(Invited) Internal Photoemission (IPE) Spectroscopy Measurement of Interfacial Energy Barriers in Operating Metal/Insulator/Metal (MIM) and Metal/Ferroelectric/Metal (MFM) Devices
Authors: Conley, Jr John 
Published: August 2024

Metal-insulator-metal (MIM) and Metal/Ferroelectric/Metal (MFM) devices find application as high speed tunnel diodes for rectenna based energy harvesting and sensing of IR radiation, hot-electron transistors, single electron transistors, resistive random access memory (RRAM), back end of line (BEOL) capacitors, and ferroelectric tunnel junction (FTJ) diodes. Precise knowledge of band-offsets / energy barriers (φBn) at metal/insulator interfaces is critical for understanding, predicting, and optimizing charge transport in these devices.

The Schottky-Mott rule, which neglects charge transfer across interfaces, states that the difference between the Fermi level of a metal (EF) and the conduction band of an insulator should vary simply with the metal vacuum work function (ΦM,vac) and the insulator electron affinity (χi) so that φBn = ΦM,vac - χi. In induced gap state theory, charge transfer at intrinsic interface traps creates an interfacial dipole that drives the EF towards the charge neutral level of the insulator (ECNL,i) the energy at which the dominant character of the interface states switches from donor-like to acceptor-like. A metal on an insulator will thus behave as if it has an effective work function (ΦM-eff) where ΦM,eff = ECNL,I + S(ΦM,vac - ECNL,i), where S = dφBn / dΦM,vac.1 Empirically, S = 1 / (1 + 0.1(εopt - 1)2), so that as the optical dielectric constant of the insulator (εhf) increases, S decreases from an ideal 1 towards 0 so that the insulator more effectively "pins" EFM at ECNL-i.2 Despite some success of MIGS theory, it is difficult to calculate or determine ECNL for a given material and it is observed that actual φBn's often deviate substantially from predictions. Indeed, φBn's depends on material processing, extrinsic effects (such as interfacial and near-interfacial trapped charge arising from point defects), dipoles (due to interfacial chemical reactions), and scavenging of oxygen from the insulator. Therefore, it is necessary to directly measure φBn for the specific metal/insulator or metal/ferroelectric combination in use. Internal photoemission (IPE) spectroscopy is an electro-optical technique that allows measurement of φBn at both metal interfaces in operating MIM & MFM devices.3-5 IPE is the only technique capable of measuring barrier heights, in-situ, in operating device structures. To date, relatively little work has been reported on IPE of these devices.6-12

In this invited talk, IPE spectroscopy measurements are described and measurements are presented of φBn's in MIM structures between insulators (Al2O3, HfO2, SiO2, NiO, CoOx) and ferroelectric HfZrOx deposited via atomic layer deposition (ALD) and various metals including ALD Ru, sputtered thin film amorphous metals (ZrCuAlNi, TaWSi, and TaNiSi), and benchmark metals (TaN, Al, and Au). Comparisons with electrical measurements on the same devices are found to be qualitatively consistent with φBns determined via IPE, even when inconsistent with Schottky model predictions, pointing to various non-idealities present in the materials and their interfaces and demonstrating the power of IPE for understanding and optimizing MIM & MFM device operation.

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