Demonstration of Self-Assembled InGaN/GaN Superlattice on GaN Template Grown by Plasma-Assisted Molecular Beam Epitaxy

Abstract

Abstract Body: The (In,Ga)N material system is attractive for photovoltaic applications as its optical absorption covers the solar spectrum. Thick InGaN films (hundreds of nm thick) with InN mole fraction larger than 10% are required for this purpose. However, the growth of high In-content InGaN films remains challenging due to 10% lattice mismatch between InN and GaN and the large difference between the thermal stability of GaN and InN. While the optimum growth temperature of GaN by plasma-assisted molecular beam epitaxy (PAMBE) is ~700 °C-730 °C, the growth temperature of InGaN is typically below 600 °C. Therefore, lower growth temperatures are required to grow InGaN films within the miscibility gap. InGaN films can be grown pseudo-morphically on GaN below a critical thickness. As the thickness increases beyond the critical thickness, the InGaN film relaxes. This relaxation may occur through different mechanisms depending on the growth technique and growth conditions (e.g., growth temperature). V-defect formation from threading dislocations is a common way of strain relaxation, especially in InGaN films grown via metal organic chemical vapor deposition, and has been observed in molecular beam epitaxy (MBE) grown InGaN films as well [1]. Pyramidal slip through the Matthews-Blakeslee mechanism and basal plane slip of misfit dislocation are other mechanisms through which strain relaxation occurs in InGaN films. Critical thickness reduces as InN mole fraction in InGaN increases. Bazioti et al [2] studied relaxation mechanism in InGaN films grown by PAMBE. They showed that at lower indium contents, strain relaxation dominantly occurred via formation of V-defects, with concurrent formation of an indium-rich interfacial zone. As indium content increased, this mechanism was replaced by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal plane stacking faults and threading dislocations in the rest of the film. In this talk, we will present growth of 700 nm-thick InGaN with an average InN mole fraction of ~8% coherently strained to GaN confirmed by XRD reciprocal space map scans. This is several times thicker than the critical thickness of In0.08Ga0.92N. Surprisingly, we did not observe generation of new threading dislocations in the InGaN films or misfit dislocations at the interface. Instead, we show that the strain is managed by self-formation of a superlattice structure. The superlattice was confirmed via scanning transmission electron microscopy and atom probe tomography. Photoluminescence (PL) measurements at room temperature revealed peak wavelength of ~450 nm, corresponding to In0.18Ga0.82N with a full width half max (FWHM) of ~36 nm. Reference: 1. C. Bazioti et al. Structure and strain variation in InGaN interlayers grown by PAMBE at low substrate temperatures Phys. status solidi 252 1155–62 (2015). 2. C. Bazioti et al. Defects, strain relaxation, and composition grading in high indium content InGaN epilayers grown by molecular beam epitaxy J. Appl. Phy. 118,155301 (2015).