Anode Contact Research Articles

Highly oriented indium tin oxide films for high efficiency organic light-emitting diodes

Highly oriented indium tin oxide (ITO) thin films were grown by pulsed-laser deposition (PLD) on glass and single-crystal yttria-stabilized zirconia (YSZ) substrates. The structural, electrical, and optical properties of these films were investigated as a function of oxygen partial pressure. Films were deposited at substrate temperature of 300 °C in mixed gases (12 mTorr of argon and 1–50 mTorr of oxygen) using a KrF excimer laser (248 nm and 30 ns full width at half maximum) at a fluence of 1.2 J/cm2. ITO films (300 nm thick), deposited by PLD on YSZ at 300 °C in a gas mixture of 12 mTorr of argon and 6 mTorr of oxygen, exhibit a low electrical resistivity (1.6×10−4 Ω cm) with a high transparency (∼74%) at 550 nm. ITO films deposited on both glass and YSZ substrates have been used as an anode contact in organic light-emitting diodes. A comparison of the device performance for the two substrates shows that the device fabricated on the ITO/YSZ has a higher external quantum efficiency than that of the device fabricated on the ITO glass.

Indium tin oxide thin films grown on flexible plastic substrates by pulsed-laser deposition for organic light-emitting diodes

Transparent conducting indium tin oxide (ITO) thin films were grown by pulsed laser deposition (PLD) on flexible polyethylene terephthalate (PET) substrates. The structural, electrical, and optical properties of these films were investigated as a function of substrate deposition temperature and background gas pressure. ITO films (200 nm thick), deposited by PLD on PET at 25 °C and 45 mTorr of oxygen, exhibit high optical transparency (∼87%) in the visible (400–700 nm) with a low electrical resistivity of 7×10−4 Ω cm. ITO films grown by PLD on PET were used as the anode contact in organic light-emitting devices. A luminous power efficiency of ∼1.6 lm/W was achieved at 100 cd/m2, slightly higher than that (∼1.5 lm/W) measured for the control device based on a sputter-deposited ITO on glass.

A study on acrylamide polymerization by anodic contact glow‐discharge electrolysis: A novel tool

AbstractContact glow‐discharge electrolysis (CGDE) is an unconventional electrolytic phenomenon in which a plasma is sustained by a direct current (dc) glow‐discharge between an electrode and the liquid electrolyte around it. A remarkable feature of CGDE is highly nonfaradaic chemical effects at the glow‐discharge electrode. During anodic CGDE of an aqueous electrolyte, non‐Faradaic yields originate mainly from reactions triggered by H• and OH• radicals generated in high local concentrations near the anodic plasma/liquid electrolyte interface during the process. The radical‐generating potentiality of anodic CGDE was explored for the polymerization of acrylamide in aqueous media. The percentage of monomer conversion, rate of polymerization, charge efficiency, and viscometric average molar mass of the polymers produced were measured as functions of the quantities of electricity passed. The charge efficiency of the polymerization of acrylamide by anodic CGDE was at least 1 order of magnitude higher than that of ordinary electrochemically initiated polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1584–1588, 2001

Fabrication and characterization of heterojunction diodes with HVPE-grown GaN on 4H-SiC

GaN/SiC heterojunctions can improve the performance considerably for BJTs and FETs. In this work, heterojunction diodes have been manufactured and characterized. The fabricated diodes have a GaN n-type cathode region on top of a 4H-SiC p-type epi layer. The GaN layer was grown with HVPE directly on off-axis SIC without a buffer layer. Mesa structures were formed and a Ti metallization was used as cathode contact to GaN, and the anode contact was deposited on the backside using sputtered Al. Both current-voltage (I-V) and capacitance-voltage (C-V) measurements were performed on the diode structures. The ideality factor of the measured diodes was 1.1 and was constant with temperature. A built in potential of 2.06 V was extracted from I-V measurements and agrees well with the built in potential from C-V measurements. The conduction band offset was extracted to 1.1 eV and the heterojunction was of type II. The turn on voltage for the diodes is about 1 V lower than expected and a suggested mechanism for this effect is discussed.

Transparent conducting Zr-doped In2O3 thin films for organic light-emitting diodes

Zirconium-doped indium oxide (ZIO) thin films (∼2000 Å thick) have been deposited by pulsed-laser deposition on glass substrates without a postdeposition anneal. The structural, electrical and optical properties of these films have been investigated as a function of substrate temperature and oxygen partial pressure during deposition. Films were deposited at substrate temperatures ranging from 25 °C to 400 °C in O2 partial pressures ranging from 0.1 to 50 mTorr. The films (∼2000 Å thick) deposited at 200 °C in 25 mTorr of oxygen show electrical resistivities as low as 2.5×10−4 Ω cm, an average visible transmittance of 89%, and an optical band gap of 4.1 eV. The ZIO films were used as a transparent anode contact in organic light emitting diodes and the device performance was studied. The external quantum efficiency measured from these devices was about 0.9% at a current density of 100 A/m2.

