Location of Residual Donors in GaN Epitaxial Layers
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ABSTRACT Optically-detected electron-nuclear double resonance (ODENDOR) studies at 24 GHz on high-resistivity GaN films grown on A120 3 have been combined with x-ray diffraction measurements to obtain information on the location of the residual shallow donors. Strong ODENDOR assigned to 69 ,71Ga lattice nuclei was detected on the g=1.951 effective-mass donor resonance found on the 2.2 eV emission bands. The x-ray studies reveal that the layers are under biaxial compression with high values of strain (- 2-3 x 10-3). The quadrupole splittings for 69Ga are smaller than those reported for strain-free samples by 15-25 %. The dominant sources of the local electric field gradient (EFG) responsible for the splittings are attributed to the wurtzite crystal structure and the strain fields that arise from the lattice constant mismatch and the difference in thermal expansion coefficients. An EFG/strain relationship of 3 x 1022 Vm-2 per unit strain at the 69,7 Ga nuclei is deduced. The ODENDOR can be described with asymmetry parameter rl=0. This provides evidence that the donors are in the crystallites rather than near grain boundaries. INTRODUCTION High-resolution transmission electron microscopy and x-ray diffraction studies [1] show that GaN/A12 0 3 films are not single crystalline but, rather, are composed of multi-connected hexagonal crystallites with sizes of - 0.2-5 gm. Within this structure, the local environment and, thus, the properties of point defects will depend on their position with respect to the grain boundaries. The motivation for this work is to determine the location of residual defects and dopants in GaN epitaxial layers. Toward this goal, optically-detected electron-nuclear double resonance (ODENDOR) experiments on two high-resistivity (HR) GaN films grown on A120 3 are correlated with high-precision x-ray measurements. The ODENDOR, detected on the effective-mass (EM) donor resonance observed in optically-detected magnetic resonance (ODMR) of the 2.2 eV "yellow" emission bands [2-5], provides a precise value of the electric field gradient (EFG) at the 697Ga lattice nuclei situated around 2.5 nm from the donors while the x-ray measurements give an average, macroscopic value of the residual strain in the films. The quadrupole splittings were measured for two films on sapphire and values were obtained from the literature for a free-standing, 220 jim-thick GaN platelet and for a bulk powder sample. The in-plane and out-of-plane lattice constants determined from room-temperature x-ray measurements indicate that the films are under biaxial compression with high values of strain (2 - 3 x 103). The combined quadrupole and x-ray data provide the electric field gradient vs. strain relationship for 69 71 ' Ga nuclei in GaN epitaxial films of 3 x 1022 Vm-2 per unit strain. The symmetry of the EFG suggests that the donors are in the crystallites. While these results are based on only two samples, further work should refine the EFG/strain relationship and confirm the location of the donors. 561 Mat. Res. Soc. Symp. Proc
