Andreas Thiessen

Nanoscale and macroscopic electrical ac transport along conductive domain walls in lithium niobate single crystals

The electrical impedance properties of UV-illuminated (λ = 310 nm) charged, conductive domain walls (CDWs) in 5 mol% magnesium-doped lithium niobate (LNO) single crystals are investigated on the nm-length scale using nanoimpedance microscopy (NIM) as well as by comparing the macroscopically measured complex impedance response between multi- and single-domain LNO samples. Similar to the case of dc conductivity, a higher conductivity of domain walls (DWs) compared to the bulk insulating matrix was observed. The contrast between DWs and bulk is most pronounced at lower frequencies (f 200 Hz) due to the large bulk capacitance at higher frequencies. Moreover, the simultaneous application of both an ac and dc bias results in an increased real part of the ac DW current. Also, equivalent circuits accurately describing both the domain and CDW contributions were developed; as a result we are able to analyze and quantify the complex dielectric conductive behavior of both bulk and CDWs in LNO within the framework of the mixed conduction model. Hopping of excited charge carriers along the CDWs was identified as the dominant charge transport process.

Multiphoton photoluminescence contrast in switched Mg:LiNbO3 and Mg:LiTaO3 single crystals

We observed a multiphoton luminescence contrast between virgin and single-switched domains in Mg-doped LiNbO3 (LNO) and LiTaO3 (LTO) single crystals with different doping levels of 0–7 mol. % and 0–8 mol. %, respectively. A luminescence contrast in the range of 3% was measured between as-grown and electrically inverted domain areas in Mg:LNO samples, while the contrast reaches values of up to 30% for the Mg:LTO case. Under annealing, an exponential decay of the domain contrast was observed. The activation energy of about 1 eV being determined for the decay allowed a comparison with reported activation energies of associated defects, clearly illustrating a strong connection between thermal contrast decay and the H+ and Li+-ion mobility. Finally, performing similar experiments on oxidized samples undoubtedly demonstrated that the origin of the reported luminescence contrast is strongly connected with lithium ions.

Multiphoton-induced luminescence contrast between antiparallel ferroelectric domains in Mg-doped LiNbO3

We report on differentiating antiparallel ferroelectric domains in congruent Mg-doped LiNbO3 (Mg:LNO) single crystals through a multiphoton photoluminescence technique. Sample illumination with femtosecond laser pulses at λ = 790 nm results in a broad multiphoton emission spectrum revealing a domain contrast of >3% between virgin and inverted domains. The contrast decreases via annealing and shows an exponential decay in the temperature range from 80 to 150 °C. Our findings give clear ground of a thermally induced structural change by surpassing a specific activation energy. Hence, the reported contrast dynamics must be closely connected to the thermal activation of charged defects, which dramatically alters the internal bias field of these defects. This explanation is also supported when using single crystal LNO of different Mg doping levels showing much lower multiphoton effects for a < 5% Mg concentration. Based on this effect of multiphoton luminescence, it becomes easy to microscopically monitor and ...

Enhancing the Domain Wall Conductivity in Lithium Niobate Single Crystals

Domain walls (DWs) in ferroelectric/ferroic materials have been a central research focus for the last 50 years; DWs bear a multitude of extraordinary physical parameters within a unit-cell-sized lateral confinement. Especially, one outstanding feature has recently attracted a lot of attention for room-temperature applications, which is the potential to use DWs as two-dimensional (2D) conducting channels that completely penetrate bulk compounds. Domain wall currents in lithium niobate (LNO) so far lie in the lower pA regime. In this work, we report on an easy-to-use and reliable protocol that allows enhancing domain wall conductivity (DWC) in single-crystalline LNO (sc-LNO) by 3 to 4 orders of magnitude. sc-LNO thus has become one of the most prospective candidates to engineer DWC applications, notably for domain wall transport both with and without photoexcitation. DWs were investigated here for several days to weeks, both before and after DWC enhancement. 2D local-scale inspections were carried out using adequate local-probe techniques, i.e., piezoresponse force microscopy and conductive atomic force microscopy, while Cerenkov second-harmonic generation was applied for mapping the DW constitution in three-dimensional space across the full LNO single crystal. The comparison between these nano- and microscale inspections allows us to unambiguously correlate the DW inclination angle α close to the sample surface to the measured domain wall current distribution. Moreover, ohmic or diode-like electronic transport characteristics along such DWs can be readily interpreted when analyzing the DW inclination profile.

