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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 2 — Jan. 27, 2014
  • pp: 1661–1666
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Bias-assisted improved lateral photovoltaic effect observed in Cu2O nano-films

Biao Zhang, Liang Du, and Hui Wang  »View Author Affiliations


Optics Express, Vol. 22, Issue 2, pp. 1661-1666 (2014)
http://dx.doi.org/10.1364/OE.22.001661


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Abstract

A large lateral photovoltaic effect (LPE) has been observed in a Cu2O/Si heterojunction structure when its surface is illuminated by a laser. Moreover, with external bias voltage, the maximal LPE sensitivity can reach up to 1114 mV/mm, which is almost 10 times larger compared with its initial non-biased value of 113 mV/mm. We ascribe this phenomenon mainly to the effect of the increased photo-generated holes caused by the bias. Giant output voltage and high sensitivity suggest the potential of Cu2O nano-films could be used in a wide variety of applications for position-sensitive photodetectors.

© 2014 Optical Society of America

1. Introduction

2. Experimental details

3. Experimental results and discussion

The dependence of the induced photovoltage on the laser position with different typical Cu2O film thickness is shown in Fig. 2(a)
Fig. 2 (a) LPV measurement in the structures of Cu2O/Si with different Cu2O film thickness (58.0 nm, 63.2 nm, 66.9 nm and 74.3 nm). The bottom inset displays the schematic circuit of the VAB measurement. LPVs in (b) PP and PN mode and (c) NN and NP mode as a function of laser position in Cu2O(66.9 nm)/Si structure. The inset shows the layout of LPV measurement. A (2 mm), B (−2 mm), C (2 mm) and D (−2 mm) mark the electrodes.
. When each sample was scanned with a He-Ne laser (632 nm and 3 mW) focused on a roughly 50-µm diameter spot at the Si surface without any background light, a large LPV can be observed between electrodes on Cu2O side. The LPV is the largest when the incident radiation spot is closest to the measurement electrodes and shows a monotonic linear decrease as the spot is scanned away from the electrodes, becoming zero at the midpoint of these two electrodes. The voltage is reversed when the light spot is moved across the center between the two contacts. Similar LPVs as a function of laser position have been observed in the Cu2O film thickness of 58.0 nm, 63.2 nm, 66.9 nm and 74.3 nm, respectively. It shows the strongest LPE at the optimum value of 66.9 nm.

Figures 2(b) and 2(c) present the values of LPV in four different modes in the Cu2O (66.9 nm)/Si structure, which are called as PN mode, PP mode, NN mode and NP mode. Here “P” and “N” represent “p-Cu2O side” and “n-Si side”, respectively. The first capital letter represents on which side the LPV is measured, and the second capital letter represents on which side the laser illuminates. Compared with the LPV measured on the Si side (NP or NN mode), the LPV measured on the Cu2O side (PP or PN) is lager due to the higher resistivity of the Cu2O thin film than that of the Si wafer. In addition, the distances from electrodes to the junction are larger if measuring from Si side, which reduces the LPV. Interestingly, when LPV is measured on the Cu2O side, PN mode produces a larger LPV than PP mode. Whereas, in the two modes that LPV is measured on the Si side, NP mode produces a larger LPV. Thus, if we change the measurement side, the laser illumination side which produces the larger LPV is altered. This result is intriguing but the mechanism is not well known and needs further investigation. As shown in Figs. 2(b) and 2(c), four modes all present a good linear LPV versus laser position, and the obtained sensitivities range from 25 mV/mm to a remarkable value of 113 mV/mm (achieved in PN mode).

