Browsing by Author "Caraman, Mihail"

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ABSORBŢIA LUMINII ŞI FOTOLUMINESCENŢA IONILOR Eu3+ ÎN CRISTALE DE CaF2
(CEP USM, 2012) Untila, Dumitru; Racoveț, Oxana; Evtodiev, Silvia; Caraman, Mihail
From optical transmittance and photoluminescence measurements of natural crystals of CaF2:Eu was established the correlation between energy states diagram of Eu3+ ion and the structure of these bands. Photon absorption in this material occurs at electronic transitions from the 7F0 level on the G, D, L configuration levels of Eu3+ ion. Because 7F1 level population is much smaller than the 7F0 fundamental level population, the photon absorption intensity which involving 7F1 level is not evident in absorption spectra at room temperature. CaF2:Eu3+ crystal photoluminescence is determined by electron radiative transitions from 5 D0 level to the low energy levels 7F0, 7F2 and, perhaps, 7F3.
ABSORBȚIA OPTICĂ ȘI FOTOLUMINESCENȚA COMPOZITULUI Ga2S3-Ga2O3
(Universitatea Tehnică din Moldova, 2018-05-24) Сaraman, Iuliana; Evtodiev, Igor; Untilă, Dumitru; Dmitroglo, Liliana; Caraman, Mihail; Evtodiev, Silvia; Palachi, Leonid
În această lucrare sunt studiate absorbția optică și fotoluminescența cristalelor de Ga2S3, obținute prin metoda CVT în atmosferă de I2, și a compozitului Ga2S3–Ga2O3, obținut prin tratament termic al monocristalelor de Ga2S3 în atmosferă normală, la temperatura 1073K. S-a determinat că în rezultatul tratamentului termic de lungă durată (12 ore) suprafața cristalelor de Ga2S3 se acoperă cu un strat granular de Ga2O3. Din măsurări ale reflexiei difuze, lățimea benzii interzise a stratului de Ga2O3 de pe suprafața monocristalului Ga2S3 a fost aproximată ca fiind egală cu 4,47 eV. La 300K, marginea benzii de absorbție a cristalelor de Ga2S3 este formată din trei sectoare în care au loc tranziții optice directe cu lățimea benzii interzise egală cu 3,020 eV, 3,178 eV și 3,312 eV, iar la 80K - cu 3,196 eV, 3,302 eV și 3,422 eV. Spectrul de FL a cristalelor de Ga2S3 conține doar o singură bandă în regiunea roșu a spectrului, ce se interpretează ca emisie radiativă a stratului de Ga2S3, iar spectrul de FL al compozitului Ga2S3–Ga2O3 pe lângă banda roșie conține și o bandă în regiunea violet–albastru a spectrului, ce se identifică ca emisie radiativă în cristalele de oxid din compozitul Ga2O3-Ga2S3.
ABSORPTION AND PHOTOCONDUCTIVITY OF PbMoO4 CRYSTALS
(2011) Mușinschi, Valeriu; Caraman, Mihail; Mușinschi, Camelia
Spectral characteristics of the fundamental absorption coefficient and photoconductivity of PbMoO4 crystals with two polarizations in a temperature range of 77 to 300 K are investigated. The measurements of light transmitted in the polarized radiation for both orientations ( E|| C4 and E ┴ C4, C4 is the [001] crystallographic direction corresponding to the optical axis) of crystals are carried out. Spectral dependence of the absorption coefficient at the edge absorption (λ ≈ 0.3-0.5 μm) shows that the edge absorption is determined by indirect assisted by phonons transitions. The measured spectral dependence of the absorption coefficient at two orientations of polarization of light and the known theoretical dependence were combined and compared for determination the types of optical transitions in the fundamental absorption of PbMoO4 crystals. Limited energies Egind and Egdir for E|| C4 and E ┴ C4 were found. A detailed investigation of the fundamental absorption of PbMoO4 crystals is presented. A simple model of band structure of crystals is proposed. The photoconductivity of PbMoO4 single crystals is investigated on E|| C4 and E ┴ C4 at 300 K in the fundamental absorption spectral region of 2.8 to 5.0 eV. The spectral dependence of the photoconductivity is studied and discussed in conjunction with reported optical data. The monopolar character of the photoconductivity was confirmed by investigation time-of-flight motion of carriers. The present study reveals only the intrinsic photoconductivity in PbMoO4 pure single crystals. Spectral responses of the photocurrent through PbMoO4 crystals showed one spectral band in a range of 3.1 to 5.0 eV with maximums at 3.45 and 3.73 eV for E|| C4 and E ┴ C4, respectively. A good correlation of the photoconductivity bands at E|| C4 and E ┴ C4 with absorption bands at also polarizations has been demonstrated. The unipolar time-of flight motion of carriers was analyzed, and drift mobility of electrons and its temperature dependence were determined.
