Reduction of dislocations in GaAs and InP epitaxial layers by quasi ternary growth and its effect on device performance

It is shown that strained isoelectronically doped buffer layers grown on GaAs and InP substrates allow one to reduce the dislocation density as well as the deep level concentration. Photoluminescence measurements show an improved near gap luminescence and a reduced deep level emission. This has been also confirmed by DLTS measurements. The higher material quality is also reflected in the device performance. GaAs Schottky diodes (n ˜0.5 · 10¹⁶Cm^-3) exhibit a leakage current of 100 pA up to 50 V reverse bias at room temperature even with large Schottky contact area of 125 μm × 60 μm. This technique allows the quality of electronic and optoelectronic devices to be improved.     

High-performing transparent photodetectors based on Schottky contacts

Transparent UV-photodetectors exhibiting very high responsivity and fast operation are discussed. Schottky contact photoelectric devices utilizing wide band gap TiO₂ absorber layer were evaluated for their performances as UV-photodetectors. Three different work function metals Cu, Mo and Ni were used to realize Schottky barrier with TiO₂. Ni Schottky contacts were found to be most suitable to fabricate high responsivity (2.034 A/W) photodetector with faster rise time (0.14 ms) and wide linear dynamic range (128 dB) operating at small applied reverse bias of −1 V. However, higher barrier height in the case of Mo/TiO₂ interface resulted in lowest dark current density of the value 2.21×10⁻⁸ A/cm² with quick fall time of 0.52 ms. The modulation of the barrier height would provide a route for designing fast and high responsive Schottky photodetector with broad linear dynamic range performance.     

High breakdown voltage Schottky rectifier fabricated on bulk n-GaN substrate

Vertical Schottky rectifiers have been fabricated on a free-standing n-GaN substrate. Circular Pt Schottky contacts with different diameters (50 μm, 150 μm and 300 μm) were prepared on the Ga-face and full backside ohmic contact was prepared on the N-face by using Ti/Al. The electron concentration of the substrate was as low as 7 × 10¹⁵ cm⁻³. Without epitaxial layer and edge termination scheme, the reverse breakdown voltages (V_B) as high as 630 V and 600 V were achieved for 50 μm and 150 μm diameter rectifiers, respectively. For larger diameter (300 μm) rectifiers, V_B dropped to 260 V. The forward turn-on voltage (V_F) for the 50 μm diameter rectifiers was 1.2 V at the current density of 100 A/cm², and the on-state resistance (R_on) was 2.2 mΩ cm², producing a figure-of-merit (V_B)²/R_on of 180 MW cm⁻². At 10 V bias, forward currents of 0.5 A and 0.8 A were obtained for 150 μm and 300 μm diameter rectifiers, respectively. The devices exhibited an ultrafast reverse recovery characteristics, with the reverse recovery time shorter than 20 ns.     

Metal-semiconductor-metal photodetectors with InAlGaP capping and buffer layers

To improve the Schottky contact performance and carrier confinement of GaAs metal-semiconductor-metal photodetectors (MSM-PDs), we employed the wide bandgap material, In/sub 0.5/(Al/sub 0.66/Ga/sub 0.34/)/sub 0.5/P, for the capping and buffer layers. We directly evaluated the Schottky contact parameters on the MSM-PD structure. The reverse characteristics of the Schottky contacts were examined by taking into account the Schottky barrier height depended on the electric field in the depletion region, and hence on the applied bias. The ideality factor and Schottky barrier height of Ti-Pt-Au contacts to In/sub 0.5/(Al/sub 0.66/Ga/sub 0.34/)/sub 0.5/P are 1.02 and 1.05 eV, respectively. Extremely low dark currents of 70 and 620 pA were obtained for these MSM-PDs when they were operated at a reverse bias of -10 V at room temperature and at 70/spl deg/C, respectively.     

Rectified Schottky diodes based on PEDOT:PSS/InGaZnO junctions

We report the successful use of the high-work-function, high-conductivity transparent conducting polymer PEDOT:PSS as the Schottky contact to form the Schottky junction (and thus Schottky diode) with the n-type semiconductor a-IGZO. The Schottky didoes exhibited a low apparent turn-on voltage, a high rectification ratio of >10⁵ at ±1 V, and a decent ideality factor of ∼1.5–1.6. Detailed junction properties were systematically analyzed from J-V and C-V characteristics of the diodes. We also demonstrated the applications of PEDOT:PSS/a-IGZO Schottky junctions to various types of Schottky diodes, including the flexible, the transparent, and the flexible transparent PEDOT:PSS/a-IGZO Schottky diodes, by using different substrates and different counter electrodes.     

