Double-positioning Boundaries Research Articles

Influence of growth conditions on the structural perfection of β-SiC epitaxial layers fabricated on 6H-SiC substrates by vacuum sublimation

We report on the influence of the surface quality of substrate, growth rate and vapour phase composition in the growth cell on, both, the size of double position twins (and correspondingly the density of double position boundaries) in β-SiC epitaxial layers and on the perfection of interfacial layers in β-SiC/6H-SiC structures. The growth of β-SiC was done by vacuum sublimation on the (0001)Si-face of 6H-SiC Lely substrates. Epitaxial layers of β-SiC, with 5–6 mm 2 double position twins and low (10 1–10 2 cm 2) defect density, have been grown. The resulting β-SiC/6H-SiC heterostructures had no damaged intermediate layers at the interface.

Characterization of structural defects in wurtzite GaN grown on 6H SiC using plasma-enhanced molecular beam epitaxy

Thin wurtzite GaN films have been grown by plasma-enhanced molecular beam epitaxy on the basal plane of 6H SiC, with and without AlN buffer layers. Threading defects, identified from high-resolution electron micrographs as double-positioning boundaries (DPBs), originate at the substrate–buffer and/or buffer–film interfaces. The density of these faults seems to be related to the smoothness of the substrate, so that their occurrence emphasizes the importance of adequate substrate preparation. Stacking faults within the GaN are often visible parallel to the SiC substrate basal plane, sometimes terminating at the DPBs. These faults are related to the particular growth conditions, with greatly decreased density obtained for lower plasma power during GaN deposition. Growth of high quality GaN without stacking faults was achieved without using AlN buffer layers by deposition directly onto a vicinal SiC surface having a miscut angle of 4°. Such stepped substrates represent a potentially useful means for controlled growth of the DPBs, which could then serve as suitable stress-relieving defects in lieu of misfit dislocations.

Epitaxial growth of SiC thin films on Si-stabilized α-SiC(0001) at low temperatures by solid-source molecular beam epitaxy

Epitaxial growth of SiC on α-SiC(0001) substrates was carried out at relatively low temperatures (900–1000°C) and high growth rates (about 1 nm/min), by means of solid-source molecular beam epitaxy controlled by a quadrupole mass spectrometry based flux meter. The films were obtained on silicon-stabilized surfaces showing (3 × 3) and (2 × 2) superstructures. The reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM) investigations show, independent of the surface orientation, that the film growth can be quantified in two steps: In the initial stage of growth, at first the Si-determined superstructure will be formed. Despite the low temperature, growth of SiC then proceeds in a layer-by-layer mode leading to flat film-substrate interfaces. This result demonstrates the significance of surface reconstruction for the growth process, corresponding to results recently obtained using gas source molecular beam epitaxy. The films grow by stacking of laminae of α-and β-SiC, respectively, which may be attributed to fluctuations in the Si adlayer thickness. Films grown on off-oriented substrates contain many defects, likely double-positioning boundaries, directly associated with surface steps. For increasing film thickness during the film growth this boundaries may be eliminated. Films grown on well-oriented substrates show only a few of these defects. In case of increasing Si excess flux during the growth, the TEM investigations, RHEED patterns and damped RHEED-oscillations indicate an abrupt change in the growth process. This is due to the formation of Si islands on the film surface and the SiC island growth by carbonization of these islands. Only β-SiC was found in this case.

Lattice mismatch measurement of epitaxial β-SiC on α-SiC substrates

The lattice mismatch in chemically vapor deposited epitaxial β-SiC (3C-SiC) films on 6H- and 15R-SiC (0001) substrates was investigated using a high-resolution x-ray diffractometer. The misfit parallel and perpendicular to the growth plane was determined to be (Δc/c)∥=−9.3×10−4 and (Δa/a)⊥=1.9×10−4 for the 3C/6H system, and (Δc/c)∥=−10.0×10−4 and (Δa/a)⊥=2.3×10−4 for the 3C/15R system. Our analysis of the lattice parameters in these three SiC polytypes revealed that the Si-C pair spacings along the c direction increased with substrate hexagonality, while the lattice spacings along the a direction decreased with hexagonality. The extent of relaxation was greater in 3C films grown on 6H substrates, a phenomenon attributed to a higher density of double position boundaries.

