Intersubband absorption in p-type Si_1-xGe_x quantum dots on pre-patterned Si substrates made by a diblock copolymer process

In this work, intersubband absorption in the valence bands of Si_1-xGe_x quantum dots is investigated. Using molecular beam epitaxy, boron-doped (10²⁰ cm^-3) Ge dots with base dimensions of 15 nm, heights of 5-7 nm, and an aerial dot density of 10¹¹ cm^-2 are grown on pre-patterned Si substrates. The patterned substrate is prepared by a diblock copolymer process. Fourier transform infrared (FTIR) spectroscopy is used to measure the infrared absorption of various structures (with different dot heights and dot compositions). As the height of quantum dots is decreased, the absorption peak is shifted to a longer wavelength region. For low germanium content (x=0.5), there is also a red shift of the absorption peak. The results suggest that we can tune optical properties of Si_1-xGe_x quantum dots for infrared detector applications. The polarization-dependent results demonstrate that these structures have significant normal-incidence absorption. This normal-incidence absorption comes from the strong quantum confinement of the small lateral size and non-zero momentum matrix elements. For low Ge content, there is very strong normal-incidence absorption. This observation suggests that we can control polarization dependence of intersubband absorption by adjusting Ge content.