Effect of invasive probes on measurement of magneto-transport in macroscopic samples: A gallium nitride case study

Abstract

Small changes in electrical conductance of quasi-2 dimensional samples often need to be measured at low temperatures, T∼1K and lower. Care needs to be taken to minimize self-heating due to the excitation current itself and the possibility of any unexpected source of variation of the electrochemical potential around the contacts need to be understood. It is not only the low resistance of the ohmic contacts but also their “noninvasive” character that must be ensured. While the importance of the “noninvasiveness” of the voltage probes is well appreciated for mesoscopic devices—it turns out that these considerations continue to remain important even in macroscopic samples that are a few millimeters in size. We have done low temperature measurements on different types of gallium nitride samples with contacts made of pure indium (popularly called indium dots), which have a superconducting transition and contacts made of Ti/Au, which remain nonsuperconducting. We show that there are qualitative differences in the measured four-terminal resistance in the two cases. Obviously, correct conclusions about phase relaxation times, localization lengths, etc., will require ensuring that there are no effects arising from the “invasiveness” of the probes.

Negative Magnetoresistivity in Highly Doped n-Type GaN

“…This scattering mechanism could also explain the observed decrease of the exponent p as the temperature decreases below 30 K. In this temperature range, the phasing coherence time τ φ could only be measured in the less doped samples, and according to paper [ 27 ] for 3D systems approaching a metal-insulator transition a weaker temperature dependency τ φ ∝ T −1 can be expected. A similar effect of decreasing of the exponential term with decreasing temperature was also observed for the GaN nanowall network [ 16 ].…”

“…In the work by Jastrzebski et al, which deals with GaN heavily doped with ferromagnetic dopants (Mn and Fe), only the existence of the nMR effect was signaled [ 15 ]. Attempts at a qualitative description of the phenomenon were made by Jain et al, who, however, did not describe bulk GaN, but a GaN nanowall network [ 16 ]. In the presented paper, we investigate the electrical transport properties of highly doped n-type GaN thick layers or bulk crystals with carrier Hall concentration n H ≥ 8 × 10 17 cm −3 .…”

Spin transport in polarization induced two-dimensional electron gas channel in c-GaN nano-wedges

“…The study further predicts remarkably high electron mobility in this channel, which arises due to the natural separation of the electrons (in the middle) from the ionized donors (at the boundaries) 21 . These predictions are also supported by the experimental findings of high conductivity 23 – 26 and long phase coherence length 24 , 27 , 28 for electrons in networks of c -oriented GaN nano-wedges. Since the 2D confinement takes place deep inside the structure, the symmetry between the conduction and valence band is intact over the entire potential profile.…”