InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110) InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) composed of InAs/GaAs and InAs/AlAs monolayer superlattices were grown using molecular beam epitaxy. The substrates were misoriented (110) InP tilting 3° toward the [00 ] direction. We grew half monolayers of AlAs and GaAs and single monolayers of InAs alternately, keeping regular arrays of single monolayer steps. The structures were evaluated by transmission electron microscopy (TEM). In a transmission electron diffraction pattern from the ( 10) cross-section, we observed two types of superstructure spot pairs double-positioned in the [001] direction, indicating the formation of the intended IPSL structures. In a cross-sectional TEM dark-field image, we observed the InGaAs/InAlAs superlattice structures formed almost in the [001] direction. The mean period of the superlattices was approximately 4 nm, which was comparable to the terrace width expected from the substrate tilt angle. However, IPSL structures were not completely formed, i.e., the lateral interfaces meandered along the growth direction, and partial disorderings were often observed. The photoluminescence spectrum from the IPSL had a peak corresponding to the InGaAs (2 nm thick)/InAlAs (2 nm thick) superlattice in addition to a peak corresponding to the In_0.5Al_0.25Ga_0.25As alloy.     

Critical thickness of the 2-dimensional to 3-dimensional transition in GaSb/GaAs(001) quantum dot growth

The phase transformation from planar to quantum dot growth is driven by strain energy reduction at the cost of surface energy. By calculating and comparing the strain energies of monolayer thick GaSb and InAs films on GaAs(001), a critical thickness for the 2-dimensional to 3-dimensional phase transformation of about 1.2 ML was derived for the GaSb/GaAs quantum dot system. This value is in agreement with the direct observation of the effectively deposited amount of material using cross-sectional scanning tunneling microscopy. Deviating experimental literature values can be traced back to the neglect of the Sb-for-As exchange process.     

Epitaxial growth and characterization of InAs/GaSb and InAs/InAsSb type-II superlattices on GaSb substrates by metalorganic chemical vapor deposition for long wavelength infrared photodetectors

We report on the epitaxial growth and characterization of InAs/GaSb and InAs/InAsSb type-?? superlattices (T2SLs) on GaSb substrates by metalorganic chemical vapor deposition. For InAs/GaSb strained T2SLs, interfacial layers were introduced at the superlattice interfaces to compensate the tensile strain and hence to improve the overall material quality of the superlattice structures. The optimal morphology and low strain was achieved via a combined interfacial layer scheme with 1 monolayer (ML) InAsSb+1 ML InGaSb layers. In contrast, the InAs/InAsSb strain-balanced T2SLs allow for a relatively easy strain management and simple precursor flow switching scheme while maintaining device-quality materials. Surface root mean square roughness of 0.108 nm and a nearly zero net strain were obtained, with effective bandgaps of 147 and 94 meV determined for two sets of InAs/InAsSb strain-balanced T2SLs.     

Transmission electron microscopic evaluation of InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110)InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) consisting of InAs/GaAs and InAs/AlAs monolayer superlattices grown on slightly misoriented (110)InP substrates by molecular beam epitaxy have been structurally evaluated by transmission electron microscopy. We used (110)InP 3° tilted toward the [00 ] direction. The ISPLs were fabricated by an alternative growth of half monolayers of AlAs and GaAs and one monolayer of InAs while maintaining regular arrays of one monolayer steps on the growth surface. In electron diffraction patterns from the ( 10) cross-section, two types of superstructure spots double-positioned in the 001 direction are observed, consistent with the existence of the IPSLs. Dark-field imaging from the superstructure spots reveal a periodic diffraction contrast with an average lateral periodicity of about 4 nm, i.e., one terrace width. However, meandering of the vertical interface and partial disordering in the IPSLs are often observed. From high resolution ( 10) cross-sectional TEM images, the presence of IPSLs was also confirmed with an atomic scale resolution, although their vertical interface are meandering. In electron diffraction patterns from the (110) plan-view, extra-spots similar to those observed in the ( 10) cross-section were seen. Dark-field images from the superstructure spots indicated that the IPSLs were formed almost exactly along the 110 direction, suggesting that the steps on the growth surface are very straight along the 110 direction.     

