Martin Hjort

Al2O3/InAs metal-oxide-semiconductor capacitors on (100) and (111)B substrates

The influence of InAs orientations and high-k oxide deposition conditions on the electrical and structural quality of Au/W/Al2O3/InAs metal-oxide-semiconductor capacitors was investigated using capacitance-voltage (C-V) and x-ray photoemission spectroscopy techniques. The results suggest that the interface traps around the conduction band edge are correlated to the As-oxide amount, while less to those of As-As bonds and In-oxides. The quality of the deposited Al oxide determines the border trap density, hence the capacitance frequency dispersion. The comparison of different processing conditions is discussed, favoring a 350 °C high-k oxide deposition on (111)B substrates followed by an annealing procedure at 400 °C.

Direct Imaging of Atomic Scale Structure and Electronic Properties of GaAs Wurtzite and Zinc Blende Nanowire Surfaces.

Using scanning tunneling microscopy and spectroscopy we study the atomic scale geometry and electronic structure of GaAs nanowires exhibiting controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal segments. We find that the nonpolar low-index surfaces {110}, {101[overline]0}, and {112[overline]0} are unreconstructed, unpinned, and without states in the band gap region. Direct comparison between Wz and Zb GaAs reveal a type-II band alignment and a Wz GaAs band gap of 1.52 eV.

Interface composition of InAs nanowires with Al2O2 and HfO2 thin films

Vertical InAs nanowires (NWs) wrapped by a thin high-κ dielectric layer may be a key to the next generation of high-speed metal-oxide-semiconductor devices. Here, we have investigated the structure and chemical composition of the interface between InAs NWs and 2 nm thick Al2O3 and HfO2 films. The native oxide on the NWs is significantly reduced upon high-κ deposition, although less effective than for corresponding planar samples, resulting in a 0.8 nm thick interface layer with an In-/As-oxide composition of about 0.7/0.3. The exact oxide reduction and composition including As-suboxides and the role of the NW geometry are discussed in detail.

Electronic and Structural Differences between Wurtzite and Zinc Blende InAs Nanowire Surfaces: Experiment and Theory

We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior.

Surface Chemistry, Structure, and Electronic Properties from Microns to the Atomic Scale of Axially Doped Semiconductor Nanowires

Using both synchrotron-based photoemission electron microscopy/spectroscopy and scanning tunneling microscopy/spectroscopy, we obtain a complete picture of the surface composition, morphology, and electronic structure of InP nanowires. Characterization is done at all relevant length scales from micrometer to nanometer. We investigate nanowire surfaces with native oxide and molecular adsorbates resulting from exposure to ambient air. Atomic hydrogen exposure at elevated temperatures which leads to the removal of surface oxides while leaving the crystalline part of the wire intact was also studied. We show how surface chemical composition will seriously influence nanowire electronic properties. However, opposite to, for example, Ge nanowires, water or sulfur molecules adsorbed on the exterior oxidized surfaces are of less relevance. Instead, it is the final few atomic layers of the oxide which plays the most significant role by strongly negatively doping the surface. The InP nanowires in air are rather insensitive to their chemical surroundings in contrast to what is often assumed for nanowires. Our measurements allow us to draw a complete energy diagram depicting both band gap and differences in electron affinity across an axial nanowire p-n junction. Our findings thus give a robust set of quantitative values relating surface chemical composition to specific electronic properties highly relevant for simulating the performance of nanoscale devices.

Nondestructive nanostraw intracellular sampling for longitudinal cell monitoring

Significance Cell content analysis has rapidly become one of the most important new tools for measuring cell phenotype and behavior. However, the central limitation of current sampling technologies is they are destructive and must lyse the cells to measure the contents. This destruction prevents knowledge of prior or future states of the cell, which is particularly important for dynamic cell processes, such as development and differentiation. Here, we show a nondestructive longitudinal sampling and analysis platform that can sample repeatedly and accurately from the same single cell or group of cells over a long time period. We demonstrate sampling of both proteins and mRNA for cell lines as well as human-derived cardiomyocytes and astrocytes. Here, we report a method for time-resolved, longitudinal extraction and quantitative measurement of intracellular proteins and mRNA from a variety of cell types. Cytosolic contents were repeatedly sampled from the same cell or population of cells for more than 5 d through a cell-culture substrate, incorporating hollow 150-nm-diameter nanostraws (NS) within a defined sampling region. Once extracted, the cellular contents were analyzed with conventional methods, including fluorescence, enzymatic assays (ELISA), and quantitative real-time PCR. This process was nondestructive with >95% cell viability after sampling, enabling long-term analysis. It is important to note that the measured quantities from the cell extract were found to constitute a statistically significant representation of the actual contents within the cells. Of 48 mRNA sequences analyzed from a population of cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs), 41 were accurately quantified. The NS platform samples from a select subpopulation of cells within a larger culture, allowing native cell-to-cell contact and communication even during vigorous activity such as cardiomyocyte beating. This platform was applied both to cell lines and to primary cells, including CHO cells, hiPSC-CMs, and human astrocytes derived in 3D cortical spheroids. By tracking the same cell or group of cells over time, this method offers an avenue to understand dynamic cell behavior, including processes such as induced pluripotency and differentiation.

