蓝宝石外延生长GaN厚膜的自分离及断裂机理研究

Group III-nitride semiconductors and their ternary and quaternary alloys with variable concentration have great application value for military and civil use because they have the widest adjustable forbidden gap. Fabricating high quality III-nitride materials needs to fabricate high quality native subatrate such as GaN and AlN. Bending and cracking often occur in fabricating GaN substrate because of the stress concentration. Therefore, it is difficult that the GaN thick-films fully separate from the substrate. The previous stress models deal with the stress in terms of a scalar quantity and can not exhibit the character of stress anistropy, and all depend on the curvature measurement. As a result， those models can not essentially interpret the phenomenon of self-separation, bending and cracking. The present project will thoroughly investigate the stress properties of the sapphire-GaN system, and explore the effect of shear deformation, lattice mismatch, thermal mismatch and nucleation coalescence on the strain of GaN thick-films, and estabish the three-dimentions stress model with tensor form by the relation of stress-strain. We will reveal the mechanism of bending, cracking, and self-separation from the sapphire substrate for GaN thick-films by investigating the mechanism of stress anistropy, the reason of their propensity to grow in island mode and the atomistic mechanism of critical thickness, the thansformation from elastic deformation to plastic deformation for the crack source, and the effect of island nucleation on stress as well as the generation mechanism of tensile stress.

III族氮化物半导体及其组分可调的三四元合金是迄今禁带宽度调制范围最宽的半导体体系，在军用和民用领域均有重大应用价值。制备高质量的III族氮化物首先需要制备出高质量的同质衬底如GaN和AlN。制备GaN衬底时由于应力的积聚经常出现翘曲甚至断裂，难以实现GaN厚膜与衬底的完整分离。以往应力模型对应力按标量处理，没有体现应力的各向异性特征，并且均依赖于曲率测量，不能从根本上解释自分离、翘曲以及断裂现象。本项目拟深入研究蓝宝石-GaN厚膜体系的应力特性，探索剪切形变、晶格失配、热失配以及形核合并对GaN厚膜应变的影响，通过应力与应变的基本关系，建立张量形式的三维应力模型；通过研究应力的各向异性机理、GaN厚膜倾向于岛状生长的原因以及临界厚度的微观原子机制、裂纹源从弹性向塑性形变的转化、岛状形核对应力的影响以及张应力的产生机制，揭示GaN厚膜与蓝宝石衬底的自分离以及GaN厚膜产生翘曲和断裂的机理。

GaN作为一种重要的半导体材料，可以应用到蓝光照明LED、半导体激光器以及高性能探测等领域。制备GaN常采用GaN衬底实现同质外延，单晶GaN主要在蓝宝石基底上生长，由于晶格失配和热膨胀系数不同，常导致GaN厚膜的翘曲和开裂，这显然与应力在厚膜内的分布特性有关。本研究考虑两条假设：（a）去除Stoney模型中膜厚远小于基底厚度的近似条件，考虑GaN膜的厚度和基底蓝宝石的厚度相当；（b）把GaN的膜厚考虑成非均匀的，随面内径向坐标r变化。并且，考虑剪切形变，从连续体力学方程出发建立了包含界面剪切应力的“三维应力模型”，计算了GaN-蓝宝石异质厚膜体系的曲率和界面剪切应力，研究了从系统中心到边缘膜厚的薄-厚-薄和厚-薄的两种变化模式。计算结果表明系统的曲率不再是常量而是随坐标r变化的变量，界面剪切应力在整个半径R范围内出现方向转变，转变点正好对应曲率取极值的点，表明曲率对界面剪切应力有重要影响。此外，基于GaN厚膜在蓝宝石基底上外延生长过程中膜厚方向的晶格常数从界面到膜表面呈现弛豫特征的实验结果，我们建立了描述膜厚方向的正应变的“弛豫模型”，进而计算和讨论了GaN膜边缘处的应力和应变沿膜厚方向的变化，GaN和基底界面处的应力在面内的变化以及中心处的应力和应变沿膜厚方向的变化。 Stoney模型是从薄膜基底的应力应变关系出发，首先得到应变能函数，从而推得膜内应力与曲率的关系。在本研究中，我们推广了Stoney模型，考虑z方向的应力和应变，以及和z方向相关的剪切应变，得出柱坐标下的任意一点的应变能函数，并考虑膜内面间的均匀失配应变，得到体系的应变能的积分表达式。