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We report a transient lateral photoeffect (LPE) in thin metallic Co films deposited on n-type Si substrates with native SiO2 surfaces. Under the nonuniform irradiation of a laser beam, the lateral phtovoltage (LPV) shows high sensitivity to the laser position in the metal film plane. This effect can be interpreted by the metal-semiconductor (MS) junction formed between metal and semiconductor. The LPV depends significantly on the thickness of Co film. The position sensitivity shows a peak value of 42.6 mV/mm for Co2.8 nm-SiO2-Si and decreases greatly with the increase of the Co film thickness. We explain that by the shorting effect of the metallic film.
©2008 Optical Society of America
1. Introduction
Since the lateral photovoltage (LPV) in response to spot illumination was first discovered by Wallmark [1] in floating Ge p+-n junctions, the lateral photoeffect (LPE) has been utilized in position-sensitive detectors (PSDs) [2–7] dut to its linear excitation position dependence. Nowadays, as the dimension of the devices is getting smaller and the production of nanodevices is increasing, PSDs with high sensitivity is of great importance. The desire to maximize the position sensitivity of PSDs has stimulated interest in the discovery and design of new photovoltaic materials exhibiting large LPV. Recent discoveries include the colossal position sensitivity as high as 1.5 mV/µm in Ti/Si amorphous superlattices [8–11] and the moderate position sensitivity with a maximum of about 20 mV/mm in some metal-semiconductor (MS) structures [12, 13]. In our previous work, we achieved a largest position sensitivity of 34.3 mV/mm in CoxMnyO/Si MS like structures [14]. The key mechanism of these systems is based on the MS junction formed at the interface between metal and semiconductor.
In MS junctions, it is vital that the ohmic contacts should be placed on the resistive side (semiconductor) instead of the conductive side (metal) to achieve appreciable LPV since the conductive side easily gets short. Most interestingly, in this study we observed large position sensitivities in Co films deposited on n-type Si substrates with a thin native SiO2 layer by placing two ohmic contacts on the metal side. Such metal-oxide-semiconductor (MOS) structures have been extensively studied as solar cell materials (transverse photovoltage) for many decades [15–20], but so far little work about MOS performed as PSDs (LPV) has ever been reported. This oxide layer, although essentially transparent to electrons [21], allows a potential difference to exist between the metal and semiconductor, resulting in an increased open-circuit transverse photovoltage under illumination [17, 22]. Thus it is reasonable to suppose that such MOS structures can also exhibit larger LPV than MS does. Here we report on an enhanced LPE in Co-SiO2-Si MOS structures. The position sensitivity depends greatly on the Co film thickness and the largest value obtained is about 42.6 mV/mm under illumination by a 3 mW He-Ne laser. We also present the laser position dependence of LPV on the Co film plane and discuss the possible mechanism of LPE with view of formation of MS junction.
2. Experimental details
The Co thin films were deposited on n-type Si (1 1 1) at room temperature by DC magnetron reactive sputtering. The thickness of the wafers is around 0.3 mm and the resistivity of the wafers is in the range of 50-80 Ωcm at room temperature. Si substrates are covered with a thin native SiO2 layer of 1.2 nm measured by transmission electron microscopy (TEM). The base pressure of the vacuum system prior to deposition was better than 5.0 × 10−5 Pa. High purity Co target (60 mm diameter) was used. An argon gas pressure of 0.64 Pa was maintained during deposition. The deposition rates, determined by stylus profile meter on thick calibration samples, were 0.4 Å/s.
Fig. 1. A three-dimensional schematic illustration of the LPV measurement using fixed electrodes and a variable light spot position.
All samples were cut into 6 mm × 6 mm rectangles and were scanned spatially with a He-Ne laser (3 mW and 632 nm) focused on a roughly 50-µm diameter spot at the surface and without any spurious illumination (e.g., background light, etc.) reaching the samples. This beam was chopped and the resulting photovoltage was measured using standard lock-in techniques. The geometry is shown in Fig. 1. All the contacts (less than 1 mm in diameter) to the films were formed by alloying indium and showed no measurable rectifying behaviour. The length between the two ohmic contacts is 3 mm. The film resistivity and I-V characteristic were determined using CIP (current in the film plane) geometry four-terminal method by physical property measurement system (PPMS, Quantum Design).
