Fractional-layer Superlattices Research Articles

Stretched-exponential decay of the luminescence in ZnSeZnTe superlattices

The photoluminescence decay of fractional-layer superlattices of ZnSeZnTe has been measured to investigate the origin of “slow-bands” (K. Suzuki et al. [4]). The decay of this band is found to be well described by a stretched exponential function. Distributions of effective life-time of excitons at several wavelength positions of the bands are derived from the stretched exponential parameters with which the decay is fitted. From the distributions, the radiative life-time of localized excitons in the slow bands are estimated to be 10–20 ns.

Excitonic optical properties in fractional-layer-superlattice wire structures

We report on the evolution of the excitonic optical features from two dimensions (2D) to one dimension (1D) in GaAs/AlAs fractional-layer-superlattice (FLS) wire structures. We demonstrate for the first time how the excitonic optical properties continuously evolve from 2D to 1D by introducing the FLS lateral compositional modulation in an AlGaAs quantum well. We also present a theory numerically analyzing the optical absorption properties in the FLS wires. Present theory accurately reproduces the experimental results using the AlAs/GaAs FLS wires and clarifies the main optical features associated with excitons in the intermediate quantum-confinement regime between 1D and 2D.

Rapid relaxation of hot carriers in GaAs fractional-layer superlattice quantum wires

We measure the hot carrier relaxation in two GaAs quantum wire arrays made from fractional-layer superlattices (FLS). Using femtosecond optical pump–probe spectroscopy, we measure the differential transmission spectra for various pump–probe time delays from which we determine the carrier thermalization times. Although the two FLS structures have different degrees of one-dimensional confinement at the band edge, we observe rapid (<2 ps) and efficient carrier relaxation in both cases with no sign of the inhibited relaxation predicted for ideal one-dimensional structures. We believe that FLS quantum wire structures avoid relaxation bottlenecks because the shape of the FLS confinement potential produces high energy states which are two dimensional in character and which facilitate rapid energy relaxation.

Optical properties in fractional-layer-superlattice quantum wires calculated by multi-band effective mass theory

We present the results of theoretical studies numerically analyzing the optical absorption and gain properties in AlGaAs fractional-layer-superlattice (FLS) quantum wires. With our calculation method we can treat any asymmetrical FLS wire with arbitrary dimensionality from 2D to 1D and to calculate not only absorption spectra but also gain properties. Our theory accurately reproduces the optical absorption anisotropy, experimentally evaluated for an AlGaAs FLS quantum wire. It is numerically shown how optical features evolve as the quantum confinement changes from 2D to 1D by varying the FLS lateral modulation. The small modulation of Al content in the AlGaAs FLS layer is found to change the optical properties markedly and improve the gain characteristics largely.

Optical Propaties of Semiconductor Quantum Wires Made from Fractional Layer Superlattices

Optical Propaties of Semiconductor Quantum Wires Made from Fractional Layer Superlattices

Quantum wire microcavity laser made from GaAs fractional layer superlattices

We report the first demonstration of lasing action in a quantum wire microcavity semiconductor laser made from an array of (AlAs)1/4(GaAs)3/4 fractional-layer superlattice (FLS) quantum wires. The FLS growth method produces uniform, densely packed, damage-free arrays of nanometer-size quantum wires which are integrated into an optical microcavity that is the size of the wavelength of the light. We obtain room temperature optically pumped lasing for wavelengths from 670 to 690 nm. The lasing output is linearly polarized parallel to the quantum wires, reflecting the higher optical gain for polarization direction parallel to the wires. The combination of a semiconductor quantum wire active material with an optical microcavity offers the possibility of ultimately compact, highly efficient laser sources.

Fractional superlattices and their device applications

Abstract ( AlAs ) 1 2 ( GaAs ) 1 2 fractional-layer superlattices (FLSs) are grown on a (001) vicinal substrate by metalorganic chemical vapor deposition (MOCVD). FLSs are new types of lateral superlattice with a period of 4–20 nm. The uniform superlattice period is observed over a large surface area by bright field transmission electron microscopy (TEM). The results suggest the ideal growth from a kink site occurs during MOCVD growth, and the distances between kink sites are equal. Polarization-dependent photoluminescence and optical absorption spectra of FLSs are also observed. Electron wave interference devices with a lateral periodic potential have been fabricated.

