Epitaxial And Physical Properties Of ZnO-based Diluted Magnetic Semiconductors Grown By Molecular Beam Epitaxy

While conventional electronic devices utilize the charge of an electron or hole to store and process data.The need for a new physical basis for digital computation is becoming increasingly obvious as we reach the physical limits of complementary metal oxide semiconductor technology.Those that are based on spin electronics,commonly referred to as spintronics,and would rely on quantum mechanical spin.Many such devices would require electrical injection of a spin-polarized current into semiconductor heterostructure in order to function.Semiconducting materials that are ferromagnetic represent an ideal means of meeting that requirement,provided the material remains ferromagnetic above room temperature and exhibits a high degree of spin polarization.Thus far,only a handful of these materials have been discovered,and transition metal-doped ZnO has the potential to be a highly multifunctional material with coexisting magnetic, semiconducting,electromechanical,and optical properties.ZnO-based magnetic semiconductors have been attracting intense interests.Today,the main method of study magnetic semiconductors is doping transition elements into semiconductor, Recently,stimulated by the success in GaMnAs and TiCoO magnetic semiconductors,ZnO-based magnetic semiconductors have been investigated by a few groups since they are predicted to realize ferromagnetic semiconductors with a high Tc above 300 K and a large magnetization.To date,the reported experimental results of ZnO-based magnetic semiconductors by these groups are different and even contradictory.It seems that the magnetism is very sensitive to the preparation methods.One key question is whether there is an intrinsic ferromagnetic phase in high quality ZnO-based DMSs at all.The main focus of this research is on understanding the epitaxial growth and properties of Zn_l-xCo_xO films and Zn_1-xMn_xO thin films grown on Al₂O₃(0001).Co-doped wurtzite ZnO(Zn_1-xCO_xO)thin films have been grown on Al₂O₃(0001)substrates by using oxygen plasma-assisted molecular beam epitaxy at the low growth temperature of 450℃.The epitaxial films of Co concentration at 0≤X≤0.12 are single crystalline,which were examined by reflection high energy electron diffraction and x-ray diffraction.Both of optical transmission spectrum and in situ.x-ray photoelectron spectroscopy studies confirmed the incorporation of Co²⁺cations into wurtzite ZnO lattice.Magnetic measurements revealed that the Zn_1-xCo_xO thin films are ferromagnetic with Curie temperature T_C above room temperature,and the ferromagnetism shows intrinsic characteristic.Based on the experiments,we can rule out possible magnetic contamination and the secondary phase of the metallic Co or CoO_x clusters as the origins of the high T_C ferromagnetism of Zn_1-xCo_xO thin films.The ferromagnetism of Zn_1-xCo_xO DMSs shows intrinsic characteristic,however,the moment of Co impurities at room temperature is relatively small.The saturation moment Ms(300 K)is 0.4μ_B per Co atom at x=0.049,if all Co atoms are assumed to contribute equally to the moment.Ms(300 K)decreases in lower and higher Co concentrations,for example,0.19μ_B per atom at x=0.009 and 0.2μ_B per atom at x=0.12.We suggest that the percolation of bound magnetic polarons (BMP)might be responsible for the origin of the high T_C ferromagnetism in Co-doped ZnO DMSs,where only those of Co²⁺located on the intersection of two BMPs contribute to long range magnetic interactions.Mn-doped ZnO films were grown on Al₂O₃(0001)substrates by oxygen-plasma assisted molecular beam epitaxy(OPAMBE).Reflection High Energy Electron Diffraction and Micro-Raman scattering measurements indicated that the Zn_1-xMn_xO films are high quality single crystal with the substitution of Zn²⁺by Mn²⁺cations,and no secondary phase was detected.Room temperature (RT)ferromagnetism was observed in Zn_1-xMn_xO films which were post annealed in oxygen plasma radical.Using temperature dependent photoluminescence,we demonstrated that the observed RT ferromagnetism was originated from the ferromagnetic interaction mediated by activated shallow acceptor in n-type Zn_1-xMn_xO films.From the PL spectra and Hall data,it is concluded that there are D⁰X and A⁰X in Mn doped ZnO films,and the dominated carrier is determined by the competive results.It is obvious that this activated acceptor in n-type conductivity plays an important role in establishing RT ferromagnetism in O-annealed ZnMnO thin film.The observed ferromagnetic exchange can not be explained by the Zener model mediated with hole mediated interaction in p-type ZnO,or by bound magnetic polarons mediated with shallow donor electrons.Recent LDSA+U calculations indicated that the nearest neighbor pairs of pure substitutional Mn in ZnO have antiferromagnetic exchange coupling. It suggests that the activated acceptor modifies the antiferromagnetic interaction between the nearest neighbor Mn²⁺ions,which may be a possible origin for observed RT ferromagnetism in ZnMnO DMS.The acceptor in O-annealed Mn-doped ZnO film was possibly induced by interstitial oxygen,Zn vacancy,or both of them.Theoretical calculation showed that interstitial oxygen has relatively lower energy,which is more favorable for thermal annealing by oxygen-plasma radical.RT ferromagnetism was only observed in O-annealed film with the saturation moment of 0.3μ_B/Mn at 300K.PL and Hall effect measurements indicated that the observed ferromagnetism was possibly originated from the ferromagnetic interaction mediated by activated shallow acceptor in n-type ZnMnO film.The shallow acceptor was induced by interstitial Oxygen with the binding energy of 0.134eV.