Photoluminescence of Ge(Si)/Si(0 0 1) self-assembled islands in the near infra-red wavelength range

The dependence of photoluminescence spectra of structures with GeSi/Si(0 0 1) self-assembled nanoislands on growth temperature has been investigated. It was shown that the redshift of the island-related photoluminescence peak with a decrease of the growth temperature is associated with suppression of Si diffusion in the islands and an increase of Ge content in them. For the first time a photoluminescence signal from SiGe islands was observed at energies much lower than the Ge band gap. The energy position of the island-related photoluminescence peak is well described by the model of optical transition, which is indirect in real space. The photoluminescence signal at 1.55 μm from GeSi/Si(0 0 1) self-assembled islands was obtained up to room temperature.     

Photoluminescence of self-assembled GeSi/Si(001) nanoislands of different shapes

The dependence of the photoluminescence spectra of structures with self-assembled GeSi/Si(001) islands on Ge deposition temperature was studied. The position of the island photoluminescence peak maximum was found to shift nonmonotonically with decreasing Ge deposition temperature. The blue shift of the island photoluminescence peak with the growth temperature decreasing from 600 to 550°C is assigned to the change in the island shape occurring in this temperature interval accompanied by a strong decrease in the average island height.     

Morphology and photoluminescence of self-assembled GeSi/Si nanoclusters grown by sublimation molecular-beam epitaxy in a GeH<Subscript>4</Subscript> ambient

The dependence of the morphology and photoluminescence spectra of GeSi/Si(001) heterostructures with self-assembled nanoclusters, obtained by sublimation molecular-beam epitaxy in a GeH₄ ambient, on the growth conditions has been analyzed. The conditions for obtaining structures with uniform nanocluster arrays exhibiting photoluminescence at room temperature have been determined.     

Photoluminescence of GeSi/Si(0 0 1) self-assembled islands with dome and hut shape

The photoluminescence spectra of GeSi/Si (0 0 1) self-assembled islands produced by solid source molecular beam epitaxy in a wide range (460–700°C) of growth temperatures were investigated. The results showed a blue shift of the island-related photoluminescence peak with a growth temperature lowering from 600°C to 550°C. The observed blue shift of the island photoluminescence peak is associated with a sharp decrease in the average height of the island, which occurs through a change of its shape from dome to hut as the growth temperature lowers from 600°C to 550°C.     

Influence of the germanium deposition rate on the growth and Photoluminescence of Ge(Si)/Si(001) self-assembled islands

The growth and photoluminescence of Ge(Si)/Si(001) self-assembled islands are investigated over a wide range of germanium deposition rates v_Ge = 0.1–0.75 Å/s at a constant growth temperature T _g = 600°C. Examination of the surface of the grown structures with an atomic force microscope revealed that, for all the germanium deposition rates used in the experiments, the dominant island species are dome islands. It is found that an increase in the deposition rate v_Ge leads to a decrease in the lateral size of the self-assembled islands and an increase in their surface density. The decrease in the lateral size is associated both with the increase in the germanium content in the self-assembled islands and with the increase in the fraction of the surface occupied by these islands. The observed shift in the position of the photoluminescence peak toward the low-energy range is also explained by the increase in the germanium content in the islands with an increase in the deposition rate v_Ge.     

Photoluminescence of Ge nanostructures grown by gas source molecular beam epitaxy on silicon (1 1 8) surface

In this paper, we present a photoluminescence (PL) study of Si/Ge/SiGe/Si structures grown by gas source molecular beam epitaxy on an (1 1 8) undulated surface with various Ge coverage. Nucleation and growth of Ge films is obtained by the Stranski–Krastanov mechanism. The influence of the substrate orientation on the changeover 2D–3D growth mode is investigated. Furthermore, we show the use of growing an SiGe wetting layer to control the uniformity of the Ge island size. The PL signal related to the Ge islands is found to be highly dependent of the power excitation and is observed up to room temperature.     

Photoluminescence of germanium quantum dots grown in silicon on a SiO<Subscript>2</Subscript> submonolayer

The photoluminescence of quantum dots in Si/Ge/SiO₂/Si and Si/Ge/Si structures is investigated as a function of temperature. The low activation energies for the temperature quenching of photoluminescence of germanium quantum dots in both structures are explained in terms of the thermally stimulated capture of holes from quantum dots to the energy levels of defects localized in their vicinity.     

