Browsing by Author "Vatavu, Sergiu"

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The analysis of current flow mechanism in CdS/CdTe heterojunction [Articol]
(Elsevier, 2007) Vatavu, Sergiu; Gașin, Petru
An analysis of current–voltage dependencies of CdS/CdTe heterojunction in the 78–370 K temperature range has been carried out. According to this analysis the current flow mechanism is determined by the tunneling processes through dislocations, which penetrate the heterojunction space charge region. The concentration of dislocations has been estimated as 2 · 105 cm− 2. The number of steps necessary for tunneling varies: 2.5 · 102–1.7 · 103. The characteristic energy has a weak temperature dependence (− 0.2 meV/K) and its value vary 120–200 meV. The increase of the annealing duration results in the decrease of the characteristic energy. The multistep tunneling processes through local centres, determined by impurity centres, interface states and defects in the space charge region, predominate at reverse biases. The number of tunneling steps is 1–4 · 102. The concentration of local centres (traps) in the heterojunction have been estimated as 2.37 · 105–1.63 · 106 cm− 3. The thermal annealing in the presence of CdCl2 up to 60 min does not modify the current flow mechanism in CdS/CdTe heterojunctions.
CdTe films by Elemental Vapor Transport [Articol]
(IEEE, 2013) Kendre, V.; Evani, V.; Khan, M.; Palekis, V.; Vatavu, Sergiu; Morel, D.; Ferekides, C.
The electro-optical properties of CdTe films deposited by Elemental Vapor Transport (EVT) are being investigated. The EVT process, unlike most processes currently used for the deposition of CdTe thin films, allows for the creation of excess Cd or excess Te conditions during the deposition, which can be used to influence the formation of defects and improve doping in CdTe. Using resistivity measurements, it has been demonstrated that the Cd/Te ratio used during the deposition process influences the incorporation of Cu in CdTe. Photoluminescence measurements have shown that the Cd/Te ratio also influences the formation of defect complexes in CdTe. Junctions formed with CdS suggest that the conductivity of CdTe can be adjusted p- or n-type by creating Te or Cd-rich conditions respectively. Structurally, the EVT-CdTe films have been found to be densely packed and highly oriented.
Composition, structure, and wear resistance of surface nanostructures obtained by electric spark alloying of 65G steel [Articol]
(2024) Yurchenco, E.; Ghilețchii, Gheorghe; Vatavu, Sergiu; Petrenco, Vladimir; Harea, Diana; Bubulincă, C.; Dikusar, Alexandr
A combination of X-ray diffraction and X-ray fluorescence analysis has shown that the strength- ened layer formed during electric spark alloying of 65G steel with a processing electrode made of the T15K6 hard alloy is a nanocrystalline material, the ratio of the crystalline and amorphous phases in which is achieved by changing the discharge energy. Since an increase in discharge energy leads to an increase in surface rough- ness and its amorphization, there is an optimal value of discharge energy at which maximum wear resistance of the resulting nanocomposites is achieved. At E = 0.2 J, the wear resistance of the hardened layer is 7– 10 times higher than the wear resistance of the untreated surface.
The copper influence on the PL spectra of CdTe thin film as a component of the CdS/CdTe heterojunction [Articol]
(Elsevier, 2009) Vatavu, Sergiu; Zhao, Hehong; Caraman, Iuliana; Gașin, Petru; Ferekides, Chris
The influence of annealing in the presence of CdCl2 and a thin copper layer deposited onto CdTe on the photoluminescence spectra of CdTe, as a component of CdS/CdTe heterojunction, has been studied for two excitation wavelengths: 0.337 μm and 0.6328 μm. The behavior of the PL was studied as a function of the measurement temperature and excitation intensity. At 0.6328 μm excitation, the interface PL consists of a known 1.43X band, and the chloride annealing enhances radiative transitions at 1.536 eV. The intensity of the 1.536 eV transitions increases when Cu is present. The PL of as-deposited CdTe films prepared in the presence of oxygen has the 1.45X band attenuated when excited with 0.337 μm excitation wavelength.
CuInSe2 nanostructures prepared by chemical close-spaced vapor transport for hybrid photovoltaic devices [Articol]
(Elsevier, 2017) Vatavu, Sergiu; von Morze, N.; Wiesner, S.; Moras, S.; Hinrichs, V.; Gasiorowski, J.; Zahn, D.R.T.; Lux-Steiner, Lux-Steiner; Rusu, M.
