Schottky Electrode Research Articles

Synthesis and Application of Metal Nitrides as Schottky Electrodes for Gallium Nitride Electron Devices

To achieve thermally-stable Schottky electrode on gallium nitride (GaN) electron devices for gate-first process and high-temperature devices, Titanium nitride (TiN) film was synthesised by magnetron reactive sputtering as the Schottky contact on n-GaN and AlGaN/GaN heterostructure field-effect transistors (HFETs). To reduce the sheet resistance, a cap layer of W/Au was deposited on the films. The ideality factor and Schottky barrier height (SBH) of the circular Schottky contact on n-GaN were 1.07 and 0.59 eV, respectively. GaN Schottky diodes and AlGaN/GaN HFETs were fabricated by annealing the Ohmic contact (Ti/Al/Ti/Au) and Schottky contact simultaneously. No obvious change was observed in the forward characteristics of the GaN Schottky diode, when the sample was annealed at 850 � C for 1 min. For the AlGaN/GaN HFETs which were annealed at 850 � C, the HFETs operated very well when they were annealed at 850 � C for 1 or 3 mins. The TiN/W/Au Schottky

Metal-Semiconductor-Metal Ultraviolet Photodiodes Fabricated on Bulk GaN Substrate

We report the demonstration of a GaN-based planar metal-semiconductor-metal (MSM) ultraviolet photodetector (PD). The MSM PD with semitransparent interdigitated Schottky electrodes is fabricated on low-defect-density GaN homoepitaxial layer grown on bulk GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the GaN homo-epilayer characterized by cathodoluminescence mapping technique is ~5×106 cm−2. The PD exhibits a low dark current density of ~4.1×10−10 A/cm2 and a high UV-to-visible rejection ratio up to 5 orders of magnitude at room temperature under 10 V bias. Even at a high temperature of 425 K, the dark current of the PD at 10 V is still <1×10−9 A/cm2 with a reasonable UV-to-visible rejection ratio more than 3×104, indicating that such kind of PDs are suitable for high temperature operation.

Electrical properties of TiN on gallium nitride grown using different deposition conditions and annealing

This study evaluates the thermal stability of different refractory metal nitrides used as Schottky electrodes on GaN. The results demonstrate that TiN, MoSiN, and MoN possess good rectification and adhesion strength, with barrier heights of 0.56, 0.54, and 0.36 eV, respectively. After thermal treatment at 850 °C for 1 min, the TiN and MoN electrodes still exhibit rectifying characteristics, while the MoSiN degrades to an ohmic-like contact. For further study, several TiN films are deposited using different N2/Ar reactive/inert sputtering gas ratios, thereby varying the nitrogen content present in the sputtering gas. Ohmic-like contact is observed with the pure Ti contact film, and Schottky characteristics are observed with the samples possessing nitrogen in the film. The average Schottky barrier height is about 0.5 eV and remains virtually constant with varying nitrogen deposition content. After examining Raman spectra and x-ray photoelectron spectroscopy results, the increase in the film resistivity after thermal treatment is attributed to oxidation and/or nitridation. Films deposited with a medium (40% and 60%) nitrogen content show the best film quality and thermal stability.

Correlation of on‐wafer 400 V dynamic behavior and trap characteristics of GaN‐HEMTs

A method for identifying the critical traps for the dynamic behavior of gallium nitride high electron mobility transistors (GaN‐HEMTs) is described. This method provides information on where the critical traps are located in the horizontal direction. At first, the on‐wafer 400 V dynamic behavior of a GaN‐HEMT was measured. Acceptor‐like trap behavior with 0.14 eV activation energy was observed using conductance analysis. Next, trap characterization was achieved using the capacitance and series conductance transients of a comb‐style Schottky barrier diode after electrical stress. Several different biases were applied to determine the locations of the detected traps, because depletion area edges can change upon imposition of different biases. Notably, 0.18 eV acceptor‐like trap behavior below the Schottky electrode, 0.44 eV donor‐like trap behavior in the access region, and 0.5eV acceptor‐like trap behavior in access region were observed. By correlating these diode results with HEMT results, it was concluded that the critical trap behavior likely occurs below the gate electrode in HEMT. As a conclusion, the information obtained with this method should provide useful information for the improvement of device processes.

