Thin InAs Layer Research Articles

Atomic-Scale Characterization of Planar Selective-Area-Grown InAs/InGaAs Nanowires.

Atomic-scale information about the structural and compositional properties of novel semiconductor nanowires is essential to tailoring their properties for specific applications, but characterization at this length scale remains a challenging task. Here, quasi-1D InAs/InGaAs semiconductor nanowire arrays were grown by selective area epitaxy (SAE) using molecular beam epitaxy (MBE), and their subsequent properties were analyzed by a combination of atom probe tomography (APT) and aberration-corrected transmission electron microscopy (TEM). Results revealed the chemical composition of the outermost thin InAs layer, a fine variation in the indium content at the InAs/InGaAs interface, and lightly incorporated element tracing. The results highlight the importance of correlative microscopy approaches in revealing complex nanoscale structures, with TEM being uniquely suited to interrogating the crystallography of InGaAs NWs, whereas APT is capable of three-dimensional (3D) elemental mapping, revealing the subtle compositional variation near the boundary region. This work demonstrates a detailed pathway for the nanoscale structural assessment of novel one-dimensional (1D) nanomaterials.

Towards Interband Cascade lasers on InP Substrate

In this study, we propose designs of an interband cascade laser (ICL) active region able to emit in the application-relevant mid infrared (MIR) spectral range and to be grown on an InP substrate. This is a long-sought solution as it promises a combination of ICL advantages with mature and cost-effective epitaxial technology of fabricating materials and devices with high structural and optical quality, when compared to standard approaches of growing ICLs on GaSb or InAs substrates. Therefore, we theoretically investigate a family of type II, “W”-shaped quantum wells made of InGaAs/InAs/GaAsSb with different barriers, for a range of compositions assuring the strain levels acceptable from the growth point of view. The calculated band structure within the 8-band k·p approximation showed that the inclusion of a thin InAs layer into such a type II system brings a useful additional tuning knob to tailor the electronic confined states, optical transitions’ energy and their intensity. Eventually, it allows achieving the emission wavelengths from below 3 to at least 4.6 μm, while still keeping reasonably high gain when compared to the state-of-the-art ICLs. We demonstrate a good tunability of both the emission wavelength and the optical transitions’ oscillator strength, which are competitive with other approaches in the MIR. This is an original solution which has not been demonstrated so far experimentally. Such InP-based interband cascade lasers are of crucial application importance, particularly for the optical gas sensing.

Inverted-Type InAlAs/InAs High-Electron-Mobility Transistor with Liquid Phase Oxidized InAlAs as Gate Insulator.

An inverted-type InAlAs/InAs metal-oxide-semiconductor high-electron-mobility transistor (MOS-HEMT) with liquid phase oxidized (LPO) InAlAs as the gate insulator is demonstrated. A thin InAs layer is inserted in the sub-channel layers of InGaAs to enhance the device performance. The proposed inverted-type InAlAs/InAs MOS-HEMT exhibits an improved maximum drain current density, higher transconductance, lower leakage current density, suppressed noise figures, and enhanced associated gain compared to the conventional Schottky-gate HEMT. Employing LPO to generate MOS structure improves the surface states and enhances the energy barrier. These results reveal that the proposed inverted-type InAlAs/InAs MOS-HEMT can provide an alternative option for device applications.

Real-time structural analysis of InGaAs/InAs/GaAs(1 1 1)A interfaces by in situ synchrotron X-ray reciprocal space mapping

Using in situ synchrotron X-ray reciprocal space mapping, this study investigates the evolution of lattice deformations in the InGaAs/GaAs(1 1 1)A structure with or without a thin InAs layer grown by molecular beam epitaxy. During the initial growth phase, InGaAs directly grown on GaAs(1 1 1)A showed an anomalous lattice shrinkage along the c-axis, with no change in the indium composition. Conversely, the InGaAs grown on InAs/GaAs(1 1 1)A showed no initial lattice distortion, but a variable indium composition. The evolution of the diffraction peak broadening was also monitored. The results confirmed that the thin InAs layer effectively improved the crystal quality during the initial growth of InGaAs.

Electrophysical and structural properties of the composite quantum wells In0.52Al0.48As/InxGa1−xAs/In0.52Al0.48As with ultrathin InAs inserts

Abstract

Evaluation of thickness and strain of thin planar layers of InAs on GaAs(001) using spectroscopic ellipsometry

We develop a technique for accurately measuring thickness of planar InAs films grown on (001) GaAs by spectroscopic ellipsometry, using bulk optical constants. We observe that the critical point structure for the E1 and E1 + Δ1 transitions extracted from the measured dielectric properties varies with strain in the layer. Transmission electron microscopy confirms the extracted thickness and measures the residual strain based on the dislocation spacing in the film. At small thickness, the E1 critical point is seen to markedly deviate from the dependence predicted by deformation potential theory and appears to be consistent with additional quantum confinement effects.

