Oktay Yilmazoglu

First Observation of Bias Oscillations in GaN Gunn Diodes on GaN Substrate

In this paper, we report on the bias oscillation of GaN-based Gunn diodes realized on a n+-GaN substrate. Different contact materials, ambient gases, and pulsewidths were used and compared with regard to device stability. A wide negative- differential-resistance (NDR) region was measured for electrical- field values E larger than a threshold field Eth of 150 kV/cm. Electrical fields much higher than the threshold value did not lead to any electromigration effects or discharging problems from the contacts. The drift velocity derived from the current-voltage characteristics, diode geometry, and doping concentration in the active layer was estimated to be 1.9 times 107 cm/s. Bias oscillations were obtained for the GaN Gunn diodes in the presence of a series inductance.

Quantum-size resonance tunneling in the field emission phenomenon

Theoretical analyses have been performed of the quantum-size (QS) resonance tunneling in the field-emission (FE) phenomenon for different models of the emitting structures. Such experimentally observed peculiarities have been considered as the enhancement of the FE current, the deviation from the Fowler-Nordheim law, the appearance of sharp current peaks, and a negative resistance. Different types of FE cathodes with QS structures (quantized layers, wires, or dots) have been studied experimentally. Resonance current peaks have been observed, from which the values of the energy-level splitting can be estimated.

Vacuum nanoelectronic devices : novel electron sources and applications

Description: Vacuum Nanoelectronic Devices introduces up–to–date coverage of research in electron field emission from nanostructures. It outlines the physics of quantum nanostructures, the basic principles of electron field emission and vacuum nanoelectronic devices operation, and offers an insight into the state–of–the–art and future research and developments. The book also evaluates the results of research and development into novel quantum electron sources, which will determine the future development of vacuum nanoelectronics. Moreover, the influence of quantum mechanical effects on high frequency vacuum nanoelectronic devices is also assessed. Key features: In–depth description and analysis of the fundamentals of quantum electron effects in novel electron sources; Comprehensive and up–to–date summary of the physics and technologies for THz sources for students of physical and engineering specialties and electronics engineers; Unique coverage of quantum physical results for electron–field emission and novel electron sources with quantum effects, relevant for many applications such as electron microscopy, electron lithography, imaging and communication systems and signal processing; New approaches for the realization of electron sources with required and optimal parameters in electronic devices such as vacuum micro and nanoelectronics. This book is an essential reference for researchers working in terahertz technology, who want to expand their knowledge of electron beam generation in vacuum and electron source quantum concepts. It will also be invaluable to advanced students in electronics engineering and physics who want to deepen their understanding of this topic. Ultimately, the progress of the quantum nanostructure theory and technology will promote the progress and development of electron sources as a main part of vacuum macro–, micro– and nanoelectronics.

Formation of conducting nanochannels in diamond-like carbon films

A sharp increase of the emission current at high electric fields and a decrease of the threshold voltage after pre-breakdown conditioning of diamond-like carbon (DLC) films have been measured. This effect was observed for DLC-coated silicon tips and GaAs wedges. During electron field emission (EFE) at high electric fields the energy barriers caused by an sp3 phase between sp2 inclusions can be broken, resulting in the formation of conducting nanochannels between the semiconductor–DLC interface and the surface of the DLC film. At high current densities and the resulting local heating, the diamond-like sp3 phase transforms into a conducting graphite-like sp2 phase. As a result an electrical conducting nanostructured channel is formed in the DLC film. The diameter of the conducting nanochannel was estimated from the reduced threshold voltage after pre-breakdown conditioning to be in the range of 5–25 nm. The presence of this nanochannel in an insulating matrix leads to a local enhancement of the electric field and a reduced threshold voltage for EFE. Based on the observed features an efficient method of conducting nanochannel matrix formation in flat DLC films for improved EFE efficiency is proposed. It mainly uses a silicon tip array as an upper electrode in contact with the DLC film. The formation of nanochannels starts at the interface between the tips and the DLC film. This opens new possibilities of aligned and high-density conducting channel formation.

Evidence of satellite valley position in GaN by photoexcited field emission spectroscopy

GaN field emitter rods with nanometer diameter were fabricated by photoelectrochemical etching on a n+-GaN substrate. Their electron field emission properties were investigated under ultraviolet (UV) illumination. The Fowler–Nordheim plots of the emission current show different slopes for nonilluminated and UV illuminated devices. A model based on the electron emission from valleys having different specific electron affinities is proposed to explain the experimental results. In the absence of illumination, the GaN rods are almost fully depleted and emission takes place only from the lower valley. Upon UV illumination and presence of a high electric field at the emitter tip, the upper valley of the conduction band appears to be occupied by electrons generated at the valence band. The energy difference between the lower and upper valleys was determined to be 1.15eV and is in good agreement with formerly published theoretical and measured values.

