Ezekiel Johnston-Halperin

Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Graphenes success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.

A 160-kilobit molecular electronic memory patterned at 10 11 bits per square centimetre

The primary metric for gauging progress in the various semiconductor integrated circuit technologies is the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. Modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 μm2. Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies in 2013 have ‘no known solution’. Promising ingredients for advances in integrated circuit technology are nanowires, molecular electronics and defect-tolerant architectures, as demonstrated by reports of single devices and small circuits. Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. Here we describe a 160,000-bit molecular electronic memory circuit, fabricated at a density of 1011 bits cm-2 (pitch 33 nm; memory cell size 0.0011 μm2), that is, roughly analogous to the dimensions of a DRAM circuit projected to be available by 2020. A monolayer of bistable, [2]rotaxane molecules served as the data storage elements. Although the circuit has large numbers of defects, those defects could be readily identified through electronic testing and isolated using software coding. The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.

A strong ferroelectric ferromagnet created by means of spin–lattice coupling

Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials that could give rise to new technologies in which the low power and high speed of field-effect electronics are combined with the permanence and routability of voltage-controlled ferromagnetism. Furthermore, the properties of the few compounds that simultaneously exhibit these phenomena are insignificant in comparison with those of useful ferroelectrics or ferromagnets: their spontaneous polarizations or magnetizations are smaller by a factor of 1,000 or more. The same holds for magnetic- or electric-field-induced multiferroics. Owing to the weak properties of single-phase multiferroics, composite and multilayer approaches involving strain-coupled piezoelectric and magnetostrictive components are the closest to application today. Recently, however, a new route to ferroelectric ferromagnets was proposed by which magnetically ordered insulators that are neither ferroelectric nor ferromagnetic are transformed into ferroelectric ferromagnets using a single control parameter, strain. The system targeted, EuTiO3, was predicted to exhibit strong ferromagnetism (spontaneous magnetization, ∼7 Bohr magnetons per Eu) and strong ferroelectricity (spontaneous polarization, ∼10 µC cm−2) simultaneously under large biaxial compressive strain. These values are orders of magnitude higher than those of any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression we turned to tensile strain. Here we show both experimentally and theoretically the emergence of a multiferroic state under biaxial tension with the unexpected benefit that even lower strains are required, thereby allowing thicker high-quality crystalline films. This realization of a strong ferromagnetic ferroelectric points the way to high-temperature manifestations of this spin–lattice coupling mechanism. Our work demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics.

Highly enhanced Curie temperature in low-temperature annealed [Ga,Mn]As epilayers

We report Curie temperatures up to 150 K in annealed Ga1−xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) suggest that the higher Curie temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material, suggesting that the free surface of Ga1−xMnxAs epilayers may be important in determining their physical properties.

(Ga,Mn)As as a digital ferromagnetic heterostructure

(Ga,Mn)As digital ferromagnetic heterostructures are grown by incorporating submonolayer planes of MnAs into GaAs using molecular beam epitaxy. Structural and magnetic measurements indicate single-crystalline superlattice structure and ferromagnetic order with Curie temperatures (TC) up to 50 K. By varying the spacing between neighboring Mn layers, we observe that TC initially decreases with increasing spacer thickness, followed by a regime with weak dependence on the spacer thickness. The persistence of ferromagnetism for interlayer spacings of at least 200 ML (∼560 A) suggests that the individual Mn layers are ferromagnetic.

Spin-polarized Zener tunneling in (Ga, Mn)As

We investigate spin-polarized inter-band tunneling through measurement of a (Ga,Mn)As based Zener tunnel diode. By placing the diode under reverse bias, electron spin polarization is transferred from the valence band of p-type (Ga,Mn)As to the conduction band of an adjacent n-GaAs layer. The resulting current is monitored by injection into a quantum well light emitting diode whose electroluminescence polarization is found to track the magnetization of the (Ga,Mn)As layer as a function of both temperature and magnetic field.

Giant spin Seebeck effect in a non-magnetic material

The spin Seebeck effect is observed when a thermal gradient applied to a spin-polarized material leads to a spatially varying transverse spin current in an adjacent non-spin-polarized material, where it gets converted into a measurable voltage. It has been previously observed with a magnitude of microvolts per kelvin in magnetically ordered materials, ferromagnetic metals, semiconductors and insulators. Here we describe a signal in a non-magnetic semiconductor (InSb) that has the hallmarks of being produced by the spin Seebeck effect, but is three orders of magnitude larger (millivolts per kelvin). We refer to the phenomenon that produces it as the giant spin Seebeck effect. Quantizing magnetic fields spin-polarize conduction electrons in semiconductors by means of Zeeman splitting, which spin–orbit coupling amplifies by a factor of ∼25 in InSb. We propose that the giant spin Seebeck effect is mediated by phonon–electron drag, which changes the electrons’ momentum and directly modifies the spin-splitting energy through spin–orbit interactions. Owing to the simultaneously strong phonon–electron drag and spin–orbit coupling in InSb, the magnitude of the giant spin Seebeck voltage is comparable to the largest known classical thermopower values.

Theory of semiconductor magnetic bipolar transistors

Bipolar transistors with a ferromagnetic base are shown theoretically to have the potential to generate almost 100% spin-polarized current injection into nonmagnetic semiconductors. Optical control of ferromagnetism and spin splitting in the base can lead to either long-lived or ultrafast switching behavior. Fringe field control of the base magnetization could be used for information transfer between metallic magnetoelectronics and conventional semiconducting electronics.

Fabrication of conducting Si nanowire arrays

The recent development of the superlattice nanowire pattern transfer technique allows for the fabrication of arrays of nanowires at a diameter, pitch, aspect ratio, and regularity beyond competing approaches. Here, we report the fabrication of conducting Si nanowire arrays with wire widths and pitches of 10–20 and 40–50 nm, respectively, and resistivity values comparable to the bulk through the selection of appropriate silicon-on-insulator substrates, careful reactive-ion etching, and spin-on glass doping. These results promise the realization of interesting nanoelectronic circuits and devices, including chemical and biological sensors, nanoscale mosaics for electronics, and ultradense field-effect transistor arrays.

Anisotropic electrical spin injection in ferromagnetic semiconductor heterostructures

A fourteen-fold anisotropy in the spin transport efficiency parallel and perpendicular to the charge transport is observed in a vertically biased (Ga, Mn)As-based spin-polarized light emitting diode. The spin polarization is determined by measuring the polarization of electroluminescence from an (In, Ga)As quantum well placed a distance d (20–420 nm) below the p-type ferromagnetic (Ga, Mn)As contact. In addition, a monotonic increase (from 0.5% to 7%) in the polarization is measured as d decreases for collection parallel to the growth direction, while the in-plane polarization from the perpendicular direction (∼0.5%) remains unchanged.