Plan-view Transmission Electron Microscopy Research Articles

Electrical properties of GaN grown on a-plane GaN template by plasma-assisted molecular beam epitaxy

a-Plane GaN templates were grown on r-plane sapphire substrate by metalorganic vapor phase epitaxy (MOVPE) and then undoped a-plane GaN layers were grown at different growth temperatures on the a-plane GaN templates by plasma-assisted molecular beam epitaxy (PA-MBE). In order to investigate the effect of growth temperature on basal stacking fault (BSF) density and the effect of BSFs on electrical properties of a-plane GaN, background carrier concentration and electron mobility were measured by Hall measurement at room temperature. BSF density and dislocation density were also evaluated by plan-view transmission electron microscope. As the growth temperature increased from 740 to 800 °C, the background carrier concentration decreased from 1.4×10 19 to 3.2×10 17 cm −3. It was observed that the electron mobility ( μ c ) along c-axis [0 0 0 1] direction is larger than that ( μ m ) along m-axis [1 1̄ 0 0] direction except the sample grown at 780 °C. The electron mobility ratio ( μ c / μ m ) is 0.98 for the sample grown at 780 °C. Anisotropic and isotropic electrical property of a-plane GaN grown by PA-MBE is related to the BSF density. As the growth temperature was increased from 740 to 800 °C, the BSF density increased from 4.5×10 5 to 7.91×10 5 cm −1. In the case of isotropic electron mobility of the sample grown at 780 °C, the maximum BSF density was observed. Therefore, isotropic electron mobility results mainly from the carrier scattering by the BSFs along c-axis direction. As the dislocation density was increased, the BSF density also increased.

Magnetic properties of Fe62Co19Ga19 films deposited on Si(100) substrates

Fe81–XCoXGa19 films, with x = 0−23 atom % Co, were deposited on Si(100) substrates, respectively, by the dc magnetron sputtering method. Film thickness (tf) ranged from 30 to 400 nm. We have done the following experiments on those films: saturation magnetostriction (λS), high-temperature magnetic hysteresis, x-ray diffraction (XRD), atomic force microscopy, Auger-depth profile analysis, and plane-view and/or cross-section transmission electron microscopy. Our main finding is that if x increases from 0 to 19 atom % Co, λS increases from 56 to 152 ppm, and if x continues to increase from 19 to 23 atom % Co, λS decreases. These results indicate that the addition of Co in the Fe81Ga19 alloy is advantageous in enhancing λS, if x is the correct value. From XRD, the Fe62Co19Ga19/Si(100) film comprises the A2 and B2 phases. The mechanism for large enhancement of λS in Fe62Co19Ga19/Si(100) is related to an intrinsic origin. The tf dependence of λS for the Fe62Co19Ga19 films is that: [1] in the range of 110 ≤ tf ≤ 400 nm, λS increases as tf decreases; [2] in the range of 30 ≤ tf ≤ 110 nm, λS decreases as tf continues to decrease. The λS behavior in the regions [1] and [2] is explained based on the modified Neel model. Other physical properties of the Fe62Co19Ga19 films include the following: saturation magnetization 4πMS = 1.8 – 2.0 T, coercivity HC = 35 – 64 Oe, Curie temperature TC = 597 °C, planar (mean) grain size DP = 29.6 nm, and columnar grain size DL ≈ tf.

Aluminum nitride grown on lens shaped patterned sapphire by hydride vapor phase epitaxy

AbstractThick c‐plane aluminum nitride (AlN) films were grown directly onto lens shaped patterned c‐plane sapphire substrates by hydride vapor phase epitaxy (HVPE). The sapphire patterning consisted of convex hemispherical lenses in a hexagonal pattern. These films were fully coalesced, planar, colorless, specular and up to 20 μm thick. Full widths at half maximum of X‐ray rocking curves for the on‐axis (0002) and off‐axis (20$ \bar 2 $1) AlN reflections for fully coalesced films reached 914 and 1502 arcsec respectively. Plan view transmission electron microscopy was used to determine the threading dislocation density which was ∼8 x 108 cm‐2. An interrupted growth series was also performed to evaluate the growth evolution of the AlN. Scanning electron microscopy was used to examine the initial growth and coalescence of the films. The AlN was determined to grow in hexagonal pillars initiated on the lenses, eventually coalescing together. These AlN films offer a promising substrate for AlGaN based UV optoelectronics thanks to both its transparency in the UV region and patterning effects. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of TbAs:GaAs nanocomposites

