Shuzhen Yu

Broadband quasi-omnidirectional antireflection AlGaInP window for III-V multi-junction solar cells through thermally dewetted Au nanotemplate

Al(Ga)InP subwavelength structures (SWS) were fabricated and optimized through thermally dewetted Au nanotemplate and ICP pattern-transfer. When λ< 900 nm, most AlGaInP nanostructures exhibit the reflectivity of less than 2% and insensitive to the incident angle up to 45°. When λ extends to 1800 nm, the reflectivity of less than 5% over 0°-45° is achieved in the optimized nanostructure, which benefits III-V multi-junction solar cells to improve their efficiency. Moreover, not only is such cost-effective nano-fabrication process completely compatible with the other processing of III-V solar cells, but their defined disordered SWS benefit the antireflection performance over broadband and wide view according to the comparison between the measurement and simulation results from AlGaInP SWS.

Single Crystal Growth and Spin Polarization Measurements of Diluted Magnetic Semiconductor (BaK)(ZnMn) 2 As 2

Recently a new diluted magnetic semiconductor, (Ba,K)(Zn,Mn)2As2 (BZA), with high Curie temperature was discovered, showing an independent spin and charge-doping mechanism. This makes BZA a promising material for spintronics devices. We report the successful growth of a BZA single crystal for the first time in this study. An Andreev reflection junction, which can be used to evaluate spin polarization, was fabricated based on the BZA single crystal. A 66% spin polarization of the BZA single crystal was obtained by Andreev reflection spectroscopy analysis.

Design and fabrication of six-volt vertically-stacked GaAs photovoltaic power converter.

A six-volt vertically-stacked, high current GaAs photovoltaic power converter (PPC) has been designed and fabricated to produce output power over 1 W under monochromatic illumination. An N++-GaAs/P++-AlGaAs tunnel junctions (TJs) structure has been used for connecting each sub-cell in this vertically-stacked PPC device. The thickness of the each GaAs sub-cell has been derived based on the calculation of absorption depth of photons with a wavelength of 808 nm using absorption coefficient obtained from ellipsometry measurements. The devices were characterized under non-uniform CW laser illumination at 808 nm with incident power up to 4.1 W. A maximum conversion efficiency of 50.2% was achieved at 0.3 W under non-uniform (coupled in optical fiber) monochromatic illumination, dropping to 42.5% at 4.1 W. The operating voltage at the maximum power point is 5.5–6.0 V, depending on the incident laser power, and an output electrical power output of 1.3 W can be extracted at a laser power of 2.9 W and the maximum electrical power output amounts to 1.72 W. The external quantum efficiency (EQE) measurement indicates that the performance of PPC can be further improved by refining the design of the thickness of sub-cells and improving TJs.

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.

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.