Boron implanted in silicon: Segregation at angular configurations of the silicon/silicon dioxide oxidation boundary

Based on computer simulation of the physicochemical segregation processes involving dopants implanted into a host material (silicon), the details of boron injection were investigated for four types of angular configurations (direct and inverse kinks and cavities of the “trench” and “square” types) of the “silicon/silicon dioxide” oxidation boundary. A complicated picture of the B distribution inside the Si and SiO₂ regions and at the SiO₂/Si front was obtained and analyzed in general terms.     

Chemical Structures of the SiO2Si Interface

As a result of considerable progress in microfabrication technology for ultra-large scale integration (ULSI), it has become necessary to control oxide formation on an atomic scale in order to produce defect-free SiO₂/Si interfaces. However, the possibility of forming an atomically flat interface by oxidizing an atomically flat silicon surface without introducing structural defects is not yet clarified. In this article the present understanding of chemical structures of SiO₂/Si interfaces and initial stage of oxidation of silicon surfaces are reviewed.     

Quantum chemical study of the properties of an SiO<Subscript>2</Subscript>/Si(100) interface implanted with boron ions

Quantum-chemical calculations of the properties of a B⁺ ion-implanted SiO₂/Si(100) interface are presented. Dependencies of the total energy of a B⁺ ion cluster system on the location of B⁺ ions in oxygen and silicon vacancies are calculated, along with the geometric and electronic characteristics of the equilibrium cluster states with implanted boron ions.     

A kinetic Monte Carlo study of the initial stage of silicon oxidation: Basic mechanisms-induced partial ordering of the oxide interfacial layer

A kinetic Monte Carlo study of the early stage of silicon oxidation is presented. The model assembles the most recently published dedicated surface mechanisms: oxygen incorporations, migrations, charge transfer effects. Simulations of the thermal oxidation at typical manufacturing temperature and pressure conditions are discussed. As revealed recently through Density Functional Theory investigations, we observe hexagonal patterns that can be here extended over the surface giving rise to a new model system of the Si/SiO₂ interface as well as new associated specific defects. We show that our simulator is able to reproduce correctly the oxidation states of the silicon atoms which are specific of the Si/SiO₂ interface.     

Role of masking oxide on the silicon surface on defect formation in SIMOX structures

The properties of Si/SiO₂ structures produced by oxygen implantation into silicon (SIMOX technology) are investigated by the high-frequency C-V method and by the electroluminescence (EL) method. The existence of electrically active and luminescence centers in the oxide layer near the interface is established. The effect of a SiO₂ masking layer on the silicon surface on defect formation in the SIMOX structure is elucidated. The dependence of the concentration of the electrically active and luminescence centers on the thickness of the masking layer is found.     

First-principles study of the segregation of boron dopants near the interface between crystalline Si and amorphous SiO2

We investigate the stability of boron dopants near the interface between crystalline Si and amorphous SiO₂ through first-principles density functional calculations. An interstitial B is found to be more stable in amorphous SiO₂ than in Si, so that B dopants tend to segregate to the interface. When defects exist in amorphous SiO₂, the stability of B is greatly enhanced, especially around Si floating bond defects, while it is not significantly affected near Si–Si dimers, which are formed by O-vacancy defects.     

Physical and mechanical properties of silicon near the SiO2/Si interface

The influence of an oxide coating on the strength characteristics of single-crystal silicon surface layers is investigated by the microindentation method. It is shown experimentally that a strengthened layer with a thickness of 0.2–0.4 μm and a microhardness of 20–35 GPa, which is two or three times as much as the microhardness of bulk single-crystal silicon, is present near the SiO₂/Si interface. The thickness and microhardness of this layer depends on the growth conditions of the oxide. The formation of this layer is most probably caused by interstitial silicon atoms formed near the SiO₂/Si interface during silicon oxidation.     

