1. Introduction

The lateral photovoltaic effect (LPE) can be used in position-sensitive detectors (PSDs) because its output photovoltage changes linearly with the distance between the laser illumination spot and the electrodes [1–3]. Since the LPE was discovered by Schottky and expanded by Wallmark [1,4], investigations have been carried out in many different structures including Ti/Si amorphous superlattices [3], AlGaAs/GaAsheterostructures [5], hydrogenated amorphous silicon Schottky barrier structures [6] and some metal–semiconductor (MS) structures [7–10]. In addition to the sensitivity of the LPE to the position of the laser illumination, both the optical response and relaxation time associated with LPE are important characteristics for PSD applications. Because the LPE originates from the lateral diffuse transport of excited carriers from the point of excitation to the electrodes [4], the photovoltage response and relaxation time will be greatly influenced by the carrier mobility of the semiconductor in the conventional p-n or p-i-n junctions. The fast response time of LPE was achieved in Si p-n junctions due to the high carrier mobility of Si single crystal, but the position sensitivity of Si p-n PSDs is much smaller than that of a metal/Si structure [11]. However, due to the shorting effect of the metal, the LPE in metal/Si Schottky structures is highly sensitive to the metal film thickness. As a result, the metal film must be extremely thin (on the order of nanometers) resulting in instability due to the oxidation [7,11,13]. In contrast, oxide-films are stable in air and are, thus, potential candidates for LPE applications in PSDs. Some semiconductor oxides, such as Cu₂O [12], show the large LPE. Recently, high position sensitivity of 32.5 mV mm⁻¹ and a fast optical relaxation time was observed in Fe₃O₄/Si Schottky junctions, which is caused by the formation of accumulation of charge (inversion layer) at the interface between the half-metallic Fe₃O₄ thin film and the lightly-doped Si substrate due to the large difference in the work functions of Si and Fe₃O₄ [14].

Ultraviolet/near-ultraviolet photodetectors had been pursued for a long time due to their practical application such as the ultraviolet environmental monitoring and communicating. The ultraviolet PSD with fast response time (3~5 µs) had been realized when the Si-based junction was covered with an optical filter, but the device performance will become worse at high temperature and the high-power laser is needed [15]. Photodetectors utilizing wide bandgap semiconductors (WBS) have good visible-blind/solar-blind performance [16,17].However, there are few reports about the ultraviolet/near-ultraviolet LPE basing on WBS due to the difficulty associated with fabrication of p-type WBS. Our previous research on the LPE in Fe₃O₄/Si Schottky junctions suggest that fast ultraviolet/near-ultraviolet PSD could be achieved in Fe₃O₄/WBS Schottky junctions with large LPE if the inversion layer could be formed and the WBS has high carrier mobility. Silicon carbide is a wide-bandgap (2.2~3.3 eV at room temperature) semiconductor that is attractive for its great potential applications in ultraviolet/near-ultraviolet optoelectronic devices because of its high carrier mobility and excellent stability at high temperature [18,19].

In this paper, we report a sensitive LPE in Fe₃O₄/3C-SiC junctions with a fast relaxation time at near-ultraviolet wavelengths. The position sensitivity has a strong dependence on the Fe₃O₄ film thickness with the largest value of 67.8 mV/mm observed for a 15 nm Fe₃O₄ film under illumination with a 405 nm laser. The optical response and relaxation times of the LPE are roughly 3.2 μs and 30 μs, respectively.

2. Sample growth and characterization

The high quality Fe₃O₄ thin films were prepared on 3C-SiC (111) substrates using pulsed laser deposition (PLD). The high purity target was prepared by pressing α-Fe₂O₃ powder into a pellet and sintering at 1000 °C for 10 h. The base pressure of the vacuum system prior to deposition was below 10⁻⁷Torr and the substrate temperature was 350 °C. The pulsed exciter laser used KrF (λ = 248 nm) and produced a laser beam with an intensity of ~150 mJ at a rate of 3 Hz. The deposition rate was approximately 1 nm/min. After the deposition, the films were annealed for 20 minutes under vacuum at 350 °C.

X-ray diffraction (XRD) data were collected using the X'Pert XRD spectrometer with Ni-filtered Cu K_α radiation. The chemical states were measured using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Transmission electron microscopy (TEM) patterns were performed using the Tecnai G2 F30 Field Emission Transmission Electron Microscopy. The current-voltage (I-V) curves were measured using the Keithley 2601. All samples were scanned spatially with a laser focused on a roughly 100 µm diameter spot. All the contacts (less than 0.5 mm in diameter) to the films were covered by indium. The time response of the LPE was collected by an oscilloscope of 500 MHz (Tektronix DPO 5054) using 400 nm femtosecond laser which is modulated using SHG method by α-Ti: sapphire regeneration amplified laser system(the pulse duration is 130 fs).

