Michael Defensor

Strong enhancement of terahertz emission from GaAs in InAs/GaAs quantum dot structures

We report on the intense terahertz emission from InAs/GaAs quantum dot (QD) structures grown by molecular beam epitaxy. Results reveal that the QD sample emission was as high as 70% of that of a p-type InAs wafer, the most intense semiconductor emitter to date. Excitation wavelength studies showed that the emission was due to absorption in strained undoped GaAs, and corresponds to a two order-of-magnitude enhancement. Moreover, it was found that multilayer QDs emit more strongly compared with a single layer QD sample. At present, we ascribe the intense radiation to huge strain fields at the InAs/GaAs interface.

Influence of As4 flux on the growth kinetics, structure, and optical properties of InAs∕GaAs quantum dots

We report the effects of variations in As4 growth flux on the evolution of molecular beam epitaxy grown InAs quantum dots (QDs) and their structures and optical properties. For InAs QDs grown under As-stable conditions, evaluated through photoluminescence and atomic force microscopy (AFM) measurements, it is evident that QD size increases with As4 pressure along with improvement in size uniformity. Furthermore, transmission electron microscopy measurements for InAs layers of critical thicknesses (∼1.7 ML) showed decreasing QD density with increasing As4 pressure accompanied by a strong reduction in photoluminescence (PL) integral intensity. These show that high As4 fluxes suppress InAs QD formation while the decreasing PL intensity seems to indicate cluster formation that features nonradiative recombination. AFM measurements show larger and denser QDs for samples grown at higher As4 pressures. These are explained on the basis of adatom condensation during surface cooling and the influence of As4 pressure ...

Interband dot-to-well transitions in InAs∕InGaAs dots in a well probed via photocurrent and electroluminescence spectroscopy

Hole states of InAs∕InGaAs dots in a well are optically probed by observing interband–quantum well (QW)–quantum dot (QD) transitions through photocurrent and electroluminescence (EL) spectroscopy. We find multiple sharp peaks with spacing in the range of 10–20meV in between the expected QD and the QW signals. The spacing and the observed temperature dependent EL suggest that the features are brought about by transitions between the QW electron ground state and the QD hole states. The data extracted from these transitions provide experimental values for the QD hole confinement energies, which can potentially aid in the verification of theoretical models in QD structures.

Transitions of epitaxially lifted-off bulk GaAs and GaAs/AlGaAs quantum well under thermal-induced compressive and tensile strain

Low temperature photoluminescence and reflectance measurements on epitaxially lifted-off (ELO) bulk GaAs and GaAs/AlGaAs multiple quantum wells (MQWs) bonded to Si and MgO substrates are reported. Photoluminescence measurements indicate no strain at room temperature for the ELO bulk GaAs film but show biaxial strain at 10 K. Si-bonded films undergo tensile strain, while films with MgO host substrates experience compressive strain. Reflectance measurements at 10 K show that light hole band is closer to the conduction band for the tensile strained film. In GaAs MQW ELO films, the separation of the heavy hole and light hole band is reduced in tensile strained films by 4.7 meV, corresponding to a strain e=−0.7±0.05×10−3 and stress X=0.9±0.05 kbar (90±5 MPa). For compressively strained films, this separation is enhanced by 3.9 meV, equivalent to a strain e=0.6±0.05×10−3 and X=0.8±0.05×10−3 kbar (80±5 MPa). The findings demonstrate that ELO is an effective technique to introduce tensile and compressive strain i...

Enhanced terahertz emission from GaAs in MBE-grown InAs/GaAs quantum dot structures

We report on intense terahertz emission from InAs/GaAs quantum dot layers grown by molecular beam epitaxy. The emission is attributed to absorption in the GaAs layer and a strong enhancement was observed from the quantum dot samples.

Surface Structure Dependent Growth of InAs/InAlAs Quantum Wires on InP(100).

Formation of quantum wires (QWRs) or quantum dots (QDs) can be controlled by in situ surface preparation. InAs QDs and QWRs were grown on InAlAs using Riber 32P molecular beam epitaxy system. By varying the growth temperature, the surface morphology of InAlAs can be tailored. Xray diffraction (XRD) and scanning electron microscopy (SEM) measurements were done to determine the mole fraction and surface morphology of the InAlAs layers. Atomic force microscopy (AFM) results verified the formation of quantum wires on a rough surface and quantum dots on a smooth surface. Low-temperature photoluminescence (PL) spectra revealed that the spectral positions of the wires are within of 1.0 – 1.3 μm and 1.189 μm for the quantum dots.