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This study describes variable-area diode data analysis of surface and bulk effects of HgCdTe infrared photodiodes
passivated with dual-layer CdTe/ZnS films. We attempt to present a general analytical relation between the zero-bias
resistance-area product and the perimeter-to-area ratio of the diodes by variable-area diode array test structures. We have
taken contributions into consideration from surface leakage between HgCdTe and passivant due to band bending, surface
generation currents in the depletion region close to the HgCdTe-passivant interface, and the bulk currents. The model we
use is based on the one put forward by Vishnu Gopal. The variable-area diode data analysis can be of great practical help
in identifying the various possible mechanism contributing to the surface leakage currents. Through data analysis and
curve fitting, we can also get some other useful parameters (like junction depth), which can be the reference to other
experiment results. The experimental samples we used range from 20μm to 200μm in size and include both square and
circular diode geometries. The conventional boron implantation was used to form the p-n junction and Au was used for
the metal pads. The insulating layers of CdTe and ZnS were both electron-beam evaporated at a rate of 1.3 Å/sec. The
fabricated diode test patterns were wire-bonded and packaged into a dewar system. I-V measurements were performed
using a Keithley 4200 parameter analyzer. The data analysis and curve fitting are all dealt with by MATLAB.
Through the results we can find that the surface leakage is nearly the same to the bulk current in diameter between
50~150μm, which indicate that surface leakage is still a dominating dark current in small dimension diode. The results
also showed that diodes from 50 to 150μm in size have better performance than the larger or smaller ones and this can be
explained by the limit of material imperfection and the limit of processing techniques.
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