Free Standing Silicon as a Compliant Substrate for SiGe

We show that SiGe grown on free-standing silicon is elastically relaxed. The free-standing Si structure consists of a ∼30 nm-thick, 5 μm-square silicon slab supported by a SiO2 pedestal at a single contact point at the center of the square (the cross-section resembles a mushroom). A matrix of free-standing structures was made by patterning a bonded silicon-on-insulator (SOI) wafer and undercutting the SiO2 to form the pedestal. Un-patterned areas of the SOI wafer and the exposed bulk Si substrate were included as reference regions. A UHVCVD Si0.8Ge0.2 film, about 200 nm-thick, was grown epitaxially on both sides of the free-standing silicon and the surrounding exposed bulk Si. The SiGe was also grown on the un-patterned SOI and bulk substrate control areas. The SiGe film grown on both SOI and bulk silicon was found to be fully strained. In contrast, the SiGe layer grown on free-standing silicon is ∼89% strain-relaxed, and the free-standing silicon film was measured to be under tensile strain. Since the same lattice mismatch was found between the SiGe layer and the Si on the free-standing silicon and on the SOI and bulk Si control regions, we conclude that the strain relaxation of the SiGe film on freestanding Si is elastic with the strain accommodated entirely by the free-standing silicon film under tensile strain. This was further confirmed by AFM measurements. The SiGe film on the control regions showed a very smooth SiGe surface with only a few surface steps originating from misfit dislocations at the SiGe/Si interface. No surface steps from misfit dislocations were observed on the surface of the SiGe film on free-standing Si. These results show that freestanding silicon serves as an ideal compliant substrate for SiGe.

Charge Trapping by Deep Donors in Si-Doped AlxGa1-xAS

The kinetics of charge capture by deep donors in AlxGa1-xAs have been measured. The time dependence indicates that a single energy cannot be used to describe the trap. A model assuming thermally activated capture into a resonance in the conduction band with a range of energies gives excellent fits to the data and provides a measure of the energy range for the trap. This model is consistent with the large lattice relaxation model for DX centers. The increase of the activation energy for capture as the Al mole fraction is decreased contradicts the model which attributes the decay of the persistent photoconductivity to tunneling through the heterojunction barrier in modulationdoped structures.

Observation of columnar microstructure in step-graded Si1-xGex/Si films using high-resolution X-ray microdiffraction

Columnar microstructure in step-graded Si(1-x)Ge(x)/Si(001) structures with low threading dislocation densities has been determined using high angular resolution (approximately 0.005 degrees ) x-ray microdiffraction. X-ray rocking curves of a 3-microm-thick strain-relaxed Si(0.83)Ge(0.17) film show many sharp peaks and can be simulated with a model having a set of Gaussians having narrow angular widths (0.013 degrees -0.02 degrees ) and local ranges of tilt angles varying from 0.05 degrees to 0.2 degrees. These peaks correspond to individual tilted rectangular columnar micrograins having similar (001) lattice spacings and average areas of 0.8 to 2.0 microm(2).

SiGe Technology: Heteroepitaxy and High-Speed Microelectronics

▪ Abstract  We review recent advances in our understanding of the epitaxial growth and properties of SiGe/Si heterostructures for applications in high-speed field-effect transistors. Improvements in computing power and experimental methods have led to new calculations and experiments that reveal the complexity of 60° misfit dislocations and their interactions, which ultimately determine the characteristics of strain-relaxed SiGe films serving as a buffer layer for strained-layer devices. Novel measurements of the microstructure of relaxed SiGe films are discussed. We also present recent work on the epitaxial growth of SiGe/Si heterostructures by ultra-high-vacuum chemical vapor deposition. This growth method not only provides device quality buffer layers, but abrupt, high-concentration phosphorous-doping profiles, and high-mobility S0.20Ge0.80/Ge composite hole channels have also been grown. These achievements enabled the fabrication of outstanding n- and p-channel modulation-doped field-effect transistors that show enormous promise for a variety of applications.