Effect of film thickness on the properties of indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) thin films (40–870 nm) were grown by pulsed laser deposition on amorphous substrates and the structural, electrical, and optical properties of these films were investigated. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2, at substrate temperature of 300 °C and 10 mTorr of oxygen pressure. For ITO films (30–400 nm thickness) deposited at 300 °C in 10 mTorr of oxygen, a resistivity of 1.8–2.5×10−4 Ω cm was observed and the average transmission in the visible range (400–700 nm) was about 85%–90%. The Hall mobility and carrier density for ITO films (40–870 nm thickness) were observed to be in the range of 24–27 cm2/V s and 8–13×1020 cm−3, respectively. The ITO films have been used as the anode contact in organic light emitting diodes and the effect of ITO film thickness on the device performance has been studied. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 60–100 nm. The device with the optimum thickness of ITO anode showed an external quantum efficiency of about 0.85% at 100 A/m2.

Nonlinear transport phenomena and mode locking in millimetre-wave Gunn-instability structures

The nonlinear behaviour of drift-diffusion transport model of the 3 µm GaAs Gunn-effect structure is analysed here. It is shown that the correct choice of the v(E) form and its exact approximation is necessary. Increase of the v(E) dependence steepness in the region above the threshold leads to dissolution of the dipole domains before anode contact. A local extremum appears on the dependence of the transit frequency on the applied dc voltage. This complicates mode-locking processes when a driving microwave signal is applied. The effective length of the transit region quasiperiodically changes, similarly to threshold modulation in relaxation oscillator models. The nonlinear interaction of oscillations leads to multiplication of the current oscillations period and complication of the terminal current shape. The quasiperiodic microwave signal outside the phase-locking regions has different upper and lower envelopes, leading to the appearance of an additional high-frequency component in the signal spectrum. The principal phase-locking region boundaries are defined by the golden mean proportions. Some results of the experimental investigations of Gunn oscillator nonlinear dynamics in the 40–55 GHz frequency band are also given here.

Characterization of 4H-SiC gate turn-off thyristor

An 800 V 4H-SiC gate turn-off thyristor (GTO) is fabricated and characterized. The switching characteristics of the SiC GTO investigated over a temperature range from 25°C to 240°C will be presented with a switched current density through anode contact up to 10,000 A/cm 2. The turn-on and turn-off times as well as the minority electron lifetime in the p-base region will also be reported as a function of temperature. The high current density capability of the GTO is consistent with the observed reproducible avalanche characteristics of the GTO’s anode p +n diode. The maximum controllable current decreases with increasing temperature as a result of carrier mobility degradation.

Transparent conducting aluminum-doped zinc oxide thin films for organic light-emitting devices

Aluminum-doped zinc oxide (AZO) thin films (∼3000 Å) with low electrical resistivity and high optical transparency have been grown by pulsed-laser deposition on glass substrates without a postdeposition anneal. Films were deposited at substrate temperatures ranging from room temperature to 400 °C in O2 partial pressures ranging from 0.1 to 50 mTorr. For 3000-Å-thick AZO films grown at room temperature in an oxygen pressure of 5 mTorr, the electrical resistivity was 8.7×10−4 Ω cm and the average optical transmittance was 86% in the visible range (400–700 nm). For 3000-Å-thick AZO films deposited at 200 °C in 5 mTorr of oxygen, the resistivity was 3.8×10−4 Ω cm and the average optical transmittance in the visible range was 91%. AZO films grown at 200 °C were used as an anode contact for organic light-emitting diodes. The external quantum efficiency measured from these devices was about 0.3% at a current density of 100 A/m2.

Novel photoconductive effect in spark-processed silicon with metal contacts

Spark-processed silicon (sp-Si) has been shown to display strong photoluminescence in the UV/blue and green spectral ranges. Furthermore, ferromagnetic behavior, electroluminescence and cathodoluminescence have been reported. The present contribution reports on a novel photoconductive effect, which has been observed in a simple device made from sp-Si. A symmetric device consisting of two metal contacts on sp-Si increases its electrical conductivity dramatically when the junction of the sp-Si and the anode contact is illuminated with UV or visible light. However, its conductivity decreases when the sp-Si/cathode contact junction is illuminated. The magnitude of both effects is proportional to the intensity of the light source, and is dependent on the wavelength of the light. There is no significant change in conductivity when the bulk of the sp-Si is illuminated. This behavior is unique and strikingly different than existing devices such as p-n, p-i-n, and Schottky diodes, as well as photoconductive devices, which all show an increase in conductivity in both directions when illuminated.

