Direct growth of GaN on GaAs substrates in an As-free environment results in a polycrystalline film with misoriented grains and inclusions. Adopting a procedure of substrate nitridation, deposition of a low-temperature GaN buffer layer, and high-temperature overgrowth significantly improves film quality. The improved films have better crystallinity, fewer defects, and a smoother surface morphology. Transmission electron microscopy analysis reveals the films have a columnar structure with straight grain boundaries and defects concentrated at the interface with the GaAs substrate.
Effect of nitridation on crystallinity of GaN grown on GaAs by MBEOleg Maksimov
1) GaN films were grown on GaAs substrates using plasma-assisted molecular beam epitaxy. The growth process involved substrate nitridation, deposition of a low-temperature buffer layer, and high-temperature film growth.
2) The effect of nitridation temperature on film crystallinity was studied using X-ray diffraction. Low-temperature (400°C) nitridation promoted the growth of c-oriented hexagonal α-GaN, while higher nitridation temperatures resulted in mis-oriented domains and inclusions of cubic β-GaN.
3) It was demonstrated that controlling the nitridation temperature is critical for achieving high quality GaN crystal growth, and nitridation must be performed at
The document summarizes research on GeSn alloys grown by molecular beam epitaxy. It was found that single crystal Ge1-xSnx alloys with tin atomic fractions up to x=0.145 were grown coherently on Ge substrates at temperatures below 250°C. Rutherford backscattering spectrometry determined the tin composition and found over 90% of the tin atoms were substitutionally incorporated into the Ge lattice in all alloys. The degree of strain and dependence of the effective unstrained lattice constant on tin composition was determined from high resolution x-ray diffraction measurements.
Al gan gan field effect transistors with c-doped gan buffer layer as an elect...Kal Tar
1. The authors grew AlGaN/GaN field effect transistor structures on carbon-doped GaN buffer layers using molecular beam epitaxy.
2. These structures demonstrated excellent device characteristics, including a high product of sheet carrier density and mobility (nsl) up to 2 × 1016 V−1s−1 and an on/off current ratio of 107.
3. Inter-device isolation measurements showed isolation currents in the low picoampere range, indicating the carbon-doped GaN buffer layer effectively suppressed parallel conduction paths.
This document discusses structural properties of boron-doped germanium-tin alloys grown by molecular beam epitaxy. It summarizes the following key points:
1) Boron-doped Ge1-xSnx alloys with tin compositions up to x=0.08 and boron concentrations of around 1018 cm-3 were grown on n-type germanium substrates.
2) Characterization using techniques like secondary ion mass spectroscopy, Rutherford backscattering spectrometry, and high-resolution x-ray diffraction showed that the alloys were single crystal, strained coherent layers with low defect densities for thicknesses up to 90 nm.
3) Rutherford backscattering spect
Red–green–blue light emitting diodes and distributed Bragg reflectors based o...Oleg Maksimov
This document summarizes research on developing red, green, and blue light emitting diodes (LEDs) and distributed Bragg reflectors (DBRs) using ZnCdMgSe alloys lattice-matched to InP substrates. LED structures emitting across the visible spectrum were grown by varying quantum well thickness and composition. Lattice-matched DBRs with reflectivity peaks in red, green, and blue were also grown. Combining these LED and DBR structures could enable the design of high-performance visible LEDs and vertical cavity surface emitting lasers across the entire visible range using nearly identical material systems.
The Isoplaten -"Thermal Performance of Electrically Heated Platens" - May 2011Acrolab Ltd.
Acrolab’s Isoplaten (“The Intuitive Platen”) redistributes the energy generated by standard cartridge heaters to give molders rapid thermal recovery and unparalleled and linear temperature distribution along the whole platen surface. Your tools operate with reduced energy demands, produce better quality molded parts and allow you to use faster curing compounds with narrower thermal processing windows.
Uniformly Cure Parts Resulting In Better Quality
• Reduce cycle time• Reduce Thermal Energy Required From Heaters• Energy Cost Savings• Save On Time• Evenly Heat Thermal Sensitive Parts• Eliminate Hot Spots• Reduce Scrap
Acrolab’s Isoplatens provide high levels of thermal stability to press platen applications and can be designed for electric, oil, or steam heating. Integral water cooling lines for fast process temperature changes are available. The standard Isoplaten is electronically heated and has an operating range of ambient to 500°F.
Acrolab engineered bi-level Isoplatens can hold a thermal uniformity of +/- 5°F over 90% of the surface of the Isoplaten. These results allow a substantial
improvement in cycle times, start up times, thermal recovery rates, and improved part quality.
The Isoplaten’s Unique thermal uniformity permits the use of one single zone temperature controller for the entire platen. No special multi-zone controls, heaters, or thermocouples are required.
Thermally induced amorphous to crystalline transformation of argon ion bombar...Kudakwashe Jakata
This document studies the thermally induced recrystallization of an amorphous layer of gallium arsenide (GaAs) created by bombarding a single crystal GaAs sample with 100 keV argon ions at a fluence of 5 × 1016 ions/cm2. Surface Brillouin scattering and Raman spectroscopy were used to investigate the structural changes during isochronal annealing. The Surface Brillouin scattering showed continuous stiffening of the layer beginning above 200°C, reaching a maximum value above 500°C. The Raman studies showed evidence of full recrystallization above 500°C, indicating the reformed layer was polycrystalline.
This document discusses the structural, vibrational, and microwave dielectric properties of double perovskite ceramics with the formula Ba2Zn1-xCaxWO6 (x = 0-0.4). The samples were sintered at temperatures between 1300-1400°C and characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The analysis showed that the tolerance factor and temperature coefficient of resonant frequency decreased with increasing calcium content. Microwave dielectric properties such as relative permittivity and quality factor were measured and found to vary with sintering temperature and calcium concentration.
Effect of nitridation on crystallinity of GaN grown on GaAs by MBEOleg Maksimov
1) GaN films were grown on GaAs substrates using plasma-assisted molecular beam epitaxy. The growth process involved substrate nitridation, deposition of a low-temperature buffer layer, and high-temperature film growth.
2) The effect of nitridation temperature on film crystallinity was studied using X-ray diffraction. Low-temperature (400°C) nitridation promoted the growth of c-oriented hexagonal α-GaN, while higher nitridation temperatures resulted in mis-oriented domains and inclusions of cubic β-GaN.
3) It was demonstrated that controlling the nitridation temperature is critical for achieving high quality GaN crystal growth, and nitridation must be performed at
The document summarizes research on GeSn alloys grown by molecular beam epitaxy. It was found that single crystal Ge1-xSnx alloys with tin atomic fractions up to x=0.145 were grown coherently on Ge substrates at temperatures below 250°C. Rutherford backscattering spectrometry determined the tin composition and found over 90% of the tin atoms were substitutionally incorporated into the Ge lattice in all alloys. The degree of strain and dependence of the effective unstrained lattice constant on tin composition was determined from high resolution x-ray diffraction measurements.
Al gan gan field effect transistors with c-doped gan buffer layer as an elect...Kal Tar
1. The authors grew AlGaN/GaN field effect transistor structures on carbon-doped GaN buffer layers using molecular beam epitaxy.
2. These structures demonstrated excellent device characteristics, including a high product of sheet carrier density and mobility (nsl) up to 2 × 1016 V−1s−1 and an on/off current ratio of 107.
3. Inter-device isolation measurements showed isolation currents in the low picoampere range, indicating the carbon-doped GaN buffer layer effectively suppressed parallel conduction paths.
This document discusses structural properties of boron-doped germanium-tin alloys grown by molecular beam epitaxy. It summarizes the following key points:
1) Boron-doped Ge1-xSnx alloys with tin compositions up to x=0.08 and boron concentrations of around 1018 cm-3 were grown on n-type germanium substrates.
