Serpentine Superlattice Research Articles

Growth of CdTe/MnTe tilted and serpentine lattices on vicinal surfaces

We have grown by atomic layer epitaxy CdTe/MnTe tilted and serpentine superlattices. These heterostructures are formed by depositing in the step-flow growth mode fractional monolayer superlattices (CdTe)m(MnTe)n, with p=m+n∼1, onto 2 °A and 2 °B Cd0.95Zn0.05Te vicinal substrates. Transmission electron microscopy images reveal a good in-plane CdTe/MnTe separation and a uniform short-range superlattice period. The very existence of those superlattices imply that Te-based vicinal surfaces present a regular array of monomolecular steps, with no important step meandering and no step bunching.

MBE growth physics: application to device technology

We present in this review a consistent picture of the current status of epitaxial growth physics, as related to molecular beam epitaxy. After a short introduction in the field we describe the different growth modes and the attributed theoretical models. Among the simulation methods used presently to describe the different growth modes, we selected the Monte-Carlo simulation for a more detailed example. The experimental part of the paper is divided in to two main sections of lattice-matched and lattice-mismatched systems. In the first part, we concentrate on the growth mechanisms on nominally oriented substrates and substrates with vicinal surfaces, which includes the description of the growth of tilted and serpentine superlattices. In the second part we discuss the concept of the critical layer thickness together with island formation and strain-induced effects. Besides these two main topics we report on the application of surface-active species, which provide a new avenue to achieve high-quality man-made microstructures against thermodynamic odds. Within this context self-organisation of epitaxial deposits is discussed. Finally, we describe material-related growth peculiarities, which are usually connected with a specific property of the constituent element, like volatility or extraordinary chemical reactivity. We briefly present the currently known growth characteristics for IV-IV heterostructures, wide-gap III–V and narrow-gap II–VI compounds.

Magneto-optical determination of exciton binding energies in quantum-wire superlattices.

Polarization-resolved photoluminescence excitation spectra of (Al, Ga)As serpentine superlattice quantum-wire arrays have been measured in magnetic fields up to 20 T. In the Faraday configuration we are able to resolve Landau-level-like magnetoexciton transitions in the low-field limit, and to directly determine the exciton binding energy. A maximum enhancement of excitonic binding by 70{percent} is observed in a 160-A-wide quantum wire array as compared to a two-dimensional reference, and the true superlattice character is evidenced by a decreased excitonic binding for a short period (54 A wide) compared to a long period quantum-wire array (160 A wide). {copyright} {ital 1996 The American Physical Society.}

Serpentine superlattice nanowire-array lasers

We report characterization and modeling of serpentine superlattice nanowire-array lasers. These samples were grown by molecular beam epitaxy on (100) n/sup +/-GaAs vicinal substrates. In-plane ridge-waveguide lasers with ridge stripes either parallel or perpendicular to the nanowire arrays have been characterized at low temperatures. The measured net gain spectra at 1.4 K showed strong optical gain anisotropy such that the TM mode gain became greater than the TE mode gain when the optical cavity was placed along the nanowire direction. This provides strong evidence that the lateral quantum confinement in the serpentine superlattice is stronger than the vertical quantum confinement. Optical gain spectra in the serpentine superlattice are calculated with consideration of coupling between wires and homogeneous line broadening. A good fit to the measured polarization-dependent gain spectra is achieved when the lateral Al segregation and the homogeneous line broadening are chosen to be 8% and 7 meV, respectively. This small but finite Al segregation in the serpentine superlattice provides lateral quantum confinement for holes, which results in significant anisotropy in the relation between net gain and injection current density,.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Temperature dependent effects on luminescence polarization and recombination lifetime in serpentine superlattice quantum wire arrays

We have measured photoluminescence excitation (PLE) spectra and radiative lifetimes as functions of temperature for serpentine superlattice quantum-wire arrays. The (Al, Ga)As arrays have lateral periods near 10 nm, and lateral confining potentials of 120 meV in the conduction band. At low temperature the excitons are strongly localized within potential fluctuations along the wires. The radiative lifetime of these localized states is 340 ps at 2 K. The degree of exciton localization decreases with increasing temperature, from which we estimate the strongly-localizing potential fluctuations to be approximately 10 meV deep. Above 80 K the excitons have sufficient thermal energy for motion along the wires. The radiative lifetimes increase with temperature, to 20 ns for free carriers at 325 K. The lateral potential barriers inhibit diffusion to non-radiative recombination sites.

