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At least 1,045 records · Page 58

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DOE OSTI2011

Experimental and theoretical study for InAs quantum dashes-in-a-step-well structure on (001)-oriented InP substrate

An InAs/In{sub 0.73}Ga{sub 0.27}As/In{sub 0.53}Ga{sub 0.47}As/In{sub 0.53}Al{sub 0.235}Ga{sub 0.235}As quantum dashes-in-a-step-well (QDSWELL) structure grown via molecular beam epitaxy has been studied. It is observed that the photoluminescence (PL) emission wavelength of such a structure can be as long as 2.12 {mu}m at room temperature (RT). This is the longest emission wavelength of InAs QDashes to be realized at RT. The electron and hole energy levels of the InAs/In{sub 0.73}Ga{sub 0.27}As/In{sub 0.53}Ga{sub 0.47}As/In{sub 0.53}Al{sub 0.235}Ga{sub 0.235}As QDSWELL structure have been calculated using effective-mass envelope-function theory. The calculated transition energy E{sub E1-HH1} (from the first electron energy level E1 to the first heavy-hole energy level HH1) agrees with the measured PL emission peak position quite well. It is found that QDSWELL is an alternative structure for realizing lasers with wavelengths beyond 2 {mu}m at RT.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Comparison between chemical vapor deposited and physical vapor deposited WSi{sub 2} metal gate for InGaAs n-metal-oxide-semiconductor field-effect transistors

We compare chemical vapor deposition (CVD) and physical vapor deposition (PVD) WSi{sub 2} metal gate process for In{sub 0.53}Ga{sub 0.47}As n-metal-oxide-semiconductor field-effect transistors using 10 and 6.5 nm Al{sub 2}O{sub 3} as dielectric layer. The CVD-processed metal gate device with 6.5 nm Al{sub 2}O{sub 3} shows enhanced transistor performance such as drive current, maximum transconductance and maximum effective mobility. These values are relatively better than the PVD-processed counterpart device with improvement of 51.8%, 46.4%, and 47.8%, respectively. The improvement for the performance of the CVD-processed metal gate device is due to the fluorine passivation at the oxide/semiconductor interface and a nondestructive deposition process.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates

We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 10{sup 8} cm{sup -2} dot density and metamorphic In{sub x}Ga{sub 1-x}As (x=0.15,0.30) confining layers result in emission wavelengths at 1.3 {mu}m. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2011

Power Conversion Efficiency of AlGaAs/GaAs Schottky Diode for Low-Power On-Chip Rectenna Device Application

A Schottky diode has been designed and fabricated on n-AlGaAs/GaAs high-electron-mobility-transistor (HEMT) structure. Current-voltage (I-V) measurements show good device rectification with a Schottky barrier height of 0.4349 eV for Ni/Au metallization. The differences of Schottky barrier height from theoretical value are due to the fabrication process and smaller contact area. The RF signals up to 1 GHz are well rectified by the fabricated Schottky diodes and stable DC output voltage is obtained. Power conversion efficiency up to 50% is obtained at 1 GHz with series connection between diode and load. The fabricated the n-AlGaAs/GaAs Schottky diode provide conduit for breakthrough designs for ultra-low power on-chip rectenna device technology to be integrated in nanosystems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2011

Self-aligned epitaxial metal-semiconductor hybrid nanostructures for plasmonics

We demonstrate self-alignment of epitaxial Ag nanocrystals on top of low-density near-surface InAs quantum dots (QDs) grown by molecular beam epitaxy. The Ag nanocrystals support a surface plasmon resonance that can be tuned to the emission wavelength of the QDs. Photoluminescence measurements of such hybrid metal-semiconductor nanostructures reveal large enhancement of the emission intensity. Our concept of epitaxial self-alignment enables the integration of plasmonic functionality with electronic and photonic semiconductor devices operating down to the single QD level.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2011

Mechanisms of edge-dislocation formation in strained films of zinc blende and diamond cubic semiconductors epitaxially grown on (001)-oriented substrates

Ninety degree edge misfit dislocations (MDs) are 'sessile' dislocations; such dislocations, however, were found in large amounts in relaxed films. The commonly accepted formation mechanism of such dislocations is an interaction of two complementary 60 deg. dislocations with appropriate Burger's vectors, for example: a/2[101] + a/2 [011] = a/2 [110]. In the present study, four possible types of interaction were analyzed: (i) random meeting of two complementary MDs; (ii) crossing of two complementary 60 deg. MDs in the vicinity of film-substrate interface in systems grown on substrates misoriented from exact (001) orientation; (iii) formation of edge MDs during cross-slipping of a secondary MD; and (iv) induced nucleation of a secondary complementary 60 deg. MD. Examples of discussed interactions are given. Contrary to the widespread opinion that edge MDs in GeSi and InGaAs films grown by MBE on Si and GaAs substrates predominantly form under elastic strains greater than 2% and at the final stage of plastic relaxation, in the present study, we show that such dislocations may also form at an early stage of plastic relaxation in films with less-than-1% lattice misfit with substrate. A necessary condition for that is a sufficient amount of 60 deg. dislocations available in the system by the moment the strained film starts growing. Dislocations (60 deg. ) can be introduced into the system using a preliminarily grown, partially or fully relaxed buffer layer. This layer serves as a source of threading dislocations for the next growing layer that favor the formation of paired complementary MDs and their 'reagents', edge MDs, at the interface with growing film.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Electron holographic tomography for mapping the three-dimensional distribution of electrostatic potential in III-V semiconductor nanowires

