Silicon Self-interstitials Research Articles

Conversion of silicon wafer type from perfect-interstitial to perfect-vacancy by rapid thermal process

Vacancies, self-interstitials, and Frenkel pairs represent the basic point defects in silicon single-crystals and are unavoidably introduced in the ingots during the manufacturing process. In semiconductor technology, several problems are caused by points defects that are not at thermodynamic equilibrium. For this reason, virtually defect-free silicon wafers having a small residual concentration of vacancies in the bulk (“Perfect-vacancy”, or Pv silicon) are solely used for the fabrication of advanced microelectronic devices, whereas similar wafers containing a residual concentration of silicon self-interstitials (Pi silicon) are often regarded as unsuitable substrates for device fabrication. This paper describes the possibility of using Pi silicon wafers, after their conversion to Pv, for those applications requiring only vacancy-rich perfect silicon as the substrate. The conversion from Pi to Pv is achieved in these wafers by using rapid thermal annealing at high temperature to produce Frenkel pairs first and to re-install a specific vacancy concentration in the bulk of the wafer later, thus reassigning a new Pv character to the silicon substrate.

Comprehensive understanding on phosphorus precipitation in heavily phosphorus-doped Czochralski silicon

Heavily phosphorus-doped Czochralski (HP-CZ) silicon is an important substrate material for manufacturing power electronic devices. The high concentration of phosphorus impurities may be supersaturated during the crystal growth of HP-CZ silicon or device manufacturing. Thus, understanding phosphorus precipitation in HP-CZ silicon is of technological significance. Herein, a panoramic view of phosphorus precipitation in HP-CZ silicon is presented in terms of crystallography, thermodynamics, and kinetics. It is found that the orthorhombic SiP precipitates can form during the crystal growth of HP-CZ silicon and also during the post-anneals of HP-CZ silicon at 450–1050 °C. Along with increasing annealing temperature, the formed SiP precipitates tend to adopt the platelet, polyhedron, and sphere-like shapes. Moreover, the excess point defects, i.e., silicon self-interstitials and vacancies, are found to affect phosphorus precipitation occurring in the low and high temperature regimes in different ways. In light of the kinetics of phosphorus precipitation at different temperatures, it is deduced that phosphorus precipitation follows a growth law in compliant with Ham's theory to a large extent. As an important output of this work, the temperature-dependent phosphorus solubilities in the dislocation-free silicon, which have been hardly acquired previously, are derived on the basis of investigating phosphorus precipitation in a set of HP-CZ silicon wafers with different phosphorus concentrations. Moreover, the derived solvus line for the phosphorus impurities in silicon could be a beneficial supplement to the existing phase diagram of the Si–P binary system in the extremely silicon-rich corner.

Formation energies of silicon self-interstitials using periodic coupled cluster theory

Formation energies of silicon self-interstitials using periodic coupled cluster theory

Investigation of the impact of amorphous silicon layers deposited by PECVD and HDP-CVD on oxide precipitation in silicon

The effect of deposited a-Si layers with different layer stress on oxide precipitation was investigated in order to find out if intrinsic point defects affecting oxide precipitation are generated at the interface a-Si/Si and if possibly hydrogen affects the oxide precipitation. A thermal cycle of nucleation at 650 °C for 4 h or 8 h followed by stabilization at 780 °C for 3 h, and growth at 1000 °C for 16 h was applied. It was found that there are no signs for the injection of intrinsic point defects from the interface a-Si/Si into the Si substrate during the applied thermal treatment. However if a-Si is deposited on 1000 nm silicon oxide, deposited previously from TEOS in a plasma process, silicon self-interstitials seem to be injected from the interface silicon oxide/Si into the silicon substrate retarding oxide precipitation in the initial stage of nucleation annealing at 650 °C. There are also no signs of any impact of the layer stress on oxide precipitation or self-interstitial injection. The concentration of hydrogen in the layers can be controlled via the RF bias power. The hydrogen concentration is reduced markedly already during annealing at 650 °C. Part of the hydrogen diffuses into the silicon substrate and enhances oxide precipitation if its initial concentration in the layers is higher than 1.5 × 1022 cm−3. For a-Si deposited on 1000 nm silicon oxide, the enhancement effect appears for hydrogen concentrations in the layer higher than approximately 2.8 × 1022 cm−3.

