Title: [Nonuniform primary photocurrent spreading in quantum well infrared photoconductors]
1Nonuniform primary photocurrent spreading in
quantum well infrared photoconductors
- M. Cook
- Solid State Physics
- Phys 8510
2Probably no electricity in the caves of Tora Bora!
Even in the darkest caves, we can easily
distinguish this man from Mr. Laden
3Background information
- Metal vs. Semiconductor
- Drift-diffusion approximation
- Analytical equations describing physical
processes in QWIP - Rate equation(s)
- Continuity equation
4Simplified model we incorporated
- Assumed the dark current could be made negligibly
small by making QWIP with large barrier width
when compared with QW width - Neglected contact effects which have been shown
to play important roles for QWIP performance.
This is especially true for detectors having a
relatively small number of QWs - thermoionically assisted injection current
- Only study steady state mode of operation
- this case is assumed to give threshold amount of
lateral spreading which would be present in any
QWIP described by our model
5Physical Processes in QWIP
What is the lateral current distribution here?
Ershov et al., J. Appl. Phys. 82, 1446 (1997)
65 QWs
GaAs / Al0.25Ga0.75As
0
Conduction band diagrams for different applied
voltages
.2 V
Bias Voltage
Bandedge profile
.5 V
Band-bending
.8 V
distance
Ershov et al., J. Appl. Phys. 82, 1446 (1997)
7Basic Equations (current density)
- Assume current density
-
- -Drift-diffusion approximation
electrons per unit volume
electron mobility
Diffusion coefficient
Electrostatic scalar potential
8Basic Eqns. of model (continuity equation for
continuum state electrons) steady state
Source terms
0
If
(exact locations of QWs)
What about when
9Source terms (continuum state electrons)
when x is any multiple of the fundamental length
a, i.e. exactly at a QW, the continuity equation
is no longer homogeneous and must now be
described by some Ij which is dependent on QW
index
Optical excitation of bounded electrons
Thermal excitation of bounded electrons
Capture of continuum state electrons
10Current density (A/cm2)
r (10-5 cm)
11J (r)
r
12Plots of average lateral smearing vs. Ds for
three separate systems
For systems with many periods, the performance of
QWIP is a strong function of Ds
lt r gt (10-5 cm)
Spreading is more pronounced for Ds 102 cm2s-1
Optimal performance of device for points of
minima on the graph
Ds (cm2s-1)
13lt r gt (10-5 cm)
Ds (cm2s-1)