The January 2000 issue (Vol. 25) of MRS Bulletin covered in its "Research/Researchers" section the recent papers published by Mattila, Wei, and Zunger in Physical Review Letters (T. Mattila, S.H. Wei and A. Zunger, "Electronic structure of sequence mutations in ordered GaInP2," Physical Review Letters 83, 2010-2013 (1999). and in Rapid Communications (T. Mattila, S.H. Wei and A. Zunger, "Localization and anticrossing of electron levels in GaAsN alloys," Phys. Rev. B. 60, R11245-R11248,1999).

NREL Researchs Explain Anomalous Photoluminescence of Ordered GaInP2 and Pressure-Dependence of Transitions in Alloys of GaAs_1-xN_x

In recent work on their study of electronic theory of metal and semiconductor alloys, researchers at the Solid State Theory Group at the National Renewable Energy Laboratory (NREL) explain the origin of luminescence properties of ordered GaInP₂ --- published in the September 6, 1999 issue of Physical Review Letters --- and the origin of anomalous phenomena exhibited by GaAs_1-xN_x alloys --- published in the Rapid Communications section of the October 15, 1999 issue of Physical Review B.

While exploring the electronic consequences fo genetic sequence errors in superlattices, the research team uncovered electron-hole charge separation in CuPt-ordered GaInP₂. Alex Zunger, Institute Research Fellow at NREL, said that within the sequence mutated In-In layer embeded in the Ga-In-Ga-In...(CuPt) structure, holes localized in the mutated segment while electrons reside in the unmutated segment. More specifically, the sequence mutation creates a spatially indirect, low-energy transition (E_LE) --- from e1 to h1 --- in addition to the spatially direct, excitonic transition (E_X) --- from e1 to h2. The researchers said that these results explain the type-II behavior (of spatially indirect band-to-band transition) exhibited from the below-bandgap luminescence emission for CuPt-ordered GaInP₂ samples. When the researchers reduced the mutated segment from 2D quantum well to 0D quantum disk, the same hole localization persisted.

The researchers said that their calculated energies "provide strong evidence that the experimentally seen peculiar luminescence prperties of ordered GaInP₂ are a consequence of quantum-disk-like microstructures formed due to sequence mutations in [111]-superlattices."

In a study conducted by the same group, T. Mattila, S-H. Wei, and Zunger account for the unusual behavior demonstrated in the two lowest interband transitions of nitrogen-poor GaAs_1-xN_x alloys in which the lowest-energy transition E_- quickly shifts to the red as the nitrogen concentration is increased, and as pressure is applied it slowly shifts to blue. Ordinarily, as pressure is increased in alloys, the energy of E_- tends to saturate. In regards to the higher energy transition E₊, increased nitrogen concentration causes it to shift to the blue, and as pressure is applied, the intensity of the transition increases and its energy shifts to the blue at a rate similar to the lowest-energy transition. Zunger said that the nitrogen impurity perturbs the lattice to such an extent that the band edge states of GaAs (Gamma_1c, L_1c, X_1c) become mixed. The interaction leads to the alloy behaviour as a function of pressure, composition, and so forth.

The researchers calcualated pressure coefficients for the GaAs Gamma_1c, L_1c, X_1c states by using the empirical psuedopotential method (EPM) compared with the experimental values and with the values obtained by first-principles linear augmented plane-wave method within the local density approximation (LDA).