V. C. Venezia

Boron diffusion in amorphous silicon and the role of fluorine

We demonstrate that boron diffuses at high concentrations during low-temperature thermal annealing in amorphous silicon pre-amorphized by germanium ion implantation. For a typical boron ultrashallow junction doping profile, concentrations as high as 2×1020 cm−3 appear to be highly mobile at 500 and 600 °C in the amorphous silicon region before recrystallization. In crystalline silicon at the same temperatures the mobile boron concentration is at least two orders of magnitude lower. We also show that boron diffusivity in the amorphous region is similar with and without fluorine. The role of fluorine is not to enhance boron diffusivity, but to dramatically slow down the recrystallization rate, allowing the boron profile to be mobile up to the concentration of 2×1020 cm−3 for a longer time.

Low-temperature diffusion of high-concentration phosphorus in silicon, a preferential movement toward the surface

We demonstrate that ultrashallow high-concentration phosphorus profiles in silicon diffuse preferentially toward the surface during low-temperature annealing at 700°C after recrystallization of an amorphous layer. In this work, we observe the preferential diffusion following a preamorphizing germanium implant, and also after a self-amorphizing phosphorus implant. This phenomenon is driven by the presence and dissolution of silicon interstitial defects. The greater the distance between the defect band and the high-concentration phosphorus profile, the less the preferential diffusion for a fixed anneal time. The overall result of this effect is a phosphorus profile that is significantly shallower and steeper than after implant.

Role of silicon interstitials in boron cluster dissolution

We present kinetic nonlattice Monte Carlo atomistic simulations to investigate the role of Si interstitials in B cluster dissolution. We show that the presence of Si interstitials from an oxidizing anneal stabilize B clusters and slow down B cluster dissolution, compared to anneal in inert ambient. We have also analyzed the influence of injected Si interstitials from end of range defects, due to preamorphizing implants, on B deactivation and reactivation processes. We have observed that the B cluster evolution can be clearly correlated to the evolution of Si interstitial defects at the end of range. The minimum level of activation occurs when the Si interstitial supersaturation is low because the end of range defects have dissolved or reach very stable configurations, such as dislocation loops.

Enhanced Low Temperature Electrical Activation of B in Si

The electrical activation of B in n-type epitaxial-Si(100) has been enhanced in the temperature range of 400–800 °C. This enhanced activation was measured for 40 keV, 2×1014 cm−2 dose of B implanted into a vacancy-rich Si region. The vacancy-rich region consists of excess vacancies (Vex) generated by a 2 MeV Si implant in the dose range of 3×1015–10×1015 cm−2. The B activation in vacancy-rich Si is found to be a factor of ∼2.4 larger with up to ∼80% of the B activated as compared to similar B implant and activation anneals carried out in the bulk Si. The dependence of B activation on Vex concentration shows that the active B concentration increases with the Vex concentration. From this dependence it was estimated that at least three vacancies are required to activate an additional B atom. This process is distinctly different from the low temperature activation that occurs during solid-phase epitaxial recrystallization of B-doped amorphous Si as no amorphous Si is produced during any step. This low tempera...

Impurity redistribution due to recrystallization of preamorphized silicon

We have studied impurity redistribution due to low-temperature crystallization of amorphous silicon. Many impurities move ahead of the amorphous-crystalline interface and relocate closer to the surface. In general, redistribution is more likely at high impurity concentrations. By investigating a wide range of concentrations for indium, lead, and antimony, we demonstrate the direct correlation between the magnitude of this redistribution effect and the impurity metastable solubility limit in crystalline silicon. At low concentrations, it is less likely for impurities to redistribute. However, in this regime we show that indium experiences concentration-independent segregation, and that boron profiles are also affected by the crystallization process.

Influence of preamorphization and recrystallization on indium doping profiles in silicon

The effect of preamorphization and solid-phase epitaxial regrowth on indium doping profiles in silicon has been investigated. It is shown that preamorphized silicon significantly reduces channeling during indium ion implantation, producing a much more abrupt doping profile. During recrystallization by thermal annealing, indium segregates in front of the moving amorphous/crystalline interface, creating a clearly visible peak in the doping profile. We establish that the physical mechanism for this phenomenon in the 1018–1019 cm−3 concentration range is segregation determined, as there is no significant concentration dependence for those doses studied in this work. We also demonstrate that this phenomenon is enhanced at lower temperatures.

Plasma-nitrided silicon-rich oxide as an extension to ultrathin nitrided oxide gate dielectrics

We have investigated the mechanism of N incorporation, during plasma nitridation, in thermally grown ultrathin (<2nm)SiO2 films and deposited silicon-rich oxide films. X-ray photoelectron spectroscopy analysis indicates that N atoms exchange mainly with O to bond with Si atoms in ultrathin plasma-nitrided oxides. Based on this understanding, we were able to increase the amount of N that can be incorporated in plasma-nitrided silicon oxides by increasing the silicon content in these films. This was achieved by depositing ultrathin substoichiometric silicon-rich oxide films. We demonstrate an increase of almost twice as much N in these ultrathin plasma-nitrided silicon-rich oxide films yielding lower gate leakage current for a given thickness.

Dopant redistribution effects in preamorphized silicon during low temperature annealing

The time evolution of B, As, and In doping profiles during and after solid phase epitaxial regrowth (SPER) was monitored for conditions applicable to sub-65 nm CMOS technologies. As and In segregate during SPER by a sweep of the regrowing interface. In the case of B, significant B diffusion in a-Si occurs during SPER, while afterwards B uphill diffusion dominates. With the aid of atomistic simulation we have identified a temperature dependent time to maximum uphill B diffusion.

Dopant diffusion in amorphous silicon

In this work we investigate the diffusion of high-concentration ultrashallow boron, fluorine, phosphorus, and arsenic profiles in amorphous silicon. We demonstrate that boron diffuses at high concentrations in amorphous silicon during low-temperature thermal annealing. Isothermal and isochronal anneal sequences indicate that there is an initial transient enhancement of diffusion. We have observed this transient diffusion characteristic both in amorphous silicon preamorphized by germanium ion implantation and also in amorphous silicon preamorphized by silicon ion implantation. We also show that the boron diffusivity in the amorphous region is similar with and without fluorine, and that the lack of diffusion for low-concentration boron profiles indicates that boron diffusion in amorphous silicon is driven by high concentrations. Ultrashallow high-concentration fluorine profiles diffuse quite rapidly in amorphous silicon, and like boron, undergo a definite transient enhancement. In contrast, ultrashallow high- concentration phosphorus and arsenic profiles did not significantly diffuse in our experiments.