Kubilay Kutlu

Conduction mechanism analysis in β-FeSi2/n-Si heterojunction through J–V–T measurement

The p-type iron disilicide (β-FeSi2) semiconductor was formed at room temperature without heat treatment due to the superiority of the employed unbalanced magnetron sputtering technique on n-type crystalline silicon (n-Si), and conduction mechanism(s) of the resulting p-β-FeSi2/n-Si heterostructure was investigated by current density–voltage–temperature (J–V–T) measurement in darkness condition under vacuum after evaporation of both chromium (Cr) and gold (Au) metals as the front electrode. Two different current mechanisms seemed to be dominant on Cr/β-FeSi2/n-Si and Au/β-FeSi2/n-Si heterostructures, respectively. The transition of one mechanism to another occurred in a particular bias voltage range: between ~3 kT/q and 0.3 V, the multistep tunneling capture emission (MSTCE) mechanism became dominant with an activation energy (EA) around 0.3 eV for both forward and reverse directions of bias and interpreted as an Fe impurity. Also, the reverse current density had a square-root dependence on reverse bias voltage, thus proposing generation current. In this frame, at an EA of 0.3 eV above the valance band edge denoted the efficient trap level for the recombination–generation mechanism in the β-FeSi2 semiconductor or at the interface of the β-FeSi2/Si heterojunction. Subsequently, as the second mechanism, space charge limited current (SCLC) started at a high forward bias voltage region (from 0.65 V to 1 V), where the power of the bias (m) changed from high to low value as the ambient temperature was increased (110 K to 380 K). A further increase in bias voltage (above 1 V) yielded a series resistance region where thermally activated current was observed, representing a conduction band offset, ΔEc. Its value was determined as 0.16 eV, consistent with the announced values in the literature.

Nano-crystal V2O5nH2O sol-gel films made by dip coating

Vanadium pentoxide (V2O5) sols were formed through a reaction of V2O5 powder with hydrogen peroxide (H2O2); the solution evolves in time, beginning from diperoxo species, decavanadic acid spedies at intermediate step and finally polymerization of V2O5 nH2O sols. From the sols, V2O5 thin films were succesfully fabricated on indium-tin-oxide (ITO) coated and corning glass substrates by sol-gel dip coating method. X-ray diffraction pattern of the films at hand exhibited a series of (00l) peaks that confirmed both some preferred orientation of a layered structure in one dimension stacking of ribbons perpendicular to the substrate and ability of employed method to produce nano crystal of V2O5 nH2O sol-gel films. The basal distance,d, (increased with an amount of water in the sols in steps of 2.8 A, attributed van der Walls water molucule diameter), was determined as 10.6-17.2 A that corresponded n in between 2-6. On the other hand, the average particle sized, D, of the precipitated nanocrystals were calculated through Sherrers formula and determined around 15-28 nm that consisted with AFM and SEM analysis. The electrochemical studies by cyclic voltametry showed the Li + intercalation capacity, reversibility and stability. Finally, the simultaneous TG and DTA on the as prepared films indicated two stage mass loss.

Influence of water expulsion on structural properties of V2O5nH2O sol-gel films

Vanadium pentoxide (V2O5) thin films were prepared by dip-coating sol-gel technique by dissolving V2O5 powder in hydrogen peroxide (H2O2), the solution, leading V2O5 sol-gel nanocrystalline films. The films under preparation were exposed different hydration states, n, varying from 6 to 2 determined via XRD measurements. Water molecules were adsorbed in layered structure of V2O5 oxide films and hence the adsorption could be described as an intercalation in which the molecules were trapped in some cavities and were not randomly oriented. (00l) pattern of X-ray diffraction analysis not only approved the one dimensional stacking of V2O5 ribbons but also served to determine basal distance from which hydration state was guessed. The basal distance were calculated from the position of 00l peak and varied from 17.2 A to 10.6 A that corresponded to n=6 to n=2 respectively. During the water departure upon heating, the basal distance reduced by steps of 2.8 A (diameter of vander Walls water molecule) towards 8.8 A v...

Ionic conduction in different hydrated V2O5 film

Because of the layered structure of vanadium pentoxide films (V2O5), approved by XRD measurement, sensitized from different hydrated V2O5.nH2O sols, demonstrated anisotropic conductivities in current voltage (I-V) measurement. Conductivity values, originated from electronic and ionic conductions, differed provided that measurements were performed in a direction parallel to the ribbons rather than perpendicular to them. The overall electrical conductivity of V2O5nH2O sols mainly depended on the hydration state n and the amount of reduced V4+ ions in which n was determined around 4-6 [1] from the basal distance (17.6 A) through XRD measurement while V4+ ions were determined through FTIR analysis. Electronic conduction prevailed in dehydrated V2O50.5H2O sols whereas non-stoichiometric vanadium pentoxide was a mixed-valence compound and its electronic properties arised from electron hopping between V4+ and V5+ ions so-called “small polaron model”. Indeed, reduction/oxidation peaks in lithium (Li+) intercalati...

