J. R. Babcock

Indium-cadmium-oxide films having exceptional electrical conductivity and optical transparency: Clues for optimizing transparent conductors

Materials with high electrical conductivity and optical transparency are needed for future flat panel display, solar energy, and other opto-electronic technologies. InxCd1-xO films having a simple cubic microstructure have been grown on amorphous glass substrates by a straightforward chemical vapor deposition process. The x = 0.05 film conductivity of 17,000 S/cm, carrier mobility of 70 cm2/Vs, and visible region optical transparency window considerably exceed the corresponding parameters for commercial indium-tin oxide. Ab initio electronic structure calculations reveal small conduction electron effective masses, a dramatic shift of the CdO band gap with doping, and a conduction band hybridization gap caused by extensive Cd 5s + In 5s mixing.

Selective-area atomic layer epitaxy growth of ZnO features on soft lithography-patterned substrates

Templated ZnO thin-film growth from the vapor phase is achieved on docosyltrichloro- silane-patterned Si substrates using atomic layer epitaxy (ALE) combined with soft lithography. Patterned hydrophobic self-assembled monolayers (SAMs) are first transferred to single-crystal Si surfaces by hot microcontact printing. Using diethylzinc and water as ALE precursors, crystalline ZnO layers are then grown selectively on the SAM-free surface regions where native hydroxy groups nucleate growth from the vapor phase. High-resolution ZnO patterns with 1.0–40 μm feature sizes are readily achieved, demonstrating that soft lithography combined with ALE is a simple and promising methodology for selective area in situ vapor phase fabrication of patterned oxide thin films.

First-principles calculations for understanding high conductivity and optical transparency in InxCd1−xO films

Abstract We investigate In x Cd 1− x O materials, where x =0.0, 0.031, 0.063 and 0.125, to understand their high electrical conductivity and optical transparency windows, using the full-potential linearized augmented plane wave (FLAPW) method. In addition, we employ the screened exchange LDA (sX-LDA) method to evaluate accurate band structures including band gap that is underestimated by the LDA calculations. The results show a dramatic Burstein–Moss shift of the absorption edge by the In doping, reflecting the small effective mass of the Cd 5s conduction band. The calculated direct band gaps, 2.36 eV for x =0.0 and 3.17 eV for x =0.063, show excellent agreement with experiment. The effective mass of the conduction band of CdO is calculated to be 0.24 m e (in the ▵ direction), in good agreement with an experimental value of 0.27m e , explaining its high electrical conductivity. The hybridization between the Cd 5s and the In 5s states yields complex many-body effects in the conduction bands: a hybridization gap in the conduction bands and a band-gap narrowing which cancels the further Burstein–Moss shift for higher In doping.

Growth, microstructure, charge transport, and transparency of random polycrystalline and heteroepitaxial metalorganic chemical vapor deposition-derived gallium-indium-oxide thin films

Gallium-indium-oxide films (Ga x In 2 - x O 3 ), where x = 0.0-1.1, were grown by low-pressure metalorganic chemical vapor deposition using the volatile metalorganic precursors In(dpm) 3 and Ga(dpm) 3 (dpm = 2,2,6,6-tetramethyl-3,5-heptanedionato). The films were smooth (root mean square roughness = 50-65 A) with a homogeneously Ga-substituted, cubic In 2 O 3 microstructure, randomly oriented on quartz or heteroepitaxial on (100) yttria-stabilized zirconia single-crystal substrates. The highest conductivity of the as-grown films was found at x = 0.12, with σ = 700 S/cm [n-type; carrier density = 8.1 × 10 1 9 cm - 3 ; mobility = 55.2 cm 2 /(V s); du/dT < 0]. The optical transmission window of such films is considerably broader than that of Sn-doped In 2 O 3 , and the absolute transparency rival or exceeds that of the most transparent conductive oxides known. Reductive annealing, carried out at 400-425 °C in a flowing gas mixture of H 2 (4%) and N 2 , resulted in increased conductivity (σ = 1400 S/cm; n-type), carrier density (1.4 x 10 2 0 cm - 3 ), and mobility as high as 64.6 cm 2 /(V s), with little loss in optical transparency. No significant difference in carrier mobility or conductivity is observed between randomly oriented and heteroepitaxial films, arguing in combination with other data that carrier scattering effects at high-angle grain/domain boundaries play a minor role in the conductivity mechanism.

Development and Implementation of New Volatile Cd and Zn Precursors for the Growth of Transparent Conducting Oxide Thin Films Via Mocvd

For the growth of thin zinc group metal oxide films [i.e. cadmium oxide (CdO), cadmium stannate (Cd 2 SnO 4 ), and zinc oxide (ZnO)] via metal-organic chemical vapor deposition (MOCVD), volatile Cd and Zn precursor families are needed. Starting with Cd, β-ketoiminates of varying substitution were prepared to elucidate structure-property relationships. The nature of the ligand substituents strongly influences the melting point (liquid precursors are desired). Unlike conventional Cd β-diketonates, these complexes are monomeric as determined by x-ray crystallography. Despite these advantageous characteristics, attempts to grow CdO thin films in a cold-wall MOCVD reactor using two such derivatives were not successful. This class of Cd complex appears to decompose thermally with time-- a likely cause of the poor performance. Therefore, a new series of more thermally stable Cd precursors was sought. Using the chelating diamine N,N,N 1 ,N 1 -tetramethylethylenediamine (TMEDA), monomeric β-diketonates were prepared. The molecular structure of Cd(hfa) 2 (TMEDA) (hfa = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) confirms the monomeric structural assignment. This series of Cd complexes is appreciably more volatile and sublime more cleanly than the aforementioned β-ketoiminates, as determined by vacuum thermogravimetric analysis (TGA). In addition to this advantageous characteristic, these complexes are easily prepared under ambient laboratory conditions from commercially available starting materials in a single step. Following the protocol established for Cd, a volatile series of Zn precursors was also prepared. For the Zn series, the melting point was effectively tuned through variation of both the β-diketonate and diamine ligands. The use of the Cd and Zn β-diketonate precursors in the successful growth of CdO and ZnO thin films, respectively, by MOCVD is also presented.

Volatile, Fluorine-Free β-Ketoiminate Precursors for MOCVD Growth of Lanthanide Oxide Thin Films

Lanthanide oxide thin films are of increasing scientific and technological interest to the materials science community. A new class of fluorine-free, volatile, low-melting lanthanide precursors for the metal-organic chemical vapor deposition (MOCVD) of these films has been developed. Initial results from a full synthetic study of these lanthanide-organic complexes are detailed.