Search query: fermi level pinning

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abstract: The Schottky barrier height in metal/Ge contacts shows weak dependence on the metal work function indicating strong Fermi level pinning close to the Bardeen limit. The pinning factor S is about 0.05 and the charge neutrality level (CNL) is only about 0.09eV above the top of the valence band. Because of this, the Fermi level in Ge lies higher than CNL in most cases of interest so that unpassivated acceptorlike gap states at the interface are easily filled, building up a net negative fixed charge. This could prevent efficient inversion of a p type Ge surface in a metal oxide semiconductor structure.
author_names: Athanasios Dimoulas

Polychronis Tsipas

A Sotiropoulos

Evangelos Evangelou
title: Fermi level pinning and charge neutrality level in germanium
year: 2006

abstract: Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2 6.1 eV. Except for the p type Schottky contact with platinum, the Fermi levels in all of the studied metal MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal MoS2 contact is shown to be unique for metal 2D semiconductor interfaces, remarkably different from the well known Bardeen pinning effect, metal induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal semiconductor junctions. At metal MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function modification by interface dipole formation due to the charge redistribution, and second by the production of gap states mainly of Mo d orbitals character by the weakened intralayer S Mo bonding due to the interface metal S interaction. This finding would provide guidance to develop approaches to form Ohmic contact to MoS2.
author_names: Cheng Gong

Luigi Pietro Maria Colombo

Robert M Wallace

Kyeongjae Cho
title: The unusual mechanism of partial Fermi level pinning at metal MoS2 interfaces.
venue: Nano letters
year: 2014

abstract: The purpose of this paper is to understand metal/germanium (Ge) junction characteristics. Electrode metals with a wide work function range were deposited on Ge. All metal/p Ge and metal/n Ge junctions have shown Ohmic and Schottky characteristics, respectively, with the strong Fermi level pinning. The charge neutrality level (CNL) at metal/Ge interface is close to the branch point calculated for the bulk Ge. Moreover, the pinning level is hardly modulated by annealing in forming gas, forming metal germanide/Ge interfaces or changing the substrate orientation. These results suggest that Fermi level at metal/Ge interface is intrinsically pinned at the CNL characterized by the metal induced gap states model.
author_names: Tomonori Nishimura

Koji Kita

Akira Toriumi
title: Evidence for strong Fermi level pinning due to metal induced gap states at metal/germanium interface
year: 2007

abstract: Photoelectron spectroscopy has been used to map out energy level alignment of conjugated polymers at various organic organic and hybrid interfaces. Specifically, we have investigated the hole injection interface between the substrate and light emitting polymer. Two different alignment regimes have been observed: (i) Vacuum level alignment, which corresponds to the lack of vacuum level offsets (Schottky Mott limit) and (ii) Fermi level pinning, where the substrate Fermi level and the positive polaronic level of the polymer align. The observation is rationalized in terms of spontaneous charge transfer whenever the substrate Fermi level exceeds the positive polaron/bipolaron formation energy per particle. The charge transfer leads to the formation of an interfacial dipole, as large as 2.1 eV.
author_names: Carl Tengstedt

Wojciech Osikowicz

William Raymond Salaneck

Ian D Parker

Che-Hung Hsu

Mats Fahlman
title: Fermi level pinning at conjugated polymer interfaces
year: 2006

abstract: Electrical metal contacts to two dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc) Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and 0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. Our results further imply that metal work functions have very little influence on contact properties of 2D material based devices. Moreover, we found that Rc is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC based semiconductors can be achieved efficiently.
author_names: Changsik Kim

