A MSM Visible-near-Infrared Wavelength Photodetector Based on PbS Quantum Dots/Porous Silicon Hybrid Structure

1. Introduction

As a silicon-based photodetector (PD), one can envision its role for operation from visible light to short-wavelength infrared range. Si-based Photodetectors, such as Complementary Metal Oxide Semiconductor (CMOS), have played a critical role in the daily life due to their advantages of low cost, high stability, high response speed, and high adaptability [1]. As the response wavelength increase from visible to near infrared range, the light collection efficiency becomes lower. To increase the absorption of near-infrared wavelength range and extend the light absorption in silicon for wavelengths longer than 1100nm, several approaches have been performed, such as metalens and microlens, which could offer good performance [2,3,4,5]. Recently, the porous silicon (PSi) template has attracted much attention due to its unique internal photoemission and filling property [6]. PSi is a mature material with low cost fabrication and its application has been expanded in recent years, such as GeSi template [7], biomarkers [8,9], solar cells [10,11] and light-emitting diodes [12,13]. PbS quantum dots (QDs) with ~3 nm diameter has a significant quantum effect, and its emission spectrum can cover the near-infrared band range. In addition, the PbS QDs has a good photoelectrical performance with high color purity, good stability and easy mass production [14], thus they have been utilized for biological and other detection [15]. Gaur et al. have investigated highly sensitive dual-mode labeled detection of biotin using reflection interference and fluorescence generated by conjugations of QDs-biotin and streptavidin probes immobilized on the PSi pore walls [16]. Bashkany et al. used the microwave-assisted chemical bath deposition (MA-CBD) method to deposit PbS on the PSi prepared by photoelectric chemical etching [17]. Nayef et al. prepared Al/TiO2 nanoparticles/PSi structure ultraviolet (UV) photodetector by coating TiO2 nanoparticles (NPs) on PSi [18]. Ismail et al. prepared photodetectors with responses at 570nm and 700nm by infiltrating CdS NPs into PSi [19]. However, the above methods for preparing NPs/PSi structures require complicated fabrication and the electrical properties of the structures in the visible-near-infrared (Vis-NIR) wavelength are not well studied. In this paper, PbS QDs with an average particle size of ~3nm and an emission wavelength of 950nm were employed to form a PbS QDs/PSi template by a drop casting method compounded with the PSi obtained by electrochemical etching. Furthermore, A metal-semiconductor-metal (MSM) device is fabricated and its response spectrum could extend to the 1200 nm at -3 V. As a silicon-based photodetector (PD), one can envision its role for operation from visible light to short-wavelength infrared range.

2. Materials and Methods

Crystal silicon(c-Si) is purchased from Suzhou Yancai Micro-nano Technology. The chemical used in the experiment is analytically pure, which is purchased from Sinopharm Chemical. PbS QDs and toluene solvents are purchased from Xingshuo Nano.

PSi is prepared using electrochemical etching of (100) oriented boron doped p-type silicon wafer (p-Si) of thickness and resistivity 400 μm and (1-10) $\mathsf{\Omega }·\mathrm{cm}$ respectively. Prior to etching, the rectangular Si of $2\times 3{\mathrm{cm}}^2$ is cleaned by RCA process followed by soaking in dilute hydrofluoric acid (HF, >40%) to remove the native oxide and rinsing with de-ionized (DI) water. The electrolyte was a mixture of HF and ethyl alcohol (EtOH, >99.9%) with a volume ratio of 1: 2. We used a self-built electrochemical etching equipment with a Teflon tank that can be manually raised and lowered in height. The square area of $2\times 2{\mathrm{cm}}^2$ was soaked in electrolyte as anode and platinum piece as cathode. The morphology of PSi was controlled by changing current density and time, as shown in Table 1. The constant current source was provided by the Keithley 2400 source meter controlled by Lab-view 2006. Finally, PSi was rinsed in EtOH with low surface tension for 5min and dried with nitrogen to protect the integrity of it.

Drop casting technique was employed to incorporating the PSi samples with PbS QDs [20,21]. Dropped 200 μl of 0.3 mg/ml PbS QDs dissolved in toluene on PSi using a pipette. The sample was placed at room temperature for 30 min until the toluene evaporated completely and QDs were physically coasted on the porous surface of PSi [22].

A 50 nm Al layer was evaporated at a rate of 1 A/s by electron beam evaporation system (FU-12PEB) on the S22 (10 mA/cm²-20 min) covered with an interdigitated shadow mask of dimension $6\times 6{\mathrm{cm}}^2$ to form an Al/PbS QDs/PSi MSM configuration [23], as shown in Figure 1.

