Si Self-interstitials Research Articles

RF loss reduction by a carbon-regulated Si substrate engineering in GaN-based HEMT buffer stacks

A carbon-regulated Si substrate engineering has been adopted to reduce the RF loss of GaN-based HEMT buffer stacks. By implanting the substrate with high-dose carbon, undersaturation of Si self-interstitials is formed, and the self-interstitial-assisted aluminum diffusion into the Si substrate during the growth can be significantly suppressed. Consequently, the formation of parasitic conductive channel is suppressed, and the RF loss of the buffer stacks can be reduced. By combining the substrate engineering with low-temperature growth, the RF loss of the buffer stacks is reduced to as low as 0.13 dB/mm at 10 GHz. In addition, the crystal quality of the buffer stacks grown on the regulated substrates does not degrade. This work shows a great potential for fabrication of high-quality and low-loss GaN-on-Si RF devices.

Microscopic origin of the acceptor removal in neutron-irradiated Si detectors - An atomistic simulation study

The improved radiation hardness of p-type Si detectors is hindered by the radiation-induced acceptor removal process, which is not fully understood yet. Through atomistic modeling of displacement damage and dopant interactions, we analyze the acceptor removal under neutron irradiation, providing physical insight into its microscopic origin. Our results show that the fast decay of the effective dopant concentration (Neff) at low irradiation fluences is due to B deactivation caused by Si self-interstitials. The intriguing increase of the acceptor removal parameter with the initial dopant concentration (Neff,0) is explained by the limited number of mobile Si self-interstitials that survive annihilation and clustering processes. The sublinear dependence of the removal parameter on Neff,0 is associated to the inhomogeneity of damage for low Neff,0 and the formation of B-interstitial clusters with several B atoms for high Neff,0. The presence of O and C modifies B deactivation mechanisms due to the key role of BiO defects and the trapping of vacancies and Si self-interstitials, but for the impurity concentrations analyzed in this work ([O] >> [C]) it has little effect on the overall amount of removed acceptors. At high irradiation fluences, the reported increase of Neff is attributed to the formation of defect-related deep acceptors. From the analysis of the defect concentrations resulting from neutron irradiation and the occupancy of small clusters with acceptor levels reported in literature, we point out the tetra-vacancy cluster as one of the main contributors to Neff with negative space charge.

Influence of Point Defects on the Equilibrium Concentration of Interstitial Oxygen in Crystalline Silicon

The effect of excess point defects on the equilibrium concentration of interstitial oxygen for the system of interstitial oxygen/SiO2 precipitates in crystalline Si is theoretically investigated. The expression for the equilibrium concentration of interstitial oxygen in Si modified by the excess point defects is derived. Excess vacancies in Si are found to decrease this concentration, while the excess Si self-interstitials have the opposite effect. The effects of different conditions for the point defect generation on the equilibrium in the system of interstitial oxygen/SiO2 precipitates in crystalline Si are studied.

W and X Photoluminescence Centers in Crystalline Si: Chasing Candidates at Atomic Level Through Multiscale Simulations

Several atomistic techniques have been combined to identify the structure of defects responsible for X and W photoluminescence lines in crystalline Si. We used kinetic Monte Carlo simulations to reproduce irradiation and annealing conditions used in photoluminescence experiments. We found that W and X radiative centers are related to small Si self-interstitial clusters but coexist with larger Si self-interstitials clusters that can act as nonradiative centers. We used molecular dynamics simulations to explore the many different configurations of small Si self-interstitial clusters, and selected those having symmetry compatible with W and X photoluminescence centers. Using ab initio simulations, we calculated their formation energy, donor levels, and energy of local vibrational modes. On the basis of photoluminescence experiments and our multiscale theoretical calculations, we discuss the possible atomic configurations responsible for W and X photoluminescence centers in Si. Our simulations also reveal that the intensity of photoluminescence lines is the result of competition between radiative centers and nonradiative competitors, which can explain the experimental quenching of W and X lines even in the presence of the photoluminescence centers.

Effects of oxygen-inserted layers on diffusion of boron, phosphorus, and arsenic in silicon for ultra-shallow junction formation

The effects of oxygen-inserted (OI) layers on the diffusion of boron (B), phosphorus (P), and arsenic (As) in silicon (Si) are investigated, for ultra-shallow junction formation by high-dose ion implantation followed by rapid thermal annealing. The projected range (Rp) of the implanted dopants is shallower than the depth of the OI layers. Secondary ion mass spectrometry is used to compare the dopant profiles in silicon samples that have OI layers against the dopant profiles in control samples that do not have OI layers. Diffusion is found to be substantially retarded by the OI layers for B and P, and less for As, providing shallower junction depth. The experimental results suggest that the OI layers serve to block the diffusion of Si self-interstitials and thereby effectively reduce interstitial-aided diffusion beyond the depth of the OI layers. The OI layers also help to retain more dopants within the Si, which technology computer-aided design simulations indicate to be beneficial for achieving shallower junctions with lower sheet resistance to enable further miniaturization of planar metal-oxide-semiconductor field-effect transistors for improved integrated-circuit performance and cost per function.

