Novel Processing and Electrical Characterization of Nanowires

This thesis investigates novel electrical nanowire characterization tools and devices. Conventional characterization methods, long available to bulk semiconductor samples, have been adapted and transferred to the nanowire geometry. The first part of the thesis describes the development of Hall effect measurements, an entirely new characterization tool for nanowires. It is shown that Hall effect measurements can be performed on InP core-shell nanowires using a self-aligned lifting layer. By combining experimental data with results from simulations of band diagrams and current distribution under the influence of a magnetic field, the carrier density in the n-type nanowire shell can be determined. We found that the nanowire shell exhibits a doping gradient along the length of the nanowire. This doping inhomogeneity is important to account for and engineer when making devices using InP nanowires. The second part of the thesis demonstrates how capacitance-voltage measurements can be performed on arrays of InAs nanowires. Using a novel device structure, the capacitance signal from the nanowires can be distinguished from parasitic capacitances. A model was developed to simulate the capacitance-voltage behavior of the nanowires and was fitted to the experimental data to extract the doping concentration. Furthermore, the hysteresis observed in the capacitance-voltage sweeps was used to calculate the trap density close to the InAs-HfO2 dielectric interface. Finally, studies of gate effects in nanowires are presented. We demonstrate how the gating efficiency is improved by means of wrapped or semiwrapped gates on nanowires. Field-effect transistors made from InP nanowires are described that exhibit both ambipolar behavior and gating efficiency only 13% lower than the theoretical limit. These properties were used to fabricate a field-effect diode, a device in which a p-n junction is formed without any doping incorporation in the active region. We also demonstrate how nanowires positioned laterally on a substrate can be equipped with a fully wrapped gate electrode. This device was developed as part of a platform to perform basic research in which a uniform gating effect is desired

Realizing lateral wrap-gated nanowire FETs: Controlling gate length with chemistry rather than lithography

An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A nanowire with a coaxial metal gate provides optimal gate-channel coupling, but has only been realized for vertically oriented nanowire transistors. We report a method for producing laterally oriented wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. It allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the sub-threshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 to 4 Kelvin, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing.Comment: 16 pages, 5 figures. Submitted version, published version available at http://http://pubs.acs.org/journal/nalef

Simplifying Nanowire Hall Effect Characterization by Using a Three-Probe Device Design

Electrical characterization of nanowires is a time-consuming and challenging task due to the complexity of single nanowire device fabrication and the difficulty in interpreting the measurements. We present a method to measure Hall effect in nanowires using a three-probe device that is simpler to fabricate than previous four-probe nanowire Hall devices and allows characterization of nanowires with smaller diameter. Extraction of charge carrier concentration from the three-probe measurements using an analytical model is discussed and compared to simulations. The validity of the method is experimentally verified by a comparison between results obtained with the three-probe method and results obtained using four-probe nanowire Hall measurements. In addition, a nanowire with a diameter of only 65 nm is characterized to demonstrate the capabilities of the method. The three-probe Hall effect method offers a relatively fast and simple, yet accurate way to quantify the charge carrier concentration in nanowires and has the potential to become a standard characterization technique for nanowires

The Risk of Cancer Among Patients Previously Hospitalized for Atopic Dermatitis

In treatment of severe atopic dermatitis, drugs with carcinogenic potentials are used to manage the disease. We therefore analyzed whether patients having severe atopic eczema had an increased cancer risk. The study population included all individuals hospitalized in Denmark with a primary diagnosis of atopic dermatitis during 1977–1996. Follow-up was conducted in 1996 in the Danish Cancer Register. A total of 6275 persons were included. Among 2030 adult patients, an increased risk of cancer was observed, standard morbidity ratio (SMR)=1.5 (95% CI: 1.2–1.9). Half the excess cases of cancer was keratinocyte carcinomas of the skin diagnosed within the first 9 y of follow–up, SMR=2.4 (95% CI: 1.4–3.9). For men, SMR=2.7 (95%CI: 1.2–5.4). In conclusion, earlier hospitalized adult atopic dermatitis patients had an increased risk of cancer. Half the excess cases of cancer were keratinocyte carcinomas. This may be a result of a detection bias or due to the carcinogenic potentials of some of the therapies of severe atopic dermatitis

