Width Lw Research Articles

Growth and photoluminescence studies of Al-rich AlN∕AlxGa1−xN quantum wells

A set of AlN∕AlxGa1−xN (x∼0.65) quantum wells (QWs) with well width Lw varying from 1to3nm has been grown by metal organic chemical vapor deposition. Low temperature photoluminescence (PL) spectroscopy has been employed to study the Lw dependence of the PL spectral peak position, emission efficiency, and linewidth. These results have shown that these AlN∕AlGaN QW structures exhibit polarization fields of ∼4MV∕cm. Due to effects of quantum confinement and polarization fields, AlN∕AlGaN QWs with Lw between 2 and 2.5nm exhibit the highest quantum efficiency. The dependence of the emission linewidth on Lw yielded a linear relationship. The implications of our results on deep ultraviolet optoelectronic device applications are also discussed.

Spontaneous and piezoelectric polarization effects in wurtzite ZnO∕MgZnO quantum well lasers

Spontaneous and piezoelectric polarization effects on electronic and optical properties of ZnO∕MgZnO quantum well (QW) structures are investigated by using the non-Markovian gain model with many-body effects. The spontaneous polarization constant for MgO determined from a comparison with the experiment is about −0.070C∕m2, which is larger than the value (−0.050C∕m2) for ZnO. The negligible internal field effect observed in the case of ZnO∕MgZnO QW structures with relatively low Mg composition (x<0.2) and thin well width (Lw<46Å) can be explained by the cancelation of the sum of piezoelectric and spontaneous polarizations between the well and the barrier. The ZnO∕MgZnO QW laser has much larger optical gain than the GaN∕AlGaN QW laser. This is attributed to the fact that the ZnO∕MgZnO QW structure has a larger optical matrix element due to the relatively small internal field, compared to the GaN∕AlGaN QW structure.

Confinement-enhanced biexciton binding energy in ZnO/ZnMgO multiple quantum wells

By employing a nanosecond pump-probe method, biexciton formation process was investigated in ZnO/Zn1−xMgxO (x=0.26) multiple quantum wells (MQWs) grown on ScAlMgO4 substrate by laser molecular-beam epitaxy. Bleaching of absorption due to the saturation of excitonic states, and induced absorption related to the exciton–biexciton transition were observed in their spectra. It is demonstrated that the pump-probe method allows us to precisely determine binding energies of exciton complexes even applied to the semiconductor quantum structures where the localization effect are not negligible. This is because a transition from free-excitonic states to free-biexcitonic states is involved in the induced absorption process. The biexciton binding energy is a monotonically decreasing function of well width (Lw). For the MQWs with Lw smaller than 2.5 nm, the biexciton binding energy is larger than 25 meV, comparable to the thermal energy of room temperature.

GaAs/GaAs 0.8 P 0.2 quantum wells grown on (n11)A GaAs substrates by molecular beam epitaxy

We investigated optical properties of GaAs/GaAs1−xPx(x≃0.2) quantum wells (QWs) with each well width (Lw) of 2.7–10.0 nm grown on (100) and (n11)A GaAs substrates (n=3, 4, 5) by molecular beam epitaxy with the use of P2 and As4 molecular beams generated from polycrystalline GaP chunk and solid As sources. It was found that the P contents (x) in GaAs1−xPx layers strongly depend on both the substrate orientation and substrate temperature (Ts). In the whole range of the Ts (535–640 °C), the largest P contents (x=0.19–0.23) were observed in the (411)A GaAs1−xPx layers, and the (100) GaAs1−xPx layers showed the smallest x (0.08–0.18). The full width at half maximum of photoluminescence (PL) peak (λ=758 nm) from the (411)A GaAs/GaAsP QW with Lw=2.7 nm was as small as 5 meV which is only 66% of that of the (100) QW probably due to improved interface flatness of the (411)A QW. Furthermore, the PL intensities of the (411)A and (311)A QWs were more than one order of magnitude larger in intensity than that of the (100) QWs.