Fabrication of GaAs/Al0.3Ga0.7As Multiple Quantum Well Nanostructures on (100) Si Substrate Using a 1-nm InAs Relief Layer

Nanometer scale thin InAs layer has been incorporated between Si (100) substrate and GaAs/Al0.3Ga0.7As multiple quantum well (MQW) nanostructure in order to reduce the defects generation during the growth of GaAs buffer layer on Si substrate. Observations based on atomic force microscopy (AFM) and transmission electron microscopy (TEM) suggest that initiation and propagation of defect at the Si/GaAs interface could be suppressed by incorporating thin (1 nm in thickness) InAs layer. Consequently, the microstructure and resulting optical properties improved as compared to the MQW structure formed directly on Si substrate without the InAs layer. It was also observed that there exists some limit to the desirable thickness of the InAs layer since the MQW structure having thicker InAs layer (4 nm-thick) showed deteriorated properties.

Modeling InAs quantum-dot formation on the side surface of GaAs nanowires

We have theoretically studied the formation of InAs quantum dots (QDs) on the side surface of GaAs nanowires (NWs). The effective energies of formation of a thin InAs layer and QDs on the NW side surface are compared with allowance for elastic stresses at the radial heteroboundary of two materials with lattice mismatch. The concept of a critical thickness of the external (wetting) layer is introduced, at which the mechanical stresses stimulate three-dimensional growth of QDs. The dependence of the critical layer thickness on the NW diameter and elastic constants of the system is determined. The phenomenon of partial filling of the NW side surface by QDs is explained by a decrease in the thickness of a deposited InAs layer with increasing height. The results of modeling agree well with the available experimental data.

Monte Carlo simulation of InAs HEMTs considering strain and quantum confinement effects

We carried out Monte Carlo simulation of In0.53Ga0.47As/strained-InAs/In0.53Ga0.47As composite channel high electron mobility transistors (HEMTs) considering strain and quantum confinement effects in very thin InAs layer. We calculated the unstrained and the strained band structures of InAs. We also considered the self-consistent analysis of 2-dimensional electron gas (2DEG) by solving Schrödinger and Poisson equations. With considering the effect of 2DEG, the drain-source current Ids decreases. However, the negative threshold voltage shift due to the short-channel effects is not affected by considering 2DEG. The threshold voltage shift occurs in the region Lg/d < ~5 (Lg: gate length, d: sum of the barrier and channel layer thicknesses). We also obtained the cutoff frequency fT values. At a gate length Lg of 20 nm, the calculated fT values were 943 GHz without 2DEG and 813 GHz with 2DEG. The trend of the fT values with Lg reflects that of the electron velocities mainly. The present simulation results indicate that the record fT might be obtained by reducing Lg to around 20 nm for In0.53Ga0.47As/strained-InAs/In0.53Ga0.47As channel HEMTs.

Thin InAs membranes and GaSb buffer layers on GaAs(001) substrates

Thin InAs layers and membranes are fabricated on GaAs substrates using GaSb buffer layers grown by MOVPE. The quality of the GaSb buffer layers is optimized and epitaxial InAs layers are grown on GaSb layers of various thickness. The best GaSb buffer layers are obtained for a nucleation temperature of 450 °C and a subsequent growth temperature of 570 °C with a V/III ratio of 3, as confirmed by both the structural (high-resolution XRD, AFM) and electrical (Hall) measurements. Furthermore, a clear relationship between the structural quality of the GaSb and InAs layers is established. Finally, free-standing InAs structures are fabricated where Hall measurements reveal a mobility that depends on the film thickness.

Improvement of the GaSb/Al2O3 interface using a thin InAs surface layer

The highly reactive GaSb surface was passivated with a thin InAs layer to limit interface trap state density (Dit) at the III–V/high-k oxide interface. This InAs surface was subjected to various cleaning processes to effectively reduce native oxides before atomic layer deposition (ALD). Ammonium sulfide pre-cleaning and trimethylaluminum/water ALD were used in conjunction to provide a clean interface and annealing in forming gas (FG) at 350°C resulted in an optimized fabrication for n-GaSb/InAs/high-k gate stacks. Interface trap density, Dit≈2–3×1012cm−2eV−1 resided near the n-GaSb conductance band which was extracted and compared with three different methods. Conductance–voltage–frequency plots showed efficient Fermi level movement and a sub-threshold slope of 200mV/dec. A composite high-k oxide process was also developed using ALD of Al2O3 and HfO2 resulting in a Dit≈6–7×1012cm−2eV−1. Subjecting these samples to a higher (450°C) processing temperature results in increased oxidation and a thermally unstable interface. p-GaSb displayed very fast minority carrier generation/recombination likely due to a high density of bulk traps in GaSb.

