Two-dimensional Variable Range Hopping Research Articles

The effect of Si3N4/Al2O3 stacked structure of AlGaN/GaN HEMTs

Excessive gate leakage current and current collapse effects are the critical reliability issues for GaN-based high electron mobility transistors (HEMTs). In this paper, Si3N4 passivation layers was grown separately on p-GaN gate HRCL-HEMTs with or without Al2O3 film, and the gate leakage and current collapse effects of the devices and their intrinsic mechanisms were investigated. It is found that the device with Si3N4/Al2O3 stacked structure has better performance in terms of gate reverse leakage current, breakdown voltage, etc. The Al2O3 film blocking layer can reduce the H plasma damage to the channel, decrease the gate leakage, and the passivation can effectively suppress the increase of gate leakage current. Through variable temperature characteristic test, it is verified that the dominant mechanism of the device gate reverse leakage current is the two-dimensional variable range hopping (2D-VRH) model. The interface state between Si3N4 passivation layer and Al2O3 film increases the trap activation energy of the device, which is the reason for the reduction of gate reverse leakage current, and also causes the current collapse effect of the device.

Gate leakage mechanisms of the AlGaN/GaN HEMT with fluorinated graphene passivation

The AlGaN/GaN high-electron-mobility transistor (HEMT) passivated by fluorinated graphene (FG) has been investigated. The performances of the HEMTs without passivation before and after F plasma treatment, with FG, Silicon nitride (SiNx), FG/SiNx passivation were contrasted. Compared with the HEMT without F plasma treatment and passivation, the gate leakage current decreases one and a half orders of magnitude after F plasma treatment and SiNx passivated, and four orders of magnitude after FG and FG/SiNx passivated. To study the leakage mechanisms, the temperature dependences of gate leakages have been tested at 323 K–473 K. Through analysis and fitting, the leakages are composed of three components, which are thermionic emission (TE) current, two-dimensional variable range hopping (2D-VRH) current, and Fowler–Nordheim tunneling (FNT) current. TE dominates in the high forward bias region. 2D-VRH dominates in low forward bias. And the chief leakage mechanisms at reverse bias are 2D-VRH and FNT. At 473 K, Schottky barrier height of the HEMT with FG passivation is 1.21 eV. The activation energy and the effective barrier height on the HEMT with FG passivation are 0.64 eV and 0.33 eV respectively. Furthermore, the reasons for the decrement of the gate leakage on the HEMT with FG and FG/SiNx passivation are the enhancement of Schottky barrier heights and augmentation of activation energy. And the improvement in gate leakages for the HEMT passivated by FG/SiNx mainly results from the effect of FG.

Study on H plasma treatment enhanced p-GaN gate AlGaN/GaN HEMT with block layer

<sec>High electron mobility transistors(HEMTs)show tremendous potentials for high mobility, high breakdown voltage, low conduction, low power consumption, and occupy an important piece of the microelectronics field. The high-resistivity-cap-layer high electron mobility transistor (HRCL-HEMT) is a novel device structure. Based on the hole compensation mechanism, the p-GaN is converted into high resistance semiconductor material by hydrogen plasma implantation. Thus, the surface of the p-GaN layer will have a serious bombardment damage under the hydrogen plasma implantation. In practical work, it is also very challenging in the accurate controlling of the hydrogen injection rate, injection depth and injection uniformity. To achieve the required depth of injection, the injected hydrogen plasma is often more than the required dose or multiple injections times. The energy of hydrogen plasma plays a huge influence on the surface of the p-GaN layer.The leakage current will be generated on the device surface, which deteriorates the electrical performance of the device.</sec><sec>In this work, to protect the surface of p-GaN layer, a 2-nm Al<sub>2</sub>O<sub>3</sub> film is deposited on the surface of the p-GaN cap layer to reduce the implantation damage caused by hydrogen plasma treatment. The research shows that after the device deposited Al<sub>2</sub>O<sub>3</sub> film prior to the hydrogen plasma treatment, the gate reverse leakage current is reduced by an order of magnitude, the ratio of <i>I</i><sub>ON</sub> to <i>I</i><sub>OFF</sub> is increased by about 3 times. Meanwhile, the OFF-state breakdown voltage is increased from 410 V to 780 V. In addition, when the bias voltage is 400 V, the values of dynamic <i>R</i><sub>ON</sub> of devices A and B are 1.49 and 1.45 respectively, the device B shows a more stable dynamic performance. To analyze the gate leakage mechanism, a temperature-dependent current<i> I</i><sub>G</sub>-<i>V</i><sub>G</sub> testing is carried out, and it is found that the dominant mechanism of gate leakage current is two-dimensional variable range hopping (2D-VRH) at reverse gate voltage. The reason for reducing the gate reverse current is analyzed, and the Al<sub>2</sub>O<sub>3</sub> film increases the activation energy of trap level and changes the surface states of HR-GaN; furthermore, the Al<sub>2</sub>O<sub>3</sub> film blocks the injection of too much H plasma, thereby reducing the density of AlGaN barrier and channel trap states, and weakening the current collapse.</sec>

