Internal Photoemission Spectroscopy Measurements of the Energy Barrier Heights between ALD Dielectrics and Ta-Based Amorphous Metals
Authors: Jenkins Melanie A. , Klarr Tyler , McGlone John M. , Wager John F. , Conley John F. 
Published: February 2020

High speed metal/insulator/metal (MIM) diodes show promise for rectenna based energy harvesting of IR radiation, for IR sensing, and as building blocks for beyond CMOS hot-electron (MIMIM) transistors. Operation of these devices is ideally based on Fowler-Nordheim tunneling, which is exponentially dependent both on electric field and the height of the energy barriers between the metal electrodes and the insulator. Smooth interfaces are thus necessary to control electric fields. The amorphous metal ZrCuAlNi (ZCAN) has been shown to function well as an ultra-smooth bottom electrode for MIM devices,1,2 but suffers from an interfacial oxide layer and thermal instability above ~300 °C. Recently, Ta based amorphous metals such as TaWSi and TaNiSi have been shown to have larger work functions and stability approaching 900 °C and above.3 Precise knowledge of metal/insulator barrier heights is critical for predicting, understanding, and optimizing MIM diode device operation. Although the Schottky-Mott model may be used to predict ideal barrier heights (φBn = ΦM - χi) from the metal vacuum work function (ΦM) and insulator electron affinity (χi), actual barrier heights depend strongly on deposition method and interfacial properties and often deviate substantially. Internal photoemission (IPE) spectroscopy may be used to directly measure barrier heights at buried interfaces. To date, there have been few reports of IPE measurements of MIM structures and none on these amorphous metals. In this work, IPE is utilized to measure the metal/insulator barrier heights in MIM stacks with Ta-based amorphous metal bottom electrodes with TaN as a reference. Results are compared with current-voltage measurements.
TaWSi and TaNiSi bottom electrodes were sputtered onto 100 nm of thermally grown SiO2 on silicon followed by deposition of various high-k insulators via atomic layer deposition. Al and Au dots were evaporated through a shadow mask to serve as semitransparent (~10 nm thick) top gates. For IPE, current was monitored under a fixed applied field while photon energy was swept from 2 to 5 eV. A range of negative and positive applied fields were used to characterize both bottom and top interfaces.
Barrier heights will be reported for all electrode/insulator combinations. Schottky plots of bias dependent barrier heights vs. square root of electric field for Al2O3 are shown in Fig. 1 for (a) positive bias (Au interface) and (b) negative bias (TaXN interface). A number of conclusions may be drawn from the results of this measurement array. First, metal/Al2O3 barrier heights are qualitatively consistent with the expected TaXN metal work function as determined as a weighted average of the constituent metals. Second, Au/insulator barrier heights are not affected by the bottom electrode. Third, the asymmetry in the current-voltage response (I(-V)/I(V)) is qualitatively consistent with the IPE determined barrier heights whereas ideal theory predictions are not always. TaWSi and TaNiSi electrodes showed consistently higher barrier heights than ZCAN electrodes and comparable performance to high-quality TaN, indicating promise as a thermally stable bottom electrode for MIM tunnel diodes.
Support from NSF Center for Sustainable Materials Chemistry, CHE-1606982.

1 N. Alimardani, S.W. King, B.L. French, C. Tan, B.P. Lampert, and J.F. Conley Jr., J. Appl. Phys. 116, 24508 (2014).

2 N. Alimardani, E.W. Cowell, J.F. Wager, J.F. Conley Jr., D.R. Evans, M. Chin, S.J. Kilpatrick, and M. Dubey, J. Vac. Sci. Technol. A 30, 01A113 (2012).

3 J.M. McGlone, K.R. Olsen, W.F. Stickle, J.E. Abbott, R.A. Pugliese, G.S. Long, D.A. Keszler, and J.F. Wager, MRS Commun. 7, 715 (2017).

Figure 1