Minna Nieminen

Deposition of Tin Oxide into Porous Silicon by Atomic Layer Epitaxy

The deposition of conformal coatings into porous silicon layers was successfully demonstrated. Tin oxide films were formed from SnCl 4 and H 2 O precursors by atomic layer epitaxy. The influence of the porous substrate structure on the deposition parameters was analyzed from the viewpoint of formation mechanism, growth rate, and layer composition. The SnO x covered porous substrates were characterized by means of Rutherford backscattering, secondary ion mass spectrometry, cross-sectional transmission electron microscopy, and ellipsometry. The mesoporous structure of the Si substrate uniquely determines the gas-phase diffusion and physisorption of the precursors. The processing parameters favoring chemisorption are more critical for porous silicon than those for a flat surface. Even a small decrease in the deposition temperature results in a considerable increase in the growth rate through gas-phase reactions, and the process becomes chemical vapor deposition-like. Conformal step coverage was obtained on extremely high (140 :1) aspect ratio pores if the deposition conditions were chosen such that chemisorption was the growth rate determining step in the process.

Surface-controlled growth of LaAlO3 thin films by atomic layer epitaxy

LaAlO3 thin films were deposited by atomic layer epitaxy (ALE) from β-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), MgO-buffered Si(100), sapphire and SrTiO3(100) substrates. The influence of the La∶Al precursor pulsing ratio on the film growth and quality in the temperature range of 325–400 °C 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 °C. 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 LaAlO3 thin films were obtained on SrTiO3(100) after annealing at 900 °C, verified by measurement of the X-ray rocking curve of the (200) reflection and the AFM surface roughness. Stoichiometric LaAlO3 films contained <1.9 atom% carbon and about 0.3 atom% hydrogen as impurities.

Growth of LaCoO3 thin films from β-diketonate precursors

LaCoO 3 thin films have been deposited in a flow-type atomic layer epitaxy (ALE) reactor from La(thd) 3 and Co(thd) 2 , (thd = 2,2,6,6-tetramethyl-3,5-heptadione) precursors using ozone as oxygen source. Films were grown on both soda lime and Corning glass substrates in the temperature range of 200-400°C. Profilometry, X-ray diffraction, Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy measurements were used to determine the thickness, crystallinity and stoichiometry of the films. The films grown below reactor temperatures of 400°C were X-ray amorphous, but became crystalline LaCoO 3 when annealed at 600°C in air. Besides LaCoO 3 , the binary oxides (La 2 O 3 and Co 3 O 4 ) were also grown as thin films.

Deposition of LaNiO3 thin films in an atomic layer epitaxy reactor

LaNiO 3 thin films have been deposited in an atomic layer epitaxy (ALE) reactor, using La(thd) 3 , Ni(thd) 2 and ozone as reactants, thereby proving the feasibility of the ALE technique to produce films of ternary oxides. Depositions were made on Corning glass in the temperature range 150–450 °C. The growth conditions were studied and the growth rate showed a linear dependence on the number of cycles. At 400 °C the growth rate was 0.24–0.26 A per cycle. The growth rate of the LaNiO 3 thin films was greatly influenced by the deposition temperature but in the temperature range 215–250 °C the growth saturated at 0.08 A cycle -1 independent of the deposition temperature, thus indicating an ALE window. As-deposited thin films were amorphous but crystallized when heated at 600 °C. Simultaneously the colour of the films changed from yellow–brown to black. Possible reasons for the colour changes are discussed. Resistivity measurements showed that the crystalline thin films were metallic, ρ=(5–20)×10 -6 Ω m. The amorphous thin films had resistivity values five orders of magnitude larger, ρ>3 Ω m. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM), the films were homogeneous and dense. The surface roughness increased on crystallisation. X-Ray photoelectron spectroscopy (XPS) and magnetic susceptibility measurements were employed in order to further characterize the amorphous and crystalline thin films.

Growth of gallium oxide thin films from gallium acetylacetonate by atomic layer epitaxy

Gallium oxide thin films have been deposited by atomic layer epitaxy (ALE) using Ga(acac)3(acac = pentane-2,4-dionate) and either water or ozone as precursors. Films were grown on silicon (100), 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 Ga2O3 films.

High-permittivity YScO3 thin films by atomic layer deposition using two precursor approaches

Amorphous YScO3 thin films have been deposited by atomic layer deposition using two types of volatile metal precursors, viz. β-diketonate-type metal complexes M(thd)3 (M = Y, Sc; thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) and organometallic cyclopentadienyl compounds tris(methylcyclopentadienyl)yttrium (C5H4CH3)3Y and tris(cyclopentadienyl)scandium Cp3Sc (Cp = C5H5). Ozone and water were used as oxygen sources in the M(thd)3 and cyclopentadienyl precursor-based processes, respectively. Deposition temperatures were 335–350 °C for the M(thd)3 precursor-based process and 300 °C for the cyclopentadienyl precursor-based process. Metal ratio and film thickness were easily controlled by varying the metal precursor pulsing ratio and the number of deposition cycles. Stoichiometric YScO3 films contained less than 1 atom% hydrogen and less than 0.2 atom% carbon regardless of the precursors used. The as-deposited stoichiometric films were smooth, amorphous and they had high permittivity (14–16). Films deposited using the cyclopentadienyl precursor-based process started to crystallize at 800 °C while films deposited using the M(thd)3 precursor-based process still remained amorphous at this temperature. Films deposited using the latter process crystallized at 1000 °C. Crystallization significantly deteriorated the dielectric properties of the films, however.

Determination of P/Al ratio in phosphorus-doped aluminium oxide thin films by XRF, RBS and FTIR

Phosphorus-doped aluminium oxide thin films were deposited in a flow-type ALE reactor from AlCl3, H2O and from either P2O5 or trimethyl-phosphate. Structural information of the films was obtained from Fourier transform infrared (FTIR) spectra. Rutherford backscattering spectroscopy (RBS) was used to quantitatively determine the composition of the films. The P/Al intensity ratios calculated from X-ray fluorescence (XRF) results were in a linear relation with the P/Al concentration ratios calculated from RBS results. For comparison, the intensity ratios of the phosphorus peak (P=O) at about 1250 cm−1 and the aluminium peak (Al-O) at about 950 cm−1 were determined from the IR absorption spectra. The calibration of FTIR peak intensities was done by plotting the intensity ratios of phosphorus and aluminium peaks against the P/Al concentration ratios measured by RBS. FTIR gave also a linear calibration curve with RBS but the method is less suitable for routine analysis of P/Al ratio than XRF.