InAs Layer Thickness Research Articles

Quantification of atomic intermixing in short-period InAs/GaSb superlattices for infrared photodetectors

Cross-sectional scanning tunneling microscopy (XSTM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are applied for characterizing the MBE-grown short-period InAs/GaSb superlattices for mid- and long-wavelength infrared detection. The focus of this study is on atomic intermixing in close proximity to the interfaces, which is a key issue for device performance. HAADF-STEM permits visualization of the anion-cation dumbbells in individual sub-layers and is even capable to resolve anions and cations separated by 0.15 nm within a dumbbell. On this basis, the interfacial width is extracted with high accuracy. XSTM technique allows us direct counting of certain kinds of atoms within atomic rows, which sheds light on compositional fluctuations and isovalent atomic intermixing. Both methods reveal rather abrupt interfaces with InAs-on-GaSb interfaces being slightly wider than GaSb-on-InAs interfaces. According to HAADF-STEM, in the set of investigated samples, the 90%-to-10% interface width is confined within 2.6–4.4 monolayers (0.8–1.3 nm). In all the samples studied, XSTM analysis showed some residual concentration (on the level of a few atomic percent) of Sb atoms in the middle of the InAs sub-layers, as a result of As/Sb substitutions during growth. The level of intermixing is diminishing with increasing the thickness of individual InAs layers.

Electronic structure and optical properties of InAs/GaSb/AlSb/GaSb superlattice

The electronic structure and optical properties of InAs/GaSb/AlSb/GaSb superlattice (M-structure) are investigated in the framework of eight-band effective-mass theory, with the interface potential considered. The calculated energy gaps agree excellently with the experimental results. Via calculations, we find that the electronic structure of M-structure strongly depends on the geometrical structure of superlattice. The electron effective mass increases notably with the thickness of GaSb and AlSb layers, and AlSb layer is more favorable to obtain large electron effective mass than GaSb layer. Increased thickness of AlSb layer also leads to larger variation range of valence band maximum (VBM) and so M-structure has more tunable VBM than InAs/GaSb superlattice. Also the VBM of M-structure rises considerably with the increment of GaSb layer thickness and is almost independent of InAs layer thickness. We further find that M-structure has no remarkable superior optical absorption coefficient over InAs/GaSb superlattice. However, with larger electron effective mass and more tunable valence band maximum compared with InAs/GaSb superlattice, M-structure can be used as barrier in InAs/GaSb superlattice infrared detector to reduce the dark current. And the quantum efficiency of infrared photodiodes will not depend on the bias voltage when the M-structure is appropriately doped and carefully designed based on the dependence of its electronic structure on the superlattice geometry.

Study of surface roughness using spectral reflectance measurements recorded during the MOVPE of InAs/GaAs heterostructures

InAs layers were elaborated on semi-insulating GaAs (100) substrates in a horizontal atmospheric pressure MOVPE reactor at a temperature of 450°C. The growth process was in situ monitored by spectral reflectance (SR) in the wavelength range 200–1100nm. The best fittings of the experimental responses using the transfer matrix method (TMM) combined with the Bruggeman effective medium approximation (EMA), permit the study of the surface roughness during the growth and the determination of the InAs layer thickness at different phases of the epitaxy. The calculations were in good agreement with the ex situ spectroscopic ellipsometry (SE) measurements. The roughness modeled by an effective layer thickness, obtained indirectly from SR and SE measurements, was well correlated with direct measurements using the atomic force microscopy (AFM). Simulated roughness closely obeys a relationship of the form of deff(SR/SE)≈1.5RMS (AFM) where RMS (AFM) is the root-mean-square roughness measured by AFM.

Optical investigation of InAs/InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy

We report on the optical characteristics of InAs quantum dots based on the InP(100) substrate grown by gas source molecular beam epitaxy without assisting any other methods. The photoluminescence was carefully investigated by adjusting the thickness of InAs layers and the growth temperature. A wide range of emitting peaks is obtained with the increase in the thickness of InAs layers. In addition, we find that the morphology and shape of quantum dots also greatly depend on InAs layers. The images of atomic force microscopy show that the quantum dots like forming into quantum dashes elongated along the [01−1] direction when the thickness of InAs layers increased. A critical thickness of formation quantum dots or quantum dash is obtained. At the same time, we observe that the growth temperature also has a great impact on the emission wavelength peaks. High qualities of InAs/InP(100) quantum dots providing their emission wavelength in 1.55μm are obtained, and good performances of quantum dots lasers are fabricated.

