Surface Miscut Research Articles

Spatial arrangements and types of dislocations in interfacial networks obtained by Si(001) wafer bonding at low twist angle: A TEM characterization

Two interfacial dislocation networks were obtained in a Si–Si material system by wafer bonding process for low twist angles around 0.2°. For one sample, one of the two initial wafers was rotated with an additional angle of 90° before the bonding. TEM analyses of samples prepared for planar and transverse views show different characteristics in terms of spatial arrangement and dislocation types. Long climbing segments slightly deviated from the [100] axis and short segments a/2<101> are arranged in the dislocation network obtained with an additional rotation and at the lowest twist angle (Ψ = 0.18°) from a <110> axis. Long screw segments a/2<110> and short partial Shockley segments are arranged in the dislocation network obtained without any additional rotation at the highest twist angle (Ψ = 0.27°) from a <110> axis. In both cases, the long and short segments form elongated hexagonal patterns, which are accurately ordered in one set paving the <100> directions or randomly ordered in two perpendicular sets paving the <110> directions, respectively. The results are discussed taking into account the arangement of the surface miscut steps in the initial wafers before the bonding.

Direct visualization and control of SrOx segregation on semiconducting Nb doped SrTiO3 (100) surface

We investigated how SrOx segregates on a Nb doped SrTiO3 (100) surface by in air annealing. Using atomic force and photoemission electron microscopes, we can directly visualize the morphology and the electronic phase changes with SrOx segregation. SrOx islands less than 2 micron meter in size and 1-5 unit cells thick nucleate first and grow in a labyrinth domain pattern. After prolonged annealing, SrOx forms a ~10 nm thick film. We show that the domain pattern can be controlled by introducing a surface miscut angle of SrTiO3. Additionally, the segregated SrOx has a lower work function, compared to that of SrTiO3. These results suggest that the control and tunability of SrOx segregation is applicable to the design of a new functional electronic devices in the semiconducting SrTiO3 based heterostructure.

(Invited) Prospects for MOCVD Growth of GaN for 10kV Power PN Diodes

GaN wide-bandgap semiconductors have superior electronic material properties compared to Si and even SiC that will enable the foundational diodes and transistors for the next revolution in power management technology. To date, GaN pn diodes with 33 μm thick, lightly n-type doped drift layers have been demonstrated with breakdown voltages of 5 kV. [1] However, applications requiring higher blocking voltages could operate with greater efficiency and higher frequency if GaN based diodes and transistors could be proven. Simulations indicate that GaN pn diodes with blocking voltages of 10 kV require drift layers at least 50 μm thick and with carrier concentration less than 3x1015 cm-3. Achieving such demanding such material requirements presents significant challenges for GaN growth processesHydride Vapor Phase Epitaxy (HVPE) is the growth process of choice for very thick GaN layers due to its high growth rate (>50 μm/hr) and is a prevalent method for producing free-standing N+ GaN substrates. This growth technique and its use of carbon free precursors is a natural choice for growth of 50+ μm thick drift layers needed for 10 kV pn diodes. However, Si from the quartz growth chamber used in conventional hot-wall HVPE system leads to difficulty in controllably achieving low carrier concentrations and thus new materials and equipment designs for the growth chamber are being developed. [2] Furthermore, it is challenging to maintain smooth surface morphology of thick HPVE grown GaN epilayers and to grow p-type GaN epilayers that are needed for continuously grown pn diodes. Anode formation by epitaxial regrowth using Metal-Organic Chemical Vapor Deposition (MOCVD) or Mg implantation and annealing have yet to demonstrate multi-kilovolt diodes with reverse leakage currents equal to the best, continuously grown diodes grown by MOCVD.The low growth rate (< ~5 μm/hr) for MOCVD and the use of carbon containing precursors make it a suspect process for producing the > 50 μm thick, lightly doped (< 3x1015 cm-3) drift layers needed for 10 kV diodes. However, large substrate capacity, high up-time MOCVD reactors running highly-efficient, mature manufacturing processes developed for commercial production of visible LEDs could prove attractive for GaN pn diodes when compared to immature, quartz free HVPE reactors that to date have much lower wafer capacity. Further increasing the growth rate of the HPVE process offers diminishing returns in reducing the overall production cycle time, while modest advances in the growth rate of the MOCVD process would significantly reduce in cycle time.Since the lower growth rate of MOCVD might not be disqualifying, the concern shifts to controlling electron concentration at the low levels (< 3x1015 cm-3) in GaN epilayers where the background carbon concentration is at a similar level due to the use of carbon containing precursors. In this presentation we will discuss MOCVD growth and Si doping of GaN epilayers up to 60 μm thick and the properties of pn diodes with 50 μm thick drift layers. We find that carrier concentration is linear with Si flow down to 1x1015 cm-3, the limit of the mercury probe capacitance voltage system, and that the carrier concentration is independent of epilayer thickness from a few microns to 60 microns showing that the MOCVD process is stable for growth of thick drift layers required for 10 kV diodes. We find that controlling the surface morphology is the greatest challenge to producing 10kV-class pn diode devices and discuss how various elements such as surface mis-cut and substrate vendor affect surface morphology. While more work is needed, our studies suggest that MOCVD growth is capable of producing epilayers required for GaN pn power diodes with breakdown voltages of 10 kV.The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES and OPEN+ programs directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. H. Ohta et al., J. J. Appl. Phys., 57, 04FG09 (2018).H. Fujikura et al., Appl Phys. Lett. 117, 012103 (2020).

