Lateral Confinement Energy Research Articles

Optical study of GaAsN/GaAs and InGaAsN/GaAs T-shaped quantum wires grown by MOVPE

GaAsN/GaAs and InGaAsN/GaAs T-shaped quantum wires (T-QWRs) grown by metal-organic vapor phase epitaxy have been investigated by low-temperature photoluminescence (LT-PL) and temperature dependent PL to verify their optical properties. The T-QWR structure was clearly observed at the intersection of their familiar quantum wells (QWs), namely QW1 and QW2. LT-PL spectra show the related QWR feature located at the lower energy side of their quantum wells related peaks. It is found that the quantum energy state in the T-QWR structure depends on the quantum energy states in QW1 and QW2. It is interpreted that they act as the second confinement barrier of QWR, which is occurred by the effect of the lateral confinement energy. In our case, the lateral confinement energy larger than 100meV was observed for all samples. This is expected to be very useful for carrier confinement at higher temperature. PL spectrum of InGaAsN/GaAs T-QWR shows smaller peak-width and higher intense than that of GaAsN/GaAs T-QWRs. It may be explained by the strain compensation from an In atom incorporation, reducing the lattice distortion and improving the alloy fluctuation in the wire region.

Photoreflectance Study of Strained GaAsN/GaAs T-junction Quantum Wires Grown by Metal-Organic Vapor Phase Epitaxy

Strained GaAsN T-junction quantum wires (T-QWRs) with different N contents grown on GaAs by two steps metal-organic vapor phase epitaxy in [001] and [110] directions, namely QW1 and QW2 respectively, have been investigated by photoreflectance (PR) spectroscopy. Two GaAsN T-QWRs with different N contents were formed by T-intersection of (i) a 6.4-nm-thick GaAs0.89N0.011 QW1 and a 5.2-nm-thick GaAs0.968N0.032 QW2 and (ii) a 5.0-nm-thick GaAs0.985N0.015 QW1 and a 5.2-nm-thick GaAs0.968N0.032 QW2. An evidence of a one-dimensional structure at T-intersection of the two QWs on the (001) and (110) surfaces was established by PR resonances associated with extended states in all the QW and T-QWR samples. It is found that larger lateral confinement energy than 100 meV in both of [001] and [110] directions were achieved for GaAsN T-QWRs. With increasing temperature, the transition energy of GaAsN T-QWRs decreases with a faster shrinking rate compared to that of bulk GaAs. Optical quality of GaAsN T-QWRs is found to be affected by the N-induced band edge fluctuation, which is the unique characteristic of dilute III-V-nitrides.

Ground-state transitions beyond the singlet-triplet transition for a two-electron quantum dot

We studied electrostatic confinement effects on correlated electronic states in vertical quantum dots in high magnetic fields. We prepared vertical quantum dots with different lateral confinement strengths and investigated the magnetic field evolution of the electronic states. We observed that the formation of the maximum density droplet state corresponding to filling factor $\ensuremath{\nu}=1$ shifts to lower magnetic field as the lateral confinement energy becomes weaker. In addition, we found a ground-state transition in the $\ensuremath{\nu}<1$ regime for the most weakly laterally confined two-electron quantum dot that is predicted to appear beyond an easily accessible magnetic field in standard vertical quantum dots more typically investigated.

Exciton radiative lifetime controlled by the lateral confinement energy in a single quantum dot

We report on time-resolved measurements on single GaAs quantum dots formed at the interface fluctuation of a $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ quantum well. We measure exciton radiative lifetimes as short as $100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, demonstrating that monolayer fluctuation quantum dots have larger oscillator strength than any other $\mathit{III}\text{\ensuremath{-}}\mathit{V}$ or $\mathit{II}\text{\ensuremath{-}}\mathit{VI}$ semiconductor quantum dots. Studying various single quantum dots, we demonstrate that the oscillator strength of a quantum dot is controlled by its lateral confinement energy.

Single-electron transport through an individual InAs SAQD embedded in a graded-dope semiconductor nano-pillar

By using the semiconductor nano-pillar with a graded-dope configuration, we implemented the measurement for a single-electron transport through an individual InAs self-assembled quantum dot (SAQD). An atomic-force microscope observation showed that the SAQD had a disk-like shape with a diameter of ∼30 nm . We succeeded in observing a significant diamagnetic shift of the Coulomb oscillation peak under the magnetic field applied perpendicular to the disk plane. The measurement gave us a lateral confinement energy of 14 meV and an electron effective mass of 0.039, which provided us with quantitative evidence that the constituent material of the observed quantum dot originates from the InAs SAQD.

InGaAs/GaAs quantum wells and quantum dots on (111)B orientation

We report a magneto optical characterization of InGaAs/GaAs quantum well (QW) and quantum dot (QD) structures grown on (111)B GaAs substrates. The photoluminescence (PL) peak shift under high excitation intensity is used to distinguish QW from QD structures together with atomic force microscopy (AFM) imaging. The binding energy in (111)InGaAs/GaAs QW is about 5 meV. The extent of the wave function obtained from the diamagnetic shift of the PL peak energy is consistent with the result calculated by the k.p method. The InGaAs/GaAs QD lateral confinement energy and the dot size are also estimated from the diamagnetic shift of the PL lines. The lateral confinement energy is estimated as 7 meV. The mean radius of the InGaAs QD is about 14 nm and the dot height is about 6 nm, which is in good agreement with the results as revealed in AFM imaging.

