Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA.
Nanoscale Res Lett. 2012 Jun 6;7(1):289. doi: 10.1186/1556-276X-7-289.
We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.
我们进行了一项比较研究，研究了在两种不同的 III 族氮化物衬底上生长的 GaN/AlN 短周期超晶格（SL）的应变弛豫，这两种衬底在薄膜中引入了不同量的补偿应变。我们通过等离子体辅助分子束外延（0001）取向的 SL 在 GaN 缓冲层上生长，该 GaN 缓冲层沉积在 GaN（厚）-蓝宝石模板和 AlN（薄）-蓝宝石模板上。对 SL 层的应变、裂纹形成、位错密度和微观结构的原位分析表明，这些结构中应变弛豫的机制取决于衬底中的残余应变，并主要由层间的晶格失配决定。对于在 AlN 薄膜上的生长，该薄膜在层中引入的补偿应变防止了裂纹的形成；然而，与在 GaN 模板上的生长相比，表面凹坑和位错的密度增加了。在 AlN 模板上的伪形生长 SL 的样品中观察到 GaN 帽层的三维生长。同时，在 GaN 模板上的非伪形生长 SL 的结构中观察到 GaN 帽层的二维阶跃流生长，表面出现了大量大裂纹。GaN 帽层的生长模式由顶层 SL 层的弛豫程度预先确定。
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