The role of muons in semiconductor research (Ch 5)

The aim of this chapter is to provide an introduction and overview of using muons to study defects in semiconductors for an audience with a background in material science. First is a general tutorial to relevant models and discussion of the muon-based techniques that have been important to the semiconductor field. The latter portion of this chapter highlights results from selected studies on semiconductors to demonstrate and describe some contributions that muon spin research (SR) techniques have made to the semiconductor community in recent years

Invited Talk/Session: Role of the Muon in Semiconductor Research

Muons are used in semiconductor research as an experimentally accessible analog to the isolated Hydrogen (H) impurity -- a complex that is very difficult (or impossible) to study by other means. Hydrogen impurities of any concentration can modify the electrical, optical or magnetic properties of the host. For instance, H can be incorporated to remove electrically active levels from the energy gap (i.e. passivation) while some can form isolated centers that tend to be responsible for the trap and release of charge carriers and participate in site and charge-state dynamics which certainly affect the electrical properties of the host. Therefore, it can be quite useful to characterize these impurities in semiconducting materials that are of interest for use in devices. A muon has the same charge and spin as a proton but a mass that is nine times lighter. When implanted in a target material, a positively charged muon can behave as a light proton or bind with an electron to form a complex known as Muonium (Mu) with properties that are very similar to that of ionic or neutral H, respectively. A result of these similarities and direct non-destructive implantation is that Mu provides a direct measure of local electronic structure, thermal stability and charge-state transitions of these impurity centers. Since any material can be subjected to muon implantation and it is the muons themselves that mimic the H impurity centers, these measurements do not depend (at all) on the host\u27s solubility of hydrogen nor do they require some minimum concentration; unlike many other techniques, such as EPR, ENDOR, NMR, or IR vibrational spectroscopy. Here we summarize major contributions muons have made to the field of semiconductor research followed by a few case studies to demonstrate the technique and detailed knowledge of the physical and electronic structures as well as dynamics (e.g.: charge-state and site transitions; local motion; long-range diffusion) of Mu/H that can be obtained

Characterization of the Magnetic Phase in Ti-Doped Vanadium Dioxide

Here we present a characterization of the recently discovered magnetic phase in titanium-doped vanadium dioxide (VO2:Ti with 1, 3 & 5 at%) via Muon Spin Relaxation (MuSR) measurements. Specifically, variations in the magnetic phase were studied as a function of titanium dopant concentration in an effort to understand the fundamental mechanism responsible for magnetism and other transitions exhibited by the material. Muons are spin 1/2 particles with an average lifetime of 2.2 us and a gyromagnetic ratio of 135.54 MHz/T that are used to study the local magnetic environment through a technique known as Muon Spin Relaxation/Rotation/Resonance (MuSR). Implanted muons precess about the field in their local environment until they decay into a positron that is preferentially aligned with the spin direction at the time of decay (and associated neutrinos). By tracking these positrons, the time evolution of the muon spin polarization is determined and used to map the local magnetic field distribution at the muon stopping site. VO2 exhibits an ultra-fast, reversible metal−semiconductor transition (MST) at TMST near 340 K. Above TMST it is metallic, reflective and electrically conductive; below, the electrical conductivity drops by several orders of magnitude, it is transparent with a bandgap of 0.7 eV. The MST can be triggered by thermal, optical, electrical or barometric means and is accompanied by a structural transition. Dopants like tungsten and Ti reduce TMST to below room temperature while having minimal effect on the electronic or optical properties of the host, whereas dopants such as fluorine and chromium can raise it to above 400 K. While VO2 has been studied since the 1960s, its low-temperature magnetic phase was first reported in 2014 by our collaboration. This contribution is part of a large-scale project aimed at understanding the fundamental mechanism responsible for transitions in VO2 compounds, a question still highly debated within the community

Characterizing the Muonium Impurity in Anatase TiO2

In this contribution, we introduce the Muon Spin Rotation, Relaxation and Resonance technique (MuSR) and discuss our current study focused on understanding the characteristics of Muonium–like states (as an analog to isolated Hydrogen) in Anatase Titanium Dioxide (TiO2). MuSR utilizes 100% spin polarized positive muons (charge +e; spin 1/2; mass 1/9 of proton), which upon being implanted in a material, precess in the local environment and decay with a positron emitted preferentially along the spin direction at the time of decay. The time evolution of the muon spin polarization is tracked as an ensemble of these decay events. The muon’s sensitivity to small magnetic field fluctuations and electronic interactions make it a great tool for studying the local environment in bulk materials. In some cases, the muon captures an electron after implantation to form Muonium (Mu): an experimentally accessible analog to an isolated Hydrogen impurity (H). Muonium is a factor of nine lighter than isolated H, but with nearly the same Bohr radius and ground state energy, it behaves very similarly to H. In rutile TiO2 specifically, Mu and H are both found with the same Oxygen bonding configurations and an identical electronic structure [R.C. Vilao, et al. PRB 92 (2015) 081202 ̃ (R)]. The Mu configuration and any associated dynamics (e.g.: charge state cycles, local motion and diffusion) in the similar anatase phase of TiO2 are the focus of this investigation. Understanding H impurities in these materials is important since H is a common and unavoidable impurity that has a very significant effect on the electrical and optical properties of TiO2 [see e.g.: Lavrov et al., Phys Rev B 93 (2016) 045204; Erdal et al., J Phys Chem C 114 (2010) 9139]. TiO2 is of particular interest due to its broad range of applications – some examples are gas–sensing systems, H storage, electrochromic devices and for photocatalysis [See e.g.: Chen et al., Chem Rev 107 (2007) 2891; Diebold, Suf Sci Rep 48 (2003) 53; Zhang et al. Phys Chem Chem Phys 16 (2014) 20382]. Contributing authors: P.W. Mengyan (NMU), R.L. Lichti (Texas Tech University), J.S. Lord (Rutherford Appleton Lab, UK)

Dynamics of Heavy Carriers in the Ferromagnetic Superconductor UGe2

Superconductivity and ferromagnetism in a number of uranium-based materials come from the same f-electrons with a relatively large effective mass, suggesting the presence of a band of heavy quasiparticles, whose nature is still a mystery. Here, UGe2 dynamics in both ferromagnetic and paramagnetic phases is studied employing high-field μ+SR spectroscopy. The spectra exhibit a doublet structure characteristic to formation of subnanometer-sized magnetic polarons. This model is thoroughly explored here and correlated with the unconventional physics of UGe2. The heavy-fermion behavior is ascribed to magnetic polarons; when coherent they form a narrow band, thus reconciling heavy carriers with superconductivity and itinerant ferromagnetism

Recent and future employment trends and their implications for Scottish electronics

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