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.

Ion-beam induced effects in α-Al2O3 of c, a, and r orientations were studied by Rutherford backscattering spectrometry (RBS) in channeling configuration using 1.4MeV He ions. 150keV Ar, 150keV K, or 80keV Na ions were step by step implanted at 15K followed immediately by the RBS analysis without changing the sample environment. Defect annealing was observed during the RBS measurement, which is attributed to the electronic energy loss of the He ions. A similar effect occurs due to the electronic energy loss of the implanted ions, resulting in a reduced defect concentration between surface and profile maximum. The electronic energy loss of ions may change the charge state of defects, thus enhancing their mobility and causing defect annealing. The results suggest that within the collision cascade of individual ions in perfect sapphire only point defects are produced, the concentration of which is well reproduced by SRIM calculations taking into account suggested values of the displacement energies of EdAl=20...

Ion-beam induced effects at 15K in α-Al2O3 of different orientations

The damage production in potassium titanyl phosphate KTiOPO 4 (KTP) was investigated for Li + , N + , Ar + and Er + ion implantation at different energies by means of the Rutherford backscattering (RBS) channeling spectrometry. Selected samples were furnace annealed at 650 ∘ C. The damage production in the case of Li + -implantation follows the profile of the nuclear energy deposition. A continuous damage evolution observed in the case of N + and Ar + implantations can be correlated with the electronic energy loss. The results indicate that damaging due to electronic excitation obviously occurs if a certain value of the electronic energy loss per incident ion is exceeded. For Er + -implantation both energy deposition processes seem to contribute to damaging. The findings are related to the binding energies reported for KTP crystals.

Investigation of radiation damage in ion implanted KTiOPO4

Because of the specific properties of GaAs, the annealing of implantation damage as well as the electric activation of dopants present more complez processes compared with silicon. Starting from the problems connected with conventional furnace annealing of GaAs, the results obtained with short time annealing in the solid and liquid phase are discussed. By means of liquid phase epitaxy using ns-laser pulses, extremely high carrier concentrations can be achieved, but the mobility of the carriers is limited by matrix-specific defects. Optical and RBS investigations give information about the defect types existing in pulsed laser irradiated GaAs layers, and the connection with the electrical properties of the layers is demonstrated. Possible ways for a reduction of the concentration of such defects as well as the improvement of the electrical properties are discussed.



Auf Grund der spezifischen Eigenschaften von GaAs ist die Ausheilung von Strahlenschaden in diesem Material ein weitaus komplexerer Prozes als im Silizium. Ausgehend von den Problemen bei der konventionellen Ofenausheilung von GaAs werden die mit Hilfe von Kurzzeitausheilverfahren im Festphasen- und Flussigphasenregime erzielten Ergebnisse diskutiert. Bei der mit ns- Laserimpulsen realisierten Flussigphasenepitaxie konnen extrem hohe Ladungstragerkonzentrationen erzeugt werden. Die Beweglichkeit der Ladungstrager wird jedoeh durch matrixspezifische Defekte begrenzt. Mit Hilfe von optischen und RBS-Messungen werden Aussagen uber die nach Flussigphasenepitaxie vorhandenen Defekttypen gemacht, der Zusammenhang mit den elektrischen Eigenschaften wird demonstriert. Mogliche Wege zur Verringerung der Konzentration derartiger Defekte sowie zur Erzielung verbesserter elektrischer Eigenschaften werden diskutiert.

Rapid annealing of ion-implanted GaAs†

We have studied the ion-beam synthesis of GaSb nanocrystals in Si by high-fluence implantation of Sb and Ga ions followed by thermal annealing. RBS, TEM/TED, RS, and photoluminescence (PL) were employed to characterize the implanted layers. It was found that the nanocrystals size increases from 5 to 60 nm in the samples annealed at 900 °C up to 20–90 nm in the samples annealed at 1100 °C. An existence of significant mechanical stresses within implanted layers has been detected. The stress values have been calculated from the shift of the Si first order Raman band. For the samples annealed at 900 °C a broad band in the spectral region of about 0.75–1.05 eV is detected in the PL spectra. The nature of this PL band is discussed.

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Structure and optical properties of silicon layers with GaSb nanocrystals created by ion‐beam synthesis

Epitaxial silicon nanowires were grown on gold (Au) implanted silicon substrates by chemical vapour deposition (CVD). We studied the gold droplet formation upon annealing of the Au implanted silicon and discuss the gold droplets capability to catalyze the vapour–liquid–solid (VLS) growth of silicon nanowires. Furthermore, we investigated the annealing behaviour of implanted gallium, indium, and aluminum in silicon and discuss why and how they differ from gold. It was found that they do not easily serve as a template for VLS nanowire growth.

Werner Wesch

Papers

Ion-beam induced effects at 15K in α-Al2O3 of different orientations

Investigation of radiation damage in ion implanted KTiOPO4

Rapid annealing of ion-implanted GaAs†

Structure and optical properties of silicon layers with GaSb nanocrystals created by ion‐beam synthesis

Silicon nanowire synthesis on metal implanted silicon substrates