SiGe Nanostructures Research Articles

Interface and surface segregation of germanium in the SiGe semiconductor

The concentration and distribution of germanium in the silicon crystal or SiGe alloy can play critical roles in affecting the performance of prepared semiconductor devices. We quantitatively investigate the segregation behavior of germanium in silicon and SiGe semiconductor based on molecular dynamics simulation using the Stillinger-Weber (SW) and Tersoff empirical potentials. The value predicted by the SW potential is 0.35 which agrees with some reported experimental results. Both potentials predict that the Ge equilibrium segregation coefficient at the solid/liquid interface of silicon is smaller than one. The germanium atoms tend to segregate to the surface in the SiGe nano structures in order to lower the surface energy. We define the surface segregation coefficient Ks to quantitively describe this behavior. It is found that Ks decreases with the increase of total germanium atomic fraction in the liquid slab. The size dependence of the surface segregation coefficient in the nanodroplet is studied and it scales with 1/R3 linearly. The germanium distribution in the SiGe solid slab and nanoparticle is largely affected by the Ge distribution in the liquid slab and nanodroplet before crystallization.

Fabrication of ultrathin poly-crystalline SiGe-on-insulator layer for thermoelectric applications

For realizing high power generator efficiency based on thermoelectricity, Si, Ge and SiGe nanostructures have attracted attention. In this paper, we have investigated a new approach to fabricate an ultrathin polycrystalline SiGe-on-insulator (pc-SGOI) substrate by a simple process based on Si and Ge deposition followed by thermal diffusion suitable for thermoelectric devices. A 45-nm-thick SGOI layer with a Ge fraction of nearly 0.45 was fabricated, and the Ge fraction was homogeneous in plane over the layer. Its thermal conductivity was 0.87 W mK−1, lower than that of a single-crystalline SGOI layer. This is caused by the enhancement of scattering of phonons at grain boundaries in the pc-SGOI layer.

(Gordon E. Moore Medal for Outstanding Achievement in Solid State Science and Technology Award Address) Towards Silicon-Based Photonic Integrated Circuits: The Quest for Compatible Light Sources

Light emission from silicon and germanium nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon optoelectronics and photonics. Both silicon and germanium possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering employing quantum wells, wires, or dots has been proposed as one way to overcome this limitation and, in the past, Si/Ge, Si/SiO2, or Si/SiGe-alloy thin-multilayer nanostructures grown on Si have been produced on this principle, and although light emission with a greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recent advances in band gap engineering technology have resulted in the development of different systems incorporating one-, two-, and three-dimensionally confined Si-Ge nanostructures that produce bright light in the visible to near infrared. Here, the novel optical properties of various such nanostructures are reviewed.

Fabrication of Homogeneous Ultra-Thin Sige-on-Insulator Layer for Thermoelectric Applications

SiGe nanostructures has attracted much attention in recent years in order to realize high-efficient thermoelectric devices due to carrier confinement effect and alloying effect [1,2]. For this purpose, in the present study, we fabricated a compositionally-homogeneous thin SGOI (SiGe on insulator) substrate by simple sputtering and physical evaporation techniques and characterized its crystallographic properties and thermal conductivity. A 30-nm-thick Si layer was sputtered on a thermally-oxidized Si wafer (SiO2 thickness: 103 nm), followed by the deposition of a thin Ge film with a thickness of 30 nm, at room temperature. Afterwards, Si-Ge diffusion was carried out by a rapid thermal annealing process at 880oC for 10 and 120 min. The prepared SGOI substrates were characterized by means of XRD, SEM, EPMA, Raman spectroscopy, XPS, and AFM, which showed the formation of a compositionally-homogeneous poly-crystalline Si1-xGex layer with x=0.45 (thickness: 60 nm). The thermal conductivity was determined by a picosecond time-domain thermoreflectance (TDTR) method at room temperature and shown in the figure, as a function of annealing time. For a reference, the measured values of our as-deposited Ge/Si layer and a commercial single-crystalline SGOI substrate (x=0.5) are also plotted. It is found that our poly-crystalline SGOI layer has lower thermal conductivity of 0.87 W/mK, compared with that of a single-crystalline SGOI 1.43 W/mK. [1] H. Kim, I. Kim, H.-J. Choi, and W. Kim, Appl. Phys. Lett., 96 (2010) 233106. [2] R. Basu, S. Bhattacharya, R. Bhatt, M. Roy, S. Ahmad, A. Singh, M. Navaneethan, Y. Hayakawa, D. K. Aswal, and S. K. Gupta, J. Mater. Chem. A, 2 (2014) 6922. Figure 1

