Y.-Y. Fang

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.

Sn-alloying as a means of increasing the optical absorption of Ge at the C- and L-telecommunication bands

The optical properties of Ge1−ySny alloys (y ~ 0.02) grown by chemical vapor deposition on Si substrates have been studied using spectroscopic ellipsometry and photocurrent spectroscopy. The system shows a 10-fold increase in optical absorption, relative to pure Ge, at wavelengths corresponding to the C-telecommunication band (1550 nm) and a 20-fold increase at wavelengths corresponding to the L-band (1620 nm). Measurements on a series of samples with different thicknesses reveal nearly identical dielectric functions, from which the composition reproducibility of the growth method is estimated to be as good as 0.1%. It is shown that a model that includes excitonic effects reproduces the measured onset of absorption using the direct band gap E0 as essentially the only adjustable parameter of the fit.

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.

Practical B and P doping via SixSnyGe1−x−y−zMz quaternaries lattice matched to Ge: Structural, electrical, and strain behavior

We describe the fabrication of B and P doped SiGeSn ternaries, lattice-matched to Ge, with compositions adjusted to independently tune the band gap. These are deposited at 320–350 °C with superior crystallinity and morphology via in situ reactions of diborane (p-type) and designer P(SiH3)3 and P(GeH3)3 precursors (n-type). Device-level carrier concentrations in the 1019–1020/cm3 range are produced yielding film resistivities and carrier mobilities comparable to those of Ge indicating negligible alloy scattering. High boron levels induce a significant and systematic contraction of the host lattice, which is compensated by an adjustment of the Sn/Si ratio in accord with a simple model based on Vegard’s law, the mismatch of covalent radii of the constituents, and the absolute hydrostatic deformation potentials for the band edges.

Direct absorption edge in GeSiSn alloys

The lowest direct absorption edge E0 in ternary Ge1‐x‐ySixSny alloys was measured in films grown lattice‐matched to Ge using spectroscopic ellipsometry and photoreflectance. The sample choice is dictated by possible applications of Ge1‐x‐ySixSny in photovoltaics as the long‐sought ∼1 eV gap material to complement Ge/InGaAs/InGaP multijunctions. The compositional dependence of the E0 transition is analyzed in detail.

Thermal expansivity of Ge{sub 1-y}Sn{sub y} alloys

The temperature dependence of the lattice parameter of Ge{sub 1-y}Sn{sub y} alloys deposited on Si substrates has been determined from an analysis of their x-ray reciprocal-space maps. It is found that over the range 0<y<0.03 the alloy thermal expansivity increases by up to 20% as a function of y. This implies a strong deviation from a linear interpolation between the end compounds since the thermal expansivities of pure Ge and {alpha}-Sn are nearly the same. Alternative interpolation formulas based on a Debye model and a mixed Debye-Einstein model of the phonon structure are tested and it is found that they also fail to explain the observed increase in thermal expansivity.

Photoresponse at 1.55 μm in GeSn epitaxial films grown on Si

In summary, epitaxial Ge_1-xSn_x films with hole mobilities as high as 600 cm²V^-1s^-1 were deposited on Si(100) substrates by UHV-CVD and used to fabricate photoconductor devices employing standard semiconductor processing steps. Performance measurements of the produced device provided feedback for improvement of the Ge_1-xSn_x material quality during the growth. Photoconductor devices showed maximum 0.5% decrease of the resistance upon irradiation by 1.55 μm laser light with 5 mW power.