Electron emission from ferroelectrics (FEE) is a promising source for electrons. Although extensive studies have shown that the emission is inititated by a variation of the spontaneous polarization, the exact underlying emission process remained unclear to date. The focus of this work is to analyze and improve the electron emission process from ferroelectric materials. To achieve low operation voltages thin films and low coercive voltage ferroelectric relaxor single crystals were used. The emission was measured under ultrahigh vacuum (UHV) conditions with a single electron detector. The ferroelectric thin films were prepared with a structured top electrode, with nanometer-sized regularly arranged apertures. The emission from lead zirconate titanate (PZT) thin films was achieved at excitation voltages as low as 10 V. The voltage dependent polarization state within the emission apertures of PZT was imaged using piezoresponse force microscopy (PFM). The PFM measurements revealed that an increased fraction of the free surface area is switched by an increased applied voltage. Additional, as a thin film electron emitter, bismuth ferrite (BFO) films were investigated. Ferroelectric relaxor lead magnesium niobate - lead titanate (PMN-PT) was used as single crystal electron emitter due to its low coercive field. The time-dependent electron emission process from PMN-PT was clarified with the help of exciting voltage pulses of variable duration. It is demonstrated that FEE from PMN-PT can be described in the framework of a random field model for relaxors, with the measured electron flux correlating with the amount of reversed polarization. The time-resolved analysis gives insight into the polarization switching and screening processes within PMN-PT. The local electron emission from PMN-PT single crystals has been investigated, in the nanometer regime, with the help of an AFM tip serving as an electron detector. Additionally, the influence of the aperture size in the top electrode on the emission has been investigated. It is found that the electron emission is strongly influenced by the electric field distribution in the aperture. An optimum aperture width for electron emission from PMN-PT, which is much smaller than the apertures used so far, was found. Comparative investigations of the electron emission process from relaxors with barium titanate showed that the emission from PMN-PT is much more complex than the emission from a conventional ferroelectric. General conclusions on the future applications of FEE can be drawn.

Electron emission from ferroelectric materials is initiated by a variation of the spontaneous polarization. It is the main focus of this work to develop ferroelectric cathodes, which are characterized by a significantly decreased excitation voltage required to initiate the electron emission process. Particular attention is paid to the impact of the polarization on the emission process. Two materials are investigated. Firstly, relaxor ferroelectric lead magnesium niobate - lead titanate (PMN-PT) single crystals are chosen because of their low intrinsic coercive field. Electron emission current densities up to 5 · 10^(−5) A/cm² are achieved for excitation voltages of 160 V. A strong enhancement of the emission current is revealed for the onset of a complete polarization reversal. Secondly, lead zirconate titanate (PZT) thin films are investigated. A new method to prepare top electrodes with sub-micrometer sized, regularly patterned apertures is introduced and a stable electron emission signal is measured from these structures for switching voltages < 20 V. Furthermore, a detailed analysis of the polarization switching process in the PMN-PT samples is given, revealing a spatial rotation of the polarization vector into crystallographic easy axes, as well as the nucleation of reversed nano-domains. Both processes are initiated at field strengths well below the coercive field. The dynamics of the polarization reversal are correlated to the electron emission measurements, thus making it possible to optimize the efficiency of the investigated cathodes. / Die Ursache für Elektronenemission aus ferroelektrischen Materialien ist eine Veränderung des Zustandes der spontanen Polarisation. Gegenstand der vorliegenden Arbeit ist eine Verringerung der dafür nötigen Anregungsspannung, wobei besonderes Augenmerk auf die Rolle der ferroelektrischen Polarisation innerhalb des Emissionsprozesses gelegt wird. Es werden zwei verschiedene Materialien untersucht. Das Relaxor-Ferroelektrikum Bleimagnesiumniobat - Bleititanat (PMN-PT) wurde aufgrund seines geringen Koerzitivfeldes ausgewählt. Es konnten Emissionsstromdichten von bis zu 5·10^(−5) A/cm² bei einer Anregungsspannung von 160 V erreicht werden. Bei Einsetzen eines vollständigen Umschaltens der Polarisation wurde eine deutliche Verstärkung des Emissionsstromes festgestellt. Desweiteren werden Untersuchungen an Bleizirkoniumtitanat (PZT) Dünnfilmen gezeigt. Eine neue Methode, eine Elektrode mit periodisch angeordneten Aperturen im Submikrometerbereich zu präparieren, wird vorgestellt. Diese Strukturen liefern ein stabiles Emissionssignal für Anregungsspannungen < 20 V. Eine detailierte Analyse des Schaltverhaltens der Polarisation der PMN-PT Proben zeigt sowohl eine Rotation des Polarisationsvektors als auch eine Nukleation umgeschaltener Nanodomänen. Beide Prozesse starten bei Feldstärken unterhalb des Koerzitivfeldes. Die ermittelte Zeitabhängigkeit des Schaltprozesses erlaubt Rückschlüsse auf den Emissionsprozess und erlaubt es, die Effizienz der untersuchten Kathoden weiter zu optimieren.