H. Weman

Serpentine superlattice: concept and first results

Abstract 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 substrates steps, which we interpret to be consistent with quasi-1D confinement.

Measuring linear polarization of photoluminescence and photoluminescence excitation using a photoelastic modulation technique

Polarization modulation is used to investigate the polarization dependencies in the low‐temperature photoluminescence (PL) and the photoluminescence excitation (PLE) of GaAs/AlGaAs quantum wire arrays. The modulation technique employing a photoelastic modulator is described and its implementation in the experimental setup of the PL and PLE experiments is presented. In the PL experiment the technique is used to analyze the emitted light with respect to its polarization, whereas for the PLE experiment the polarization of the exciting light is modulated, probing the polarization dependence of the absorption of the light. Since the modulation of the light is restricted to the polarization, the polarization dependence can be measured simultaneously with the PL or PLE intensity. The versatility and the sensitivity of the technique is exemplified by presenting results of polarized PL and PLE obtained on quantum wire samples grown on the vicinal (100) surface of GaAs by molecular‐beam epitaxy that show a considerable anisotropy in the linear polarization for both the PL and PLE.Polarization modulation is used to investigate the polarization dependencies in the low‐temperature photoluminescence (PL) and the photoluminescence excitation (PLE) of GaAs/AlGaAs quantum wire arrays. The modulation technique employing a photoelastic modulator is described and its implementation in the experimental setup of the PL and PLE experiments is presented. In the PL experiment the technique is used to analyze the emitted light with respect to its polarization, whereas for the PLE experiment the polarization of the exciting light is modulated, probing the polarization dependence of the absorption of the light. Since the modulation of the light is restricted to the polarization, the polarization dependence can be measured simultaneously with the PL or PLE intensity. The versatility and the sensitivity of the technique is exemplified by presenting results of polarized PL and PLE obtained on quantum wire samples grown on the vicinal (100) surface of GaAs by molecular‐beam epitaxy that show a consider...

Photoluminescence study of lateral carrier confinement and compositional intermixing in (Al,Ga)Sb lateral superlattices

We have compared the photoluminescence properties of an (Al,Ga)Sb lateral superlattice (LSL) quantum well to those of an (Al,Ga)Sb alloy quantum well, with respect to recombination energy and polarization dependence. From the results we have deduced the compositional intermixing and lateral carrier confinement present in the LSL structure. We found that the LSL well luminesces at 36 meV lower than the alloy well, and that emitted light from the LSL well is more than twice as intense when its electric field is polarized parallel versus perpendicular to the LSL ‘‘wires.’’ From these data we calculate that the lateral content of the LSL varies periodically between approximately 24% and 42% AlSb, and the maximum:minimum carrier density ratios are about 4:1 and 6:1 for electrons and heavy holes, respectively.

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

Abstract 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.

Luminescence polarization anisotropy in (Al,Ga)As serpentine quantum wire arrays

Serpentine superlattices (SSL) with a parabolic-crescent cross section defining the wells and barriers, have been grown on vicinal GaAs substrates by molecular beam epitaxy. The SSL structures have been studied by photoluminescence (PL) and photoluminescence excitation (PLE) measurements at 1.4 K, showing a strong polarization anisotropy in both PL and PLE. The carrier confinement has been characterized by measuring the linear polarization dependence of the PL from the surface as well as from the cleaved edges by using a photoelastic modulation technique. Calculations of the conduction band and valence band electronic structure describe the polarization dependence as a function of segregation into lateral wells and barriers. We find that about 30% of the Al intended for the barriers end up in the well giving AlxGa1-xAs wells and barriers of x equals 0.12 and 0.21, instead of the nominally intended values of 0.00 and 0.33, corresponding to a lateral conduction band barrier of 70 meV. Linear polarized PLE has been used to reveal the laterally induced heavy-light hole splitting. PL decay time measurements of the serpentine emission, shows a longer decay time than for a reference alloy-well structure, indicating a reduced carrier relaxation in the serpentine structure. The linear polarization of the PL is found to be rather constant over large areas of the wafer indicating uniform quantum wire like states, showing the intended advantage of the serpentine structure over tilted superlattices.

Implementation of a polarization modulation technique in a photoluminescence and photoluminescence excitation measurement setup

We describe a polarization modulation technique employing a photoelastic modulator and present its implementation in the experimental setup of a photoluminescence (PL) and a photoluminescence excitation (PLE) experiment. In the PL experiment the technique is used to analyze the emitted light with respect to its polarization, whereas for the PLE the polarization of the exciting light is modulated, probing the polarization dependence of the absorption of the light. Since the modulation of the light is restricted to the polarization, the polarization dependence can be measured simultaneously with the PL or PLE intensity. The versatility and the sensitivity of the technique is exemplified by presenting results of polarized PL and PLE obtained on quantum wire samples grown on the vicinal (100) surface of GaAs by molecular beam epitaxy (MBE) that show a considerable anisotropy in the linear polarization for both the PL and PLE at low temperatures.

Epitaxial Growth of GaAs-AIGaAs Quantum Wire Superlattices on Vicinal Surfaces.

This paper reviews the problems that are generic to the direct epitaxy of quantum wire superlattices on {100} and {110} vicinal surfaces using molecular beam epitaxy.