Ge Hut Research Articles

An initial phase of Ge hut array formation at low temperature on Si(001)

We report a direct STM observation of Ge hut array nucleation on the Si(001) surface during ultrahigh vacuum molecular-beam epitaxy at 360 ∘C. Nuclei of pyramids and wedges have been observed on the wetting layer M×N patches starting from the coverage of about 5.1 Å (∼3.6 ML). Further development of hut arrays consists of simultaneous growth of the formerly appeared clusters and nucleation of new ones resulting in gradual rise of hut number density with increasing surface coverage. Huts nucleate reconstructing the patch surface from the usual c(4×2) or p(2×2) structure to one of two recently described formations composed by epitaxially oriented Ge dimer pairs and chains of four dimers.

CMOS-compatible dense arrays of Ge quantum dots on the Si(001) surface: hut cluster nucleation, atomic structure and array life cycle during UHV MBE growth.

We report a direct observation of Ge hut nucleation on Si(001) during UHV molecular beam epitaxy at 360°C. Nuclei of pyramids and wedges were observed on the wetting layer (WL) (M × N) patches starting from the coverage of 5.1 Å and found to have different structures. Atomic models of nuclei of both hut species have been built as well as models of the growing clusters. The growth of huts of each species has been demonstrated to follow generic scenarios. The formation of the second atomic layer of a wedge results in rearrangement of its first layer. Its ridge structure does not repeat the nucleus. A pyramid grows without phase transitions. A structure of its vertex copies the nucleus. Transitions between hut species turned out to be impossible. The wedges contain point defects in the upper corners of the triangular faces and have preferential growth directions along the ridges. The derived structure of the {105} facet follows the paired dimer model. Further growth of hut arrays results in domination of wedges, and the density of pyramids exponentially drops. The second generation of huts arises at coverages >10 Å; new huts occupy the whole WL at coverages ~14 Å. Nanocrystalline Ge 2D layer begins forming at coverages >14 Å.

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.

Nucleation of Ge quantum dots on the Si(001) surface

A direct observation of nucleation of Ge hut clusters formed by ultrahigh-vacuum molecular-beam epitaxy is reported. The nuclei of the pyramidal and wedgelike clusters have been observed on the wetting layer blocks and found to have different structures. The growth of the clusters of both species goes on following different scenarios: Formation of the second atomic layer of the wedgelike cluster results in rearrangement of its first layer. Its ridge structure does not replicate the structure of the nucleus. The pyramidal cluster grows without phase transitions. The structure of its vertex copies the structure of the nucleus. The wedgelike clusters contain point defects on the triangular faces and have preferential directions of growth along the ridges.

Classification of Ge hut clusters in arrays formed by molecular beam epitaxy at low temperatures on the Si(001) surface

Ge hut clusters forming quantum dot arrays on the Si(001) surface in the process of low-temperature ultrahigh-vacuum molecular beam epitaxy are morphologically investigated and classified using in situ scanning tunnelling microscopy. It is found that two main Ge hut cluster types—pyramidal and wedge-shaped—have different atomic structures, and it is concluded that shape transitions between the two are impossible. Derivative cluster species — obelisks (or truncated wedges) and accreted wedges — are revealed and investigated for the first time and shown to start dominating at high Ge coverages. The uniformity of cluster arrays is shown to be controlled by the scatter in the lengths of wedge-like clusters. At low growth temperatures (360 °C), cluster nucleation during the growth of the array is observed for all values of Ge coverage except for a particular point at which the arrays are more uniform than at higher or lower coverages. At higher temperatures (530 °C), no cluster nucleation is observed after the initial formation of the array.

Klassifikatsiya hut-klasterov Ge v massivakh, formiruemykh na poverkhnosti Si(001) metodom molekulyarno-luchevoi epitaksii pri nizkikh temperaturakh

Классификация hut-кластеров Ge в массивах, формируемых на поверхности Si(001) методом молекулярно-лучевой эпитаксии при низких температурах, Арапкина Л.В., Юрьев В.А.

Nucleation, growth, and deformation of one-dimensional Ge nanostructures on the Si(5 5 12)-2 × 1 surface

Evolution of the strained Ge-nanofacet, self-assembled on the Si(5 5 12)-2 × 1 template held at 530 °C, has been observed in the atomic scale using scanning tunneling microscopy. Initially, a Ge π-bonded chain is formed on the preferential adsorption site, a dimer–adatom row at the (2 2 5) subunit in a (5 5 12) unit, which results in subunit-switching between (3 3 7) and (2 2 5). By repeated subunit-switchings, the seed of sawtooth-like facet, that is, a single (1 1 3) subunit, is sorted out at the center of double (2 2 5) subunits. Then, the stable (1 1 3) facet expands until the (1 1 2) [or (3 3 5)] facet on the other side of sawtooth-like facet borders on the edge of the next (1 1 3) facet, and, in the end, the one-dimensional (1D) nanofacet with high aspect-ratio and well-defined cross-section (average width: 14 nm, and height: 0.58 nm) is completed at 1.5 monolayer (ML) of Ge coverage. At higher substrate temperature, 600 °C, the cross-section of nanofacet becomes smoother with additional faces (1 1 1) and (2 2 5), while its height becomes less uniform. When the anisotropic stress relaxation arrives at the limit with additional Ge, the (1 1 3) facet starts to have 2 × 2 structure and lose 1D symmetry. Finally, at 4 MLs of Ge coverage, it converts into a Ge hut with faces, (1 1 3), (1 1 2), (15 3 23), and (3 15 23).

