Ground State Subband Research Articles

Effect of silicon delta-doping density on optical properties of type-II InAs/GaAsSb quantum dots

We have investigated the optical properties of type-II InAs/GaAs 0.83 Sb 0.17 quantum dots (QDs) with different silicon delta-doping densities of 5×10 10 , 5×10 11 , and 2×10 12 cm −2 using photoluminescence (PL). The PL spectra of the QD ground state (GS) emission peaks for the samples are blueshifted at a slower rate with increasing the doping density due to the enhanced radiative recombination rate of the carriers. The PL intensity ratio of the GS emission to the first excited state emission increases with the doping density, which is indicative of the faster radiative recombination at the GS subbands with the doping density. The redshift rate of the GS emissions becomes faster at a high temperature (>130 K) as the doping density increases up to 5×10 11 cm −2 resulting from the quantum confined Stark effect by the electric field of the ionized dopants, and decreases at an increased doping density of 2×10 12 cm −2 due to the enhanced QD size uniformity. Time-resolved PL exhibits that the QD sample doped at 5×10 10 cm −2 has a longer total radiative lifetime than the undoped sample, and a further increase in the doping density to 2×10 12 cm −2 decreases the lifetime due to the enhancement of the radiative recombination through fast carrier relaxation. • The optical properties of InAs/GaAsSb quantum dots doped with silicon are studied. • The sample doped at 5×10 10 cm −2 has the longest carrier lifetime among the samples. • The improved carrier lifetime is beneficial to intermediate band solar cells.

Theoretical Study of the Gate Leakage Current in Sub-10-nm Field-Effect Transistors

Scaling field-effect transistors (FETs) with conventional semiconductor channels requires a reduction of the body or fin thickness or of the diameter of the nanowire (NW) channel. We present a theoretical study showing that the increase of the ground-state sub-band energy induced by the quantum confinement associated with this scaling results in a dramatic reduction of the barrier at the channel/gate-insulator interface, which causes an increase of the leakage current across the gate insulator. We have studied the problem using scaling rules extracted from the 2011 International Technology Roadmap for Semiconductors (ITRS)-Roadmap (finding them excessively relaxed) and using more strict scaling rules from the literature confirmed by simulations of 5-nm gate-length III-V FETs. Employing local empirical pseudopotentials to calculate the electronic structure of Si and InAs thin bodies, the leakage gate current in the ON-state is shown to reach worrisome values at gate lengths of about 5 nm. This suggests that 1-D channels (i.e., NWs), but especially channels based on intrinsically 2-D (e.g., graphene/graphane or transition-metals dichalcogenides) or 1-D (e.g., carbon nanotubes) structures, are required to push scaling toward the 5-nm node.

Ground state terahertz quantum cascade lasers

A terahertz quantum cascade laser (THz QCL) architecture is presented in which only the ground state subbands of each quantum well are involved in the transport and lasing transition. Compared to state-of-the art THz QCLs based on the resonant-phonon scheme, ground state QCLs employ narrower wells so that all high-energy subbands are pushed up far above the occupied subband levels, significantly reducing parasitic interactions. Data on the experimental realization of two types of ground state QCLs are presented, in which the result of lasing above 5 THz is demonstrated.

Laser dressed donor impurities in free-standing GaAs films under an electric field

Abstract The laser-field dependence of the shallow donor states in a free-standing thin GaAs film under an external static field is studied within the effective mass approximation. The laser dressing effects are considered for the confinement potential of the well as well as for the impurity Coulomb interaction distorted by the dielectric mismatch at interfaces. We found that (i) the increase of the laser intensity dramatically modifies the electron potential energy, which establishes the quantum confinement; (ii) the ground state subband energy is significantly enhanced by the electrostatic self-energy arising from the interaction between the electron and its images; (iii) the impurity binding is much larger than those of the dielectrically homogenous case and it becomes stronger sensitive to the laser intensity variation; (iv) under an electric field parallel to the growth direction, the inversion symmetry with respect to the quantum well center is broken and a red/blue-shift of the binding energy, depending on the impurity position along the field direction, occurs. Therefore, the shallow donor energy levels in the free-standing thin films can be tuned in a wide range by proper tailoring of the structure parameters (well size, impurity position) as well as by varying the external applied fields.

