We have confirmed the k-dependent spin splitting in wurtzite Al x Ga 1-x N/GaN heterostructures. Anomalous beating pattern in Shubnikov-de Haas measurements arises from the interference of Rashba and Dresselhaus spin-orbit interactions. The dominant mechanism for the k-dependent spin splitting at high values of k is attributed to Dresselhaus term which is enhanced by the ∆ C1 -∆ C3 coupling of wurtzite band folding effect.

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In this study, dye-sensitized solar cells (DSSCs) were fabricated using nanocrystalline titanium dioxide (TiO2) nanoparticles as photoanode. Photoanode thin films were prepared by doctor blading method with 420 kg/cm2 of mechanical compression process and heat treatment in the air at 500°C for 30 min. The optimal thickness of the TiO2 NP photoanode is 26.6 μm with an efficiency of 9.01% under AM 1.5G illumination at 100 mW/cm2. The efficiency is around two times higher than that of conventional DSSCs with an uncompressed photoanode. The open-circuit voltage of DSSCs decreases as the thickness increases. One DSSC (sample D) has the highest conversion efficiency while it has the maximum short-circuit current density. The results indicate that the short-circuit current density is a compromise between two conflict factors: enlargement of the surface area by increasing photoanode thickness and extension of the electron diffusion length to the electrode as the thickness increases.

The surface morphology of GaN epitaxial films grown by plasma-assisted molecular-beam epitaxy has been investigated. We found that the surface morphology was sensitive to the N to Ga flux ratio (N/Ga) when grown at a high temperature (i.e., 788 °C). At that temperature, we did not observe large sized Ga droplets on the surface even at Ga-rich conditions. Furthermore, we found a transition from two-dimensional (2D) to three-dimensional (3D) growth in the intermediate Ga-stable regime. The slope of the growth rate was different: Slope=(0.39±0.06) was observed in the 2D-growth mode and (0.14±0.03) in the 3D-growth mode. In the high N/Ga ratio, the total dislocation concentration was reduced, and the mixed threading dislocation concentration had a minimum value at N/Ga=22.5. By comparing with the Hall carrier concentration results, we found that the mixed threading dislocations influence the number of electronic carriers.

In this study, we prepared different shapes of gold nanoparticles by seed-mediated growth method and applied them on the photoelectrodes of dye-sensitized solar cells (DSSCs) to study the surface plasma resonant (SPR) effect of gold nanoparticles on the photoelectrodes of dye-sensitized solar cells. The analyses of field emission scanning electron microscopy show that the average diameter of the spherical gold nanoparticles is 45 nm, the average length and width of the short gold nanorods were 55 and 22 nm, respectively, and the average length and width of the long gold nanorods were 55 and 14 nm, respectively. The aspect ratio of the short and long gold nanorods was about 2.5 and 4, respectively. The results of ultraviolet–visible absorption spectra show that the absorption wavelength is about 540 nm for spherical gold nanoparticles, and the absorption of the gold nanorods reveals two peaks. One is about 510 to 520 nm, and the other is about 670 and 710 nm for the short and long gold nanorods, respectively. The best conversion efficiency of the dye-sensitized solar cells with spherical gold nanoparticles and short and long gold nanorods added in is 6.77%, 7.08%, and 7.29%, respectively, and is higher than that of the cells without gold nanoparticles, which is 6.21%. This result indicates that the effect of gold nanoparticles on the photoelectrodes can increase the conductivity and reduce the recombination of charges in the photoelectrodes, resulting in the increase of conversion efficiency for DSSCs. In addition, the long gold nanorods have stronger SPR effect than the spherical gold nanoparticles and short gold nanorods at long wavelength. This may be the reason for the higher conversion efficiency of DSSCs with long gold nanorods than those of the cells with spherical gold nanoparticles and short gold nanorods.

A new mechanism (∆ C1 -∆ C3 coupling) is accounted for the spin splitting of wurtzite GaN, which is originated from the intrinsic wurtzite effects (band folding and structure inversion asymmetry). The band-folding effect generates two conduction bands (∆ C1 and ∆ C3 ), in which p-wave probability has tremendous change when k z approaches anti-crossing zone. The spin-splitting energy induced by the ∆ C1 -∆ C3 coupling and wurtzite structure inversion asymmetry is much larger than that evaluated by traditional Rashba or Dresselhaus effects. When we apply the coupling to GaN/AlN quantum wells, we find that the spin-splitting energy is sensitively controllable by an electric field. Based on the mechanism, we proposed a p-wave-enhanced spin-polarized field effect transistor, made of In x Ga 1-x N/In y Al 1-y N, for spintronics application. _____________________ Keywords: GaN, Spintronics, Spin-field effect transistor, Rashba effect, Dresselhaus effect. PACS numbers: 71.15.Ap, 72.25.Dc, 73.21.Fg 2 I. IntroductionGate-controlled spin splitting in two dimensional electron system has been investigated in many zinc-blende III-V semiconductor quantum wells. 1,2 The gate-controlled spin splitting is arisen from the spin-orbit coupling 3 in zinc-blende structure with respect to inversion asymmetry.Carriers confined in asymmetric quantum wells will experience an effective magnetic field that may induce spin precession. 4 The manipulation of electron spins in a semiconductor is one of the key problems in the field of spintronics, in which additional degrees of freedom executed by electron spins are expected to play important roles in future nano-scaled electronic devices. 5,6 The spin splitting in zinc-blende III-V compound is induced either by a bulk inversion asymmetry of crystal potential (the k 3 -term, called Dresselhaus effect), 7 or by a structure inversion asymmetry of electrostatic confinement potential (the linear-k term, named Rashba effect). 8 Ganichev et al. have demonstrated the spin-orbital Hamiltonian of Rashba or Dresselhaus effects for zinc-blende InAs quantum well (QW) in terms of a k-dependent effective crystal magnetic field B eff (k); e.g., H SO = σ‧B eff (k), where k is the electron wave vector and σ the vector of Pauli matrices. The presence of B eff (k) implies that the spin orientation of electrons depends on the k-dependent Rashba and Dresselhaus terms. 4 Recently, Tsubaki et al. 9 and our group 10 independently observed a large spin-splitting energy (greater than 5 meV) in the 2DEG of GaN/AlGaN wurtzite heterostructures. The wurtzite GaN-based QW can be a potential candidate to realize the gate-controlled spin-polarized devices. However, the spin-splitting energy of wurtzite GaN, calculated by a traditional Rashba model (~1 meV), is much smaller than the measured values. 11,12 Reviewing the spin-orbital interaction in wurtzite semiconductors, Lew Yan Voon et al. have pointed out that, in addition to Dresselhaus k 3 -term, there exists a linear-k term spin-splitting energy caused by an intrinsic struc...

The edge and threading dislocations of M-plane GaN epilayers grown on γ-LiAlO2 have been studied by high-resolution transmission electron microscope. We found that edge dislocations were grown in [11¯00] direction while threading dislocations were generated along a1 or −a2 axes. We also observed a single stacking fault in the M-plane GaN epilayer.