Ge incorporation in HfO2 thin films deposited on Ge substrates by electron-beam evaporation was investigated during dry/wet thermal annealing in normalN2 ambient. The structural and chemical changes and the electrical properties of HfO2 were monitored by angle-resolved X-ray photoemission spectroscopy and current–voltage characteristics. Considerable Ge incorporation as GeOx in HfO2 on the Ge substrate during the deposition and annealing processes led to the formation of germanate of hafnium and the deterioration of the leakage current after thermal annealing. The oxidation degree of the incorporated GeOx increased with wet annealing time. It was indicated that Ge incorporated in HfO2 came from Ge upward diffusion from the Ge substrate and the GeO evaporation from the uncovered back side of the Ge substrate.

Metamorphic In Ga P/In Ga As/Ge triple-junction (3J-MM) solar cells are grown on Ge (100) substrates via metal organic chemical vapor deposition. Epi-structural analyses such as high resolution x-ray diffraction, photoluminence, cathodoluminescence and HRTEM are employed and the results show that the high crystal quality of 3J-MM solar cells is obtained with low threading dislocation density of graded buffer (an average value of 6.8 10 /cm . Benefitting from the optimized bandgap combination, under one sun, AM0 spectrum, 25 C conditions, the conversion efficiency is achieved about 32%, 5% higher compared with the lattice-matched In Ga P/In Ga As/Ge triple junction (3J-LM) solar cell. Under 1-MeV electron irradiation test, the degradation of the EQE and - characteristics of 3J-MM solar cells is at the same level as the 3J-LM solar cell. The end-of-life efficiency is 27.1%. Therefore, the metamorphic triple-junction solar cell may be a promising candidate for next-generation space multi-junction solar cells.

The InAs/GaAs quantum dots structure embedded in GaInP/Ga(In)As/Ge triple junction solar cell with and without Ga0.90In0.10As strain reducing layer was investigated. Conversion efficiency of 33.91% at 1,000 suns AM 1.5D with Ga0.90In0.10As strain reducing layer was demonstrated. A 1.19% improvement of the conversion efficiency was obtained via inserting the Ga0.90In0.10As strain reducing layer. The main contribution of this improvement was from the increase of the short-circuit current, which is caused by the reduction of the Shockley–Read–Hall recombination centers. Consequently, there was a decrease in open circuit voltage due to the lower thermal activation energy of confined carriers in Ga0.9In0.1As than GaAs and a reduction in the effective band gap of quantum dots.

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