Recently, there has been interest in semimetallic rare earth monopnictide nanoparticles epitaxially embedded in III-V semiconductors due to the drastic changes brought about in these materials’ electrical and thermal properties. The properties of terbium codeposited with gallium arsenide by molecular beam epitaxy are discussed here. These new materials were characterized by x-ray diffraction, Rutherford backscattering spectrometry, resistivity measurements, photoluminescence, time-domain thermoreflectance thermal conductivity measurements, optical absorption spectroscopy, and plan-view high-angle annular dark-field scanning transmission electron microscopy. Results revealed successful formation of randomly distributed nanoparticles with an average diameter of ∼1.5 nm, reduction of thermal conductivity by a factor of about 5, and consistency with theoretical predictions of mid-band-gap Fermi level pinning and behavior of past similar materials. The success of these TbAs:GaAs materials will lead the way for growth of similar materials [TbAs:InGa(Al)As] which are expected to exhibit highly desirable thermoelectric properties.

Structural characterization of ZnO nanopillars grown by atmospheric-pressure metalorganic chemical vapor deposition on vicinal 4H-SiC and SiO2/Si substrates

The structural characteristics of ZnO nanocrystals epitaxially grown on p-type (0001) 4H-SiC substrates were studied by transmission electron microscopy (TEM). The nanocrystallites were grown by atmospheric-pressure metalorganic chemical vapor deposition. The ZnO nanocrystals were formed at terraces introduced by vicinal 4H-SiC substrates toward the [112¯0] direction. They had the shape of hexagonal nanopillars, with their edges parallel to the 〈112¯0〉 directions and a top c-plane facet, reflecting the crystal symmetry of ZnO. The free surface between the hexagonal nanopillars was covered by a very thin and highly defected epitaxial ZnO film, which strongly suggests the Stranski-Krastanov mode of growth. The ZnO/SiC interface was systematically studied by plane view TEM and cross sectional high resolution TEM. The residual strain in the thin continuous film as well as in the nanopillars was estimated from Moiré patterns and by geometrical phase analysis. ZnO was also deposited on the SiO2/Si substrate for comparison. The films were polycrystalline exhibiting strong preferred orientation, with the c-axes of the grains almost perpendicular to the substrate resulting in the formation of nanopillars. The differences of nanopillar formation in the two substrates, 4H-SiC and SiO2 is also discussed.

Defects in m-plane ZnO epitaxial films grown on (112) LaAlO3 substrate

The crystallographic orientations of m-plane ZnO on (112) LaAlO3 (LAO) substrate are [1¯21¯0]ZnO∥[111¯]LAO and [0001]ZnO∥[1¯10]LAO. The defects in m-plane ZnO have been systematically investigated using cross section and plan-view transmission electron microscopy (TEM). High-resolution TEM observations in cross section show misfit dislocations and basal stacking faults (BSFs) at the ZnO/LAO interface. In the films, threading dislocations (TDs) with 1/3⟨112¯0⟩ Burgers vectors are distributed on the basal plane, and BSFs have 1/6⟨202¯3⟩ displacement vector. The densities of dislocations and BSFs are estimated to be 5.1×1010 cm−2 and 4.3×105 cm−1, respectively. In addition to TDs and BSFs, plan-view TEM examination also reveals that stacking mismatch boundaries mainly lie along the m-planes and they connect with planar defect segments along the r-planes.

Capillarity-driven migration of a thin Ge wedge in contact with a bicrystalline Au film

We have investigated the retraction of a single-crystalline Ge wedge in epitaxial contact with a bicrystalline Au film using in situ electron microscopy. The rate of retraction was close to that predicted for capillarity-driven surface diffusion, following kinetics proportional to t n , with n = 0.22–0.35, but crystal anisotropy caused migration to be significantly faster along 〈1 0 0〉 directions than along 〈1 1 0〉. The bicrystalline Au substrate was not inert, but underwent abnormal grain growth in the area swept by the receding Ge wedge. Cross-sections made from plan-view transmission electron microscopy samples revealed that this was related to ridge formation during the retraction process. In situ observations of the process in an inclined orientation showed direct evidence of substrate grain boundaries being dragged by the receding Ge wedge. The results can be understood in the framework of capillarity models for isotropic solid-state wedges and reactive wetting in high-temperature liquid–solid experiments.

