Development of a spin-injection device utilising Gadolinium Nitride

<p>In this thesis, the first steps in creating a realisable spin-injection transistor using ferromagnetic semiconductor electrodes are detailed. A spin-injection device utilising the ferromagnetic semiconductor gadolinium nitride has been designed, fabricated and electrically tested. In addition, an experimental setup for future measurements of a spin current in spin-injection devices was adapted to our laboratory-based off one developed by the Shiraishi group at Kyoto University. Issues encountered during fabrication were identified, and an optimal method for fabricating these devices was determined. Gadolinium nitride and copper were used to make the devices on Si/SiO2 substrates. The electrical integrity and applicability of the devices for future measurements of injected spin-current was determined through electrical device testing. Resistance measurements of electrical pathways within the device were undertaken to determine the successful deposition of the gadolinium nitride and copper. IV measurements to determine if the devices could withstand the current required for spin current measurements were done. The durability of the devices through multiple measurement types was observed. It was determined that although spin-injection devices utilising gadolinium nitride can be successfully fabricated, more work needs to be done to ensure that the electrical pathways through the copper and gadolinium nitride can be consistently reproducible to allow spin-injection measurements to be done.</p>

Development of a spin-injection device utilising Gadolinium Nitride

<p>In this thesis, the first steps in creating a realisable spin-injection transistor using ferromagnetic semiconductor electrodes are detailed. A spin-injection device utilising the ferromagnetic semiconductor gadolinium nitride has been designed, fabricated and electrically tested. In addition, an experimental setup for future measurements of a spin current in spin-injection devices was adapted to our laboratory-based off one developed by the Shiraishi group at Kyoto University. Issues encountered during fabrication were identified, and an optimal method for fabricating these devices was determined. Gadolinium nitride and copper were used to make the devices on Si/SiO2 substrates. The electrical integrity and applicability of the devices for future measurements of injected spin-current was determined through electrical device testing. Resistance measurements of electrical pathways within the device were undertaken to determine the successful deposition of the gadolinium nitride and copper. IV measurements to determine if the devices could withstand the current required for spin current measurements were done. The durability of the devices through multiple measurement types was observed. It was determined that although spin-injection devices utilising gadolinium nitride can be successfully fabricated, more work needs to be done to ensure that the electrical pathways through the copper and gadolinium nitride can be consistently reproducible to allow spin-injection measurements to be done.</p>

Investigation of the electron and thermal transport in rare-earth nitrides

<p>Gadolinium nitride (GdN) and samarium nitride (SmN) have been widely studied to understand their ferromagnetic ordering and electronic structure, and for their promise in spintronics applications. This thesis presents experimental magnetotransport studies of GdN and SmN films in which experimental results have been compared with the existing band structure calculation. Three GdN films have been prepared in different conditions, among them two films are epitaxial quality and one film is polycrystalline in nature, and two films of SmN were also studied. Their magnetic properties were probed by SQUID magnetometry and they are found to be ferromagnetic. The transition temperature differs from sample to sample and this behaviour has been attributed to the presence of magnetic polarons that nucleate around nitrogen vacancies and give rise to an inhomogeneous ferromagnetic state. The charge transport results have been discussed for all GdN and SmN films. A full set of charge/heat transport results are obtained on only one epitaxial GdN. The difference of resistivity among these samples is noticeable. The Hall effect results show the presence of different carrier concentration with at most only weak temperature dependence. We also have noticed the presence of anomalous Hall effect in the paramagnetic region for a lower-concentration epitaxial GdN. The thermopower in both GdN and SmN was measured to provide further insight into the material’s electronic properties. In this thesis we present the first experimental investigation of the thermopower of epitaxial gadolinium nitride and samarium nitride films, measured using an experimental set-up designed for measuring the temperature dependent thermopower of thin films. Our result shows a negative thermopower for both GdN and SmN films and simple, though strong temperature dependence. At low temperatures we observe a peak near the ferromagnetic transition temperature in GdN. The results are interpreted in terms of the diffusion thermopower. Overall the results suggest that the nitrogen vacancy concentration controls the carrier concentration and plays a significant role towards the transport properties. We conclude that all films are either heavily, moderately or weakly doped semiconductors with a metallic characteristic.</p>

