Confinement in self-assembled InAs/InP quantum wires studied by magneto-photoluminescence

We studied two InAs/InP quantum wire samples with different growth conditions. The photoluminescence of the first sample reveals up to six distinct peaks, while the second has only two pronounced photoluminescence peaks that are attributed to flat wires with heights that differ by exactly one monolayer. Despite the large band offsets in this system, the photoluminescence energy shift of these peaks with a magnetic field applied in the plane of the wires shows that the extent of the exciton wave function in the growth direction is much larger than the wire height, i.e. the wave function spills over into the InP. Moreover, the exciton wave function shrinks for increasing wire height. The wave function spill-over is qualitatively confirmed in the first quantum wire sample.     

Polaronic exciton in quantum wells wires and nanotubes

We investigate the effect of the longitudinal-optical phonon field on the binding energies of excitons in quantum wells, well-wires and nanotubes based on ionic semiconductors. We take into account the exciton-phonon interaction by using the Aldrich-Bajaj effective potential for Wannier excitons in a polarizable medium. We extend the fractional-dimensional method developed previously for neutral and negatively charged donors to calculate the exciton binding energies in these heterostructures. In this method, the exciton wave function is taken as a product of the ground state functions of the electron polaron and hole polaron with a correlation function that depends only on the electron-hole separation. Starting from the variational principle we derive a one-dimensional differential equation, which is solved numerically by using the trigonometric sweep method. We find that the potential that takes into account polaronic effects always give rise to larger exciton binding energies than those obtained using a Coulomb potential screened by a static dielectric constant. This enhancement of the binding energy is more considerable in quantum wires and nanotubes than in quantum wells. Our results for quantum wells are in a good agreement with previous variational calculations. Also, we present novel curves of the exciton binding energies as a function of the wire and nanotubes radii for different models of the confinement potential.     

可用于红外探测器的自组织量子线及其带间和子带间光学跃迁

研究了自组织量子线Ga1-xInxAs的结构、应力分布及其光学性质.模拟了微应力导致的横向成序及其导引短周期超晶格形成量子线的过程，并计算出量子线在原子尺度上的微应力分布.这里考虑了价带各向异性、带间混合及局域应力分布对光学性质的作用.研究发现自组织量子线具有应用于正入射红外探测器的良好光学特性.结果显示当量子线的周期长度为15到30nm时，导带子带间跃迁波长处在10到20μm，这正是红外探测器的理想工作范围.同时，带间吸收波长在中红外范围，它提供了红外探测器的另一个窗口. 关键词： 自组织 微应力 红外探测器 量子线     

Electronic structure of quantum spheres and quantum wires

The electronic structures of quantum spheres and quantum wires are studied in the framework of the effective-mass theory. The spin-orbital coupling (SOC) effect is taken into account. On the basis of the zero SOC limit and strong SOC limit the hole quantum energy levels as functions of SOC parameter λ are obtained. There is a fan region in which the ground and low-lying excited states approach those in the strong SOC limit as λ increases. Besides, some theoretical results on the corrugated superlattices (CSL) are given.     

0.7 structure in long quantum wires

While quantized conductance steps in short quantum wires are understood through a single electron picture, additional structure often observed in high-quality one-dimensional systems near g=0.7×(2e²/h) is commonly interpreted as arising due to many-body interactions. Most studies of conductance structure below 2e²/h use short one-dimensional wires where transport is known to be ballistic. We report transport measurements for both short (0.5 μm) and long (5 μm) quantum wires, and use both conductance and nonlinear transport to explore the behavior of one-dimensional wires.     

Generalized path integral method for Elliott-Yafet spin relaxations in quantum wells and narrow wires

We generalized the semiclassical path integral method originally used in the D'yakonov-Perel' mechanism to study the spin relaxation of the Elliott-Yafet mechanism in low-dimensional systems. In quantum wells, the spin properties calculated by this method confirmed the experimental results. In two-dimensional narrow wires, size and impurity effects on the Elliott-Yafet relaxation were predicted, including the wire-width-dependent relaxation time, the polarization evolution on the sample boundaries, and the relaxation behavior during the diffusive-ballistic transition. These properties were compared with those of the D'yakonov-Perel' relaxation calculated under similar conditions. For ballistic narrow wires, we derived an exact relation between the Elliott-Yafet relaxation time and the wire width, which confirmed the above simulations.     

