Molecular Beam Epitaxial Growth of GaAs on Porous Si

在Si衬底上生长GaAs作为联合收割机硅集成电路与GaAs电子和光学器件结合的一种有前途的方法引起了人们极大的兴趣。由于晶格失配引起的位错密度高和热膨胀系数差异大，无法获得结晶性好的GaAs层。为了降低位错密度，采用分子束外延（MBE）方法在多孔硅衬底上生长GaAs层。多孔硅的柔性有望缓解由于大的失配和大的热膨胀系数差异引起的应力。卢瑟福背散射谱发现，在GaAs/多孔Si界面处以及GaAs中仍存在许多缺陷。对多孔硅上GaAs层形成初期的研究表明，多孔硅表面形貌的变化是导致粗糙界面和缺陷GaAs层形成的原因。通过轻微氧化多孔硅避免了多孔硅表面形貌的变化。在此氧化多孔衬底上生长薄Si层，然后在Si层上生长GaAs层。与传统的GaAs on Si样品相比，这种样品的曲率大大减小。然而，在GaAs层中仍然存在大量的位错，这被发现是由于氧化多孔硅衬底的不完全清洁引起的。在此基础上，我们采用了新的生长工艺，包括在p^+衬底上生长薄n-Si层、选择性形成多孔层、多孔层的氧化和GaAs层的生长。我们现在通过改变生长条件来研究这些GaAs层中的缺陷。

Growth of GaAs on Si substrates has attracted considerable interest as a promising way to combine silicon integrated circuits with electronic and optical GaAs devices. Because of high density of dislocation due to lattice mismatch and large difference in the thermal expansion coefficients, GaAs layers with good crystallnity have not been obtained. To reduce the dislocation density, GaAs layers are grown on porous Si substrates by MBE. Flexibility of porous Si is expected to relax the stresses due to the large misfit and large difference in thermal expansion coefficients. It is found that many defects still exist at the GaAs/porous Si interfaces as well as in the GaAs by Rutherford backscattering spectrometry. Studies of the initial stages of GaAs layers on porous Si reveal that the change in morphology of the porous Si causes the rough interfaces and defective GaAs layers. The change in surface morphology of the porous Si is avoided by slightly oxidizing porous Si. Thin Si layers are grown on this oxidizing porous substrates and then GaAs layers are grown on the Si layers. Curvature of this sample is greatly reduces compared with that of the conventional GaAs on Si sample. However, there still exist a large number of dislocations in GaAs layers, which are found to be induced by imperfect cleaning of oxidized porous Si substrates. Then, we adopt the new growth processes which include the growth of thin n-Si layers on p^+ substrates, the selective formation of porous layers, the oxidation of porous layers and the growth of GaAs layers. We are now studying the defects in these GaAs layers by changing the growth conditions.