Dopants diffusion, activation and pile-up due to rapid thermal annealing of implanted Al and B in a thin (∼200Å) Si cap layer on top of Si 1-x-y Ge x C y layer were studied. Experimental results show that both the lattice strain and differential diffusion flux can cause atomic pile-up at the interface and the evidences of those effects were shown independently to each other in this paper. In addition, the pile-up can be extended from the interface to the surface by incorporating C in the underlying layer where B diffusion is much less than in the cap Si. Material analysis shows that both B atomic and activated concentrations in the Si cap layer are increased by 50 %, which suggests that the dopant activation can be increased and junction depth can be decreased at the same time using the inserted Si 1-x-y Ge x C y diffusion blocking layer.

We present a first-principles study of the structure and dynamics of small As-interstitial complexes (AsI2, As2I2, AsI3, and As2I3) in crystalline Si. These complexes can be important components of stable As-interstitial clusters or play a key role in interstitial-mediated formation of As-vacancy clusters. Neutral AsI2 and As2I2 are identified as fast-diffusing species that contribute to As transient enhanced diffusion. We demonstrate that the extended defect configuration As2I3ext is a stable configuration with a binding energy of 2.64 eV. As2I3ext can serve as a nucleation site and facilitate the formation of larger As-interstitial clusters in presence of excess Si interstitials and high As concentration. We also discuss the implications of our findings on As transient enhanced diffusion and clustering and highlight the role of small As-interstitial complexes in ultrashallow junction formation.

The behavior of arsenic defect complexes at amorphous SiO2/Si(110) interfaces has been studied using density-functional theory calculation. We find that arsenic defect complexes that are stable in bulk Si show moderate energy gain in SiO2/Si interface region due to the interface-induced strain effect. We have identified three arsenic defect complex configurations, As-it, As2I2I, and As2I2II, which exist only at SiO2/Si interface. These interface arsenic defect complexes are highly stabilized due to their unique bonding configurations at SiO2/Si interface. Therefore, they could contribute to arsenic segregation as both initial stage precursor and dopant trapping sites. Our calculation indicates that arsenic atoms trapped in such interface complexes are electrically inactive. Finally, the formation and evolution dynamics of interface arsenic defect complexes are discussed.

A kinetic arsenic-interstitial interaction model has been developed to study and predict arsenic transient enhanced diffusion (TED) and deactivation behavior during ultrashallow junction (USJ) formation. This model is based on density functional theory and has been verified by previous experiments in which the significant role of interstitial mechanism in arsenic TED was revealed. The mechanism of enhanced and retarded arsenic diffusion in different point defect environments is investigated by utilizing this model in kinetic Monte Carlo simulation. The arsenic-interstitial pair, with low binding energy and low migration energy, is shown to be the major contributor to arsenic TED in silicon interstitial-rich situations. In addition, by using this model, we demonstrate the transient existence of arsenic-interstitial clusters (AsnIm) during postimplant annealing and propose their possible role in deactivation for short time annealings such as laser annealing and spike annealing. Moreover, we have developed a novel surface-trap based kinetic Monte Carlo model to simulate arsenic uphill diffusion in proximity of the Si∕SiO2 interface. The simulation results show that the activation behavior of the uphill portion of arsenic has considerable impact on the junction sheet resistance. The activation behavior of this arsenic is expected to become more important when USJ depth is scaled down further.

Using density-functional theory calculations we identified an interstitial-based fast boron diffusion mechanism in amorphous silicon. We found that interstitial-like point defects, omnipresent in as-implanted silicon, to be very stable in an amorphous network and can form highly mobile pair with Boron atoms. The transient existence of such point defects in amorphous silicon is suggested to play an important role in boron diffusion. We found the activation energy for this pathway to be 2.73 eV, in good agreement with experimental results. In addition, this mechanism is consistent with the experimentally reported transient and concentration-dependent features of boron diffusion in amorphous silicon.

