Molybdenum disulphide (MoS2) is one of the transition metal dichalcogenide (TMD) materials which has attracted attention due to its various interesting properties. MoS2 is very promising for electronic and optoelectronic devices due to its indirect band gap (∼1.2 eV) for few layer and direct band gap (∼1.8 eV) for monolayer MoS2. In MoS2 based Schottky devices, Schottky barrier height depends on the thickness of MoS2 because of its tunable electronic properties. Here, we have used DC sputtering technique to fabricate metal-semiconductor junction of MoS2 with platinum (Pt) metal contacts. In this work, MoS2 thin film (∼10 nm) was deposited on p-Silicon (111) using DC sputtering technique at optimized parameters. Schottky metallization of Pt metal (contact area ∼ 0.785x10-2 cm2) was also done using DC sputtering. Current-voltage (I-V) characteristics of the Pt/MoS2 Schottky junction have been investigated in the temperature range 80-350K. Forward I-V characteristics of Pt/MoS2 junction are analysed to calculate different Schottky parameters. Schottky barrier height increases and ideality factor decreases on increasing the temperature from 80-350K. The I-V-T measurements suggest the presence of local inhomogeneities at the Pt/MoS2 junction. Schottky barrier inhomogeneities occur in case of rough interface. In such cases, the Schottky barrier height does not remain constant and vary locally. Current transport through the Schottky junction is a thermally activated process. As temperature increases, more and more electrons overcome the spatially inhomogeneous barrier height. As a result, the ideality factor becomes close to unity and apparent barrier height increases due to increase in temperature.

Thermoelectronic energy conversion can potentially provide an exceptionally efficient way to convert heat into electric power. Key components of such converters are materials with designed, small work functions. We present the principles of thermoelectronic energy conversion and discuss the advantages and challenges of the conversion process, as well the state of the art of the respective research.

Electron emission represents the key mechanism enabling the development of devices that have revolutionized modern science and technology. Today, science still relies on advanced electron-emission devices for imaging, electronics, sensing, and high-energy physics. New generations of emission devices are continuously being improved based on innovative materials and the introduction of novel physical concepts. Recent advances are highlighted by emerging low-work-function and low-dimensional materials with unusual electronic and thermal properties. Nanotubes, nanowires, graphene, and electron-emission models are discussed in this issue, as well as original mechanisms, such as the thermoelectronic effect, thermionic emission, and heat trap processes. Advances in electron-emission materials and physics are driving a renaissance in the field, both opening up new applications, such as energy conversion and ultrafast electronics, as well as improving traditional applications in electron imaging and high-energy science.

Light-induced generation of free electrons is of interest for a wide variety of vacuum electronic devices and systems. The properties of nanomaterials, stemming from their geometry and the strong manifestation of quantum phenomena in them, have opened up new avenues for developing new cathodes and exploring and exploiting electron emission. This article presents the heat trap effect—efficient localized heating of carbon nanotube arrays using light, leading to electron emission through the thermionic mechanism. This process requires unexpectedly modest amounts of optical power—available from sources such as handheld lasers—and dramatically simplifies the electron emitter. Potential applications, including thermionic and thermoelectric conversion for solar-energy harvesting and simple electron-beam systems, are also highlighted.

The theories of thermionic emission and field emission (also known as the Richardson–Dushman [RD] and Fowler–Nordheim [FN] Laws, respectively) were formulated more than 80 years ago for bulk materials. In single-layer graphene, electrons mimic massless Dirac fermions and follow relativistic carrier dynamics. Thus, their behavior deviates significantly from the nonrelativistic electrons that reside in traditional bulk materials with a parabolic energy-momentum dispersion relation. In this article, we assert that due to linear energy dispersion, the traditional thermionic emission and field emission models are no longer valid for graphene and two-dimensional Dirac-like materials. We have proposed models that show better agreement with experimental data and also show a smooth transition to the traditional RD and FN Laws.

Nanoscale electron sources with high electron-emitting performance are of great interest in vacuum nanoelectronics. Resembling traditional thermionic emission sources based on a hot tungsten filament, a hot carbon nanotube or graphene can function as a nanoscale electron source because of its excellent thermal stability and electrical conductivity. In this article, studies of thermionic emission from single hot carbon nanostructures are overviewed, emphasizing their differences in physics from macroscopic thermionic emission as well as potential applications in vacuum nanoelectronics. Due to their low dimensionality, nanoscale size, and nonequilibrium electron distribution, Richardson’s Law, which governs thermionic emission from macroscopic metals, breaks down in the case of thermionic emission from single carbon nanostructures, and an internal electric field in a carbon nanostructure can contribute directly to its thermionic emission. Graphene-based nanoscale thermionic emission sources, source arrays, and vacuum transistors have been fabricated and demonstrated to exhibit the advantages compared to those based on field emission. The advances imply the promise of realizing high-performance nanoscale electron sources and vacuum electronic devices based on thermionic emission.

