Leszek S. Wielunski

In situ infrared spectroscopy of hafnium oxide growth on hydrogen-terminated silicon surfaces by atomic layer deposition

The interface formation between HfO2 and H-terminated Si(111) and Si(100) is studied by in situ infrared absorption spectroscopy during atomic layer deposition using alternating tetrakis-ethylmethylamino hafnium (TEMAH) and deuterium oxide (D2O) pulses. The HfO2 growth is initiated by the reaction of TEMAH with Si–H rather than D2O, and there is no evidence for SiO2 formation at moderate growth temperatures (∼100°C). Although Rutherford backscattering shows a linear increase of Hf coverage, direct observations of Si–H, Si–O–Hf, and HfO2 phonons indicate that five cycles are needed to reach the steady state interface composition of ∼50% reacted sites. The formation of interfacial SiO2 (∼0.7nm) is observed after postdeposition annealing at 700°C in ultrapure nitrogen.

Lattice constant and substitutional composition of GeSn alloys grown by molecular beam epitaxy

Single crystal epitaxial Ge1−xSnx alloys with atomic fractions of tin up to x = 0.145 were grown by solid source molecular beam epitaxy on Ge (001) substrates. The Ge1−xSnx alloys formed high quality, coherent, strained layers at growth temperatures below 250 °C, as shown by high resolution X-ray diffraction. The amount of Sn that was on lattice sites, as determined by Rutherford backscattering spectrometry channeling, was found to be above 90% substitutional in all alloys. The degree of strain and the dependence of the effective unstrained bulk lattice constant of Ge1−xSnx alloys versus the composition of Sn have been determined.

Ferromagnetism in Fe-implanted a-plane ZnO films

Fe ions of dose 5×1016cm−2 were implanted at 200keV into a-plane ZnO epitaxial films. The epitaxial quality of the postannealed samples was verified by x-ray diffraction ω-rocking curves and φ scans, whereas x-ray absorption spectroscopy identified the presence of both Fe2+ and Fe3+ ions, as well as changes in their relative concentration during postannealing. Superconducting quantum interference device measurements show that the as-implanted and postannealed films are ferromagnetic at room temperature. The saturation magnetization reduces during annealing possibly due to the decrease in the number of oxygen vacancies.

Photoelectron generation by photosystem II core complexes tethered to gold surfaces

By using a nondestructive, ultrasensitive, fluorescence kinetic technique, we measure in situ the photochemical energy conversion efficiency and electron transfer kinetics on the acceptor side of histidine-tagged photosystem II core complexes tethered to gold surfaces. Atomic force microscopy images coupled with Rutherford backscattering spectroscopy measurements further allow us to assess the quality, number of layers, and surface density of the reaction center films. Based on these measurements, we calculate that the theoretical photoelectronic current density available for an ideal monolayer of core complexes is 43 microA cm(-2) at a photon flux density of 2000 micromol quanta m(-2) s(-1) between 365 and 750 nm. While this current density is approximately two orders of magnitude lower than the best organic photovoltaic cells (for an equivalent area), it provides an indication for future improvement strategies. The efficiency could be improved by increasing the optical cross section, by tuning the electron transfer physics between the core complexes and the metal surface, and by developing a multilayer structure, thereby making biomimetic photoelectron devices for hydrogen generation and chemical sensing more viable.

Infrared characterization of hafnium oxide grown by atomic layer deposition using ozone as the oxygen precursor

HfO2 growth on H-terminated Si surfaces by atomic layer deposition (ALD) is studied with in situ Fourier transform infrared spectroscopy and ex situ Rutherford backscattering, using tetrakis-(ethyl-methyl-amino) hafnium and ozone as the hafnium and oxygen precursors, and compared to water-vapor-based ALD growth. The reaction pathways are different for the two oxygen precursors, leading to a lower growth rate for ozone (∼0.05nm∕cycle) than for water-based growth and to incorporation of different impurities in the HfO2 film. Furthermore, interfacial SiO2 is readily formed with ozone at the growth temperature (∼100°C), in contrast to water-based HfO2 growth.

