Preparation of polycrystalline CdZnTe thick film Schottky diode for ultraviolet detectors

High resistivity (3 × 10⁹ Ω cm) polycrystalline CdZnTe thick films with thickness of 25 μm–150 μm were grown on SnO₂: F-coated glass substrates by close-spaced sublimation method. The properties of polycrystalline CdZnTe films were studied by XRD, SEM and EDS, respectively. A CdZnTe film Schottky diode detector was also fabricated and investigated using current–voltage and capacitance–voltage methods. The photo-current density of the device was about 1508.69 nA/mm² under light illumination (λ = 260 nm), at an applied negative voltage of 15 V. The results showed that polycrystalline CdZnTe thick film was suitable for application in ultraviolet detectors.     

In-line deposition of organic light-emitting devices for large area applications

Light-emitting diodes based on organic semiconductors show promising features for display and lighting applications. A vertical in-line deposition technique for organic light-emitting diode (OLED) manufacturing was developed.OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime. The preparation of p-i-n type devices was performed with the in-line fabrication tool resulting in highly efficient OLED with low operating voltage. The lowest operating voltage was achieved for green diodes with 2.9 V for 100 cd/m². This demonstrates that the p-i-n device concept can be applied under manufacturing conditions. In-line manufactured highly efficient red, green and blue OLED are presented.One important aspect for fabrication cost is the used ground contact, which is commonly made by indium tin oxide (ITO). For low cost fabrication an alternative for ITO has to be used. In this work, ITO was replaced by aluminium doped zinc oxide (ZAO). The results are comparable to OLEDs using ITO as transparent conductive oxide.     

Nanoscale Channel Gate-Tunable Diodes Obtained by Asymmetric Contact and Adhesion Lithography on Fluoropolymers

Adhesion lithography offers to fabrication of coplanar asymmetric nanogap electrodes with a low-cost and facile process. In this study, a gate-tunable diode with coplanar asymmetric nanogap is fabricated using adhesion lithography. A fluoropolymer material is introduced to the adhesion lithography process to ensure a manufacturing patterning process yield as high as 96.7%. The asymmetric electrodes formed a built-in potential, leading to rectifying behavior. The coplanar electrode structure allowed the use of a gate electrode in vertical contact with the channel, resulting in gate-tunable diode characteristics. The nanoscale channel induced a high current density (3.38 × 10⁻⁷ A∙cm⁻¹), providing a high rectification ratio (1.67 × 10⁵ A∙A⁻¹). This rectifier diode is confirmed to operate with pulsed input signals and suggests the gate-tunability of nanogap diodes.     

Voltage tunable Ba0.6Sr0.4TiO3 thin films and coplanar phase shifters

Not only the ferroelectric properties of Ba_0.6Sr_0.4TiO₃ thin films but also the design, fabrication and microwave properties of coplanar waveguide (CPW) phase shifter were investigated. When the applied voltage changes from zero to 20 V range at a frequency of 1 kHz, the dielectric constant, tunability, remanent polarization (2P_r) and coercive electric field (Ec) of the BST films are 1266, 29.5%, 2.29 μC/cm² and 22.27 kV/cm, respectively. The structure of CPW phase shifter was designed by ANSOFT High Frequency Structure Simulator (HFSS) and Agilent Advanced Design System (ADS). The designed phase shifter consists of 56 same sections. It is observed from the photograph of the fabricated device that the lines of electrodes are regular and the widths of bottom electrodes overlapped by top electrodes are about 5 μm. It was found that the phase shift gradually increases as the voltage increases from 0 V to 20 V, which may be due to the tunability of BST films varies with external dc field. About 168° phase shift was achieved at 28 GHz with a bias voltage of 20 V.     

