The role of transport processes of nonequilibrium charge carriers in radiative properties of arrays of InAs/GaAs quantum dots

The results of time-resolved photoluminescence studies of heterostructures containing monolayer arrays of InAs/GaAs quantum dots are presented. A two-component time dependence of intensity of photoluminescence from the ground state of quantum dots, with characteristic times of the slow component up to hundreds of nanoseconds and those of rapid one several nanoseconds, is studied. It is shown that the slow component is determined by the transport of nonequilibrium charge carriers between the quantum dots. At low temperatures, the time of the slow component is determined by tunneling, and at high temperatures by thermal escape of nonequilibrium charge carriers. The ratio of the contributions of tunneling and thermal escape is determined by the degree of isolation of quantum dots. A theoretical model is constructed that describes the effect of the dynamics of carrier transport on the emergence and decay of the slow component of photoluminescence.     

Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.     

Tunnel charge transport within silicon in reversely-biased MOS tunnel structures

The features of the electrical behaviour of a MOS tunnel structure, which arise from the tunnel carrier transport in semiconductor, are considered. For the explicitely given band diagram, the total current increases due to the contribution of electrons in energy range where the only-oxide tunneling is impossible. The resonance transport via the discrete levels in the quantum well may introduce steps in the reverse current-voltage characteristic. The band-to-band tunneling, which is to be treated as semiconductor tunneling, perturbates the balance of minority carriers in the inversion layer, modifying the charge state of a MOS structure. The stationary non-equilibrium support of a large surface carrier concentration becomes therefore possible, and the voltage partitioning in the MOS structure is distorted.     

Resonances related to an array of InAs quantum dots and controlled by an external electric field

Photoluminescence of multilayer structures with InAs quantum dots grown in the p-n junction in GaAs by molecular-beam epitaxy is studied. Formation of vertical columns of quantum dots is verified by the data of transmission electron microscopy. It is shown that a natural increase in the size of quantum dots from layer to layer brings about their vertical coalescence at the upper part of a column. An unbalance of electronic levels caused by the enlargement of quantum dots was compensated by an external electric field, so that the resonance of ground electronic states in the column was attained. The onset of resonances was checked by the methods of steady-state and time-resolved photoluminescence. It is shown that, in the case of a resonance, the photoluminescence intensity and the radiative lifetime of excitons increase (up to 0.6–2 ns), while the time of tunneling of charge carriers becomes shorter (shorter than 150 ps). Outside the resonances, tunneling of electrons is appreciably enhanced owing to the involvement of longitudinal optical phonons. If only these phonons are involved, the time of nonresonance tunneling between quantum dots becomes shorter than the time of relaxation of charge carriers from the barrier (100 and 140 ps, respectively).     

Analysis of mechanisms of carrier emission in the p-i-n structures with In(Ga)As quantum dots

With the help of the photocurrent spectroscopy, the mechanism of emission of charge carriers from energy levels of the (In,Ga)As/(Al,Ga)As quantum dots of different design are studied. Thermal activation is shown to be the main mechanism of carrier emission from the quantum dots for the isolated layer of quantum dots separated by wide (Al,Ga)As spacer layers. At a small width of the (Al,Ga)As spacer layer, when electron binding of separate layers of the quantum dots in the vertical direction takes place, the role of the tunneling mechanism of carrier emission between the vertically coupled quantum dots increases.     

The resonant tunneling of holes through double-barrier structures with InAs QDs at the center of a GaAs quantum well

The effect of InAs quantum dots (QDs) grown in the center of a GaAs quantum well on the tunneling characteristics of resonant-tunneling diodes based on p-AlAs/GaAs/AlAs heterostructures is studied. The introduction of QDs results in a shift and broadening of resonance peaks in the current-voltage characteristics of the diodes; however, this effect is found to be strongly dependent on the number of the 2D subband involved in the tunneling. The obtained dependence is attributed to origination of the fluctuation potential in the vicinity of the QD layer.     

Charge carrier interference in one-dimensional semiconductor rings

Interference of the ballistic charge carriers in one-dimensional (1D) rings formed by two quantum wires in the self-ordered silicon quantum wells was investigated for the first time. The charge carrier transmission coefficient, which is dependent on the carrier energy, is calculated as a function of the length and modulation depth of the parallel quantum wires. The wires can be linked to the two-dimensional reservoirs either by the common source-drain system or by the quantum point contacts. It is predicted that the conductance of a 1D ring in the first case is four times higher than in the second due to the carrier interference. The calculated dependences manifest themselves in the conductance oscillations observed in the 1D silicon rings upon varying the source-drain voltage or the external magnetic field. The results obtained made it possible to design an Aharonov-Bohm interferometer based on a 1D silicon ring in the weak localization mode; its characteristics are demonstrated in the studies of the phase coherence in the tunneling of single charge carriers through the quantum point contact.     

