Valence band features affecting carrier transport in III-V superlattice nBn detectors.
David R. Rhiger and Edward P. Smith,
Raytheon Vision Systems.
We have investigated non-ideal features occurring in the valence band profile of nBn detectors which affect the transport of minority-carrier holes representing the IR signal. The objectives are to reduce dark currents and improve quantum efficiency. The nBn device consists of an n-type absorber several microns thick, plus a very thin electron barrier B and a thin n-type collector (top contact region). In this investigation, the absorber and collector were built with the InAs/InAsSb superlattice. For normal operation, holes generated by photons in the absorber must flow to the collector. Current is promoted by a combination of diffusion and electric field drift. However, in some cases the transport of holes is limited by (1) absorber-barrier valence band misalignment, (2) bandgap difference between collector and absorber, or (3) possible localization sites in the absorber due to compositional fluctuations. These characteristics, when combined with the known limitations of hole diffusion length, can adversely affect the quantum efficiency, and require the application of an operating bias that is larger than otherwise necessary. We have been able to identify and measure these valence band features by analyzing device characteristics as a function of temperature, bias voltage, photon flux, and wavelength dependence of the response. Examples will be presented.
This work was supported by Dr. Meimei Tidrow of NVESD, Contract Number W15P7T-06-D-E402, Task BD30, Agreement No. S08-092256, Purchase Order P000006939.