Heyward G. Robinson

Modeling uphill diffusion of Mg implants in GaAs using suprem‐iv

Transient, uphill diffusion of implanted Mg in GaAs during a 900 °C anneal is simulated using suprem‐iv. The diffusion is believed to occur via the substitutional‐interstitial‐diffusion (SID) mechanism, with excess interstitials and vacancies produced by the implantation process causing this abnormal diffusion behavior. The SID mechanism is shown to be equivalent, in terms of the governing equations, to the interstitial‐dopant pair diffusion model used in suprem‐iv. This allows one to use suprem‐iv, a silicon process simulator that includes dopant–point‐defect interactions, to model uphill diffusion once the appropriate diffusivity and defect parameters are included. The profiles of excess interstitials and vacancies produced by the implantation process are obtained from Boltzmann transport equation calculations. The transient uphill diffusion phenomenon can be well simulated using the diffusion model in suprem‐iv, with the dopant diffusing from the region of excess interstitials toward the surface and the region of excess vacancies. Once the defect concentrations return to their steady‐state levels, either by diffusion, recombination, or capture by sinks, the normal concentration‐dependent diffusion into the substrate occurs.

Diffusion of implanted beryllium in n‐ and p‐type GaAs

The diffusion of ion‐implanted Be in GaAs is studied by comparing the diffusion of implanted Be in undoped GaAs and in GaAs uniformly doped with Zn or Si. The Si‐doped sample exhibits much less Be diffusion compared to the undoped case, while the Zn‐doped sample shows much more Be diffusion. The diffusion in the doped substrate cases can be simulated with a constant Be diffusivity, as opposed to a concentration‐dependent diffusivity in the undoped case. The results are consistent with the substitutional‐interstitial diffusion mechanism, which predicts a diffusivity that is dependent on the net hole concentration.

Damage‐induced uphill diffusion of implanted Mg and Be in GaAs

The redistribution of Be and Mg implants upon post‐implant annealing is studied in order to evaluate the influence of implant damage on the diffusion process. Rapid uphill diffusion is observed in the peak of Mg implants in GaAs, whereas Be implants show only uniform, concentration‐dependent diffusion. This behavior is explained by the substitutional‐interstitial‐diffusion mechanism and computer simulations of damage‐produced point defects. In the region of uphill diffusion, the dopants diffuse from areas of excess interstitials toward areas of excess vacancies. A critical concentration of point defects is necessary to initiate uphill diffusion. Uphill diffusion can be induced in Be implants by co‐implanting with a heavier element such as Ar.

Extended defects of ion‐implanted GaAs

Ion‐implantation‐induced extended defect formation and annealing processes have been studied in GaAs. Mg, Be, Si, Ge, and Sn ions were implanted between 40 and 185 keV over the dose range of 1×1013–1×1015/cm2. Furnace annealing after capping with Si3N4 was performed between 700 and 900 °C for times between 5 min and 10 h. Plan‐view and cross‐sectional transmission electron microscopy results were correlated with secondary‐ion‐mass spectroscopy profiles. The results indicate subthreshold (type‐I) defect formation occurs at a dose of 1×1014/cm2 for high‐energy, light (Mg, Be) ions but not for heavier ions (Si, Ge, Sn) at shallower projected ranges (<500 A). Si and Ge implants at a dose of 1×1015/cm2 both show extended defect formation upon annealing that is believed to be precipitation related (type‐V defects). For Si implants, these dislocation loops are eliminated after 10 h at 900 °C. Upon annealing 1×1015/cm2 Sn implants, unusual precipitate motion both toward the surface and into the crystal was observ...

Diffusion of implanted beryllium in gallium arsenide as a function of anneal temperature and dose

The diffusion of implanted Be in GaAs was studied by annealing samples of GaAs implanted with low and high doses of Be. The high‐dose (1×1014 cm−2) samples show an increase in diffusion with increasing anneal temperature from 700 to 900 °C. However, the low‐dose (2×1013 cm−2) samples show a decrease in diffusion as the temperature is increased. The temperature dependence of the low‐dose case can be reversed by coimplantation of 1×1014 cm−2 boron. This behavior is explained in terms of the substitutional‐interstitial diffusion mechanism and the relative concentrations of Be interstitials and Be substitutionals for the different cases.

