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Engineering topics

Salamo, G. J.

Publications and source records attributed to Salamo, G. J..

9 records

All sources NASA NTRS DOE OSTI

DOE OSTI2016

Intensity-dependent nonlinearity of the lateral photoconductivity in InGaAs/GaAs dot-chain structures

Photoelectric properties of laterally correlated multilayer InGaAs/GaAs quantum dots (QDs) heterostructures are studied. The response of the photocurrent to increasing excitation intensity is found to be nonlinear and varying with excitation energy. The structures are photosensitive in a wide range of photon energies above 0.6 eV. The spectral dependence of the photoconductivity (PC) is caused by strong interaction between the bulk GaAs and the lower energy states of the wetting layer, the QDs, as well as the defect states in the GaAs band gap. In particular, a mechanism for the participation of deep electron trap levels in the photocurrent is clarified. These structures also demonstrate a high sensitivity of up to 10 A/W at low excitation intensities. However, at higher excitation intensities, the sensitivity reduces exhibiting a strong spectral dependence at the same time. The observed sublinear PC dependence on excitation power results from a direct electron-hole recombination both in the QDs and in GaAs host. The solution of rate equations included the contributions of QD ground and exited states, bulk GaAs states and the states of defects within the GaAs bandgap describes well the experimental data.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Carrier transfer in vertically stacked quantum ring-quantum dot chains

The interplay between structural properties and charge transfer in self-assembled quantum ring (QR) chains grown by molecular beam epitaxy on top of an InGaAs/GaAs quantum dot (QD) superlattice template is analyzed and characterized. The QDs and QRs are vertically stacked and laterally coupled as well as aligned within each layer due to the strain field distributions that governs the ordering. The strong interdot coupling influences the carrier transfer both along as well as between chains in the ring layer and dot template structures. A qualitative contrast between different dynamic models has been developed. By combining temperature and excitation intensity effects, the tuning of the photoluminescence gain for either the QR or the QD mode is attained. The information obtained here about relaxation parameters, energy scheme, interlayer and interdot coupling resulting in creation of 1D structures is very important for the usage of such specific QR–QD systems for applied purposes such as lasing, detection, and energy-harvesting technology of future solar panels.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Measuring the depth profiles of strain/composition in AlGaN-graded layer by high-resolution x-ray diffraction

Here, we demonstrate X-ray fitting through kinematical simulations of the intensity profiles of symmetric reflections for epitaxial compositionally graded layers of AlGaN grown by molecular beam epitaxy pseudomorphically on [0001]-oriented GaN substrates. These detailed simulations depict obvious differences between changes in thickness, maximum concentration, and concentration profile of the graded layers. Through comparison of these simulations with as-grown samples, we can reliably determine these parameters, most important of which are the profiles of the concentration and strain which determine much of the electrical properties of the film. In addition to learning about these parameters for the characterization of thin film properties, these fitting techniques create opportunities to calibrate growth rates and control composition profiles of AlGaN layers with a single growth rather than multiple growths as has been done traditionally.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Deep level centers and their role in photoconductivity transients of InGaAs/GaAs quantum dot chains

The in-plane photoconductivity and photoluminescence are investigated in quantum dot-chain InGaAs/GaAs heterostructures. Different photoconductivity transients resulting from spectrally selecting photoexcitation of InGaAs QDs, GaAs spacers, or EL2 centers were observed. Persistent photoconductivity was observed at 80 K after excitation of electron-hole pairs due to interband transitions in both the InGaAs QDs and the GaAs matrix. Giant optically induced quenching of in-plane conductivity driven by recharging of EL2 centers is observed in the spectral range from 0.83 eV to 1.0 eV. Conductivity loss under photoexcitation is discussed in terms of carrier localization by analogy with carrier distribution in disordered media.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Band filling effects on temperature performance of intermediate band quantum wire solar cells

Detailed studies of solar cell efficiency as a function of temperature were performed for quantum wire intermediate band solar cells grown on the (311)A plane. A remotely doped one-dimensional intermediate band made of self-assembled In{sub 0.4}Ga{sub 0.6}As quantum wires was compared to an undoped intermediate band and a reference p-i-n GaAs sample. These studies indicate that the efficiencies of these solar cells depend on the population of the one-dimensional band by equilibrium free carriers. A change in this population by free electrons under various temperatures affects absorption and carrier transport of non-equilibrium carriers generated by incident light. This results in different efficiencies for both the doped and undoped intermediate band solar cells in comparison with the reference GaAs p-i-n solar cell device.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Low temperature magneto-photoluminescence of GaAsBi /GaAs quantum well heterostructures

