H. Lichtenberger

Initial stage of the two-dimensional to three-dimensional transition of a strained SiGe layer on a pit-patterned Si(001) template

We investigate the initial stage of the 2D-3D transition of strained Ge layers deposited on pit-patterned Si(001) templates. Within the pits, which assume the shape of inverted, truncated pyramids after optimized growth of a Si buffer layer, the Ge wetting layer develops a complex morphology consisting exclusively of {105} and (001) facets. These results are attributed to a strain-driven step-meandering instability on the facetted side-walls of the pits, and a step-bunching instability at the sharp concave intersections of these facets. Although both instabilities are strain-driven, their coexistence becomes mainly possible by the geometrical restrictions in the pits. It is shown that the morphological transformation of the pit surface into low-energy facets has strong influence on the preferential nucleation of Ge islands at the flat bottom of the pits.

Lateral quantum dots in Si∕SiGe realized by a Schottky split-gate technique

Lateral quantum dots are formed in the two-dimensional electron gases of a high-mobility Si∕SiGe heterostructures by means of split Schottky gates. Palladium gates, defined by e-beam lithography and lift-off, show Schottky barriers with very well controlled leakage currents. At low temperatures we observe Coulomb-blockade and stability diamonds on lateral quantum dots containing a total charge of about 25 electrons. The experiments demonstrate that, in contrast to recent reports, Schottky gates are a feasible approach for the fabrication and integration of single electron transistors in the strained Si∕SiGe heterosystem.

Stranski-Krastanow growth of tensile strained Si islands on Ge (001)

Stranski-Krastanow island growth is demonstrated for tensile strained silicon epilayers on Ge (001) substrates over a wide range of growth temperatures. Small, Si-rich islands show sidewall faces near {1,1,10}, whereas larger islands are {113}-terminated truncated pyramids with an aspect ratio near 0.1. In contrast to compressively strained Ge on Si, we find for Si on Ge a significantly thicker wetting layer of >8 ML and coexistence of islands and dislocations.

Ordering of Ge islands on hill-patterned Si(001) templates

We studied the nucleation and ordering mechanism of Ge islands on hill-patterned Si (001) templates. Like in the case of pit-patterned substrates, the initial Ge wetting layer decorates the inclined surfaces of the hill pattern with corrugations consisting entirely of {105} faceted prisms and {001} terraces. Upon further Ge deposition, islands nucleate as pairs in the V-shaped troughs between neighboring hills, and subsequently merge into a single island near the center of each trough. Finite element calculations show that island nucleation and the subsequent movement toward the center of the trough are governed by elastic energy minimization.

Quantitative determination of Ge profiles across SiGe wetting layers on Si (001)

The peak positions in photoluminescence spectra of Ge wetting layers (WL) deposited at 700 °C were measured versus the Ge coverage with an extremely high relative resolution of 0.025 monolayers. A nearly linear redshift of the peaks with increasing Ge coverage is observed. We derived quantitative WL composition profiles by fitting this shift, and its dependence on the deposition temperature of the capping layer (Tc), to results of band structure calculations. Despite the high growth temperature, the Ge content in the WL exceeds 80%. It is shown that the composition profile is dominated by surface segregation of Ge on Si.

Ordering of Si0.55Ge0.45 islands on vicinal Si(001) substrates: Interplay between kinetic step bunching and strain-driven island growth

We synchronized the kinetic and the strain-driven growth instabilities in the SiGe∕Si(001) heterosystem to implement a two-stage self-organization scheme for the fabrication of long-range-ordered SiGe islands. In the first step, a homoepitaxial Si buffer is grown under optimized step-bunching conditions on a 4° miscut Si(001) substrate. In the second step, SiGe is deposited under conditions that yield three-dimensional island growth in registry with the underlying step-bunching template. By varying the deposition temperature of the SiGe layer, the nucleation phase of the islands could be resolved: At 4° miscut along [110] the slope of the step-bunching areas is most favorable for their disintegration into trains of adjacent (1¯05) and (01¯5) facets upon pseudomorphic overgrowth with SiGe. This strain-driven step meandering instability is a precursor of the SiGe islands, which evolve during further coarsening. At high enough deposition temperatures, we find face-centered, rectangular island ordering that i...

Transient-enhanced Si diffusion on native-oxide-covered Si(001) nanostructures during vacuum annealing

We report on the transient-enhanced shape transformation of nanostructured Si(001) surfaces upon in vacuo annealing at relatively low temperatures of 900–950 °C for a few minutes. We find dramatic surface mass transport concomitant with the development of low-energy facets on surfaces that are covered by native oxide. The enhanced surface mass transport ceases after the oxide is completely desorbed, and it is also not observed on surfaces where the native oxide had been removed by HF before annealing.

Spin relaxation in SiGe islands

We investigate spin relaxation of electrons confined in SiGe structures produced by MBE growth of Ge islands on unstructured and structured Si substrates that cause strain in a Si layer above. These structures are investigated by photoluminescence spectroscopy and electron spin resonance both in continuous wave and pulsed mode. These structures show an ESR line‐width and a g‐factor of conduction electrons in Si with inhomogeneous broadening and no 2D anisotropy.

Self ‐ Organized Si Dots On Ge Substrates

The epitaxial growth conditions for silicon on germanium substrates were investigated as a function of growth temperature and monolayer coverage. Island formation was observed for the hole studied temperature range, although strong alloying with the substrate occurred for the highest temperatures. Carbon pre‐deposition offers suitable nucleation centers for the Si island and reduction of alloying. pre‐structured Ge substrates were prepared to enhance islanding and to achieve ordering.

Electron‐Hole Liquid in Si/SiGe Heterostructures

In thin SiGe layers of Si/Si1−xGex/Si heterostructures electron‐hole liquid (EHL) photoluminescence is found out. The threshold density of excitation necessary for the EHL formation is considerably lower and the threshold temperature is noticeably higher than in bulk alloy of the same composition owing to accumulation of the photoexcited charge carriers in the potential well of the SiGe layer. The analysis of the form of luminescence lines in the SiGe layer allows us to estimate the EHL density (n0 = 2.6⋅1018 cm−3) and the binding energy with respect to gas of excitons (φ = 5.2 meV). At a high level of excitation and temperature exceeding 30 K the line of radiation of electron‐hole plasma (EHP) in SiGe layer is observed in spectra.In thin SiGe layers of Si/Si1−xGex/Si heterostructures electron‐hole liquid (EHL) photoluminescence is found out. The threshold density of excitation necessary for the EHL formation is considerably lower and the threshold temperature is noticeably higher than in bulk alloy of the same composition owing to accumulation of the photoexcited charge carriers in the potential well of the SiGe layer. The analysis of the form of luminescence lines in the SiGe layer allows us to estimate the EHL density (n0 = 2.6⋅1018 cm−3) and the binding energy with respect to gas of excitons (φ = 5.2 meV). At a high level of excitation and temperature exceeding 30 K the line of radiation of electron‐hole plasma (EHP) in SiGe layer is observed in spectra.