Schottky Barrier Devices Research Articles

Studies on electrical and photoelectrical behaviour of ITO/ArV/In Schottky barrier device

The electrical and photoelectrical properties of aryl viologen (ArV), chemically known as 1,1′-diphenyl-4,4′-bipyridinium dichloride, in the form of thin film, sandwiched between ITO and In electrode were studied. The current–voltage ( J– V) characteristics in dark show the rectification effect due to the formation of Schottky barrier at In–ArV interface. The diode quality factor of the device, greater than unity, indicates the recombination of electron-hole in depletion region. Ohmic conduction in low voltage range and space charge limited conduction (SCLC) controlled by an exponential distribution of traps above the valence band edge, for higher voltage region have been observed. Various electrical parameters were calculated from the analysis of J– V and capacitance–voltage ( C– V) characteristics at different temperatures and discussed in details. At higher frequencies, the device exhibit voltage independent capacitance, which is explained in terms of the extremely slow kinetics of space charge and low mobility of charge carriers. The photoaction spectra of the device and absorption spectra of the ArV thin film reveal that the fraction of light, which is absorbed near or within the diffusion length of exciton, is responsible for producing the free charge carriers. The photovoltaic parameters were calculated from the J– V characteristics under illumination through ITO and discussed in detail.

Influence of dioxygen and annealing process on the transport properties of nickel phthalocyanine Schottky-barrier devices

Schottky-barrier cells of the type (Al/p-NiPc/Au) are fabricated by successive vacuum depositions of p-NiPc thin films and aluminium fingers onto an ohmic gold electrode. The electrical conductivity has been measured both after exposure to oxygen for 10 days and after annealing at temperature up to 423 K. These cells showed high rectification in the oxygen-doped process. Current density–voltage characteristics under forward bias (aluminium electrode negative) are found to be due to space-charge-limited conduction controlled by a discrete trap level at lower voltage sections and by an exponential distribution of traps at higher voltage regions. Under reverse bias, the conduction processes are interpreted in terms of a transition from electrode-limited Schottky emission to the bulk-limited Poole–Frenkel effect. The effect of annealing showed similar behaviour and resulted in lowering the current density due to oxygen desorption. The linearity of the C −2–V dependence for oxygen-doped devices is associated with a homogenous distribution of the impurities inside the space-charge region and also provided evidence of uniform doping.

Microstructural and electronic transport properties of Pt xSi/p-Si(100) metal–semiconductor composite films

Sputtered Pt x Si/p-Si(100) films were characterized to assess their potential for applications in buried Schottky barrier devices. Transmission electron microscopy (TEM), X-ray diffraction and Hall measurements were used to measure structural and electronic transport properties and particle distribution. The findings presented in this paper provide insight into the potential device applications of these structures as photodetector devices operating in the 3–5 μm range.

Comparison of the performance of quantum well and conventional bulk infrared photodetectors

Investigations of the performance of quantum well infrared photodetectors (QWIPs) as compared o other types of semiconductor infrared detectors are presented. Two types of QWIPs are considered: GaAs/AlGaAs intersubband quantum well photoconductors and type II staggered InAs/InGaSb photodiodes. In comparative studies the HgCdTe photoconductors and photodiodes, PbSnTe photodiodes, Schottky barrier photoemissive detectors and doped silicon detectors are considered. It is assumed that the performance of HgCdTe and PbSnTe photodiodes is due to thermal generation governed by the Auger mechanism in the base regions. Investigations of the fundamental physical limitations of HgCdTe photodiodes indicate better performance of this type of detectors in comparison with GaAs/AlGaAs QWIPs operated in the range 35 to 77 K. In practice however, in temperature range below 50 K the performance of the HgCdTe photodiodes is determined by trap-assited tunneling. As a result, advantage of GaAs/AlGaAs QWIPs increases in the wider spectral range (below 14 μm) and temperature below 50 K. This observation plus the maturity of GaAs/AlGaAs technology and its radiation hard characteristics promise that QWIPs technology can produce high quality focal plane arrays (FPAs) for space applications. The operating temperature for HgCdTe detectors is higher than for other types of photondetectors. HgCdTe detectors with background limited performance operate with thermoelectric coolers in the medium wavelength range, instead the long wavelength detectors operate at ≈ 100 K. HgCdTe is characterized by high absorption coefficient and quantum efficiency and relatively low thermal generation rate compared to extrinsic detectors, silicide Schottky barriers and GaAs/AlGaAs QWIPs. However, the cooling requirments for GaAs/AlGaAs QWIPs with cutoff wavelengths below 10 μm are less stringent in comparison with extrinsic detectors and Schottky barrier devices. The theoretically predicted performance of long wavelength InAs/GaInSb photodiodes are comparable with HgCdTe photodiodes. The high performance of InAs/InGaSb detectors is due to suppression of band-to-band Auger recombination rate.

