Sarmita Majumder

Residual Stress, Defects, and Electrical Properties of Epitaxial Copper Growth on GaAs

The residual stress in Cu films epitaxially grown on GaAs(001) single crystalline substrates has been compared to polycrystalline Cu growth on (111) textured Au substrates, both grown via the same galvanostatic electrodeposition process. Optimized Cu/GaAs epitaxial nucleation and growth was obtained with a substrate pre-etch in dilute ammonium-hydroxide followed by electrodeposition in a pure Cu sulfate aqueous electrolyte at elevated temperatures. The resulting films are single crystalline, and strain relaxed, as measured by X-ray and electron diffraction. The (001) surfaces developed square pyramidal facets that increase in average size with increasing current density. In situ wafer curvature measurements found that the Cu/GaAs films followed the commonly observed change from compressive to tensile and finally to compressive stress (-200 MPa) that quickly relaxed once growth was interrupted. In contrast, polycrystalline Cu/Au films developed a smaller and constant tensile stress (+10 MPa) that relaxed more slowly. Given the similar growth rates of the two systems, differences in residual stress are related to differences in the density and nature of the coalescence boundaries and associated surface adatom processes. The resulting electrical properties of Cu/GaAs diodes show an interfacial capacitance that is consistent with interdiffusion and the reaction layer detected by electron microscopy.

Epitaxial Fe x Ni1 − x Thin Film Contacts to GaAs via Electrochemistry

bSandia Livermore Laboratories, Livermore, California 94551, USA The galvanostatic electrodeposition of epitaxial FexNi1�x films on n-GaAs001 substrates from aqueous metal ammonium sulfate solutions is reported. Structural measurements using X-ray diffraction and transmission electron microscopy indicate that the films have single crystalline or highly oriented body-centered cubic bcc or face-centered cubic fcc structure at Fe and Ni-rich compositions, respectively. The preparation of the substrate surface via ammonium hydroxide as well as the usage of an ammonium sulfate electrolyte buffer facilitates the initial nucleation of aligned metal islands on GaAs. The ratio of the Fe atomic concentration in the fcc films to that in the electrolyte is close to unity 1.1, whereas a preferential incorporation occurs for the bcc phase such that this ratio increases linearly with Fe atomic composition at a rate of 3.7. bcc FexNi1�x/GaAs diodes have uniform Schottky barrier heights as measured by current-voltage measurements independent of Fe composition consistent with a high interfacial state density. The observed magnetic properties of FexNi1�x are consistent with single crystalline material with distributed inhomogeneities. The epitaxy via electrodeposition suggests the importance of ammonium sulfate for technical applications.

Large Magnetoresistance at Room Temperature in Ferromagnet/Topological Insulator Contacts

We report magnetoresistance for current flow through iron/topological insulator (Fe/TI) and Fe/evaporated-oxide/TI contacts when a magnetic field is used to initially orient the magnetic alignment of the incorporated ferromagnetic Fe bar, at temperatures ranging from 100 K to room temperature. This magnetoresistance is associated with the relative orientation of the Fe bar magnetization and spin-polarization of electrons moving on the surface of the TI with helical spin-momentum locking. The magnitude of the observed magnetoresistance is relatively large compared to that observed in prior work.

Epitaxial Fe on free-standing GaAs nanowires

Epitaxial Fe contacts have been fabricated onto the top half of free-standing, Te-doped GaAs nanowires (NWs) via electrodeposition. Electrical isolation from the substrate via a polymeric layer enabled the measurement of electrical transport through individual wires. Using a fixed probe within a scanning electron microscope, an average metal-semiconductor diode barrier height of 0.69 ± 0.03 eV (ideality factor 1.48 ± 0.02) was found.

Interfacial reactions at Fe/topological insulator spin contacts

The authors study the composition and abruptness of the interfacial layers that form during deposition and patterning of a ferromagnet, Fe on a topological insulator (TI), Bi2Se3, Bi2Te3, and SiOx/Bi2Te3. Such structures are potentially useful for spintronics. Cross-sectional transmission electron microscopy, including interfacial elemental mapping, confirms that Fe reacts with Bi2Se3 near room temperature, forming an abrupt 5 nm thick FeSe0.92 single crystalline binary phase, predominantly (001) oriented, with lattice fringe spacing of 0.55 nm. In contrast, Fe/Bi2Te3 forms a polycrystalline Fe/TI interfacial alloy that can be prevented by the addition of an evaporated SiOx separating Fe from the TI.

Hanle measurements of electrodeposited Fe/GaAs spin tunnel contacts

We report spin transport in electrodeposited Fe/n-GaAs tunnel diodes via three-terminal Hanle measurements. For temperatures between 20 K and 150 K, the spin resistance was up to 20 times higher than expected from theoretical calculations and 1000 times larger compared to a vacuum-deposited counterpart. This higher spin resistance was correlated with a higher contact resistance, and a higher concentration of oxygen impurities in the electrodeposited Fe film and interface, as detected via x-ray photoelectron and Auger spectroscopies, and inferred from Fe film nucleation rates. These results can be explained via a small effective tunnel-contact area of 5%, but extra spin filtering via interfacial states or magnetic oxide layers cannot be ruled out. The spin diffusion times (8.5 ± 0.4 ns to 1.8 ± 0.4 ns, for 20 K to 150 K) extracted from Lorentzian fits were in good agreement with values obtained from earlier 4-terminal Hanle measurements (7.8 ± 0.4 ns to 3.2 ± 0.4 ns, for 25 K to 77 K), both 10 times slower than reported vacuum-deposited contacts.

Inhomogeneous magnetization processes in electrodeposited iron thin films on GaAs

Electrodeposited thin films of Fe on GaAs with (100), (110), and (111) orientation are composed of quasisingle crystalline mosaic nanoregions, probably because of defects arising from the coalescence of separately nucleated epitaxial regions. The epitaxy is promoted by the use of an ammonium sulfate electrolyte buffer and a substrate oxide etch in dilute ammonium hydroxide. Samples with these different orientations have been studied using x rays, transmission electron microscopy, and magnetization measurements. The observations of magnetic hysteresis with minor loops that extend outside the major loop are among the interesting features of the electrodeposited films. The major hysteresis loops do not correspond to models with uniform rotation plus domain wall motion. The evident uniaxial anisotropy preferring the ⟨100⟩ axes does not explain the results unless it is added inhomogenously into the modeling. For the (110) substrates, the inhomogeneous distribution of uniaxial anisotropy generates patterns of ...

Detection of current induced spin polarization in epitaxial Bi2Te3 thin film

We electrically detect charge current induced spin polarization on the surface of a molecular beam epitaxy grown Bi2Te3 thin film in a two-terminal device with a ferromagnetic MgO/Fe contact and a nonmagnetic Ti/Au contact. The two-point resistance, measured in an applied magnetic field, shows a hysteresis tracking the magnetization of Fe. A theoretical estimate is obtained for the change in resistance on reversing the magnetization direction of Fe from coupled spin-charge transport equations based on the quantum kinetic theory. The order of magnitude and the sign of the hysteresis are consistent with the spin-polarized surface state of Bi2Te3.