Deposition of Potassium Niobate Thin Films by Metalorganic Chemical Vapor Deposition and their Nonlinear Optical Propert
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183 Mat. Res. Soc. Symp. Proc. Vol. 392 0 1995 Materials Research Society
Table I. Oxide substrates used in the deposition of KNbO 3.
*
Lattice Parameter
% Mismatch % Mismatch % Mismatch a* = 3.973 A b* = 4.027 A c* = 4.045 'A
Substrate
Crystal Structure
LaAlO 3
Perovskite
3.793
A
-4.75
-6.17
-6.64
MgAI 2 0 4
Spinel
8.083
A
1.69
0.36
-0.09
5.70
4.41
3.99
MgO Rock Salt 4.213 A Lattice constants of orthorhombic KNbO 3
EXPERIMENTAL Potassium niobate thin films were prepared by low pressure metalorganic chemical vapor deposition in a two-zone horizontal quartz reactor.' 3 The metalorganic precursors employed were potassium tert-butoxide ([KOC(CH,)3] 4 ) and niobium pentaethoxide ([Nb(OCH 2 CH3 )5]). Ultra high purity argon was used as the carrier gas, and ultra high purity oxygen served as the reactant gas. Layers were deposited on single crystal lanthanum aluminate (LaA10 3), spinel (MgAl20 4), and magnesium oxide (MgO). Substrates were degreased prior to deposition by boiling successively in dichloroethane, acetone, and methanol. The MgO substrates received an in situ oxygen anneal above 900'C for 10 minutes prior to deposition to improve the surface quality. The reactor and susceptor were etched in dilute hydrofluoric acid between every growth to prevent the redeposition of potassium. Film thicknesses were typically between 100 nm and 1100 nm with deposition rates varying from 60 - 240 nm/hr. Deposition temperatures between 700'C - 850°C and pressures of 4 - 5 Torr were used. RESULTS AND DISCUSSION Microstructure and composition studies Potassium niobate thin films were deposited on LaA10 3, MgA120 4, and MgO substrates. The selection of these oxide substrates provides a variety of film/substrate mismatch conditions over which to examine the development of the thin film microstructure. Table I lists the lattice parameter mismatch between KNbO 3 and the substrates. Epitaxial KNbO 3 films deposited on perovskite (001)cubic LaA10 3 substrates experiences at least a 4.5% compressive strain. Films deposited on MgA120 4 are nearly lattice matched with the substrate with a mismatch of less than 1%. Coherent KNbO 3 films deposited on single crystal MgO substrates have a tensile lattice mismatch of at least 4%. Figure 1 shows the theta-two theta x-ray diffraction patterns for layers deposited on the three types of substrates. Figure 1(a) shows the pattern for a 450 nm thick epitaxial KNbO 3 thin film deposited on a (100) MgAI204 substrate. For clarity, the pseudo-cubic indices for potassium niobate will be used in place of the orthorhombic indices, where [101]cubic = [100]ono, [TO1]cubic = [010]oo, and [0 10]cubic = [ 0 0 1]ortho. The pattern shows only the (001)cubic reflections. In contrast figure I(b) shows the theta-two theta x-ray diffraction pattern for an 1100 nm thick epitaxial KNbO 3 thin film deposited on a (100) MgA120 4 substrate. Both the (001)cubic and (j00)cubc KNbO 3 reflection are present. The thicker films appear to have relaxed by formation of both (001)cubic and (i 0 0 )ocuborien
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