Lattice-mismatched Layers Research Articles

Van der Waals epitaxy of tunable moirés enabled by alloying.

The unique physics in moiré superlattices of twisted or lattice-mismatched atomic layers holds great promise for future quantum technologies. However, twisted configurations are thermodynamically unfavourable, making accurate twist angle control during growth implausible. While rotationally aligned, lattice-mismatched moirés such as WSe2/WS2 can be synthesized, they lack the critical moiré period tunability, and their formation mechanisms are not well understood. Here, we report the thermodynamically driven van der Waals epitaxy of moirés with a tunable period from 10 to 45 nanometres, using lattice mismatch engineering in two WSSe layers with adjustable chalcogen ratios. Contrary to conventional epitaxy, where lattice-mismatch-induced stress hinders high-quality growth, we reveal the key role of bulk stress in moiré formation and its unique interplay with edge stress in shaping the moiré growth modes. Moreover, the superlattices display tunable interlayer excitons and moiré intralayer excitons. Our studies unveil the epitaxial science of moiré synthesis and lay the foundations for moiré-based technologies.

Thermal oxidation of lattice mismatched Al1-xInxN films on GaN

Lattice-mismatched Al1-xInxN layers grown on GaN and with varying x are thermally oxidized to understand how alloy content affects the oxidation process and oxide films. The samples are oxidized in a horizontal tube furnace at 830 oC and 900 oC for 2 h under O2. The samples are characterized using atomic force microscopy to determine root mean square roughness before and after oxidation. The oxide thickness for each sample is determined by spectroscopic ellipsometry. The AlInN layers with less indium produce smoother oxide layers, and the oxidation rate of the samples increases with increasing indium content. Energy dispersive X-ray spectroscopy of scanning transmission electron microscopy images of the oxide layers show the In collects on or near the surface of the oxide layer. Overall, the results indicate that oxides formed from AlInN layers with less indium produce smoother oxide films and are more suitable for device applications.

Low-noise X-ray PIN photodiodes made of perovskite single crystals by solution-processed dopant incorporated epitaxial growth

X-ray photodiodes made of metal halide perovskites (MHPs) which directly convert X-ray photons into electron-hole pairs have shown advantages in low-cost and high X-ray detection sensitivity. However, lattice-mismatched blocking layers deposited by spin-coating or evaporation cannot fully limit the dark current and noise current under large reverse bias. In this work, Solution-processed dopant incorporated epitaxial growth is employed to form sufficient blocking barriers through epitaxially growing lattice-matched n-type MHPs (bismuth-doped) and p-type MHPs (silver-doped) on opposite faces of intrinsic CH3NH3PbBr2.5Cl0.5 MHP single crystals. Through energy structure design, electron/hole blocking layers can form sufficient charges barriers, the X-ray PDs exhibit a low leakage dark current density of ~ 11 nA cm−2 under large reverse electrical field (over 500 V cm−1) and noise current of 47 fAHz−0.5 (1 mm2, over 500 V cm−1). The large external electrical field enables a state-of-art response speed (fall) of 750 ns, large X-ray detection sensitivity of 36.0μCmGy−1cm−2 and the lowest detectable dose rate of 16 nGys−1(40 kVp). This work will motivate new strategies to fabricate high-performance devices based on perovskites using solution-processed methods. These founding also explore a new generation of low dose and high dynamic X-ray detectors based on MHPs.

Bandlike and localized states of extended defects in n-type In0.53Ga0.47As

In0.53Ga0.47As p + n diodes with different densities of extended defects have been analyzed by detailed structural and electrical characterization. The defects have been introduced during Metal-Organic Vapor Phase Epitaxy (MOVPE) growth by using a lattice-mismatched layer on a semi-insulating InP or GaAs substrate. The residual strain and indium content in the n-type In0.53Ga0.47As layer have been determined by high-resolution X-ray diffraction, showing nearly zero strain and a fixed indium ratio of 0.53. The deep levels in the layer have been characterized by Deep Level Transient Spectroscopy. The mean value of electron traps at 0.17 ± 0.03 eV below the conduction band minimum EC is assigned to the “localized” states of α 60° misfit dislocations; another broad electron trap with mean activation energies between EC− 0.17 ± 0.01 and 0.39 ± 0.04 eV, is identified as threading dislocation segments with “band-like” states. A high variation of the pre-exponential factor KT by 7 orders of magnitude is found for the latter when changing the filling pulse time, which can be explained by the coexistence of acceptor-like and donor-like states in the core of split dislocations in III-V materials. Furthermore, two hole traps at EV+ 0.42 ± 0.01 and EV+ 0.26 ± 0.13 eV are related to the double acceptor of the Ga(In) vacancy (VGa/In3-/2-) and 60° β misfit dislocations, respectively. Finally, the dislocation climbing mechanism and the evolution of the antisite defects AsGa/In are discussed for n-type In0.53Ga0.47As.

