Subir Parui

Temperature dependent transport characteristics of graphene/n-Si diodes

Realizing an optimal Schottky interface of graphene on Si is challenging, as the electrical transport strongly depends on the graphene quality and the fabrication processes. Such interfaces are of increasing research interest for integration in diverse electronic devices as they are thermally and chemically stable in all environments, unlike standard metal/semiconductor interfaces. We fabricate such interfaces with n-type Si at ambient conditions and find their electrical characteristics to be highly rectifying, with minimal reverse leakage current (<10−10 A) and rectification of more than 106. We extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83 eV for the CVD graphene devices at room temperature. The temperature dependent electrical characteristics suggest the influence of inhomogeneities at the graphene/n-Si interface. A quantitative analysis of the inhomogeneity in Schottky barrier heights is presented using the potential fluctuation model proposed by Werner and Guttler.

A molecular spin-photovoltaic device

A spin-valve solar cell Electronic spin currents can be measured with a spin valve—a device that injects charge carriers from one ferromagnetic electrode to another through a semiconductor layer. Some organic semiconductors can have long spin-carrier lifetimes and can also generate charge carriers through the photovoltaic effect. Sun et al. fabricated a spin valve based on C60 and showed that the spin current could be modulated by the photocurrent. At certain light intensities, the sign of the photocurrent could be changed using an applied magnetic field, an effect that could potentially be harnessed for sensing applications. Science, this issue p. 677 A spin valve with a C60 semiconductor layer could integrate charge carriers generated through its photovoltaic response. We fabricated a C60 fullerene–based molecular spin-photovoltaic device that integrates a photovoltaic response with the spin transport across the molecular layer. The photovoltaic response can be modified under the application of a small magnetic field, with a magnetophotovoltage of up to 5% at room temperature. Device functionalities include a magnetic current inverter and the presence of diverging magnetocurrent at certain illumination levels that could be useful for sensing. Completely spin-polarized currents can be created by balancing the external partially spin-polarized injection with the photogenerated carriers.

Probing electron transport across a LSMO/Nb:STO heterointerface at the nanoscale

We investigate electron transport across a complex oxide heterointerface of La0.67Sr0.33MnO3 (LSMO) on Nb:SrTiO3 (Nb:STO) at different temperatures. For this, we employ the conventional current-voltage method as well as the technique of ballistic electron emission microscopy (BEEM), which can probe lateral inhomogeneities in transport at the nanometer scale. From current-voltage measurements, we find that the Schottky barrier height (SBH) at the LSMO/Nb:STO interface decreases at low temperatures accompanied by a larger than unity ideality factor. This is ascribed to the tunneling dominated transport caused by the narrowing of the depletion width at the interface. However, BEEM studies of such unbiased interfaces do not exhibit SBH lowering at low temperatures, implying that this is triggered by the modification of the interface due to an applied bias and is not an intrinsic property of the interface. Interestingly, the SBH at the nanoscale, as extracted from BEEM studies, at different locations in the device is found to be spatially homogeneous and similar both at room temperature and at low temperatures. Our results highlight the application of BEEM in characterizing electron transport and its homogeneity at such unbiased complex oxide interfaces and yield insights into the origin of the temperature dependence of the SBH at biased interfaces. DOI: 10.1103/PhysRevB.87.085116

Hot electron transport in a strongly correlated transition-metal oxide

Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La0.7Sr0.3MnO3 (LSMO)- Nb-doped SrTiO3 (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at −1.9 V, increasing to 2.02 ± 0.16 u.c. at −1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

Probing hot electron transport across an epitaxial Schottky interface of SrRuO3/Nb:SrTiO3

SrRuO3 (SRO), a conducting transition metal oxide, is commonly used for engineering domains in BiFeO3. Oxide devices can be envisioned by integrating SRO with an oxide semiconductor as Nb doped SrTiO3 (Nb:STO). Using a three-terminal device configuration, we study vertical transport in a SRO/Nb:STO device at the nanoscale and find local differences in transport which originate due to the high selectivity of SRO growth on the underlying surface terminations in Nb:STO. This causes a change in the interface energy band characteristics and is explained by the differences in the spatial distribution of the interface-dipoles at the local Schottky interface.

Hot electron transmission in metals using epitaxial NiSi2/n-Si(111) interfaces

We have investigated hot electron transmission across epitaxial metal-disilicide/n-Si(111) interfaces using ballistic electron emission microscopy (BEEM). Different crystal orientations of epitaxial NiSi2 were grown on a Si(111) substrate using molecular beam epitaxy. The presence of different interfaces of NiSi2 on Si(111) were confirmed by high resolution transmission electron microscopy. Electrical transport measurements reveal a clear rectifying Schottky interface with a barrier height of 0.69 eV. However, using BEEM, three different regions with different transmissions and Schottky barrier heights of 0.65 eV, 0.78 eV, and 0.71 eV are found. The addition of a thin Ni film on the NiSi2 layer strongly reduces the transmission in all the three regions and interestingly, almost equalizes the transmission across them.

Energy Level Alignment at Metal/Solution-Processed Organic Semiconductor Interfaces

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in-device hot-electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n-type polymeric semiconductors. This study measures and compares metal/solution-processed electron-transporting polymer interface energy barriers by in-device hot-electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in-device hot-electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.

Spin doping using transition metal phthalocyanine molecules

Molecular spins have become key enablers for exploring magnetic interactions, quantum information processes and many-body effects in metals. Metal-organic molecules, in particular, let the spin state of the core metal ion to be modified according to its organic environment, allowing localized magnetic moments to emerge as functional entities with radically different properties from its simple atomic counterparts. Here, using and preserving the integrity of transition metal phthalocyanine high-spin complexes, we demonstrate the magnetic doping of gold thin films, effectively creating a new ground state. We demonstrate it by electrical transport measurements that are sensitive to the scattering of itinerant electrons with magnetic impurities, such as Kondo effect and weak antilocalization. Our work expands in a simple and powerful way the classes of materials that can be used as magnetic dopants, opening a new channel to couple the wide range of molecular properties with spin phenomena at a functional scale.

Reliable determination of the Cu/n-Si Schottky barrier height by using in-device hot-electron spectroscopy

We show the operation of a Cu/Al_2O_3/Cu/n-Si hot-electron transistor for the straightforward determination of a metal/semiconductor energy barrier height even at temperatures below carrier-freeze out in the semiconductor. The hot-electron spectroscopy measurements return a fairly temperature independent value for the Cu/n-Si barrier of 0.66

Frequency driven inversion of tunnel magnetoimpedance and observation of positive tunnel magnetocapacitance in magnetic tunnel junctions

\pm