Mao Liu

Low-temperature synthesis of Mn3O4 hollow-tetrakaidecahedrons and their application in electrochemical capacitors

Mn3O4 hollow-tetrakaidecahedron micro/nano structures are synthesized basing on a structure-shrinkage-assisted Kirkendall effect under the hydrothermal condition. The Mn powders are transformed into Mn(OH)2 micro tetrakaidecahedron and substantially oxidized to Mn3O4 during the filtration under the flow of oxygen. The Kirkendall effect was proposed to have important responsibility on the formation of voids. Due to the structural shrinkage while transformation, the diffusion of substances from core to shell was found to be accelerated and thus larger holes formed. Electrochemical behavior of such hollow structure electrode was 148 F g−1. Mn2O3 and Mn5O8 micro hollow tetrakaidecahedron could also be obtained by annealing the corresponding Mn3O4 tetrakaidecahedrons in the air at different temperature.

Interface control and modification of band alignment and electrical properties of HfTiO/GaAs gate stacks by nitrogen incorporation

Effects of nitrogen incorporation on the interface chemical bonding states, optical dielectric function, band alignment, and electrical properties of sputtering-derived HfTiO high-k gate dielectrics on GaAs substrates have been studied by angle resolved X-ray photoemission spectroscopy (ARXPS), spectroscopy ellipsometry (SE), and electrical measurements. XPS analysis has confirmed that the interfacial layer of a HfTiO/GaAs gate stack is suppressed effectively after nitrogen incorporation. Analysis by SE has confirmed that reduction in band gap and increase in refractive index are observed with the incorporation of nitrogen. Reduction in valence band offset and increase in conduction band offset have been observed for a HfTiON/GaAs gate stack. Electrical measurements based on metal-oxide-semiconductor (MOS) capacitors have shown that the MOS capacitor with a HfTiON/GaAs stacked gate dielectric annealed at 600 °C exhibits low interface-state density (2.8 × 1012 cm−2 eV−1), small gate leakage current (2.67 × 10−5 A cm−2 at Vg = Vfb + V), and large dielectric constant (25.8). The involved mechanisms may originate from the decrease in the interface state density and the increase in the conduction band offset. The appropriate band offset relative to GaAs and excellent interface properties render HfTiON/GaAs as promising gate stacks in future III–V-based devices.

Effect of post deposition annealing on the optical properties of HfOxNy films

We report in this letter that Hafnium oxynitride (HfOxNy) gate dielectrics were prepared by radio frequency (RF) reactive magnetron sputtering and optical properties of the HfOxNy thin films were investigated by spectroscopic ellipsometry with photon energy of 0.75–6.5eV at room temperature. The investigations showed that the annealing temperatures have a strong effect on the optical properties of HfOxNy thin films. With increased annealing temperature, the refractive index n is observed to increase, while the extinction coefficient k decreases, respectively. The changes of the complex dielectric functions and the optical band gap Eg with the annealing temperature are also discussed.

The effect of nitrogen concentration on the band gap and band offsets of HfOxNy gate dielectrics

The effect of N concentration on the band gap and band offsets of HfOxNy films has been systematically investigated. It was found that the band gap as well as the band offsets of HfOxNy films decreased with the increase of N concentration. When the N concentration reached 16.3%, the conduction band offset (ΔEc) reduced to be 0.88eV, which is smaller than the minimal requirement of ΔEc values for high-k dielectrics and, thus, leads to unacceptably high leakage currents. Therefore, nitrogen concentration should be carefully controlled to guarantee excellent properties of nitrogen incorporated high-k dielectrics.

Effect of Ti incorporation on the interfacial and optical properties of HfTiO thin films

Interfacial and optical properties of HfTiO films with different Ti concentration grown by radio frequency reactive magnetron sputtering have been investigated by x-ray photoelectron spectroscopy (XPS) and spectroscopy ellipsometry (SE). XPS spectra indicate that interfacial layer is formed unavoidably for Ti doped and undoped HfO2 thin films and the interfacial structure is stable for Ti doped films. The composition of interfacial layer is most likely silicate and SiOx. Meanwhile, SE results indicate that the band gap of HfTiO thin films decreases with the increase in Ti concentration. Further results show that the valence band offset (ΔEv) decreases from 2.32 to 1.91 eV while the conduction band offset (ΔEc) decreases from 2.05 eV to 0.99 with the increase in Ti content. The optical constants consist of refractive index and extinction coefficient have also been investigated to provide the valuable references to prepare and select the HfTiO thin films for future high-k gate dielectrics.