Tuning of the electronic and transport properties of phosphorene nanoribbons by edge types and edge defects

By performing first-principle quantum transport calculations, we investigated the effects of the edge types and edge defects on the electronic and transport properties of phosphorene nanoribbons (PNRs). The calculated band structures show the PNRs with the zigzag and cliff edges are all metallic. The conductance of the cliff phosphorene nanoribbon (CPNR) is higher than that of the zigzag phosphorene nanoribbon (ZPNR). The low bias negative differential resistance behavior is only found in the ZPNR and the peak-to-valley current ratio is up to 10². More over, we found the carrier transport channels under low bias of ZPNR and CPNR mainly locate on the edges. The current-voltage characteristics show the defects induced by removing the phosphorus atoms from the edge can decrease the conductance of the ZPNR and CPNR obviously. The low bias negative differential resistance behavior of the ZPNR also can be weakened or removed by the edge defects.     

Gate controlled 2-DEG varactor for VCO applications in microwave circuits

A novel gate controlled Schottky diode varactor is introduced. The three-terminal varactor is a modulation-doped heterostructure of AlGaAs/GaAs with two Schottky contacts, similar to a metal–semiconductor–metal (MSM) diode. Schottky metal contacts are made to a two-dimensional electron gas (2-DEG). The third contact, the gate contact is formed from highly doped n⁺ GaAs material to allow an open optical window that can be used for optical gating and mixing. Structure capacitance is less than 1 PF and a change of more than 30% from the zero bias capacitance is observed with the applied gate voltage. On the basis of our quasi two-dimensional C–V model, the layer structure and device dimensions can be optimized and scaled to cover a wide range of operations in the microwave and millimeter wave regimes.     

Fabrication and electrical characterization of Al/p-ZnIn2Se4 thin film Schottky diode structure

Polycrystalline thin films of ternary ZnIn₂Se₄ compound with p-type conductivity were deposited on a pre-deposited aluminium (Al) film by a flash evaporation technique. A Schottky diode comprising of Al/p-ZnIn₂Se₄ structure was fabricated and characterized in the temperature range 303–323 K in dark condition. The Schottky diode was subjected to current (I)-voltage (V) and capacitance (C)-voltage (V) characterization. The Al/p-ZnIn₂Se₄ Schottky diode showed behaviour typical of a p-n junction diode. The devices showed very good diode behaviour with the rectification ratio of about 10⁵ at 1.0 V in dark. The Schottky diode ideality factor, barrier height, carrier concentration, etc. were derived from I-V and C-V measurements. At lower applied voltages (V≤0.5 V), the electrical conduction was found to take place by thermionic emission (TE) whereas at higher voltages (V>0.5 V), a space charge limited conduction mechanism (SCLC) was observed. An energy band diagram was constructed for fabricated Al/p-ZnIn₂Se₄ Schottky diode.     

Controllable Switching between Highly Rectifying Schottky and p–n Junctions in an Ionic MoS2 Device

Semiconductor junctions are of great significance for the development of electronic and optoelectronic devices. Here, controllable switching is demonstrated from a Schottky junction to a p–n junction in a partially ionic liquid-gated MoS₂ device with two types of metal contacts. Excellent rectification behavior with a current on-off ratio exceeding 10⁶ is achieved in both Schottky and p–n junction modes. The formation of Schottky junction at the Pd electrode/MoS₂ contact and p–n junction at the p-MoS₂/n-MoS₂ interface is revealed by spatially resolved photocurrent mappings. The switching between the two junctions under ionic gate modulation is correlated with the evolution of the energy band, further validated by the finite element simulation. The device exhibits excellent photodetection properties in the p–n junction mode, including an open circuit voltage up to 0.84 V, a responsivity of 0.24 A W⁻¹, a specific detectivity of 1.7 × 10¹¹ Jones, a response time of hundreds of microseconds and a linear dynamic range of up to 91 dB. The electric field control of such high-performance Schottky and p–n junctions opens up fresh perspectives for studying the behavior of junction and the development of 2D electronic devices.     