Signatures of self-interstitials in laser-melted and regrown silicon

Photoluminescence spectroscopy investigates epitaxially regrown silicon single crystals after pulsed laser melting for atomic-level lattice defects. The measurements identify a transition from a regime free of defect-related spectral lines to a regime in which spectral lines appear originating from small self-interstitial clusters. This finding of self-interstitial clusters indicates supersaturated concentrations of self-interstitials within the regrown volume. Molecular dynamics simulations confirm that recrystallization velocities vre ≈ 1 m/s after laser melting lead to supersaturation of both self-interstitials and vacancies. Their concentrations ci and cv in the regrown volumes are ci ≈ cv ≈ 1017 cm−3. An analytical model based on time-dependent nucleation theory shows a very strong dependence of self-interstitial aggregation to clusters on the cooling rate after solidification. This model explains the transition identified by photoluminescence spectroscopy.

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into VSi+ or VSi−, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec−1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T≲600 °C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T≳1200 °C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q=+1 and q=+3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.

SIMULATION OF BORON DIFFUSION IN THE NEAR-SURFACE REGION OF SILICON SUBSTRATE

The mechanism of boron-enhanced diffusion from a thin boron layer deposited on the surface in the case of silicon crystal doping is proposed and investigated. It was supposed that lattice contraction occurs in the vicinity of the surface due to the difference between the atomic radii of boron and silicon. This lattice contraction provides a stress-mediated diffusion of silicon self-interstitials from the near-surface region to the bulk of a semiconductor. Due to the stress-mediated diffusion, the near-surface region is depleted of silicon self-interstitials, and simultaneous oversaturation of this species occurs in the bulk. In this way, a strong nonuniform distribution of silicon self-interstitials in the vicinity of the surface is formed without regard to the large migration length of this species. The oversaturation of the bulk of a semiconductor with nonequilibrium self-interstitials allows one to explain the boron-enhanced diffusion of impurity atoms. The strong nonuniform distribution of these point defects also results in a specific form of boron concentration profile in the vicinity of the surface. Good agreement of the calculated boron profile with the experimental data for the entire doped region was obtained within the limit of the proposed model.

Formation of thermal donor enhanced by oxygen precipitation in silicon crystal

The formation of thermal donors in silicon is investigated using Czochralski silicon crystals grown with different grown-in defect regions, such as voids and nuclei of oxidation-induced stacking faults. It was found that the formation rate of thermal donors during annealing at 450 °C increases with an increase in the density of oxide precipitates in the regions containing different grown-in defects. The thermodynamic model for the formation of thermal donors shows that the electrically inactive oxygen trimers as nuclei of thermal donors in an as-grown crystal increase with an increase in the density of oxide precipitates, suggesting that the formation of the nuclei is enhanced due to silicon self-interstitials emitted by oxygen precipitations during the crystal growth.

Enhanced diffusion of boron by oxygen precipitation in heavily boron-doped silicon

The enhanced diffusion of boron has been investigated by analyzing out-diffusion profiles in the vicinity of the interface between a lightly boron-doped silicon epitaxial layer and a heavily boron-doped silicon substrate with a resistivity of 8.2 mΩ cm and an oxide precipitate (O.P.) density of 108–1010 cm−3. It is found that the boron diffusion during annealing at 850–1000 °C is enhanced with the increase of the oxide precipitate density. On the basis of a model for boron diffusion mediated by silicon self-interstitials, we reveal that the enhanced diffusion is attributed to self-interstitials supersaturated as a result of the emission from oxide precipitates and the absorption by punched-out dislocations. In addition, the temperature dependence of the fraction of the self-interstitial emission obtained analyzing the diffusion enhancement well explains the morphology changes of oxide precipitates reported in literature.