The high carrier concentrations typically reported for nanowire devices indicate that when Schottky barrier transport is present, it occurs in the thermionic field emission regime with a substantial but not exclusive tunneling component. Analysis by thermionic field emission is difficult due to its multivariate nature. In recent work, we developed a mathematical stability approach that greatly simplified the evaluation of the multivariate thermionic field emission parameters. This is a general method with potentially wide applicability, requiring only the effective mass m* and relative dielectric constant εr for a given semiconductor as inputs. In the present work, we investigate the influence of the materials properties effective mass m* and relative dielectric constant εr on stability for a range of real and simulated semiconductor nanowires. A further investigation of temperature sensitivity and regime trends is presented.

Diamond films with good electron emission properties show great potential for applications such as electron sources. For single-crystalline diamond, the negative electron affinity at hydrogen-terminated surfaces enables efficient emission of conduction electrons into vacuum. Although electrons are not naturally present in the diamond conduction band, p–n junction diode structures make this possible; electrons are injected from n-type diamond to the conduction band of p-type diamond, giving rise to electron emission with efficiencies exceeding 1%. Alternatively, impacting electron beams can be used to inject “secondary” electrons into the conduction band, resulting in high emission gain. For ultrananocrystalline diamond (UNCD) films with versatile granular structure, enhanced electron field emission (EFE) properties can be achieved by altering the granular structure of the films. Utilization of nanoscale tips as templates for growing UNCD film or direct reactive ion etching of the film further enhances their EFE behavior. On the other hand, the release of electrons through application of thermal energy can be utilized in a thermionic energy converter to directly transform heat into electricity. With the addition of ion current from doped diamond emitters to the thermionic electron current, power output enhancement of the converter can be realized.

In this work, we re-evaluate the temperature coefficient in multi-junction solar cells by including the effects of radiative coupling (i.e. re-absorption of emitted photons within the cell) and eliminating radiative losses towards the substrate (i.e. use of a back mirror). The model developed is for two-junction devices, and it takes into account: the number of terminals, the energy bandgaps of the sub-cells, the light concentration, and whether a back mirror is used or not. The temperature coefficients obtained are compared with the case where no luminescent effects are considered. The results show that, in two-terminal and four-terminal devices, the sensitivity to temperature is almost the same whether luminescent effects are taken into account or not. However, these effects are most significant in three-terminal devices. In four-terminal devices, the results show that these effects depend to a large extent on the materials used, the design of the system, i.e. on the effectiveness of radiative exchange between the cells involved.

The output power-density and the efficiency of thermo-tunnel devices are examinedas a function of inter-electrode separation, electrode work-function, and temperature. We find that these physical parameters dramatically influence the device characteristics, and under optimal conditions a thermo-tunnel device is capable of delivering a very high output power-density of ∼ 103Wcm−2. In addition, at higher temperatures, the heat-conversion efficiency of the thermo-tunnel device approaches ∼ 10%, comparable to that of a thermoelectric generator. We therefore propose that thermo-tunnel devices are promising for solid-state thermal energy conversion.

We have investigated, using density functional simulations, the energetics and the electronic properties of oxides of selected transition metals, TMs, adsorbed onto a dia-mond (001) surface. We find that stoichiometric oxides of TMs, particularly Ti and Zn,influence the electron affinity of diamond strongly. The electron affinities of stoichiomet-ric oxides of Ti and Zn are calculated to be around −3 eV, significantly higher than 1.9 eV of commonly used H–termination. The reactions of TMs with an oxygenated diamond are found to be highly exothermic. Based upon the energetics and the electronic properties, we propose that in the regime of ultra thin films, oxides of TMs are promising options for surface coating of diamond–based electron emitters, as these coatings are compatible with semiconductor device fabrication processes, while having the benefit of inducing a large negative electron affinity.

