Second-order surface-plasmon assisted responsivity enhancement in germanium nanophotodetectors with bull’s eye antennas Fang-Fang Ren,1,2,* Wei-Zong Xu,1 Jiandong Ye,1,2 Kah-Wee Ang,3 Hai Lu,1 Rong Zhang,1 Mingbin Yu,3 Guo-Qiang Lo,3 Hark Hoe Tan,2 and Chennupati Jagadish2 2
1 School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia 3 Institute of Microelectronics, A*STAR (Agency of Science and Technology Research), 11 Science Park Road, Singapore Science Park II, 117685, Singapore * [email protected]
Abstract: The enhancement of photo-response in nanometer-scale germanium photodetectors through bull’s eye antennas capable of supporting 2nd-order Bloch surface plasmon modes is demonstrated in theory and experiment. A detailed numerical investigation reveals that the presence of surface wave and its constructive interference with the directly incident light are incorporated into the main mechanisms for enhancing transmission through the central nanoaperture. With a grating period of 1500 nm, the area-normalized responsivity can be enhanced up to 3.8 times at 2 V bias for a 780 nm laser. It provides an easier fabrication path for ultra-short wavelength operations especially in devices using optically denser materials. ©2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (230.5160) Photodetectors; (050.1220) Apertures.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15949
15. T. Jiang, L. Shen, X. Zhang, and L. Ran, “High-order modes of spoof surface plasmon polaritons on periodically corrugated metal surfaces,” Prog. Electromagn. Res. 8, 91–102 (2009). 16. A. Paul, D. Solis, Jr., K. Bao, W.-S. Chang, S. Nauert, L. Vidgerman, E. R. Zubarev, P. Nordlander, and S. Link, “Identification of higher order long-propagation-length surface plasmon polariton modes in chemically prepared gold nanowires,” ACS Nano 6(9), 8105–8113 (2012). 17. Y.-C. Chen, Y.-T. Chang, H.-H. Chen, F. T. Chuang, C.-H. Cheng, and S.-C. Lee, “Enhanced transmission of higher order plasmon modes with random Au nanoparticles in periodic hole arrays,” IEEE Photon. Technol. Lett. 25(1), 47–50 (2013). 18. F. J. Rodríguez-Fortuño, R. Ortuño, C. García-Meca, J. Martí, and A. Martínez, “High order standing-wave plasmon resonances in silver u-shaped nanowires,” J. Appl. Phys. 112(10), 103104 (2012). 19. F. J. Rodríguez-Fortuño, C. García-Meca, R. Ortuño, J. Martí, and A. Martínez, “Modeling high-order plasmon resonances of a U-shaped nanowire used to build a negative-index metamaterial,” Phys. Rev. B 79(7), 075103 (2009). 20. G. Sun, J. B. Khurgin, and C. C. Yang, “Impact of high-order surface plasmon modes of metal nanoparticles on enhancement of optical emission,” Appl. Phys. Lett. 95(17), 171103 (2009). 21. J. H. Weaver and H. P. R. Frederikse, CRC Handbook of Chemistry and Physics (CRC Press, 2001). 22. C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature 445(7123), 39–46 (2007).
1. Introduction The enhanced light-matter interaction on metallic nanostructures or nanoaperture antennas has stimulated a broad interest for achieving high-performance optoelectronic devices [1,2]. Among the different strategies being explored, bull’s eye (BE) structure, which consists of a nanoaperture surrounded by concentric surface corrugations (or grooves) has demonstrated its ability to enhance the absorption of a nanometer-scale photodetector by converting input light into surface plasmons (SPs) and inducing extremely strong optical intensity inside an ultrasmall region for the intent to achieve simultaneously high-speed response and sufficient responsivity [3–6]. Advanced considerations were made by integrating the concentric ring gratings with localized resonance elements for even higher performance [7,8]. In most cases, the propagating surface waves launched by periodically modulated electric permittivity of metal are Bloch SP waves since the generation is under Bragg coupling condition 2π
λ
ε mε d 2π 2π = sin θ + m a εm + εd λ
(1)
where λ is the free-space wavelength and a the period of grating. The parameters ε m and ε d represent the relative permittivity of the metal and dielectric, respectively. The integer index, m characterizes the order of Bloch SP modes at a specific incident angle, θ. So far, the fundamental or first-order ( m = 1 ) Bloch SP modes are of most studied modes both in theory and experiment [3–12], and only a limited number of reports focus on higher orders ( m > 1 ) mainly due to their weaker near-field intensity/confinement. However, the higher order modes have the unique advantage of being able to be excited with a much longer grating period, since they have a larger wave vector in magnitude and the effective wavelength of SPs is thus smaller than the grating period. It suggests an easier fabrication for subwavelength applications where optically denser materials are required, since the advances in shorter wavelengths are increasingly dependent on the ability to precisely control the morphology of metal nanostructures [13]. Recent progress on higher order Bloch SPs has been made in linear gratings/waveguides [14,15] or plasmon resonances in metallic nanowires/nanoparticles [16–20] but not on a BE structure and its variations. It would be of interest to analytically investigate the collective response features of high-order modes in these BE-like structures and comparing them to the fundamental modes or other designs, since the variation of geometrical parameters or periodicity breaking might cause different changes in the extraordinary optical transmission spectra, including the spectral position and transmissivity. It will consequently affect light trapping and absorption that may be of use in photodetection applications. In this paper, we conduct a comprehensive study of the 2nd-order Bloch SP mode in a nanometer-scale germanium (Ge) photodetector with a BE or split BE antenna. Particular
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15950
emphasis is given to the geometrical optimization to understand how such plasmonic antennas enhance and focus the transmitted light, since it is essential for energy buildup in an ultrasmall absorbing area. Finally, we observed the photocurrent enhancement for a 780 nm light at normal incidence, which confirms the contribution of 2nd-order SP modes. 2. Principle and simulation The cross-sectional view of our Ge photodetector with a basic BE antenna is sketched in Fig. 1(a), where the antenna consists of shallow annular corrugations (period a, depth d, width b) surrounding a single aperture (diameter w) milled in an opaque aluminum film (thickness h). A circular Ge mesa with diameter of 1 µm and thickness of 200 nm is placed directly beneath the central aperture. When illuminated, the periodic structure acts like an antenna to collect and couple the optical signals into SPs at a resonant wavelength, which gives rise to a very high transmission into the active region under appropriate conditions. By separating a BE structure into halves with a Φ-shape nanogap (width g), we can form a split BE with the potential advantage of merging the SP waves and the local resonances. Technically, all the geometrical features and structural parameters are interlinked and could have influence on the transmission resonances [9]. However, to illustrate the basic idea, we carry out separate analyses on the optical properties of groove arrays and central hole, since it allows us to differentiate amongst the different mechanisms involved in the extraordinary transmission process.