2) Characterization using techniques like secondary ion mass spectroscopy, Rutherford backscattering spectrometry, and high-resolution x-ray diffraction showed that the alloys were single crystal, strained coherent layers with low defect densities for thicknesses up to 90 nm.
3) Rutherford backscattering spect
Red–green–blue light emitting diodes and distributed Bragg reflectors based o...Oleg Maksimov
This document summarizes research on developing red, green, and blue light emitting diodes (LEDs) and distributed Bragg reflectors (DBRs) using ZnCdMgSe alloys lattice-matched to InP substrates. LED structures emitting across the visible spectrum were grown by varying quantum well thickness and composition. Lattice-matched DBRs with reflectivity peaks in red, green, and blue were also grown. Combining these LED and DBR structures could enable the design of high-performance visible LEDs and vertical cavity surface emitting lasers across the entire visible range using nearly identical material systems.
The Isoplaten -"Thermal Performance of Electrically Heated Platens" - May 2011Acrolab Ltd.
Acrolab’s Isoplaten (“The Intuitive Platen”) redistributes the energy generated by standard cartridge heaters to give molders rapid thermal recovery and unparalleled and linear temperature distribution along the whole platen surface. Your tools operate with reduced energy demands, produce better quality molded parts and allow you to use faster curing compounds with narrower thermal processing windows.
Uniformly Cure Parts Resulting In Better Quality
• Reduce cycle time• Reduce Thermal Energy Required From Heaters• Energy Cost Savings• Save On Time• Evenly Heat Thermal Sensitive Parts• Eliminate Hot Spots• Reduce Scrap
Acrolab’s Isoplatens provide high levels of thermal stability to press platen applications and can be designed for electric, oil, or steam heating. Integral water cooling lines for fast process temperature changes are available. The standard Isoplaten is electronically heated and has an operating range of ambient to 500°F.
Acrolab engineered bi-level Isoplatens can hold a thermal uniformity of +/- 5°F over 90% of the surface of the Isoplaten. These results allow a substantial
improvement in cycle times, start up times, thermal recovery rates, and improved part quality.
The Isoplaten’s Unique thermal uniformity permits the use of one single zone temperature controller for the entire platen. No special multi-zone controls, heaters, or thermocouples are required.
Thermally induced amorphous to crystalline transformation of argon ion bombar...Kudakwashe Jakata
This document studies the thermally induced recrystallization of an amorphous layer of gallium arsenide (GaAs) created by bombarding a single crystal GaAs sample with 100 keV argon ions at a fluence of 5 × 1016 ions/cm2. Surface Brillouin scattering and Raman spectroscopy were used to investigate the structural changes during isochronal annealing. The Surface Brillouin scattering showed continuous stiffening of the layer beginning above 200°C, reaching a maximum value above 500°C. The Raman studies showed evidence of full recrystallization above 500°C, indicating the reformed layer was polycrystalline.
This document discusses the structural, vibrational, and microwave dielectric properties of double perovskite ceramics with the formula Ba2Zn1-xCaxWO6 (x = 0-0.4). The samples were sintered at temperatures between 1300-1400°C and characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The analysis showed that the tolerance factor and temperature coefficient of resonant frequency decreased with increasing calcium content. Microwave dielectric properties such as relative permittivity and quality factor were measured and found to vary with sintering temperature and calcium concentration.
Epitaxial deposition involves growing a crystalline layer that matches the structure of the substrate. It allows for the creation of high purity, abrupt interface layers useful for applications with stringent quality demands like semiconductor devices. Vapor phase epitaxy is a common method that uses gas precursors and high temperature to grow ordered, crystalline layers through thermodynamically and kinetically controlled processes. Epitaxial layers are used in applications such as engineered wafers, III-V semiconductor devices, and transistors to enable their operation.
This document summarizes a numerical simulation of a rotary kiln. The simulation models turbulent combustion, heat transfer, and granular flow inside the kiln. It seeks to understand formation of rings within the kiln and optimize production. The model considers chemical reactions, turbulence, radiation heat transfer, and NOx formation. It was used to test adjusting the fuel-air ratio to reduce temperature peaks that cause rings. Increasing the ratio from 10 to 12 successfully destroyed an existing ring over 24 hours by lowering the liquid phase temperature below self-sustaining levels. The model will further explore configurations to prevent or counteract ring formation and reduce NOx emissions.
Structural properties of SrO thin films grown by molecular beam epitaxy on La...Oleg Maksimov
The document summarizes research on growing and characterizing strontium oxide (SrO) thin films on lanthanum aluminate (LaAlO3) substrates using molecular beam epitaxy (MBE). Key findings include:
1) 500 angstrom thick SrO films grown on LaAlO3 substrates by MBE were relaxed and exhibited smooth, atomically flat surfaces based on reflection high-energy electron diffraction (RHEED) measurements.
2) The SrO films were found to be epitaxially aligned with the LaAlO3 substrates with an in-plane 45° rotation, minimizing lattice mismatch between the materials and leading to high crystalline quality verified by RHEED and x-ray
One-step hydrothermal synthesis of graphene decorated V2O5 nanobelts for enha...suresh kannan
This document summarizes a study that synthesized graphene-decorated vanadium pentoxide (V2O5) nanobelts through a one-step hydrothermal method for use in supercapacitors. Graphene oxide acted as both a mild oxidant to form the V2O5 nanobelts and was reduced to graphene, enhancing conductivity. The graphene-decorated V2O5 nanobelts showed improved specific capacitance compared to other compositions due to pseudocapacitance from V2O5 and double layer capacitance from graphene. A vanadium-rich composite exhibited the best performance with high specific capacitance and capacity retention over 5000 cycles.
Epitaxial growth aims to produce defect-free single crystal layers, but structural and electrical defects can still form. Dislocations are usually undesirable as they can cause electron-hole recombination. Misfit and threading dislocations occur due to lattice mismatch. Point defects like vacancies and anti-site defects are also commonly observed. Stacking faults are another common defect where the ABCABC stacking sequence is mistaken in cubic crystal structures. Lattice mismatch can lead to misfit dislocations at the interface. Antiphase disorder can also occur due to symmetry in unit cells, particularly for heteroepitaxial growth.
recent developments on Graphene oxide based membranesKishan Kasundra
This document discusses recent developments in graphene oxide (GO) based membrane technology. It begins with an introduction to GO and its advantages over polymeric membranes. It then describes methods for preparing and characterizing GO, as well as different approaches for fabricating GO membranes, including free-standing GO membranes, supported GO membranes, and GO-modified composite membranes. Specific examples are provided for each membrane type and their applications in water treatment and separation processes. The document concludes that GO is a promising nano-material for membrane applications due to its unique properties and that the membrane structure and performance depends on the fabrication method.
Determination of nonlinear absorption (β) and refraction (n2)by the Z-scan me...IOSR Journals
Potassium Pentaborate nonlinear optical (NLO) material was synthesized by the solution growth method. The grown crystals were subjected to structural, optical and mechanical property studies. Crystal with excellent transparency were grown with maximum size of 9mm×8mm×5mm and the grown crystals were characterized by single crystal Single crystal XRD, FT-IR, TGA-DTA&DSC, and UV–vis-NIR studies. The crystal belongs to orthorhombic with a space group of mm2 having unit-cell dimensions a = 11.068Åb= 11.175Å c = 9.058Åand α = 90°; β = 90°; and γ =90°; Z=4, at 298(2) K. The second-order nonlinear optical property of the polycrystalline sample has been confirmed by Kurtz-Perry powder SHG analysis. Third order nonlinear optical properties were also studied by Z-scan techniques. Nonlinear absorption and nonlinear refractive index were found out and the third order bulk susceptibility of compound was also calculated.
This document discusses epitaxial deposition, which is the deposition of a crystalline layer that matches the crystalline structure of the substrate. It describes the mechanisms and methods of epitaxial growth, including vapor phase epitaxy. Epitaxial layers have applications in engineered wafers, III-V semiconductor devices, and other applications that require high quality crystalline layers with abrupt interfaces and controlled doping profiles.