Electron mobility and drift velocity in selectively doped InAlAs/InGaAs/InAlAs heterostructures

An increase in the electron mobility and drift velocity in high electric fields in quantum wells of selectively doped InAlAs/InGaAs/InAsAs heterostructures is obtained experimentally via controlling the composition of semiconductors forming the interface. The electron mobility at the interface in the In0.8Ga0.2As/In0.7Al0.3As metamorphic structure with a high molar fraction of In (0.7–0.8) is as high as 12.3 × 103 cm2 V−1 s−1 at room temperature. An increase in the electron mobility by a factor of 1.1–1.4 is attained upon the introduction of thin (1–3 nm) InAs layers into a quantum well of selectively doped In0.53Ga0.47As/In0.52Al0.48As heterostructures. A maximal drift velocity attains 2.5 × 107 cm/s in electric fields of 2–5 kV/cm. The threshold field Fth for the intervalley Γ-L electron transfer (the Gunn effect) in the InGaAs quantum well is higher than in the bulk material by a factor of 2.5–3. The effect of two- to threefold decrease in the threshold field Fth in the InGaAs quantum well is established upon increasing the molar fraction of In in the InAlAs barrier, as well as upon the introduction of thin InAs inserts into the InGaAs quantum well.

High quality InAs and GaSb thin layers grown on Si (1 1 1)

We have investigated growth of thin epitaxial layers of InAs on Si (1 1 1) substrates by metalorganic vapor phase epitaxy (MOVPE) and successfully grown GaSb layers on InAs. Furthermore, we have studied the effect of growth temperature and material flow on the nucleation stage and also nucleated GaSb nanowires on the underlying GaSb layer. The InAs layers are grown by a standard two-step growth approach. The main parameter under investigation is the effect of incorporating more nucleation layers into the growth process. The addition of more nucleation layers clearly correlates with the surface morphology and quality of the resulting InAs layer. Morphological and structural characterizations prove that a perfect quality InAs layer is achieved by incorporating 4 nucleation layers. Deposited layers with 250 nm thickness and 0.4 nm atomic step roughness have a mobility of 2400 cm 2/Vs and a carrier concentration of 2.7×10 18 cm −3. Moreover, we have utilized the InAs film to grow a GaSb layer and observed atomic step roughness for a thin film of ∼300 nm. Finally we demonstrate the growth of GaSb nanowires on the GaSb layer.

Electron transport in modulation-doped InAlAs/InGaAs/InAlAs and AlGaAs/InGaAs/AlGaAs heterostructures

The possibilities of enhancing the electron mobility and high-field drift velocity in channels of modulation-doped InAlAs/InGaAs and AlGaAs/InGaAs quantum wells by tuning interaction of electrons with interface phonons are tested and reviewed. A large increase in the mobility is achieved in a novel metamorphic In0.7Al0.3As/In0.8Ga0.2As structure with the high InAs content in the InAlAs barrier layer as well as in the AlxGa1-xAs/In0.2Ga0.8As structure with the low AlAs content in the AlGaAs barrier layer. An enhancement in the electron mobility by inserting thin InAs layers into the In0.52Al0.48As/In0.53Ga0.47As quantum well is obtained. The electron drift velocity saturates at high electric fields of 1.5–5 kV/cm and achieves a maximal value of 2.5 . 107 cm/s in the InGaAs quantum well with the thin InAs and GaAs inserts.

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.

Effects of group-III elements on the growth kinetics of shape-engineered InAs/InAlGaAs quantum dots

We studied the influences of the group-III elements on the shape-engineered InAs/InAlGaAs quantum dots (SEQDs) by photoluminescence (PL) spectroscopy. By alternately depositing a thin InAs layer and a thin InAlGaAs layer on an InAlGaAs buffer layer ( so called alternate growth method, AGM), the shape of QDs, especially height, was significantly manipulated. To optically investigate the effect of the introduction of Al and Ga atoms to InAs/InAlGaAs SEQDs (SEQD1), InAs/GaAs (SEQD2) and InAs/AlAs (SEQD3) SEQDs were respectively grown by using the same AGM. The emission peak of the InAs/InAlGaAs SEQDs was 1427 nm with a linewidth broadening of 36 meV at 15 K. The emission peak of the InAs/GaAs SEQDs was red-shifted by 215 nm from the SEQD1 sample. On the other hand, the emission peak for the InAs/AlAs SEQDs was blue-shifted by 111 nm from the SEQD1 sample. From the temperature-dependent PL measurements, the emission peak for the SEQD1, SEQD2, and SEQD3 samples were respectively red-shifted by 18, 5, and 40 nm with increasing temperature. The different behavior in the PL results for the SEQD1, SEQD2, and SEQD3 samples can be attributed to the different atomic distribution of the group-III elements inside the SEQDs.

