The role of planar symmetry and scattering-enhanced tunneling in resonant transport

Although there has been active research on resonant transport in planar semiconductor structures for more than 30 years, there still is no general framework for understanding when resonant transport will dominate a structure. Here we present the development of such a framework. Our technique is based on capacitance-voltage spectroscopy, and it allows us to directly determine whether the transport is dominated by resonant, momentum-conserved tunneling or scattering-enhanced tunneling. We measure the timeconstant associated with tunneling in and out of a given 2D system, and we relate this timeconstant to the quasi-bound state lifetime. The key feature of our method is the ability to measure the lifetime while varying the energy of the quasi-bound state. This is achieved by applying a DC bias to the sample and varying the carrier concentration of the two-dimensional electron gas. The response of the lifetime to changes in DC bias indicates which transport mechanism dominates in a given device. By measuring the transport characteristics of several different sample structures, we are able to determine which structures are more or less sensitive to disrupted planar symmetry and scattering-enhanced tunneling. Within certain sample structures, the dominant transport mechanism can switch from resonant tunneling to scattering-enhanced tunneling. One way to cause this change is by varying the carrier concentration within the two-dimensional electron gas. A less obvious way is to apply a magnetic field perpendicular to the layers. Increasing the magnetic field sweeps the chemical potential alternately through the cyclotron orbitals of Landau Levels and the edge states that surround defects. As the chemical potential crosses a Landau Level, the quasi-bound state lifetime jumps by nearly two orders of magnitude. This, we suggest, is a planar-tunneling analogue of the Integer Quantum Hall effect. After identifying the relevant sample parameters, a simple scaling allows us to map out a diagram of transport regimes dominated by resonant or scattering-enhanced tunneling. This provides a simple framework to predict which mechanism will dominate in a given sample and which sample structures provide the most robust resonant-tunneling dominated transport.