Browsing by Subject "PECVD"

Now showing 1 - 6 of 6

Open Access
A figure of merit for optimization of nanocrystal flash memory design
(2008) Dâna, A.; Akca, I.; Aydınlı, Atilla; Turan, R.; Finstad, T. G.
Nanocrystals can be used as storage media for carriers in flash memories. The performance of a nanocrystal flash memory depends critically on the choice of nanocrystal size and density as well as on the choice of tunnel dielectric properties. The performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. We present a model that describes the charge/discharge dynamics of nanocrystal flash memories and calculate the effect of nanocrystal, gate, tunnel dielectric and substrate properties on device performance. The model assumes charge storage in quantized energy levels of nanocrystals. Effect of temperature is included implicitly in the model through perturbation of the substrate minority carrier concentration and Fermi level. Because a large number of variables affect these performance measures, in order to compare various designs, a figure of merit that measures the device performance in terms of design parameters is defined as a function of write/erase/discharge times which are calculated using the theoretical model. The effects of nanocrystal size and density, gate work function, substrate doping, control and tunnel dielectric properties and device geometry on the device performance are evaluated through the figure of merit. Experimental data showing agreement of the theoretical model with the measurement results are presented for devices that has PECVD grown germanium nanocrystals as the storage media. Copyright
Open Access
Formation of Ge nanocrystals with CW laser irradiation of Siox:Ge thin films
(2015) Gümüş, Melike
Germanium and silicon are the materials which have effective absorption in the visible and near infrared region of electromagnetic spectrum; therefore they are preferred for optoelectronic device and solar cell applications. Si and Ge are the material of choice when it comes to solar cell applications due to their being low cost, widely available and inert. They have indirect bandgap and the absorption coefficient of indirect bandgap materials is lower than direct ones. It is known that decreasing dimensions of materials to nanometric sizes cause transition from indirect bandgap to direct bandgap behavior along with increasing band gap. Therefore decreasing their dimensions both a shift of the band gap toward the blue as well as an increase in absorption can be achieved. In this work, thin films of SiOx:Ge were fabricated with different germanium concentrations and annealed with CW Ar+ laser operating at 488 nm that resulted in formation of Ge nanocrystals in the SiOx matrix. Composition analysis of as grown samples were done by Rutherford Backscattering Spectroscopy, optical properties were determined by ellipsometry. Nanocrystal formation within laser irradiated samples was confirmed by Raman spectroscopy. Data were also collected about crystal formation by scanning surface texture with stylus surface profilometer. As a result of all the analysis, it was shown that crystal formation depends on germanium concentration in the SiOx matrix and laser irradiation power density
Open Access
Germanium alloys for optoelectronic devices
(2008) Erbil, Ayşe
Silicon has been the backbone of the mainstream electronics of the last fifty years. It is however, used in conjunction with other matierals, mainly with its oxides and nitrides. Germanium, on the other hand, is also a group IV element and has been used in the early stages of transistor and detector development. In addition to Si/Ge heterojunctions, bandgap engineering through SiGe alloys has also been used in photodetectors. Recent progress in light emitting devices utilizing Si nanocrystals suggest the use of Ge1-xNx layers as barriers due to its suitable band offsets [1]. Experiments have shown that Ge1-xNx is also a promising material for applications in photodiodes, amplifiers, optic fibers, protective coatings, etc [1]. Both Si and Ge are, however indirect bandgap semiconductors, lacking efficient light emission. On the other hand, strong light emission observed in Si nanocrystals has made the study of semiconductor nanocrystals an expanding field of interest due to potential applications in novel optoelectronic devices [2]. These nanocrystals exhibit strong luminescence and nonlinear optical properties that usually do not appear in the bulk materials [2]. SiGe nanocrystals attract attention due to the possibility of a tunable band gap with composition. In this study, formation of Ge1-xNx thin films and SiGe nanocrystals by plasma enhanced chemical vapor deposition (PECVD) reactor has been studied. We present the growth conditions and experimental characterization of the resulting thin films and nanocrystals. We used ellipsometry, Raman Spectrometry, Fourier Infrared Spectrometry (FTIR) and X-ray photoelectron Spectroscopy (XPS). For SiGe nanocrystals, 4 peaks in the Raman Spectra were observed around 295 cm-1, 400 cm -1, 485 cm-1 and 521 cm-1. These peaks are assigned to the Ge-Ge, Si-Ge, local Si-Si and crystalline Si-Si vibrational modes, respectively [3]. For the Ge1-xNx thin films FTIR spectrum showed the existence of the Ge-N bonds and its band offsets determined by XPS confirm its suitability for optoelectronic devices.
