Chiyu Zhu

Comparative band alignment of plasma-enhanced atomic layer deposited high-k dielectrics on gallium nitride

Al2O3 films, HfO2 films, and HfO2/Al2O3 stacked structures were deposited on n-type, Ga-face, GaN wafers using plasma-enhanced atomic layer deposition (PEALD). The wafers were first treated with a wet-chemical clean to remove organics and an in-situ combined H2/N2 plasma at 650 °C to remove residual carbon contamination, resulting in a clean, oxygen-terminated surface. This cleaning process produced slightly upward band bending of 0.1 eV. Additional 650 °C annealing after plasma cleaning increased the upward band bending by 0.2 eV. After the initial clean, high-k oxide films were deposited using oxygen PEALD at 140 °C. The valence band and conduction band offsets (VBOs and CBOs) of the Al2O3/GaN and HfO2/GaN structures were deduced from in-situ x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The valence band offsets were determined to be 1.8 and 1.4 eV, while the deduced conduction band offsets were 1.3 and 1.0 eV, respectively. These values are compared with the theoretical calculations b...

Combined visible light photo-emission and low temperature thermionic emission from nitrogen doped diamond films

This study reports a photoemission threshold of ∼1.5 eV from nitrogen-doped nanocrystalline diamond, which ranks among the lowest photo-threshold of any non-cesiated material. Diamond films on molybdenum substrates have been illuminated with light from 340 to 550 nm, and the electron emission spectrum has been recorded from ambient to ∼320 °C. The results display combined thermionic and photo-electron emission limited by the same low work function and indicate that the two emission processes are spatially separated. These results indicate the potential for a solar energy conversion structure that takes advantage of both photoemission and thermionic emission.

Band alignment of zinc oxide as a channel layer in a gate stack structure grown by plasma enhanced atomic layer deposition

A gate stack structure with a thin ZnO layer between an oxidized Si(100) surface and an alloyed hafnium and lanthanum oxide (HfO2-La2O3) layer was prepared by plasma enhanced atomic layer deposition at ∼175 °C. High resolution electron microscopy indicated an amorphous structure of the deposited layers. The electronic properties were characterized with x-ray and ultraviolet photoemission spectroscopy. A significant amount of excess oxygen was observed in the as-deposited ZnO and (HfO2-La2O3) layers. A helium plasma postdeposition treatment can partially remove the excess oxygen in both layers. The band alignment of this structure was established for an n-type Si substrate. A valence band offset of 1.5 ± 0.1 eV was measured between a thin ZnO layer and a SiO2 layer. The valence band offset between HfO2-La2O3 (11% HfO2 and 89% La2O3) and ZnO was almost negligible. The band relationship developed from these results demonstrates confinement of electrons in the ZnO film as a channel layer for thin film transis...

Band alignment of vanadium oxide as an interlayer in a hafnium oxide-silicon gate stack structure

Vanadium oxide (VO2) is a narrow band gap material (Eg = 0.7 eV) with a thermally induced insulator-metal phase transition at ∼343 K and evidence of an electric field induced transition at T < 343 K. To explore the electronic properties of VO2, a sandwich structure was prepared with a 2 nm VO2 layer embedded between an oxidized Si(100) surface and a 2 nm hafnium oxide (HfO2) layer. The layer structure was confirmed with high resolution transmission electron microscopy. The electronic properties were characterized with x-ray and ultraviolet photoemission spectroscopy, and the band alignment was deduced on both n-type and p-type Si substrates. The valence band offset between VO2 and SiO2 is measured to be 4.0 eV. The valence band offset between HfO2 and VO2 is measured to be ∼3.4 eV. The band relation developed from these results demonstrates the potential for charge storage and switching for the embedded VO2 layer.

Low temperature growth of high-k Hf-La oxides by remote-plasma atomic layer deposition: Morphology, stoichiometry, and dielectric properties

In this work, we investigated the growth of Hf oxide, La oxide, and alloyed Hf–La oxide films using remote-plasma atomic layer deposition at temperatures ranging from ∼80 to ∼250 °C. The relative composition and atomic bonding structure of the film were determined by in situ x ray photoelectron spectroscopy (XPS). Atomic force microscopy and transmission electron microscopy were implemented to characterize the morphology and crystalline structure. The XPS results indicated that for low temperature Hf oxide growth, a significant amount of excess oxygen species was observed in the deposited film. This oxygen could lead to instabilities and adversely affect the function of thin film transistors. The authors established that a He plasma post deposition treatment can partially remove the excess oxygen. In addition, the pure Hf oxide films show a surface morphology with protruding islands over a smooth surface which reflects the crystallized nature of the Hf oxide domains. In order to suppress the crystallizati...

Band alignment of a HfO2-VO2-HfO2 confined well structure on silicon

Vanadium dioxide (VO2) is a narrow band gap material that undergoes a metal-insulator phase transition at ∼343 K with evidence of an electric-field induced transition at T < 343 K. In this study, a sandwich-type dielectric structure is prepared consisting of two ∼1.5 nm hafnium oxide (HfO2) layers with a ∼1.0 nm VO2 interlayer grown on an oxidized n-type silicon substrate. The electronic properties of the sample were characterized by in-situ x-ray and ultraviolet photoelectron spectroscopy after each layer was deposited. The band alignment was analyzed after each growth step. The SiO2/HfO2 interface valence band offset is found to be 0.7 eV, and the HfO2/VO2 interface valence band offset is determined to be 3.4 eV.

Cu film thermal stability on plasma cleaned polycrystalline Ru

The first part of this study examined oxide stability and cleaning of Ru surfaces. The surface reactions during H2 plasma exposure of Ru polycrystalline films were studied using x-ray photoelectron spectroscopy (XPS). The ∼2 monolayer native Ru oxide was reduced after H-plasma processing. However, absorbed oxygen, presumably in the grain boundaries, remains after processing. A vacuum thermal anneal at 150 °C substantially removes both surface oxide and absorbed oxygen which is attributed to a reduction by carbon contamination. The second part of the study examined the thermal stability of Cu on a Ru layer. The thermal stability or islanding of the Cu film on the Ru substrate was characterized by in situ XPS. After plasma cleaning of the Ru adhesion layer, the deposited Cu exhibited full coverage. In contrast, for Cu deposition on the Ru native oxide substrate, Cu islanding was detected and was described in terms of grain boundary grooving and surface and interface energies. The oxygen in the grain boundar...