Optical emission and Raman scattering in modulation-doped gaAs-AlGaAs quantum wires and dots

Magneto-optical properties and resonant Raman spectroscopy of modulation doped GaAs-AlGaAs quantum well wires are reported. Their properties are compared with similar undoped quantum well wires to investigate the many electron effects in nanostructures. In undoped samples, the quantised energy levels observed by luminescence excitation spectroscopy are in good agreement with a particle-in-a-box model. In doped samples, the carrier confinement is explicitly revealed by magneto-luminescence and depolarised resonant Raman scattering. The calculated spectra in a Hartree model are in reasonable agreement with experiment.PACS: 78.66.Fd, 78.30.Fs, 78.55.Cr, 73.20.Dx     

Canted antiferromagnetic phase in a double quantum well in a tilted quantizing magnetic field

We investigate the double-layer electron system in a parabolic quantum well at filling factor nu=2 in a tilted magnetic field using capacitance spectroscopy. The competition between two ground states is found at the Zeeman splitting appreciably smaller than the symmetric-antisymmetric splitting. Although at the transition point the system breaks up into domains of the two competing states, the activation energy turns out to be finite, signaling the occurrence of a new insulator-insulator quantum phase transition. We interpret the obtained results in terms of a predicted canted antiferromagnetic phase.     

Theory of g-factor enhancement in narrow-gap quantum well heterostructures

We report on the study of the exchange enhancement of the g-factor in the two-dimensional (2D) electron gas in n-type narrow-gap semiconductor heterostructures. Our approach is based on the eight-band k?p Hamiltonian and takes into account the band nonparabolicity, the lattice deformation, the spin-orbit coupling and the Landau level broadening in the δ-correlated random potential model. Using the 'screened' Hartree-Fock approximation we demonstrate that the exchange g-factor enhancement not only shows maxima at odd values of Landau level filling factors but, due to the conduction band nonparabolicity, persists at even filling factor values as well. The magnitude of the exchange enhancement, the amplitude and the shape of the g-factor oscillations are determined by both the screening of the electron-electron interaction and the Landau level width. The 'enhanced' g-factor values calculated for the 2D electron gas in InAs/AlSb quantum well heterostructures are compared with our earlier experimental data and with those obtained by Mendez et?al (1993 Phys.?Rev.?B 47 13937) in magnetic fields up to 30?T.     

Effects of extreme disorder on electronic properties in δ-doped semiconductor microstructures—a realistic computer simulation

Impurity induced disorder is a key feature of strongly doped semiconductor microstructures. We present a theoretical approach which allows the realistic and efficient calculation of localized quantum states in layered, delta-doped systems and the resulting properties of the quasi-2D multisubband electron/hole gas. The random Coulomb potential is directly computed from the impurity distribution without any simplifying assumptions. Electron-electron interaction is treated self-consistently on the Hartree level. The extreme cases of the doping superlattice (strong disorder) and the modulation doped quantum well (weak disorder) are studied as example device structures. Intersubband absorption spectra are then calculated for both types of systems and studied as a function of the electron filling factor. Striking differences are found between the linewidths of potential fluctuations and absorption spectra. These results are explained on the basis of system geometry, nonlinear screening and intersubband correlations. Finally, we discuss possible future applications and extensions of the method.     

Analysis of metamaterials using transmission line models

We use simple transmission line models with lumped elements of inductance and capacitance to interpret optical transmission and reflection spectra of cut wires and cut-wire pairs in the near infrared region. The numerical values of the elements are obtained by fitting experimental or numerical simulated reflectance and transmittance spectra. The scattering parameters and the retrieved effective material parameters calculated from the transmission line models show good agreements with those obtained from experiments or numerical simulations. This indicates that transmission line theory is a powerful tool for designing and analyzing metamaterials at optical frequencies. PACS 41.20.Jb; 78.67.-n; 78.66.Sg     

A continuous-wave and pulsed electron spin resonance spectrometer operating at 275 GHz

An electron paramagnetic resonance (EPR) spectrometer is described which allows for continuous-wave and pulsed EPR experiments at 275 GHz (wavelength 1.1 mm). The related magnetic field of 9.9 T for g approximately 2 is supplied by a superconducting solenoid. The microwave bridge employs quasi-optical as well as conventional waveguide components. A cylindrical, single-mode cavity provides a high filling factor and a high sensitivity for EPR detection. Even with the available microwave power of 1 mW incident at the cavity a high microwave magnetic field B1 is obtained of about 0.1 mT which permits pi/2-pulses as short as 100 ns. The performance of the spectrometer is illustrated with the help of spectra taken with several samples.     

