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Abstract
Au-hole array and Au-GeSn grating structures were designed and incorporated in GeSn metal-semiconductor-metal (MSM) photodetectors for enhanced photo detection at 2 µm. Both plasmonic structures are beneficial for effective optical confinement near the surface due to surface plasmon resonance (SPR), contributing to an enhanced responsivity. The responsivity enhancement for Au hole-array structure is insensitive to the polarization direction, while the enhancement for Au-GeSn grating structure depends on the polarization direction. The responsivity for GeSn photodetector with Au hole-array structure has ∼50% reinforcement compared with reference photodetector. On the other hand, Au-GeSn grating structure benefits a 3× enhanced responsivity of 0.455 A/W at 1.5V under TM-polarized illumination. The achieved responsivity is among the highest values for GeSn photodetectors operating at 2 µm. The plasmonic GeSn photodetectors in this work offer an alternative solution for high-efficiency photo detection, manifesting their great potentials as candidates for 2 µm optical communication and other emerging applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The exponentially increasing capacity demand due to the emerging applications of cloud computing, 5G and big data brings grand challenges for conventional single mode fibers [1–3]. Owing to the development of low-loss hollow-core photonic bandgap fibers and thulium doped fiber amplifiers [4–7], a novel communication band at 2 µm has been proposed as a promising solution for the forthcoming capacity crisis. To enable the practical application of 2 µm communication band, photodetectors operating at such wavelength are indispensable. Si-based GeSn photodetector is a promising candidate for optical transceiver operating at 2 µm due to its CMOS compatibility [8–16]. However, GeSn photodetectors suffer from relatively low responsivity at 2 µm compared with that of III-V photodetector. To enhance the optical absorption of GeSn photodetector, myriads of approaches have been proposed. Waveguide photodetectors are able to simultaneously realize high speed and high efficiency by decoupling varying routes for photon absorption and carrier collection [17,18]. Photon-trapping photodetectors offer another solution by turning the normal incident light into lateral propagation in photodetectors through well-designed hole-array structures [19–21]. Resonant cavity enhanced photodetectors can achieve high external quantum efficiency at certain wavelengths by elongating the effective absorption length through resonance cavity structure [22,23].
Surface plasmon polariton (SPP) is able to localize the optical field at metal/dielectric interface and has been widely applied in solar cells and molecular sensors [24–26]. The spatial overlap between the penetration depth of SPR and the absorption region of photodetector is beneficial for effective absorption and provides another approach to enhance the detection performance. Recently, Au disk structure, Au hole-array and U-shaped Au slots structures have been proved effective for reinforced absorption in photodetectors operating at middle infrared range [27–30]. In addition, Au-Ge grating structure was applied in Ge MSM photodetectors and 1.4× enhancement in optical response was achieved at 1,550 nm [31].
In this work, plasmonic structures were incorporated in GeSn photodetectors for the first time. The influence of Au hole-array and Au-GeSn grating structures on GeSn MSM photodetectors was investigated and compared. The GeSn MSM photodetector with Au-hole array structure shows a polarization-independent enhancement in optical response. For the photodetector with a hole periodicity of 560 nm, a ∼50% increased responsivity of 0.0787 A/W at 1V was achieved compared with the reference photodetector. Although the SPR provides significant optical confinement in the Au hole-array structure, the weak electric field perpendicular to the direction of diffusion process hampers the effective collection of photon-generated carriers. Regarding GeSn MSM photodetectors with Au-GeSn grating structure, a 3× improved responsivity under TM-polarized illumination was achieved compared the responsivity under TE-polarized illumination. The highest responsivity achieved on the GeSn photodetector with a grating periodicity of 1.4 µm and a width of 0.7 µm is 0.455 A/W at 1.5 V, demonstrating its high performance in photo detection at 2 µm. The polarization-dependent enhancement mainly attributes to the enhanced absorption in the GeSn layer which benefits the excitation of electron-hole pairs, while the contribution from internal photoemission in the Au layer is negligible.