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.     

Conductance through two dots coupled in series: application of the exact solution

Using the exact Bethe-ansatz solution, we study the conductance through symmetric and asymmetric double-dot (DD) systems in the Kondo regime, where two quantum dots are coupled in series. The application of the Ward-Takahashi identity enables us to compute the conductance at low temperatures. For strong inter- and intra-dot Coulomb repulsions with weak inter-dot tunneling, we discuss how the Kondo effect evolves for an asymmetric DD system as well as a symmetric DD system in a magnetic field. In particular, we clarify that the conductance is decreased (increased) in the asymmetric DD (symmetric DD in a field), reflecting that the Kondo effect due to inter-dot charge fluctuations is suppressed (enhanced).     

Simulation of quantum transport in monolithic ICs based on In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As RTDs and HEMTs with a quantum hydrodynamic transport model

A new quantum hydrodynamic transport model based on a quantum fluid model is used for numerical calculations of different quantum sized devices. The simulation of monolithic integrated circuits of resonant tunneling structures and high electron mobility transistors (HEMT) based on In/sub 053/Ga/sub 0.47/As-In/sub 052/Al/sub 0.48/As-InP is demonstrated. With the new model, it is possible to describe quantum mechanical transport phenomena like resonant tunneling of carriers through potential barriers and particle accumulation in quantum wells. Different structure variations, especially the resonant tunneling diode area and the gate width of the HEMT structure, show variable modulations in the output characteristics of the monolithic integrated device.     

Nonequilibrium population of charge carriers in structures with InGaN deep quantum dots

Electronic and optical properties of ensembles of quantum dots with various energies of activation from the ground-state level to the continuous-spectrum region were studied theoretically and experimentally with the InGaN quantum dots as an example. It is shown that, depending on the activation energy, both the quasi-equilibrium statistic of charge carriers at the levels of quantum dots and nonequilibrium statistic at room temperature are possible. In the latter case, the position of the maximum in the emission spectrum is governed by the value of the demarcation transition: the quantum dots with the transition energy higher than this value feature the quasi-equilibrium population of charge carriers, while the quantum dots with the transition energy lower than the demarcation-transition energy feature the nonequilibrium population. A model based on kinetic equations was used in the theoretical analysis. The key parameters determining the statistic are the parameters of thermal ejection of charge carriers; these parameters depend exponentially on the activation energy. It is shown experimentally that the use of stimulated phase decomposition makes it possible to appreciably increase the activation energy. In this case, the thermal-activation time is found to be much longer than the recombination time for an electron-hole pair, which suppresses the redistribution of charge carriers between the quantum dots and gives rise to the nonequilibrium population. The effect of nonequilibrium population on the luminescent properties of the structures with quantum dots is studied in detail.     

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.     

Observation of the multiple negative-differential-resistance ofmetal-insulator-semiconductor-like structure with step-compositioned InxGa1-xAs quantum wells

An interesting multiple negative-differential-resistance (MNDR) device, based on an AlGaAs-InGaAs-GaAs metal-insulator-semiconductor (MIS)-like structure, has been fabricated and demonstrated. Three and six switching phenomena have been observed at room temperature and -105°C, respectively. The impressive MNDR behaviors are believed to be caused by the sequential accumulation process of carriers at In_x Ga_1-xAs subwells and the successive barrier lowering and potential redistribution effects. These effects yield the step by step enhancement of tunneling through the “insulated” AlGaAs barrier. It is known that, from experimental results, the temperature variation plays an important role on carriers transport and experimental current-voltage (I-V) characteristics     

Current bistability in InGaAs quantum wire p-i-n heterostructures

We report a clear evidence of bistability in the current-voltage (I-V) characteristics of p-i-n heterostructures containing InGaAs V-shaped quantum wires. The observed phenomenon is explained in the framework of a single carrier transport model in which the quantum wires act like traps for the vertical current. The charge trapping phenomenon is indeed demonstrated by capacitance-voltage (C-V) and photocurrent experiments.     

