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.     

Self-patterned Si surfaces as templates for Ge islands ordering

We propose a two-step process, which is based on substrate nano-patterning by means of growth instabilities in a first step and self-assembling of Ge dots on the top of surface instabilities in a second step. We used the instabilities that develop during the growth of Si(Ge) layers on both nominal and vicinal Si (1 1 1) or (0 0 1) surfaces. Depending on the growth conditions (Ge concentration, growth temperature, thickness), various growth instability regimes were observed: pure kinetic regime, kinetically activated strain-induced regime and pure strain-driven regime. In the case of Si/Si growth, kinetic instabilities developed at different growth temperatures depending on the surface orientation. The critical exponents describing evolution with time have been determined: amplitude At^β and wavelength Lt^α. Experimental results show that each instability regime appears for a given growth temperature range that critically depends on the concentration of Ge. Evolution with time also depends on the Ge concentration. But in all cases, we evidence discrepancies between the experimental critical exponents and those predicted by classical modelling. We also give some examples of Ge dots self-organization on substrates nano-patterned (periodically undulated) by means of the different growth instabilities described above. In all cases, we observe Ge dots ordering along the substrate undulations due to step and/or strain effects. On kinetic instabilities (Si/Si(0 0 1) vicinal), Ge islands preferentially nucleate on step bunches. On SiGe(0 0 1) template layers, Ge dots nucleate on top of the SiGe undulations. In that case, strain gradients improved island ordering. The best ordering was achieved using SiGe(0 0 1) 10° off misoriented template layers as a result of almost perfect anisotropic morphology.     

Formation of Ge islands from a Ge layer on Si substrate during post-growth annealing

We have deposited a 12 nm thick Ge layer on Si(1 0 0) held at 200 °C by thermal evaporation under high vacuum conditions. Upon subsequent thermal annealing in vacuum, self-assembled growth of nanostructural Ge islands on the Ge layer occurred. Atomic force microscopy (AFM) and grazing incidence small-angle X-ray scattering (GISAXS) were used to characterize such layers. GISAXS measurements evidenced the formation of cylinder shaped structures upon annealing at 700 °C, which was confirmed by AFM measurements with a very sharp tip. A Ge mass transport from the layer to the islands was inferred by X-ray reflectivity and an activation energy of 0.40 ± 0.10 eV for such a process was calculated.     

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     

Small and high-density GeSiC dots stacked on buried Ge hut-clusters in Si

Self-assembled GeSiC dots stacked on a Ge hut-cluster layer buried in Si have been investigated. The critical thickness for formation of GeSiC dots is reduced owing to the strain fields from the buried hut-clusters. By utilizing the stacked structure, the dot size is decreased and the uniformity is improved. The highest density of the GeSiC dots with stacked structures is 7.4×10¹⁰ cm⁻², which is six times larger than that of single GeSiC dots. The formation of the self-assembled GeSiC dots is strongly influenced by being stacked with buried Ge dots as well as C incorporation.     

Electrical peculiarities in Al/Si/Ge/…/Ge/Si and Al/SiGe/Si structures

The current–voltage (I–V) and capacitance–voltage (C–V) behaviour of different Si/Ge multilayers and SiGe single layers prepared on p-type Si substrates by magnetron sputtering and annealing, has been studied in the temperature range of 80–320 K by using Al Schottky contacts as test structures. Although a significant influence of the microstructure of the Si/Ge multilayers and SiGe layers was obtained on the electrical behaviour of the structures, the structures exhibited similar specific features.     

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.     

Dome-to-pyramid transition induced by alloying of Ge islands on Si(001)

Received: 19 October 1998/Accepted: 19 October 1998     

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.     

Relaxation of mechanical stresses in an array of Ge quantum dots obtained in Si

Raman scattering on optical phonons in Si/Ge/Si structures with Ge quantum dots grown by molecular beam epitaxy at low temperatures 200–300°C has been investigated. A pseudomorphic state of an array of Ge quantum dots to a Si matrix with an ideally sharp interface has been obtained. Features associated with the inelastic relaxation of mechanical stresses have been revealed in the Raman spectrum. Two mechanisms of stress relaxation are separated. It has been shown that the spectrum of the electronic states of the array differs significantly from the set of the discrete levels of a single quantum dot, because the relaxation is inhomogeneous.     

