Effect of substrate misorientation on the InAs/InAlAs/InP nanostructure morphology and lateral composition modulation in the InAlAs matrix

The authors report the self-organized growth of InAs/InAlAs quantum wires on nominal (001) InP substrate and (001) InP substrates misoriented by 2 degrees, 4 degrees, and 8 degrees towards both [-110] and [110]. The influence of substrate misorientation on the structural and optical properties of these InAs/InAlAs quantum wires is studied by transmission electron microscopy and photoluminescence measurements. Compared with that grown on nominal (001) InP substrate, the density of InAs/InAlAs quantum wires grown on misoriented InP(001) substrates is enhanced. A strong lateral composition modulation effect take place in the InAlAs buffer layers grown on misoriented InP substrates with large off-cut angles (4 degrees and 8 degrees), which induces a nucleation template for the first-period InAs quantum wires and greatly improve the size distribution of InAs quantum wires. InAs/InAlAs quantum wires grown on InP (001) substrate 8 degrees off cut towards [-110] show the best size homogeneity and photoluminescence intensity. (c) 2007 American Institute of Physics.

Effect of GaAS(100) 2 degrees surface misorientation on the formation and optical properties of MOCVD grown InAs quantum dots

The influence of GaAS(1 0 0)2 degrees substrate misorientation on the formation and optical properties of InAs quantum dots (QDs) has been studied in compare with dots on exact GaAs(1 0 0) substrates. It is shown that, while QDs on exact substrates have only one dominant size, dots on misoriented substrates are formed in lines with a clear bimodal size distribution. Room temperature photoluminescence measurements show that QDs on misoriented substrates have narrower FWHM, longer emission wavelength and much larger PL intensity relative to those of dots on exact substrates. However, our rapid thermal annealing (RTA) experiments indicate that annealing shows a stronger effect on dots with misoriented substrates by greatly accelerating the degradation of material quality. (c) 2005 Elsevier B.V All rights reserved.

Formation, microstructure and development of the localized shear deformation in low-carbon steels

An investigation has been made into the effect of microstructural parameters on the propensity for forming shear localization produced during high speed torsional testing by split Hopkinson bar with different average rates of 610, 650 and 1500 s(-1) in low carbon steels. These steels received the quenched, quenched and tempered as well as normalized treatments that provide wide microstructural parameters and mechanical properties. The results indicate that the occurrence of the shear localization is susceptible to the strength of the steels. In other words, the tendency of the quenched steel to form a shear band is higher than that of the other two steels. It is also found that there is a critical strain at which the shear localization occurs in the steels. The critical strain value is strongly dependent on the strength of the steels. Before arriving at this point, the material undergoes a slow work-hardening. After this point, the material suffers work-softening, corresponding to a process during which the deformation is gradually localized and eventually becomes spatially correlated to form a macroscopic shear band. Examinations by SEM reveal that the shear localization within the band involves a series of sequential crystallographic and non-crystallographic events including the change in crystal orientation, misorientation, generation and even perhaps damage in microstructures such as the initiation, growth and coalescence of the microcracks. It is expected that the sharp drop in the load-carrying capacity is associated with the growth and coalescence of the microcracks rather than the occurrence of the shear localization, but the shear localization is seen to accelerate the growth and coalescence of the microcracks. The thin foil observations by TEM reveal that the density of dislocations in the band is extremely high and the tangled arrangement and cell structure of dislocations tends to align along the shear direction. The multiplication and interaction of dislocations seems to be responsible for work-hardening of the steels. The avalanche of the dislocation cells corresponds to the sharp drop in shear stress at which the deformed specimen is broken. Double shear bands and kink bands are also observed in the present study. The principal band develops first and its width is narrower than that of the secondary band.

An analysis of the effects of symmetrical tilt grain boundaries on the plastic deformation

A numerical investigation on the simple polycrystals containing three symmetrical tilt grain boundaries (GBs) is carried out within the framework of crystal plasticity which precisely considers the finite deformation and finite lattice rotation as well as elastic anisotropy. The calculated results show that the slip geometry and the redistribution of stresses arising from the anisotropy and boundary constraint play an important role in the plastic deformation in the simple polycrystals. The stress level along GB is sensitive to the load level and misorientation, and the stresses along QB are distributed nonuniformly. The GB may exhibit a softening or strengthening feature, which depends on the misorientation angle. The localized deformation bands usually develop accompanying the GB plastic deformation, the impingement of the localized band on the GB may result in another localized deformation band. The yield stresses with different misorientation angles are favorably compared with the experimental results.

