Bulk Crystalline $4H$-Silicon through a Metastable Allotropic Transition

We report the synthesis of bulk, highly oriented, crystalline $4H$ hexagonal silicon ($4H\text{−}\mathrm{Si}$), through a metastable phase transformation upon heating the single-crystalline ${\mathrm{Si}}_{24}$ allotrope. Remarkably, the resulting $4H\text{−}\mathrm{Si}$ crystallites exhibit an orientation relationship with the ${\mathrm{Si}}_{24}$ crystals, indicating a structural relationship between the two phases. Optical absorption measurements reveal that $4H\text{−}\mathrm{Si}$ exhibits an indirect band gap near 1.2 eV, in agreement with first principles calculations. The metastable crystalline transition pathway provides a novel route to access bulk crystalline $4H\text{−}\mathrm{Si}$ in contrast to previous transformation paths that yield only nanocrystalline-disordered materials.

Figure 1

(a) 2D XRD pattern of a multicrystalline $4H\text{−}\mathrm{Si}$ sample produced after heating a ${\mathrm{Si}}_{24}$ single crystal at $300°\mathrm{C}$ for four days. The sixfold hexagonal symmetry is emphasized by green dashed lines which are at 60° intervals. Reflections in addition to those along [001] are observed as the samples were rotated with respect to the x-ray beam. The upper inset shows the single-crystalline nature of ${\mathrm{Si}}_{24}$ precursor oriented along [001], and the lower inset shows a model of the $4H\text{−}\mathrm{Si}$ structure with ABCB stacking generated using vesta-v3 software. [46] (b) Powder diffraction pattern produced by averaging multiple sample orientations (black-dots) with Le Bail fit using GSAS-II (purple line). Allowed Bragg peaks for the $4H\text{−}\mathrm{Si}$ structure are indicated by the red tick marks below the pattern. A $4H\text{−}\mathrm{Si}$ powder pattern simulated using atomic positions from reference [35] is shown above the experimental data for comparison of powder averaging. A magnified high-angle region of the experimental diffraction pattern with the calculated pattern overlayed is shown in the inset.

Figure 2

Raman spectra of a ${\mathrm{Si}}_{24}$ crystal and $4H\text{−}\mathrm{Si}$ powder compared to a calculated $4H\text{−}\mathrm{Si}$ spectrum. The calculated spectrum is shown with Gaussian peaks of an arbitrary width. (inset) Regions of the Raman spectra showing that the weaker ($E_{2g}$, $E_{1g}$, and $A_{1g}$) peaks from the $4H\text{−}\mathrm{Si}$ calculation match the experimental observations.

Figure 3

(a) Bright-field TEM image of a lamella from annealed ${\mathrm{Si}}_{24}$ crystal. (b) Dark-field TEM image taken from the (002) reflection (indicated by a green circle) of the SAED pattern (c) which has been indexed to the $⟨100⟩$ zone of $4H\text{−}\mathrm{Si}$. (d) High-resolution TEM image of a $4H\text{−}\mathrm{Si}$ crystal. (e) Fourier-filtered image of a select region of (d). The inset is an intensity profile from the region inside the red rectangle which shows the 12.59(7) Å periodic spacing between layers in the $4H\text{−}\mathrm{Si}$ crystal. Note that all images and diffraction patterns are viewed along the $⟨100⟩$ axis.

Figure 4

(a) NIR–VIS transmission measurement of a piece of $4H\text{−}\mathrm{Si}$ with a change in slope between 1.15–1.2 eV highlighting an absorption edge (red arrow). (b) Calculated band structure for $4H\text{−}\mathrm{Si}$ displaying an indirect band gap ($\mathrm{\Gamma }-M$) of 1.2 eV. (c) A ball-and-stick schematic showing the lowest-energy transition pathway from ${\mathrm{Si}}_{24}$ (blue) to $4H\text{−}\mathrm{Si}$ (green) generated using vesta-v3 [46]. Translucent blue spheres indicate atomic displacements during the transition.