Efficiency measurement of GaN-based quantum well and light-emitting diode structures grown on silicon substrates

The optical efficiency of GaN-based multiple quantum well (MQW) and light emitting diode (LED) structures grown on Si(111) substrates by metal-organic vapor phase epitaxy was measured and compared with equivalent structures on sapphire. The crystalline quality of the LED structures was comprehensively characterized using x-ray diffraction, atomic force microscopy, and plan-view transmission electron microscopy. A room temperature photoluminescence (PL) internal quantum efficiency (IQE) as high as 58% has been achieved in an InGaN/GaN MQW on Si, emitting at 460 nm. This is the highest reported PL-IQE of a c-plane GaN-based MQW on Si, and the radiative efficiency of this sample compares well with similar structures grown on sapphire. Processed LED devices on Si also show good electroluminescence (EL) performance, including a forward bias voltage of ∼3.5 V at 20 mA and a light output power of 1 mW at 45 mA from a 500×500 μm2 planar device without the use of any additional techniques to enhance the output coupling. The extraction efficiency of the LED devices was calculated, and the EL-IQE was then estimated to have a maximum value of 33% at a current density of 4 A cm−2, dropping to 30% at a current density of 40 A cm−2 for a planar LED device on Si emitting at 455 nm. The EL-IQE was clearly observed to increase as the structural quality of the material increased for devices on both sapphire and Si substrates.

Increased critical thickness for high Ge-content strained SiGe-on-Si using selective epitaxial growth

The onset of misfit dislocation formation, i.e., the critical thickness for heteroepitaxy, is studied for selective epitaxial growth of high Ge-content, strained SiGe on oxide-patterned Si wafers. Misfit dislocation spacing was analyzed as a function of film thickness using plan-view transmission-electron microscopy. For selective epitaxial growth at 450 °C, the critical thickness for Si0.33Ge0.67 is found to be 8.5 nm. This is a twofold increase compared to the 4.0 nm theoretical equilibrium critical thickness and the 4.5 nm critical thickness measured for growth on bare Si wafers. The misfit dislocation density for selective epitaxial growth is strongly influenced by the shape and orientation of the growth area.

Self-assembled In0.5Ga0.5As quantum dots on GaP

We demonstrate the growth and luminescence of coherently strained In0.5Ga0.5As self-assembled quantum dots on GaP. Cross-sectional and planar-view transmission electron microscopy confirmed the dislocation-free nature of the In0.5Ga0.5As quantum dots and GaP cap layers. Intense photoluminescence from the quantum dots was measured at 80 K and was visible to the unaided eye in ambient lighting. The photoluminescence results show that emission energy can be controlled by varying the In0.5Ga0.5As deposition thickness. In combination with recent advances in the growth of GaP on Si, the In0.5Ga0.5As quantum dots demonstrated here could enable monolithic optoelectronic integration on Si.

III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy

Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {11¯01} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to ∼20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650–780 °C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs.

Fabrication and characterization of highly textured Nd–Fe–B thin film with a nanosized columnar grain structure

We have fabricated a highly textured Nd–Fe–B thin film with a hard magnetic performance: Mr=1.39 T, Hc=827 kA/m, and (BH)max=358 kJ/m3. The microstructure of the film was characterized in detail by cross-sectional and plane-view transmission electron microscopy observations. The film consisted of nanosized columnar grains with an average size of 40 nm and included a strong diffraction contrast along the grain boundary. A high resolution energy filtered image indicated that Nd was enriched discontinuously along the grain boundary, causing an unsuitable decoupling among the Nd2Fe14B grains, which is in agreement with the pinninglike feature of the initial magnetization curve.

High quality single-crystal germanium-on-insulator on bulk Si substrates based on multistep lateral over-growth with hydrogen annealing

Germanium-on-insulator (GOI) is desired for high performance metal-oxide-semiconductor transistors and monolithically integrated optoelectronics. We demonstrate a promising approach to achieve single-crystal defect-free GOI by using lateral over-growth through SiO2 window. The dislocations due to the lattice mismatch are effectively terminated and reduced in SiO2 trench by selective area heteroepitaxy combined with hydrogen annealing. Low defect density of 4×106 cm−2 and low surface roughness of 0.7 nm (root-mean-square) on GOI are confirmed by plan-view transmission electron microscopy and atomic force microscopy analysis. In addition, the excellent metal-semiconductor-metal diode electrical characteristics fabricated on this GOI confirm Ge crystal quality. The selectively grown GOI structure can provide the monolithic integration of SiGe based devices on a Si very large scale integration (VLSI) platform.

