Experiments on Si-rich SiGe layers show an exponential increase in Ge difFusion and an exponential decrease in B difFusion as a function of compressive strain, indicating a linear dependence of activation energy on strain. The efFect arises &om the structural relaxation of the lattice around the defect mediating difFusion (inward for a vacancy, outward for an interstitial). We infer the mechanisms of Ge and B difFusion in strain-f'ree and compressively strained Si(Ge) at T ( 1030'C, and draw some general conclusions on strain-modified diffusion in cryst&»ne solids. PACS numbers: 66.30.Jt Strained SiGe/Si heterostructures and superlattices are an essential component of many advanced Si-based devices, but the kinetic mechanisms of SiGe layer relaxation during thermal annealing are still poorly understood. Most previous work on strained-layer relaxation has focused on the nucleation, growth, and multiplication of dislocation loops during growth and subsequent thermal annealing. However, some workers have reported an alternative, diffusive relaxation process [1,2). In particular, Iyer and LeGoues have reported enhanced Si-Ge interdiffusion which is quenched on formation of a high density of dislocations [2]. This observation was attributed to strain-assisted difFusion, based on the thermodynamic analysis of spinodal decomposition by Cahn and Hilliard [3], but no specific physical mechanism for the enhanced Si-Ge interdifFusion has so far been proposed. This remains a significant challenge for our understanding of difFusion in Si and related materials. Recently, enhanced As difFusion [4] and retarded B diffusion [5,6] have been reported in compressively strained Si-rich SiGe layers. In particular, Moriya et al. presented extensive data showing a large reduction (up to a factor of 10) in the intrinsic difFusivity of B in Si(Ge) under compressive strain. By making the critical assumption that B difFusion is mediated by positively charged point defects, Moriya et al. were able to explain their result in terms of band-gap narrowing [6]. Although this assumption is consistent with early diffusion data [7], it appears to be incorrect. More extensive diffusion studies, using isoconcentration p-type and n-type backgrounds, have shown that the contributions of charged and neutral point defects to intrinsic B difFusion are of similar magnitude [ 8,9]. This conclusion rules out a strong reduction in intrinsic B difFusion due to band-gap narrowing, and points to a more drastic strain-related phenomenon.A hint as to the nature of this phenomenon can be found in recent total-energy calculations [10,11). Antonelli and Bernholc computed the formation energies for self-interstitials (b, Eyl) and vacancies (EEyv) in Si as a function of hydrostatic pressure. A linear increase in AEfl and decrease in bEyv were found with increasing pressure, corresponding to an outward relaxation of the lattice around the interstitial, and an inward relaxation of the vacancy. More recently, the same authors computed the effect of tensile strain in a Si l...

No abstract

The recently observed phenomenon of boron uphill diffusion during low-temperature annealing of ultrashallow ion-implanted junctions in silicon has been investigated. It is shown that the effect is enhanced by preamorphization, and that an increase in the depth of the preamorphized layer reduces uphill diffusion in the high-concentration portion of boron profile, while increasing transient enhanced diffusion in the tail. The data demonstrate that the magnitude of the uphill diffusion effect is determined by the proximity of boron and implant damage to the silicon surface.

The formation of ultra-shallow junctions (USJs) for future integrated circuit technologies requires preamorphization and high dose boron doping to achieve high activation levels and abrupt profiles. To achieve the challenging targets set out in the semiconductor roadmap, it is crucial to reach a much better understanding of the basic physical processes taking place during USJ processing. In this paper we review current understanding of dopant-defect interactions during thermal processing of device structures – interactions which are at the heart of the dopant diffusion and activation anomalies seen in USJs. First, we recall the formation and thermal evolution of End of Range (EOR) defects upon annealing of preamorphized implants (PAI). It is shown that various types of extended defect can be formed: clusters, {113} defects and dislocation loops. During annealing, these defects exchange Si interstitial atoms and evolve following an Ostwald ripening mechanism. We review progress in developing models based on these concepts, which can accurately predict EOR defect evolution and interstitial transport between the defect layer and the surface. Based on this physically based defect modelling approach, combined with fully coupled multi-stream modelling of dopant diffusion, one can perform highly predictive simulations of boron diffusion and de/re-activation in Ge-PAI boron USJs. Agreement between simulations and experimental data is found over a wide range of experimental conditions, clearly indicating that the driving mechanism that degrades boron junction depth and activation is the dissolution of the interstitial defect band. Finally, we briefly outline some promising methods, such as co-implants and/or vacancy engineering, for further down-scaling of source-drain resistance and junction depth.