2. Material characterization
The GeSn film was grown on a Ge-buffered Si substrate using the reduced pressure chemical vapor deposition (RPCVD). The thickness of the GeSn film is ∼964 nm, which was confirmed by high-resolution transmission electron microscope (HRTEM). As illustrated in Fig. 1(a), the threading dislocations are mostly confined at the Ge/Si interface. The top 500 nm GeSn layer near the surface maintains high material quality, which is beneficial for high-performance MSM photodetector. The strain and Sn concentration in the as-grown GeSn film was investigated by X-ray diffraction (XRD) analysis. As shown in Fig. 1(b), the left peak on the rocking curve corresponds to GeSn layer with a Sn concentration of 8%. Although the as-grown GeSn has exceeded the critical thickness, there is still a residual compressive strain of ∼0.24% in the GeSn layer.
Fig. 1. (a) High-resolution TEM image of the GeSn film grown on a Ge-buffered Si substrate. (b) XRD rocking curve of the as-grown GeSn film at (004) orientation.
Taking the Sn concentration and compressive strain into consideration, the bandgap of the GeSn film was theoretically calculated. For unstrained GeSn, the bandgap can be estimated using Vegard’s Law: Eg(Ge1-xSnx)=(1-x)Eg(Ge)+xEg(Sn)-bGeSn(1-x)x, where x is the Sn concentration and bGeSn is the bowing parameter of GeSn. The application of strain additionally shifts the valence band and conduction band. The energy shifts caused by strain for each band can be expressed using the following equations:
Here, ac(av) is the hydrostatic deformation potential for conduction(valence) band, ε∥(ε⊥) is the in-plane(normal) stain, bv is the shear deformation potential and Δ0 is the split-off energy. The calculated direct and indirect bandgap of the as-grown GeSn film is 0.538 and 0.540 eV (∼2300 nm), demonstrating that the as-grown GeSn film is capable of efficient photo detection at 2 µm.
3. GeSn photodetector with an Au hole-array structure
The two-dimensional metal hole-array structures has been widely used in III-V material for enhanced photo detection in the middle infrared range (3-5 µm). The design of such structure follows the following equation under normal incidence [30].
Here, λi,j is surface plasmon resonance wavelength with mode orders (i, j), p is periodicity of the metal hole-array structure and εm(εd) is the permittivity of corresponding metal(dielectric) material. In this work, Au was chosen as the metal, and its permittivity can be calculated using the Drude model [28]. The permittivity of GeSn was calculated using εd=n2+k2, where the n, k parameters were obtained from ellipsometer measurement and fitting. To realize enhanced photo detection at 2 µm, the fundamental plasmon mode λ10 or λ01 was set to 2 µm. The width of each square hole was fixed as half of the periodicity. The 3D schematic diagram of proposed GeSn MSM photodetector with Au hole-array structure is shown in Fig. 2(a). In the fabrication, two electron beam lithography and lift-off steps were used to form Au hole-array structure and Ti/Al electrode separately. The Schottky contacts were formed between Au and GeSn without special treatment. The top-view scanning electron microscope (SEM) image of the fabricated device is shown in Fig. 2(b). The spacing between the interdigitated electrode fingers is fixed at 6 µm. As illustrated in the inset, uniform and well-defined square hole-array structures can be observed clearly.
Fig. 2. (a) 3D schematic diagram of GeSn MSM photodetector Au hole-array structure. (b) Top-view SEM image of the GeSn MSM photodetector. The inset is the zoomed-in view of the hole-array structure.
The current-voltage characteristics for both photodetectors with and without Au hole-array structures were measured. Due to the symmetry of I-V characteristics, only half of the curves were shown here for easy comparison. As shown in Fig. 3(a), the incorporation of Au hole-array structure has negligible influence on dark current. The high dark current of ∼25 mA at 1 V is attributed to the large active area of 1.56×10−4 cm2 and lack of effective surface passivation. In the future, a thin Al2O3 layer (∼1.5 nm) will be grown on the GeSn film to reduce the dark current by alleviating the Fermi pinning effect. The photo currents for photodetectors with varying Au hole-array periodicities were plotted in Fig. 3(b). A fiber-coupled Fabry-Pérot laser diode operating at 2 µm (Thorlabs FPL2000S) was used in the measurement and the incident power was fixed at 11.89 mW. Compared with reference photodetector without Au hole-array structure, only the GeSn photodetector with the Au hole-array periodicity of 560 nm has an enhanced photo current. The responsivities for all photodetectors were extracted and compared in Fig. 4(a). A ∼50% improved responsivity of 0.0787 A/W at 2 µm was achieved on GeSn photodetector with Au hole-array periodicity of 560 nm. It should be noted that the enhancement is polarization-independent due to the symmetric square hole-array structure.