This work focuses on the fabrication of stoichiometric CuInSe2 nanostructures with controllable physical parameters of the nanocrystals suitable for hybrid organic/inorganic photovoltaics. CuInSe2 nanostructures were prepared by the chemical close-spaced vapor transport (CCSVT) method onto Mo/barrier/glass substrates by using an In2Se3 source material and Cu precursors. The In2Se3 source material was volatilized in H2 ambience with the addition of HCl vapors at 550 °C. Three different types of Cu precursors were used: (i) Cu thin films (6–250 nm thick) deposited by e-beam, (ii) Cu nanoparticles prepared by spray pyrolysis and (iii) Cu nanostructures formed by applying the nanosphere lithography (using a monolayer of 450 nm nanospheres). The CCSVT process parameters were varied to reveal the optimum conditions for the preparation of secondary phases free CuInSe2 nanostructures. The structural characterization by x-ray diffraction in both grazing incidence and Θ-2Θ configurations revealed the formation of CuInSe2 chalcopyrite phase independently on the applied precursor type. The elemental composition of the as-prepared CuInSe2 nanostructures was analyzed by laser ablation-inductively coupled plasma mass-spectrometry. In non-optimised processes, an excess of Se compared to stoichiometric composition was detected and attributed to the formation of molybdenum selenide and indium selenide phases. The formation of the latter secondary phases was suppressed by tuning the CCSVT deposition parameters.
Dispozitive din fosfură de indiu bazate pe efectul fotovoltaic [Articol]
(CEP USM, 2021) Botnariuc, Vasile; Gorceac, Leonid; Coval, Andrei; Vatavu, Sergiu; Cinic, Boris; Rotaru, Corneliu; Raevschi, Simion
Homo- și heterojoncțiunile din p - InP și n - CdS au fost confecționate aplicând metoda epitaxiei din faza gazoasă în volum deschis, în sistem de cloruri, metoda HVPE (Hydride Vapour Phase Epitaxy) și tehnologia în volum cvasiînchis, în hidrogen. S-a stabilit că eficiența celulelor fotovoltaice pe bază de heterojoncțiuni n+CdS-p-p+InP cu suprafața fotoactivă de 3 cm2 și pe homojoncțiuni n+- p- p+InP (1 cm2) constituie 12% și,respectiv, 7,3% în condiții de iluminare standard, AM1 (1000 Wm-2). Eficiența cuantică externă maximală constituie 75-80% pentru heterojoncțiunea n+CdS - p- p+ InP și 70% pentru homojoncțiunea n+-p-p+InP în intervalul (600-900) nm al spectrului electromagnetic. Fotosensibilitatea absolută maximă de 0,51 A/W este caracteristică pentru heterojoncțiunea n+CdS - p- p+InP cu strat epitaxial intermediar(p = 6,51016 cm-3). Astfel de heterojoncțiuni pot fi utilizate pentru elaborarea fotodetectorilor în intervalul VIS.
The effect of Cu and annealing treatments on CdS/CdTe heterostructures studied with QE and photocurrent relaxation techniques [Articol]
(IEEE, 2008) Zhao, H.; Vatavu, Sergiu; Caraman, Iu. M.; Gasin, P. A.; Morel, D. L.; Ferekides, C. S.
The photo-emf (photocurrent) decay al light impulses excitation (in longitudinal configuration) has been used to determine the lifetime of the non-equilibrium charge carriers recombining in different regions of the CdS/CdTe Cu containing heterojunction at different temperatures and biases. The photosensitivity spectral distributions have been investigated for samples with different technological variations.
Fotodetector pe heterojoncțiune din fosfură de indiu [Articol]
(CEP USM, 2020) Gorceac, Leonid; Botnariuc, Vasile; Coval, Andrei; Vatavu, Sergiu; Cinic, Boris; Raevschi, Simion; Rotaru, Corneliu
The investigation of TCO/CdS/CdTe heterojunctions by C-U and C-f measurements: Experiment and modeling [Articol]
(IEEE, 2013) Rotaru, Corneliu; Vatavu, Sergiu; Fedorov, Vladimir; Gașin, Petru; Lux-Steiner, Martha; Ferekides, Chris; Rusu, Marin
The paper presents the analysis of C-U and C-f data obtained in the 1 kHz-10 MHz range for CdS/CdTe heterojunctions prepared by CSS onto (ZnO:Al, ZnO:Al/i-ZnO)/Glass substrates. An attempt to model the physical properties of the heterojunctions is made. A small signal analysis has been carried out to establish the equivalent circuit of device. The unusual capacitance behavior of the ZnO based heterojunction is attributed to presence of the barrier at the back contact.
