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.

Hexagonal 6H– and 4H–SiC wafers were implanted with (1−1.5) × 1016 cm−2 germanium ions at room temperature and at 700 °C with subsequent annealing between 1000 and 1600 °C. Structural changes in the SiC matrix were studied in detail by means of transmission electron microscopy (TEM). After implantation at room temperature the hexagonal SiC matrix becomes amorphous and, after annealing, recrystallizes into cubic SiC. The latter process was accompanied by the creation of voids and cracks. In case of high-temperature (700 °C) implantation, where amorphization was avoided, no polytype change in as-implanted and annealed SiC wafers was observed. In annealed samples nanocrystalline precipitates with high Ge content were observed in high-resolution TEM images.

Transmission electron microscopy study of Ge implanted into SiC

Patterning of LiNbO3 by means of ion irradiation using the electronic energy deposition and wet etching

GaAs and InP crystals were implanted with 1.6 MeV Ar+ and 2.0 MeV Se2+ ions in the dose range of 1 × 1012 cm−2 up to 3 × 1015 cm−2 at room temperature. The investigation of the resulting damage after implantation by means of Rutherford backscatttering spectrometry shows that the same amount of nuclear energy deposition leads to different damage in the two materials for the two ion species. In GaAs pronounced self-annealing occurs which is different for the different depth regions of the implanted layer. Part of the samples were short time annealed and show an activation of 40% and high mobilities in the low ion fluence region.

Comparison of MeV-implanted GaAs and InP

We report on the rapid implantation-induced amorphization of the ternary InxGa1−xAs alloys Unlike AlxGa1−xAs, InxGa1−xAs did not exhibit amorphization kinetics intermediate between the two binary-alloy extremes Instead, our investigation of the crystalline-to-amorphous phase transformation over the entire stoichiometry x range demonstrated that InxGa1−xAs alloys with x= 006–053 were rendered amorphous at fluences less than that required for both InAs and GaAs Implantation-induced disorder was quantified with Rutherford backscattering spectroscopy in the channeling configuration and fit to the Hecking model to yield the probabilities of direct-impact and stimulated amorphizations The phase transformation was dominated by stimulated amorphization, which was a maximum at the stoichiometry x= 034 most easily amorphized, while the probability of direct-impact amorphization was effectively stoichiometry independent From extended x-ray-absorption fine-structure spectroscopy measurements of unimplanted InxGa1−xAs alloys, separate stoichiometry-dependent In-As and Ga-As bond lengths were measured Distortion in both the bond-length and bond-angle distributions was apparent though structural perturbation was primarily accommodated in the latter as consistent with measured deviations from the tetrahedral bond angle We attribute the relative ease with which the InxGa1−xAs alloys were amorphized to the presence of localized regions of strain due to structural distortion Equivalently, atomistic configurations comprised of strained bond lengths and bond angles represent pre-existing and preferential sites for stimulated amorphization To demonstrate the general applicability of our model, we report on preliminary measurements with the InxGa1−xP alloys which also exhibit a bimodal bondlength distribution and distortion in the bond-angle distribution Comparable amorphization behavior to that of the InxGa1−xAs alloys has been observed

Rapid ion-implantation-induced amorphization of InxGa1−xAs relative to InAs and GaAs

Various semiconductors (GaAs, GaP, InAs, InP, Ge, Si) were implanted under identical conditions with 200 keV Ar + ions and 300 keV Se + ions at 80 and 300 K over a wide dose range. It is shown that significant differences occur between GaAs and InAs on the one hand and the other materials on the other hand, which cannot be explained by the ionicity of the materials. The high mobility of Frenkel defects seems to be responsible for the substantial influence of defect annealing and transformation in the arsenides, whereas in the other materials a higher stability of the primarily produced defect clusters can be assumed.

Werner Wesch

Papers

Transmission electron microscopy study of Ge implanted into SiC

Patterning of LiNbO3 by means of ion irradiation using the electronic energy deposition and wet etching

Comparison of MeV-implanted GaAs and InP

Rapid ion-implantation-induced amorphization of InxGa1−xAs relative to InAs and GaAs

Investigation of the amorphization process in ion implanted AIIIBV compounds