Dual ground-state pulse generation from a passively mode-locked InAs/InGaAs quantum dot laser

In this paper, we present experimental results related to tunable dual wavelength passive mode locking of two independent ground state sub-bands in a multi section InAs/InGaAs quantum dot laser. The emission of these sub-bands is related to gain suppression at the center of the ground state emission, whereas their wavelength separation is tunable with injection current. Through the mechanism of passive mode locking two independent pulses were obtained, with typical pulse width in the order of 17 ps.

Sequential tunneling transport characteristics of GaN/AlGaN coupled-quantum-well structures

Vertical electronic transport in periodic GaN/AlGaN multiple-quantum-well structures grown on free-standing GaN substrates is investigated. Highly nonlinear current-voltage characteristics are measured, displaying a clear transition from a high-resistance state near zero applied bias to a low-resistance state as the voltage is increased. The measurement results, including their temperature dependence and the variations in turn-on voltage with subband structure and bias polarity are in full agreement with a picture of sequential tunneling through the ground-state subbands of adjacent coupled quantum wells. Scattering-assisted tunneling due to interface roughness or structural defects appears to be the dominant transport mechanism. The potential role of photon-assisted tunneling is also investigated.

Wetting Layer Effect on Optical Gain of Strained CdTe/ZnTe Pyramidal Quantum Dots

The optical properties of strained CdTe/ZnTe pyramidal quantum dots (QDs) are investigated as a function of the wetting layer thickness using an eight-band strain-dependent k·p Hamiltonian. The ground-state subband energies in the conduction and valence bands rapidly decreases with the increasing wetting layer thickness. This is attributed to the reduction of subband energies in both the conduction and the valence bands due to the strain effect. The optical gain peak on the shorter wavelength side decreases with the increasing wetting layer thickness. On the other hand, the gain peak on the longer wavelength side is nearly independent of the wetting layer thickness. The decrease in the gain peak on the shorter wavelength side is related to the decrease in matrix elements corresponding to transitions between higher subbands such as (3,4) and (4,3).

Laser-induced reshaping of the density of impurity states in GaAs/AlGaAs nanowires

Abstract The binding energy of shallow-donor impurities in a cylindrical quantum well wire irradiated by an intense non-resonant laser field is calculated within the effective mass approximation by using a variational procedure. Accurate laser-dressing effects are considered for both the confinement potential of the wire and the Coulomb potential of the impurity. The computation of the ground state subband energy eigenfunctions for different laser field intensities is based on a bidimensional finite element method. Important changes of the electron probability density under intense laser field conditions are predicted. The study reveals that the laser field compete with the quantum confinement and breaks down the degeneracy of states for donors symmetrically positioned within the nanostructure. A proper analysis of the density of impurity states is found to be essential for controlling the optical emission related to shallow donors in semiconductor quantum wires.

Intense THz laser effects on off-axis donor impurities in GaAs–AlGaAs coaxial quantum well wires

The binding energy of an off-axis shallow-donor impurity in a GaAs coaxial cylindrical quantum well-wire under the action of an intense, high-frequency laser field is calculated using a variational procedure within the effective-mass approximation. We take into account the laser dressing effects on both the impurity Coulomb potential and the confinement potential. Numerical calculations of the ground state subband energy levels based on a finite element method are performed for different barrier thicknesses and laser field parameters. Our model indicates a possible tuning of the well potential shape along the laser polarization direction. Strong distortions of the electron probability density under intense laser field conditions are also predicted. The study proves that the presence of the laser field partially breaks down the degeneracy of the states for donors symmetrically positioned within the structure. The results obtained show that the impurity energy levels in coaxial quantum wires can be significantly modified and controlled by intense laser fields.

Experimental Observation of Sequential Tunneling Transport in GaN/AlGaN Coupled Quantum Wells Grown on a Free-Standing GaN Substrate

AbstractA GaN/AlGaN multiple-quantum-well structure based on an asymmetric triple-quantum-well repeat unit was grown by molecular beam epitaxy, and its vertical electrical transport characteristics were investigated as a function of temperature. To minimize the density of dislocations and other structural defects providing leakage current paths, homoepitaxial growth on a free-standing GaN substrate was employed. The measured vertical-transport current-voltage characteristics were found to be highly nonlinear, especially at low temperatures, consistent with sequential tunneling through the ground-state subbands of weakly coupled adjacent quantum wells. Furthermore, different turn-on voltages were measured depending on the polarity of the applied bias, in accordance with the asymmetric subband structure of the sample repeat units.

