【作者】 徐大庆；

【导师】 张义门；

【作者基本信息】 西安电子科技大学 ， 微电子学与固体电子学， 2009， 博士

【摘要】 稀磁半导体(Dilute Magnetic Semiconductors,DMSs)作为一种优良的自旋电子学的后备材料,迅速成为当今自旋电子学材料研究的热点。它有着半导体的能带结构,而且晶格常数也与基体半导体类似,不仅在制造器件时能够很好的和现有的半导体技术兼容,而且兼有磁性材料的特性。但是DMSs的研究受到低的居里温度和磁性掺杂元素固溶度等问题的困扰。2000年Dietl和他的合作者基于Zener模型从理论上预测GaN基的稀磁半导体的居里温度Tc可以达到室温以上。这一理论预测引起了人们对GaN基稀磁半导体材料的关注。GaN基稀磁半导体可以利用扩散法、分子束外延(MBE)、金属有机物化学气相淀积(MOCVD)和离子注入等方法制备。由于利用扩散法将磁性金属元素掺入GaN仍然受到固溶度的限制,并且需要较高的温度和较长的时间,所以不具有实用价值。而对于MBE和MOCVD,如何解决掺杂磁性元素固溶度问题一直是一个难题。由于离子注入本身的技术特点和没有固溶度的限制等优点,因此离子注入是制备DMSs的一种有效的手段。对利用离子注入制备GaN基DMSs,虽然也有过研究报道,但绝大多数也只是对样品的磁特性进行了简单的报道,而且基本都是根据理论预测对Mn掺杂P型GaN进行研究,而对Mn离子注入非故意掺杂GaN的研究非常少,尤其是结合材料微结构的变化特征对样品的磁学特性进行分析仍然是一个有待深入研究的课题。在此背景下,本文利用基于密度泛函理论的第一性原理平面波赝势方法对Mn掺杂GaN的电子结构和光学性质进行了计算分析,对Mn离子注入制备的GaN基稀磁半导体的微结构、光学、磁学及电学特性进行了系统的测试研究,获得的主要成果如下:(1)首先采用基于密度泛函理论的第一性原理平面波赝势方法对Mn掺杂GaN的能带结构、电子态密度和光学性质进行了计算,分析了掺杂后相关性质的改变。计算表明,Mn掺杂后由于Mn3d与N2p轨道杂化,产生自旋极化杂质带,材料表现为半金属性,如果杂质带中的载流子有足够的移动性,从GaMnN可以产生高极化率的自旋极化载流子注入。此外,Mn离子的掺入在费米面附近提供了大量的载流子,改变了电子在带间的跃迁,对GaN的介电函数产生影响。计算表明,Mn掺杂GaN后,由于Mn掺杂产生的杂质带中不同态之间的带内跃迁,光吸收谱中出现了新的吸收峰。计算结果还表明GaMnN的电子结构更适合于自旋极化电荷的传输,是一种合适的自旋注入源。(2)采用蒙特卡罗方法,借助Trim模拟软件对不同能量下Mn离子注入GaN的平均射程、标准偏差和浓度分布进行了统计计算,模拟得到了不同注入能量下Mn离子注入GaN中的浓度分布;设计了Mn离子注入制备的GaN基稀磁半导体的注入工艺参数(能量和注入剂量)及退火条件。(3)研究了Mn离子注入GaN的微结构和光学特性。首先对Mn离子注入非故意掺杂GaN的微结构和光学特性进行了研究。借助XRD测试,发现了与Mn替代Ga原子或Ga-Mn相及Mn-N化合物相关的特征。采用显微Raman谱对离子注入前后和退火前后样品微结构的变化进行了研究,除了观察到GaN的特征峰外,样品中还出现了一些新的声子模并且在特征峰E 2high峰两侧显现出了双肩效应。分析认为新出现的声子模分别是与无序激活相关的拉曼散射(DARS),Ga、N空位相关缺陷的振动模以及由Gax-Mny相关的局域振动(LVM)引起的。利用基于洛伦兹变换的分峰拟合方法和约化质量模型,分析认为E 2high峰的左右肩分别由与MnxNy局域结构相关的局域振动(LVM)和(Ga,Mn)N中Mn离子的LVM引起的。利用光致发光谱(PL谱)对样品的光学特性进行了研究,测试显示PL谱中出现了两个与离子注入相关的位于2.53eV和2.92eV的新的发光峰,分析认为位于2.92eV处的发光带是由导带或浅施主能级向深受主能级的跃迁产生的复合辐射,而这一深受主能级可能是与VGa相关的复合体,该复合体的能级大约位于能隙中价带顶以上0.4eV的位置。对于位于2.53eV的绿光发光峰,认为是由浅施主到深受主的辐射复合跃迁产生的。研究了Mn离子注入Mg掺杂GaN样品的微结构和光学特性。Raman谱的测试结果和非故意掺杂样品的测试结果基本是一致的。PL谱测试结果显示除了位于2.54eV和2.9eV的这两个峰外,在1.69eV出现了另一个新的发光峰。结合Mg掺杂GaN的特点并通过对样品PL谱中2.9eV峰和2.54eV峰的峰强比随退火温度变化的分析,认为位于2.9eV的发光峰是与MgGa-VN复合体(Dd)和Mg的浅受主形成的深施主-浅受主对之间跃迁相关的辐射复合,并据此分析认为位于1.69eV的发光峰可能是基于MgGa-VN复合体深施主能级(Dd)和VGa复合体相关的深受主能级(Ad)之间的辐射复合。(4)研究了离子注入导致的GaN表面损伤及不同退火温度下损伤的修复。借助AFM对离子注入导致的GaN表面损伤及退火修复进行了分析,表明因为GaN表面在高温下的热分解,限制了通过采用更高的热退火温度对样品进行有效的损伤修复。研究也表明采用热靶注入是一种降低离子注入损伤的有效方法。通过对Raman谱中A1(LO)和E 2high峰的峰形及半峰宽随退火温度演进的研究,认为离子注入引起的晶格损伤的修复可以分为三个阶段:当退火温度不高于800℃时,离子注入样品开始出现再结晶,由离子注入引起的晶格损伤开始得到修复,随着退火温度从800℃逐步升高到900℃,晶格损伤得到进一步的修复并且离子注入产生的缺陷也逐步减少。当退火温度升高至900℃以上后,GaN外延层的表面开始分解。从晶格修复和铁磁特性两方面同时考虑,认为最佳的离子注入后样品快速热退火处理的温度应控制在800℃至900℃之间。(5)研究了Mn离子注入非故意掺杂GaN样品的磁学特性和电学特性。对不同退火温度下样品的磁化特性和磁滞回线的测试表明,经过800℃退火处理后的样品获得了最高的磁化强度,而且室温下样品依然表现出清晰的磁滞回线,表明材料具有室温铁磁特性。分析认为样品的铁磁特性主要来源于(Ga,Mn)N,而GaxMny相一方面由于Ga空位的形成,能够引起参与调节铁磁相互作用的空穴浓度的增加;另外GaxMny相也增强了样品的铁磁性。磁化强度随温度的变化曲线进一步验证了本实验制备的材料的居里温度高于室温,测试表明样品磁化强度随温度的变化趋势明显分为两部分。这一结果进一步验证了前面做出的(Ga,Mn)N和GaxMny相对材料铁磁性的贡献的推理。样品的C-V测试和霍尔测试表明离子注入引入的缺陷一方面对载流子的浓度产生了影响,另一方面也降低了载流子的迁移率。降低离子注入产生的缺陷,减小缺陷对稀磁半导体特性的影响是一项需要继续研究的问题。(6)研究了Mn离子注入Mg掺杂GaN样品的磁学特性和电学特性。磁学特性测试结果表现出与Mn离子注入非故意掺杂GaN样品相似的结果,样品在800℃退火后获得了最高的磁化强度并显示样品具有室温铁磁性。测试结果显示样品的磁化强度明显高于Mn离子注入非故意掺杂GaN样品。样品的M-T曲线的变化趋势虽然也分为两部分,但和Mn离子注入非故意掺杂GaN样品比较这两部分曲线斜率的变化明显变小。分析认为这主要是由于Mg掺杂GaN样品的高空穴浓度确保了(Ga,Mn)N对样品铁磁特性的主导作用。由于使用的Mg掺杂GaN外延片只是在700℃进行了弱激活处理,所以进行退火处理时会对掺杂的Mg离子产生二次激活,因此样品经过800℃、900℃退火处理后,载流子浓度有了一定程度的增加,但当退火温度高于900℃后,样品表面分解产生的N空位引入的电子,使得空穴的浓度有所降低。电学测试数据的变化趋势也基本反映了Mn离子注入Mg掺杂GaN样品的这一特点。更多还原

