Growth of Ge islands on prepatterned Si (0 0 1) substrates

We report on the growth and properties of Ge islands grown on (0 0 1) Si substrates with lithographically defined two-dimensionally periodic pits. After thermal desorption and a subsequent Si buffer layer growth these pits have an inverted truncated pyramid shape. We observe that on such prepatterned substrates lens-like Ge-rich islands grow at the pit bottoms with less Ge deposition than necessary for island formation on flat substrates. This is attributed to the aggregation of Ge at the bottom of the pits, due to Ge migration from the pit sidewalls. At the later stages of growth, dome-like islands with dominant {1,1,3} or {15,3,23}, or other high-index facets [i.e. {15,3,20} facets] are formed on the patterned substrates as shown by surface orientation maps using atomic force microscopy. Furthermore, larger coherent islands can be grown on patterned substrates as compared to Ge deposition on flat ones.     

Luminescence study of Si/Ge quantum dots

We present a photoluminescence (PL) study of Ge quantum dots embedded in Si. Two different types of recombination processes related to the Ge quantum dots are observed in temperature-dependent PL measurements. The Ge dot-related luminescence peak near 0.80 eV is ascribed to the spatially indirect recombination in the type-II band lineup, while a high-energy peak near 0.85 eV has its origin in the spatially direct recombination. A transition from the spatially indirect to the spatially direct recombination is observed as the temperature is increased. The PL dependence of the excitation power shows an upshift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is ascribed to band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.     

Growth and optical properties of Ge/Si quantum dots formed on patterned SiO2/Si(0 0 1) substrates

Single and stacked layers of Ge/Si quantum dots were grown in SiO₂ windows patterned by electron-beam lithography on oxidized Si (0 0 1) substrates. The growth of a silicon buffer layer prior to Ge deposition is found to be an additional parameter for adjusting the Ge-dot nucleation process. We show that the silicon buffer layer evolves towards [1 1 3]-faceted pyramids, which reduces the area of the topmost (0 0 1) surface available for Ge nucleation. By controlling the top facet area of the Si buffer layers, only one dot per circular window and a high cooperative arrangement of dots on a striped window can be achieved. In stacked layers, the dot homogeneity can be improved through the adjustment of the Ge deposited amount in the upper layers. The optical properties of these structures measured by photoluminescence spectroscopy are also reported. In comparison with self-assembled quantum dots, we observed, both in single and stacked layers, the absence of the wetting-layer component and an energy blue shift, confirming therefore the dot formation by selective growth.     

Femtosecond pulsed laser deposition of Ge quantum dot growth on Si(1 0 0)-(2 × 1)

Ge quantum dots were grown on Si(1 0 0)-(2 × 1) by femtosecond pulsed laser deposition at various substrate temperatures using a femtosecond Ti:sapphire laser. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy were used to analyze the film structure and morphology. The morphology of germanium islands on silicon was studied at different coverages. The results show that femtosecond pulsed laser deposition reduces the minimum temperature for epitaxial growth of Ge quantum dots to ∼280 °C, which is 120 °C lower than previously observed in nanosecond pulsed laser deposition and more than 200 °C lower than that reported for molecular beam epitaxy and chemical vapor deposition.     

Phonons in Ge/Si superlattices with Ge quantum dots

Ge/Si superlattices containing Ge quantum dots were prepared by molecular beam epitaxy and studied by resonant Raman scattering. It is shown that these structures possess vibrational properties of both two-and zero-dimensional objects. The folded acoustic phonons observed in the low-frequency region of the spectrum (up to 15th order) are typical for planar superlattices. The acoustic phonon lines overlap with a broad emission continuum that is due to the violation of the wave-vector conservation law by the quantum dots. An analysis of the Ge and Ge-Si optical phonons indicates that the Ge quantum dots are pseudoamorphous and that mixing of the Ge and Si atoms is insignificant. The longitudinal optical phonons undergo a low-frequency shift upon increasing laser excitation energy (2.54–2.71 eV) because of the confinement effect in small-sized quantum dots, which dominate resonant Raman scattering.     

