Luminescence properties of LiGaO2 crystal

The study was supported by the Latvia-Lithuania-Taiwan research project “Nonpolar ZnO thin films: growth-related structural and optical properties” (Latvia: LV-LT-TW/2016/5 , Lithuania: TAP LLT 02/2014 , Taiwan: MOST 103-2923-M-110-001-MY3 ).The comprehensive spectral study of lithium metagallate LiGaO2 crystal has been done including methods of pump-probe techniques, optical absorption, photoluminescence, luminescence kinetics, thermoluminescence and polarised luminescence in broad temperature region. Luminescence spectrum of the crystal contains the main emission bands at 4.43, 3.76, 2.38 and 1.77 eV. The novel data on luminescence excitation spectra including VUV area, kinetics and polarization are presented. The correlation between pump-probe experiment results and luminescence properties is found. Conclusions are done about the recombination character of all the observed emission bands, implying tunnel recombination of donor-acceptor pairs.Ministry of Science and Technology 103-2923-M-110-001-MY3; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Investigation of wide-band-gap semiconductor photoelectric properties by using optical techniques with temporal and spatial resolution

The thesis is dedicated to investigation of carrier dynamics in SiC, GaN and diamond by using light-induced transient gratings, differential transmittivity, differential reflectivity and photoluminescence techniques. Also new absorption coefficient measurement and carrier lifetime microscopy techniques are presented. The experimental studies were performed in a wide range of excess carrier densities and temperatures under single- and two- photon excitation conditions. Strong diffusion coefficient temperature and injection dependences were determined. They were explained by phonon scattering, inter-carrier interaction processes and degeneracy. It was determined, that in indirect-gap SiC and diamond semiconductors at low injections lifetime is limited by point and surface defects, while in GaN recombination on grain boundaries prevails. Also nonlinear Auger recombination coefficient was observed. It was Coulombically enhanced at low injections and screened at high ones. Photoluminescence efficiency revealed radiative recombination coefficient reduction with injection in GaN. On the other hand, in SiC radiative coefficient was injection independent. Compensating defect (aluminum in SiC and boron in diamond) concentrations and activation energies were determined from their absorption saturation and recovery rate temperature dependences

Long-lived excitons in thermally annealed hydrothermal ZnO

Applying thermal annealing to hydrothermal ZnO crystals an enhancement of exciton lifetime from 80 ps to 40 ns was achieved boosting PL quantum efficiency of the UV luminescence up to 70 %. The lifetime improvement is related to the reduced density of carrier traps by a few orders of magnitude as revealed by the reduction of the slow decay tail in pump probe decays coupled with weaker defects-related PL. The diffusion coefficient was determined to be 0.5 cm2/s, providing a large exciton diffusion length of 1.4 μm. The UV PL lifetime drop at the lowest exciton densities was explained by capture to traps. Release of holes from acceptor traps provided delayed exciton luminescence with ∼200 μs day time and 390 meV thermal activation energy. Pump-probe decays provided exciton absorption cross-section of 9 × 10−18 cm2 at 1550 nm wavelength and verified the PL decay times of excitons. Amplitudes and decay times of the microsecond slow decay tails have been correlated with the trap densities and their photoluminescence. A surface recombination velocity of 500 cm/s and the bimolecular free carrier recombination coefficient 0.7 × 10−11 cm3/s were calculated. Therefore, the properly annealed hydrothermally grown ZnO can be a viable and integral part of many functional devices as light-emitting diodes and lasers

Time-Resolved Photoluminescence in GeSn Film by New Infrared Streak Camera Attachment Based on a Broadband Light Upconversion

GeSn coatings on commercial Si substrates have gained increased interest for application in infrared detectors and lasers. The characterization of these materials is crucial for their quality assessment and in describing device performance for commercialization. The time-resolved photoluminescence is an efficient method for contactless, time-resolved characterization of GeSn optoelectronic properties. For this purpose, in this work, we developed an infrared streak camera attachment based on the broadband upconversion of infrared photoluminescence by using powerful nanosecond 1064 nm pulses. The attachment achieved picosecond time resolution, being limited by the laser pulse duration, jitter, and temporal resolution of the streak camera. The spectral range for time-resolved photoluminescence in the 1100–2400 nm interval was obtained, outperforming the range of commercial infrared InGaAs streak cameras. The developed setup was applied for excitation-dependent time-resolved photoluminescence decay measurements in a GeSn and compared to the conventional upconversion technique with the optically delayed picosecond gate pulses. The new setup provided 2D spectro-temporal images for analysis. The photoluminescence decay times in the 30–80 ps range were obtained in the GeSn layer depending on the excitation pulse energy and spectral emission wavelength. Carrier thermalization was observed as a redshift of the photoluminescence spectra with time