ABSORPTION SPECTRA AND EXTRINSIC PHOTOCONDUCTIVITY OF Cu AND Cd DOPED GaSe SINGLE – CRYSTAL FILMS
(2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Rusu, George G.
GaSe single crystal films were doped during growth process with Cu or Cd atoms with concentration ranged between 0.05 and 0.50 at. %. Single crystal films (with thickness about 0.5 μm) were obtained by mechanical splitting of bulk single crystals. Impurity concentration was determined using atomic emission spectroscopy. Spectral dependences of absorption coefficient and photoconductivity were studied in the range 1.50 eV – 3.70 eV. It was experimentally established that the absorption spectra have an additional absorption band and its corresponding energy depends on the nature (Cu or Cd) and concentration of the doping atoms. Also, independently on the presence of the dopant, other two absorption bands in the IR region are present.
CARACTERIZAREA STRUCTURALĂ A NANOFORMAȚIUNILOR DE Ga2O3 OBȚINUTE PRIN TRATAMENT TERMIC AL MONOCRISTALELOR Ga2S3
(Universitatea Tehnică din Moldova, 2018-05-24) Untilă, Dumitru; Sprincean, Veaceslav; Caraman, Mihail; Cojocaru, Ala; Lupan, Oleg; Tighineanu, Ion; Palachi, Leonid; Caraman, Iuliana
Prin tratament termic (TT), în atmosferă normală, al monocristalelor β-Ga2S3, cu rețea cristalină monoclinică, la temperaturi din intervalul 970-1170 K, timp de 3-6 ore, suprafața acestora se acoperă cu un strat omogen de β-Ga2O3, cu rețea cristalină monoclinică. La interfața monocristal-oxid se formează nanocristalite de Ga2O3 și Ga2S3. Structura cristalină și dimensiunile medii ale cristalitelor sunt analizate prin măsurări XRD, SEM și Raman.
CERCETAREA SPECTRELOR DE REFLECTANȚĂ A MICROOBIECTELOR BIOLOGICE LA DISTANȚA
(CEP USM, 2020) Chiriţa, Arcadii; Caraman, Mihail; Sprincean, Veaceslav; Paladi, Florentin
COMPORTAREA NICHELULUI ÎN ANTIMONIDUL DE GALIU CA DOPANT
(2016) Gheorghiță, Andrian; Gheorghiță, Eugeniu; Guțuleac, Leonid; Caraman, Mihail; Korolevski, Boris; Untilă, Pantelei; Melinte, Victoria
În lucrare se analizează particularitățile proprietăților fizice ale antimonidului de galiu dopat cu nichel pentru un diapazon vast de concentrații, până la 3% atomare și temperaturi (2÷300)K. Din analiza în ansamblu a efectelor galvanomagnetice, optice și de iradiere se evidențiază în premieră un șir de particularități ale proprietăților fizice determinate de interacțiunea purtătorilor de sarcină cu momentul magnetic excitat de structura energetică a substratului electronic incomplet 3d.
