Hole Tunneling Times Research Articles

Time-Resolved Observation of Hole Tunneling in van der Waals Multilayer Heterostructures.

We reported a time-resolved study of quantum-mechanical tunneling of holes between two MoSe2 monolayers that are separated by a monolayer WS2 energy barrier. Four-layer heterostructures of MoSe2/WS2/MoSe2/graphene, as well as control samples, were fabricated by mechanical exfoliation and dry transfer techniques. To time-resolve the hole tunneling process, an ultrashort laser pulse was used to excite electrons and holes in both MoSe2 layers. By utilization of the graphene layer to eliminate carriers in the third MoSe2 layer, the first MoSe2 layer is selectively populated with the holes, which then tunnel to the third MoSe2 layer. By monitoring decay of the hole population with an ultrashort probe pulse, we measure a hole tunneling time of about 20 ps, which is found to slightly increase with the injected carrier density. Besides the fundamental interests of real-time observation of the quantum-mechanical tunneling effect across a nanometer barrier, these results provide quantitative understanding on tunneling mechanisms of charge transfer in van der Waals heterostructures, which is useful for designing sophisticated van der Waals multilayer heterostructures.

Effect of barrier width between GaAs/InGaAs/GaAs double coupled quantum wells on bipolar transport and terahertz radiation by hot carriers in lateral electric field

The dependences of the current and integral intraband terahertz electroluminescence intensity on the electric field in the n-InGaAs/GaAs heterostructures with asymmetric double tunnel-coupled quantum wells under the conditions of bipolar lateral transport are established to differ qualitatively for different interwell barrier widths. In the case of the thick (∼50 Å) barriers at electric fields less than those corresponding to the Gunn instability, high-frequency current oscillations are observed, and the emission intensity monotonously increases with increasing electric field strength over the entire studied range up to 3 kV/cm. At small barrier widths (∼30 Å), the current oscillations are practically absent, and the emission intensity substantially increases with the field strength between 1.5 and 2 kV/cm. The emission intensity increase is explained by the addition of direct electron and hole transitions between the size-quantization subbands to the indirect intrasubband transitions. The observed differences may be explained by the different relationships between the interband radiative recombination time of the minority charge carriers (holes) in the narrow wells and the hole tunneling time into the wide wells, dependent on the interwell barrier width.

Tuning the carrier tunneling in a single quantum dot with a magnetic field in Faraday geometry

We report on an increase in the carrier tunneling time in a single quantum dot (QD) with a magnetic field in Faraday geometry using photocurrent spectroscopy. A nearly 60\% increase in hole tunneling time is observed with an applied magnetic field equal to 9 T. For a truncated pyramid QD, hole tunnels out faster at the lateral edge of the QD due to the reduced barrier height. The magnetic field in Faraday geometry shrinks the hole wave function at the center of QD plane, which weakens the tunneling at lateral edge and increases the average tunneling time. This mechanism also works for electron but the effect is smaller. The electron wave function is more localized at the center of the QD due to the uniform confining potential, therefore the relatively weak shrinkage caused by the magnetic field does not reduce the tunneling rate significantly.

Giant Photocurrent Enhancement by Coulomb Interaction in a Single Quantum Dot for Energy Harvesting

Understanding the carrier excitation and transport processes at the single-charge level plays a key role in quantum-dot-based solar cells and photodetectors. Here, we report on Coulomb-induced giant photocurrent enhancement of positive charged trions (\emph{X$^+$}) in a single self-assembled InAs/GaAs quantum dot embedded in an \emph{n-i-}Schottky device by high-resolution photocurrent (PC) spectroscopy. The Coulomb repulsion between the two holes in the \emph{X$^+$} increases the tunneling rate of the hole, and the remaining hole can be reused as the initial state to regenerate \emph{X$^+$} again. This process brings the PC amplitude of \emph{X$^+$} up to 30 times larger than that of the neutral exciton. The analysis of the hole tunneling time gives the equivalent change of hole tunnel barriers caused by Coulomb interaction between two holes with a value of 8.05 meV during the tunneling process. Our work brings a fundamental understanding of energy conversion for solar cells in nanoscale to improve internal quantum efficiency for energy harvesting.

Fast Hole Tunneling Times in Germanium Hut Wires Probed by Single-Shot Reflectometry.

Heavy holes confined in quantum dots are predicted to be promising candidates for the realization of spin qubits with long coherence times. Here we focus on such heavy-hole states confined in germanium hut wires. By tuning the growth density of the latter we can realize a T-like structure between two neighboring wires. Such a structure allows the realization of a charge sensor, which is electrostatically and tunnel coupled to a quantum dot, with charge-transfer signals as high as 0.3 e. By integrating the T-like structure into a radiofrequency reflectometry setup, single-shot measurements allowing the extraction of hole tunneling times are performed. The extracted tunneling times of less than 10 μs are attributed to the small effective mass of Ge heavy-hole states and pave the way toward projective spin readout measurements.

