L. Dimitrocenko

Characterization of Crystalline Structure and Morphology of Ga2O3 Thin Film Grown by MOCVD Technique

Growth of gallium oxide thin film was realized with MOCVD on (0001) sapphire substrate. Structural and compositional properties of thin film were studied employing trimethylgallium and water as precursors, carrier gases were H2 and N2. Obtained film is polycrystalline and predominantly consisted of (201) oriented β-Ga2O3. Sample exhibited blue luminescence which is attributed to oxygen vacancies. H2 gas proved to have beneficial effect on film quality and overall growth process.

Laser scribing on HOPG for graphene stamp printing on silicon wafer

Highly oriented pyrolytic graphite (HOPG) was scribed by pulsed laser beam to produce square patterns. Patterning of HOPG surface facilitates the detachment of graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted stamp printing of a few-layers graphene was compared. Printed graphene sheets were visualized by optical and scanning electron microscopy. The number of graphene layers was measured by atomic force microscopy. Glue-assisted stamp printing allows printing relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method is easier to implement and is more flexible than the majority of existing ways of placing graphene sheets onto a substrate.

Ex situ investigations of MOCVD-grown gallium nitride nanowires using reflection high energy electron diffraction

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on sapphire substrates, via metal organic chemical vapor deposition (MOCVD). We present ex situ investigations on orientation and structure of grown GaN nanowires on GaN(0001) surface using reflection high energy electron diffraction (RHEED). Both ordered and randomly oriented GaN crystalline structures have been detected.

Dynamics of exciton creation and decay processes in composition – disordered InGaN thin films

In the GaN-based ternary alloys, InGaN crystals have been recognized as key materials for e-h plasmas-exciton dynamics, because of large exciton binding energies (24.8 meV in GaN). We report investigations of creating and recombination dynamics of excitons in commercially important InxGa1-xN composition range from x = 0.1 to 0.18 in which nanoscale indium composition fluctuation occurs and formation of indium rich clusters acting as quantum dots (QD) can be expected. Three MOCVD grown samples having x = 0.1; 0.14 and 0.18 have been investigated using 3D picosecond transient PL spectroscopy. It has been found that the band to band photo excitation at 15 K in whole composition range results in creating of complex luminescence band represented by three close overlapping Gaussian shape single exciton bands. All three exciton bands show fast decay time constants in a picosecond range. For all the samples the PL intensity dependence on excitation pulse power for each exciton band is different. An increase of the excitation pulse power density results in the linear growth of the band at the higher photon energy side. The next energy bands with lower photon energy show correspondingly quadratic and cubic dependences on laser pulse energy and are caused by the formation of biexcitons and triexcitons. Fast decay kinetics and excitation of multi-excitonic complexes are evident for important role of quantum dots by quantum capture of excitons in the considered composition range.

Growth temperature influence on the GaN nanowires grown by MOVPE technique

GaN nanowires (NWs) were successfully grown by Vapor-Liquid-Solid (VLS) growth mechanism on GaN template using metal-organic vapor phase epitaxy (MOVPE) with diameters ranging from 20 to 200 nm and length up to few microns. The characterization by scanning electron microscopy (SEM) reveals an optimum growth temperature at 790°C and X-ray diffraction (XRD) investigations indicates oriented crystallinity of grown NWs.

EPR of radiation defects in lithium-oxyfluoride glass ceramics

We studied oxyfluoride composites based on lithium silicate glasses with yttrium fluorides and rare-earth dopants. The electron paramagnetic resonance (EPR) has been used to obtain information about radiation induced defects in these materials. Spectra have been measured before and after X-ray irradiation at room temperature and at liquid nitrogen temperature. Fluoride crystallites within samples were created by means of thermal treatment at specific temperatures. EPR spectra of radiation induced defects in oxyfluoride glass ceramics, in which crystallites have not been yet created, show no explicit hfs interaction of fluorine nuclei. However, in glass ceramics, which already contains fluoride crystallites, the hfs characteristic to fluorine nuclei appears in the EPR spectra. EPR hyperfine structure could be explained within a model of an F-type centre in YF3 crystalline phase.

Localization dynamics of exciton luminescence in InxGa1−xN epitaxial films

Picosecond time resolved photoluminescence (PL) spectroscopy of excitonic processes in MOCVD grown InxG1−xN mixed films with the In concentration in range from x=0.1 to 0.18 under the band-to-band excitation are considered. It is stated that by an In content in alloy up to 12% the band-band photo excitation at 8 K results in creating of localized excitons and biexcitons represented by close overlapping Gaussian shape luminescence bands having FWHM 27 and 8.7 meV, respectively. PL decay kinetics of both bands involves two exponential decay stages. Excitons and biexcitons in unperturbed lattice positions causes fast decay with τ~ 10 ps, whereas their transfer to the metastable state due to relaxation of In – Ga local configuration causes slow decay τ ~ 90 ps At increased In content up to 19% both the localized excitons and biexcitons are represented by Gaussian type luminescence bands being non-uniform broadened to the high energy side. The continuous distribution of excitons and biexcitons in transition energies is stated by analysis of the red-shift of luminescence bands during the spectra decay. An additional new narrow low energy PL band arises at low energy side expected to be caused by excitons at InN clusters.

