P-type Regime Research Articles

Electrical properties of BaZr0.5Ce0.3Y0.1Yb0.1O3-δ proton conductor for reversible proton-conducting solid oxide electrochemical cells

Proton-conducting oxide-based reversible solid oxide electrochemical cells (protonic-SOEC, P-SOEC) have a great potential to be the most efficient transition technology operating at intermediate temperatures. Here, we have introduced a perovskite-type proton conductor BaZr0.5Ce0.3Y0.1Yb0.1O3-δ (BZCYYb5311) and evaluated its electrochemical and transport properties. The chemical stability of BZCYYb5311 was confirmed by thermo-gravimetric analysis (TGA) under pure CO2 conditions and X-ray diffraction analysis. The total and partial conductivities were obtained using DC four-probe conductivity measurements under various pO2 and pH2O conditions. The ratio of proton conductivity to ionic conductivity including the oxygen-ion conductivity of this material was 98% at 600 °C, indicating that it is a relatively pure proton conductor with most of the ionic conduction almost exclusively carried out by protons. Chemical diffusivities were calculated from Fick's 2nd law by non-linear least squares fitting during oxidation/reduction and hydration/dehydration processes and were faster than other proton conductor BZCY (Y-doped BaCeO3-BaZrO3) series, which have a similar zirconium contents. In addition, the theoretical performance and efficiency of the reversible P-SOEC were determined from the spatial distribution of oxygen and hydrogen partial pressure in BZCYYb5311, calculating the maximum power density (4.5 Wcm−2 at 600 °C, 3% of water vapor pressure) for the fuel cell mode and current density (12.3 Acm−2 at 1.5 V, 600 °C, and 20% of water vapor pressure) for the electrolysis mode. The fuel consumption efficiency is high (99.1% at 0.6 V at 600 °C), while the electrolysis efficiency is low (72.3% at 1.6 V) due to the dominant p-type regime in the electrolyte during electrolysis. This low electrolysis efficiency can be enhanced by recovering the electrolyte regime by increasing pH2O or decreasing pO2 in the air electrode; however, it will still have low efficiency. Consequently, BZCYYb5311 is not an efficient electrolyte material for electrolyzers, but is promising for fuel cells because of its superior chemical stability, exceptional pure proton conductivity, and better bulk diffusion than those of the BZCY system.

Temperature-dependent electrical transport, Hall effect, and Seebeck properties of bulk chemically reduced graphene oxide with bipolar charge carrier materials

The temperature-dependent electrical transport, Hall effect, and Seebeck properties of bulk-reduced graphene oxide (rGO) prepared by a chemical reduction process were investigated in a temperature range of 310–475 K. The bulk rGO contained bipolar charge carriers with p-type to n-type switching at a temperature of 420 K. The materials illustrated a p-type characteristic in the temperature range of 310–420 K and n-type characteristic in the temperature range of 420–475 K. The charge transport mechanism was that of the graphene-derived 2D material in the p-type regime and governed by polaronic charge carriers.

Gate-tunable magnetoresistance in six-septuple-layer MnBi2Te4

The recently discovered antiferromagnetic topological insulator MnBi2Te4 hosts many exotic topological quantum phases such as the axion insulator and the Chern insulator. Here we report on systematic gate-voltage-dependent magneto-transport studies in six-septuple-layer MnBi2Te4. In the p-type carrier regime, we observe positive linear magnetoresistance (MR) when MnBi2Te4 is polarized in the ferromagnetic state by an out-of-plane magnetic field. Whereas in the n-type regime, distinct negative MR behaviors are observed. The behaviors of magnetoresistance in both regimes are highly robust against temperature up to the Néel temperature. Within the antiferromagnetic regime, the behavior of MR exhibits a transition from negative to positive under the control of gate voltage. The boundaries of the MR phase diagram can be explicitly marked by the gate-voltage-independent magnetic fields that characterize the processes of the spin-flop transition. The rich transport phenomena demonstrate the intricate interplay between topology, magnetism and dimensionality in MnBi2Te4.

Massive and Topological Surface States in Tensile-Strained HgTe.

Magneto-transport measurements on gated high-mobility heterostructures containing a 60 nm layer of tensile-strained HgTe, a three-dimensional topological insulator, show well-developed Hall quantization from surface states both in the n- as well as in the p-type regime. While the n-type behavior is due to transport in the topological surface state of the material, we find from 8-orbital k·p calculations that the p-type transport results from massive Volkov-Pankratov states. Their formation prevents the Dirac point and thus the p-conducting topological surface state from being accessible in transport experiments. This interpretation is supported by low-field magneto-transport experiments demonstrating the coexistence of n-conducting topological surface states and p-conducting Volkov-Pankratov states at the relevant gate voltages.