The surface morphology of the prepared samples was completed by ultra-high resolution cold field emission scanning electron microscopy (HR-SEM, Regular 8100) and transmission electron microscopy (TEM, JEOL-2100 plus), operating voltages are 10 kV and 200 kV. TEM samples were prepared using a focused ion beam (FIB) dual beam system (Helios Nanolab 600i) with an operating voltage of 0.1-30kV. To explored the photoluminescence (PL) characteristics of the sample, a PL spectrometer (RMP BLUE) equipped with 226 nm and 532 nm lasers were employed. Ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrum (Agilent, Cary 5000) was used to characterize the refractive index of the sample. I-V spectra of dark and illumination were measured by Keithley 4200 source meter, in which the test light source in illumination was provided by 905 nm laser (effective output power of 15 mW). Response spectra was achieved with a Fourier transform infrared (FT-IR) spectrometer (Bruker GmbH, Vertex-70v).

3. Results and discussion

Figure 2 shows the SEM cross-sectional image of PSi with and without PbS QDs, respectively. Dense, uniform and vertically sponge-like nano-structures with ~700 nm thickness formed on Si substrates are shown in Figure 2(a), which is mainly caused by the <100> crystal orientation of crystalline silicon [24]. QDs can be easily infiltrated into pores due to the hydrophobicity of the PSi structure and the weak surface tension of toluene. PSb QDs with a diameter of 3 nm can freely pass through the high-porosity layer with a pore size at least three times its size (9 nm) until be blocked by the low-porosity layer with the pore size reduced below 3 nm [25]. Figure 2(b) shows that a part of the PbS QDs remain on the surface of PSi, forming a layer of QDs with a relatively uniform thickness (~170 nm). This may be due to the small pores size of PSi that make some clusters gather on its surface. The 768nm PSi layer increases by 69nm compared to a single PSi, which is due to the less controlled electrochemical anode etching resulting in uneven etching. The supple skeleton of the PSi layer reduced the stress that was induced in the evaporation phase and limited the formation of cracks in the PbS layers [26].

To determine the distribution of PbS QDs in the hybrid structure, energy dispersive spectroscopy (EDS) analysis of the cross-section is carried out, as shown in Figure 3. It can be clearly seen that there are Pb and S atoms in the PSi, which means that PbS QDs are successfully infiltrated into PSi. Meanwhile, the thin PbS layer on PSi surface indicates that a uniform surface layer of QDs is formed.

After that, TEM is employed to explore the microstructure of PbS QDs/PSi hybrid structure in detail. Figure 4(a) is a cross-sectional view of QDs/PSi, which shows that the thickness of PSi is ~847 nm and the PbS QDs on the surface is ~180 nm, which is consistent with the SEM results. Platinum (Pt) film can be found on the surface of the PbS QDs/PSi structure because Ga⁺ in FIB has a certain beam width rather than a geometric straight line, which will damage the upper surface when cutting the sample. The high hardness properties of Pt can protect the upper surface. A 100 nm transition region is found at the interface of PbS and PSi. To explore the microscopic morphology of this region, a high-magnification TEM as shown in Figure 4(b) is performed. A layer of black spots is scattered near the interface of PbS QDs and PSi, which may be PbS QDs infiltrated into the pores of PSi.

To explore the element distribution in the PbS QDs/PSi structure, dark field EDS of the hybrid region of PbS QDs and PSi is carried out. According to Figure 5, it can be concluded that PbS has partially penetrated into PSi, which is attributed to the brighter appearance of heavy particles (Pb) in the dark field mode. Through EDS analysis, it can demonstrate that a part of PbS is coated on the surface of PSi, and the other part infiltrates into the pores of PSi, which is consistent with our previous conjecture.

Optical properties of the PSi and PbS QDs studied through UV-Vis-NIR and PL spectra are recorded. Figure 6 compares the absolute UV-Vis-NIR (300-800 nm) reflectance spectra of c-Si and PSi. From the figure, it can be observed that c-Si exhibits 35-45% reflectivity, while PSi decreases reflectivity to 15-25%. These results indicates that absorption of PSi in the absolute spectral range is about 2 times higher than that of c-Si, which results from the porous structure with rough surface [27]. In addition, because the surface of PSi has more defects than that of c-Si, trap levels are formed at the band edge, which is easy for electron transmission and conducive to allowing more photons to input [28]. Figure 6(b) shows PL and Vis-NIR absorption spectra of PbS QDs, it can be seen that PbS QDs show onset and offset absorption band edge at ~950 nm and ~700 nm respectively in Vis-NIR region. As shown in Figure 6(b), PbS QDs has a strong PL peak appears in ~950 nm corresponding to absorption spectra, thereby it can be used as one of the candidates for near-infrared response of multi-wavelength detectors.

