CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Physics * Mathematics

Volume 52 No. 3 September 2003

Special issue on atomic layer deposition

Preface; 243–244

Arnold Rosental

Precursor design of vapour deposited cubic boron nitride versus diamond; 245–256

Karin Larsson

Abstract. The similarities and dissimilarities in the growth of diamond vs. cubic boron nitride (c-BN) were studied using quantum mechanical calculations. Great similarities were observed when considering the surface stabilization by H atoms. Very great similarities were recorded when considering the adsorption of various growth species to these materials. It was found necessary to avoid mixtures of B- and N-containing species in the gas phase during c-BN growth, since they should most probably result in a mixture of these species also on the surfaces. Greater dissimilarities were observed when studying the surface migrations on the diamond and c-BN surfaces and nucleation of the cubic phases on the corresponding hexagonal ones. Nucleation of diamond/c-BN on graphite/h-BN was found to be energetically feasible. This was calculated to be especially the situation for the armchair edge of the basal plane of h-BN and of the zigzag edge of the basal plane of graphite. These theoretical results can be used as guidelines in the strive towards thin film deposition of c-BN using gentle chemical vapour deposition methods like atomic layer deposition.

Key words: atomic layer deposition, theoretical, quantum chemistry, growth, surface processes, diamond, boron nitride.

High-temperature atomic layer epitaxy of TiO₂ from TiCl₄ and H₂O₂–H₂O; 257–265

Ahti Niilisk, Arnold Rosental, Aivar Tarre, and Teet Uustare

Abstract. Epitaxial TiO₂ films were grown on off-cut a-Al₂O₃ (R-plane sapphire) substrates by gas-phase atomic layer deposition at 550–750 °C. X-ray diffraction and reflection high-energy electron diffraction measurements showed that the films had a two-domain textured rutile structure. The domains made a straight angle about the normal to the substrate plane; those quantitatively superior were 3° inclined with respect to the plane, resulting in the overall orientation relationship ||(3°)_rutile. An account of the two-domain growth was given. The epitaxial quality worsened with the increase in the deposition temperature.

Key words: atomic layer deposition, epitaxy, rutile titanium dioxide, R-plane sapphire, X-ray diffraction, X-ray fluorescence, reflection high-energy electron diffraction.

Atomic layer deposition: a key technology for the controlled growth of oxide thin films for advanced applications; 266–276

Lauri Niinistö

Abstract. Atomic layer deposition (ALD), also referred to as atomic layer epitaxy (ALE), was originally developed to process thin film electroluminescent structures for flat panel displays, which include as core components also insulating oxide layers such as alumina and aluminium titanium oxide. Another early application of oxide ALD was the processing of overlayers for catalysts. More recently, a major breakthrough for the ALD technology is the use of oxide thin films in microelectronics as gate and dynamic random access memory capacitor dielectrics. Besides giving a brief introduction to the ALD/ALE technology, the paper will address the present status of depositing binary and more complex (i.e. perovskite-type) metal oxides emphasizing precursor chemistry.

Key words: atomic layer deposition, atomic layer epitaxy, metal oxides, microelectronics, thin films, volatile precursors.

Atomic layer deposition of ZnO thin films and dot structures; 277–288

Marek Godlewski, Krzysztof Kopalko, Andrzej Szczerbakow, Elżbieta Łusakowska, Michał M. Godlewski, Ewa M. Goldys, Kenneth Scott A. Butcher, and Matthew R. Phillips

Abstract. Successful growth of thin films and quantum dots of ZnO by atomic layer deposition (ALD) is reported. Properties of ZnO films produced by four different ALD-procedures and by oxidation of ALD-grown ZnS films are discussed. We also shortly describe the use of thin ZnO films as buffer layers for GaN deposition.

Key words: atomic layer deposition, ZnO, GaN, thin films, quantum dots.

