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The dielectric functions of germanium–tin alloy thin-films, deposited by molecular beam epitaxy on bulk Ge substrates, with relatively high Sn contents from 15 to 27 at. %, were measured by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 μm, using a combination of ultraviolet-visible and infrared ellipsometers. The band structure critical point energies, specifically the E1 and E1 + Δ1 optical transitions, were extracted from the measurements by a method of parametric oscillator modeling and second derivative analysis. With increasing Sn content, the transitions shifted to lower energies, and for alloys with less than 20% Sn, the numerical values agreed reasonably with predictions based on deformation potential theory that accounted for film strain. For the higher Sn alloys, the critical point energies from measurements agreed less well with deformation potential theory. These results provide information on the band structure of GeSn alloys with high Sn contents, which are increasingly important for long-wave infrared devices and applications.

We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75 eV) into the vacuum ultraviolet (8.5 eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Γ-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24 me) at the bottom of the Γ-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24 me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Γ-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8 meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements ( e ∞ = 1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].

Crack-free AlN films with threading dislocation density (TDD) below 109 cm−2 are needed for deep-UV optoelectronics. This is typically achieved using pulsed lateral overgrowth or very thick buffer layers (>10 μm), a costly and time-consuming approach. A method for conventional metalorganic chemical vapor deposition growth of AlN/SiC films below 3 μm with greatly improved quality is presented. Focusing on substrate pretreatment before growth, we reduce average film stress from 0.9 GPa (tension) to −1.1 GPa (compression) and eliminate cracking. Next, with optimized growth conditions during initial deposition, AlN films with x-ray rocking curve widths of 123 arc-sec ( 0002) and 304 arc-sec ( 20 2 ¯ 1) are developed, and TDD is confirmed via plan view transmission electron microscopy (TEM) to be 2 × 108 cm−2. Film stress measurements including x-ray 2θ-ω, reciprocal space mapping, and curvature depict compressively stressed growth of AlN on 4H-SiC due to lattice mismatch. The thermal expansion coefficient mismatch between AlN and SiC is measured to be Δ α = α AlN − α SiC = 1.13 × 10 − 6 ° C − 1 and is found to be constant between room temperature and 1400 °C. TEM confirms the existence of dense misfit dislocation (MD) networks consistent with MD formation near SiC step edges and low MD density regions attributed to nearly coherent AlN growth on SiC terraces. These low-TDD, crack-free AlN/SiC buffers provide a platform for deep-UV optoelectronics and ultrawide bandgap electronics.

Transient dielectric functions with a 120 fs time resolution of Ge, Si, and InP were acquired from 1.7 to 3.5eV with a femtosecond pump-probe rotating-compensator ellipsometer. The intensity of the pump laser (with 1.55, 3.10, or 4.65eV photon energy) was adjusted to create an initial near-surface carrier density of 10(20)cm(-3). In Ge, there is a significant (similar to 15%) decrease in the E-1 and E-1+Delta(1) critical point absorption and a Kramers-Kronig consistent change in the refractive index because photoexcited electrons at L block these transitions and reduce their amplitudes. Only a small redshift of the E-1 critical point is observed, which we attribute to lattice heating and exchange-correlation effects. Minimal changes were found for Si and InP, where electrons near Delta and Gamma do not participate in interband transitions between 1.7 and 3.5eV.

Tin-rich Sn1−xGex alloys with Ge contents up to 6% were grown pseudomorphically on InSb (001) substrates by molecular beam epitaxy at room temperature. The alloys show a germanium-like lattice and electronic structure and respond to the biaxial stress within continuum elasticity theory, which influences bands and interband optical transitions. The dielectric function of these alloys was determined from 0.16 to 4.7 eV using Fourier-transform infrared and spectroscopic ellipsometry. The E1 and E1 + Δ1 critical points decrease with the increasing Ge content with a bowing parameter similar to the one established for Ge-rich Sn1−xGex alloys. On the other hand, the inverted direct bandgap E¯0 is nearly independent of the Ge content, which requires a bowing parameter of about 0.8 eV, much lower than what has been established using photoluminescence experiments of Ge-rich relaxed Sn1−xGex alloys.

Using Fourier-transform infrared ellipsometry, the authors provide spectroscopic evidence about the valence band (VB) structure of diamond-like α-tin. The mid-infrared dielectric function of α-tin grown pseudomorphically on InSb or CdTe by molecular beam epitaxy shows a strong E¯0 peak near 0.41 eV. This peak is assigned to allowed intravalence band transitions from the Γ7− (electron-like) VB to the Γ8+v heavy hole VB and/or interband transitions from Γ7− to the Γ8+c light “hole” conduction band. The strength of this peak requires a hole density in the mid-1018 cm−3 range at room temperature, which might be caused by unintentional doping, by thermal electron-hole pair generation, or by the possibility that the L6+ conduction band might have an energy slightly lower than the Γ8+ VB maximum. Alternatively, this E¯0 peak might be enhanced by the M-shape of the Γ7− VB caused by interactions with the Γ7+ split-off hole VB. A sum-rule analysis of the dielectric function between 0.16 and 6.5 eV is consistent with a high-frequency dielectric constant of 24, which has at most a weak temperature dependence between 100 and 300 K.

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.