Role of Schottky barrier height at source/drain contact for electrical improvement in high carrier concentration amorphous InGaZnO thin film transistors

We report the fabrication of bottom-gate thin film transistors (TFTs) at various carrier concentrations of an amorphous InGaZnO (a-IGZO) active layer from ~10¹⁶ to ~10¹⁹ cm⁻³, which exceeds the limit of the concentration range for a conventional active layer in a TFT. Using the Schottky TFTs configuration yielded high TFT performance with saturation mobility (μ_sat), threshold voltage (V_TH), and on off current ratio (I_ON/I_OFF) of 16.1 cm²/V s, −1.22 V, and 1.3×10⁸, respectively, at the highest carrier concentration active layer of 10¹⁹ cm⁻³. Other carrier concentrations (<10¹⁹ cm⁻³) of IGZO resulted in a decrease of its work function and increase in activation energy, which changes the source/drain (S/D) contact with the active layer behavior from Schottky to quasi Ohmic, resulting in achieving conventional TFT. Hence, we successfully manipulate the barrier height between the active layer and the S/D contact by changing the carrier concentration of the active layer. Since the performance of this Schottky type TFT yielded favorable results, it is feasible to explore other high carrier concentration ternary and quaternary materials as active layers.     

Comparative studies of Pt and Ir schottky contacts on undoped Al0.36Ga0.64N

Schottky contacts of Pt and Ir on undoped Al_0.36Ga_0.64N have been fabricated and the ideality factor, the built-in voltage and the reverse bias current were determined using current–voltage measurements to make a comparison.The smallest ideality factors, the lowest reverse bias current and the highest built-in voltages have been obtained for Ir Schottky contacts.We have studied the effect of an annealing for Pt and Ir Schottky contacts, on the ideality factor, the built-in voltage and the reverse bias current. A decrease of the ideality factor and the reverse bias current associated to an increase of the built-in voltage have been obtained except for high annealing temperature (T > 400 °C).Reductions of 37% and 43% of the ideality factor and improvements of 24% and 41% of the built-in voltage have been obtained for Pt and Ir Schottky contacts, respectively, after an annealing performed at 350 °C during 30 min.Two different electrical stresses have also been applied on the ohmic and Schottky contacts during 164 h to study the reliability of the employed technology. In a first time, the devices have been stressed with a drain-to-source voltage V_DS of 20 V and a gate-to-source voltage V_GS of −5 V to submit the devices to an electrical field only and not to a thermal effect induced by the electrical current. In a second time, the aging stress has been applied for a V_DS of 20 V and for a V_GS of 0 V in order to study the impact of the electrical field and the thermal effect induced by the drain current on the electrical behaviours of Al_0.36Ga_0.64N/GaN transistors. This study has also shown the existence of electrical traps in the device structure and proved the good reliability of the involved technology.These comparative studies demonstrate that Ir is a better candidate than Pt for the realisation of Schottky contacts on undoped Al_0.36Ga_0.64N.     

Electrical characteristics of high performance Au/n-GaN schottky diodes

Au/n-GaN Schottky diodes with the Au electrode deposited at low temperature (LT=77K) have been studied. In comparison, the same chip of GaN epitaxial layer was also used for room temperature Schottky diodes. The low temperature Schottky diodes exhibit excellent performance. Leakage current density as low as 2.55×10⁻¹¹ A·cm⁻² at −2.5 V was obtained in the LT Schottky diodes. The linear region in the current-voltage curve at forward bias extends more than eight orders in current magnitude. Current-voltage-temperature measurements were carried out to study the characteristics of the LT Schottky diodes. A typical barrier height of about 1.32 eV for the LT diode, which is the highest value ever reported, was obtained. The obvious enhancement in electrical performance makes the LT processing a very promising technique for GaN device application although the detailed mechanisms for the LT Au/n-GaN Schottky diodes are still under investigation.     

