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

It is widely accepted that ZnO is aâ…¡-â…¥semiconductor with a band gap energy (Eg)of 3.37eV and large excitonic binding energy of 60 meV at room temperature.It has great potential for applications in short-wavelength optoelectronics,light-emitting diodes,and lasers.Although high quality n-ZnO for device applications has been produced,it is well known that high resistivity,low carrier concentrations, low mobility and stablility of P-type ZnO remains a big challenge.Several research groups proposed a co-doping method where N and group III elements such as Ga and Al are used as dopants to obtain p-typeZnO.However,the process for p-type ZnO production was not reproducible.Recent first-principles calculations suggested that the solubility of group-I elements such as Li and Na would be greatly enhanced by hydrogenation, and a subsequent annealing process leads to the activation of passivated acceptors. 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 P has the smallest difference with O.first-principles calculations prove that the doping with 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-doped p-type ZnO has many problems,such as:high resistivity,low carrier concentrations,low mobility and the lack of understanding of phosphorus doping mechanism.In this work, p-ZnO thin films were prepared by sputtering a ZnO target doped with P2O5 at high temperatures followed by a thermal annealing process. Combining X-ray diffraction (XRD), photoluminescence (PL) measurement, X-ray photoelectron spectroscopy (XPS),EDX,room-temperature Hall effect and low-temperature photoluminescence,the nature of phosphorus doping is explored.We study the structures,optoelectronic properties and mechanism 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 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.We study the influence of annealing atmosphere,annealing temperature and buffer on the structural,electrical and optical of phosphorus-doped ZnO.In this study ,we found:the as-grown samples is high resistivity, After grown, a series of annealing process has been employed. The crystallization, electrical and optical properties of the ZnO:P films has been improved after the annealing due to release of the residual stress. Annealing atmosphere and temperature have significant influence on the concentration of intrinsic defects, acceptor and conductivity types.When the film without buffer layer was annealed at 800OC air RTA (rapid thermal annealing),due to the large number of native defects such as oxygen vacancies which act as donors,so show n-type characteristics in ZnO films.When the film was annealed at 850OC N2 RTA, acceptor defects increase gradually and compensate donor defects partly, so the conductivity type is n-p type. When the film was annealed at 800OC Ar RTA, P is activated and released O compensate large number of oxygen vacancies,hole concentration can compensate electron concentration effectively. So the conductivity type of sample is p-type.When the film with buffer layer was annealed at 750OC N2 RTA (B),lattice constants of c-axes is larger than standard ZnO.This is attributed to P substitutes at an O site.But the conductivity type is n-type.It is because that PO is a deep acceptor ,can not compensate donor defects effectively,result in donor are dominative(VO),so the conductivity type is n-type. When the film was annealed at 800OC N2 RTA(C), acceptor defects increase gradually and compensate donor defects partly, so the conductivity type is n-p type.sample (B)and (C)were annealed at the same Annealing atmosphere and different Annealing temperature result in different conductivity type.this attributes to Annealing temperature having significant influence on conductivity type .When the film was annealed at 800OC air RTA(D), P acceptor is activated,and P-O is opened up adequately,so released O compensate large numbers of oxygen vacancies (Vo).hole concentration can compensate electron concentration effectively. So the conductivity type of sample D is p-type.sample (C)and (D)were annealed at the same Annealing temperature and different Annealing atmosphere result in different conductivity type .this attributes to Annealing atmosphere having significant influence on conductivity type.The temperature dependent PL spectra of a p-type film without buffer layer was analysed.The AOX emission at 3.326eV was found at low temperature.The acceptor energy of the phosphorus dopant was estimated from the free electron to acceptor lever transition(FA) at 3.306eV PL spectra of p-type ZnO:P. The shallow acceptor level was estimated to be located at 131 meV above the valence band . The temperature dependent PL spectra of a p-type film with buffer was analysed.The AOX emission at 3.339eV was found at low temperature.The acceptor energy of the phosphorus dopant was estimated from the DAP transition at 3.268eV PL spectra of p-type ZnO:P. The shallow acceptor level was estimated to be located at 127meV above the valence band.127meV has little difference from the former 131meV,may be due to two factors.On the one hand,the buffer can reduce lattice mismatch and heat mismatch between substrate and film,and reduce the residual stress,so the top of valence band move up resulting in reducing the distance between the top of valence band and acceptor level.On the other hand,may be due to the difference of calculations method.Either 127meV or 131meV indicates Phosphorus is a promising p-type dopant.The investigation of PL,XPSå'ŒXRD on p-type samples reveal the phosphorus doping mechanism is that Phosphorus substitute Zn of the ZnO film and form shallow acceptor complex PZn-2VZn.we also found buffer layer can reduce lattice mismatch and heat mismatch between substrate and film by comparation. Quality of ZnO film is improved consequently.