(a)
(c)
Device A central hole
b Al
a
s
w z
d
Ge y Si Substrate
h SiO2
Area:10 µm x 10 µm
(d)
Device C Φ-shape gap
θ
(b) b Al
a z
d y
h
Fig. 1. Structure of the Ge photodetectors. (a) Schematic cross-sectional view of devices with conventional BE antenna. (b) Schematic of a 2D linear grating. (c,d) SEM images of devices A and C with conventional or split BE antenna.
The parameter optimization for the groove arrays can be started from a two-dimensional (2D) linear reflection grating model as shown in Fig. 1(b), which is infinite in the y-direction but uniform in the x-direction. The TM-polarized incoming light with electric field in the y-z plane is introduced at an incident angle θ from air down to the opaque surface corrugations. Based on the permittivity values of Al given in [21], the solutions to Eq. (1) as a function of the grating period a for m = 1, 2, and 3 are partly shown in Fig. 2(a) when θ = 0. They coincide with the energetic positions of SP modes in the limit of groove depth d→0. However, the actual value of a should be slightly smaller since the real grating has a nonzero depth and a finite period number. Here, we are concerned with the 2nd-order SP mode excitation at the operation wavelength of 780 nm with θ = 0, where the grating period a is ~1500 nm. At oblique incidence, the eigenmodes become non-degenerate and show a strong dependence on the excitation angle, resulting in the dispersion of reflection spectra as shown in Fig. 2(b), which is a direct proof of the gratings providing the necessary momentum and energymatching conditions. The optimization for the groove width b, depth d, and the metal
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15951
m=1 m=2 m=3
3.7
Grating period, a (μ m)
2 m=3
0
5.5
m=2
60 50
0
9.0
1 m=1
(b)
40 30 20
0.3
0.88 0.24
10 0 0.4
0.4
Channel depth, d (μ m)
0
o
(a)
Incident angle, θ ( )
thickness h can be performed through detecting the dips in the optical reflection spectra of the 2D linear grating by using a semi-analytical method based on the rigorous coupled-wave analysis (RCWA) for faster calculation as compared to the finite-difference time-domain (FDTD) method or the finite-element method [13]. For instance, with a = 1500 nm, h = 400 nm, we vary the groove depth d from 0 to 395 nm and the groove width b from 0 to 600 nm for the 2nd SP modes with θ = 0 as shown in Fig. 2(c). It is found that the lowest reflection occurs when d is in the range of 50 to 100 nm, which is reduced as compared to the fundamental mode (d required from 120 to 220 nm). In the case of 3rd-order, the required groove depth can be further reduced to 30 nm. We believe it is due to the higher orders that are more tightly localized on the metal surface in the z-direction as illustrated in the insets of Fig. 2(a), which plot the profiles of Ez component for SP modes with m = 1, 2, and 3 in the reflection region of a 2D grating as shown in Fig. 1(b) with a = 1500 nm, d = 100 nm, and b = h = 400 nm. Further calculation illustrates that the resonant positions have periodical dependence on the groove depth, d in terms of wavelength, provided that the metal layer thickness, h is sufficiently thick. Figure 2(c) also shows that the lower portion of the reflectivity plot can span a wider range of groove width b from 225 to 500 nm, which is similar to the properties of the fundamental modes as previously reported in [6]. For a split BE structure, the parameters of groove arrays, especially the grating period a, should be slightly adjusted due to the added Φ-shape gap. Based on the FDTD method, we obtained an optimal design for a split BE structure with a = 1450 nm, b = 250 nm, d = 100 nm, and h = 200 nm. The gap distance, g is retained as 500 nm as used in [7] for comparison.
0.9 1.4 1.9 Wavelength, λ (μm)
(c)
0.89 0.55
0.2 0.1
0.4 0.6 0.8 1.0 1.2 1.4 Wavelength, λ (μm)
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Channel width, b (μm)
Fig. 2. Numerical calculations for optimizing the groove arrays in a BE structure. (a) Estimate of the 1st, 2nd, and 3rd-order Bloch SP modes for an air-Al grating interface at normal excitation angle. The insets show the profiles of Ez-field intensity for SP modes for m = 1, 2, and 3 in the reflection region of a 2D grating with a = 1500 nm, d = 100 nm, b = h = 400 nm. (b) Dependence of the reflection spectra on the incident angle in the 2D grating. (c) Dependence of the reflectivity of the 2nd SP modes on the channel width and depth in the 2D grating.
Other structural parameters in a BE-like antenna, including the diameter of the central hole w, the period number on each side of the aperture N (counting from the 1st groove as shown in Fig. 1(a)), and the distance from the central hole to its nearest groove (the 1st groove) s, would also have an influence on the SP excitation and the efficient tunneling of surface waves through the central hole. We performed the corresponding simulations based on a 3D fullfield FDTD code since the RCWA method is only suitable for perfect periodic structures. The SP modes are numerically observed by detecting the resonant peaks in transmission spectra of light tunneling through the aperture into Ge active region. Close to such resonances, the SP waves launched by the grooves can efficiently couple to the incoming light, leading to a large enhancement of the optical near-field intensity on the exit side of the aperture, and thus
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15952
enhance the photon-generated carriers within the ultra-small Ge area which is depleted under the reverse-bias condition. In a conventional BE structure with N = 4, we calculated the transmission with w increasing from 50 to 950 nm at a wavelength of 780 nm, by assuming s = a = 1490 nm, d = 100 nm, b = 400 nm, and h = 200 nm. The results show two apparent features as depicted in Fig. 3(a). Firstly, the high transmission associated with 2nd-order SP modes will not take place when w less than 350 nm due to the occurrence of cut-off condition. However, no cutoff wavelength was observed in the Fig. 2(d) in [7] as the calculation is approximated by a 2D model based on cylindrical symmetry. In that situation, the light with perpendicular polarization can always propagate through the central slit no matter how narrow it is [22]. Secondly, no visible resonant shift is observed around 780 nm in Fig. 3(a) since the SP mode position is mainly determined by the annular grooves as discussed above. However, the transmitted energy appears remarkably enhanced with increasing diameter because of gradual exposure of the Ge area. Here, we chose w = 550 nm to ensure high transmission and good contact for both conventional and split BE antennas. In addition, since the SP frequency will slightly blue-shift when N increases, this effect should be considered when designing the structure for higher N, as will be discussed in the next paragraphs.