Investigation of the electron and thermal transport in rare-earth nitrides

<p>Gadolinium nitride (GdN) and samarium nitride (SmN) have been widely studied to understand their ferromagnetic ordering and electronic structure, and for their promise in spintronics applications. This thesis presents experimental magnetotransport studies of GdN and SmN films in which experimental results have been compared with the existing band structure calculation. Three GdN films have been prepared in different conditions, among them two films are epitaxial quality and one film is polycrystalline in nature, and two films of SmN were also studied. Their magnetic properties were probed by SQUID magnetometry and they are found to be ferromagnetic. The transition temperature differs from sample to sample and this behaviour has been attributed to the presence of magnetic polarons that nucleate around nitrogen vacancies and give rise to an inhomogeneous ferromagnetic state. The charge transport results have been discussed for all GdN and SmN films. A full set of charge/heat transport results are obtained on only one epitaxial GdN. The difference of resistivity among these samples is noticeable. The Hall effect results show the presence of different carrier concentration with at most only weak temperature dependence. We also have noticed the presence of anomalous Hall effect in the paramagnetic region for a lower-concentration epitaxial GdN. The thermopower in both GdN and SmN was measured to provide further insight into the material’s electronic properties. In this thesis we present the first experimental investigation of the thermopower of epitaxial gadolinium nitride and samarium nitride films, measured using an experimental set-up designed for measuring the temperature dependent thermopower of thin films. Our result shows a negative thermopower for both GdN and SmN films and simple, though strong temperature dependence. At low temperatures we observe a peak near the ferromagnetic transition temperature in GdN. The results are interpreted in terms of the diffusion thermopower. Overall the results suggest that the nitrogen vacancy concentration controls the carrier concentration and plays a significant role towards the transport properties. We conclude that all films are either heavily, moderately or weakly doped semiconductors with a metallic characteristic.</p>

Device Applications of Rare-Earth Nitrides

<p>In this thesis the properties of thin film spintronic devices are investigated. These devices incorporate rare-earth nitrides as the active elements in a geometry with vertical transport perpendicular to the layers. Many rare-earth nitrides are ferromagnetic semiconductors with a rich range of magnetic properties arising from their 4-f magnetic moments. These magnetic moments contain both spin and orbital contributions, in contrast to the quenched, spin-based magnetism frequently exploited in spintronic devices based on transition metals. Magnetic tunnel junctions are demonstrated with the ferromagnetic electrodes made from the intrinsic ferromagnetic semiconductor GdN surrounding GaN and AlN barriers. Fitting of the current-voltage characteristics of a GdN/GaN/GdN device determines a barrier height of 1.5 eV at room temperature. This puts the GdN Fermi level close to the GaN mid-gap, consistent with recent theoretical predictions of the band alignment at the GdN/GaN interface [Kagawa et al., Phys. Rev. Applied 2, 054009 (2014)]. This barrier height is found to scale with the band gap of the group-III nitride barrier, being approximately twice as large for AlN barriers. It was observed that the barrier height reduces as the AlN barrier thickness increases, signalling the formation of Schottky barriers at the interface. These polycrystalline junctions exhibit a tunnel magnetoresistance of a few percent but do not show clear signs of homogeneous switching. The transport properties of the GdN/GaN/GdN junctions are heavily influenced by the electronic structure of the semiconducting GdN layers, making junctions based on rare-earth nitrides promising candidates for further investigation. A fully semiconductor-based magnetic tunnel junction that uses spin-orbit coupled materials made of intrinsic ferromagnetic semiconductors is then presented. Unlike more common approaches, one of the electrodes consists of a near-zero magnetic moment ferromagnetic semiconductor, samarium nitride, with the other electrode comprised of the more conventional ferromagnetic semiconductor gadolinium nitride. Fabricated tunnel junctions exhibit magnetoresistances as high as 200%, implying strong spin polarisation in both electrodes. In contrast to conventional tunnel junctions, the resistance is largest at high fields, a direct result of the orbital-dominant magnetisation in samarium nitride that requires the spin in this electrode aligns opposite to that in the gadolinium nitride when the magnetisation is saturated. The magnetoresistance at intermediate fields is controlled by the formation of a twisted magnetisation phase in the samarium nitride, a direct result of the orbital-dominant ferromagnetism. Thus, new functionality can be brought to magnetic tunnel junctions by use of novel electrode materials, in contrast to the usual focus on tuning the barrier properties. Finally, highly resistive GdN films intentionally doped with Mg are demonstrated. These films are found to have increased resistivities and decreased carrier concentrations, with no observed degradation in crystal quality as compared with undoped films. An increase of the Curie temperature in conductive films is observed which is consistent with the existence of magnetic polarons centred on nitrogen vacancies. The prospect of doping rare-earth nitride films in this manner promises greater control of the material properties and future device applications.</p>

