T-shaped Quantum Wires Research Articles

Optical properties of T-shaped quantum wires considering impurity, temperature and pressure

Optical properties of T-shaped quantum wires considering impurity, temperature and pressure

Mid-infrared emission in InxGa1−xAs/GaAs T-shaped quantum wire lasers and its indium composition dependence

In this work, the emission wavelength has been extended out to 1.3, 1.5, and 2.2 μm for InxGa1−xAs/GaAs T-shaped quantum wire (TQWR) using multi-band k.p model and variational formalism. We have investigated the impact of the indium composition on the performance of a series of TQWR through a calculation of the optical gain and transition energies. It is found that the optical gain and the emission wavelength are more influenced taking into account the effect of the indium concentration and persisted up at room temperature (RT). The results could open the way to the development of laser communication systems operating at long wavelengths and fabricated from TQWRs structure.

Novel T-shaped GaSb/InAsN quantum wire for mid-infrared laser applications

In this work, we investigate GaSb/InAs1−xNx T-shaped quantum wire active region in mid-infrared laser. Multi-band k.p model and variational formalism are applied to find the confinement energies, the band structures, and optical gain. We then present a method of numerical calculation that is suited to any T-shaped quantum wire. By tuning the quantum wire thickness, the TE- and TM-polarized optical gain up to 21×103 cm−1 can be obtained for λ=3.11 μm at room temperature (RT), which is very promising to serve as an alternative active region for high-efficiency mid-infrared laser applications.

Anomalous excitation-power-dependent photoluminescence of InGaAsN/GaAs T-shaped quantum wire

The selected InGaAsN/GaAs T-shaped quantum wire (T-QWR) fabricated by metal organic vapor phase epitaxy has been investigated by microphotoluminescence (µ-PL) and excitation-power-dependent µ-PL. The optical characteristics of one-dimensional structure were taken at low-temperature (4 K) and room temperature (RT) to clarify the intersection of two familiar quantum wells (QWs) in the [001] and [110] directions, named QW1 and QW2, respectively. For the excitation-power-dependent measurement, the intensity of the excitation source was used in the range of 0.001I0 to I0. The result shows that all of emissions related to QW1 and QWR peaks have a nonsymmetric line shape as evidenced by the tailing on the lower-energy side. All peaks shift to higher-energy side (blueshift) with the increase of the excitation power intensity. The blueshift and the low-energy tailing of PL peaks are attributed to the alloying effect. However, the emission peak related to QWR region shows a larger blueshift rate than that of QW1 on increasing of the excitation power intensity. This is an anomalous characteristic for the low-dimensional structure, affected by the large fluctuation state in the QWR region. This fluctuation state is combined of both edges of QWs (QW1 and QW2).

Refractive index change and absorption coefficient of T shaped quantum wires: comparing with experimental results

In this work, we have studied the optical properties of a GaAs/Al $$_{x}$$ Ga $$_{1-x}$$ As T-shaped quantum wire. In this regard, we have obtained the refractive index change and absorption coefficient. We have also studied the temperature effect on absorption coefficient. The results show that the refractive index change and absorption coefficient decrease and shift towards higher energies when temperature increases. Finally, to check our results, we have compared the absorption coefficient obtained from this work with experimental data at 5 K. Our theoretical prediction has some deviations with experimental data.

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.

Quantitative absorption spectra of quantum wires measured by analysis of attenuated internal emissions

An absorption-spectroscopy method that utilizes internal emissions as the source of the probe light was used to measure the absorption spectra of quasi-one-dimensional (q-1D) excitons in T-shaped quantum wires embedded in an optical waveguide. The modal absorption area of the 1D ground-state excitons was estimated to be 0.39 eV cm−1 and was almost independent of temperature in the range 4–150 K. Quantitative evaluation using the absorption spectra revealed that the absorption cross-section per unit length at resonance peak and the spectrally integrated absorption cross-section area per unit length of the 1D ground-state excitons were 1.0 nm and 2.5 × 10−3 eV nm, respectively.

MOVPE growth and optical characterization of InGaAsN T‐shaped quantum wires lattice‐matched to GaAs

AbstractInGaAsN T‐shaped quantum wire (T‐QWR) lattice‐matched to GaAs was grown by metal organic vapour‐phase epitaxy (MOVPE) and its optical property was investigated. The InGaAsN T‐QWR was observed at the intersection of two InGaAsN quantum wells (QWs) on (001) and (110) surfaces, namely QW1 and QW2, respectively. In, N contents and well‐thickness in QW1 and QW2 were determined by high resolution X‐ray diffraction (HRXRD). Photoluminescence (PL) was taken to obtain excitonic states in the one‐dimensional T‐QWR as well as QW1 and QW2. High PL intensity reveals an achievement of high quality T‐QWR. At low‐temperature, InGaAsN T‐QWR exhibits an emission at around 1.141 eV with a large lateral confinement of about 61 and 99 meV for QW1 and QW2, respectively. This method for producing InGaAsN T‐QWR may be useful for optical or electronic device applications.