Low-temperature deposition of stoichiometric HfO2 on silicon: Analysis and quantification of the HfO2/Si interface from electrical and XPS measurements

An understanding of the exact structural makeup of dielectric interface is crucial for development of novel gate materials. In this paper a study of the HfO₂/Si interface created by the low-temperature deposition ultrathin stoichiometric HfO₂ on Si substrates by reactive sputtering is presented. Analysis, quantification and calculation of layer thickness of an HfO₂/Hf-Si-O_x/SiO₂ gate stack dielectrics have been performed, using X-ray photoelectron spectroscopy (XPS) depth profile method, angle resolved XPS and interface modeling by XPS data processing software. The results obtained were found to be in good agreement with the high frequency capacitance-voltage (C-V) measurements. The results suggest a development of a complex three layer dielectric stack, including hafnium dioxide layer, a narrow interface of hafnium silicate and broad region of oxygen diffusion into silicon wafer. The diffusion of oxygen was found particularly detrimental to the electrical properties of the stack, as this oxygen concentration gradient leads to the formation of suboxides of silicon with a lower permittivity, κ.     

Formation of epitaxial SiC nanocrystals

Epitaxial 3C-SiC grains are formed at 1190 °C in the top region of silicon, when Si wafers coated by SiO₂ are annealed in CO atmosphere. The formed SiC grains are 40-50 nm high and 100 nm wide in cross-section and contain only few defects. Main advantage of the method is that the final structure is free of voids.The above method is further developed for the generation of SiC nanocrystals, embedded in SiO₂ on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO₂ close to the Si/SiO₂ interface by a two-step annealing of oxide covered Si: first in a CO, than in a pure O₂ atmosphere. The first (carbonization) step created epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moved the interface beyond them. Conventional and high resolution cross-sectional electron microscopy showed pyramidal Si protrusions at the Si/SiO₂ interface under the grains. The size of the grains, as well as their distance from the Si/SiO₂ interface (peak of pyramids) can be controlled by the annealing process parameters. The process can be repeated and SiC nanocrystals (oriented in the same way) can be produced in a multilevel structure.     

Effect of High Temperature—Pressure on Buried Silicon Dioxide in Simox and Soi Structures

The effect of annealing at 1520–1570 K under high pressure (HP, up to 1.2GPa) on the structure of SiO₂ in oxygen implanted silicon (Si:O) and in silicon with buried SiO₂ layer (SOI) was investigated by TEM, X-Ray and FTIR methods. Depending on the implantation and treatment parameters, SiO₂ precipitates or continuous SiO₂ layers, sometimes with defects at the SiO₂/Si boundary, are created. A stress dependent shift of asymmetric stretching vibration mode associated with Si-O bonds towards lower frequencies is detected for SiO₂ in the HT—HP treated Si:O and SOI samples.     

Correlation of size and oxygen bonding at the interface of Si nanocrystal in Si–SiO2 nanocomposite: A Raman mapping study

Raman spectroscopy/mapping is used to investigate the variation of Si phonon wavenumbers, i.e., lower wavenumber (LW ~ 495–510 cm⁻¹) and higher wavenumber (HW ~ 515–519 cm⁻¹) phonons, observed in Si–SiO₂ multilayer nanocomposite (NCp) grown using pulsed laser deposition. Sensitivity of Raman spectroscopy as a local probe to surface/interface is effectively used to show that LW and HW phonons originate at surface (Si–SiO₂ interface) and core of Si nanocrystals, respectively. The consistent picture of this understanding is developed using Raman spectroscopy monitored laser heating/annealing and cooling experiment at the site of the desired wavenumber, chosen with the help of Raman mapping. Raman spectra calculations for Si₄₁ cluster with oxygen and hydrogen termination show strong mode at 512 cm⁻¹ for oxygen terminated cluster corresponding to the vibration of surface Si atoms. This supports our attribution of LW phonons to be originating at the Si–SiO₂ surface/interface. These results along with XPS show that nature of interface (oxygen bonding) in turn depends on the size of nanocrystals and LW phonons originate at the surface of smaller Si nanocrystals. The understanding developed can conclude the ongoing debate on large variation in Si phonon wavenumbers of Si–SiO₂ NCps in the literature. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhancement of the room temperature oxidation of silicon by very thin predeposited gold layers

The oxidation of Si(111) surfaces covered with very thin layers of gold is studied by Auger and electron energy loss spectroscopies under ultra high vacuum conditions. It is found that by exposing the Au covered surface to an oxidizing atmosphere, formation of silicon dioxide occurs at room temperature on top of the substrate and the presence of SiO₄ tetrahedra is clearly seen on electron energy loss spectra. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the formation of an oxygen monolayer and no structure corresponding to Si-O bonds in SiO₄ tetrahedra are observed. This enhancement of the oxidation is attributed to a change in the hybridization state of Si atoms in a gold environment.     