Quantifying the electrical transport characteristics of electron-doped La0.7Ce0.3MnO3 thin films through hopping energies, Mn valence, and carrier localization length

Presently, cerium-doped LaMnO3 is vividly discussed as an electron-doped counterpart prototype to the wellestablished hole-doped mixed-valence manganites. Here, La0.7Ce0.3MnO3 thin films of different thicknesses, degrees of CeO2 phase segregation, and oxygen deficiency, grown on SrTiO3 single crystal substrates, are compared with respect to their resistancevs.-temperature (R vs. T) behavior from 300 K down to 90 K. While the variation of the film thickness (and thus the degree of epitaxial strain) in the range between 10 nm and 100 nm has only a weak impact on the electrical transport, the degree of oxygen deficiency as well as the existence of CeO2 clusters can completely change the type of hopping mechanism. This is shown by fitting the respective R-T curves with three different transport models (adiabatic polaron hopping, Mott variable-range hopping, Efros-Shklovskii variable-range hopping), which are commonly used for the mixed-valence manganites. Several characteristic transport parameters, such as the hopping energies, the carrier localization lengths, as well as the Mn valences are derived from the fitting procedures. PACS numbers: 71.30.h, 75.47.Lx, 73.61.Ng

The Mn2+/Mn3+ state of La0.7Ce0.3MnO3 by oxygen reduction and photodoping

Films of cerium-doped LaMnO3, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La0.7Ce0.3MnO3 (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO2 segregation. Charge-injection from the photoconductive SrTiO3 substrate into the Mn eg band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.

Conductivity and magnetoresistance of La0.7Ce0.3MnO3−δ thin films under photoexcitation

La0.7Ce0.3MnO3 thin films of different thicknesses, degrees of CeO2-phase segregation and oxygen deficiency, grown on SrTiO3 single crystal substrates, were comparatively investigated with respect to both their spectral and temperature-dependent photoconductivity (PC) and their magnetoresistance (MR) behaviour under photoexcitation. While as-grown films were insensitive to optical excitation, oxygen reduction appeared to be an effective way to decrease the film resistance, but the film thickness was found to play a minor role. However, from the evaluation of the spectral behaviour of the PC and the comparison of the MR of the LCeMO/substrate-samples with a bare substrate under illumination we find that the photoconductivity data reflects not only contributions from (i) photogenerated charge carriers in the film and (ii) carriers injected from the photoconductive substrate (as concluded from earlier works), but also (iii) a decisive parallel photoconduction in the SrTiO3 substrate. Furthermore--also by analyzing the MR characteristics--the unexpected occurence of a strong electroresistive effect in the sample with the highest degree of CeO2 segregation and oxygen deficiency could be attributed to the electroresistance of the SrTiO3 substrate as well. The results suggest a critical reconsideration and possibly a reinterpretation of several previous photoconductivity and electroresistance investigations of manganite thin films on SrTiO3.

Mn

Films of cerium-doped LaMnO3, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La0.7Ce0.3MnO3 (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO2 segregation. Charge-injection from the photoconductive SrTiO3 substrate into the Mn eg band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron-hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.

^{2+}

Surface photovoltage (SPV) spectroscopy, which is a versatile method to analyze the energetic distribution of electronic defect states at surfaces and interfaces of wide-bandgap semiconductor (hetero-)structures, is applied to comparatively investigate heterostructures made of 5-unit-cell-thick LaAlO3 films grown either on TiO2- or on SrO-terminated SrTiO3. As shown in a number of experimental and theoretical investigations in the past, these two interfaces exhibit dramatically different properties with the first being conducting and the second insulating. Our present SPV investigation reveals clearly distinguishable interface defect state distributions for both configurations when interpreted within the framework of a classical semiconductor band scheme. Furthermore, bare SrTiO3 crystals with TiO2 or mixed SrO/TiO2 terminations show similar SPV spectra and transients as do LaAlO3-covered samples with the respective termination of the SrTiO3 substrate. This is in accordance with a number of recent works t...