Hole diameter, w (μm)
0.8 0.7
(a)
max 40
min
0.6 0.5 0.4 0.3 0.2 0.1 0.7 0.8 0.9 1.0 Wavelength, λ (μm)
Transmission enhancement
0.9
(b)
30
20
10 BE split BE 0
0
2 4 6 8 10 12 14 Period number, N
Fig. 3. (a) Dependence of transmission spectra on the central hole diameter for a BE structure with N = 4. (b) Dependence of transmission enhancement on the grating period number for the conventional (black) or split (red) BE structure at λ = 780 nm.
To investigate how the transmission enhancement depends on the grating period number, N by reference to a single hole without concentric grooves, we define the transmitted power, P by surface integration of the Poynting vector of the light entering into the Ge region, i.e. Re( EH ∗ dA) , normalized by the incident wave flux. The transmitted power through a single
hole with a diameter of 550 nm is set to P0. The transmission enhancement of a conventional or split BE antenna is thus calculated as the ratio of P/P0. As shown in Fig. 3(b), the split BE antenna requires 9 periods to reach maximum enhancement, which is less than the conventional one. It is known that saturation occurs when re-radiation (plasmons to photons) is balance with the material absorption. After saturation, the enhancement factor drops because the SPP propagation loss by re-radiation becomes dominant rather than material absorption [4,7]. Therefore, we speculate that the fewer period number required in the split design is a result of increased SPP propagation loss due to the periodicity breaking, which finally shortens the propagation length when saturation occurs, i.e., reduces the period number to reach saturation. The value of s plays a key role in the coherent optical property when the SP wave launched by the grooves interferes with the direct transmission of light at the central aperture. In the case of N = 9, we simulated the transmission enhancement with the value of s = la/12, (l
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15953
= 5, 6, ……, 24). Here we start with the value of l from 5 in order to clearly separate the first groove from the central hole. As shown in Fig. 4(a), the resonant peak appears at the wavelength of 780 nm if the value of s is close to 6a/12, 12a/12, 18a/12, or 24a/12, but will disappear when s is close to 9a/12, 15a/12, or 21a/12. If we continuously increase s from 0.4a to 2a with λ fixed at 780 nm as shown in Fig. 4(b), we can clearly see that the 2nd-order SP modes are periodically dependent on the distance s with a period of a/2. It can be well understood based on light interference between the directly transmitted light and the SP wave. By considering the optical route difference as δ ≈ s + Na , we can infer that maximum transmission occurs when the phase difference 2πδ/λSP is close to an integer multiple of 2π, which gives rise to s ≈ pa / m , where λSP is the SP wavelength, p an integer, and m the order of SP mode. The maximal points with respect to the 2nd-order modes are, therefore, given by the condition of s = pa/2. For the case of split BE structure, the behavior is similar as shown in Figs. 4(c) and 4(d). Such a theoretical analysis reveals that collective resonance launched by the corrugation will not always lead to resonant transmission if the SP waves radiated by the grooves do not interfere constructively with the photons at the central aperture. In the special case of s = a, the conditions for constructive interference are satisfied for all orders. yo ffset=10
160 140 120 100 80 60 40 20 0 0.7
0.8
0.9
1.5
1.0
BE, N = 9 0.5
1.0
25
20
220 200
s/a
s=24/12a 23/12a 22/12a 21/12a 20/12a 19/12a 18/12a 17/12a 16/12a 15/12a 14/12a 13/12a 12/12a 11/12a 10/12a 09/12a 08/12a 07/12a 06/12a 05/12a
180
2.0
(b)
15
10
5
Wavelength, λ (μm) Transmission Enhancement
(c)
y of fset=10 s= 24/12a 23/12a 22/12a 21/12a 20/12a 19/12a 18/12a 17/12a 16/12a 15/12a 14/12a 13/12a 12/12a 11/12a 10/12a 09/12a 08/12a 07/12a 06/12a 05/12a
180 160 140 120 100 80 60 40 20 0
0.7
0.8
0.9
Wavelength,λ (μm)
2.0
(d)
1.5
s/a
(a)
Transmission Enhancement
Transmission Enhancement
200
1.0
Split BE, N = 9 0.5 1.0 40
20
0
Transmission Enhancement
Fig. 4. (a,c) Transmission enhancement spectra of a conventional and split BE structures with the value of s varying from 5a/12 to 24a/12. The spectra are shifted upwards for clarity with an offset of 10. (b,d) Dependence of the transmission enhancement on the value of s at λ = 780 nm for the conventional and split BE structures.
3. Experiment and analysis To demonstrate the contribution of 2nd-order SP modes for high performance, we fabricate Ge photodetectors A (a = 1500 nm), B (a = 820 nm) with conventional BE antennas, and C (a = 1450 nm) with split BE antenna as listed in Table 1. According to the above theoretical analyses, the transmission peak associated with 2nd-order SP mode will take place at the wavelength of 780 nm in devices A and C, but will shift to 432 nm in the device B (its fundamental mode located at 885 nm). The devices were fabricated on an 8 in. Si wafer with a thickness of 750 μm. A circular Ge region was first formed with diameter of 1 μm and thickness of 200 nm using blanked Ge epi and mesa-cut approach. This was followed by the deposition and polishing of a passivation oxide using chemical mechanical polishing and wetetch to expose the Ge active region. An Al layer was then deposited and partially etched on the top surface to form the concentric grating. Finally, the Al layer was treated with a deep etch to form the single aperture or the Ф-shape gap. The scanning electron microscopy (SEM) images of detectors A and C with BE or split BE shaped antenna are shown in Figs. 1(c) and 1(d), respectively.