DC electrical and photovoltaic studies on Schottky barrier devices using the tetra aza difurazano decalin (TADFD) thin films

1, 4, 5, 8 tetra aza difurazano-(3, 4-c) (3, 4-h) decalin (TADFD) a fused ring system, was synthesised by 2:1 condensation of diaminofurazan with glyoxal in warm hydrochloric acid (HCl). Absorption spectra were recorded between 200 and 800 nm in dimethyl formamide (DMF). The electronic transition in this molecular system is n-π ∗ transition. The electrical and photovoltaic effects observed with the thin films of TADFD sandwiched between indium tin oxide (ITO) and Al electrodes are explained in terms of the P-type semiconducting behaviour of the organic film and its formation of a blocking contact (Schottky barrier) with Al electrode. At low voltages, the dark current in forward bias corresponding to a negative potential at Al electrode, varies exponentially with voltage, and Ohm's law is followed. The thermally activated hole concentration p 0 = 10 16m −3 and hole mobility μ p = 2.5×10 −5cm 2/V s and room temperature conductivity σ 0 = 4.0×10 −8(Ω m) −1 were obtained. At high voltage there is a space-charge-limited conductivity (SCLC) controlled by a single trap level. The derived trap level parameters are for e.g. the total trap concentration N t = 2.0×10 20 m −3, under reverse bias current, are identified with the Schottky emission over a potential barrier of 1.09 eV. The low-frequency capacitance-voltage ( C- V) characteristics have been employed to characterise the Schottky barrier device. The width of the depletion layer decreases with the increase of the temperature of the device. On illumination through ITO electrode with monochromatic light of wavelength 600 nm and light intensity P i = 10 mW/cm 2, the typical photovoltaic parameters were calculated. The action spectra were also explained in detail.

A comparison of the hydrogenated amorphous Si schottky barrier and the hydrogenated amorphous Si p—i—n dark forward-bias current density—Voltage characteristics

Abstract Using detailed computer simulations we explore the origin of the current density differences experimentally observed in the dark current density—voltage (J—V) characteristics of hydrogenated amorphous Si (a-Si:H) Schottky barriers and a-Si:H p—i—n homojunctions. Schottky barrier devices show higher dark current densities at low forward biases and p—i—n devices show higher current densities at high forward biases. Computer modelling shows that at low forward voltages the transport mechanism limiting the total current density is drift diffusion over the barrier in Schottky barriers and recombination through localized states in p—i—n diodes. On the other hand at high forward bias voltages the limiting transport mechanism is electron space-charge-limited current in both devices. We found that the non-blocking nature of the p—i—n front contact is the ultimate cause for the higher current densities observed experimentally in a-Si:H p—i—n structures at high forward biases. In this paper we also compare ...

Partial Depletion Region Collapse and its Impact on Transient Capacitance Measurements

AbstractWe investigate capacitance transients under various experimental conditions on a number of different amorphous silicon Schottky barrier device structures. The samples differ mainly in their back contact configuration, which is non-ohmic for one type of sample and ohmic for the second type of samples. Anomalous transients can be observed for all samples regardless of the back contact if the depletion region is only partially collapsed by partial voltage pulsing. We find that samples without an ohmic back contact show a slowly rising capacitance during the filling pulse. These samples show anomalous capacitance transients for short filling pulses.

Infrared Detectors - New Trends

Recent trends in infrared detectors are towards large, electronically ad- dressed two-dimensional arrays and higher operating temperature. This will lead both to higher performance and to smaller, lighter and more afford- able IR systems. Cooling requirements are the main obstacle to the more widespread use of infrared systems based on semiconductor photodetectors, particularly in the civil field. Fundamental limitations to performance of IR photodetectors are due to the statistical nature of generation—recombination processes and resulting noise. Ways to overcome the limitations are discussed including use of the optical immersion and optical resonant cavity. Finally, the progress in a mode of operation is presented, in which the thermal gen- eration of carriers is suppressed by the use of stationary depletion of narrow gap semiconductors. The practical near room temperature narrow gap semi- conductor photodetectors are reported. Competitive technologies based on Schottky barrier devices and low dimensional solids are considered briefly.

