SiGe Quantum Dots Research Articles

Ether-like Si–Ge hydrides for applications in synthesis of nanostructured semiconductors and dielectrics

Hydrolysis reactions of silyl-germyl triflates are used to produce ether-like Si-Ge hydride compounds including H(3)SiOSiH(3) and the previously unknown O(SiH(2)GeH(3))(2). The structural, energetic and vibrational properties of the latter were investigated by experimental and quantum chemical simulation methods. A combined Raman, infrared and theoretical analysis indicated that the compound consists of an equal mixture of linear and gauche isomers in analogy to the butane-like H(3)GeSiH(2)SiH(2)GeH(3) with an exceedingly small torsional barrier of approximately 0.2 kcal mol(-1). This is also corroborated by thermochemistry simulations which indicate that the energy difference between the isomers is less than 1 kcal mol(-1). Proof-of-principle depositions of O(SiH(2)GeH(3))(2) at 500 degrees C on Si(100) yielded nearly stoichiometric Si(2)Ge(2)O materials, closely reflecting the composition of the molecular core. A complete characterization of the film by RBS, XTEM, Raman and IR ellipsometry revealed the presence of Si(0.30)Ge(0.70) quantum dots embedded within an amorphous matrix of Si-Ge-O suboxide, as required for the fabrication of high performance nonvolatile memory devices. The use of readily available starting materials coupled with facile purification and high yields also makes the above molecular approach an attractive synthesis route to H(3)SiOSiH(3) with industrial applications in the formation of Si-O-N high-k gate materials in high-mobility SiGe based transistors.

Transient Hole Trapping in Individual GeSi Quantum Dot Grown on Si(001) Studied by Conductive Atomic Force Microscopy

Electrical properties of individual self-assembled GeSi quantum dots grown on Si substrates are investigated by using conductive atomic force microscopy at room temperature. By controlling the bias voltage sweep in a certain fast sweep rate range, a novel current peak is observed in the current–voltage characteristics of the quantum dots. The current peaks are detectable only during the backward voltage sweep immediately after a forward sweep. The current peak position and intensity are found to depend strongly on the voltage sweep conditions. This kind of current–voltage characteristic under fast sweep is very different from the ordinary steady state current behaviour of quantum dots measured previously. The origin of this phenomenon can be attributed to the transient hole trapping in the potential well formed by the quantum dot sandwiched between the native oxide layer and the bottom Si substrate.

SiGe Quantum Rings by Ultra-high Vacuum Chemical Vapor Deposition

SiGe quanrum rings (QRs) grown at 500oC and 600oC were observed on SiGe quantum dots (QDs) capped with Si. Average depth and diameter are 9 nm and 185 nm, respectively, for QRs at 500oC, while those are 0.9 nm and 84 nm for QRs at 600oC. Ge out-diffusion mechanism is proposed to be responsible for nanorings formation in 500oC, and Si surface diffusion toward strain-free edges is proposed to be responsible for nanorings formation in 600oC. Raman spectroscopy demonstrates that formation of QRs in 600oC is close correlated with a strain-driven process. QRs grown in 600oC are the metastable states and can be only observed in very limited conditions. Thick cap and high thermal budget can both destroy SiGe nanorings structures.

Fabrication of Well-Organized and Densely Packed Si Nanopillars Containing SiGe Nanodots by Using Block Copolymer Templates

We present a method to obtain a dense array of well-organized SiGe quantum dots in Si nanopillars on a large substrate area by using a PS-b-PMMA block copolymer template. First, the 2D order of the template is evaluated using a pair correlation function and a bond-orientational correlation function. It is shown that the holes contained in the polystyrene template form a structure presenting a hexatic order. This template is then used to fabricate the nanopillars using the following method: in a first step, the copolymer template serves as a deposition mask to fabricate a hexagonal array of Pt dots. In a second step, the Pt dot pattern is transferred to an alternation of Si and SiGe layers on a silicon substrate with an anisotropically etching using Cl2/O2 plasma.

SiGe/Si Quantum-Dot Infrared Photodetectors With ${\bm \delta}$ Doping

The multicolor absorption of MOS SiGe/Si quantum-dot (QD) infrared photodetectors is demonstrated using the boron delta-doping in Si spacers. The energy-dispersive X-ray spectroscopy shows that the Ge concentration in the wetting layers is much smaller than that in QDs. Most holes stay at the ground state in QDs instead of wetting layers. The energy band structure in QDs is calculated to understand the absorption spectrum. The absorption at 3.7-6 mum is due to the intersubband transition in the SiGe QDs. The other absorption at 6-16 mu m mainly comes from the intraband transition in the boron delta-doping wells. Since the broadband spectrum covers most of the atmospheric transmission windows for infrared, the broadband detection is feasible using this device.