Current-injection T-shaped quantum wire lasers with perpendicular doping layers operating at 100 K

We demonstrated lasing from current-injection T-shaped GaAs/AlGaAs quantum wire (T-wire) lasers with perpendicular p- and n-doping layers. The T-wire laser showed lasing between 5 and 110 K, and showed the optimal threshold current of 2.1 mA and differential quantum efficiency of 0.9% at 100 K. In microscopic-EL imaging measurements, we observed the emissions from the outside of optical core region, which indicated overflow of holes from the active region.

Thermal transport in a dielectric T-shaped quantum wire

We investigate the thermal transport in a special ballistic system, the dielectric T-shaped quantum wire. It is found that the transmission coefficient of phonons exhibits the oscillation behavior similar to that of electrons. Due to the wide energy range involved in thermal transport, thermal conductance changes smoothly with temperature, which is different from the electrical conductance. We find that thermal conductance is a monotonic function of temperature at any system. In the low temperature limit, thermal conductance tends to $\frac{{\ensuremath{\pi}}^{2}{k}_{B}^{2}T}{3h}$ against all system parameters, even though the transmission coefficient is not unity.

Exciton-plasma crossover with electron-hole density in T-shaped quantum wires studied by the photoluminescence spectrograph method

We investigated the evolution of photoluminescence (PL) spectra with the electron-hole $(e\text{\ensuremath{-}}h)$ pair density in a single T-shaped quantum wire of high quality grown by a cleaved-edge overgrowth method with molecular-beam epitaxy. By using a spectrograph imaging method for the PL measurements, we obtained PL spectra free from carrier migration effects for the one-dimensional (1D) $e\text{\ensuremath{-}}h$ system in the T wire for a wide range of $e\text{\ensuremath{-}}h$ pair densities from $5\ifmmode\times\else\texttimes\fi{}{10}^{1}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. In the low $e\text{\ensuremath{-}}h$ pair density region below $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, PL only from 1D excitons in the wire was observed. At a pair density of $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, a new peak characteristic of biexcitons with a binding energy of $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ appeared below the exciton peak. At a pair density of $1\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, the biexciton peak started broadening without a peak energy shift and completely changed to an $e\text{\ensuremath{-}}h$ plasma at $6\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The transition from the dilute exciton gas to the $e\text{\ensuremath{-}}h$ plasma is, therefore, a gradual crossover via biexcitons, which indicates the importance of biexcitonic effects in 1D $e\text{\ensuremath{-}}h$ systems. Moreover, we found that during the continuous exciton-plasma crossover, the band edge of 1D excitons showed no significant energy shift at an $e\text{\ensuremath{-}}h$ pair density as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, where the biexciton PL peak had already started broadening in linewidth and changing to the degenerate $e\text{\ensuremath{-}}h$ plasma peak. The level crossing between the band edge and the exciton was not observed experimentally.

The evolution of bound states in quantum wires under potential modulation

We calculate the bound state energies in various quantum wires under potential modulation, including crossed, T-shaped and L-shaped quantum wires. It is found that the bound state energy rapidly increases with the potential height in the lower potential region while it slowly approaches a limit in the higher potential region. A fit formula that describes the dependence of the bound state energy and the potential height is obtained. Based on the formula and the contour plots of electron probability density, the relation and evolution of bound states in different quantum wire systems are shown. We also find that the bound state in a quantum dot, in some potential-modulation region, turns into a quasibound state with finite lifetime.

Bandgap renormalization in T-shaped quantum nanowires

Bandgap renormalization for the geometry of T-shaped quantum wires is calculated as a function of the electron–hole plasma density and quantum wire width in the random phase approximation. Considering a suitable confinement potential, a numerical scheme has been proposed to calculate the screened Coulomb potential, profile of charge density with various quantum wire widths and relative renormalization of gap energy. We will show that carrier concentration, screened confinement potential and relative bandgap renormalization are functions of the ratio of well width in the x and y directions. We also show that increasing temperature leads to more relative renormalization of gap energy.

Room-temperature excitonic absorption in quantum wires

We measured the absorption spectra of T-shaped quantum wires at room temperature using waveguide-transmission spectroscopy. A strong and narrow room-temperature one-dimensional-exciton absorption peak was observed, which indicates a peak modal absorption coefficient of 160cm−1 per 20 wires with a Γ-factor of 4.3×10−3, a width of 7.2meV, and strong polarization anisotropy.

