Al Schottky Contacts Research Articles

Boosting the photosensitivity of organic phototransistors with photomultiplication via Corbino configurations

An organic phototransistor (OPT) with a photomultiplication mechanism is studied, where a bulk-heterojunction of C70 and 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane is used as a channel and a light-absorbing layer with an Al Schottky contact as source/drain electrodes. The light absorption in the source-side region is found to dominate the photomultiplication effect. In addition, the source electrode area that is up to 100 μm away from the channel is identified as the most effective region in the OPTs with contact-limited operation used in this study. Therefore, to maximize the light-sensing performance of the contact-limited OPTs, we propose Corbino source/drain geometries that have a larger source electrode area than that of the drain electrode. The Corbino-type OPTs show improved photo-response properties proportional to the relative increment of the source-side area, reaching 1.7-fold higher performance in terms of photosensitivity compared to the conventional lateral-type OPTs.

CuIn0.7Ga0.3Se2 thin films\u2019 properties grown by close-spaced vapor transport technique for second-generation solar cells

In this paper, CuIn0.7Ga0.3Se2 (CIGS) thin films are deposited on both glass (SLG) and glass/SnO2:F (SLG/FTO) substrates, by close-spaced vapor transport technique. The Hall effect measurements are performed in the temperature range (300–438 K) for the two SLG/CIGS samples namely CIGS1 and CIGS2, grown at substrate temperature (Ts) of 470 °C and 510 °C, respectively, to investigate the temperature effect on the electrical parameters such as hole concentration (p), conductivity (σ) and mobility (µ). As results, from Arrhenius diagram of (p) and (σ), bandgap energy (Eg) of about 1.38 eV and 1.24 eV are extracted for CIGS1 and CIGS2, respectively. Besides, activation energies (Ea) at 563.9 meV and 239.4 meV are determined for CIGS1 whereas values at 584.2 meV and 72.7 meV are obtained for CIGS2. Furthermore, average mobilities of 1.83 cm2/V s and 1.77 cm2/V s are achieved for CIGS1 and CIGS2 thin films, respectively. Pure aluminum (Al) Schottky contacts are deposited on the front side of FTO/CIGS thin film-devices by physical vapor deposition. Current–voltage (I–V) characteristics are measured and used to extract the electrical parameters of FTO/CIGS/Al Schottky diode using the one diode model. The electrical parameters including series resistance (Rs) of about 93.7 Ω and an ideality factor (n) around 3.47 indicate that the generation–recombination mechanism is predominant.

Current–voltage and capacitance–voltage characteristics of Al Schottky contacts to strained Si-on-insulator in the wide temperature range

The electrical characteristics of Al/strained Si-on-insulator (sSOI) Schottky diode have been investigated using current–voltage (I–V) and capacitance–voltage (C–V) measurements in the wide temperature range of 200–400K in steps of 25K. It was found that the barrier height (0.57–0.80eV) calculated from the I–V characteristics increased and the ideality factor (1.97–1.28) decreased with increasing temperature. The barrier heights determined from the C–V measurements were higher than those extracted from the I–V measurements, associated with the formation of an inhomogeneous Schottky barrier at the interface. The series resistance estimated from the forward I–V characteristics using Cheung and Norde methods decreased with increasing temperature, implying its strong temperature dependence. The observed variation in barrier height and ideality factor could be attributed to the inhomogeneities in Schottky barrier, explained by assuming Gaussian distribution of barrier heights. The temperature-dependent I–V characteristics showed a double Gaussian distribution with mean barrier heights of 0.83 and 1.19eV and standard deviations of 0.10 and 0.16eV at 200–275 and 300–400K, respectively. From the modified Richardson plot, the modified Richardson constant were calculated to be 21.8 and 29.4Acm−2K−2 at 200–275 and 300–400K, respectively, which were comparable to the theoretical value for p-type sSOI (31.6Acm−2K−2).

Fermi-Level Depinning of Ge Schottky Junction Using Se Treatment

Depinning of the Fermi-level at metal/Ge Schottky junctions was achieved by Se treatment at room temperature. Due to the strong Fermi-level pinning close to the valence band edge in Ge, Schottky and Ohmic behaviors were observed in conventional Al Schottky contact s to n- and p-type Ge, respectively. However, Al Schottky contacts to n- and p-type Ge with Se treatment became Ohmic and Schottky, respectively. The transmission electron microscopy (TEM) examination combined with X-ray photoemission spectroscopy (XPS) analysis showed that Se treatment at room temperature yielded the formation of a fairly uniform Se-Ge alloy film with partially ionic binding nature caused by the chemical reaction between Se and Ge. This led to the reduction of surface states of Ge surface along with the passivation of dangling bonds by the incorporation of Se atoms, which was responsible for Fermi-level depinning at Al/Ge junctions.