Influence of Step-edge induced antiphase boundary density on the magnetoresistance behavior of epitaxial Fe3O4 thin films

We have studied the influence of step-edge (SE) induced antiphase-boundaries (APB) density on the magnetoresistance (MR) behavior of epitaxial magnetite (Fe3O4) films. The APB density was varied through the step-terrace periodicity of vicinal MgO (100) surface (miscut angle between 1 and 5°). The contribution from the SEs induced APBs on the MR, ΔMR, in Fe3O4 films increases with the decrease in temperature and peaks at the Verwey transition. Magnitude of the ΔMR is found to increase with miscut angle (up to 2°) and decreases for larger miscut angles, which is related to the influence of APB related strain-fields on the degree of spin-polarization of Fe3O4 regions on terraces.

Kinetics of Structural Changes on GaSb(001) Singular and Vicinal Surfaces During the UHV Annealing

The dynamics of processes of antimony desorption was investigated on the singular and vicinal GaSb(001) surface by RHEED method. The role of the terraces edges was determined during antimony evaporation in Langmuir desorption mode. It is shown that the structural transition (2x5) -> (1x3) is a complex of two transitions - order -> disorder and disorder -> order. The influence of the degree of surface miscut from the singular face on the dimension of the transition (2x5) -> DO was studied. The activation energies of structural transitions ex(2x5) -> (2x5), (2x5) -> DO and DO -> (1x3) on singular and vicinal faces GaSb(001) were determined.

Kinetics of structural changes on GaSb(001) singular and vicinal surfaces during the UHV annealing

AbstractThe dynamics of processes of antimony desorption was investigated on the singular and vicinal GaSb(001) surface by RHEED method. The role of the terraces edges was determined during antimony evaporation in Langmuir desorption mode. It is shown that the structural transition (2 × 5) → (1 × 3) is a complex of two transitions— order → disorder and disorder → order. The influence of the degree of surface miscut from the singular face on the dimension of the transition (2 × 5) → DO was studied. The activation energies of structural transitions ex (2 × 5) → (2 × 5), (2 × 5) → DO and DO → (1 × 3) on singular and vicinal faces GaSb(001) were determined.

High uniaxial magnetic anisotropy of the Fe1−xSix films synthesized by MBE

The structure and the magnetic anisotropy of the films obtained by simultaneous deposition of iron and silicon on n-Si(111) 7×7 at 130°C are investigated. It is found the uniaxial magnetic anisotropy field for the Fe1−xSix films with x=0.25 (Fe3Si stoichiometric ratio) deposited on Si(111) 7×7 depends on both the surface miscut angle and the oblique sputtering direction and changes from 0.82Oe up to 117.26Oe.

