Si/compound semiconductor heterojunctions are assumed to be one of the key components in realizing functional devices composed of these materials such as tandem cells. Heterojunctions made of p-Si and n-GaAs substrates were fabricated by using hybrid approaches (surface-activated bonding). Their structural and electrical properties were investigated by SEM/EDS observations, I-V, and C-V measurements.
FE/SEM and EDS observations of Si/GaAs heterojunctions were performed. No mechanical deficits appeared at the interfaces. The EDS results indicated that atomic percentages of oxygen at the interfaces (a2 below) were close to those in the Si and GaAs substrates (a1, a3), which suggested that no residual oxide layers remained at the interfaces.
The p-Si/n-GaAs heterojunctions revealed unsymmetric I-V characteristics similar to those in conventional pn junctions. Their turn-on voltage at room temperature (0.38 V) was lower than expectations. The current for reverse-bias voltages increased and the I-V characteristics got closer to those with ohmic properties as the temperature was raised. The measurements of the capacitance-voltage characteristics showed the built-in potential of 1.6 V, indicating the conduction band discontinuity of 0.6 eV.
(Note) This work was jointly performed with NTT Photonics Laboratories.