Improvement of white organic light emitting diode performances by an annealing process

White organic light emitting diode (OLED) devices with the structure ITO/PHF:rubrene/Al, in which PHF (poly(9,9-di-n-hexylfluorenyl-2,7-diyl)) is used as blue light emitting host and rubrene (5,6,11,12-tetraphenylnapthacene) as an orange dye dopant, have been fabricated. Indium tin oxide (ITO) coated-glass and aluminium were used as anode and cathode, respectively. The devices were fabricated with various rubrene-dopant to obtain a white light emission. The OLED device that composed of several concentrations of rubrene-doped PHF film was prepared in this study. It was found that the concentration of rubrene in the PHF-rubrene thin film matrix plays a key role in producing the white color emission. In a typical result, the device composed of 0.06 wt.% rubrene-dopant produced the white light emission with the Commission Internationale de L'Eclairage (CIE) coordinate of (0.30,0.33). The turn-on voltage and the brightness were found to be as low as 14.0 V and as high as 6540 cd/m², respectively. The annealing technique at relatively low temperature (50 °C, 100 °C, and 150 °C) was then used to optimize the performance of the device. In a typical result, the turn-on voltage of the device could be successfully reduced and the brightness could be increased using the annealing technique. At an optimum condition, for example, annealed at 150 °C, the turn-on voltage as low as 8.0 V and the brightness as high as 9040 cd/m² were obtained. The mechanism for the improvement of the device performance upon annealing will be discussed.     

Effect of thickness of ZnO active layer on ZnO-TFT's characteristics

We have investigated the electrical characteristics of ZnO thin film transistors with respect to the thickness of ZnO active layers. The ZnO layers with the thickness of 30 nm to 150 nm were deposited on bottom gate patterned Si substrate by RF sputtering at room temperature. The low-temperature oxide served as gate dielectric. As ZnO channel layer got thicker, the leakage current at V_DS = 30 V and V_G = 0 V greatly increased from 10^− 10 A to 10^− 6 A, while the threshold voltage decreased from 15 V to 10 V. On the other hand, the field effect mobility got around 0.15 cm²/V s except for the 30-nm-thick channel. Overall, the 55-nm-thick ZnO channel layer showed the best performance.     

Electrical and photocurrent characteristics of Au/PVA (Co-doped)/n-Si photoconductive diodes

In this work, we investigate the some main electrical and photocurrent properties of the Au/PVA(Co-doped)/n-Si diodes by using current–voltage (I–V) measurements at dark and various illumination intensity. Two types of diodes with and without polyvinyl alcohol (PVA) (Co-doped) polymeric interfacial layer were fabricated and measured at room temperature. Results show that the polymeric interfacial layers and series resistance (R_s) strongly affect the main electrical parameters of these structures. Also, metal/polymer/semiconductor (MPS) diode with PVA (Co-doped) interfacial organic layer is very sensitive to the light such that the current in reverse bias region increase by 10³–10⁴ times with the increasing illumination intensity. The open circuit voltage V_oc and short-circuit current I_sc values of this MPS diode under 100 mW/cm² illumination intensity were found as 0.28 V and 19.3 μA, respectively.     

Fabrication and characterization of all-oxide heterojunction p-CuAlO2 + x/n-Zn1 − xAlxO transparent diode for potential application in “invisible electronics”

Transparent p-n heterojunction diodes have been fabricated by p-type copper aluminum oxide (p-CuAlO_2 + x) and n-type aluminum doped zinc oxide (n-Zn_1 − xAl_xO) thin films on glass substrates. The n-layers are deposited by sol-gel-dip-coating process from zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and aluminum nitrate (Al(NO₃)₃·9H₂O). Al concentration in the nominal solution is taken as 1.62 at %. P-layers are deposited onto the ZnO:Al-coated glass substrates by direct current sputtering process from a prefabricated CuAlO₂ sintered target. The sputtering is performed in oxygen-diluted argon atmosphere with an elevated substrate temperature. Post-deposition oxygen annealing induces excess oxygen within the p-CuAlO_2 + x films, which in turn enhances p-type conductivity of the layers. The device characterization shows rectifying current-voltage characteristics, confirming the proper formation of the p-n junction. The turn-on voltage is obtained around 0.8 V, with a forward-to-reverse current ratio around 30 at ± 4 V. The diode structure has a total thickness of 1.1 μm and the optical transmission spectra of the diode show almost 60% transmittance in the visible region, indicating its potential application in ‘invisible electronics’. Also the cost-effective procedures enable the large-scale production of these transparent diodes for diverse device applications.     

