Radiative efficiency in low-dimensional semiconductor structures

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.

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.

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.

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.

: 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. .

Influence of indium content and carrier density on spontaneous emission spectra of wurtzite InGaN/GaN quantum wires with screening effects

We theoretically investigate the optical properties of conventional, normal (type A) parabolic and novel (type B) parabolic InGaN quantum well (QW) for blue light emitters. Two specially designed active layer structures by parabolic‐shaped QW are proposed, and the optical characteristics of these two parabolic QW structures are calculated and compared to those of conventional QW structures. The electron–hole wavefunction overlap (Γe–hh) of type‐B parabolic QWs is 2.8 times (69.6%) that in the conventional QW (24.8%), and the spontaneous emission rate is ninefold that of conventional QWs. The transparency carrier density of type‐B parabolic QWs is much smaller than type‐A parabolic or conventional QW. These results can be attributed to a higher indium index in the center of the type‐B parabolic QWs, and that leads to better confinement of carriers wavefunctions.

We discuss in detail a new mechanism of nonlinearity of the light-current characteristic (LCC) in heterostructure lasers with reduced-dimensionality active regions, such as quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs). It arises from: 1) noninstantaneous carrier capture into the quantum-confined active region and 2) nonlinear (in the carrier density) recombination rate outside the active region. Because of 1), the carrier density outside the active region rises with injection current, even above threshold, and because of 2), the useful fraction of current (that ends up as output light) decreases. We derive a universal closed-form expression for the internal differential quantum efficiency /spl eta//sub int/ that holds true for QD, QWR, and QW lasers. This expression directly relates the power and threshold characteristics. The key parameter, controlling /spl eta//sub int/ and limiting both the output power and the LCC linearity, is the ratio of the threshold values of the recombination current outside the active region to the carrier capture current into the active region. Analysis of the LCC shape is shown to provide a method for revealing the dominant recombination channel outside the active region. A critical dependence of the power characteristics on the laser structure parameters is revealed. While the new mechanism and our formal expressions describing it are universal, we illustrate it by detailed exemplary calculations specific to QD lasers. These calculations suggest a clear path for improvement of their power characteristics. In properly optimized QD lasers, the LCC is linear and the internal quantum efficiency is close to unity up to very high injection-current densities (15 kA/cm/sup 2/). Output powers in excess of 10 W at /spl eta//sub int/ higher than 95% are shown to be attainable in broad-area devices. Our results indicate that QD lasers may possess an advantage for high-power applications.

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

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.

Effect of p-AlGaN electron blocking layers on the injection and radiative efficiencies in InGaN/GaN light emitting diodes

Homologous recombination plays an essential role in processes involved in genome stability/instability, such as molecular evolution, gene diversification, meiotic chromosome segregation, DNA repair and chromosomal rearrangements. p53 devoid cells exhibit predisposition to neoplasia, defects in G1 checkpoint and high genetic instability but a normal rate of point mutations. We investigated the effect of a p53 mutation, on spontaneous homologous recombination between intrachromosomal direct repeat sequences, in mouse L cells. In these cells, wild type for the p53 gene, we have overexpressed the mutant p53(175(Arg>His)) protein leading to a p53 mutant phenotype, as verified by the absence of a G1 arrest after gamma-irradiation. We show that the rate of spontaneous recombination is increased from five- to 20-fold in the mutant p53 lines. Moreover, this increase is observed in gene conversion as well as in deletion events. Our results provide new insights into the molecular mechanisms of genetic instability due to a defect of p53.

We report on the development of surface plasmon (SP) enhanced AlGaN-based multiple quantum wells (MQWs) ultraviolet (UV) light-emitting diodes (LEDs) grown on silicon (111) substrates. In order to generate SP-coupling with the radiating dipoles in MQWs, an aluminum layer is selectively deposited in holes etched in the top p-AlGaN to p-GaN layers. After flip-chip bonding and substrate removal, an optical output power of ∼1.2 mW is achieved at an emission wavelength of 346 nm; the output power of these UV LEDs with Al layer is increased by 45% compared to that of conventional UV LEDs without Al layer. This enhancement can be attributed to an increase in the spontaneous emission rate and improved internal quantum efficiency via resonance coupling between excitons in MQWs and SPs in the aluminum layer.

Nonresonant carrier tunneling dynamics in GaAs/AlGaAs double quantum wells as a function of carrier density was investigated by using time‐resolved pump and probe measurements. At low temperatures, tunneling time increases with increasing carrier density. At high temperatures, no dependence of tunneling time on carrier density was shown. This unique behavior is well explained by the thermodynamic effect be‐ tween excitons and free carriers as a function of carrier density on the tunneling process. At low temperatures, a larger fraction of excitons at a higher carrier density, which have a relatively slow tunneling time in contrast to free electrons, leads to a slow tunneling time. The independence of the tunneling time at high temperatures is attributed to the insensitivity of the exciton population to the increased carrier density. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)