Electrical and Surface Properties of InAs/InSb Nanowires Cleaned by Atomic Hydrogen

We present a study of InAs/InSb heterostructured nanowires by X-ray photoemission spectroscopy (XPS), scanning tunneling microscopy (STM), and in-vacuum electrical measurements. Starting with pristine nanowires covered only by the native oxide formed through exposure to ambient air, we investigate the effect of atomic hydrogen cleaning on the surface chemistry and electrical performance. We find that clean and unreconstructed nanowire surfaces can be obtained simultaneously for both InSb and InAs by heating to 380 ± 20 °C under an H2 pressure 2 × 10(-6) mbar. Through electrical measurement of individual nanowires, we observe an increase in conductivity of 2 orders of magnitude by atomic hydrogen cleaning, which we relate through theoretical simulation to the contact-nanowire junction and nanowire surface Fermi level pinning. Our study demonstrates the significant potential of atomic hydrogen cleaning regarding device fabrication when high quality contacts or complete control of the surface structure is required. As hydrogen cleaning has recently been shown to work for many different types of III-V nanowires, our findings should be applicable far beyond the present materials system.

Doping profile of InP nanowires directly imaged by photoemission electron microscopy

InP nanowires (NWs) with differently doped segments were studied with nanoscale resolution using synchrotron based photoemission electron microscopy. We clearly resolved axially stacked n-type and undoped segments of the NWs without the need of additional processing or contacting. The lengths and relative doping levels of different NW segments as well as space charge regions were determined indicating memory effects of sulfur during growth. The surface chemistry of the nanowires was monitored simultaneously, showing that in the present case, the doping contrast was independent of the presence or absence of a native oxide.

Atomic Scale Surface Structure and Morphology of InAs Nanowire Crystal Superlattices: The Effect of Epitaxial Overgrowth

While shell growth engineering to the atomic scale is important for tailoring semiconductor nanowires with superior properties, a precise knowledge of the surface structure and morphology at different stages of this type of overgrowth has been lacking. We present a systematic scanning tunneling microscopy (STM) study of homoepitaxial shell growth of twinned superlattices in zinc blende InAs nanowires that transforms {111}A/B-type facets to the nonpolar {110}-type. STM imaging along the nanowires provides information on different stages of the shell growth revealing distinct differences in growth dynamics of the crystal facets and surface structures not found in the bulk. While growth of a new surface layer is initiated simultaneously (at the twin plane interface) on the {111}A and {111}B nanofacets, the step flow growth proceeds much faster on {111}A compared to {111}B leading to significant differences in roughness. Further, we observe that the atomic scale structures on the {111}B facet is different from its bulk counterpart and that shell growth on this facet occurs via steps perpendicular to the ⟨112⟩B-type directions.

Epitaxial growth and surface studies of the Half Heusler compound NiTiSn (001)

The Half Heuslers are currently an attractive family of compounds for high temperature thermoelectrics research, and recently, there has been renewed interest since some of these compounds are proposed to be topological insulators. NiTiSn belongs to the family of 18 valence electron Half Heuslers that are predicted to be semiconducting, despite being composed entirely of metallic elements. The growth of the Half Heusler compound NiTiSn by molecular beam epitaxy is demonstrated. The NiTiSn films are epitaxial and single crystalline as observed by reflection high-energy electron diffraction and x-ray diffraction. Temperature dependent transport measurements suggest the films may be semiconducting, but with a high background carrier density indicative of a high density of electrically active defect states. Methods of protecting the sample surface for synchrotron-based photoemission measurements are explored. These methods may be applied to the study of surface electronic structure in unconventional materials.