Electron Microscopy of As Supersaturated Silicon

Abstract

Silicon specimens have been implanted with 1 • 1016 and 5 • 1016 As + ions/cm 2 and annealed with a pulsed ruby laser, thus giving rise to a supersaturated solution. Subsequent heat-treatments carried out in an inert ambient at either 450 ~ or 900~ led to a strong deactivation of the dopant. The physical nature of the inactive arsenic has been investigated by transmission electron microscopy (TEM), both in the weak beam and the high resolution imaging modes. For both doses and annealing temperatures, the presence of precipitates, having a diameter ranging from 1.5 to 3 nm, has been observed. Other defects, such as dislocations, perfect and faulted loops, and rod-like defects have also been detected in our TEM investigations. Despite the high density of the detected precipitates, the corresponding amount of arsenic atoms, as deduced by assuming an SiAs composition, can account for only a fraction of the inactive dopant. A possible explanation is that particles in the subnanometer range are invisible even in the high resolution images. This hypothesis is supported by the absence of contrast found in corresponding simulated high resolution images.Arsenic, due to the high solubility and low diffusivity, is the silicon n-type dopant more commonly used in VLSI circuits, both for the source and drain of MOS transistors and the emitter of bipolar transistors.Several works performed in recent years have investigated the physical nature of electrically inactive As, which is present in silicon at high doping levels. Most of these authors suggest the formation of complex point defects or clusters in thermal equilibrium with the As in solution (1-6). The existence of arsenic clusters, however, is presently not more than a hypothesis. Chu and Masters (7), from-angle-scanned channeling studies, reported results that are compatible with cluster formation, but there is not, up to now, direct evidence of the arsenic clusters.Recent work (8, 9) performed on ion-implanted and laser-annealed silicon, doped with As in a wide range of compositions, provided evidence that the electrically active concentration depends only on temperature, a finding leading to the conclusion that a two-phase equilibrium takes place. The formation of a second phase was also indicated by the isochronal heating behavior, showing a reverse annealing typical of a precipitation process (8). Precipitates were, in fact, detected by small angle x-ray scattering (SAXS), a technique that is suitable because of the noticeably different atomic numbers of Si and As. These determinations provided information on the size distribution and shape of these precipitates that was found to be in agreement with the classical theory of nucleation, which predicts a decrease in particle density and an increase in particle size as the supersaturation decreases (9). Moreover, most of these particles should be coherent with the silicon matrix, as expected from the RBS experiments performed in channeling conditions (7, 10), showing that the off-axis concentration is markedly lower than...

Point defects and dopant diffusion in silicon

“…Two more pieces of experimental data have recently been reported which have attempted to reveal the atomistic identity of electrically inactive As. One is the detection by transmission electron microscopy of very small (15 -30 A) As precipitates (Armigliato, Nobili, Solmi, Bourret, and Werner, 1986), although in concentrations too low to account for all of the electrically inactive As. The other is the use of extended x-ray-absorption finestructure spectroscopy (EXAFS) of heavily As-doped silicon (Erbil, Cargill, and Boehme, 1985;Erbil, Weber, Cargill, and Boehme, 1986), indicating localized regions of Si and As on alternating lattice sites, similar to the arrangement of Ga and As atoms in GaAs.…”

Contrast of small SiX particles in silicon by computed HREM images

“…The formation of a second phase upon heating an S i p or SiAs supersaturated solid solution has been demonstrated [1 to 31. I n case of arsenic implanted silicon, we have recently observed by weak-beam E M and HREM the presence of precipitates having a diameter ranging from 1.5 u p t o 3 nm, but their density was not sufficiently large to account for the amount of inactive As, as deduced from electrical me asurements [4]. To solve this discrepancy, we have determined a visibility criterion for small SiAs particles, according t o which in areas thicker than 10 nm these precipitates are invisible [5].…”

Oxygen Precipitation in Highly Doped Silicon Substrates

“…In the literature, the formation of arsenic clusters/precipitates has been studied mainly in silicon wafers implanted with arsenic at levels higher than the solubility limit, up to 4 × 10 21 cm −3 , where they are considered responsible for a fraction of electrically inactive arsenic. [19][20][21][22] In the case of as-grown silicon, no direct evidence of arsenic clusters was found by examining samples with TEM, 23 however their presence can be considered possible on the basis of sectional X-ray topography results in silicon samples doped with arsenic to a concentration of 4 × 10 19 cm-3. 24 In our case, arsenic precipitates may be formed by agglomeration of minority interstitial arsenic atoms As i that coexists with the majority substitutional species As s .…”