The Preparation By Using PLD And Study On Properties Of ZnO-based Dilute Magnetic Semiconductors Thin Film

Semiconductor industry based on carry charges transmitting had set off third technicalrevolution, which has greatly promoted social development and sci tech progress. So far, therunning speed and storage capacity of electronic device is getting closer and closer to itstheoretical limit, which has become bottleneck for the development of semiconductortechnology, information technique and electronic products. Nowadays, electronic device inuse overlooked other a degree of freedom of electron—spin. Compared with traditionalsemiconductor devices, spin electronic device show the advantages of rapid reactive rate,small size, low energy cost, non volatility etc. by applying two properties of electron—chargeand spin. Therefore, the application of spin electronic device will undoubtedly bring a newlook of semiconductor technology, information technique and electronic products. The key ofsuccessfully making spin electronic device is the preparation of dilute magneticsemiconductors (DMSs) with room temperature or higher ferromagnetism.The research of scholars in the past few decades has gradually observed and recognizedlots of novel physical properties of DMSs materials. But Curie temperature of as preparedDMSs materials was very low, for a moment study on DMSs was in silence. Renewed interestin DMSs with room temperature or higher ferromagnetism has been stimulated greatly sinceDietl et al. predicted that room temperature ferromagnetism probably occurred inwide band gap metal oxide semiconductor doped with transition metal ions such as Co ionsetc. in 2000.Nowadays, DMSs materials, prepared by wide band gap non magnetic semiconductorsdoped with 3d transition metal element such as Mn and Co etc., were studied. Although agreat achievement in DMSs has been gained, research results of many groups wereinconsistent or even controversial. ZnO, direct and wide band gap semiconductor, has apromising application prospect in the fields of short wavelength optoelectronic devices, high frequency filter, resonator, and optical waveguide etc. The magnetic element dopedZnO based DMSs will probably be promising multifunctional devices materials withmagnetic, semiconductor, piezoelectric, optoelectronic properties. The crystal structure ofZn_1-xCo_xO thin film, formed by doping Co into ZnO, is close to that of perfect ZnO, duo tothe radius of Zn²⁺is close to that of Co²⁺. Therefore, Co doped ZnO thin film was selected tostudy in this paper.Techniques of preparing thin film fall into two general categories: physical and chemicaltechniques. Physical techniques include evaporation, direct current, and high frequency or RFsputtering, ion beam sputtering, pulsed laser deposition (PLD) and molecule beam epitaxy(MBE) etc. Chemical techniques include chemical vapor deposition (VCD), sol gel etc. PLDhas many advantages such as being of great benefit for thin film growth of refractorymaterials, easily doping, easily growing single crystal epitaxial film at low temperature,simple equipment and easy operation etc. The Zn_1-xCo_xO thin films on sapphire or siliconwere prepared by PLD in this paper.The properties of Zn_1-xCo_xO thin films were very sensitive to growth condition and Coconcentration. Effect of growth condition and Co concentration on structure, morphology,optical and magnetic properties of thin films etc was studied. The major work and resultswere as follows:1 major work:(1) Zn_1-xCo_xO thin films were fabricated under condition as follow:i Target: Zn_1-xCo_xO (x=0.05) ceramic target; Substrates: sapphire and silicon; Distancebetween target and substrate: 45 mm; Laser energy: 200mJ/pulse; Repetition frequency:10 Hz; Deposition pressure: 5.0×10^-4Pa; Substrate temperature: 300℃, 400℃, 500℃,600℃, 700℃.ii Target: Zn_1-xCo_xO (x=0.05) ceramic target; Substrates: sapphire and silicon; Distancebetween target and substrate: 45 mm; Laser energy: 200mJ/pulse; Repetition frequency:10 Hz; Substrate temperature: 500℃; Deposition pressure: 5.0 x 104Pa, 1.0 x 102Pa, 1.0×101Pa, 1 Pa, 10 Paiii Target: Zn_1-xCo_xO (x=0.05) ceramic target; Substrates: sapphire and silicon; Distance between target and substrate: 45 mm; Repetition frequency:10 Hz; Substrate temperature:500℃;Deposition pressure: 5.0 x 104Pa, 1.0 x 102Pa, 1.0 x 101Pa, 1 Pa, 10 Pa Laserenergy: 150mJ/pulse, 200mJ/pulse, 250mJ/pulse, 300mJ/pulse, 350mJ/pulse.iv Target: Zn_1-xCo_xO (x=0.01, 0.05, 0.10, 0.15); Substrate: sapphire; Distance betweentarget and substrate: 45 mm; Repetition frequency: 10 Hz; Deposition pressure: 5.0 x10^-4Pa; Substrate temperature: 500℃; Laser energy: 200mJ/pulse(2) The properties of samples were detected and characterized by X ray diffraction (XRD),atomic force microscope (AFM), UV–VIS–NIR spectrophotometer, X ray photoelectronspectroscopy (XPS), Fluorescence Spectroscopy, and alternating gradient magnetometer(AGM).2 major results(1) The XRDθ2θscanning pattern of Zn_1-xCo_xO thin films showed a stronger peakcorresponding to the ZnO (002) diffraction peak. All as prepared samples had wurtzitehexagonal structure of ZnO with preferred c axis orientation. The crystal structures of ZnOwere not distorted by Co doping. The trace of Co₃O₄was not detected for limit of sensitivityof XRD. In as grown samples, partial Co incorporated into ZnO matrix and substituted for Znsites. While a very small amount of Co₃O₄existed in samples.(2) In the as deposited films, island like grains completely covered the whole film surface,and the surface was compact and smooth. Grain sizes of thin film evaluated by AFM weresomewhat larger than those calculated by the Scherrer equation.