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Stark Effect (stark + effect)
Selected AbstractsInGaAs/GaAs quantum wells and quantum dots on GaAs(11n) substrates studied by photoreflectance spectroscopyPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2007J. S. Rojas-Ramirez Abstract Pseudomorphic InGaAs/GaAs quantum wells (QWs) and self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on GaAs(11n)A substrates. Photoreflectance spectroscopy was employed to investigate the transitions in the heterostructures. The transitions in QWs have two contributions, a blue shift due to the compressive strain, and a red shift due to the quantum confined Stark effect produced by the piezoelectric field. A traditional theoretical interpretation of the QWs transitions employing a simple well model with sharp interfaces shows discrepancies with the experimental results. In order to satisfactorily explain the transitions we proposed to include segregation effects of Indium at the wells interfaces. The matrix transfer method was implemented to numerically solve the Schrödinger equation taking into account In segregation effects by including an asymmetric potential well with a profile depending on the details of the In incorporation. With segregation effects included, the calculated transitions fit very well the PR spectra. On the other hand, the transitions in self-assembled QDs were obtained by fitting the PR spectra employing a first derivative line-shape function. For n = 2, 4, 5, two functions were required to fit the spectra. For n = 3 only one function was required, in agreement with the more uniform QDs size distribution observed by atomic force microscopy on GaAs(113)A. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Strong ultraviolet emission from non-polar AlGaN/GaN quantum wells grown over r -plane sapphire substratesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2003W. H. Sun Abstract GaN and GaN/Al0.25Ga0.75N multiple quantum wells (MQWs) over c - and r -plane sapphire substrates have been grown by metal-organic chemical vapor deposition. A comparative study of photoluminescence (PL) in GaN epitaxial layers and AlGaN/GaN MQWs on these two types of substrates is reported. At low excitation levels, the measured room temperature PL signal in GaN layers grown over r -plane sapphire was more than order of magnitude lower than in GaN on c -plane substrates. In contrast, the emission intensity from AlGaN/GaN MQWs grown over r -plane substrates was almost 30 times stronger than in the structures grown over c -plane sapphire. Furthermore, with excitation power density up to 1 MW/cm2, the PL peak position for the non-polar MQWs kept completely stable whereas the one for the c -plane structures exhibited a blue shift as large as 250 meV. We attribute this large difference in the ultraviolet emission intensity to the suppression of a strong quantum Stark effect in the AlGaN/GaN MQWs on the r -plane sapphire. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Transverse Stark effect of electrons in a semiconducting quantum wirePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2003G. J. Vázquez Abstract We investigate the effect of an electric field applied tranversely to the axis of cylindrical symmetry of a cylindrical quantum wire on the ground-state energy of the electrons in the wire using an infinite confining potential well model. For low electric fields, we find a quadratic shift of the energy levels with the electric field; while for strong fields, the Stark shift of the ground-state energy increases almost linearly with the electric field. This increase is greater for wide wires, but for narrow wires, the Stark shift of the ground-state energy does not change much with the electric field. Also, at higher electric fields, the Stark shift of the ground-state energy increases with increasing wire radius. This will lead to the decrease of the effective bandgap of a semiconducting quantum wire with electric field. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Piezoelectric effects in sidewall quantum wires grown on patterned (311)A GaAs substratePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2003D. Alderighi Abstract Time-resolved photoluminescence measurements have been performed on sidewall InGaAs/AlGaAs quantum wires and quantum wells. Experimental data show a band filling of quantum wells and quantum wires that is dominant in the first 400 ps after the excitation pulse, then a dynamical screening of the built-in piezoelectric field (Fp) by means of fast injection of photo-generated charges is observed allowing an efficient radiative recombination. At longer time delay, during the regime when the Fp unscreened value is recovered, a strong quantum confined Stark effect is observed. A good agreement is obtained for the energy shift and the overlap integrals of electrons and heavy holes by means of discrete element calculations. [source] Piezoelectric Field Influence on GaN/AlxGa1,xN Quantum Well Optical PropertiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2003S. Fanget Abstract The absorption and luminescence properties of hetero-polarization GaN/AlxGa1,xN (x = 0.12 and 0.165) quantum well (QW) structures are studied by photoreflectivity, photoluminescence excitation spectroscopy (PLE), and photoluminescence at low temperature. The QW transition energy as a function of well thickness exhibits a quantum-confined Stark effect (QCSE) due to the presence of a strong built-in electric field (piezoelectricity and spontaneous polarization). An electric field strength of 120 kV/cm in the barrier and between 600 and 800 kV/cm in the well are obtained from the analysis of Franz-Keldysh oscillations in photoreflectivity spectra. These values are in good agreement with results from the fit of the QW transition energy versus the thickness, using the electric field as a parameter. [source] Electronic structure and transport properties of quantum dotsANNALEN DER PHYSIK, Issue 5 2004M. Tews Abstract The subject of this paper are electronic properties of isolated quantum dots as well as transport properties of quantum dots coupled to two electronic reservoirs. Thereby special focus is put on the effects of Coulomb interaction and possible correlations in the quantum dot states. First, the regime of sequential tunneling to the reservoirs is investigated. It is shown that in case degenerate states participate in transport, the resonance positions in the differential conductance generally depend on temperature and the degree of degeneracy. This effect can be used to directly probe degeneracies in a quantum dot spectrum. A further effect, characteristic for sequential tunneling events, is the complete blocking of individual channels for transport. A generalisation of the well known spin blockade is found for correlated dot states transitions through which are not directly spin-forbidden. In the second part, the electronic structure of spherical quantum dots is calculated. In order to account for correlation effects, the few-particle Schrödinger equation is solved by an exact diagonalization procedure. The calculated electronic structure compares to experimental findings obtained on colloidal semiconductor nanocrystals by Scanning Tunneling Spectroscopy. It is found that the electric field induced by the tunneling tip is gives rise to a Stark effect which can break the spherical symmetry of the electronic ground state density which is in agreement with wave-function mapping experiments. The symmetry breaking depends on the competition between exchange energy and the Stark energy. Moreover, a systematic dependence on particle number is found for the excitation energies of optical transitions which explains recent experimental findings on self-organized quantum dots. In the last part, co-tunneling in the Coulomb blockade regime is studied. For this end the tunneling current is calculated up to the forth order perturbation theory in the tunnel coupling by a real-time Green's function approach for the non-equilibrium case. The differential conductance calculated for a quantum dot containing up to two interacting electrons shows complex signatures of the excitation spectrum which are explained by a combination of co-tunneling and sequential tunneling processes. Thereby the calculations show a peak structure within the Coulomb blockade regime which has also been observed in experiment. [source] | ||||||||||