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 Si_0.5Ge_0.5 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 Si_0.5Ge_0.5 buffer.     

Segregation of phosphorus and germanium to grain boundaries in chemical vapor deposited silicon-germanium films determined by scanning transmission electron microscopy

The segregation of phosphorus and germanium to grain boundaries in P implanted, Si_0.87Ge_0.13 films deposited by chemical vapor deposition (CVD), was investigated using energy dispersive x-ray (EDX) micro-analysis. A quantitative analysis of the x-ray spectra obtained at grain boundaries showed that the excess amount of P varied with the crystallography of the boundary but that the segregation always followed an equilibrium process with an activation energy of 0.28 eV. On the other hand, Ge did not segregate to grain boundaries in either P implanted Si_0.87Ge_0.13 films nor in intrinsic Si_1−xGe_x films, containing 2, 13 and 31 at.% Ge. Possible reasons for the absence of Ge segregation are discussed.     

Interfacial reactions and electrical properties of Hf/p-Si0.85Ge0.15

Interfacial reactions and electrical properties of Hf/p-Si_0.85Ge_0.15 as a function of the annealing temperature were studied. Hf₃(Si_1−xGe)₂ and Hf(Si_1−xGe)₂ were initially formed at 500°C and 600°C, respectively. At temperatures above 400°C, Ge segregation out of the reacted layers associated with strain relaxation of the unreacted Si_0.85Ge_0.15 films appeared. At 780°C, agglomeration occurred in the Hf(Si_1−xGe_x)₂ films. All the as-deposited and annealed Hf/p-Si_0.85Ge_0.15 samples showed the formation of an ohmic contact. The lowest specific contact resistance around 10⁻⁵ ω cm² could be obtained for the Hf₃ (Si_1−xGe_x)₂ contacts to p-Si_0.85Ge_0.15 formed at 500°C. Below 500°C, the decrease of specific contact resistance with the annealing temperature is mainly caused by the formation of Hf₃(Si_1−xGe_x)₂ and an interfacial Ge-rich layer between the Hf₃(Si_1−xGe_x)₂ and unreacted Si_0.85Ge_0.15 films, while above 600°C, the increase of specific contact resistance may be due to the formation of Hf(Si_1−xGe_x)₂ and SiC as well as the roughness of the Hf(Si_1−xGe_x)₂ films.     

Low temperature heteroepitaxial growth of Si1−xGexon-Si by photo-enhanced ultra high vacuum chemical vapor deposition using Si2H6 and Ge2H6

Strained-layer Si_1×Ge_x-on-Si heteroepitaxy has been achieved by photolytic decomposition of disilane (Si₂H₆) and digermane (G e₂H₆) in an ultra high vacuum (UHV) chamber at substrate temperatures as low as 275°C. An ArF excimer laser (193 nm) shining parallel to the Si substrate was used as the UV light source to avoid surface damage and substrate heating. The partial pressures of the source gases in the reactor were chosen to vary the Ge mole fraction x from 0.06 to 0.5 in the alloy. The Si₂H₆ partial pressure was kept at 10 mTorr and the Ge₂H₆ partial pressure was varied from 0.13 to 2 mTorr with the laser intensity fixed at 2.75 × 10¹⁵ photons/cm²·pulse. To fit the Si_1−xGe_x growth rate and Ge mole fraction data, the absorption cross section of Ge₂H₆ at 193 nm was set to 1 × 10⁻¹⁶ cm², which is 30 times larger than that of Si₂H₆ (3.4 × 10⁻¹⁸ cm2). For Si_1−xGe_x alloy growth, the deposition rate of Si increases with Ge mole fraction, resulting in increased Si_1−xGe_x alloy growth rates for higher Ge content. The increase of the Si growth rate was attributed to the enhanced adsorption rate of Si₂H₆ pyrolytically in the presence of Ge, rather than due to photolytic decomposition reaction. The Ge mole fraction in Si_1−xGe_x alloys can be predicted by a new model for Si and Ge pyrolytic and photolytic growth. The model describes the increased growth rate of Si_1−xGe_x alloys due to a Ge₂H₆ catalytic effect during photo-enhanced chemical vapor deposition.     

