W. Anwand

Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction

Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emitting p-n junction was subsequently formed by postimplantation annealing in air. Deep level transient spectroscopy was used to investigate deep level defects induced by N+ implantation and the effect of air annealing. The N+ implantation enhanced the electron trap at EC−(0.31±0.01)eV (E3) and introduced another one at EC−(0.95±0.02)eV (D1), which were removed after annealing at 900 and 750°C, respectively. Another trap D2 (Ea=0.17±0.01eV) was formed after the 750°C annealing and persisted at 1200°C.

Arsenic doped p-type zinc oxide films grown by radio frequency magnetron sputtering

As-doped ZnO films were grown by the radio frequency magnetron sputtering method. As the substrate temperature during growth was raised above ∼400 °C, the films changed from n type to p type. Hole concentration and mobility of ∼6×1017 cm−3 and ∼6 cm2 V−1 s−1 were achieved. The ZnO films were studied by secondary ion mass spectroscopy, x-ray photoelectron spectroscopy (XPS), low temperature photoluminescence (PL), and positron annihilation spectroscopy (PAS). The results were consistent with the AsZn–2VZn shallow acceptor model proposed by Limpijumnong et al. [Phys. Rev. Lett. 92, 155504 (2004)]. The results of the XPS, PL, PAS, and thermal studies lead us to suggest a comprehensive picture of the As-related shallow acceptor formation.

Defect studies of ZnO single crystals electrochemically doped with hydrogen

Various defect studies of hydrothermally grown (0001) oriented ZnO crystals electrochemically doped with hydrogen are presented. The hydrogen content in the crystals is determined by nuclear reaction analysis and it is found that already 0.3at.% H exists in chemically bound form in the virgin ZnO crystals. A single positron lifetime of 182ps is detected in the virgin crystals and attributed to saturated positron trapping at Zn vacancies surrounded by hydrogen atoms. It is demonstrated that a very high amount of hydrogen (up to ∼30at.%) can be introduced into the crystals by electrochemical doping. More than half of this amount is chemically bound, i.e., incorporated into the ZnO crystal lattice. This drastic increase of the hydrogen concentration is of marginal impact on the measured positron lifetime, whereas a contribution of positrons annihilated by electrons belonging to O–H bonds formed in the hydrogen doped crystal is found in coincidence Doppler broadening spectra. The formation of hexagonal shape ...

Electrical properties of ZnO nanorods studied by conductive atomic force microscopy

ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, we present a complementary electrical characterization of individual upright standing and lying ZnO nanorods using conductive atomic force microscopy (C-AFM). Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps. The current maps were recorded simultaneously with the topography acquired by contact mode AFM. Further, C-AFM was utilized to determine the local current-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs. Current-voltage characterization revealed a characteristic similar to that of a Schottky diode. Detailed discussion of the electrical properties is based on local I-V curves, as well as on the 2D current maps recorded from specific areas.

Rise and fall of defect induced ferromagnetism in SiC single crystals

6H–SiC (silicon carbide) single crystals containing VSi–VC divacancies are investigated with respect to magnetic and structural properties. We found that an initial increase in structural disorder leads to pronounced ferromagnetic properties at room temperature. Further introduction of disorder lowers the saturation magnetization and is accompanied with the onset of lattice amorphization. Close to the threshold of full amorphization, also divacancy clusters are formed and the saturation magnetization nearly drops to zero.

Hydrogen-mediated ferromagnetism in ZnO single crystals

We investigated the magnetic properties of hydrogen-plasma-treated ZnO single crystals by using superconducting quantum interferometer device magnetometry. In agreement with the expected hydrogen penetration depth, we found that ferromagnetic behavior is present in the first 20nm of the H-treated surface of ZnO with magnetization at saturation up to 6emug 1 at 300K and a Curie temperature of Tc & 400K. In the ferromagnetic samples, a hydrogen concentration of a few atomic per cent in the first 20nm of the surface layer was determined by nuclear reaction analysis. The saturation magnetization of H-treated ZnO increases with the concentration of hydrogen. After a large number of studies and different kinds of efforts, experimental and theoretical work of the last few years indicates that defect-induced magnetism remains key to triggering ferromagnetism in ZnO (as well as in other non-magnetic oxides) with a Curie temperature above 300K. It is not doping with magnetic elements that appears to be a successful and reproducible method to trigger magnetic order in this oxide, but the introduction of a certain defect density of the order of a few per cent, such as O (1) or Zn vacancies (2) with or without doping with non-magnetic ions such as C (3), N (4), Li (5) or Cu (6, 7). In general, however, the achieved magnetization values are still too low, indicating that the magnetic order is most 4 Author to whom any correspondence should be addressed.