Impact of template variations on shape and arrangement of Si∕Ge quantum dot arrays

Templated self-assembly allows the fabrication of quantum dot (QD) arrays for use in nanoelectronic devices. Here, we show the strong dependence of the shape and arrangement of QDs on the template structures. Arrays of etched pits are patterned on Si (100) substrates by extreme ultraviolet interference lithography on which Si∕Ge layers are grown in a molecular beam epitaxy system. Single Ge dome clusters or quantum molecules consisting of four Ge hut clusters are obtained by a change of the pit diameter. Both arrays exhibit a narrow size distribution and exact alignment of the dots. In addition, multiple stacking of these arrays is demonstrated.

Modified growth of Ge quantum dots using C 2H 4 mediation by ultra-high vacuum chemical vapor deposition

C 2H 4 mediations were used to modify the Stranski–Krastanow growth mode of Ge dots on Si(0 0 1) at 550 °C by ultra-high vacuum chemical vapor deposition. With appropriate C 2H 4-mediation to modify the Si surface, the elongated Ge hut clusters can be transformed to highly uniform Ge domes with a high Ge composition at the core. These C 2H 4-mediated Ge dots, almost bounded by {1 1 3} facets, have an average diameter and height of 55 and 9 nm, respectively. We propose two major mechanisms to depict the formation of these C 2H 4-mediated Ge dots: (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation, and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. This work provides a useful scheme to tune the topography of Ge dots in an UHV/CVD condition for possible optoelectronic applications.

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S A and rebonded S B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along ( 5 ¯ 0 1 ) direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and [ 5 ¯ 0 1 ] directions.

First-principles study of indium-stabilized {103} facets in Ge quantum dots

It has been repeatedly observed that at different formation conditions, the {103} facet of Ge hut clusters appears to be stable. Numerous structural models for the In-covered {103} facet have been studied using first-principles total energy calculations. Our extensive calculations show that the {103} facet can be strongly stabilized by adsorbing indium adatoms. The indium coverage is found to be two indium adatoms per unit cell for the $\mathrm{In}∕\mathrm{Ge}(103)$ $1\ifmmode\times\else\texttimes\fi{}1$ reconstruction, corresponding to the model proposed by Seehofer et al. [L. Seehofer et al., Phys. Rev. B 54, 11062 (1996)]. The simulated scanning tunneling microscopy (STM) image of the model is in agreement with the experimental STM observations. Furthermore, the registry of atomic positions is in agreement with the surface x-ray diffraction (SXRD) data.

Order- N first-principles calculations with the conquest code

We summarize the principles underlying the conquest code for first-principles modeling of systems containing many thousands of atoms. Very recent technical developments implemented in the code are outlined. We give illustrations of physical systems currently being studied with the code, ranging from biologically important molecules to Ge hut clusters on Si (001). Our studies of hut clusters require structural relaxation of systems of over 20,000 atoms using electronically self-consistent density-functional theory.

Is the Elongation of Ge Huts in the Low-Temperature Regime Governed by Kinetics?

Contrary to pyramids and domes, elongated huts on a Si(001) substrate are commonly considered as kinetically limited structures. In this work, however, based on detailed scanning tunneling microscopy observations of Ge huts growing on Si(001) at temperatures below 550 degrees C and finite element analysis, the possibility of an equilibrium-driven elongation is raised, where hut-preceding pits are thought to lift the energetic degeneracy of {105} facets and cause elongation along energetically preferred directions.

Germanium growth on Br-terminated Si(1 0 0)

The consequences of Ge deposition on Br-terminated Si(1 0 0) were studied with scanning tunneling microscopy at ambient temperature after annealing at 650 K. One monolayer of Br was sufficient to prevent the formation of Ge huts beyond the critical thickness of 3 ML. This is possible because Br acts as a surfactant whose presence lowered the diffusivity of Ge adatoms. Hindered mobility was manifest at low coverage through the formation of short Ge chains. Further deposition resulted in the extension and connection of the Ge chains and gave rise to the buildup of incomplete layers. The deposition of 7 ML of Ge resulted in a rough surface characterized by irregularly shaped clusters. A short 800 K anneal desorbed the Br and allowed Ge atoms to reorganize into the more energetically favorable “hut” structures produced by conventional Ge overlayer growth on Si(1 0 0).

