Theoretical And Experimental Study Of Doped AlN

AlN is a direct wide-bandgap(～6.2 eV)â…¢-â…¤-semiconductor material,which have important application in optoelectronic devices using short wavelength light.However,a significant challenge to the widespread exploitation of AlN-related materials in photelectronic applications is the difficulty in achieving effective p-type layers.Using the first-principles full-potential linearized augmented plane-wave(FP-LAPW) method,we have investigated the native defect properties and p-type doping efficiency in AlN doped with group-â…¡A elements such as Be,Mg,and Ca.It is shown that nitrogen vacancies(V_N) have low formation energies and introduce deep donor levels in wurtzite AlN,while in zinc blende AlN and GaN,these levels are reported to be shallow.The calculated acceptor levelsε(0/-) for substitutional Be(Be_Al),Mg(Mg_Al),and Ca(Ca_Al) are 0.48,0.58,and 0.95 eV, respectively.In p-type AlN,Be interstitials(Be_i),which act as donors,have low formation energies,making them a likely compensating center in the case of acceptor doping.Whereas, when N-rich growth conditions are applied,Be_i are energetically not favorable.It is found that p-type doping efficiency of substitutional Be,Mg,and Ca impurities in w-AlN is affected by atomic size and electronegativity of dopants.Among the three dopants,Be may be the best candidate for p-type w-AlN.N-rich growth conditions help to increase the concentration of Be_Al,Mg_Al,and Ca_Al.Recently dilute magnetic semiconductors(DMS) have attracted intense interest.In this thesis,we study the magnetic properties of 6.25%Mg and Ca substituted AlN(Al₁₅XN₁₆, X=Ca/Mg) using FP-LAPW method.The Ca-doped AlN is found to be a half-metallic ferromagnet with 100%carrier spin polarization at the Fermi level.The magnetic moments are localized within the CaN₄ tetrahedron,and a net magnetic moment of 1μB is found per Ca.At a Ca concentration of 12.5%(Al₁₄Ca₂N₁₆),total energy calculations show that the ferromagnetic state is 32.6 meV lower than that of antiferromagnetic state,and Curie temperature around 300 K is estimated.Mg doped AlN has very similar properties to Ca doped AlN,but the latter has a larger half-metallic gap and more stable FM phase,which indicate Ca-doped AlN is more suitable for spin injection applications.Since there is no magnetic element,Mg/Ca-doped AlN appears to be a promising dilute magnetic semiconductor free from magnetic precipitate and may have potential applications in the field of spintronics.Previous studies have show that luminescence temperature quenching of rare-earth (RE) doped semiconductor strongly depends on the band gap of the host materials.It was found that for larger band gap energy,there is less temperature quenching of RE³⁺ luminescence occurring.Therefore,RE doped AlN semiconductors offer the prospect of efficient,temperature-insensitive luminescence device.In Chapter 5,we focus on the optical and structural properties of AlN and AlN doped with Er,Eu and Tm films.The films were deposited by conventional magnetron sputtering in a mixed atmosphere of pure nitrogen and argon.For Er doped AlN,X-ray diffraction indicates that the films are amorphous.The films exhibit strong room-temperature Er³⁺-related photoluminescence(PL).The strongest green emissions(～539 and 560 nm) from the AlN:Eu films are due to the intra-4fⁿ transition of Er³⁺ from ²H₁₂ and ⁴S_3/2 to ⁴I_15/2 transitions.For Eu doped AlN,the photoluminescence spectra of samples prepared under different growth and thermal treatment condition were investigated.Phototexcitation resulted in not only red(～613 nm) but also violet(～407 nm) emission.The red emissions correspond to the intra-4fⁿ transition of Eu³⁺,while the violet emission is identified as 5d-4f transition of Eu²⁺.AlN doped with Tm sandwich films are also prepared by magnetron sputtering.The XRD data reveal that the annealed AlN:Tm films are hexagonal wurtzite structure.The blue light emissions(～460 nm) from ¹D₂-³F₄,which are considerably enchanced after thermal annealing,are observed on all the films.