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Defects in a nitrogen implanted and thermally annealed zinc oxide thin film (n-type conducting) and reference samples were studied. Space charge regions realised by fabrication of semitransparent palladium Schottky contacts enabled the application of capacitance spectroscopic methods and photo - current measurements. We report on the formation of a deep level, in the following labelled TN1. It is 580 meV below the conduction band edge, probably related to nitrogen, and must be distinguished from the well known intrinsic deep level E4 at almost equal energetical depth. Capacitance measurements in combination with optical excitation, conducted at different temperatures, as well as photo-current measurements revealed the existence of two states approximately 60 meV and 100 meV above the valence band edge for the nitrogen implanted sample. These states cause an acceptor compensation degree larger than 0.9. The thermal emission of holes from these states into the valence band was observed by optical deep level transient spectroscopy.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3369-3371, Cu(In,Ga)Se2 (CIGSe) solar cells on flexible polyimide substrates were produced in an ion beam assisted low temperature deposition process with a maximum efficiency of 14 %. The selenium ion energy supports the growth of the CIGSe layer by an enhanced mobility of the adatoms which should result in smoother surfaces, less tension and increased adhesion to the back contact. However higher selenium ion energies concurrently cause ion beam induced defects reducing the solar cell efficiency. The aim of this work is to analyse the effect of varying ion beam energies by quantum efficiency measurements and calculated luminescence spectra. We show that the Urbach energy as a measure of structural disorder and the luminescence intensity of a defect allocated peak increases with higher ion energy. Furthermore we show that with increasing selenium ion energy losses in VOC are caused mainly by non-radiative recombination.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2053-2055, Strong correlation between the pre-breakdown sites visible in dark lock-in thermography due to local heating and the intensity of spatially resolved electroluminescence of reverse-biased solar cells was observed. By comparing differently texturised structurally similar solar cells we could show that the positions of the observed prebreakdown sites are identical for acidic and alkaline texturised solar cells and therefore are not correlated with the surface morphology. The pre-breakdown sites are directly related to bulk defects in the wafer. Nevertheless, the breakdown voltage is lower for acidic texturised solar cells. The quantitative parameters of breakdown are influenced by the texture. Also pre-breakdown sites at bulk defects are observed in areas without specific surface features for alkaline texturised solar cells.

NiO has great potential applications in gas sensors, optical fibers, solar thermal absorbers, or in non-volatile resistive random memories. In our study NiO, NiMnO, and NiMnLiO have been grown on double-side polished r-plane sapphire substrates by pulsed laser deposition. In contrast to the antiferromagnetic behaviour of bulk NiO, we probed weak ferromagnetism with a coercivity ranging between 150 and 250 Oe by means of SQUID magnetometry. We measured the complex Voigt angle using a HeCd laser, a Glan Taylor polarizer, a Hinds PEM-100 and two LockIns. The polarization state of light after transmission through a sample consisting of ca. 1 μm thick, weak ferromagnetic and diamagnetic NiO thin films on diamagnetic r-plane sapphire substrates has been modelled using the 4*4 matrix formalism in dependence of an external magnetic field applied in-plane. The modelling results revealed that for the bare diamagnetic substrate the Voigt angle depends parabolically on the external magnetic field and that the weak ferromagnetic and diamagnetic NiO thin films changed the parabolic dependence of the Voigt angle in the range of ±0.1 T to a flat-top shape in agreement with the experimentally determined Voigt angle.

We have investigated the magnetic properties of pure ZnO thin films grown under N2 pressure on a-, c- and r-plane Al2O3 substrates by pulsed laser deposition. The substrate temperature and the N2 pressure were varied from room temperature to 570 °C and from 0.007 mbar to 1.0 mbar, respectively. The magnetic properties of bare substrates and ZnO films were investigated by SQUID magnetometry. ZnO films grown on c- and a-plane Al2O3 substrates did not show significant ferromagnetism. However, ZnO films grown on r-plane Al2O3 showed reproducible ferromagnetism at 300 K when grown at 300¡400 °C and 0.1-1.0 mbar N2 pressure. Positron annihilation spectroscopy measurements as well as density functional theory calculations suggest that the ferromagnetism in ZnO films is related to Zn vacancies.

Spin-polarized tunnel magnetoresistance (TMR) effects occur when two ferromagnets are separated by a thin insulator. The resistance of the tunneling current changes with the relative magnetization orientation of the magnetic bottom and top electrode. The research in this field is fuelled by the demanding of magnetoresistive random access memory (MRAM) devices. Novel MRAM cells are based on magnetic tunnel junctions with current-induced switching. It has been shown that semiconductors need a current pulse for switching which is two orders of magnitude smaller in comparison to metals. Using wide-gap magnetic semiconductors, e.g. ZnO, the magnetic tunnel structure may be transparent and may possess a Curie temperature above room temperature. In this talk, we report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as the bottom electrode and Co as the top electrode prepared by pulsed laser deposition and thermal evaporation [1], respectively. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 separation layer and tunnelled through the amorphous part of the Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices. [1] Q. Xu et al., Phys. Rev. Lett. 101, 076601 (2008)

