InAs Monolayer Research Articles

Spin relaxation in submonolayer and monolayer InAs structures grown in a GaAs matrix

Electron-spin dynamics in InAs/GaAs heterostructures consisting of a single layer of InAs (1/3--1 monolayer) embedded in (001) and (311)A GaAs matrix was studied by means of time-resolved Kerr rotation spectroscopy. The spin-relaxation time of the submonolayer InAs samples is significantly enhanced, compared with that of the monolayer InAs sample. The electron-spin-relaxation time and the effective $g$ factor in submonolayer samples were found to be strongly dependent on the photogenerated carrier density. The contribution from both the D'yakonov-Perel' mechanism and Bir-Aronov-Pikus mechanism are discussed to interpret the temperature dependence of spin decoherence at various carrier densities.

Investigation of the effect of varying growth pauses on the structural and optical properties of InAs/GaAs quantum dot heterostructures

Effect of growth pause or ripening time on structural and optical properties of self-assembled InAs/GaAs quantum dot (QD) heterostructures grown by solid state molecular beam epitaxy (MBE ) technique with two different growth rates of InAs (0.032 MLs −1 and 0.197 MLs −1) has been investigated. The QD heterostructures were grown at 520 ∘C with InAs monolayer coverage of 2.7 ML. The results were explained on the basis of high angle annular dark field scanning transmission electron microscope (HAADF-STEM), scanning electron microscope (SEM) and photoluminescence (PL) measurements. Introduction of growth pause leads the QD system towards a thermodynamic equilibrium state which in turn makes interesting changes on the morphology of the samples. Coagulation of some smaller dots occurs because of ripening to produce evolved QDs and the dot density reduces with growth pause.

Modulation spectroscopy of InAs semiconductor nanostructures grown on (631) high index substrates

AbstractThe molecular beam epitaxial (MBE) growth of InAs nanostructures on GaAs(631)‐oriented substrates is studied by photoluminescence (PL) and photoreflectance spectroscopy (PR). First, a corrugated surface conformed by regularly spaced grooves aligned along the [$ \bar 5 $93] azimuth was formed by the GaAs homoepitaxial growth on the (631) substrate. On this template, we proceeded with the deposition of InAs at several thicknesses in the range of 1 to 4.5 monolayers (MLs). An atomic force microscopy (AFM) analysis of samples without GaAs capping, revealed that assemblage of QDs occurs only after the deposition of the equivalent to ∼1.9 ML of InAs. On these samples, we observed changes on the PR line‐shape in the near‐bandgap GaAs region linked to the quantity of InAs deposited. The intensity of the built in electric fields was correlated with the strain state at the heterointerface, as a consequence of the self induced piezoelectric effect, typical from high index surfaces. On the other hand, when the samples were capped with a 100 Å thick GaAs layer, strong emission of the nanostructures occurs even for deposited quantities of InAs as low as 1 ML. Since for this InAs thickness the self‐assemblage of QDs is not observed, the optical transitions observed were associated with the optical emission of self assembled semiconductor quantum wires, promoted by surface diffusion anisotropy, characteristic of the (631) plane. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hole Spin Relaxation in an Ultrathin InAs Monolayer

We investigate the spin relaxation time of holes in an ultrathin neutral InAs monolayer (1.5 ML) and compare with that of electrons, using polarization-dependent time-resolved photoluminescence (TRPL) experiments. With excitation energies above the GaAs gap, we observe a rather slow relaxation of holes (τ1h = 196 ± 17 ps) that is in the magnitude similar to electrons (τ1e = 354 ± 32 ps) in this ultrathin sample. The results are in good agreement with earlier theoretical prediction, and the phonon scattering due to spin-orbit coupling is realized to play a dominant role in the carrier spin kinetics.

