Understanding the interaction of group V impurities with intrinsic defects in ZnO is important for developing p-type material. We have studied N-doped ZnO thin films and N-doped bulk ZnO crystals, with positron annihilation spectroscopy, in contrast to earlier studies that have concentrated on N-implanted ZnO crystals. We show that the introduction of N impurities into ZnO, irrespective of whether it is done during the growth of thin films or bulk crystals or through implantation and subsequent thermal treatments, leads to the formation of stable vacancy clusters and negative ion-type defects. Interestingly, the stability of these vacancy clusters is found almost exclusively for N introduction, whereas single Zn vacancy defects or easily removable vacancy clusters are more typically found for ZnO doped with other impurities.

The angle dependence of the phonon modes of a-plane (11 (2) over bar0) and c-plane (0001) faced ZnO crystals in wurtzite structure was studied at room temperature by Raman spectroscopy in backscattering geometry. The samples were rotated by 360 degrees about the axis defined by the excitation laser light coming in at normal incidence. A polarization filter combined with a lambda/2 plate was used in the optical path of the scattered light from the sample to select parallelly or perpendicularly polarized light with respect to the linearly polarized 532 nm excitation laser. The origin of the observed Raman modes is discussed in detail, and their variations in intensity are compared to calculated scattering intensities of allowed phonon modes in wurtzite crystals, yielding the Raman tensor elements of several modes normalized to that of the E-2(high) phonon mode: vertical bar a/d vertical bar = 0.6 and vertical bar b/d vertical bar = 0.5, vertical bar c/d vertical bar = 0.4, and vertical bar a/d vertical bar = 0.1 for the A(1)(TO), E-1(TO), and A(1)(LO) phonon modes, respectively.

We report on the optical properties of nitrogen acceptor-doped ZnO epilayers in the medium and high doping regimes using temperature and excitation power-dependent, as well as time-resolved photoluminescence experiments. The epilayers were doped with ammonia during homoepitaxial growth on ZnO single-crystal substrates with different surface polarities. Significant differences in the optical characteristics of the epilayers are observed between growth on nonpolar a-plane, polar c-plane Zn-face substrates and polar c-plane O-face substrates, which demonstrates different incorporation of the nitrogen acceptor depending on the substrate polarity. The incorporation of nitrogen into the ZnO films ranges between 10(19) and 10(21) cm(-3) as determined by secondary ion mass spectrometry. Within this doping range the samples change from lightly compensated to highly doped compensated. We discuss the unique photoluminescence features of nitrogen-doped ZnO epilayers within the concept of shallow donor-acceptor-pair recombinations and at the highest doping level by the appearance of potential fluctuations.

We report on the successful growth of ZnS epilayers on GaP (001) substrates using a CVD setup. The crystal structure of our layers is cubic zinc blende. The structural aspects and surface morphologies of ZnS films with different thicknesses and growth temperatures have been investigated using X-ray diffraction (XRD) and atomic force microscopy (AFM). Furthermore, low temperature photoluminescence (PL) at 4 K was used to determine optical properties. The appearance of two acceptor related transistions was found to correlate with our analysis of impurity incorporation by secondary ion mass spectrometry (SIMS). The high structural quality and low impurity levels are promising for the further development of the material system, aiming at an application as transparent conductive material.

Cu2O thin films were grown on sapphire (0001) and MgO (100) substrates by chemical vapor deposition. The crystalline, vibrational and electrical properties of the layers and the amount of incorporated background impurities have been examined. X-ray diffraction measurements revealed, that the polycrystalline films grew in (111) and (100) orientation on sapphire and in (100) orientation on MgO. Raman measurements indicated the presence of CuO inclusions in the films. The electrical properties are dominated by an acceptor level located 150 meV above the valence band. This level may originate from unintentionally incorporated silicon impurities. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We reconsider acceptor doping of ZnO with Li and Cu published nearly 40 years ago by comparing it with the behaviour of nitrogen in ZnO. While Cu plays an exceptional role due to the d-shell configuration (acceptor level at 190 meV below conduction band) Li and N as single acceptors give rise to deep distorted acceptors with binding energies between 700 and 800 meV above valence band. With these binding energies no hole conductivity at room temperature can be expected, having in mind that typical background donor densities in ZnO are between 10(16) and 10(17) cm(-3). We propose a defect model to explain the role of lithium and nitrogen doping in ZnO which is based on two acceptor-one donor pairing as proposed by theory in the framework of co-doping. Hydrogen is a key candidate for the donor role. The luminescence properties in ZnO: N can be understood on the basis of the calculation of the hole densities without and with compensation assuming pair formation. We compare our experimental findings with published results on nitrogen doped ZnO explaining the limitations of p-type doping of ZnO with nitrogen. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons (Y lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the Y-0 transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers (I lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all Y lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II-VI and III-V semiconductors.