Computational Fermi level engineering and doping-type conversion of Mg:Ga2O3 via three-step synthesis process

Gallium oxide (Ga2O3) is being actively explored for electronics that can operate at high power, temperature, and frequency as well as for deep-ultraviolet optoelectronics and other applications due to its ultra-wide bandgap (UWBG) and low projected fabrication cost of large-size and high-quality crystals. Efficient n-type doping of monoclinic beta-phase of Ga2O3 has been achieved, but p-type doping faces fundamental obstacles due to compensation, deep acceptor levels, and the polaron transport mechanism of free holes. However, aside from the challenges of achieving p-type conductivity, plenty of opportunity exists to engineer the position of the Fermi level for improved design of Ga2O3-based devices. We use first-principles defect theory and defect equilibrium calculations to simulate a three-step growth-annealing-quench synthesis protocol for hydrogen-assisted Mg doping in β-Ga2O3. The simulations take into account the gas phase equilibrium between H2, O2, and H2O, which determines the H chemical potential. We predict Ga2O3 doping-type conversion to a net p-type regime after growth under reducing conditions in the presence of H2 followed by O-rich annealing, which is a similar process to Mg acceptor activation by H removal in GaN. For equilibrium annealing with re-equilibration of compensating O vacancies, there is an optimal temperature that maximizes the Ga2O3 net acceptor density for a given Mg doping level; the acceptor density is further increased in the non-equilibrium annealing scenario without re-equilibration. After quenching to operating temperature, the Ga2O3 Fermi level drops below mid-gap down to about 1.5 eV above the valence band maximum, creating a significant number of uncompensated neutral MgGa0 acceptors. The resulting free hole concentration in Ga2O3 is very low even at elevated operating temperature (∼108 cm−3 at 400 °C) due to the deep energy level of these Mg acceptors, and hole conductivity is further impeded by the polaron hopping mechanism. However, the Fermi-level reduction and suppression of free electron density in this doping-type converted (NA > ND) Ga2O3 material are important for improved designs of Ga2O3 electronic devices. These results illustrate the power of computational predictions not only for new materials but also for their synthesis science.

Gate Tunable Supercurrent in Josephson Junctions Based on Bi2Te3 Topological Insulator Thin FilmsSupported by the Basic Science Center Project of the National Natural Science Foundation of China (Grant No. 51788104), and the National Key R&D Program of China (Grant No. 2017YFA0302900).

We report transport measurements on Josephson junctions consisting of Bi2Te3 topological insulator (TI) thin films contacted by superconducting Nb electrodes. For a device with junction length L = 134 nm, the critical supercurrent I c can be modulated by an electrical gate which tunes the carrier type and density of the TI film. I c can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state. In the p-type regime the Josephson current can be well described by a short ballistic junction model. In the n-type regime the junction is ballistic at 0.7 K < T < 3.8 K while for T < 0.7 K the diffusive bulk modes emerge and contribute a larger I c than the ballistic model. We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p–n junctions. Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.

Synthesis of anthradithiophene containing conjugated polymers via a cross-coupling strategy.

New conjugated polymers that incorporate dihexylanthradithiophene (DHADT) in the main chain were prepared by Stille, Sonogashira, and Yamamoto cross-coupling polymerization reactions. The polymerization chemistry is enabled by a soluble 5,11-dibromodihexylanthradithiophene monomer that is capable of cross-coupling reactions. Five readily soluble DHADT containing polymers were prepared and characterized experimentally and computationally. These polymers possess HOMO energies of −5.18 eV to −5.43 eV and LUMO energies of −3.0 eV to −2.82 eV. The notable optical features include broad absorption and band gaps ranging from 1.62 eV to 2.15 eV. Polymers were tested in organic field effect transistors and were found to operate in the p-type regime.

Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films

How the morphology of a macroscopic assembly of nanoobjects affects its properties is a long-standing question in nanomaterials science and engineering. Here, we examine how the thermoelectric properties of a flexible thin film of carbon nanotubes depend on macroscopic nanotube alignment. Specifically, we have investigated the anisotropy of the Seebeck coefficient of aligned and gated single-wall carbon nanotube thin films. We varied the Fermi level in a wide range, covering both the p-type and n-type regimes, using electrolyte gating. While we found the electrical conductivity along the nanotube alignment direction to be several times larger than that in the perpendicular direction, the Seebeck coefficient was found to be fully isotropic, irrespective of the Fermi level position. We provide an explanation for this striking difference in anisotropy between the conductivity and the Seebeck coefficient using Mott's theory of hopping conduction. Our experimental evidence for an isotropic Seebeck coefficient in an anisotropic nanotube assembly suggests a route toward controlling the thermoelectric performance of carbon nanotube thin films through morphology control.