Excitons in Ge/Si double quantum dots

The spatial structure of excitons and the oscillator strength characterizing the intensity of interband optical transitions in vertically coupled Ge/Si quantum dots have been theoretically studied. It has been found that the probability of the exciton transition under certain conditions (the sizes of the quantum dots, the separation of the dots) can be much larger (up to a factor of 5) than the value for the case of single quantum dots. It is expected that the results will make it possible to approach the creation of efficient light-emitting and photoreceiving devices based on Si and Ge indirect-band semiconductors.     

Folded acoustic phonons in Si/Ge superlattices with Ge quantum dots

The spectra of Raman scattering by folded acoustic phonons in Si/Ge superlattices with pseudomorphic layers of Ge quantum dots (QDs) grown by low-temperature (T = 250°C) molecular beam epitaxy are studied. New features of the folded phonon lines related to the resonant enhancement and unusual intensity ratio of the doublet lines that cannot be explained by the existing theory have been observed. The observed modes are shown to be related to the vibrations localized to the QDs and induced by the folded phonons of the Si spacer layers. The calculations performed in the model of a one-dimensional chain of atoms have allowed the nature of the localization of acoustic phonons attributable to a modification of the phonon spectrum of a thin QD layer to be explained. The observed intensity ratio of the folded phonon doublet lines is caused by asymmetry of the relief of the QD layers.     

Longitudinal conductivity of Ge/Si heterostructures with quantum dots

The conductance along an island layer of Ge quantum dots buried in silicon was investigated. The sizes of the islands varied in the range D ≈ 12−19 nm. It was found that the charge transport is characterized by two activation energies. The first one is associated with the thermal emission of holes from Ge quantum wells into the valence band of Si. The second one is due to the tunneling of holes between islands under Coulomb blockade conditions and is determined by the electrostatic charging energy of a quantum dot. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 6, 423–426 (25 March 1996)     

Ge quantum dots growth on nanopatterned Si(0 0 1) surface: Morphology and stress relaxation study

The self-organized growth of germanium quantum dots on square nanopatterned Si(0 0 1) substrates is investigated by scanning tunnelling microscopy (STM) and grazing incidence X-ray diffraction (GIXRD) techniques. A regular surface patterning in the 10-100 nm period range is obtained by etching an interface dislocation network obtained by the controlled molecular bonding of Si substrates. The depth of the silicon surface profile is increased by a double etching process. Growth experiments are performed by solid source molecular beam epitaxy (MBE), and for deep trenches, germanium growth conditions are optimized to obtain one Ge dot per Si mesa. It is shown that the trench depth and the mesa profile strongly affect the dot size and its coincidence with the initial regular surface network. Anomalous GIXRD measurements are performed to highlight the Ge elastic relaxation and intermixing during heteroepitaxial growth. We report a significant modification in the stress state of Ge dots as a function of thermal annealing after growth.     

Absorption of terahertz radiation in Ge/Si(001) heterostructures with quantum dots

The terahertz spectra of the dynamic conductivity and radiation absorption coefficient in germanium-silicon heterostructures with arrays of Ge hut clusters (quantum dots) have been measured for the first time in the frequency range of 0.3–1.2 THz at room temperature. It has been found that the effective dynamic conductivity and effective radiation absorption coefficient in the heterostructure due to the presence of germanium quantum dots in it are much larger than the respective quantities of both the bulk Ge single crystal and Ge/Si(001) without arrays of quantum dots. The possible microscopic mechanisms of the detected increase in the absorption in arrays of quantum dots have been discussed.     

Spatial separation of electrons in Ge/Si(001) heterostructures with quantum dots

It is shown experimentally that the excitation of interband optical transitions in arrays of Ge/n-Si(001) quantum dots leads to a decrease in the concentration of electrons in the conduction band. The phenomenon observed is due to the formation of negatively charged exciton complexes in Ge islands and represents the first experimental confirmation of the spatial separation of electrons in the silicon matrix surrounding the islands.     

