Effect of mechanical compression on Cu(In,Ga)Se films : micro-structural and photoluminescence analysis

Cu(In,Ga)Se (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 °C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties

Narrow-gap piezoelectric heterostructure as IR detector

Narrow-gap mercury cadmium telluride thin films grown by MBE and LPE methods onto various substrates (HgCdTe/Si, HgCdTe/GaAs, HgCdTe/CdZnTe) were investigated as a piezoelectric heterostructure for IR detection. The photoresponse, infrared transmittance spectra, parameters of the charge carrier transport, and mechanical properties were studied. Mechanical stresses at the layer-substrate interface were analyzed. HgCdTe-based infrared device is considered, operating in the middle (3–5 μm) infrared spectral range without cryogenic cooling to achieve performance level D* = 2.6 ⋅ 10⁹ ⋅ W⁻¹ cm ⋅Hz¹/² The possibility to detect infrared radiation is thought to be based on the possibility of the spatial separation of the non-equilibrium carriers in the strained semiconductor heterostructure with piezoelectric properties

Optical spectroscopy studies of Cu2ZnSnSe4 thin films

Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor–acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis

Influence of the growth method on the photoluminescence spectra and electronic properties of CuInS2 single crystals

A comparative analysis of free and bound excitons in the photoluminescence (PL) spectra of CuInS2 single crystals grown by the traveling heater (THM) and the chemical vapor transport (CVT) methods is presented. The values of the binding energy of the A free exciton (18.5 and 19.7 meV), determined by measurements of the spectral positions of the ground and excited states, allowed the Bohr radii (3.8 and 3.7 nm), bandgaps (1.5536 and 1.5548 eV) and dielectric constants (10.2 and 9.9) to be calculated for CuInS2 crystals grown by THM and CVT, respectively

Diamagnetic shifts of free excitons in cuins2 in magnetic fields

Single crystals of CuInS2 were studied with reflectance (RF) and photoluminescence (PL) at 4.2 K in magnetic fields up to 10 T. The diamagnetic energy shifts observed for the free excitonic lines in both the RF and PL spectra were used to calculate the reduced masses (0.141m0, 0.142m0, and 0.114m0), binding energies (18.46, 18.59, and 14.90 meV), Bohr radii (3.82, 3.79, and 4.73 nm), and hole effective masses (1.20m0, 1.28m0, and 0.40m0) for the ALPB, AUPB, and BC excitons, respectively, using a low-field perturbation approach

Excited states of the A free exciton in CuInS2

High quality CuInS2 single crystals, grown by the traveling heater method in an indium solvent, were studied using reflectance and photoluminescence at 4.2 K. The first, EAn=2=1.5494 eV, and second, EAn=3=1.5532 eV, excited states of the A free exciton have been observed in the photoluminescence spectra. Accurate values of the A exciton binding energy EFE A =18.5 meV and Bohr radius aB A=3.8 nm, bandgap Eg=1.5540 eV at 4.2 K and static dielectric constant =10.2 have been derived assuming a hydrogenic model

Heart rhythm safety of the mononuclear cord blood stem cells transplantation in patients with refractory ischemic heart failure

The aim – to evaluate the safety of mononuclear cord blood stem cells transplantation regarding influence upon heart rhythm in patients with refractory heart failure Materials and methods. The work is based on the analysis of Holter-ECG monitoring data in 20 patients with refractory heart failure and reduced myocardial contractility. With the purpose of improvement of systolic function patients underwent transplantation of mononuclear cord blood stem cells. Results. Analysis of the probable proarrhythmogenic influence of mononuclear cord blood stem cells was performed in terms of follow-up during up to 9 months. Results demonstrated that administration of the cell preparation «Cryopreserved human cord blood» is not associated with increased risk of life-threatening cardiac arrhythmias. Conclusions.Transplantation of mononuclear cord blood stem cells should not be considered as an additional arrhythmogenic risk factor in patients with refractory heart failure