Gabriel Agnello

Observation of crystallite formation in ferromagnetic Mn-implanted Si

Mn-implanted Si was investigated using transmission electron microscopy to gain insight into the structure of the implanted region. Diffraction contrast images, selected area diffraction patterns, and high resolution images of the samples were acquired before and after postimplant annealing at 800°C. The images of the annealed samples revealed the formation of nanometer size precipitates distributed throughout the implanted region. Analysis of the selected area diffraction pattern determined that the most prominent lattice spacing of the crystallites is 2.15A. This spacing indicates that the most probable phase of the crystallites is MnSi1.7 and this is consistent with the Mn:Si binary phase diagram. This phase is paramagnetic at room temperature with a Curie temperature of 47K and cannot readily account for the high Curie temperature of the material.Mn-implanted Si was investigated using transmission electron microscopy to gain insight into the structure of the implanted region. Diffraction contrast images, selected area diffraction patterns, and high resolution images of the samples were acquired before and after postimplant annealing at 800°C. The images of the annealed samples revealed the formation of nanometer size precipitates distributed throughout the implanted region. Analysis of the selected area diffraction pattern determined that the most prominent lattice spacing of the crystallites is 2.15A. This spacing indicates that the most probable phase of the crystallites is MnSi1.7 and this is consistent with the Mn:Si binary phase diagram. This phase is paramagnetic at room temperature with a Curie temperature of 47K and cannot readily account for the high Curie temperature of the material.

In situ monitoring of formation of InAs quantum dots and overgrowth by GaAs or AlAs

With a goal of development of a high performance medium for optoelectronic devices, formation of InAs self-assembled quantum dots (QDs) in GaAs∕AlAs short period superlattice was studied using primarily in situ molecular beam epitaxy techniques including reflection high-energy electron diffraction and Auger electron spectroscopy. The processes of formation, decay of QDs and overgrowth of wetting InAs layers were analyzed with the emphasis on the role of AlAs or GaAs capping layers. InAs critical coverage for QD formation on GaAs and AlAs surfaces was measured as a function of temperature, and diffusion coefficients of In adatoms were estimated. QD decay was studied, activation energy for the process was found to be 0.8 and 2.3 eV for GaAs and AlAs surfaces, respectively, indicating that QD decay process is likely driven by In intermixing with the substrate for GaAs surface and by evaporation of In from the AlAs surface. Intermixing of InAs with the capping layers was studied at growth temperatures. Typica...

Room-temperature defect tolerance of band-engineered InAs quantum dot heterostructures

Using photoluminescence (PL) at 77–420K and high-energy proton implantation (1.5MeV, dose up to 3×1014cm−2) we have studied the thermal quenching of PL and defect tolerance of self-assembled shape-engineered InAs quantum dots (QDs) embedded into GaAs quantum wells (QWs). At room temperature, QDs appeared to withstand two orders of magnitude higher proton doses than QWs without PL degradation. A simple dynamic model was used to account for both dose and temperature dependence of PL efficiency. At low temperatures, the defect-related quenching is mainly controlled by a reduction in the density of defect-free QDs. At and above room temperature, both thermal and defect-related quenching of PL are due to the escape of carriers from dots to wells that act as barriers with low damage constants. A relatively large barrier for escape (450meV) as well as low nonradiative recombination rate in QDs is shown to account for unsurpassed room-temperature defect tolerance and high PL efficiency at room and elevated temper...

Nano-Engineering Approaches to Self-Assembled InAs Quantum Dot Laser Medium

A number of nano-engineering methods are proposed and tested to improve optical properties of a laser gain medium using the self-assembled InAs quantum dot (QD) ensemble. The laser characteristics of concern include higher gain, larger modulation bandwidth, higher efficiency at elevated temperatures, higher thermal stability, and enhanced reliability. The focus of this paper is on the management of QD properties through design and molecular beam epitaxial growth and modification of QD heterostructures. This includes digital alloys as high-quality wide-bandgap barrier; under- and overlayers with various compositions to control the dynamics of QD formation and evolution on the surface; shape engineering of QDs to improve electron-hole overlap and reduce inhomogeneous broadening; band engineering of QD heterostructures to enhance the carrier localization by reduction of thermal escape from dots; as well as tunnel injection from quantum wells (QWs) to accelerate carrier transfer to the lasing state. Beneficial properties of the developed QD media are demonstrated at room temperature in laser diodes with unsurpassed thermal stability with a characteristic temperature of 380 K, high waveguide modal gain >50 cm−1, unsurpassed defect tolerance over two orders of magnitude higher than that of QWs typically used in lasers, and efficient emission from a two-dimensional (2-D) photonic crystal nanocavity.

