Ge Nanodots Research Articles

Three-Dimensional Self-Ordered Multilayered Ge Nanodots on SiGe

Three-dimensional (3D) self-ordered Ge nanodots in cyclic epitaxial growth of Ge/SiGe superlattice on Si0.4Ge0.6 virtual substrate (VS) were fabricated by reduced pressure chemical vapor deposition. The Ge nanodots were formed by Stranski-Krastanov mechanism. By the Ge/SiGe superlattice deposition, dot-on-dot alignment and 〈100〉 alignment were obtained toward the vertical and lateral direction, respectively. Facets and growth mechanism of Ge nanodots and key factors of alignment were studied. Two types of Ge nanodots were observed, diamond-like nanodots composed of {105} and dome-like nanodots composed of {113} and {519} or {15 3 23} facets. The Ge nanodots tend to grow directly above the nanodots of the previous period as these regions show a relatively higher tensile strain induced by the buried nanodots. Thus, this dot-on-dot alignment is sensitive to the SiGe spacer thickness, and it degrades when the SiGe spacer becomes thicker. The Ge content of the SiGe spacer ranging from 45 to 52% affects the lateral alignment and the size uniformity of Ge nanodots because of the strain balance between the superlattice and the VS. By maintaining the strain balance, ordering of the 3D aligned Ge nanodots can be improved.

Vertical alignment control of self-ordered multilayered Ge nanodots on SiGe

Self-ordered multilayered Ge nanodots with SiGe spacers on a Si0.4Ge0.6 virtual substrate are fabricated using reduced-pressure chemical vapor deposition, and the mechanism of vertical ordering is investigated. The process conditions of Ge and SiGe layer deposition are H2-GeH4 at 550 °C and H2-SiH4-GeH4 at 500 °C–550 °C, respectively. By depositing the SiGe at 550 °C or increasing Ge content, the SiGe surface becomes smooth, resulting in vertically aligned Ge nanodots to reduce strain energy. Ge nanodots prefer to grow on the nanodot where the SiGe is relatively tensile strained due to the buried Ge nanodot underneath. By depositing at 500 °C and lowering Ge content, checkerboard-like surface forms, and the following Ge nanodots grow at staggered positions to reduce surface energy. The Ge nanodots are laterally aligned along the elastically soft 〈100〉 direction without pre-structuring resulting from the strain distribution.

