Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-Type Germanium

Abstract

A metal-interlayer-semiconductor (M-I-S) structure with excellent thermal stability and electrical performance for a nonalloyed contact scheme is developed, and considerations for designing thermally stable M-I-S structure are demonstrated on the basis of n-type germanium (Ge). A thermal annealing process makes M-I-S structures lose their Fermi-level unpinning and electron Schottky barrier height reduction effect in two mechanisms: (1) oxygen (O) diffusion from the interlayer to the contact metal due to high reactivity of a pure metal contact with O and (2) interdiffusion between the contact metal and semiconductor through grain boundaries of the interlayer. A pure metal contact such as titanium (Ti) provides very poor thermal stability due to its high reactivity with O. A structure with a tantalum nitride (TaN) metal contact and a titanium dioxide (TiO) interlayer exhibits moderate thermal stability up to 400 °C because TaN has much lower reactivity with O than with Ti. However, the TiO interlayer cannot prevent the interdiffusion process because it is easily crystallized during thermal annealing and its grain boundaries act as diffusion path. A zinc oxide (ZnO) interlayer doped with group-III elements, such as an aluminum-doped ZnO (AZO) interlayer, acts as a good diffusion barrier due to its high crystallization temperature. A TaN/AZO/n-Ge structure provides excellent thermal stability above 500 °C as it can prevent both O diffusion and interdiffusion processes; hence, it exhibits Ohmic contact properties for all thermal annealing temperatures. This work shows that, to fabricate a thermally stable and low resistive M-I-S contact structure, the metal contact should have low reactivity with O and a low work-function, and the interlayer should have a high crystallization temperature and a low conduction band offset to Ge. Furthermore, new insights are provided for designing thermally stable M-I-S contact schemes for any semiconductor material that suffers from the Fermi-level pinning problem.

Schottky Barrier Height Engineering for Electrical Contacts of Multilayered MoS2 Transistors with Reduction of Metal-Induced Gap States

“…Mechanism of SBH Control: Fermi-Level Unpinning by MIGS Reduction. The SBH control by insertion of the interlayer can be explained by three mechanisms: (1) Fermilevel unpinning by the MIGS reduction, [23][24][25][26][27][28][29][30][31][32]35,36 (2) Fermilevel unpinning by the metal/semiconductor interface passivation, 27,28,44 and (3) interface dipole formation. 23,26,33,34,36 First, the two Fermi-level unpinning effects and the interface dipole effect can be separated from the SBH vs contact metal work function plot as shown in Figure 6a.…”

“…These MIGS have been identified as the main cause of the FLP in conventional semiconductor materials. Therefore, a metal–interlayer–semiconductor (MIS) structure has been developed to alleviate the FLP and reduce the electron SBH by reducing the MIGS in Si, , Ge, − and III–V. − Ultrathin interlayers such as TiO 2 − ,,, and ZnO − ,, which exhibit a small conduction band offset (CBO) to semiconductor materials are inserted between the metal contact and the semiconductor as the Fermi-level unpinning layer that minimizes the increase in the interlayer resistance. Several researchers have previously developed the MIS structure for the MoS 2 to control the electron SBH of its electrical contacts by using Ta 2 O 5 , TiO 2 , − MgO, and h -BN , interlayers.…”

“…In this work, the SBH controllability is improved by using an MIS structure on multilayered MoS 2 FETs, and its SBH controlling mechanism is systematically investigated by analyzing three possible mechanisms: (1) Fermi-level unpinning by the MIGS reduction, − ,, (2) Fermi-level unpinning by the metal/semiconductor interface passivation, ,, and (3) interface dipole formation. ,,,, Four metals with various work-function values, titanium (Ti), copper (Cu), gold (Au), and platinum (Pt), are adopted as contact metals with an ultrathin titanium dioxide (TiO 2 ) interlayer, and the transfer characteristics of multilayered MoS 2 FETs and SBH of their S/D contacts are analyzed to demonstrate the SBH-controlling effect. The mechanism of the SBH control is investigated by comparing each of the three possible mechanisms.…”

Threshold Switching in Single Metal‐Oxide Nanobelt Devices Emulating an Artificial Nociceptor

“…With Ti electrodes, the theoretical difference between the Ti work function (4.33 eV) and the TiO 2 Fermi level (4.2 eV) is small enough to lead to an Ohmic contact. Nevertheless, the sweeping performance is similar to that observed for the Pt and Au electrodes, indicating that at least, partial Fermi level pinning [50][51][52][53][54][55] may be occurring due to the surface-dominating nature of the TiO 2 nanobelts. A similar transition from a capacitive behavior to a capacitivecoupled memristive behavior is observed for the device with Au-Au electrodes ( Figure S5, Supporting Information).…”

Combinatorial Large-Area MoS2/Anatase–TiO2 Interface: A Pathway to Emergent Optical and Optoelectronic Functionalities

“…The FLP factor S = ∂ϕ B /∂ϕ m has an explicit relation with the interfacial density of states D s = (1 – S )­ε i ε 0 / Sq 2 δ i 2 , which could be modulated by the optimal thickness (δ i ) of an appropriate dielectric layer with dielectric constant ε i . For conventional semiconductors such as Ge , or Si, the insertion of an ultrathin interlayer of high- k oxides between the metal and the semiconductor is considered to be the most promising expedient to assuage the FLP. This oxide layer results in a smaller SBH and lower contact resistances by dwindling both the virtual MIGS and the real interface states .…”