Initial stage of aluminum nitride film growth on 6H-silicon carbide by plasma-assisted, gas-source molecular beam epitaxy

The initial stage of AlN film growth on 6H-SiC(0001) substrates by plasma-assisted, gas source molecular beam epitaxy (PAGSMBE) has been investigated in terms of growth mode and interface defects. Cross-sectional high resolution transmission electron microscopy (HRTEM) was used to observe the microstructure of the deposited films and the AlN/SiC interfaces. Surface morphologies and interface atomic structures were compared between films grown on vicinal and on-axis surfaces. Essentially atomically flat AlN surfaces were obtained using on-axis substrates. This is indicative of two-dimensional growth to a thickness of ∼15 Å. Islandlike features were observed on the vicinal surface. The coalescence of these features at steps gave rise to double positioning boundaries (DPBs) as a result of the misalignment of the Si/C bilayer steps with the Al/N bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of DPBs formed at the outset of growth.

Terrace growth and polytype development in epitaxial β-SiC films on α-SiC (6H and 15R) substrates

Epitaxial β-SiC (3C) films were grown on (0001) 6H-SiC and 15R-SiC substrates by chemical vapor deposition (CVD). TEM characterization revealed that films on both substrates exhibited large areas of atomically flat, coherent interfaces. However, when 3C-SiC films were grown on 6H substrates, double position boundaries (DPB's) were frequently observed, and islands of 6H were occasionally embedded in the predominantly 3C film. In contrast, films of 3C-SiC grown on 15R substrates exhibited relatively few DPB's and only occasional islands of 15R. A model of interlay er interactions in SiC was applied to predict the atomic structures at both 3C/6H and 3C/s15R interfaces, and these predictions were consistent with experimental observations of the interfaces by TEM. The observed interface structures and defect distributions were attributed to a microscopic kinetic mechanism of terrace growth. Consideration of step energies and growth kinetics led to the prediction that DPB's can be avoided by growing 3C-SiC films on 15R-SiC substrates.

Interface structures of epitaxial β-SiC on α-SiC substrates

Epitaxial β-SiC (3C) films were grown on (0001) 6H-SiC and 15R-SiC substrates by chemical vapor deposition (CVD). Transmission electron microscopy (TEM) characterization revealed that both films exhibited large areas of atomically flat, coherent interfaces. These areas were sometimes separated by structural ledges that were integral numbers of α-SiC unit cells in height. When 3C-SiC films were grown on 6H substrates, the primary defects were double position boundaries (DPBs), and islands of 6H were occasionally embedded in the predominantly 3C film. Films of 3C-SiC grown on 15R substrates exhibited relatively few grain boundaries, although islands of 15R were sometimes observed. The effect of substrate surface terraces on the observed interfacial microstructures and on defect formation mechanisms is discussed.

Initial Stages of Growth of Thin Films of III-V Nitrides and Silicon Carbide Polytypes by Molecular Beam Epitaxy

ABSTRACTThe morphology and interface chemistry occurring during the initial deposition of BN, AlN and GaN films via metal evaporation and N2 decomposition under UHV conditions have been determined. FTIR spectroscopy and TEM revealed the consecutive deposition of an initial 20Å layer of a-BN, 20–60Å of oriented h-BN, and a final layer of polycrystalline c-BN. This sequence is attributed primarily to increasing intrinsic compressive stress in the films. XPS analysis revealed the growth of GaN on sapphire to occur via the Stranski-Krastanov mode; growth on SiC showed characteristics of three-dimensional growth. AlN grew layer-by-layer on both substrates. Vicinal 6H-SiC(0001) substrate surfaces contain closely spaced, single bilayer steps. During deposition of Si and C at 1050°C, 6H layers initially form and step bunching occurs. The latter phenomenon results in more widely spaced steps, the nucleation of 3C-SiC both on the new terraces and at the larger steps and formation of double position boundaries. The C/Si ratio in the gaseous reactants also affects the occurrence of these three phenomena.

