Abstract: The work presents a comparative study on the AlGaN/GaN type-II heterostructures grown on c-plane sapphire (Al2O3) and Si (111) by Plasma Assisted Molecular Beam Epitaxy (PA-MBE). The in depth structural characterizations of these samples were performed by High-Resolution X-Ray Diffraction (HRXRD), X-ray Reflectivity (XRR) and Field Emission Scanning Electron Microscopy (FESEM). On the basis of HRXRD measurements, the stress-strain of the layers was examined. The c- and a-lattice parameters of the epilayers as well as in-plane and out-of plane strains were determined from the ω-2θ for symmetric scan and ω- Xθ (X represents the coupling coefficient) for asymmetric scan. Strain, tilt and correlation lengths were calculated from Williamson-Hall (W-H) plots. Moreover, the twist angle was measured from skew symmetric scan of (102), (103), and (105) plane along with (002) symmetric plane by fit with model of Srikant et al. The composition and strain/relaxation state of the epilayers were observed in details by reciprocal space mapping (RSM). The symmetric (002) triple axis RSM (TARSM) and asymmetric (105) double axis (DA) RSM of grazing incidence (GI) were carried out on each sample. The defect density were measured from HRXRD curves of skew symmetric (002) and (102) reflection plane. The Al mole fraction and strain states of the layers were calculated by fitting the experimental curves with computer simulations and compared with theoretical findings based on elastic theory. The thicknesses of the layers and roughness of the interfaces were measured from simulation of the nominal structure by fitting with XRR experimental curves.
The Ga(Al) terminated GaN/AlGaN/GaN on Al2O3 and Si (111) substrate were grown by plasma assist molecular beam epitaxy. The solid sources were used for Al and Ga on the other hand die nitrogen (N2) is used as a plasma source for nitride element. The (002) ω-2θ scan shows the presence of GaN, AlGaN and AlN layer of proposed heterostructure. The XRR and cross sectional FESEM image confirm that the thickness of individual layer for GaN/ AlGaN on Al2O3heterostructure with AlN nucleation layer are GaN=1.2 μm, AlGaN=20 nm ,GaN cap = 3 nm and AlN = 20 nm. For GaN/AlGaN/GaN on Si (111), the thickness of individual layer are GaN cap =3 nm, AlGaN = 20 nm, GaN channel = 1.2 μm, and AlN/GaN/AlN stack for buffer layer are 0.23 μm, 0.27 μm and 0.05 μm respectively. The measured Al mole fraction is 30 % for both structure which were measured from peak separation between GaN and AlGaN and also fit with simulations. The details HRXRD analysis shows from RSM that AlGaN layer exhibit pseudomorphic in nature on the other hand GaN layer is nearly relaxed, only residual strain is present for different thermal expansion coefficient between epilayer and substrate. The screw and edge defect density were measured from FWHM of skew symmetric scan of (002) and (102) plane respectively.