Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 1994
We describe the fabrication of nanostructures and the physics of the devices formed in InGaAs-bas... more We describe the fabrication of nanostructures and the physics of the devices formed in InGaAs-based modulation doped heterostructures using Ga focused ion beam implantation. The two-dimensional electron gas in this heterostructure has a high electron density, a small effective mass, and a high mobility at high temperature, all of which are advantageous for low-dimensional quantum devices and ballistic devices. Ballistic transport at high temperature, quantized conductance with a large spacing of one-dimensional subbands, and a single electron transistor action with nonperiodic Coulomb oscillations are demonstrated.
We have studied the negative bend resistance phenomena in a side-gated crossed-wire junction in t... more We have studied the negative bend resistance phenomena in a side-gated crossed-wire junction in the fractional quantum Hall regime. The dip near zero magnetic field due to electrons and that near the filling factor 1 2 due to composite fermions are both enhanced as the channel is squeezed. The width of the dips in the magnetic field exhibits opposite behavior as a function of gate voltage, implying that the effective potential that the composite fermions experience is different from that of the electrons. The gradual variation of the electron density due to the lateral potential depletion is anticipated to result in a deviation of the effective magnetic field in the vicinity of the channel boundary. We numerically examine the effects of a nonuniform magnetic field on the transmission properties.
The exciton population and the transfer of excitons was investigated for growth islands in quantu... more The exciton population and the transfer of excitons was investigated for growth islands in quantum wells (QW's) by cw-photoluminescence (PL), time-resolved PL, and photoluminescence excitation spectroscopy (PLE). For high-quality GaAs/Al, Ga, "AsQW's with 13 nm QW thickness we found a strong dependence of the exciton population of three observed different island regions on excitation wavelength and excitation intensity. In a wide range of excitation wavelengths, the intensity ratios of the corresponding exciton transitions vary strongly with excitation intensity and, in general, do not reflect the area ratios of these islands. This behavior is caused by the different relaxation processes at the different excitation wavelengths when the lowest-energy states of the island regions are populated. By resonantly exciting the subbands in the QW, the created excitons remain localized and relax within the island regions. By nonresonantly exciting the exciton continuum of the confined subbands, the islands with the lowest energy are effectively populated within a time smaller than the lUminescence decay-time constant. We determined the area ratios of the islands and the normalized exciton-transfer rates. A quantitative analysis of the intensity ratios of the cw measurements and time-resolved PL measurements revealed a surprisingly long excitontransfertime constant in the order of 2 ns, which is dependent on the potential-energy difference between neighboring islands.
Anisotropic Hall mobilities of a two-dimensional electron gas are observed in modulation-doped Al... more Anisotropic Hall mobilities of a two-dimensional electron gas are observed in modulation-doped Al Gay As/GaAs heterostructures grown by molecular-beam epitaxy on a (001) GaAs substrate. The mobility in the [110]direction is larger than that in the [110]direction. An anisotropic interface roughness is proposed to account for the observed anisotropic Hall mobilities. The dependences of the anisotropic mobilities on the electron concentration are explained well by the theoretical calculation, which assumes the existence of interface islands longer in the [110] direction than in the [110] direction. This assumption is consistent with previous reports on in situ measurement of growing surfaces by scanning tunneling microscope and electron diffraction. The dominant electron-scattering process in modulation-doped structures at low temperatures is thought to be ionized-impurity scattering. When the ionized impurities are far from the conducting channel, there appears a new regime of scattering, where
Physica E-Low-Dimensional Systems & Nanostructures, 2010
We report a systematic study of the contact resistance present at the interface between a metal (... more We report a systematic study of the contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate that the contact resistance is quantitatively the same for single-, bi-, and tri-layer graphene (∼ 800 ± 200Ωµm), and is in all cases independent of gate voltage and temperature. We argue that the observed behavior is due to charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality point.