高压扭转铜试样的微观组织与压缩性能

通过高压扭转对铜试样施加不同程度的变形，研究了样品扭转面（ND面）和纵截面（TD面）上微观组织特征．对ND面，在较小的剪应变下，原始晶粒形貌模糊，晶粒内部形成等轴状的位错胞及亚晶结构；随变形量的增大，亚晶间取向差及亚晶内部的位错密度增大，最后形成亚微米尺度的等轴晶粒．对TD面，变形初期原始晶粒被拉长，晶粒内部为位错墙分割成的层状结构，层内为拉长的位错胞；随变形程度的增大，拉长晶粒的宽度减小，与剪切方向的夹角减小，晶内层状组织间距减小，并逐渐演化成拉长的亚晶组织；进一步增大变形，晶粒拉长痕迹消失，变形组织与ND面相似，为等轴状亚微米晶粒．压缩实验表明，经16圈扭转后，整个试样上的压缩性能基本均匀，σ0．2达到385MPa，应变率敏感性指数增大至0．021．

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520 degrees C, and then exhibits a sudden decrease at 535 degrees C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates. (c) 2006 American Institute of Physics.

High-power AlGaInP laser diodes with current-injection-free region near the laser facet

A high-power AlGaInP laser diode with current-injection-free region near the facet is successfully fabricated by metaorganic chemical vapor deposition (MOCVD) using the (100) direction n-GaAs substrates with a misorientation of 15 deg toward the (011) direction. The maximum continuous wave output power is about 90 mW for the traditional structure. In comparison, the maximum output power is enhanced by about 67%, and achieves 150 mW for LDs with current-infection-free regions. The fundamental transverse-mode operation is obtained up to 70 mW. Output characteristics at high temperatures are also improved greatly for an LD with a current-injection-free region, and the highest operation temperature is 70 C at 50 mW without kink. The threshold current is about 33 mA, the operation current and the slope efficiency at 100 mW are 120 mA and 0.9 mW/mA, respectively. The lasing wavelength is 658.4 nm at room-temperature 50 mW. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

Two-dimensional ordering of self-assembled InxGal-xAs quantum dots grown on GaAs(311)B surfaces

Two-dimensional (2D) ordering of self-assembled InxGa1-xAs quantum dots (QDs) fabricated on GaAs(311)B surface by molecular beam epitaxy (MBE) are reported. The QDs are aligned into rows deferring from the direction of the misorientation of the substrate, and strongly dependent on the mole In content x of InxGa1-xAs solid solution. The ordering alignment deteriorates significantly as the In content is increased to above 0.5. The 2D ordering can be described as a centered rectangular unit mesh with the two sides parallel to [01 (1) over bar] and [(2) over bar 33], respectively. Their relative arrangement seems to be determined by a combination of the strongly repulsive elastic interaction between neighbouring islands and the minimization of the strain energy of the whole system. The ordering also helps to improve the size homogeneity of the InGaAs islands. Photoluminescence (PL) result demonstrates that QDs grown on (311)B have the narrowest linewidth and the strongest integrated intensity, compared to those grown on (100) and other high-index planes under the same condition. (C) 1999 Elsevier Science B.V. All rights reserved.

Two-dimensional ordering of self-assembled InxGa1-xAs quantum dots grown on GaAs(311)B surfaces

The two-dimensional (2D) ordering of self-assembled InxGa1-xAs quantum dots (QDs) fabricated on GaAs(3 1 1)B surface by molecular beam epitaxy (MBE) are reported. The QDs are aligned into rows differing from the direction of the misorientation of the substrate, and strongly dependent on the mole In content x of InxGa1-As-x solid solution. The ordering alignment deteriorates significantly as the In content is increased to above 0.5. The 2D ordering can be described as a centered rectangular unit mesh with the two sides parallel to [0 1 (1) over bar] and [(2) over bar 3 3], respectively. Their relative arrangement seems to be determined by a combination of the strongly repulsive elastic interaction between the neighboring islands and the minimization of the strain energy of the whole system. The ordering also helps to improve the size homogeneity of the InGaAs islands. The photoluminescence (PL) result demonstrates that QDs grown on (3 1 1)B have the narrowest linewidth and the strongest integrated intensity, compared to those on (1 0 0) and other high-index planes under the same condition. (C) 1999 Elsevier Science B.V. All rights reserved.