C-Si (n +)/a-Si Alloy/Pd Schottky Barrier Device for the Effective Evaluation of Photovoltaic Performance of a-Si Alloy Materials

ABSTRACTThe Schottky barrier device with a metal/a-Si (n+) /a-Si alloy/metal structure has been widely used as an alternative evaluation tool for the photovoltaic performance of a-Si alloy material since it more reliably reflects the carrier transport in a solar cell than the conventional material characterization tool such as PDS, CPM, and SSPG, and is easier to be fabricated compared with a complete nip solar cell. However, a multiple chamber device making system is still needed to fabricate such a device since one does not want to deposit the a-Si intrinsic material to be studied together with an n+ layer in the same chamber. We have explored the use of a Schottky barrier device deposited on heavily doped n-type crystalline wafer substrate, c-Si (n+) /a-Si alloy/metal, as an evaluation tool for a-Si alloy materials. In this device, besides the evaporation of a thin semi-transparent metal layer, only the active a-Si alloy layer needs to be deposited using the plasma enhanced or other deposition techniques. We have compared the performance of such a device with that of reference n-i-p solar cells deposited at the same time and demonstrated that the FF measured under weak red light show a good correlation between these two types of devices. Therefore the c-Si (n+) /a-Si alloy/metal device can be used as a convenient technique to reliably evaluate the material performance in a solar cell device.

Accuracy of nonoscillating one-port noise measurements

This paper presents a detailed analysis of the accuracy of nonoscillating one-port device noise measurements. A new expression is given for the calculation of the noise temperature from parameters that can be measured directly by using either a power sensor or a noise meter with the capability of making power measurements. A general expression for the DUT noise temperature error function is also obtained, which enables the authors to study how the noise measurement accuracy is affected by a number of different factors. This expression has been found very useful in order to study how the accuracy can be improved in a given noise measurement system. The error function has been applied to evaluate the uncertainty of Schottky barrier device noise measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

New trends in semiconductor infrared detectors

Recent efforts in semiconductor IR detector research have been directed toward improving the performance of single-element devices,large electronically scanned arrays, and higher operating temperature. Another important aim is to make IR detectors cheaper and more convenient to use. New trends in semiconductor IR detector technologies are discussed, including HgCdTe photodiodes, Schottky-barrier photoemissive devices, alternatives to HgCdTe ternary alloys, monolithic lead-chalcogenide photodiodes, GaAs/AlGaAs intersubband quantum well photoconductors, and ways to improve the performance of nearroom-temperature detectors. A comparison of different types of detectors with the present stage of HgCdTe technology achievements is undertaken.

A short wavelength infrared CCD detector of the optical sensor installed in the JERS-1 satellite

Abstract High resolution image sensing detector is presented which is capable of detecting radiations reflected on the terrestrial surfaces in the range of short wavelength infrared. The detector has four spectral band rows on single semiconductor chip which is composed of Pt-Si Schottky barrier device. The scanning of the detector is achieved by electrical method using CCD devices consolidated within the chip. The detector is integrated into the optical sensor system (OPS) and the OPS is installed in the JERS-1 satellite.

The origin of quantum efficiencies greater than unity in a-Si:H Schottky barriers

A photogating effect, recently predicted with computer simulations and subsequently observed in a-Si:H p-i-n structures, is shown to be also present in a-Si:H Schottky barrier devices. it is experimentally demonstrated that this photogating effect can be so significant that it can lead to quantum efficiency values greater than unity in Schottky barrier a-Si:H devices even in the annealed state. This photogating effect is found to be present under blue bias light and, for the annealed state, at low forward biasing conditions. Using our computer simulations, we show that, as in p-i-n devices, this photogating effect is caused by modulation of the field with blue bias light present near the front contact. We show also that this photogating effect is dependent on material and device parameters.

Structure and properties of interphase boundaries of gallium arsenide-metal (dielectric)

To study the nature and properties of potential barriers in gallium arsenide devices, we have investigated structural phase transitions in GaAs contacts with multilayer films containing refractory transition-metal borides (TiB2, LaB6). We verified the important role in degrading Schottky barrier device performance played by local mechanical stresses introduced at the interface by lateral nonuniformities in interphase interactions. We examine the electrical properties of MIS gallium arsenide devices, taking into account the high density of electronic surface states (ESS). We show it is possible to control the density of ESS by selecting the dielectric, and we discuss its deposition and annealing with a pulsed laser. We discuss the nature of potential barriers in gallium arsenide devices, drawing upon our data and previously published data and modern theoretical models.