We propose new structural configurations and novel diffusion mechanisms for neutral di-arsenic interstitial (As2I2) in silicon with a first-principle density functional theory simulation within the generalized gradient approximation. With an assumption of excess silicon interstitials and high arsenic concentrations, neutral As2I2 is expected to be favorable and mobile with low-migration barrier. Moreover, because the diffusion barrier of arsenic interstitial pairs (AsI) is very low (< 0.2eV) under the same conditions, As2I2 can be easily formed and likely intermediate stage of larger arsenic interstitial clusters.

We report that arsenic diffusion can be enhanced and retarded by surrounding interstitial rich and vacancy rich environments created by Si point defect engineering implant. The enhancement and retardation can be attributed to the dominant arsenic interstitial diffusion mechanism during post-implant anneal. Kinetic Monte Carlo simulations with newly implemented models show good match with experiments. Our study suggests the importance of arsenic interstitial mechanism and a possible approach for n-type ultra shallow junction fabrication.

Arsenic enhanced or retarded diffusion is observed by overlapping the dopant region with, respectively, interstitial-rich and vacancy-rich regions produced by Si implants. Enhanced diffusion can be attributed to interstitial-mediated diffusion during postimplant annealing. Two possible mechanisms for diffusion retardation, interstitial-vacancy recombination and dopant clustering, are analyzed in additional experiments. The point defect engineering approach demonstrated in this letter could be applied to fabrication of n-type ultrashallow junctions.

Using density functional theory calculations within the generalized gradient approximation we have examined structure and dynamics of neutral Si vacancies at Si/SiO2 interface. We show that Si/SiO2 interface may serve as a limited sink for Si vacancies. Single vacancy and vacancy cluster defects are substantially more stable at c-Si/a-SiO2 interface compared to the bulk c-Si layers away from interface, mainly due to termination of dangling bonds with bridging O atoms and reduction of interface strain

Introduction Much interest has been focused in recent years on nonvolatile memories using nanocrystal charge storage due to improved reliability and process simplification [1]. Different technology applications bring new sets of device requirements, including module scaling [2]. For medium density embedded flash applications, concerns include speed and reliability as well as low power and module size. Source-side injection (SSI) is an attractive programming method due to its low current compared to hot electron injection (HCI) [3], and has garnered increasing interest [4-6]. Since a nanocrystal memory has independent gate control, SSI voltages can be scaled significantly compared to the corresponding floating gate device. Lowering the natural threshold voltage (Vt,nat) of the bitcell would furthermore allow for peripheral circuitry scaling by use of logic I/O transistors as decoders. In this work, we use SSI programming and incorporate threshold voltage lowering to create a 90nm memory bitcell that has good performance, low programming current, and acceptable disturb characteristics.

Introduction As CMOS technology is scaled to the 90nm node and beyond, silicon nanocrystal nonvolatile memories are receiving increased attention as a replacement for floating gate nonvolatile memories [1, 2]. The thin dielectrics in these memories can lead to excessive gate disturb during the read operation. Of primary concern is the loss of electrons of the program state to the gate through the top oxide overlying the nanocrystals. This loss is the result of tunneling due to the high electric field between the gate and the nanocrystals. It has been shown that reducing the natural threshold voltage (Vt,nat) of the memory cell leads to a reduction in gate disturb [3]. Simple reduction of the Vt,nat by decreasing the substrate doping concentration can result in severely degraded short channel performance, as well as degraded hot carrier injection (HCI) performance during the program operation. Thus, it is desired to construct a substrate doping profile with a light surface concentration to obtain a low Vt,nat, and a heavy doping concentration just below the surface to provide robust short channel performance and good HCI programmability.

We study B diffusion in the presence of Ge by using a first principles density functional theory calculation. We investigate the relative stability and migration barriers of Si and Ge interstitials as well as binding energy and diffusion pathway of Boron-Interstitial (BI) pair comprised of Boron and Si or Ge interstitials. We find that Ge interstitials are more stable but less mobile compared to Si interstitials, leading to higher population of interstitials in the implanted Si1-xGex. However, BI pair comprised of Ge interstitial and Boron is less stable compared to Si interstitial –Boron pair and migration barrier of BI pair in presence of Ge is increased, leading to less TED.