Fermi-Level Depinning of Ge Schottky Junction Using Se Treatment

Depinning of the Fermi-level at metal/Ge Schottky junctions was achieved by Se treatment at room temperature. Due to the strong Fermi-level pinning close to the valence band edge in Ge, Schottky and Ohmic behaviors were observed in conventional Al Schottky contacts to n- and p-type Ge, respectively. However, Al Schottky contacts to n- and p-type Ge with Se treatment became Ohmic and Schottky, respectively. Electron and hole barrier heights calculated using Richardson plots were found to be 0.15 eV for of Al contacts to Se-treated n- type Ge, and 0.33 eV for Al contacts to Se-treated p-type Ge, respectively. The transmission electron microscopy (TEM) examination combined with X-ray photoemission spectroscopy (XPS) analysis showed that Se treatment at room temperature yielded the formation of a fairly uniform Se-Ge alloy film with partially ionic binding nature caused by the chemical reaction between Se and Ge. This led to the reduction of surface states of Ge surface along with the passivation of dangling bonds by the incorporation of Se atoms, which was responsible for Fermi-level depinning at Al/Ge junctions.

Effect of Indium–Tin-Oxide Schottky Contact on the Resistance Switching of NiO Film

The effect of electrode materials on resistance switching was evaluated on the Pt/NiO/electrode (EL) structures where the EL contacts were Pt, Al, and indium–tin-oxide (ITO). It was confirmed that ohmic Pt contact needs to induce the effective electric field for resistance switching across the NiO film. For the Pt/NiO/Al structure, the barrier height of the Al Schottky contact was measured as 0.66 eV and no resistance switching was observed owing to a large voltage drop at the rectifying interface induced by the reduction of NiO resulting from the formation of Al oxide. In the ITO (EL)/NiO/Pt structure, the barrier height of the Schottky contact between ITO and NiO was about 0.52 eV and it did not show any resistance switching, either. Through the depth-profile study by X-ray photoelectron spectroscopy, chemical reactions at the interface ITO/NiO was identified to be not too much evolved compared with that of NiO/Al, which might due to be abundant oxygen on the ITO surface. Such Schottky barrier heights 0.52–0.66 eV were considered too high to induce a sufficient electric field in the NiO film causing the resistance switching.

Multiple Module Pixellated CdTe Spectroscopic X-Ray Detector

A pixellated CdTe detector system comprising 2×2 detector modules has been developed for high energy spectroscopic X-ray imaging applications and has an active area of 16 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The detector modules are made from the HEXITEC 80×80 ASIC and 1 mm thick CdTe with Al-Schottky contacts. The CdTe has 250 μm pitch pixels with an outer guard ring on the same pitch. The single HEXITEC 80×80 detectors have an average energy resolution (FWHM) of 800 eV at 59.9 keV. Limitations in the multiple module DAQ system mean that the energy resolution of the pixels in the 2×2 detector array is 2.0 keV at 59.9 keV. The spacing between the tiled detector modules is 150 μm which results in an inactive area equivalent to 3 pixels, including the guard ring on the edge of the detectors. The modular detector configuration demonstrates the potential to create large area detector arrays in the future.

DLTS of p-type Czochralski Si wafers containing processing-induced macropores

The deep levels present in p-type Czochralski silicon with processing-induced macropores in the depletion region have been studied by the deep-level transient (DLT) spectroscopy technique. It is shown that a broad band is present for a bias pulse close to the interface with the Al Schottky contact, which exhibits anomalously slow hole capture and is ascribed to the internal interface states of the macropores. For depths beyond the pore region, other deep levels, associated with point defects—possibly metal contamination during the high-temperature annealing step under H2 ambient–-have been observed. The impact of the observed defects on the lifetime of thin-film solar cells, fabricated using macropore-based layer transfer is discussed. Finally, it is shown that the presence of pores in the depletion region, which also affects the DLT-spectrum, alters the capacitance–voltage characteristics.

A proposed organic Schottky barrier photodetector for application in the visible region

The fabrication and characterization of an organic photodetector (OPD) in the form of ITO coated glass/polycarbazole (PCz)/Al Schottky contact is reported. The device has been fabricated in our laboratory for the first time using the polymer synthesized by us. The device has been subsequently characterized in respect of electrical and optical properties in order to explore its potential for possible use as a detector in the visible region at 650 nm. It is observed that the detector exhibits a reasonably high value of peak detectivity (∼6 × 10 6 cm Hz 1/2 W −1) near zero bias voltage at V = 0.2 V.