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15954
Table 1. Structural parameters of devices A-C device
a (nm)
b (nm)
d (nm)
h (nm)
w (nm)
g (nm)
N
A
1500
400
100
200
550
–
9
B
820
200
100
200
550
–
9
C
1450
250
100
200
550
500
9
(a)
λ = 780 nm
0
Current ( μA)
10
-1
10
-9
10 -10 10 -11 10 -12 10
Device A Device B Device C Device C dark Device A/B dark
Area-nor mali zed current ( μA/μm2 )
The room-temperature current-voltage (I-V) curve of devices was measured with the devices illuminated by a 780 nm diode laser at a power of 6.63 mW. As shown in Fig. 5(a), the dark current of device C with split BE antenna (0.67 nA @ 2 V bias) is higher than that of device A or B (0.20 nA at 2 V bias) mainly due to the larger effective active area in device C. With a reverse bias voltage of 2 V, a photocurrent of 0.714 or 1.59 μA can be obtained from device A or C, which is ~3.76 or 8.37 times of the value of 0.190 μA measured from device B. If surface reflection is neglected and simply considering the input light power as 6.63 mW, the corresponding responsivities of devices A, B, and C can be estimated to be 0.11, 0.029, and 0.24 mA/W, respectively. It indicates that the area-normalized responsivity of devices with grating period a = 1.5 µm can be enhanced by 3.8 times at the operation wavelength of 780 nm.
(b) 10
10
0
-1
-9
10-10 10 -11 10 -12 10 0.0
Normalized to the exposed Ge area
0.5 1.0 1. 5 2.0 0.0 0.5 1.0 1.5 2.0 Rever se bias voltage (V)
Fig. 5. (a) Measured current-voltage (I-V) characteristics from devices A (blue circle), B (black square), and C (red triangle). (b) Current-voltage curve normalized to the Ge area of each detector.
As predicted, the enhancement from devices A and C is mainly due to the excitation of 2nd-order SP mode and also its constructive interference with photons when tunneling through the central aperture. However, the further improvement from device A to C is not as high as the case of λ = 1310 nm that we previously reported in [7]. To understand this phenomenon and get a physical insight, we plot the normalized and time-averaged Ez field at the metal antenna-Ge layer interface of devices A and C in Figs. 6(a) and 6(b), respectively. Ez is the perpendicular component of electric fields on the antenna surface that yields SP waves. The insets show a more detailed description of Ez-field distribution surrounding the active regions. One important observation is that the split BE antenna has a better light concentration performance than the basic BE scheme, which should significantly contribute to the additional photo-response improvement in device C. However, the split BE structure also has disadvantages at λ = 780 nm as compared to 1310 nm, which consequently lead to the lower area-normalized photocurrent from device C than device A as shown in Fig. 5(b). For example, the maximum electric field density in device C appears along the gap edges but becomes weaker in the Ge regime, which limits the lensing ability of antennas in this
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15955
orientation at this wavelength. Still in device C, the optical energy confinement in the active region is not under the electrodes, which will reduce the overlap between optical energy density and electric fluxline. To improve the performance of split BE antennas, one should configure a more efficient active area in terms of the working wavelength and its related nearfield electric profiles.
(a)
2.5
(b)
0
10
0
Device C
Device A Area: 15.7μm x 15.7μm
Fig. 6. (a,b) Normalized and time-averaged Ez-field intensity distribution at the interface of Al and Ge layer (plane of z = 0) for devices A and C. Insets show a more detailed description of the Ez-field distribution surrounding the active region with an area of 2 µm × 2 µm. The white dotted circles in the insets represent the boundary of Ge region. The white solid curve describes the hole edge or the Φ-shape gap.
4. Conclusion In conclusion, we demonstrated the enhancement of photo-response in nano-scale Ge photodetectors by employing BE-like antennas which enable 2nd-order SP modes at 780 nm. The geometric parameters, including the grooves, central holes, the spacing between them, etc, are identified and optimized based on theoretical simulations for optimum efficient channeling of optical energy into the central aperture. The constructive interference between surface waves and the direct transmission of light also plays an important role in yielding maximum transmission. These results show that the application of high-order SP modes could provide an easier fabrication path for ultra-short wavelength operations. Acknowledgments This work was supported by the Australian Research Council Discovery Early Career Researcher Award (DE130101700), the National Natural Science Foundation of China (Nos. 11104130, 61274058, 61322403, 60825401, and 60936004), the Basic Research Program of Jiangsu Province (Nos. BK2011556, BK2011437, and BK20130013), and the State Key Program for Basic Research of China (Nos. 2010CB327504 and 2011CB301900).
