Surface plasmon enhanced nanopillar photodetector array

P. N. Senanayake, C. H. Hung, J. Shapiro, A. Lin, and D. L. Huffaker

Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA 90095, USA.

California Nano-Systems Institute, University of California, Los Angeles, California 90095, USA email:

pradeeps@ee.ucla.edu, huffaker@ee.ucla.edu

We demonstrate surface plasmon-enhaced nanopillar (NP) photodetector arrays in the near IR spectral range. Light coupling into the NPs takes place via surface plasmons through subwavelength, self-aligned metal

holes. A novel fabrication technique produces elongated subwavelength holes in the gold top electrode such that the holes are self-aligned to the NPs. Surface plasmons excited on the metal/NP and metal/polymer

interfaces couple into the NP resulting in electric field intensity “hot spots”. The measured responsivity shows

the typical cos2(θ) dependence with respect to incident light polarization showing the antenna effect of the

nanohole array. The shape resonance of the elongated metal hole produces polarization sensitive light

absorption1. Surface plasmon enhanced nanopillar detector arrays are a compelling technology for small foot

print subwavelength photodetectors.

The device consists of patterned p-doped In0.4Ga0.7As NPs grown on n+ doped GaAs substrates by selective area epitaxy (SAE) as shown in Figure 1a. The diameter, pitch and pattern geometry of the nanohole

array is controlled by the electron beam lithography. A more detailed description of substrate patterning and NP epitaxy are published elsewhere

2. The InGaAs NPs are 2.5 µm in height, 150 nm in diameter and arranged

in a square lattice with a pitch of 1 µm.

A schematic of the fully processed NP photodetector array is shown in Figure 1b. Following epitaxy,

the NP array is planarized using a hard cured polymer and etched back to expose ~500 nm of the NP tips. Figure 1b shows an SEM image of the sample surface after the top metal contact (Cr: 20 nm and Au: 200 nm)

is deposited at a 55o tilt. The angled beam coats the entire array surface including the top and one exposed side

of the NP (as shown in Figure 1c). The NP “shadow” leaves the other pillar side uncoated and forms a self-

aligned nanohole adjacent to each NP. In this work, the resulting nanoholes are 260 nm wide and 315 nm

long. Figure 1d shows a high-resolution SEM image of the self aligned nanohole array (SANA).

After fabrication, the NP photodetector is wire bonded to a leadless chip carrier for electrical and optical characterization. The NP photodiode (500 µm x 500 µm) exhibits I-V characteristic typical of a

photovoltaic detector as shown in Fig. 2a. The ideality factor of the diode is 1.9, while the rectification ratio at +/-1V is 10

4. A reverse leakage current of 30 nA at -5 V indicates that no threading dislocations are formed at

the InGaAs NP-substrate interface. Figure 2b shows the polarization sensitive spectral response of the NP

photodetector. Light from a quartz tungsten halogen lamp is dispersed by a grating monochromator and is polarized from 0

o to 360

o. The spectral response is normalized with respect to the source power at each

polarization. The devices show highest responsivity at 30o from the long edge of the nanohole at 1.1 µm as

shown in Figure 2b. The control sample which consists of the InGaAs NPs with ITO top contact shows no dependence of the responsivity on the polarization angle. We believe that this polarization sensitive light

absorption is due to shape resonance of the nanohole.

To further understand the underlying physics of the light absorption in the NPs through SANA, the electric field intensity of the structure is modeled by the finite difference time domain method (FDTD) using

the commercial software, Lumerical. The incident light source is polarized at 30o

from the long side of the

nanohole. The source excitation is a plane wave at 1.1µm. A 3D unit cell is defined such that periodic boundary conditions are used in the x-y boundaries, while a perfectly matched layer (PML) boundary

condition is used in the z – boundary. GaAs material parameters are used to define the NPs, and gold is modeled by the material parameters described by Johnson et al

3. Fig. 3 shows the electric field intensity in the

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x-z plane of the NP at 1.1 m for polarized light at 30o with respect to the long side of the nanohole. It can be

seen that light has coupled through the nanohole into the NP where it has caused an electric field enhancement

or “hot spot”. This electric field enhancement results from the coupling of surface plasmons in the metal hole.1

Surface plasmon enhanced nanopillar pn photodetectors offer higher photocurrent densities, and lower

capacitance than planar photodetectors and is a compelling technology for subwavelength devices.4

Figure 1: a) Tilted SEM of In0.3Ga0.7As NPs grown on n+ GaAs by SAE b) Schematic of the final device structure c) Top

view SEM of the metal nanohole next to the nanopillar d) Tilted SEM of the self aligned nanopillar array.

References 1

K. J. K. Koerkamp, S. Enoch, F. B. Segerink, N. F. van Hulst, and L. Kuipers, Phys. Rev. Lett. 92, 4 (2004). 2

J. N. Shapiro, A. Lin, P. S. Wong, A. C. Scofield, C. Tu, P. N. Senanayake, G. Mariani, B. L. Liang, and D. L. Huffaker, Applied Physics Letters 97, 243102 (2010).

3 P. B. Johnson and R. W. Christy, Physical Review B 6, 4370 (1972).

4 L. Tang, S. Latif, and D. A. B. Miller, Electron. Lett., 30 (2009).

Figure 3: Electric field intensity in the x-z plane in the nanopillar for 30o

Figure 2: a) Dark and light I-V characteristics at 659nm illumination at 4mWcm-2

b) Polarization dependent

responsivity. Maximum and minimum responsivity observed at 30o

and 120o

. c) Polarization dependent spectral

response at -0.5V showing at 30o

and 120o

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