Preferential nucleation, guiding, and blocking of self-propelled droplets by dislocations

Lattice-mismatched layers of GaAs/InGaAs are grown on GaAs(001) using molecular beam epitaxy and subsequently heated in vacuum while the surface is imaged in situ using low-energy electron microscopy, in order to study (i) the nucleation of group-III droplets formed as a result of noncongruent sublimation and (ii) the dynamics of these self-propelled droplets as they navigate the surface. It is found that the interfacial misfit dislocation network not only influences the nucleation sites of droplets, but also exerts unusual steering power over their subsequent motion. Atypical droplet flow patterns including 90° and 180° turns are found. The directions of these dislocations-guided droplets are qualitatively explained in terms of in-plane and out-of-plane stress fields associated with the buried dislocations and the driving forces due to chemical potential and stress gradients typical of Marangoni flow. The findings would benefit processes and devices that employ droplets as catalysts or active structures such as droplet epitaxy of quantum nanostructures, vapor-liquid-solid growth of nanowires, or the fabrication of self-integrated circuits.

High-Resistivity Semi-insulating AlSb on GaAs Substrates Grown by Molecular Beam Epitaxy

Thin-film structures containing AlSb were grown using solid-source molecular beam epitaxy and characterized for material quality, carrier transport optimization, and room-temperature radiation detection response. Few surface defects were observed, including screw dislocations resulting from shear strain between lattice-mismatched layers. Strain was also indicated by broadening of the AlSb peak in x-ray diffraction measurements. Threading dislocations and interfacial misfit dislocations were seen with transmission electron microscopy imaging. Doping of the AlSb layer was introduced during growth using GaTe and Be to determine the effect on Hall transport properties. Hall mobility and resistivity were largest for undoped AlSb samples, at 3000 cm2/V s and 106 Ω cm, respectively, and increased doping levels progressively degraded these values. To test for radiation response, p-type/intrinsic/n-type (PIN) diode structures were grown using undoped AlSb on n-GaAs substrates, with p-GaSb cap layers to protect the AlSb from oxidation. Alpha-particle radiation detection was achieved and spectra were produced for 241Am, 252Cf, and 239Pu sources. Reducing the detector surface area increased the pulse height observed, as expected based on voltage–capacitance relationships for diodes.

Effects of dislocation strain on the epitaxy of lattice-mismatched AlGaInP layers

Strain fields arising from a non-uniform distribution of misfit dislocations in an underlying compositionally graded buffer are shown to be sufficiently strong to modify indium incorporation in III-phosphide light emitting layers. Composition fluctuations (xIn±0.02) in lattice-mismatched (AlyGa1−y)xIn1−xP thin films with length-scales of 5–10μm and a broadened light emission spectra are observed. Cathodoluminescence, photoluminescence and wavelength dispersive x-ray spectroscopies are used in this analysis to generate spatial maps of luminescence spectra and element distributions in metal-organic chemical vapor deposition (MOCVD) grown films. It is seen that these fluctuations due to misfit dislocations are hard to eliminate via growth-kinetics alone but can be lowered through the use of miscut substrates or spacer layers between the graded buffer layer and the active layer. A link between crosshatch surface-roughness and group-III atom distribution under group-V rich growth conditions in both AlInP and GaInP films is also demonstrated. In summary, the interaction of the dislocation strain field with the growth surface can affect the optical characteristics of lattice-mismatched LEDs even if the final threading dislocation density is low.

Determination of heteroepitaxial layer relaxation at growth temperature from room temperature X-ray reciprocal space maps

Lattice-mismatched heteroepitaxial growth in compound semiconductor layer structures, e.g. metamorphic buffers, is frequently used to combine materials with favorable properties. A detailed understanding of the strain relaxation during growth is important to optimize the buffer layers and to achieve high material quality. We present a method to determine the epilayer relaxation at growth temperature from room temperature X-ray diffraction measurements. For this the lattice parameters are measured with (004) and (224) reciprocal space maps along two orthogonal <110> directions to account for anisotropic misfit strain relaxation. Dissimilar thermal expansion coefficients of substrate and epilayer are taken into consideration when calculating the lattice parameters and epilayer relaxation parameters at growth temperature. The application of this method is discussed for GaAsxP1−x metamorphic buffer structure on Si with eight lattice-mismatched layers showing a relaxation parameter of over 130% at room temperature. All layers were found to be nearly 100% relaxed at growth temperature. The use of this method for other compound semiconductors is discussed.

