Hung Wei Yu

Threading Dislocation Blocking in Metamorphic InGaAs/GaAs for Growing High-Quality In0.5Ga0.5As and In0.3Ga0.7As on GaAs Substrate by Using Metal Organic Chemical Vapor Deposition

High quality In0.3Ga0.7As and In0.51Ga0.49As epilayers have been successfully grown on the GaAs substrate by MOCVD. A cross-sectional study by transmission electron microscopy showed that the threading dislocations (TDs) have been successfully contained and limited within the buffer layers designed to stop the elongation of TDs into the In0.3Ga0.7As and In0.51Ga0.49As epilayers. A TD density of 1?106 cm-2 in a fully relaxed In0.51Ga0.49As epilayer was achieved. The measurement of lifetimes of n- and p-type In0.51Ga0.49As has been done by using time-resolved photoluminescence. A great reduction in the number of recombination centers in the InGaAs epilayer has been shown.

High-Quality 1 eV In0.3Ga0.7As on GaAs Substrate by Metalorganic Chemical Vapor Deposition for Inverted Metamorphic Solar Cell Application

In0.3Ga0.7As layers were grown by metalorganic chemical vapor deposition using step graded buffer layers on different misoriented GaAs(001) substrates. Smooth-surface In0.3Ga0.7As film with a root mean square roughness of 1.9 nm was obtained with the growth temperature of 490 °C using a 10-step graded parabolic-like indium profile buffer layer on the surface with the 6°-off cut toward the [111] direction. The threading dislocation density in the film was determined to be 1.2×106 cm-2 by transmission electron microscopy. The photoluminescence results obtained at 300 and 77 K indicate that very low recombination centers existed in the epilayer.

Ti/Al/Ti/Ni/Au ohmic contacts on AlGaN/GaN high electron mobility transistors with improved surface morphology and low contact resistance

Optimizing surface morphology of ohmic contacts on GaN high electron mobility transistors continues to be a challenge in the GaN electronics industry. In this study, a variety of metal schemes were tested under various annealing conditions to obtain contacts with optimal qualities. A Ti/Al/Ti/Ni/Au (20/120/40/60/50u2009nm) metal scheme demonstrated the lowest contact resistance (Rc) and a smooth surface morphology, and the mechanisms were investigated by materials analysis. A Ti/Al/Ti/Ni/Au metal scheme with optimized Ti and Ni thicknesses can result in formation of a larger proportion of Al-Ni intermetallics and a continuous TiN interlayer, which results in smooth surface and low Rc.

Low resistance copper-based ohmic contact for AlGaN/GaN high electron mobility transistors

Ti/Al/Ni/Cu ohmic contact for AlGaN/GaN structure has been fabricated. The Ni layer played an important role in achieving low specific contact resistance (rc), smooth morphology, and excellent edge acuity. With a 50-A Ni layer, a rc of 1.35u2009×u200910−6 Ω-cm2 and a root-mean-square roughness of 7.65u2009nm have been realized. The characterization results indicated that no evidence of Cu diffusion into the semiconductor layers. The formation of Al-Cu and Ti-Cu alloys might have confined the Cu within the ohmic metal. In the absence of gold, the surface roughening caused by Au-Al alloy in conventional Ti/Al/Ni/Au structure was also prevented.

Impact of interfacial misfit dislocation growth mode on highly lattice-mismatched InxGa1-xSb epilayer grown on GaAs substrate by metalorganic chemical vapor deposition

Highly lattice-mismatch (over 8%) ternary InxGa1-xSb alloy directly grown on GaAs substrates was demonstrated by metalorganic chemical vapor deposition (MOCVD). The influence of growth parameters, such as growth temperature, indium vapor composition, and V/III ratio, on the film properties was investigated, and it was found that the growth temperature has the strongest effect on the surface morphology and the crystal quality of the InxGa1-xSb epilayer. An optimized growth temperature of ∼590u2009°C and a V/III ratio of 2.5 were used for the growth of the InxGa1-xSb epilayer on GaAs that displays a lower surface roughness. High-resolution transmission electron microscopy micrographs exhibit that InxGa1-xSb epilayer growth on GaAs was governed by the interfacial misfit dislocation growth mode. Furthermore, the variation of the intermixing layer thickness at the InxGa1-xSb/GaAs heterointerface was observed. These results provide an information of growing highly lattice-mismatched epitaxial material systems by MO...

