K. Eberl

Growth and strain compensation effects in the ternary Si1−x−yGexCy alloy system

Strain compensation is an important aspect of heterostructure engineering. In this letter, we discuss the synthesis of pseudomorphic Si1−yCy and Si1−x−yGexCy alloy layers on a silicon (100) substrate by molecular beam epitaxy using solid sources and the controlled strain compensation that results from the introduction of the ternary system. The introduction of C into substitutional sites in the crystal lattice is kinetically stabilized by low‐temperature growth conditions (400–550 °C) against thermodynamically favored silicon‐carbide phases. The lattice constant in Ge is about 4% larger than in Si, whereas in diamond it is 52% smaller. Consequently, the compressive strain caused by 10.8% Ge in a pseudomorphic Si1−xGex alloy can be compensated by adding about 1% carbon into substitutional lattice sites of the film assuming Vegard’s law of linear change of the lattice constant in the alloy as a function of the composition. Using x‐ray diffraction, we observe a partial strain compensation in Si0.75−yGe0.25Cy...

Synthesis of Si1−yCy alloys by molecular beam epitaxy

We have synthesized pseudomorphic Si1−yCy (y≤0.05) alloys and strained layer superlattices on silicon by molecular beam epitaxy using solid sources for carbon and silicon. The introduction of C into substitutional sites in the silicon lattice is kinetically stabilized by low‐temperature growth conditions (500–600 °C) and relatively high Si fluxes, against extremely low C solubility (10−6 at 1420 °C) and the thermodynamically favored silicon carbide phases. Higher temperature growth leads to an islanded morphology. At lower temperatures, disruption of epitaxy occurs via the formation of highly twinned layers or even amorphous growth. The temperature window for alloy growth is reduced as the C concentration is increased. X‐ray diffraction, transmission electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy confirm the growth of pseudomorphic, tetragonally strained alloy layers with no detectable silicon carbide precipitation. These alloy layers allow for the engineering of Si‐based lat...

Thermal stability of Si1−xCx/Si strained layer superlattices

The thermal stability of epitaxial silicon‐carbon alloys grown by molecular beam epitaxy on (001) silicon was investigated using high resolution x‐ray diffraction, transmission electron microscopy, and secondary ion mass spectroscopy measurements. Different superlattices, with alloy compositions of Si0.997C0.003, Si0.992C0.008, and Si0.985C0.015, all nominally 6 nm thick, with 24 nm Si spacer layers were employed. Annealing studies determined that there are different pathways to strain relaxation in this material system. At annealing temperatures of 900 °C and below, the structures relax only by interdiffusion, indicating that these layers are stable during typical device processing steps. At temperatures of 1000 °C and above, SiC precipitation dominates with enhanced precipitation in the Si1−xCx layers with the highest initial carbon content.

Raman spectroscopy of CySi1−y alloys grown by molecular beam epitaxy

Raman spectroscopy has been used to observe the C local mode in epitaxial CySi1−y layers grown by molecular beam epitaxy. The scattering cross section per C atom is independent of alloy concentration for y<2%. Lattice relaxation about substitutional C sites induces an extra peak near 475 cm−1.

Relaxation by the modified Frank–Read mechanism in compositionally uniform thin films

Relaxation of compositionally graded SiGe films during growth has recently been shown to occur through the multiplication of dislocations by the so‐called modified Frank–Read mechanism. Here, we show that the same mechanism is observed, during ex situ annealing of highly metastable, compositionally uniform layers. However, unlike graded layers, threading dislocations penetrate the upper layers of the film after relaxation. We explain the occurrence of the modified Frank–Read mechanism in ungraded layers by the lack of dislocation nucleation sites. The threading dislocations result from the fact that, in compositionally uniform layers, all dislocations tend to be located at (or very near) the plane of the interface, generating numerous pinning sites.