【Abstract】 As good alternative materials for spintronics, Dilute Magnetic Semiconductors (DMSs) has become a hot point for spintronic material research. DMSs have a band structure of semiconductors and also a similar lattice constant to the host material. In addition they can be well compatible with existing semiconductor manufacture technologies. DMSs also have the characteristics of magnetic materials.The major obstacles for DMSs research are to obtain the low Curie temperature and the limitation of solubility of magnetic elements. In 2000, T. Dietl and his colleague predicted the Curie temperatures (TC) for Mn doped GaN can be above the room temperature using the Zener’s model. These predictions for GaN have led to the great experimental efforts. GaN-based DMSs can be synthesized by the diffusion method, molecular beam epitaxy(MBE), metal organic chemical vapor deposition (MOCVD) and ion implantation. The diffusion of magnetic elements into GaN is hampered by their solubility and also requires high temperature and long processing time, making it impractical. For the MBE and MOCVD, doping solubility of magnetic elements is always limited in GaN. It is well known that ion implantation has many advantages including independent control of the doping level, selective area doping, and the ability to fabricate planar devices and self aligned structures without any limitation of dopant solubility. Ion implantation is a very effective method for the formation of DMSs. At present, although some experimental studies on the magnetic characteristics of GaN-based DMSs prepared by ion implantation have been reported, little is known about unintentionally doped GaN epilayers implanted with Mn ions.The magnetic ion related properties and the effect of microstructure change on both magnetic and optical properties of Mn-implanted GaN are still not clear and required to be investigated further.Under this background, the electronic structure and optical properties of Mn-doped GaN have been calculated and analyzed using the first-principles plane-wave pseudo-potential approach based on density functional theory. The microstructure, optical, magnetic and electrical properties of GaN based DMSs prepared by Mn ion implantation have been studied systematically. The main results and contributions of this dissertation are as follows:1) The band structure, the density of states, and the optical properties of Mn-doped GaN were calculated and analyzed using the first-principles plane-wave pseudo-potential approach based on density functional theory. It has shown that Mn-doping changes the properties of the films. The calculated results also reveal a spin polarized impurity band in the band structure of GaMnN due to hybridization of Mn 3d and N 2p orbitals. This band renders the material half metallic. Carriers injected from the GaMnN layer will have high spin-polarization if charge carries within it are sufficiently mobile. The dopants can provide large numbers of carriers near the Fermi energy and change the properties of the interband transition of electrons. This unique feature, together with the previously suggested high Curie temperature and inherent compatibility with GaN technology, makes GaMnN a potentially