Transport Measurement of Ultra-dilute 2D-Holes in High Quality (100) GaAs HIGFETs

We have studied transport properties of high purity p-channel GaAs HIGFETs and observed a surprising temperature dependence of conductivity ($\sigma$) at ultra-low densities from $p=2\times10^{10}cm^{-2}$ to $p=8\times10^{8}cm^{-2}$. For $p>4\times10^{9}cm^{-2}$, as the temperature is decreased $\sigma$ first decreases linearly with T, then exhibits a clear upward bending which signifies a metallic behavior. The temperature at which the bending occurs shifts to a lower value as $p$ is decreased. However, the up-turning of $\sigma$ weakens as the density goes lower until, at $4\times10^{9}cm^{-2}$, it becomes almost independent of T between 80mK and 40mK. As $p$ is decreased from $4\times10^{9}cm^{-2}$ to $2\times10^{9}cm^{-2}$, $\sigma$ continues to show little temperature dependence and tend to saturate at some finite values below $80mK$. Meanwhile, the high temperature linear regions become almost parallel to each other. As $p$ is further decreased below $2\times10^{9}cm^{-2}$, the results are even more striking: the linear T-dependence persists even though the slope of the linear region starts to decrease. Remarkably, $\sigma$ maintains the weak T-dependence below $80mK$. At $8\times10^{8}cm^{-2}$, $\sigma$ retains a finite value of $0.045e^{2}/h$ at $40mK$. Consistent results were obtained through measuring three different samples. These observations contrast sharply with the metal-insulator-transition model which would predict a low-density insulating state, whereas we found no evidence of localization all the way down to the lowest density of $8\times10^{8}cm^{-2}$.