3-Dimensional Self-Ordered Multilayered Ge Nanodots on SiGe

Multilayered Ge nanodots have drawn much attention due to their potential applications in optoelectronics, such as photodetectors and lasers. Many groups studied multilayered Ge nanodots with Si spacers on Si(001) grown by Stranski-Krastanov (SK) growth mode and vertically aligned by local tensile strain induced by buried Ge nanodots (1-2). However, to avoid plastic relaxation caused by a 4.2% lattice mismatch between Si and Ge, thick nanodots and/or large layer numbers are challenging. Additionally, laterally-aligned Ge nanodots without pre-structuring have not been reported. In this study, we demonstrate 3-dimensional (3D) self-ordered Ge nanodots on SiGe virtual substrate (VS) by SiGe/Ge cyclic epitaxial growth and show the effects of fabrication parameters.The 3D self-ordered Ge nanodots were fabricated by reduced-pressure chemical vapor deposition. A Si0.4Ge0.6 VS with step-graded buffer deposited on Si(001) wafer was used. This VS was post-annealed at 1000°C, followed by chemical-mechanical polishing. After HF dip, the substrate was baked at 850°C in H2, then cooled down to 550°C for epitaxial growth. A 52-82 nm thick Si0.48Ge0.52 layer was deposited using a H2-SiH4-GeH4 gas mixture, then a self-assembled Ge nanodots layer via SK growth mechanism was deposited with 7.5-15.0 nm Ge coverage using a H2-GeH4 gas mixture. This Si0.48Ge0.52/Ge deposition cycle was repeated 5 to 20 times to fabricate the 3D self-ordered Ge nanodot stack. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the morphology and alignment of the Ge nanodots. The facets of the Ge nanodots were studied by analyzing the cross-section cuts of AFM images and confirmed by scanning transmission electron microscopy (STEM). Nano-beam diffraction (NBD) was used to study strain in the superlattice.Fig. 1(a-c) show the AFM images of the 5-cycle superlattices with Ge coverage 7.5 to 15.0 nm. In fig. 1(a), we can see mainly two types of nanodots, diamond (32%) and dome (68%). The height of the diamond-like nanodots is 9 nm with a standard deviation (s) of 2.6 nm while that of the dome-like nanodots is 23 nm with s=1.8 nm. With increasing Ge coverage, dome-like nanodots dominate (fig. 1(b)) and some nanodots merge with the adjacent nanodots (fig. 1(c)). Since the dome-like nanodots show a lower s in height than the diamond-like nanodots do, engineering of self-ordering is more feasible with the dome-like nanodot.Fig. 2(a-b) show the angle view of SEM images of Ge nanodots on 5- and 20-cycle superlattice of 12.5 nm-Ge/52 nm-SiGe. The dome-like nanodots are dominant and laterally aligned. The alignment and the uniformity improve with the increasing cycle number of the superlattice. However, when the thickness of Si0.48Ge0.52 spacer increases, the lateral- and vertical alignment of nanodots become random, and the amount of diamond-like nanodots increases (not shown).To study facets of two types of nanodots, the tilt of each facet was calculated from the cross-section cuts of AFM images. Fig. 3(a) shows an AFM image of the dome-like nanodot. Fig. 3(b) shows the cross-section cuts of fig. 3(a) along <110>. These cross-section cuts are well-overlapped with the high-angle annular dark-field (HAADF) STEM image as shown in the background of fig. 3(b). Therefore, it is possible and reliable to estimate the facets from the cross-section cuts of our AFM images. By this method, we confirmed that the diamond-like nanodot is composed of {105} and the dome-like nanodot is composed of {113} and {159}. This is consistent with a study of Ge dot on Si substrate except for {159} facet (3). Instead of {159} facet, a relatively similar facet {3 15 23} was reported. This difference may result from the less compressive strain in our Ge nanodots because SiGe VS was used.To explain the vertical correlation of Ge nanodots, a HAADF STEM image and in- and out-of-plane strain distributions measured by NBD are shown in fig. 4(a-c). The nanodots are vertically aligned (fig. 4(a)). The SiGe on the nanodot shows a relatively higher lattice parameter along <110> (fig. 4(b)) and lower lattice parameter along <001> (fig. 4(c)) compared to that on Ge wetting layer, indicating tensile strain. This tensile strain area is the preferred position for Ge nanodot formation because of less lateral lattice mismatch. Consequently, the nanodots tend to grow above the buried nanodots.3D self-ordered multilayered Ge nanodots on SiGe VS were successfully fabricated, and the facets and the vertical correlation of Ge nanodots were studied. References P.S. Chen et al. Materials Science and Engineering B 108 (2004) 213-218K.L. Wang et al. Proceeding of the IEEE 95 (2007) 1866J.T. Robinson et al. Nanotechnology 20 (2009) 085708 Figure 1

3-Dimensional Self-Ordered Multilayered Ge Nanodots on SiGe

Three-dimensional (3D) self-ordered Ge nanodots in cyclic epitaxial growth of Ge/SiGe superlattice on Si0.4Ge0.6 virtual substrate (VS) were fabricated by reduced pressure chemical vapor deposition. By the Ge/SiGe superlattice deposition, dot-on-dot alignment and <100> alignment were obtained toward the vertical and lateral direction, respectively. Facets and growth mechanism of Ge nanodots and key factors of alignment were studied. Two types of Ge nanodots were observed, diamond-like nanodots composed of {105} and dome-like nanodots composed of {113} and {159} facets. The Ge nanodots tend to grow directly above the nanodots of the previous period as these regions show a relative higher tensile strain induced by the buried nanodots. Thus, this dot-on-dot alignment is sensitive to the SiGe spacer thickness, and it degrades when the SiGe spacer is over 82 nm. The Ge content of the SiGe spacer ranging 45-52% affects the lateral alignment and the size uniformity of Ge nanodots because of the strain balance between the superlattice and the VS. When the strain is balanced, 3D aligned Ge nanodots can be achieved.

