Winston V. Schoenfeld

Cavity-quantum electrodynamics using a single InAs quantum dot in a microdisk structure

We investigate cavity-quantum electrodynamics (QED) effects in an all-semiconductor nanostructure by tuning a single self-assembled InAs quantum dot (QD) into resonance with a high quality factor microdisk whispering gallery mode (WGM). The stronger temperature dependence of the QD single-exciton (1X) resonance allows us to change the relative energy of the WGM and the 1X transitions by varying the sample temperature. The two coupled resonances exhibit crossing behavior due to the weak coupling cavity-QED regime. We demonstrate exciton lifetime reduction by 6 due to the Purcell effect by tuning the QD into resonance with the WGM. Our experiments also show that single-exciton lifetime is independent of temperature up to 50 K.

High-Performance TiO2 -Based Electron-Selective Contacts for Crystalline Silicon Solar Cells.

Thin TiO2 films are demonstrated to be an excellent electron-selective contact for crystalline silicon solar cells. An efficiency of 21.6% is achieved for crystalline silicon solar cells featuring a full-area TiO2 -based electron-selective contact.

Photoluminescence up-conversion in InAs/GaAs self-assembled quantum dots

We report up-converted photoluminescence in a structure with InAs quantum dots embedded in GaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both the dots and the wetting layer. The intensity of the up-converted luminescence is found to increase superlinearly with the excitation density. The results suggest that the observed effect is due to a two-step two-photon absorption process involving quantum dot states.

Carrier concentration dependence of acceptor activation energy in p-type ZnO

The characteristics of an acceptor level in Sb-doped, p-type ZnO were studied using cathodoluminescence (CL) spectroscopy as a function of hole concentration. Variable-temperature CL measurements allowed us to estimate the activation energy of an Sb-related acceptor from temperature-induced decay of CL intensity. The values of activation energy of about 212±28, 175±20, 158±22, and 135±15meV were obtained for samples with carrier concentrations of 1.3×1017, 6.0×1017, 8.2×1017, and 1.3×1018cm−3, respectively. The involvement of acceptor levels is supported by the temperature-dependent hole concentration measurements. The possible origins of the strong temperature dependence are discussed.

Hybrid CdZnO/GaN quantum-well light emitting diodes

We report on the demonstration of light emission from hybrid CdZnO quantum-well light emitting diodes. A one-dimensional drift-diffusion method was used to model the expected band structure and carrier injection in the device, demonstrating the potential for 90% internal quantum efficiency when a CdZnO quantum well is used. Fabricated devices produced visible electroluminescence that was found to redshift from 3.32 to 3.15 eV as the forward current was increased from 20 to 40 mA. A further increase in the forward current to 50 mA resulted in a saturation of the redshift.

Bandgap engineering of sol-gel synthesized amorphous Zn1−xMgxO films

Amorphous Zn1−xMgxO (α-Zn1−xMgxO) ternary alloy thin films across the full compositional range were synthesized by a low-cost sol-gel method on quartz substrates. The amorphous property of the α-Zn1−xMgxO films was verified by x-ray diffraction, and atomic force microscopy revealed a smooth surface with sub-nanometer root-mean square roughness. The current phase segregation issue limiting application of crystalline Zn1−xMgxO with 38% < x < 75% was completely eliminated by growing amorphous films. Optical transmission measurements showed high transmissivity of more than 90% in the visible and near infrared regions, with optical bandgap tunability from 3.3 eV to more than 6.5 eV by varying the Mg content.

Photoluminescence of a single InAs quantum dot molecule under applied electric field

We study the electronic coupling between two vertically stacked InAs quantum dots, which are embedded in the center of a n-i-n structure. We use a microphotoluminescence setup to optically isolate a single quantum dot pair and measure the time-averaged photoluminescence under an applied vertical electric field. We find that field tunable coupling between excited states of the two quantum dots leads to charge transfer from one dot to the other. We model the spectra including simultaneously the field-dependent charge transfer and exciton capture rates, and the many-body spectra of the quantum dot molecule for different carrier configurations.

Luminescence quenching in InAs quantum dots

We report how photoluminescence from self-assembled InAs quantum dots depend on pumping power and vertical electric field. The InAs dots, which are embedded in a capacitor-like structure, act as efficient trapping centers for excitons. At a high enough electric field, however, the photoexcited electrons tunnel out of the dots fast enough to quench the emission. For samples with two adjacent layers of vertically aligned dots, we find that the threshold voltage for quenching depends very strongly on the optical pumping power. In total contrast to this, we find no comparable effect for samples grown with a single layer of dots. We explain this in terms of efficient storage of electrons and holes in the double-layer samples.

Argon ion damage in self-assembled quantum dots structures

The effects of radiation damage exposure on InGaAs quantum wells and InAs quantum dots are compared using luminescence spectroscopy techniques. A large increase in the radiation resistance of the InAs quantum dots is observed and attributed to exciton localization in the quantum dots and a point defect strain gettering effect.

Magneto-optical properties of ring-shaped self-assembled InGaAs quantum dots

Abstract We report on measurements of the magneto-optical properties of excitons confined in ring-shaped self-assembled semiconductor quantum dots. The rings are embedded in a field-effect structure that allows the number of confined electrons to be set electrostatically. In addition, electron-hole pairs are generated optically. The resulting photoluminescence spectra of neutral, singly and doubly charged excitons were measured at 4.2 K as a function of the applied magnetic field (0– 9 T ). The emission energy shows a diamagnetic shift as well as a Zeeman splitting. We measured the emission of different charge states of the exciton in many individual dots. In a few of the measured rings, a new behavior was observed, namely a clear departure from the low field diamagnetic dispersion for fields larger than 6 T .