Synthesis And Application Properties Of CdO-based Nano Structure Thin Films

Cadmium Oxide (CdO) is n type and II-VI semiconductor oxide with a directand narrow band gap. At room temperature, it has a cubic NaCl crystal structure. CdOfilm has a lot of intrinsic point defects, such as oxygen vacancies (Vo) and interstitialcadmium atoms (Cdi), they act as shallow donors and provide a lot of carriers, theymake the undoped CdO film has lower resistivity (10-3~10-4Ω cm). CdO has a bandgap of about2.2eV, and it is near the maximum energy of solar spectrum in thevisible spectrum. Therefore, CdO has many unique properties, such as high carrierconcentration, low resisitivity, and high transmittance in visible range and it is widelyused as a transparent conducting electrode, solar cells, phototransistor and gas sensor,etc. However, the optical band gap of CdO is relatively small to strictly limit theadhibition of photoelectric devices. In order to compete with other TCOs, on the basisof improving the conductivity, we increase the transmittance in visible range and theoptical band gap by doping with other elements or adjusting the technologicalparameters.This thesis systematically sums up the optical and electrical properties and gassensitivities of CdO thin film, and introduces the various application developmentsand preparation methods in detail, and provides an outlook of research directions. TheCdO-based films were prepared by the radio frequency magnetron sputtering (RFMS)and pulsed laser deposition (PLD) methods. The good electrical conductivity and hightransmittance in visible range of CdO thin films can be obtained by adjusting theproportion of intrinsic point defects or doping transition elements and rare earthelements, such as gadolinium (Gd), cerium (Ce), and indium (In). In addition, theCdO thin films prepared with Gd and In were deposited on p-type silicon (p-Si)substrates, the p-n heterojunctions were formed. And the probability of the CdO thin film acting as the solar thin-flim cells was investigated. The periodic nano stripestructure on the surface of CdO thin film was induced by the single-beamfemtosecond laser technology, and this film was used to prepare the CdO/p-Siheterojunction diode. We hope these studies can extend the applications inoptoelectronic devices and solar cells for the CdO nano materials. The structure,composition and microstructure of CdO-based thin films were characterized byX-radial Diffractometer (XRD), X-ray photoelectron emission microscope (XPS),transmission electron microscopy (TEM), High Resolution Transmission ElectronMicroscopy (HRTEM), field emission scanning electron microscope (FE-SEM) andEnergy Disperse Spectroscopy (EDS), the optical and electrical properties werecharacterized by Ultraviolet-visible (UV-VIS)Absorption Spectroscopy and HallEffect measurement, and the electrical and photovoltaic propersities of heterojunctiondiodes were characterized by the Current-voltage (I-V) characteristic curves. Themain research contents and results were summarized as follows:1. The undoped cadmium oxide (CdO) thin films were deposited on glasssubstrates by PLD method with different laser energy density (10~25J/cm2) at a fixedoxygen pressure of10Pa. The experiment temperature was25℃. The targes weremade with the CdO powder, and its purity is99.99%. The microstructures, opticaland electrical performances of the CdO thin films were studied. Results show that allthe films were polycrystalline with a cubic NaCl crystal structure, and they highlygrew along [200] preferred orientation. With the decrease in the laser energy density,the average grain size significantly decreases from108.3nm to21.1nm. The decreasein the grain size leads to the interior stress increasing and the diffusion rates of Cd andO atom changing, and the oxygen vacancies (Vo) increase in the films. These defectlevels can increase the concentrations of free electron. The film with smallest grainsize has high optical transmittance (~92%) in the visible region, the significantlywidened band gap (3.33eV), and the highest carrier concentration (1.25×1021cm-3)and lowest resistivity (2.8×10-4cm). The results show that the CdO film withsmaller grain size has extensive application prospects in solar cells, transparentconducting electrode and other optoelectronic equipment. 