Atomic Step Formation on Sapphire Surface in Ultra-precision Manufacturing.

Surfaces with controlled atomic step structures as substrates are highly relevant to desirable performances of materials grown on them, such as light emitting diode (LED) epitaxial layers, nanotubes and nanoribbons. However, very limited attention has been paid to the step formation in manufacturing process. In the present work, investigations have been conducted into this step formation mechanism on the sapphire c (0001) surface by using both experiments and simulations. The step evolutions at different stages in the polishing process were investigated with atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). The simulation of idealized steps was constructed theoretically on the basis of experimental results. It was found that (1) the subtle atomic structures (e.g., steps with different sawteeth, as well as steps with straight and zigzag edges), (2) the periodicity and (3) the degree of order of the steps were all dependent on surface composition and miscut direction (step edge direction). A comparison between experimental results and idealized step models of different surface compositions has been made. It has been found that the structure on the polished surface was in accordance with some surface compositions (the model of single-atom steps: Al steps or O steps).

Surface structure of the Ag-In-(rare earth) complex intermetallics

We present a study of the surface structure of the Ag-In-RE (RE: rare-earth elements Gd, Tb, and Yb) complex intermetallics using scanning tunneling microscopy and low-energy electron diffraction. The surface of the Ag-In-Yb approximant prepared by sputter-annealing methods under ultrahigh-vacuum conditions produces a flat (100) surface with no facets. However, the Ag-In-Gd and Ag-In-Tb 1/1 approximants, which have a surface miscut of about ${12}^{\ensuremath{\circ}}$ relative to the (100) plane, develop surface facets along various crystallographic directions. The structure of each facet can be explained as a truncation of the rhombic triacontahedral clusters, i.e., the main building blocks of these systems. Despite their differences in atomic structure, symmetry, and density, the facets show common features. The facet planes are In rich. The analysis of the nearest-neighbor atom distances suggests that In atoms form bonds with the RE atoms, which we suggest is a key factor that stabilizes even low-density facet planes.

Electroluminescence characteristics of blue InGaN quantum wells on m-plane GaN “double miscut” substrates

InxGa1−xN quantum wells (QWs) grown by metalorganic chemical vapor deposition on bulk m-plane GaN substrates with surface miscut −1° toward [0001] (“−1° c-miscut”) currently suffer from low indium uptake and broad luminescence linewidth in the blue spectrum. New surface “double miscuts” with miscut components in the combined a- and c-directions exhibit more uniform step flow growth in homoepitaxy than coloaded −1° c-miscut substrates. QWs grown on double miscut surfaces exhibit reproducibly higher indium uptake, narrower and more symmetric electroluminescence lineshapes in the blue spectrum, and higher output power at the peak wavelength than those grown on −1° c-miscut surfaces.

Surface-step-terrace tuned magnetic properties of epitaxial LaBaCo2O5.5+δthin films on vicinal (La,Sr)(Al,Ta)O3substrates

Interface-engineered double-perovskite structural LaBaCo2O5.5+δ thin films were highly epitaxially fabricated on (001) (La,Sr)(Al,Ta)O3 substrates with various miscut angles of 0°, 1°, 3° and 5° using a pulsed laser deposition system. All the LaBaCo2O5.5+δ thin films have a c-axis orientation with a highly epitaxial crystalline quality. Moreover, the effect of the interface tuning mechanism on the magnetic properties has been systematically investigated using physical properties measurement systems. Larger strain can be obtained by increasing the substrates’ surface miscut angles and can further enhance the magnetic properties of the LaBaCo2O5.5+δ thin films. All these results reveal that interface engineering using surface-step-terraces is an effective way of tuning/controlling the magnetic properties of LaBaCo2O5.5+δ films.

