Semiconductor Spintronic Devices Research Articles

Heavily Fe-doped quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

We study epitaxial growth and physical properties of heavily Fe-doped quaternary-alloy ferromagnetic semiconductor (In0.84−x ,Ga x ,Fe0.16)Sb thin films (Ga content x = 2%–10%, Fe content fixed at 16%). The (In0.84−x ,Ga x ,Fe0.16)Sb films have a zinc-blende-type crystal structure without any other second phase, and all the samples exhibit intrinsic ferromagnetism with high Curie temperature (>300 K). The carrier type of the (In0.84−x ,Ga x ,Fe0.16)Sb films is found to change by varying x, and a carrier-type phase diagram of (In,Ga,Fe)Sb is presented. These results suggest that (In,Ga,Fe)Sb is a promising material for semiconductor spintronic devices, such as ferromagnetic p-n junctions, operating at RT.

Two-dimensional HfCr2N4 semiconductor with intrinsic room-temperature ferromagnetism and enhanced conductivity via electrostatic doping

Two-dimensional semiconductors with room-temperature intrinsic ferromagnetism and superior conductivity are crucial components for low-dimensional semiconductor spintronic devices. Based on density functional theory and Boltzmann transport theory, we investigate the electromagnetic and transport properties of the HfCr2N4 monolayer and modulate these properties via electrostatic doping. The HfCr2N4 monolayer is an intrinsic ferromagnetic semiconductor with a narrow band gap of 0.18 eV. The notably high Curie temperature of 719 K is highly desired for the practical applications of two-dimensional ferromagnetic materials. Under electrostatic doping, the HfCr2N4 monolayer maintains its ferromagnetic ground state, easy magnetization xy-plane, and high Curie temperature. Interestingly, owing to the narrow band gap of the HfCr2N4 monolayer, a significant enhancement in conductivity is observed at lower carrier doping concentrations, leading to a transition from a semiconductor to a half-metal. These unique features can be utilized to represent binary states ‘0’ and ‘1’. Our results indicate that the HfCr2N4 monolayer emerges as an ideal candidate for dual-gate field effect transistors.

Realization of Robust and Ambient-Stable Room-Temperature Ferromagnetism in Wide Bandgap Semiconductor 2D Carbon Nitride Sheets.

Due to their weak intrinsic spin-orbit coupling and a distinct bandgap of 3.06 eV, 2D carbon nitride (CN) flakes are promising materials for next-generation spintronic devices. However, achieving strong room-temperature (RT) and ambient-stable ferromagnetism (FM) remains a huge challenge. Here, we demonstrate that the strong RT FM with a high Curie temperature (TC) up to ∼400 K and saturation magnetization (Ms) of 2.91 emu/g can be achieved. Besides, the RT FM exhibits excellent air stability, with Ms remaining stable for over 6 months. Through the magneto-optic Kerr effect, Hall device, X-ray magnetic circular dichroism, and magnetic force microscopy measurements, we acquired clear evidence of magnetic behavior and magnetic domain evolutions at room temperature. Electrical and optical measurements confirm that the Co-doped CN retains its semiconductor properties. Detailed structural characterizations confirm that the single-atom Co coordination and nitrogen defects as well as C-C covalent bonds are simultaneously introduced into CN. Density functional theory calculations reveal that introducing C-C bonds causes carrier spin polarization, and spin-polarized carrier-mediated magnetic exchange between adjacent Co atoms leads to long-range magnetic ordering in CN. We believe that our findings provide a strong experimental foundation for the enormous potential of 2D wide bandgap semiconductor spintronic devices.

Half‐Metallic Heusler Alloy/GaN Heterostructure for Semiconductor Spintronics Devices

AbstractBecause spin‐orbit coupling in wurtzite semiconductors is relatively weak compared with that in zincblende ones, the III‐nitride semiconductor GaN is a promising material for high‐performance optical semiconductor spintronic devices such as spin lasers. For the purpose of reducing the operating power of spin lasers, it is necessary to demonstrate highly efficient electrical spin injection from a ferromagnetic material into GaN with a low‐resistance contact. Here, an epitaxial half‐metallic Heusler alloy Co2FeAlxSi1−x(CFAS)/GaN heterostructure is developed by inserting an ultrathin Co layer between the CFAS and GaN. The CFAS/n+‐GaN heterojunctions clearly show tunnel conduction with very small rectification and a low resistance‐area product of ≈3.8 kΩµm2, which is several orders of magnitude smaller than those reported in previous work, at room temperature. Nonlocal spin signals and a Hanle effect curve are observed at low temperatures using lateral spin‐valve devices with the CFAS/n+‐GaN contacts, suggesting pure spin current transport in bulk GaN. The spin transport is observed at temperatures as high as room temperature, with a high spin polarization of 0.2 at a low bias voltage less than 2.0 V. This study is expected to open a path to GaN‐based spintronic devices with highly spin‐polarized and low‐resistance contacts.

