Some results on HgCdTe surface passivation

HgCdTe photodiodes have been successfully passivated by a thin native oxide film. In this work, the diode characteristics and the results of Metal-Insulator-Semiconductor interface property measurements will be presented. n⁺-p mesa photodiodes were fabricated by boron implantation into HgCdTe epitaxial layers. Their active area is about 2 × 10^-4cm²and the spectral cut-off wavelengths at 77K range from 4.7 µm to 6.9 µm. The temperature dependence of the RoA products was measured both before and after the passivation process as a means of monitoring the surface leakage. For the 6.9 µm spectral cut-off diode before passivation, the initial RoA (77K) was 300 Ω-cm²and subsequently degraded to a value of 20 Ω-cm². The RoA vs 1/T plot showed that the leakage current at 120K or lower was surface dominated. After passivation, the RoA product increased dramatically to 5500 Ω-cm². Similarly, for other diodes with spectral cut-offs at 5.6 µm and 4.7 µm (77K), RoA values increased from 2 × 10⁴Ω-cm²and 10⁴Ω-cm²to 10⁶Ω-cm²and 3 × 10⁷Ω-cm²after passivation, respectively. The I-V characteristics of the passivated devices indicate that at 77K the leakage current was mainly G-R limited. Unlike the bare surface diodes, these devices are stable in vacuum after repeated thermal cycling to low temperature. MIS diodes with this native oxide show C-V curves closely following the ideal capacitance model. They have a sharp transition from accumulation to inversion; the flatband voltage is less than 0.5 volts and the hysteresis is less than 0.2 volts. This is an indication of excellent interface properties. The estimated fast interface state density is in the range of low 10¹¹cm^-2eV^-1.