Device Applications of Rare-Earth Nitrides

<p>In this thesis the properties of thin film spintronic devices are investigated. These devices incorporate rare-earth nitrides as the active elements in a geometry with vertical transport perpendicular to the layers. Many rare-earth nitrides are ferromagnetic semiconductors with a rich range of magnetic properties arising from their 4-f magnetic moments. These magnetic moments contain both spin and orbital contributions, in contrast to the quenched, spin-based magnetism frequently exploited in spintronic devices based on transition metals. Magnetic tunnel junctions are demonstrated with the ferromagnetic electrodes made from the intrinsic ferromagnetic semiconductor GdN surrounding GaN and AlN barriers. Fitting of the current-voltage characteristics of a GdN/GaN/GdN device determines a barrier height of 1.5 eV at room temperature. This puts the GdN Fermi level close to the GaN mid-gap, consistent with recent theoretical predictions of the band alignment at the GdN/GaN interface [Kagawa et al., Phys. Rev. Applied 2, 054009 (2014)]. This barrier height is found to scale with the band gap of the group-III nitride barrier, being approximately twice as large for AlN barriers. It was observed that the barrier height reduces as the AlN barrier thickness increases, signalling the formation of Schottky barriers at the interface. These polycrystalline junctions exhibit a tunnel magnetoresistance of a few percent but do not show clear signs of homogeneous switching. The transport properties of the GdN/GaN/GdN junctions are heavily influenced by the electronic structure of the semiconducting GdN layers, making junctions based on rare-earth nitrides promising candidates for further investigation. A fully semiconductor-based magnetic tunnel junction that uses spin-orbit coupled materials made of intrinsic ferromagnetic semiconductors is then presented. Unlike more common approaches, one of the electrodes consists of a near-zero magnetic moment ferromagnetic semiconductor, samarium nitride, with the other electrode comprised of the more conventional ferromagnetic semiconductor gadolinium nitride. Fabricated tunnel junctions exhibit magnetoresistances as high as 200%, implying strong spin polarisation in both electrodes. In contrast to conventional tunnel junctions, the resistance is largest at high fields, a direct result of the orbital-dominant magnetisation in samarium nitride that requires the spin in this electrode aligns opposite to that in the gadolinium nitride when the magnetisation is saturated. The magnetoresistance at intermediate fields is controlled by the formation of a twisted magnetisation phase in the samarium nitride, a direct result of the orbital-dominant ferromagnetism. Thus, new functionality can be brought to magnetic tunnel junctions by use of novel electrode materials, in contrast to the usual focus on tuning the barrier properties. Finally, highly resistive GdN films intentionally doped with Mg are demonstrated. These films are found to have increased resistivities and decreased carrier concentrations, with no observed degradation in crystal quality as compared with undoped films. An increase of the Curie temperature in conductive films is observed which is consistent with the existence of magnetic polarons centred on nitrogen vacancies. The prospect of doping rare-earth nitride films in this manner promises greater control of the material properties and future device applications.</p>

Electrical and Magnetic Investigations of Magnesium-doped Epitaxial Gadolinium Nitride Thin Films

<p>Mg-doped epitaxial GdN thin films with various Mg-doping levels were grown using molecular beam epitaxy, and their electric, magnetic and optoelectronic properties were investigated. Characterisation through X-ray diffraction technique showed that there is no systematic variation in the crystallographic structure of the films with increasing level of Mg-doping, for Mg concentrations up to ~5 x 10¹⁹ atoms/cm³. However, from Mg concentration ~2 x 10²⁰ atoms/cm³ a clear deterioration in the crystalline quality was seen. We observed an increase in the resistivity of the films from 0.002 Ωcm to 600 Ωcm at room temperature when increasing the Mg-doping level, resulting in semi-insulating films for Mg concentrations up to 5 x 10¹⁹ atoms/cm³. Hall effect measurements revealed that the n-type carrier concentration was reduced from 7 x 10²⁰ cm⁻³ for an undoped film to 5 x 10¹⁵ cm⁻³ for a heavily doped film, demonstrating electron compensation in GdN via Mg-doping. Magnetic measurements exhibited substantial contrasts in the films, with a Curie temperature of ~70 K for an undoped film reduced down to ~50 K for a heavily Mg-doped film. Finally, photoconductivity measurements showed that films with higher level of Mg-doping displaying a faster photoconductive response. The decay time of 13000 s for an undoped film was reduced to 170 s with a moderate level of Mg-doping, which raises the possibility of Mg impurities providing hole traps that act as recombination centres in n-type GdN films.</p>

