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Abstract
PbSe has attracted considerable attention due to its promising applications in optoelectronics and energy harvesting. In this work, we explore the lateral photovoltaic effect (LPE) of PbSe films with a simple PbSe/Si heterostructure under nonuniform light illumination and zero-bias conditions. The LPE response is strongly dependent on the thickness of the PbSe film, but always shows a linear dependence on the laser spot position in an ultra-large working size of 5 mm and exhibits a wide photoresponse ranging from visible to near-infrared. The maximum position sensitivity can reach up to 190 mV/mm for the 15-nm-thick PbSe device at 1064 nm and nonlinearity is less than 4%, demonstrating its new potential application in novel position sensitive detectors (PSDs). Besides, the device also shows an ultrafast response speed, with the rise and fall time of ∼40 µs and ∼105 µs, respectively, and excellent reproducibility. These results bring great inspirations for developing high-performance broadband and self-powered PSDs based on the PbSe/Si heterostructure.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lead selenide (PbSe) thin films, with favorable electronic band structures, high carrier mobility, and dielectric constants, have long been used to perform infrared (IR) detectors, laser diodes, solar cells, and thermoelectric devices, and attracted tremendous research interest over the last few years [1–6]. In recent decades, PbSe-based optoelectronic devices such as PbSe photodetectors (PDs) and quantum dot (QD) devices have stimulated intensive research [7,8]. From the previous results, it has been demonstrated that researchers have mainly focused on the vertical separation of the photo-generated carriers at the heterojunction interfaces and tried many different methods to improve the interface field and thus the photoelectric conversion efficiency. However, besides the longitudinal separation, there also exists an important physical process of transverse diffusion, and the photo-generated carriers involved in the transverse diffusion process can induce an important phenomenon of lateral photovoltaic effect (LPE) and also provide a new utilization in position-sensitive detectors (PSDs) for PbSe-based heterojunctions [9–13]. The LPE originates from nonuniform illumination of a heterojunction, which can cause photogenerated electron-hole pairs to be separated upward and downward by the built-in electric field and then diffused laterally along the interface away from the illuminated region; at last, these carriers are collected by the electrodes prepared on both sides of the illumination spot. Once the carriers concentration collected in the two electrodes is different, then a lateral photovoltage (LPV) is generated, and the output LPV changes enormously with small displacements of the illumination spot and has a linear relationship with the laser spot position [14–16]. More importantly, the LPE response is not only dependent on the longitudinal separation of the photo-excited carriers but also largely determined by their subsequent transverse diffusion. Therefore, the modulation mechanism of the LPE should be quite different from that of the traditional photoelectric effects. Besides, it is suggested that the PSDs based on the LPE can usually operate without any external power and exhibit fast response speed, the performances of which are urgently needed for next-generation nano-devices. Considering these unique properties, till now, a variety of architectures have been built and investigated as PSDs extensively in recent years, containing hydrogenated amorphous silicon materials [17], metal-oxide-semiconductor structures [15,18,19], two-dimensional (2D) materials [20–23], and perovskite materials [24,25]. However, the LPE in the PbSe structures has not been studied so far, and the working mechanism and influencing factors in them are still unknown.
In this work, we fabricated a series of single phase PbSe films with pulsed laser deposition (PLD) technique and studied the LPE responses in a PbSe/n-Si heterostructure. The results show that the LPE of the heterojunction is strongly dependent on the thickness of the PbSe film, and exhibits a wide wavelength response range from 300 to 1100 nm, with the maximum position sensitivity of 190 mV/mm and excellent nonlinearity of <4% for the 15-nm-thick PbSe device under illumination at 1064 nm. More importantly, no any external bias is needed here to realize the good performances, indicating the great potential of this heterostructure for use in self-powered PSD applications. Besides, the linear working size of this heterostructure is as large as 5 mm, and the device exhibits an ultrafast response speed by modulating the chopper frequency, with a rise and fall time of ∼40 µs/∼105 µs. These results disclose that the PbSe/Si heterostructure may be a priority candidate for optoelectronic device applications in PSDs.