The effect of linear and non-linear diffusion on exciton energies in quantum wells

This paper considers the technique of investigating diffusion processes via monitoring spectroscopically the ground state energy of an exciton confined in a quantum well. It is shown that the change in the exciton energy E−E0 during linear diffusion, can be described by an empirical relationship E−E0=(Ebg−E0)(1−exp{−γ√Dt/lw}), where Ebg is the band gap of the initial barrier material, D the diffusion constant and t the time. Detailed calculations accounting for the changes in the exciton binding energy have shown that the parameter γ∼1.5 for all wells of width lw≳40 Å regardless of the material system. It is proposed that this relationship could be used to determine the linear diffusion coefficient D. Once D has been determined the relationship could then be utilized as a predictive tool, e.g., to determine the annealing time necessary to produce a given energy shift for a particular quantum well width. The paper goes on to discuss the effects non-linear diffusion processes could have on exciton energies in quantum wells. In particular, it is shown how detailed spectroscopy and annealing experiments when coupled with accurate modelling could be used to distinguish between constant and concentration dependent diffusion coefficients.

Interface-Roughness Scattering in GaAs/AlxGa1-xAs Superlattices

The electron mobility limited by the interface-roughness scattering in a GaAs/AlxGa1-xAs superlattice is studied as a function of the period (the well width LW and the barrier thickness LB) and the temperature. If the superlattice period is short and the electronic states resemble that of the 3D state, the mobility is limited by the interface-roughness scattering at low temperatures and insensitive to the temperature, in good agreement with experimental results. Using a finite potential-barrier height, it is shown that the variation of the mobility as a function of the quantum well width LW is not so steep as that of the single-quantum-well structure assuming an infinite potential-barrier height.

THEORETICAL ANALYSIS OF THE OPTICAL-ABSORPTION TAIL IN AMORPHOUS SEMICONDUCTOR SUPERLATTICES

We propose a two-band tight-binding model Hamiltonian to describe the behaviour of electrons in amorphous semiconductor superlattices. Our model Hamiltonian is characterized by its off-diagonal terms υz in direction z which is perpendicular to layers in a superlattice; we define υz as a parameter which represents the dimension of well layers accompanying the change in a well-layer width Lw under the condition that a barrier-layer width is fixed. On the basis of this model, we calculate the density of states D(E) and the absorption spectra I(E), from which we estimate the exponential-tail widths E0 and Eu of D(E) and I(E), respectively. We discuss the ways in which these widths E0 and Eu are influenced (i) by the dimension of the system, (ii) by the dispersion of well-layer widths and (iii) by the degree of disorder. We also propose the possibility to improve the photovoltaic efficiency in solar cells composed of amorphous semiconductor superlattices.

Near Wake Properties of a Strumming Marine Cable: An Experimental Study

Resonant flow-induced oscillations of a flexible cable can occur when the damping of the cable system is sufficiently small. The changes in the flow field that occur in the near wake of the cable during these resonant oscillations are closely related to the changes in the fluid forces that accompany these oscillations. The present wind tunnel experiments were undertaken to examine the effects that forced synchronized vibration and the helically-wound cross section of the cable have on near wake vortex shedding-related parameters; specifically the shedding frequency, vortex formation length Lf, reduced velocity Ur, vortex strength and the wake width Lw. The range of flow speeds over which the vortex shedding was locked on to the vibration frequency varied directly with the vibration amplitude. The helical cross section and the synchronized vibration caused significant changes in the near wake development that could be directly related to the increase in hydrodynamic forces associated with unforced synchronized vibration.

軟鋼薄板材の疲労き裂近傍における転位構造の実験的考察

The Sheet specimens of mild steel were fatigued at room temperature by plane bending stressing. The dislocation structures during the process of fatigue crack initiation and near the tips of fatigue cracks were observed using a transmission electron microscope and the dislocation density and the size of cell structure were obtained quantitatively. The relationship between the dislocation structure and the plastic deformation zone near the crack tips was examined.The main results obtained are as follows:(1) During the process of crack initiation, the total dislocation density ρt increased to a saturation with progress of fatigue. The dominant structure at high stress amplitude was cell structure, and that at low stress amplitude was loop patch structure. The cell width lw (or the bundle spacing ls) and the free dislocation density ρf in the cell structure decreased gradually during this process.(2) From the observation of dislocation structure near the crack tip, the mean size of cell structure lm and the free dislocation density ρf in the cell structure were found to decrease with approaching the tip. They showed almost the same distribution in all directions from the crack tip, and this influence of decreasing lm and ρf extended to the distance of about 1000μm from the crack tip.(3) The plastic deformation region near the crack tip determined by Vickers hardness tests was larger than the area where the mean cell size lm was found to be below about 4μm.