Synergistic phonon scattering in epitaxial silicon multilayers with germanium nanodot inclusions

Temperature-dependent thermal conductivity of epitaxial silicon (Si)/ultrathin silica multilayers film with epitaxial germanium (Ge) nanodot inclusions is measured over the range of temperature from 50 K to room temperature using time-domain thermoreflectance. The measured thermal conductivity with 5-nm Ge nanodots is much smaller than the reported values for Si/Ge superlattices, bulk SiGe, and nanostructured SiGe in the entire temperature range. The thermal conductivity of the film is analyzed with a kinetic model incorporating multiple phonon scattering processes, where intrinsic three-phonon scattering inside the Si layers is calculated by first principles, boundary scattering at the ultrathin silica layer is calculated by the atomistic Green's function, and scattering by the Ge nanodots is approximated with nanovoids. The analysis reveals that summing the multiple scattering rates by Matthiessen's rule cannot explain the extremely low thermal conductivity. The Monte Carlo ray tracing calculation that incorporates the multiple scattering effect reveals that the synergistic effect of ultrathin silica interfaces and Ge nanodots enhances phonon scattering. This suggests the merit in synergistically designing multiple nanostructures to reduce thermal conductivity, which is beneficial for developing thermoelectric materials.

Phonon transport in the nano-system of Si and SiGe films with Ge nanodots and approach to ultralow thermal conductivity.

Phonon transport in the nano-system has been studied using well-designed nanostructured materials to observe and control the interesting phonon behaviors like ballistic phonon transport. Recently, we observed drastic thermal conductivity reduction in the films containing well-controlled nanodots. Here, we investigate whether this comes from the interference effect in ballistic phonon transport by comparing the thermal properties of the Si or Si0.75Ge0.25 films containing Ge nanodots. The experimentally-obtained thermal resistance of the nanodot layer shows peculiar nanodot size dependence in the Si films and a constant value in the SiGe films. From the phonon simulation results, interestingly, it is clearly found that in the nanostructured Si film, phonons travel in a non-diffusive way (ballistic phonon transport). On the other hand, in the nanostructured SiGe film, although simple diffusive phonon transport occurs, extremely-low thermal conductivity (∼0.81 W m-1 K-1) close to that of amorphous Si0.7Ge0.3 (∼0.7 W m-1 K-1) is achieved due to the combination of the alloy phonon scattering and Ge nanodot scattering.

Ambipolar Plasmon‐Enhanced Photodetector Built on Germanium Nanodots Array/Graphene Hybrid

AbstractGraphene with gate‐tunable electronic properties is promising as the channel materials in ambipolar multifunctional photodetectors. However, owing to the zero bandgap and weak light absorption, photodetectors fabricated on pristine graphene have low photoresponsivity. Herein, an ambipolar multifunctional photodetector with improved photocurrent and response speed is constructed on germanium (Ge) nanodots array‐decorated graphene. The photocurrent map and simulated electric field distributions reveal that the enhanced photo‐response is attributed to the localized surface plasmonic resonance effects by trapping light around the Ge nanodots. The photodetector exhibits ambipolar photo‐response and the photocurrents can be tuned from negative to positive by applying different gate voltages due to the gate‐tunable Fermi level of graphene. This ambipolar photodetector with fast response has excellent potential in imaging and sensing arrays as well as multifunctional photodetectors.

Stimulated Raman Scattering in Ge Nanowires.

Investigating group-IV-based photonic components is a very active area of research with extensive interest in developing complementary metal-oxide-semiconductor (CMOS) compatible light sources. However, due to the indirect band gap of these materials, effective light-emitting diodes and lasers based on pure Ge or Si cannot be realized. In this context, there is considerable interest in developing group-IV based Raman lasers. Nevertheless, the low quantum yield of stimulated Raman scattering in Si and Ge requires large device footprints and high lasing thresholds. Consequently, the fabrication of integrated, energy-efficient Raman lasers is challenging. Here, we report the systematic investigation of stimulated Raman scattering (SRS) in Ge nanowires (NWs) and axial Al-Ge-Al NW heterostructures with Ge segments that come into contact with self-aligned Al leads with abrupt metal–semiconductor interfaces. Depending on their geometry, these quasi-one-dimensional (1D) heterostructures can reassemble into Ge nanowires, Ge nanodots, or Ge nanodiscs, which are monolithically integrated within monocrystalline Al (c-Al) mirrors that promote both optical confinement and effective heat dissipation. Optical mode resonances in these nanocavities support in SRS thresholds as low as 60 kW/cm2. Most notably, our findings provide a platform for elucidating the high potential of future monolithically integrated, nanoscale low-power group-IV-based Raman lasers.

