Mn Doping Of Ga <sub> 2 </ Sub> O <sub> 3 </ Sub> Thin Films, Structural And Optical Properties

Gallium oxide(Ga₂O₃) is a wide bandgap semiconductor with the optical bandgap between 4.2 and 4.9 eV.Much attention has long been attracted by its excellent conductive and luminescence properties.Gallium oxide is known to have five polymorphs includingα-Ga₂O₃,β-Ga₂O₃,γ-Ga₂O₃,ε-Ga₂O₃ andδ-Ga₂O₃.The monoclinicβ-Ga₂O₃ is the most stable structure at room temperature.It can be widely applied to diluted magnetic semiconductors(DMS),phosphor materials,gas sensors, antireflective coatings of GaAs solar cells,deep ultroviolet transparent conductive oxides, ammoniation for preparing high quality GaN epitaxial thin films,etc.As one of the widest bandgap transparent conductive semiconductors,it has a transparency of more than 80%from visible to ultraviolet region.Therefore,it will be potentially used as the electrode of ultraviolet photoelectric devices and ultraviolet detecting materials.Theoretical calculations indicate that Mn-doping to Ga₂O₃ can bring the generation of extra energy level above the valence band of Ga₂O₃.Thus,the bandgap of Mn-doped Ga₂O₃ film is narrowed.The bandgap can be modulated according to the quantities of Mn dopant.Therefore the Mn-doped Ga₂O₃ is expected to be applied in tunable ultraviolet photoelectric detectors and ultraviolet filters corresponding to different wavelengths.In addition,Mn-doping to Ga₂O₃ could generate spin polarization and bring about ferromagnetism with high Curie temperature at room temperature or above.Therefore, high-quality Ga₂O₃ thin film prepared by appropriate process is expected to achieve Ga₂O₃-based high-temperature diluted magnetic semiconductors.Since the preparation of Ga₂O₃ thin film is compatible with modern coating technology,preparing high quality Ga₂O₃ thin film by appropriate processes control will be of great importance to photo-electrical detection and spintronics,etc.In this paper,Mn-doped Ga₂O₃ thin films were prepared by DC and RF magnetron co-sputtering method,and were charactered by scanning electron microscope(SEM), X-ray diffraction(XRD),UV-vis spectroscopy and spectroscopic ellipsometer.The preparation technics as well as the impact of annealing temperature and Mn dopant on the microstructure and optical properties of Ga₂O₃ thin films were studied.The main conclusions are the following: 1.Near stoichiometric Mn-doped Ga₂O₃ thin films can be prepared with stable magnetic co-sputtering course.The optimum conditions are:workpiece rotating speed 60 rpm,substrate temperature 200-250℃,sputtering voltage(current) of Ga₂O₃ target 850V(0.2A),sputtering gas pressure 1.3 Pa,oxygen partial-pressure 0.6 Pa.2.Appropriate annealing temperature could improve the surface morphology,grains distribution and crystal structure of Mn-doped Ga₂O₃ thin films.(ⅰ) The grains of the unannealed Ga₂O₃ thin films are small.The size and shape distribution of crystal grains are quite inhomogeneous.As the annealing temperature rises,the size of crystal grains increase,the grains distribution and roughness of the thin films are improved evidently. (ⅱ) The Ga₂O₃ thin films before annealing are amorphous.The crystallinity of Ga₂O₃ thin films is improved obviously after annealing.From this experiment we find that the phase transformation temperature fromγ-Ga₂O₃ toβ-Ga₂O₃ is between 600℃and 750℃.The annealing temperature above 750℃is beneficial to the oriented growing ofβ-Ga₂O₃. 1000℃is an appropriate annealing temperature.3.A proper amount of Mn-doping to the Ga₂O₃ thin films could inhibit the grain from expanding,induce the directional growing of the crystal grains and then form polyhedral grains.1.05 wt.%Mn-doping could promote the Ga₂O₃ thin films to grow along <-401>,<-202>,<111> and <401> crystal orentation.1.92 wt.%Mn-doping could promote the growth along <-712> crystal orientation.The promotion is weakened at excessive or deficient amount of Mn-doping.4.Mn-doping to the Ga₂O₃ thin films could bring the generation of extra energy level above the valence band of Ga₂O₃.Thus,the Mn-doped Ga₂O₃ film has narrower bandgap,resulting in the red shift of absorption edge and the increase of absorptivity in near ultraviolet band.Both the refractive indexs of the samples before and after doping meet the normal dispersion relation.With the addition of manganese,the refractive index and the extinction coefficient are increased.This indicates that the optical constants of Mn-doped Ga₂O₃ thin films can be controlled by varing manganese content,which lead to potential applications in tunable ultraviolet photoelectric detectors and ultraviolet filters corresponding to different wavelengths.