Near-infrared metal-semiconductor-metal photodetector
based on semi-insulating GaAs and
interdigital electrodes
A. I. Nusir,
1,
* A. M. Hill,
2
M. O. Manasreh,
1
and J. B. Herzog
2
1
Electrical Engineering Department, University of Arkansas, Fayetteville, Arkansas 72701, USA
2
Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA
*Corresponding author: ainusir@email.uark.edu
Received November 19, 2014; revised November 24, 2014; accepted November 25, 2014;
posted December 1, 2014 (Doc. ID 224963); published December 12, 2014
Metal-semiconductor-metal photodetectors on semi-insulating GaAs with interdigital electrodes showed signifi-
cant enhancement in the spectral response in the near-infrared region as the electrode spacing is reduced. The
photocurrent for the device with 5 μm interdigital spacing is five orders of magnitude higher than the dark
current, and the room temperature detectivity is on the order of 2.4 × 10
12
cmHz
1∕2
W
−1
at 5 V bias. Furthermore,
the spectral response of this device possesses strong dependence on the polarization of incident light showing
potential plasmonic effects with only microscale dimensions. These experimental data were analyzed using
optical simulation to confirm the response of the devices. © 2014 Chinese Laser Press
OCIS codes: (230.5160) Photodetectors; (260.5430) Polarization; (250.5403) Plasmonics.
http://dx.doi.org/10.1364/PRJ.3.000001
1. INTRODUCTION
Recently, various photodetection schemes and designs
have been proposed [1–5]. The metal-semiconductor-metal
(MSM) configuration is among the simplest structures that
were implemented in many applications such as high-speed
on-chip optical interconnects [6], and optical communication
systems [7]. The planar MSM structure consists of metallic
electrodes deposited on a semiconductor. With this structure
incident photons are detected by the excitation of carriers that
drift under the influence of an electric field applied between
the electrodes. Metallic electrodes with different shapes and
sizes were embedded in the design of photodetectors to en-
hance their performance [8–10]. The implementation of inter-
digital metallic electrodes in MSM photodetectors led to an
efficient light absorption while also downsizing the device ac-
tive area [11]. Furthermore, the electrical properties of the
photodetector can be improved by increasing the photocur-
rent, which is achieved by enhancing the rates of photogen-
erated carriers [12], and reducing the dark current, which
is due to the Schottky barriers formed between the metal
and the semiconductor [13,14]. Plasmonic enhancement seen
in metallic structures may lead to promising applications in-
cluding surface-enhanced Raman spectroscopy [15], photo-
voltaics [16,17], and photodetection [18,19]. Typically
plasmonic devices consist of nanoscale optical antennas that
use surface plasmons to collect and focus light in ultra-small
volumes. Properly designing interdigital electrodes not only
enhances the electronic characteristics of the photodetectors,
but also improves the light absorption of the photodetectors
through plasmonic effects [20–23].
The photodetectors investigated in this work operate in the
near-infrared region and consist of microscale interdigital
metallic electrodes on semi-insulating GaAs. The interdigital
electrodes were designed to enhance the electrical and optical
properties of the photodetector. Both experimental and sim-
ulation studies were performed to fully understand the effect
of interdigital electrodes on the device performance. The pho-
tocurrent of the devices was found to be several orders of
magnitude higher than the dark current. Significant enhance-
ment in the spectral response was observed as the spacing
between the interdigital electrodes and the electrode finger-
widths are reduced. The spectral response of the devices
showed dependence on the polarization of the incident light,
which showed maximum spectral response when the incident
light was polarized perpendicular to the lengths of the elec-
trode fingers. The experimental results were compared to
the results obtained from an optical model.
2. EXPERIMENTAL
The GaAs photodetector device consists of two interdigital
finger-like gold electrodes separated by a spacing channel,
similar to the structure used in previous work [24]. Figure 1
shows the design schematic of one of the devices. The top
view of the device pattern as well as the polarization angle
and the polarization directions of the incident light are illus-
trated in Fig. 1(a). The cross section of the device is repre-
sented in Fig. 1(b) and shows metallic electrodes with a
width of w spaced by distance d. The ratio between the width
of the electrodes and the spacing was designed to be w 2d
for all the devices in this work. Devices with different elec-
trode spacings (d 5, 10, 20, and 50 μm) were fabricated
to investigate the effect of reducing the electrodes spacing
on the device performance. The interdigital pattern was pre-
pared on a semi-insulating GaAs substrate by standard optical
photolithography procedures inside a Class 100 clean room.
A photomask was designed for this purpose to generate
Nusir et al. Vol. 3, No. 1 / February 2015 / Photon. Res. 1
2327-9125/15/010001-04 © 2015 Chinese Laser Press
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