Low-frequency noise in the low dimensional semiconductor structures

By taking the influence of optical phonon modes into account, this paper adopts the dielectric continuum phonon model and force balance equation to investigate the electronic mobility parallel to the interfaces for AlAs/GaAs semiconductor quantum wells (QWs) under hydrostatic pressure. The scattering from confined phonon modes, interface phonon modes and half-space phonon modes are analysed and the dominant scattering mechanisms in wide and narrow QWs are presented. The temperature dependence of the electronic mobility is also studied in the temperature range of optical phonon scattering being available. It is shown that the electronic mobility reduces obviously as pressure increases from 0 to 4GPa, the confined longitudinal optical (LO) phonon modes play an important role in wide QWs, whereas the interface optical phonon modes are dominant in narrow QWs, the half-space LO phonon modes hardly influence the electronic mobility expect for very narrow QWs.

Electron mobility and terahertz absorption due to interactions with confined acoustic phonons in a free-standing quantum well

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.

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.

Due to inter-quantum well tunneling, coupled double quantum wells (DQWs) contain an extra degree of electronic freedom in the growth direction, giving rise to new transport phenomena not found in single electron layers. This report describes work done on coupled DQWs subject to inplane magnetic fields B{sub {parallel}}, and is based on the lead author`s doctoral thesis, successfully defended at Oregon State University on March 4, 1997. First, the conductance of closely coupled DQWs in B{sub {parallel}} is studied. B{sub {parallel}}-induced distortions in the dispersion, the density of states, and the Fermi surface are described both theoretically and experimentally, with particular attention paid to the dispersion anticrossing and resulting partial energy gap. Measurements of giant distortions in the effective mass are found to agree with theoretical calculations. Second, the Landau level spectra of coupled DQWs in tilted magnetic fields is studied. The magnetoresistance oscillations show complex beating as Landau levels from the two Fermi surface components cross the Fermi level. A third set of oscillations resulting from magnetic breakdown is observed. A semiclassical calculation of the Landau level spectra is then performed, and shown to agree exceptionally well with the data. Finally, quantum wires and quantum point contacts formed in DQW structures are investigated. Anticrossings of the one-dimensional DQW dispersion curves are predicted to have interesting transport effects in these devices. Difficulties in sample fabrication have to date prevented experimental verification. However, recently developed techniques to overcome these difficulties are described.

Optical-phonon modes and Fröhlich potential in quantum dots

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.

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.

Electronic mobility limited by optical phonons in symmetric MgxZn1-xO/ZnO quantum wells with mixed phases

The electrical transport properties of typical hexagonal metals are anisotropic because of their anisotropic lattice structures. Unexpectedly, we show that the electron transport properties in hexagonal close-packed (hcp) noble metals are almost isotropic. Although the electron transport properties of an individual electronic band are highly anisotropic, the total contributions are almost equal in different crystalline orientations because of the complementary contributions of different bands. The electron transport is severely limited by phonons for metals with multisheet Fermi surfaces and optical phonon polarizations. However, it is found the electron-phonon interactions are weak in hcp noble metals, although their Fermi surfaces and phonon spectra are complicated. The electronic structure acts as a phonon filter, resulting in small electron-phonon scattering rates. The weak electron-phonon interactions are beneficial to electron and thermal transport, indicating hcp noble metals have great potential to be used in electronics and solar cells.

Shallow GaAs/InGaAs/GaAs quantum well structures with and without a three-monolayer thick AlAs central barrier have been investigated for different well widths and Si doping levels. The transport parameters are determined by resistivity measurements in the temperature range 4–300 K and magnetotransport in magnetic fields up to 12 T. The (subband) carrier concentrations and mobilities are extracted from the Hall data and Shubnikov–de Haas oscillations. We find that the transport parameters are strongly affected by the insertion of the AlAs central barrier. Photoluminescence spectra, measured at 77 K, show an increase of the transition energies upon insertion of the barrier. The transport and optical data are analysed with the help of self-consistent calculations of the subband structure and envelope wavefunctions. Insertion of the AlAs central barrier changes the spatial distribution of the electron wavefunctions and leads to the formation of hybrid states, i.e., states which extend over the InGaAs and the delta-doped layer quantum wells.

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

The absorption of linearly polarized light in low-dimensional semiconductor structures is investigated. It is shown that the absorption under consideration can give rise to spin orientation of free carriers. A theory of this optical orientation by linearly polarized light is developed for resonant intersubband optical transitions in n-type quantum wells. It is demonstrated that, in the vicinity of the resonance, the optical orientation undergoes spectral inversion, namely, the electron spin orientation reverses sign with increasing frequency. This behavior can be accounted for by the spin-orbit subband splitting, which is linear in the wave vector, and by the energy and quasi-momentum conservation laws.

In this paper we will investigate the origin of low-frequency excess noise in GaN-based LEDs. We will present experimental results on the impact of material growth processes on the properties of the low-frequency noise. Based on the results we will present a model on the underlying process for low-frequency excess noise in GaN-based LED structures and their relationship with the degradation of the device due to hot-electron stressing. We also demonstrate the use of low-frequency noise measurement as a characterization tool for the optimization of the growth parameters for the multiple quantum wells.

Electronic transport is theoretically investigated in laterally confined semiconductor superlattices using the formalism of nonequilibrium Green's functions. Velocity-field characteristics are calculated for nanowire superlattices of varying diameters, from the quantum dot superlattice regime to the quantum well superlattice regime. Scattering processes due to electron-phonon couplings, phonon anharmonicity, charged impurities, surface and interface roughness, and alloy disorder are included on a microscopic basis. Elastic scattering mechanisms are treated in a partial coherent way beyond the self-consistent Born approximation. The nature of transport along the superlattice is shown to depend dramatically on the lateral dimensionality. In the quantum wire regime, the electron velocity-field characteristics are predicted to deviate strongly from the standard Esaki-Tsu form. The standard peak of negative differential velocity is shifted to lower electric fields, while additional current peaks appear due to integer and fractional resonances with optical phonons.