Figure 1(a) shows the XRD pattern of a 20 nm Fe₃O₄ film on 3C-SiC substrate. Only the reflections from the (111) family for Fe₃O₄ are clearly observed indicating that a single-phase Fe₃O₄ film with cubic inverse-spinel structure is formed. X-ray photoelectric spectroscopy (XPS) was performed to determine the valence states of Fe in the Fe₃O₄ film. Clear evidence for the presence of Fe³⁺ (709.5 eV and 722.5 eV) and Fe²⁺ (711.1 eV and 724.2 eV) was observed with a content ratio of 2:1 for Fe³⁺: Fe²⁺, as shown in Fig. 1(b), confirming the high quality of the Fe₃O₄ film we obtained [20].The high-resolution cross-sectional transmission electron microscopy (HRTEM) image shown in Fig. 1(c) gives the clear structural features of the Fe₃O₄/SiC structure. The lattice constant a is 8.406 Å, which is consistent with the XRD results (a = 8.350 Å) and the lattice spacing of Fe₃O₄ (8.396 Å) [21].

 

Fig. 1 (a) The XRD pattern of the Fe₃O₄ film on 3C-SiC. (b) The Fe 2p XPS spectrum of Fe₃O₄ on 3C-SiC substrate. (c) The high-resolution TEM pattern of Fe₃O₄/3C-SiC structure. (d) Transmission spectrum of 3C-SiC wafer. We can estimate that the bandgap of 3C-SiC is about 2.36 eV from the (αhv)^1/2 versus hv curve because the 3C-SiC is an indirect band gap semiconductor.

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3. Results

The longitudinal I–V characteristic of the Fe₃O₄/3C-SiC junction for a 15 nm Fe₃O₄ film was measured at room temperature. Two indium electrodes were placed on the surface of the Fe₃O₄ film and 3C-SiC substrate, as shown in the inset of Fig. 2(a). It is clear that the longitudinal I–V curve exhibits a backward diode-like rectifying feature [14,22]. The junction current increases dramatically with increasing forward bias voltage, but the current remains very low for the reverse bias voltage even when the voltage reaches −3 V, indicating that the Fe₃O₄/3C-SiC structure is a typical Schottky junction. The electron concentration of 3C-SiC (111) is n ≈4.5 × 10¹⁶ cm⁻³ and E_F-E_C≈-0.083 eV which is calculated from n = N_C exp[-(E_C-E_F)/k_BT] where N_C(SiC) = 1 × 10¹⁸ cm⁻³ at 300 K [23]. The work function of 3C-SiC is Φ_S = χ_SiC + (E_C-E_F) = 3.73 eV where χ_SiC = 3.65 eV is the electron affinity of SiC and E_F-E_V = 2.27 eV [24]. On the other hand, the work function for clean Fe₃O₄ is Φ_M = 5.3 eV [25]. Therefore, the difference between the work functions of Fe₃O₄ and 3C-SiC is Φ_M-Φ_S = Φ_MS = 1.57 eV, which will induce an upwards band-bending at the SiC/SiO₂ interface leading to accumulation and bounding of holes in a quasi-triangular potential well. The metal-to-semiconductor barrier height is very close to the experimental value of 1.6 V, as shown in the Fig. 2(a). The energy band diagram of the Fe₃O₄/3C-SiC structure is shown in Fig. 2(b). Once the Fe₃O₄ film has been deposited on the 3C-SiC substrate, a large build-in electric field forms and the Schottky junction is produced which increases the separation of light-induced carrier-pairs under the radiation of the laser.

 

Fig. 2 (a) The longitudinal I–V curve for the Fe₃O₄ (15nm and 10 nm)/3C-SiC junction, the inset shows the schematic circuit. (b) Energy band diagram of the Fe₃O₄/3C-SiC structure.

