Research For The Growth And The ECV Of ZnO Thin Films

ZnO is a broadband gap semiconductor after GaN.Its large exciton binding energy is about 60 meV. Large exciton can be used to fabricate many useful devices, such as emitting diode, lasers and high-speed optical switch. Many methods have been used to deposit ZnO films such as Molecule Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD), Pulsed laser Deposition (PLD),Sputtering, etc. Here ZnO films are grown by MOCVD.It was found that the flow rate of DEZn strongly influenced on the structural properties and surface morphology of ZnO films. The film grown at 3SCCM had only one intense (002) diffraction, and had the biggest grain size. Increasing the flow rate of DEZn, the intensity of (002) and other diffraction faces was decreased, when it reaches 9SCCM, the crystalline quality of the ZnO films was degraded to poly crystalline. Considering thermodynamics, when the flow rate of DEZn is above 3SCCM, the growth is under nonequilibrium reaction condition, so it is polycrystalline.Previous researches had established that ZnO was always n-type, because it was also known that the crystal growth was typically under Zn-rich condition. The dominant donor was almost always identified as either the O vacancy VO or the Zn interstitial ZnI. This model was strongly challenged in the year 2000 when Kohan et al. showed theoretically that both VO and ZnI had high formation energies in n-type ZnO and that furthermore, both wewe deep, not shallow donors. Also in the year 2000, the defect-donor model was further challenged by Van de Walle's theoretical result that H was always a donor in ZnO, that it was easily ionized, and that it had a low enough formation energy to be abundant. However, no matter which model is dominant in ZnO films, the self-compensation of the donor is the key point to prevent the formation of the p-type conductivity. Now, high quality p-type ZnO films have formed in many groups, but the state of the acceptor impurity has not been conformed. Take As atoms for example, some persons suggested that the Aso impurity was the reason of the p-type conductivity in ZnO films, then on the country, other persons suggested that the compound AsZn-2VZn impurity was the dominant acceptor. So making sure the state of the acceptor-like dopant is important.High-quality p-type ZnO films are still difficult to obtain, and the measurement of p-type ZnO films is also essential. For Hall measurements, when ZnO film is measured in the dark,it can be p-type, but it can be a n-type after several minutes in light measurement, it is because electrons in the valence band were excitated to the conduction on the light, and the equivalent of holes were produced in the valence band. As identified in the Hall measurement, the conductivity type of samples is based on the p-type relationship: pμ_p~2 > nμ_n~2Where p,n,μpandμn are the hole concentration, electronic concentration, hole mobility and e-mobility respectively. It is usually thatμn≥μp. For the weak p-type sample, whenΔp-hole andΔn electron were increased at the same time, the majority carriers of the samples were still holes. But the measurement of the Hall suggested it was n-type. Hall may have inaccurate factors ,so the electric properties of the ZnO films were measured by the electrochemical capacitance–voltage profiling(ECV). In this paper we use ECV method to test the GaAs sample with electrolyte of Tiron. We get the good figure of the carrier concentration profiling, intuitively reflecting the homogeneity inside the sample. Then we use ECV method to measure ZnO film. During measuring, because the front contact is too big, we choose the way to make electrode on ZnO film to resolve. Choosing nickel,aluminium,aurum- zinc,etc. to make experiment with electrode, finally choosing aluminium to make pole can get good results. And the electrolyte was chosen among the reagent of acetic acid,NaOH-EDTA,sulfuric acid,Tiron,ZnCl2, etc. Confirming the ZnCl2 is the electrolyte we want. we use ECV to get the result of concentration of carriers in difference depth of the ZnO film.