Sidewall Quantum Wires Research Articles

Sidewall Quantum Wires on GaAs (001) Substrates

We study the structural, optical, and transport properties of sidewall quantum wires on GaAs(001) substrates. The QWRs are grown by molecular beam epitaxy (MBE) on GaAs(001) substrates prepatterned with shallow ridges. They form as a consequence of material accumulation on the sidewalls of the ridges during the overgrowth of a quantum well (QW) on the patterned surface. The QWRs are approximately 200 nm-wide and have emission energies red-shifted by 27meV with respect to the surrounding QW. Spatially resolved spectroscopic photoluminencence studies indicate that the QW thickness reduces around the QWRs, thus creating a 4 meV energy barrier for the transfer of carriers from the QW to the QWR. We show that the QWRs act as efficient channels for the transport of optically excited electrons and holes over tens of {\mu}m by a high-frequency surface acoustic wave (SAW). These results demonstrate the feasibility of efficient ambipolar transport in QWRs with sub-micrometer dimensions, photolithographically defined on GaAs substrates.

Optical properties of stacked InGaAs sidewall quantum wires in InGaAsP∕InP

We report on the optical properties of threefold stacked InGaAs sidewall quantum wires (QWires) with quaternary InGaAsP barriers grown on shallow-patterned InP (311)A substrates by chemical beam epitaxy. Temperature dependent photoluminescence (PL) reveals efficient carrier transfer from the adjacent quantum wells (QWells) into the QWires at low temperature, thermally activated repopulation of the QWells at higher temperature, and negligible localization of carriers along the QWires. Strong broadening of power dependent PL indicates enhanced state filling in the QWires compared to that in the QWells. Clear linear polarization of the PL from the QWires confirms the lateral quantum confinement of carriers. These results demonstrate excellent optical quality of the sidewall QWire structures with room temperature PL peak wavelength at 1.55μm for applications in fiber-based optical telecommunication systems.

(In,Ga)As sidewall quantum wires on shallow-patterned InP (311)A

(In,Ga)As sidewall quantum wires (QWires) are realized by chemical beam epitaxy along [01-1] mesa stripes on shallow-patterned InP (311)A substrates. The QWires exhibit strong lateral carrier confinement due to larger thickness and In composition compared to the adjacent quantum wells, as determined by cross-sectional scanning-tunneling microscopy and microphotoluminescence (micro-PL) spectroscopy. The PL of the (In,Ga)As QWires with InP and quaternary (Ga,In)(As,P) barriers reveals narrow linewidth, high efficiency, and large lateral carrier confinement energies of 60–70meV. The QWires are stacked in growth direction with identical PL peak emission energy. The PL emission energy is not only controlled by the (In,Ga)As layer thickness but also by the patterned mesa height. Stacked (In,Ga)As QWires with quaternary barriers exhibit room temperature PL emission at 1.55μm in the technologically important wavelength region for telecommunication applications.

Carrier recombination kinetics in (3 1 1)A InGaAs sidewall quantum wires

We present a study of the optical properties and carrier dynamics in strained InGaAs sidewall quantum wires (QWR) on patterned GaAs (3 1 1)A substrates by means of picosecond time-resolved photoluminescence (PL). A pronounced dynamical red shift of the QWR-PL band when increasing the delay time after the pulse excitation is observed. In addition, time-resolved data show a significant shortening of the PL decay time from the wire at short delay and when high excitation power is used. The data are compared with theoretical predictions. The results, i.e. the dynamical red shift observed in the wire emission and the shortening of the PL decay with increasing the excitation density, are interpreted in terms of a dynamical screening effect of the piezoelectric field.