Traditional electronics can be greatly stimulated by a combination of magnetic and semiconducting properties where spins provide an additional degree of freedom. Recently theoretical works predict that some semiconductors (e.g. ZnO) doped with transition metal are diluted magnetic semiconductors (DMS) [1]. In binary DMS materials, transition metal ions substitute cation sites of the host semiconductor and are coupled by free carriers resulting in ferromagnetism. One of the main obstacles in creating DMS materials is the formation of secondary phases because of the solid-solubility limit. Ion implantation is a non-equilibrium method, and has been successfully used to create GaAs-based DMS [2]. In our study transition metal ions were implanted into ZnO single crystals with the peak concentrations of 0.5-10 at.%. We established a correlation between structural and magnetic properties. By synchrotron radiation X-ray diffraction secondary phases (Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as the source for ferromagnetism [3-6]. Due to their different crystallographic orientation with respect to the host crystal these nanocrystals in some cases are very difficult to be detected by a simple Bragg-Brentano scan. This results in the pitfall of using XRD to exclude secondary phase formation in DMS materials. For comparison, the solubility of Co diluted in ZnO films amounts to 10 at.% using pulsed laser deposition (PLD) [7]. Such diluted, Co-doped ZnO films show paramagnetic behaviour. However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d exchange interaction as compared to Co-implanted ZnO single crystals. References [1] T. Dietl, et al., Science 2000; 287:1019. [2] M. A. Scarpulla, et al, Appl. Phys. Lett. 2003; 82:1251. [3] K. Potzger, et al., Appl. Phys. Lett. 2006; 88 :052508. [4] Shengqiang Zhou, et al., J. Appl. Phys. 2008; 103:023902. [5] Shengqiang Zhou, et al., Phys. Rev. B 2008 ; 77:035209. [6] Qingyu Xu, et al., Phys. Rev. B 2006; 73:205342.

24th European Photovoltaic Solar Energy Conference, 21-25 September 2009, Hamburg, Germany; 2100-2102, The authors compare electrical properties of defects contained in standard p-type multi-crystalline silicon (mc- Si) and upgraded metallurgical silicon (umg-Si) material and in solar cells processed from these wafers by a commercial production line. For that thermal admittance spectroscopy allowing the determination of a defects capture cross-section and thermal activation energy, both being fingerprints of defects, was used. Further, the density of defect levels can be estimated. The authors found, that umg-Si contains more defect levels compared to mc-Si, however, the processed solar cells have similar defect composition. For mc-Si wafers and mc-Si solar cells the authors studied the impact of the dislocation density on the existence and density of electrically active defect states.

Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e. ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.

CuCr0.95Mg0.05O2 has been reported to be p-type oxide semiconductor with the highest conductivity determined by the Seebeck effect [1]. We prepared conductive, polycrystalline and amorphous CuCr0.95Mg0.05O2 and CuCr0.93Mg0.05Mn0.02O2 films on a-plane sapphire substrates by pulsed laser deposition under different O2 partial pressure and substrate temperature. Hall measurements were performed to study the majority carrier type in these films. The polycrystalline CuCr0.95Mg0.05O2 and CuCr0.93Mg0.05Mn0.02O2 films are n-type conducting up to 290 K, while in amorphous CuCr0.95Mg0.05O2 and CuCr0.93Mg0.05Mn0.02O2 films the type of majority charge carriers changes from n-type to p-type around 270 K. Interestingly, the structure has little influence on the magnetic properties of the films. The well-known antiferromagnetic to paramagnetic transition was observed in both polycrystalline and amorphous CuCr0.95Mg0.05O2 films at 25 K, while the CuCr0.93Mg0.05Mn0.02O2 films revealed no antiferromagnetic ordering below 25 K. [1] R. Nagarajan et al. J. Appl. Phys. 89, 8022 (2001)

Series of Co-doped Al-codoped ZnO films with electron concentration at 5 K ranging from 8.31017 cm-3 to 9.91019 cm-3 were prepared by pulsed laser deposition under different O2 pressure and substrate temperature. The magnetoresistance (MR) effect was studied between 5 K and 290 K with fields up to 6 T, showing large electron concentration and temperature dependence. A large positive MR of 124 % has been observed in the film with the lowest electron concentration of 8.31017 cm-3, while only negative MR of –1.9 % was observed in the film with an electron concentration of 9.91019 cm-3 at 5 K. The positive MR is attributed to the quantum correction on the conductivity due to the s-d exchange interaction induced spin-splitting of the conduction band [1]. The negative MR is attributed to the magnetic field suppressed weak localization [1]. The modelled superimposed positive and negative MR agrees well with the experimentally observed MR and hints towards the physical origin of MR in Co-doped ZnO [2]. [1] P. A. Lee and T. V. Ramakrishnan, Rev. Mod. Phys. 57, 287 (1985) [2] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, D. Spemann, and M. Grundmann, Phys. Rev. B 76, 134417 (2007)