SiGe quantum dot molecules grown on patterned Si (001) substrates

SiGe quantum dot molecules (QDMs) grown on patterned Si (001) substrates by molecular beam epitaxy were studied. Experimental results showed that the density, the dimension, and the dimension distribution of the SiGe QDMs grown in the windows were dependent on the window size. When the thickness of the Si0.8Ge0.2 film was 40 nm, QDMs only appeared in the unpatterned areas of the Si substrate and none could be found inside the windows of 6×6 μm2 on the same substrate. However, when the thickness of Si0.8Ge0.2 film was increased to 80 nm, QDMs appeared both inside the windows and in the unpatterned areas, and the density of QDMs was reduced with the decrease in the window size. We attribute these results to the different strain relaxations in different size windows, which are caused by the edge effect of the epitaxial film in the window. Based on these experimental results we discuss the formation and the size stability of the QDMs and conclude that the formation of the SiGe QDM originates from an intrinsic cause of the strain relief mechanism. This work also shows that by means of the edge induced strain relaxation of the epitaxial film in the window, it is possible to reveal the influence of the strain on some physical properties of the SiGe film without changing its Ge atomic fraction.

Small SiGe quantum dots obtained by excimer laser annealing

In this paper, we obtain SiGe quantum dots with the diameters and density of 15–20 nm and 1.8×10 11 cm −2, respectively, by 193 nm excimer laser annealing of Si 0.77Ge 0.23 strained films. Under the excimer laser annealing, only surface atoms diffusion happens. From the detailed statistical information about the size and shape of the quantum dots with different annealing time, it is shown that the as-grown self-assembled quantum dots, especially the {1 0 5}-faceted dots, are not stable and disappear before the appearance of the laser-induced quantum dots. Based on the calculation of surface energy and surface chemical potential, we show that the {1 0 3}-faceted as-grown self-assembled quantum dots are more heavily strained than the {1 0 5}-faceted ones, and the heavy strain in the dot can decrease the surface energy of the dot facets. The formation of the laser-induced quantum dots, which is also with heavy strain, is attributed to kinetic constraint.

Multiple scattering effects on the EXAFS of Ge nanocrystals

We present a detailed extended x-ray absorption fine structure (EXAFS) spectroscopystudy on the influence of multiple scattering effects on the analysis of bulk polycrystallineGe (c-Ge) and of four Ge nanocrystal (NC) distributions with mean sizes from 4 to 9 nm. Acomplete description of the EXAFS signal up to the third shell of nearest neighbours forboth c-Ge and Ge NCs is only achieved by including at least two double scattering and onetriple scattering path. Unlike reports for bulk semiconductors and Ge–Si quantum dots, ourresults show that including only the most prominent double scattering path is insufficientfor accurately ascertaining the structural parameters of the second and third shells, leadingto unphysically small coordination numbers for the NCs. The same is observed when nomultiple scattering paths are taken into account. The size dependence of theinteratomic distance distributions up to the third shell of nearest neighbours has beendetermined for the first time. A greater reduction in coordination numbers and higherstructural disorder were observed for the outer shells, reflecting the increase of thesurface-to-volume ratio and reinforcing the presence of an amorphous Ge layer between theSiO2 matrix and the NCs.

Carrier gas effects on the SiGe quantum dots formation

SiGe quantum dots (QDs) grown by ultra-high vacuum chemical vapor deposition using H 2 and He carrier gases are investigated and compared. SiGe QDs using He carrier gas have smaller dot size with a better uniformity in terms of dot height and dot base as compared to the H 2 carrier gas. There is a higher Ge composition and less compressive strain in the SiGe QDs grown in He than in H 2 as measured by Raman spectroscopy. The Ge content is higher for He growth than H 2 growth due to hydrogen induced Si segregation and the lower interdiffusivity caused by the more strain relaxation in the He-grown SiGe dots. The photoluminescence also confirms more compressive strain for H 2 growth than He growth. Hydrogen passivation and Ge–H cluster formation play an important role in the QDs growth.

