A. D. Li

The effect of acoustic phonon scattering on the carrier mobility in the semiconducting zigzag single wall carbon nanotubes

Carrier mobilities of the semiconducting single wall carbon nanotubes (SWCNTs) have been studied by using the first-principles calculations with the deformation potential approximation, which only considers the scattering by the longitudinal acoustic phonons based on the adapting Bardeen–Shockley theory [J. Bardeen and W. Shockley, Phys. Rev. 80, 72 (1950)] to one-dimensional case. From the band structures of the semiconducting SWCNTs, we calculated the effective masses, the stretching modulus, the deformation potential constants. We demonstrated that the calculated intrinsic carrier mobility can reach 106 cm2/V s at room temperature, and the carrier mobilities of the semiconducting SWCNTs show the intriguing alternating behavior.

Optical and electrical properties study of sol-gel derived Cu2ZnSnS4 thin films for solar cells

The fabrication of environmental-friendly Cu2ZnSnS4 (CZTS) thin films with pure kesterite phase is always a challenge to researchers in the field of solar cells. We introduce a simple non-vacuum sol-gel method to fabricate kesterite CZTS films. Ethylenediamine is used as the chelating agent and stabilizer and plays an important role in preparing stable precursor. X-ray diffraction, Raman and scanning electron microscopy studies suggest that the microstructure and optical properties of CZTS films depend strongly on annealing temperatures. The temperature dependence of conductivity of 500 °C annealed CZTS film shows that the Mott law dominates in the low temperature region and thermionic emission is predominant at high temperatures.

High-resolution electron microscopy investigations on stacking faults in SrBi2Ta2O9 ferroelectric thin films

Structural planar defects in the SrBi2Ta2O9 (SBT) films with 10 mol % excess Bi grown in Pt/TiO2/SiO2/Si substrates by metalorganic deposition have been observed by high-resolution electron microscopy. It was found that these stacking defects were planar defects with extra Bi–O planes normal to the c axis. These structural defects are expected to effectively improve the ferroelectric response and fatigue-resistance characteristics of SBT films because of the extra Bi–O planes having higher strucutral flexibility and alleviating the mechanical stresses and strains as well as injected-charge problems.

Remarkable charge-trapping efficiency of the memory device with (TiO2)0.8(Al2O3)0.1 composite charge-storage dielectric

A memory device p-Si/SiO2/(TiO2)0.8(Al2O3)0.1(TAO-81)/Al2O3/Pt was fabricated, in which a composite of two high-k dielectrics with a thickness of 1 nm was employed as the charge-trapping layer to enhance the charge-trapping efficiency of the memory device. At an applied gate voltage of ±9 V, TAO-81 memory device shows a memory window of 8.83 V in its C-V curve. It also shows a fast response to a short voltage pulse of 10−5 s. The charge-trapping capability, the endurance, and retention characteristics of TAO-81 memory device can be improved by introducing double TAO-81 charge-trapping layers intercalated by an Al2O3 layer. The charge-trapping mechanism in the memory device is mainly ascribed to the generation of the electron-occupied defect level in the band gap of Al2O3 induced by the inter-diffusion between TiO2 and Al2O3.

Comparison of the interfacial and electrical properties of HfAlO films on Ge with S and GeO2 passivation

We report the characteristics of HfAlO films deposited on S- and GeO2-passivated Ge substrates at 150 °C by atomic layer deposition technique using Hf(NO3)4 and Al(CH3)3 as the precursors. The x-ray photoelectron spectroscopic analyses reveal that GeO2 passivation is more effective to suppress GeOx formation than S passivation. It is demonstrated that the capacitors with GeO2 passivation exhibit better electrical properties with less hysteresis, improved interface quality, and reduced leakage current. These results indicate that using GeO2 as an interfacial layer may be a promising approach for the realization of high quality Ge-based transistor devices.

