Growth Of Ge Research Articles

Ge/si系の成長と歪緩和のメカニズム( ヘテロエピタキシーと界面構造制御)

Ge/si系の成長と歪緩和のメカニズム( ヘテロエピタキシーと界面構造制御)

(Invited) Atomically Controlled Processing for Dopant Segregation in CVD Silicon and Germanium Epitaxial Growth

High performance Si-based devices require atomically ordered interface of heterostructures and doping profiles as well as strain engineering, which is obtained by the introduction of Ge into Si. In this work, dopant (P and B) segregation during in-situ doping in CVD Si and Ge epitaxial growth is reviewed. The epitaxial growth of in-situ doped Si and Ge films either on Si(100) or on a few nm-thick Si0.5Ge0.5/Si(100) was performed at 550oC and 350oC, respectively. In the case of P doping, the P atoms segregate to the Si and the Ge surfaces and a part of them are incorporated into the grown Si and Ge cap layers. The P segregation during Si growth is larger than that during Ge growth. In the case of B doping, the B atoms scarcely segregate to the grown Si and Ge surface. Based on these experimental results, the in-situ doping processes are explained by the modified Langmuir-type adsorption and reaction scheme.

(Invited) Electrodeposition of Germanene

Germanene is the Ge analogue Silicene or Graphene. A number of ab-initio calculations in the literature suggest that Germanene sheets should exist. Those reports suggest that the material would have a buckled structure, where half the atoms are about 0.07 nm higher than the other half. Micro Raman, electrochemistry and in-situ scanning tunneling microscopy (EC-STM) have been used to better understand the electrochemical growth of Ge from aqueous solutions, from the first atomic layers on Au(111). Our initial studies of the electrodeposition of Ge resulted in some interesting, though not clearly understood results, such as why only about 2-3 ML of Ge could be formed by direct electrodeposition on Au, depending on the pH. Some reports in the literature indicate that thicker deposits of Ge can be formed by electrodeposition on electrode surfaces other than Au. It is not yet clear why Au would be different. In-situ STM studies of the first atomic layers of Ge have shown a number of processes which indicate that nanoscopic atomically flat layers are forming on the surface, which demonstrate that Germanene is being formed. Micro Raman has also demonstrated the presences of Germanene grown on Au(111), which displays a Raman band near 295 cm-1. These results will be described. References Liang, et al., Langmuir, 26(4) (2010) 2877-2884 Xuehai Liang and John L. Stickney, Chemistry of Materials, 23(7)(2011)1742-1752 Xuehai Liang, et al., J. Am. Chem. Soc., 133(2011)8199-8204 M. A. Ledina, et al., ECS Transactions, 66 (6) 129-140 (2015)

Highly tensile-strained sub-monolayer Ge nanostructure on GaSb studied by scanning tunneling microscopy

Highly tensile-strained sub-monolayer Ge nanostructures on GaSb have been grown by molecular beam epitaxy and studied by ultrahigh-vacuum scanning tunneling microscopy. Four different coverage rates of Ge nanostructures on GaSb are achieved and investigated. It is found the growth of Ge on GaSb follows 2D growth mode. The crystal lattice of the sub-monolayer Ge nanostructures is coherent with that of the GaSb, inferring as large as 7.74% tensile strain in the Ge nanostructures.

Heterogeneous growth of single crystals on polycrystals

This work discloses a surprising, previously unknown heterogeneous growth mode. Namely, large-area, thin sheets of single-crystalline Ge were observed to grow laterally on top of a polycrystalline Al substrate, covering as many as tens of differently oriented Al grains at low temperatures. The observation of the Ge crystal-growth process by in situ heating transmission electron microscopy demonstrates an intriguing type of ``faceted'' growth: the growth of single-crystalline Ge thin sheets proceeding Al-grain by Al-grain on top of the polycrystalline Al substrate. The crystalline Ge growth front tends to align along the lines of intersection of the Al grain boundaries with the Al surface. Such an unusual heterogeneous growth mode has been shown to be a consequence of the strong anisotropy of the energy of the crystalline/crystalline (here: c-Ge/c-Al) interfaces.

