1. NC STATE UNIVERSITY Direct observation and characterization of domain-patterned ferroelectrics by UV Photo-Electron Emission Microscopy Woochul Yang, Brian J. Rodriguez, Alexei Gruverman, and Robert J. Nemanich Department of Physics, and Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-8202 Summary We observed polarity contrast of domain-patterned ferroelectrics with UV-PEEM. Enhanced emission from negative domains is attributed to differences in electron affinity. The photothreshold for negative domains of LNO and PZT were found to be 4.6eV and less than 4.3eV, respectively. Future Work Compare PEEM of an as-loaded and clean surface of ferroelectrics Include screening charge (internal and external) into model XPS/UPS to measure surface Fermi level position How are these differences manifested in other interfaces (GaN growth on PPLN)Acknowledgements Duke University Free Electron Laser Laboratory ONR and AFOSR P sp Negative domain Positive domain ---- --- - - - - - + + + + +++ + + + + + E -------- E - + Polarization-induced bound charges + - Absorbates (accumulated charges) -- Electrons emitted from valence band P sp ---- --- - - - - - + + + + +++ + + + + + ---- ---- Emission model Surface dipole induced by adsorbates changes surface electron affinity. Electron affinity for negative domain is lower and thus emission is more intense. PEEM contrast of ferroelectrics Negative domain E th = E g +  s -  Positive domain E th * = E g +  s +  * E vac EcEc EFEF EvEv P sp E th  ss EgEg + + + + + + E vac EcEc EFEF EvEv P sp E th * ss EgEg  * + + + + + + _ _ _ _ _ _  eff  eff * Energy band diagram of polar domains Difference in electron affinity due to the surface dipole causes a PEEM polarity contrast between the positive and negative end domains of ferroelectrics surfaces. PZT -  s ~ 3.5eV (Jpn. J. Appl. Phys. 38, 2272 (1999)), E g ~ 3.4 eV  E th ~ 6.9eV - PEEM measurement: E th ~ < 4.3eV,  eff =  s -  = E th – E g = < 0.9 eV E th * ~ > 6.0eV,  eff * =  s +  * = E th * – E g = > 2.6 eV LNO -  s ~ 1.1eV (Sov. Phys. Solid State 25, 1990 (1983)), E g ~ 3.9 eV  E th ~ 5.0 eV - PEEM measurement: E th ~ < 4.6eV,  eff =  s -  = E th – E g = < 0.7 eV E th * ~ > 6.2eV,  eff * =  s +  * = E th * – E g = > 2.3 eV Motivation Precise control of ferroelectric domains has become important as a new approach to the self-assembly of complex nanostructures. Direct information about local polarization, charge distribution, and potential of the ferroelectric surface is necessary to control the local electronic structures and to influence the chemical reactivity. UV-FEL PEEM can allow us to image ferroelectric domain structures with high resolution (~ 10nm) and to obtain local polarization and surface electronic structures through the variation in work function on the surface. Goals PEEM observation of ferroelectrics with polarity patterned domains Understanding PEEM polarity contrast of ferroelectric materials PEEM measurement of photothresholds of ferroelectrics to understand local electronic properties Sample (-20kV) Anode (ground) Channel Plate P-screen CCD Objective lens Intermediate lens hv Projective lens Concept of PEEM Illuminate sample with UV photons just above the photoelectron threshold. Accelerate photo-electrons with an immersion lens and image the surface with conventional electron optics with high magnification. Advantages of PEEM Good surface sensitivity: depth(1-20 nm) High spatial resolution: ~10nm In situ, real-time characterization of film surfaces Non-destructive imaging method Measurement of surface work function and electronic structures of materials (UV-FEL PEEM) What is PEEM? Coherent FEL radiation Spontaneous Radiation 4.0-6.3eV UV-XUV FEL OK-4 System e-beam Injection Laser Mirror 1 GeV Duke Storage Ring Evaporator CCD AES MBE Lens Column computer & image processor e t  hv : 4 – 6.3 eV P a ~ 2mW, P p ~ 20W  : 100 psec t : 6 nsec n p : 5 x 10 15 photon/sec  E/E : ~1% UV-FEL Parameters FEL-PEEM : PEEM (~10nm resolution) + FEL (tunable, high-intensity, polarized light) UV-PEEM at Duke FEL Measure piezoelectric properties by detection of sample deformation Positive domainNegative domain P sp ---- --- - +++ + + + + + Phase contrast of ferroelectric domain polarity In phase: Brighter (negative domain) Out of phase: darker (positive domain) LNO 50  m Piezoresponse Force Microscopy positive domain negative domain 10  m PEEM PFM AFM hv = 4.8eV In PEEM, the brightness contrast displays different polar domains. PFM measurement confirms that the bright regions are negative domains. Polarity-patterned PbZrTiO 3 (PZT) thin films 150 o C 250 o C300 o C The polarity contrast disappears at near the Curie temperature of ~ 300 o C 10  m hv = 4.9eV PEEM images of PZT during annealing hv=4.3eV4.8eV5.0eV 5.2eV5.5eV Emission threshold of negative domains is less than 4.3eV - + 6.3eV PEEM images of PZT-photon energy scan 10  m Before etching After etching PEEM PFM AFM PFM The brighter (negative) domains is wider than darker (positive) domains. Chemical etching (negative domain) and PFM measurement confirm that the emission from the negative domains is more intense. + - + - + - + - Polarity patterned LiNbO 3 (LNO) crystal hv=4.5eV4.6eV4.7eV 5.2eV Emission threshold of negative domains is less than 4.6eV 5.9eV6.2eV PEEM images of LNO - photon energy scan E vac EgEg  h Semiconductor Surfaces Minimum Escape Energy h =  + E g Ferroelectric Semiconductor Surfaces: local variations in  (electron affinity), E g (band gap), band bending, doping density will change the minimum escape energy and lead to PEEM contrast. PEEM Image Contrast: Photo-threshold difference EgEg ** 11 22 Contrast between two regions can be obtained by choosing  2 < hv <  1 Photo electrons will be emitted from region 2 (bright) but not from region 1 (dark) 10  m 22 11