In-situ FTIR study of atomic layer deposition (ALD) of copper metal films

Growth mechanisms of atomic layer deposition of copper films on various substrates using a novel metal precursor [Cu( s Bu-amd)]2 and molecular H2 are investigated by in-situ transmission Fourier transform infrared spectroscopy (FTIR). The Cu-precursor reacts with SiO2 and Al2O3 surfaces by forming chemical bonds with the surface. Upon reduction by H2, Cu atoms agglomerate, yielding additional reactive sites for more Cu-precursors. Cu agglomeration is relatively weaker on nitrided Si surfaces. H-terminated Si surfaces show a minimal reaction with the Cu precursor. The growth rates of the Cu films on all these surfaces are all less than 1 A per cycle.

Ion Beam Mixing for Processing of Nanostructure Materials

Ion beam mixing (IBM) has been used to process various nanostructure materials and thin films for applications in microelectronics and optoelectronics. In this paper, a study of alloy formation of Si-Ge, processed at shallow depths followed by oxygen implantation, is presented. The mixture is annealed to form Si-GeO2-Si, wherein the germanium oxide may form alone or as a matrix with the source of excitation. Characterization techniques used in this study include investigations of the structural variations due to argon ion-beam irradiation by Rutherford backscattering (RBS) and shallow defects and deep trapping level states by thermoluminescence (TL) measurements. Fourier transform infrared (FTIR) spectroscopy is used to analyze the thin film/islands of GeO2 formed in the matrix.

a-plane MgxZn1−xO films deposited on r-sapphire and its surface acoustic wave characteristics

Piezoelectric zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (MgxZn1−xO) films are deposited on r-plane (011¯2) sapphire (Al2O3) substrates using the hybrid deposition technology by combining metal-organic chemical vapor deposition (MOCVD) and sputtering. An ultra-thin ZnO buffer is first grown on r-Al2O3 by MOCVD technique, followed by a thick piezoelectric MgxZn1−xO (0⩽x⩽0.3) film deposited using RF sputtering. The sputtering targets are made by mixing ZnO and MgO powders in appropriate composition ratio, and nickel oxide (NiO) powder (2wt%) is added as the compensation dopant to achieve piezoelectricity. The as-deposited MgxZn1−xO films have a-plane (112¯0) orientation in a wurtzite crystal structure. The crystallinity of the films is further improved by annealing at 600–700°C in oxygen ambient. It is found that a ZnO thin buffer layer and post-deposition annealing process significantly improve the film’s piezoelectric properties. The c-axis of the MgxZn1−xO film lies in the plane of the s...

Influence of oxygen diffusion through capping layers of low work function metal gate electrodes

This letter evaluates Ru and W capping layers for MoTa metal gate electrodes in MOS capacitor applications. The authors report that the oxygen diffusion from the capping layer plays an important role in determining the MoTa alloy effective work function value on SiO/sub 2/. A MoTa alloy metal gate with Ru capping exhibits stable effective work function up to 900/spl deg/C annealing but is not stable with W capping. Auger electron spectroscopy and Rutherford backscattering spectroscopy analyses show minimal oxygen diffusion into MoTa gate stacks with Ru capping while severe oxygen diffusion into the gate is observed with W capping metal after 900/spl deg/C annealing. Current-voltage analysis also demonstrates different barrier heights of MoTa on SiO/sub 2/ with Ru or W capping layer after 900/spl deg/C annealing, confirming the effective work function value change.

Fabrication and characterization of compact silicon oxynitride waveguides on silicon chips

We investigate silicon oxynitride (SiON) waveguides for long optical delay lines on a silicon chip. With the choice of a moderately low refractive index contrast, a balance can be achieved between compact waveguide cross-section and low loss. The material composition and refractive index are characterized by Rutherford backscattering spectrometry and ellipsometry. High-temperature annealing is performed after waveguide fabrication so as to simultaneously remove light absorbing bonds in the materials and smooth the sidewall roughness at the core‐cladding interface. A meter-long SiON waveguide is demonstrated on a centimeter scale chip.