Evolution of InAs quantum dots grown on cross‐hatch substrates

AbstractInAs quantum dots (QDs) are grown on In0.2Ga0.8As/GaAs cross‐hatch layer by molecular beam epitaxy. The self‐assembled QDs form with the deposition of approximately 0.8 monolayer (ML) of InAs. The QDs distribute on the cross‐hatch surface unevenly: more are found along the [1‐10] crystallographic direction than the orthogonal [110] direction, while there exists even lower density of QDs on the flat areas. Height analyses indicate that QDs first form at the hatches' intersections, then they form along the [1‐10] direction, the [110] direction, and on the flat areas, respectively. This orderly evolution of QDs is attributed to the characteristic strain distribution of the cross‐hatch surface, giving rise to position‐ and orientation‐dependent critical thickness for QD formation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin Relaxation in InAs Columnar Quantum Dots

We have investigated carrier spin dynamics in InAs columnar quantum dots (CQDs) by time-resolved photoluminescence (PL) measurement. The CQDs were formed by depositing a 1.8 monolayer InAs seed dot layer and a short-period GaAs/InAs superlattice (SL). The spin relaxation time was found to be prolonged from 1.6 to 5.3 ns by increasing the number of SL periods from 3 to 35. The PL decay time also increased from 0.93 to 1.9 ns, indicating a decrease in the spatial overlap of electron and hole wave functions. The changes in both the spin relaxation time and the PL decay time suggest that the Bir–Aronov–Pikus process is the main spin relaxation mechanism.

Investigation of the effect of growth interruption on the formation of InAs/GaAs quantum dot superlattice near the InAs critical thickness

Two kinds of superlattices (i) with and (ii) without growth interrupt (GI) after deposition of 1.77 monolayers (ML) of InAs on GaAs (001) were grown by solid-source molecular beam epitaxy (MBE) and assessed by transmission electron microscopy (TEM) techniques, double crystal X-ray diffraction (DCXRD) and photoluminescence (PL) measurements in order to gain an understanding of the structural and compositional properties. In case (i) formation of coherent dislocation free self-organized quantum dots (SOQDs) with 2.8–3.2nm height and 13–16nm lateral size was observed, whereas in case (ii) no quantum dots had formed. In order to better understand the implication of growth interruption for the formation mechanism, highly localised assessment of the composition of the QD was carried out via atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS).

Structural and optical properties of stacked self-assembled InAs∕InGaAs quantum dots on graded Si1−xGex∕Si substrate

We have investigated the effects of InAs monolayer coverage on the structural and optical characteristics of stacked InAs quantum dot (QD) layers on graded Si1−xGex∕Si substrate. No preferential InAs QDs nucleation was observed along the cross-hatched lines on the graded Si1−xGex∕Si substrate. Employing alternate-beam molecular beam epitaxy, InAs QDs with areal density as high as 7×1010cm−2 was achieved. Temperature dependence of the InAs QDs optical properties is discussed. The InAs QDs show room-temperature photoluminescence at 1.3μm with full width at half-maximum of 65nm. The results are significant for potential realization of III-V QD devices on silicon-based platforms.

Adsorption of indium on an InAs wetting layer deposited on the GaAs(001) surface

In this work, we perform a first-principles study of the adsorption properties of an In adatom deposited on a 1.75 ML( monolayer) InAs, forming a wetting layer on GaAs(001) with the ${\ensuremath{\alpha}}_{2}(2\ifmmode\times\else\texttimes\fi{}4)$ or ${\ensuremath{\beta}}_{2}(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction. Thus, we are addressing here the important case of the wetting layer at a high In coverage, just before the occurrence of the two-dimensional to three-dimensional transition. The structural properties of these reconstructions have been studied: we determine the equilibrium geometry of the surfaces and their stability for various growth conditions. We have then carried out a detailed study of the potential energy surface (PES) for an In adsorbate, finding the minima and the saddle points. The main characteristics of the PES and the bonding configurations of the In adatom on the surface are analyzed by comparing them with analogous studies reported in the literature, trying to extract the effects due to (i) the compressive strain to which the InAs adlayer is subjected, (ii) the particular surface reconstruction, and (iii) the wetting layer composition. We found that, in general, stable adsorption sites are located at (i) locations beside the As in-dimers, (ii) positions bridging two As in-dimers, (iii) between two adjacent ad-dimers (only in ${\ensuremath{\beta}}_{2}$), and (iv) locations bridging two As ad-dimers. We find also other shallower adsorption sites which are more reconstruction specific due to the lower symmetry of the ${\ensuremath{\alpha}}_{2}$ reconstruction compared to the ${\ensuremath{\beta}}_{2}$ reconstruction. We point out that the ${\ensuremath{\alpha}}_{2}$ reconstruction has the lowest symmetry among the reconstructions studied so far for this system.

