E. Nykänen

Growth of titanium dioxide thin films by atomic layer epitaxy

Abstract Titanium dioxide thin films were deposited by atomic layer epitaxy using TiCl4 and water as reactants. The film growth was performed over the temperature range 150–600 °C in order to study the effects of temperature on the growth rate. The effect of the substrate material on the growth rate and crystal structure was also investigated. Spectrophotometry, X-ray diffraction, Rutherford backscattering spectroscopy and nuclear reaction analysis were used to determine the chemical and physical characteristics of the films. The growth mechanism is discussed on the basis of literature and results obtained.

Nitrides of titanium, niobium, tantalum and molybdenum grown as thin films by the atomic layer epitaxy method☆

Abstract Cubic δ-TiN, cubic δ-NbN, cubic TaN, tetragonal Ta 3 N 5 , cubic γ-Mo 2 N and hexagonal δ-MoN were grown at 773 K from the corresponding chlorides and ammonia by the atomic layer epitaxy method. With the exception of tantalum nitrides the films show a preferred orientation of either (111) type (NbN and Mo 2 N) or (200) type (MoN and TiN). The nitrogen contents of the films were determined by the nuclear resonance broadening technique and compared with structure and other properties. The δ-NbN films with 55 at.% nitrogen had at room temperature a resistivity of 200 μΩ cm became superconducting below 10 K.

Preparation and characterization of phosphorus-doped aluminum oxide thin films

Aluminum oxide thin films with or without phosphorus doping were deposited by the atomic layer epitaxy (ALE) technique. The source materials for aluminum were aluminum chloride and aluminum n-propoxide and for oxygen, water and 2-methyl-2-propanol. The phosphorus source was P2O5. The films were analyzed by Rutherford backscattering spectroscopy (RBS), nuclear resonance broadening (NRB), X-ray diffraction (XRD), X-ray fluorescence (XRF), and Fourier transform infrared (FTIR) spectroscopy for chemical composition and structure. The results show that phosphorus can be incorporated in a wide range of concentration levels into the aluminum oxide layers. Greater than 20 wt% doping, however, led to the formation of crystalline aluminum phosphate when the oxygen source was 2-methyl-2-propanol. The phosphorus doping also increased the growth and H3PO4 etch rates of the films.

Growth and characterization of aluminium oxide thin films deposited from various source materials by atomic layer epitaxy and chemical vapor deposition processes

Abstract Aluminium oxide thin films were prepared by the ALE process using AICI3 and A1(OR)3 (R = Et,Pr) as aluminium source and H2O, O2 or various aliphatic alcohols as oxygen source. The process was also operated in the CVD mode and the deposition rates and mechanisms were compared. The films were characterized by Auger and XRF spectrometries for trace element impurities while the chloride residues were quantitatively determined by chemical analysis. FTIR was used to study the residual OH groups. The measured electrical constants and environmental stability indicate that the films are suitable for practical applications as insulating and protective layers.

Thermoanalytical and mass spectrometric studies on volatile β-diketone chelates

Abstract Thd-chelates (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) of Y, Ce, Pr, Sm, Eu, Gd, Tb, Tm and Yb and hexafluoroacetylacetonates (hfa) of Pr and Eu were studied by TG, DSC and MS techniques. TG and DSC curves of the complexes indicated the sublimation between 100 and 300°C. According to the DSC measurements the sublimation temperatures of rare earth thd chelates decrease with the cation size being 223°C for Pr and 170°C for Yb. The sublimation temperatures of fluorine containing complexes were 100 degrees lower than those of thd-chelates. The mass spectra of the rare earth-thd chelates were very similar. The main peak in each spectrum corresponded Ln(thd)2- The alkaline earth-thd chelates exist as M2(thd)3 species in the UHV conditions. The strongest peaks in the MS of the fluorocomplexes were the following: Pr(hfa)3· H2O and Eu(hfa)3· DMF.

