Amorphous defect clusters of pure Si and type inversion in Si detectors

Si detectors subjected to energetic particle bombardment are known to undergo a deleterious type inversion from $n$ type to $p$ type. The effect is due to defects that trap electrons but the identity of the main responsible traps remains unknown. Using a combination of classical molecular-dynamics simulations and large-scale density-functional theory calculations, we show that amorphous defect clusters formed under particle bombardment are strong acceptors of electrons and may as such well explain the phenomenon.

Figure 1

(Color online) Example of the structure of a system in each of the six studied categories.

Figure 2

(Color online) Distribution of the first donor and acceptor levels within the experimental band gap of Si as calculated from total-energy differences. Here both the annealed and nonannealed systems were considered as a single category for each composition, resulting in 40 systems for each of the three categories shown. The midgap position is marked by the vertical line. $E_v$ and $E_c$ signify the top of the valence band and the bottom of the conduction band, respectively.

Figure 3

(Color online) Visualizations (Ref. 48) of the isosurface of the electron density of the first acceptor state of three sample systems at 50% (upper row) and 10% (lower row) of the maximum electron density of the orbital. The first column represents an $\text{a-Si}+\text{c-Si}$ system, the second column an annealed $\text{a-Si}+\text{c-Si}+V$ system, and the third column shows an $\text{a-Si}+\text{c-Si}+V_2$ system.