InAs Quantum Wells Research Articles

Ultrathin InAs and modulated InGaAs layers in GaAs grown by MOVPE studied by photomodulated reflectance spectroscopy

Photomodulated reflectance spectroscopy (PR) and X-ray diffraction (XRD) were used for the characterization of highly strained ultrathin InAs quantum wells and modulated InGaAs layers in GaAs grown by metal-organic vapor phase epitaxy (MOVPE). Structures were grown in AIXTRON 200 reactor at 500 °C on (1 0 0) oriented GaAs substrates by sequential growth of InAs and GaAs layers. Various PR spectral features corresponding to optical transitions between ground and excited states in the layers were identified by means of simulation of electronic states in these structures using nextnano 3 quantum simulator. Different models of InAs layer growth were used to explain both the XRD and PR data. Results show that the Gaussian distribution of In atoms within few monolayers gives the best fit for our MOVPE grown ultrathin InAs layers.

Growth and characterization of GaAsSb metamorphic samples on an InP substrate

Buffer layers of GaAs1−xSbx were grown on an InP substrate starting at x=0.49 (lattice matched to InP) and increasing x in steps of 0.03–1.0 (GaSb), followed by a 0.5μm thick GaSb metamorphic layer. A 10nm thick InAs quantum well was grown on top and capped with a 100nm GaSb. All layers were grown using gas source molecular beam epitaxy by reducing the As flux in small steps while keeping the Sb flux fixed. The substrate temperature during growth was ∼490°C. High-Resolution x-ray diffraction analysis showed that the metamorphic layer was almost fully relaxed. Cross-sectional transmission electron microscopy images show that a postgrowth anneal at 600°C for 30s successfully reduces the number of dislocations threading through the metamorphic layer. Hence the top layer can be used satisfactorily as a pseudosubstrate for subsequent layer growths assuming a GaSb lattice constant.

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

Abstract Formation of a multimodal quantum dot (QD) ensemble by strained layer epitaxy of InAs on GaAs near the critical value for the onset of the 2D–3D transition is studied. Reflection anisotropy spectroscopy is employed to confirm that a smooth surface is maintained during strained layer growth prior to QD formation. Instantaneous capping after deposition leads to InAs quantum wells with some thickness flucuations. Multimodal QD InAs ensembles form after an at least short growth interruption prior to cap layer deposition. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers. Related exciton energies indicate a simultaneous increase of both height and lateral extension, i.e. a shell-like increase of sizes. The formation of the multimodal QD ensemble is described by a kinetic approach. A growth scenario is presented where QDs having initially shorter base length stop vertical growth at a smaller height, accounting for the experimentally observed shell-like sub-ensemble structure.

Effect of toroidal moment on a macroscopic self-organization of electrons in the quantum Hall regime

We have studied CR lineshape of terahertz-light-induced current in InAs quantum wells in tilted quantizing magnetic fields. We have observed dramatic modification of the lineshape with increasing of in-plane component of magnetic field as well as with increasing of transverse built-in electric field in the well. Scenario of the modification shows that the energy spectrum asymmetry is determined by so-called toroidal moment of the system and is a function of Landau quantum number. Macroscopic self-organization of electrons under the conditions of quantum Hall effect has also been directly demonstrated in both linear and saturation regimes of the light absorption.

Spectra of Persistent Photoconductivity in InAs∕AlSb Quantum-Well Heterostructures

Persistent photoconductivity at T = 4.2 K in AlSb/InAs/AlSb heterostructures with two-dimensional (2D) electron gas in InAs quantum wells is studied. Under illumination by IR radiation (ℏω = 0.6–1.2 eV), positive persistent photoconductivity related to the photoionization of deep-level donors is observed. At shorter wavelengths, negative persistent photoconductivity is observed that originates from band-to-band generation of electron-hole pairs with subsequent separation of electrons and holes by the built-in electric field, capture of electrons by ionized donors, and recombination of holes with 2D electrons in InAs. It is found that a sharp drop in the negative photoconductivity takes place at ℏω > 3.1 eV, which can be attributed to the appearance of a new channel for photoionization of deep-level donors in AlSb via electron transitions to the next energy band above the conduction band.

Low-frequency noise in AlSb∕InAs high-electron-mobility transistor structure as a function of temperature and illumination

Measurements of the low-frequency noise in AlSb∕InAs high-electron-mobility transistor structures over the temperature range between 60 and 300K are reported. Without illumination, the slope of the noise level with frequency was found to be close to 1∕f with a Hooge parameter, αH, of 9×10−3 at room temperature. With broad-spectrum visible-light illumination at lower temperatures, however, the noise level increases greatly and displays a strong Lorentzian component with the characteristic 1∕f2 slope above the corner frequency. The associated sheet resistance also increases greatly, consistent with previously observed negative photoconductivity in AlSb∕InAs quantum wells.