To further study the LPE in this structure, an external negative bias voltage VBD ( = VB-VD) is applied on electrodes B and D. Figure 3
Fig. 3 The LPV (VAB) as a function of laser position in Cu2O (66.9 nm)/Si structure when a bias voltage of −3 V is applied on electrodes B and D. The inset displays the schematic setup for LPV measurement, where A (2 mm), B (−2 mm), C (2 mm), and D (−2 mm) denote the electrodes.
illustrates the LPV (VAB) as a function of laser position when the bias is −3 V. Without laser irradiation, the value of VAB in this biased mode is about 2.7 V. When the laser spot is scanned from electrode D to C on the Si surface, however, a bias-assisted large LPV is observed. The range of VAB reaches up to 1500 mV, whereas with no bias the initial range is 400 mV as illustrated in Fig. 2(b). Generally, the process of LPV enhancement with laser position shows two phases: (a) in the part where the laser spot is close to the D electrode (we call it region 1), VAB rises rapidly from 2.2 V to 3.0 V in the range of 0.8 mm; (b) the changing rate of VAB drops when the laser spot is close to the C electrode (region 2). Compared with no biased mode, the linearity in region1 deteriorates as the correlation coefficient [11

11. 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]

] drops from 0.98 to 0.92. However, the dramatically enhanced sensitivity (974 mV/mm) indicates that a very small displacement of laser spot can lead to the giant change of VAB.

In addition, we also investigate the characteristic of VAB in response to different bias voltages (VBD from −1 V to −6 V). Table 1

Table 1. (Sensitivities measured on AB side in different bias voltage)

table-icon
View This Table
shows the sensitivity measurements when laser spot is scanned through two regions between electrodes C and D. The sensitivities increase with rising of VBD which implies they can be modulated by the bias voltage. When bias VBD is −6 V, in region 1, the maximal LPV sensitivity 1114mV/mm is 10 times larger than its initial value of 113 mV/mm. To our knowledge such high sensitivity has not reached ever before. In region 2 the rising magnitude is much smaller as the bias increases.

The dependence of LPV on the distance between two electrodes with a bias voltage of −3V is shown in Fig. 4
Fig. 4 The experimental results of LPV (VAB) measured in structure of Cu2O(66.9 nm)/Si with a negative bias voltage of −3 V, in which the distances between AB ( = 2L) are 3.0 mm, 4.0mm, 5.0 mm and 6.0 mm, respectively. The inset shows the schematic setup for LPV measurement with a bias voltage of −3 V.
. The range of LPV amplifies with the decrease of L and short L can boost the position sensitivity in region 1 greatly. Moreover, the turning point between two regions varies when the electrodes’ distance changes. The width of region 1 with large LPV changing rate will occupy more proportion of the whole distance between two electrodes as the L decreases.

The fully understanding of why LPV changing rate presents the two-region pattern and the sensitivity in region 1 increases significantly is not well available. After a careful inspection, we have tentatively generalized a qualitatively physical explanation. When there is no bias voltage on the electrodes B and D, the LPV sensitivity between two lateral electrodes can be presented as [16

16. C. Q. Yu, H. Wang, S. Q. Xiao, and Y. X. Xia, “Direct observation of lateral photovoltaic effect in nano-metal-films,” Opt. Express 17(24), 21712–21722 (2009). [CrossRef] [PubMed]

,17

17. C. Q. Yu and H. Wang, “Large lateral photovoltaic effect in metal-(oxide-) semiconductor structures,” Sensors (Basel) 10(11), 10155–10180 (2010). [CrossRef] [PubMed]

]
κ=2KfN0λfexp(Lλf),
(1)
where Kfis the proportional coefficient, N0is the transition holes from Si substrate to Cu2O film, L is the half-distance between two electrodes as shown in the inset of Fig. 2, λfis the diffusion length of holes. Furthermore, N0 can be expressed as n0[1P(τp/n0)], where p is the laser power, τis the life time of diffusion carriers, andn0is the density of light-excited carriers. P represents the possibility for holes to recombine with electrons in Si substrate.