Studies of point defect/dislocation loop interaction processes in silicon

Abstract Studies of the interactions between point defects introduced during semiconductor processing and dislocation loops are reviewed. The processing steps studied include oxidation, ion implantation and silicidation. By using doped marker layers it is shown that the interaction kinetics between the point defects and the dislocation loops is strongly diffusion limited. It is also shown that these dislocation loops can be used to quantitatively measure the flux of point defects introduced. This has provided a novel means of better understanding the process of defect injection as well as the effect these dislocations have on the excess point defect concentrations.

ION DEPENDENT INTERSTITIAL GENERATION OF IMPLANTED MERCURY CADMIUM TELLURIDE

The creation of Hg interstitials during ion implantation of mercury cadmium telluride is found to strongly depend on the preferred lattice position of the element implanted. Elements that substitute onto the cation sublattice create significantly more interstitials than elements that sit interstitially or on the anion sublattice. In particular, implants of column II elements Mg and Zn produced much larger interstitial concentrations than implants of column VI elements S and Se. Implants of B, which resides mostly as an interstitial, produced Hg interstitial concentrations intermediate between the column II and column VI ions. Recoils from implant damage also contributed to Hg interstitial formation in heavier mass implants (Zn and Se), but appear to have negligible influence on interstitial generation in implants of lighter ions.

Hole‐dependent diffusion of implanted Mg in GaAs

Magnesium implants in GaAs exhibit two types of diffusion during annealing: uphill diffusion in the peak of the implant and concentration‐dependent diffusion into the bulk. The uphill diffusion predominates at short times and low temperatures, while the concentration‐dependent diffusion is dominant at long times and high temperatures. By studying implants that were annealed at temperatures where no uphill diffusion occurs, diffused profiles could be modeled and an expression for the Mg diffusivity obtained. The activation energy for this process is 1.77 eV. Results from Fermi level experiments show that the diffusivity is hole dependent rather than concentration dependent. The hole‐dependent exponent is unity for Mg implanted into semi‐insulating substrates, but may change to two at high hole concentrations.

Modeling co-implanted silicon and beryllium in gallium arsenide

Abstract The implantation and diffusion of Si and Be are modeled using SUPREM 3.5, a GaAs process simulator. According to the new model used for Be diffusion with a hole-dependent effective diffusivity, the presence of the Si should decrease the Be diffusion in the overlap region due to a Fermi-level effect, and the experimental results confirm this. The damage created by the Si implantation is also seen to affect the Be diffusion, as confirmed by Ar co-implantation, but the relatively simple diffusion models in SUPREM 3.5 do not currently model this effect. The Be segregation coefficient is seen to be a function of implant energy. The diffusion of Si, which is negligible to begin with, is not affected by the presence of Be. The implantation profiles of both Be and Si are not appreciably affected by the implantation damage caused by the other dopant and are well modeled by Pearson IV parameters.

Ion Implantation Related Defects in GaAs

Extended defect formation is studied in ion implanted GaAs. A number of different species including Si{sup +}, Al{sup +}, Mg{sup +}, Ge{sup +}, As{sup +}, and Sn{sup +} have been investigated. Cross-sectional TEM studies have been done comparing the as-implanted structure (amorphous or crystalline) with the final defect location and morphology. The defects are identified by the same classification scheme used for implanted and annealed silicon. It is found that the threshold dose for type 1 defect formation is very sensitive to the implant energy for heavier ion masses. Type 2, 3, and 4 defects are unstable at annealing temperatures below 900{degrees}C. Type 5 defects are of a loop morphology for Si{sup +} and Ge{sup +} implants. The source of the interstitials may be a kickout process as the implanted species moves onto substitutional sites. Type 5 defects for Sn implants appear as precipitates which at the annealing temperature appear to be migrating in the liquid phase. Upon cooling the Sn precipitates, in many cases, solidify as grey ({alpha}) Sn.