Atomic structure and band alignment at Al2O3/GaN, Sc2O3/GaN and La2O3/GaN interfaces: A first-principles study

The atomic structures, chemical bonding and band alignment at trivalent oxides X2O3 (where X = Al, Sc and La) and GaN interface are studied based on the density functional supercell calculations. The insulating interfaces with small roughness and a clean bandgap are built based on the electron counting rule. The results prove that GaO bonds dominate the interfacial chemical bonding for all the interfaces, and the calculated oxide/GaN band alignment consistent with the experimental values. All the oxides are proved to have the type-I band alignment with GaN with hybrid functional calculation. For the Al2O3 interface, the calculated valence band offset is 1.17 eV, while that for the Sc2O3 and La2O3 interface are 0.81 eV and 0.95 eV, respectively. The calculated conduction band offsets are all larger than 1 eV, and as large as 1.8 eV for the Al2O3 interface. The theoretically calculated band alignments indicate that the studied trivalent oxides Al2O3, Sc2O3 and La2O3 are all suitable gate insulators for GaN-based MOSFET applications

Atomic structure and electronic structure of disordered graphitic carbon nitride

Random networks of sp2 bonded amorphous graphitic carbon nitride (g-CN) have been created by density functional molecular dynamics calculations. A direct molecular dynamics approach was found to create a network with too many like-atom bonds so that an indirect method via an h-BN random network is used. The resulting network possesses the local units of melems found in the crystalline g-C3N4 lattice. The networks have the electron affinity and ionization potential values compatible with photocatalytic water splitting. They are found to possess too many defects so that the band gap is smaller than found experimentally

Role of the third metal oxide in In–Ga–Zn–O4 amorphous oxide semiconductors: Alternatives to gallium

We study the role of the third metal oxide in In–Ga–Zn-type oxides (IGZO), Ga2O3, by comparing the calculated electronic properties of various alternatives (Al, Y, Hf, Ti, Si, and W) with Ga. It is found that Ga2O3 causes little disorder in the conduction band minimum (CBM) energy based on In or Zn oxides, and it has a large O vacancy suppression effect, which benefits both a high mobility and a low OFF current of IGZO. However, other alternatives give a pronounced conduction band disorder potential due to their higher CBM energies and thus are not ideal components in amorphous oxide semiconductors. Si and W may reduce the negative bias illumination stress instability by lowering hydrogen-induced states to below the bulk valence band maximum, but Si is not beneficial for mobility. Their role in back-end-of-line transistors is also noted

Termination-dependence of Fermi level pinning at rare-earth arsenide/GaAs interfaces

The properties of metal/semiconductor interfaces are generally described by the metal-induced gap states (MIGS) model. However, rare-earth (RE) arsenide interfaces are found not to follow the MIGS model in having very different Schottky barrier heights (SBHs) for the Ga- or As-terminations of polar (100) or (111) RE-As/GaAs interfaces. Density function supercell calculations find this effect is due to localized defect interface states located on the mis-coordinated atoms of these interfaces that pin their SBHs at very different energies for each termination as determined by the anion sublattice bonding. Band offsets of semiconducting ScN/GaN interfaces also depend on their termination as determined by the same defect interface states. This pinning mechanism dominates any MIGS mechanism when it arises. Nonpolar (110) interfaces have little change in bonding, so they have no defect interface states, and we find their SBH is pinned by MIGS at the charge neutrality level. Hence, traditional MIGS models should be extended to include such interface states in a more general description

Phase dependence of Schottky barrier heights for Ge–Sb–Te and related phase-change materials

The large difference of dielectric functions between the amorphous and crystalline phases of Ge–Sb–Te based phase-change materials (PCMs) used in memory storage devices also affects their Schottky barrier heights (SBHs) and thus their electrical device properties. Here, the SBHs of each phase of Ge2Sb2Te5, GeTe, GeSe, and SnTe are found by density functional supercell calculations. The Fermi level pinning factor S calculated for the crystalline phases (with a larger dielectric constant) is smaller than their amorphous phases, agreeing well with the empirical relationship linking SBH to a dielectric constant. The relatively large dielectric constant of crystalline PCMs arises from their resonant bonding (metavalent bonding), but their pinning factor is not always as small as empirically expected. The results are useful for optimizing the design of metal contacts for Ge–Sb–Te type phase-change memory devices

High-Throughput Electronic Structures and Ferroelectric Interfaces of HfO2 by GGA+U(d,p) Calculations

