Admittance Spectroscopy Measurements Research Articles

Influence of annealing atmosphere on formation of electrically-active defects in rutile TiO2

Electronic states in the upper part of the bandgap of reduced and/or hydrogenated n-type rutile TiO2 single crystals have been studied by means of thermal admittance and deep-level transient spectroscopy measurements. The studies were performed at sample temperatures between 28 and 300 K. The results reveal limited charge carrier freeze-out even at 28 K and evidence the existence of dominant shallow donors with ionization energies below 25 meV. Interstitial atomic hydrogen is considered to be a major contributor to these shallow donors, substantiated by infrared absorption measurements. Three defect energy levels with positions of about 70 meV, 95 meV, and 120 meV below the conduction band edge occur in all the studied samples, irrespective of the sample production batch and the post-growth heat treatment used. The origin of these levels is discussed in terms of electron polarons, intrinsic point defects, and/or common residual impurities, where especially interstitial titanium atoms, oxygen vacancies, and complexes involving Al atoms appear as likely candidates. In contrast, no common deep-level defect, exhibiting a charge state transition in the 200–700 meV range below the conduction band edge, is found in different samples. This may possibly indicate a strong influence on deep-level defects by the post-growth heat treatments employed.

Characterization of electronic properties of natural type IIb diamonds

Precision admittance spectroscopy measurements were carried out over wide temperature and frequency ranges for a set of natural single crystal type IIb diamond samples. Peaks of conductance spectra vs. temperature and frequency were used to compute the Arrhenius plots, and activation energies were derived from these plots. The capacitance-voltage profiling was used to estimate the majority charge carrier concentration and its distribution into depth of the samples. Apparent activation energies between 315 and 325meV and the capture cross section of about 10−13cm2 were found for samples with uncompensated boron concentrations in the range of 1 to 5×1016cm−3 (0.06–0.3ppm). The obtained boron concentrations are in good coincidence with FTIR results for the samples. Also, a reason for the difference between the observed admittance activation energy and the previously reported ionization energy for the acceptor boron in diamond (0.37eV) is proposed.

Investigation of the hole transport characterization and mechanisms in co-evaporated organic semiconductor mixtures

The hole transport characteristics in small molecule semiconductor mixtures of HAT-CN : NPB, HAT-CN : TAPC and HAT-CN : CBP in the ratio of 2 : 1 have been investigated by admittance spectroscopy measurements.

Structural and Electrical Characterization of SiO2 Gate Dielectrics Deposited from Solutions at Moderate Temperatures in Air.

Silicon dioxide (SiO2) is the most widely used dielectric for electronic applications. It is usually produced by thermal oxidation of silicon or by using a wide range of vacuum-based techniques. By default, the growth of SiO2 by thermal oxidation of silicon requires the use of Si substrates whereas the other deposition techniques either produce low quality or poor interface material and mostly require high deposition or annealing temperatures. Recent investigations therefore have focused on the development of alternative deposition paradigms based on solutions. Here, we report the deposition of SiO2 thin film dielectrics deposited by spray pyrolysis in air at moderate temperatures of ≈350 °C from pentane-2,4-dione solutions of SiCl4. SiO2 dielectrics were investigated by means of UV-vis absorption spectroscopy, spectroscopic ellipsometry, XPS, XRD, UFM/AFM, admittance spectroscopy, and field-effect measurements. Data analysis reveals smooth (RRMS < 1 nm) amorphous films with a dielectric constant of about 3.8, an optical band gap of ≈8.1 eV, leakage current densities in the order of ≈10-7 A/cm2 at 1 MV/cm, and high dielectric strength in excess of 5 MV/cm. XPS measurements confirm the SiO2 stoichiometry and FTIR spectra reveal features related to SiO2 only. Thin film transistors implementing spray-coated SiO2 gate dielectrics and C60 and pentacene semiconducting channels exhibit excellent transport characteristics, i.e., negligible hysteresis, low leakage currents, high on/off current modulation ratio on the order of 106, and high carrier mobility.

Fe substitution for Mn in PrSrCaMnO - The electrical and dielectrical properties of Pr0.6Sr0.3Ca0.1Mn1-xFexO3 perovskite

We have performed admittance spectroscopy measurements on a series of $Pr_{0.6}Sr_{0.3}Ca_{0.1}Mn_{1-x}Fe_{x}O_{3}$ (PSCMO) compounds with $ x=0$ , 0.025, 0.050 and 0.075. The results show that all samples exhibit a metal-insulator transition at a temperature which decreases with the iron content. The conduction mechanism is described by thermally activated hopping of small polaron. Activation energy changes at specific temperature T s, probably due to the contribution of the polarization effect to the conductivity. The dielectric permittivity increases with the temperature until a temperature T d near the specific temperature T s and then begins to decrease. This is explained by the fact that at a certain temperature, the free carriers acquire sufficient energy to overcome potential barriers, and so they enhance the conductivity and decrease the polarization. The dielectric permittivity measurements are well described by a modified Curie-Weiss law.