(3) In the PL spectra, five emission peaks at about 383 nm (3.24 eV), 486 nm (2.55 eV), 495nm (2.5 eV), 505 nm (2.45 eV), and 521nm (2.38 eV) were observed. The strong emissionpeak at 383 nm (3.24 eV) was ultraviolet emission (UV), and attributed to near the near bandedge emission. The emission peak at 486 nm (2.55 eV) was assigned to electron transitionfrom the Zn interstitials to Zn vacancies; the emission peak at 495 nm (2.5 eV) was ascribedto electron transition from Zn interstitials to O interstitials; the emission peak at 521nm (2.38eV) came from electron transition from O vacancies to Zn interstitials; the emission peak at505 nm (2.45 eV) was related to O vacancies. In Zn_1-xCo_xO (x=0.15), the emission peak at383 nm (3.24 eV) attributed to ultraviolet (UV) near band edge emission of ZnO was quenched, duo to high Co concentration.(4) The sp–d exchange interaction between the localized d electrons of Co²⁺ions and bandelectrons of ZnO occured. The s–d and p–d exchange interactions gave rise to a negative andpositive correction to the conduction band and the valence band edges, respectively, leading toband gap narrowing. Therefore, the absorption edge in UV visible spectra showed red shift.Co²⁺incorporating into ZnO lattice and the energy level splitting of valence band and/orconduction band induced that the absorption edge was gentler than that of ZnO. It was foundthat three different absorption peaks in the optical transmission spectra located at 566, 611,665nm, which was typical indicator of d d electron transitions of high spin Co²⁺ions intetragonal crystal field.(5) Compared with sapphire substrate, lattice mismatch between silicon and Zn_1-xCo_xO thinfilm was lower. Therefore, structures, morphologies, luminescence properties of samples onsilicon were better than on sapphire under same condition. But the results were contrary tothose under oxygen pressure 10Pa condition, due to silicon surfaces to oxidation.(6) Magnetism originated from Co²⁺Co²⁺exchange interaction, was related to the defectssuch as O vacancies, dislocation, grain boundary etc. magnetic properties were differentbetween samples on sapphire and silicon under same condition.(7) Substrate temperatures influenced energies of particles in plasma flow when they came tosubstrate. When substrate temperatures were too low, the energy of particles would quicklyloss after they reached substrate, which led ad atoms not to arrive appropriate sites due toinadequate energy. When substrate temperatures were too high, ad atoms sputtered orevaporated again. Therefore appropriate substrate temperature was important for thin growth.In this paper, Zn_1-xCo_xO thin films were synthesized at substrate temperature from 300℃to700℃. The structures, smoothness, and optical properties became better, and worse withsubstrate temperature rising. Crystalline, optical properties of sample on silicon and sapphirewere best at substrate temperature 400℃and 500℃, respectively. All sample had roomtemperature magnetism. Magnetism and residual stress were sensitive to growth temperature,but they have not regularity.(8) The motion velocity of particles in plasma changed due to collision with oxygen, which changed energies of particles. More oxygen incorporated into crystal structure with oxygenincreasing. Appropriate oxygen pressure is important for preparation of high crystallineZn_1-xCo_xO thin films. In this paper, Zn_1-xCo_xO thin films were prepared under oxygenpressure 5.0×10^-4Pa 10Pa condition. The structures, smoothness, and optical propertiesbecame better, and worse with oxygen pressure rising, and crystalline, optical properties ofsample were best when 1.0×101Pa. All samples had room temperature magnetism whenoxygen pressure was below 10 Pa. The average saturation moment decreased with oxygenpressure rising. The samples were paramagnetic when oxygen pressure was 10 Pa.(9) If laser energy was too low, target evaporated from interaction of laser with target, it wasimpossible to achieve the thin film on substrate same with target in component, due todifferent saturation vapor pressure between components in target. So laser energy influencedthe quality of samples. If laser energy was increased to inappropriate value, energies ofparticles in plasma were low, so ad atoms did not get to appropriate sites due to inadequateenergy. If laser energy was too high, excessive particles reach substrate in a time unit,although energies of particles were enough, particles did not reach appropriate sites due toinsufficient time. Meanwhile, new hole came into being on thin film due to atoms sputteringwhen high velocity particles reach the surface of thin film. Then defects such as dislocation,grain boundary occurred, due to the migration of holds. In addition, substrate or surface ofthin film was damaged owing to high velocity particles sputtering. In this paper, samples weregrown by using laser energy 150mJ/pulse, 200mJ/pulse, 250mJ/pulse, 300mJ/pulse, and350mJ/pulse, respectively. Preferred orientation, grain size, residual stress, morphology,optical property increased, and then decreased with laser energy rising. Quality and opticalproperty was best when laser energy was 200mJ/pulse. All samples deposited by usingdifferent laser energy had room temperature ferromagnetism, and ferromagnetism wassensitive to laser energy.(10) Crystalline of thin films decreased with Co concentration going up. Co glided easily toequilibrium position due to Co easily diffusing, which decreased growth index and fluctuationof thin film. Consequently, the degree of density and smoothness of thin films became betterwith Co concentration rising. Too high Co concentration led to surface roughness increasing. The average saturation moment decreased with Co concentration rising. Crystalline, grain size,optical property of Zn_1-xCo_xO (x=0.01) thin films were best in this paper.