Quantification of germanium-induced suppression of interstitial injection during oxidation of silicon

The oxidation of silicon is known to inject interstitials, and the presence of silicon–germanium (SiGe) alloys at the Si/SiO2 interface during oxidation is known to suppress the injection of silicon self-interstitials. This study uses a layer of implantation-induced dislocation loops to measure interstitial injection as a function of SiGe layer thickness. The loops were introduced by a 50 keV 2 × 1014 cm−2 P+ room-temperature implantation and thermal annealing. Germanium was subsequently introduced via a second implant at 3 keV Ge+ over a range of doses between 1.7 × 1014 cm−2 and 1.4 × 1015 cm−2. Results show that upon oxidizing at 850 °C for 3 h or 900 °C for 70 min to condense the germanium at the Si/SiO2 interface, where if forms a Si0.5Ge0.5 alloy. Upon subsequent oxidations of 850 °C for 6 h or 900 °C for 2 h, partial suppression of interstitial injection can be observed for sub-monolayer doses of germanium, and more than three monolayers of Si0.5Ge0.5 (1.4 × 1015 cm−2) are necessary to suppress interstitial injection below the detection limit during oxidation. These results show that low-energy implantation of germanium can be used to eliminate or modulate injection of oxidation-induced interstitials.

Elimination and Quantification of Oxidation Induced Interstitial Injection via Ge Implants

The presence of Silicon-Germanium (SiGe) alloys at the Si/SiO2 interface during oxidation is known to suppress the injection of silicon self-interstitials that normally accompanies silicon oxidation and lead to observed effects such as Oxidation Enhanced Diffusion (OED) and stacking fault growth. This study uses a layer of implantation induced dislocation loops to measure interstitial injection as a function of SiGe layer thickness. The loops were introduced by implanting phosphorus and thermal annealing, and Germanium was subsequently introduced via a second implant at 3 keV over a range of doses between 1.7 x1014 cm-2 and 1.4 x1015 cm-2. Results show that partial suppression of interstitial injection can be observed for sub-monolayer doses of germanium, and that more than three monolayers of SiGe are necessary to fully suppress interstitial injection below our detection limit during oxidation. They further show that low energy implantation of germanium opens up possibilities to eliminate or modulate injection of interstitials during thermal processing of future devices.

On concentration dependence of arsenic diffusivity in silicon

An analysis of the equations used for modeling thermal arsenic diffusion in silicon has been carried out. It was shown that for arsenic diffusion governed by the vacancy-impurity pairs and the pairs formed due to interaction of impurity atoms with silicon self-interstitials in a neutral charge state, the doping process can be described by the Fick’s second law equation with a single effective diffusion coefficient which takes into account two impurity flows arising due to interaction of arsenic atoms with vacancies and silicon self-interstitials, respectively. Arsenic concentration profiles calculated with the use of the effective diffusivity agree well with experimental data if the maximal impurity concentration is near the intrinsic carrier concentration. On the other hand, for higher impurity concentrations a certain deviation in the local regions of arsenic distribution is observed. The difference from the experiment can occur due to the incorrect use of effective diffusivity for the description of two different impurity flows or due to the formation of nonuniform distributions of neutral vacancies and neutral self-interstitials in heavily doped silicon layers. We also suppose that the migration of nonequilibrium arsenic interstitial atoms makes a significant contribution to the formation of a low concentration region on thermal arsenic diffusion.

A DLTS study of hydrogen doped czochralski-grown silicon

In this study we examine proton implanted and subsequently annealed commercially available CZ wafers with the DLTS method. Depth-resolved spreading resistance measurements are shown, indicating an additional peak in the induced doping profile, not seen in the impurity-lean FZ reference samples. The additional peak lies about 10–15μm deeper than the main peak near the projected range of the protons. A DLTS characterization in the depth of the additional peak indicates that it is most likely not caused by classical hydrogen-related donors known also from FZ silicon but by an additional donor complex whose formation is assisted by the presence of silicon self-interstitials.

