Mid-infrared photocurrent measurements on self-assembled Ge dots in Si

We report on mid-infrared photocurrent spectroscopy of p-doped Ge quantum dots in Si. The Ge dots were fabricated by self-assembling in the Stranski–Krastanov growth mode using molecular beam epitaxy. Cross-sectional transmission electron microscopy reveals a lens-like dot shape with a lateral size of about 70 nm. Photocurrent is obtained between 200 meV (View the MathML sourcem) and 600 meV (View the MathML sourcem) in waveguide as well as normal incidence geometry for measurement temperatures up to 100 K. The strong high-energy tail is mainly attributed to intravalence band transitions from states bound in the Ge dots to continuum states. This is supported by the polarization dependence of the photocurrent as well as by interband photoluminescence.