Formation of deep acceptor centers in AlGaN alloys

AlGaN alloy thin film materials are of high interest for light emitting diodes (LED of the ultraviolet (UV) spectral region. Origin of the deep intrinsic and impurity Si states in the AlxGa1-xN (0 < x < 0.35) epilayer structures grown by metalorganic chemical vapor deposition (MOCVD) technique have been considered. Effects of the lattice mismatch and Si-doping in the heterostructures of epilayers with different alloy composition are investigated using time resolved photoluminescence (PL) of donor - deep acceptor (DA) pairs. It is shown that the undoped AlGaN alloys, grown on a GaN buffer layer, due to the lattice mismatch contain the increased concentration of cation vacancy (Vcation) defects acting as a deep acceptor centers and responsible for PL. Si-doping results in both the additional increase of Vcation concentration and the formation in cation sub lattice of new (VcationSication) deep acceptor complexes. It is shown that by increase of the Al content in the AlGaN alloy the composition disorder of both deep acceptor centers Vcation and (VcationSication) complex appears. The corresponding broad PL bands are resolved in number of subbands. It is stated that deposition of Si-doped AlGaN alloy on undoped GaN results in formation of Si-doped GaN interlayer.

Temperature Effects in Up-Conversion Processes of Erbium - Ytterbium Doped Oxyfluoride Silicate Glass

Temperature Effects in Up-Conversion Processes of Erbium - Ytterbium Doped Oxyfluoride Silicate Glass Er3+ - Yb3+ doped oxyfluoride glass has been synthesized and investigated. It was found that the up-conversion spectrum of the sample excited by a 980 nm laser diode is highly dependent on the temperature of the sample. Using fluorescence intensity ratio technique the green up-conversion emissions at 525 nm and 550 nm were studied in detail in the temperature range 120-600 K. The potential applicability of the obtained results in the field of the temperature sensing is discussed. Temperatūras Efekti Daudzfotonu Ierosmes Procesā AR Erbiju UN Iterbiju Legētā Oksifluorīdu Silikāta Stiklā Tika sintezēts un pētīts ar Er3+ un Yb3+ legēts oksifluorīda stikls. Ierosinot paraugu daudzfotonu procesā ar 980 nm starojumu, novērota luminiscences spektra būtiska atkarība no temperatūras, intervālā 120-600 K. Izmantojot luminiscences intensitāšu attiecības metodi, spriests par iespējamo oksifluorīda stikla pielietojumu temperatūras sensoru jomā.

AlGaN-InGaN-GaN Near Ultraviolet Light Emitting Diode

AlGaN-InGaN-GaN Near Ultraviolet Light Emitting Diode A 382-nm InGaN/AlGaN light-emitting diode (LED) was made on a sapphire substrate by metal-organic vapour phase deposition (MOCVD) technique. Growing of the undoped and Si-doped GaN and AlxGa1-xN monocrystalline layers with a surface roughness of < 1 nm required for making light emitting devices has been carried out. To enhance the LED emission efficiency, a modified symmetric composition of an active single quantum well (SQW) structure was proposed. In addition to the conventional p-doped AlGaN:Mg electron overflow blocking barrier, an n-doped AlGaN:Si SQW barrier layer in the structure was formed that was meant to act as an additional electron tunneling barrier. AlGaN-InGaN-GaN Gaismas Diode Tuvajam Ultravioletam Rajonam Izmantojot metālorganikas ķīmisko tvaiku uzklāšanas metodi (MOCVD) uz safīra pamatnes, izveidota InGaN/AlGaN gaismas diode (LED) ar emisijas maksimumu pie 382 nm. Izstrādāta metodika monokristālisku nelegētu, Si- un Mgdopētu GaN un AlxGa1-xN, kur x<0.3 slāņu audzēšanai ar virsmas raupjumu mazāku par 1 nm, kuri nepieciešami LED multislāņu struktūras audzēšanai. Lai palielinātu LED gaismas emisijas efektivitāti, izveidota modificēta vienas kvantu akas (SQW) struktūra. Papildus konvenciālam p-legētam AlGaN:Mg elektronu pārplūšanu bloķējošam barjēras slānim SQW ietverts n-dopēts AlGaN:Si barjeras slānis, kurš rada papildus elektronu tunelēšanas barjeru.