2. The cerium (Ce) and gadolinium (Gd) co-doped cadmium oxide thin filmswith different Ce and Gd concentrations were deposited on glass substrates by RFMSmethod at400°C. The total pressure was10Pa, and the gas flow of argon (Ar) andoxygen (O2) was40sccm and10sccm, respectively. The ceramic target material wasprepared by the powder mixture of CdO, CeO2and Gd2O3, the purity of all the poweris99.99%. The microstructures, optical and electrical properties of the CdO filmsprepared were studied at different Ce and Gd concentrations. The doping of Ce andGd can promote the growth of the films along [200] preferred orientation, the (200)diffraction angle slightly shifts towards lower values and its lattice constants increaseas the Cd content increasing. When the concentrations of Ce and Gd reach to0.4at%and0.8at.%, respectively, the as-doposited CdO film has the best optical andelectrical properties. It is highly transparent (~85%) in the visible region, the opticalband gap was2.99eV and lowest resistivity was3.3×10-4Ωcm. However, with afurther increase of Ce and Gd concentrations to0.5at.%and1.0at.%, respectively,the enhanced grain boundary scattering and ionization impurity scattering effects canlead to the degradation of these optical and electrical performances. Therefore, theappropriate doping of Ce and Gd can evidently improve the optical and electricalproperties of CdO, and endow that the as-deposited CdO thin films have importantapplications in transparent conducting materials.3. The gadolinium (Gd) and indium (In) co-doped cadmium oxide thin filmswith different Gd and In concentrations were deposited on quartz glass substrates byPLD method at200°C. The fixed oxygen pressure was10Pa. The ceramic targetmaterial was prepared by the powder mixture of CdO, In2O3and Gd2O3, the purity ofall power is99.99%. XRD analysis confirms that all the films were cubic NaClstructure, and all the films grew along (200) preferred orientation, and XPS analysisindicates that the doping of In induces the formation of In2O3. With the increase inconcentrations of Gd and In, the grain size gradually decreases from40nm to23nm.All the films have high transmittance (~85%) in visible range. When theconcentrations of Gd and In reach to2at%and4at.%, respectively, the as-depositedCdO thin film shows high transmittance (92%) in visible range and widened band gap (3.56eV). Under the same experimental conditions, these thin films weredeposited on p-Si substrates, and the CdO/p-Si thin film heterojunctions were formed.The Current-Voltage (I-V) characteristic curves show that all the heterojunctions werediodes and have obvious photovoltaic effects in illumination. The CdO thin filmdoped with3at.%Gd and6at.%In shows best photovoltaic properties. Thephotoelectric conversion efficiency reaches7.5%, the fill factor is63%, the Amaximum saturation current density (Jsc) and open circuit voltage (Voc) are11.4mA/cm2and1.04V, respectively. These results show that the CdO film doped withGd and In has potential applications in solar thin-film cells.4. The undoped cadmium oxide thin films were deposited on glass substrates byPLD method at25℃with the laser energy density of10J/cm2. The fixed oxygenpressure was10Pa. The prepared films were put in the muffle furnace for annealingtreatment at200℃for30min. The treated CdO films were ablated by thesingle-beam femtosecond laser with different pulse repetition frequency to induce theperiodic nanometer stripe structure. The central wavelength of exported laser pulsewas800nm, the pulse width was100fs, and the adjustable repetition frequency was1~2500Hz, single pulse energy was2mJ. The FE-SEM images show that the CdOfilm was irradiated by2000pulses, its surface appears short periodic stripemicrostructure, stripe cycle is about150~170nm. When the pulse numbers weregreater than2000pluses, fluffy nano foam microstructure can be seen on the surfaceof CdO thin films. UV-VIS measurement with an integrating sphere shows that theseCdO films have the broadband and high absorption properties. And Hall Effect testsshow that the CdO thin films ablated over2000pulses have the highest carrierconcentration of1.9×1021cm-3and the lowest resistivity was1.3×10-5Ω cm. On thisbasis, the CdO film with femtosecond laser micro structure was uses to prepare theCdO/p-Si heterojunction. The Current-Voltage (I-V) characteristic curves show goodrectification characteristics under dark and in illumination, respectively. The I-Vcharacteristic curves in illumination show photovoltaic characteristics, the maximumopen circuit voltage (Voc) is0.55V, the short circuit current (Jsc) is6.8mA/cm2. Thefill factor is29.1%, and the conversion efficiency is1.1%. The broadband and high absorption properties for the femtosecond laser micro structure make these thin filmshave potential application prospects in many optoelectronic devices, such asphotovoltaic devices, charge-couple device and optical detector.