Epitaxial growth of both pure and doped CdO thin films has been achieved on MgO (111) substrates using pulsed laser deposition. A maximum conductivity of 42 000 S/cm with mobility of 609 cm2/V s is achieved when the CdO epitaxial film is doped with 2.5% Sn. The pure CdO epitaxial film has a band gap of 2.4 eV. The band gap increases with doping and reaches a maximum of 2.87 eV when the doping level is 6.2%. Both grain boundary scattering and ionized impurity scattering are found to contribute to the mobility of CdO films.

Materials with high electrical conductivity and optical transparency are needed for future flat panel display, solar energy, and other opto-electronic technologies. In xCd1-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 S͞cm, carrier mobility of 70 cm 2 ͞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.

The growth and properties of transparent conducting oxides (TCOs) have recently been the subject of intense academic and industrial investigation. [1] This reflects a plethora of technologically important TCO applications ranging from photovoltaic (PV) cells to flat-panel displays and organic light-emitting diodes. While tin-doped indium oxide (ITO) has been heavily studied as the TCO layer for a variety of opto-electronic applications (including transparent electrodes for PVs), [1] and reports have documented the growth of ITO films via numerous techniques, ITO is not ideally suited for use in future PV systems. Reasons include less than optimum conductivity and transparency, high cost, and cation diffusion between ITO and the Cd chalcogenide PV-active layers. [2] To address this problem, thin films of the Cd-containing TCO, cadmium stannate (Cd 2 SnO 4 ), have recently been grown by rf sputtering. [3] These films exhibit promising electrical and optical properties, including a high carrier mobility (59.6 cm 2 V ±1 s ±1 ). [3] However, the lack of suitable Cd precursors has so far impeded the growth of Cd 2 SnO 4 , or any other Cd-based TCO thin film grown by efficient CVD techniques. Thin films of the parent TCO, CdO, have been extensively studied and are known to be highly conductive, primarily due to various defect structures. While CdO has a modest intrinsic bandgap (~2.3 eV), [4] it serves as an excellent model material for the development of TCO CVD processes. Furthermore, previous work in this laboratory and elsewhere has shown that n-type aliovalent doping of CdO has profound effects on the electronic structure, significantly enhancing both the conductivity and the bandgap by introducing n-type charge carriers [5±8] and therefore blueshifting the band edge via a Burstein±Moss shift. [5,6,9] Metal±organic (MO) CVD is a widely used film growth process which complements physical vapor deposition techniques such as rf sputtering, and certain characteristics of MOCVD are particularly attractive for TCO thin film growth. Growth conditions are closer to ambient, growth at higher O 2 partial pressures is possible, conformal coverage over complex three-dimensional features can be achieved, and the process is amenable to very large-scale depositions. All these attractions afford a technique engineered for maximum overall growth efficiency and diverse applications. However, a crucial feature for a useful MOCVD growth process is the necessity of highly volatile, thermally stable, easy to handle, metal±organic precursors. Such precursors must decompose cleanly at the substrate surface during film growth since premature decomposition causes involatile metal species that remain in the precursor reservoir, while complexes that are too thermally stable decompose incompletely, thereby contaminating the reactor and resultant films.To date, growth of Cd-containing films via MOCVD has only been achieved using dimethylcadmium (CdMe 2 ) or its derivatives as precursors. While CdMe 2 can be used for the growth of the heavier C...

Rb 2 Bi 8 Se 13 (I), Cs 2 Bi 8 Se 13 (II), CsBi 3.67 Se 6 (III), and BaBi 2 Se 4 (IV) were synthesized by direct combination reactions of the A/Se (A ) Rb, Cs, Ba) and Bi 2 Se 3 at g650 °C. Their structures were determined by single-crystal X-ray diffraction. Rb 2 Bi 8 Se 13 and Cs 2 Bi 8 Se 13 are isostructural and crystallize in the monoclinic space group P2 1 /m (No. 11) with a ) 13.4931(4) Å, b ) 4.1558(3) Å, c ) 24.876(2) Å, β ) 96.571(4)°, R1 ) 0.0577, and wR2 ) 0.1159 [I > 2σ(I)] for I and a ) 13.704(1) Å, b ) 4.1532(4) Å, c ) 25.008(2) Å, β ) 96.848(2)°, R1 ) 0.0497, and wR2 ) 0.1123 [I > 2σ(I)] for II. CsBi 3.67 Se 6 crystallizes in the orthorhombic space group Pnma (No. 62) with a ) 23.421(4) Å, b ) 4.1877(8) Å, c ) 13.710(3) Å, R1 ) 0.0611, and wR2 ) 0.1384 [I > 2σ(I)]. BaBi 2 Se 4 crystallizes in the hexagonal space group P6 3 /m (No. 176) with a ) 26.157(1) Å, c ) 4.3245(3) Å, R1 ) 0.0371, and wR2 ) 0.0817 [I > 2σ(I)]. The structure of A 2 Bi 8 Se 13 features a three-dimensional framework consisting of wide rectangular NaCl-type infinite rods, running parallel to the b-axis, which are stitched together by CdI 2 -and Sb 2 Se 3 -type fragments. The NaCl-type blocks are aligned parallel to each other, and between them are rows of alkali metal ions. CsBi 3.67 Se 6 consists of narrower NaCl-type infinite rods, which share edges. The cesium metal ions reside in the space between these rods. The bismuth sites that connect the NaCl-type rods are partially occupied. The [Bi 2 Se 4 ] 2framework in BaBi 2 Se 4 contains tunnels running along the c-axis that are occupied by Ba atoms. All compounds are narrow band-gap semiconductors. Electrical conductivity and thermoelectric power measurements show that I-IV exhibit n-type charge transport. Compounds I and II, however, can also exhibit p-type behavior. The thermal conductivity for I and IV is low with room-temperature values of ∼1.6 W/(m‚K) for I and ∼1.2 W/(m‚K) for IV. The optical band gaps of all compounds range between 0.3 and 0.6 eV.