The effects of ultra-thin cerium fluoride film as the anode buffer layer on the electrical characteristics of organic light emitting diodes

In this study, the efficiency of organic light-emitting diodes (OLEDs) was enhanced by depositing a CeF3film as an ultra-thin buffer layer between the indium tin oxide (ITO) electrode and α-naphthylphenylbiphenyldiamine (NPB) hole transport layer, with the structure configuration ITO/CeF3 (0.5, 1, and 1.5nm)/α-naphthylphenylbiphenyl diamine (NPB) (40nm)/tris(8-hydroxyquinoline) aluminum (Alq3) (60nm)/lithium fluoride (LiF) (1nm)/Al (150nm). The enhancement mechanism was systematically investigated via several approaches. The X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy results revealed the formation of the UV-ozone treated CeF3 film. The work function increased from 4.8eV (standard ITO electrode) to 5.22eV (0.5-nm-thick UV-ozone treated CeF3 film deposited on the ITO electrode). The surface roughness of the UV-ozone treated CeF3 film was smoother than that of the standard ITO electrode. Further, the UV-ozone treated CeF3 film increased both the surface energy and polarity, as determined from contact angle measurements. In addition, admittance spectroscopy measurements showed an increased capacitance and conductance of the OLEDs. Accordingly, the turn-on voltage decreased from 4.2V to 3.6V at 1mA/cm2, the luminance increased from 7588cd/m2 to 24760cd/m2, and the current efficiency increased from 3.2cd/A to 3.8cd/A when the 0.5-nm-thick UV-ozone treated CeF3 film was inserted into the OLEDs.

The effects of UV-ozone treated ultra-thin Li2CO3-doped NiO film as the anode buffer layer on the electrical characteristics of organic light-emitting diodes

In this study, the efficiency of organic light-emitting diodes (OLEDs) was enhanced by depositing a Li2CO3-doped NiO (LNO) film as a buffer layer between the indium tin oxide (ITO) electrode and α-naphthylphenylbiphenyl diamine (NPB) hole transport layer, with the structure configuration ITO/NiO:Li2CO3 (X wt%, Y nm) (X = 0, 5 and 10 wt%; Y = 0.5, 1, and 1.5 nm)/NPB (40 nm)/tris(8-hydroxyquinoline) aluminum (Alq3) (60 nm)/lithium fluoride (LiF) (1 nm)/Al (150 nm). The enhancement mechanism was systematically investigated the X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy results revealed the formation of the UV-ozone treated Li2CO3-doped NiO film. The work function increased from 4.8 eV (standard ITO electrode) to 5.45 eV (1-nm-thick UV-ozone treated Li2CO3-doped NiO film deposited on the ITO electrode with 5 wt%). The surface roughness of the UV-ozone treated Li2CO3-doped NiO film was smoother than that of the standard ITO electrode. Further, the UV-ozone treated Li2CO3-doped NiO film increased both the surface energy and polarity as determined from contact angle measurement and admittance spectroscopy measurement showed that increased capacitance and conductance of the OLEDs. In addition, the carrier injection increased in the space-charge region when the UV-ozone treated Li2CO3-doped NiO buffer layer was inserted. Moreover, the turn-on voltage decreased from 4.2 V to 3.5 V at 1 mA/cm2, the luminance increased from 7588 cd/m2 to 23240 cd/m2, and the current efficiency increased from 3.2 cd/A to 3.4 cd/A when the 1-nm-thick UV-ozone treated Li2CO3-doped NiO film with 5 wt% was inserted of the OLEDs.