Strong optical anisotropy is observed in the emission from a GaAs{sub 1−x}Bi{sub x} (x ∼ 0.04) quantum well grown by low temperature molecular beam epitaxy on (001) GaAs by means of low temperature magneto-photoluminescence (MPL) taken at 2 K in Faraday geometry for magnetic fields, B, up to 10 T. A significant diamagnetic shift (∼2.5 meV) develops for magnetic fields above ∼8 T, which is accompanied by a narrowing of the emission bandwidth and a substantial increase in the difference between the integrated intensities of the σ{sup +} and σ{sup −} polarizations in the MPL spectra. This, along with a peculiar spectral dependence of the polarization degree which evolves with increasing magnetic field, is interpreted in terms of bound and free magneto excitons in the system where Bi-related levels become hybridized to different extents with the valence and conduction bands of the GaAs host material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Strong excitation intensity dependence of the photoluminescence line shape in GaAs{sub 1-x}Bi{sub x} single quantum well samples

A set of high quality single quantum well samples of GaAs{sub 1-x}Bi{sub x} with bismuth concentrations not exceeding 6% and well widths ranging from 7.5 to 13 nm grown by molecular beam epitaxy on a GaAs substrate at low temperature is studied by means of photoluminescence (PL). It is shown that the PL line shape changes when the exciton reduced mass behavior changes from an anomalous increase (x < 5%) to a conventional decrease (x > 5%). Strongly non-monotonous PL bandwidth dependence on the excitation intensity is revealed and interpreted in terms of optically unresolved contributions from the saturable emission of bound free excitons.

36 MATERIALS SCIENCE↗

DOE OSTI2012

Effect of dimensionality and morphology on polarized photoluminescence in quantum dot-chain structures

Change of the photoluminescence (PL) polarization is studied by changing the excitation intensity and temperature for aligned In(Ga)As quantum dot (QD) structures with varying inter-dot distances grown by molecular beam epitaxy on semi-insulating GaAs (100) substrates. An unusual increase of the polarization ratio is observed by increasing the temperature and/or excitation intensity throughout a low temperature (T < 70 K) and low intensity (I{sub ex} < 1 W/cm{sup 2}) range. This increase as well as the general behavior of the polarized PL are the results of the exciton dynamics and the peculiarities of the system morphology. They are due to the varying inter-dot distances which change the system from zero-dimensional comprised of isolated QDs to one-dimensional comprised of wire-like structures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2012

Structural, morphological, and magnetic characterization of In{sub 1-x}Mn{sub x}As quantum dots grown by molecular beam epitaxy

In this paper, we present a method to order low temperature (LT) self-assembled ferromagnetic In{sub 1-x}Mn{sub x}As quantum dots (QDs) grown by molecular beam epitaxy (MBE). The ordered In{sub 1-x}Mn{sub x}As QDs were grown on top of a non-magnetic In{sub 0.4}Ga{sub 0.6}As/GaAs(100) QDs multi-layered structure. The modulation of the chemical potential, due to the stacking, provides a nucleation center for the LT In{sub 1-x}Mn{sub x}As QDs. For particular conditions, such as surface morphology and growth conditions, the In{sub 1-x}Mn{sub x}As QDs align along lines like chains. This work also reports the characterization of QDs grown on plain GaAs(100) substrates, as well as of the ordered structures, as function of Mn content and growth temperature. The substitutional Mn incorporation in the InAs lattice and the conditions for obtaining coherent and incoherent structures are discussed from comparison between Raman spectroscopy and x-ray analysis. Ferromagnetic behavior was observed for all structures at 2 K. We found that the magnetic moment axis changes from [110] in In{sub 1-x}Mn{sub x}As over GaAs to [1-10] for the ordered In{sub 1-x}Mn{sub x}As grown over GaAs template.

77 NANOSCIENCE AND NANOTECHNOLOGY↗