(Invited) Isotopically Programmed Group IV Semiconductors: A Versatile Platform for Quantum Technologies

The introduction of stable isotopes as an additional degree of freedom in the growth of semiconductor films and quantum structures provides a wealth of opportunities to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. In this presentation, I will describe the historical context and outline the recent progress in the area of isotopically engineering group IV semiconductors with focus on nanoscale and quantum structures and devices. This ability to isotopically program semiconductor structures has been a powerful paradigm to investigate and manipulate some of the important physical properties of semiconductors and exploit them in innovative device structures [1-13]. Isotopes of an element differ in the number of neutrons in the nucleus. This creates differences between the isotopes in their lattice dynamics and nuclear properties. For instance, the slight difference in zero-point motion leads to a difference in atomic volume between the isotope atoms, which influences the lattice constant [3]. Also, the difference in electron-phonon coupling between crystals of different isotopic composition was found to affect the electronic band gap [4]. The nuclear spin is another significant difference between stable isotopes. For instance, natural silicon (Si) has three stable isotopes: 28Si, 29Si, and 30Si, with isotopic abundances of 92.23%, 4.67%, and 3.10%, respectively. Among these three isotopes, only 29Si has a nuclear spin of ½, whereas 28Si and 30Si are nuclear spin-free. This property has been crucial in the realization of Si-based quantum information devices [5-8]. One of the most drastic isotope related effect in semiconductors is found in phonon properties [9-13]. Mass fluctuation induced by isotope disorder acts as a substitutional defect in a crystal thus affecting the phonon mean free path and consequently the phononic thermal conductivity. Measurements on isotopically pure Ge [9] and Si [10] crystals showed an enhanced thermal conductivity as compared to their natural counterparts. Also, lower thermal conductivity was recently demonstrated in Si isotope superlattices [11]. All the properties of semiconductor stable isotopes have been investigated and exploited in bulk materials or thin films. Herein, we will describe the new opportunities emerging from the combinations of the isotope effects with size-related effects in nanoscale materials [14-18]. More specifically, we will discuss phonon engineering in metal catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane and german precursors 28SiH4, 29SiH4, 30SiH4, 74GeH4 and 76GeH4, with purity better than 99.9%. Isotopically mixed nanowires 28Six 30Si1-x with a composition close to the highest mass disorder (x ~ 0.5) were used as a playground to elucidate the interplay between nanoscale interface phenomena and heat transport [16]. We will show how isotopically engineered nanowire homo-junctions can be introduced to realize innovative phononic devices such as thermal diodes and thermal transistors. Additionally, we will also discuss the use of nuclear spin-full 29Si to engineer novel quantum devices in nuclear spin-free SiGe nanostructures. Finally, atomistic-level investigations of isotopically programmed nanoscale materials will be presented based on laser-assisted atom probe tomography [15,17].

Progress in Light Emission from Silicon and Germanium Nanostructures

Light emission from silicon and germanium nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon optoelectronics and photonics. Both silicon and germanium possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering employing quantum wells, wires, or dots has been proposed as one way to overcome this limitation and, in the past, Si/Ge, Si/SiO2, or Si/SiGe-alloy thin-multilayer nanostructures grown on Si have been produced on this principle, and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recent advances in band gap engineering technology have resulted in the development of new systems incorporating one-, two-, and three-dimensionally confined Si-Ge nanostructures that produce bright light in the near infrared. Here, the novel optical properties of recently-investigated various such nanostructures are reviewed.

Thermal conduction in Si and SiGe phononic crystals explained by phonon mean free path spectrum

Thermal phonon transport in single-crystalline Si, amorphous SiGe, and poly-SiGe nanostructures was investigated experimentally at room temperature. The characteristic length dependence of thermal conductivity was compared across these three materials by changing the shortest distance between the circular holes of phononic crystals formed in the membranes. The dependences clearly differ for these materials, and these differences can be explained by the thermal phonon mean free path spectra of the three materials. Nanostructuring has a larger impact on thermal conductivity reduction when the characteristic length is comparable to that in the region where the thermal phonon mean free path spectrum is dense. The results suggest that thermal phonon mean free path spectra can be estimated qualitatively by thermal conductivity measurements with characteristic length sweeps.

