AVS2014 Session EM2-WeM: High-K Dielectrics from Non-Classical Channels

High-κ gate dielectrics have been proposed as a means of producing high performance field effect devices with low gate leakage on GaN-based substrates for low static power consumption, improved transconductance, and higher output power capabilities [1-3]. However, because the surface of GaN may contain defects such as dangling bonds and contaminants [4], understanding the effect of varying surface preparation prior to atomic layer deposition (ALD) of the high-κ gate dielectrics on GaN is of great importance for the advancement of field effect devices. Surface defects and contaminants such as carbon and oxygen may have detrimental effects on optical quality and device performance of GaN based devices. Several methods to improve GaN surface and interface quality have been proposed [4-8]. They include surface cleaning procedures using aqueous (NH₄)₂S, and acid/base treatments such as HCl, HF, NaOH and NH₄OH.

We present a detailed study on the influence of surface preparations pre-atomic layer deposition of Al₂O₃ on GaN metal oxide semiconductor capacitors. The electrical, chemical, and luminescence characteristics of MOS structures prepared on both chemically treated and as-received GaN substrates are reported. Aqueous NH₄OH cleaning shows promise for providing an enhanced starting surface for atomic layer deposition of Al₂O₃ layers on GaN.

[1] P.D. Ye et al., Appl. Phys. Lett. 86, 063501 (2005).

[2] O.I. Saadat et al., IEEE Electron. Dev. Lett. 30 1254-1256 (2010).

[3] D.J. Meyer et al., Solid-State Electron.5 1098 (2010).

[4] R.D. Long et al., Materials. 5, 1297-1335 (2012).

[5] Diale et al., Appl.Surf.Sci. 246, 279–289 (2005).

[6] Lee et al., J. Electrochem. Soc. 147, 3087–3090 (2000).

[7] Hattori et al. Surf. Sci. 2010, 604, 1247–1253.

[8] Y. Koyama et al.,Solid State Electron.43,1483–1488 (1999).

There is growing interest in the exploring complex oxide semiconductors as functional elements in solid state devices. this is in part created by the inevitable limits to use of traditional semiconductors in highly scaled devices and also expanding interest in integrating multiple functionalities at the chip-level. Dielectrics with engineered defects and correlated oxides could be potentially interesting in this regard as switchable, adaptive materials for interconnects, logic and memory. There are a number of fundamental issues from the materials and interface aspects that remain poorly understood. For example, how can we develop a quantitative understanding of the electrical aspects of the high-k / correlated oxide interface where in almost all cases, such phase change materials show drastic frequency dependent properties? How can we design gate stacks to modulate carrier density approaching that of metallic state in such oxides? In this presentation, I will address these problems, with emphasis on studies conducted in our laboratory on rutile (e.g. VO2) and perovskite (e.g. SmNiO3) structured oxide thin films that undergo insulator-metal transitions.

Novel methods of deposition developed over the past decade or so, enabled fabrication of thin films of ferroelectric materials, such as BaTiO₃ (BTO), of very high crystal quality. This has resulted in renewed interest in ferroelectric field effect transistors and in addition, led to new device architectures, such as negative capacitance devices. Thanks to very high Pockels coefficient, thin films of BTO may find applications in Si nano-photonics.

In this talk I will describe our recent efforts on integration of BTO (and other ferroic oxides) on semiconductors using a SrTiO₃ (STO) buffer. More specifically, I will describe integration of BaTiO₃ on Si (001) and Ge (001) using molecular beam epitaxy (MBE) and atomic layer deposition (ALD). We employ first principles modeling to both guide the crystal growth and analyze the characterization data. By modeling core level spectroscopy and comparing it with the x-ray photoemission data we are able to identify the Zintl growth template for STO on Si and Ge. Comparing theoretical spectral functions with the angle resolved photoemission spectra (ARPES), provides us with a better understanding of the SrTiO₃ buffer surface. Using this strategy we stabilized ferroelectric state with out-of-plane polarization in BaTiO₃ (BTO) grown on Si with an STO buffer. And we demonstrate both out-of-plane in-plane polarized BTO growth on Ge (001). Annular dark field microscopy is used to elucidate the atomic structure of the semiconductor/oxide interface that is used in subsequent first principles calculations of the band alignment at the interface. We use a combination of polarization force and microwave impedance microscopies to investigate the ferroelectric response and field effect in our structures.

