Direction Of Substrate Misorientation Research Articles

Formation of edge misfit dislocations in GexSi1−x(x∼0.4–0.8) films grown on misoriented (001)→(111) Si substrates: Features before and after film annealing

The dislocation structure at the initial stage of relaxation of GexSi1−x films (x∼0.4–0.8) grown on Si(001) substrates tilted at 6° to the nearest (111) plane is studied. One of the directions along which edge dislocations are formed is no longer in the plane of the interface but crosses the latter at the angle of miscut. Therefore, long edge misfit dislocations (MDs) cannot exist in the direction of substrate misorientation, because their ends move away from the interface during dislocation propagation. Two different mechanisms of formation of short segments of edge (90°) MDs in the direction of substrate misorientation are found. The first mechanism is the correlated nucleation of complementary 60° dislocation half-loops manifested in the form of the so-called Y-center consisting of a short segment of the 90° MD and 60° MDs diverging from this segment in form of two rays in the miscut direction. In the second mechanism, the 90° MD segment is formed owing to intersection of already existing complementary 60° MDs slipping in mirrorlike inclined planes {111}. Samples annealed at higher temperatures contain MD segments, which lose their contrast on one of the TEM images taken to detect edge dislocation segments in the two-wave diffraction mode but do not coincide with the direction [110] (they pass along the intersections of the mirrorlike inclined planes {111} with the surface of the misoriented Si substrate). To explain this phenomenon, a model is proposed, where one of the pair of complementary 60° MDs propagating at an angle to each other continuously approaches the other by the cross-slip mechanism. Formation of a chain of edge MD segments connected by a pair of 60° MDs becomes possible. The averaged direction of such a dislocation structure coincides with the direction of one of the 60° MDs.

Systematic Investigation of c-Axis Tilt in GaN and AlGaN Grown on Vicinal SiC(0001) Substrates

High-resolution X-ray diffraction measurements of GaN and AlGaN grown on 4H- and 6H-SiC(0001) vicinal substrates with misorientation angles of up to 9° are presented. Growth of (Al)GaN was carried out by plasma-assisted molecular beam epitaxy. The c-axis tilt, i.e., inclination of the (Al)GaN c-axis relative to that of SiC, was systematically investigated. The inclination angle clearly depended on the SiC substrate misorientation angle, while it was independent of the (Al)GaN growth temperature, SiC polytype, and substrate misorientation direction. The behavior observed for both GaN and AlGaN is in excellent agreement with the model proposed previously by Nagai for the InGaAs/GaAs system.

Formation of edge misfit dislocations in Ge xSi 1−x ( x∼0.4–0.5) films grown on misoriented (0 0 1)→(1 1 1) Si substrates

We have studied the dislocation structure at the initial stage of relaxation of Ge x Si 1− x films ( x∼0.4–0.5) grown on Si substrates tilted 6° about the 〈0 1 1〉 axis. It is demonstrated that edge misfit dislocations (MDs) in the miscut direction arise in the form of short segments on intersections of 60° MDs. As a result, the total length of the edge MDs in the direction of substrate misorientation becomes substantially smaller than that in the direction of the miscut axis. Substrate misorientation from the singular plane made it possible to discover the MD configurations consisting of a short segment of an edge MD and only two 60° MDs diverging from this segment in the miscut direction. This configuration is assumed to start forming from simultaneous nucleation of two dislocation half-loops, which form a short edge MD in the interface and then propagate in only one direction in the form of two diverging branches of 60° MDs.

Morphological variations on surface topography of YBa 2Cu 3O 7−δ thin films on SrTiO 3, with respect to the substrate misorientation direction

The surface topography of YBa 2Cu 3O 7− δ thin films deposited on SrTiO 3 substrates has been evaluated by atomic force and scanning tunneling microscopy. The observed correlation between the direction of the substrate misorientation and the surface morphology of the films produced is reported: when using substrates with offcut lying only along the [1 0 0] plane, smoother and highly ordered film surfaces are produced, while for substrates misoriented along both the [1 0 0] and [0 1 0] planes, island growth reappears in the resulting films. The findings demonstrate that the direction of substrate offcut influences the morphology of the films deposited, enabling better thin film engineering at nano-scale.