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Effect on SiGe Light Emission

The nature of the interfaces between SiGe nanostructures and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one, two, and three dimensions. The interface sharpness is influenced by many factors including growth conditions, strain, and thermal processing, which can make it difficult to attain the desired structures. This is certainly the case for nanostructure confinement in one dimension. However, axial Si/Ge nanowire heterojunctions with a Si/Ge nanowire diameter in the range 50–120 nm produce a strong photoluminescence signal associated with band-to-band electron-hole recombination at the nanowire heterojunction that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, experiments show that two quite different SiGe nanostructures incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic non-exponential decay time that is remarkably shorter (as much as 1000 times) than that found in conventional Si/SiGe nanostructures.

Reduction of the surface roughness of Ge-on-insulator layers up to sub-nanometer range by chemical mechanical polishing

We are investigating the thermoelectric characteristics of Ge and SiGe nanostructures for realizing high power generator efficiency. In this paper, we investigated the influence of the thinning process on the surface roughness of a direct wafer-bonded p-type Ge-on-insulator (GOI) layer in order to realize an ultra-thin GOI substrate with extremely low surface roughness for the fabrication of Ge and SiGe nanostructures. The wafer thinning process was performed by chemical mechanical polishing (CMP) and wet chemical etching (WCE). A very smooth GOI layer with sub-nanometer (0.3 nm) surface roughness, suitable for nanostructure fabrication, was achieved using CMP compared to WCE process.

Si/SiGe Interfaces in Three-, Two-, and One-Dimensional Nanostructures and Their Influence on SiGe Light Emission

The nature of the interfaces between SiGe nanostructures (NSs) and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one, two, and three dimensions. The interface sharpness is influenced by many factors including growth conditions, strain, and thermal processing, which can make it difficult to attain the desired structures. This is certainly the case for NS confinement in one dimension. However, axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50–120 nm produce a strong PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, experiments show that two quite different SiGe NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic non-exponential decay time that is remarkably shorter (as much as 1000 times) than that found in conventional Si/SiGe NSs.

Features of SOI substrates heating in MBE growth process obtained by low-coherence tandem interferometry

Differences in heating of silicon and silicon-on-insulator (SOI) substrates in molecular beam epitaxy were revealed by low-coherence tandem interferometry. Using this technique the interference effects which impede the correct evaluation of SOI substrate temperature by infrared pyrometers can be eliminated and so the reliable temperature readout can be achieved. It was shown that at the same thermocouple and heater power settings the real temperature of SOI substrates is higher than of silicon ones and the difference may be as high as 40–50°C at temperatures close to 600°C. It is supposed that such effect is caused by the additional absorption of heater radiation by the buried oxide layer in the mid-infrared range. Independent proof of this effect was obtained by growing on both types of substrates a series of structures with self-assembled Ge nanoislands whose parameters are known to be very temperature sensitive. The proposed low-coherence interferometry technique provides precise real-time control of the growth temperature and so allows formation of SiGe nanostructures with desired parameters.

Composition and stress of SiGe nanostructures on curved substrates

Recent experimental studies of Ge nanoislands on silicon-on-insulator (SOI) substrates have provided a defect-free strain relaxation mechanism through the bending of the substrate. Here, using atomistic Monte Carlo simulations and analytical modeling, we couple this relaxation mechanism with interdiffusion and alloying and observe composition profiles that are completely different from those observed in flat nanoislands. Moreover, for comparable SOI and island thicknesses, intermixing can be greatly reduced and Ge content in the islands is highly preserved.

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Impact on SiGe Light Emission

Fast optical interconnects together with an associated light emitter that are both compatible with conventional Si-based complementary metal-oxide- semiconductor (CMOS) integrated circuit technology is an unavoidable requirement for the next-generation microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures, which can emit light at wavelengths within the important optical communication wavelength range of 1.3 – 1.55 μm, are already compatible with standard CMOS practices. However, the expected long carrier radiative lifetimes observed to date in Si and Si/Si1-xGex nanostructures have prevented the attainment of efficient light-emitting devices including the desired lasers. Thus, the engineering of Si/Si1-xGex heterostructures having a controlled composition and sharp interfaces is crucial for producing the requisite fast and efficient photoluminescence (PL) at energies in the range 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe nanostructures and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in three dimensions (corresponding to the case of quantum dots), two dimensions (corresponding to quantum wires), and one dimension (corresponding to quantum wells). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for nanostructure confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex nanostructures incorporated into a Si0.6Ge0.4 wavy superlattice structure display PL of high intensity while exhibiting a characteristic decay time that is up to 1000 times shorter than that found in conventional Si/SiGe nanostructures. The non-exponential PL decay found experimentally in Si/SiGe nanostructures can be interpreted as resulting from variations in the separation distance between electrons and holes at the Si/SiGe heterointerface. The results demonstrate that a sharp Si/SiGe heterointerface acts to reduce the carrier radiative recombination lifetime and increase the PL quantum