Comparative experiments under different etching time and current density are set up to explore the effect of pore on PL of PSi. Figure 7 is the physical and schematic diagram taken during the PL test of S22 (10 mA/cm²-20 min) under the 266 nm laser. It can be clearly seen that the obtained PSi appears red under the laser. Figure 8(a) compares the PL of PSi under the etching time of 10 min, 20 min and 40 min respectively. Finally, an emission peak is appeared at ~700 nm, and PL shifts blue with the increase of etching time. Since the etching time mainly affects the depth of PSi, it can be considered that PL shifts blue with the increase of depth. The structure of the nano-PSi layer is actually a stack of nano-Si grains. The size of nano-grains gradually decreases with the increase of etching time, which makes the distance between them gradually increase and eventually manifested as an increase in depth. The PL of PSi is mainly due to the nano-Si grains in it. Therefore, according to the quantum confinement effect, the increase of etching time will cause the band gap widening and PL blue shifting [29]. Figure 8(b) shows the PL of PSi at different current densities. We can find that the PL of PSi undergoes a blue shift as the current density increases. The current density mainly affects the pore size of PSi, so we can determine that PSi has a blue shift with the increase of pore size. With the increase of current density, the aggravation of etching results in a decrease in the size of Si nanoparticles and an increase in the distance between them, which leads to the PL blue shift of PSi [30]. The change of current density on pores is mainly manifested in pore size, which is different from that of etching time which mainly affects pore depth. In general, PL peak of PSi will have a blue shift with the increase of pore size and depth. From Figure 8(a) and (b), a PL emission peak of PSi is found at ~700 nm, which is not available in c-Si. It proves that PSi has changed from an indirect bandgap to a direct bandgap compared with c-Si [31]. This is attribute to the quantum confinement effect of the nano-Si grains inside the porous silicon and the Si-H dangling bonds attached to the surface of the pores resulting in defect energy levels at the edge of the Si bandgap and widening the bandgap. The PL of PbS QDs/PSi is shown in Figure 8(c), it exhibits two emission peaks at ~650 nm and ~925 nm corresponding to PSi and PbS QDs respectively, which proves that QDs successfully recombine with PSi to form a heterojunction [32,33]. The pore size can be controlled by adjusting the experimental conditions to adjust the PL emission band, and compound with the PL band of QDs [34].

To analyse electrical properties of the prepared Al/PbS QDs/PSi MSM configuration, current-voltage (I-V) characteristics under dark and 905nm laser in the applied bias range of -5 V to 5 V and responsivity characteristics in the applied bias of -3 V are recorded for MSM electrode configuration. As can be seen from Figure 9(a), I-V spectra shows almost symmetric enhancement of rectification in both forward and reverse bias condition with a photocurrent to dark-current ratio of 3.07 and 1.10 at ±3 V respectively. There is no zero-point shift phenomenon of self-powered detector which is in agreement with the symmetry of MSM device, and indicates that etching of PSi is relatively uniform [35]. The dark current of Al/QDs/PSi configuration is ~5×10-7 A, which shows that it has a weak dark current. It generates a current of ~1×10-6 A under the irradiation of 905 nm laser, and the symmetry of its I-V curve remains good, which further shows that the device has excellent stability under illumination. The small particle size characteristics of PbS QDs also contribute to the photocurrent, due to the fact that for larger clusters, the band arrangement at the PbS QDs/PSi interface is not conducive to carrier transfer [36]. The response spectra of MSM configuration is shown in Figure 9(b), it can be observed that there is a certain optical response at 850 nm and 1150 nm, which may correspond to a combination of PSi, PbS QDs, and c-Si substrates [37]. In brief, Vis-NIR wavelength detectors of PSi have been successfully prepared, while its corresponding light intensity is weak, which may be attribute to the low uniformity of PSb QDs infiltrated into PSi and the oxidation layer between the Al electrode and the PSi [38,39].

4. Conclusions

In conclusion, we have successfully fabricated the PbS/PSi hybrid structure and thus a MSM device for Vis-NIR detector is demonstrated. The experimental results indicate that part of the PbS QDs were infiltrated into PSi to a depth of ~100 nm, while other parts were clustered on the surface. The response spectra shows that there are two response peaks at~850 nm and~1150 nm in the visible and near-infrared region. This work could provide a valuable reference for the future research of PSi application in the field of infrared detection.