Nitrogen dependence of band alignment and electrical properties of HfTiON gate dielectrics metal-oxide-semiconductor capacitor

The effect of nitrogen incorporation on the band alignment and electrical properties of HfTiO gate dielectric complementary metal-oxide-semiconductor (MOS) capacitors has been investigated by spectroscopy ellipsometry, x-ray photoelectron spectroscopy, and electrical measurements. Reduction in optical band gap and band offsets has been detected. However, improved electrical properties have been achieved from HfTiON gate dielectric MOS capacitors attributed to the nitrogen-induced reduction in oxygen-related traps and the improved interface quality. Although there is a nitrogen-induced reduction in band gap and band offset, the sufficient barrier heights still makes HfTiON a promising high-k gate dielectric candidate for proposed near-future complementary metal-oxide-semiconductor applications.

HfO2–GaAs metal-oxide-semiconductor capacitor using dimethylaluminumhydride-derived aluminum oxynitride interfacial passivation layer

In this letter, treatment of GaAs surface by using dimethylaluminumhydride-derived AlON passivation layer prior to HfO2 deposition is proposed to solve the issue of Fermi level pinning. It has been found that AlON passivation layer effectively suppresses the oxides formation and leads to the Fermi level unpinning at the interface between GaAs and HfO2. Based on analysis from metal-oxide-semiconductor capacitors of Au/HfO2/AlON/GaAs stack, excellent capacitance-voltage characteristics with saturated accumulation capacitance and reduced leakage current have been achieved, which may originate from the decrease in the interface state density and the increase in the conduction band offset.

Thickness-modulated optical dielectric constants and band alignments of HfOxNy gate dielectrics

Thickness-modulated optical dielectric constants and band alignments of HfOxNy films grown by sputtering have been investigated by spectroscopic ellipsometry (SE) and x-ray photoelectron spectroscopy. Based on SE measurements, it has been noted that an increase in optical dielectric constant and band gap has been observed as a function of the film thickness. Analyses of thickness-dependent band alignment of the HfOxNy/Si system indicate that the valence band offset increases, but only slight change in the conduction band offset, resulting from the thickness-induced change in the structure. The suitable optical dielectric constants and band offsets relative to Si make sputtering-derived HfOxNy film a promising candidate for high-k gate dielectrics.

Thermal nitridation passivation dependent band offset and electrical properties of AlOxNy/GaAs gate stacks

Fermi level pinning attributed to oxides at the GaAs/high-k interface is a major obstacle to develop GaAs-based metal-oxide-semiconductor devices with high performance. In this letter, thermal nitridation treatment of GaAs surface prior to the high-k deposition is proposed to solve the issue of interface pinning. Results have confirmed that nitridation passivation effectively suppresses the oxides formation and leads to a shift in the Fermi level toward the conduction band minimum on the GaAs surface, which increases the conduction band offset at the GaAs/AlOxNy interface, followed by a saturated accumulation capacitance with reduced gate leakage current.

Effect of thermal treatment on the band offsets and interfacial properties of HfOxNy gate dielectrics

Band offsets and interfacial properties of HfOxNy films grown on Si(1?0?0) through radio-frequency reactive magnetron sputtering were investigated by x-ray photoelectron spectroscopy. The changes in the conduction band offset, the valence band offset, as well as in the interfacial properties were obtained as a function of the annealing temperature. The interfacial layer is unavoidably formed for the deposited films and the composition of the interfacial layer is most likely Hf-silicate and SiOx. With the increase in the annealing temperature, it is confirmed that the reaction between SiOx and HfOxNy films formed more Hf-silicate interfacial layer. The valence band offset (?Ev) shifts upwards gradually with the increase in annealing temperature due to the decrease in N elements. After thermal treatment, the obtained conduction band offset (?Ec) increased from 1.20?eV at 500??C to 1.25?eV at 800??C.