GaAs Quantum-Wire Laser Using Fractional Layer Superlattice

In (AlAs)k/n(GaAs)l/n (k+l=n) fractional layer superlattice (FLS) growth on the (001) GaAs vicinal surface 2° off toward [1̄10] by metal-organic chemical vapor deposition, using a (AlAs)m(GaAs)n (m, n>3) short period superlattice as the buffer layer very much improve the uniformity of lateral periodicity in the FLS near the FLS/buffer interface compared with using binary (GaAs and AlAs) and ternary AlGaAs as the buffer layers. This is due to the improvement in the uniformity of the terrace width on the surface of a short period superlattice by step-ordering. As a result, a large polarization anisotropy of about 50% is observed in the photoluminescence spectrum for the quantum-wire array structure oriented toward [110] in which a 4-nm-thick (AlAs)1/4(GaAs)3/4 FLS layer is sandwiched by (AlAs)3(GaAs)3 short period superlattice layers. Separated confinement heterostructures consisting of a 12-nm-thick undoped (AlAs)1/4(GaAs)3/4 FLS quantum-wire active layer, doped (AlAs)1(GaAs)2 carrier confinement layers, and doped Al0.6Ga0.4As optical confinement layers are grown. To prevent the disordering of the FLS layer induced by the diffusion of intentionally doped impurity atoms during the growth and the processing, low-diffusive Si and C atoms are used for n- and p-type doping into carrier and optical confinement layers. Lasing operation is obtained by current injection below 195 K for the electrode with simple stripe geometry. The lasing wavelength depends on the orientation of the stripe electrode. The electrode with [110] orientation which is parallel to the quantum wire array has a lasing wavelength of 662 nm, and the [1̄10] orientation which is perpendicular to the array has a lasing wavelength of 667 nm at 10 K.

Step ordering during fractional-layer superlattice growth on GaAs(001) vicinal surfaces by metalorganic chemical vapor deposition

Step ordering during fractional-layer superlattice (FLS) growth is observed by trans- mission electron microscopy. A cross-sectional transmission electron micrograph of an (AlAs)1/2(GaAs)1/2FLS shows that the steps of unequal spacings observed on a GaAs surface develop a sequence of uniformly spaced steps of single monolayer height during the growth. Comparison of the observed FLS structure with a numerical simulation based on the assumption of unequal incorporation probabilities of adatoms into up steps and down steps shows that a net flux into the up steps 2%–4% larger than that into the down steps is enough to reproduce the FLS growth. It is demonstrated that the FLS growth provides a unique opportunity to observe the step ordering process directly and determine the anisotropy of diffusion quantitatively.

Polarization properties of a vertical cavity surface emitting laser using a fractional layer superlattice gain medium

We investigate the polarization properties of a vertical cavity surface emitting laser that uses an (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional-layer superlattice (FLS) as an anisotropic gain medium. The anisotropy in the gain enables us to both control and switch the polarization state of the optically pumped lasing output. We obtain room-temperature lasing for wavelengths from 690 to 720 nm. The output is linearly polarized and the polarization direction is fixed, either parallel or perpendicular to the FLS layers. By tuning the cavity resonance wavelength, we demonstrate high contrast switching between two orthogonal linear polarization states in the FLS surface emitting laser.

MOCVD methods for fabricating GaAs quantum wires and quantum dots

Semiconductor low-dimensional structures such as quantum wires and quantum dots fabricated by metalorganic chemical vapor deposition (MOCVD) are reported. GaAs/AlAs quantum wire arrays including fractional-layer superlattices were grown on (001) GaAs vicinal surfaces. Uniform lateral superlattices were observed by transmission electron microscopy. However, polarization-dependent photoluminescence and the optical absorption spectra suggest that significant alloying occurs in AlAs/GaAs vertical interfaces. GaAs tetrahedral quantum dots buried in AlGaAs were also fabricated on a SiO 2 masked (111)B GaAs substrate partially etched to a triangular shape. First, AlGaAs truncated tetrahedral structures with three {110} facets were grown on GaAs triangular areas. Next, GaAs dot structures were sequentially grown on top of the AlGaAs truncated tetrahedron. Finally, AlGaAs was over-grown on {110} sidewall facets with a different growth condition. We observed large quantum size effects for about 100 nm TQD in low temperature photoluminescence, which is in good agreement with calculations.

Donor impurity states on structured semiconductor interfaces.

Ground-state energies of shallow states of donor impurities on periodically structured interfaces formed by two semiconductors, such as GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As and Si/${\mathrm{SiO}}_{2}$ interfaces, are calculated variationally with the approximation that interfaces represent infinitely high potential barriers. The results show that the ground-state energies of the interface impurity states can be strongly affected by interface structures in GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As lateral surface superlattices produced by molecular-beam-epitaxy growth of AlAs/GaAs fractional-layer superlattices on [001] vicinal GaAs substrates and on ${\mathrm{SiO}}_{2}$/Si interfaces for Si with (001) orientation. The effects of the interface defects on the interface impurity states are negligible for Si/${\mathrm{SiO}}_{2}$ interfaces when the density of interface defects is less than ${10}^{10}$/${\mathrm{cm}}^{2}$.

Density of states of AlAs/GaAs fractional layer superlattice quantum wires in a modulation doped structure

The density of states (DOS) of AlAs/GaAs fractional layer superlattice (FLS) quantum wires is found to consist of one-dimensional and zero-dimensional states. The FLS quantum wires are fabricated by electron beam lithography from the AlAs/GaAs FLS modulation doped heterostructure. The periodicity of the FLS quatum wires is 8 nm and the width is 0.25 μm. Drain current oscillations with negative transconductance is observed below 7.5 K due to a reduction in wire width. The density of states is estimated from the temperature dependence of the threshold voltage ( V th). Since the DOS of the FLS quantum wire structure has sharper peaks than that of the unconfined FLS modulation-doped structure, which has both one-dimensional and two-dimensional electron states, we can conclude that the additional sub-micron confinement reduces these states to zero-dimensional and one-dimensional, respectively.

