【作者】 屈媛；

【导师】 班士良；

【作者基本信息】 内蒙古大学 ， 理论物理， 2010， 博士

【摘要】 纤锌矿氮化物半导体具有禁带宽度大、临界场强高、热导率大、饱合漂移速度高、稳定性好等特点,是制作高频大功率器件的重要材料.目前,GaN基高电子迁移率晶体管(HEMT)的理论和实验研究是一个热点方向.纤锌矿氮化物量子阱是GaN-HEMT的基本结构,强自发和压电极化效应使体系在界面处产生高浓度的二维电子气(2DEG).2DEG的浓度和迁移率决定HEMT的电学特性.本文基于介电连续模型、单轴晶体模型和雷-丁平衡方程方法,讨论在光学声子起主要散射作用的温度区域,纤锌矿氮化物量子阱中的电子迁移率.主要研究内容和所得结果概括如下：(1) AlN/GaN/AlN量子阱中电子迁移率的尺寸效应.结果表明,在室温和常压下,在窄阱和宽阱时,界面光学声子散射对电子的迁移率起主导作用；在中间阱宽时,局域光学声子散射对电子的迁移率起主导作用.电子迁移率随阱宽的增加先从一小值减小至低谷,然后增加,最终在某一阱宽时达到饱合值.对给定阱宽,有限垒宽情形的电子迁移率大于无限垒宽情形.减小阱区的内建电场可增大电子迁移率.量子阱的尺寸会改变体系的内建电场强度及2DEG分布,尺寸的优化可提高电子迁移率,从而改进HEMT的性能.(2) AlN/GaN/AlN量子阱中电子迁移率的流体静压力效应.计算发现,随着流体静压力的增大,一方面材料的有效质量、高频介电常数、光学声子频率、体系的带阶和内建电场均增大；另一方面体系中声子数却减少.结果表明,电子迁移率随着流体静压力的增大而微弱增大.(3) AlxGa1-xN/GaN/AlxGa1-xN量子阱中光学声子模的三元混晶(TMC)效应以及对电子迁移率的影响.结果发现,由于声子色散的各向异性,量子阱中各类光学声子模随组分而变化,声子的局域模,界面模,半空间模以及传播模存在于特定组分和频率范围内,且同种声子模色散关系也随组分而改变.随着Al组分的增加,对称光学声子对电子的作用减弱,反对称光学声子对电子的作用则增强.电子迁移率随Al组分和温度的增加而减小,随电子面密度的增大而增大.(4)在AlN/GaN/AlN量子阱中引入InGaN凹槽层对电子迁移率的影响.结果显示,InGaN/GaN界面的引入和TMC效应使体系的声子模式发生改变.当In组分增加到使导带上InGaN/GaN界面处的能量低于AIN/GaN界面处的能量时,2DEG将由GaN阱转移至InGaN凹槽层.当电子主要分布在阱区时,随着In组分的增大,电子迁移率先增后减,但总是高于不含InGaN凹槽层之情形；当2DEG转移至InGaN凹槽层时,迁移率则突然降低,此后又随In组分的增大而逐渐增大.上述结论对于相关实验和HEMT的设计有指导作用.更多还原

【Abstract】 Due to their large bandgap, high breakdown field, large heat conductivity, high electron saturation velocity and well stability, wurtzite nitrides are important candidates for devices with high frequency and high power. At present, many theoretical and experimental works focus on the GaN based high electron mobility transistors (HEMTs). Wurtzite nitride quantum wells (QWs) are the basic structures in GaN-HEMTs. The effect of strong spontaneous and piezoelectric polarization induces a two dimensional electron gas (2DEG) with high sheet density near the interfaces in the structures. The electric properties of a HEMT can be controlled by the mobility and sheet density of a 2DEG. In this thesis, the electron mobility in wurtzite nitride QWs in the temperature range, in which the scattering from optical phonons has a main effect, is discussed based on the dielectric continuous model, uniaxial model and the force balance equation of Lei and Ding. Main contents and results obtained are generalized as follows:(1) Size effect on electron mobility in wurtzite AIN/GaN/AIN QWs. The results show that under normal pressure, the main contribution to electron mobility at room temperature (RT) is from the scattering of interface optical phonons for cases of narrow and wide wells, whereas it is from that of confined optical phonons for cases of intermediate well width. As well width increases, electron mobility first decreases from a small value to a valley, then increases and reaches a saturated value. For a given well width, electron mobility in QWs with finite thick barriers is greater than that with infinite thick barriers. The decrease of built-in electric field (BIF) in AIN/GaN/AIN QWs enhances electron mobility. The size of QWs affects the BIF and distribution of 2DEG. The optimization of the size can enhance electron mobility and accordingly improve the properties of HEMTs.(2) Pressure effect on electron mobility in AIN/GaN/AIN QWs. It is found that, on one hand, the electron effective mass, phonon frequency, BIF, band offset and high-frequency dielectric constant increase with increase of hydrostatic pressure, on the other hand, the number of phonons in the system reduces. As a result, the electron mobility increases slightly as hydrostatic pressure increases.(3) Effect of ternary mixed crystals (TMC) of optical phonon modes in AlxGa1-xN/GaN/AlxGa1-xN QWs and its influence on electron mobility. It is found that optical phonon modes in QWs vary with the components of TMC. Phonon modes such as localized modes, interface modes, half-space modes, and propagating modes exist in certain regions of the component and frequency due to the anisotropy of phonon dispersion in wurtzite nitrides. At the same time, the dispersion relation of the same phonon modes also varies with the component. The results show that the influence on electrons from anti-symmetric phonons becomes relatively stronger while that from symmetric phonons becomes weaker with increase of Al component. Electron mobility decreases with increase of Al component and temperature, whereas it increases obviously with increase of the sheet density of electrons.(4) Influence of the introduction of an InxGa1-xN nanogroove in a strained wurtzite AIN/GaN/AIN QWs on electron mobility. The results show that the optical phonon modes can be changed by the introduction of InGaN/GaN interfaces and the TMC effect. It can also be found that electron wave function will shift to InGaN layer as long as the conductor band energy at GaN/InGaN interface is lower than that at AIN/GaN interface. When electrons mainly distribute in the well region, electron mobility is greater than the case without an InGaN nanogroove and it first increases and then decreases as In component increases. After the 2DEG transfers to the InGaN layer, electron mobility drops sharply and then increases with the increase of In component.The above conclusions may direct the related experiments and design of HEMTs.更多还原