Olof C. Hellman

New SiGe dielectrics grown at room temperature by low‐energy ion beam oxidation and nitridation

New dielectric materials based on SiGe have been formed at room temperature by direct ion beam oxidation and nitridation. Si0.8Ge0.2 layers were deposited by molecular beam epitaxy on Si(100) and then exposed to a low‐energy ion beam of 18O+2 to form oxides and 14N+2 to form nitrides. The ion energies investigated ranged from 100 eV to 1 keV. Thin films of SiGe oxide and SiGe nitride were formed at all energies used as evidenced by in situ x‐ray photoelectron spectroscopy analysis. They were found to be insulating by ex situ scanning electron microscopy observations. During the ion beam processing, the Ge content of the alloy layer decreases, due to preferential sputtering of Ge and the Ge compounds. However, as the ion energy is decreased, the concentration of Ge in the alloy remains closer to the original content. The thermal stability of these new SiGe dielectrics was also assessed up to 500 °C.

A quantitative model of point defect diffusivity and recombination in ion beam deposition and combined ion and molecular deposition

We are investigating the use of low energy ions (<1 keV) in low temperature thin film growth techniques, ion beam deposition (IBD) and combined ion and molecular deposition (CIMD). In IBD, a thin film is directly grown from a low energy ion beam as the only source of material, while in CIMD, low temperature growth of thin films is achieved by depositing materials simultaneously from a low energy ion beam and one or several molecular beams. A simple model of the IBD process has been developed and accounts for atomic collisions and thermal diffusion during thin film growth. Computer simulation of IBD of Si on Si have been conducted as a function of ion energy to support more quantitatively this physical description of IBD. The results show that the IBD growth mechanism is mediated by the fast diffusing interstitials and establish a low energy limit to achieve epitaxial growth by IBD that depends on the point defect diffusivities. The defect generation has to be confined in the subsurface region in order to ...

Role of ion energy in ion beam oxidation of semiconductors : experimental study and model

Ion beam oxidation (IBO) is a low temperature growth technique where a directional low energy (≤1 keV) ion beam introduces the oxygen into the substrate and athermally activates the chemical reaction leading to the oxide growth. In this work, IBO of Si, Ge, Si1−xGex was investigated experimentally as a function of ion energy from 100 eV to 1 keV. The results show a strong dependence of the materials properties such as phase formation, stoichiometry, and thickness upon the ion energy. To investigate the kinetics of IBO and to account for the observed relationship between ion energy and films properties, three models were successively developed taking progressively into account: (1) ion implantation and sputtering (model IS), (2) replacement and relocation events, i.e., ion beam mixing effects (model ISR) and (3) oxygen diffusion (model ISRD). The simulation results show that the model IS based only on implantation and sputtering cannot explain the oxide thickness dependence upon ion energy observed experim...

Comparative study of low energy ion beam oxidation of Si(100), Ge/Si(100) and Si1−xGex/Si(100)

Abstract Low energy ion beam oxidation (IBO) of Si(100) and germanium and Si1−xGex grown by molecular beam epitaxy on Si(100) was investigated at room temperature using 18O2+ ion beams with energies E ion ranging from 100 eV to 1 keV. The dependence of phase formation and film properties upon ion energy was established. In the case of silicon, thin films of stoichiometric SiO2 are formed at each energy studied and their thickness increases from 39 to 70 A with increasing E ion . Insulating GeO2 can only be formed for E ion ⩽ 200 eV . Under IBO, both silicon and germanium in Si0.8 Ge0.2 are fully oxidized. At each energy investigated, thin SiGe dioxide films are formed and found to be insulating by scanning electron microscopy. This contrasts with IBO of elemental germanium and shows that the presence of silicon surrounding the germanium in the SiGe alloy enhances the oxidation of germanium under IBO.