Preparation of lead sulfide thin films by the atomic layer epitaxy process

Abstract Preparation of lead sulfide thin films on different substrates by the Atomic Layer Epitaxy process has been studied. Sulfur source was in all experiments H 2 S, but as lead source the following compounds were tested: bromide, iodide and acetate as well as thd (2,2,6,6-tetramethyl-3,5-heptanedione) and diethyldithiocarbamate chelates. The last complex gave the highest growth rate. The growth experiments were carried out at 300–350°C and the film thickness varied between 0.1 and 1 μm. The films were characterized by XRD, XRF, SEM, and by photoconductivity and Hall measurements. The results showed that the films were polycrystalline and randomly oriented. The conductivity was p-type and the carrier concentration and mobility were comparable with those found in films deposited by traditional chemical methods.

An XPS study of SrS:Ce thin films for electroluminescent devices

Abstract Cerium-doped SrS thin films were analyzed using X-ray photoelectron spectroscopy (XPS) in order to optimize the doping conditions of real electroluminescent thin films. The films were deposited by atomic layer epitaxy (ALE) on soda lime glass substrates using β -diketonate complexes of the metals and H 2 S as precursors. The composition and the stoichiometry of the SrS:Ce films as well as the chemical state of strontium, sulfur and cerium were determined. In addition, the structure of the films was studied using X-ray diffraction (XRD) and the average cerium content of the films was measured by X-ray fluorescence spectroscopy. XRD measurements showed that the SrS:Ce thin films were well-crystallized having oriented cubic structure. According to the XPS measurements, cerium occurred as trivalent both at the surface and in the bulk of the films even in high concentrations. Strontium was mainly present as strontium sulfide at the film surface. In addition to this, strontium carbonate, strontium hydroxide and strontium sulfite were detected. Sulfur appeared mainly as sulfide at the surface but traces of oxidized sulfur were observed, too.

Growth of PbS thin films from novel precursors by atomic layer epitaxy

Two lead tert-butoxide complexes (1)[Pb(Obut)2]m(m= 3 or 2 in the solid and gaseous phase, respectively) and (2) Pb4O(OBut)6 were used as precursors for atomic layer epitaxy (ALE) deposition of PbS thin films. The growth on soda lime glass, with and without an alumina coating, was studied by varying the source furnace and the substrate temperatures as well as the total number of cycles. Pb(thd)2(3) and Pb(dedtc)2(4) were used for comparison while H2S served in all experiments as the sulfur source. The films obtained were smooth and generally highly crystalline. The substrate temperature had a strong effect on the growth rate of PbS thin films. Nevertheless, in the self-controlled region of ALE growth the tert-butoxide complexes gave a significantly higher growth rate than the other source chemicals, with a maximum of 0.9 A per cycle at 150 °C. Upon sublimation 1 is converted to 2, which contains four Pb atoms in a tetrahedral arrangement; this may cause the higher growth rate. Thermogravimetry/differential thermal analysis curves and mass spectrometric data were measured for all precursors. As the butoxide and thd complexes (1–3) are thermally unstable the useful ALE prosessing windows (temperature/pressure) are narrow compared to the much more stable dedtc complex (4).

The effect of gaseous SiCl4 on the ALE growth of CaS, SrS and SrS:Ce

Abstract The deposition of alkaline earth sulfide thin films by atomic layer epitaxy (ALE) and the effect of an additional dopant (SiCl 4 ) on the growth have been studied. The precursors used in the deposition were H 2 S and thd-chelates of calcium, strontium and cerium (thd = 2,2,6,6-tetramethyl-3,5-heptanedione). The flow rate and pulse time of SiCl 4 , as well as the substrate temperature, affected the diffraction pattern and the growth rate of the films. AFM studies revealed differences in the crystal morphology between SiCl 4 doped and undoped films. Chlorine was a main impurity in the films.

Deposition of lanthanum sulfide thin films by atomic layer epitaxy

Abstract Lanthanum sulfide thin films prepared using atomic layer epitaxy technique were analysed by X-ray diffraction and X-ray photoelectron spectrometry. The films grown from a β-diketonate precursor and H 2 S were amorphous below 400°C. The first traces of crystallization appeared above 400°C, and at 500°C films of well-defined cubic γ-La 2 S 3 structure could be deposited. XPS analysis revealed the presence of La 2 S 3 both in amorphous and crystallized samples. Upon annealing at 510–730°C the material reacted with residual oxygen in the bulk and La 2 O 2 S and other oxysulfides were formed.