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Optical Characterization of Semiconductor Nanostructures

Thesis Info

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Author

Begum, Narjis

Supervisor

Arshad Saleem Bhatti

Program

PhD

Institute

COMSATS University Islamabad

City

Islamabad

Province

Islamabad.

Country

Pakistan

Thesis Completing Year

2010

Thesis Completion Status

Completed

Subject

Physics

Language

English

Link

http://prr.hec.gov.pk/jspui/handle/123456789/624

Added

2021-02-17 19:49:13

Modified

2024-03-24 20:25:49

ARI ID

1676726812242

Similar


Self assembled semiconductor nanostructures, such as Quantum wells, nanowires and Quantum Dots, offer a variety of novel properties different from the bulk material. The new properties of low dimensional structures make them a potential candidate in optoelectronic industry. Efforts are now being made to reveal the underlying physics and phenomena of quasi one-dimensional and zero dimensional structures. The work presented herein deals with optical characterization of III-V semiconductor nanowires and III-N-V based emitters i.e., quantum wells and quantum dots, in long wavelength range. Spintronic is an emerging field where dilute magnetic semiconductors are used to achieve magnetic properties. Nanowires with magnetic impurity is considered to be a step towards one-dimensional spintronic devices. Au is the most commonly used catalyst for the VLS growth of NWs. But it also introduces the deep acceptor levels. One way to avoid deep acceptor levels and induce a magnetic impurity is the use of Mn as catalyst. In this thesis, gold (Au) and manganese (Mn) catalyzed self-assembled GaAs and InAs nanowires (NWs) were characterized. The samples were fabricated by molecular beam epitaxial (MBE) technique, on various substrates, at various temperature (540 to 620) oC. Scanning electron microscope (SEM) images revealed high density one-dimensional nanostructures with diameters in the range of 20 to 200 nm and lengths of few microns. Mn was found to diffuse into the stem of wires. HRTEM images show the presence of defects (stacking faults) in nanowires. Raman spectroscopy was used for optical characterization of nanowires and thus to determine the quality of these wires. Defects (stacking faults) were analyzed as the violation of Raman selection rules, which resulted in the asymmetrical broadening and the downshift of the LO and TO modes. We also observed some peaks at the low energy side of the TO peak of the GaAs and InAs NWs, irrespective of the catalyst used for the growth of NWs due to the oxide layer that surrounds the NWs. Surface optical phonons (SO) were found to be activated in both GaAs and InAs NWs. Phonon confinement model (PCM) was used to fit the LO phonon peaks, which also takes into account the contribution for asymmetry in the line shape caused by the presence of SO phonons and structural defects. This allowed to determine the correlation lengths in these wires, the average distance between defects and the defect density in these nanowires. Influence of these defects on SO phonon was also investigated. A good agreement between the experimental results and calculated for SO phonon mode by using the model presented by Ruppin and Englman was obtained. Statistical analysis ofthe data showed a distribution pattern of correlation length related to the growth conditions. Both Au and Mn catalyzed nanowires were found to exhibit similar quality, which indicates that Mn can replace Au catalyst resulting in magnetic impurity in the nanowires and giving us the opportunity to avoid the Au activated deep acceptor levels. To obtain the optical communication wavelength of 1.31 and 1.55 μm on GaAs substrates, InGaAs(N)/GaAs quantum wells and InAs(N)/GaAs(N) quantum dot structures were studied using photoluminescence spectroscopy. The samples were grown by MBE with a proper design of the samples by using stepped barriers to improve carrier trapping efficiency. Comparison of the luminescence from InAsN/GaAs and InAsN/GaAsN quantum dots was made with InGaNAs/GaAs quantum wells, grown under the same experimental conditions. Quantum dot emitters were found to exhibit higher thermal stability. The use of GaAsN barriers as opposed to GaAs barriers provided for narrower and more intense quantum-dot luminescence. Efficient room temperature emission of 1.41 μm (0.88 eV) has been obtained.
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ہر روز