Einzelne Elektronen surfen auf einer Schallwelle Schaltkreise, die mit einzelnen Elektronen arbei... more Einzelne Elektronen surfen auf einer Schallwelle Schaltkreise, die mit einzelnen Elektronen arbeiten, können die Leistungsfähigkeit von Computern enorm steigern. Sie benötigen aber einen Mechanismus, um einzelne Elektronen zu befördern. Zwischen Quantenpunkten ist dies nur möglich, wenn diese sehr eng beieinander liegen. Am Institut-Néel CNRS Grenoble ist es unserer Forschergruppe zusammen mit Kollegen aus Tokio und Bochum gelungen [1, 2], einzelne Elektronen mit einer Schallwelle zwischen zwei benachbarten Quantenpunkten zu befördern. Dies bedeutet einen großen Schritt in Richtung Quanten-Informationstechnologie mit einzelnen Elektronen.
We describe photon assisted tunneling (PAT) in single and coupled quantum dot systems which have ... more We describe photon assisted tunneling (PAT) in single and coupled quantum dot systems which have a relatively large zero-dimensional (0D) level separation. A microwave electric field applied across the tunneling barrier leads to the formation of a sideband structure. For a single quantum dot system, a PAT current peak is clearly observed in addition to the Coulomb oscillation peak. For a weakly-coupled double-dot system, we observe resonant 0D-0D PAT. The resonant PAT current appears only when two discrete quantum levels in the neighboring dots are exactly separated by the microwave photon energy. The PAT current shows a narrower peak than the main resonant tunneling current in accordance with time-dependent tunneling theory. The photon stimulated emission associated with PAT from a higher-lying occupied level to a lower-lying empty level is obtained at a drain voltage sufficient that population inversion takes place.
Si-based single electron transistors has been measured for the ®rst time in 2± 12 MHz at 1.5 K wi... more Si-based single electron transistors has been measured for the ®rst time in 2± 12 MHz at 1.5 K with a lock-in technique. The magnitude of the shot noise normalized by the full shot noise, 2eI, clearly shows oscillatory structures as a function of the gate voltage, having a minima at the Coulomb oscillation peaks and a maxima in the Coulomb blockade region. The value of the maxima is approximately unity, re¯ecting a tunneling event with no correlation. On the other hand, the value of the minima is signi®cantly smaller than half the full shot noise, which is smaller than in the previous theoretical and experimental reports and suggests that some current regulation mechanism is present.
Transport phenomena in a specially optimized vertical asymmetric sub-micron Al,z8Ga, ,? As/ GaAs ... more Transport phenomena in a specially optimized vertical asymmetric sub-micron Al,z8Ga, ,? As/ GaAs double barrier structure with modulation-doped barriers is investigated by applying a bias to a special Schottky side gate, which allows the effective area of the conducting channel to be finely "tuned". For a sufficiently small device, the electrical properties of the controllable quantum dot bounded by well defined heterostructure barriers and an adjustable side wall potential are expected to be governed by single electron charging, and single electron resonant tunneling. Single electron transistor (SET) operation is possible because the number of electrons in the dot, n, can be varied one-by-one with the side gate. The drain current Rowing through the conducting channel in response to a small drain voltage is strongly modulated by the gate voltage close to "pinch-off' as n approaches zero, and oscillations in the drain current persist up to about 25 K. A gate modulated zero current region of Coulomb blockade, step-like features, and resonances exhibiting negative differential resistance are clearly observed at low bias. Although this technology is very promising for the realization of SET operation at temperatures well above 4.2 K, the temperature dependence of the Coulomb blockade is an important limitation. We describe the evolution of the conductance oscillations and the degradation of Coulomb blockade with temperature from 0.3 K up to 25 K, and propose that co-tunneling in a small system containing just a "few" electrons in which the zero-dimensional energy level spacing is significant and comparable to the Coulomb charging energy is important, and is likely to be more complex than that documented for large planar dot structures containing "mung" electrons 1. INTRODtJCTION
Spin-orbit interaction in InGaAs/InAlAs quantum wires is investigated by measuring conductance fl... more Spin-orbit interaction in InGaAs/InAlAs quantum wires is investigated by measuring conductance fluctuations at temperatures ranging from 2 K down to 0.06 K. The resistance of an InGaAs/InAlAs wire in a zero magnetic field decreases as the temperature decreases below 1.6 K. This temperature dependence is reproduced by the theory of antilocalization in the strong spin-orbit interaction regime. In the same temperature range, the conductance fluctuation amplitude increases nonmonotonically with decreasing temperature. This behavior is attributed to the temperature-induced transition from the weak to the strong spin-orbit scattering regime in the mesoscopic disordered system.