Low-density InAs QDs with subcritical coverage obtained by conversion of In nanocrystals

We report growth of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy with low density of 2 μm^?2 by conversion of In nanocrystals deposited at low temperatures. The total amount of InAs used is about one monolayer, which is less than the critical thickness for conventional Stranski–Krastanov QDs. We also demonstrate the importance of the starting surface reconstruction for obtaining uniform QDs. The QD emission wavelength is easily tunable upon post-growth annealing with no wetting layer signal visible for short anneals. Microphotoluminescence measurements reveal well separated and sharp emission lines of individual QDs.     

Nucleation and ripening of seeded InAs/GaAs quantum dots

We have investigated the nucleation and ripening of pairs of InAs/GaAs quantum dot layers separated by thin (2–20 nm) GaAs spacer layers. Reflection high energy electron diffraction (RHEED) measurements show that the 2D–3D transition in the second layer can occur for less than 1 monolayer deposition of InAs. Immediately after the islanding transition in the second layer chevrons were observed with included angles as low as 20° and this angle was seen to increase continuously to 45±2° as more material was deposited. Atomic force microscopy showed the dot density in both layers to be the same. It is proposed that surface morphology can radically alter processes that determine the nucleation and ripening of the 3D islands.     

In-situ fabrication of three-dimensionally confined GaAs and InAs volumes via growth on non-planar patterned GaAs(001) substrates

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 100 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, >5 ML for mesa size 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.     

Characterization of InAs quantum dots on lattice-matched InAlGaAs/InP superlattice structures

Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images.     

Indium surface segregation in InGaAs-based structures prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

We present a comparative study on In surface segregation in InGaAs/GaAs structures prepared by molecular beam epitaxy (MBE) and atomic layer MBE (ALMBE) at different growth temperatures. The effect of segregation is evaluated by the energy position of exciton transitions in pseudomorphic 10 ML thick In_xGa_1−xAs/GaAs (0.15≤x≤0.30) and in 1 ML thick InAs/GaAs quantum wells. We show that: (i) In segregation decreases with the growth temperatures and is minimized at ALMBE and MBE growth temperatures lower than 260 and 340°C, respectively, and (ii) the segregation is more effective in ALMBE structures than in the MBE counterparts. The growth conditions that have been singled out allow the preparation of structures with high photoluminescence efficiencies even at the low growth temperatures required to minimize In segregation.     

Determination of the electrical properties of 2.5 nm thick silicon-based dielectric films: thermally grown SiOx

Ultra-thin (<5 nm thick) thermal oxide and oxynitride films with different compositions are candidates for complementary metal/oxide/semiconductor technology in ultra-large-scale integration (ULSI) applications. The latter are expected to offer the best compromise between nitrides and oxides. The aim of this work is to measure the electrical properties of a leaky 2.5 nm thick thermally grown oxide film using the high frequency capacitance–voltage (HF C(V)) measurements. The cleanliness and the surface roughness of the Si(1 0 0) surface were measured prior to in situ oxidation by means of, respectively, Auger electron spectroscopy (AES) and atomic force microscopy (AFM). The physical–chemical properties of the thermal oxide film were measured by AES (film thickness, composition), Fourier transform infrared spectroscopy (FTIR) (composition, vibration modes), cross-sectional transmission electron microscopy (TEM) (film thickness, homogeneity) and electron energy loss spectroscopy (EELS) (gap width determination). The results are compared to those obtained for the native oxide film and a chemical oxide film. The latter was first grown on the silicon substrate to prevent contamination and surface disorder after flash heating in vacuum prior to oxide growth. The substrate Si(1 0 0) surface cleaned in ultra-high vacuum (UHV) was then oxidized in a 10⁻³ mbar oxygen (O₂) gas pressure at 900°C to get the 2.5 nm thick oxide film. The grafting of a self-assembled insulating monolayer (SAM) of organic molecules on the grown oxide film permits us to obtain analysable capacitance as a function of voltage data. Indeed, this monolayer made up of octadecyltrichlorosilane molecules leads to a reduction of the leakage current through the aluminium/self-assembled monolayer/oxide/silicon heterostructure. The resulting current as a function of voltage data were compared to those of a standard 2.5 nm thick oxide film. The method proposed here to extract the electrical parameters of the thermal oxide film is demonstrated to be valid. We show mainly that the reduction of the leakage current through the aluminium/self-assembled monolayer/thermal oxide/silicon heterostructure is seven orders of magnitude bigger than in the case of the native oxide film.     

