Sio2 içinde yarıiletken nanokristallerin iyon ekme tekniği ile oluşturulması

In this study, we used ion implantation technique to synthesize semiconductor (Ge, Si) nanocrystals in SiO2 matrix. Ge and Si nanocrystals have been successfully formed by Ge and Si implantation and post annealing. Implanted samples were examined by characterization techniques such as TEM, XPS, EDS, SAD, SIMS, PL, Raman and FTIR spectroscopy and the presence of Ge and Si nanocrystals in the SiO2 matrix has been evidenced by these measurements. It was shown that implantation dose, implantation energy, annealing temperature, annealing time and annealing ambient are important parameters for the formation and evolution of semiconductor nanocrystals embedded in SiO2 matrix. The size and size distribution of Ge and Si nanocrystals were estimated successfully by fitting Raman and PL spectra obtained from Ge and Si implanted samples, respectively. It was demonstrated that Si implanted and post annealed samples exhibit two broad PL peaks at ~ 625 and 850 nm, even at room temperature. Origin of these peaks was investigated by temperature, excitation power and excitation wavelength dependence of PL spectrum and etch-measure experiments and it was shown that the peak observed at ~ 625 nm is related with defects (clusters or chain of Si located near the surface) while the other is related to the Si nanocrystals. As an expected effect of quantum size phenomenon, the peak observed at ~ 850 nm was found to depend on the nanocrystal size. Finally, the formation and evolution of Ge and Si nanocrystals were monitored by FTIR spectroscopy and it was shown that the deformation in SiO2 matrix caused by ion implantation tends to recover itself much quicker in the case of the Ge implantation. This is a result of effective segregation of Ge atoms at relatively low temperatures.Ph.D. - Doctoral Progra

Si Alttaş Üzerine Tip II InAs/GaSb Süperörgü Yapıların Moleküler Demet Epitaksi Yöntemiyle Büyütülmesi

Tip II InAs/GaSb süperörgü (T2SL) yapılar geniş aralıkta (3-30 µm) kızılötesi algılayıcılığı olan ve özellikle askeri alanlar olmak üzere uydu/uzay çalışmalarında, sağlık ve çevresel alanlarda kullanılan yapılardır. T2SL yapılar, maliyet ve tekrarlanabilirlik göz önüne alındığında kızılötesi dedektörler arasında en yaygın olarak kullanılan II-VI grubu Cıva-Kadmiyum-Tellür (MCT) yapılara karşı ciddi bir alternatif oluşturmaktadırlar. MCT yapılara göre diğer avantajlarının yanında maddi avantajlar da sağlayan T2SL yapıların günümüzde daha yaygın olarak kullanılabilmeleri için bu yapıların maliyetlerinin daha da azaltılması ve özellikle günümüz elektronik endüstrisinin en önemli malzemesi olan Silisyumla (Si) bütünleştirilmesi gerekmektedir. Bu proje kapsamında, Si alttaşlar üzerine kaliteli tampon GaSb katmanlar ve sonrasında da bu yapının üzerine T2SL yapılar büyütülmüştür. Moleküler demet epitaksi yöntemiyle büyütülen yapılar yüksek çözünürlüklü X-ışını kırınımı, taramalı elektron mikroskobu, yüksek çözünürlüklü geçirimli elektron mikroskobu, atomik kuvvet mikroskobu, fotolüminesans ve fototepki teknikleri ile incelenmiştir. Si üzerine büyütülen GaSb epikatmanlardan şu ana kadar raporlarmış en iyi yüzey pürüzlülüğü değerleri elde edilmiştir.Type II InAs/GaSb superlattice (T2SL) structures have a wide range (3-30 µm) infrared photoresponse and they can be used in satellite/space applications, health and environmental applications and especially in the military fields. Taking into account their cost and reproducibility, T2SL structures are practical alternative to the Mercury-Cadmium-Telluride (MCT) which is a group II-VI material and the most commonly used infrared detector among others. As well as other benefits compare to the MCT structures, T2SL structures are cost effective structures. However, it is required to reduce their costs and fabricate integrated devices with Silicon (Si) which is a well-known and widely used material in electronic industry to extend the application of T2SL structures. In this project, high quality GaSb buffer layers were grown on Si substrates and then T2SL structures were grown on those buffer layers. The structures were grown by using molecular beam epitaxy and characterized by using high resolution X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy, photoluminescence and photoresponse. The best surface roughness values reported upto now are obtained from the GaSb epilayers grown on Si

Direct growth of type II InAs/GaSb superlattice MWIR photodetector on GaAs substrate

We report on the direct growth and characterization of type-II InAs/GaSb superlattice (T2SL) MWIR photodetector structure grown on a GaAs substrate by molecular beam epitaxy. The designed photodetector structure contains 140 period of 8.0 ML InAs/8.3 ML GaSb p-i-n SL structure with a 50% cutoff wavelength of 3.78 pm. We achieved a peak specific detectivity (D*) and differential resistance area product at zero bias (R(0)A) of 1.3 x 10(12)cm Hz(1/2) W-1 and 10(4) Omega cm(2) at 80 K, respectively. The obtained D* value is the best value reported up to now for a T2SL MWIR p-i-n photodetector grown on a GaAs substrate. The crystalline quality and the uniformity of the grown structure were verified by high resolution X-ray diffraction method by measuring three different spots on grown structure on a full 4 inch SI GaAs substrate