Coherent Hopping Transport and Giant Negative Magnetoresistance in Epitaxial CsSnBr3

Single-crystal inorganic halide perovskites are attracting interest for quantum device applications. Here we present low-temperature quantum magnetotransport measurements on thin film devices of epitaxial single-crystal CsSnBr$_{3}$, which exhibit two-dimensional Mott variable range hopping (VRH) and giant negative magnetoresistance. These findings are described by a model for quantum interference between different directed hopping paths and we extract the temperature-dependent hopping length of charge carriers, their localization length, and a lower bound for their phase coherence length of ~100 nm at low temperatures. These observations demonstrate that epitaxial halide perovskite devices are emerging as a material class for low-dimensional quantum coherent transport devices.

Variable range hopping mechanism and modeling of isolation leakage current in GaN-based high-electron-mobility transistors

Off-state leakage current of AlGaN/GaN high-electron-mobility transistors (HEMTs) with implant and mesa isolation was studied. Comparison of isolated pad structures with HEMTs shows that isolation leakage flowing through contact pads is the major leakage source in the studied GaN-based HEMTs whose gate finger is connected to an Ohmic contact pad. Then, circular metal-oxide-semiconductor field-effect-transistor devices were used to identify this isolation leakage path as surface isolation leakage at the SiN/nitride interface rather than bulk leakage in the buffer layer or implanted region. The temperature-dependent measurement shows that the two-dimensional variable range hopping mechanism dominates both the implant and mesa isolation leakage current. Mesa isolation results in a larger hopping probability and isolation leakage current than implant isolation. The isolation leakage current through gate and drain contact pads results in a non-zero switch of gate current in rectangular devices with either a Schottky gate or a metal-oxide-semiconductor gate, which is not observed in circular devices. Gate voltage for the switch of gate leakage current is linearly correlated with drain bias voltage, and the slope of the linear model represents the influence of drain bias on gate leakage current. This empirical model is independent of the fabrication process, provided that the same layout is used. With an increase in the source–drain distance from 4 μm to 10 μm, the ratio of gate–drain isolation resistance to gate–source isolation resistance increases from 1.38 to 2.33, leading to a decrease in the slope of empirical lines from 0.42 to 0.30.

Ultrathin molecule-based magnetic conductors: A step towards flexible electronics

ABSTRACTOrganic-inorganic hybrid materials have shown a remarkable and rapid development during the past decade because they can be tailored to obtain new device concepts with controlled physical properties. Here, we report on the electronic and magnetic properties of multilayer organic-inorganic hybrid films. Electrical transport properties arising from the π electrons in the organic layer are characteristic of a metallic state at high temperature and evolve into a state described by two-dimensional variable range hopping when temperature decreases below 150 K. The intrinsic electronic behavior of the hybrid films was further studied via the optical properties in the IR range. The optical response confirms the metallic character of the hybrid films. In the second part, the magnetic properties are discussed. A long-range ferromagnetic order with an ordering temperature of ∼ 1 K is revealed in the Gd-based hybrid film. The Cu-based hybrid film, however, shows more extended ferromagnetic exchange interactions than the Gd-based hybrid LB film.

Observation of Different Charge Transport Processes and Origin of Magnetism in rGO and rGO-ZnSe Composite

The effect of nanoparticle incorporation inside reduced graphene oxide (rGO) layers on the electrical transport properties has been investigated by temperature-dependent resistivity measurements on rGO and rGO-zinc selenide (rGO-ZnSe) thin films in a wide temperature range of 84–473 K. The fraction of ZnSe in rGO-ZnSe composite has been varied from 26 to 98%. Resistance curve derivative analysis reveals a conduction mechanism consistent with Mott two-dimensional (2D) variable range hopping (VRH) in rGO films, whereas different rGO-ZnSe samples exhibit three-dimensional (3D) VRH at lower temperatures (84–280 K). At higher temperatures (290–473 K), Arrhenius-like transport was observed for all of the samples. A model where ZnSe nanoparticles are incorporated inside wrinkled rGO layers to facilitate interlayer connections, thus 3D charge transport, has been proposed. Room-temperature carrier mobility values, calculated using trap-free space charge limited current conduction model in the dark, reach a maximum...