Temperature admittance spectroscopy of boron doped chemical vapor deposition diamond

Precision admittance spectroscopy measurements over wide temperature and frequency ranges were carried out for chemical vapor deposition epitaxial diamond samples doped with various concentrations of boron. It was found that the experimentally detected boron activation energy in the samples decreased from 314 meV down to 101 meV with an increase of B/C ratio from 600 to 18000 ppm in the gas reactants. For the heavily doped samples, a transition from thermally activated valence band conduction to hopping within the impurity band (with apparent activation energy 20 meV) was detected at temperatures 120–150 K. Numerical simulation was used to estimate the impurity DOS broadening. Accurate determination of continuously altering activation energy, which takes place during the transformation of conduction mechanisms, was proposed by numerical differentiation of the Arrhenius plot. With increase of boron doping level the gradual decreasing of capture cross section from 3 × 10−13 down to 2 × 10−17 cm2 was noticed. Moreover, for the hopping conduction the capture cross section becomes 4 orders of magnitude less (∼2 × 10−20 cm2). At T &amp;gt; Troom in doped samples the birth of the second conductance peak was observed. We attribute it to a defect, related to the boron doping of the material.

A comparative study of Cd‐ and Zn‐compound buffer layers on Cu(In1−x,Gax)(Sy,Se1−y)2 thin film solar cells

AbstractThis paper reports a comparative study of Cu(In,Ga)(S,Se)2 (CIGSSe) thin‐film solar cells with CBD‐CdS, CBD‐ZnS(O,OH) and ALD‐Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD‐CdS/CIGSSe, CBD‐ZnS(O,OH)/CIGSSe and ALD‐Zn(O,S)/CIGSSe solar cells exceeded 18%, for a cell area of 0.5 cm2. The solar cells underwent a heat‐light soaking (HLS) post‐treatment at 170 °C under one‐sun illumination in the air; among the three condtions, the ALD‐Zn(O,S)/CIGSSe solar cells showed the highest cell efficiency of 19.78% with the highest open‐circuit voltage of 0.718 V. Admittance spectroscopy measurements showed a shift of the N1 defect's energy position toward shallower energy positions for ALD‐Zn(O,S)/CIGSSe solar cells after HLS post‐treatment, which is in good agreement with their higher open‐circuit voltage and smaller interface recombination than that of CBD‐ZnS(O,OH)/CIGSSe solar cells. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Temperature dependent current–voltage and admittance spectroscopy on heat-light soaking effects of Cu(In,Ga)Se2 solar cells with ALD-Zn(O,S) and CBD-ZnS(O,OH) buffer layers

Heat-light soaking effects on Cu(In,Ga)Se2 (CIGS) solar cells with atomic layer deposition (ALD)-Zn(O,S) and chemical bath deposition (CBD)-ZnS(O,OH) buffer layers were investigated using temperature-dependent current–voltage and admittance spectroscopy measurements. Both CBD-ZnS(O,OH)/CIGS and ALD-Zn(O,S)/CIGS solar cells showed a significant increase in the cell performance after the combined heat and light soaking (HLS) post-treatment. Temperature-dependent current–voltage measurements showed a reduced roll-over of current density–voltage curve after the HLS post-treatment, suggesting a reduced recombination at the absorber/buffer interface. Admittance spectroscopy measurement revealed a remarkable shift towards shallower energy positions for the defect N1 after HLS post-treatment in both CIGS solar cells fabricated using CBD and ALD methods. By optimizing the HLS and the deposition conditions for each buffer layer, CBD-ZnS(O,OH)/CIGS and ALD-Zn(O,S)/CIGS solar cells yielded total efficiencies of 18.8 and 18.7%, respectively.

Understanding the Effect of MgO Interfacial Layer on ZnO/High-K/Fto Transparent Thin Film Transistor for Large-Area Transparent Electronics Applications

