PROCEEDINGS ARTICLE | May 7, 2009
Proc. SPIE. 7298, Infrared Technology and Applications XXXV
KEYWORDS: Staring arrays, Readout integrated circuits, Avalanche photodetectors, Mercury cadmium telluride, Cadmium, Dispersion, Photons, Diffusion, Doping, Medium wave
The optimization of HgCdTe avalanche-photo-diodes (e-APD) and focal plane arrays (FPA) are reported. The gain
performances was measured in planar homo-junction APDs with Cd compositions between xCd=0.3 to 0.41.
Exponentially increasing gain, synonym of exclusive electron multiplication, was observed in all the devices up to
M>100. The high gain at high xCd opens the path to thermo-electric cooled active imaging at T=200K. This perspective is
corroborated by the demonstration of high gains and low excess noise factor F=1.2 in extrinsically doped MW-APDs,
which enables reduced dark-current at high temperatures. The equivalent input dark current (Ieq_in) decreased from 200
fA to 1 fA, when the cut-off wavelength was reduced from λ
c=5.2 μm to 4.1 μm at M=100 and T=80 K. This shows that
sensitivity can be optimized by increasing xCd, at the cost of increased reverse bias. A new horizontal-gain-well (HGW)
hetero-structure was processed to optimize the sensitivity at high gain and low bias. The first HGW-APDs had gains
comparables with MW e-APDs and 50 times lower dark-current at T=200 K. They did also display surprisingly high
quantum efficiency in the MWIR range, ηpeak=30%, which enables thermal imaging at high operating temperature. The
high performance of MW-APDs was confirmed by the characterizations of a first 320x256 30μm pitch APD-FPA,
yielding a 99.8 % operability, low gain dispersion (<10%) and low noise equivalent photons (NEPh=3 at tint=1 μs) for
gains up to M=70. The maturity of the DEFIR HgCdTe e-APD-FPA technology was highlighted by the first
demonstration of passive amplified imaging.