Specific features of plastic relaxation of a metastable Ge x Si1 − x layer buried between a silicon substrate and a relaxed germanium layer

Heterostructures Ge/Ge _x Si_{1 ? x} /Si(001) grown by molecular beam epitaxy have been investigated using atomic scale high-resolution electron microscopy. A germanium film (with a thickness of 0.5–1.0 μm) grown at a temperature of 500°C is completely relaxed. An intermediate Ge_0.5Si_0.5 layer remains in a strained metastable state, even though its thickness is 2–4 times larger than the critical value for the introduction of 60° misfit dislocations. It is assumed that the Ge/GeSi interface is a barrier for the penetration of dislocations from a relaxed Ge layer into the GeSi layer. This barrier is overcome during annealing of the heterostructures for 30 min at a temperature of 700°C, after which dislocation networks having different degrees of ordering and consisting predominantly of edge misfit dislocations are observed in the Ge/GeSi and GeSi/Si(001) heteroboundaries.     

C-induced Ge dots: enhanced light-output from Si-based nanostructures

Pre-deposition of a fraction of a monolayer of C on an Si (0 0 1) substrate causes the formation of extremely small islands after the growth of only 2 monolayers (ML) of Ge. We demonstrate that these CGe dots exhibit particularly intense photoluminescence (PL) compared to a variety of different but comparable structures. Although grown at low temperatures (460°C), the CGe islands show a ten times more intense PL signal than conventionally grown self-assembled Ge islands, grown at 700°C. We show that the initial stage of CGe dot formation is likely to be governed by strain compensation effects. In a series of samples, where we have kept the total C and Ge amounts constant but varied the deposition sequence, we show that the specific CGe dot growth order of pre-grown low surface mobility C, followed by high surface mobility Ge leads to a distinct nanostructure within the SiGeC material system, exhibiting typical ‘dot-like' and intense PL. An almost strain compensated 50 layer stack of CGe dots is shown to emit intense PL at 0.99 eV.     

Forming Dislocation Pairs in the Ge/GeSi/Si(001) Heterostructure

Physics of the Solid State - In the Ge/LTGe/GeSi/Si(001) heterostructures, the GeSi buffer layer remains pseudomorphic in a certain range of the heterostructure parameters and growth regimes, while...     

Electromodulation of the interband and intraband absorption of Ge/Si self-assembled islands

We have investigated the interband and the intraband absorption properties of Ge/Si self-assembled islands. The investigated structure consists of a p–i–n junction containing Ge/Si self-assembled islands embedded in a Si_0.98Ge_0.02 waveguiding layer. The variation of transmission associated with carrier injection under forward bias is monitored both in the near-infrared and in the mid-infrared spectral ranges. We show that the carrier injection leads to an absorption resonant at 185 meV which is polarized along the growth axis of the islands. This transition corresponds to an intraband optical transition from the island ground states to the two-dimensional wetting layer states. This assignment is supported by a two-dimensional band structure calculation performed in a 14 band k·p formalism. Meanwhile, the carrier injection leads to a bleaching of the interband absorption. We show that this electroabsorption spectroscopy is a useful tool for the study of self-assembled islands that is complementary of standard photoluminescence, electroluminescence or absorption spectroscopies.     

Surface mobility difference between Si and Ge and its effect on growth of SiGe alloy films and islands

Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Ge(001) surface the diffusivity of Ge is 10(2)-10(3) times higher than that of Si in the temperature range of 300 to 900 K, while on a tensile surface, the two diffusivities are comparable. Consequently, the growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands.     

Visible photoluminescence of nanometer-sized SiGe/Si heterostructure fabricated by ion implantation

An innovative fabrication technique for the nanometer-sized SiGe/Si heterostructure was developed in this study. Ge was induced in Si substrate by two-step ion implantation. The spherical SiGe nanoclusters are self-assembled in the Si substrate by subsequent rapid thermal annealing at 1,100 °C. The diameter of the spherical SiGe nanoclusters is 5–7 nm. Visible photoluminescence from this nanometer-sized SiGe/Si heterostructure at room temperature was investigated. We found three peak energies of visible luminescence spectra at 1.97, 2.13, and 2.16 eV, respectively. The luminescence intensity depends on the number of the nanoclusters and will be decreased because of the micro-defects around the heterostructure, which is discussed in detail.     

Evolution of self-assembled Ge/Si island grown by ion beam sputtering deposition

The effect of temperature and Ge coverage on the evolution of self-assembled Ge/Si islands grown by ion beam sputtering deposition is studied. Atomic force microscopy and Raman spectroscopy are used to analyze the island morphology and the intermixing between Si and Ge. The experiments are presented in two aspects. First, when the temperature is increased, intermixing is promoted, resulting in the reappearance of low aspect ratio islands. Second, a different evolution pathway is observed, in which short islands initially don’t grow along the constant ratio of 11:1 (diameter:height) and the islands always grow faster in vertical direction. In summary, the interdiffusion, surface diffusion, and amount of Ge determines the evolution of Ge/Si islands.     

