InAs Quantum Wells Research Articles

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.

Deep Levels in Type-II Superlattices

AbstractQuantum confinement in superlattices affects shallow levels and band edges considerably (length scale of order 100 Å), but not deep levels (length scale of order 5 Å). Thus by band-gap engineering, one can move a band edge through a deep level, causing the defect responsible for the level to change its doping character. For example, the cation-on-anion-site defect in AlxGa1−xSb alloys is predicted to change from a shallow acceptor to a deep acceptor-like trap as the valence band edge passes through its T2 deep level with increasing At alloy content x. In a, Type-II superlattice, such as InAs/AlxGa1−xSb for x>0.2, where the conduction band minimum of the InAs should lie energetically below the antisite defect's T2 level in bulk AlxGa1−xSb, the electrons normally trapped in this deep level (when the defect is neutral) remotely dope the InAs n-type in the superlattice, leaving the defect positively charged. Thus a native defect that is thought of as an acceptor can actually be a donor and control the n-type doping of InAs quantum wells. The physics of such deep levels in superlattices and in quantum wells is summarized, and related to high-speed devices.

Optical properties of InAs quantum wells emitting between 0.9 mu m and 2.5 mu m

InAs/AlxGa0.48-xIn0.52As single and multiple quantum wells (QWS) grown in InP by modified solid source molecular beam epitaxy are investigated by photoluminescence (PL) spectroscopy. By adjusting the QW thickness between 2 and 16 monolayers (ML) and the barrier composition between Al0.48In0.52As and Ga0.47In0.53As, the PL emission can be tuned from 1.0 to 2.2 mu m. The excitonic nature of the low-temperature recombinations and the observations of luminescence at room temperature demonstrate the high quality of the samples and the potential of the InAs/AlxGa0.48-xIn0.52As system for optoelectronic device applications.

Optical Characterization of Ultra-Thin InAs Strain-Layer Quantum Wells Grown on (511) GaAs Substrate

Absorption, photoluminescence, photoreflectance and transmission electron microscopy measurements have been performed on 2 and 3-monolayer thin InAs quantum wells(QW) (with 250A wide GaAs barriers) grown on (511) GaAs substrate. For comparison, similar samples grown on (100) substrates have also been studied. (511)-grown thin quantum wells may have possible quantum wire configuration. However, polarization studies show a small anisotropic absorption from the (511) sample, which indicates that the optical property of the (511)-quantum well is different than either that of a quantum wire or that of a (l00)-grown quantum well structures. A theoretical calculation, making a unitary transformation of the valence band k•P Hamiltonian matrix to the (511) base, and using a perturbation method to determine the new wavefunctions, yield an anisotropic absorption comparable with the experimental result. We have also compared the transition energies from PL data with the calculated one using the conventional effective mass approximation.

InAs/Ga0.47In0.53As quantum wells: A new III-V materials system for light emission in the mid-infrared wavelength range

We propose the use of strained InAs/Ga0.47In0.53As quantum wells (QWs) for light emission in the technologically important mid-IR wavelength range. Temperature dependent photoluminescence measurements on single QWs demonstrate that light emission at room temperature is obtained from all samples having InAs QW widths between 2 and 23 monolayers. Luminescence up to 2.4 μm is obtained at 300 K, which is the longest wavelength achieved so far for QWs grown on InP. These results demonstrate the potential of the InAs/Ga0.47In0.53As QW materials system for the fabrication of optoelectronic devices operating in the mid-IR.

Accurate determination of effective quantum well thickness: Infrared absorption by transverse-optical phonons

We have studied far infrared transmission spectra of various AlSb/InAs/AlSb single quantum wells, where the well width ranges from 60 to 200 Å. Because the quantum well is thin, the far infrared absorption due to transverse-optical phonons is not saturated. We obtained excellent fitting to the transmission spectra by taking into account the complex dielectric functions with a transfer matrix formalism. The thickness of InAs quantum wells can be determined with monolayer resolution. High-resolution transmission electron microscopy shows clear lattice images in a region away from the AlSb/InAs interfaces. The thickness of the pure nonintermixed InAs region is in excellent agreement with our far infrared absorption results. Phonon absorption can therefore provide a nondestructive method to effectively determine the number of pure InAs monolayers, in spite of interdiffusion occurring at the interface.

