J. Borme

Direct observation of electron confinement in epitaxial graphene nanoislands.

One leading question for the application of graphene in nanoelectronics is how electronic properties depend on the size at the nanoscale. Direct observation of the quantized electronic states is central to conveying the relationship between electronic structures and local geometry. Scanning tunneling spectroscopy was used to measure differential conductance dI/dV patterns of nanometer-size graphene islands on an Ir(111) surface. Energy-resolved dI/dV maps clearly show a spatial modulation, indicating a modulated local density of states due to quantum confinement, which is unaffected by the edge configuration. We establish the energy dispersion relation with the quantized electron wave vector obtained from a Fourier analysis of dI/dV maps. The nanoislands preserve the Dirac Fermion properties with a reduced Fermi velocity.

Introduction of Si PERC Rear Contacting Designto Boost Efficiency of Cu(In,Ga)Se2 Solar Cells

Recently, Cu(In,Ga)Se2 (CIGS) solar cells have achieved 21% world-record efficiency, partly due to the introduction of a postdeposition potassium treatment to improve the front interface of CIGS absorber layers. However, as high-efficiency CIGS solar cells essentially require long diffusion lengths, the highly recombinative rear of these devices also deserves attention. In this paper, an Al2O3 rear surface passivation layer with nanosized local point contacts is studied to reduce recombination at the standard Mo/CIGS rear interface. First, passivation layers with well-controlled grids of nanosized point openings are established by use of electron beam lithography. Next, rear-passivated CIGS solar cells with 240-nm-thick absorber layers are fabricated as study devices. These cells show an increase in open-circuit voltage (+57 mV), short-circuit current (+3.8 mA/cm2), and fill factor [9.5% (abs.)], compared with corresponding unpassivated reference cells, mainly due to improvements in rear surface passivation and rear internal reflection. Finally, solar cell capacitance simulator (SCAPS) modeling is used to calculate the effect of reduced back contact recombination on high-efficiency solar cells with standard absorber layer thickness. The modeling shows that up to 50-mV increase in open-circuit voltage is anticipated.

Integration of Magnetoresistive Biochips on a CMOS Circuit

Since 2006, fully scalable matrix-based magnetoresistive biochips have been proposed. This integration was initially achieved with thin film switching devices and moved to complementary metal-oxide-semiconductor (CMOS) switching devices and electronics. In this paper, a new microfabrication process is proposed to integrate magnetoresistive sensors on a small CMOS chip (4 mm2). This chip includes a current generator, multiplexers, and a diode in series with a spin valve as matrix element. In this configuration, it is shown that the fabricated spin-valves have similar magnetic characteristics when compared to standalone spin valves. This validates the successfulness of the developed microfabrication process. The noise of each matrix element is further characterized and compared to the noise of a standalone spin valve and a portable electronic platform designed to perform biological assays. Although the noise is still higher, the spin valve integrated on the CMOS chip enables an increase in density and compactness of the measuring electronics.

Graphene field-effect transistor array with integrated electrolytic gates scaled to 200 mm

Ten years have passed since the beginning of graphene research. In this period we have witnessed breakthroughs both in fundamental and applied research. However, the development of graphene devices for mass production has not yet reached the same level of progress. The architecture of graphene field-effect transistors (FET) has not significantly changed, and the integration of devices at the wafer scale has generally not been sought. Currently, whenever an electrolyte-gated FET (EGFET) is used, an external, cumbersome, out-of-plane gate electrode is required. Here, an alternative architecture for graphene EGFET is presented. In this architecture, source, drain, and gate are in the same plane, eliminating the need for an external gate electrode and the use of an additional reservoir to confine the electrolyte inside the transistor active zone. This planar structure with an integrated gate allows for wafer-scale fabrication of high-performance graphene EGFETs, with carrier mobility up to 1800 cm(2) V(-1) s(-1). As a proof-of principle, a chemical sensor was achieved. It is shown that the sensor can discriminate between saline solutions of different concentrations. The proposed architecture will facilitate the mass production of graphene sensors, materializing the potential of previous achievements in fundamental and applied graphene research.

Scanning tunneling spectroscopy of epitaxial graphene nanoisland on Ir(111).

Scanning tunneling spectroscopy (STS) was used to measure local differential conductance (dI/dV) spectra on nanometer-size graphene islands on an Ir(111) surface. Energy resolved dI/dV maps clearly show a spatial modulation, which we ascribe to a modulated local density of states due to quantum confinement. STS near graphene edges indicates a position dependence of the dI/dV signals, which suggests a reduced density of states near the edges of graphene islands on Ir(111).