In the case of applying a −3V bias on the B and D electrodes, VAB and VCD are measured as 2.7V and −62 mV. Thus, the voltage VAC on the A and C electrodes is −23.8 mV, much smaller than BD bias. In region 1, due to the larger bias VBD, the recombination rate of photo-induced electron-hole pairs in Si wafer decreases (P becomes smaller) so that more holes are injected into the Cu2O film, which meansN0increases and thus sensitivity κimproved greatly. However, the density of transition holes into Cu2O film changes a little in region 2 because of small bias VAC. We consider the different bias of VBD and VAC has a main and significant impact on this sensitivity variation in region 1 and 2.

Another factor which may influence this phenomenon is the variation of holes transport length in Cu2O film. With the bias voltage, an extra electric field from electrode A to B will be formed in the lateral direction. The external electric field decays from electrode B to A because of the resistance effect. For the sake of simplicity, we suppose the electric fields in region 1 and 2 are approximately constant and the field in region 1 is much larger than that in region 2. Therefore the diffusion lengthλfis different in the same direction and in the reversed direction of electric field. We note λ1 and λ2 as the holes transport length in the direction of BA and AB. So, in region 1 close to electrode B, the electric field is larger so that λ1 decreases remarkably and λ2 increases. In region 2, however, both λ1 and λ2 have a small change since the electric field is comparatively low. The effect on the sensitivityκin virtue of the competition between the reduction of λ1 and the increase of λ2is difficult to evaluate theoretically. Nevertheless, it is reasonable to think this change of holes transport lengths plays a minor role in LPV sensitivity compared with the above-mentioned increasing transition holesN0, since the variations of λ1and λ2 have a reversed effect on the sensitivity according to Eq. (1).

4. Conclusion

In this study, we report a giant lateral photovoltaic effect modulated by external bias in a Cu2O/Si structure as its surface is illuminated by a laser. The experimental results have shown that LPE sensitivity greatly increases with the applied biased voltage. Although the nonlinearity increases as the distance L reduces, its high position sensitivity implies the potential utilizing in high accuracy measurement and position detection by signal processing. A mechanism about the effect of increased photo-generated holes has been presented to explain this result. The field-induced variation of diffusion length of these carriers is also discussed. We believe this remarkable LPV and high sensitivity are useful for the development of novel and multifunctional photo sensors.

Acknowledgments

This work was supported in part by the National Nature Science Foundation under Grants 11374214, 10974135 and 60776035 and in part by the National Minister of Education Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).

References and links

1.

R. Neskovska, M. Ristova, J. Velevska, and M. Ristov, “Electrochromism of the electroless deposited cuprous oxide films,” Thin Solid Films 515(11), 4717–4721 (2007). [CrossRef]

2.

J. T. Zhang, J. F. Liu, Q. Peng, X. Wang, and Y. D. Li, “Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors,” Chem. Mater. 18(4), 867–871 (2006). [CrossRef]

3.

D. U. Kim and M. S. Gong, “Thick films of copper-titanate resistive humidity sensor,” Sens. Actuators B Chem. 110(2), 321–326 (2005). [CrossRef]

4.

A. Mittiga, E. Salza, F. Sarto, M. Tucci, and R. Vasanthi, “Heterojunction solar cell with 2% efficiency based on a Cu2O substrate,” Appl. Phys. Lett. 88(16), 163502 (2006). [CrossRef]

5.

K. P. Musselman, A. Marin, L. Schmidt-Mende, and J. L. MacManus-Driscoll, “Incompatible length scales in nanostructured Cu2O solar cells,” Adv. Funct. Mater. 22(10), 2202–2208 (2012). [CrossRef]

6.

Z. F. Hu, Z. J. Li, L. Zhu, F. J. Liu, Y. W. Lv, X. Q. Zhang, and Y. S. Wang, “Narrowband ultraviolet photodetector based on MgZnO and NPB heterojunction,” Opt. Lett. 37(15), 3072–3074 (2012). [CrossRef] [PubMed]

7.