ideal material for spin injection applications.2) Based on the analysis of ion-implantation, the ion implantation range, location of peak concentration and longitudinal straggling of Mn are calculated with the Monte Carlo simulator TRIM. The process to form GaN based DMSs with Mn ion implantation has been proposed, including the implantation energy and dose, annealing conditions.3) The structural and optical properties of Mn-Ion implanted GaN have been investigated. First, the properties of Mn-Ion implanted unintentionally doped GaN have been studied. From XRD measurements, the samples display the characters of Mn-Ion implanted GaN with a contribution of the Mn occupying the Ga sites or the Ga-Mn phases and the Mn-N compounds. Micro-Raman spectra have demonstrated that in addition to GaN like phonon modes, the most significant features of Raman scattering from the present Mn doped GaN layers are new phonon modes and the appearance of left and right shoulders (SL, SR) around E 2high. The new phonon modes have been analyzed firstly by applying the Lorentzian formula to be useful for multi-peaks cases, and it can be seen that there are at least two peaks constituting each shoulder SL and SR respectively. Using the reduced mass model, the phonon modes constituting each shoulder are attributed to the LVM of different MnxNy localized structures and the LVM of Mn atoms in the (Ga,Mn)N. The results of photoluminescence(PL) measurement showed that optical transitions related to ion-implantation appear at 2.53eV and 2.92eV. It can be obtained from the analysis that the peak at 2.92eV is ascribed to a conduction band to deep acceptor transition or a shallow donor to deep acceptor transition, where the deep acceptor level is located about at Ev+0.4eV. It is also believed that the peak at 2.53eV is a transition between a shallow dopant level and a deep acceptor level.The properties of Mn-Ion implanted Mg doped GaN have also been investigated. Micro-Raman spectra showed the similar results as Mn-Ion implanted unintentionally doped GaN. The results of PL measurement showed that a new emission peaking at 1.69eV besides the transitions at 2.54eV and 2.9eV. Considering the characteristics of Mg-doped GaN, the peak at 2.9eV is ascribed to a MgGa-VN complex related deep donor(Dd) to Mg related shallow acceptor transition. It is believed that the peak at 1.69 eV is a transition between the a MgGa-VN complex related deep donor(Dd) level and a VGa related deep acceptor (Ad)level.4) The surface damage induced by ion implantation and damage repaired by annealing were studied. The results showed that the thermal decomposition of GaN surface at high temperatures limits the application of a higher annealing temperature for repairing the damage of the ion-implanted samples. The study also showed that Hot Target in ion implantation process is an effective manner to reduce ion implantation damage.Using different annealing temperatures we have observed the gradual recovery of the crystalline features based on the Raman spectra of Mn-implanted GaN samples. The evolution of the Raman spectra with annealing temperature(TA) suggests the existence of three stages in the lattice recovery process. Firstly, for TA below 800°C, the implanted