Binding energies of donor impurities in modulation-doped double quantum wells under an electric field

In this study, we have investigated theoretically the binding energies of shallow donor impurities in modulation-doped GaAs/Al_0.33Ga_0.67As double quantum wells (DQWs) under an electric field which is applied along the growth direction for different doping concentrations as a function of the impurity position. The electronic structure of modulation-doped DQWs under an electric field has been investigated by using a self-consistent calculation in the effective-mass approximation. The results obtained show that the carrier density and the depth of the quantum wells in semiconductors may be tuned by changing the doping concentration, the electric field and the structure parameters such as the well and barrier widths. This tunability gives a possibility of use in many electronic and optical devices.     

Optical spectroscopy of GaAsGaAlAs quantum wells under an external electric field

We have studied by means of low temperature photoluminescence (PL) and photocurrent spectroscopy the effects of an external electric field on the excitons in GaAs quantum wells confined between GaAlAs. Increasing the field causes a Stark shift of the excitons toward lower energies with a simultaneous quenching in the PL intensity. At moderate fields, we find very good agreement (better than 0.5 meV) between the light- and heavy-hole exciton energies obtained by PL and photocurrent measurements. A significant deviation in energy of the PL relative to the photocurrent is observed at high fields, manifesting the increase in the contributions of impurity-bound excitons to the PL lineshape. A detailed PL study of the Stark shift as a function of well thickness has also been performed. The results show an increasing Stark shift with increasing well thickness, amounting to 110 meV for a 230 Å-wide well at a field of 10⁵ V/cm. For very wide wells (∼ 1000 Å) the behavior of bulk GaAs is recovered: the excitons become ionized before large Stark shifts can be observed. Variational calculations have been carried out and shown to account for the experimental observations of both the Stark shift and the quenching of the PL. In this light, we will discuss the mechanisms governing the optical properties of quantum wells under an external electric field.     

Linear and nonlinear optical absorption coefficients in inverse parabolic quantum wells under static external electric field

In the present theoretical study, the linear and third-order nonlinear optical absorption coefficients have been calculated in GaAs/Ga_1−x Al _x As inverse parabolic quantum wells (single and double) subjected to an external electric field. Our calculations are based on the potential morphing method in the effective mass approximation. The systematic theoretical investigation contains results with all possible combinations of the involved parameters, as quantum well width, quantum barrier width, Al concentration at each well center and magnitude of the external electric field. Our results indicate that in most cases investigated, the increase of the electric field blue-shifts the peak positions of the total absorption coefficient. In all cases studied it became apparent that the incident optical intensity considerably affects the total absorption coefficient.     

Multiphonon capture of carriers in parabolic quantum wells in a constant electric field

The zero-radius potential model is used to investigate multiphonon (radiationless) transitions between bound states of a parabolic quantum well (PQW) in a constant electric field whose intensity vector is perpendicular to the PQW surface. It is shown that thermal-capture thicknesses depend significantly on the magnitude and direction of the electric field and on the position of the impurity in the spatially bounded system. Fiz. Tverd. Tela (St. Petersburg) 40, 1126–1129 (June 1998)     

Exciton states in asymmetric GaInNAs/GaAs coupled quantum wells in an applied electric field

The electronic and optical properties of exciton states in GaInNAs/GaAs coupled quantum well (CQW) structure have been theoretically investigated by solving the Schrödinger equation in real space. The effect of well width on the exciton states has been also studied by varying the well width from 5?nm to 10?nm in asymmetric structures. The electron, hole and exciton states are calculated in the presence of an applied electric field. It is found that there are two direct (bright) exciton states with the largest oscillator strengths. Their energies weakly depend on the electric field due to the compensation between the blue shift and red shift of the electron–hole pair states. In addition, these two states are overlap in the case of symmetric CQWs and one of them is then shifted to higher energy in asymmetric CQWs. The ground state exciton has the binding energy of approximately 7.3?meV and decrease to around 3.0?meV showing the direct to indirect transition of the ground state. The direct–indirect crossover is observed at different electric field for different structure. It happens at the electric field when the e1–e2 electron anticrossing or h1–h2 hole anticrossings is observed, so that the crossover can be controlled by the well width of CQWs structure.     