Controlled growth of 3C-SiC and 6H-SiC films on low-tilt-angle vicinal (0001) 6H-SiC wafers

We have found that, with proper pregrowth surface treatment, 6H-SiC single-crystal films can be grown by chemical vapor deposition (CVD) at 1450 °C on vicinal (0001) 6H-SiC with tilt angles as small as 0.1°. Previously, tilt angles of greater than 1.5° were required to achieve 6H on 6H at this growth temperature. In addition, 3C-SiC could be induced to grow within selected regions on the 6H substrate. The 3C regions contained few (or zero) double-positioning boundaries and a low density of stacking faults. A new growth model is proposed to explain the control of SiC polytype in this epitaxial film growth process.

Growth of improved quality 3C-SiC films on 6H-SiC substrates

Previously reported chemical vapor deposition of 3C-SiC on 6H-SiC has resulted in films with a high density of double positioning boundaries (DPBs). We have found that growth on as-grown faces of 6H-SiC crystals can yield films that are largely free of DPBs. The (111) 3C-SiC films, up to 12 μm thick, were evaluated by optical and electron microscopy and low-temperature photoluminescence (LTPL). The LTPL spectra of the films were similar to those of high quality Lely-grown 3C-SiC.

Heteroepitaxial growth and characterization of GaAs on silicon-on-sapphire and sapphire substrates

A comparative study pertaining to the molecular beam epitaxial growth of GaAs directly on silicon-on-sapphire and (101̄2) sapphire substrates has been made. Thermal strain is significantly reduced in these heteroepitaxial GaAs films as compared with GaAs on silicon; however, the (111) GaAs epitaxy on nominal (101̄2) sapphire contains double-position boundaries. Hall effect measurements and current-voltage characteristics from metal-semiconductor contacts show that GaAs grown on silicon-on-sapphire is superior to GaAs grown on (101̄2) sapphire under the experimental conditions employed.

Deep-Level Dominated Current-Voltage Characteristics of Novel Semiconductor Heterostructures

AbstractCurrent-voltage characteristics of Au contacts formed on buried implanted oxide silicon-on-insulator structures and molecular beam epitaxially grown GaAs on (1012) sapphire and silicon-on-sapphire substrates indicate that the dominant transport mechanism in these films is space-charge-limited current conduction in the presence of deep-level states. The deep-level parameters, determined using an analysis of the current-voltage characteristics, appear to be sensitive to the nature of crystallographic defects present in the grown layers. Conduction in the GaAs film on SOS was dominated by one discrete state located ~ 0.28eV below the conduction band-edge, which is close to the El center uniquely observed in the molecular beam epitaxially grown GaAs-on-Si. Discrete levels are also observed in annealed buried implanted oxide silicon-on-insulator films. In contrast, the GaAs films deposited directly on (1012) sapphire substrates and rapid-thermally annealed high-dose As implanted buried oxide SOI films appear to have a continuous distribution of states. The distributed states in GaAs films deposited directly on sapphire probably arise from the electrical activity of the double-position boundaries present in this material system.

An examination of double positioning boundaries and interface misfit in beta-SiC films on alpha-SiC substrates

Beta-SiC thin films epitaxially grown on 6H-SiC (0001) substrates were examined via scanning-reflection x-ray topography (XRT) and x-ray rocking curve analysis. The lattice misfit between the (111) plane of the β-SiC epitaxial layer and the (0001) plane of the 6H-SiC substrate was determined to be 7.6×10−4. Boundaries separating regions of the β-SiC film rotated from each other by 60° were observed and identified by XRT as double positioning boundaries (DPBs), which are a special type of twin boundary. The XRT maps showed that the mosaic structure examined via optical microscopy was also caused by the DPBs. Finally, many stacking faults were generated at the boundaries as revealed by plan-view transmission electron microscopy, indicating the high internal energy of these DPBs. The formation mechanisms of the DPBs is also discussed.