Strain sensitivity of AlGaN/GaN HEMT structures for sensing applications

Sensing elements based on AlGaN/GaN HEMT and Schottky diode structures have been investigated in relation with the strain sensitivity of their characteristics. Piezoresistance of the Al 0.3 Ga 0.7 N/GaN HEMT-channel as well as changes in the current-voltage characteristics of the Schottky diodes have been observed with gauge factor (GF) values ranging between 19 and 350 for the selected biasing conditions. While a stable response to strain was measured, the observed temperature dependence of the channel resistance demonstrates the need for a systematic characterisation of the sensor properties to allow compensation of the observed temperature effects.

Gunn effect in field-emission phenomena

The peculiarities of electron field emission from nanostructured GaN surface have been investigated. The current–voltage characteristics of emission current in Fowler–Nordheim plot show two parts with different slopes. There are emission current oscillations in the changing slope region. As an explanation for the experimental results a model based on the electron-emission analysis from lower (Γ) valley, upper (U) valley, and electron transition between valleys due to heating in electric field has been proposed. The electron affinities for the emission from Γ and U valleys have been determined. The decreased affinities from there valleys have been estimated for quantization in nanostructured GaN.

Patterned growth of ultra long carbon nanotubes. Properties and systematic investigation into their growth process

High aspect ratio carbon nanotube arrays were grown using a hydrogen/water assisted catalytic chemical vapor deposition method using ethylene as precursor which achieves a growth rate of 25 to 30 μm carbon nanotube length per minute. The influence of the growth parameters determining the growth rate and the growth height of the carbon nanotubes arrays was studied in detail. Water and hydrogen concentration were varied systematically and it was found that they have to be in a precise ratio in order to establish ultralong growth together with high quality (no contamination with carbonaceous side products) of the arranged carbon nanotubes. For given growth conditions, the hydrogen content in the feed gas stream controls the carbon nanotube length, the purity level and the number of graphitic walls of carbon nanotubes grown. It was found that water acts as a weak oxidant keeping the mixed aluminium/iron catalyst active for ultrahigh length growth. Its concentration should be established very precisely in the process to achieve a maximum growth rate of carbon nanotubes. Furthermore a straightforward, non-costly mask technique is presented to achieve highly structured carbon nanotube growth. Finally the field emission characteristics of such ultrahigh, well arranged and structured carbon nanotube blocks was studied and this revealed low turn-on electric and threshold field values of 0.34 V μm−1 and 0.71 V μm−1 respectively.

Two mechanisms of negative dynamic conductivity and generation of oscillations in field-emissions structures

There is great interest in the generation and amplification of high-frequency oscillations and their use in vacuum microelectronic devices. The main advantages of such devices are radiation hardness and reduced switching times due to electron transport in vacuum. In this presentation two proposed mechanisms for the generation of high-frequency oscillations are investigated and analysed. The first mechanism is connected with the generation of oscillations in field emission structures based on silicon (or metal) tips with multilayer film coatings. Several types of silicon-based structures are investigated, namely silicon tips coated with multilayer films including a Si delta-doped layer (Si-SiO 2 -Si-SiO 2 ) and coated with ultrathin (<10 nm) diamond-like carbon (DLC) films. Theoretical calculations have shown possibilities of resonant tunnelling in the electron transport mechanism through such coatings when electron field (FE) emission takes place. The current-voltage characteristics of a FE with negative differential conductivity are experimentally observed. Estimations suggest the possibility of generation of oscillations with frequencies between 200 MHz and 100 THz in such structures. The second mechanism is based on direct generation of oscillations in GaAs Gunn cathodes with emission to an anode. GaAs vertical and lateral electrodes have been produced and investigated. Oscillation in the current-voltage characteristics was observed. In this case there is no limitation related to transit-time effects in the input region; this is because the emitted electrons are not under the influence of an oscillating electric field.

Electron field emission from wide bandgap semiconductors under intervalley carrier redistribution

Electron field emission phenomena from semiconductors (and, in particular, wide band gap materials) are analyzed theoretically for the general case, i.e., by taking into consideration aspects that have not been considered earlier such as two (or more) valleys of the energy band structure, nondegenerated statistics for the free electrons, heating of conduction band electrons, intervalley carrier redistribution under applied electrical fields, size quantization of electron band spectra, and change in the field emission characteristics. Comparisons with experiments performed on the highly structured (micro- and nano) surfaces of the GaN wide bandgap semiconductor have been made. The influence of the above factors on the current-voltage Fowler–Nordheim characteristics was demonstrated by theory and experiment. From theoretical and experimental results the intervalley energy difference (ΔE) for GaN quantum-sized cathodes was estimated to be 0.8 eV, which is considerably less than that predicted for bulk semico...