Carrier recombination kinetics in (311)A InGaAs sidewall quantum wires

We present a study of the optical properties and carrier dynamics in strained InGaAs sidewall quantum wires (QWR) on patterned GaAs (3 1 1)A substrates by means of picosecond time-resolved photoluminescence (PL). A pronounced dynamical red shift of the QWR-PL band when increasing the delay time after the pulse excitation is observed. In addition, time-resolved data show a significant shortening of the PL decay time from the wire at short delay and when high excitation power is used. The data are compared with theoretical predictions. The results, i.e. the dynamical red shift observed in the wire emission and the shortening of the PL decay with increasing the excitation density, are interpreted in terms of a dynamical screening effect of the piezoelectric field.

Dynamical nonlinearity in strained InGaAs (311)A sidewall quantum wires

Significant optical nonlinearity has been found in InGaAs (311)A sidewall quantum wires by means of time resolved photoluminescence measurements. A strong reverse quantum confined Stark effect has been observed and attributed to the dynamical screening of both the internal piezoelectric field and the Coulomb interaction between carriers. The time evolution of the quantum wire emission has been reproduced by means of self-consistent calculations that take into account excitonic effects, strain, and induced piezoelectric charges.

Piezoelectric effects in sidewall quantum wires grown on patterned (311)A GaAs substrate

Time-resolved photoluminescence measurements have been performed on sidewall InGaAs/AlGaAs quantum wires and quantum wells. Experimental data show a band filling of quantum wells and quantum wires that is dominant in the first 400 ps after the excitation pulse, then a dynamical screening of the built-in piezoelectric field (Fp) by means of fast injection of photo-generated charges is observed allowing an efficient radiative recombination. At longer time delay, during the regime when the Fp unscreened value is recovered, a strong quantum confined Stark effect is observed. A good agreement is obtained for the energy shift and the overlap integrals of electrons and heavy holes by means of discrete element calculations.

Novel semiconductor nanostructures by functional self-organized epitaxy

We review our recent results on GaAs sidewall quantum wires, quantum dots and coupled wire–dot arrays fabricated by functional selforganized epitaxy on patterned GaAs(3 1 1) A . The distinct selectivity of (Al,Ga)As growth on patterned GaAs(3 1 1) A allows for strong lateral confinement of photogenerated carriers up to room temperature. Quasi-planar arrays of these semiconductor nanostructures are of unprecedented uniformity, and they can be positioned on the wafer with high accuracy.

Patterned growth on high-index GaAs (311)A substrates

The selectivity of growth on patterned GaAs (311)A substrates qualitatively differs from that on low-index (100) and (111) substrates. During molecular beam epitaxy (MBE) of (AlGa)As, [01−1] oriented mesa stripes develop a fast-growing convex sidewall. For shallow mesa height, this new growth mechanism produces quasi-planar quantum wires with excellent optical properties. Nanometer-scale self-faceting in atomic hydrogen-assisted MBE transforms these sidewall quantum wires into a linear array of quantum dots with minimized size fluctuations. Upon rotating the mesa stripe from [01−1], a continuous transition occurs from the fast-growing sidewall to the slow-growing sidewall along the perpendicular [−233] direction. This allows their systematic combination at the corner or edge of intersecting mesa stripes appropriately inclined from [01−1], thus offering a novel degree of flexibility for the design of lateral functional semiconductor nanostructures.

Selective electroluminescence from a single stack of sidewall quantum wires on patterned GaAs (311)A substrates

A p-i-n light-emitting diode (LED) with a single stack of sidewall quantum wires in the center of the intrinsic region has been fabricated by molecular-beam epitaxy on patterned GaAs (311)A substrates with all-Si n- and p-type doping. For small injection currents, the electroluminescence (EL) measured at low temperatures solely originates from the quantum wires without emission from the surrounding quantum wells within the patterned LED of 220 μm diameter. The selectivity of the EL emission is gradually reduced with increasing temperature, but the emission intensity per unit area in the wire regions is still two orders of magnitude larger than that in the well regions at room temperature. A model based on the lateral diffusion of injected electrons and holes is proposed to explain the selective carrier injection into the quantum wires.

