Germanium oxide passivation for Ge absorber

In photovoltaic industry, Ge material also plays an important role for III-V solar cells since Ge with smaller band-gap than Si is used as the bottom junction to absorb the infrared (IR) light, so the Ge layer passivation becomes more important. GeO₂ is a suitable material to passivate the surface due to the decrease of the interface defect density (D_it) and the introduction of positive fixed charge. The excess carriers in the bulk Ge are generated without nonradiative recombination since the front surface recombination velocity (SRV) is reduced by GeO₂ layer passivation. The metal oxide semiconductor (MOS) capacitors show almost no frequency dispersion from 1 K to 1 MHz at room temperature for oxidation time of 1 and 5 hour. The minimum value of D_it with 5 hour oxidation treatment is obtained around 1.8×10¹¹ cm^-2eV^-1 by high-low frequency method. Furthermore, adding Al₂O₃ capping layer on GeO₂ layer is used to avoid moisture in the air.