High Work Function Contact Research Articles

Tailoring Contacts for High-Performance 1D Ta2Pt3S8 Field-Effect Transistors.

Materials with van der Waals (vdW) unit structures rely on weak interunit vdW forces, facilitating physical separation and advancing nanomaterial research with remarkable electrical properties. Recently, there has been growing interest in one-dimensional (1D) vdW materials, celebrated for their advantageous properties, characterized by reduced dimensionality and the absence of dangling bonds. In this context, we synthesize Ta2Pt3S8, a 1D vdW material, and assess its suitability for field-effect transistor (FET) applications. Spectroscopic analysis and electrical characterization confirmed that the band gap and work function of Ta2Pt3S8 are 1.18 and 4.77 eV, respectively. Leveraging various electrode materials, we fabricated n-type FETs based on Ta2Pt3S8 and identified Cr as the optimal electrode, exhibiting a high mobility of 57 cm2 V-1 s-1. In addition, we analyzed the electron transport mechanism in n-type FETs by investigating Schottky barrier height, Schottky barrier tunneling width, and contact resistance. Furthermore, we successfully fabricated p-type operating Ta2Pt3S8 FETs using a molybdenum trioxide (MoO3) layer as a high work function contact electrode. Finally, we achieved Ta2Pt3S8 nanowire rectifying diodes by creating a p-n junction with asymmetric contact electrodes of Cr and MoO3, demonstrating an ideality factor of 1.06. These findings highlight the electronic properties of Ta2Pt3S8, positioning it as a promising 1D vdW material for future nanoelectronics and functional vdW-based device applications.

The working principle of hybrid perovskite gamma-ray photon counter

Gamma-ray spectroscopy that quantifies the gamma-ray energies is a critical technology widely needed in astrophysics, nuclear material detection and medical treatment. The key is to precisely count gamma-ray photons using sensitive detectors. In this paper, we investigate the operational principles of chlorine-doped methylammonium lead tribromide (MAPbBr3−xClx) perovskite single crystal detectors that can efficiently count gamma-ray photon events with electrical pulses. Specifically, we find the main dark current originates from the thermally activated electron injection from the impurities, and using high work function contacts can block out the dark noise thus allows for efficient pulse collection at higher electrical fields ∼500 V/cm. As a result, we observe strong electrical pulses when exposing the detector under radioactive sources emitting gamma-ray photons at various energies. Our results also reveal the fundamental issues that prevent the reliable observation of photo-electric peak. This work suggest pathway towards energy resolved gamma-ray spectroscopy using perovskite crystal detectors.

Tunable high workfunction contacts: Doped graphene

Contact electrodes with high work functions can enable significant enhancement in optoelectronic device performance due to their important role in efficient extracting/injecting carriers especially from optically active materials with high electron affinity or deep valence band edge, such as CdTe and some perovskites. With such materials becoming increasingly important in emerging solar cell technologies, the need for high work function electrodes has become of timely importance. In this work, p-doped graphene is investigated using first principle calculations, as a potential high work function contact electrode material for optoelectronic device applications. We found that chemical doping based on the adsorption of different non-metallic adatoms on graphene allows tuning the work function which can reach as high as 5.76 eV. A range of p-dopant adatoms, with varying doping concentrations, was investigated and we showed that the largest change of the work function is caused by chlorine and bromine dopants: a 4% concentration of Cl and Br dopants results in an increase of the work function of graphene from 4.38 eV to 5.76 eV and 5.71 eV, respectively. Furthermore, the calculations show that this significant increase in graphene’s work function is mainly due to the charge transfer from graphene to p-dopant adatoms, which increases the concentration of holes at the graphene surface, and hence, increases its work function. We also analyzed the stability conditions for absorbed halogen adatoms on graphite. In particular, we found that halogen molecules formation process should be significantly inhibited by electrostatic repulsion between charged adatoms which provides additional barrier for them to get closer to react. These findings provide valuable guidance to experimental efforts towards the realization of tunable high work function graphene-based electrodes.

Investigating the contact material influence on didodecyloxy-PPE OFETs

In this contribution we discuss the impact of the contact formation between semiconductor and source and drain contact on the device performance of bottom-contact top-gate organic field-effect transistors (OFETs) based on fibrillary semiconducting networks of poly(2,5-didodecyloxy-para-phenyleneethynylene) (D-OPPE). We show that the density and strength of the junctions between the D-OPPE fibers and the electrodes is likewise essential for the device performance as the electronic match of the involved materials and can even predominate the electronical effect. Due to the large bandgap of D-OPPE, high work function contacts are required to allow for efficient hole injection. MoO3 interlayers are known to improve the hole injection and thus commonly the device performance of p-channel OFETs. Indeed, photoelectron spectroscopy revealed a significant hole injection barrier reduction of 0.7 eV once the employed Cr/Au contacts are covered with MoO3. At first glance, it is thus not surprising that the device performance of p-channel OFETs with MoO3 modified contacts is superior. However, pure Cr/Au contacts can perform similarly once a smoothly applicated dielectric as Parylene C is used instead of PMMA. This cannot be explained by the above-mentioned electronical argument. To understand this finding, microscope techniques were used to address morphological effects rather than electronical properties. Our investigations showed that the semiconductor fibrils between channel and bottom contacts are thicker but the density of these interconnects between semiconductor fibrils and bottom contact is substantially lower for the bare Cr/Au contact. Depending on the successive deposition of the gate dielectric this density might be reduced further essentially destroying the injection pathway of carriers.