Stability of Chlorine Termination on Ge(100) and Ge(111) Surfaces

The different cleaning solution; HCl and HF solution are used to remove the suboxide and oxide component on Ge surface. The HCl cleaning results chlorine (Cl) termination on Ge surface whereas no Fluorine (F) termination was observed just after HF cleaning. The growth of Ge oxide is studied after treated with HCl cleaning on two surface orientations; (100) and (111), respectively in dry oxygen ambient and cleanroom air by spectroscopic ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). A clear step and terrace trend was observed for the oxidation growth of Ge (100) and Ge (111) in dry oxygen ambient compared to in clean room air. This trend shows the difference in surface reaction of Ge oxidation as humidity varies. The stability of chlorine termination of Ge (111) than Ge (100) explains the slower growth of oxidation in dry oxygen ambient.

On the correlation of growth, structural and electrical properties of epitaxial Ge grown on Si by solid source molecular beam epitaxy

We report growth, structural, and electrical properties of epitaxial Ge layers on Si (001) wafers for next generation complementary metal oxide semiconductor devices. The epi-Ge layers were grown by solid source molecular beam epitaxy (MBE) at substrate temperatures (TG) varying from 200 °C to 500 °C. A two-step growth process, where an initial layer of thickness ∼30 nm is grown at a substrate temperature of 250 °C (except those grown below/at 250 °C), and the remaining layer is grown at a higher temperature, was found to be an efficient approach to improve the crystal quality of the Ge layers. The epi-Ge on Si exhibits bulk hole-mobility as high as 736 cm2/V-s at room temperature. Ti/Ge/Ti metal-semiconductor-metal (MSM) back-to-back Schottky diodes, fabricated on these epitaxial Ge layers, show excellent electrical properties. Further, metal oxide semiconductor (MOS) capacitors fabricated with HfO2 as the gate oxide exhibit low leakage current density of 4.7 × 10−2 A/cm2 (at Vg−VFB = 1 V) and mid-gap interface trap density of 5.0 × 1012cm−2eV−1.

Mazes and meso-islands: Impact of Ag preadsorption on Ge growth on Si(111)

The preadsorption of Ag on Si(111) drastically changes the growth of Ge. In a temperature range from $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $650{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, Ag adsorption on Si leads to the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ reconstruction that exhibits a maze-like morphology on the mesoscopic scale, as observed by low-energy electron diffraction (LEED) and low-energy electron microscopy. This maze morphology can be attributed to a surface roughening on an atomic scale, induced by the re-arrangement of top layer atoms during the $7\ifmmode\times\else\texttimes\fi{}7$ to $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ transition. The subsequent deposition of Ge results in the formation of a wetting layer, the evolution of which has been found to be governed by the Ag/Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ template's maze structure, as the latter offers a high density of heterogeneous nucleation sites. Upon further Ge growth, three-dimensional islands with diameters in the micrometer range are formed, which exhibit a large and flat (111) top facet. X-ray photoemission electron microscopy reveals that during Ge growth, Ag is segregating to the surface very efficiently. Grazing-incidence x-ray diffraction and transmission electron microscopy have been used to study the composition, strain state and defect structure of the Ge islands in dependence of the growth temperature. The strain induced by lattice mismatch is found to be largely relaxed (80--90% relaxation) in the investigated growth temperature range from 400 to $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is confirmed by high-resolution LEED measurements. As a main relaxation mechanism, the formation of interfacial misfit dislocations has been identified. Interdiffusion of Si into the Ge islands becomes more and more pronounced for increasing growth temperature, whereas the formation of twinned Ge regions can drastically be suppressed at higher temperature.

