N Type Doping of Strained Epitaxial Germanium Films Using Ion Implantation Followed by Nanosecond Pulse Laser Melting

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Abstract

The last few years have seen much interest in developing CMOS compatible, Germanium lasers for applications in optical computing, telecommunications, and IR photonics. Band engineering of Ge using a combination of tensile strain and high n-type doping has been experimentally shown to facilitate efficient recombination through the direct bandgap (Gamma-valley (111)), producing population inversion and gain. The gain has been theorized and experimentally shown to increase linearly with active n-type doping. Simulations suggest that for lasing emission near 1550nm in 0.25% tensile strained Ge, an active dopant concentration of up to 10^20/cm3 is the optimal value for maximum gain. However, in-situ Phosphorus (n-type) doping during Ge epilayer growth using Ultra High Vacuum Chemical Vapor Deposition (UHVCVD) is limited to dopant concentrations of 1.2×10^19/cm3. This limit is a consequence of the trade-off between out-diffusion of P during the growth process and the Ge growth temperature required to obtain high quality films. To achieve higher active doping concentrations in Ge films, non-thermal equilibrium techniques are preferred.The first demonstration of an electrically pumped Ge laser relied upon delta doping to achieve the high active dopant concentration required for lasing. Here we will discuss results investigating a potentially more scalable doping method to achieve higher active dopant concentration: a combination of ion implantation and nanosecond Pulsed Laser Melting (PLM). Ion implantation can introduce a high impurity concentration into the Ge. Non-equilibrium nanosecond PLM can be used to recover lattice crystallinity while preserving and activating high dopant concentration.In this work, we report the fabrication of tensile strained epi-Ge layers co-implanted with phosphorus doses of 1.8×10^15/cm2 and fluorine (F) doses of 10^14/cm2 and PLM processed at different fluences with a nanosecond 355 nm Nd:YAG laser. F was used to passivate the vacancies and prevent the vacancy-dopant complexes (E-center) that are known to be an important inhibitor of the dopant activation. We present ion implantation and laser induced melting simulations of the Ge films. We show by means of Scanning Electron Microscopy and Raman spectroscopy a high lattice recovery after PLM processes. However, Raman spectroscopy shows a significant loss of epi-Ge layer strain after PLM. We detect local vibrational modes related with substitutional P sites. This peak has been observed previously only for P dopant concentrations between 2×10^20/cm3 and 10^21/cm3. Photoluminescence (PL) measurements provide further information on the active dopant concentration by determining bandgap narrowing (BGN) due to the high n-type doping.

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