Alex Zunger, National Renewable Energy Laboratory

Localized-to-delocalized transition in GaAsN alloys: Calculated conduction band wavefunction isosurfaces of the GaAsN alloy from 14,000 atom supercell plane-wave calculations. At low nitrogen concentrations (a), the lowest energy wavefunctions are highly nitrogen localized (shown in red). With increased nitrogen concentration (b), the lowest energy conduction levels consist of both localized (red) and delocalized levels (green). For higher nitrogen concentrations (c), the conduction levels become increasingly delocalized (green).

Research Objectives
Our calculations allow us to predict in detail the effect of nano-scale 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 properties and many-body (configuration interaction) treatments.

Computational Approach
We use a combination of methods to bridge the length and computational cost scales from the 100–1,000 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 atom nanostructure regime, where the optoelectronic properties are determined by the near gap conduction and valence states.

We use local density approximation based methods for small systems, and empirical pseudopotential based methods, such as the folded spectrum and linear combination of bulk bands methods, for large-scale nanostructures. Our optimized pseudopotential methods allow us to study million-atom systems with quantum mechanical accuracy. Using the single particle wave functions, we are able to treat many-body effects, important for optical properties and effects such as Coulomb blockade, by means of a configuration interaction based approach.

Accomplishments
In FY2000 we successfully studied several classes of nanostructure systems:

1. Alloy dots, arrays: We predicted the excitonic exchange splitting of Si dots, predicted the electron-addition spectra of InP and CdSe dots, and developed a theory of lens-shaped self-assembled InAs/GaAs dots. Using our many-body configuration-interaction approach, we predicted failures of both Hund’s rule and the Aufbau principle in quantum dots.

2. Nitrides: We developed the first theory of localization in InGaN alloys and successfully explained anomalous pressure effects in GaAsN.

3. Short-range order: We developed the first quantitative theory of alloy precipitate shape in metal alloys, successfully predicted the size and shape of precipitates versus temperature of Al-Zn, and demonstrated the first quantitative theory of phase-stability of brass (Cu-Zn).

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 their potential application to lasers, sensors, photovoltaics and novel quantum devices. These structural features occur on distance scales of ~100–500 Å, thus encompasing 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
A. Franceschetti and A. Zunger, “Inverse band-structure problem of finding an atomic configuration with given electronic properties,” Nature 402, 60 (1999).

S. B. Zhang, S. H. Wei, and A. Zunger, “Microscopic origin of the phenomenological equilibrium ‘doping limit rule’ in n-Type III-V semiconductors,” Phys. Rev. Lett. 84, 1232 (2000).

A. Franceschetti and A. Zunger, “Hund’s rule, spin blockade, and the Aufbau principle in strongly confined semiconductor quantum dots,” Europhysics Letters 50, 243 (2000).

http://www.sst.nrel.gov