We propose and numerically demonstrate a high absorption hybrid-plasmonic-based metal semiconductor metal photodetector (MSM-PD) comprising metal nanogratings, a subwavelength slit and amorphous silicon or germanium embedded metal nanoparticles (NPs). Simulation results show that by optimizing the metal nanograting parameters, the subwavelength slit and the embedded metal NPs, a 1.3 order of magnitude increase in electric field is attained, leading to 28-fold absorption enhancement, in comparison with conventional MSM-PD structures. This is 3.5 times better than the absorption of surface plasmon polariton (SPP) based MSM-PD structures employing metal nanogratings and a subwavelength slit. This absorption enhancement is due to the ability of the embedded metal NPs to enhance their optical absorption and scattering properties through light-stimulated resonance aided by the conduction electrons of the NPs.
©2013 Optical Society of America
The interest in designing and developing new nanoscale high-speed photodetectors has greatly increased in past few decade for application in integrated electronics-photonics systems. Metal-semiconductor-metal photodetectors (MSM-PDs) have emerged as a promising candidate due to their high speed (ultra-low intrinsic capacitance), high spectral bandwidth, easy fabrication process, and high bandwidth-responsivity product, compared to their conventional PIN photodetector counterparts [1,2].
The ultralow capacitance of an MSM-PD enables them to attain a response time of few tens of picoseconds, and this can be further reduced by decreasing the finger spacing down to the optical diffraction limit [3,4]. However, downsizing the finger spacing decreases the active area of the MSM-PD, thus lowering its sensitivity. During the past decade, subwavelength slit arrays exhibiting extraordinary light transmission have inspired the development of high-sensitivity MSM-PD structures of small finger spacing [5–7]. Plasmonic metal nanograting nanostructures have also been implemented into MSM-PD structures to act as microlenses that focus and confine the light into the subwavelength slits, thus significantly reducing the metal-induced reflection losses [8,9]. Detailed simulations and studies on surface plasmon polariton (SPP) propagation in MSM-PD structures has been carried out demonstrating very substantial improvement in light transmission and absorption [10–12]. Recently, optimized MSM-PD structures have been successful fabricated, demonstrating a few fold absorption enhancements at near infrared region [9,13,14]. However the light absorption of such SPP-based MSM-PD structures is significant only at the metal-substrate interface in the vicinity of the subwavelength slit (only around few hundred nanometers), limiting the overall responsivity of the MSM-PDs. The use of double plasmonic metal nanogratings on both the top and the bottom of the metal contacts (as shown in Fig. 1(b)) has recently been proposed to further improve the absorption of the plasmonic MSM-PD through triggering the SPP mode of the bottom metal nanograting, thus increasing the absorption area of the semiconductor substrate . Unfortunately, due to the high refractive index of the semiconductor substrate, the period of the bottom metal nanograting must be extremely small, making it impractical to fabricate using conventional fabrication processes. It is also hard to accurately align the top and bottom metal nanogratings etched onto the metal contacts with the subwavelength slit.
Motivated by the above-mentioned limitations, we propose a practical hybrid plasmonic MSM-PD structure incorporating plasmonic metal gratings in conjunction with localized surface plasmon resonance (LSPR) through embedded metal nanoparticles (NPs). Simulation results using Finite Difference Time Domain (FDTD) software demonstrate that this hybrid plasmonic MSM-PD structure yields 28-fold and 3.5-fold enhancement in absorption compared to conventional non-plasmonic and plasmonic MSM-PD structures, respectively. This absorption enhancement is mainly due to the light-stimulated resonance induced by the conduction electrons of the embedded NPs. This hybrid plasmonic MSM-PD structure can easily be fabricated using conventional cleanroom fabrication processes. Rigorous Coupled Wave Analysis (RCWA) was also adopted to design and verify that the NP-induced LSPR peak is in the mid infrared region.
2. Hybrid plasmonic MSM-PD structure
2.1 Plasmonic metal nanograting and subwavelength slit
Figure 1 shows schematic diagrams of Fig. 1(a) a conventional plasmonic MSM-PD structure using a single metal nanograting, Fig. 1(b) a plasmonic MSM-PD structure comprising double metal nanogratings, and Fig. 1(c) the proposed hybrid plasmonic MSM-PD structure employing a single metal nanograting in conjunction with embedded metal NPs.
The hybrid plasmonic MSM-PD structure shown in Fig. 1(c) can be divided into two separated substructures, namely (i) the top substructure, which comprises the top metal nanograting and the subwavelength slit and (ii) the bottom substructure which includes the metal NPs, embedded material and substrate. The top metal grating consists of a perfect conductor of parallel grooves (along the x direction). For a metal grating period of, the wave vector of the SPPs is given by 10]. While a substantial absorption enhancement was achieved with the top substructure alone, this enhancement was observed within a small substrate area of few hundred nanometer depth below subwavelength slit. The proposed hybrid plasmonic MSM-PD structure addresses the latter limitation through the incorporation of the bottom substructure, which further expands the light absorption area (hence the responsivity) of the MSM-PD structure.