3. Results and discussions
Fig. 2. (online colour at: www.pss-a.com) AFM images (1 µm × 1 µm) of Co films with different thicknesses deposited on SiO2/Si substrates: (a) 1.6 nm; (b) 3.2 nm; (c) 6.4 nm and (d) 12 nm.
Fig. 3. Three-dimensional plot of the LPV as a function of the laser spot in the Co film plane for Co3.2 nm-SiO2-Si.
The topography of the samples was measured using atomic force microscopy (AFM) in tapping mode. Figure 2 shows the AFM images of the 16 Å, 32 Å, 64 Å and 120 Å thick samples. The average surface roughness of the 16 Å, 32 Å, 64 Å and 120 Å samples is 0.368 nm, 0.424 nm, 0.707 nm and 1.22 nm respectively. Obviously, both the average surface roughness and the grain size increase with the increment of the Co film thickness. Furthermore, from the micrographs, we can see that all the films are continuous and approximately uniform.
Figure 3 shows the three-dimensional exterior view of the LPV as a function of the laser position in the Co film plane for Co3.2 nm-SiO2-Si. The LPV shows an approximately linear dependence as the spot is scanned along the lines y = constant in the Co film plane, becoming null at the midline x = 0. The photovoltage sign reverses when the laser spot is moved across the center line between the two contacts. The signal is roughly plane symmetric on a plane normal to the y axis at y = 0.
Usually, only the LPV as a function of the laser spot scanned along the line between two ohmic contacts was often discussed. This is because the LPV obtained this way can achieve the highest position sensitivity and an excellent linearity. Therefore, in the following discussion, we only present the LPV varying with the laser spot scanned along the line y = 0.
Fig. 4. LPV as a function of the laser position (x) observed in Co film planes with different thickness for y = 0. The lines are guides for the eye.
The dependence of the induced photovoltage on the position of light scanned along the line y = 0 at room temperature for Co-SiO2-Si MOS structures with different Co thickness is shown in Fig. 4. When their film surfaces are illuminated by a He-Ne laser with a wavelength of 632 nm and a power of 3 mW, a transient LPE can be observed. We find that 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 contacts, becoming zero at the midpoint of these two contacts. The position dependence of LPV is quite similar to that of the traditional MS junctions [8–13]. However, the LPE in MS junctions was often observed by placing two ohmic contacts on the semiconductor side in order to get a relatively high LPV, whereas in our MOS samples the LPV was obtained by placing two contacts on the metal side. Furthermore, the LPV depends significantly on the thickness of the metallic film. The largest open-circuit position sensitivity is 42.6 mV/mm for Co2.8 nm-SiO2-Si. The correlation coefficients [9, 10], which measure the linearity of the device output, are close to 1.000 for all samples. This indicates a perfect linearity for our samples. According to the extensive work done by Fortunato, et al. [23–25], there are two other figures of merit of a PSD. The first one is its nonlinearity, with an acceptable device having nonlinearities less than 15% [24]. This quantity, also known as position-detection error, is defined [23, 25] as follows:
and it is a measure of the distortion of the sensor output. The second one is the spatial resolution, which indicates the minimum distance that can be measured when the light spot is moved from one position to another. The spatial resolution of devices is measured by calculating the nonlinearity of devices for data produced using different increments. We measured over increment of 100 µm. Results showing the difference in performance of the MOS structures with different thickness Co films are shown in Table 1. As can be seen, all the MOS structures show good to excellent nonlinearities for 100 µm spatial resolution. In addition, we choose SiO2/Si (substrate with no Co film deposited) and Co48 nm-SiO2-Si with a very thick Co film as two control samples in contrast to other samples. No obvious LPV was found in these two control samples.
Table 1. Results of Co-SiO2-Si MOS structures with different Co film thicknesses.
