Magnetotransport in an aluminum thin film on a GaAs substrate grown by molecular beam epitaxy
© Liang et al; licensee Springer. 2011
Received: 7 August 2010
Accepted: 26 January 2011
Published: 26 January 2011
Magnetotransport measurements are performed on an aluminum thin film grown on a GaAs substrate. A crossover from electron- to hole-dominant transport can be inferred from both longitudinal resistivity and Hall resistivity with increasing the perpendicular magnetic field B. Also, phenomena of localization effects can be seen at low B. By analyzing the zero-field resistivity as a function of temperature T, we show the importance of surface scattering in such a nanoscale film.
Aluminum has found a wide variety of applications in heat sinks for electronic appliances such as transistors and central processing units, electrical transmission lines for power distribution, and so forth. As a result, it is highly desirable to prepare high-quality aluminum materials for practical device applications. In particular, the epitaxial growth of Al thin films on GaAs substrates has attracted much interest because of its relevance to the field of electronic interconnects [1, 2]. Fundamental limitations on the speed of interconnects are the various scattering processes [3, 4] occurring in low-dimensional systems. In order to fully utilize it in the integrated circuits consisting of GaAs-based high electron mobility transistors, investigations of the scattering mechanism on an Al thin film grown on a GaAs substrate are necessary.
One of the most important issues regarding the power dissipation and the speed of the device is the inelastic process such as electron-phonon scattering and electron-electron scattering. It is also important for the illustrations of quantum interference phenomena [5–12], one of which is weak localization [WL]. In the WL regime, phase-coherent loops formed by the paths of electrons undergoing multiple scattering events and the time-reversed ones lead to constructive interference at the original position of electrons at zero magnetic field under the assumption that the inelastic scattering time is much larger than the elastic one. However, phase coherence would be destroyed under a perpendicular B and lead to the negative magnetoresistance [NMR]. Positive magnetoresistance [PMR] can also be observed in the WL regime if the spin-orbit scattering [6, 8, 12] is strong enough.
Here, we review the temperature dependences of resistivity for various scattering mechanisms [13, 14] that are generally observed in bulk materials. At low temperatures, T (lower than the Debye temperature), electron-phonon scattering is usually the dominant one, which is expected to give a Bloch-Gruneisen T 5 contribution to the resistivity. However, for the materials with complex Fermi surfaces or are suffering from interband scattering, Umklapp process [13–15] should be taken into account, leading to the T 3 dependence instead. Umklapp process means that the crystal momentum is not conserved after an electron-phonon scattering event. A reciprocal lattice vector is added after this process, possibly leading to a large-angle scattering [15–17]. That is, the resistivity would not decrease as rapidly as T 5, which introduces an additional factor of T 2 for the low-angle phonon scattering at low T. Also, the T 2 term expected for electron-electron scattering may possibly appear at low T[13, 15], while at extremely high T (much larger than the Debye temperature), the resistivity follows AT , where A is a constant depending on the properties of the system.
It is well known that electronic transport is significantly affected by surface scattering [18–20], in addition to electron-electron scattering and electron-phonon scattering, as the thickness of a system is reduced to become comparable to the electron mean free path. There are several theories dealing with surface scattering.
As proposed by Olsen , neglecting the Umklapp process, low-angle scattering of electrons by phonons is important in a thin film where electrons are deflected by low-energy phonons to the surface [22, 23] more easily than that in the bulk sample. That is, surface scattering occurs frequently in a thin film. A more careful treatment for the size effects considering the surface conditions is proposed by Soffer . Here, we use Soffer's theory as the beginning of our analyses for the zero-field resistivity.
An Al thin film is investigated in our experiments especially for its special properties. With increasing B, a crossover from electron- to hole-dominant transport occurs as a result of its non-simple Fermi surface [25–28]. Also, it is a good material for the investigations of quantum phenomena in low-dimensional systems ascribed to its long inelastic scattering time .
Result and discussion
Here, it is worth mentioning that the electron-phonon impurity interference also leads to the T 2 contribution to the resistivity [32–34], which should be smaller than the residual resistivity. However, in our results, the difference between ρ(T = 78 K) and ρ(T = 30 K) is approximately equal to 0.059 Ω, which is larger than ρ(T = 4.8 K) = 0.025 Ω, taken as the residual resistivity, inconsistent with the requirement for the correction term. Also, there are several experimental results indicating that such a mechanism is not the dominant one for a relatively pure metal. Therefore, we can safely neglect the influence of the electron-phonon impurity interference in our Al thin film.
In conclusion, we have performed magnetotransport measurements on an aluminum thin film grown on a GaAs substrate. A crossover from electron- to hole-dominant transport can be inferred from both longitudinal resistivity and Hall resistivity with increasing B, characteristic of the complex Fermi surface of aluminum. The existence of positive magnetoresistance at T ≧ 20 K indicates that the spin-orbit scattering should be taken into account for the exact treatment of localization effects. The observed surface caused T 2 term for ρxx demonstrates that surface Umklapp scattering is important. With decreasing T, a tendency toward a T 3 dependence suggests that an Umklapp process occurring in the interior is more important than that occurring at the surface. Such a crossover is consistent with Olsen's argument for low-angle electron-phonon scattering qualitatively. All these experimental results show that the nature of the interface between the Al thin film and the GaAs substrate would significantly affect the electrical properties of such a nanoscale film.
The authors declare that they have no competing interests. This work was funded by the NSC, Taiwan.
STL and CC performed the low-temperature experiments on the Al film and drafted the manuscript. KYC and MRY performed the low-temperature experiments on the Al film. SDL and CTL conceived of the study. JYW fabricated the Al samples. SWL prepared the Al samples and performed the AFM and X-Ray measurements. All authors read and approved the final manuscript.
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