2000 surface polymerization of epitaxial sb wires on si(001)pmloscholte
This document reports on a study of the local density of states of epitaxial antimony nanostructures on silicon 001 using scanning tunneling spectroscopy. The key findings are:
1) Epitaxial antimony lines self-assemble on the silicon surface when antimony is deposited at low rates and the sample is annealed. The lines are two atoms wide and extend up to 12 antimony dimers in length.
2) Spectroscopy shows the electronic structure in the center of the antimony lines is similar to the silicon surface, indicating the lines are nonmetallic. In contrast, the ends of the lines show a finite density of states at the Fermi level, consistent with theoretical predictions of a "radical
Al gan aln_gan_sic hemt structure with high mobility gan thin layer as channe...Kal Tar
The document summarizes the growth and characterization of an AlGaN/AlN/GaN HEMT structure with a high mobility GaN channel layer grown on SiC substrates by MOCVD. Key results include:
1) The structure achieved a high room temperature 2DEG mobility of 2215 cm2/V·s and 2DEG concentration of 1.044×1013 cm-2.
2) 0.35 μm gate length HEMT devices exhibited a maximum drain current density of 1300 mA/mm, transconductance of 314 mS/mm, and current gain cutoff frequency of 28 GHz.
3) The devices demonstrated a maximum output power density of 4.10
The document reports on an experiment measuring the interstitial iron concentration in multicrystalline silicon wafers using photoconductance and photoluminescence techniques. Samples underwent chemical treatment and passivation but yielded unexpectedly low minority carrier lifetimes (<20 μs) preventing analysis. Possible reasons for this include contamination during passivation from an unclean substrate holder or imperfections introduced during chemical treatment. Further experiments with a cleaned chamber could provide different results.
The document discusses the benefits of exercise for mental health. Regular physical activity can help reduce anxiety and depression and improve mood and cognitive functioning. Exercise causes chemical changes in the brain that may help protect against mental illness and improve symptoms.
This document shows the process of transcription and translation in DNA replication. The DNA unwinds and RNA polymerase copies the coding region of DNA into a messenger RNA. The mRNA then moves to the ribosome where it is translated into a strand of amino acids that fold to create a protein.
Un plano de la planta física de Alkaedas Inc. muestra la ubicación de la sala de sistemas en el segundo piso. El plano fue creado por Cristian Vergara, Sebastian Moreno, Oscar López y Santiago Rendón del grado 11-3.
Drei Dutzend kleine Tipps und Inspirationen für euer eigenes Blog. Querbeet aus den Bereichen Content, Design, Usability, Accessibility, SEO, Performance, Gimmicks…
Nach dem Motto: Mein Blog muss schöner / besser werden.
Ob ein Tipp für euch in Frage kommt oder nicht, hängt vom Blog ab. Pickt euch einfach raus, was euch sinnvoll erscheint.
März 2013 | Nicolai Schwarz | @textformer
Falls ich die Zeit finde, gibt es das Thema demnächst auch etwas ausführlicher unter http://nicolaischwarz.de.
Iryna Gerasymenko completed an online course in Critical Thinking in Global Challenges from the University of Edinburgh. The introductory undergraduate course aimed to help students develop their understanding and skills in critical thinking by practicing and enhancing their abilities. The course was taught by Professor Mayank Dutia and Dr Celine Caquineau from the University of Edinburgh.
Nanoengineered Curie temperature in laterally patterned ferromagnetic semicon...Oleg Maksimov
This document summarizes research on manipulating the Curie temperature of ferromagnetic semiconductor heterostructures through nanolithography. The key findings are:
1) Patterning buried (Ga,Mn)As layers into nanowires exposes free surfaces along the sidewalls, allowing Mn interstitials to diffuse out during annealing.
2) Annealing nanowired samples leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples, due to the removal of Mn interstitials.
3) The enhancement of Curie temperature is larger for narrower nanowires, as the smaller width facilitates the removal of Mn interstitials during annealing for a fixed time.
Epitaxial deposition involves growing a crystalline layer that matches the structure of the substrate. It allows for the creation of high purity, abrupt interface layers useful for applications with stringent quality demands like semiconductor devices. Vapor phase epitaxy is a common method that uses gas precursors and high temperature to grow ordered, crystalline layers through thermodynamically and kinetically controlled processes. Epitaxial layers are used in applications such as engineered wafers, III-V semiconductor devices, and transistors to enable their operation.
This document summarizes a numerical simulation of a rotary kiln. The simulation models turbulent combustion, heat transfer, and granular flow inside the kiln. It seeks to understand formation of rings within the kiln and optimize production. The model considers chemical reactions, turbulence, radiation heat transfer, and NOx formation. It was used to test adjusting the fuel-air ratio to reduce temperature peaks that cause rings. Increasing the ratio from 10 to 12 successfully destroyed an existing ring over 24 hours by lowering the liquid phase temperature below self-sustaining levels. The model will further explore configurations to prevent or counteract ring formation and reduce NOx emissions.
Structural properties of SrO thin films grown by molecular beam epitaxy on La...Oleg Maksimov
The document summarizes research on growing and characterizing strontium oxide (SrO) thin films on lanthanum aluminate (LaAlO3) substrates using molecular beam epitaxy (MBE). Key findings include:
1) 500 angstrom thick SrO films grown on LaAlO3 substrates by MBE were relaxed and exhibited smooth, atomically flat surfaces based on reflection high-energy electron diffraction (RHEED) measurements.
2) The SrO films were found to be epitaxially aligned with the LaAlO3 substrates with an in-plane 45° rotation, minimizing lattice mismatch between the materials and leading to high crystalline quality verified by RHEED and x-ray
One-step hydrothermal synthesis of graphene decorated V2O5 nanobelts for enha...suresh kannan
This document summarizes a study that synthesized graphene-decorated vanadium pentoxide (V2O5) nanobelts through a one-step hydrothermal method for use in supercapacitors. Graphene oxide acted as both a mild oxidant to form the V2O5 nanobelts and was reduced to graphene, enhancing conductivity. The graphene-decorated V2O5 nanobelts showed improved specific capacitance compared to other compositions due to pseudocapacitance from V2O5 and double layer capacitance from graphene. A vanadium-rich composite exhibited the best performance with high specific capacitance and capacity retention over 5000 cycles.
Epitaxial growth aims to produce defect-free single crystal layers, but structural and electrical defects can still form. Dislocations are usually undesirable as they can cause electron-hole recombination. Misfit and threading dislocations occur due to lattice mismatch. Point defects like vacancies and anti-site defects are also commonly observed. Stacking faults are another common defect where the ABCABC stacking sequence is mistaken in cubic crystal structures. Lattice mismatch can lead to misfit dislocations at the interface. Antiphase disorder can also occur due to symmetry in unit cells, particularly for heteroepitaxial growth.
recent developments on Graphene oxide based membranesKishan Kasundra
This document discusses recent developments in graphene oxide (GO) based membrane technology. It begins with an introduction to GO and its advantages over polymeric membranes. It then describes methods for preparing and characterizing GO, as well as different approaches for fabricating GO membranes, including free-standing GO membranes, supported GO membranes, and GO-modified composite membranes. Specific examples are provided for each membrane type and their applications in water treatment and separation processes. The document concludes that GO is a promising nano-material for membrane applications due to its unique properties and that the membrane structure and performance depends on the fabrication method.
