Single molecular precursor metal-organic chemical vapor deposition of MgAl 2 O 4 thin films
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Single molecular precursor metal-organic chemical vapor deposition of MgAl2O4 thin films Jiming Zhang, Gregory T. Stauf, Robin Gardiner, Peter Van Buskirk, and John Steinbeck Advanced Technology Materials, Inc., Danbury, Connecticut 06810 (Received 5 November 1993; accepted 18 February 1994)
MgAl 2 O 4 films have been grown epitaxially on both Si(100) and MgO(lOO) by a novel single source metal-organic chemical vapor deposition (MOCVD) process. A single molecular source reagent [magnesium dialuminum isopropoxide, MgAl2(OC3H7)8] having the desired Mg: Al ratio was dissolved in a liquid solution and flash-vaporized into the reactor. Both thermal and plasma-enhanced MOCVD were used to grow epitaxial MgAl 2 O 4 thin films. The Mg: Al ratio in the deposited films was the same as that of the starting compound (Mg: Al = 1:2) over a wide range of deposition conditions. The deposition temperature required for the formation of crystalline spinel was found to be significantly reduced and crystallinity was much improved on Si by using a remote plasma-enhanced MOCVD process. The epitaxial nature of the MgAI 2 O 4 films was established by x-ray pole figure analysis.
Magnesium aluminum spinel (MgAl 2 O 4 ) is a promising electronic material with electrical and optical properties that have great technological importance.1"4 The combination of low dielectric constant (e = 7.5), low loss tangent (tan 5 = 4 X 10~4), and the close match of the oxygen ion lattice structure of MgAl 2 O 4 with Si and a number of other important oxide systems makes MgAl 2 O 4 particularly attractive for integrated electronic devices.5"7 The lattice parameters of spinel and several important oxides are listed in Table I. Both single crystal Si6 and GaAs 7 have been epitaxially grown on (100) MgAl 2 O 4 substrates. The epitaxial growth of perovskitetype oxides (PbTiO3, BaTiO3, and SrTiO3) on MgAl 2 O 4 coated Si has also been recently demonstrated.8'9
MgAl 2 O 4 thin films have been grown by a halogen CVD technique reacting metal chlorides with CO 2 and H2 at elevated temperatures to give MgAl 2 O 4 , with CO and HC1 by-products.8 The main shortcoming of this halogen CVD technique is the high growth temperature (980 °C) needed to provide the activation energy to drive the reaction. The corrosive and toxic by-products are also strong deterrents for scaling-up this process. Metal-organic chemical vapor deposition (MOCVD) has been shown to be a promising technique for the fabrication of device quality compound semiconductors, highTc superconductors, and ferroelectric thin films. Key issues for any CVD process are the selection of suitable precursors and a method of repro-
TABLE I. Lattice parameters of MgA^C^ and several important oxides. Material
Crystal structure
MgAl 2 O 4 BaTiO 3
Cubic Tetragonal
PbTiO 3
Tetragonal
PLZT SrTiO 3 YBa 2 Cu 3 O 7
Cubic Cubic Orthorhombic
MgO
Cubic
Lattice constant (A)
Oxygen distance (A)
Lattice mismatch (%)
8.08 3.99 4.03 3.90 4.15 4.09 3.91 a: 3.84 b: 3.88 4.21
2.86 2.79 2.86 2.76 2.93 2.89 2.76 2.72 2.74 2
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