On the radiative recombination and tunneling of charge carriers in SiGe/Si heterostructures with double quantum wells

For SiGe/Si(001) epitaxial structures with two nonequivalent SiGe quantum wells separated by a thin Si barrier, the spectral and time characteristics of interband photoluminescence corresponding to the radiative recombination of excitons in quantum wells are studied. For a series of structures with two SiGe quantum wells different in width, the characteristic time of tunneling of charge carriers (holes) from the narrow quantum well, distinguished by a higher exciton recombination energy, to the wide quantum well is determined as a function of the Si barrier thickness. It is shown that the time of tunneling of holes between the Si0.85Ge0.15 layers with thicknesses of 3 and 9 nm steadily decreases from ~500 to <5 ns, as the Si barrier thickness is reduced from 16 to 8 nm. At intermediate Si barrier thicknesses, an increase in the photoluminescence signal from the wide quantum well is observed, with a characteristic time of the same order of magnitude as the luminescence decay time of the narrow quantum well. This supports the observation of the effect of the tunneling of holes from the narrow to the wide quantum well. A strong dependence of the tunneling time of holes on the Ge content in the SiGe layers at the same thickness of the Si barrier between quantum wells is observed, which is attributed to an increase in the effective Si barrier height.

Enhanced photocurrent readout for a quantum dot qubit by bias modulation

We demonstrate coherent control of a quantum dot exciton using photocurrent detection with a sinusoidal reverse bias. Optical control is performed at low bias, where tunneling-limited coherence times are long. Following this step, the tunneling rates are increased to remove the long-lived hole, achieving a high photocurrent signal. For a detection efficiency of 68%, electron and hole tunneling times during optical control of 200 ps and 20 ns can be achieved, compared to 120 ps and 7 ns for the constant bias case, respectively.

Electrical Control of Interdot Electron Tunneling in a Double InGaAs Quantum-Dot Nanostructure

We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5 ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8 meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Carrier storage and confinement in GaAs‐based self‐organized quantum dots

We studied the carrier storage and confinement of electrons and holes in InGaAs/GaAs QDs with deep level transient spectroscopy (DLTS) and time-resolved tunneling capacitance spectroscopy (TRTCS). For InGaAs/GaAs QDs with an additional AlGaAs barrier below the QD layer we observe in charge-selective DLTS experiments a hole ground state activation energy of about 580 meV. This activation energy leads to a storage time of about 5 ms at room temperature. With the TRTCS method we studied in detail the electric field dependence of the electron and hole tunneling time from InGaAs QDs without an AlGaAs barrier. The dependence of the tunneling time on the applied electric field allows us to determine the hole and electron localization energy to E H1loc = (210 ± 20) meV and E E1loc = (260 ± 20) meV, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Selective measurement of hole tunneling times through AlGaAs barriers based on the quantum confined Stark effect

We demonstrate a method able to measure selectively and over a wide dynamic range the tunneling time of electrons or holes through a given potential barrier in a semiconductor heterostructure. The method relies on appropriate band gap engineering of the intrinsic region of a $p\text{\ensuremath{-}}i\text{\ensuremath{-}}n$ diode, and on the quantum confined Stark effect. As an example, the hole tunneling time through thick ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ potential barriers has been measured over six orders of magnitude in $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ structures. For $x=0.6$, ultralong hole tunneling times are demonstrated in fair agreement with a semiclassical tunneling model. This model however turns out to be inapplicable for $x\ensuremath{\leqslant}0.4$, overestimating the tunneling times by several orders of magnitude. This suggests the formation of intrinsic leakage current paths through the AlGaAs barriers at these concentrations.

Single-chip device for tunneling time measurements

A single-chip mesoscopic device is proposed for the measurement of the tunneling time of electrons or holes through various mesoscopic structures. The method is based on the possibility of generating spatially separated electrons and holes that are transported with a controlled velocity and separation over a quantum wire. The electrons and holes, subsequently separated in distinct waveguides, are allowed to recombine after propagating through carefully controlled distances. The electron and hole coincidence requirement is satisfied by compensating the tunneling time of one type of carrier through an engineered mesoscopic structure with the magnetic field induced delay of the other carrier type.

Orientation effect on intersubband tunneling of quantum wells grown on high-index GaAs( n11)A ( n≤4) substrates

The effect of the orientation on intersubband tunneling process in Al 0.35Ga 0.65As/GaAs asymmetric double quantum wells (ADQWs) grown on GaAs( n11)A ( n≤4) substrates was studied by cw photoluminescence (PL), time-resolved photoluminescence (TRPL) at low temperatures and transmission electron microscope (TEM) measurements. The red-shift of two PL peaks from the wide quantum well (WW) and the narrow quantum well (NW) for substrate orientation far from the (100) plane is attributed to the large anisotropy of the heavy-hole band in the different GaAs orientations. TRPL experimental results show the decay time τ of the excitation from the NW is about 100 ps for (111)A and (211)A samples, which is 50% smaller than that of the other samples. This fast tunneling process may be understood such that the large anisotropy of the heavy-hole band for (111)A and (211)A planes causes the resonant tunneling of holes between the WW and NW. The optical transition dependence of the orientation in ADQWs provides a useful method to estimate the tunneling time of holes through a barrier.

