Epitaxial Growth And Critical Thickness Of GaN On Substrates Under Different-dimensionally Confinement

Gallium nitride(GaN)is a promising wide band-gap semiconductor materials.Its large saturation electron mobility,high thermal conductivity,high temperature resistance and strong radiation make it an ideal material for power electronic devices.However,as it is difficult to obtain high-quality single-crystal GaN substrate,the performance of GaN-based devices is still far from its theoretical value.Currently,as the homo-epitaxy process of GaN is not mature,GaN is generally prepared by heteroepitaxial growth.However,during the hetero-epitaxial process,there are lattice mismatch and thermal expansion coefficient mismatch between the epilayer and the substrate.When the thickness of the GaN epilayer exceeds the critical thickness,dislocations are generated to release the mismatch strain.These dislocations seriously affect optical and electrical performance of GaN material.Under this background,this dissertation systematically studies the critical thickness of GaN epilayer on substrates under different-dimensionally confinement,and designs two types of novel two-dimensional unconfined substrates to improve the critical thickness and crystal quality of GaN materials.The main achievements of this work are summarized as follows:1.Calculation of the critical thickness of GaN on one-dimensional confined and two-dimensional confined substratesBased on the fundamental theories of critical thickness,the critical thickness of the one-dimensional confined trench substrate and the two-dimensional confined smallhole substrate are calculated.The calculation results reveal that both the trench and the small-hole substrates can effectively increase the critical thickness of the GaN materials and the critical thickness of GaN increases as the dimensions of trench and small-hole substrate decreases.It is found that the critical thickness of GaN is more obviously improved by the small-hole substrate.These results are consistent with experimental results.The calculation model of this study can be used to predict the occurrence of dislocations in GaN epilayers with different width,length(or radius)and thickness.2.Preparation of two-dimensional unconfined T-shaped ultrathin film and epitaxial growth of GaNThe T-shaped ultrathin film is designed based on the two-dimensional unconfined ultrathin film model.Finite element simulation indicates that compared to the ordinary SOI substrate,the T-shaped ultrathin film can effectively reduce the thermal stress(42%)of the GaN epitaxial layer.A 20 nm ultrathin Si film is successfully prepared on the SOI substrate by exploring processes.Experiment results indicate that T-shaped ultrathin film can improve the surface morphology,crystal quality and optical properties of GaN,and effectively reduce the tensile stress inside GaN.This study provides an effective method for the preparation of high-quality GaN on ultrathin films.3.Preparation of two-dimensional unconfined suspended ultrathin film and epitaxial growth of GaNIn terms of shortcomings of T-shaped films,we firstly propose a novel structure named two-dimensional unconfined suspended ultrathin film structure.Finite element simulation reveals that the suspended thin film can effectively reduce(44.4%)thermal stress of GaN epilayer.By exploring peocess,a 20 nm suspended ultrathin Si film is successfully prepared on the SOI substrate.Compared with the ordinary SOI substrate,GaN material on the effective region has no cracks and the surface morphology is smoother.The overall quality of GaN on the suspended ultrathin film is better than that of ordinary SOI substrates.These experimental results are consistent with the simulation results.This research provides a novel structure for the preparation of highquality GaN on ultrathin films,which can be applied to GaN-based MEMS and sensors.