Facultatea de Fizică şi Inginerie / Faculty of Physics and Engineering
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Item Fotoluminescenţa straturilor nanolamelare de GaSe obţinute prin intercalarea cu Cd [Articol](2012) Dmitroglo, Liliana; Evtodiev, Igor; Caraman, Iuliana; Nedeff, Valentin; Dafinei, AdrianS-au analizat spectrele de emisie fotoluminescentă a lamelor monocristaline p-GaSe cu concentraţia golurilor3•1014 cm-3 şi a lamelor intercalate cu Cd în fază de vapori la temperatura 500 °C. Durata tratamentului termic a fost de 20 şi 24 ore. Spectrul de emisie al cristalelor de GaSe conţine liniile excitonilor direcţi localizaţi cu energia de legătură ~6 meV, prima repetare fononică a acestora (ħωf =20 meV) şi banda de emisie a excitonilor indirecţi cu emisia fononilor cu energia 15 meV. Spectrul de emisie a compozitelor obţinute prin intercalarea lamelor de GaSe cu Cd se obţine în rezultatul suprapunerii benzilor de emisie a compusului CdSe şi banda impuritară a monoseleniurii de galiu. Structura spectrului FL depinde de durata tratamentului termic. La majorarea timpului de tratament se amplifică subbanda corespunzătoare compusului CdSe.Item Photoluminescence study of ZnO nanostructures grown on silicon by MOCVD [Articol](Elsevier, 2012) Sirkeli, Vadim; Nedeoglo, DmitriiZnO nanostructures with a size ranging from 20 to 100 nm were successfully deposited on (1 0 0)-Si substrates at different temperatures (500–800 °C) using MOCVD. It could be confirmed that the size of ZnO nanostructures decreased with increasing growth temperature. From photoluminescence (PL) studies it was found, that intensive band-edge PL of ZnO nanostructures consists of emission lines with maxima at 368.6 nm, 370.1 nm, 373.7 nm, 383.9 nm, 391.7 nm, 400.7 nm and 412 nm. These lines can be dedicated to free excitons and impurity donor-bound excitons, where hydrogen acts as donor impurity with an activation energy of about 65 meV. A UV shift of the band-edge PL line with increasing growth temperature of ZnO nanostructures was observed as a result of the quantum confinement effect. The results suggest that an increase of growth temperature leads to increased band-edge PL intensity. Moreover, the ratio of band-edge PL intensity to green- (red-) band intensity also increases, indicating better crystalline quality of ZnO nanostructures with increasing growth temperature.Item Anisotropy of the Exciton Processes in GaSe Crystals with Low S and Te Concentrations [Articol](American Scientific Publishers, 2009) Evtodiev, IgorThe anisotropy of the excitonical processes in the GaSe crystals and GaSe with small quantities of GaS(GaSe0.99S0.01) crystals has been studied through the optical specters (SO) and through the photoluminescence (PL) from the perpendicular surface on the symmetry axis C6 (E⊥C polarization) and from the flat surface parallel with the C6axis (E∥C and E⊥C polarization). The edge of the fundamental band of the GaSe crystals as well as of the GaSe0.99S0.01 and GaSe0.99Te0.01 crystals is formed at T = 78 K of the direct excitons' band. The width of the free excitons' band is determined by the processes of interaction between the excitons and optical and acoustic phonons. Phonons with energy of 17 meV and 27 meV participate to the formation of the edge towards small energies of the excitonic band in the GaSe crystals. The average energy of the phonons that participate to the formation of the excitonic absorbtion band in the GaSe crystals with small concentrations of S and Te equals 17 meV. Due to the mechanism of interaction of the excitons and phonons the integral absorption coefficient for the studied crystals (polarized E⊥C) is in small increase once with the temperature whilst the integral absorption coefficient in the maximum of the direct excitons' band. The n = 1 state is in diminution. For example, for the GaSe0.99Te0.01 crystals, α increases from 2700 at T = 78 K to 2025 cm−1 at 220 K. The edge towards small energies of the free excitons' band in the GaSe crystals and GaSe crystals with small quantities of S and Te is in a great concordance with Toyozowa's theory. The constant of interaction between the free excitons with phonons with an average energy of 135 cm−1 equals 0.9. Using the spectral characteristic of the reflection coefficient from the surface parallel to the C6 axis, there has been