J. W. Chai

Reaction of SiO2 with hafnium oxide in low oxygen pressure

A dynamic process consisting of a series of reactions during deposition of HfO2 films on SiO2-covered silicon under oxygen-deficient conditions is identified. The oxygen-deficient HfOx<2 layer absorbs the oxygen in the SiO2 layer to form fully oxidized metal oxide film. As a result, there is no silicate and silicon oxide formed at the interface with silicon substrate. Thermodynamic analysis indicates that even if there is a layer of silicate forming at the initial stage of deposition, the silicate layer will be decomposed with the progress of HfOx<2 deposition.

Density functional study on ferromagnetism in nitrogen-doped anatase TiO2

We report first principles calculations on the magnetism and electronic structures for nitrogen-doped anatase TiO2 (N:TiO2). Our calculations indicate that magnetic state is the ground state for N:TiO2 systems. An isolated N atom produces a total magnetic moment of 1.00μB and introduces spin-polarized 2p states in the band gap. The origin of the magnetic moments is the holes in N 2p π band of the N dopant. Several doping configurations studied suggest the existence of ferromagnetic coupling between N dopants. The ferromagnetism in N:TiO2 can be attributed to the hole-mediated double exchange through the strong p-p interaction between N and O.

Effect of nitrogen incorporation on the electronic structure and thermal stability of HfO2 gate dielectric

The effect of nitrogen incorporation on the electronic structure and thermal stability of HfO2 gate dielectric was investigated by using photoemission study and first-principles calculation. Hafnium oxynitride (HfON) dielectric shows higher thermal stability in comparison to pure HfO2 on Si. Atomic N can passivate O vacancies in the dielectrics during nitridation process, but the N atoms incorporated into interstitial sites cause band gap reduction. Postnitridation annealing is required to activate interstitial N atoms to form stable N–Hf bonds, which will increase the band gap and band offset of as-nitrided dielectric film.

Effect of nitrogen doping on optical properties and electronic structures of SrTiO3 films

The nitrogen-doping induced changes in optical properties and electronic structures of SrTiO3 films have been investigated by using spectroscopic ellipsometry and x-ray photoemission spectroscopy. Combined with the first-principles calculations, it is found that the localized N 2p states above O 2p states are attributed to the new absorption edge at 500nm and the photoactivity in the visible light region. Our results are consistent with both recent experimental and theoretical studies on nitrogen-doped TiO2, where the visible light responses arise from the localized N 2p states slightly above the valence-band edge rather than the band gap narrowing.

Thermal stability and band alignments for Ge3N4 dielectrics on Ge

Ge3N4 dielectrics were prepared on Ge surface by in situ direct atomic source nitridation. The thermal stability and band alignments for Ge3N4∕Ge interfaces have been studied by using high-resolution x-ray photoemission spectroscopy. The in situ thermal treatment shows that Ge3N4 film has higher temperature thermal stability up to 550°C in vacuum. The conduction- and valence-band offsets at Ge3N4∕Ge interface are quite asymmetrical with the values of 2.22 and 1.11eV, respectively.

Impact of oxide defects on band offset at GeO2/Ge interface

High quality GeO2 dielectrics were prepared on Ge(001) surface by direct atomic source oxidation. The band alignments have been studied by using high resolution x-ray photoemission spectroscopy. The valence and conduction band offsets at GeO2/Ge(001) interface are 4.59±0.03 and 0.54±0.03 eV, respectively. The calculated projected density of states indicate that the formation of germanium and oxygen vacancies at different oxidation stages might result in the reduction of valence band offsets, which clarified the varied experimental results of valence band offset [M. Perego et al., Appl. Phys. Lett. 90, 162115 (2007) and V. V. Afanas’ev and A. Stesmans, Appl. Phys. Lett. 84, 2319 (2004)].

Impact of interface structure on Schottky-barrier height for Ni∕ZrO2(001) interfaces

The Schottky-barrier heights for the Ni and ZrO2 interfaces have been determined using the x-ray photoemission method. Depending on the surface treatment of ZrO2, the variation of Schottky-barrier heights at Ni∕ZrO2 interfaces was found as large as 0.76±0.10eV. The p-type Schottky-barrier height for the oxygen-rich (oxygen-deficient) interface was measured to be 2.60eV (3.36eV). First-principles calculations provide a microscopic explanation of such variation, which was attributed to the different interface dipole formed by interfacial Ni–O, Ni–Zr bonds, or oxygen vacancies.

Band offsets of HfO2/ZnO interface: In situ x-ray photoelectron spectroscopy measurement and ab initio calculation

High quality HfO2 dielectrics have been grown on ZnO (0001) substrates. The valence- and conduction-band offsets for HfO2/ZnO (0001) heterojunctions have been determined to be 0.14±0.05 and 2.29±0.05 eV, respectively, by using in situ x-ray photoemission spectroscopy. First-principles calculations show that the valence-band offset at the HfO2/ZnO (0001) interface of the most energetically favorable interface structure is 0.40 eV, which is consistent with the experimental results.

Energy-band alignments at LaAlO3 and Ge interfaces

The energy-band alignments for LaAlO3 films on p-Ge(001) with and without GeOxNy interfacial layer have been studied using photoemission spectroscopy. The valence-band offsets at LaAlO3∕GeOxNy∕Ge and LaAlO3∕Ge interfaces were measured to be 2.70 and 3.06eV, respectively. The effect of interfacial GeOxNy layer on the band alignments is attributed to the modification of interface dipoles. The conduction-band offsets at LaAlO3∕Si(001) and LaAlO3∕Ge interfaces are found to have the same value of 2.25±0.05eV, where the shift of valence-band top accounts for the difference in the energy-band alignment at two interfaces.

Band alignment and thermal stability of HfO2 gate dielectric on SiC

The band alignment and thermal stability for HfO2 films on SiC with and without nitridation have been studied by using photoemission spectroscopy. The valence- and conduction-band offsets at HfO2/4H-SiC interfaces were measured to be 1.02 and 1.53 eV, respectively. The atomic source nitridation improves interface thermal stability with nitrogen passivation for the oxygen vacancies in dielectric films and for the defects on SiC surface, but induces band gap reduction for the HfO2 dielectric layer and band alignment shift at the interface. Postnitridation annealing helps to improve the band offsets of dielectric film to have sufficient injection barrier.