Room-temperature plasma doping without bias power for introduction of Fe, Au, Al, Ga, Sn and In into Si

It is demonstrated by use of secondary ion mass spectroscopy that some impurities, including Fe, Au, Al, Ga, Sn and In, can be doped into Si wafers with depths of tens nanometer but quite high densities in radio frequency (RF)-excited plasma without any bias power at room temperature. This process is referred to as plasma doping without bias (PDWOB). In PDWOB, the quantity and depth of an impurity doped into the Si wafer depend on the character of the impurity, power of the RF that excites the plasma and the processing time of the PDWOB. The good fitting of the complementary error function distribution with the experimental data of the concentration distributions of the impurities doped into Si wafers indicates that PDWOB is a result of room-temperature diffusion of impurities in Si stimulated by vacancies and Si self-interstitials induced by plasma. The application prospects of the PDWOB, including doping ultra-thin films, ultra-shallow junctions and two-dimensional materials, are emphasized.

Effect of carbon situating at end-of-range defects on silicon self-diffusion investigated using pre-amorphized isotope multilayers

The effect of implanted carbon (C) on silicon (Si) self-diffusion has been investigated using pre-amorphized 28Si/natSi multilayers. The isotope multilayers were pre-amorphized by Ge implantation followed by C implantation, and annealed at 950 °C. Because of the presence of C, the Si self-diffusion was slower in 30 min annealing than the self-diffusion without C. This was attributed to the trapping of Si self-interstitials by C. On the other hand, the Si self-diffusion with C was faster in 2 h annealing than the self-diffusion without C, except in the end-of-range (EOR) defect region. The cause of this enhanced diffusion was understood as the retardation of Ostwald ripening of EOR defects by C trapped at the defects. In the EOR defect region, however, Si self-diffusion was slower than the self-diffusion without C in both 30 min and 2 h annealing owing to the presence of C. Relaxation of the tensile strain associated with the EOR defects by the trapped C was proposed to be the main cause of the retarded diffusion in the EOR region.

Observation of silicon self-diffusion enhanced by the strain originated from end-of-range defects using isotope multilayers

Si self-diffusion in the presence of end-of-range (EOR) defects is investigated using natSi/28Si isotope multilayers. The isotope multilayers were amorphized by Ge ion implantation, and then annealed at 800–950 °C. The behavior of Si self-interstitials is investigated through the 30Si self-diffusion. The experimental 30Si profiles show further enhancement of Si self-diffusion at the EOR defect region, in addition to the transient enhanced diffusion via excess Si self-interstitials by EOR defects. To explain this additional enhanced diffusion, we propose a model which takes into account enhanced diffusion by tensile strain originated from EOR defects. The calculation results based on this model have well reproduced the experimental 30Si profiles.

Simultaneous observation of the diffusion of self-atoms and co-implanted boron and carbon in silicon investigated by isotope heterostructures

Diffusion of self-atoms and co-implanted carbon (C) and boron (B) in silicon (Si) has been simultaneously observed using natSi/28Si isotope heterostructures. The supersaturation of Si self-interstitials (I’s) is investigated through the 30Si diffusion. The experimental results showed that Si self-diffusion was enhanced, that is, the I is more severely supersaturated, while B diffusion near the kink region was reduced with higher C dose. We also found slower dissolution of immobile BI clusters. C diffusion was not significant, indicating the formation of immobile CI clusters. These results indicate that the reduction of B diffusion is not due to the trapping of I by C, if any, but due to the retardation of BI cluster dissolution by the presence of C to decrease the amount of mobile B. The Si self-diffusion is enhanced by the dissolution of CI clusters to emit I.

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.

Process modeling of stress and chemical effects in SiGe alloys using kinetic Monte Carlo

In state-of-the-art silicon based process technologies, strained and relaxed SiGe, strained-silicon layers, and process-induced stress are widely present. Based on a literature review, we developed and calibrated continuum and kinetic Monte Carlo process models for chemical and stress effects in SiGe (Zographos et al. in AIP Conf. Proc. 1496:212---216, 2012). In this paper, we explain in full detail the corresponding kinetic Monte Carlo models and calibration. The models take into account the effects on band gap, amorphization, recrystallization, point defect generation and diffusion, extended defect evolution, dopant diffusion and clustering, and dopant segregation. The influence of Ge concentration and strain profile on Si self-interstitials and vacancies properties are deducted from experimental data as well as from ab-initio studies. The {311} interstitial clusters are less stable in the presence of Ge or compressive hydrostatic pressure, and the transformation of {311} defects into dislocation loops is faster. The corresponding parameter adjustments have been calibrated based on experimental data generated within the ATOMICS research project. The effects of Ge and stress on dopant diffusion have been calibrated for boron, arsenic and phosphorus taking into account that in experiments using epitaxial layers of strained SiGe embedded in Si, or strained silicon embedded in relaxed SiGe, boron and phosphorus have been found to segregate at Si/SiGe interfaces.