Quantum-engineering of III–V semiconductor structures

We review some of the recent advances and developments in the field of III–V semiconductor quantum structures. We emphasize particularly the recent development of laterally confined structures in which self-assembly processes play an important role. The well eontrolled eritaxy of films of the III–V materials permits size-engineering of the materials in the growth direction. Further lateral confinement can be obtained using lithographic techniques to their limits or, going beyond these limits, by using the self-organizing motion of the atoms during the growth, such as in the case of V-grooves, patterned substrate, cleaved edge overgrowth, tilted or serpentine superlattices, or self-organizing quantum dots formed by the spontaneous coherent islanding effect during the growth of highly strained semiconductors. The influence of atomic-scale size-fluctuations on the confined energy levels will be examined in such quantum structures.

Optical gain anisotropy in serpentine superlattice nanowire-array lasers

We report optical gain measurement from a serpentine superlattice nanowire-array laser sample, grown by molecular beam epitaxy on a 2°-off (100)GaAs vicinal substrate. Gain spectra, obtained from in-plane ridge-waveguide lasers with stripes either parallel or perpendicular to the nanowire arrays at 1.4 K, showed that the optical gain for the TM mode became greater than that of the TE mode when the optical cavity was placed along the nanowire direction. This provides strong evidence that the lateral quantum confinement in the serpentine superlattice is stronger than the vertical quantum confinement.

Observation of higher confined exciton states in serpentine superlattices by linear polarized excitation spectroscopy

(Al,Ga)As serpentine superlattice (SSL) quantum-wire arrays with a parabolic-crescent cross section have been grown on vicinal GaAs substrates by molecular beam epitaxy. The quantum wires have dimensions on the order of 5 nm and 1D confined valence band states have been reported previously. Here we report on subsequent SSL samples having photoluminescence (PL) linewidths of 5 meV and displaying well-resolved excitation spectra. The data were obtained at 1.4 K and include linearly-polarized PL and PL excitation spectra (PLE) taken normal to the vicinal (100) surface. These data were fitted with effective-mass calculations that include the SSL geometry, valence band mixing, and imperfect lateral composition modulation. In linear polarized PLE (polarized parallel and perpendicular to the quantum wires), we have been able to resolve a SSL-induced heavy-light hole splitting of the ground state exciton. The observed heavy-light hole splitting is of the order of 5 meV with an electron-heavy hole subband separation of 15-20 meV. We extract a lateral composition modulation between the barrier and the wire to be 15% from the observed heavy-light hole splitting corresponding to a lateral potential-energy difference in the conduction band of about 120 meV

Optical properties of quantum-wire arrays in (Al,Ga)As serpentine-superlattice structures.

Serpentine superlattices with a built-in quantum-wire array have been grown on vicinal (100) GaAs substrates by molecular-beam epitaxy. The quantum wires have parabolic cross sections with confinement dimensions on the order of 100 \AA{}. The goal of this paper is to make a comprehensive optical study of two different serpentine-superlattice samples. The serpentine structures have been characterized by low-temperature cathodoluminescence, photoluminescence, and photoluminescence-excitation measurements. In photoluminescence, a rather sharp peak (typically 7 meV full width at half maximum) is attributed to excitonic recombination in the built-in quantum-wire array. The linear polarization dependence of the serpentine-superlattice emission has been measured with a photoelastic modulation technique, showing a pronounced polarization anisotropy in both photoluminescence and photoluminescence excitation. The serpentine-superlattice photoluminescence emission normal to the vicinal surface shows a linear polarization along the wires of up to about 30% due to the lateral confinement. The carrier confinement has been further characterized by measuring the linear polarization dependence of the photoluminescence normal to the cleaved edges. The measured polarization anisotropy has been compared with the calculated polarization dependence as a function of intermixing between the lateral barriers and wells. It is found that there is a substantial intermixing between the barriers and wells, with at least 30% of the Al intended for the barrier ending up in the well.This results in hole states confined to one dimension, while the electron states are two dimensional due to coupling through the lateral barriers. Linearly polarized photoluminescence excitation has been used to reveal the laterally induced heavy-hole--light-hole splitting. Photoluminescence decay-time measurements of the serpentine-superlattice emission (390 ps) show a longer radiative decay time at low temperature than for a reference alloy-well structure (260 ps), indicating a reduced recombination rate in the serpentine structure. The linear polarization of the photoluminescence is found to be rather constant over large areas of the wafer even though the growth rate varies by a few percent. This indicates that uniform quantum-wire-like states are formed and demonstrates the intended advantage of the serpentine superlattice, which avoids the tilt sensitivity of the tilted superlattice.

