Phonon Kinetics and Heat Removal from Low Dimensional Semiconductor Structures

Improved technologies for growing semiconductor thin films of different structural and electronic properties, and even with layer thickness approaching atomic dimensions, have provided new opportunities for basic scientific studies and device applications. New fabrication technologies have also made it possible to reduce device dimensions to the point where quantum size effects must be considered in order to realistically describe the device operation and reliably predict their performance for electronic and optical applications. In this chapter, we shall present a partial summary of advances in low-dimensional semiconductor materials and their device applications.

We present here the results on electron transport and noise (diffusion) in low‐dimensional semiconductor structures obtained self‐consistently by the Monte Carlo technique. Electron scattering by confined and localized optical phonons in quasi‐one‐dimensional quantum wires and quasi‐two‐dimensional quantum wells is taken into account. In spite of the considerable differences in phonon spectra and their coupling with electrons, and in spite of electron quantization, the diffusivity (noise) and transport parameters in low‐dimensional structures only slightly differ from corresponding bulk material characteristics at room temperature. At low lattice temperatures the existence of several optical phonon modes in low‐dimensional structures causes electron streaming at several frequencies.

Recently, low dimensional semiconductor structures such as metallic nanoparticles, have attracted significant attention for photovoltaics. In this work we study the low-temperature formation of metallic nano-dots in an indium tin oxide (ITO) antireflection coating for solar cells after an appropriate hydrogen plasma treatment. Atomic force microscopy (AFM), scanning spreading resistance microscopy (SSRM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the analysis of the morphology, electrical properties and composition of ITO layers deposited on Corning glass at different temperatures and subsequently subjected to hydrogen plasma. Formation of highly conductive nano-structures at hydrogenation temperatures below 250degC is observed. It is shown that the nano-structures formation can be controlled by the time and the temperature of the hydrogenation

Organic materials have revolutionized optoelectronics by their processability, flexibility and low cost, with application to light-emitting devices for full-colour screens, solar cells and lasers. Some low-dimensional organic semiconductor structures exhibit properties resembling those of inorganics, such as polarized emission and enhanced electroluminescence. One-dimensional metallic, III-V and II-VI nanostructures have also been the subject of intense investigation as building blocks for nanoelectronics and photonics. Given that one-dimensional polymer nanostructures, such as polymer nanofibres, are compatible with sub-micrometre patterning capability and electromagnetic confinement within subwavelength volumes, they can offer the benefits of organic light sources to nanoscale optics. Here we report on the optical properties of fully conjugated, electrospun polymer nanofibres. We assess their waveguiding performance and emission tuneability in the whole visible range. We demonstrate the enhancement of the fibre forward emission through imprinting periodic nanostructures using room-temperature nanoimprint lithography, and investigate the angular dispersion of differently polarized emitted light.

Bandgap calculation in Si quantum dot arrays using a genetic algorithm

Electronic states of a hydrogenic donor impurity in semiconductor nano-structures

We demonstrate the monolithic integration of AlGaAs/GaAs 2DEGs with short carrier lifetime photoconductive LT-GaAs, allowing the investigation of THz frequency range magnetoplasmon resonances by the direct injection of ~picosecond duration pulses through the system. The evolution of time domain signals with magnetic field were measured, and the results found to be in good agreement with theoretical calculations made using a mode-matching approach for frequencies above 0.1 THz. Our experimental work introduces a generic technique suitable for the study of ultrafast carrier dynamics in low-dimensional semiconductor nanostructures at THz frequencies

