Lyudmila V. Goncharova

Quantum confinement in Si and Ge nanostructures

We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Genanostructures including quantum wells(QWs), wires (Q-wires), and dots(QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses, and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium, and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), or photoluminescence(PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy(TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

Atomic layer deposition of hafnium oxide on germanium substrates

Germanium combined with high-κ dielectrics has recently been put forth by the semiconductor industry as potential replacement for planar silicon transistors, which are unlikely to accommodate the severe scaling requirements for sub-45‐nm generations. Therefore, we have studied the atomic layer deposition (ALD) of HfO2 high-κ dielectric layers on HF-cleaned Ge substrates. In this contribution, we describe the HfO2 growth characteristics, HfO2 bulk properties, and Ge interface. Substrate-enhanced HfO2 growth occurs: the growth per cycle is larger in the first reaction cycles than the steady growth per cycle of 0.04nm. The enhanced growth goes together with island growth, indicating that more than a monolayer coverage of HfO2 is required for a closed film. A closed HfO2 layer is achieved after depositing 4–5HfO2 monolayers, corresponding to about 25 ALD reaction cycles. Cross-sectional transmission electron microscopy images show that HfO2 layers thinner than 3nm are amorphous as deposited, while local epita...

Lanthanum silicate gate dielectric stacks with subnanometer equivalent oxide thickness utilizing an interfacial silica consumption reaction

A silicate reaction between lanthana and silica layers has been utilized to eliminate interfacial silica in metal-insulator-semiconductor devices and to obtain devices with very low equivalent oxide thickness EOT. This provides a simple process route to interface elimination, while producing a silicate dielectric with a higher temperature stability of the amorphous phase. The La2O3 layers in this study are deposited by reactive evaporation on 001 Si covered by a 0.8– 1.0-nm-thick SiO2 chemical oxide, and are capped in situ with a Ta gate, followed by a reaction anneal, which lowers the EOT from greater than 1.5 nm for the as-deposited bilayer stack to as low as 0.5 nm. Electron energy-loss spectroscopy and medium-energy ion scattering are used to show that a temperature of 400 °C is sufficient for the formation of the silicate gate dielectric. Gate leakage currents as low as 0.06 A / cm 2 are obtained for stacks having an EOT of 0.63 nm, orders of magnitude below that of SiO2 having the same EOT value. Electrical breakdown is observed at applied fields above 16 MV/ cm.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminesc...

Interface structure and thermal stability of epitaxial SrTiO3 thin films on Si (001)

We have used medium energy ion scattering, temperature programmed desorption, and atomic force microscopy to study the interface composition and thermal stability of epitaxial strontium titanate thin films grown by molecular-beam epitaxy on Si (001). The composition of the interface between the film and the substrate was found to be very sensitive to the recrystallization temperature used during growth, varying from a strontium silicate phase when the recrystallization temperature is low to a Ti-rich phase for a higher recrystallization temperature. The films are stable towards annealing in vacuum up to ∼550°C, where SrO desorption begins and the initially flat film starts to roughen. Significant film disintegration occurs at 850°C, and is accompanied by SiO and SrO desorption, pinhole formation, and finally titanium diffusion into the silicon bulk.

Superior Stable and Long Life Sodium Metal Anodes Achieved by Atomic Layer Deposition

Na-metal batteries are considered as the promising alternative candidate for Li-ion battery beneficial from the wide availability and low cost of sodium, high theoretical specific capacity, and high energy density based on the plating/stripping processes and lowest electrochemical potential. For Na-metal batteries, the crucial problem on metallic Na is one of the biggest challenges. Mossy or dendritic growth of Na occurs in the repetitive Na stripping/plating process with an unstable solid electrolyte interphase layer of nonuniform ionic flux, which can not only lead to the low Coulombic efficiency, but also can create short circuit risks, resulting in possible burning or explosion. In this communication, the atomic layer deposition of Al2 O3 coating is first demonstrated for the protection of metallic Na anode for Na-metal batteries. By protecting Na foil with ultrathin Al2 O3 layer, the dendrites and mossy Na formation have been effectively suppressed and lifetime has been significantly improved. Furthermore, the thickness of protective layer has been further optimized with 25 cycles of Al2 O3 layer presenting the best performance over 500 cycles. The novel design of atomic layer deposition protected metal Na anode may bring in new opportunities to the realization of the next-generation high energy-density Na metal batteries.