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15956
1 School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia 3 Institute of Microelectronics, A*STAR (Agency of Science and Technology Research), 11 Science Park Road, Singapore Science Park II, 117685, Singapore * [email protected]
Abstract: The enhancement of photo-response in nanometer-scale germanium photodetectors through bull’s eye antennas capable of supporting 2nd-order Bloch surface plasmon modes is demonstrated in theory and experiment. A detailed numerical investigation reveals that the presence of surface wave and its constructive interference with the directly incident light are incorporated into the main mechanisms for enhancing transmission through the central nanoaperture. With a grating period of 1500 nm, the area-normalized responsivity can be enhanced up to 3.8 times at 2 V bias for a 780 nm laser. It provides an easier fabrication path for ultra-short wavelength operations especially in devices using optically denser materials. ©2014 Optical Society of America OCIS codes: (240.6680) Surface plasmons; (230.5160) Photodetectors; (050.1220) Apertures.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
K. R. Catchpole and A. Polman, “Plasmonic solar cells,” Opt. Express 16(26), 21793–21800 (2008). M. K. Kwon, J. Y. Kim, B. H. Kim, I. K. Park, C. Y. Cho, C. C. Byeon, and S. J. Park, “Surface-plasmonenhanced light-emitting diodes,” Adv. Mater. 20(7), 1253–1257 (2008). T. Ishi, J. Fujikata, K. Makita, T. Baba, and K. Ohashi, “Si nano-photodiode with a surface plasmon antenna,” Jpn. J. Appl. Phys. 44(12), L364–L366 (2005). R. D. R. Bhat, N. C. Panoiu, S. R. J. Brueck, and R. M. Osgood, Jr., “Enhancing the signal-to-noise ratio of an infrared photodetector with a circular metal grating,” Opt. Express 16(7), 4588–4596 (2008). J. Fujikata, D. Okamoto, K. Nishi, and K. Ohashi, “Si/Ge nano-photodiode with a surface-plasmon antenna,” in Proceedings of the 4th IEEE International Conference on Group IV Photonics (IEEE, 2007), pp. 291–293. F.-F. Ren, K.-W. Ang, G.-Q. Lo, and D.-L. Kwong, “Nanometer germanium photodetector with aluminum surface plasmon antenna for enhanced photo-response,” Proc. SPIE 7719, 77191U (2010). F.-F. Ren, K.-W. Ang, J. D. Ye, M. B. Yu, G.-Q. Lo, and D.-L. Kwong, “Split bull’s eye shaped aluminum antenna for plasmon-enhanced nanometer scale germanium photodetector,” Nano Lett. 11(3), 1289–1293 (2011). D. X. Wang, T. Yang, and K. B. Crozier, “Optical antennas integrated with concentric ring gratings: electric field enhancement and directional radiation,” Opt. Express 19(3), 2148–2157 (2011). O. Mahboub, S. C. Palacios, C. Genet, F. J. Garcia-Vidal, S. G. Rodrigo, L. Martin-Moreno, and T. W. Ebbesen, “Optimization of bull’s eye structures for transmission enhancement,” Opt. Express 18(11), 11292–11299 (2010). A. Karar, C. L. Tan, K. Alameh, Y. T. Lee, and F. Karouta, “Metal nano-grating optimization for higher responsivity plasmonic-based GaAs metal-semiconductor-metal photodetector,” J. Lightwave Technol. 31(7), 1088–1092 (2013). E. Laux, C. Genet, T. Skauli, and T. W. Ebbesen, “Plasmonic photon sorters for spectral and polarimetric imaging,” Nat. Photonics 2(3), 161–164 (2008). E. Altewischer, M. P. van Exter, and J. P. Woerdman, “Plasmon-assisted transmission of entangled photons,” Nature 418(6895), 304–306 (2002). A. Kobyakov, A. Mafi, A. R. Zakharian, S. A. Darmanyan, and K. B. Sparks, “Fundamental and higher-order Bloch surface plasmons in planar bimetallic gratings on silicon and glass substrates,” J. Opt. Soc. Am. B 25(9), 1414–1421 (2008). J. Park, H. Kim, and B. Lee, “High order plasmonic Bragg reflection in the metal-insulator-metal waveguide Bragg grating,” Opt. Express 16(1), 413–425 (2008).
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15949
15. T. Jiang, L. Shen, X. Zhang, and L. Ran, “High-order modes of spoof surface plasmon polaritons on periodically corrugated metal surfaces,” Prog. Electromagn. Res. 8, 91–102 (2009). 16. A. Paul, D. Solis, Jr., K. Bao, W.-S. Chang, S. Nauert, L. Vidgerman, E. R. Zubarev, P. Nordlander, and S. Link, “Identification of higher order long-propagation-length surface plasmon polariton modes in chemically prepared gold nanowires,” ACS Nano 6(9), 8105–8113 (2012). 17. Y.-C. Chen, Y.-T. Chang, H.-H. Chen, F. T. Chuang, C.-H. Cheng, and S.-C. Lee, “Enhanced transmission of higher order plasmon modes with random Au nanoparticles in periodic hole arrays,” IEEE Photon. Technol. Lett. 25(1), 47–50 (2013). 18. F. J. Rodríguez-Fortuño, R. Ortuño, C. García-Meca, J. Martí, and A. Martínez, “High order standing-wave plasmon resonances in silver u-shaped nanowires,” J. Appl. Phys. 112(10), 103104 (2012). 19. F. J. Rodríguez-Fortuño, C. García-Meca, R. Ortuño, J. Martí, and A. Martínez, “Modeling high-order plasmon resonances of a U-shaped nanowire used to build a negative-index metamaterial,” Phys. Rev. B 79(7), 075103 (2009). 20. G. Sun, J. B. Khurgin, and C. C. Yang, “Impact of high-order surface plasmon modes of metal nanoparticles on enhancement of optical emission,” Appl. Phys. Lett. 95(17), 171103 (2009). 21. J. H. Weaver and H. P. R. Frederikse, CRC Handbook of Chemistry and Physics (CRC Press, 2001). 22. C. Genet and T. W. Ebbesen, “Light in tiny holes,” Nature 445(7123), 39–46 (2007).
