Strained InAs Quantum Research Articles

Stress-engineered spatially selective self-assembly of strained InAs quantum dots on nonplanar patterned GaAs(001) substrates

The lattice-mismatch stress-induced two-dimensional-to-three-dimensional morphology change is combined with interfacet adatom migration to selectively assemble parallel chains of InAs islands on top of [11̄0] oriented stripe mesas of sub-100-nm widths on GaAs(001) substrates. On such mesa stripes, prepared in situ via size-reducing epitaxy, deposition of InAs amounts subcritical for island formation on planar GaAs (001) is shown to allow self-assembly of three, two, and single chains of InAs three-dimensional island quantum dots selectively on the stripe mesa tops for widths decreasing from 100 nm down to 30 nm.

Intermixing and shape changes during the formation of InAs self-assembled quantum dots

The initial stages of GaAs overgrowth over self-assembled coherently strained InAs quantum dots (QDs) are studied. For small GaAs coverages (below 5 nm), atomic force microscopy (AFM) images show partially covered island structures with a regular size distribution which are elongated in the [011] direction. Analysis of the AFM profiles show that a large anisotropic redistribution of the island material is taking place during the initial GaAs overgrowth. Short time annealing experiments together with photoluminescence spectroscopy on annealed QDs are consistent with a Ga and In intermixing during the overgrowth. Surface QDs capped with 5 nm or more GaAs show a strong luminescence intensity indicating that surface QDs are remarkably insensitive to surface recombination effects.

Arrays of strained InAs quantum dots in an (In Ga)As matrix, grown on InP substrates by molecular-beam epitaxy

Arrays of strained InAs islands in an (In, Ga)As matrix on an InP(100) substrate are synthesized by molecular-beam epitaxy, and their structural and optical properties are investigated. According to transmission electron microscope and high-energy electron diffraction data, the critical thickness corresponding to the onset of island growth is 3 monolayers. The resulting InAs islands are coherently strained, and their base diameter varies from 20 nm to 90 nm. The formation of islands produces in the photoluminescence spectra a dominant long-wavelength line, which shifts toward lower energies as the effective thickness of the InAs increases. The radiation emitted by the InAs islands spans a wavelength range of 1.65–2 µm.

Spatial dependence of the strain-induced coupling in highly strained quantum wells.

The strain-induced modifications of the orbital components of the valence-band wave functions in strained GaAs quantum wells grown on GaP substrates and strained InAs quantum wells grown on GaAs substrates are studied. The relative weight of the orbital components is analyzed by comparing the piezoreflectance and the derivative of the reflectance spectra. Depending on the localization of the wave function the strain-induced changes are very different. \textcopyright{} 1996 The American Physical Society.

Highly confined two-dimensional electron gas in an In0.52AI0.48As/In0.52Ga0.47As modulation-doped structure with a strained InAs quantum well

This paper examines a detailed analysis by Shubnikov-de Haas measurements of the effective mass of two-dimensinal electron gas (2DEG) in an In0.52Al0.48As/ In0.53Ga0.47As modulation-doped (MD) structure with an InAs quantum well inserted into the InGaAs channel (InAs-inserted channel). The measured effec-tive mass of 2DEG in the InAs-inserted-channel MD structure is in good agreement with the calculated one of the strained InAs layer on In0.53Ga0.47As. This indicates that almost all of the 2DEG forms in the strained InAs quantum well. These results show that the InAs-inserted-channel MD structure improves the electron confinement, since the 2DEG is confined in the InAs quantum well with the thickness of 4 nm.

Superconducting transistors using InAs-inserted-channel InAlAs/InGaAs inverted HEMTs

A newly fabricated Josephson field effect transistor (JOFET) is coupled with a two-dimensional electron gas (2DEG) in a strained InAs quantum well inserted into an inverted modulation-doped structure with an HEMT-type gate. We indicate the improved characteristics of the JOFET with the HEMT-type gate, instead of the MIS-type gate. The superconducting critical current as well as the junction's normal resistance can be completely controlled via a gate voltage of about -1 V; this provides voltage gain over unity, the first time for a JOFET.