Top–down SiGe nanostructures on Ge membranes realized by e-beam lithography and wet etching

SiGe nanostructures on Ge membranes have been fabricated by electron beam lithography and anisotropic wet-chemical etching, starting from SiGe/Ge heterostructures epitaxially deposited on Si substrates. Two different top–down approaches have been studied in order to obtain the best freestanding structures. We find that the process in which the Ge membrane is suspended after the lithography of the SiGe nanostructures leads to high quality SiGe nanostructures without damage to either the SiGe nanostructures or the Ge membrane. The structures have been systematically analyzed at every step of the fabrication process, by scanning electron microscopy and by atomic force microscopy.

Towards promising modification of GeSi nanostructures via self-assembly on miscut Si(001) substrates

Self-assembled GeSi nanostructures on miscut Si(001) substrates are studied systematically with regard to the miscut angle and azimuth, the amount of Ge and the growth temperature. The comprehensive dependence of the spatial arrangement, which can exhibit one- and two-dimensional (1D and 2D) ordering, as well as the shape and density, of GeSi nanostructures on the miscut angle is observed. The orientation and side-walls of the 1D ordered in-plane GeSi nanowires on miscut Si(001) substrates are intimately associated with the miscut azimuth towards the 〈110〉 or 〈010〉 directions. Furthermore, the unique evolution of the GeSi nanostructures with the amount of Ge and the growth temperature on miscut Si (001) substrates towards the 〈010〉 direction is discovered. Such promising features of self-assembled GeSi nanostructures on miscut Si (001) substrates are explained in terms of the thermodynamics and growth kinetics, which are both affected significantly by the substrate vicinality. These results demonstrate that the miscut substrates offer a promising degree of freedom for the feasible modification of self-assembled nanostructures.

Lasing from Glassy Ge Quantum Dots in Crystalline Si.

Semiconductor light-emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices. Light sources based on group IV elements would be SIT-compatible, but suffer from the poor optoelectronic properties of bulk Si and Ge. Here we demonstrate that epitaxially grown Ge quantum dots (QDs) in a defect-free Si matrix show extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In contrast to conventional SiGe nanostructures, these QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem.

Investigating the Composition and Conductance Distributions on Highly GeSi Mixed Quantum Dots and Inside Oxidation Problem.

With the help of a nanoscale trench, the composition and conductance distributions of single GeSi quantum dots (QDs) are obtained by conductive atomic force microscopy combined with selective chemical etching. However, the obtained composition and current distributions are unwonted and inconsistent on the QDs grown at 680 °C. With a series of confirmatory experiments, it is suggested that a thick oxide layer is formed and remains on the QDs’ surface after etching. Though this selective chemical etching has already been widely applied to investigate the composition distribution of GeSi nanostructures, the oxidation problem has not been concerned yet. Our results indicate that the oxidation problem could not be ignored on highly GeSi mixed QDs. After removing the oxide layer, the composition and conductance distributions as well as their correlation are obtained. The results suggest that QDs’ current distribution is mainly determined by the topographic shape, while the absolute current values are influenced by the Ge/Si contents.

(Invited) Bright Light Emitting Silicon/Germanium Nanostructures

Light emission from Si nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon opto-electronics and photonics. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering employing quantum wells or quantum dots has been proposed as one way to overcome this limitation and Si/Ge or Si/SiGe-alloy thin-multilayer nanostructures grown on Si have been produced on this principle, and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recent advances in band gap engineering technology have resulted in the development of new systems incorporating one-, two-, and three-dimensionally confined Si-Ge nanostructures that produce bright light in the near infrared. Here, the novel optical properties of three such nanostructures – super unit cells, nanowires, and nanocluster layers – are explored.

Local uniaxial tensile strain in germanium of up to 4% induced by SiGe epitaxial nanostructures

We show that a relatively simple top-down fabrication can be used to locally deform germanium in order to achieve uniaxial tensile strain of up to 4%. Such high strain values are theoretically predicted to transform germanium from an indirect to a direct gap semiconductor. These values of strain were obtained by control of the perimetral forces exerted by epitaxial SiGe nanostructures acting as stressors. These highly strained regions can be used to control the band structure of silicon-integrated germanium epilayers.

Engineering thermal transport in SiGe-based nanostructures for thermoelectric applications

Abstract