Dark current suppression of MgZnO metal-semiconductor-metal solar-blind ultraviolet photodetector by asymmetric electrode structures

The application of asymmetric Schottky barrier and electrode area in an MgZnO metal-semiconductor-metal (MSM) solar-blind ultraviolet photodetector has been investigated by a physical-based numerical model in which the electron mobility is obtained by an ensemble Monte Carlo simulation combined with first principle calculations using the density functional theory. Compared with the experimental data of symmetric and asymmetric MSM structures based on ZnO substrate, the validity of this model is verified. The asymmetric Schottky barrier and electrode area devices exhibit reductions of 20 times and 1.3 times on dark current, respectively, without apparent photocurrent scarification. The plots of photo-to-dark current ratio (PDR) indicate that the asymmetric MgZnO MSM structure has better dark current characteristic than that of the symmetric one.

Impact of Electrode Oxidation on the Current Transport Properties at Platinum/(Niobium-Doped Strontium-Titanate) Schottky Junctions

The current transport properties and resistance-switching (RS) behavior at platinum/niobium-doped strontium-titanate (Pt/NSTO) Schottky junctions were investigated for junctions with different degrees of oxidation (PtOx) in the Schottky electrode. It was found that the rectifying current-voltage (I–V) profile strongly depends on the heat-treatment of the NSTO crystal and oxidation of the Pt electrode. The hysteresis in the I–V relationships was larger for the PtOx/NSTO junction than for the pure Pt/NSTO junction, and heat-treatment of the NSTO crystal suppresses the hysteresis in I-V and increases the leakage current. The change in the rectifying I–V profiles can be reasonably explained by the increase in the built-in potential caused by partial oxidation of the electrode interface, as determined by soft X-ray photoemission spectroscopy. The experimental results indicate that control of the built-in potential and the partial oxidation of the electrode, which affects the homogeneity of the potential profile, is the key to improving and controlling the rectification and RS properties in Pt/NSTO junctions.

Unidirectional AlGaN/GaN-on-Si HFETs with reverse blocking drain

We have developed a novel unidirectional AlGaN/GaN-on-Si heterojunction field-effect transistor (HFET) with reverse blocking drain. A recessed Schottky contact was incorporated into a conventional ohmic drain electrode to prevent the undesired reverse current flow while reducing the turn-on voltage in forward current characteristics. The combination of recessed Schottky and ohmic electrodes significantly reduced the turn-on voltage in comparison with a conventional Schottky drain. A turn-on voltage of 0.4 V, an off-state forward breakdown voltage of 615 V, and a reverse blocking drain voltage of −685 V were achieved for the gate-to-drain distance of 12 µm.

GaN MSM UV Sensor Using Multi-Layer Graphene Schottky Electrodes

We demonstrated a metal-semiconductor-metal type GaN UV sensor for the first time by using multi-layer graphene as a Schottky electrode. Multi-layer graphene shows good Schottky electrode characteristic and fabricated UV sensor shows good UV response characteristics. The maximum dark current density and photo-responsive current density were 6.42 × 10-9 A/cm2 and 5.57 × 10-5 A/cm2 at the 10 V bias, respectively. UV/visible rejection ratios were higher than 103 with each applied bias from 1 V to 15 V.

Dual-wavelength sensitive AlGaN/GaN metal-insulator-semiconductor-insulator-metal ultraviolet sensor with balanced ultraviolet/visible rejection ratios

We proposed and fabricated a metal-insulator-semiconductor-insulator-metal type dual-wavelength sensitive UV sensor by using an AlGaN/GaN hetero-structure layer epitaxially grown on a sapphire substrate and a thin Al2O3 layer inserted between AlGaN and Ni Schottky electrodes to reduce dark current and improve the UV/visible rejection ratio. The proposed sensor shows high photo-responsive current to both UV wavelength regimes with a significantly improved UV/visible rejection ratio under the regime of the GaN-related UV response. Cut-off wavelengths can be controlled by changing the bias below and above 10 V.

Vertical Diamond Schottky Barrier Diode Fabricated on Insulating Diamond Substrate Using Deep Etching Technique

Diamond Schottky barrier diodes (SBDs) with vertical channel structures are fabricated on an insulating diamond substrate. The deep etching technique is used to make a vertical channel structure in the diamond stacking layers of the p+/p-/insulating substrate. Mo Schottky electrodes with the Al2O3 field plate structure are also fabricated to realize a high breakdown voltage. As a result, I-V characteristics with a high forward current density >103 A/cm2 and a high breakdown voltage >700 V are obtained at 25°C and 250 °C. Baliga's figure of merit is 332 MW/cm2, which is equivalent to the high value previously reported for diamond SBDs with lateral channel or pseudovertical channel structures, and is also ten times higher than the limit of Si SBDs. High-power diamond SBDs are expected to be realized on a large area insulating diamond substrate using this technique.

