Investigation of the presence of metal droplets after pulsed InN and GaN epitaxial growth using atomic force microscopy and nanoindentation

The purpose of this study was to detect the presence of excess Ga- or In-metal on the surface of GaN or InN epitaxial layers, using AFM ¹ and nanoindentation. We ascertain that in this case AFM phase-contrast images could not be used for actual material composition distribution mapping due to the large topography variation of the surface. Force spectroscopy on droplets shows observable hysteresis between approach and retraction curves as well as the presence of adhesion forces on the retraction curves. This indicates the presence of compliant material - Ga- or In-metal in our case. Etching away these droplets with HCl solution confirmed their metallic consistence. The presence of In-metal is confirmed by XRD ² measurements on the InN sample with the presence of the (1 0 1)In reflection plane peak, which disappears after etching. The method becomes extremely useful for detection of Ga-metal which cannot be detected by XRD. We have also observed that this technique is also able to identify the situation where a metal droplet is partially nitrided on the droplet surface, so that it cannot be removed by etching, a situation where the droplets presence might not otherwise be observed by phase-contrast measurements, or by other techniques.