It has been well established that the mechanism of transverse photovoltage in such MOS structures with a native thin oxide layer less than 25 Å is similar to that of MS structures [15, 17]. Similarly, the operation mechanism of LPE in such MOS structures can also be attributed to the MS junction, which results in the formation of a depletion region almost in the semiconductor region. When light is uniformly incident onto the metal surface, electron-hole pairs are generated inside or within the minority-carrier diffusion length of the depletion region almost in the semiconductor. The minority holes in the depletion region are swept into the metallic film by the Schottky field with electrons remaining in the n-region. This leads to the development of a transverse photovoltage, as is observed in a MS or MOS solar cell. If now the light impinges at one point on the surface, the presence of the excess remaining electrons and injected holes will give rise to a non-equilibrium distribution because of all other points on the film surface without any illumination, generating a gradient between the illuminated and the nonilluminated zones. So the excess holes in the metallic layer and the excess electrons (majority carriers) in the Si layer move laterally away from the illuminated spot. If the lateral distance of the laser spot from each electrode is different, then the quantity of the collected carriers at the two contacts is different. A lateral field is therefore set up, as well as the LPV. Ideally, there should be a linear relationship between the LPV output and the position of the light spot. This linear behaviour indicates the sensitivity, or the usefulness of the sensor [25].
Fig. 5. Schematic simple equilibrium energy-band diagram of the Co-SiO2-Si MOS system with a n-type semiconductor substrate. Egi and Egs denote the bandgap of SiO2 and Si respectively. Φmi is the metal-to-insulator barrier height and is related to work function of Co. ΦSi is the semiconductor-to-insulator barrier height and is related to work function of Si. The semiconductor from interface (SiO2/ Si) to bulk can be divided into three regions: the inversion layer, depletion region and neutral region as illustrated.
Figure 5 shows the simple energy-band diagram for our Co-SiO2-Si MOS system. In MOS solar cells providing the insulator (SiO2) is kept thin enough (≤20 Å), tunneling through the insulator is the transport mechanism [17, 26, 27]. In our case, the native SiO2 film is about 1.2 nm. According to the tunnel MOS theory [28, 29], the metal-to-insulator barrier height Φmi control the degree of the inversion of the n-type base semiconductor. To invert the surface of a n-type semiconductor to p-type requires Φmi to be higher than an approximate value of 3.6 eV. Note that the work function (about 4.45 eV) of Co is much larger than the demarcation value, so the conductivity type of the semiconductor surface underneath the interfacial layer is inverted. Consequently a pseudo p-n junction is formed and conversion of radiation to charge carriers occurs in the bulk of the base semiconductor. This may be the difference in photogenerated carriers mechanism between our MOS structures and the classical MS structures.
In order to better understand the Co-film-thickness dependence of LPV, we compare the position sensitivity with the effective resistivity for all samples in Fig. 6(a). As seen, the effective resistivity shows quite a large quantity (nonmetallic behaviour) when Co film thickness is less than 5 nm and then behaves metallic characteristic with further increasing the Co film thickness. In MOS structures, the effective resistivity measured on the metal film is strongly determined by whether the conducting channel switches from the upper metallic film to the Si inversion layer (This inversion layer is shown in Fig. 5) [30–33]. That is, the effective resistance is composed of two parallel channels. One is the metal film channel; the other is the Si inversion layer channel. The thicker the metallic film, the lower is its resistance compared to the constant inversion layer resistance, and therefore the metal film channel should play a major role in the effective resistivity, which leads to a smaller effective resistance. On the other hand, the position sensitivity shows a peak value of 42.6 mV/mm at about 2.8 nm of Co thickness. With further increasing the thickness of Co film, the position sensitivity drops as dramatically as the effective resistivity does. This is because too thick film can short the two ohmic contacts which establish the LPV measurement. The thicker the metallic film, the more remarkably the metallic film shorts the two ohmic contacts, leading to a smaller LPV value. A simple physical model of shorting effect is shown in the inset of Fig. 6(a). The LPV is composed of two parts: the minority carriers (holes) diffuse laterally in the metal film and then are collected by two ohmic contacts; the majority carriers (electrons) diffuse laterally in the Si inversion layer and then are collected by two ohmic contacts. Evidently, the resistivity of the inversion layer (semiconductor) is relatively large. Therefore, C and D are open. If the metal film is thin enough (usually less than 5 nm), the resistance of the metal film is so large that A and B are also open. As a result, a large LPV can be measured. With increasing the Co film thickness, however, the Co film exhibits a more and more pronounced metallic behaviour, which can easily short A, B and C, D. Consequently a more and more small LPV till to zero can be obtained, as observed.