Spin transport characteristics and photoelectric properties of magnetic semiconductor NiBr<sub>2</sub> monolayer

Magnetic semiconductor materials have potential applications in spintronic devices. In this work, some nano-device structures based on the magnetic semiconductor NiBr<sub>2</sub> monolayer (NiBr<sub>2</sub>-ML) are designed, their spin-resolved transport and photoelectric properties are studied by using density functional theory combined with non-equilibrium Green’s function method. The results show that both the NiBr<sub>2</sub>-ML PN-junction diodes and sub-3 nanometer PIN-junction field-effect transistors (FETs) exhibit the significant rectification and spin filtering effects in either the armchair or the zigzag direction. The gates can obviously tune the electron transmission of the PIN-junction FETs. The current is significantly suppressed with the increase of gate voltage. In addition, NiBr<sub>2</sub>-ML has a strong response to the blue and green light, thus its phototransistor can generate a strong photocurrent under the irradiation of blue and green light. The research results in this paper reveal the multifunctional characteristics of NiBr<sub>2</sub>-ML, which provides an important reference for the application of nickel-based dihalides in semiconductor spintronic devices and optoelectronic devices.

Triggering phase-coherent spin packets by pulsed electrical spin injection across an Fe/GaAs Schottky barrier

The precise control of spins in semiconductor spintronic devices requires electrical means to generate spin packets with a well-defined initial phase. We demonstrate a pulsed electrical scheme that triggers the spin ensemble phase in a similar way as circularly polarized optical pulses generate phase coherent spin packets. Here, we use fast current pulses to initialize phase coherent spin packets, which are injected across an Fe/GaAs Schottky barrier into $n$-GaAs. By means of time-resolved Faraday rotation, we demonstrate phase coherence by the observation of multiple Larmor precession cycles for current pulse widths down to 500 ps at 17 K. We show that the current pulses are broadened by the charging and discharging time of the Schottky barrier. At high frequencies, the observable spin coherence is limited only by the finite bandwidth of the current pulses, which is of the order of 2 GHz. These results therefore demonstrate that all-electrical injection and phase control of electron spin packets at microwave frequencies is possible in metallic-ferromagnet--semiconductor heterostructures.

Ultralong lifetime for fully photogenerated spin-polarized current in two-dimensional ferromagnetic/nonmagnetic semiconductor heterostructures

Photogenerated fully spin-polarized current has attracted considerable attention in spintronics owing to its high efficiency for information processing but low energy consumption. However, the short spin carrier lifetime of the primary materials significantly impedes the practical application of spin-polarized current and presents a major challenge. Here, on the basis of density functional theory and nonadiabatic molecular dynamics simulations, we design two prototypes of two-dimensional (2D) ferromagnetic/nonmagnetic semiconductor heterostructures ($\mathrm{Cr}{\mathrm{I}}_{3}/\mathrm{Mo}{\mathrm{Se}}_{2}$ and $\mathrm{Cr}{\mathrm{Br}}_{3}/\mathrm{Mo}{\mathrm{S}}_{2}$) with demonstrated ultralong photogenerated spin carrier lifetime. It is shown that the photogenerated spin holes can be effectively injected from the ferromagnetic layer to the nonmagnetic layer, and fully spin-polarized electrons will be confined in the ferromagnetic layer due to their type-II spin-polarized channel and large spin-flip gap. More surprisingly, the photogenerated spin carrier lifetime of these heterostructures is up to the time scale of nanoseconds. Such an ultralong spin carrier lifetime is mainly originated from small nonadiabatic coupling induced by weak spin electron-phonon interaction and slow nuclear velocity. Our results provide insights into the design of 2D semiconductor spintronic devices with an ultralong fully spin-polarized current lifetime.

Application of k $$\cdot $$ p method on band structure of GaAs obtained through joint density-functional theory

The structural and electronic properties of zinc-blende GaAs were calculated within the framework of plane-wave density-functional theory code JDFTx by using Becke 86 in 2D and PBE exchange-correlation functionals from libXC. The standard optimized norm-conserving Vanderbilt pseudopotentials were used to calculate optimized lattice constants, band gap and spin–orbit (SO) split-off parameter. The calculated values of optimized lattice constants and direct band gap are in satisfactory agreement with other published theoretical and experimental findings. By including SO coupling, conduction bands and valence bands were studied under parabolicity to calculate effective masses. The calculated values of effective masses and SO split-off parameter are in satisfactory agreement with most recent findings. This study will be useful for more computational studies related to semiconductor spintronic devices.

