Pure HfO Research Articles

Rapid formation of nanocrystalline HfO 2 powders from amorphous hafnium hydroxide under ultrasonically assisted hydrothermal treatment

Peculiarities of hafnium hydroxide hydrothermal decomposition were studied by in situ heat flux calorimetry for the first time. It was shown that this process occurs in one exothermal stage (Δ H = −17.95 kJ mol −1) at 180–250 °C resulting in complete crystallization of amorphous phase with formation of pure monoclinic HfO 2. It was found that the rate of m-HfO 2 formation can be significantly increased by combining hydrothermal treatment with simultaneous ultrasonic activation.

Synthesis and characterization of hafnium oxide and hafnium aluminate ultra-thin films by a sol–gel spin coating process for microelectronic applications

Currently there are intense industry-wide efforts in searching for new high dielectric constant (high- k) materials for use in future generations of ultra-large scale integrated circuits (ULSI). There are number of requirements for the new high- k materials, such as high dielectric constant, thermal stability (400 °C or higher), high mechanical strength, and good adhesion to neighboring layers. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological significance. Spinels exhibit a wide range of electronic and magnetic properties in particular nickel, hafnium, cobalt, containing spinels. In the present investigation, crack free, dense polycrystalline monoclinic structure of pure HfO 2, and Al 2HfO 5 ultra-thin films have been prepared by a simple and cost effective sol–gel spin coating method. The formation of the monoclinic HfO 2 phase at 600 °C and complete formation of the single phase Al 2HfO 5 at 800 °C has been reported. The composition of the annealed films has been measured and found to be 70 at.% of O, 30 at.% of Hf for HfO 2 and 22 at.% of Al, 12 at.% of Hf and 66 at.% of O for Al 2HfO 5 films, which are close to the stoichiometry of the HfO 2 and Al 2HfO 5 thin films.

X-ray photoelectron spectroscopy of erbium-activated-silica–hafnia waveguides

X-ray photoelectron spectroscopy (XPS) has been used in the study of sol gel-derived Er 3+-activated xHfO 2–(100 − x)SiO 2 ( x = 10, 20, 30, 40, 50 mol) planar waveguides. The analysis of Si 2p and O 1s core lines were related to the Hf/Si molar ratio to assess the role of hafnia in modifying the silica network. Increasing the HfO 2 content brings about a change of the Si 2p and O 1s binding energy respect to those from pure silica. This trend is explained with a formation of hafnium silicate in the matrix with successive phase separation between HfO 2 and SiO 2 rich phases. XPS results show that hafnia is well dispersed in the silica matrix for molar concentration below 30%. Formation of pure HfO 2 domains was detected at higher hafnia concentrations in agreement with previous spectroscopic analyses.

Electrical property of HfO xN y–HfO 2–HfO xN y sandwich-stack films

The effects of HfO x N y on the electrical property of HfO x N y –HfO 2–HfO x N y sandwich-stack (signed as SS) films were investigated. Excellent electrical performances were achieved in SS films, with a high dielectric constant of 16 and a low leakage current of ∼2 × 10 −8 A/cm 2 at 1 MV/cm. Schottky (SK) emission and Frenkel–Poole (PF) emission are found to be the dominant mechanisms for the current conduction behavior. After a long time stress, the flat-band voltage shift in the SS film is much smaller than that in a pure HfO x N y film indicating fewer charge traps existed in the SS film. Based on the experiments, the new SS structure is more favorable for the improvement of electrical performances than a pure HfO x N y or HfO 2 structure.

Characterization of HfO xN y gate dielectrics using a hafnium oxide as target

Hafnium oxynitride (HfO x N y ) gate dielectric has been deposited on Si (1 0 0) by means of radio frequency (rf) reactive sputtering using directly a HfO 2 target in N 2/Ar ambient. The thermal stability and microstructural characteristics for the HfO x N y films have been investigated. XPS results confirmed that nitrogen was successfully incorporated into the HfO 2 films. XRD analyses showed that the HfO x N y films remain amorphous after 800 °C annealing in N 2 ambient. Meanwhile the HfO x N y films can also effectively suppress oxygen diffusion during high temperature annealing and prevent interface layer from forming between HfO x N y films and Si substrates. AFM measurements demonstrated that surface roughness of the HfO x N y films increase slightly as compared to those pure HfO 2 films after post deposition annealing. By virtue of building reasonable model structure, the optical properties of the HfO x N y films have been discussed in detail.

Microstructure and interfacial properties of HfO 2–Al 2O 3 nanolaminate films

High- k HfO 2–Al 2O 3 composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO 2–Al 2O 3 films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO 2–Al 2O 3 composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO 2 thin films. FTIR characterization indicates that the growth of the interfacial SiO 2 layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO 2–Al 2O 3 composite films has been increased compared to pure HfO 2 films. Al 2O 3 as a passivation barrier for HfO 2 high- k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.

