Active Defect States Research Articles

Tailoring optical and electrical properties of PVP-I2 complexes through gamma irradiation-induced structural defect Engineering for optoelectronic applications

This study investigates the complexation of iodine with polyvinylpyrrolidone (PVP) subjected to gamma irradiation at doses of 0, 10, 20, and 30 kGy. The complex formation involves I2 acting as a Lewis acid with electron-rich PVP as a Lewis base through a charge transfer mechanism. UV–visible spectroscopy revealed a red shift of the absorption peak at 337 nm for irradiated PVP-I2 samples, indicating stronger interactions between the triiodide ion (I3¯) and irradiated PVP. FTIR analysis confirmed the formation of new C≡N moieties arising from PVP-I2 complexation. The FT-Raman analysis revealed a new C≡N peak at 2360 cm−1 in irradiated PVP-I2 samples, with increasing intensity correlating to higher irradiation doses, indicating enhanced electron delocalization due to complex formation. XRD analysis showed that PVP-I2 samples irradiated at 0 and 10 kGy exhibited higher crystallinity than those irradiated at 20 and 30 kGy, suggesting that high-dose irradiation causes PVP radiolysis and ring opening. Dielectric measurements demonstrated enhanced polarizability and ionic conductivity of irradiated PVP-I2 films, related to structural disorder promoting charge dissociation and ion mobility. The direct bandgap decreased from 2.75 eV for unirradiated samples to 2.55 eV for samples irradiated at 30 kGy, indicating the creation of new optically active defect states. Urbach energy increased from 0.85 eV to 1.55 eV with increasing irradiation dose, confirming increased disorder in the PVP structure. These findings reveal that gamma irradiation significantly alters the optical and electrical properties of PVP-I2 films by creating defects and disrupting structure, offering new insights for tailoring these materials for optoelectronic applications.

Decoration of BiVO4/ZnO Photoanodes with Fe‐ZIF‐8 to Simultaneously Enhance Charge Separation and Hole Transportation for Efficient Solar Water Splitting

AbstractFacilitating the separation and transport efficiency of photogenerated electron–hole pairs is a key factor in realizing efficient photoelectrochemical (PEC) reactions. Herein, a novel metal–organic framework (MOF)‐derived cocatalyst of iron‐substituted ZIF‐8 (Fe‐ZIF‐8) is fabricated, which is attached to a BiVO4/ZnO (BZ) photoanode. The formation of Fe‐ZIF‐8 is based on ZnO nanoparticles in BZ (BiVO4/ZnO : Fe‐ZIF‐8, BZZF) through a ligand‐exchange method. Fe‐ZIF‐8 can extract holes from BZ, prolong the lifetime of charge carriers at the photoanode/electrolyte interface, and enhance the built‐in electric field to drive the separation of photogenerated charge carriers. In addition, more active sites and surface defect states are induced by the decoration with Fe‐ZIF‐8. As a result, the prepared BZZF photoanode achieves enhanced separation and transport efficiency under AM 1.5G solar light illumination (100 mW cm−2). The PEC performance achieves 4.14 mA cm−2 with a maximum applied bias photon‐to‐current efficiency of 1.10 % at 0.71 V vs. RHE, which is superior to that of BiVO4. This work suggests that the semiconductor combined with MOF‐derived materials is a rational design to facilitate charge separation and increase hole transport efficiency simultaneously to advance PEC water splitting systems.

Enhancing the Fluorescence Activity of Graphitic Carbon Nitride Nanosheets by Eliminating the Active Defect States: Implications for Fluorescent Sensing and Biological Imaging

The fluorescence (FL) activity of graphitic carbon nitride nanosheets (CNNSs) is dependent on both their band gap and surface defect states. It is known that the band gap of CNNSs is difficult to tune; however, studies on the regulation of the FL activity of CNNSs by surface defect states are still few. Herein a facile strategy to enhance the FL activity of CNNSs is developed by alkali-mediated passivation of the active defect states (dangling bonds) on CNNSs. With alkali treatment, the active defect states of CNNSs are eliminated and replaced with the passivated −NH2 groups, leading to significant enhancement of the FL quantum yields. We believe that this work will promote an understanding of the structure–FL properties of CNNSs, advancing an understanding of the luminous mechanisms and the application of CNNSs in FL sensing as well as biological imaging.