Growth of ferromagnetic Co[formula omitted]FeSi films on flexible Ge(111)

For flexible semiconductor spintronic devices, we explore the growth of one of the ferromagnetic full-Heusler alloys, Co2FeSi, on a (111)-oriented pseudo-single-crystalline Ge/polyimide flexible template. Using a low-temperature molecular beam epitaxy (MBE) technique, crystalline Co2FeSi films are obtained on the flexible Ge at less than 80 °C. An MBE-grown Ge layer on the flexible Ge enables to improve magnetic properties of the Co2FeSi films. This work is an important step of high-performance flexible spintronics with ferromagnetic full-Heusler alloys.

Epitaxial growth of Sb-doped Ge layers on ferromagnetic Fe3Si for vertical semiconductor spintronic devices

By combining solid phase epitaxy and molecular beam epitaxy with Sb doping, we can form n-type Ge layers on one of the ferromagnetic Heusler alloys, Fe3Si. Two-dimensional epitaxial growth of the Sb-doped Ge layers can be achieved on the Si-terminated Fe3Si surface at 175 ◦C. Electrical properties of the Au-Ti/Sb-doped Ge/Fe3Si/p-Ge/Al vertical devices indicate that the Sb-doped Ge layer is an n-type semiconductor. We also show a high-quality CoFe/n-Ge/Fe3Si trilayer structure for vertical semiconductor spintronic devices.

Electrical control of ferromagnetism in the n-type ferromagnetic semiconductor (In,Fe)Sb with high Curie temperature

By studying the electrical control of the magnetic properties of ferromagnetic semiconductors (FMSs), we can understand many fundamental aspects of carrier-induced ferromagnetism and explore the possibilities of device applications. Previous experiments on the electrical control of ferromagnetism in Mn-doped FMSs were limited to very low temperatures due to their low Curie temperature (TC). Here, we demonstrate electrical control ferromagnetism at high temperature (210 K) in an electric double layer transistor with an n-type high-TC FMS (In0.89,Fe0.11)Sb thin film channel. A liquid electrolyte is used instead of a conventional solid gate to obtain a large change (40%) of the electron density in the (In0.89,Fe0.11)Sb channel. By applying a small gate voltage (0 → +5 V), TC of the (In,Fe)Sb thin film can be changed by 7 K, indicating that the magnetization as well as ferromagnetic phase transition in (In,Fe)Sb can be controlled at high temperature by the gate electric field despite a small change of electron concentration Δn = 2.2 × 1017 cm−3. Our result paves a way for realizing semiconductor spintronic devices operating at room temperature with low power consumption.

Role of cobalt cations in short range antiferromagnetic Co3O4 nanoparticles: a thermal treatment approach to affecting phonon and magnetic properties

We report the phonon and magnetic properties of various well-stabilized Co3O4 nanoparticles. The net valence in cobalt (II)/(III) cation can be obtained by subtracting the Co2+ ions in tetrahedral interstices and Co3+ ions in the octahedral interstices, respectively, which will possess spatial inhomogeneity of its magnetic moment via Co2+ in tetrahedra and Co3+ in octahedral configurations in the normal spinel structure. Furthermore, the distribution of Co2+/Co3+ governed by various external (magnetic field and temperature) and internal (particle size and slightly distorted CoO6 octahedra) sources, have led to phenomena such as a large redshift of phonon-phonon interaction and short-range magnetic correlation in the inverse spinel structure. The outcome of our study is important in terms of the future development of magnetic semiconductor spintronic devices of Co3O4.

A Two-Dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature

Practical applications of semiconductor spintronic devices necessitate ferromagnetic behavior at or above room temperature. In this paper, we demonstrate a two-dimensional manganese gallium nitride surface structure (MnGaN-2D) which is atomically thin and shows ferromagnetic domain structure at room temperature as measured by spin-resolved scanning tunneling microscopy and spectroscopy. Application of small magnetic fields proves that the observed magnetic domains follow a hysteretic behavior. Two initially oppositely oriented MnGaN-2D domains are rotated into alignment with only 120 mT and remain mostly in alignment at remanence. The measurements are further supported by first-principles theoretical calculations which reveal highly spin-polarized and spin-split surface states with spin polarization of up to 95% for manganese local density of states.

A theoretical calculation of the electronic and magnetic behaviour of C-doped zincblende AlN

Based on structural spin-polarized density functional theory, the electronic and magnetic properties of carbon-doped zincblende AlN were investigated. The calculations were carried out by means of the pseudopotential method employed exactly as implemented in Quantum Espresso code. For 0.625% C-doped AlN, we found a ferromagnetic and half-metallic behaviour. Per supercell, the AlC0.0625N0.9375 has a net magnetic moment of 1.0μβ. The main contribution to the magnetic moment comes from the 2p-C orbitals, with a lesser contribution of the 2p-N and 2p-Al orbitals. Our results suggest that C-doped AlN has potential applications in semiconductor spintronic devices.