... which is ideal for investigating single electron charging of, and single electron tunnelling ... more ... which is ideal for investigating single electron charging of, and single electron tunnelling through, a ... Substantial regulation of the current flowing through a gated vertical AlGaAs/GaAs double-barrier ... of very small circular DBRTT mesas with a scanning electron microscope, we ...
Tunneling conductance through a quantum dot is calculated around the local spin singlet-triplet c... more Tunneling conductance through a quantum dot is calculated around the local spin singlet-triplet crossover region including the Kondo effect. The calculation is carried out using the numerical renormalization group method. When the potential on the dot deepens, two electrons filling a lower energy orbital redistribute to gain Hund's coupling energy. This redistribution induces a bump in the conductance between the Coulomb peaks. The Kondo temperature on the bump is high due to the fluctuation on the singlet-triplet crossover region. The behaviors agree well with recent experiment.
We determine the contributions from the direct Coulomb and exchange interactions to the total int... more We determine the contributions from the direct Coulomb and exchange interactions to the total interaction in semiconductor artificial atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. We find that electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle state degeneracy, and find that the spin-configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 1994
We describe the fabrication of nanostructures and the physics of the devices formed in InGaAs-bas... more We describe the fabrication of nanostructures and the physics of the devices formed in InGaAs-based modulation doped heterostructures using Ga focused ion beam implantation. The two-dimensional electron gas in this heterostructure has a high electron density, a small effective mass, and a high mobility at high temperature, all of which are advantageous for low-dimensional quantum devices and ballistic devices. Ballistic transport at high temperature, quantized conductance with a large spacing of one-dimensional subbands, and a single electron transistor action with nonperiodic Coulomb oscillations are demonstrated.
We have studied the negative bend resistance phenomena in a side-gated crossed-wire junction in t... more We have studied the negative bend resistance phenomena in a side-gated crossed-wire junction in the fractional quantum Hall regime. The dip near zero magnetic field due to electrons and that near the filling factor 1 2 due to composite fermions are both enhanced as the channel is squeezed. The width of the dips in the magnetic field exhibits opposite behavior as a function of gate voltage, implying that the effective potential that the composite fermions experience is different from that of the electrons. The gradual variation of the electron density due to the lateral potential depletion is anticipated to result in a deviation of the effective magnetic field in the vicinity of the channel boundary. We numerically examine the effects of a nonuniform magnetic field on the transmission properties.
The exciton population and the transfer of excitons was investigated for growth islands in quantu... more The exciton population and the transfer of excitons was investigated for growth islands in quantum wells (QW's) by cw-photoluminescence (PL), time-resolved PL, and photoluminescence excitation spectroscopy (PLE). For high-quality GaAs/Al, Ga, "AsQW's with 13 nm QW thickness we found a strong dependence of the exciton population of three observed different island regions on excitation wavelength and excitation intensity. In a wide range of excitation wavelengths, the intensity ratios of the corresponding exciton transitions vary strongly with excitation intensity and, in general, do not reflect the area ratios of these islands. This behavior is caused by the different relaxation processes at the different excitation wavelengths when the lowest-energy states of the island regions are populated. By resonantly exciting the subbands in the QW, the created excitons remain localized and relax within the island regions. By nonresonantly exciting the exciton continuum of the confined subbands, the islands with the lowest energy are effectively populated within a time smaller than the lUminescence decay-time constant. We determined the area ratios of the islands and the normalized exciton-transfer rates. A quantitative analysis of the intensity ratios of the cw measurements and time-resolved PL measurements revealed a surprisingly long excitontransfertime constant in the order of 2 ns, which is dependent on the potential-energy difference between neighboring islands.
