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Radiative Recombination Processes in Transition Metal Disulfide Crystals 2H-WS2 and 2H-MoS2


Author: Dumitru Dumcenco
Degree:doctor of physics and mathematics
Speciality: 01.04.10 - Semiconductors physics and engineering
Year:2006
Scientific adviser: Leonid Culiuc
doctor habilitat, professor, Institute of Applied Physics of the ASM
Institution:

Status

The thesis was presented on the 15 March, 2006
Approved by NCAA on the 27 April, 2006

Abstract

Adobe PDF document0.34 Mb / in romanian
Adobe PDF document0.48 Mb / in russian

Thesis

CZU 621.315.592

Adobe PDF document 2.39 Mb / in russian
132 pages


Summary

The thesis present the results of the complex study of the radiative properties of the single transition metal disulphides (TX2) 2H-WS2:(Br2,I2) and 2H-MoS2:Cl2. TX2 single crystals were obtained by chemical vapour transport method using I2, Br2 and Cl2 as transport agents. The diatomic halogen molecules were incorporated into Van der Waals gap of the 2H-TX2 crystals during the growth process.

The photoluminescence (PL) investigation of the obtained samples showd the presence of two distinct spectral regions: the excitonic region, located in the vicinity of the indirect band gap and consisting of several sharp zero-phonon lines and their phonon replica, and the vibronic broad band located at the low energy region. The PL spectra of the natural, undoped molybdenite has no excitonic region.

There were observed and identified at least three zero-phonon lines for each investigated compound, located in the energy ranges 1.32 – 1.34eV for 2H-WS2:(Br2, I2) and 1.17 – 1.19eV for 2H-MoS2:Cl2 These lines appear due to the radiative recombination of the excitons bound to the neutral centres formed by halogens molecules (Br2, I2 or Cl2). These neutral centres create energy levels placed about 0.1eV below the indirect band gap energy and display properties similar to those of the acceptor like isoelectronic traps in GaP or Si, providing an efficient radiative recombination. The temperature increase up to about 50K leads to the redistribution of intensities among the excitonic lines. At T>50K an exponential thermal quenching of the overall exciton emission with activation energy of 0.1 eV occurs.

The maxima of the vibronic band are located at 0.97 eV and 0.95 eV for 2H-WS2 and 2H-MoS2 respectively. This band has been attributed to the radiative recombination of the nonequilibrium carriers via intrinsic defects (deep centres) of the lamellar crystals. The analysis of the temperature behaviour of the excitonic and vibronic broad band intensities proved that the radiative and nonradiative recombinations through deep centres act as a shunting channels leading to the thermal quenching of the exciton emission.

The single configuration coordinate diagram of the deep center placed at 0.41eV and 0.27eV below the conduction band for 2H-WS2 and 2H-MoS2 respectively was constructed. A kinetic model numerically solved for thermal equilibrium conditions was developed to describe the temperature non-monotonic behavior of the excitonic spectral lines intensities and observed radiative decay times. This model can explain also the thermal redistribution of the excitonic lines intensities taking into account the existence of the nonradiative recombination channel via deep centres. The observed atypical increase of the excitonic emission decay time observed for 2H-WS2:(Br2,I2) at 2 - 7K is explained by the existence of a excitonic sublevel with a long life time located at 0.3meV above the lowest level.

It was proved that the luminescent properties of the synthetic single crystals TX2 have their origin in halogens molecules intercalated into lamellar crystal lattice due to the perfect match of the halogens molecules dimensions to the sizes of the tetrahedral coordinated interstitial sites of the 2H-TX2 crystals.