Stranski-Krastanov formation of InAs quantum dots monitored during growth by reflectance anisotropy spectroscopy and spectroscopic ellipsometry

In this study the successful application of reflectance anisotropy spectroscopy (RAS) and spectroscopic ellipsometry (SE) for the in-situ investigation of InAs quantum dot growth on GaAs (001) in Stranski-Krastanov growth mode is reported. Both techniques provide the precise determination of the growth mode transition from two-dimensional InAs layer to three-dimensional island growth. In order to optimize the growth conditions, spectral and time-resolved measurements were performed for different growth parameters (temperatures, growth rates and V/III ratios). For high temperature deposition large additional anisotropies, caused by clusters elongated in the [110] direction were found. Decreasing the deposition rate (0.5 to 0.125 ML/s) also results in the formation of large clusters, as decreases in reflectivity due to larger stray light losses prove. Finally, increasing the AsH₃ partial pressure leads to an earlier onset of island formation and an enhanced tendency for cluster formation.     

Alloy phase separation in InAs/InAlAs/InP nanostructure superlattices studied by finite element calculation

The alloy phase separation effect in InAlAs spacer layer of InAs/InAlAs nanostructure superlattices was studied by two-dimensional finite element calculation. The calculation results showed that InAs islands with wide top would prefer to induce “V”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer, while InAs islands with narrow top would promote the formation of “I”-like In-rich InAlAs arms above InAs islands in InAlAs spacer layer which corresponded well with the experimental results reported in Ref. [9].     

Formation of Ge-diffusion-suppressed GaAs layers and InAs quantum dots on Ge/Si substrates

Crystal growth of GaAs layers and InAs quantum dots (QDs) on the GaAs layers was investigated on Ge/Si substrates using ultrahigh vacuum chemical vapor deposition. Ga-rich GaAs with anti-site Ga atoms grown at a low V/III ratio was found to suppress the diffusion of Ge into GaAs. S-K mode QD formation was observed on GaAs layers grown on Ge/Si substrates with Ga-rich GaAs initial layers, and improved photoluminescence from 1.3 μm-emitting InAs QDs was demonstrated.     

X-ray analysis of Self-Organized InAs/InGaAs Quantum Dot Structure

We report on an X-ray study of an InAs/InGaAs/GaAs multi quantum dot stack grown by metalorganic chemical vapor deposition using grazing incidence reflectometry, high-resolution X-ray diffraction, reciprocal space mapping and pole figure analysis. No direct signal from the quantum dots is found by the high-resolution techniques. All rocking curves on different symmetric and asymmetric Bragg reflections can be simulated within the framework of dynamical theory assuming a perfect tretragonally distorted InAs/InGaAs/GaAs multiquantum well system. A pole figure analysis in the vicinity of the (113) and (022) reflections, however, reveals a signal from the quantum dots. There is a considerable indium enrichment in the quantum dots as compared to the wetting layer indicating a strong In-diffusion during their formation. Moreover, a strongly anisotropic diffuse scattering distribution with respect to the [110] and [1-10] directions is found.     

Structural and magnetic properties of Ni2MnIn Heusler thin films grown on modulation-doped InAs heterostructures with metamorphic buffer

This paper reports on the morphological, structural, magnetic, and magneto-optic properties of Ni₂MnIn Heusler films grown on InAs-high electron-mobility transistor structures (HEMT) with metamorphic buffers for spintronic applications. Similar to our previous results on the Ni₂MnIn/InAs (001) system, the Heusler layer is found to have a (110) orientation relative to the (001) InAs-HEMT surface. We observe almost equal spin-polarizations for Heusler films on (001) InAs-HEMT as well as on (001) InAs. In addition, we find further support for interfacial intermixing previously reported for the Ni₂MnIn/InAs (001) system. On the other hand, the Heusler/InAs-HEMT system shows distinct morphologic, structural, and magnetic properties as compared to the Ni₂MnIn/InAs (001) system. In particular, more rapid and complex plastic deformation effects resulting in a high surface density of pin-holes in the Heusler films are found. We report on complex mutual deformation effects between the Heusler films and the underlying InAs-HEMT structure. Furthermore, a hysteresis loop squareness close to 1 for a 50 nm Heusler film on InAs-HEMT is observed. We tentatively associate these phenomena with the higher mismatch strain of the Ni₂MnIn/InAs-HEMT system compared to Ni₂MnIn films grown on (001) InAs.     

