Vertical Current Transport Research Articles

Vertical Current Transport in Monolayer MoS<sub>2</sub> Heterojunctions with 4H-SiC Fabricated by Sulfurization of Ultra-Thin MoO<sub>x</sub> Films

In this paper, we report on the growth of highly uniform MoS2 films, mostly consisting of monolayers, on SiC surfaces with different doping levels (n- SiC epitaxy, ~1016 cm-3, and n+ SiC substrate, ~1019 cm-3) by sulfurization of a pre-deposited ultra-thin MoOx films. MoS2 layers are lowly strained (~0.12% tensile strain) and highly p-type doped (<Nh>≈4×1019 cm−3), due to MoO3 residues still present after the sulfurization process. Nanoscale resolution I-V analyses by conductive atomic force microscopy (C-AFM) show a strongly rectifying behavior for MoS2 junction with n- SiC, whereas the p+ MoS2/n+ SiC junction exhibits an enhanced reverse current and a negative differential behavior under forward bias. This latter observation, indicating the occurrence of band-to-band-tunneling from the occupied states of n+ SiC conduction band to the empty states of p+ MoS2 valence band, is a confirmation of the very sharp hetero-interface between the two materials. These results pave the way to the fabrication of ultra-fast switching Esaki diodes on 4H-SiC.

Work function of van der Waals topological semimetals: Experiment and theory

The work function (WF) of a material governs the back and forth movement of the charge carriers across the hetero-interface of two materials. Therefore, for optimum device performance, precise knowledge of the WF is prerequisite while employing any new material in electronic devices. In this work, using metal oxide semiconductor capacitors, we experimentally determine the WF of layered van der Waals topological semimetals (TSMs) 1T′-MoTe2, 1T-PtSe2, and Td-WTe2 as 4.87, 5.05, and 4.82 eV, respectively. The experimentally obtained results are corroborated with density functional theory calculations. Furthermore, by analyzing the vertical current transport across the metal oxide semiconductor stack using Fowler–Nordheim tunneling formalism, the barrier height between the TSMs and the gate insulator (SiO2) is experimentally calculated. The obtained barrier heights are also following the same trend as that of WF for three TSMs. These TSMs host unique topological nontrivial phases potentially useful for the development of emerging quantum technologies, and therefore, the findings of this study are significant for designing the future quantum devices.

Low-resistivity vertical current transport across AlInN/GaN interfaces

Effects of n-type doping of Al0.82In0.18N/GaN heterostructures on the conduction band (CB) profile have been investigated. Doping concentrations well above 1019 cm−3 are required to reduce the large barriers in the CB. Experimentally, Si- and Ge donor species are compared for n-type doping during metalorganic vapor phase epitaxy. For Si doping, we find substantial interface resistivity that will strongly contribute to total device resistivity. Doping of AlInN is limited by either the onset of a self-compensation mechanism (Si) or structural degradation of the AlInN (Ge). Only by Ge doping, purely ohmic behavior of periodic AlInN/GaN layer stacks could be realized.

Vertical current transport processes in MOS-HEMT heterostructures

The nature of vertical electron transport through MIS structures has not been fully understood until now. This applies all the more to metal/Al2O3/GaN/AlGaN/GaN heterostructures used in GaN-based MOS-HEMTs. Devices based on such heterostructures are utilized in power electronics applications and are frequently intended for use at very high temperatures. Regrettably, there are only few works dealing with the effect of heat on the current transport in such devices. Therefore, we describe a novel model for vertical electron transport through GaN-based MOS-HEMT structures, simulate the gate currents, and compare the results with experimental high-temperature I-V curves. In the present paper we show that trap-assisted tunnelling (TAT) between the metal and the surface of the semiconductor is the main transport mechanism in such MOS structures.

Vertical Current Transport in AlGaN/GaN HEMTs on Silicon: Experimental Investigation and Analytical Model

We investigate the vertical leakage mechanism in metal–organic chemical vapor deposition-grown carbon (C)-doped AlGaN/GaN High Electron Mobility Transistors (HEMTs) on 6-in silicon wafer. Substrate bias polarity-dependent ${I}$ – ${V}_{s}$ , temperature-dependent fitting, and band diagram analysis pointed to the Poole–Frenkel (P–F) type of conduction mechanism for vertical transport in the devices with breakdown as high as 580 V for a buffer of $\textsf {4}~\mu \text{m}$ . Trap activation energy of 0.61 eV was estimated from the P–F fitting which matches well with values reported in the literature. We propose that higher dislocation density leads to shallower traps in the buffer and build an analytical model of dislocation-mediated vertical leakage around this. The variation in leakage as a function of dislocation density at a given field is predicted and is found to be the most abrupt in the range from $\sim 10^{\textsf {7}}$ to $\sim 10^{\textsf {9}}$ cm $^{-\textsf {2}}$ of dislocation density. This can be attributed to a sharp decrease in trap activation energy in the above range of dislocation density, possibly due to complex formation between point defects and dislocations.