Effects of dopants on the growth of oxidation-induced stacking faults in heavily doped n-type Czochralaki silicon

Through comparative investigation on the growth of oxidation-induced stacking faults (OSFs) in heavily antimony (Sb)-doped and phosphorus (P)-doped Czochralaki (Cz) silicon wafers with almost the same resistivity, effects of dopants on the growth of OSF in heavily doped n-type Cz silicon are studied experimentally. Moreover, the influences of Sb and P atoms on the recombination of self-interstitials and vacancies are also explored on the basis of the first-principles calculations. It is shown experimentally that all the OSF lengths are almost identical regardless of the type and density of OSF nucleation centers, such as copper precipitates and mechanical scratches etc.. However, it is found that the OSF length of heavily Sb-doped Cz silicon wafer is larger than that of heavily P-doped Cz silicon wafer under the same oxidation condition. Essentially, the OSFs are formed by the aggregation of silicon self-interstitials refleased at the Si/SiO2interface during the oxidation. Therefore, a longer OSF implies that a higher quantity of silicon self-interstitials remains after the recombination of vacancies and silicon self-interstitials in the heavily Sb-doped Cz silicon wafer. The first-principles calculations based on density functional theory (DFT) indicate that Sb atoms combine with vacancies more readily than P atoms. This is actually due to the fact that Sb has a much larger atomic size than P. In other words, as compared with P atoms, the Sb atoms are the moreflefficient vacancy-trapping centers, thus retarding the recombination of vacancies and silicon self-interstitials. Consequently, the silicon self-interstitials remain after recombination with the vacancies that are much more in heavily Sb-doped Cz silicon wafer than in heavily P-doped counterpart when undergoing the same oxidation. In turn, the OSFs in heavily Sb-doped silicon wafers are relatively longer.

Carbon related defects in irradiated silicon revisited.

Electronic structure calculations employing hybrid functionals are used to gain insight into the interaction of carbon (C) atoms, oxygen (O) interstitials, and self-interstitials in silicon (Si). We calculate the formation energies of the C related defects Ci(SiI), CiOi, CiCs, and CiOi(SiI) with respect to the Fermi energy for all possible charge states. The Ci(SiI)2+ state dominates in almost the whole Fermi energy range. The unpaired electron in the CiOi+ state is mainly localized on the C interstitial so that spin polarization is able to lower the total energy. The three known atomic configurations of the CiCs pair are reproduced and it is demonstrated that hybrid functionals yield an improved energetic order for both the A and B-types as compared to previous theoretical studies. Different structures of the CiOi(SiI) cluster result for positive charge states in dramatically distinct electronic states around the Fermi energy and formation energies.

Quantification of the number of Si interstitials formed by hydrogen implantation in silicon using boron marker layers

H implantation results in the appearance of tensile out-of-plane strain in the implanted region which evolves during further annealing. VnHm complexes and/or larger platelets, both co-precipitates of vacancies and H atoms, are believed to be responsible for strain generation. However, during H+ implantation, Frenkel pairs i.e., both vacancies and interstitials are generated. Silicon self-interstitials have been rarely detected and thus their possible role in strain generation has been ignored so far. In this work, we demonstrate that Si interstitials are actually present in large measurable quantities in such implanted layers. For this, we have studied by Secondary Ions Mass Spectrometry the diffusion of boron delta layers during annealing at 350°C, 550°C and 850°C after H implantation at 12keV with a fluence of 1×1016H+/cm2. The Si self-interstitial supersaturations were extracted by comparison with simulations. Frank dislocation loops, i.e., precipitates of Si atoms, were observed by Transmission Electron Microscopy growing by Ostwald ripening during 850°C annealing. The supersaturation of Si self-interstitials in dynamical equilibrium with these loops was extracted showing consistency with the values found from the diffusion experiments. These results and more generally the role of interstitials in the strain build up are discussed.

