SiGe Nanodots Research Articles

Optical Properties of Multilayered Staggered SiGe Nanodots Depending on Si Spacer Growth Temperature

The optical properties of the multilayered staggered SiGe nanodots (NDs) embedded in the Si spacers fabricated at varying Si growth temperatures are clarified in conjunction with the effect of strain by Photoluminescence (PL) and Raman spectroscopy. We found that compressive strain is induced in the SiGe NDs, with higher growth temperatures of the Si spacer resulting in stronger compressive strain. PL spectra indicate that the higher growth temperature leads to a wider bandgap. This transition energy behavior is caused by not only the strain in the SiGe NDs but also the Ge segregation. Furthermore, luminescence from the SiGe NDs was observed along with luminescence originated at the Si/SiGe interface in the SiGe NDs with a small dot size. This phenomenon was caused by the small energy difference of the conduction-band minima between the SiGe NDs and the tensile-strained Si spacers and the small volume of the tensile-strained Si spacers.

Evaluation of Thermal Transport Characteristics of Three-Dimensional Self-Ordered Multilayered Silicon-Germanium Nanodots

1. Background and purpose Silicon-germanium (SiGe) alloys have much lower thermal conductivity than Si and Ge single crystals and are expected to be candidates for next-generation thermoelectric device materials of internet of things society. It is important to achieve both high carrier mobility and low thermal conductivity on the basis of the dimensionless figure of merit [1]. There are several studies on the thermoelectric properties of SiGe with low-dimensional structures such as superlattice and nanodots [2,3] to dramatically decrease thermal conductivity. However, the relationship between the nanoscale thermal properties and the phonon scattering mechanism of self-ordered multilayer SiGe nanodots with high uniformity grown by Stranski-Krastanov [4] mode is unclear. In this study, we demonstrated the thermal transport characteristics of the self-ordered multilayer SiGe nanodots. 2. Experiments Si/SiGe superlattice and nanodots samples were fabricated by reduced pressure chemical vapor deposition. We set to 20 cycles of Si0.65Ge0.35 / Si superlattice, the staggered Si0.65Ge0.35 nanodots (alternately stacked) and the dot-on-dot Si0.65Ge0.35 nanodots (vertically stacked) by changing growth temperature and precursors for Si spacer growth [4], as shown in Fig. 1. Thermal properties were evaluated by the Time Domain Thermoreflectance (TDTR) method. The strain states and Ge fraction were evaluated by Raman spectroscopy. The focal length and wavenumber resolution of the Raman spectrometer were 2,000 mm and 0.1 cm-1, respectively. The excitation light source was a green laser (wavelength: 532 nm). We obtained the Raman spectra of Si-Si, Si-Ge, and Ge-Ge vibrational modes derived from the SiGe regions. 3. Results and discussion Figure 2 shows the thermal conductivity for the SiGe region of the superlattice and nanodots (dot-on-dot and staggered structures) obtained by TDTR method. We found that the SiGe nanodots with the dot-on-dot structure have the highest thermal conductivity, while the SiGe nanodots with the staggered structure and SiGe superlattice have almost the same thermal conductivity. We consider that the heat flow easily penetrates from the top (surface) to the bottom (Si substrate interface) in the dot-on-dot structures. On the other hand, it is possible that the staggered SiGe nanodots can play a sufficient role as a phonon scatterer compared to the dot-on-dot structures since the superlattice structure and the staggered SiGe nanodots have almost the same thermal properties. In conclusion, we demonstrated the possibility of controlling phonon scattering by changing the arrangement of the self-ordered multilayer SiGe nanodots. Acknowledgement This work was supported by JSPS KAKENHI Grant Number 21K14201, Japan.

Strain and optical characteristics analyses of three-dimentional self-ordered multilayered SiGe nanodots by photoluminescence and Raman spectroscopy

The strain state, optical properties, and band structure of the self-ordered multilayered silicon-germanium (SiGe) nanodots, which are staggered and dot-on-dot alignment and embedded by Si spacer, were evaluated by Raman spectroscopy and low-temperature photoluminescence (PL). These results suggest that the compressive strain applied to the staggered nanodots is smaller than that of the dot-on-dot nanodots, which contributes to the shrinking of the bandgap of the staggered nanodots. Strong PL intensity was observed from the nanodots compared to the single crystalline bulk SiGe due to the carrier confinement and high crystal quality of the nanodots. The stack-controlled nanodots showed a redshift of the PL peaks compared to the bulk SiGe and the effect of strain induced in SiGe nanodots might not be enough to explain this phenomenon. The cause of the redshift was clarified by considering the hetero band structure of the nanodots and the tensile strained spacer.

