Synthesis and characterization of group II-Vi semiconductor nanowires

Semiconductor nanowires are an emerging class of nanostructures that represent attractive building blocks for nanoscale electronic and photonic devices. To the present, nanowires are synthesized on a small scale by experimentally demanding gas phase deposition techniques. This thesis describes a simple and cost-efficient solution-based synthesis towards high-quality nanowires of Group II-VI compound semiconductors. The effects of various reaction parameters and of doping on the wires' structural, electrical and magnetic properties are investigated. The first part of this thesis is dedicated to a novel variant of the solvothermal synthesis of CdS nanowires employing a single-source precursor. The wires are of single-crystalline quality and can be indexed to wurtzite-type bulk CdS with the preferential growth direction along the axis. As-grown nanowires have a smooth surface, are up to 40 µm long and exhibit aspect ratios up to 1000. The wires' aspect ratio can be increased by choosing a low precursor concentration. Time-dependent experiments reveal the wires to axially grow at a rate of ≈1µm/h while lateral growth only occurs during the initial reaction phase. As shown by photoluminescence spectroscopy the surface defect density can be significantly lowered by raising the reaction temperature from 180 to 200 °C. Electrically contacted CdS nanowires are highly resistive in the dark, but become more conductive by 4 orders of magnitude upon illumination above the bandgap energy (λ

Field-Effect Transistors Based on ZnO Nanowires

Daniel Dominik Kälblein

Semiconductor nanowires are an emerging class of materials with great potential for applications in future electronic devices. The small footprint and the large charge-carrier mobilities of nanowires make them potentially useful for applications with high-integration density, but also to replace thin-film transistors in large-area electronics. This thesis investigates the use of wet-chemically grown ZnO nanowires for the fabrication of high-performance, low-voltage field-effect transistors (FETs). The first part of this thesis addresses the electrical characteristics of the wet-chemically grown ZnO nanowires. The as-grown ZnO nanowires are highly conductive due to a large charge-carrier concentration caused by dopants unintentionally incorporated into the ZnO nanowires during the synthesis. This large charge-carrier concentration makes it difficult to modulate the conductivity of the nanowires by an external electric field, so that the as-grown wires are not suitable for FETs. A post-growth annealing step is employed to dramatically reduce the charge-carrier concentration of the nanowires which makes it possible to fabricate FETs with useful characteristics. ZnO-nanowire FETs in a global back-gate geometry based on annealed ZnO nanowires have a transconductance of 300 nS, an on/off current ratio of 107 and a subthrehold slope of 500 mV/decade. The maximum field-effect mobility of the annealed ZnO nanowires is around 50 cm2/Vs. An important requirement to obtain good FET characterisitcs is to minimize the influence of the source and drain contact resistance on the total device resistance. The material used for the source and drain contacts in this thesis is aluminum. It is demonstrated that the use of a plasma treatment in the contact regions prior to the evaporation of the aluminum contacts can greatly influence the contact properties between ZnO and aluminum. An argon-plasma treatment locally increases the charge-carrier concentration in the ZnO. It is shown that the doping effect of the argon-plasma treatment can be exploited to reduce the contact resistance between alumiunm and ZnO and improve the electrical performance of the FETs. In the second part of this thesis, the influence of the ambient air on the electrical characteristics of the ZnO-nanowire FETs is investigated. The electrical conductivity of the FETs is strongly affected by the distinct atmospheric conditions, which makes it difficult to obtain reliable FET characteristics. The potential of a self-assembled monolayer (SAM) based on fluoroalkylphosphonic acid molecules to passivate the ZnO nanowires against the undesirable effects of the ambient and to stabilize the electrical performance of ZnO-nanowire FETs is demonstrated. The stabilizing effect is attributed to the formation of a densely packed, hydrophobic SAM on the surface of ZnO and aluminum oxide. The quality of the hydrophobic SAMs is confirmed by means of water-contact-angle measurements on SAM-covered ZnO single crystals that show a contact angle of more than 110°. The third part of this thesis is dedicated to the fabrication of high-performance ZnO-nanowire FETs with patterned metal top-gate electrodes. A top-gate fabrication process is developed that uses a very thin gate dielectric consisting only of an alkylphosphonic acid SAM that can be deposited from solution. The insulating quality of the SAM is investigated for top-gate FETs that utilize either gold or aluminum for the top-gate electrode. Gold top-gate FETs show a transconductance of 1 µS, an on/off current ratio of 107, and a steep subthreshold slope of 90 mV/decade. The thin gate dielectric makes it possible to operate the gold top-gate FETs at low voltages of 1 V and simultaneously reduces the undesired gate current by more than three orders of magnitude compared to gold top-gate FETs that have been fabricated without a gate dielectric (MESFETs). The gate current of the gold top-gate FETs with SAM gate dielectric is as low as 1 pA for gate-source voltages between -1 V and 0.5 V. When aluminum is used for the top-gate electrode, a hybrid dielectric is formed at the interface between the SAM and the aluminum, consisting of the SAM and a spontaneously formed aluminum oxide layer. Owing to the hybrid gate dielectric, the aluminum top-gate FETs are operated with gate currents below 1 pA for voltages up to 3 V. The top-gate fabrication process does not require temperatures above 160 °C, which makes it possible to fabricate FETs on unconventional substrates that cannot tolerate high temperatures, such as glass or flexible plastics. Integrated circuits on single ZnO nanowires that are fabricated on glass and plastic substrates are demonstrated. The maximum switching frequency of the inverters is 1 MHz.

