2636 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 Interfacial Spin Filtering and Temperature Variation of Copper/GaMnAs Contact Resistance K. F. Eid , B. Paudel , N. Opondo , C. Otieno , G. Riley , X. Liu , and J. K. Furdyna Department of Physics, Miami University, Oxford, OH 45056 USA Department of Physics, University of Notre Dame, Notre Dame, IN 46556 USA We studied the temperature dependence of the specific contact resistance of the copper/GaMnAs interface in the range from 15 K to 290 K. This range includes the Curie temperature of the GaMnAs film of about 145 K. is typically as low as cm , and decreases slowly with decreasing temperature . However, as approaches Curie temperature jumps to about double its value. We suggest that this behavior arises from the suppression of one of the two spin conduction channels, resulting in substantial spin polarization. This might offer a convenient scheme to estimate the spin polarization at a single ferromagnetic/nonferromagnetic interface using all-electrical measurements. Index Terms—Contact resistance, magnetic semiconductors. T HE ferromagnetic semiconductor GaMnAs has been extensively investigated, largely because of its potential for studying a variety of spin-related device concepts [1]–[7]. The study of this material near its Curie temperature , which is typically between 50 and 150 K, provides an excellent op- portunity for observing the effects of ferromagnetic ordering on spin-related phenomena. GaMnAs is also a leading candidate for achieving efficient spin injection into nonferromagnetic semiconductors [8], [9]. A key parameter for spin injection is the contact resistance at the ferromagnetic/nonmagnetic materials interface [10], [11]. Since GaMnAs is a heavily doped p-type semiconductor, it has been used in Esaki-diode-type structures to inject and/or detect spin into n-doped GaAs, utilizing a -GaAs layer [12]–[14]. Here, it is useful to introduce the specific contact resistance , defined as the effective contact area times the contact resistance . It is needed to analyze simple all-electrical measurements on GaMnAs/metal interfaces to directly obtain information on spin polarization in the current transport process across the interface [15]–[17]. Yet, in GaMnAs/metal interfaces has not been studied thoroughly so far. In this paper, we will use the circular transmission line method (TLM) [18] to find of GaMnAs/copper for a wide range of temperatures above and below of GaMnAs. Nonalloyed el- emental copper will be used, because it gives a clean interface and its spin transport parameters are well studied [10]. Starting with 100 nm films of MBE-grown GaMnAs on semi-insulating GaAs substrates, we first anneal the samples in air at 180 C for 92 hour to improve their ferromagnetic properties by removing most of the interstitial Mn atoms from the crystal [19]. We then use standard photolithography to fabricate copper contacts on the GaMnAs film. Copper deposition is performed in a background pressure of about mBar after the native oxide film on GaMnAs has been stripped off by sulfuric acid. The copper con- tacts are in the form of circular discs with a 300 m or 200 m Manuscript received February 21, 2011; revised April 22, 2011; accepted April 30, 2011. Date of current version September 23, 2011. Corresponding author: K. F. Eid (e-mail: eidkf@muohio.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2153190 diameter separated from the surrounding copper area by a cir- cular gap of different widths varying from 8 m to 40 m, as shown in Fig. 1(a). Both D.C. and A.C. resistance measure- ments are then performed using a four-point technique, where two wires are connected to the inner copper disk and two wires to the outer copper structure using indium soldering. The elec- tric current flows radially in the gap from the inner copper disk to the outer copper terminal going through the GaMnAs. Due to this simple device geometry, the total resistance to current flow is given by modified Bessel functions, as discussed by Marlow and Das [18]. Fig. 1(b) displays the dependence of the current in the device on the applied D.C. voltage, showing a linear (i.e., Ohmic) be- havior without any rectifying properties. The doping density of GaMnAs (close to cm ) is so high that the depletion zone at the GaMnAs/copper interface is extremely thin ( nm), al- lowing for efficient carrier tunneling and leading to a low resis- tance linear (Ohmic) current-voltage behavior. Since the inter- face has no rectifying effects, a standard, low frequency lock-in technique with a low bias current A has been used to measure the resistance of the devices in zero applied magnetic field [14]. The resistance of four different samples is shown in Fig. 2(a) as a function of temperature, displaying the typ- ical temperature dependence characteristic of the resistivity of GaMnAs, with a peak corresponding to of about 145 K. Each curve was collected in a separate cooling, but cooling was done in an identical manner each time. It is seen from the figure that the peaks of all curves occur at the same temperature of 145 K, which is evidence that there are no significant differ- ences in measured sample temperature or temperature-related errors from run to run. When is much less than the sheet resistance of the semiconductor (and thus when , as will be shown below), the total resistance of a circular-contact device becomes [20], [21] where (1) 0018-9464/$26.00 © 2011 IEEE