Laser MBE for Atomically Defined Ceramic Film Growth
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High-Tc Cuprate Superconductors (Oxygen Deticient Perovskite)
CuO, TIO, HgO
Bi202, T1202, Cu202 (A'=Ba, Sr)
BO
Perovskite : ABO3 Versatile properties Insulator - Metal - Superconductor Ferro-, Piezo, Pyro-electricity Ferromagnetism, Giant magnetic resistivity Optical Functions
1
/ 17 B02 * z11117AO
AO Cu02
(A=Ca, Rare earth) A 2] (n=l, 2) LCuO2J•
[_
___
A'O
9
BOx (11 00) (Oxygen ions are not vritten)
•
B02
Asite ion
SB site ion 0Oxygen ion
Fig. 1 Layer lattice image (atomic layer model) commonly applicable to high T(. (>77K) superconducting oxides, as compared with typical perovskite oxide. can be chosen from A= alkaline and rare earths, A'= Ba and Sr, and B= Cu, Cu 2i 2, TI, T12, Hg. There can be a lot more combinations among these elements as well as by choosing other elements. Thermodynamics restricts the formation of lattices to be formed in reality by the conventional bulk sintering process. Non-equilibrium processes to deposit thin films from gas phase may fix metastable lattice structures stacked by atomically controlled layer-by-layer growth. Lower deposition temperature should be desirable for this purpose. Since each atomic layer cannot always be charge-neutralized, more stable lattice growth is expected if we can set such conditions to deposit a few atomic layers simultaneously into a charge neutral molecular layer. RHEED pattern and intensity oscillation, 'which is indispensable for digital control of growth unit, should be diagnosed in-situ. Other reasons for molecular layer epitaxy of oxides include the fabrications of high T, Josephson tunnel junction and oxide-base all epitaxial electronic devices and the exploration of new properties in artificially designed new oxide materials [7]. Keeping these features into mind, we examined the method to achieve the molecular layer epitaxy of oxides. Pulsed excimer laser deposition (PLD) was presumed to be superior to sputtering and vacuum evaporation in the controllability of composition, cleanness of system due to the absence of vaporization device in the chamber, and excess kinetic energy in the laser ablated species to decrease the epitaxy temperature. In order to install in situ RHEED and grow films in high vacuum conditions, however, we had to abandon the big advantage of using relatively high oxygen pressures in PLD. This problem was suggested to be overcome by using such an activated oxidant as NO 2 and ozone from our thermodynamic calculation [8]. Requisites for molecular layer epitaxy of oxide thin films by laser MBE are summarized in Table 1. Based on these considerations, we designed and constructed a laser MIBE systems [9]. Table 1. Item 1 2 3 4
Elementary processes and characterization of molecular Process Oxidation in UHV Epitaxial growth Lateral growth Atomic & valence control in as grown film & interface
146
layer epitaxy of oxides by laser MBE Characterization Thermodynamics & Kinetics, XPS RHEED pattern, XRD RHEED oscillation XPS (in-situ)
HARDWARE: LASER MBE SYSTEM One of our laser MBE systems is illustrated in Fig. 2
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