Li-Kang Wang

Characterization of atomic-layer-deposited Al2O3∕GaAs interface improved by NH3 plasma pretreatment

Al2O3 thin films were deposited by atomic layer deposition on HF-cleaned and NH3 plasma-treated GaAs surfaces, respectively. The precursors used for Al2O3 films are trimethylaluminum and water. Effects of NH3 plasma pretreatment on the electrical and structural properties of Al2O3∕GaAs interface were investigated by C-V measurements, high-resolution transmission electron microscopy, and x-ray photoelectron spectroscopy measurements. The C-V measurements showed that the electrical property is improved after NH3 plasma pretreatment. X-ray photo electron spectroscopy analyses confirmed that GaAs oxides and elemental As are greatly decreased and the GaAs surface can be efficiently protected during NH3 plasma pretreatment and atomic layer deposition of Al2O3.

First principles calculations of oxygen vacancy passivation by fluorine in hafnium oxide

The fluorine incorporation into HfO2 with oxygen vacancies has been investigated using first principles calculations. The authors show that atomic fluorine can efficiently passivate the neutral oxygen vacancy with excess energies of 4.98 and 4.39eV for threefold- and fourfold-coordinated oxygen vacancy sites, respectively. The introduction of fluorine elevates the vacancy induced state into conduction band by transferring the neutral vacancy state to positively charged state, and thus removes the gap state which causes trap-assisted tunneling. The HfO2 band gap is not narrowed after fluorine incorporation.

High-temperature conduction behaviors of HfO2/TaN-based metal-insulator-metal capacitors

High-temperature (∼90–150 °C) conduction mechanisms of metal-insulator-metal (MIM) capacitors with atomic-layer-deposited HfO2 dielectric are studied. In the low field range, the Schottky emission current is dominant, and the deduced dielectric constant is close to the static one of HfO2. In the high field range, the resulting leakage current complies with the Poole-Frenkel (PF) emission, which is demonstrated by the fact that the extracted dielectric constant equals the optical frequency one (i.e., square of refractive index) of HfO2. The underlying mechanisms are discussed based on carrier velocities under different electric fields. Further, the deduced Schottky barrier height is ∼0.251–0.274 eV in the low field range, which relates to the contributions from high density traps in the HfO2 film and the nonideal TaN/HfO2 interface, etc. The extracted trap potential well depth for the PF effect is ∼1.11–1.37 eV in the high field range.

Atomic-layer-deposited Al2O3-HfO2 laminated and sandwiched dielectrics for metal–insulator–metal capacitors

Metal–insulator–metal (MIM) capacitors with atomic-layer-deposited Al2O3–HfO2 laminated and sandwiched dielectrics have been fabricated and electrically compared for analog circuit applications. The experimental results indicate that the laminated dielectrics exhibit much better leakage and breakdown characteristics than the sandwiched ones while maintaining higher capacitance densities and acceptable voltage linearity. In respect of the 1 nm Al2O3 and 10 nm HfO2 laminated dielectric, the resulting capacitor offers an extremely low leakage current of 2.4 × 10−9 A cm−2 at 8 V and a breakdown electric field of ~3.3 MV cm−1 at 125 °C together with a capacitance density of ~3.1 fF µm−2 and voltage coefficients of capacitance of 100 ppm V−2 and −80 ppm V−1 at 100 kHz. The superiority of the laminated dielectrics correlates with inhibition of HfO2 crystallization, discontinuity of the grain boundary channels from the top to the bottom and changes of the dielectric electronic properties due to the bonding and polarization effects at the multi-interfaces.

Spectroscopic and electrical properties of atomic layer deposition Al2O3 gate dielectric on surface pretreated Si substrate

Interfacial layer (IL) suppression is a big concern in atomic layer deposition (ALD) of high-k gate dielectrics. We have studied two kinds of surface pretreatments for ALD Al2O3 deposition: surface Al(CH3)3 (TMA) pretreatment (ST) and surface nitridation (SN). High resolution transmission electron microscopy images show that the IL has been effectively suppressed after performing surface pretreatments. X-ray photoelectron spectroscopy results indicate that both surface pretreatments are beneficial for compact Al2O3 deposition, however, the sample with SN shows much better interface thermal stability. And electrical characterizations demonstrate that the capacitor with SN exhibits better interface property, lower leakage current, and smaller capacitance-voltage (C-V) hysteresis. Based on transient capacitance measurement (C-t) and C-V measurement with different gate bias sweep ranges, a “shallow level trap” model is proposed to explain well the difference of C-V hysteresis between ST and SN.