A Josephson field effect transistor using an InAs-inserted-channel In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure

A Josephson field effect transistor (JOFET) was coupled with a two-dimensional electron gas in a strained InAs quantum well inserted into an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure. The characteristics of this JOFET are much improved over previous devices by using a high electron mobility transistor (HEMT)-type gate instead of the usual metal-insulator- semiconductor (MIS)-type gate. The superconducting critical current as well as the junction normal resistance are completely controlled via a gate voltage of about −1 V; this provides voltage gain over 1 for a JOFET.

Superconducting junctions using a 2DEG in a strained InAs quantum well inserted into an InAlAs/InGaAs MD structure

A newly fabricated three-terminal Josephson junction is coupled with a two-dimensional electron gas (2DEG) in a strained InAs quantum well inserted into an InAlAs/InGaAs modulation-doped structure. The 2DEG is confined in the InAs quantum well and has a maximum mobility of 155000 cm/sup 2//Vs at a sheet-carrier density of 1.86/spl times/10/sup 12/ cm/sup -2/ at 10 K. The supercurrent flows through the 2DEG and can be controlled by adjusting the gate voltage. The critical current and normal resistance are measured as a function of the gate voltage, and the sheet-carrier density dependence of the critical current is obtained. We also measure the temperature dependence of the critical current at different gate voltages. The results indicate that, when using this junction, the superconducting characteristics can range between the clean and dirty limits. >

Optimized InAs quantum effect device structures grown by molecular beam epitaxy

We have investigated InAs quantum effect devices based on both antimonides and arsenides. In an InAs quantum point contact device based on antimonides (InAs/AlGaSb), we have successfully reduced the leakage currents and observed quantum effects at around 77 K by optimizing the heterostructure growth and mesa-etched split-gate approach. Strained InAs quantum dots based on arsenides (AlInAs/AlAs/InAs/InGaAs/AlInAs) were successfully fabricated by MBE growth and mesa-etching. Blue-shifted photoluminescence was obtained from millions of quantum dots with an average lateral size of approximately 2000 Å square.

Intersubband transitions in InAs/AlSb quantum wells studied by resonant Raman scattering.

We have used resonant Raman scattering to study intersubband transitions in InAs/AlSb quantum wells. For optical excitation in resonance with the ${\mathit{E}}_{1}$ band gap of InAs, both high- and low-frequency coupled longitudinal-optical phonon-intersubband plasmon modes are observed. From the measured energies of these plasmon modes the single-particle transition energies between the first and second confined electron subband were deduced as a function of the InAs well width. Good agreement was found with subband spacings predicted by theory including the effects of strain and nonparabolicity. InAs/AlSb surface quantum wells, where a pseudomorphically strained InAs quantum well is grown on an AlSb buffer layer without an AlSb top barrier, also show well-resolved intersubband plasmon modes, indicating higher-electron mobilities than those typically found in the surface inversion region of thick InAs layers.

Structural and optical behavior of strained InAs quantum boxes grown on planar and patterned GaAs (100) substrates by molecular-beam epitaxy

Strained, coherent InAs islands were grown on GaAs (100) planar substrates by molecular-beam epitaxy (MBE). These coherent islands were buried under GaAs cap layers grown by MBE and migration enhanced epitaxy at low temperature (∼ 400 °C), the latter case exhibiting an order of magnitude stronger photoluminescence (PL). The PL and PL excitation spectra reveal the discrete nature of the density of states of three-dimensionally confined electronic states in the islands. The possibility of placing InAs on a regular array of prepatterned square mesas, size-reduced via buffer layer growth to a ≤150 nm mesa top, was explored. The InAs on mesas with sub-100 nm lateral dimensions is found to have an enhanced critical thickness for two-dimensional to three-dimensional island morphology transition, going from ∼1.8 monolayers (ML) for growth on planar substrate to ≥5 ML for 70 nm mesas. Spectrally and spatially resolved cathodoluminescence studies show these InAs volumes to be optically active.

Improving the mobility of an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure by inserting a strained InAs quantum well

The mobility of two-dimensional electrons in an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure improved by inserting an InAs quantum well into the InGaAs channel. This letter addresses the main cause of this mobility improvement. By optimizing the thickness of the InAs quantum well, its distance from the underlying InAlAs spacer layer, and the InAlAs spacer-layer thickness, maximum mobilities of 16 500 cm2/V s at 300 K and 155 000 cm2/V s at 10 K are attained. The improvement in mobility is attributed to a decrease in scattering caused by ionized impurities, interface-roughness, and trap impurities. This decrease is a result of the superior confinement of two-dimensional electron gas in the InAs quantum well.