Open Access
Plasma enhanced chemical vapor deposition of low-loss as-grown germanosilicate layers for optical waveguides
(SPIE, 2004) Ay, Feridun; Agan, S.; Aydınlı, Atilla
We report on systematic growth and characterization of low-loss germanosilicate layers for use in optical waveguides. Plasma enhanced chemical vapor deposition (PECVD) technique was used to grow the films using silane, germane and nitrous oxide as precursor gases. Chemical composition was monitored by Fourier transform infrared (FTIR) spectroscopy. N-H bond concentration of the films decreased from 0.43 ×1022 cm -3 down to below 0.06x 1022 cm-3, by a factor of seven as the GeH4 flow rate increased from 0 to 70 seem. A simultaneous decrease of O-H related bonds was also observed by a factor of 10 in the same germane flow range. The measured TE loss rates at λ=632.8 nm were found to increase from are 0.20 ± 0.02 to 6.46 ± 0.04 dB/cm as the germane flow rate increased from 5 to 50 seem, respectively. In contrast, the propagation loss values for TE polarization at λ-1550 nm were found to decrease from 0.32 ± 0.03 down to 0.14 ± 0.06 dB/cm for the same samples leading to the lowest values reported so far in the literature, eliminating the need for high temperature annealing as is usually done for these materials to be used in waveguide devices.
Open Access
Silicon based dielectrics : growth, characterization, and applications in integrated optics
(2005) Ay, Feridun
In recent years, growing attention has been paid to silicon based dielectrics, such as silicon oxynitrides, silicon nitrides, and semiconductor doped silicon oxides, all combined under the name silica on silicon technology. This attention has been motivated mainly due to their excellent optical properties such as well controlled refractive index and high transparency over a wide range of wavelength. In accordance with the main goal of this study that relied on the utilization of silicon based dielectrics and their optimization for applications in integrated optics, an emphasis was given to optimize the compositional and optical properties of these materials. A detailed quantitative compositional analysis using Fourier transform infrared spectroscopy resulted in identification of the germanosilicate dielectrics as the most promising candidates for use in integrated optics. The first reported systematic study of propagation losses for different-index planar waveguides by using prism coupling method was correlated with the compositional analysis. This study had an important outcome for planar waveguides fabricated with germanosilicate core layers resulting in the lowest propagation loss values reported so far for as deposited CVD-grown films at λ=1.55 µm, eliminating the need for costly and cumbersome annealing process. An improvement of the prism coupling technique led to a new approach for elasto-optic characterization of thin polymer films. This completely new method allows one to determine the optical anisotropy and out-of-plane mechanical properties and to correlate both in order to obtain the elasto-optical properties of thin polymer films, for the first time. Of interest as potential electro-optic material, we have concentrated on thermally poled germanosilicate films deposited on fused-silica substrates by PECVD. As a result of an optimization study, we demonstrated a record peak nonlinear coefficient of ∼1.6 pm/V, approximately twice as strong as the highest reliable value reported in a thermally poled fused silica glass. Finally, we have demonstrated several applications of this technology in the field of integrated optics. Since optical waveguides constitute the building blocks of many integrated optical devices, we had first concentrated on design and optimization of waveguides employing germanosilicates as the core layers. The final step of our work concentrated on design and implementation of microring resonator devices based on germanosilicate layers.
Open Access
Silicon oxynitride layers for applications in optical waveguides
(2000-09) Ay, Feridun
Silicon oxynitride layers, aimed to serve as the core material for optical waveguides operating at l.55µm, v.-ere grown by a PECVD technique using SiH4, N20, and NH3 as precursor gases. The films were deposited at 350 °c, 13.56 MHz RF frequency, and 1 Torr pressure by varying the flow rates of N20 and l\"H3 gases. The resulting refractirn indices of the layers varied between 1.47 and 2.0. The compositional properties of the layers were analyzed by FTIR and ATR infrared spectroscopy techniques. A special attention was given to the N-H bond stretching absorption at 3300-3400 cm-1, since its first overtone is known to be the main cause of the optical absorption at l.55µm. An annealing study was performed in order to reduce or eliminate this bonding type. For the annealed samples the corresponding concentration was strongly reduced as verified by FTIR transmittance and ATR methods. A correlation between the N-H concentration and absorption loss was verified for silicon oxynitride slab waveguides. Moreover, a single mode waveguide with silicon oxynitride core layer was fabricated. lts absorption and insertion loss values were determined by butt-coupling method, resulting in low loss waveguides.