Summary form only given. Like no other technology, integrated electronics has changed our daily life during the past 50 years. Integrated circuits have become the indispensable resource of the modern knowledge based society: For example, without ICs, the rich multimedia experience we enjoy when using the Internet, mobile phones or digital video and audio would not have been possible. This revolutionary progress is based primarily on one major development: The continuous miniaturization - The power of small! At the beginning, structures in integrated circuits were approximately 10 mum large in lateral size, which corresponds to the size of a human blood cell. Present state-of-the-art technologies are utilizing a feature size of 100 nm and below. Today, micro-electronics is in the transition to nano-electronics and device structures are smaller as a virus. Due to the continuous downscaling, integrated circuits with steadily increasing functionality and complexity have been realized. System performance has greatly enhanced, but at the same time the cost-per-function has dropped substantially. - The power of small! Also, integrated electronics has become a major driver of the economic progress worldwide: With an average annual growth rate of 15 % per year for the past three decades, the worldwide annual market of electronics with euro800 billion exceeded the global automotive market in 2004. - The power of small! The end of the silicon roadmap has been announced already many times. However, conventional semiconductor-based transistors have been continuously shrinking at a pace which has brought us today's cheap and powerful electronic products. Nevertheless, it is obvious that the traditional top-down technologies are becoming more and more impractical with the nanoscale. Beyond the Moore's law era of silicon, new bottom-up methods will be mandatory to implement e.g. low-dimensional semiconductor nanostructures, like carbon nanotubes (CNTs), which offer unique possibilities such as extremely low power dissipation, high surface sensitivity and low fabrication cost. It is generally expected, that nanotechnology as an interdisciplinary area of research which cuts across many fields - electronics, chemistry, physics, biology and engineering is capable to fill this technology-gap. This contribution will emphasize on nanotechnology for electronic applications. An attempt is made to briefly summarize the past development of macro-/micro-electronics and discuss promising technological developments and future challenges towards nanoelectronics as well.

Hydrostatic pressure effect on photoionization cross section of a trion in quantum dots

Due to their strong spin-orbit coupling III-V semiconductor nanowires are excellent candidates for electrical spin manipulation. Therefore, a major goal is to tailor spin-orbit coupling in these devices. Direct electrical spin injection into quasi one-dimensional nanowires is demonstrated. Furthermore, the weak antilocalization effect was investigated in InAs nanowires. The quantum corrections to the conductivity are interpreted by developing a quasi-one-dimensional diffusive model. It turns out that by means of doping and electric gating the spin-lifetimes can be tuned significantly. By creating few-electron quantum dots inside these devices the impact of the confinement on the spin relaxation properties is investigated.

Doping-related point defect engineering in low-dimensional semiconductor nanostructures is important to regulate their optical and electronic properties. The substitutional or interstitial location of heterovalent dopants is critical and has not been controlled effectively yet. Herein, we carefully control the kinetics of reverse cation exchange between CuxS 2D nanosheets and ligand-coordinated Cd2+ cations to control the Cu doping sites in CdS nanosheets (NSs). The substitutional and interstitial Cu dopants were directly confirmed by spherical aberration-corrected TEM (SACTEM) and their X-ray absorption spectroscopy (XAS) coordination investigation. Density functional theory (DFT) calculations and their experimental conductivities and dopant luminescence performance demonstrated the dramatic differences that are due to the location of different Cu dopants. These findings provide deeper insights on dopants' location regulation in a nanostructured host semiconductor.

Low-dimensional semiconductor nanostructures are of great interest in high performance electronic and photonic devices. ZnO is considered to be a multifunctional material due to its unique properties with potential in various applications. In this work, 3-nm ZnO nanoislands are deposited by Atomic Layer Deposition (ALD) and the electronic properties are characterized by UV-Vis-NIR Spectrophotometer and X-ray Photoelectron Spectroscopy. The results show that the nanostructures show quantum confinement effects in 1D. Moreover, Metal-Oxide-Semiconductor Capacitor (MOSCAP) charge trapping memory devices with ZnO nanoislands charge storage layer are fabricated by a single ALD step and their performances are analyzed. The devices showed a large memory window at low operating voltages with excellent retention and endurance characteristics due to the additional oxygen vacancies in the nanoislands and the deep barrier for the trapped holes due to the reduction in ZnO electron affinity. The results show that the ZnO nanoislands are promising in future low power memory applications.

Cathodoluminescence (CL) is a very powerful technique for studying the optical properties of semiconductor structures, especially low-dimensional structures (structures with sub-micron features). We will give a number of examples of studies, where CL has been used to study a variety of low-dimensional semiconductor structures. We will also compare the CL technique with other techniques for studies of luminescence with high spatial resolution.

The ratio of the spontaneous emission and Auger recombination rates for quantum wells and wires is compared theoretically with that for bulk structures. Dramatic improvements in radiative efficiency are not predicted for these novel structures unless carrier densities are greatly reduced.

On boundary condition-induced states in low-dimensional semiconductor structures