Room temperature deposited vanadium oxide thin films for uncooled infrared detectors

We demonstrate the room temperature deposition of vanadium oxide thin films by pulsed laser deposition (PLD) technique for application as the thermal sensing layer in uncooled infrared (IR) detectors. The films exhibit temperature coefficient of resistance (TCR) of 2.8%/K implies promising application in uncooled IR detectors. A 2-D array of 10-element test microbolometer is fabricated without thermal isolation structure. The IR response of the microbolometer is measured in the spectral range 8-13 μm. The detectivity and the responsivity are determined as ∼6×10⁵ cm Hz^1/2/W and 36 V/W, respectively, at 10 Hz of the chopper frequency with 50 μA bias current for a thermal conductance G∼10-3 W/K between the thermal sensing layer and the substrate. By extrapolating with the data of a typical thermally isolated microbolometer (G∼10⁻⁷ W/K), the projected responsivity is found to be around 10⁴ V/W, which well compares with the reported values.     

Edge sensitivity of “edgeless” silicon pad detectors measured in a high-energy beam

We report measurements in a high-energy beam of the sensitivity of the edge region in “edgeless” planar silicon pad diode detectors. The edgeless side of these rectangular diodes is formed by a cut and break through the contact implants. A large surface current on such an edge prevents the normal reverse biasing of this device above the full depletion voltage, but we have shown that the current can be sufficiently reduced by the use of a suitable cutting method, followed by edge treatment, and by operating the detector at a low temperature. A pair of these edgeless silicon diode pad sensors was exposed to the X5 high-energy pion beam at CERN, to determine the edge sensitivity. The signal of the detector pair triggered a reference telescope made of silicon microstrip detector modules. The gap width between the edgeless sensors, determined using the tracks measured by the reference telescope, was then compared with the results of precision metrology. It was concluded that the depth of the dead layer at the diced edge is compatible with zero within the statistical precision of ±8 μm and systematic error of ±6 μm.     

ZnO nanobelt/nanowire Schottky diodes formed by dielectrophoresis alignment across au electrodes

Rectifying diodes of single nanobelt/nanowire-based devices have been fabricated by aligning single ZnO nanobelts/nanowires across paired Au electrodes using dielectrophoresis. A current of 0.5 microA at 1.5 V forward bias has been received, and the diode can bear an applied voltage of up to 10 V. The ideality factor of the diode is approximately 3, and the on-to-off current ratio is as high as 2,000. The detailed IV characteristics of the Schottky diodes have been investigated at low temperatures. The formation of the Schottky diodes is suggested due to the asymmetric contacts formed in the dielectrophoresis aligning process.     

Performance enhancement of polymer Schottky diode by doping pentacene

Schottky diodes have been fabricated using pentacene-doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as a semiconducting material. To understand the fundamental properties of the pentacene-doped PEDOT:PSS, ultraviolet visible (UV) absorption spectroscopy was employed. It was found that a significant amount of pentacene can dissolve in n-methylpyrrolidone solvent. No characteristic absorption peak of pentacene was observed in the UV-visible spectra of PEDOT:PSS films doped with pentacene,. However, the absorption intensity of the doped PEDOT:PSS films increased as the pentacene concentration increased in particular in the UV region. The atomic force microscope images show that the surface roughnesses of PEDOT:PSS films increased as the pentacene concentration increased. Three-layer Schottky diodes comprising Al/PEDOT:PSS/Au or Al/PEDOT:PSS-pentacene/Au were fabricated. The maximum forward currents of non-doped and doped Schottky diodes were 4.8 and 440 µA/cm² at 3.3 MV/m, respectively. The forward current increased nearly two orders of magnitude for Schottky diode doped with 11.0 wt.% of pentacene.     