Over the past few years, thermoelectric (TE) materials have been receiving an increasing amount of attention owing to their promising potential for energy conversion and thermal management applications. Thermal characterisation techniques offer a powerful tool in investigating and optimizing the TE device performance. In addition, they can provide a better understanding of the underlying fundamental principles such as Peltier effects at the interfaces of the active medium. In this paper, we present the design and thermal characterisation of integrated highspeed microcoolers based on SiGe superlattices. The electrode metalisation is laid out as a coplanar waveguide, enabling to supply electrical pulses with short rise times to the coolers. We employ a variety of CCD-based transient thermoreflectance imaging methods to perform an extensive dynamic thermal analysis. These techniques provide 2-D temperature maps of the chip surface with ∼100ns temporal and submicron spatial resolution without the need to scan the sample. Net cooling in the 2 degree range is observed, with response times well below 1μs. This is almost two orders of magnitude faster compared to the best in the literature. The obtained images also confirm the previous observations that the Peltier cooling term responds faster than the Joule heating term, in agreement with their expected locality and associated thermal mass. This provides potential to study ultrafast electron-phonon interactions during Peltier effects.

We use a state-of-the-art non-equilibrium quantum transport simulation code, NEMO-1D, to address the device physics and performance benchmarking of cross-plane superlattice Peltier coolers. Our findings show quantitatively how barriers in cross-plane superlattices degrade the electrical performance, i.e. power factor. The performance of an In0.53Ga0.47As/In0.52Al0.48As cross-plane SL Peltier cooler is lower than that of either a bulk In0.53Ga0.47As or bulk In0.52Al0.48As device, mainly due to quantum mechanical effects. We find that a cross-plane SL device has a Seebeck coefficient vs. conductance tradeoff that is no better than that of a bulk device. The effects of tunneling and phase coherence between multi barriers are examined. It is shown that tunneling, SL contacts, and coherency only produce oscillatory behavior of Seebeck coefficient vs. conductance without a significant gain in PF. The overall TE device performance is, therefore, a compromise between the enhanced Seebeck coefficient and degraded conductance.

We use a Thermoreflectance Thermal Imaging technique to study the transient cooling of SiGe-based microrefrigerators. Thermal imaging with submicron spatial resolution, 0.1C temperature resolution and 100 nanosecond temporal resolution is achieved. Transient temperature profiles of SiGe-based superlattice microrefrigerator devices of different sizes are obtained. The dynamic behavior of these microrefrigerators, show an interplay between Peltier and Joule effects. On the top surface of the device, Peltier cooling appears first with a time constant of about 10-30 microseconds, then Joule heating in the device starts taking over with a time constant of about 100-150 microseconds. The experimental results agree very well with the theoretical predictions based on Thermal Quadrupoles Method. The difference in the two time constants can be explained considering the thermal resistance and capacitance of the thin film. In addition this shows that the Joule heating at the top metal/semiconductor interface does not dominate the microrefrigerator performance or else we would have obtained the same time constants for the Peltier and Joule effects. Experimental results show that under high current values, pulse-operation the microrefrigerator device can provide cooling for about 30 microseconds, even though steady state measurements show heating. Temperature distribution on the metal leads connected to the microrefrigerator’s cold junction show the interplay between Joule heating in the metal as well as heat conduction to the substrate. Modeling is used to study the effect of different physical and geometrical parameters of the device on its transient cooling. 3D geometry of heat and current flow in the device plays an important role. One of the goals is to maximize cooling over the shortest time scales.

We report the influence of surface treatment, annealing temperature and metal bilayer thickness on the specific contact resistance (ρc) of Au/Ni ohmic contacts to p-GaN and p-AlGaN. Ohmic contact on p-GaN with a hole concentration of 6.5 x 1017 cm-3, shows the lowest ρc of ˜9.2 x 10-6 Ω cm2, when GaN was treated in HCl:H2O (3:1) solution before metal deposition and annealed at 500°C for 10 minutes in 90% N2 and 0% O2 atmosphere. Similar procedure applied on p-AlxGa1-xN (x = 5-7%), with a hole concentration of 2.3 x 1017 cm-3, yields a ρc of 1.8 x 10-4 Ω cm2. An increase is observed in ρc when Mg doping exceeds 4 x 1019 cm-3 in both p-GaN and p-AlGaN. This is attributed to Mg self compensation. This increase is more pronounced in AlGaN which we attribute to the presence of residual native aluminum oxides.

Commercial scandium oxide doped thermionic cathodes have demonstrated current densities over 100 A/cm2. In order to understand the effect of Sc- and Ba- oxides on the emissivity of these cathodes we have imaged thin films of scandium oxide and barium oxide on tungsten foils using photoelectron emission microscopy and thermionic emission microscopy. Arrays of 100 um × 100 um squares of scandium and 25 um × 25 um squares of barium, 200 nm thick, were sputter deposited onto 50 um thick sheets of tungsten foil. Imaging squares of different sizes gives an unequivocal identification of each material and a completely consistent comparison of each material and cathode structure under identical conditions in one image.

