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
Cuprous oxide (Cu2O), which is a p-type semiconductor with a band gap of 2.0 eV, has been widely studied due to its merits such as nontoxic nature, abundant availability and low-cost production. Cu2O thin films are generally applied to electrochromic devices , oxygen and humidity sensors [2,3] and particularly heterojunction solar cells [4,5]. Although many early works associated with this oxide were focused on the application based on the longitudinal photovoltaic effect across the heterojunction [6,7], few study has been reported for the lateral photovoltaic effect (LPE) measured on the surface of Cu2O nano-films. Since the LPE in response to spot illumination was first discovered by Wallmark , similar studies have been conducted in many different systems and LPE has been used in a wide application for various of optical transducers and sensors, including position sensitive detectors, shaft encoders, optical alignment sensors and so on [9–13]. Compared with the previous researches, the structure of Cu2O nano-films deposited on Si substrate reported in this work has presented a significantly giant lateral photovoltage (LPV) with the highest sensitivity reaching up to 113 mV/mm in PN mode. More interestingly, when an external voltage is applied, LPV in this system can be modulated and the position sensitivity is improved dramatically, almost 10 times as large as its initial value, which provides a huge potential prospect for manufacture of high-sensitivity sensors.
2. Experimental details
In this paper, the Cu2O thin films were deposited on n-type Si (1 1 1) at room temperature by DC magnetron reactive sputtering. The thickness of the Si wafers is around 0.3 mm and the resistivity is in the range of 50-80 Ωcm. 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 2.3 Pa and an O2 gas pressure of 0.7 Pa were maintained during deposition. The deposition rates, determined by stylus profile meter on thick calibration samples, were 3.7 Å/s. Serial Cu2O films with a variety of thicknesses ranging from 10 nm to 100 nm were fabricated. These Cu2O samples were later confirmed by a Bruker Discovery D8 X-ray diffractometer (XRD), as shown in Fig. 1. It is found that two XRD peaks at 2θ = 36.8。and 61.9。appeared, owning to the diffraction of the (111) and (220) planes of Cu2O. The results were consistent with the previous work . All the electrodes (about 1 mm in diameter) to the films were formed by alloying indium and showed perfect ohmic contact. Other experimental details are similar with our published papers on LPE .
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). 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 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  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 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. 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 [16,17]Fig. 2, is the diffusion length of holes. Furthermore, can be expressed as where p is the laser power, is the life time of diffusion carriers, andis 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 meansincreases and thus sensitivity improved 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 lengthis different in the same direction and in the reversed direction of electric field. We note and 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 decreases remarkably and increases. In region 2, however, both and have a small change since the electric field is comparatively low. The effect on the sensitivityin virtue of the competition between the reduction of and the increase of is 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 holes, since the variations of and have a reversed effect on the sensitivity according to Eq. (1).
In addition, as two electrodes are close to each other, which means L is shorter, the LPV sensitivity increases. Moreover, the region affected by the bias voltage is comparatively enlarged within a short L, thus implying region 1 with large LPV changing rate will occupy more proportion of the distance between two electrodes as shown in Fig. 4. The turning point of LPV curve is probably determined by the instinctive conductive character of material and the thickness of Cu2O . The systematic search of this property is expected to be further investigated.
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.
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).
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