High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid Organic-Inorganic Perovskite Solar Cells.

We investigate the effect of high work function contacts in halide perovskite absorber-based photovoltaic devices. Photoemission spectroscopy measurements reveal that band bending is induced in the absorber by the deposition of the high work function molybdenum trioxide (MoO3). We find that direct contact between MoO3 and the perovskite leads to a chemical reaction, which diminishes device functionality. Introducing an ultrathin spiro-MeOTAD buffer layer prevents the reaction, yet the altered evolution of the energy levels in the methylammonium lead iodide (MAPbI3) layer at the interface still negatively impacts device performance.

Ohmic Contacts to 2D Semiconductors through van der Waals Bonding

High contact resistances have blocked the progress of devices based on MX2 (M = Mo, W; X = S, Se, Te) 2D semiconductors. Interface states formed at MX2/metal contacts pin the Fermi level, leading to sizable Schottky barriers for p‐type contacts in particular. It is shown that i) one can remove the interface states by covering the metal by a 2D layer, which is van der Waals‐bonded to the MX2 layer, and ii) one can choose the buffer layer such that it yields a p‐type contact with a zero Schottky barrier height. Possible buffer layers are graphene, a monolayer of h‐BN, or an oxide layer with a high electron affinity, such as MoO3. The most elegant solution is a metallic M′ X′2 layer with a high work function. A NbS2 monolayer adsorbed on a metal yields a high work function contact, irrespective of the metal, which gives a barrierless contact to all MX2 layers.

Molybdenum and tungsten oxide: High work function wide band gap contact materials for hole selective contacts of silicon solar cells

The high work function metal oxides, tungsten oxide (WOx) and molybdenum oxide (MoOx), were investigated regarding their ability to form a hole-selective contact for a crystalline silicon absorber. We show that in principle both materials have the potential to (i) either replace the p-type amorphous silicon thin films typically used as the high work function contact material in silicon based heterojunction solar cells or (ii) to assist the hole extraction if used as an additional contact layer placed between the p-type amorphous silicon and the TCO electrode. For an integral evaluation of the actual loss mechanisms limiting the contact characteristics both the ability of the contact scheme to passivate the absorber and to selectively extract the excess holes from the absorber are analyzed.

Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures

Indium tin oxide (ITO), Au and Pt are materials of interest as high work function contacts for organic semiconductor devices. In this paper the relative energy level line-up of these materials is investigated both as bare surfaces or part of a polymer/conductor interface. Kelvin probe (KP) measurements show that the estimated work function for Au and Pt surfaces, evaporated under normal high vacuum (HV) fabrication conditions and measured in air, can be significantly lower (by ∼0.2 to 0.9 eV) than those of clean surfaces evaporated and measured in the more demanding and clean ultra high vacuum (UHV), that are often used as reference values. The work function of all surfaces increases significantly (from ∼0.4 to >1 eV) after an oxygen plasma, but then decreases upon air exposure, with different rates for different materials. The effect of the plasma wears off most rapidly for Au whilst it is more resilient for ITO. Most interestingly, via KP and electroabsorption measurements of the built-in potential on polymer/conductor and conductor/polymer/conductor structures, we demonstrate that a plasma-induced enhancement of the work function is “frozen in” by the application of a polymer semiconductor layer over the plasma-treated surfaces and can be made to last for years by proper device encapsulation. These results have strong implications on the understanding, fabrication, design and stability of organic semiconductor devices.

Device modeling of light-emitting ambipolar organic semiconductor field-effect transistors

Recent experiments have demonstrated ambipolar channel conduction and light emission in conjugated polymer field-effect transistors (FETs). The devices have source∕drain contacts fabricated using metals with different work functions. Negative charge carriers are injected from a low work-function metal contact and positive charge carriers from a high work-function contact. In the ambipolar mode of operation, the gate potential lies between the potentials of the electron and hole injecting contacts, so that electrons dominate the channel conductance near the electron injecting contact and holes dominate channel conductance near the hole injecting contact. The injected charge carriers propagate along the FET channel and recombine in regions where both types of carriers are present. The location and intensity of maximum recombination and light emission is controlled by the voltages applied to the transistor terminals. In this paper a device model for ambipolar organic field-effect transistors based on the gradual channel approximation is presented. The model includes the effect of charge carrier trapping through density dependent mobilities. The resulting nonlinear differential equation for the channel potential is solved numerically. The results of the device model are in good agreement with the published experimental data.

Time-dependent polarization of CdTe gamma-ray detectors

A decrease in the counting rate and charge collection efficiency of p-type CdTe gamma-ray detectors, starting from the time of first application of the electric field, has been observed. Momentary removal of the bias returns the detector to its initial state. The behavior can be understood in terms of deep acceptors which detrap holes and decrease the thickness of the space charge region. Non-polarizing detectors can be made by using either high work function contacts with appropriate surface preparation or by periodic removal of the bias.