K. Xue

Structural evidence of secondary phase segregation from the Raman vibrational modes in Zn1−xCoxO(0<x<0.6)

Micro-Raman measurements were performed to study the influence of Co doping on the lattice dynamic properties of the host ZnO. The structural evidence of secondary phase segregation was detected from two distinct phonon vibrational modes at around 472 and 678cm−1 in Zn1−xCoxO (0<x<0.6) ternary alloys with x above 0.098. In addition, an intense, broad, and symmetric phonon vibration was apparent at about 530cm−1 for alloys with x not more than 0.155. The authors suggest the shallow donor defects bound on the Co sites rather than the local vibrational mode involving Co motion as the origin.

Control of interfacial silicate between HfO2 and Si by high concentration ozone

By high concentration ozone oxidation at low temperature, the Hf-silicate interfacial layer between HfO2 and silicon substrate is effectively controlled. This is evident by investigating the chemical shifts of the Hf4f and Si2p core-level spectra with depth by using x-ray photoelectron spectroscopy. The improved interfacial microstructure is further confirmed by high-resolution cross-sectional transmission electron microscopy. The capacitance-voltage curves, obtained from the metal-oxide-semiconductor capacitors using the ozone oxidized HfO2 as the gate dielectric, show a negligible hysteresis of about 5mV and a low fixed charge density.

Effects of plasma immersion ion nitridation on dielectric properties of HfO2

Plasma immersion ion nitridation is used to produce thin HfO2 films with improved thermal and electrical properties. The film composition is investigated by examining the chemical shifts of the Hf 4f, Si 2p, and N 1s core-level spectra using x-ray photoelectron spectroscopy. The improved thermal stability and interfacial microstructure are further confirmed by high-resolution cross-sectional transmission electron microscopy. Electrical studies show an equivalent oxide thickness of about 1.25nm, a negligible hysteresis of about 5mV, and a low fixed charge density.

Local study of thickness-dependent electronic properties of ultrathin silicon oxide near SiO2/Si interface

We have used scanning tunnelling microscopy and spectroscopy (STM/STS) to investigate the electronic properties of ultrathin silicon oxide films on silicon, e.g. surface band gap and band offset versus silicon oxide thickness near the interface. The length of the interfacial electronically transitional region is measured to be about 0.9 nm, in which the silicon oxide surface band gap increases gradually with thickness and its evolution is meticulously observed. Both the conduction and valence band offsets between silicon oxide and silicon increase by about 1.0 eV with only a ~0.3 nm steric difference in this region. Furthermore, in an oxide thinner than ~0.6 nm, oxide gap states can be discerned from STS which is attributed to the electronic states extending from the silicon substrate over the Si oxide. On the basis of local layer-resolved electronic properties, we corroborate that a usable oxide thickness should be no less than ~0.9 nm on the Si(1 1 1) substrate, which corresponds to only three initial oxide layers.

Effects of an oxygen environment on the electrical properties of a single CdS nanobelt device.

The influence of the oxygen environment on the transport behavior of a cadmium sulfide (CdS) single crystal nanobelt is investigated by device performance under various light illuminations and oxygen partial pressures. The CdS nanobelt shows superior photo response in the visible light region and the conductance is sensitive to the oxygen environment. The results show that when exposed to oxygen, the surface chemisorbed oxygen species and their interactions with surface defects will significantly affect the conductivity by decreasing the carrier concentrations and reducing the mobility. The effect is explained by surface band bending which is observed by surface potential measurement. Furthermore, we show that the height of the nanobelt is one of the critical factors that greatly affects the conductance due to the intrinsic belt-like geometry.

Co doped ZnO(0001)-Zn by diffusion method and its magnetic properties

The diffusion behaviors of Co clusters on clean ZnO(0001)-Zn single crystal surface and their magnetic properties are studied. Co clusters are deposited on the clean ZnO(0001)-Zn surface at room temperature and then undergone ultrahigh vacuum annealing until fully reconstructed. The replacement of Zn2+ by Co2+ is confirmed by scanning tunneling microscopy and x-ray photoelectron spectroscopy. The Co doped ZnO shows a weak ferromagnetism at room temperature with a saturation magnetic moment of 1.08 μB/Co. Our observations indicate that surface Zn vacancies facilitate Co diffusion, and the interplay of Co ion with internal O vacancies leads to the ferromagnetism.

In situ fabrication and characterization of tungsten nanodots on SiO2/Si via field induced nanocontact with a scanning tunnelling microscope

Tungsten nanodots were reproducibly fabricated on ultrathin SiO2/Si by a novel technique in which repeatedly large-scale voltage ramps were applied between a scanning tunnelling microscope (STM) tip and samples. These nanodots have a similar geometry with a height of 1.0 ± 0.2 nm and diameter of 4.0 ± 1.0 nm. Nanodot formation processes were inspected simultaneously during the fabrication by means of scanning tunnelling spectroscopy (STS). The appearance of high conductivity I–V curves indicates a nanocontact formed during the nanodot fabrication. The fabrication mechanism is believed to be the sequential process of the field induced tip elongation, nanocontact formation and nanocontact breaking. The behaviour of the electron transport through the nanocontact was scrutinized by fitting the I–V curves to a direct tunnelling model in the low bias range. A tunnelling barrier of 1.2 ± 0.3 eV between tungsten and SiO2 was deduced. We also present the feasibility of probing the STM tip apex by the nanodots, which could be applied in in situ monitoring of tip apex variation during STM operation.

In situ characterization of initial growth of HfO2

The initial growth of HfO2 on Si (111) is monitored in situ by ultrahigh vacuum (UHV) scanning probe microscopy. UHV scanning tunneling microscopy and UHV atomic force microscopy reveal the topography of HfO2 films in the initial stage. The chemical composition is further confirmed by x-ray photoelectron spectroscopy. Scanning tunneling spectroscopy is utilized to inspect the evolution of the bandgap. When the film thickness is less than 0.6 nm, the bandgap of HfO2 is not completely formed. A continuous usable HfO2 film with thickness of about 1.2 nm is presented in this work.

Fabrication HfO x nanopatterns by atomic force microscopy lithography

The HfOx nanopatterns are fabricated with the aid of atomic force microscopy (AFM), which shows the great capacity of the local anodic oxidation (LAO) on the potential field of nano-scale device fabrications. Moreover, the kinetic mechanisms of the anodic HfOx growth are discussed in detail.

Electron interferometry in the proximity of amorphous ultrathin SiO2∕Si

Electron standing waves (ESWs) have been excited in the proximity of amorphous ultrathin SiO2∕Si using ultrahigh vacuum scanning tunneling microscope. Distinct ESW characteristics have been discerned in conductance spectra obtained in the vicinity of ultrathin (∼1nm) SiO2 films. And these features are similar to those obtained on Si surface, showing that both the interference and coherence of electron waves degrade only slightly due to the oxide presence. In a thicker oxide (>1.5nm), no ESW features are observed. The results indicate that the ESW is a very locally confined phenomenon and attainable in amorphous films with sufficiently slender thickness and low defect density.