Fig. 6. The position sensitivity and effective resistivity as functions of the thickness of the metal film for (a) Co-SiO2-Si and (b) Cu-SiO2-Si structures. The inset in (a) shows a schematic simple physical mode for the shorting effect of the metal film. The arrows show the lateral diffusions of photo-generated carriers.
We also performed experiments on Cu-SiO2-Si structures due to Cu’s low resistivity, 1.68 × 10−8 Ωm, which is almost a quarter of Co’s resistivity (6.64 × 10−8 Ωm). The position sensitivity and effective resistivity as functions of the thickness of the Cu film for Cu-SiO2-Si structures are shown in Fig. 6(b). Since Cu’s work function (about 4.4 eV) is much larger than the demarcation value, the conductivity type of the semiconductor surface underneath the interfacial layer is also easily inverted. Therefore, a large LPV measured on the metal side can be obtained. As seen, the position sensitivity also shows a peak value of 72.2 mV/mm at about 1.4 nm of Cu thickness and drops dramatically with increasing the Cu film thickness from the peak point. The consistent droping pace between the position sensitivity and effective resistivity is quite similar to Co-SiO2-Si structures. However, the dropping pace for Cu-SiO2- Si is more abrupt than that for Co-SiO2-Si. Furthermore, the LPV measured on the metal side for the former is appreciable at Cu thickness ranging from 1 nm to 5 nm while the latter is appreciable at Co thickness ranging from 1 nm to 12 nm. It indicates that the shorting effect of the former is more pronounced than that of the latter and this is consistent with the fact that the resistivity of Cu is much lower than that of Co. Apparently, the Cu-SiO2-Si structures used as PSDs have a more rigorous range of the metal film thickness than the Co-SiO2-Si structures. This is the reason why we focus on Co-SiO2-Si structures in this paper.
Figure 7 shows the I-V characteristics of Co (3.2 nm and 6.4 nm) films deposited on SiO2/Si substrates measured at 300 K. The I-V curve for Co6.4 nm-SiO2-Si is representative of metallic behaviour, which suggests that the metal film channel should play a major role in the effective resistivity. For Co3.2 nm-SiO2-Si, however, the I-V curve is clearly nonlinear. At low currents, the sample exhibits low resistance, and at high currents it switches to the high resistance state. This implies that the effective resistance in Co3.2 nm-SiO2-Si is determined by both the metal film channel and the Si inversion layer channel. This vivid contrast between these two I-V curves further supports our interpretation for the Co-film-thickness dependence of LPV.
Since this inversion layer can show interesting effect like anomalous large magnetoresistance (MR) at room temperature [30–33], the combination of LPE and MR in a same structure should be realized. In practice, we have achieved the coexistence of both effects in such Co-SiO2-Si MOS structures [34]. Moreover, the LPV obtained at the metal side is strongly influenced by the resistivity of the inversion layer, which can be affected by the external magnetic field, so one can predict that it is possible to realize the magnetic field modulation of LPE and the light modulation of MR in such MOS structures. A further study on the relation between LPE and MR is underway.
However, with decreasing the thickness of metal film from the peak point, the position sensitivities for both Co-SiO2-Si and Cu-SiO2-Si structures diminish greatly, till to zero when there is no deposition of metal film. The reason may be that too thin film cannot form a junction to produce LPV. Such a thin film (less than 1 nm) can not either be uniform in any case or fully cover the substrate.
4. Conclusions
In summary, an enhanced transient lateral photoeffect which significantly depends on the thickness of the Co film was observed in Co-SiO2-Si MOS structures in comparison with that in MS structures. Under the nonuniform illumination of a laser, the lateral photovoltage shows high sensitivity to the laser spot on the metal surface. There exists an optimized thickness of Co film for the peak value of position sensitivity (42.6 mV/mm for the 2.8 nm thick Co film) obtained on the film side instead of Si Side. No matter to increase or decrease the Co film thickness from the peak point, the LPV just diminishes. In addition to applications in solar cells, such MOS structures can also be used in PSDs. In consideration of future combination with the film side MR property, this study may supply a new way to optimize PSDs and stimulate researches on the correlations between LPE and MR effect.
Acknowledgments
This work has been supported by National Nature Science Foundation (Grant No. 60378028 and 60776035) and supported in part by the National Minister of Education Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT).
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