MAGNETIC RESONANCE AND MAGNETORESISTANCE FOR THE UNDERSTANDING OF DEFECT CHEMISTRY AND SPIN-TRANSPORT IN AMORPHOUS SEMICONDUCTORS AND DIELECTRICS

Open Access
Author:
Mutch, Michael James
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 24, 2016
Committee Members:
  • Patrick M. Lenahan, Dissertation Advisor
  • Patrick M. Lenahan, Committee Chair
  • Mark Horn, Committee Member
  • S. Ashok, Committee Member
  • Jerzy Ruzyllo, Outside Member
Keywords:
  • Magnetic Resonance
  • electrical engineering
  • materials engineering
  • electronic transport
  • quantum tunneling
  • trap-assisted tunneling
  • defects
  • low-k dielectrics
  • silicon nitride
  • magnetoresistance
Abstract:
Amorphous semiconductors and dielectrics are widely utilized throughout the microelectronics industry. Applications include thin-film transistors (TFTs) utilized in flat panel displays and solar cells, oxides in metal-oxide-semiconductor field-effect transistors (MOSFETs) for microprocessors and memory-cell access devices, low-dielectric constant (κ) interlayer dielectrics which separate back-end of line interconnects, and several others. Like other systems, reducing performance limiting defects within amorphous systems is critical for microelectronic integrated circuit (IC) development and improvement. Perhaps the most powerful analytical tool for studying point defects in electronic materials is electron paramagnetic resonance (EPR). However, EPR is not sensitive enough to measure defects in state-of-the-art microelectronic devices, since its sensitivity is limited to 1010 total defects. Additionally, EPR spectra of defects within amorphous systems often are often featureless due to the inherent disorder of these systems. This work utilizes an EPR based approach, electrically detected magnetic resonance (EDMR), to study defect chemistry in amorphous semiconductors and dielectrics even when featureless spectra are present. EDMR is the electrically detected analog of EPR in which EPR induced changes in device current are detected. In this study, EDMR is detected via changes in amorphous semiconductor or dielectric tunneling current via spin-dependent trap assisted tunneling (SDTAT) events. Due to the nature of SDTAT, defects detected are directly linked to electronic transport; an additional benefit of EDMR relative to EPR. Unlike EPR, SDTAT/EDMR may also be detected at any field/frequency combination without loss of sensitivity. As will be explained, this field/frequency independence allows for a distinction between EDMR line width contributions from electronic g tensor components or electron iv nuclear hyperfine interactions, thus providing insight into defect chemistry when featureless spectra are present. Additionally, performing EDMR measurements at multiple biases and comparing with MIS band diagrams allows for a rudimentary understanding of defect energy levels. Finally, we utilize EDMR to understand near-zero-field magnetoresistance (MR) phenomena, which are of interest for spin-transport applications such as spin-valves and magnetometers. The EDMR techniques utilized in this study are relatively new, and have not been exploited to study a wide range of electronic materials. Thus, in Chapter 4, baseline EDMR measurements are provided in relatively simple amorphous systems including a-Si:H and a-C:H. We find that EDMR spectra in a-Si:H and a-C:H systems are due to silicon and carbon dangling bonds, respectively. Additionally, we utilize multiple frequency EDMR to provide additional information regarding contributions of line width due to the breadth of g tensor components in the featureless a-Si:H and a-C:H EDMR spectra. By providing a measurement of g tensor breadth, Δg, we develop a baseline for distinguishing between silicon and carbon dangling bonds in more complex systems, such as low-κ dielectrics a SiOC:H and a-SiCN:H, in which silicon and/or carbon dangling bonds may be present. Low-κ dielectric constant materials are critical for reducing parasitic capacitances due to the scaling of back-end of line interconnects. In Chapter 4, we first utilize conventional EPR measurements to study a variety of porous low-κ dielectric powders. Via conventional EPR on these low-κ powders, we are able to analyze the effects of UV radiation and remote hydrogen plasma upon the low-κ systems. Our results indicate that UV treatments, which are utilized to eliminate sacrificial porogens to introduce pores, significantly increase defect density. Remote hydrogen plasma (RHP) treatments are found v to decrease dangling bond concentration. However, due to the featureless EPR spectra, we are unable to provide insight into defect chemistry via conventional EPR. Thus, we utilize multiple field/frequency EDMR in these low-κ systems, and compare Δg measurements with previous baseline measurements, to provide insight into defect chemistry which was previously unavailable. We find a multitude of silicon and carbon dangling bonds in a SiOCH and a-SiCN:H dielectrics. Defect chemistry seems to depend upon precursor chemistry. Additionally, EDMR measurements confirm that UV treatments in low-κ systems introduce silicon dangling bonds, suggesting that these treatments may be damaging the Si-O-Si network in a-SiOC:H systems. Finally, we perform EDMR measurements at multiple biases to get a general understanding of defect energy levels in these systems. Band gaps are calculated via reflected electron energy loss spectroscopy (REELS), and band offsets are calculated via X-ray photoelectron spectroscopy (XPS). We find that carbon dangling bonds in a-SiOC:H systems have levels near the middle of the a SiOC:H band gap, and silicon dangling bonds in a-SiCN:H systems have levels near the upper-middle part of the a-SiCN:H band gap. In Chapter 5, we analyze silicon nitride (a-SiN:H) thin films, which are widely utilized in the electronics industry as gate dielectrics for TFTs. However, defects and electronic transport in these systems are not fully understood. We utilize multiple frequency EDMR and variable bias EDMR to better understand defect chemistry and energy levels in a-SiN:H systems. It is found that K centers, which have been previously observed in a-SiN:H via EPR and electron nuclear double resonance (ENDOR), are primarily responsible for transport in these systems. Additionally, we find that K centers vi are about 3.1 eV above the a-SiN:H valence band edge, in agreement with previous theoretical calculations. In Chapter 6, we illustrate that near-zero field MR phenomena are ubiquitous in amorphous semiconductors and dielectrics. We link the MR and EDMR responses by measuring response amplitude for each technique versus bias. The observed EDMR and MR versus bias trends are nearly identical, suggesting that the defects responsible for each technique correspond to similar energy levels. Though circumstantial, our measurements provide strong evidence that the defects whose chemistry is plausibly identified via multiple frequency EDMR are primarily responsible for MR in the amorphous semiconductors and dielectrics in this study.