2. Experimental
Different PbSe films with the thicknesses of 5,10, 15, 20, 30 and 50 nm were fabricated on the n-type Si (001) substrates with a 1.5-nm-thick native SiO2 surface layer by using the PLD technique. The target was compounded by mixing the same molar mass of high-purity Pb (99.99%) and Se (99.99%) raw powders and pressing them into a pellet. The thickness of the Si substrate was approximately 500 µm and the resistivity was about 1-10 Ω·cm. Prior to the deposition process, the Si wafers were cleaned in acetone and alcohol for 5 min in sequence and the base pressure was pumped below 5×10−5 Pa. During the preparation, Ar pressure was controlled at 0.1 Pa and the substrate temperature was keep at 300°C. A XeCl (λ=308 nm) excimer laser was used with an intensity of 1.2 J/cm2 and a pulse frequency of 1 Hz. The deposition speed was about 5 nm/min, thus different thicknesses of PbSe films could be obtained by simply controlling the deposition time. Finally, two 0.5 mm diameter indium pads were prepared onto the PbSe film with different distances to act as the electrodes. To reduce the tolerance and improve the uniformity, there were 5 samples (area within 5 mm×10 mm) for each thickness in the same batch. The difference of the LPV values were small and any one LPV value can be chosen as the result of this certain thickness.
The crystal structure, chemical state analyses, surface morphology and elementary composition of PbSe film were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS), respectively. The optical performances of the film were studied using an ultraviolet-visible spectrophotometer. The LPV characteristics were identified using the Keithley 2700 voltmeter combining with a 3-dimensional linear motor stage, meanwhile, the focused laser beam illuminated perpendicularly on the sample surface. The LPV versus time signals were recorded on a digital oscilloscope and a chopper-modulated pulse laser at different frequencies. Current-voltage (I-V) characterization curves were tested using a Keithley 4200 Sourcemeter.
3. Results and discussion
Figure 1(a) shows the XRD pattern of the PbSe films with different thicknesses on the n-type Si substrates. Only the (200) and (400) diffraction peaks of the PbSe were observed at 29.1° and 60.5°, respectively, along with the (400) diffraction peak at 69° for the Si substrate, indicating that the deposited PbSe films can be identified as the cubic PbSe phase (JCPDS card No. 06-0354) and exhibits an a-axis preferred orientation. Figure 1(b) presents the absorption spectral result of the PbSe film with a thickness of 15 nm. The absorption range from 300 nm to 1100 nm can be clearly observed in the graph. Such wide absorption range demonstrates the excellent prospects of the PbSe film for broadband photodetection applications. The band gap (Eg) of the PbSe film was calculated from the absorption spectra based on the Tauc relation [26]:
where h is known as Planck’s constant, $\mathrm{\nu }$ is the incident light frequency, $\mathrm{\alpha }$ represents the absorption coefficient, B is a constant coefficient, and n is an exponent, which has a value of 1/2 for a direct transition [27]. By deducing the linear part in the curve with incident beam energy ${(h}\mathrm{\nu }{)}$, a band gap of 1.16 eV was obtained, as plotted in the inset of Fig. 1(b). Interestingly, this value is quite different from bulk PbSe materials with a ${{E}_{g}}$ of 0.28 eV at 300K as it has been demonstrated that the optical bandgap of the PbSe materials is strongly dependent on the deposition conditions and the film thickness [28].
Fig. 1. (a) The XRD patterns of PbSe films on the Si substrate. (b) The absorption spectrum of the PbSe film on the glass with the inset is the relationship of incident beam energy (${h}\mathrm{\nu }$) with ($\mathrm{\alpha }{h\nu }$)2. (c) SEM surface morphology of PbSe film with a thickness of 15 nm. (d) EDXS spectra and (e) the Pb and Se elemental mapping results. (f) XPS spectra of Pb (4f) and Se (3d) core levels of the PbSe thin film.