2.2 Embedded metal nanoparticles
The absorption area of a plasmonic MSM-PD structure can be increased through the use of metal NPs embedded onto the semiconductor substrate, as illustrated in Fig. 1(c). It is well known that the conduction electrons of such metal NPs induce light-stimulated resonance that enhances the optical absorption. This phenomenon is known as localized surface plasmon resonance (LSPR), because it occurs within the NPs . However this LSPR is sensitive to the size, type, shape and refractive index of the metal NPs . Adopting the conventional dipole model, the absorption and scattering cross-sections of a nanoparticles, having a diameter much smaller than the wavelength, of the incident light, can be expressed as follows [18,19]:20]. Au NPs and Ag NPs embedded onto Ge have LSPR peak at 990 nm and 920 nm, respectively, leading to stronger absorption at 850 nm through the light-stimulated resonance induced by the conduction electrons within the NPs. Detailed simulation of the proposed hybrid plasmonic MSM-PD structure embedding metal NPs is discussed in Section 4.
3. Simulation setup
3.1 FDTD simulation
The 2D hybrid plasmonic MSM-PD structure shown in Fig. 1(c) was simulated using the Opti-FDTD software package developed by Optiwave. In the design the refractive indices of Au and Ag, , were obtained from the Lorentz-Drude model , while those of the GaAs and Ge, , were assumed to be 3.66 + 6.28e−2i and 4.653 + 0.298i, respectively, as reported in . For the FDTD simulation, we used a grid step size and a time step . This high-resolution sampling yielded convergent solutions at reasonable computation times. A periodic boundary condition was assumed along the x-direction for an incident light wave propagated along the normal direction. Anisotropic perfectly matched layer (APML) boundary condition was assumed along the z-direction to accurately simulate the absorption of the light reflected from the bottom as well as light transmitted from the top boundaries of the simulated hybrid plasmonic MSM-PD structure.
Based on the simulation setup described above, parameters such as metal grating period (Λ), thickness () and duty cycle () were optimized for maximizing the absorption enhancement, as will be discussed in Section 4. Also, the subwavelength slit thickness (L) and width (SW) adopted throughout the FDTD simulation were taken from published high-impact journals [7,9,11]. The embedded NP parameters that were varied in the simulations were the height (), diameter () and spacing between the NPs (). The default simulation parameters used for the optimization of the hybrid plasmonic MSM-PD structure are shown in Table 1. The absorption enhancement factor is defined as the ratio of the normalized power absorbed in the active area (200 nm below the metal contact) to the normalized power absorbed in a similar active area of a conventional MSM-PD structure without a metal nanograting or metal NPs. The normalized power absorbed in the active area is calculated by subtracting the total power at 200 nm below the metal contact from the total power at the interface between the subwavelength slit and the embedded material.
3.2 RCWA simulation
As discussed in Section 2.2, LSPR is sensitive to the size, type, shape and refractive index of the metal NPs. In order to determine and verify that germanium (Ge) or amorphous silicon (a-Si) embedded gold (Au) or silver (Ag) NPs induce an LSPR peak in the mid infrared region, a two-dimensional RCWA method was used to evaluate the LSPR induced by different metal NPs for different embedded materials . For simplicity, the NPs were assumed to be hemispheroid and their cross sections were assumed to be semicircles. In the RCWA modeling, we considered two identical Au or Ag nanoparticles of 120 nm and 160 nm respectively, separated by a distance of 50 nm and embedded into four different materials namely, air, silicon dioxide, Ge and also a-Si . Au and Ag NPs have LSPR peaks at 560 nm and 470 nm, respectively, when the NP host material is air (refractive index () = 1). The LSPR peak was red shifted when the refractive index of the NP host material increased. The refractive indices () of the Au and Ag NPs embedded into Ge and a-Si were assumed to be 4.653 + 0.298i and 3.673 + 0.005i, respectively, as reported in Ref. .
4. Results and discussions
4.1 Plasmonic grating and subwavelength slit
The accuracy of the FDTD simulations was first verified by evaluating the absorption enhancement factor versus the subwavelength slit thickness, L, shown in Fig. 2, and comparing it with the reported simulation results. The simulation results shown in Fig. 2 are in excellent agreement with the results published in  and our previous publication . These results confirm that the maximum absorption enhancement occurs when the relation is satisfied, where m is the order of the resonance, and are the reflection phases of the top and bottom of the subwavelength slit, respectively, and is the propagation wave vector along the z-direction. The results displayed in Fig. 2 show a sinusoidal relationship between the subwavelength slit thickness and the absorption enhancement factor. It is obvious that by decreasing the subwavelength slit thickness the light intensity coupled from the metal nanograting to the subwavelength slit increases.
