Study On The Ohmic Contact To Zno Film

ZnO semiconductor has a band gap of3.37eV and exciton binding energy of60meV. ZnO has good thermal stability and chemical stability in the air ambient, these properties make it very popular in the fabrication of light-emitting diodes, laser diodes and solar cells and other optoelectronic devices. In order to make the application in the device better, the ohmic contact with low resistance between the electrode and p-type and n-type ZnO films needs to be studied and prepared. If there is a large contact resistance between the electrode and the semiconductor film, the performance of devices would be significantly decreased due to heat or poor contact. Therefore, ohmic contact with low resistance and thermal stability is very critical. In addition, for the ZnO-based light-emitting devices, improving the efficiency of the light transmission of the electrode can make the optical output efficiency of the device higher. When using a metal electrode, if it is too thick, it will lead to the reduction of the transmittance; if it is too thin, its thermal stability and reliability get worse, its current diffusion becomes non-uniform, which leads to decreased device lifetime. ITO material has a very good performance in the light transmission rate, it is suitable material for n-ZnO electrode with concerns of the conductivity, the substrate binding energy, hardness and chemical stability.This paper studies the impact of different annealing temperatures on the ITO electrode fabricated by radio-frequency magnetron sputtering. TheI-V curves were measured to study the electrical properties of the contact, the contact resistivity was calculated with the circular transmission line method (CTLM), the surface morphology of the ITO electrode was observed with the atomic force microscope (AFM), the light transmittance spectrum of ITO annealed at different temperatures was measured with UV spectrophotometer. The lowest contact resistivity,6.0×10-3Ocm2, was obtained when annealed at300°C under the protection of high purity N2gas. AFM test results showed that the ITO electrode annealed at300°C had the best surface morphology, in addition, the light transmittance test results showed the efficiency of light transmittance through ITO electrodes was up to92%near wavelength corresponding to the band gap of ZnO.Ohmic contacts to n-ZnO:Al films were also investigated, firstly Al electrode was prepared, and then the cover layer of Au was introduced, the double-layer electrode of Al/Au electrode was prepared. The resistivity of as-deposited contact was8.68x10-3Ocm2. The resistivity of contact annealed at200°C was6.68×10-3Ocm2. The resistivity of contact annealed at300°C was8.03×10-4Ocm2, the ohmic contact annealed at400°C degraded. The I-V curve was rectified. On contrast, the I-ν curve of Al/Au electrode annealed at up to500°C remains a straight line, but the contact resistivity increases with the elevated annealing temperatures. The as-deposited contact resistivity was2.5x10-3Ocm2. The lowest contact resistivity3×1O-4Ocm2was obtained after annealing at200°C. The surface morphology of Al/Au electrode annealed at500°C degraded severely.