physica status solidi (b)Volume 248, Issue 5 Back Cover Back Cover: A model for acceptor doping in ZnO based on nitrogen pair formation (Phys. Status Solidi B 5/2011) S. Lautenschlaeger, Corresponding Author S. Lautenschlaeger [email protected] Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanyPhone: +49 641 99 33107; Fax: +49 641 99 33109Search for more papers by this authorM. Hofmann, M. Hofmann Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorS. Eisermann, S. Eisermann Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorG. Haas, G. Haas Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorM. Pinnisch, M. Pinnisch Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorA. Laufer, A. Laufer Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorB. K. Meyer, B. K. Meyer Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this author S. Lautenschlaeger, Corresponding Author S. Lautenschlaeger [email protected] Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanyPhone: +49 641 99 33107; Fax: +49 641 99 33109Search for more papers by this authorM. Hofmann, M. Hofmann Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorS. Eisermann, S. Eisermann Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorG. Haas, G. Haas Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorM. Pinnisch, M. Pinnisch Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorA. Laufer, A. Laufer Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this authorB. K. Meyer, B. K. Meyer Physics Institute, Justus-Liebig-University Giessen, 35393 Giessen, Heinrich-Buff-Ring 16, GermanySearch for more papers by this author First published: 27 April 2011 https://doi.org/10.1002/pssb.201190015AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Graphical Abstract The back cover image shows the essence of this issue's Editor's Choice article by Stefan Lautenschlaeger et al. (pp. 1217-1221). A complex model for shallow acceptors in ZnO, involving two group V acceptors (i.e., two nitrogen atoms) and one donor (in this case hydrogen), is presented and discussed. As you can see, three different nitrogen confi gurations are presented. One is the isolated NO which might, according to recent results and theoretical calculations, lead to deep acceptor states. The second configuration is the neutral N-H complex. The third possible configuration consists of an acceptor-donor-acceptor complex and may lead to the observed shallow acceptor. The authors discuss the photoluminescence data obtained by ammonia-doped samples with different amounts of nitrogen and hydrogen with respect to their acceptor model. Volume248, Issue5May 2011 RelatedInformation

ZnO crystals grown by the vertical Bridgman technique were comprehensively characterized in view of the impurities and intrinsic defects in the material. It is shown that residual Al is the cause of the residual n-type conductivity and intrinsic defects play only a minor role in the samples. Annealing the samples at 1100 degrees C for 1 h in oxygen atmosphere improves the surface properties of the samples without having negative effects on their optical and electrical properties. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Zinc oxide (ZnO) is regarded as a promising material for optoelectronic devices, due to its electronic properties. Solely, the difficulty in obtaining p-type ZnO impedes further progress. In this connection, the identification and quantification of impurities is a major demand. For quantitative information using secondary ion mass spectrometry (SIMS), so-called relative sensitivity factors (RSF) are mandatory. Such conversion factors did not yet exist for ZnO. In this work, we present the determined RSF values for ZnO using primary (ion implanted) as well as secondary (bulk doped) standards. These RSFs have been applied to commercially available ZnO substrates of different surface termination (a-plane, Zn-face, and O-face) to quantify the contained impurities. Although these ZnO substrates originate from the same single-crystal, we observe discrepancies in the impurity concentrations. These results cannot be attributed to surface termination dependent RSF values for ZnO.

physica status solidi (a)Volume 208, Issue 1 p. 11-15 ContentsFree Access Contents: (Phys. Status Solidi A 1/2011) First published: 12 January 2011 https://doi.org/10.1002/pssa.201121802AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Volume208, Issue1January 2011Pages 11-15 RelatedInformation

We report on the growth of non-polar a-plane ZnO by CVD on r-plane-sapphire-wafers, a-plane GaN-templates and a-plane ZnO single-crystal substrates. Only the homoepitaxial growth approach leads to a Frank–van-der–Merwe growth mode, as shown by atomic force microscopy. The X-ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the ZnO substrate. The morphological and the structural quality of the homoepitaxial films is comparable to the best results for the growth on c-plane ZnO-substrates. The impurity incorporation, especially of group III elements, seems to be reduced when growing on the non-polar a-plane surface compared to the c-plane films as demonstrated by secondary ion mass spectrometry (SIMS). Optical properties have been investigated using low temperature photoluminescence measurements. We employed capacitance–voltage measurements (C–V) to measure the background carrier density and its profile from substrate/film interface throughout the film to the surface. In thermal admittance spectroscopy (TAS) specific traps could be distinguished, and their thermal activation energies and capture cross sections could be determined.