The electronic structure, vacancy symmetry, defect levels, ferroelectric phases, and interface properties of HfO2 are studied using a GGA + U(d,p) approach, a simplified version of the ACBN0 method. Introducing an on-site Coulomb interaction to both Hf 5d orbitals and O 2p orbitals reproduces the experimental bandgap, gives band energies similar to those of hybrid functionals, gives the correct symmetry for the oxygen vacancy, and describes the Schottky barriers at the metallic contacts like TiN correctly. The energetics of phase energies and strain arising from different ferroelectric–electrode interfaces are tested. The GGA + U(d,p) approach is a useful tool to study various HfO2 configurations by rapid ab initio molecular dynamics calculations

Band Structure, Band Offsets, and Intrinsic Defect Properties of Few-Layer Arsenic and Antimony

We present a detailed first-principle study of few-layer arsenic and antimony electronic structures. The band structures of 2D arsenic and antimony are calculated by a hybrid functional with the spin–orbital coupling. The results show that the band gaps of arsenene (monolayer arsenic) and antimonene (monolayer antimony) are 1.93 and 1.52 eV, respectively. It is observed that the band gaps narrow in trilayer arsenic and bilayer antimony. The band alignment with HfO2 and other 2D materials is calculated to show that HfO2 is a good candidate as a gate oxide in field effect transistors. It is found that point defects such as a single vacancy or adatom will introduce several defect states in arsenene in the middle of the band gap. Meanwhile, the defect formation energy becomes negative when the Fermi level is close to the band edges. By comparison, the defect formation energy in antimonene is always positive so that the Fermi level pinning should be suppressed in contact with the reactive metal

A Framework for Temperature Imaging using the Change in Backscattered Ultrasonic Signals

Hyperthermia is a cancer treatment that elevates tissue temperature to 40 to 43oC. It would benefit from a non-invasive, safe, inexpensive and convenient thermometry to monitor heating patterns. Ultrasound is a modality that meets these requirements. In our initial work, using both prediction and experimental data, we showed that the change in the backscattered energy: CBE) is a potential parameter for TI. CBE, however, was computed in a straightforward yet ad hoc manner. In this work, we developed and exploited a mathematical representation for our approach to TI to optimize temperature accuracy. Non-thermal effects of noise and motion confound the use of CBE. Assuming additive white Gaussian noise, we applied signal averaging and thresholding to reduce noise effects. Our motion compensation algorithms were also applied to images with known motion to evaluate factors affecting the compensation performance. In the framework development, temperature imaging was modeled as a problem of estimating temperature from the random processes resulting from thermal changes in signals. CBE computation was formalized as a ratio between two random variables. Mutual information: MI) was studied as an example of possible parameters for temperature imaging based on the joint distributions. Furthermore, a maximum likelihood estimator: MLE) was developed. Both simulations and experimental results showed that noise effects were reduced by signal averaging. The motion compensation algorithms proved to be able to compensate for motion in images and were improved by choosing appropriate interpolation methods and sample rates. For images of uniformly distributed scatterers, CBE and MI can be computed independent of SNR to improve the temperature accuracy. The application of the MLE also showed improvements in temperature accuracy compared to the energy ratio from the signal mean in simulations. The application of the framework to experimental data requires more work to implement noise reduction approaches in 3D heating experiments. The framework identified ways in which we were able to reduce the effects of both noise and motion. The framework formalized our approaches to temperature imaging, improved temperature accuracy in simulations, and can be applied to experimental data if the noise reduction approaches can be implemented for 3D experiments

Impact of the interface vacancy on Schottky barrier height for Au/AlN polar interfaces

Schottky barrier heights (SBHs) at the Au/AlN interface are systemically studied by density functional calculations. Two types of interfaces, including Al- and N-polar interfaces, are constructed to examine the relationship between the SBH dependence on the interfacial atom species. An in-depth exploration is conducted by introducing interfacial aluminum vacancy or nitride vacancy. The results show that the calculated p-type SBH of the Al-polar interface (2.30 eV) is higher than that of the N-polar interface (1.23 eV). Results also show that the SBH of the interface with Al or N vacancies would be higher. More obvious metal-induced gap states (MIGS) can be observed after the introduction of interfacial vacancy site, leading to a stronger Fermi-level pinning at the contact. The derived SBHs are within the reported measurement range. The findings provide an insightful hint for AlN-based devices where Schottky contact matters