Single-frequency laser spectroscopy of the boron bound exciton in ${}^{28}\text{S}\text{i}$

While the first comparison of shallow bound exciton photoluminescence between natural Si and highly enriched ${}^{28}\text{S}\text{i}$ dramatically demonstrated the importance of inhomogeneous isotope broadening, the transitions in ${}^{28}\text{S}\text{i}$ were in fact too narrow to be resolved with the then available instrumental resolution of $0.014{\text{cm}}^{-1}$. We report results for the boron bound exciton transition in highly enriched ${}^{28}\text{S}\text{i}$ using a novel apparatus for photoluminescence excitation spectroscopy based on a tuneable single-frequency laser source with sub-MHz resolution. Twenty well-resolved doublets, exhibiting a ${}^{10}\text{B}–{}^{11}\text{B}$ isotope splitting, are observed in the new spectra for ${}^{28}\text{S}\text{i}$ with isotopic enrichment $>99.99\%$. Linewidths as narrow as $0.0012{\text{cm}}^{-1}$ (150 neV) full width at half maximum are observed for the most highly enriched sample.

Figure 1

Photoluminescence excitation spectrum of the no-phonon boron bound exciton transitions for a sample of ${}^{28}\text{S}\text{i}$ with isotopic enrichment of 99.995%. Twenty transitions are observed in this spectrum with each transition consisting of a doublet with an intensity ratio reflecting the 80/20 natural abundance ratio of ${}^{11}\text{B}/{}^{10}\text{B}$. The stronger ${}^{11}\text{B}$ components are labeled 1–20 in order of increasing energy.

Figure 2

Photoluminescence excitation spectrum of Lines 14 and 15, revealing a difference in the boron-isotope splitting for two nearby transitions. Line 14 has a boron isotope splitting of $0.0100\left(2\right){\text{cm}}^{-1}$ while Line 15 has a larger splitting of $0.0119\left(3\right){\text{cm}}^{-1}$.

Figure 3

Photoluminescence excitation spectrum of Lines 6 and 13, for the four ${}^{28}\text{S}\text{i}$ samples. The remaining isotopic inhomogeneity, which increases from top to bottom, is seen to produce only a broadening for Line 13, but both a broadening and a splitting for Line 6.

Figure 4

(Color online) Plot of the FWHM of Line 7 versus He bath temperature for the ${}^{28}\text{S}\text{i}$ a sample. A curve fit to the data (shown by the solid curve) using the expression: $\text{FWHM}=\sqrt{A^2+{\left(\frac{B}{e^{C/k_{B}T}-1}\right)}^2}$, yields a value for $C$ of $2.1{\text{cm}}^{-1}$.

Figure 5

The bottom spectrum shows the photoluminescence excitation signal of the boron bound exciton ground state to acceptor ground-state transitions for the ${}^{28}\text{S}\text{i}$ a sample under an applied magnetic field of 490 Gauss parallel to the [001] axis with at least 76 transitions being resolved. The region enclosed by the bracket is expanded and displayed in the upper spectrum to show the remarkable density of the transitions.

Figure 6

Photoconductivity spectrum of the no-phonon boron bound exciton transitions for the ${}^{28}\text{S}\text{i}$ a sample. This spectrum exhibits the same fine structure that was previously seen in the PLE spectrum in Fig. 1.