Group IV nanodots: synthesis, surface engineering and application in bioimaging and biotherapy.

Group IV nanodots (NDs) mainly including carbon (C), silicon (Si), germanium (Ge) have aroused much attention as one type of important nanomaterials that are widely studied in optoelectronics, semiconductors, sensors and biomedicine-related fields owing to the low cost of synthesis, good stability, excellent biocompatibility, and some attractive newly emerged properties. In this review, the synthesis, surface engineering and application in bioimaging and biotherapy of group IV NDs are summarized and discussed. The recent progress in the rational synthesis and functionalization, specific therapy-related properties, together with in vivo and in vitro bioimaging are highlighted. Their new applications in biotherapy such as photothermal therapy (PTT) and photodynamic therapy (PDT) are illustrated with respect to C, Si and Ge NDs. The current challenges and future applications of these emerging materials in bioimaging and biotherapy are presented. This review provides readers with a distinct perspective of the group IV NDs nanomaterials for synthesis and surface engineering, and newly emerging properties related to applications in biomedicine.

Study on the formation mechanism of bismuth-mediated Ge nanodots fabricated by vacuum evaporation

Germanium (Ge) nanodots are expected for various applications. We have proposed a unique fabrication method for them with bismuth (Bi) mediation, and we have successfully formed high-density crystalline Ge nanodots by vacuum evaporation followed by low-temperature annealing. In this paper, we show the experimental results, such as dependence of the process condition on the nanodot structure and some results of the analyses of the nanodot samples. Then we discuss and propose a schematic formation mechanism for the nanodots as follows. First, during the deposition process, sequentially deposited Bi and Ge layers are mixed by solid phase diffusion and form an amorphous-alloy layer. Next, during the annealing process, crystallization of the Ge nanodots and desorption of the Bi proceeds; during this time, Bi and Ge exist as a solid–liquid coexistence phase in the alloy system, and this condition may enhance the phenomena.

Thermoelectric power factor enhancement based on carrier transport physics in ultimately phonon-controlled Si nanostructures

We thoroughly investigated carrier and phonon transports related to thermoelectric properties using absolutely-controlled Si nanostructures, namely Si films containing epitaxial nanodots (NDs). In the Si nanostructure, various ND materials were introduced selectively such as Ge and β-FeSi2. This brings the ND/Si hetero-interface control; interface energy band offset and lattice mismatch strain in Si films. Ultrahigh density stacking faults can also be intentionally introduced using ultrathin SiO2 films. We reduced thermal conductivity (<2 Wm−1K−1) close to amorphous Si due to phonon transport control in absolutely-controlled Si nanostructures: scattering both by NDs and intentionally-doped atomic-scale impurities, namely wide-scale length phonon scattering. The electron transport related to Seebeck coefficient and electrical conductivity was manipulated by intentionally-introduced nanostructures: ND/Si hetero-interfaces and stacking faults. In the epitaxial Si thin films containing β-FeSi2 NDs, energetic carrier transports through stacking faults with several tens of meV barrier height enhanced the power factor. In the strained epitaxial Si thin films containing Ge NDs, it was found that strain-induced band splitting leading to high electron mobility enhanced the power factor. These results demonstrated that thermoelectric performance determined by electron and phonon transports were strongly dependent on the crystal characters (strain, crystal defects and the interfaces) relating to intentionally-introduced nanostructures. This suggests that it is important to design nanostructured materials on nanoscale and atomic-scale for enhancement of thermoelectric performance: introduction of stacking faults with proper energy barriers, NDs and dopants scattering phonons, and the less point defects. This also demonstrates that its nanostructuring methodology opens a load to realization of thermoelectric Si thin films.

