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

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

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

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

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

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)

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

(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

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

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

J (r) r

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)

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