Interaction of nanometer-sized gold nanocrystals with rutile (110) surface steps revealed at atomic resolution

We show that progress in atomic resolution Z-contrast imaging now enables a detailed understanding of nanocrystal (NC) interaction with surface steps. The interaction is measured based on the shape, orientation, strain and interfacial energy of Au NCs on surface steps of rutile (TiO2) (110) surfaces with a small miscut. Au NCs with the approximate Au(111)[−110] || TiO2(110)[001] epitaxial relationship are selected for study. Results show that the presence of surface steps induces a small rotation in the NCs in an amount less than the surface miscut angle. Atomic displacements measured from atomic resolution images show significant strain near the surface steps inside Au NCs. The shape of Au NCs on low miscut surfaces, however, is approximately similar. From this, an analysis of the average NC shape is performed using the modified Wulff–Kaishew theorem. Compared to Au NCs on flat surfaces, the measurements show a large height/width ratio, lowered interfacial energy and increased triple line energy for NCs observed on surface steps.

Accurate determination of surface orientation of single-crystal wafers using high-resolution X-ray rocking curve measurements

Theoretical models of the variations of the peak positions of X-ray rocking curves as a function of the azimuthal angle of a single-crystal wafer have been proposed. These models completely describe the variations of the peak positions both when the surface normal of a wafer is parallel and when it is not parallel to the rotation axis of the goniometer used. Based on the models, an accurate measurement method for the surface orientation of a single-crystal wafer with a small surface miscut of &lt;∼3° has been proposed through rocking curve measurements using a high-resolution X-ray diffractometer. The method measures the misalignment of the sample surface normal with respect to the rotation axis of the goniometer as well as the surface orientation of the wafer. The surface orientation has been measured for a 6 inch (152.4 mm) single-crystal sapphire wafer, and the misalignment of the surface normal from the rotation axis was determined. The results were compared with those from a different method. In addition, a simple and accurate method to obtain the surface orientation of a wafer is proposed by measuring only four rocking curves, two each at two sample azimuths 180° apart.

Anisotropic relaxation and crystallographic tilt in BiFeO3 on miscut SrTiO3 (001)

Epitaxial BiFeO3 thin films on miscut (001) SrTiO3 substrates relax via mechanisms leading to an average rotation of the crystallographic axes of the BiFeO3 layer with respect to the substrate. The angle of the rotation reaches a maximum in the plane defined by the surface normal of the film and the direction of the surface miscut. X-ray microdiffraction images show that each BiFeO3 mosaic block is rotated by a slightly different angle and contains multiple polarization domains. These effects lead to a complicated overall symmetry in BiFeO3 thin films. This relaxation mechanism can be extended to other complex oxides.

Transmission Electron Microscopy and Luminescence Studies of Quantum Well Structures Resulting from Stacking Fault Formation in 4H-SiC Layers

The formation of 3C-SiC staking fault inclusions in higher band-gap other SiC polytypes provides a unique possibility to obtain quantum well (QW) structures without changing of chemical composition of epitaxial layers during growth. [1] In this report we present low temperature microphotoluminescence and transmission electron microscopy (TEM) studies of 3C-SiC stacking faults inclusions in 4H-SiC homoepitaxial layers, grown by molecular vapor deposition (MOCVD) on 4H-SiC substrates with an 8o surface miscut from the basal plane. TEM investigations showed that basal plane inclusion with cubic 3CSiC stacking faults present in the substrate propagate into the 4H-SiC layer material, forming inclined QW structures. The distance between SF was found to vary from 2 nm to 1.5 μm depending on the position on the layer. The presence of SF in the layer was confirmed by photoluminescence studies. They showed that in addition to excitonic emission typical for high quality 4H-SiC layers, well defined group of several emission lines around energy of 2.9 eV, which we attribute to QW formation induced by 3C-SiC staking faults [2]. In contrary to the emission due to nitrogen-bound excitons (observed at higher energy), which remains practically unchanged when changing position on the sample, the energy of spectral lines related to SF emission shows quasi-periodic jumps between well defined values when scanning on the sample surface (laser spot on the sample is of about 1-2 μm). The observed intensity pattern changes in a similar way. The obtained experimental data are discussed in terms of single, double and multiple quantum wells formed form neighboring 3C-SiC SF layers. It is confirmed that a concept of “structure-only” QW is promising from the point of view applications, provided precise control of FS formation during the growth of SiC based nanostructures is attainable. Our results show that, at least the formation of SF related QW structures can be successfully initiated by appropriate substrate preparation.

