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Frontiers of Optoelectronics

Front Optoelec    2012, Vol. 5 Issue (3) : 256-260     DOI: 10.1007/s12200-012-0235-4
Direct band gap luminescence from Ge on Si pin diodes
1. Institut für Halbleitertechnik (IHT), University of Stuttgart, Stuttgart 70569, Germany; 2. Joint Lab IHP/BTU Cottbus, Cottbus 03013, Germany
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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     
Corresponding Authors: KASPER E.,   
Issue Date: 05 September 2012
 Cite this article:   
M. OEHME,J. WERNER,T. AGUIROV, et al. Direct band gap luminescence from Ge on Si pin diodes[J]. Front Optoelec, 2012, 5(3): 256-260.
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Fig.1  Basic scheme of Ge pin structure and of luminescence experiments. MD misfit dislocation network; TD threading dislocation; photoluminescence (PL) with 532 nm laser excitation; electroluminescence (EL) via recombination of injected electrons (e) and holes (h)
Fig.2  PL spectrum of the Ge-pin photodiode (mesa diameter: 160 μm). Main lines are assigned (from left) to dislocations, narrowed direct band gap luminescence from n+ top layer (0.73 eV), and direct band gap luminescence from intrinsic Ge (0.80 eV)
Fig.3  EL spectrum of Ge-pin photodiode. Forward biased to 200 mA; Main line (0.80 eV) is from the direct gap of intrinsic Ge
Fig.4  Occupation scheme of the Ge conduction band with direct gap (Γ). The Fermi energy is assumed to be above the L valley (see text)
Tab.1  Density ratio of direct /indirect states (ln/) given as function of the normalized carrier density /. The model (Eq. (6)) is assumed with Δ = 136 meV and = 300 K
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