Fig. 3. (a) Dark currents of GeSn MSM photodetector with and without Au hole-array structures. (b) Photo currents of GeSn MSM photodetector with and without Au hole-array structures at fixed optical power of 11.89 mW.
Fig. 4. (a) Extracted responsivities of GeSn MSM photodetectors with and without Au-hole array structure. (b) Simulated electric field density distribution in a unit cell of Au hole-array structure using FDTD method.
To investigate the weak enhancement of responsivity, finite-difference time-domain (FDTD) method was used to simulate the electric field distribution in the structure. As shown in Fig. 4(b), the cross-sectional view manifests the effective optical confinement due to SPR effect. Since the Au hole-array structure is able to provide enhanced optical absorption, the problem could be with the effective collection of photon-generated carriers. It should be mentioned that the interdigitated Ti/Al electrodes were placed laterally on the two sides of Au hole-array structure. The photon-generated carriers will be collected in the lateral direction through the drift process. However, most electron-hole pairs are generated near the surface due to the SPR effect. The penetration depth δd of surface plasmon polaritons can be expressed as [30]:
Here k0 is free space wavevector, εm’ is the real permittivity of Au and εd is the permittivity of GeSn. The calculated δd for GeSn photodetector with Au hole-array structure is only 280 nm at 2 µm. Therefore, the photon-generated carriers will diffuse from the surface to the inside of GeSn film. It is noteworthy that the diffusion direction is perpendicular to the drift direction. In addition, the spacing distance between the interdigitated electrodes is 6 µm. The weak electric field in the lateral dimension made the collection of the photon-generated carries more inefficient. Many carriers will recombine in the diffusion process before they are collected, which results in a weak enhancement in responsivity for Au hole-array structure. A similar phenomenon was also observed on InAsSb photodiode in Ref. [27] due to the inefficient carrier collection. Theoretically, such drawback can be compensated by decreasing the spacing distance between the interdigitated electrodes. Nevertheless, the spacing distance between electrodes needs to be large enough to provide a periodical distribution of Au hole-array structure in this direction. The Au hole-array structure is more suitable for vertical p-i-n photodetector, which has the same direction for carrier drift and diffusion.
4. GeSn photodetector with an Au-GeSn grating structure
Au-GeSn grating structure was also incorporated in GeSn MSM photodetectors to enhance the optical absorption through SPR effect. The 3D schematic diagram of GeSn MSM photodetector with Au-GeSn grating is shown in Fig. 5(a). The 100 nm Au layer was firstly patterned using electron beam lithography and lift-off processes. Subsequently, the Au layer was used as a hard mask for the etching of the underneath GeSn layer to form the Au-GeSn grating structure. Chlorine-based reactive ion etching (RIE) under high power was utilized to form straight GeSn sidewalls with a high aspect ratio. The Au layer was served as the electrode and plasmonic metal simultaneously. The two junctions between Au electrode and GeSn film are both Schottky contacts. GeSn photodetectors with various grating periodicity P and width W were fabricated for comparison, while the etching depth was fixed as 280 nm. The top-view SEM image of the fabricated device is shown in Fig. 5(b). The inset shows the distinct Au-GeSn grating structure with uniform periodicity.
Fig. 5. (a) 3D schematic diagram of GeSn MSM photodetector with Au-GeSn grating structure. (b) Top-view SEM image of the fabricated GeSn MSM photodetector. The inset is a zoomed-in view of Au-GeSn grating structure.
The performance of GeSn photodetectors with different Au-GeSn grating parameters were compared. The I-V characteristics shown here were measured when the electrode with grating structure was applied positive bias. The proposed plasmon-enhanced GeSn photodetector has a grating periodicity of 1.3 µm and a width of 0.68 µm, while the reference GeSn photodetector has a grating periodicity of 4 µm and a width of 2 µm without specific design. The current-voltage characteristics for these two photodetectors were plotted in Fig. 6(a). In the measurement setup, the fiber-coupled Fabry-Pérot laser diode was connected with a 3-paddle polarization controller to manually change the polarization direction of the incident light. As illustrated in the inset of Fig. 6(a), the TE polarization is defined as the electric field direction of the incident light is parallel to the bridge direction of the grating. The TM polarization represents a perpendicular electric field compared with the defined TE polarization. For the plasmon-enhanced GeSn photodetector, a slightly lower dark current of 25.2 mA was obtained at 1 V compared with the reference photodetector. The reason for the high dark current is similar to the previous GeSn MSM photodetectors with Au-mesh structure, which attributes to the large active area of ∼2.92×10−4 cm2 and lack of surface passivation. It is noteworthy that the photocurrent curves for the plasmon-enhanced photodetector show significant difference under TE-polarized illumination and TM-polarized illumination. The responsivities as a function of bias voltage for both photodetectors were extracted at 2 µm in Fig. 6(b). It can be observed that the extracted responsivities saturate when the bias voltage is higher than 1.4 V for both GeSn photodetectors. When the surfaces were illuminated by TE-polarized light, these two photodetectors have a similar responsivity of ∼0.153 A/W at 1.5 V. Under TM-polarized illumination, the plasmon-enhanced GeSn photodetector has a 3× enhanced responsivity of 0.441 A/W compared with that of the reference GeSn photodetector at 1.5 V.
