Investigation On P-type Doping And Room Temperature Diluted Magnetic Propteries Of ZnO

Wurtzite ZnO is a promising semiconductor for short wavelength optoelectronicdevices because of its wide band gap of3.37eV at and a large excition binding energyof60meV at room temperature （RT）. However, its application suffers from achievingstable, controllable and high crystalline quality p-type ZnO. On the other hand,considering the RT ferromagnetism might be realized in3d transition metal （Mn, Fe, Co,Ni, etc.） doped ZnO, ZnO based diluted magnetic semiconductors （DMSs） are attractingmuch more attention. The theoretical calculations predict that RT ferromagnetism ofMn-doped ZnO would be realized in p-type condition. However, so far, theferromagnetic coupling of Mn-N co-doped ZnO system is still not clear. Theexplanations for understanding the ferromagnetism of ZnO:Mn-N are also controversial.Hence, according to the present hotspots and difficulties on ZnO, in thisdissertation, our works are focused on N ion implanted ZnO, Mn mono-doped andMn-N codoped ZnO DMS. On experiments, using XRD, SEM, Raman, PL, Hall,SQUID, etc, we study the effects of doping doses and post-annealing on the structural,electrical and optical properties of N+-implanted ZnO single crystals, the influences ofMn doping concentration and annlealing temperature on the structural, composition,optical, electrical and magnetic properties of Mn mono-doped and Mn-N codoped ZnOfilms. Theoretically, the electronic structure and magnetic behivor of Mn-doped andMn-N codoped ZnO are investigated by first-priciples density functional calculations,and the effect of native defects on the magnetic properties has also been analysed. Themain obtained results are as following:①The influences of the doping doses and post-annealing on the structural andoptical properties of N+-implanted ZnO single crystals have been investigated. Thelattice defects and the reduced luminescence of ZnO induced by the energetic N+ionscould be recovered significantly upon thermal annealing. The intensity of the near-bandemission for the sample which was annealed at600-800°C is improved about10-20times in comparing with that of the as-implanted one.②The implanted N+ions can be effectively activated around650°C in a N2ambient. In the process of the post-annealing, it is found that the substituted N molecule（N-N）o donors and H can be removed and most importantly the interstitial N can bemoved to the right lattice sites, accordingly p-type ZnO sample with a hole concentration of7.81×1016cm-3has been achieved. The thermal ionization energy of thenitrogen acceptor is estimated to be135meV. N+-implantation induced interstitial Znrelated defects in ZnO and they are stable in the process of the thermal annealing at thetemperatures500-800°C. The authors propose that compensation from interstitial Znrelated defects might lead to the low hole concentration of the p-type ZnO:N.③Mn-doped ZnO films with different Mn contents （0⁶.72at.%） were grown onfused quartz substrates by radio-frequency （RF） magnetron sputtering. All the filmshave the single-phase wurtzite structure with c-axis preferred orientation. The doped Mnions are in divalent states. Mn incorporation results the increase of lattice constant andband gap energy of ZnO, and also induces the structural quality deteriorated and thelower transmittance. In addition, the increment of Mn doping also induced severesuppressions of electron carriers and mobility in ZnO films. At room temperature,Mn-doped ZnO films show paramagnetic behavior.④The first-principles calculations suggest that the Mn ions tend to cluster togetheras dimmers via an intervening O atom （-Mn-O-Mn-） in the Mn-doped ZnO. Thecoupling between two Mn ions is dominated by the sup-exchange interaction via theintermediate O ion and favoring in the antiferromagnetic （AFM）. In the presence ofdonor defects, such as oxygen vacancies and interstitial Zn, the interactions remainantiferromagnetic, whereas in case of acceptor defects like Zn vacancies, ferromagneticinteractions are observed. Therefore, the authors propose that the being of Zn vacanciescan induce the room-temperature ferromagnetism in Mn mono-doped ZnO.⑤Experimentally, we find that the codoping N plays an crucial rule in activatingthe RT ferromagnetism in ZnO:Mn films, and the ferromagnetism is depended on thesubstituting No concentrations. The mechanism of ferromagnetic coupling in codopedZnO is discussed based on a bound magnetic polaron model. Using first-principlecalculations, the interaction between Mn²⁺ions in the presence of N has been analyzed.It is found that the incorporation of Mn decreases the formation of substituting No inZnO and the most stable configurations are to be-O-Mn-N-Mn-O-. The codoping N is apromising approach to enhance the ferromagnetic coupling between thenearest-neighboring Mn ions due to the strong hybridization between the N:2p andMn:3d states. Moreover, the spin-polarized states induced by N are so extended thatthey can mediate long-ranged ferromagnetic exchange interactions beyond thenext-neighboring case. However, the H passivation on No leads the coupling betweentwo Mn²⁺ions bridged by a N atom changes to be AFM again. ⑥Experimentally, we find that the total magnetic moments of the samples wouldreach a limit under a certain N doping condition even if more Mn ions are incorporatedinto the ZnO lattice. First-principles calculations suggest that a codoping N atom canonly modulate two Mn²⁺ions. In contrast to the high solubility （up to35%） of Mn inZnO, the solubility limit of N in ZnO is as low as¹%, thus the authors conclude thatthe magnetic moment will reach a limit in （Mn, N） co-doped ZnO system.