Misfit dislocation formation via pre-existing threading dislocation glide in (112¯2) semipolar heteroepitaxy

Cathodoluminescence (CL) was used to study the onset of mechanical stress relaxation in low indium composition semipolar (112¯2) InxGa1−xN lattice-mismatched layers grown on bulk GaN substrates. Monochromatic CL of short interfacial misfit dislocation (MD) segments showed a single threading dislocation (TD) associated with each MD segment—demonstrating that the initial stage of MD formation in semipolar III-nitride heterostructures proceeded by the bending and glide of pre-existing TDs on the (0001) slip plane. The state of coherency as determined by panchromatic CL is also compared to that determined by x-ray diffraction analysis based on crystallographic epilayer tilt and Matthew-Blakeslee’s critical thickness calculations.

Fabrication of Hinged Mirrors Using a Strain-Driven Self-Assembly Method on a GaAs Substrate

Hinged mirror arrays are widely utilized for display applications and optical communication. They can be fabricated by an self-assembly technique using the strain in lattice-mismatched epitaxial layers. A multilayer structure including a strain-compensated layer, a digital alloy sacrificial layer and a strain bilayer, is grown by molecular-beam epitaxy. Self-assembly hinged mirrors on a GaAs substrate have been successfully fabricated by photolithography and selective etching. The hinge fabrication method with a strain bilayer is simple and flexible. Structures formed by multiple hinged plates will enable the self-assembly of more complex three-dimensional microstructures.

Low-misfit epilayer analyses using in situ wafer curvature measurements

Several benefits of in situ wafer curvature monitoring on simple structures with low misfit are discussed. The misfit of lattice-mismatched layers is measured during pseudomorphic growth, allowing for experiments that test relationships between misfit and growth conditions. As an example, Bi incorporation in GaAs is quantified by varying the substrate temperature throughout growth while using curvature measurements to continuously calculate the composition. Results agree well with x-ray diffraction measurements on individual GaAsBi samples, demonstrating the utility of this technique for the study of incorporation in mismatched systems. Once relaxation begins, the strain and dislocation energetics of low-misfit epilayers are determined from changes in wafer curvature. The authors perform several analyses on GaInAs epilayers with different misfits using an anisotropic thin film approximation. Substantial information on dislocation formation and motion is derived from the wafer curvature. Potential applications of this technique include the study of devices that utilize coherently strained layers and structures that intentionally use dislocations to relieve strain. Wafer curvature is a powerful method for comparing strain evolution in mismatched materials.

Exact Analytical Solution for the Critical Layer Thickness of a Lattice-Mismatched Heteroepitaxial Layer

Based on the Lambert W-function, an exact analytical solution for the critical thickness of a lattice-mismatched heteroepitaxial layer is presented. The new expression in exact and algebraic closed form eliminates the need for complex iterative computation. Its high accuracy is proved by comparison of the calculated critical thickness versus fractional atomic content of an alloy epilayer with the respective numerical solution.

Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight

A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell is shown to provide current-matching of all three subcells and thus composes a device structure with virtually ideal band gap combination. We demonstrate that the key for the realization of this device is the improvement of material quality of the lattice-mismatched layers as well as the development of a highly relaxed Ga1−yInyAs buffer structure between the Ge substrate and the middle cell. This allows the metamorphic growth with low dislocation densities below 106 cm−2. The performance of the approach has been demonstrated by a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5d ASTM G173–03).

Density functional calculations on the intricacies of Moiré patterns on graphite

Scanning tunneling microscopy (STM) imaging has detected a wealth of puzzling features on the surface of highly oriented pyrolytic graphite, among them, anomalously large superperiodicities, called Moir\'e patterns, caused by the lattice-mismatched top layer of graphite. Exactly, the top graphene layer rotates with respect to the graphite substrate. Such rotation gives rise to different types of local stackings in the different surface graphite regions. As STM mapping is highly dependent on the differences of local density of states of the graphite surface at the Fermi level, variations in brightness differentiate graphite regions with different local stackings. Bright areas (visible graphite areas) correspond to $AABABAB\dots{}$ local graphite stackings, whereas dark areas (hidden graphite areas) to $BABABAB\dots{}$ or $CABABAB\dots{}$ ones. We have programmed an algorithm which first built systematically the whole range of Moir\'e structures and afterwards quantified the percentages of the different local graphite stackings. Finally, periodic density functional theory calculations have been performed on a selection of Moir\'e structures in order to draw the energy profile of the rotation between two graphene layers.