Optical Detection of Cyclotron Resonance in Serpentine Superlattice Quantum-Wire Arrays

We report low-temperature studies of microwave-induced cyclotron resonance of photo-generated carriers in (A1,Ga)As serpentine superlattice quantum-wire arrays. The geometric size of the parabolic-crescent cross-section of the quantum wires was of the order of 100 A x 50 A, depending on the angle of the vicinal substrate and the amount of parabolic curvature. Comparing the obtained spectra, we estimate the relative degree of carrier confinement in the ordered AlGaAs structure.

Growth and characterization of serpentine superlattices in the GaSb-AlSb system

We report the growth of GaSb/AlSb serpentine superlattices (SSLs) on vicinal GaAs and GaSb substrates. Cross-sectional transmission electron microscopy confirms the SSL structure and shows excellent lateral uniformity, better than previous arsenide-SSLs. Photoluminescence (PL) measurements indicate a good-quality lateral superlattice with a spectral linewidth between 13 and 15 meV. Polarization-dependent PL measurements give a normalized linear polarization around 60%, the strongest that has been seen for SSL structures. Preliminary estimates suggest much better segregation between the Ga-rich and Al-rich regions than arsenide-SSLs, with the change in aluminum concentration Δx≊0.35.

Magneto-luminescence study of quantum wire arrays in (Al,Ga)As serpentine superlattice structures

(Al,Ga)As serpentine superlattices (SSLs) with a built-in quantum wire array have been grown on vicinal GaAs substrates by molecular beam epitaxy. The geometric size of the parabolic-crescent cross section of the quantum wires is on the order of 160 Å × 50 Å, depending on the angle of the vicinal substrate and the amount of parabolic curvature. The low-temperature magneto-luminescence properties of these quantum wire arrays have been studied in magnetic fields up to 13 T. The diamagnetic shift from the serpentine structure was found to be smaller than from a reference alloy-well sample, indicating an increased binding energy of the exciton due to the lateral confinement in the serpentine structure. A diamagnetic shift anisotropy is observed when the magnetic field is applied in the three principal perpendicular directions of the wire. In particular we see a difference when the magnetic field is applied parallel to the the wire and perpendicular to the wire, in qualitative agreement with theoretical calculations. This is a direct observation of the one-dimensional properties of the excitonic wave function. The linewidth is found to increase with the applied magnetic field, with the amount of the increase critically dependent on the direction of the magnetic field. This is explained by the way the exciton volume is probing the potential variations in the ordered SSL structure.

Structure of serpentine superlattice quantum-wire arrays determined by transmission electron microscopy

The structure of serpentine superlattice (SSL) quantum-wire arrays is analyzed with transmission electron diffraction and microscopy. The lateral SSL barriers are nominally formed as a digital alloy of (AlAs)r(GaAs)p partial layers in the vertical direction. Micrographs demonstrate the predicted SSL shape and its insensitivity to systematic deposition rate errors. A diffraction pattern is synthesized to explain diffraction features and order-sensitive SSL images, which are compared with chemical contrast images. The lateral structure is observed to be incompletely segregated into wells and barriers. The length of the quantum wires is estimated to be at least on the order of 100 nm.

Optical properties of serpentine superlattices on GaAs vicinal substrates for quantum wire laser applications

In this letter, the first detailed theoretical study of optical gain in serpentine superlattice quantum wire arrays grown on GaAs vicinal substrates is presented. In the calculations, the complex nature of the miniband structure due to coupling between wires and the valence band intermixing are taken into account. In addition to the ideal structure, the effects of imperfect Al segregation between GaAs wires and AlGaAs barriers are also investigated.

Serpentine superlattice quantum-wire arrays of (Al,Ga)As grown on vicinal GaAs substrates.