A. Dhanabalan, J. K. J. van Duren, P. A. van Hal, J. L. J. van Dongen, and R. A. J. Janssen, “Synthesis and characterization of a low bandgap conjugated polymer for bulk heterojunction photovoltaic cells,” Adv. Funct. Mater. 11(4), 255–262 (2001). [CrossRef]

8.

J. T. Wallmark, “A new semiconductor photocell using lateral photoeffect,” Proc. IRE45(4), 474–483 (1957). [CrossRef]

9.

J. Henry and J. Livingstone, “Thin-film amorphous silicon position-sensitive detectors,” Adv. Mater. 13(12–13), 1022–1026 (2001). [CrossRef]

10.

W. S. Park and H. S. Cho, “Measurement of fine 6-degrees-of-freedom displacement of rigid bodies through splitting a laser beam: experimental investigation,” Opt. Eng. 41(4), 860–871 (2002). [CrossRef]

11.

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]

12.

K. Zhao, K. J. Jin, H. B. Lu, Y. H. Huang, Q. L. Zhou, M. He, Z. H. Chen, Y. L. Zhou, and G. Z. Yang, “Transient lateral photovoltaic effect in p-n heterojunctions of Lasub0.7Sr0.3MnO3 and Si,” Appl. Phys. Lett. 88(14), 141914 (2006). [CrossRef]

13.

S. Salvatori, G. Mazzeo, and G. Conte, “Voltage division position sensitive detectoron photoconductive materials; part I: principle of operation,” IEEE Sens. J. 8(2), 188–193 (2008). [CrossRef]

14.

H. L. Zhu, J. Y. Zhang, C. Z. Li, F. Pan, T. M. Wang, and B. B. Huang, “Cu2O thin films deposited by reactive direct current magnetron sputtering,” Thin Solid Films 517(19), 5700–5704 (2009). [CrossRef]

15.

L. Du and H. Wang, “Infrared laser induced lateral photovoltaic effect observed in Cu2O nanoscale film,” Opt. Express 18(9), 9113–9118 (2010). [CrossRef] [PubMed]

16.

C. Q. Yu, H. Wang, S. Q. Xiao, and Y. X. Xia, “Direct observation of lateral photovoltaic effect in nano-metal-films,” Opt. Express 17(24), 21712–21722 (2009). [CrossRef] [PubMed]

17.

C. Q. Yu and H. Wang, “Large lateral photovoltaic effect in metal-(oxide-) semiconductor structures,” Sensors (Basel) 10(11), 10155–10180 (2010). [CrossRef] [PubMed]

18.

K. A. M. Scott, A. K. Sharma, C. M. Wilson, B. W. Mullins, S. F. Soares, and S. R. J. Brueck, “High resolution Si position sensor,” Appl. Phys. Lett. 62(24), 3141–3143 (1993). [CrossRef]

OCIS Codes
(040.5160) Detectors : Photodetectors
(040.5350) Detectors : Photovoltaic
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Electrooptics

History
Original Manuscript: November 12, 2013
Revised Manuscript: December 5, 2013
Manuscript Accepted: December 20, 2013
Published: January 16, 2014

Citation
Biao Zhang, Liang Du, and Hui Wang, "Bias-assisted improved lateral photovoltaic effect observed in Cu2O nano-films," Opt. Express 22, 1661-1666 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-2-1661