Microstructure and electric properties of (104)/(014)-oriented Bi3.15Nd0.85Ti3O12 films on Pt (111)/Ti/SiO2/Si by sol-gel method

Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films with predominant (104)/(014) orientation were fabricated directly on (111)Pt/Ti/SiO₂/Si substrates through a sol-gel process. The volume fraction of (104)/(014)-oriented grains in the film was estimated to be about 65% according to X-ray pole figure. The BNdT film is dense and uniform and consists of columnar grains penetrating the whole film thickness. The (104)/(014)-oriented BNdT film capacitors showed excellent ferroelectric properties with 2P_r = 46.4 μC/cm² and E_c ≈ 140 kV/cm. The films also exhibit excellent piezoelectric property, with high piezoelectric coefficient d₃₃ ≈ 17 pm/V.     

Enhancement-mode thin film transistor with nitrogen-doped ZnO channel layer deposited by laser molecular beam epitaxy

The thin film transistors (TFTs) based on nitrogen doped zinc oxide (ZnO) were investigated by laser molecular beam epitaxy. The increase of ZnO films' resistivity by nitrogen doping was found and applied in enhancement mode ZnO-TFTs. The ZnO-TFTs with a conventional bottom-gate structure were fabricated on thermally oxidized p-type silicon substrate. Electrical measurement has revealed that the devices operate as an n-channel enhancement mode and exhibit an on/off ratio of 10⁴. The threshold voltage is 5.15 V. The channel mobility on the order of 2.66 cm² V^− 1 s^− 1 has been determined.     

Study of polarization phenomena in Schottky CdTe diodes using infrared light illumination

Schottky CdTe diode detectors suffer from a polarization phenomenon, which is characterized by degradation of the spectral properties over time following exposure to high bias voltage. This is considered attributable to charge accumulation at deep acceptor levels. A Schottky CdTe diode was illuminated with an infrared light for a certain period during a bias operation, and two opposite behaviors emerged. The detector showed a recovery when illuminated after the bias-induced polarization had completely progressed. Conversely, when the detector was illuminated before the emergence of bias-induced polarization, the degradation of the spectral properties was accelerated. Interpretation of these effects and discussion on the energy level of deep acceptors are presented.     

Indium zinc oxide semiconductor thin films deposited by dc magnetron sputtering at room temperature

Amorphous indium zinc oxide (IZO) thin films were prepared on glass substrates by dc magnetron sputtering at room temperature. The resistivity of IZO films could be controlled between 3.8 × 10⁻³ and 2.5 × 10⁶ Ω cm by varying the oxygen partial pressure during deposition, while keep the average transmittance over 83%. With IZO films as channel layers, whose surface root-mean-square roughness was less than 1 nm, thin film transistors were fabricated at room temperature, showing enhanced mode operation with good saturation characteristics, mobility of 5.2 cm² V⁻¹ s⁻¹, threshold voltage of 0.94 V and on/off ratio of ∼10⁴.     

Schottky and heterojunction diodes based on poly(3-octylthiophene) and poly(3-methylthiophene) films of high tensile strength

Schottky and heterojunction diodes were fabricated using high tensile strength polymers. The heterojunction diode was fabricated by sequential electrochemical polymerisation of 3-methyl thiophene and 3-octyl thiophene on an indium-tin oxide (ITO) coated glass substrate. The high tensile strength enabled the bilayer (used in heterojunction diodes) or the poly 3-octyl thiophene films (used in the Schottky diodes) to be peeled of from the substrate and sandwich it between any two desired metals. It was found that the Schottky diodes of ITO (or Si)/POT/Al (or Zn) exhibit moderate rectifying behaviour and ITO (or Si)/POT/Cu devices exhibit ohmic contact. The POT/PMT heterojunction diode showed excellent rectification effect when sandwiched between any two metals irrespective of their work function. This shows that the results observed were solely due to the polymer/polymer interface. The Cu/POT/PMT/Cu heterojunction system was used in this study. The carrier-flow of the two semiconductors in the Cu/POT/PMT/Cu heterojunction diode was discussed in details. The rectification ratio, the barrier height, and the ideality factor for the heterojunction diode were found to be 64 (±1.2 V), 0.81 eV, and 5.7 under ambient conditions, respectively. Some of the important energy band parameters were also determined.     

