The optoelectronic properties of Ge 1−x−y Si x Sn y /Ge 1−x Sn x /Ge 1−x−y Si x Sn y double heterostructures pseudomorphically grown on a Ge substrate were investigated. The photoluminescence (PL) intensity of the sample with Ge 0.66 Si 0.23 Sn 0.11 cladding layers is three times larger compared to PL from structure with a Ge cladding layer, which can be attributed to higher energy band offsets at both conduction and valence band edges at the Ge 0.91 Sn 0.09 /Ge 0.66 Si 0.23 Sn 0.11 interface. The PL spectrum of the sample with the Ge 0.66 Si 0.23 Sn 0.11 cladding layer at room temperature can be deconvoluted into four components, and the origins of these components can be assigned to direct and indirect transitions by measuring the temperature dependence of each component's intensity. In addition, we examined the formation and characterization of strain-relaxed Ge 1−x−y Si x Sn y /Ge 1−x Sn x /Ge 1−x−y Si x Sn y double heterostructures to relieve the compressive strain in the Ge 1−x Sn x layer. Stacking faults were observed in the Ge 1−x Sn x and Ge 1−x−y Si x Sn y layers. The PL peak intensity of the strainrelaxed Ge 1−x Sn x layer decreases by a factor of 1/20 compared to the PL peak intensity of the double heterostructure pseudomorphically grown on a Ge(001) substrate. In addition, PL intensity can be increased by post-deposition annealing owing to decreasing defects.

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We have investigated the formation and optoelectronic properties of strain relaxed Ge 1−x−y Si x Sn y /Ge 1−x Sn x /Ge 1−x−y Si x Sn y double heterostructures on ion-implanted Ge substrates. The strain relaxation of Ge 1−x−y Si x Sn y and Ge 1−x Sn x epitaxial layers was achieved using an ionimplanted Ge substrate. The maximal degree of strain relaxation (DSR) of the Ge 1−x Sn x layers was evaluated to be 46%. In addition, we obtained a sharp and strong peak in the photoluminescence (PL) spectra from the sample with a DSR of 46%, while no strong peak was detected from a sample with a smaller DSR (22%). From the theoretical calculation of the energy band structure and the measurement temperature dependence of the PL intensity, the sharp and strong peak can be explained by the transition from an indirect to direct bandgap semiconductor due to the increase of the DSR and a concomitant increase of the Γ-valley electron population. Moreover, the PL intensity increases by the improvement of the crystallinity by a post deposition annealing process.

The energy band alignment of Ge 1−x Sn x /Ge 1−x−y Si x Sn y heterostructures was investigated, and control of the valence band offset at the Ge 1−x Sn x /Ge 1−x−y Si x Sn y heterointerface was achieved by controlling the Si and Sn contents in the Ge 1−x−y Si x Sn y layer. The valence band offset in the Ge 0.902 Sn 0.098 /Ge 0.41 Si 0.50 Sn 0.09 heterostructure was evaluated to be as high as 330 meV, and its conduction band offset was estimated to be 150 meV by considering the energy bandgap calculated from the theoretical prediction. In addition, the formation of the strain-relaxed Ge 1−x−y Si x Sn y layer was examined and the crystalline structure was characterized. The epitaxial growth of a strain-relaxed Ge 0.64 Si 0.21 Sn 0.15 layer with the degree of strain relaxation of 55% was examined using a virtual Ge substrate. Moreover, enhancement of the strain relaxation was demonstrated by post-deposition annealing, where a degree of strain relaxation of 70% was achieved after annealing at 400 °C. These results indicate the possibility for enhancing the indirect-direct crossover with a strained and high-Sn-content Ge 1−x Sn x layer on a strain-relaxed Ge 1−x−y Si x Sn y layer, realizing preferable carrier confinement by type-I energy band alignment with high conduction and valence band offsets.