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References

  1. R. Neskovska, M. Ristova, J. Velevska, M. Ristov, “Electrochromism of the electroless deposited cuprous oxide films,” Thin Solid Films 515(11), 4717–4721 (2007). [CrossRef]
  2. J. T. Zhang, J. F. Liu, Q. Peng, X. Wang, Y. D. Li, “Nearly monodisperse Cu2O and CuO nanospheres: preparation and applications for sensitive gas sensors,” Chem. Mater. 18(4), 867–871 (2006). [CrossRef]
  3. D. U. Kim, M. S. Gong, “Thick films of copper-titanate resistive humidity sensor,” Sens. Actuators B Chem. 110(2), 321–326 (2005). [CrossRef]
  4. A. Mittiga, E. Salza, F. Sarto, M. Tucci, R. Vasanthi, “Heterojunction solar cell with 2% efficiency based on a Cu2O substrate,” Appl. Phys. Lett. 88(16), 163502 (2006). [CrossRef]
  5. K. P. Musselman, A. Marin, L. Schmidt-Mende, J. L. MacManus-Driscoll, “Incompatible length scales in nanostructured Cu2O solar cells,” Adv. Funct. Mater. 22(10), 2202–2208 (2012). [CrossRef]
  6. Z. F. Hu, Z. J. Li, L. Zhu, F. J. Liu, Y. W. Lv, X. Q. Zhang, Y. S. Wang, “Narrowband ultraviolet photodetector based on MgZnO and NPB heterojunction,” Opt. Lett. 37(15), 3072–3074 (2012). [CrossRef] [PubMed]
  7. A. Dhanabalan, J. K. J. van Duren, P. A. van Hal, J. L. J. van Dongen, R. A. J. Janssen, “Synthesis and characterization of a low bandgap conjugated polymer for bulk heterojunction photovoltaic cells,” Adv. Funct. Mater. 11(4), 255–262 (2001). [CrossRef]
  8. J. T. Wallmark, “A new semiconductor photocell using lateral photoeffect,” Proc. IRE45(4), 474–483 (1957). [CrossRef]
  9. J. Henry, J. Livingstone, “Thin-film amorphous silicon position-sensitive detectors,” Adv. Mater. 13(12–13), 1022–1026 (2001). [CrossRef]
  10. W. S. Park, H. S. Cho, “Measurement of fine 6-degrees-of-freedom displacement of rigid bodies through splitting a laser beam: experimental investigation,” Opt. Eng. 41(4), 860–871 (2002). [CrossRef]
  11. J. Henry, J. Livingstone, “Optimizing the response of Schottky barrier position sensitive detectors,” J. Phys. D Appl. Phys. 37(22), 3180–3184 (2004). [CrossRef]
  12. K. Zhao, K. J. Jin, H. B. Lu, Y. H. Huang, Q. L. Zhou, M. He, Z. H. Chen, Y. L. Zhou, G. Z. Yang, “Transient lateral photovoltaic effect in p-n heterojunctions of Lasub0.7Sr0.3MnO3 and Si,” Appl. Phys. Lett. 88(14), 141914 (2006). [CrossRef]
  13. S. Salvatori, G. Mazzeo, G. Conte, “Voltage division position sensitive detectoron photoconductive materials; part I: principle of operation,” IEEE Sens. J. 8(2), 188–193 (2008). [CrossRef]
  14. H. L. Zhu, J. Y. Zhang, C. Z. Li, F. Pan, T. M. Wang, B. B. Huang, “Cu2O thin films deposited by reactive direct current magnetron sputtering,” Thin Solid Films 517(19), 5700–5704 (2009). [CrossRef]
  15. L. Du, H. Wang, “Infrared laser induced lateral photovoltaic effect observed in Cu2O nanoscale film,” Opt. Express 18(9), 9113–9118 (2010). [CrossRef] [PubMed]
  16. C. Q. Yu, H. Wang, S. Q. Xiao, Y. X. Xia, “Direct observation of lateral photovoltaic effect in nano-metal-films,” Opt. Express 17(24), 21712–21722 (2009). [CrossRef] [PubMed]
  17. C. Q. Yu, H. Wang, “Large lateral photovoltaic effect in metal-(oxide-) semiconductor structures,” Sensors (Basel) 10(11), 10155–10180 (2010). [CrossRef] [PubMed]
  18. K. A. M. Scott, A. K. Sharma, C. M. Wilson, B. W. Mullins, S. F. Soares, S. R. J. Brueck, “High resolution Si position sensor,” Appl. Phys. Lett. 62(24), 3141–3143 (1993). [CrossRef]

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