Terahertz Photon-Assisted Tunneling in InAs Quantum Dots

We have investigated electron transport in a single self-assembled InAs quantum dot (QD) coupled to nanogap metal electrodes under terahertz (THz) radiation. The fabricated QD samples operated as single electron transistors in a few electron regime, exhibiting clear shell structures. Under the THz radiation, in addition to the original Coulomb oscillation peaks, new side-peaks showed up. The dependence of the new side-peak current on the THz power follows the prediction of the photon-assisted tunneling (PAT) theory. Moreover, two types of PAT processes were observed in the THz range; the ground state resonance and the photon-induced excited state resonance, depending on the relative magnitude between the orbital quantization energy of the QDs and the THz photon energy. Furthermore, a very high coupling efficiency between the THz waves and the QDs was realized in our system and we observed multi-photon absorption up to the fourth-order during the tunneling process, resulting in almost complete lifting of the Coulomb blockade. This high coupling efficiency between THz wave and electrons in QDs opens a way to the manipulation of single electron charge/spin states in the THz frequency range.     

Electron-hole Coulomb interaction in InGaN quantum dots

In contrast to group-III arsenide-based quantum dots, group-III nitride-based quantum dots are much smaller in size and have less conduction band offset. For this reason, it is important to take into account the electron-hole Coulomb interaction in which the spherical quantum dot is substituted for the cube-shaped quantrum dot, which in this paper is treated within the Hartree approximation. In addition to making the electron binding energy several times larger, this strong Coulomb interaction between carriers in quantum dots makes conventional distribution functions invalid, requiring their recalculation. Such calculations reveal that while the one-particle distribution functions are donor-like, the electron-hole function is quite different from its predecessors. Fiz. Tekh. Poluprovodn. 32, 1235–1239 (October 1998)     

Inverted bistability in the current-voltage characteristics of a resonant tunneling device

The current-voltage characteristics of an asymmetric double barrier resonant tunneling device show a butterfly-shaped hysteresis loop in which, for a range of voltage, the off-resonant current exceeds the resonant current. This “inverted bistability” is due to the effects of space charge buildup in the quantum well. Magnetoquantum oscillations in the tunnel current with | are used to investigate the distribution of charge within the device and the intersubband scattering processes which control the charge buildup.     

Optical characterization of individual semiconductor nanostructures using a scanning tunneling microscope

By injecting low-energy minority carriers from the tip of a scanning tunneling microscope (STM) and analyzing the light emitted from the tip-sample gap of the STM, it is possible to study the optical and electronic properties of individual semiconductor nanostructures with an extremely high spatial resolution close to the atomic scale. This technique has been applied to investigate the transport properties of hot electrons injected into AlGaAs/GaAs quantum well structures and the optical properties of single self-assembled InAs/AlGaAs quantum dots. The physical principles, usefulness and future expectations of this novel technique are discussed.     

Nanoelectronic architecture for Boolean logic

A nanoelectronic implementation of Boolean logic circuits is described where logic functionality is realized through charge interactions between metallic dots self-assembled on the surface of a double-barrier resonant tunneling diode (RTD) structure. The primitive computational cell in this architecture consists of a number of dots with nearest neighbor (resistive) interconnections. Specific logic functionality is provided by appropriate rectifying connections between cells. We show how basic logic gates, leading to combinational and sequential circuits, can be realized in this architecture. Additionally, architectural issues including directionality, fault tolerance, and power dissipation are discussed. Estimates based on the current-voltage characteristics of RTD's and the capacitance and resistance values of the interdot connections indicate that static power dissipation as small as 0.1 nW/gate and switching delay as small as a few picoseconds can be expected. We also present a strategy for fabricating/synthesizing such systems using chemical self-organizing/self-assembly phenomena. The proposed synthesis procedure utilizes several chemical self-assembly techniques which have been demonstrated recently, including self-assembly of uniform arrays of close-packed metallic dots with nanometer diameters, controlled resistive linking of nearest neighbor dots with conjugated organic molecules and organic rectifiers     

Optical properties of blue light-emitting diodes in the InGaN/GaN system at high current densities

The current-voltage and brightness-voltage characteristics and the electroluminescence spectra of blue InGaN/GaN-based light-emitting diodes are studied to clarify the cause of the decrease in the emission efficiency at high current densities and high temperatures. It is found that the linear increase in the emission intensity with increasing injection current changes into a sublinear increase, resulting in a decrease in efficiency as the observed photon energy shifts from the mobility edge. The emission intensity decreases with increasing temperature when the photon energy approaches the mobility edge; this results in the reduction in efficiency on overheating. With increasing temperature, the peak of the electroluminescence spectrum shifts to lower photon energies because of the narrowing of the band gap. The results are interpreted taking into account the fact that the density-of-states tails in InGaN are filled not only via trapping of free charge carriers, but also via tunneling transitions into the tail states. The decrease in the emission efficiency at high currents is attributed to the suppression of tunneling injection and the enhancement of losses via the nonradiative recombination channel “under” the quantum well.