Determination of nonlinear absorption (β) and refraction (n2)by the Z-scan me...IOSR Journals
Potassium Pentaborate nonlinear optical (NLO) material was synthesized by the solution growth method. The grown crystals were subjected to structural, optical and mechanical property studies. Crystal with excellent transparency were grown with maximum size of 9mm×8mm×5mm and the grown crystals were characterized by single crystal Single crystal XRD, FT-IR, TGA-DTA&DSC, and UV–vis-NIR studies. The crystal belongs to orthorhombic with a space group of mm2 having unit-cell dimensions a = 11.068Åb= 11.175Å c = 9.058Åand α = 90°; β = 90°; and γ =90°; Z=4, at 298(2) K. The second-order nonlinear optical property of the polycrystalline sample has been confirmed by Kurtz-Perry powder SHG analysis. Third order nonlinear optical properties were also studied by Z-scan techniques. Nonlinear absorption and nonlinear refractive index were found out and the third order bulk susceptibility of compound was also calculated.
This document discusses epitaxial deposition, which is the deposition of a crystalline layer that matches the crystalline structure of the substrate. It describes the mechanisms and methods of epitaxial growth, including vapor phase epitaxy. Epitaxial layers have applications in engineered wafers, III-V semiconductor devices, and other applications that require high quality crystalline layers with abrupt interfaces and controlled doping profiles.
2000 surface polymerization of epitaxial sb wires on si(001)pmloscholte
This document reports on a study of the local density of states of epitaxial antimony nanostructures on silicon 001 using scanning tunneling spectroscopy. The key findings are:
1) Epitaxial antimony lines self-assemble on the silicon surface when antimony is deposited at low rates and the sample is annealed. The lines are two atoms wide and extend up to 12 antimony dimers in length.
2) Spectroscopy shows the electronic structure in the center of the antimony lines is similar to the silicon surface, indicating the lines are nonmetallic. In contrast, the ends of the lines show a finite density of states at the Fermi level, consistent with theoretical predictions of a "radical
Al gan aln_gan_sic hemt structure with high mobility gan thin layer as channe...Kal Tar
The document summarizes the growth and characterization of an AlGaN/AlN/GaN HEMT structure with a high mobility GaN channel layer grown on SiC substrates by MOCVD. Key results include:
1) The structure achieved a high room temperature 2DEG mobility of 2215 cm2/V·s and 2DEG concentration of 1.044×1013 cm-2.
2) 0.35 μm gate length HEMT devices exhibited a maximum drain current density of 1300 mA/mm, transconductance of 314 mS/mm, and current gain cutoff frequency of 28 GHz.
3) The devices demonstrated a maximum output power density of 4.10
The document reports on an experiment measuring the interstitial iron concentration in multicrystalline silicon wafers using photoconductance and photoluminescence techniques. Samples underwent chemical treatment and passivation but yielded unexpectedly low minority carrier lifetimes (<20 μs) preventing analysis. Possible reasons for this include contamination during passivation from an unclean substrate holder or imperfections introduced during chemical treatment. Further experiments with a cleaned chamber could provide different results.
The document discusses the benefits of exercise for mental health. Regular physical activity can help reduce anxiety and depression and improve mood and cognitive functioning. Exercise causes chemical changes in the brain that may help protect against mental illness and improve symptoms.
This document shows the process of transcription and translation in DNA replication. The DNA unwinds and RNA polymerase copies the coding region of DNA into a messenger RNA. The mRNA then moves to the ribosome where it is translated into a strand of amino acids that fold to create a protein.
Un plano de la planta física de Alkaedas Inc. muestra la ubicación de la sala de sistemas en el segundo piso. El plano fue creado por Cristian Vergara, Sebastian Moreno, Oscar López y Santiago Rendón del grado 11-3.
Drei Dutzend kleine Tipps und Inspirationen für euer eigenes Blog. Querbeet aus den Bereichen Content, Design, Usability, Accessibility, SEO, Performance, Gimmicks…
Nach dem Motto: Mein Blog muss schöner / besser werden.
Ob ein Tipp für euch in Frage kommt oder nicht, hängt vom Blog ab. Pickt euch einfach raus, was euch sinnvoll erscheint.
März 2013 | Nicolai Schwarz | @textformer
Falls ich die Zeit finde, gibt es das Thema demnächst auch etwas ausführlicher unter http://nicolaischwarz.de.
Iryna Gerasymenko completed an online course in Critical Thinking in Global Challenges from the University of Edinburgh. The introductory undergraduate course aimed to help students develop their understanding and skills in critical thinking by practicing and enhancing their abilities. The course was taught by Professor Mayank Dutia and Dr Celine Caquineau from the University of Edinburgh.
Nanoengineered Curie temperature in laterally patterned ferromagnetic semicon...Oleg Maksimov
This document summarizes research on manipulating the Curie temperature of ferromagnetic semiconductor heterostructures through nanolithography. The key findings are:
1) Patterning buried (Ga,Mn)As layers into nanowires exposes free surfaces along the sidewalls, allowing Mn interstitials to diffuse out during annealing.
2) Annealing nanowired samples leads to an enhanced Curie temperature and reduced resistivity compared to unpatterned samples, due to the removal of Mn interstitials.
3) The enhancement of Curie temperature is larger for narrower nanowires, as the smaller width facilitates the removal of Mn interstitials during annealing for a fixed time.
High reflectivity symmetrically strained ZnxCdyMg1ÀxÀySe-based distributed Br...Oleg Maksimov
This summary provides the key details from the document in 3 sentences:
The document describes the growth and characterization of distributed Bragg reflector (DBR) structures made of alternating ZnCdSe and ZnCdMgSe layers grown on InP substrates by molecular beam epitaxy. Undoped DBR structures with up to 24 periods achieved a maximum reflectivity of 99%, while a chlorine-doped 12-period DBR achieved 89% reflectivity. Structural characterization confirmed high crystalline quality and layer thicknesses close to the designed optical thickness of λ/4n, demonstrating the suitability of these materials for applications requiring highly reflective mirrors such as vertical cavity surface emitting lasers.
High crystalline quality ZnBeSe grown by molecular beam epitaxy with Be}Zn co...Oleg Maksimov
This summary provides the key details from the document in 3 sentences:
The document discusses the growth of high crystalline quality ZnBeSe layers on GaAs substrates using molecular beam epitaxy. It finds that using Be-Zn co-irradiation of the GaAs surface before growing a ZnSe buffer layer or using a BeTe buffer layer improves the crystalline quality of the ZnBeSe layers, resulting in narrower X-ray linewidth and lower etch pit density. Near lattice-matched ZnBeSe layers grown with Be-Zn co-irradiation exhibited an X-ray linewidth as low as 23 arcseconds and a photoluminescence linewidth of about 2.5 meV at 13K.
The document summarizes research on Ga adlayer coverage and surface morphology evolution during GaN growth by plasma-assisted molecular beam epitaxy (PAMBE). Key findings include:
1) Ga adlayer coverage increases continuously from 0 to a maximum of 2.5 monolayers as the Ga/N flux ratio increases from N-rich to Ga-rich conditions.
2) A laterally contracted Ga bilayer with 2.5 monolayer coverage stabilizes the GaN surface, producing the minimum pit density and planar morphology.
3) The critical Ga incident flux needed to achieve the optimized Ga bilayer coverage without Ga droplets can be determined from kinetic rates of Ga adsorption and desorption.
Investigation of As-doped ZnO films synthesized via thermal annealing of ZnSe...Oleg Maksimov
The document summarizes research investigating As-doped ZnO films synthesized via thermal annealing of ZnSe/GaAs heterostructures. Key findings include:
1) XRD and Raman analysis showed the films were polycrystalline c-axis oriented ZnO containing nanometer-sized As clusters.
2) SEM images showed the surface consisted of dense, uniformly sized grains.
3) Photoluminescence spectra exhibited a sharp band edge emission line at 3.262 eV, indicating high optical quality.
4) Auger electron spectroscopy detected uniform As diffusion through the ZnO film and at the surface, suggesting As incorporation into the ZnO lattice.