Epitaxial growth of InGaAs on misoriented GaAs(1 0 0) substrate by metal-organic vapor phase epitaxy

In 0.35Ga 0.65As layers were grown on GaAs(1 0 0) substrates by metal-organic vapor phase epitaxy using graded buffer layers. GaAs(1 0 0) substrates misoriented toward (1 1 1)A and (1 1 1)B and exactly oriented GaAs(1 0 0) substrates were used. Surface morphology of InGaAs layers strongly depended on the substrate misorientation direction. The lowest root mean square of surface roughness was obtained for the layer on the substrate misoriented toward (1 1 1)A under optimized conditions. Strain relaxation in InGaAs layers was influenced by substrate misorientation. Threading dislocation densities were 6.4×10 6, 3.6×10 6, and 2.0×10 6 cm −2 in InGaAs layers on the exactly oriented substrate and substrates misoriented toward (1 1 1)A and (1 1 1)B, respectively.

Correlation of anisotropic strain relaxation with substrate misorientation direction at InGaAs/GaAs(001) interfaces

We have investigated the effects of substrate misorientation towards (111)A, (111)B, and (011) on asymmetries in the strain relaxation of InxGa1−xAs, grown on (001) GaAs substrates by molecular beam epitaxy. For epilayers grown under conditions of two-dimensional growth, we find large anisotropies in bulk strain relaxation and epilayer rotation about an in-plane axis (epilayer tilt) in proportion to the degree of (111)A misorientation. The residual strain asymmetry is largest for the (111)A misoriented substrate (≳50%) and smallest for the (111)B misoriented substrate (<15%). At higher growth temperatures, the bulk strain relaxation becomes isotropic while the epilayer tilt remains sensitive to the offcut direction. At all temperatures, a preference for epilayer tilt toward the [110] direction for (011) misorientations is observed.

Effects of Substrate Misorientation Direction on Strain Relaxation at InGaAs/GaAs(001) Interfaces

ABSTRACTWe have investigated the effects of substrate misorientation direction on strain relaxation at InGaAs/GaAs(001) interfaces. Calculations of the shear stresses due to the misfit strain, resolved on the glide plane in the glide direction, suggest that the dislocation glide force and the activation energy for dislocation nucleation are essentially identical for the α and β slip systems. However, experimental results indicate that asymmetries in strain relaxation are sensitive to A-type misorientation and/or step-edge densities. Thus, a dislocation nucleation source or glide velocities sensitive to step densities or local roughness may explain these results.

Influence of Substrate Misorientation on Current Gain in AlGaAs/GaAs Heterojunction Bipolar Transistors Grown by Metal Organic Chemical Vapor Deposition

The influence of substrate misorientation on the current gain characteristics of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is systematically investigated. The current gain is found to increase by a factor of up to three when the substrate is tilted from (100). The behavior of the minority electron lifetime determined separately in thick C-doped GaAs layers grown on the same set of misoriented substrates correlates very well with that of the current gain. This is consistent with the result of base current ideality factors, which are close to unity for all devices. These improvements in the current gain and the minority electron lifetime are commonly observed in layers grown on misoriented substrates regardless of the substrate misorientation direction. These results can be interpreted in terms of the incorporation of nonradiative recombination centers such as native defects, where the substrate misorientation can effectively suppress the formation of these centers.

Atomic ordering in InAs0.5P0.5 grown by organometallic vapor phase epitaxy

InAsP epilayers grown by organometallic vapor phase epitaxy have been investigated using transmission electron microscopy. Electron diffraction studies using 〈110〉 cross sections indicate the formation of CuPt-like ordering on the group V sublattice. Only two of the four possible ordered variants are observed for epilayers grown on the exactly (001) oriented InP substrates. All the order-induced diffraction spots for InAsP are found to occur on the [110] cross section. Thus, the variants found in InAsP are 1/2(1̄11) and 1/2(11̄1), exactly the same as those found in GaInP, an alloy with CuPt ordering on the group III sublattice. This result is in agreement with recent studies on GaAsP and is contradictory to expectations based on the bond-length model proposed previously for GaInP alloys. The direction of substrate misorientation has a strong effect on the formation of ordered structures for normally (001) oriented InP substrates.

