Browsing by Subject "Photoluminescence Spectroscopy"

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Open Access
Infrared luminescence of annealed germanosilicate layers
(Elsevier, 2014-03) Tokay, M. S.; Yasar, E.; Agan, S.; Aydınlı, Atilla
In the light of growing importance of semiconductor nanocrystals for photonics, we report on the growth and characterization of annealed germanosilicate layers used for Ge nanocrystal formation. The films are grown using plasma enhanced chemical vapor deposition (PECVD) and post-annealed in nitrogen at temperatures between 600 and 1200 degrees C for as long as 2 h. Transmission electron microscopy (TEM), Raman scattering and photoluminescence spectroscopy (PL) has been used to characterize the samples both structurally and optically. Formation of Ge precipitates in the germanosilicate layers have been observed using Raman spectroscopy for a variety of PECVD growth parameters, annealing temperatures and times. Ge-Ge mode at similar to 300 cm(-1) is clearly observed at temperatures as low as 700 degrees C for annealing durations for 45 min. Raman results indicate that upon annealing for extended periods of time at temperatures above 900 degrees C; nanocrystals of few tens of nanometers in diameter inside the oxide matrix and precipitation and interdiffusion of Ge, forming SiGe alloy at the silicon and oxide interface take place. Low temperature PL spectroscopy has been used to observe luminescence from these samples in the vicinity of 1550 nm, an important wavelength for telecommunications. Observed luminescence quenches at 140 K. The photoluminescence data displays three peaks closely interrelated at approximately 1490,, 1530 and 1610 nm. PL spectra persist even after removing the oxide layer indicating that the origin of the infrared luminescent centers are not related to the Ge nanocrystals in the oxide layer. (C) 2013 Elsevier B.V. All rights reserved.
Open Access
Size controlled germanium nanocrystals in dielectrics : structural and optical analysis and stress evolution
(2017-08) Bahariqushchi, Rahim
Group IV semiconductor nanocrystals, namely silicon and germanium have attracted much interest in the past two decades due to their broad applications in photovoltaic, memory, optoelectronic, medical imaging and photodetection devices. Generally, there are two major features of semiconducor nanocrystals: First, spatial confinement of charge carriers which leads to the significant changes in optical and electronic properties of materials as a function of size. This effect gives the possibility to use the size and shape of the nanocrystals to tune the energy of electronic energy states. Second feature of nanocrystals, is the increased of surface area to volume ratio of the nanocrystal with reducing size. This leads to an enhanced role of the effects related to surface and interface of the nanocrystal. Furthermore, stress on the nanocrystals can lead modification of the band structure as well as in uencing the crystallization of the nanomaterials. Recent works show that measurement and control of the stress can open the way for strain engineering of the electronic band structure, thereby opening the way for new physics and applications. In this thesis, we first carry out a study on the synthesis of germanium embedded in silicon nitride and oxide matrices. In uence of the annealing method as well as germanium concentration on the formation of nanocrystals is discussed. It was found that Ge concentration and annealing play important roles in the formation of the Ge nanocrystals. With crystallographic data obtained from high resolution transmission electron microscopy, quantitative analysis of stress state of germanium nanocrystals have been done by analyzing Raman peak shift of embedded nanocrystals taking into account the phonon confinement effect. Finally, using stressors as buffer layers, superlattices of Ge nanosheets were studied to understand the effects of the stressors on the stress state of Ge nanocrystals. We demonstrate that it is possible to tune the stress on the Ge nanocrystals from compressive to tensile. Finally we showed a three dimensional Ge quantum solid that can be used in optoelectronic applications.