A serpentine superlattice quantum-wire array avoids the tilt sensitivity of a tilted superlattice and promises significant 1D confinement at the 10 nm scale. Transmission electron micrographs confirm the intended geometry. Three independent polarization measurements, an energy shift, and th light-hole-heavy-hole splitting at 1.4 K, when compared with theory, demonstrate 1D valence band states and 2D conduction band states. All five measurements are explained by lateral Al x Ga 1-x As wells and barriers of x=0.12 and 0.21, respectively, thus characterizing the step-flow growth

Electronic structure and optical properties of serpentine superlattice quantum-wire arrays.

The subband structure and band-to-band optical matrix elements are computed for serpentine superlattice quantum-wire arrays. The subband dispersion and bandwidths are examined as a function of the curvature of the confining barriers and of the barrier height separating the wires. The optical matrix elements show a strong polarization dependence, even when the coupling between wires in the array is sufficiently strong that the states are not quasi-one-dimensional.

Making quantum wires and boxes for optoelectronic devices

Abstract This paper will review current techniques used to fabricate low dimensional quantum structures for optoelectronic applications. Work has included the fabrication of quantum wire structures using molecular beam epitaxy (MBE) growth of tilted superlattices on vicinal substrates, observation and modeling of self-ordering effects that occur during this process, extension to antimony-containing III–V compounds, and quantum wire formation in “serpentine superlattices”. In addition, we will discuss results obtained using focused ion beam implantation techniques to “write” quantum wires and boxes. Direct growth by both MBE and metal-organic chemical vapor deposition (MOCVD) on lithographically patterned substrates, presently underway at Bellcore, will also be described, along with the electron-beam patterning and MOCVD regrowth of phosphide-based quantum wire and box configurations carried out at the Tokyo Institute of Technology. Application of these various fabrication techniques to semiconductor lasers and high-speed photodetectors will be discussed.

Serpentine superlattice: concept and first results

We present an epitaxial growth technique for vicinal substrates which avoids the geometric tilt sensitivity inherent to the cyclic growth of such structures as tilted superlattices. This cyclical deposition and growth technique produces a serpentine superlattice (SSL) (M.S. Miller et al., in: Proc. 20th Intern. Conf. on Physics of Semiconductors, Thessaloniki, August 1990). Purposefully varying the per-cycle growth rate produces a lateral superlattice with a variable, meandering tilt, with 1 D, microscopically confined electronic states formed where the superlattice bends over. The quantum wire electronic confinement is principally determined by the SSL shape near these widest regions, near places with a vertical tangent. This shape is accurately established through the smooth adjustment of the fractional monolayer growth rates. If a large enough range of growth rates is spanned, then a particular confinement structure is guaranteed: With a SSL, quantum wire arrays with particular energy levels may be precisely realized. Not only is this true at a particular place on the substrate, but the growth rate variations across a substrate are automatically accommodated and uniform wires are realized over the entire wafer. Calculations of the two-dimensional subband structure were performed for (Ga,Al)As SSLs to confirm the 1D electronic states, determine optimal structure parameters and explain experimental results. Such (Ga,Al)As SSLs have been prepared by MBE and characterized with photoluminescence. We find emitted light in several samples that is polarized parallel to the direction of the vicinal substrate's steps, which we interpret to be consistent with quasi-1D confinement.

MBE growth of tilted superlattices: advances and novel structures

The vicinal surface ordering required for the TSL deposition has been studied as a function of temperature and composition for Ga xAl 1−xAs, GaSb and AlSb surfaces. The interface sharpness observed during the Al-Ga co-deposition and self-organization on a vicinal surface has been modeled using a stochastic kinetics method. The modeling reproduces well the observations. To avoid critical effects of the tilt parameter variations, a novel structure, the serpentine superlattice, has been proposed. This structure has a “build in” two-dimensional confinement and yields uniform luminescence properties over large wafer areas. Finally, the TSL concept has been demonstrated for a novel system, the GaSb-AlSb system.

Novel approaches in 2 and 3 dimensional confinement structures: Processing and properties

In this paper we review two novel types of quantum structures. The first, aimed at producing during growth quantum wire superlattices relies on the deposition of tilted superlattices. Some of the difficulties associated with the growth of tilted superlattices and the novel serpentine superlattice have been discussed and solutions proposed. The second type of quantum structures aimed at producing zero dimensional confinement structures relies on the formation of an antidot lattice. The transport properties of antidot lattices with various periodicities are presented.