Optimized InAs quantum effect device structures grown by molecular beam epitaxy

We have investigated InAs quantum effect devices based on both antimonides and arsenides. In an InAs quantum point contact device based on antimonides (InAs/AlGaSb), we have successfully reduced the leakage currents and observed quantum effects at around 77 K by optimizing the heterostructure growth and mesa-etched split-gate approach. Strained InAs quantum dots based on arsenides (AlInAs/AlAs/InAs/InGaAs/AlInAs) were successfully fabricated by MBE growth and mesa-etching. Blue-shifted photoluminescence was obtained from millions of quantum dots with an average lateral size of approximately 2000 å square.     

In原子在GaAs(001)表面的成核与扩散研究

近年来,半导体量子点特别是InAs量子点的基本物理性质和潜在应用得到了广泛研究。许多研究者利用InAs量子点结构的改变以调制其光电特性。本文采用液滴外延法在GaAs(001)表面沉积了不同沉积量的In(3 ML、4 ML、5 ML),以研究In的成核机制和表面扩散。实验发现,随着In沉积量的增加,液滴尺寸(包括直径、高度)明显增大。不仅如此,在相同的衬底温度下,沉积量越大,液滴密度越大。利用经典成核理论,计算了GaAs(001)表面In液滴形成的临界厚度为0.57 ML,计算的结果与已报道的实验一致。从In原子在表面的迁移和扩散,以及衬底中Ga和液滴中的In之间的原子互混原理解释了In液滴形成和形貌演化的机理。实验中得到的In液滴临界厚度以及In液滴在GaAs(001)上成核机理,可以为制备InAs量子点提供实验指导。     

Growth of InAs on GaAs(1 0 0) by low-pressure metalorganic chemical vapor deposition employing in situ generated arsine radicals

InAs was grown by low-pressure metalorganic chemical vapor deposition on vicinal GaAs(1 0 0) substrates misoriented by 2° toward [0 0 1]. We observed InAs crystal growth, at substrate temperatures down to 300°C, employing in situ plasma-generated arsine radicals as the arsenic source. The in situ generated arsine was produced by placing solid arsenic downstream of a microwave driven hydrogen plasma. Trimethylindium (TMIn) feedstock carried by hydrogen gas was used as the indium source. The Arrhenius plot of InAs growth rate vs. reciprocal substrate temperature displayed an activation energy of 46.1 kcal/mol in the temperature range of 300–350°C. This measured activation energy value is very close to the energy necessary to remove the first methyl radical from the TMIn molecule, which has never been reported in prior InAs growth to the best of authors’ knowledge. The film growth mechanism is discussed. The crystallinity, infrared spectrum, electrical properties and impurity levels of grown InAs are also presented.     

Effect of atomic hydrogen in highly lattice-mismatched molecular beam epitaxy

Effects of atomic hydrogen on the growth of lattice-mismatched InAs/GaAs and GaAs/InP systems have been investigated. The irradiation of atomic H has resulted in a delay of the onset of formation of three-dimensional islands maintaining flat surface morphology and increase of the critical layer thickness (CLT) from 4 to 10 å in the case of the InAs/GaAs system. The effect of atomic H was shown to be dependent on the growth conditions such as the growth temperature and V/III flux ratio. The increase of CLT with atomic H irradiation may be explained by the uniform distribution of the total misfit stress in the plane of the surface as a result of enhanced two-dimensional growth by atomic H acting as a surfactant.     

Facilitating growth of InAs–InP core–shell nanowires through the introduction of Al

InAs nanowires were grown on GaAs substrates by the Au-assisted vapour–liquid–solid (VLS) method in a gas source molecular beam epitaxy (GSMBE) system. Passivation of the InAs nanowires using InP shells proved difficult due to the tendency for the formation of axial rather than core–shell structures. To circumvent this issue, Al_xIn_1?xAs or Al_xIn_1?xP shells with nominal Al composition fraction of x=0.20, 0.36, or 0.53 were grown by direct vapour–solid deposition on the sidewalls of the InAs nanowires. Characterisation by transmission electron microscopy revealed that the addition of Al in the shell resulted in a remarkable transition from the VLS to the vapour–solid growth mode with uniform shell thickness along the nanowire length. Possible mechanisms for this transition include reduced adatom diffusion, a phase change of the Au seed particle, and surfactant effects. The InAs–AlInP core-shell nanowires exhibited misfit dislocations, while the InAs–AlInAs nanowires with lower strain appeared to be free of dislocations.