High-resolution X-ray diffraction analysis of AlxGa1−xN/InxGa1−xN/GaN on sapphire multilayer structures: Theoretical, simulations, and experimental observations

Abstract

The work presents a comparative study on the effects of In incorporation in the channel layer of AlGaN/GaN type-II heterostructures grown on c-plane sapphire by Plasma Assisted Molecular Beam Epitaxy. The structural characterizations of these samples were performed by High-Resolution X-Ray Diffraction (HRXRD), X-ray Reflectivity (XRR), Field Emission Scanning Electron Microscopy, and High Resolution Transmission Electron Microscopy. The two-dimensional electron gas in the AlGaN/GaN and AlGaN/InGaN interface was analyzed by electrochemical capacitance voltage and compared with theoretical results based on self-consistent solution of Schördinger–Poisson equations. The carrier profile shows enhanced confinement in InGaN channel (1.4393 × 1013 cm−2 compared to 1.096 × 1013 cm−2 in GaN). 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 plots, whereas stress was examined from modified plot of the same data assuming Uniform Stress Deformation Model. Moreover, the twist angle was measured from skew symmetric scan of (102), (103), and (105) plane along with (002) symmetric plane. The composition and strain/relaxation state of the epilayers were observed in detail by reciprocal space mapping (RSM). The symmetric (002) triple axis RSM and asymmetric (105 and 114) double axis RSM of grazing incidence and exit geometry were carried out on each sample. The defect density was measured from HRXRD curves of skew symmetric (002) and (102) reflection plane. The Al and In 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 HRXRD measured thicknesses of the layers were further confirmed by cross sectional electron micrographs.

Structural, optical, and transport properties of AlGaN/GaN and AlGaN/InGaN heterostructure on sapphire grown by plasma assisted molecular beam epitaxy

“…Thus, the confinement of the electrons (solved by self-consistent solution of Schr€ odinger-Poisson equations) is indeed higher for the AlGaN/InGaN interface in S2 rather than in AlGaN/GaN interface of S1. 29 Theoretical estimations also confirm this enhancement of carrier confinement at the AlGaN/InGaN interface with respect to the AlGaN/GaN due to both the larger band offset and the higher lattice mismatch strain, which significantly enhances the piezoelectric field at the barrier layer. Additionally, the intrinsic nature of strong spontaneous polarization effect of the InGaN layer also increases the carrier density at the interface.…”

“…29 The thick InGaN layer is also nearly relaxed according to M-B critical layer thickness model and has been experimentally confirmed. 29,47 The measurements show that in-plane strain observed in the AlGaN barrier for both S1 and S2 samples were tensile in nature. As a result, the piezoelectric polarization is also negative for both AlGaN/GaN and AlGaN/ InGaN structure.…”

“…The strain tensor is measured from RSM. 29 In mole-fraction (see Table II). Now, the biaxial stress is related to the in-plane strain e a by r a ¼ Ye a , where Y is the biaxial modulus and can be expressed as c 11 þ c 12 À 2…”

“…31,32 The lattice constants and in-plane strain were measured from symmetric TA (002), (006) and double axis (105), (114) asymmetric reciprocal space mapping (RSM) (detailed analysis was given in Ref. 29). The measured in-plane strain on AlGaN layer from the RSM shows that AlGaN barrier is pseudomorphic on the GaN (S1 sample) and InGaN (S2 sample) buffer layer, respectively.…”

“…[ 29 The RTPL was carried out using a RPM 2000 equipped with 266 nm Q-Switch laser with 10.61 W/cm 2 power densities. Raman scattering measurements were carried out at the center region of each sample with ZðXXÞ Z back-scattering geometry with the incident light along the [0001] direction using a fiber-coupled micro-Raman spectrometer (model TRIAX550, JY) at room temperature.…”