InAs Monolayer Research Articles

Structure of the InAlAs/InP interface by atomically resolved energy dispersive spectroscopy

The structure of epitaxially grown InAlAs/InP interfaces was studied using atomically resolved x-ray energy dispersive spectroscopy in scanning transmission electron microscopy. As and P sublattices show sharp termination on the interface. The In sublattice is continuous across the interface. The study has shown the depletion of the Al concentration at the interface; at the last atomic columns of the InAlAs, In occupancy is close to 100%, while Al occupancy is almost zero. A monolayer of InAs at the interface is consistent with substitution of As for P at the surface preceding growth.

Atomic structure and optical properties of InAs submonolayer depositions in GaAs

Using cross-sectional scanning tunneling microscopy and photoluminescence spectroscopy, the atomic structure and optical properties of submonolayer depositions of InAs in GaAs are studied. The submonolayer depositions are formed by a cycled deposition of 0.5 monolayers InAs with GaAs spacer layers of different thicknesses between 1.5 and 32 monolayers. The microscopy images exhibit InAs-rich agglomerations with widths around 5 nm and heights of up to 8 monolayers. A lateral agglomeration density in the 1012 cm−2 range is found. During the capping of the InAs depositions a vertical segregation occurs, for which a segregation coefficient of ∼0.73 was determined. In the case of thin GaAs spacer layers, the observed segregation forms vertically connected agglomerations. The photoluminescence spectra exhibit peaks with linewidths below 10 meV and show a considerable dependence of the peak energy on the spacer thickness, even up to 32 monolayers GaAs, indicating a long range electronic coupling.

In situ STM observation during InAs growth in nano holes at 300 °C

We have successfully confirmed that In atoms were favored to congregate inside hole structures, during In and As4 irradiations, by a STMBE system which was a scanning tunneling microscope located inside a molecular beam epitaxy growth chamber. After forming 1.5 monolayer of InAs wetting layer (WL) on a GaAs(001) surface, we applied voltage at a particular site on the WL during As4 irradiation at 300°C, creating hole structures (widths: 33–66.1nm, depths: 4.9–9.7nm). With the In and As4 irradiations, spontaneously, In atoms on the WL were congregated inside the holes, decreasing the volume of the hole structures. It was found that InAs growth rates inside the hole structures were 23.1–217 times larger than that at the WL growth region near the holes.

The enhanced binding energy for biexcitons in InAs quantum dots

We observed that the biexciton binding energy in InAs quantum rhombic disks (QRDs) is enhanced by twice compared with that for InAs quantum dots (QDs) so far reported around 1.24 μm nearby the telecommunication wavelength. The heterodyne-detected four-wave-mixing detected the exciton-biexciton quantum beat superposed on photon echo decay, giving the biexciton binding energy of 3.4 meV to 3 monolayer (ML) InAs QRDs and 4.1 meV to 4 ML InAs QRDs, respectively. The largest biexciton binding energy of 4.1 meV in InAs QDs is ascribed to increased electron-hole overlap in confined geometry with a minimized strain distribution.

Noise Characterization of Midwave Infrared InAs/GaSb Superlattice pin Photodiode

We report on noise characterization of a midwave infrared (MWIR) InAs/GaSb superlattice (SL) single detector. The SL structure was made of eight InAs monolayers (MLs) and eight GaSb MLs, with a total thickness of 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> (440 SL periods). This structure exhibits a cut-off wavelength of 4.8 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> at 77 K. Extracted from current–voltage characteristics, zero-bias resistance area product <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${R} _{0}{A}$</tex> </formula> above <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$5\times 10 ^{5} ~\Omega \cdot \hbox{cm}^{2}$</tex></formula> at 80 K was measured. Noise measurements were also performed under dark conditions. The measurements reveal the absence of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$ 1/{f}$</tex></formula> noise above 30 Hz. Moreover, the detector under test remains Schottky noise-limited up to a bias voltage of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$-$</tex></formula> 200 mV typically, which confirms the quality of the MWIR SL pin photodiode.

Tuning the structural properties of InAs nanocrystals grown by molecular beam epitaxy on silicon dioxide

We report on InAs nanocrystals ( nc-InAs) grown on silicon dioxide (SiO 2) by solid-source molecular beam epitaxy. We show that the growth parameters influence the properties of the nc-InAs in terms of density, size, and crystallinity. The growth temperature influences mainly the density of the nc-InAs, whereas their size can be controlled by the number of deposited InAs monolayers. Using an adequate set of parameters, we show that the nc-InAs properties are tunable in a range where the crystal structure presents zero defects. These nc-InAs grown on SiO 2 have high crystalline quality, making them perfectly suitable for advanced electronic devices.