Evolution of Ge and SiGe Quantum Dots under Excimer Laser Annealing

We present different relaxation mechanisms of Ge and SiGe quantum dots under excimer laser annealing. Investigation of the coarsening and relaxation of the dots shows that the strain in Ge dots on Ge films is relaxed by dislocation since there is no interface between the Ge dots and the Ge layer, while the SiGe dots on Si0.77Ge0.23 film relax by lattice distortion to coherent dots, which results from the obvious interface between the SiGe dots and the Si0.77Ge0.23 film. The results are suggested and sustained by Vanderbilt and Wickham's theory, and also demonstrate that no bulk diffusion occurs during the excimer laser annealing.

Three-Dimensional Si/Ge Quantum Dot Crystals

Modern nanotechnology offers routes to create new artificial materials, widening the functionality of devices in physics, chemistry, and biology. Templated self-organization has been recognized as a possible route to achieve exact positioning of quantum dots to create quantum dot arrays, molecules, and crystals. Here we employ extreme ultraviolet interference lithography (EUV-IL) at a wavelength of lambda = 13.5 nm for fast, large-area exposure of templates with perfect periodicity. Si(001) substrates have been patterned with two-dimensional hole arrays using EUV-IL and reactive ion etching. On these substrates, three-dimensionally ordered SiGe quantum dot crystals with the so far smallest quantum dot sizes and periods both in lateral and vertical directions have been grown by molecular beam epitaxy. X-ray diffractometry from a sample volume corresponding to about 3.6 x 10(7) dots and atomic force microscopy (AFM) reveal an up to now unmatched structural perfection of the quantum dot crystal and a narrow quantum dot size distribution. Intense interband photoluminescence has been observed up to room temperature, indicating a low defect density in the three-dimensional (3D) SiGe quantum dot crystals. Using the Ge concentration and dot shapes determined by X-ray and AFM measurements as input parameters for 3D band structure calculations, an excellent quantitative agreement between measured and calculated PL energies is obtained. The calculations show that the band structure of the 3D ordered quantum dot crystal is significantly modified by the artificial periodicity. A calculation of the variation of the eigenenergies based on the statistical variation in the dot dimensions as determined experimentally (+/-10% in linear dimensions) shows that the calculated electronic coupling between neighboring dots is not destroyed due to the quantum dot size variations. Thus, not only from a structural point of view but also with respect to the band structure, the 3D ordered quantum dots can be regarded as artificial crystal.

The intermixing and strain effects on electroluminescence of SiGe dots

Secondary-ion mass spectroscopy, energy dispersion spectrometry, and Raman spectroscopy reveal that SiGe dots grown by ultrahigh-vacuum chemical vapor deposition at 600°C exhibit significant intermixing with an average Ge composition of ∼50%. Raman spectroscopy shows the top SiGe quantum dots of the 20-layer sample to be more relaxed than those of the 5-layer samples. As a result, the electroluminescence from the top SiGe quantum dots of the 20-layer sample has the higher peak energy at ∼0.84eV as compared to ∼0.82eV for the 5-layer sample. The external tensile mechanical strain can compensate the built-in compressive strain of SiGe quantum dots and increase electroluminescence energy.

Magnetoacoustic studies as a probe of electron systems

The pulse-echo technique was used to probe the electron systems in solids. The measurements were performed in the frequency range 10–500 MHz, at temperatures down to 0.3 K and in the magnetic fields up to 18 T. (a) Acoustic quantum oscillations were investigated in URu2Si2, AuZn, and Sr2RuO4. Extracted parameters of the electron systems are in good agreement with data known from other experiments and theoretical calculations. Hydrostatic pressure effect on superconductive transition in Sr2RuO4 was studied. It is found that the sound speed increases, critical temperature and critical fields decreases, and transition becomes broader under the pressure. Magnetic field dependencies of ultrasound attenuation are compared with theoretical predictions. (b) Surface acoustic waves were used for contactless measurements of conductivity in low dimensional structures. The value of complex ac conductivity was extracted from simultaneous measurements of the sound attenuation and velocity. Such measurements allowed to study, for example, mechanisms of conductivity in a dense array of SiGe quantum dots and localization of the 2D carries in GaAs/AlGaAs and Si/SiGe heterostructures in the extreme quantum limit. NHMFL is supported by the NSF Cooperative Agreement No. DMR-0084173 and the State of Florida. The ultrasonic research at the NHMFL is supported by the In-House Research Program.

