Hao Lee

Determination of the shape of self-organized InAs/GaAs quantum dots by reflection high energy electron diffraction

We report a reflection high energy electron diffraction study of InAs self-organized quantum dots grown on GaAs (001). We observe facet reflections along the [310] and [130] azimuths, which indicate that the quantum dot shape is pyramidal with bounding facets corresponding to a family of four {136} planes. The determined structure, which possesses C2v symmetry, is quite different both from square-base pyramidal geometries which have been assumed in recent electronic structure calculations, and from previously proposed structures which have been based upon incomplete reflection high energy electron diffraction data.

FORMATION OF INAS/GAAS QUANTUM DOTS BY MOLECULAR BEAM EPITAXY : REVERSIBILITY OF THE ISLANDING TRANSITION

We report a study of the dynamics of coherent island formation in InAs/GaAs films grown by molecular beam epitaxy. A comparison of the temperature dependence of the critical layer thickness for islanding between the migration-enhanced and the continuous growth modes confirms that surface adatom diffusion and indium desorption are the controlling processes which determine the variation of the measured critical layer thickness with temperature. We find that under conditions in which indium desorption is significant, the islanding transition is reversible, which provides a new way to study the dynamics of the islanding transition. Applying this technique, we find that the size distribution of the three-dimensional islands evolves into a bimodal distribution during the reverse process.

Photoluminescence study of in situ annealed InAs quantum dots: Double-peak emission associated with bimodal size distribution

We report a photoluminescence study of self-assembled InAs islands subjected to in situ annealing prior to the growth of a capping layer. A distinctive double-peak feature is observed in the photoluminescence spectra of annealed samples. The power dependence of the photoluminescence spectra reveals that the double-peak emission is associated with the ground-state transition of islands in two different size branches. This observation agrees with a previous study, which demonstrated that the InAs island size distribution bifurcates during post-growth annealing. The temperature dependence of the photoluminescence intensities from samples with bimodal island size distributions illustrates that different thermal activation energies for carrier emission are associated with islands in different size branches.

Correlation between photoluminescence and infrared absorption spectra of oxidized nanoscale silicon clusters

We report in situ photoluminescence and ex situ Fourier transform infrared spectra of nanoscale silicon clusters exposed to atomic hydrogen, molecular oxygen, and humidified argon. Comparisons between infrared absorption spectra of fresh and aged samples indicate that photoluminescence efficiency is correlated with a stoichiometric oxide shell and the presence of Si dangling bond passivants at the core/oxide interface. Photoluminescence quenching is demonstrated in efficiently luminescing samples upon exposure to atomic hydrogen with recovery of photoluminescence occurring upon subsequent exposure to air. The photoluminescence quenching and recovery is correlated with a partial quenching and recovery of absorption due to interfacial silane groups. The correlations between photoluminescence and infrared absorption spectra, together with the hydrogen quenching results, provide evidence that radiative recombination in these samples is associated with interfacial oxide-related defects.

Optical spectra of crystalline silicon particles embedded in an amorphous silicon matrix

Abstract Deuterated amorphous silicon films deposited by DC reactive magnetron sputtering were measured by Raman spectroscopy and TEM imaging. The films were found to consist of silicon crystallites embedded in an amorphous silicon matrix. The sub-band-gap optical spectra of these films were recorded using photocapacitance and transient photocurrent spectroscopy. These spectra reveal an amorphous silicon sub-band-gap spectrum together with a unique optical transition in the embedded c-Si particles. This transition corresponds to valence band electrons being optically inserted into empty levels lying within the amorphous silicon mobility gap. We believe these empty levels are associated either with the conduction band of the c-Si particles or with defect states at the crystalline–amorphous boundary.

Visible electroluminescence from porous silicon/hydrogenated amorphous silicon pn‐heterojunction devices

We report the fabrication and characterization of a p‐type porous silicon/ n‐type hydrogenated amorphous silicon (a‐Si:H) pn‐heterojunction electroluminescent device structure. The devices exhibit electroluminescence in forward bias, demonstrating minority carrier injection from n‐type a‐Si:H into p‐type porous silicon.

Electronic structure and optical polarization anisotropy of self-organized InAs/GaAs quantum dots

The lack of precise information regarding the shape of self-organized InAs/GaAs QDs has been a key obstacle in the development of optoelectronics technology based upon these structures. This knowledge is necessary to quantitatively model the electronic structure, optical spectra, and relaxation dynamics -- all key parameters in the design of QD-based optoelectronic devices. We have recently determined the shape of self-organized InAs/GaAs quantum dots (QDs), based upon an intensive study utilizing RHEED, TEM and atomic force microscopy. Our results indicate that the bounding facets of InAs/GaAs quantum dots are of the {136} family. The resulting pyramidal structure possesses a parallelogram base that is substantially elongated along [1-10] and thus is characterized by C2v symmetry, quite different from square-base pyramidal or lens-shaped geometries which have been previously assumed. In this paper the relationship between the shape and the optical properties of self-organized InAs quantum dots is discussed. PL spectra exhibit multiple excited state transitions, each of which are linearly polarized, an experimental fact which is inconsistent with previous shape models of InAs/GaAs QDs. A coupled-band electronic structure calculation of {136}-bounded QDs is presented to show that the optical polarization anisotropy can be understood quantitatively by consideration of the true shape of the QD structures. Our main conclusion is that the spectral polarization anisotropy provides a signature which is uniquely sensitive to shape; polarization measurements provide a more stringent test of electronic structure models than simple comparison to PL peak positions.