Transport through T-shaped quantum wires under potential modulation: Lattice Green’s function approach

By developing the lattice Green's function approach, we study transport properties of T-shaped quantum wires (TQWR) under potential modulation. It is found that the potential barrier in the vertical arm induces a dip-peak structure in conductance straight through and around the bend. The increase of potential thickness results in the dip-peak couples being more pronounced, while the number of dip-peak couple also increases with the thickness. The dip-peak structures are related to the quasibound states localized at the junction of the TQWR. We numerically analyze the dependence of the quasi-bound state energy on the potential height and width. Different conductance profiles can be selected by the potential modulation on coupled TQWR.

One-dimensional excitonic states and lasing in highly uniform quantum wires formed bycleaved-edge overgrowth with growth-interrupt annealing

High quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth withmolecular beam epitaxy on an interface improved by a growth-interrupt high temperatureannealing. Microphotoluminescence and photoluminescence (PL) excitation spectroscopy atlow temperatures reveals the formation of quantum wires with unprecedentedly highquality, and intrinsic structures of one-dimensional (1D) free excitons, exciton excitedstates, and 1D continuum states. At high pumping levels, the PL evolves from showing asharp free exciton peak via exhibiting a biexciton peak to a Coulomb correlatedelectron–hole plasma PL band. Lasing has been achieved with a low lasing threshold, andits emission patterns are measured in imaging experiments. The lasing energy is in theplasma PL band and is about 5 meV below the free exciton level. The observed shiftexcludes the possibility of free excitons in the lasing, and suggests a contribution from theelectron–hole plasma. Single T-wire samples such as a single-quantum-wire laser and afield-effect-transistor-type doped single quantum wire are fabricated and studied optically.

Strongly confined quantum wire states in strained T-shaped GaAs/InAlAs structures

The confinement energy of T-shaped quantum wires (QWRs), which were fabricated by the cleaved edge overgrowth technique in a way that the QWRs form at the intersection of In 0.2Al 0.8As stressor layers and the overgrown (1 1 0) GaAs quantum well (QW), is examined using micro-photoluminescence spectroscopy. Photoluminescence (PL) signals from individual QWRs can be spatially resolved, since the strained films are separated by 1 μm wide Al 0.3Ga 0.7As layers. We find that due to the tensile strain being transmitted to the QW, the confinement energy of the QWRs rises systematically up to 40 meV with increasing thickness of the stressor layers. By reducing the excitation power to 0.1 μW the QWR PL emission occurs 48 meV redshifted with respect to the QW. All QWR peaks exhibit smooth lineshapes, indicating the absence of pronounced exciton localization.

Ballistic transport through coupled T-shaped quantum wires

The ballistic conductance of a coupled T-shaped semiconductor quantum wire (CTQW) is studied. Two types of CTQW are considered, one of which is a Π-shaped quantum wire ( ΠQW) which consists of two vertical arms on the same side of the horizontal arm and the other a Π-clone quantum wire ( ΠCQW) which consists of two vertical armes on the opposite sides of the horizontal arm. The mode matching method and Landauer-Buttiker theory are employed to study the energy dependence of the ballistic conductance. Most of transmission profiles of ΠQW and ΠCQW are found to be distinguishable for large separation d between the two vertical arms. The transmission probability manifests oscillatory behavior when d is increased. When a potential is applied to the connection region, it results in decoupling or coupling effects between the two T-shaped quantum wires according to whether it is positive or negative. When magnetic field is applied to CTQW, the transmission profiles are found to be affected prominently even if the electron passes through the field free region only.

Imaging of emission patterns in a T-shaped quantum wire laser

Spatially and spectrally resolved microscopic images of spontaneous and stimulated emissions are imaged at the mirror facets of a GaAs T-shaped quantum wire laser with high uniformity. Laser emission from the one-dimensional ground state reveals a circular image located at the core of a T-shaped optical waveguide but significantly smaller in area than the low power spontaneous emission from the same waveguide. These images unambiguously allow assignment of all spontaneous and laser emissions to the wire ground state and respective intersecting wells in the structure.

Polarization-dependent photoluminescence-excitation spectra of one-dimensional exciton and continuum states in T-shaped quantum wires

We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shaped quantum wires at 5 K. We attribute one peak to the one-dimensional- (1D-) exciton ground state and the continuous absorption band to 1D continuum states. These had similar polarization dependences. We also observed some other peaks above the 1D-exciton ground state and attribute them to exciton states consisting of excited hole subbands. These results show good agreement with a model calculation of a single electron–hole pair in T-shaped geometry with exact diagonalizations of the Coulomb interaction.