Characteristics of Germanium n⁺/p junctions formed by phosphorus diffusion from on indium-gallium-phosphide layer

Although numerous studies have been previously reported for the formation of Ge p-n junctions, there is still a lack of research on Ge junctions formed by solid-phase diffusion doping, which typically uses the diffusion phenomenon of phosphorus (P) atoms from InGaP for the fabrication of a Ge n⁺/p subcell in a III-V multi-junction cell. Here, we investigate the characteristics of Ge n⁺/p junctions achieved by the InGaP-based diffusion technique at 450-650 °C with SIMS, ECV, and J-V analyses. In addition, through a multiple error function fitting method, diffusivity, peak position, and activation energy values are accurately estimated from raw In/Ga/P/Ge SIMS profiles. The extracted activation energy values for In/Ga/P atoms are much lower than previously reported, indicating that a faster diffusion phenomenon occurs during the simultaneous diffusion of In/Ga/P into Ge. A non-annealed InGaP-deposited junction shows Ohmic behavior with a high current density because of leakage currents by many interfacial point defects. After a 550 °C anneal, the current density is reduced by 3-4 orders of magnitude and a small on/off-current ratio is obtained. Compared to this 550 °C annealed junction, a current density increases ∼10 times in the 650 °C sample due to an increased n-type carrier concentration.