An efficient wet-cleaning of SiGe virtual substrates and of thick, pure Ge layers on Si(0 0 1) after a chemical mechanical planarization step

The aim of this study is to propose an efficient wet cleaning of the surfaces of the SiGe virtual substrates just after a chemical mechanical polishing step. We have first of all studied the chemical compatibility of miscellaneous solutions, such as the standard cleaning 1 (SC1), the Standard Cleaning 2 (SC2), the CARO one etc with SiGe. A definite, logarithmic-like increase of the etch rate with the Ge content has been obtained for the SC1, the SC2 and the CARO solutions (with values 1000-10,000 those of Si evidenced for pure Ge), making them unsuitable for Ge contents above 30%. We have thus investigated the efficiency of new cleaning sequences (named “DDC-SiGe” for SiGe and “HF/O₃” for pure Ge) that call upon diluted HF and ozone solutions spiked with HCl, on SiGe and pure Ge. The overall material consumption of those cleaning sequences, which increases from 10 Å for pure Si up to 130 Å for pure Ge, is quite low. The particle removal efficiency of such cleanings is around 99% for Si_0.8Ge_0.2 and Si_0.7Ge_0.3. It drops down to 83% for Si_0.5Ge_0.5 and to 65% for pure Ge. This is most probably due to pre-existing epitaxy defects which are revealed during the wet cleaning then wrongly assimilated to particles by our surface inspection tool. The metallic contaminants present on the surface after the use of our wet cleaning sequences have a surface density lower than 10¹⁰ atoms cm⁻², this whatever the Ge content of the underlying layer.     

Luminescent Si-Ge solid solution layers ER-doped in molecular-beam epitaxy

Erbium-doped Si_1?x Ge_x epitaxial layers have been grown by sublimation molecular-beam epitaxy (SMBE) in an atmosphere of germane (GeH₄). Doping with erbium was done during growth, with single-crystal Si:Er as the source of Er. The Si-Si_1?x Ge_x (0.01≤x≤0.09) interface was studied by secondary-ion mass spectrometry. Er and Ge show concentration profiles with abrupt boundaries and a significant decrease in their surface segregation. This means that hydrogen acts as a “surfactant.” Data on the luminescent properties of the Er-doped Si/Si_1?x Ge_x samples are reported.     

Role of surface segregation in formation of abrupt interfaces in Si/Si1−x Gex heterocompositions grown by molecular-beam epitaxy with combined sources

The coefficients of segregation of germanium atoms were measured for the Si_1?x Ge_x system grown by molecular-beam epitaxy with combined Si-GeH₄ sources under the conditions of efficient filling of surface bonds by the products of the decomposition of hydrides. In their turn, these measurements made it possible to determine for the first time the ratio between the coefficients of the incorporation of Si and Ge atoms into the growing Si_1?x Ge_x layer using the developed kinetic model of growth. For the Si-Si_1?x Ge_x structures grown by molecular-beam epitaxy with the Si-GeH₄ combined sources, the role of various mechanisms (pyrolysis, segregation, etc.) in the formation of the profile of metallurgical layer interfaces was compared for a wide range of technological parameters.     

Special features of the formation of Ge(Si) islands on the relaxed Si1−xGex/Si(001) buffer layers

The results of studying the growth of self-assembled Ge(Si) islands on relaxed Si_1?xGe_x/Si(001) buffer layers (x≈25%), with a low surface roughness are reported. It is shown that the growth of self-assembled islands on the buffer SiGe layers is qualitatively similar to the growth of islands on the Si (001) surface. It is found that a variation in the surface morphology (the transition from dome-to hut-shaped islands) in the case of island growth on the relaxed SiGe buffer layers occurs at a higher temperature than for the Ge(Si)/Si(001) islands. This effect can be caused by both a lesser mismatch between the crystal lattices of an island and the buffer layer and a somewhat higher surface density of islands, when they are grown on an SiGe buffer layer.     