Improved flux pinning in YBa2Cu3O7 with nanorods of the double perovskiteBa2YNbO6

We report significantly enhanced critical current densities (Jc) and flux pinning forces (Fp) in applied magneticfields for YBa2Cu3O7 (YBCO)films with embedded Ba2YNbO6 (BYNO) nanorods. The films were grown by pulsed laser deposition with a target consisting ofYBa2Cu3Oy with five molar percent additions of BaNbOy and Y2O3. With this composition, deposited films were found to contain a high density ofBYNO nanorods that frequently traversed the entire thickness of the film (up to 1 µm), depending upon the deposition conditions. EnhancedJc performance occurs primarily for applied field orientations near thec-axis of the YBCO, which is nominally along the growth direction of the BYNOnanorods. The threading nanorod density of one film of the present workwas measured by plan-view transmission electron microscopy to be 710–850nanorods µm − 2. For approximately1 µm thickfilms, typical Jc(75.6 K, sf) and values were ∼ 4.5 MA cm − 2 and 1.3–1.5 MA cm − 2,respectively. For a 0.5 µm thick film, was achieved, and values of Fp in excess of 30 and 120 GN m − 3 were achieved at 75.5 K and 65 K, respectively.

Dislocation reduction in GaN grown on Si(111) using a strain-driven 3D GaN interlayer

In this paper we demonstrate a strain-driven GaN interlayer method to reduce dislocation densities in GaN grown on (111) oriented silicon by metal organic vapour phase epitaxy (MOVPE). In order to achieve crack-free GaN layers of reasonable thicknesses and dislocation densities it is crucial to integrate both dislocation reduction and strain management layers. In contrast to techniques like FACELO or nanoELO we show the in situ formation of GaN islands directly on the AlN nucleation layer without the need to deposit a SiO 2 or SiN x mask. A graded AlGaN layer for strain management can be grown on top of this dislocation reducing 3D GaN inter-layer in order to achieve crack-free GaN layers grown on top of the AlGaN strain management layer. Furthermore, an additional SiN x layer for subsequent dislocation reduction can also be incorporated into the structure and is shown to efficiently reduce the dislocation density down to the low 10 9 cm -2 . The structural properties of the 3D GaN island buffer layer and overgrown samples are studied by means of SEM, cross-sectional, and plan view TEM. Cathodoluminiscence in an SEM is employed to correlate the dislocation microstructure as observed by plan view TEM with luminescent properties.

Growth and Characterization of Self-Assembled InAs/InP Quantum Dot Structures

InAs quantum dots (QDs) are grown on InP or lattice matched GaInAsP buffers using horizontal flow metal-organic chemical vapor deposition (MOCVD) at a pressure of 180 mbar. A range of techniques, such as photoluminescence (PL), atomic force microscopy, and plan-view transmission electron microscopy is used to characterize the QD and other semiconductor layers. The effects of different growth parameters, such as V/III ratio and growth time, and the effects of buffer layers, interlayers, and cap layers are investigated and the optimized growth conditions are discussed. In the case of the QDs grown on InP buffers, the As/P exchange reaction is found to be prominent. A very thin (0.6 nm) GaAs interlayer grown between the buffer and the QD layers consumes segregated indium and minimizes the As/P exchange reaction. As a result, the QD PL emission energy increases, the PL intensity improves, and the PL linewidth decreases. The experimental results show that by changing the thickness of a GaAs interlayer (0.3-0.6 nm), the emission wavelength/energy of the QDs grown on a lattice matched GaInAsP buffer can be tuned over a wide range covering 1550 nm. However, further increase in the thickness of the GaAs interlayer results in the agglomeration of the QDs and the deterioration of the QD optical properties. Detailed microscopy studies show that capped QDs have higher density and are smaller in size on average compared to uncapped QDs, which undergo coalescence during cooling of the sample after growth. Overall, the QDs grown for shorter time with a smaller V/III ratio (approximately 8) show improved PL intensity and narrower PL linewidth.