COMPOSITION AND SURFACE OPTICAL PROPERTIES OF GASE:EU CRYSTALS BEFORE AND AFTER HEAT TREATMENT
(2024) Sprincean, Veaceslav; Haoyi, Qiu; Tjardts, Tim; Lupan, Oleg; Untila, Dumitru; Aktas, Oral Cenk; Adelung, Rainer; Leontie, Liviu; Cârlescu, Aurelian; Gurlui, Silviu; Caraman, Mihail
This work studies the technological preparation conditions, morphology, structural char- acteristics and elemental composition, and optical and photoluminescent properties of GaSe single crystals and Eu-doped β–Ga2O3 nanoformations on ε–GaSe:Eu single crystal substrate, obtained by heat treatment at 750–900 ◦C, with a duration from 30 min to 12 h, in water vapor-enriched atmosphere, of GaSe plates doped with 0.02–3.00 at. % Eu. The defects on the (0001) surface of GaSe:Eu plates serve as nucleation centers of β–Ga2O3:Eu crystallites. For 0.02 at. % Eu doping, the fundamental absorption edge of GaSe:Eu crystals at room temperature is formed by n = 1 direct excitons, while at 3.00 at. % doping, Eu completely shields the electron–hole bonds. The band gap of nanostructured β–Ga2O3:Eu layer, determined from diffuse reflectance spectra, depends on the dopant concentration and ranges from 4.64 eV to 4.87 eV, for 3.00 and 0.05 at. % doping, respectively. At 0.02 at. % doping level, the PL spectrum of ε–GaSe:Eu single crystals consists of the n = 1 exciton band, together with the impurity band with a maximum intensity at 800 nm. Fabry–Perrot cavities with a width of 9.3 μm are formed in these single crystals, which determine the interference structure of the impurity PL band. At 1.00–3.00 at. % Eu concentrations, the PL spectra of GaSe:Eu single crystals and β–Ga2O3:Eu nanowire/nanolamellae layers are determined by electronic transitions of Eu2+ and Eu3+ ions.
CRYSTALLINITY AND OPTICAL PROPERTIES OF β-Ga2O3/Ga2S3 LAYERED STRUCTURE OBTAINED BY THERMAL ANNEALING OF Ga2S3 SEMICONDUCTOR
(2021) Sprincean, Veaceslav; Lupan, Oleg; Caraman, Iuliana; Untila, Dumitru; Postica, Vasile; Cojocaru, Ala; Gapeeva, Anna; Palachi, Leonid; Adeling, Rainer; Tiginyanu, Ion; Caraman, Mihail
In this work, the β-Ga2O3 nanostructures were obtained by thermal annealing in air of β-Ga2S3 single crystals at relatively high temperatures of 970 K, 1070 K and 1170 K for 6 h. The structural, morphological, chemical and optical properties of β-Ga2O3–β-Ga2S3 layered composites grown at different temperatures were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) as well as photoluminescence spectroscopy (PL) and Raman spectroscopy. The results show that the properties of obtained β-Ga2O3–β-Ga2S3 composites were strongly influenced by the thermal annealing tem- perature. The XRD and Raman analyses confirmed the high crystalline quality of the formed β-Ga2O3 nano- structures. The absorption edge of the oxide is due to direct optical transitions. The optical bandwidth was estimated to be approximately 4.34-4.41 eV, depending on the annealing temperature. Annealing of the β-Ga2S3 monocrystals at a higher temperature of 1170 K showed the complete conversion of the surface to β-Ga2O3. These results demonstrate the possibility to grow high quality β-Ga2O3–β-Ga2S3 layered composites and β-Ga2O3 nanostructures in large quantities for various applications such as gas sensing, non-toxic biomedical imaging, nonlinear optical, as well as power device applications. Micro and nanocrystallites present on the surface of the Ga2O3 layer contribute to a diffusion of the incident light which leads to an increase of the absorption rate allowing thus to reduce the thickness of the Ga2O3 layer, in which the generation of unbalanced charge carriers takes place. By decreasing the Ga2O3 layer thickness in such layered composites, the efficiency of photovoltaic cells based on such junctions can be increased.