Kinetics of photoconductivity and photoluminescence of CdS/CdTe heterojunctions [Articol]
(2005) Vatavu, Sergiu; Caraman, Iuliana; Gashin, Peter A.
The photoluminescence and absoption spectral distribution close to the edge of fundamental band were studied in the CdS and CdTe films components of the CdS/CdTe heterojunctions. Recombination level energetic position was determined. The annealing of the CdS/CdTe heterojunctions in presence of CdCl2 results in formation of new recombination levels, revealed by a luminescent band in the energy range of 1.6-1.7 eV and by the shift of the impurity band maximum to the red wavelength region by 50 meV.
Kinetics of photoconductivity and photoluminescence of CdS/CdTe heterojunctions [Articol]
(2005) Vatavu, Sergiu; Caraman, Iuliana; Gasin, Petru
The photoluminescence and absoption spectral distribution close to the edge of fundamental band were studied in the CdS and CdTe films components of the CdS/CdTe heterojunctions. Recombination level energetic position was determined. The annealing of the CdS/CdTe heterojunctions in presence of CdCl2 results in formation of new recombination levels, revealed by a luminescent band in the energy range of 1.6-1.7 eV and by the shift of the impurity band maximum to the red wavelength region by 50 meV.
Metalorganic aerosol deposition technique [Articol]
(CEP USM, 2022-11-10) Şapoval, Oleg; Belenciuc, Alexandr; Vatavu, Sergiu
Metalorganic aerosol deposition: the building of oxide films [Articol]
(CEP USM, 2022-11-10) Şapoval, Oleg; Belenciuc, Alexandr; Vatavu, Sergiu
Negative differential resistance in ZnO-based resonant tunneling diodes [Articol]
(IEEE, 2019) Sirkeli, Vadim; Vatavu, Sergiu; Yilmazoglu, Oktay; Preu, Sascha; Hartnagel, Hans
We present the results of a simulation study of resonant tunneling transport of non-polar m-plane ZnO/ZnMgO quantum structures with double and triple quantum barriers. It is found that in current density-voltage characteristics of such devices a region is present with negative differential resistance and this feature can be used for the generation of terahertz waves. The best performance at room temperature with output power of 912 μW @ 1 THz is derived for the non-polar m-plane ZnO/ZnMgO structures with triple quantum barriers and optimized design.
Oxygen-dependent formation of VO2 thin films by metalorganic aerosol deposition [Articol]
(Universitatea Tehnică din Moldova, 2018-05-24) Belenchuk, Alexandr; Shapoval, Oleg M.; Zasavitsky, E.; Vatavu, Sergiu; Kiritsa, Arcady; Moshnyaga, V.
We report on the growth and properties of vanadium dioxide (VO2) on amorphous and oriented substrates by the low-cost and industry-oriented method of metalorganic aerosol deposition (MAD). X-ray diffraction and temperature- dependent Raman spectroscopy confirm formation of single phase tin films. Abrupt change of resistance by 5×103 times and steep drop of infrared transmission from 49 to 10% at 1700 nm pave the way for application of MAD technique in fabrication of VO2-based optoelectronic and thermochromic devices, such as “smart windows”.
Particularități structurale ale filmelor ZnSnN2 [Articol]
(CEP USM, 2024) Ghilețchii, Gheorghe; Narolschi, Igor; Rotaru, Corneliu; Rusu, Marin; Vatavu, Sergiu
ZnSnN2 is composed of common non-toxic elements. It exhibits promising optoelectronic properties for application in photoelectric conversion and electromagnetic radiation detection devices. However, the available data on its physical properties are incomplete or contradictory. In this work, we prepare ZnSnN2 thin films and study in detail their structural properties as functi on of deposition conditions. DC magnetron sputtering was used because it allows the preparation of thin polycrystalline ZnSnN2 films on large areas and it can be easily upscaled. The films were prepared using targets of various atomic [Zn]/[Sn] ratios in nitrogen atmosphere at temperatures ranging from 30 to 300°C. Structural analysis, using Grazing Incidence X-ray Diffraction (GI XRD) and X-ray Reflectivity (XRR), revealed the formation of polycrystalline ZnSnN2 films with a wurtzite crystal structure. The obtained structural parameters were found to be influenced by the substrate temperature and elemental concentration in the target. These findings will be used for optimization of the manufacturing process for desired film characteristics.