Anisotropic Hall mobilities of a two-dimensional electron gas are observed in modulation-doped Al... more Anisotropic Hall mobilities of a two-dimensional electron gas are observed in modulation-doped Al Gay As/GaAs heterostructures grown by molecular-beam epitaxy on a (001) GaAs substrate. The mobility in the [110]direction is larger than that in the [110]direction. An anisotropic interface roughness is proposed to account for the observed anisotropic Hall mobilities. The dependences of the anisotropic mobilities on the electron concentration are explained well by the theoretical calculation, which assumes the existence of interface islands longer in the [110] direction than in the [110] direction. This assumption is consistent with previous reports on in situ measurement of growing surfaces by scanning tunneling microscope and electron diffraction. The dominant electron-scattering process in modulation-doped structures at low temperatures is thought to be ionized-impurity scattering. When the ionized impurities are far from the conducting channel, there appears a new regime of scattering, where
Physica E-Low-Dimensional Systems & Nanostructures, 2010
We report a systematic study of the contact resistance present at the interface between a metal (... more We report a systematic study of the contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on 11 graphene flakes we demonstrate that the contact resistance is quantitatively the same for single-, bi-, and tri-layer graphene (∼ 800 ± 200Ωµm), and is in all cases independent of gate voltage and temperature. We argue that the observed behavior is due to charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality point.
Einzelne Elektronen surfen auf einer Schallwelle Schaltkreise, die mit einzelnen Elektronen arbei... more Einzelne Elektronen surfen auf einer Schallwelle Schaltkreise, die mit einzelnen Elektronen arbeiten, können die Leistungsfähigkeit von Computern enorm steigern. Sie benötigen aber einen Mechanismus, um einzelne Elektronen zu befördern. Zwischen Quantenpunkten ist dies nur möglich, wenn diese sehr eng beieinander liegen. Am Institut-Néel CNRS Grenoble ist es unserer Forschergruppe zusammen mit Kollegen aus Tokio und Bochum gelungen [1, 2], einzelne Elektronen mit einer Schallwelle zwischen zwei benachbarten Quantenpunkten zu befördern. Dies bedeutet einen großen Schritt in Richtung Quanten-Informationstechnologie mit einzelnen Elektronen.
We describe photon assisted tunneling (PAT) in single and coupled quantum dot systems which have ... more We describe photon assisted tunneling (PAT) in single and coupled quantum dot systems which have a relatively large zero-dimensional (0D) level separation. A microwave electric field applied across the tunneling barrier leads to the formation of a sideband structure. For a single quantum dot system, a PAT current peak is clearly observed in addition to the Coulomb oscillation peak. For a weakly-coupled double-dot system, we observe resonant 0D-0D PAT. The resonant PAT current appears only when two discrete quantum levels in the neighboring dots are exactly separated by the microwave photon energy. The PAT current shows a narrower peak than the main resonant tunneling current in accordance with time-dependent tunneling theory. The photon stimulated emission associated with PAT from a higher-lying occupied level to a lower-lying empty level is obtained at a drain voltage sufficient that population inversion takes place.
