【作者】 张勇；

【导师】 牛憨笨； 刘文；

【作者基本信息】 华中科技大学 ， 光学工程， 2008， 博士

【摘要】 氮化铝（AlN）是直接带隙宽禁带（～6.2 eV）Ⅲ-Ⅴ族半导体材料,在短波长光电子器件领域具有重要应用价值。但是,实现AlN材料在光电器件上的广泛应用还面临重大挑战—难以获得有效的p型层。利用第一性原理的全势—线性缀加平面波（FP-LAPW）方法,我们研究了纤锌矿AlN材料的本征缺陷及ⅡA族元素（Be,Mg和Ca）掺杂AlN的p型效率。结果表明氮空位（V_N）在AlN材料中具有低的形成能并引入较深的施主能级,而在闪锌矿AlN和GaN中,V_N引入浅施主能级。计算得到的Be_Al,Mg_Al和Ca_Al在AlN中引入的受主能级深度分别为0.48,0.58和0.95 eV。在p型AlN中,处于间隙位置的Be（Be_i）表现为施主且具有较低的形成能,这使得它们很可能成为p型AlN中空穴的复合中心,然而,在富氮（N-rich）生长条件下Be_i的形成能明显提高。研究结果表明,Be,Mg和Ca在AlN中的p型掺杂效率受到杂质本身原子尺寸和电负性的影响;三种杂质中,Be可能是制备p型AlN更有效的杂质;N-rich生长条件有助于提高杂质在AlN中的含量。近年来,稀磁半导体受到越来越多的注意。本论文运用FP-LAPW方法研究了6.25%含量的Mg和Ca掺杂AlN(Al₁₅XN₁₆,X=Ca/Mg)的磁性质。Ca掺杂AlN的电子态密度在费米能级附近100%自旋极化,具有半金属磁性质。磁矩主要分布在由杂质Ca和其最近邻的四个N构成的CaN₄四面体内部,得到的总磁矩为1μ_B/Ca。对12.5%掺杂量的AlN(Al₁₄Ca₂N₁₆)总能量计算结果表明晶胞的铁磁态总能量比反铁磁态总能量低约32.6 eV,由此估算出的铁磁居里温度在300 K附近。Mg掺杂AlN的性质与AlN:Ca很相似,但是后者具有更大的半金属带隙和更稳定的铁磁基态,这表明AlN:Ca比AlN:Mg更适合作为自旋注入材料。既然杂质本身没有磁性,用Mg/Ca掺杂AlN将可避免通常用磁性杂质掺杂所带来的磁沉积问题,在自旋电子学领域将具有潜在应用价值。已有的研究结果表明,稀土掺杂半导体发光材料的温度淬灭效应严重受基体材料禁带宽度的影响—在宽带隙能的基体材料中稀土发光的温度淬灭效应更小。因此,稀土掺杂宽禁带的AlN半导体有望可以得到高效、受温度影响小的发光器件。在本论文中,我们重点研究了AlN及稀土（Er,Eu和Tm）掺杂AlN的结构和光学性质。薄膜制备采用通常的磁控溅射。对于制备的Er掺杂AlN薄膜,X-射线衍射测试表明所制备的样品为无定型薄膜,样品展示了强烈的室温光致发光光谱,可见光范围最强的绿光发射（～539和560 nm）源自Er³⁺ 4fⁿ轨道的²H₁₂和^S_3/2到⁴I_15/2能级的跃迁;对于AlN:Eu,我们研究了不同生长条件和退火温度下样品的PL光谱。在激励光辐射下,样品不仅发出了红光（～613 nm）,还出现了波峰位于～407 nm附近的紫光发射。红光对应于Eu³⁺ 4fⁿ轨道内部能级跃迁,而紫光可能与Eu²⁺离子的5d-4f能级跃迁有关。用磁控溅射我们还制备了夹层结构的Tm掺杂AlN薄膜。XRD测试表明制备的样品经退火处理后出现了六方AlN的衍射峰。光致发光测试观察到了源自Tm³⁺¹D₂-³F₄能级跃迁的光（～460 nm）发射,该发射峰强度受退火温度影响。更多还原

【Abstract】 AlN is a direct wide-bandgap（～6.2 eV）Ⅲ-Ⅴ-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-ⅡA 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.更多还原