Semiconductor Alloys, Amorphous and Impurities

Semiconductor alloys and impurities are ways to change their electronic properties. Without defects and impurities, semiconductors cannot be used in practical applications. Alloying with different cation and anion is another way to tune the properties (e.g., the band gap) of a semiconductor material. Sometime due to large lattice mismatch, when two semiconductors mix heavily (with large composition percentages), the system become amorphous (lose its long range ordering). One example is GaAs_xN_(1-x).

Using large scale ab initio calculations (e.g., one thousand atoms), we have studied alloy atomic structures, band gap bowing coefficients, phase transitions in compounds. We have also studied defect formation energies, defect levels (including both deep and shallow impurity levels). In particular, we are interested in using alloys and impurity for photovoltaic and photo induced water splitting.

A few examples are given here for the systems studied in this group.

The O impurity induced state (left) and conduction band state (right) of ZnTe:O with 6% O. This alloy system (with O replacing Te) is proposed to be used for intermediate state solar cell.

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GaN:ZnO atomic structure at different temperature. Only the Zn(gray) O(red) atoms are shown. At the 1100K synthesis temperature, there are significant short range ordering. This GaN:ZnO alloy is the best single material visible light water splitting material.

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The following figure shows the conduction band of the GaAsAs alloy is very similar to the Gamma point state of a pure crystal (virtual crystal approximation). We call this: majority representation.

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This figure shows the bandwidth of an alloy InGaAs. While it still has the band structure, the width for each band line is finite, representing its finite life time.

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This figure represents a In impurity in bulk Si (one In replacing one Si). The eigen energy of such shallow donor is only converged after a 64,000 atom supercell is used.

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The followings are the typical defect formation energies in different synthesis condition (this is for Cu in ZnS, and the related nature defects).