Effect of wet-chemical substrate smoothing on passivation of ultrathin-SiO<Subscript>2</Subscript>/n-Si(111) interfaces prepared with atomic oxygen at thermal impact energies

Ultrathin SiO₂ layers for potential applications in nano-scale electronic and photovoltaic devises were prepared by exposure to thermalized atomic oxygen under UHV conditions. Wet-chemical substrate pretreatment, layer deposition and annealing processes were applied to improve the electronic Si/SiO₂ interface properties. This favourable effect of optimized wet-chemical pre-treatment can be preserved during the subsequent oxidation. The corresponding atomic-scale analysis of the electronic interface states after substrate pre-treatment and the subsequent silicon oxide layer formation is performed by field-modulated surface photovoltage (SPV), atomic force microscopy (AFM) and spectroscopic ellipsometry in the ultraviolet and visible region (UV-VIS-SE).     

A model for phosphorus segregation at the silicon-silicon dioxide interface

A new model for phosphorus segregation at the Si-SiO₂ interface is derived and verified by experimental data. The model considers for the first time, a third phase, the interface layer itself, in addition to the Si and SiO₂ phases, and the dynamics of the three-phase system is described in terms of rate equations. In particular, the phosphorus compound formation in the interface layer (phosphorus pile-up), which renders the dopant electrically inactive to a large extent, is described as a competition of the dopant in silicon and in silicon dioxide in filling and depleting a constant density of interface traps. Our model allows an unambiguous correlation of the dopant concentration on both sides of the interface with the integral dose of the interface phosphorus pile-up. Experimental data for different phosphorus concentrations, different temperatures, and different oxidation ambients, including inert anneals, are fitted by a single curve.     

Resonant photoemission at the oxygen K edge as a tool to study the electronic properties of defects at SiO2 /Si and SiO2 /SiC interfaces

Silicon is by far the most important material used in microelectronics, partly due to the excellent electronic properties of its native oxide (SiO₂), but substitute semiconductors are constantly the matter of research. SiC is one of the most promising candidates, also because of the formation of SiO₂ as native oxide. However, the SiO₂/SiC interface has very poor electrical properties due to a very high density of interface states which reduce its functionality in MIS devices. We have studied the electronic properties of defects in the SiO₂/Si and SiO₂/SiC interfaces by means of XAS, XPS and resonant photoemission at the O 1s and the Si 2p edges, using silicon dioxide thermally grown with thicknesses below 10 nm. Our XAS data are in perfect agreement with literature; in addition, resonant photoemission reveals the resonant contributions of the individual valence states. For the main peaks in the valence band we find accordance between the resonant behaviour and the absorption spectra, except for the peaks at −15 eV binding energy, whose resonant photoemission spectra have extra features. One of them is present in both interfaces and is due to similar defects, while another one at lower photon energy is present only for the SiO₂/SiC interface. This is related to a defect state which is not present at the SiO₂/Si interface.     

An isotopic labeling study of the diffusion mechanism during oxidation of Si(100) in water vapor by successive oxidation

The diffusion mechanism during the wet oxidation of Si(100) at 1373 K was investigated by successive oxidations finally containing isotopic water. SiO₂ was first thermally grown on Si in non-labeled oxidizing ambient (dry O₂ or H₂O) followed by isotopic water (H₂¹⁸O) to trace ¹⁸O species in SiO₂. The distributions of ¹⁶O and ¹⁸O in the oxide film were analyzed by means of secondary ion mass spectroscopy (SIMS). SIMS depth profiles show that there was a wide overlap of both isotopes (¹⁸O and ¹⁶O) throughout the SiO₂ layer, no matter whether the first oxidation step was carried out in dry O₂ or H₂O, and the concentration gradient of ¹⁸O decreased with increasing oxidation time at the second oxidation step by H₂¹⁸O. The results suggest that the diffusion mechanism in SiO₂ during water vapor oxidation is exchange diffusion; H₂O related oxidizing species diffuse through the network with significant exchange with the pre-existing oxygen in it.     