Inyong Moon

Daeyeong Lee

Min Sup Choi

Faisal Ahmed

Seunggeol Nam

Yeonchoo Cho

Hyeon-Jin Shin

Seongjun Park

Won Jong Yoo
title: Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides.
venue: ACS nano
year: 2017

abstract: The Schottky barrier for carrier injection into 2D semiconductors can be effectively tuned by using 2D metals. Two dimensional (2D) semiconductors have shown great potential for electronic and optoelectronic applications. However, their development is limited by a large Schottky barrier (SB) at the metal semiconductor junction (MSJ) which is difficult to tune by using conventional metals because of the effect of strong Fermi level pinning (FLP) We show that this problem can be overcome by using 2D metals, which are bounded with 2D semiconductors through van der Waals (vdW) interactions. This success relies on a weak FLP at the vdW MSJ, which is attributed to the suppression of metal induced gap states. Consequently, the SB becomes tunable and can vanish with proper 2D metals (for example, H NbS2) This work not only offers new insights into the fundamental properties of heterojunctions but also uncovers the great potential of 2D metals for device applications.
author_names: Yuanyue Liu

Paul Stradins

Suhuai Wei
title: Van der Waals metal semiconductor junction: Weak Fermi level pinning enables effective tuning of Schottky barrier
venue: Science Advances
year: 2016

abstract: The passivation of interface states remains an important problem for III V based semiconductor devices. The role of the most stable bound native oxides GaOx (0.5<=x<=1.5) is of particular interest. Using monochromatic x ray photoelectron spectroscopy in conjunction with controlled GaAs(100) and InGaAs(100) surfaces, a stable suboxide (Ga2O) bond is detected at the interface but does not appear to be detrimental to device characteristics. In contrast, the removal of the Ga 3+ oxidation state (Ga2O3) is shown to result in the reduction of frequency dispersion in capacitors and greatly improved performance in III V based devices.
author_names: Christopher L Hinkle

M Milojevic

Barry Brennan

A M Sonnet

F S Aguirre-Tostado

Greg Hughes

Eric M Vogel

Robert M Wallace
title: Detection of Ga suboxides and their impact on III V passivation and Fermi level pinning
year: 2009

abstract: Due to the strong Fermi level pinning close to the germanium (Ge) valence band edge, all metal/p type Ge contacts show Ohmic characteristics, while metal/n type Ge contacts exhibit rectifying behaviors. In this paper, we report a simple method to alleviate this Fermi level pinning effect by inserting a thin layer of aluminum oxide (Al2O3) formed by oxidation of aluminum (Al) between the metal/Ge interface. The effective Schottky barrier heights of nickel (Ni)/n type Ge, cobalt (Co)/n type Ge, and iron (Fe)/n type Ge decrease from 0.54, 0.62, and 0.61eV to 0.39, 0.23, and 0.18eV, respectively, with this thin layer of Al2O3. The tunneling oxide significantly suppresses the Fermi level pinning, and yet does not restrict the current density. This method seems promising to realize low resistance metal contact to n type Ge, which is essential to realize n channel Ge complementary metal oxide semiconductor field effect transistor with metal source and drain.
author_names: Yi Zhou

Masaaki Ogawa

Xinhai Han

Kang L Wang
title: Alleviation of Fermi level pinning effect on metal/germanium interface by insertion of an ultrathin aluminum oxide
year: 2008

abstract: Various mechanisms for the Fermi level pinning of p gate metals on HfO2 are analyzed in detail. It is found that for Fermi energies below the Si valence band, HfO2 can oxidize Si by creating positively charged O vacancies. The band bending due to this vacancy concentration can account for the observed Fermi level pinning of p metals on HfO2.
author_names: John Robertson

Onise Sharia

Alexander A Demkov
title: Fermi level pinning by defects in HfO2 metal gate stacks
year: 2007

abstract: It is predicted that Schottky barriers of the transition metal dichalcogenides MoSe2, MoTe2, WS2, WSe2, and WTe2 will suffer less from Fermi level pinning by chalcogen vacancies than does MoS2, because their vacancy formation energies are larger. The reduction in vacancy numbers will allow a greater degree of Schottky barrier height tuning by varying metal work function of the contacts in these compounds. The vacancy levels of WS2, WSe2 and MoSe2, and MoTe2 are also calculated to lie nearer midgap, so that ambipolar conduction will be easier in these compounds than in MoS2.
author_names: Yuzheng Guo

Dehong Liu

John Robertson
title: Chalcogen vacancies in monolayer transition metal dichalcogenides and Fermi level pinning at contacts
year: 2015