Formation of nanoelectrostatic quantum dots and two-dimensional subbands by the random potential of Mn donors in <Emphasis Type="Italic">p-i-n</Emphasis> resonant tunneling heterosystems

The influence of the random potential of Mn impurities in p-i-n resonant tunneling structures on the electronic spectrum has been studied. The possibility of the formation of zero-dimensional states in nanoelectrostatic quantum dots, as well as two-dimensional subbands in the region of GaAs quantum well, by the minima of this potential has been shown.     

Energy levels of hydrogenic impurity states in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of the ground state and of four excited states of a hydrogenic impurity in quantum well structures consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. The ground-state binding energy is calculated as a function of the barrier potential for a given size of the GaAs quantum well and is found to be linearly dependent on the inverse of the square root of the barrier potential except for very small potentials. The variation of the binding energies of all five states as a function of the size of the GaAs quantum well are also calculated and their behavior is discussed.     

Fano resonances and electron localization in heterobarriers

A study is made of electron tunneling in semiconductor heterostructures having a complex dispersion law. A generalized Fabry-Perot approach is used to describe tunneling across the barrier. Mixing of electron states at the heterojunctions is responsible for the asymmetric resonant structure of the transmission which is characterized by a resonance-antiresonance pair. The resonance corresponds to a pole while the antiresonance corresponds to a zero of the scattering amplitude in the complex energy plane, i.e., near the pole and the zero the transmission of the heterobarrier has a Fano resonance structure. It is shown that for certain barrier parameters the resonances may collapse and localized states may appear in the heterobarrier, which is observed on the current-voltage characteristics of the barriers. The two-valley model of a GaAs/Al_xGa_1?x As/GaAs heterostructure is considered as an example. An analysis is made of the resonance structure in the barrier as a function of the type of boundary conditions used for the heterojunctions. The low-temperature current-voltage characteristic of the barrier is calculated.     

Binding energies of wannier excitons in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of Wannier excitons in a quantum well structure consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. Due to reduction in symmetry along the axis of growth of these quantum well structures and the presence of band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed leading to two exciton systems, namely, the heavy hole exciton and the light hole exciton. The variations of the binding energies of these two excitons as a function of the size of the GaAs quantum wells for various values of the heights of the potential barrier are calculated and their behavior is discussed.     

Quantum box resonant tunneling spectroscopy: Experiments and modelisation

We have calculated the potential profile and the electronic levels in resonant tunneling double barrier structures with nanometric lateral dimensions (≤ 500 nm) for various contact doping. At biases for which the box states (laterally confined quantum well) are resonant with the emitter Fermi level, fine structures are expected in the resonant tunneling current. Comparison with I(V) characteristics measured on nanometric GaAs/GaAlAs and GaAs/GaAlAs/InGaAs resonant tunneling diodes shows that our model accounts for the resonance bias voltage and explains the shape of the current peak. The fine structure observed in the current peak provides a spectroscopy of the confined states in the quantum box.     

Linear and nonlinear intersubband optical absorption and refractive index changes of asymmetric double semi-parabolic quantum wells

The optical properties of the asymmetric double semi-parabolic quantum wells (DSPQWs) are investigated numerically for typical GaAs/Al_xGa_1−xAs. Optical properties are obtained using the compact density matrix approach. In this work, effects of the structure parameters such as the barrier width and the well widths on the optical properties of the asymmetric DSPQWs are investigated. The results show that the linear and nonlinear optical properties of asymmetric DSPQW are non-monotonic functions of these structure parameters. The behavior of the refractive index changes of asymmetric DSPQW with the variation of the barrier width is different substantially with that of symmetric DSPQW. Results reveal that the resonant peak values of the total absorption coefficient of asymmetric DSPQW is usually greater than that of symmetric DSPQW. Our calculations also show that the total absorption coefficient of asymmetric DSPQW is larger than that of asymmetric double square quantum well.     

Resonant tunneling in double superlattice barrier heterostructures

We have investigated resonant tunneling in double barrier heterostructures in which the tunnel barriers have been replaced by short period superlattices, and have shown for the first time quantum well confinement in a single quantum well bounded by superlattices. These results also demonstrate the first utilization of short period binary superlattices as effective tunnel barriers to replace the conventional Al_xGa_1−xAs barriers. The superlattice structure does not exhibit the asymmetry around zero bias in the electrical characteristics normally observed in the conventional Al_xGa_1−xAs barrier structures, suggestive of reduced roughness at the inverted interface by superlattice smoothing. The superlattice barrier also exhibits an anomalously low barrier height. The performance of this symmetric superlattice structure is compared with an intentionally constructed asymmetric double barrier superlattice structure, which exhibits pronounced asymmetry in the electrical characteristics. The observed behavior supports the view that resonant enhancement occurs in the quantum well.     