Strain relaxation and dopant distribution in the rapid thermal annealing of Co with Si/Si1−xGex/Si heterostructure

The reaction of cobalt with the Si-sacrificial cap in the strained Si/Si_1−xGe_x/Si MBE grown heterostructure was studied. The Si-cap is added to prevent the relaxation of the SiGe and to guarantee uniform and reliable silicidation reaction. The Si_1−xGe_x epilayer, with Ge content between 18 and 28 at%, was highly B doped, while the Si-cap was undoped or B doped either during growth or by ion implantation. Cobalt evaporation was followed by rapid thermal annealing at 450–700°C for 30 sec in N₂ or Ar+10%H₂. When the silicide penetrated the Si-cap/Si_1−xGe_x interface, noticeable out-diffusion of Ge and B to the surface was observed. In spite of the presence of the Si-cap significant strain relaxation was observed in three cases: (1) in the implanted samples, although the implantation was confined to the Si-cap, (2) when the Co layer was too thick, such that the silicide penetrated the SiGe layer and (3) when the Ge content in the SiGe layer was relatively high (27.5%).     

A GeSi-buffer structure for growth of high-quality GaAs epitaxial layers on a Si substrate

A SiGe-buffer structure for growth of high-quality GaAs layers on a Si (100) substrate is proposed. For the growth of this SiGe-buffer structure, a 0.8-μm Si_0.1 Ge_0.9 layer was first grown. Because of the large mismatch between this layer and the Si substrate, many dislocations formed near the interface and in the low part of the Si_0.1Ge_0.9 layer. A 0.8-μm Si_0.05Ge_0.95 layer and a 1-μm top Ge layer were subsequently grown. The strained Si_0.05Ge_0.95/Si_0.1Ge_0.9 and Ge/Si_0.05Ge_0.95 interfaces formed can bend and terminate the upward-propagated dislocations very effectively. An in-situ annealing process is also performed for each individual layer. Finally, a 1–3-μm GaAs film was grown by metal-organic chemical vapor deposition (MOCVD) at 600°C. The experimental results show that the dislocation density in the top Ge and GaAs layers can be greatly reduced, and the surface was kept very smooth after growth, while the total thickness of the structure was only 5.1 μm (2.6-μm SiGe-buffer structure +2.5-μm GaAs layer).     

Solid phase reaction of Ti with Si−Ge layers prepared by Ge-implantation

Titanium germanosilicide films from thin Ti films (∼27.5 nm) are formed by solid phase reaction of Ti/Si_0.62Ge_0.38 bilayer at different annealing temperatures ranging from 600°C to 800°C. The effect of crystallographic state of Si−Ge alloy film on the reaction, phase formations, and polymorphic phase transformations, stability of germanosilicides have been investigated by x-ray diffraction, atomic force microscope, and sheet resistance measurements. Both amorphous and relaxed epitaxial Si_0.62Ge_0.38 films are prepared by Ge-multiple implantations into Si wafers with appropriate dose and energy followed by different post-implantation RTA schemes comprising alternative implantation and annealing in on case, and single final annealing in another one. XRD results indicate that the reaction sequence in both cases is found to be Ti/Si like with the formation of C49-Ti(Si−Ge)₂ as a precursor to the low resistivity C54-Ti(Si−Ge)₂. The films formed on amorphous alloy layer exhibit lower polymorphic transition temperature (∼750°C), smoother surface, lower sheet resistance and less agglomeration as compared to those on c:Si−Ge films. These characteristics are due to enhanced nucleation of C54 phase as a result of greater number of nucleation sites in the reaction with amorphous films. The formation of Ti(Si−Ge)₂ films is, however, accompanied by the decrease of Ge content in Ti(Si−Ge)₂ films formed on both amorphous and crystalline alloy films and indicates possible segregation/diffusion effects during the germanosilicidation.     