Lateral ordered InGaAs self-organized quantum dots grown on (311) GaAs by conventional molecular beam epitaxy

Self-assembled InxGa1-xAs quantum dots (QDs) on (311) and (100) GaAs surfaces have been grown by conventional solid source molecular beam epitaxy. Spontaneously ordering alignment of InxGa1-xAs QDs with lower In content around 0.3 has been observed on As-terminated (B type) surfaces. The direction of alignment orientation of the QDs formation differs from the direction of misorientation of the (311) B surface, and is strongly dependent upon the In content x. The ordering alignment becomes significantly deteriorated as the In content is increased to above 0.5 or as the QDs are formed on (100) and (311) Ga-terminated (A type) substrates.

In composition dependence of lateral ordering in InGaAs quantum dots grown on (311)B GaAs substrates

Self-assembled InxGa1-xAs quantum dots (QDs) on (311)A/B GaAs surfaces have been grown by molecular beam epitaxy (MBE). Spontaneously ordering alignment of InxGa1-xAs with lower In content around 0.3 have been observed. The direction of alignment orientation of the QDs formation differs from the direction of misorientation of the (311)B surface, and is strongly dependent upon the In content x. The ordering alignment become significantly deteriorated as the In content is increased to above 0.5 or as the QDs are formed on (100) or (311)A substrates. (C) 1999 Elsevier Science B.V. All rights reserved.

Triclinic deformation and anisotropic strain relaxation of an InAs film on a GaAs(001) substrate measured by a series of symmetric double crystal X-ray diffraction

We presented a series of symmetric double crystal X-ray diffraction (DCXD) measurements, (0 0 4), (2 2 0) and (2 - 2 0) diffraction, to investigate the strain relaxation in an InAs film grown on a GaAs(0 0 1) substrate. The strain tensor and rotation tensor were calculated according to the DCXD results. It is found that the misfit strain is relaxed nearly completely and the strain relaxation caused a triclinic deformation in the epilayer. The lattice parameter along the [1 1 0] direction is a little longer than that along the [1 - 1 0] direction. Furthermore, a significant tilt, 0.2 degrees, towards the [1 1 0] direction while a very slight one: 0.002 degrees, towards [1 - 1 0] direction were discussed. This anisotropic strain relaxation is attributed to the asymmetric distribution of misfit dislocations, which is also indicated by the variation of the full-width at half-maximum (FWHM) of (0 0 4) diffraction along four azimuth angles. (C) 1998 Elsevier Science B.V. All rights reserved.

A theoretical model for the tilt of the GaAs/Si epilayers

The crystallographic tilt of the epilayers with respect to their substrates has been observed in many heteroepitaxial systems. Many models have been proposed to explain this phenomenon, but none of them is suitable for the large mismatched system, such as GaAs/Si. Here a new model is proposed for GaAs/Si epilayers, which can also be used in other large mismatched systems. The magnitude of the tilt calculated from this model coincide well with the experimental results. Especially, this model can correctly predict the tilt direction of the GaAs/Si epilayers.

Assessment of the structural properties of GaAs/Si epilayers using X-ray (004) and (220) reflections

We improved the method previously used to determine the lattice constants and misorientation of GaAs/Si by recording the patterns of X-ray (004) and (220) reflections. The (220) reflection was measured from the (110) cross section of a GaAs/Si epilayer. The structural properties of the GaAs/Si epilayers grown by metal-organic chemical-vapor deposition (MOCVD) using an ultrathin a-Si buffer layer were investigated. The rotation angle of GaAs/Si epilayers grown by MOCVD using an a-Si buffer layer is very small and the lattice constants of these GaAs/Si epilayers agree quite well with elastic theory.