(Sn,Al)Ox Films Grown by Atomic Layer Deposition

Tin oxide (SnO2) is a transparent semiconductor with a wide band gap and electrical resistivity as low as 2 10 4 Ω 3 cm and high infrared reflectivity, over 90%. 4 These properties are achieved using n-type doping by substituting fluorine for about 1% of the oxygen. The low electrical resistance and optical transparency in SnO2 are widely used in applications such as solar cells, displays, touch controls, and defrosting windows. Its high infrared reflectivity provides its energy saving properties in lowemissivity windows. Some potential applications of SnO2 would require high instead of low electrical resistance. For example, transparent thin film transistors for displays, 11 microchannel electron multiplier plates, 14 and hole-blocking layers in solar cells would require highly resistive SnO2 layers that still maintain high electron mobility. Native defects, such as oxygen vacancies and hydroxyl groups, normally contribute electrons to the conduction band. However, high resistivity requires a low concentration of electrons in the conduction band, so electrons from these native defects need to be trapped in order to obtain highly resistive SnO2. Electrons could be trapped by substituting nitrogen for oxygen, or substituting trivalentmetals for tin. In this paper, we took the latter approach, and substituted aluminum (Al3þ) for tin (Sn4þ). Thus by adding insulating Al2O3 into SnO2, we explored the possibility of controlling its film resistivity over a wide range, forming (Sn,Al)Ox composite materials. One advantage of using Al2O3 for modulation of the conducting layer is that it can also be used as the electron emission layer in a microchannel electron multiplier plate. Atomic layer deposition (ALD) can produce multicomponent films with good control of their stoichiometry. This control of stoichiometry even extends to highly conformal films on complex structures with high aspect ratios, such as narrow holes. ALD involves sequential and self-limiting chemical reactions of precursor pulses on the surface of a growing film. 22 As a result, the films can be extremely smooth and continuous without pinholes. These unique characteristics have made ALD one of the most popular techniques for new application areas of various nanotechnologies. 25 Multicomponent ALD with the use of more than two cation precursors involves repetitive exposure of different precursors to heterogeneous surfaces. Differing chemisorption amounts of precursors on different oxide surfaces make it difficult to predict the composition of the films. 29 Therefore, in-depth understanding of chemisorption of precursors on heterogeneous surfaces is necessary to control the composition and the electrical, optical, magnetic, andmechanical properties of the resulting films. 34 One process for ALD of SnO2 at low temperature has been reported, but the films were amorphous as deposited, and needed a high-temperature anneal to obtain crystalline material with significant mobility. A recently reported ALD process overcomes this difficulty by using a more reactive tin source that deposits well-crystallized SnO2 films with reasonably high electron mobility at temperatures as low as 100 C. There have been several efforts to investigate the chemisorption behavior of trimethylaluminum (Al(CH3)3, TMA) on ZnO 34,37 and TiO2 30 surfaces. Elam et al. reported retarded nucleation of both diethylzinc (Zn(CH2CH3)2, DEZ) and TMA following the other subcycles by using in situ quartz crystal microbalance and additional ex situ measurements of film properties. Na et al. conducted a similar experiment on Aldoped ZnO and also confirmed the retarded nucleation of DEZ on Al2O3 surfaces. 33 For TiO2 films, Kim et al. reported retarded adsorption of titanium tetrakis(isopropoxide), Ti(O-i-C3H7)4,