Si-based single electron transistors has been measured for the ®rst time in 2± 12 MHz at 1.5 K wi... more Si-based single electron transistors has been measured for the ®rst time in 2± 12 MHz at 1.5 K with a lock-in technique. The magnitude of the shot noise normalized by the full shot noise, 2eI, clearly shows oscillatory structures as a function of the gate voltage, having a minima at the Coulomb oscillation peaks and a maxima in the Coulomb blockade region. The value of the maxima is approximately unity, re¯ecting a tunneling event with no correlation. On the other hand, the value of the minima is signi®cantly smaller than half the full shot noise, which is smaller than in the previous theoretical and experimental reports and suggests that some current regulation mechanism is present.
Transport phenomena in a specially optimized vertical asymmetric sub-micron Al,z8Ga, ,? As/ GaAs ... more Transport phenomena in a specially optimized vertical asymmetric sub-micron Al,z8Ga, ,? As/ GaAs double barrier structure with modulation-doped barriers is investigated by applying a bias to a special Schottky side gate, which allows the effective area of the conducting channel to be finely "tuned". For a sufficiently small device, the electrical properties of the controllable quantum dot bounded by well defined heterostructure barriers and an adjustable side wall potential are expected to be governed by single electron charging, and single electron resonant tunneling. Single electron transistor (SET) operation is possible because the number of electrons in the dot, n, can be varied one-by-one with the side gate. The drain current Rowing through the conducting channel in response to a small drain voltage is strongly modulated by the gate voltage close to "pinch-off' as n approaches zero, and oscillations in the drain current persist up to about 25 K. A gate modulated zero current region of Coulomb blockade, step-like features, and resonances exhibiting negative differential resistance are clearly observed at low bias. Although this technology is very promising for the realization of SET operation at temperatures well above 4.2 K, the temperature dependence of the Coulomb blockade is an important limitation. We describe the evolution of the conductance oscillations and the degradation of Coulomb blockade with temperature from 0.3 K up to 25 K, and propose that co-tunneling in a small system containing just a "few" electrons in which the zero-dimensional energy level spacing is significant and comparable to the Coulomb charging energy is important, and is likely to be more complex than that documented for large planar dot structures containing "mung" electrons 1. INTRODtJCTION
Spin-orbit interaction in InGaAs/InAlAs quantum wires is investigated by measuring conductance fl... more Spin-orbit interaction in InGaAs/InAlAs quantum wires is investigated by measuring conductance fluctuations at temperatures ranging from 2 K down to 0.06 K. The resistance of an InGaAs/InAlAs wire in a zero magnetic field decreases as the temperature decreases below 1.6 K. This temperature dependence is reproduced by the theory of antilocalization in the strong spin-orbit interaction regime. In the same temperature range, the conductance fluctuation amplitude increases nonmonotonically with decreasing temperature. This behavior is attributed to the temperature-induced transition from the weak to the strong spin-orbit scattering regime in the mesoscopic disordered system.
... which is ideal for investigating single electron charging of, and single electron tunnelling ... more ... which is ideal for investigating single electron charging of, and single electron tunnelling through, a ... Substantial regulation of the current flowing through a gated vertical AlGaAs/GaAs double-barrier ... of very small circular DBRTT mesas with a scanning electron microscope, we ...
Tunneling conductance through a quantum dot is calculated around the local spin singlet-triplet c... more Tunneling conductance through a quantum dot is calculated around the local spin singlet-triplet crossover region including the Kondo effect. The calculation is carried out using the numerical renormalization group method. When the potential on the dot deepens, two electrons filling a lower energy orbital redistribute to gain Hund's coupling energy. This redistribution induces a bump in the conductance between the Coulomb peaks. The Kondo temperature on the bump is high due to the fluctuation on the singlet-triplet crossover region. The behaviors agree well with recent experiment.
We determine the contributions from the direct Coulomb and exchange interactions to the total int... more We determine the contributions from the direct Coulomb and exchange interactions to the total interaction in semiconductor artificial atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. We find that electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle state degeneracy, and find that the spin-configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.
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Papers by S. Tarucha