Lars Hartmann

University of Leipzig , Leipzig, Saxony, Germany

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Publications (14)38.66 Total impact

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    ABSTRACT: The breakdown characteristics of flexible Cu(In,Ga)Se2 solar cells were analyzed with respect to the influence of sodium. Higher sodium contents lead to lower breakdown voltages, which were deduced from reverse bias JV-measurements. For a locally resolved analysis of this effect, voltage dependent dark lock-in thermography images were recorded. It can be shown that enhanced sodium concentration does not increase the number of local shunts but their voltage dependence.
    27th European Photovoltaic Solar Energy Conference and Exhibition; 09/2012
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    ABSTRACT: Laser patterning of thin-film solar cells is essential to perform external serial and integrated monolithic interconnections for module application and has recently received increasing attention. Current investigations show, however, that the efficiency of thin-film Cu(In,Ga)Se2 (CIGS) modules is reduced due to laser scribing also with ultrashort laser pulses. Hence, to investigate the reasons of the laser-induced material modifications, thin-film CIGS solar cells were laser-scribed with femto- and picosecond laser pulses using different scribing procedures and laser processing parameters. Besides standard electrical current voltage (I–V) measurements, additional electrical and optical analysis were performed such as laser beam-induced current (LBIC), dark lock-in thermography (DLIT), and electroluminescence (EL) measurements to characterize and localize electrical losses due to material removal/modifications at the scribes that effecting the electrical solar cell properties. Both localized as well as distributed shunts were found at laser scribe edges whereas the laser spot intensity distribution affecting the shunt formation. Already laser irradiation below the ablation threshold of the TCO film causes material modification inside the thin film solar cell stack resulting in shunt formation as a result of materials melting near the TCO/CIGS interface that probably induces the damage of the pn-junction.
    Optics & Laser Technology 09/2012; 44(6):1753–1757. · 1.37 Impact Factor
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    ABSTRACT: The human (h) P2X(3) receptor and its mutants deficient in one out of four N-glycosylation sites were expressed in HEK293 cells. Concentration-response curves were generated by whole-cell recordings of alpha,beta-methylene ATP (alpha,beta-meATP)-induced currents. A gradual change of external pH from the alkaline 8.0 to the acidic 5.0 successively decreased the maximum current amplitude (E(max)) without affecting the EC(50) value. The replacement of Asn-139 and -170 by Asp (N139D, N170D) abolished the pH sensitivity of the wild-type (WT) hP2X(3) receptor. In the case of N194D, the E(max) was again the highest at the alkaline pH value with no change from 7.4 to 6.5, whereas in the case of N290D, there was an inverse pH sensitivity, with an increase of E(max) in the acidic range. However, this effect appeared to be due to enhanced protonation by the insertion of Asp into the receptor, because replacement of Asn by the neutral Thr resulted in a comparable potency of alpha,beta-meATP at any of the pH values investigated. In accordance with the reported finding that His-206 is involved in the modulation of WT P2X(3) receptors by protons, we showed that the normal change of E(max) by an acidic, but not alkaline pH was abolished after substitution of this His by Ala. However, the double mutant H206A + N290D did not react to acidification or alkalinization with any change in E(max). In conclusion, only fully N-glycosylated P2X(3) receptors recognize external pH with a modified sensitivity towards alpha,beta-meATP.
    Journal of Neurochemistry 10/2008; 107(5):1216-24. · 3.97 Impact Factor
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    ABSTRACT: We report the clearly observed tunneling magnetoresistance at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom ferromagnetic electrode and Co as a top ferromagnetic electrode prepared by pulsed laser deposition. Spin-polarized electrons were injected from Co-doped ZnO to the crystallized Al2O3 and tunnelled through the amorphous Al2O3 barrier. Our studies demonstrate the spin polarization in Co-doped ZnO and its possible application in future ZnO-based spintronics devices.
    Physical Review Letters 09/2008; 101(7):076601. · 7.73 Impact Factor
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    ABSTRACT: Zn90Mn7.5Cu2.5O100 films have been prepared on a-plane sapphire substrates by pulsed laser deposition. Paramagnetism mainly caused by Mn2+ ions was observed in the films from room temperature down to 2 K. Magnetotransport properties (magnetoresistance (MR) and Hall effect) were studied from 5 K to 290 K up to a field of 6 T. Negative MR was observed at temperature above 100 K. Low field positive MR and high field negative MR was observed at 5 K. Clear anomalous Hall effect with a kink at low field was observed below 20 K, indicating that there exist two different scattering mechanisms for the Mn2+ and Cu2+ ions.
