Yongming Zhao

Characteristics of InP on GaAs substrate using Zn-doped Al(Ga)InAs metamorphic buffers

We have investigated the effects of Zn doping on the surface morphology and the dislocation movement of the metamorphic Al(Ga)InAs buffers grown by metal–organic chemical vapor deposition (MOCVD) on (001) GaAs substrate with 15° miscut toward (111)A. It is found that surface morphology and roughness are governed by the amount of In atoms and their diffusion on the surface, and Zn-doping suppresses the In segregation to weaken the formation of phase separation which prevents the dislocations gliding and increases the threading dislocation density. In addition, the strong bond strength between Zn impurity atoms and sublattice site atoms plays an important role for dislocation motion. In other words, propagating dislocation in Al(Ga)InAs metamorphic buffers will be stopped at a position adjacent to the Zn impurities to decrease the interaction between the surface morphology and dislocation pile-up to reduce the threading dislocation density in InP layer.

Control of threading dislocations by Al(Ga)InAs reverse-graded buffers grown on GaAs substrates

High-quality strain-relaxed InP layers with undulating step-graded Al(Ga)InAs buffers were grown on GaAs substrates by metal–organic chemical vapor deposition. Transmission electron microscopy, high-resolution electron microscopy (HREM), atom force microscopy, and photoluminescence were carried out to characterize the metamorphic buffers. V-shaped dislocations in [001] Al(Ga)InAs reverse-graded layers were observed by HREM and the behavior of reverse-graded layers was simulated theoretically using analytical models. Both the experimental and theoretical results indicated that the insertion of reverse-graded layers with appropriately designed thicknesses and In grading coefficients promotes the annihilation and coalescence reactions between threading dislocations and reduces threading dislocations density.

Effect of High-Temperature Pregrowth Treatment on the Surface Morphology of GaInP Epilayers on Ge Grown by Metal-Organic Vapor-Phase Epitaxy

Surface morphologies of GaInP films grown on germanium (Ge) by metal–organic vapor-phase epitaxy (MOVPE) have been investigated for different pre-growth treatments. A high temperature pre-annealing to the Ge substrate results in a smooth surface of GaInP, while a surface degradation in the case of a low temperature annealing is obtained, which might come from an incomplete carbon and/or oxide desorption. An improved surface morphology of GaInP with silicon doping and with increasing Ge substrate misoriented angle was also observed, which indicates that the highly disordered GaInP film and the neat interface between Ge and GaInP were formed with the assistance of a high temperature pre-growth treatment.

Characterizations of high-voltage vertically-stacked GaAs laser power converter

Six-junction vertically-stacked GaAs laser power converters (LPCs) with n+-GaAs/p+-Al0.37Ga0.63As tunnel junctions have been designed and grown by metal-organic chemical vapor deposition for converting the power of 808 nm lasers. The LPC chips are characterized by measuring current–voltage (I–V) characteristics under 808 nm laser illumination, and a maximum conversion efficiency ηc of 53.1% is obtained for LPCs with an aperture diameter of 2 mm at an input laser power of 0.5 W. In addition, the characteristics of the LPCs are analyzed by a standard equivalent-circuit model, and the reverse saturation current, ideality factor, series resistance and shunt resistance are extracted by fitting of the I–V curves.

Four-junction AlGaAs/GaAs laser power converter

Four-junction AlGaAs/GaAs laser power converters (LPCs) with n + -GaAs/p + -Al 0.37 Ga 0.63 As heterostructure tunnel junctions (TJs) have been designed and grown by metal-organic chemical vapor deposition (MOCVD) for converting the power of 808 nm lasers. A maximum conversion efficiency η c of 56.9% ± 4% is obtained for cells with an aperture of 3.14 mm 2 at an input laser power of 0.2 W, while dropping to 43.3% at 1.5 W. Measured current–voltage ( I – V ) characteristics indicate that the performance of the LPC can be further improved by increasing the tunneling current density of TJs and optimizing the thicknesses of sub-cells to achieve current matching in LPC.

Dislocation distributions and tilts in Al(Ga)InAs reverse-graded layers grown on misorientated GaAs substrates

Compositionally undulating step-graded Al(Ga)In x As (x = 0.05?0.52) buffers with the following InP layer were grown by metal?organic chemical vapor deposition (MOCVD) on (001) GaAs with a 15? miscut. The dislocation distribution and tilts of the epilayers were examined using x-ray rocking curve and (004) reciprocal space maps (RSM) along two orthogonal 110 directions. The results suggested that such reverse-graded layers have different effects on ? and ? dislocations. A higher dislocation density was observed along the [110] direction and an epilayer tilt of ?1.43? was attained in the [1-10] direction when a reverse-graded layer strategy was employed. However, for conventional step-graded samples, the dislocation density is normally higher along the [1-10] direction.

Low specific contact resistivity to graphene achieved by AuGe/Ni/Au and annealing process*

Low metal–graphene contact resistance is important in making high-performance graphene devices. In this work, we demonstrate a lower specific contact resistivity of Au0.88Ge0.12/Ni/Au–graphene contact compared with Ti/Au and Ti/Pt/Au contacts. The rapid thermal annealing process was optimized to improve AuGe/Ni/Au contact resistance. Results reveal that both pre- and post-annealing processes are effective for reducing the contact resistance. The specific contact resistivity decreases from 2.5 × 10−4 to 7.8 × 10−5 Ωcm2 by pre-annealing at 300 °C for one hour, and continues to decrease to 9.5 × 10−7 Ωcm2 after post-annealing at 490 °C for 60 seconds. These approaches provide reliable means of lowering contact resistance.

Control of symmetric properties of metamorphic In0.27Ga0.73As layers by substrate misorientation

Asymmetry in dislocation density and strain relaxation has a significant impact on device performance since it leads to anisotropic electron transport in metamorphic materials. So it is preferred to obtain metamorphic materials with symmetric properties. In this paper, we grew metamorphic In0.27Ga0.73As epilayers with symmetric low threading dislocation density and symmetric strain relaxation in two 〈110〉 directions using InAlGaAs buffer layers on 7° misoriented GaAs (001) substrates. To understand the control mechanism of symmetric properties of In0.27Ga0.73As layers by the substrate miscut angles, In0.27Ga0.73As grown on 2° and 15° misoriented substrates were also characterized as reference by atomic force microscopy, transmission electron microscopy, and high resolution triple axis x-ray diffraction. The phase separation and interaction of 60° misfit dislocations were found to be the reasons for asymmetry properties of In0.27Ga0.73As grown on 2° and 15° substrates, respectively. Photoluminescence results proved that the In0.27Ga0.73As with symmetric properties has better optical properties than the In0.27Ga0.73As with asymmetric properties at room temperature. These results imply that high quality metamorphic In0.27Ga0.73As can be achieved with controllable isotropic electron transport property.

Effects of Substrate Miscut on the Quality of In0.3Ga0.7As Layers Grown on Metamorphic (Al)GaInP Buffers by Metal–Organic Chemical Vapor Deposition

In0.3Ga0.7As layers were grown by metal–organic chemical vapor deposition utilizing compositionally step-graded (Al)GaInP buffers on different misorientated GaAs substrates. The substrate miscut toward the [110] direction promotes α dislocation glide along the [110] direction while it exerts an adverse effect on β dislocations in the perpendicular direction with increasing the miscut degree. In comparison with the 2° and 7° samples, a better surface morphology with a RMS roughness of 5.77 nm and a lower density of threading dislocations indicated by the photoluminescence and transmission electron microscopy results were obtained in the 15° sample.