A. Vaitkevičius

Influence of quantum-confined Stark effect on optical properties within trench defects in InGaN quantum wells with different indium content

The trench defects in InGaN/GaN multiple quantum well structures are studied using confocal photoluminescence (PL) spectroscopy and atomic force microscopy. A strong blueshift (up to ∼280 meV) and an intensity increase (by up to a factor of 700) of the emission are demonstrated for regions enclosed by trench loops. The influence of the difference in the well width inside and outside the trench loops observed by transmission electron microscopy, the compositional pulling effect, the strain relaxation inside the loop, and corresponding reduction in the built-in field on the PL band peak position and intensity were estimated. The competition of these effects is mainly governed by the width of the quantum wells in the structure. It is shown that the PL band blueshift observed within the trench defect loops in the InGaN structures with wide quantum wells is mainly caused by the reduction in efficiency of the quantum-confined Stark effect due to strain relaxation.

Correlation between structure and photoluminescence properties in InGaN epilayers with thicknesses below and above critical thickness

The layer strain and its relaxation effects on the photoluminescence (PL) of InGaN layers are studied using confocal microscopy. The relaxation imposed structural changes are studied by X-ray diffraction (XRD) reciprocal space mapping and atomic force microscopy. Initial layer relaxation generated misfit dislocations were observed by confocal microscopy as intersecting parallel lines of lower PL intensity. The splitting of the PL spectrum into several PL bands indicated an onset of changes in the layer structure, which were confirmed by XRD measurements. The PL bands were attributed to two sub-layers of the sample: A relaxed upper sub-layer and a strained sub-layer underneath. Bright spots, approximately 250 nm in diameter, were observed on the background of the inhomogeneous PL intensity distribution due to fluctuations of In content. The bright spots correspond to column-like structures with relaxed lattice, In content as in the initial strained layer, and lower density of nonradiative recombination cen...

Influence of defects and indium distribution on emission properties of thick In-rich InGaN layers grown by the DERI technique

We report on the spatial variation of optical properties in thick, In-rich InGaN layers, grown by a novel droplet elimination by radical beam irradiation (DERI) technique. The increase of layer thickness causes layer relaxation and results in double-peaked photoluminescence spectra. Spatially resolved measurements show that the defects in the strained sub-layer are distributed inhomogeneously. An increase in the layer thickness results in faster nonradiative recombination due to increasing density of nonradiative recombination centers, as evidenced by time-resolved free carrier absorption, and facilitates larger indium incorporation in the upper part of the layer.

Non-Linear Optical Phenomena in Detecting Materials as a Possibility for Fast Timing in Detectors of Ionizing Radiation

The time resolution of the detectors currently in use is limited by 50-70 ps due to the spontaneous processes involved in the development of the response signal, which forms after the relaxation of carriers generated during the interaction. In this study, we investigate the feasibility of exploiting sub-picosecond phenomena occurring after the interaction of scintillator material with ionizing radiation by probing the material with ultra-short laser pulses. One of the phenomena is the elastic polarization due to the local lattice distortion caused by the displacement of electrons and holes generated by ionization. The key feature of the elastic polarization is its short response time, which makes it prospective for using as an optically detectable time mark. The nonlinear optical absorption of femtosecond light pulses of appropriate wavelength is demonstrated to be a prospective tool to form the mark. This study was aimed at searching for inorganic crystalline media combining scintillation properties and non-linear absorption of ultra-short laser pulses. The nonlinear pump-and-probe optical absorption technique with 200 fs laser pulses was used to study the effects in lead tungstate, garnet-type, and diamond scintillator crystals.

Potential of non-linear optical phenomena for fast timing in detectors of ionizing radiation

After the discovery of the Higgs boson at CERN, the development of new experiments at future colliders remains the main stream in experimental high energy physics. Since pile up shows significant increase with the increase of collider luminosity, timing becomes the key issue for the future detection of rare events. Unfortunately, the time resolution of the detectors currently used in high energy physics experiments is limited to 50-70 ps. That occurs due to the spontaneous processes involved in the development of the light response signal, which is generated after the relaxation of carriers, created during the interaction. In this study, we consider a possibility to exploit some phenomena for ionizing radiation detection which are carried on in parallel with relaxation of carriers in the very first few ps while ionization starts. One is the elastic polarization due to local lattice distortion as the electrons are displaced and holes generated at ionization. The key feature of the elastic polarization is its short response time, which makes it promising to use as an optically detectable time mark. Nonlinear optical absorption of femtosecond light pulses of appropriate wavelength is considered to be a tool to generate time mark. The study was targeting at searching of the inorganic scintillating materials combining scintillation properties and non-linear absorption of ultra-short laser pulses. The nonlinear pump-and-probe optical absorption technique with of 200 fs laser pulses was used to study the effects in lead tungstate and garnet type scintillator crystals.