Storage of electrons and holes in self-assembled InAs quantum dots

We report spectroscopic measurements of charge-tunable quantum dots. The samples contain vertically aligned double dots which we can fill with electrons from a back contact. We show how we can also accumulate holes in the dots by illuminating the samples with below band gap radiation when a large negative bias is applied. We argue that this is possible through a large disparity in the electron and hole tunneling times. Interband spectroscopy reveals a strong reduction in the quantization energy for the dots in the second layer.

The excition tunneling in ZnCdSe/ZnSe asymmetric double quantum well

Photoluminescence spectra of asymmetric double-quantum-well structure are studied in this paper. We show the excitation power dependence of exciton tunneling. Due to the different tunneling time of electrons and holes, space-charge effect is observed.

Effect of strain on hole tunneling dynamics in double barrier heterostructures

A study of the effects of strain due to external stress and lattice mismatch on hole tunneling times in double barrier heterostructures is presented. The band structure of holes is calculated based on a k·p method within the envelope function approximation, including band mixing effects. The phase delay time is obtained from the energy derivative of the total phase shift of the wave function upon tunneling and is used to estimate the hole tunneling time. The results demonstrate that strain can be utilized to tailor hole tunneling times by changing the energy separation and, consequently, the mixing between heavy hole and light hole states in the quantum well.

Effects of band mixing on hole tunneling times in GaAs/AlAs double-barrier heterostructures

The tunneling time of holes in GaAs/AlAs double-barrier heterostructures is calculated within the envelope function approximation, including band-mixing effects. The phase delay time is obtained from the energy derivative of the total phase shift of the wave function upon tunneling. The results show clearly that mixing plays a very important role in hole tunneling. In particular, our study suggests that the conventional effective mass models (without band mixing) can significantly overestimate the heavy hole tunneling time and thus are not suitable for estimation of the hole tunneling time. It is also demonstrated that the effect of band mixing on hole tunneling times becomes more important for thicker barriers.

Tunneling through Single AlGaAs Barriers

AbstractTunneling transfer through Al0.35Ga0.65As barriers is studied in asymmetric double quantum well structures by time‐resolved photoluminescence measurements in the pico‐ and femtosecond regime. A large variety of electron and hole resonances is detected when electric fields of both signs are externally applied. The ground state resonance shifts, when the electrons tunnel in the reverse direction, revealing the importance of excitonic effects. Longitudinal optical phonon assisted tunneling plays a minor role for narrow quantum wells in comparison to impurity or interface roughness assisted transfer. Resonant electron tunneling times depend exponentially on the integrated square root of the tunneling barrier height and are an order of magnitude faster than resonant hole tunneling times. The n = 2 to n = 1 electronic intersubband scattering time in a 10 nm quantum well is determined to be 550 fs measuring the transfer time through a thin barrier.

Effect of band mixing on hole-tunneling times in GaAs/AlAs double-barrier heterostructures.

We examine the influence of band mixing on hole-tunneling times in GaAs/AlAs double-barrier heterostructures using the eight-band effective bond-orbital model. We find that mixing of heavy-hole and light-hole states in the valence subbands can substantially reduce hole-tunneling times from the values predicted by the Kronig-Penney model, which does not account for band mixing. These results are in agreement with the experimental measurements of Jackson et al. [Appl. Phys. Lett. 54, 552 (1989)], indicating that hole-tunneling times are much shorter than predicted by the Kronig-Penney model, and comparable in fact to electron-tunneling times. We also compare our calculation with an earlier phenomenological model for incorporating band-mixing effects in the calculation of hole-tunneling times.

Tunneling of minority holes through a double-barrier resonant-tunneling structure under applied bias

Time-resolved photoluminescence of an AlAs/GaAs double barrier resonant tunneling structure has been measured under bias in the region of the first electron resonance, both for exciton recombination inside the quantum well, and in the n-type GaAs contact layers. The characteristic time evolution of the confined exciton and n-GaAs emission reveals the processes of accumulation of photocreated holes and of sequential hole tunneling through the first and second AlAs barrier, for which nearly equal tunneling times are observed. The hole tunneling time is found to decrease monotonically with increasing voltage, as expected for tunneling through a single barrier.

Measurement of the exciton-formation time and the electron- and hole-tunneling times in a double-quantum-well structure.

Demonstration experimentale de la nature bimoleculaire de la formation des excitons a partir des porteurs libres dans une structure de double puits quantique asymetrique a effet tunnel, GaAs. Obtention d'un coefficient de formation bimoleculaire de 6×10 −12 cm 2 /PS. Determination simultanee des durees d'effet tunnel des electrons et des trous sous les memes conditions experimentales de l'echantillon. Utilisation de la technique de la correlation croisee de la photoluminescence pour mesurer ces durees de canalisation