Fig. 6. (a) Current-voltage (I-V) characteristics of GeSn MSM photodetector with Au-GeSn grating structure. The optical power is fixed at 4.9 mW. The insets illustrate the defined TE and TM polarization directions. (b) The relationship between responsivities and bias voltage for GeSn MSM photodetectors under TE and TM polarized illumination.
The responsivities for plasmon-enhanced GeSn photodetectors with grating periodicities ranging from 1.3 to 1.5 µm and widths ranging from 0.66 to 0.70 µm were measured and compared in Fig. 7(a). For all plasmon-enhanced photodetectors, the achieved responsivities show significant difference under TE-polarized and TM-polarized illumination. The highest responsivity under TM-polarized illumination is 0.455 A/W for GeSn photodetector with a grating periodicity of 1.4 µm and a width of 0.7 µm. For the same device, the responsivity under TE-polarized illumination is 0.177 A/W. The performances for plasmon-enhanced photodetectors feature no distinct variation when the grating parameters vary in a small range. As shown in Fig. 7(b), the peak position in the simulated absorption spectra shifts slightly when the periodicity or width of grating structure changes in a small range. The simulated absorption at 2 µm remains stable within a range from 50% to 57%. That explains the similar performance for the fabricated plasmon-enhanced GeSn photodetectors. Moreover, compared with reference GeSn photodetector with a grating periodicity of 4 µm and a width of 2 µm, all the plasmon-enhanced photodetectors show ∼4× reinforcement in responsivity, which is consistent with experimental results.
Fig. 7. (a) Responsivities of GeSn MSM photodetectors with Au-GeSn grating structures. The grating periodicity ranges from 1.3 to 1.5 µm and the width ranges from 0.66 to 0.70 µm. (b) Simulated absorption spectra from 1.9 to 2.1 µm when the grating periodicity or width varies.
The responsivity for GeSn photodetector with a grating periodicity of 1.4 µm and a width of 0.7 µm was benchmarked in Fig. 8(a) [9–11,16,21,23,32–34]. It can be observed that the responsivities achieved in this work are among the highest values for GeSn photodetectors operating at 2 µm. The reflection spectra for plasmon-enhanced GeSn photodetector under TE-polarized and TM-polarized illumination were also measured using Fourier Transform Infrared Spectroscopy (FTIR). The output light from FTIR was firstly focused using a magnification microscope objective and subsequently passed through a polarizer film to change the polarization direction. The reflection spectra were calibrated using a standard Au sample. As shown in Fig. 8(b), a significant higher reflection was observed for TE-polarized illumination. Compared with the reflection of ∼0.133 a.u. under TM-polarized illumination, the reflection is ∼0.700 a.u. under TE-polarized illumination at 2 µm. The ratio of the arbitrary unit obtained in FTIR measurement to percentage is assumed to 1 (the actual ratio ranges from 0.8 to 1.2) and the transmission is assumed to be negligible. Therefore, the absorption can be roughly estimated using the formula: A=1-R, where A is absorption and R is reflection. The estimated absorption under TM polarized illumination is ∼86.7%, which is ∼3× higher than the estimated absorption of ∼30% under TE polarized illumination. Since the responsivity is proportional to the optical absorption, the FTIR results explain that the ∼3× enhanced responsivity is attributed to the enhanced optical absorption at 2 µm.
Fig. 8. (a) Benchmark of responsivity at 2 µm for GeSn photodetectors with varying Sn concentrations. (b) Reflection spectra of GeSn MSM photodetectors with Au-GeSn grating structure from 1.6 to 2.4 µm under TE and TM polarized illumination.
