K. Goksen

Donor–acceptor pair recombination in gallium sulfide

Low temperature photoluminescence of GaS single crystals shows three broad emission bands below 2.4 eV. Temperature and excitation light intensity dependencies of these bands reveal that all of them originate from close donor–acceptor pair recombination processes. Temperature dependence of the peak energies of two of these bands in the visible range follow, as expected, the band gap energy shift of GaS. However, the temperature dependence of the peak energy of the third band in the near infrared shows complex behavior by blueshifting at low temperatures followed by a redshift at intermediate temperatures and a second blueshift close to room temperature, which could only be explained via a configuration coordinate model. A simple model calculation indicates that the recombination centers are most likely located at the nearest neighbor lattice or interstitial sites.

Dispersive optical constants and temperature tuned band gap energy of Tl2InGaS4 layered crystals

The optical properties of Tl2InGaS4 layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 400–1100 nm. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.35 and 2.54 eV, respectively. Transmission measurements carried out in the temperature range of 10–300 K revealed that the rate of change of the indirect band gap with temperature is γ = −4.70 × 10−4 eV K−1. The absolute zero value of the band gap energy was obtained as Egi(0) = 2.45 eV. The dispersion of the refractive index is discussed in terms of the Wemple–DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 5.73 eV, 31.46 eV, 11.72 × 1013 m−2 and 2.55, respectively. From x-ray powder diffraction study, the parameters of the monoclinic unit cell were determined.

Photoluminescence Spectra of GaS0.75Se0.25 Layered Single Crystals

Photoluminescence (PL) spectra of GaS 0.75 Se 0.25 layered single crystals have been studied in the wavelength region of 500-850 nm and in the temperature range of 10-200 K. Two PL bands centered at 527 ( 2.353 eV, A-band) and 658 nm (1.884 eV, B-band) were observed at T = 10 K. Variations of both bands have been studied as a function of excitation laser intensity in the range from 8×10 -3 to 10.7 W cm -2 . These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.043 and 0.064 eV below the bottom of conduction band to acceptor levels located 0.088 and 0.536 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively.

Below and above bandgap excited photoluminescence in Tl4InGa3S8 layered single crystals

Photoluminescence (PL) spectra of Tl 4 InGa 3 S 8 layered single crystals grown by the Bridgman method have been studied in the 550-710 nm wavelength and 80-300 K temperature ranges with below bandgap excitation (λ exc = 532 nm), and in the 420-600 nm wavelength and 30-300 K temperature ranges with above bandgap excitation (λ exc = 325 nm). The broad emission bands centered at 580 nm (2.14 eV) and 496 nm (2.49 eV) were observed at T = 80 and 30 K for below and above bandgap excitation processes, respectively. Variations in emission spectra have been studied as a function of excitation laser intensity in the 10.3-429.7 mW cm -2 range for below bandgap excitation. Radiative transitions from the donor levels located at 0.02 and 0.37 eV below the bottom of the conduction band to the deep acceptor level located at 0.20 eV above the top of the valence band were proposed to be responsible for the observed PL bands. From x-ray powder diffraction and EDS analysis, the monoclinic unit cell parameters and compositional parameters, respectively, were determined.

Low-temperature photoluminescence study of GaS0.5Se0.5 layered crystals

Abstract Photoluminescence (PL) spectra of GaS 0.5 Se 0.5 layered single crystals have been studied in the wavelength range of 565–860 nm and in the temperature range of 15–170 K. Two PL bands centered at 585 nm (2.120 eV) and 640 nm (1.938 eV) were observed at T = 15 K. Variations of both bands were studied as a function of excitation laser intensity in the range from 10 −3 to 15.9 W cm −2 . These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located at 0.029 and 0.040 eV below the bottom of the conduction band to deep acceptor levels located 0.185 and 0.356 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the PL spectra, respectively. A simple energy level diagram explaining the recombination process is proposed.