The surface morphology characterization of the PbSe film was shown in Fig. 1(c). The film has a homogenous and dense spherical particle surface without cracks or holes. To determine the composition and the elemental distribution of the PbSe film, EDXS and compositional mappings of the Pb and Se elements are identified, as shown in Fig. 1(d) and (e), respectively. The weight percentages were transformed into atomic percentages by calculating the atomic weights of both the Pb and Se atoms to estimate the stoichiometric character of the PbSe film. The calculated atomic ratio of Pb:Se ≈ 0.94:1 from the Fig. 1(d) is nearly consistent with the constituent of the synthesis target. Moreover, from the selected region in Fig. 1(c), the Pb and Se elements are homogeneously distributed for compositional mappings, as presented in Fig. 1(e).
Qualitative determination of the electronic structures and chemical states of the PbSe film was performed using XPS. The double peak of the Pb (4f) and Se (3d) in the XPS spectrum shown in Fig. 1(f), suggests that the PbSe thin film is mainly composed of a PbSe stoichiometric compound. The four fitted peaks were located at binding energies of 138.3 [Pb (4f7/2)], 143.2 [Pb (4f5/2)], 137.4 (a1), and 142.3 (a2) eV for the double peak features of Pb (4f) [29]. The peaks at the binding energies of 137.4 and 142.3 eV correspond to the core levels of the Pb2+ cations associated with the formation of PbO [29]. From the peaks at 54.1 and 55 eV were the binding energies of Se 3d5/2 and Se 3d3/2 that indicate the Se2+ of purely PbSe. In general, the XPS spectra results indicate that the deposited thin film is constituted of PbSe together with very trivial traces of the PbO compound.
Figure 2(a) presents the transverse I-V characteristics of the PbSe/Si heterojunction measured under dark conditions. With the increase in the film thickness from 5 nm to 50 nm, the I-V curves show linear behaviors, demonstrating that the Ohmic relationship is formed, thus the influence of the contact between the electrodes and PbSe films can be excluded [30]. Then the longitudinal I-V properties of the PbSe/Si heterojunction were measured for a typical 15-nm-thick sample and the results are presented in Fig. 2(b). Good backward diode-like rectification characteristics are clearly seen from the longitudinal I-V curve. The heterojunction behaves an excellent photodetection capability as the current increases substantially with increasing light source intensity even to 25 mW for 1064 nm laser illumination. Figure 2(c) shows the schematic diagram of the LPE measurement in the PbSe/Si heterojunction. The laser beam was fixed and perpendicularly illuminated on the film surface. The film sample can move horizontally by using an electric control displacement station, with the voltage variation between the two electrodes is measured using a source meter. When the light illuminates the PbSe/Si heterojunction, the photons are absorbed in the PbSe, the PbSe/Si interface, and the Si substrate, which leads to the excitation of electron-hole pairs in these layers. The generated carriers at the interface of the heterojunction are then separated into the PbSe and Si layer attributable to the large built-in electric field. The corresponding energy band diagram is shown in Fig. 2(d). From the previous results, the electron affinity and the energy band gap of PbSe are ≈4.21 eV and ≈1.16 eV, respectively. The holes would diffuse from PbSe into Si substrate and electrons would move in a reverse manner, thus leading to the energy bands in the PbSe to bend downward and in the Si to bend upward, respectively. Then a barrier is created at the interface. When the heterojunction is illuminated by a laser beam, the excited holes would be swept into the PbSe film (and the electrons would be swept into the Si substrate), subsequently diffusing laterally from the illuminated position into the unilluminated regions because of the density gradient. This will result in the generation of a LPV between the two electrodes, which has a linear dependence on the laser position, as shown in Fig. 2(e). Based on carrier diffusion theory, the positional dependence of the LPV can be expressed as follows [31,32]:
Fig. 2. (a) Transverse I-V characteristics of the PbSe film with different thicknesses. The inset shows a schematic of the circuit used to perform the transverse measurements. (b) Longitudinal I-V characteristics of PbSe (15 nm)/Si heterojunction under dark conditions and various illumination powers. The inset shows a schematic of the circuit used to perform the longitudinal measurements. (c) Schematic diagram of the LPE measurement in the PbSe/Si heterojunction. (d) Energy band diagrams for the PbSe/Si heterojunction. (e) Laser position dependence LPV curve and theoretical fitting (PbSe of 15 nm under laser wavelength of 1064 nm and 5 mW). (f) Laser position dependence LPV curves of the PbSe heterojunction with different PbSe thicknesses.