Thin film metal-insulator-metal (MIM) tunnel devices are gaining interest for applications such as hot electron transistors, diodes for optical rectenna based IR energy harvesting, IR detectors, large area macroelectronics, and selector diodes to avoid the sneak leakage in RRAM crossbar arrays. For many of these applications, figures of merit include high asymmetry and strong nonlinearity of current vs. voltage (I-V) behavior at low turn on voltages (V_ON). The common strategy to achieving rectification in MIM devices relies on Fowler-Nordheim tunneling (FNT) conduction in conjunction with the use of dissimilar work function metal electrodes to produce an asymmetric, polarity dependent electron tunneling barrier. The properties of single layer MIM diodes are dominated by the choice of insulator. Performance is limited by the workfuction difference that can be achieved between the electrodes as well as the metal-insulator band offsets. Wide bandgap oxides are limited by high V_ON. Narrow bandgap dielectrics such as Ta₂O₅ and Nb₂O₅ are attractive because the small barrier heights allow for low turn-on voltages. However, because conduction in these materials is based on emission rather than tunneling, they may not be suitable for high speed rectification. Recently, we showed that a nanolaminate pair of insulators (Al₂O₃/HfO₂) can be used to form MIIM diodes with enhanced performance over single layer MIM diodes and demonstrated that observed enhancements in low voltage asymmetry are due to "step tunneling," a situation in which an electron may tunnel through only the larger bandgap insulator instead of both.¹

In this work, we show that MIIM diodes may require only one of the insulators to be dominated by tunneling and thus allow use of narrow band gap insulators for tunnel devices. Atomic layer deposition (ALD) was used to deposit nanolaminate insulators on smooth amorphous metal bottom electrodes. We demonstrate that Ta₂O₅, a narrow bandgap dielectric dominated by thermal emission, may be combined with Al₂O₃, a wide bandgap dielectric dominated by tunneling, to achieve high asymmetry, low V_ON MIIM diodes whose overall performance is dominated by tunneling. The performance of a variety of other bilayer MIIM diodes (HfO₂/Ta₂O₅, ZrO₂/Ta₂O₅, Al₂O₃/ZrO₂, and HfO₂/ZrO₂) will be discussed as well. These results advance the understanding needed to engineer thin film tunnel devices for microelectronics applications.

1. N. Alimardani and J.F. Conley, Jr., Appl. Phys. Lett. 102, 143501 (2013).

As scaling of Si based devices approaches fundamental limits, thin film metal-insulator-metal (MIM) tunnel diodes are attracting interest due to their potential for high speed operation. Because operation of these devices is based on tunneling, electrode / interfacial roughness is critical. Recently, we showed that combining ultra-smooth bottom electrodes with insulators deposited via atomic layer deposition (ALD) enabled reproducible fabrication of MIM diodes with stable I-V behavior.¹ Key performance parameters of MIM diodes include high I-V asymmetry and low turn-on voltage. The standard way to achieve asymmetry relies on the use of non-equivalent workfunction metal electrodes to induce a built-in field that creates polarity dependent electron tunneling barrier.² Assessment of metal-insulator barrier heights is therefore critical for predicting diode performance.

In this work, we report the first use of internal photoemission spectroscopy (IPE) to measure barrier heights between an amorphous ZrCuAlNi (ZCAN) metal bottom electrode and several high-k dielectrics. MIM stacks were fabricated on Si substrates capped with 100nm of thermally grown SiO₂ and a 150nm thick ZCAN amorphous metal bottom electrode deposited via DC magnetron sputtering. Al₂O₃ and HfO₂ were deposited via thermal ALD at 250ºC using H₂O and TMA or TEMA-Hf, respectively. SiO₂ was deposited using plasma-enhanced ALD (PEALD) at 200ºC using O₂ and bis-diethylaminosilane (BDEAS). For IPE measurements, semitransparent top electrodes were formed by electron beam evaporation of Al (0.04mm²) and patterned by a multistep photolithography process. In IPE, the conduction band offset between two materials is characterized by measuring the additional current created by photo-excitation of carriers under an applied bias (V_app). Devices were tested in a custom built IPE system in which incident photon energy (E_ph) from a broadband 150W xenon lamp source was swept from 1.5 to 5eV while the increase in current (photoemission yield) was monitored. The V_app polarity was such that photoemission occurs at the ZCAN/insulator interface. The photoemission yield^1/2 was plotted vs. E_ph to determine the spectral threshold at each V_app. Finally, a Schottky plot of spectral threshold vs. V_app^1/2 was used to estimate the zero field barrier heights from the y-axis intercept. Initial analysis indicates barriers of 3.4, 3.2, and 2.7 eV for SiO₂, Al₂O₃, and HfO₂, respectively. Additional dielectrics and metals are under investigation. IPE results will be compared to electrical methods of barrier extraction.

1. N. Alimardani et al, JVSTA 30, 01A113 (2012).

2. J.G. Simmons, JAP 34, 2581 (1963); JAP 34, 1793 (1963).