Ge distribution in Si0.9Ge0.1 alloy ingot grown from thin melt layer

We have studied experimentally and theoretically the possibility to obtain a uniform single crystal of SiGe alloy enriched at the Si side. The content of the second component in a crystal 15mm in diameter and 40mm in length grown by the modified floating zone technique from the charge of 79.8 at.% Si and 20 at.% Ge composition with 0.2% B admixture has been investigated using selected area X-ray analysis in different points and in line scanning mode along and across the crystal axis. The longitudinal changes in the germanium concentration proved to be well described by the analytical equation previously derived for conditions of Sb (Ga) doped Ge growth from a thin melt layer in the presence of a heater submerged into the melt. For a more accurate description of the experimental data we have made allowance for the change in the melt layer thickness between the growing crystal and the bottom of the submerged heater. The lateral distribution of the second component not exceeding 5% over the crystal diameter can be significantly improved by reducing the curvature of the phase interface during the growth.

Two-dimensional growth of germanium under a diffusion limited aggregation environment

The discovery of graphene has triggered immense interest in two-dimensional (2D) nanomaterials. However, the 2D growth of several layerstructured crystals such as graphene, MoS2, and black phosphorus is difficult and limited. Here, we report the gas-phase 2D growth of germanium (Ge) with a cubic structure to form Ge nanosheets (GeNSs) using the chemical vapor deposition method. Our investigation revealed that a diffusion limited aggregation (DLA) environment is essential for the 2D growth of Ge that induces a dendritic growth in the direction and suppresses the growth in the [111] direction. The growth behavior was similar to the 2D growth of silicon reported previously. Thus, it can be concluded that a DLA environment is essential for the 2D growth of cubic structured materials. The electron density and mobility of GeNSs were found to be 1.3 × 1015 cm−3 and 792 cm2/Vs, respectively, and their resistivity varied with the intensity of light.

Effects of Ge growth rate and temperature on C-mediated Ge dot formation on Si (100) substrate

Effects of Ge growth rate and growth temperature were investigated on C-mediated formation of Ge quantum dots (QDs) on Si (100) substrate. The samples were prepared by a solid-source molecular beam epitaxy. Ge dots were scaled down and dot density increased with increasing Ge growth rate up to about 2nm/min, and dot size and density were saturated beyond that. The effect of Ge growth temperature was strong at slow Ge growth rate, while it was insensitive at fast Ge growth rate. Consequently, minimum mean dot diameter of about 25nm and maximum dot density of about 1.1×1011cm−2 were obtained at growth temperature of 450°C. These results suggest that the Ge growth condition of fast growth rate or low growth temperature acts effectively to increase the nucleation probability of Ge adatoms and consequently enables to form Ge QDs by maximizing the effects of C-mediated surface reconstruction.

Cerium ion doping into self-assembled Ge using three-dimensional dot structure

Rare earth element, Ce, doped Ge (Ge:Ce) three-dimensional crystal is fabricated on Si (001) via a solid source low-temperature molecular beam epitaxy. Ce affects the formation of the self-assembled Ge dot, including its structure, initial growth and the density because of the self-limited process to suppress the total energy contained in the surface, the interface and bulk stress. Owing to the compressive strain from Si substrate, the out-of-plane lattice parameters of Ge:Ce dots are larger than that of bulk Ge, while in-plane lattice constants are smaller. Based on the lattice constant change and the full width at half maximum of the Raman peaks found as a function of Ce concentration, the solid solubility limit of Ce in the substitutional sites of Ge is as high as 0.7at%. The magnitude of Raman shift does not depend upon Ce concentration, which suggests the growth or incorporation of Ge:Ce dots on Si (001) is limited by the stress.

Effects of carbon coverage on Ge quantum dots formation on Si(100) using C-Si reaction and transition of Ge growth mode

Effects of carbon (C) coverage on C-mediated Ge quantum dots (QDs) formation on a Si(100) substrate changing a state of surface reconstruction were investigated by using solid-source molecular beam epitaxy. For C=0–2.0 monolayers (MLs), the Ge QD scaled down and its density increased with C coverage. In addition, growth mode of Ge QDs changed from Volmer-Weber (VW) mode without a wetting layer to Stranski-Krastanov (SK) mode with the wetting layer for C=0.50–0.75ML. This transition was induced by decrease in interfacial energy between Ge and Si surface due to the formation of C-Ge bonds near the Ge/Si interface. For C≥2.5MLs, the Ge QD enlarged slightly and its density decreased with increasing C coverage, and he Ge growth mode went back to the VW mode. The Raman spectroscopy and X-ray photoelectron spectroscopy revealed the formation of a mixture of amorphous C and nano-crystalline graphite on the Si surface. Thus, the formation of a large amount of C–C (sp2) bonds induced the growth transition of QDs from the SK mode to the VW mode due to the decrease in surface energy of C.