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously ( n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along [ 1 ¯ 1 0 ] and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.

High-density and narrow size-distribution InAs quantum dots formed by a modified two-step growth

We develop a modified two-step method of growing high-density and narrow size-distribution InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. In the first step, high-density small InAs QDs are formed by optimizing the continuous deposition amount. In the second step, deposition is carried out with a long growth interruption for every 0.1 InAs monolayer. Atomic force microscope images show that the high-density (∼5.9×1010 cm−2) good size-uniformity InAs QDs are achieved. The strong intensity and narrow linewidth (27.7 meV) of the photoluminescence spectrum show that the QDs grown in this two-step method have a good optical quality.

Spatially Localized Formation of InAs Quantum Dots on Shallow Patterns Regardless of Crystallographic Directions

AbstractWe report on the ability to grow InAs quantum dots into patterns of any shape. We specifically demonstrate the spatial localization of InAs quantum dots on mesa and trench patterns varying from line, square and triangle patterns on GaAs (100) substrates by molecular beam epitaxy. Based on the underlying science, this growth approach enables the localization of InAs QDs on GaAs (100) by controlling the sidewall facets and InAs monolayer coverage.

Growth and characterization of InAs columnar quantum dots on GaAs substrate

The growth of InAs columnar quantum dots (CQDs) on GaAs substrates by molecular beam epitaxy was investigated. The CQDs were formed by depositing a 1.8 monolayer (ML) InAs seed dot layer and a short period GaAs/InAs superlattice (SL). It was found that the growth of the CQDs is very sensitive to growth interruption (GI) and growth temperature. Both longer GI and higher growth temperature impact the size dispersion of the CQDs, which causes the broadening of photoluminescence (PL) spectrum and the presence of the additional PL peak tails. By properly choosing the GI and the growth temperature, CQDs including GaAs (3 ML)/InAs (0.62 ML) SL with period number up to 35 without plastic relaxation were grown. The corresponding equivalent thickness of the SL is 41 nm which is two times higher than the theoretical critical thickness of the strained InGaAs layer with the same average In composition of 16%. The increase of the critical thickness is partially associated with the formation of the CQDs. Based on a five-stack CQD active region, laser diodes emitting around 1120 nm at room temperature were demonstrated, indicating a high material quality. CQDs with nearly isotropic cross section (20 nm×20 nm dimensions) were formed by depositing a 16-period GaAs (3 ML)/InAs (0.62 ML) SL on an InAs seed dot layer, indicating the feasibility of artificial shape engineering of QDs. Such a structure is expected to be very promising for polarization insensitive device applications, such as semiconductor optical amplifiers.

Influence of InAs deposition thickness on the structural and optical properties of InAs quantum wires

Abstract The influence of InAs deposition thickness on the structural and optical properties of InAs/InAlAs quantum wires (QWR) superlattices (SLS) was studied. The transmission electron microscopy (TEM) results show that with increasing the InAs deposited thickness, the size uniformity and spatial ordering of InAs QWR SLS was greatly improved, but threading dislocations initiated from InAs nanowires for the sample with 6 monolayers (MLs) InAs deposition. In addition, the zig-zag features along the extending direction and lateral interlink of InAs nanowires were also observed. The InAs nanowires, especially for the first period, were laterally compact. These structural features may result in easy tunneling and coupling of charge carriers between InAs nanowires and will hamper their device applications to some extent. Some suggestions are put forward for further improving the uniformity of the stacked InAs QWRs, and for suppressing the formation of the threading dislocations in InAs QWR SLS.

Optical properties of strain-compensated InAs/InGaAsN/GaAsN superlattices

The optical properties of heterostructures comprising InAs/InGaAsN quantum wells in strain-compensated GaAsN/InGaAsN superlattices have been studied. It is demonstrated that, using such superlattices of various design and thickness and with additional InAs monolayer spacers, it is possible to control the wavelength of room-temperature emission from InGaAsN quantum wells within 1.3–1.6 μm without deteriorating the output radiation characteristics, which opens additional prospects for the development of lasers on GaAs substrates for telecommunication applications.

