C. G. Van de Walle

Carbon impurities and the yellow luminescence in GaN

Using hybrid functional calculations we investigate the effects of carbon on the electrical and optical properties of GaN. In contrast to the currently accepted view that C substituting for N (CN) is a shallow acceptor, we find that CN has an ionization energy of 0.90 eV. Our calculated absorption and emission lines also indicate that CN is a likely source for the yellow luminescence that is frequently observed in GaN, solving the longstanding puzzle of the nature of the C-related defect involved in yellow emission. Our results suggest that previous experimental data, analyzed under the assumption that CN acts as a shallow acceptor, should be re-examined.

Why nitrogen cannot lead to p-type conductivity in ZnO

Based on electronic structure and atomic size considerations, nitrogen has been regarded as the most suitable impurity for p-type doping in ZnO. However, numerous experimental efforts by many different groups have not resulted in stable and reproducible p-type material, casting doubt on the efficacy of nitrogen as a shallow acceptor. Based on advanced first-principles calculations we find that nitrogen is actually a deep acceptor, with an exceedingly high ionization energy of 1.3 eV, and hence cannot lead to hole conductivity in ZnO. In light of this result, we reexamine prior experiments on nitrogen doping of ZnO.

Oxygen vacancies and donor impurities in β-Ga2O3

Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.

Effect of Si doping on strain, cracking, and microstructure in GaN thin films grown by metalorganic chemical vapor deposition

The effect of Si doping on the strain and microstructure in GaN films grown on sapphire by metalorganic chemical vapor deposition was investigated. Strain was measured quantitatively by x-ray diffraction, Raman spectroscopy, and wafer curvature techniques. It was found that for a Si concentration of 2×1019 cm−3, the threshold for crack formation during film growth was 2.0 μm. Transmission electron microscopy and micro-Raman observations showed that cracking proceeds without plastic deformation (i.e., dislocation motion), and occurs catastrophically along the low energy {11_00} cleavage plane of GaN. First-principles calculations were used to show that the substitution of Si for Ga in the lattice causes only negligible changes in the lattice constant. The cracking is attributed to tensile stress in the film present at the growth temperature. The increase in tensile stress caused by Si doping is discussed in terms of a crystallite coalescence model.

Quantum computing with defects

Identifying and designing physical systems for use as qubits, the basic units of quantum information, are critical steps in the development of a quantum computer. Among the possibilities in the solid state, a defect in diamond known as the nitrogen-vacancy (NV-1) center stands out for its robustness—its quantum state can be initialized, manipulated, and measured with high fidelity at room temperature. Here we describe how to systematically identify other deep center defects with similar quantum-mechanical properties. We present a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate defect systems. To illustrate these points in detail, we compare electronic structure calculations of the NV-1 center in diamond with those of several deep centers in 4H silicon carbide (SiC). We then discuss the proposed criteria for similar defects in other tetrahedrally coordinated semiconductors.

Large band gap bowing of InxGa1−xN alloys

Band gap measurements have been performed on strained InxGa1−xN epilayers with x⩽0.12. The experimental data indicate that the bowing of the band gap is much larger than commonly assumed. We have performed first-principles calculations for the band gap as a function of alloy composition and find that the bowing is strongly composition dependent. At x=0.125 the calculated bowing parameter is b=3.5 eV, in good agreement with the experimental values.

Electrostatic interactions between charged defects in supercells

Most theoretical calculations for point defects employ the supercell approach. The supercell consists of a few dozen or 100 atoms of the bulk material with a single defect, and is subject to periodic boundary conditions. However, the large density and periodic arrangement of the defects introduce artifacts. They need to be corrected for to extrapolate to the isolated-defect limit. This is particularly important for electrostatic interactions between charged defects, which decay only very slowly (asymptotically like L ―1 ) with increasing supercell lattice constant L. In this paper, we summarize the underlying electrostatics in condensed matter. A novel defect scheme is derived from this analysis. It overcomes limitations of previous schemes with respect to applicability, systematic improvement, and formal justification. Good performance is demonstrated for vacancies in diamond and GaAs.

Native defects in Al2O3 and their impact on III-V/Al2O3 metal-oxide-semiconductor-based devices

Al2O3 is a promising material for use as a dielectric in metal-oxide-semiconductor devices based on III-V compound semiconductors. However, the presence of deep levels and fixed charge in the Al2O3 layer is still a concern, with native defects being a possible cause of traps, leakage, and fixed charge. We report hybrid density functional calculations for vacancies, self-interstitials, and antisites in Al2O3. The energetic positions of defect levels are discussed in terms of the calculated band alignment at the interface between the oxide and relevant III-V materials. We find that oxygen vacancies are the defects most likely to introduce gap levels that may induce border traps or leakage current in a gate stack. In addition, both self-interstitials and aluminum vacancies introduce fixed charge that leads to increased carrier scattering in the channel and shifts the threshold voltage of the device.

Mechanism of Visible‐Light Photocatalysis in Nitrogen‐Doped TiO2

IO N Semiconductor-based photocatalysis is growing at an unprecedented rate, with TiO 2 leading the way as an important material in applications ranging from the degradation of pollutants to water splitting. [ 1 , 2 ] The basic mechanism is the creation of an electron-hole pair by exciting an electron from the valence to the conduction band through light absorption. Since rutile and anatase have bandgaps of 3.1 eV and 3.2 eV, respectively, only a small fraction of the solar spectrum is absorbed and great efforts have been devoted to extending the TiO 2 photoabsorption to the visible region of the spectrum. Adding N impurities has been shown to enhance visible-light absorption, leading to enhanced photochemical activity. [ 3–7 ] The behavior of N in TiO 2 has been widely discussed, [ 3 , 8–10 ] but the fundamental mechanisms underlying the visible-light absorption remain unclear. It is generally accepted that the visible-light transitions involve electrons from N-related states in the gap to the conduction band. Nonetheless, whether the predominant active species are N atoms at interstitial sites or on substitutional O sites (N O ), and whether the behavior of N in rutile is different from that in anatase are still open issues. In this work we address the stabilities of the relevant N-related defects in both polymorphs, fi nding that impurity-band transitions from N O are the origin of the lower absorption threshold in N-doped titania. Early experimental work on N-doped TiO 2 focused on the correlation between visible-light absorption, photocatalytic activity, and X-ray photoelectron spectroscopic (XPS) measurements of the N 1 s state. [ 3 ] Two main peaks attributed to the N 1 s have been correlated with photocatalytic activity, [ 3 , 9 , 10 ] with one at ∼ 396 eV and the other at ∼ 400 eV. More recent reports on rutile and anatase fi lms grown by plasma-assisted molecular-beam epitaxy have indicated that N incorporates as N O with a characteristic peak at 396.6 eV seen by XPS. [ 11 ] We note that different groups have suggested different causes for each peak, with N O , interstitial N, and N–H complexes as proposed sources. [ 3 , 9–12 ]

Dangling-bond defects and hydrogen passivation in germanium

The application of germanium in complementary metal-oxide semiconductor technology is hampered by high interface-state densities. Using first-principles calculations, we investigate the effects of dangling bonds (DBs) and their interaction with hydrogen. We find that Ge DBs give rise to electronic levels below the valence-band maximum. They therefore occur exclusively in the negative charge state, explaining why they cannot be observed with electron spin resonance. The associated fixed charge is likely responsible for threshold-voltage shifts and poor performance of n-channel transistors. We also find that passivation of DBs by hydrogen will be ineffective because interstitial hydrogen is also stable exclusively in the negative charge state.