M.J.P. Hopstaken

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.

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.

Experimental Investigation of the Impact of Implanted Phosphorus Dose and Anneal on Dopant Diffusion and Activation in Germanium

Germanium has regained attention in the semiconductor industry for MOSFET application because of the higher mobility of carriers – two times higher mobility for electrons and four times for holes – as compared to silicon. In the opposite of the Silicon, the major issue with Germanium is to limit the n-dopant diffusion. Usual n-dopants (Phosphorus and Arsenic for example) are not electrically activated at an acceptable level without a large diffusion of the doping profile and a substantial dose loss. In this work, we have studied the influence of low energy and dose implant (15KeV to 40KeV @ 8E13 to 1E15at.cm −2 ) and low temperature anneal (515°C to 600°C) on diffusion, exodiffusion and activation of the phosphorus dopant into Germanium. The annealing steps were made in RTP furnace, the chemical profile and electrically active profiles were extracted by using Secondary-Ion-Mass Spectroscopy (SIMS) and sheet resistance measurement (Rs). To investigate the implantation-induced defects in depth, cross-sectional micrographs were made by using Transmission Electron Microscopy (TEM). Experimental results show that we achieved an efficient activation level by tuning both dose implant and anneal temperature, limiting the exodiffusion with pratically no diffusion of the dopant. We also show that very abrupt profile can be achieved with appropriate implant and thermal annealing conditions. To limit the leakage current in devices, we suppose we have to limit the defects generated during the implantation. Specially for dopant activation temperature anneal below 550°C, we have shown and observed by cross-sectional micrograph that the defects are totally removed by addition of a pre step of annealing at 400°C.

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.

Experimental Study on the Mechanism of Carbon Diffusion in Silicon

CVD-grown lightly C-doped superlattices with peak C concentrations of 2.10 18 /cm 2 and 2.10 19 /cm 2 were annealed in NH3, N2/H2, N2, and O2 ambient gases to investigate the influence of a range of point-defect conditions on C diffusion at the nanometer scale. C profiles were measured by secondary-ion mass spectroscopy. The profiles exhibit exponential-like diffusion consistent with a ‘long hop’ diffusion process with a characteristic migration length λ (=19 ± 3 nm at 850 °C). Within experimental errors the value of λ is the same for all the ambient gases used, whereas the migration frequency g increases by two orders of magnitude as the ambient gas is changed from NH3 ambient (interstitial undersaturation) to O2 ambient (interstitial supersaturation), and decreases as a function of C concentration in the as-grown superlattice. The results confirm that C diffuses predominantly by a kick out mechanism under nearequilibrium diffusion conditions. Initial results support the chemical-pump model for suppression of diffusion in C-doped silicon.

Diffusion Suppression in Silicon by Substitutional C Doping

The role of C as a suppressor of B, P, and In diffusion is widely known but the mechanisms involved are still poorly understood. This paper presents novel results on the diffusion of C at the nanometer scale, which clearly show that the suppression of diffusion arises from the expulsion of interstitials from the Cdoped region, caused by long-range migration of interstitial C atoms. Fundamental parameters for C diffusion (migration frequency and jump length) are presented and compared with existing data for B diffusion, and the results are placed in the context of a unified model of impurity diffusion.