A. Portavoce

SiGe nanostructures: new insights into growth processes

During the last decade, Si/Si1−xGex heterostructures have emerged as a viable system for use in CMOS technology with the recent industrial production of heterojunction bipolar transistor-based integrated circuits. However, many key problems have to be solved to further expand the capabilities of this system to other more attractive devices. This paper gives a comprehensive review of the progress achieved during the last few years in the understanding of some fundamental growth mechanisms. The discrepancies between classical theories (in the framework of continuum elasticity) and experimental results are also specially addressed. In particular, the major role played by kinetics in the morphological evolution of layers is particularly emphasized. Starting from the unexpected differences in Si1−xGex morphological evolution when deposited on (001) and on (111), our review then focuses on: (1) the strain control and adjustment (from fully strained to fully relaxed 2D and 3D nanostructures)—in particular, some original examples of local CBED stress measurements are presented; (2) the nucleation, growth, and self-assembly processes, using self-patterned template layers and surfactant-mediated growth; (3) the doping processes (using B for type p and Sb for type n) and the limitations induced by dopant redistribution during and after growth due to diffusion, segregation, and desorption. The final section will briefly address some relevant optical properties of Si1−xGex strained layers using special growth processes.

Composition measurement of the Ni-silicide transient phase by atom probe tomography

The transient phase observed during the reaction of a Ni(Pt) alloy with a (001)Si substrate is studied using in situ x-ray diffraction and atom probe tomography microscopy. This transient phase exhibits a diffraction peak at 47°, and is found to have a uniform composition corresponding to Ni0.6Si0.4. During the reaction, it is located between a δ-Ni2Si layer and a thin (nanocrystalline or amorphous) NiSi layer in contact with Si. The Pt atoms are found in the δ-Ni2Si grain boundaries, while they have not been detected in the NiSi layer.

Ge dots self-assembling: Surfactant mediated growth of Ge on SiGe (118) stress-induced kinetic instabilities

The ordering of islands on naturally or artificially nanostructured surfaces is one of the most recent objectives among actual nanotechnology challenges. We show in this letter that, by a combination of two approaches, i.e., a two-step molecular beam epitaxy (MBE) deposition process and surfactant-mediated growth, we are able to obtain chains of nicely ordered ultrasmall islands of lateral size below 50 nm. The two-step MBE process consists of vicinal Si(001) surface self-patterning by SiGe growth instability and Ge dot ordering by subsequent Ge deposition on a SiGe template layer. The surfactant-mediated growth consists of submonolayer Sb deposition prior to Ge growth, in order to reduce the island size up to 25 nm. The best ordering of Ge islands is obtained when the island size matches the wavelength of the template layer.

Sb segregation in Si and SiGe: effect on the growth of self-organised Ge dots

The segregation and incorporation coefficients of antimony (Sb) in Si 1-x Ge x buried doped layers were investigated simultaneously using specific temperature sequences. We first showed an exponential kinetic evolution of Sb surface segregation in Si. In contrast such an evolution could not be observed in Si 1-x Ge x because of the Sb thermal desorption, at growth temperatures of 550°C. We also showed an increased surface segregation increasing with the partial Ge concentration in Si 1-x Ge x alloys, which was explained by a decrease of the kinetic barrier for Sb atoms mobility. It was, therefore, possible to determine the growth conditions to obtain a Si 1-x Ge x doped layer with a controlled incorporation level and a negligible surface segregation obtained by the thermal desorption of the Sb surface coverage. Finally, using Sb surfactant mediated growth, we found Ge dots with lateral sizes reduced by a factor of 2.8 and density multiplied by a factor of four as compared to dots directly deposited on Si(001).