Investigation of atomic-layer-deposited ruthenium nanocrystal growth on SiO2 and Al2O3 films

The growth of ruthenium (Ru) nanocrystals on the SiO2 and Al2O3 films has been investigated using atomic layer deposition (ALD) method, indicating that much higher density Ru nanocrystals are grown on the Al2O3 film compared with the SiO2 film. Ru nanocrystals with a density of 9×1010cm−2 are obtained on the Al2O3 film in the present experiment. The typical nanocrystal height increases with ALD cycles; however, the resulting nanocrystal density decreases for long deposition time (e.g., 600cycles) on the Al2O3 film, going with inferior size uniformity. Postdeposition annealing treatments at both 800 and 900°C cause a decrease in the nanocrystal density and an increase in the medial transverse dimension of nanocrystals. Prolonged annealing time at 900°C leads to a shrinkage of the medial transverse dimension of nanocrystals due to the formation of ball-like nanocrystals driven by minimizing the total surface energy. X-ray photoelectron spectroscopy analyses reveal metallic Ru nanocrystals surrounded by RuO2...

High density and program-erasable metal-insulator-silicon capacitor with a dielectric structure of SiO2∕HfO2–Al2O3nanolaminate∕Al2O3

We demonstrate a program-erasable metal-insulator-silicon capacitor with a dielectric structure of SiO2∕HfO2–Al2O3 nanolaminate (HAN)∕Al2O3. The memory capacitor exhibits a high capacitance density of 4.5fF∕μm2, a large memory window of 1.45 V in the case of +12Vprogram∕−12Verase for 5 ms, nearly symmetrical positive and negative flatband voltages under the program/erase operations with the same magnitudes of voltage and time, and no erase saturation. This is attributed to the fact that the introduction of atomic-layer-deposited high-dielectric-constant HAN∕Al2O3 layers increases the electric field across the tunnel oxide and reduces that across the blocking layer, hence, preventing effectively Fowler-Nordheim tunneling current through the blocking layer. Additionally, we find that the HAN is a promising charge storage layer with sufficient trapping centers for electrons and holes.

Quantum chemical study of the initial surface reactions of HfO2 atomic layer deposition on the hydroxylated GaAs(001)-4×2 surface

Initial surface reaction mechanism for atomic layer deposition of HfO2 on the hydroxylated GaAs(001)-4x2 surface using HfCl4 and H2O as precursors is investigated using hybrid density functional theory. The reaction between HfCl4 and H2O with the hydroxylated GaAs(001)-4x2 surface consists of two half-reactions: (1) HfCl4 with GaAs-OH sites and (2) H2O with Hf-Cl sites. The two half-reactions proceed through the formation of stable chemisorbed states, resulting in high activation barriers of 17.1 and 17.6 kcal/mol for HCl formation, respectively. Additional energies are also needed to desorb the respective physisorbed states HCl from the substrate surfaces.

Comparative study of passivation mechanism of oxygen vacancy with fluorine in HfO2 and HfSiO4

It has been experimentally observed that the incorporated fluorine will greatly improve the reliability of high-permittivity gate dielectric based transistors, but the role of fluorine passivation on leakage current change through gate is still a debated issue. The authors performed first-principles calculations to study the role of fluorine in oxygen vacancies in HfO2 and HfSiO4. They found that fluorine behaves completely differently in these two gate dielectrics. Fluorine can passivate the gap states of HfO2 completely but has no effect on the passivation of oxygen vacancies in HfSiO4, which is a possible explanation to the controversy surrounding varying leakage current.

Effects of chlorine residue in atomic layer deposition hafnium oxide: A density-functional-theory study

Hafnium tetrachloride is one of the most commonly used precursors for atomic layer deposition of HfO2. According to the experimental result, chlorine residue is almost unavoidably incorporated during the deposition process. We performed first-principles calculation to study the effects of chlorine residue in HfO2 and found that chlorine at the interstitial site serves as a source of negative fixed charge while chlorine at the oxygen substitutional site changes its charge state depending on the position of the electron chemical potential within the band gap of HfO2. Moreover, chlorine also reduces the band gap of HfO2 by raising the valence band maximum.