B. Singh, and K. S. Narayan, “Semiconducting-polymer-based position-sensitive detectors,” Appl. Phys. Lett. 85(21), 5073–5075 (2004). [CrossRef]
6. J. Henry, and J. Livingstone, “A comparison of layered metal-semiconductor optical position sensitive detectors,” in Proceedings of IEEE Sensors (IEEE, 2002), pp. 836–840.
7. N. Tabatabaie, M. H. Meynadier, R. E. Nahory, J. P. Harbison, and L. T. Florez, “Large lateral photovoltaic effect in modulation-doped AlGaAs/GaAs heterostructures,” Appl. Phys. Lett. 55(8), 792–794 (1989). [CrossRef]
8. J. Cárabe, J. J. Gandia, N. Gonzalez, E. Galiano, and M. T. Gutierrez, “A simple amorphous-silicon photodetector for two-dimensional position sensing,” Appl. Phys. Lett. 69(22), 3408–3410 (1996). [CrossRef]
9. S. Salvatori, G. Mazzeo, and G. Conte, “Voltage division position sensitive detectors based on photoconductive materials; Part I: Principle of operation,” IEEE Sens. J. 8(2), 188–193 (2008). [CrossRef]
10. D. Kabra, S. Shriram, N. S. Vidhyadhiraja, and K. S. Narayan, “Charge carrier dynamics in organic semiconductors by position dependent optical probing,” J. Appl. Phys. 101(6), 064510–064517 (2007). [CrossRef]
11. J. Henry and J. Livingstone, “Improved position sensitive detectors using high resistivity substrates,” J. Phys. D Appl. Phys. 41(16), 165106 (2008). [CrossRef]
12. J. Henry, and J. Livingstone, “High sensitivity optical position sensitive detectors fabricated from high resistivity substrates,” Proc. SPIE 7003, (2008).
13. J. Henry and J. Livingstone, “Aging effects of Schottky barrier position sensitive detectors,” IEEE Sens. J. 6(6), 1557–1563 (2006). [CrossRef]
14. J. Henry and J. Livingstone, “A comparison of Schottky barrier position-sensitive detectors as a function of light wavelength,” IEEE Sens. J. 3(4), 519–524 (2003). [CrossRef]
15. J. Henry and J. Livingstone, “A comparison of layered metal-semiconductor optical position sensitive detectors,” IEEE Sens. J. 2(4), 372–376 (2002). [CrossRef]
16. D. W. Boeringer and R. Tsu, “Lateral photovoltaic effect in porous silicon,” Appl. Phys. Lett. 65(18), 2332–2334 (1994). [CrossRef]
17. E. Fortunato, G. Lavareda, R. Martins, F. Soares, and L. Fernandes, “Large-area 1D thin-film position-sensitive detector with high detection resolution,” Sens. Actuators A Phys. 51(2–3), 135–142 (1996). [CrossRef]
18. S. Q. Xiao, H. Wang, Z. C. Zhao, Y. Z. Gu, Y. X. Xia, and Z. H. Wang, “The Co-film-thickness dependent lateral photoeffect in Co-SiO2-Si metal-oxide-semiconductor structures,” Opt. Express 16(6), 3798–3806 (2008). [CrossRef] [PubMed]
19. S. Q. Xiao, H. Wang, Z. C. Zhao, and Y. X. Xia, “Large lateral photoeffect observed in metal-semiconductor junctions of CoxMnyO films and Si,” J. Phys. D Appl. Phys. 40(18), 5580–5583 (2007). [CrossRef]
20. C. Q. Yu, H. Wang, and Y. X. Xia, “Giant lateral photovoltaic effect observed in TiO2 dusted metal-semiconductor structure of Ti/TiO2/Si,” Appl. Phys. Lett. 95(14), 141112–141113 (2009). [CrossRef]
22. M. Jörger, E. Tsitsishvili, T. Fleck, and C. Klingshirn, “Infrared absorption by excitons in Cu2O,” Phys. Status Solidi B 238(3), 470–473 (2003). [CrossRef]
23. M. Jörger, T. Fleck, C. Klingshirn, and R. von Baltz, “Midinfrared properties of cuprous oxide: high-order lattice vibrations and intraexcitonic transitions of the 1s paraexciton,” Phys. Rev. B 71(23), 235210 (2005). [CrossRef]
24. R. Huber, B. A. Schmid, Y. R. Shen, D. S. Chemla, and R. A. Kaindl, “Stimulated terahertz emission from intraexcitonic transitions in Cu2O,” Phys. Rev. Lett. 96(1), 017402 (2006). [CrossRef] [PubMed]
25. A. R. Rastkar, A. R. Niknam, and B. Shokri, “Characterization of copper oxide nanolayers deposited by direct current magnetron sputtering,” Thin Solid Films 517(18), 5464–5467 (2009). [CrossRef]
26. J. H. Hsieh, P. W. Kuo, K. C. Peng, S. J. Liu, J. D. Hsueh, and S. C. Chang, “Opto-electronic properties of sputter-deposited Cu2O films treated with rapid thermal annealing,” Thin Solid Films 516(16), 5449–5453 (2008). [CrossRef]
27. J. Henry and J. Livingstone, “Optimizing the response of Schottky barrier position sensitive detectors,” J. Phys. D Appl. Phys. 37(22), 3180–3184 (2004). [CrossRef]
28. R. Martins and E. Fortunato, “Role of the resistive layer on the performances of 2D a-Si:H thin film position sensitive detectors,” Thin Solid Films 337(1–2), 158–162 (1999). [CrossRef]
29. E. Fortunato, G. Lavareda, M. Vieira, and R. Martins, “Thin film position sensitive detector based on amorphous silicon p–i–n diode,” Rev. Sci. Instrum. 65(12), 3784–3786 (1994). [CrossRef]
30. A. P. Young and C. M. Schwartz, “Electrical conductivity and thermoelectric power of Cu2O,” J. Phys. Chem. Solids 30(2), 249–252 (1969). [CrossRef]
31. S. Leinss, T. Kampfrath, K. Volkmann, M. Wolf, J. T. Steiner, M. Kira, S. W. Koch, A. Leitenstorfer, and R. Huber, “Terahertz coherent control of optically dark paraexcitons in Cu2O,” Phys. Rev. Lett. 101(24), 246401 (2008). [CrossRef]
32. K.-J. Jin, K. Zhao, H.-B. Lu, L. Liao, and G.-Z. Yang, “Dember effect induced photovoltage in perovskite p-n heterojunctions,” Appl. Phys. Lett. 91(8), 081906 (2007). [CrossRef]