Optoelectronic devices based on type II polytype tunnel heterostructures

We describe a new family of optoelectronic devices based on the unique properties of combinations of semiconductor tunnel junctions with type II polytype heterostructures. A typical light-emitting device structure consists of AlSb (thin barrier)/InAs(quantum well)/AlSb(thin barrier) structure clad by AlInAsSb emitter and GaSb collector. The emission source is the intersubband radiative transition between the first two subbands in the InAs well. Since the conduction-bank minimum of InAs is energetically lower than the valence-band maximum of GaSb, the device can be designed in such a way that electrons are injected only into the higher subband and extracted out only from the lower subband, both via tunneling, leading to an intraband population inversion and highly efficient radiative transitions. The same operation principle can be applied to design detector structures.

Photoluminescence of virtual-surfactant grown InAs/Al0.48In0.52As single quantum wells

Strained InAs/Al0.48In0.52As single quantum wells (QWs) grown by solid-source molecular beam epitaxy on InP substrates are studied by photoluminescence spectroscopy. The thickness of the InAs QW lies between 2 and 16 monolayers, corresponding to an intrinsic emission wavelength between 0.9 and 1.8 μm at 6 K. We demonstrate that the growth of the InAs QW under virtual-surfactant conditions, i.e., by keeping the As shutter closed throughout the growth of the well, gives a striking improvement of the optical properties, as compared to conventionally grown samples. Finally, we report on the first room-temperature luminescence of this QW system.

One-dimensional electron transport on the surface channel of InAs quantum wells

Utilizing the unique feature of Fermi-level pinning at the InAs surface, one-dimensional electron transport has been achieved at the edge of confined InAs layers. At low temperatures, the transport is characterized by a much enhanced conductance with a flat temperature dependence in comparison with that in the hulk. Negative magnetoresistance, Shubnikov-dc Haas oscillations and conductance fluctuations are further manifestations of one-dimensional surface conduction.

Reflection high-energy electron diffraction and optical measurements on the molecular-beam epitaxial growth of one and two monolayers of InAs on GaAs

We report here on the reflection high-energy electron diffraction, photoluminescence (PL), and photoluminescence excitation study of the direct growth by molecular-beam epitaxy of one and two monolayers (MLs) of InAs on GaAs. InAs can be grown pseudomorphically up to a thickness of 1 ML, deposition of 2 MLs results in three-dimensional nucleation. In the latter case our results suggest that the first ML is disrupted by the growth of the second. This finding is in contradiction of the Stranski–Krastanov growth mode. A 1-ML InAs quantum well (QW) with GaAs barriers produces intense luminescence at 1.465 eV, whereas for the intended 2-ML InAs QW sample, PL characteristic of InAs clusters with a most probable thickness of 3 MLs is observed.

Observation of photoluminescence from InAs surface quantum wells grown on InP(100) by molecular beam epitaxy

Photoluminescence (PL) measurements are presented for thin epitaxial layers of InAs, 2.5 Å<d <36 Å, grown on InP(100) by molecular beam epitaxy. The combination of efficient carrier capture and PL redshift with increasing InAs thickness clearly indicate the formation of InAs quantum wells on the InP surface. Data are also presented for InAs/InP structures capped with strained layers of either GaAs or In0.5 Al0.5 As. Since radiative recombination within the InAs layers can be distinguished from PL arising from both bulk and surface defects, this system allows us to monitor the quality of both the InAs/InP and InAs/air interfaces via their influence on the InAs quantum well luminescence.

Exciton localization in submonolayer InAs/GaAs multiple quantum wells.

We study the optical transitions and their relation to the crystal potential in ultrathin InAs/GaAs multiple quantum wells. Photoluminescence, photoluminescence excitation, and photoreflectance measurements evidence strong localization of the heavy- and light-hole excitons in submonolayer wide InAs quantum wells. A tight-binding model in the linear-chain approximation explains these results in terms of particle localization at the two-dimensional potential discontinuity produced by the insertion of the InAs plane in the host GaAs matrix.

Resonant interband tunneling through a 110 nm InAs quantum well

The mechanism of resonant interband tunneling in polytype heterostructures of GaSb/AlSb/InAs gives excellent peak-to-valley current ratios due to the band-gap blocking of the nonresonant current components. Using InAs as the base in a double-barrier polytype heterostructure, it is possible to demonstrate resonant tunneling at room temperature through a quantum well as wide as 110 nm. At this width, which is about 20 times larger than that typically used in resonant tunneling diodes in the GaAs/AlGaAs system, the peak-to-valley ratio is 44:1 (77 K). Significant negative differential resistance is observed even for 240 nm wells. The projected device response time for a resonant tunneling transistor with a wide InAs quantum base is more than five times faster than for a GaAs device, due to the reduced base resistance.

