| The ability to integrate the optical properties of III–V semiconductors with the current capabilities of Si microelectronics on a single substrate has been of great interest for many years. In addition, III–V/Si integration is also of great interest for space photovoltaics applications in order to combine high performance space cells with a strong, lightweight and inexpensive substrate. However, due to fundamental materials incompatibilities, namely the 4% lattice mismatch between GaAs and Si and the >63% mismatch in thermal expansion coefficient, epitaxial GaAs/Si integration has been largely unsuccessful due to uncontrolled threading dislocation (TD) nucleation resulting in thread densities greater than 109 cm−2. Although many integration techniques have been able to successfully reduce TD's to ∼3–7 × 106 cm−2 (including thermal cycle annealing and the insertion of various III–V buffer layers), none has achieved device performance equivalent to homoepitaxial GaAs due to the residual threading dislocation density. Recently, GeSi graded buffers have provided an integration technique, which has successfully reduced threading dislocation densities (TDD's) below 1 × 106 cm−3 for a 100% Ge cap on a Si substrate. This integration approach is unique in the respect that it does not attempt to provide strain management or dislocation engineering within the III–V layers (similar to other techniques) but rather through the gradual and controlled introduction of strain during a GexSi1−x grade from a 100% Si to a 100% Ge surface.; In this research, we investigate the application of the GeSi graded buffer for GaAs/Si integration, specifically for application to high efficiency single junction solar cells. Through atomic control of the GaAs/GeSi interface via MBE, the elimination of anti-phase boundary and TD nucleation as well as the minimization of atomic diffusion at the GaAs/GeSi interface was achieved for the GaAs/GeSi system. Combined with the low TDD enabled by the GeSi graded buffer, record GaAs/Si minority carrier lifetimes in excess of 10 ns have been achieved, more than tripling the best lifetime achieved by any other GaAs/Si integration technique to date, ∼3ns. Extending this material quality to devices, Voc's in excess of 1040 mV were achieved for single junction GaAs solar cells, more than a 10% improvement over the previous best achieved for GaAs/Si (940mV). These results demonstrate the promise of this integration technique and will result in the realization of GaAs/Si solar cells with efficiencies exceeding 20% AM0, suggesting that the GaAs/GeSi epitaxial integration methodology is a viable and beneficial alternative for space solar applications. |
