The effects of vacuum baking on theⅠ-Ⅴcharacteristics of LWIR HgCdTe photodiodes

摘要

Vacuum bake-out, for out-gassing is a required process in packaging of devices which are designed to operate at cryogenic temperatures. This process may be problematical for HgCdTe devices, even at relatively low temperatures, since the material is sensitive to heat induced changes. The effect of vacuum baking on HgCdTe photodiode characteristics and performance is investigated throughⅠ-Ⅴand spectral responsivity measurements. The photodiodes were fabricated on LPE grown HgCdTe on lattice matched CdZnTe substrates. The surface was passivated with thermally evaporated CdTe and the p-n junction was formed by plasma induced p-to-n type conversion. TheⅠ-Ⅴcharacteristics of the devices were then measured and the devices were baked under vacuum for varying times at 80 ℃. This simulates the vacuum bakeout required in vacuum packaging of the devices in cryogenic dewars. The results indicate that the vacuum baking process can significantly modify the Ⅰ-Ⅴ characteristics of the photodiodes. There is an initial improvement in device characteristics after a 6 hour bake at 80 ℃, with a five times increase in the zero-bias dynamic resistance. Further baking sees the dynamic resistance decrease slightly. An insight into the mechanisms and parameters that are affected by the vacuum baking process is also gained by studying the Ⅰ-Ⅴ characteristics of the fabricated photodiodes before and after baking. It is observed that tunnelling dark currents are significantly reduced after baking.