K. Strzałkowski

Thermal characterization of II–VI binary crystals by photopyroelectric calorimetry and infrared lock-in thermography

In this paper, a complete thermal characterization (measurement of all static and dynamic thermal parameters) of some selected II–VI binary crystals was carried out. The semiconductors under investigation were grown from the melt by high-pressure/high-temperature-modified Bridgman method. The contact photopyroelectric (PPE) method in back configuration and non-contact infrared lock-in thermography technique were used in order to get the thermal diffusivity of the investigated crystals. The thermal effusivity of the samples was obtained using the PPE technique in the front configuration, together with the thermal wave resonator cavity method. Knowing the values of the thermal effusivity and thermal diffusivity, the remaining two thermal parameters, i.e., thermal conductivity and specific heat were calculated.

Complementary photothermal techniques for complete thermal characterization of porous and semi-transparent solids

The paper is focused on the ability of using two complementary photothermal techniques for the measurement of all thermal parameters of some porous and/or semi-transparent solid samples whose composition, structure, and geometry do not allow a complete thermal characterization using a single technique. In this work, we combine a contact technique, photopyroelectric (PPE) calorimetry, with a non-contact one, infrared lock-in IR thermography (IRT), in order to investigate some solid samples such as dental composites, building materials, drugs, and semiconductors. The composition and the geometry of the investigated samples make the PPE method (in “front” detection configuration together with thermal-wave resonator cavity (TWRC) technique as scanning procedure) suitable for thermal effusivity measurements and IRT for thermal diffusivity investigations. In such a way, this combination of methods leads to a complete thermal characterization of the investigated materials.

An active thermography approach for thermal and electrical characterization of thermoelectric materials

The enhancement of figure of merit (ZT) of thermoelectrics is becoming extremely important for an efficient conversion of thermal energy into electrical energy. In this respect, reliable measurements of thermal and electrical parameters are of paramount importance in order to characterize thermoelectric materials in terms of their efficiency. In this work, a combined theoretical-experimental active thermography approach is presented. The method consists of selecting the right sequential interdependence between the excitation frequency and the sampling rate of the infrared camera, by computing a temporal Fourier analysis of each pixel of the recorded IR image. The method is validated by using a reference sample which is then applied to a recent synthesized titanium trisulphide thermoelectric material (TiS3). By combining AC and steady-state experiments, one can obtain information on both thermal and electrical parameters of TE materials (namely thermal diffusivity, Seebeck coefficient). The thermal diffusivity and thermal conductivity of TiS3 are also measured using photothermal radiometry technique (PTR) and the resulting values of these parameters are α = 9.7*10−7 m2 s−1 and k = 2.2 W m−1 K, respectively. The results obtained with the two techniques are in good agreement. In the case of TE materials, the main benefit of the proposed method is related to its non-contact nature and the possibility of obtaining the electric potential and temperature at the same probes. The Seebeck coefficient obtained by active IR thermography (S = −554 μV K−1) is consistent with the one obtained using an ULVAC-ZEM3 system (S = −570 μV K−1). For a large number of users of thermographic cameras, which are not equipped with a lock-in thermography module, the present approach provides an affordable and cheaper solution.

Photoacoustic method of determination of quantum efficiency of luminescence in Mn2+ ions in Zn1−x−yBexMnySe crystals

This paper presents step by step the procedure of determination of the quantum efficiency of luminescence of Mn2+ ions in the Zn1−x−yBexMnySe crystals. The method is based on the photoacoustic spectroscopy approach. In the paper, the experimental spectra of absorbance, transmission, absorption and photoacoustic spectra of the samples are presented and analyzed from the point of view of the possibility of determination of the quantum efficiency of Mn2+ ion luminescence at room temperature. It was determined experimentally that in the investigated crystals the quantum efficiency of luminescence in the Mn2+ ions is about 35%, 40%, 32% for the absorption peaks at 430 nm, 470 nm, and 510 nm, respectively, for Zn0.75Be0.2Mn0.05Se crystal.

Energy efficiency of near infrared cobalt luminscence in ZnSe:Co determined by a photoacoustic method

The paper presents results of computations of the energy efficiency of the cobalt luminescence in ZnSe:Co determined by the photoacoustic method. The transmission spectra, photoacoustic experimental and theoretical spectra, and the frequency dependence on the photoacoustic amplitude characteristics are presented. From them, the energy efficiency of Co2+ the near infrared luminescence (3200 nm) was computed in the frame of new proposed photoacoustic model of computations of the luminescence energy efficiency.

