Xiao-Jie Liu

First-Principles Study on Electronic Structure of Gd-Doped HfO2 High k Gate Dielectrics

The 6.25 mol% Gd-doped HfO2 high k gate dielectrics were investigated by first-principles calculations, based on the density functional theory, using the code of Vienna ab-initio simulation package (VASP). Equilibrium crystal structures and electronic structures were characterized. The simulation cell consists of a cubic structure with 96 atoms (32 Hf atoms and 64 O atoms), two Gd substituted atoms for Hf (GdHf). To satisfy the charge neutrality of the system, we setup a complex defect [(GdHf)2VO]0 by removing an O atom near the two Gd atoms. There are four sevenfold coordination Hf atoms in the optimized structure of the supercell with 96 atomic sites. This is beneficial to the stabilization of the cubic phase with high dielectric constant and the decrease of oxygen vacancies. Meanwhile, this complex defect increases the band gap of Gd-doped HfO2. In addition, the charged defect [(GdHf)2VO]− was also explored. The d-d coupling of Hf 5d and Gd 5d antibond state electrons through an O atom (Gd-O-Hf) is confirmed theoretically, which is considered as the main reason for the broadening of the band gap. The calculation results are in accord with experimental data.

Growth of high-density Ir nanocrystals by atomic layer deposition for nonvolatile nanocrystal memory applications

A careful investigation is made of the growth of Ir nanocrystals (NCs) on Al2O3 by atomic layer deposition (ALD), and a charge trapping memory device using ALD-grown Ir NCs as the charge trapping layer and ALD-grown Al2O3/HfO2 as the tunneling/blocking layers is fabricated. It is found that the ex situ nucleation of Ir NCs on ALD-grown Al2O3 is difficult, though in situ growth can produce pure metallic Ir NCs with a face-centered cubic crystalline phase directly on ALD-grown Al2O3 at the initial growth stage, which follows the nucleation incubation model. The growth of these metallic Ir NCs is attributed to the presence of a uniform coverage of reactive groups (hydroxyl or dimethylaluminum) on the as-deposited fresh ALD-grown Al2O3 surface, which greatly promotes the uniform nucleation of Ir. Electrical measurements of p-Si/Al2O3/Ir NCs/HfO2 memory cells exhibit a large memory window of 4.2 V at the sweeping gate voltage of ±10 V, and a ∼76% retention property after 104 s at 75 °C. Also, a stable memory w...

Effect of annealing on interfacial and band alignment characteristics of HfO2/SiO2 gate stacks on Ge substrates

The authors have investigated the effect of 500 °C annealing for 60 s in N2 on interfacial and band alignment characteristics of HfO2/SiO2 gate stacks on n-type Ge substrates. X-ray photoelectron spectroscopy analyses reveal that the SiO2 interlayer can effectively suppress Ge outdiffusion during HfO2 growth and subsequent postdeposition annealing process. The electrical measurement shows that capacitance equivalent thickness of 1.75 nm and a leakage current density of 3.9 × 10−3 A/cm2 at gate bias of flatband voltage (Vfb) + 1 V was obtained for the annealed sample. The conduction band offsets at the HfO2/SiO2/Ge with and without annealing are found to be 2.22 and 2.07 eV, respectively.

Effects of Postannealing Temperature on the Band Alignments and Interfacial Properties of Atomic Layer Deposited Al2O3 on Ge Substrates

The effect of postannealing temperature (500, 600 and 700°C) on the band alignments and interfacial structures of Al2O3 films grown on Ge by atomic layer deposition was investigated by X-ray photoelectron spectroscopy and electrical measurements. With increasing postannealing temperature, the reaction between Al2O3 and Ge becomes serious and the valence-band offsets of Al2O3/Ge interface is observed to upshift. The conduction-band offsets at the interface of Al2O3/Ge without and with postannealing temperatures at 500, 600 and 700°C are estimated to be 2.90, 2.75, 2.59 and 2.41 eV, respectively.