1. Introduction The enhanced light-matter interaction on metallic nanostructures or nanoaperture antennas has stimulated a broad interest for achieving high-performance optoelectronic devices [1,2]. Among the different strategies being explored, bull’s eye (BE) structure, which consists of a nanoaperture surrounded by concentric surface corrugations (or grooves) has demonstrated its ability to enhance the absorption of a nanometer-scale photodetector by converting input light into surface plasmons (SPs) and inducing extremely strong optical intensity inside an ultrasmall region for the intent to achieve simultaneously high-speed response and sufficient responsivity [3–6]. Advanced considerations were made by integrating the concentric ring gratings with localized resonance elements for even higher performance [7,8]. In most cases, the propagating surface waves launched by periodically modulated electric permittivity of metal are Bloch SP waves since the generation is under Bragg coupling condition 2π
λ
ε mε d 2π 2π = sin θ + m a εm + εd λ
(1)
where λ is the free-space wavelength and a the period of grating. The parameters ε m and ε d represent the relative permittivity of the metal and dielectric, respectively. The integer index, m characterizes the order of Bloch SP modes at a specific incident angle, θ. So far, the fundamental or first-order ( m = 1 ) Bloch SP modes are of most studied modes both in theory and experiment [3–12], and only a limited number of reports focus on higher orders ( m > 1 ) mainly due to their weaker near-field intensity/confinement. However, the higher order modes have the unique advantage of being able to be excited with a much longer grating period, since they have a larger wave vector in magnitude and the effective wavelength of SPs is thus smaller than the grating period. It suggests an easier fabrication for subwavelength applications where optically denser materials are required, since the advances in shorter wavelengths are increasingly dependent on the ability to precisely control the morphology of metal nanostructures [13]. Recent progress on higher order Bloch SPs has been made in linear gratings/waveguides [14,15] or plasmon resonances in metallic nanowires/nanoparticles [16–20] but not on a BE structure and its variations. It would be of interest to analytically investigate the collective response features of high-order modes in these BE-like structures and comparing them to the fundamental modes or other designs, since the variation of geometrical parameters or periodicity breaking might cause different changes in the extraordinary optical transmission spectra, including the spectral position and transmissivity. It will consequently affect light trapping and absorption that may be of use in photodetection applications. In this paper, we conduct a comprehensive study of the 2nd-order Bloch SP mode in a nanometer-scale germanium (Ge) photodetector with a BE or split BE antenna. Particular
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15950
emphasis is given to the geometrical optimization to understand how such plasmonic antennas enhance and focus the transmitted light, since it is essential for energy buildup in an ultrasmall absorbing area. Finally, we observed the photocurrent enhancement for a 780 nm light at normal incidence, which confirms the contribution of 2nd-order SP modes. 2. Principle and simulation The cross-sectional view of our Ge photodetector with a basic BE antenna is sketched in Fig. 1(a), where the antenna consists of shallow annular corrugations (period a, depth d, width b) surrounding a single aperture (diameter w) milled in an opaque aluminum film (thickness h). A circular Ge mesa with diameter of 1 µm and thickness of 200 nm is placed directly beneath the central aperture. When illuminated, the periodic structure acts like an antenna to collect and couple the optical signals into SPs at a resonant wavelength, which gives rise to a very high transmission into the active region under appropriate conditions. By separating a BE structure into halves with a Φ-shape nanogap (width g), we can form a split BE with the potential advantage of merging the SP waves and the local resonances. Technically, all the geometrical features and structural parameters are interlinked and could have influence on the transmission resonances [9]. However, to illustrate the basic idea, we carry out separate analyses on the optical properties of groove arrays and central hole, since it allows us to differentiate amongst the different mechanisms involved in the extraordinary transmission process.
(a)
(c)
Device A central hole
b Al
a
s
w z
d
Ge y Si Substrate
h SiO2
Area:10 µm x 10 µm
(d)
Device C Φ-shape gap
θ
(b) b Al
a z
d y
h
Fig. 1. Structure of the Ge photodetectors. (a) Schematic cross-sectional view of devices with conventional BE antenna. (b) Schematic of a 2D linear grating. (c,d) SEM images of devices A and C with conventional or split BE antenna.
The parameter optimization for the groove arrays can be started from a two-dimensional (2D) linear reflection grating model as shown in Fig. 1(b), which is infinite in the y-direction but uniform in the x-direction. The TM-polarized incoming light with electric field in the y-z plane is introduced at an incident angle θ from air down to the opaque surface corrugations. Based on the permittivity values of Al given in [21], the solutions to Eq. (1) as a function of the grating period a for m = 1, 2, and 3 are partly shown in Fig. 2(a) when θ = 0. They coincide with the energetic positions of SP modes in the limit of groove depth d→0. However, the actual value of a should be slightly smaller since the real grating has a nonzero depth and a finite period number. Here, we are concerned with the 2nd-order SP mode excitation at the operation wavelength of 780 nm with θ = 0, where the grating period a is ~1500 nm. At oblique incidence, the eigenmodes become non-degenerate and show a strong dependence on the excitation angle, resulting in the dispersion of reflection spectra as shown in Fig. 2(b), which is a direct proof of the gratings providing the necessary momentum and energymatching conditions. The optimization for the groove width b, depth d, and the metal
#211104 - $15.00 USD (C) 2014 OSA
Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15951
m=1 m=2 m=3
3.7
Grating period, a (μ m)
2 m=3
0
5.5
m=2
60 50
0
9.0
1 m=1
(b)
40 30 20
0.3
0.88 0.24
10 0 0.4
0.4
Channel depth, d (μ m)
0
o
(a)
Incident angle, θ ( )
thickness h can be performed through detecting the dips in the optical reflection spectra of the 2D linear grating by using a semi-analytical method based on the rigorous coupled-wave analysis (RCWA) for faster calculation as compared to the finite-difference time-domain (FDTD) method or the finite-element method [13]. For instance, with a = 1500 nm, h = 400 nm, we vary the groove depth d from 0 to 395 nm and the groove width b from 0 to 600 nm for the 2nd SP modes with θ = 0 as shown in Fig. 2(c). It is found that the lowest reflection occurs when d is in the range of 50 to 100 nm, which is reduced as compared to the fundamental mode (d required from 120 to 220 nm). In the case of 3rd-order, the required groove depth can be further reduced to 30 nm. We believe it is due to the higher orders that are more tightly localized on the metal surface in the z-direction as illustrated in the insets of Fig. 2(a), which plot the profiles of Ez component for SP modes with m = 1, 2, and 3 in the reflection region of a 2D grating as shown in Fig. 1(b) with a = 1500 nm, d = 100 nm, and b = h = 400 nm. Further calculation illustrates that the resonant positions have periodical dependence on the groove depth, d in terms of wavelength, provided that the metal layer thickness, h is sufficiently thick. Figure 2(c) also shows that the lower portion of the reflectivity plot can span a wider range of groove width b from 225 to 500 nm, which is similar to the properties of the fundamental modes as previously reported in [6]. For a split BE structure, the parameters of groove arrays, especially the grating period a, should be slightly adjusted due to the added Φ-shape gap. Based on the FDTD method, we obtained an optimal design for a split BE structure with a = 1450 nm, b = 250 nm, d = 100 nm, and h = 200 nm. The gap distance, g is retained as 500 nm as used in [7] for comparison.
0.9 1.4 1.9 Wavelength, λ (μm)
(c)
0.89 0.55
0.2 0.1
0.4 0.6 0.8 1.0 1.2 1.4 Wavelength, λ (μm)
0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Channel width, b (μm)
Fig. 2. Numerical calculations for optimizing the groove arrays in a BE structure. (a) Estimate of the 1st, 2nd, and 3rd-order Bloch SP modes for an air-Al grating interface at normal excitation angle. The insets show the profiles of Ez-field intensity for SP modes for m = 1, 2, and 3 in the reflection region of a 2D grating with a = 1500 nm, d = 100 nm, b = h = 400 nm. (b) Dependence of the reflection spectra on the incident angle in the 2D grating. (c) Dependence of the reflectivity of the 2nd SP modes on the channel width and depth in the 2D grating.