Self-assembled lateral InAs quantum dot molecules: Dot ensemble control and polarization-dependent photoluminescence

By proper control of underlying templates, lateral InAs quantum dot molecules (QDMs) can be grown with a desired number of quantum dots (QDs) per molecule. We demonstrate that by capping as-grown InAs QDs with lattice-mismatched GaAs layer and by varying the capping temperature between 430 and 470 °C, we are able to control the span of nanoholes template to between 150 and 300 nm. InAs regrowth on this template result in nanopropellers (0.6 ML regrowth) or QDMs (1.2 ML), the latter with 4–12 QDs per molecule depending on the span of the underlying nanoholes template. All QDM samples grown share a characteristic feature with a clear preference to orient along the [ 1 1 ¯ 0 ] crystallographic direction. Room-temperature photoluminescence measurements show a strong polarization-dependent characteristic, in good agreements with the QDM’s geometry.

Regrowth of self-assembled InAs quantum dots on nanohole and stripe templates

Self-assembled InAs quantum dots (QDs) are grown on two types of templates by molecular beam epitaxy (MBE). A nanohole template is prepared by first growing InAs QDs on a GaAs substrate by a standard MBE process, then capping the QDs with a thin lattice-mismatched layer of GaAs. Subsequent annealing of the nanohole template results in a stripe template. We are able to transfer the patterns of self-assembled QDs onto these two types of templates: regrowth on the nanohole template results in uniform QDs situated in the nanoholes, while regrowth on the stripe template results in chains of QDs aligned along the stripes. Our results imply that self-assembled QDs can be grown onto in-situ prepared patterned substrates, with limited degree of randomness.

Self-forming silicide/SiGe-based tube structure on Si(001) substrates

Silicide/SiGe-based tube structures have been fabricated onto silicon by precise transformation from two-dimensional structures to three-dimensional objects. By using the strain in a pair of lattice-mismatched epitaxy layers, a method was developed to create the tube structure by their release from a substrate. The tube structures combining semiconductor (SiGe) and metallic silicide (NiSi 2) may find applications in advanced devices.

Quantum dot structures: Fabrication technology and control of parameters

Quantum dot (QD) semiconductor heterostructures for device applications are currently synthesized using the effect of spontaneous transformation of the growth surface at the initial stage of heteroepitaxy of lattice-mismatched layers. When a certain critical layer thickness is reached, the planar growth surface is transformed into an array of nanoscale islands, as was first demonstrated for an InAs/GaAs system. For various device applications, it is desirable to control the shape and size of individual QDs. This is achieved by variation of the effective thickness of the deposited InAs layer, deposition of several QD layers, the use of various matrix materials and a metamorphic buffer layer, and the addition of a small amount of nitrogen into QDs and the matrix material.

Growth of InAs Quantum Dots on a Low Lattice-Mismatched AlGaSb Layer Prepared on GaAs (001) Substrates

Optical communication wavelength emissions from the quantum dots (QDs) structures prepared on (001)-oriented GaAs substrates are discussed. A new growth technique of low-stressed InAs QDs on the AlGaSb layer in a low lattice-mismatched (1.3%) InAs/AlGaSb system is presented. The average height and diameter of the 4-ML InAs QDs on AlGaSb are evaluated to 5.8 nm and 45.2 nm respectively with an average density of 2.18 x 1010 /cm2 using atomic force microscope (AFM) measurements. There is structural selectivity between the QDs layer and the flat hetero-interface under changing growth conditions in the InAs/AlGaSb system. Long-wavelength PL emissions around 1.3 µm and 1.55 µm can be achieved by embedding InAs QDs in AlGaSb layers. Therefore it is expected that low-stressed InAs QDs grown on a AlGaSb layer prepared on a GaAs substrate will be useful in the fabrication of novel QDs devices for optical-communication networks.

Roughness-induced piezoelectric scattering in lattice-mismatched semiconductor quantum wells

We present a theory of the mobility of electrons in real semiconductor quantum wells (QW's) made from lattice-mismatched epitaxial layers. In the case of zinc-blende structure QW's, we prove that besides the conventional scattering mechanisms, e.g., impurity doping, surface roughness, and alloy disorder there exists an ad hoc scattering source, which is due to a large fluctuating density of roughness-induced piezoelectric charges. Scattering by their piezoelectric field is found to be a new important scattering mechanism limiting the electron mobility of real strained QW's, especially those with a well thickness of the order of or greater than 50 \AA{}. By incorporating this scattering into the theory, we are able to provide a perfect explanation for the low-temperature electron mobility measured in lattice-mismatched InGaAs-based QW's, which has not been understood starting from the so far-known scattering sources. The possibility of applying our theory to other lattice-mismatched systems such as Si/SiGe heterostructures and nitride-based QW's is outlined.