Scanning vector Hall probe microscopy

Scanning vector Hall probe microscopy

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 272–276 (2004) 2141–2143 Scanning vector Hall probe microscopy a ! V. Cambela,*, D. Gre...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) 2141–2143

Scanning vector Hall probe microscopy a ! V. Cambela,*, D. Gregus$ova! a, J. Fedora, R. Kudela , S.J. Bendingb a

! Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravsk a! cesta 9, Bratislava 841 04, Slovakia b Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK

Abstract We have developed a scanning vector Hall probe microscope for mapping magnetic field vector over magnetic samples. The microscope is based on a micromachined Hall sensor and the cryostat with scanning system. The vector Hall sensor active area is B5  5 mm2. It is realized by patterning three Hall probes on the tilted faces of GaAs pyramids. Data from these ‘tilted’ Hall probes are used to reconstruct the full magnetic field vector. The scanning area of the microscope is 5  5 mm2, space resolution 2.5 mm, field resolution B1 mT Hz1/2 at temperatures 10–300 K. r 2003 Elsevier B.V. All rights reserved. PACS: 07.07.Df; 07.55.Ge Keywords: Magnetic microscope; Hall probe

Within the past decade several experiments have been performed that utilize highly sensitive Hall sensors in close proximity to magnetic samples in order to observe interesting physical phenomena. The experimental configurations include magnetic field mapping using Hall probe arrays [1], ballistic Hall micromagnetometry [2], and scanning Hall probe microscopy [3]. In these experiments the Hall probes are sensitive to the perpendicular component of the magnetic field only, so a part of the information is lost. Here we present a scanning vector Hall probe microscopy based on a new kind of vector Hall sensor with B5 mm spatial resolution. The vector Hall sensor is realized by patterning Hall probes on the tilted faces of pyramidal-shaped mesa structures following epitaxial overgrowth of the active layer, sensor definition and Ohmic contact preparation. Data from three of these ‘tilted’ Hall probes are used to reconstruct the full magnetic field vector. Vector Hall sensors of this type not only have much smaller active volumes and better spatial resolution than existing *Corresponding author. Tel.: +4212-54775820-2440; fax: +4212-54775816. E-mail address: [email protected], [email protected] (V. Cambel).

comparable sensors, but are also compatible with monolithic integration techniques. They may form the basis of magnetic micro-gradiometers or may be incorporated into complete vector scanning systems in their own right. Our GaAs vector Hall sensor technology consists of several non-standard fabrication steps applied sequentially on GaAs substrates; the formation of smooth high-mesa structures and mesa overgrowth with the active layer, followed by non-planar photolithography operations. It can be summarized as follows: (1) GaAs pyramid patterning: The formation of suitable high GaAs mesas was of critical importance and had to give us enough scope for subsequent complicated non-planar processing. For this purpose we have developed a method for the formation of B10 mm-high symmetric GaAs pyramidal structures suitable for further epitaxial overgrowth [4]. Ti-masked GaAs samples with embedded AlAs layers were etched with 1H3PO4:1H2O2:8H2O solution. The AlAs layer controls the lateral etching rate and influences the slope of GaAs mesa, which can be precisely tuned over the range 20 –60 by the H2O2 content in the solution as well as the AlAs layer thickness. (2) The overgrowth of the pyramids: The MOCVD epitaxial overgrowth of GaAs mesas was realized in the

0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.865

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AIX200 horizontal low-pressure IR-heated reactor. The best surface quality was achieved for a growth rate of 0.6 mm/h at 640 C and a V/III ratio of 383. The doping concentration of the 600 nm thick layer was 3  1017 cm3 and the 300 K electron mobility was B3000 cm2 V1 s1. (3) Sensor definition on the sidewalls of the mesa: This was realized using optical lithography with 4562 AZ positive-tone resist. After hard-baking the resist for 5 min at 120 C, the sensor structure with 5 mm linear dimensions was defined in 2% bromine in methanol at 26 C. (4) The Ohmic contact metallization: This was prepared by lift-off following the evaporation of the Au–Ge–Ni metallization at a pressure of 2  105 Pa. Before contact deposition the samples were treated in an oxygen plasma for 25 s and etched in a 1NH4OH : 10H2O solution. The metallization was finally annealed at 490 C for 3 min in a forming gas atmosphere. The completed sensor is shown in Fig. 1 where the GaAs pyramid with top common contact and three Hall sensors defined on the sidewalls is visible. The critical figures-of-merit for the vector Hall sensor are the sensitivity, linearity and resolution, defined by the properties of three individual probes in an external magnetic field. The sensitivities of the individual probes are S1 ¼ 3:17 mV/T, S2 ¼ 4:94 mV/T, S3 ¼ 3:73 mV/T for a bias current of 100 mA at 300 K and typical linearity error for all three probes is less then 1% for the magnetic field interval70.15 T. The magnetic field resolution of the sensor is B1 mT Hz1/2 at 10 kHz. The realized sensor lies at the heart of the developed scanning vector Hall probe microscope. It consists of x;y

Fig. 1. Vector Hall sensor realized on the sides of GaAs mesa.

stepper motors with 2.5 mm precision and a scan range B5  5 mm2. The z movement uses a piezoelectric positioner with precision o10 nm and range 25 mm, combined with a mechanical moving system (screw). During measurement the top of the pyramid serves as a tunnelling contact and helps to control the sensor— sample separation. The sample is placed in a helium cryostat with temperature control in the range 10–300 K. An external magnetic field is controlled electronically in the range 7200 mT. The time to capture one frame is B1 h for 256  256 pixels, which is similar to existing SQUID microscopes. The whole system is PC compatible and differs from most other existing systems in the achievable scan range at low temperatures. In contrast to most other similar systems we are able to scan an area B5  5 mm2 at 10 K with a spatial resolution of 2.5 mm. We have tested first the scanning vector Hall probe microscope at 300 K. Fig. 2 shows vector Hall sensor scans of the remanent magnetic field of two sharp ferromagnetic needles at a distance of B30 mm. Data from three probes are first collected (Fig. 2 left, B1; B2; B3—is the perpendicular component of the magnetic field at each of the probes). Using simple

Fig. 2. Magnetic field over two sharp magnetized needles. Left—field perpendicular to each of three Hall probes, right— calculated components of the magnetic field vector.

ARTICLE IN PRESS V. Cambel et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2141–2143

transformations the Cartesian components Bx ; By ; Bz (Fig. 2, right) are then computed from B1; B2; B3: This research is sponsored by the NATO’s Scientific Affairs Division within the framework of the Science for Peace Programme.

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