Transport Properties Of Electronic Devices Research Articles

Work function modulation of electrodes contacted to molybdenum disulfide using an attached metal pad

The transport properties of electronic devices fabricated using two-dimensional materials are severely affected by the Schottky barrier at the contact of an electrode. The Schottky barrier height exhibits a strong correlation with the work function of the electrode. We observed rectifying current–voltage characteristics for a back-gated field effect transistor with a channel of molybdenum disulfide and Al electrodes, where one of the electrodes is attached to a Au pad. This result is explained in terms of the increase in the effective work function of the Al electrode attached to the Au pad. The dependence of a photocurrent on the bias voltage exhibited an opposite tendency to the current–voltage characteristics; this is also attributed to the work function modulation of the electrode, thus resulting in the variation in the Schottky barrier height.

Electronic properties and structure of single crystal perylene.

The transport properties of electronic devices made from single crystalline molecular semiconductors typically outperform those composed of thin-films of the same material. To further understand the superiority of these extrinsic device properties, an understanding of the intrinsic electronic structure and properties of the organic semiconductor is necessary. An investigation of the electronic structure and properties of single crystal α-phase perylene (C20H12), a five-ringed aromatic molecule, is presented using angle-resolved ultraviolet photoemission, x-ray photoelectron spectroscopy (XPS), and field-effect transistor measurements. Key aspects of the electronic structure of single crystal α-perylene critical to charge transport are determined, including the energetic location of the highest occupied molecular orbital (HOMO), the HOMO bandwidth, and surface work function. In addition, using high resolution XPS, we can distinguish between inequivalent carbon atoms within the perylene crystal and, from the shake-up satellite structure in XPS, gain insight into the intramolecular properties in α-perylene. From the device measurements, the charge carrier mobility of α-perylene is found to depend on the device structure and the choice of dielectric, with values in the range of 10-3 cm2 V-1 s-1.

Large rectifying ratio in a nitrogen-doped armchair graphene device modulated by the gate voltage

Using a combination of first-principles density functional theory and the non-equilibrium Green's function method, we have investigated the electronic transport properties of devices based on an orderly nitrogen-doped armchair graphene nanoribbon under gate voltages. The results show that the gate voltage strongly affects the electronic transport properties of the devices. Strong rectifying behavior is observed and it can be modulated by the gate voltage. The maximum rectifying ratio reaches the order of 106 at a specific gate voltage, which is two orders of magnitude higher than the device without applying a gate voltage. The mechanism for the rectifying behavior was investigated from the calculated transmission spectra, potential drop, and local device density of state. The results indicate that the devices regulated by the gate voltage can potentially be applied to high-performance rectifiers.

Thermodynamic considerations on interfacial reactivity concerning carrier transport characteristics in metal/p-Zn3P2 junctions

Chemical reaction at the metal/semiconductor interfaces, which have a significant impact on carrier transport properties of electronic devices, can be systematically discussed based on chemical potential diagrams.

Enhanced superconductivity in atomically thin TaS2.

The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this top–down approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulfide flakes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we find a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electron–phonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials so far.

High performance bipolar spin filtering and switching functions of poly-(terphenylene-butadiynylene) between zigzag graphene nanoribbon electrodes

Using the nonequilibrium Green’s function method combined with spin-polarized DFT, we investigate the spin-resolved electronic transport properties of devices made of poly-(terphenylene-butadiynylene) (PTB) between zigzag graphene nanoribbon (ZGNR) electrodes.