Experimental separation of Rashba and Dresselhaus spin splittings in semiconductor quantum wells.

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

Asymmetric AlAsSb/InAs/CdMgSe quantum wells grown by molecular-beam epitaxy

Asymmetric (6.7–300)-nm-thick InAs quantum wells (QWs) confined between AlAsSb and CdMgSe barriers have been fabricated by molecular-beam epitaxy. A special procedure of the CdMgSe-on-InAs growth initiation, exploiting an ex situ S passivation of InAs and in situ deposition of an ultrathin ZnTe buffer layer, results in the fabrication of high quality structures with a density of extended defects below 106 cm2. QW photoluminescence studies demonstrate a confinement effect and confirm the type I band alignment at the heterovalent InAs/CdMgSe interface mediated by the ZnTe interlayer. Observation of Shubnikov de Haas oscillations of magnetoresistance for an asymmetric 19-nm-thick InAs QW indicates an existence of the two-dimensional electron gas with the low-temperature sheet electron density of 1.3×1012 cm−2 and the mobility as high as ∼10 000 cm2/V s.

Effect of the spin-orbit interaction on the cyclotron resonance of two-dimensional electrons

In the cyclotron resonance (CR) spectra of two-dimensional (2D) electrons in InAs quantum wells, the CR line splitting is observed. The splitting is found to be an oscillating function of magnetic field. The oscillations do not correlate with the filling factor. The experimental results are interpreted in terms of the spin-orbit splitting in the presence of a built-in electric field appearing due to the asymmetry of the quantum-well potential. From the splitting of the CR line, the spin-orbit coupling constant αso is determined. The resulting value agrees well with the value obtained for the same sample from the Shubnikov-de Haas oscillations. The role of the resonance interaction of charge carriers in the well with the interface donor states is discussed.

II–VI/III–V structures with a heterovalent interface in the active region: New opportunities in band engineering

The paper presents an overview of recent activity in fabrication by molecular beam epitaxy and study of AlGaSbAs/InAs/(ZnTe)/Cd(Mg)Se hybrid pseudomorphic heterostructures with an InAs/II–VI heterovalent interface either in the active region (in case of mid-IR laser diodes) or in the InAs quantum well (QW). Different approaches to fabrication of the defect-free InAs/II–VI interface are discussed, as well as their effect on crystalline properties and an electronic band structure at the interface. The hybrid mid-IR laser diodes are characterized by a dramatically improved carrier (hole) and optical confinement in the InAs-based active region. Clear confining effects are observed in the photoluminescence from a ∼7-nm-thick InAs QW with CdMgSe and AlAsSb barriers. An existence of 2D electron gas in such QWs of ∼16 nm in a thickness is proved by observation of Shubnikov-de Haas oscillations. The obtained results are prospective both for mid-IR optoelectronics and spintronics. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MBE growth and studies of hybrid heterostructures with II–VI/InAs heterovalent interfaces in the active region

InAs quantum wells (QWs) with AlAsSb and CdMgSe asymmetric barriers have been fabricated by a two-step molecular beam epitaxy on InAs(100) substrates. A 5-monolayer-thick ZnTe layer has been introduced between the InAs and CdMgSe layers as a buffer of the II–VI growth initiation. Ex-situ S-passivation technique applied to the uncapped InAs surface provides formation of defect free III–V/II–VI heterovalent interface. Photoluminescence study proves a strong type I band line-up at the ZnTe-mediated InAs/CdMgSe interface with the conduction and valence band offsets of about 0.6 eV and 1.0 eV, respectively. Transport properties of two-dimensional electron gas confined in a 16-nm-thick AlAsSb/InAs/ZnTe/CdMgSe QWs grown on GaAs (100) are compared with those of pure III–V 2DEG structures. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of spin–orbit interaction on cyclotron resonance in InAs quantum wells

AbstractWe present the study of cyclotron resonance (CR) in a two‐dimensional electron gas in AlSb/InAs/AlSb quantum wells. The CR splitting, resolved at magnetic fields as low as 1.6 T, is found to be independent of the filling factor. We identify the origin of the splitting as related to the spin–orbit interaction. Spin–orbit coupling constants of 1.9 × 10–9 eV cm and 2.8 × 10–9 eV cm are obtained for samples with different band‐edge profiles. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrically tunable mid-infrared electroluminescence from graded cascade structures

Mid-infrared electroluminescence (EL) is observed from multiperiod bilayer type-II InAs/AlGaSb structures with the effective interlayer separation controlled by bias. The emission with powers in the microwatt range is characterized by a linear dependence of the photon energy on the bias. By measuring the temperature and current dependence of EL, we find evidence that the EL emission results from recombination of holes in AlGaSb quantum wells (QWs) with electrons occupying two different quantum states in InAs QWs.

