The Preparation And Ohmic Contact Characteristics Of AZO Thin Film On P-type GaN

GaN is attractive material for optoelectronic devices,such as, light emit diode, laser diode. Nowadays, blue light-emitting diodes (LEDs) based on GaN-based material usually epitaxially grow on insulative sapphire substrate, and conventionally Ni/Au metal are deposited on p-GaN as the electrode. However, poor optical transmittance of thick Ni/Au metal electrode will decrease the light extraction efficiency of LED, thin semitransparent Ni/Au layer will influence on uniform current spreading and thermal stability. Therefore, how to obtain ohmic contact on p-GaN with low contact resistance and high transmittance, which has became a hot research topic of researcher. In order to improve light efficiency of LED and uniform current spreading, instead of changing package structure and optical design, one way is to apply the transparent conductive film as the contact electrode on p-GaN. In this thesis, the latest progress of transparent electrode was presented, and the low-cost Al-doped ZnO transparent conductive film deposited on p-GaN was proposed, and studied on the contact characteristics.In this paper, Al doped ZnO thin film was prepared by using DC reactive magnetron sputtering on p-GaN substrate, the structure, electrical properties of the AZO/p-GaN have been investigated by AFM, XRD and Hall Measurement. Besides, the optical of the AZO thin film deposited on glass substrate was analyzed by double-light ultraviolet and visible photometer. Because AZO thin film directly deposited on p-GaN, which was difficult to achieve ohmic contact, instead of it, Ni/AZO scheme was proposed. The Ni and AZO thin film were deposited on p-GaN by using electron beam evaporation and dc reactive magnetron sputtering, respectively, which were applied as transparent electrode, then circular (c-TLM) contact patterns formed on the GaN using standard lithography and wet etching process, and the I-V characteristics of Ni/AZO contacts have been investigatedThe results indicate that AZO thin films on p-GaN substrate displayed a high preferential (0002) orientation; the surface of the annealed samples becomes rougher; the annealing treatment resulted in the increase of the electrical conductivity of the thin films, the highest carrier concentration is 6.16×10²⁰cm^-3 and lowest resistivity is 9.07×10^-4Ω.cm at 573K. Besides, the optical transmittance of AZO thin films on glass substrate are higher than 80% in the visible region, for the annealed AZO film, the optical transmittance increases and the optical band gap becomes wide. In addition, the optical band gap of AZO thin film was calculated by Swanepoel method and mechanism was analyzed. Owing to the excellent electrical and opitcal properties of AZO thin film, it can be a promising material as transparent electrode in the GaN-based optoelectronic devices. Therefore, the I-V characteristics of Ni/AZO contacts on p-GaN have been investigated, It was found that a linear behavior of the I-V characteristics can be obtained after rapid thermal annealed at 400℃and 500℃in air, which indicates the formation of an ohmic contact a further increase in the annealing temperature deteriorated the contact resistivity, the I-V curves were rectified in the sample annealed at 550℃, the specific contact resistances (ρ_c) is 2.2927×10^-2Ω.cm² after annealed at 400℃.Although the Ni/AZO contact to p-GaN resulted in ohmic behavior after annealing, the optical characterization of these films was another important issue, the transmittance spectra against wavelength for the Ni/AZO samples annealed at various annealed temperature under air conditions, the light transmission was lower than 50% for the pre-annealed sample, and increased to 60⁷0% at 400⁶00nm for the sample annealed at 400℃in air.