Large single-crystal Ge-on-insulator by thermally-assisted (~400 °C) Si-seeded-pulse-laser annealing

Low temperature (≤400°C) formation of orientation-controlled large (≥10µm) Ge-on-insulator (GOI) structures is desired to fabricate 3-dimensional large-scale integrated circuits (LSIs), where Ge-based functional devices are stacked on Si-LSIs. For this purpose, Si-seeded pulse-laser annealing (PLA) combined with low-temperature substrate heating (≤400°C) has been developed. Here, a-Ge stripes on Si substrates partially covered with insulating films are subjected to PLA, where single edges of the a-Ge stripes directly contact Si-seeding substrates through opening windows of insulating films. PLA at room temperature generates lateral growth of Ge layers from Si-seeding substrates. However, the growth length is short (~1µm), which is attributed to very short melting time. To increase the melting time, low-temperature (≤400°C) substrate heating during PLA is examined. As a result, very large (~20µm) orientation-controlled GOI is obtained by combining substrate heating (400°C) with PLA. Detailed electron microscopy analysis reveals very high crystallinity of the grown layers. Consequently, high-quality rapid-melting growth of Ge becomes possible at a low processing temperature of ~400°C. This thermally-assisted (~400°C) Si-seeded PLA will facilitate realization of 3-dimensional LSIs.

Controlling Nucleation and Crystal Growth of Ge in a Liquid Metal Solvent

The electrochemical liquid–liquid–solid (ec-LLS) deposition of crystalline germanium (Ge) in a eutectic mixture of liquid gallium (Ga) and indium (In) was analyzed as a function of liquid metal thickness, process temperature, and flux. Through control of reaction parameters, conditions were identified that allow selective nucleation and growth of crystalline Ge at the interface between e-GaIn and a crystalline Si substrate. The crystal growth rates of Ge by ec-LLS as a function of process temperatures were obtained from time-dependent powder X-ray diffraction measurements of crystalline Ge. The driving force, Δμ, for crystal formation in ec-LLS was estimated through analyses of the experimental data in conjunction with predictions from a finite-difference model. The required Δμ for Ge nucleation was tantamount to a supersaturation approximately 102 larger than the equilibrium concentration of Ge in e-GaIn at the investigated temperatures. These points are discussed both in the context of advancing new, lo...

Fully coherent Ge islands growth on Si nano-pillars by selective epitaxy

Our recent experimental results of Ge nanoheteroepitaxy (NHE) on Si nanopillars (NPs) are reviewed to confirm the possibility of relaxed Ge growth on Si without misfit dislocations (MDs) formation by elastic deformation. Selective Ge growth is performed by using reduced pressure chemical vapor deposition (CVD) on two types of Si NPs with thermal SiO2 or CVD SiO2 sidewalls and on Si nanoislands (NIs) on SiO2. By using thermal SiO2 sidewall, compressive strain is generated in the Si pillar and fixed by the thermal SiO2. This results in an incoherent Ge growth on Si NPs due to MD formation. By using CVD SiO2 sidewall, tensile strain formation due to thermal expansion during prebake for Ge epi process is observed. However, strain in Si due to Ge growth is not dominant. By introducing a Si0.5Ge0.5 buffer layer, no MD and stacking faults are observed by cross section TEM. The shape of Ge on Si NPs becomes more uniform due to improved crystal quality. On Si NIs on SiO2, a clear compliance effect is observed after Ge growth. Coherent growth of Ge on Si is also realized on Si NIs by using Si0.5Ge0.5 buffer.