Photoluminescence studies of the interface of CdS/CdTe heterojunctions [Articol]
(2009) Vatavu, Sergiu; Zhao, Hehong; Caraman, Iuliana; Gașin, Petru; Morel, Don; Ferekides, Chris
The photoluminescence analysis of CdS/CdTe interface layer of CdS/CdTe heterojunction has been carried out in the 15- 100 K temperature range. An attempt to correlate observed PL features with the CdSxTe1–x layer presence at the interface of the heterojunctions was made. It is assumed the 1.525 eV band has an excitonic origin and 1.37X PL band is a characteristic impurity band due to CdSTe layer. Comparative plots showing SnO2/CdTe PL spectra are given as well.
Photoluminesence studies of CdTe/SnO2 and CdTe/CdS heterojunctions: The influence of oxygen and the CdCl2 heat treatment [Articol]
(Elsevier, 2011) Vatavu, Sergiu; Zhao, Hehong; Caraman, Iuliana; Gasin, Petru; Ferekides, Chris
The influence of oxygen and annealing in the presence of CdCl2 on the photoluminescence (PL) spectra of CdTe, component of SnO2/CdTe heterojunction (HJ), has been studied in a temperature range of 17–100 K. The changes in the photoluminescence spectra were studied as a function of excitation intensity. Analysis of the PL spectra was carried out with considerations of spectra obtained from CdS/CdTe heterojunctions. CdTe side PL (SnO2/CdTe HJ) consisted of 1.450 eV-DA defect band and 1.243 eV band (17 K). Annealing resulted in the disappearance of 1.243 eV band in oxygen containing samples. Interface PL for the unannealed samples consisted of mainly the 1.264 eV and a trace of the defect band. The CdCl2 treatment is responsible for an almost symmetrical 1.416 eV band.
Proprietățile fizice ale straturilor de ZnO preparate pe Si prin metoda hidrotermală [Articol]
(CEP USM, 2020) Raevschi, Simion; Gorceac, Leonid; Botnariuc, Vasile; Braniște, Tudor; Vatavu, Sergiu
Straturi subțiri, de nucleație și proprii, de ZnO au fost sintetizate pe Si prin metoda hidrotermală din soluțiile compușilor zincului folosindu-se solvenții: apă, apă + etanol, apă + metanol, apă + propanol, apă + acetonă, etanol, propanol, metanol.La prepararea straturilor de nucleație s-a folosit acetatul de zinc dihidrat, Zn(CH3COO)2·2H2O. Depunerea straturilor proprii de ZnO pe structurile nucleatea avut loc prin fierberea lor în soluție apoasă de Zn(NO 3)2+ KOH. În lucrare sunt date caracterizări structurale și morfologice ale straturilor buffer obținute,fiind demonstrată eficiența stratului buffer în prepararea straturilor de GaN.
Radiative recombination mechanisms in CdTe thin films deposited by elemental vapor transport [Articol]
(Elsevier, 2015) Collins, Shamara; Vatavu, Sergiu; Evani, Vamsi; Khan, Md; Bakhsh, Sara; Palekis, Vasilios; Rotaru, Corneliu; Ferekides, Chris
A photoluminesence (PL) study of the radiative recombination mechanisms for CdTe films deposited under different Cd and Te overpressure by elemental vapor transport is presented. The experiment and analysis have been carried out in the temperature range of 12–130 K. The intensity of the PL laser excitation beam was varied by two orders of magnitude. It has been established that the bands in the 1.47–1.50 eV are determined by transitions involving shallow D and A states and the 1.36x–1.37x eV band is due to band to level transitions. Deep transitions at 1.042 eV and 1.129 eV are due to radiative transitions to levels determined by CdTe native defects.