Nicholas G. Rudawski

Nanostructured ion beam-modified Ge films for high capacity Li ion battery anodes

Nanostructured ion beam-modified Ge electrodes fabricated directly on Ni current collector substrates were found to exhibit excellent specific capacities during electrochemical cycling in half-cell configuration with Li metal for a wide range of cycling rates. Structural characterization revealed that the nanostructured electrodes lose porosity during cycling but maintain excellent electrical contact with the metallic current collector substrate. These results suggest that nanostructured Ge electrodes have great promise for use as high performance Li ion battery anodes.

Analysis of trap state densities at HfO2/In0.53Ga0.47As interfaces

HfO2 was deposited on n- and p-type In0.53Ga0.47As by chemical beam deposition. Interface trap densities (Dit) and their energy level distribution were quantified using the conductance method in a wide temperature range (77 to 300 K). A trap level close to the intrinsic energy level caused the Dit to rise above 1013 cm−2 eV−1. The trap level at midgap gives rise to false inversion behavior in the capacitance-voltage curves for n-type channels at room temperature. The apparent decrease of the Dit close to the band edges is discussed.

TaN interface properties and electric field cycling effects on ferroelectric Si-doped HfO2 thin films

Ferroelectric HfO2-based thin films, which can exhibit ferroelectric properties down to sub-10 nm thicknesses, are a promising candidate for emerging high density memory technologies. As the ferroelectric thickness continues to shrink, the electrode-ferroelectric interface properties play an increasingly important role. We investigate the TaN interface properties on 10 nm thick Si-doped HfO2 thin films fabricated in a TaN metal-ferroelectric-metal stack which exhibit highly asymmetric ferroelectric characteristics. To understand the asymmetric behavior of the ferroelectric characteristics of the Si-doped HfO2 thin films, the chemical interface properties of sputtered TaN bottom and top electrodes are probed with x-ray photoelectron spectroscopy. Ta-O bonds at the bottom electrode interface and a significant presence of Hf-N bonds at both electrode interfaces are identified. It is shown that the chemical heterogeneity of the bottom and top electrode interfaces gives rise to an internal electric field, whic...

Stressed multidirectional solid-phase epitaxial growth of Si

The study of the solid-phase epitaxial growth (SPEG) process of Si (variously referred to as solid-phase epitaxy, solid-phase epitaxial regrowth, solid-phase epitaxial crystallization, and solid-phase epitaxial recrystallization) amorphized via ion implantation has been a topic of fundamental and technological importance for several decades. Overwhelmingly, SPEG has been studied (and viewed) as a single-directional process where an advancing growth front between amorphous and crystalline Si phases only has one specific crystallographic orientation. However, as it pertains to device processing, SPEG must actually be considered as multidirectional (or patterned) rather than bulk in nature with the evolving growth interface having multiple crystallographic orientations. Moreover, due to the increasingly ubiquitous nature of stresses presented during typical Si-based device fabrication, there is great interest in specifically studying the stressed-SPEG process. This work reviews the progress made in understan...

Comparing Electron Recombination via Interfacial Modifications in Dye-Sensitized Solar Cells

Establishing a blocking layer between the interfaces of the photoanode is an effective approach to improve the performance of dye-sensitized solar cells (DSSCs). In this work, HfO2 blocking layers are deposited via atomic layer deposition (ALD) onto tin-doped indium oxide (ITO) and TiO2. In both cases, addition of the blocking layer increases cell efficiencies to greater than 7%. The improved performance for a HfO2 layer inserted between the ITO/TiO2 interface is associated with an energy barrier that reduces electron recombination. HfO2 blocking layers between the TiO2/dye interface show more complex behavior and are more sensitive to the number of ALD cycles. For thin blocking layers on TiO2, the improved device performance is attributed to the passivation of surface states in TiO2. A distinct transition in dark current and electron lifetime are observed after 4 ALD cycles. These changes to performance indicate thick HfO2 layers on TiO2 formed an energy barrier that significantly hinders cell performance.

Effects of buffer layers on the structural and electronic properties of InSb films

We have investigated the effects of various buffer layers on the structural and electronic properties of n-doped InSb films. We find a significant decrease in room-temperature electron mobility of InSb films grown on low-misfit GaSb buffers, and a significant increase in room-temperature electron mobility of InSb films grown on high-misfit InAlSb or step-graded GaSb+InAlSb buffers, in comparison with those grown directly on GaAs. Plan-view transmission electron microscopy (TEM) indicates a significant increase in threading dislocation density for InSb films grown on the low-misfit buffers, and a significant decrease in threading dislocation density for InSb films grown on high-misfit or step-graded buffers, in comparison with those grown directly on GaAs. Cross-sectional TEM reveals the role of the film/buffer interfaces in the nucleation (filtering) of threading dislocations for the low-misfit (high-misfit and step-graded) buffers. A quantitative analysis of electron mobility and carrier-concentration de...

Effect of postdeposition anneals on the Fermi level response of HfO2/In0.53Ga0.47As gate stacks

The electrical characteristics, in particular interface trap densities, oxide capacitance, and Fermi level movement, of metal oxide semiconductor capacitors with HfO2 gate dielectrics and In0.53Ga0.47As channels are investigated as a function of postdeposition annealing atmosphere. It is shown, using both conductance and Terman methods, that the Fermi level of nitrogen annealed stacks is effectively pinned at midgap. In contrast, samples annealed in forming gas show a large band bending in response to an applied gate voltage and a reduced midgap interface trap density compared to those annealed in nitrogen.

Nanoscale manipulation of Ge nanowires by ion irradiation

Nanowires have generated considerable interest as nanoscale interconnects and as active components of both electronic and electromechanical devices. However, in many cases, manipulation and modification of nanowires are required to fully realize their potential. It is essential, for instance, to control the orientation and positioning of nanowires in some specific applications. This work demonstrates a simple method to reversibly control the shape and the orientation of Ge nanowires using ion beams. Crystalline nanowires were amorphized by 30 keV Ga+ implantation. Subsequently, viscous flow and plastic deformation occurred causing the nanowires to bend toward the beam direction. The bending was reversed multiple times by ion implanting the opposite side of the nanowires, resulting in straightening and subsequent bending into that opposite direction. This effect demonstrates the detailed manipulation of nanoscale structures is possible through the use of ion irradiation.

Characterization of the gate oxide of an AlGaN/GaN high electron mobility transistor

A subnanometer thick interfacial oxide layer present between the Ni/Au gate metal stack and semiconducting epilayers of an AlGaN/GaN high electron mobility transistor was characterized using high-angle annular dark-field scanning transmission electron microscopy and laser-assisted atom probe tomography. It was revealed that the oxide is composed of distinct Ni-oxide-rich and Al-oxide-rich layers with no Ga-oxide detected. The results provide information that is of potential importance in determining failure mechanisms and improving reliability of AlGaN/GaN high electron mobility transistors.

Effect of uniaxial stress on solid phase epitaxy in patterned Si wafers

The effect of uniaxial stress on solid phase epitaxy in patterned {001} Si wafers after ion implantation and annealing was investigated. It was found that mask edge defect formation was suppressed when tensile stresses greater than 100MPa were applied along the ⟨110⟩ direction. The application of compressive stress retarded ⟨001⟩ regrowth up to ∼6% and enhanced ⟨110⟩ regrowth up to ∼6%, while tensile stress enhanced ⟨001⟩ regrowth up to ∼60% and retarded ⟨110⟩ regrowth up to ∼40%. A stress-dependent regrowth velocity model qualitatively agrees with the observed trends in the ratio of ⟨001⟩ and ⟨110⟩ regrowth velocities.