Growth of heterojunctions in Si–Ge alloy nanowires by altering AuGeSi eutectic composition using an approach based on thermal oxidation

Understanding the growth mechanism of heterojunctions in silicon–germanium alloy (Si–Ge) nanowires is helpful for designing adequate physical properties in the material for device applications. We examine the formation of the heterojunction in low Ge-content Si–Ge nanowires by an approach of thermal oxidation, which produces an atomically abrupt interface with an obvious concentration change. Forming heterojunctions in Si–Ge nanowires by this approach involves more complicated reaction routes than direct growth of heterojunction nanowires using the vapor–liquid–solid method. At the beginning of the oxidation process, the AuGeSi eutectic liquid at the nanowire tip significantly etches the Si–Ge alloy nanowires. Selective oxidation of Si results in a change of the relative amount of Ge to Si in the eutectic liquid, which further modulates the solubility of Ge and Si atoms. The compositional variation in the Au–Ge–Si ternary alloy system during the oxidation process accounts for the observed concentration profile in the heterojunction nanowire. The thermal oxidation approach is applied on a low Ge-content Si–Ge thin film that is coated with Au nanoparticles. Si–Ge nanodots, which exhibit a higher Ge concentration, are precipitated epitaxially in the film, as a result of compositional modulation in the AuGeSi eutectic liquid.

Large Scale Self-Organization of 2D Hexagonal Ge and Au Nanodots on Patterned TiO2 for Optoelectronic Applications

We report a new strategy for the ordering of 2D arrays of Ge and Au nanodots on a silicon wafer using a patterned titanium oxide layer. In a rst step, a TiO 2 layer is prepared by block-copolymer-micelles-assisted sol-gel deposition on a full Si wafer, followed by a thermal annealing. The process leads to hexagonally positioned perforations of homogeneous size and spacing. In a second step, these TiO 2 Inorganic NanoPatterns (INPs) are used as templates for the organization of Ge and Au nanodots. Germanium adatoms deposited by Molecular Beam Epitaxy on INPs, diuse and self-assemble into nanodots, located within the INPs pores. They form homogeneous sub-20 nm Ge nanodots in epitaxy on the silicon substrate and regularly distributed with one dot per perforation. The same approach is used for the formation of Au nanodots. In this case, a gentle mechanical polishing is requiered to suppress the dots seating at 1 the top of the TiO 2 network. The process developed in this study paves the way to the large scale self-organisation of quantum dots that are highly interesting for various applications, such as opto-electronics, and microelectronics.

Self-Ordered Ge Nanodot Fabrication by Using Reduced Pressure Chemical Vapor Deposition

Ge nanodot formation on Si surface and its three dimensional alignment is investigated using a reduced pressure chemical vapor deposition system. By exposing GeH4 on Si (001) surface at 550°C, a smooth wetting Ge layer is deposited for the first ∼0.9 nm, and then Ge nanodot formation occurs according to a Stranski-Krastanov growth mechanism. The Ge nanodots are randomly distributed with density of ∼6 × 1010 cm−2. By postannealing at 600°C, the Ge nanodots are coalesced. The size and density become ∼60 nm diameter 5 nm height and ∼1.5 × 1010 cm−2, respectively. By exposing GeH4 followed by postannealing at 600°C on checkerboard mesa structured Si surface which is fabricated by embedded body-centered tetragonal (BCT) Si0.6Ge0.4 nanodots, the Ge nanodot formation occurs at concave regions of the checkerboard mesa. By repeating Ge nanodot formation followed by Si spacer deposition by two step epitaxy using SiH4 at 600°C and using SiH2Cl2 at 700°C, vertical alignment of the Ge nanodots is observed. The lateral periodicity of the Ge nanodots is the same as that of the BCT Si0.6Ge0.4 nanodot template. The driving force of the self-ordered alignment is tensile strain of Si spacer surface above the Ge nanodots. By using the checkerboard mesa structured Si substrate by embedded BCT Si0.6Ge0.4 nanodot stack deposition as a template, vertically and laterally aligned 3D Ge nanodot stack fabrication is demonstrated without photolithography process.

Self-Ordered Ge Nanodot Fabrication By Reduced Pressure Chemical Vapor Deposition