Thermoelectric properties of higher manganese silicide films deposited by radio frequency magnetron co-sputtering

Higher manganese silicide (HMS) MnSiγ films with varying γ (1.65 ≤ γ ≤ 3.23) were deposited on oxidized Si wafers at 703 K by radio frequency (RF) magnetron co-sputtering of Mn and Si targets. As-deposited films were annealed at 1273 K for 3min using a lamp annealing system, and the thermoelectric (TE) properties were measured in the temperature range from room temperature to 1073 K. All the MnSiγ films were p-type below about 550 K, but the polarity converted to n-type at higher temperatures. Moreover, the power factor (PF) of the polarity-inverted films was exceptionally high, reaching the maximum value of 12.3 mW/K2m in the annealed MnSi2.04 film. In case of the as-deposited films, the PF in the p-type regime (PFp) grew larger as the Si/Mn molar ratio γ approached the stoichiometric value of 1.73, while the PFn became vanishingly small. In contrast, the PFn of the annealed films remained very high even in the Mn-rich film (γ < 1.73), such that 9.6 mW/K2m was obtained from the MnSi1.65 film. Hall measurement at elevated temperatures (≤773 K) showed that electron mobility was greatly increased after the annealing, which has produced unusually high PFn. The HMS phase was composed of Mn4Si7 and Mn11Si19, but regions of unidentified phase were also confirmed by transmission Kikuchi diffraction (TKD) analysis, the proportion of which was larger in the Si-rich films. The modification of the band structure induced by defects and structural disorder of the Mn and/or Si sublattice are supposed to be the origin of this novel phenomenon.

Thermodynamic Quantities and Defect Chemical Properties of La0.8Sr0.2FeO3-δ

In this work, the defect structure analysis of La0.8Sr0.2FeO3-δ (LSF82) was presented. Thermogravimetric measurements were performed to determine change in oxygen nonstoichiometry (Δδ) with oxygen partial pressure in 10−19⩽ ⩽0.21 and 750⩽(T/°C)⩽900 range. LSF82 showed a clear electronic stoichiometric point around δ≈0.1. The negative sign of partial molar enthalpy and entropy indicated that the incorporation of oxygen was an exothermic process and showed that the experimentally observed variations in and with δ fitted well with the statistical thermodynamic analysis. The defect diagram analysis showed that in n-type regime Fe/Fe concentration varied with whereas in p-type regime Fe•Fe concentration varied with . The electrical conductivity relaxation (ECR) experiments were performed to extract partial ionic conductivity and oxygen self diffusivity. The oxygen self-diffusivity (DO) increased from 1.20 × 10−8 cm2⋅s−1 at 800°C to 3.54 × 10−8 cm2⋅s−1 at 900°C with an activation energy of 1.17 ± 0.23 eV. Dilatometry was performed for expansion measurements as function of and temperature. The chemical expansion model based on the relative change in mean ionic radius vs. δ showed that Fe3+ existed as a mixture of high-spin and low-spin states and made a transition from low-spin to high-spin state with increasing δ.

Gate-Controlled Spin-Valley Locking of Resident Carriers in WSe_{2} Monolayers.

Using time-resolved Kerr rotation, we measure the spin-valley dynamics of resident electrons and holes in single charge-tunable monolayers of the archetypal transition-metal dichalcogenide (TMD) semiconductor WSe_{2}. In the n-type regime, we observe long (∼130 ns) polarization relaxation of electrons that is sensitive to in-plane magnetic fields B_{y}, indicating spin relaxation. In marked contrast, extraordinarily long (∼2 μs) polarization relaxation of holes is revealed in the p-type regime, which is unaffected by B_{y}, directly confirming long-standing expectations of strong spin-valley locking of holes in the valence band of monolayer TMDs. Supported by continuous-wave Kerr spectroscopy and Hanle measurements, these studies provide a unified picture of carrier polarization dynamics in monolayer TMDs, which can guide design principles for future valleytronic devices.

Ambipolar Graphene-Quantum Dot Hybrid Vertical Photodetector with a Graphene Electrode.

