Study of the fundamental linewidths of 1S→nPdonor transitions in ultrapure germanium

The fundamental linewidths of dipole transitions of donor-bound electrons in ultrapure germanium were investigated. The experiments were performed on the stress-insensitive D(H,O) donor complex which is present in concentrations of ${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}3}$. Various transitions (n=2,3,4,5,6) were tuned with a variable magnetic field to the frequencies of several far-infrared laser lines and the photothermal response was recorded. The transitions 1S→2$P_{\mathrm{-}}$ and 1S→3$P_{\mathrm{-}}$ lead to Lorentzian lines with a full width at half maximum (FWHM) of 8.2±0.4 μeV. Higher excited states produce lines with a FWHM of around 6.4 μeV, the narrowest lines for electronic transition ever recorded. The experiments show that the observed linewidths do not depend on the sample temperature below 9 K, the applied electric field below 1.4 eV ${\mathrm{cm}}^{\mathrm{-}1}$, or band-edge light. The last process creates free electrons and holes which in turn are captured by ionized donors and acceptors. Stark broadening by random Coulomb fields from ionized impurities is thereby greatly reduced. The experimentally determined linewidths are dominated by the excited-state lifetime which results from the electron–acoustic-phonon interaction. This interaction couples the bound excited state to all the other nearby states. The excited-state lifetimes derived from the linewidths are in agreement with the values predicted by the theory of Barrie and Nishikawa.