A versatile multifaceted resistive switching memory activated by light and ion irradiation in poly (3-octylthiophene)-zinc oxide hybrids

Abstract The resistive switching memory behavior activated by simulated solar light and swift heavy ion irradiation in poly (3-octylthiophene)-zinc oxide hybrid devices are reported. The current-voltage characteristics show the appearance of a remarkable hysteresis in reverse bias and a considerable in a forward bias upon illumination by simulated solar light results in memory behavior. Ion irradiation exhibits strong enhancement of the memory behavior under both dark and light illuminations. Thus, the memory response is activated and/or enhanced upon illumination by light and irradiation along with the cyclic endurance of the devices. This is attributed due to the formation of well built-in conducting filament pathways through the trapping of injected and photo-generated charges via irradiation-induced electrically active defect and trap states and well explained by a schematic energy band diagram. The defect density induced upon ion irradiation works as a dominant factor and thus the tailoring of the interfacial properties of the hybrid devices leads to the strong strengthening of memory formations. Thus, the hybrid devices can be activated and/or enhanced by light and irradiation for potential fast resistive switching memory applications.

(Invited) 20 Years of Carbon Nanotube Evolution

From the advent of individualized carbon nanotubes nearly 20 years ago to the current ability to control their properties at multiple levels, the world of CNTs has changed tremendously. Through the lens of my group’s efforts, I will provide a retrospective on how our understanding of CNTs has changed through this period. An overview will be given of our efforts on Raman and photoluminescence (PL) spectroscopy to identify and characterize fundamental CNT properties. While spectroscopic separation enabled many revelations on CNT photophysics, the limitations imposed by mixed-chirality samples ultimately were overcome with the advent of effective and efficient methods for chirality enrichment. Examples will be given of advances in photophysics established through Raman performed on pure chirality samples. New separations techniques presented new questions on nanotube surface chemistry, ultimately leading to studies of surface chemistry, surface structure, and defects, at the ensemble and single tube levels. Most recently, development of functionalization methods to introduce optically active defect states has opened an exciting new direction in nanotube studies. An overview of the new photophyics and quantum light emission enabled by defects will be given, along with some perspective on future outlook.

Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum photonic technologies. The ability to tailor quantum emitters via site-selective defect engineering is essential for realizing scalable architectures. However, a major difficulty is that defects need to be controllably positioned within the material. Here, we overcome this challenge by controllably irradiating monolayer MoS2 using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion exposed MoS2 flake with high-quality hBN reveals spectrally narrow emission lines that produce photons in the visible spectral range. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron–hole complexes at defect states generated by the local helium ion exposure. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and interacting exciton lattices that may allow the exploration of Hubbard physics.

Enhanced photocatalytic hydrogen production over conjugated polymer/black TiO2 hybrid: The impact of constructing active defect states

Utilization of noble metal (e.g. Pt, Au) as co-catalyst seems always indispensable for a photocatalytic H2 production catalyst because most semiconductors lack sufficient active sites. Herein, some non-noble metal active states, Ti3+/Ov (oxygen vacancies) and surface amorphization structure, are introduced into anatase TiO2 nanoparticles by ball milling them with TiH2 to prepare black TiO2. And then, an organic/inorganic hybrid (BE-black TiO2) containing a conjugated polymer (BE, a modified poly- benzothiadiazole flake) and black TiO2 is fabricated and investigated for photocatalytic H2 production without noble metal co-catalyst. Compare to BE-TiO2 catalyst, the BE-black TiO2 exhibit a ∼ 30% enhancement for H2 production activity under visible light (λ > 420 nm) irradiation. Combining the structural characterizations, comparison of H2 production rate and electron transfer behavior analysis, it is found that these active states play key role in the enhancement of photoactivity through narrowing the band-gap of TiO2, accelerating photoelectrons transfer from BE to TiO2, acting as electron traps and reaction sites for H2 production.

Growth of Eshelby twisted ZnO nanowires through nanoflakes & nanoflowers: A room temperature ammonia sensor

Insight into the controlled growth features of nanowires has been in the prominent spotlight for engineering the material properties. Defect and dislocation induced nanowire growth has been emerging as a robust model by dwindling the conventional growth models. In this context, we have proposed a screw dislocated Eshelby twist origin in cationic assimilated ZnO nanowires synthesized via Successive Ionic Layer Absorption and Reaction (SILAR) technique. The growth of twisted nanowires occurred through a subsequent transformation from nanoflakes to nanoflowers. Presence of twist contours in various zone axis pattern provided strong validation of Eshelby origin in twisted nanowires. The preferential plane orientation of (0 0 0 2) confirmed the twisted growth along c-axis orientation. Presence of screw tail at the twisted end of nanowire confirmed the influence of Peach-Kohler force acted on the screw axis. Active vibrational modes and surface defect states of nanoflowers and nanowires were investigated and reported. Twisted ZnO nanowires showed maximum sensing response of 291 towards 100 ppm of ammonia at room temperature with the lowest detection limit of 5 ppm. The response and recovery times were found to be 39 and 17 s. Influence of grain alignment, grain orientation and potential barrier height on ammonia sensing signatures are reported.