Influence of the Si/SiO<Subscript>2</Subscript> interface on the charge carrier density of Si nanowires

The electrical properties of Si nanowires covered by a SiO₂ shell are influenced by the properties of the Si/SiO₂ interface. This interface can be characterized by the fixed oxide charge density Q_f and the interface trap level density D_it. We derive expressions for the effective charge carrier density in silicon nanowires as a function of Q_f, D_it, the nanowire radius, and the dopant density. It is found that a nanowire is fully depleted when its radius is smaller than a critical radius a_crit. An analytic expression for a_crit is derived. PACS 68.65.-k; 61.46.+w; 81.10.Bk     

Suppression of boron segregation by interface Ge atoms at SiGe/SiO2 interface

We investigate the migration pathway and barrier for B diffusion at SiGe/SiO₂ interface through first-principles density functional calculations. Similar to the diffusion mechanism reported for Si/SiO₂ interface, a substitutional B, which initially forms a B-self-interstitial complex in SiGe, diffuses to the interface and then to the oxide in form of an interstitial B. At the defect-free interface, where bridging O atoms are inserted to remove interface dangling bonds, it is energetically more favorable for the interstitial B to intervene in the Ge–O bridge bond rather than the Si–O bridge bond at the interface. As a result of the B intervention, interface Ge atoms significantly enhance the stability of B-related defects in the interface region and thereby act as traps for B dopants. At the interface with the Ge–O bridge bond, the overall migration barrier for B diffusion from SiGe to SiO₂ is estimated to be about 3.7 eV, much higher than the reported value of about 2.1 eV at Si/SiO₂ interface. Our results provide a clue to understanding the experimental observation that B segregation toward the oxide is suppressed in SiGe/SiO₂ interface.     

Phase formation and redistribution of Xe implanted in the Ni-Si system after fast melting and solidification

The behaviour of Xe implanted at the Ni-Si interface and irradiated with Nd-laser pulses is studied in details and compared with Xe implantation into NiSi₂ and into pure Si. Ion beam mixing followed by laser irradiation is able to form good quality epitaxial NiSi₂ layer on Si.An inward segregation of Xe is observed with retention of Xe at a depth of 30 nm inside pure silicon. Implantation of Xe into NiSi₂ or pure Si causes broadening and loss of Xe, as generally observed for implantation into pure materials. The different behaviour of Xe at the Si/NiSi₂ interface must thus be ascribed to peculiar characteristics of the interface itself.     

Embedded silicon nanocrystal interface structure and strain

The structure of nanocrystal-matrix interface and strain in embedded nanocrystals are studied using large-scale atomistic simulations, with the examples of Si nanocrystal embedded in amorphous matrix of SiO₂. Photoluminescence from silicon nanocrystals embedded in a dielectric matrix like SiO₂ and Si₃N₄ are promising for Si-based optical devices. The nanocrystal-matrix interface plays a crucial role in understanding its optical and electrical properties. Nanocrystals with diameters varying from 2.17 to 4.56 nm are studied. A detailed quantitative analysis of the variation of Si/SiO₂ interface structure and strain distribution with nanocrystal diameter is reported. A linear variation of the interface width with nanocrystal diameter is observed with thinner interfaces for larger nanocrystals. Local deformation analysis reveals that the smaller nanocrystals are highly strained, whereas the strain in the larger ones shifts to the interface. This is in accordance with observed increase in total percentage of defect states in the interface from 39 to 70% for diameter increasing from 2.17 to 4.56 nm. Moreover, based on the atomic arrangements at the interface, optically active defects like Pb centres, E centres and non-bridging oxygen centres are identified and a dominance of Pb centres is observed for all the nanocrystals. The detailed structural characterization-related investigations using the proposed simulation approach will find useful application in designing system-level response of embedded nanocrystals and also to correlate various experimental observations.