Nalin Fernando

Properties of pseudomorphic and relaxed germanium1−xtinx alloys (x < 0.185) grown by MBE

Epitaxial layers of Ge1−xSnx with Sn compositions up to 18.5% were grown on Ge (100) substrates via solid-source molecular beam epitaxy. Crystallographic information was determined by high resolution x-ray diffraction, and composition was verified by Rutherford backscattering spectrometry. The surface roughness, measured via atomic force microscopy and variable angle spectroscopic ellipsometry, was found to scale with the layer thickness and the Sn concentration, but not to the extent of strain relaxation. In addition, x-ray rocking curve peak broadening was found not to trend with strain relaxation. The optical response of the Ge1−xSnx alloys was measured by spectroscopic ellipsometry. With increasing Sn content, the E1 and E1 + Δ1 critical points shifted to lower energies, and closely matched the deformation potential theory calculations for both pseudomorphic and relaxed Ge1−xSnx layers. The dielectric functions of the high Sn and strain relaxed material were similar to bulk germanium, but with slightl...

Optical constants of germanium and thermally grown germanium dioxide from 0.5 to 6.6eV via a multisample ellipsometry investigation

Thermal GeO2 oxides up to 136 nm thickness were produced by annealing Ge wafers in pure oxygen at 550 °C and 270 kPa pressure for up to 10 h. The oxidation kinetics followed the Deal–Grove law. Using multisample spectroscopic ellipsometry for a series of five thermal oxides with different thicknesses, the complex dielectric functions of Ge and GeO2 were determined from 0.5 to 6.6 eV, for thin-film metrology applications in Ge-based microelectronics and photonics. The dispersion of the GeO2 layer was modeled with a simple Tauc-Lorentz oscillator model, but a more complicated dispersion with eight parametric oscillators was required for Ge. A reasonable fit to the ellipsometric angles could be obtained by assuming that all thermal oxides can be described by the same dielectric function, regardless of thickness, but a slight improvement was achieved by allowing for a lower density oxide near the surface of the thickest films. The authors compare their results with literature data for Ge and bulk and thin-fil...

Band gap and strain engineering of pseudomorphic Ge1−x−ySixSny alloys on Ge and GaAs for photonic applications

The authors report the compositional dependence of the direct and indirect band gaps of pseudomorphic Ge1−x−ySixSny alloys on Ge and GaAs with (001) surface orientation determined from deformation potential theory and spectroscopic ellipsometry measurements. The effects of alloying Ge with Si and Sn and the strain dependence of the band gaps at the Γ, Δ, and L conduction band minima are discussed. Deformation potential theory predicts an indirect to direct crossover in pseudomorphic Ge1−y−xSixSny alloys on Ge or GaAs only for very high Sn concentrations between 15% and 20%. No indirect to direct cross-over in pseudomorphic Ge1−ySny alloys (x = 0) on Ge or GaAs was found for practically approachable Sn compositions (y < 25%). The predictions for the compositional dependence of the E0, E1, and E1 + Δ1 band gaps were validated for pseudomorphic Ge1−ySny alloys on Ge using spectroscopic ellipsometry. The complex pseudodielectric functions of pseudomorphic Ge1−ySny alloys grown on Ge by molecular beam epitaxy ...

Infrared dielectric response, index of refraction, and absorption of germanium-tin alloys with tin contents up to 27% deposited by molecular beam epitaxy

The dielectric spectral response of Ge1-xSnx thin film alloys with relatively high Sn contents (0.15 ≤ x ≤ 0.27) and thickness from 42 to 132 nm was characterized by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 μm. The Ge1-xSnx thin films were deposited on Ge substrates by molecular beam epitaxy using an electron-beam source for Ge to achieve a substrate temperature below 150 °C to prevent the surface segregation of Sn. From the measured dielectric function, the complex refractive index was calculated indicating an increase in the real index with the Sn content at mid-infrared wavelengths. The ellipsometry revealed that the band structure critical point energies red-shifted with the increasing Sn content. The optical absorption coefficient was calculated from the imaginary index and showed a strong absorption into, and beyond, the mid-infrared with the increasing Sn content.The dielectric spectral response of Ge1-xSnx thin film alloys with relatively high Sn contents (0.15 ≤ x ≤ 0.27) and thickness from 42 to 132 nm was characterized by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 μm. The Ge1-xSnx thin films were deposited on Ge substrates by molecular beam epitaxy using an electron-beam source for Ge to achieve a substrate temperature below 150 °C to prevent the surface segregation of Sn. From the measured dielectric function, the complex refractive index was calculated indicating an increase in the real index with the Sn content at mid-infrared wavelengths. The ellipsometry revealed that the band structure critical point energies red-shifted with the increasing Sn content. The optical absorption coefficient was calculated from the imaginary index and showed a strong absorption into, and beyond, the mid-infrared with the increasing Sn content.

Band structure and optical properties of pseudomorphic Ge 1−x−y Si x Sn y on Ge

Ge is an indirect band gap material. The band structure of Ge is a strong function of strain and alloy composition, and a transition from an indirect to a direct band gap has been observed for y~6-10% for relaxed Ge1_ySny indicating the possibility of widespread applications of Ge-based photonic devices. The pseudomorphic nature of the Ge-based alloy layer on a substrate is important to keep dislocation densities low at the interface to improve the performance of the device. Band gap engineering of Ge by controlling strain and alloying with Si and Sn has attracted great interest since Ge1-x-ySixSny ternary alloy with two compositional degrees of freedom allows decoupling of the lattice constant and electronic structures. Hence the knowledge of the compositional and strain dependence of the Ge1-x-ySixSny band structure is critical for the design of photonic devices with the desired interband transition energies.