High-Performance Ultraviolet Photodetectors Enabled by van der Waals Schottky Junction Based on TiO2 Nanorod Arrays/Au-Modulated Ti3C2Tx MXene

Two-dimensional transition metal carbides and nitrides (MXenes) show tremendous potential for optoelectronic devices due to their excellent electronic properties. Here, a high-performance ultraviolet photodetector based on TiO₂ nanorod arrays/Ti₃C₂T_x MXene van der Waals (vdW) Schottky junction by all-solution process technique is reported. The Ti₃C₂T_x MXene modulated by the Au electrode increases its work function from 4.41 to 5.14 eV to form a hole transport layer. Complemented by the dangling bond-free surface of Ti₃C₂T_x, the Fermi-level pinning effect is suppressed and the electric-field strength of the Schottky junction is enhanced, which promotes charge separation and transport. After applying a bias of −1.5 V, the photovoltaic effect is favorably reinforced, while the hole-trapping mechanism (between TiO₂ and oxygen) and reverse pyroelectric effect are largely eliminated. As a result, the responsivity and specific detectivity of the device with FTO/TiO₂ nanorod arrays/Ti₃C₂T_x/Au structure reach 1.95 × 10⁵ mA W⁻¹ and 4.3 × 10¹³ cm Hz^1/2 W⁻¹ (370 nm, 65 mW cm⁻²), respectively. This work provides an effective approach to enhance the performance of photodetectors by forming the vdW Schottky junction and choosing metal electrodes to modulate MXene as a suitable charge transport layer.     

High-Performance Flexible Schottky DC Generator via Metal/Conducting Polymer Sliding Contacts

Dynamic Schottky direct-current (DC) generators hold great promise for ambient mechanical energy harvesting as it overcomes the low-current output limitation in conventional approaches. However, the lack of a fundamental understanding of DC generation in conducting polymer-based Schottky generators has hindered their application for self-powered wearable and implantable electronics. Here, a high-performance, flexible Schottky DC generator with metal/conducting polymer sliding contact system is demonstrated, which exhibits a large current density (J) up to 20 A m^–2 for single contact geometry and a scaled-up DC output reaching 200 µA (J = 0.73 A m^–2) and 0.8 V. The design of flexibility in such a Schottky DC generator is inherited from the long-chain polymer concept, leading to the demonstration of a variety of device configuration of free-standing thin film, supported thin film and nanocomposite prototype toward practical applications. It is revealed that the sliding junctions may exhibit a different mechanical energy conversion mechanism compared to the compressive conducting polymer Schottky junctions. It is also proven that the magnitude and polarity of DC generation is determined by the Schottky contact formation and interfacial electric field. The concept of a flexible Schottky generator not only shows great promise for next-generation, self-powered wearable devices, but also provides potential mechanisms for developing novel wearable sensors.     

Effects of annealing treatment on the high temperature performance of 4H-SiC metal-semiconductor-metal ultraviolet photodiodes

The high temperature performance plays a crucial role in the high-temperature harsh environment detection. In this paper, the electrical and optical characteristics of 4H-SiC metal-semiconductor-metal (MSM) ultraviolet photodiodes (PDs) were investigated at high temperatures. The C-V measurement indicates that the 4H-SiC Schottky barrier diode is partially depleted at 40 V bias. Analysis of I-V data based on the thermionic emission theory demonstrates that the annealing treatment at 400 °C can effectively improve the homogeneity of Ni/4H-SiC Schottky barrier height. Experimental results confirm that the annealing treatment is beneficial not only to reduce the dark current and improve the photoresponse, but also to enhance the sensitivity for 4H-SiC MSM PDs. The sensitivity of 400 °C annealed MSM PDs (6.2 × 10³) is five times larger than that of as-deposited MSM PDs (1.3 × 10³) at 200 °C.     

Charge carrier mobility and lifetime of organic bulk heterojunctions analyzed by impedance spectroscopy

Charge carrier diffusion and recombination in an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with indium tin oxide (ITO) and aluminium contacts have been analyzed in the dark by means of impedance spectroscopy. Reverse bias capacitance exhibits Mott–Schottky-like behavior indicating the formation of a Schottky junction (band bending) at the P3H:PCBM-Al contact. Impedance measurements show that minority carrier (electrons) diffuse out of the P3HT:PCBM-Al depletion zone and their accumulation contributes to the capacitive response at forward bias. A diffusion–recombination impedance model accounting for the mobility and lifetime parameters is outlined. Electron mobility results to be 2 × 10⁻³ cm² V⁻¹ s⁻¹ and lifetime lies within the milliseconds timescale.     