AlGaN/AlN/GaN高电子迁移率器件的电容电压特性的经验拟合

利用蓝宝石衬底的AlGaN/AlN/GaN 高电子迁移率器件(HEMT)的电容电压(C-V)特性,对电子费米能级与二维电子气面密度的经验关系进行表征,其结果对器件电荷控制模型的建立,跨导及电容表达式的简化有重要意义.文章创新性地提出参数α用于表征二维势阱对沟道电子限制能力,并认为α越小则二维势阱的沟道电子限制能力越强.利用上述经验关系来拟合电容,可以获得与实测电容很好的一致性. 关键词： HEMT 费米能级 C-V特性')" href="#">C-V特性 二维势阱的电子限制能力     

Capacitance dispersion in electrochemical impedance spectroscopy measurements of iodide adsorption on Au(1 1 1)

Electrochemical interfaces that display dispersive characteristics do not present the purely capacitive behaviour predicted by the theory of ideally polarised interfaces. For interfaces involving solid electrodes, capacitance dispersion phenomena in the double layer region are usually attributed to the structural characteristics of the electrode surface as well as to the interfacial region. This paper presents a study of the dispersive characteristics, in the double layer potential region, related to the iodide adsorption on an Au(1 1 1) electrode. The study was performed by using electrochemical impedance spectroscopy, and the corresponding spectra fitted with an equivalent circuit containing a constant phase element (CPE). The fitting results are compared with capacitance curves obtained by chronocoulometry, in order to analyse the relationship between the CPE and the interfacial capacitance. It was observed the occurrence of dispersive behaviour in the potential regions associated with phase transition processes in the adsorbed layer and to the potential induced reconstruction phenomena. On the other hand, in the potential regions where such phenomena do not occur, the interface presents almost pure capacitive behaviour. These observations provide evidence of the strong contribution of the solution properties to the capacitance dispersion.     

栅控电子枪中轮辐栅网截止放大系数的研究

本文首先建立轮辐栅网结构模型, 分区计算其屏蔽系数和单个栅格内栅丝半径与该栅格面积之比, 结合 Spangenberg书中的结论给出了轮辐栅网截止放大系数的新表达式, 然后详细地研究了栅丝半径、各环区所对球心角以及径向栅丝数目对截止放大系数的影响, 并计算了温度升高之后截止放大系数的变化, 最后给出了设计轮辐栅网的步骤和具体实例. 结果显示, 根据新表达式设计的栅网具有更好的稳定性和可靠性, 能够很好地解决平板正方形栅格近似中存在的问题.     

Quantum Hall effect in a system with an electron reservoir

Precise measurements of the magnetic-field and gate-voltage dependences of the capacitance of a field-effect transistor with an electron system in a wide GaAs quantum well have been carried out. It has been found that the capacitance minima caused by the gaps in the Landau spectrum of the electron system become anomalously wide when two size-quantization subbands are occupied. The effect is explained by retention of the chemical potential in the gap between the Landau levels of one of the subbands owing to redistribution of electrons between the subbands under a change in the magnetic field. The calculation taking into account this redistribution has been performed in a model of the electron system formed by two two-dimensional electron layers. The calculation results describe both the wide capacitance features and the observed disappearance of certain quantum Hall effect states.     

Electron–hole states in the fractional quantum Hall regime probed by photoluminescence

The electron–hole states in the fractional quantum Hall regime is investigated with a back-gated undoped quantum well by photoluminesccence in magnetic fields. The evolution of the photoluminescence spectra is discussed depending on the electron density. We find anomalies of the photoluminescence at the integer as well as the fractional filling factors.     

Fabrication and optical properties of quantum dots and wires

We describe photoluminescence measurements made on mesa geometry quantum dots and wires with exposed side walls fabricated by laterally patterning undoped GaAs/AlGaAs quantum wells using electron beam lithography and dry etching. At low temperature the photoluminescence efficiency of many but not all of the GaAs quantum dot arrays scales with the volume of quantum well material down to lateral dimensions of 50nm. This behaviour contrasts with that found in wires produced at the same time where the intensity falls off rapidly with decreasing wire width for dimensions below 500nm but is recovered by overgrowth with indium tin oxide, possibly as a result of strain. Narrow overgrown wires exhibit anisotropy in polarized excitation spectra which is discussed in relation to strain and lateral confinement effects.     

Shallow Donor Impurities in Quantum Well WiresIEffects of Image potential

We calculate the shallow donor impurity binding.energies including image potential in quasione- dimensional GaAs/AIAs quantum well wires with rectangular cross section using the variational approach. The results we have obtained show that when impurity ion image potential is considered the variations of binding energy are remarkable, and when the image potentials of impurity ion and electron are considered simultaneously the corresponding variations of binding energy are small. The results also show that the image potential is important, especially when the cross section dimensions of quantum wires become small.     