Stimulated emission from monolayer thick quantum-well heterostructures

Summary form only given. Stimulated emission from clearly defined quantum-well transitions has been observed from single quantum wells as thin as one monolayer. These results are unexpected, since previous experimental and theoretical work has indicated that if the well width is comparable to or smaller than the scattering path length of the electrons or holes, carrier collection becomes inefficient and the quantum well cannot support stimulated emission. Well-defined confined particle transitions were observed from the GaAs quantum wells for thicknesses as thin as two monolayers (5.65 AA) and from pseudomorphic InAs quantum wells for thicknesses as thin as one monolayer (3 AA). Continuous-wave laser thresholds as low as 820 W/cm/sup 2/, J/sub eq/=340 A/cm/sup 2/ from the pseudomorphic samples and 1740 W/cm/sup 2/, 720 A/cm/sup 2/ from the lattice-matched samples have been measured at 77 K. Data are presented which demonstrate that monolayer thick quantum wells can operate efficiently as lasers at low thresholds despite previous experimental and theoretical predictions to the contrary. >

Heteroepitaxy of InAs quantum wells

Recent experimental results of InAs quantum wells clad with GaSb are described. It is shown that high quality GaSb is critical to the formation of the electron-hole system. The same results also apply to quantum wells with GaSb ternary alloys, where the densities of carrier can be controlled by varying the alloy composition.

Optical properties of some III–V strained-layer superlattices

We discuss some features of strained-layer superlattices built from III–V direct gap semiconductors. First, the effects of built-in strains on the optical properties of two examples of moderately strained structures, In x Ga 1− x As GaAs and In yAl 1−yAs GaAs on GaAs substrates are described. We report, in a second part, experimental data obtained on samples containing highly strained layers. In narrow InAs quantum wells in GaAs, the interface broadening due to indium segregation and InAs film roughness is shown to influence strongly the variations of the optical transitions energies with the well thickness. Finally, we present structural and optical data on (InAs) m(GaAs) n short period superlattices grown by Migration Enhanced Epitaxy on InP substrate. We show from Raman scattering and optical spectra as well as X-ray diffraction profiles that high quality material (either in thick layers or used in multi Quantum Well structures) can be obtained using this technique.

High quality ultrathin InAs/GaAs quantum wells grown by standard and low-temperature modulated-fluxes molecular beam epitaxy

We report on the optical study of ultrathin (1–3 monolayers) InAs quantum wells in GaAs. The samples have been grown either by standard molecular beam epitaxy (MBE) or low-temperature (350 °C) modulated-fluxes MBE. The latter technique enlarges the range of pseudomorphic deposition of strained InAs layers on GaAs. Both types of samples display sharp (down to 6 meV spectral width) and intense low-temperature excitonic photoluminescence, and a high in-plane homogeneity. Higher energy optical transitions have been observed in both absorption and photoluminescence excitation experiments, performed on multiple quantum well structures.

Optical investigations of the band structure of strained InAs/AlInAs quantum wells

We report photoluminescence and photoconductivity measurements on single InAs quantum wells under a 3.4% biaxial compression. The photoluminescence line, which is in the 1.2–1.6 μm wavelength range depending on well thickness, is interpreted as recombination between electrons and holes distributed over a range of confined energy states associated with quantum well width fluctuations. Photoconductivity is found to provide spectroscopic measurements of the optical transitions even for single, extremely thin quantum wells. We find excellent agreement between the measured transition energies and the result of an envelope function calculation taking into account the effect of both strain and quantum confinement.

Magnetic-field-induced transitions in InAs/Ga 1− xAl xSb heterostructures

We have studied the two-dimensional properties of high-mobility electron-hole systems in InAs quantum wells bounded by Ga 1− x Al x Sb. By changing the alloy composition x, we are able to vary the system from quasi-semimetallic (with unequal electron and hole carrier concentrations) to semiconducting (with only electrons). Furthermore, we have observed a magnetic-field-induced semimetal-to-semi-conductor transition which manifests itself in the magnetotransport properties of samples with both carriers. This transition can be understood by considering the continuity of the Fermi energy across the InAs/Ga 1− x Al x Sb interface and the interdependence of the electron and hole densities.

Long-lived excitons in InAs quantum wells under uniaxial stress.

Exciton states of InAs quantum wells under uniaxial stress are studied with a multiband effective-mass theory. For InAs quantum wells with compressive uniaxial stress, the first valence subband can have a maximum at k\ensuremath{\ne}0 several meV above the zone-center energy. When the indirect valence-band maximum is high enough to offset the difference in binding energies between the direct and indirect excitons, the formation of long-lived indirect excitons becomes both energetically and radiatively favorable. The lifetimes of the indirect excitons are found to be 3 to 5 orders of magnitude longer than those of the direct excitons.