Enhancement of Curie temperature in Ga1Oleg Maksimov
1) Ga1-xMnxAs films grown on InyGa1-yAs buffers have higher as-grown Curie temperatures (TC) than those grown on GaAs, ranging from 105-125 K versus typically <110 K.
2) Annealing Ga1-xMnxAs/InyGa1-yAs films leads to only small increases in TC, in contrast to larger increases seen in Ga1-xMnxAs/GaAs films.
3) This suggests Ga1-xMnxAs/InyGa1-yAs films have fewer Mn interstitial defects initially, which are believed to reduce with annealing by migrating to surfaces/interfaces. The
1. Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. It has excellent electrical and thermal conductivity properties.
2. The document discusses various methods for synthesizing graphene, including chemical vapor deposition (CVD) using carbon precursors like methane and ethylene. Solid carbon sources like camphor can also be used.
3. Characterization techniques like Raman spectroscopy are used to analyze graphene samples and confirm the number of layers. Properties vary depending on whether the graphene is mono-layer or multi-layer.
This document is a seminar report submitted by Sunamro Sarkar discussing compounds for pocket-sized data storage. It introduces graphene and its properties, methods for preparing graphene like scotch tape method and chemical vapor deposition. The report discusses how an organometallic compound containing zinc and graphene fragments sandwiched between cobalt and copper layers can store and read digital data at -23°C using molecular spintronics and electron spin orientation. It achieves data storage capacity of 1000 terabytes per square inch using this mechanism.
Graphene is synthesized from graphite through chemical reactions and thermal reduction to create a single layer of carbon atoms. Transmission electron microscopy is used to characterize the structure of graphene oxide and graphene, showing they are thin, crystalline flakes. Graphene nanosheets can then be incorporated into polymers as a nanocomposite to enhance the polymer's properties, such as increasing its thermal conductivity for applications in thermal management.
This document summarizes an experiment to engineer the growth of germanium nanowires by tuning the supersaturation of gold-germanium binary alloy catalysts. The researchers deposited gold films on one half of silicon substrates and gold-germanium bilayer films on the other half. Upon annealing, the films formed catalyst particles under identical conditions. Nanowires grown from the bilayer catalysts were significantly longer, due to a reduced incubation time for nucleation. Increasing the thickness of the germanium layer in the bilayers further increased nanowire length and growth rate, demonstrating that tuning the catalyst composition can control supersaturation and nanowire growth kinetics. In-situ TEM experiments showed thinner
Surface modification and properties modulation of r go film by short duration...Journal Papers
This document discusses the effects of hydrogen (H2) plasma and ammonia (NH3) plasma treatments on the surface and properties of reduced graphene oxide (rGO) films. Low-power H2 plasma at low and medium temperatures further reduced oxygen content in the rGO films but slightly reduced conductivity. Low-power H2 plasma at high temperature introduced defects that significantly reduced conductivity. Medium-power NH3 plasma did not effectively dope the rGO with nitrogen but did reduce conductivity through etching. Higher-power NH3 plasma treatments doped the rGO with nitrogen but also significantly etched the films, greatly reducing hole concentration and conductivity. The highest-power NH3 plasma treatment produced an electron-dominated film with the lowest
Graphene is a one atom thick layer of carbon atoms arranged in a honeycomb lattice. It has excellent mechanical and electrical properties. The document discusses the use of graphene and chemically modified graphene as catalysts. Graphene can be modified through doping with nitrogen or boron to introduce a band gap and alter its conductivity. These doped graphene materials show potential as metal-free catalysts for organic reactions, fuel cells through oxygen reduction, and nitrogen fixation through electrochemical nitrogen reduction. Doped graphene catalysts offer advantages over traditional metal catalysts including lower cost and stability.
This work studied the manipulation of ferromagnetic behavior in gallium manganese nitride (GaMnN) epilayers and heterostructures. Key findings include:
1) GaMnN/p-GaN heterostructures exhibited room temperature ferromagnetism that increased with p-type GaN thickness and hole concentration.
2) GaMnN i-p-n structures showed electric field controlled ferromagnetism at room temperature, with magnetization decreasing under reverse bias due to p-GaN layer depletion.
3) Ferromagnetism in the structures depended on p-GaN layer thickness and GaMnN thickness, demonstrating manipulation of room temperature ferromagnetic properties.
CVD grown nitrogen doped graphene is an exceptional visible-light driven phot...Pawan Kumar
This document summarizes research on nitrogen-doped graphene (NGr) and its potential as a visible-light photocatalyst. Key points:
- NGr was synthesized using chemical vapor deposition with acetonitrile as the carbon and nitrogen source. Nitrogen doping introduces a bandgap and increases the chemical reactivity of graphene.
- Silver nanoparticles were synthesized to form NGr/Ag nanohybrids. Under visible light, plasmonic effects in silver nanoparticles and excitonic effects in NGr interact, generating "hot electrons" that increase photocatalytic activity.
- The photocatalytic performance of NGr, NGr/Ag nanohybrids, and
CVD grown nitrogen doped graphene is an exceptional visible-light driven phot...Pawan Kumar
The photocatalytic potential of large area CVD grown nitrogen doped graphene (NGr) has been explored though the chemical transformation of 4-nitrobenzene thiol into p,p'-dimercaptoazobenzene. Decoration of NGr with Ag nanocubes with rounded edges to form NGr/Ag nanohybrids resulted in a slight increase in the work-function and a decrease in the n-type character of NGr due to ground state transfer of negative charge from NGr to Ag. The Ag nanocubes exhibited a localized surface plasmon resonance (LSPR) at ~425 nm. When the NGr/Ag nanohybrids were illuminated with visible light of wavelength close to the LSPR peak, Kelvin probe force microscopy (KPFM) indicated a dramatic change in surface potential of −225 mV and Raman spectra detected electron accumulation in NGr, which are attributed to a high local field enhancement-mediated hot electron injection into NGr and the formation of long-lived charge separated states. Pristine nitrogen doped graphene and its coupled system with plasmonic Ag nanoparticles showed superior photocatalytic performance compared to bare plasmonic Ag catalyst. While standalone Ag NPs were unable to complete the transformation of 4-NBT into DMAB even at a laser power of 10 mW, NGr/Ag nanohybrids completed this transformation at a laser power of 1 mW, pointing to the high photoreduction strength of NGr/Ag. Density functional theory (DFT) based computational modeling was used to examine the electronic structure of graphene doped with graphitic, pyridinic and pyrrolic nitrogen dopant atoms. DFT results indicated an enhanced chemical reactivity of NGr due to stronger localization of charge at the dopant sites and a pronounced difference in the projected density of states (PDOS) for carbon atoms in proximity to, and distant from, the nitrogen dopant sites.
Graphene: the world's first 2D material. Since graphene's isolation in 2004, it has captured the attention of scientists, researchers, and industry worldwide.
Magneto-transport properties of MnGeP2 and MnGeAs2 filmsIJERA Editor
MnGeAs2 and MnGeP2 thin films were deposited on GaAs and Si substrates. For these film samples, roomtemperature ferromagnetism was observed from magnetization and resistance measurements and verified from hysteresis in magnetization measurements. Hysteresis as well as anomalous behavior in Hall effect measurements was found in the deposited MnGeAs2 and MnGeP2 films, implying spin polarization of the mobile carriers in the films. The Hall resistance measurements above the ferromagnetic transition temperature showed that the carriers are n-type in MnGeAs2 and p-type in MnGeP2.