Magnetic Response and Spin Polarization of Bulk Cr Tips for In-Field Spin-Polarized Scanning Tunneling Microscopy

The magnetic properties of bulk Cr tips have been investigated by spin-polarized scanning tunneling spectroscopy (SP-STS). To extract the properties of the Cr tips, we performed low-temperature SP-STS measurements on a well-known model system: nanometric Co islands on Cu(111). Our experiments indicate the existence of uncompensated magnetic moments at the apex of the Cr tips, which rotate in the direction of the applied vertical magnetic field and become aligned with it at approximately 2 T. We extracted a tip spin polarization of 45% at the Fermi energy. We showed that the tip spin polarization can change with a modification of the tip apex.

High power and low critical current density spin transfer torque nano-oscillators using MgO barriers with intermediate thickness

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωμm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (Pout) in the range of 1–40 nW. Here, Pout values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωμm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (JSTT) was observed, without any further change in the magnetic configuration.

InGaZnO Thin-Film-Transistor-Based Four-Quadrant High-Gain Analog Multiplier on Glass

This letter presents a novel high-gain four-quadrant analog multiplier using only n-type enhancement indium-gallium-zinc-oxide thin-film-transistors. The proposed circuit improves the gain by using an active load with positive feedback. A Gilbert cell with a diode-connected load is also presented for comparison purposes. Both circuits were fabricated on glass at low temperature (200°C) and were successfully characterized at room temperature under normal ambient conditions, with a power supply of 15 V and 4-pF capacitive load. The novel circuit has shown a gain improvement of 7.2 dB over the Gilbert cell with the diode-connected load. Static linearity response, total harmonic distortion, frequency response, and power consumption are reported. This circuit is an important signal processing building block in large-area sensing and readout systems, specially if data communication is involved.

Impact of MgO Thickness on the Performance of Spin-Transfer Torque Nano-Oscillators

A magnetic tunnel junction (MTJ) stack incorporating an MgO wedge was deposited over a 200 mm wafer and nanofabricated into circular nanopillars with a diameter of 200 nm. Due to the variable MgO thickness, we could obtain MTJ nanopillars with resistance x area (RA) ranging from less than 1 Ωμm2 up to 15 Ωμm2 and tunnel magnetoresistance ratios up to ~100%. It was observed that the ferromagnetic coupling (HF) displayed by the transfer curves increases for smaller values of RA. This variation is attributed to the orange-peel coupling, which is caused by the MgO roughness, with a larger impact for thinner barriers. The RF output of the nanopillars caused by the spin-transfer torque-induced magnetic precession of the free layer was measured as a function of bias current and external magnetic fields for nanopillars with different RA values. Results concerning typical devices in the thin MgO region (1.3 Ωμm2) and in a thicker MgO region (2.7 Ωμm2) are shown to illustrate the dependence found. It was observed that the higher output power (~300 nW) and the smaller linewidth (~90 MHz) could be obtained with the higher RA region. These results suggest that there must be an optimum RA value for the production of nano-oscillator devices, which depends on the tradeoffs between the need for large breakdown voltages and strong spin polarization of the bias current (favored for larger RA values) and between the need of endurance under large current densities (favored for smaller RA values). Provided the breakdown voltage of the MgO barrrier can be made large enough, nanopillars based on MTJ stacks with an intermediate RA can provide a solution for the production of large output power spin-transfer nano-oscillators.

Magnetoresistive Sensors for Surface Scanning

This chapter provides an overview on several techniques used for surface imaging, including SQUIDs, Hall-effect sensors, Giant magnetoimpedance sensors, and magnetoresistive (MR) sensors. Among all magnetic field sensors, only SQUIDs and MR devices have the potential to localize buried and non-visual field sources (such as defects in integrated circuits or magnetic field sources in biological environments. In particular, we describe how MR sensors have been used with advantage for integrated circuit (IC) mapping, with resolution below 500 nm and sensitivity to detect currents as low as 50 nA and have been used for many applications requiring low magnetic field detection. Challenges and experimental considerations on integration of MR sensors on a commercial analysis tool are provided here. Examples obtained with real devices demonstrate how Scanning Magnetic Microscopy has become an established failure analysis technique for visualizing current paths in microelectronic devices.