Electrical and Magnetic Investigations of Magnesium-doped Epitaxial Gadolinium Nitride Thin Films

<p>Mg-doped epitaxial GdN thin films with various Mg-doping levels were grown using molecular beam epitaxy, and their electric, magnetic and optoelectronic properties were investigated. Characterisation through X-ray diffraction technique showed that there is no systematic variation in the crystallographic structure of the films with increasing level of Mg-doping, for Mg concentrations up to ~5 x 10¹⁹ atoms/cm³. However, from Mg concentration ~2 x 10²⁰ atoms/cm³ a clear deterioration in the crystalline quality was seen. We observed an increase in the resistivity of the films from 0.002 Ωcm to 600 Ωcm at room temperature when increasing the Mg-doping level, resulting in semi-insulating films for Mg concentrations up to 5 x 10¹⁹ atoms/cm³. Hall effect measurements revealed that the n-type carrier concentration was reduced from 7 x 10²⁰ cm⁻³ for an undoped film to 5 x 10¹⁵ cm⁻³ for a heavily doped film, demonstrating electron compensation in GdN via Mg-doping. Magnetic measurements exhibited substantial contrasts in the films, with a Curie temperature of ~70 K for an undoped film reduced down to ~50 K for a heavily Mg-doped film. Finally, photoconductivity measurements showed that films with higher level of Mg-doping displaying a faster photoconductive response. The decay time of 13000 s for an undoped film was reduced to 170 s with a moderate level of Mg-doping, which raises the possibility of Mg impurities providing hole traps that act as recombination centres in n-type GdN films.</p>

The Magnetic Properties of Selected Rare Earth Nitrides Grown by Pulsed Laser Deposition

<p>Gadolinium nitride (GdN) and samarium nitride (SmN) are grown by pulsed laser deposition on yttria stabilised zirconia substrates. Surface and structural characterisation shows that the thin films are epitaxial with crystallites of up to 30 nm in diameter and a very large in plane coherence length. A novel oxide layer is observed at the substrate-film interface, caused by oxygen in the substrate reacting with the deposited rare earth element. GdN is found to be ferromagnetic below 70 K with a saturation moment of 7 Bohr magneton per ion. The relationship between the crystal structure and the magnetisation is investigated using ferromagnetic resonance and a weak easy axis along the [111] azimuth is reported. Hall effect measurements show the carriers are electrons, present in concentrations of 1020/cm3. Magnetic measurements on SmN show the presence of metallic droplets, but correcting for these, the Curie temperature is found to be 30 K.We report on preliminary growths of europium nitride and show the valence of the Eu is 3+, solving an outstanding theoretical question.</p>

The Magnetic Properties of Selected Rare Earth Nitrides Grown by Pulsed Laser Deposition

<p>Gadolinium nitride (GdN) and samarium nitride (SmN) are grown by pulsed laser deposition on yttria stabilised zirconia substrates. Surface and structural characterisation shows that the thin films are epitaxial with crystallites of up to 30 nm in diameter and a very large in plane coherence length. A novel oxide layer is observed at the substrate-film interface, caused by oxygen in the substrate reacting with the deposited rare earth element. GdN is found to be ferromagnetic below 70 K with a saturation moment of 7 Bohr magneton per ion. The relationship between the crystal structure and the magnetisation is investigated using ferromagnetic resonance and a weak easy axis along the [111] azimuth is reported. Hall effect measurements show the carriers are electrons, present in concentrations of 1020/cm3. Magnetic measurements on SmN show the presence of metallic droplets, but correcting for these, the Curie temperature is found to be 30 K.We report on preliminary growths of europium nitride and show the valence of the Eu is 3+, solving an outstanding theoretical question.</p>