  Physical models of Ge energy band structure and material loss were
implemented in LASTIP(TM), a 2D simulation tool for edge emitting laser diodes.
The model calculation is able to match experimental data available. Important
design parameters of a Fabry-Perot Ge laser, such as the cavity length,
thickness, width, polycrystalline Si cladding layer thickness were studied and
optimized. The laser structure optimizations alone were shown to reduce Ith by
22-fold and increase the differential efficiency by 11 times. The simulations
also showed that improving the defect limited carrier lifetime is critical for
achieving an efficient and low-threshold Ge laser. With the optimized structure
design (300 micron for the cavity length, 0.4 micron for the cavity width, 0.3
micron for the cavity thickness, and 0.6 micron for the polycrystalline Si
cladding layer thickness) and a defect limited carrier lifetime of 100 ns, a
wall-plug efficiency of 14.6% at 1mW output is predicted, where Jth of 2.8
kA/cm2, Ith of 3.3 mA, I_1mA of 9 mA, and differential efficiency of 23.6% can
also be achieved. These are tremendous improvements from the available
experimental values at 280 kA/cm2, 756 mA, 837 mA and 1.9%, respectively.