Electrical spin transport in cylindrical silicon nanowires with CoFeB/MgO contacts

We examined electrical spin transport in cylindrical silicon nanowires (Si NWs) using the lateral nonlocal spin-valve (NLSV) geometry with CoFeB/MgO contacts. The use of a thin MgO layer as the tunnel barrier in the NLSV devices provided an optimum resistance-area product for spin transport measurements in the Si NWs. A robust NLSV spin signal of over 3.95 kΩ and clear minor loops were observed at 1.8 K in the Si NWs heavily doped with phosphorous. Furthermore, the NLSV magnetoresistance was strongly influenced by the local magnetizations resulting from the ferromagnetic (FM) electrodes being attached to the cylindrically shaped Si NW, with these magnetizations differing from those of bulk ferromagnets. These local micro-magnetic configurations of the FM electrodes led to intriguing NLSV spin signals associated with the Hanle effect. Our study of spin transport in the heavily doped Si NWs provides a sound basis for developing applications of nanoscale semiconductor spintronic devices.

Spin-dependent transport in GaAs nanowire-based devices

The transport properties of GaAs nanowire (NW)-based devices are investigated using non-equilibrium Green’s function technique combined with density functional theory (DFT). Two types of NW-based devices are studied, which include the electrodes that are made of half-metallic GaMnAs NWs grown along the [0001] direction, and the scattering region that consists of a tunneling structure (GaMnAs/GaAs/GaMnAs) or a conducting structure (GaMnAs/GaAs:Be/GaMnAs), respectively. The proposed nanostructures both exhibit robust transport properties including spin-filtering, negative differential resistance (NDR) and giant magnetoresistance (GMR) effects, which are further analyzed with carrier channels and transmission spectra. These structures imply potential applications for low dimensional semiconductor spintronic devices such as spin valve.

Enhancing s, p–d exchange interactions at room temperature by carrier doping in single crystalline Co0.4Zn0.6O epitaxial films

Magnetic doping of semiconductors has been actively pursued because of their potential applications in the spintronic devices. Central to these efforts is a drive to control the mutual interactions between their magnetic properties (supported by d electrons of the magnetic ions) and their semiconductor properties (supported by s and/or p electrons) at room temperature (RT). Despite the long, intensive efforts, the experimental evidence of thermally robust s, p–d coupling in a semiconductor remains scarce and controversial. Here, we report the enhancement of RT ferromagnetic s, p–d exchange interaction by means of carrier doping in single crystalline Co0.4Zn0.6O epitaxial films with a high Co concentration. Magneto-transport measurements reveal that spin-polarized conducting carriers are produced at RT and are increased with the carrier density through Ga3+ doping, owing to the s, p–d coupling between Ga (4s), O (2p), and Co (3d) orbitals. With the ability to individually control carrier density and magnetic doping, single crystalline Ga(Co, Zn)O films can lay a solid foundation for the development of practical semiconductor spintronic devices operable at RT.

Spin Diffusion in the Finite Magnetic Heterojunction

Spin diffusion in the finite magnetic heterojunction was explored considering the spin dependent conductivity. In organic semiconductor spintronic devices, the up-spin and down-spin polarons have different density once spin injection happens from ferromagnetic electrodes into organic semiconductors. The difference results in the spin dependent conductivity. The calculations show that the spin injection efficiency is dependent on the spin dependent conductivity and the size of the layers. The spin dependent conductivity has great influence on the spin injection efficiency in the finite magnetic heterojunction, when the spin polarization of the organic semiconductors is moderate.

Graphene and monolayer transition-metal dichalcogenides: properties and devices

Abstract

Synthesis of quasi-core–shell Co-doped ZnO/graphene nanoparticles

Co-doped ZnO/graphene quasi-core–shell hybrid nanoparticles with a reduced graphene oxide (RGO) shell around the Zn0.98Co0.02O core are synthesized by a facile chemical process. X-ray diffraction (XRD) analysis reveals that nanoparticles consist of wurtzite ZnCoO and graphene without the impurity phase. The successful doping of Co ions into the ZnO host is illustrated by the XRD lines shifting and XPS spectra of Co ions. High-resolution transmission electron microscopy (HRTEM) image shows that the Zn0.98Co0.02O is fully covered with the RGO shell, verifying that Zn0.98Co0.02O/RGO hybrid structure is quasi-core–shell structure. Raman spectra indicate that graphene oxide is reduced to graphene successfully. The red-shift of the characteristic E2H peak and 2D peak of Zn0.98Co0.02O/RGO imply the electron interaction between Zn0.98Co0.02O and RGO. Magnetization measurement demonstrates that the Zn0.98Co0.02O/RGO nanoparticles present canted antiferromagnetic behavior, which has a promising application in semiconductor spintronic devices.