Determination of the density of the defect states in Hf0.5Zr0.5O2 high-k film Deposited by using rf-magnetron sputtering technique

A memory structure Pt/Al2O3/Hf0.5Zr0.5O2/Al2O3/p-Si was fabricated by using atomic layer deposition and rf-magnetron sputtering techniques, and its microstructure has been investigated by using the high resolution transmission electron microscopy (HRTEM). By measuring the applied gate voltage dependence of the capacitance for the memory structure, the planar density of the trapped charges in Hf0.5Zr0.5O2 high-k film was estimated as 6.63 × 1012 cm−2, indicating a body defect density of larger than 2.21 × 1019 cm−3. It is observed that the post-annealing in N2 can reduces the defect density in Hf0.5Zr0.5O2 film, which was ascribed to the occupancy of oxygen vacancies by nitrogen atoms.

Metalorganic Chemical Vapor Deposition of Lanthanum Aluminate Thin Films for Gate Dielectrics

Amorphous lanthanum aluminate (LAO) gate dielectric has been grown on silicon substrates by low-pressure metalorganic chemical vapor deposition. The structure, morphology, thermal stability, bandgap, and electrical properties have been characterized by various techniques. The LAO films exhibit excellent thermal stability and keep noncrystalline structure even after 900°C rapid thermal anneal. The smooth surface and larger bandgap are also confirmed. By optimizing the processing, the minimume value of equivalent oxide thickness around 0.9 nm of LAO/Si has been achieved.

Ta 2 O 5 -TiO 2 Composite Charge-trapping Dielectric for the Application of the Nonvolatile Memory

The charge-trapping memory devices with a structure Pt/Al2O3/(Ta2O5)x(TiO2)1−x/Al2O3/p-Si (x = 0.9, 0.75, 0.5, 0.25) were fabricated by using rf-sputtering and atomic layer deposition techniques. A special band alignment between (Ta2O5)x(TiO2)1−x and Si substrate was designed to enhance the memory performance by controlling the composition and dielectric constant of the charge-trapping layer and reducing the difference of the potentials at the bottom of the conduction band between (Ta2O5)x(TiO2)1−x and Si substrate. The memory device with a composite charge storage layer (Ta2O5)0.5(TiO2)0.5 shows a density of trapped charges 3.84 × 1013/cm2 at ± 12 V, a programming/erasing speed of 1 µs at ± 10 V, a 8% degradation of the memory window at ± 10 V after 104 programming/erasing cycles and a 32% losing of trapped charges after ten years. The difference among the activation energies of the trapped electrons in (Ta2O5)x(TiO2)1−x CTM devices indicates that the retention characteristics are dominated by the difference of energy level for the trap sites in each TTO CTM device.

Comparative study on the charge-trapping properties of TaAlO and ZrAlO high-k composites with designed band alignment

The charge-trapping memory (CTM) structures Pt/Al2O3/TaAlO/Al2O3/p-Si and Pt/Al2O3/ZrAlO/Al2O3/p-Si were fabricated by using rf-sputtering and atomic layer deposition techniques, in which the potentials at the bottom of the conduction band (PBCB) of high-k composites TaAlO and ZrAlO were specially designed. With a lower PBCB difference between TaAlO and p-Si than that between ZrAlO and p-Si, TaAlO CTM device shows a better charge-trapping performance. A density of trapped charges 2.88 × 1013/cm2 at an applied voltage of ±7 V was obtained for TaAlO CTM device, and it could keep about 60% of initially trapped charges after 10 years. It was suggested that the PBCB difference between high-k composite and p-Si dominates their charge-trapping behaviors.

Preparation and Electro-Optic Properties of Poled Nanocrystals and Polymer Composite BaTiO3/PC Films

BaTiO 3 /polycarbonate (BT/PC) composite thin films were prepared by spin coating method. Ultra-fine BaTiO 3 (BT) (∼40–50 nm) nanocrystals with pure perovskite tetragonal phase were synthesized by hydrothermal method. The structure of BT nanocrystals and composite films were studied by means of x-ray diffraction and transmission electron microscopy. The poling condition was optimized through the dielectric properties of BT and PC polymer and effective field intensity theory. The EO coefficients of 20% wt doped BT nanocrystals composite films were evaluated to 34 pm/v. The orientation degree of nanocrystals in PC polymer was estimated by measuring the transparency spectra of unpoled and poled composite thin films.