Effects of the strain relaxation of an AlGaN barrier layer induced by various cap layers on the transport properties in AlGaN/GaN heterostructures

The strain relaxation of an AlGaN barrier layer may be influenced by a thin cap layer above, and affects the transport properties of AlGaN/GaN heterostructures. Compared with the slight strain relaxation found in AlGaN barrier layer without cap layer, it is found that a thin cap layer can induce considerable changes of strain state in the AlGaN barrier layer. The degree of relaxation of the AlGaN layer significantly influences the transport properties of the two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is observed that electron mobility decreases with the increasing degree of relaxation of the AlGaN barrier, which is believed to be the main cause of the deterioration of crystalline quality and morphology on the AlGaN/GaN interface. On the other hand, both GaN and AlN cap layers lead to a decrease in 2DEG density. The reduction of 2DEG caused by the GaN cap layer may be attributed to the additional negative polarization charges formed at the interface between GaN and AlGaN, while the reduction of the piezoelectric effect in the AlGaN layer results in the decrease of 2DEG density in the case of AlN cap layer.     

高温AlN插入层对AlGaN/GaN异质结材料和HEMTs器件电学特性的影响

研究了在GaN缓冲层中插入40 nm厚高温AlN层的GaN外延层和AlGaN/GaN异质结材料, AlN插入层可以增加GaN层的面内压应力并提高AlGaN/GaN高电子迁移率晶体管（HEMTs）的电学特性. 在精确测量布拉格衍射角的基础上定量计算了压应力的大小. 增加的压应力一方面通过增强GaN层的压电极化电场, 提高了AlGaN/GaN异质结二维电子气（2DEG）面密度, 另一方面使AlGaN势垒层对2DEG面密度产生的两方面影响相互抵消. 同时, 这种AlN插入层的采用降低了GaN与AlGaN层之间的 关键词： 高温AlN插入层 AlGaN/GaN异质结 二维电子气 应力     

The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

AlGaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates (0001) are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance--voltage measurement and the Van der Pauw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Furthermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.     

背势垒层结构对AlGaN/GaN双异质结载流子分布特性的影响

首先通过一维自洽求解薛定谔/泊松方程,研究了AlGaN/GaN双异质结构中AlGaN背势垒层Al组分和厚度对载流子分布特性的影响.其次利用低压MOCVD方法在蓝宝石衬底上生长出具有不同背势垒层的AlGaN/GaN双异质结构材料,通过汞探针CV测试验证了理论计算的正确性.理论计算和实验结果均表明,随着背势垒层Al组分的提高和厚度的增加,主沟道中的二维电子气面密度逐渐减小,寄生沟道的二维电子气密度逐渐增加;背势垒层Al组分的提高和厚度的增加能有效的增强主沟道的二维电子气限域性,但是却带来了较高的 关键词： AlGaN/GaN 双异质结构 限域性 寄生沟道     

High-electric-field-stress-induced degradation of SiN passivated AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated by employing SiN passivation, this paper investigates the degradation due to the high-electric-field stress. After the stress, a recoverable degradation has been found, consisting of the decrease of saturation drain current I_Dsat, maximal transconductance g_m, and the positive shift of threshold voltage V_TH at high drain-source voltage V_DS. The high-electric-field stress degrades the electric characteristics of AlGaN/GaN HEMTs because the high field increases the electron trapping at the surface and in AlGaN barrier layer. The SiN passivation of AlGaN/GaN HEMTs decreases the surface trapping and 2DEG depletion a little during the high-electric-field stress. After the hot carrier stress with V_DS=20 V and V_GS=0 V applied to the device for 10⁴ sec, the SiN passivation decreases the stress-induced degradation of I_Dsat from 36% to 30%. Both on-state and pulse-state stresses produce comparative decrease of I_Dsat, which shows that although the passivation is effective in suppressing electron trapping in surface states, it does not protect the device from high-electric-field degradation in nature. So passivation in conjunction with other technological solutions like cap layer, prepassivation surface treatments, or field-plate gate to weaken high-electric-field degradation should be adopted.