The second electron state of an InGaAs quantum dot (blue), calculated using the empirical pseudopotential method (EPM). Shown in purple is the wavefunction obtained by more approximate methods, while a cross section of the EPM wavefunction is shown in red, revealing the rich and phenomenologically important atomistic detail.

Alex Zunger, Paul Kent, Gabriel Bester, Marco Califano, Priya Mahadevan, and Cetin Kilic, National Renewable Energy Laboratory

Research Objectives
Our calculations allow us to predict in detail the effect of nanoscale atomic structure on the electronic and optical properties of semiconductor systems. Using atomistic models in conjunction with quantum mechanical methods, we are able to interpret excitonic spectra, provide feedback to experiment, and predict new properties to be engineered and investigated. Our studies focus on both one-body electronic structure and many-body (configuration interaction) treatments.

Computational Approach
Our computational approach is based on accurate quantum mechanical calculations for the electronic, optoelectronic, and thermodynamic properties of semiconductor nanostructures using detailed fully relaxed atomistic models of semiconductor nanostructures. We use a combination of methods to bridge the length and computational cost scales from the 100-1000 atom microstructural scale, where we obtain thermodynamic information, and compute fully relaxed geometries of complex structures such as impurity complexes and surfaces, to the 100,000-1,000,000 nanostructure regime, where the optoelectronic properties are determined by the near gap conduction and valence states.

We use local density approximation (LDA) density-functional methods for small systems; for large-scale nanostructures, we use empirical pseudopotential methods, such as the folded spectrum method (FSM), as well as the linear combination of bulk bands method. Our pseudopotential methods are optimized for application to nanostructures, and allow us to study million-atom systems with quantum mechanical accuracy, without the approximations and pathologies inherent in conventional nanostructure calculation approaches such as k.p.

Accomplishments
In FY2001 we successfully studied several classes of nanostructure systems:

Quantum dots: We extended our theory of lens and pyramidal-shaped embedded InGaAs quantum dots to include composition variation, and utilized the spectroscopic signature of current grown dots to accurately determine their size, shape, and composition profile.

Nitride alloys and alloy microstructure: In dilute nitride alloys, we demonstrated how small nitrogen clusters result in below band-gap cluster states, drastically altering these materials' optical properties. In the related InGaAsN system, we established a quantitative theory of spatial correlation, where ordering leads to significant changes in the optical properties and electronic localization.

Metal alloys: We developed our first-principles theory of brass (Cu-Zn), predicting the low-temperature ground states, finite-temperature phase diagram, and short range order.

Significance
The electronic, optical, transport, and structural properties of semiconductor nanostructures (films, quantum dots, and quantum wires) and microstructures in alloys are important because of the novel physical properties exhibited by these systems (state localization, Coulomb blockade, quantum confinement, exchange enhancement, and shape-dependent spectroscopy) and because of their application to lasers, sensors, photovoltaics, and new novel quantum devices. These structural features occur on distance scales of ~100-500 Å, thus encompassing 10⁴-10⁵ atoms. Ours is the only available pseudopotential-based theory which can address this size scale. Understanding the underlying physical phenomena in these systems is essential to designing nanoscale devices with custom-made electronic and optical properties.

Publications
P. R. C. Kent and A. Zunger, "Evolution of III-V nitride alloy electronic structure: The localized to delocalized transition," Physical Review Letters 86, 2609 (2001).

K. Kim and A. Zunger, "Spatial correlations in GaInAsN alloys and their effects on band gap enhancement and electron localization," Physical Review Letters 86, 2605 (2001).

F. A. Reboredo and A. Zunger, "L-to-X crossover in the conduction band minimum of Ge quantum dots," Physical Review B 62, R2275 (2000).

http://www.sst.nrel.gov