EPFL2011

Nanoscale Organic / Inorganic Hybrid Photovoltaic Devices

Thomas Dufaux

There is an increasing trend of using various types of nanostructures as components of solar cells. This highlights the need to gain a deeper understanding of the nanoscale interface in such devices. The present thesis aims to study individual photoactive junctions as the elementary functional unit of bulk photovoltaic devices, and exploit the gained knowledge toward further improving the solar cells' efficiency. Along this direction, Schottky-contacts to cadmium sulphide nanowires were thoroughly studied using scanning photocurrent microscopy (SPCM). In conjunction with theoretical simulations of the measured photocurrent profiles, it was found that charge carriers can be very efficiently extracted out of a few micrometer long nanowire segments below the metal contact. Moreover, the carrier combination rate in this section could be determined as a function of an applied bias or backgate voltage. These findings provide valuable clues about how electric fields are distributed within semiconductor nanowire-based devices. Moreover, SPCM was employed to explore the local photoresponse along graphene/CdS heterojunctions, representing a first step toward implementing graphene as an active acceptor material into solar cells. The short circuit current of such devices could be enhanced by two orders of magnitudes through chemical tailoring of the graphene-CdS interface. In addition, evidence was obtained that surface plasmon excitation in the metal contacts can make significant contributions to the generated photocurrent. This effect was exploited in the fabrication of novel surface plasmon detectors, which support different plasmon modes depending on the polarization direction of the incoming light. Finally, it was attempted to realize photovoltaic devices comprising a pn-junction implemented into a π-conjugated organic polymer. While the electric-field assisted exciton dissociation in this system could be successfully modelled in the framework of the Braun-Onsager model, photoinduced charge separation at the interface between differently gated regions of the polymer could not be achieved due to insufficient electron conduction of the material.

EPFL2012

Growth and optical, electrical and mechanical characterization of zinc oxide nanowires

Martial Duchamp

The topic of this dissertation is embedded into the new-born field of inorganic nanowires. The research was focused on zinc oxide (ZnO) nanowires, in particular, which besides posing fundamental questions in physics, promise broad range of applications. ZnO nanowires were grown by chemical-vapour-deposition (CVD) using the vapour-liquid-solid (VLS) mechanism with gold particles as catalysts. Oxygen was introduced in the argon carrier gas and zinc was carboreduced from the ZnO/C mixture. We produced micrometer long ZnO nanowires and their diameter was controlled by the size of the gold catalyst particles. The diameter range of our ZnO nanowires is between 30 to 100nm. After growth, the alloyed particles were found at the top of the nanowires. ZnO nanowires are composed of single crystals of ZnO in the wurtzite phase. Growth direction is defined by their epitaxial relation with the substrate. From photoluminescence (PL) and cathodoluminescence (CL) characterizations, a deep impurity energy level at 2.3eV was found in a band gap of 3.3 eV, which could be related to oxygen off-stoichiometry. PL measurements showed that this energy level does not depend on the oxygen concentration used during the growth. Scanning-Tunnelling-Microscopy-based CL (STM-CL) measurements were made on single ZnO nanowires and showed that this energy level is likely to come from curved nanowires. Electrical measurements were performed on single nanowires, by using dielectrophoresis to place individual ZnO nanowires at a predefined location. E-beam lithography was used to deposit four metallic contacts on top of individual micrometer long ZnO nanowires. In order to avoid oxidation and the Shockley effect, we used chromium nitride (CrN) as electrical contacts. By measuring the temperature dependence on the resistance we extracted the activation energy of 2eV for charge transport, possibly arising from the same impurity level, as measured in PL. In the field-effect-transistor (FET) configuration, by using PMMA as dielectric layer and Cr as gate electrode, we extracted the charge carrier mobility of an individual ZnO nanowire as being 9.5 cm2/(Vs). In order to discover the origin of the energy level in the band gap, we measured the relaxation time of the current, after UV-illumination was switched off, and we found out that the surface states have significant effects. The elastic properties of individual ZnO nanowires were studied as a function of diameter and point defect/dislocation concentration. Using dielectrophoresis, nanowires were placed over a hole of a microfabricated Si3N4 membrane and, from bending measurements using an atomic force microscope (AFM) the Young's modulus was extracted. The diameter dependence of the Young's modulus showed a maximal value for a diameter of 50nm. Annealing under argon atmosphere showed an improvement of the Young's modulus which is explained by the removal of the defects/dislocations left during the growth. For comparison, the elastic properties of ZnS nanotubes produced by atomic layer deposition (ALD) were also measured. A clear dependence of the Young's modulus on the diameter and the wall thickness were found and attributed to the variation of the crystal structure of the nanotubes.

EPFL2010