Electric field breakdown of lateral-type Schottky diodes formed on lightly doped homoepitaxial diamond

The reverse current of lateral-type Schottky diodes fabricated on p-type homoepitaxial diamond was analyzed by changing the distance between Schottky and Ohmic electrodes and the metal materials in the Schottky electrodes. The maximum electric field at breakdown was 0.56 MV cm −1 for the Au Schottky contact and less than 0.26 MV cm −1 for the Al Schottky contact. The breakdown voltage depended on the electrode distance when the diamond surface was revealed in vacuum, whereas the Schottky diodes sustained the applied voltage of 500 V, corresponding to 0.69 MV cm −1, after covering of the diamond surface with an insulating liquid. Diamond surface protection is an indispensable technique for fabrication of high-voltage Schottky diodes based on diamond.

Properties of Al Schottky Contacts on CdTe(111)Cd Surface Treated by He and H2 Plasmas

We investigated the effect of plasma treatments using He and H2 gases on the electrical properties of Schottky CdTe diodes. The composition of the initial CdTe surface was Te-rich owing to Br-methanol etching. However, after the plasma treatments, the Te-rich layer was removed from the CdTe surface and the surface composition became stoichiometric. The plasma treatment improved the rectification properties of the Al Schottky CdTe diode. Furthermore, by adopting a guard-ring electrode, the reverse current of the CdTe diode decreased by more than one order compared with that of the CdTe diode without a guard-ring electrode. We evaluated the barrier height and ideality factor from current–voltage characteristics. The barrier heights of the CdTe diodes after He and H2 plasma treatments were 0.75 and 0.73 eV, respectively.

Schottky contact analysis of photovoltaic chalcopyrite thin film absorbers

Abstract Current–voltage and capacitance–voltage measurements serve to analyze thermally evaporated Al Schottky contacts on Cu(In, Ga)Se 2 based photovoltaic thin film devices, either taken as grown or etched in a bromine-methanol solution. The characteristics of the Schottky contacts on the as-grown films give evidence for some dielectric layer developing between the metal and the semiconductor. Etching the semiconductor surface prior to evaporation of the Al front contact yields a pure metal-semiconductor behavior, including effects that can be attributed to an additional diode at the Mo contact. Simulations confirm the experimental results.

Effect of He Plasma Treatment on the Rectification Properties of Al/CdTe Schottky Contacts

We investigated the effect of He plasma treatment on the surface composition of CdTe and the electrical properties of Al/CdTe Schottky contacts. The composition of the initial CdTe surface is Te-rich due to Br-methanol etching. When Al Schottky contacts are formed on the CdTe surfaces with the Te-rich layer, the barrier height is low and the rectification property is not good. In this paper, we propose the He plasma treatment method to remove the Te-rich layer. From X-ray photoelectron spectroscopy measurement, it was found that the plasma treatment can remove the Te-rich layer. The rectification property of the Al Schottky contacts on the plasma-treated surfaces is improved, and their barrier heights are estimated to be about 0.65 eV. In γ-ray spectrometry, a high-energy resolution of 1.6 keV full width at half maximum at 59.5 keV was obtained from the plasma-treated Al/CdTe/Pt detector. The results indicate that the plasma treatment of CdTe surfaces significantly improves the energy resolution of Schottky-type Al/CdTe/Pt radiation detectors.

Thermal stability of Ru, Pd and Al Schottky contacts to p ‐type 6H‐SiC

In this paper, a comparative study of the thermal stability of Ru, Pd and to a lesser extent Al Schottky contacts to p-type 6H-SiC and the influence of a hydrogen plasma treatment on these systems is presented. Ohmic contacts were prepared by thermally evaporating Al (180 nm) followed by Ti (50 nm) on the back surface of the SiC. Schottky contacts were subsequently formed on the polished top surface by evaporating ∼0.8 mm diameter dots, 40 nm in thickness, through a metal contact mask. Isochronal (15 min) anneals revealed a general improvement in the SBD quality for temperatures up to 400 °C in the case of Ru and 300 °C for Pd, with progressive deterioration in the quality above these temperatures. Above 450 °C the Ru Schottky contacts became unusable whereas Pd was still functioning at 600 °C. In contrast Al SBDs showed inferior thermal stability. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Classification of charge relaxation processes in diamond on silicon based devices

Diamond thin films on p-type Si prepared via a bias enhanced chemical vapor deposition technique and equipped with Al Schottky contacts are studied by charge deep-level transient spectroscopy. Three charge relaxation processes can be resolved in Q-DLTS spectra: (i) a spectrum of relaxation times centered about a thermal energy of 0.33 eV has been assigned to hole traps induced by Al/diamond interface dipoles. These are metastable when heating the diodes to 450 K during a Q-DLTS scan, (ii) quasi discrete energy levels at 0.82 and 0.98 eV are indicative of an amorphous (hydrogenated) Si (a-Si:H) interlayer between the Si substrate and diamond film. This conclusion results from a direct comparison with Q-DLTS spectra of a-Si:H based devices and (iii) a slow charge relaxation with a non-exponential kinetics responsible for a Q-DLTS signal tail independent of the rate window for longer times of observation. This phenomenon is known from frequency-domain observations as ‘nearly constant loss’.