Implementation of an algorithmic spectrometer using Quantum Dot Infrared Photodetectors

A series of Quantum Dot Infrared Photodiodes (QDIPs) incorporating a dot-in-a-well structure was grown by Molecular Beam Epitaxy. These QDIPs were grown with 2.4 monolayers of InAs sandwiched between two 1 nm In 0.15Ga 0.85As layers which were in turn grown in a 10.85 nm GaAs quantum well with 50 nm Al 0.1Ga 0.9As barriers. Thirty stacks were grown with the quantum dot doping density of 0, 1 and 2 electron/dot. Very low dark current density below 10 −4 A cm −2 was measured at voltage range of −1.9 V to +1.5 V at room temperature in the undoped QDIPs, this is the lowest dark current reported for QDIPs at room temperature. Incorporation of doping was found to increase the dark current density at room temperature by 2–4 orders of magnitude in the QDIPs with 1 and 2 electron/dot, respectively. The undoped QDIPs were further characterised. A high peak detectivity value of 6.7 × 10 10 cm W/Hz 1/2 with a peak absorption wavelength of 6.3 μm was measured at a bias of +20 V and temperature of 77 K. Our QDIPs exhibit highly tuneable spectral response across the voltage range of −21 to +22 V, making them an excellent candidate for an algorithm based infrared spectrometer. A polyethylene sheet was used to filter the black body radiation incident on the QDIPs. Responsivity measurements as a function of voltage was incorporated into a post processing algorithm which assigns appropriate weighting factors to each responsivity value. The spectra characteristics of the polyethylene sheet with an absorption feature as narrow as 0.3 μm was successfully reconstructed using our QDIPs and algorithmic spectrometer demonstrating the potential of our QDIPs for compact, cheap and highly flexible multi and hyperspectral infrared imaging.

Effects of growth parameters on the surface morphology of InAs quantum dots grown on GaAs/Ge/Si1−xGex/Si substrate

We present the heterogeneous growth of InAs quantum dots (QDs) on Si platform using GaAs/Ge/Si1−xGex/Si substrate. Self-assembled InAs QDs were grown on GaAs/Ge/Si1−xGex/Si substrate by employing molecular beam epitaxy (MBE). The areal density, width and height of QDs were characterized using atomic force microscopy (AFM). The undulation originating from the graded Si1−xGex/Si substrate causes no noticeable change in dot densities and dot dimensions across the undulated surface. The effects of V/III ratio and InAs coverage on structural properties of the QDs were investigated. The dot density increases with increasing V/III ratio and QDs with high density of 1011cm−2 were obtained, attributed to the reduced diffusion length of the adatoms. Lateral dimension of the QDs increases as the InAs coverage increases. The QDs coalesce at 3.0 monolayer (ML) InAs coverage. This work is beneficial to those working on III–V on Si integration.

In situ STM observation of site‐ controlled InAs nano‐dot growth on GaAs(001) during In and As4 irradiations

AbstractWe have successfully fabricated site‐controlled InAs nano dots during In and As4 irradiations, observing by a STMBE: in situ scanning tunneling microscopy (STM) during molecular beam epitaxy (MBE) growth. After 1.5 monolayers (ML) of InAs wetting layer (WL) growth by ordinal Stranski‐Krastanov (S‐K) dot fabrication procedures, we applied voltage at a particular site on InAs WL during As4 irradiation at 300 °C, creating the site where In atoms, which were migrating on the WL, favored to congregate. Then, we started supplying In atoms again. After supplying 1.52 ML (1.5 + 0.02 ML) of InAs, spontaneously, In atoms congregated this site, forming a dot structure without breaking a normal MBE growth. After further continually supplying 1.66 ML (1.5 + 0.16 ML) of InAs, we also confirmed that ordinal S‐K dots were appeared. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Atomic Structures of a Monolayer of AlAs, GaAs, and InAs on Si(111)

We study atomic structures of a monolayer of AlAs, GaAs, and InAs on a Si(111) substrate from first-principles. The surface with the stacking sequence of ...SiSiMAsSiAs is energetically more stable than the surface with the stacking sequence of ...SiSiSiAsMAs, where M is Al, Ga, or In. The atomic structure of the three top layers of the low-energy surfaces are quite robust, irrespective of M, and the atomic structure of the AlAsSiAs terminated surface and that of the GaAsSiAs terminated surface are very similar. For the high-energy AsMAs terminated surfaces, the broken local tetrahedral symmetry plays an important role in the atomic structures. The calculated atomic structures of InAs on the Si(111) substrate depart most from the structure of crystalline Si.

