U. Denker

Long-range ordered lines of self-assembled Ge islands on a flat Si (001) surface

Self-assembled growth in combination with prepatterning yields ordered lines of Ge islands on a planar Si (001) surface. The self-assembled Ge nanostructures are grown on top of a 15-period Si/SiGe superlattice, which is deposited on a prepatterned Si substrate. The pattern consists of 10 nm deep trenches with a period of 250 nm. The superlattice translates the surface modulation of the substrate into a strain-field modulation at the growth front of the superlattice. This strain field modulation provides the template for the ordered nucleation of self-assembled Ge islands. Our method gives rise to the long-range ordering of perfectly passivated nanostructures and can in principle be applied to any other strained material system.

Photoluminescence of ultrasmall Ge quantum dots grown by molecular- beam epitaxy at low temperatures

Low-temperature epitaxial growth of Si‐Ge heterostructures opens possibilities for synthesizing very small and abrupt low-dimensional structures due to the low adatom surface mobilities. We present photoluminescence from Ge quantum structures grown by molecular-beam epitaxy at low temperatures which reveals a transition from two-dimensional to three-dimensional growth. Phononless radiative recombination is observed from ^105& faceted Ge quantum dots with height of approximately 0.9 nm and lateral width of 9 nm. Postgrowth annealing reveals a systematic blueshift of the Ge quantum dot’s luminescence and a reduction in nonradiative recombination channels. With increasing annealing temperatures Si‐Ge intermixing smears out the three-dimensional carrier localization around the dot.

Photoluminescence investigation of phononless radiative recombination and thermal-stability of germanium hut clusters on silicon(001)

Intense photoluminescence (PL) originating from single layers of germanium hut clusters grown on silicon (001) is investigated using PL spectroscopy. We propose that the luminescence originates from phononless recombination within a spatially indirect, type-II neighboring confinement structure. Enhanced no-phonon (NP) luminescence is attributed to exciton localization at the Ge/Si interfaces. The PL intensity is sensitive to the growth temperature during interface formation, as well as to post-growth thermal annealing, illustrating the influence of atomic-level Si–Ge intermixing on exciton localization and NP enhancement.

Effect of overgrowth temperature on the photoluminescence of Ge/Si islands

Ge/Si islands grown with molecular-beam epitaxy at 630 °C are overgrown with Si at different temperatures Tcap, and their photoluminescene spectra are recorded. Both the island-related and wetting-layer-related energy transitions redshift with lowered Tcap, which is explained by reduced material intermixing. The mandatory growth interruption, which is introduced during the temperature drop, causes island ripening and shifts the island (wetting layer) photoluminescence peaks slightly to lower (higher) energies. The growth interruption quenches the quantum efficiency of the wetting layer by more than an order of magnitude, whereas the island-related photoluminescence intensity even slightly increases. The island’s superior resistance against growth interruptions, and hence interface contamination, is explained by effective carrier localization in the Ge nanostructures. Room-temperature photoluminescence is reported for Ge islands overgrown at 460 °C.

Composition of self-assembled Ge/Si islands in single and multiple layers

The degree of Si alloying in vertically aligned self-assembled Ge islands increases with the number of stacked layers. We find that the Si–Ge interdiffusion coefficient increases by more than two orders of magnitude for stacked hut clusters. Furthermore, we determine the composition profiles through the center of dome-shaped islands, capped with Si. These profiles exhibit a plateau near the base and a Ge enrichment near the apex of the islands. In this case, too, the upper dome island experiences a state of increased alloying with Si.

Trench formation around and between self-assembled Ge islands on Si

Investigations on near-surface diffusion during the formation of self-assembled Ge islands on Si(001) are presented. We measure the detailed shape of trenches around islands that are formed due to short range, strain enhanced diffusion. It is found that these trenches have anisotropic shape which we explain in terms of the intrinsic anisotropy of the elastic properties for the Si crystal. At high growth temperatures, long-range depletion of the substrate and trench formation between neighboring islands due to strong in-diffusion of Si into the nominally pure Ge islands is observed. A simple diffusion model which predicts trench depths as a function of island distance fits well to our experimentally observed data. Calculated diffusion lengths from this model are comparable to the average island distance on the surface.

Ge hut cluster luminescence below bulk Ge band gap

We report on the photoluminescence (PL) properties of Ge hut cluster islands on Si(001) that were overgrown at temperatures as low as 250 °C. We find that the island-related photoluminescence systematically redshifts as the overgrowth temperature is reduced from 500 to 360 °C, which is attributed to a reduced Ge segregation. For even lower overgrowth temperatures, the emission energy saturates at 0.63 eV or 1.96 μm, more than 110 meV smaller than the band gap of unstrained bulk Ge. We report a PL peak centered at 2.01 μm at low excitation power, in good agreement with the estimated transition energy for a spatially indirect transition between holes confined in the strained Ge island and electrons confined in the surrounding Si matrix. PL is observed up to a temperature of 185 K and an activation energy of 40 meV is deduced from fitting the temperature-dependent peak intensity. Annealing experiments reveal a systematic blueshift of the hut cluster-related PL, thus verifying unambiguously, that the PL signa...

Electroluminescence of self-assembled Ge hut clusters

We have fabricated Si-based light-emitting diodes operating in the near infrared. The active layers of the devices consist of either one or ten layers of Ge/Si self-assembled hut clusters grown by molecular-beam epitaxy. Luminescence is observed in the spectral range between 1.4 and 1.5 μm. For the ten layer stack of Ge islands, electroluminescence is observed up to room temperature. A direct comparison with a pure Si reference p-i-n diode allows us to attribute the luminescence to radiative recombinations between holes localized in the Ge islands and electrons localized in the strained Si above and below the islands.

Resonant tunneling diodes made up of stacked self-assembled Ge/Si islands

Multiple layers of self-assembled Ge/Si islands are used for resonant tunneling diodes (RTDs). The extremely closely stacked Ge nanostructures form vertical channels with energetically deep thermalization layers and high Si double barriers. Two resonances are found in the RTD current–voltage curve, which are attributed to the heavy-heavy hole (hh) and heavy-light hole (lh) transition. The lh resonance shows negative differential resistance up to 50 K. With increasing magnetic field, the lh resonance slightly shifts to higher voltages.

Effect of overgrowth temperature on shape, strain, and composition of buried Ge islands deduced from x-ray diffraction

We have investigated a series of samples containing SiGe islands capped at different growth temperatures. A layer of islands formed by deposition of 5 ML of pure Ge was capped with Si, deposited at temperatures of 460, 540, and 630 °C, respectively. The Ge composition profile and the shape of the buried islands are deduced from x-ray diffraction data. While for capping at high substrate temperatures a significant dilution of the Ge content and a flattening of the islands occur, capping at low temperatures maintains a high aspect ratio and a high Ge content of the islands. The maximum in-plane strain in the island remains as high as 0.005 for capping at low temperatures.