Figure 3 shows the absorption enhancement spectrum for different subwavelength slit widths. As the subwavelength slit width decreases, the absorption enhancement increases. This is because reducing the subwavelength slit width increases the effective refractive index experienced by the incident light wave, thus resulting in a larger amount of the electric field being guided by the metal nanograting and coupled into the slit . Note also that decreasing the subwavelength slit width also increases the speed of the MSM-PD.
Optimizing the pitch, , of the first metal nanograting is crucial for maximizing the absorption of the hybrid plasmonic MSM-PD structure because affects the phase between the SSP and incident light wave, and thus the overall power transmitted through the slit. Figure 4 shows the absorption enhancement spectrum for different values of . It is obvious that maximum absorption enhancement is obtained when is around 600 nm and that for a given wavelength, constructive or destructive interference may occur, depending on the value of , which affects the phase between the SSP and the incident light waves [25,26].
The absorption enhancement spectra for different metal nanograting heights and duty cycles are shown in Fig. 5(a) and Fig. 5(b), respectively. It is obvious that the maximum absorption enhancement is attained for a metal nanograting height and a duty cycle of around 100 nm and 60%, respectively. Figure 5(a) show that metal grating height is greatly affect the absorption enhancement phase as due to change on the reflection phases of the top and bottom of the total subwavelength slit.
4.2 Embedded metal nanoparticles
As discussed earlier, in plasmonic MSM-PD structure the intensity of the light transmitted through the subwavelength slit is typically concentrated in a small area below the top metal grating. Embedded metal NPs into a semiconductor layer below the top metal nanograting increase the light absorption through the LSPR phenomenon that was discussed earlier. Selection of the NP metal type, size and shape as well as the host material is crucial for enhancing the LSPR-induced absorption enhancement. Figure 6(a) and Fig. 6(b) show the simulated transmission spectra of the hybrid plasmonic MSM-PD structure shown in Fig. 1(c) for Ag and Au NPs, respectively, embedded into different host materials, namely air, silicon dioxide (SiO2), germanium (Ge) and amorphous silicon (a-Si). These simulation results were obtained using Rsoft (RCWA simulation) software [22,27]. The simulation results for NPs embedded in air agree very well with the experimental result reported in [28,29]. It is worthwhile to notice from Fig. 6 that LSPR peak is red shifted as the refractive index of the host material increases. Figure 6 shows also that the materials a-Si and Ge are well suitable for hosting the Au NPs and Ag NPs because the LSPR peak is shifted to around 900 nm, thus enhancing the absorption of the substrate through the 850 nm light-stimulated resonance induced by the conduction electrons within the NPs. For all the FDTD simulation, Ge is used to embedded the Au and Ag NPs and also host material for all the conventional MSM-PD structure.
The absorption enhancement spectra for different NP spacing () are shown in Fig. 7(a) and Fig. 7(b), for Au NPs and Ag NPs, respectively, with the NPs size () being 120 nm. It is noticed that the optimum Au and Ag NP spacings that maximize the absorption enhancement are 50 nm and 20 nm, respectively. However, it is noticed from Fig. 7 that the impact of on the absorption enhancement is not as significant as the impact of the nanoparticles diameter on the absorption enhancement. This is due to the light-stimulated resonance induced by the metal NPs which only occurs at the interface between the metal NPs embedded within the host material and the substrate, thus the spacing between the NPs has a smaller impact on the absorption enhancement in comparison to the impact of the NPs size.
The impact of the Au and Ag NP diameter () on the absorption enhancement is shown, respectively, in Fig. 8(a) and Fig. 8(b). It is noticed that the NP diameters have significant impact on the absorption enhancement of hybrid plasmonic MSM-PDs, and that the absorption enhancement increases as the NP diameter increases. This is attributed to the increase in the overall surface of the NPs interfacing the host medium. The simulation results are in excellent agreement with theoretical calculations using Eq. (1). As the volume of the NPs increases light scattering rather than absorption becomes dominant, enhancing the absorption of the substrate since the light path length increases, leading to strong absorption around the NPs. It is important to note, however, that increasing the NP diameter also widens the absorption enhancement spectrum due to multimode light-stimulated resonance induced by the conduction electrons within the NPs. This spectral bandwidth broadening reaches a saturation point that leads to the appearance of two resonance peaks as shown in Fig. 8. For the simulation results shown in Fig. 8, the optimum Au and Ag NP diameters that maximize the absorption enhancement are 120 nm and 160 nm, respectively.