Detection of single photons by a resonant tunneling heterostructure with a quantum dot layer

Light absorption by GaAs/AlAs heterostructures with a layer of self-assembled InAs quantum dots (QDs) at resonant tunneling through an energy-selected QD has been investigated. A high sensitivity of the current through this selected tunneling channel to the absorption of single photons with a wavelength λ ≲ 860 nm up to a temperature of 50 K is demonstrated; this sensitivity is caused by the Coulomb effect of the photoexcited holes captured by surrounding QDs on the resonance conditions. It is shown that single-photon absorption can discretely change the current through the system under study by a factor of more than 50. The captured-hole lifetimes have been measured, and a model has been developed to qualitatively describe the experimental data. It is also demonstrated that the InAs monolayer can effectively absorb photons. The properties of the heterostructure studied can be used not only to detect photons but also to design logical valves and optical memory devices.

Atomic structure and strain of the InAs wetting layer growing on GaAs(001)-c(4×4)

Using scanning tunneling microscopy, the authors studied the wetting layer evolution of InAs on GaAs(001)-c(4×4) and unraveled the different surface reconstructions during this process. At low coverages the deposited InAs material is first stored at defects and then at the hollow sites of the GaAs(001)-c(4×4) reconstruction. Close to an InAs coverage of 2/3 monolayer (ML), the whole surface abruptly reconstructs into an In2/3Ga1/3As monolayer, showing mainly a (4×3) reconstruction. Further deposited InAs is arranged in three different InAs(001)-(2×4) reconstructions on top of the In2/3Ga1/3As layer. After quantum dot occurrence above about 1.4 ML of InAs, a material transport away from the wetting layer is observed by a partial reappearance of the underlying (4×3) reconstruction. A detailed analysis of the observed reconstructions clearly shows that their specific atomic arrangements lead to a reduction of strain, while increased amounts of strain at the wetting layer start to build up above about 1.4 ML of deposited InAs, thus leading to quantum dot formation.

Low-Density InAs Quantum Dots Grown on InP(001) Using Solid-Source Molecular Beam Epitaxy with a Post-Growth Annealing Process

Low-density InAs quantum dots (QDs) with a large lateral size were grown on InP(001) by solid-source molecular beam epitaxy with a post-growth annealing process. A decrease in QD density with the amount of InAs deposited was shown by atomic force microscopy, and a density of 5×108 QDs/cm2 was obtained for 0.8 monolayer InAs deposition. Moreover, we found that the growth mechanism of QDs changed significantly depending on the amount of InAs deposited in the vicinity of critical thickness, even if the same annealing condition was applied. Near-field photoluminescence spectroscopy of single QDs demonstrated the high optical qualities of low-density QDs.

Analysis by high-resolution electron microscopy of elastic strain in thick InAs layers embedded in Ga 0.47In 0.53As buffers on InP(0 0 1) substrate

Elastic strain has been investigated by transmission electron microscopy in nanometric InAs layers grown on Ga 0.47In 0.53As/InP(0 0 1) by molecular beam epitaxy using a residual Sb flux. Deposits of 10 and 15 monolayers of InAs (3 and 4.5 nm) remain elastically stressed with a two-dimensional growth mode. The out-of-plane strain in the layers is analyzed by cross-sectional high-resolution electron microscopy. A distortion of the substrate below and on top of the InAs layers is detected and is attributed to a significant surface relaxation effect due to thinning. Surface relaxation is modeled by three-dimensional finite element modeling. An additional relaxation effect is obtained when the sample is not infinite along the direction perpendicular to the thinning. This effect enhances the buffer distortion of the buffers below and on top of the strained layers. Taking into account thin foil effects, the experimental out-of-plane strain is in excellent agreement with the theoretical value calculated for a pure InAs layer (i.e. 0.035), demonstrating the high level of strain and stress in the layers.

Quantitative investigation of the onset of islanding in strained layer epitaxy of InAs/GaAs by X-ray mapping in STEM

The Stranski-Krastanow transition from two- to three-dimensional growth describes the onset of roughening and the formation of islands. For InAs/GaAs, islanding occurs abruptly for a deposition of about one unit cell of InAs. We combine scanning transmission electron microscopy and X-ray elemental mapping to measure the layer thicknesses and chemical compositions of thin layers of InAs deposited on GaAs(001) for nominal thicknesses of 1.6, 1.8 and 2.0 monolayers of InAs. Different methods to analyse ratios of such elemental maps have been compared to measure quantitatively the indium content within these ultra-thin layers. The most accurate and reliable of the three methods investigated is shown to be the analysis of the inverse slope of plots of As/In ratio maps versus scan window size perpendicular to the layers.

