E. V. Monakhov

Annealing behavior between room temperature and 2000 °C of deep level defects in electron-irradiated n-type 4H silicon carbide

The annealing behavior of irradiation-induced defects in 4H-SiC epitaxial layers grown by chemical-vapor deposition has been systematically studied by means of deep level transient spectroscopy (DL ...

Zinc oxide: bulk growth, role of hydrogen and Schottky diodes

Zinc oxide (ZnO) is a wide band gap semiconductor material with attractive features for light emitting devices, photovoltaics, chemical sensors and spintronics. In the past 10 yr ZnO has attracted tremendous interest from the materials science and semiconductor physics research communities, and in this review recent progress in (i) bulk growth, (ii) understanding of the role of hydrogen and (iii) formation of high-quality Schottky barrier (SB) diodes, are discussed for single crystalline ZnO. In (i), the emphasis is put on hydrothermally grown material and how the concentration of intentional and unintentional impurities, such as In and Li, can be controlled and modified by high temperature treatment and defect engineering involving vacancy clusters. In (ii), different possible configurations of hydrogen as a shallow donor are evaluated based on results from calculations employing the density-functional-theory as well as from experimental studies of local vibrational modes using Fourier transform infrared spectroscopy. Further, hydrogen is demonstrated to be very reactive and the interaction with zinc vacancies, group I and group V elements, and transition metals are elucidated. Moreover, the diffusion of hydrogen is found to be rapid and limited by the concentration of traps in hydrothermal samples, and it is argued that isolated (free) hydrogen is not very likely to exist in ZnO at room temperature. In (iii), a compilation of the literature data illustrates that the SB heights for metals deposited on n-type samples have no correlation with the metal work function, violating the fundamental Schottky–Mott model. The role of surface preparation cannot be overestimated and in several cases an oxidation of the surface prior to metal deposition is shown to be beneficial for the formation of high barrier SB diodes. The effects of near-surface defects, such as oxygen vacancies, and contact inhomogeneity are also addressed. However, in spite of the significant progress made in the past 5–7 years, a thorough understanding of the SB formation to ZnO is still lacking. Finally, results from characterization of electrically active point defects employing the SB contacts and junction spectroscopic techniques are reviewed and the identification of some prominent bandgap states is critically evaluated.

Electrical characteristics of palladium Schottky contacts to hydrogen peroxide treated hydrothermally grown ZnO

The formation of Schottky barrier contacts to hydrogen peroxide treated ZnO has been investigated. Low resistivity hydrothermally grown single crystal ZnO wafers of n-type were used. Pd contacts deposited on organic solvent cleaned O face (0001¯) showed Ohmic behavior, while on the H2O2 treated O face up to nine orders of magnitude in rectification of the current was obtained for biases of −2 and +2V. Concurrently, the surface roughness increases from 1.0±0.5 up to 2.0±0.5nm due to the H2O2 treatment. A majority of the contacs deposited were stable or improved their performance by annealing in air at 200°C for 30min. However, the contacts both before and after the annealing exhibited ideality factors of at least ∼1.8 at +0.5V suggesting that the current transport cannot be described as purely thermionic. Finally, results of capacitance versus voltage and capacitance versus temperature measurements are discussed and show a dominant electron state at ∼0.32eV below the conduction band edge.

Surface passivation and interface reactions induced by hydrogen peroxide treatment of n-type ZnO (0001¯)

X-ray photoemission spectroscopy and electrical measurements have been employed to study O-face (0001¯) n-type ZnO samples treated by hydrogen peroxide (H2O2). A highly resistive and oxygen-rich surface layer is revealed, presumably caused by a high concentration of zinc vacancies and/or adsorbed O2 molecules. As a result, the surface exhibits upward energy band bending (∼0.4 eV) promoting the formation of high barrier Schottky contacts and suppressing the surface leakage current. Furthermore, after Pd deposition an enhanced formation of PdO is found at the Pd/ZnO interface for the H2O2-treated samples, and this is also expected to increase the resulting Schottky barrier height (∼0.6 eV), which yields up to seven orders of magnitude in current rectification between forward and reverse bias voltage.

The oxygen dimer in Si: Its relationship to the light-induced degradation of Si solar cells?

