Browsing by Author "Oral, Ahmet"

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Open Access
Construction of a scanning tunneling microscope and first results
(1990) Oral, Ahmet
In this thesis, construction of a Scanning Tunneling Microscope in air is explained. A step motor sample approach mechanism and a tripod scanner are used in the construction. Atomic resolution images of graphite samples are obtained in both constant current and constant height modes. Loss of trigonal symmetry in some Graphite images are also observed. This anomaly is attributed to the multiple atom tip or slipped top layer of Graphite.
Open Access
Investigation of Si1-xGex alloy formation by using STM
(1994) Oral, Ahmet
Open Access
Linear measurements of nanomechanical phenomena using small-amplitude AFM
(Materials Research Society, 2004) Hoffmann, P. M.; Patil, S.; Matei, G.; Tanülkü, A.; Grimble, R.; Özer, Ö.; Jeffery, S.; Oral, Ahmet; Pethica, J.
Dynamic Atomic Force Microscopy (AFM) is typically performed at amplitudes that are quite large compared to the measured interaction range. This complicates the data interpretation as measurements become highly non-linear. A new dynamic AFM technique in which ultra-small amplitudes are used (as low as 0.15 Angstrom) is able to linearize measurements of nanomechanical phenomena in ultra-high vacuum (UHV) and in liquids. Using this new technique we have measured single atom bonding, atomic-scale dissipation and molecular ordering in liquid layers, including water.
Open Access
Nano and micro Hall-effect sensors for room-temperature scanning hall probe microscopy
(Elsevier, 2004) Sandhu, A.; Okamoto, A.; Shibasaki, I.; Oral, Ahmet
GaAs/AlGaAs two-dimensional electron gas (GaAs-2DEG) Hall probes are impractical for sub-micron room-temperature scanning Hall microscopy (RT-SHPM), due to surface depletion effects that limit the Hall driving current and magnetic sensitivity (Bmin). Nano and micro Hall-effect sensors were fabricated using Bi and InSb thin films and shown to be practical alternatives to GaAs-2DEG probes for high resolution RT-SHPM. The GaAs-2DEG and InSb probes were fabricated using photolithography and the Bi probes by optical and focused ion beam lithography. Surface depletion effects limited the minimum feature size of GaAs-2DEG probes to ∼1.5 μm2 with a maximum drive current Imax of ∼3 μA and Bmin∼0.2 G/Hz. The B min of 1.5 μm2 InSb Hall probes was 6×10 -3 G/Hz at Imax of 100 μA. Further, 200 nm×200 nm Bi probes yielded good RT-SHPM images of garnet films, with Imax and sensitivity of 40 μA and ∼0.80 G/Hz, respectively.
Open Access
Nanomechanics using an ultra-small amplitude AFM
(Cambridge University Press, 2001) Hoffmann, P. M.; Jeffery, S.; Oral, Ahmet; Grimble, R. A.; Özer, H. Özgür; Pethica, J. B.
A new type of AFM is presented which allows for direct measurements of nanomechanical properties in ultra-high vacuum and liquid environments. The AFM is also capable to atomic-scale imaging of force gradients. This is achieved by vibrating a stiff lever at very small amplitudes of less than 1 Å (peak-to-peak) at a sub-resonance amplitude. This linearizes the measurement and makes the interpretation of the data straight-forward. At the atomic scale, interaction force gradients are measured which are consistent with the observation of single atomic bonds. Also, atomic scale damping is observed which rapidly rises with the tip-sample separation. A mechanism is proposed to explain this damping in terms of atomic relaxation in the tip. We also present recent results in water where we were able to measure the mechanical response due to the molecular ordering of water close to an atomically flat surface.
Open Access
Room-temperature scanning Hall probe microscope (RT-SHPM) imaging of garnet films using new high-performance InSb sensors
(IEEE, 2002) Oral, Ahmet; Kaval, Murat; Dede, Münir; Masuda, H.; Okamoto, A.; Shibasaki, I.; Sandhu, A.
The room-temperature scanning Hall probe microscopy (RT-SHPM) imaging of garnet films using high-performance InSb sensors was discussed. The high-performance InSb micro-Hall sensors were fabricated by optical lithography. It was found that the room-temperature noise figure of the InSb sensors was 6-10 mG/√Hz, which was an order of magnitude better than GaAs-AlGaAs two-dimensional electron gas sensors.
Open Access
RT-SHPM imaging of permalloy microstructures and garnet films using new high performance INSB sensors
(IEEE, 2002) Oral, Ahmet; Kaval, Murat; Dede, Münir; Sandhu, A.
