High-density InAs/GaAs_1-xSb_x quantum-dot structures grown by molecular beam epitaxy for use in intermediate band solar cells

InAs quantum-dot structures were grown using a GaAs_1-xSb_x matrix on a GaAs(001) substrate. The use of GaAs_1-xSb_x for the buffer and cap layers effectively suppressed coalescence between dots and significantly increased the dot density. The highest density (∼3.5 × 10¹¹/cm²) was obtained for a nominal 3.0 monolayer deposition of InAs with an Sb composition of x = 13-14% in the GaAs_1-xSb_x matrix. When the Sb composition was increased to 18%, the resulting large photoluminescent red shift (∼90 meV) indicated the release of compressive strain inside the quantum dots. For x > 13%, we observed a significant decrease in photoluminescence intensity and an increase in the carrier lifetime (≥4.0 ns). This is attributed to the type-II band alignment between the quantum dots and matrix material.