Large-Scale Simulations of $a$-Si:H: The Origin of Midgap States Revisited

Large-scale classical and quantum simulations are used to generate $a$-Si:H structures. The bond-resolved density of the occupied electron states discloses the nature of microscopic defects responsible for levels in the gap. Highly strained bonds give rise to band tails and midgap states. The latter originate mainly from stretched bonds, in addition to dangling bonds, and can act as hole traps. This study provides strong evidence for photoinduced degradation (Staebler-Wronski effect) driven by strain, thus supporting recent work on $a$-Si, and sheds light on the role of hydrogen.

Figure 1

$a$-Si:H with 5% H: Density of states projected on the Wannier orbitals (Wannier centers) related to all defects (black), and analyzed in SBs [long (solid brown); short (dashed brown)] and DBs (blue). The vertical dashed line corresponds to the estimated position (see text) of the mobility edge $E_T$. The zero of the energy scale corresponds to the highest occupied state $E_F$.

Figure 2

$a$-Si:H with 5%.H: $J_W$ (see text) relative to the DB, vs Si-Si bond length d (Å). The vertical dashed line corresponds to the average bond length $d_t$ of the “normal” tetrahedral Si-Si bonds. The $J_W$ integral is taken either over the whole interval (from $E_T$ to $E_F$: solid line) or the lowest half (I) (dash-dotted line: $E_T->E_F-0.3\mathrm{eV}$) or the highest half (II) (dashed line: $E_T+0.3\mathrm{eV}->E_F$). Symbols as in Fig. 1.

Figure 3

Spin density (shaded areas) for the $a$-Si:H model structure (5% H), both neutral (purple) and with a single hole (green). The network of strained bonds is shown together with the corresponding Wannier centers (filled circles). Red (green) denotes stronger (weaker) weight in the $E_T-E_F$ interval ($J_W/J_{\mathrm{DB}}$ larger (smaller) than 0.25).