Semiconductor structures and devices including strained material layers having impurity-free zones, and methods for fabricating same. Certain regions of the strained material layers are kept free of impurities that can interdiffuse from adjacent portions of the semiconductor. When impurities are present in certain regions of the strained material layers, there is degradation in device performance. By employing semiconductor structures and devices (e.g., field effect transistors or “FETs”) that have the features described, or are fabricated in accordance with the steps described, device operation is enhanced.

Semiconductor structures employing strained material layers with defined impurity gradients and methods for fabricating same

Circuits for processing radio frequency (“RF”) and microwave signals are fabricated using field effect transistors (“FETs”) that have one or more strained channel layers disposed on one or more planarized substrate layers. FETs having such a configuration exhibit improved values for, for example, transconductance and noise figure. RF circuits such as, for example, voltage controlled oscillators (“VCOs”), low noise amplifiers (“LNAs”), and phase locked loops (“PLLs”) built using these FETs also exhibit enhanced performance.

RF Circuits Including Transistors Having Strained Material Layers

Ultrathin silicon dioxide films have been prepared on a Mo(110) substrate at a relatively low temperature (<100 °C) by evaporating silicon in an oxygen background. The SiO2 films were characterized by Auger electron, electron energy loss, and IR reflection absorption spectroscopies. At low substrate temperatures, the SiO2 films are amorphous, consisting of networks of [SiO4] tetrahedra units with limited size. Upon post‐annealing, the small networks of [SiO4] fuse together, and become more ordered. In turn, the electronic properties of the SiO2 films approach those of vitreous silica.

New approach to the preparation of ultrathin silicon dioxide films at low temperatures

The self‐limiting oxidation of silicon by a low‐energy ion beam (40–120 eV) is described by an implantation‐sputtering model. The thin oxide (40–50 A) is grown primarily by a surface implantation process which leads to a logarithmic increase of oxide thickness with dose in the absence of sputtering. At higher energies (100 eV), the sputtering of the growing film leads to net self‐limiting growth. The model, which does not include adjustable parameters, is used to describe the dose evolution of the oxide growth as a function of beam energy. The implantation‐sputtering model is found to be in excellent agreement with experimental observations.

Oxidation of silicon by a low‐energy ion beam: Experiment and model

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.

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

Bombardment of silicon (100) surfaces at room temperature by an oxygen‐containing low‐energy ion beam is studied as an alternative to thermal oxidation to produce ultrathin oxide films. A self‐limiting oxide thickness of about 50 A is obtained by using ions with energy 100 eV or lower. Auger electron spectroscopy depth profiles of an ion‐beam grown oxide and a thermally grown oxide show very similar composition. Grazing angle x‐ray photoelectron spectroscopy indicates the presence of lower oxides of silicon near the surface. The capacitance–voltage characteristics of ion‐beam grown oxides compare favorably with those of thermally grown oxides.

C. F. Yu

Papers

Characterization of ultrathin SiO2 films formed by direct low‐energy ion‐beam oxidation

Direct formation of dielectric thin films on silicon by low energy ion beam bombardment