Paper NS+EM-MoM6
Polarization Engineered 1-Dimensional Electron Gas

Monday, October 31, 2011, 10:00 am, Room 203

One-dimensional electron gas (nanowire) based devices are of great interest due to their promise in high-performance electronics and other future device applications. However, synthesis and patterning of arrays of nanowires is a challenge in all material systems since both bottom-up and top-down approaches have their own merits and demerits.

Here we report on the demonstration of pure 1-dimensional arrays of electrons with current density up to 130 mA/mm and carrier confinement greater than 100 meV using lateral polarization engineering in N-polar vicinal AlGaN/GaN heterostructures. The width of the atomic terraces characteristic of vicinal surfaces defines the dimensions of the nanowires which are found to exhibit sharp and clear signatures of 1-dimensionality at room temperature making them promising for novel device applications.

We report on devices fabricated on MOCVD grown N-polar AlGaN/GaN HEMT structures on vicinal sapphire substrate (4⁰ miscut towards a-plane) with anisotropy in current and channel pinch-off voltages. Channels parallel to the miscut direction pinched off at higher negative gate biases than those perpendicular to the steps and carried more charge as measured by direction-dependent C-V profiling. An electrostatic model which predicts a saw-tooth energy band profile in the lateral direction has been proposed to explain the charge anisotropy. Each atomic terrace characteristic of the surface morphology of vicinal GaN with its corresponding saw-tooth energy profile is proposed to exhibit quasi-1D confinement. We will discuss the heterostructure/polarization design of structures demonstrating pure 1-D transport in direction parallel to steps.

Gated structures were fabricated to investigate the physics of the system as the Fermi occupation function is varied by varying gate bias. To confirm that the carriers are indeed 1-dimensional, we used direction-dependent small-signal capacitance voltage measurements to probe the density of state function and hence dimensionality of electrons as a function of gate bias. We developed a 2-band model consisting of one 1-D and one 2-D subband to describe the behavior of these wires at room temperatures. The variation of capacitance as well as charge density for a pure 1-D and a pure 2-D system as a function of applied gate bias as predicted by our 2-band model based on density of states matches very well with the data measured experimentally for 1-D and 2DEG respectively. This confirms that the channels created are indeed 1-dimensional in nature. Since 1-D channels are atomic terrace defined, they are promising for eliminating the disadvantages of both bottom-up and top-down approaches.