Tunnel quantum dot/well InAs/InGaAs Structures as active medium for laser diodes

Structures of tunnel pairs consisting of InGaAs quantum well (QW) and self-assembled InAs quantum dots (QDs) were employed to improve gain medium in laser diodes. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the influence of the relative position of ground states (GS) energies of QW and QDs as well as structure design on the properties of tunnel structures and the characteristics of multi-layer lasers. QDs on QW structures with different GS relative separation were grown by variation of In concentration in QWs with fixed growth process of QDs. An 1160 nm edge-emitting lasers with 4 pairs of QDs-on-QW as active medium showed higher (than in a similar multilayer QD lasers) maximum saturated gain, 26 cm-1, with low minimum threshold current density Jth = 95 A/cm2. First attempt of a triple-pair tunnel QW-on-QDs laser emitting at 1125 nm exhibited a saturated modal gain more than 50 cm-1. These lasers demonstrated broad multi-peak emission spectra with minimum threshold current density Jth = 255 A/cm2 and with lasing from intermediate states between QDs and QW GS transitions. RF small signal modulation characteristics of the 3x(QW-on-QDs) lasers were measured. From the damping factor and resonance frequency dependence, maximum possible 3dB cut-off frequency for this QW-on-QD media can be estimated as 13 GHz.

Structural and Optical Effects of Capping Layer Material and Growth Rate on the Properties of Self-Assembled InAs Quantum Dot Structures

In order to develop nanoengineering methods to control electronic spectrum of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy, we have utilized atomic force microscopy (AFM), photoluminescence (PL) and TEM methods to investigate the effects of capping layer growth on the physical/chemical properties as well as the optical/electronic performance of QD device structures. Capping layer material choice (or its absence all together) has been found to directly influence QD dimensions (size, height), and subsequently, to affect QD emission wavelength. We report results of QD lateral size and height as well as densities of InAs QDs capped with 2ML (monolayers) of AlAs or GaAs grown at various rates. Our AFM results are complemented by PL measurements, where the optical properties of capped versus non-capped QDs have been explored and direct correspondence between structural differences induced by capping and the electronic/optical properties of QDs is demonstrated. Analysis of the data shows that the results can be explained by two competing surface processes. The first of which is the redistribution of indium between QDs on top of the 2D wetting layer, resulting in the increase of QD size with time. The second effect is the diffusion of indium out of the QDs and onto the top of the capping layer. TEM with multislice image simulation has supported our AFM and PL observations with the demonstration of “indium driven” alloy intermixing in the overlayer as well as significant alloying in the InAs wetting layer.

Nanoengineered InAs quantum dot active medium for laser diodes

Nanoengineering approach was used to develop an efficient active medium based on self-assembled InAs/GaAs quantum dots (QDs) for laser diodes operating at elevated temperatures. Photoluminescence (PL), transmission electron microscopy, and electroluminescence were used to study the influence of an overgrowth procedure on the properties of multiple-layer QDs. Optical properties of QDs were optimized by the adjustment of a GaAs overlayer thickness prior to a heating step, responsible for the truncation of the pyramid-shaped QDs. Triple-layer QD edge-emitting lasers with 1220 nm emitting wavelength exhibited a maximum saturated modal gain of 16 cm-1. To use truncated QD active medium for vertical cavity surface emitting lasers, seven layers of QDs with 20 nm of short period superlattice barriers between layers was developed. A wavelength of 1190 nm edge-emitting lasers with 120 nm total thickness 7xQDs active medium showed almost two times higher maximum saturated gain, 31 cm-1. Unfortunately, these lasers with closer distance between QD layers in active medium demonstrated stronger temperature dependence (with To = 110 K) of threshold current density and lasing wavelength. A record high characteristic temperature for lasing threshold, To = 380 K up to 55 C, was measured for edge-emitting laser diodes, which contained triple-layer truncated QD active medium. We believe that AlAs capping in combination with truncation procedure result in significant suppression of carrier transport between QDs within the layer as well as between QD layers.

Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser

The influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with two-monolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T 0 = 380 K up to 55 °C, as well as a maximum saturated gain of 16 (5.3 per layer) cm −1 , were measured for a 1.22 μm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm −1 , and T 0 = 110 K have been demonstrated in a 1.19 μm lasers with truncated seven layer QD medium.

Room-Temperature Defect Tolerance of Shape Engineered Quantum Dot Structures

A quantum dot (QD) medium is expected to demonstrate superior performance in various devices when compared with quantum wells (QWs). One area of interest has been the improved defect tolerance of QD media, though it was demonstrated at low temperatures so far. In this study, the defect tolerance of shape-engineered QD structures is compared with that of a QW structure at temperatures up to 300 K. To create high defect densities both QD and QW structures were irradiated with high energy (1.5 MeV) protons (with doses up to 3×10 14 cm -2 ). Then, the relative luminescence efficiency was measured by variable temperature photoluminescence. The shape-engineered QD structure withstood two orders of magnitude higher defect density than the QWs at room temperature. This improvement is correlated with the activation energy for thermal evaporation of 390 meV, acquired through a kinetic model.