Growth, optical and structural investigation of sige nanostructures

• Autores: Ioana Carmen Marcus
• Directores de la Tesis: María Isabel Alonso Carmona (dir. tes.), Javier Rodríguez Viejo (dir. tes.), Alejandro Rodolfo Goñi (dir. tes.)
• Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2012
• Idioma: inglés
• Tribunal Calificador de la Tesis: Anna Roig Serra (presid.), Frank Güell Vilà (secret.), Xavier Cartoixà Soler (voc.)
• Materias:
  • Física
    • Física del estado sólido
      • Semiconductores
• Enlaces
  • Tesis en acceso abierto en: DDD
• Resumen

  • Summary The main purpose of this thesis has been to develop a new process in order to achieve reproducible control of the dimensions and spatial location of Ge nanostructures (islands and nanowires) formed on Si substrates. In this context, our primary objective was to form bi-dimensional arrays of Au nanoclusters using mass-filtered focused ion beam (FIB) with Au2+ ions and to use them as patterns to nucleate the Ge nanostructures. The first chapter is dedicated to a review of: 1) Si and Ge fundamental properties, 2) generalities about the SiGe alloy layers, and 3) growth of the self assembled Ge islands and nanowires. Details about the nanostructure growth mode are exposed. The nanowire growth mechanism ¿ vapor liquid solid (VLS) ¿ is described along with the Au behavior as a catalyst. Chapter 2 gives an overview of the patterning (focused ion beam) and growth (molecular beam epitaxy) techniques. Besides them, methods for sample characterization from a morphological and optical point of view are described. The third chapter is devoted to investigate the influence of Ge nanoclustering on the composition and strain determination of Si1-xGex/Si alloys. The cause of the large scatter of the phonon frequency values measured by Raman for relaxed alloy layers that is found in the literature was revealed as due to Ge nanoclustering effects. This phenomenon occurs in SiGe alloy layers as a result of employing nonequilibrium epitaxial growth methods such as molecular beam epitaxy. The obtained alloy layers presenting Ge nanoclusters were thermally treated and the effect of the cumulative annealings of randomizing the Ge atom distribution within the alloy layer was demonstrated. Additionally, we proved that the strain and composition of the SiGe alloy layers can be evaluated and determined by performing a single Raman measurement. An analytical/graphical method to estimate the Ge composition and strain status within the SiGe alloy layers independent of their strain status was elaborated. The growth of the Ge nanostructures on patterned Si substrates was investigated and is presented in chapter 4. For this purpose a three step process was developed, based mainly on: i) focused ion beam (FIB) patterning, ii) annealing and iii) molecular beam epitaxy growth. Si substrates with different crystallographic orientations [(001) and (111)] were patterned using a FIB equipment with Au2+ ions. Pattern evolution as a consequence of the employed FIB parameters was examined, and their progress with the thermal treatments performed using a large range of temperatures and times was explored. The formation of AuSi clusters inside the FIB patterned areas during the annealings was achieved. Depending on the ion dose used to pattern the substrates, after Ge deposition, both islands and/or nanowires were formed. The effect of the Au amount and hole dimensions on the growth mode, morphology, and two-dimensional ordering of the nanostructures was investigated.