Experimental And Theoretical Research On Optical And Electrical Properties Of Alkali Or Alkaline-earth Metal Doped CuAlO₂

P-type transparent conducting oxides?TCOs?have been of great interest in the field of semiconductor materials at present.The p-type TCO thin film of high quality is the key to realize the fabrication of transparent oxide optoelectronic devices,such as transparent p-n junction diodes and transistors.Based on the theory of“chemical modulation of the valence band”,the delafossite CuAlO₂ is firstly discovered as a p-type TCO,which is a great breakthrough due to its essential weakening of strong localization of the hole carries around the oxygen ions within oxides.However,the electrical conductivity of CuAlO₂ is too low to be applied in optoelectronic devices.Therefore,improving electrical conductivity is an important issue for realizing CuAl O₂-based transparent optoelectronic devices.Doping is an available approach to improve the p-type conductivity of CuAlO₂.Up to now,a plenty of doping experiments have been reported for CuAlO₂,but its conductivity is still less than 20 S/cm.It is difficult to figure out which element is most appropriate to be doped in CuAlO₂ for those existing experimental reports for the qualities of their CuAlO₂ films prepared in different conditions are not uniform or even of great difference.Additionally,some conflicting conclusions are drawn from those reports,which implies that the reliability should be tested.It is adverse to the further research of CuAlO₂ if these issues remain unsolved.In recent two decades,benefiting from the development of calculational material science,significant progress has been made in first-principles calculations based on the density functional theory,which can simulate a wide variety of materials and output a relatively reasonable result for their properties such as crystal structure parameters,band structure and density of states.The formation energies of some point defects can be also derived from the first-principles calculations.The theoretical simulation greatly supports some experiments and can sometimes provide a profitable guidance for them,which will facilitate the experimental research to a large extent.In this present work,we did some research on the fabrication and doping issues in CuAl O₂ by combining the experiment with first-principles calculations and some progresses have been achieved as follows.1.The bulk Na-doped Cu AlO₂ was synthesized by conventional solid-state reaction.It was demonstrated that the Na-related band-tail emission at 3.08 eV in the photoluminescence spectra of Na-doped CuAlO₂ was caused by a donor-acceptor compensated complex of Na_Al-2Na_i by combining the experimental results and first-principles calculations.The Na_Al-2Na_i complex would introduce extra occupied bands above the valence band maximum?VBM?of CuAlO₂,equivalently pushing its VBM to higher energy,which is beneficial to reducing the ionization energy of acceptor.2.First-principles calculations based on density functional theory were performed to investigate the behavior of alkaline-earth metal M?M=Be,Mg,Ca?and/or N in CuAlO₂.It was found that Mg_Al shallow acceptor is easier to be formed than either of Be _Al and Ca_All due to its lowest formation energy in CuAlO₂.The nitrogen prefers to substitute O site rather than to occupy the interstitial site in CuAlO₂ but the p-type conductivity of CuAlO₂ can hardly be enhanced by N doping due to the high formation energy of N_O defect.In addition,the alkaline-earth metal M and N co-doping in CuAlO₂is not feasible to form dual-acceptor complex since the incorporation of M_Al with N_O is unstable.The substitution of alkaline-earth metal for Al sites in CuAlO₂ may cause the photon absorption in the visible light and thus degrades the transmittance of CuAlO₂.3.The delafossite Mg-doped CuAlO₂ thin films were deposited on the c-axis oriented single-crystal sapphire by RF magnetron sputtering with CuAl_1-xMg_xO₂ targets synthesized by solid-state reaction.Hall effect measurements indicate that the hole concentration of CuAlO₂ was greatly improved by Mg doping and the best room temperature hole density and conductivity are 1.79×10¹⁸ cm^-3 and 8.0×10^-2 S/cm for CuAl_0.94Mg_0.06O₂.Experimental results show that Mg dopant substitutes for Al site in CuAl O₂ to form Mg_Al shallow acceptor.4.The p-Cu-Al-Mg-O/n-ZnO heterojunction was successfully fabricated on sapphire by RF magnetron sputtering with the CuAl_0.94Mg_0.06O₂ and ZnO targets.Although the conductivity and transmittance of the amorphous Cu-Al-Mg-O layer in the heterojunction are both very low,the heterojunction shows a p-n junction rectifying characteristic.