Inorganic–Organic Coating via Molecular Layer Deposition Enables Long Life Sodium Metal Anode

Metallic Na anode is considered as a promising alternative candidate for Na ion batteries (NIBs) and Na metal batteries (NMBs) due to its high specific capacity, and low potential. However, the unstable solid electrolyte interphase layer caused by serious corrosion and reaction in electrolyte will lead to big challenges, including dendrite growth, low Coulombic efficiency and even safety issues. In this paper, we first demonstrate the inorganic-organic coating via advanced molecular layer deposition (alucone) as a protective layer for metallic Na anode. By protecting Na anode with controllable alucone layer, the dendrites and mossy Na formation have been effectively suppressed and the lifetime has been significantly improved. Moreover, the molecular layer deposition alucone coating shows better performances than the atomic layer deposition Al2O3 coating. The novel design of molecular layer deposition protected Na metal anode may bring in new opportunities to the realization of the next-generation high energy-density NIBs and NMBs.

Oxygen diffusion and reactions in Hf-based dielectrics

Oxygen transport in and reactions with thin hafnium oxide and hafnium silicate films have been investigated using medium energy ion scattering in combination with O218 isotopic tracing methods. Postgrowth oxidation of Hf-based films in an O218 atmosphere at 490–950°C results in O exchange in the film. The exchange rate is faster for pure hafnium oxides than for silicates. The amount of exchanged oxygen increases with temperature and is suppressed by the SiO2 component. Films annealed prior to oxygen isotope exposure show complex incorporation behavior, which may be attributed to grain boundary defects, and SiO2 phase segregation.

Elastic recoil detection studies of near-surface hydrogen in cavity-grade niobium

Recent studies of the quality factor degradation mechanisms in superconducting RF niobium cavities at high surface magnetic fields revealed that RF performance may depend on the total hydrogen content in the 40 nm thick near-surface layer. Hydrogen distribution in niobium and its near-surface content variations after different chemical surface treatments has been addressed in previous studies. However, only chemical treatments were studied while heat treatments are equally important. In this work we use the elastic recoil detection (ERD) technique to systematically study the distribution of hydrogen in niobium sheet and cavity cutout samples subjected to chemical and heat treatments typically performed on niobium cavities. Our results indicate the near-surface segregation of hydrogen at the niobium oxide/niobium interface, and do not show any significant variation in hydrogen content after various heat and chemical treatments. We do not observe a direct correlation between total hydrogen content and the high field Q slope. Consequences of the observed hydrogen segregation are discussed in the framework of the NbHx phase diagram. (Some figures in this article are in colour only in the electronic version)

Ferromagnetism on the unpolished surfaces of single crystal metal oxide substrates

It is shown that a variety of single crystal substrates (Al2O3, LaAlO3, SrTiO3, TiO2, and ZnO), purchased from commercial suppliers, contain a ferromagnetic component to the magnetization in addition to the expected linear magnetic response. This ferromagnetic contribution is only observed on the unpolished surfaces and can be eliminated by either polishing or annealing at 600 °C in air, but not by annealing at 600 °C in a vacuum of 5 × 10−6 Torr. Particle induced x-ray emission spectra demonstrate that there is excess Fe on the unpolished surfaces of these single crystal substrates. While defect related ferromagnetic signals have been reported in some of these substrates, and while our results do not exclude this origin of ferromagnetism, we clearly show that the ferromagnetic signals observed in our samples are largely due to excess iron on the unpolished surfaces, possibly in the form of a mixture of Fe, Fe3O4, and or γ-Fe2O3.