Optical characterization of Zn0.96Be0.04Se and Zn0.93Mg0.07Se mixed crystals

A detailed optical characterization of Zn0.96Be0.04Se and Zn0.93Mg0.07Se mixed crystal samples grown by the modified high-pressure Bridgman method has been carried out via photoluminescence (PL), contactless electroreflectance (CER), and photoreflectance (PR) in the temperature range of 15–400 K. With these optical diagnostic tools we observed features originating from exciton emission, an edge emission due to recombination of shallow donor-acceptor pairs (DAPs), and a broad band related to recombination through deep-level defects, as well as band-edge and spin-orbit splitting critical points interband transitions in CER and PR experiments. The optical properties of these two mixed crystals are compared. Our comparative study revealed that the beneficial effect of beryllium (Be) incorporation is superior to that of magnesium (Mg), in the sense that a Be-containing ternary alloy resulted in better crystalline quality with more intense excitonic emission line and weaker DAPs and deep-level defect-related ba...

Temperature dependence of energies and broadening parameters of near band edge interband excitonic transitions in wurtzite ZnCdMgSe

An optical characterization of two wurtzite ZnCdMgSe crystalline alloys grown by the modified high-pressure Bridgman method was carried out by temperature-dependent photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 10–300 K and photoreflectance (PR) measurements between 300–400 K. Low temperature PL spectra of the investigated samples consisted of the excitonic line, the “edge-emission” due to radiative recombination of shallow donor-acceptor pairs and a broad band related to recombination through deep level defects. Three excitonic features, A, B, and C, in the vicinity of band edge were observed in the CER and PR spectra. The peak positions of band edge excitonic features in the PL spectra were shifted slightly toward lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase in the CER-PL shift with the increasing of Mg content in the investigated crystals was explained by the rising of ...

Temperature dependent electromodulation characterization of Zn1−x−yBexMgySe mixed crystals

We report a detailed investigation of the temperature dependence of the band-edge excitonic transitions of three Zn1−x−yBexMgySe mixed crystals using contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15–400 K. The samples were grown by the modified high pressure Bridgman methods. The fundamental transition energy E0 and broadening parameters are determined via a lineshape function fit to the CER and PR spectra. The parameters that describe the temperature dependence of the band-edge excitonic transition energy and broadening parameters are evaluated and discussed.

Photoluminescence and surface photovoltage spectroscopy characterization of Zn1−x−yBexMgySe mixed crystals

This paper presents an optical characterization of three Bridgman-grown Zn1−x−yBexMgySe mixed crystals in the near-band-edge interband transitions using temperature-dependent photoluminescence (PL) in the temperature range of 10–300 K and surface photovoltage spectroscopy (SPS) at room temperature. PL spectra at low temperatures of the investigated samples consist of an excitonic line, an edge emission due to radiative recombination of shallow donor-acceptor pairs, and a broad band related to recombination through deep level defects. The anomalous S-shape temperature dependence of the exciton emission peak for Zn1−x−yBexMgySe crystals with high Mg content (y=0.26) can be explained as due to localization of excited carriers caused by statistical fluctuations of local composition. The peak positions of the excitonic emission lines in PL spectra correspond quite well to the energies of the fundamental transitions determined from SPS measurements. The parameters that describe the temperature dependence of the...

Atomic Structure Imaging in ZnSe and Mixed Zn0.74Mn0.2Be0.06Se Crystals with X-ray Fluorescence Holography

Abstract X-ray fluorescence holograms of ZnSe and Zn0.74Mn0.2Be0.06Se single crystals were measured by means of a table-top laboratory setup. Holograms were recorded simultaneously for Zn K, Se K and, in the case of the mixed crystal, for Mn K X-ray fluorescence. Analysis of the holograms was based on a linear regression algorithm preceded by the matrix effects correction. It enabled the determination of real and imaginary parts of the structure factors from which atomic structures around different fluorescing atoms could be reconstructed. In particular, direct imaging of Mn lattice sites in Zn0.74Mn0.2Be0.06Se unambiguously demonstrated that the majority of Mn atoms occupy substitutional Zn positions. However, it was shown that the precise analysis of Mn lattice site distribution can be hindered due to extinction effects which have to be included in the analysis of XFH in the case of non-centrosymmetric samples.