Quantum description of transport phenomena: Recent progress

Over 100 years ago, atoms were believed to be the smallest indivisible particles and no one knew what was inside an electric current. For instance, what was cathode ray made of had puzzled many. In 1897, J. J. Thomson, based on his refined and newly designed experiments, discovered that cathode rays were made of electrons! Fifty years later, William Shockley, John Bardeen, and Walter Brattain invented a device called transistor that operated on the flow of electrons. A revolution of information technology started. Exactly 67 years after the invention of transistor, here we are at the east campus of Renmin University of China in Beijing, enjoying a beautiful November afternoon. Almost every student walking by is busy tapping some hand held device. In a modern city like Beijing, probably, every person has a computer, a pad, a smartphone or some electronic gadget each containing billions of transistors. The materials of building transistors have evolved from a piece of germanium, plastic and gold in 1947 to today’s silicon, high-k oxide and copper, and will inevitably move forward to a “post-silicon” era with emerging electronic materials. The information carrier, originally electric charge, now includes spin, possibly “valley”, and perhaps other more exotic quantum degrees of freedom by the on-going effort of physicists. Profiting from nanofabrication, the characteristic size of a transistor has decreased from the millimeter scale to micrometer scale and now to nanometer scale, closely following the Moore’s Law. The perspective of eventually reaching the single atom or molecule scale is exciting. Semiconductor field effect transistor (FET) is the most widely used transistor technology today. In an FET, electrons flow through the semiconductor channel in a controlled manner. For long channels, electron motion is interrupted by frequent random collisions with such things as impurities, defects and vibrations of the material lattice, and transport can be well described by diffusion. On the other hand, for very short channels, electrons sip through without enough time to suffer any collision, and transport is ballistic. To move the device technology forward, achieving a basic understanding of carrier transport is of critical importance. There has been a long history of theoretical description of transport processes. Boltzmann was the first one thinking of combining Newtonian mechanics with entropy driven processes to analyze transport phenomenon. The Boltzmann transport equation which combines classical mechanics with statistical mechanics is the standard semi-classical theory for predicting transport properties of electronic devices. Similarly, a full quantum transport theory requires some kind of combination of quantum mechanics with statistical mechanics, and is conveniently done by the non-equilibrium Green’s function (NEGF) formalism established by the pioneering work of Martin, Schwinger, Kadanoff, Baym, Keldysh, and the others. More recently, Landauer, Buttiker, and Datta contributed significantly on our current understanding of quantum transport at the nanoscale [1]. Taylor, Guo, and Wang were the first to establish first principles materials calculation by density functional theory (DFT) within the framework of NEGF [2] and produced the first NEGF-DFT quantum transport package named McDCal [2] abbreviation of McGill Devices Calculator, offering full quantum description of transport phenomena for real materials. After 14 years of the first report of NEGF-DFT formalism and code, this approach has become the de facto method for parameter-free simulations of quantum transport and device physics. Many implementations of NEGF-DFT, including MatDCal, NanoDCal, Transiesta, ATK, SMEAGOL and several others, are now widely used by physicists, chemists, materials sci-

Nonequilibrium coherent potential approximation for electron transport

Accounting for the effects of disorder on the transport properties of electronic devices is indispensable for comparison with experiment. However, theoretical treatment of disorder presents essential difficulty because the disorder breaks the periodicity of the system. The coherent potential approximation (CPA) solves this problem by replacing the disordered medium with a periodic effective medium. However, calculating the electron current within CPA requires summing scattering diagrams to infinite order called vertex corrections. In this work we reformulate CPA for nonequilibrium electron transport. This approach, based on the nonequilibrium Green's function formalism, greatly simplifies the treatment of disordered transport by eliminating the vertex corrections.

Temperature-dependent contact resistances in high-quality polymer field-effect transistors

Contact resistances between organic semiconductors and metals can dominate the transport properties of electronic devices incorporating such materials. We report measurements of the parasitic contact resistance and the true channel resistance in bottom contact poly(3-hexylthiophene) field-effect transistors with channel lengths from 400 nm up to 40 μm, from room temperature down to 77 K. For fixed gate voltage, the ratio of contact to channel resistance decreases with decreasing temperature. We compare this result with a recent model for organic semiconductor-metal contacts. Mobilities corrected for this contact resistance can approach 1 cm2/V s at room temperature and high gate voltages.

Phase measurement in a quantum dot via a double-slit interference experiment

The transport properties of electronic devices are usually characterized on the basis of conductance measurements. Such measurements are adequate for devices in which transport occurs incoherently, but for very small devices—such as quantum dots1,2—the wave nature of the electrons plays an important role3. Because the phase of an electron's wavefunction changes as it passes through such a device, phase measurements are required to characterize the transport properties fully. Here we report the results of a double-slit interference experiment which permits the measurement of the phase-shift of an electron traversing a quantum dot. This is accomplished by inserting the quantum dot into one arm of an interferometer, thereby introducing a measurable phase shift between the arms. We find that the phase evolution within a resonance of the quantum dot can be accounted for qualitatively by a model that ignores the interactions between the electrons within the dot. Although these electrons must interact strongly, such interactions apparently have no observable effect on the phase. On the other hand, we also find that the phase behaviour is identical for all resonances, and that there is a sharp jump of the phase between successive resonance peaks. Adequate explanation of these features may require a model that includes interactions between electrons.