ZnO and doped-ZnO based thin film transistors (TFT) have been widely studied for transparent electronics applications [1-2]. However, majority of the previous work have evaluated the performance of TFT using Indium Tin Oxide (ITO) as transparent gate electrodes which is expensive and may not be sustainable for large-area transparent electronics applications, such as transparent electronics integrated glass windows and glass walls in buildings [3-4]. In addition, replacing ITO with low-cost and scalable material such as Fluorine-Doped Tin Oxide (FTO) coated glass substrates also has potential to reduce the cost of displays in consumer electronic devices. Towards addressing this issue and extending the application of ZnO-TFT in large-area transparent electronics that is cost-effective and sustainable, we have evaluated the performance of ZnO TFT with commercially available FTO as gate electrodes. ZnO was deposited at room temperature using RF magnetron sputtering while reactively sputtered HfO2, deposited at 300◦C, was used as high-k gate dielectric. The devices indicated well-behaved transistor characteristics with field effect mobility of 0.84 cm2V-1s-1, the ON/OFF current ratio of 2.5×103 and sub-threshold swing 140 mV/decade. To understand the cause of low field effect mobility and large sub-threshold slope, the defect characteristics at ZnO/HfO2 interface was studied using admittance spectroscopy (Gp/ω vs. ω) measurements. The interface state density was found to be on the order of 1013 eV-1 cm-2 which is 1-2 orders of magnitude higher than typically reported for ZnO-TFT [5-6]. This work will present our systematic understanding on the role played by FTO electrode in governing the FTO/HfO2 and HfO2/ZnO interface characteristics which finally plays an important role in governing the performance of the transistor. In particular, the role of fluorine diffusion in the stack and possibility of engineering the FTO/HfO2 and HfO2/ZnO interfaces via the insertion of interfacial layers such as MgO will be presented and supported with experimental results. [1] R. L. Hoffman, B. J. Norris, and J. F. Wager, Appl. Phys. Lett. 82, 475 (2003) [2] J. S. Seo, J. H. Jeon, Y. H. Hwang, H. Park, M. Ryu, S. H. Park and B.S. Bae, Nat. Sci. Rep, 3, 2085 (2013) [3] L. Zhang, J. Li, X. W. Zhang, X. Y. Jiang, and Z. L. Zhang, Appl. Phys. Lett. 95, 072112 (2003) [4] J. Zhang, Z. Li, L. Shen, C. Ye, B. Wang, and H. Wang, Thin Solid Films 544, 281 (2013) [5] P. F. Carcia, R. S. McLean, and M. H. Reilly, Appl. Phys. Lett, 88, 123509 (2006) [6] J. J. Siddiqui, J. D. Phillips, K. Leedy, and B. Bayraktaroglu, IEEE Electron Devices Lett.32, 1713 (2011)

Influence of iron on the performance of CIGS thin-film solar cells

In the last years steel foil has become an attractive candidate for use as a flexible substrate material for Cu(Inx,Ga1−x)Se2 thin-film solar cells (CIGS). It is stable at the high temperatures present during CIGS processing and is also commercially available. However, one disadvantage is the diffusion of substrate elements into the CIGS layer, which are detrimental for the performance of solar cells. We found a direct correlation between the iron (Fe) content in the CIGS layer and corresponding solar cell parameters. A deep defect related to Fe impurities could be detected by admittance spectroscopy measurements. The solar cell parameters could be well fitted by simulation with recombination at an acceptor like deep defect in the bulk of the CIGS layer. The simulated density of deep defect states correlates nicely with the Fe concentration in the CIGS layer. From this we conclude that Fe replaces an In (or Ga) site in the CIGS lattice and creates an FeIn2+ (or FeGa2+) deep acceptor state in the bulk of CIGS layers, which is detrimental already at a low concentration in the sub ppm range. The simulations enabled us to estimate the maximum Fe concentration in CIGS layers which is tolerable without disturbing the performance of high-efficiency CIGS solar cells.

Electrical characterization of a buried GaSb p‐n junction controlled by native defects

Buried GaSb junctions were induced in Te‐doped GaSb bulk crystals by growing a heavily Zn‐doped GaAs layer on GaSb. However, the p‐n junction resulted to be located much more deeply with respect to the Zn diffusion front and originated by a local rising up of native acceptor density, which controls the p‐type conductivity conversion of the GaSb substrate for about 1 μm beyond the Zn penetration depth. Admittance spectroscopy measurements supported the identification of such defects with the double native acceptor GaSb. Current‐voltage characteristics of the p‐n junction, performed after the removal of the GaAs layer, were here analyzed as a function of the temperature for different Zn doping levels and resulted consistent with the model previously proposed to explain the formation of the junctions.

Analysis of the relationship between the kink effect and the indium levels in MOS transistors

Abstract In this work, we investigate the effects of indium ion implantation on the channel of nMOSFETs. Deep level transient spectroscopy (DLTS) and admittance spectroscopy (AS) measurements have been made on a series of indium doped silicon N+P structures and MOS capacitors. To analyse the indium-related levels in nMOSFETs, we used a low frequency (LF) output conductance dispersion analysis, which is based on the Gain-Phase versus frequency at different temperatures. These experiences show that the indium level when operated at low temperatures at which the majority of carriers freeze-out exhibit a kink effect. The effects of indium doping on the kink were studied using the variation of channel conductance gd and transconductance gm versus temperature in the kink zone. The excess drain current versus drain and gate voltage show the maximums of both conductance gd and transconductance gm at around T = 124 K when the indium level is activated.