sample starts its recrystallization and restores the crystalline quality. Then, for TA from 800°C to 900°C, there is better recovery of the crystalline quality and the decrease of some lattice imperfections in Mn-implanted samples after annealing. Finally, for TA above 900°C, the surface of GaN epilayers begins to decompose.Our results suggest that the optimal lattices recovery and ferromagnetic properties are achieved by RTA at 800°C~900°C.5) The magnetic and electrical properties of unintentionally doped GaN have been studied. The room-temperature FM in unintentionally doped GaN epilayers implanted with Mn ions was achieved after being annealed at 700°C, 800°C and 900°C. The highest magnetization was obtained in the samples annealed at 800°C. We believe that the (Ga,Mn)N can be mainly responsible for the observed FM behavior of Mn-implanted unintentionally doped GaN epilayers. The GaxMny phases produced after annealing Mn-implanted unintentionally doped GaN epilayers at 700°C and 800°C provide holes and also enhance the FM. It is further confirmed by the temperature dependency of the Magnetization that Curie temperature of the material is higher than the room temperature. The tests showed that the trends of temperature dependency of the Magnetization has been divided into two parts, which also approved the deduction of the contribution of (Ga,Mn)N and GaxMny phases for the observed FM. The results of C-V and Hall tests showed that the defects introduced by ion implantation have an impact on the concentration of carrier on one hand, and also reduce the carrier’s mobility on the other hand. All of these have a significant impact on the properties of DMSs prepared by ion implantation, so should be considered in order to obtain good properties of the materials.6) The magnetic and electrical properties of Mn-Ion implanted Mg doped GaN have also been studied. The characteristics of the magnetic test showed similar results to Mn-Ion implanted unintentionally doped GaN. The room-temperature FM in the sample was achieved after annealing. The highest magnetization was also obtained after annealing at 800°C. The results show that the magnetizations are significantly higher than Mn ion implantation unintentionally doped GaN. Although the trend of M-T curves of the samples is divided into two parts, the change is much smaller. It is believed that the difference between the Mn-ion implanted Mg-doped GaN and the Mn-ion implanted undoped GaN was mainly due to the high concentration of holes in Mn-implanted Mg-doped GaN epilayers, which ensures that the (Ga,Mn)N can be mostly responsible for the observed FM behavior. A weak activation of Mg-doped GaN epitaxial films was carried out at 700℃. When the samples were annealed after ion implantation, there will be a second activation of the Mg ions. So after being annealed at 800℃and 900℃, the concentration of carriers has increased to some extent. As the annealing temperature above 900°C, the surface of GaN begins to decompose, leading to the formation of N vacancies. Thus, some parts of the holes compensate with electrons generated from N vacancies, resulting in the reduction of the hole concentration. This characteristic is also confirmed by the electrical test.更多还原