Technologies of group IV semiconductor fabrication enable new artificial materials such as superlattice and nanodot structures (1). In particular, three dimensional (3D) ordered SiGe or Ge structures are promising materials to improve quantum dot lasers that can be integrated into existing CMOS platforms (2). In order to control regularly ordered nucleation sites, usually pre-patterning of the substrate is required. In the case of 3D stacking of Ge nanodots on Si substrates, the Ge nanodots tend to grow vertically aligned, due to the elastic strain field of the buried Ge nanodots (3). In a previous study, we reported a self-ordered body-centered tetragonal (BCT) SiGe nanodot formation using reduced pressure (RP) CVD, by repeating SiGe and Si layer stack deposition (4). By repeating several cycles of the SiGe and Si layer stack deposition, checkerboard Si mesas are formed on embedded SiGe dots and next SiGe dots are formed in concave region between the Si mesas. In this study we discuss the influence of Si surface roughness on the Ge nanodot formation and present a self-ordered 3D Ge nanodot formation. 3D Ge nanodot layer stack fabrication is carried out by using a single wafer RPCVD system. Standard Si (100) wafers are used. After H2 bake in the reactor, GeH4 is injected at 550°C to form Ge nanodots. Then a Si cap is deposited using H2-SiH4 at 600°C followed by H2-SiH2Cl2 at 700°C. In order to investigate the influence of surface roughness and strain on the Ge nanodot formation, a checkerboard Si mesa with ~7 nm height is fabricated by BCT Si0.6Ge0.4 nanodot layer stacks (4) for selected samples. Surface morphology is measured by AFM and SEM. Alignment of the Ge nanodots is measured by cross section SEM and TEM. Periodicity is measured by X-ray diffraction. By GeH4 exposure at 550°C Ge nanodots (Diameter: ~50, height: 2 nm) with ~6×1010 cm-2 were randomly formed on the Si surface. By postannealing at 600°C, migration of the Ge nanodots occurs resulting in larger Ge nanodots (Diameter: ~60 nm, height: 5 nm) and lower density (~1.5×1010 cm-2) as shown in Fig. 1. On the other hand, in the case of the checkerboard mesa structured Si surface fabricated by BCT Si0.6Ge0.4 nanodot underneath (RMS: 1.2 nm) (Fig. 2a), Ge nanodots are formed in concave region (Fig. 2b). This phenomenon is also observed for SiGe nanodot (4). The position of the Ge nanodot formation seems to be determined by a balance of strain field over embedded SiGe dots and surface energy. In this case, surface energy reduction seems to be more dominant compared to strain at the Si surface transferred from embedded Si0.6Ge0.4. In Fig 3, a cross section STEM image of 6 layers of Ge nanodot with Si spacer layer stack on the staggered aligned SiGe nanodot is shown. The Ge nanodots are vertically aligned. As shown in Fig. 2b, the first Ge nanodots are formed in the concave regions of the Si surface, therefore the first Ge nanodots are aligned to embedded SiGe nanodots underneath. By depositing a Si cap layer on the first Ge nanodots, a smoother Si surface (RMS: 0.5 nm) is formed compared to the initial checkerboard structure. By following GeH4 exposure, a second Ge nanodot formation occurred on the embedded Si surface. The aligned Ge nanodot formation seems to be driven by the tensile strain field at the Si surface above embedded Ge nanodots, because of a smooth Si surface. In Fig. 4a and 4b, nano beam diffraction (NBD) images of out-of-plane and in-plane strain distribution of the vertically aligned Ge nanodot structure are shown, respectively. The positions of the Ge nanodots are visible by higher vertical lattice parameter (Fig. 4a). In the Si part, smaller vertical lattice parameters are observed above the Ge nanodots compared to other area, indicating that the Si on the Ge nanodot is tensile strained. In the lateral lattice parameter distribution (Fig. 4b), only a periodic strain variation is visible, but to identify the position of the Ge nanodot is not possible, indicating that the 3D Ge nanodot stack is pseudomorphically grown. These NBD results support the assumption of Ge nanodot alignment discussed in Fig. 3. In this study we demonstrated self-ordered and vertically aligned Ge nanodot layer stack fabrication. Reference (1) K. L. Wang et al., Mat. Sci. in Semicond. Proc. 8 (2005) 389 (2) G. Capellini et al., Appl. Phys. Lett. 82 (2003) 1772 (3) Y. Nakaumura et al., Nanotechnology 21 (2010) 095305 (4) Y. Yamamoto et al., Nanotechnology 28 (2017) 485303 Figure 1

Self-Ordered Ge Nanodot Fabrication by Reduced Pressure Chemical Vapor Deposition

