【作者】 王昊；

【导师】 张进成；

【作者基本信息】 西安电子科技大学 ， 微电子学与固体电子学， 2011， 硕士

【摘要】 以GaN为代表的第三代(宽禁带)半导体材料因其禁带宽度大、击穿场强高、热导率高、耐腐蚀和抗辐照等优势,特别是GaN异质结构具有高密度和高迁移率的二维电子气,被誉为是研制微波功率器件的理想材料。近年来,随着外延技术的不断进步,GaN外延材料的结晶质量也逐步提升,加上器件制造工艺的不断成熟,AlGaN/GaN HEMT器件性能不断提高。不过仍然存在一些关键问题制约器件性能与可靠性,如GaN缓冲层漏电问题和最佳异质结构问题。GaN缓冲层漏电直接使得器件的夹断特性变差,器件击穿电压不高,将严重降低器件的功率特性;GaN异质结构参数的优化问题也很严重。它们都与器件工作特性息息相关。本文首先从GaN缓冲层中杂质分布研究出发,分析认为缓冲层漏电可分为两种情况,一种是聚集有极高浓度载流子的掩埋电荷层,另一种是分布在整个GaN缓冲层中的背景载流子。在通过采用优化条件的HT-AlN成核层生长后,实现了将衬底中氧杂质的扩散抑制在了3D成核岛中,并且背景载流子浓度也控制在了1014cm-3量级。然而,GaN基微波功率器件工作时结温一般超过150℃,在这种情况下,GaN材料会本征激发出大量的背景电子,另外,工作在负栅压下的GaN基HEMT器件沟道中二维电子也会大量溢出至GaN缓冲层中,这都会严重影响GaN缓冲层的高阻特性。这时,需要通过适量的Fe掺杂在GaN缓冲层中形成深能级陷阱来束缚住这些背景载流子,以保证GaN基器件在工作时缓冲层仍然为高阻态。其次,本文通过分析GaN材料中存在的多种散射机制对沟道二维电子输运特性的影响,认为在GaN基HEMT器件正常工作的情况下,影响2DEG迁移率的主要散射机制为合金无序散射、界面粗糙度散射以及位错散射。本文中主要从优化常规的AlGaN/GaN异质结构着手来实现更高的沟道2DEG迁移率。通过对AlN插入层、AlGaN势垒层以及GaN帽层的优化,分析沟道2DEG浓度与迁移率的函数关系,来降低由合金无序散射和界面粗糙度散射对迁移率的限制作用,并最终实现器件性能的提升。再次,GaN材料作为一种极性半导体材料,沟道中二维电子与势垒层应变程度直接相关。而GaN外延层中残留大量应力时对器件的长期可靠性是极为不利的,受逆压电效应的作用,存有应力的GaN器件在高偏压工作时容易导致无法修复的损伤。最后,从成本以及材料的利用率来讲,GaN材料必然会向着大直径化的方向发展,通过对自主研发的MOCVD320系统结构的改进,成功的实现了在3、4inch蓝宝石和碳化硅衬底上高质量的GaN薄膜外延,整个外延片也具有较好的均匀性,基于此缓冲层外延的AlGaN/GaN异质结其电特性和均匀性也都能够满足器件制造的需要。更多还原

【Abstract】 Gallium Nitride (GaN), the representative of the third-generation compound semiconductor materials, owing to its high breakdown field, good thermal conductivity, corrosion resistance, and radiation hardness, had been known as the ideal material for the development of microwave power devices. In recent years, with the improving of the GaN epitaxial equipment, the crystalline quality of GaN material increasing gradually, coupled with the maturity of the device manufacturing process, the performance of AlGaN/GaN high electron mobility transistors had been able to meet the needs of the defense electronics and communication applications fields. There are two issues of concern in the traditional hetero-junction, one is the high resistivity of GaN buffer layer, and another is the optimized hetero-structure. Both of them are related wity the device operating characteristics closely.Firstly, this paper starting from the studies of the impurities distribution in GaN buffer layer, the current leakage of buffer layer can be divided into two cases. One is the buried layers which have gathered high concentration of carriers, and another is background carriers which have distributed throughout the GaN buffer layer. After the optimized growth conditions have been used in high-temperature AlN (HT-AlN) nucleation layer, we have controlled the spread of oxygen impurities in the 3D nucleation islands and the background carriers’concentration also have been controlled in the order of 1014cm-3. However, at the GaN-based microwave power device is working, the junction temperature always more than 150 degrees. In this case, a large number of electrons will be intrinsic inspired in the GaN buffer layer. In addition, a large number of two-dimensional electrons located in the channel will overflow to the GaN buffer layer when the GaN-based device working in the negative gate voltage. These would seriously effect on the impedance characteristics of GaN buffer layer. At this time, we need to bind these background carriers by deep level traps which through the appropriate amount of Fe doping in the GaN buffer layer. Only this way can ensure the impedance characteristics of buffer layer at the GaN-based microwave power devices working.Secondly, we have researched the most optimized structure of conventional AlGaN/GaN hetero-structure. By analyzing the impact of the multiple scattering mechanisms existed in GaN material to the 2DEG transport properties of AlGaN/GaN channel. Combined with the test results of our samples, we have optimized the AlN interlayer, AlGaN barrier layer and GaN cap layer.Third, as polar semiconductor material, the 2DEG density at the GaN channel is directly related to the strain of AlGaN barrier layer. But the residual stress at the GaN epitaxial layer is extremely unfavorable to the reliability of device. Affected by the inverse piezoelectric effect, the GaN-based devices which existed residual stress will lead to be damaged and could not be repaired.Last, in order to reduce costs and improve material utilization, the diameter of GaN wafer will be increasing inevitably. In our laboratory, we had implemented high quality GaN films growing on 3, 4 inch sapphire and Sillcon Carbon substrates, and the entire wafer had a better uniformity. The AlGaN/GaN hetero-structure also had been grown on these templates, which electrical characteristics and uniformity are also able to meet the needs of the device manufacturing.更多还原