Point Defect Charge‐State Effects on Transient Diffusion of Dopants in Si

Abstract

Ion implantation damage produces point defects during annealing which can significantly enhance dopant diffusion. This diffusion at low temperatures was studied for P, As, and B implants in St. Enhanced diffusion was observed below certain doping concentrations which depend only on temperature. For As this concentration corresponds to As solid solubility. For P, enhanced diffusion occurs below the well-known kink concentration. For B, diffusion occurs below n~ for temperatures below 850~ A model is presented which correlates the self-interstitial donor level position, EI +, in the Si bandgap with the dominance of B diffusion via neutral self-interstitials. It is shown that when self-interstitials are extremely generated, the resulting changes in the free energy of self-interstitial formation force the B diffusion via I ~ to be 10-100 times greater than B diffusion via I § This results in significant enhanced diffusion of B in intrinsic Si relative to B layers doped above n, At temperatures at or above 850~ thermally-assisted, concentration-dependent diffusion via I + begins to dominate over enhanced diffusion which occurs with an ever decreasing time constant.

Physical mechanisms of transient enhanced dopant diffusion in ion-implanted silicon

“…The present study also shows that the pinning of the B concentration during TED does not necessarily require the presence of extended defects such as rodlike defects or stacking faults, in contrast to what was proposed in the literature. 9 Furthermore, earlier studies 20 have pointed out that the demarcation level between mobile and immobile B appears to be correlated with the intrinsic carrier concentration n i . This observation has occasionally been used as a reference point in process simulations to model the immobile B fraction during TED.…”

“…Alternatively, the anomalous B diffusion was ascribed to the trapping of B at extended defects, 9 or was explained in terms of an enhanced coupling between B and neutral interstitials. 20 The latter model was based on the observation that the kink in the dopant profiles after TED appears to be correlated with the intrinsic carrier concentration. In a recent article on process simulations, 21 the rather unphysical assumption was made that the incomplete B activation results from the formation of immobile, isolated B interstitials through a kick-out reaction with Si interstitials.…”

Modelisation of boron diffusion from ultra-low-energy implantation in crystalline silicon

“…The data reported in this work refer to the case of 500 eV B implant and annealed at 900 C for 20 s. In the first approach, we have used the model proposed by Uematsu [5] and tested it to simulate correctly boron diffusion after ultra-lowenergy implantation. This model is based on kickout mechanism including substituted dopants (B s ), neutral boron interstitials (B i ), positive and neutral silicon self-interstitials (I + ,I 0 ) based on Fair's estimate [6,7]. B precipitation above the solubility limit [5] is taken into account.…”

Characterization of low-energy (100 eV–10 keV) boron ion implantation

“…2, a 5ϫ10 14 cm Ϫ2 B implantation at different low energies is shown for three conditions: a 7°tilt angle, a 0°tilt angle and an implant into a 205 nm deep Ge preamorphized Si wafer. 9 Recent evidence has emerged that this link is fortuitous. A preamorphized wafer will give no channeling.…”