Structural and Magnetic Properties of Epitaxial Fe and Ni Thin Films Grown on n-AlGaAs(001) Using Electrodeposition

Abstract

We report epitaxial growth of body-centered cubic Fe and face-centered cubic Ni thin films on n-type AlGaAs͑001͒ substrates using electrochemical deposition with the cubic axes of Fe oriented parallel to the substrate lattice while the Ni is rotated in-plane 45°. The magnetic properties in either case are dominated by the fourfold crystalline anisotropy, a high remanence, and saturation magnetization values similar to bulk properties. The growth of such ferromagnetic contacts on AlGaAs might find applications in magneto-electronics.The discovery of spin-dependent transport 1-3 has recently stimulated wide interest in designs for memory and logic devices. In particular, the proposal of a spin-polarized field effect transistor 4 has triggered great attention on the issue of integration of ferromagnetic materials with semiconductors. Devices of this kind would require the growth of epitaxial ferromagnetic films with well-defined crystallographic, magnetic, and interface properties. Electrochemical deposition ͑ECD͒ in this regard offers important advantages as it is a low-energy, room-temperature deposition process which can limit the interdiffusion between the film and the substrate, thus enabling the growth of high-quality epitaxial layers. 5,6 In this article, we report on the electrochemical epitaxial growth of Fe and Ni films from sulfate electrolytes onto n-AlGaAs and on the structural and magnetic properties of the grown films. ExperimentalThe substrates were single-crystalline, n-doped GaAs͑001͒ on which a 200 nm thick Al 0.08 Ga 0.92 As:Si layer was grown by molecular beam epitaxy with 1 ϫ 10 18 cm −3 n-doping. The electrical contact to the substrate used for ECD was made using a Ga-In eutectic on the back side of the substrates. Prior to deposition, the substrates were cleaned in a 10% ammonia solution for 2 min and subsequently rinsed in deionized water prior to deposition. The surface area exposed for deposition typically was about 20 mm 2 .Electrodeposition was carried out under galvanostatic control ͑current density of 2.5 mA/cm 2 for Fe and 3.5 mA/cm 2 for Ni, using 0.1 M sulfate solutions with pH 2.5 in both cases͒ at 25°C in a prismatic cell with vertical parallel electrodes and a calomel reference electrode using a potentiostat EG&G, model 273A. In order to reduce the effect of any resistance variation of the back contact to the substrate, galvanostatic deposition was preferred over potentiostatic control. Graphite was used as the counter electrode. After deposition, the samples were rinsed with deionized water and blow dried. Film thicknesses are 50 nm for Ni and about 90 nm for Fe. These values were estimated from X-ray reflectivity for Ni and from saturation magnetic moments for Fe. Fe films were too rough to allow thickness measurements based on X-ray reflectivity. Hysteresis loops and angular remanence curves were measured using a Digital Measurement Systems vibrating sample magnetometer ͑VSM, DMS model 990͒. In-plane angular ferromagnetic resonance ͑FMR͒ was measured at 25.6 GHz with the dc fi...

Epitaxial Fe[sub x]Ni[sub 1−x] Thin Film Contacts to GaAs via Electrochemistry

“…Plan-view transmission electron microscopy ͑TEM͒ investigations found that nucleation and coalescence of Fe films electrodeposited under similar condition occurs within 1 s. 18 The linear thickness vs time behavior reported here is that expected for a galvanostatic process. 29 The deviation from this linear behavior at longer times is likely related to the depletion of Fe ions and the…”

“…17 There is also a recent report of the growth of epitaxial Fe and Ni on AlGaAs. 18 In all cases these metals were electrodeposited from conventional, highly acidic electrolyte to inhibit the formation of oxides.…”

“…3,26 Pure Ni ͑fcc͒ is reported to grow with a reduced preference for ͑111͒ on ͑001͒ GaAs 17 and with a preferred ͑111͒ oriented on GaAs͑011͒ surfaces 16 but the most single crystalline epitaxy is reported when Ni is introduced with a second metal, e.g., NiCu as mentioned earlier 14 or apparently on AlGaAs substrates. 18 To reduce the lattice mismatch, these films grow with their ͗110͘ directions aligned with the GaAs͗100͘. The iron-rich Fe x N 1−x ͑x ജ 0.9͒ alloy has been reported to grow via electrodeposition on GaAs͑110͒ substrates in an acidic sulfate electrolyte, but with an unknown crystallinity and orientation.…”

Role of Hydrogen Evolution during Epitaxial Electrodeposition of Fe on GaAs

“…Electrodeposition of epitaxial Fe films on GaAs has been proposed as an efficient alternative to physical deposition methods. [8][9][10][11][12][13][14] The low-energy character of the electrodeposition technique means that interface intermixing 15 is largely avoided. In general, Fe deposition on GaAs proceeds via island nucleation, coalescence, and subsequent film growth, which for electrodeposition is typically at rates of 2-6 nm/s, perhaps 100 times faster than MBE.…”

“…Furthermore, at the point of coalescence, the thickness lies above 20 nm for electrodeposition, 10 in stark contrast to about 3 atomic layers for MBE. 16 Therefore, most studies on electrodeposited Fe on GaAs [8][9][10][11][12][13] have focused on the fabrication of films of several 100 nm, and the structural, magnetic 12 and spin transport characterization 13 of these. Recently, we demonstrated that epitaxial Fe nanocuboids can be achieved when the deposition is stopped prior to coalesce of the nuclei in a lower-concentration electrolyte with reduced deposition rate.…”