A spectroscopic detecting system for measuring the temperature distribution of silver breaking arc using a CCD color camera

This paper describes a spectroscopic detecting system that utilizes a CCD color camera to observe the distributions of two spectral intensities along the axis of an arc column. Following this, the distributions of Ag I 421 nm and Ag I 547 mm spectra observed along the axis of an arc column and the arc temperature calculated from the two spectra when silver contacts interrupt a circuit of dc 50 V and 3.3 A are presented. The spectral intensities of excited silver atoms are strongest near the cathode and become weaker with distance from the cathode contact. The temperature in the cross section of an arc column is highest in the axis of the arc column. The temperature along the axis of arc column is highest near the cathode and becomes weaker with distance from the cathode. Furthermore, in this study it is clarified that CN band spectra exist near the anode contact.

Enhanced Hole Injection and Brightness of Organic Electroluminescent Devices with Indium Tin Oxide Surface Modification Using Oxygen Plasma Treatment

The improvement of indium tin oxide (ITO) anode contact to organic electroluminescent devices via oxygen plama treatment is investigated. Enhanced hole-injection efficiency improved the performance of organic light-emitting devices. The injection current increased by 1-2 orders of magnitude and the turn-on voltages droped several volts. Much higher injection current could be applied to achieve much higher brightness. Maximum brightness and luminous efficiency can reach to about 4000 cd/m2 and 0.3lm/W for the structure of ITO/TPD/Alq3/Al, respectively.

Ultra-Fast NiSi2 Formation in p+-Si by High Current Densities

ABSTRACTAn ultra-fast lateral formation of epitaxial NiSi2 line in heavily doped p+-Si diffused tub was created at room temperature ambient under the application of current densities of 106 A/cm2. A NiSi2 line of 140 μm long and I μm wide linking the cathode and anode contacts to p+-Si was formed within 1 second. Shorter lines were formed with lower current densities. The formation of shorter lines was again completed within 1 second and there was very little subsequent growth upon further stressing. The line length strongly depends upon applied current which tend to suggest that the line formation is limited by driving force rather than by kinetics. We propose that the ultra-fast silicide formation is a result of Ni interstitial diffusion in Si induced by electron-hole recombination heating and electromigration.

Polarity effect on failure of Ni and Ni2Si contacts on Si

Stability of contacts in shallow junction devices against high current density is a reliability issue for very large scale integration technology. We have observed a strong polarity effect on failure at nickel and nickel silicide contacts on both n- and p-type Si under high stress conditions. In a pair of cathode and anode contacts the Ni/n+-Si contact pair fails at the anode, while the Ni/p+-Si pair fails at the cathode. The Ni/Ni2Si/n+-Si and Ni/Ni2Si/p+-Si were found to fail preferentially at the cathode. Microbeam Rutherford backscattering spectrometry and Auger electron spectroscopy depth profiles show that a silicide reaction occurs between Ni and Si during current stressing, especially at the failed contacts. In situ resistance data indicate that the resistance of the failed contact increases with time while that of the other contact in the pair remains constant. Transmission electron microscopy shows that the silicide formation is not uniform at the damaged contacts. A mixture of dominant epitaxial NiSi2 and a minor amount of polycrystalline NiSi2 phases was identified. We have proposed mechanisms to explain the polarity effect on failure: wear-out mechanism for the damaged positive contacts of Ni/n+-Si, electromigration enhanced silicide formation for the damaged negative contacts of Ni/Ni2Si/n+-Si and electron-hole recombination mechanism for the damaged negative contacts of Ni/p+-Si and Ni/Ni2Si/p+-Si.

Surface modification of indium tin oxide by plasma treatment: An effective method to improve the efficiency, brightness, and reliability of organic light emitting devices

We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from >20 to <10 V, the external electroluminescence quantum efficiency (backside emission only) increases by a factor of 4 (from 0.28% to 1%), a much higher drive current can be applied to achieve a much higher brightness (maximum brightness ∼10,000 cd/m2 at 1000 mA/cm2), and the forward-to-reverse bias rectification ratio increases by orders of magnitude (from 102 to 106–107). The lifetime of the device is also enhanced by two orders of magnitude.