UNPASSIVATED HIGH OPERATING TEMPERATURE GaInAsSb INFRARED PHOTODETECTOR GROWN ON GaAs SUBSTRATE

Ga0.87In0.13As0.4Sb0.96 photodiode structure was grown on semi-insulating 4” GaAs substrate by molecular beam epitaxy. The composition, crystal quality and dislocation density of epilayers were determined by high resolution X-ray diffraction rocking curve measurements. The threading dislocation density of the photodetector structure was calculated from the rotational broadening as ~2.5x108 cm-2. The cutoff wavelength and the peak responsivity of the photodetector were determined as around 2.15 μm and 0.08 A/W at 300 K, respectively. By applying reverse bias (-100 mV) the responsivity value of the photodetector increases more than an order (~0.96 A/W) which is the best value reported up to now. Those results indicate that although there is a large lattice mismatch (~8.4%) between GaAs substrate and the photodetector structure, an acceptable photodetector performance was achieved which is important for reducing photodetector costs

Evolution of SiO2 matrix during the formal-ion of Ge and Si nanocrystals by ion implantation

Fourier transformed infrared spectroscopy (FTIR) has been employed to observe and understand structural variations in SiO2 matrix during the formation of Ge and Si nanocrystals by ion implantation as a function of processing parameters. The Si-O asymmetric stretching peak of absorption spectra were used to monitor the evolution of SiOx (x < 2) films during the annealing process. It was shown that the recovery process in Si-O network is quite different in Ge and Si implanted samples and the deformation caused by Ge atoms in the SiO2 matrix can be recovered by annealing the implanted samples at lower temperatures than that by Si atoms. This is in agreement with the formation kinetics of the Si and Ge nanocrystals in SiO2 as observed by Raman spectroscopy and photoluminescence measurements of the same samples

The quantum confined Stark effect in silicon nanocrystals

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model

Comparative evaluation of InAs/GaSb superlattices for mid infrared detection: p-i-n versus residual doping

We report on the opto-electronic characterization of an InAs/GaSb superlattice (SL) midwave infrared p-i-n photodetector structure (pin-SL) in comparison with the same structure with no intentional doping (i-SL). Both structures were grown on an n-GaSb substrate using molecular beam epitaxy. The nominally undoped structure (i-SL) presented p-i-n like behavior and showed a photovoltaic mode photoresponse due to the residual doping and native defects in this material system. For similar to 77 K operation, 0.76 and 0.11 A W-1 responsivity values were obtained at 4 mu m from the pin-SL and i-SL structures, respectively. Activation energy analysis showed that the recombination current was dominant in both structures but different recombination centers were involved. The same i-SL structure was also grown on a semi-insulating (SI)-GaAs substrate to study the contribution of the substrate to the carrier density in the SL layers. Temperature dependent Hall effect measurements showed that the nominally undoped structure presented both n-type and p-type conductivities; however, the temperature at which the carrier type switched polarity was observed to be at higher values when the i-SL structure was grown on the SI-GaAs substrate. In addition, a higher carrier density was observed for i-SL on the GaSb substrate than on the GaAs substrate

Performance evaluation of InAs/GaSb superlattice photodetector grown on GaAs substrate using AlSb interfacial misfit array

We report on the growth and opto-electronic characterization of type-II InAs/GaSb superlattice (SL) mid-wavelength infrared pin photodetector grown on a GaAs substrate. AlSb interfacial misfit array was employed at the GaAs buffer/GaSb epilayer interface to reduce the dislocation density of the SL structure grown on the lattice mismatched GaAs substrate. Optical and electrical performance of this sample (SL-GaAs) were then compared with the reference sample of the same structure grown on a GaSb substrate (SL-GaSb). At 80 K, the dark current density and the detectivity values of the pin photodetectors were recorded as 5.40. x. 10-3 A cm-2 and 2.34. x. 1010 cm Hz0.5W(-1) for the SL-GaAs and 9.50. x. 10(-4) A cm(-2) and 4.70. x. 1010 cm Hz0.5W(-1) for the SL-GaSb, respectively

Electrical and magnetic properties of Si ion implanted YBa 2Cu3O7-δ thin films and microbridges

Fabrication of superconducting bilayer YBa2Cu3O 7-δ (YBCO) thin film structure by Si ion implantation and properties of microbridge patterned on that are presented. YBCO thin film of 150 nm thickness was grown on single crystal (100) SrTiO3 substrate by inverted cylindrical magnetron sputtering. The sample was implanted with 100 keV, 1×1016 Si ions/cm2. Upon implantation with Si, the sample lost its electrical conductivity and diamagnetism while its crystalline structure was preserved after the annealing of the sample. The implanted ions do not alter the overall crystal structure of high temperature superconductor film. This allows the growth of epitaxial superconducting second layer YBCO film on top of the implanted area without using any buffer layer, thus providing an effective method of fabricating multilayer structures. The second layer film and the microbridge patterned by laser writing technique, showed the superconducting properties similar to those of pure YBCO base layer with a reduced critical current density