Shallow-donor impurities in indium selenide investigated by means of far-infrared spectroscopy.

Abstract

Shallow impurities in n-type indium selenide (InSe) have been investigated by means of Fourier-transform spectroscopy in the far-infrared region. Three electric dipole transitions have been identified: 1s-2${\mathit{p}}_{\ifmmode\pm\else\textpm\fi{}}$, 1s-2${\mathit{p}}_{0}$, and 1s-3${\mathit{p}}_{\ifmmode\pm\else\textpm\fi{}}$, corresponding to electrons bound to native donors and tin-, silicon-, and chlorine-related donors, whose ionization energies (17.6, 18.1, 18.8, and 19 meV, respectively) have been determined through the Guerlach-Pollmann model. That model was also used to calculate the oscillator strengths of those dipole transitions, and then to estimate the shallow-donor concentrations in each sample. Native donors turn out to be the most hydrogenic ones, and the energies of their related transitions are used to determine a more accurate value of the low-frequency dielectric-constant product. An absorption line is observed in the low-energy side of the spectra (80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) for samples with a donor concentration higher than ${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, which is attributed to impurity pairing. A mechanism is proposed to explain the large value of the full width at half maximum associated with the 1s-2${\mathit{p}}_{\ifmmode\pm\else\textpm\fi{}}$ absorption line (\ensuremath{\approxeq}6.6 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the purest samples): Longitudinal-acoustic phonons polarized parallel to the c axis create dielectric-constant waves that modulate the dipole transition energies of shallow donors. In highly doped samples, compensating acceptors give rise to internal electric fields that largely broaden the absorption lines.