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

Wednesday, November 12, 2014 8:00 AM in Room 314

Wednesday Morning

Time Period WeM Sessions | Abstract Timeline | Topic EM Sessions | Time Periods | Topics | AVS2014 Schedule

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8:00 AM EM2-WeM-1 The Influence of Surface Preparation pre-Atomic Layer Deposition of Al2O3 on GaN Metal Oxide Semiconductor Capacitors
Dmitry Zhernokletov (Stanford University)

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).

8:20 AM EM2-WeM-2 Low Voltage Nonlinearity Metal-Insulator-Insulator-Metal (MIIM) Capacitors using Plasma Enhanced Atomic Layer Deposition of SiO2 and Al2O3
Dustin Austin (Oregon State University); Derryl Allman, David Price, Sallie Hose (On Semiconductor); John Conley (Oregon State University)
Back end of line (BEOL) metal-insulator-metal (MIM) capacitors reduce the need for discrete off-board components and have become a core passive device in integrated circuits. Applications include analog-to-digital converters, analog noise filters, DC voltage decoupling, and electrostatic discharge (ESD) protection. To enable continued scaling, capacitance density must be increased, either by introducing higher dielectric constant (κ) materials or by reducing the insulator film thickness. However decreasing insulator film thicknesses increases both leakage current density and voltage nonlinearity (characterized by the quadratic voltage coefficient of capacitance or αVCC). In addition high-κ materials typically have a large positive αVCC. Although a promising route to simultaneously meeting these competing requirements is to use a nanolaminate of insulators, which allows for combining of layers with complementary properties. Previous work has demonstrated nanolaminate MIIM devices with high capacitance density, low leakage current density, and low αVCC using PECVD SiO2 or uncommon materials. 1–3
In this work, MIIM capacitors using bilayers of Al2O3 and SiO2 were deposited sequentially using plasma enhanced atomic layer deposition (PEALD). PEALD allows for low deposition temperatures, precise thickness control, and conformal coverage over high aspect ratio structures. Al2O3 and SiO2 are attractive due to their common usage in IC fabrication, large metal-insulator barrier heights, and high dielectric breakdown strength. In addition SiO2 is one of the few materials that exhibits a negative αVCC. Spectroscopic ellipsometry was used to characterize the growth rate and nucleation delay on TaN and Si substrates. The dielectric constants of Al2O3 and SiO2 were found to be 4.6 and 8.7, respectively. αVCC values were plotted as a function of thickness and fit with a power law. Appropriate layer thicknesses were chosen to offset the negative αVCC of SiO2 with the positive αVCC of Al2O3 in order to minimize the effective αVCC for a given capacitance density. The initial results for 8 nm Al2O3 / 3.5 nm SiO2 MIIM devices show capacitance density of 5.4 fF/μm2, 2 nA/cm2 leakage at 1V, and αVCC of 70 ppm/V2, simultaneously meeting the ITRS 2014 requirements for capacitance density (> 5 fF/μm2), leakage current density (< 10 nA/cm2 at 1V), and voltage nonlinearity (< 100 ppm/V2). Current work is underway to optimize this nanolaminate to meet the ITRS 2017 requirements.
1 S. Van Huylenbroeck et al, Electron Device Lett. IEEE 23, 191 (2002).
2 S.J. Kim et al, Electron Device Lett. IEEE 25, 538 (2004).
3 T.H. Phung et al, Electrochem. Soc. 158, H1289 (2011).
8:40 AM EM2-WeM-3 Metal-Insulator Transitions, Resistive Switches and Oxide Electronics
Shriram Ramanathan (Harvard University)

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.

9:20 AM EM2-WeM-5 Complex Oxide Devices
Suman Datta (Penn State University)
Strongly correlated electronic phases encountered in complex oxides exhibit collective carrier dynamics that if properly harnessed can enable novel functionalities and perhaps even new computation paradigms. In this talk, we will present our recent understanding of electronically triggered charge oscillations in a prototypical metal insulator transition (MIT) system, vanadium dioxide. We show that the key to such oscillatory behavior lies in the ability to stabilize a spontaneously reversible phase transition in the complex oxide devices using a negative feedback mechanism. We also explore the synchronization dynamics of such oscillators via experiment and simulation, and investigate its potential for coupled oscillator based non-Boolean associative computing.
10:00 AM BREAK - Complimentary Coffee in Exhibit Hall
11:00 AM EM2-WeM-10 Ferroelectric Devices
Alexander Demkov (The University of Texas at Austin)

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.

This work is done in collaboration with Patrick Ponath, Kurt Fredrickson, Agham Posadas, John Ekerdt, David Smith, Martin Frank, Vijay Narayanan, Catherine Dubourdieu, Sergei Kalinin and Keji Lai. It is supported by the Air Force Office of Scientific Research under grant FA9550-12-10494, Office of Naval Research (ONR) under grant N000 14-10-1-0489, National Science Foundation under grant DMR- 1207342, and Texas Advanced Computing Center.
11:40 AM EM2-WeM-12 Enhanced Performance Metal/Insulator/Insulator/Metal (MIIM) Tunnel Diodes
Nasir Alimardani, JohnF. Conley, Jr. (Oregon State University)

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 (VON). 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 VON. 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 VON 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).

12:00 PM EM2-WeM-13 Assessment of Barrier Heights between ZrCuAlNi Amorphous Metal and SiO2, Al2O3, and HfO2 using Internal Photoemission Spectroscopy
Tyler Klarr (Oregon State University); Li Wei, Nhan Nguyen, Oleg Kirillov (National Institute of Standards and Technology (NIST)); John McGlone, John Wager, John Conley (Oregon State University)

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 (Vapp). Devices were tested in a custom built IPE system in which incident photon energy (Eph) from a broadband 150W xenon lamp source was swept from 1.5 to 5eV while the increase in current (photoemission yield) was monitored. The Vapp polarity was such that photoemission occurs at the ZCAN/insulator interface. The photoemission yield1/2 was plotted vs. Eph to determine the spectral threshold at each Vapp. Finally, a Schottky plot of spectral threshold vs. Vapp1/2 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).

Time Period WeM Sessions | Abstract Timeline | Topic EM Sessions | Time Periods | Topics | AVS2014 Schedule