Effects of Silicon Doping and Threading Dislocation Density on Stress Evolution in AlGaN Films
- PDF / 2,666,154 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 60 Downloads / 227 Views
Effects of Silicon Doping and Threading Dislocation Density on Stress Evolution in AlGaN Films Joan M. Redwing,1,2 Ian C. Manning,1 Xiaojun Weng,2 Sarah M. Eichfeld,3 Jeremy D. Acord,3 Mark A. Fanton3 and David W. Snyder3 1 Department of Materials Science and Engineering, 2Materials Research Institute, 3ElectroOptics Center, The Pennsylvania State University, University Park, PA 16802 USA ABSTRACT In-situ wafer curvature measurements were used to study the effect of Si doping on intrinsic growth stress during the metalorganic chemical vapor deposition (MOCVD) growth of AlxGa1-xN (x=0-0.62) layers on SiC substrates. Post-growth transmission electron microscopy (TEM) characterization was used to correlate measured changes in stress with changes in film microstructure. Si doping was found to result in the inclination of edge-type threading dislocations (TDs) in AlxGa1-xN which resulted in a relaxation of compressive stress and generation of tensile stress. The experimentally measured stress gradient was similar to that predicted by an effective climb model. Dislocation inclination resulted in a reduction in the TD density for Si-doped layers compared to undoped AlxGa1-xN likely due to increased opportunities for dislocation interaction and annihilation. The TD density, which increased with increasing Alfraction, was found to significantly alter the stress gradients in the films. Film stress was also observed to play a role in TD inclination. In undoped AlxGa1-xN, TD inclination was observed only when the film grew under a compressive stress while in Si-doped AlxGa1-xN, TD inclination was observed independent of the sign or magnitude of the film stress. Si dopants are believed to alter the concentration of surface vacancies which gives rise to dislocation jog via a surfacemediated climb mechanism. INTRODUCTION There is significant interest in the fabrication of high Al fraction AlxGa1-xN alloys for UV emitter and detector applications. 1 The ionization energy of donors in AlxGa1-xN increases with increasing Al fraction, 2 consequently, high concentrations of Si dopants are required in order to obtain low resistivity n-type layers. Si doping of GaN and AlxGa1-xN produces tensile strain in the film 3 which is relaxed via the formation of channeling cracks. Film cracking generally becomes more problematic as the Al fraction and Si doping level are increased which is problematic for device fabrication. While strain mitigating layers such as strained-layer superlattices 4 have been employed to reduce cracking in AlxGa1-xN, there is interest in understanding the origin of Si-induced tensile stress and its relation to film composition. There are several mechanisms that can give rise to a tensile stress in AlxGa1-xN films. Tensile stress can arise due to epitaxial mismatch such as when AlxGa1-xN is grown on GaN. Differences in the coefficient of thermal expansion (CTE) between the AlxGa1-xN film and the substrate can also give rise to a tensile film stress upon cooling from the growth temperature. This is a particular problem f
Data Loading...
