Dislocation density reduction of virtual Ge substrates by CW diode laser treatment

Ge wafers have been employed as substrates for high efficiency III-V multi-junction solar cells because of the almost ideal lattice match. However, the cost of Ge wafers is high and the Ge bandgap is too small which limits its contribution to the overall voltage output. Since Si has much higher bandgap and lower cost than Ge, using epitaxial Ge on Si as virtual substrates for III-V material growth is a promising solution. However, the challenge is the high threading dislocation density (TDD) in the Ge layer generated by the large lattice mismatch (4%) between Ge and Si. Thermal annealing has been intensively investigated in order to reduce TDD in the Ge layer deposited on Si. Two-step low temperature - high temperature Ge deposition followed by thermal annealing and cycled epitaxial Ge growth and annealing have been studied using CVD systems. Compared with conventional thermal processes, the CW diode laser treatment offers a fast and low-cost alternative to effectively reduce the TDD in Ge films. By laser scanning the sample, the Ge layer can be melted and recrystallized. After the laser treatment, the TDD of Ge films can be reduced by three orders of magnitude to 10⁶ cm^-2 which may be suitable for fabrication of III-V solar cells. Raman spectroscopy and Transmission Electron Microscopy measurements are employed to investigate the TDD reduction of the virtual Ge substrates.