Strain symmetrization effects in pseudomorphic Si1−yCy/Si1−xGex superlattices

We report on strain and stability measurements on pseudomorphic Si1−yCy/Si1−xGex superlattices which are synthesized by solid source molecular beam epitaxy on silicon (100) substrate. The strain in the superlattices alternates between tensile and compressive in the individual Si1−yCy and Si1−xGex alloy layers, respectively. A symmetrical strain distribution can be achieved directly on silicon by adjusting the carbon and the germanium content and/or the thickness of the individual layers. X‐ray diffraction and transmission electron microscopy are applied to investigate the structural properties and the thermal stability.

Atomic layer doping for Si

Abstract We report on initial results of the potential of self-limiting surface reactions for the doping of Si layers using molecular beam epitaxy (MBE) and atmospheric pressure chemical vapor deposition (APCVD). In MBE experiments using Sb as an n-type dopant a self-limiting process is obtained at a coverage of half a monolayer. No evidence of a self-limiting process has yet been found for p-type doping using B2H6 above 400 °C. In the case of MBE growth at temperatures below 400 °C the B is only partly activated (10%–20%). In APCVD grown samples B surface coverage leads to significant growth inhibition of the subsequent deposition of Si from SiCl2H2. Finally, preliminary results of atomic layer doping using AsH3 in APCVD indicate a self-limitation of chemisorption of AsH3 at about 0.1 monolayer at a temperature of 600 °C; however, subsequent growth of Si leads to a smearing out of the As due to segregation and to the residence time of As in the system.

In situ study of relaxation of SiGe thin films by the modified Frank–Read mechanism

We have studied the dynamics of thermal relaxation of highly metastable films of SiGe/Si(100) in situ in the transmission electron microscope (TEM). This makes it possible to study the early stages of strain relaxation, and thus obtain information about the nucleation of dislocations. We find that, when care is taken not to introduce artifacts during sample preparation, relaxation occurs by the multiplication of ‘‘precursor dislocations’’ through a mechanism similar to the Frank–Read mechanism. An individual nucleation site is observed, confirming the model previously proposed.

Piezo-optical response of Si1−yCy alloys grown pseudomorphically on Si (001)

Abstract We have measured the dielectric functions of three Si 1− y C y alloy layers ( y ≤ 1.4%) grown pseudomorphically on Si (001) substrates using molecular beam epitaxy at low temperatures. From the numerical derivatives of the measured spectra, we determine the critical point energies E ′ 0 and E 1 as a function of y ( y ≤ 1.4%) using a comparison with analytical line shapes and analyze these energies in terms of the expected shifts and splittings due to negative pressure, shear stress, and alloying. Our data agree well with the calculated shifts for E 1 , but the E ′ 0 energies are lower than expected.

Si1-yCy Alloys — Extending Si-Based Heterostructure Engineering

Si1− y C y alloy single layers and strained layer superlattices on Si (100) with C concentrations of up to a few percent have been synthesized using Solid Source Molecular Beam Epitaxy. While the presence of even small surface contaminants containing carbon disrupts Si epitaxy, we show that co-evaporated Si and C can yield defect-free epitaxial films with C concentrations up to 5 atomic %. Critical parameters include growth temperature, the total Si growth rate and the atom flux ratio. Outside the acceptable process window, the films are highly twinned, and in some cases amorphous. In addition, growth temperature also plays a significant role in preventing twinning and islanding. Low growth temperature also suppresses the precipitation of β-SiC and leads to the formation of pseudomorphic Si1− y C y random alloys. Secondary Ion Mass Spectroscopy, X-ray Diffraction, Raman Spectroscopy, and electron microscopy have been used to verify that the layers are indeed substitutional alloys without silicon carbide precipitation. We have also studied the thermal stability of these layers and find that the layers degrade by qualitatively different mechanisms: interdiffusion at low temperatures, and silicon carbide precipitation at high temperatures. The ability to synthesize and process these alloy layers in combination with Si and Si1− x Ge x layers, allows for exploitation of the ternary system and the possibility of more flexible bandgap engineering in a Si-based technology.