The band structure of self-assembled Si/Ge quantum dot structures deposited by molecular beam epitaxy in the Stranski Krastanov growth mode is characterized by optical and electrical spectroscopy. Interband and intraband absorption, photocurrent, photoluminescence, Raman and admittance spectroscopy of structures with quantum dots of about 20 nm lateral size offer insight into the discrete level scheme within the valence band, the optical transitions and the lifetime of localized hole states. The results are discussed with respect to their possible applications in infrared light detection, storage and quantumlogic devices.

Ge quantum dots in Si were fabricated by molecular beam epitaxy in the Stranski Krastanow growth mode at low substrate temperatures and were investigated by optical and electrical spectroscopy. The dot size is about 20 nm in width and 2 nm in height for a substrate temperature of about 510 C. The effective valence band structure of such Si/Ge quantum dots is consistently analyzed by type-II interband photoluminescence transitions between electrons in the Si host and holes localized within the Ge dots. by bound-to-quasibound intra-valence band transitions of localized holes in the mid-infrared spectral range and by C-F and admittance spectroscopy. Holes localized in the dot ground states reveal in ionization energy of about 350 meV corresponding to the effective Si/Ge dot valence band offset. The energy separation of zero-dimensional ground and first excited levels clue to lateral confinement is about 40 meV, and the Coulomb charging energy for the second hole within the ground state is about 15 meV. A slow thermal excitation rate of holes out of the dots at low temperature strongly affects optoelectronic properties like photocurrent response, The impact of elastic and electronic coupling of dots in self-aligned stacks on band structure is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

We studied the vertical correlation of small hut-like Ge islands in Si–Ge multilayers grown by molecular beam epitaxy (MBE) at 510°C in the modified Stranski–Krastanov growth mode. The fundamental, structural and optical properties of stacked Ge islands embedded in Si are analyzed by in-situ RHEED, TEM, photoluminescence (PL), and photocurrent (PC) spectroscopy. Ge islands of about 20 nm width and 2 nm height reveal a reduction in the critical Ge coverage for correlated growth for Si spacer widths below about 14 nm. This value is much smaller than observed for dome-like islands of about 80 nm size deposited at higher temperatures. Such a scaling of island separation for stacked growth with island diameter is suggested by finite element simulations of elastic strain relaxation in islands. The correlation affects island size, island density, and local strain fields which may have a direct impact on band offsets and the electronic coupling of stacked islands. This is important for possible application in devices like NIR Si–Ge dot photodetectors and tunneling structures.