Photoluminescence Polarization Anisotropy in Quasi-Planar Sidewall Quantum Wires on GaAs (311)A

We investigate the polarization anisotropy of the excitonic photoluminescence (PL) of quantum wells (QWs) and of [0-11]-oriented sidewall quantum wires (QWRs) grown on (311) GaAs substrates. We demonstrate that vertical (i.e., along [311]) and lateral (along [-233]) confinements in these QWRs yield contributions of opposite signs to the PL anisotropy. The anisotropy of the QWRs initially reduces and then changes sign with decreasing wire widths.

Cathodoluminescence investigations of vertically stacked, sub-μm arrays of sidewall quantum wires on patterned GaAs (311)A substrates

Vertically stacked arrays of GaAs/(AlGa)As sidewall quantum wires (Qwires) were successfully fabricated on GaAs (311)A substrates patterned with 0.5μm-pitch gratings. The Qwires exhibit a lateral confinement potential as large as 210meV and high luminescence efficiency up to 300K. The distinct carrier transfer and loss mechanisms are studied by temperature dependent, spectrally and spatially resolved cathodoluminescence. Despite the almost perfect carrier capture in the Qwires, non-radiative recombination within the connecting quantum well (Qwell) regions usually cannot be neglected even at low temperatures. For temperatures approaching 300K, reemission of carriers out of the Qwell into the vertical (AlGa)As barriers contributes increasingly to the reduction of the carrier transfer and luminescence efficiency of the Qwires without notable repopulation of the connecting Qwells.

Enhanced optical band-gap modulation in strained (InGa)As sidewall quantum wires on patterned GaAs (3 1 1)A substrates

We study the optical non-linearity of strained (InGa)As sidewall quantum wires grown by MBE on patterned GaAs (3 1 1)A substrates. The wires are formed at the sidewall of shallow mesa stripes along [0 1 −1] due to the preferential migration of surface group-III atoms from the mesa top and bottom toward the sidewall. Strained In 0.2Ga 0.8As quantum wires exhibit a blue shift of the photoluminescence of 17 meV with increasing excitation power which is about one order of magnitude larger compared to that of the adjacent quantum well of 1–2 meV. The blue shift is reduced from 7 meV for an In composition of 0.1 to zero for unstrained GaAs wires. This enhanced optical non-linearity is assigned to additional lateral optical band-gap modulation due to internal piezoelectric fields effective in the present quantum-wire structure.

Device quality submicron arrays of stacked sidewall quantum wires on patterned GaAs (311)A substrates

Three-dimensional arrays of vertically stacked sidewall quantum wires are fabricated by molecular beam epitaxy on GaAs (311)A substrates patterned with 500-nm-pitch gratings. The cathodoluminescence spectra at low temperature are dominated by the emission from the quantum wires with narrow linewidth accompanied by a very weak emission from the connecting thin quantum wells due to localization of excitons at random interface fluctuations. When the carriers in the quantum well become delocalized at elevated temperature, only the strong emission from the quantum-wire array is observed revealing perfect carrier capture into the quantum wires without detectable thermal repopulation of the quantum well up to room temperature. Thus, unpreceded device quality of this quantum-wire structure is demonstrated.

Enhancement of optical nonlinearity in strained (InGa)As sidewall quantum wires on patterned GaAs (311)A substrates

Strong enhancement of the optical nonlinearity occurs in strained (InGa)As sidewall quantum wires grown on patterned GaAs (311)A substrates by molecular beam epitaxy. The wires are formed on the fast growing sidewall of mesa stripes along [01-1] and have a smooth, convex-curved surface profile. The blue shift of the photoluminescence peak energy of the wire in dependence on the excitation power is one order of magnitude larger compared to that of the surrounding well. This enhanced optical nonlinearity is assigned to additional lateral optical band gap modulation due to internal piezoelectric fields effective in the present quantum-wire structure.