Large Active Area AlGaN Solar-Blind Schottky Avalanche Photodiodes with High Multiplication Gain

We report the fabrication and performance of solar-blind AlGaN Schottky avalanche photodiodes grown on sapphire substrates. An increased active donor density is found near the surface, leading to an enhanced electric field adjacent to the Schottky electrode. Multiplication gain over 2000 has been achieved in the fabricated devices with a mesa diameter of 200 μm. The measured dark I—V curves at different temperatures show strong temperature dependence, suggesting that the gain mechanism in our devices is primarily due to impact ionization. Peak responsivity of 66.3mA/W is obtained at 260nm and at zero bias, corresponding to an external quantum efficiency of 31.6%.

Photoelectron characteristics of diode structures based on quantum-well GaAs/InGaAs heteronanostructures with a Mn δ-doped layer

A GaAs layer over Mn is found to acquire p-type conductivity in Au(Ni)/n-GaAs/InGaAs diode heteronanostructures due to laser δ-doping of Mn with a density of 0.15–1.5 monolayers, yielding the formation of a p-n junction in the Schottky barrier. The effect of the shunting of diode structures by the ohmic resistance of the Mn δ-doped layer and a p-n junction outside the Schottky electrode is revealed. This effect is found to disappear after the formation of mesadiodes. The current-voltage characteristics of the mesadiodes with Mn cannot be described by the mechanism of thermionic emission through the barrier, and the current in these mesadiodes flows with the participation of thermal-field and field emission. A compensated region in the n-GaAs buffer layer, adjacent to the δ-doped layer, formed due to the laser deposition of a coating layer is found. The length of this region decreases with reduction in the layer-production temperature. A substantial increase in the effective surface potential barrier in the semiconductor (up to 1.05 V) in structures with Mn was revealed, and the photoelectric activity and depth of Mn layer occurrence in the forbidden band of GaAs energies is revealed.

Effect of electric field on exciton in high‐purity GaAs epilayer measured at room temperature

The excitonic electroabsorption has been investigated for a high-purity GaAs epilayer of 10 µm thickness at room temperature and clear red-shift (≃1.44 nm) of the excitonic absorption peak has been observed. An extinction ratio over 10 dB has been obtained with an applied voltage of 11 V, which is nearly five times larger than the theoretical estimation without considering the exciton. This may be the first observation for of a surface normal structure with a polyaniline as a transparent Schottky electrode. The insertion loss is estimated to be 3 dB. This relative low driving voltage for a bulk configuration without quantum wells is due to high-purity GaAs.

Localized mid-gap-states limited reverse current of diamond Schottky diodes

Electric properties of the lateral-type tungsten carbide/p-diamond (100) Schottky diodes were investigated. At the lower voltage below 10 V, the reverse current density JR increased steeply and then became saturate at a certain value. This saturated JR value was kept constant in the wider voltage range of 10–500 V. A large variation of ∼5 orders of magnitude in the constant JR value was observed among the electrodes examined, whereas there was a clear correlation between the constant JR values and the current level of the shoulder component in the forward characteristics. The generation current induced by the spatially localized mid-gap states was presumed as the transport mode of this constant JR. The fitting procedure indicates the mid-gap states localized ∼2 μm far from the Schottky electrode edge. The maximum density of mid-gap states divided by the effective carrier lifetime was estimated to be ∼1017 cm−2s−1. The mid-gap states locating energetically at 0.12 eV and 0.62 eV above the valence-band top were detected in the temperature range of 300–700 K.

Optical coupling optimization in a novel metal—semiconductor—metal ultraviolet photodetector based on semicircular Schottky electrodes

A novel semicircular electrode metal-semiconductor-metal (SEMSM) ultraviolet detector is modeled, investigated and characterized with a self-consistent numerical calculation method. For the purpose of model and performance verification, a comprehensive comparison of the SEMSM detector and a conventional electrode MSM detector is carried out with experimental data. The results indicate that the physical models are able to predict the enhanced device features. Moreover, the structural parameters have been adjusted appropriately to optimize the SEMSM detector. The findings show that a device with a 2 μm finger radius and 3 μm spacing exhibits outstanding characteristics in terms of a peak responsivity of 0.177 A/W at 290 nm, a maximum external quantum efficiency of over 75%, and a comparable normalized photocurrent to dark current ratio of 1.192 × 1011 W−1 at 0.3 V bias. These results demonstrate that the SEMSM detector has excellent performance for optoelectronic integrated circuit applications.