Temperature dependence of hole spin relaxation in ultrathin InAs monolayers

The temperature dependence of hole spin relaxation time in both neutral and n-doped ultrathin InAs monolayers has been investigated. It has been suggested that D'yakonov–Perel (DP) mechanism dominates the spin relaxation process at both low and high temperature regimes. The appearance of a peak in temperature dependent spin relaxation time reveals the important contribution of Coulomb scatterings between carriers to the spin kinetics at low temperature, though electron–phonon scattering becomes dominant at higher temperatures. Increased electron screening effect in the n-doped sample has been suggested to account for the shortened spin relaxation time compared with the undoped one. The results suggest that hole spins are also promising for building solid-state qubits.

Comparison of single-layer and bilayer InAs/GaAs quantum dots with a higher InAs coverage

AbstractEpitaxially grown self-assembled InAs quantum dots (QDs) have found applications in optoelectronics. Efforts are being made to obtain efficient quantum-dot lasers operating at longer telecommunication wavelengths, specifically 1.3 μm and 1.55 μm. This requires narrow emission linewidth from the quantum dots at these wavelengths. In InAs/GaAs single layer quantum dot (SQD) structure, higher InAs monolayer coverage for the QDs gives rise to larger dots emitting at longer wavelengths but results in inhomogeneous dot-size distribution. The bilayer quantum dot (BQD) can be used as an alternative to SQDs, which can emit at longer wavelengths (1.229 μm at 8 K) with significantly narrow linewidth (∼16.7 meV). Here, we compare the properties of single layer and bilayer quantum dots grown with higher InAs monolayer coverage. In the BQD structure, only the top QD layer is covered with increased (3.2 ML) InAs monolayer coverage. The emission line width of our BQD sample is found to be insensitive towards post growth treatments.

Emission wavelength tuning of interband cascade lasers in the 3–4 μm spectral range

GaSb-based type-II quantum well (QW) structures and interband cascade lasers (ICLs) are investigated with regards to the dependence of emission wavelength on active QW thicknesses. Experimentally derived photoluminescence data and electrically driven ICL device data accompanied by theoretical calculations yield an average tuning rate of 0.55 μm per monolayer InAs in the range between 2.97 and 4.16 μm. Together with a temperature dependent ICL tuning behavior of 1.88 nm/K, the presented results provide the means for reliable and accurate emission wavelength control of ICLs in the 3–4 μm wavelength span which is of major importance for gas sensing applications.

Atomic structure of the (4×3) reconstructed InGaAs monolayer on GaAs(001)

Abstract We investigated the atomic structure of a surface layer forming at about 2/3 of a monolayer of InAs deposited on the GaAs(0 0 1)-c(4 × 4) surface using in situ scanning tunneling microscopy. The samples were grown by molecular beam epitaxy under arsenic-rich growth conditions at a temperature of about 450 °C. We found a surface layer terminated by three arsenic top dimers oriented along the [1 1 0] direction—similar to the arsenic dimers of the GaAs(0 0 1)-c(4 × 4) reconstruction—with additional two arsenic dimers oriented in a trench along the [ 1 ¯ 1 0 ] direction, all together forming a (4 × 3) surface unit cell. The (4 × 3) surface unit cells are arranged in-line as well as in a brick-lined alignment, then forming a c(4 × 6) reconstruction. The periodicity offset between both reconstructions along the [ 1 ¯ 1 0 ] direction is locally interrupted by an η (6 × 3) surface unit cell, composed of two pseudo-(3 × 3) unit cells beneath each other, each exhibiting only two top dimers and both together sharing three trench dimers. Rarely, also single η (2 × 3) surface unit cells were observed, characterized by only one top dimer and one trench dimer. The (4 × 3) surface unit cell consists of eight indium atoms, located at the end of the backbonds of the two arsenic trench dimers, and of four gallium atoms underneath the top dimer rows. Thus, for this layer an In 2/3 Ga 1/3 stoichiometry results. Occasionally, also single hetero-dimers are found, containing one arsenic and one group-III atom. The observed structural arrangement is further supported by previous density functional theory calculations of this surface. The occasional breaking of the periodicity along the [ 1 ¯ 1 0 ] direction also explains the often reported (1 × 3) patterns observed in reflection high-energy electron diffraction.

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.