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The LPE in the Fe₃O₄/3C-SiC junctions was investigated in detail. Two indium electrodes were placed on the surface of the Fe₃O₄ film (2L = 3.6 mm), as shown in Fig. 3(a). When the Fe₃O₄ surface was partially illuminated with a laser (405 nm and 10 mW) spot, a large LPE was observed. Most of the light was absorbed by the 3C-SiC substrate because the probability of absorption in an ultrathin Fe₃O₄ film is very smaller than that in 3C-SiC from the transmission spectrum (See the Fig. 1(d) and the Ref [14]). The photon-excited electrons and holes will be generated in the junction and then separated into Fe₃O₄ and 3C-SiC by the build-in electric field, respectively. Figure 3(a) shows the dependence of the induced photovoltage on the laser position for the Fe₃O₄/3C-SiC junctions. The largest LPE is observed when the incident radiation spot is the closest to the electrodes and it shows a monotonic linear decrease as the spot is moved away from the contacts. According to the Ref [26], this structure with lateral photovoltaic effect can be served as a position-sensitive device (PSD) without applied gate-voltage. The largest open-circuit position sensitivity of 67.83 mV mm⁻¹ is observed in the junction with 15 nm Fe₃O₄ film. The nonlinearity of the LPE on the Fe₃O₄ side of the junction is approximately 1.64% indicating a pretty linear response can be achieved in this junction.

 

Fig. 3 (a) The dependence of the lateral photovoltage on the laser position for the Fe₃O₄/3C-SiC junctions. (b) LPE voltage on the 3C-SiC side. (c) LPE sensitivities as a function of Fe₃O₄ thickness. (d) LPE measurement in Fe₃O₄ (15 nm)/3C-SiC as a function of laser power with different light wavelengths

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The energy of the photons emitted by the 405 nm laser is sufficient to excite electron-hole pairs in 3C-SiC and Fe₃O₄. The excited electrons can move to the Fe₃O₄ side of the junction because of the build-in electric field of the Fe₃O₄/3C-SiC structure. The electric field gradient between the illuminated and the non-illuminated zones results in excess electron diffusion from the illuminated spot toward the electrodes. The position dependence of LPE can be fitted using the diffusion theory [7,13]:

A small LPE (11 mV) with a large nonlinearity is observed on the 3C-SiC side of the Fe₃O₄ (15 nm)/3C-SiC structure, as shown in Fig. 3(b), which is much smaller than the value of ~120 mV on the Fe₃O₄ side. We connected contact A (or C) and contact B (or D) with the positive and negative probes of a Keithley 2000 multimeter, respectively. When the laser illuminates the Fe₃O₄ surface near contact B, the light-induced voltage between A and B is positive, suggesting the lateral diffusion of electrons occurs. Meanwhile, the signal between C and D is also positive, suggesting that the LPE on the 3C-SiC side originates from the lateral diffusion of electrons instead of holes, because the light-excited holes are bound within the 3C-SiC semiconductor. Similar results have been reported for La_0.9Sr_0.1MnO₃ /SrNb_0.01Ti_0.99O₃ and La_0.7Sr_0.3MnO₃/Si junctions and explained using the Dember effect [27,28]. Most of the light-induced electrons will have a probability (1−P) of moving into the Fe₃O₄ side of the junction through the Schottky barrier due to the existence of a build-in electric field, but some of the rest electrons can still diffuse within the 3C-SiC semiconductor with the probability, P, calculated from quantum tunneling theory. In a word, the LPE in both the Fe₃O₄ and 3C-SiC sides originate from the light-excited electrons’ lateral diffusion.

Wang et al. showed that the highest vertical photovoltage (VPE) could be observed for a critical thickness of La_0.9Sr_0.1MnO₃ (LSMO) film in a LSMO/Si heterojunction [29]. Likewise, the LPE shows a significant dependence on the thickness of the Fe₃O₄ film because the interface region plays a crucial role on the separation of the excited electron-hole pairs. The highest LPE sensitivity of 67.83 mV/mm observed in the Fe₃O₄ (15 nm)/3C-SiC device can be understood as follows. The VPE is first induced by the separation of photon-excited electron-hole pairs through the built-in electric field near the interface. Then, the excited electrons laterally diffuse away from the laser spot and the LPE is observed. The short of the distance of excited electrons from 3C-SiC in a thinner Fe₃O₄ film can greatly reduce the incidence of electron-hole recombination, which results in the enhancement of photovoltage with decreasing Fe₃O₄ thickness. However, when the Fe₃O₄ layer is thinner than the critical thickness, the built-in electric field will weakened with decreasing thickness of the Fe₃O₄ film, reducing the separation of photon-excited electron-hole pairs. An imperfect diode-like rectifying character for the longitudinal I-V curve of the Fe₃O₄ (10 nm)/3C-SiC structure is shown in Fig. 2(a), confirming that the built-in field was reduced with reducing thickness of the Fe₃O₄ film. Therefore, there is an optimum thickness for the Fe₃O₄ film to obtain the highest LPE, as shown in Fig. 3(c).

