Characterization of p-type wide band gap transparent oxide for heterojunction devices
Transparent p-type CuCr1-xMgxO2 wide band gap oxide semiconductor thin films were deposited over quartz substrates by chemical spray pyrolysis technique using metalloorganic precursors. A mechanism of synthesis of CuCrO2 films involving precursor decomposition, oxidation, and reaction between constituent oxides in the spray deposition process is presented. Crystalline single phase CuCrO2 delafossite structure was dominant in ≥700oC argon ambient annealed films but the as-deposited films contained spinel CuCr2O4 mixed phases as shown by XRD and XPS studies. Spin-orbital energy ∼9.8eV in Cr 2p electron spectra consistent with Cr3+ valence state and Cr 2p3/2 resolved peaks show mixed valence state on Cr4+ /Cr6+ confirming CuCr1-xMgxO 2 compound phase in the films. Effect of substrate temperature, film thickness, and acceptor Mg2+ doping on crystallographic structure, optical, electrical conductivity and thermoelectric coefficient was investigated. The invariance of the α- and increase of the c-lattice parameter with Mg concentration suggests that Mg2+ ions are introduced at the Cr3+ site. Highly transparent ≥80% CuCr 0.93Mg0.07O2 films with direct and indirect optical band gaps 3.08 and 2.58eV for 155 nm and 3.14 and 2.79eV for 305nm thin films, respectively were obtained. Photoluminescence emission bands at 532 and 484nm interpreted to arise from 3d94s1 and 3d 10 Cu+ intra-band transitions.
Electrical conductivity of CuCr0.93Mg0.07O 2 films ranged from 0.6−1.0 Scm-1 and exhibits activation energies ∼0.11eV in 300-420K and ∼0.23eV in ≥ 420K region ascribed to activated conduction and grain boundary trap assisted conduction, respectively. Restricted by the Mg solubility, the substituted Mg dopants limited to x≤0.05 are only able to contribute to the optimum hole carrier in the range ∼2−4×1019cm-3 and thus no substantial increase of electrical conductivity could be realized with increased Mg concentration. A major fraction of Mg atoms do not act as acceptor, but beyond the low solubility limit act as defect centers. Transparent p-CuCrO2/n-ZnO heterojunction diodes showing rectifying current-voltage characteristics were fabricated. The forward to reverse bias current ratio was estimated as ∼128 and ∼108 at ±1V and ±2V, respectively. Built-in voltages of p-CuCrO2/n-ZnO heterojunctions discerned from small signal capacitance measurement are 1.27 and 0.67V, attributed to the alignment of Fermi level and distribution of interface trap states, respectively.
0544: Electrical engineering
0794: Materials science