Shiyu Sun

Surface Termination and Roughness of Ge(100) Cleaned by HF and HCl Solutions

Oxide removal from Ge(100) surfaces treated by HCl and HF solutions with different concentrations are systematically studied by synchrotron radiation photoelectron spectroscopy (SR-PES). SR-PES results show that clean surfaces without any oxide can be obtained after wet chemical cleaning followed by vacuum annealing with a residual carbon contamination of less than 0.02 monolayer. HF etching leads to a hydrogen-terminated Ge surface whose hydrogen coverage is a function of the HF concentration. In contrast, HCl etching yields a chlorine-terminated surface. Possible etching mechanisms are discussed. Surface roughness after HF and HCl treatments is also investigated by atomic force microscopy which shows that HF treatment leaves a rougher surface than HCl.

Chemical states and electronic structure of a HfO2∕Ge(001) interface

We report the chemical bonding structure and valence band alignment at the HfO{sub 2}/Ge (001) interface by systematically probing various core level spectra as well as valence band spectra using soft x-rays at the Stanford Synchrotron Radiation Laboratory. We investigated the chemical bonding changes as a function of depth through the dielectric stack by taking a series of synchrotron photoemission spectra as we etched through the HfO{sub 2} film using a dilute HF-solution. We found that a very non-stoichiometric GeO{sub x} layer exists at the HfO{sub 2}/Ge interface. The valence band spectra near the Fermi level in each different film structure were carefully analyzed, and as a result, the valence band offset between Ge and GeO{sub x} was determined to be {Delta}E{sub v} (Ge-GeO{sub x}) = 2.2 {+-} 0.15 eV, and that between Ge and HfO{sub 2}, {Delta}E{sub v} (Ge-HfO{sub 2}) = 2.7 {+-} 0.15 eV.

Photoemission studies of passivation of germanium nanowires

The surface of single crystal germanium nanowires grown by cold-wall chemical vapor deposition was studied by synchrotron radiation photoemission spectroscopy and also by conventional x-ray photoelectron spectroscopy. The surfaces of the nanowires are not oxidized from which we infer that they are hydrogen passivated as-grown. Exposure to laboratory atmosphere leads to germanium oxide growth with oxidation states of Ge1+, Ge2+, Ge3+, while exposure to UV light leads to a predominance of the Ge4+ oxidation state. Most of the surface oxide could be removed readily by aqueous HF treatment which presumably leaves the nanowire surface hydrogen terminated. The HF-treated surface has more limited stability in air. Alternatively, chlorine termination could be achieved by aqueous HCl treatment of the oxide-coated nanowires. This chlorine termination was found to be more stable in air than the putative hydrogen termination achieved by aqueous HF treatment.

Nitrogen doping and thermal stability in HfSiOxNy studied by photoemission and x-ray absorption spectroscopy

We have investigated nitrogen-doping effects into HfSiO{sub x} films on Si and their thermal stability using synchrotron-radiation photoemission and x-ray absorption spectroscopy. N 1s core-level photoemission and N K-edge absorption spectra have revealed that chemical-bonding states of N-Si{sub 3-x}O{sub x} and interstitial N{sub 2}-gas-like features are clearly observed in as-grown HfSiO{sub x}N{sub y} film and they decrease upon ultrahigh vacuum (UHV) annealing due to a thermal instability, which can be related to the device performance. Annealing-temperature dependence in Hf 4f and Si 2p photoemission spectra suggests that the Hf-silicidation temperature is effectively increased by nitrogen doping into the HfSiO{sub x} although the interfacial SiO{sub 2} layer is selectively reduced. No change in valence-band spectra upon UHV annealing suggests that crystallization of the HfSiO{sub x}N{sub y} films is also hindered by nitrogen doping into the HfSiO{sub x}.