Magnetism as a probe for wave function localization in GaSb/AlGaSb quantum wells

Il Nuovo Cimento D - The localization of the wave function on the scale length of a single monolayer has been studied by magnetophotoluminescence in GaSb/AlGaSb quantum wells. The studied range of...     

Weak localization of Dirac fermions in HgTe quantum wells

Weak localization in a system of gapless two-dimensional Dirac fermions in HgTe quantum wells with thickness d = 6.6 nm, which corresponds to the transition from a normal to an inverted spectrum, has been investigated experimentally. A negative logarithmic correction to the conductivity of the system has been observed both at the Dirac point and in the vicinity of this point. The anomalous magnetoresistance of two-dimensional Dirac fermions is positive. This indicates that weak localization in the system of two-dimensional Dirac fermions occurs owing to localization and interaction effects in the presence of rapid spin relaxation.     

Kinetic theory of semiconductor cascade laser based on quantum wells and wires

The paper presents a numerical solution of a system of nonlinear equations for the electron distribution functions in the upper and lower subbands between which lasing transitions occur and the number of nonequilibrium optical phonons in semiconducting cascade lasers based on quantum wells and wires. For the case of quantum wells, we propose an analytical solution of this system of equations, which is a generalization of the previously found solution [V. F. Elesin and Yu. V. Kopaev, Zh. éksp. Teor. Fiz. 108, 2186 (1995) [JETP 81, 1192 (1995)]; V. F. Elesin and Yu. V. Kopaev, Sol. St. Commun. 96, 897 (1995)] in a wider range of injection rates. The threshold injection rate can be significantly reduced owing to reabsorption and accumulation of nonequilibrium optical phonons, nonparabolicity of the subbands and different effective masses of electrons in different subbands. In the case of quantum wires, the threshold injection rate is considerably lower, and its decrease is even larger than in quantum wells. It is remarkable that, owing to the lower electron-electron relaxation rate in the one-dimensional case, the decrease in the threshold injection rate may be two or three orders of magnitude. The relation between the density of states and threshold current has also been studied. Zh. éksp. Teor. Fiz. 111, 681–695 (February 1997)     

Electronic structure and polarizability of quantum metallic wires

The electronic structure and the linear response to an external electric field of simple metal wires with a quantum-size cross-section have been studied within the density-functional theory and the “jellium” model. It is found that an increase in the wire radius leads to a nonmonotonic change in the work function and static polarizability of the wire. The photoabsorption spectra of Na wires with different cross-sections are obtained. The effect of a dielectric environment on the properties of metallic wires is investigated. An increase in the permittivity of a medium brings about a decrease in the static polarizability of metallic wires. It is demonstrated that the surface plasma resonance in the photoabsorption cross-section for Na wires placed in a dielectric matrix is shifted from the continuous spectrum toward the range somewhat below the ionization threshold.     

Electronic structure of ZnO wurtzite quantum wires

The electronic structure and optical properties of ZnO wurtzite quantum wires with radius R≥3 nm are studied in the framework of six-band effective-mass envelope function theory. The hole effective-mass parameters of ZnO wurtzite material are calculated by the empirical pseudopotential method. It is found that the electron states are either two-fold or four-fold degenerate. There is a dark exciton effect when the radius R of the ZnO quantum wires is in the range of [3,19.1] nm (dark range in our model). The dark ranges of other wurtzite semiconductor quantum wires are calculated for comparison. The dark range becomes smaller when the |Δ_so| is larger, which also happens in the quantum-dot systems. The linear polarization factor of ZnO quantum wires is larger when the temperature is higher.     

Fabrication and characterization of a vertical pillar structure including a self-assembled quantum dot and a quantum well

We fabricate and characterize a novel vertical pillar structure including a self-assembled InAs quantum dot (QD) and an InGaAs quantum well (QW). The vertical current through both the InAs QD and an electrostatically defined QD made in the InGaAs QW can be measured by adjusting the position of the InGaAs QD in the QW plane relative to the InAs QD with two side-gate voltages applied independently. We study optical response of the current through the vertical double QD by irradiating light, which is assumed to be mainly absorbed in the InAs QDs. We successfully probe a time-dependent energy level shift due to the Coulomb interaction from holes trapped in the vicinity of the pillar.