Exciton spin splitting in ultrathin InAs layers

Unique spin splitting behaviors in ultrathin InAs layers, which show very different spin splitting characteristics between the InAs monolayer (ML) and submonolayer (SML) have been observed. While distinct spin splitting is observed in an InAs ML, no visible spin splitting is found in a 1∕3 ML InAs SML. In addition, the spin relaxation time in the 1∕3 ML InAs is found to be much longer than that in the 1 ML sample. These results are in good agreement with the theoretical prediction that the interexcitonic exchange interaction plays a dominant role in energy splitting, while the intraexciton exchange interaction controls the spin relaxation.

Enhancement of electron drift velocity in a quantum well by confinement of polar optical phonons

AbstractNew approaches to decrease the electron–polar optical (PO) phonon scattering rate in double heterostructures by using phonon barriers (reflective to PO phonons and transparent for electrons) are proposed. Twofold decrease of the electron–PO phonon scattering rate in the AlGaAs/GaAs/AlGaAs quantum well by inserting the InAs phonon barriers is achieved. The enhancement of high electric field drift velocity in the AlGaAs/GaAs/AlGaAs heterostructure with the inserted one monolayer InAs phonon barriers compared with that heterostructure without barriers is observed experimentally. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modification of InAs quantum dot structure during annealing

The structural modification of InAs/GaAs(0 0 1) quantum dots (QDs) grown with 2.0 or 2.7 monolayers (ML) of InAs during annealing was investigated by a combination of in situ X-ray diffraction (XRD), reflection high-energy electron-beam diffraction (RHEED) and ex situ atomic force microscopy (AFM). For 2.0 ML in coverage, QD size increased due to the incorporation of Ga atoms into the QDs, after which QD ripening occurred. For 2.7 ML in coverage, on the other hand, the change in the strain energy due to enhanced intermixing of In and Ga atoms induced a morphological transition from three dimension (3D) to two dimension (2D). These results yielded some useful information about control of the SK QD structure during growth interruption.

Spin relaxation dynamics in InAs monolayer and submonolayer

By using time-resolved photoluminescence and time-resolved Kerr rotation, we have studied the unique electron spin dynamics in InAs monolayer (ML) and submonolayer (SML), which were sandwiched in GaAs matrix. Under non-resonant excitation, the spin relaxation lifetimes of 3.4ns and 0.48ns were observed for 1/3ML and 1ML InAs samples, respectively. More interestingly, the spin lifetime of the 1/3ML InAs decreased dramatically under resonant excitation, down to 70ps, while the spin lifetime of the 1ML sample did not vary much, changing only from 400 to 340ps. These interesting results come from the different electron-hole interactions caused by different spatial electron-hole correlation, and they provide a direct evidence of the dominant spin relaxation process, i.e. the BAP mechanism. Furthermore, these new results may provide a valuable enlightenment in controlling the spin relaxation and in seeking new material systems for spintronics application.

Interface analysis of InAs/GaSb superlattice grown by MBE

We report on the structural characterization of short period type-II InAs/GaSb superlattices (SLs) adapted for mid-infrared detection. These structures, grown by molecular beam epitaxy (MBE) on n-type GaSb substrates, are made up of 10 InAs monolayers (MLs) and 10 GaSb MLs and a strain balanced condition is obtained by inserting an InSb ML at the interface between InAs and GaSb in each superlattice's period. From cross-sectional transmission electron microscopy (TEM) measurements, the interface structure is analysed in detail. Very homogeneous and smooth InAs and GaSb layers all over a 50 periods SL structure are observed and high-resolution images bring out the InSb ML inserted between InAs and GaSb. Room temperature absorbance spectroscopy measurements have been performed on strain-compensated SLs including 50, 100, 300 and 400 periods, for a total absorption zone thickness of 0.32, 0.64, 1.92 and 2.56 μm, respectively. The spectra display reproducible and well-defined features with an absorption coefficient varying between 2×10 3 and 4×10 3 cm −1 in the 3–5 μm mid-infrared domain, a sign of high-quality samples suitable for detection.