Interstitial-type defects away from the projected ion range in high energy ion implanted and annealed silicon

Defects in high energy ion implanted silicon have been investigated, especially in the depth range around half of the projected ion range RP/2 after annealing at temperatures between 700 and 1000 °C. Preferable trapping of metals just in this depth range proves the existence of defects there. No vacancy-like defects could be detected by variable energy positron annihilation spectroscopy after annealing at temperatures T>800 °C. Instead, interstitial-type defects were observed in the RP/2 region using cross section transmission electron microscopy of a specimen prepared under special conditions. The results indicate the presence of small interstitial agglomerates at RP/2 which remain after high temperature annealing.

Non-destructive characterization of vertical ZnO nanowire arrays by slow positron implantation spectroscopy, atomic force microscopy, and nuclear reaction analysis

ZnO nanorods, grown by a hydrothermal method, have been characterized by slow positron implantation spectroscopy (SPIS) and atomic force microscopy (AFM). It has been demonstrated that such non-destructive characterization techniques can provide a comprehensive picture of the nanorod structure (including its length, shape, orientation, and seed layer thickness), as well as provide additional information about defects present in the structure. Nanorods were also characterized by scanning electron microscopy (SEM) and x-ray diffraction (XRD), and it was found that the SPIS/AFM combination is more sensitive to the nanorod orientation and the thickness of the seed layer. To obtain still more information about defects in the nanorods, as well as to confirm the findings on the sample structure, nuclear reaction analysis (NRA) was performed and a large concentration of bound hydrogen was found. The results obtained by different characterization techniques are discussed.

Design and Construction of a Slow Positron Beam for Solid and Surface Investigations

On the basis of the design and construction of the slow positron beam SPONSOR at the Helmholtz-Centre Dresden-Rossendorf an example is given how to build-up a simple slow positron beam for solid surface investigations within a short time and without high financial costs. The system uses a 22Na source and consists of three main parts: (1) the source chamber with a thin film tungsten moderator used in transmission, and a pre-accelerator stage, (2) the vacuum system with magnetic transport, a bent tube for energy selection and an accelerator, (3) the sample chamber with a sample holder, Ge detectors and (4) facilities for remote control and data acquisition. These parts are described in detail. The paper is preferentially addressed to beginners in the field of slow positron beam techniques and other readers being generally interested in positron annihilation spectroscopy.

DAMAGE IN SILICON CARBIDE INDUCED BY RUTHERFORD BACKSCATTERING ANALYSIS

Abstract Damage in silicon carbide generated by ion implantation or irradiation is usually analyzed by Rutherford backscattering spectroscopy in combination with channeling (RBS/C) of MeV He + ions, a technique which is considered to be largely non-destructive. In this paper we report on swelling of 6H–SiC induced by He + implantation at doses commensurate with, or lower than those commonly used for obtaining RBS/C spectra of desirable statistics. The swelling increases by about 40% if the He + ions are implanted in a non-channeling direction. The formation of high concentrations of deep-reaching (μm range) defects due to RBS/C is confirmed by slow positron implantation spectroscopy (SPIS) measurements. An optical damage depth-profile, with distinct optical properties corresponding to the regions of electronic and nuclear stopping, is obtained from a fit to polarized infrared reflection spectroscopy (PIRR) data and compared to TRIM calculation. Auger electron spectroscopy (AES) shows that the specific color of the implanted area is not due to the deposition of a thin surface film during He + implantation, and the swelling is not related to chemical reactions in the near-surface region. The formation of additional disorder from RBS/C may corrupt the respective data obtained subsequently by SPIS and PIRR. Therefore, RBS/C measurements should always be carried out last, i.e. following analytical techniques which are certainly non-destructive.