Spin splitting in narrow InAs quantum wells with In0.75Ga0.25As barrier layers

Using two independent magnetotransport experiments, i.e., thermal activation and the coincidence method in tilted fields, we determine the g factor in a two-dimensional electron system in a 4-nm-wide InAs quantum well. From these independent techniques we deduce consistently an absolute value |gexp|≅6. This is considerably smaller if compared to |g|=14.8 for bulk InAs. Nonparabolicity in InAs cannot fully explain the reduced g factor. We argue that the penetration of the wave function into the In0.75Ga0.25As barriers and into the In0.75Al0.25As spacer layer plays an additional role.

Rashba-effect-induced spin dephasing in n-type InAs quantum wells

We perform a many-body investigation of the spin dephasing in n-type InAsquantum wells under moderate magnetic fields in the Voigt configuration byconstructing and solving numerically the kinetic Bloch equations. Weobtain the spin dephasing time due to the Rashba effect together with thespin-conserving scattering such as electron–phonon, electron–nonmagneticimpurity as well as electron–electron Coulomb scattering. By varying the initialspin polarization, temperature, impurity density, applied magnetic fieldand the interface electric field, we are able to study the spin dephasingtime under various conditions. For the electron density and quantum wellwidth that we study, the many-body effect dominates the spin dephasing.Moreover, we find an anomalous resonance peak in the spin dephasing timefor high initial spin polarization under moderate magnetic fields.

A novel InAs quantum wire system

We report here a new technique in patterning high quality low-dimensional electrons in single InAs quantum wells. Grown by molecular beam epitaxy, the single InAs quantum well is sandwiched between AlSb barriers and capped by a thin layer of InAs. When the InAs cap layer is patterned by electron beam lithography and selectively removed, electrons are induced in the InAs quantum well below due to the different surface Fermi level pinning voltages on the exposed AlSb layer from the InAs cap. One-dimensional quantum wires can thus be conveniently defined by lithography and nm-shallow etching. We demonstrate that these one-dimensional electrons posses a long elastic mean free path (>1.4 μm) and a long coherence length (>3 μm) at 2 K .

Wave-packet oscillations in a strongly driven InAs quantum well

In this work, we have numerically integrated in space and time the effective-mass nonlinear Schrödinger equation for an electron wave packet in a strongly driven InAs quantum well. Considering a time-dependent Hartree potential, we have calculated the tunneling dynamics between the two quantum well barriers when an external ac bias is applied. At certain ac oscillation periods, we have obtained a time-varying wave-function penetration in the barriers with an amplitude which is also oscillating with time. In this way, the possible time-dependent effects of electronic spins in a strongly driven InAs quantum well could be also investigated.

Reduction of the unintentional background electron density in AlSb/InAs/AlSb quantum wells

InAs quantum wells clad by AlSb barriers contain an unintentional electron density of approximately 1×10 12 cm −2. This report concerns the compensation of these electrons by p-type modulation doping with Be. The electron concentration in the InAs quantum well decreases linearly with increasing Be doping as expected from lever rule arguments. Hall measurements show that the electron mobility increases with increasing electron concentration at temperatures between 10 and 300 K. These findings need to be considered in the design of field-effect transistors based on this material system.

Characterization of ultrathin InAs quantum wells by TEM–cathodoluminescence and TEM techniques

AbstractEmission from ultrathin quantum wells of different thicknesses is studied. In particular, for structures with doped substrate the unintentional deposition of Cu droplets is used as a probe of electron diffusion inside the material. A large diffusion length (L = 8 ± 3 µm) is found in spite of the presence of quantum dots. In particular, the contribution to the diffusion from re‐emission of electrons from quantum dots is evaluated. Copyright © 2003 John Wiley & Sons, Ltd.

High-Frequency Noise Sources in Quantum Wells

Hot-electron noise is investigated for InGaAs and InAs quantum wells containing a two-dimensional electron gas channel in a pulsed electric field applied parallel to the interfaces. Noise sources resulting from hot-electron thermal motion, electron temperature fluctuations, and real-space transfer are observed. The experimental results on hot-electron thermal noise are used to estimate energy relaxation time in the field range where other sources do not play any important role. Measurements of noise anisotropy in the plane of electron confinement are used to discuss real-space-transfer noise. High-frequency noise technique is used to study hot-electron trapping, and trap location in InAlAs/InGaAs/InAlAs heterostructure channels is determined.