Preparation And Properties Of Free-standing AIN Film By PLD

With the emergence and rapid development of wearable,increasing attention has been paid to flexible electronic devices.In order to meet the demands from wearable market,semiconductor devices with small area,low consumption,high reliability,and more features are required.Attributed to its wide band gap of 6.2 eV,photodetector based on AlN is able to detect the deep ultraviolet,and AlN shows many peculiar properties,like high piezoelectric coefficient,high sound wave velocity,and good chemical stability for the use of piezoelectric,surface acoustic wave and other flexible device.Conventionally,flexible device is based on superior flexible semiconductor materials.Since directly epitaxial growth of semiconductor on flexible substrates has to face constraints from lattice mismatch,the preferred method to gain flexible material is firstly stripping free-standing film from original substrate and then transferring it onto flexible substrate.In this paper,frees-standing AlN film is stripped off after epitaxial growth by PLD and it shows many novel properties and promising applications.Although the preparation of single AlN buck remains a difficult problem,many techniques have been developed to epitaxial growth AlN film.Reactive sputter,MOCVD,PLD and MBE are mainly selected among all choice,while they all have respective advantages and disadvantages.For the execution of chemical etching to get free-standing films,PLD is selected due to its superiority in preparing multi-layer films.To seek the growth condition of PLD preparing AlN film,MgO?111?substrate is be chosen for its lower lattice mismatch between AlN.Contradistinctive samples were prepared at different temperatures,with different lase energy and under different atmosphere pressure,and optimal growth condition was confirmed according to the comparison between their XRD scan results.Then under this condition,extra AlN films were successfully epitaxial grown on Si?004?,Al₂O₃?006?and ZnO?002?substrates.With a trade-off between lattice mismatch and substrate cost,MgO or ZnO were pre-deposited on Al₂O₃ as buffer layer to improve the quality of AlN by PLD.XRD rocking curve and AFM scan revealed that the AlN film on MgO has better crystallization while the AlN film on ZnO has lower surface smoothness.To confirm the component of AlN film deposited under vacuum,XPS analysis was carried and revealed that except a little impurity of O on the surface,the ration of Al to N was 1 to0.99 within the normal range.Then from the fitting result of ellipsometer,the thickness of films and growth rate of PLD was calculated,furthermore refractive index of AlN agreed with the result reported before and the band gap of AlN fitted is about 6.1 eV.The surface electrical morphology of AlN film remained uniform observed by SEM,and the thickness of films from cross section SEM was consistent with previous result.Finally TEM was employed to reveal good crystalline of AlN thin film.MgO or ZnO inserted between AlN and Al₂O₃ substrate also acted a sacrificial layer etched by?NH₄?₂SO₄ to obtain free-standing AlN film transferred onto any substrate.Without the constraint,the free-standing films combined with certain substrate shows many excellent properties approaching or even exceeding intrinsic material.The AlN film transferred on quartz showed high transmittance in visible light and shield percentage in ultraviolet light from the result of spectrophotometer,and the band gap of AlN fitted was very closed to the original value.The piezoelectric properties of AlN film on conducting substrate ware characterized by PFM.Significant piezoelectric effect was found and the d₃₃ calculated was higher than reported.Compared to photodetector with interdigital electrode,photodetector based on free-standing AlN film generated 2 orders of magnitude larger current excited by 193nm ultraviolet light and much steeper,proving that the photodetector based on transferred AlN film is more sensitive to UV light.In this thesis,MgO or Zn O was pre deposited as buffer layer and sacrifice layer by PLD,to get a high quality free-standing AlN film.This free-standing thin film can be transferred onto other substrate,and reveal several outstanding properties,which provides a variety of promising applications,for instance,flexible and wearable ultraviolet photo detectors,clamping free SAW devices and so on.