Direct band gap luminescence from Ge on Si pin diodes

Abstract

Germanium (Ge) pin photodiodes show clear direct band gap emission at room temperature, as grown on bulk silicon in both photoluminescence (PL) and electroluminescence (EL). PL stems from the top contact layer with highly doped Ge because of strong absorption of visible laser light excitation (532 nm). EL stems from the recombination of injected carriers in the undoped intrinsic layer. The difference in peak positions for PL (0.73 eV) and EL (0.80 eV) is explained by band gap narrowing from high doping in n⁺-top layer. A superlinear increase of EL with current density is explained by a rising ratio of direct/indirect electron densities when quasi Fermi energy level rises into the conduction band. An analytical model for the direct/indirect electron density ratio is given using simplifying assumptions.

Keywords

photoluminescence (PL) / electroluminescence (EL) / germanium (Ge) / direct band gap

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Acknowledgements

The authors thank O. Kirfel, M. Kaschel and M. Schmid for assistance with growth, device processing and measurements. Discussions with J. Schulze are acknowledged. Thanks to K. Ye for help with manuscript preparation. This work was supported by DFG (German Research Foundation).

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2014 Higher Education Press and Springer-Verlag Berlin Heidelberg