Fast and Slow Vacancies in Silicon

Vacancies (and probably also self-interstitials) in silicon appear to exist in several forms (atomic configurations) some of them being fast diffusers and other slow diffusers. The data on enhanced self-diffusivity under proton irradiation, on vacancy and oxide precipitate profiles installed by Rapid Thermal Annealing, and on the self-diffusivity under equilibrium conditions suggest that there are at least two kinds of vacancy: 1) Vw- a fast-diffusing localized vacancy manifested in electron irradiated samples (Watkins vacancy), 2) Vs- a slow-diffusing extended vacancy manifested under hot proton irradiation. In RTA experiments, these two species behave as one equilibrated subsystem of a moderate effective diffusivity intermediate between those of Vwand Vs. There is also strong evidence in favor of a third kind of vacancy: Vfa fast extended species, which controls the grown-in voids in silicon crystals.

Characterization of the OSF-band structure in n-type Cz-Si using photoluminescence-imaging and visual inspection

Oxygen induced stacking faults (OSFs) are mainly seen in oxygen rich wafers from the seed end of Cz-silicon crystals. In wafers this ring shaped OSF-region delineates a border between two defect regions; usually silicon self-interstitials dominate outside and vacancies inside this ring. High temperature treatment (>800°C) leads to oxygen precipitation in the border region. These precipitates act as nucleation sites for stacking faults. The standard procedure for characterizing the OSF-rings is to expose an oxidized sample to a preferential etchant, e.g. the highly toxic Wright solution. In this work photoluminescence-imaging is compared to preferential etching and visual inspection. Vertical samples from an n-type Cz-crystal containing an OSF-boundary are studied before and after wet oxidation. High resolution PL-images, which allow for a close inspection of the OSF-area, reveal a complex band-structure of this border region.

Kinetics of self-interstitials reactions in p-type silicon irradiated with alpha particles

New findings on the self-interstitial migration in p-type silicon are presented. They are based on experimental studies of the formation kinetics of defects related to interstitial carbon after irradiation with alpha particles. The main parameters characterizing the interaction rate of silicon self-interstitials with substitutional carbon atoms have been determined. A preliminary interpretation of the experimental data is given. The interpretation takes into account different diffusivities of self-interstitials in their singly and doubly ionized states.

Combinedab initioand classical potential simulation study on silicon carbide precipitation in silicon

Atomistic simulations on the silicon carbide precipitation in bulk silicon employing both, classical potential and first-principles methods are presented. The calculations aim at a comprehensive, microscopic understanding of the precipitation mechanism in the context of controversial discussions in the literature. For the quantum-mechanical treatment, basic processes assumed in the precipitation process are calculated in feasible systems of small size. The migration mechanism of a carbon $\ensuremath{\langle}1\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0\ensuremath{\rangle}$ interstitial and silicon $\ensuremath{\langle}11\phantom{\rule{0.16em}{0ex}}0\ensuremath{\rangle}$ self-interstitial in otherwise defect-free silicon are investigated using density functional theory calculations. The influence of a nearby vacancy, another carbon interstitial and a substitutional defect as well as a silicon self-interstitial has been investigated systematically. Interactions of various combinations of defects have been characterized including a couple of selected migration pathways within these configurations. Most of the investigated pairs of defects tend to agglomerate allowing for a reduction in strain. The formation of structures involving strong carbon--carbon bonds turns out to be very unlikely. In contrast, substitutional carbon occurs in all probability. A long range capture radius has been observed for pairs of interstitial carbon as well as interstitial carbon and vacancies. A rather small capture radius is predicted for substitutional carbon and silicon self-interstitials. Initial assumptions regarding the precipitation mechanism of silicon carbide in bulk silicon are established and conformability to experimental findings is discussed. Furthermore, results of the accurate first-principles calculations on defects and carbon diffusion in silicon are compared to results of classical potential simulations revealing significant limitations of the latter method. An approach to work around this problem is proposed. Finally, results of the classical potential molecular dynamics simulations of large systems are examined, which reinforce previous assumptions and give further insight into basic processes involved in the silicon carbide transition.