Bi surfactant mediated growth for fabrication of Si,Ge nanostructures and investigation of Si,Ge intermixing by STM

Heteroepitaxial growth of Ge on Si is a prototypical system for studying strained Stranski-Krastanow growth.The technological drawback is that it turns out to be difficult to grow Ge on Si in a controlled layer-by-layer manner. After a few smooth layers have been grown epitaxially, the growth then proceeds by the formation of islands. This is known as the Stranski-Krastanov growth mode. It has been found, however, that deposition of a single monolayer of As, Sb or Bi onto the silicon surface before growth of Ge begins, can overcome the problem. The Ge then grows epitaxially in a two-dimensional mode, while the surfactant (As, Sb or Bi) atoms apparently rise up through the growing Ge layers and segregate on the top of the film. This is called surfactant mediated growth (SME). Surfactant mediated growth of Ge on Si(111) has been well studied in the last decade. Initial investigations used As and Sb as surfactants. In fact, Ge p-channel MOSFETs have already been fabricated using the Sb surfactant. However for the Bi surfactant, the studies were not so thorough. In the thesis work presented here, we show that Bi is more promising surfactantmaterial than Sb. We demonstrate that by using Bi as a terminating layer on Ge/Si surface, it is possible to distinguish between Si and Ge in Scanning tunnelling microscope (STM). Something which was very difficult in the past. Using this property, it is possible to create Ge/Si nanostructures in a controlled manner. Moreover, it is also possible to study Ge/Si intermixing in surface layers in some detail. Any attempt to utilize surfactant mediated growth must be preceded by a thorough study of its effect on the the system being investigated. Thus, the third chapter of this thesis deals with an extensive study of the Bi surfactant mediated growth of Ge on Si(111) surface as a function of Ge coverage. The growth is investigated from the single bilayer Ge coverage till the Ge coverage of about 15 BL when the further Ge deposition leads to two-dimensional growth. In the fourth chapter, the unique property of Bi terminating layer on Ge/Si surface to result in an STM height contrast between Si and Ge is explained with possible explanations given for the reason of this apparent height contrast. The controlled fabrication of Ge/Si nanostructures such as nanowires and nanorings is demonstrated. A study on Ge-Si diffusion in the surface layers by a direct method such as STM was impossible previously because of the similar electronic structure of Ge and Si. Since with the Bi terminating surface layer, one is able to distinguish between Ge and Si, the study of intermixing between them is also possible using STM. This method to distinguish between Si and Ge allows one to study intermixing on the nanoscale and to identify the fundamental diffusion processes giving rise to the intermixing. In Chapter 5 we discuss how this could prove useful especially as one could get a local probe over a very narrow Ge -Si interface. On one hand it is possible to study the displacement of the Si and Ge atoms when factors like temperature and deposition rate are varied during growth. On the other hand, a post growth study like annealing already grown Ge-Si wires over a period of time could also be performed. A new model is proposed to estimate change in the Ge concentration in the surface layer with time. The values of the activation energies of Ge/Si exchange and Si/Ge exchange are estimated by fitting the experimental data with the model. During step flow growth of Ge wires along Si step edges, the step speed is seen to strongly affect the Ge/Si intermixing. Ge wires grown at faster step speeds appear higher than Ge wires grown at slower step speeds. In chapter 5, the Ge/Si intermixing has been studied on a surface having 1 ML Bi reconstruction. In Chapter 6, we discuss the Ge-Si intermixing on surfaces with different reconstruction, such as the 1/3 ML Bi reconstruction and the Si reconstruction. The vertical Ge-Si intermixing is more in the surface with the Si reconstruction as compared to the surface with 1/3 ML Bi reconstruction. This is due to the reason that the Bi layer inhibits Ge atoms from exchanging with Si substrate atoms during Ge growth. In the last chapter, an attempt has been made to elucidate the need for utilizing two dimensional Bi surfactant Ge/Si surfaces for industrial applications as transistors by demonstrating the quick, efficient and complete removal of Bi surfactant monolayer from thick Ge layers by ion beam sputtering without damaging the underlying Ge/Si layer.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2007

Author dc:creator

• Paul, Neelima

Contributors dc:contributor

• Voigtländer, Bert

• info:eu-repo/classification/ddc/530
• Rastertunnelmikroskop
• Grenzflächenaktiver Stoff
• Mischen
• Wachstum
• Physik
• Si/Ge growth
• nanostructure
• intermixing
• STM
• Bi surfactant

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124101%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62537