PHYSICAL LIMITATIONS IN THE MATERIAL GROWTH PROCESS THAT AFFECT MAGNETO TRANSPORT PROPERTIES IN MULTILAYER MATERIALS

Magneto transport properties are of significant importance and provide one of the direct methods in the study of semiconductor physics and devices (1977). Numerous physical properties of semiconductor materials can be explored by investigating the longitudinal current, the temperature and the magnetic field dependence of the magnetic resistance voltage Vρ(B, T, I) and the Hall voltage VH (B, T, I) (Chu & Sher, 2008). This work reports on the physical limitations that affect the magneto transport properties in the material growth process. A few of these limitations are briefly described in the rest of the report.
Mixed-Conduction: Mixed-conduction effects have been considered to almost always have an influence on the magneto transport properties of narrow bandgap infrared (IR) semiconducting materials (Antoszewski et al., 1995). In case where multiple carrier species are present, mixed-conduction effects become a limiting factor and have a strong influence on the magneto transport properties of not only multilayer materials, but also widely on other semiconductor materials, including bulk samples, thin films, and quantum wells (Kasap & Acar, 2004). Fig.1 shows Mobility spectrum using QMSA at 80 and 110K, and Beck and Anderson envelope at 11OK and experimental points and calculated curves of the conductivity tensor components. A great deal of information can be extracted if the magneto transport experiments are performed as a function of magnetic field. This is because the data can then be easily de-convolved to obtain densities and mobilities for each carrier species present in the sample (Antoszewski et al., 1995).
Impurity levels: In a study conducted by (Kasap & Acar, 2004) resistivity, magnetoresistivity and Hall-effect measurements were carried out as a function of temperature (14-350 K) and magnetic field (0-1.35 T) in n-type Te-doped InSb grown by the LEC technique. The measurement analyses were carried out using the conventional and quantitative mobility spectrum methods. It was found that the impurity levels arising from LEC technique and intentionally dopants can become a limiting factor for the magneto transport properties. An impurity level with the energy E1 ≈ 3 meV has been observed from the low temperature zero-field resistivity analysis. As the temperature decreases, the temperature dependence of magnetoresistivity coefficient greatly exhibits an increase with representing two minima. It was shown (see Fig.2) from the quantitative mobility spectrum analysis that at the temperatures where two minima are occur, the existence of extra carriers are observed which are caused by impurities.
Spin-flip scattering effects: In various approaches to giant magneto resistance (MR) in magnetic multilayers and magnetic granular composites, generally only the collinear magnetization configuration is considered. As such, all ferromagnetic (FM) layers or ferromagnetic granules are assumed to have only two possible magnetization directions, parallel or anti parallel to a fixed spin quantization axis. Gu et al (1998) proposed an extended Boltzman equation approach, with non diagonal components of the electron distribution function, in order to study the spin-flip effect on the magneto resistance (MR). It was found that the presence of spin-flip scattering reduces the MR which leads to decreased deviation of the MR from linear dependence of sin2(θ/2) where θ represents the angle between magnetization of successive magnetic films. Fig.3 presents the angular dependence of MR, in the presence of spin-flip scattering, for a super-lattice composed of FM and nonmagnetic (NM) layers. The spin-flip scattering is shown to take place only in the intervals with λsf (I) = 2dI (solid line), 7dI (dashed line) and 50 dI (dotted line). The authors have argued with the help of results that if the spin diffusion length is comparable with the thickness of layer, the spin-flip scattering plays a vital role in the magneto transport in magnetic multilayer.
Growth-Temperature dependence: In another study by Song et al (2005) a synthesis of MnAs/GaAs hybrid multilayer structures is presented. The authors have reported on the growth temperature dependence of the magnetic and magneto transport properties. A Quantum Design SQUID magnetometer was used to measure the magnetic properties and the magneto transport measurements were taken on cryostat Hall-bar patterned samples. The epitaxial MnAs/GaAs multilayers were fabricated in their experiments by using molecular beam epitaxy, with a total thickness of 300nm. They kept the periodicity of the samples fixed at 5nm/5nm. All studied samples were reported to have exhibited ferromagnetism above room temperature, as shown in Fig.4. Song et al (2005) have argued that the magnetic anisotropy and magneto transport properties are strongly dependent on the growth temperature. There results show that it is possible to manipulate the magnetic and magneto transport properties by changing the growth temperature.
Magnetic Structure and resistor values: Yao et al. (2007) have reported on magneto transport properties by using Monte Carlo simulations and resistor-network method. They constructed an anisotropic layered structure based on Ising model as an approach to make realistic bi-layered manganites. There results show that the anisotropy of the transport behaviours is strong. The authors have argued that not only the magnetic anisotropy resulting from anisotropic interactions contributes to the transport anisotropy, but also the value of resistors in the resistor-network can affect the magneto transport behaviour of the system. In Fig.5 the light grey bars indicate metallic resistor of ρM(T), the deep grey ones denote insulating resistors type-1 of ρ11(T), and the black ones represents insulating resistors type-2 of ρI2(T). The resistors in the line-groups and between the line groups play different roles for the magneto transport behaviour of the system.
Other Physical Limitation: Feng (2006) has reported that numerous important physical parameters for AlGaN/GaN 2DEG structures can be extracted from low temperature magneto transport measurements. These physical parameters include carrier concentration, scattering times, and effective mass. The magneto transport lifetime has been reported to be mainly limited by short-range potentials such as the alloy disorder potential and ionized centres in the multilayer materials. The author has also shown that other factors such as surface morphology and scattering by roughness potential with a large correlation length (grain size>100nm) is predominantly small angle scattering with a little influence on the magneto transport lifetime.