Ge nanodot formation on Si surface and its three dimensional alignment is investigated using a reduced pressure chemical vapor deposition (RPCVD) system. By exposing GeH4 on Si (001) surface at 550oC, a smooth wetting Ge layer is deposited for the first ~0.9 nm, and then Ge nanodot formation occurs as Stranski-Krastanov growth mechanism. The Ge nanodots are randomly distributed with density of ~6×1010 cm-2. By postannealing at 600oC, the Ge nanodots are coalesced. The size and density become ~60 nm diameter 5 nm height and ~1.5×1010 cm-2, respectively. By exposing GeH4 followed by postannealing at 600oC on checkerboard mesa structured Si surface which is fabricated by embedded body-centered tetragonal (BCT) Si0.6Ge0.4 nanodot, the Ge nanodot formation occurs at concave regions of the checkerboard mesa. By repeating Ge nanodot deposition and Si spacer deposition by two step epitaxy using SiH4 at 600oC and using SiH2Cl2 at 700oC, vertical alignment of the Ge nanodots is observed. The lateral periodicity of the Ge nanodots is the same as that of the BCT Si0.6Ge0.4 nanodot template. The driving force of the self-ordered alignment is tensile strain of Si spacer surface above the Ge nanodots.

THERMAL OXIDATION EFFECT ON ELECTRICAL PROPERTIES OF MOS CAPACITORS WITH EMBEDDED Ge NANOCRYSTALS

A 2D layer of spherical, crystalline Ge nanodots embedded in a SiO 2 layer was formed by low pressure chemical vapor deposition combined with furnace oxidation and rapid thermal annealing. The samples were characterized structurally by using transmission electron microscopy in plan-view and cross-section geometries. It was found that the formation of highdensity Ge dots took place due to oxidation induced by the Ge segregation. Electrical properties were controlled by measuring C–V and I–V characteristics after the formation of MOS capacitors in different oxidation conditions and the ambient medium. A strong evidence of the charge storage effect on the crystalline Ge-nanodot layer was demonstrated by the hysteresis behavior of the high-frequency C–V curves. It is shown that dry oxidation followed by its reduction increases the hysteresis value compared to wet oxidation conditions. This hysteresis behavior is discussed taking into account the decrease in the Ge concentration and a possible effect of low temperature GeO evaporation is followed by wet oxidation.

Deep Level Assessment of n-Type Si/SiO2 Metal-Oxide-Semiconductor Capacitors with Embedded Ge Quantum Dots

Device structures with silicon or germanium nanodots embedded in an oxide matrix may find application in the field of Non-Volatile Memories (NVMs) [1,2] or photonics [3,4]. In the former case, the nanodots can become charged by tunneling of electrons or holes from the silicon substrate through a tunnel oxide. Successful operation requires that the charge does not leak away by the assistance of traps or defects in the material stack. The latter can be probed by techniques like Admittance Spectroscopy [5] or Deep-Level Transient Spectroscopy (DLTS) [2,5]. In the present work we report on a systematic investigation of Metal-Oxide-Semiconductor (MOS) capacitors fabricated on n-type Czochralski silicon substrates and containing Ge nanodots with different size. Ge nanodots with average diameter of 1, 2 or 3 nm have been deposited by Molecular Beam Epitaxy (MBE) on a 5 nm tunnel oxide, thermally grown on n-type Czochralski silicon wafers. An Atomic Force Microscopy (AFM) picture is shown in Fig. 1. Next, 50 nm of SiO2 is deposited. MOS capacitors are prepared by thermal evaporation of 2 mm diameter Al gate contacts, while InGa+In foil ohmic contacts are prepared on the silicon substrate side. The device structure is schematically shown in Fig. 2. Capacitance-Voltage (C-V) characterization is performed at a fixed frequency f=1 MHz, using both a forward and reverse gate voltage sweep. As can be seen in Fig. 3, the presence of the Ge nanodots has a pronounced impact on the C-V characteristics: there is a shift towards positive flat-band voltage, compared with the zero dot reference, a strong increase of the accumulation capacitance and, for the nanodot samples, the hysteresis increases with the increase in nanodot size. For temperature (T-) scan DLTS, the capacitors are mounted in a helium contact gas cryostat cooled with liquid nitrogen. Measurements at different bias pulses from reverse bias (VR) to the pulse bias (VP) are performed in parallel in order to probe the electron traps in different parts of the structure. An example is given in Fig. 4, for the 1 nm nanodots sample. It shows that in deep depletion, there is a broad peak between 150-250 K which could stem from the response of filled interface traps. The increase found at room temperature may originate from minority carrier generation in the depletion region [2,5,6]. When probing closer to the interface, for more positive VR, a pronounced but broad peak around 200 K dominates the spectra. From the Arrhenius plot shown in Fig. 5, it is concluded that it corresponds to the dangling bond (Pb) acceptor level at about 0.31 eV from the conduction band [2,5,6]. Maximum densities are estimated in the range of 5x1011 cm-2eV-1. A comparison of the spectra near the interface for the different samples studied is provided in Fig. 6 and will be discussed in more detail in the conference paper.