Critical conditions for SiGe island formation during Ge deposition on Si(100) at high temperatures

The strain relaxation during the Ge growth on Si(100) occurs vikia surface diffusion and Si-Ge intermixing at temperatures below 800°C. The Ge diffusion into the Si substrate is an additional process at higher temperatures. We found that, if its rate is higher than the Ge deposition rate, the island formation is not realized. We determined the critical Ge deposition rate as a function of the temperature in the range of 840–960°C, at which the dynamic equilibrium between the growth of islands and their decay through the diffusion takes place. The islands grown in the conditions close to the dynamic equilibrium are ordered with a distance between them of about 1µm and they form a smoothed surface morphology. These are indicative of the surface layer strain uniformity. The islands have a SiGe composition which, in the direction parallel to the sample surface, is more uniform in comparison with the islands grown at lower temperatures. The results show that the use of high temperatures essentially improves the conditions for the heterostructure self-organization.

Selective epitaxial growth of Ge1−xSnx on Si by using metal-organic chemical vapor deposition

Selective epitaxial growth of Ge and Ge1−xSnx layers on Si substrates was performed by using metal-organic chemical vapor deposition (MOCVD) with precursors of tertiary-butyl-germane (t-BGe) and tri-butyl-vinyl-tin (TBVSn). We investigated the effects of growth temperature and total pressure during growth on the selectivity and the crystallinity of the Ge and Ge1−xSnx epitaxial layers. Under low total pressure growth conditions, the dominant mechanism of the selective growth of Ge epitaxial layers is the desorption of the Ge precursors. At a high total pressure case, it is needed to control the surface migration of precursors to realize the selectivity because the desorption of Ge precursors was suppressed. The selectivity of Ge growth was improved by diffusion of the Ge precursors on the SiO2 surfaces when patterned substrates were used at a high total pressure. The selective epitaxial growth of Ge1−xSnx layer was also realized using MOCVD. We found that the Sn precursors less likely to desorb from the SiO2 surfaces than the Ge precursors.

Different growth mechanisms of Ge by Stranski-Krastanow on Si (111) and (001) surfaces: An STM study

Structural and morphological features of the wetting layer formation and the transition to the three-dimensional Ge growth on (111) and (100) Si surfaces under quasi-equilibrium growth conditions were studied by means of scanning tunneling microscopy. The mechanism of the transition from the wetting layer to the three-dimensional Ge growth on Si was demonstrated. The principal differences and general trends of the atomic processes involved in the wetting layers formation on substrates with different orientations were demonstrated. The Ge growth is accompanied by the Ge atom redistribution and partial strain relaxation due to the formation of new surfaces, vacancies and surface structures of a decreased density. The analysis of three-dimensional Ge islands sites nucleation of after the wetting layer formation was carried out on the (111) surface. The transition to the three-dimensional growth at the Si(100) surface begins with single {105} facets nucleation on the rough Ge(100) surface.

Reduced-Pressure Chemical Vapor Deposition Growth of Isolated Ge Crystals and Suspended Layers on Micrometric Si Pillars.

In this work, we demonstrate the growth of Ge crystals and suspended continuous layers on Si(001) substrates deeply patterned in high aspect-ratio pillars. The material deposition was carried out in a commercial reduced-pressure chemical vapor deposition reactor, thus extending the "vertical-heteroepitaxy" technique developed by using the peculiar low-energy plasma-enhanced chemical vapor deposition reactor, to widely available epitaxial tools. The growth process was thoroughly analyzed, from the formation of small initial seeds to the final coalescence into a continuous suspended layer, by means of scanning and transmission electron microscopy, X-ray diffraction, and μ-Raman spectroscopy. The preoxidation of the Si pillar sidewalls and the addition of hydrochloric gas in the reactants proved to be key to achieve highly selective Ge growth on the pillars top only, which, in turn, is needed to promote the formation of a continuous Ge layer. Thanks to continuum growth models, we were able to single out the different roles played by thermodynamics and kinetics in the deposition dynamics. We believe that our findings will open the way to the low-cost realization of tens of micrometers thick heteroepitaxial layer (e.g., Ge, SiC, and GaAs) on Si having high crystal quality.