Si (001) surface preparation for the antiphase domain free heteroepitaxial growth of GaP on Si substrate

The present paper summarizes results of thermal annealing steps on epitaxially grown Si buffer morphology in dependence of Si offcut angle as well as miscut direction. A Si buffer layer was grown on an undulated Si substrate by vapour phase epitaxy, which provides due to its roughness deliberate off-orientations. An optimized annealing procedure under a hydrogen pressure of 950 mbar was used after growth. Atomic force investigation shows that there is a clear tendency to form bi-atomic steps along both <110> directions as long as the surface miscut is low in angle. For the off-orientations in between these two directions the surface steps delimit mono-layer high terraces. An exact (001) Si substrate with a low miscut of 0.12° towards [11̄0] was treated by the same annealing condition. The Si surface is strongly dominated by one kind of sublattice but not completely double-stepped. Using this Si wafer as a substrate for GaP heteroepitaxy by metal organic vapour phase epitaxy, an antiphase domain free GaP layer is achieved after 50 nm of overgrowth, proven by transmission electron microscopy investigation.

Self-organized growth of microsized Ge wires on Si (111) surfaces

Microsized Ge wires can appear spontaneously when grown on a vicinal Si (111) surface miscut by 4 along the [11-2] direction by using molecular-beam epitaxy. Time-resolved in situ grazing incidence small-angle scattering of x rays, atomic force microscopy, and micro-Raman scattering show that the formation of Ge microwires is due to coalescence of islands along the step edges and ripening of the structures accompanied by a partial consumption of the wetting layer.

Domain ordering of strained 5 ML SrTiO3 films on Si(001)

High resolution x-ray diffraction data indicate ordered square shaped coherent domains, ∼1200Å in length, coexisting with longer, ∼9500Å correlated regions in highly strained 5 ML SrTiO3 films grown on Si(001). These long range film structures are due to the Si substrate terraces defined by the surface step morphology. The silicon surface “step pattern” is comprised of an “intrinsic” terrace length from strain relaxation and a longer “extrinsic” interstep distance due to the surface miscut.

Early stage of Ge growth on Si(001) vicinal surfaces with an 8° miscut along[110]

The atomistic pathway towards the growth of semiconductor heterostructures on vicinal surfaces is investigated in a special experiment. A step-by-step study of the early stages of Ge deposition at T=600 degrees Celsius on a 8 degrees off Si(001) surface miscut along [110] is performed by scanning tunneling microscopy (STM). The microscopic processes occurring during growth are identified. Highly resolved STM images show how double height steps, which characterize the clean substrate, evolve by a step flow process generated by ad-dimer chains located at specific positions. This process leads to the formation of metastable single domains until the development of {105} faceted ripples extending along the whole surface in the miscut direction.

Self-organized 2D nanopatterns after low-coverage Ga adsorption on Si (1 1 1)

The evolution of the Si(1 1 1) surface after submonolayer deposition of Ga has been observed in situ by low-energy electron microscopy and scanning tunnelling microscopy. A phase separation of Ga-terminated -R 30° reconstructed areas and bare Si(1 1 1)-7 × 7 regions leads to the formation of a two-dimensional nanopattern. The shape of this pattern can be controlled by the choice of the surface miscut direction, which is explained in terms of the anisotropy of the domain boundary line energy and a high kink-formation energy. A general scheme for the nanopattern formation, based on intrinsic properties of the Si(1 1 1) surface, is presented. Experiments performed with In instead of Ga support this scheme.