Carrier loss mechanisms in type II quantum wells for the active region of GaSb-based mid-infrared interband cascade lasers

Thermal quenching of photoluminescence has been measured for InAs/GaInSb broken gap system quantum wells as a function of the thickness of the conduction and valence band wells. The primary activation energy appears too small for the electrons to leave the well, and insensitive to even significant changes in the InAs layer thickness and hence electron confinement potential. On the contrary, it varies strongly with the modification of the confinement of holes in the GaInSb layer. Thus, tunneling-assisted escape of holes into the GaSb layers has been recognized as the dominant carrier loss mechanism in these type II quantum wells.

Effect of layer separation, InAs thickness, and rapid thermal annealing on the optical emission from a multi-layer quantum wire structure

Multi-layer InAs quantum wire stacks with different layer separations (8, 15, and 25 nm) and InAs thicknesses (3, 4, 5, and 7 monolayers [ML]) were grown on and embedded in In0.53Ga0.27Al0.20As barrier/spacer layers lattice-matched to an InP substrate. For the samples with 4 ML of InAs and different layer separations, double peak photoluminescence was observed from quantum wire stacks separated by 8 nm, and with a 15 nm spacer layer a long wavelength component was observed extending from the main peak. Only a single peak was found as the spacer layer thickness was increased to 25 nm. For the quantum wire stacks with different InAs layer thicknesses and a separation of 8 nm, double peak photoluminescence spectra were observed in the sample with 4 ML of InAs, and a main peak with a long wavelength component was obtained from the sample with 3 ML of InAs. Only a single peak was detectable for the InAs layer thicknesses of 5 and 7 ML. The optical emission features were studied via temperature and excitation laser power dependent photoluminescence. Based on the photoluminescence and transmission electron microscopy observations, photoluminescence spectral features can be attributed to a bi-modal height distribution in certain samples. In order to extend the optical emission to room temperature, the sample with 5 ML of InAs and an 8 nm spacer layer was subjected to post-growth rapid thermal annealing at different temperatures. The emission wavelength was tunable from 1.63 to 1.72 μm at room temperature.

Effects of a InGaAs Strained Layer on Structures and Photoluminescence Characteristics of InAs Quantum Dots

The influences of a thin InGaAs layer grown on GaAs(100) substrate before deposited InAs self-assembled quantum dots(SAQDs) were experimentally investigated. Scanning electronic microscope (SEM) measurements show that the InGaAs strained layer may release the strain between wetting layer and QDs, and then enlarge size of QDs. When the thickness of InAs layer is small, the QDs are chained. Temperature dependent photoluminescence (TDPL) measurements show that the PL peaks of InAs QDs with In0.1Ga0.9As show much more red shift compared with the QDs directly deposited on GaAs matrix, and PL integral intensity enhances as T rises from 50K to 90K. We attribute this enhancement to the small potential barrier between WL and QDs produced by the InGaAs stained layer.

Electron transport properties of InAs ultrathin films obtained by epitaxial lift-off and van der Waals bonding on flexible substrates

We investigated InAs ultrathin films on flexible substrates. InAs layers grown on GaAs(001) are separated by epitaxial lift-off (ELO), followed by van der Waals bonding (VWB) on flexible substrates. We employed “normal” and “inverted” VWB; in the former, top and bottom sides are maintained during ELO and VWB, while inverted in the latter. From the InAs on flexible substrates, we fabricated Hall-bar devices with recess etch-thinning, using which electron transport properties depending on InAs layer thickness were characterized. For the inverted VWB, we observe very high electron mobilities of InAs ultrathin films, such as ∼10 000 cm2/V s for ∼100 nm thickness and ∼7000 cm2/V s for ∼20 nm. These carrier mobilities are highest not only for thin films on flexible substrates but also for InAs thin films; higher than those of InAs films grown on GaAs(111)A and membranes fabricated from them.

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.

Interfacial intermixing in InAs/GaSb short-period-superlattices grown by molecular beam epitaxy

The unique properties of the noncommon-atom InAs/GaSb short-period-superlattices (SPSL) strongly depend on the interface structure. These interfaces are characterized using transmission electron microscopy (TEM). The compositional sharpness is obtained from the comparison of the experimental contrast in g002 two-beam dark-field TEM images with simulated intensity profiles, which are calculated assuming that the element distribution profiles are described by sigmoidal functions. The interfacial intermixing, defined by the chemical width, is obtained for SPSL with different periods and layer thicknesses, even in the extreme case of nominally less than 3 ML thick InAs layers. Nominal 1 ML InSb layers intentionally inserted are also identified.