Investigation of the chemical state of ultrathin Hf–Al–O films during high temperature annealing

Al 2O 3 incorporated HfO 2 films grown by atomic layer deposition (ALD) were investigated by high-resolution X-ray photoelectron spectroscopy (HRXPS). The core level energy state of a 15 Å thick film showed a shift to higher binding energy, as the result of a silicate formation and Al 2O 3 incorporation. The incorporation of Al 2O 3 into the HfO 2 film had no effect on silicate formation at the interface between the film and Si, while the ionic bonding characteristics and hybridization effects were enhanced compared to a pure HfO 2 film. The dissociation of the film in an ultrahigh vacuum (UHV) is effectively suppressed compared to a pure HfO 2 film, indicating an enhanced thermal stability of Hf–Al–O. Any dissociated Al 2O 3 on the film surface was completely removed into the vacuum by vacuum annealing treatment over 850 °C, while HfO 2 contributed to Hf silicide formation on the film surface.

A study of mixtures of HfO 2 and TiO 2 as high- k gate dielectrics

Hf x Ti 1− x O 2 films have been deposited by chemical vapor deposition. Permittivities of approximately 50 have been obtained. The films are stable up to 1000 °C. The interface properties and charge trapping appear to depend on composition. For films with approximately equal Hf and Ti concentration, transistor characteristics and inversion layer mobilities are comparable to those obtained when using pure HfO 2.

Elaboration, structural and spectroscopic properties of rare earth-doped yttrium–hafnium sol–gel oxide powders for scintillation applications

Hafnium dioxide (HfO 2) presents a high crystalline density (≈10 g/cm 3) which makes it attractive for host lattice activated by rare earths (RE) for applications as scintillating materials. The potentiality to prepare Eu 3+ and Tb 3+ activated HfO 2 sol–gel powders, with high scintillation yield, is explored. The powders are heat-treated at 1000 °C before analyses. The incorporation of yttrium (Y 3+) in various concentrations is conducted to vary the lattice phase and to stabilize the trivalent terbium ions. The influence of Y 3+ on the microstructure and then on the scintillation properties of the material is presented. A high concentration of Y 3+ (20 mol%) stabilizes pure HfO 2 tetragonal phase whatever RE (1 mol%) doping. The powders with the highest relative scintillation yield are Eu 3+:HfO2 without Y 3+ incorporation and crystallized into the monoclinic phase and Y 3+ (20 mol%): Tb 3+:HfO2 crystallized into the tetragonal phase. Sequential energy transfer process is assumed to explain these results.

Étude thermodynamique des systèmes Hf-I-H-C-O: Préparation du carbure et de l'oxyde de hafnium par dépôt chimique à partir d'une phase vapeur

Conditions for the chemical vapor deposition of pure hafnium carbide and hafnium dioxide were determined by a thermodynamic investigation of Hf-I-H-C-O systems. A quantitative deposition of carbon free HfC can be obtained above 1500 K from a mixture of methane with an excess of HfI 4. Pure HfO 2 can also be quatitatively deposited from HfI 4-H 2-CO 2 mixtures if hydrogen and carbon dioxide are in excess.

Metal oxide analogue of metal alloy catalysts

Because of ion size, valence and oxide structure, hafnium and zirconium mixed oxide catalysts are ideal for study as a metal alloy analogue. Furthermore, hafnium oxide and zirconium oxide exhibit quite different selectivity for the conversion of 2-alcohols to alkene products. The alkene selectivity changed abruptly from that of pure HfO 2 to that of pure ZrO 2 at a composition with ca. 90 mol% Zr. Characterization of the catalysts by ESCA, Auger, and ISS spectroscopy show that the surface composition is very similar to the bulk composition and, thus, changes uniformly throughout the composition range rather than abruptly as does the alkene selectivity. Pure ZrO 2 impregnated with Hf to produce catalysts with the full range of Hf-Zr surface compositions showed a catalytic selectivity that paralleled the selectivity of the bulk composition catalysts. Zr or Th impregnated onto alumina serve as catalyst poisons; however, these two metals alter the selectivity in a quite different manner. The data indicate that the catalytic selectivity of these mixed oxide catalysts is determined by ligand, rather than bulk electronic, effects.

Characterization of metastable tetragonal hafnia

Conditions for retaining metastable tetragonal HfO 2 at room temperature were determined. Mechanical properties were measured as a function both of the fraction of retained metastable HfO 2 and of the porosity. The metastable phase was retained by 1) thermal decomposition of pure Hf(OH) 4 or HfOCl 2 and 2) hot pressing of HfO 2 + 1% Er 2O 3. The critical size for retaining pure tetragonal HfO 2 at room temperature in a stress free condition is 100 Å, whereas the critical size for the 1% Er 2O 3 hot pressed composition is significantly larger. The elastic moduli were found to vary linearly with porosity and showed a small effect due to the metastable phase presence.