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.     

Binding energy of impurity states in an inverse parabolic quantum well under magnetic field

 We have investigated the effects of the magnetic field which is directed perpendicular to the well on the binding energy of the hydrogenic impurities in an inverse parabolic quantum well (IPQW) with different widths as well as different Al concentrations at the well center. The Al concentration at the barriers was always x_max=0.3. The calculations were performed within the effective mass approximation, using a variational method. We observe that IPQW structure turns into parabolic quantum well with the inversion effect of the magnetic field and donor impurity binding energy in IPQW strongly depends on the magnetic field, Al concentration at the well center and well dimensions.     

The electronic structure of a quantum well under an applied electric field

The effects of an applied electric field on quantum well subband energies are calculated variationally within the effective mass approximation for model potential profiles. The concept of a quasi-bound state is examined critically. For higher electric field values it is shown that the quasi-bound state approximation for the ground and first excited state of the electron, and for the ground state of the hole is valid.     

Spatially nonlocal effects on optical absorption properties in coupled quantum wells with an applied electric field

Based on the microscopic nonlocal optical response theory, the intersubband optical absorption properties in AlGa As/Ga As couple quantum wells（CQWs） are investigated for p-polarized states. The numerical results show that spatial nonlocality of optical responses can induce a radiation shift on optical absorption spectra due to nonlocal effects. The dependence of the radiation shift on the CQW structure and the applied electric field is clarified. It is also demonstrated that the maximal radiation shift and the least optical absorbance can be obtained by adopting an appropriate CQW structure and a suitable applied electric field. This work may provide some methods of designing the nanomaterials with controllable nonlocality and observing the spatial nonlocal effects in experiment.     

Laterally coupled circular quantum dots under applied electric field

The optical response of a system of two laterally coupled quantum dots with circular cross-sectional shape is investigated within the effective mass approximation, taking into account the effects of the change in the geometrical configuration, the application of an external static electric field, and the presence of a donor impurity center. The first-order dielectric susceptibility is calculated in order to derive the corresponding light absorption and relative refractive index coefficients. The possibility of tuning these optical properties by means of changes in the quantum dot symmetry and the electric field intensity is particularly discussed.     

Binding energies and oscillator strengths of excitons in shallow quantum wells

Il Nuovo Cimento D - A model for calculating exciton binding energies in quantum wells (QWs) is presented, which can be applied to situations in which one of the two band discontinuities is large,...     

Lateral electric field effects on quantum size confinement in cylindrical quantum dot under parabolic potential

Within the effective mass approximation, we investigated theoretically the ground-state energy of a single particle and the binding energy of the neutral donor impurity (D⁰) affected by a lateral electric field in a parabolic quantum dot (QD). The results show that the electron and the hole ground-state energy and the band to band transition energies shift to lower values (red shift) by increasing the field intensity. The quantum Stark shift (QSS) for the electron increases rapidly in the quasi spherical QD (QSQD) by increasing the lateral field, whereas for the hole it increases monotony. In the cylindrical QDs (CQDs), we found that the QSS for electron and hole increase monotonically. The quantum size, lateral electric field and impurity position effect on the binding energy of neutral donor (D⁰) is studied. Unexpected behavior of D⁰ in quantum well limit (QW), the binding energy of D⁰ is increasing (blue shift) with increasing QD radius R$R$ at the presence of a lateral electric field. It appears that for a fixed size of the QD, the off-center binding energy decreases when the impurity ion is displaced from the center to the QD borders, while it is shifted to lower energy with increasing the field.     

Effect of electric field and central-cell corrections on the spectrum of shallow donors in parabolic quantum wells

The effects of the electric field and of the central-corrections on the binding energies of shallow donors in a Ga As/Ga_1−x Al _x As parabolic quantum well are studied. The effectivemass approximation within a variational scheme is adopted, and central-cell corrections are calculated by using a model potential with an adjustable parameter. For great values of the parabolic parameter, relatively large corrections are obtained for the shallow donors studied.