The metal squares oxidize in air before imaging. Each sample was heated in situ in a Bauer-Telieps style LEEM/PEEM used primarily in the ThEEM mode. The barium oxide squares emit below 875 K, and diffuse over the scandium below 875 K. Thermionic emission from scandium oxide squares is observed at temperatures significantly larger than 875 K. Failure of the barium oxide film cathode is through barium desorption. AES spectra show that the Sc does not desorb.

While the origin of reduced emission temperature is commonly believed to be a result of a low work function monolayer of Ba and Sc oxides, in our study, the benefits of a combined Ba/Sc cathode are present in a thick (multi-layer), layered structure of barium oxide on top of a thick scandium oxide layer.

Erbium(III) doped TiO2 nanofibers (Er2Ti2O7) have been synthesized by electrospinning mixtures of polymers, metal-containing materials, and erbium acetate. These electrospun nanofibers were subsequently annealed at temperatures of 550, 750, 950, and 1150 oC to remove the organics and leave behind the metal oxides. The crystal structure and optical properties of the nanofiber pyrochlores were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transformation IR (FTIR) spectroscopy. Different crystal structures were formed by controlling the annealing conditions. XRD data are compared with near-IR spectra to better understand the effects of annealing temperature on the Er (III) thermally-excited selective optical emission process.

Schottky barrier diodes fabricated on Silicon carbide have been demonstrated as gas sensors for deployment in extreme environments. It has been shown that the interfacial layer formed at the Metal – Semiconductor junction, determines both the sensitivity and the reliability of the device. Hence, accurate knowledge of the thickness and interfacial trap density of this layer is required to make predictions of the behaviour of the sensor in the environment under investigation and to predict its variation with time. Diode parameters, such as the ideality factor, barrier height and series resistance have been extracted from experimental measurements on Palladium Schottky Barrier diodes on 4H SiC, over a range of temperatures. The comparison of the parameters extracted from modified Norde function, Cheung's method and Thermonic Emission model has been performed. The variation in the barrier height obtained is quite marked between the different techniques. The reverse I-V characteristics have been used to extract thickness of the interfacial layer, by fitting to the experimental data using the TEBIL model to extract the value of Dit from ä and the ideality factor, assuming the interfacial layer is stoichiometric SiO2 . This allows a comparison between the effective interfacial layer behaviour for the different parameter extraction techniques and demonstrates that knowledge of this interfacial layer is influenced by the technique selected.

GaN layers on sapphire substrates were grown by metalorganic chemical vapor deposition using in situ porous SiNx nano-network. Crystalline quality of epilayers was characterized by X-ray rocking curve scans, and the full width at hall maximum values for (002) and (102) diffractions were improved from 252 arc sec and 405 arc sec, respectively, in control samples to 216 arc sec and 196 arc sec when SiNx was used. Ni/Au Schottky diodes (SDs) were fabricated and the SD performance was found to be critically dependent on the SiNx coverage (fewer and farther the pores the better the results) which is consistent with the trends of XRD and photoluminescence data. A 1.13eV barrier height was achieved when 5min SiNx layer was used compared with 0.78 eV without any SiNx nanonetwork. Furthermore, the breakdown voltage improved from 76 V to 250V when SiNx nanonetwork was used in otherwise identical structures.

A brief overview of the research activities at the Thermionic Energy Conversion (TEC) Center is given. The goal is to achieve direct thermal to electric energy conversion with >20% efficiency and >1W/cm2 power density at a hot side temperature of 300–650C. Thermionic emission in both vacuum and solid-state devices is investigated. In the case of solid-state devices, hot electron filtering using heterostructure barriers is used to increase the thermoelectric power factor. In order to study electron transport above the barriers and lateral momentum conservation in thermionic emission process, the current-voltage characteristic of ballistic transistor structures is investigated. Embedded ErAs nanoparticles and metal/semiconductor multilayers are used to reduce the lattice thermal conductivity. Cross-plane thermoelectric properties and the effective ZT of the thin film are analyzed using the transient Harman technique. Integrated circuit fabrication techniques are used to transfer the n- and p-type thin films on AlN substrates and make power generation modules with hundreds of thin film elements. For vacuum devices, nitrogen-doped diamond and carbon nanotubes are studied for emitters. Sb-doped highly oriented diamond and low electron affinity AlGaN are investigated for collectors. Work functions below 1.6eV and vacuum thermionic power generation at temperatures below 700C have been demonstrated.