Hydrogen-Free Amorphous Silicon with No Tunneling States

The ubiquitous low-energy excitations, known as two-level tunneling systems (TLSs), are one of the universal phenomena of amorphous solids. Low temperature elastic measurements show that $e$-beam amorphous silicon ($a\text{−}\mathrm{Si}$) contains a variable density of TLSs which diminishes as the growth temperature reaches $400°\mathrm{C}$. Structural analyses show that these $a\text{−}\mathrm{Si}$ films become denser and more structurally ordered. We conclude that the enhanced surface energetics at a high growth temperature improved the amorphous structural network of $e$-beam $a\text{−}\mathrm{Si}$ and removed TLSs. This work obviates the role hydrogen was previously thought to play in removing TLSs in the hydrogenated form of $a\text{−}\mathrm{Si}$ and suggests it is possible to prepare “perfect” amorphous solids with “crystal-like” properties for applications.

Figure 1

(a) $Q_{\text{osc}}^{-1}$ of DPOs carrying $e$-beam $a\text{−}\mathrm{Si}$. The two films with $T_{\text{sub}}=400°\mathrm{C}$ with their respective backgrounds (solid lines) are shown in a $y$-axis shifted view. The inset shows $G_{\text{film}}$ vs $T_{\text{sub}}$. The dashed line is a guide to the eye. (b) $Q_{\text{film}}^{-1}$ of the $e$-beam $a\text{−}\mathrm{Si}$, and a 107 nm thick dry thermal oxide for comparison. The label “contamination peak” denotes that a large portion of $Q_{\text{film}}^{-1}$ for films with $T_{\text{sub}}=350°\mathrm{C}$ and $400°\mathrm{C}$ includes contamination induced loss in the DPO substrate, not related to the $a\text{−}\mathrm{Si}$ films. The double arrow denotes the “glassy range.” See the text for details.

Figure 2

$\mathrm{\Delta }v/v_0$ of the $e$-beam $a\text{−}\mathrm{Si}$ films, and a 107 nm thick dry thermal oxide film for comparison. The straight lines indicate the linear temperature dependence explained in the text.