Polarized photoluminescence of fractional layer superlattices

The polarization dependences of photoluminescence of ( AlAs) 1 2 ( GaAs) 1 2 and ( Al 0.5 Ga 0.5As ) 1 2 ( GaAs) 1 2 fractional layer superlattices were measured at room temperature. Anisotropy in light emission was observed for the first time. The photoluminescence spectra depend on its polarization. This shows that quantum carrier confinement occurs in the direction parallel to the surface, i.e., lateral quantum carrier confinement.

Step-flow growth and fractional-layer superlattices on GaAs vicinal surfaces by MOCVD

(AlAs) 1/2(GaAs) 1/2 fractional-layer superlattices are grown on various GaAs (001) vicinal substrates by metalorganic chemical vapor deposition (MOCVD). When the substrate is misoriented to [ 1 10] by 1.92° and [110] by 0.10°, uniform superlattice periods are observed over a large surface area by bright field transmission electron microscopy (TEM). These results suggest that ideal growth from a kink site occurs during MOCVD growth, and that the distances between kink sites are equal. On a substrate misoriented to [110] by 1.90°, the superlattice periods exhibit an undualtion. This shows that kink flow mode growth is not dominant in the [110] direction. On a substrate misoriented to [010] by 2.0°, no superlattice period were observed. The growth mechanisms are discussed on the basis of these results.

Step-density dependence of growth rate on vicinal surface of MOCVD

The dependence of the growth rate on the step density is reported for the first time. We accurately measured the growth rate on GaAs (001) vicinal substrates with various misorientation angles. The substrates were tilted toward the [ 1 10] or [110] direction with the misorientation angle ranging from 1.0° to 4.0°. The growth rate was derived from the superlattice period of the (AlAs) 1/2(GaAs) 1/2 fractional layer superlattice (FLS) measured by X-ray diffraction satellite peaks. We obtained the step density dependence of the growth rate. The results reveal that on [ 1 10] misoriented surfaces the growth rate decreases slightly with decreasing misorientation angle, i.e., with decreasing step density. The growth rate decreases by 5% as the misorientation angle decreases from 4.0° to 1.0°. On [110] misoriented surface with the same misorientation angle, the growth rates scatter, ranging about ±2% from run to run. These results for [ 1 10] misoriented surfaces can be explained by a simple adsorption/desorption rate model, taking into account the rate for the group III atoms to desorb from the surface (1/τ d), as well as the rate for the group III atoms to hop to adjoining sites (1/τ s). As the misorientation angle decreases, the terrace width increases, and the mean time of migration of group III atoms to reach the site steps after adsorption at the surface increases. Therefore, the possibility of desorption increases, and the growth rate decreases slightly. A Monte Carlo simulation using this model fits the experimental results.

Polarization-dependent optical nonlinearities in fractional-layer superlattices

We measure the room-temperature polarization-dependent nonlinear absorption and refractive-index spectra of a (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional-layer superlattice (FLS) structure grown by metalorganic chemical vapor deposition. The anisotropic nonlinear effects between the directions parallel and perpendicular to the superlattice give rise to a nonlinear optical birefringence in the plane of the growth surface. From our measurements using a femtosecond optical pulse, we derive the magnitude and spectral shape of the nonlinear optical birefringence. We describe the basis of an all-optical polarization rotation switch using the FLS structure.

Anisotropy in photoluminescence and absorption spectra of fractional layer superlattices

Polarization-dependent photoluminescence (PL) and optical absorption spectra of (AlAs)1/2(GaAs)1/2 and (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional layer superlattices (FLSs) were measured at room temperature. For the first time, it was observed that the spectra are dependent on their polarizations. The anisotropies in the PL and absorption spectra show that quantum confinement unambiguously occurs in the direction parallel to the surface, i.e., lateral quantum carrier confinement effect appears. This finding proves that FLSs are promising for novel optical devices.

Direct observation of optical anisotropy in a GaAs/AlAs quantum well wire array

Using reflectance difference spectroscopy and transmittance difference spectroscopy, optical anisotropy in a quantum well wire array is directly observed at room temperature. The quantum wire array is a fractional-layer superlattice grown by metalorganic chemical vapor deposition on a vicinal (001)GaAs substrate. The size for quantum confinement of the wires is nominally 4×4 nm. Polarization dependence of optical absorption and refractive index is clearly observed between the directions parallel and perpendicular to the wires.

Electron wave interference device with fractional layer superlattices

Modulation-doped Al0.3Ga0.7As/GaAs heterostructure electron wave interference devices with fractional layer superlattices are fabricated. The periods of the fractional layer superlattice in the electron wave interference devices are 16, 12, and 8 nm, respectively. These devices show drain current oscillation due to electron wave interference at 4.2 K. The oscillation period is determined by the period of the fractional layer superlattice. From the analysis of the drain current oscillation, the peaks of the structure function agree with the multiples of the periods of the fractional layer superlattice.