Oscar D. Dubon

Low-temperature homoepitaxial growth on Si(111) through a Pb monolayer

A monolayer of Pb mediates high-quality homoepitaxial growth on Si (111) surfaces at temperatures where growth with other overlayer elements or on bare surfaces leads to amorphous or highly defective crystalline films. Nearly defect-free epitaxy proceeds for film thicknesses up to 1000 A with no sign that this is an upper limit. The minimum temperature for high-quality epitaxy depends on the substrate miscut. For a 0.2° miscut, the minimum temperature is 340 °C. Films grown on substrates miscut 2.3° towards [112] show good crystalline quality down to 310 °C.

Doping by metal-mediated epitaxy: Growth of As delta-doped Si through a Pb monolayer

In molecular-beam epitaxy a monolayer of Pb on the Si(111) surface induces single-crystal growth at temperatures well below those required for similar growth on a bare surface. We demonstrate that the suppression of dopant segregation at the lower temperatures attainable by Pb-mediated growth allows the incorporation of As donors at concentrations reaching a few atomic percent. When Pb and Si are deposited on an As-terminated Si(111) substrate at 350 °C, the Pb segregates to the surface without doping the Si film while the As is buried within nanometers of the substrate–film interface. The resulting concentration of electrically active As, 1.8×1021 cm−3, represents the highest concentration of As donors achieved by any delta-doping or thin-film deposition method.

Low-Temperature Si (111) Homoepitaxy and Doping Mediated by a Monolayer of Pb

The codeposition of Pb during Si (111) molecular beam homoepitaxy leads to high-quality crystalline films at temperatures for which films deposited on bare Si (111) are amorphous. Like other growth mediating elements-commonly called surfactants-Pb segregates to the film surface. Ion channeling and transmission electron microscopy reveal nearly defect-free epitaxy for a Pb coverage of one monolayer and temperatures as low as 310 °C. We have deposited films up to 1000 Å in thickness with no indication that this is an upper limit for high-quality epitaxy. However, a decrease in the Pb coverage during growth by only one tenth of a monolayer leads to highly defective films at these temperatures. The codeposition of both As and Pb results in a striking enhancement of the film quality as well. In this case, while the Pb again segregates to the film surface, the As is incorporated into the film with no apparent segregation. Lead-mediated Si epitaxy on As-terminated Si (111) produces high-quality films in which the As remains buried at the substrate-film interface. These results show Pb-mediated Si (111) homoepitaxy to be a promising strategy for the synthesis of layered structures having abrupt nanoscale dopant profiles.

Comment on “Low-temperature homoepitaxial growth on high-miscut Si(111) mediated by thin overlayers of Pb” [Appl. Phys. Lett. 75, 2954 (1999)]

Wei and Su recently published a letter in this journal on low-temperature homoepitaxial growth of silicon mediated by thin overlayers of Pb. A significant portion of the experimental work reported in their letter was performed in our laboratory ~depositions, Rutherford backscattering analysis, and electron microscopy!. Those experiments were poorly controlled and produced major inconsistencies that made the work unfit for publication. A subsequent, more systematic, investigation revealed the origin of these inconsistencies and established the precise conditions for high-quality growth. Our report on the latter work appeared in this journal several months prior to submission of Ref. 1. The purpose of this comment is to correct the factual errors reported in Ref. 1. Foremost among these is the claim that high-quality Si films can be grown on vicinal Si~111! for Pb coverages of 0.8–1.0 ML (1 ML57.83310 atoms cm) by first depositing the Pb overlayer and subsequently growing the Si film without continuously supplying the sample surface with additional Pb. As reported in Ref. 2, the growth of arbitrarily thick, high-quality Si films requires a Pb coverage of 1.0 60.1 ML which cannot be achieved under the conditions outlined in Ref. 1 because Pb desorbs from the surface of the growing film. We have observed evaporation of Pb from vicinal surfaces including those from the same substrate material used by Wei while working in our laboratory. To maintain a constant Pb coverage at substrate temperatures of 280 °C or higher, one must deposit Pb during the growth of the Si film. For example, at 295 °C a Pb flux of 0.12 ML min is needed to maintain a Pb coverage of 1.0 ML on the sample surface. The Pb coverages reported in Ref. 1 were measured at the end of the deposition of Si. By wrongly assuming that the final Pb coverages they measured represent the actual coverages during growth, Wei and Su have overlooked the effect of Pb desorption on their results. The misinterpretation of ion channeling spectra in Ref. 1 undermines the authors’ claim that high-quality films can be