Current transport and formation of energy structures in narrow Au/n-GaAs Schottky diodes

The current transport and formations of potential barrier height in narrow Au/n-GaAs Schottky diodes (SD) with a contact surface in length of 200 μm, width of 1 and 4 μm have been investigated.It was determined that features of current transport are in good agreements with the thermionic emission theory in the forward bias as like high-quality conventional (flat) SD. Features of current transport in the reverse bias also is well described by thermionic emission theory, but it has specific features unlike I–V characteristics flat SD.Forward bias of narrow SD current–voltage (I–U) characteristics are represented by straight lines in semi-logarithmic scale in a wide range, nearly nine order of current up to 0.7 V with near unit ideality factor. In the beginning of the reverse voltage, the current practically was extremely low, by increasing in voltage the current jump in steps approximately for 3–4 order in voltage of 3–4 V, then current increases linear for 3–5 order in semi-logarithmic scale by increasing in voltage up to nearly 7 V.Numerical values of parameters such as the saturation currents, the operating barrier height, ideality factor, dimensionless factor are obtained. The correlations between ideality factor and dimensionless factor were meaningful.The energy diagrams of narrow SD have been drawn in absence and presence of forward and reverse voltage. It is found that electronic processes in narrow SD are well described by energy model of real narrow metal–semiconductor contacts. The additional electric field arising in near contact area of the semiconductor because of creating contact potential difference between contact surface and to it adjoining free surfaces of the metal and semiconductor.     

Schottky barrier height modification in Au/n-type 6H-SiC structures by PbS interfacial layer

We have prepared the Au/PbS/n-6H-SiC Schottky diodes with interface layer and the reference Au/n-6H-SiC/Ni Schottky diodes without interface layer to realize Schottky barrier height (SBH) modification in the Au/SiC Schottky diodes. The BH reduction has been succeeded by the PbS interlayer to modify the effective BH by influencing the space charge region of the SiC. The PbS thin layer on the SiC was formed by the vacuum evaporation. The SBH values of 0.97 and 0.89 eV for the samples with and without the interfacial PbS layer were obtained from the forward bias current-voltage (I-V) characteristics. X-ray diffraction (XRD) study was carried out to determine the structural formation of the PbS on SiC. The reduction of the BH in the Au/PbS/n-6H-SiC Schottky diodes has been attributed to the fact that the interface states have a net positive interface charge in metal/n-type semiconductor contact, and thus the positive space charge Q_sc in the Au/PbS/n-6H-SiC Schottky diodes becomes smaller than if the interface state charges Q_ss were absent. The experimental carrier concentration value of 4.73 × 10¹⁷ cm⁻³ obtained from the forward and reverse bias capacitance-voltage characteristics for the Au/PbS/n-6H-SiC contacts is lower than the value of 5.52 × 10¹⁷ cm⁻³ obtained for the reference diode, and this is an evidence of the reduction of the BH by the modification of the space charge density of the SiC.     

High-speed organic single-crystal transistors gated with short-channel air gaps: Efficient hole and electron injection in organic semiconductor crystals

Short-channel, high-mobility organic filed-effect transistors (OFETs) are developed based on single crystals gated with short-channel air gaps. The high hole mobility of 10 cm²/Vs for rubrene, and high electron mobility of 4 cm²/Vs for PDIF-CN₂ crystals are demonstrated even with a short channel length of 6 μm. Such performance is due to low contact resistance in these devices estimated to be as low as ～0.5 kΩ cm at gate voltage of ?4 V for rubrene. With the benefit of the short channel length of 4.5 μm in a new device architecture with less parasitic capacitance, the cutoff frequency of the rubrene air–gap device was estimated to be as high as 25 MHz for drain voltage of ?15 V, which is the fastest reported for p-type OFETs, operating in ambient conditions.     

Low-noise solar-blind AlxGa1-xN-based metal-semiconductor-metal ultraviolet photodetectors

We report the growth, fabrication, and characterization of high performance Schottky metal-semiconductor-metal solar-blind photodetectors fabricated on epitaxial Al_0.4Ga_0.6N layers grown by metalorganic chemical vapor deposition. The devices exhibited low dark current (<2 pA at 30 V) and a gain-enhanced ultraviolet (UV) photocurrent for bias voltages >40 V. The gain was corroborated by external quantum efficiency measurements reflecting a quantum efficiency as high as 49% (at=272 nm) at 90 V bias, with a corresponding responsivity R=107 mA/W. A visible-to-UV rejection factor of more than three orders of magnitude was demonstrated. Time-domain and frequency-domain speed measurements show a 3-dB bandwidth of ∼100 MHz. Low-frequency noise measurements have determined a detectivity (D^*) as high as 3.3 10¹⁰ cm·Hz^1/2/W for a 500 Hz bandwidth at 37 V bias.