Reduction in dislocation density and strain in GaN thin films grown via maskless pendeo-epitaxy
Maskless pendeo-epitaxy (PE) involves the lateral and, commonly, the vertical growth of cantilevered "wings" of material from the side-walls of unmasked etched forms. Cross-sectional SEM micrographs revealed that films grown at 1020degreesC exhibited similar vertical (0001) and lateral (1120) growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(1120). The (1120) surface was atomically smooth under all growth conditions with an RMS = 0.17 nm. High resolution X-ray diffraction (HRXRD) and atomic force microscopy of the PE films confirmed transmission electron microscopy results regarding the reduction in dislocation density in the wings. Measurement of strain indicated that the wing material is crystallographically relaxed as evidenced by the increase in the c-axis lattice parameter and the upward shift of the E2 Raman line frequency. However, tilting of the wings of less than or equal to0.15degrees occurred due to the tensile stresses in the stripes induced by the mismatch in the coefficients of thermal expansion between the GaN and the underlying substrate.