It is widely believed that the light induced degradation of crystalline silicon solar cells is due to the formation of a BsO2i recombination center created by the optically excited migration of the oxygen dimer (charge-state-driven motion). In this letter the concentration dependence of the neutral state of O2i on [Oi] in p- and n-type Cz–Si has been determined using infrared absorption. A systematic search for the absorption signature of the dimer in the doubly positively charged state has been unsuccessful. These data strongly suggest that charge-state-driven motion (Bourgoin–Corbett mechanism) of the oxygen dimer cannot occur in typical solar silicon and hence bring into question the accepted degradation mechanism.

Evolution of deep electronic states in ZnO during heat treatment in oxygen- and zinc-rich ambients

Hydrothermally grown ZnO samples have been annealed in Ar, Zn-rich, and O-rich ambients and investigated by deep level transient spectroscopy (DLTS). The DLTS measurements reveal up to 6 different defect levels in the band gap after different annealing conditions. A clear correlation has been found between the annealing treatment and the formation/suppression of two deep defect levels at ∼0.2 and ∼0.5 eV below the conduction band edge (Ec). As a result, the Ec-0.5eV level is assigned to a Zn-rich defect while the Ec-0.2eV level is due to a O-rich defect, where the latter shows donor behavior as revealed by a distinct Poole-Frenkel effect.

Hydrogen implantation into ZnO for n+-layer formation

Bulk ZnO crystals were implanted using 100keV H+ ions with doses 5×1016 and 2×1017cm−2 and subsequently annealed at 200–600°C to study the evolution of the implanted H by employing secondary ion mass spectrometry and scanning spreading resistance microscopy. It is shown that the heat treatment results in a decrease of H concentration in the implanted region, while no significant broadening of the H profiles is observed. This suggests that the implanted H is trapped in immobile complexes which dissociate during annealing with subsequent outdiffusion of H from the implanted region. The formation of a highly conductive n+-layer is observed in the implanted region, and the n+-layer is found to be stable up to 600°C. A correlation between electrical activity of H and presence of radiation damage is discussed.

Deuterium diffusion and trapping in hydrothermally grown single crystalline ZnO

Secondary ion mass spectrometry is employed to investigate diffusion of H2 implanted in hydrothermally grown single crystal ZnO. Diffusion profiles have been studied after 30 min isochronal heat treatments from 100 to 400 °C and evaluated using three different models: the infinite source model, a solid solubility limited model, and a trap limited model. Only the latter one reproduces closely the measured values. From this model an activation energy Ea=0.85 eV is extracted, and it is speculated that trapping may be a source of the discrepancies between the reported values of Ea in the literature.

On the identity of a crucial defect contributing to leakage current in silicon particle detectors

The annealing kinetics of the so-called E4/E5 center or E4a/E4b center in electron-irradiated Si particle detectors has been studied at four different temperatures from 23to65°C using deep-level transient spectroscopy (DLTS). The center gives rise to two energy levels at 0.37 and 0.45eV below the conduction-band edge, and the annealing process is found to be of dissociative nature with an energy barrier of 1.1–1.2eV. A striking similarity of the annealing rates (and kinetics) is revealed with that obtained for the 936-cm−1 infrared absorption band, studied by Fourier-transform infrared spectroscopy using identical type of Si material as in the DLTS study but irradiated with neutrons. The result strongly suggest that the E4/E5 levels and the 936-cm−1 band originate from the same defect, and the latter has been attributed to a di-interstitial-oxygen (I2O) complex. The E4/E5 center plays a crucial role for the detrimental leakage current in irradiated Si particle detectors, and an assignment of E4/E5 to I2O ...

The work function of n-ZnO deduced from heterojunctions with Si prepared by ALD

Highly doped n-type ZnO films have been grown on n-type and p-type Si substrates by atomic layer deposition (ALD). Transmission electron microscopy shows columnar growth of the ZnO films with randomly oriented grains and a very thin interfacial layer of SiOx(x ≤ 2) with a thickness below 0.4 nm to the Si substrate. Current–voltage and capacitance–voltage measurements performed at temperatures from 50 to 300 K reveal a strong rectifying behaviour on both types of substrates with an ideality factor close to unity between 180 and 280 K. Using the classical approach of thermionic emission, the barrier heights of the ZnO/n-Si and ZnO/p-Si junctions have been deduced and consistent values are obtained yielding a work function of n-type ZnO close to 4.65 eV.