N.M. Gasanly

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.

Donor-acceptor pair recombination in AgIn5S8 single crystals

Photoluminescence (PL) spectra of AgIn5S8 single crystals were investigated in the 1.44–1.91 eV energy region and in the 10–170 K temperature range. The PL band was observed to be centered at 1.65 eV at 10 K and an excitation intensity of 0.97 W cm−2. The redshift of this band with increasing temperature and with decreasing excitation intensity was observed. To explain the observed PL behavior, we propose that the emission is due to radiative recombination of a donor-acceptor pair, with an electron occupying a donor level located at 0.06 eV below the conduction band, and a hole occupying an acceptor level located at 0.32 eV above the valence band.

Thermally stimulated current analysis of shallow levels in TlGaS2 layered single crystals

We have carried out thermally stimulated current measurements on as-grown TlGaS2 layered single crystals in the temperature range of 10–60 K with various heating rates. We found experimental evidence for the presence of three trapping centres in TlGaS2, located at 6, 12 and 26 meV. We have determined the trap parameters using various methods of analysis, and these agree well with each other. The retrapping process is negligible for these levels, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping.

Radiative donor-acceptor pair recombination in TlInS2 single crystals

Photoluminescence (PL) spectra of TlInS2 layered single crystals were investigated in the 500-860 nm wavelength region and in the 11.5-100 K temperature range. We observed two PL bands centred at 515 nm (2.41 eV, A band) and 816 nm (1.52 eV, B band) at T = 11.5 K and an excitation intensity of 7.24 W cm-2. A detailed study of the A band was carried out as a function of temperature and excitation laser intensity. A red shift of the A band position was observed for both increasing temperature and decreasing excitation laser intensity in the range from 0.12 to 7.24 W cm-2. Analysis of the data indicates that the A band is due to radiative transitions from the moderately deep donor level located at 0.25 eV below the bottom of the conduction band to the shallow acceptor level located at 0.02 eV above the top of the valence band. An energy-level diagram for radiative donor-acceptor pair transitions in TlInS2 layered single crystals is proposed.

Crystal data, electrical resistivity, and Hall mobility of n-type AgIn5S8 single crystals

The X-ray diffraction has revealed that AgIn 5 S 8 is a single phase crystal of cubic spinel structure. The value of the unit cell parameter for this crystal is 1.08286 nm. The electrical resistivity and Hall effect of n-type AgIn 5 S 8 crystals are measured in the temperature range of 50-400 K. A carrier effective mass of 0.20 m 0 , an acceptor to donor concentration ratio of 0.8 and an acoustic phonons deformation potential of 20 eV are identified from the Hall effect measurement. The Hall mobility data are analyzed assuming the carrier scattering by acoustic and polar optical phonons, and ionized impurities.

Hall effect, space-charge limited current and photoconductivity measurements on TlGaSe2 layered crystals

TlGaSe2 layered crystals are studied through dark electrical conductivity, Hall mobility, space-charge limited current and illumination- and temperature-dependent photoconductivity in the temperature ranges 120–350 K, 220–350 K, 260–350 K and 120–350 K, respectively. The Hall effect measurements revealed the extrinsic p-type conduction. The Hall mobility increase with decreasing temperature is limited by the thermal lattice scattering. The space-charge limited current and dark conductivity measurements predicted the existence of a single discrete trapping level located at 330 meV with a trap concentration of (1.4–2.2) × 1013 cm−3. The dark electrical conductivity and photoconductivity measurements reflect the existence of three other energy levels located at 95, 46 and 26 meV at high, moderate and low temperatures, respectively. The photocurrent is observed to increase with increasing temperature up to a maximum temperature of 320 K. The illumination dependence of photoconductivity is found to exhibit sublinear, linear and supralinear recombinations at high, moderate and low temperatures, respectively. The change in recombination mechanism is attributed to the exchange in the behaviour of sensitizing and recombination centres.

Temperature-?and excitation intensity-dependent photoluminescence in TlInSeS single crystals

Photoluminescence (PL) spectra of TlInSeS layered single crystals were investigated in the wavelength region 460–800 nm and in the temperature range 10–65 K. We observed one wide PL band centred at 584 nm (2.122 eV) at T = 10 K and an excitation intensity of 7.5 W cm−2. We have also studied the variation of the PL intensity versus excitation laser intensity in the range from 0.023 to 7.5 W cm−2. The red shift of this band with increasing temperature and blue shift with increasing laser excitation intensity was observed. The PL was found to be due to radiative transitions from the moderately deep donor level located at 0.243 eV below the bottom of the conduction band to the shallow acceptor level at 0.023 eV located above the top of the valence band. The proposed energy-level diagram permits us to interpret the recombination processes in TlInSeS layered single crystals.

Temperature effect on dark electrical conductivity, Hall coefficient, space charge limited current and photoconductivity of TlGaS2 single crystals

The dark electrical conductivity, Hall coefficient, space charge limited current, and illumination and temperature dependences of the photocurrent of TlGaS2 single crystals in the temperature regions of 100–350, 200–350, 200–290 and 100–350 K, respectively, have been measured and analysed. The Hall coefficient measurements revealed the extrinsic type of conduction with conductivity-type conversion from p- to n-type at a critical temperature of 315 K. The temperature dependence of the dark electrical conductivity exhibits activation behaviour with activation energies (0.360 ± 0.005) eV and (0.240 ± 0.005) eV at high and low temperatures, respectively. The space charge limited current analysis has shown that the energy level of (0.240 ± 0.005) eV is a trapping state with trap density of (2.2–3.9) × 1012 cm−3. The data analysis of the photocurrent–temperature dependence has revealed two photoconductivity activation energies of (0.660 ± 0.005) eV and (0.360 ± 0.005) eV in the temperature regions of 290–350 K and 220–280 K, respectively. The illumination dependence of photoconductivity is found to exhibit linear and supralinear recombination mechanisms above and below 290 K, respectively.

Low-temperature phase transitions in TlGaS2 layer crystals

Abstract Polarized Raman scattering spectra of TlGaS2 layer crystals have been studied for the first time as a function of temperature between 8.5 and 295 K. No evidence for a soft mode behaviour has been found. The anomalies observed in the temperature dependence of low- and high-frequency phonon modes at ∼ 250 and ∼ 180 K, respectively, are explained as due to the phase transitions. It is supposed that the phase transitions are caused by the deformation of structural complexes GaS4, rather than by slippage of Tl atom channels in [110] and [1 1 0] directions, which is mainly responsible for the appearance of the low-temperature ferroelectric phase transitions in other representatives of TlBX2 layer compounds.

Polymorphism of InS at high pressures

Abstract In situ X-ray diffraction has been used to study high-pressure polymorphism of InS up to ∼ 13 Gpa in the 293–573 K temperature range. The phase transition InS I⌉arr2;InS II is found under isothermal compression at pt = 7.5 ± 0.5 GPa and T = 293 K; at pt = 6.0 ± 0.5 GPa and T = 573 K. InS II crystallizes in the structural type of Hg2Cl2: a = 3.823 ± 0.008 A ; c = 10.868 ± 0.030 A ; c/a = 2.843; Z = 4; D4h17(I4/mmm); Vp/V0 = 0.85; p = 10 GPa, T = 293 K. X-ray powder data indicate a continuous change of the orthorhombic structure of InS I with increasing pressure associated with the transition to the tetragonal phase InS II.