Capacitance–voltage studies of Al-Schottky contacts on hydrogen-terminated diamond

The electrical properties of Al contacts on hydrogen-terminated diamond have been characterized. Capacitance–voltage experiments were carried out in order to obtain further insight into the origin of the Schottky contact behavior of Al. Schottky diodes with different areas and peripheries were fabricated and analyzed. We have found that the capacitance of these structures is not proportional to the area but to the length of the contact periphery, supporting the two-dimensional nature of the surface charge layer in hydrogen-terminated diamond. A model based on the negative electron affinity of hydrogen-terminated diamond and the work function of Al is used to explain the in-plane capacitance of the Al Schottky contacts.

Electrical peculiarities in Al/Si/Ge/…/Ge/Si and Al/SiGe/Si structures

The current–voltage ( I– V) and capacitance–voltage ( C– V) behaviour of different Si/Ge multilayers and SiGe single layers prepared on p-type Si substrates by magnetron sputtering and annealing, has been studied in the temperature range of 80–320 K by using Al Schottky contacts as test structures. Although a significant influence of the microstructure of the Si/Ge multilayers and SiGe layers was obtained on the electrical behaviour of the structures, the structures exhibited similar specific features.

Space charge spectroscopy of diamond

Capacitance–voltage (CV) and deep level transient spectroscopy (DLTS) experiments (thermally and optically excited) are applied to explore various properties of boron-doped type IIb high pressure high temperature diamond and of homoepitaxially grown boron doped CVD diamond: (a) properties of Al Schottky contacts; (b) acceptor densities and hydrogen boron interactions; and (c) energy distributions and densities of compensating defects. Two distinct excitation energies at 0.9 and 1.25 eV are detected by DLTS and optically excited DLTS. Carbon implantation experiments reveal that the defect with optical excitation energy of 1.25 eV is the positively charged vacancy. Annealing measurements show that in the temperature regime of 525–600 K a significant part of the vacancy density is annealed which is attributed to recombination of carbon interstitial with vacancies.

Aluminum, magnesium, and gold contacts to contamination free n-GaN surfaces

Current–voltage (I–V) characteristics of Al, Mg, and Au Schottky contacts to atomically clean n-GaN(0001)-1×1 surfaces prepared in an ultrahigh vacuum were investigated. The Al/n-GaN contact is rectifying at room temperature and becomes Ohmic after annealing at 500 °C. Coupled with previous photoemission spectroscopy data, this result demonstrates that the origin of the Ohmicity is the reaction-induced doping of the interface. For nonannealed interfaces, the Schottky barrier heights determined from I–V characteristics are in qualitative agreement with the results obtained by photoemission spectroscopy. We find that the ideality factor of the barrier is close to unity for the unreactive interface i.e. Au/GaN, but significantly higher for the reactive interfaces, i.e., Al/GaN and Mg/GaN. Our experimental results suggest that the reaction-induced defects and thermionic field emission play an important role in the electrical behavior of these interfaces.

The CDMS II Z-sensitive ionization and phonon germanium detector

The last year saw major progress in the development of the nuclear-recoil dark matter detector for the Cryogenic Dark Matter Search (CDMS II) experiment. The basic γ-ray background discrimination is based on ionization yield. However, as shown in separate experiments, surface events and especially electrons also results in low ionization yield due to incomplete charge collection. The two-fold strategy to reduce these unwanted backgrounds was to improve charge collection at the detector surface and to employ fast phonon sensors on Ge. A 250 g Ge detector with an Al Schottky contact on an amorphous Si blocking layer and a Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge (QET) phonon sensor has been characterized. The phonon collection efficiency of the new detector is similar to that of the latest 100 g Si Fast Large Ionization and Phonon (FLIP) detector, which has the same phonon sensor design. The fall times of the phonon pulses are longer and consistent with simulations. The charge collection at the surface is high and very similar to that of smaller test devices and the last revision of Berkeley Large Ionization and Phonon (BLIP) detectors. The four fast non-thermal phonon sensors yield x, y position resolution similar to the Si FLIP detector. A rise time surface effect was demonstrated on both sides of the detector. The effect results in a much more effective rejection of surface events than the rejection based on charge yield alone and therefore improves the sensitivity for a dark matter search.