Other structural parameters in a BE-like antenna, including the diameter of the central hole w, the period number on each side of the aperture N (counting from the 1st groove as shown in Fig. 1(a)), and the distance from the central hole to its nearest groove (the 1st groove) s, would also have an influence on the SP excitation and the efficient tunneling of surface waves through the central hole. We performed the corresponding simulations based on a 3D fullfield FDTD code since the RCWA method is only suitable for perfect periodic structures. The SP modes are numerically observed by detecting the resonant peaks in transmission spectra of light tunneling through the aperture into Ge active region. Close to such resonances, the SP waves launched by the grooves can efficiently couple to the incoming light, leading to a large enhancement of the optical near-field intensity on the exit side of the aperture, and thus
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15952
enhance the photon-generated carriers within the ultra-small Ge area which is depleted under the reverse-bias condition. In a conventional BE structure with N = 4, we calculated the transmission with w increasing from 50 to 950 nm at a wavelength of 780 nm, by assuming s = a = 1490 nm, d = 100 nm, b = 400 nm, and h = 200 nm. The results show two apparent features as depicted in Fig. 3(a). Firstly, the high transmission associated with 2nd-order SP modes will not take place when w less than 350 nm due to the occurrence of cut-off condition. However, no cutoff wavelength was observed in the Fig. 2(d) in [7] as the calculation is approximated by a 2D model based on cylindrical symmetry. In that situation, the light with perpendicular polarization can always propagate through the central slit no matter how narrow it is [22]. Secondly, no visible resonant shift is observed around 780 nm in Fig. 3(a) since the SP mode position is mainly determined by the annular grooves as discussed above. However, the transmitted energy appears remarkably enhanced with increasing diameter because of gradual exposure of the Ge area. Here, we chose w = 550 nm to ensure high transmission and good contact for both conventional and split BE antennas. In addition, since the SP frequency will slightly blue-shift when N increases, this effect should be considered when designing the structure for higher N, as will be discussed in the next paragraphs.
Hole diameter, w (μm)
0.8 0.7
(a)
max 40
min
0.6 0.5 0.4 0.3 0.2 0.1 0.7 0.8 0.9 1.0 Wavelength, λ (μm)
Transmission enhancement
0.9
(b)
30
20
10 BE split BE 0
0
2 4 6 8 10 12 14 Period number, N
Fig. 3. (a) Dependence of transmission spectra on the central hole diameter for a BE structure with N = 4. (b) Dependence of transmission enhancement on the grating period number for the conventional (black) or split (red) BE structure at λ = 780 nm.
To investigate how the transmission enhancement depends on the grating period number, N by reference to a single hole without concentric grooves, we define the transmitted power, P by surface integration of the Poynting vector of the light entering into the Ge region, i.e. Re( EH ∗ dA) , normalized by the incident wave flux. The transmitted power through a single
hole with a diameter of 550 nm is set to P0. The transmission enhancement of a conventional or split BE antenna is thus calculated as the ratio of P/P0. As shown in Fig. 3(b), the split BE antenna requires 9 periods to reach maximum enhancement, which is less than the conventional one. It is known that saturation occurs when re-radiation (plasmons to photons) is balance with the material absorption. After saturation, the enhancement factor drops because the SPP propagation loss by re-radiation becomes dominant rather than material absorption [4,7]. Therefore, we speculate that the fewer period number required in the split design is a result of increased SPP propagation loss due to the periodicity breaking, which finally shortens the propagation length when saturation occurs, i.e., reduces the period number to reach saturation. The value of s plays a key role in the coherent optical property when the SP wave launched by the grooves interferes with the direct transmission of light at the central aperture. In the case of N = 9, we simulated the transmission enhancement with the value of s = la/12, (l
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15953
= 5, 6, ……, 24). Here we start with the value of l from 5 in order to clearly separate the first groove from the central hole. As shown in Fig. 4(a), the resonant peak appears at the wavelength of 780 nm if the value of s is close to 6a/12, 12a/12, 18a/12, or 24a/12, but will disappear when s is close to 9a/12, 15a/12, or 21a/12. If we continuously increase s from 0.4a to 2a with λ fixed at 780 nm as shown in Fig. 4(b), we can clearly see that the 2nd-order SP modes are periodically dependent on the distance s with a period of a/2. It can be well understood based on light interference between the directly transmitted light and the SP wave. By considering the optical route difference as δ ≈ s + Na , we can infer that maximum transmission occurs when the phase difference 2πδ/λSP is close to an integer multiple of 2π, which gives rise to s ≈ pa / m , where λSP is the SP wavelength, p an integer, and m the order of SP mode. The maximal points with respect to the 2nd-order modes are, therefore, given by the condition of s = pa/2. For the case of split BE structure, the behavior is similar as shown in Figs. 4(c) and 4(d). Such a theoretical analysis reveals that collective resonance launched by the corrugation will not always lead to resonant transmission if the SP waves radiated by the grooves do not interfere constructively with the photons at the central aperture. In the special case of s = a, the conditions for constructive interference are satisfied for all orders. yo ffset=10
160 140 120 100 80 60 40 20 0 0.7
0.8
0.9
1.5
1.0
BE, N = 9 0.5
1.0
25
20
220 200
s/a
s=24/12a 23/12a 22/12a 21/12a 20/12a 19/12a 18/12a 17/12a 16/12a 15/12a 14/12a 13/12a 12/12a 11/12a 10/12a 09/12a 08/12a 07/12a 06/12a 05/12a
180
2.0
(b)
15
10
5
Wavelength, λ (μm) Transmission Enhancement
(c)
y of fset=10 s= 24/12a 23/12a 22/12a 21/12a 20/12a 19/12a 18/12a 17/12a 16/12a 15/12a 14/12a 13/12a 12/12a 11/12a 10/12a 09/12a 08/12a 07/12a 06/12a 05/12a
180 160 140 120 100 80 60 40 20 0
0.7
0.8
0.9
Wavelength,λ (μm)
2.0
(d)
1.5
s/a
(a)
Transmission Enhancement
Transmission Enhancement
200
1.0
Split BE, N = 9 0.5 1.0 40
20
0
Transmission Enhancement
Fig. 4. (a,c) Transmission enhancement spectra of a conventional and split BE structures with the value of s varying from 5a/12 to 24a/12. The spectra are shifted upwards for clarity with an offset of 10. (b,d) Dependence of the transmission enhancement on the value of s at λ = 780 nm for the conventional and split BE structures.