The direct observation of atomic surface structure and inclined planar defects in Au(111) films

With a high resolution electron microscope it is possible to image directly the atomic surface lattice of vapor-deposited Au(111) films. The contrast of the surface atoms can be either black or white relative to the background which is characteristic of close-packed planes of atoms in the stacking sequence ABCABC…. An analysis of these images was performed using multislice dynamical diffraction computations of 256 × 256 = 65 536 reciprocal space points and subsequent image simulations. The effects of top and bottom surface roughness, different termination layers and the contrast compared with single-atom imaging are considered. The identification of atomic structure detail at incoherent double-position boundaries in these (111) films was also achieved. Surface layer terminations, variations in the choice of every third matching plane and the possibility that twinning planes produce boundaries with jogs were investigated. Multislice computer programs were designed to calculate the diffraction effects from these inclined boundaries using two-dimensional fast Fourier transform and shift techniques of the projected potential. This is the first time that inclined planar defects have been analyzed in this manner.

The Direct Observation of Atomic Surface Structure and Inclined Planar Defects in Au(111) Films

With a high resolution electron microscope it is possible to image directly the atomic surface lattice of vapor-deposited Au(111) films. The contrast of the surface atoms can be either black or white relative to the background which is characteristic of close-packed planes of atoms in the stacking sequence ABCABC.... An analysis of these images was performed using multislice dynamical diffraction computations of 256 × 256 = 65 536 reciprocal space points and subsequent image simulations. The effects of top and bottom surface roughness, different termination layers and the contrast compared with single-atom imaging are considered.The identification of atomic structure detail at incoherent double-position boundaries in these (111) films was also achieved. Surface layer terminations, variations in the choice of every third matching plane and the possibility that twinning planes produce boundaries with jogs were investigated. Multislice computer programs were designed to calculate the diffraction effects from these inclined boundaries using two-dimensional fast Fourier transform and shift techniques of the projected potential. This is the first time that inclined planar defects have been analyzed in this manner.

The structure and growth of PbTe films on mica substrates

Abstract Epitaxial films of PbTe were produced by vacuum evaporation onto air-cleaved mica. Electron and x-ray diffraction shows that the films are well oriented with {111} planes parallel to the substrate, although transmission electron micrographs indicate the presence of double-positioning boundaries. Shadowed carbon replicas were used to examine the mechanism of film growth. Both film structure and electrical mobility were studied as a function of substrate temperature during deposition. At growth temperatures above 400°c, the mobility of molecules on the surface of PbTe films becomes very high, and the surface breaks up into a system of {100} facets. The deposition temperature giving the highest Hall mobility is about 250°c.

Epitaxial Growth of Gold on Cadmium Iodide

AbstractThe epitaxial growth of gold on (0001) surfaces of cadmium iodide is studied by transmission electron microscopy. A method for the preparation of suitable substrates is given. Dark field studies of different stages of film growth show that the deposit nucleates with a particular orientation, and during further growth this orientation changes by 30°. The diffraction pattern contains many extra twin and double diffraction spots which arise from twins and double position boundaries introduced during island coalescence.

Moiré patterns and coherent double-positioning boundaries in {111} epitaxial gold films

Abstract The double-positioning structure of discontinuous evaporated {111} gold layers grown on MoS2 substrates has been studied in detail. The two twin-related orientations of the deposit are nucleated independently on the substrate. At a later stage of growth, after coalescence has occurred, deposit islands are seen to contain both of the orientations, separated by a double-positioning boundary. It is shown, by a detailed analysis of moire patterns formed between the Au and MoS2 crystals, that double-positioning boundaries frequently lie parallel to the substrate surface and so exist as coherent twin boundaries.