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Gallium oxide thin films have been deposited by atomic layer epitaxy (ALE) using Ga(acac), (acac = pentane-2,4-dionate) and either water or ozone as precursors. Films were grown on silicon (loo), soda lime and Corning glass substrates. The influence of the deposition parameters (e.g. pulse duration, growth and source temperatures) on film growth were studied and by a proper choice of the parameters a self-controlled growth was demonstrated around 370 "C. Spectrophotometry, X-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS) were used to determine the refractive index, thickness, crystallinity and stoichiometry of the films. All the films were amorphous and highly uniform with only small thickness variations. The films deposited with water contained a considerable amount of carbon as an impurity whereas ozone as an oxidizer gave stoichiometric Ga203 films.Gallium oxide is a thermally and chemically stable material which is insulating at room temperature but semiconducting at higher temperatures. The n-type semiconducting property of gallium oxide is due to a slight oxygen deficit in the crystal lattice. Ga203 is the single stable oxidation state of gallium under normal conditions. It exists in several crystalline forms, of which the monoclinic, high temperature P-Ga203 modification is the most stable one. Because of its optical and electrical properties, gallium oxide has found applications in metal-insulator structures on GaAs' and facet coatings for GaAs-based lasers. Recently, the use of gallium oxide thin films as gas sensors has attracted increasing i n t e r e ~t . ~-~ At high temperatures (800-1000°C) the films can be used as oxygen sensors while at lower temperatures (<7OO"C) they can be used to make sensors for reducing gases.

Scandium oxide thin film deposition by atomic layer epitaxy was studied at 175-500 °C using Sc(thd) 3 (thd ) 2,2,6,6-tetramethyl-3,5-heptanedione) and (C 5 H 5 ) 3 Sc as scandium precursors. A constant deposition rate of 0.125 Å (cycle) -1 was observed at 335-375 °C on Si(100) and soda lime glass substrates with Sc(thd) 3 and O 3 . The use of H 2 O 2 as an additional oxidizer slightly increased the deposition rate to 0.14 Å (cycle) -1 . When (C 5 H 5 ) 3 Sc and H 2 O were used as precursors, the growth rate of Sc 2 O 3 was significantly higher, viz., 0.75 Å (cycle) -1 at 250-350 °C. The effects of growth parameters such as reactant pulsing times were investigated in detail to confirm the surface-controlled growth mechanism. The crystallinity and surface morphology of the films were characterized by XRD and AFM, while ion-beam analysis (time-of-flight elastic recoil detection analysis) was used to determine the stoichiometry and impurity levels. Crystalline thin films with (111) as the dominant orientation were obtained on Si(100) when depositions were carried out at 300 °C or above from Sc(thd) 3 and O 3 precursors, while films deposited from (C 5 H 5 ) 3 Sc and H 2 O were polycrystalline regardless of the deposition temperature. When films were deposited at 300 °C onto Si(100), the preferred orientation changed from ( 111) to (100) when the film thickness exceeded 200 nm. Films were stoichiometric when deposited from Sc(thd) 3 /O 3 below 450 °C or from (C 5 H 5 ) 3 Sc/H 2 O at 250 °C or above. When the films were deposited from Sc(thd) 3 /O 3 , the carbon content was below 0.1 atom % regardless of the deposition temperature, whereas the hydrogen content decreased to below 0.1 atom % when the deposition temperature was increased to 375 °C. The C and H contents of the films, deposited from (C 5 H 5 ) 3 Sc/H 2 O at 300-400 °C, were around 0.1 and 0.5-0.3 atom %, respectively.

LaAlO 3 thin films were deposited by atomic layer epitaxy (ALE) from b-diketonate-type precursors La(thd) 3 and Al(acac) 3 . Ozone was used as an oxygen source. Films were grown on soda lime glass, Si(100), MgObuffered Si(100), sapphire and SrTiO 3 (100) substrates. The influence of the La : Al precursor pulsing ratio on the film growth and quality in the temperature range of 325-400 uC was studied in detail. Stoichiometry and impurity levels were measured using RBS, TOF-ERDA and XPS while the chemical type of carbon impurity was identified by FTIR. XRD and AFM were used to determine crystallinity and surface morphology. The films were transparent and uniform and their thickness could be accurately controlled by the number of deposition cycles. The as-deposited films were amorphous but became crystalline upon annealing at 900 uC. The annealed films grown on Si(100) and MgO(111)-buffered Si(100) substrates had a preferred (110) orientation whereas those grown on MgO(100)-buffered Si(100) substrates showed a preferred (100) orientation. Epitaxial and smooth LaAlO 3 thin films were obtained on SrTiO 3 (100) after annealing at 900 uC, verified by measurement of the X-ray rocking curve of the ( 200) reflection and the AFM surface roughness. Stoichiometric LaAlO 3 films contained v1.9 atom% carbon and about 0.3 atom% hydrogen as impurities.