determined the refraction index placed in the center of the excitons n = 1 which equals 2.62 for GaSe and 2.58 and 2.55 respectively for the GaSe0.99S0.01 and GaSe0.99Te0.01 crystals. The shifting of the reflection specters towards big energies like ∼10 meV in a E⊥C polarization comparing to E∥C is determined by the difference of the oscillators' strength in these polarizations. The PL at T = 78 K specters from the surface parallel with the C6 axis (polarized E∥C) confirm the difference between the forces of the excitons' oscillators in the E∥C and E⊥C polarization. The intensity of PL bands, at the (001) surface as well as at the (100) surface depends on the excitation intensity by a function of a I = Ln force towards the emission bands of the direct and indirect free excitons the force factor is overlinear, and for the impurity nature bands it represents ∼0.5. The parameters that determine the width of the bands of excitonic PL is determined, considering the strong concentration of the structural faults at the (100) surface of the GaSe and GaSe0.99Te0.01 and GaSe0.99S0.01 GaSe crystals. Out of the spectral analysis I(L) the nature of the impurity bands has been determined, and from the PL specter structure there has been determined the energy of the accepting level which equals 93 meV from the maximum of the valence band of the GaSe crystals. Out of the analysis of the PL specter in a E∥C and E⊥C polarization (the (100) surface) it was stated that the process of emissional annihilation of the indirect excitons in the E∥C polarization takes place once with the emission of the phonons of a 38 meV energy whilst at the E⊥C polarization there are emitted phonons with an energy of 17 meV. GaSe with small concentrations of GaS and GaTe leads to the forming of a considerable concentration of localizing centers of the direct excitons and at the same time to the shifting towards small and big energies of the excitonic emission band (state n = 1) comparing to the GaSe crystals with a stoikiometric composition.Item Shallow donor states induced in ZnNe :au single crystals by lattice deformation [Articol](American Institute of Physics, 2008) Nedeoglo, Natalia; Nedeoglo, Dmitrii; Laiho, R.; Sirkeli, Vadim; Lähderanta, E.Photoluminescence (PL) spectra are investigated in n-ZnSe single crystals at different temperatures from 4.4 to 300 K immediately after doping with Au from melt of Se+Au or Zn+Au and after storage of the doped samples for 4 years in the dark under normal room conditions. Due to the formation of Aui interstitial donors in the n-ZnSe:Se:Au crystals with time, the origin of the near band edge PL changes from acceptor-bound to donor-bound excitons. Taking into account the results of PL characterization, we proposed that the Aui donors are generated by displacement of Au ions from regular lattice sites to interstitial sites with the help of lattice deformation forces. Transport measurements show dramatic increase in the electrical conductivity and the free electron concentration after storage of the n-ZnSe:Zn:Au crystals, thus confirming the proposed model.Item Effect of annealing of ZnSe:Cr crystals in Bi(Zn) melt on the intensity of radiation bands of Cr ions [Articol](2010) Kulyuk, Leonid; Nedeoglo, Dmitrii; Siminel, Anatolii; Sushkevich, ConstantinThe photoluminescence in visible and near-IR spectral range of ZnSe:Cr as-grown and annealed in Bi(Zn) melt crystals was investigaded at room temperature. It was established that heat treatment of the crystals in Bi melt does not lead to chrome extraction, but their annealing in Zn melt results in partial extraction into melt of chrome and some other shallow impurities.Item Optical properties of compounds with submicron points obtained through Ga2S3 intercalation with Cd [Articol](Universitatea de Stat „Alecu Russo“ din Bălţi, 2012) Racoveț, O.; Evtodiev, I.; Caraman, Iu.; Rotaru, I.; Lazăr, G.Luminescence and optical absorption spectra of Ga2S3 single crystals were investigated at temperatures of 78 K and 293 K. Optical band gap is equal to 3.27 eV and 3.457 eV at 293 K and 78 K respectively. Luminescence spectrum of single crystal lamellas at temperature of 78K consists of three bands with peaks at 2.04 eV, 1.84 eV and 1.66 eV.Native structural defects form deep recombination and electronic capture levels localized within the Ga2S3 band gap.