Imaging Schottky Barriers and Ohmic Contacts in PbS Quantum Dot Devices

We fabricated planar PbS quantum dot devices with ohmic and Schottky type electrodes and characterized them using scanning photocurrent and photovoltage microscopies. The microscopy techniques used in this investigation allow for interrogation of the lateral depletion width and related photovoltaic properties in the planar Schottky type contacts. Titanium/QD contacts exhibited depletion widths that varied over a wide range as a function of bias voltage, while the gold/QD contacts showed ohmic behavior over the same voltage range.

Copper Oxide Edge-Termination for GaN Schottky Barrier Diodes with Low Turn-on Voltage

ABSTRACTIn this study, we propose copper oxide (CuOx) edge-termination for GaN-based Schottky barrier diodes (SBDs) with low turn-on voltage. CuOx fabricated by thermal oxidization of sputtered Cu film at 275°C consisted mainly of Cu2O which is known as a p-type semiconductor. We applied CuOx edge-termination to GaN SBDs with tantalum (Ta) Schottky electrode which has low work function of 4.25 eV. The experimental results of current-voltage characteristics insisted that CuOx edge-termination structure was effective to increase breakdown voltage of GaN SBDs with keeping low turn-on voltage of 0.29 V at 10 A/cm2.

Electronic properties of dislocations introduced mechanically at room temperature on a single crystal silicon surface

This paper focuses on the effects of temperature and environment on the electronic properties of dislocations in n-type single crystal silicon near the surface. Deep level transient spectroscopy (DLTS) analyses were carried out with Schottky electrodes and p+–n junctions. The trap level, originally found at EC−0.50eV (as commonly reported), shifted to a shallower level at EC−0.23eV after a heat treatment at 350K in an inert environment. The same heat treatment in lab air, however, did not cause any shift. The trap level shifted by the heat treatment in an inert environment was found to revert back to the original level when the specimens were exposed to lab air again. Therefore, the intrinsic trap level is expected to occur at EC−0.23eV and shift sensitively with gas adsorption in air.

Metal–Semiconductor–Metal Ultraviolet Avalanche Photodiodes Fabricated on Bulk GaN Substrate

We report the first demonstration of a GaN-based planar metal-semiconductor-metal (MSM) ultraviolet (UV) avalanche photodiode (APD). The MSM APD with semitransparent interdigitated Schottky electrodes is fabricated on a low-defect-density GaN homoepitaxial layer grown on a bulk GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the GaN homoepilayer characterized by a cathodoluminescence mapping technique is ~ 5 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The photodiode exhibits a low dark current density of ~ 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a high UV-to-visible rejection ratio up to five orders of magnitude under 20-V bias. At high bias, a room-temperature avalanche gain of more than 1100 is achieved under 365-nm UV illumination. The breakdown voltage of the APD shows a positive temperature coefficient of 0.15 V/K, confirming that the high-voltage gain is dominated by the avalanche breakdown mechanism.

Schottky contact on ZnO nano-columnar film with H2O2 treatment

The surface treatment with boiling hydrogen peroxide (H2O2) solution on the surface of ZnO nano-columnar film was investigated. Field emission-SEM and TEM analysis revealed that amorphous ZnO2 layer covers the ZnO nano-column surface through the H2O2 treatment at 100°C for 1 min. X-ray photoemission spectroscopy (XPS) has been conducted on the H2O2 treated ZnO surface. The surface exhibits high resistive conductivity after the H2O2 treatment, suggesting that the treatment promotes a compensation effect. We demonstrate that dramatic improvement in the rectifying behavior on the Schottky diodes can be achieved by inserting a ZnO2 interface layer between the Pt Schottky electrode and the ZnO nano-column film. The ZnO2 interface layer promotes surface passivation and suppresses the surface leakage current. This is expected to increase the Schottky barrier height to 0.78 eV. The H2O2 treated Schottky diode showed five orders of magnitude in current rectification between forward and reverse bias at 3 V.