Influence of patterning on the nucleation of Ge islands on Si and SiO2 surfaces

Surface patterning is expected to influence the nucleation site of deposited nanostructures. In the present study, clean Si and SiO₂ surfaces were patterned by a nanolithographic process using a Focused Ion Beam (FIB). Ge was evaporated in ultra high vacuum at 873 K on these substrates, resulting in the formation of island arrays. Based on scanning tunneling microscopy and atomic force microscopy images, a statistical analysis was performed in order to highlight the effect of patterning on the size distribution of islands compared to a non-patterned surface. We find that the self-organization mechanism on patterned substrates results in a very good arrangement and positioning of Ge nanostructures, depending on growth conditions and holes distance, both on Si and SiO₂ surfaces.     

Thermoelectric transport in strained Si and Si/Ge heterostructures

The anisotropic thermoelectric transport properties of bulk silicon strained in the [111]-direction were studied by detailed first-principles calculations focusing on a possible enhancement of the power factor. Electron and hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high-temperature and high-doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of a [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole doping was drastically suppressed due to quantum-well effects, while under electron doping an enhanced power factor was found. For this, we state figures of merit of ZT?=?0.2 and 1.4 at T?=?300?and 900?K for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features.     

Features of atomic processes at the formation of a wetting layer and nucleation of three-dimensional Ge islands on Si(111) and Si(100) surfaces

The intermediate stages of the formation of a Ge wetting layer on Si(111) and Si(100) surfaces under quasiequilibrium grow conditions have been studied by means of scanning tunneling microscopy. The redistribution of Ge atoms and relaxation of mismatch stresses through the formation of surface structures of decreased density and faces different from the substrate orientation have been revealed. The sites of the nucleation of new three-dimensional Ge islands after the formation of the wetting layer have been analyzed. Both fundamental differences and common tendencies of atomic processes at the formation of wetting layers on Si(111) and Si(100) surfaces have been demonstrated. The density of three-dimensional nuclei on the Si(111) surface is determined by changed conditions for the surface diffusion of Ge adatoms after change in the surface structure. Transition to three-dimensional growth on the Si(100) surface is determined by the nucleation of single {105} faces on the rough Ge(100) surface.     

Identification of Ge/Si intermixing processes at the Bi/Ge/Si(111) surface

The chemical contrast between Si and Ge obtained by scanning tunneling microscopy on Bi-covered Si(111) surfaces is used as a tool to identify two vertical Ge/Si intermixing processes. During annealing of an initially pure Ge monolayer on Si, the intermixing is confined to the first two layers approaching a 50% Ge concentration in each layer. During epitaxial growth, a growth front induced intermixing process acting at step edges is observed. Because of the open atomic structure at the step edges, relative to the terraces, a lower activation barrier for intermixing at the step edge, compared to the terrace, is observed.     

Optical modulator based on a Si0.75Ge0.25/Si/Si0.5Ge0.5 asymmetric superlattice structure

An optical modulator is designed and fabricated based on a Si_0.75Ge_0.25/Si/Si_0.5Ge_0.5 asymmetrical superlattice structure. The device comprises a p-i-n diode made on the asymmetrical superlattice integrated with a 920-μm-long Fabry-Perot (F-P) cavity. Parameters of the rib waveguide are designed to satisfy only the fundamental-TE mode transmission. Here, 65 and 40-pm red shifts of the peak resonant were measured under the applied bias of 2.5 and ?32.0 V, respectively. The analysis shows that, besides the thermal-optical and plasma dispersion effects, the Pockels effect also contributes to such a peak shift. The corresponding calculated effective Pockels coefficient is about 0.158 pm/V.

     

Preferential ordering of self-assembled Ge islands on focused ion-beam patterned Si(100)

We report the growth of Ge islands on Si (001) substrates with lithographically defined two-dimensionally periodic pits using focused ion-beam patterning and molecular beam epitaxy. The formation of circularly ordered Ge islands has been achieved by means of nonuniform strain field around the periphery of the holes due to ion bombardment. Lateral ordering of the Ge islands have been controlled by both the pit size and pit separation. Preferential growth at the pit sites has also been achieved by using appropriate pattern shape and size.