Structure and Photoluminescence Investigations of Self-Organized InAs/GaAs Quantum Wires

Self –organized InAs quantum wires (QWRs) were fabricated on the step edges of GaAs (331)A surface by molecular beam epitaxy (MBE). The atomic force microscopy (AFM) results show that the lateral size of InAs QWRs is 90 nm while the size along the step lines increasing with the thicknesses of InAs layers, amounting to 1100nm. The height of InAs QWRs varies from 7.9nm to 13nm. Photoluminescence (PL) measurements on the two samples were explored and an obvious PL peak around 967 nm was observed at 25 K. The PL intensity decreases as the temperature increases, and it will vanish above 60 K. However, the QWR sample with thicker InAs layer emits a long emission of 1100 nm -1400 nm as the temperature rises above 50 K, and a longer emission of 1400-1600nm as the temperature approaches to 100 K. We considered that the complex photoluminescence spectra were originated from the multiple energy steps. The carrier migration among the different QWRs structures intensified with temperature, and the chance rate from the higher energy levels to the lower ones which generated a stronger emission of long wavelength. The carrier dynamics of QWR samples were measured by using time resolved PL (TRPL) spectra from 25 K to 100 K. The PL decay time in the QWR structure at longer emission was found to be independent of the temperature as T<100 K, showing a typical dynamical behavior of the localized excitons.

Temperature dependent and time-resolved photoluminescence studies of InAs self-assembled quantum dots with InGaAs strain reducing layer structure

Four types of self-assembled InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy and studied via temperature-dependent and time-resolved photoluminescence (PL) spectroscopy measurements. A thin InGaAs stain reducing layer (SRL) is adopted which extends the emission wavelength to 1.3 μm and the influence of strain on QDs is investigated. The SRL releases the strain between the wetting layer and QDs, and enlarges the size of QDs, as shown by atomic force microscopy measurements. As the thickness of InAs layer decreases to 1.7 ML, the QDs with the SRL are chained to strings and the density of QDs increases significantly, which leads to an abnormal redshift of 1.3 μm PL peak at room temperature. PL peaks of InAs QDs with the SRL show redshift compared with the QDs directly deposited on GaAs matrix. The dependences of PL lifetime on the QD size, density and temperature (T) are systematically studied. It is observed that the PL lifetime of QDs is insensitive to T below 50 K. Beyond 50 K, increases and then drops at higher temperature, with a peak at TC, which was determined by the SRL and the thickness of InAs. We have also observed an obvious PL spectral redshift of the QDs with 1.7 ML InAs coverage on SRL at low T as the measuring time delays. The PL lifetime of QDs with the SRL is smaller than that of QDs without the SRL. The QDs with different densities have different PL lifetime dependence on the QDs size. These observations can be explained by the competition between the carrier redistribution and thermal emission.

Growth of vertical InAs nanowires on heterostructured substrates

We investigate the Au-assisted growth of InAs nanowires on two differentkinds of heterostructured substrates: GaAs/AlGaAs structures capped by a50 nm thick InAs layer grown by molecular beam epitaxy and a2 µm thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological andstructural properties of substrates and nanowires are analyzed by atomic force andtransmission electron microscopy. Our results indicate a promising direction for theintegration of III–V nanostructures on Si-based electronics as well as for the development ofnovel micromechanical structures incorporating nanowires as their active elements.

Optical properties of GaSb-based type II quantum wells as the active region of midinfrared interband cascade lasers for gas sensing applications

Photoreflectance and photoluminescence, supported by the energy level calculations in the eight-band k⋅p model including strain, have been used to study the optical properties of GaSb/AlSb/InAs/InGaSb/AlSb/GaSb type II quantum wells (QWs). The broad emission wavelength tunability in the midinfrared range has been demonstrated by the control of InAs layer thickness. The temperature dependent measurements have shown that the emission can still be efficient at room temperature in such structures, and that the temperature shift of the fundamental type II optical transition between 10 and 300 K can be significantly smaller than for type I QW systems.

Photoluminescence and lasing properties of InAs/GaAs quantum dots grown by metal-organic chemical vapour deposition

Photoluminescence (PL) and lasing properties of InAs/GaAs quantum dots (QDs) with different growth procedures prepared by metalorganic chemical vapour deposition are studied. PL measurements show that the low growth rate QD sample has a larger PL intensity and a narrower PL line width than the high growth rate sample. During rapid thermal annealing, however, the low growth rate sample shows a greater blueshift of PL peak wavelength. This is caused by the larger InAs layer thickness which results from the larger 2–3 dimensional transition critical layer thickness for the QDs in the low-growth-rate sample. A growth technique including growth interruption and in-situ annealing, named indium flush method, is used during the growth of GaAs cap layer, which can flatten the GaAs surface effectively. Though the method results in a blueshift of PL peak wavelength and a broadening of PL line width, it is essential for the fabrication of room temperature working QD lasers.

