Investigation On Electrical, Optical Properties And Device Physics Of Mn-Co-Ni-O Thin Films

Mn-Co-Ni-O spinel transition metal oxides are excellent NTCR (negativetemperature coefficient of resistance) materials, which have broad application andgreat promise in many fields, including uncooled infrared detectors, thermal sensors,rectifiers, magneto-electric coupling devices, and so on.In recent years, researchers have made a great progress in synthesizingMn-Co-Ni-O thin films by a novel chemical solution deposition (CSD) method,enabling the application of integrated Mn-Co-Ni-O devices and promoting the basicresearch works of the optical, electrical and magnetic properties of Mn-Co-Ni-O thinfilms. Radio frequency (RF) sputtering deposition method is an important synthesismethod. Comparing to CSD method, RF sputtering method exhibits advantages ofhigh production efficiency, precise control in growth parameters and compatible withmicroelectronics applications. However, most Mn-Co-Ni-O thin films obtained by RFsputtering method show inferior quality to thin films obtained by CSD method andsintering materials. Therefore, it is necessary to develop high quality Mn-Co-Ni-Othin films by using RF sputtering method. In this paper, we prepare high qualityMn-Co-Ni-O thin films by RF sputtering method, and we investigate the electrical,optical properties of the films, as well as the device physics of the infrared detectors.The detailed research works can be summarized as follows:1. Preparation of highly (220) oriented Mn1.4Co1.0Ni0.6O4(MCN) thin films:We prepare Mn1.4Co1.0Ni0.6O4(MCN) thin films on amorphous Al2O3substrateby using RF deposition method and annealed the films in air afterwards. We find thatthe films show highly (220) preferred orientation after annealing at750°C, and theintensity of the (220) peak enhances with growing post annealing time. Thedependence between the origin of preferred orientation and sputtering condition isdiscussed.2. Electrical properties of Mn1.4Co1.0Ni0.6O4(MCN) thin films:We compare the morphology, microstructure, and electrical properties ofMn1.4Co1.0Ni0.6O4(MCN) thin films grown at80W,100W, and120W. The resistivity of as-grown MCN thin films at295K are at the range of930-1100Ω cm, the smallpolaron hopping factors (p factors) of the films are0.23,0.47and0.36, respectively.The RT resistivity of each sample decreases by70%after annealing at750°C,90min, and the activation energy decreases by about6%. The reproducibility of the filmresistivity is improved after the post annealing process, and all p factors are about0.5,well matched with variable range hopping (VRH) model. The electrical properties ofMCN films with different post annealing processes are investigated, and it is foundthat the resistivity of the films are about220Ω cm and the TCR coefficients are about-3.7%K-1@300K when the post annealing time exceeds18minutes.3. Optical properties of Mn1.4Co1.0Ni0.6O4(MCN) thin films:X-ray photoelectron spectroscopic (XPS) investigations are made for as-grownand post annealed Mn1.4Co1.0Ni0.6O4thin films. The results indicate that the oxygenstoichiometry has been improved after post annealing process, and Mn4+to Mn3+ratioincreases by43%. Spectroscopic ellipsometry is used to investigate the opticalproperties in the infrared and UV-VIS-NIR wavelength range. The average effectivecharge of the film increases from1.84to2.23after the post annealing process,supporting the results from XPS spectra. By using double Lorentz plus Tauc-Lorentzmodel, we obtained the optical constants in the range of300-1000nm. The opticalabsorption structure at1.7eV is probably attributed to the charge transfer (CT)transition from O2-(2p) to Mn4+(3d). The2.4-2.6eV structure is probably owing tothe CT transition from O2-(2p) to Co3+(3d). The3.5-4.0eV structure is probablyowing to the CT transition from O2-(2p) to Mn3+(3d).4. Excess noise of and device performance of MCN thin film infrared detectors:After proper post annealing process in air, the excess noise of MCN thin filmsdecreases. After post annealing for18to90minutes, the values of normalized noisepower γ/n are found to be on the order of (2~6)×10-21cm3. MCN thin film infrareddetectors are made to investigate the responsivity (RV) and detectivity (D*) as afunction of frequency. At295K and±10V bias, the responsivity is about9.5V/W@10Hz, and the detectivity is about0.21×107cm·Hz0.5/W@10Hz. Byimproving bolometer design by using micromachining technique, the performance of the detector is estimated to be RV~1.8×104V/W, D*~0.63×109cm Hz0.5/W@10Hz.5. Fabrication and performance of Mn-Co-Ni-O thin film infrared detectors:By using a composite etching technique that involving both dry etching and wetetching techniques, we fabricate Mn-Co-Ni-O thin film infrared detectors. Theimmersed detector is fabricated by using a double-faced masking technique. Theresponsivity of the detector is4.4103V/W@10Hz，and the detectivity is about5108cm Hz0.5/W@10Hz at room temperature. The time constant is18ms.Mn1.56Co0.96-xNi0.48CuxO4(x=0,0.08,0.16,0.24) thin film detectors are fabricated.The typical time constants of the detectors are about20to40ms. The detectivity ofthe detectors are3.6×106,4.3×106,5.7×106and8.3×106cm·Hz0.5W-1@11Hz (x=0,0.08,0.16,0.24). The electrical drift of the detector increases with the increment ofCu ratio, while proper post annealing process promotes the stability of the resistance.