Optical and electrical properties of amorphous GdxGa0.4−xO0.6 films have been systematically investigated as a function of Gd mole fraction x (0⩽x⩽0.31). The GdxGa0.4−xO0.6 films are part of GdxGa0.4−xO0.6/Ga2O3 gate dielectric stacks grown onto the GaAs(001) surface by molecular-beam deposition. When the Gd mole percent is raised from 0 to 25 at. % (0⩽x⩽0.25), the refractive index n and the optical band gap increase from 1.908 to 1.987 and from 4.22 to 5.08 eV, respectively. The electrical properties of GdxGa0.4−xO0.6 films improve dramatically with increasing Gd concentration: The leakage current falls from 7.8×10−1 to 9.7×10−10 A/cm2 (measured at an electric field of 1 MV/cm) when the Gd mole percent is raised from 8.8 to 31 at. %, and the dielectric breakdown field exceeds 4 MV/cm for x>0.21.

We report growth of device-quality, single-crystal SnxGe1−x alloys (with x=0.02–0.2) directly on Si via chemical vapor deposition with deuterium-stabilized Sn hydrides. The high Sn-content materials are stabilized with ordered superstructures that gives rise to a layered structure adjacent to the Si substrate. Density functional theory simulations were used to elucidate the structural and bonding behavior of this material. Optical determinations show a Ge-like band structure that is substantially redshifted compared to that of elemental Ge. Thus, these systems are excellent candidates for a new generation of infrared devices, with the critical advantage that they can be grown directly on Si.

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.

Hexagonal AlN layers were grown on Si(111) by gas-source molecular-beam epitaxy with ammonia. The transition between the (7×7) and (1×1) silicon surface reconstructions, at 1100 K, was used for in situ calibration of the substrate temperature. The initial deposition of Al, at 1130–1190 K, produced an effective nucleation layer for the growth of AlN. The Al layer also reduced islands of SiNx that might be formed due to background NH3 on the silicon surface prior to the onset of epitaxial growth. The transition to two-dimensional growth mode, under optimum conditions, was obtained after the initial AlN thickness of ∼7 nm.

We report on the quantum confinement in SrTiO3 (STO) quantum wells (QWs) grown by molecular beam epitaxy. The QW structure consists of LaAlO3 (LAO) and STO layers grown on LAO substrate. Structures with different QW thicknesses ranging from two to ten unit cells were grown and characterized. Optical properties (complex dielectric function) were measured by spectroscopic ellipsometry in the range of 1.0 eV–6.0 eV at room temperature. We observed that the absorption edge was blue-shifted by approximately 0.39 eV as the STO quantum well thickness was reduced to two unit cells. This demonstrates that the energy level of the first sub-band can be controlled by the QW thickness in a complex oxide material.

Spectroscopic ellipsometry was used to measure the dielectric functions of epitaxial and bulk Ge at photon energies from 1.5 to 5.2 eV. The epitaxial Ge was grown at 400 °C by molecular beam epitaxy on (001) Si substrates. The optical response and the interband critical‐point parameters of Ge on Si were found to be indistinguishable from that of bulk single crystal Ge, indicating high optical quality. Dislocation density measurements using an iodine etch verified low surface defect densities. We conclude that epitaxial Ge grown on Si at relatively low temperatures is suitable for optical device applications.

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

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...

The broadenings of the E1 and E1+Δ1 interband critical points can be understood as lifetime effects due to the ultrafast relaxation of the photoexcited hot holes. The contributions to these broadenings arising from the electrons in the conduction band are small, as intervalley scattering times are rather long. We have measured such broadenings in Si, Ge, α‐tin, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, and InSb with spectroscopic ellipsometry and compare them with calculations based on the deformation potential‐type electron‐phonon interaction in the rigid pseudo‐ion approximation.

Since the conduction‐band structure of InP is still controversial, we perform an ab initio linear muffin tin orbital (LMTO) band structure calculation to obtain the valence bands with high accuracy. The LMTO is based on the local density approximation (LDA) which suffers from the well known ‘‘band‐gap problem.’’ We therefore combine our calculated valence‐band energies with carefully evaluated optical data from the literature and confirm the interconduction‐band separation energies ΔEΓL=0.86±0.02 eV and ΔEΓX=0.96±0.02 eV. We also give new empirical local pseudopotential form factors adjusted to reproduce these experimental values.

The ability to modulate the bandgap of a material without altering its functional properties is crucial for fabricating heterojunctions for device applications. Here, we explore experimentally and theoretically the effect of the substitution of Ti with Al on the bandgap of perovskite oxide SrTiO3. We grow Al-doped SrTiO3 films directly on Si(100) and show from electron energy loss spectra that the bandgap is increased by approximately 0.3 eV over undoped SrTiO3. This bandgap increase is confirmed by spectroscopic ellipsometry measurements on identical films grown on LaAlO3 substrates, which show a 0.3 eV blue shift in the steep increase of the absorption edge. Current vs. voltage measurements show a reduction in leakage current by six orders of magnitude at a field of 1 MV/cm. We use density functional theory to explain how Al replacing Ti modifies the conduction band edge density of states resulting in the experimentally observed increase in the bandgap.

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.

Analytical models with eight parameters have been found which describe the dielectric functions of native or electrochemically grown oxide layers on the compound semiconductors GaP, GaAs, GaSb, InP, InAs, and InSb in the 1.6 to 5.6 eV photon energy range. These model functions agree with the experimental optical constants to within the accuracy of the measurements. They will be useful for the design and analysis of semiconductor laser structures, solar cells, and other electro‐optical devices.