Advances in Low-Dimensional Semiconductor Structures

: Our research on electron-phonon and phonon-phonon processes in low-dimensional semiconductor nanostructures has been focused on the effects of a surfaces and interfaces on the electron energy momentum relaxation rates due to acoustic phonon emission, new specific mechanism of electron interaction with acoustic phonons in semiconductor heterostructures, and phonon-induced processes of charge can loosing phase coherence in heterostructures. Our research provides the necessary basis for the control of 1 dissipation and for the design of nanodevices with electron-phonon relaxation and dephasing rates required specific applications. .

This issue of physica status solidi (b) provides an account of the most recent and interesting topical work presented at the 11th International Conference on High Pressure Semiconductor Physics (HPSP‐11), which was held in Berkeley, California, from 2 to 5 August 2004. This is a satellite of the 27th International Conference on the Physics of Semiconductors held in Flagstaff, Arizona, from 26 to 30 July 2004. About 65 registered participants from different countries gathered at the Clark Kerr Campus, Berkeley, for a lively and highly enjoyable conference both from the scientific and social point of view. In comparison to the previous conferences in this series, held at Guildford (2002), Sapporo (2000), Thessaloniki (1998) and Schwäbisch Gmünd (1996), the attendance was low, owing to the “post‐9/11 reality”. Several potential participants were discouraged to go through the difficult and time‐consuming procedures to obtain a visa while a few others were unable to get their visas on time. In the interest of scientific exchange and progress, we do sincerely hope that the situation improves soon enough, so the international scientific community is not forced to restrict the exchange of research results.As an advanced announcement and prelude to these proceedings, a Conference Report written by Prof. M. Cardona appeared in the October issue of physica status solidi (b) [1] and is reprinted at the end of this print edition. This issue begins with an historical article by Prof. William Paul [2], one of the pioneers of high‐pressure research in semiconductors, in which he describes the application of extremely high pressures as a powerful tool in solid‐state physics. His studies spanning over a decade (1950's and 60's) helped not only to unravel the behavior of solids, particularly semiconductors, under extreme conditions of pressure and temperature but also to obtain a deeper insight into ambient pressure properties, leading to a better understanding of a variety of physical phenomena. This conference covered several fundamental aspects of high‐pressure physics concerning the electronic band structure, the optical and vibrational properties and structural stability of many different semiconductor materials and compounds. Low‐dimensional semiconductor structures, such as quantum well heterostructures, quantum wires and self‐assembled quantum dot systems were discussed in several papers. Carbon nanotubes and organic semiconductors captured the attention of the audience for two morning sessions. Papers dealing with pressure effects on the performance and properties of diode lasers and materials like manganites and diluted magnetic semiconductors that are used in applications as storage devices were also presented at the meeting. Two prizes, one each for the best presentation in the two poster sessions, were awarded; the recipients are: (1) S. X. Li, J. Wu, E. E. Haller, W. Walukiewicz, W. Shan, Hai Lu, and W. J. Schaff, a joint collaboration between the University of California, Berkeley, and the Lawrence Berkeley National Laboratory; (2) V. Domnich, D. Ge, T. Juliano, and Y. Gogotsi, Drexel University, Philadelphia.This conference would not have been possible without the financial support from the Department of Physics, University of California, Berkeley, and from the Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley. We gratefully acknowledge the excellent secretarial support of Kate Hug and Chia‐Hua Yu as well as the cooperation of Andy Prins in all technical aspects of the meeting.The next conference on High Pressure Semiconductor Physics will be held in 2006 near Barcelona. We look forward to having another stimulating and enjoyable meeting in Spain and welcome all interesting suggestions to the planning in the two years ahead.

The engineering of the architecture of the quantum wires has shown a real challenge in the scientific community owing to their fascinating and auspicious application potential in the field of optoelectronics. The modulation of the morphology and structure of the quantum wires may give rise to the modulation of the energy levels and band offset positions to enhance the charge carriers transfer through any electronic device and improve the overall performance for the future application in the field of spintronics and photonics. Here, we proposed, for the first time, a novel rectangular architecture based AlxGa1-xAs and GaxIn1-xAs quantum wires to engineer the electron energy spectrum according to a wide range of applications in electronics and optical devices. The electron energy levels in rectangular AlxGa1-xAs and GaxIn1-xAs quantum wires with infinite potential barrier were calculated at different x values and different cross-section areas to explore the role of dopant and compared with the cylindrical shape. The calculations of the electron confinement energy in the first and second energy levels indicate that the energy value in cylindrical quantum wire is less than its value in rectangular one while for E3 the energy value in cylindrical quantum wire is larger than its value in rectangular one for all values of x. The confinement energy was found to be inversely proportional to the ratio of the doped material. The electron energy dispersion in AlxGa1-xAs and GaxIn1-xAs quantum wires of 100 nm2 cross-section area, x = 0.4 for E1, E2 and E3 with the wave vector value has been investigated. The calculations of the first and second energy levels indicated that the energy value in cylindrical quantum wire is less than its value in rectangular one for E1 and E2 while for E3 the energy value in cylindrical quantum wire is larger than its value in rectangular with a distinct value for each wave vector value for all x values. These unique features of the proposed novel architecture may open a new avenue for the future applications in photonics, spintronics and waveguides.