Defect formation under high temperature dark-annealing compared to elevated temperature light soaking

In the last years, significant progress has been made regarding an understanding of light induced degradation at elevated temperature observed on PERC solar cells (LeTID). Nevertheless, the detailed root cause is still under discussion. Latest results show that a similar degradation occurs by annealing lifetime samples in the dark without carrier injection. In this work, we show that mc-Si PERC cells degrade and recover at high temperature without carrier injection. As the lateral appearance and the recovery behaviour agree with what is known about LeTID, it is likely that the same defect is observed. However, even after recovery the treatment in the dark does not result in LeTID stable cells. A subsequent illumination leads to a further power loss. This subsequent degradation differs from the first degradation in its kinetics and its lateral appearance. Based on these results it is concluded that two recombination active defect states are activated by LeTID. These recombination active defect states can be distinguished by annealing the samples without carrier injection before illuminating the samples. However, a high temperature anneal activates also additional defects, which might lead to a more pronounced degradation. Thus, a high temperature treatment is not recommended for LeTID testing neither as substitution nor as pretreatment prior the LeTID test.

Defects induced by solid state reactions at the tungsten-silicon carbide interface

Defects introduced by the solid state reactions between tungsten and silicon carbide have been studied using deep level transient spectroscopy (DLTS) and Laplace DLTS. W/4H-SiC Schottky barrier diodes were isochronally annealed in the 100–1100 °C temperature range. Phase composition transitions and the associated evolution in the surface morphology were investigated using x-ray diffraction (XRD) and scanning electron microscopy (SEM). After annealing at 1100 °C, the E0.08, E0.15, E0.23, E0.34, E0.35, E0.61, E0.67, and E0.82 defects were observed. Our study reveals that products of thermal reactions at the interface between tungsten and n-4H-SiC may migrate into the semiconductor, resulting in electrically active defect states in the bandgap.

(Invited) The Inversion Behaviour of Narrow Band Gap Mos Systems: Experimental Observations, Physics Based Simulations and Applications