(Invited) Nanostructure Design for Control of Phonon and Electron Transports

Thermoelectric materials are required to high electrical conductivity and low thermal conductivity for high performance. The independent control of electrons and phonons is difficult due to their correlation resulting in the difficulty of the enhancement of thermoelectric performance. Recently, nanostructures have drawn much attention in thermoelectric material study. One of the mechanisms of its enhancement is the thermal conductivity reduction due to the phonon scattering at the interfaces introduced by nanostructuring [1]. In some case, however, its interfaces can scatter the carriers leading to the reduction of the carrier conductivity. In this study, we are proposing the novel nanostructure for independent control of electrons and phonons using nanodots. The nanostructure is ultra-small epitaxial Ge nanodots are embedded in Si layers. It is expected that carrier transports in Si layers with high electrical conductivity and high Seebeck coefficient, while phonons are scattered at the interfaces between nanodots and Si layers [2,3]. Ultra-small Ge nanodots in this nanostructure were grown by our original technique: ultrathin SiO2 film technique. First, we formed the ultrathin SiO2 films with one monolayer thickness by oxidizing clean Si surfaces at~500ºC after introducing the Si substrates into molecular beam epitaxy chamber at the base pressure of ~10-8 Pa. We deposited Ge on the ultrathin SiO2 film to form Ge nanodtos. At the first stage, of Ge deposition the reaction of Ge+SiO2 →GeO↑+SiO↑ occurred to form nanowindows in the ultrathin SiO2 films. Subsequently deposited Ge atoms were trapped at the nanowindows, which worked as nucleation sites. Then, ultras-mall nanodots were formed. The nanodots contacted with Si substrates through nanowindows leading to the epitaxial growth. Si was deposited on the epitaxial Ge nanodots to form Si layers. These formations of the ultrathin SiO2 films, epitaxial Ge nanodots, and epitaxial Si layers were repeated to form the nanostructure that is Si films including Ge nanodots. The thermal conductivity measurements of the nanostructure showed the drastic reduction of thermal conductivity. Hall effect measurements exhibited a similar high electrical conductivity to that of bulk Si. Electrons transports through the interfaces easily by band engineering. On the other hand, ultras-mall nanodots work as phonon scatterers leading to the drastic reduction of thermal conductivity. This phonon scattering is consistent with wave scattering theory. These results demonstrated nanostructure design brought to the independent control of carrier and phonon transport using the Si layers with ultrasmall epitaxial Ge nanodots This work was partially supported by JSPS KAKENHI Grant Number 16H02078 for Scientific Research (A) and 15K13276 for Challenging Exploratory Research. In part, this work was supported by CREST-JST program. Reference [1] Y. Nakamura, et al., Nano Energy 12, 845 (2015). [2] S. Yamasaka, et al., Sci. Rep. 5, 14490 (2015). [3] S. Yamasaka, et al., Sci. Rep. 6, 22838 (2016).

(Invited) Nanostructure Design for Control of Phonon and Electron Transports

Simultaneous achievement of low thermal conductivity κ and high electrical conductivity σ is a promising route for realizing Si-based thermoelectric materials. Although several attempts have been made, this requirement has still been bottlenecked by their correlated nature. In order to realize low κ and high σ simultaneously, we have proposed a nanoarchitecture that is Si films including epitaxial Ge nanodots. This nanoarchitecture was fabricated based on molecular beam epitaxy, where Ge nanodots and Si layer were stacked alternately on Si substrates using unique ultrathin SiO2 film technique. In this study, we investigated the impact of Ge nanodot incorporation and doping on the κ and σ. The κ was significantly reduced by Ge nanodot incorporation whereas the σ exhibited high value coming from Si property. This demonstrated the accomplishment of the nanostructure design for the independent control of carrier and phonon transport using the ultrasmall epitaxial Ge nanodots.