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Register a new userOrder of Epitaxial Self-Assembled Quantum Dots: Linear Analysis
تحليل خطي لنظام النقاط الكمية المجمعة تلقائيا بشكل الإبتاعي
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Authors
Lawrence H. Friedman
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تتأثر النقاط الكمية المجمعة تلقائياً الإبتاعية (SAQDs) بالإهتمام للأجهزة الإلكترونية والإشعاعية المنحنية الصغيرة مثل الليزرات ومستشعرات الضوء والمنطق الصغير الحجم. النظام المكاني والنظام الحجمي للنقاط الكمية المجمعة تلقائياً هما مهمان لتطوير الأجهزة القابلة للاستخدام. من المحتمل أن هذين النوعين من النظام يرتبطان بشدة؛ لذلك، يشكل الدراسة المكانية أيضاً تأثيراً قوياً على النظام الحجمي. يتم هنا الدراسة المكانية باستخدام تحليل خطي لنموذج معتاد لإنشاء النقاط الكمية المجمعة تلقائياً بناء على التحرك السطحي. وتتم العثور على صيغ الجمع الحسابية لوظائف الارتفاع الأفلام التي تشخص النظام المكاني للنقاط الكمية وأطوال الارتباط المقابلة التي تقييم النظام. وتؤدي الانحرافات الأولية الذاتية الحجم الذري إلى أطوال ارتباط قليلة جداً (حوالي نقطتين) عندما يكون تأثير الجذب الرطوبة صفراً؛ ومع ذلك، تؤدي الأطوال الارتباط إلى الانحراف عندما يسمح لإنشاء SAQDs في ارتفاع أفلام قريب من الحالة الحرارية النقطية. يعزز هذا العمل النتائج السابقة حول البعدية والنظام SAQD ويقدم محركاً واضحاً ومحسناً للترتيب مع المعادلات التحليلية المقابلة. بالإضافة إلى ذلك، فإن إنشاء SAQD يعتبر عملية استثنائية بطبيعتها، ويتم عرض الجوانب الرياضية المختلفة حول التحليل الإحصائي لإنشاء SAQD والنظام.
Epitaxial self-assembled quantum dots (SAQDs) are of interest for nanostructured optoelectronic and electronic devices such as lasers, photodetectors and nanoscale logic. Spatial order and size order of SAQDs are important to the development of usable devices. It is likely that these two types of order are strongly linked; thus, a study of spatial order will also have strong implications for size order. Here a study of spatial order is undertaken using a linear analysis of a commonly used model of SAQD formation based on surface diffusion. Analytic formulas for film-height correlation functions are found that characterize quantum dot spatial order and corresponding correlation lengths that quantify order. Initial atomic-scale random fluctuations result in relatively small correlation lengths (about two dots) when the effect of a wetting potential is negligible; however, the correlation lengths diverge when SAQDs are allowed to form at a near-critical film height. The present work reinforces previous findings about anisotropy and SAQD order and presents as explicit and transparent mechanism for ordering with corresponding analytic equations. In addition, SAQD formation is by its nature a stochastic process, and various mathematical aspects regarding statistical analysis of SAQD formation and order are presented.
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Epitaxial self-assembled quantum dots (SAQDs) represent an important step in the advancement of semiconductor fabrication at the nanoscale that will allow breakthroughs in electronics and optoelectronics. In these applications, order is a key factor. Here, the role of crystal anisotropy in promoting order during early stages of SAQD formation is studied through a linear analysis of a commonly used surface evolution model. Elastic anisotropy is used a specific example. It is found that there are two relevant and predictable correlation lengths. One of them is related to crystal anisotropy and is crucial for determining SAQD order. Furthermore, if a wetting potential is included in the model, it is found that SAQD order is enhanced when the deposited film is allowed to evolve at heights near the critical surface height for three-dimensional film growth.
We investigate the electronic structure of the InAs/InP quantum dots using an atomistic pseudopotential method and compare them to those of the InAs/GaAs QDs. We show that even though the InAs/InP and InAs/GaAs dots have the same dot material, their electronic structure differ significantly in certain aspects, especially for holes: (i) The hole levels have a much larger energy spacing in the InAs/InP dots than in the InAs/GaAs dots of corresponding size. (ii) Furthermore, in contrast with the InAs/GaAs dots, where the sizeable hole $p$, $d$ intra-shell level splitting smashes the energy level shell structure, the InAs/InP QDs have a well defined energy level shell structure with small $p$, $d$ level splitting, for holes. (iii) The fundamental exciton energies of the InAs/InP dots are calculated to be around 0.8 eV ($sim$ 1.55 $mu$m), about 200 meV lower than those of typical InAs/GaAs QDs, mainly due to the smaller lattice mismatch in the InAs/InP dots. (iii) The widths of the exciton $P$ shell and $D$ shell are much narrower in the InAs/InP dots than in the InAs/GaAs dots. (iv) The InAs/GaAs and InAs/InP dots have a reversed light polarization anisotropy along the [100] and [1$bar{1}$0] directions.
Epitaxial self-assembled quantum dots (SAQDs) are of both technological and fundamental interest, but their reliable manufacture still presents a technical challenge. To better understand the formation, morphology and ordering of epitaxial self-assembled quantum dots (SAQDs), it is essential to have an accurate model that can aid further experiments and predict the trends in SAQD formation. SAQDs form because of the destabilizing effect of elastic mismatch strain, but most analytic models and some numerical models of SAQD formation either assume an elastically homogeneous anisotropic film-substrate system or assume an elastically heterogeneous isotropic system. In this work, we perform the full film-substrate elastic calculation. Then we incorporate the elasticity calculation into a stochastic linear growth model. We find that using homogeneous elasticity can cause errors in the elastic energy density as large as 26%, and for typical modeling parameters lead to errors of about 11% in the estimated value of average dot spacing. We also quantify the effect of elastic heterogeneity on the order estimates of SAQDs and confirm previous finding on the possibility of order enhancement by growing a film near the critical film height.
We study spin dynamics of excitons confined in self-assembled CdSe quantum dots by means of optical orientation in magnetic field. At zero field the exciton emission from QDs populated via LO phonon-assisted absorption shows a circular polarization of 14%. The polarization degree of the excitonic emission increases dramatically when a magnetic field is applied. Using a simple model, we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the absence and presence of magnetic field, respectively. With increasing temperature the polarization of the QD emission gradually decreases. Remarkably, the activation energy which describes this decay is independent of the external magnetic field, and, therefore, of the degeneracy of the exciton levels in QDs. This observation implies that the temperature-induced enhancement of the exciton spin relaxation is insensitive to the energy level degeneracy and can be attributed to the same excited state distribution.
The spin polarization of electrons trapped in InAs self-assembled quantum dot ensembles is investigated. A statistical approach for the population of the spin levels allows one to infer the spin polarization from the measure values of the addition energies. From the magneto-capacitance spectroscopy data, the authors found a fully polarized ensemble of electronic spins above 10 T when $mathbf{B}parallel[001]$ and at 2.8 K. Finally, by including the g-tensor anisotropy the angular dependence of spin polarization with the magnetic field $mathbf{B}$ orientation and strength could be determined.
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