Impedance spectroscopy of the metal-oxide semiconductor (MOS) system has played a central role in the development of silicon-based complementary MOS (CMOS) technology over the past 50 years [1, 2]. With current research interest into alternative semiconductor channels to silicon for MOSFET and tunnel FET technologies, the measurement and interpretation of the overall impedance of the MOS structure requires detailed analysis to separate and quantify the contribution of interface states, and near interface traps (border traps), on the capacitance and conductance response, and to separate the contribution of these electrically active defect states from the ac response of minority carriers in the case of genuine inversion of the semiconductor/dielectric interface. There has been considerable progress in recent years in reducing the interface state density (Dit) [3], [4], [5], in narrow gap InxGa1-xAs MOS structures to the point where genuine surface inversion can be observed for both n- and p-type In0.53Ga0.47As MOS capacitors, which is confirmed by analysis of the minimum capacitance of n- and p-type In0.53Ga0.47As MOS structure with doping concentration ranging from approximately 1x1016 to 1x1018 cm-3 [6]. In this presentation we will provide an overview of the experimental relationship between specific functions of the capacitance (C) and conductance (G) for the case of narrow band gap III-V MOS structures which exhibit genuine surface inversion, where the capacitance and conductance of the MOS system as a function of ac angular frequency (ω) are related, and in particular, the peak values of G/ω and −dC/dloge(ω) (≡− ωdC/dω) are equal, and that these peak magnitudes occur at the same value of ω [7]. The relationship is also confirmed by physics based ac simulations of MOS structures and through analysis of the equivalent circuit model in inversion. Results will be presented for InGaAs and InGaSb MOS structures (Al2O3 and Al2O3/HfO2 ALD oxides) where genuine inversion of the III-V/oxide surface is confirmed by the G/ω and −dC/dloge(ω) functions, which have peak values of Cox 2/2(Cox+Cd) when the surface is inverted (where Cox is the gate oxide capacitance and Cd is the maximum capacitance of the semiconductor surface in inversion). In the case of p type InGaSb MOS structures it is also notable that the accumulation frequency dispersion is very low (<0.7%/decade) indicating a reduced density of border traps at energies in the Al2O3 gate oxide aligning with the In0.3Ga0.7Sb valence band edge, which is approximately aligned with the lowest energy in the In0.53Ga0.47As conduction band gamma valley. Finally, experimental results and physics based simulations will be presented, which indicate that the peak values of G/ω and −dC/dloge(ω) in inversion for any MOS system which demonstrates an inversion response within the typically range of temperatures and ac signal frequencies employed in experiments, opens a route to determine the gate oxide capacitance in inversion (where the gate oxide field and gate leakage are reduced), and that the angular frequency associated with the peak values of the G/ω and −dC/dloge(ω) functions allows the determination of the minority carrier generation rate in the semiconductor, which is relevant to the leakage currents in MOSFETs and the optical performance in photonic applications. [1] E. H. Nicollian and J. R. Brews, MOS Physics and Technology. New York, NY, USA: Wiley, 1982 [2] E. H. Nicollian and A. Goetzberger, “The Si-SiO2 interface—Electrical properties as determined by the metal-insulator-silicon conductance technique,” Bell Syst. Tech. J., vol. 46, no. 6, pp. 1055–1133, 1967. [3] É. O’Connor et al., J. Appl. Phys., vol. 109, no. 2, p. 024101, 2011. [4] H.-D. Trinh et al., Appl. Phys. Lett., vol. 97, no. 4, pp. 042903-1–042903-3, 2010. [5] T. D. Lin et al., “Realization of high-quality HfO2 on In0.53Ga0.47As by in-situ atomic-layer-deposition,” Appl. Phys. Lett., vol. 100, no. 17, p. 172110, 2012. [6] E. O'Connor, K. Cherkaoui, S. Monaghan, B. Sheehan, I. M. Povey, and P. K. Hurley, Appl. Phys. Lett 110, 032902 (2017) [7] Scott Monaghan, Éamon O’Connor, Rafael Rios, Fahmida Ferdousi, Liam Floyd, Eimear Ryan, Karim Cherkaoui, Ian M. Povey, Kelin J. Kuhn, and Paul K. Hurley, IEEE Transaction on Electron Devices, 61, 4176 (2014)

Composition Related Electrical Active Defect States of InGaAs and GaAsN

This paper discusses results of electrically active defect states - deep energy level analysis in InGaAs and GaAsN undoped semiconductor structures grown for solar cell applications. Main attention is focused on composition and growth condition dependent impurities and the investigation of their possible origins. For this purpose a widely utilized spectroscopy method, Deep Level Transient Fourier Spectroscopy, was utilized. The most significant responses of each sample labelled as InG2, InG3 and NG1, NG2 were discussed in detail and confirmed by simulations and literature data. The presence of a possible dual conduction type and dual state defect complex, dependent on the In/N composition, is reported. Beneficial characteristics of specific indium and nitrogen concentrations capable of eliminating or reducing certain point defects and dislocations are stated.

(Invited) An Investigation of the InGaAs MOS System for Future High Mobility Channel Applications

As silicon devices reach the limit of dimensional scaling there is a growing interest in the use of high electron mobility channels, such as InxGa1-xAs, in conjunction high dielectric constant (high-k) gate oxides for n-channel Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) in FinFET structures [1] and in 3D integration schemes [2]. The understanding and control of electrically active defect states at the high-k/InxGa1-xAs interface and of charges within the atomic layer deposited (ALD) high-k films will be essential for the successful implementation of high mobility channel materials in MOSFET or tunnel FET configurations. The objective of this presentation will be to provide an overview of the current understanding of the density and distribution of electrically active defects at the high-k/InGaAs interface and of defect states located in the interfacial oxide transition region which are manifest as accumulation frequency dispersion and capacitance-voltage hysteresis. The presentation will focus on InGaAs with a 53% Indium concentration. The electrically active interface state density distribution is determined from fully fabricated InGaAs MOSFET structures based on the full gate capacitance in conjunction with the Maserjian Y-function and Poisson-Schrodinger simulations [3]. Comparisons to simulations will be discussed in terms of identifying potential candidates for the physical origin of the interface defects. Very significant progress has been made in recent years in the passivation and intrinsic elimination of high-k/In0.53Ga0.47As interface defects to the point where genuine surface inversion for n and p type InGaAs MOS structures can be achieved and this research will be reviewed [4]. The characteristic signatures of capacitance and conductance, which indicate genuine surface inversion for an InGaAs MOS structure in inversion, will also be discussed [5]. While significant attention has been focussed on interface states and fixed charges in the InGaAs MOS system, the issue capacitance-voltage (CV) hysteresis has received relatively little attention to date [4]. This is now becoming a more pressing issue as progress is being made in controlling interface defect levels. Results will be presented covering CV hysteresis for the HfO2/InGaAs and Al2O3/InGaAs MOS structures with a range of high-k oxide thicknesses. The experimental results indicate that the defect states responsible for the hysteresis are located in the oxide transition region near the InGaAs/high-k interface and that engineering of this interface region, which contains native oxides of In, Ga and As, is the key to the control of CV hysteresis and accumulation frequency dispersion [7]. Finally, the presentation will cover the application of CV analysis to the understanding and quantification of charged defect components for InGaAs on insulator structures for 3D integration [8]. The authors acknowledge the financial support of Science Foundation Ireland through the INVENT project (SFI/09/IN.1/I2633) and the EU through the project COMPOSE3 (FP7-ICT- 2013-11-619325). [1] M. Radosavljevic et al., Tech.Dig.-Int. Electron Devices Meet, 126 (2010) [2] L. Czornomaz, et al., Tech.Dig.-Int. Electron Devices Meet. 6479088, (2012) [3] V. Djara, et al., Microelectronic Engineering 109 182–188 (2013) [4] N. Goel, D. et al., Tech. Dig. - Int. Electron Devices Meet. 363, (2008) [5] É. O’Connor, et al., Appl. Phys Lett 99, 212901 (2011) [6] Scott Monaghan, et al., IEEE Transaction on Electron Devices, 61, 4176 (2014) [7] Jun Lin, et al., J. Appl. Phys. 114, 144105 (2013) [8] K. Cherkaoui et al., Microelectronic Engineering 147 63–66 (2015)

Understanding Photovoltage in Insulator-Protected Water-Splitting Half-Cells

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis. ALD-TiO2 remains the most widely used material because of its excellent stability under water oxidation conditions and potential for high electrical conductivity both as an ultrathin film and with thicknesses exceeding 100 nm [1-3]. However, the most conductive ALD-TiO2films exhibit poor photovoltages of ~ 400 mV and less [3]. Even assuming near-ideal fill factors, these voltages fall far short of the values needed for viable water splitting devices. Photovoltage optimization is especially difficult in MOS photoanodes due to the range of factors which influence the conductivity and stability of the protection layer as well as the presence of electrically active defect states at the semiconductor/oxide interface. Therefore, understanding how to optimize photovoltage and stability is of utmost concern for the advancement of the field. Here we report a novel observation of photovoltage loss associated with charge transfer in these metal-oxide protected devices, and by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported to date for single-junction water-splitting silicon cells. The loss mechanism is systematically probed in MOS Schottky junction cells compared to buried junction p+n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A capacitor model that predicts this loss is developed, and is related to the dielectric properties of the protective oxide, achieving excellent agreement with the data. From these findings, we extract design principles for simultaneous optimization of charge transfer resistance and interface quality to maximize the photovoltage of metal-oxide protected MOS water splitting devices. [1] Y.W. Chen, et al. Nature Mat. 2011, 10, 539-544. [2] A. G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487–2496. [3] S. Hu, et al. Science 2014, 344, 1005−1009. Figure 1 | Charge transfer in three cell types for water splitting applied to silicon: The Type 0 Semiconductor-Liquid (SL), Type 1 Metal-Insulator-Semicondcutor (MIS), and Type 2 p+n junction. The density of states on either side of the oxides and the excition splitting position with respect to these layers play a crucial role in mediating efficient charge transfer. These effects are so strong that Type 0 protected silicon cells exhibit essentially no photovoltage, Type 1 nSi cells show a linear photovoltage loss with oxide thickness, and Type 2 cells--where the hole concentration on the Si/SiO2 interface is always high--exhibit record photovoltages at all oxide thicknesses and pH values studied. Figure 1

Epitaxial growth and surface studies of the Half Heusler compound NiTiSn (001)

The Half Heuslers are currently an attractive family of compounds for high temperature thermoelectrics research, and recently, there has been renewed interest since some of these compounds are proposed to be topological insulators. NiTiSn belongs to the family of 18 valence electron Half Heuslers that are predicted to be semiconducting, despite being composed entirely of metallic elements. The growth of the Half Heusler compound NiTiSn by molecular beam epitaxy is demonstrated. The NiTiSn films are epitaxial and single crystalline as observed by reflection high-energy electron diffraction and x-ray diffraction. Temperature dependent transport measurements suggest the films may be semiconducting, but with a high background carrier density indicative of a high density of electrically active defect states. Methods of protecting the sample surface for synchrotron-based photoemission measurements are explored. These methods may be applied to the study of surface electronic structure in unconventional materials.

Investigations on the structural and optical properties of the swift heavy ion irradiated 6H [sbnd]SiC

Single crystal 6H SiC wafers have been irradiated with 150 MeV Ag 12+ ions with fluences ranging from 1 × 10 11 to 1 × 10 13 ions/cm 2 at 300 K. The defect accumulation as a function of fluence was studied to determine changes in structural and optical properties. The variation in the fundamental Raman modes of the crystalline 6H SiC due to irradiation has been correlated with the disorder accumulation. The creation of defect states due to irradiation in the bandgap affects the blue-green photoluminescence emission in the irradiated samples. The UV–Visible absorption studies support the existence of defect states in the bandgap which is observed by the shift in the absorption edge towards the lower energy side with increasing fluence. Time Correlated Single Photon Counting photoluminescence decay results suggest that the existing defect states are radiative, exhibiting three lifetimes when irradiated with a fluence 5 × 10 11 ions/cm 2. The total number of lifetime components was reduced for a fluence 1 × 10 13 ions/cm 2 as the defect states produced increase the non-radiative defect centres. These results suggest that the accumulation of defects due to irradiation at fluences 5 × 10 11 and 1 × 10 13 ions/cm 2 are degenerate configurations which exhibit multiple lifetimes in photoluminescence studies. It is inferred that the optically active defect states influence the transition rate of charge carriers in this device material.

Negative differential resistance in isolated GaN nanowires with focused electron beam deposited platinum contacts

Straight and zig-zag isolated GaN nanowires were investigated for their electrical properties using focused ion/electron beam (FIB/FEB) deposited platinum contacts. The straight and smooth nanowire with FIB contacts was non-ohmic and relatively high leakage current was observed under reverse bias. The straight and smooth nanowire with FEB contact shows ohmic behaviour whereas the zig-zag nanowire shows non-ohmic with negative differential resistance (NDR). The peak-to-valley current ratio was about 4.5 at room temperature under forward bias. The NDR observed in the zig-zag nanowires has been explained based on inelastic tunnelling through electrically active acceptor-like defect states present within the bandgap.

A scanning Kelvin probe study of charge trapping in zone-cast pentacene thin filmtransistors

We have used scanning Kelvin probe microscopy (SKPM) as a local probe tostudy charge trapping in zone-cast pentacene field effect transistors on bothSiO2 and benzocyclobutene (BCB) substrates. Annealing at130 °C was found to reduce the threshold voltage, susceptibility to negative gate bias stress andtrapping of positive charges within single pentacene grains. We conclude that oxygen isable to penetrate and disassociatively incorporate into crystalline pentacene, chemicallycreating electrically active defect states. Screening of a positive gate bias caused by electroninjection from Au into pentacene was directly observed with SKPM. The rate of screeningwas found to change significantly after annealing of the film and depended on the choice ofgate dielectric.

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ∼10nm). Using this technique, the position of the surface Fermi level EFs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ∼1015states/cm3 can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100nm) layers, and the minimum achievable distance of EFs from the conduction band is ∼360mV for doping with 104ppm PH3. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.

Direct observation of infrared photoluminescence of spray-deposited CdS:In thin films

Infrared photoluminescence spectra (in the range 0.9–1.4 eV) of the as-deposited CdS:In thin films prepared by the spray pyrolysis technique were recorded at different laser powers and different film temperatures in the range 24–130 K. The spectra show an infrared band centred at 1.06 eV, which have a structure and asymmetry. The structure might be attributed to active defect states which are produced through the growth of the film and might be partially due to coupling to longitudinal phonon. Gaussian peaks were used to deconvolute the spectrum by using nonlinear square fit. The Gaussian peaks used in the fit are expected to fit the spectra taken at different laser powers and different temperatures. These results are discussed in view of the importance of CdS as a window layer for photovoltaic heterojunction solar cells.