Electron holographic tomography (EHT), the combination of off-axis electron holography with electron tomography, is a technique, which can be applied to the quantitative 3-dimensional (3D) mapping of electrostatic potential at the nanoscale. Here, we show the results obtained in the EHT investigation of GaAs and GaAs-AlGaAs core-shell nanowires grown by Au-catalysed metalorganic vapor phase epitaxy. The unique ability of EHT of disentangling the materials mean inner potential (MIP) from the specimen projected thickness allows reconstruction of the nanowire 3D morphology and inner compositional structure as well as the measurement of the MIP.

43 PARTICLE ACCELERATORS↗

DOE OSTI2011

Electron Spin Dynamics in Semiconductor Quantum Dots

An electron spin confined to a semiconductor quantum dot is not subject to the classical spin relaxation mechanisms known for free carriers but it strongly interacts with the nuclear spin system via the hyperfine interaction. We show in time resolved photoluminescence spectroscopy experiments on ensembles of self assembled InAs quantum dots in GaAs that this interaction leads to strong electron spin dephasing.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Temperature Dependence of Photoluminescence Peak Energy in Ga(In)NAs

Introducing normal coordinate system, theoretical approach to band gap alteration with temperature is described. The derived expression is applied to a GaNAs system having the nitrogen composition of about 1.4%. It is found that the band gap narrowing in this system is originated in the inter-band mixing assisted by an LO phonon at a {Gamma} point through electron--phonon interaction.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Chemical composition and thermal stability of GaAs oxides grown by AFM anodic oxidation for site-controlled growth of InAs quantum dots

We have fabricated GaAs oxides by using atomic force microscope (AFM)-assisted anodic oxidation at various bias voltages, V{sub ox}, and studied their chemical compositions and thermal stabilities. The oxides grown at bias voltages less than 30 V desorbed after standard thermal cleaning in molecular beam epitaxy, while the oxide patterns fabricated at V{sub ox}{>=}40 V survived on the surface. We have further investigated the chemical composition of the oxides by X-ray photoemission spectroscopy. It has been found that the AFM oxides grown at V{sub ox}{approx}10 V predominantly consist of Ga{sub 2}O and GaO, whereas those grown at V{sub ox}{approx}50 V contain a Ga{sub 2}O{sub 3}-component. This result indicates that the better thermal stability of AFM oxides grown at V{sub ox}{>=}40 V can be attributed to the formation of Ga{sub 2}O{sub 3}. We grew a GaAs buffer layer on the oxide nanomasks and obtained nanoholes. After supplying InAs, selective dot nucleation took place in the nanoholes, resulting in successful formation of site-controlled QDs.

36 MATERIALS SCIENCE↗

DOE OSTI2011

On the Ga interdiffusion in InAs free-standing nanowires grown by molecular beam epitaxy

Free-standing InAs nanowires can be grown on GaAs (111)B substrates by several epitaxial growth techniques. Despite what should be expected from the general accepted Vapor-Liquid-Solid growth model.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Coulomb Interaction between InAs/GaAs Quantum Dots and Adjacent Impurities

Defects positioned close to a plane of quantum dots (QDs) are shown to be influenced by coulomb interaction effect when the quantum dots are charged by electrons. Signals from deep level transient spectroscopy (DLTS) measurement give rise to a mirror effect in the spectrum depending on movement of the defect energy level in relation to the Fermi-level as a result of the electron traffic at the QDs.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Theoretical And Experimental Studies Of The Effects Of Rapid Thermal Annealing In GaAs/AlGaAs Quantum Dots Grown By Droplet Epitaxy

We fabricated low-density GaAs/AlGaAs quantum dots for single photon source by droplet epitaxy. We investigated the emission energies of the dots and underlying superlattice by using photoluminescence and cathodoluminescence measurements. By forming a mesa etched structure, we distinguished the transitions from the superlattice and the dots. And we calculated the diffusion length in this system from the peak shift of the superlattice, and applied the diffusion to the dots to investigate the emission energy shift of the QDs.z

36 MATERIALS SCIENCE↗

DOE OSTI2011

Quantum Dot Charging By Means Of Temperature And Magnetic Field

A micro-photoluminescence study of individual InAs/GaAs quantum dots is presented. It is demonstrated that by varying the strength of an applied magnetic field and/or the temperature, the charge state of a quantum dot can be tuned. The charge tuning mechanism is shown to be due to the modification of the electron and hole transport in the wetting layer plane prior to their capture into the quantum dot.