Nanostructured CeO2−x films have been grown by electron beam evaporation (EBE) and ion beam assisted deposition (IBAD). The CeO2−x consists of grains with size 9–28 nm and nanoscale voids enhancing the surface and quantum-size effects. We investigate the sizedependent electronic properties of nanostructured CeO2−x films and we establish relations between them and the film structure, composition and morphology. We investigate how the structure and morphology variations affect the absolute values of the dielectric function at the UV-Vis and IR spectral regions. The nature of the fundamental gap Eg of CeO2−x is also considered. In addition we study in detail the light absorption mechanisms in the UV spectral regions and we correlate them with the reported band structures of the pure CeO2 and Ce2O3. We also employ nanoindentation testing to compare the mechanical response of CeO2−x films grown by EBE and IBAD. Correlations between the mechanical properties of the films and the growth parameters have been made providing the possibility of tailoring films with good mechanical performance for optical device 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.

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

The transient grating spectroscopy is widely used to determine the diffusion coefficients of valley excitons or spins in low-dimensional semiconductor materials. Here, we present the investigation on the diffusion dynamics of the valley excitons in a high-quality large-scale mechanically exfoliated tungsten diselenide (WSe2) monolayer by this technique at room temperature. Collinearly polarized laser excitation (at a photon energy of 1.66 eV resonant to the energy of valley A-excitons) was used to introduce a spatially periodic density of valley excitons. Through probing the spatial and temporal evolution of the initial density of valley excitons, we find that the signals of transient grating exhibit an nonexponential decay, and its decay rate is independent of the period of optical grating Λ. Combined with the transient reflection measurements, we show that the exciton-exciton annihilation plays a key role in decay processes of the transient grating spectroscopy, which results in the distortion of sinusoidal gratings. Based on Einstein relationship, we estimate the diffusion coefficient of valley exciton DX = 0.7 cm2/s.

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

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

Low-dimensional semiconductor structures, often referred to as nanocrystals or quantum dots, exhibit fascinating behavior and have a multitude of potential applications, especially in the field of communications. This book examines in detail the optical properties of these structures, giving full coverage of theoretical and experimental results, and discusses their technological applications. The author begins by setting out the basic physics of electron states in crystals (adopting a 'cluster-to-crystal' approach), and goes on to discuss the growth of nanocrystals, absorption and emission of light by nanocrystals, optical nonlinearities, interface effects, and photonic crystals. He illustrates the physical principles with references to actual devices such as novel light-emitters and optical switches.

We demonstrate theoretically that a coherent manipulation of electron spins in low-dimensional semiconductor structures with a spin–orbit coupling by infrared radiation is possible. The proposed approach is based on using a dipole force acting on a two-level system in a nonuniform optical field, similar to that employed in the design of the cold atoms diode. For ballistic electrons the spin-dependent force, proportional to the intensity of external radiation, leads to a spin-Hall effect and the resulting spin separation even if the spin–orbit coupling itself does not allow for these effects. Achievable spatial separation of electrons with opposite spins can be of the order of several tenths of a micron; an order of magnitude larger than that can be produced by the charged impurity scattering in the diffusive regime.

One of the most interesting parameters of semiconductors and devices is the carrier mobility in the valence and conduction band because the mobility can result in a limitation of the speed of the device. From the development of ultrafast measuring techniques new questions arise — because the carrier mobility is different at different energy states in the band or subband and the mobility can be influenced by external parameters like field strength applied, temperature, stress etc. Especially in low-dimensional semiconductor structures (quantum wells, quantum wires or quantum dots) we have a quantized energy structure. Furthermore, there are many physically different dynamical processes (intraband and interband transitions, different scattering mechanisms etc.). To perform mobility measurements it seems to be necessary to develop measuring techniques running on a time scale faster than the fastest dynamical process to be studied. Time-resolved photoconductivity seems to be a powerful tool to solve these problems down to a picosecond time scale.KeywordsCarrier DensityCarrier MobilityElectrical PulseShallow TrapMobility EdgeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.