Linear and nonlinear intersubband optical absorption in symmetric double semi-parabolic quantum wells

The linear and the nonlinear intersubband optical absorption in the symmetric double semi-parabolic quantum wells are investigated for typical GaAs/AlxGa1−xAs. Energy eigenvalues and eigenfunctions of an electron confined in finite potential double quantum wells are calculated by numerical methods from Schrödinger equation. Optical properties are obtained using the compact density matrix approach. In this work, the effects of the barrier width, the well width and the incident optical intensity on the optical properties of the symmetric double semi-parabolic quantum wells are investigated. Our results show that not only optical incident intensity but also structure parameters such as the barrier and the well width really affect the optical characteristics of these structures.     

Energy levels of single nonabrupt GaAs/AlxGa1-xAs quantum wells

Energy levels of electrons in nonabrupt GaAs/Al_xGa_1-xAs single quantum wells are calculated with and beyond the constant interfacial effective mass approximation (CIEMA), and compared with those of abrupt GaAs/Al_xGa_1-xAs quantum wells. For a given interface width, the energy levels calculated with the CIEMA are higher than those calculated beyond it, but both are higher than those of the abrupt semiconductor quantum well. The shifts of the energy levels increase with the interfacial width of the nonabrupt quantum well, as well as with the degree of interfacial asymmetry.     

Intensity-dependent nonlinear optical properties in a modulation-doped single quantum well

In the present work, the changes in the intersubband optical absorption coefficients and the refractive index in a modulation-doped quantum well have been investigated theoretically. Within the envelope function approach and the effective mass approximation, the electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger-Poisson equations. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results GaAs/Al_xGa_1−xAs are presented for typical modulation-doped quantum well system. The linear, third-order nonlinear and total absorption and refractive index changes depending on the doping concentration are investigated as a function of the incident optical intensity and structure parameters, such as quantum well width and stoichiometric ratio. The results show that the doping concentration, the structure parameters and the incident optical intensity have a great effect on the optical characteristics of these structures.     

Multiple wavelength electroluminescence and laser generation in p-i-n resonant tunneling heterostructures

We report a design and electroluminescence (EL) investigation of a p-i-n resonant tunneling device based on an Al_0.4Ga_0.6As/GaAs graded-index waveguide heterostructure. The intrinsic region of the structure consists of a quantum well (QW) surrounded by multiple barrier energy filters providing simultaneous resonant occupation of electron and heavy-hole second excited subbands in the QW. Several peaks are observed in the EL spectra, confirming occupation of the excited subbands. The EL efficiency displays a resonant behavior accompanied by an S-shaped negative differential resistance region in the voltage–current characteristic. Current bistability is demonstrated, leading to bistability in the EL and laser generation spectra.     

Transmission in symmetrical GaAs/AlxGa1-xAs double-barriers with compositionally nonabrupt interfaces

Transmission properties of electrons through GaAs/Al_xGa_1-xAs symmetrical double-barriers with abrupt and nonabrupt interfaces are calculated and compared. The interface potential and carrier effective-mass are obtained assuming a linear variation of the aluminium molar fraction in the transition regions GaAs ⇔ Al_xGa_1-xAs. When the electron energy E_0,-1,e is smaller than the double-barrier height V_x0 , changes in the internal interfaces widths shift tunneling resonances, while changes in the external interfaces increase the energy widths of the resonant transmission peaks. When E_0,-1,e > V_x0, both external and internal interfaces modify remarkably the transmission of nonabrupt GaAs/Al_xGa_1-xAs double-barriers when compared with the abrupt double-barrier, even if their widths are as small as two GaAs lattice parameters. However, the first transmission peaks of abrupt and nonabrupt GaAs/Al_xGa_1-xAs double-barriers are very similar, except when the interface widths are greater than four lattice parameters.     

Characterization of aluminium concentration in shallow quantum wells AlxGa1−xAs/GaAs types

We report a reflectance study on series of shallow quantum wells GaAs/Al_xGa_1−xAs types with different aluminium concentration. The observed barrier exciton reflectance line shape depends strongly on the shift in aluminium concentration in the two barriers, with the appropriate choice of the cap layer thickness. This observation was based on the reflectivity line shape analysis of anti-Bragg structures.     

Binding energies of excitons in symmetric and asymmetric quantum wells in a magnetic field

The binding energy of the exciton in the symmetric and asymmetric GaAs/Ga_1 − xAl_xAs quantum wells is calculated with the use of a variational approach. Results have been obtained as a function of the potential symmetry, and the size of the quantum well in the presence of an arbitrary magnetic field. The applied magnetic field is taken to be parallel to the axis of growth of the quantum well structure. The role of the asymmetric barriers, magnetic field, and well width in the excitonic binding is discussed as the tunability parameters of the GaAs/Ga_1 − xAl_xAs system.     

