Tech Talk Tuesday: Internal Photoemission Spectroscopy Measurement of Energy Barrier Heights at Interfaces of ALD Materials in Metal/Insulator/Metal (MIM) Device Structures

Metal-insulator-metal (MIM) devices find application as high-speed tunnel diodes, hot-electron transistors, single electron transistors, resistive random access memory (RRAM), and capacitors. Precise knowledge of metal/insulator energy barrier heights, φBn, is critical for predicting and understanding charge transport and optimizing MIM device operation. The simplest theoretical model predicts that φBn's should vary linearly with both the vacuum work function of the metal, ΦM,vac, and the insulator electron affinity, χi, so that φBn = ΦM,vac - χi. In the more sophisticated induced gap state theory, charge transfer at intrinsic (induced) interface states is considered to create an interfacial dipole that drives the metal Fermi level, EF, towards the charge neutral level of the insulator, ECNL,i, which is 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, that is different from ΦM,vac, and the dependence of φBn on ΦM,vac varies with how effectively the insulator "pins" EF at the ECNL-i. In practice, it is observed that actual φBn's depend strongly on processing and can deviate substantially from predictions due to extrinsic effects such as interfacial and near-interfacial trapped charge arising from point defects, charge dipoles due to interfacial chemical reactions, and remote scavenging of oxygen from the opposing electrode. It is therefore necessary to directly measure φBn for the metal-insulator combination in use. The only technique capable of measuring φBn in-situ (in operating devices) is internal photoemission (IPE) spectroscopy.

In this talk, after a brief overview of research in the Conley research group, IPE measurements of φBn's in MIM structures between various insulators (including Al2O3, HfO2, SiO2, NiO, CoOx, and ferroelectric HfZrOx,) deposited via atomic layer deposition (ALD), and metals including ALD Ru, sputtered amorphous metals ZrCuAlNi, TaWSi, and TaNiSi, as well as benchmark metals such as TaN, Al, and Au will be described. Comparisons with electrical measurements are qualitatively consistent with IPE determined φBn, even when inconsistent with the theoretical model predictions. IPE is a powerful tool for understanding and optimizing MIM device operation.