A strategy to fabricate an ambipolar near-infrared vertical photodetector (VPD) by sandwiching a photoactive material as a channel film between the bottom graphene and top metal electrodes was developed. The channel length in the vertical architecture was determined by the channel layer thickness, which can provide an ultrashort channel length without the need for a high-precision manufacturing process. The performance of VPDs with two types of semiconductor layers, a graphene-PbS quantum dot hybrid (GQDH) and PbS quantum dots (QDs), was measured. The GQDH VPD showed better photoelectric properties than the QD VPD because of the high mobility of graphene doped in the channel. The GQDH VPD exhibited excellent photoresponse properties with a responsivity of 1.6 × 104 A/W in the p-type regime and a fast response speed with a rise time of 8 ms. The simple manufacture and the promising photoresponse of the GQDH VPDs reveal that an easy and effective way to fabricate high-performance ambipolar photodetectors was developed.

Electrical Stabilization of Surface Resistivity in Epitaxial Graphene Systems by Amorphous Boron Nitride Encapsulation.

Homogeneous monolayer epitaxial graphene (EG) is an ideal candidate for the development of millimeter-sized devices with single-crystal domains. A clean fabrication process was used to produce EG-based devices, with n-type doping level of the order of 1012 cm–2. Generally, electrical properties of EG, such as longitudinal resistivity, remain unstable when devices are exposed to air due to adsorption of molecular dopants, whose presence shifts the carrier density close to the Dirac point (<1010 cm–2) or into the p-type regime. Here, we report experimental results on the use of amorphous boron nitride (a-BN) as an encapsulation layer, whereby EG can maintain its longitudinal resistivity and have its carrier density modulated. Furthermore, we exposed 12 devices to controlled temperatures of up to 85 °C and relative humidity of up to 85% and reported that an approximately 20 nm a-BN encapsulation thickness is sufficient to preserve their longitudinal resistivity to within 10% of the previously measured value. We monitored the electronic properties of our encapsulated and nonencapsulated EG samples by magnetotransport measurements, using a neodymium iron boron magnet. Our results have essential importance in the mass production of millimeter-scale graphene devices, with stable electrical properties.

Effect of Polymer Gate Dielectrics on Charge Transport in Carbon Nanotube Network Transistors: Low-k Insulator for Favorable Active Interface.

Charge transport in carbon nanotube network transistors strongly depends on the properties of the gate dielectric that is in direct contact with the semiconducting carbon nanotubes. In this work, we investigate the dielectric effects on charge transport in polymer-sorted semiconducting single-walled carbon nanotube field-effect transistors (s-SWNT-FETs) by using three different polymer insulators: A low-permittivity (εr) fluoropolymer (CYTOP, εr = 1.8), poly(methyl methacrylate) (PMMA, εr = 3.3), and a high-εr ferroelectric relaxor [P(VDF-TrFE-CTFE), εr = 14.2]. The s-SWNT-FETs with polymer dielectrics show typical ambipolar charge transport with high ON/OFF ratios (up to ∼105) and mobilities (hole mobility up to 6.77 cm2 V-1 s-1 for CYTOP). The s-SWNT-FET with the lowest-k dielectric, CYTOP, exhibits the highest mobility owing to formation of a favorable interface for charge transport, which is confirmed by the lowest activation energies, evaluated by the fluctuation-induced tunneling model (FIT) and the traditional Arrhenius model (EaFIT = 60.2 meV and EaArr = 10 meV). The operational stability of the devices showed a good agreement with the activation energies trend (drain current decay ∼14%, threshold voltage shift ∼0.26 V in p-type regime of CYTOP devices). The poor performance in high-εr devices is accounted for by a large energetic disorder caused by the randomly oriented dipoles in high-k dielectrics. In conclusion, the low-k dielectric forms a favorable interface with s-SWNTs for efficient charge transport in s-SWNT-FETs.

High-performance ambipolar self-assembled Au/Ag nanowire based vertical quantum dot field effect transistor

Most lateral PbSe quantum dot field effect transistors (QD FETs) show a low on current/off current (Ion/Ioff) ratio in charge transport measurements. A new strategy to provide generally better performance is to design PbSe QD FETs with vertical architecture, in which the structure parameters can be tuned flexibly. Here, we fabricated a novel room-temperature operated vertical quantum dot field effect transistor with a channel of 580 nm, where self-assembled Au/Ag nanowires served as source transparent electrodes and PbSe quantum dots as active channels. Through investigating the electrical characterization, the ambipolar device exhibited excellent characteristics with a high Ion/Ioff current ratio of about 1 × 105 and a low sub-threshold slope (0.26 V/decade) in the p-type regime. The all-solution processing vertical architecture provides a convenient way for low cost, large-area integration of the device.