The room temperature scanning Hall probe microscopy (RT-SHPM) imaging of permalloy microstructures and garnet films was discussed. The high performance InSb Hall sensors were used for this purpose. It was shown that the InSb Hall probes were highly sensitive and low noise alternatives to GaAs sensors for RT-SHPM.
Open Access
Scanning Hall probe microscopy (SHPM) using quartz crystal AFM feedback
(American Scientific Publishers, 2007) Dede, Münir; Ürkmen, Koray; Girişen, Ö.; Atabak, Mehrdad; Oral, Ahmet; Farrer, I.; Ritchie, D.
Scanning Hall Probe Microscopy (SHPM) is a quantitative and non-invasive technique for imaging localized surface magnetic field fluctuations such as ferromagnetic domains with high spatial and magnetic field resolution of ∼50 nm and 7 mG/Hz 1/2 at room temperature. In the SHPM technique, scanning tunneling microscope (STM) or atomic force microscope (AFM) feedback is used to keep the Hall sensor in close proximity of the sample surface. However, STM tracking SHPM requires conductive samples; therefore the insulating substrates have to be coated with a thin layer of gold. This constraint can be eliminated with the AFM feedback using sophisticated Hall probes that are integrated with AFM cantilevers. However it is very difficult to micro fabricate these sensors. In this work, we have eliminated the difficulty in the cantilever-Hall probe integration process, just by gluing a Hall Probe chip to a quartz crystal tuning fork force sensor. The Hall sensor chip is simply glued at the end of a 32.768 kHz or 100 kHz Quartz crystal, which is used as force sensor. An LT-SHPM system is used to scan the samples. The sensor assembly is dithered at the resonance frequency using a digital Phase Locked Loop circuit and frequency shifts are used for AFM tracking. SHPM electronics is modified to detect AFM topography and the frequency shift, along with the magnetic field image. Magnetic domains and topography of an Iron Garnet thin film crystal, NdFeB demagnetised magnet and hard disk samples are presented at room temperature. The performance is found to be comparable with the SHPM using STM feedback.
Open Access
Variable temperature Scanning Hall Probe Microscopy (SHPM) using quartz crystal AFM feedback
(IEEE, 2006) Dede, Münir; Ürkmen, Koray; Oral, Ahmet; Farrer, I.; Ritchie, D. A.
Scanning Hall Probe Microscopy (SHPM) is a quantitative and non-invasive technique for imaging localized surface magnetic field fluctuations such as ferromagnetic domains. In this work, we have eliminated the difficulty in the cantilever-Hall probe integration process, just by gluing a Hall Probe chip to a quartz crystal tuning fork force sensor. The resultant SHPM can operate in variable temperature environment, 77-300 K.
Open Access
Variable temperature-scanning Hall probe microscopy with GaN/AlGaN two-dimensional electron gas (2DEG) micro Hall sensors in 4.2-425 K range using novel quartz tuning fork AFM feedback
(IEEE, 2008) Akram, Rizwan; Dede, Münir; Oral, Ahmet
In this paper, we present the fabrication and variable temperature (VT) operation of Hall sensors, based on GaN/AlGaN heterostructure with a two-dimensional electron gas (2DEG) as an active layer, integrated with quartz tuning fork (QTF) in atomic force-guided (AFM) scanning Hall probe microscopy (SHPM). Physical strength and a wide bandgap of GaN/AlGaN heterostructure makes it a better choice to be used for SHPM at elevated temperatures, compared to other compound semiconductors (AlGaAs/GaAs and InSb), which are unstable due to their narrower bandgap and physical degradation at high temperatures. GaN/AlGaN micro Hall probes were produced using optical lithography and reactive ion etching. The active area, Hall coefficient, carrier concentration, and series resistance of the Hall sensors were ∼1 × 1 μm, 10 mΩ/G at 4.2 K, 6.3 × 10 12 cm -2 and 12 kΩ at room temperature and 7 mΩ/G, 8.9 × 10 12 cm -2 and 24 kΩ at 400 K, respectively. A novel method of AFM feedback using QTF has been adopted. This method provides an advantage over scanning tunneling-guided feedback, which limits the operation of SHPM the conductive samples and failure of feedback due to high leakage currents at high temperatures. Simultaneous scans of magnetic and topographic data at various pressures (from atmospheric pressure to high vacuum) from 4. to 425K will be presented for different samples to illustrate the capability of GaN/AlGaN Hall sensors in VT-SHPM.