Pulsed laser deposition and processing of wide band gap semiconductors and related materials

The present work describes the novel, relatively simple, and efficient technique of pulsed laser deposition for rapid prototyping of thin films and multi-layer heterostructures of wide band gap semiconductors and related materials. In this method, a KrF pulsed excimer laser is used for ablation of polycrystalline, stoichiometric targets of wide band gap materials. Upon laser absorption by the target surface, a strong plasma plume is produced which then condenses onto the substrate, kept at a suitable distance from the target surface. We have optimized the processing parameters such as laser fluence, substrate temperature, background gas pressure, target to substrate distance, and pulse repetition rate for the growth of high quality crystalline thin films and heterostructures. The films have been characterized by x-ray diffraction, Rutherford backscattering and ion channeling spectrometry, high resolution transmission electron microscopy, atomic force microscopy, ultraviolet (UV)-visible spectroscopy, cathodoluminescence, and electrical transport measurements. We show that high quality AIN and GaN thin films can be grown by pulsed laser deposition at relatively lower substrate temperatures (750-800 degrees C) than those employed in metalorganic chemical vapor deposition (MOCVD), (1000-1100 degrees C), an alternative growth method. The pulsed laser deposited GaN films (similar to 0.5 mu m thick), grown on AlN buffered sapphire (0001), shows an x-ray diffraction rocking curve full width at half maximum (FWHM) of 5-7 are-min. The ion channeling minimum yield in the surface region for AIN and GaN is similar to 3%, indicating a high degree of crystallinity. The optical band gap for AIN and GaN is found to be 6.2 and 3.4 eV, respectively. These epitaxial films are shiny, and the surface root mean square roughness is similar to 5-15 nm. The electrical resistivity of the GaN films is in the range of 10(-2)-10(2) Omega-cm with a mobility in excess of 80 cm(2)V(-1)s(-1) and a carrier concentration of 10(17)-10(19) cm(-3), depending upon the buffer layers and growth conditions. We have also demonstrated the application of the pulsed laser deposition technique for integration of technologically important materials with the III-V nitrides. The examples include pulsed laser deposition of ZnO/GaN heterostructures for UV-blue lasers and epitaxial growth of TiN on GaN and SiC for low resistance ohmic contact metallization. Employing the pulsed laser, we also demonstrate a dry etching process for GaN and AlN films.