Deliang Wang

AlGaN-GaN double-channel HEMTs

We present the design, fabrication, and characterization of AlGaN-GaN double-channel HEMTs. Two carrier channels are formed in an AlGaN-GaN-AlGaN-GaN multilayer structure grown on a sapphire substrate. Polarization field in the lower AlGaN layer fosters formation of a second carrier channel at the lower AlGaN-GaN interface, without creating any parasitic conduction path in the AlGaN barrier layer. Unambiguous double-channel behaviors are observed at both dc and RF. Bias dependent RF small-signal characterization and parameter extraction were performed. Gain compression at a high current level was attributed to electron velocity degradation induced by interface scattering. Dynamic IV measurement was carried out to analyze large-signal behaviors of the double-channel high-electron mobility transistors. It was found that current collapse mainly occurs in the channel closer to device surface, while the lower channel suffers minimal current collapse, suggesting that trapping/detrapping of surface states is mainly responsible for current collapse. This argument is supported by RF large-signal measurement results.

Mechanical characterization of suspended GaN microstructures fabricated by GaN-on-patterned-silicon technique

The mechanical properties of high-quality suspended GaN microstructures fabricated by GaN-on-patterned-silicon technique are characterized. Micro-Raman scattering is used to study the stress distribution in the GaN microstructures, and the measured results show that the stress in GaN microbeams decreases 47% when the silicon underneath the microbeams is removed. Microbeam bending test is used to measure the Young’s modulus of GaN films grown on silicon (111) substrate, yielding a Young’s modulus of 330 GPa.

Dye-sensitized solar cells with modified TiO2 surface chemical states: The role of Ti3+

Dye-sensitized solar cells (DSSCs) with TiO2 electrodes, which were modified to have different surface chemical states, were fabricated. The DSSCs had an ultra-flat TiO2 electrode, on which only a mono-layer dye was attached. The cell I-V measurement showed that Ti3+ ion had a dramatic effect on cell performance. The efficiency of a DSSC was shown to have a strong correlation with the concentration of the Ti3+ surface state, it decreased almost linearly with increased Ti3+ concentration. The oxygen vacancy-Ti3+ defect served as electron recombination center and decreased both the open-circuit voltage and the fill factor.

Thin film CdTe solar cells with an absorber layer thickness in micro- and sub-micrometer scale

CdTe thin film solar cell with an absorber layer as thin as 0.5 μm was fabricated. An efficiency of 7.9% was obtained for a 1-μm-thick CdTe solar cell. An increased intensity of deep recombination states in the band gap, which was responsible for the reduced open-circuit voltage and fill factor for ultra-thin solar cells, was induced due to the not-well-developed polycrystalline CdTe microstructure and the CdS/CdTe heterojunction and the presence of Cu in the back contact. The experimental results presented in this study demonstrated that 1-μm-thick absorber layer is thick enough to fabricate CdTe solar cell with a decent efficiency.

Growth of hexagonal GaN on Si(111) coated with a thin flat SiC buffer layer

Hexagonal GaN films were grown on Si(111) covered with a thin flat SiC buffer layer under both N- and Ga-rich growth conditions. A flat 2.5-nm-thick SiC layer was an effective buffer layer for GaN growth. The growth mode and microstructure of GaN depended strongly on the Ga/N flux ratios. Under N-rich growth conditions, the growth mode was three dimensional; GaN showed statistical roughening of the surface and a characteristic columnar structure. Under Ga-rich conditions, the GaN growth mode was two dimensional; GaN films with a flat surface and an almost stacking-fault-free microstructure were obtained. The two-dimensional growth mode was facilitated by strong wetting between Ga and SiC(111) at the first Ga-layer deposition on SiC.

Enhanced Performance of Perovskite CH3NH3PbI3 Solar Cell by Using CH3NH3I as Additive in Sequential Deposition.