Impact of dopant-induced band tails on optical spectra, charge carrier transport, and dynamics in single-crystal CdTe

Cadmium telluride (CdTe) semiconductors are used in thin-film photovoltaics, detectors, and other optoelectronic applications. For all technologies, higher efficiency and sensitivity are achieved with reduced charge carrier recombination. In this study, we use state-of-the-art CdTe single crystals and electro-optical measurements to develop a detailed understanding of recombination rate dependence on excitation and temperature in CdTe. We study recombination and carrier dynamics in high-resistivity (undoped) and arsenic (As)-doped CdTe by employing absorption, the Hall effect, time-resolved photoluminescence, and pump-probe in the 80–600 K temperature range. We report extraordinarily long lifetimes (30 µs) at low temperatures in bulk undoped CdTe. Temperature dependencies of carrier density and mobility reveal ionization of the main acceptors and donors as well as dominant scattering by ionized impurities. We also distinguish different recombination defects. In particular, shallow As Te and deep V Cd −As Cd acceptors were responsible for p-type conductivity. AX donors were responsible for electron capture, while nonradiative recombination centers (V Cd −As Te , As 2 precipitates), and native defects (V Cd −Te Cd ) were found to be dominant in p-type and n-type CdTe, respectively. Bimolecular and surface recombination rate temperature dependencies were also revealed, with bimolecular coefficient T −3/2 temperature dependence and 170 meV effective surface barrier, leading to an increase in surface recombination velocity at high temperatures and excitations. The results of this study allowed us to conclude that enhanced crucible rotation growth of As-doped CdTe is advantageous to As activation, leading to longer lifetimes and larger mobilities and open-circuit voltages due to lower absorption and trapping

Luminescence properties of LiGaO2 crystal

The study was supported by the Latvia-Lithuania-Taiwan research project “Nonpolar ZnO thin films: growth-related structural and optical properties” (Latvia: LV-LT-TW/2016/5 , Lithuania: TAP LLT 02/2014 , Taiwan: MOST 103-2923-M-110-001-MY3 ).The comprehensive spectral study of lithium metagallate LiGaO2 crystal has been done including methods of pump-probe techniques, optical absorption, photoluminescence, luminescence kinetics, thermoluminescence and polarised luminescence in broad temperature region. Luminescence spectrum of the crystal contains the main emission bands at 4.43, 3.76, 2.38 and 1.77 eV. The novel data on luminescence excitation spectra including VUV area, kinetics and polarization are presented. The correlation between pump-probe experiment results and luminescence properties is found. Conclusions are done about the recombination character of all the observed emission bands, implying tunnel recombination of donor-acceptor pairs.Ministry of Science and Technology 103-2923-M-110-001-MY3; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Morphological and optical property study of Li doped ZnO produced by microwave-assisted solvothermal synthesis

ZnO materials have been at the centre of many studies for decades. Doping of ZnO by lithium atoms is a prospective approach for compensation of n-type conductivity in the unintentionally doped ZnO aimed at obtaining p-type semiconductor. In this study, we have synthesized ZnO rod-like powders doped with lithium ions (0–0.65 atom%) by the new microwave-assisted solvothermal method in order to obtain greater photoluminescence intensity of ZnO emissions in the UV region. The obtained powders contain nanoparticles from 20 nm up to 250 nm depending on Li content. X-ray diffractometry and Raman spectroscopy were employed to characterise the structure of ZnO powders, scanning electron microscopy was used to determine the morphology, and most importantly the photoluminescence technique was used to investigate the optical properties of the obtained materials. As a result, the photoluminescence intensity has been increased up to an order of magnitude with the Li content in the ZnO crystals. The increase of the excitation laser power has led to an increased carrier lifetime and photoluminescence response intensity that is observed for all the samples