Effects of charge density on the luminescence energy shift of GaN quantum wells: A variational approach

Abstract The influence of free-carrier screening on the luminescence energy shift in GaN quantum wells (QWs) mainly in relation to a quantum-confined Stark effect is investigated theoretically. This effect substantially modifies the electronic states in the QW and the emission energy in the photoluminescence (PL) spectra. In this paper, the influence of the built-in electric field on the PL emission is calculated by considering a variational electron and hole wave function to obtain the ground-energy transitions inside the active region in QW. Furthermore, the contribution between the interaction of the electron with the charge distribution of electrons and holes in the QW is taken into account in the Schrodinger equation, and this energy potential also satisfies the Poisson’s equation with the charge density in the ground-state subband as the source term. The transition energy calculations between the confined electron and hole states as a function of the carrier density and the well thickness are in agreement with the PL experiments reported in the literature.

Spin magnetotransport in two-dimensional hole systems

Spin current of two-dimensional holes occupying the ground-state subband in an asymmetric quantum well and interacting with static disorder potential is calculated in the presence of a weak magnetic field $\mathbf{H}$ perpendicular to the well plane. Both spin-orbit coupling and Zeeman coupling are taken into account. It is shown that the applied electric field excites both the transverse (spin-Hall) and diagonal spin currents, the latter changes its sign at a finite $H$ and becomes greater than the spin-Hall current as $H$ increases. The effective spin-Hall conductivity introduced to describe the spin response in Hall bars is considerably enhanced by the magnetic field in the case of weak disorder and demonstrates a nonmonotonic dependence on $H$.

Major features of frequency sweeping of emission from heterojunction quantum lasers

We have used numerical modeling to establish the major features of the variation (sweeping) of the “ instantaneous” laser frequency of heterojunction quantum lasers as a function of the pump current modulation frequency and tuning of the lasing frequency within the gain band. The active medium is described within a two-band model with identical distribution of levels for the electron and hole subbands, assuming transitions with no selection rule between the ground-state subbands. Sweeping of the laser frequency occurs due to variation of the refractive index of the active medium, as a result of the variation in the concentration of nonequilibrium charge carriers. Laser frequency sweeping does not occur for low current modulation frequencies, corresponding to realization of the quasisteady-state lasing regime. In the other limiting case of relatively high current modulation frequencies, the modulation depth of the laser output also tends toward zero. The magnitude of the sweep is greatest in the intermediate current modulation frequency region. For a specified current modulation frequency, the dynamic shift of the laser mode depends on the position of the lasing frequency within the gain band.

Specific features of 2D electron distribution over the subbands of the quantum well of a single heavily doped heterojunction

The electron distribution over the quantum confinement subbands (Em is the ground-state subband and Ep is the excited subband) in the quantum well of a heavily doped heterojunction is discussed. The functional relation between the concentrations nm and np in the Em and Ep subbands, as well as the threshold value nc, agrees with the theory only qualitatively. This fact can be due to the existence of the density-of-states tails and to the quasi-two-dimensionality of the electrons in the Ep subband. The measured nonzero concentration np in the subthreshold region nm < nc is explained by the effect of pushing the np electrons to the heterojunction plane by the internal electric field.

Correlation Kohn-Sham potential for quasi-two-dimensional electron gases

The optimized effective potential (OEP) method of density functional theory (DFT) has been developed for the study of the Kohn-Sham (KS) correlation potential of quasi-two-dimensional electron gases, for zero temperature and in an open configuration. The correlation energy functional has been generated by second-order perturbation theory, using the KS Hamiltonian as the unperturbed Hamiltonian. Its more important feature is that it is an implicit functional of the density, but an explicit functional of all quasi-two-dimensional electron gas (2DEG) subbands, occupied and unoccupied. Numerical results are presented for the physically relevant one-subband regime, where all electrons are in the ground-state subband. Main findings are as follows: (i) the correlation potential presents four types of contributions, with one of them predominant over the other three. This dominant contribution corresponds to a situation where all electronic transitions are within the ground-state subband; (ii) the correlation potential exhibits the correct dimensionality crossover from the quasi-two-dimensional to the strict two-dimensional regime; and (iii) in the asymptotic limit far from the two-dimensional electron gas the correlation potential is finite.

