C. C. Fulton

Structural, microstructural, and electrical properties of gold films and Schottky contacts on remote plasma-cleaned, n -type ZnO{0001} surfaces

Current–voltage measurements of Au contacts deposited on ex situ cleaned, n-type ZnO(0001) [(0001¯)] surfaces showed reverse bias leakage current densities of ∼0.01(∼0.1)A∕cm2 at 4.6 (3.75) V reverse bias and ideality factors >2 (both surfaces) before sharp, permanent breakdown (soft breakdown). This behavior was due primarily to the presence of (1.6–2.0)±0.1[(0.7–2.6)±0.1] monolayers (ML) of hydroxide, which forms an electron accumulation layer and increases the surface conductivity. In situ remote plasma cleaning of the (0001) [(0001¯)] surfaces using a 20vol%O2∕80vol%He mixture for the optimized temperatures, times, and pressure of 550±20°C(525±20°C), 60 (30) min, and 0.050 Torr reduced the thickness of the hydroxide layer to ∼0.4±0.1ML and completely eliminated all detectable hydrocarbon contamination. Subsequent cooling of both surfaces in the plasma ambient resulted in the chemisorption of oxygen and a change from 0.2 eV of downward band bending for samples cooled in vacuum to 0.3 eV of upward band ...

Measurement of the band offsets of SiO2 on clean n- and p-type GaN(0001)

The band alignment at the SiO2-GaN interface is important for passivation of high voltage devices and for gate insulator applications. X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy have been used to observe the interface electronic states as SiO2 was deposited on clean GaN(0001) surfaces. The substrates, grown by metallorganic chemical vapor deposition, were n- (1×1017) and p-type (2×1018) GaN on 6H-SiC(0001) with an AlN(0001) buffer layer. The GaN surfaces were atomically cleaned via an 860 °C anneal in an NH3 atmosphere. For the clean surfaces, n-type GaN showed upward band bending of 0.3±0.1 eV, while p-type GaN showed downward band bending of 1.3±0.1 eV. The electron affinity for n- and p-type GaN was measured to be 2.9±0.1 and 3.2±0.1 eV, respectively. To avoid oxidizing the GaN, layers of Si were deposited on the clean GaN surface via ultrahigh vacuum e-beam deposition, and the Si was oxidized at 300 °C by a remote O2 plasma. The substrates were annealed at 650 °C for d...

In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces

A layer containing an average of 1.0 monolayer (ML) of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (0001) and Zn-terminated (0001) surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(0001) in pure oxygen at 600–650 °C±20 °C reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O2 at 700 °C and 0.100±0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (∼0.03 ML=∼0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO...

Conduction band-edge States associated with the removal of d-state degeneracies by the Jahn-Teller effect

X-ray absorption spectroscopy (XAS) is used to study band edge electronic structure of high-/spl kappa/ transition metal (TM) and trivalent lanthanide rare earth (RE) oxide gate dielectrics. The lowest conduction band d/sup */-states in TiO/sub 2/, ZrO/sub 2/ and HfO/sub 2/ are correlated with: 1) features in the O K/sub 1/ edge, and 2) transitions from occupied Ti 2p, Zr 3p and Hf 4p states to empty Ti 3d-, Zr 4d-, and Hf 5d-states, respectively. The relative energies of d-state features indicate that the respective optical bandgaps, E/sub opt/ (or equivalently, E/sub g/), and conduction band offset energy with respect to Si, E/sub B/, scale monotonically with the d-state energies of the TM/RE atoms. The multiplicity of d-state features in the Ti L/sub 2,3/ spectrum of TiO/sub 2/, and in the derivative of the O K/sub 1/ spectra for ZrO/sub 2/ and HfO/sub 2/ indicate a removal of d-state degeneracies that results from a static Jahn-Teller effect in these nanocrystalline thin film oxides. Similar removals of d-state degeneracies are demonstrated for complex TM/RE oxides including Zr and Hf titanates, and La, Gd and Dy scandates. Analysis of XAS and band edge spectra indicate an additional band edge state that is assigned Jahn-Teller distortions at internal grain boundaries. These band edges defect states are electronically active in photoconductivity (PC), internal photoemission (IPE), and act as bulk traps in metal oxide semiconductor (MOS) devices, contributing to asymmetries in tunneling and Frenkel-Poole transport that have important consequences for performance and reliability in advanced Si devices.

