Dmitri O. Klenov

Thermoelectric power factor in semiconductors with buried epitaxial semimetallic nanoparticles

We have grown composite epitaxial materials that consist of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix both as superlattices and randomly distributed throughout the matrix. The presence of these particles increases the free electron concentration in the material while providing scattering centers for phonons. We measure electron concentration, mobility, and Seebeck coefficient of these materials and discuss their potential for use in thermoelectric power generators.We have grown composite epitaxial materials that consist of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix both as superlattices and randomly distributed throughout the matrix. The presence of these particles increases the free electron concentration in the material while providing scattering centers for phonons. We measure electron concentration, mobility, and Seebeck coefficient of these materials and discuss their potential for use in thermoelectric power generators.

Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces

Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately ½ electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution, and the influence of different electrostatic boundary conditions are obtained.

ZrO2 gate dielectrics produced by ultraviolet ozone oxidation for GaN and AlGaN∕GaN transistors

We investigated the suitability of ZrO2 as a high-k dielectric for GaN material systems. Thin Zr films (4nm) were deposited by electron-beam evaporation at room temperature on n-type GaN and Al0.22Ga0.78N(29nm)∕GaN high electron mobility transistor (HEMT) structures. The Zr-coated samples were subsequently oxidized at temperatures in the range of 200–400 °C in an ozone environment. Atomic force microscopy studies after oxidation show that the ZrO2 forms a conformal layer on the underlying GaN template. Cross-section transmission electron microscopy studies showed little intermixing of the ZrO2 with the AlGaN∕GaN. The relative dielectric constant of the ZrO2 was determined to be 23. In comparison with HEMTs with bare gates (no dielectric between the gate metal and AlGaN), the HEMTs with ZrO2 showed two to three order of magnitude reduction in gate leakage current. Optimization of the HEMT process on sapphire substrates with ZrO2 under the gates yielded devices with powers of 3.8W∕mm and 58% power-added eff...

Influence of strain on the dielectric relaxation of pyrochlore bismuth zinc niobate thin films

Bi1.5Zn1.0Nb1.5O7 (BZN) films were deposited by rf magnetron sputtering on different substrates to systematically vary the film stress due the thermal mismatch between BZN and the substrate. Substrates included Pt/SiO2 covered silicon, vycor glass, magnesium oxide, and sapphire. The BZN film microstructures (orientation, grain size, and roughness) were similar on the different substrates. Measurements of the permittivity and dielectric loss tangent were carried out between 80 and 300 K at frequencies between 10 kHz and 10 MHz. Films that were under a moderate tensile stress showed a low-temperature dielectric relaxation, associated with a dielectric loss peak and drop in permittivity, at ∼100 K. In contrast, the dielectric relaxation was shifted to temperatures below 80 K in films on vycor that were under a large tensile stress. This shift reflected a lowering of the activation energy of the dielectric relaxation processes due to tensile stress. It is expected that films under large tensile stress require...

Impact of stress on oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films

We investigate oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films grown by sputter deposition on (001) LaAlO3 and (001) SrTiO3. After annealing at 500 °C under oxygen partial pressures greater than those used during deposition, films transform to a long-range oxygen vacancy ordered structure with orthorhombic symmetry. Observed orientation variants of the oxygen vacancy ordered structures are different for the two substrates. We discuss the relationship between film stress due to lattice and thermal mismatch with the substrate, and vacancy ordering.

Electrical and material characterizations of high-permittivity HfxTi1−xO2 gate insulators

High-permittivity hafnium titanate (HfxTi1−xO2) films of various compositions have been investigated. Chemical-vapor deposited thin films have well-controlled composition and a smooth surface. Thicker films show large, highly anisotropic grains with their long axis in the plane of the film. Electrical measurements showed that the interfacial layer equivalent oxide thickness (EOT) decreases linearly with increasing Hf content. Electron energy-loss spectroscopy in scanning transmission electron microscopy indicates that the interfacial layer is primarily SiO2. In the composition range of 0.3<x<0.5, the permittivity of the high-κ films increases linearly with increasing Ti content. For values of x<0.3, the film’s permittivity is close to that of pure HfO2(∼20), while for Ti-rich films the permittivity is close to that of pure TiO2(∼50). For films with a physical thickness less than 10 nm, the leakage current decreases as Ti increases for a fixed EOT.

The Interface between Single Crystalline (001) LaAlO3 and (001) Silicon

Atomic resolution high-angle annular dark-field imaging in scanning transmission electron microscopy is used to determine atomic arrangements at LaAlO3/Si interfaces, which were obtained by growing Si films epitaxially on (001) LaAlO3 single crystals. An unusual 3 ×1 interface reconstruction, in which every third La column is removed from the interface plane, is observed. The interface atomic structure is discussed in the context of electrically favorable interfacial bonding between the ionic oxide and Si.

Interface atomic structure of epitaxial ErAs layers on (001) In0.53Ga0.47As and GaAs

High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to determine the atomic structure of interfaces between epitaxial ErAs layers with the cubic rock salt structure and In0.53Ga0.47As and GaAs, respectively. All layers were grown by molecular-beam epitaxy. We show that the interfacial atomic arrangement corresponds to the so-called chain model, in which the zinc blende semiconductor is terminated with a Ga layer. Image analysis was used to quantify the expansion between the first ErAs plane and the terminating Ga plane. In the HAADF images, a high intensity transfer from the heavy Er columns into the background was observed in the ErAs layer, whereas the background in In0.53Ga0.47As was of much lower intensity.

Extended defects in epitaxial Sc2O3 films grown on (111) Si

Epitaxial Sc2O3 films with the cubic bixbyite structure were grown on (111) Si by reactive molecular beam epitaxy. High-resolution transmission electron microscopy (HRTEM) revealed an abrupt, reaction-layer free interface between Sc2O3 and Si. The ∼10% lattice mismatch between Si and Sc2O3 was relieved by the formation of a hexagonal misfit dislocation network with Burgers vectors of 1∕2⟨1¯10⟩Si and line directions parallel to ⟨112¯⟩Si. A high density of planar defects and threading dislocations was observed. Analysis of lattice shifts across the planar defects in HRTEM showed that these faults were likely antiphase boundaries (APBs). ABPs form when film islands coalesce during growth because films nucleate with no unique arrangement of the ordered oxygen vacancies in the bixbyite structure relative to the Si lattice.

Strain relaxation in transistor channels with embedded epitaxial silicon germanium source/drain

We report on the channel strain relaxation in transistors with embedded silicon germanium layer selectively grown in source and drain areas on recessed Si(001). Nanobeam electron diffraction is used to characterize the local strain in the device channel. Our results show that strain is reduced in the device channel regions after implantation and thermal anneal.