Microstructure and stoichiometry dependence of ion beam nitrides as a function of energy and temperature: A comparative study between Si and SiGe

The microstructure and stoichiometry of nitrides formed by direct low‐energy ion beam nitridation has been investigated as a function of ion energy and substrate temperature for Si(100) and SiGe/Si(100) films. Cross‐sectional transmission electron microscopy, Rutherford backscattering spectroscopy combined with ion channeling and in situ x‐ray photoelectron spectroscopy were used. It was established that a substrate temperature of 700 K produces a homogeneous amorphous nitride layer, whereas lower substrate temperatures decrease the incorporation of nitrogen in the film, while causing the formation of a nitrogen‐poor amorphous layer beneath the nitride film. The N‐to‐Si or N‐to‐(Si+Ge) atomic ratio is found be close to 1.33 at 1 keV and decreases with ion energy. Effects of chemically enhanced physical sputtering of germanium are observed.

Semiconductor‐based heterostructure formation using low energy ion beams: Ion beam deposition (IBD) & combined ion and molecular beam deposition (CIMD)

In our previous work, we investigated the use of ion beam deposition (IBD) to grow epitaxial films at temperatures lower than those used in thermal processing (less than 500°C). Presently, we have applied IBD to the growth of dense (6.4×1022 atom/cm3) silicon dioxide thin films at 400°C. Through these experiments we have found several clues to the microscopic processes leading to the formation of thin film phases by low energy ions. Using Monte‐Carlo simulations, we have found that low energy collision cascades in silicon have unique features such as a high probability of relocation events that refill vacancies as they are created. Our results show that the combination of a low defect density in low energy collision cascades with the high mobility of interstitials in covalent materials can be used to athermally generate atomic displacements tha can lead to ordering. These displacements can lead to epitaxial ordering at substrate temperatures below the minimum temperature necessary for molecular beam epita...

Structure and properties of silicon nitride and SixGe1-x nitride prepared by direct low energy ion beam nitridation

Abstract Thin films of silicon nitride, germanium nitride and silicon germanium nitride were formed using direct low energy ion beam nitridation. In this process a monoenergetic nitrogen ion beam directly impinges on the material to be nitrided, in the present work Si(100), Ge/Si(100) and Si 0.89 Ge 0.11 /Si(100). The energies investigated ranged from 100 eV to 1 keV. Germanium and SiGe alloy were grown on Si(100) using molecular beam epitaxy. The kinetic energy of the ion beam introduces activated nitrogen species athermally into the substrate, and allows the formation of nitrides at low temperatures (20–420°C). Properties of the films such as stress, stoichiometry and microstructure are found to depend strongly on ion energy and substrate temperature. Film stress is highly compressive for samples deposited at room temperature, but decreases with temperature and becomes tensile at 420 °C. Film thicknesses, as measured by cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry, were found to be much greater than the projected range of the ions. The creation of an amorphous layer beneath the amorphous nitride films is observed, and is found to be a strong function of ion energy, temperature and nitrogen ion dose.

Buried reconstruction inhibition of solid phase epitaxy of Ge on Si (111)

We study the epitaxial crystallization of a thin film of amorphous Ge deposited at room temperature on Si (111). Silicon surface features which are buried beneath the Ge film are seen to affect the rate of crystallization. In particular, solid phase growth is observed to be enhanced at surface steps and defects in the surface reconstruction. We demonstrate that control of Ge crystallization morphology is possible through manipulation of Si surface structure.

Role of point defect diffusion and recombination in low-temperature growth of semiconductor heterostructures using low-energy ion beams

We are investigating dircctdeposition oflow energy (10 - 500 eV) ions to grow elemental and compound thin films at low temperature (R.T. - 700 K). We have developed a model to describe layer growth by Ion Beam Deposition (IBD) that takes into account not only atomic collision processes but also thermally-activated diffusion and recombination ofpoint defects during ion bombardment. Numerical simulations of our experimental conditions using this model have given us new insight into growth mechanisms during IBD. More specifically, we show in this work that the IBD growth rate is not limited by the sputtering yield only, but also by the recombination rate ofpoint defects at the surface; this rate depends on the depth distribution of the defects, which is deternined by the ion energy.