Optical investigation of defects in semi-insulating ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ single crystals

Native defect levels in ternary compound ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ single crystals were studied by low-temperature photoluminescence (PL) and photoconductivity (PC) measurements. From the PL measurements, a broad emission band centered at 1.64 eV was observed at low temperatures. The peak can be decomposed using two standard Gaussian functions to reveal two emission bands at 1.55 and 1.66 eV. The PL peak at 1.55 eV is attributed to donor-acceptor pair recombination between a sulphur vacancy $\left({\mathrm{V}}_{\mathrm{S}}\right)$ deep donor $(E_d=0.57\text{eV})$ and an antisite defect $\left({\mathrm{S}}_{\mathrm{I}}\right)$ shallow acceptor $(E_a=20\text{meV})$. The 1.66-eV emission band is attributed to self-activated luminescence involving a defect complex and is described using a configuration coordinate model. Within this framework, the 1.66-eV emission band is associated with a S vacancy donor bound to a Tl vacancy acceptor that forms a ${\mathrm{V}}_{\mathrm{S}}\text{−}{\mathrm{V}}_{\mathrm{Tl}}$ Schottky pair. The photoconductivity spectra show the presence of a deep donor level located at 0.46 eV below the conduction-band edge, in good agreement with that measured by PL spectroscopy.

Figure 1

(a) The crystal structure of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ represented by a ball-and-stick model. Silver, purple, and green balls correspond to Tl, I, and S atoms, respectively. Tl atoms are present in two different lattice sites. (b) Square of absorption coefficient vs. photon energy of a ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ sample. (Inset) Transmission and reflection spectra of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$.

Figure 2

PL spectra of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ measured at 24 K. (Inset) The deconvoluted broad peak at 1.64 eV showing two emission bands at 1.55 and 1.66 eV, respectively. No near band edge emission is observed.

Figure 3

(a) Excitation intensity dependence of the PL spectra of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ at 24 K. (b) Excitation intensity dependence of the peak PL intensity of the 1.55- and 1.66-eV bands.

Figure 4

Excitation laser intensity vs peak energy at 24 K. The solid curve gives the theoretical fit using Eq. (5).

Figure 5

(a) PL spectra of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ measured in the energy region of 1.4–2.1 eV and in the temperature range of 24–100 K at constant excitation intensity. (b) Integrated intensity of the 1.55- and 1.66-eV peaks as a function of reciprocal temperature.

Figure 6

(a) The temperature dependence of the peak energy of the 1.55- and 1.66-eV bands. (b) The temperature dependence of the FWHM line width of the 1.66-eV emission band. The inset shows the FWHM of the 1.55-eV band.

Figure 7

(a) PC spectra of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ single crystals at 100 K and in the energy range of 1.6–2.15 eV. (b) Variation of ln $\left(I_{\mathrm{ph}}\right)$ with 1000 ${\mathrm{T}}^{-1}$ for a ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ single crystal.

Figure 8

(a) The photoluminescence process responsible for the observed 1.55-eV emission band. (b) The configuration coordinate diagram describing the 1.66-eV luminescence of the defect complex. Also shown are the band gap, and donorlike $\left({\mathrm{V}}_{\mathrm{S}}\right)$ and acceptorlike $\left({\mathrm{V}}_{\mathrm{Tl}}\right)$ levels of zero-point energies of the ground and excited states, respectively, which lie within the band gap. $E_{\mathrm{ab}}$ and $E_{\mathrm{em}}$ are the absorption and emission energies, respectively.

Figure 9

Crystal structure of ${\mathrm{Tl}}_6$${\mathrm{I}}_4\text{S}$ depicting the types of point defects that can occur in the compound. These include vacancies $({\mathrm{V}}_{\mathrm{Tl}},{\mathrm{V}}_{\mathrm{S}}$, and ${\mathrm{V}}_{\mathrm{I}})$ and antisite disorder $({\mathrm{I}}_{\mathrm{S}}$ and ${\mathrm{S}}_{\mathrm{I}})$.

Figure 10

Energy band schematics (not to scale) deduced from the photoluminescence, photoconductivity, and DFT results showing the optical processes at 24 K responsible for the observed peaks at 1.55, 1.61, and 1.66 eV.