Investigation of energy barrier heights within metal-insulator-metal devices via internal photoemission spectroscopy

Metal-insulator-metal (MIM) and dual-insulator MIM (MIIM) devices are used in rectennas, hot-electron transistors, single electron transistors, resistive random access memory (RRAM), and capacitors. The performance of these devices rely heavily on the energy barrier height at each metal-insulator interface. Thus, determination of the in-situ electron energy barrier at each interface is critical to accurately predicting charge transport and confidently integrating new materials into microelectronic devices. Internal photoemission (IPE) spectroscopy is a well-established electro-optical technique that allows for direct measurement of interfacial energy barriers within a device structure. Although IPE has widely been used to characterize the interfaces between various polycrystalline elemental metals and oxides within MOS structures, there have been relatively few reports of IPE within MIM structures.

First, MIM structures with amorphous metal bottom electrodes are investigated via IPE. Amorphous metals are attractive for use in the aforementioned devices owing to their ultra-low roughness which gives rise to a highly uniform electric field in the ultrathin sandwiched insulator(s). IPE is used to measure the energy barrier heights in MIM device structures between either amorphous metal ZrCuAlNi, Ta-based amorphous metals (TaNiSi and TaWSi), or polycrystalline TaN and insulators deposited via atomic layer deposition (ALD). It is found that the Ta-based amorphous metals exhibit the largest barrier heights. The effect on the barrier height of a number of interfacial non-idealities are explored, including an interfacial ZrOx layer on ZrCuAlNi. A comparison is also made between Al and Au top electrodes for devices with a TaWSi bottom electrode, unexpectedly showing an effect of the top electrode on the bottom electrode barrier height. It is found that IPE energy barriers are consistent with current-voltage asymmetry of MIM diodes, whereas ideal Schottky model predictions of barrier heights were inconsistent.

Next, ALD metal top electrodes are investigated in both MIM and MOS structures. ALD metals such as ruthenium are promising electrode materials with growing interest for applications that require conformal, pinhole free conductive films, particularly for high aspect-ratio structures. Ru, due to relatively low bulk resistivity, high work function, a conductive oxide (RuO2), and ease of etching, is of interest as a gate electrode for MOS transistors, metal-insulator-metal (MIM) capacitors, RRAM, and tunnel diodes, as well as a conductive Cu diffusion barrier/liner for Cu interconnects. IPE is used to directly measure the φBn between ALD dielectrics Al2O3 and HfO2 and ALD Ru, both as-deposited and after a post-metallization anneal. Large barrier heights are found, supporting use of Ru as a gate electrode. It is found that barrier heights are relatively unaffected by the post-metallization anneal. The effects of interfacial oxides and interfacial charge trapping on the measured barrier heights are discussed.

Finally, IPE is utilized to study ALD ferroelectric HfZrOx. There has been increasing interest in ferroelectric materials for non-volatile memory applications. Barrier heights at the interface of ALD ferroelectric HfZrOx films and various metals are determined within MIM structures on TaN bottom electrodes. Knowledge of these technologically relevant barrier heights will assist in integration of ALD ferroelectric HfZrOx into non-volatile memory devices.

Major Advisor: John Conley
Committee: Larry Cheng
Committee: Alan Wang
Committee: Greg Herman
GCR: Janet Tate