3. Experiment and analysis To demonstrate the contribution of 2nd-order SP modes for high performance, we fabricate Ge photodetectors A (a = 1500 nm), B (a = 820 nm) with conventional BE antennas, and C (a = 1450 nm) with split BE antenna as listed in Table 1. According to the above theoretical analyses, the transmission peak associated with 2nd-order SP mode will take place at the wavelength of 780 nm in devices A and C, but will shift to 432 nm in the device B (its fundamental mode located at 885 nm). The devices were fabricated on an 8 in. Si wafer with a thickness of 750 μm. A circular Ge region was first formed with diameter of 1 μm and thickness of 200 nm using blanked Ge epi and mesa-cut approach. This was followed by the deposition and polishing of a passivation oxide using chemical mechanical polishing and wetetch to expose the Ge active region. An Al layer was then deposited and partially etched on the top surface to form the concentric grating. Finally, the Al layer was treated with a deep etch to form the single aperture or the Ф-shape gap. The scanning electron microscopy (SEM) images of detectors A and C with BE or split BE shaped antenna are shown in Figs. 1(c) and 1(d), respectively.
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15954
Table 1. Structural parameters of devices A-C device
a (nm)
b (nm)
d (nm)
h (nm)
w (nm)
g (nm)
N
A
1500
400
100
200
550
–
9
B
820
200
100
200
550
–
9
C
1450
250
100
200
550
500
9
(a)
λ = 780 nm
0
Current ( μA)
10
-1
10
-9
10 -10 10 -11 10 -12 10
Device A Device B Device C Device C dark Device A/B dark
Area-nor mali zed current ( μA/μm2 )
The room-temperature current-voltage (I-V) curve of devices was measured with the devices illuminated by a 780 nm diode laser at a power of 6.63 mW. As shown in Fig. 5(a), the dark current of device C with split BE antenna (0.67 nA @ 2 V bias) is higher than that of device A or B (0.20 nA at 2 V bias) mainly due to the larger effective active area in device C. With a reverse bias voltage of 2 V, a photocurrent of 0.714 or 1.59 μA can be obtained from device A or C, which is ~3.76 or 8.37 times of the value of 0.190 μA measured from device B. If surface reflection is neglected and simply considering the input light power as 6.63 mW, the corresponding responsivities of devices A, B, and C can be estimated to be 0.11, 0.029, and 0.24 mA/W, respectively. It indicates that the area-normalized responsivity of devices with grating period a = 1.5 µm can be enhanced by 3.8 times at the operation wavelength of 780 nm.
(b) 10
10
0
-1
-9
10-10 10 -11 10 -12 10 0.0
Normalized to the exposed Ge area
0.5 1.0 1. 5 2.0 0.0 0.5 1.0 1.5 2.0 Rever se bias voltage (V)
Fig. 5. (a) Measured current-voltage (I-V) characteristics from devices A (blue circle), B (black square), and C (red triangle). (b) Current-voltage curve normalized to the Ge area of each detector.
As predicted, the enhancement from devices A and C is mainly due to the excitation of 2nd-order SP mode and also its constructive interference with photons when tunneling through the central aperture. However, the further improvement from device A to C is not as high as the case of λ = 1310 nm that we previously reported in [7]. To understand this phenomenon and get a physical insight, we plot the normalized and time-averaged Ez field at the metal antenna-Ge layer interface of devices A and C in Figs. 6(a) and 6(b), respectively. Ez is the perpendicular component of electric fields on the antenna surface that yields SP waves. The insets show a more detailed description of Ez-field distribution surrounding the active regions. One important observation is that the split BE antenna has a better light concentration performance than the basic BE scheme, which should significantly contribute to the additional photo-response improvement in device C. However, the split BE structure also has disadvantages at λ = 780 nm as compared to 1310 nm, which consequently lead to the lower area-normalized photocurrent from device C than device A as shown in Fig. 5(b). For example, the maximum electric field density in device C appears along the gap edges but becomes weaker in the Ge regime, which limits the lensing ability of antennas in this
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15955
orientation at this wavelength. Still in device C, the optical energy confinement in the active region is not under the electrodes, which will reduce the overlap between optical energy density and electric fluxline. To improve the performance of split BE antennas, one should configure a more efficient active area in terms of the working wavelength and its related nearfield electric profiles.
(a)
2.5
(b)
0
10
0
Device C
Device A Area: 15.7μm x 15.7μm
Fig. 6. (a,b) Normalized and time-averaged Ez-field intensity distribution at the interface of Al and Ge layer (plane of z = 0) for devices A and C. Insets show a more detailed description of the Ez-field distribution surrounding the active region with an area of 2 µm × 2 µm. The white dotted circles in the insets represent the boundary of Ge region. The white solid curve describes the hole edge or the Φ-shape gap.
4. Conclusion In conclusion, we demonstrated the enhancement of photo-response in nano-scale Ge photodetectors by employing BE-like antennas which enable 2nd-order SP modes at 780 nm. The geometric parameters, including the grooves, central holes, the spacing between them, etc, are identified and optimized based on theoretical simulations for optimum efficient channeling of optical energy into the central aperture. The constructive interference between surface waves and the direct transmission of light also plays an important role in yielding maximum transmission. These results show that the application of high-order SP modes could provide an easier fabrication path for ultra-short wavelength operations. Acknowledgments This work was supported by the Australian Research Council Discovery Early Career Researcher Award (DE130101700), the National Natural Science Foundation of China (Nos. 11104130, 61274058, 61322403, 60825401, and 60936004), the Basic Research Program of Jiangsu Province (Nos. BK2011556, BK2011437, and BK20130013), and the State Key Program for Basic Research of China (Nos. 2010CB327504 and 2011CB301900).
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Received 29 Apr 2014; revised 5 Jun 2014; accepted 6 Jun 2014; published 20 Jun 2014 30 June 2014 | Vol. 22, No. 13 | DOI:10.1364/OE.22.015949 | OPTICS EXPRESS 15956