Formation of SiCH6-mediated Ge quantum dots with strong field emission properties by ultrahigh vacuum chemical vapor deposition

Pretreatment of silicon surface with SiCH6 was used to modify the Stranski-Krastanow growth mode of Ge on Si(001) at 550°C by ultrahigh vacuum chemical vapor deposition. With the appropriate SiCH6 mediation, the elongated Ge hut clusters can be transformed to highly uniform multifaceted domes with a high Ge composition at the core. These SiCH6-mediated Ge dots have an average diameter and height of 38 and 7 nm, respectively. The modified growth mode for the formation of SiCH6-mediated Ge dots can be attributed to (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. The results also demonstrate that SiCH6-mediated dots exhibit the improved field emission characteristics compared to shallow Ge huts.

First-principles study of strain stabilization of Ge(105) facet on Si(001)

Using the first-principles total energy method, we calculate surface energies, surface stresses, and their strain dependence of the Ge-covered Si (001) and (105) surfaces. The surface energy of the Si(105) surface is shown to be higher than that of Si(001), but it can be reduced by the Ge deposition, and becomes almost degenerate with that of the $\mathrm{Ge}∕\mathrm{Si}(001)$ surface for three-monolayer Ge coverage (the wetting layer), leading to the formation of the {105}-faceted Ge hut. The unstrained Si and Ge (105) surfaces are unstable due to the large tensile surface stress originated from the surface reconstruction, but they can be largely stabilized by applying an external compressive strain, such as by the deposition of Ge on Si(105). Our study provides a quantitative understanding of the strain stabilization of $\mathrm{Ge}∕\mathrm{Si}(105)$ surface, and hence the formation of the {105}-faceted Ge huts on Si(001).

Germanium hut nanostressors on freestanding thin silicon membranes

The heteroepitaxial growth of Ge on thin Si membranes can lead to significant bending under self-assembled Ge hut nanostructures. Undercut silicon-on-insulator mesas approximate a Si freestanding membrane and serve as a crystalline substrate for the growth of Ge huts. Synchrotron x-ray microdiffraction shows a local curvature on the lateral scale of the size of the hut and an overall bending of the freestanding region. In comparison with conventional mechanically rigid substrates, the freestanding film can bend significantly. We have found a local radius of curvature of 6μm beneath huts on 30-nm-thick Si membranes.

Towards Quantitative Understanding of Formation and Stability of Ge Hut Islands on Si(001)

We analyze Ge hut island formation on Si(001), using first-principles calculations of energies, stresses, and their strain dependence of Ge/Si(105) and Ge/Si(001) surfaces combined with continuum modeling. We give a quantitative assessment on strain stabilization of Ge(105) facets, estimate the critical size for hut nucleation or formation, and evaluate the magnitude of surface stress discontinuity at the island's edge and its effect on island stability.

Effects of the temperature and of the amount of Ge on the morphology of Ge islands grown by reduced pressure–chemical vapor deposition

We have studied the formation mechanisms and the structural features of Ge islands grown by Reduced Pressure–Chemical Vapor Deposition onto Si(001) substrates. The size, the shape, and the density of Ge islands change drastically when altering parameters such as the growth temperature or the Ge coverage (in-between 3 and 15 monolayers (ML)). For temperatures either equal to 600 °C or 650 °C, pyramids with {105} facets are nucleated first. They then gradually change into larger size domes as the amount of deposited Ge increases. Most probably because of longer Ge atoms diffusion lengths, the islands are less numerous (by a factor of 5) and larger (25% increase in diameter and height) at 650 °C than at 600 °C. At 550 °C, Ge hut clusters are nucleated first; then, small domes appear as the number of Ge monolayers increases. Those islands, although much denser, are rather smaller than the counterparts at higher temperatures. We have also studied Si-capped Ge islands (grown at 600 °C) in photoluminescence. The Ge No Phonon (NP) and Transverse Optical (TO) lines shift to lower energies as the amount of Ge increases, which is most probably due to bigger islands which are richer in Ge when capped. For 15.3 ML, the NP line and its TO replica are centered at 9.5 K on 835 meV and 795 meV, respectively.

Strain relief during Ge hut cluster formation on Si(001) studied by high-resolution LEED and surface-stress-induced optical deflection

The kinetics of Ge hut cluster formation and the evolution of film stress have been studied during Ge deposition at 400 and $500\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ by high-resolution low-energy electron diffraction and surface-stress-induced optical deflection. The hut clusters grow coherent to the Si substrate but show an increased layer distance of 4% due to strain-induced tetragonal distortion. The distortion of the Ge unit cell adds up to more than 8%. The sudden onset of hut formation is observed at 3.5 monolayers of Ge. Individual hut clusters instantly grow to a width of $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The strain relief of the order of 15%--20% is maximized by complete dissociation of the Ge wetting layer. Together with this Ge only two additional monolayers of Ge are necessary to cover the entire surface with fully evolved hut clusters.