Sequential deposition is a widely adopted method to prepare CH3NH3PbI3 on mesostructured TiO2 electrode for organic lead halide perovskite solar cells. However, this method often suffers from the uncontrollable crystal size, surface morphology, and residual PbI2 in the resulting CH3NH3PbI3, which are all detrimental to the device performance. We herein present an optimized sequential solution deposition method by introducing different amount of CH3NH3I in PbI2 precursor solution in the first step to prepare CH3NH3PbI3 absorber on mesoporous TiO2 substrates. The addition of CH3NH3I in PbI2 precursor solution can affect the crystallization and composition of PbI2 raw films, resulting in the variation of UV-vis absorption and surface morphology. Proper addition of CH3NH3I not only enhances the absorption but also improves the efficiency of CH3NH3PbI3 solar cells from 11.13% to 13.37%. Photoluminescence spectra suggest that the improvement of device performance is attributed to the decrease of recombination rate of carriers in CH3NH3PbI3 absorber. This current method provides a highly repeatable route for enhancing the efficiency of CH3NH3PbI3 solar cell in the sequential solution deposition method.

Heteroepitaxial growth of cubic GaN on Si(001) coated with thin flat SiC by plasma-assisted molecular-beam epitaxy

High-quality cubic GaN films were grown on Si(001) coated with flat ultrathin SiC under different Ga/N flux ratios. The 2.5-nm-thick cubic SiC film proved to be an effective buffer layer for cubic GaN growth on Si(001). Under a Ga-rich condition, films with local atomically flat surfaces were obtained, and the x-ray diffraction full-width at half maximum of (002) peak was 19 min for a 0.82-μm-thick film. The reduced SiC surface roughness decreased the defect density in the GaN epilayers. Under a N-rich condition, the GaN films showed statistical roughening of the surface and a characteristic columnar structure. Under the Ga-rich condition, the columns grew up and then laterally coalesced, so that an atomically flat surface with flat areas in size from 0.05 to 0.40 μm was formed.

High-efficiency CdTe thin-film solar cell with a mono-grained CdS window layer

High-crystalline-quality CdS thin films with a mono-grained layer of submicron grain size were successfully fabricated. CdS thin films prepared by chemical bath deposition were re-crystallized under different chemical atmospheres. The microstructure of the CdS films, the hetero-junction interface CdS/CdTe, and therefore the solar cell performance were critically dependent on the film process history of the window CdS layers. Heat treatment of a CdS precursor film coated with a CdCl2 layer and under a high CdCl2 vapor pressure reduced over-oxidation at the grain surface and promoted in-plane grain coalescence along the CdS/FTO (F-doped SnO2) interface. A high-crystalline-quality, mono-grain CdS layer ensured homogenous intermixing of CdS and CdTe at the junction interface. A short-circuit current as high as 25.1 mA cm−2 was obtained for a mono-grain-CdS/CdTe solar cell. The corresponding solar cell efficiency is 14.6%.

LATTICE VIBRATION FUNDAMENTALS IN NANOCRYSTALLINE ANATASE INVESTIGATED WITH RAMAN SCATTERING

A detailed nano-size and temperature-dependent Raman scattering study of the Eg(1) mode was carried out from 83 to 723 K for nanocrystalline anatase with sizes of 5.6, 8.6, and 19.4 nm. The Eg(1) Raman spectra were fitted and calculated on the basis of a combined model of both anharmonic coupling and phonon confinement. The temperature-dependent lattice vibration fundamentals are essentially the same for all these three nanocrystallites. Both the three- and four-phonon processes are needed to obtain excellent fitting, and the main contribution comes from the three-phonon processes. The anharmonic-decay-related phonon lifetime increases with decreasing nanocrystallite size, and the smaller nanoparticles have slower anharmonic decay. The phonon confinement has a comparative effect on both the linewidth broadening and the peak frequency shift, and it is responsible for a much shorter phonon lifetime at low temperatures for the 5.6 nm nanocrystallite.

AlGaN-GaN HEMTs on patterned silicon (111) substrate

We report the AlGaN-GaN high-electron mobility transistors (HEMTs) grown and fabricated on patterned silicon (111) substrates. A crack-free AlGaN-GaN HEMT heterostructure was grown on top of rectangular silicon ridges patterned on the silicon substrate. Fabrication of HEMT on the ridges was demonstrated using a polyimide planarization process. Maximum drain current density of 1.05 A/mm and peak transconductance of 150 mS/mm were achieved with 1.0 /spl mu/m gate-length. The current gain cutoff frequency and maximum frequency of oscillation were 9.7 and 20.5 GHz, respectively, for the 1 /spl mu/m /spl times/ 300 /spl mu/m devices.