Determination of electron mobility in tris(8-hydroxy-quinolinato) aluminum by admittance spectroscopy

The electron mobilities of tris(8-hydroxy-quinolinato) aluminum (Alq3) thin films at various thicknesses (70–280 nm) have been determined by using admittance spectroscopy measurements. Our results show that the electric field dependence of electron mobilities exhibits different behaviors at different thicknesses of Alq3 films. Clearly, when the thickness of Alq3 film is less than 150 nm, the electron mobilities slightly decrease with increasing the electric field. For the thickness of Alq3 film more than 150 nm, however, the mobilities increase as the electric field increases. The phenomena are well discussed by energetic disorder. Using temperature dependent I–V characteristics, we further calculated the density of traps.

Defect spectroscopy of Cu(In,Ga)Se2-based thin film solar cells on polyimide substrate

Cells made on a polyimide substrate, with sodium supplied at different stages of the absorber deposition, have been studied by defect spectroscopy methods. In all of the cells, admittance spectroscopy measurements revealed the presence of two defect levels. Discussion on the origin of the observed levels, based on the correlation with free carrier concentration profiles and photocurrent results, is presented. We attribute one of the levels as belonging to interface defects, and the second one to a bulk deep defect. A distinctive feature of defects' sensitivity to a hole concentration, which leads to unusual behaviour of defect parameters is discussed.

Modelling and measurement of the metastable defect distribution in chalcopyrite-based thin film solar cells

Metastable behaviour in chalcopyrite-based solar cells is often related to the presence of intrinsic defects in the absorber layer which can change configuration through thermally activated transitions. The facilities to model these defects have recently been introduced in the software SCAPS (a one-dimensional solar cell simulator developed at the University of Gent and freely available to the research community). The presence of these defects leads to strongly non-uniform defect densities which can explain admittance spectroscopy measurement results under different bias conditions. In particular the appearance of a peak with a characteristic energy of about 0.13eV which only appears when the spectrum is measured under forward bias conditions can be related to the presence of the IIICu-defect.

The effects of tris(2-phenylpyridine) iridium on the hole injection and transport properties of 4,4′,4″-tri(N-carbazolyl)-triphenylamine thin films

The effects of tris(2-phenylpyridine) iridium [Ir(ppy)3] on the hole injection and transport properties of 4,4′,4″-tri(N-carbazolyl)-triphenylamine (TCTA) thin films have been investigated by current–voltage characteristic and admittance spectroscopy measurement. Dramatically improved hole injection can be achieved by inserting either pure Ir(ppy)3 or Ir(ppy)3 doped TCTA film between the indium–tin-oxide anode and TCTA layer, because of the hole transport ability and relatively higher-lying highest occupied molecular orbital (HOMO) level of Ir(ppy)3. When doping Ir(ppy)3 into TCTA, it acts as a role of trapping center and greatly reduces the hole mobility. Also, the hole transport properties in TCTA:Ir(ppy)3 thin films are greatly affected by doping concentration and show a nontrivial fluctuant dependence on the doping concentration. The origin of such dependence is discussed. This study provides insight into determining the charge carrier mobility of phosphorescent material doped organic semiconductor film.

Design Rules for Charge-Transport Efficient Host Materials for Phosphorescent Organic Light-Emitting Diodes

The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies, the alignment of levels with respect to the emitter, the ability to form and sustain amorphous order, material processability, and an adequate charge carrier mobility. A possible design strategy is to choose a π-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents, however, induce large spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole-Frenkel slope, supporting our conclusions. By linking electronic structure, molecular packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.

Characterization of defects in Mg doped GaN epitaxial layers using conductance measurements

Mg-doped GaN films have been grown on sapphire by metalorganic vapour phase epitaxy. Two different buffer layer schemes between the film and the sapphire substrate were used, giving rise to differing threading dislocation densities (TDDs) in the GaN.Frequency-dependent capacitance and conductance measurements have been used to study the electronic states associated with the Mg doping, and to determine how these are affected by the TDD. Current–voltage and admittance spectroscopy measurements were carried out using Al/Au Schottky contacts and Ni/Au Ohmic contacts fabricated in a dot-and-ring pattern. Frequency-dependent measurements on these devices were corrected to remove the series resistance effects arising from the well known high resistivity of Mg-doped GaN using formulae derived from the equivalent parallel circuit model. Thermal admittance spectroscopy, in which the conductance is monitored as a function of temperature, verified the existence of a single impurity-related acceptor level in samples with different TDDs at 165±10meV. For all of the samples the extrapolated capture cross-section values were very small, in the range of ~10−20cm−2, suggesting that the acceptor level could be very repulsive. Since admittance spectroscopy detects only majority carrier traps, we believe these defect signature values are most probably associated with the Mg acceptor state as they are very close to the results previously reported by other research groups using different techniques.