Integration of vertical InAs nanowire arrays on insulator-on-silicon for electrical isolation

Vertical and electrically isolated InAs nanowires (NWs) are integrated with Si in a technique that bypasses structural defects and transport barriers at the Si–III–V NW interface. Smart-cut® technique is used to transfer a thin InAs layer onto SiO2/Si and is subsequently used for ordered organometallic vapor phase epitaxy of InAs NWs. The InAs layer in the regions between the InAs NWs is etched resulting in ordered, vertical, and electrically isolated InAs NW arrays. This transfer and fabrication technique enables heteroepitaxy of three dimensional III–V structures on Si and allows the realization of vertical devices with unprecedented control over their architectures.

Growth-interruption-induced low-density InAs quantum dots on GaAs

We investigate the use of growth interruption to obtain low-density InAs quantum dots (QDs) on GaAs. The process was realized by Ostwald-type ripening of a thin InAs layer. It was found that the optical properties of the QDs as a function of growth interruption strongly depend on InAs growth rate. By using this approach, a low density of QDs (4 dots/μm2) with uniform size distribution was achieved. As compared to QDs grown without growth interruption, a larger energy separation between the QD confined levels was observed, suggesting a situation closer to the ideal zero-dimensional system. Combining with an InGaAs capping layer such as In-rich QDs enable 1.3 μm emission at 4 K.

A comparison of transmission electron microscopy methods to measure wetting layer thicknesses to sub-monolayer precision

Electron microscopic imaging methods usually provide only indirect chemical information on thin layers. Energy-dispersive X-ray spectroscopy in a transmission electron microscope is a more direct analytical method of microanalysis but lacks atomic spatial resolution. Hence, information on ultra-thin layers must be interpreted carefully. Two approaches are compared here. They are based on either recording, in scan mode, line profiles across thin layers or, in transmission imaging mode, series of spectra with different electron beam diameters centred on the same position. The accuracy of quantification is judged using Monte Carlo simulations of electron beam broadening within a GaAs specimen. It is shown for a 0.28 nm thin InAs layer that the indirect approach based on linear regression analysis of series of spectra reliably yields sub-monolayer accuracy in the effective chemical layer width.

Pseudopotential calculations of band gaps and band edges of short-period(InAs)n∕(GaSb)msuperlattices with different substrates, layer orientations, and interfacial bonds

The band edges and band gaps of ${(\mathrm{In}\mathrm{As})}_{n}∕{(\mathrm{Ga}\mathrm{Sb})}_{m}$ $(n,m=1,20)$ superlattices have been theoretically studied through the plane-wave empirical pseudopotential method for different situations: (i) different substrates, GaSb and InAs; (ii) different point group symmetries, ${C}_{2v}$ and ${D}_{2d}$; and (iii) different growth directions, (001) and (110). We find that (a) the band gaps for the (001) ${C}_{2v}$ superlattices on a GaSb substrate exhibit a nonmonotonic behavior as a function of the GaSb barrier thickness when the number of ${(\mathrm{In}\mathrm{As})}_{n}$ layers exceed $n=5$; (b) substrate effects: compared with the GaSb substrate, the different strain field generated by the InAs substrate leads to a larger variation of the band gaps for the (001) ${C}_{2v}$ superlattices as a function of the InAs well thickness; (c) effect of the type of interfacial bonds: the In-Sb bonds at the interfaces of the (001) ${D}_{2d}$ superlattices partially pin the band edge states, reducing the influence of the confinement effects on electrons and holes, and lowering the band gaps as compared to the (001) ${C}_{2v}$ case. The valence band maximum of the (001) ${D}_{2d}$ superlattices with Ga-As bonds at the interfaces are shifted down, increasing the band gaps as compared to the (001) ${C}_{2v}$ case; (d) effect of layer orientation: the presence of In-Sb bonds at both interfaces of the (110) superlattices pin the band edge states and reduces the band gaps, as compared to the (001) ${C}_{2v}$ case. An anticrossing between the electron and hole levels in the (110) superlattices, for thin GaSb and thick InAs layers, leads to an increase of the band gaps, as a function of the InAs thickness; (e) superlattices vs random alloys: the comparison between the band edges and band gaps of the superlattices on a GaSb substrate and those for random alloys, lattice matched to a GaSb substrate, as a function of the In composition, shows that the random alloys present almost always higher band gaps and give a clear indication of the effect of superlattice's ordering and period on the behavior of the band gaps and band edges. Inclusion of interfacial interdiffusion, using the approach of Magri and Zunger [Phys. Rev. B 65, 165302 (2002)], is shown to significantly increase the band gaps relative to the predictions for abrupt superlattices, bringing the results closer to experiment. It is noteworthy that $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model fit instead measured gaps corresponding to interdiffused interfaces using a chemically abrupt model.