US EPA

2072626 

Technical Report 

Concentration Fluctuation Degradation of FPA'S 

Sher, A; Muller, MW 

1999 

NTIS/02930757_a 

GRA and I 

GRA and I 

Semiconductor alloys like Hg(1-x)Cd(x)Te and Al(1-x)Ga(x)As, where there is a close lattice constant match between the constituents, are nearly random. However, for mesoscopic size scales of radius r, that are large compared with a lattice constant ((caret)25 A and lt;r and lt;(caret)250 A), but small compared with typical device dimensions, the number of substitutional sites is small enough so the root-mean-square concentration, (Delta x)sup 2 sup 1/2, is sufficiently large to produce random arrays of mesoscopic 'quantum dots' that can adversely affect many device properties. The influence of the adverse effects differ among various properties for example, electron and hole mobilities, lifetimes, and so on but in general are expected to be worse the smaller the fundamental gap becomes. These kinds of fluctuations are suppressed in lattice-mismatched alloys like Hg(1-x)Zn(x)Te and Ga(1-x)In(x)As because there is a long-range strain energy penalty associated with them. Thus, lattice-mismatched alloys are more spatially uniform than lattice-matched alloys. In this paper, a calculation of the impact of concentration fluctuations, among regions large enough so it is appropriate to assign them a band structure ((caret)390x780x780 A cubed), on tunneling currents is presented. An enhancement of (caret)1.20 ((caret)20%) in the average tunneling current density for LWIR material is found, and its root mean square deviation is 0.90. For 25 micrometers sized pixels this translates to 0.3% variations among pixels due to this effect. This estimate does not include the impact of smaller regions that must be treated quantum mechanically. As pixel sizes decrease the impact of this variation will become more significant. Surprisingly, the effect of fluctuations on the leakage current characteristics of VLWIR (20 micrometers cut off) pixels is also small.