This document summarizes a study analyzing the composition and structure of near-lattice-matched AlGaInN epitaxial layers. Four ~80 nm thick AlGaInN layers were grown on GaN buffers using metalorganic chemical vapor deposition. Rutherford backscattering spectrometry combined with ion channeling allowed for a self-consistent and depth-resolved determination of the compositional and strain profiles without standards. The layers grew pseudomorphically with excellent crystal quality and minimum channeling yields down to 3%. Parasitic Ga incorporation was observed for certain growth conditions, leading to compositional gradients. Composition and strain profiles were corroborated using x-ray diffraction and wavelength dispersive x-ray microanalysis, which also confirmed spatial homogeneity across
Iaetsd synthesis and investigation of properties in ga asxn1-xIaetsd Iaetsd
The document summarizes research on the synthesis and properties of GaAsxN1-x nano ternary semiconductor thin films. Key findings include:
- GaAsxN1-x layers grown on GaAs substrates were cubic phase while those on GaP substrates were hexagonal phase. Growth conditions like annealing and temperature affected phase and crystallinity.
- Nano rods of GaAsxN1-x were successfully synthesized using a pyrolysis route with organic precursors for gallium and iron catalyst.
- Transmission electron microscopy showed the nano rods had diameters of 25-50nm and were straight and uniform. The tips were polycrystalline while the rods were single crystalline.
-
Similar to Growth of GaN films on GaAs substrates in an As-free environment (20)
Giant LO oscillation in the Zn1yxBex(Se,Te) multi-phonons percolative alloysOleg Maksimov
This document discusses Raman scattering experiments on ZnBe(Se,Te) alloys. It finds that within the percolation regime of 0.19<x<0.81:
1) The low-frequency LO component is overdamped.
2) The high-frequency LO component exhibits a red asymmetry and apparent blue shift compared to predictions.
3) These anomalies can be explained by a discrete multi-mode description for the low- and high-frequency components, where coupling between modes via the macroscopic polarization field results in a transfer of oscillator strength building a "giant" LO oscillation.
Observation of free-to-acceptor-type photoluminescence in chlorine-doped Zn(B...Oleg Maksimov
The document summarizes photoluminescence studies of chlorine-doped Zn1-xBexSe alloys. At low temperatures, photoluminescence is dominated by neutral donor-bound excitons. At high temperatures, a free-to-acceptor transition emerges, involving recombination of free electrons and holes localized at acceptors. The acceptor ionization energy increases with beryllium concentration, suggesting an effective mass defect.
The Indices of Refraction of Molecular-Beam Epitaxy–Grown BexZn1–xTe Ternary ...Oleg Maksimov
This document discusses a study that uses a combination of prism-coupling, reflectivity, and ellipsometric techniques to determine the indices of refraction (n) of molecular-beam epitaxy grown BexZn1-xTe thin films. Initially, prism-coupling measurements were used to obtain discrete n values and film thicknesses at specific wavelengths. Reflectivity data was then analyzed using the prism-coupling results to determine n dispersion below the bandgap. Ellipsometry was finally used to model n dispersion both below and above the bandgap, guided by the prior thickness and below-bandgap n data from the other techniques. The combination of these three methods allowed for accurate determination of n over a wide wavelength
Properties of MBE-Grown ZnBeSe: Study of Be Isoelectronic Traps and of Dopant...Oleg Maksimov
This document summarizes a study of the properties of MBE-grown ZnBeSe, including its undoped, Cl-doped, and N-doped forms. Key findings include:
1) PL from undoped ZnBeSe is attributed to isoelectronic bound excitons formed by a Be-related isoelectronic trap, based on its effective mass behavior under temperature and pressure.
2) For Cl- and N-doped ZnBeSe, the binding energies of donor and acceptor impurities increase with Be concentration, following hydrogenic behavior.
3) Reported values for the ZnBeSe bandgap from one reference seem too low to explain the observed PL peaks
Temperature Dependence of the Band-Edge Transitions of ZnCdBeSeOleg Maksimov
This study characterized the temperature dependence of band-edge transitions in three ZnCdBeSe films with varying concentrations of beryllium (Be) using contactless electroreflectance (CER) and piezoreflectance (PzR) measurements from 15-450K. The CER and PzR spectra showed doublet features near the band edge, indicating light-hole and heavy-hole excitonic transitions. Comparing the relative intensities of the PzR and CER spectra allowed identification of the transitions. Analysis of the temperature dependence provided information on how the energy and broadening of the transitions varied with temperature and Be concentration. The results showed that incorporating Be effectively reduced the rate of temperature variation of the energy
Temperature dependence of the energy gap of MgxZnyCd1–x–ySe alloyOleg Maksimov
This document reports on a study of the temperature dependence of the band gap energy in MgxZnyCd1–x–ySe alloys. Reflectivity measurements were taken between 4.2K and 300K. The data was fitted to a Bose-Einstein relationship. A similar temperature dependence was observed for MgxZnyCd1–x–ySe and Zn0.5Cd0.5Se, indicating that quantum confinement in MgxZnyCd1–x–ySe/Zn0.5Cd0.5Se quantum wells does not significantly change with temperature. This stability of quantum confinement could simplify the design of light emitting devices using these materials.
Efficient free exciton emission at room temperature in Zn0.5Cd0.5Se/MgxZnyCd1...Oleg Maksimov
This document summarizes research on efficient free exciton emission at room temperature in Zn0.5Cd0.5Se/MgxZnyCd1KxKySe quantum wells. Photoluminescence spectroscopy shows that at low temperatures, luminescence is dominated by bound exciton emission, but free exciton emission becomes dominant above 100 K and persists up to room temperature at 40% of the low-temperature intensity. This is attributed to the deep confinement potential stabilizing excitons and preventing thermal escape. Calculations estimate the increase in exciton binding energy due to quantum well confinement.
Reflectance and photoluminescence characterization of BexZn1Oleg Maksimov
This document summarizes research on the optical properties of BexZn1-xTe thin films grown by molecular beam epitaxy. Reflectance and photoluminescence measurements were performed on the epilayers as a function of BeTe content and temperature. An increase in emission line broadening was observed with higher BeTe content, which is attributed to alloy disorder. Temperature-dependent reflectance data showed a reduction in the variation of band gap energy with increasing BeTe content. This effect is proposed to be due to lattice hardening from BeTe decreasing the lattice contribution to the temperature dependence.
Exciton localization in MgxZnyCd1–x–ySe alloyOleg Maksimov
The document summarizes photoluminescence and reflectivity measurements of MgxZnyCd1–x–ySe epitaxial layers with varying Mg content (x). As Mg content increases, significant emission line broadening, an increase in activation energy, and a large Stokes shift are observed. For samples with high Mg content (x > 0.3), an anomalous temperature dependence of emission energy and line broadening is seen, attributed to exciton localization. Exciton localization on statistical CdSe clusters within the alloy is proposed as the most likely localization mechanism.
Optical properties of molecular-beam-epitaxy-grown InGaMnAs thin filmsOleg Maksimov
The document summarizes research on the optical properties of molecular beam epitaxy grown (InGa)MnAs thin films with varying Mn concentration. Spectroscopic ellipsometry was used to determine the dielectric functions of the films. Analysis showed the fundamental band gap energy (E0 critical point) blueshifted with increasing Mn concentration, while the E1 critical point showed a redshift. This indicates the electronic structure is changing with Mn incorporation. Specifically, the band structure transitions in the Brillouin zone are affected by the Mn doping level in this quaternary semiconductor system.
Percolation-based vibrational picture to estimate nonrandom N substitution in...Oleg Maksimov
This document discusses using Raman spectroscopy to estimate the degree of nonrandom nitrogen (N) substitution in GaAsN alloys. It finds evidence of two distinct Ga-N vibrational modes: one at around 475 cm-1 representing isolated Ga-N bonds, and another lower frequency mode at around 428 cm-1 representing Ga-N bonds within nitrogen-rich regions. This is interpreted using a model of mechanical contrast between the nitrogen-rich and nitrogen-poor regions, similar to models used for Be-chalcogenide alloys. Estimating the relative strengths of the two Ga-N modes suggests the nitrogen-rich regions incorporate around 30% of the total nitrogen, much higher than the approximately 4% seen in random Be
Magnetoresistance anomalies in (Ga,Mn)As epilayers with perpendicular magneti...Oleg Maksimov
This document summarizes magnetoresistance anomalies observed in tensile-strained (Ga,Mn)As epilayers with perpendicular magnetic anisotropy. The researchers observed "spikes" in the longitudinal magnetoresistance that are antisymmetric with respect to the direction of the magnetic field. These anomalies occur during magnetization reversal and are accompanied by a change in sign of the anomalous Hall effect. Angular sweeps of the magnetic field reveal that the anomalies have an antisymmetric dependence on the helicity of the field sweep. The data suggest that the antisymmetric anomalies originate from anomalous Hall effect contributions to the longitudinal resistance when domain walls are located between the voltage probes.