Reliability and micro-structural properties of GaAs Schottky diodes for submillimeter-wave applications : A. Grub, V. Krozer, A. Simon and H. L. Hartnagel. Solid-State Electronics, 37, 12, 1925 (1994)

Abstract Whisker contacted GaAs Schottky barrier diodes are the standard devices for mixing and multiplier applications in the THz frequency range. This is mainly due to their minimum parasitics and mature technology. But with the decreasing size of the anode contact, which is required for operation at high frequencies (up to approx. 3 THz), the reliability and the micro-structural understanding of the Schottky barrier becomes increasingly important. This contribution presents new results concerning the reliability of Schottky diodes and the physical properties of small-area Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fabricated with different epilayer doping concentrations and anode diameters. Measured I / V characteristics show that the diode current deviates considerably from the ideal thermionic current behavior with decreasing diode diameter. This deviation shows an exponential dependence on the diode voltage and is a function of the doping concentration of the active layer. For a given doping concentration in the epi-layer and decreasing anode diameter, this phenomenon shifts the minimum of the ideality factor towards higher current densities. An explanation is given in terms of a difference of the cyrstallinity of the polycrystalline platinum films on the GaAs for decreasing SiO 2 aperture size in connection with a reduced Pt mobility in the electrolyte. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results show that the barrier height and the ideality factor of the fabricated structures are not affected by thermal stress. Electrical stress induced by large forward currents up to a current density of 10 kA/mm 2 even leads to a slight increase of the barrier height and a reduction of the series resistance.

Fourier transform IR monitoring of porous silicon passivation during post-treatments such as anodic oxidation and contact with organic solvents

The surface passivation of porous silicon is a determining factor in the emission efficiency of the material. The hydrogen surface coverage has been shown to provide very efficient passivation. In this work, we have performed Fourier transform IR (FTIR) measurements to monitor the Si-H surface coverage, which is readily obtained after the layer formation in HF, during different post-treatments (anodic oxidation and contact with organic solvent) and to relate it to the emission efficiency. FTIR studies, performed at different steps of the electrochemical oxidation, indicate that, during the anodic treatment, the hydrogen surface coverage is preserved and that the oxidation takes place on the back bonds of the surface silicon atoms. The importance of the hydrogen coverage is also shown by the analysis of porous layers treated in boiling CCl 4 after formation. This treatment provokes the desorption of the hydrogen atoms and results in a drastic decrease in the photoluminescence. When samples are immersed in boiling methanol after formation, FTIR analyses show that there is also a partial loss of the hydrogen coverage, but accompanied with an oxidation of the material, so that no significant changes in the emission efficiency can be observed.

Reliability and micro-structural properties of GaAs Schottky diodes for submillimeter-wave applications

Whisker contacted GaAs Schottky barrier diodes are the standard devices for mixing and multiplier applications in the THz frequency range. This is mainly due to their minimum parasitics and mature technology. But with the decreasing size of the anode contact, which is required for operation at high frequencies (up to approx. 3 THz), the reliability and the micro-structural understanding of the Schottky barrier becomes increasingly important. This contribution presents new results concerning the reliability of Schottky diodes and the physical properties of small-area Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fabricated with different epilayer doping concentrations and anode diameters. Measured I/ V characteristics show that the diode current deviates considerably from the ideal thermionic current behavior with decreasing diode diameter. This deviation shows an exponential dependence on the diode voltage and is a function of the doping concentration of the active layer. For a given doping concentration in the epi-layer and decreasing anode diameter, this phenomenon shifts the minimum of the ideality factor towards higher current densities. An explanation is given in terms of a difference of the cyrstallinity of the polycrystalline platinum films on the GaAs for decreasing SiO 2 aperture size in connection with a reduced Pt mobility in the electrolyte. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results show that the barrier height and the ideality factor of the fabricated structures are not affected by thermal stress. Electrical stress induced by large forward currents up to a current density of 10 kA/mm 2 even leads to a slight increase of the barrier height and a reduction of the series resistance.

Composite anode contact for planar transferred electron devices

In order to prevent the anode contact of transferred electron devices (TEDs) from burning out, a Schottky contact is frequently used. Unfortunately, this approach gives rise to a decrease in efficiency due to the built-in voltage of the Schottky contact. A possible way of avoiding this decrease is to use a composite contact. By combining a short stripe of a Schottky metal with an ohmic contact, the loss of DC power dissipated in the Schottky anode contact is eliminated, thereby maintaining to some extent the depletion layer necessary for preventing the formation of a stationary high field domain at the anode contact. With this approach, a 30% increase in efficiency of a CW operated field effect controlled transferred electron device (FECTED) oscillator at a frequency of 35 GHz has been obtained. The achieved efficiency of 3.8% is, to the authors' knowledge, the highest efficiency obtained with a planar transferred electron oscillator (TEO) at 35 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Current-voltage characteristics of high resistivity CdTe

Current-voltage characteristics of high resistivity CdTe, with consideration to Schottky barrier height at the contact and defect levels in the band gap were investigated by experimental and numerical simulation methods. From the electron injection investigation, the density of electron traps corresponded to the doped Cl concentration in the crystal. Numerical simulation of current-voltage characteristics was made based on Schockley Read Hall (SRH) statistics. In the low bias region, calculated characteristics were in fairly close agreement with the experimental results for Pt/CdTe/Pt diode. The electric field was not uniform along the biased direction and an almost neutral region was present in the vicinity of anode contact even when the bias was applied.