Temperature dependence of the compressibility of two-dimensional electron systems with long-range potential fluctuations in the quantum Hall effect regime

The temperature dependence of the compressibility of two-dimensional electron systems (2DESs) in GaAs/AlGaAs heterostructures in the quantum Hall effect regime have been studied both experimentally and theoretically. The compressibility was determined using the capacitance spectroscopy technique and the measurements of a low-frequency electric field penetrating through the 2DES. The measured temperature dependences of the 2DES compressibility are quantitatively described using a model taking into account inhomogeneity of the electron density at a finite temperature. Changes in the chemical potential of the 2DES in the vicinity of even filling factors determined from the capacitive and transport measurements are mutually consistent and agree with the results of finite-temperature calculations.     

Optimization of two parallel wires of laser plasma filaments for channeling electromagnetic energy

We theoretically investigated the electromagnetic wave (EMW) transmission along two parallel wires of laser plasma filaments produced by the filamentation of ultrafast laser pulses in air. Many factors, such as wire diameter and separation, electron density, and operating frequency are shown to influence the propagation loss. By taking into consideration the radiation and transmission effects of the wires, the calculations of the two parallel filament wires reasonably agrees with that of the standard commercial twin-lead wire. Specifically, the optimum separation of the two wires is determined for a given frequency and an effective electron density of the wires. When compared with free-space propagation, transmission enhancement of tens dB is obtained using optimized wire configurations. Thus, the two plasma wires may be a potential channel for point to point directed delivery of EM energy or communication of pulsed-modulated EM radiation.     

Study of the electric capacitance spectra on symmetric quantum-dot pattern

We have calculated the ground-state energy of the symmetric quantum-dot pattern by the ab initio calculation method, i.e. unrestricted Hartree-Fock-Roothaan (UHFR) method based on the Gaussian basis, and studied their electric capacitance spectra, assuming each quantum dot of quantum-dot pattern to be confined in a three-dimensional spherical potential well of finite depth. For the systems in question, our results show that our method and theoretical model not only give the electric capacitance peaks similar to s-shell and p-shell atom-like quantum dot, but also show some new fine-structure of electric capacitance in the symmetric quantum-dot pattern system. This method might be a feasible tool to study few-electron problems on the symmetric quantum-dot pattern system.     

Structure and photoluminescence study of type-II GaAs quantum wires and dots grown on nano-faceted (3 1 1)A surface

(3 1 1)A GaAs/AlAs corrugated superlattices (CSLs) and satellite (3 1 1)B and (1 0 0) SLs were studied using Raman spectroscopy, high-resolution transmittance electron microscopy (HRTEM) and photoluminescence (PL). The thickness of GaAs layers was varied from 1 monolayer (ML) to 10 ML, the thickness of AlAs barriers was 10 ML in (3 1 1) direction. The strongest modification of the Raman spectra is found for the case of partial (<1 nm) GaAs filling of the AlAs surface. The calculated and experimental Raman spectra demonstrated a good agreement for both complete (1 nm) and partial (<1 nm) GaAs filling of the AlAs surface. According to Raman and HRTEM data, in the case of partial filling of (3 1 1)A AlAs surface, GaAs forms quantum well wires of finite length (quantum dots). A drastic difference of PL from grown side-by-side (3 1 1)A and (3 1 1)B SLs was observed. A strong room temperature PL in the green–yellow spectral region was observed in GaAs/AlAs (3 1 1)A CSLs containing GaAs type-II quantum dots.     

Plasmons in the modulated and confined 2DEG: A Raman scattering study

Lateral wire arrays have been fabricated from a single modulation-doped GaAs quantum well employing reactive ion etching. Depending on the etch depth, the two-dimensional electron gas (2DEG) in the well acquires different degrees of modulation up to complete confinement. The different regimes are identified by their unique photoluminescence and Raman spectra. Deep-etched wires show plasmon resonances down to a width of 100nm.     

Intriguing electron correlation effects in the photoionization of metallic quantum-dot nanorings

We report detailed results on ionization in metallic quantum-dot (QD) nanorings described by the extended Hubbard model at half filling obtained by exact numerical diagonalization. In spite of very strong electron correlations, the ionization spectra are astonishingly scarce. We attribute this scarcity to a hidden quasi-symmetry, generalizing thereby similar results on optical absorption recently reported [Phys. Rev. B 75, 125323 (2007); 77, 165339 (2008)]. Numerical results indicate that this hidden quasi-symmetry of the extended Hubbard model does not evolve into a true (hidden) symmetry but remains a quasi-symmetry in the case of the restricted Hubbard model as well. Based on the observation on the number of significant ionization signals per each spatial symmetry, we claim the existence of a one-to-one map between the relevant ionization signals of the correlated half-filled nanorings and the one-hole and two-hole-one-particle processes possible in the noninteracting case. Similar to the case of optical absorption, numerous avoided crossings (anticrossings) are present in the ionization spectra, which often involve more than two states. The present results demonstrate that ionization could be a useful tool to study electron correlations in metallic QD-nanoarrays, providing information that is complementary to optical absorption.