We investigate B diffusion in strained Si by using density functional theory calculations. We calculate the migration barriers and formation energies of the B-Si complexes at different charge states in the biaxial tensile strained {001} Si layer. The migration barriers and formation energies overall intend to decrease under strain at all charge states. For neutral and negatively charged B-Si complexes, the migration barrier reduces along the strain plane while the barrier in the direction perpendicular to strain plane remains unchanged, but there is no anisotropy in B diffusion for positively charged B-Si complexes.

Precise control of dopant redistribution and activation in the vicinity of the semiconductor-dielectric interface has become crucial for fabrication of deep sub-micron metal-oxide-semiconductor field-effect transistor devices. During the process of ion implantation and thermal oxidation a great number of native defects (such as vacancies and interstitials) can be created in the substrate. These defects are known to be mainly responsible for transient enhanced diffusion and electrical activation/deactivation of dopant impurities. In this work we seek to develop a detailed understanding of the exact mechanisms of defect annihilation, and dopant diffusion and clustering/dissolution in complex systems such as Si surfaces and amorphous-crystalline Si-Si and Si-SiO2 interfaces using density functional theory total energy calculations. Si(001) surface: (1) We examine structure, energetics, and bonding of vacancies and interstitials on the clean and terminated Si(001) surface and its subsurface layers. (2) We propose mechanism of vacancy stabilization at the surface and subsurface layers. (3) We find Si(001) surface to be an effective sink for vacancies and interstitials, irrespective of surface passivation. (4) We present diffusion pathways and barriers of vacancies at and in the vicinity of the clean surface. (5) We have demonstrated that the stability of native defects within the top-most three subsurface layers is greatly influenced by surface passivation. Amorphous-crystalline Si interface: (1) We present native defect configurations, energetics and the origin of their stabilization at amorphous-crystalline Si interface and in amorphous Si. (2) A continuous random network model is employed in the construction of a realistic a-c interface structures. (3) We propose the ‘sponge-like’ behavior of the amorphous phase toward native defects. Si/SiO2 interface: (1) We present stable Si interstitial structures at interface and in the oxide. (2) We propose mechanism of interstitial diffusion from Si into a-SiO 2. (3) We consider Boron-Interstitial pair behavior in vicinity of interface. (4) We propose a novel mechanism of vacancy stabilization and vacancy clustering at interface. We believe the findings we present here show the importance of understanding the role of surfaces and interfaces in affecting defect and dopant behavior in their vicinity. This research leads to an understanding of the broad range of phenomena applicable to modern microelectronics device fabrication and process modeling.

We investigate B diffusion in strained Si by using first-principles density functional theory calculations. An enhancement and an anisotropy of B diffusion in biaxial tensile strained Si are found. The diffusion barrier along the strain plane (channel) is decreased while the barrier in the vertical direction (depth) remains unchanged. This anisotropy comes from the orientation dependence of the saddle point in the diffusion pathway. The formation enthalpy of B-I pair also decreases in strained Si. According to our calculations, for strained Si on a Si0.8Ge0.2 buffer layer, which is widely used in strained metal oxide semiconductor field-effect-transistor, an enhancement of B diffusivity along the channel by a factor ∼4 and a factor ∼2 in the vertical direction are expected for typical rapid thermal anneals.

Using density functional theory calculations within the generalized gradient approximation, we have investigated the structure, energetics, bonding, and diffusion behavior of Si interstitials and boron-interstitial pairs at the Si/SiO2 interface. We find that interstitials are significantly stabilized at the Si/SiO2 interface and prefer to reside on the SiO2 side rather than the Si side. Due to the interstitial stabilization, boron-interstitial pairs are likely to be easily dissociated in the vicinity of the Si/SiO2 interface. This study provides valuable insight into interstitial annihilation and boron precipitation at the interface.