Germanium thin film integration on silicon substrates via oxide heterostructure buffers

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Titel: Germanium thin film integration on silicon substrates via oxide heterostructure buffers
Autor(en): Giussani, Alessandro
Erstgutachter: Prof. Dr. Joachim Wollschläger
Zweitgutachter: Dr. Thomas Schroeder
Zusammenfassung: Germanium-on-Insulator (GeOI) substrates combine the potential of the Silicon-on-Insulator (SOI) technology with the superior properties of Ge over Si in terms of a) charge carrier mobilities (relevant for CMOS), b) optical bandgap and absorption coefficient (of impact for infra-red photodetectors and high-bandwidth optical interconnects), and c) lattice and thermal match with GaAs (of interest for integration of III-V based optoelectronics and photovoltaics on the mainstream Si platform). Several techniques are under study for the achievement of GeOI structures, such as layer transfer, Ge condensation, and Ge epitaxial overgrowth of Si via crystalline oxide templates. Following the GeOI heteroepitaxial approach, Ge was deposited by molecular beam epitaxy (MBE) on PrO2(111) / Si(111) support systems, and the initial growth stages were studied by means of in-situ reflection high energy electron diffraction (RHEED), and x-ray and ultra-violet photoelectron spectroscopy (XPS and UPS, respectively). It was shown that in the first evaporation stages an amorphous GeO2-like layer forms as a result of the Ge adatom interaction with the PrO2 substrate, namely the diffusion of lattice oxygen from the dielectric into the growing semiconductor deposit. In consequence the PrO2(111) buffer oxide is fully reduced to an oxygen-deficient cub (cubic) Pr2O3(111) film structure. Since no oxidizing species are available in the process anymore, the Ge oxide layer converts under continuous Ge evaporation to GeO, which is volatile at the deposition temperature (~550°C). The sublimation of GeO uncovers the cub-Pr2O3(111) surface, which finally provides a thermodynamically stable template for the heteroepitaxial growth of elemental Ge. A Volmer-Weber growth mode is initially observed, which, by properly tuning the deposition parameters, results after island coalescence in the formation of a closed and flat Ge / cub-Pr2O3 / Si heterostructure. Ge epilayer thickness (in the range 20-1000 nm) and morphology were studied ex-situ by means of x-ray reflectivity (XRR) and secondary electron microscopy (SEM). Dynamic secondary ion mass spectroscopy (D-SIMS) was employed to study the chemical compositions of the Ge films, which turned out to be free from Si and Pr impurities at the sensitivity of some parts-per-billion (ppbs), even after supplying a high thermal budget. This is an important achievement, because in most applications (i.e., optoelectronics), there is the demand for ultra-pure Ge epilayers. Then, laboratory- and synchrotron-based x-ray diffraction (XRD) analyses were performed to assess the epitaxial relationship and the defect structure of the Ge epifilms. It was demonstrated that the Ge layers grow single crystalline with (111) orientation and an exclusive type-A stacking configuration on the type-B cub-Pr2O3(111) / Si(111) support system. Furthermore, the Ge epifilms are fully relaxed in the thickness range 20-1000 nm. Finally, XRD techniques combined with transmission electron microscopy (TEM) permitted the identification and the quantification of three main types of defects at work during the growth of the Ge epi-layers, namely rotation twins, stacking faults and microtwins across {11-1} net-planes. These structural flaws were studied as a function of Ge film thickness and after annealing at 825°C for 30 min in ultrahigh vacuum. It turned out that rotation twins constitute less than 1% of the Ge matrix, are located at the Ge(111) / cub-Pr2O3(111) interface and their amount can be lowered by the thermal treatment. Microtwins across {11-1} were detected only in closed Ge films, after Ge island coalescence. The fraction of Ge film volume affected by microtwinning is constant within the thickness range 20–260 nm. Beyond 260 nm, the density of microtwins is clearly reduced, resulting in thick layers with a top part of higher crystalline quality. Microtwins were found to be insensitive to the post-deposition annealing (PDA). Instead, the density of stacking faults across {11-1} planes decreased after the thermal treatment. In conclusion, the defect density was proved to diminish with increasing Ge thickness and after annealing. A defect density of 10^8-10^9 per cm^2 was estimated in case of a ~ 1000 nm-thick Ge film after PDA.
Schlagworte: germanium; silicon; praseodymia; oxide buffer; semiconductor ; MBE; epitaxy
Erscheinungsdatum: 16-Apr-2010
Enthalten in den Sammlungen:FB04 - E-Dissertationen

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