    Thin Solid Films 01/2008; 516(6):1160–1163. · 1.87 Impact Factor
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    ABSTRACT: Mn-doped ZnO films with preferred c-axis growth orientation were prepared by pulsed laser deposition under N2 atmosphere on a-plane sapphire substrates. Large positive magnetoresistance amounting to 60% was observed at 5 K. Clear anomalous Hall effect was observed at 20 K. Ferromagnetism with Curie temperature higher than 290 K has been observed, and a deep acceptor trap due to Zn vacancies with a thermal activation energy amounting to 0.815 eV has been detected by deep-level transient spectroscopy. For comparison, only paramagnetism was observed in Mn-doped ZnO films with donor traps prepared under O2 atmosphere. Their results clearly demonstrate that the ferromagnetism in Mn-doped ZnO originates from the parallel alignment of magnetic moments mediated by acceptor defects.
    Applied Physics Letters 08/2007; 91(9):092503-092503-3. · 3.79 Impact Factor
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    ABSTRACT: The Co valence state and the location of Co dopant atoms in ZnO:Co thin films revealing anomalous Hall effect below 50 K have been determined by electron energy-loss spectroscopy (EELS) measurements and atom location by channeling enhanced microanalysis (ALCHEMI), respectively. The method of ALCHEMI-EELS to investigate the dopant in materials was brought. It was used to determine that the intrinsic ferromagnetism in ZnO:Co thin films derives from the +2 Co which substitute for Zn atoms at Zn sites in the ZnO matrix. The divalent Co state in ZnO:Co has been unambiguously proven by modeling the corresponding EELS data.
    Applied Physics Letters 04/2007; 90(15):154101-154101-3. · 3.79 Impact Factor
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    ABSTRACT: Magnetotransport measurements were performed on n -type conducting Co-doped ZnO and Mn-doped ZnO films prepared by pulsed laser deposition on a -plane sapphire substrates, and positive magnetoresistance (MR) was observed at low temperature. The positive MR decreases drastically with the free electron concentration n exceeding 10<sup>19</sup> cm <sup>-3</sup> and reveals almost the same dependency on n for Co-doped ZnO and Mn-doped ZnO. This hints towards a similar s-d exchange constant in both types of magnetic ZnO films. For Co-doped ZnO, the saturated anomalous Hall resistivity increases with decreasing electron concentration. No anomalous Hall effect was observed in Mn-doped ZnO. Within a free electron approximation the positive MR may be related with the spin polarization of conducting electrons due to s-d exchange interactions. The modeled spin splitting of the conduction band is smaller than 10 meV .
    Journal of Applied Physics 04/2007; · 2.21 Impact Factor
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    ABSTRACT: The magnetoresistance (MR) effect in Co-doped ZnO films prepared by pulsed laser deposition on a -plane sapphire substrates with electron concentration at 5K ranging from 8.3×1017cm-3to9.9×1019cm-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 8.3×1017cm-3 , while only a negative MR of -1.9% was observed in the film with an electron concentration of 9.9×1019cm-3 at 5K . 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.
    Physical Review B 01/2007; 76(13). · 3.66 Impact Factor
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    ABSTRACT: Co-doped ZnO films with Co content above 7% have been grown by pulsed laser deposition on a-sapphire substrates. Temperature and magnetic field dependent magnetotransport measurements have been performed on samples with electron concentration n around the critical electron concentration nc~4.9×1019 cm-3, where the metal-insulator transition (MIT) occurs. At 5 K we observed positive magnetoresistance (MR) in the insulating range (nnc). The MIT was determined from the MR and Hall effect at 5 K to occur at the electron concentration nc~4×1019 cm-3. In the vicinity of the MIT (n~nc) we observed negative MR at low field and positive MR at high field. Only for n
    AIP Conference Proceedings 01/2007; 893:1187-1188.
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    ABSTRACT: ZnCoO : Al, ZnCoO : (Al,Cu), and reference samples without Co have been grown by pulsed laser deposition to investigate the influence of Co-dopants and Cu-codopants on the magneto transport properties of ZnO. Positive magnetoresistance and anomalous Hall effect have been observed for ZnCoO : (Al,Cu). Versatile theoretical approaches for modelling positive magnetoresistance in ZnO-based diluted magnetic semiconductors are still to be developed. Negative magnetoresistance due to the scattering of spin-polarized charge carriers at isolated magnetic impurities has been observed in ZnO : (Al,Cu) without Co-dopants and successfully modelled. Cu-codopants enhance the positive and negative magnetoresistance in ZnCoO : Al and ZnO : Al, respectively, by one order of magnitude.
    Journal of Physics D Applied Physics 11/2006; 39(23):4920. · 2.53 Impact Factor
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    ABSTRACT: Zn <sub>0.90</sub> Co <sub>0.10</sub> O films of different thicknesses (689, 408, 355 nm ) doped with 0.5 at. % Al were prepared by pulsed laser deposition on a -plane sapphire substrates. At 290 K the resistivity increases drastically with decreasing film thickness, while the electron concentration and mobility decrease. Magnetoresistance (MR) effects were measured in the temperature range of 5–290 K . At low temperature, the positive MR increases with decreasing film thickness. Positive MR decreases rapidly with increasing temperature. With increasing temperature, the MR of the thicker film changes to negative, while positive MR was still observed for the 355 nm thick film at 290 K . Anomalous Hall effect was observed in the 355 nm thick film at 20 K , indicating the possible ferromagnetism in Zn <sub>0.90</sub> Co <sub>0.10</sub> O .