Application of coordinate transformation and finite differences method for electron and hole states calculations

In this work we are particularly interested for GaAs/Ga_1−xAl_xAs V-groove quantum wires. The paper presents an efficient and simple method for energy spectra and wave function calculations of electrons and holes in V-groove quantum wires. The method is based on the coordinate transformation of the V-groove quantum wire structure and the computational domain using a function proposed by Inoshita. Then, the Hamiltonian followed by implementation of the FDM (Finite Difference Method) in the new computational space leads to an eigenvalues problem resolved using specialized LAPACK’s routines. The influence of the parameters introduced in the mathematical function, is studied on the energy levels of electrons and holes as well as the oscillator strengths.     

Impurity absorption of light involving resonant states of shallow donors in quantum wells

The energy spectrum of localized and resonant states of shallow donors in heterostructures GaAs/Al_xGa_1?xAs with quantum wells is calculated. The widths of the resonant states belonging to the second size quantization subband are determined. It is shown that the width of a resonance level is mainly determined by the interaction with optical phonons. The spectrum of impurity absorption of light due to electron transitions from the ground state of the donor to the resonant states belonging to the second size quantization subband is calculated.     

Effect of the Ge content on the Schottky barrier height in structures based on Si1 − x Ge x solid solution

The subject of investigation is the influence of the Ge content on the Schottky barrier height in Au-p-i-n/Al structures based on Si_{1 ? x} Ge _x (0 < x < 0.26) solid solutions grown by electron-beam floating-zone melting. In Au-i-Si_{1 ? x} Ge _x , Au-αGe/Si(Li), Au-αGe/Si, and Au-n-Si_{1 ? x} Ge _x structures, the height of the barrier is about 1 eV, which exceeds the average value of this parameter for silicon available in the literature. It is shown that an excess Ge concentration in the near-surface region of the Si_{1 ? x} Ge _x crystal causes self-passivation of the surface, which leads to a rise in the barrier height. In the Au-i-Si_{1 ? x} Ge _x structures, the Schottky barrier height increases from 0.97 to 1.03 eV as Ge content x increases from 0 to 0.11.     

Nonlinear optical rectification in asymmetric coupled quantum wells

The optical rectification (OR) in the asymmetric coupled quantum wells (ACQWs) is calculated theoretically. The dependence of the OR on the width of the right-well and the barrier is studied. The analytical expression of the optical rectification coefficient is obtained by using the compact density-matrix approach and the iterative method, and the numerical calculations are presented for a typical GaAs/Al_xGa_1 − xAs ACQW. The results obtained show that the OR efficient can reach the magnitude of 10⁻⁴ m/V in this ACQW system, which is 1-2 orders higher than that in single quantum systems. Moreover, the OR coefficient is strongly dependent on the widths of the barrier and the right-well of the ACQWs. An appropriate choice for the width of the barrier and the right-well of the ACQWs can induce a larger OR coefficient.     

Diamagnetic susceptibility of hydrogenic donor impurity in low-dimensional semiconducting systems

The binding energies of a hydrogenic donor both in the parabolic and non-parabolic conduction band model within the effective mass approximation have been computed for the low-dimensional semiconducting systems (LDSS) like quantum well, quantum well wire and quantum dot taking GaAs/Al_xGa_1−xAs systems as an example. It is observed that the effect of non-parabolicity is not effective when the system goes to lower dimensionality. The diamagnetic susceptibility of a hydrogenic donor impurity has also been computed in these LDSS in the infinite barrier model. Since no theoretical or experimental works on the diamagnetic susceptibility of LDSS are available in the literature, as a realistic case the diamagnetic susceptibility has been computed in the finite barrier model (x=0.3) for a quantum well and the results are discussed in the light of semiconductor-metal transition.     

Magneto-tunneling spectroscopy of GaAs---AlGaAs double barrier tunneling devices in pulsed high magnetic fields up to 40T

The magneto-tunneling effect was investigated in GaAs---AlGaAs double barrier resonant tunneling devices in pulsed high magnetic fiels up to 40T applied parallel(B) and perpendicular (B) to the barrier layers. In a sample with , oscillatory structures due to the 2D electrons in the emitter and the LO phonon assisted resonant tunneling were observed when the magnetic field (B) was swept at constant bias voltages. A large drop of the current was found in the quantum limit at applied voltages below the negative differential conductivity region. A striking hysteresis was observed in the voltage-current (V - I) curves. In a wide well sample with , rich structures were observed in the V - I curve for B, corresponding to the tunneling to different cyclotron orbits from the emitter.