DETERMINAREA STADIULUI DE VEGETAȚIE , A CALITĂȚII ȘI TIPULUI SOLULUI PRIN MĂSURĂRI ALE SPECTRELOR OPTICE
(CEP USM, 2020) Spincean, Veaceslav; Caraman, Mihail; Chirița, Arcadii; Paladi, Florentin
Sunt cercetate spectrele de reflexie difuză și de fotoluminescență pentru probe de sol și de frunze colectate la diferite stadii de dezvoltare. Stadiul de vegetație a frunzelor și tipul lor poate fi determinat atât după marginea benzii de reflexie difuză în intervalul 460÷520 nm, cât și după valoarea numerică a coeficientului de reflexie difuză în regiunea pantei dependenței Rd(λ).Este determinat intervalul spectral în care se poate stabili calitatea și tipul solului după mărimea coeficientului de reflexie difuză.
EFECTE OPTICE ÎN BICRISTALELE ALIAJELOR Bi1-xSbx (0,06≤x≤0,20)
(2006) Gheorghiță, Andrian; Postolachi, Igor; Caraman, Mihail
Sunt prezentate rezultatele studiului experimental al efectelor optice în monocristale şi bicristale Bi1-xSbx (0,06≤x≤0,20) pentru diapazonul spectral (1-6) eV. Comparându-se spectrele optice ridicate de la suprafeţele monocristalelor şi bicristalelor cu aceeaşi compoziţie, se evidenţiază un şir de particularităţi cauzate de influenţa suprafeţei de delimitare a cristalelor cu diferite unghiuri de dezorientare.
EFFECT OF OPTICAL COATING IN THE THIN-FILM SYSTEM OF CHALCOGENIDE GLASSY SEMICONDUCTOR-DIELECTRIC WHEN RECORDING THE HOLOGRAPHIC OPTICAL INFORMATION
(2015) Chirita, Arcady; Prilepov, Vladimir; Popescu, Mihai; Andries, Ion; Caraman, Mihail; Jidcov, Iurie
In this paper we show that the possibility of obtaining the high values of efficiency of relief-phase holographic gratings formed in the photo-thermoplastic recording process holds not only due to the non-sinusoidal profile form, but also due to the effect of optical coating that appears in the two-layer system of photo-semiconductor – deformed thermoplastic. Analyzed are the conditions, under which takes place the antireflection optical coating, reducing unwanted reflections from surfaces and thereby enhancing efficiency. It is shown that by careful choice of the thicknesses and refractive indices of the layers materials the efficiency of holographic gratings can be improved by 2-5 %.
EFFECTS OF IMPURITY BAND IN HEAVILY DOPED ZnO:HCl
(Elsevier, 2019) Colibaba, Gleb; Avdonin, A.; Shtepliuk, I.; Caraman, Mihail; Domagała, J.; Inculeț, Ion
A comparative study of properties of ZnO:HCl single crystals obtained by various methods is presented. Characterization by photoluminescence, optical and electrical measurements in the wide temperature range has allowed to analyze the energy spectra of Cl-containing stable defects in ZnO. Presence of shallow Cl donors, deeper donor complexes, incorporating several Cl atoms or stable H-Cl pairs and presence of compensating deep acceptors, attributed to VZnClO centers, are demonstrated. The presence of shallow donor impurity band, as well as strong dependence of its activation energy on the doping level is shown. The controversy of various models for estimation of this dependence is discussed. It is demonstrated, that 90% of this dependence is caused by feature of temperature dependence of Hall coefficient related to conductive impurity band, and a more correct equation for activation energy is suggested. An abnormally low efficiency of neutral impurity scattering of charge carriers and strong optical absorption in the near-IR spectral range are demonstrated and attributed to upper conductive impurity band of negatively charged donors with an extra electron.
