A. V. G. Chizmeshya

Ge-Sn semiconductors for band-gap and lattice engineering

We describe a class of Si-based semiconductors in the Ge1−xSnx system. Deuterium-stabilized Sn hydrides provide a low-temperature route to a broad range of highly metastable compositions and structures. Perfectly epitaxial diamond-cubic Ge1−xSnx alloys are grown directly on Si(100) and exhibit high thermal stability, superior crystallinity, and crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are completely characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction (rocking curves), as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. Ab initio density functional theory simulations are also used to elucidate the structural and spectroscopic behavior.

Perfectly tetragonal, tensile-strained Ge on Ge1−ySny buffered Si(100)

High-quality, tensile-strained Ge layers with variable thickness (>30nm) have been deposited at low temperature (350–380°C) on Si(100) via fully relaxed Ge1−ySny buffers. The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable tensile strains up to 0.25% for y=0.025. Combined Raman analysis and high resolution x-ray diffraction using multiple off-axis reflections reveal unequivocally that the symmetry of tensile Ge is perfectly tetragonal, while the strain state of the buffer (∼200nm thick) remains essentially unchanged. A downshift of the direct gap consistent with tensile strain has been observed.

Molecular-Based Synthetic Approach to New Group IV Materials for High-Efficiency, Low-Cost Solar Cells and Si-Based Optoelectronics

Ge(1-x-y)Si(x)Sn(y) alloys have emerged as a new class of highly versatile IR semiconductors offering the potential for independent variation of band structure and lattice dimension, making them the first practical group IV ternary system fully compatible with Si CMOS processing. In this paper we develop and apply new synthetic protocols based on designer molecular hydrides of Si, Ge, and Sn to demonstrate this concept from a synthesis perspective. Variation of the Si/Sn ratio in the ternary leads to an entirely new family of semiconductors exhibiting tunable direct band gaps (E(o)) ranging from 0.8 to 1.2 eV at a fixed lattice constant identical to that of Ge, as required for the design of high-efficiency multijunction solar cells based on group IV/III-V hybrids. As a proof-of-concept demonstration, we fabricated lattice-matched Si(100)/Ge/SiGeSn/InGaAs architectures on low-cost Si(100) substrates for the first time. These exhibit the required optical, structural, and thermal properties, thus representing a viable starting point en route to a complete four-junction photovoltaic device. In the context of Si-Ge-Sn optoelectronic applications, we show that Ge(1-x-y)Si(x)Sn(y) alloys serve as higher-gap barrier layers for the formation of light emitting structures based on Ge(1-y)Sn(y) quantum wells grown on Si.

Optical and structural properties of SixSnyGe1−x−y alloys

Single-phase SixSnyGe1−x−y alloys (x⩽0.25,y⩽0.11) were grown on Si using chemical vapor deposition. First principles simulations predict that these materials are thermodynamically accessible and yield lattice constants as a function of Si/Sn concentrations in good agreement with experiment. An empirical model derived from experimental SixGe1−x and SnyGe1−y binary data also provides a quantitative description of the composition dependence of the lattice parameters. Spectroscopic ellipsometry of selected samples yields dielectric functions indicating a band structure consistent with highly crystalline semiconductor materials of diamond symmetry. Incorporation of Si into SnyGe1−y leads to an additional reduction of the E2 critical point, as expected based on the E2 values of Si and Ge.

Weak Binding Potentials and Wetting Transitions

We present ab initio calculations of the adsorption potentials V(Z) of inert gases and hydrogen on the surfaces of various metals. The ratio of the adsorption well depth to that of the adsorbate pair potential is ∼ 3.5, 2, 1.5, 1, 0.9 and 0.9 for adsorption on Mg, Li, Na, K, Rb, and Cs, respectively, with some variation between gases (always smallest for Ne). When this ratio is small, a wetting transition occurs; we predict the wetting temperature Twusing a model of Cheng et al. Comparison is made with other calculations and with experiments.