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.

Alignment control of self-ordered three dimensional SiGe nanodots

Alignment control of three dimensional (3D) SiGe nanodot arrangements is investigated using a reduced pressure chemical vapor deposition system. Several cycles of SiGe layers with 30% Ge content and Si spacers are deposited by SiH4-GeH4 at 550 °C and SiH4 or SiH2Cl2 at 700 °C, respectively, to form a 3D SiGe nanodot structure. By using SiH4 as a precursor for the Si spacer deposition, SiGe nanodots are aligned at staggered positions resulting in a body-centered tetragonal (BCT) structure, because a checkerboard mesa structured Si surface is formed and the next SiGe nanodot formation occurs at the concave region to reduce surface energy. On the other hand, after planarizing the Si surface with checkerboard structure by chemical mechanical polishing (CMP), the new SiGe nanodot formation occurs directly above the embedded SiGe nanodot located nearest to the Si surface (dot-on-dot). The driving force seems to be local tensile strain formed at the Si surface above the embedded SiGe nanodot. By using SiH2Cl2 as precursor for the Si spacer deposition, a smooth Si surface can be realized on BCT SiGe nanodot structures without CMP process resulting in a vertically aligned SiGe nanodot formation. The local tensile strain formation in Si above SiGe nanodots is confirmed by nano beam diffraction analysis.

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

A self-ordered, body-centered tetragonal superlattice of SiGe nanodot growth by reduced pressure CVD

Self-ordered three-dimensional body-centered tetragonal (BCT) SiGe nanodot structures are fabricated by depositing SiGe/Si superlattice layer stacks using reduced pressure chemical vapor deposition. For high enough Ge content in the island (>30%) and deposition temperature of the Si spacer layers (T > 700 °C), we observe the formation of an ordered array with islands arranged in staggered position in adjacent layers. The in plane periodicity of the islands can be selected by a suitable choice of the annealing temperature before the Si spacer layer growth and of the SiGe dot volume, while only a weak influence of the Ge concentration is observed. Phase-field simulations are used to clarify the driving force determining the observed BCT ordering, shedding light on the competition between heteroepitaxial strain and surface-energy minimization in the presence of a non-negligible surface roughness.

Thermal transport through short-period SiGe nanodot superlattices

The cross-plane thermal conductivity κ of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far κ can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of κ is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane κ of Ge/Si superlattices with period lengths down to 1.5 nm. Although κ tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for κ. To assess the role played by the nanodots in the reduction of κ we studied Ge/Si superlattices with nanodot densities varying from 0 to ∼8×1010 cm−2 and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with ‘‘pyramid’’-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1−xGex (with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the κ by ∼20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics.

Formation of Si or Ge nanodots in Si3N4 with in-situ donor modulation doping of adjacent barrier material

Conventional doping of small nanodots does not provide majority carriers due to self-purification effects and much increased ionisation energies of dopants presenting point defects. Adjacent barrier layers to Si-rich Si3N4 can be doped by excess Si in-situ with the segregation anneal for Si nanodot formation. Donor doping of AlxGa1−xN (0 ⩽ x ⩽ 1) with Si is an established process. Material properties and process compatibility of AlxGa1−xN render it suitable as barrier for Si nanodot superlattices. Ab-initio calculations showed that Ge also works as a donor in AlN, extending the material range to Ge and SiGe nanodots in Si3N4.

Structural Analysis of Si-Based Nanodot Arrays Self-Organized by Selective Etching of SiGe/Si Films

Atomic force microscopy (AFM) revealed the surface structures of nanodot arrays fabricated by selective etching of SiGe/Si films that have pure-edge dislocation networks at the SiGe–Si interface. By varying the etching time, it is possible to control the composition of the nanodot arrays, allowing either SiGe or Si nanodot arrays to be fabricated. The surface roughness of the nanodot arrays does not originate from the nanodots themselves, but from the spatial variation in the depth of dislocations in the SiGe/Si films prior to etching. Individual nanodots in SiGe nanodot arrays have smoother profiles than those in Si nanodots although the nanodots in SiGe and Si nanodot arrays have similar heights and diameters. The rougher surface of individual Si nanodots is due to the lower etching selectivity, which results from the smaller dislocation influence on the etching reaction in Si substrates far from the interface. Transmission electron microscopy images reveal the formation of arrays of epitaxial SiGe nanodots with smooth surfaces, which is consistent with the AFM results.