Stress analysis of Si1−xGex embedded source/drain junctions

The purpose of this paper is to evaluate the impact of the geometry of embedded Si_1−xGe_x source/drain junctions on the stress field. Stress simulations were performed using TSUPREM4 2D software to further investigate the elastic strain relaxation as a function of Si_1−xGe_x alloy active size, in the regime where no plastic relaxation is present. Moreover, the role of the epilayer thickness and the Ge content on the stress levels is also discussed. The work is complemented with experimental Raman spectroscopy.     

Room temperature oxidation of Cu3Ge and Cu3(Si1−xGex) on Si1−xGex

Room temperature oxidation of Cu₃(Si_1−xGe_x) and Cu₃Ge films grown on Si_1−xGe_x at a temperature of 200–400°C was studied using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS). For Cu₃(Si_1−xGe_x) and Cu₃Ge films grown at 200°C and then exposed to air, room temperature oxidation occurred. The Cu₃Ge film was superior to the Cu₃(Si_1−xGe_x) film in reducing the oxidation rate because of its higher Ge concentration. Annealing at higher temperatures such as 400°C resulted in Ge segregation out of the Cu₃(Si_1−xGe_x) film or Si diffusion from the Si_1−xGe_x substrate into the Cu₃Ge overlayer, and hence enhanced the oxidation rate of Cu₃(Si_1−xGe_x) and Cu₃Ge films. The present study shows that upon exposure to air even the Cu₃Ge film grown on Si_1−xGe_x is subject to room-temperature oxidation, revealing that the use of Cu₃Ge contacts on Si_1−xGe_x may be limited by some strict conditions.     

Critical thickness for the Stranski-Krastanov transition treated with the effect of segregation

A new estimation of the critical thickness for the 2D–3D transition during epitaxial growth of the Ge _x Si_{1 ? x} /Si(001) and In _x Ga_{1 ? x} As/GaAs(001) systems is suggested. In the estimation, the segregation of atoms in the process of growth is taken into account. For the criterion of such a Stranski-Krastanov transition, the balance between the gain in the energy of elastic strains in the system due to relaxation of the island and the loss in surface energy because of the increase in the surface area is used. In contrast to calculations for previously known criteria, the energy of elastic strains is calculated taking into consideration all deposited layers rather than only one upper layer. The segregation is described in the model of thermally activated exchange of atoms between the surface and the upper layer. A comparison of the critical thickness, calculated for different compositions of the deposited alloy at different temperatures and rates of growth, with the experimental data shows rather good agreement for both systems. The transition mechanisms corresponding to the criterion suggested in the study are discussed.     

Characterization of reduced pressure chemical vapor deposited Si0.8Ge0.2/Si multi-layers

We have investigated the Si_0.8Ge_0.2/Si multi-layer grown directly onto the Si (001) substrates using reduced pressure chemical vapor deposition. The thicknesses of the Si_0.8Ge_0.2/Si multi-layer were determined using transmission electron microscopy. From the results of energy-dispersive X-ray spectroscopy and X-ray diffraction analyses on the Si_0.8Ge_0.2/Si multi-layer, Ge composition in the Si_1?xGe_x layers was determined as ～20% and the value of residual strain ε of the Si_0.8Ge_0.2 layer is calculated to be 0.012. Three peaks are observed in Raman spectrum, which are located at approximately 514, 404, and 303 cm^?1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence spectrum originates from the radiative recombinations both from the Si substrate and the Si_0.8Ge_0.2/Si multi-layer. For the Si_0.8Ge_0.2/Si multi-layer, the transition peaks related to the quantum well region observed in the photocurrent spectrum were preliminarily assigned to e–hh and e–lh fundamental excitonic transitions.     