InGaP/GaAs dual-junction solar cell with AlGaAs/GaAs tunnel diode grown on 10° off misoriented GaAs substrate

þþ -AlGaAs/N þþ -GaAs TDs grown on 10 � off GaAs substrates not only show a higher external quantum efficiency (EQE) but also generate a higher peak current density (Jpeak) at higher concentration ratios (185� ) than the solar cells with P þþ -GaAs/ N þþ -InGaP TDs grown on 6 � off GaAs substrates. Furthermore, the cell design with P þþ -AlGaAs/N þþ -GaAs TDs grown on 10 � off GaAs substrates

Effect of Two-Step Metal Organic Chemical Vapor Deposition Growth on Quality, Diameter and Density of InAs Nanowires on Si (111) Substrate

High-density (∼xa080/um2) vertical InAs nanowires (NWs) with small diameters (∼xa028xa0nm) were grown on bare Si (111) substrates by means of two-step metal organic chemical vapor deposition. There are two critical factors in the growth process: (1) a critical nucleation temperature for a specific In molar fraction (approximately 1.69xa0×xa010−5xa0atm) is the key factor to reduce the size of the nuclei and hence the diameter of the InAs NWs, and (2) a critical V/III ratio during the 2nd step growth will greatly increase the density of the InAs NWs (from 45xa0μm−2 to 80xa0μm−2) and at the same time keep the diameter small. The high-resolution transmission electron microscopy and selected area diffraction patterns of InAs NWs grown on Si exhibit a Wurtzite structure and no stacking faults. The observed longitudinal optic peaks in the Raman spectra were explained in terms of the small surface charge region width due to the small NW diameter and the increase of the free electron concentration, which was consistent with the TCAD program simulation of small diameter (<xa040xa0nm) InAs NWs.

The growth of pine-leaf-like hierarchical SnO2 nanostructures

Pine-leaf-like SnO2 hierarchical nanostructures (NSs) were grown by a two-step vapour transport deposition process with a combination of vapour–solid and vapour–liquid–solid mechanisms at the primary and secondary processes, respectively. This type of hierarchical structure consisted of SnO2 trunk with homo-branching nanowires (NWs). The branched NWs connected the trunk NWs at included angles of 56° and 90° for two different types of hierarchical NSs. Based on the thermodynamic calculation, the formation of branched NWs at those angles are all energetically favourable.

Performance improvement of highly mismatched GaSb layers on GaAs by interfacial-treatment-assisted chemical vapor deposition

Strain-relieved GaSb quantum dots on GaAs can be achieved by either periodic interfacial misfit (IMF) or the conventional Stranski–Krastanov (SK) growth modes by changing the growth parameters. In this study, the Sb interfacial treatment was employed to improve the GaSb crystal quality including low defect density, smooth surface morphology, and high hole mobility. This technique yields two-dimensional (2D) islands with a height as low as 1.7xa0nm and width up to 190xa0nm in the IMF growth mode. In contrast to the interfacial treatments conventionally employed in the initial strain relaxation of GaSb/GaAs hererostructure, the Sb treatment promotes the formation of strong Ga-Sb bonds on the surface of the grown island, which effectively reduces the interfacial free energy and thus promotes the formation of 2D islands. With the Sb interfacial treatment, a high-relaxation 100-nm GaSb epilayer was grown on the GaAs substrate, the epilayers was strain relaxed and exhibited enhanced electrical properties with a high hole mobility of ~667xa0cm2xa0V−1xa0s−1 and with superior optical properties as evidenced by the photoluminescence B-line peak. The results of this study demonstrate an effective interfacial-treatment growth technique to relax the initial strain for the highly mismatched GaSb layers grown on a GaAs substrate.

Hydrothermal growth of ZnO nanotubes on InGaP/GaAs/Ge solar cells

A new design where ZnO nanotubes were grown on the antireflection (AR) layer coated on triple-junction (T-J) solar cell devices to enhance the light conversion efficiency. Compared to the bare T-J solar cells (without an AR layer), the performance of Si3N4 AR coated solar cell showed improvement. The sample with a layer of ZnO nanotubes grown in top of AR layer showed the lowest light reflection compared with the bare and solely AR coated T-J solar cell especially in the spectrum range of 350-500 nm. The use of ZnO nanotubes have increased the conversion efficiency by 4.9% compared with the conventional T-J solar cell. While the Si3N4 AR coated sample only increased the conversion efficiency by 3.2%. This result is quite encouraging as further refinement and variation in the experiment procedures could possibly bring more exciting performance in the future.