Pulsed laser deposited ZnO films were imlpanted with vanadium ions using ion energies between 30 and 250 keV with different fluences yielding vanadium concentrations in the range between 0.8 and 5 at. %. After annealing under oxygen ambient at 800°C, a broad luminescence band observed by photoluminescence covers nearly the total visible spectral region. This luminescence is a superposition of different bands triggered by the incoporated V and remaining implantation defects. The visual impression of the bright whitish emission of the implanted ZnO has been quantified using the color space map of the Commission internationale de l'Eclairage. Furthermore, the intensity of the white emission strongly increases with increasing V concentration, whereas Ar-implanted reference sample shows only weak white emission.

The main motivation for mixing a small amount of Ge into Si for example by the self-organized growth of a stack of Ge dots into Si [1], is the controlled modification of the electronic band structure in strained Si namely charge carrier mobility and optical transition probabilities [2]. We report on charge accumulation in fivefold stacks of Ge quantum dots embedded in the n-region of a p+n-Si diode. By means of thermal admittance spectroscopy, capacitance voltage and deep level transient spectroscopy measurements [3] electron confinement in the type II Ge/Si-heterostructures, barrier effects of the quantum wells and wetting layers and defect states in the n-region of the sample associated with the surrounding Si-matrix have been probed. By relating these results to the self consistently modeled electronic band-structure and capacitance voltage characteristic charge accumulation in quantum confined electron states in the investigated type II Ge/Si-heterostructures is clearly revealed. [1] A. Malachias, T. H. Metzger, M. Stoffel, O.G. Schmidt and V. Holy, Thin Solid Films 515, 5587 (2007) [2] O.G. Schmidt and K. Eberl, Phys. Rev. B 61, 20 (2000) [3] M. Gonschorek, H. Schmidt, J. Bauer, G. Benndorf, G. Wagner, G.E. Cirlin and M. Grundmann, Phys. Rev. B 74, 115312 (2006)

Nd and Mn were codoped into ZnO films which have been grown on a-plane sapphire substrates by pulsed laser deposition with the thickness ranging between 46 and 971 nm. The room temperature resistivity of the codoped films is independent of the film thickness. Large positive magnetoresistance and clear anomalous Hall effect were observed at 5 K. Ferromagnetism with clear hysteresis up to 290 K was observed. Codoping is suggested to be an efficient method to introduce energy levels in the ZnO band gap to mediate electron spins of the magnetic doping ions.

The magnetoresistance (MR) effect in Co-doped ZnO films prepared by pulsed laser deposition on a-plane sapphire substrates with electron concentration at 5 K ranging from 8.3×10^(17) cm^(−3) to 9.9×10^(19) cm^(−3) has been studied experimentally and theoretically. A large positive MR of 124% has been observed in the film with the lowest electron concentration of 8.3×10^(17) cm^(−3), while only a negative MR of −1.9% was observed in the film with an electron concentration of 9.9×10^(19) cm^(−3) at 5 K. The positive MR is attributed to the quantum correction on the conductivity due to the s-d exchange interaction induced spin splitting of the conduction band. The negative MR is attributed to the magnetic field suppressed weak localization. The presented modeling of superimposed positive and negative MR well agrees with the experimentally observed MR and hints at the physical origin of MR in Co-doped ZnO.

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High quality ZnO single crystals of dimensions 10 x 10 x 0.5 mm3, grown by a hydrothermal approach, have been implanted by 40 keV N+ ions to a fluence of 1 x 1015 cm-2 at room temperature. Their proper-ties revealed by positron annihilation and Hall effect measurements are given in the as-grown and as-irradiated states, and after post-implantation annealing in an oxygen ambient at 200 °C and 500 °C.

High quality single crystals of ZnO in the as-grown and N+ ion-implanted states have been investigated using a combination of three experimental techniques - namely, positron lifetime/slow positron implantation spectroscopy accompanied by theoretical calculations of the positron lifetime for selected defects, temperature-dependent Hall (TDH) measurements and deep level transient spectroscopy (DLTS). The positron lifetime in bulk ZnO is measured to be (151 + 2) ps and that for positrons trapped in defects (257 + 2) ps. On the basis of theoretical calculations the latter is attributed to Zn+O divacancies, existing in the sample in both neutral and single negative charge states, and not to the Zn vacancy proposed in previous experimental work. From TDH measurements the concentrations of negatively-charged and neutral Zn+O divacancies are estimated to lie between 4 × 1015 and 6.3 × 1016 cm-3. DLTS revealed the creation of the defect E1 and an increase in concentration of the defect E3 after N+ ion implantation. Furthermore, a p-conducting layer is formed at the surface after the implantation/annealing procedure that can be used to inject holes during a DLTS measurement. In this way, the acceptor traps A2 and A3 with thermal activation energies of about 150 and 280 meV, respectively, have been detected for the first time by DLTS.