Next, the LPE response of the PbSe/Si heterojunction with film thickness of 15 nm is investigated in detail. Figure 3(a) shows the LPV curves measured under the different laser powers ranging from 0.5 mW to 60 mW with a 1064 nm laser. The LPV curves show highly linear relationships over the entire power range and the LPV response increases gradually with enhancing the laser power, indicating that the heterojunction has large power response range. Then by extracting the LPV curves, the position sensitivity results are gotten and plotted in Fig. 3(b). The sensitivity increases from 36.83 mV/mm to 117 mV/mm when the power increases from 0.5 mW to 60 mW. However, the changing rate is quite different in the whole power range as the sensitivity increases initially very fast at the low powers, and then improves gradually slow at high powers; the phenomenon of which can be ascribed to the competition between the increasing numbers of photo-generated carriers and the increasing recombination probability of electron-hole [34].
Fig. 3. (a) Laser spot position dependent LPV of the 15-nm-thick PbSe film at various laser powers of 1064 nm. (b) Extracted power-dependent sensitivities and nonlinearities. (c) Laser spot position dependent LPV for various laser wavelengths under illumination at 5 mW, the inset showing the maximum LPV at electrode position under each laser wavelength. (d) Extracted position sensitivity of laser power for different lasers wavelength.
Nonlinearity is another important parameter for PSDs and is known as position-detection error. Because of the limited diffusion lengths of the carriers or the presence of inhomogeneous sample surface [35], generally, the LPV curve is not accurately linearly dependent on the laser spot position. The quantity of this nonlinearity is described by the following equation [9]:
To detect the LPE response range and the relative response strength of the PbSe/Si heterojunction, the LPV curves were measured at various wavelengths of 405 nm, 532 nm, 671 nm, 808 nm, 1064 nm, and 1550 nm under the illumination of 5 mW, as plotted in Fig. 3(c). Obviously, the LPV response increases with the increasing of laser wavelength until it reaches to 1064 nm, and reduces nearly to zero for the 1550 nm laser, which can be directly observed from the extracted maximal LPV shown in the inset to Fig. 3(c). The reason for this phenomenon can be attributed to the absorption result of the Si substrate and the transmittance results of the PbSe film, which can be seen in Fig. S1 in the Supporting Information. For the PbSe/Si heterojunction with a thin PbSe (15 nm) layer, the transmittance of PbSe film increase gradually with the wavelength increasing from 300 nm to 1200 nm, and the Si substrate has a higher absorption in the range of 500 nm to 1100 nm. Therefore, the LPV improves gradually with the laser wavelength due to a gradual increasing number of photo-generated electron-hole pairs. However, the absorption property in both the PbSe and the Si is very bad when the laser wavelength reaches 1550 nm, so that a nearly zero LPV is observed at this wavelength. The PbSe/Si heterojunction shows the highest LPV response and excellent linearity at the wavelength of 1064 nm, thus demonstrating the significant prospects of this heterostructure for use in near-infrared photodetection applications. Subsequently, the laser power dependent LPE response at the different laser wavelengths was also characterized, the results are shown in Fig. S2(a)–(d) in the Supporting Information. With the variation of the laser powers, all the LPVs under different wavelengths show well linear dependence to the laser spot position. Then the position sensitivities were extracted from Fig. S2 and plotted in Fig. 3(d). When the laser power increases from 0.5 mW to 60 mW, the position sensitivity of the LPV under different wavelengths shows a similar increasing tendency. Moreover, the position sensitivity increases with the laser wavelength from 405 nm to 1064 nm under a certain power, confirming the intrinsic wavelength dependence of the LPE in the PbSe/Si heterojunction.