Structural Modification of Sol-Gel Synthesized V2O5 and TiO2 Thin Films with/without Erbium Doping

Comparative work of with/without erbium- (Er-) doped vanadium pentoxide (V2O5) and titanium dioxide (TiO2) thin films were carried out via sol-gel technique by dissolving erbium (III) nitrate pentahydrate (Er(NO3)3·5H2O) in vanadium (V) oxoisopropoxide (OV[OCH(CH3)2]3) and titanium (IV) isopropoxide (Ti[OCH(CH3)2]4). Effect of Er doping was traced by Fourier transform IR (FTIR), thermogravimetric/differential thermal (TG/DTA), and photoluminescence measurements. UV-Vis transmission/absorption measurement indicated a blue shift upon Er doping in V2O5 film due to the softening of V=O bond while appearance of typical absorption peaks in Er-doped TiO2 film. Granule size of the films increased (reduced) upon Er substitution on host material compared to undoped V2O5 and TiO2 films, respectively.

Excess Capacitance Due to Minority Carrier Injection in CrSi2/p-Type Crystalline Si Isotype Junction

Excess current and capacitance phenonema were observed for the first time on a CrSi2/p-type crystalline silicon junction produced by cathodic arc physical vapor deposition. The heterojunction was investigated by current–voltage–temperature (I–V–T) and capacitance (conductance)–voltage/temperature (C,G–V/T) measurements for the purpose of studying transport and storage features. Excess current, manifested as a crossover at a large forward bias, was observed in I–V–T curves since minority carriers injected into the quasi-neutral region of p-c-Si were neutralized by majority carriers supplied from the p-c-Si semiconductor side. This phenomenon, known as conductivity modulation, appeared distinctly as a hump in C–V/T curves (storage property); a sharp rise in capacitance towards a maximum value as forward bias increased and the subsequent fall after a specific value. For reverse and low forward bias regions, where minority carrier injection was negligible, geometrical junction capacitance and a shoulder in C–V/T curves were observed. In the voltage range where the peak was observed in C–V/T measurements, trap-assisted tunneling recombination generation and space-charge-limited current (SCLC) mechanisms were determined in the CrSi2/p-c-Si isotype junction. Traps introduced during tunneling were identified as bulk point defects due to the chromium–boron (Cr–B) complex for the CrSi2/p-c-Si junction on the Si side by I–V–T and C(G)–T analyses. This finding seemed to be in agreement with a recent DLTS [Deep Level Transient Spectroscopy] measurement in terms of both energy depth (0.26 eV) and bulk nature. Finally, the shoulder in C–V/T curves indicated Cr–B point defects in the measurement.

Effect of ion beam modifications on the surface and structural properties of β-FeSi2 thin films

We have investigated the effect of ion bombardment during and after deposition on the structure and surface properties of semiconducting β-FeSi2 thin films grown on n-Si(1 0 0) substrates at room temperature by unbalanced magnetron sputtering. The properties of β-FeSi2 thin films were characterized with field emission gun scanning electron microscopy, atomic force microscopy, x-ray diffraction analysis and Raman spectroscopy. Ion bombardment of the films after deposition resulted in grain size refinement and decreased the crystallinity of the films. However, ion bombardment during deposition increased the crystallinity and grain size of the coatings. These modifications induced by ion bombardment are also expected to affect the photovoltaic performance.

Influence of Heat Treatment on Structure and Charge Capacity of Sol–Gel Produced TiO2 Films

Titanium dioxide thin films were synthesized by sol–gel route from titanium isopropoxide (TIP) with acetic acid. Prior to the heat treatment, the films were amorphous phase. Above 400 °C, phase transition took place from amorphous into anatase phase. Electrochromic properties of each phase indicated reversible coloration upon Li+ ion intercalation in cyclic voltammetric measurements. Nevertheless, both charge capacity and energy band gap of films begun to decrease with increase in annealing temperature due to the crystallization.

Effect of Erbium Doping on Sol-Gel Synthesized Vanadium Pentoxide and Titanium Dioxide Thin Films

Comparative work of erbium doped vanadium pentoxide and titanium dioxide thin films were carried out via sol gel technique by dissolving erbium (III) nitrate pentahydrate in vanadium (V) oxoisopropoxide and titanium (IV) isopropoxide. Fourier Transform IR and thermogravimetric/differential thermal measurements were performed to find out erbium substitution. UV-Vis. spectroscopy indicated a blue shift upon Er doping in V2O5 film due to the softening of V=O bond. The similar behavior was expected in TiO2 film and the prediction shall be shown only if annealing of the film above 600°C, resulting oxygen deficiency in anatase TiO2 while Ti deficiency in rutile TiO2 film. Due to such impact of erbium on structure, granule size of the films, determined by AFM, increased yielding more space for intercalation of ion in host materials and monitored through cyclic voltammetry measurements.

Admittance analysis in (PPE-PPV) polymer (AnE-PVstat) light emitting diodes

In this work, admittance analysis of organic light emitting diode (OLED) (anode/active layer/cathode) was performed at room temperature within the frequency range of 1 kHz-1MHz to find out transport properties of both injected carriers from each side. Moreover, by proper chosen metals, electron or hole only OLED devices were prepared and the measurement was resumed to identify the governed transport path of injected carrier. Mobility of injected carriers followed the Poole-Frenkel type conduction mechanism and distinguished in the frequency range due to the difference of transit times in admittance measurement. Beginning of light output and onset of negative capacitance took place at the turn-on voltage (or flat band voltage), 1.8 V, which was the difference of energy band gap of polymer and two barrier offsets between metals and polymer. The proposed analytical model for admittance, derived for the frequency dependent space charge limited behavior leading negative capacitance issues, was applied on the measured data for the present OLED device.