Microphotoluminescence characterization of cleaved edge overgrowth T-shaped InxGa1−xAs quantum wires

Microphotoluminescence (micro-PL) characterization was performed for T-shaped InxGa1−xAs quantum wires (T-QWRs) fabricated by the cleaved edge overgrowth method with molecular beam epitaxy. The spatial distribution of optical properties in InxGa1−xAs T-QWRs was examined by means of PL intensity imaging and scanning micro-PL spectroscopy with about 1 μm spatial resolution. In the successfully fabricated 3.5-nm-scale In0.17Ga0.83As T-QWRs with Al0.3Ga0.7As barriers, uniform PL image and PL spectra from QWRs along the wire direction were observed, which indicates spatially uniform and high quality QWRs were formed. The effective lateral confinement energy of one-dimensional excitons was found to be 34 meV, showing the usefulness of InxGa1−xAs as a well material in T-QWR structures. On the other hand, in the unsuccessfully fabricated 4-nm-scale In0.09Ga0.91As T-QWRs with Al0.3Ga0.7As barriers, PL image and spectra were inhomogeneous. These results demonstrate the importance of flat cleaved surface and optimized overgrowth condition to fabricate uniform T-QWRs.

Photoluminescence study of lateral confinement energy in T-shapedInxGa1−xAsquantum wires

A study of high-quality ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ T-shaped quantum wires (T-QWR's) via photoluminescence spectroscopy to characterize the lateral confinement effect is reported. The effective lateral confinement energy of excitons in 3.5-nm-scale ${\mathrm{In}}_{0.17}{\mathrm{Ga}}_{0.83}\mathrm{As}/{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ T-QWR's is found to be as large as 34 meV. The value has been examined in comparison with the previous results on ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ and GaAs/AlAs T-QWR's.

A theoretical analysis of quantum-wire fabrication by vacancy-enhanced interdiffusion of quantum wells

The fabrication of quantum-wire structures using vacancy enhanced interdiffusion of quantum wells is analyzed theoretically. A phenomenological equation is used to describe the effects of strain on vacancy diffusion. The quantum-wire confinement potentials are studied as a function of the trench opening widths and the separation distances from the SiO/sub 2/-AlGaAs interface. The lateral confinement energy is not a monotonic function of the trench opening width. It first increases and then decreases when the trench opening width is decreased. When the separation distance of the quantum wire from the interface is reduced, the confinement potential is changed from a nonsquare profile to a square-well profile.

Optical studies of free-standing single InGaAs/GaAs quantum dots

Using electron beam lithography and wet chemical etching we have developed single InGaAs/GaAs quantum dots with diameters between 200 nm and 40 nm. The lithographic definition permits the dot shape as well as the dot size to be controlled. In addition to circular dots, which allow the transition between two- and zero-dimensional systems to be studied, we have fabricated rectangular dots which can be used to study the changeover from one- to zero-dimensional structures. Photoluminescence spectra of the dots show a lateral quantization-induced increase of the bandgap energy as well as emission of higher dot states. The experimental results for the lateral confinement energies can be described well by numerical calculations. The average number of electron - hole pairs in the dot can be derived from different features of the spectra. With decreasing dot diameter we observe a continuous reduction of the luminescence linewidths in our structures. This variation is consistent with a decrease of elastic scattering processes by well widths and compositional fluctuations for reduced dot size.

Far-infrared study of a quasi-one-dimensional electron gas formed on (100)GaAs facets with hole gas sidegates on a (311)A GaAs substrate

A new type of quasi-one-dimensional electron gas has been realized by using molecular beam epitaxy to grow a high mobility heterostructure on a (311)A GaAs substrate selectively etched to expose (100) facets. The electron gas formed on the (100) facets is confined in one lateral dimension by the p–n junctions formed with the adjacent two-dimensional hole gases on (311)A, thereby forming a p–n–p structure. Far-infrared cyclotron resonance spectra demonstrate the dimensionality of such structures and yield typical lateral confinement energies of 22.3 cm−1 and electronic widths of ∼900 nm. These estimates are supported by cathodoluminescence data.

Lateral quantization effects in lithographically defined CdZnSe/ZnSe quantum dots and quantum wires

Quantum dots and quantum wires based on CdZnSe/ZnSe single quantum well heterostructures have been achieved using electron beam lithography and wet chemical etching. Photoluminescence spectra of the dot and wire structures show a blue shift due to lateral quantization for lateral dimensions below 40 nm. For the dot ground state, a lateral confinement energy of 16 meV is obtained for 28 nm diameter structures. For wires with widths on the order of 20 nm, lateral confinement energies of about 5 meV are observed. The dot diameter and wire width dependence of the emission energies can be described based on a square well potential and the measured sizes of the structures.

Far-infrared study of a quasi-one-dimensional electron gas formed by molecular beam epitaxial regrowth on patterned GaAs

A quasi-one-dimensional electron gas has been realized by using molecular beam epitaxy to grow a high electron mobility transistor (HEMT) on a patterned GaAs structure consisting of alternate layers of p-GaAs and n-GaAs. Independently contacting the p-GaAs and n-GaAs creates a patterned backgate, which is used to electrostatically induce a lateral modulation of the electron gas in the HEMT. Far-infrared cyclotron resonance spectra demonstrate that lateral confinement energies over the range 1.3–3.3 meV may be selected via tuning of the biases applied to the p-GaAs and n-GaAs layers.

Optical investigation of the one-dimensional confinement effects in narrow GaAs/GaAlAs quantum wires

We show optical data obtained at 8 K on narrow GaAs/GaAlAs quantum wires, with width down to 15 nm, fabricated by reactive ion etching and metal organic chemical vapor deposition overgrowth. Lateral confinement energies (up to 23 meV) and polarization effects are evidenced in the photoluminescence excitation spectra. These experimental results are in good agreement with calculated absorption spectra, which include the effects of wire width fluctuations, yielding, for our fabrication technique, a value of ±5 nm for these size fluctuations.