Interaction of low-temperature surfactant-grown InAs superlattice layers with arsenic precipitates

Double crystal X-ray diffraction (DCXRD), transmission electron microscopy (TEM) and photoluminescence (PL) measurements were used to study the effect of post-growth annealing temperature on the structural properties of novel low growth (250°C) and normal growth (450°C) temperature InAs surfactant-mediated grown materials. Under conditions of “arsenic-free” growth, high-quality InAs–GaAs superlattices are obtained at low temperatures (LT) even at thicknesses as high as 3 monolayers (MLs) for InAs. The interaction of the built-in strain fields and the point defects in LT-GaAs both before and after annealing has been studied in detail. DCXRD studies show that the thickness of the LT-grown InAs layers decreased by up to 0.7ML as the annealing temperature increased to 550°C. There is evidence from the DCXRD and TEM that the LT InAs–GaAs superlattice structure starts to distort at annealing temperatures above 450°C. In comparison, the sample grown at normal temperature, 450°C, still retained the periodicity of the superlattice layers up to an annealing temperature of 650°C without any change in the thickness of either the InAs (wetting layer) or GaAs.

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.

Controlling the Aspect Ratio of Quantum Dots: From Columnar Dots to Quantum Rods

We demonstrate the feasibility and flexibility of artificial shape engineering of epitaxial semiconductor nanostructures. Novel nanostructures including InGaAs quantum rods (QRs), nanocandles, and quantum dots (QDs)-in-rods were realized on a GaAs substrate. They were formed by depositing a short-period GaAs/InAs superlattice (SL) on a seed QD layer by molecular beam epitaxy growth. The InAs layer thickness in the SL plays an important role in obtaining the QRs. The growth of the QRs is very sensitive to growth interruption and growth temperature. By properly choosing both growth parameters, QRs with length of 41 nm corresponding to an extremely large aspect ratio of 4.1 were obtained. The evolution from a 0-D to 1-D confinement type is evidenced in the optical properties. The origin of the optical transitions from the QRs was understood by calculations of the electronic states within a fully 3-D approach in the eight-band k <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex>$\cdot$</tex> </formula> p approximation. The QRs are embedded in a GaAs matrix and are therefore free from surface traps. This feature enables high material quality and consequently their application in real devices. At room temperature, laser diodes based on QR active regions lasing around 1120 (or 1130) nm are demonstrated.

The formation of self-assembled InAs∕GaAs quantum dots emitting at 1.3μm followed by photoreflectance spectroscopy

Photoreflectance (PR) modulation spectroscopy, supported by photoluminescence (PL) and atomic force microscopy, was applied to the study of the optical properties of InAs∕GaAs structures at the transition from the typical two-dimensional epitaxial growth to three-dimensional Stranski-Krastanov mode of InAs self-assembled quantum dot (QD) formation. Room temperature photoreflectance was measured on several molecular-beam epitaxy (MBE) grown structures, with growth conditions optimized for the 1.3μm emission (an important window for telecommunication applications), differing in the nominal thickness of InAs layer from 1 to 2.5 ML (monolayer). The evolution of optical transitions connected with energy levels confined in a very thin InAs∕GaAs quantum well was observed. For a small InAs nominal layer thickness (up to the critical thickness for the formation of three-dimensional islands), the heavy (light)-hole level to electron level transitions shift towards lower energy, indicating the increase in the quantum well width. When the nominal InAs layer thickness exceeds the critical value, the transition energies remain constant. It implies the formation of the so-called wetting layer, whose thickness is independent of the amount of deposited InAs material (fully driven by the strain). Its energy level structure was calculated (exploiting the effective mass approximation, with strain effects) in order to determine the actual wetting layer thickness, which was found to be approximately 1.6 ML. The features connected with the transitions between levels confined in QDs appear in PR and PL spectra for the amount of InAs material exceeding this number. The energies of the QD transitions shift to the red when the InAs layer nominal thickness is increased from 1.7 to 2 ML (indicating the increase in the average dot sizes) but remain constant for thicker layers, which is the evidence that the additional InAs material increases the density of QDs, but not their sizes. It points out at the existence of size limitation effect in the MBE growth of self-assembled QDs.

Effects of a Shell on the Electronic Properties of Nanowire Superlattices

We compute the structural and electronic properties of core-shell InAs/GaAs nanowire superlattices using Keating's valence force field and a sp3d5s* tight-binding model. We show that the GaAs shell limits strain relaxation but homogenizes the hydrostatic strain distribution in the InAs layers. This prevents the formation of a strain-induced well in the conduction band at the surface of the nanowires, which was shown to trap the electrons in thin InAs layers (Phys. Rev. B 2006, 74, 155304). The shell, however, enhances the piezoelectric field, which increases the separation between the electrons and holes in thick InAs layers. These results emphasize the intricate links between the structural and electronic properties of strained nanowire heterostructures.