EV Concentration Research Articles

Preparation and Characterization of Electrodeposited-Annealed CuInSe2 Thin Films for Solar Cells

CuInSe2 (CIS) films with good crystalline quality were synthesized by electrodeposition followed by annealing in Se vapor at 530 °C. The morphology, composition, crystal structure, optical and electrical properties of the CIS films were investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, UV-VIS-NIR spectroscopy, and admittance spectroscopy. The results revealed that the annealed CIS films had chalcopyrite structure and consisted of relatively large grains in the range of 500–1000 nm and single grain of films extend usually through the whole film thickness. The band gap of CIS films was 0.98 eV and carrier concentration was in the order of 1016 cm−3 after etching the Cu-Se compounds on the film surface. Solar cells with the structure of AZO/i-ZnO/CdS/CIS/Mo/glass were fabricated. Current density vs. voltage test under standard reported condition showed the solar cells with an area of 0.2 cm2 had a conversion efficiency of 0.96%. The underlying physics was also discussed.

Spectroscopy of the tungsten plasma produced by pulsed plasma-ion streams or laser beams

The paper reports on experiments, which concerned studies of plasma produced from a tungsten (W) target bombarded by powerful (ca. 5 μs, 1–5 MW/cm 2) plasma-ion streams in RPI-IBIS plasma accelerator, and a similar target irradiated with intense Nd:YAG laser pulses (0.5 J, 3 ns, ca. 5.3 × 10 9 W/cm 2) in another vacuum chamber. In both experiments optical measurements were performed with a Mechelle ®900 spectrometer, which enabled the spectrum from 300 nm to 1100 nm to be recorded, and different WI- and WII-lines to be identified. From space- and time-resolved measurements of those lines, basic W-plasma parameters were estimated. During W-plasma expansion the electron temperature was found to be 0.8–1 eV and electron concentration (2–8) × 10 16 cm −3. The emission of higher-ionized W-ions (up to W +6) was confirmed by measurements with an ion-energy analyzer. Structural changes in the irradiated targets were investigated with an optical microscope and SEM.

Deep-Level Defects in Nitrogen-Doped 6H-SiC Grown by PVT Method

ABSTRACTAn effect of the nitrogen concentration on the concentrations of deep-level defects in bulk 6H-SiC single crystals is investigated. Six electron traps labeled as T1A, T1B, T2, T3, T4 and T5 with activation energies of 0.34, 0.40, 0.64, 0.67, 0.69, and 1.53 eV, respectively, were revealed. The traps T1A (0.34 eV) and T1B (0.40 eV), observed in the samples with the nitrogen concentration ranging from ∼2×1017 to 5×1017 cm−3, are attributed to complexes formed by carbon vacancies located at various lattice sites and carbon antisites. The concentrations of traps T2 (0.64 eV) and T3 (0.67 eV) have been found to rise from ∼5×1015 to ∼1×1017 cm−3 with increasing the nitrogen concentration from ∼2×1017 to ∼2.0×1018 cm−3. These traps are assigned to complexes involving silicon vacancies occupying hexagonal and quasi-cubic sites, respectively, and nitrogen atoms. The trap T4 (0.69 eV) concentration also substantially rises with increasing the nitrogen concentration and it is likely to be related to complexes formed by carbon antisites and nitrogen atoms. The midgap trap T5 (1.53 eV) is presumably associated with vanadium contamination. The presented results show that doping with nitrogen involves a significant change in the defect structure of 6H-SiC single crystals.

Magneto-optical and transport studies of ZnO-based dilute magnetic semiconductors

Thin film samples of ZnO doped with V were grown on sapphire substrates by pulsed laser deposition (PLD). The magnetization was measured by SQUID magnetometry and the films were found to be ferromagnetic at room temperature. The transmission, Faraday rotation and magnetic circular dichroism were measured as a function of frequency at room temperature over an energy range of 1.5–4.0 eV and carrier concentrations were determined from Hall effect measurements. Clear magneto-optical signals that are ferromagnetic in origin were observed at the ZnO band edge and the optimal conditions for observing large ferromagnetic magneto-optic signals are discussed.

