|Advisor:||Paul R. Berger|
|Students:||Wei Gao (graduated with Ph.D. in September 1995)|
|Michael McCarthy (graduated with Master's thesis in November 1996)|
|Sean L. Rommel (Ph.D. candidate on another project now)|
|David Erby (graduated with Bachelor's in May 1998)|
|Collaborators:||George Zydzik (AT&T Bell Labs)|
|H. M. O'Bryan (AT&T Bell Labs)|
|D. Sivco (AT&T Bell Labs)|
|A. Y. Cho (AT&T Bell Labs)|
|J. P. Lorenzo (Rome Lab, Hanscom AFB)|
|K. Vaccaro (Rome Lab, Hanscom AFB)|
|S. M. Spaziani (Rome Lab, Hanscom AFB)|
Importance of the Problem:
Photodetectors are being used in more regions of everyday life from the
bar code scanner at the grocery store, to the receiver for your remote
control on the VCR, to the photoreceiver at the end of a fiber optic cable
in a communication system. Since MSM diodes have such simple technology, any
improvement in responsivity, the current limiting factor to their
widespread use, will allow this class of photodetectors to supplant
existing photodetectors in the marketplace.
Metal-semiconductor-metal (MSM) photodetectors offer an attractive benefit
over alternative photodetectors such as conventional p-i-n photodiodes.
An MSM photodetector consists of interdigitated Schottky metal
contacts on top of an active (absorption) layer. An MSM photodetector
is inherently planar and requires only a single photolithography step
which is compatible with existing field effect transistor (FET) technology.
MSM photodetectors are very high speed devices due to their low
capacitance, and they typically have very low dark currents (current
produced without incident light).
However, the responsivity (total signal produced from a
from a given optical input) is quite low compared to p-i-n photodiodes.
The main causes for the low responsivity is the reflection from the
surface metals and semiconductor surface, the finite carrier
lifetime as the carriers traverse the gap between the electrodes before
being collected, absorption of incident light outside
the region in which photogenerated carriers can be collected by the
electrodes, and surface recombination currents and deep traps within
the semiconductor material which may lower the detected optical signal.
We are investigating ways to improve the
responsivity of MSM photodiodes. By increasing the responsivity, simple
easy to manufacture photodiodes sensitive to low-light levels would be
feasible. This could be accomplished by
investigations to suppress surface recombination through
passivation, to minimize surface reflections and therefore
collect a greater percentage of the incident light, to improve
the carrier lifetime, and to better understand the internal gain
Brief Description of Work and Results:
A metal-semiconductor-metal (MSM) In0.53Ga0.47As photodiode
using a transparent cadmium tin oxide (CTO) layer for the interdigitated
electrodes was investigated.
CTO has greater transmission properties at the wavelengths appropriate
for In0.53Ga0.47As photodiodes over conventional transparent
conductors such as indium tin oxide (ITO). The CTO functions as a
Schottky contact, an optical window and an anti-reflection (AR) coating.
The transparent contact prevents shadowing of the active layer by the
electrodes, thus allowing greater collection of incident light.
The MSM photodiodes had 1 m wide electrodes and 2 m spacings
and a 200 Å cap layer of In0.52Al0.48As for Schottky
barrier enhancement. The 2000 Å CTO electrodes were reactively
sputtered with an argon/oxygen plasma. MSM photodiodes with CTO electrodes
and active area 7575 m2 exhibited leakage currents of
4.8 A at 10 V and soft breakdown voltages of
Similar MSM photodiodes with Ti/Au electrodes had leakage currents of
270 nA at 10 V and soft breakdown voltages of
Elevated dark current of the CTO photodiode is attributed to
defect-related tunneling through the thin 200 Å
In0.52Al0.48As layer which was damaged by the sputtering process.
The barrier height ()
of CTO on i-In0.52Al0.48As
was determined to be 0.47 eV, while the Ti/Au barrier height
was 0.595 eV. The reduced barrier height for CTO is caused by
tunneling through the sputter-damaged cap layer.
Responsivity for 1.3 m incident light was 0.49 A/W and 0.28 A/W,
respectively, for the CTO and Ti/Au MSM photodiodes. No anti-reflection
(AR) coating was utilized over the bare semiconductor surface. The CTO MSM
photodiode shows a factor of almost two improvement in responsivity over
conventional Ti/Au MSM photodiodes.
The Schottky barrier height was measured for five different materials on
undoped In0.52Al0.48As grown by molecular beam epitaxy (MBE).
Of the materials tested, two were transparent conductors, indium-tin-oxide
(ITO) and cadmium tin oxide (CTO) and for comparison, three were opaque
metals, (Au, Ti and Pt). The barrier heights were measured using I-V
measurements. Due to the high series resistance created by the undoped
In0.52Al0.48As, the Norde method [J. Appl. Phys., 50, 5052 (1979)] was used to
plot the I-V characteristics and extract the Schottky barrier height.
The Schottky barrier heights were determined to be 0.639 eV, 0.637 eV,
0.688 eV, 0.640 eV, and 0.623 eV for ITO, CTO, Au, Ti, and Pt,
respectively. Previously published results for Schottky barriers on
In0.52Al0.48As are compared with our measurements.
Metal-semiconductor-metal (MSM) photodiodes with an
In0.53Ga0.47As active region were investigated
using a transparent cadmium tin oxide (CTO) layer for the
interdigitated electrodes to improve the low responsivity
of conventional MSM photodiodes with opaque electrodes.
CTO is suitable as a Schottky contact, an optical window
and an anti-reflection (AR) coating.
Responsivity of CTO-based MSM photodiodes without AR coating
between the electrodes was twice (0.62 A/W) that of a similar
MSM photodiodes with Ti/Au electrodes (0.30 A/W).
A thin 800 Å In0.52Al0.48As layer is inserted
below the electrodes to elevate the electrode Schottky barrier
height. A digitally graded superlattice region (660 Å) was
also employed to reduce carrier trapping at the
heterointerface which acts to degrade photodiode bandwidth.
Bandwidth was elevated nearly an order of magnitude over a
previous MSM photodiode design with an abrupt heterointerface.
Metal-semiconductor-metal (MSM) photodiodes with electrodes fabricated from the transparent conductor cadmium tin oxide (CTO) have been shown to double photoresponsivity. Their bandwidths, however, are significantly lower than those of MSMs fabricated with standard Ti/Au contacts. Though MSMs are generally believed to be limited by the transit time of electrons, it is possible the larger resistivity of CTO has become a significant factor, making the MSMs RC time constant limited instead. Previous models of MSMs only account for one of the two back-to-back Schottky diodes. A new model which takes into account both the forward and reverse biased junctions has been developed from the small signal model of a Schottky diode. This new model was fit to data obtained from S-parameter measurements, and incorporates both the transit time response and RC time constant response.
For further information contact:
Nanofabrication and Materials Processing Center (NanoMPC)
Nanoscale Patterning Laboratory
Nanoelectronics and Optoelectronics Laboratory (NOEL)
Polymer Device Laboratory (PDL)
201 Caldwell Laboratory
Department of Electrical and Computer Engineering
The Ohio State University
205 Dreese Laboratory
2015 Neil Avenue
Columbus, OH 43210 USA
Direct phone: (614) 247-6235
EE Dept. FAX: (614) 292-7596
University of Delaware Research Foundation (UDRF)
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