The Risk of Cancer Among Patients Previously Hospitalized for Atopic Dermatitis

In treatment of severe atopic dermatitis, drugs with carcinogenic potentials are used to manage the disease. We therefore analyzed whether patients having severe atopic eczema had an increased cancer risk. The study population included all individuals hospitalized in Denmark with a primary diagnosis of atopic dermatitis during 1977–1996. Follow-up was conducted in 1996 in the Danish Cancer Register. A total of 6275 persons were included. Among 2030 adult patients, an increased risk of cancer was observed, standard morbidity ratio (SMR)=1.5 (95% CI: 1.2–1.9). Half the excess cases of cancer was keratinocyte carcinomas of the skin diagnosed within the first 9 y of follow–up, SMR=2.4 (95% CI: 1.4–3.9). For men, SMR=2.7 (95%CI: 1.2–5.4). In conclusion, earlier hospitalized adult atopic dermatitis patients had an increased risk of cancer. Half the excess cases of cancer were keratinocyte carcinomas. This may be a result of a detection bias or due to the carcinogenic potentials of some of the therapies of severe atopic dermatitis

Doping Incorporation in InAs nanowires characterized by capacitance measurements

Sn and Se dopedInAsnanowires are characterized using a capacitance-voltage technique where the threshold voltages of nanowirecapacitors with different diameter are determined and analyzed using an improved radial metal-insulator-semiconductor field-effect transistor model. This allows for a separation of doping in the core of the nanowire from the surface charge at the side facets of the nanowire. The data show that the doping level in the InAsnanowire can be controlled on the level between 2×10¹⁸ to 1×10¹⁹ cm¯³, while the surface charge density exceeds 5×10¹² cm¯² and is shown to increase with higher dopant precursor molar fraction.This work was supported by the Swedish Research Council, the Swedish Foundation for Strategic Research, VINNOVA, the EU-project NODE 015783 and the Knut and Alice Wallenberg Foundation

Doping Incorporation in InAs nanowires characterized by capacitance measurements

Sn and Se doped InAs nanowires are characterized using a capacitance-voltage technique where the threshold voltages of nanowire capacitors with different diameter are determined and analyzed using an improved radial metal-insulator-semiconductor field-effect transistor model. This allows for a separation of doping in the core of the nanowire from the surface charge at the side facets of the nanowire. The data show that the doping level in the InAs nanowire can be controlled on the level between 2×1018 to 1×1019 cm−3, while the surface charge density exceeds 5×1012 cm−2 and is shown to increase with higher dopant precursor molar fraction

InAs nanowire metal-oxide-semiconductor capacitors

We present a capacitance-voltage study for arrays of vertical InAs nanowires. Metal-oxide-semiconductor (MOS) capacitors are obtained by insulating the nanowires with a conformal 10 nm HfO2 layer and using a top Cr/Au metallization as one of the capacitor's electrodes. The described fabrication and characterization technique enables a systematic investigation of the carrier density in the nanowires as well as of the quality of the MOS interface

Scanning gate imaging of quantum dots in 1D ultra-thin InAs/InP nanowires

We use a scanning gate microscope (SGM) to characterize one-dimensional ultra-thin (diameter ≈ 30 nm) InAs/InP heterostructure nanowires containing a nominally 300 nm long InAs quantum dot defined by two InP tunnel barriers. Measurements of Coulomb blockade conductance versus backgate voltage with no tip present are difficult to decipher. Using the SGM tip as a charged movable gate, we are able to identify three quantum dots along the nanowire: the grown-in quantum dot and an additional quantum dot near each metal lead. The SGM conductance images are used to disentangle information about individual quantum dots and then to characterize each quantum dot using spatially resolved energy-level spectroscopy.Physic

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Nanowires have the potential to play an important role for next-generation light-emitting diodes. In this work, we present a growth scheme for radial nanowire quantum-well structures in the AlGaInP material system using a GaInP nanowire core as a template for radial growth with GaInP as the active layer for emission and AlGaInP as charge carrier barriers. The different layers were analyzed by X-ray diffraction to ensure lattice-matched radial structures. Furthermore, we evaluated the material composition and heterojunction interface sharpness by scanning transmission electron microscopy energy dispersive X-ray spectroscopy. The electro-optical properties were investigated by injection luminescence measurements. The presented results can be a valuable track toward radial nanowire light-emitting diodes in the AlGaInP material system in the red/orange/yellow color spectrum