The absorption enhancement factors of the optimized hybrid plasmonic MSM-PD structure and the conventional plasmonic MSM-PD structure normalized to the absorption enhancement of the conventional MSM-PD structure (i.e., without metal gratings or metal NPs), shown in Fig. 1, are displayed in Fig. 9. A 28-fold absorption enhancement is achieved with the hybrid plasmonic MSM-PD compared to the conventional MSM-PD structure and 3.5-fold compared to the plasmonic MSM-PD structure without NPs. The latter absorption enhancement is mainly due to the ability of the metal NPs to enhance the optical absorption and scattering through light-stimulated resonance induced by the conduction electrons within the NPs. It is also noticed that the hybrid plasmonic MSM-PD with embedded Ag NPs has higher absorption enhancement compared to that with embedded Au NPs. This is because Au NPs have higher light absorption and lower scattering than those of Ag NPs.
Figure 10(a) and Fig. 10(b) show the simulated electric field distributions for the conventional plasmonic MSM-PD and the hybrid plasmonic MSM-PD structures, respectively. It is important to notice from Fig. 10(a) the formation of an SPP wave along the metal grating and the penetration of the electric field within the GaAs substrate of the plasmonic MSM-PD. However, this SPP wave is only concentrated at the exit of the subwavelength slit and exponentially decreases as it propagates away from the subwavelength slit. On the other hand, Fig. 10(b) reveals that the light-stimulated resonance (red highlighted area) within the NPs extends the penetration of the electric field within substrate of the hybrid plasmonic MSM-PD. The absorption within the substrate and the NP host material mainly increases because of the back-scattering characteristics of the metal NPs and the long propagation path experienced by the incident light within the substrate.
The impact of the metal NPs on the light absorption enhancement for the hybrid plasmonic MSM-PD can be clearly evaluated by investigating the amplitudes of the electric field, Ez, along the z-direction and the magnetic field, Hy, along the y-direction at the interface between the subwavelength slit and the material hosting the embedded NPs as well as at the interface between the metal NPs and GaAs substrate. Figure 11(a) and Fig. 11(b) show the magnitude of Hy at the interface between subwavelength slit and the embedded material and at the interface between the metal NPs and GaAs substrate, respectively. Figure 11(c) and Fig. 11(d) show the magnitude of Ez at the interface between subwavelength slit and the embedded material and at the interface between the metal NPs and GaAs substrate, respectively, for the conventional, plasmonic and hybrid plasmonic MSM-PD structures. For the results shown in Fig. 11, optimum MSM-PD parameters were used.
Figure 11(c) and Fig. 11(d) reveal more than 1.3 times improvement in Ez for the hybrid plasmonic MSM-PD in comparison with the conventional MSM-PD, due to light penetration within the substrate through light-stimulated resonance induced by the metal NPs. Figure 11(b) and Fig. 11(d) reveal that, at the interface between the metal NPs and GaAs substrate, Ez and Hy exhibit random spikes. These are due to light-stimulated resonance discussed earlier. The high amplitudes of random spikes result in absorption enhancement at the interface between the metal NPs and the GaAs substrate. It is also worth noticing that, at the interface between the subwavelength slit and the embedded material as shown in Fig. 11(a) and Fig. 11(c), the amplitudes Ez and Hy also increase with the presence of the metal NPs. This is attributed to the back scattering induced by the metal NPs, which also increases the absorption at the substrate between the interface of subwavelength slit and the NP host material.
We have proposed and investigated the performance of a new high absorption hybrid plasmonic metal semiconductor metal photodetector (MSM-PD) employing metal nanogratings, a subwavelength slit and embedded Au and Ag nanoparticles (NPs). FDTD simulations have been carried out to optimize the different parameters of the proposed structure, including the subwavelength slit width and depth, the metal nanograting dimensions, the metal NP size and spacing, for maximizing the optical absorption of the hybrid plasmonic MSM-PD. The simulation results have been validated through excellent agreement with results published in high-impact journals. Simulation results have shown that Ge and Si are suitable host materials for embedding Au and Ag NPs where localized surface plasmon resonance (LSPR) induced by the conduction electrons of the embedded metal NPs shifts the absorption peak to around 900 nm, thus enhancing the substrate absorption beyond 850 nm. The optimization of the parameters of the proposed hybrid plasmonic MSM-PD structure have revealed that 28-fold and 3.5-fold absorption enhancement can be attained in comparison to conventional and plasmonic MSM-PD structures, respectively, and that this absorption enhancement is mainly due to the ability of the embedded metal NPs to enhance their optical absorption and scattering properties through the light-stimulated resonance induced by the conduction electrons of the NPs.
This work was partly supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2011-0017606), by the World Class University (WCU) program at GIST through a grant provided by MEST of Korea (R31-20008-000-10026-0), and by the Gwangju Institute of Science & Technology in 2012 “Systems biology infrastructure establishment grant”.
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