Ordered Lattices of Quantum Dots on Ultrathin SOI Nanomembranes

Undercut or fully released silicon template layers of ultrathin silicon-on-insulator are structurally compliant and allow long- range mechanical interactions that are impossible on supported SOI, thick SOI, or on bulk surfaces. SiGe quantum dots create and respond to strain in these freestanding Si substrates. We show that elastic effects lead to long-range order in the positions of quantum dots grown on both sides of a free- standing thin SOI substrate. A statistical analysis of the distribution of quantum dots shows that the ordered lattice of dots is coherent over distances of at least several hundred of nanometers. X-ray microdiffraction probes the structure of the SOI and finds that curvature in the Si lattice is consistent with the proposed deformation of the substrate by quantum dots.

Can quantum dots be analysed with macrobeam RBS?

One of the greatest challenges to the continuing relevance of RBS for modern materials development is the decreasing scale of structures that need to be characterised. While continuing the development of newer and better hardware is essential, this work follows a different approach: we develop physical models to describe the influence of inclusions in RBS spectra, and implement them in the standard data analysis code NDF. We show that, while complementary information from other techniques is almost always necessary, macrobeam RBS can contribute significant information. We apply the models developed to Ge quantum dots in Si.

Photocurrent spectroscopy of indirect transitions in Ge/Si multilayer quantum dots at room temperature

Lateral photoconductivity spectra of multilayer Ge/Si heterostructures with Ge quantum dots were studied in the work proposed at room temperature. The photocurrent with minimal energy 0.48–0.56 eV that is smaller than Ge band gap was observed from such structures at the geometry of waveguide excitation. Generation of the photocurrent with the limit energy 0.48–0.56 eV was explained by spatially indirect electron transitions from heavy hole states of SiGe valence band into Δ 2-valley of the conduction band of Si surrounding. It was found out that the limit energy of such transitions decreased, as the number of SiGe quantum dot layers increased.

Effect of doping location on photoconductive spectrum of SiGe quantum dots

AbstractComparative analysis of photoconductive (PC) spectra obtained from SiGe quantum dots grown on Si shows large dependence on the location of doping. The sample with doping in the silicon barrier has much larger response than the sample in which the doping was placed in the wetting layer, i.e. in the self assembled dots. This is due to the penetration of the wave‐function into the barrier in the first structure, as shown by the 1‐D simulation. Several peaks in the infrared were observed, ranging from 70 to 220 meV, associated with inter‐level transitions of holes in the valance band of the dot or the wetting layer. The overall spectra and the relative intensity of the various peaks change drastically with bias magnitude and polarity. The PC signals increase super‐linearly with bias, indicating that the carriers are released into the quasi‐continuum through tunneling. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Broadband SiGe∕Si quantum dot infrared photodetectors

The broadband absorption of metal-oxide-semiconductor SiGe∕Si quantum dot infrared photodetectors is demonstrated using boron δ doping in the Si spacer. The peak at 3.7–6μm results from the intersubband transition in the SiGe quantum dot layers. The other peak at 6–16μm mainly comes from the intraband transition in the boron δ-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and 7.5–14μm, broadband detection is feasible using this device. The δ doping in SiGe quantum dots and Si0.9Ge0.1 quantum wells is also investigated to identify the origin of the absorption.

The stability of faceted SiGe quantum dots capped with a thin Si layer

Detailed surface morphologies and facets of partially capped Ge/Si(001) quantum dotswere investigated by atomic force microscopy for samples capped with differentthicknesses and subsequently annealed in a desiccator for a period of 12 months. Thepyramid-shaped quantum dots bounded by {103}, {104} and {105} facets were observedin as-capped samples. After annealing, the {104} and {105} facets remained, whilethe {103} facets changed their structural profile to {104} facets. Extensive atomicforce microscopy and transmission electron microscopy observations confirmedthis finding. It is believed that the high surface free energy of the {103} facets andpre-existing Ge atoms in the wetting layer are responsible for this surface evolution.

SiGe quantum dot single-hole transistor fabricated by atomic force microscope nanolithography and silicon epitaxial-regrowth

A SiGe quantum dot single-hole transistor passivated by silicon epitaxial regrowth with extremely stable Coulomb blockade oscillations has been demonstrated. The quantum dot was defined by atomic force microscopy nanopatterning technique and subsequently passivated by the epitaxial regrowth of silicon. Such passivation of the dot avoids any potential defect states on the dot associated with the Si∕SiO2 interface. Coulomb blockade oscillations controlled by side planar gating at ∼0.3K are reproducible, in sharp contrast with the noisy and irreproducible I-V characteristics of unpassivated SiGe quantum dot devices. An additional top gate was used to further tune the Coulomb blockade oscillations, enabling a shift in side-gate voltage of up to three periods.