We have performed a comprehensive first-principles investigation of native point defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native point defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of point defects in ZnO and their influence on the control of $p$-type doping.

Native point defects in ZnO

Silicon carbide (SiC), a material long known with potential for high-temperature, high-power, high-frequency, and radiation hardened applications, has emerged as the most mature of the wide-bandgap (2.0 eV ≲ Eg ≲ 7.0 eV) semiconductors since the release of commercial 6HSiC bulk substrates in 1991 and 4HSiC substrates in 1994. Following a brief introduction to SiC material properties, the status of SiC in terms of bulk crystal growth, unit device fabrication processes, device performance, circuits and sensors is discussed. Emphasis is placed upon demonstrated high-temperature applications, such as power transistors and rectifiers, turbine engine combustion monitoring, temperature sensors, analog and digital circuitry, flame detectors, and accelerometers. While individual device performances have been impressive (e.g. 4HSiC MESFETs with fmax of 42 GHz and over 2.8 W mm−1 power density; 4HSiC static induction transistors with 225 W power output at 600 MHz, 47% power added efficiency (PAE), and 200 V forward blocking voltage), material defects in SiC, in particular micropipe defects, remain the primary impediment to wide-spread application in commercial markets. Micropipe defect densities have been reduced from near the 1000 cm−2 order of magnitude in 1992 to 3.5 cm−2 at the research level in 1995.

Status of silicon carbide (SiC) as a wide-bandgap semiconductor for high-temperature applications: A review

A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t...

Ion and electron irradiation-induced effects in nanostructured materials

A fractal model for predicting the effective thermal conductivity of liquid with suspension of nanoparticles

Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4–4, 3–5 and 2–6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions1,2. On the other hand, its liquid form has much in common, thermodynamically, with water and other ‘tetrahedral network’ liquids that show density maxima3,4,5,6,7. Proper study of the ‘liquid–amorphous transition’, documented for non-crystalline silicon by both experimental and computer simulation studies8,9,10,11,12,13,14,15,16,17, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger–Weber potential18, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar10. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction3 that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.

Liquid-liquid phase transition in supercooled silicon.

Results are presented of the RBS/channeling study of the structural defect behavior in ion bombarded InxGa1−xAsyP1−y (0 ⩽ x, y ⩽ 1) compounds at temperatures ranging from 15 K (LT) to 300 K (RT). Experiments consisted of implantation with different ions to fluences ranging from 4 × 1013 to 5 × 1015 at./cm2 at different temperatures followed by in situ RBS/channeling measurements. Successive measurements of LT implanted samples were performed during warming up to RT. Broad recovery stage beginning at 100 K for all compounds was revealed. It was attributed to the defect mobility in the group III sublattice. Steep damage buildup up to amorphisation with increasing ion dose was observed. The defect production efficiency is much higher at LT than at RT. The consequences of defect mobility at RT for ion implanted semiconductor structures are discussed.

Damage buildup and recovery in III–V compound semiconductors at low temperatures

Electron-energy-loss spectra have been recorded from cross-sectional transmission electron microscopy samples of Li + -irradiated KTiOPO 4 single crystals. By evaluating the intensity in the pre-edge shoulder of the OK ionization edge, the depth distribution of O vacancies in radiation-damaged KTiOPO 4 was monitored. Li + irradiation at 295K and Li + irradiation at 100K generate very similar defect profiles except for an additional superficial maximum in the room-temperature-irradiated sample. Vacancy distributions in the O sublattice can be monitored on the nanometre scale by the method introduced. The methodology is potentially applicable to a wide range of materials, when pertinent defect-sensitive features in electron-energy-loss spectra are evaluated, and its sensitivity is expected to become further improved with the availability of monochromated electron sources in transmission electron microscopes.

Depth distribution of oxygen vacancies in 1 MeV Li+-irradiated KTiOPO4

GaAs was irradiated with 10 MeV or 17.5 MeV Au ions at 16 K and subsequently measured at the same temperature. Under these conditions ion-beam induced processes can be studied whilst thermal effects are widely excluded. It is found that the measured defect concentration scales with the number of displacements per lattice atom (representing the nuclear energy deposition) independent of the ion energy and of the depth. This result demonstrates that, for the chosen ion energies, the electronic energy deposition itself does not influence the damage production in GaAs. Contrary in GaAs implanted with high-energy ions (10 MeV range) and analysed at room temperature, the defect concentration close to the surface was found to be lower than expected from theoretical simulations. The explanation for this previously observed effect is a preferred annealing of lightly damaged areas during warming the samples to room temperature for measurement.

High-energy Au implantation of GaAs at 16 K

In order to investigate the formation of Mn/Sb clusters embedded in crystalline silicon, sequential ion implantation with fluences of 1 × 1016 at cm−2 and 2 × 1016 at cm−2, respectively, was used to incorporate Mn and Sb ions at high concentrations into Si(0 0 1). Based on investigations with Rutherford backscattering spectroscopy (RBS) and corresponding channelling measurements (RBS/c), we report on a temperature dependent redistribution of the implanted species during the rapid thermal annealing process governed by the radiation-induced defects. Additionally performed cross-sectional TEM analyses, including EDX measurements, clearly show the presence of hexagonal shaped elementary Sb precipitates as well as compound clusters consisting of Mn and Sb, which are aligned to the crystal structure of the host silicon. In electron magnetic resonance measurements many samples exhibit broad resonance bands persisting up to approximately 60 K. For out-of-plane rotations, the bands show a weak angular dependence of the resonance field but a strong angular dependence of the intensity. Zero-field-cooled and field-cooled magnetization curves were measured on selected samples with a SQUID magnetometer between 10 and 400 K at different applied fields. The curves show a weak magnetic signal generated by different magnetic phases while at least one can be ascribed to superparamagnetic nanoparticles of MnSb.

https://iopscience.iop.org/article/10.1088/0022-3727/42/3/035406/pdf

Ion beam synthesis of Mn/Sb clusters in silicon

Rutherford Backscattering Spectrometry/Channeling and Extended X-ray Absorption Fine Structure measurements have been combined to investigate the amorphization of InxGa1−xP alloys at 15 and 300 K for selected stoichiometries representative of the entire stoichiometric range. The amorphization kinetics differs considerably for the two temperatures: at 15 K, the amorphization kinetics of InxGa1−xP is intermediate between the two binary extremes while at 300 K, InxGa1−xP is more easily amorphized than both InP and GaP. Direct impact and stimulated amorphization both contribute to the amorphization process at 15 K. Dynamic annealing via thermally induced Frenkel pair recombination reduces the influence of direct impact amorphization at 300 K such that the stimulated amorphization is dominant. At this temperature, stimulated amorphization in ternary InxGa1−xP alloys is supported by the structural disorder inherent from the bimodal bond length distribution.

/pdf/ion-implantation-induced-amorphization-of-inxga1-xp-alloys-5wonnoy2h3.pdf

Werner Wesch

Papers

Damage buildup and recovery in III–V compound semiconductors at low temperatures

Depth distribution of oxygen vacancies in 1 MeV Li+-irradiated KTiOPO4

High-energy Au implantation of GaAs at 16 K

Ion beam synthesis of Mn/Sb clusters in silicon

Ion-implantation-induced amorphization of InxGa1−xP alloys as functions of stoichiometry and temperature