Magnetotransport in type-enriched single-wall carbon nanotube networks

Single-wall carbon nanotubes (SWCNTs) exhibit a wide range of physical phenomena depending on their chirality. Nanotube networks typically contain a broad mixture of chiralities, which prevents an in-depth understanding of SWCNT ensemble properties. In particular, electronic-type mixing (the simultaneous presence of semiconductor and metallic nanotubes) in SWCNT networks remains the single largest hurdle to developing a comprehensive view of ensemble nanotube electrical transport, a critical step toward their use in optoelectronics. Here, we systematically study temperature-dependent magnetoconductivity (MC) in networks of highly enriched semiconductor and metal SWCNT films. In the semiconductor-enriched network, we observe two-dimensional variable-range hopping conduction from 5 to 290 K. Low-temperature MC measurements reveal a large, negative MC from which we determine the wave-function localization length and Fermi energy density of states. In contrast, the metal-enriched film exhibits positive MC that increases with decreasing temperature, a behavior attributed to two-dimensional weak localization. Using this model, we determine the details of the carrier phase coherence and fit the temperature-dependent conductivity. These extensive measurements on type-enriched SWCNT networks provide insights that pave the way for the use of SWCNTs in solid-state devices.

Gate leakage mechanisms in normally off p-GaN/AlGaN/GaN high electron mobility transistors

In this letter, gate leakage mechanisms in different gate contact normally off p-GaN/AlGaN/GaN high electron mobility transistors (HEMTs) have been studied by the temperature-dependent current-voltage (IG-VG) measurement. It is found that two-dimensional variable range hopping is responsible for gate leakage current at the reverse gate bias and low forward gate bias in both high-leakage and low-leakage Schottky gate contact devices. At high forward gate bias, in the case of high-leakage Schottky contact, the dominant current conduction mechanism is found to be thermionic field emission while it is Poole–Frenkle emission (PFE) for the case of low-leakage Schottky contact and the activation energy of trap states for PFE current is derived as 0.6 eV. Besides, related models are also proposed to describe the gate leakage current in p-GaN gate HEMTs and they match well with the experimental gate leakage current within a wide range of temperatures and gate biases.

Preparation, Structure Evolution, and Metal-Insulator Transition of Na xRhO2 Crystals (0.25 ≤ x ≤ 1).

The triangular lattice Na xRhO2 contains a 4d Rh element with large spin-orbit coupling, and the electron-electron correlation effect is expected to have some novel physical properties. Here we report NaRhO2 crystal growth by Na2CO3 vapor growth and a series of Na xRhO2 (0.25 ≤ x ≤ 1) crystals prepared using the chemical desodiation method. Na xRhO2 reveals a layer structure with the space group R3̅ m, and the lattice parameter a evolves from 3.09 to 3.03 Å and c from 15.54 to 15.62 Å when x decreases from 1.0 to 0.2. Decreasing potassium concentration leads to a contraction of the RhO6 octahedral layers, which may be attributed to a higher covalency of Rh-O bonds. More important, the metal-insulator transition in Na xRhO2 was observed in resistivity along the ab plane. The conducting mechanism of Na xRhO2 is strongly dependent on x. Two-dimensional variable range hopping (VRH) mechanisms (0.67 ≤ x ≤ 1) and metallic behaviors (0.42 and 0.47) are observed in temperature-dependent resistivity. The origin of this metal-insulator transition was discussed on the basis of the Ioffe-Regel criterion. Our work demonstrates the strong correlation between sodium concentration and physical properties of Na xRhO2.

Low temperature resistivity studies of [formula omitted]: Observation of two-dimensional variable-range hopping conductivity

We studied electrical resistance of a single-crystalline SmB6 sample with a focus on the region of the “low-temperature resistivity plateau”. Our observations did not show any true saturation of the electrical resistance at temperatures below 3K down to 70mK. According to our findings, temperature dependence of the electrical conduction in a certain temperature interval above 70mK can be decomposed into a temperature-independent term and a temperature-activated term that can be described by variable-range hopping formula for two-dimensional systems, exp[−(T0/T)1/3]. Thus, our results indicate importance of hopping type of electrical transport in the near-surface region of SmB6.

