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Electron transport in semiconductors

What do I work on?

The movement of electrons in a material is called transport. In most materials, electrons move like balls in a pinball machine, colliding with the impurities present in the material and losing their momentum in the process. This gives rise to Ohm’s law and resistance of the material. This type of transport is called diffusive transport. 

Recently scientists have been able to fabricate materials which are “ultraclean”- almost no impurities. In absence of impurities, electrons have two options – either they can collide with the boundaries of the device or they can collide with each other. This can lead to novel transport regimes, namely- ballistic and hydrodynamic transport regime. In ballistic regime, electrons scatter predominantly against the boundaries of the device. In hydrodynamic regime, electrons start interacting with each other, leading to collective electron transport resembling that of water flowing in a pipe. 

Movie depicting evolution of electron flow in the ballistic regime.  The vortices form over time merely due to reflections of the device boundaries.

My research deals with understanding the conditions under which these non-diffusive regimes can occur. I do so by making tiny devices on the an ultraclean semiconductor material (GaAs/AlGaAs) which can map the movement of electrons. Interestingly, we have discovered that both the regimes share strikingly similar characteristics  despite one being interaction dominated while other being interaction free. We have detected negative nonlocal resistance in both the regimes – a signature usually considered the hallmark of hydrodynamic regime. Our simulations have revealed the formation of current vortices in both the regimes. The formation of vortices in ballistic regime is particularly surprising because vortices are usually associated with fluids like whirlpools forming in water.

Why is this research important?

The major advantage of ballistic and hydrodynamic regimes is that momentum of the system remains conserved while for the diffusive regimes, momentum is lost to the lattice. This makes these regimes inherently more efficient. These regimes thus have potential of creating new electronic devices are more efficient in terms of power, speed,  noise etc. than conventional silicon based electronic devices. Crucially, because the underlying physics in these regimes is fundamentally different than present devices and is largely unexplored, more research into these regimes can open up possibilities of engineering devices with unprecedented functionalities.  

How do I actually fabricate tiny devices to study?

The GaAs/AlGaAs heterostructure is grown using highly optimized MBE by our collaborators (Manfra group) at Purdue university. We then fabricate tiny devices on the material using a multistep cleanroom fabrication process which includes lithography techniques (photolithography and electron beam lithography), etching techniques (wet etching, dry etching), vapor deposition and ohmic contacting.  After fabrication, we perform low temperature transport measurements using low-frequency lock-in measurement technique. The experimental data is then analyzed using high-resolution supercomputer simulations. 

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