Photovoltaic and interface state density properties of the Au/n-GaAs Schottky barrier solar cell

The electrical and photovoltaic properties of the Au/n-GaAs Schottky barrier diode have been investigated. From the current-voltage characteristics, the electrical parameters such as, ideality factor and barrier height of the Au/n-GaAs diode were obtained to be 1.95 and 0.86 eV, respectively. The interface state distribution profile of the diode as a function of the bias voltage was extracted from the capacitance-voltage measurements. The interface state density D_it of the diode was found to vary from 3.0 × 10¹¹ eV⁻¹ cm⁻² at 0 V to 4.26 × 10¹⁰ eV⁻¹cm⁻² at 0.5 V. The diode shows a non-ideal current-voltage behavior with the ideality factor higher than unity due to the interfacial insulator layer and interface states. The diode under light illumination exhibits a good photovoltaic behavior. This behavior was explained in terms of minority carrier injection phenomenon. The photovoltaic parameters, such as open circuit voltage and short circuit current density were obtained to be 362 mV and J_sc = 28.3 μA/cm² under AM1, respectively.     

Characterization and improvement of thin natural diamond detectors for spectrometry of heavy ions below 1 MeV/amu

In our previous paper [V.Kh. Liechtenstein, N.V. Eremin, R. Golser, W. Kutschera, A.A. Paskhalov, A. Priller, P. Steier, Ch. Vockenhuber, S. Winkler, Nucl. Instr. and Meth. A 521 (2004) 203], first results on the evaluation of thin natural diamond-based detectors (NDDs) as an energy spectrometer for heavy ions in the energy range below 1 MeV/amu were presented. Although results were promising, the energy resolution of the detector was limited by an unexpected high-energy loss in the “dead layer” of the entrance window. In this paper, we report a significant improvement in the spectrometric properties of two highly selected and carefully treated NDDs with electrical contacts made of carbon and gold films as thin as about 10 and 20 μg/cm², respectively, instead of much thicker aluminum contacts used before. In particular, for the NDD with thin carbon contact an energy resolution of 7.6% for ¹⁹⁷Au-ions at 20.6 MeV was obtained. The energy cut-off of the detectors was reduced to 0.9 and 1.5 MeV for carbon and gold contact, respectively. The measured data on energy cut-off for different projectiles are compared with calculations, which yields an estimate of the thickness of the dead layers. Long-term irradiation runs proved stable spectroscopic performance of the detectors, in spite of the inherent “pumping” effects and imperfections of pulse height distributions. Our data suggest that NDD-based spectrometers might outperform other detector types in applications where very fast detectors with high radiation tolerance are required.     

Enhanced variant designs and characteristics of the microstructured solid-state neutron detector

Silicon diodes with large aspect ratio perforated microstructures backfilled with ⁶LiF show a dramatic increase in neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in this work are advancements in the technology with increased microstructure depths and detector stacking methods to increase thermal neutron detection efficiency. The highest efficiency devices thus far have delivered over 37% intrinsic thermal neutron detection efficiency by device-coupling stacking methods. The detectors operate as conformally diffused pn junction diodes with 1 cm² square-area. Two individual devices were mounted back-to-back with counting electronics coupling the detectors together into a single dual-detector device. The solid-state silicon device operated at 3 V and utilized simple signal amplification and counting electronic components. The intrinsic detection efficiency for normal-incident 0.0253 eV neutrons was found by calibrating against a ³He proportional counter and a ⁶LiF thin-film planar semiconductor device. This work is a part of on-going research to develop solid-state semiconductor neutron detectors with high detection efficiencies and uniform angular responses.