Vapor-Liquid-Solid 成長を用いて作製した人工ピン導入 REBa2Cu3O7−y 膜の微細組織

In order to improve a critical current density under applied magnetic fields, an addition of BaMO3 (BMO; M=Zr, Sn) nanorods into REBa2Cu3Oy (REBCO) films is actively discussed. Although superconducting properties of the REBCO films are dramatically enhanced by self-assembled BMO nanorods, the growth mechanisms of the BMO nanorods have not been clarified yet. In this study, in order to clarify the growth mechanisms of the BZrO3 (BZO) nanorods and to further improve the superconducting properties, we fabricated a BZO-doped Sm1+xBa2−xCu3Oy (Sm+BZO) film by using modified Vapor-Liquid-Solid (VLS) technique (VLS-Sm+BZO/i). From the surface morphologies, there are BZO particles grown at step in the spiral growth measured by Br-etched VLS films doped with BZO. In general the impurity-related phase was pinning of growth fronts on REBCO films on substrate. Thus, we can conclude the variation of the surface morphology in VLS films is explained, not by the difference the step energy, but impurity phase could be responsible for the pinning and bending of different growth fronts, resulting in the formation of screw dislocations. From the plan-view TEM images, VLS-SmBCO+BZO film is approximately 9 nm in diameter of BZO nanorods, whereas VLS-SmBCO+BZO/i film is 7-8 nm. To create the BZO nano island/dots on the seed layer introduced the increase of the density of BZO nanorods. From the cross-sectional TEM images, columns of BZO nanorods aligned along [001] or the c-axis of SmBCO are clearly seen in the VLS films. Columnar BZO structures growing from the substrate to the surface have been observed. Nano BZO particles grown on the seed layer remained after the VLS growth. Nano BZO rods separated by 5 nm SmBCO spacer layers are shown to exhibit self-organized growth along c-axis of SmBCO.

Structural property of m-plane ZnO epitaxial film grown on LaAlO 3 (1 1 2) substrate

The microstructure of m-plane (1 0 1¯ 0) ZnO grown on LaAlO 3 (1 1 2) (LAO (1 1 2)) substrate by pulsed laser deposition method has been investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD shows that ZnO grown on LAO(1 1 2) appears to be oriented in pure m-plane. TEM electron diffraction patterns in cross section illustrate that m-plane ZnO is in epitaxy with LAO (1 1 2), and the orientation relationships are determined to be [1 2¯ 1 0] ZnO//[1¯ 1¯ 1] LAO and [0 0 0 1] ZnO//[1¯ 1 0] LAO. Also, TEM shows that most of threading dislocations (TDs) in a-type are mainly distributed as wiggle-like lines. From the observations in plan-view TEM, the densities of TDs and basal stacking faults are approximately estimated to be 5.1×10 10 cm −2 and 4.3×10 5 cm −1, respectively.

Defect structure study of epitaxial InN films by transmission electron microscopy and X-ray diffraction

The structural and morphological properties of InN epilayers grown on Si(1 1 1) substrates by plasma-assisted molecular beam epitaxy were studied. The mosaic characteristics such as grain size, twist and tilt angles were extracted by performing symmetric and asymmetric rocking curves and reciprocal space mapping. According to X-ray diffraction analysis, the dislocation densities reduced from 1×10 11 to 2.27×10 10 cm −2, when the substrate temperature was increased from 440 to 525 °C. Edge-type dislocation densities estimated both by fitting the FWHM of ω-scans of various lattice planes based on cosine theorem of spherical trigonometry and by grazing incidence (1 1 2¯ 0) diffraction agree with plan-view transmission electron microscopy (TEM) results. Surface morphology revealed grains with flat or terrace-like features separated by deep trenches. Faceted pits associated with screw-type threading dislocations are observed and their densities are in good consistence with the results obtained from X-ray diffraction. Improved film quality is achieved by applying highest growth temperature within InN dissociation limit.

Morphological and microstructural evolution in the two-step growth of nonpolar a-plane GaN on r-plane sapphire

In this paper, we report a detailed study on the evolution of surface morphology and microstructure of nonpolar a-plane GaN (a-GaN) through controlled growth interruptions. Microscopy imaging shows that the two-step a-GaN growth went through a roughening-recovery process. The first-step growth (under high V/III and high pressure) produced a rough surface with tall mesas separated by voids. The second-step growth (under low V/III and low pressure) promoted the lateral growth and filled up the voids. Striations that formed during the island coalescence persisted throughout the second-step growth, but could be relieved by an additional third-step growth. The morphological evolution was explained according to the kinetic Wulff plots. The microstructure of the a-GaN films was investigated by transmission electron microscopy (TEM) and x-ray rocking curve analysis. Most of the extended defects observed in the plan-view TEM images were I1 type basal-plane stacking faults (BSFs) and their associated partial dislocations (PDs). It is found that the bending of PDs (at the inclined/vertical growth fronts) within the basal plane toward the m-axes was responsible for the substantial reduction in threading PDs and the increase in BSF dimension. Based on a careful correlation between the morphological evolution and the microstructure development, we proposed a model explaining the possible mechanisms for the great reduction in defect density during the two-step growth process.