Direct-to-indirect band gap crossover for the BexZn1-xTe alloyOleg Maksimov
The document investigates the growth and optical properties of BexZn1-xTe epitaxial layers with Be concentrations ranging from 0-0.7. Reflectivity and photoluminescence spectra were used to determine the direct-indirect band gap crossover. The results indicate the crossover occurs at around x=0.28 Be concentration. Above this concentration, BexZn1-xTe behaves as an indirect semiconductor, while below it has direct band gap characteristics. The direct band gap increases linearly with Be content, while the indirect gap can be modeled by a quadratic equation related to composition.
High-Brightness 9XX-nm Pumps with Wavelength StabilizationOleg Maksimov
This document summarizes the development of high-power, high-brightness laser diode pumps for fiber lasers. It describes pumps capable of outputting over 100W of continuous wave power while maintaining high efficiency and beam quality. Key points discussed include:
1. Single emitter pumps have advantages over bar stacks in cost, reliability, and performance for applications requiring kilowatt-class output.
2. New pumps achieve over 65% efficiency and output over 100W while maintaining a beam numerical aperture below 0.13.
3. Wavelength stabilized pumps provide over 50W within a 1.5nm window, suitable for pumping specific gain media.
4. 100W-class pumps demonstrate high reliability with
This document summarizes research on developing high-power, high-brightness fiber coupled diode pumps. [1] New diode chips with cavity lengths of 4.5mm instead of the previous 3mm allow output power over 20W continuously while maintaining high efficiency and reliability. [2] Optimization of the layered semiconductor structure and ridge waveguide geometry achieved a peak power efficiency of over 72% and average efficiency over 60% up to 14W output. [3] Further optimization targeted high output power, demonstrating over 20W continuously with good efficiency at room temperature operation.
Hanle Effect Measurements of Spin Lifetime in Zn0.4Cd0.6Se Epilayers Grown on...Oleg Maksimov
This document summarizes a study that used the Hanle effect to measure spin lifetimes in ZnCdSe epilayers grown on InP substrates with varying n-doping levels. Four samples were studied: three Zn0.4Cd0.6Se samples with carrier densities of 8.0×1016 cm-3, 4.3×1017 cm-3, and 1.1×1018 cm-3, as well as an undoped Zn0.5Cd0.5Se sample. Measurements showed that spin lifetime varied non-monotonically with carrier density, reaching a maximum of ~10.5 ns for the sample near the metal-insulator transition.
Patterned three-color ZnCdSe/ZnCdMgSe quantum-well structures for integrated ...Oleg Maksimov
This document summarizes research on growing and characterizing patterned quantum well (QW) structures for full-color light emitting applications. Three different QWs emitting in the red, green, and yellow regions of the visible spectrum were integrated on a single indium phosphide substrate using sequential selective area epitaxy with a shadow mask. Micro-photoluminescence measurements showed well-defined excitonic emission peaks from each patterned region, demonstrating the feasibility of fabricating full-color light emitting diodes and lasers using this zinc cadmium magnesium selenide material system and growth technique.
Exchange biasing of the ferromagnetic semiconductor Ga,Mn As by MnOOleg Maksimov
This document summarizes research on exchange biasing of the ferromagnetic semiconductor Ga1−xMnxAs with an antiferromagnetic MnO layer. Characterization using RBS, XPS, TEM, and XRR showed the formation of a MnO layer on Ga1−xMnxAs. SQUID magnetometry measurements exhibited an exchange field and enhanced coercivity, indicating exchange bias. The exchange and coercive fields varied with temperature and cooling field. Both cases where the Néel temperature of MnO was similar to or greater than the Curie temperature of Ga1−xMnxAs were investigated.
Exchange biasing of the ferromagnetic semiconductor Ga1−xMnxAsOleg Maksimov
1) Researchers demonstrated exchange coupling between a ferromagnetic semiconductor (Ga1-xMnxAs) and an overgrown antiferromagnet (MnO).
2) Samples were grown by molecular beam epitaxy and consisted of a 10nm Ga1-xMnxAs layer capped with a thin Mn layer that oxidized to MnO.
3) Magnetization measurements showed clear exchange biasing effects, including a shifted hysteresis loop and enhanced coercivity, when the sample was cooled in a magnetic field through the blocking temperature of MnO (48± 2 K) and Curie temperature of Ga1-xMnxAs (55.1± 0.2 K).
Annealing Dependence of Exchange Bias in MnO/Ga1−xMnxAs HeterostructuresOleg Maksimov
The document discusses the annealing dependence of exchange bias in MnO/Ga1-xMnxAs heterostructures. It finds that as-grown Mn overlayers do not produce exchange coupling, and annealing is essential to create exchange bias signatures like loop shifting. X-ray photoelectron spectroscopy shows the cleanest exchange bias arises when the Mn overlayer is completely converted to MnO during annealing. Rapid thermal annealing of samples bonded with indium provides sufficient heating to oxidize the Mn layer to MnO and produce exchange bias, while unannealed samples do not exhibit this behavior.
Ellen Burstyn: From Detroit Dreamer to Hollywood Legend | CIO Women MagazineCIOWomenMagazine
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Adani Group's Active Interest In Increasing Its Presence in the Cement Manufa...Adani case
Time and again, the business group has taken up new business ventures, each of which has allowed it to expand its horizons further and reach new heights. Even amidst the Adani CBI Investigation, the firm has always focused on improving its cement business.
Presentation by Herman Kienhuis (Curiosity VC) on Investing in AI for ABS Alu...Herman Kienhuis
Presentation by Herman Kienhuis (Curiosity VC) on developments in AI, the venture capital investment landscape and Curiosity VC's approach to investing, at the alumni event of Amsterdam Business School (University of Amsterdam) on June 13, 2024 in Amsterdam.
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AI Transformation Playbook: Thinking AI-First for Your BusinessArijit Dutta
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Unlocking WhatsApp Marketing with HubSpot: Integrating Messaging into Your Ma...Niswey
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2. 1672 Maksimov et al.: Growth of GaN films on GaAs substrates 1672
FIG. 2. XRD -2 scans of ϳ1.5 m thick GaN films grown on GaAs ͑A͒
by direct deposition, ͑B͒ after nitridation, and ͑C͒ after nitridation on the
low-temperature buffer layer.
