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Measurement of Interface Traps in HgCdTe and
InAs/GaSb Superlattice Metal-InsulatorSemiconductor Structure
Michael McGovern
UIC REU 2006
August 3rd, 2006
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 Current consists of the flow of
“conduction band” electrons and
“valence band holes”
 The p-i-n photodiode has 3 layers:
mostly holes (p), mostly electrons (n)
and a region of negligible carrier
concentration (i)
 Ideally no current can flow under
“reverse bias”
 Optically generated carriers can cause
current flow
 As reverse bias is increased, avalanche
effect begins and significant current
flows
 Carriers are multiplied in the avalanche
effect leading to increased signal
p-i-n Photodiode
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Passivation
The surface of the semiconductor has
dangling bonds and allows Hg to
evaporate
Dangling bonds and Hg evaporation
reduce device sensitivity
The surface can be passivated to
reduce these problems
Passivation itself leads to other
problems
Surface states will appear at the
interface that will allow carrier
recombination
•HgCdTe Infrared Detector Material: History, Status and Outlook, Rogalski, A
InAs/GaSb Superlattice
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The InAs/GaSb superlattice
consists of alternating layers
of InAs and GaSb
The band gap of InAs is
about .8eV, while that of GaSb
is about .4eV
The result is a periodic
potential
~50Å
~100Å
InAs/GaSb Superlattice
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The potential resembles a “particle in a well”
The Schrödinger equation can be used to find the electron wave
function
The solution can be found exactly. While not difficult, this is
tedious and the resulting expression is very long
The result is that there is a single lowest allowed energy level for
electrons in the conduction band that can be altered by varying the
superlattice geometry
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HgCdTe
• Hg1-xCdxTe has a narrow band gap that
varies with x
• Band gap corresponds to energies in the
IR spectrum
• “Direct band gap”: High absorption
coefficient
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C-V Measurements
Interface trapped charges can
exchange charges with the
semiconductor layer
This increases the
capacitance under negative
voltage
Physics of Semiconductor Devices, Second Edition, Sze, S., Chap. 7,
John Wiley and Sons, New York, 1981
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Terman Method
The Terman method can extract
trap density information from C-V
data:
When a voltage is applied across
a Metal-Iinsulator-Semiconductor
(MIS) structure, part of the voltage
drop appears across the insulator
and part occurs between the bulk
and surface of the semiconductor.
The second contribution is the
“surface potential”
From the charge that
accumulates at the surface, the
surface potential can be
determined
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Terman Method
Some of the charge will come from the
normal doping of the semiconductor,
while some will come from interface
traps
A surface potential function for the
ideal case of no traps can be
constructed
The rate at which the difference
between the actual applied voltage for a
given capacitance and the ideal voltage
at the same capacitance changes with
respect to surface potential is
proportional to the interface traps
exchanging charge at that potential
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Results
16.00
15.00
Capacitance (pF)
14.00
13.00
Foward
12.00
Reverse
11.00
10.00
9.00
-30.00
-20.00
8.00
-10.00
0.00
10.00
20.00
30.00
Voltage (V)
Graph shows CV data for MIS device with 4000Å ZnS passivant
layer
Results
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16.00
15.00
Capacitance (pF)
14.00
13.00
Foward
12.00
Reverse
Ideal
11.00
10.00
9.00
-30.00
-20.00
8.00
-10.00
0.00
10.00
20.00
30.00
Voltage (V)
An ideal curve was constructed in order to use the Terman method
Results
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Dit(traps/eVcm^2)
3E+15
2E+15
1E+15
0
-1.00E-01
0.00E+00
-1E+15
1.00E-01
2.00E-01
3.00E-01
-2E+15
-3E+15
-4E+15
The maximum trap density is about 2*10^15 traps/eVcm^2
The best results previously obtained on MCT gave 5*10^12 traps/eVcm^2. We have a
long way to go
The density is still negative at some points, which does not make sense
Results
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Dit
2E+15
1.5E+15
1E+15
Dit
5E+14
0
-1.00E-01
0.00E+00
-5E+14
1.00E-01
2.00E-01
Same set of data, reverse pass
3.00E-01
InAs/GaSb C-V Curve
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Capacitance (pF)
6.20E+01
6.10E+01
6.00E+01
5.90E+01
5.80E+01
5.70E+01
5.60E+01
5.50E+01
5.40E+01
-2.00E -1.50E -1.00E -5.00E 0.00E+ 5.00E+ 1.00E+
+01
+01
+01
+00
00
00
01
Voltage (V)
The capacitance grows again after reaching a minimum rather than saturating
at a low value. This may indicate that the frequency is not high enough
There is a bump on the C-V curve. The cause of this is unknown
The Terman method can not be used on this data because it assumes high
frequency
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Acknowledgements
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NSF CTS-0630470 & 0434201 GOALI: Atomic-scale
Investigation of High Dielectric Constant Thin Films Using In Situ
and Other Techniques, (Director C.G. Takoudis)
NSF EEC-0453432 Grant, Novel Materials and Processing in
Chemical and Biomedical Engineering (Director C.G. Takoudis),
funded by the DoD-ASSURE and NSF-REU Programs
Shubhrangshu Mallick
Dr. An
Professor Ghosh
Professor Takoudis