Prof. Jacek Zakrzewski

20. April 2022.

You are cordially invited to the seminar of the Photonics Center, Laboratory for photoacoustics, which will be held on Thursday, 24 October 2019 at 11:00 in the “Zvonko Marić” hall, Institute of Physics Belgrade. The talk entitled

Photothermal Spectroscopy of Bulk Crystals and Heterostructures

will be given by Prof. Jacek Zakrzewski (Institute of Physics, Nicolaus Copernicus University in Torun, Poland).

Abstract of the talk:

The presentation shows the latest results obtained in the Applied
Physics Group in Torun (Poland) by piezoelectric photothermal
spectroscopy. Thermal and optical parameters of CdZnSe mixed crystals
and AlGaAs/GaAs heterostructures were characterized by this method.

The investigated bulk crystals were grown by the high-pressure Bridgman
method under argon overpressure. The crystals were cut into 1-1.2 mm
thick plates and mechanically grounded, polished and chemically etched.
The influence of zinc content and preparation method on the amplitude
and phase spectra is observed.
Mechanical, chemical and thermal treatment of the surface can create the
damaged subsurface layer which introduces additional defects states in
the material. The increase of the zinc content can also be associated
with the creation of volume defects in the material. Several features
of the crystals were revealed from the spectra, with the increase of
zinc content: a shift of the energy gap, broadening of the absorption
band, increase of the surface absorption.
For semiconductors, there are three main sources of photothermal signal:
thermal, plasma waves (associated with diffusion and recombination of
carriers). In the photoacoustic investigation of transport in
semiconductors one can also take into account immediate thermalization
of carriers and nonradiative surface recombination. Their influence on
the character of amplitude and phase spectra is showed and discussed.

Piezoelectric photothermal spectroscopy was also used to characterize
AlGaAs/GaAs heterostructures. The two-layer model proposed by Fernelius
and the interference model of Malinski were chosen to analyze the phase
and amplitude of PPS spectra. For each type of experimental setup
configuration, the theoretical models were fitted to experimental data
in order to obtain the thermal and optical parameters of the
investigated material.