Correlation of the structural properties with the device characteristics of hydrogenated amorphous silicon deposited at different substrate temperatures

The structural properties of hydrogenated amorphous silicon (a-Si:H) have a profound effect on the characteristics of the devices based on this material. The structural properties of the films were varied by depositing the samples at different substrate temperatures and characterized by secondary neutral mass spectroscopy, hydrogen evolution technique, and photothermal deflection spectroscopy. The results were correlated with the primary photocurrent data of a-Si:H Schottky barrier devices. It was observed that there is an optimum in both the rigidity of network and hydrogen content for the realization of the best film and device properties.

Hydrogenated amorphous silicon films prepared at high substrate temperature: Properties and light induced degradation

Of the different deposition parameters, the substrate temperature Ts has a profound effect on the microstructure and optoelectronic properties of hydrogenated amorphous silicon (a-Si:H). A detailed study was done to evaluate a-Si:H materials deposited at high substrate temperatures (≥325 °C). Their characteristics and nature of light induced degradation were compared to a-Si:H deposited at 200 °C. Electrical properties were studied with coplanar electrode structure as well as on Schottky barrier devices. Absorption measurements in the visible and infrared regions and spin-density measurements were carried out. For high Ts (≥325 °C) the presence of acceptorlike defects are indicated in addition to the neutral dangling bonds. Annealing recovery from the light soaked state is slower as compared to a film deposited at 200 °C. The results have been discussed in connection with the role of hydrogen motion in the annealing of light induced defects.

Charge transport in poly(3-methylthiophene) schottky barrier diodes

Schottky-barrier devices were formed from electropolymerised films of poly (3-methylthiophene) (PMeT). Thermal annealing of a partially undoped film led to diodes with rectification ratios as high as 5900 at 1 V and 50,000 at 2.5 V and ideality factors slightly above 2. The temperature dependence of ac loss tangent and forward currents are identical suggesting that bulk effects dominate device behaviour event at very low forward voltages. Below 250 K forward currents are essentially independent of temperature. Preliminary TSC measurements show the presence of at least two trapping levels in the devices.

GaAs revisited as a room temperature X-ray detector

In recent years there has been a renewal of interest from many disciplines, including high energy physics, particle astrophysics and medicine, in developing GaAs as a detector. This work reports on the assessment of some Schottky barrier devices, made from LEC starting material for particle physics experiments [1], as X-ray detectors. The detectors measured 3.0 mm × 5.0 mm × 135 μ m with a small, 1 mm diameter. Schottky contact in the centre. In the best devices a charge separation efficiency above 90% was achieved and measured resolutions at 122 keV and 60 keV were ∼8% and ∼6% HWHM respectively. Carrier trapping and device instabilities are the dominant limitations to better performance.

Molecular heterostructure devices composed of langmuir-blodgett films of conducting polymers

Organic heterostructures have been fabricated by alternating the deposition of monoand multilayers of undoped poly(3-hexylthiophene) and doped polypyrrole prepared by the Langmuir-Blodgett technique. The dopant profiles of the structures have been electrically characterized by incorporating the polymer layers as the active semiconductive material into metal-insulator-semiconductor and Schottky barrier device configurations. The dopant concentrations were evaluated by capacitance-voltage measurements. The results indicate modulated dopant profiles and a possible transfer of dopants. The dopant concentrations differ by about an order of magnitude between the undoped and doped layers. The Schottky barriers become thinner the closer the doped polypyrrole monolayer lies to the Schottky electrode.

High-speed infrared camera

We have developed a staring-mode, high-speed infrared camera based on a RCA 160×244 PtSi Schottky-barrier charge-coupled device image sensor. The camera uses a unique method to achieve greater temporal resolution of rapidly varying thermal phenomena in the 3–5 μm band. Rather than increase the imager’s frame rate, we decrease the image integration time while maintaining a frame rate that is compatible with standard RS-170 video. This technique allows us to display and record repetitive events that occur at frequencies up to 5 kHz with conventional video equipment. In this article we describe the theory and operation of this system.