Masakazu Ichikawa

Visible photoluminescence of Ge dots embedded in Si/SiO2 matrices

Ge island formation on ultrathin SiO2 films enabled us to fabricate multilayer structures of Ge dots ∼6–7 nm in diameter and with an extremely high dot density of 2×1012 cm−2. Each dot had a boundary with the SiO2 film and a Si spacer layer. The multilayer structures exhibited photoluminescence (PL) with a maximum in the range of 2–3 eV depending on the excitation energy. The PL was associated with recombination between holes confined within Ge dots and electrons localized in the radiative defect centers at the Ge-dot/SiO2 interfaces. The results suggest that this recombination is much more effective than that at the Si/SiO2 interface and supported by the hole migration from the Si spacer layers to the Ge dots.

Void nucleation in thin HfO2 layer on Si

We examined void nucleation in a thin HfO2 film on Si at 820–920 °C in an ultrahigh vacuum. The clustering of mobile species on the HfO2 surface led to the opening of micron-scale voids containing Hf silicide. The incubation period observed for void nucleation exhibited transition of decomposition process with activation energies of 2.3 eV ( 890 °C). We propose that the former energy corresponds to the creation of mobile species and the latter to the decomposition of the HfO2 film under the cluster.

Effect of Al2O3 capping layer on suppression of interfacial SiO2 growth in HfO2/ultrathin SiO2/Si(001) structure

We investigated the effect a 1.2-nm-thick Al2O3 capping layer had on suppressing interfacial Si oxidation in a 2.6-nm-HfO2/0.35-nm-SiO2/Si(001) structure during postdeposition annealing in an oxygen ambient. An incubation period (IP) was initially observed during which the HfO2/Si interface exhibited remarkable stability without any interfacial SiO2 growth. This was then followed by very slow interface oxidation. Our detailed study suggested that low oxidant diffusion through the capping layer determined the effective IP. Furthermore, HfO2/Si interface oxidation, which proceeded through a two-step process that was similar to an uncapped structure, was severely constrained by the limited availability of oxygen at the Al2O3/HfO2 interface.

Investigation of the effect of high-temperature annealing on stability of ultrathin Al2O3 films on Si(001)

We investigated the stability of a uniform and stoichiometric 0.6-nm-thick Al2O3 film on a Si(001) surface during high-temperature annealing in ultrahigh vacuum (UHV), under low oxygen pressure (2×10−6, 5×10−6, and 2×10−5 Torr O2), and under high oxygen pressure (5×10−5 Torr O2) conditions. UHV annealing of the Al2O3/Si(001) system at 900 °C drastically degraded the Al2O3 film quality and caused atomic-scale roughness at the Al2O3/Si(001) interface. Voids formed in the oxide film as annealing progressed. A low oxygen pressure ambient during annealing, while more or less maintaining the film stoichiometry, caused atomic-scale roughness at the interface. A high oxygen pressure ambient during annealing maintained the film stoichiometry and thickness. However, this processing condition led to the formation of interfacial Si oxide, which caused substantial SiO volatilization and etching of the Si substrate at the Al2O3/Si(001) interface, thereby inducing nanometer-scale roughness at the interface. These result...

Analysis of interfacial silicates and silicides formed by annealing ultrathin Hf on SiO2: Effect of Hf/SiO2 thickness ratio

The annealing of two different ultrathin Hf/SiO2 stacks, i.e., Hf rich (1.7 ML/0.3 nm) and SiO2 rich (1 ML/1 nm) is investigated in situ in an ultrahigh vacuum (UHV) by using scanning tunneling microscopy and x-ray photoelectron spectroscopy. To describe the interface structure formed in practical high-k processes, this approach conjectures the effects of underlying SiO2 on the stability of metal–silicon and metal–oxygen bondings, which would subsequently determine the interfacial phases. The annealing of these film stacks causes silicate formation, but the relative thickness ratio between Hf and SiO2 is found to greatly affect a phase stability of interfacial silicates in a high-temperature (⩾780 °C) regime. As the underlying SiO2 thickens, the Hf–Si bondings are expected to be replaced with Hf–O–Si (silicate) bonding units, even at room temperature deposition in an UHV. In the Hf-rich stack (Hf–Si bonding dominant), phase separation into silicides was observed at a relatively low temperature (∼780 °C) c...

Effect of oxygen pressure on the structure and thermal stability of ultrathin Al2O3 films on Si(001)

We were successful in the growth of uniform, stoichiometric, and ultrathin Al₂O₃ film with an atomically abrupt interface with Si(001). High-pressure reoxidation (HPO)oxidation led to the formation of interfacial SO₂ film, which grew in a layered manner. The oxygen pressure of the ambience plays an important role in the transport, chemical reactions and stability of the Al₂O₃/Si(001) interface at various substrate temperatures.

Interface stability during the growth of Al2O3 films on Si(001)

We grew thin Al2O3 films on Si(001)-2×1 surfaces using three different growth procedures and investigated the Al2O3/Si(001) interface structure and stability for each case. We observed that stacked Al2O3 film grew with an atomically abrupt interface on Si(001). However, depositing a relatively thick initial Al film on Si(001) followed by oxidation, resulted in Al2O3 films being formed having a significantly roughened interface with the Si(001). The interfacial roughness was attributed to the Si–Al interdiffusion near the interfacial region, which with increasing oxidation time, resulted in a nonuniform interfacial region being formed with Al–O–Si compounds. In the growth of Al2O3 film on an Al2O3 prelayer/Si(001) system by depositing Al in an oxygen ambient, about one layer of roughening of the Si substrate occurred at the interface, which was attributed to nonuniform oxidation of the Si substrate. Furthermore, the Al2O3 film growth rate was very slow in this case. These results indicate that the growth p...

Electrical damage of an ultrathin Si oxynitride layer induced by scanning tunneling spectroscopy

Ultrathin Si oxynitride layers were examined by using scanning tunneling microscopy (STM) and spectroscopy (STS). These techniques revealed that a structural change from an intrinsic defect (Si–Si bond) to a damaged structure (Si cluster) takes place under conventional STM/STS conditions. Comparison of the damaged structures formed in the oxynitride with those in the oxide indicated that nitrogen atoms suppress the expansion of the damaged regions. It was also found that nitrogen incorporation enhances both the defect density and the atomic-scale roughness at the oxynitride/Si interface. We suggested that this degradation is related to a local strain produced by the N≡Si3 structures at the oxynitride/Si interface. On the contrary, a normal oxynitride structure had a higher resistance to an electrical stress than an intrinsic defect, but, when the constant electrical stress was applied, the normal oxynitride structure was also damaged. This damage proceeds in two steps: creation of charge traps, and then f...

Selective thermal desorption of ultrathin aluminum oxide layers induced by electron beams

The mechanism of electron-beam-induced selective thermal desorption of ultrathin aluminum-oxide layer (∼0.4 nm) on Si(001) surface was investigated by using scanning reflection electron microscopy, reflection high-energy electron diffraction, and Auger electron spectroscopy. We found that the change in the aluminum-oxide layer composition induced by electron-stimulated oxygen desorption accounted for the selective thermal desorption of the oxide layer. A systematic increase in the vacuum-annealing temperature to 500 °C, 600 °C and 720 °C resulted in the formation of three-dimensional metal aluminum clusters, desorption of these clusters, and creation of a nanometer-scale clean Si(001)-2×1 open window in the selected electron-beam-irradiated area.