Surface passivation and 1/f noise phenomena in HgCdTe photodiodes

Abstract

1/f noise currents have been measured in HgCdTe photodiodes and gate-controlled diodes as a function of gate bias, diode voltage, and dark (leakage) currents. The diodes are fabricated by ion implanting boron (n+) on bulk p-type material with x=0.22. Native anodic sulfide in combination with deposited ZnS is used for surface passivation. The measurements demonstrate that the dominant mechanism that produces 1/f noise in HgCdTe photodiodes is tunneling. With the gated diodes it is possible to separate bulk and surface related mechanisms responsible for the 1/f noise currents. 1/f noise currents are produced by surface-induced tunneling at pinched-off depletion regions adjacent to accumulated surfaces and by tunneling across field-induced junctions underneath inverted surfaces. Bulk trap-assisted tunneling that limits the R0A product below 50 K but is masked by diffusion at 77 K, is responsible for the 1/f noise currents at optimized surface potentials, (i.e., approximately flat-band conditions) at low bias voltages. For nonoptimized surface potentials, the tunneling currents are either band to band or trap assisted, depending on material properties. The noise associated with band to band tunneling is modeled with In=αt(It)1/2( f )−1/2 where It is the measured tunneling current. The proportionality factor αt between the noise current and the tunneling current in gate-controlled devices is in the range αt=4×10−7√A to αt=1×10−8√A. HgCdTe photodiodes with the same passivation technology but without a gate, fabricated in higher-quality bulk material, operating at high reverse bias, yield αt values that range between αt=2×10−7√A for small area diodes (junction area equals to 6×10−5 cm2) to αt=6×10−8√A for large area diodes (with junctions area equal to 1.45×10−3 cm2). The noise associated with trap-assisted tunneling is modeled with In=αt(It)β( f )−1/2 where β is approximately 0.75. The numerical values of αt are strongly dependent upon material properties and the dimensions are (A)1−β.