Ultrathin films based on III-nitrides have become a principal element of many optoelectronic, high temperature or high frequency devices. Quality of the interfaces between a deposited thin film and substrate or between individual layers is a crucial parameter for the operational performance and lifetime of these devices. Improvements of the technological steps leading to the higher purity of the interface and thin film quality are therefore under strong focus of electronic industry. Seeking new industrially applicable substrates (e.g. silicon) for low-cost production and alternative growth techniques to the most widespread ones (e.g. MOCVD) has been a subject of intensive studies as well.
In this contribution the study of initial periods of GaN ultrathin films grown on Si(111) and Si(100) substrates is presented. GaN is synthetized under UHV condition by concurrent nitrogen low-energy ion and gallium atomic beams. Nitrogen ions of hyperthermal energies (20 - 200 eV) impinging on the surface bring an excess energy into the reactions on the substrate surface and thus reduce a need for high substrate temperatures typical for conventional GaN deposition methods (~ 1000 °C). In this way the synthesis runs at significantly lower substrate temperatures (< 600 °C).
The growth of ultrathin films of a several monolayer thickness was controlled by ion impact energy, substrate temperature and ion-to-atom arrival ratio. In comparison with our previous experiments, the deposition process was improved by the installation of a Wien filter for mass separation of nitrogen ion beam. It resulted in higher purity of nitrogen ion beam and, hence, in better conditions for preparation of oxygen and carbon free GaN layers under UHV environment.
The chemical composition of the grown layers was analysed in situ by XPS, structure topography and morphology by LEED and ex-situ AFM, respectively. In the last experiments, the maximum content of Ga-N bonds in the films was increased up to 90 % at T < 600 °C.
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