ABSTRACT
Abstract. The wing deformation resulting from the thermal stress distributions in the GaN film obtained by the air-bridged lateral epitaxial growth (ABLEG) on the sapphire substrates has been simulated by finite element method (FEM) based on the two-dimensional orthotropic linear elastic theory. The compressive stress exists in the GaN-seed layer due to large mismatch of the thermal expansion coefficient (ocraN <ocsapphire)· In the ABLEG-GaN, large compressive stress concentrates at the coalescence boundary region and the tensile stress exists at the back side of the wings. As a result, the wings bend and the coalescence boundary becomes higher than the seed region. The tilt angle of the ABLEG-GaN film, whose thickness is ~3pm, is about 0.2 degrees, which is in good agreement with the experimental results. 1
1. Introduction
Group III nitrides are highly promising for applications in blue and ultraviolet optoelectronic devices, high-temperature, high-power transistors, because of the wide bandgap varying from 1.9eV for InN to 6.3eV for AIN. Recent studies on the various selective area growth (SAG) techniques, such as lateral epitaxial overgrowth (LEO) or Pendeo-epitaxy of group III nitrides on sapphire and SiC substrates via metalorganic vapor phase epitaxy (MOVPE) have shown that they were extremely effective in reducing the threading dislocation density and drastically improved the performance of the devices, such as violet laser diodes[l-3]. However, there is a problem that the wing tilt (about 1 degree) exits. It is supposed that the wing tilt originates from the stress at the interface between the wing and the mask, and the thermal stress due to the mismatch of thermal expansion coefficients between GaN-layer and a substrate. The wing tilt induces the dislocations near the coalescence region of the wings [4]. We have developed a new LEO structure, namely
“Air-Bridged Lateral Epitaxial Grown (ABLEG)”-GaN [5-6], which has no contact between the wing and the mask, as shown in Fig.l. Since the wings in the ABLEG-GaN have no contact with the masks, there is no stress in the interface between them. Therefore, the wing tilt drastically decreased in the ABLEG-GaN. However, there is a little tilt of the wings in the ABLEG-GaN. It is supposed that the wing tilt is due to the thermal stress generated between the GaN-seed layer and the sapphire substrate, not the stress in the interface between wings and masks. The local stress analysis of the epitaxially laterally-overgrown GaN deposited on S1O2 masks was reported [7]. However, there are few reports on the quantitative analysis of the deformation induced from only the thermal stress in the SAG.