Self-organization of two-dimensional SiGe nanodot arrays using selective etching of pure-edge dislocation network

A self-organization technique of two-dimensional SiGe nanodot arrays was developed using selective etching of SiGe/Si substrates that have a pure-edge dislocation network at the interface. The pure-edge dislocation network was selectively etched by a water solution of CrO3 and HF. This produced SiGe nanodot arrays with nanometer-scale periodicities. The periodicities and the compositions of the SiGe nanodot arrays correspond with that of the interfacial pure-edge dislocation network and that at the interface, respectively, in the SiGe/Si substrates prior to etching. We found that the etchant temperature strongly affects the etching rate and selectivity. We demonstrated self-organized formation of an array with a periodicity of 17 nm of dome-shaped Si0.82Ge0.18 nanodots with an aspect ratio of 0.12.

Modeling of Self-Assembled GeSi Nano-Dots and Corresponding Admittance Spectroscopy

We examine two techniques to determine experimentally and theoretically the strength of quantum confinement in SiGe nano-dots. A simple theory for admittance spectroscopy in a quantum dot is developed in conjunction with our experiments and experimental findings. Using a mass-balance equation approach based on the Boltzmann equation in which the hole-phonon interaction in a SiGe nano-dot is considered, we can successfully reproduce those observed experimentally in the admittance spectroscopy measurements. Thus, we are able to understand the interesting features of the electronic properties in SiGe nano-dots, especially the dependence of the quantum confinement on the size of the dots.

SiGe nanostructure fabrication through selective epitaxial growth using self-assembled nanotemplates

Ordered SiGe nanostructures including nanodots (NDs) and nanowires (NWs) were fabricated via selective epitaxial growth (SEG) of SiGe using ultrahigh vacuum chemical vapour deposition (UHV-CVD) on Si openings fabricated using self-assembled nanotemplates of anodic anluminum oxide (AAO) and diblock copolymer (DBC) of polystyrene-block-polymethylmethacrylate (PS-b-PMMA), exhibiting hexagonally arranged nanoholes. SiGe SEG was processed through repeating the unit cycle composed of two separated steps of SiGe growth using disilane (Si2H6) and Germane (GeH4) and chlorine (Cl2 exposure. Cl2 was used to improve the selectivity of SiGe SEG between the Si openings and the oxide area. Ordered SiGe NDs and NWs were fabricated through SiGe SEG of 20 cycles and 400 cycles on AAO/Si, respectively. In addition, ordered SiGe NDs were obtained via SiGe SEG of 20 cycles on SiO2 template, fabricated through pattern transfer of nanoholes of PS-b-PMMA to SiO2/Si. SiGe nanostructure fabrication using both AAO and PS-b-PMMA showed good replication of the nanohole size of the nanotemplates.An erratum to this article was added by the author on 18 May 2010. The text of the erratum is appended to the PDF.

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.

Properties and applications of SiGe nanodots

The use of SiGe strained layers for BiCMOS technology has been prevailing in the market place for high-speed and high-frequency mixed mode applications. The advantages of using SiGe are due to its compatibility with mainstream Si CMOS processing. SiGe has also been gradually making its way to the mainstream strained layer CMOS as well. In this paper, we will discuss SiGe quantum structures, which have the unique properties due to their nanosize effects and the presence of strain. We will describe the growth of SiGe quantum dots on Si. First, for dot growth, issues such as the growth conditions and their effects on the dot morphology will be reviewed. We will focus on the dependence of growth temperature on the size and also show the mechanisms of uniform dot formation. The basic issues such as vertical correlation and dot morphology evolution will be addressed in relation to the critical thickness of Ge quantum dot superlattices. Both interband and intersubband photoluminescence properties of Ge quantum dot superlattices will be discussed as functions of dot size, excitation power, growth temperature, superlattice period, and Ge concentration. We will also discuss quantum dot pin photodetectors for 1.3–1.55 μm for integration with Si technology for communications applications. Potential light emission from dots will be described and the characteristics of fabricated LED devices will be reported. Likewise, we will show the results of intersubband properties of boron-doped and modulation-doped Ge quantum dot superlattices in reference to intersubband absorption as studied by FTIR. We will describe the quantum dot pip photodetectors for 3–5 and 8–12 μm applications.