Haojie Ji

Determination of excitonic size with sub-nanometer precision via excitonic Aharonov-Bohm effect in type-II quantum dots

A spectral analysis of the Aharonov-Bohm (AB) oscillation in magneto-photoluminescence intensity was performed for stacked type-II ZnTe/ZnSe quantum dots (QDs). Very narrow AB oscillations (∼0.3 T) allowed for probing of both the lateral size distribution in the stack ensemble of QDs and the size of type-II excitons as determined by the electronic orbit with sub-nanometer precision. Two sets of stacks with excitonic size of 18.2 and 17.5 nm are determined to be present in the sample.

Determination of shape anisotropy in embedded low contrast submonolayer quantum dot structures

We describe a procedure for the morphological characterization of hard-to-image submonolayer quantum dot structures. This procedure employs high resolution x-ray diffraction based reciprocal space mapping, accompanied by rigorous diffraction modeling for precise determination of the morphology of submonolayer quantum dots. Our modelling results and experimental data clearly show that the investigated quantum dots are anisotropically elongated along the [110] orientation. Complementary polarization dependent photoluminescence measurements, combined with our previously reported magneto-photoluminescence data, confirm this conclusion. Our formalism enables direct extraction of structural information of complex embedded three-dimensional structures, which, due to their low electron density contrast with respect to the surrounding host matrix, cannot be readily investigated by traditional electron diffraction techniques.

Determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer quantum dots via spectral analysis of optical signature of the Aharanov-Bohm excitons

For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.

Decoherence in semiconductor nanostructures with type-II band alignment: All-optical measurements using Aharonov-Bohm excitons

I. L. Kuskovsky,1,2 L. G. Mourokh,1,2 B. Roy,1,2 H. Ji,1,2 S. Dhomkar,1,2 J. Ludwig,3,4 D. Smirnov,3,4 and M. C. Tamargo2,5 1Department of Physics, Queens College of The City University of New York (CUNY), Flushing, New York 11367, USA 2The Graduate Center of CUNY, New York, New York 10016, USA 3National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 4Department of Physics, Florida State University, Tallahassee, Florida 32306, USA 5Department of Chemistry, The City College of CUNY, New York, New York 10031, USA (Received 30 May 2015; revised manuscript received 1 January 2017; published 26 April 2017)

Spin dynamics of ZnSe-ZnTe nanostructures grown by migration enhanced molecular beam epitaxy

We study the spin dynamics of ZnSe layers with embedded type-II ZnTe quantum dots using time resolved Kerr rotation (TRKR). Three samples were grown with an increasing amount of Te, which correlates with increased quantum dot (QD) density. Samples with a higher quantum dot density exhibit longer electron spin lifetimes, up to ∼1 ns at low temperatures. Tellurium isoelectronic centers, which form in the ZnSe spacer regions as a result of the growth conditions, were probed via spectrally dependent TRKR. Temperature dependent TRKR results show that samples with high QD density are not affected by an electron-hole exchange dephasing mechanism.

Optical anisotropy in type-II ZnTe/ZnSe submonolayer quantum dots

Linearly polarized photoluminescence is observed for type-II ZnTe/ZnSe submonolayer quantum dots (QDs). The comparison of spectral dependence of the degree of linear polarization (DLP) among four samples indicates that the optical anisotropy is mostly related to the elongation of ZnTe QDs. Numerical calculations based on the occupation probabilities of holes in px and py orbitals are performed to estimate the lateral aspect ratio of the QDs, and it is shown that it varies between 1.1 and 1.4. The value of anisotropic exchange splitting for bright excitonic states is found to be ∼200 μeV from the measurement of the degree of circular polarization as a function of the magnetic field. The results also show that heavy-light hole mixing ratio is about 0.16.