Vertical ordering of islands in Ge-Si multilayers

Growth and ordering of GeSi islands in Ge-Si multilayer systems during deposition by Low-Pressure Chemical Vapour Deposition (LPCVD) at 700°C on Si (001) substrates have been investigated for different layer distances by transmission electron microscopy of cross-section and plane-view specimens. Vertical ordering of GeSi islands with almost perfect correlation is observed for distances between the Ge layers of 100 nm. At larger interlayer distances, a continuous decrease of the correlation is found. Vertical ordering in the multilayer system is modelled in terms of the elastic interaction between island nuclei in a newly forming layer and close islands in a buried layer below. Lateral ordering parallel to < 100 >, as observed previously in larger Ge-Si multilayer systems is not found in our systems, consisting of two Ge layers. This difference indicates that lateral ordering in the upper Ge layers of a large multilayer system is triggered by vertical ordering.     

Shape, facet evolution and photoluminescence of Ge islands capped with Si at different temperatures

We investigate the embedding of Ge islands in a Si matrix by means of atomic force microscopy and photoluminescence (PL) spectroscopy. The Ge islands were grown between 360°C and 840°C and subsequently capped with Si at different temperatures. For the highest Ge growth temperature (840°C), we show that the surface flattens at high Si capping temperatures while new facets can be identified at the island base for intermediate capping temperatures (650–450°C). At low capping temperatures (300–350°C), the island morphology is preserved. In contrast to the observed island shape changes, the decreasing Si capping temperature causes only a small redshift of the island related PL signal for islands grown on high temperatures. This redshift increases for Ge islands grown at lower temperatures due to an increased Ge content in the islands. By applying low-temperature capping (300°C) on the different island types, we show that the emission wavelength can be extended up to 2.06 μm for hut clusters grown at 400°C. Further decreasing of the island growth temperature to 360°C leads to a PL blueshift, which is explained by charge carrier confinement in Ge quantum dots.     

Modeling the formation kinetics of wedge-shaped germanium quantum dots on silicon

A mathematical model for calculating the dependences of the parameters of self-organized germanium quantum dots (QDs) on silicon on the conditions of growth in molecular beam epitaxy is described. The energies of formation for pyramidal and wedge-shaped islands in the Ge/Si(001) system are calculated with allowance for the contributions from surface energy and elastic stress relaxation, and the drop in the energy of attraction between atoms and the substrate.     

Perfect alignment of self-organized Ge islands on pre-grown Si stripe mesas

Self-organized Ge islands grown on patterned Si(001) substrates have been investigated. Selective epitaxial growth (SEG) of Si is carried out with gas-source molecular beam epitaxy to form Si stripe mesas followed by subsequent Ge island growth. Self-aligned Ge islands with regular spacing are formed on the <110>-oriented ridges of the Si mesas. The regular spacing is driven by the repulsive interaction between the neighbor islands through the substrates. A mono-modal distribution of the islands has been observed on the ridges of the Si mesas. The spatial confinement as well as the preferential nucleation is believed to be the mechanism of this alignment of the self-organized Ge islands. Received: 16 July 1999 / Accepted: 6 August 1999 / Published online: 24 March 2000     

Size distribution and optical properties of self-assembled Ge on Si

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ge islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 °C), therefore the diffusion length of adatoms is high (∼100 μm) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38–1.55 μm. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up to 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Photoelectric properties of the GeSi/Si heterostructures with self-assembled nanoclusters grown by sublimation molecular beam epitaxy in a GeH4 medium

The photosensitivity (PS) spectra of the GeSi/Si(001) heterostructures with self-assembled nanoclusters grown by sublimation molecular beam epitaxy in a GeH₄ medium have been studied by photo-emf spectroscopy at the semiconductor/electrolyte junction (PSE) and by photo-emf and photocurrent spectroscopy of Schottky barriers including the temperature dependence of the PS spectra in the temperature range of 10–300 K. The bands related to the phonon assisted and phonon-less interband of spatially indirect optical transitions in the GeSi nanoclusters have been observed in the PS spectra even at 300 K. The scatter effect of the GeSi nanoclusters in size and/or in composition on the PS spectrum’s edge shape has been theoretically considered. The emission theory of the photoexcited carriers from the quantum wells of the GeSi/Si nanoclusters built-in to a Schottky barrier (p-tn junction, semiconductor/electrolyte junction) has been developed.