To better understand the LPE, we investigated the influence of laser wavelength and power on the LPE. Figure 3(d) shows the LPE voltage as a function of laser power at different wavelengths (532, 405 and 400 nm) in the Fe₃O₄ (15 nm)/3C-SiC structure. The average intensity was measured with a calibrated photometer and maintained by adjusting the power of the laser. The colored lines are the theoretical results. The LPE sensitivity is proportional to the power when the applied power is low and then slowly saturates as the power is increased. The power at which saturation occurs depends on the laser wavelength. The saturation is likely attributed to the rapidly increasing recombination rate of the carriers in the region of irradiation with increasing light intensity [8].Furthermore, the lateral voltage at 532 nm is quite small, which means the LPV could be only observed under near-ultraviolet light in Fe₃O₄ (15 nm)/3C-SiC junctions.

The time response of the LPE for a Fe₃O₄/3C-SiC junction was further investigated using a 400 nm femtosecond laser system. Figure 4(a) shows the diagram of the experimental setup. Figure 4(b) shows the typical variation of the open-circuit LPE voltage with time. The rise time is approximately 80 µs and the full width at half maximum (FWHM) is approximately 500 µs when the LPE is measured directly. The FWHM indicates the relaxation time for the LPE. To reduce the influence of the circuit in the measurement, some resistors with different values were connected, as shown in the inset of Fig. 4(c). Interestingly, a fast response and relaxation time of the LPE was observed. When a resistor with 1.3 kΩ was parallel connected with the oscilloscope, the rise time dramatically reduced to roughly 3.2 µs and the relaxation time is also decreased to about 30 µs.

 

Fig. 4 (a) Diagram of the experimental set-up for the optical response measurement. (b) The variation of the open-circuit LPE with time, the schematic circuit of the measurement is shown in the inset. (c) The variation of the LPE with time, different resistances were connected in parallel to the Fe₃O₄ film, the schematic circuit of the measurement is shown in the inset. (d) The transverse current-voltage curve on the Fe₃O₄ surface of the Fe₃O₄ (15nm)/3C-SiC junction.

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Once the Fe₃O₄ film was deposited on the 3C-SiC substrate, a large build-in filed forms at the Fe₃O₄/3C-SiC interface due to the difference of work-function and bandgap between Fe₃O₄ and 3C-SiC. Thus, the accumulation of charge appears easily in interface of the Fe₃O₄/3C-SiC, which has a higher conductivity than that of semiconductor. The higher conductivity layer of Si had been named as inversion layer and the transverse I-V curve measured on the surface of Fe₃O₄ film is an effective way to verify the existence of inversion layer because the Fe₃O₄/3C-SiC structure is similar to the metal-oxide-semiconductor (MOS). The nonlinear increase in current with increasing voltage confirms the existence of the inversion layer in the Fe₃O₄/3C-SiC junction, as shown in Fig. 4(d) [7,14,30–32]. The inversion layer provides a low resistive path for carrier transport along the interface of the Fe₃O₄/3C-SiC junction and thus the measured resistance on the surface of the film is the parallel resistivity of the film and the inversion layer in the Fe₃O₄/3C-SiC structure. Figure 5 shows a schematic circuit of the lateral flow of the excited electrons away from the laser illumination spot in the Fe₃O₄ film and the inversion layer at the Fe₃O₄/3C-SiC interface. When the laser illuminates the junction, the excited electrons are driven towards the Fe₃O₄ side of the junction by the build-in electric field, and then laterally diffuse away from the laser spot. The laser-excited electrons could travel in Fe₃O₄ and the inversion layer at the Fe₃O₄/3C-SiC interface. However, the inversion layer has a lower resistance compared with the Fe₃O₄ film, and thus most of the excited-electrons diffuse laterally in the inversion layer at the Fe₃O₄/3C-SiC interface rather than in the Fe₃O₄film. The relaxation time is greatly influenced by the carrier mobility between the two electrodes because it originates from the lateral flow of the excited electrons away from the laser spot. It’s well-known that the electron mobility in the inversion layer, i.e. at the SiO₂/SiC interface (~100 cm²V⁻¹s⁻¹), is much larger than the electron mobility of Fe₃O₄ (~0.48 cm²V⁻¹s⁻¹) [33,34].The I-V curve for the Fe₃O₄ film clearly suggests parallel circuits for the Fe₃O₄ film and inversion layer, therefore, the fast response and relaxation time of the LPE are due to rapid movement of electrons in the inversion layer at the interface of Fe₃O₄/3C-SiC. Although the relaxation time (~30 µs) of LPE in Fe₃O₄/3C-SiC junctions is longer than that (3~5 µs) of Si-based ultraviolet PSD covered with an optical filter, but the excellent stability at high temperature and the anti-irradiation properties of SiC make the great potential applications in ultraviolet or near-ultraviolet PSD under particular environmental conditions.