Enhanced and Retarded SiO2 Growth on Thermally Oxidized Fe-Contaminated n-Type Si(001) Surfaces

At the beginning of the oxidation of Fe-contaminated n-type Si(001) surfaces, Fe reacted with oxygen (O2) on the silicon (Si) substrate to form Fe2O3 and oxygen-induced point defects (emitted Si + vacancies). SiO2 growth was mainly enhanced by catalytic action of Fe. At 650 °C, SiO2 growth of the contaminated samples was faster than in reference samples rinsed in RCA solution during the first 60 min. However, it substantially slowed and became less than that of the reference samples. As the oxidation advanced, approximately half of the contaminated Fe atoms became concentrated close to the surface area of the SiO2 film layer. This Fe2O3-rich SiO2 layer acted as a diffusion barrier against oxygen species. The diffusion of oxygen atoms toward the SiO2/Si interface may have been reduced, and in turn, the emission of Si self-interstitials owing to oxidation-induced strain may have been decreased at the SiO2/Si interface, resulting in the retarded oxide growth. These results are evidence that emitted Si self-interstitials are oxidized not in the Fe2O3-rich SiO2 layer, but at the SiO2/Si interface in accordance with a previously proposed model. A possible mechanism based on the interfacial Si emission model is discussed. The activation energies for the oxide growth are found to be in accord with the enhanced and reduced growths of the Fe-contaminated samples.

Depth-resolved cathodoluminescence spectroscopy of silicon supersaturated with sulfur

We investigate the luminescence of Si supersaturated with S (Si:S) using depth-resolved cathodoluminescence spectroscopy and secondary ion mass spectroscopy as the S concentration is varied over 2 orders of magnitude (1018–1020 cm−3). In single-crystalline supersaturated Si:S, we identify strong luminescence from intra-gap states related to Si self-interstitials and a S-related luminescence at 0.85 eV, both of which show a strong dependence on S concentration in the supersaturated regime. Sufficiently high S concentrations in Si (>1020 cm−3) result in complete luminescence quenching, which we propose is a consequence of the overlapping of the defect band and conduction band.

Formation Energy of Intrinsic Point Defects in Si and Ge and Implications for Ge Crystal Growth

The formation energies of intrinsic point defects in Si and Ge were calculated by means of Density Functional Theory applying thin film models. The obtained formation energy for a Ge vacancy is close to the value calculated by means of a hybrid exchange correlation potential (HSE06). Well accepted experimental data exist for the sum of the formation and migration energies of vacancies and self-interstitials in Si. Taking into account recent values for the migration energies of point defects, the results of the present study are in excellent agreement with the experimental data. Applying the thin film model for both Si and Ge, one can compare formation energies of intrinsic point defects for both semiconductors. The thermal equilibrium concentration of Ge self-interstitials is more than one order of magnitude lower than that of Si interstitials while the vacancy concentrations are of similar magnitude. Contrary to Si, the diffusivity of Ge self-interstitials cannot recover the unbalance of the thermal equilibrium concentrations even if the growth rate is reduced to very low values. Therefore, Ge crystals grown from a melt are always vacancy-rich which results in void formation independent of the growth condition.

Stability and Segregation of B and P Dopants in Si/SiO2 Core–Shell Nanowires

Using molecular dynamics simulations, we generate realistic atomic models for oxidized Si nanowires which consist of a crystalline Si core and an amorphous SiO(2) shell. The amorphous characteristics of SiO(2) are well reproduced, as compared to those for bulk amorphous silica. Based on first-principles density functional calculations, we investigate the stability and segregation of B and P dopants near the radial interface between Si and SiO(2). Although substitutional B atoms are more stable in the core than in the oxide, B dopants can segregate to the oxide with the aid of Si self-interstitials which are generated during thermal oxidation. The segregation of B dopants occurs in the form of B interstitials in the oxide, leaving the self-interstitials in the Si core. In the case of P dopants, dopant segregation to the oxide is unfavorable even in the presence of self-interstitials. Instead, we find that P dopants tend to aggregate in the Si region near the interface and may form nearest-neighbor donor pairs, which are energetically more stable than isolated P dopants.