Process-dependent band structure changes of transition-metal (Ti,Zr,Hf) oxides on Si (100)

In this study, we have deposited Ti, Zr, and Hf oxides on ultrathin (∼0.5 nm) SiO2 buffer layers and have identified metastable states which give rise to large changes in their band alignments with respect to the Si substrate. This results in a potential across the interfacial SiO2 layer, significant band bending, and large shifts of the high-k valence band. The magnitude of the shift differs for the three materials and is dependant on both the SiO2 buffer layer thickness and annealing temperature. We propose a model where excess oxygen accumulates near the high-k-SiO2 interface providing electronic states, which are available to electrons that tunnel from the substrate.

Band offset measurements of the GaN (0001)/HfO2 interface

Photoemission spectroscopy has been used to observe the interface electronic states as HfO2 was deposited on clean n-type Ga-face GaN (0001) surfaces. The HfO2 was formed by repeated deposition of several monolayers of Hf followed by remote plasma oxidation at 300 °C, and a 650 °C densification anneal. The 650 °C anneal resulted in a 0.6 and 0.4 eV change in band bending and valence band offset, respectively. The final annealed GaN/HfO2 interface exhibited a valence band offset of 0.3 eV and a conduction band offset of 2.1 eV. A 2.0 eV deviation was found from the electron affinity band offset model.

Band offset measurements of the Si3N4/GaN (0001) interface

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were used to measure electronic states as Si3N4 was deposited on clean GaN (0001) surfaces. The n-type (2×1018) and p-type (1×1017) GaN surfaces were atomically cleaned in NH3 at 860 °C, and the n-and p-type surfaces showed upward band bending of ∼0.2±0.1 eV and downward band bending of 1.1±0.1 eV, respectively, both with an electron affinity of 3.1±0.1 eV. Layers of Si (∼0.2 nm) were deposited on the clean GaN and nitrided using an electron cyclotron resonance N2 plasma at 300 °C and subsequently annealed at 650 °C for densification into a Si3N4 film. Surface analysis was performed after each step in the process, and yielded a valence band offset of 0.5±0.1 eV. Both interfaces exhibited type II band alignment where the valence band maximum of GaN lies below that of the Si3N4 valence band. The conduction band offset was deduced to be 2.4±0.1 eV, and a change of the interface dipole of 1.1±0.1 eV was observed for Si3N4/GaN interfac...

Electronic states at the interface of Ti–Si oxide on Si(100)

The requirement for high K dielectrics for Si devices includes both a low interface state density and a band alignment that blocks both electrons and holes. Titanium dioxide materials are known to exhibit dielectric constants of 80 or higher depending on the crystal structure and, as such, are prime candidates for gate dielectrics. We employ an ultrathin layer of SiO2 prior to the formation of a Ti oxide to limit the density of defect states. The electronic structure is observed during the stepwise growth of the oxide using x-ray and ultraviolet photoemission spectroscopy. Measurements indicate Ti oxide states at approximately 2 eV below the Si valence band maximum suggesting that the TiO2 conduction band aligns with the Si conduction band. The results indicate nearly flat bands in the silicon consistent with a low interface state density.

Electronic properties of the Zr-ZrO2-SiO2-Si(100) gate stack structure

The interface electronic structure of a layered Zr–ZrO2–SiO2–Si(100) system was studied with x-ray (hν=1254eV) and ultraviolet (hν=21.2eV) photoemission spectroscopies. In situ growth and characterization allow the structures to be deposited and studied in a stepwise manner without the risk of contamination. This study discusses the electronic properties including electron affinities and work functions, valence band maxima, band bending in the Si, and internal fields in a layered high-κ gate stack. With this information the band alignments can be reconstructed and compared to predictions of the vacuum alignment models (i.e., the Schottky-Mott model for metal-semiconductor interfaces or the electron affinity model for heterojunctions) and the interface induced gap states model. The vacuum alignment models are first order approaches to determine the electronic barrier height for a heterojunction, and interface bonding can contribute to charge transfer across the interface, affecting the dipole contribution ...

Local Bonding Analysis of the Valence and Conduction Band Features of TiO2

An analysis of the valence and conduction band electronic structure of TiO2 as studied by ultraviolet photoemission spectroscopy (UPS) and x-ray absorption spectroscopy (XAS) using synchrotron radiation is reported. Valence band spectra from UPS have been deconvolved using a five-peak model. The spectra are interpreted based on the peak assignments to the XAS data and the symmetries of the valence band states. The interpretation is consistent with theoretical calculations of molecular orbitals found in the literature. The removal of the d-state degeneracies that arise from a collective Jahn–Teller splitting of the crystal field split t2g and eg states is observed and scales with the conduction band results from the absorption data. These Jahn–Teller derived energy separations are present in the O K1 and Ti L3 spectra but are not resolved in the photoemission valence band spectra. Two defect states are clearly observed ∼0.7 and 2.0 eV above the valence band edge and are attributed to the presence of oxygen...