A model for the I-V characteristics of Ge and GeSn epitaxial pin diodes

A quantitative model is presented for the I-V characteristics of GeSn pin diodes. GeSn alloys, with band gaps ranging between those of pure Ge and zero, are viewed as intriguing Si-compatible alternatives for infrared applications. This has motivated sustained efforts to understand the properties of GeSn devices.

The new model incorporates deep and shallow SRH recombination centers, as observed in DLTS studies. Tunneling-assisted-transitions associated with these defects account for the voltage dependence of the reverse bias current. The validity and universality of the approach is initially demonstrated by reproducing the I-V curves of epitaxial Ge-on-Si diodes in the literature. The model is also shown to be in good agreement with the measured I-V characteristics of Ge solar cells under illumination.

To test the applicability of the model to the GeSn system, GeSn pin diodes have been fabricated on Si using CVD techniques that combine stannanes and polygermanes with several precursors compatible with low temperature in situ doping. The compositional dependence of the material parameters is acconted for using experimental values or known extrapolation schemes. It is found that the I-V curves of GeSn diodes are reproduced with no loss of accuracy with respect to pure Ge diodes.

The model presented here represents a significant step toward the development of analytical tools for the rational optimization of GeSn technology.