Atomic ordering in GaAsP

CuPt type ordering, which consists of a monolayer compositional modulation along one of the 4 〈111〉 directions in the lattice, was studied using transmission electron microscopy for GaAs1−xPx with values of x extending from 0.25 to 0.85. The samples were grown by organometallic vapor phase epitaxy on nominal (001) GaAs substrates that were misoriented by varying amounts in three directions. No CuPt type ordering was observed for GaAs1−xPx with x ≤0.35, while ordering was found to occur for 0.4≤x≤0.85. The direction of substrate misorientation has a major effect on the determination of which of the four possible CuPt variants are formed for 0.4≤x≤0.85. Two variants, with ordering on the (1̄11) and (11̄1) planes, appear for epilayers grown on substrates oriented exactly on the (001) plane and for substrates misoriented by 6° towards the [110] direction. Only one variant, with ordering on the (1̄11) plane, appears for epilayers grown on substrates misoriented by 6° towards [1̄10]. These ordering-induced spots observed in transmission electron diffraction (TED) patterns for GaAsP occur only for the [110] cross section. From TED studies of GaInP grown on similar substrates, we conclude that the CuPt variants in GaAsP are exactly the same as for GaInP. Further evidence supporting this conclusion was obtained by growing first a layer of GaInP followed by a layer of GaAsP. High-resolution dark field electron micrographs show domains of the same variants in both layers. A mechanism describing the formation of the specific ordered variant for both GaAsP and GaInP is proposed. From studies of ordering in a strain-layer superlattice, the strain due to lattice mismatch was found to play no significant role in the propagation of ordered domains. Microtwins, also generated due to lattice mismatch, can act as domain boundaries and prevent the propagation of the ordered domains.

Effects of substrate misorientation on ordering in GaAs0.5P0.5 grown by organometallic vapor phase epitaxy

Effects of (001) GaAs substrate misorientation on the formation of the group V sublattice {111} (CuPt) ordered structure are studied for the first time for GaAs0.5P0.5. It is found that the direction of substrate misorientation has a strong effect on the determination of which variants are formed. Two of the four possible ordered variants appear for epilayers grown on exact (001) substrates. The same two variants also appear for the epilayers grown on the (001) substrates misoriented by 6° towards [110]. Only one variant appears on epilayers grown on (001) substrates misoriented by 6° towards the [1̄10] direction. Most significantly, all the ordered-induced diffraction spots in GaAsP are found to occur on the [110] cross section. Thus, the variants found in GaAsP are exactly the same as for GaInP, an alloy with CuPt ordering on the group III sublattice. This result is contradictory to expectations based on the bond-length model proposed previously for GaInP alloys. In addition, for all the 6° misorientated GaAs substrates, independent of the direction of misorientation, large ordered domains with dimensions on the order of micrometers are found in the GaAsP alloys. This has not been reported for other ternary or quaternary alloy systems.

Nucleation and Growth of CrSi2 on Si(111)

AbstractChromium disilicide layers have been grown on Si(111) in a commercial molecular beam epitaxy machine. Islands are found to nucleate with more than one epitaxial orientation, and three different orientations have been identified: CrSi2(0001)//Si(111) with CrSi2 [1010]//Si[110]. CrSi2(0001)//Si(111) with CrSi2[ll20]//Si[1l0], and CrSi2(1100)//Si(111) with CrSi2[000l]//Si[112]. Very thin (2 nm) layers have been studied under a variety of growth conditions to determine factors controlling the orientation of nuclei. The degree and direction of substrate misorientation, the growth technique, and the use of a CoSi2 buffer layer are all factors affecting the morphology of CrSi2 islands.