A physically motivated model that accounts for the spatial and temporal evolution of self-interstitial agglomerates in ion-implanted Si is presented. For the calibration of the model, a genetic algorithm is used to find the optimum set of physical parameters from experimental data. Mean-size evolution of {113} defects obtained by transmission electron microscopy and self-interstitial oversaturation results measured in the vicinity of extended defects are combined in the same fitting procedure. The calibration of parameters shows that binding energies of small self-interstitial clusters exhibit strong maxima, as reported in other investigations. Results of the calibrated model are compared to experimental data obtained in complementary investigations. It is demonstrated that the model is able to predict a wide variety of physical phenomena, from the oversaturation of self-interstitials via the mean-size evolution of {113} defects to the depth distribution of the density of the latter

The time evolution of B diffusion and electrical activation after ion implantation and annealing at 800 and 900 °C is studied using secondary-ion mass spectrometry and spreading-resistance profiling. The time evolution at 800 °C is observed in both crystalline and post-amorphized samples. Amorphized samples show near-normal concentration enhanced diffusion. Crystalline samples show anomalous transient diffusion, with a rapidly diffusing low-concentration region and a static peak region above a critical concentration Cenh=3.5×1018 cm−3. The peak region above Cenh is shown to be electrically inactive. The static, inactive B is released over a period of many hours, compared with the transient diffusion enhancement which relaxes to near-normal within 30 min. The time evolution of B diffusion at 900 °C is observed as a function of implantation dose. A critical concentration for transient diffusion, Cenh=8×1018 cm−2, independent of dose, is observed at this temperature. The transient diffusion enhancement in the diffusing part of the B profile increases with dose, up to a dose of ∼5×1014 cm−3, and saturates at higher doses. A comparison with published data shows that Cenh∼ni within a factor 2 over the temperature range 550–900 °C. We interpret our observations in terms of a nonequilibrium point-defect model of diffusion and intermediate defect formation.

B diffusion measurements are used to probe the basic nature of self-interstitial 'point' defects in Ge. We find two distinct self-interstitial forms -a simple one with low entropy and a complex one with entropy ~30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket -we name it a morph. Computational modelling suggests that morphs exist in both self-interstitial and vacancy-like forms, and are crucial for diffusion and defect dynamics in Ge, Si and probably many other crystalline solids.A vast array of crystalline material properties arises from the behavior of atomic-scale 'point'defects, yet these defects are poorly understood. Knowledge of simple point defects -single atoms added interstitially to, or missing from, an otherwise undisturbed lattice -is well established from quantum theoretical calculations and low-temperature experiments, but diffusion experiments hint that more complex entities may be involved at high temperatures relevant to industrial processing [1][2][3][4][5]. This Letter provides the first definitive evidence for these elusive complex defects and presents a specific physical model for their structure and diffusion. 2 Recent interest in Ge-based nano-electronics has led to basic studies on diffusion [5][6][7][8][9] and implantation defects [10,11] in crystalline Ge. Most dopants in Ge are found to diffuse by vacancy mechanisms, with activation energies below that of vacancy-mediated self-diffusion (≈ 3.1 eV), but boron diffusion is an exception with an activation energy of ≈ 4.65 eV [6,12].Experiments [5,[7][8][9] show that boron diffuses via the reaction B + I  BI, where 'B' represents substitutional boron, 'I' the self interstitial, and 'BI' a mobile dopant-interstitial complex. The energetics involved is illustrated in Figure 1.The reduction in free energy on forming BI enables it to migrate a mean projected distance λ before dissociating to B and I. The mean number of jumps before dissociation depends on the energy difference between migration and dissociation of BI and the diffusional entropies of I and BI. In general,A is the impurity (here, boron), X the point defect driving AX diffusion (here, I), a the capture radius for the forward reaction, f AX the diffusion correlation factor (~1), E AX , S AX , E self,X , S self,X the activation energies and entropies of impurity diffusion and self-diffusion via the species AX and Fig. 1. Schematic of total energy versus configuration for the reaction mediating B diffusion in Ge. Also shown are energies inferred from previous experiments. E BI and E self,I are the respective energies of BI and I at their migration saddle points, relative to that of substitutional B. Under RED conditions (dashed curve) the fitted values of E λ shift 0.025 eV in the negative direction. This could be accounted for by a reduction of 0.05 eV in the migration energy of BI under H irradiation. T (°C)4To test this idea we have repeated the experiments...

No abstract