 

Fig. 5 The photo-excited electron motion profile in Fe₃O₄/3C-SiC structure.

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4. Physical mechanism

We propose a following model to explain the mechanism of LPE in Fe₃O₄/3C-SiC structure. When a laser illuminates on the junction, light-excited carriers are generated within the 3C-SiC at laser-spot position because the probability of absorption in an ultrathin Fe₃O₄ film is very smaller than that in 3C-SiC from the transmission spectrum (See the Ref 14 and the Fig. 1(d)). Then these light-excited electrons will have a possibility of (1-P), which can be estimated from the Schrodinger equation, to transit from the 3C-SiC into the upper layer (the inversion layer at the Fe₃O₄/3C-SiC interface) through the Schottky barrier due to the build-in field, and the rest electrons, possibility of P, can laterally diffuse within the 3C-SiC [35].

Based on this model, the light-induced electrons will diffuse laterally along the inversion layer away from the illuminated spot toward two different sides and thus generate a gradient laterally between the illuminated and the non-illuminated zones. One-dimensional situation in x direction was considered. According to diffusion theory [7,13],

The lateral photo-voltage can be obtained by calculating the difference of the quasi-Fermi level between the two contact electrodes position A and B in Fig. 5. So,

If L<<λ_m, the nonlinearity will be very small, which means the LPE will show a very linear characteristic response to the laser position.

Equation (6) suggests that the LPE sensitivity is also determined by the electron diffusion length. The electrons diffusion length is related to the effective region of Schottky field, so we can suppose that the electrons diffusion length is proportional to the Fe₃O₄ film thickness, and thus it can be written as λ_m = C(t−t₀), where C is a proportional coefficient and t₀ is the threshold thickness of Fe₃O₄ films [13,36,37]. The position sensitivity of LPE in Fe₃O₄ side can be written as:

From the Eq. (3) and Eq. (6), the sensitivity of LPE in Fe₃O₄ side can be written as:

It can be clearly seen that the resistivity and the Fermi level of the thin-film material are the two crucial factors of the sensitivity of LPE.

Generally, a large laser power will result in more excited-electrons tunneling from semiconductor to upper layer because the recombined electrons have more opportunity to be re-excited by photons. Thus the density of excited-electrons in the inversion layer at the laser spot position can be written as: N₀ = n₀[1−P^τp/no], where p is the laser power, τ is the life time of diffusion carriers, and n₀ is the density of light-excited carriers [37,38]. So, in the linear region, the voltage can be signed as:

The LPE sensitivity will increase and then saturate with increasing the laser power, which is consistent with our experimental results in Fe₃O₄/3C-SiC junction, as shown in Fig. 3(d). And the density of light-excited carriers strongly depends on the wavelength of laser and the bandgap of semiconductor, n₀ = K[hν−E_g]^α, where K is a proportional coefficient, E_g is the bandgap of the semiconductor, and α is an exponential coefficient (0.4<α<0.7) [39]. Most of the photon was absorbed by 3C-SiC, and the electrons with a large rest energy of (hv−E_g) after transition from 3C-SiC possess a longer diffusion length (λ_m) in Fe₃O₄ film that can be written as: λ_m = γ(hν−E_g)^β, where γ is a proportional coefficient and β is another related coefficient [39,40]. So,

The LPE sensitivity will be in the maximum when the wavelength of laser is close to the value: λ_max = hc/{E_g + [(L/γ)/ (1−α/β)]^1/β}.

5. Conclusion

In conclusion, we have fabricated high quality Fe₃O₄/3C-SiC junctions and investigated their LPE properties. Both rectifying behavior and a large LPE were observed. The dependence of the LPE on the laser spot position shows high sensitivity (67.8 mV mm⁻¹) and good linearity. The optical response and relaxation times caused by the formation of the inversion layer at the interface of Fe₃O₄/3C-SiC were 3.2 μs and 30 μs, respectively. The high positional sensitivity and fast response speed together make Fe₃O₄/3C-SiC junctions a promising candidate for a wide range of ultraviolet/near-ultraviolet device applications.