Epitaxial Ge layers on Si via GexSi1-xO2 reduction: The roles of the hydrogen partial pressure and the Ge content

The epitaxial growth of an epi-Ge layer via Ge_xSi_1-xO₂ reduction in hydrogen annealing is reported. Ge_xSi+_1-x alloys with x = 0.52 and 0.82 were first grown epitaxially on Si substrates. They were then oxidized in a wet ambient and subsequently annealed in 5% or 100% H₂. The reduction of Ge from its oxide state is observed in both samples with both ambients. However, an epitaxial Ge growth is only observed in the sample with x = 0.82 after the 5% H₂ annealing. The other three cases result in the formation of polycrystalline Ge. The roles of the hydrogen partial pressure and the Ge content are discussed and conditions under which this novel mode of solid-phase epitaxy can occur are explained.     

Characterization of CMOS sub-65 nm metallic contact by laser scattering: Thermal stability of Ni(Si1−xGex)

Very efficient in particles detection, light scattering also offers fast non-invasive full-mapping wafer surface state. This sensitivity was used in the case of germano-silicide process development. As a matter of fact, we report on haze measurement performances, compared to the usual methods used to investigate thermal stability of Ni(Si_1−xGe_x), such as sheet resistance (SR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). We observed defectivity related to thermal agglomeration and Ge-segregation of Ni(Si_1−xGe_x) on strain Si_1−xGe_x (x ? 30%) by haze measurement (like SEM observations) earlier than SR measurement. Moreover, we noticed that a high Ge content affects at lower temperature the stability of Ni(Si_1−xGe_x) with a segregation phenomena.     

Control of strain relaxation behavior of Ge1−xSnx buffer layers

We have investigated how to realize a strain-relaxed Ge_1−xSn_x layer with large in-plane lattice constant as a buffer layer for a tensile-strained Ge layer. This paper reports the dependence of strain relaxation behavior in Ge_1−xSn_x layers on the misfit strain at the interface between Ge_1−xSn_x layers and substrates. We examined control of the misfit strain by growth of Ge_1−xSn_x layers on bulk-Si and virtual Ge substrates. Large misfit strain between the Ge_1−xSn_x layer and the Si substrate leads to strain relaxation during growth and high degree of strain relaxation after annealing. However, it also leads to interfacial mixing and surface roughening with annealing. As a result, the Ge_1−xSn_x layer having a Sn content of 9.2% was achieved, and it has a potential to induce a tensile strain of 0.99% in Ge layer.     

Confocal Raman microscopy of self-assembled GeSi/Si(001) Islands

The method of confocal Raman microscopy is used for the first time to study the spatial distribution of elemental composition and elastic strains in self-assembled GexSi_{1 − x} /Si(001) islands grown by the method of sublimation molecular-beam epitaxy in the GeH₄ ambient. The lines related to vibrational modes Si-Si, Ge-Ge, and Si-Ge are identified in the Raman spectra measured in the regions with dimensions <100 nm on the surface of the samples. The spatial distribution of the Ge atomic fraction x in the Ge _x Si_{1 − x} alloy and of the elastic strain ɛ (averaged in depth over the island layer) have been calculated from the maps of the Raman shifts of the corresponding lines over the sample surface. The dependences of x and ɛ on the islands’ growth temperature and on the nominal thickness of the deposited Ge layer have been studied.     

Optical study of germanium nanostructures grown on a Si(118) vicinal substrate

Photoluminescence (PL) measurements were carried out on Si/Ge(n)/Si_0.7Ge_0.3/Si structures (n is varying from 1 to 7 ML) deposed by gas source molecular beam epitaxy (GS-MBE) on Si(100) surfaces and high index Si(118) vicinal surfaces. Ge nanostructures were confined on the top of the undulation of the Si_0.3Ge_0.7 wetting layer, according to the Stranski–Krastanov growth mode. PL measurements reveal a correlation between the substrate orientation and the island morphology: square dots for (001) and wires for (118) surface orientation. The results suggest that the SiGe wetting layer is required to ensure a good dot size uniformity. The dependence of the luminescence on the excitation power and the PL decay time indicate that the luminescence transitions likely occur in a type-II band line up. Finally, the dot-related PL persists up to room temperature which is very promising for optoelectronic device applications.