Of Nio Based P-type Transparent Conductive Oxide Film Diode

The p-type transparent conducting oxide (TCO) thin films have been widely researched in recent years. However, until now, the properties of reported p-type TCO films can't meet the commercial applications yet. The condution mechanism, synthetization method of the materials and device fabrication need to be further studied. In this paper, we study Cu-doped and K-doped NiO p-type transparent conducting films by a newly pulsed plasma deposition (PPD) method, and further the diodes composed of the p-type NiO-based thin films. NiO is a kind of typical wide band gap p-type semiconductor. The conductivity of NiO could be increased by doping with appropriate elements. PPD is a kind of deposition technique based on the ablation of the target. The study on p-type doped NiO films by PPD method is a meaningful work for exploring newly p-type TCO materials systems and preparation methods. Main experimental results and conclusions are as follows:The energy level of Cu 3d10 is comparable to that of O 2p6 and the formation of covalent bonding is possible, which could lead to large dispersion in the valence band and reduction in localization of positive holes. The study on Cu-doped NiO transparent p-type conducting films was first performed. The Ni1-xCuxO(x=0～0.9) thin films were prepared by PPD method. The conductivity will be improved with high content of dopant while the transparency will be deteriorated. The conduction mechanism of NiO was mainly ascribed to the existence of Ni3+. Ni3+ could be formed by the excess oxygen in films. The conductivity would be decreased due to the decreased amount of Ni3+, which is mainly caused by low depositon temperature and working pressure. The Ni0.9Cu0.1O film exhibits the highest conductivity of 5.17 S cm-1, with an average transmittance of 60% in the visible region (400～700 nm).K-doped NiO transparent p-type conducting films was studied. The Nii-xKxO (x =0～0.4) films were prepared by PPD method. There was no impurity peaks detected in films. With the deposition temperature increased, the diffraction peaks would become stronger, which indicate the better crystalline structure. The conductivty of the films were promoted due to the K doping. The transparency was improved with the increase of deposition temperature while the conductivity was deteriorated due to the decrease amount of Ni3+. Both the electrical and optical properties of the films would be improved if deopsited in a high oxygen atmosphere. The Ni0.75K0.25O film deposited at room temperature exhibits the highest conductivity of 4.25 S cm-1, with an average transmittance of～60% in the visible region.First principles based density functional theory have been performaned for Cu-doped and K-doped NiO supercells, respectively. The calculation results showed that both the two doped elements make the fermi level of NiO into the valence band maxium (VBM), which was a typical characteristic of the p-type semiconductors. Besides, the doped elements didn't introduce the deep level in the gap. The VBM of Cu-doped NiO was mainly composed of Ni 3d, Cu 3d and O 2p states, respectively, while the conduction band minimun (CBM) consisted mainly of Ni 4s states; The VBM of K-doped NiO was mainly composed of Ni 3d, K 2p and O 2p states, respectively. The impurity level introduced by the K doping contributed less to the states of density near the fermi level in contrast to Cu doping. The 3d level introduced by Cu doping were hybridized with the energy levels from VBM. The formed hybridized orbits would weaken the localization of O 2p level. The results indicate that Cu doping have a better effect on the increase of p-type conductivity.Transparent oxide hetero-junction of P-Ni0.9Cu0.1O/n-IWO was investigated. Tungsten doped indium oxide (IWO) is a newly kind of TCO material with low resistivity, high mobility and high transparency in both visible and near infrared region (700～2500nm). IWO films have potential applications in solar cells and near infrared sensors. The resistivity of IWO film could be controlled to match the p-Ni0.9Cu0.1O layer by adjusting the oxygen partial pressure during the sputtering process, with an average transmittance of 85% in visible region and energy band gap of 3.65 eV. The diode with the structure of IWO/n-IWO/p-Ni0.9Cu0.1O/Al was fabricated by dc magnetron sputtering, PPD method and thermal evaporation. Current-voltage (Ⅰ-Ⅴ) curve of the diode exhibits nonlinear and rectifying characteristics. The ratio of forward current to the reverse current is about 90. The ohmic nature of the contacts between electrode layer and semiconductive layer was confirmed. The equilibrium energy band diagram of the pn heterojunction was given. The value of the built-in voltage calculated theoretically is approximately identical with the threshold voltage. The results give the reference for the next step of fabrication of amorphous transparent all oxide film diode at room temperature.Amorphous tranparent conducting oxides are highly favorable for applications in flexible displays because they have many inherent advantages such as robust properties with regard to lattice mismatch and low temperature deposition on large substrates. The optoelectronic devices composed of such materials should be fabricated to enlarge the applications in the field of flexible electronics. In this paper, amorphous transparent all oxide film diode with structure ofα-IWO/n-IZO/p-Ni0.9Cu0.1O/α-IWO was fabricated at room temperature by dc magnetron sputtering and PPD technique. Each layer was studied respectively to obtain the optimal deposition condition. IWO film as electrode layer showed a resistivity of 5.75×10-4Ω. cm, with an average transmittance over 80% in both visible and near infrared region. Indium zinc oxide (IZO) was used as n-type semiconductive layer. The resistivity of IZO films could be adjusted by the oxygen partial pressure during deposition. Each layer of the diode showed amorphous structure and smooth surface. Diode composed of p-Ni0.9Cu0.1O and n-IZO layers with matched properties obtained the better nonlinear and rectifyingⅠ-Ⅴcharacteristics, with a ratio of 40 of forward current to the reverse current and an average transmittance of～50% in visible region. The results indicate a promosing application in flexible transparent electronic devices.