Epitaxial Growth Of NdBCO Superconducting Films By Laser CVD

NdBCO superconducting films possess not only high critical transition temperature?Tc?and high critical current density?Jc?under strong magnetic field,but also better crystal quality and surface stability than YBCO films which are widely studied at present.Therefore,NdBCO films have great application potential in the fields of superconducting power technology,superconducting magnet technology,junction devices,microwave applications and photon detection.In this paper,c-axis-oriented NdBCO films were firstly prepared on LAO?100?by self-designed MOCVD system using Nd?DPM?₃,Ba?DPM/TMOD?₂,Cu?DPM?₂as precursors,Ar as carrier gas and O₂ as reaction gas.Based on the previous exploration on MOCVD,laser CVD was adopted to enhance the deposition process by introducing laser into the system,enabling the fast fabrication of c-axis-oriented NdBCO films.The effects of process parameters on the phase,growth orientation,micro morphology,deposition rate(R_dep)and superconductivity?T_c?were investigated.The effects of precursor volatile ratios,deposition temperature(T_dep),deposition pressure(P_tot)and deposition distance?d?on the phase,orientation,morphology,crystal quality and deposition rate(R_dep)of NdBCO films were investigated in MOCVD.The results showed:The films composition close to 1:2:3 standard stoichiometric ratio can be achieved by setting the volatile ratio of the Nd source,Ba source,and Cu source precursor to 1:3.5-4.5:1.5;With T_dep increasing,the orientation was sequentially changed from a,c-axis to c-axis,then back to a,c-axis.When T_depep was between 1050-1080 ^oC,NdBCO films were c-axis-oriented epitaxial with the in-plane arrangement of the a-and b-axes of NdBCO grains parallel to the a-and b-axes of LAO;The increase of P_tot would reduce the R_dep of NdBCO films and cause the orientation changed from c-axis to a,c-axis and then to a,c-axis mixed with other orientations;On the one hand,increasing deposition distance would rapidly decline R_dep,but on the other hand,the crystal quality and the epitaxy of the films would be improved appropriately.However,among the NdBCO epitaxial thick films with the thickness exceeding 1.3?m prepared by MOCVD,two different cross-sectional structures were commonly found near the substrate end and the surface end,and the quality of the films were poor.Based on the previous MOCVD investigation,the effects of deposition temperature,oxygen partial pressure and thickness on the phase,orientation,morphology,deposition rate,superconductivity and strain relaxation of NdBCO films were investigated in laser CVD.The results showed:With increasing T_dep,the trend of film orientation change was similar to that in MOCVD,which was changed from a,c-axis to c-axis then to a,c-axis.Highly c-axis-oriented NdBCO films with the T_c of87 K can be prepared at 760°C,with the in-plane arrangement of NdBCO[100]//LAO[010]and NdBCO[010]//LAO[001];Low oxygen partial pressure was favorable for c-axis-oriented epitaxial growth,but excessive reduction of oxygen partial pressure would increase the surface roughness of the film.In addition,it was easy to form a-axis-oriented grains in the film under high oxygen partial pressure;As the film thickness increased,the growth orientation would change from the c-axis to a,c-axis with the formation of the second phase Nd₄Cu₂O₇.Meanwhile,the compressive strain of the ab plane of the NdBCO film and the tensile strain along the c-axis were released;Compared with MOCVD,the R_dep of laser CVD has been greatly enhanced,and the highest Rdep was 62.95?m/h.More importantly,the film prepared by laser CVD still maintained a dense and uniform cross-sectional structure even at the thickness of 2.9?m,indicating the great advantage of laser CVD in the commercial preparation of ReBCO thick films.