1. [Nonuniform primary photocurrent spreading in quantum well infrared photoconductors]
M. Cook
Solid State Physics
Phys 8510

2. Probably no electricity in the caves of Tora Bora!
$ 25,000,000.00
Probably no electricity in the caves of Tora Bora!
Even in the darkest caves, we can easily distinguish this man from Mr. Laden

3. Background information
Metal vs. Semiconductor
Drift-diffusion approximation
Analytical equations describing physical processes in QWIP
Rate equation(s)
Continuity equation

4. Simplified 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

5. Physical Processes in QWIP
What is the lateral current distribution here?
Ershov et al., J. Appl. Phys. 82, 1446 (1997)

6. Bias Voltage 5 QWs Bandedge profile “Band-bending” distance
GaAs / Al0.25Ga0.75As
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)

7. Basic Equations (current density):
Assume current density:
-Drift-diffusion approximation
electrons per unit volume
electron mobility
Diffusion coefficient
Electrostatic scalar potential

8. (exact locations of QWs)
Basic Eqns. of model (continuity equation for continuum state electrons) – steady state
Source terms If
(exact locations of QWs)
What about when

9. Source 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

10. Current density (A/cm2)
r (10-5 cm)

11. J (r) r

12. Plots 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
< r > (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)

13. < r > (10-5 cm)
Ds (cm2s-1)