N and p-type properties in organo-metal halide perovskites studied by Seebeck effects

Organo-metal halide perovskites can exhibit co-existed electrical polarizations and semiconducting properties respectively from organic and inorganic components. Here, we find that the Seebeck coefficient can be changed between positive and negative values when the concentration of chloride ions is varied between single-halide (CH3NH3PbI3) and mixed-halide structures (CH3NH3PbIxCl3−x). This indicates that varying the concentration of chloride ions can tune the semiconducting properties between the n-type and p-type regimes in the organo-metal halide perovskites. Our temperature-dependent capacitance measurement shows that increasing temperature can cause a change on internal electrical polarization. As a result, we can propose that the internal polarization functions as the underlying mechanism responsible for large temperature-dependent Seebeck coefficients in organo-metal halide perovskites operating between n-type and p-type regimes.

Band Structures and Transport Properties of Half-Heusler Compounds Nb&lt;i&gt;M&lt;/i&gt;Sb (&lt;i&gt;M&lt;/i&gt; = Fe, Ru)

The band structures and transport properties of Half-Heusler compounds NbFeSb and NbRuSb were studied using ab initio calculations and the Boltzmann transport equation with constant scattering time approximation (CSTA). Both compounds were identified as good p-type thermoelectric materials because of the combination of heavy and light bands in the valence band maximum (VBM). The Seebeck coefficients for NbRuSb were lower than that for NbFeSb; while the electrical conductivities for NbRuSb were little higher than that for NbFeSb. Consequently, the power factors in the p-type regimes for both compounds were similar at a given temperature. NbFeSb and NbRuSb could be efficient materials for thermoelectric generators based on the results in the present investigation.

One-step and controllable bipolar doping of reduced graphene oxide using TMAH as reducing agent and doping source for field effect transistors

Simultaneous reduction and doping of the graphene oxide (GO) is an important issue for low temperature processed flexible electronic devices. A low temperature method for reduction and ambipolar doping has been developed which yield the doped reduced GO with wide range of work function with a mass production using tetra-methyl ammonium hydroxide (TMAH). The doping type of obtained reduced GO is tuned with TMAH concentration. XPS analysis revealed that the graphitic N is converted to oxidized N with increase of TMAH concentration. The work function is tuned via wide range variation in the carrier concentration in neutral (rGO-A, 4.46 eV), n-type (rGO-B, 3.90 eV) and p-type (rGO-C, 5.29 eV) regimes. The obtained Dirac voltages of field effect devices are −1 V, −31 V and +35 V with active layer of rGO-A, rGO-B and rGO-C, respectively. The n-type doping is due to incorporation of graphitic N, whereas, oxidized N acts as electron withdrawing group which causes p-type doping.

Ambipolar quantum dots in intrinsic silicon

We electrically measure intrinsic silicon quantum dots with electrostatically defined tunnel barriers. The presence of both p- and n-type ohmic contacts enables the accumulation of either electrons or holes. Thus, we are able to study both transport regimes within the same device. We investigate the effect of the tunnel barriers and the electrostatically defined quantum dots. There is greater localisation of charge states under the tunnel barriers in the case of hole conduction, leading to higher charge noise in the p-type regime.

Switch the n-type to ambipolar transfer characteristics by illumination in n-type pentacene-based organic field-effect transistors

This work investigates the suppression of n-channel and the switch of transfer characteristics (from n-type to ambipolar) by illumination in n-type pentacene-based organic field-effect transistors (OFETs). The illumination outcomes differently on the output characteristics of OFETs, which markedly decreases the magnitude of drain current (n-channel) and shifts the turn-on voltage to a higher positive bias in the n-type regime, but induces the formation of p-channel in the p-type regime. We attribute that the trapped negative charges in the device as induced by illumination electrostatically shield the effective electrical field applied to the gate with source/drain electrodes and modulate the device performance. The result of quasi-static capacitance–voltage measurement agrees well with the modulations of the transfer characteristics for n-type OFETs by illumination. In addition, the de-trapping of charges recovers the n-type only output characteristics of pentacene-based OFETs. This study highlights the unique photo responses of n-type pentacene-based OFETs to the development of phototransistors of distinct output characteristics operated in n- and p-type regime.