To understand the dependence relationship of position sensitivity on the laser power, an expression of the position sensitivity is deduced based on the Eq. (2), as follows [38]:
For the application of PSDs, a large response size is another important target that must be considered for practical use. Thus the contact distance-dependent LPE response is also investigated in the PbSe/Si heterojunction. Figure 4(a) shows the laser position dependence of the LPVs at five different electrode distances (0.6 mm, 1.2 mm, 2 mm, 3.2 mm, and 5 mm) under the illumination of 5 mW with a 1064 nm laser. Relatively good linearity of the LPV curves can be obtained for the electrode distances below 3.2 mm. However, with the distance increasing to 5 mm, the linearity gets worse with a nonlinearity of 13.32%, but it is still below the allowable value of 15%, indicating the large working distance of this heterojunction. The contact distance-dependent nonlinearity can be understood using the diffusion model presented in Eq. (2) and (3). When the electrode distance is close to or exceeds the carrier diffusion length ($\textrm{L }\;$≥ $ {\mathrm{\lambda }_\textrm{m}}$), there is a possibility that the separated holes may return back and recombine with the electrons in the region far away from the illumination region and cannot diffuse further and be collected by the two electrodes, that may be why the nonlinearity appears as the LPV response in the central region is much less than that in the two sides for the large electrode distance of 5 mm. Moreover, the LPV response (position sensitivity) decreases considerably with increasing the electrode distance. To verify whether the LPV of different electrode distances varies to the laser powers, the LPV curves of other electrode distances of 0.6 mm, 2 mm, 3.2 mm, and 5 mm were investigated under different laser powers. The results are shown in Fig. S3(a)–(d) in the Supporting Information. The LPV curves acquired under the different laser powers show similar trends and the linearity deteriorates considerably with the distance increasing from 0.6 mm to 5 mm. Figure 4(b) shows the extracted sensitivities for the five different electrode distances under 1064 nm laser illumination. Obviously, the position sensitivities of different electrode distances have the similar increasing tendencies with the increase of laser powers, and increment rates are more rapid for the shorter electrode distances. This behavior can be explained well using Eq. (5) as the position sensitivity has an inverse relationship to the electrode distance. The highest saturated position sensitivity of 190 mV/mm can be achieved for the electrode distance of 0.6 mm, suggesting that this heterostructure will be highly important for application in visible-near IR PSDs. To further present the good performance parameters of the PbSe/Si heterojunction PSD, Table 1 summarizes the comparison of the other heterostructure PSDs based on self-powered mode. The position sensitivity and detection range of PbSe/Si heterojunction from visible to near-infrared have clearly advantages comparing with other self-powered heterojunction PSDs.
Fig. 4. (a) Laser spot position dependent LPV curves in a PbSe/Si heterojunction for various electrode distances. (b) Dependence of position sensitivities on the laser power for the different electrode distances under laser wavelength of 1064 nm.
Table 1. Key parameters comparison of the PbSe/Si heterostructure for self-powered PSDs.
The response time is another key factor used to evaluate the LPE performance. The dynamic optical response of the LPV and related relaxation time are studied in-depth in the PbSe/Si heterojunction. The heterojunction was illuminated by a continuous-wave (CW) laser modulated with a mechanical chopper and the LPV versus time (LPV-t) signal was generated and recorded using an oscilloscope. The schematic diagram of LPV-t measurement was shown in Fig. 5(a). During the measurements, the PbSe/Si heterojunction is illuminated under a 1064 nm laser of 5 mW with a constant illumination spot position (x=−0.5 mm). The chopper frequency is varied from 100 to 2000 Hz, with results as shown in Fig. 5(b). No reduction or increase in the LPV was observed over the entire frequency range (as shown in Fig. S4 in the Supporting Information), indicating that a very broad frequency response range and ultrafast optical detection capability of PbSe/Si heterojunction. The response times (i.e., the rise and fall times) extracted from Fig. 5(b), is used to evaluate the time intervals of the LPV response. From the magnified response curve of 2000 Hz shown in Fig. 5(c), the rise time of 40 µs and the fall time of 105 µs was obtained. Figure 5(d) shows the frequency-dependent response time. The rise time decreases rapidly from 0.73 ms to 85 µs and the fall time drops from 1.01 ms to 140 µs when the pulse frequency increases from 100 to 1000 Hz; these times subsequently decrease slightly from 85 µs to 40 µs and from 140 µs to 105 µs, respectively, when the frequency changes from 1000 to 2000 Hz. The slow response speeds observed at lower frequencies could result from the longer laser on or off times as the incidence of the laser spot has a specific area and it’s on or off is controlled via rotation of an optical chopper at a specified frequency. The time required for the laser spot to illuminate the PSD would be shorter at higher frequencies. This phenomenon further indicates the significant effects of the pulse width or frequency on the PSD response speed (i.e., the longer laser on or off times may be reduced the response speed). Therefore, the response times obtained at high pulse frequencies can represent the actual response speed for the PbSe/Si heterojunction.
Fig. 5. (a) The schematic diagram used to study the PSD time response. (b) Time-dependent LPV under illumination at 1064 nm at different chopper frequencies. (c) Magnified LPV curve of single response cycle at the frequency of 2000 Hz. (d) Extracted response times at different frequencies. (e) LPV-t characteristics under illumination at different laser wavelengths (frequency of 2000 Hz at a power of 5 mW). (f) Extracted response times for each laser wavelength.
Additionally, to determine whether the response time of this heterojunction is dependent on the laser wavelength, the LPV-t response was also determined at different laser wavelengths under the frequency of 2000 Hz with the laser power of 5 mW, as plotted in Fig. 5(e). The transient LPV still keeps stable at the laser on and off stages for each wavelength and increases gradually along with the wavelength extending from 405 nm to 1064 nm; this behavior agrees well with the corresponding steady LPV results shown in Fig. 4(c). Figure 5(f) illustrates the extracted wavelength-dependent response times. Both the rise and fall time are nearly independent of the illuminated wavelength, demonstrating the minimal effect of the wavelength on the response speed of the PbSe/Si heterojunction.
4. Conclusion
In conclusion, we have prepared a series of high-quality PbSe/n-Si heterojunction using the PLD technique and investigated their LPE responses characteristics. The heterojunctions exhibit broadband LPE responses ranging from at least 300 to 1100 nm and can operate with a self-powered mode. The results show that the LPV in the PbSe/n-Si heterostructure has a very wide response range of visible to near-infrared and exhibits a strong dependence on the thickness of the PbSe film, with the maximum position sensitivity reaching up to 190 mV/mm and nonlinearity less than 4% for the 15-nm-thick device at 1064 nm. Besides, the linear working distance is as large as 5 mm in this heterostructure, and the LPV response decreases gradually with increasing distance, which is consistent with the theoretical results. At last, by modulating the chopper frequency, an ultrafast optical response speed of the LPV is obtained with the rising and falling time of 40 µs/105 µs. This work suggests that the PbSe/n-Si heterojunction is a promising candidate for optoelectronic device applications in PSDs.
Funding
Post-graduate’s Innovation Fund Project of Hebei Province (CXZZBS2021013); Postdoctoral Science Foundation of Hebei Province (B2020003018); China Postdoctoral Science Foundation (2020M670679); Natural Science Foundation of Educational Department of Hebei Province (QN2020156); Natural Science Foundation of Hebei Province (216Z1703G, A2017201104, F2018201198); National Natural Science Foundation of China (11604073, 51972094, 62175058, U20A20166).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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