蓝宝石衬底上GaN/AlxGa1-xN超晶格插入层对AlxGa1-xN外延薄膜应变及缺陷密度的影响

室温300K下,由于Al_xGa_1-xN的带隙宽度可以从GaN的3.42eV到AlN的6.2eV之间变化,所以Al_xGa_1-xN是紫外光探测器和深紫外LED所必需的外延材料.高质量高铝组分Al_xGa_1-xN材料生长的一大困难就是Al_xGa_1-xN与常用的蓝宝石衬底之间大的晶格失配和热失配.因而采用MOCVD在GaN/蓝宝石上生长的Al_xGa_1-xN薄膜由于受张应力作用非常容易发生龟裂.GaN/Al_xGa_1-xN超晶格插入层技术是释放应力和减少Al_xGa_1-xN薄膜中缺陷的有效方法.研究了GaN/Al_xGa_1-xN超晶格插入层对GaN/蓝宝石上Al_xGa_1-xN外延薄膜应变状态和缺陷密度的影响.通过拉曼散射探测声子频率从而得到材料中的残余应力是一种简便常用的方法,Al_xGa_1-xN外延薄膜的应变状态可通过拉曼光谱测量得到.Al_xGa_1-xN外延薄膜的缺陷密度通过测量X射线衍射得到.对于具有相同阱垒厚度的超晶格,例如4nm/4nm,5nm/5nm,8nm/8nm的GaN/Al_0.3Ga_0.7N超晶格,研究发现随着超晶格周期厚度的增加Al_xGa_1-xN外延薄膜缺陷密度降低,Al_xGa_1-xN外延薄膜处于张应变状态,且5nm/5nmGaN/Al_0.3Ga_0.7N超晶格插入层Al_xGa_1-xN外延薄膜的张应变最小.在保持5nm阱宽不变的情况下,将垒宽增大到8nm,即十个周期的5nm/8nmGaN/Al_0.3Ga_0.7N超晶格插入层使Al_xGa_1-xN外延层应变状态由张应变变为压应变.由X射线衍射结果计算了Al_xGa_1-xN外延薄膜的刃型位错和螺型位错密度,结果表明超晶格插入层对螺型位错和刃型位错都有一定的抑制效果.透射电镜图像表明超晶格插入层使位错发生合并、转向或是使位错终止,且5nm/8nmGaN/Al_0.3Ga_0.7N超晶格插入层导致Al_xGa_1-xN外延薄膜中的刃型位错倾斜30°左右,释放一部分压应变.

The Influence of GaN/AlxGa1-xN Superlattice(SLs) Interlayer(IL) on the Strain and Threading Dislocations(TDs) Density of AlxGa1-xN Grown on GaN/Sapphire

AlxGa1-xN material has a direct band gap between 3.42 eV and 6.2 eV at room temperature,the corresponding cut-off wavelength flexible from 365 nm to 200 nm.It also covers the spectrum obtained with conventional gas and solid-state ultraviolet(UV) lasers,therefore it is necessary for UV photo-detector and deep-ultraviolet light emitting diodes(deep UV-LED).For the fabrication of these devices,thick AlxGa1-xN layer with high Al composition and high crystalline quality are essential.A major obstacle concerning the growth of such AlxGa1-xN layer is the large mismatches in the lattice constants and thermal expansion coefficients between the AlxGa1-xN and the commonly used substrate of sapphire.Crack start to be generated because of tensile strain at a thickness of less than 100 nm for Al0.25Ga0.75N once it directly grown on GaN/Sapphire.The production of crack-free thick AlxGa1-xN ternary layer with high Al composition has also been a challenging issue.It has been reported that low temperature AlN(LT-AlN) IL,or high temperature AlN(HT-AlN) IL,or AlN/ AlxGa1-xN SLs IL is introduced to improve the quality of AlxGa1-xN layer.Introduction of GaN/AlxGa1-xN superlattices(SLs) interlayer(IL) is an effective technique for releasing stress in AlxGa1-xN and decreasing TDs density in AlxGa1-xN epitaxial film.The influence of GaN/AlxGa1-xN superlattice(SLs) interlayer(IL) on the strain and threading dislocations(TDs) density of AlxGa1-xN epitaxial film grown on GaN/sapphire were investigated.Probing of phonon frequency by Raman scattering is one of the most convenient methods for charactering residual stress.The strain in AlGaN epitaxial film is obtained via Raman spectroscopy.Edge TDs density and screw TDs density of AlxGa1-xN film are obtained by X-Ray Diffraction(XRD).As thickness of superlattices period increase(well and barrier have the same thickness),both the edge TDs and the screw TDs density of AlxGa1-xN film decrease,and minimum tensile strain of AlxGa1-xN film is obtained for the sample with ten periods 5 nm/5 nm GaN/Al0.3Ga0.7N SLs IL.When the well thickness of GaN/Al0.3Ga0.7N superlattices is keeped and the barrier thickness of superlattices is increased to 8 nm,namely using ten periods 5 nm/8 nm GaN/Al0.3Ga0.7N SLs IL,compressive strain in AlxGa1-xN epitaxial film is achieved.Minimum TDs density of AlxGa1-xN film is obtained for the sample with ten periods 8nm/8nm GaN/Al0.3Ga0.7N SLs IL.The TDs density indicates that SLs IL can not only restrain edge TDs but also restrain screw TDs partly.Transmission electron microscopy(TEM) measurement demonstrates that the SLs IL can bend or end the TDs or make TDs incorporate,which can lead to TDs density reduction in AlxGa1-xN epitaxial film.And the inclination of edge TDs induced by relaxation strain in AlxGa1-xN film was observed,which resulted in a compressive strain relaxation of AlGaN layer.The strain relaxation of compressively strained AlxGa1-xN by inclined threading dislocation is calculated using TEM results.Above all,the AlxGa1-xN epitaxial film with compressive strain and low TDs density is obtained by the technique of GaN/AlxGa1-xN SLs IL.It is a foundation for the growth of AlxGa1-xN epitaxial film with high crystalline quality and high Al composition.