Morphology and photoluminescence of ultrasmall size of Ge quantum dots directly grown on Si(0 0 1) substrate

High density and ultrasmall size of Ge quantum dots (QDs) have been achieved directly on Si(0 0 1) (2 × 1) reconstruction surface. Their detailed morphology was observed by atomic force microscope (AFM) and shows that small pyramids, small domes, huts, and multi-headed large domes coexist in the film grown at 400 °C, while small domes and multi-headed large domes formed at 450 °C. Their low temperature photoluminescence (PL) showed that a very strong non-phonon (NP) peak with a large blue shift of 0.19 eV at 14 K, which can be attributed to their very high areal density, 5.2 × 10¹¹ cm⁻², and sub-10-nm mean size, 7.6 ± 2.3 nm.     

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.     

Ge dots on Si (1 1 1) and (1 0 0) surfaces with SiO2 coverage: Raman study

Germanium quantum dots formed on Si (1 1 1) and (1 0 0)-oriented surfaces coated with ultra-thin oxide layers are studied using Raman spectroscopy technique. Some structural properties (height, stoichiometry and mechanical stresses) of the dots were estimated from Raman data. For analysis of the experimental data, the Raman spectra of Ge nanoclusters containing some hundreds of Ge atoms were calculated numerically. The effects of the resonance enhancement of the intensity of Raman scattering in the Ge-nanoclusters–SiO₂–Si system were discussed. The influence of the lateral sizes of Ge nano-clusters on the frequencies of phonons localized in them was studied using numerical simulation. The influence of multi-layer growth on the structure of the Ge quantum dots was investigated.     

Atomic structure of Ge quantum dots on the Si(001) surface

In situ morphological investigation of the “105” faceted Ge islands on the Si(001) surface (hut clusters) have been carried out using an ultra high vacuum instrument integrating a high resolution scanning tunnelling microscope and a molecular beam epitaxy vessel. Both species of hut clusters-pyramids and wedges-were found to have the same structure of the “105” facets which was visualized. Structures of vertexes of the pyramidal clusters and ridges of the wedge-shaped clusters were revealed as well and found to be different. This allowed us to propose a crystallographic model of the “105” facets as well as models of the atomic structure of both species of the hut clusters. An inference is made that transitions between the cluster shapes are impossible.     

Suppression of hole relaxation in small Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photocurrent, photoconductive gain, and hole capture probability in ten-period p-type Ge/Si quantum dot heterostructures. The dot dimensions are varied by changing the Ge coverage during molecular beam epitaxy of Ge/Si(001) system in the Stranski–Krastanov growth mode while keeping the deposition temperature to be the same. A device with smaller dots is found to exhibit a lower capture probability and a higher photoconductive gain and photoresponse. The integrated responsivity in the mid-wave atmospheric window (λ = (3–5) μm) is improved by a factor of about 8 when the average in-plane dot dimension changes from 18 to 11 nm. The decrease in the dot size is expected to reduce the carrier relaxation rate due to phonon bottleneck by providing strong zero-dimensional quantum mechanical confinement.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Spin states of holes in Ge/Si nanowire quantum dots

We investigate tunable hole quantum dots defined by surface gating Ge/Si core-shell nanowire heterostructures. In single level Coulomb-blockade transport measurements at low temperatures spin doublets are found, which become sequentially filled by holes. Magnetotransport measurements allow us to extract a g factor g approximately 2 close to the value of a free spin-1/2 particle in the case of the smallest dot. In less confined quantum dots smaller g factor values are observed. This indicates a lifting of the expected strong spin-orbit interaction effects in the valence band for holes confined in small enough quantum dots. By comparing the excitation spectrum with the addition spectrum we tentatively identify a hole exchange interaction strength chi approximately 130 microeV.     

Micro-Raman scattering by laser-modified structures with Ge/Si quantum dots

Structures with self-assembled Ge/Si quantum dots grown by molecular-beam epitaxy are exposed to pulsed radiation of a picosecond laser. Changes in the vibrational spectrum of nanostructures under an external action are studied by Raman spectroscopy. An analysis of the Raman spectra measured with a micron spatial resolution along the exposed region indicates a mixing of Ge and Si atoms and a change in the induced mechanical stresses in quantum dots.