FIG. 1. RHEED patterns for ͑A͒ GaAs substrate after oxide desorption, ͑B͒ However, ␣-GaN misoriented grains ͑͗1011͘, ͗1012͘, ͗1120͘,
GaN film grown by direct deposition, ͑C͒ nitridated GaAs, ͑D͒ annealed ͗1013͒͘ and -GaN inclusions ͑͗002͒͘ are also present. The
nitridated GaAs, ͑E͒ GaN film grown at 600 ° C after nitridation, and ͑F͒
GaN film grown at 750 ° C after nitridation on the low-temperature buffer surface of the film is very rough and exhibits a grain struc-
layer. ture, as shown in Figs. 3͑a͒ and 3͑b͒. Cross-sectional SEM
image ͑Fig. 4͒ reveals that GaN grows in columnar crystals,
in agreement with other attempts to deposit GaN without
by RHEED. The substrate temperature is measured by a ther- stabilizing As flux.12
mocouple in contact with the backside of the mounting It is suggested that low-temperature “nitridation” ͑the wa-
block. The wafer is rotated during the deposition for the fer is exposed to nitrogen plasma͒ followed by high-
growth uniformity. temperature annealing can produce a smooth single-crystal
The crystalline quality is studied ex situ by x-ray diffrac- GaN layer.13 This layer can be used as a template for further
tion ͑XRD͒, scanning electron microscopy ͑SEM͒, and trans- GaN growth. To adopt this strategy we nitridate GaAs for
mission electron microscopy ͑TEM͒. For TEM studies, 15 min at 400 ° C and anneal it for half an hour at 600 ° C.
cross-sectional specimens are prepared using conventional Surface reconstruction disappears during the first few min-
mechanical thinning followed by argon-ion milling. TEM, utes of GaAs wafer exposure to nitrogen plasma, suggesting
high-resolution TEM, and selected area diffraction ͑SAD͒ formation of an amorphous GaAsN layer at the surface. We
are carried out in a JEOL 2010F transmission electron mi- observe an arc pattern after approximately 5 min, indicating
croscope operating at 200 keV. The surface morphology of development of a preferred orientation in a disordered layer.
the films is examined by atomic force microscopy ͑AFM͒ Spotlike features with hexagonal symmetry develop by the
with scanned areas up to 10ϫ 10 m2. end of nitridation ͓Fig. 1͑c͔͒. Annealing at 600 ° C sharpens
diffraction spots ͓Fig. 1͑d͔͒, demonstrating recrystallization
of GaN phase. The diffraction spots are relatively broad, sig-
III. RESULTS AND DISCUSSION nifying that a very defective GaN layer forms at the begin-
Oxide desorption typically results in a slightly distorted ning. However, they become significantly sharper and elon-
GaAs surface and the RHEED pattern is characterized by gated during the MBE growth ͓Fig. 1͑e͔͒, indicating that the
modulated intensity, twofold reconstruction along the ͓011͔ final GaN film has a better crystalline quality and a smoother
direction, and, occasionally, faceting along the ͓0-11͔ direc- surface. An XRD -2 scan of a film grown on the nitridated
tion. Kikuchi lines are clearly evident, indicating that GaAs substrate is shown in Fig. 2͑b͒. The main diffraction peak is
surface is free of oxide layer ͓Fig. 1͑a͔͒. ͗0002͘ ␣-GaN. A very weak ͗002͘ -GaN diffraction is also
When we deposit GaN directly on such a surface, we present. It may be attributed to cubic grains at the
observe an arc RHEED pattern, characteristic for the poly- GaN / GaAs interface that can develop during the nitridation-
crystalline growth ͓Fig. 1͑b͔͒. An XRD -2 scan of a film annealing process. AFM images ͓Figs. 3͑c͒ and 3͑d͔͒ show
grown at 600 ° C is shown in Fig. 2͑a͒. It reveals that the film that GaN surface is much smoother and the grains coalesce
is a highly textured ␣-GaN with ͗0001͘ preferred orientation. better. Cross-sectional SEM images, not shown, also demon-
J. Vac. Sci. Technol. B, Vol. 24, No. 3, May/Jun 2006
3. 1673 Maksimov et al.: Growth of GaN films on GaAs substrates 1673
FIG. 3. AFM images of ϳ1.5 m thick GaN films grown on GaAs ͓͑A͒ and ͑B͔͒ by direct deposition, ͓͑C͒ and ͑D͔͒ after nitridation, and ͓͑E͒ and ͑F͔͒ after
nitridation on the low-temperature buffer layer.
strate that the film is denser, the GaN columns are more thick interfacial GaN layer at 600 ° C after nitridation. Next,
uniform, and the boundaries between them are less evident. we raise the wafer temperature to 750 ° C for consequent
It is reported that significant improvement of the quality GaN growth. When the growth is initiated, we observe a
of the GaN films on ͓001͔ Si substrates can be achieved by a streaky ͑1 ϫ 1͒ RHEED pattern ͓Fig. 1͑f͔͒. An XRD -2
two-step growth process. A thin buffer layer is grown at a scan of this film is shown in Fig. 2͑c͒. The spectrum, similar
relatively low temperature; the rest of the film is grown at a
to the previous, is dominated by ͗0002͘ ␣-GaN diffraction.
higher temperature.14,15 This procedure is successfully ap-
However, the intensity ratio of ͗0002͘ ␣-GaN / ͗002͘ GaAs
plied to other substrates including ␣-Al2O3, 6H-SiC, and
ZnO.16,17 To investigate this approach, we deposit a 50 nm peaks, that can serve as an estimate of the crystallinity, in-
JVST B - Microelectronics and Nanometer Structures
4. 1674 Maksimov et al.: Growth of GaN films on GaAs substrates 1674
axis bright-field image. Similar to previous films, GaN dis-
plays columnar structure. The columns are separated by
straight boundaries and have well-defined surface facets. We
present a high-resolution TEM image of two neighboring
columns in Fig. 5͑d͒. The image, collected close to the film
surface along the ͓11-20͔ GaN zone axis, indicates that grain
boundaries consist of an amorphous GaN. Figure 6 shows
SAD pattern collected from the top part of the film along the
͓11-20͔ zone axis. It displays both a bright wurtzite pattern
and an additional set of weak spots, most probably, originat-
ing from planar defects or double diffraction. In addition, the
two columns have slightly different contrasts, possibly, due
to the twist and tilt between them.
Figure 5͑b͒ shows a ͓1-100͔ two-beam dark-field image of
FIG. 4. Cross-sectional SEM image of an ϳ1.5 m thick GaN film grown
GaN film with the top part milled away. It is evident that the
by direct deposition on GaAs. film consists of two different regions separated, for clarity,
by a dashed line. Unlike the top part, the bottom part of the
film is not columnar and contains a higher density of defects.
creases from 3 to 20, indicating higher quality of the film. Based on the thickness, ϳ70 nm, we assign it to the low-
AFM images ͓Figs. 3͑e͒ and 3͑f͔͒ also show superior surface temperature buffer layer. We suggest that, similar to
morphology, in agreement with streaky RHEED pattern ob- MOCVD on Al2O3, low-temperature buffer serves as a
served during the growth. nucleation layer for prismatic GaN growth.18 A high-
We use TEM for detailed analysis of GaN grown under resolution TEM image of the interface between epilayer and
the last conditions. Figure 5͑a͒ shows a large scale on-zone- substrate is presented in Fig. 5͑c͒. The image, collected along
FIG. 5. ͑A͒ On-zone-axis bright-field image showing the GaN / GaAs heterostructure. ͑B͒ ͓1-100͔ two-beam dark-field image of GaN / GaAs. The top part of
the film is milled away. ͑C͒ HRTEM image collected near the GaN / GaAs interface along the ͓110͔ GaAs zone axis. ͑D͒ HRTEM image collected along the
͓11-20͔ GaN zone axis, showing two neighboring columns.
J. Vac. Sci. Technol. B, Vol. 24, No. 3, May/Jun 2006
5. 1675 Maksimov et al.: Growth of GaN films on GaAs substrates 1675
talline ␣-GaN containing misoriented domains and cubic in-
clusions. The surface is very rough and consists of coalesc-
ing grains. We use a three-step process to improve the film
quality. The procedure consists of substrate nitridation, depo-
sition of a GaN buffer layer at a low temperature, and epi-
taxial regrowth at an elevated temperature. These steps sup-
press misorientation and improve surface morphology of the
film. TEM demonstrates the presence of a 5 nm thick amor-
phous layer at the GaAs/ GaN interface. We suggest that it is
due to the incomplete recrystallization of GaAsN layer and
believe that further improvement of nitridation conditions
should resolve this problem.
ACKNOWLEDGMENT
The work is supported by ONR ͑Dr. C. E. C. Wood͒.
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JVST B - Microelectronics and Nanometer Structures