ZnO shows a great variety of richly structured luminescence lines in a very narrow energy range of 30 meV below the free $A$-exciton line. At very low temperatures the majority of these lines can be explained by radiative recombination of excitons bound to neutral and ionized donors. With increasing temperature even more photoluminescence (PL) lines appear with activation energies which indicate the involvement of excited states of the bound exciton complexes. Based on high-resolution temperature-dependent luminescence and luminescence excitation experiments we investigate the excited states properties of donor bound excitons in ZnO. Several possible configurations of excited states could be distinguished: (i) excitons which involve a hole from the $B$- instead of the $A$-valence band, (ii) vibrational-rotational excited states of the excitons, and (iii) electronic excited states of the excitons. Magneto-PL measurements of the ground and vibrational-rotational excited states corroborate the identification of the excited states with comparable $g$-factors for all shallow donor bound excitons. In addition to the excited states, energy transfer processes via the ionized bound excitons and free exciton polaritons are observed. The experimental results are supported by theoretical calculations and demonstrate that ZnO is a unique compound semiconductor where the basics of atomic and molecular physics can be studied and understood in a solid state matrix.

The wide bandgap polar semiconductors GaN and ZnO and their related alloys exhibit fascinating properties in terms of bandgap engineering, carrier confinement, internal polarisation fields, and surface terminations. With a small lattice mismatch of ~1.8 % between GaN and ZnO and the possibility to grow MgZnO lattice-matched to GaN, the system AlGaN/MgZnO offers the opportunity to design novel optoelectronic devices circumventing the problem of p-type doping of ZnO. In such AlGaN/MgZnO heterostructures with either hetero- or isovalent interfaces, tuning of band offsets is possible in various ways by polarisation fields, surface termination, strain, and composition. These aspects need to be fully understood to be able to make full use of this class of heterostructures. We report on the growth of ZnO films by chemical vapor deposition on p-type GaN and AlGaN grown by metal-organic vapor deposition on sapphire templates and on the fabrication of corresponding light-emitting diode (LED) structures. Electrical and optical properties of the n-ZnO/p-GaN and n-ZnO/p-AlGaN LEDs will be compared and the observed differences will be discussed in terms of the band alignment at the heterointerface.

Bipolar conduction of electron and holes is mandatory for many electronic and optoelectronic applications of ZnO. Among the impurity atoms suited for acceptor formation nitrogen is the prime candidate. Controlling the incorporation without the formation of deep donors and acceptors and avoiding the deterioration of the materials quality are the main goals for homo- and hetero-epitaxially grown ZnO films. We report on the sputter deposition of N-doped ZnO layers and will show by SIMS and Raman spectroscopy how the substrate type and substrate temperature influence the nitrogen incorporation.Hydrogen in ZnO plays an unusual role since it acts as a shallow donor and may control the n-type conductivity in nominally undoped material. However, it can also be used as an n-type dopant. We will demonstrate this behavior using Hall and optical transmission measurements made on ZnO films prepared by RF-magnetron sputtering. The incorporation of hydrogen as a function of the substrate temperature as well as its thermal stability was investigated by SIMS. (C) 2009 Elsevier B.V. All rights reserved.

ZnO thin films have been deposited on GaN and ZnO substrates at substrate temperatures up to 750 degrees C by radio-frequency sputtering using ZnO ceramic targets in pure argon or in a mixture of argon and oxygen. By optimizing the sputter parameters, such as sputtering power, Ar/O-2 sputtering gas ratio and temperature of the substrates high quality films were obtained as judged from the X-ray rocking curve half width and luminescence line width. The crystallinity of the ZnO films increases with increasing substrate temperature. Yet there are distinct differences between films grown on GaN templates and on O- and Zn-polar ZnO substrates. (C) 2009 Elsevier B.V. All rights reserved.

ZnO:N thin films have been deposited on oxygen and zinc terminated polar surfaces of ZnO. The nitrogen incorporation in the epilayers, using NH(3) as doping source, was investigated as a function of the growth temperature in the range between 380 degrees C and 580 degrees C. We used Raman spectroscopy and low temperature photoluminescence to investigate the doping properties. It turned out that the nitrogen incorporation strongly depends on both, the surface polarity of the epitaxial films and the applied growth temperatures. In our CVD process low growth temperatures and Zn-terminated substrate surfaces clearly favour the nitrogen incorporation in the ZnO thin films. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim