Study of effect of inserted pentacene layer in ITO/P(VDF-TrFE)/α-NPD/Au capacitor using electric-field-induced optical second-harmonic generation and displacement current

The effect of inserted pentacene layers on the interface optimization was investigated in the indium-tin oxide (ITO)/Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/N, N′-bis(1-naphthyl)-N,N′-diphenyl-1, 1′-biphenyl-4,4′-diamine (α-NPD)/Au capacitors, by using displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement. Results show that the inserted pentacene layer between the α-NPD and P(VDF-TrFE) layers effectively improves the contact between the semiconductor and the ferroelectric P(VDF-TrFE) and it further enhances dipolar switching, resulting in three peaks in the DCM, while the insertion of a pentacene layer between the α-NPD and the Au electrode can effectively acquire holes from the Au electrode to be conductive, only leading to peak shift in the DCM with only two peaks. It is concluded that pentacene is a good candidate available for the interface optimization of organic devices.     

Probing carrier behavior in organic semiconductor device by electric field induced optical second harmonic generation measurement

Electric field induced optical second harmonic generation (EFISHG) measurement is capable of probing carrier motions in organic devices. By measuring nonlinear polarization induced in active layer of organic devices, the carrier motion is visualized. After summarizing the conventional SHG measurement as a tool for material characterization, we show that the EFISHG is a way for probing carrier motions, and thus extends the potentiality of the conventional SHG measurement. We employ the EFISHG measurement to probe carrier behaviors in double-layer metal-insulator–metal diodes, e.g., Au/TIPS-pentacene/PI/ITO diodes, and demonstrate how the time-resolved SHG measurement probes carrier motion in the diodes, in terms of the I–V and C–V characteristics. Results give an insightful picture on the carrier injection, and the succeeding carrier transport.     

Electrical and optical analyses of trapping phenomenon with temperature dependence of organic device

Carrier mechanism in actual organic devices is not simple, owing to the dielectric nature of active organic semiconductor layers, the complexity of the organic device interface, carrier trapping effects by stress biasing, and others. By coupling the conventional electrical measurement, e.g., current–voltage, capacitance–voltage and capacitance-frequency measurements, with optical charge modulation spectroscopy (CMS) measurement, we studied the hysteresis behavior and the temperature dependence of indium tin oxide/polyimide/6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS-pentacene)/Au diodes to understand the effect of carrier trapping caused by injected carriers. The coupled electrical and optical measurements were very helpful to clarify carrier injection that was followed by carrier trapping in the diode, in terms of hysteresis behavior. CMS measurement was used to observe energetic states of carriers in TIPS-pentacene double-layer diode. Finally, the carrier mechanism in organic diodes was discussed by analyzing the diodes as a Maxwell-Wagner effect element.     

并五苯有机薄膜晶体管电学性能研究

制作了以并五苯为半导体有源层材料的有机薄膜晶体管。用热氧化的方法制备了一层230nm的二氧化硅栅绝缘层并用原子力显微镜(AFM)分析了表面形貌。研究了器件的电学性能,得到的并五苯有机薄膜晶体管器件载流子迁移率为8.9×10-3cm2/V.s,器件的阈值电压和开关电流比分别为-8.2V和1.0×104。     

New method for the determination of the surface barrier heights in MIS tunnel diodes (solar cells)

The methods commonly applied for the determination of the surface barrier heights of Schottky diodes are usually extended for the MIS tunnel structures characterization, although, in such cases both, their range of application and accuracy are often strongly reduced.In this paper a new photoelectric method for the determination of the surface barrier heights in MIS tunnel diode is proposed together with experimental results supporting its validity. The presence of the ultra-thin oxide layer between the metal and semiconductor does not reduce method's applicability as long as the direct semiconductor-metal tunneling is dominant current flow mechanism through the oxide layer. In the contrary to the previous methods it allow us to measure directly the barrier height for minority carriers, and therefore, is especially recommended for minority carrier dievices.     

Minority carrier MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion—II. Experiment

The theory of metal-insulator-semiconductor (MIS) tunnel diodes has been described in a companion paper for the case where the insulating layer is so thin that large tunnel currents can flow between the metal and the semiconductor. Of particular interest was the case where the dominant component of tunnel current is between the metal and the minority carrier energy band in the semiconductor (minority carrier diodes). In the present paper, these diodes are investigated experimentally. The differences between minority and majority carrier diodes are demonstrated. Minority carrier diodes are shown to possess properties similar to p-n junction diodes as predicted theoretically. The effectsof different metal contacts, insulator thicknesses, and substrate resistivities are investigated and confirm previous theory. The application of the minority carrier MIS tunnel diodes to energy conversion employing the electron-voltaic effect is investigated experimentally. The diodes were found to be more efficient in this application than p-n junction diodes. Other possible applications of the diodes are as photo-voltaic energy converters, as injecting contacts, and as photo-diodes or elements of photo-diode arrays.     

Electroluminescence of InGaAs/GaAs quantum-size heterostructures with (III,Mn)V and Ni ferromagnetic injectors

Electroluminescence characteristics of light-emitting diodes based on InGaAs/GaAs quantum well heterostructures with an injector layer made of ferromagnetic metal (Ni), semimetal compound (MnSb), or magnetic semiconductor (InMnAs) were comparatively studied. The general feature is electroluminescence quenching as the spacer layer thickness between a quantum well and a magnetic injector decreases. It was found that the temperature dependence of the electroluminescence in diodes with Ni and MnSb is caused by thermal ejection of carriers from the quantum well; in diodes with InMnAs, it is caused by the temperature dependence of the carrier concentration in magnetic semiconductor and thermal ejection of carriers from the quantum well in the high-temperature region.     

Minority carrier MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion—I. Theory

If the insulating layer in a metal-insulator-semiconductor (MIS) diode is very thin ($\text{<60}\text{A}\text{?}$ for AlSiO₂Si), measureable tunnel current can flow between the metal and the semiconductor. If the insulating layer is even thinner ($\text{<30}\text{A}\text{?}$), tunnel currents are so large that they can significantly disturb the semiconductor from thermal equilibrium. Under such conditions, MIS diodes exhibit properties determined by which of the following tunneling processes is dominant; tunneling between the metal and the majority carrier energy band in the semiconductor, between the metal and the minority carrier energy band, or between the metal abd surface state levels. In the present paper, minority carrier MIS tunnel diodes are analysed using a very general formulation of the tunneling processes through the insulator, transport properties in the semiconductor, and surface state effects. Starting from solutions for diodes with relatively thick insulating layers where the semiconductor is essentially in thermal equilibrium, solutions are obtained for progressively thinner insulating layers until non-equilibrium effects in the semiconductor are observed. It is shown that such minority carrier MIS tunnel diodes with very thin insulating layers possess properties similar to p-n junction diodes including exponential current-voltage characteristics which approach the “ideal diode” law of p-n junction theory. The theory adequately describes the observed properties of experimental devices reported in a companion paper. The diodes have application as injecting contacts, as photodiodes or elements of photodiode arrays, and as energy conversion devices employing the electron- or photo-voltaic effects.     

Study of blocking effect of Cu-phthalocyanine layer in zinc oxide/pentacene/CuPc/C60/Al organic solar cells by electric field-induced optical second harmonic generation measurement

By employing electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the blocking effect of sandwiched Cu-phthalocyanine (CuPc) layer on IZO/pentacene/Cu-phthalocyanine (CuPc)/C₆₀/Al organic solar cells (OSCs). Results evidently showed that the Maxwell–Wagner type interfacial charging appears at both pentacene/CuPc and CuPc/C₆₀ interfaces, depending on the CuPc layer thickness. When the thickness of CuPc layer was 8 nm, the charging at the two interfaces was significantly suppressed and the I–V characteristic was improved. CuPc layer is available as a blocking layer for the regulation of interfacial charging induced in IZO/pentacene/C₆₀/Al organic solar cells (OSCs). The blocking effect of the CuPc layer was discussed on the basis of Maxwell–Wagner model analysis.     

Charge Storage in Pentacene/Polymethylmethacrylate Memory Devices

The electrical behavior of organic metal–insulator–semiconductor (MIS) structures incorporating a layer of self-assembled metallic nanoparticles is described. These have been based on thermally evaporated pentacene (semiconductor) and spin-coated polymethylmethacrylate (insulator). The MIS devices containing the nanoparticles exhibited significant hysteresis in their capacitance-versus-voltage and conductance-versus-voltage characteristics, which was attributed to the charging and discharging of the nanoparticles. The memory structure reported here offers a useful advance in the development of flexible organic memory structures.      

Orders‐of‐Magnitude Reduction of the Contact Resistance in Short‐Channel Hot Embossed Organic Thin Film Transistors by Oxidative Treatment of Au‐Electrodes

In this study we report on the optimization of the contact resistance by surface treatment in short‐channel bottom‐contact OTFTs based on pentacene as semiconductor and SiO₂ as gate dielectric. The devices have been fabricated by means of nanoimprint lithography with channel lengths in the range of 0.3 μm < L < 3.0 μm. In order to reduce the contact resistance the Au source‐ and drain‐contacts were subjected to a special UV/ozone treatment, which induced the formation of a thin AuO_x layer. It turned out, that the treatment is very effective (i) in decreasing the hole‐injection barrier between Au and pentacene and (ii) in improving the morphology of pentacene on top of the Au contacts and thus reducing the access resistance of carriers to the channel. Contact resistance values as low as 80 Ω cm were achieved for gate voltages well above the threshold. In devices with untreated contacts, the charge carrier mobility shows a power‐law dependence on the channel length, which is closely related to the contact resistance and to the grain‐size of the pentacene crystallites. Devices with UV/ozone treated contacts of very low resistance, however, exhibit a charge carrier mobility in the range of 0.3 cm² V^–1 s^–1 < μ < 0.4 cm² V^–1 s^–1 independent of the channel length.     

Influence of Al2O3 barrier on the interfacial electronic structure of Au/Ti/n-GaAs structures

The Au/Ti/n-GaAs structures with and without Al₂O₃ interfacial layer have been fabricated.The Al₂O₃ interfacial layer has been formed on the GaAs substrate by atomic layer deposition.The effects of the interfacial layer on the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the devices have been investigated in the temperature range of 60-300 K.It has been seen that the carrier concentration from C-V characteristics for the MIS (metal/insulating layer/semiconductor) diode with Al₂O₃ interfacial layer has a higher value than that for the reference diode without the Al₂O₃ interfacial layer (MS).Such a difference in the doping concentration has been attributed not to doping variation in the semiconductor bulk but to the presence of the Al₂O₃ interfacial layer because both diodes have been made on the pieces cut from the same n-type GaAs wafer.The temperaturedependent I-V characteristics of the MIS diode do not obey the thermionic emission current theory because of the presence of the Al₂O₃ layer.An electron tunneling factor,aδ(χ)^1/2,value of 20.64 has been found from the I-V-T data of the MIS diode.An average value of 0.627 eV for the mean tunneling barrier height,χ,presented by the Al₂O₃ layer has been obtained.     

Analytical integration of small-signal transport equations in an MIS structure at flat-band condition

The small-signal transport equations are solved analytically in a homogeneously doped MIS structure at flat-band condition. We can establish exact expressions of ac carrier density, electric field, and potential distributions in the MIS structure for any frequency. The plot of these expressions, in the frequency range from 10^-3to 10⁹Hz, proves the existence of majority and minority modes. A complete impedance expression is sufficient to determine all equivalent circuit components of the MIS structure at flat-band condition. The proposed method is valid for any bulk lifetime of carriers and for any semiconductor thickness.     

Electrical characteristics of GaAs MIS Schottky diodes

The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of GaAs metal-insulator-semiconductor (MIS) Schottky barrier diodes are investigated over a wide temperature range and compared with MS diodes. The effects of the insulating layer on barrier height and carrier transport are delineated by an activation energy analysis. Excess currents observed at low forward and reverse bias have also been analyzed and their cause identified. A capacitance anomaly consistently noticed in MIS Schottky barriers is resolved by stipulating a non-uniform interfacial layer, and a self-consistent model of the GaAs MIS Schottky barrier is developed by analyzing I-V and C-V data of both MIS and MS diodes.     

On the mechanisms limiting mobility in InP/InGaAs buried channel nMISFETs

MISFETs incorporating the InP/InGaAs buried channel showed a high peak mobility of 5500 cm²/V s. Spillover of the inversion carriers from the buried channel to the MIS interface on the InP barrier layer caused drastic mobility degradation as the carrier concentration was increased. The spillover was evidenced by observing a negative transconductance and a kink in split capacitance-voltage curves. Remote scattering by the trapped charges at the MIS interface also reduced the mobility when the InP barrier layer was as thin as 2 nm.     

Reverse breakdown behavior in organic pin-diodes comprising C60 and pentacene: Experiment and theory

Charge carrier transport under reverse voltage conditions is of major relevance in devices like organic photo-detectors, organic solar cells (tandem cells), organic light emitting diodes (generation contacts), and organic Zener diodes. We present organic pin-diodes comprising molecular doped layers of pentacene and C60 with an adjustable and reversible reverse breakdown behavior. We discuss the electric field and temperature dependence of the breakdown mechanism and propose a coherent charge transport scenario to describe the experimental findings. Within this model a field assisted tunneling of charge carriers over a rather large distance from valence to conductance states (and vice versa) governs the breakdown behavior. This is in accordance to experimental observations where charge carriers can overcome a layer thickness of 110 nm in the breakdown regime.     

The Effect of Gate‐Dielectric Surface Energy on Pentacene Morphology and Organic Field‐Effect Transistor Characteristics

The effects of the surface energy of polymer gate dielectrics on pentacene morphology and the electrical properties of pentacene field‐effect transistors (FETs) are reported, using surface‐energy‐controllable poly(imide‐siloxane)s as gate‐dielectric layers. The surface energy of gate dielectrics strongly influences the pentacene film morphology and growth mode, producing Stranski–Krastanov growth with large and dendritic grains at high surface energy and three‐dimensional island growth with small grains at low surface energy. In spite of the small grain size (≈ 300 nm) and decreased ordering of pentacene molecules vertical to the gate dielectric with low surface energy, the mobility of FETs with a low‐surface‐energy gate dielectric is larger by a factor of about five, compared to their high‐surface‐energy counterparts. In pentacene growth on the low‐surface‐energy gate dielectric, interconnection between grains is observed and gradual lateral growth of grains causes the vacant space between grains to be filled. Hence, the higher mobility of the FETs with low‐surface‐energy gate dielectrics can be achieved by interconnection and tight packing between pentacene grains. On the other hand, the high‐surface‐energy dielectric forms the first pentacene layer with some voids and then successive, incomplete layers over the first, which can limit the transport of charge carriers and cause lower carrier mobility, in spite of the formation of large grains (≈ 1.3 μm) in a thicker pentacene film.     

Limitations of the MIS capacitance method resulting from semiconductor properties

The influence of doping profiles, traps and grain boundaries in the semiconductor on the C-V characteristics of ideal MIS diodes is calculated. In addition, it is shown that losses and frequency dispersion as well as hysteresis effects can result from traps in the semiconductor. If these semiconductor properties are ignored misinterpretations in terms of interface state density will arise which can be about 10¹² cm⁻². In the case of decreasing doping profiles and traps in a n-type semiconductor one will get an apparent positive surface charge while an apparent negative surface charge will result from increasing profiles and grain boundaries. A method for the experimental determination of the C-V characteristics of ideal MIS diodes is developed. This method is applicable to stable diodes with only shallow traps in the semiconductor.     

Electroluminescence from MIS silicon-based light emitters with arrays of self-assembled Ge(Si) nanoislands

We report on the electroluminescence from silicon-based metal–insulator–semiconductor (MIS) diodes with arrays of self-assembled Ge(Si) nanoislands. Aluminum oxide (Al₂O₃) is used as an insulator material in the MIS contact. Variations in the electroluminescence spectra caused by changing the metal work function are examined. The intense electroluminescence from Ge(Si) nanoislands localized at a distance of 50 nm from the insulator–semiconductor interface is observed at room temperature. The emission spectrum is found to be controlled by choosing the design of the semiconductor structure and the barrier height for injected carriers.     

Structural phase transition in pentacene caused by molecular doping and its effect on charge carrier mobility

The structural properties and charge carrier mobility of pentacene doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 2,2-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) are studied by X-ray diffraction, scanning electron microscopy, field effect transistor measurements, and space charge limited currents (SCLC). We observe the presence of polycrystalline and amorphous domains within the doped pentacene film grown under co-deposition conditions. The appearance of the amorphous phase is induced by the molecular dopants F4-TCNQ and F6-TCNNQ. A strong drop of crystallite size is obtained at a doping concentration of around 7 and 4 wt.%, respectively. The loss of the polycrystalline structure is correlated to a strong decrease of the charge carrier mobility in pentacene in horizontal and vertical film structures. We discuss typical scenarios of charge transport for polycrystalline and amorphous thin films in order to explain the observed loss of mobility originated by the doping induced structural phase transition. In this way an optimum doping concentration for highest conductivity with acceptable mobility is determined which can help to improve the performance of organic solar cells and organic high-frequency rectification diodes.