Roles of oxygen and water vapor in the oxidation of halogen terminated Ge(111) surfaces

The initial stage of the oxidation of Cl and Br terminated Ge(111) surfaces is studied using photoelectron spectroscopy. The authors perform controlled experiments to differentiate the effects of different factors in oxidation, and find that water vapor and oxygen play different roles. Water vapor effectively replaces the halogen termination layers with the hydroxyl group, but does not oxidize the surfaces further. In contrast, little oxidation is observed for Cl and Br terminated surfaces with dry oxygen alone. However, with the help of water vapor, oxygen oxidizes the surface by breaking the Ge–Ge back bonds instead of changing the termination layer.

Angular Dependence of the Photoelectron Energy Distribution of InP(100) and GaAs(100) Negative Electron Affinity Photocathodes

Energy distribution of the photoelectrons from InP(100) photocathodes are investigated with a photon energy range from 0.62eV to 2.76eV. When the photon energy is less than 1.8eV, only electrons emitted from the Gamma valley are observed in the energy distribution curves (EDC). At higher photon energies, electrons from the L valley are observed. The angular dependence of the electron energy distributions of InP and GaAs photocathodes are studied and compared. The electrons emitted from the L valley have a larger angular spread than the ones from the Gamma valley due to the larger effective mass of the L valley minimum.

Study on mechanism of crystallization in HfO2 films on Si substrates by in-depth profile analysis using photoemission spectroscopy

We have investigated the effect of HfO2 layer thickness on crystallization in thin HfO2 films using photoemission spectroscopy and x-ray absorption spectroscopy (XAS). O K-edge XAS spectra depending on annealing temperature indicate that crystallization in thin HfO2 films is suppressed upon annealing for thin HfO2 film, which in turn has a beneficial effect on the performance of devices. The annealing-temperature dependence of the depth profile of HfO2 films suggests that diffusion of Si atoms into the HfO2 layer plays only a minor role in the suppression of crystallization, indicating that HfO2 films can restrain a decrease in dielectric constant.

The Effectiveness of HCl and HF Cleaning of Si0.85Ge0.15 Surface

The cleaning of Si{sub 0.85}Ge{sub 0.15} surfaces using HCl and HF solutions is studied using synchrotron radiation photoelectron spectroscopy. The HF solution is found to be effective in removing both the Si oxide and the Ge oxide while the HCl solution can only remove part of the Ge oxide. For samples treated with HF, four spectral components are needed to fit the Ge 3d photoemission spectra. One is the bulk component and the other three are attributed to the surface Ge atoms with mono-hydride, di-hydride and tri-hydride terminations, respectively.

The distribution of oxide species in the Cs/O activation layer on InP(100) negative electron affinity photocathodes

The atomic arrangement of Cs oxides in the activation layer of an InP photocathode is investigated using angle dependent photoemission spectroscopy. Two distinct peaks in the O1s core level and in valence band spectra have led to the discovery of two molecular oxygen species incorporated in the thin activation layer: Cs peroxide (Cs2O2) and Cs superoxide (CsO2). The different angular dependences of these oxides observed in the photoemission spectra are caused by different vertical locations of the oxygen molecules in each Cs oxide in the activation layer. The thickness of the activation layer, which is about 7A, suggests lateral distribution of Cs peroxide and Cs superoxide. The quantum efficiency of InP photocathodes in our ultra high vacuum system decreases with time due to the chemical transformation of the Cs oxides and subsequent substrate oxidation, as deduced from an observation of the peak evolution in the photoemission spectra, and supported by the thermodynamic stability of Cs superoxide as comp...

Effects of Al doping and annealing on chemical states and band diagram of Y2O3∕Si gate stacks studied by photoemission and x-ray absorption spectroscopy

The authors have investigated the effects of Al doping and annealing on the photoemission spectra and thermal stability of Y2O3∕Si gate stacks by photoemission spectroscopy and x-ray absorption spectroscopy. They have found that the SiO2 components diffuse into the Y2O3 layer by annealing, resulting in the formation of Y silicate; however, the formation of metallic Y silicide is not observed. The changes in valence- and conduction-band offsets by doping Y2O3 with Al with respect to both Al concentration and annealing temperature have been systematically investigated. With an increase in the Al concentration, the band offsets and band gaps increase and the conduction-band edges change nonlinearly.