غمِ روز گار کے جھمیلوں میں ہر روز
اعضائے جسم اپنے درد سے کراہتے ہیں

تو ہم بھی اک یاد کے بستر پر
روز ہی سر رکھ کر سو جاتے ہیں
فکریں تو ذہن میں سو رہتی ہیں
پر خواب میں محور بدل جاتے ہیں

یوں ہماری شب کٹتی ہے اور صبح
کسی بے نوا آواز کے طائر جگاتے ہیں
سرابِ و صل کے محل بکھرتے ہیں
اور ہم اس تلخ حالتِ اصل میں آ جاتے ہیں

پھر چکر چلتا ہے
پیٹ لاتیں مارتا ہے
ہم کام میں لگ جاتے ہیں
سب بھول جاتے ہیں

یوں اب ہم کام کے وقت کام
اور یاد کے وقت یاد کرتے ہیں
گویا کہ اب ہم سمجھ دار ہوتے جاتے ہیں

Education Curriculum of Learning In Children

To develop in children a broad range of skills, including the problem solving, interpersonal and communication skills that are essential for successful living in a rapidly changing society.  The curriculum encourages student initiative by providing children with materials, equipment, and time to pursue activities they choose.  At the same time, it provides teachers with a framework for guiding children’s independent activities toward sequenced learning goals. There are seven specific types of learning styles. Visual learners prefer to learn mathematics through pictures, diagrams etc.  A well-balanced intelligent child is able to develop all the types of learning styles. The students have to understand and accept their type of learning style earlier so that learning becomes easier and less stressful in the future. But it is important to train and practice the other types of learning styles so that the children can utilize them as effectively as possible. The teacher plays a key role in instructional activities by selecting appropriate, developmentally sequenced material and by encouraging children to adopt an active problem-solving approach to learning. This teacher-student interaction teachers helping students achieve developmentally sequenced goals while also encouraging them to set many of their own goals uniquely distinguishes the High/Scope Curriculum from direct-instruction and child-centered curricula (high/Scope Educational Research Foundation, 1989).  Teachers keep notes about significant behaviors, changes, statements, and things that help them better understand a child’s way of thinking and learning.  Teachers use two mechanisms to help them collect data: the key experiences note form and a portfolio.  The High/Scope Child Observation Record is also used to assess children’s development.  According to Ronald Barnett, learning may or may not take place when a subject is taught.  While discussing this point he has presented two contrasting images of quality.  They are institutional performance and student experience, student learning or student achievement.  The teacher in his opinion is central to higher education.  Teaching may be able to improve the quality of student’s learning but the teacher should remind himself that it may also impair the quality of student’s learning.  This is partly because student’s learning strategies vary under two polarities, one between deep and surface understanding and the other between holistic and atomistic understanding of their learning experiences.  He goes on to add that for a student, learning has three distinct aspects: learning style, motivation and curriculum demands.  Therefore teachers have to pursue, beyond teaching strategies to enable their students to attain certain specific skills.

Impact of Technical Analysis on Karachi Stock Exchange of Pakistan

This thesis examines the impact of Technical Analysis on Karachi Stock Exchange by investigating the tools used for Technical Analysis for the sample period of 1997 to 2014. The KSE-100 index was examined to represent the market over the sample period for the following three aspects. First the results indicate that KSE-100 index do not follow random walk model by applying the Wright„s rank and sign variance ratio test. Secondly the study compared a variety of extremely popular technical trading rules based on simple moving averages, exponential moving averages, relative strength index and stochastic RSI to find the predictive ability of these indicators. The thesis also employed generalized regression neural network (GRNN) for stock prediction. The results show that these trading rules have predictive power over future price behavior. It is also evidenced that the inclusion of oscillators like RSI and RSIStochastic increase the performance in generating above average return. The combination of GRNN with simple moving averages also produced significant return. Based on these trading rules, the study proposed two trading strategy in order to know that whether investor beat buy-and- hold strategy. The results indicate that strategy based onthese rules have the ability to outperform the buy-andhold strategy. The results are significant even after considering the transactional cost. Technical Analysis is very effective for the investors in creating excess return for the sample period.