Electrical analysis and interface states evaluation of Ni Schottky diodes on 4H‐SiC thick epilayers

AbstractThis work has been focused on characterization of thick 4H‐SiC layers produced by sublimation epitaxy. Nickel Schottky contacts have been fabricated in order to characterize the grown material and evaluate the interfacial layer between metal and semiconductor. The characterization study includes current–voltage and capacitance–voltage high temperature measurements, from which Schottky barrier, net donor concentration and on‐state resistance values have been extracted. The diodes show a typical behavior of J–V and C–V curves with temperature, with Schottky barrier heights of 1.3 eV ÷ 1.4 eV and net donor concentration of 4 × 1015 cm–3 ÷ 1 × 1016 cm–3. From the Bardeen's model on reverse J–V, the density of states of the interfacial layer has been estimated to 7 × 1011 eV–1 cm–2 ÷ 8 × 1011 eV–1 cm–2, a value that is similar to the density of states of oxide layers in deliberated MOS structures realized on the same epilayers. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoconductive and photovoltaic properties of CVD diamond films

Free-standing undoped polished MPCVD diamond films (200–300 μm thick) have been used to fabricate photoconductive and photodiode structures for UV light detection. Multifinger planar Au/Cr electrodes of the photoconductive structures were realized on the diamond surface by lithographic technique. The sandwich semitransparent Ni electrodes of the photodiode structures were deposited by a magnetron sputtering on both sides of the diamond films. The films showed conductivity less then 10 −15 S/cm at room temperatures with activation energy close to 1 eV and low concentration of the electrically active defects according to charge-based deep-level transient spectroscopy (Q-DLTS). The distinct trap levels with activation energy of 0.5, 0.7 and ∼1.3 eV were evaluated. Planar photoconductive detectors with electrodes on the growth side showed the dark current below 1 pA at 10 V bias voltage with spectral discrimination ratio higher than 5×10 5 (at 50 V bias) in the 210–270 nm wavelength range. Two orders of magnitude lower responsivity were found on the nucleation side. The sandwich photodiodes showed a sharp cut-off in photoresponse at 220 nm and photovoltage as high as 2.3 V in open circuit regime.

Electrical conductivity and Hall mobility in p-type TlGaSe 2 crystals

Systematic dark electrical conductivity and Hall mobility measurements have been carried out in the temperature range of 200–350 K on p-type TlGaSe 2 crystals. The analysis of the temperature-dependent electrical conductivity and carrier concentration reveals the extrinsic type of conduction with an acceptor impurity level located at 0.33 eV, and donor and acceptor concentrations of 9.0×10 15 and 1.3×10 16 cm −3, respectively. A hole and electron effective masses of 0.520 m 0 and 0.325 m 0, respectively, with a donor to acceptor compensating ratio of 0.69 are also being identified. The Hall mobility is found to be limited by the hole–phonon short-range interactions scattering with a hole–phonon coupling constant of 0.17.

Carrier Scattering Mechanisms in GaS0.5Se0.5 Layered Crystals

Systematic dark electrical resistivity and Hall mobility measurements have been carried out in the temperature range 150-400 K on n-type GaS 0.5 Se 0.5 layered crystals. The analysis of temperature dependent electrical resistivity and carrier concentration reveals the extrinsic type of conduction with a donor impurity level located at 0.44 eV, donor and acceptor concentrations of 3.4×10 17 and 4.1×10 16 cm - 3 , respectively, and an electron effective mass of 0.41 m 0 . The Hall mobility is limited by the electron-phonon short-range interactions scattering at high temperatures combined with the ionized impurity scattering at low temperatures. The electron-phonon short-range interactions scattering mobility analysis reveals an electron-phonon coupling constant of 0.25 and conduction band deformation potential of 5.57 eV/A.

Influence of the screening effect on passivation of p-type silicon by hydrogen

Passivation of p-Si by hydrogen through its diffusion was simulated by solving diffusion-kinetic equations with allowance made for hydrogen-acceptor-pair formation, internal electric field, and the screening effect. Screening of hydrogen and acceptor ions by free carriers leads to a decrease in the radius of interaction between the ions and to the weakening of the concentration dependence of hydrogen diffusivity in heavily doped Si. At a binding energy of the pairs of 0.70–0.79 eV, calculated and experimental concentration profiles of holes and the hydrogen-acceptor pairs are in agreement over a wide range of boron concentrations, from 4×1014 to 1.2×1020 cm−3. The radius of the Coulomb interaction of hydrogen and boron ions is 35 A in lightly doped Si and decreases as the dopant concentration increases.

Growth of spinel zinc thioindionate thin film by single-source MOCVD

Binuclear complex Et2In(S2CNEt2)·ZnEt(S2CNEt2) was prepared by the reaction of indium triethyl (InEt3) with zinc bis(diethyldithiocarbamate) (Zn(S2CNEt2)2). This complex was an excellent single-source precursor for low-pressure MOCVD growth of spinel ZnIn2S4 thin layers. Optimal growth conditions for strongly 〈111〉 oriented spinel ZnIn2S4 thin film were as follows: substrate temperature Tsub, 400°C; source temperature Tso, 60°C; carrier gas N2; carrier flow rate Rflow (total pressure 1.3 kPa), 0.3 l/min. The spinel ZnIn2S4 thin film grown on an Si(111) substrate showed an n-type conduction nature and excellent semiconductor properties, such as optical bandgap energy (Eg)opt, 2.72 eV, mobility μ, 25.4 cm2/Vs, and carrier concentration N, 3.7×1016 cm−3.