Using Femtosecond Laser Irradiation to Enhance the Vertical Electrical Properties and Tailor the Morphology of a Conducting Polymer Blend Film.

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.

Effect of compositional interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors grown on SiC

We have investigated the effect of strain-compensating interlayers on the vertical electrical conductivity of Si-doped AlN/GaN distributed Bragg reflectors (DBRs). Samples with 10.5 mirror pairs were grown through plasma-assisted molecular beam epitaxy on SiC. Room-temperature current–voltage characteristics were measured vertically in mesas through 8 of the 10.5 pairs. The sample with no interlayers yields a mean specific series resistance of 0.044 Ω cm2 at low current densities, while three samples with 5/5-Å-thick, 2/2-nm-thick, and graded interlayers have resistivities between 0.16 and 0.34 Ω cm2. Thus, interlayers impair vertical current transport, and they must be designed carefully when developing conductive DBRs.

Hot Electron Transistors Based on Graphene/AlGaN/GaN Vertical Heterostructures

This paper presents a study of the vertical current transport in a graphene (Gr) heterostructure with AlxGa1-xN/GaN, which represent the main building block of a novel high frequency device, the hot electron transistor (HET) with Gr base. The morphological and electrical properties of this heterostructures have been investigated at nanoscale by atomic force microscopy (AFM) and conductive atomic force microscopy (CAFM). In particular, local current-voltage measurements by the CAFM probe revealed the formation of a Schottky contact with low barrier height (∼0.41 eV) and excellent lateral uniformity between Gr and AlGaN. Basing on the electrical parameters extracted from this characterization, the theoretical performances of a HET formed by a metal/Al2O3/Gr/AlGaN/GaN stack have been evaluated.

Silicon quantum dot based solar cells: addressing the issues of doping, voltage and current transport

AbstractSilicon quantum dot (Si QD) solar cells offer the potential to tune the effective band gap through quantum confinement and hence allow fabrication of optimised tandem devices in one growth run in a thin film process. Previous work in our group has shown how such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. Doping multilayers with P and B allows formation of a rectifying junction with an effective band gap of 1.8 eV, which can give an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have big but opposite effects on QD crystallisation, with P(B) forming larger (smaller) QDs than for undoped material. Two possible models for the doping mechanisms in these materials are explored: one relying on doping of a sub‐oxide region around the Si QDs and the other based on the differing nucleation effects of P and B. In addition initial results on hetero‐interfaces in the QD superstructure are assessed as a means to improve vertical current transport, and the effects of hydrogenation on improving VOC by passivating defects. Routes to incorporating these explanations for doping and other structural improvements are discussed as means of optimising the performance of these Si QD cells. Copyright © 2010 John Wiley & Sons, Ltd.

Operation of Silicon Carbide BJTs Free from Bipolar Degradation

The mechanisms of bipolar degradation in silicon carbide BJTs are investigated and identified. Bipolar degradation occurs as result of stacking fault (SF) growth within the low-doped collector region. A stacking fault blocks vertical current transport through the collector, driving the defective region into saturation. This results in considerable drop of emitter current gain if the BJT is run at a reasonably low collector-emitter bias. The base region does not play any significant role in bipolar degradation. Long-term stress tests have shown full stability of large-area high-power BJTs under minority carrier injection conditions provided the devices are fabricated using low Basal Plane Dislocation (BPD) material. However, an approximately 20% current gain compression is observed for the first 30-60 hours of burn-in under common emitter operation, which is related to instability of surface recombination in the passive base region.

Three-dimensional nanoscale W-GaAs heterostructures

Layers of semi-insulating GaAs have been realised by epitaxial overgrowth over matrices of nm-sized tungsten discs. By fabricating and characterising structures with different metal disc densities, the influence of metal-induced carrier depletion on the resistivity in GaAs is demonstrated. The vertical current transport in this material shows a distinct decrease as the metal discs are spaced closer than 400 nm, which indicates an onset of overlapping Schottky depletion regions. From the temperature dependence of the current transport at different bias conditions, we deduce the height of the internal barriers, which are built-up in the semiconductor material between the discs, and their variation with the applied voltage. Finally, the actual metal–semiconductor Schottky barrier height is obtained from the spectral distribution of a photoresponse, which we attribute to electron excitation in the buried discs in the otherwise semi-insulating GaAs layer.