Hung-Wei Shiu

Multilength-Scale Chemical Patterning of Self-Assembled Monolayers by Spatially Controlled Plasma Exposure: Nanometer to Centimeter Range

We present a generic and efficient chemical patterning method based on local plasma-induced conversion of surface functional groups on self-assembled monolayers (SAMs). Here, spatially controlled plasma exposure is realized by elastomeric poly(dimethylsiloxane) (PDMS) contact masks or channel stamps with feature sizes ranging from nanometer, micrometer, to centimeter. This chemical conversion method has been comprehensively characterized by a set of techniques, including contact angle measurements, X-ray photoelectron spectroscopy (XPS), scanning photoelectron microscopy (SPEM), scanning electron microscopy (SEM), and scanning Kelvin probe microscopy (SKPM). In particular, XPS and SPEM can be used to distinguish regions of different surface functionalities and elucidate the mechanism of plasma-induced chemical conversion. In the case of an octadecyltrichlorosilane (OTS) monolayer, we show that exposure to low-power air plasma causes hydroxylation and oxidation of the methyl terminal group on an OTS-covered Si surface and generates polar functional groups such as hydroxyl, aldehylde, and carboxyl groups, which can allow subsequent grafting of dissimilar SAMs and adsorption of colloid nanoparticles onto the patterned areas with an achievable resolution down to the 50 nm range.

Is electron accumulation universal at InN polar surfaces

Recent experiments indicate the universality of electron accumulation and downward surface band bending at as-grown InN surfaces with polar or nonpolar orientations. Here, we demonstrate the possibility to prepare flatband InN ( 000 1 ¯ ) surfaces. We have also measured the surface stoichiometry of InN surfaces by using core-level photoelectron spectroscopy. The flatband InN ( 000 1 ¯ ) surface is stoichiometric and free of In adlayer. It implies that the removal of In adlayer at the InN ( 000 1 ¯ ) surface leads to the absence of downward surface band bending. On the other hand, the stoichiometric InN (0001) surface still exhibits surface band bending due to the noncentrosymmetry in the wurtzite structure.

Valence band offset and interface stoichiometry at epitaxial Si3N4/Si(111) heterojunctions formed by plasma nitridation

Ultrathin {beta}-Si{sub 3}N{sub 4}(0001) epitaxial films formed by N{sub 2}-plasma nitridation of Si(111) substrates have been studied by photoelectron spectroscopy using synchrotron radiation. The valence band offset at the {beta}-Si{sub 3}N{sub 4}/Si interface was determined by valence-band photoelectron spectra to be 1.8 eV. Furthermore, the Si 2p core-level emissions were analyzed for nitride (Si{sup 4+}) and subnitride (Si{sup 3+} and Si{sup +}) components to characterize the interface stoichiometry. In contrast to the interfaces formed by ammonia thermal nitridation and N{sub 2}-plasma nitridation at room temperature, the interface formed by N{sub 2}-plasma nitridation at high substrate temperature is very close to subnitride free with an abrupt composition transition.

Natural band alignments of InN/GaN/AlN nanorod heterojunctions

Valence band alignments of wurtzite III-nitride semiconductorheterojunctions are investigated using cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically −c-axis-aligned heterostructurenanorod array. The nonpolar measurement geometry and near fully relaxed lattice structure allow for the determination of “natural” band alignments without the influence of spontaneous and piezoelectricpolarization fields. The valence band offsets of InN/GaN, GaN/AlN, and InN/AlN are measured to be 0.8 ± 0.1, 0.6 ± 0.1, and 1.4 ± 0.1 eV, respectively. These results are in good agreement with previous data for heteroepitaxial films and obey the expected transitivity rule.

Direct imaging of GaN p-n junction by cross-sectional scanning photoelectron microscopy and spectroscopy

uted by them to the existence of high-density steps and defects at the cleavage surface. In this work, we provide direct evidence for the unpinned nature of cleaved a-plane GaN surfaces. Furthermore, we report on the application of XSPEM/S technique for direct imaging of GaN p-n junction. The sample structure for this study is N-polar p-GaN 1.5 m /n-GaN 1.5 m /AlN 25 nm /Si 3 N 4 /Si111, which was grown by plasma-assisted molecular beam epitaxy PA-MBE on a Si wafer. Details of the growth technique can be found elsewhere. 11 The n- and p-type doping was performed by using high-purity Si and Mg solid cells during the PA-MBE growth process. The Mg concentration as determined by secondary ion mass spectroscopy is 7

Synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy of n-ZnO:Al/p-GaN:Mg heterojunction

Al-doped ZnO (AZO) deposited by radio frequency co-sputtering is formed on epitaxial Mg-doped GaN template at room temperature to achieve n-AZO/p-GaN heterojunction. Alignment of AZO and GaN bands is investigated using synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically c-axis aligned heterostructure. It shows type-II band configuration with valence band offset of 1.63 ± 0.1 eV and conduction band offset of 1.61 ± 0.1 eV, respectively. Rectification behavior is clearly observed, with a ratio of forward-to-reverse current up to six orders of magnitude when the bias is applied across the p-n junction.

Graphene reduction dynamics unveiled

The reduction dynamics of micron-sized defects created on chemical vapor deposition- (CVD) grown graphene through scanning probe lithography (SPL) is reported here. CVD-grown graphene was locally oxidized using SPL and subsequently reduced, making use of a focused beam of soft x-rays. During this whole process, the reduction dynamics was monitored using a combination of micro-Raman spectroscopy (μ-RS) and micro-x-ray photoelectron spectroscopy (μ-XPS). After x-ray reduction, the graphene film was found to be chemically identical (μ-XPS) but structurally different (μ-RS) from the original graphene. During reduction the population of C–C bonds was found to first increase dramatically and then decrease exponentially. By modeling the dynamics of the C=O → C–O → C–C → C=C reduction process with four coupled-rate equations and three rate constants, the conversion from C–C to C=C bonds was found to be the limiting rate.