Junctions for Deep Sub-100 NM MOS: How Far will Ion Implantation Take Us?

Why Te Doping Can Break the Traditional n-Type Doping Limit of Silicon

We report a theoretical investigation of the impact of hyperdoping with chalcogens (Se and Te) and pnictogens (P and As) on free-carrier concentrations of Si, employing density functional theory calculations. Our results illustrate that isolated substitutional chalcogens in moderately doped Si function as deep donors that are difficult to ionize at room temperature, unlike isolated substitutional pnictogens. The pairing of substitutional defects is found to be energetically favorable for every dopant element, implying that the concentration of substitutional pairs can be significant in hyperdoped Si. However, chalcogen-substitutional pairs have the capability to increase the carrier concentration, whereas pnictogen-substitutional pairs serve only as compensators for n-type doping. By evaluating the carrier concentrations for Te- and P-hyperdoped Si, we demonstrate the importance of substitutional Te pairs of Te-hyperdoped Si in breaking the traditional n-type doping limit observed in pnictogen-hyperdoped Si. Our work elucidates the underlying microscopic mechanisms that give rise to substantial carrier densities in chalcogen-hyperdoped Si, which will pave the way for the development of high-performance silicon-based devices.

Depth-dependent imaging of individual dopant atoms in silicon

We have achieved atomic-resolution imaging of single dopant atoms buried inside a crystal, a key goal for microelectronic device characterization, in Sb-doped Si using annular dark-field scanning transmission electron microscopy. In an amorphous material, the dopant signal is largely independent of depth, but in a crystal, channeling of the electron probe causes the image intensity of the atomic columns to vary with the depths of the dopants in each column. We can determine the average dopant concentration in small volumes, and, at low concentrations, the depth in a column of a single dopant. Dopant atoms can also serve as tags for experimental measurements of probe spreading and channeling. Both effects remain crucial even with spherical aberration correction of the probe. Parameters are given for a corrected Bloch-wave model that qualitatively describes the channeling at thicknesses 20 nm, but does not account for probe spreading at larger thicknesses. In thick samples, column-to-column coupling of the probe can make a dopant atom appear in the image in a different atom column than its physical position.

Imaging individual atoms inside crystals with ADF-STEM

The quantitative imaging of individual impurity atoms in annular dark-field scanning transmission electron microscopy (ADF-STEM) requires a clear theoretical understanding of ADF-STEM lattice imaging, nearly ideal thin samples, and careful attention to image processing. We explore the theory using plane-wave multislice simulations that show the image intensity of substitutional impurities is depth-dependent due to probe channeling, but the intensity of interstitial impurities need not be. The images are only directly interpretable in thin samples. For this reason, we describe a wedge mechanical polishing technique to produce samples less than <50 A thick, with low surface roughness and no amorphous surface oxide. This allows us to image individual dopants as they exist within a bulk-like silicon environment. We also discuss the image analysis techniques used to extract maximum quantitative information from the images. Based on this information, we conclude that the primary nanocluster defect responsible for the electrical inactivity of Sb in Si at high concentration consists of only two atoms.

Atomic-scale imaging of individual dopant atoms and clusters in highly n-type bulk Si

As silicon-based transistors in integrated circuits grow smaller, the concentration of charge carriers generated by the introduction of impurity dopant atoms must steadily increase. Current technology, however, is rapidly approaching the limit at which introducing additional dopant atoms ceases to generate additional charge carriers because the dopants form electrically inactive clusters. Using annular dark-field scanning transmission electron microscopy, we report the direct, atomic-resolution observation of individual antimony (Sb) dopant atoms in crystalline Si, and identify the Sb clusters responsible for the saturation of charge carriers. The size, structure, and distribution of these clusters are determined with a Sb-atom detection efficiency of almost 100%. Although single heavy atoms on surfaces or supporting films have been visualized previously, our technique permits the imaging of individual dopants and clusters as they exist within actual devices.