Diffusion of co-implanted carbon and boron in silicon and its effect on excess self-interstitials

Diffusion of co-implanted carbon (C) and boron (B) in silicon (Si) and its effect on excess Si self-interstitials (I’s) after annealing at 800 and 1000 °C were investigated by means of secondary ion mass spectrometry. The experimental results showed that C diffusion was not significant at 800 and 1000 °C but later became visible for longer annealing times at 1000 °C. B diffusion was reduced by the presence of C when no significant C diffusion was observed, but it was enhanced when C diffusion was observed. These results indicate that all implanted C atoms form immobile CI clusters with excess I in the amount of implanted C and that these CI clusters are stable and trap I to reduce B diffusion. On the contrary, CI clusters are dissolved to emit I for longer annealing times at 1000 °C and both B and C diffusion are enhanced. Diffusion simulation based on these models fits the experimental profiles of B and C.

Ab initio study of boron segregation and deactivation at Si/SiO 2 interface

Abstract We perform first-principles density functional calculations to investigate the stability of various B-related defects near Si/SiO 2 interface, and propose a mechanism for boron segregation to the interface. In Si, a substitutional B is energetically very stable and does not diffuse into the oxide in the absence of Si self-interstitials. Under nonequilibrium conditions, where self-interstitials are abundant, B dopants diffuse via the formation of a defect pair which consists of a B dopant and a self-interstitial. It is found that diffusing B dopants further segregate toward the oxide near the interface in form of positively charged interstitials, resulting in the suppression of activated dopants.

Formation and evolution of small B clusters in Si: Ion channeling study

B off-lattice displacement in B-doped Si was observed under Si self-interstitials $(Is)$ supersaturation induced by ion irradiation at room temperature. B lattice location has a characteristic channeling mark and was studied by nuclear reaction analyses and ion channeling technique, through the comparison of the performed angular scans along the $⟨100⟩$ and $⟨110⟩$ crystal axes and the simulated scans by FLUX code. Solid and liquid-phase epitaxies and molecular beam epitaxy were used to prepare B-doped Si samples in order to investigate samples with B concentration in the range between ${10}^{19}$ and ${10}^{21}\text{ }\text{at}/{\text{cm}}^{3}$. B off-lattice displacement is limited by the fluence of excess $Is$ per B atom. Small $\text{B-}Is$ clusters (BICs) were formed as consequence of the interaction with $Is$ produced during the ion irradiation. Clusters structures were investigated by simulating the channeling angular scans of cluster configurations predicted by theoretical calculations. In the early stage of $Is$ injection, experimental observations are consistent with the presence of the predicted ${\text{B}}_{2}I$ clusters. These small BICs evolved into different structures under further ion irradiation.

Ab initio study of the diffusion mechanisms of gallium in a silicon matrix

We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si using ab initio techniques. Five stable neutral configurations for single and multi-atom defects are identified by density-functional theory (DFT) calculations within the local density approximation and using a localized basis set as implemented in the SIESTA package. Formation energy (EF) calculations on these stable structures show the most likely neutral single-atom defect to be the Ga substitutional, with an EF of 0.7 eV in good agreement with previous work. Charge state studies show the Ga tetrahedral interstitial defect to be in a + 1 state for most doping conditions. They also indicate the possibility for a gallium substitutional-tetrahedral interstitial complex to act as a deactivating center for the Ga dopants except in n-doped regime, where the complex adopts a − 1 charge state. Migration pathway calculations using SIESTA coupled with the activation relaxation technique (ART nouveau) allow us to determine possible migration paths from the stable configurations found, under various charge states. In general, diffusion barriers decrease as the charge state becomes more negative, suggesting that the presence of Si self-interstitials can enhance diffusion through the kicking out of substitutional Si and by adding negative charge carriers to the system. An overall picture of a possible Ga diffusion and complex formation mechanism is presented based on these results.

Indirect Diffusion Mechanism of Boron Atoms in Crystalline and Amorphous Silicon

ABSTRACTThe diffusion of B atoms in crystalline and amorphous Si has been experimentally investigated and modeled, evidencing the indirect mechanism of these mass transport phenomena. The migration of B occurs after interaction with self-interstitials in crystalline Si (c-Si) or with dangling bonds in amorphous Si (a-Si). In the first case, an accurate experimental design and a proper modeling allowed to determine the microscopic diffusion parameters as the B-defect interaction rate, the reaction paths leading to the diffusing species and its migration length. Moreover, by changing the Fermi level position, B atoms are shown to interact preferentially with neutral or doubly positively charged self-interstitials. As far as the amorphous case is concerned, B diffusion is revealed to have a marked transient character and to depend on the B concentration itself. In particular, boron atoms can move after the interaction with dangling bonds whose density is transiently increased after ion implantation or permanently enhanced by the presence of boron atoms themselves. Unexpectedly, B diffusivity in a-Si is seen to be orders of magnitude above than in c-Si and to depend on the thermal history, i.e. the relaxation status of the amorphous phase. These data are presented and their implications discussed.