Mechanism of leakage of ion-implantation isolated AlGaN/GaN MIS-high electron mobility transistors on Si substrate

In this paper, we systematically investigated the leakage mechanism of the ion-implantation isolated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) on Si substrate. By means of combined DC tests at different temperatures and electric field dependence, we demonstrated the following original results: (1) It is proved that gate leakage is the main contribution to OFF-state leakage of ion-implantation isolated AlGaN/GaN MIS-HEMTs, and the gate leakage path is a series connection of the gate dielectric Si3N4 and Si3N4-GaN interface. (2) The dominant mechanisms of the leakage current through LPCVD-Si3N4 gate dielectric and Si3N4-GaN interface are identified to be Frenkel–Poole emission and two-dimensional variable range hopping (2D-VRH), respectively. (3) A certain temperature annealing could reduce the density of the interface state that produced by ion implantation, and consequently suppress the interface leakage transport, which results in a decrease in OFF-state leakage current of ion-implantation isolated AlGaN/GaN MIS-HEMTs.

Charge transfer-induced magnetic exchange bias and electron localization in (111)- and (001)-oriented LaNiO3/LaMnO3 superlattices

High-quality lattice-matched LaNiO3/LaMnO3 superlattices with monolayer terrace structure have been grown on both (111)- and (001)-oriented SrTiO3 substrates by pulsed laser deposition. In contrast to the previously reported experiments, a magnetic exchange bias is observed that reproducibly occurs in both (111)- and (001)-oriented superlattices with the thin single layers of 5 and 7 unit cells, respectively. The exchange bias is theoretically explained by charge transfer-induced magnetic moments at Ni atoms. Furthermore, magnetization data at low temperature suggest two magnetic phases in the superlattices, with Néel temperature around 10 K. Electrical transport measurements reveal a metal-insulator transition with strong localization of electrons in the superlattices with the thin LaNiO3 layers of 4 unit cells, in which the electrical transport is dominated by two-dimensional variable range hopping.

Millimeter-scale layered MoSe2 grown on sapphire and evidence for negative magnetoresistance

Molecular beam epitaxy technique has been used to deposit a single layer and a bilayer of MoSe2 on sapphire. Extensive characterizations including in-situ and ex-situ measurements show that the layered MoSe2 grows in a scalable manner on the substrate and reveals characteristics of a stoichiometric 2H-phase. The layered MoSe2 exhibits polycrystalline features with domains separated by defects and boundaries. Temperature and magnetic field dependent resistivity measurements unveil a carrier hopping character described within two-dimensional variable range hopping mechanism. Moreover, a negative magnetoresistance was observed, stressing a fascinating feature of the charge transport under the application of a magnetic field in the layered MoSe2 system. This negative magnetoresistance observed at millimeter-scale is similar to that observed recently at room temperature in WS2 flakes at a micrometer scale [Zhang et al., Appl. Phys. Lett. 108, 153114 (2016)]. This scalability highlights the fact that the underlying physical mechanism is intrinsic to these two-dimensional materials and occurs at very short scale.

Electrical and Thermoelectric Transport by Variable Range Hopping in Thin Black Phosphorus Devices.

The moderate band gap of black phosphorus (BP) in the range of 0.3-2 eV, along a high mobility of a few hundred cm(2) V(-1) s(-1) provides a bridge between the gapless graphene and relatively low-mobility transition metal dichalcogenides. Here, we study the mechanism of electrical and thermoelectric transport in 10-30 nm thick BP devices by measurements of electrical conductance and thermopower (S) with various temperatures (T) and gate-electric fields. The T dependences of S and the sheet conductance (σ□) of the BP devices show behaviors of T(1/3) and exp[-(1/T)(1/3)], respectively, where S reaches ∼0.4 mV/K near room T. This result indicates that two-dimensional (2D) Mott's variable range hopping (VRH) is a dominant mechanism in the thermoelectric and electrical transport in our examined thin BP devices. We consider the origin of the 2D Mott's VRH transport in our BPs as trapped charges at the surface of the underlying SiO2 based on the analysis with observed multiple quantum dots.