Tungsten diffusion in silicon

Two doses (1013 and 1015 cm−2) of tungsten (W) atoms were implanted in different Si(001) wafers in order to study W diffusion in Si. The samples were annealed or oxidized at temperatures between 776 and 960 °C. The diffusion profiles were measured by secondary ion mass spectrometry, and defect formation was studied by transmission electron microscopy and atom probe tomography. W is shown to reduce Si recrystallization after implantation and to exhibit, in the temperature range investigated, a solubility limit close to 0.15%–0.2%, which is higher than the solubility limit of usual metallic impurities in Si. W diffusion exhibits unusual linear diffusion profiles with a maximum concentration always located at the Si surface, slower kinetics than other metals in Si, and promotes vacancy accumulation close to the Si surface, with the formation of hollow cavities in the case of the higher W dose. In addition, Si self-interstitial injection during oxidation is shown to promote W-Si clustering. Taking into account these observations, a diffusion model based on the simultaneous diffusion of interstitial W atoms and W-Si atomic pairs is proposed since usual models used to model diffusion of metallic impurities and dopants in Si cannot reproduce experimental observations.

Differential scanning calorimetry measurements of kinetic factors involved in salicide process

Experimental procedure allowing the measurement of kinetic factors controlling the reaction of a nanometric film with a monocrystalline substrate by differential scanning calorimetry (DSC) is described. This technique is shown to be of great interest for characterization of silicide formation during microelectronic industrial processes. Combining both in situ x-ray diffraction measurements (crystal structure) and DSC measurements (formation enthalpy), the interface reaction coefficient Kr and the effective diffusion coefficient Kd characterizing Pd2Si growth have been measured.

Nanometric size effect on Ge diffusion in polycrystalline Si

The nanosize effect on Ge diffusion (850≤T≤1000 °C) in polycrystalline Si layers is investigated. The Ge diffusion coefficients in microcrystalline and nanocrystalline Si layers made of 30 μm and 40 nm wide grains, respectively, are measured and compared. In the microcrystalline Si layer, the Ge diffusion coefficient in micrograin boundaries is measured using a conventional analytical solution of Fick’s equations corresponding to the Fisher model. In the nanocrystalline Si layer, the Ge diffusion coefficients in nanograins and in nanograin boundaries are measured via a method based on two-dimensional simulations using the Fisher model geometry. The diffusivities in nanograins and nanograin boundaries are one order of magnitude higher than in micrograins and micrograin boundaries, respectively. However, the nanosize effect appears to be different in grains and grain boundaries; despite that the activation energy for diffusion in 40 nm wide grains is at least 1 eV lower than in Si bulk. The activation energ...

Triple-junction contribution to diffusion in nanocrystalline Si

The influence of triple-junctions on experimental Ge diffusion profiles (850–1000 °C) in nanocrystalline Si is investigated using three-dimensional finite element simulations. We found that triple-junction diffusion is not negligible in nanocrystalline Si made of 40 nm wide grains. Ge triple-junction diffusion coefficient follows the Arrhenius law 5.72×104 exp(−3.24 eV/kT)cm2 s−1. It is approximately 4.7×102 times higher than grain boundary diffusion coefficient, even though diffusion in triple-junction and in grain boundary exhibits similar activation energy.

Transition from anomalous kinetics toward Fickian diffusion for Si dissolution into amorphous Ge

Over the last years, several experimental and theoretical studies of diffusion kinetics on the nanoscale have shown that the time evolution (x∝tkc) differs from the classical Fickian law (kc=0.5). However, all work was based on crystalline samples or models, so far. In this letter, we report on the diffusion kinetics of a thin amorphous Si layer into amorphous Ge to account for the rising importance of amorphous materials in nanodevices. Employing surface sensitive techniques, the initial kc was found at 0.7±0.1. Moreover, after some monolayers of Si dissolved into the Ge, kc changes to the generally expected classical Fickian law with kc=0.5.

Experimental insights into Si and SiGe growth instabilities: Influence of kinetic growth parameters and substrate orientation

In this paper, the origin of Si(SiGe) growth instabilities has been experimentally addressed. Depending on the growth conditions (Ge concentration, growth temperature, thickness), various growth instability regimes were observed: pure kinetic regime, kinetically activated strain-induced regime and pure strain-driven regime. Also by comparing morphological evolution of layers grown either on nominal or on vicinal (001) and (111) surfaces, the important role of nature and density of surface steps was evidenced. In the end, some examples are given to illustrate the potential use of self-patterned substrates by means of different Si (SiGe) growth instabilities as templates for Ge islands ordering.