Defect states in strain-relaxed Si0.7Ge0.3 layers grown at low temperature

Two shallow hole traps dominate the deep level transient spectroscopy (DLTS) data for strain-relaxed Si0.7Ge0.3 layers grown on Si(100) by ultrahigh vacuum chemical vapor deposition at temperatures ⩽560 °C. The trap energy levels are at Ev+0.06 and Ev+0.14 eV and trap concentrations are ⩽5×1014 cm−3 in relaxed layers having threading dislocation densities of 2–4×107 cm−2. A logarithmic dependence of the filling rate indicates that these traps are associated with extended defects and this is confirmed by their absence in a sample having no dislocations. The annealing temperature of the DLTS peaks is consistent with the interpretation of these traps as states of defect complexes at dislocations, rather than intrinsic dislocation states or isolated defect complexes. The trap concentrations are proportional to the oxygen concentration in the film, suggesting that oxygen may be a constituent of the defect complex.

Characterization of high fluence neutron induced defect levels in high resistivity silicon detectors using a laser deep level transient spectroscopy (L-DLTS)

Neutron irradiated high resistivity (4–6 kΩ-cm) silicon detectors in the neutron fluence ( Φ n) range of 5 × 10 11 n/cm 2 to 1 × 10 14 n/cm 2 have been studied using a laser deep level transient spectroscopy (L-DLTS). It has been found that the A-center (oxygen-vacancy, E c = 0.17 eV) concentration increases with neutron fluence, reaching a maximum at Φ n ≈ 5×10 12 n/cm 2 before decreasing with Φ n. A broad peak has been found between 200 K and 300 K, which is the result of the overlap of three single levels: the V-V − ( E c = 0.38 eV), the E-center (P-v, E c = 0.44 eV), and a level at E c = 0.56 eV that is probably V-V 0. At low neutron fluences ( Φ n < 5 × 10 12 n/cm 2), this broad peak is dominated by V-V − and the E-centers. However, as the fluence increases ( Φ n ≥ 5 × 10 12 n/cm 2), the peak becomes dominated by the level of E c = 0.56 eV.

Hole traps in Be-doped MBE InGaP

Deep level transient spectroscopy has been used to characterise p-type In/sub 0.48/Ga/sub 0.52/P grown by gas-source molecular beam epitaxy. Three hole traps were detected in Be-doped material, with thermal emission energies of approximately 0.2-0.3, 0.7-0.8, and 1.0-1.3 eV and trap concentrations less than 4*10/sup 14/, 9*10/sup 14/, and 10/sup 15/ cm/sup -3/, respectively.

Optical Properties of MoRe Alloys

AbstractMeasurements of the reflectivity [R(pure) − R(alloy)] of 3000 Å thick Mo‐Re films for photon energies between 0.54 and 6 eV and Re concentrations around 4 at% are reported. The resistivity of the films is also measured and 2.2 μΩ cm/at% is found for the change in resistivity of the films upon alloying. The frequency dependence of the optical dielectric function, ϵ = ϵ1 + iϵ2, of the pure and alloy films is found from a Kramers‐Kronig analysis of the reflectivity data. The results for the differential optical conductivity ω[ϵ2(alloy) minus;ϵ2(pure)] are discussed in terms of the changes in the electronic structure of Mo upon alloying with Re. It is concluded that the rigid band model is inapplicable in this case. The results of the optical data and the resistivity are consistent with the formation of localized Re d‐levels 0.8 eV below the Fermi level where pure Mo has a low density of electronic states. However, this interpretation is not unique and further studies are suggested.

Virtual-Bound-State—Induced Optical Absorptivity in CuNi Alloys

The optical absorptivity of dilute CuNi alloys at 4\ifmmode^\circ\else\textdegree\fi{}K has been measured for photon energies between 0.1 and 3 eV and Ni concentrations between 0.5 and 4 at.%. The data are interpreted in terms of the Anderson model of magnetic impurities. A virtual bound state is found 0.75\ifmmode\pm\else\textpm\fi{}0.02 eV below the Fermi energy with a half-width of 0.27\ifmmode\pm\else\textpm\fi{}0.02 eV. The extra absorptivity of the alloy is found to arise predominantly from the scattering of the host conduction electrons in the presence of the virtual bound state.