Dependence of InAs phonon energy on misfit-induced strain

The transverse-optical (TO) phonon energy in strained InAs quantum wells has been investigated by using far-infrared absorption. We observe that the TO phonon energy decreases when the misfit-induced biaxial tension in the InAs single quantum well is increased. Our result shows a stronger phonon energy dependence on the strain than the one reported by Cerdeira et al. [Phys. Rev. B 5, 580 (1972)]. The discrepancy may be explained by stress relaxation near the surface in their experiment. The application of our result will be discussed.

Strained InAs/Al xGa 0.48 − xIn 0.52As heterostructures: a tunable quantum well materials system for light emission from the near-IR to the mid-IR

We report on spontaneous and simulated emission from strained InAs quantum wells (QWs) embedded in an Al xGa 0.48 − xIn 0.52As matrix grown on InP substrates by solid-source molecular-beam epitaxy. By adjusting the QW width between 2 and 23 monolayers and/or the barrier composition between Al 0.48In 0.52As and Ga 0.47In 0.53As, the low temperature photoluminescence can be tuned between 1.0 μm and 2.2 μm. Efficient room temperature luminescence is achieved up to 2.4 μm which is the longest wavelength reported for QWs grown on InP substrates. Finally, we demonstrate the first stimulated emission near 1.3 μm and 1.8 μm at 80 K in this materials system. These results thus enlarge the wavelength range of applications achievable with InP-based materials.

Low-density band-filling in strained InAs quantum wells

We study the low-temperature photoluminescence (PL) of strained InAs single quantum wells (SQWs) embedded in a Ga0.47In0.53As matrix grown on InP substrates by modified solid-source molecular beam epitaxy. The spectra are interpreted in the frame of a two-level rate equation model describing the carrier dynamics in the structures. We show that band-filling occurs in these QWs for an excitation power as low as 30 Wcm−2. Moreover, the spectra reveal that the band-filling results from the rapid population of the hole subband. This observation highlights the low in-plane heavy-hole mass in the compressively strained film. Our results therefore demonstrate the high potential of InAs/Ga0.47In0.53As QW nonlinear optical devices operating in the mid-IR wavelength range.

Observation of InGaAs / Inaias Surface Quantum Wells by Photoreflectance and Photoluminescence Excitation Spectroscopies

The samples under study consist of strained InAs/In0.52Al0.48As single quantum wells, grown on InP substrates by molecular beam epitaxy. All samples were capped by a 15 nm-thick lattice matched InGaAs layer. By photoreflectance investigations, transitions which do not arise from the strained InAs quantum well have been observed. They have been interpreted as transitions from a surface quantum well formed by the InGaAs cap layer confined between the InAIAs barrier layer on one side and the sample surface on the other side. Photoluminescence excitation on the photoluminescence peak attributed to the first level of this well has been performed at 5 K. The obtained spectra are typical of a two-dimensional system. A modelisation of this structure is proposed and theoretical calculations of the expected quantum levels have been performed and fit well with experimental results. Finally, etching of the InGaAs cap layer has been done to confirm the origin of the observed transitions.

Raman study of a single InP/InAs/InP strained quantum well

We report on the first observation of both confined optical phonons and interface phonons in a single strained InAs quantum well grown on InP. Quasi resonant Raman measurements allow for the observation of interface modes. Informations about the mismatch induced strain in the InAs layer are obtained. The effect of the strain and the confinement on the longitudinal and transverse optical InAs modes are estimated.

Stimulated emission from ultrathin InAs/GaAs quantum well heterostructures grown by atomic layer epitaxy

We report the first observation of stimulated emission (16 K) in material grown by atomic layer epitaxy (ALE). The active region of our laser structure consists of six strained InAs quantum wells (6.6 Å thick, 7.4% strain) separated by 500 Å of GaAs, which makes these the thinnest and most highly strained quantum wells ever reported to support stimulated emission. These results demonstrate that the ALE process can be used to grow laser quality material with highly reproducible layer thickness.