    Journal of Applied Physics 08/2006; · 2.21 Impact Factor
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    ABSTRACT: ZnO films with different 3d transition metal doping (Ti, Co, Mn) were prepared by pulsed laser deposition. The occurrence of positive magnetoresistance (MR) was studied in dependence on temperature. Large positive MR was observed in Co- and Mn-doped ZnO films at 5 K. Zn0.98Mn0.02O films show a positive MR up to ∼ 31% at 5 K. The positive MR for Co- and Mn-doped ZnO films decreases drastically from 5 K to 50 K. Positive MR was also observed in Zn0.9998Ti0.0002O films at 290 K, while negative MR was observed at 5 K. Clear stripe-like magnetic domain pattern was observed by magnetic force microscopy for the Zn0.9998Ti0.0002O films, indicating the possible ferromagnetism at room temperature due to Ti3+-ions in a polarized spin state.
    Thin Solid Films 01/2006; 515(4):2549-2554. · 1.87 Impact Factor
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    ABSTRACT: The magnetotransport properties magnetoresistance MR and Hall effect of Co-doped ZnO films prepared by pulsed laser deposition have been investigated around the metal-insulator transition MIT as a function of temperature from 5 to 290 K under a maximum magnetic field strength of 6 T. From the MR behavior measured at 5 K we conclude that the MIT occurs at the critical electron concentration n c 4 10 19 cm −3 . At 5 K we observed positive MR in the insulating regime n n c and negative MR in the metallic regime n n c . Furthermore, in the transition regime of the MIT n n c negative MR at low magnetic field and positive MR at high field was observed. We consider the critical electron concentration n c as an important material parameter because n c does not depend on film thickness or Co content. The anomalous Hall effect being of importance for future spintronic materials was only clearly observed in Co-doped ZnO with n n c . Diluted magnetic semiconductors DMSs have attracted much attention for their potential areas of applications in spintronics. 1 Following the theoretical prediction of room-temperature ferromagnetism in ZnO-based DMSs, 2 3d transition-metal-TM-doped ZnO has been studied inten-sively to achieve room-temperature ferromagnetism. Besides ferromagnetic magnetization, the ZnO-based DMSs also ex-hibit interesting magnetotransport phenomena. Both positive and negative magnetoresistance MR were observed, de-pending on TM doping, film thickness, and measuring temperature. 3,4 The anomalous Hall effect AHE was also observed in Co-and Mn-doped ZnO films. 4,5 A spin coher-ence time as long as 1 ns has been found by measuring low-temperature MR on undoped and Mn-doped ZnO and a cou-pling constant amounting to so = 4.4± 0.4 10 −11 eV cm. 6 However, more work is still needed to understand the mag-netotransport properties in ZnO-based DMSs at elevated de-vice operating temperatures. It is known that magnetotransport properties of magnetic semiconductors strongly depend on the charge carrier concentration, 7,8 which are electrons in intrinsically n-conducting ZnO. With decreasing electron concentration, at the metal-insulator transition MIT, the character of wave functions changes from delocalized to localized. In this pa-per, we study the MR and Hall effect of Co-doped ZnO films in dependence on the free-electron concentration n. For a critical electron concentration n c we probed the MIT and related changes of MR in Co-doped ZnO. The Co-doped ZnO films were grown from a Zn 0.945 Co 0.05 Al 0.005 O pulsed laser deposition PLD target on 10 10 mm 2 a-plane sapphire substrates by pulsed laser deposition using a KrF excimer laser. The distance between PLD target and substrate amounted to 10 cm. The film thick-ness was controlled by the number of the laser pulses with an energy density of 2 J cm −2 and ex situ determined by mod-eling spectral ellipsometry data measured in the energy range of 1 – 4 eV. 9 Different temperatures at the substrate holder and film thickness were chosen to vary the electron concen-tration by several orders of magnitude around the critical electron concentration n c . The PLD target was prepared by mixing and pressing appropriate amounts of ZnO 99.9% , CoO 99.999% , and Al 2 O 3 99.998% powders. 0.5 mol % Al was included in the PLD target to fabricate conductive thin films. The resulting composition of the films was deter-mined by combined Rutherford backscattering spectrometry and particle-induced x-ray emission measurements and is given together with substrate temperature in Table I. Due to the underlying substrate, the Al content in the Co-doped ZnO films could not be determined. When decreasing the sub-strate holder temperature from 820 to 390 ° C, the Co content in the deposited films decreases gradually from 10% to 7%, nevertheless being larger than the nominal Co content in the target. The crystal structure of the films was characterized by x-ray diffraction measurements with -2 scans using a Cu K source. Only 002 and 004 peaks of wurtzite ZnO were observed, indicating that the Co-doped ZnO films are
    Physical Review B 01/2006; 73:205342. · 3.66 Impact Factor