Structural and Electrical Properties of MoTe2 and MoSe2 Grown by Molecular Beam Epitaxy.

We demonstrate the growth of thin films of molybdenum ditelluride and molybdenum diselenide on sapphire substrates by molecular beam epitaxy. In situ structural and chemical analyses reveal stoichiometric layered film growth with atomically smooth surface morphologies. Film growth along the (001) direction is confirmed by X-ray diffraction, and the crystalline nature of growth in the 2H phase is evident from Raman spectroscopy. Transmission electron microscopy is used to confirm the layered film structure and hexagonal arrangement of surface atoms. Temperature-dependent electrical measurements show an insulating behavior that agrees well with a two-dimensional variable-range hopping model, suggesting that transport in these films is dominated by localized charge-carrier states.

Hall and field-effect mobilities in few layered p-WSe₂ field-effect transistors.

Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm2/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm2/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics.

Transport mechanism of reverse surface leakage current in AlGaN/GaN high-electron mobility transistor with SiN passivation**Project supported by the National Natural Science Foundation of China (Grant Nos. 61334002, 61106106, and 61474091), the Opening Project of Science and Technology on Reliability Physics and Application Technology of Electronic Component Laboratory, China (Grant No. ZHD201206), the New Experiment Development Funds for Xidian University, China

The transport mechanism of reverse surface leakage current in the AlGaN/GaN high-electron mobility transistor (HEMT) becomes one of the most important reliability issues with the downscaling of feature size. In this paper, the research results show that the reverse surface leakage current in AlGaN/GaN HEMT with SiN passivation increases with the enhancement of temperature in the range from 298 K to 423 K. Three possible transport mechanisms are proposed and examined to explain the generation of reverse surface leakage current. By comparing the experimental data with the numerical transport models, it is found that neither Fowler–Nordheim tunneling nor Frenkel–Poole emission can describe the transport of reverse surface leakage current. However, good agreement is found between the experimental data and the two-dimensional variable range hopping (2D-VRH) model. Therefore, it is concluded that the reverse surface leakage current is dominated by the electron hopping through the surface states at the barrier layer. Moreover, the activation energy of surface leakage current is extracted, which is around 0.083 eV. Finally, the SiN passivated HEMT with a high Al composition and a thin AlGaN barrier layer is also studied. It is observed that 2D-VRH still dominates the reverse surface leakage current and the activation energy is around 0.10 eV, which demonstrates that the alteration of the AlGaN barrier layer does not affect the transport mechanism of reverse surface leakage current in this paper.

Temperature evolution of magnetic and transport behavior in 5d Mott insulator Sr2IrO4: significance of magneto-structural coupling

We have investigated the temperature evolution of magnetism and its interrelation with structural parameters in the perovskite-based layered compound Sr2IrO4, which is believed to be a Jeff = 1/2 Mott insulator. The structural distortion plays an important role in this material and induces a weak ferromagnetism in an otherwise antiferromagnetically ordered magnetic state with a transition temperature around 240 K. Interestingly, at low temperatures, below around 100 K, a change in the magnetic moment has been observed. Temperature dependent x-ray diffraction measurements show that sudden changes in structural parameters around 100 K are responsible for this. Resistivity measurements show insulating behavior throughout the temperature range across the magnetic phase transition. The electronic transport can be described with Mott's two-dimensional variable range hopping (VRH) mechanism, however, three different temperature ranges are found for VRH, which is a result of varying the localization length with temperature. A negative magnetoresistance (MR) has been observed at all temperatures in contrast to positive behavior generally observed in strongly spin-orbit coupled materials. The quadratic field dependence of MR implies the relevance of a quantum interference effect.

Temperature dependent electrical transport of disordered reduced graphene oxide

We report on the simple route for the synthesis of chemically reduced graphene oxide (rGO) using ascorbic acid (a green chemical) as a reducing agent. Temperature-dependent electrical transport properties of rGO thin films have been studied in a wide range (50 K T 400 K) of temperature. Electrical conduction in rGO thin films was displayed in two different temperature regimes. At higher temperatures, Arrhenius-like temperature dependence of resistance was observed indicating a band gap dominating transport behavior. At lower temperatures, the rGO sample showed a conduction mechanism consistent with Mottʼs two-dimensional variable range hopping (2D-VRH). An unsaturated negative magnetoresistance (MR) was observed up to 3 T field. A decrease in negative MR at high temperatures is attributed to the phonon scattering of charge carriers.