The mechanism for the responsivity enhancement was further investigated using FDTD method. The electric field density distributions in the Au-GeSn grating structure under TE-polarized and TM-polarized illumination are shown in Fig. 9(a) and Fig. 9(b). When the Au-GeSn grating structure is illuminated under TE-polarized light, most light is reflected back, degrading the photo response. For the TM-polarized illumination, obvious SPR occurs at the Au/GeSn interface. The effective optical confinement in the GeSn layer contributes to more photon-generated carries, increasing the optical response. On the other hand, the hot carriers generated in the Au layer are able to jump over the Schottky barrier between Au and GeSn, benefitting the enhanced responsivity at 2 µm further [31]. The Schottky barrier between Au and GeSn can be extrapolated from the dark current curve [35,36]. The relationship between dark current and forward bias can be expressed as:
Here Rs is series resistance and I0 is saturation current, which can be extracted from the intercept of lnI at V=0 V. The extracted I0 for GeSn photodetector with Au-GeSn grating structure is 3.07×10−5 A. Subsequently, the Schottky barriers can be estimated using the following equation:
Here A is the active area of the fabricated device and A* is the Richardson constant for GeSn. The dominant carriers in intrinsic GeSn are holes due to the lattice defects. The estimated Schottky barrier between Au and GeSn for electrons and holes are ∼0.39 and ∼0.44 eV, respectively. The laser diode used in the measurement has a central wavelength at 2 µm, which corresponds to a photon energy of 0.62 eV. The hot carriers generated in the Au layer have enough energy to jump through the Schottky barrier.
Fig. 9. Simulated electric field distribution in Au-GeSn grating structure under (a) TE and (b) TM polarized illumination. Periodical boundary condition was adopted in the simulation.
To investigate the respective contributions from Au layer and GeSn layer under TM polarized illumination, the absorption spectra in Au layer and GeSn layer were simulated separately. As shown in Fig. 10, the optical absorption mainly concentrates in the GeSn layer across the entire spectra range. At 2 µm, the absorption in GeSn layer is 57.5% which is ∼7× higher compared with the absorption of 7.9% in Au layer. The respective absorption percentages are consistent with the electric field density distribution in Fig. 9(b). In addition, the probability of internal photoemission between metals and semiconductors is extremely low due the diffidence in density of states and the requirement of momentum reservation [37,38]. Taking both absorption and internal photoemission probability into consideration, the hot carriers in Au layer contribute negligibly to the enhanced responsivity under TM polarized illumination. Compared with polarization-independent Au hole-array GeSn photodetector, the polarization-sensitive characteristic for GeSn photodetector with Au-GeSn grating structure enables its unique application in military navigation, high-contrast polarizer and optical switches [39].
5. Conclusion
For the first time, plasmonic structures were incorporated in GeSn MSM photodetectors to localize the optical field near the surface. Both Au-hole array and Au-GeSn grating structures were designed and implemented with varying degrees of reinforcement in responsivity. For GeSn MSM photodetectors with Au hole-array structure, polarization-independent enhanced responsivity of 0.0787 A/W was achieved at 2 µm. The weak enhancement (∼50%) attributes to the ineffective collection of photon-generated carriers. On the other hand, GeSn photodetectors with Au-GeSn grating structure presents remarkable responsivity enhancement up to 3× under TM-polarized illumination. A high responsivity of 0.455 A/W was accomplished on GeSn photodetector with a grating periodicity of 1.4 µm and a width of 0.7 µm. The polarization-dependent enhancement primarily benefits from the localized optical field in the GeSn layer which allows for generation of more carries under TM polarized illumination. In these two plasmonic GeSn photodetectors, SPR is proven to be highly effective for the optical confinement near Au/GeSn interface at 2 µm. In the future, surface passivation such as insertion of a thin Al2O3 layer will be adopted to decrease the dark current. This work demonstrates that it is promising to incorporate plasmonic structures with GeSn photodetectors for high-efficiency optical communication at 2 µm.
Funding
National Research Foundation Singapore (NRF–CRP19–2017–01); Ministry of Education - Singapore (2019-T1-002-040 RG147/19 (S)).
Acknowledgements
The authors acknowledge Ms. Xiaohong Yang and Ms. Xin Guo in Nanyang Technological University for the assistance in electron beam lithography and measurement setup.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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.
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