Versatile buffer layer architectures based on Ge1−xSnx alloys

We describe methodologies for integration of compound semiconductors with Si via buffer layers and templates based on the GeSn system. These layers exhibit atomically flat surface morphologies, low defect densities, tunable thermal expansion coefficients, and unique ductile properties, which enable them to readily absorb differential stresses produced by mismatched overlayers. They also provide a continuous selection of lattice parameters higher than that of Ge, which allows lattice matching with technologically useful III-V compounds. Using this approach we have demonstrated growth of GaAs, GeSiSn, and pure Ge layers at low temperatures on Si(100). These materials display extremely high-quality structural, morphological, and optical properties opening the possibility of versatile integration schemes directly on silicon.

New classes of Si-based photonic materials and device architectures via designer molecular routes

Ge/Sn-based group IV semiconductors with tunable band gaps across the wide IR range were synthesized using designer hydrides with tailored Si, Ge and Sn stoichiometries and structures. GeSn, SiGeSn, SiSn and SiGeSn/Ge heterostructures undergo indirect to direct band gap transitions via strain engineering and alloy composition tuning, providing the basis for integration of microelectronics with optical components into a single chip. SiGeSn systems also enable buffer layer technologies with unprecedented lattice and thermal matching capabilities for applications in monolithic integration of III–V semiconductors with Si electronics.

Low temperature chemical vapor deposition of Si-based compounds via SiH3SiH2SiH3 : Metastable SiSn/GeSn/Si(100) heteroepitaxial structures

Growth of Si1−xSnx alloys on Ge1−ySny-buffered Si(100) was achieved via reactions of SnD4 and SiH3SiH2SiH3 at 275°C. Kinetic studies indicate that unprecedented low growth temperatures are made possible by the highly reactive SiH2 groups. The authors obtain supersaturated metastable compositions (y∼25%) near the indirect to direct band gap crossover predicted by first principles simulations. Extensive characterizations of composition, structure, and morphology show that the SiSn∕GeSn films grow lattice matched via a “compositional pinning” mechanism. The initial Raman observations of Si–Sn bond vibrations in a condensed phase are discussed in the context of simulated bond distributions in the alloys.

Chemical routes to Ge∕Si(100) structures for low temperature Si-based semiconductor applications

The authors describe very low temperature (350–420°C) growth of atomically smooth Ge films (0.2–0.4nm roughness) directly on Si(100) via gas-source molecular beam epitaxy. A carefully tuned admixture of (GeH3)2CH2, possessing unique pseudosurfactant properties, and conventional Ge2H6 provides unprecedented control of film microstructure, morphology, and composition. Formation of edge dislocations at the interface ensures growth of virtually relaxed monocrystalline Ge films (∼40–1000nm thick) with a threading dislocation density less than 105cm−2 as determined by etch pit measurements. Secondary ion mass spectroscopy showed no measurable carbon incorporation indicating that C desorbs as CH4, consistent with calculated chemisorption energies.

First‐principles calculation of the equation‐of‐state, stability, and polar optic modes of CaSiO3 perovskite

We report the results of a first-principles LAPW calculation of the equation-of-state, dynamic stability, and infrared-active transverse optic vibrational mode frequencies of CaSiO3 perovskite. A Birch-Murnaghan fit to the computed energy-volume relation of the cubic phase yields values of Vo=45.62 A³, Ko=227 GPa, and Ko′=4.29 for the thermally-corrected equation-of-state parameters. These values are in excellent agreement with recent quasi-hydrostatic compression data to 10 GPa, but significantly differ from values derived from higher pressure non-hydrostatic compression data. We calculate the volume dependence of the infrared-active TO mode frequencies using a frozen-phonon approach. The lowest frequency ferroic mode is predicted to occur near 228 cm−1 at ambient pressure and displays classic soft-mode behavior in the tensile regime, in quantitative agreement with earlier molecular and lattice dynamical calculations, based on empirical potentials. These established a link between the low frequency ferroic mode and the thermally activated crystalline-amorphous transition in a model CaSiO3 perovskite system. Our present calculations also reveal that the static cubic perovskite structure is unstable with respect to small octahedral rotations, corresponding to Brillouin zone edge dynamical instabilities, at ambient pressure and throughout the pressure range of the lower mantle. We speculate that coupling of the low frequency ferroic mode with octahedral tilting modes and strain lower the activation energy for the crystalline-amorphous transition.