Lateral tunneling, ballistic transport, and spectroscopy in a two-dimensional electron gas

Abstract

We report a direct observation, via electron energy spectroscopy, of lateral tunneling and lateral ballistic electron transport in a two-dimensional electron gas (2D EG). This was accomplished through the use of a novel transistor structure employing two potential barriers, induced by 50-nm-wide metal gates deposited on a GaAs/AlGaAs selectively doped heterostructure. Hot electrons with very narrow energy distributions ( -5 meV wide) have been observed to ballistically traverse 2D EG regions ==170 nm wide with a mean free path of about 480 nm. PACS numbers: 73.40.Gk, 73.20.Dx Ballistic transport of hot electrons was established recently in n "'"-type GaAs by the use of energy spectroscopy in a hot-electron structure. l These experiments utilized an injector at one end of a transport region and a spectrometer at the other end, with the electrons moving normal to the plane of the layers (vertical transport). This technique proved to be very powerful since it permitted the energy distribution and the mean free path of the ballistic electrons to be determined. The very recent demonstration of quantized resistance in a confined quasi-two-dimensional electron gas (2D EG) 2,3 strongly suggests ballistic transport of electrons near equilibrium parallel to the interface between the layers (lateral transport). We report here the first utilization of an energy spectroscopy technique to establish directly lateral tunneling through an induced potential barrier and the existence of lateral ballistic transport in a 2D EG. This was done by inducing two closely spaced potential barriers in the 2D EG via two narrow Schottky metal gates deposited on the surface of the structure. One barrier was employed as a tunnel injector and the second as a spectrometer. We have measured narrow hot-electron distributions ballistically traversing lateral 2D EG regions 170 nm wide, and have estimated their mean free path.Several structures were made on a selectively doped GaAs/AlGaAs heterostructure grown by molecularbeam epitaxy. On top of an undoped GaAs buffer layer, an undoped AlGaAs layer (50 nm thick, AlAs mole fraction x=34%) was grown, followed by a thin heavily doped GaAs cap layer (15 nm thick). A sheet of Si atoms, with an areal density of -2x 10 12 cm ~2, was deposited under overpressure of As when growth of the AlGaAs was interrupted (planar doping), 30 nm away from the GaAs buffer layer; these supply the electrons in the 2D EG [ Fig. 1(a)]. The 2D EG had a carrier density of 3x10 ll cm -2 and a mobility of 3xl0 5 cm 2 /Vsec at 4.2 K. Two parallel AuPd gates, each 52 nm wide and 0.5 jam long, were patterned 93 nm apart using electron-beam nanolithography, on a 5-^m-wide isolated 2D EG line [ Fig. 1(b)]. Ohmic contacts were made to the three regions defined by both gates. Biasing the gates negatively with respect to the central region between them (called the base) depleted the 2D EG underneath the gates and prevented the free motion of the equilibrium electrons among the three regions [emitter (E), base (B), and collector (C)]. The...

Coulomb scattering lifetime of a two-dimensional electron gas

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A 2015 perspective on the nature of the steady-state and transient electron transport within the wurtzite phases of gallium nitride, aluminum nitride, indium nitride, and zinc oxide: a critical and retrospective review

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