Nonequillibrium Green’s function analysis of interwell transport and scattering in monopolar lasers

Transport and scattering is examined in a simple coupled double-well model of infrared monopolar lasers via a nonequilibrium Green's function (NEGF) based analysis. Roughly speaking, in such lasers a more or less three level system is formed where electrons are injected into the first excited state subband of a leading well, decay via photon emission to intermediate ideally resonant-state subbands resulting from coupling of the ground state subband of the leading well to the first excited state subband of the trailing well, and subsequently, preferably quickly to allow population inversion between the initial and intermediate state, decay via phonon emission to the ground state subband of the trailing well. Golden Rule based analysis is widely used to model scattering including in this system. Implicit in its use is a random-phase approximation among the final states. However when scattering processes appear to produce not only changes in energy states but also real-space transport as between the wells here, this approximation can become suspect. In this work the affects of this approximation on scattering-induced population and depopulation of intermediate level(s) are addressed and overcome using a NEGF technique that allows consideration of transport and scattering absent the Golden Rule and associated random phase approxmiations. It is found that as the barrier becomes thick, the Golden Rule approximation can overestimate the depopulation rate of the intermediate levels. Through changes in the homogeneous broadening of the photon transition associated with changes in the depopulation rates of the intermediate level(s), the variations in barrier thickness could also have additional effects on population inversion and gain not apparent through Golden Rule calculations. Accordingly, barrier thickness is found to also be a potentially critical parameter for optimizing device performance.

Structural and optical properties of Cd xZn 1−xTe/ZnTe quantum dots grown on Si(1 0 0) substrates by using molecular beam epitaxy

Abstract We have studied structural and optical properties of self-assembled Cd 0.6 Zn 0.4 Te/ZnTe quantum dots (QDs) grown on Si(1 0 0) substrates by using molecular beam epitaxy. X-ray diffraction patterns indicated that the ZnTe buffer layers grown on the Si substrates were hetero-epitaxial films with the (1 0 0) orientation. The atomic force microscopy images showed that Cd 0.6 Zn 0.4 Te/ZnTe QDs were formed on Si(1 0 0) substrates. The photoluminescence spectra at 32 K showed the dominant excitonic peaks corresponding to the interband from the ground-state electronic sub-band to the ground-state heavy-hole band in the Cd 0.6 Zn 0.4 Te/ZnTe QDs. The present results can help to improve the understanding of the structural and interband transition properties in Cd 0.6 Zn 0.4 Te/ZnTe QDs grown on Si(1 0 0) substrates.

Frequency sweeping of emission from heterojunction quantum lasers with pump current modulation

We have used numerical modeling to study the dynamic shift of the “instantaneous” laser frequency (sweeping) as a function of the pump current modulation frequency for heterojunction quantum lasers when the lasing frequency is tuned within the gain band. As the model for the active medium, we used a two-band model with identical distribution of levels for the electron and hole subbands, in which we take into account direct transitions between the ground-state subbands. We have established that the variation in the sweep qualitatively corresponds to the behavior of the amplitude detuning characteristics of the laser. Depending on the parameters of the system during emission of pulses, the variation in the laser frequency can be monotonic or nonmonotonic. By selecting the pump current modulation frequency and setting the laser frequency within the gain band, we can realize the cases of maximum or minimum dynamic shift of the lasing frequency.

Atomistic Approach for Nanoscale Devices at the Scaling Limit and Beyond– Valley Splitting in Si

Band-structure effects on channel carrier density in the ultrathin-body end of the ITRS roadmap silicon (100) n-type metal oxide semiconductor field effect transistors (MOSFETs) are assessed here using a semi-empirical nearest-neighbor sp3d5s* tight-binding model with spin-orbit interaction. The calculations focus on the body thickness range between 10 and 18 atomic layers (∼1.5–2.5 nm). At this range, the standard effective mass approach is limited by its inability to capture the conduction band nonparabolicity effects and the subband splitting. The tight-binding simulations show interesting effects of ground-state subband splitting in this thickness range, and as a result of this, the channel charge density was found to fluctuate by as much as 30%. Additionally, it was observed that strict process tolerance is necessary in this thickness range in order to maintain an acceptable threshold voltage variation.

Modification of the energy spectrum and magnetic breakdown in a system of 2D holes at the GaAs/Al0.5 Ga0.5As heterojunction upon uniaxial compression

Uniaxial compression of the p-type GaAs/Al0.5Ga0.5 As heterostructures induces magnetic breakdown between the spin-split ground-state subbands of two-dimensional heavy holes. This phenomenon serves as direct experimental evidence of a strong qualitative modification of the energy spectrum of these structures upon uniaxial deformation. This modification has been revealed by numerical calculations, and, according to it, the subband spin splitting decreases upon compression, while the contours of the hole Fermi surface in both subbands touch one another in the compression direction in a pressure range of 2.5 kbar.