ELECTRONIC TRANSPORT AND PHOTOCONDUCTIVITY OF POLYCRYSTALLINE CdSe THIN FILMS
(2006) Baban, Cristian Ioan; Caraman, Mihail; Rusu, Gheorghe Ioan
In this paper we have studied the electrical properties and photoconductivity of CdSe thin films, prepared by vacuum evaporation using the quasi-closed volume technique. It was found that the films were polycrystalline with a hexagonal (würtzite) structure and the crystallites are highly oriented with the (002) planes paralel to substrate. The film surface was investigated by means of SEM. The temperature dependence of electrical conductivity and Seebeck coefficient was studied. The films have n-type conduction. The spectral characteristics of photoconductivity were studied at 78 K and 300 K in the wavelength range 300–1100 nm. The bandgap energy (1.53–1.63 eV) calculated by using the Moss rule is smaller than those determined from absorption spectra. For the diffusion length, determined form absorption spectra and spectral dependence of photoconductivity, values between 0.22 and 0.29 μm were found.
ELLIPSOMETRIC STUDIES OF NANOMETRIC CdS AND CdTe FILMS
(2005) Caraman, Mihail; Evtodiev, Igor; Cuculescu, Elmira; Rusu, Marin; Salaoru, Iurie
The thickness of the layers CdS and CdTe grown on the Si, Ge, GaAs was determined from the analysis of the polarization ellipse of the reflected light from the surfaces of structures thin layers substrate. The thermal treatment of the samples CdS-CdTe in the presence of CdCl2 leads to the increasing of the crystalline phase from layers.
EXCITONIC AND RADIATIVE EFFECTS IN GaSe-C 60 STRUCTURES
(2007) Evtodiev, Igor; Lozovanu, Petru; Cuculescu, E.; Caraman, Mihail
The absorption and photoluminescence spectra of GaSe intercalated in C60 fullerene has been investigated at 293 K and 78 K. C60 and C60-toluene molecules intercalated in-between GaSe stratified “packing” cause the appearance of localized levels revealed in low energy wing of n=1 excitonic of the absorption and determine the exciton-phonon interaction. It has been established that C60 and C60 -toluene molecules form new recombination levels in GaSe crystals and determine the 1.7÷1.75 eV photoluminescence band.
FOTOLUMINESCENȚA – METODĂ EXPERIMENTALĂ DE CARACTERIZARE A FACTORILOR DE MEDIU
(CEP USM, 2024) Sprincean, Veaceslav; Chirița, Arcadi; Caraman, Mihail
The excitation and photoluminescent emission mechanisms of the substrates in atomic, molecular, and solid state are analyzed. Experimental methods for the research of nanostructured material and composites based on GaS/GaSe layered semiconductors and β-Ga2O3 oxides doped with Eu and biological objects, leaves of fruit trees, and elements of clean fish (Leuciscus Squalius) are proposed. The influence of UVB radiation on the FL spectral structure of the studied materials was determined.
FOTOREZISTOR PENTRU REGIUNEA ULTRAVIOLETĂ PE BAZĂ DE STRAT DIN NANOFIRE DE β-Ga2O
(CEP USM, 2022-11-10) Vatavu, Elmira; Sprincean, Veaceslav; Dmitroglo, Liliana; Gurău, Virginia; Caraman, Mihail
IMPACTUL ECONOMIC ȘI DE MEDIU AL PLATFORMEI eALERT PENTRU MONITORIZAREA ÎN TIMP REAL A CALITĂȚII AERULUI
(CEP USM, 2022-11-10) Leu, Alexei; Jalencu, Marian; Savva, Marianna; Caraman, Mihail; Paladi, Florentin; Sprîncean, Veaceslav
INVESTIGATION OF ENERGETIC STATES, DETERMINED BY Cu AND Cd IMPURITY ATOMS, AT SURFACE OF GaS AND GaSe MONOCRYSTALLINE LAYERS
(2007) Evtodiev, Igor; Cuculescu, E.; Postolachi, Vitalie; Caraman, Mihail
The reflection, absorption, photoconductivity, and thermally stimulated luminescence (TSL) spectra of GaS:Cu and GaSe:Cu, Cd single crystals have been studied. As a result of these investigations, the energy diagram of GaS and GaSe crystals after doping was determined.