Spectroscopic characterization of ZrO₂ thin films grown by atomic layer deposition; 289–298

Jaan Aarik, Hugo Mändar, and Marco Kirm

Abstract. Zirconium dioxide films grown by atomic layer deposition from ZrCl₄ and H₂O at substrate temperatures of 500–870 K were characterized using spectroscopic methods. A significant influence of the phase composition on absorption and photoluminescence spectra was observed. The band-gap energies determined from absorption spectra of films with monoclinic structure grown at 670–870 K ranged from 5.25 to 5.28 eV. Low-temperature (10 K) photoexcitation of these films resulted in a non-elementary emission band with the maximum at 4.25–4.35 eV, tentatively assigned to the radiative decay of self-trapped excitons. The low-energy edges of the excitation spectra coincided with the edges of intrinsic absorption. A film that was grown at 500 K and contained tetragonal ZrO₂ with large amounts of impurities had an absorption edge at 5.26 eV and wide emission band at 3.0 eV, which could be excited at as low photon energy as 4.9 eV.

Key words: zirconium dioxide, atomic layer deposition, structure, absorption, photoluminescence.

Analysis of Zn(O,S) films for Cu(In,Ga)Se₂ solar cells; 299–307

Charlotte Platzer-Björkman, John Kessler, and Lars Stolt

Abstract. Thin films of Zn(O,S), deposited by atomic layer deposition (ALD), were investigated as buffer layers in Cu(In,Ga)Se₂ solar cells. The oxygen to sulphur ratio of these films was varied by alternating diethylzinc (DEZ)/H₂S and DEZ/H₂O cycles. Solar cell efficiencies of up to 14% were achieved, with the best result obtained for 20% DEZ/H₂S cycles. Properties of films deposited on glass were studied, showing a variation of resistivity, structure, and band gap for varying sulphur content. A coupled X-ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy–ALD system was used to study band alignment at the Cu(In,Ga)Se₂/Zn(O,S) interface. Conduction and valence band offsets were determined for ZnS and Zn(O,S) deposition with 20% DEZ/H₂S cycles and compared to previously determined values for ZnO deposition.

Key words: atomic layer deposition, solar cells, Cu(In,Ga)Se₂, Zn(O,S) buffer layer.

Effects of growth temperature on the properties of HfO₂ films grown by atomic layer deposition; 308–320

Giovanna Scarel, Claudia Wiemer, Sandro Ferrari, Grazia Tallarida, and Marco Fanciulli

Abstract. A relatively high dielectric constant  20–25), wide band gap and conduction band offset (6.0 eV and 1.5 eV, respectively), and good thermal stability upon contact with silicon indicate hafnium dioxide as one of the most promising candidates to substitute silicon dioxide as dielectric gate in complementary metal-oxide-semiconductor devices. To investigate the properties of thin films suitable for application in microelectronics, HfO₂ films were grown by atomic layer deposition. Hafnium tetrachloride (HfCl₄) and water (H₂O) were used as precursors. Film structural, morphological, and compositional properties were then investigated focusing on their dependence on growth temperature in the range between 150 °C and 350 °C. A modification of the film structure with growth temperature is expected because the density of the reactive OH sites is known to decrease with increasing temperature. The extent and consequences of these modifications were investigated using X-ray diffraction and reflectivity, and atomic force microscopy. Time of flight–secondary ion mass spectrometry was used to study film composition.

Key words: atomic layer deposition, HfO₂, ZnO₂, dielectric oxides.

Optical coatings grown by atomic layer deposition for high-power laser applications; 321–329

Shin-ichi Zaitsu, Shinji Motokoshi, Takahisa Jitsuno, Masahiro Nakatsuka, and Tatsuhiko Yamanaka

Abstract. We prepared optical coatings with low (Al₂O₃) and high (TiO₂) refractive index materials using the sequential chemical reaction process of atomic layer deposition (ALD). Also, we examined the laser damage thresholds of the films for high-power laser applications. The highest damage thresholds were obtained for amorphous films grown at room temperature. For TiO₂ and Al₂O₃ films they equalled 5 and 5.2 J/cm², respectively. Finally, we employed ALD for growing desired refractive index coatings consisting of alternating nanoscale Al₂O₃–TiO₂ laminated layers. The refractive index of the stack of these layers could be varied linearly from 1.61 to 2.39 by adjusting the thickness of the component layers.

Key words: atomic layer deposition, optical coating, laser-induced damage, refractive index.

Instructions to authors; 330–332