Preparation And Properties Of P-doped P-type Zinc Oxide Thin Films

It is widely accepted that ZnO is one of the most promising materials for producing an ultraviolet laser at room temperature due to its wide direct band gap (Eg=3.37eV) and large excitonic binding energy of 60 meV, which was testified by the results of optically pumped stimulated emission and lasing from ZnO thin films. The growth temperature of ZnO is only a half value of that of GaN, which reduces the atomic diffusion between the film and substrate when grow in high temperature. Like GaN compound, the reasearch about ZnO has burned white hot in the range of wide band gap. Since the realization of stimulated emission of ZnO multicrystal films at room temperature by optical pumping in 1997, researches on ZnO and its related materials have become one of the most promising and attractive aspects in the optoelectronic region.It is well known that a few groups have reported the p-type ZnO is obtained,but the growth of reproducible p-type ZnO remains a big challenge. Otherwise,the low carrier concentrations is the most significant obstacle for ZnO-based optoelectronic devices practically. The key to obtain the high quality device is producible low resistivity and reproducible p-type ZnO.It is well known that a few groups have reported that the reliable p-type ZnO:P is obtained. This is because of among group-V dopant, in addition to N, the atom radius of phosphorus(P) has the small difference with O .The stability of p-type ZnO:P is well than N-doped, and then there is no phenomenon of p-type material changes back to n-type over time.The theory for large-size-mismatched impurities by first-principles calculations has been presented, which prove that the doping with other group V elements (P,As,Sb) could result in a deep acceptor level and hence p-type ZnO is perceived to be difficult to obtain. Contrary to the theoretical prediction, many experiments prove that they can form shallow acceptor and they are the promising p-type dopant. Phosphorus has been considered as a possible dopant for p-type. While by simple instrument of radio-frequency sputtering can produce high quality, reproducible and reliable p-type ZnO films. However, the p-type conduction in P-doped ZnO are under debate, someone think that the acceptor level of P dopant can be given by forming PZn-2VZn complex or Po.The study of ZnO is a hot and hard problem. To explore it, the details of the series studies have been exhibited successively. We focus on the fabrication, structures, and optoelectronic properties of p-type phosphorus-doped ZnO after annealing process. The films were prepared on quartz substrates by r.f. magnetron sputtering method using a 2 wt % P₂O₅ (99.99%) mixed ZnO (99.99%) target at 500℃in Ar/O₂ for P-doped. The process of fabricated the target: mixing the ZnO and the P₂O₅ uniformity, pressing under the 45 MPa for 10 minutes, putting it into the stove at 300℃for 5 hours, crushing it and then pressing the target , sintering at 1000℃for 12 hours .In this study , all the samples were obtained by this target.The system studies of the annealing method , sputtering gas ratio and annealing temperature influence on the electrical and optical of phosphorus-doped ZnO.The structural, optical and electrical properties of the ZnO thin films was systematically studied through X-ray diffraction (XRD), photoluminescence (PL) measurement, X-ray photoelectron spectroscopy (XPS) and Hall Effect measurement.The P-doped ZnO films were polycrystalline in nature with preferred (002) orientation. The as-grown samples is high resistivity, After grown, a series of annealing process has been employed. The greatly improved crystallization property of the ZnO:P films has been found during the annealing due to release of the residual stress. The electrical and optical properties were also improved and the p-type was realized after air RTA at 750℃. The concentration of intrinsic defects and acceptor was determined to be changeable while under different annealing process. The conductivity types would be changed after annealing. When sample is annealed in vacuum, a lot of P would diffuse from the film,impurity and defect decrease,the conductivity type is n-type. When the film was annealed at 750℃air RTA (rapid thermal annealing), P is activated and hole concentration is increasing , oxygen vacancies (VO) donor defects decrease dramatically, so the characteristic of sample is p-type. The investigation of XPS and EDX on n-and p-type samples reveal that the phosphorus has been doped into the ZnO film and the acceptor PZn-2VZn has been formed.The temperature dependent PL spectra of a p-type film was analysed.The AoX emission at 3.345eV was found at low temperature.The acceptor energy of the phosphorus dopant was estimated from the DAP transition at 3.282eV PL spectra of p-type ZnO:P. The shallow acceptor level was estimated to be located at 113–125 meV above the valence band . P seems to be induced shallow acceptor and an efficient p-type dopant .The effect of the Ar/ O₂ gas ratios on the film structure,optical and electrical properties have also been predicted. It reveals that the conductivity type of the films is changing with the increasing of the O₂ content in the growth process,from n-p complex to p-type, then come back to n-type. We found that the optimal ratio of the Ar and O₂ is 1 to 0.05 in this study. ZnO:P films grown under a pure Ar ambient showed the n- p complex types of characteristics , even after air RTA activation, due to the large number of native defects such as oxygen vacancies which act as donors and show n-type characteristics in ZnO films. However, ZnO:P films grown under an Ar/O₂ gas ratio from 1/3 to pure O₂ showed n-type conductivity after the annealing process. This n-type conductivity can be attributed to the formation of complex defects involving O₂-induced defects that acts as donor, which can compensate for the hole carriers generated by phosphorus dopants. When Ar/O₂=1/0.05, we can observe the obvious violet light, along with the O₂ content increasing, violet peak disappear gradually. The band gap become narrow due to the P content is decreasing in the sample.Great effect of the annealing temperature on the film quality and electrical properties have also been predicted. The appropriate annealing temperature exists in which air RTA can induce the conversion of ZnO:P characteristics.Those annealed temperature is low were dominated by zinc interstitials (Zni) and oxygen vacancies (VO) donor defects, the P acceptor was not activated which result in n-tpye ZnO.. With the higher annealing temperature, zinc interstitials (Zni) and oxygen vacancies (VO) donor defects decrease, and the increased P accepter defects will fight over the donor defects Zni and Vo, lead n-p mixed to p-type. When the temperature keeps going up, the concentrations of Vo will increase dramatically. The Vo together with the Zni exert a compensation for P acceptor and turn the sample back to n-type. This explained why the n-type at 450℃and 550℃, n-p mixed type at 650℃, p-type at750℃and n-type again at 850℃. When the annealing temperature is higher , compression stress in the lattice play important role .With increasing annealing temperature, the lattice constant c changed small and band gap became wide. The violet peak intensity in PL spectrum is increasing and FWHM became narrow, the crystal quality is improved.